Bi_0.5Na_0.5TiO₃-Bi_3.25La_0.75Ti₃O₁₂ Lead-Free Thin Films for Energy Storage Applications through Nanodomain Design

Abstract

Dielectric capacitors have received increasing attention due to their high power density. The Bi-based Aurivillius phase compound Bi_3.25La_0.75Ti₃O₁₂ (BLT) is considered a potential material in the field of energy storage due to its excellent ferroelectric properties and good fatigue resistance, and temperature stability. In this paper, 0.4Bi_0.5Na_0.5TiO₃-0.6Bi_3.25La_0.75Ti₃O₁₂ (0.4NBT4BNT-0.6BLT)-thin films were prepared on Pt/Ti/SiO₂/Si substrates with the sol-gel method. The addition of BNT destroys the long-range ferroelectric order of BLT and forms nanodomains. By increasing the BNT content, the BLT is transformed from a ferroelectric state to a relaxed state, and its application in the field of energy storage is realized. The recoverable energy density is 42.41 J/cm³, and the recoverable energy storage density is relatively stable in the range of 25–200 °C with good thermal stability. The energy storage efficiency is 75.32% at ~2663 kV/cm. The leakage current density at 300 kV/cm is 1.06 × 10⁻⁹ A/cm².

1. Introduction

With the rapid development of the electronic industry and the trend of miniaturization and integration of integrated circuits, dielectric capacitors with high energy storage density and energy storage efficiency have attracted increasing attention [1,2,3,4,5]. Typically, the recoverable energy storage density (U_rec) and energy efficiency (ŋ) is calculated by integrating the hysteresis loop as follows [6,7,8]:

$$U_{rec}=\underset{P_r}{\overset{P_{max}}{{\displaystyle \int }}}EdP$$

$$\mathrm{ŋ}=U_{rec}/(U_{rec}+U_{loss})$$

Here, E is the applied electric field, P_max is the maximum polarization, P_r is the remnant polarization. Therefore, materials with a large P_max, small P_r, and high breakdown field strength are potential candidates for high energy storage performance. Previously, commercial energy storage materials were mainly lead-based. However, the volatilization of lead in lead-based materials would not only cause harm to the human body but also pollute the environment [9,10,11]. Therefore, it is imperative to replace lead-based materials with lead-free materials. Lead-free thin-film materials have also achieved many exciting results. For example, Zhang et al. achieved enhanced energy storage performance in 0.9Na_0.5Bi_0.5TiO₃-0.1BiFeO₃ (0.9NBT-0.1BFO) thin films prepared by adding BFO to the lead-free material system NBT, with a recoverable energy density of 44 J/cm³ [12]; Song et al. obtained an energy storage density of 47.6 J/cm³ and an energy storage efficiency of 65.68% by doping 2% Mn into lead-free 0.7Sr_0.7Bi_0.2TiO₃-0.3BiFeO₃ film to reduce the oxygen vacancy content of the material [13]. In order to meet the application of power devices in various fields, it is necessary to further develop lead-free dielectric capacitors with high energy storage performance.

At present, the main ways to improve the energy storage properties of materials are domain engineering [14,15,16], structural design [4,6,17,18], and element doping [13,19,20,21]. In general, slender hysteretic loops with low energy dissipation can be achieved by customizing the ferroelectric (FE) or anti-ferroelectric (AFE) domain sizes to the micron or nanoscale. Due to a local random field, the relaxation ferroelectric (RFE) with a polar nanoregion requires a very high applied electric field to construct a saturated long-range order, resulting in enhanced energy storage densities. Song et al. reported that the long-range order of anti-ferroelectric domains could be destroyed by introducing lanthanum (La) into AgNbO₃ ceramics. Thus, the micron-level anti-ferroelectric domains were transformed into anti-ferroelectric nanodomains, resulting in a lower residual polarization, larger breakdown electric field strength, and higher energy density and efficiency [22]. By adding SrTiO₃(STO) into BiFeO₃(BFO), Pan et al. transformed its ferroelectric microdomains into nanodomains with high dynamic polarity, from ferroelectric to relaxor ferroelectric and obtained a high energy storage density and efficiency through domain engineering [23]. All the results show that domain engineering effectively improves the dielectric capacitor’s storage characteristics. However, these energy storage materials can only maintain good energy storage performance at low temperatures (<120 °C), so it is necessary to explore a kind of energy storage material that can achieve good energy storage performance through domain engineering and make it work stably at a high temperature.

Bi₄Ti₃O₁₂ (BIT) belongs to the Aurivillius family of oxides. It consists of a (Bi₂O₂)²⁺ layer and a perovskite (A_n−1B_nO_3n+1)²⁻ layer, which has excellent fatigue resistance and temperature stability, and its improved ferroelectric properties can be found in the study of its A-site doping La, which has broad application prospects in non-volatile ferroelectric memories [24,25,26,27]. In addition, its large polarization, small leakage current density, excellent anti-fatigue properties, and temperature stability determine its application in the field of energy storage [28]. However, its large residual polarization and low breakdown strength limit its application in the field of energy storage. Therefore, it is necessary to reduce its residual polarization and improve its breakdown field strength.

In this work, we selected another Bi-based material with a higher Curie temperature (Tc = 320 °C)-Bi_0.5Na_0.5TiO₃(BNT), to be incorporated into the BLT. It causes compositional and chemical disorder in 0.4BNT-0.6BLT films, which breaks the long-range FE order of BLT. With the addition of BNT, the long-range ferroelectric order of BLT is destroyed, resulting in nanodomains. The RFE state induced by domain engineering achieves suppressed residual polarization and enhanced breakdown strength, and maintains a large polarization value of BNT. Excellent energy storage performance, a small leakage current density, and good temperature stability (25–200 °C) are obtained in 0.4BNT-0.6BLT thin films.

2. Experiments

0.4Bi_0.5Na_0.5TiO₃-0.6Bi_3.25La_0.75Ti₃O₁₂ (0.4BNT-0.6BLT) thin films were grown on Pt/Ti/SiO₂/Si substrates with the sol-gel method. The precursor solution uses bismuth(III) acetate, lanthanum acetate, sodium acetate, and tetrabutyl titanate as raw materials, propionic acid as a solvent, and ethanolamine as a stabilizer. Bismuth acetate, lanthanum acetate, sodium acetate, and tetrabutyl titanate were weighed according to the stoichiometric ratio. To compensate for the volatilization of bismuth and sodium during annealing, a 5% excess of bismuth acetate and sodium acetate was used to prepare the precursor solution. First, bismuth acetate, sodium acetate, and lanthanum acetate were added to propionic acid in turn, and stirred at room temperature until fully dissolved. Tetrabutyl titanate was added, and a certain amount of ethanolamine was added to stabilize the colloid and stirred at room temperature for one day to obtain a golden yellow precursor solution. The solution was spun at 6000 rpm for 20 s and then pyrolyzed on a hot stage at 400 °C for 6 min; this process was repeated a certain number of times to obtain thin film samples with a certain thickness. Finally, the samples were rapidly annealed in an air atmosphere in an infrared annealing furnace at 700 °C. The same process was used to prepare the BLT film.

The crystallinity and phase of the samples were characterized by performing X-ray diffraction (XRD), the elemental distribution of the sample was tested with an energy dispersive spectroscopy (EDS), and the domain structure of the samples was characterized by piezoelectric atomic force microscopy (PFM). The ferroelectric properties of the samples were investigated using a ferroelectric tester (Radiant Technology, Albuquerque, NM, USA) before electrical testing. Au top electrodes with a diameter of 0.5 mm were deposited on the films by magnetron sputtering through a reticle to obtain Au/BNT-BLT/Pt capacitors.

3. Results and Discussion

The XRD patterns of 0.4BNT-0.6BLT films grown on Pt/Ti/SiO₂/Si substrates with the sol-gel method are shown in Figure 1. Compared with the (117) preferred orientation of pure BLT films, 0.4BNT-0.6BLT showed no obvious preference orientation. Moreover, all the characteristic peaks of BNT-BLT were assigned to the Bi₄Ti₃O₁₂ phase. Energy dispersive spectroscopy confirmed the formation of a homogeneous NBT-BLT solid solution. As shown in Figure 1b, bismuth, lanthanum, titanium, and sodium elements were uniformly dispersed in the film without apparent aggregation.

The typical morphology (AFM) images of the 0.4BNT-0.6BLT films are shown in Figure 2. All films exhibit good surface density and roughness. The root-mean-square (RMS) of surface roughness of the BLT film was calculated with Igor Pro software, and was found to be 7.62; the RMS value of the 0.4BNT-0.6BLT film is 5.29 nm, which indicates that all the films show a very smooth surface. The suitable microstructure and surface morphology indicate that the electrical properties of the films improved, including a reduced leakage current and tolerable electric field strength, which are discussed in the next section.

Figure 3 shows the phase image of the random region of the BNT-BLT thin film. Regions with bright contrast are polarized in opposite directions to regions with dark contrast, where bright and dark regions represent upper and lower polarizations, respectively. Figure 3a shows the phase map zoomed in at random regions, and a linear domain structure with a width of nanometers is found. Figure 3b shows the phase in the region corresponding to the red line in Figure 3a [29], indicating that in the BNT-BLT thin film material system, the addition of BNT can disrupt the long-range ferroelectric order state in the BLT matrix, reducing the macrodomain size and the transformation to the nanodomain structure.

Figure 4a,b show the hysteresis loops of the BNT-BLT films under different electric fields. It can be found that the BLT films exhibit typical ferroelectricity, while the BNT-BLT films exhibit relaxed ferroelectricity. This is because the addition of BNT disrupts the long-range ferroelectric order of BLT, inducing a nanoscale domain structure and polar nanoscale regions (PNRs). The appearance of nanodomains and PNRs leads to a decrease in remnant polarization and thinner P-E rings. The reduction in domain size can enhance the relaxation behavior of ferroelectrics, in which more dipoles return to the disordered state when the applied field is removed. Therefore, the remanent polarization of BNT-BLT is maintained at a low level. Figure 4c shows the P_max and P_r of BNT-BLT as a function of the electric field, respectively. As the electric field increases, P_max and P_r increase at different growth rates, and ΔP also increases in this case (ΔP = P_max − P_r), which is beneficial for energy storage. A higher breakdown strength is also necessary for energy storage. BNT-BLT exhibits good breakdown strength, and the maximum breakdown voltage can reach ~2663 kV/cm, which is related to its smaller leakage current shown in Figure 5. According to the hysteresis loop, the energy storage characteristics of BNT-BLT are calculated. As shown in Figure 4d, when 0.4BNT-0.6BLT is at 2662.675 kV/cm, the recoverable energy density is 42.41 J/cm³, and the energy efficiency is 75.32%.

The leakage current density and electric field (J-E) characteristics of BLT and 0.4BNT-0.6BLT films are shown in Figure 5. Compared with the BLT film, the leakage current of the 0.4BNT-0.6BLT film is significantly reduced. Its leakage current density at 300 kV/cm is 1.06 × 10⁻⁹ A/cm² and for BLT it is about 10⁻⁸ A/cm². It may be due to the increase in the number of grain boundaries, which is due to the decrease in grain size. The increased grain boundaries hinder the onset of defect diffusion and also contribute to reducing the current density [30].

Figure 6a shows the hysteresis loops of the 0.4BNT-0.6BLT film measured from 25 to 200 °C under an applied electric field of 1500 kV/cm. A typical P-E curve is obtained. Figure 6b shows the variation of P_max and P_r with the electric field at different temperatures. When the temperature is below 100 °C, P_max decreases slightly. When it is above 100 °C, P_max begins to increase greatly, while P_r increases with temperature. This is because domain wall motion is easier when the temperature increases. However, the obvious increase in P_r is slightly larger than P_max, resulting in the decrease in ΔP, which is not good for the energy storage performance. When the temperature increased from 25 to 200 °C, U_rec decreased from 18.10 to 17.08 J/cm³, the change of U_rec was only 5.65%, and ŋ decreased from 84.25 to 76.64%, indicating that 0.4BNT-0.6BLT has good thermal stability.

4. Conclusions

In summary, BLT and 0.4BNT-0.6BLT thin films were prepared on Pt/Ti/SiO₂/Si substrates using the sol-gel method. The introduction of BNT destroys the long-range order of the BLT ferroelectric domain structure, leading to the transitions from microdomains to nanodomains. A lower remnant polarization and a larger remnant polarization were obtained, a recoverable energy density of 42.41 J/cm³ was obtained at ~2663 kV/cm, the energy storage efficiency was 75.32%, and the leakage current density at 300 kV/cm was 1.06 × 10⁻⁹ A/cm². There is no obvious change in the recoverable energy storage density in the range of 25–200 °C, which indicates that it has good thermal stability. These properties indicate that the lead-free material 0.4BNT-0.6BLT has a good application prospect in the field of energy storage. This work provides a reference for preparing dielectric energy storage materials with excellent energy storage properties by domain engineering.