Epoxy‐Functional Hyperbranched Polysiloxane Hybridized Water‐Synthesized Polyimide Dielectrics for High‐Temperature Capacitive Energy Storage

ABSTRACT Water‐synthesized polymer dielectrics with reliable high‐temperature energy storage capability are highly desirable for sustainable power electronic applications. Herein, a topology‐engineered organic‐inorganic hybrid dielectric is constructed by integrating epoxy‐functionalized hyperbranched polysiloxane (EPHPSi) into water‐synthesized fluorinated polyimide (W‐PI). During thermal imidization, the epoxy groups of EPHPSi undergo ring‐opening reactions with the polymer matrix, forming a robust cross‐linked hybrid network. The hyperbranched siloxane nanoarchitecture regulates molecular packing, suppresses segmental relaxation, and introduces additional charge‐trapping sites, thereby helping to reduce charge transport and improve the dielectric performance under elevated temperatures. As a result, the optimized W‐PI EPHPSi ‐10% film exhibits a high breakdown strength of 669 MV m −1 at room temperature and retains 630 MV m −1 at 200°C, with a relatively narrow Weibull distribution. Meanwhile, the composite film achieves a high discharge energy density of 8.75 J cm −3 with a charge‐discharge efficiency of 94.7% at room temperature. More importantly, an ultrahigh discharge energy density of 8.52 J cm −3 is maintained at 200°C under 600 MV m −1 . This work provides a sustainable strategy for developing environmentally friendly polymer dielectrics for high‐temperature capacitive energy storage.