High energy density of polyimide films employing an imidization reaction kinetics strategy at elevated temperature

Abstract

Polymer dielectrics have been widely used in electrical energy storage devices. However, the relatively low operating temperature hinders their applications in harsh environments. Herein, the molecular structure of polyimide (PI) is optimized by adjusting the reaction kinetics of polyamic acid, which improves the energy storage performance of PI at elevated temperature. The PI film with an optimal imidization degree achieves a simultaneous increase in dielectric permittivity (ε_r) and breakdown strength (E_b), resulting in a maximum discharged energy density (U_e) of 6.9 J cm⁻³ with a charge–discharge efficiency of 90.0% at room temperature and a high U_e of 3.9 J cm⁻³ at 150 °C. The introduced –COOH/–CN–OH– polar groups increase the ε_r through the enhancement of dipole polarization. Moreover, an appropriate number of –COOH/–CN–OH– groups as deep traps reduce the mobility of carriers, thereby increasing the E_b. Finite element simulation reveals that PI with an appropriate imidization degree exhibits suppressed space charge accumulation and improved electric field distortion. This method reduces the PI processing temperature, which effectively reduces the production energy consumption. For verifying the universality of the design strategy, two other PIs prepared from different monomers are also demonstrated to have simultaneous improvement in ε_r and E_b.Finally, this production process of only reducing the preparation temperature without adjusting the original commercial PI production equipment is crucial for the practical application of capacitor films in the future.