High-Temperature Energy Storage Performance of Polar Small-Molecule Graft-Modified Polypropylene Films

With the rapid advancement of new energy technologies and the trend towards higher power density in power electronic equipment, there is an increasing demand for dielectric capacitors to reliably store energy under high-temperature and high-electric-field conditions. Polypropylene (PP) film is widely utilised due to its low dielectric loss and high breakdown strength. However, its electrical conductivity exhibits exponential growth at elevated temperatures, leading to deterioration in insulation strength and a significant decline in energy storage performance. This severely limits its application in high-temperature operating environments. To enhance the high-temperature energy storage stability of PP film, this study employs an ultraviolet irradiation grafting process to graft acrylic acid (AA) monomers onto the PP backbone. Using benzophenone (BP) as the photoinitiator, the systematic investigation examines the influence of UV irradiation power on the microstructure and dielectric properties of PP-g-AA capacitor film. Moderate irradiation power effectively promotes the grafting reaction of acrylic acid molecules, enhancing the polarisation modification efficiency of polypropylene chains. This results in a denser, more ordered crystalline structure, thereby improving the film’s energy storage capacity and electrical field tolerance. However, excessively high irradiation power induces β-scission and oxidative degradation in polypropylene chains, shortening chain length and reducing regularity. This disrupts crystalline domain integrity, diminishes film crystallinity, and increases interfacial defects, consequently elevating dielectric loss and reducing breakdown strength. Experimental results demonstrate that under moderate irradiation power conditions, the grafted film maintains excellent energy storage performance in high-temperature environments. At 125 °C, its discharge energy density reaches 2.52 J/cm3, with charge-discharge efficiency exceeding 95%.