Multiple roles of superoxide on oxygen reduction reaction in Li+-containing nonaqueous electrolyte: Contribution to the formation of oxide as well as peroxide

Abstract

Oxygen reduction reaction (ORR) on carbon in lithium-ion-containing nonaqueous electrolytes is supposed to follow a three-step mechanism: oxygen (O2 0) to superoxide (O2 - or LiO2) to peroxide (O2 2- or Li2O2) to oxide (O2- or Li2O). 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 LiClO4 or LiPF6 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, Li2CO3, was observed only in electrolyte with LiClO4