To Break or Not to Break: Solution and Surface Mechanisms of DMSO Decomposition in Aprotic Li-O2 Battery Electrolytes

Abstract

The aprotic Li-O2 battery offers an appealing opportunity to make use of atmospheric oxygen (together with metallic lithium) to achieve batteries with truly remarkable energy densities. However, oxygen is reduced to superoxides and peroxides, which react irreversibly with most materials; non-aqueous electrolytes, binders, and cathodes – hindering stable and rechargeable battery operation. Therefore, preventing parasitic reactions, in particular developing stable electrolytes, is critical for progress. Dimethyl sulfoxide (DMSO) is a promising solvent for Li-O2 battery electrolytes. It has been scrutinized both experimentally and computationally, but there are conflicting opinions on the stability of DMSO. Experimental work by Kwabi et al. [1] and computational results by Laino et al. [2] suggest that DMSO is readily oxidized to dimethyl sulfone (DMSO2) at Li2O2 surfaces, also forming LiOH. These results have, however, been challenged by Schroeder et al. [3], claiming that DMSO is sufficiently stable in the presence of Li2O2 – as long as there are no other sources of acidic protons present that can initiate decomposition by forming more reactive hydroperoxy species. More fundamental research on the reaction mechanisms of DMSO with reduced oxygen species is needed to resolve this contradiction. In this work we have used DFT-MD to model DMSO decomposition by LiO2 in solution and by Li2O2 surfaces. We present reaction energies and barriers to reactions for alternative decomposition mechanisms; proton abstraction (DMSO-H), methyl abstraction (DMSO-CH3), and addition reactions (DMSO2) with the aim of better understanding the relative importance of surface and solution reactions, the reacting oxygen species, and the mechanisms of reaction.