  Quantum chemical calculations have been employed to investigate the solvation
of lithium cations in ethylene carbonate/propylene carbonate and propylene
carbonate/dimethyl carbonate mixed electrolytes. The impact of the presence of
the counteranion on the solvation of Li+ in pure propylene carbonate and
dimethyl carbonate was also studied. The calculations revealed small
free-energy changes for the transitions between different preferred structures
in mixed solvents. This implies that transitions between distinct local
arrangements can take place in the mixtures. The addition of dimethyl carbonate
causes a significant increase of the dipole moment of solvation clusters,
indicating important molecular-scale modifications when dimethyl carbonate is
used as a co-solvent. The presence of an anion in the solvation shell of Li+
modifies the intermolecular structure comprising four carbonate molecules in
dilute solutions, allowing only two carbonate molecules to coordinate to Li+.
The bidentate complexation of Li+ with the anion electron donor atoms, however,
maintains the local tetrahedral structure on interatomic length scales. The
neutralization of the solvation shell of Li+ due to contact ion pair formation
and the consequent implications on the underlying mechanisms provide a rational
explanation for the ionic conductivity drop of electrolyte solutions at high
salt concentrations.
