Cation−Anion Interactions in 1-Ethyl-3-Methylimidazolium Trifluoromethanesulfonate-Based Ionic Liquid Electrolytes

An important step in developing ionic-liquid-based electrolytes for lithium rechargeable batteries is obtaining
a molecular-level understanding of the ionic interactions that occur in these systems. In this study, 1-ethyl-3-methylimidazolium trifluoromethansulfonate ([C2mim]CF3SO3) is complexed with LiCF3SO3, and the local
structures of the CF3SO3- and [C2mim]+ ions are investigated with infrared and Raman spectroscopy. The
isolation and subsequent refinement of a Li[C2mim](CF3SO3)2 crystal provides further insight into the structure
of the [C2mim]CF3SO3−LiCF3SO3 solutions. Minor changes are observed in the infrared and Raman spectra
of dilute [C2mim]CF3SO3−LiCF3SO3 solutions compared to pure [C2mim]CF3SO3. However, a suspension
of very small Li[C2mim](CF3SO3)2 crystallites forms at a solution composition of [C2mim]CF3SO3:LiCF3SO3 = 10:1 (mole ratio), placing an upper limit on the solubility of LiCF3SO3. Essentially no changes are
observed in the vibrational modes of the [C2mim]+ cations over the entire range of LiCF3SO3 compositions
studied, suggesting that the addition of these compounds does not significantly perturb the local structure of
the [C2mim]+ cations. The salt used in this study has a common anion with the ionic liquid; thus, the ion
cloud surrounding the [C2mim]+ ions, which must be primarily composed of CF3SO3- anions, is not significantly
altered with the addition of LiCF3SO3.