Electrochemical studies of lithium-oxygen reactions for lithium-air battery applications

Abstract

Fundamentally understanding reaction mechanisms during Li-O₂ cell operation is critical for implementing Li-air batteries with high reversibility and long cycle life. In this thesis, the rotating ring disk electrode (RRDE) technique has been used to probe the influence of different electrolyte solvents on the stability of the superoxide radical produced on planar glassy carbon and Au electrodes. It was found that the fraction of oxygen reduction reaction current attributable to superoxide generation exhibits a solvent-invariant potential dependence on carbon, with a higher fraction of superoxide produced at lower discharge overpotentials. This trend is in support of a proposed growth model for different Li-O₂ morphologies, where Li-O₂ growth is governed primarily by disproportionation of superoxide at low overpotentials and direct electron transfer at high overpotentials. On Au, superoxide stability exhibits a strong solvent dependence, which can be explained in terms of the effect of the electrolyte solvent basicity on the stability of the Li+-O₂- ion pair. This study highlights the potential use of RRDE as a tool to gain insights into Li-O₂ reaction and growth mechanisms and the contribution of soluble intermediate species to parasitic reactions in practical Li-air batteries.