To Break or not to break: Mechanisms of DMSO decomposition in aprotic LiO2 battery electrolytes

Paper in proceeding, 2016

Aprotic Li-O2 batteries offer an appealing opportunity to make use of our immediate environment; harvesting the air for oxygen and further reducing and combining it with Li+ to form LiO2 or Li2O2 at the cathode/electrolyte interface. Although the electrochemistry of such a device could in principle be operated reversibly, side-reactions interfere with the main reactions and limit the lifetime of practical Li-O2 cells – so far operated only in pure O2. Preventing these parasitic reactions, in particular by developing more stable solvents/electrolytes, is critical for progress. Dimethyl sulfoxide (DMSO) is a promising solvent for Li-O2 battery applications [1], but there are conflicting opinions on the long-term stability of DMSO. Experimental work by Kwabi et al. [2] and computational results by Laino et al. [3] suggest that DMSO is readily oxidized to dimethyl sulfone (DMSO2) at Li2O2 surfaces – also forming LiOH. These results have, however, recently been challenged by Schroeder et al. [4], claiming that DMSO is sufficiently stable in the presence of Li2O2, as long as there are no sources of acidic protons present (e.g. from water impurities or carbon electrodes) 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 make use of quantum chemistry calculations to model alternative DMSO decomposition mechanisms in gas, solution phase, and at surfaces. We present reaction energies and barriers to reactions for proton abstraction (DMSO-H), methyl abstraction (DMSO-CH3), and addition reactions (DMSO2) with the aim of better understanding the relative importance of different reaction pathways and the impact of the reactants immediate environment.