Ultrahigh Energy Density in Dielectric Polymers Near Glass Transition Temperature by Molecular Twisting Conformation Locking.

Abstract

Dielectric polymers with high operating temperatures (T_o) for capacitive energy storage applications are urgently needed in new energy vehicles and power electronics. Polymers with high glass transition temperatures (T_g), such as Kapton polyimide (T_g≈360 °C), suffer low T_o (< 150 °C) due to electron delocalization between donor and acceptor units. Here, a molecular twisting conformation-locking strategy is proposed for high-temperature dielectric polymers to block intrachain and interchain electron migration pathways. Density functional theory (DFT) calculations indicate that the elevated leakage current in polyimides originates from enhanced electron delocalization induced by intrachain imide ring planarization and interchain donor-acceptor (D-A) face-to-face stacking. The molecular twisting conformation-locking disrupts intrachain imide ring planarization and the face-to-face stacking of interchain D-A units. As a result, the designed polymer exhibits an ultrahigh resistivity of 6.8 × 10¹³ Ω m^-1 at 250 °C (close to its T_g), surpassing the 2.8 × 10¹³ Ω m^-1 of PEI at 50 °C. Simultaneously, it achieves an ultrahigh discharge energy density of 4.3 J cm^-3, outperforming existing high-T_g dielectric polymers. This study introduces a design paradigm to address the challenge of dielectric polymers that do not function properly as ambient temperatures approach their T_g.