**3. Results**

Figure 1 displays the X-ray diffraction (XRD) patterns of the CBTi-BFO films annealing at four different temperatures and standard JCPDS cards of the target phases. From Figure 1a, as the temperature is at a lower value of 450 ◦C, most of the diffraction peak of Aurivillius phases have not appeared yet or just a bump, indicating that some amorphous phases formed due to insufficient heat energy. Note that the CBTi-BFO films annealed at 500 and 550 ◦C show the (119) and (200) diffraction peaks accompanied with another peak with low intensity, which are consistent with the reported results of other CBTi films with Aurivillius phase, demonstrating a polycrystalline bismuth layered structure without any second phase [31–33]. As the annealing temperature continued to rise to 600 ◦C, apart from the Aurivilius phase, two diffraction peaks that do not belong to the characteristic of the bismuth layered ferroelectric appeared near 15◦ and 30◦, which is due to the formation of the pyrochlore phase of Bi2Ti2O7 [34–37]. Generally, the pyrochlore phase can be formed frequently due to the bismuth element volatilize as the annealing temperature increases, resulting in the ratio of bismuth ions to titanium ions approaching 1:1. As shown in Figure 1b, the diffraction peak of (119) slightly shifts to a large angle with the increase of the annealing temperature, which may be caused by the release of surface residual stress [38,39].

**Figure 1.** (**a**) X-ray diffraction (XRD) patterns in the 2*θ* range of 10–60◦ of the CBTi-BFO films annealed at various temperatures. (**b**) Enlarged XRD patterns of the diffraction peaks of 2*θ* at around 30◦.

The SEM images clearly display the surface and the cross-sectional morphologies of the CBTi-BFO thin films annealing at different temperatures. In Figure 2, the inset images reveal that the thickness of all samples is approximately 550 nm. Meanwhile, all CBTi-BFO thin films present compact and pore-free surface, which is favorable to energy storage performance. As shown in Figure 2a, when the film is annealing at 450 ◦C, relatively uniform fine grains can be noticed on the surface. As the temperature rises to 500 ◦C, the grains absorb heat energy and thus, increase uniformly (Figure 2b). In sharp contrast, the CBTi-BFO film annealing at 550 ◦C possesses different grains with a wide range of grain size from ~17 to ~70 nm in Figure 2c. As the annealing temperature rises up to 600 ◦C, the grain size further increases in the range of ~20 to ~85 nm (Figure 2d). The phenomenon can be ascribed to: (i) different grain shapes corresponding to different orientations [40]; (ii) an inhomogeneous nucleation and grain-growth at a high annealing temperature [41].

It is well known that thin film with a high dielectric constant (*ε*r) typically achieves tremendous recoverable energy density and energy efficiency [27,42]. Figure 3a presents the room-temperature frequency dependence of dielectric constant (*ε*r) and the dissipation factor (tan*δ*) of the CBTi-BFO thin films annealing at different temperatures. The *ε*<sup>r</sup> value of each film slightly decreases with the frequency raising, and increases obviously with the annealing temperature increasing. The values of *ε*<sup>r</sup> for the films annealing at 450 and 500 ◦C have slightly changed with the frequency increases, indicating that the samples annealed at these two temperatures have better frequency stability. Generally, the dielectric properties of ferroelectric film contain the intrinsic and extrinsic contributions, which could be influenced by different factors, such as the grain size, preferred orientation, and so on [43,44]. The reason that in the changes of *ε*<sup>r</sup> with increasing annealing temperature may be due to the fact that the increased annealing temperature resulted in the increased grain size and reduced grain boundaries, leading to the enhancement of *ε*<sup>r</sup> [27]. Moreover, the dielectric loss (tan*δ*) gradually increases with increasing frequency in all samples. Besides, all samples possess smaller loss (tan*δ* < 0.08) at 10 kHz.

**Figure 2.** The scanning electron microscopic (SEM) images of the CBTi-BFO thin films annealed at (**a**) 450 ◦C, (**b**) 500 ◦C, (**c**) 550 ◦C and (**d**) 600 ◦C, and the inset shows their corresponding crosssectional micrographs.

**Figure 3.** CBTi-BFO films annealed at 450, 500, 550 and 600 ◦C: (**a**) Frequency dependence of dielectric properties. (**b**) Weibull distributions dielectric breakdown strengths. (**c**) Leakage current density as a function of applied electric filed. (**d**) The bipolar *P-E* loops at 2500 kV·cm−1. (**e**) The bipolar *P-E* loops around *E*b, The inset shows the variation of *P*max, *P*r, and *P*max-*P*<sup>r</sup> as a function of annealing temperature. (**f**) The variations of *W*rec, *W*loss and *η* values.

Generally, *E*<sup>b</sup> is analyzed by two parameter Weibull statistics, which is closely affected the energy storage performance of dielectric materials [45], as displayed in Figure 3b. Meanwhile, the *E*<sup>b</sup> endurance of four films with different annealing temperatures are

demonstrated in the inset of Figure 3b. The Weibull distribution of *E*<sup>b</sup> can be expressed by the following formulas:

$$X\_{\mathbf{i}} = Ln(E\_{\mathbf{i}}) \tag{4}$$

$$Y\_i = Ln(-Ln(1 - \frac{i}{n+1}))\tag{5}$$

where *E*<sup>i</sup> is the breakdown electric field for each sample, *i* signifies the number of test samples and *n* denotes the total number of test samples. Based on the Weibull distribution function, the mean *E*<sup>b</sup> for each film can be obtained from the intersection of the fitted lines with the horizontal axis at Y*<sup>i</sup>* = 0. *β* represents Weibull shape parameter. It can be observed that *E*<sup>b</sup> increases rapidly with the annealing temperature decreasing, as shown in the inset of Figure 3b. The average breakdown strength of the dielectric film increases from 3241 kV·cm−<sup>1</sup> to 3984 kV·cm<sup>−</sup>1, with the annealing temperature decrease from 600 to 450 ◦C. The enhancement of dielectric breakdown strength can be ascribed to the following factor. It is well-known that electric breakdown field strength is inversely proportional to the grain size (*G*), which can be manifested by the following formula:

$$E\_{\mathbf{b}} \propto (G)^{-a} \tag{6}$$

where *a* is the exponent values, being in the range of 0.2–0.4 [6,46]. It can be seen that the dielectric breakdown strength increases with the decreased grain size, which is aligned with the results in Figures 2 and 3b. That is owing to grain boundaries producing depletion regions similar to Schottky barriers located in semiconductor interfaces. Then, the grain boundaries depletion layers establish important barriers for the cross-transport of ionic and electronic charges [47]. Therefore, the *E*<sup>b</sup> for CBTi-BFO film annealing at 450 ◦C is superior to the films that have higher annealing temperature. The slope parameter *β*, related to the scatter of *E*<sup>b</sup> data, increases from 12.88 to 17.71 with the annealing temperature decrease from 600 ◦C to 450 ◦C, indicating an enhancement in dielectric reliability by annealing temperature decreasing. At the same time, the *β* of all films based on the linear fitting is higher than 12, suggestive of all samples possessing high reliability.

Figure 3c represents the leakage current of all films, which are measured by applying 0–185 kV·cm−<sup>1</sup> to the electrodes. The CBTi-BFO films annealed at 450 and 500 ◦C illustrate well insulating properties with leakage current densities < 7 × <sup>10</sup>−<sup>7</sup> A cm−<sup>2</sup> under an applied electric field of 185 kV·cm<sup>−</sup>1. The CBTi-BFO films annealed at 550 and 600 ◦C exhibit higher leakage currents, which is ascribed to the greater grain size and the simultaneously declined number of the grain boundary.

Presented in Figure 3d are bipolar polarization-electric filed (*P-E*) loops of the CBTi-BFO films annealing at different temperatures applying the electric field of 2500 kV·cm−<sup>1</sup> at frequency of 10 kHz. With the annealing temperature decreasing, a monotonous decrease of polarization (*P*max) is observed, from 33 μC cm−<sup>2</sup> of 450 ◦C to 63 μC cm−<sup>2</sup> of 600 ◦C.

Figure 3e shows *P-E* loops of CBTi-BFO films near their respective *E*<sup>b</sup> annealing at different temperatures. It can be seen that the CBTi-BFO thin film annealing at 600 ◦C presents a practically saturated *P-E* loop with a remnant polarization *P*<sup>r</sup> of 20 μC/cm2, which is lower than previous reports [32,33], suggesting that the derived CBTi-BFO thin films have somewhat discrepant ferroelectric properties. For the CBTi-BFO thin films annealing at lower temperatures, the obtained *P-E* hysteresis loops exhibit slim shape, which can be ascribed to the function of lower leakage current and small crystallite size [48–50]. Figure 3f presents the corresponding energy storage parameters of *W*rec, *W*loss and *η* determined from *P-E* loops about all CBTi-BFO films. The value of *W*rec and *η* for the CBTi film annealing at <sup>500</sup> ◦C, respectively, reaches 82.8 J·cm−<sup>3</sup> and 78.3% due to an integration of remarkable *<sup>E</sup>*<sup>b</sup> of 3596 kV·cm−<sup>1</sup> and a large polarization disparity of 52.3 <sup>μ</sup>C cm−<sup>2</sup> (inset of Figure 3e). It is well known that the dielectric film can induce outstanding energy performance due to high *P*max, low *P*r and high electric breakdown strength. The lower annealing temperature leads to more slender electric hysteresis loops and lower *W*loss. Meanwhile, *E*<sup>b</sup> is enhanced with the decrease of the annealing temperature.

Figure 4a–d show the Nyquist plots of films measured in the frequency range of 100 Hz to 1 MHz at a series of temperatures (225–300 ◦C). As is known to all, a low-frequency arc means the dielectric response of grain boundaries, while a high-frequency arc means the dielectric response of grains. One semicircular arc is observed for the samples. As the grain boundary dielectric relaxation of the material mainly contributes to the total impedance, only the semi-circular arc corresponding to the grain boundary response is observed. The intercept of impedance semicircular arcs on the Z'-axis can represent the total resistance (*R*b) values of the film. Clearly, the value of *R*<sup>b</sup> gradually decreased with the increasing measured temperature in each film, showing a negative temperature characteristic. This phenomenon can be attributed to the fact that the mobility of the space charge becomes easier, and more charge carriers will accumulate at the grain boundaries with the increasing measured temperature, thus resulting in increased electrical conductivity and decreased grain boundary resistance [51,52].

**Figure 4.** Nyquist diagrams of complex-plane of the CBTi-BFO films with annealing temperature of (**a**) 450, (**b**) 500, (**c**) 550, (**d**) 600 ◦C measured at 20 V under four measured temperatures.

In order to compare the impedance differences of the four samples in more detail, we separately take out the impedance diagrams tested at 300 ◦C and showed them in Figure 5a. It can be seen that *R*<sup>b</sup> of CBTi-BFO films gradually increases with a decreasing annealing temperature, indicating that the charge carrier concentration decreases in low-temperature annealed samples [53].

Generally, the energy required for carriers to cross the energy barrier is called conductive activation energy (*E*a), which can be calculated by Arrhenius formula:

$$
\sigma = \sigma\_0 \exp(-E\_\mathrm{a}/k\_\mathrm{B}T) \tag{7}
$$

where *T*, *σ*0, *E*<sup>a</sup> and *k*<sup>B</sup> are the measuring temperature on the Kelvin scale, preexponential factor, activation energy and Boltzmann constant, respectively. The plots of ln(*σ*) as a function of 1000/T for the films and linear fittings are shown in Figure 5b. The activation energies (*E*a) obtained from the slopes of fitting lines for films annealing at 450, 500, 550, 600 ◦C are estimated to be 1.21, 0.84, 0.68, 0.66 eV, respectively. Generally, the higher value of activation energy means fewer defects exist in the film [54]. Thus, we can deduce that the sample with the lowest annealing temperature has the least defects. Combining the above two points, it can be seen that CBTi-BFO with lowest annealing temperature possesses the smallest charge carrier concentration and fewer defects, which contribute greatly to the observed highest intrinsic breakdown field in this film.

**Figure 5.** (**a**) The impedance and fit result of CBTi-BFO films measured at 300 ◦C. (**b**) Arrhenius plots of CBTi-BFO films annealing at different temperatures.

For practical application, the requirement of reliability and stability is emphasized, such as under high/low temperature, high/low frequency and long-term working environments. Thus, in view of the CBTi-BFO film annealing at 500 ◦C possessing large *W*rec and *η* among all samples, further investigations are carried out on it under an electric field of 2500 kV·cm−1. Firstly, the temperature stability of the CBTi-BFO film annealing at 500 ◦C is measured. The bipolar *P-E* loops of the film measured from relatively low temperature of −25 ◦C to ultrahigh temperature of 200 ◦C are displayed in Figure 6a. For the purpose of expressing the variation of polarization more clearly, the unipolar hysteresis loop diagrams' dependence on temperature is drawn in Figure 6d. It can be seen that the *P*max value varies slightly from 42.62 to 45.89 μC cm<sup>−</sup>2. Correspondingly, *<sup>W</sup>*rec values are slightly increased by 2% from 44.88 to 45.84 J·cm−<sup>3</sup> and *η* is reduced by 6% from 80.04 to 75.19% with temperature increasing, as shown in Figure 6g. The aforesaid energy storage performance of the film shows good temperature stability, which is adequate to meet the demands of applications in extreme environments (capacitors used in underground industrial instruments need to work at temperatures higher than 150 ◦C and the inverter must work at about 140 ◦C in HEV) [55–57]. Besides, frequency dependence of the energy storage behavior is also researched at 2500 kV·cm<sup>−</sup>1. Figure 6b, e are the bipolar and unipolar hysteresis loops measured at the frequency range of 500 Hz to 20 kHz, respectively; and the corresponding energy performance *W*rec and *η* are shown in Figure 6h. Clearly, the *P-E* loop can maintain a slim feature and no discernible decline in energy storage performance can be discovered. Even though the frequency rises from 500 Hz to 20 kHz, the *W*rec and *η* values slightly drop from 48.71 to 43.80 J·cm−<sup>3</sup> and 83.94 to 77.47%, respectively, indicating that a good frequency stability can be realized. Furthermore, to assess the long-term charging–discharging stability of dielectric capacitors, the fatigue endurance should be investigated. The *P-E* loops of CBTi-BFO film annealing at 500 ◦C over 10<sup>7</sup> charge–discharge cycles are exhibited in Figure 6c, f. It can be seen that there is no obvious change in the hysteresis loops. The corresponding *W*rec and an excellent *η* present a negligible degradation with 1% and 0.3% as shown in Figure 6i. The observed superior antifatigue feature is closely related to the (Bi2O2) 2+ layer in the Aurivillius phase, which has a good insulating effect inhibiting the flow and generation of leakage charges during the repeated polarization process. Thus,

the Aurivillius phase can more effectively suppress the electrical breakdown during the fatigue process and improve the anti-fatigue performance.

**Figure 6.** CBTi-BFO thin film annealed at 500 ◦C: (**a**–**c**) bipolar *P-E* hysteresis loops measured at different temperatures/frequencies/switching cycles; (**d**–**f**) unipolar *P-E* hysteresis loops of CBTi-BFO films at different temperatures/frequencies/switching cycles; (**g**–**i**) the changes of *W*rec and *η* depending on temperature/frequencies/switching cycles.

Figure 7 summarizes a serious of results on the energy storage performance of Aurivillius ferroelectric films [12,13,27,28,58–60]. In this study, the CBTi-BFO thin film annealing at 500 ◦C possesses a relatively high *η* (~78%) superior than BaLa0.2Bi3.8Ti4O15 (BLBT~60%) and Bi3.25La0.75Ti3O12/BiFeO3/Bi3.25La0.75Ti3O12 (BLT/BFO/BLT~74%), but inferior to Ba2Bi4Ti5O18 (BBT~92%), 0.6BaTiO3-0.4Bi3.25La0.75Ti4O12 (0.6BT-0.4BLT~84%), CaBi2Nb2O9 (CBNO~82%), Sr2Bi4Ti5O18 (SBT~81%), Bi3.25La0.75Ti3O12-BiFeO3 (BLT-BFO~80%). In contrast, its *W*rec clearly outperforms the surveyed dielectric systems with a high *η*. That is, the CBTi-BFO thin film annealing at 500 ◦C has excellent comprehensive properties, namely, a good balance between *<sup>W</sup>*rec and *<sup>η</sup>*. Meanwhile, the film has a great *<sup>E</sup>*<sup>b</sup> (~3596 kV·cm<sup>−</sup>1) at a high level. Taken together, the CBTi-BFO thin film annealing at 500 ◦C is a good candidate for application in energy storage devices.

**Figure 7.** Summary of recently reported the core parameters of *E*b, *W*rec and *η* for energy storage properties of representative Aurivillius ferroelectrics film [12,13,27,28,58–60].

### **4. Conclusions**

In this work, a series of CBTi-BFO thin films are prepared by chemical solution deposition under different annealing temperatures. We found that by optimizing the microstructures and crystalline structure of films via adjusting the annealing temperature, the electrical properties and energy storage performance can be greatly improved, especially in the film with a relatively lower annealing temperature. In the annealing temperature of 500–550 ◦C, CBTi-BFO thin films demonstrates a polycrystalline bismuth layered structure without any second phase. The CBTi-BFO with the lowest annealing temperature has the smallest carrier concentration and fewer defects, which greatly contribute to the highest intrinsic breakdown field of this film. Here, ultrahigh energy storage density of *W*rec (~82.8 J·cm<sup>−</sup>3) and *<sup>η</sup>* (~78.3%) are achieved in the CBTi-BFO film that annealed at 500 ◦C. The excellent energy storage performance can be ascribed to its uniform fine grain size. The film also shows superior thermal stability (from −25 to 200 ◦C), frequency stability (from 500 Hz to 20 kHz) and fatigue endurance (after 10<sup>7</sup> switching cycles). In a word, annealing temperature plays an important part in performance tuning, which is a vital factor needed to be considered for preparing thin film capacitors with high energy storage characteristics.

**Author Contributions:** Conceptualization, C.Y., S.H. and L.L.; methodology, C.Y.; investigation, T.L., W.W., J.Q. and Q.L.; resources, C.Y.; data curation, T.L., W.W. and M.F.; writing—original draft preparation, T.L. and W.W.; writing—review and editing, C.Y. and S.H.; supervision, C.Y., S.H. and L.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (Grant Nos. 51972144, and U1806221), the Shandong Provincial Natural Science Foundation (Grant No. ZR2020KA003), the Project of "20 Items of University" of Jinan (Grant Nos. T202009 and T201907), and the Synergetic Innovation Center Research Project (Grant No. WJGTT-XT2), the Introduction Program of Senior Foreign Experts (G2021024003L).

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
