The realization of secondary lithium-oxygen batteries (Li-O-2 batteries, LOBs) with large gravimetric energy density requires the development of an innovative electrolyte with high chemical stability that allows the charge-discharge reaction to proceed with low overvoltage. In this study, we evaluated the potential of an electrolyte solvent, N,N-dimethylethanesulfonamide (DMESA) with a sulfonamide functional group, at a current density of 0.4 mA cm(-2) and a capacity of 4 mA h cm(-2). The voltage at which CO2 was generated during charging was substantially higher than that of a tetraglyme (G4)-based electrolyte with redox mediators, which is one of the standard electrolytes used for LOBs. Experiments using a C-13-containing positive electrode revealed that CO2 generated during charging mainly originated from the decomposition of the positive electrode. The analyses of the charging profile in conjunction with differential electrochemical mass spectrometry suggested the formation of highly degradable lithium peroxide (Li2O2) in the DMESA-based electrolyte. The formation of highly degradable Li2O2 enables a reduction of the charging voltage, leading to further suppression of the electrolyte decomposition.