Growing Li2O2 surfaces on discharge cause electrolyte degradation and capacity loss in Li–O2 batteries

Abstract

Ether-based solvents have shown promise as the most stable candidates for the electrolyte solution in the Li–O₂ battery. However, the yield of Li₂O₂ after discharge is less than 100%, despite achieving a near-ideal ratio of charge passed to O₂ consumed (2e⁻/O₂) for the reduction of O₂ to Li₂O₂. The loss of Li₂O₂ leads to the observed capacity fade on cycling and is associated with electrolyte degradation. Here we investigate the chemical formation of Li₂O₂ from superoxide in two commonly used ether solutions, dimethoxyethane and tetraethylene glycol dimethyl ether. The results indicate that it is the freshly growing Li₂O₂ surface reacting with the electrolyte solution that is the dominant source of Li₂O₂ loss and electrolyte degradation. Additionally, we quantify common side products from the degradation, including Li₂CO₃, HCO₂Li, CH₃CO₂Li and H₂O, and identify the composition of ethylene oxides formed during the reaction.