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.