Designing of Ferroelectric/Linear Dielectric Bilayer Films: An Effective Way to Improve the Energy Storage Performances of Polymer-Based Capacitors

In this paper, poly­(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)/polyimide (abbreviated as P­(VDF-TrFE-CFE)/PI) bilayer films are prepared by a casting method, where the PI with linear polarization characteristics is used as the bottom layer and P­(VDF-TrFE-CFE) with nonlinear ferroelectric polarization characteristics is used as the top layer. The microstructure, polarization behavior, and energy storage performances of the bilayer composite films have been systematically investigated. The results show that the interfaces between the P­(VDF-TrFE-CFE) layer and PI layer are uniform and continuous. On comparing with individual films, it is observed that the dielectric constant and breakdown field strength of bilayer films are higher than those of PI films, leading to the improved discharge energy density, and the residual polarization of bilayer films is effectively suppressed compared to that of P­(VDF-TrFE-CFE), resulting in the enhanced energy storage efficiency. Excellent energy storage performances have been obtained by regulating the volume content of PI in P­(VDF-TrFE-CFE)/PI bilayer films, which possess a discharge energy density of 9.6 J/cm3 and an energy storage efficiency of 58% with a PI content of 50 vol %. The results of this work indicate that constructing the ferroelectric/linear bilayer films is an effective way to improve the energy storage performances of flexible polymer-based capacitors.