Unusual Li⁺Ion Solvation Structure in Bis(fluorosulfonyl)amide Based Ionic Liquid

Abstract

Raman spectra of 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide [C2mIm+][FSA–] ionic liquid solutions dissolving LiFSA salt of various concentrations were measured at 298 K. FSA– ((FSO2)2N–) is an analogue anion of bis(trifluoromethanesulfonyl)amide ((CF3SO2)2N–; TFSA–). We found that a solvation number of the Li+ ion in [C2mIm+][FSA–] is 3, though it has been well established that Li+ ion is solvated by two TFSA– anions in the corresponding ionic liquids below the Li+ ion mole fraction of xLi+ < 0.2. To yield further insight into larger solvation numbers, Raman spectra were measured at higher temperatures up to 364 K. The Li+ ion solvation number in [C2mIm+][FSA–] evidently decreased when the temperature was elevated. Temperature dependence of the Li+ ion solvation number was analyzed assuming an equilibrium between [Li(FSA)2]− and [Li(FSA)3]2–, and the enthalpy ΔH° and the temperature multiplied entropy TΔS° for one FSA– liberation toward a bulk ionic liquid were successfully evaluated to be 35(2) kJ mol–1 and 29(2) kJ mol–1, respectively. The ΔH° and ΔS° suggest that the Li+ ion is coordinated by one of bidentate and two of monodentate FSA– at 298 K, and that the more weakly solvated monodentate FSA– is liberated at higher temperatures. The high-energy X-ray diffraction (HEXRD) experiments of these systems were carried out and were analyzed with the aid of molecular dynamics (MD) simulations. In radial distribution functions evaluated with HEXRD, a peak at about 1.94 Å appeared and was attributable to the Li+–O(FSA–) correlations. The longer Li+–O(FSA–) distance than that for the Li+–O(TFSA–) of 1.86 Å strongly supports the larger solvation number of the Li+ ions in the FSA– based ionic liquids. MD simulations at least qualitatively reproduced the Raman and HEXRD experiments.