Soluble Semi-Alicyclic Fluorinated Copolyimide Dielectrics for High-Temperature Capacitive Energy Storage

Polyimides are renowned for their excellent thermal stability and insulating properties, making them ideal candidates for high-temperature dielectric energy storage. However, traditional polyimides often suffer from a narrow bandgap and limited solubility, which complicates processing and restricts their widespread application in high-performance energy storage. In this work, we propose a molecular design strategy by copolymerizing rigid, low-polarity cyclobutane-1,2,3,4-tetracarboxylic dianhydride (CBDA), 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with a bulky fluorinated bridge group, and 2,2′-bis(trifluoromethyl)benzidine (TFMB) to form wide-bandgap CBDA/6FDA/TFMB copolyimides, achieving a balanced performance with excellent solubility and high-temperature dielectric properties. Their electronic structure, molecular aggregation behavior, thermal stability, solubility, dielectric behavior, electrical breakdown strength, and high-temperature energy storage performance were systematically investigated as a function of 6FDA content. The results demonstrate that the copolyimide with 30% 6FDA content maintains good solubility while exhibiting excellent high-temperature dielectric polarization, low electrical conduction loss, and high breakdown strength. This leads to outstanding performance in high-temperature energy storage performance, with an energy storage density of 2.11 J/cm3 and a charge–discharge efficiency of 90.3% at 150 °C and 350 MV/m. Overall, this work demonstrates the successful design and synthesis of semi-alicyclic fluorinated copolyimides with tailored solubility and dielectric properties, underscoring their promising potential for high-temperature dielectric energy storage applications.