Significantly Enhancing the Energy-Storage Properties of Polypropylene Films by Physically Manipulating Their Permittivity and Crystalline Behavior with Polar Organic Molecules

Abstract

To meet the increasing demands of modern power electronics for high-temperature resistance and energy storage performance and avoid the trade-off between high energy storage (U_e) performance and prominent processability, a strategy to modify polypropylene (PP) by introducing polar electron-deficient 8-hydroxyquinoline (8-HQ) physically during melt extrusion granulation is proposed. 8-HQ molecules are initially designed to capture charges injected under a high electric field and depress the leakage current density. Unexpectedly, they are found to reside at PP grain boundaries, promoting grain growth and thereby enhancing PP films' mechanical strength. Both effects may address the enhanced breakdown strength (E_b) up to 814 MV m⁻¹. Besides, 8-HQ increases the permittivity of modified PP films. Due to simultaneously enhanced E_b and dielectric constant, an impressive U_e of 9.87 J cm⁻³ with a discharge efficiency above 90% is obtained in the optimal sample, and an U_e of 6.96 J cm⁻³ at 83% efficiency is well retained up to 125 °C, far exceeding the previously reported results. This study offers a novel strategy to modify PP film physically by manipulating its crystalline behavior for high-pulse energy storage capacitor applications.