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 (ri) of 90% was improved from similar to 2.9 J/cm3 for amorphous PLLA to similar to 5.7 J/cm3 for the crystallized PLLA film at room temperature. Under 200 MV/m at 85 degrees C (the operation conditions of commercial biaxially oriented polypropylene-based capacitors in HEVs), Urec of 0.82 J/cm3 with ri 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 degrees 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.