Styrene Content Determined Charge Trap Modulation Towards Enhanced Dielectric Energy Storage Characteristics of Ethylene-Styrene Gradient Copolymers

Film capacitors are widely used in high-voltage transmission converter stations, new energy vehicles, advanced electromagnetic weapons and other fields because of their superior characteristics of high withstand voltage, high instantaneous charging and discharging current, and long service life. However, the relatively low energy density as well as high dielectric loss limit the development of film capacitors towards miniaturization and lightweight. Ethylene/styrene gradient copolymer, reported as a high-energy-density dielectric material, is expected to be promoted in engineering applications due to its low production cost and good homogeneity. In this paper, based on the structures of ethylene/styrene gradient copolymers, we focus on modulating the benzene ring density by varying the styrene content, which affects the polymer's trap density, towards excellent electrical breakdown and energy storage properties. With a styrene content of 35 mol%, an energy storage density of 3.0 J/cm³ as well as a charge-discharge efficiency of 81.2% are realized under the condition of 450 kV/mm, room temperature, which is comparable to the commercially applied Biaxially Oriented Polypropylene. Furthermore, we utilized molecular dynamics simulations to elucidate the energy storage properties of gradient copolymers with varied styrene contents, and established the relationship between molecular structures and trap densities, revealing the mechanism by which the gradient polymeric structure can enhance the breakdown field strength and energy storage density. This work facilitates the use of theoretical simulations in order to probe high-performance molecular chain structures, contributing the commercialization and maturation of gradient copolymers.