Designing of surface modification and sandwich structure: effective routs to improve energy storage property in polyimide-based composite films

The energy density of the capacitors is determined by the breakdown strength and the relative dielectric constant. Polyimide (PI) has high breakdown strength, linear polarization property and high energy efficiency. However, due to the low dielectric constant of PI, its energy storage density is not high enough to satisfy the requirement. Therefore, seeking a method to improve the dielectric constant while ensuring its breakdown strength is the key to solve the problem of the low energy storage of PI. In this study, the core–shell structured barium zirconate titanate calcium (BCZT) and silica (SiO 2 ) composited fibers (SiO 2 @BCZT) have been constructed and modified by silane coupling agent (KH550), the SiO 2 @BCZT inorganic fillers are beneficial to improve the dielectric constant of the PI-based composites. In order to further improve the breakdown strength of the composites, the sandwich-structured composite films have been designed, where the PI filled with SiO 2 @BCZT films act as dielectric reinforced top and bottom layers, and the PI filled with hydroxylated boron nitride nanosheet (BN) films serve as breakdown strength enhanced inter-layer. The microstructure and electrical properties of the composite films have been systematically investigated. For the sandwiched composite films, the discharged energy density and energy storage efficiency reach to 3.37 J/cm 3 and 84.89%, respectively. This study provides an effective way to improve the energy storage properties for PI-based composite films.