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.