Superior Energy Storage Performance Induced by Cross‐Scale Electric Field Modulation Utilizing Hybrid Hierarchical Micro–Nano Fillers in PEI‐based Composites

Abstract Polymer‐based composites with superior energy storage capabilities are indispensable components for realizing the lightweight architecture of pulsed power systems. Nevertheless, they confront an intrinsic challenge of the diminution in breakdown strength ( E b ) under extreme conditions of high temperature and/or strong electric field, consequently undermining energy storage efficacy. Herein, a cross‐scale electric field modulation strategy is successfully developed in the sandwich‐structured PEI‐based composites, as characterized by hybrid hierarchical barium titanate (BT) particles in the middle layer, whereas boron nitride nanosheets (BNNSs) in outermost layers. Through this innovative structure, hierarchical BT particles not only enhance dielectric properties but also work together with BNNSs to create unevenly distributed electric fields. Additionally, it markedly improves insulation and mitigates Joule heat, ultimately achieving systematic modulation of dielectric and breakdown properties at high temperatures. Consequently, the composite achieves an ultrahigh energy density ( U e ) of 21.80 J·cm −3 with a remarkable efficiency ( η ) of 96.89% at 620 MV·m −1 , surpassing most previously reported polymer‐based composites. Moreover, it demonstrates exceptional cycling stability and maintains robust energy storage performance at 150 °C, obtaining an outstanding U e of 11.98 J·cm −3 and a η of 87.1% at 565 MV·m −1 . This strategy provides a simple yet highly effective pathway for designing polymer‐based composites.