Polyphenylene Oxide Film Sandwiched between SiO2 Layers for High-Temperature Dielectric Energy Storage

The commercial capacitor using dielectric biaxially oriented polypropylene (BOPP) can work effectively only at low temperatures (less than 105 °C). Polyphenylene oxide (PPO), with better heat resistance and a higher dielectric constant, is promising for capacitors operating at elevated temperatures, but its charge–discharge efficiency (η) degrades greatly under high fields at 125 °C. Here, SiO2 layers are magnetron sputtered on both sides of the PPO film, forming a composite material of SiO2/PPO/SiO2. Due to the wide bandgap and high Young's modulus of SiO2, the breakdown strength (Eb) of this composite material reaches 552 MV/m at 125 °C (PPO: 534 MV/m), and the discharged energy density (Ue) under Eb improves to 3.5 J/cm3 (PPO: 2.5 J/cm3), with a significantly enhanced η of 89% (PPO: 70%). Furthermore, SiO2/PPO/SiO2 can discharge a Ue of 0.45 J/cm3 with an η of 97% at 125 °C under 200 MV/m (working condition in hybrid electric vehicles) for 20,000 cycles, and this value is higher than the energy density (∼0.39 J/cm3 under 200 MV/m) of BOPP at room temperature. Interestingly, the metalized SiO2/PPO/SiO2 film exhibits valuable self-healing behavior. These results make PPO-based dielectrics promising for high-temperature capacitor applications.