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 ([C(2)mim]CF3SO3) is complexed with LiCF3SO3, and the local structures of the CF3SO3- and [C(2)mim](+) ions are investigated with infrared and Raman spectroscopy. The isolation and subsequent refinement of a Li[C(2)mim](CF3SO3)(2) crystal provides further insight into the structure of the [C(2)mim]CF3SO3-LiCF3SO3 solutions. Minor changes are observed in the infrared and Raman spectra of dilute [C(2)mim]CF3SO3-LiCF3SO3 solutions compared to pure [C(2)mim]CF3SO3. However, a suspension of very small Li[C(2)mim](CF3SO3)(2) crystallites forms at a solution composition of [C(2)mim]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 [C(2)mim](+) 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 [C(2)mim](+) cations. The salt used in this study has a common anion with the ionic liquid; thus, the ion cloud surrounding the [C(2)mim](+) ions, which must be primarily composed of CF3SO3 anions, is not significantly altered with the addition of LiCF3SO3.