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.

Molecular dynamics simulations of equimolar mixtures of glymes (triglyme and tetraglyme) and Li[TFSA] (lithium bis(trifluoromethylsulfonyl)amide) show that the glyme chain length affects the coordination geometries of Li(+), which induces the changes in interactions between the [Li(glyme)](+) complex and [TFSA](-) anions and diffusion of ions in the equimolar mixtures.

Equimolar mixtures of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and tetraglyme (G4: CH3O-(CH2CH2O)4-CH3) yield the solvate (or chelate) ionic liquid [Li(G4)][TFSA], which is a homogeneous transparent solution at room temperature. Solvate ionic liquids (SILs) are currently attracting increasing research interest, especially as new electrolytes for Li-sulfur batteries. Here, we performed neutron total scattering experiments with (6/7)Li isotopic substitution to reveal the Li(+) solvation/local structure in [Li(G4)][TFSA] SILs. The experimental interference function and radial distribution function around Li(+) agree well with predictions from ab initio calculations and MD simulations. The model solvation/local structure was optimized with nonlinear least-squares analysis to yield structural parameters. The refined Li(+) solvation/local structure in the [Li(G4)][TFSA] SIL shows that lithium cations are not coordinated to all five oxygen atoms of the G4 molecule (deficient five-coordination) but only to four of them (actual four-coordination). The solvate cation is thus considerably distorted, which can be ascribed to the limited phase space of the ethylene oxide chain and competition for coordination sites from the TFSA anion.

Thermal properties and mixing states of ethylene glycol (EG)−water binary solutions in the entire mole fraction range of EG, 0 ≤ x EG ≤ 1, have been clarified by using differential scanning calorimetry (DSC), large-angle X-ray scattering (LAXS), and small-angle neutron scattering (SANS) techniques. The DSC curves obtained have shown that the EG−water solutions over the range of EG mole fraction 0.3 ≤ x EG ≤ 0.5 are kept in the supercooling state until ∼100 K, and those in the range of 0.6 ≤ x EG ≤ 0.8 are vitrified, and those in the ranges of 0 < x EG ≤ 0.2 and 0.9 ≤ x EG < 1 are crystallized. The radial distribution function (RDF) for pure EG obtained from the LAXS measurements has suggested that a gauche conformation of an EG molecule is favorable in the liquid. The RDFs for the EG−water solutions have shown that the structure of the binary solutions moderately changes from the inherent structure of EG to the tetrahedral-like structure of water when the water content increases. The SANS intensities for deuterated ethylene glycol (HOCD2CD2OH) (EGd 4)−water solutions at x EG = 0.4 and 0.6 have not been significantly observed in the temperature range from 298 to 173 K, showing that EG and water molecules are homogeneously mixed. On the other hand, the SANS intensities at x EG = 0.2 and 0.9 have been strengthened when the temperature decreases due to crystallization of the solutions. On the basis of all the present results, a relation between thermal properties of EG−water binary solutions and their mixing states clarified by the LAXS and SANS measurements has been discussed at the molecular level.

Herein, we report on a structural study for characterizing unique solution structures in the salt-concentrated electrolytes, which are promising new lithium (Li)-ion battery electrolytes. A combination of high-energy X-ray total scattering (HEXTS) experiments with all-atom molecular dynamics (MD) simulations was performed on the salt-concentrated electrolytes that were based on Li bis­(trifluoromethanesulfonyl)­amide (LiTFSA) and N,N-dimethylformamide (DMF). The radial distribution functions obtained from the HEXTS and MD approaches were in good agreement in the current LiTFSA/DMF solutions. We found that in the local structure: (1) the Li-ions were coordinated with both the DMF molecules and the TFSA anions in the concentrated solutions and (2) specific Li+···Li+ correlations were present in the radial distribution function over the r range of 3 Å–15 Å. The Li+···Li+ correlations originated from the extended multiple Li-ion complexes, that is, polymerized [Li+···TFSA–···Li+] n complexes so that they were highly structurally ordered. We concluded that this type of an ion-ordered structure plays a crucial role in the electrochemical stability and the ion-conducting mechanism, resulting in a unique LIB performance employing these salt-concentrated electrolytes.

We report a new approach for investigating polymer structures in solution systems, including polymer–solvent interactions at the molecular level. The solvation structure of poly­(benzyl methacrylate) (PBnMA) in an imidazolium-based ionic liquid (IL) has been investigated at the molecular level using high-energy X-ray total scattering (HEXTS) with the aid of all-atom molecular dynamics (MD) simulations. The X-ray radial distribution functions derived from both experimental HEXTS and theoretical MD (G exp(r) and G MD(r), respectively) were in good agreement in the present PBnMA/IL system. The G(r) functions were successfully separated into two components for the inter- and intramolecular contributions. Here, the former corresponds to polymer solvation (or polymer–solvent interactions) and the latter to polymer structure, such as conformation and interactions between side chains (benzyl groups) in PBnMA. The intermolecular G MD inter(r) revealed that the side chains are preferentially solvated by imidazolium cations rather than anions. On the other hand, the intramolecular G MD intra(r) suggested that PBnMA is also stabilized by interactions among the aromatic side chains (π–π stacking). Thus, polymer (benzyl group)–cation interactions and benzyl group stacking within a PBnMA chain coexist in the PBnMA/IL system to give a more ordered solution structure. This behavior might be ascribed to negative mixing entropy in the solution state, which is key to the lower critical solution temperature (LCST)-type phase behavior in the PBnMA/IL solutions.

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