Superior High-Temperature Energy Storage Performance of Poly(ether imide) Capacitive Films by Incorporating Core–Shell Structured BN@MgO Fillers

Dielectric capacitors are widely used in electrical engineering due to their excellent insulation, high safety, and high power density. However, under high-temperature conditions, injected electric charges from metal electrodes, along with thermally generated charges, lead to a rapid increase in leakage current and severe degradation of insulation performance in polymer capacitive films, limiting long-term and stable operation. To address this issue, core–shell structured BN@MgO inorganic fillers were prepared using the magnetron sputtering method, with MgO as the core and BN as the shell. Results demonstrate that BN@MgO fillers significantly reduce conduction loss in poly(ether imide) (PEI) films by forming charge traps at interfacial regions between the fillers and the PEI matrix. The built-in electric field between BN and MgO efficiently captures mobile charges, enhancing the electrical insulation properties at elevated temperatures. Optimizing the doping content and BN shell thickness leads to a discharge energy density of 3.9 J/cm3 at 150 °C, with a charge/discharge efficiency of 90% for PEI/BN@MgO-1.5(2.5h) composite films. Additionally, excellent cyclic reliability and performance are maintained after 50,000 charge/discharge cycles. The study presents an efficient method for constructing core–shell BN@MgO fillers and clarifies mechanisms for restricting charge mobility.