The structure of bis(trifluoromethanesulfonyl) imide (TFSI-) in the liquid state has been studied by means of Raman spectroscopy and DFT calculations. Raman spectra of 1-ethyl-3-methylimidazolium (EMI+) TFSI- show relatively strong bands arising from TFSI- at about 398 and 407 cm(-1). Interestingly, the 407 cm(-1) band, relative to the 398 cm(-1) one, is appreciably intensified with raising temperature, suggesting that an equilibrium is established between TFSI- conformers in the liquid state. According to DFT calculations followed by normal frequency analyses, two conformers of C2 and C1 symmetry, respectively, constitute global and local minima, with an energy difference 2.2-3.3 kJ mol(-1). The wagging omega-SO2 vibration appears at 396 and 430 cm(-1) for the C1 conformer and at 387 and 402 cm(-1) for the C2 one. Observed Raman spectra over the range 380-440 cm(-1) were deconvoluted to extract intrinsic bands of TFSI- conformers, and the enthalpy of conformational change from C2 to C1 was evaluated. The enthalpy value is in good agreement with that obtained by theoretical calculations. We thus conclude that a conformational equilibrium is established between the C1 and C2 conformers of TFSI- in the liquid EMI+TFSI-, and the C2 conformer is more favorable than the C1 one.

We demonstrate that Li + hopping conduction, which cannot be explained by conventional models i.e., Onsager's theory and Stokes' law, emerges in highly concentrated liquid electrolytes composed of LiBF 4 and sulfolane (SL). Self-diffusion coefficients of Li + (D Li ), BF 4 − (D BF 4 ), and SL (D SL ) were measured with pulsed-field gradient NMR. In the concentrated electrolytes with molar ratios of SL/LiBF 4 ≤ 3, the ratios D SL /D Li and D BF 4 /D Li become lower than 1, suggesting faster diffusion of Li + than SL and BF 4 − , and thus the evolution of Li + hopping conduction. X-ray crystallographic analysis of the LiBF 4 /SL (1:1) solvate revealed that the two oxygen atoms of the sulfone group are involved in the bridging coordination of two different Li + ions. In addition, the BF 4 − anion also participates in the bridging coordination of Li + . The Raman spectra of the highly concentrated LiBF 4 −SL solution suggested that Li + ions are bridged by SL and BF 4 − even in the liquid state. Moreover, detailed investigation along with molecular dynamics simulations suggests that Li + exchanges ligands (SL and BF 4 − ) dynamically in the highly concentrated electrolytes, and Li + hops from one coordination site to another. The spatial proximity of coordination sites, along with the possible domain structure, is assumed to enable Li + hopping conduction. Finally, we demonstrate that Li + hopping suppresses concentration polarization in Li batteries, leading to increased limiting current density and improved rate capability compared to the conventional concentration electrolyte. Identification and rationalization of Li + ion hopping in concentrated SL electrolytes is expected to trigger a new paradigm of understanding for such unconventional electrolyte systems.

The solvation structure of the lithium ion in room-temperature ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EMI(+)TFSI(-)) and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMP(+0TFSI(-)) has been studied by Raman spectroscopy and DFT calculations. Raman spectra of EMI(+)TFSI(-) and BMP(+)TFSI(-) containing Li(+)TFSI(-) over the range 0.144-0.589 and 0.076-0.633 mol dm(-3), respectively, were measured at 298 K. A strong 744 cm-1 band of the free TFSI(-) ion in the bulk weakens with increasing concentration of the lithium ion, and it revealed by analyzing the intensity decrease that the two TFSI(-) ions bind to the metal ion. The lithium ion may be four-coordinated through the O atoms of two bidentate TFSI(-) ions. It has been established in our previous work that the TFSI(-) ion involves two conformers of C(1) (cis) and C(2) (trans) symmetries in equilibrium, and the dipole moment of the C(1) conformer is significantly larger than that of the C(2) conformer. On the basis of these facts, the geometries and SCF energies of possible solvate ion clusters [Li(C(1)-TFSI(-))(2)](-), [Li(C(1)-TFSI(-))(C(2)-TFSI(-))](-), and [Li(C(2)-TFSI(-))(2)](-) were examined using the theoretical DFT calculations. It is concluded that the C(1) conformer is more preferred to the C(2) conformer in the vicinity of the lithium ion.

To develop a basic understanding of a new class of ionic liquids (ILs), "solvate" ILs, the transport properties of binary mixtures of lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) and oligoethers (tetraglyme (G4), triglyme (G3), diglyme (G2), and monoglyme (G1)) or tetrahydrofuran (THF) were studied. The self-diffusion coefficient ratio of the solvents and Li(+) ions (Dsol/DLi) was a good metric for evaluating the stability of the complex cations consisting of Li(+) and the solvent(s). When the molar ratio of Li(+) ions and solvent oxygen atoms ([O]/[Li(+)]) was adjusted to 4 or 5, Dsol/DLi always exceeded unity for THF and G1-based mixtures even at the high concentrations, indicating the presence of uncoordinating or highly exchangeable solvents. In contrast, long-lived complex cations were evidenced by a Dsol/DLi ∼ 1 for the longer G3 and G4. The binary mixtures studied were categorized into two different classes of liquids: concentrated solutions and solvate ILs, based on Dsol/DLi. Mixtures with G2 exhibited intermediate behavior and are likely the borderline dividing the two categories. The effect of chelation on the formation of solvate ILs also strongly correlated with electrolyte properties; the solvate ILs showed improved thermal and electrochemical stability. The ionicity (Λimp/ΛNMR) of [Li(glyme or THF)x][TFSA] exhibited a maximum at an [O]/[Li(+)] ratio of 4 or 5.

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 liquid 1-ethyl-3-methylimidazolium (EMI+) salts, EMI(+)BF4-, EMI(+)PF6-, EMI(+)CF3SO3-, and EMI(+)N(CF3SO2)2-, were measured over the frequency range 200-1600 cm(-1). In the range 200-500 cm(-1), we found five bands originating from the EMI+ ion at 241, 297, 387, 430, and 448 cm(-1). However, the 448 cm(-1) band could hardly be reproduced by theoretical calculations in terms of a given EMI+ conformer, implying that the band originates from another conformer. This is expected because the EMI+ involves an ethyl group bound to the N atom of the imidazolium ring, and the ethyl group can rotate along the C-N bond to yield conformers. The torsion energy for the rotation was then theoretically calculated. Two local minima with an energy difference of ca. 2 kJ mol(-1) were found, suggesting that two conformers are present in equilibrium. Full geometry optimizations followed by normal frequency analyses indicate that the two conformers are those with planar and nonplanar ethyl groups against the imidazolium ring plane, and the nonplanar conformer is favorable. It elucidates that bands at 241, 297, 387, and 430 cm(-1) mainly originate from the nonplanar conformer, whereas the 448 cm(-1) band does originate from the planar conformer. Indeed, the enthalpy for conformational change from nonplanar to planar EMI+ experimentally obtained by analyzing band intensities of the conformers at varying temperatures is practically the same as that evaluated by theoretical calculations. We thus conclude that the EMI+ ion exists as either a nonplanar or planar conformer in equilibrium in its liquid salts.

Time-of-flight neutron diffraction measurements were carried out for 6Li/7Li isotopically substituted 10 mol % LiPF6-propylene carbonate-d6 (PC-d6) solutions, in order to obtain structural information on the first solvation shell of Li+. Structural parameters concerning the nearest neighbor Li+...PC and Li+...PF6- interactions were determined through least-squares fitting analysis of the observed difference function, DeltaLi(Q). It has been revealed that the first solvation shell of Li+ consists in average of 4.5(1) PC molecules with an intermolecular Li+...O(PC) distance of 2.04(1) A. The angle Li+...O=C bond angle has been determined to be 138(2) degrees.

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.