Synergistic multi-crosslinking and bandgap engineering elevate high-temperature polymer capacitor performance

High-temperature dielectric polymers face inherent limitations in capacitive energy storage due to thermally activated carrier transport and irreversible electromechanical breakdown. Here, we present a molecular design paradigm involving terminal functionalization of polyetherimide (PEI) with π-conjugated phenylacetyl moieties, constructing a multi-crosslinked network with dual functionality: an elevated glass transition temperature (T_g = 242 °C) restricts chain segment mobility under thermal-electrical stress; Density functional theory reveals that π-conjugated triple bonds widen the bandgap compared to conventional PEI, establishing deep charge traps (0.6 eV below conduction band) that suppress carrier injection kinetics. The optimized films achieve an ultrahigh breakdown strength of 573.8 MV m⁻¹ at 150 °C (46 % above pristine PEI) with leakage current reduced by two orders of magnitude, enabling a discharged energy density of 4.3 J cm⁻³ at 90 % efficiency, surpassing commercial high-temperature dielectrics. This molecular engineering strategy provides a generalizable framework for developing extreme-condition capacitive materials through atomic-scale electronic structure modulation and macromolecular conformation control.