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.

References

[1] D.G. Kwabi et al., J. Phys. Chem. Lett. 5 (2014) 2850.

[2] T. Laino et al., New J. Phys. 15 (2013) 095009.

[3] M.A. Schroeder et al., ACS Appl. Mater. Interfaces. 7 (2015) 11402.

Figure Free energy of methyl abstraction reaction between DMSO and LiO₂ in solution phase (left). Highlight of transition state geometry (right). DFT-MD, PBE functional, T=353K.