Biaxially oriented films of grafted-polypropylene with giant energy density and high efficiency at 125 °C

Abstract

The urgent demand for next-generation high-temperature film capacitors with excellent energy storage properties originates from electrical-power applications under harsh environments. However, the state-of-the-art commercial capacitor dielectric biaxially oriented polypropylene (BOPP) suffers from a rapid decline in energy density and charge–discharge efficiency at elevated temperatures due to remarkably increased leakage current and reduced breakdown strength. Here, we report a scalable high-temperature capacitor film based on grafted polypropylene, in which the polar methyl methacrylate group is uniformity grafted by a facile and green water-solid phase suspension grafting method. The polypropylene is continuously grafted, melt cast, extruded, and biaxially stretched to form thin and uniform capacitor films, which are compatible with the present capacitor film industrial production line with high productivity. The grafted group keeps excellent interaction with the polypropylene during the biaxial stretching process without producing any perceptible defects. The experimental result and density functional theory simulation indicate that the grafted group can serve as a deep trap impeding the transit of charge carriers, leading to a one-order-of-magnitude decrease in the leakage current and a remarkable increase in breakdown strength from 537 MV m⁻¹ of commercial BOPP to 764 MV m⁻¹ of the grafted polypropylene at 125 °C. Consequently, the maximum discharged energy density above 90% efficiency of the grafted polypropylene (4.5 J cm⁻³) is significantly enhanced with respect to commercial BOPP (0.1 J cm⁻³). The approach reported in this work provides the possibility for large-scale production of capacitor films with high energy storage density under elevated temperature and high electric field.