High‐Temperature Energy Storage Performance of Polyetherimide‐based Composites Enhanced by Core–Shell Structured Nanofillers

Functioning as the fundamental building blocks of new energy power systems, high‐temperature dielectric materials critically affect device durability and efficiency under extreme conditions. However, conventional polymer composites often suffer from increased dielectric loss and reduced insulation strength at high temperatures, degrading energy storage performance and limiting high‐temperature capacitor applications. To address this, core–shell Bi 6 Ti 5 WO 22 @MgO (BTWO@MO) nanofillers via chemical precipitation are designed. The inner core BTWO demonstrates high polarizability, while the wide bandgap MgO shell effectively suppresses interfacial charge accumulation. By incorporating BTWO@MO into polyetherimide (PEI), composites with high insulation strength and low dielectric loss are achieved. Experimental results show that at 150 °C and 580 MV·m −1 , the composite achieves a remarkable discharged energy density ( U d ) of 6.24 J·cm −3 (a 197% improvement compared to pure PEI) under breakdown electric fields, with an efficiency retention of 76.35%. More importantly, at 400 MV·m −1 , the U d of the composite is 4.80 J·cm −3 , and the efficiency is as high as 87.51%. Additionally, the composite exhibits excellent cycling stability under practical operating conditions involving both elevated temperatures and strong electric fields. This study develops high‐performance high‐temperature dielectric materials through an innovative core–shell filler design, offering a novel solution for extreme‐environment dielectric applications.