Characteristics of Lithium Ions and Superoxide Anions in EMI-TFSI and Dimethyl Sulfoxide

To
clarify the microscopic effects of solvents on the formation
of the Li+-O2– process of a Li–O2 battery, we studied
the kinetics and thermodynamics of these ions in dimethyl sulfoxide
(DMSO) and 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide
(EMI-TFSI) using classical molecular dynamics simulation. The force
field for ions–solvents interactions was parametrized by force
matching first-principles calculations. Despite the solvation energies
of the ions are similar in both solvents, their mobility is much higher
in DMSO. The free-energy profiles also confirm that the formation
and decomposition rates of Li+-O2– pairs are greater in DMSO than
in EMI-TFSI. Our atomistic simulations point out that the strong structuring
of EMI-TFSI around the ions is responsible for these differences,
and it explains why the LiO2 clusters formed in DMSO during
the battery discharge are larger than those in EMI-TFSI. Understanding
the origin of such properties is crucial to aid the optimization of
electrolytes for Li–O2 batteries.