Biodegradable Poly(l‑Lactic Acid) Films with Excellent Cycle Stability and High Dielectric Energy Storage Performance

Polymer-based film capacitors play key roles in numerous applications, such as converter/inverter systems in hybrid electric vehicles (HEVs), smart grids, and pulsed power sources. However, nearly all actively studied dielectric polymers are nondegradable. In this work, we prepare flexible biodegradable poly(l-lactic acid) (PLLA) films via a simple solvent casting method and achieve enhanced dielectric performances by polymer crystallization. The recoverable energy density (Urec) with charge–discharge efficiency (η) of 90% was improved from ∼2.9 J/cm3 for amorphous PLLA to ∼5.7 J/cm3 for the crystallized PLLA film at room temperature. Under 200 MV/m at 85 °C (the operation conditions of commercial biaxially oriented polypropylene-based capacitors in HEVs), Urec of 0.82 J/cm3 with η of 95% is achieved in the crystallized PLLA film, which is much higher than that of BOPP (below 0.5 J/cm3). In particular, the remarkable cyclic stability of the crystallized PLLA film is demonstrated by charge–discharge tests for 20 000 cycles at both room temperature and 85 °C under 200 MV/m. Moreover, the low C/(H+O) atom ratio helps metalized PLLA films exhibit a valuable self-healing ability after breakdown. With excellent recoverable energy density, high efficiency, good cyclic reliability, low-cost preparation method, self-healing ability, and eco-friendliness, the crystallized biodegradable PLLA film provides an eco-friendly and high-performance candidate to develop high-energy-storage capacitors

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