Enhancing High‐Temperature Energy Storage of Multilayer Polymer Composites Using Various Functions of Single Fillers

Polymer dielectrics for high‐temperature energy storage are limited by the trade‐off between high dielectric constant and high breakdown strength. In this work, BN nanosheets are incorporated into polyetherimide (PEI) to construct composites with tunable dielectric behavior. The dielectric constant and breakdown strength of single‐layer x vol.% BN NSs/PEI composites exhibit different dependencies on filler content, which are governed by interfacial polarization and charge transport behavior, respectively. Based on this, a sandwich composite is designed by combining a high‐polarization layer with x = 0.3 and high‐breakdown layers with x = 0.7. The optimized composite achieves a high energy storage density of 7.6 J cm −3 at 150°C and obtains an energy storage density of 4.9 J cm −3 ( η > 90%). This superior performance is attributed to the synergistic effects of interfacial polarization and interfacial barriers, which enhance polarization and suppress electrical tree propagation. This work provides an effective strategy for balancing dielectric constant and breakdown strength in polymer dielectrics for high‐temperature energy storage applications.