Design of Sandwich Structures for High Energy Density Film Capacitors Based on Interface Polarization and Field Redistribution

ABSTRACT To overcome the challenge of simultaneously enhancing breakdown strength and dielectric constant in polymer dielectrics, this study proposes a simple layer‐by‐layer casting method to prepare a three‐layer film. The structure features outer layers of PMMA/g‐C 3 N 4 /PVDF for high breakdown strength and a central g‐C 3 N 4 /PVDF layer for high dielectric constant. This “high‐breakdown–high‐dielectric–high‐breakdown” design promotes electric field redistribution and interfacial polarization, maintaining a high dielectric constant (7.26 at 100 Hz) even with 50 wt% PMMA. Moreover, the insulating PMMA outer layers introduce deep charge traps, while embedded g‐C 3 N 4 nanosheets extend carrier migration paths, synergistically inhibiting electrical treeing. Consequently, the three‐layer film achieves a breakdown strength of 647.57 MV/m at 50 wt% PMMA, about 1.15 times that of a single‐layer PCP film (568.1 MV/m). More importantly, the simultaneous enhancement of polarization and breakdown strength yields an energy density of 8.22 J/cm 3 , 2.3 times that of pure PVDF (3.51 J/cm 3 ). This work demonstrates a structural design strategy for breakthrough polymer dielectric performance, offering a new approach to high‐performance organic dielectric films.