⇦Chinese Physics Letters, 2020, Vol. 37, No. 11, Article code 118401 Design of Lead-Free Films with High Energy Storage Performance via Inserting a Single Perovskite into Bi$_{4}$Ti$_{3}$O$_{12}$ Qiong Wu (吴琼), Xin Wu (邬新), Yue-Shun Zhao (赵悦顺), and Shifeng Zhao (赵世峰)* Affiliations Inner Mongolia Key Lab of Nanoscience and Nanotechnology, and School of Physical Science and Technology, Inner Mongolia University, Hohhot 010021, China Received 31 July 2020; accepted 23 September 2020; published online 8 November 2020 Supported by the National Natural Science Foundation of China (Grant Nos. 11864028 and 12074204).
*Corresponding author. Email: zhsf@imu.edu.cn Citation Text: Wu Q, Wu X, Zhao Y S and Zhao S F 2020 Chin. Phys. Lett. 37 118401    Abstract We report a distinctive way for designing lead-free films with high energy storage performance. By inserting different single perovskite cells into Bi$_{4}$Ti$_{3}$O$_{12}$, $P$–$E$ hysteresis loops present larger maximum polarization, higher breakdown strength and smaller slim-shaped area. We prepared 0.15Bi$_{7}$Fe$_{3}$Ti$_{3}$O$_{21}$-0.5Bi$_{4}$Sr$_{3}$Ti$_{6}$O$_{21}$-0.35Bi$_{4}$Ba$_{3}$Ti$_{6}$O$_{21}$ solid solution ferroelectric films employing the sol-gel method, and obtained high energy storage density of 132.5 J/cm$^{3}$ and efficiency of 78.6% while maintaining large maximum polarization of 112.3 μC/cm$^{2}$ and a high breakdown electric field of 3700 kV/cm. Moreover, the energy storage density and efficiency exhibit stability over the temperature range from 20 ℃ to 125 ℃, and anti-fatigue stability maintains up to 10$^{8}$ cycles. The films with a simple preparation method and high energy storage performance are likely to become candidates for high-performance energy storage materials. DOI:10.1088/0256-307X/37/11/118401 PACS:84.60.Ve, 68.55.-a, 77.80.-e © 2020 Chinese Physics Society Article Text Turning clean energy sources, such as solar and wind power, into electric energy for human use has become an indispensable part of daily life, such as Chinese three gorges hydropower station and wind power generation, which are associated with energy storage. Reliable and efficient energy storage is the basis for efficient use of these clean energy sources. For decades, ferroelectric materials have been an important branch of energy storage materials.[1–4] For ferroelectrics, the energy density can be expressed as follows:[1] $$ W=\int_0^{D_{\max } } {EdD},~~ \tag {1} $$ where $D$ is the electrical displacement in the ferroelectric layer and $E$ is the applied external electric field. The formula $D=\varepsilon_{0}E+P$ is often used for ease of calculation. Thus, Eq. (1) can be rewritten as $$ W=\int_0^{P_{\max } } {EdP},~~ \tag {2} $$ where $P$ is the polarization of the ferroelectric layer at electric field of $E$. The energy density obtained from Eq. (2) consists of two parts: one is the loss in the charging and discharging process, which represents the part enclosed by the hysteresis loop, and the other is the recycled energy $W_{\rm re}$, which can be calculated by $$ W_{\rm re} =\int_{P_{\rm r} }^{P_{\max } } {EdP},~~ \tag {3} $$ where $P_{\rm r}$ represents the remanent polarization of the ferroelectric layer. Therefore, the energy storage efficiency can be derived from $\eta =W_{\rm re}/W$. Great efforts have been devoted to improving the energy storage performances of ferroelectrics through different strategies. A typical method is to induce relaxor properties in ferroelectrics or antiferroelectrics by the substitution of elements to obtain the hysteresis loop with a slim shape, which results in a high energy storage efficiency.[5–7] However, this method sacrifices the polarization so that $W_{\rm re}$ is not satisfactory. Another classic method is to combine ferroelectrics with paraelectric materials.[8–10] On the one hand, the long-range ferroelectric orders can be broken to form nanodomains. On the other hand, the paraelectric materials can improve the breakdown strength because of its intrinsic properties.[1,11] At the same time, the low polarization of paraelectrics is also detrimental to $W_{\rm re}$. Few novel strategies are as follows: Polymorphic nanodomains can reduce the polarization anisotropy and energy barrier between different phases, which leads to ultrahigh energy storage performance with a $W_{\rm re}$ of 112 J/cm$^{3}$ and efficiency of $\sim $80%, while the maximum polarization $P_{\rm m}$ is only 69 µC/cm$^{2}$.[12] What is more, the Jujube–Cake structure of the segregation particles is conducive to the breakdown strength and polarization, which results in a $W_{\rm re}$ of 94.1 J/cm$^{3}$ and efficiency of 84.5%.[13] Meanwhile, the value of $P_{\rm m}$ is still less than 90 µC/cm$^{2}$. Actually, it is much better and easier to improve $W_{\rm re}$ by increasing $P_{\rm m}$ than by increasing the breakdown strength. In other words, as shown in Fig. 1, it is expected to get a large green area. Mathematically, the area is the largest only when the maximum values of polarization and electric field are very close. However, the reality is very different. It seems that $P_{\rm m}$ is more important than the breakdown strength. Most previous studies have been focused on how to improve breakdown strength, and few focused on $P_{\rm m}$. Small $P_{\rm m}$ is one of the main reasons for the low $W_{\rm re}$. Bismuth layer structured ferroelectrics consist of oxygen octahedral (perovskite-like) blocks interleaved with (Bi$_{2}$O$_{2})^{2+}$ layers, which are named as an Aurivillius structure. (Bi$_{2}$O$_{2})^{2+}$ layers act as an insulating layer, so this structure always shows a high breakdown electric field and excellent stability against repetitive polarization switching. Bi$_{4}$Ti$_{3}$O$_{12}$ is a representative Aurivillius structure compound. Some single perovskite cells can be inserted into Bi$_{4}$Ti$_{3}$O$_{12}$ to regulate the properties of Bi$_{4}$Ti$_{3}$O$_{12}$. For example, Bi$_{4}$Ti$_{3}$O$_{12}$ inserted with two layers BaTiO$_{3}$ can increase its relaxor properties.[14] These advantages make this structure very suitable for the energy storage applications.
cpl-37-11-118401-fig1.png
Fig. 1. The schematic diagram of the main idea of this work. Inserting BiFeO$_{3}$, SrTiO$_{3}$ and BaTiO$_{3}$ into Bi$_{4}$Ti$_{3}$O$_{12}$ makes the hysteresis loop have larger $P_{\rm m}$, higher breakdown strength and smaller slim-shaped area (white). The shadow area represents the recoverable energy storage density.
In this study, we use a distinctive strategy, that is, Bi$_{4}$Ti$_{3}$O$_{12}$ with an Aurivillius structure can be inserted by perovskite cells,[7,14,15] hoping to design lead-free films with large $P_{\rm m}$, high breakdown electric field and smaller slim-shape area, as shown in Fig. 1. Different perovskite cells are inserted into Bi$_{4}$Ti$_{3}$O$_{12}$ for different purposes. The polarization in the Aurivillius structure mainly depends on the pseudo perovskite layer.[16] Therefore, BiFeO$_{3}$ is inserted into Bi$_{4}$Ti$_{3}$O$_{12}$ to obtain the large $P_{\rm m}$. Typical paraelectric SrTiO$_{3}$ is inserted into Bi$_{4}$Ti$_{3}$O$_{12}$ for the high breakdown strength. Classical relaxor ferroelectric BaTiO$_{3}$ is inserted into Bi$_{4}$Ti$_{3}$O$_{12}$ to get a $P$–$E$ hysteresis loop with a slim shape.[17,18] This idea can be more visually understood through Fig. 1. Finally, 0.15Bi$_{7}$Fe$_{3}$Ti$_{3}$O$_{21}$-0.5Bi$_{4}$Sr$_{3}$Ti$_{6}$O$_{21}$-0.35Bi$_{4}$Ba$_{3}$Ti$_{6}$O$_{21}$ (BFSBT) films were prepared. The solvents were all mixtures of glycol and glycol methyl ether with a volume ratio of 1 : 1 and all processes were performed at room temperature. (CH$_{3}$COO)$_{2}$Ba was dissolved in 10 mL solvent. After it was completely dissolved, Sr(NO$_{3})_{2}$ was added and continued to dissolve for 5 h. Then Bi(NO$_{3})_{3}$$\cdot$5H$_{2}$O and Fe(NO$_{3})_{3}$$\cdot$9H$_{2}$O were added to the preparation solution, and the precursors were continuously stirred with a magnetic stirrer for 2 h to form solution 1. We took 1 mL solvent and 0.5 mL acetylacetone into another beaker. C$_{16}$H$_{36}$O$_{4}$Ti was added to this beaker and stirred for 1 min to form solution 2. Finally, solution 2 was slowly dripped into solution 1. The concentration of the sol was adjusted to 0.04 mol/L by adding the solvent. After the solution was aged for 120 h, it was crystallized on the Pt(111)/Ti/SiO$_{2}$/Si substrate. First, the solution was dripped onto the substrate, holding for 15 s at 500 r/min and then for 35 s at 7000 r/min. Then the films were dried at 320 ℃ for 3 min and crystallized at 750 ℃ for 3 min using the rapid thermal annealing. The above steps were repeated until the films reached a certain thickness ($\sim $500 nm) and finally annealed at 750 ℃ for 15 min. X-ray diffraction (XRD, Kratos Amicus, UK) with Cu $K_\alpha$ radiation was used for the structural characterization of the BFSBT films. A scanning electron microscope (SEM, Hitachi SU4800) was used to measure the film thickness. The $P$–$E$ hysteresis loops and leakage properties of the films were characterized by a MultiFerroic tester system (MultiFerroic, RT, USA) and a temperature control system (Instec mK2000) was used to change temperature.
cpl-37-11-118401-fig2.png
Fig. 2. (a) X-ray diffraction patterns of BFSBT films, with S standing for the substrate. (b) Energy dispersive x-ray date of the BFSBT films. The inset shows the element proportions of the films. Surface morphology of BFSBT films measured with SEM (c) and PFM (d). The inset of (c) is the cross-sectional SEM image.
To confirm that BiFeO$_{3}$, SrTiO$_{3}$ and BaTiO$_{3}$ were inserted into the Bi$_{4}$Ti$_{3}$O$_{12}$ matrix, x-ray diffraction of the BFSBT films was measured as shown in Fig. 2(a). The diffraction peaks marked in red prove that Bi$_{4}$Ti$_{3}$O$_{12}$ is inserted into three layers of single perovskite.[14,19–24] The diffraction peaks coincide with standard diffraction peaks (Code: 155931) in the Inorganic Crystal Structure Database (ICSD). We further compare the patterns with the standard structures of BiFeO$_{3}$, SrTiO$_{3}$ and BaTiO$_{3}$ in the ICSD. The diffraction peaks are different from this data. Therefore, there is no single perovskite phase in the films. This indicates that BiFeO$_{3}$, SrTiO$_{3}$ and BaTiO$_{3}$ were inserted into the Bi$_{4}$Ti$_{3}$O$_{12}$ matrix. Once the ratio of the single perovskite reaches a threshold, the entire structure becomes unstable.[25] The (119) diffraction peak marked in blue is the phase where one layer of BaTiO$_{3}$ or SrTiO$_{3}$ is inserted into Bi$_{4}$Ti$_{3}$O$_{12}$.[26–28] Based on the above discussion, the main compounds in BFSBT films are Bi$_{7}$Fe$_{3}$Ti$_{3}$O$_{21}$, Bi$_{4}$Sr$_{3}$Ti$_{6}$O$_{21}$ and Bi$_{4}$Ba$_{3}$Ti$_{6}$O$_{21}$ and a small amount of Bi$_{4}$SrTi$_{4}$O$_{15}$ or Bi$_{4}$BaTi$_{4}$O$_{15}$ appears. To further demonstrate the composition of the films, the energy dispersive x-ray measurement of the films was carried out, as shown in Fig. 2(b). The ratio of elements is $14.73\!:\!1.50$: $4.32\!:\!3.46$ (Bi : Fe : Sr : Ba), which is very close to the theoretical value (4.45 : 0.45 : 1.5 : 1.05). If there are more impure phases, Bi content should be much higher than 14.73. Such a result further confirms existence of 0.15Bi$_{7}$Fe$_{3}$Ti$_{3}$O$_{21}$-0.5Bi$_{4}$Sr$_{3}$Ti$_{6}$O$_{21}$-0.35Bi$_{4}$Ba$_{3}$Ti$_{6}$O$_{21}$ films. Figures 2(c) and 2(d) show the surface morphology of the BFSBT film measured by SEM and piezoresponse force microscopy. It can be seen that the film surface is smooth and dense. The film thickness is about 500 nm.
cpl-37-11-118401-fig3.png
Fig. 3. (a) Leakage properties of the BFSBT films. The inset shows the relationship between $\ln E$ and $\ln J$. (b) Weibull distributions of the breakdown electric fields. The characteristic breakdown electric field $E_{0}=3745$ kV/cm. The inset shows the Weibull modulus $\beta$. The larger the value, the better the uniformity of the films. (c) Temperature-dependent dielectric permittivities and loss tangents at frequencies of 0.01, 0.1 and 1 MHz. The inset shows the relaxor factor $\gamma$ of the BFSBT films calculated by the Curie–Weiss law. (d) Domains of the BFSBT films measured at 1 µm scale.
Figure 3(a) shows the leakage performance of the BFSBT films. It is well known that the leakage current density under a high applied electric field can more accurately reflect the dielectric properties of the films.[29] As can be seen, the leakage current density of the BFSBT films under 1600 kV/cm is still less than 10$^{4}$ A/cm$^{2}$, indicating good leakage performance, which reduces the possibility of thermal breakdown and avalanche breakdown.[1] The space charge limited conduction (SCLC) mechanism, interface limited Schottky excitation (ILSE) mechanism and ohmic conduction mechanism are the three main leakage mechanisms in the dielectrics.[12,30] As we can see in the inset of Fig. 3(a), the value of the slope is around 1, which indicates that the ohmic mechanism is dominant under low applied electric fields. However, the leakage mechanism is mainly ILSE at high applied electric fields because the slope is much larger than 2. That is to say, there is no space charge limited conduction through the whole applied electric field. This means that there are few space charges such as oxygen vacancies in the films. During the preparation of films, adequate oxygen atmosphere depresses the formation of oxygen vacancies and the accumulation of space charge.[31] In order to test the uniformity of the breakdown electric field, the breakdown electric field is measured at 10 points, evenly selected on the films, and the evaluation is made with weber distributions:[12,32] $$ F=1-\exp[-(E / {E_{0} })^{\beta }],~~ \tag {4} $$ where $E$ is the measured breakdown electric field, $E_{0}$ is characteristic breakdown electric field, and $\beta$ is the Weibull modulus. The larger the value of $\beta$, the better the uniformity of the films. Here $F$ is the failure probability of the films under $E$, which can be calculated by $$ F=\frac{i-0.44}{n-0.25}\times 100\%,~~ \tag {5} $$ where $n$ is the total number of experimental samples, which is equal to 10 here. Put 10 values of $E$ in ascending order, and $i$ is what the rank of each $E$ is. The results are shown in Fig. 3(b). The characteristic breakdown electric field is $E_{0}=3745$ kV/cm and the Weibull modulus is $\beta =67.5$. This data shows that the BFSBT films have good homogeneity. The breakdown electric field at most points of the BFSBT films is around 3745 kV/cm, indicating very low probability of a point deviating from its breakdown electric field of 3745 kV/cm. Figure 3(c) shows the temperature-dependent dielectric permittivities and loss tangents of the films at frequencies of 0.01, 0.1 and 1 MHz. It can be seen that the variation of the dielectric permittivities becomes dispersive as the frequency increases. The inset of Fig. 3(c) shows the relaxor factor $\gamma$ derived from the modified Curie–Weiss law:[12] $$ \frac{1}{\varepsilon_{\rm r} }-\frac{1}{\varepsilon_{\rm r,m} }=\frac{(T-T_{\rm m})^{\gamma }}{C},~~ \tag {6} $$ where $\varepsilon _{\rm r}$ is the permittivity, $T_{\rm m}$ is the temperature at which $\varepsilon _{\rm r}$ reaches the maximum $\varepsilon _{\rm r,m}$. $C$ is a constant. The value of $\gamma$ always varies from 1 (conventional ferroelectrics) to 2 (ideal relaxor ferroelectrics). The $\gamma$ value of the present films is 1.78, which means that the BFSBT films are more like relaxor ferroelectrics. This can be confirmed by domains of the films shown in Fig. 3(d) since the domains do not form large areas but many small irregular regions with sizes of less than 100 nm.
cpl-37-11-118401-fig4.png
Fig. 4. (a) $P$–$E$ hysteresis loops of the BFSBT films under different applied electric field. The inset shows $P$–$E$ hysteresis loops of Bi$_{4}$Ti$_{3}$O$_{12}$ films. (b) The change rule of $W_{\rm re}$ and efficiency with the increasing applied electric field. (c) $P$–$E$ hysteresis loops at 20, 60, 100 and 125 ℃. (d) $W_{\rm re}$ and efficiency of the BFSBT films versus temperature from 20 ℃ to 125 ℃ under electric field of 2000 kV/cm. (e) Leakage properties of the BFSBT films at 20, 60, 100 and 125 ℃. (f) The fatigue properties of BFSBT films.
Figure 2(c) shows the cross-sectional SEM image of the films. The thicknesses of the BFSBT films are controlled at about 500 nm because the breakdown electric field is highest near this thickness.[33] Finally, we have obtained a $P$–$E$ hysteresis loop [Fig. 4(a)] with a large $P_{\rm m}$ of 112.3 µC/cm$^{2}$ due to the insertion of BiFeO$_{3}$. For Bi$_{4}$Ti$_{3}$O$_{12}$, spontaneous polarization along the $a$-axis can reach almost 50 µC/cm$^{2}$.[34] Meanwhile, the large polarization is obtained in BiFeO$_{3}$ since the relative displacement of Bi$^{3+}$ with respect to the Fe–O octahedron is along the direction of [111].[35] Therefore, the polarization component of BiFeO$_{3}$ on the $a$-axis can promote the polarization of Bi$_{4}$Ti$_{3}$O$_{12}$. In addition, BiFeO$_{3}$ also contributes to the polarization along the $c$-axis because Bi$_{4}$Ti$_{3}$O$_{12}$ still has a small spontaneous polarization along the $c$-axis.[36] As was reported previously, the most high applied electric field of Bi$_{7}$Fe$_{3}$Ti$_{3}$O$_{21}$ is only 300 kV/cm, while $P_{\rm m}$ reaches as high as 30 µC/cm$^{2}$.[22] However, it is well known that the low breakdown strength of Bi$_{7}$Fe$_{3}$Ti$_{3}$O$_{21}$ makes it difficult to achieve ideal polarization.[22,23] In other words, extremely high polarization is not possible at low applied electric fields. High polarization at high electric field is most useful for energy storage application. Therefore, SrTiO$_{3}$ is inserted in order to obtain a high applied electric field. In addition, the Bi$_{2}$O$_{2}^{2+}$ layer also plays the role of an insulating layer.[7,37] It is the joint action of Bi$_{2}$O$_{2}^{2+}$ and SrTiO$_{3}$ that makes the system reach a high applied electric field of up to 3700 kV/cm. This further enables high polarization to be achieved at a high applied electric field. Therefore, the insertion of BiFeO$_{3}$ and SrTiO$_{3}$ endows the BFSBT films a very satisfactory polarization at a high applied electric field. These advantages enable $W_{\rm re}$ of BFSBT films to reach 132.5 J/cm$^{3}$, and the efficiency of 78.6%. The inserting BaTiO$_{3}$ plays a crucial role in the high efficiency. The mechanical stress caused by the point charge and the uneven distribution of positive ions lead to the formation of random electric fields, which effectively breaks down the long-range interaction and prevents macroscopic ferroelectric orders.[38] The inset of Fig. 4(a) shows the $P$–$E$ hysteresis loop of Bi$_{4}$Ti$_{3}$O$_{12}$ films. It can be seen that the hysteresis loop has a very fat shape and the applied electric field is only 350 kV/cm. It is difficult for the pure Bi$_{4}$Ti$_{3}$O$_{12}$ film to be a candidate for the energy storage application. Bi$_{4}$BaTi$_{4}$O$_{15}$ is the representative of Bi$_{4}$Ti$_{3}$O$_{12}$-based films for energy storage through element substitution such as La$^{3+}$ or Pr$^{3+}$.[7,39] However, although this strategy increases the breakdown strength, it greatly reduces the polarization. This results in their energy storage density below 50 J/cm$^{3}$. Also, the energy storage densities of both Bi$_{4}$Ba$_{2}$Ti$_{5}$O$_{18}$ and Bi$_{4}$Sr$_{2}$Ti$_{5}$O$_{18}$ were less than 40 J/cm$^{3}$.[14] What is more, for BiFeO$_{3}$-based, BaTiO$_{3}$-based and SrTiO$_{3}$-based films, large leakage current, low breakdown strength and low polarization have bad effects on energy storage performance. Although some complex strategies can improve their energy storage performance, the results are not satisfactory.[8,10] To further verify the necessity of inserting three layers of single perovskite cells, we prepared the solid solution films with one-layer and two-layer single perovskite cells inserted in solid solution films (Bi$_{5}$FeTi$_{3}$O$_{15}$-Bi$_{4}$SrTi$_{4}$O$_{15}$-Bi$_{4}$BaTi$_{4}$O$_{15}$ and Bi$_{6}$Fe$_{2}$Ti$_{3}$O$_{18}$-Bi$_{4}$Sr$_{2}$Ti$_{5}$O$_{18}$-Bi$_{4}$Ba$_{2}$Ti$_{5}$O$_{18}$) in the same proportion. As can be seen in $P$–$E$ hysteresis loops at the same electric field (1000 kV/cm) (Fig. S1 in the Supplementary Material), with the increase of the number of inserted single layers, $P_{\rm m}$ rises sharply to 50.3 µC/cm$^{2}$, which enhances $W_{\rm re}$ from 5.8 J/cm$^{3}$ to 18.5 J/cm$^{3}$. Furthermore, the $P$–$E$ hysteresis loops at the highest electric fields for one- and two-layer single perovskite cell inserted Bi$_{4}$Ti$_{3}$O$_{12}$ are shown in Fig. S2. $W_{\rm re}$ and $\eta$ of the former are only 19.6 J/cm$^{3}$ and 31.5%, respectively, at the breakdown electric field of 2200 kV/cm, while $W_{\rm re}$ and $\eta$ of the latter are 46.5 J/cm$^{3}$ and 55.5%, respectively, at the breakdown electric field of 2600 kV/cm. Their energy storage performance is not comparable to that of the BFSBT films. Figure 4(b) shows the energy storage density and efficiency of the BFSBT films as a function of the applied electric field. The energy storage density increases with the increase of the applied electric field. The efficiency is around 80% as the applied electric field increases. It is worth noting that the $W_{\rm re}$ of the BFSBT films still exceeds 80 J/cm$^{3}$ with the efficiency of 83.6% even in the electric field of 2700 kV/cm because of the large polarization. We conducted a thermal stability test for the BFSBT films. Energy storage density and efficiency at intervals of 5 ℃ from 20 ℃ to 125 ℃ under the applied electric field of 2000 kV/cm were calculated. The $P$–$E$ hysteresis loops at 20, 60, 100 and 125 ℃ are selected as the representatives as shown in Fig. 4(c). As the temperature increases, the remanent polarization of the films increases and $P_{\rm m}$ decreases slightly, which lead to a slight decrease in the energy storage efficiency. The overall pattern can be seen in Fig. 4(d). As the temperature increases from 20 ℃ to 125 ℃, $W_{\rm re}$ drops slowly from 51.9 J/cm$^{3}$ to 47 J/cm$^{3}$. The reduction of $W_{\rm re}$ is less than 8%. What is more, the efficiency is still above 71% and the reduction is less than 13%. This data indicates that the BFSBT films have good thermal stability so they can be safely applied over a wide temperature region. It is supposed that the increase of free electrons in the films at high temperature leads to the decreases in the energy storage efficiency and density. Therefore, the leakage properties of the BFSBT films were measured as shown in Fig. 4(e). The leakage current density of the films at 1000 kV/cm increases greatly from $\sim $$ 10^{-6}$ A/cm$^{2}$ to $\sim $$10^{-2}$ A/cm$^{2}$ with the increasing temperature. This indicates a significant increase of free electrons in the films. Furthermore, the logarithms of the leakage current and electric field were conducted, as shown in Fig. S3, to explore the leakage mechanism. The values of the slope obtained by fitting each curve are listed in Table S1. At 20 ℃, there are two slopes that are all around 1, which indicates that the ohmic mechanism is the main one in the films under both low and high electric fields. However, there are three slopes at 60, 100 and 125 ℃ as listed in the table. Slope 2 is close to 2, indicating that the ohmic mechanism is gradually transiting to the SCLC in this range of electric field. At a higher electric field, slope 3 is obviously larger than 2, indicating that the leakage mechanism is mainly ILSE. Therefore, the increase of temperature contributes to SCLC and ILSE. This causes the leakage current in the films to increase thousands of times. It is the reason why $W_{\rm re}$ and efficiency decrease at high temperature. What is more, we tested the fatigue properties of the films [Fig. 4(f)] under 2000 kV/cm. After 10$^{8}$ cycles, $W_{\rm re}$ drops slightly from 57.1 to 55 J/cm$^{3}$ and efficiency drops from 82% to 76.6%. Their changes were less than 7%, which shows that BFSBT films have good anti-fatigue properties. In summary, we have designed and prepared BFSBT films based on the characteristics of Bi$_{4}$Ti$_{3}$O$_{12}$. BFSBT films have large $P_{\rm m}$ of 112.3 µC/cm$^{2}$ and high breakdown electric field of 3700 kV/cm, which lead to the high $W_{\rm re}$ of 132.5 J/cm$^{3}$. At the same time, BFSBT films also have high efficiency of 78.6%. The films present good thermal stability between 20 ℃ and 125 ℃. What is more, the films have good fatigue resistance. These films are likely to become candidates for high-performance energy storage materials because of their simple preparation method and high energy storage performance. 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