Effect of Annealing Temperature on Energy Storage Performance of SrBi_3.25La_0.75Ti₄O₁₅ Thin Films

Abstract

Dielectric capacitors, characterized by ultra-fast charge/discharge speeds and high power densities, are widely used in modern electronic power systems. However, their low energy density and poor thermal stability limit applications. In this study, SrBi_3.25La_0.75Ti₄O₁₅ (SBLT) ferroelectric thin films were prepared by the sol–gel method. We systematically investigated the effect of annealing temperature on microstructural evolution, electrical properties, and energy storage performance. The SBLT film annealed at 700 °C exhibited optimal performance, achieving a balanced enhancement in polarization and breakdown strength, with an energy storage density of 48.66 J cm⁻³ and an efficiency of 78%. The material also demonstrated excellent thermal stability (30–175 °C) and frequency stability (0.1–100 kHz). These findings not only validate the potential of SBLT as a next-generation energy storage dielectric but also provide a practical solution for applications in semiconductor technology.

1. Introduction

With the rapid development of advanced electronic and electrical systems, the demand for improved energy and power densities has become increasingly urgent, driving the continuous improvement of energy storage devices. Dielectric capacitors, due to their ultra-fast charge/discharge speeds, are considered one of the most promising candidates for capacitors [1,2,3,4,5,6,7]. However, compared to electrochemical batteries and supercapacitors, dielectric capacitors have relatively low energy densities, limiting their applications [8,9]. The recoverable energy density (U_e) of dielectric capacitors depends on their dielectric polarization (P) and breakdown strength (BDS) under an applied electric field (E) [10,11,12,13]. Specifically, U_e = ${\int }_{P_{\mathrm{r}}}^{P_{\mathrm{m}}}E\mathrm{d}P$, where P_m and P_r are maximum polarization and remnant polarization, respectively. Additionally, polarization hysteresis also determines energy storage efficiency (η), defined as η = U_e/(U_e + U_loss). Thus, enhancing BDS is crucial for improving overall energy storage performance [14,15,16,17,18,19,20,21].

Various methods have been attempted to improve the breakdown field strength. For example, high-entropy design can effectively refine grain sizes and induce the formation of an amorphous phase, thereby significantly enhancing BDS in the high-entropy-stabilized perovskite structure [22,23,24], with reported BDS increasing from 2.88 MV cm⁻¹ to 6.4 MV cm⁻¹ [23]. Similarly, interface engineering can suppress the expansion of the breakdown path, also contributing to the improvement of BDS [25,26,27], with reported BDS increasing from 3.3 MV cm⁻¹ to 4.7 MV cm⁻¹ [26]. Compared to these more intricate and complex design strategies, almost amorphous dielectric materials, lacking long-range order, often achieve higher BDS more easily [28,29,30,31,32,33], such as an increase from 2.0 MV cm⁻¹ to 8.3 MV cm⁻¹ [28]. The linear polarization characteristics of amorphous dielectrics also favor high η. For example, Huang et al. reported high η (88.3%) in nearly amorphous 0.92BaTiO₃-0.08Bi (Ni_0.5Zr_0.5)O₃ films [29]; Wang et al. also achieved 90% efficiency in nearly amorphous (Ba_0.95, Sr_0.05) (Zr_0.2, Ti_0.8)O₃ films [31]. However, the energy density of amorphous dielectrics is limited by their low polarization. Therefore, controlling an appropriate crystalline–amorphous phase ratio is crucial for optimizing overall performance.

SrBi₄Ti₄O₁₅ (SBT), a typical Aurivillius-phase layered ferroelectric material [34], the crystal structure is formed by alternating (Bi₂O₂)²⁺ layers and (SrBi₂Ti₄O₁₃)²⁻ perovskite layers. Owing to its high Curie temperature, excellent fatigue resistance, and environmental friendliness, SBT holds promising prospects for applications in high-temperature electronics, information storage, and microwave communications. Doping with La³⁺ stabilizes the A-site and reduces the concentration of oxygen vacancies generated by the volatilization of Bi³⁺, thereby lowering leakage current density in SBT films and enhancing breakdown strength and operational stability [35,36,37,38,39]. Therefore, La-doped SBT films have significant potential for energy storage applications that require high thermal stability [40,41]. Yu et al. [42] reported that doping SBT with Er³⁺ not only produces strong green upconversion luminescence but also significantly enhances the ferroelectric and piezoelectric properties (e.g., achieving a remnant polarization of 14.92 μC/cm² at an erbium doping level of x = 0.008). This results in a high-performance material integrating both optical and electrical functionalities. In another study, Sun et al. [43] fabricated SBT thin films with Sr-deficient and Bi-excess composition (Sr_0.8Bi_4.13Ti₄O₁₅), which exhibited a markedly increased remnant polarization up to 36.6 μC/cm², along with excellent fatigue endurance (up to 4.4 × 10¹⁰ switching cycles). However, the generally observed large remnant polarization (2Pr > 14 μC/cm²) in SBT-based systems typically leads to high hysteresis losses, fundamentally limiting the energy storage efficiency and thus constraining their application in the field of dielectric energy storage. In this study, SrBi_3.25La_0.75Ti₄O₁₅ (SBLT) films were prepared via the sol–gel method. By adjusting the annealing temperature to optimize the crystalline–amorphous phase ratio, the SBLT films annealed at 700 °C exhibited high relaxation characteristics and enhanced BDS. The balance between polarization and breakdown strength was well optimized, achieving a high U_e of 48.66 J cm⁻³ and η of 78%. Additionally, the films demonstrated good temperature stability (30–175 °C), providing a new solution for improving the energy storage performance of dielectric capacitors.

2. Experimental Section

2.1. Film Preparation

SrBi_3.25La_0.75Ti₄O₁₅ (SBLT) films were prepared on Pt/Ti/SiO₂/Si substrates using the sol–gel method. The precursor solution was prepared using bismuth (Ⅲ) acetate (99.99%, Aladdin, Shanghai, China), lanthanum acetate (99.99%, Aladdin, Shanghai, China), strontium acetate (99.97%, Aladdin, Shanghai, China), and titanium tetrabutoxide (99%, Aladdin, Shanghai, China) as raw materials, with propionic acid (99.99%, Aladdin, Shanghai, China) as the solvent and ethanolamine (99.99%, Aladdin, Shanghai, China) as the stabilizer. The bismuth (Ⅲ) acetate, lanthanum acetate, strontium acetate, and titanium tetrabutoxide were dissolved sequentially in propionic acid at 50 °C, followed by the addition of an appropriate amount of ethanolamine to stabilize the colloid. To compensate for bismuth volatilization during heat treatment, 5% excess bismuth was added to the solution. The solution was stirred at room temperature for 24 h. After aging the solution at room temperature for 48 h, it was filtered using a 0.22 μm filter and spin-coated onto the Pt/Ti/SiO₂/Si substrates at 6000 rpm for 20 s. The wet film was baked at 200 °C for 5 min to evaporate the organic solvent. The spin-coating and baking steps were repeated multiple times to increase the thickness. Finally, the films were annealed at different temperatures (550 °C, 600 °C, 650 °C, 700 °C, 750 °C, and 800 °C) for 25 min in a rapid thermal annealing furnace (RTP-3S01, Jouleyacht, Wuhan, China) with a heating rate of 10 °C/s.

2.2. Characterization of Materials

The crystallinity and phase of the samples were analyzed by X-ray diffraction (XRD, PANalytical EMPYREAN, Almelo, The Netherlands). The surface morphology and thickness of the samples were measured using scanning electron microscopy (SEM, TESCANCLARA, Brno, Czech Republic). The dielectric properties of the samples were characterized using an LCR tester (DMS2000, BALAB, Wuhan, China). P–E hysteresis loops were measured by a ferroelectric testing system (Huace FE-2000, Huace, Beijing, China).

3. Results and Discussion

3.1. Structure and Microstructure

XRD and SEM were employed to characterize the phase structure and surface morphology of SBLT films with annealing temperature. As shown in Figure 1a, the XRD results indicate that when annealed at 550 °C, the pattern exhibits only a broad and diffused diffraction peak within the 30–35°, suggesting that the film is predominantly amorphous [29]. As the annealing temperature increases to 800 °C, multiple distinct diffraction peaks emerge, demonstrating a significant improvement in crystallinity. The previously amorphous regions have transformed into an ordered crystalline structure, and the diffraction peaks further confirm the formation of a typical Aurivillius phase (ICDD PDF No. 00-043-0973) in the film. In addition, as shown in Table 1, the crystallinity (Xc) estimated from XRD—calculated via the semi-quantitative formula X_c = [I_crystalline/(I_crystalline + I_amorphous)] × 100%, where I_crystalline and I_amorphous are the integrated intensities of the crystalline diffraction peaks and the amorphous halo, respectively—increases monotonically with annealing temperature (≈3% at 550 °C, ≈5% at 600 °C, ≈20% at 650 °C, ≈27% at 700 °C, ≈43% at 750 °C, and ≈62% at 800 °C), consistent with the progressive peak sharpening and intensity enhancement. Moreover, the enlarged low-angle region (Figure 1b) clearly shows the gradual appearance and strengthening of the characteristic low-angle reflections with increasing temperature, further evidencing the development of an ordered layered Aurivillius structure. These results illustrate that with increasing annealing temperature, the SBLT film gradually transitions from an amorphous state to a crystalline state.

Figure 1. (a) XRD patterns of SBLT thin films annealed at different temperatures (550–800 °C) and the bare Pt/Ti/SiO₂/Si substrate; (b) enlarged view of the XRD patterns in the low-angle region (10–37°). The symbol “#” denotes diffraction peaks originating from the substrate (Pt/Ti/SiO₂/Si).

Table 1. Crystallinity of SBLT thin films annealed at different temperatures.

Annealing Temperature (°C)	Crystallinity (%)
550	3
600	5
650	20
700	27
750	43
800	62

The microstructure of SBLT films was characterized by SEM images, as shown in Figure 2. As the annealing temperature increased, the film gradually transformed from an amorphous to a crystalline state. Compared to crystalline films, the surface of amorphous films was smoother and flatter. Under annealing at 550 °C, no obvious crystalline particles were observed on the film surface, indicating that the film was primarily in an amorphous matrix form. When the annealing temperature reached 800 °C, clear grains and grain boundaries appeared on the surface, indicating a significant reduction in the amorphous matrix and the dominance of an ordered crystalline phase. This morphological evolution is consistent with the structural changes reflected in the X-ray diffraction results. Therefore, by adjusting the annealing temperature, films with varying crystalline–amorphous phase ratios can be effectively obtained.

Figure 2. Surface and cross-sectional morphologies of SBLT films at different annealing temperatures: (a) 550 °C, (b) 600 °C, (c) 650 °C, (d) 700 °C, (e) 750 °C, (f) 800 °C.

As shown in Figure 3a, the EDX elemental mapping images of the SBLT thin film annealed at 700 °C reveal a homogeneous spatial distribution of Bi, La, Sr, and Ti across the investigated area. No obvious elemental segregation or compositional inhomogeneity is observed, indicating that the sol–gel process combined with annealing at 700 °C ensures good chemical uniformity of the film. As shown in Figure 3b, the Bi concentration (wt%) exhibits a gradual decrease with increasing annealing temperature from 550 to 800 °C. The nearly monotonic reduction in the relative Bi content suggests that Bi becomes progressively deficient at elevated temperatures. Considering the high volatility of Bi-containing species during thermal treatment, this trend is plausibly associated with partial Bi loss (e.g., volatilization) upon high-temperature annealing, which may drive a deviation from the nominal composition. Notably, the drop becomes more pronounced at 750–800 °C, implying that Bi loss is accelerated in this temperature range. Overall, the mapping result in (a), together with the compositional trend in (b), indicates that annealing at 700 °C provides a favorable balance between chemical uniformity and Bi retention, whereas higher annealing temperatures may increase the risk of Bi deficiency and compositional fluctuation.

Figure 3. (a) EDX elemental mapping images of the sample annealed at 700 °C, showing the spatial distributions of Bi, La, Sr, and Ti. (b) Variation in Bi concentration (wt%) as a function of annealing temperature for samples annealed at different temperatures.

3.2. Dielectric Properties

Figure 4a,b show the frequency dependence of the dielectric constant and loss of thin films. As the annealing temperature increases, the dielectric constant of the SBLT films gradually increases. The dielectric loss remains low across 0.1 kHz to 1 MHz, indicating that SBLT films have good dielectric frequency stability. Figure 4c shows the dielectric constant and dielectric loss (1 kHz) of all films. The dielectric constants of the films were 68, 73, 148, 237, 269, and 329 for annealing temperatures from 550 to 800 °C, primarily due to the formation and growth of crystalline regions. Meanwhile, the dielectric loss of the films at different annealing temperatures was 0.0116, 0.0057, 0.0116, 0.0079, 0.0142, and 0.0431, respectively. Except for the film annealed at 800 °C, all other samples had dielectric loss levels below 0.02, which is beneficial for their energy storage performance. Specifically, the samples annealed at relatively low temperatures may retain organic residues and exhibit residual porosity, both of which can contribute to an elevated loss. As the annealing temperature increases into the intermediate regime, enhanced densification reduces extrinsic loss pathways and thus lowers the dielectric loss. With further heating, the onset of crystallization can introduce crystalline/amorphous interfaces as well as additional defects, and the associated interfacial polarization and defect-related scattering lead to a rebound in loss. Around ~700 °C, densification and a moderate degree of crystallization appear to reach a balance, giving rise to a comparatively low dielectric loss. At even higher temperatures, however, Bi volatilization and the concomitant increase in oxygen vacancies and defect concentrations substantially enhance conduction- and defect-mediated loss, resulting in a pronounced increase in dielectric loss. The significant increase in dielectric loss for the 800 °C sample is likely due to bismuth volatilization and increased defect density [44,45].

Figure 4. (a) Frequency-dependent dielectric constant and (b) dielectric loss of SBLT films at room temperature. (c) Dielectric constant and loss (1 kHz) of SBLT films at different annealing temperatures. (d) Temperature dependence of dielectric constant and loss of the film annealed at 700 °C.

Figure 4d shows the temperature-dependent dielectric properties of SBLT films annealed at 700 °C. The temperature range tested was from 30 °C to 200 °C. Both dielectric constant and loss remained stable with minimal variation, demonstrating excellent thermal stability. The slight increase in loss at higher temperatures may be attributed to enhanced lattice vibrations and conduction losses.

Figure 5a shows the Weibull distribution of BDS of all films. All shape factors (β) exceeded 9.4, indicating reliable fitting. The BDS initially increased with annealing temperature, reaching a maximum of 3410 kV cm⁻¹ at 700 °C due to improved crystallinity and densification [46]. Further increasing the temperature to 800 °C leads to a decrease in the BDS due to increased defect concentration caused by Bi volatilization, which results in a reduction in the breakdown field strength [47,48]. Figure 5b shows the leakage current density. Under 300 kV cm⁻¹, the leakage current density first decreased and then increased with annealing temperature. When the annealing temperature is below 700 °C, the decrease results from the removal of organic residues and increased film density. Above 700 °C, bismuth volatilization led to a sharp increase in oxygen vacancy concentration and leakage current, consistent with the observed decline in BDS. The film annealed at 700 °C exhibited the lowest leakage current.

Figure 5. (a) Weibull distribution of BDS and (b) leakage current density of SBLT thin films annealed at different temperatures.

Figure 6a shows unipolar P–E hysteresis loops of SBLT thin films annealed at different temperatures under 1000 kV cm⁻¹ and 10 kHz. Ferroelectricity strengthened with annealing temperature due to increased crystallinity. The 800 °C sample showed large P_r, detrimental to efficiency [49]. Figure 6b,c show the P–E hysteresis loops under their respective E_m and the corresponding P_m, P_r, and P_m − P_r values for all SBLT films. SBLT films annealed at 700 °C and 750 °C exhibit larger P_m − P_r, which is beneficial for improving U_e and η. However, the high leakage current in the 750 °C sample limited BDS and U_e. Figure 6d shows the U_e and η as a function of electric field for SBLT films annealed at different temperatures. Among them, the film annealed at 700 °C exhibited the best performance, reaching a high U_e of 48.66 J cm⁻³ and η of 78%. This optimum stems from its high BDS, low leakage current, and enhanced polarization due to appropriate crystallization.

Figure 6. (a) Unipolar P–E hysteresis loops at 1000 kV cm⁻¹ and 10 kHz; (b) unipolar P–E hysteresis loops at E_m. and 10 kHz. (c) Comparison of P_m, P_r, and ΔP (P_m − P_r) of SBLT thin films at E_m. (d) U_e and η as functions of electric field.

The reliability of the energy storage performance of SBLT films annealed at 700 °C at different frequencies and temperatures was evaluated. As shown in Figure 7a,b, at 2000 kV cm⁻¹, when the frequency increased from 100 Hz to 100 kHz, P_m increased from 29.88 μC cm⁻² to 30.60 μC cm⁻², while U_e and η increased by 2.1 J cm⁻³ and 9.6%, respectively. With the temperature rising from 30 to 175 °C (Figure 7c,d), P_m gradually decreased due to enhanced lattice vibrations that hinder domain switching. At 175 °C, U_e decreased by 5.18 J cm⁻³ and η dropped by 18.3% relative to room temperature, which is usually due to the contribution of thermally activated charge carriers to conductivity at higher temperatures [50]. This indicates that the energy storage performance of the 700 °C annealed SBLT films has good frequency and temperature stability.

Figure 7. SBLT films annealed at 700 °C, (a) frequency-dependent P–E hysteresis loops at 2000 kV cm⁻¹, and (b) corresponding U_e and η of SBLT film. (c) Temperature-dependent P–E hysteresis loops at 2000 kV cm⁻¹ and 10 kHz, and (d) corresponding U_e and η.

4. Conclusions

In conclusion, SrBi_3.25La_0.75Ti₄O₁₅ (SBLT) thin films were successfully synthesized on Pt/Ti/SiO₂/Si substrates via sol–gel processing. By controlling the annealing temperature to tune the crystalline–amorphous phase ratio, the energy-storage performance was optimized. The film annealed at 700 °C exhibited the most compact microstructure, low dielectric loss and leakage current, and the highest BDS. Coupled with enhanced polarization from appropriate crystallization, it achieved a high U_e of 48.66 J cm⁻³ with 78% η. Furthermore, this film demonstrated good frequency stability (0.1–100 kHz) and thermal stability (30–175 °C). This work demonstrates that balancing the crystalline and amorphous phases through annealing is an effective strategy for developing high-performance dielectric energy-storage films, providing a solid foundation for next-generation capacitor applications.