**1. Introduction**

With the rapid development of the global economy and a rising population, the search for efficient and clean energy and energy storage technologies has become a priority worldwide. Because of its exceptionally fast energy conversion rate, long life, and environmental friendliness, dielectric energy storage technology has been used in applications for the electronics and power industries such as wearable electronic devices, hybrid vehicles, and weapon systems [1]. As the trend toward high-performance miniaturized electronic devices continues, the demand for dielectric materials with high energy storage density (*Ue*) is increasing. *Ue* is an important parameter to measure the energy storage performance of dielectric materials:

$$\mathbf{U}\_e = \mathbf{1}/2\varepsilon\_r \varepsilon\_o \mathbf{E}\_b^{\ 2} \tag{1}$$

where *ε<sup>r</sup>* is the permittivity of material and *ε<sup>o</sup>* is the permittivity of free space (8.85 � <sup>10</sup>�<sup>12</sup> F m�<sup>1</sup> ) [2]. This requires that the dielectric material has a high ε<sup>r</sup> while having a low dielectric loss and a high breakdown strength.

Commonly known high-energy storage dielectric materials are mainly biaxially oriented polypropylene (BOPP), polyester, polycarbonate (PC), polyphenylene disulfide, polyurea, polyurethane, and polyvinylidene fluoride [3]. Among many polymers, polyimide (PI) is a type of polymer containing an imide ring on the main chain [4]. PI is widely used in packaging materials, insulation layers, circuit boards, and interlayer dielectrics due to its high tensile strength, excellent mechanical properties, high glass transition temperature (*Tg*), and good solvent resistance and thermal stability [5]. However, the ε<sup>r</sup> of polyimide is not sufficiently high (usually less than 10) to meet the requirements of the applications of high-energy density film capacitors.

The chemical groups of a dielectric medium contribute to its molar polarization; as the molar polarization increases, the *ε<sup>r</sup>* increases. The dielectric properties (including *ε<sup>r</sup>* and dielectric loss) of polymers are mainly related to molecular polarization, which includes electron polarization, vibration polarization (or atomic polarization), orientation polarization (or dipole polarization), ion polarization, and interfacial polarization. However, low-quality/purity polar molecules can reduce the dielectric properties of PI materials [6].

Ma et al. [7] used high-throughput density functional theory (DFT) to rationally design high *ε<sup>r</sup>* and band gaps and linked experimental and theoretical results to changes in PI to demonstrate the relationship between chemical functionality and dielectric properties. Currently, researchers usually use two methods to prepare polyimide film capacitors with high *εr*, low dielectric loss, and high breakdown strength. One method is directly based on the molecular design of polyimide: Polar groups, conjugated components, or electron-rich groups are introduced into the main polymer chain to increase molecular polarizability, thereby increasing the *ε<sup>r</sup>* [8]. The other method, which is currently the most studied, prepares a composite material by introducing high-*ε<sup>r</sup>* ferroelectric materials such as TiO2, BaTiO3 (BT), Pb (Zr, Ti)O3, etc. into the polymer matrix to significantly improve the dielectric properties [5].
