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 the 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. It shows 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 discharging process, which can improve the discharged energy density and the charge-discharge efficiency of the linear dielectric capacitors.

To simoutinusly achieve improved breakdown strength (BDS) and suppressed dielectric loss, a modified ternary nanocomposite composed of polypropylene (PP), maleic anhydride‐grafted PP and core‐shell structure SiO2@ZrO2 nanoparticles is proposed. By tailoring the thickness of SiO2 shell from 3 to 11 nm and the nanofillers content from 0.5 to 5 wt%, the particles disparsion is improved and meanwhile both the deep trap density and the interaction force between molecules are enhanced. In consequence, the restricted transportation of electrons along molecule chains as well as between chains together raise the DC BDS, that the BDS of optimum sample film reaches 430.8 kV/mm, 23% higher than neat PP. Besides, the ternary nanocomposites with SiO2@ZrO2 less than 3 wt% maintain the tanδ below 0.3% at a broad frequency ranging from 0.1 Hz to 100 kHz, which is an extremely low level same as neat PP.

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To improve the energy storage density while maintaining low dielectric loss is crucial for the miniaturization of capacitors. In the present study, we proposed a ternary nanocomposite system comprised of polypropylene (PP), acrylic acid grafted polypropylene (PP-g-AA), and nano-ZrO2. The PP-g-AA was firstly prepared by a free radical grafting with the proportion fixed to 50 wt%, and the content of nano-ZrO2 ranged from 0.5 wt% to 5 wt% to tailor the ternary composite. The transmission electron microscope (TEM) observation showed that PP-g-AA could significantly benefit the dispersion of nanoparticles and the differential scanning calorimetry (DSC) results indicated the superior compatibility of the ternary nanocomposites. Accordingly, with the optimum nano-ZrO2 content of 0.5 wt%, the ternary nanocomposites achieved the permittivity of 2.7 and DC breakdown strength of 456.2 kV/mm, resulting in the enhanced energy density of 2.77 J/cm3 which was 60% higher than neat PP, while the dielectric loss was kept lower than 0.003. Moreover, even at the temperature of 120°C, the ternary nanocomposites maintained a high-performance energy storage density of 2.28 J/cm3 (with energy storage efficiency above 90%) , which was 670% of the neat PP.

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