Influence of Nonaqueous Solvents on the Electrochemistry of Oxygen in the Rechargeable Lithium−Air Battery

Abstract

A fundamental study of the influence of solvents on the oxygen reduction reaction (ORR) in nonaqueous electrolytes has been carried out for elucidating the mechanism of the oxygen electrode processes in the rechargeable Li−air battery. Using either tetrabutylammonium hexafluorophosphate (TBAPF6) or lithium hexafluorophosphate (LiPF6) electrolyte solutions in four different solvents, namely, dimethyl sulfoxide (DMSO), acetonitrile (MeCN), dimethoxyethane (DME), and tetraethylene glycol dimethyl ether (TEGDME), possessing a range of donor numbers (DN), we have determined that the solvent and the supporting electrolyte cations in the solution act in concert to influence the nature of reduction products and their rechargeability. In solutions containing TBA+, O2 reduction is a highly reversible one-electron process involving the O2/O2− couple. On the other hand, in Li+-containing electrolytes relevant to the Li−air battery, O2 reduction proceeds in a stepwise fashion to form O2−, O22−, and O2− as products. These reactions in the presence of Li+ are irreversible or quasi-reversible electrochemical processes, and the solvents have significant influence on the kinetics, and reversibility or lack thereof, of the different reduction products. The stabilization of the one-electron reduction product, superoxide (O2−) in TBA+ solutions in all of the solvents examined can be explained using Pearson’s hard soft acid base (HSAB) theory involving the formation of the TBA+---O2− complex. The HSAB theory coupled with the relative stabilities of the Li+−(solvent)n complexes existing in the different solvents also provide an explanation for the different O2 reduction products formed in Li+-conducting electrolyte solutions. Reversible reduction of O2 to long-lived superoxide in a Li+-conducting electrolyte in DMSO has been shown for the first time here. Our results provide a rational approach to the selection of organic electrolyte solutions for use in the rechargeable Li−air battery.