Synergistic Interfacial Engineering via In Situ Covalent Bonding for High‐Temperature Polymer Dielectric Energy Storage

Polymer dielectrics for high‐temperature capacitors are critically limited by thermally induced charge transport and breakdown. Herein, we introduce synthesized novel Au coated with Si‐O‐Si network (SONET) nanoparticles into a polyetherimide (PEI) matrix. The design leverages synergistic interfacial barrier effects—Coulomb blockade effect from the Au nanoparticle core and charge‐blocking effect from the wide‐bandgap SONET shell—while in situ covalent bonding (Au─S and C─O─Si) ensures robust interfaces and eliminates severe agglomeration. Furthermore, tailoring the particle size distribution modulates the Young's modulus and rigid amorphous fraction (RAF) formation, enabling a bimodal size distribution that maximizes the trapping efficiency. Thus, the composite achieves an outstanding U d of 4.14 J cm −3 with η of 90% and a maximum U d of 5.07 J cm −3 at 200°C alongside the improved E b of 604 MV m −1 . This work offers a generalizable paradigm for covalently engineered polymer nanocomposites toward high‐performance capacitive energy storage at extreme temperatures.