Stepwise Molecular Design of Epoxy Dielectric Films toward High-Temperature, High-Efficiency Energy Storage

The growing demand for compact, high-power electronics in electric vehicles, renewable energy converters, and other harsh-environment platforms calls for dielectric films that can simultaneously deliver high energy density and high efficiency at elevated temperatures. However, the operating temperature ceiling of state-of-the-art BOPP films makes it difficult to satisfy the related requirements, motivating the exploration of alternative high-temperature polymer dielectrics. In this work, epoxy dielectric films are developed through a stepwise molecular design strategy that systematically balances thermal capability, polarization response, and high-field loss. A sulfone-containing curing agent is first introduced to improve high-temperature efficiency while maintaining strong polarization. Resin functionality is then tuned via progressive substitution with a tetrafunctional epoxy to extend the operating temperature window, revealing that high-field loss behavior, rather than <i>T</i>_g alone, governs efficiency at elevated temperatures. Finally, a small fraction of hydrogenated epoxy is incorporated to disrupt loss-active microstructural organization and concurrently suppress dielectric loss and leakage current. The final optimized film achieves 4.3 J/cm³ at 200 °C with efficiency above 90%. This study establishes a stepwise design framework that links molecular tuning to comprehensive performance, offering guidance for high-temperature dielectric energy storage films.