Thin Film Deposition Techniques in Surface Engineering Strategies for Advanced Lithium-Ion Batteries

Abstract

Recent progress in the fabrication of controlled structures and advanced materials has improved battery performance in terms of specific capacity, rate capability, and cycling stability. However, interfacial problems such as increased resistance and contact instability between the electrodes and solid/liquid electrolytes still put pressure on the controllable formation of structures and the improvement of performance as well as safety. Here, we first briefly introduce the deposition techniques in terms of working mechanism and experimental process, then illustrate the associated advantages/disadvantages of the surface engineering methods based on deposition techniques (physical vapor deposition and chemical reaction deposition) to the provision of reference for researchers selecting the appropriate approach. Second, we exemplify the Si/LiCoO₂/LiPON/Li to demonstrate the main progress made in lithium-ion batteries, elaborating on the efforts in engineering the reactive surface utilizing the deposition techniques. Finally, general conclusions and prospects for future advanced thin film deposition techniques in the field of lithium-ion batteries are presented.

1. Introduction

Portable and wearable electronics and micro-electro-mechanical systems, e.g., micro-robots and micro-sensors, are currently moving toward the integrated multi-functionality of on-chip, thin and lightweight features. The ever-increasing demands for smart and miniaturized electronics have led to an urgent need for on-chip energy storage systems with high performance. Benefiting from its lightweight, high energy density, no memory effect, and environmental friendliness, lithium-ion batteries have been used in many fields, especially in portable and tiny actuators, making it possible for microsystems to work autonomously [1]. In recent years, with the rapid development of electric vehicles and smart electronic devices, various industries have put forward higher requirements for the energy density of lithium-ion batteries with high safety [2]. Currently, lithium-ion batteries using liquid organic electrolytes and graphite anodes are approaching their theoretical energy density limit (300 Wh·kg⁻¹) [3]. Furthermore, silicon has a specific capacity far exceeding that of graphite [4]. However, a series of problems is caused by the huge volume expansion after lithiation, which hugely limits its commercial application [5]. Using lithium metal with high specific energy and low potential as the anode can increase the energy density, but lithium dendrites during cycling may puncture the separator and cause a short circuit in the battery [6]. In addition, the bottleneck of lithium-ion batteries is no longer simply improving the ionic conductivity of the electrolyte, but building a good electrode-electrolyte interface [7]. Moreover, the energy density would be dramatically improved with the assembling appropriate cathode. Fortunately, recognizing the powerful usage of thin film deposition techniques, researchers began to explore the surface modification of lithium-ion batteries, which promoted the application of lithium-ion batteries and solved the interface problem between electrodes and electrolytes [8,9,10,11,12]. More and more research results have proved that interface engineering through surface modification by thin film coatings may significantly suppress the dissolution of cations [13], alleviate the decomposition of electrolytes [14], increase the structural stability of the electrodes [15], and enhance the ionic, electronic conductivities of the electrode material [16,17,18,19], etc.

However, there is still a lack of systematic comparisons of various vapor deposition methods utilized in lithium-ion batteries. In this review, we classify the fabrication techniques into two categories including (i) physical vapor deposition (PVD) and (ii) reactive vapor deposition (RVD). On the other hand, the anode, cathode, and solid-state electrolyte (SSE) often encounter surface-related problems such as increased surface resistance and excessive surface-side reactions. From the perspective of film deposition design, it can be realized by wet chemical methods, mechanical milling, and vapor deposited techniques [20]. Despite the low-cost and industrial manufacturing superiority, solution-based technique and mechanical milling may lead to inhomogeneous deposition of coating (the coating thickness may differ from batch to batch) and give a chance for active materials to be exposed to air/moisture in consideration of most of the active materials in battery system are sensitive to moisture, which is incompatible with the manufacturing of Microelectro Mechanical Systems (MEMS). Fortunately, the vapor deposition technique enables uniform deposition in a vacuum or low-pressure working environment. Thereby, the vapor deposition device can be integrated with other devices such as gloveboxes [21], fluidized beds [22], rotating substrate traps [23], etc. Therefore, mastering advanced thin film deposition techniques and applying them to lithium-ion batteries may suppress lithium dendrites and other problems. Understanding advanced thin film deposition techniques is a base tool for resolving interface problems for lithium-ion batteries.

Here, we focus on the further understanding of surface modification and interfacial interactions of electrode/electrolyte and give insight into selecting proper modification techniques by introducing thin film deposition techniques from the aspects of working mechanisms, experimental parameters, advantages, and disadvantages, etc. Subsequently, we point out the preparation methods, morphological characteristics, and electrochemical performance by exemplifying the commonly used materials (silicon, lithium cobalt oxide, LiPON, and Li) utilizing thin film deposition techniques. Finally, the prospects of surface decoration and interfacial engineering in lithium-ion batteries are demonstrated.

2. Presentation of Film Deposition Method

The functional layer is an important part of the storage mechanism in lithium-ion batteries, as it helps to improve the performance and stability of the interface and allows for the efficient transfer of ions and electrons. The film deposition techniques are vital for the development of lithium-ion batteries by providing a functional layer at the interface between electrode and electrolyte, which fabricates a solid-state battery with a different approach from the traditional methods. Considering the unfriendly effect of water-based strategies on environmental pollution regardless of their excellent uniformity, we choose two kinds of film deposition techniques here, namely, physical and chemical processes. In fact, in recent years, the number of papers using thin film deposition techniques to treat the surface of lithium-ion batteries has maintained a high degree of concern (Figure 1), which confirms its important role in lithium-ion batteries.

2.1. Physical Vapor Deposition

Physical Vapor Deposition (PVD) techniques can help in surface modification by changing the physical properties of the surface. PVD is a process that involves the deposition of a thin film onto a substrate. PVD techniques can change the physical properties of the surface by altering its composition, structure, or morphology. In the case of lithium-ion batteries, PVD can be used to deposit a thin film of a conductive material, such as a metal or metal oxide, onto the electrode surface to improve its electronic conductivity and reduce its impedance. During PVD, target material was emitted through an energy supply with the form of heat, pulsed laser, and protons subsequently traveled in a controlled environment, then condensed on a substrate. The deposition rate and film properties are influenced by a range of processing factors including power density, chamber atmosphere, vacuum degree, gas pressure, target-substrate distance, and substrate temperature. With their excellent film-forming quality in a relatively short time, in this section, we selectively introduce common magnetron sputtering and pulsed laser deposition techniques. It is undeniable that other PVD techniques such as thermal evaporation made a great contribution to the field of lithium-ion batteries [24,25]. However, restricted by the disadvantages of thermal evaporation, there are limited works have been reported for the modification of lithium-ion batteries. For example, many alloys and combinations are difficult to deposit due to the different melting points. The deposited film is less uniform at several nanometers of thickness and has less controllable variables [26].

2.1.1. Magnetron Sputtering

Direct current (DC) and radio frequency (RF) magnetron sputtering (MS) are effective techniques for surface engineering because they allow for precise control of the film thickness, composition, and microstructure. Magnetron sputtering was first observed as the loss of cathode material in a glow discharge device by Grove in 1852 [27]. In this process, the surface of the target is eroded by particle bombardment through high voltage and then landed on the substrate after traveling in the chamber for a while. The presence of a magnetic field facilitates the generation of Ar⁺ and further inhibits the heat up of substrate via controlling secondary electrons moving circularly around the target. Therefore, the whole process includes [28,29]: ① generation of Ar⁺, ② bombardment of Ar⁺ toward the target by electric field acceleration, ③ escapement of target particle from the surface, ④ film formation when in contact with the substrate.

It is worth noting that a wide range of materials could be sputtered regardless of conductivity. High conductive targets such as metal or alloy sputtered by DC MS, while the sputtering of insulators and semiconductors is realized by RF MS. For example, ceramic with low conductivity would be sputtered under RF sputtering [30]. The choice of materials for individual processes is typically driven by the specific properties of the material being deposited. The choice of materials for individual processes is typically driven by the specific properties of the deposited material. In the case of lithium-ion batteries, the choice of material for surface modification is often driven by the need to improve the performance and stability of the electrode-electrolyte interface, which may require the deposition of a conductive or protective film.

Sputtering techniques facilitate the investigation of the battery with the fabrication of a binder-free electrode for a thin-film battery or modification of the electrode/electrolyte in lithium-ion batteries. The deposition rate of magnetron sputtering is generally 1–10 nm/s [31], which is mainly related to the parameters of equipment operation such as chamber pressure, the distance between target and substrate, etc. Additionally, the film properties can be tuned by varying deposition parameters such as substrate temperature, power, time, working pressure, etc. Compared to other film deposition methods, sputtering is characterized by several remarkable advantages and disadvantages [32]:

Advantages

First, the sputtering process can be achieved at low temperatures, which is friendly for flexible substrates. The rigid substrates can be heated to promote deeper diffusion of the particles and provide additional energy for bonding with the substrate [33,34]. Second, sputtering enables good adhesion of films on substrates due to the highly energetic collision with accelerating speed via transforming kinetic energy into binding force partially [35]. Third, the ability of stoichiometric transfer enables the same composition of target and as-deposited film [36,37]. Fourth, the deposition rate is mainly controlled by a power supply, while a faster deposition rate can sometimes be achieved at high power [38]. Fifth, sputtering provides the possibility for scalable fabrication from laboratory up to industrial scale, with the ability to exactly control sputtering parameters (deposition rate(1–10 nm/s), uniformity, particle size) [39]. In industry, sputtering is favorable for its easy working mechanism and reproducible film deposition on a large scale.

Disadvantages

Low availability of target: circle groove would be formed in the surface of the target and would be deeper after a long period of utilization due to secondary electron damage, which in turn resulted in irreversible usage [40].

2.1.2. Pulsed Laser Deposition

As a most promising PVD technique, the energy of pulsed laser deposition (PLD) was supplied from laser pulses and focused on a target leading to material melting, evaporation, and ionization [41]. This produces a plasma plume that expands rapidly away from the target surface. The ablated material is collected on a substrate upon which it condenses and forms a thin film with a deposition rate of 0.03–0.07 nm/pulse [24]. Reasonably, the PLD process can be divided into three steps [42,43]: ① the generation of plasma, ② expansion of plasma plume, ③ nucleation of vapor and film growth.

The commonly used lasers include Nd: YAG (Neodymium-doped Yttrium Aluminum Garnet, λ = 266 nm), ArF (λ = 193 nm), and KrF (λ = 248 nm), while the mean diameter of the particle would increase with wavelength [44]. During the deposition, the film growth rate is controlled by the pulsing duration of the laser [41]. The deposition rate will increase when employing high repetition and high-power lasers [45]. When depositing lithium-containing material, excess lithium is required due to the high vapor pressure of lithium and its easy scattering character during deposition [46]. During the PLD process, depositing conditions such as gas pressure, deposition temperature, laser power, and post-annealing process all influence the morphology, structure, and consequent properties (e.g., electrical and ion conductivities) of the films [47].

To summarize, activated by laser energy, the plasma including ions, electrons, and atoms is ejected from the target surface forming glow zones with high temperature, which in turn provide enough energy for melting and evaporating of the target material to the vapor phase. The availability of various experimental parameters permits film growth in a wide range of characteristics. Accordingly, film characteristics such as epitaxy, phase purity, density, roughness, morphology, composition, and thickness would be tuned. Furthermore, both the substrate and target were rotated to promise uniform deposition, and the laser beam was scanned to avoid the puncture of the target. The main difference in the working mechanism between sputtering and pulsed laser deposition was the type of supplied energy as well as the mechanism of vapor generation. Namely, it can be considered that the velocity energy of plasma is transferred to the target during sputtering and the heat of laser irradiation induce vaporization of the target in PLD. Subsequently, as a PVD technique, pulsed laser deposition resembles sputtering in many aspects. In this configuration, the characteristic of pulsed laser deposition was briefly introduced in comparative perspectives as follows.

Advantages

Firstly, the high energy supplied by laser pulse may enable film crystallization during a deposition without post-annealing (e.g., a strong temperature gradient leads to the formation of amorphous-crystalline core-shell silicon nanostructures at room temperature) [48]. Secondly, PLD has been considered a clean deposition method without introducing precursor into the chamber [49]. Correspondingly, the high energy from the laser enables stoichiometry transfer from the localized target without chamber contamination. Additionally, the in-situ introduction of reaction gas allowed the formation of the heteroepitaxial film [50].

Disadvantages

In contrast to sputtering, compactness and adhesion of film were inferior due to the lower kinetic energy in the PLD process. Besides, the PLD possesses a lower deposition rate, due to the deposition rate being proportional to laser power, frequency, and fluency [45].

Notably, the introduction of reactive gas (O₂ or N₂) in the chamber is an alternative approach toward depositing a composite film, which was called reactive deposition [51,52]. The vaporized target particles would react with gas to form a compound, which may enable elemental doping. Additionally, deposition from multi-target in one chamber was observed with the rotation of substrate for uniform film deposition [53]. For instance, a solid electrolyte with multi-element was deposited by alternative sputtering of Al, Li₂O, and LLZO (Li₇La₃Zr₂O₁₂) target [54]. Normally, an excessive amount of lithium was added to the target to compensate for lithium loss during sputtering [55]. Besides, the nitrogen-doped carbon was enabled by bombarding graphite targets in the nitrogen atmosphere [56].

Despite the numerous advantages of the physical vapor deposition process, it also shows disadvantages in practical application. PLD and MS need targets with a suitable diameter and an optimized composition. Additionally, if the chamber is not under a high vacuum, prepared films may be contaminated. More seriously, due to the line-of-sight deposition feature, vapor particles hardly attach to the inner side of a porous structure. It is also difficult to deposit film in the “shadow” area of the uneven surface with pinholes generated during deposition. To compensate for the disadvantage of PVD techniques, the reactive vapor reaction process was often conducted to make a wide range of usage.

2.2. Reactive Vapor Deposition

To differentiate and unify the chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes, here, we called them reactive vapor deposition (RVD). The RVD process would be witnessing the breaking and binding of chemical bonds excited by supplied energy and forming a new material, which is the main difference from PVD. Another divergence of RVD should be the non-line of sight deposition feature derived from the pervading ability of vapor phase precursor in pores and voids mixed in a carrier gas [57]. Activation energy is required in the reaction process for bond breaking and linking with the form of heat, photons, and plasma [58,59,60]. Correspondingly, low-temperature deposition is possible in the vapor reaction process assisted by plasma [61,62].

2.2.1. Chemical Vapor Deposition

In the CVD process, reactions are conducted in a quartz tube with the favor of carrier gas in higher temperatures (200–1100 °C) [63,64,65]. Whereas, the ultrahigh temperature (>2000 °C) is adopted for the growth of certain materials [66]. Notably, a wide range of materials can be synthesized with various morphology. For example, a two-dimensional material with a monolayer or few layers would be realized by CVD, such as graphene, MoS_2, and PtSe [67,68,69]. There are several processes involved in CVD [70,71]: ① The precursor in a vapor state was dispersed and adhered onto the surface of a substrate with the aid of carrier gas. ② The precursor decomposed into a smaller unit to react with the substrate and then form a thin film. ③ The gas-phase byproduct was purged by a vacuum pump. The precursor can be solid-state placed at the entrance of carrier gas or gas phase would adhere at the substrate followed by carrier gas from solid precursor placed at the entrance of carrier gas or gas phase filled in a bottle. The coating can be conducted at both the smooth silicon substrate and three-dimensional framework [72,73]. Affected by the chemical reaction rate of different film deposition, equipment models, and other factors, the CVD deposition rate is generally 1–100 nm/s [74]. Film thickness can be controlled by tuning physical parameters such as gas flow, temperature, and reaction time.

Advantages

The complex-structured substrate with pores and voids can be coated due to the vapor precursor can spread into them mixed in a carrier gas, which is the main difference between PVD and RVD.

Disadvantages

The major disadvantage of CVD is stemmed from its slow deposition rate. Another disadvantage is associated with the toxicity or flammability nature of precursor or exhausted gas. Additionally, a higher temperature is required to vaporize the precursor from a solid/liquid state. However, sometimes, the required energy can be lowered through the introduction of a laser or initiator [75].

2.2.2. Atomic Layer Deposition

ALD has been demonstrated to be an effective method to either synthesize electrode/electrolyte materials for micro/thin-film batteries or modify particles and electrodes/electrolytes through depositing thin film in a layer-by-layer process based on the sequential and saturated reaction of gaseous precursors with a cyclic manner [76]. Attributing to the limited number of active sites in the as-depositing substrate, reactive gas remained for seconds to react entirely (saturatively) when reactive gas entered into the reaction chamber, then the redundant reactive gas and by-product will be purged away by a carrier gas (N₂). In the ALD process, one cycle also consists of four steps [77]: ① Release precursor in the reaction chamber, ② Purge away excess precursors and byproducts through carrier gas, ③ Exposure of a second precursor, ④ Evacuation of the excess precursor and byproduct. The desired film thickness is often realized by the controlling cycle number as well as deposition time (about 0.1 nm film was deposited in one cycle).

Advantages

Despite the similar point of the CVD process in conformal deposition, ALD possesses self-limiting nature due to its limited active sites and precursor in each cycle, resulting in thin films with uniform and controlled thickness. Additionally, the film thickness can be exactly controlled on the atomic scale with a low deposition rate of few angstroms in each cycle, which provides precise control over the film thickness, resulting in uniform and conformal thin films. Meanwhile, ALD requires a lower reaction temperature (100–300 °C) than the CVD process, while the lower temperature sacrificed the reaction rate as well as film growth. Higher temperatures (above 300 °C) compromise the growth rate since the rapid decomposition of precursors. Notably, gas-phase precursors enable penetration of pores, voids, and grain boundaries of the target material.

Disadvantages

It is suffered from a low deposition rate with few angstroms in one cycle. Moreover, few materials can be deposited through ALD the process due to the limited choice of precursors.

Therefore, RVD enables excellent step coverage on high aspect ratio structures. The reasonably, uniform and dense thin film would be achieved attributed to the smaller participating unit down to molecule size. Consequently, RVD is seldom applied in the fabrication of thin-film batteries restricted by the low deposition rate. However, it is commonly observed that the coating of the electrode surface with a functional layer and the formation of the electrode is assisted by a sacrificial template. In an endeavor to obtain full coverage with uniform coating on powder, researchers introduce fluidized beds in the film deposition process [78,79]. This method of rationally matching the required modules into advanced thin film deposition equipment provides a new idea for the next generation of industrial production of lithium-ion batteries coated with functional thin films. We outlined the main characteristics of as-discussed four types of deposition techniques (Figure 2). It is not difficult to find that these methods have their own advantages and disadvantages as well as corresponding restrictions. From a practical point of view, researchers need to use functional thin films to compensate for the shortcomings of existing electrodes, in an attempt to obtain more comprehensive and excellent material properties. Kotlarski et al. [80] found that W deposited on aluminum and copper surfaces exhibited α-W and β-W, respectively. Among them, the resistivity of β-W is much higher than that of α-W, and the β-W can be changed into α-W after thermal annealing treatment. Under the condition of applying a contact pressure of 40 N after depositing a thickness W of 6 um, the contact resistance value of the aluminum material decreased from 9 to 6 mΩ, and the contact resistance of copper increased from 0.12 to 0.65 mΩ. Furthermore, the anti-corrosion ability of the film after the deposition of W has been improved, and the performance of α-W is better than that of β-W [81]. Certainly, in addition to the characteristics of the deposited material itself, the quality of the film obtained after deposition is also crucial to the electrode. Parameters such as adhesion, crystal phase of film materials, impurity content, and roughness are often related to factors such as deposition method, deposition environment, and sample surface cleanliness [82,83]. This review does not address these factors in detail.

Apart from the above-mentioned coating method, there also exist other approaches such as vacuum evaporation, molecular beam epitaxy(MBE), and ion plating [84], extensively employed in lithium-ion batteries. The working mechanism of vacuum evaporation involves the heating of the material to its boiling point, causing it to evaporate and form a thin film on the substrate. The deposition rate is controlled by the heating rate of the material and the pressure in the vacuum chamber; The working mechanism of MBE involves the formation of a thin film on the substrate through the deposition of neutral particles. The deposition rate is controlled by the flux of particles and the substrate temperature; The working mechanism of ion implantation involves the modification of the surface composition and structure through the implantation of ions. The implantation depth and concentration are controlled by the energy of the ions and the dose of ions implanted into the surface. In general, each of these techniques offers different advantages and disadvantages, and the choice of technique will depend on the specific requirements of the application. However, all of these techniques can be used to deposit thin films with precise control over the film thickness, composition, and microstructure, making them useful for a variety of applications, including surface modification of lithium-ion batteries.

Impressively, the small and low-cost vacuum-based coating equipment is welcomed in a laboratory for cutting-edge investigation. Certainly, coating techniques with new working principle, high coating efficiency and exactly controlling film quality is highly desired. Next, some representative cases of using advanced thin film deposition techniques to improve the performance of lithium-ion batteries are presented.

3. Engineering of Interface and Formation of Active Material via Thin Film Deposition Techniques

The interface of the battery is formed when the electrode and electrolyte have direct contact. It is important due to its rate-limiting part during lithium-ion transfer between electrode and electrolyte. In a liquid-based battery system, the interface is facing issues such as high impedance and instability [85]. The high impedance is stemmed from three reasons: First, the low ionic conductivity of the electrode/electrolyte [86]. Second, inefficient contact between electrolyte and electrode is probably due to poor wettability/hydrophobic in liquid electrolytes and the existence of pores in a solid-state battery [87]. Third, the formation of by-products during the charge/discharge process is due to an electrochemical reaction or decomposition [88]. Additionally, a solid-state battery usually suffered from high interfacial impedance owing to grain boundary/lattice mismatch at the interface [89]. The specific presentation forms of interface problems may be as follows: Lithium Plating: Lithium plating occurs when lithium ions are deposited at the electrode during charging form metal lithium instead of lithium compounds. This can cause a short circuit within the battery, which can be hazardous; Side Reactions: Side reactions can occur at the electrode/electrolyte interface, leading to the formation of solid electrolyte interphase (SEI) layers, which can impact the battery’s performance and stability; Capacity Fading: Over time, the interface between the electrode and electrolyte can degrade, leading to a decrease in the battery’s capacity and its ability to hold a charge; Electrode-Electrolyte Interfacial Resistance: The interfacial resistance at the electrode/electrolyte interface can impact the battery’s performance, as it can slow down the transfer of lithium ions between the electrode and electrolyte; Dendrite Formation: During repeated charge and discharge cycles, dendrites can form at the electrode/electrolyte interface, which can lead to a short circuit within the battery and its eventual failure. The schematic illustration for those problems is presented in Figure 3.

To address this issue, researchers use the interface tailoring approach that modifies the electrode/electrolyte interface to reduce impedance and improve battery performance. So, it is important for interface tailoring in lithium-ion battery applications. Interface tailoring refers to the manipulation of the interface between the electrode and electrolyte to enhance stability and performance. Strategies commonly used for interface customization of Lithium-ion batteries include surface modification (electrodes can be treated to modify the surface chemistry and reduce interfacial reactivity) [90], electrolyte additives (additives can be added to the electrolyte to improve stability and reduce interfacial reactions) [91], membrane separation (a separator membrane can be used to physically separate the electrode and electrolyte and reduce interfacial reactivity) [92], and electrolyte design(the composition and formulation of the electrolyte can be tailored to enhance stability) [93], which can improve the stability and performance of battery systems. Here, we exemplify the commonly used lithium-ion battery materials including silicon, lithium metal, LiCoO₂, and LiPON for illustrating the superior character in modifying the surface/interface.

3.1. Silicon

It is well established that silicon show promise for next-generation energy storage device due to its highest theoretical specific capacity (4200 mAh g⁻¹) (>10 times that of graphite) [94,95] and abundance in the earth’s crust [96]. However, the problem of low conductivity and huge volume change (>300%) [97] hampered its further development. which leads to expanding in volume, causing electrode swelling, mechanical fracture, or pulverization of the Si anode during longer cycles [98,99,100]. In view of this situation, researchers began to study composite systems composed of carbon/silicon and some metal silicide impurities [101,102], SiOx electrodes [103], new nanostructures [104,105,106,107], etc., and made great efforts to avoid the shortcomings of pure silicon electrodes. The two main strategies used for purpose of ameliorating intrinsic weakness are incorporation with a conductive agent and introduction of nanostructure. Commonly, these two strategies are used in parallel in hopes of protecting the larger surface area exposed in the liquid electrolyte, in which the flexible use of various film deposition techniques is critical.

The construction of nanostructure can be realized by either a sacrificial template or non-sacrificial core-shell conductive support. Such an approach has been demonstrated by Lotfabad et al., where 20–35 nm silicon film was deposited on ZnO nanorods then followed by the removal of ZnO by reduced to Zn in 50% H₂ in Ar and evaporation of Zn at 600 °C [108]. Subsequently, artificial solid electrolyte interphase (TiO₂, TiN, and Al₂O₃) was covered on the outer and inner surfaces of the silicon nanotube by ALD. The optimum performance was achieved for 1.5 nm TiO₂ coated on both sides of silicon nanotubes (1700 mA h/g vs. 1287 mA h/g for the uncoated baseline, after 200 cycles at 0.2 C). The improved electrochemical performance is attributed to the high ion and electron conductive TiO₂ thin layer on both sides of the silicon nanotube, therefore highly effective in enabling rapid lithiation and delithiation. Similar reasoning and coating procedure were done in other works, such as silicon coated on carbon fiber, carbon nanotube, via CVD using a precursor of SiH₄ and H₂ or directly deposited from the target by MS [109].

Another application of a film deposition device is the deposition of a thin film on active material anticipating performing various functions. For example, Haruta et al. attempt to inhibit electrolyte decomposition on silicon surfaces by coating LiF (2–16 nm) amorphous film via RF MS [110]. Afterward, visualization of solid electrolyte interphase was performed using in-situ atomic force microscopy. Moreover, improved columbic efficiency in the first few cycles was observed suggesting suppressed electrolyte decomposition. Focusing on the overall improvement of cycling and rate performance, Han et al., combined silicon particles with conductive graphene sheets, thereby achieving a long cycling life (500 cycles with capacity retention of 80.1% at 2 A/g) and an outstanding rate capability (457.9 mA h/g at 20 A/g) [111]. Moreover, Fang et al., proposed the dual design process to improve mechanical strength and lithium-ion conductivity via ALD/MLD (molecular layer deposition) approach [112]. As presented in Figure 4a,b,e,f, the bare silicon anode and monotonous modification would lead to fracture after cycling. While the incorporation of ALD/MLD yields controllably deposited nano-pores to compact dual film and the cycled electrode exhibited decreased fracture on the surface as shown in Figure 4c,d,g,h. Meanwhile, the finite elemental analysis reveals the difference in stress distribution across the surface as depicted in Figure 4i–l. The result indicates the dual film design change stress distribution to gradient stress distribution from the unevenly concentrated point-point region in bare silicon and equally distributed face-face region in a single ALD TiO₂-Si. In addition, by electrochemical impedance spectroscopy (EIS) (Figure 4m), it can be obtained that the charge–transfer resistance (R_ct) for the Si@zincone/TiO₂ and the Si@titanicone/TiO₂ electrodes are 70.0 and 75.1 Ω, respectively. which are a little larger than those of the pure silicon electrode (R_ct = 65.6 Ω). The establishment of this dual-film coated electrode can significantly enhance the cycling performance of silicon anodes, facilitate electron/ion transport kinetics and ensure high power output(Figure 4n). The above results illustrate that the highly uniform, conformal, and ultrathin coatings produced by ALD/MLD contribute to the formation of more stable, uniform, and well-defined interfaces between electrodes and electrolytes, thereby enhancing device performance and stability. Furthermore, Zhu et al. developed a new surface coating material, aluminum oxynitride (AlO_xN_y), using plasma-enhanced atomic layer deposition with trimethylaluminum and plasma N₂/H₂ as precursors [107]. With the optimal AlO_xN_y coating thickness (~2 nm), the reversible capacity after 140 cycles was improved from 331 mAh g⁻¹ for the bare Si electrode to 1297 mAh g⁻¹ for the AlO_xN_y-coated one, which corresponds to the capacity retention changes from 13% to 72%.

3.2. LiCoO₂

As the first commercialized cathode material, LiCoO₂ still has a residual problem to be solved associated with rate capability and cycling. Fast charge ability mainly depends on lithium-ion migration kinetics and a fast diffusion rate would lead to high-rate capability. For example, ultrahigh rate changeability was achieved via ferroelectric BaTiO₃ nanodot decoration on epitaxial LiCoO₂ thin film cathode [113]. The BaTiO₃ nanodot (height of <3 nm, average diameter about 10 nm, and surface coverage of 5%) decorated LiCoO₂ (130 nm) cathode was fabricated as shown in Figure 5a, delivering excellent rate capability in the ultrahigh rate of 50 C and 100 C with 67% and 50% capacity retention of value at 1C (LiPF₆ (EC:DEC = 3:7 v/v) as the electrolyte and lithium metal as an anode). However, the discharge capacity of bare LiCoO₂ and planer BaTiO₃-LiCoO₂ cathode were decreased precipitously and then drop to null at a higher rate (50 C and 10 C, respectively) as depicted in Figure 5d. The presence of dot BaTiO₃ (Dot-BTO) enables the formation of current concentration around the interface as observed in Figure 5b, which is believed to be favorable for rate capability. By comparing three kinds of models in Figure 5c, the result demonstrates the enhanced high-rate charge ability and cyclability for Dot-BTO (Figure 5d,e). It accelerates lithium-ion penetration when passing through a high electric field and results in a robust lithium-ion intercalation/de-intercalation pathway in the vicinity of TPI (triple-phase interface). In contrast, the poor performance at a high rate of planer BaTiO₃-LiCoO₂ cathode was related to the fully covered surface and limited reaction sites of the BaTiO₃ grain boundary. In order to obtain a stable interlayer structure and improve conductivity, Sivajee Ganesh et al. [114] used the magnetron sputtering method to realize Zr doping of LiCoO₂ material to obtain LiZr_xCo_1-xO₂ (x = 0, 0.02). Compared with LiCoO₂, the average discharge capacity of LiZr_0.02Co_0.98O₂ thin-film cathode increased from 47.6 to 60.2 uAh cm⁻²um⁻¹ at 1 C. After EIS measurement, it was found that compared with LiCoO₂, the R_ct of LiZr_0.02Co_0.98O₂ decreased from 156 to 60 Ω, which suggests that Zr doping in LiCoO₂ can decrease the charge transfer resistance and enhance its electrochemical performance.

Increasing the working voltage up to a higher value would enhance energy density. However, the higher voltage may result in a serious problem at the interface. So, it is reasonable to coat the electrode surface with a suitable thin film. Since passive or too thick coating may block the lithium-ion transportation pathway. It was reported that coating 2 ALD cycles AlF₃ film (about 0.2 nm) on vacuum filtrated LiCoO₂ (60μm) freestanding electrode (LiCoO₂/multiwall carbon nanotube/nanocellulose) pushed the operating potential to 4.7 V and delivered a high specific capacity of 216 mAh g⁻¹ corresponding to a volumetric density of 720 mAh cm⁻³ [115]. The initial capacity retention of 75.7% and 70% were maintained after 100 and 160 cycles respectively.

Figure 5. (a) Schematic of the calculated model structure of BTO-LCO with ANSYS HFSS. (b) Calculated current density mapping on an LCO surface. (c) Schematic three-dimensional images of Bare, Planar BTO, and Dot BTO. (d) Discharge capacities as a function of C−rate, which increases from low 1 C to ultra-high 100 C in a five−repeated measurement sequence for each C−rate. Black, blue, and red circles correspond to Bare, Planar BTO, and Dot BTO, respectively. (e) Discharge capacities of Bare at 5 C, Dot BTO at 5 C, and Dot BTO at 50 C repeating 800 times. Reprinted with permission from Ref. [113]. Copyright 2019, American Chemical Society. (f) Schematic of the thin film solid electrolyte/thin film cathode architecture investigated. (g) Cross−section SEM image of the thin-film half−cell stack (Si/MgO/Ti/Pt/LCO/Li-Nb-O/LLZO). (h) Discharge capacities at different C−rates (1 C, 2 C, 4 C, 10 C, 20 C, 40 C) of the LCO, LCO/LLZO, and LCO/Li−Nb−O/LLZO half-cell stacks. Reprinted with permission from Ref. [116]. Copyright 2020, American Chemical Society.

Figure 5. (a) Schematic of the calculated model structure of BTO-LCO with ANSYS HFSS. (b) Calculated current density mapping on an LCO surface. (c) Schematic three-dimensional images of Bare, Planar BTO, and Dot BTO. (d) Discharge capacities as a function of C−rate, which increases from low 1 C to ultra-high 100 C in a five−repeated measurement sequence for each C−rate. Black, blue, and red circles correspond to Bare, Planar BTO, and Dot BTO, respectively. (e) Discharge capacities of Bare at 5 C, Dot BTO at 5 C, and Dot BTO at 50 C repeating 800 times. Reprinted with permission from Ref. [113]. Copyright 2019, American Chemical Society. (f) Schematic of the thin film solid electrolyte/thin film cathode architecture investigated. (g) Cross−section SEM image of the thin-film half−cell stack (Si/MgO/Ti/Pt/LCO/Li-Nb-O/LLZO). (h) Discharge capacities at different C−rates (1 C, 2 C, 4 C, 10 C, 20 C, 40 C) of the LCO, LCO/LLZO, and LCO/Li−Nb−O/LLZO half-cell stacks. Reprinted with permission from Ref. [116]. Copyright 2020, American Chemical Society.

To investigate the cathode/electrolyte interface, the thin-film architecture is established as shown in Figure 5f,g [116]. After introducing an in-situ lithiated Nb₂O₅ diffusion layer, LLZO/LCO interface resistance is lowered and demonstrates fast charge transfer (Figure 5h). The interface between LiCoO₂ and the solid electrolyte was investigated in the mode)l system by sputtering 500 nm LiCoO₂ film on NASICON type solid electrolyte (Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP)) [117]. After heat treatment (500 °C for crystallization of the LiCoO₂ cathode), solid cell (Pt (50 nm)/LiCoO₂ (500 nm)/LICGC (150 μm)/LIPON (1 μm)/Li foil) was assembled and exhibited a stable interface without extensive interdiffusion of element for 10 cycles when cycled between 3.3–4.2 V at 0.01 C and 30 °C. Unfortunately, the irreversible capacity loss occurred at the first cycle (columbic efficiency of 68%) due to structural defects in the annealed LiCoO₂ film. However, the chemically stable interface was established between sputtered LiCoO₂ and NASICON-type electrolytes after thermal annealing. In addition, some researchers have used the CVD method to grow Poly(3,4-ethylenedioxythiophene) (PEDOT) on the surface of LiCoO₂ [118]. The theoretical calculations and experimental results illustrate that the PEDOT coating exhibits enhanced Li⁺ kinetics, resulting in increased current uniformity in the LiCoO₂ electrode, thereby improving the rate capability of the LiCoO₂ battery and increasing cycle life by over 1700%.

3.3. LiPON

LiPON is one of the most competitive solid electrolytes due to its wide voltage window (0-5.5 V) and compatibility with lithium metal except for its low ionic conductivity (1−3 × 10⁻⁶ S cm⁻¹ at 25 °C) [119]. To enhance the ionic conductivity of LiPON electrolytes, the approach of lithium compensation was employed from a lithium-rich target in the sputtering process [55]. As a result, moderate enhancement of ionic conductivity was evidenced for Li-LiPON with 3.6 × 10⁻⁶ S cm⁻¹, compared to 2.4 × 10⁻⁶ S cm⁻¹ for normal LiPON sputtered from Li₃PO₄. Consequently, rate performance was improved in the structure of LiCoO₂ (0.45 μm)/Li-LiPON (1.49)/Li with 98% capacity retention when the current rate decreased from 4 C to 0.1 C, compared to 90% for normal LiPON. The improved rate capability originated from increased ionic conductivity and an augmented amount of lithium ions. In addition, it is interesting that this research work found that the lithium-ion conductivity (σ) of the LiPON electrolyte layer gradually increased when the working pressure decreased from 0.5 to 0.2 Pa; and when the working pressure decreased to 0.08 Pa, σ decreased significantly (Figure 6a). According to the corresponding equivalent circuit (Figure 6b), LiPON sputtered from Li_3.3PO₄ is indeed lower in resistance than Li₃PO₄. Sputtering techniques enable the conformal coating on the surface of the particles as presented in Figure 6c, preserving clear boundaries and uniform elemental dispersion (Figure 6d,e) [120]. Moreover, a nanometer LiPON layer was deposited on the surface of LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) powder via sputtering and exhibited improved electrochemical performance compared to bare-NMC811 [121]. The film thickness was controlled by sputtering time and power. The optimized condition was obtained (25 W, 6 min) in Figure 6f with 64% capacity retention in solid-state batteries. The rate performance was improved after lithium-ion conducting LiPON solid electrolyte coating as shown in Figure 6g. In the full-cell configuration, it performed well under a cutoff voltage of 4.1, 4.2, and 4.3 V (Figure 6h). However, the thicker coating would block lithium-ion migration and degrade cell performance.

Another appealing area of solid-state electrolytes is integration with a flexible substrate, which requires a delicate design at low temperatures with the presence of the polymer substrate. It was reported that a lithium-ion-based flexible battery was established on a polyimide substrate with the structure of LiV₂O₅ (90 nm)/LiPO₂N (100 nm)/Si (40 nm) [122]. In which, pinhole-free LiPO₂N solid electrolyte was grown at 300 °C via ALD technique with an ionic conductivity of (6.51 ± 0.36) × 10⁻⁷ S cm⁻¹ at 35 °C. Importantly, the electrolyte film was electrochemically stable in the voltage range of 0–0.53 V versus Li/Li⁺, when assembled in the full cell (solid electrolyte was grown at 250 °C) delivered a stable capacity of 1.6 μAh cm⁻² after bending for 10 times, indicating tolerance to mechanical bending due to conformal coating ability of ALD with the formation of intimate contact.

3.4. Lithium Metal

Lithium metal was recognized as the ultimate anode material for lithium-ion batteries due to its advantages such as high theoretical capacity (3840 mAh g⁻¹), lowest anode potential (−3.04V vs. standard hydrogen electrode), and light-weight (0.53 g cm⁻³), shedding light on developing batteries with high energy density [123]. Unfortunately, the safety problem related to the uncontrolled formation of dendrite due to inhomogeneous lithium-ion deposition during charging prevents its further industrialization. Therefore, it is demanding to employ surface modification techniques for the improvement of safety and performance utilizing powerful surface engineering techniques.

Due to the low melting point and simple elemental composition of lithium anode (180 °C), the vacuum evaporation method is usually applied in the construction of lithium anode for micro/thin-film batteries [124,125]. However, in the case of surface modification for lithium anode with multi-elemental composition, sputtering was conducted favored by its stoichiometric transferring characteristic. For instance, Wang et al. demonstrate the sputtering of an amorphous Li₃PO₄ layer (30 nm) on lithium foil (200 μm) and successfully hinders surface oxidation after air exposure (Figure 7a) [126]. The Li₃PO₄ (30 nm) film (electronic conductivity of 1.4 × 10⁻¹⁰ S cm⁻¹ and ionic conductivity of 2.8 × 10⁻⁸ S cm⁻¹) also extends the lifespan of lithium metal with stable cycled above 30 h due to the chemical stability and conformal coating (Figure 7b). Liping et al. [127]. used magnetron sputtering to deposit 20 nm Al₂O₃ on the surface of lithium foil, which can endure for 1186 h at a current density of 0.5 mA/cm² with a capacity of 0.5 mAh/cm² in a carbonate liquid electrolyte battery and 660 h at a current density of 0.1 mA/cm² with a capacity of 0.1 mAh/cm² in an all-solid-state battery. In the 96-h EIS test, the Al₂O₃ modified electrode maintained better stability than the unmodified one (the former changed from 653 to 705 Ω, and the latter changed from 424 to 1080 Ω). Similarly, Kozen et al. reported the protecting effect of 14 nm Al₂O₃ on lithium metal (750 μm) via ALD from corrosion of atmosphere (laboratory environment with CO₂ and H₂O, 25 °C, 40% humidity), organic solvent (propylene carbonate) and sulfur/DME (dimethyoxylane) respectively [128]. Zhang et al. reported an ultrathin Li₃N film-modified separator with controllable thickness by physical vapor deposition (PVD) of lithium metal and subsequent nitridation at room temperature, which can homogenize Li ions and protect the Li metal anode [129]. With this functional separator, the Li/Li symmetrical cell could achieve a long cycle with low overpotential for 1000 h at a current density of 1 mA cm⁻².

To engineer the surface of lithium metal with a functional layer, an ionic conductive sulfide-based solid electrolyte (Li_xAl_yS) with ionic conductivity of 2.5 × 10⁻⁷ S cm⁻¹ was coated by ALD [130]. The precisely controlled thickness and composition enable interface stabilization as well as doubling the lifetime of Li-Cu asymmetric cells. Besides, the resistance increase was retarded when contact with a commercial carbonate-based electrolyte in a symmetric cell for 68 h with R_SEI (resistance across solid-electrolyte interface) of 500 Ω for protected lithium and 2500 Ω for pristine. Significantly, the Li_xAl_yS-lithium anode delivered stable columbic efficiency over 170 cycles, much better than pristine lithium with a gradual decrease in the first 90 cycles and a precipitous drop at around 135 cycles (Figure 7c). The enhanced cycling performance originated from the stable surface as well as the high ionic conductivity benefit from Li_xAl_yS solid electrolyte thin film. Additionally, the ultrathin ZnO layer was deposited on the Cu foam to fabricate a lithium metal host (Figure 7d) [131]. Consequently, the ZnO deposited symmetric cell operated for over 900 h without fluctuation as seen in Figure 7e. The lipophilic ZnO film enables uniform nucleation of lithium and inhibits dendrite formation.

Advanced thin film deposition techniques have significantly improved the performance of lithium-ion battery materials such as silicon, lithium metal, LiCoO₂, and LiPON (typical materials for anode, cathode, and electrolyte of lithium-ion batteries), especially in reducing interface impedance, increasing corrosion resistance, and improving ionic conductivity. This proves that the use of advanced thin film deposition techniques has positive effects on the surface engineering of lithium-ion batteries. MS, PLD, CVD, and ALD, as representatives of advanced thin film deposition techniques, together provide the possibility of mass production for surface modification of lithium-ion batteries. Various thin film deposition techniques should be effectively selected to deal with different demands. In general, planar substrates are suitable for all deposition techniques considering step coverage. At this time, considering the time cost, MS or PLD with a faster deposition rate can be selected. For substrates with nanostructures, it is suitable for CVD or ALD. ALD is more suitable for materials with large specific surface areas or when ultra-fine film thickness control is required. Typically, process parameters directly affect the properties of the deposited film, such as temperature, pressure, and gas flow rate. Occasionally, the same material deposited on different substrates forms films with different properties (W deposited on aluminum and copper). In addition, the effects brought about by different ratios of doping elements are also different (Zr doping of LiCoO₂). Therefore, further research is needed to fully understand the influence of thin films on lithium-ion batteries under different parameters.

4. Conclusions

To conclude, the film deposition techniques are introduced in terms of related processes, mechanisms, and advantages/disadvantages. Based on the understanding of the electrode/electrolyte interface, we exemplified the works for addressing interface problems by engineering techniques. A great contribution has been acquired by introducing film deposition techniques for surface coating. Besides, the film deposition techniques have the functions of doping, nanostructuring, and formation of active materials [132,133,134,135,136,137,138,139,140]. Therefore, from the perspective of advanced thin film deposition techniques, it is particularly significant for the development of lithium-ion batteries to deeply investigate the related works to emphasize the understanding as well as smart applications.

Significant progress has been achieved for lithium-ion batteries in past decades with the aid of film coating techniques. Favored by its tailoring effect, the electrochemical performance of lithium-ion batteries was increasingly enhanced in terms of cycling number, and rate capability and even widened the voltage window by effectively modifying the interface. However, there still exist crucial problems preventing further industrialization as well as scalable fabrication. The reason behind this includes but is not limited to: firstly, the uniform film is just restricted to a small area substrate due to the inherent defect of the coating device, especially for PLD. Secondly, the deposition rate is relatively slow in ALD and CVD. Thirdly, most of the devices are pricey and not suitable for large-scale applications. With the consideration of the significant role of the coating technique, vapor coating strategies still require further development to meet specific applications. It is worth mentioning that the low-cost, scalable, and effective coating technique is anticipated to contribute to the lithium-ion batteries market.

Engineering the interface between electrode and electrolyte is given increasing attention arising of its important role played in electrochemical devices. Although the above-mentioned modification has been certified to be significant in advancing the performance, the major drawback of film deposition techniques for further application is the high price of the device and target/precursors. One promising strategy to relieve such a problem is to selectively adopt of those methods. Frontier research should be given priority to utilize those techniques and explore deeper working factors. On the other hand, seeking inexpensive alternative approaches is beneficial for industrial applications. In terms of solid-state micro-battery, vapor deposition techniques should be promoted for medical, smart, and intelligent applications. Even though there is still much room for further improvement in overall performance, the freestanding or binder-free electrode structures also show great potential application in flexible devices. It is expected that, with extensive efforts, high energy/power density and safety lithium-ion batteries can provide better service in practical applications. In addition, further research is needed to be focused on the design of manufacturing devices based on deposition techniques for all-solid-state batteries with low cost toward high-end applications such as an in-vivo actuator, artificial intelligence, and micro-robots.