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 degrees 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 (eta) degrades greatly under high fields at 125 degrees 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 degrees C (PPO: 534 MV/m), and the discharged energy density (U-e) under E-b improves to 3.5 J/cm(3) (PPO: 2.5 J/cm(3)), with a significantly enhanced eta of 89% (PPO: 70%). Furthermore, SiO2/PPO/SiO2 can discharge a U-e of 0.45 J/cm(3) with an eta of 97% at 125 degrees C under 200 MV/m (working condition in hybrid electric vehicles) for 20,000 cycles, and this value is higher than the energy density (similar to 0.39 J/cm(3) 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.