Continuous self-assembled BNNS layer on/within polymer film significantly enhances high-temperature capacitive energy storage

High-temperature polymer dielectric capacitors are essential for modern electronic and electrical systems, yet their energy density and charge-discharge efficiency degrade sharply at elevated temperatures due to increased conduction losses from charge injection and transport. Despite significant progress in enhancing high-temperature dielectric performance, most approaches address only specific types of conduction losses, with limited efficacy beyond 150 °C. Here, a universal strategy that enables the continuous self-assembly of boron nitride nanosheet (BNNS) layers on both the surface and within the interior of polyetherimide (PEI) films, which architecture concurrently suppresses charge injection at the electrode/dielectrics interface and bulk charge transport within the dielectrics. Experimental characterization and computational simulations reveal that the continuous distribution of small, wide-bandgap BNNS layers effectively reduces conduction losses, achieving energy storage density of 5.2 J cm^-3 at 150 °C and 2.8 J cm^-3 at 200 °C, with charge-discharge efficiencies exceeding 90 % and cycling stability over 10,000 cycles, outperforming most reported dielectrics. Better yet, common materials, low filler, scalable film fabrication provides a foundation for designing next-generation large-scale polymer dielectrics.