Effect of Solvents on the Behavior of Lithium and Superoxide Ions in Lithium-Oxygen Battery Electrolytes.

The molecular life of intermediates, namely, O2- and Li+ , produced during the discharge of aprotic Li-O2 batteries was investigated by molecular dynamics simulation. This work is of potential interest in the development of new electrolytes for Li-air batteries. We present the results on the structure and stability of the Li+ and O2- solvation shells and the thermodynamics and kinetics of the ion-association reaction in solvents such as dimethyl sulfoxide (DMSO), dimethoxyethane (DME), and acetonitrile (ACN). The residence time of solvent molecules in the Li+ solvation shell increases with the solvent donor number and is 100 times larger in DMSO than in ACN. In DMSO and DME, the Li+ ion diffuses with its solvation shell as a whole. On the contrary, in ACN it diffuses as a "bare" ion because of weak solvation. The rate constant for the association of the lithium ion with the superoxide anion in DMSO is two orders of magnitude slower than that in ACN due to fact that the free-energy barrier is 2.5 times larger in DMSO than in ACN. In addition, we show that despite the strong dependence of the Li+ shell stability on donor number, the rate of association does not necessarily correlate with this solvent property.