Interface engineering of polymer composite films for high-temperature capacitive energy storage

Polymer film capacitors encounter challenges in harsh conditions of high temperatures and electric fields because of large conduction loss and degraded breakdown strength (Eb). Herein, an interface engineering strategy is proposed to fluorinate the interfaces between the polyetherimide (PEI) matrix and wide bandgap boron nitride nanosheets (BNNSs) fillers. The composite films exhibit high-performance capacitive energy storage with a remarkable energy density of 5.73 J/cm3 and an ultrahigh efficiency of 91.22 % in conditions of 575 kV/mm and 150 degrees C. By adopting interfacial fluorination, the band structure of BNNSs is tailored to achieve a type II band alignment with PEI, promoting the dual trapping for both electrons and holes. It highly suppresses leakage current and reduces conduction loss. Typically, introducing fillers can compromise the properties of interfacial layers, creating weak points that trigger breakdown. Conversely, fluorinated interfaces exhibit an increased Young's modulus and a reduced dielectric constant. According to the electromechanical breakdown theory, the interfacial Eb is increased. The breakdown phase propagation along the interfaces is thereby impeded, ultimately resulting in a further increase in overall Eb, reaching up to 589 kV/mm at 150 degrees C. Fluorinated interface engineering addresses interfacial challenges posed by fillers, enabling high-temperature energy storage capability in PEI-based capacitors.