Tailoring Polar Groups in Ethylene–Tetrafluoroethylene Copolymer for Enhanced Dielectric Energy Storage Performance

Polymer dielectric films that exhibit a high energy density and discharge efficiency are of great importance for advanced electronic and electrical systems. However, achieving a simultaneous enhancement in both the dielectric constant and breakdown strength to attain excellent energy storage performance at elevated temperatures remains a significant challenge. Herein, poly(methyl methacrylate) (PMMA) side chains containing polar groups were successfully grafted onto ethylene–tetrafluoroethylene copolymer (ETFE) via a photocatalytic C–F bond activation strategy. The incorporation of polar groups not only increases the dielectric constant of the graft copolymer but also creates trap sites that hinder charge carrier mobility, thereby suppressing the leakage current and enhancing breakdown strength. As a result, the graft copolymer (optimized at 7 wt % PMMA) achieves a discharge energy density of 6.4 J/cm³, along with a discharge efficiency of 87%. Moreover, the material retains excellent thermal stability at elevated temperatures, exhibiting a discharge energy density of 6.6 J/cm³ and an efficiency of 84% even at 100 °C. This work provides a feasible strategy for modifying high-temperature-resistant fluoropolymers to achieve superior energy storage performance, offering valuable insights for the development of polymer dielectric materials.