Advancements in power electronics necessitate dielectric polymer films capable of operating at high temperatures and possessing high energy density. Although significant strides have been achieved by integrating inorganic fillers into high-temperature polymer matrices, the inherently low dielectric constants of these matrices have tempered the magnitude of success. In this work, we report an innovative nanocomposite based on sulfonylated polyimide (SPI), distinguished by the incorporation of sulfonyl groups within the SPI backbone and the inclusion of wide bandgap hafnium dioxide (HfO₂) nanofillers. The nanocomposite has demonstrated notable enhancements in thermal stability, dielectric properties, and capacitive performance at elevated temperatures. Detailed simulations at both molecular and mesoscopic levels have elucidated the mechanisms behind these improvements, which could be attributed to confined segmental motion, an optimized electronic band structure, and a diminished incidence of dielectric breakdown ascribed to the presence of sulfonyl groups. Remarkably, the SPI-HfO₂ nanocomposite demonstrates a high charge-discharge efficiency of 95.7% at an elevated temperature of 150 °C and an applied electric field of 200 MV/m. Furthermore, it achieves a maximum discharged energy density of 2.71 J/cm³, signalling its substantial potential for energy storage applications under extreme conditions.

Future electronic devices toward high integration and miniaturization demand reliable operation of dielectric materials at high electric fields and elevated temperatures. However, the electrical deterioration caused by Joule heat generation remains a persistent challenge to overcome. Here, the solution-processed polyimide (PI) nanocomposites with unique two-dimensional (2D) alumina nanoplates are reported. Substantial improvements in the breakdown strength, charge–discharge efficiency and discharged energy density at elevated temperatures have been demonstrated in the composites, owing to simultaneously suppressed conduction loss and increased thermal conductivity upon the incorporation of 2D Al₂O₃ nanofillers possessing excellent dielectric insulation and thermophysical properties. The predominance of Al₂O₃ nanoplates in enhancing thermal stability and high-temperature capacitive performance over nanoparticles and nanowires is validated experimentally and is further rationalized via finite element simulations. Notably, the Al₂O₃ nanoplates filled PI nanocomposite exhibits a high-temperature capability up to 200 °C and remarkable efficiency (e.g. ≥ 95% at 200 MV/m) over a wide temperature range, which outperforms commercial dielectric polymers and rivals the state-of-the-art polyimide nanocomposites.