A unified model for conductivity, electric breakdown, energy storage, and discharge efficiency of linear polymer dielectrics

Polymer dielectric capacitors are widely used as high-power-density energy storage devices. However, their energy storage density is relatively low and they cannot meet the requirements for high temperature resistant and high energy density dielectric capacitors. In order to clarify the key factors affecting the energy storage performance and improve the energy storage density and energy efficiency synergistically, it is urgent to establish a unified model to simultaneously study the volt-ampere characteristics, space charge distribution, breakdown strength, discharged energy density, and charge-discharge efficiency of linear dielectrics. Based on the bipolar charge transport (BCT) model, we establish a unified model by a comprehensive consideration of charge injections from electrodes, carrier migration, trapping effects of exponentially distributed deep traps, and damage caused by energy gain. The BCT unified model is first used to simulate the breakdown strengths at different temperatures, the discharged energy densities, and charge-discharge efficiencies at different voltages and temperatures for biaxially oriented polypropylene (BOPP) film and SiO2-coated BOPP multilayer film. The simulation results are consistent with the experiments. They show that carrier injection and transport are key factors to determine the conductivity, electric breakdown, and energy storage performance for linear dielectrics. Coating a layer of SiO2 on BOPP film can increase the injection barrier and reduce the charge injection, which can reduce the conductivity and Joule heat, and can alleviate the electric field distortion, resulting in the improvement of the breakdown strength. Meanwhile, reducing the space charge accumulation during the charging process by suppressing the charge injection can elevate the voltage at the beginning of the discharging process, which can improve the discharged energy density and the charge-discharge efficiency of linear dielectric capacitors.