High-temperature energy storage performances of “isomer-like” polyimide and its thermoplastic polyurethane blending system

Abstract

With the rapid development of high-temperature energy storage devices, polyimide films with excellent thermal stability have become candidates for capacitor materials. However, the energy density (U_e) and efficiency (η) of the most commonly used polyimide (common PI) decrease exponentially at high temperatures. In this work, the “isomer-like” of common PI (i-PI) is prepared by optimizing the structure of the repeating unit of common PI. Compared with the common PI film (U_e of 0.59 J cm⁻³ and η of 24% at 150 °C), the i-PI film obtains a high U_e of 3.48 J cm⁻³ with an η of 54% at 150 °C. The adjustment of the position of the ether bond reduces the interchain space of the polymer, which decreases the free volume for electron acceleration resulting in a better insulation performance. Then, flexible thermoplastic polyurethane (TPU) with high dielectric permittivity is introduced into i-PI, which further reduces the interchain space. The obtained TPU/i-PI film achieves a synergistic improvement in dielectric permittivity and breakdown strength. At 150 °C, the high U_e of 4.23 J cm⁻³ with η of 73% is achieved. Compared with those of common PI, U_e and η of TPU/i-PI film are improved by 617% and 204%, respectively. Therefore, reducing the interchain space by optimizing the polymer chains is a feasible method to enhance the high-temperature capacitive performance.