Modulating electron traps of PEI-based nanocomposites for superb capacitive performance over a broad temperature range

• Novel wide-bandgap 2D core-shell structured BNNSs@AO are prepared. • Outstanding discharged energy density of 5.74 J/cm 3 along with efficiency of 91.5% are achieved at 150 °C. • The 2D BNNSs@AO introduction into PEI matrix could simultaneously promote the heat diffusion and improve breakdown strength. The thermal breakdown is a crucial factor leading to the failure of dielectric polymers owing to the production of a large amount of Joule heat derived from conduction loss and thermal accumulation, which becomes a technical bottleneck restricting the development of high-temperature electronic devices and equipment. Nevertheless, it is tough to minimize conduction loss and prevent premature heat degradation simultaneously when designing high-temperature resistant polymer-based energy storage dielectrics. Herein, PEI-matrix nanocomposites which core-shell structures of 2D BNNSs core and Al 2 O 3 shell are prepared. Our result reveals that introducing the wide-band gap core-shell design into polymer generates trap energy levels, leading to a diminished conduction loss and a strengthened Schottky barrier. And high-thermal conductivity 2D nanofiller further promotes the dissipation of Joule heat and faster heat transfer, which eases deferring thermally assisted electrical breakdown of the dielectric giving rise to an excellent high-temperature capacitive performance. As a result, an outstanding discharged energy density ( U d ) of 5.74 J/cm 3 is achieved at 525 MV/m and 150 °C while efficiency ( η ) is 91.5%, which outperforms most commercial high-temperature polymers. This work promotes the development and application of polymeric nanocomposite films appliable elevated temperatures.