Multiple Roles of Superoxide on Oxygen Reduction Reaction in Li+-Containing Nonaqueous Electrolyte: Contribution to the Formation of Oxide as Well as Peroxide

Oxygen reduction reaction (ORR) on carbon in lithium-ion-containing nonaqueous electrolytes is supposed to follow a three-step mechanism: oxygen (O 2 0 ) to superoxide (O 2 – or LiO 2 ) to peroxide (O 2 2– or Li 2 O 2 ) to oxide (O 2– or Li 2 O). This work attempts to solve three controversial issues: (1) whether the superoxide is really formed; (2) whether the superoxide can be stable or immediately converts to peroxide; (3) whether the formation of oxide is feasible at the typical discharge potentials of lithium–air battery or the discharge product is only peroxide. ORR on carbon was studied in LiClO 4 or LiPF 6 containing dimethyl sulfoxide. The staircase cyclic voltammetry combined with Fourier transform electrochemical impedance spectroscopy (SCV-FTEIS) was used for the in situ investigation of ORR during potential scans. Oxygen was quasi-reversibly reduced to superoxide in the first step. Superoxide showed chemical stability in electrolyte. The superoxide was further reduced to surface-adsorbed peroxide, and the reduction proceeded to produce oxide at higher overpotentials. We identified a novel chemical route resulting in oxide formation even at not-enough overpotentials: peroxide is reduced to oxide by superoxide as an in situ formed one-electron reducing agent. The electrolyte/electrode decomposition product, Li 2 CO 3, was observed only in electrolyte with LiClO 4 .