Polypropylene-Based Dielectrics for High-Temperature and High-Field Capacitive Energy Storage via Radiation Melt Grafting Synergies

At present, there is an immediate necessity for the development of polymer-based capacitor films that can maintain excellent energy storage performance in harsh environments, such as elevated temperatures and powerful electric fields. In this study, polypropylene grafted hydroxyethyl methacrylate (PP-g-PHEMA) composite films with varying grafting contents were successfully prepared using high-energy electron beam irradiation and the melt grafting method, which significantly enhanced the dielectric and high-temperature energy storage properties of the polymer films. Experimental tests and computational simulations confirmed that grafted PHEMA introduced deeper trap energy levels and restricted the migration of many charge carriers, while diminishing the impact of high-energy electrons on the molecules, thus enhancing the breakdown strength and energy storage density of the films. The results indicated that the PP-g-PHEMA composite films exhibited higher dielectric constants and lower dielectric losses compared to the pure PP films. Meanwhile, the PP-g-PHEMA (2 wt %) composite film had an energy density of 5.68 J/cm3 and a charge/discharge efficiency of more than 95% at 640 MV/m. Moreover, after 50,000 charge–discharge cycles at 400 MV/m, it maintained an energy density of 2.6 J/cm3 and a charge/discharge efficiency of 98%. Notably, the grafting-modified dielectric exhibited significantly improved dielectric and capacitive energy storage properties at elevated temperatures. At 120 °C, the PP-g-PHEMA (2 wt %) film still achieved a high discharge energy density of 3.81 J/cm3, which was 136% of that of the pure PP film, and maintained excellent cyclic dielectric stability at 400 MV/m. This research provides a straightforward, safe, and efficient method for the design and development of PP-based dielectrics to enhance their high-temperature energy storage capabilities.