Effect of Cation Size on Solvation and Association with Superoxide Anion in Aprotic Solvents.

Influence of cation size on solvation strength, diffusion, and kinetics of the association reaction with anions O2- in aprotic solvents, such as acetonitrile and dimethyl sulfoxide, has been investigated by means of molecular dynamics simulations. The work is motivated by the need to understand the molecular nature of the solvent-induced changes in capacity of Li-air batteries. We have shown that the dependence of the solvation shell stability on the cation size has a maximum at a particular ion radius that corresponds to a solvent coordination number of 4. The shell stability maximum coincides with the diffusion coefficient minimum. The variation of the cation shell stability has a crucial impact on the kinetics of the cation-O2- association. We have demonstrated that profound inhibition of the association reaction for Li+ in dimethyl sulfoxide is a result of the lock-and-key effect that cannot be described in the framework of Hard Soft Acid Base theory.

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.