Side-Chain Sulfonated Polyimide Films with Intrinsic High Dielectric Constant and Breakdown Strength for Efficient Energy Storage

High dielectric constant (high-k) polymer films are ideal dielectrics for high energy density capacitors. Although polyimides (PIs) exhibit excellent thermal stability, their relatively low dielectric constant limits their ability to simultaneously fulfill the dual requirements of high energy density and high-temperature endurance. To address this challenge, in this work, we designed and synthesized a novel diamine monomer (S-ODA) containing highly polar sulfonated groups via a sequence of coupling, bromination, sulfonation, and reduction reactions. A series of modified PI films with sulfonated side chains (S–PI) were subsequently prepared through polycondensation. The S–PI films exhibit a high dielectric constant (5.3–7.5) with remarkably stable dielectric properties (both constant and loss) over a broad temperature range of 25–200 °C. These films also possess excellent thermal stability (Td5 > 400 °C, Td10 > 424 °C, Tg = 287 °C) and mechanical properties (tensile strength up to 113.7 MPa, elongation at break of 12.1%). Furthermore, the S–PI films achieve a high breakdown strength of 453 MV/m and a discharged energy density of 5.0 J/cm3 with a charge–discharge efficiency of 91%. Remarkably, even at an elevated temperature of 150 °C, a discharged energy density of 4.2 J/cm3 and an efficiency of 79% are maintained. This study provides a viable strategy for developing high-performance PI films that integrate a high dielectric constant, high energy density, and high efficiency, offering new perspectives for advancing high-temperature polymer dielectrics.