In Situ Grown Inorganic Covalent Surface Barriers Enable High‐Temperature Capacitive Energy Storage

ABSTRACT Polymer dielectrics are crucial for high‐energy‐density electrostatic capacitors yet suffer from severe high‐temperature performance degradation due to escalated conduction loss and diminished breakdown strength. While conventional construction of nanoscale inorganic surface layers on dielectric polymer films can partially suppress conduction losses at elevated temperatures, poor interfacial adhesion coupled with complex, expensive fabrication technology hinders industrial scalability. Herein, we develop a solution‐processed nanoscale inorganic covalent surface barrier (SNICS) strategy. Specifically, scalable ultrathin SiO x barrier layers featuring covalently bonded Si─C transition interphases are conformally fabricated on both surfaces of polyetherimide (PEI) films via facile ultrasonic spray‐coating (USC) of perhydropolysilazane (PHPS) precursor solution followed by room‐temperature ultraviolet (UV) irradiation. The SNICS architecture ensures robust interfacial cohesion while effectively suppressing conduction loss through: 1) substantial elevation of the Schottky emission barrier height; 2) introduction of deep charge traps by the high‐electron‐affinity Si─C interphase; and 3) enhanced thermal/mechanical stability from the covalent bonding, outperforming traditional magnetron‐sputtered counterparts. Consequently, SNICS‐engineered films with optimized 186 nm SNICS achieve an exceptional discharged energy density ( U d ) of 5.05 J cm −3 with 90 % efficiency at 200 °C, which is significantly higher than the reported values obtained in the composites fabricated with multilayered structures, with along siding excellent self‐healing capability and outstanding cyclability. This surface engineering presents a scalable pathway for high‐temperature high‐performance polymer capacitors.