Improved High-Temperature Polarization and Charge–Discharge Efficiency in Fluorinated Polyimide Copolymers

Dielectric film capacitors have received extensive attention because of their fast release rate and great electrical reliability. A polymer film with large energy density and charge–discharge efficiency is required to expand the application of film capacitors at high temperatures. In this work, a series of ternary fluorinated polyimides (FPI) with different fluorine-containing ratios have been copolymerized to investigate the high-temperature electrical displacements. The fluoro-dianhydride monomer was introduced into the regular 4,4′-diaminodiphenyl ether polymerization system with accompanied thermal imidization. The FPI film exhibits large energy capability with intrinsic breakdown strength compared with pristine PI, which is ascribed to the strong electronegative effect and hydrophobic feature of –CF3 as well as the increase of free volume in bulk. The energy density of optimal FPI film reaches 20.2 J/cm3 at 659 MV/m, and the high-temperature energy storage property is improved, e.g., the energy density up to 11.5 J/cm3 with an external field of 525 MV/m at 100 °C. Low molar polarizability between C–F bonds also contributes to the decreased leakage current, which results in the large charge–discharge efficiency under high temperatures. The PI film with tailored fluorostructure demonstrates the competing candidate for the application of a high-temperature polymer film capacitor.