Elucidating the Mechanism of Oxygen Reduction for Lithium-Air Battery Applications

Abstract

Unlocking the true energy capabilities of the lithium metal negative electrode in a lithium battery has until now been limited by the low capacity intercalation and conversion reactions at the positive electrodes. Abraham et al. (Abraham, K. M.; Jiang, Z. J. Electrochem. Soc. 1996, 143, 1−5) overcame this limitation by removing these electrodes and allowing lithium to react directly with oxygen in the atmosphere, forming the Li-air battery. The Li/O2 battery redox couple has a theoretical specific energy of 5200 W h/kg and represents the ultimate, environmentally friendly electrochemical power source. In this work, we report for the first time the intimate role of electrolyte, in particular the role of ion conducting salts, in determining the reversibility and kinetics of oxygen reduction in nonaqueous electrolytes designed for such applications. Such fundamental understanding of this high energy density battery is crucial to harnessing its full energy potential. The kinetics and mechanisms of O2 reduction in solutions of hexafluorophosphate of the general formula A+PF6−, where A = tetrabutylammonium (TBA), K, Na, and Li, in acetonitrile are reported on glassy carbon electrodes using cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques. The results show that the cations in the electrolyte strongly influence the reduction mechanism of O2. Larger cations represented by TBA salts displayed reversible O2/O2− redox couple, in contrast to those containing the smaller Li (and other alkali metal) cations, where an irreversible one-electron reduction of O2 to LiO2, and other alkali metal superoxides, is shown to occur as the first process. It was also found the LiO2 formed initially decomposes to Li2O2. Electrochemical data support the view that alkali metal oxides formed via electrochemical and chemical reactions passivate the electrode surface, making the processes irreversible. The O2 reduction mechanisms in the presence of the different cations have been supplemented by kinetic parameters determined from detailed analyses of the CV and RDE data. The Lewis acid characteristics of the cation appear to be crucial in determining the reversibility of the system. The results of this study are expected to contribute to the rapid development of the Li-air battery.