Significantly improved breakdown strength and energy density in high-temperature polymer dielectric enabled by bilayered nanocoatings

Dielectric materials with superior high-temperature energy storage capability are of critical importance for the development of modern power electronic systems. However, polymer dielectrics generally suffer from rapid performance degradation at high temperature and under a strong electric field due to charge injection, which is particularly severe for highly aromatic, high-temperature-resistant polymers such as polyimide. Owing to their intrinsically low bandgap and limited charge-injection barriers, pronounced conductive losses can occur even under moderate electric fields. Conventional methods to address this issue involve coating the polymer surface with highly insulating nanomaterials; while this can suppress charge injection, it simultaneously triggers space charge accumulation and local electric-field distortion. To address these challenges, a novel bilayer nanocoated polyimide (Kapton PI)-based multilayer architecture is designed in this work to synergistically integrate charge-blocking and charge-dissipation functionalities. The boron nitride nanosheets (BNNSs) layer adjacent to the electrodes effectively increases the charge-injection barrier, while the montmorillonite (MMT) nanosheets layer near the polymer substrate facilitates in-plane charge dissipation and alleviates local electric-field distortion. Plasma treatment is strategically employed to optimize the interfacial chemistry, further enhancing interfacial polarization and interfacial interactions to ensure a robust synergistic effect. Benefiting from this comprehensive interfacial engineering, the bilayer nanocoatings simultaneously achieve suppressed electrical breakdown and enhanced polarization at elevated temperatures. Remarkably, they deliver a high discharged energy density (Ue) of 1.6 J cm−3 with an outstanding charge–discharge efficiency (η) of 90.1% at 200 °C under a low electric field of 250 MV m−1. This work demonstrates an effective strategy based on bilayer nanocoatings for improving the high-temperature energy storage performance of polymer films, showing great potential for harsh environment applications.