Instability of Poly(ethylene oxide) upon Oxidation in Lithium-Air Batteries

The instability of aprotic and polymer electrolytes in Liair batteries limits the development of these batteries for practical use. Here, we investigate the stability of an electrolyte based on poly(ethylene oxide) (PEO), which has been used extensively for polymer Li-ion batteries, during discharge and charge of LiO2 batteries. We show that applying potentials greater than open circuit voltage (OCV, similar to 3 VLi), which is typically required for Li-O-2 battery charging, increases the rate of PEO auto-oxidation in an oxygenated environment, with and without prior discharge. Analysis on the rate of reaction, extent of oxidation, and the oxidation products allows us to propose that rate of spontaneous radical formation in PEO is accelerated at applied potentials greater than OCV. We also suggest that the phenomena described here will still occur in ether-based electrolytes at room temperature, albeit at a slower rate, and that this will prevent the use of such electrolytes for practical long-lived Li-air batteries. Therefore, PEO-based electrolytes are unsuitable for use in Li-air batteries.