Cooling-Regulated Crystalline Structure for Enhanced High-Temperature Capacitive Energy Storage in PTFE Dielectric Films

Polytetrafluoroethylene (PTFE) possesses outstanding thermal stability, low dielectric loss and an excellent insulating property, showing significant application potential in high-performance energy storage film capacitors. However, the conventional natural cooling process after high-temperature heat treatment tends to induce large fibrous crystalline structures accompanied by pores and pits, which limits the energy storage performance of PTFE films at high temperatures. Herein, flexible PTFE films with different crystalline morphologies are prepared by solution casting. By optimizing the cooling process, particularly through ice water bath cooling, crystalline growth is effectively suppressed, leading to a refined, homogeneous and dense crystalline structure. Simulation results demonstrate that the optimized crystalline structure effectively mitigates local electric field distortions, heat accumulation and stress concentration to improve the overall electrical–thermal–mechanical response. Consequently, the PTFE film prepared by ice water bath cooling exhibits an enhanced energy density of 2.33 J/cm3 with an efficiency of 92% at 200 °C under an electric field of 480 MV/m. The present work suggests that regulating the crystalline structure through the cooling process is a simple and low-cost strategy for effectively enhancing the high temperature energy storage performance of polymer dielectric films.