Tuning Diamine Architecture in 6FDA-Based Polyimides for Enhanced High-Temperature Capacitive Energy Storage

Fluorinated polyimides (6FDA-based polyimides) were investigated as high-temperature dielectric materials for capacitive energy storage. This study synthesizes a series of 6FDA-based polyimide films via solution polymerization by modulating diamine monomers with distinct spatial configurations and segmental stiffness. Systematic investigations reveal that incorporating high-rigidity diamine monomers increases the glass-transition temperature (Tg) to 340.41 °C and enhances chain packing density, as evidenced by a reduction in d-spacing to 3.87 Å. Density functional theory calculations further revealed that PI-BP possesses the largest LUMO-HOMO energy gap of 3.423 eV among the three PIs, suggesting a relatively high energetic barrier for electronic excitation. Benefiting from these structural and electronic characteristics, PI-BP exhibited a breakdown strength of 800 MV/m and a discharged energy density of 7.04 J/cm3 at room temperature. At 150 °C, PI-BP maintained a breakdown strength of 600 MV/m and an energy-storage efficiency of 85.7%, whereas commercial Kapton PI exhibited only 300 MV/m and 20.3% under the same conditions. Moreover, after 106 charge-discharge cycles at 150 °C and 200 MV/m, the variations in discharged energy density and efficiency were limited to 1.8 and 1.5%, respectively. These results demonstrate that tailoring the diamine architecture provides an effective molecular-design strategy for simultaneously regulating chain packing, electronic structure, and high-temperature capacitive energy-storage performance in fluorinated polyimides.