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 (E_b) of this composite material reaches 552 MV/m at 125 °C (PPO: 534 MV/m), and the discharged energy density (U_e) under E_b 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 U_e 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.