Synergistically ultrahigh energy storage density and efficiency in designed sandwich-structured poly(vinylidene fluoride)-based flexible composite films induced by doping Na0.5Bi0.5TiO3 whiskers

High-performance energy storage dielectrics have been the key to solve energy problems in the context of energy crisis. Designing multilayered structures is an effective approach to break the paradox between high dielectric constant and high breakdown strength existing in polymer-based composite films to enhance energy storage performance. Herein, a designed sandwich-structured poly(vinylidene fluoride) (PVDF)-based composite film, with a characteristic of the inner PVDF layer filled with Na0.5Bi0.5TiO3 (NBT) whiskers and the outer layers of pure PVDF, is proposed. An ultrahigh discharged energy density of 30.55 J cm(-3) and an outstanding discharged efficiency of 80.26% can be obtained in the optimized composition with the inner layer containing 6 vol% NBT whiskers, which outperforms all the previously reported PVDF-based composite films. Furthermore, it reveals that such a sandwich structure with NBT whiskers is able to adjust dielectric properties and reform electric field distribution, as evidenced by dielectric and insulation properties as well as the dielectric breakdown model by finite element analysis, which prompts the enhancement of polarization and breakdown strength synergistically, and thus, contributes to excellent energy storage performance. This work provides a strategy for developing high-end dielectrics for electrostatic energy storage applications.