Certain molten complexes of Li salts and solvents can be regarded as ionic liquids. In this study, the local structure of Li(+) ions in equimolar mixtures ([Li(glyme)]X) of glymes (G3: triglyme and G4: tetraglyme) and Li salts (LiX: lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]), lithium bis(pentafluoroethanesulfonyl)amide (Li[BETI]), lithium trifluoromethanesulfonate (Li[OTf]), LiBF4, LiClO4, LiNO3, and lithium trifluoroacetate (Li[TFA])) was investigated to discriminate between solvate ionic liquids and concentrated solutions. Raman spectra and ab initio molecular orbital calculations have shown that the glyme molecules adopt a crown-ether like conformation to form a monomeric [Li(glyme)](+) in the molten state. Further, Raman spectroscopic analysis allowed us to estimate the fraction of the free glyme in [Li(glyme)]X. The amount of free glyme was estimated to be a few percent in [Li(glyme)]X with perfluorosulfonylamide type anions, and thereby could be regarded as solvate ionic liquids. Other equimolar mixtures of [Li(glyme)]X were found to contain a considerable amount of free glyme, and they were categorized as traditional concentrated solutions. The activity of Li(+) in the glyme-Li salt mixtures was also evaluated by measuring the electrode potential of Li/Li(+) as a function of concentration, by using concentration cells against a reference electrode. At a higher concentration of Li salt, the amount of free glyme diminishes and affects the electrode reaction, leading to a drastic increase in the electrode potential. Unlike conventional electrolytes (dilute and concentrated solutions), the significantly high electrode potential found in the solvate ILs indicates that the solvation of Li(+) by the glyme forms stable and discrete solvate ions ([Li(glyme)](+)) in the molten state. This anomalous Li(+) solvation may have a great impact on the electrode reactions in Li batteries.

Raman spectra of 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide [C 2 mIm + ][FSA − ] ionic liquid solutions dissolving LiFSA salt of various concentrations were measured at 298 K. FSA − ((FSO 2 ) 2 N − ) is an analogue anion of bis(trifluoromethanesulfonyl)amide ((CF 3 SO 2 ) 2 N − ; TFSA − ). We found that a solvation number of the Li + ion in [C 2 mIm + ][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 x Li + < 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 [C 2 mIm + ][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.

The structure and interactions of different (Li salt + glyme) mixtures, namely equimolar mixtures of lithium bis(trifluoromethylsulfonyl)imide, nitrate or trifluoroacetate salts combined with either triglyme or tetraglyme molecules, are probed using Molecular Dynamics simulations. structure factor functions, calculated from the MD trajectories, confirmed the presence of different amounts of lithium-glyme solvates in the aforementioned systems. The MD results are corroborated by S(q) functions derived from diffraction and scattering data (HEXRD and SAXS/WAXS). The competition between the glyme molecules and the salt anions for the coordination to the lithium cations is quantified by comprehensive aggregate analyses. Lithium-glyme solvates are dominant in the lithium bis(trifluoromethylsulfonyl)imide systems and much less so in systems based on the other two salts. The aggregation studies also emphasize the existence of complex coordination patterns between the different species (cations, anions, glyme molecules) present in the studied fluid media. The analysis of such complex behavior is extended to the conformational landscape of the anions and glyme molecules and to the dynamics (solvate diffusion) of the bis(trifluoromethylsulfonyl)imide plus triglyme system.

Liquids with no ions! Raman analysis and quantum calculations suggest that electrically neutral molecular species predominantly exist in an N-methylimidazole and acetic acid equimolar mixture, and that ionic species are rather minor. Nevertheless, the mixture has significant ionic conductivity, and shows "good ionic" or "superionic" behavior (see figure). It may be suitable to call such liquids "pseudo-ionic liquids" rather than "ionic liquids".