  Rechargeable lithium ion batteries are an attractive alternative power source
for a wide variety of applications. To optimize their performances, a complete
description of the solvation properties of the ion in the electrolyte is
crucial. A comprehensive understanding at the nanoscale of the solvation
structure of lithium ions in nonaqueous carbonate electrolytes is, however,
still unclear. We have measured by femtosecond vibrational spectroscopy the
orientational correlation time of the CO stretching mode of Li+-bound and
Li+-unbound ethylene carbonate molecules, in LiBF4, LiPF6, and LiClO4 ethylene
carbonate solutions with different concentrations. Surprisingly, we have found
that the coordination number of ethylene carbonate in the first solvation shell
of Li+ is only two, in all solutions with concentrations higher than 0.5 M.
Density functional theory calculations indicate that the presence of anions in
the first coordination shell modifies the generally accepted tetrahedral
structure of the complex, allowing only two EC molecules to coordinate to Li+
directly. Our results demonstrate for the first time, to the best of our
knowledge, the anion influence on the overall structure of the first solvation
shell of the Li+ ion. The formation of such a cation/solvent/anion complex
provides a rational explanation for the ionic conductivity drop of
lithium/carbonate electrolyte solutions at high concentrations.
