Surface-gradient-structured polymer films with restricted thermal expansion for high-temperature capacitive energy storage

The capacitive performance of existing dielectric polymers deteriorates significantly at elevated temperatures, although their thermal stability far exceeds, which remains a major challenge for efficient dielectric energy storage under extreme conditions. Here, a material design inspired by the cross-property connection phenomena, which bridges the seemingly unrelated material properties through similar or relevant determining microscopic factors, is reported to achieve substantially improved high-temperature capacitive performance in dielectric polymers. A high consistency is unveiled between the high-temperature electrical properties and thermal expansion of dielectric polymers, based on which a surface-gradient crosslinking structure is designed to inhibit the thermal distortion. It is confirmed by both experimental results and computational simulations that the restricted thermal expansion gives rise to reduced free volume as well as suppressed β-relaxation, which account for the marked improvements in high-temperature capacitive performances. At the optimal composition, the resultant polymer exhibits an ultrahigh discharged energy density up to 4.9 J/cm³ at 200 °C with a charge-discharge efficiency of 90 %, which is superior to all the existing polymer films based on the surface modification. This work highlights the significance of correlating variations in different physical properties for the design of high-energy-density polymer dielectrics capable of operating under harsh environments.