Ultrahigh Energy Density of Scalable Thermal‐Crosslinked Polyetherimides at 250 °C

Abstract Dielectric polymers must exhibit low conductivity and thermal stability to realize high‐performance capacitive energy storage under high‐temperature conditions. However, conventional heat‐resistant polymers suffer from deteriorated insulation at elevated temperatures, limiting their practical applications. Here, it is proposed and demonstrated that a self‐driven thermal crosslinking approach, without crosslinker, is highly efficient in breaking the adverse correlation between the insulation and thermal stability of dielectric polymers at high temperatures. By decarboxylation‐driven crosslinking within polyetherimide (PEI) molecular chains, a robust network is formed, effectively suppressing charge carrier migration. The synergistic combination of decarboxylation‐induced structural densification and residual carboxyl‐enabled deep charge trapping synergistically reduces the electrical conductivity of the modified PEI by 3 orders of magnitude relative to conventional polyimide (Kapton PI) at 250 °C. Consequently, a significantly enhanced discharge energy density of 3.7 J cm −3 with a charge‐discharge efficiency of 91% is achieved at 250 °C, outperforming existing pure polymer dielectrics. Furthermore, the scalability of this approach is demonstrated by producing a 100‐meter‐long modified film through an industrial‐scale roll‐to‐roll solution‐casting process, highlighting its compatibility with mass production. This strategy also proves to be universal to other dielectric polymers to achieve high insulation and heat resistance at elevated temperatures.