The stability of the electrolyte solution toward superoxide radical ions (O2-) presents a particular challenge to the development of lithium-oxygen (Li-O2) batteries. Electrochemical oxygen reduction and evolution reactions (ORR/OER) on the surfaces of two electrodes, a gold electrode and a well-defined graphene electrode, in cyclic carbonate solvents of propylene carbonate (PC) and ethylene carbonate (EC), have been systematically investigated using electrochemistry in combination with sum frequency generation (SFG) vibrational spectroscopy, surface-enhanced Raman scattering (SERS), and UV-vis absorption spectroscopy, in comparison with the typical aprotic solvent dimethyl sulfoxide (DMSO), under in situ conditions. These measurements demonstrated that the cyclic carbonate solvents are unstable during the ORR. Neither O2- nor lithium peroxide (Li2O2) was found in these carbonate solutions during the ORR. Several ORR products of peroxodicarbonate (C2O62-) and lithium carbonate (Li2CO3) were observed on the electrode surface in the PC-based electrolyte solution, depending on whether or not the Li ion is included in the solution. The present comprehensive spectroscopic characterizations provide essential information for the reaction mechanism between the cyclic carbonates and O2- during the ORR.
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