Record-High Capacitive Energy Density in Polyimide Dielectrics at 300°C via Conformational-Vibrational Engineering.

Polymer dielectrics are crucial for capacitors in various applications, including electric vehicles, geothermal instrumentation, and aerospace systems. However, polymers tend to exhibit strongly nonlinear conduction at high temperatures and under high electric fields, leading to a rapid increase in leakage current and typically limiting the stable operating temperature to around 250°C. Here, we report a coupled conformational and vibrational engineering strategy in polyimide (PI) dielectrics that weakens electronic coupling and suppresses specific molecular vibrations governing thermally assisted charge transport. This dual-action design effectively delays the onset of high-field nonlinear conduction and suppresses leakage current at extreme temperatures. Consequently, the resulting PI dielectric films achieve a maximum discharged energy density of 5.9 J cm-3 at 300°C and maintain 2.8 J cm-3 with 90% efficiency, while sustaining over 105 charge-discharge cycles at 300°C, surpassing state-of-the-art polymer dielectrics in operating temperature and cycling reliability. This strategy provides a fundamentally different route from conventional bandgap engineering approaches for high energy density polymer dielectrics operating at 300°C.