The chemical stability of Li metal in a rechargeable Li-O-2 cell was examined by investigating the physicochemical changes that occurred during storage of Li in an electrolyte comprising dimethyl sulfoxide (DMSO) with dissolved O-2. During prolonged storage of Li in the oxygenated electrolyte, the Li surface became moss-like and its interfacial resistance increased. Analyses of reaction products using XPS and FT-IR revealed that the bis(trifluoromethyl sulfonyl) imide (TFSI) anions and DMSO solvent could have decomposed significantly through a further reaction path induced by O-2. Furthermore, the formation of an unstable solid-electrolyte interphase by O-2 causes degradation of the Li metal and deterioration of the electrolyte. This investigation shows that the Li anode should be protected from O-2 by modification of Li and the separator to ensure long-term stability of the Li-O-2 cell. (C) 2014 Elsevier Ltd. All rights reserved.