Sharply Improved Electrical Insulation of Polyimide Dielectrics at Elevated Temperatures by Charge Reassignment Engineering

The electrical insulation of traditional polyimides (PI) dielectrics for capacitive energy storage will be significantly destroyed at elevated temperatures due to enhanced conductive loss caused by strong conjugated effect and charge transfer (CT) interactions. To address this issue, novel PI dielectrics are designed and synthesized based on charge reassignment engineering. Beneficial from non-planar dianhydride and poor conjugated diamine, the main chain of synthesized PI (semi-aromatic NA₂-alt-OB₁) significantly reduces the conjugated effect. Alicyclic dianhydride structural unit endows NA₂-alt-OB₁ with a large bandgap width of 5.22 eV and a high LUMO energy level (-0.79 eV), making the polymer resistant to both excited and injected electrons. More importantly, this work intelligently combines trifluoromethyl and dicyclohexyl to achieve weak charge reassignment on donor-acceptor electron sites in the polymer chain, effectively decreasing CT effect. Consequently, a super-high Weibull breakdown strength value of 862 and 691 MV m^-1 at room temperature and 230 °C respectively is achieved. Semi-aromatic NA₂-alt-OB₁ obtains a high discharge energy density of 6.43 J cm^-3 at 230 °C, which is greater than that of lots of reported PI dielectrics. This work offers an important strategy to break the adverse correlation between electrical insulation and heating resistance of PI dielectrics, promoting their application in capacitive energy storage at elevated temperatures.