**1. Introduction**

At present, energy and environmental issues are the focus of social attention. The vigorous development of green and clean energy (such as wind and solar energy) is one of the future trends. The instability and intermittency of green energy put forward higher requirements for energy storage technology [1–5]. Dielectric capacitors typically display ultrafast charge–discharge rates and long life-time, temperature/frequency stability, fatigue resistance, which play key roles in various modern electrical and electronic systems, such as hybrid electric vehicle, aircraft and military [6–9]. Film capacitors offer a smaller size and higher energy storage density, making them easier to integrate into circuits than other devices such as ceramic capacitors [10]. Currently, most commercial dielectrics are mainly made of organic polymers, such as biaxially oriented polypropylene (BOPP), which have been widely used as the dielectric layer in power inverter capacitor systems, making the storage system bulky due to the low energy density (<5 J·cm3). Furthermore, the operating temperature of BOPP cannot be higher than 80 ◦C, which increases the difficulty of structure

**Citation:** Liu, T.; Wang, W.; Qian, J.; Li, Q.; Fan, M.; Yang, C.; Huang, S.; Lu, L. Excellent Energy Storage Performance in Bi(Fe0.93Mn0.05Ti0.02)O3 Modified CaBi4Ti4O15 Thin Film by Adjusting Annealing Temperature. *Nanomaterials* **2022**, *12*, 730. https:// doi.org/10.3390/nano12050730

Academic Editors: Dong-Joo Kim and Alain Pignolet

Received: 29 January 2022 Accepted: 18 February 2022 Published: 22 February 2022

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design due to the need for an extra cooling system [11,12]. By contrast, inorganic dielectric film capacitors have the advantages of relatively high energy densities, better thermal stability in wider operating temperature ranges, and long-term endurance. Among this, inorganic ferroelectric film capacitors (TFFCs) are considered as good candidates for energy storage due to their large polarization and high temperature resistance [13,14]. However, low energy storage density and efficiency limit its further development in energy storage applications; thus, further improvements are needed.

For film capacitors, two important energy storage parameters, the recoverable energy storage density (*W*rec) and energy storage efficiency (*η*), can be calculated from the measured hysteresis loops adopting the following equations [15,16]:

$$\mathcal{W}\_{\text{rec}} = \int\_{P\_{\text{r}}}^{P\_{\text{m}}} E \, dP \tag{1}$$

$$\mathcal{W}\_{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\cdots}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}$$
}}}}}}}

$$m = \frac{\mathcal{W}\_{\text{rec}}}{\mathcal{W}\_{\text{t}}} \times 100\% \tag{3}$$

where *E*, *W*t, *P*m and *P*r are the applied electric field, total energy storage density, the maximum polarization and remanent polarization during the discharge process, respectively. Therefore, *W*rec can be improved by increasing the difference between *P*m and *P*r, and the electric breakdown strength (*E*b). It is well known that the *E*<sup>b</sup> of dielectric materials is mainly contingent on its microstructure, such as grain size and degree of densification. Therefore, increasing *E*<sup>b</sup> by reducing grain size is an effective way to improve energy storage performance [17]. Wang et al. sort out the relationship between grain size and electric breakdown strength, confirming the optimization effect of energy storage via grain sizeengineering [6]. As is well known, the annealing temperature has an immense impact on the quality of films prepared by chemical solution deposition (CSD). For instance, Wang et al. unveil a large value of *<sup>W</sup>*rec up to 91.3 J·cm−<sup>3</sup> at 4993 kV·cm−<sup>1</sup> for Pb0.88Ca0.12ZrO3 (PCZ) antiferroelectric thin films by designing a nanocrystalline structure of the pyrochlore phase by optimizing the annealing temperature to 550 ◦C [18]. However, the negative effect caused by the application of lead-containing dielectrics to human health and environmental sustainability cannot be ignored, and the exploration of lead-free energy storage materials is raised in the agenda. For example, Zuo et al. investigate that a high *<sup>W</sup>*rec of 8.12 J·cm−<sup>3</sup> and a great *η* of ∼90% are obtained simultaneously in BiFeO3-BaTiO3-NaNbO3 ceramics, which can be attributed to the significantly enhanced *E*<sup>b</sup> of BiFeO3-based ternary solid solutions originating from the increased resistivity and refined grain size [19].

Bismuth layer-structured ferroelectric (BLSF) compounds, such as SrBi2Nb2O9 (SBN), SrBi2Ta2O9 (SBT), Bi4Ti3O12 (BIT), CaBi4Ti4O15 (CBTi), belong to a large category of ferroelectric materials [13,20–22]. They have the advantages of excellent anti-fatigue property, large dielectric constant and small dielectric loss, high resistivity and low leakage current density, high ferroelectric Curie transition temperature, and so on [23–26]. Those traits show a good application prospect in the field of dielectric energy storage, but there is little research on BLSF compounds in this field [27,28]. This is mainly due to their intrinsic shortcomings, namely, relatively low polarization and high coercive field, which lead to lower energy density and higher losses in energy storage applications [29]. Recently, Pan et al. presented a composition modification method in ferroelectric Aurivillius Bi3.25La0.75Ti3O12 by introducing BiFeO3 to increase the polarization value and optimize hysteresis loops, in which *<sup>W</sup>*rec (113 J·cm−3) and *<sup>η</sup>* (80.4%) are observed. Yang et al. prepared a series of 0.6BaTiO3-0.4Bi3.25La0.75Ti3O12 thin films, and the modified thin film also shows higher dielectric breakdown strength and polarization. CBTi is also a representative BLSF compound, which exhibits distinct advantages including being lead-free and fatigue-free. Meanwhile, it possesses a high Curie point of about 790 ◦C to be used in relatively high temperature applications [30]. However, it also faces troubles of low spontaneous polarization.

In this work, we select Bi(Fe0.93Mn0.05Ti0.02)O3 introduced into CBTi, namely, CBTi-BFO, to reduce leakage current and enhance breakdown field strength. In order to further optimize the energy storage performance of CBTi-BFO thin films, the effect of annealing temperature on their energy storage capacity has been studied in detail. We found that the microstructures of the CBTi-BFO thin films can be dominated by adjusting the annealing temperature. The CBTi-BFO film annealing at 500 ◦C possesses an excellent *<sup>W</sup>*rec of 82.8 J·cm−<sup>3</sup> and *<sup>η</sup>* of 78.3%, simultaneously, due to the obviously enhanced *<sup>E</sup>*<sup>b</sup> of 3596 kV·cm<sup>−</sup>1. Meanwhile, the film shows outstanding temperature/frequency stability up to 150 ◦C and superior fatigue stability after 107 switch cycles. The findings overcome the shortcomings of organic thin films in energy storage, including low energy storage density and low application temperature, unveiling an effective way towards high performance lead-free and eco-friendly ferroelectric materials for energy storage applications.
