Electrolyte-Directed Reactions of the Oxygen Electrode in Lithium-Air Batteries

Abstract

Non-aqueous electrolytes play a prominent role in the redox reactions of the oxygen electrode in the non-aqueous Li-air battery. In all electrolytes the initial O2 reduction reaction (ORR) product is superoxide, O2−, whose stability is determined by the Lewis acidity of the ion-pairing cations present in the medium as governed by the Hard Soft Acid Base (HSAB) theory. Our results suggest that depending on the basicity of the solvent as measured by its Donor Number (DN), the superoxide will be stabilized to varying lengths of time before transforming to O22− via a chemical or an electrochemical reaction. Organic solvents decrease the Lewis acidity of Li+ through the formation of solvates, Li+(solvent)n. As a result, high DN solvents such as dimethyl sulfoxide (DMSO), which decrease Li+ acidity more than low DN solvents, provide longer life-time for the soft base O2− by forming the ion pair, Li+(DMSO)n–O2−. In low DN solvents such as 1, 2-dimethoxy ethane (DME) and tetraethyleneglycol dimethyl ether (TEGDME), the LiO2 rapidly decomposes to the peroxide, Li2O2. As the ionic bond strength between the O2 reduction product and the conducting salt cation becomes stronger, its rechargeability becomes poorer. Solvent electron donor property affects ORR catalysis also.