**Pulsed Laser Deposition of Aluminum Nitride Films: Correlation between Mechanical, Optical, and Structural Properties**

#### **Lilyana Kolaklieva 1, Vasiliy Chitanov 1, Anna Szekeres 2, Krassimira Antonova 2, Penka Terziyska 2, Zsolt Fogarassy 3, Peter Petrik 3, Ion N. Mihailescu <sup>4</sup> and Liviu Duta 4,\***


Received: 9 February 2019; Accepted: 13 March 2019; Published: 17 March 2019

**Abstract:** Aluminum nitride (AlN) films were synthesized onto Si(100) substrates by pulsed laser deposition (PLD) in vacuum or nitrogen, at 0.1, 1, 5, or 10 Pa, and substrate temperatures ranging from RT to 800 ◦C. The laser parameters were set at: incident laser fluence of 3–10 J/cm2 and laser pulse repetition frequency of 3, 10, or 40 Hz, respectively. The films' hardness was investigated by depth-sensing nanoindentation. The optical properties were studied by FTIR spectroscopy and UV-near IR ellipsometry. Hardness values within the range of 22–30 GPa and Young's modulus values of 230–280 GPa have been inferred. These values were determined by the AlN film structure that consisted of nanocrystallite grains, strongly dependent on the deposition parameters. The values of optical constants, superior to amorphous AlN, support the presence of crystallites in the amorphous film matrix. They were visualized by TEM and evidenced by FTIR spectroscopy. The characteristic Reststrahlen band of the *h*-AlN lattice with component lines arising from IR active phonon vibrational modes in AlN nanocrystallites was well detectable within the spectral range of 950–500 cm−1. Control X-ray diffraction and atomic force microscopy data were introduced and discussed. All measurements delivered congruent results and have clearly shown a correlation between the films' structure and the mechanical and optical properties dependent on the experimental conditions.

**Keywords:** aluminum nitride; pulsed laser deposition; nanoindentation testing; TEM imaging; FTIR spectroscopy; ellipsometry; complex refractive index

#### **1. Introduction**

Pulsed laser-assisted coatings represent a clean and fast route applied for surface modification and controlled micro-structuring of a wide range of materials. When compared to other physical vapor deposition methods, i.e., thermal evaporation or sputtering, pulsed laser deposition (PLD) stands out as a simple, versatile, rapid, and cost-effective method, which can enable precise control of thickness and morphology for the fabrication of high-quality thin films [1,2]. Amorphous or crystalline, extremely adherent, stoichiometric, dense, or porous structures from various complex materials can be synthesized, even at relatively low deposition temperatures, by simply varying the experimental parameters, mainly related to the (*i*) laser (fluence, wavelength, pulse-duration, and repetition rate) and (*ii*) deposition conditions (target-to-substrate distance, substrate temperature, nature, and pressure of the environment) [2–4].

Thin though hard coatings have proven invaluable for the production of mechanical parts or tools due to their hardness and wear-resistance characteristics [5,6]. In this respect, for the last couple of years, a great interest in using nitride-based films as protective coatings, due to their physical, chemical, electronic, thermal, or mechanical properties, has been reported [7–10]. In particular, aluminum nitride (AlN) coatings possess such characteristics, which make them suitable candidates for a wide range of applications, including insulating and buffer layers, photodetectors, light-emitting diodes, laser diodes, acoustic devices, or designs of self-sustainable opto- and micro-electronical devices [11–16]. Hard protective AlN coatings in multi-layered systems as AlN/TiN and CrN/AlN were intensively studied for tribological applications [8,9,17,18]. AlN is also commonly used in piezoelectric thin films [19,20], for the fabrication of micro-electro-mechanical system (MEMS) devices [21].

Depending on the deposition techniques and technological protocols, the AlN film structure can vary from fully-amorphous to nanocrystalline, with a tendency to decrease the volume fraction of grain boundaries [22–25]. This may significantly modify the physical, chemical, and mechanical properties of films with nano-sized crystalline structure in comparison to polycrystalline materials, which have grain sizes usually in the range of 100–300 μm [26]. Highly *c*-axis-oriented AlN films exhibit a large piezoelectric coefficient and are attractive for electroacoustic devices via surface acoustic waves [12,13]. Therefore, the fabrication of hard coatings based on properly-oriented nanocrystalline AlN layers requires a good understanding of their microstructure as a function of deposition conditions. However, obtaining AlN films with a definite structure and crystalline quality still remains a challenge for most deposition techniques. The PLD method has the main advantage of ensuring the growth of thin AlN films with good crystallinity and stoichiometry at relatively low temperatures [27]. Furthermore, PLD for AlN film synthesis proved to be one of the methods resulting in superior mechanical properties of the material [28]. There is still no straightforward theoretical or experimental model of the processes during deposition and the resulting film properties. Hence, the characterization of film growth and the mechanisms governing the film synthesis are important tasks in all application areas of AlN films.

Thin AlN films synthesis by the PLD technique is also the subject of our research. We focused during the years on the influence of the technological parameters, such as the assisting nitrogen gas pressure, incident laser fluence, repetition rate of laser pulses, substrate temperature, and the presence of an additional matching sub-layer, on the physical properties of AlN films synthesized by PLD [23,29–36] onto Si(100) substrates. Physical properties, such as surface roughness, microstructure, composition, amorphous-to-polycrystalline phase ratio, and optical constants appropriate for various applications, have been systematically studied. A systematization of the experimental results and finding the correlation between the structure and properties of the PLD AlN films and their preparation conditions would allow for the optimization of the deposition process in order to fabricate AlN films with the desired quality.

We resume with this paper the investigations with special attention to new, previously-unstudied phenomena, in the trial to better understand the quite complicated physical and chemical PLD process. Thus, by depth-sensing nanoindentation, the mechanical properties of the PLD AlN films, fabricated at substrate temperatures ranging from room temperature (RT) up to 800 ◦C and, varying other deposition parameters such as ambient environment, gas pressure, laser incident fluence, and laser pulse frequency (LPF), were studied. Complementary results obtained by transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, and UV-near IR ellipsometry are also reported, with the aim of finding the relationship between the structural properties of films and their mechanical properties.

#### **2. Experimental Details**

#### *2.1. AlN Film Preparation*

AlN films were synthesized onto Si(100) substrates by laser ablation of a polycrystalline stoichiometric AlN target using a pulsed KrF\* excimer laser source COMPex Pro205 (Coherent, Göttingen, Germany, λ = 248 nm, νFWHM ≤ 25 ns). The laser beam was oriented at 45◦ with respect to the target surface. The laser pulse energy was ~360 mJ, and the incident laser fluence was set at ~3, 4, 4.8, or 10 J/cm2, respectively. The separation distance between the target and Si substrate was 5 cm. The PLD process was performed in vacuum (~10−<sup>4</sup> Pa) or at different N2 gas pressures of 0.1, 1, 5, or 10 Pa, respectively. Before each experiment, the irradiation chamber was evacuated down to a residual pressure of ~10−<sup>5</sup> Pa.

Prior to deposition, the Si substrates were cleaned in diluted (5%) hydrogen fluoride solution in order to eliminate the native oxide layer. The target was cleaned by baking at 800 ◦C for 1 h in a vacuum followed by a short multipulse laser ablation with 1000 pulses. A shutter was interposed in this case between the target and substrate to collect the expelled impurities.

During deposition, the target was continuously rotated with 0.4 Hz and translated along two orthogonal axes to avoid piercing and allow for the growth of uniform thin films. The substrate was heated either at 800, 450, 400, and 350 ◦C or was maintained at RT. The chosen temperature was kept constant with the help of a PID-EXCEL temperature controller (Excel Instruments, Gujarat, India).

For the deposition of one thin film, 15,000, 20,000, or 25,000 consecutive laser pulses were applied, with a corresponding LPF of 40, 10, or 3 Hz, respectively.

#### *2.2. Nanoindentation Testing*

The mechanical properties of the synthesized AlN films were investigated by a depth-sensing indentation method using Compact Platform CPX-MHT/NHT equipment (CSM Instruments/Anton-Paar, Peseux, Switzerland). Nanoindentation was performed with a triangular diamond Berkovich pyramid having a facet angle of 65.3◦ ± 0.3◦ (CSM-Instruments SA certificate B-N 41), in the loading interval starting from 5–100 mN. The nanohardness and elastic modulus were determined from the load/displacement curves applying the Oliver and Phar method [37].

#### *2.3. Transmission Electron Microscopic Measurements*

The structure of the PLD AlN films was investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) with a Philips CM-20 (Amsterdam, The Netherlands) operated at a 200-kV accelerating voltage and a JEOL 3010 (Tokyo, Japan) operated at a 300-kV accelerating voltage. The cross-sectional TEM samples were prepared by ion beam milling.

#### *2.4. Optical Measurements*

The influence of the deposition conditions on the films' complex refractive index (*ñ* = *n* − *jk*, where *n* is the refractive index and *k* is the extinction coefficient) was studied by spectroscopic ellipsometry (SE) measurements on an M1000D ellipsometer from J.A. Woollam Co., Inc. (Lincoln, NE, USA) working in the spectral range of 193–1000 nm. In the SE data analysis, the Complete EASE J.A. Woollam Co., Inc. software (version 5.08) was used [38]. The experimental SE spectra were taken at RT and different angles of light incidence of 60◦, 65◦, and 70◦. In data simulation, a two-layer optical model (substrate–1st layer (film bulk)–2nd layer (surface roughness)) was applied. In the spectral range of 400–1000 nm, the data were fitted by the Cauchy model to obtain the films' thickness values. The ellipsometric data were fitted by a Tauc–Lorentz general oscillator model. The surface roughness layer was modeled as a mixture of 50% material (film) and 50% voids (air) and was calculated by applying Bruggeman's effective medium approximation theory.

FTIR reflectance spectra were obtained in a linearly-polarized incidence beam by using a Bruker Vertex 70 instrument (Billerica, MA, USA) equipped with a reflectance accessory A513/Q. Both *s* and *p* irradiation polarizations were exploited at an incident angle of 70◦. In this geometry, it is more correct to consider the orientation of *E* with respect to the normal to the film surface *z* instead of the optical nanocrystalline axis *c*, which could be oriented in a certain direction with a probability depending on the deposition conditions. Furthermore, it should be underlined that during every measurement, the components of the electric vector *E* oriented along the *x*, *y*, and *z* directions were presented with different weights at different temperatures. Thus, all electric field components contributed to the phonon-polariton modes in randomly-oriented AlN nanocrystallites. The spectral resolution was 2 cm<sup>−</sup>1, and the total number of scans per each measurement was 64.

#### **3. Results and Discussion**

#### *3.1. Nanoindentation Testing*

For all AlN films, the measured load-penetration depth curves with maximum indentation loads were smooth, with no discontinuities. The smooth loading nature testifies to the good film uniformity and adherence to the Si substrate. Even for the highest displacement load of 100 mN, when the indentation depth was close to the film thickness, there were no signs of cracking or peeling, which demonstrates the good interface quality. In Figure 1, a typical load versus indenter displacement curve is presented, corresponding to a test performed on an AlN film deposited at 800 ◦C, in 0.1 Pa N2 pressure and at a LPF of 40 Hz. The main parameters used for the analysis are marked on the graph. *F*<sup>m</sup> is the peak load corresponding to a maximum nominal penetration depth *h*max, which depends on the hardness and, consequently, on the film structure. The stiffness *S* results from the slope of the tangent to the unloading curve. The measured depth *h* verifies the relation *h* = *h*<sup>s</sup> + *h*c/ε, where *h*<sup>s</sup> is the displacement of the surface at the perimeter of the contact, *h*c is the vertical distance along which the contact is made, and ε is an indenter constant.

**Figure 1.** Typical load-displacement curve at the maximum load of 15 mN in the case of an AlN film deposited at 800 ◦C, in 0.1 Pa N2 pressure and with a laser pulse frequency of 40 Hz.

The area between the loading and unloading curves defines the plastic deformation work *W*p, while that between the unloading curve and perpendicular to the maximum penetration depth, *hmax*, is a measure of the elastic deformation work, *W*e. The ratio *W*e/(*W*<sup>e</sup> + *W*p) defines the elastic recovery of the coating after indentation and is associated with the coating ability to go back after deformation. For the studied films, this ratio varied from 49–67%, depending on the deposition conditions. This implies a very good coating recovery after mechanical deformation.

The measured load and displacement curves were analyzed, and the nanohardness *H* and elastic modulus *E* were evaluated [37]. The hardness was estimated from the relation *H* = *F*max/*A*, where *A* is the projected contact area of the indentation. By fitting the unloading curve, i.e., the stiffness *S* = d*F*/d*h*, the projected area *A* can be determined. The Young's modulus *E* is determined from the relation (1 − <sup>ν</sup>2)/*<sup>E</sup>* = (2*A*1/2/*S*π1/2) − (1−ν*<sup>i</sup>* 2)/*Ei* [37], where *Ei* and ν*<sup>i</sup>* are the elastic modulus and Poisson's ratio of the indenter (ν was assumed equal to 0.22).

The dependence of the nanohardness and elastic modulus on the indentation depth corresponding to the applied load in the interval from 5–100 mN is presented in Figure 2. Below the loading value of 5 mN, the hardness determination with sufficient accuracy is limited by the surface roughness [39]. Our PLD AlN films exhibited a considerably smooth and uniform surface morphology, with a root mean squared roughness in the range of 0.24–2.5 nm, depending on the deposition conditions [35,36,40].

**Figure 2.** Nanohardness *H* and elastic modulus *E* as a function of the maximum nominal penetration depth, *h*max, of the PLD AlN films obtained using the deposition conditions given in the inset.

The variation of hardness with the indenter penetration depth points to a region of Δ*h*max ≈ (150–200) nm, corresponding to 10–15 mN loading, where the *H* values of the films could be recorded with the weak influence of the Si substrate on the test measurements. With the further increase of the applied load, i.e., the increase of the maximum penetration depth, the nanohardness value dropped rapidly below 20 GPa, followed by a smooth decrease to values that approached the Si substrate hardness of ≈15 GPa. The latter implies an increasing influence of the substrate [41]. Taking the observed dependence into account, the further presented results correspond to the load of 15 mN, for which the influence of the Si substrate on the *H* values was similar. The observed variation of *H* values with substrate temperature can be assigned to a change in the microstructure of films. Elevated temperature facilitates the crystallization process, and thus, a less defective structure with a larger amount and size of *h*-AlN crystallite grain boundaries was growing, characterized by higher nanohardness values.

Our recent investigations on PLD AlN films have established that the variation of the nitrogen pressure, on one hand [23,30,33], and LPF, on the other [24,31,36], had the strongest influence on the formation of the AlN microstructure. The effect of laser incident fluence can be compensated by the variation of those two parameters. Correspondingly, in Figure 3, the *H* values are represented as a function of LPF (Figure 3a) and N2 pressure (Figure 3b) at other PLD parameters given in the insets. The AlN films were deposited under different conditions as either the N2 pressure was kept constant at 0.1 Pa while varying the laser fluence, LPF, and substrate temperature (Figure 3a) or the substrate temperature was kept at 800 ◦C (Figure 3b) while varying the nitrogen pressure, laser fluence, and LPF. The observed hardness behavior is closely related to the processes of film growth and the resulting film microstructure, which yielded variation in the film hardness values. Nevertheless, all *H* values were within 22–30 GPa range, superior to the ones registered in the case of films obtained by other deposition techniques [22,42–47].

**Figure 3.** Nanohardness as a function of laser pulse repetition frequency (**a**) and N2 gas pressure (**b**) for PLD AlN films deposited with variation of other PLD parameters (as given in the insets).

In general, a higher deposition temperature enhances the reaction at the surface of the substrate and promotes the formation of crystallites in the growing film [48]. As a result, AlN films deposited at 800 ◦C possessed higher hardness values (Figure 3a). When the deposition was performed at low N2 pressure, a high laser fluence of 10 J/cm2, and a low LPF of 3 Hz, the species evaporated from the polycrystalline AlN target acquired a much higher kinetic energy. This excess energy was transferred to adatoms when reaching the surface of the growing film, obstructing the ordering in a crystalline network. AlN films formed in these conditions were amorphous, as previously revealed by our TEM and XRD studies [23].

With increasing the LPF from 3 to 10 and 40 Hz, the multiple, consecutive vaporization "cleaned up" the space between the target and substrate. Consequently, the atoms ejected from the target had much more energy when reaching the substrate, contributing to the boost of the mobility of adatoms and surface diffusion. As one can observe in Figure 3a, the forming microstructure could be however more defective with lowered microhardness. When increasing the nitrogen pressure (Figure 3b), the particles ejected from the target in the plasma plume lost their energy in collisions with nitrogen particles. Accordingly, they could not significantly contribute to the thermally-induced mobility promoted by heating the substrate, but bound to their impinging sites without further surface migration. As a result, the formed film structure was less crystalline and more defective, which was reflected in the lower hardness values (Figure 3b).

As known [49,50], the hardness and elastic modulus are important material parameters that indicate the resistance to elastic/plastic deformation and could be used for the estimation of the coating wear behavior. The *H*/*E* ratio characterizes the elastic strain to failure resistance, while the *H*3/*E*<sup>2</sup> ratio evaluates the coating resistance to plastic deformation at sliding contact load. Both ratios are associated with the coating toughness, a key parameter for the evaluation of the tribological properties of materials [50]. Hence, the improvement of the tribological behavior can be achieved by increasing the coating hardness and decreasing the elastic modulus. In Figure 4, the resistance for elastic strain to failure (*H*/*E*) and to plastic deformation (*H*3/*E*2) of AlN films versus deposition temperature is presented.

From the dependence of these ranking parameters, one can state that the studied PLD AlN films had a very high *H3/E2* ratio compared to other AlN coatings [43,44].

**Figure 4.** Resistance for elastic strain to failure (*H*/*E*) and plastic deformation (*H*3/*E*2) of the PLD AlN films as a function of substrate temperature during deposition.

#### *3.2. TEM Observations*

Four types of significantly different AlN structures were revealed in previous TEM studies of PLD films [23,30,36]. Amorphous AlN layers are mostly forming at RT or in a growth environment where the mobility of the atoms after reaching the substrate surface is limited. When increasing the temperature, nano-sized crystalline grains in an amorphous matrix emerged. This case is well visible in Figure 5a, where the HR-TEM image of the AlN film, deposited at 450 ◦C, 0.1 Pa N2 pressure, LPF of 40 Hz, and incident laser fluence of 3 J/cm2, revealed hexagonal nanocrystallites surrounding with amorphous AlN. The reduced crystallinity was due to the relatively low substrate temperature of 450 ◦C. Here, AlN crystallites were hexagonal (*h*-AlN), but the metastable cubic (*c*-AlN) phase can also grow in the amorphous matrix [23,33,36]. The hardness of such AlN films may vary significantly due to the variation in the thickness of the amorphous matrix between the crystalline particles [51] and/or voids possibly incorporated into the layer, which may significantly reduce the film's hardness.

**Figure 5.** HR-TEM image of nano-sized crystalline grains in amorphous matrix (**a**) and bright field cross-sectional TEM image (**b**) of the PLD AlN films deposited at 450 and 800 ◦C, respectively. The other PLD parameters were identical: N2 pressure of 0.1 Pa, LPF of 40 Hz, and laser fluence of <sup>≈</sup>3 J/cm2.

The third type of AlN layer consists of columnar crystals with a highly crystalline *h*-AlN structure, mostly with the (001) texture [48]. A similar crystalline structure was observed for the AlN films grown at 800 ◦C. This is illustrated in Figure 5b, where the bright-field (BF) cross-sectional TEM

image of AlN film deposited at 800 ◦C, 0.1 Pa N2 pressure, LPF of 40 Hz, and incident laser fluence of 3 J/cm2 is shown. The columnar grains with a crystalline *h*-AlN structure are well seen. In the case of AlN films deposited at a higher temperature (800 ◦C), but in vacuum [23], a highly-ordered crystalline film structure was observed, where the *h*-AlN crystallites had grown epitaxially in a columnar orientation perpendicular to the Si substrate (Figure 6a,b). Although an epitaxial growth is achieved (as shown in Figure 6c), the layer is not a single crystal because *h*-AlN crystals grow with two preferred orientations, rotated from each other by 30◦ due to the growth of the *h*-AlN (001) plane onto the cubic Si lattice. The dark-field cross-sectional TEM image in Figure 6b was prepared from two dark-field images (separated from each other by color), which were recorded from spots with two possible epitaxial orientations. The selected area electron diffraction patterns in Figure 6c were taken from the cross-sectional TEM image in Figure 6a. In the first pattern (Figure 6c1), the Si(100) substrate is shown, while the other two patterns (Figure 6c2,c3) show two possible epitaxially-oriented areas in the AlN film.

**Figure 6.** Bright-field (**a**) and dark-field (**b**) cross-sectional TEM images of the PLD AlN film deposited in vacuum (10−<sup>4</sup> Pa) at a temperature of 800 ◦C, laser fluence of 10 J/cm2, and LPF of 3 Hz. In (**c**), the corresponding selected area electron diffraction (SAED) patterns from (a) are shown: SAED pattern of the Si(100) substrate (**c1**) and SAED patterns of AlN films (**c2**,**c3**) taken from two possible epitaxially-oriented areas.

TEM observations correlated well with the results of our earlier studies of PLD AlN films by X-ray diffraction (XRD, Bruker Corporation, Billerica, MA, USA) [23,24,30,35,52]. Our analysis revealed that a stable *h*-AlN phase was forming with predominant (002) *c*-axis orientation, for films deposited at 450 and 800 ◦C, low laser fluence (<10 J/cm2), small nitrogen pressure (vacuum or 0.1 Pa), and high LPF (10 or 40 Hz). For a higher laser fluence of 10 J/cm2, nitrogen pressure of 0.1 Pa, and LPF of 3 Hz, films were amorphous. At intermediate values of PLD parameters, the coexistence of hexagonal

and cubic AlN crystallites occurred [52]. The average grains size was 10–60 nm, as determined with the Scherrer equation. We mention that high-quality AlN (002) films were synthesized by PLD on (La,Sr)(Al,Ta)O3 substrates [53]. According to [54], higher laser fluence and substrate temperature and lower ambient pressure are beneficial for PLD synthesis of AlN thin films with the (002) orientation.

The structural changes ensuing from the variation of the PLD conditions were reflected in the alteration of the surface morphology of the AlN films. The latter has been studied by atomic force microscopy (AFM) and discussed in detail elsewhere [35,36,40]. The obtained results can be briefly summarized as follows. The smoothest surface (RMS roughness of ~0.46 nm) was found in the case of AlN films deposited in nitrogen at low pressure (0.1 Pa), 450 ◦C, and a LPF of 3 Hz, for which the TEM imaging detected the amorphous AlN phase only. On the other hand, the highest surface roughness (RMS roughness of ~2.5 nm) was obtained in the case of films deposited at 800 ◦C, for which better crystallinity and larger-sized crystallites coming up to the surface were detected [36]. The influence of nitrogen pressure on the surface roughness of the PLD AlN films has been reported in [40]. It was shown that deposition at a substrate temperature of 800 ◦C in vacuum (~10−<sup>4</sup> Pa) resulted in considerably high surface roughness (RMS roughness of ~1.8 nm), while increasing the nitrogen pressure up to 10 Pa yielded minimal roughness values (RMS roughness of ~0.24 nm).

The hardness values of the AlN films as a function of the film structure are shown in Figure 7. The data demonstrate well the sensitivity of the AlN film structure to the PLD conditions. As can be seen, the PLD AlN films with the amorphous structure possessed the lowest hardness values. The reason is that the amorphous material is characterized by a short-range order with a distribution in bond lengths that generally results in lower stiffness, as compared to the corresponding crystalline phase [55]. The higher the stiffness of the atomic bondings, the higher the material's hardness is. This explains the observed increased hardness of the PLD AlN films when the degree of crystallinity increased for example by enhancing the substrate temperature from 350 to 800 ◦C or increasing the LPF from 3 to 40 Hz, respectively.

**Figure 7.** Variation of the hardness values with the film structure obtained at different PLD conditions, given in the inset.

One can observe in Figure 7 that the appearance of nanocrystallites in the amorphous matrix increased the AlN film's hardness. The size and amount of crystallite grains are determinative in the hardness level of coatings [56–58]. However, when two phases coexist in films, the hardness values can be greatly influenced by the thickness of the amorphous matrix separating the nanocrystals. Moreover, when the crystalline particles are forming in the gas space, it is easier to involve cavities (voids) from their environment, which can greatly reduce the hardness of the layer.

The highest hardness values were registered for the PLD films with epitaxial-like growth of AlN on the Si(100) substrates, i.e., when the PLD process proceeded in vacuum at the highest temperature (800 ◦C) (see Figure 7). In this case, the largest size of nanocrystallites (10–20 nm), growing in a columnar grain structure with preferred grain orientations and in a negligible amount of amorphous matter, was observed (Figures 5b and 6). Such an ordered structure is characterized by a strongly-reduced amount of defects in grain boundaries and, consequently, a higher *H* value, as was observed.

#### *3.3. FTIR Reflectance Spectra Analysis*

FTIR reflectance spectra are given in Figure 8 for the case of *p*-polarized (Figure 8a) and *s*-polarized (Figure 8b) incident beams recorded at a radiation angle of 70◦. The results in Figure 8 correspond to AlN films deposited with a laser fluence of 3 J/cm<sup>2</sup> and different temperatures and LPF of 40 Hz. For higher temperatures, the spectra exhibited a complex and broad band within the 950–500 cm−<sup>1</sup> region. The complexity of the spectral envelope can be assigned to the nanocrystallites' disorientation. The Berreman effect was registered in *p*-polarization, which allows for identification of the longitudinal (LO) phonon vibrational modes [59]. This gives the possibility to characterize thin films' microstructure directly from IR spectroscopy. A comparison of the spectra taken in both *s*- and *p*-polarization points to a clear difference in the high frequency end of the band (Figure 8).

**Figure 8.** FTIR reflectance spectra of PLD AlN films, investigated in a linearly *p*-polarized (**a**) and *s*-polarized (**b**) incident beam.

The 950–500 cm−<sup>1</sup> region is characteristic for the Reststrahlen band of the *h*-AlN crystal with component lines peaking around 611, 670, 890, and 912 cm<sup>−</sup>1, arising from A1(TO), E1(TO), A1(LO), and E1(LO) IR active phonon vibrational modes, respectively [60–63]. For samples prepared at low substrate temperatures (RT and 350 ◦C), the deconvolution of the measured Reststrahlen band in *p*-polarized radiation was not possible. For higher substrate temperatures, the position of peaks was determined by the Levenberg–Marquardt deconvolution method with a fitting mean square error of 10<sup>−</sup>3. The peaks and their assignments are collected in Table 1. When decreasing the substrate temperature, a major decrease of frequencies was observed for the E1(TO) and A1(LO) phonon-polariton modes. At a large angle of *p*-polarized incidence radiation such as 70◦, the A1(LO) mode, which is polarized parallel to the nanocrystallite *c*-axis, will be the most sensitive to the orientation of the crystal phase (Figure 8a). Any deviation of *c*-axis from the surface normal leads to a structure disorientation that is equivalent to a dumping of the phonon-polariton resonance vibration [64]. The enhanced structure disordering at lower temperatures also influences the E1(TO) mode, which is polarized parallel to the *a*-axis, i.e., is parallel to the substrate surface in a good *c*-axis-oriented layer. Consequently, the resonance frequency decrease was more evident in the spectra measured in *s*-polarization (see Figure 8b). Besides, this mode is two-fold degenerated, i.e., it cumulates vibrations of two sets of atoms with the same

frequency [65]. Thus, an increasing disorder with the temperature decrease will cause the peak's widening (as observed for all components), which leads to an increase of the entire Reststrahlen band half width. This is illustrated in Figure 9 for the AlN films deposited at 0.1 Pa N2 pressure and LPF of 40 Hz. The incident laser fluence was kept within the range of 3–4 J/cm2.

It should be mentioned that the features around 620–610 cm−<sup>1</sup> in both sets of spectra in Figure 8 could hardly be assigned to the phonon mode A1(TO) of *h*-AlN only. Indeed, the vibrational modes of Si substrate [66] and those of possible AlO*<sup>x</sup>* phases [67] were also present in the above-mentioned region. Possible AlO*x* bonds could be formed either during film preparation or storage of the samples under atmospheric conditions. In our opinion, the latter assumption is more likely to occur.

**Table 1.** Peak position of the phonon-polariton modes in the Reststrahlen band, registered with *p*-polarized radiation in the AlN films (Figure 8a). TO, transverse.


**Figure 9.** Reststrahlen band half width in *p*-polarized radiation (Figure 6a) as a function of substrate temperature during AlN film deposition at N2 pressure of 0.1 Pa, LPF of 40 Hz, and laser fluence of 3–4 J/cm2.

From the presented results, one can conclude that despite the poor crystalline phase, revealed by TEM, the FTIR spectra of AlN thin films deposited at temperatures higher than 350 ◦C clearly exhibited the characteristic Reststrahlen band of the AlN crystal with a hexagonal lattice. This band was originating from the *h*-AlN nanocrystallites, the size and ordering of which were increasing with the substrate temperature. For the AlN films synthesized at a substrate temperature of 350 ◦C, the spectra did not preserve the shape of a Reststrahlen band, and therefore, if nanocrystals were formed, their contribution could be negligible. At RT, a completely amorphous layer was grown. According to the SE results, the optical thickness of this layer was relatively small with respect to the wavelengths of the measured spectral region (~λ/20), and the recorded FTIR spectrum was flat. In such a thin amorphous film, neither a Reststrahlen band, nor the multiple interference effect could be observed in the FTIR spectra [68].

#### *3.4. Spectroscopic Ellipsometry*

The ellipsometric results revealed a clear dependence on technological conditions, in good agreement with TEM and FTIR investigations. We note that each AlN film yielded a certain thickness, which was within the 400–1000-nm range (corresponding to an estimated deposition rate of ~2.8 × <sup>10</sup><sup>−</sup>2–7 × <sup>10</sup>−<sup>2</sup> nm/pulse), depending on the PLD technological protocol. For illustration purposes, in Figure 10, the optical constants *n* and *k* are shown for AlN films deposited in ambient nitrogen at a pressure of 0.1 Pa and laser fluence of 3–4 J/cm2, by varying the substrate temperature and LPF. These values are characteristic for the corresponding AlN structures and correlated well with TEM observations. The refractive index values either coincided or were superior to those of amorphous AlN and remained inferior to those of high-quality polycrystalline *h*-AlN films. This suggests the coexistence of crystalline and amorphous AlN phases. Independently of the substrate's deposition temperature, films deposited at LPF of 3 Hz (Figure 10) possessed *n* values characteristic to an amorphous AlN structure. In accordance with the TEM results, larger LPF yielded nanostructured films with better ordering at LPF of 10 Hz, which reflects slightly higher index values. The exception is the AlN film deposited at RT (data represented by black dots in Figure 10), which was completely amorphous, as revealed by TEM, but its *n* values were close to those of *nc*-AlN. Additional compositional study of this sample by energy dispersive spectroscopy (EDS), performed in a scanning electron microscopy (SEM) system, has disclosed an over-stoichiometric AlN with an average Al/N ratio of 1.14. One can notice from the SEM-EDS results in Table 2 that at elevated temperatures, the films' composition was close to the stoichiometric AlN. When deposited at RT, AlN films contained an excess amount of Al atoms, which could contribute to the observed higher index values.

**Figure 10.** Dispersion curves of the refractive index (**a**) and extinction coefficient (**b**) of the studied AlN films deposited in the conditions presented in the insets.

**Table 2.** SEM-EDS data for AlN films deposited at different substrate temperatures in ambient N2 at a pressure of 0.1 Pa, incident laser fluence of 3 J/cm2, and LPF of 40 Hz.


PLD AlN films were transparent in the 400–1000-nm spectral region, as the *k* values, dependent on substrate temperature and LPF (Figure 10b), approached zero. Below 400 nm, because of reaching the absorption edge, the extinction coefficient increased, and its value varied with the deposition conditions. A large shift of the absorption edge to higher wavelengths was observed for the RT deposited AlN film, suggesting a strong reduction of the optical bandgap in comparison to those deposited at elevated temperatures.

#### **4. Conclusions**

Aluminum nitride (AlN) films with different structural features were synthesized onto Si(100) substrates by pulsed laser deposition in vacuum and ambient nitrogen, at various pressures, substrate temperatures, laser incident fluences, and laser pulse frequencies. From the results of nanoindentation tests, transmission electron microscopy, X-ray diffraction, atomic force microscopy, Fourier transform infrared spectroscopy, and spectroscopic ellipsometry, the correlation between the mechanical properties, film structure, and optical parameters, dependent on deposition conditions, was studied.

The growth process and resulting film microstructures yielded variation in the film hardness within 22–30 GPa. Elevated substrate temperatures facilitated the crystallization process and, thus, a less defective structure for which increased nanohardness values were reached. Enhanced hardness values, in the range of 22–27 GPa, were observed for AlN films with a structure that consisted of nanocrystallite grains of 5–50 nm embedded in an amorphous matrix, strongly dependent on the deposition conditions. These values were superior to those obtained by other deposition techniques or reported for crystalline AlN. The refractive index value, superior to that of amorphous AlN, supported the existence of crystallites inside the film volume. In the case of PLD AlN films deposited at temperatures higher than 350 ◦C, the FTIR results evidenced vibrational bands within the characteristics Reststrahlen band of 950–500 cm−1, which were assigned to hexagonal AlN crystallites. For lower temperatures, the Reststrahlen band gradually vanished, and the PLD film at room temperature exhibited an FTIR spectrum characteristic of a completely-amorphous AlN material.

The mechanical and optical properties of the synthesized AlN films conformed to the applied PLD technological parameters.

**Author Contributions:** Conceptualization, A.S. and L.D.; methodology, P.T., Z.F., and L.D.; validation, L.K., A.S., K.A., P.P., and L.D.; formal analysis, K.A., and P.T.; investigation, L.K., V.C., K.A., Z.F., and L.D.; resources, I.N.M.; writing, original draft preparation, L.K., A.S., K.A., and L.D.; writing, review and editing, A.S., P.P., I.N.M., and L.D.; visualization, L.D.; supervision, A.S.; project administration, A.S. and L.D.

**Funding:** The Bulgarian co-authors thank the European Regional Development Fund, the Ministry of Economy of Bulgaria, Operational Programme "Development of the Competitiveness of the Bulgarian economy" 2007–2013, Contract No. BG161PO003-1.2.04-0027-C0001. The Romanian co-authors acknowledge the support of the Core Programme, Contract 16N/2019. Liviu Duta thanks the support from the grant of the Ministry of Research and Innovation, CNCS-UEFISCDI, Project Number PN-III-P1-1.1-PD-2016-1568 (PD 6/2018), within PNCDI III. Peter Petrik is grateful for the support from OTKA Grant No. K115852.

**Acknowledgments:** All authors acknowledge with thanks the support of this work by the Bulgarian, Hungarian, and Romanian Academies of Sciences under the 2014–2017 Collaboration Agreements.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Review* **Pulsed Laser Deposited Films for Microbatteries**

#### **Christian M. Julien \* and Alain Mauger**

Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Campus Pierre et Marie Curie, Sorbonne Université, CNRS UMR 7590, 4 Place Jussieu, 75005 Paris, France; alain.mauger@upmc.fr

**\*** Correspondence: christian.julien@upmc.fr; Tel.: +33-673-404-684

Received: 10 May 2019; Accepted: 10 June 2019; Published: 14 June 2019

**Abstract:** This review article presents a survey of the literature on pulsed laser deposited thin film materials used in devices for energy storage and conversion, i.e., lithium microbatteries, supercapacitors, and electrochromic displays. Three classes of materials are considered: Positive electrode materials (cathodes), solid electrolytes, and negative electrode materials (anodes). The growth conditions and electrochemical properties are presented for each material and state-of-the-art of lithium microbatteries are also reported.

**Keywords:** PLD films; energy storage; thin-film electrodes; thin-film solid electrolyte; lithium microbatteries

#### **1. Introduction**

It has been widely demonstrated that pulsed-laser deposition (PLD) based on the process of the transportation of a material (laser ablation) is a successful technique for the growth of stoichiometric multicomponent oxide films [1,2]. Indeed, PLD has shown unique advantages for the formation of dense films for energy storage and conversion, namely a high reproducibility, easy control of the growth rate, and a high film purity with a variety of substrates, such as amorphous glass, oriented silicon [3], stainless steel [4], (001)Al2O3 [5], indium tin oxide (ITO)- and ZnO-coated glass, and ITO-coated Upilex polymer [6]. Generally, the stoichiometry of the target phase is preserved in PLD films of oxides but a deviation is observed for lithiated material that implies an Li-enriched target. Consequently, the loss of volatile Li during deposition is compensated for by using about a 15 wt.% excess of Li2O [7,8]. Accurate stoichiometry can be obtained by controlling several parameters of the process. The typical set-up for the fabrication of PLD films consists of a stainless-steel vacuum chamber evacuated down to a residual pressure less than 1 <sup>×</sup> 10−<sup>4</sup> Pa before material deposition. Energy (laser fluence) in the range of 1.0 to 3.0 J·cm−<sup>2</sup> is generated by a pulsed-laser beam, which falls onto the target surface with an incidence angle of approximately 45◦ (see Table 1 for laser characteristics). Indeed, four PLD parameters are of prime importance for the growth of films: The laser fluence; type of substrate; orientation and lattice parameters, which must match with those of the film for an efficient epitaxy process; substrate temperature (*T*s); and the oxygen partial pressure (*P*O2 ). In addition, as the capacity of the microbattery depends on the electrode thickness, the duration of the deposition (*t*p) must also be considered. The activity of a thin-film electrode, i.e., specific discharge energy, is proportional to the thickness, thus an increase of the film thickness leads to a power limitation because of the slow transport kinetic of Li<sup>+</sup> ions. Consequently, PLD is a popular technique due to the growth of a compact and dense film, which is replaced by a thick and porous film. Another advantage of the physical vacuum-like deposition techniques is the possibility of depositing a thin layer on top of the microbattery, which protects the device against a reactivity toward moisture. Moreover, due to the well-defined surface area of PLD films, a direct comparison of the electrochemical activity of materials can be done for different morphologies, from amorphous to single-crystalline materials [9].


**Table 1.** Typical laser beams for PLD films of transition-metal (TM) oxides for energy storage.

(a) Frequency doubled; (b) Fourth harmonic.

Due to their high energy and power densities, lithium-ion batteries (LIBs) are initial power sources that are widely used in portable devices (laptops, mobile phones, cameras, etc.) and are now employed for sustainable transportation, such as full electric vehicles (EVs) and hybrid electric vehicles (HEVs). The fabrication of electrochemical cells with a thin-film architecture allows the development of microbatteries for powering micro-scaling devices, such as stand-alone sensor systems, medical implants and devices, labs-on-chip, credit cards, etc. In addition to technological applications, positive (cathode) and negative (anode) electrodes in the thin-film form are useful for studying the intrinsic properties of the material without the use of a polymeric binder and carbonaceous additive [15]. The use of thin-film technology may offer various advantages, such as: (i) Thin films are well suited for the design of devices; (ii) thinning of the layers provides a lower resistance in the transverse direction for weakly semiconducting materials; (iii) a reduction of the thickness of the solid electrolyte film allows the use of glassy materials with a low ionic conductivity; (iv) a reduction of the charge-transfer resistance of the electrolyte–electrode interface; (v) easy manufacture of microbatteries using the same technique that is currently used in the microelectronics industry; and (vi) the construction of microbatteries is realized in almost any two-dimensional shape. However, the fabrication of microbatteries also contains many difficulties, which are comprehensively discussed below [16].

In the present review paper, we present the properties of pulsed-laser deposited films used as components of energy storage devices (i.e., batteries, supercapacitors, electrochromics, etc.). The remainder of the article is organized as follows. First, the state-of-the-art of lithium microbatteries using PLD films are summarized in Section 2, providing the characteristics of the best lithium microbatteries fabricated so far. In Sections 3–5, the three classes of active materials constituting electrochemical microdevices, realized via the PLD technique, are considered: (i) Positive electrode materials (cathodes), (ii) electrolytes, and (iii) negative electrode materials (anodes). For each material, the growth conditions and electrochemical properties are presented. Finally, in Section 6, we compare and discuss the growth conditions that allow the best electrochemical performance of each electrochemically active component of microbatteries.

#### **2. Lithium Microbatteries**

The concept of a thin-film solid-state battery is quite old [17]. The subject of thin-film microbatteries has been discussed in the scientific literature for many years. The review by Kennedy is a good source for work prior to 1977 [18]. The concept and design of all-solid-state planar thin-film microbatteries have been patented by Bates et al. [19–24], who reported on micropower sources using lithium phosphate, lithium phosphorus oxynitride, and lithium phosphorus lithium oxide as a solid thin-film electrolyte. Julien investigated the electrochemical performance of individual layers in a microbattery in relation to the growth mechanism and thin-film structure [25]. Dudney addressed how to build a battery layer-by-layer by vapor deposition [26]. More recently, Oudenhoven et al. reviewed the concepts of three-dimensional (3D) microbatteries [27]. In 2015, Wang et al. discussed the choice of materials for lithium and lithium-ion microbatteries and reviewed the chemistry and electrochemistry for applications in microelectronic devices [28]. Ferrari et al. highlighted the importance of 3D microarchitecture electrodes to fabricate microgenerators for micro-electromechanical systems (MEMSs) [29]. In the

presentation of in situ analytical microprobes, Meng et al. described PLD-produced thin-film lithium microbatteries using the PLD technique and showing the production of a multilayer structure with dense and smooth films [30].

There are many variations on the general scheme of microbatteries outlined in the literature. Two principal options are shown in Figure 1 [25]. Figure 1a shows a four-layer design on a conducting substrate (i.e., oriented silicon wafer) that can act as a current collector. Figure 1b shows a six-layer stack incorporating two metallic current collectors. There are two fast-ion conductor (FIC) layers in this design: A thick layer, which is the solid electrolyte itself, and a thin buffer film that acts as an electrolyte layer between the FIC and the Li metal film to prevent interface passivation. In 1992, a thin-film solid-state microbattery with an overall thickness of approximately 10 μm, including the TiS2 cathode, oxide-sulfide solid electrolyte, LiI buffer, and Li metal anode, was developed at the Technology Laboratory of Eveready Battery Company (EBC) [31]. Laïk et al. evaluated the performance of three 4-V commercial all-solid-state lithium microbatteries (200-μm thick) with a nominal capacity of 700 μAh based on a LiCoO2 cathode material [32]. Note that a typical 1-mWh battery weighs 2.5 mg and has a volume less than 1 μL, providing a specific energy and power of 400 Wh·kg−<sup>1</sup> and 1 kWh·L<sup>−</sup>1, respectively [26].

**Figure 1.** Design principles for lithium microbatteries composed of a lithium film (Li), fast-ion conductor (FIC), mixed ionic-electronic conductor (MIC), current collector(s) (CC), silicon substrate (Si), glass substrate (Sub), and buffer layer (Buf). (**a**) four-layer design on a conducting substrate and (**b**) six-layer stack incorporating two metallic current collectors (Reproduced with permission from [25]. Copyright 2000 Springer).

Regarding the manufacture of thin-film batteries, several start-up companies have marketed micropower sources. Enfucell developed a thin, printable, and flexible SoftBattery® used in various wearable electronics products [33]. Cymbet Corporation fabricates the EnerChipTM battery, which is a battery bare die and can be embedded with other integrated circuits [34]. Excellatron announced a pilot production line (10,000 cells/month) of thin-film solid-state batteries (approximately 0.3 μm thick) made of cathode films of LiCoO2 or LiMn2O4, LiPON as the electrolyte, and Li metal or Sn3N4 as the anode based on the technology developed at Oak Ridge National Labs. Using a 2-μm thick positive electrode, these microbatteries have been cycled in excess of 2000 cycles [35]. Some industrial developments of thin film microbatteries are listed in Table 2.



A sequential PLD technique was applied for the fabrication of a rechargeable thin-film lithium battery (2-μm thick, area of 0.23 cm2) with partially crystallized LCO as the cathode, an Li6.1V0.61Si0.39O5.36 (LVSO) glassy electrolyte, and SnO film anode [41]. The ablation beam produced by a Q-switched Nd:YAG laser (λ = 266 nm, repetition rate of 10 Hz) was used at the fluence of 3.5 mJ·cm<sup>−</sup>2. A single phase LCO film was obtained by post annealing at 600 ◦C for 1 h in air and the amorphous LVSO film exhibited an ionic conductivity of ca. 10−<sup>7</sup> S·cm−<sup>1</sup> at room temperature. Such a Li microbattery cycled at 44 <sup>μ</sup>A·cm−<sup>2</sup> in the voltage range of 0.7 to 3.0 V delivered a capacity of 9.5 Ah·cm<sup>−</sup>2. After 100 cycles, the capacity retention was 45% of that of the first cycle. Characteristics of solid-state lithium microbatteries fabricated by the PLD technique reported in the literature are listed in Table 3. Most of the microcells use LiCoO2 as the positive electrode, providing a nominal voltage of ~3.9 V vs. Li+/Li. Thus, among the fabricated all-solid-state thin-film lithium batteries, the electrochemical chain of Li-In/80Li2S–20P2S5/LiCoO2 with an average potential of 3.5 V exhibits the best performance in terms of energy density.

Sakuda et al. reported the construction of an SSLMB based on an LCO positive electrode with an SE coating, a highly conductive 80Li2S–20P2S5 solid electrolyte, and an Li-In alloy as the anode [42]. Such a microbattery delivered a specific capacity of 95 mAh·g−<sup>1</sup> at a current density of 0.13 mA·cm<sup>−</sup>2. By using LCO thin films prepared from a Li*y*CoO<sup>δ</sup> target containing 15% Li2O, Xia et al. fabricated thin-film microbatteries by the successive deposition of an LCO cathode on a Pt/Ti/SiO2 (amorphous)/Si composite substrate and amorphous Si anode [12]. Recently, the analysis by impedance spectroscopy of the microcell Li/LiPON/Li4Ti5O12 (nominal voltage of 1.5 V), in which LiPON is an amorphous lithium phosphorus oxynitride (i.e., nitrogen-modified Li3PO4), has clarified the debate on the interface stability with lithium; it was clearly shown that LiPON forms a well-conducting solid electrolyte interface (SEI) layer [43]. Despite the low ionic conductivity (1 <sup>μ</sup>S·cm−1) and the rather large contribution to the internal cell resistance, LiPON can be used as a solid electrolyte film with a thickness of ~1 μm or less. Therefore, use of the LiPON-LiCoO2 system is very popular in the construction of thin-film lithium microbatteries [26].


**Table 3.** Solid-state lithium microbatteries fabricated by the PLD technique.

#### **3. Positive Electrode PLD Films**

The main parameter for a microbattery is the delivered specific capacity. Rather than being expressed as the conventional unit of mAh·g<sup>−</sup>1, due to the uncertainty in the film density, technologists prefer the stored charge, *Q* (expressed in μAh or in coulomb), per film surface area and the film thickness, i.e., <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> of mC·cm−2·μm<sup>−</sup>1.The relation between the gravimetric capacity, *Qm*, of the material and the volumetric capacity of a film, *Qf*, is given by:

$$Q\_f = 0.36 \, d \, Q\_m \tag{1}$$

where *Qf* is expressed in mC·cm−2·μm<sup>−</sup>1, *Qm* in mAh·g<sup>−</sup>1, and *<sup>d</sup>* is the density of the material in g·cm<sup>−</sup>3. Table 4 summarizes the energetic quantities for the studied cathode compounds.


**Table 4.** Characteristics of the oxide materials used as a positive electrode in Li batteries. Δ*x*<sup>m</sup> is the quantity of electrons transferred (or Li uptake).

<sup>a</sup> after Ozuku, T.; Ueda, A. *J. Electrochem. Soc*. **1994**, *141*, 2972.

#### *3.1. LiCoO2 (LCO)*

Having a lamellar structure, LiCoO2 (LCO) is the prototypal positive electrode material commonly used in Li-ion batteries that yields a practical specific capacity of 135 mAh·g−<sup>1</sup> in the voltage range from ~3.8 V (fully lithiated state) to ~4.2 V (charge state at Li0.5CoO2) [46]. Since the early work in 1996 by Berkeley's group [47], numerous studies have been devoted to the growth of LiCoO2 thin films prepared by the PLD technique due in large to their high electrochemical performances. Further investigations of dense and well-defined PLD films described the phase evolution during Li extraction and the kinetics of Li<sup>+</sup> ions in the host lattice, which eventually found applications in the fabrication of the cathode element in microbattery stacks [48]. The two crystal forms, HT- and LT-LiCoO2 phases, with the rock-salt (rhombohedral, *R*-3*m* space group) and spinel (cubic, *Fd*3*m* space group) structure, respectively, have been synthesized by pulsed-laser deposition. It was pointed out that the crystallographic texture for LCO films differs from one deposit technique to another, i.e., PLD versus sputtering, which influences the electrochemical properties due to the diffusion plane orientation [49]. Julien et al. stated that well-crystallized PLD-grown LCO thin films with a single layered structure can be obtained at substrate temperatures (*T*s) as low as 300 ◦C [3].

The first growth of single phase LCO films by the PLD method was realized by Antaya et al. [4]. Films deposited on unheated stainless-steel substrates were amorphous but crystallized readily with heat treatment in air above 500 ◦C. Later, Striebel et al. [47] demonstrated the promise of PLD-grown films as cathodes for rechargeable lithium cells. Crystalline (003)-textured LCO films with thicknesses ranging from 0.2 to 1.5 μm were prepared without postdeposition treatment, which displayed a specific capacity of films of 62 <sup>μ</sup>Ah·cm2·μm−<sup>1</sup> and an Li diffusion coefficient of 1 <sup>×</sup> 10−<sup>10</sup> cm2·s−1. Highly dense LCO films were first elaborated by the PLD process using a KrF laser under oxygen flow rates of 30 sccm and the pressure was maintained at 260 Pa on (200)-textured F-doped SnO2 on fa used silica substrate maintained at *T*<sup>s</sup> = 700 ◦C [49]. As-prepared LCO thin films were (00l) textured and had a density of 85% of the single crystal. The charge–discharge profile of the films was typical of the LCO bulk and presented an ~18% capacity loss for a single cycle to 4.15 V. In the potential range of 4.14 to 4.19 V, the measured chemical diffusion coefficients ranged from 1.7 <sup>×</sup> 10−<sup>12</sup> to 2.6 <sup>×</sup> 10−<sup>9</sup> cm2·s−<sup>1</sup> for as-deposited films and films annealed at 700 ◦C, respectively. Structural analysis of nanostructured LCO films prepared with PLD has been conducted by several research groups. Julien et al. [3,8,50] analyzed changes of the stoichiometry (i.e., the absence of the Co3O4 amorphous phase) as a function of the growth conditions using Raman spectroscopy. The inclusion of Co3O4 impurity is detected by analysis of the Raman intensity of the *A*1g modes. Impurity-free films exhibit a specific capacity as high as 195 mC·cm−2·μm−<sup>1</sup> for polycrystalline films grown from an Li-rich target (i.e., excess of 15% Li2O). The work by Okada et al. revealed that a decrease of the amount of inclusions can be obtained by a lower laser fluence and lower *T*<sup>s</sup> [51]. Figure 2 presents the relationship between the impurity inclusions and growth conditions of PLD-grown LCO films established from spectroscopic Raman data. In this figure, the Co3O4 phase is grown under the conditions of high *P*O2 , i.e., above the gray dashed line.

**Figure 2.** The relationship between impurity inclusions and growth conditions of PLD-grown LCO films established from spectroscopic Raman data (Reproduced with permission from [51]. Copyright 2017 AIP Publishing).

Ohnishi et al. [52,53] stated that suppression of the Co3O4 spinel phase can be ensured by the growth under a relatively low oxygen partial pressure. Zhang et al. [54] discussed the effect of the deposition conditions on the structure and morphology. The advantages of the preferential orientation of LCO films prepared by PLD has been discussed by numerous groups with the conclusion that dense uniaxial textured (003)-oriented films are obtained by a well-chosen substrate [6,53,55–59]. However, Xia et al. [59] stated that the fast transport of Li<sup>+</sup> ions is obtained for LCO films with a random orientation, in contrast with the results obtained with films having (003)-preferred orientation. Contrastingly, Nishio et al. claimed an excellent electrochemical performance for epitaxially grown LCO (77-nm thick) with a (104)-orientation on a (100) Nb:SrTiO3 substrate that exhibited a discharge capacity of 26 mAh·g−<sup>1</sup> even at high rates up to 100C [60]. Huo et al. stated that the film orientation is strongly dependent on the thickness and size of grains and demonstrated that films structured with parallel (003) planes are grown for thicknesses up to 300 nm [11]. Liu et al. showed that under certain PLD conditions, such as a high repetition rate of 35 Hz and low oxygen partial pressure of *P*O2 = 1 Pa, LCO films tend to grow LCO films with a random orientation [61].

Epitaxial LCO thin films deposited on (001)-Al2O3 substrates remained in a single phase in a narrow range, 250 ≤ *T*<sup>s</sup> ≤ 300 ◦C, whereas secondary phases appeared at *T*<sup>s</sup> > 300 ◦C, i.e., Co2O3, Co3O4, and LiCo2O4 [5]. Xia et al. established that thin LCO films can be easily grown with a (003) orientation because of the lowest surface energy for the (003) plane, while the minimized strain energy in thick LCO films allows preferential (101) and (104) textures. It seems that this last type of orientation favors the electrochemical performance of the LCO cathode [12]. The reduction of the laser fluence results in a decrease of the surface roughness of LCO films. With post annealing at 400 ◦C and optimized deposition conditions, LCO films exhibit an initial discharge capacity of 36 <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> and a cycleability of 94% [57]. Composition control was monitored to prepare stoichiometric LCO films using an Li-enriched target with a high-rate growth via an increase of the laser fluence to 0.29 J·cm−<sup>2</sup> and an adjustment of the *P*O2 to scatter the excess lithium. Ohnishi et al. showed that by using a Li1.1CoO2+<sup>δ</sup> target, the deposition of stoichiometric LCO with the highest crystallinity can be realized at the rate of 0.06 Å per pulse at the *P*O2 of 0.1 Pa and *T*<sup>s</sup> = 800 ◦C (Figure 3) [52,62].

Recently, Nishio et al. claimed that a high deposition rate of 1.2 Å·s−<sup>1</sup> tends to form oxygen-deficient LCO films due to the destabilization of Co3<sup>+</sup> cations and showed that post-annealing in air cancels the impurity phase [63]. Funayama et al. studied the effects of mechanical stress applied to LCO films (200 nm thick) deposited on Li-glass ceramic by the PLD method at 600 ◦C under a 20 Pa oxygen partial pressure for 1 h. Due to the lattice volume change, the generated electromotive force was 6.1 <sup>×</sup> <sup>10</sup>−<sup>12</sup> <sup>V</sup>·Pa−<sup>1</sup> [64]. The electrode behavior shows an increase of the discharge capacity from 10 to 40 mAh·g−<sup>1</sup> at a 2C rate with an increase of the *<sup>T</sup>*<sup>s</sup> from 600 to 750 ◦C, whereas a *<sup>T</sup>*<sup>s</sup> <sup>=</sup> <sup>800</sup> ◦C worsens the performance. Studies of the physico-chemistry of PLD-grown LCO thin films report the structural, surface morphology, optical, and electrical properties [65–67].

**Figure 3.** Schematic representation of the fast-laser ablation growth according to the Li/Co ratio variation of the plume for a stoichiometric (**a**) and Li-enriched target (**b**). (Reproduced with permission from [62]. Copyright 2012 IOP Publishing).

The electrochemical properties, i.e., thermodynamics and kinetics, of lithium intercalation in PLD LCO thin films have been widely investigated by the structure–electrochemistry relationship [59,68–70]; structural evolution upon charge-discharge cycles [71]; effect of doping by Ti, Al, and Mg [49,72–74]; characterization of the electrode/electrolyte interface [75]; and lithium-ion kinetics vs. basal plane orientation [76–80]. Highly (003)-oriented impurity-free LCO thin films grown by PLD on a stainless-steel substrate display an initial discharge capacity of 52.5 <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> and a capacity loss of 0.18% per cycle at a moderate current density of 12.7 <sup>μ</sup>A·cm−2. These films show a very small lattice expansion upon charge, i.e., 0.09 Å for a charge of 4.2 V [81]. Figure 4 shows the typical electrochemical features of PLD LCO thin films grown on an Si wafer maintained at different temperatures. Both the specific discharge capacity and the mid-voltage increased with increasing *T*s. For a film deposited at *T*<sup>s</sup> = 300 ◦C under *<sup>P</sup>*O2 <sup>=</sup> 15 Pa, the discharge capacity reached a value ~140 mC·cm−2·μm<sup>−</sup>1.

**Figure 4.** Electrochemical features of PLD-grown LCO thin films: specific discharge capacity and discharge mid-voltage vs. substrate temperature.

The electrochemical behavior of doped LCO thin films show that the voltage plateau at 3.65 V disappears in the charge curve of LiTi0.05Co0.95O2 due to the doping effect, which cancels the semiconductor-metal-like transition of the LCO framework [81]. The Al-doped LCO film (LiCo0.5Al0.5O2) exhibits a steady increase in the voltage vs. Li extraction with the absence of a voltage plateau as observed in stoichiometric LCO films; however, such films suffered from a limited capability and an upper bound of the diffusion coefficient of Li (*D*\* <sup>=</sup> <sup>9</sup> <sup>×</sup> <sup>10</sup>−<sup>13</sup> cm2·s<sup>−</sup>1) was observed [49]. Recent studies report on the improved electrochemical behavior of surface-modified LCO films using lithium tantalate (LTaO) and lithium niobite (LNbO). Coating with LNbO preserves the LCO surface and decreases the interfacial resistance, which indicates fast lithium transport [82]. LCO films modified by amorphous tungsten oxide (LWO) fabricated by PLD show a high capacity retention of *Q*<sup>r</sup> = 80% at a high rate of 20C, against *Q*<sup>r</sup> = 0% for bare LCO films cycled at the same C-rate. A slight increase of the superficial diffusion coefficient of Li<sup>+</sup> ions from 2.2 <sup>×</sup> <sup>10</sup>−<sup>13</sup> and 3.0 <sup>×</sup> <sup>10</sup>−<sup>13</sup> cm2·s−<sup>1</sup> was also observed, owing to the surface modification [83–86]. Note that LWO as well as LNBO are lithium ion conductors, which act as an efficient buffer between the electrolyte and LCO cathode. The structural degradation of cycled LCO films was investigated by Raman spectroscopy over 400 cycles, showing microstructural modification due to nanocrystallization and phase separation [87].

All-solid-state lithium microbatteries (SSLMB) using LiCoO2 films have been developed using various inorganic solid electrolyte (SE) films, i.e., LiPON, Li2S–P2S5, and amorphous Li3PO4. The thin-film battery with an electrochemical chain Li/amorphous Li3PO4/LCO/Pt shows a columnar-like LCO cathode (see the cross-sectional SEM image in Figure 5a) [88]. The excellent electrochemical performance is displayed in Figure 5b. This microcell exhibited an increase in capacity of up to 240 <sup>μ</sup>Ah·cm−<sup>2</sup> when increasing the LCO thickness to 6.7 <sup>μ</sup>m, which is 54% of the theoretical specific capacity of LCO (69 <sup>μ</sup>Ah·cm−2·μm−1). Shiraki et al. fabricated an SSLMB with an epitaxial LCO thin-film cathode (200 nm thick) by using PLD with a polycrystalline Li1.1CoO2 target ablated at a laser fluence of 1 J·cm<sup>−</sup>2, LiPON solid electrolyte (2 <sup>μ</sup>m thick), and Li film as the anode (0.5 <sup>μ</sup>m thick) [89]. The authors reported cyclic voltammograms with six redox peaks, which drastically changed upon cycling but did not display the galvanostatic charge–discharge profile of the SSLMB.

**Figure 5.** (**a**) SEM image cross-section of a Li/amorphous Li3PO4/LCO thin-film microbatteries. (**b**) Discharge curves for various current densities in the voltage range of 3.0 to 4.5 V vs. Li+/Li. (Reproduced with permission from [88]. Copyright 2014 Elsevier).

#### *3.2. LiNiO2 (LNO)*

PLD-grown thin films of lithium nickel oxide (LNO), i.e., Li*x*Ni1−*<sup>x</sup>*O and stoichiometric LiNiO2, are applied as electrochromic and/or battery electrodes. In an early work, Rubin et al. established the complex relationship of the surface morphology and chemical composition of Li*x*Ni1−*x*O thin films vs. the deposition oxygen partial pressure, substrate temperature, and substrate–target distance as well [90]. LNO films produced at *T*<sup>s</sup> < 600 ◦C immediately absorb CO2 and H2O when exposed to air, whereas they show long-term stability for *T*<sup>s</sup> = 600 ◦C. LNO film with a composition of Li0.5Ni0.5O (cubic rock-salt *Fd*-3*m* structure instead of the rhombohedral *R*-3*m* structure for LiNiO2) was obtained under

a deposition atmosphere of *P*O2 = 60 mTorr. This film (150-nm thick) showed excellent electrochemical reversibility as an electrochromic item in the range of 1.0 to 3.4 V vs. Li+/Li. An electrochromic device using WO3 as the opposite electrode and PEO/LiTFSI as the solid polymer electrolyte (250-μm thick) showed an optical transmission range of ≈70% at 550 nm. Bouessay et al. optimized the PLD conditions to prepare NiO films, i.e., *P*O2 = 0.1 mbar and *T*<sup>s</sup> = 25 ◦C, and analyzed the electrochromic reversibility associated with the Ni3+/Ni2<sup>+</sup> redox couple [91]. Using a laser fluence of 1 to 2 J·cm−2, which corresponded to an ablation rate of 0.9 Å·s<sup>−</sup>1, NiO films with a cubic rock-salt structure (*Fm*-3*<sup>m</sup>* space group) were formed. Porous PLD NiO films were prepared using nickel foil as the target in a low oxygen atmosphere (*P*O2 = 50 Pa) [92,93] and were applied as the electrode for supercapacitors, showing a high specific capacitance of 835 F·g−<sup>1</sup> at a 1 A·g−<sup>1</sup> current density.

Similarly to LCO, the PLD growth of stoichiometric LiNiO2 with an α-NaFeO2 layered structure requests an Li-enriched target, i.e., LiNiO2 + 15%Li2O. López-Iturbe and coworkers attempted to avoid Li loss by using an Ar atmosphere of *P*Ar = 10 mTorr and laser fluence of 15 J·cm−<sup>2</sup> [94], while Rao et al. introduced pure oxygen (*P*O2 = 0.1 Torr) in the PLD chamber and ablated the target at a laser fluence of 10 J·cm−<sup>2</sup> [95]. The LNO films prepared at *<sup>T</sup>*<sup>s</sup> <sup>=</sup> <sup>700</sup> ◦C exhibited an initial discharge capacity of 175 mC·cm−2·μm−1. Yuki et al. used, as the oxygen evolution reaction (OER), electrocatalysts, which were prepared LNO films from a target composed of Li2O and NiO2 sintered at 1000 ◦C for 8.5 h in air ablated by a Nd:YAG laser operating at 532 nm [96]. Recently, PLD Li*x*Ni2−*<sup>x</sup>*O2 thin films with 0.15 ≤ *x* ≤ 0.45 deposited on a glass substrate under a pressure of 0.1 Pa and annealed at 350 ◦C were grown by PLD with an LiNiO2 structure. The films appeared to be entirely made of particles even in the cross-section (grain size of 95 nm for *x* = 0.45). The average surface roughness estimated from the AFM measurements decreased with an increasing *x*, reaching a value of 0.615 nm for *x* = 0.45 [97].

#### *3.3. LiNi1*−*yCoyO2 (NCO)*

The LiNi1−*y*Co*y*O2 (0<*y*<1) system with a layeredα-NaFeO2 structure belongs to a LiCoO2-LiNiO2 solid solution with a higher reversible capacity than LCO and better cycleability than LNO. Among these substituted oxides, Ni-rich LiNi0.8Co0.2O2 (NCO) has been identified as one the most attractive cathodes [98]. In this context, several works investigated the growth of NCO thin films using pulsed-laser deposition. Dense PLD NCO films grown at *T*<sup>s</sup> > 400 ◦C exhibited a gravimetric density of 4.8 g·cm−<sup>3</sup> [99]. Ramana et al. grew NCO films deposited on an Ni foil substrate at temperatures of 25 ≤ *T*<sup>s</sup> ≤ 500 ◦C under *P*O2 = 6 to 18 Pa from Li-rich ceramic (15 mol% Li2O excess to avoid NiO or Co3O4 impurity phases) [100]. At *T*<sup>s</sup> ≤ 300 ◦C, the PLD film showed the highest intensity of the (00*l*) reflection, which indicates that the *c*-axis was normal to the film surface. The XRD (003) diffraction peak at 2θ = 18.5◦ corresponds to an interplanar spacing of 0.145 nm. Phase diagram mapping (Figure 6) was proposed to highlight the effect of the growth temperature on the microstructure of PLD LiNi0.8Co0.2O2 films. Galvanostatic titration carried out at a rate of *C*/30 in the potential range of 2.5 to 4.2 V showed a discharge capacity of 82 <sup>μ</sup>Ah·cm−<sup>2</sup> <sup>μ</sup>m<sup>−</sup>1, which compares with the theoretical value of <sup>136</sup> <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> (490 mC·cm−2·μm<sup>−</sup>1) [101].

Hirayama et al. fabricated NCO films using the standard PLD conditions (Φ = 100–220 mJ, *P*O2 = 3.3 Pa, target composition Li/Ni(Co) = 1.3, and *T*<sup>s</sup> = 600–650 ◦C) at a deposition rate of 0.3 nm·min−<sup>1</sup> on an oriented SrTiO3 (STO) substrate. Microstructural analysis shows a misfit of ca. 5% and roughness of 1 to 3 nm for the film grown with an in-plane orientation at *T*<sup>s</sup> = 600 ◦C. AFM imaging revealed the surface modification for films cycled in the voltage range of 2 to 5 V [102]. PLD NCO films (0.62 μm thick) were electrochemically characterized by galvanostatic titration (GITT), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). Films grown at *T*<sup>s</sup> = 600 ◦C under *P*O2 = 13 Pa with a laser fluence of Φ = 450 mJ per pulse exhibited an average specific capacity of ~60 <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> and the Li<sup>+</sup> diffusion coefficient varied from 3 <sup>×</sup> 10−<sup>13</sup> to 2 <sup>×</sup> 10−<sup>10</sup> cm2·s−1. After 100 cycles, the electrode showed a capacity retention of 85% [103]. The kinetics of the Li-ion intercalation in PLD NCO films grown on an Nb-doped STO substrate at *T*<sup>s</sup> = 600 ◦C under *P*O2 = 3.3 Pa were investigated by EIS [104]. Nyquist plots showed changes of the electrode impedance as a function

of the Li extraction/insertion with a larger value at a potential of 4.2 V. Baskaran et al. prepared NCO films on Pt and Si substrates heated at *T*<sup>s</sup> = 500 ◦C under a low oxygen partial pressure of 0.21 Pa [105]. The 40-min deposited films (120-nm thick) displayed a specific discharge capacity of 69.6 <sup>μ</sup>Ah cm−2·μm−<sup>1</sup> (145 mAh·g<sup>−</sup>1) after 10 cycles. Based on these results, a Li-ion microbattery was fabricated with a LNCO/Li3.4V0.6Si0.4O4(LVSO)/SnO configuration with a thickness of ~1.5 μm. Such a microcell delivered a capacity of 16.1 <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> after 20 cycles.

**Figure 6.** Phase diagram of microstructure development in PLD LiNi0.8Co0.2O2 films as a function of the growth temperature. (Reproduced with permission from [100]. Copyright 2006 American Chemical Society).

#### *3.4. LiNi1*−*yMnyO2 (NMO)*

The substitution of Mn for Ni has been demonstrated to be beneficial for LiNiO2 cathode materials. LiNi0.5Mn0.5O2 (NMO) thin films were prepared on stainless steel and gold substrates from an Li-enriched target with an Li/(Ni + Mn) ratio of 1.5 in the NiO + MnO2 mixture. Under standard conditions (<sup>Φ</sup> = 2 J·cm−2, *T*<sup>s</sup> = 550 ◦C, and *P*O2 = 266 Pa), impurity-free and (00l)-textured PLD films (300–500 nm thick) were obtained after an annealing process at 650 ◦C [106]. Galvanostatic charge–discharge tests showed that NMO films deposited on stainless steel displayed an electrochemical response, with a large voltage plateau between 2.5 and 3 V attributed to the presence of spinel phases (i.e., LiMn2O4 and LiNi0.5Mn1.5O4), while NMO films prepared on Au substrate showed the typical fingerprint of the LiMO2 layered compound with a single plateau at ca. 3.7 V. The analysis of the kinetics from CV measurements in the 2.5 to 4.5 voltage range led to a diffusion coefficient of the Li+-ions of *<sup>D</sup>*\* <sup>=</sup> 3.13 <sup>×</sup> <sup>10</sup>−<sup>13</sup> cm2·s−<sup>1</sup> for the Li insertion and 7.44 <sup>×</sup> <sup>10</sup>−<sup>14</sup> cm2·s−<sup>1</sup> for the Li extraction. GITT results showed that *D*\* is highly dependent on the electrode potential in the range of 10−<sup>12</sup> to 10−<sup>16</sup> cm2·s−<sup>1</sup> [107]. Sakamoto et al. addressed the growth of epitaxial PLD NMO films with an orientation of the basal layered plane (BLP) that depends on the SrTiO3 (STO) substrate plane: The BLP is parallel to the STO(110) substrate, while the BLP is perpendicular to the STO(111) substrate. The relationship between the lattice parameters and applied voltage highlights the charge–discharge processes for both orientations [108]. In a second article, Sakamoto et al. examined the structural properties of the surface and bulk of LiNi0.5Mn0.5O2 epitaxial thin films during an electrochemical reaction using in situ X-ray scattering [109]. In normal conditions, the two-dimensional diffusion of Li during (de)intercalation proceeds for the (110) plane. However, 3D-diffusion activity can be observed for a high degree of cation mixing (Ni2<sup>+</sup> in Li(3*a*) sites).

#### *3.5. Li(Ni, Co, Al)O2 (NCA)*

The growth of LiNi0.8Co0.15Al0.05O2 (NCA) has been envisaged because Al-doping provides excellent structural and thermal stability for the electrode with the suppression of phase transitions. NCA thin films were prepared on Ni substrates at *T*<sup>s</sup> = 500 ◦C by PLD with an energy and laser-pulse repetition rate of 300 mJ and 10 Hz, respectively, under *P*O2 = 18 Pa. The PLD target (i.e., pellet pressed at 1.5 to 5.0 <sup>×</sup> 10<sup>3</sup> kg·cm−2), optimized by using bulk NCA Li-enriched with 15 mol% Li2O as the

precursor, was sintered at 800 ◦C for 24 h [110]. The Li//NCA microcells delivered an initial specific capacity of 92 <sup>μ</sup>Ah·cm−2·μm<sup>−</sup>1. The kinetics of Li<sup>+</sup> ions in PLD films measured by the GITT method in the voltage range of 2.50 to 4.25 V vs. Li+/Li revealed a diffusion coefficient of 4 <sup>×</sup> <sup>10</sup>−<sup>11</sup> cm2·s−<sup>1</sup> with a maximum of 1 <sup>×</sup> 10−<sup>10</sup> cm2·s−<sup>1</sup> for the composition of Li0.7Ni0.8Co0.15Al0.05O2. Figure 7 displays the specific capacity of Li//LiNi0.8Co0.15Al0.05O2 cells as a function of the substrate temperature. PLD films were grown onto various substrates at 25 ≤ *T*<sup>s</sup> ≤ 500 ◦C under a controlled O2 atmosphere (*P*O2 = 50 mTorr). NCA thin films prepared onto Ni foil at *T*<sup>s</sup> = 500 ◦C exhibited the best specific discharge capacity of 100 <sup>μ</sup>A·cm−2·μm<sup>−</sup>1.

**Figure 7.** The specific capacity of LiNi0.8Co0.15Al0.05O2 thin films deposited onto Ni foil, Si wafer, and ITO-coated glass as a function of the substrate temperature.

#### *3.6. Li(Ni, Mn, Co)O2 (NMC)*

Lithiated nickel-manganese-cobalt oxides, LiNi1−*y*−*z*Mn*y*CozO2 (NMC), is a complex LiNiO2- LiMnO2-LiCoO2 solid solution, which displays the same structure as rock salt, α-NaFeO2, with a valence state of cations as Ni2<sup>+</sup>, Mn4<sup>+</sup>, and Co3<sup>+</sup>. LiNi1/3Co1/3Mn1/3O2 (NMC333) thin film electrodes were prepared by pulsed laser deposition on a Pt/Ti/SiO2/Si substrate (where the 150-nm thick Pt and the 100-nm thick Ti films act as the current collector and buffer layer, respectively) at room temperature under a *P*O2 of 6.6 Pa. The deposition rate was about 4.4 nm·min−1. Impurity-free NMC333 films were obtained using an Li-enriched target (15% excess Li2O). The charge–discharge profiles strongly depended on the film morphology that was tuned by increasing the annealing temperature from 400 to 500 ◦C. The best electrochemical features were obtained for annealing at 450 ◦C, showing a discharge plateau of about 3.7 and 3.6 V [111]. Epitaxial and highly textured LiNi0.5Mn0.3Co0.2O2 (NMC532) thin film cathodes were deposited by a one-step PLD process [112]. Using a laser fluence of <sup>Φ</sup> <sup>=</sup> 6 J·cm<sup>−</sup>2, *T*<sup>s</sup> = 750 ◦C, and an Li-enriched target (20% Li2CO3 excess), the films were deposited on silicon (111), stainless steel (SS), and c-cut sapphire (0001) substrates. It was stated that highly dense and epitaxial films with a strong (003) reflection peak intensity are achieved at high *T*s. Films grown on an Au-SS substrate delivered 125 mAh·g−<sup>1</sup> at 0.5C and demonstrated a 72% capacity retention after 100 cycles. Furthermore, 3D-electrode architectures were fabricated, and are applicable to power hearing aids. Structured NMC333 film electrodes (50–80 μm thick) were prepared by laser printing using a pulsed laser (<sup>λ</sup> = 355 nm) at a fluence of 50 to 100 mJ·cm<sup>−</sup>2. Such films exhibit a stable discharge capacity at a low C-rate (0.1C), but for a current density of 1C, the electrode reached 40% of the initial capacity [113]. Recently, Abe et al. elucidated the deterioration mechanism of pristine ZrO2-coated NMC333 thin films prepared by PLD on a (110)STO substrate maintained at two temperatures of 25 and 700 ◦C; the oxygen pressure ranged from 3.3 to 10 Pa [114]. Characterization was performed by cyclic voltammetry, in situ X-ray diffraction, and in situ neutron reflectometry. As a result, ZrO2 coating suppressed the low activity of the spinel phase formed at the surface and hence improved the cycleability of the thin film electrode.

#### *3.7. Li-Rich Layered Oxides*

The growth of Li-rich layered oxide Li1.2Mn0.55Ni0.15Co0.1O2 on SRO/STO(100) and SRO/STO(111) substrate (where SRO and STO are SrRuO3 and SrTiO3, respectively) heated at *T*<sup>s</sup> = 600 ◦C was reported by Bendersky et al. [115]. The transmission electron microscopy (TEM) images recorded using a high-angle annular dark field (HAADF) mode showed a predominant Li2(Mn,Ni,Co)O3 monoclinic phase. Johnston-Peck et al. confirmed the growth of monoclinic Li-rich thin films (*C*2/*m* space group) using a target with the composition of Li1.2Mn0.55Ni0.15Co0.1O2 and displayed the CV response with a non-aqueous electrolyte at a scan rate of 0.1 mV·s−<sup>1</sup> [116]. Yan et al. intended to develop a PLD thin film electrode using an Li-rich layer structured oxide with a composition of Li1.2Mn0.54Ni0.13Co0.13O2 (or written as 0.55Li2MnO3·0.45LiNi1/3Co1/3Mn1/3O2) [117]. Films deposited at *T*<sup>s</sup> = 650 ◦C under *P*O2 = 46 Pa and annealed at 800 ◦C showed the best electrochemical performance with an initial specific discharge capacity of 70 <sup>μ</sup>Ah·cm−2·μm<sup>−</sup>1. However, the differential capacity curves, d*Q*/d*V*, indicated that the layered structure gradually changed to the spinel phase during the charge–discharge cycling.

#### *3.8. Li2MO3 (M* = *Mn, Ru)*

Pulsed-laser deposited Li2MnO3 thin films at various thicknesses (12 < δ < 48 nm) were grown using a KrF excimer laser on Nb:SrTiO3(111) substrates from an Li-rich Li3.2MnO3 target. With synthetic conditions (*T*<sup>s</sup> = 650 ◦C, *P*O2 = 75 Pa, and <sup>Φ</sup> = 0.8–1.1 J·cm−2) and a laser frequency of *f* = 1 to 5 Hz, PLD Li2MnO3(111) films exhibited a single-phase with the *C*2/*m* symmetry. The results of ICP analysis gave a composition of Li1.90MnIVO2.95, which indicates lithium and oxygen vacancies. The highest discharge capacity of 300 mAh·g−<sup>1</sup> was delivered after 50 cycles by a 12.6-nm thick film [118]. PLD epitaxial Li2RuO3 thin films were successfully prepared with a (010) and (001) orientation on STO(110) and (111) substrates heated at *T*<sup>s</sup> = 500 to 550 ◦C under *P*O2 = 3.3 Pa. The initial charge–discharge capacities calculated using a theoretical density of 5.15 g·cm−<sup>3</sup> were 120 and 105 mAh·g−<sup>1</sup> at a 0.5C rate for the (010) and (001) orientation, respectively [119,120].

#### *3.9. LiMn2O4 (LMO)*

Since the early report in 1996 [47], numerous studies have been devoted to LiMn2O4 (LMO) thin films grown by laser ablation. A spinel structure (*Fd*3*m* space group) was successfully prepared using different PLD conditions and applied as a positive electrode in thin-film lithium microbatteries (see, for example, [121]). Most prior works focused on the fundamental properties of PLD-grown LMO cathode films, aiming to deposit the highly structured and porous morphology required for a good operating electrode [7,122–136]. Morcrette et al. analyzed PLD film composition as a function of *T*s and *P*O2 using the Rutherford backscattering method and nuclear reaction analysis [122]. A stoichiometric LMO film was obtained at *T*<sup>s</sup> = 500 ◦C and *P*O2 = 20 Pa, while the Li/Mn ratio decreased with *T*<sup>s</sup> and increased with *P*O2 . The film of Li0.6Mn2O3 (20% oxygen loss) was obtained under vacuum. The Mn3O4 + MnO and Mn3O4 + LiMn2O4 mixed phases were successively grown in the range of 0.01 ≤ *P*O2 ≤ 1 Pa. Julien et al. defined the conditions of the disposition of LMO films grown on Si substrates. Impurity-free well-crystallized samples with a crystallite size of 300 nm were obtained at *T*<sup>s</sup> as low as 300 ◦C and *P*O2 = 10 Pa using an Li-enriched target (15% Li2O excess) to avoid Li deficiency in the film [123]. The electrochemical features of the Li cell showed a specific capacity as high as 120 mC·cm−2·μm−<sup>1</sup> in the voltage range of 3.0 to 4.2 V vs. Li+/Li, which was attributed to the high degree of film crystallinity [7,123]. Simmen et al. studied the relationship between Li1<sup>+</sup>*x*Mn2O4−<sup>δ</sup> thin films and Li excess in the target and concluded that a film deposited from a composite target of Li1.03Mn2O4-<sup>δ</sup> + 7.5 mol% Li2O was the best, exhibiting a discharge capacity of <sup>42</sup> <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> [127]. Various substrates were successfully used for the epitaxial growth of LiMn2O4 (LMO) spinel thin films, such as Pt, Si, Au, MgO, Al2O3, and SrTiO3. Gao et al. [128] reported a detailed mechanism of the epitaxial LMO film/substrate (current collector) interface formation. A coherent hetero-interface was formed with the substrate but a tetragonal Jahn–Teller distortion was

observed, induced by oxygens' non-stoichiometry and the lattice misfit strain. PLD epitaxial LMO thin films were deposited on oriented Nb:SrTIO3 substrates maintained at 950 ◦C from a sintered target with 100 wt.% excess Li2O with different surface morphologies and orientations, such as (100)-oriented pyramidal, (110)-oriented rooftop, or (111)-oriented flat structure. The pyramidal-type LMO was cycled at a 3.3C rate, demonstrating a specific capacity of 90 mAh·g−<sup>1</sup> after 1000 cycles [129]. Using oriented substrates, i.e., (111)Nb:SrTiO3 (STO) and (001)Al2O3 single crystal, LMO films were grown with the (111) orientation under the following synthesis conditions: Li-enriched target (Li/Mn = 0.6), *T*<sup>s</sup> = 650 ◦C, and *P*O2 = 30 Pa [130]. Electrochemical tests emphasized the interactions between the films and substrate, and showed a plateau voltage at 3.6 to 3.8 V for LMO/STO and 3.8 to 4.1 V for LMO/alumina. Canulescu et al. [131] investigated the mechanisms of laser-plume expansion during the PLD of LMO films under *P*O2 ranging between 10−<sup>4</sup> and 20 Pa. The Li deficiency occurs as a result of the different behavior of the species at elevated *T*s. Hussain et al. [132,133] obtained highly oriented LiMn2O4 thin films on oriented Si substrates heated in the range of 100 ≤ *T*<sup>s</sup> ≤ 600 ◦C under *<sup>P</sup>*O2 <sup>=</sup> 10 Pa and with a laser fluence of <sup>Φ</sup> <sup>=</sup>10 J·cm<sup>−</sup>2. Grains with a spherical shape (around 230 nm in diameter) changed to a flake-like structure at *T*<sup>s</sup> = 600 ◦C (Figure 8a). The grain size varied almost linearly with the substrate temperature (Figure 8b).

**Figure 8.** (**a**) SEM images of LiMn2O4 films deposited at *T*<sup>s</sup> = 300 ◦C and *T*<sup>s</sup> = 600 ◦C. (**b**) The grain size of PLD thin films as a function of *T*s. (Reproduced with permission from [133]. Copyright 2007 Springer).

By applying an elevated-temperature PLD technique, Tang et al. [134] studied the influence of the substrate temperature (*T*s) and the oxygen partial pressure (*P*O2 ) on LMO film crystallinity. LMO thin films deposited on Si (001)/0.2 μm-SiO2 substrates at 575 ◦C under 13 Pa oxygen had a flat and smooth surface and exhibited mainly a (111) out-of-plane preferred texture (Figure 9a). Such films of the 300 nm thickness showed a very dense cross-section (density ~4.3 g·cm−3) (see Figure 9b) [135]. The effect of stoichiometric deviations on the electrochemical performance of an LMO thin-film cathode was investigated by Morcrette et al. [136], while the kinetics of Li<sup>+</sup> ions in the LMO thin-film framework were documented by Yamada and coworkers [137]. A high-activation barrier of 50 kJ·mol−<sup>1</sup> for Li-ion transfer was identified at the electrode/electrolyte interface for films deposited at *T*<sup>s</sup> = 700 ◦C and *P*O2 = 27 Pa. Albrecht et al. [138] reported the minimum crystallization temperature of spinel LMO thin films in a narrow annealing temperature range of around 700 ◦C. Electrochemical tests carried out with the galvanostatic cycling with the potential limits (GCPL) method proved that Li-ions are (de-)intercalated in different tetrahedral sites for which the processes occur at potentials that are slightly shifted by *U* ≈ 100 mV, which is similar to the previous results by Julien et al. [7].

**Figure 9.** (**a**) Surface morphology and (**b**) cross-sectional picture of PLD-grown LMO thin film deposited on an Si(001) substrate covered by a 0.2 μm SiO2 layer at 575 ◦C under a 13 Pa oxygen partial pressure. (Reproduced with permission from [135]. Copyright 2008 Elsevier).

Studies of the structure and electrochemical reactivity of heteroepitaxial LiMn2O4/La0.5Sr0.5CoO3 (LMO/LSCO) bilayer thin films deposited on crystalline SrTiO3 substrates show that LSCO reduced the lattice misfit strain with the substrate and favored a lower LMO surface roughness. However, a decrease of the electrical conductivity occurred during the electrochemical test (after first cycle) due to the lattice oxygen loss at the outermost layer (40 nm) [139]. Tang et al. reported a comparative investigation of the structures, morphologies, and properties of Li insertion for LMO films with different crystallizations. At *T*<sup>s</sup> = 400 ◦C, LMO films consisted of nanocrystallites < 100 nm in size with rough surfaces that exhibited a discharge capacity of 61 <sup>μ</sup>Ah·cm2·μm−<sup>1</sup> with a capacity loss of 0.032% per cycle up to 500 cycles, while for *T*<sup>s</sup> = 600 ◦C and *P*O2 = 10 Pa, highly crystallized films showed an initial discharge capacity of 54.3 <sup>μ</sup>Ah·cm2·μm−<sup>1</sup> [134]. The intrinsic properties of PLD-grown LMO have been investigated by several techniques. Electrical measurements of LMO films showed that the conductivity is sensitive to *T*s, as the activation energy that followed the Mott's rule increased with *T*s up to *E*<sup>a</sup> = 0.64 eV at *T*<sup>s</sup> = 600 ◦C [133]. Singh et al. characterized the crystallinity and texture of LMO films deposited at *T*<sup>s</sup> = 650 ◦C. Here, (111)-oriented films were grown on a doped Si substrate, while films deposited on a stainless-steel substrate exhibited a (001) orientation [140]. The thermo-power (or Seebeck coefficient) of PLD LMO films was reported to be 70 <sup>μ</sup>V·K−<sup>1</sup> [141].

PLD LMO films were subjected to an overcharge (5 V vs. Li+/Li), which did not modify the structure and preserved the well-resolved voltage peaks at 4.1 and 4.2 V, while an overdischarge (2 V vs. Li+/Li) led to a loss of capacity due to the structural disorder associated with the tetragonal transition, i.e., Jahn–Teller distortion [142]. Singh patented the fabrication of PLD Li1−*xMy*Mn2−2*z*O4 films, where *M* is a doping element (*M* = Al, Ni, Co, Cr, Mg, etc.) and *x*, *y*, and *z* vary from 0.0 to 0.5 [143]. These defective spinel structures enhanced the oxygen content as compared to LiMn2O4 crystal. In particular, the oxygen-rich Li1−<sup>δ</sup>Mn2−<sup>δ</sup>O4 films were superior cathode films, leading to excellent rechargeable battery performances. It is claimed that a high discharge rate of 25C produces only a 25% capacity loss and a specific capacity >150 mAh·g−<sup>1</sup> remains after 300 cycles. Rao et al. reported the preparation of well-crystallized LMO films at a high substrate temperature of *T*<sup>s</sup> = 700 ◦C and *P*O2 = 13 Pa that delivered a capacity of 133 mC·cm−2·μm−<sup>1</sup> at a very slow C/100 rate [144]. Several workers reported the evolution of the thin-film electrode/electrolyte interface, as the planar form of the film is the ideal geometry for such investigations [145–148]. Room temperature impedance measurements were carried out to identify the formation of the solid electrolyte interface (SEI) layer on a PLD LMO film cathode and the degradation mechanism during cycling in an aprotic electrolyte containing LiPF6 salt. A reversible disproportionation reaction was suggested with the formation of the Li2Mn2O4 and λ-MnO2 phases at the surface [146]. Using epitaxial-film model electrodes, Hirayama studied the surface reaction and the formation of the SEI layer and the interfacial structural reconstruction during an initial battery process using in situ surface X-ray diffraction and reflectometry [147]. TEM images confirmed the surface reconstruction that occurred during the first charge, i.e., when a potential was applied. After 10 cycles, the SEI layer was observed on both the (111) and (110) surfaces and Mn dissolution appeared at the (110) surface [148]. Inaba et al. [149] investigated the surface morphology evolution of PLD LMO thin films

grown on a PT substrate at *T*<sup>s</sup> = 600 ◦C by electrochemical scanning tunneling microscopy (STM) with voltage cycling in the range of 3.5 to 4.25 V. The original LMO grains of 400 nm in size coexisted with small particles 120 to 250 nm in size, which appeared after 20 cycles and decreased to ~70 nm after 75 cycles through a kind of dissolution/precipitation process. LMO thin-film electrodes with a grain size of <100 nm deposited on a stainless steel substrate at *T*<sup>s</sup> = 400 ◦C under a 26 Pa oxygen partial pressure displayed an excellent capacity of 62.4 <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> when cycled at a 20 <sup>μ</sup>A·cm−<sup>2</sup> current density in the voltage range of 3.0 to 4.5 V. A very low capacity fading was recorded for up to 500 cycles at 55 ◦C. Li+-ion diffusion coefficients evaluated from EIS measurements were around 2.7 <sup>×</sup> <sup>10</sup>−<sup>12</sup> cm2·s−<sup>1</sup> for an electrode charged at 4.0 V and 2.4 <sup>×</sup> 10−<sup>11</sup> cm2·s−<sup>1</sup> for 4.2 V [150]. Xie et al. [151] investigated the Li+-ion transport in LMO thin films (~100 nm thick) grown on Au substrates at 600 ◦C at a deposition rate of 0.14 nm·min−1. The chemical diffusion coefficients determined by the EIS, GITTm, and PITT methods were in the range of 10−<sup>14</sup> to 10−<sup>11</sup> cm2·s−<sup>1</sup> in the voltage range of 3.9 to 4.2 V. Table 5 lists some typical results on the kinetics of Li<sup>+</sup> ions in pulsed-laser deposited LMO thin films.

**Table 5.** Diffusion coefficients of Li<sup>+</sup> ions in PLD LMO thin film frameworks.


The electrochemical behavior of Li-rich spinel Li1.1Mn1.9O4 thin films grown by PLD on an Au substrate was reported by several workers [156,157]. The best performance was reported at a discharge current density of the 36C-rate for LMO films deposited for 30 min in *P*O2 = 30 Pa and *T*<sup>s</sup> = 600 ◦C with an Nd:YAG laser (266 nm) adjusted to an energy fluence of 1 J·cm−<sup>2</sup> [156]. Nanocrystalline LMO films with grains less than 100 nm were deposited on a stainless-steel substrate at *T*<sup>s</sup> = 400 ◦C and *P*O2 = 26 Pa using a PLD pulse power of 100 mJ at the frequency of 10 Hz. The film cycled over 100 cycles delivered a specific capacity of 118 mAh·g−<sup>1</sup> at a current density of 100 A·cm−<sup>2</sup> [157]. Using reflectometry measurements, Hirayama et al. [158] studied the structural modifications at the electrode/electrolyte interface of a lithium cell, in which the LMO electrodes were prepared as epitaxial films by the PLD method with different orientations. The respective orientation of the LMO film corresponded to that of the substrate plane, i.e., the (111), (110), and (100) planes of the SrTiO3 substrate. No density change was observed for the (110) and (100) planes, whereas a defect layer was detected in the (111) plane. ZrO2-modified LiMn2O4 thin films prepared via PLD consisting of amorphous ZrO2 formed on the grain boundary and the outer layer of the LMO matrix [159]. The high capacity retention of 82% after 130 cycles of films of *x*ZrO2-(1−*x*)LiMn2O4 (*x* = 0.025) monitored at the 4C rate was attributed to the decrease of the charge transfer resistance (*R*ct).

Epitaxial LiMn2O4/La0.5Sr0.5CoO3 (LMO/LSCO) bilayer thin films with sub-nano flat interfaces were deposited on (111)-oriented STO substrates at *T*<sup>s</sup> = 650 ◦C in *P*O2 = 4 Pa. After the first charge–discharge cycle, the decrease of the electrical conductivity of the LSCO buffer layer due to lattice oxygen loss induced capacity fading [139]. The PLD growth of a multilayer LMO thin film electrode demonstrated the compensation of lithium loss during deposition [160]. Such a sample prepared in the PLD conditions (*T*<sup>s</sup> = 650 ◦C, <sup>Φ</sup> = 530 mJ·cm−2, and *P*O2 = 1.3 Pa) showed the typical two pairs of voltammetry peaks at 0.82 and 1.02 V vs. Ag/AgCl in an aqueous cell. Kim et al. [161] prepared a Li0.17La0.61TiO3/LiMn2O4 (LLTO/LMO) hetero-epitaxial electrolyte/electrode by PLD with an energy fluence of <sup>Φ</sup> = 1.7 J·cm−<sup>2</sup> in a *P*O2 = 6.6 Pa atmosphere. The typical herostructure is composed of a 17.5-nm thick LMO, 7-nm thick interfacial layer, and 26.5-nm thick LLTO deposited on a (111)-oriented

STO substrate. Voltammograms of the first and second cycles displayed redox peaks around 3.8 V attributed to an oxygen-deficient LMO and around 4.0 and 4.2 V, which are the typical redox voltages of the LMO spinel. Suzuli et al. [162] prepared multi-layer epitaxial LiMn2O4/SrRuO3 (LMO/SRO) thin film electrodes deposited for 30 min via PLD on (111)STO substrates heated at 650 ◦C using an Li1.2Mn2O4 target in *P*O2 = 6.6 Pa. The LMO(33 nm)/SRO(38 nm) film exhibited a discharge capacity of 125 mAh·g−<sup>1</sup> with the typical plateau regions of LMO in the charge–discharge reaction. Yim et al. substituted Sn for Mn in PLD LMO thin films [163]. The LiSn*x*/2Mn2−*<sup>x</sup>*O4 films were prepared on a Pt/Ti/SiO2/Si(100) substrate in the conditions of *T*<sup>s</sup> = 450 ◦C, *P*O2 = 26.7 Pa, <sup>Φ</sup> = 4.6 J·cm−2, 10 Hz pulse frequency, and 4 cm target-substrate distance. XPS and EXAFS measurements showed that Sn2<sup>+</sup> cations replace Mn3<sup>+</sup> ions, which resulted in an increase of the valence of Mn in the spinel lattice. A high specific capacity of <sup>∼</sup>120 mAh·g−<sup>1</sup> and cycleability with a capacity retention <sup>&</sup>gt;81% at the 4C rate after 90 cycles was attributed to the Mn-deficient structure. A multi-layer PLD process was utilized to deposit LMO films (90 nm thick) on Si-based substrates coated with Pt as the current collector [164]. A reversible capacity of 2.6 <sup>μ</sup>Ah·cm−<sup>2</sup> (corresponding to a specific capacity of <sup>≈</sup>28 <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> or 66 mAh·g−<sup>1</sup> assuming a dense film with 4.3 g·cm−3) was reached at an extremely high current density of 1889 <sup>μ</sup>A·cm−<sup>2</sup> (equivalent to the 348C rate) with a capacity retention of 86% over 3500 cycles. A significant non-diffusion-controlled contribution (pseudocapacitive-like) was evidenced by cyclic voltammetry; however, the two typical voltage plateaus in the GCD of LMO (around 4 V) indicates that the faradaic redox reaction is the main process. For an easy comparison, Table 6 lists the electrochemical properties of PLD-prepared LMO thin film electrodes.



#### *3.10. LiNi0.5Mn1.5O4 (LNM)*

LiNi*x*Mn2−*<sup>x</sup>*O4 is a substituted oxide spinel that operates at high voltages >4.5 V upon Li extraction. Substituted spinel films of Li*x*Mn2−*yMy*O4 where *M* = Ni, Co and 0 ≤ *y* ≤ 0.25 as-prepared with a crystalline morphology (0.3 μm thick) showed the typical features of high-voltage electrodes without carbon additive or binder materials in the range of 2.0 to 5.8 V vs. Li+/Li [165]. Cyclic voltammetry showed that: (i) PLD LiMn1.9Ni0.1O4 films charged at 5.7 V do not show capacity fading; (ii) LiMn2O4 and LiMn1.75Co0.25O4 films present a good stability to 5.6 and 5.4 V vs. Li+/Li, respectively; and (iii) below 3 V the films exhibit the typical Jahn–Teller distortion. The compound, LiNi0.5Mn1.5O4 (LNM), is more interesting because of the oxidation state of cations: Ni2<sup>+</sup> can be oxidized twice (i.e., 2e<sup>−</sup> transfer) during charge while Mn4<sup>+</sup> is electrochemically inactive. Xia et al. showed that laser-ablated LNM films 0.3 to 0.5 μm thick deposited on a stainless steel substrate heated at 600 ◦C under an oxygen partial pressure of 26 Pa exhibit excellent capacity retention (i.e., ~120 mAh·g−<sup>1</sup> after 50 cycles) in the voltage range of 3 to 5 V vs. Li+/Li [166]. Well-crystallized oxygen deficient LiMn1.5Ni0.5O4−<sup>δ</sup> films deposited by PLD at a controlled fluence of <sup>Φ</sup> = 2 J·cm−2, *T*<sup>s</sup> = 600 ◦C, and *P*O2 = 26 Pa for 40 min exhibited a stepwise voltage profile near 4.7 V and a small plateau in the 4 V region. These

disordered spinel structures had a stable specific capacity of 55 <sup>μ</sup>A h cm−2·μm−<sup>1</sup> in the voltage range of 3 to 5 V vs. Li+/Li. The good rate capability was due to the high kinetics for Li diffusion in the range of 10−<sup>12</sup> to 10−<sup>10</sup> cm2 s−<sup>1</sup> measured by the potentiostatic intermittent titration technique (PITT). These values are comparable to that of layered LCO [167,168]. Epitaxial LNM films were grown on single-crystal oriented SrTiO3 (STO) substrates from an Li-enriched target with Li/(Ni + Mn) = 0.6. The film orientation, i.e., (100)-, (110)-, and (111)-oriented, were the replica of those of the STO substrates [169]. Depending on the film orientation and thickness, the discharge profiles exhibited two to three plateaus around 3.9, 4.5, and 4.7 V vs. Li+/Li, which were attributed to the Mn<sup>3</sup>+/Mn4+, Ni<sup>2</sup>+/Ni3<sup>+</sup>, and Ni<sup>3</sup>+/Ni4<sup>+</sup> redox couples, respectively. Note that the emergence of the Mn<sup>3</sup>+/Mn4<sup>+</sup> redox couple was due to the introduced oxygen vacancies [170].

Other 5-V class cathode thin films include LiCoMnO4. PLD LiCoMnO4 films prepared under standard conditions (i.e., *<sup>T</sup>*<sup>s</sup> <sup>=</sup> <sup>500</sup> ◦C, *<sup>P</sup>*O2 <sup>=</sup> 20–100 Pa, and <sup>Φ</sup> <sup>=</sup> 2 J·cm<sup>−</sup>2) had a composition of Li:Co:Mn = 0.99:0.98:1. These films were tested in the voltage range of 3.0 to 5.5 V vs. Li+/Li in SSMB consisting of Li/Li3PO4/LiCoMnO4 fabricated on Pt/Cr/SiO2 substrates [171]. Cyclic voltammetry showed that the higher capacity in the 5-V region was obtained for the film grown under *P*O2 = 100 Pa (Figure 10). A specific discharge capacity of 90 mAh·g−<sup>1</sup> remained after 20 cycles. Epitaxial Li0.92Co0.65Mn1.35O4 film with a cubic spinel structure was grown on a SrTiO3(111) single-crystal substrate using a layer-by-layer technique, which consisted of repeating a Li1.2Mn2O4/Li1.4CoO2 deposition process at *T*<sup>s</sup> = 650 ◦C and *P*O2 = 6.6 Pa using a KrF excimer laser (λ = 248 nm) [172]. For a film area of 0.7 mm2, thickness of 33.4 nm, and density of 4.38 g cm<sup>−</sup>3, the specific discharge capacity was 340 mA·g−<sup>1</sup> at the second cycle. A capacity retention of 80% was observed after 20 cycles.

**Figure 10.** Cyclic voltammogram recorded at a 0.5 mV·s−<sup>1</sup> scan rate of a Li/Li3PO4/LiCoMnO4 thin film battery. The LiCoMnO4 film cathode was grown under *P*O2 = 100 Pa. (Reproduced with permission from [171]. Copyright 2014 Elsevier).

#### *3.11. MnO2*

Due to its environmental compatibility and low cost, manganese oxides are promising candidate materials for supercapacitor applications using neutral aqueous solution as the electrolyte (i.e., 0.5 mol·L−<sup>1</sup> K2SO4) [173]. Xia et al. prepared a dense Mn3O4 spinel thin film grown by PLD, which transformed to nanoporous MnO*<sup>x</sup>* after electrochemical lithium insertion/extraction. After 2000 cycles, the MnO*<sup>x</sup>* film deposited at *<sup>T</sup>*<sup>s</sup> <sup>=</sup> <sup>600</sup> ◦C under *<sup>P</sup>*O2 <sup>=</sup> 26 Pa demonstrated a specific capacitance of 193 F·g−<sup>1</sup> at a current density of 5 A·g−<sup>1</sup> [174]. The fundamental aspects of the redox reaction were investigated on amorphous MnO*<sup>x</sup>* and crystalline Mn2O3 films prepared by PLD onto Si and (316)-stainless steel substrates [175]. Using standard conditions (<sup>Φ</sup> = 2–3 J·cm<sup>−</sup>2, *P*O2 = 13 Pa, *<sup>T</sup>*<sup>s</sup> <sup>=</sup> 200–500 ◦C) Co-doped MnO*<sup>x</sup>* films were grown from a hybrid Co3O4/Mn3O4 target. Non-doped and 3% Co-doped amorphous films (i.e., MnO*<sup>x</sup>* and Mn0.97Co0.03O*<sup>x</sup>* film) exhibited a specific capacitance of 45 and 99 F·g−<sup>1</sup> at a 5 mV·s−<sup>1</sup> scan rate, respectively. The V2O5/Mn3O4 target was used to prepare PLD-grown V-doped MnO*<sup>x</sup>* thin films with a

V content in the range of 3.3 to 10 atm. %. The specific capacitance of the crystalline PLD Mn2O3 film reached the value of 290 F·g−<sup>1</sup> at a 1 mV·s−<sup>1</sup> scan rate, while the 10 atm. % V-doped film (Mn0.9V0.1O2) had a higher specific current value [176].

#### *3.12. LiMPO4 (M* = *Fe, Mn) Olivines*

LiFePO4 (LFP) thin-film electrodes have been successfully fabricated by pulsed-laser deposition [156,177–179]. It was shown that, due to the film thickness and carbon content, the electrochemical performances are very sensitive, i.e., electronic conductivity and Li-ion diffusion. Iriyama et al. reported the PLD growth of olivine structured LFP thin films and their electrochemical properties characterized by cyclic voltammetry and charge–discharge tests [180,181]. The typical olivine features were evidenced by CV measurements in the range of 2.0 and 5.0 V vs. Li+/Li, i.e., a single couple of anodic and cathodic peaks at ~3.4 V. Song et al. synthesized PLD LFP films with a low carbon content (<1 wt.%) on stainless steel substrates utilizing an Ar atmosphere [182]. The 75-nm thick films showed reversible cycling of more than 80 mAh·g−<sup>1</sup> after 60 cycles. Furthermore, 156-nm thick films grown using a target–substrate distance reduced to 5 cm had a layered surface texture and delivered more than 120 mAh·g−<sup>1</sup> with a good capacity retention. LFP thin films with a needle-like morphology were prepared by an off-axis PLD technique [183]. The effect of the substrate on the structure and morphology was examined by Palomares et al. for PLD film deposited under argon gas kept at a pressure of 8 Pa [184]. Stainless steel was demonstrated to be the best substrate for the single-phase olivine (*Pnma* space group) with a temperature set at 500 ◦C.

LiFePO4 deposited by pulsed-laser deposition proved to be effective as a thin film electrode. Tang et al. stated that a well-crystallized pure olivine phase was grown using optimized deposition parameters (*T*<sup>s</sup> =500 ◦C,*P*Ar =20–30 Pa, pulse power of 120mJ, pulse frequency of 10 Hz, λ = 248 nm) [185]. An electrochemical capacity of 38 <sup>μ</sup>Ah cm−2·μm−<sup>1</sup> at the C/20 rate (36 <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> at a rate of C/4) was measured at 25 ◦C. High substrate temperatures (500 <sup>≤</sup> *T*<sup>s</sup> <sup>≤</sup> 700 ◦C) favored the presence of Fe3<sup>+</sup> impurities, i.e., Li3Fe2(PO4)3 and Fe4(P2O7)3. In a second article, the same group analyzed the kinetics of Li<sup>+</sup> ions in PLD LFP films using CV, GITT, and EIS measurements [179]. CV data provided average *D*\* values of 10−<sup>14</sup> cm2·s−1, while *D*\* deduced from both GITT and EIS techniques was in the range of 10−<sup>14</sup> to 10−<sup>18</sup> cm2·s<sup>−</sup>1. A maximum *D*\* value was observed at *x* = 0.5 for Li*x*FePO4. Lu et al. prepared different composite thin films, i.e., LiFePO4–Ag and LiFePO4–C, with the aim of enhancing the electronic conductivity [177,178]. It was found that films grown with 2 mol% carbon and annealed at 600 ◦C for 6 h had an improved coulombic efficiency. Well-crystallized olivine-type structure LFP films were obtained by PLD coupled with high temperature annealing of 650 ◦C. The first discharge capacity was 27 mAh·g−<sup>1</sup> with a retention of only 49% after 100 cycles. The low reversible capacity and poor cycling performance was attributed to the existence of an Fe2O3 impurity produced by the high temperature treatment and poor intrinsic conductivity [186]. Sauvage et al. published several reports on the electrochemical properties of PLD LFP thin films grown in different configurations [187–190]. First, it was shown that well-crystallized and homogeneous 300-nm thick LFP films deposited on Pt-capped Si substrates have intrinsic Li insertion properties evaluated both in aqueous and non-aqueous electrolytes, i.e., voltage plateau at 3.42 V vs. Li+/Li [187]. Second, the influence of the film thickness was studied in the range of 12 to 600 nm [188]. Third, the effect of the texture on the electrochemical performance was analyzed for PLD films deposited on a polycrystalline α-Al2O3 substrate coated with a 20-nm thick Pt layer from an LiFePO4 pellet as the target. The standard PLD conditions were used (i.e., (<sup>Φ</sup> = 2 J·cm−2, *P*Ar = 8 Pa, *T*<sup>s</sup> = 600 ◦C) [189]. Finally, the electrochemical stability of LFP films was analyzed as a function of the exposition to the most common lithium salt and for different current collectors (i.e., Si, Pt, Ti, Al, and (304)-stainless steel) [190]. A 270-nm thick film tested by CV at a 2 mV·s−<sup>1</sup> scanning rate in 1 mol·L−<sup>1</sup> LiClO4 in EC/DMC solution delivered a specific capacity of 1.52 μAh cm−<sup>2</sup> after 150 cycles. Recently, Raveendran et al. reported the properties of FeSe and LiFeO2/FeSe bi-layers prepared by PLD as cathode materials [191]. Mangano-olivine LiMnPO4 (LMP) thin films were fabricated on Pt-coated SiO2 glass substrates using PLD parameters, e.g., <sup>Φ</sup> <sup>=</sup> 1.58 J·cm<sup>−</sup>2, *<sup>T</sup>*<sup>s</sup> <sup>=</sup> 400–700 ◦C, and *<sup>P</sup>*Ar <sup>=</sup> 2–100 Pa [45]. LMP

films (50-nm thick, 0.09 cm2 area) were applied in Li/Li3PO4/LiMnPO4 microbatteries for 500 cycles. From the CV measurement, a capacity of 28 mAh·g−<sup>1</sup> at 20 mV min−<sup>1</sup> was reported.

*3.13. V2O5*

Another candidate material for the cathodes of microbatteries is V2O5, in which about 1 mol of Li<sup>+</sup> ions can be inserted and extracted without the phase transformation of V2O5, leading to a theoretical specific capacity of 147 mAh·g−1. Due to its stable layered structure and its ability to accommodate large amounts of Li ions, V2O5 has been widely studied for the development of electrochromic displays, color memory devices, and lithium-battery cathodes [192]. Extensive works have evidenced the advantages of PLD for the preparation of V2O5 films with a good reproducible stoichiometry similar to the target material [193–207]. The first work related to the growth of V2O5 thin film by PLD as an electrode for a thin-film lithium battery was reported by the National Renewable Energy Labs (USA) [193] followed by Julien's group [194,195]. A major advantage of laser ablation deposits is that it is possible to prepare thin layers of crystallized V2O5 under oxygen at a relatively low temperature of 200 ◦C [193]. The growth mechanism of PLD V2O5 thin films has been proposed by Ramana and coworkers [196]. It was reported that the grain size, surface roughness, and global morphology are highly sensitive to the nature and temperature of the substrate for films deposited in an oxygen partial pressure of *P*O2 = 13 Pa. The functional influence of the growth temperature on the grain size for films deposited onto various substrates was also evidenced. Two main features should be pointed out: (i) The exponential variation of the grain size over the substrate temperature range of 25 to 500 ◦C; (ii) the variation is dependent on the substrate material, which is larger for the Si(00) wafer. McGraw et al. reported that pulsed-laser deposited V2O5 films can be grown on a number of low-cost substrates, including SnO2-coated glass, on which highly textured (001) films are obtained at *T*<sup>s</sup> = 500 ◦C under *P*O2 in the range of 0.2 to 0.5 Pa [52,197,198]. PLD thin films of V2O5 were prepared for applications in lithium batteries using a ceramic V2O5 target and a KrF laser of a wavelength of 248 nm. Depending on the temperature of the substrates and the oxygen pressure during deposition, amorphous or crystallized layers are obtained. PLD-grown amorphous films exhibited a low capacity loss of ~2% over 100 discharge–charge cycles in the voltage range 4.1 to 1.8 V compared to 20% for crystalline film [45,199]. Thin layers of V2O5 were also prepared using a V2O3 target [200]. By making deposits at 200 ◦C with the same V2O3 target, amorphous layers were obtained in the absence of oxygen and layers crystallized in the presence of oxygen. Madhuri et al. [201] reported the successful crystallization of laser-ablated V2O5 thin films at *T*<sup>s</sup> = 200 ◦C. These films were grown in the orthorhombic structure and exhibited a predominant (001) orientation. The growth of crystalline thin dense films without post-deposition annealing was claimed and the good electrochemical performance of PLD films was demonstrated. Iida et al. [202] addressed the electrochromic properties of V2O5 films deposited onto ITO glass as a function of the PLD parameters. The film recrystallization occurred in the range of 400 ≤ *T*<sup>s</sup> ≤ 500 ◦C and the best morphology was obtained for *P*O2 = 13.3 Pa. McGraw et al. deposited thin films of V2O5 for applications in lithium batteries using a ceramic V2O5 target and a KrF laser, with a wavelength of 248 nm. Depending on the temperature of the substrates and the oxygen pressure during deposition, amorphous or crystallized layers were obtained [193,199]. Thin layers of V2O5 were also prepared using a V2O3 target. By making deposits at 200 ◦C with the same V2O3 target, amorphous layers were obtained in the absence of oxygen and layers crystallized in the presence of oxygen. Stoichiometric amorphous V2O5 films can be grown onto substrates maintained at low temperatures (*T*<sup>s</sup> < 100 ◦C) using a sintered V2O5 target. Ramana et al. revealed that stoichiometric V2O5 films can be grown with a layered structure onto amorphous glass substrates at temperatures as low as 200 ◦C and an oxygen partial pressure of 100 mTorr [203]. The onset of crystallization occurred at 200 ◦C with an activation energy of 0.43 to *n*0.55 eV [204]. Correlations between the growth conditions, microstructure, and optical properties were investigated for V2O5 thin films deposited over a wide substrate temperature range of 30 to 500 ◦C by Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and UV−vis−NIR

spectral measurements. As shown in Figure 11, the film grain size follows a power law of the substrate temperature and the optical energy bandgap decreases from 2.47 to 2.12 eV with the increase of *T*s from 30 to 500 ◦C [205]. Bowman and Gregg investigated the effect of the applied strain on the resistance of V2O5 thin films grown from both metallic vanadium and a ceramic V2O5 target using a laser fluence of ~3.0 and ~1.5 J cm−2, respectively [208]. Deng et al. compared the growth of V2O5 films using femtosecond (f-PDL) and nanosecond (n-PDL) pulsed laser deposition using SEM, XRD, and Raman spectroscopy. Prior to annealing, f-PLD films showed a rougher texture and nano-crystalline character, while n-PLD films were much smoother and predominantly amorphous [209].

**Figure 11.** Variation of the grain size in V2O5 thin films as a function of the substrate temperature. (Reproduced with permission from [205]. Copyright 2005 American Chemical Society).

The PLD growth conditions were refined by an analysis of the surface properties for the production of high-quality V2O5 films. The investigations were carried out by AFM, SEM, FTIR, and XRD. AFM measurements showed a surface roughness of ~12 nm with a Gaussian-like height distribution of surface grains for films deposited at *T*<sup>s</sup> = 200 ◦C under *P*O2 = 10 Pa [210]. The local structure of 0.3-μm thick films grown on Si(100) substrates was characterized by Raman spectroscopy [211]. The influence of the deposition temperature on the microstructure was investigated by an examination of the rigid layer-like mode at 145 cm−1, which showed a frequency shift with increasing *T*s. The ability of the V2O5 thin film lattice to accommodate Li<sup>+</sup> ions was also investigated by Raman spectroscopy. The appearance of the δ- and γ-phases of Li*x*V2O5 gave additional insight into the structural changes of lithiated films. From the photoluminescence spectra, Iida et al. evidenced a blue shift of the vanadyl <sup>V</sup> <sup>=</sup> <sup>O</sup> peak upon Li<sup>+</sup> insertion for different electric charge in the range of 0 <sup>≤</sup> *Q* <sup>≤</sup> 20 mC of Li<sup>+</sup> [212]. From X-ray diffraction and Raman spectroscopy data, Shibuya et al. derived a *P*O2 – *T*<sup>s</sup> phase diagram for V-O films grown on Si(100) substrates (Figure 12). The composition of V-O films was as follows: (i) A VO2 monoclinic phase was formed at *T*<sup>s</sup> ≥ 450 ◦C and *P*O2 in the range of 5 to 20 mTorr; (ii) a V2O5 orthorhombic phase was obtained under oxidative conditions, i.e., at high *P*O2 ; (iii) a V6O13 phase was grown under *P*O2 between oxidative and reductive conditions; and (iv) metastable V4O9 and VO2(B) phases were formed for lower *T*<sup>s</sup> (≤400 ◦C) and lower *P*O2 (≤30 mTorr) [213].

V2O5 thin films have been widely used as electrochromic electrodes but few reports are devoted to PLD-grown films. Fang et al. obtained thin films deposited on In2O3:SnO2 (ITO)-coated glass and (111)Si wafer from a V2O5 target using an XeCl laser with a wavelength of 308 nm for applications in electrochromic devices [214,215]. Electrochromic tests over 60,000 cycles showed that a significant change in the optical density (bleached and colored states) was evaluated to be 0.13 at λ = 600 nm for as-prepared films at *T*<sup>s</sup> = 200 ◦C. Crystallized c-axis oriented V2O5 films were obtained under oxygen and at a substrate temperature of 200 ◦C. The durability without long-term degradation of the electrochromic V2O5 films was tested over 8000 cycles in the voltage range of 1.2 to 1.4 V [216]. Ti-doped V2O5 thin films prepared by the pulsed laser ablation technique at *<sup>T</sup>*<sup>s</sup> <sup>=</sup> <sup>200</sup> ◦C and <sup>Φ</sup> <sup>=</sup> 2 J·cm−<sup>2</sup> were studied as the electrode for an electrochromic display that exhibits a neutral brownish blue color.

The long-term durability was verified over 8000 cycles of a voltage cycled in the range from −1.0 to <sup>+</sup>1.0 V vs. SCE showing a charge of 35 mC·cm<sup>−</sup>2. The good cycleability was attributed to the layered structure of PLD crystalline films with a parallel orientation to the substrate, suitable for Li+-ions' transport [215]. PLD thin films of the system, WO3-V2O5, were prepared with a laser fluence of 1 to 2 J·cm−<sup>2</sup> on SnO2/F-coated glass substrates at *T*<sup>s</sup> = 25 ◦C under *P*O2 = 0.1 mbar. Such films with low V contents cycled in the protonic medium. The true color neutrality is the main advantage of V-based WO3 thin films; however, the cell capacity and coloration efficiency decrease with an increase of the V content [217]. The orthorhombic V2O5 phase is also applied as electrodes for sensors. Huotari reported that pure PLD films were obtained at Φ = 2.6 J cm−2, *T*<sup>s</sup> = 400 ◦C, and *P*O2 = 1.0 Pa with a post-annealing treatment at 400 ◦C for 1 h in normal ambient conditions [218]. The efficient response to NH3 at part-per-billion levels. indicates these films use as possible sensing materials for ammonia gas [219].

**Figure 12.** *P*O2 vs. 1000/*T*<sup>s</sup> phase diagram for films of vanadium oxides laser-pulse deposited on silicon substrates. (Reproduced with permission from [213]. Copyright 2015 AIP Publishing).

The electrochemical properties of V2O5 thin-film cathode material have been widely studied in cells with aprotic electrolytes (typically LiClO4 dissolved in propylene carbonate). The electrochemical charge–discharge profiles of PLD V2O5 films were also found to be dependent on *T*s, exhibiting a marked difference for V2O5 films grown at *T*<sup>s</sup> < 200 ◦C when compared to those grown at *T*<sup>s</sup> ≥ 200 ◦C. The effect of the substrate temperature and hence the microstructure on the kinetics of the lithium intercalation process in V2O5 films is remarkable. The applicability of the grown PLD V2O5 films in lithium microbatteries indicates that PLD V2O5 films in the temperature range of 200 to 400 ◦C offer better electrochemical performance than films grown at other temperatures due to their excellent structural quality and stability [25,220]. As an experimental fact, pulsed laser deposited V2O5 thin films exhibit a higher initial voltage than the crystalline material, i.e., ~4.1 vs. ~3.5 V (Li+/Li). For instance, V2O5 thin-film cathodes, deposited from a V6O13 target at a fluence of ~12 J cm−<sup>2</sup> on SnO2-coated glass at *T*<sup>s</sup> = 200 ◦C, were efficient for Li+-ion incorporation. In (*h*00)-textured films, the specific capacity reached values between 50% and 80% of the theoretical value. On the other hand, amorphous films display a stable capacity corresponding to 1.2 F mol−<sup>1</sup> in the voltage range of 4.1 to 1.5 V. Prior textured V2O5 films discharged beyond the threshold to 2.0 V vs. Li+/Li showed an immediate and continuous capacity fading and a quasi-total amorphization after 10 cycles [193,197]. The chemical diffusion coefficient of Li<sup>+</sup> ions, *<sup>D</sup>*\*, measured by PITT was found to be in the range of 1.7 <sup>×</sup> <sup>10</sup>−<sup>12</sup> to 5.8 <sup>×</sup> <sup>10</sup>−<sup>15</sup> cm2·s−<sup>1</sup> in crystalline V2O5 films, which compares well to the value found in Li*x*V2O5 phases, whereas *D*\* displayed a smooth and continuous decrease as the Li content increased in amorphous films [198].

In an attempt to apply PLD V2O5 films in SSMB, a thin-film microbattery was constructed using a glassy Li1.4B2.5S0.1O4.9 electrolyte film with an ionic conductivity of 5 <sup>×</sup> 10−<sup>6</sup> S·cm−<sup>1</sup> at 25 ◦C and an Li anode film. This Li/Li1.4B2.5S0.1O4.9/V2O5 cell delivered a capacity of ~400 mC·cm−2·μm−<sup>1</sup> at a current density of 15 <sup>μ</sup>A·cm−<sup>2</sup> [221]. Ag0.3V2O5 and LiPON thin films with a smooth surface were grown by PLD in an N2 and O2 atmosphere, respectively. The Li/LiPON/Ag0.3V2O5 SSMB displayed good cycleability at a current density of 7 <sup>μ</sup>A·cm−<sup>2</sup> in the voltage window of 1.0 to 3.5 V. The specific capacity was maintained at 40 <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> after 100 cycles [222]. Recently, amorphous vanadium oxide a-VO*<sup>x</sup>* PLD films (650 nm thick) were grown on stainless steel substrates from a V2O5 PLD-target under *P*O2 in the range of 0 to 30 Pa. Films prepared under *P*O2 = 13 Pa had a smooth surface and bore an O/V atomic ratio of 2.13 with a higher atomic percentage of V5<sup>+</sup> than that of V4+. Electrochemical tests carried out in Li cells with 1 mol·L−<sup>1</sup> LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume) as the electrolyte showed a reversible specific capacity as high as 300 mAh·g−<sup>1</sup> at the C/10 current rate and a capacity retention of 90% after 100 cycles [223]. Such studies were initiated by Zhang et al. in 1997 to obtain VO*<sup>x</sup>* films PLD grown at 200 ◦C and exhibiting a specific capacity of 340 mAh·g−<sup>1</sup> at a current density of 0.1 mA·cm−<sup>2</sup> and a capacity loss <sup>&</sup>lt;2% at the end of 100 cycles [201]. A summary of the electrochemical properties of PLD-grown vanadium oxide thin film electrodes is given in Table 7.

**Table 7.** Electrochemical properties of PLD-prepared vanadium oxide thin film electrodes. *J* is the current density, δ is the film thickness, and Δ*C*<sup>c</sup> is the capacity fading per cycle.


#### *3.14. V6O13*

With the ability of vanadium cations (two V4<sup>+</sup> every V5+) to be reduced, the mixed-valence vanadium oxide, V6O13, the structure of which is formed by alternated single and double layers of VO6 units, can insert reversibly about 6 mol of Li, giving a specific capacity of 311 mAh·g−1. It makes this compound a good candidate for the cathode material of rechargeable batteries [224]. V6O13 films were fabricated by the PLD technique using a pulsed KrF excimer laser (λ = 248 nm, 20 ns pulse duration, 10 Hz frequency, and 4 J·cm−<sup>2</sup> laser fluence). A (100)-oriented Si substrate was maintained at a temperature of 500 ◦C. During the PLD process, the formation of crystalline V6O13 films (dark-bluish color) and the vanadium oxidation state (+2.166) was monitored by controlling the processing temperature and O2 partial pressure. The (002)-oriented V6O13 thin films (50 nm thick) were obtained after a post annealing at 400 ◦C under an oxygen partial pressure of 100 Pa [221]. The discharge profiles for Li//V6O13 thin-film cells were recorded in the voltage range of 3.3 to 2.5 V at a current density of 5 μA cm−<sup>2</sup> (Figure 13). A film as-grown at *T*<sup>s</sup> = 250 ◦C exhibited a steady discharge curve with an insertion uptake of 6Li per V6O13 formula unit, whereas the cell voltage decay was faster for a film deposited at *T*<sup>s</sup> = 25 ◦C. However, the films deposited at *T*<sup>s</sup> = 250 ◦C and annealed at 300 ◦C in an Ar atmosphere displayed a stepped discharge profile with the appearance of a voltage plateau at ca. 3.02 and 2.85 V vs. Li+/Li.

**Figure 13.** Discharge profiles vs. lithium uptake for Li//V6O13 thin-film microbatteries. Active cathode films were grown with: (**a**) *T*s = 250 ◦C, as-deposited; (**b**) *T*s = 250 ◦C, annealed at 300 ◦C in Ar; (**c**) *T*s = 25 ◦C, annealed at 300 ◦C in Ar.

#### *3.15. FeF2*

Iron fluoride is a conversion-type cathode material with a high theoretical specific capacity of 571 mAh·g<sup>−</sup>1. Several groups reported electronic additive-free FeF2 films grown by the PLD technique at low temperatures [225–228]. The electrochemical properties of FeF*<sup>x</sup>* films were reported to be dependent on the substrate temperature. Using an FeF3 target, crystallized-like FeF2 film (*P*42/*mnm* space group) was obtained at *T*<sup>s</sup> = 600 ◦C, while a mixed FeF3-FeF2 phase was grown at *T*<sup>s</sup> = 25 ◦C and single FeF3 phase was prepared at *T*<sup>s</sup> = −50 ◦C [226]. FeF*<sup>x</sup>* deposited on stainless steel substrates under vacuum (5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> Pa) exhibited a capacity of ~600 mAh·g−<sup>1</sup> at a current density of 0.56 <sup>μ</sup>A·cm<sup>−</sup>2. Santos-Ortiz et al. reported the PLD growth of polycrystalline FeF2 thin films on oxide-etched Si(100) and glass substrates using standard conditions (*T*<sup>s</sup> = 400 ◦C, <sup>Φ</sup> = 8 J·cm−2, growth rate of ~6 nm·min<sup>−</sup>1) [227]. A 50-nm thick PLD FeF2 film on stainless steel substrates held at 400 ◦C showed an initial specific discharge capacity of 167 mAh·g−<sup>1</sup> when cycled 200 times in the potential range of 1 to 4 V vs. Li+/Li at the 1C current rate [228].

#### *3.16. MoO3*

MoO3 is an attractive cathode material for microbattery technology from several standpoints: (i) The orthorhombic a-phase is a layered structure favorable for Li insertion between slabs; (ii) Mo has the highest +6 oxidation state, making the high structural stability; (iii) the lattice can be reversibly inserted up to 1.5Li per mole of oxide, yielding a specific capacity of 280 mAh·g<sup>−</sup>1; and (iv) the capacity of the dense film can reach a value of <sup>≈</sup>130 <sup>μ</sup>Ah·cm−2·μm<sup>−</sup>1, almost twice the value for LiCoO2 [229]. In addition to the use as cathode batteries, MoO3 is a material applied in electrochromics, gas sensors, and electro-optics. For certain applications, high-quality films grown by PLD are required.

Currently, PLD MoO3 thin films are grown using a KrF excimer laser (λ = 248 nm) with a fluence of 2 J cm−<sup>2</sup> (energy of 300 mJ per pulse) and deposited on various substrates heated in the range of 25 ≤ *T*<sup>s</sup> ≤ 500 ◦C under an atmosphere of O2 flow maintained at a pressure of 0.1 ≤ *P*O2 ≤ 20 Pa. In the prior report, Julien et al. showed that the structure analyzed by optical spectroscopy strongly depends on *T*s: For *T*<sup>s</sup> < 150 ◦C, an amorphous phase is formed, the β-MoO3 phase grows at *T*<sup>s</sup> ≈ 200 ◦C, and the layered α-MoO3 phase appears at *T*<sup>s</sup> = 300 ◦C [230–233]. Al-Kuhaili et al. reported the growth of polycrystalline MoO3 films on unheated substrates using both XeF and KrF excimer lasers. By tuning the annealing temperature in the range of 300 to 500 ◦C, both the grain size and surface roughness increased. Films formed using the XeF laser (λ = 351 nm) and annealed at 400 ◦C have the best stoichiometry of MoO2.95 [233]. Analyzing the growth mechanism, Ramana and Julien concluded that the thermochemical reaction during ablation strongly influences the structural characteristics of PLD MoO3 films. Above *T*<sup>s</sup> = 400 ◦C, the formation of compositional defects induces structural disorder, i.e., α-β-MoO3−*<sup>x</sup>* phase mixture [234,235].

The applicability of PLD films to an Li microbattery was demonstrated by the best electrochemical features: A discharge capacity of 90 μAh cm−<sup>2</sup> μm−<sup>1</sup> was obtained for *T*<sup>s</sup> = 400 ◦C, while only <sup>53</sup> <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> was delivered for *<sup>T</sup>*<sup>s</sup> <sup>=</sup> <sup>200</sup> ◦C [236]. Puppala et al. investigated the microstructure and morphology of PLD MoO3−*<sup>x</sup>* thin films' growth for catalytic applications using a femtosecond laser (f-PLD) and a nanosecond excimer-laser (n-PLD). Substantially textured films with a partially crystalline phase prior to annealing were obtained by the f-PDL laser, while the n-PLD-grown MoO3−*<sup>x</sup>* films were predominantly amorphous with a smooth surface [237]. Sunu et al. claimed that as-deposited PLD films (*T*<sup>s</sup> <sup>=</sup> <sup>400</sup> ◦C, <sup>Φ</sup> <sup>=</sup> 4–5 J·cm<sup>−</sup>2, repetition rate of 15 to 20 Hz, and *<sup>P</sup>*O2 <sup>=</sup> 500 Pa) are suboxide-like, i.e., mixture of η-Mo4O11 and χ-Mo4O11, which transformed to MoO3 after annealing at 500 ◦C in air for 5 h [238]. Several works reported the PLD growth of films (MoO3)1−*<sup>x</sup>*(V2O5)*<sup>x</sup>* with 0.0 ≤ *x* ≤ 0.3 prepared at room temperature under an oxygen pressure of 13.3 Pa. The effect of the V2O5 content on the coloring switching properties for thermochromic, gasochromic, photochromic, and electrochromic applications was investigated [239,240]. Contrary to pure MoO3, the electrochromism of MoO3-V2O5 films showed that the Mo oxidation state (+6) did not change considerably upon Li<sup>+</sup> insertion, while V5<sup>+</sup> was reduced considerably to V4<sup>+</sup> [239]. A similar improvement of the gas-sensing properties, i.e., the shortest response time and highest transmittance change, was observed for V2O5-doped MoO3 films under an H2 atmosphere [240].

#### *3.17. WO3*

Tungsten oxide (WO3) belongs to the class of "chromogenic" materials, i.e., materials exhibiting coloration effects through electro-, photo-, gas-, laser-, and thermochromism processes, which requires the high homogeneity provided by the PLD technique. Preliminary studies of the growth of WO3 thin films by PLD were first attempted by Haro-Poniaowski et al. [233] in 1998. Later, Rougier et al. reported the PLD conditions for the growth of efficient WO3 films as electrochromics (EC) components [241]. The microstructure of films deposited on SnO2:F coated glass substrate is strongly sensitive to both the oxygen pressure and substrate temperature: (i) Crystallized films are formed for *T*<sup>s</sup> = 400 ◦C and *P*O2 = 10 Pa; (ii) amorphous films are obtained for *P*O2 = 1 Pa at any *T*s; (iii) for *T*<sup>s</sup> = 25 ◦C and *P*O2 = 1 Pa, WO3 films are blue colored and conductive; and (iv) colorless insulator films are grown for *T*<sup>s</sup> = 25 ◦C and *P*O2 = 10 Pa, which display the best electrochromic properties. Qiu and Lu showed that oxygen deficient WO3−<sup>δ</sup> films with a deviated monoclinic structure were produced using PLD parameters as 2.5 J·cm−2, *P*O2 = 26 Pa, and a target-Si(100) substrate distance of *d* = 5 cm [242]. Ramana et al. investigated the structural transformations of PLD WO3 as a function of the annealing treatment. Using standard conditions (<sup>Φ</sup> = 2 J·cm−2, *T*<sup>s</sup> = 300 ◦C, *P*O2 = 13.3 Pa), films deposited on glass substrates (200–500 nm thick) showed an atomic ratio of O/W ≈ 2.96 ± 0.05. The monoclinic phase of the as-prepared film transformed to an orthorhombic phase at 350 ◦C and to a hexagonal phase at 500 ◦C [243,244]. By varying the substrate temperature in the range of 150 to 800 ◦C and the oxygen pressure from 1 to 40 Pa, Mitsugi et al. obtained WO3 films with a different microstructure: Amorphous, crystallized tetragonal, and triclinic phases [245]. Hussain et al. obtained amorphous, polycrystalline, and nanocrystalline WO3 phases, and iso-epitaxial WO3(00*l*) thin films deposited on single-crystal SrTiO3 substrates at 600 ◦C and under *P*O2 = 18 Pa [246]. Suda et al. deposited PLD WO3 thin films on flexible ITO substrates. They showed that films, prepared at *T*<sup>s</sup> < 300 ◦C, are amorphous and polycrystalline phases were obtained at *T*<sup>s</sup> > 400 ◦C, while the crystallinity of the film on glass substrates was not dependent on *P*O2 [247]. Films deposited at 400 ◦C were porous with a nanocrystalline triclinic structure and showed the best cycleability [216,248,249].

The suitability of PLF WO3 films for EC applications was investigated as a function of the partial oxygen pressure during deposition. Studies of the texture and morphology of PLD 30-nm thick WO3 films deposited on Si(100) and SrTiO3(100) substrates under an O2 background of 2.5 Pa showed that: (i) The laser fluence (in the range of 5 to 15 J·cm<sup>−</sup>2) strongly influences the texture, (ii) the films grown on STO are biaxially textured with a smooth surface, and (iii) films deposited on Si are granular [250]. The fabrication of WO3 thin films with color neutrality for applications as EC materials was realized by

the deposition of films containing 20% of vanadium onto SnO2:F coated glasses at *T*<sup>s</sup> = 20 ◦C under *P*O2 = 10 Pa. The blue color in the reduced state (−0.4 V) of the W-O-V films lost intensity and turned grey-blue (transmittance of 50%) as the V concentration increased [251]. Highly transparent WO3 films exhibiting strong coloration and fast and full bleaching were prepared under PLD conditions (<sup>Φ</sup> <sup>=</sup> 1 J·cm<sup>−</sup>2, *T*<sup>s</sup> = 250 ◦C, *P*O2 = 16 Pa, and *d* = 40 mm) [252]. WO3 films were also prepared using similar PLD parameters for applications in gas sensors [253–255].

#### **4. Solid Electrolyte PLD Films**

For the development of solid-state thin film batteries, thin films of solid electrolytes with excellent performances, i.e., high ionic conductivity (σi), good stability against the lithium anode, large electrochemical window (Δ*V*), and poor electronic conductivity (σe), are currently required. To fulfill these requirements, the thin films of oxide-, phosphate-, or sulphide-based solid electrolytes were grown by the PLD technique [256–258]. The facile manufacture of such thin films is due to the easy control of the PLD chamber's atmosphere. Table 8 lists some typical solid electrolyte thin films prepared by PLD [41,259–264].



#### *4.1. LiPON*

In the early 1990s, Bates et al. prepared Li3PO4 thin films using a sputter-deposition technique in the presence of N2 gas that resulted in a nitrogen-doped lithium phosphate (called LiPON) of a typical chemical composition, Li3.3PO3.9N0.17 to Li2.9PO2.9N0.7. The structure consists of doubly and triply coordinated nitrogen atoms, which form cross-links between the phosphate chains [20]. LiPON displays a high chemical stability and an ionic conductivity of 2 <sup>×</sup> 10−<sup>6</sup> S·cm−<sup>1</sup> at 25 ◦C [265]. The growth of LiPON thin films by pulsed-laser deposition is also realized in nitrogen partial pressure with a moderate laser power influence [259,266].

Zhao et al. reported the growth LiPON thin films on three different substrates (i.e., Si wafer, Au-coated Si, and Al-coated glass plate) by reactive PLD in an N2 gas atmosphere in the range of 50 to 200 mTorr using a Li3PO4 target. The target was ablated by the beam of a Nd:YAG laser at the fluence of 5 to 20 mJ·cm−2. The influence of the ambient N2 pressure and the laser fluence on the ionic conductivity was systemically examined and the best result of 1.6 <sup>×</sup> 10−<sup>6</sup> S·cm−<sup>1</sup> with an activation energy of 0.58 eV at 25 ◦C was obtained for a film prepared under 200 mTorr at <sup>Φ</sup> <sup>=</sup> 15 J·cm<sup>−</sup>2. The mechanism of the nitridation of Li3PO4 was carried out by XPS measurements, showing that σ<sup>i</sup> increases with the N/P ratio [259]. West et al. showed that a 17-nm thick layer of LiPON deposited at the solid electrolyte–electrode interface decreased the charge-transfer resistance from 4470 to 760 cm−<sup>2</sup> in a Li/LiPON/LNM cell. The PLD amorphous films with <sup>σ</sup><sup>i</sup> <sup>=</sup> 1.5 <sup>×</sup> <sup>10</sup>−<sup>8</sup> <sup>S</sup>·cm−<sup>1</sup> at 25 ◦C were deposited from a crystalline Li2PO2N target under the flow of N2 gas at *P*N2 = 1 Pa [267].

#### *4.2. LixLa2*/*3*<sup>+</sup>*yTiO3*−*<sup>d</sup> (LLTO)*

Solid electrolytes, such as lithium lanthanum titanium oxides, Li*x*La2/3+*y*TiO3−<sup>δ</sup> (LLTO), based on a perovskite-like structure can accept vacancies at the Li (or La) and oxygen sites and show properties depending on the composition, with an electronic conductivity when Ti3<sup>+</sup> cations (instead of Ti4<sup>+</sup>) are present and an ionic conductivity for Li-rich material. The typical growth of LLTO thin films fabricated by the laser ablation technique is obtained at deposition temperatures in the range of 600 to 800 ◦C under a controlled oxygen pressure from 0.1 to 100 Pa [268]. LLTO films, such as Li3*x*La(2/3)−*<sup>x</sup>*TiO3, exhibit a high ionic conductivity of up to 10−<sup>5</sup> S·cm−<sup>1</sup> when deposited with pulsed laser deposition [269–271]. Li0.5La0.5TiO3 (LLTO) PLD thin films, prepared at 400 to 600 ◦C, are amorphous and show an ionic conductivity of ~2 <sup>×</sup> 10−<sup>5</sup> S·cm−<sup>1</sup> at room temperature. Contrary to crystalline films, the amorphous LLTO exhibits good stability in contact with lithium metal anodes. Half-cells based on LiCoO2 films covered with LLTO films deposited by pulsed laser deposition could be cycled for hundreds of cycles [269]. Furusawa et al. prepared amorphous LLTO films at *T*<sup>s</sup> = 25 ◦C with a uniform thickness (0.46–0.63 μm) using a laser energy of 180 mJ per pulse at 10 Hz [270]. The authors stated a controlled pressure of ~10−<sup>6</sup> Torr but did not mention the presence of O2 gas. The highest <sup>σ</sup><sup>i</sup> of 1.2 <sup>×</sup> <sup>10</sup>−<sup>3</sup> <sup>S</sup>·cm−<sup>1</sup> (*E*<sup>a</sup> = 0.35 eV) obtained for Li0.5La0.5TiO3 films deposited on an Ag substrate was due to the absence of grain boundaries. Maqueda optimized the PLD growth parameters to prepare La0.57Li0.29TiO3 dense films at *T*<sup>s</sup> = 700 ◦C under *P*O2 = 15 Pa with smooth surfaces [271]. The obtained nano-crystalline films exhibited domains, which are cubic and tetragonal modifications of the perovskite phase. Transport measurements showed an ionic conductivity of 8.2 <sup>×</sup> <sup>10</sup>−<sup>4</sup> <sup>S</sup>·cm−<sup>1</sup> at 25 ◦C with *<sup>E</sup>*<sup>a</sup> <sup>=</sup> 0.34 eV. Epitaxial Li0.33La0.56TiO3 solid electrolyte thin films were grown on NdGaO3(110) by PLD at *T*<sup>s</sup> higher than <sup>900</sup> ◦C under *<sup>P</sup>*O2 <sup>=</sup> 5 Pa [272]. These films showed a conductivity <sup>σ</sup><sup>i</sup> of 3.5 <sup>×</sup> <sup>10</sup>−<sup>5</sup> <sup>S</sup>·cm−<sup>1</sup> at 25 ◦C with *E*<sup>a</sup> = 0.35 eV (Figure 14).

**Figure 14.** Temperature of the in-plane ionic for conductivity epitaxial Li0.33La0.56TiO3 solid electrolyte thin films (36-nm tick) deposited on NdGaO3(110) by PLD at *T*<sup>s</sup> higher than 900 ◦C under *P*O2 = 5 Pa. (Reproduced with permission from [272]. Copyright 2012 Elsevier).

The influence of different substrates and excess lithium in the target on the microstructure and ionic conductivity of PLD LLTO thin films was examined by Aguesse et al. [273] Despite a large lattice mismatch of up to +8.8% with the substrate, the epitaxial growth of LLTO is possible on different (001) oriented LaAlO3, SrTiO3, and MgO substrates using a sintered Li0.37La0.54TiO3 target and PLD parameters, such as *T*<sup>s</sup> = 750–880 ◦C, *P*O2 = 4–20 Pa, and laser fluence of 1.07 J cm−2. An ionic conductivity as high as 19.2 <sup>×</sup> <sup>10</sup>−<sup>3</sup> mS·cm−<sup>1</sup> at 25 ◦C was obtained for 170-nm thick LLTO films grown on an STO substrate from an ablated 10 mol% lithium excess target. PLD LLTO films with a σ<sup>i</sup> of 3 <sup>×</sup> 10−<sup>4</sup> S·cm−<sup>1</sup> and <sup>σ</sup><sup>e</sup> of 5 <sup>×</sup> 10−<sup>11</sup> S·cm−<sup>1</sup> were obtained by controlling the background *P*O2 and *T*s. Amorphous LLTO films were utilized in SSMB cycled up to 4.8 V vs. Li+/Li with high voltage LiNi0.5Mn1.5O4 spinel cathode thin films [274].

Another class of LLTO electrolytes consists of Ti-based solid electrolytes with a garnet-like structure, first reported by Weppner et al. [275]. Li6BaLa2Ta2O12 thin films were deposited on an MgO(100) substrate by the ablation of a target with a 5 mol% Li2O excess. In standard PLD conditions (*T*<sup>s</sup> = 550 ◦C, *P*O2 = 5 Pa, laser fluence of <sup>Φ</sup> = 2 J·cm−2, 40,000 laser pulses), an ionic conductivity of <sup>2</sup> <sup>×</sup> <sup>10</sup>−<sup>6</sup> <sup>S</sup>·cm−<sup>1</sup> at 25 ◦C with an activation energy of 0.42 eV was obtained. This is comparable with the <sup>σ</sup><sup>i</sup> of LiPON. The electronic conductivity varied from 2.87 <sup>×</sup> <sup>10</sup>−<sup>13</sup> to 3.47 <sup>×</sup> <sup>10</sup>−<sup>10</sup> <sup>S</sup>·cm−<sup>1</sup> in the range of the polarization voltage from 2.8 to 4.3 V [273]. Saccoccio et al. fabricated garnet Li6.4La3Zr1.4Ta0.6O12 films via PLD and studied the impact of PLD parameters (fluence of 1 to 4 J·cm−2, *T*<sup>s</sup> in the range of 50 to 700 ◦C, and a post-annealing process) on the structural and transport properties. The ionic conductivity was measured by impedance spectroscopy. It was concluded that σ<sup>i</sup> is not dependent on *T*<sup>s</sup> but is strongly affected by the laser fluence [276]. Li7La3Zr2O12 (LLZO) garnet-like thin films were deposited on Si3N4/Si substrates at temperatures in the range of 50 ≤ *T*<sup>s</sup> ≤ 750 ◦C under a fixed background of *P*O2 = 1.3 Pa with a KrF excimer laser set at 0.6 J·cm−<sup>2</sup> [277]. The best material, which exhibited an ionic conductivity of 6.3 <sup>×</sup> <sup>10</sup>−<sup>3</sup> S cm−<sup>1</sup> at 400 ◦C (*E*<sup>a</sup> <sup>=</sup> 0.6 eV), was obtained at *<sup>T</sup>*<sup>s</sup> <sup>=</sup> <sup>300</sup> ◦C. The review of garnet-like solid electrolyte thin films grown via PLD is summarized in Table 9.

**Table 9.** Literature review of garnet-like solid electrolyte thin films grown via PLD.


#### *4.3. P- and Si-Based Electrolytes*

Several phosphorus- or silicon-based oxides and sulfides are solid electrolytes for lithium batteries, such as Li3PO4, Li4SiO4-Li3PO4, Li2S-P2S5 glass ceramic, Li2<sup>+</sup>2*x*Zn1−*<sup>x</sup>*GeO4 (LiSICON), Li3.25Ge0.25P0.75S4 (thio-LiSICON), etc., that can be prepared as thin films. Kuwata et al. prepared high quality Li3PO4 thin films by PLD for applications in Li/Li3PO4/LiCoO2 all-solid-state thin-film batteries. The Li3PO4 film exhibited an ionic conductivity of 4 <sup>×</sup> <sup>10</sup>−<sup>7</sup> <sup>S</sup>·cm−<sup>1</sup> at 25 ◦C and an activation energy of 0.58 eV. This solid electrolyte showed an electrochemical stability in the potential range of 0.0 to 4.7 V vs. Li+/Li and was applied in Li/Li3PO4/LiCoO2 cells [14,282]. Amorphous PLD thin films of LiSICON display higher conductivities than that of Li4SiO4 and Li3PO4 films. The solid electrolyte 0.5Li4SiO4–0.5Li3PO4 dense films deposited on an Si wafer at <sup>Φ</sup> = 2–6 J·cm−<sup>2</sup> under an argon gas of *P*Ar = 0.01–5 Pa had an ionic conductivity of 1.6 <sup>×</sup> 10−<sup>6</sup> S·cm−<sup>1</sup> at 25 ◦C and an activation energy of 52 kJ·mol−<sup>1</sup> [283]. Nakagawa et al. determined that PLD Li2SiO3 films stable to CO2 have an ionic conductivity of 2.5 <sup>×</sup> 10−<sup>8</sup> S·cm−<sup>1</sup> at 25 ◦C lower than that of Li2SiO3 films (i.e., 4.1 <sup>×</sup> 10−<sup>7</sup> S·cm−1), which are unstable to CO2 [284]. PLD thin films of lithium meta-silicate (LSO) deposited at a growth rate of 0.17 Å per pulse on various substrates (i.e., SiO2, quartz, sapphire, Al2O3 ceramic, and MgO) from an Li2SiO3 sintered tablet were grown in the amorphous state. The ionic conductivity slightly depends on the substrate species with the best results (σ<sup>i</sup> = 4.5 <sup>×</sup> 10−<sup>4</sup> S·cm−<sup>1</sup> at 300 ◦C, *E*<sup>a</sup> = 0.88 eV) found for an 80-nm thick film deposited on SiO2 glass [262,285]. The PLD conditions for the growth of thio-LiSICON Li3.25Ge0.25P0.75S4 solid electrolyte thin films were carefully chosen (especially the Li content of 3.2 in the target, which maintains the number of Li vacancies) to obtain a high σ<sup>i</sup> value of 1.7 <sup>×</sup> <sup>10</sup>−<sup>4</sup> <sup>S</sup>·cm−<sup>1</sup> at 25 ◦C [265]. PLD 80Li2S-20P2O5 thin film prepared under *<sup>P</sup>*Ar <sup>=</sup> 5 Pa exhibited an ionic conductivity and activation energy of 7.9 <sup>×</sup> 10−<sup>5</sup> S·cm−<sup>1</sup> and 43 kJ mol−<sup>1</sup> at 25 ◦C, respectively. Heat treatment increased the <sup>σ</sup><sup>I</sup> to 2.8 <sup>×</sup> 10−<sup>4</sup> S·cm−<sup>1</sup> [286]. To avoid the formation of a Li-deficient phase, such as Li4P2S6, an Li2S-enriched Li3PS4 target was used to grow PLD solid-electrolyte thin films. Using an Li3.42PS4.21 target, PLD Li3PS4 films exhibited a higher ionic conductivity of 5.3 <sup>×</sup> <sup>10</sup>−<sup>4</sup> <sup>S</sup>·cm−<sup>1</sup> at 20 ◦C [287].

#### *4.4. PLD Electrolyte as Bu*ff*er Layers*

Solid-state electrolyte (SSE) thin films have been used as a conductive buffer layer for the reduction of high resistance at the electrode/SSE interface of high-power all-solid-state lithium batteries. Coating the Li3PO4 thin films on electrode materials by the PLD method was found to be efficient for this purpose. Konishi et al. [288] reported the effect of surface Li3PO4 coating on LiNi0.5Mn1.5O4 epitaxial thin film electrodes. Amorphous Li3PO4 film (1–4 nm thick) was deposited at 25 ◦C with a laser energy of 150 mJ under *P*O2 = 3.3 Pa. It was also pointed out that such a coating reduces the Mn dissolution in the non-aqueous electrolyte. Yubuchi et al. [289] fabricated the same coated electrode with Φ = 2 J cm−<sup>2</sup> but under a lower oxygen gas pressure of 0.01 Pa. With a 100-nm thick Li3PO4 deposit, the total resistance of the Li cell decreased from 15 to 350 Ω. A PLD protective coating of 80Li2S–20P2S5 solid electrolytes on LiCoO2 particles was performed at room temperature under Ar gas at *P*Ar = 5 Pa with a fluence of ca. 2 J·cm−<sup>2</sup> (200 mJ per pulse). After SSE deposition for 120 min, the deposited film was ~150-nm thick, corresponding to 3 wt.% LiCoO2. Annealing the SSE deposit at 200 ◦C increased the capacity of the all-solid-state cell [42]. Another example of the buffer function of the Li2S−P2S5 solid electrolyte is given by the PLD coating of NiS-carbon fiber composite electrodes. The high ionic conductivity of 80Li2S−20P2S5 film deposited on an Si wafer was 7.9 <sup>×</sup> <sup>10</sup>−<sup>5</sup> <sup>S</sup>·cm−<sup>1</sup> at 25 ◦C [290]. A capacity of 300 mAh·g−<sup>1</sup> was delivered after 50 cycles at a current density of 3.8 mA·cm−<sup>2</sup> (1C-rate). This SSE coating favors the lithium ion and electron conduction paths in the NiS framework. Ito et al. [291] successfully deposited Li2S–GeS2 thin films as the buffer electrolyte (σ<sup>i</sup> = 1.8 <sup>×</sup> 10−<sup>4</sup> S·cm−1) on LiCoO2 particles by the PLD technique. The amorphous 78Li2S–22GeS2 solid electrolyte thin films prepared using standard PLD conditions exhibited an ionic conductivity of 1.8 <sup>×</sup> <sup>10</sup>−<sup>4</sup> <sup>S</sup>·cm−<sup>1</sup> at 25 ◦C. These SSE films were applied to form an electrode–electrolyte buffer interface with LiCoO2 [291]. The coating of a LiNbO3 SSE buffer coated onto the LiMn2O4 cathode resulted in an enhancement of the high rate capability and cycling stability of the electrode [292]. A similar process ensured a high thermal stability for the LiNi0.8Co0.15Al0.05O2 electrode operating over 500 charge–discharge cycles at 150 ◦C [293].

#### *4.5. Li2O-V2O5-Si2O (LVSO)*

PLD films of Li2.2V0.54Si0.46O3.4 are amorphous solid-state electrolytes of the system, Li2O-V2O5-Si2O (LVSO), which exhibits a conductivity of ~2.5 <sup>×</sup> 10−<sup>7</sup> S·cm−<sup>1</sup> at 25 ◦C [44]. Li4SiO4 thin films were successfully deposited by PLD using both an Nd:YAG laser (λ = 266 nm) and ArF excimer laser (<sup>λ</sup> = 193 nm) at the fluence of 2.5 J·cm−<sup>2</sup> in the flow of O2 gas at *P*O2 = 0.2 Pa. Having a conductivity of 4.1 <sup>×</sup> 10−<sup>7</sup> S·cm−<sup>1</sup> at 25 ◦C, thermally activated with *E*<sup>a</sup> = 0.52 eV, these films were applied in SSMBs [44]. Zhao et al. prepared PLD Li–V–Si–O thin films' electrolytes on an Si wafer and Al-coated glass as substrates placed 4 cm from the target. The film deposition was carried out at a fluence of 1.2 J cm−<sup>2</sup> under an ambient of *P*O2 = 6 Pa [294]. For *T*<sup>s</sup> = 300 ◦C, the Li–V–Si–O film exhibited <sup>σ</sup><sup>i</sup> <sup>=</sup> 3.98 <sup>×</sup> <sup>10</sup>−<sup>7</sup> <sup>S</sup>·cm−<sup>1</sup> at 25 ◦C and *<sup>E</sup>*<sup>a</sup> <sup>=</sup> 0.55 eV. Workers at Kawamura's lab reported the PLD growth of several LVSO solid electrolytes. The Li2.2V0.54Si0.46O3.4 film deposited with a continuous flow of O*<sup>2</sup>* gas maintained at *P*O2 = 0.2 Pa displayed an ionic conductivity of 2.5 <sup>×</sup> 10−<sup>7</sup> S·cm−<sup>1</sup> at 25 ◦C with an activation energy of 0.54 eV [41]. PLD amorphous 0.6(Li4SiO4)–0.4(Li3VO4) films deposited on Si(111) or fused silica plate exhibited an ionic conductivity of 10−<sup>7</sup> S cm−1·at 25 ◦C, which is one order higher than the value for PLD Li2TiO3 film [295]. All-solid-state thin film batteries were fabricated using both LCO and LMO PLD film cathodes and amorphous LVSO solid electrolytes as shown in Figure 15 [121].

**Figure 15.** SEM cross-sectional images of solid-state thin film lithium batteries, (**a**) SnO/LVSO/LCO and (**b**) SnO/LVSO/LMO. (Reproduced with permission from [121]. Copyright 2006 Elsevier).

#### *4.6. LiNbO3*

Because of its high room-temperature ionic conductivity and low electronic conductivity (10−<sup>5</sup> and 10−<sup>11</sup> S·cm−1, respectively), LiNbO3 (*R*3*c* crystal structure) is considered as a good SSE for electrode coating [296]. LiNbO3 was applied as a buffer layer between an LCO cathode and thio-LISICON electrolyte (Li3.25Ge0.25P0.75S4). The resultant electrochemical cell showed low interfacial resistance and a high-rate capability [297]. A high quality was obtained for PLD LiNbO3 thin films deposited at 730 ◦C on sapphire substrates by using a relatively high oxygen partial pressure of *P*O2 = 133 Pa and a laser fluence of 3 to 5 J cm−<sup>2</sup> [298]. Contrastingly, Perea et al. prepared PLD LiNbO3 films using a lower laser fluence (0.8 to 1.6 J·cm<sup>−</sup>2) in a residual pressure of <sup>≈</sup><sup>4</sup> <sup>×</sup> <sup>10</sup>−<sup>4</sup> Pa [299].

#### **5. Negative Electrode PLD Films**

#### *5.1. TiO2*

Due to the theoretical capacity of ~335 mAh·g−<sup>1</sup> of titanium dioxide (comparable to ~372 mAh·g−<sup>1</sup> for graphite and the small volume expansion (~4% for anatase)) significant interest has been devoted to the applied anode material in Li-ion batteries. The tetragonal anatase polymorph of TiO2 is a good anode candidate due to its insertion potential of around 1.5 V vs. Li+/Li [300]. Several works of the literature report the growth of TiO2 thin films with either a rutile or anatase structure fabricated by the PLD technique [301,302]. The growth conditions were studied on TiO2 films deposited by PLD using an Nd:YAG laser (532 nm wavelength beam) and a rutile-type TiO2 target. The effects of the substrate temperature (*T*s) and oxygen partial pressure (*P*O2 ) were investigated by Raman spectroscopy [13]. The parameters of *T*<sup>s</sup> = 300 ◦C and *P*O2 = 50 mTorr were optimized to obtain crystalline TiO2 films with a preferential (110) orientation. Kim et al. discussed the effects of the target morphology and target density on the size and distribution density of crystalline in PLD rutile-type TiO2 films deposited on (100)-oriented Si wafers maintained at 700 ◦C in a chamber with an oxygen partial pressure of 1.33 Pa [303]. A nearly particulate-free film was obtained from a dense target and the laser shots were adjusted for clear ripple patterns from the target surface. The optical bandgap energies of TiO2 PLD films grown on an α-Al2O3 (0001) substrate with an anatase and rutile structure were evaluated to be 3.22 and 3.03 eV, respectively [304]. Inoue et al. reported that films deposited at *T*<sup>s</sup> = 150 ◦C have an anatase structure, while *T*<sup>s</sup> = 300 ◦C provides rutile-type TiO2 films [305]. Choi et al. [306] prepared anatase TiO2 thin films with nanograins of 11 to 28 nm using a TiC target with *T*<sup>s</sup> = 500 ◦C under 4 Pa O2 gas.

#### *5.2. Li4Ti5O12 (LTO)*

Li4Ti5O12 (LTO) cubic structure (Li[Li1/3Ti5/3]O4 in spinel notation), considered as a "zero-strain" anode material, exhibits the advantage of very minor volumetric changes (<0.2%) upon cycling. This electrode displays a large voltage plateau at ~1.5 V vs. Li+/Li and a theoretical specific capacity of 175 mAh·g−<sup>1</sup> [307]. The first PLD growth of LTO thin films deposited onto Pt/Ti/SiO2/Si substrates using a KrF excimer laser beam (248 nm, 250 mJ) were reported by Deng et al. [308]. Films annealed at 800 ◦C (410 nm thick) exhibited a cubic structure with a lattice constant 8.375 Å larger than that of the LTO crystal (8.359 Å). The SEM cross-section image (Figure 16a) revealed the porous morphology induced by the high temperature treatment. The discharge specific capacity was the largest for films annealed at 700 ◦C due to the optimized adhesion strength between the film and substrate (Figure 16b). The anode films discharged at a current density of 10 <sup>μ</sup>A·cm−<sup>2</sup> (0.58C rate) showed excellent cycleability; the discharge capacity remained as 149 mAh·g−<sup>1</sup> after 50 cycles. Li4Ti5O12 films (545 nm thick) deposited on conducting fluorine-doped tin oxide (LTO/FTO) with a crystallite size of 50 to 80 nm were investigated as electrochromic active material with the highest contrast at a wavelength of 705 nm (transmittance change of ~48%) [309]. Epitaxial LTO thin-film grown on SrTiO3 single crystal from an Li-rich target, Li5.2Ti5O12, have a structural orientation identical to the substrate and are impurity-free when deposited at *T*<sup>s</sup> = 700 ◦C. The electrochemical features of LTO film anodes (20 nm thick) exhibited discharge capacities of ~200 and ~250 mAh·g−<sup>1</sup> for the (100)- and (111)-orientation, respectively [310]. Kim et al. prepared nano-sized epitaxial LTO(110) deposited on Nb:SrTiO3(110) substrate. These films (~28 nm thick) were tested by cyclic voltammetry at a scan rate of 1 mV·s−<sup>1</sup> and exhibited redox peaks at 1.53 and 1.60 V, corresponding to the insertion and extraction of Li<sup>+</sup> ions. As-deposited films at a substrate temperature of 700 ◦C in a 6.6 Pa oxygen partial pressure exhibited a high initial capacity (~200 mAh·g−1) but poor stability [311]. Kumatani et al. investigated the PLD growth process of epitaxial LTO films deposited on an MgAl2O4 (111) substrate. With *T*<sup>s</sup> = 800 ◦C and *P*O2 = 1 <sup>×</sup> 10−<sup>3</sup> Torr, LTO films had excellent crystallinity and a low resistivity of 3.3 <sup>×</sup> 10−<sup>4</sup> <sup>Ω</sup> cm. at 25 ◦C. At lower *P*O2 , the PLD LiTi2O4 film was formed, while at higher *P*O2 , Ti was segregated as TiO2 rutile and Li0.74Ti3O6 [312].

**Figure 16.** (**a**) SEM cross-section image of LTO film (410 nm thick) heat treated at 800 ◦C. (**b**) Charge–discharge profiles recorded at 20 μA cm−<sup>2</sup> (i.e., ~1.15 C) current density in the voltage range of 1 to 2 V vs. Li+/Li of PLD films heated at various temperatures. (Reproduced with permission from [308]. Copyright 2009 Elsevier).

Studies of the electrochemical performance and kinetic behavior of PLD LTO films deposited on Pt/Ti/SiO2/Si substrates were reported by Deng et al. [313]. Using an Li-rich target (i.e., excess 5 wt.% Li2O), the films annealed at 700 ◦C for 2 h in air were well-crystallized items with densely packed grains. The galvanic charge–discharge plateau was observed around 1.56 V and an initial specific capacity of 159 mAh g−<sup>1</sup> was delivered with a retention of 93.7% after 20 cycles. The diffusion coefficient of Li<sup>+</sup> ions in such an LTO framework was in the range of 10−<sup>15</sup> to 10−<sup>12</sup> cm2·s<sup>−</sup>1. The energy barrier of the diffusion of lithium ions was estimated to be *E*a = 0.11 eV in LTO (111)-oriented PLD thin films (190 nm thick) grown on a spinel MgAl2O4 (111) substrate [314].

Zhao et al. reported the optical properties of epitaxially grown LTO films on (001)-oriented MgAl2O4 substrate. The optical bandgap of 3.14 eV was measured for 86 nm thick films (surface roughness of 4.61 nm) [315]. Schichtel et al. fabricated all-solid-state microbatteries with LTO as the positive electrode. PLD films were obtained on various substrates at *T*<sup>s</sup> = 650 ◦C under a 0.3 Pa pure oxygen atmosphere using a commercially available LTO powder. As-prepared films (650 nm thick) revealed columnar growth that allowed a coulombic efficiency >97% after the second cycle and a discharge capacity of 33 <sup>μ</sup>Ah·cm−<sup>2</sup> at a 3.5 <sup>μ</sup>A cm−<sup>2</sup> current density [43]. Pfenninger et al. demonstrated that LTO thin films deposited by PLD on an MgO substrate kept at 500 ◦C using a dense Li7.1Ti5O12 target sintered at 1000 ◦C for 12 h are compatible with the Li6.25Al0.25La3Zr2O12 electrolyte pellet. Such films display a stable structure and cycleability almost close to 175 mAh·g<sup>−</sup>1. The typical voltage plateau at 1.57 V (oxidation) and 1.53 V (reduction) was observed at a rate of 2.5 mA·g−<sup>1</sup> [316]. Among the Li1<sup>+</sup>*x*Ti1−*<sup>x</sup>*O4 ternary system, LiTi2O4 thin films were grown by the PLD route in the temperature range of 400 to 800 ◦C using a target with a higher Li/Ti ratio of 0.8 [317]. Chopdekar et al. grew epitaxial PLD LiTi2O4 thin films on various crystalline-oriented substrates, such as single crystalline substrates of MgAl2O4, MgO, and SrTiO3 [318]. The authors state the PLD conditions with *T*<sup>s</sup> held at 450 to 600 ◦C in a vacuum of better than 5 <sup>×</sup> <sup>10</sup>−<sup>6</sup> Torr without any mention of the oxygen partial pressure, while Kumatani determined that stoichiometric LiTi2O4 thin films were obtained at a *<sup>P</sup>*O2 of 5 <sup>×</sup> <sup>10</sup>−<sup>6</sup> Torr with *<sup>T</sup>*<sup>s</sup> <sup>=</sup> <sup>800</sup> ◦C [312]. Recently, PLD LTO films grown on Nd-doped oriented STO substrates at *T*<sup>s</sup> = 700 ◦C under *P*O2 = 20 Pa showed high discharge capacities of 280 to 310 mAh·g<sup>−</sup>1. The best rate performance of 30C was obtained for the (100)-oriented Li4Ti5O12 films [319].

#### *5.3. LiNiVO4*

Amorphous LiNiVO4 thin-film anodes for microbatteries were grown by pulsed laser deposition using a sintered Li1.2NiVO4 target. The film grown at *T*<sup>s</sup> = 25 ◦C and *P*O2 = 8 mTorr showed the best electrochemical performance with a retainable capacity as high as 410 <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> after 50 cycles [320].

#### *5.4. TiNb2O7*

An alternative to LTO, titanium-niobium oxide, TiNb2O7 (TNO), is considered a promising anode material for long life Li-ion batteries, due to its high Li<sup>+</sup> ion transport, average voltage of 1.66 V, and theoretical capacity of <sup>∼</sup>387 mAh·g−<sup>1</sup> [321]. Fabrication of PLD TiNb2O7 thin films as anode electrodes for Li-ion micro-batteries was demonstrated by the ablation of a Nb2O5 + TiO2 mixture as a target at a laser fluence of 4.6 J·cm<sup>−</sup>2. Pure monoclinic TNO films were deposited on Pt/TiO2/SiO2/Si(100) substrates at 750 ◦C under an O2 gas of *P*O2 = 6–13 Pa. The 380-nm thick films grown at *P*O2 = 13 Pa delivered an initial specific capacity of 142 <sup>μ</sup>Ah cm−<sup>2</sup> <sup>μ</sup>m−<sup>1</sup> at a current density of 50 <sup>μ</sup>A·cm−<sup>2</sup> with a 58% capacity retention after 25 cycles [322]. Recently, the same co-workers reported a high specific discharge capacity of 226 <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> (~460 mAh·g−1) at a current density of 17 <sup>μ</sup>A·cm−<sup>2</sup> for amorphous TNO films grown by PLD. Li<sup>+</sup> diffusion coefficient of <sup>≈</sup>10−<sup>13</sup> cm2·s−<sup>1</sup> and an electronic conductivity of <sup>≈</sup>10−<sup>9</sup> <sup>S</sup>·cm−<sup>1</sup> were also reported [323].

#### *5.5. Silicon*

Pulsed-laser deposited silicon thin films have been widely studied for applications in opto-electronics. With a large theoretical capacity (4200 mAh·g−1), silicon is also considered as a promising anode material for the replacement of graphite anode (LiC6, 372 mAh·g−1) for Li-ion batteries [324]. Despite the huge volume expansion of >300% during lithiation up to Li22Si5, it is possible to obtain anodes with Si thin films grown by physical vapor deposition (PVD), reaching a cycling life of up to 3000 cycles due to the limited volume change in the 2D film [325]. For example, a film deposited on Ni foil maintained a capacity of 3000 mAh·g−<sup>1</sup> at a 12C rate over 1000 cycles [326]. PLD-grown Mg2Si thin film (30–380 nm thick) exhibited electrochemical activity with a stable cycling behavior in the voltage range of 0.1 to 1.0 V vs. Li+/Li; however, the initial irreversible capacity loss increased with the film thickness. The superior capacity of the 30-nm thick film was attributed to the formation of Li-Si alloys at the Si-rich surface [327]. Park et al. prepared PLD amorphous Si (a-Si) thin films on a stainless-steel substrate at temperature of 500 ◦C under an Ar gas pressure *P*Ar = 6.5 mPa. Furthermore, 1.5-μm thick a-Si films were obtained at the growth rate of 25 nm·min<sup>−</sup>1. Electrochemical tests carried out in the voltage range of 0.005 to 1.5 V showed a first discharge capacity of 9 to 0.7 <sup>μ</sup>Ah·cm−<sup>2</sup> with a 54.4% coulombic efficiency. Although, after 70 cycles, the 1-μm thick Si film exhibited a good cyclic performance [328]. Xia et al. reported the growth of a-Si using the standard conditions (*T*<sup>s</sup> = 25 ◦C, *P* = 1.3 mPa, fluence of 150 to 160 mJ per pulse, deposition time of 30 min). Electrochemical tests showed that 120-nm thick a-Si films exhibited an initial charge capacity of ~64 <sup>μ</sup>Ah·cm−<sup>2</sup> at a current density of 100 <sup>μ</sup>A·cm<sup>−</sup>2, a discharge capacity of ~50 <sup>μ</sup>Ah cm−<sup>2</sup> was maintained after 40 cycles, and the diffusion coefficient of Li ions determined from the cyclic voltammograms was ~10−<sup>13</sup> cm2·s−<sup>1</sup> [329]. Some Si-based composite thin films prepared by PLD combine the advantages of both components. The most popular are the carbon-based composites [330–333]. Chou et al. obtained a flexible anode material by the deposition of Si film onto single-wall carbon nanotubes (SWCNTs) using standard PLD conditions (<sup>λ</sup> = 248 nm, *T*<sup>s</sup> <sup>≈</sup> 30 ◦C, <sup>Φ</sup> = 3 J·cm−2, *P*Ar = 13 Pa, target–substrate distance of 50 mm). After 50 cycles, this Si/SWCNT nanocomposite delivered a specific capacity of 163 mAh·g−<sup>1</sup> at a 25 mA·g−<sup>1</sup> current density, which is 60% higher than for CNT [330]. The ultrathin film of Si grown by PLD was deposited on multilayer graphene (MLG) by CVD on an Ni foam substrate. The specific capacity of this binder-free Si/MLG anode appeared to be stable at ca. 1400 mAh·g−<sup>1</sup> [331]. Silicon nitride SiN0.92 thin films were prepared by PLD and investigated as a negative electrode in lithium batteries. A 200-nm thick film grown on buffed stainless-steel substrates kept at 25 ◦C from an Si3N4 pellet as the target delivered a high specific capacity of 1300 mAh·g−<sup>1</sup> after 100 cycles [334].

#### *5.6. Graphene*

Most of the commercial lithium batteries have a carbon anode. Graphene is the most conductive form of carbon, and as such, it is considered as a promising electrode, especially when it is doped with nitrogen [229]. A recent review was devoted to PLD-graphene synthesized from a solid carbon source [335]. Since nitrogen modifies the intrinsic properties of graphene, it is important to control its concentration. PLD, which allows for a one-step synthesis of N-doped carbon films, is particularly suited to this purpose. The first N-doped amorphous carbon film (a-C:N) synthesized by PLD dates from 2013 [336]. It contained 2 at. % of nitrogen. More recently, using the same approach, the upper nitrogen concentration in PLD a-C:N film was raised to 3.2 at.% [337]. These films, however, were not used as electrodes. On the other hand, a N-doped graphene (NG) electrode prepared by PLD coupled with in-situ thermal annealing (PLD-TA) was achieved by Fortgang et al. [338]. More recently, Bourquard et al. used the PLD-TA process to form an N-doped graphene film by high temperature condensation of the laser-induced carbon plasma plume onto the Si electrode previously covered by an Ni catalytic film [339], using a protocol published by the same group [340] Carbon was ablated at 780 ◦C from the graphite target using a femtosecond laser (λ = 800 nm, pulse width of 60 ns, repetition rate of 1 kHz, and <sup>Φ</sup> <sup>=</sup> 5 J·cm<sup>−</sup>2) at a distance of 36 mm from the graphite target, with *<sup>P</sup>*<sup>N</sup> <sup>=</sup> 10 Pa in the vacuum chamber. The electrochemical properties were measured with the thus-obtained 40 nm-thick film with a nitrogen concentration of 1.75% as the working electrode and an active area of 0.07 cm2, saturated calomel electrode as the reference electrode, and platinum as the counter electrode, in a 0.5 mol·L−<sup>1</sup> 1,1 ferrocene-dimethanol solution of 0.1 mol·L−<sup>1</sup> NaClO4. Aiming to detect H2O2 in 0.1 mol L−<sup>1</sup> phosphate buffer saline (PBS) solution (pH 7.4), linear sweep voltammetry was used in the anodic range from 0 to 1000 mV with a scan rate of 100 mV·s−1. The electrode showed excellent reversibility, 60 mV, close to the theoretical value, and a detection limit of 1 mM of hydrogen peroxide, which constitutes a major improvement of the electroanalytical oxidation of H2O2 in comparison with undoped graphite electrodes. Such results are recent, and to our knowledge, no such electrode has been tested yet as an anode for lithium batteries.

#### *5.7. Other PLD Anodes*

Significant efforts have been devoted to the design and development of new PLD-grown anode materials for SSMBs, yielding a high energy density (from 500 to 1500 mAh·g<sup>−</sup>1) and electrochemical stability [341–354]. Table 10 summarizes the PLD-growth conditions and electrochemical properties of some new anodes proposed for lithium microbatteries. All transition-metal oxide M*x*O*y* materials are subjected to electrochemical lithiation via a conversion reaction, which implies a two-step process, i.e., first, fine metallic (*M*) nanoparticles embedded in an insulating matrix, such as Li2O, are in situ formed during the first (irreversible) discharge, and secondly, an alloying reversible reaction (Li-*M*) occurs on subsequent cycles [341].

Recently, Wu et al. proposed a novel anode consisting of a Li3P-VP nanocomposite fabricated by PLD [353] using a 5-Hz Nd:YAG laser (<sup>λ</sup> = 355 nm, <sup>Φ</sup> = 2 J·cm−2) concentrated on the target surface with an incident angle of 45◦, with *P*Ar =10 Pa in the vacuum chamber. The stainless-steel substrate was placed 3 cm from the surface and kept at 400 ◦C. The weight of the film thus obtained (without the substrate) was 0.10 mg·cm−2. The excessive lithium in this composite was used to stabilize the VP2 structure after the first charge. Electrochemical tests were made with this film as the working electrode, while lithium metal sheets were used as counter and reference electrodes with 1 mol·L−<sup>1</sup> LiPF6 in EC:DMC (1:1 in volume) electrolyte. When cycled in the range of 0.01 and 4 V vs. Li+/Li at a current density of 5 <sup>μ</sup>A·cm−2, a capacity of 1040 mAh·g−<sup>1</sup> was delivered at the second discharge, 987 mAh·g−<sup>1</sup> at the 50th cycle.


**Table 10.** Growth conditions and electrochemical properties of new anode materials.

#### **6. Discussion**

There is general agreement on the beneficial results of the pulsed-laser deposition of thin films used as active materials in lithium microbatteries. This is a technique prone to fabricate thin film rapidly, from a small amount of target material, while maintaining the ideal stoichiometry by the control of different growth parameters. We observed (Table 2) that there is a strong trend to develop microcell technologies using LiCoO2 film (typical thickness of 4-μm) as the cathode, LiPON thin film (typical thickness of 1 μm) as the solid electrolyte, and Li thin film anode, which may have advantages in terms of the following key requirements: High energy density, high voltage, sustainability, and easy fabrication. For such microbatteries, let us recall the energy units used by authors. For a comparison of the volumetric specific energy, one generally refers to that of the cathode material (i.e., the limiting electrode); thus, considering a dense LCO film (*d* = 4.3 g·cm−3), a gravimetric specific energy of 100 mAh·g−<sup>1</sup> is equivalent to 43 <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> or 154.8 mC·cm−2·μm<sup>−</sup>1.

Let us compare and discuss the growth conditions that allow the best electrochemical performance of each component, i.e., cathode, anode, and electrolyte. Regarding the growth of lithiated oxides used as cathode materials, an excess of lithium (at least ~15 wt.%) is mandatory for any material to compensate the loss of volatile lithium species during the ablation process. Given the different materials' available candidates for the cathode, LiCoO2 appears to be the most electrochemically efficient. From Figure 17, comparing the Ragone plots of LiCoO2 and LiMn2O4 thin films, each curve displays a series of discharge profiles for a lithium microbattery with the cathode thickness (in μm). As the capacity delivered by a cathode is proportional to the mass of the material, thicker films provide high energies, but often at the expense of the power performance [26]. The specific energy for a planar Li//LiCoO2 cell with a thick cathode can reach 500 <sup>μ</sup>Wh·cm−2·μm<sup>−</sup>1. However, the rate capability depends strongly on the plane orientation of the film, which can be controlled by the nature of the substrate. Thus, the preferred orientation is the (003)-plane parallel [29]. However, a surface–electrolyte interface (SEI) is formed on epitaxial LiCoO2 films with different orientations of (104), (110), and (003) that result in anisotropic reactions of intercalation activity [102]. It has also been demonstrated that the minimized strain energy in thick LCO films allows preferential (101) and (104) textures [32]. Nevertheless, the best well-crystallized LCO thin films were fabricated in the following PLD conditions: *<sup>T</sup>*<sup>s</sup> <sup>=</sup> <sup>500</sup> ◦C, *<sup>P</sup>*O2 <sup>=</sup> 13 Pa, <sup>Φ</sup> <sup>=</sup> 2 J·cm<sup>−</sup>2, substrate–target distance of 30 to 40 mm, and laser beam-target incident angle of 45◦. Interesting results reported in Ref. [53] showed impurity-free LCO films, highly (003)-oriented with a very small lattice expansion during charge (at 4.2 V), when grown on stainless steel substrates at relatively low temperatures (*T*<sup>s</sup> ≈ 300 ◦C). In this case, the films had a texture between the amorphous and well-crystalline state with very small grains, which is suitable for short pathways for electrons and ions during the (de)intercalation reaction.

**Figure 17.** Ragone plot (normalized by the active cell area) for lithium thin-film microbatteries fabricated with crystalline LiCoO2 (black lines), crystalline LiMn2O4 (blue lines), and nanocrystalline Li*x*Mn2−*<sup>y</sup>*O4 (red lines) cathode materials with different thicknesses (in μm).

To avoid the poor performance of LIBs derived from hindered lithium-ion diffusion at the interface between the LCO positive electrode and electrolyte, modifications of the cathode surface have been realized by the deposition of a thin layer of a fast-ionic Li<sup>+</sup> conductor, such as amorphous Li2WO4 or Li3PO4. This layer reduces the interfacial Li<sup>+</sup>-ion transfer resistance that results in a rapid charge–discharge rate. The a-Li2WO3/LCO/Pt/Cr/SiO2 electrode cycled at a high rate of 20C with a high capacity retention [84]. Another electrode exhibiting a fast charge–discharge rate as high as 348C has been fabricated by the multilayer PLD technique, but in this case, the LMO thin film exhibited a significant pseudocapacitive behavior (non-diffusion-controlled) instead of a faradaic mechanism. An additional promising improvement is the fabrication of an LCO thin film sandwiched between a PLD-prepared SrRuO3 film as the electronic conductor and the film of Li3PO4 (3.2 nm thick) as the ionic conductor with the result being limited surface structural change in the high voltage range (4.4 V) [71].

The influence of the PLD conditions on the texture of LiMn2O4 thin films has shown that *T*<sup>s</sup> = 500 ◦C and *P*O2 = 20 Pa are the optimum values that maintain the Li/Mn ratio close to 1, when an Li-enriched target is used, and obtains the best mass transfer [122]. It was also noticed that any substrate does not strongly influence the stoichiometry, but affects the out-of-plane preferred texture. The applicability of the PLD-grown V2O5 films in lithium microbatteries has been evidenced that, in the range of 200 < *T*<sup>s</sup> < 400 ◦C under *P*O2 = 6 Pa, the films offer better electrochemical performance than those grown at other temperatures in terms of their structural quality and stability. Only two works were devoted to the fabrication of solid-state thin-film batteries with vanadium oxide as the cathode materials: The Li/Li1.4B2.5S 0.1O4.9/V2O5 cell delivered a capacity of ~400 mC·cm−2·μm−<sup>1</sup> [221], while the Li/LiPON/Ag0.3V2O5 maintained a specific capacity of 40 <sup>μ</sup>Ah·cm−2·μm−<sup>1</sup> after 100 cycles [222], but the low current density was due to the poor electronic conductivity of the positive electrode. Recently, a solid-state thin-film battery, Li/Li3PO4/LiMnPO4, was successfully fabricated by PLD [48]. Such a cell delivered a modest specific capacity of 10 <sup>μ</sup>Ah·cm−2·μm<sup>−</sup>1, which was limited by the slow chemical diffusion coefficient of the Li<sup>+</sup> ion in the olivine framework (3 <sup>×</sup> <sup>10</sup>−<sup>17</sup> cm2·s<sup>−</sup>1).

Lithium phosphates, i.e., Li3PO4 and LiPON, are the most widely used solid electrolytes in microbatteries; they are easily fabricated by PLD using an ArF excimer laser and show a good ionic conductivity. The LiPON electrolyte is known to exhibit a better chemical stability than Li3PO4 [282]. However, the electrochemical stability of PLD-prepared Li3PO4 thin films is greater than 4.7 V [265].

Few works have attempted to replace the lithium metal thin-film anode by other lithiated materials (i.e., intercalation compound or alloy) for the fabrication of microbatteries. The most stable insertion compound should be Li4Ti5O12 spinel with minor volumetric changes but the high voltage plateau of 1.5 V is a great penalty for high energy density. The In/80Li2S–20P2S5/LiCoO2 microbattery developed by Sakuda et al. seems to be promising as the Li-In alloy allows a high specific discharge capacity at moderate current density of 0.13 mA·g−<sup>1</sup> [42].

#### **7. Concluding Remarks**

The results of the intensive research on the growth of thin films by pulsed laser deposition in recent years have been reviewed. Due to careful investigations of the mechanism of the sample preparation, optimized materials with adequate properties for energy storage and conversion have been obtained. The PLD technique is considered to be suitable for improving the density and adhesion properties of films. A huge effort has been mainly concentrated on the deposition of lithiated oxides, which require specific conditions due to the volatile character of lithium vapor species during the PLD process. Due to the outstanding performance of the conventional cathode materials, LiCoO2 and LiMn2O4, PLD films exhibiting a specific capacity close to the theoretical one are the most popular. The progress concerns mainly the epitaxial films grown with an orientation favorable to a high rate of transport of Li ions at the electrode/electrolyte interface. For instance, the pyramidal-type LiMn2O4 films cycled at the 3.3C rate demonstrate a specific capacity of 90 mAh·g−<sup>1</sup> after 1000 cycles.

The PLD technique has proved to also be efficient for the preparation of thin films of anode materials. The best example is the production of LTO, which is a "zero-strain" compound. Other anode thin film materials, such as silicon and conversion-type oxides, are attractive due to their high specific capacity and easy PLD fabrication.

So far, solid-electrolyte thin-films have been fabricated essentially by thermal vacuum evaporation and rf-sputtering. The manufacture of solid-electrolyte thin films by PLD has brought improvements in their intrinsic properties. For example, the electronic conductivity of PLD films is small in comparison with rf-sputtered films. LiPON and Li–V–S–O are the most popular solid-electrolyte films.

In recent years, due to a strong demand for smaller power sources, the interest in rechargeable micro-batteries has gradually increased. The progress on lithium microbatteries is remarkable, mainly due to the PLD growth of high-quality, pinhole-free, solid-state electrolyte thin films, such as Li6.1V0.61Si0.39O5.36. The rechargeable thin-film lithium-ion battery designed by the Japanese group at Tohoku University was fabricated using the sequential PLD technique. This microcell delivered a specific capacity of 9.5 Ah cm−<sup>2</sup> discharged at a current density of 44 <sup>μ</sup>A·cm−<sup>2</sup> using an Li-Sn alloy film as the anode and showed good reversibility over 100 cycles.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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