  High temperature polymer-based dielectric capacitors are crucial for
application in electronic power systems. However, the storage performance of
conventional dielectrics polymer dramatically deteriorates due to the thermal
breakdown under concurrent high temperatures and electric fields, and there are
hardly reports on the causes of thermal breakdown from the aspects of the high
temperature conduction loss and Joule heat dissipation. Herein, a combined
strategy of crosslinking and compositing for polyimide-based nanocomposites is
proposed, which minimizes the thermal breakdown by significantly inhibiting the
high-temperature conduction loss and enhancing the high thermal conductivity.
Furthermore, the rationale of the strategy was theoretically and experimentally
verified from multiple perspectives. The charge-trapping effect is directly
observed and quantitatively probed by Kelvin probe force microscopy with nano
level resolution, indicating that the crosslinking network introduces local
deep traps and effectively suppresses the charge transport. The thermal
conductivity of the nanocomposites inhibits the high temperature thermal
breakdown, which is confirmed by phase field simulations. Consequently, the
optimized nanocomposites possess an ultra high discharge energy density(Ud) of
5.45 J/cm3 and 3.54 J/cm3 with a charge discharge efficiency, respectively,
which outperforms the reported polyimide based dielectric nanocomposites. This
work provides a scalable direction for high temperature polymer based
capacitors with excellent performance.
