High-Energy Storage Performance in La-Doped Lead Zirconate Films on Flexible Mica Substrates

Abstract

Flexible thin-film capacitors have gained a lot of attention in energy storage applications because of their high energy storage densities and efficient charge–discharge performances. Among these materials, antiferroelectric compounds with low residual polarization and strong saturation polarization have shown great promise. However, their comparatively low breakdown strength continues to be a major issue restricting further developments in their energy storage performance. While La³⁺ doping has been explored as a means to enhance the energy storage capabilities of antiferroelectric thin films, the specific influence of La³⁺ on breakdown strength and the underlying mechanism of phase transitions have not been thoroughly investigated in existing research. In this study, Pb_1−3x/2La_xZrO₃ thin films were successfully synthesized and deposited on mica substrates via the sol–gel process. By varying the concentration of La³⁺ ions, a detailed examination of the films’ microstructures, electrical properties, and energy storage performances was carried out to better understand how La³⁺ doping influences both breakdown strength and energy storage characteristics. The results show that doping with La³⁺ significantly improves the breakdown strength of the films, reduces the critical phase transition electric field (E_F-E_A), and enhances their energy storage capabilities. Notably, the Pb_0.91La_0.06ZrO₃ thin film achieved an impressive energy storage density of 34.9 J/cm³ with an efficiency of 58.3%, and at the maximum electric field strength of 1541 kV/cm, the recoverable energy density (W_rec) was 385% greater than that of the PbZrO₃ film. Additionally, the film still maintains good energy storage performance after 10⁷ cycles and 10⁴ bending cycles. These findings highlight the potential of flexible antiferroelectric Pb_0.91La_0.06ZrO₃ thin films for future energy storage applications.

1. Introduction

As global energy demand keeps rising and climate change brings more and more challenges, the development of effective and environmentally friendly energy storage systems has turned into a crucial technological approach to dealing with energy shortages and lessening the impacts of climate change. In the area of energy storage, as a prospective means of electrical energy storage, inorganic dielectric capacitors have attracted much attention. Because of their extraordinary efficiency in charge–discharge processes, and high energy density, dielectric capacitors are known for their high efficiency in charge–discharge processes as well as their outstanding stability [1,2,3,4,5]. Despite their excellent charge–discharge rates and cycling stability, the application of dielectric capacitors in large-scale engineering projects is still restricted by their relatively low energy storage capacity [6,7,8,9,10,11]. Consequently, increasing the energy storage density of dielectric capacitors has turned into a major focus of research [12,13,14]. Energy storage density is an important parameter for assessing the energy storage performance of dielectric capacitors. It represents the quantity of energy stored per unit volume of the dielectric material under an applied electric field [15,16]. In the case of nonlinear dielectrics, the equations provided below can be used to calculate the efficiency (η) using the overall energy storage density (W_tot) and the density of recoverable energy (W_rec) [17].

$$W_{\mathrm{tot}}={\displaystyle {\int }_0^{P_{\mathrm{m}}}EdP}$$

(1)

$$W_{\mathrm{rec}}={\displaystyle {\int }_{P_{\mathrm{r}}}^{P_{\mathrm{m}}}EdP}$$

(2)

$$\eta ={\displaystyle \frac{W_{\mathrm{rec}}}{W_{\mathrm{tot}}}}={\displaystyle \frac{W_{\mathrm{rec}}}{W_{\mathrm{rec}}+W_{\mathrm{loss}}}}$$

(3)

P_m and P_r represent the saturation polarization and remnant polarization, respectively; W_loss is the energy loss during the charge–discharge process, and E is the applied electric field [18].

Lead zirconate (PbZrO₃), a well-established antiferroelectric material, is commonly used in dielectric capacitors due to its near-zero remnant polarization and high saturation polarization when subjected to an external electric field [19,20]. However, its relatively low breakdown strength remains a significant challenge to improving its energy storage performance. In recent years, element doping has attracted significant interest as a promising approach to improving the energy storage performance and stability of thin films [21,22]. For instance, Ye et al. synthesized Eu-doped PbZrO₃ thin films using the sol–gel technique and demonstrated that the incorporation of Eu influenced both the Curie temperature and the phase transition electric field [23]. At a doping concentration of 3%, the maximum energy storage density achieved was 18.8 J/cm³. Sa et al. investigated the antiferroelectric–ferroelectric phase transition process by incorporating tungsten (W) elements into PbZrO₃ [24]. As the W content increased, lattice distortion occurred, the film orientation shifted from (111) to (110), and the Curie temperature progressively decreased. However, while ion doping has improved the performance of thin films, enhancing their flexibility and sustainability remains a critical challenge. Mica, a layered silicate mineral with exceptional mechanical flexibility and thermal stability, has become an ideal substrate for flexible films due to its superior physical properties [17,25]. Flexible inorganic energy storage thin films fabricated on Mica substrates not only retain the flexibility of Mica but also integrate the excellent energy storage performance of PbZrO₃, enabling efficient energy storage [26]. In this study, Pb_1−3x/2La_xZrO₃ antiferroelectric thin films were fabricated on Mica substrates using the sol–gel method, aiming to improve the films’ energy storage density and achieve flexibility. By adjusting the La³⁺ doping concentration (x = 0, 0.04, 0.06, 0.08), the effects of La³⁺ doping on the microstructure and antiferroelectric properties of the PbZrO₃ thin films were thoroughly investigated. The mechanisms through which La³⁺ doping affects the energy storage performance of the films were analyzed, and the changes in energy storage behavior under external electric fields were investigated.

The results demonstrate that the breakdown strength of the Pb_0.91La_0.06ZrO₃ thin film reached a maximum value of 1541.0 kV/cm, which is 3.1 times higher than that of the PbZrO₃ thin film (492.5 kV/cm). Additionally, the film exhibited optimal energy storage performance, with a W_rec of 34.9 J/cm³ and an η of 58.3%. The objective of this research is to investigate the effect of La³⁺ doping on the breakdown strength, dielectric properties, and energy storage performance of PbZrO₃ thin films. While previous studies have explored the impact of doping on the energy storage performance of antiferroelectric materials, the specific mechanisms behind the enhancement of breakdown strength and phase transitions remain inadequately understood. This study aims to fill this gap by systematically varying the La³⁺ doping concentration and examining its influence on the material properties of the films, thereby providing a deeper understanding of the role of La³⁺ doping in enhancing the performance of PbZrO₃ thin films.

2. Experimental Section

2.1. Film Preparation

The Pb_1−3x/2La_xZrO₃ precursor solution was synthesized by dissolving stoichiometric quantities of Pb(CH₃COO)₂·5H₂O (99.99%, Beijing InnoChem Science & Technology Co., Ltd., Beijing, China) and La(NO₃)₃·6H₂O (99%, Beijing InnoChem Science & Technology Co., Ltd.) in a binary solvent system comprising CH₃OCH₂CH₂OH (99%, Beijing InnoChem Science & Technology Co., Ltd.) and CH₃COOH (99.5%, Beijing InnoChem Science & Technology Co., Ltd.), with zirconium isopropoxide (C₁₂H₂₈O₄Zr (70%, Beijing InnoChem Science & Technology Co., Ltd.)) introduced as the liquid-phase precursor. To compensate for lead loss during high-temperature annealing, an additional 10% of lead was incorporated. The mixture was stirred at ambient temperature for one hour, followed by storage at a cool temperature for 24 h, resulting in a clear and transparent Pb_1−3x/2La_xZrO₃ precursor solution [27,28,29]. For substrate preparation, the Mica was meticulously peeled using tweezers to achieve a smooth surface. It was then ultrasonically cleaned in anhydrous ethanol to eliminate surface impurities. After drying, a Pt electrode was deposited on the substrate via magnetron sputtering. The precursor solution was applied to the Mica substrate through spin coating, initially at 1000 rpm for 10 s, followed by 3000 rpm for 20 s to form a wet film. This film was then dried on a hot plate at 400 °C for 3 min to ensure complete removal of the organic solvents. The spin-coating process was repeated until the desired thickness of the inorganic thin film was achieved. Finally, the dried film was transferred to a rapid thermal annealing furnace for crystallization. The furnace’s temperature program was configured to rapidly reach the target annealing temperature, which was maintained for 3 min to ensure complete crystallization of the thin film. Figure S1 shows a flowchart of the thin film preparation method. The preparation of precursors and inorganic energy storage films is described in detail.

2.2. Characterization

The phase structures of the thin films and ceramic powders were analyzed using a PANalytical XRD-600 X-ray diffractometer (Almelo, The Netherlands). Raman scattering spectra for the thin-film samples were obtained with a Renishaw inVia Raman spectrometer (Renishaw, Gloucestershire, UK), covering a frequency range from 100 cm⁻¹ to 800 cm⁻¹. The cross-sectional morphology and thickness of the thin films were examined with a field-emission scanning electron microscope (SEM, SU8020, Hitachi, Tokyo, Japan). The surface topography and roughness of the thin films were studied using an atomic force microscope (AFM, Dimension Icon, Bruker, Bremen, Germany). To evaluate the ferroelectric properties, the hysteresis loops and leakage current density of the thin-film samples were measured using a Radiant Premier II ferroelectric performance testing system (Radiant, Redmond, WA, USA).

3. Results and Discussion

Figure 1a displays the XRD spectra of the Pb_1−3x/2La_xZrO₃ films (x = 0, 0.04, 0.06, 0.08). The results demonstrate that films with varying La³⁺ doping concentrations all exhibit a perovskite structure, preferentially oriented along the (110) direction. When the La³⁺ doping concentration reaches 0.08, the intensity of the (110) diffraction peak of the Pb_0.88La_0.08ZrO₃ film decreases, possibly due to changes in La³⁺ occupancy as the doping content increases [30,31,32,33]. Figure 1b displays the Raman spectrum of the Pb_1−3x/2La_xZrO₃ films. The Raman peak at 664 cm⁻¹ corresponds to the Zr-O bond stretching vibration mode, and the intensity of this characteristic peak gradually decreases as the La³⁺ doping concentration increases. Figure S2 presents the complete Raman spectra. In this study, Pb_1−3x/2La_xZrO₃ ceramic powders were prepared, and the XRD spectrum is shown in Figure 1c. No secondary peaks were observed in the XRD patterns, confirming that La³⁺ has been fully solid-solubilized. Figure 1d displays the detailed spectrum of the (110) diffraction peak, with a scanning range of 30–32°. As the La³⁺ doping concentration increases, the (110) diffraction peak shifts to higher angles. When the La³⁺ doping concentration reaches 0.08, the diffraction peak shifts to lower angles. According to the Bragg equation, nλ = 2dsinθ, the interplanar spacing (d) first increases and then decreases as the La³⁺ concentration increases. The ionic radius of La³⁺ is 0.103 nm, that of Pb²⁺ is 0.119 nm, and that of Zr⁴⁺ is 0.072 nm. At low doping concentrations, La³⁺ tends to occupy the Pb²⁺ sites, resulting in a reduction in the lattice constant. However, when the doping concentration reaches 0.08, the occupancy of La³⁺ may change, with La³⁺ preferentially occupying the Zr⁴⁺ sites, resulting in an increase in the interplanar spacing. Figure S3 shows a transmission electron microscope image of Pb_0.88La_0.08ZrO₃ film. Through measurements, it was found that the interplanar spacing of Pb_0.88La_0.08ZrO₃ is 0.35 nm, which is significantly larger than the interplanar spacing of PbZrO3, which is 0.31 nm [17].

Figure 2a–d display the cross-sectional SEM images of Pb_1−3x/2La_xZrO₃ films. The images indicate that the cross-sectional structure of the films is intact, with no significant defects such as pores or cracks, and the film thickness is approximately 260 nm. Figure 2e–h present the surface AFM images of Pb_1−3x/2La_xZrO₃ films. The particle size is uniform, and the film surface appears dense. As La³⁺ is introduced, the surface roughness of the films tends to increase [34,35]. The root mean square (RMS) surface roughness values of the Pb_1−3x/2La_xZrO₃ films (x = 0, 0.04, 0.06, 0.08) are 7.2 nm, 33.1 nm, 28.7 nm, and 45.6 nm, respectively [36,37]. Figure S4 shows the elemental mapping of the Pb_1−3x/2La_xZrO₃ film. The presence of various constituent elements in the Pb0.91La0.06ZrO3 thin film can be clearly observed in the elemental mapping images.

Figure 3 presents the P-E loops of the Pb_1−3x/2La_xZrO₃ (x = 0, 0.04, 0.06, 0.08) films. The figure indicates that the films display characteristic double hysteresis loops. As the La³⁺ doping concentration increases, the breakdown strength of the films improves, and the P_m first increases and then decreases with higher doping concentrations [38,39].

Figure 4 presents the polarization current and hysteresis loops of the Pb_1−3x/2La_xZrO₃ films under an identical electric field. The Pb_1−3x/2La_xZrO₃ films display four current peaks, consistent with their antiferroelectric characteristics. E_F and E_A correspond to the critical electric fields for the antiferroelectric-to-ferroelectric and ferroelectric-to-antiferroelectric phase transitions of the films, respectively. Furthermore, the incorporation of La³⁺ results in a decrease in the polarization current [19].

Figure 5 illustrates the variation in the polarization and critical phase transition electric fields of the films with varying La³⁺ doping concentrations under an identical electric field. The P_m and P_r exhibit a distinct decreasing trend with an increasing La³⁺ doping concentration [40,41,42,43]. Concurrently, the Pₘ–Pᵣ value also decreases, which is attributed to the ion vacancies generated during the doping process. As the La³⁺ doping concentration increases, the values of E_F and E_A gradually increase, which contributes to the enhancement of the film’s W_rec. Furthermore, the E_F–E_A value gradually decreases, which is consistent with the trend observed in the film’s P-E loops shown in Figure 3. The decrease in the E_F–E_A value indicates that the energy loss during the field-induced phase transition process is relatively small, which is advantageous for improving the η of the films.

Figure 6a,b present a comparison of the leakage current of the Pb_1−3x/2La_xZrO₃ films at room temperature and 140 °C. At room temperature, as the La³⁺ doping concentration increases, the leakage current first decreases and then increases [44]. On the one hand, La³⁺ doping captures mobile charge carriers, leading to a decrease in leakage current. However, when the doping concentration reaches 0.08, the leakage current increases. This is attributed to a significant increase in the number of oxygen vacancies at higher doping concentrations, which enlarge the conduction path for electrons, resulting in a decline in their insulating properties [45,46]. At 140 °C, the leakage current for the film with a La³⁺ concentration of x = 0.04 is relatively high, likely due to the insufficient effect of the lower La³⁺ concentration on conductivity. This allows charge carriers to migrate more freely at elevated temperatures, leading to higher conductivity and, consequently, an increased leakage current. Conversely, the leakage current for the film with a La³⁺ concentration of x = 0.06 is the lowest, which can be attributed to the doping concentration approaching the “saturation point”. At this concentration, La³⁺ ions induce significant lattice distortion, charge recombination, and local electric field effects, thereby minimizing the leakage current [47,48,49]. Figure 6c–f present a comparison of leakage current in Pb_1−3x/2La_xZrO₃ films at different temperatures. As the temperature increases, the leakage current exhibits an upward trend, but the change in leakage current with temperature is minimal, demonstrating the excellent temperature stability of Pb_1−3x/2La_xZrO₃ films [50].

Figure 7a,b present the Weibull distribution of the E_b of Pb_1−3x/2La_xZrO₃ films. The results indicate that following La³⁺ doping, the E_b of the films increases significantly. Specifically, the breakdown strengths of the Pb_1−3x/2La_xZrO₃ (x = 0, 0.04, 0.06, 0.08) films are 492.5 kV/cm, 1363.0 kV/cm, 1541.0 kV/cm, and 1210 kV/cm, respectively, with shape parameters (β) of 6.6, 10.5, 14.1, and 12.8. The Pb_0.91La_0.06ZrO₃ film exhibits the highest E_b and β. The trend in breakdown strength with increasing La³⁺ doping is consistent with the trend observed for leakage current [51,52]. A lower doping concentration benefits the insulating properties of the films, while excessive doping leads to a gradual deterioration in their breakdown strength. Figure 7c,d present the W_rec of Pb_1−3x/2La_xZrO₃ films at the maximum electric field strength. The W_rec values are 9.07 J/cm³, 25.2 J/cm³, 34.9 J/cm³, and 24.8 J/cm³, respectively. Among these, the W_rec of Pb_0.91La_0.06ZrO₃ films is 385% higher than that of PbZrO₃. At the maximum electric field strength, the η of Pb_1−3x/2La_xZrO₃ films are 72.6%, 43.4%, 58.3%, and 60.1%, respectively. The improvement in energy storage performance can be attributed to two primary factors [53,54]. On the one hand, La³⁺ doping enhances the breakdown strength of the films, thereby increasing their W_rec. On the other hand, the decrease in the critical phase transition electric field (E_F-E_A) of the films leads to an increase in η. Figure S5 shows results of the bending and electrocyclic stability test for the Pb_0.91La_0.06ZrO₃ film. After 10⁴ bending cycles, the fluctuations in W_rec and η are minimal. After 10⁷ cycles, the energy storage performance of the film capacitor shows almost no change, indicating that the film capacitor exhibits excellent cycling and bending stability.

4. Conclusions

In summary, Pb_1−3x/2La_xZrO₃ films were successfully fabricated on Mica substrates using the sol–gel method, and their microstructures, electrical properties, and energy storage performances were systematically examined. Pb_0.91La_0.06ZrO₃ films achieved a remarkable W_rec of 34.9 J/cm³ and an η of 58.3%, and at the maximum electric field strength of 1541 kV/cm, the W_rec is 385% higher than that of the PbZrO₃ film. The remarkable energy storage performance of Pb_0.91La_0.06ZrO₃ films can be attributed to the improved breakdown strength and the reduced E_F-E_A. The specific mechanism for the enhanced breakdown strength and phase transition is explained as follows. On the one hand, the La³⁺ doping process enhances the breakdown strength of the films, thereby increasing their W_rec. On the other hand, the reduction in the E_F-E_A of the films leads to an increase in η. Additionally, the film still maintains a good energy storage performance after 10⁷ electric cycles and 10⁴ bending cycles. These exceptional energy storage properties highlight the significant potential of flexible Pb_1−3x/2La_xZrO₃ thin-film capacitors in future energy storage device applications.