Li-O2 Battery Degradation by Lithium Peroxide (Li2O2): A Model Study

The chemical stability of the Li-O-2 battery components (cathode and electrolyte) in contact with lithium peroxide (Li2O2) was investigated using X-ray photoelectron spectroscopy (XPS). XPS is a versatile method to detect amorphous as well as crystalline decomposition products of both salts and solvents. Two strategies were employed. First, cathodes including carbon, alpha-MnO2 catalyst, and Kynar binder (PVdF-HFP) were exposed to Li2O2 and LiClO4 in propylene carbonate (PC) or tetraethylene glycol dimethyl ether (TEGDME) electrolytes. The results indicated that Li2O2 degrades TEGDME to carboxylate containing species and that the decomposition products, in turn, degraded the Kynar binder. The alpha-MnO2 catalyst was unaffected. Second, Li2O2 model surfaces were kept in contact with different electrolytes to investigate the chemical stability and also the resulting surface layer on Li2O2. Further, the XPS experiments revealed that the Li salts such as LiPF6, LiBF4, and LiClO4 decomposed to form LiF or LiCl together with P-O or B-O bond containing compounds when exposed to Li2O2. PC decomposed to carbonate and ether based species. The degradation of the electrolytes increased from short to long exposure time indicating that the surface layer on Li2O2 became thicker by increasing time. Overall, it was shown that a mixture of ethylene carbonate and diethyl carbonate (EC/DEC) is more robust in contact with Li2O2 compared to PC.