Multinuclear NMR Studies on Translational and Rotational Motion for Two Ionic Liquids Composed of BF₄ Anion

Two ionic liquids (ILs) based on the BF₄^– anion are studied by ¹H, ¹¹B, and ¹⁹F NMR spectroscopy by measuring self-diffusion coefficients (<i>D</i>) and spin–lattice relaxation times (<i>T</i>₁). The cations are 1-ethyl-3-methylimidazolium (EMIm) and 1-butyl-3-methylimidazolium (BMIm). Since two NMR nuclei (¹¹B and ¹⁹F) of BF₄^– exhibit narrow lines and high sensitivity, the ¹¹B and ¹⁹F NMR measurements of <i>D</i>_BF4 and <i>T</i>₁(BF₄) were performed in a wide temperature range. The temperature-dependent behaviors of <i>T</i>₁(¹⁹F) and <i>T</i>₁(¹¹B) were remarkably different, although the values of <i>D</i>_BF4(¹⁹F) and <i>D</i>_BF4(¹¹B) almost agreed. Since the Arrhenius plots of <i>T</i>₁’s for ¹H, ¹⁹F, and ¹¹B exhibited <i>T</i>₁ minima, the correlation times τ_c(¹H), τ_c(¹⁹F), and τ_c(¹¹B) were evaluated. The <i>D</i>(cation) and <i>D</i>(BF₄) were plotted against 1/τ_c(¹H) and 1/τ_c(¹⁹F), respectively, and the relationships between translational and rotational motion are discussed. The translational diffusion of the cations is related to molecular librational motion and that of BF₄ is coupled with reorientational motion. The τ_c(¹¹B) derived from ¹¹B <i>T</i>₁ can be attributed to a local jump. From the plots of the classical Stokes–Einstein (SE) equation, the empirical <i>c</i> values, which were originally derived by theoretical boundary conditions, were estimated for each ion. The empirical <i>c</i>(BF₄) was about 4.4₅, while the <i>c</i> values of the cations were smaller than 4

Dissociation and Diffusion of Glyme-Sodium Bis(trifluoromethanesulfonyl)amide Complexes in Hydrofluoroether-Based Electrolytes for Sodium Batteries

Physicochemical properties and battery performance of [Na­(glyme)]­[TFSA] complexes (TFSA: bis­(trifluoromethanesulfonyl)­amide) dissolved in a hydrofluoroether (HFE) were investigated. Glyme (tetraglyme (G4) or pentaglyme (G5)) coordinates to Na⁺ to form a 1:1 complex [Na­(G4 or G5)]⁺ cation. Raman spectroscopy revealed that the complex structure of [Na­(glyme)]⁺ is maintained in the HFE solution, and free (uncoordinated) glymes are not liberated on adding HFE. HFE molecules are scarcely involved in the first solvation shell of Na⁺ because of their low electron-pair-donating ability. Raman spectra of the [TFSA]⁻ anion suggests that the attractive interaction between the complex [Na­(glyme)]⁺ cation and [TFSA]⁻ anion is enhanced on adding HFE. The population of contact ion-pair (CIP) and/or aggregate (AGG) is smaller for the G5 system than for the G4 one, and the [Na­(G5)]­[TFSA]/HFE has higher ionic conductivity. The self-diffusion coefficients of the [Na­(glyme)]⁺ complex and [TFSA]⁻ were measured by pulsed field gradient (PFG) NMR, and the dissociativity of [Na­(glyme)]­[TFSA] was assessed. The dissociativity of the G5 system is greater than that of the G4 one, and the dissociativity can be correlated with the attractive interaction between [Na­(glyme)]⁺ and [TFSA]⁻, as evaluated by ab initio calculations. The dissociativity of the complexes gave significant effects on the battery performance

Static and Transport Properties of Alkyltrimethylammonium Cation-Based Room-Temperature Ionic Liquids

We have measured physicochemical properties of five alkyltrimethylammonium cation-based room-temperature ionic liquids and compared them with those obtained from computational methods. We have found that static properties (density and refractive index) and transport properties (ionic conductivity, self-diffusion coefficient, and viscosity) of these ionic liquids show close relations with the length of the alkyl chain. In particular, static properties obtained by experimental methods exhibit a trend complementary to that by computational methods (refractive index ∝ [polarizability/molar volume]). Moreover, the self-diffusion coefficient obtained by molecular dynamics (MD) simulation was consistent with the data obtained by the pulsed-gradient spin–echo nuclear magnetic resonance technique, which suggests that computational methods can be supplemental tools to predict physicochemical properties of room-temperature ionic liquids

Surface Analysis of Ionic Liquids with and without Lithium Salt Using X‑ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) was applied to a neat ionic liquid 1-ethyl-3-methylimidazolium bis­(trifluoromethanesulfonyl)­imide [EMI⁺]­[Tf₂N^–] and its lithium salt solution at room temperature to clarify the composition and structure of its near-surface region. Core level peaks were recorded for Li 1s, N 1s, C 1s, F 1s, O 1s, S 2s, and S 2p. Valence band XPS spectra (0–40 eV binding energy) were also studied. The XPS spectra were analyzed using DV-Xα calculations. Results show that the planar type isomer of the EMI⁺ cation is dominant at the near-surface region of EMI-Tf₂N. Results of XPS measurements show a spectrum of Li 1s in Li/EMI-Tf₂N. The proposed models for the preferred orientation of the ions exhibit good agreement with results obtained from the DV-Xα calculations

Chelate Effects in Glyme/Lithium Bis(trifluoromethanesulfonyl)amide Solvate Ionic Liquids. I. Stability of Solvate Cations and Correlation with Electrolyte Properties

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 (<i>D</i>_sol/<i>D</i>_Li) 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, <i>D</i>_sol/<i>D</i>_Li 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 <i>D</i>_sol/<i>D</i>_Li ∼ 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 <i>D</i>_sol/<i>D</i>_Li. 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 (Λ_imp/Λ_NMR) of [Li­(glyme or THF)_<i>x</i>]­[TFSA] exhibited a maximum at an [O]/[Li⁺] ratio of 4 or 5

Li⁺ Solvation and Ionic Transport in Lithium Solvate Ionic Liquids Diluted by Molecular Solvents

An equimolar mixture of lithium bis­(trifluoromethanesulfonyl)­amide (Li­[TFSA]) and either triglyme (G3) or tetraglyme (G4) yielded stable molten complexes: [Li­(G3)]­[TFSA] and [Li­(G4)]­[TFSA]. These are known as solvate ionic liquids (SILs). Glyme-based SILs have thermal and electrochemical properties favorable for use as lithium-conducting electrolytes in lithium batteries. However, their intrinsically high viscosities and low ionic conductivities prevent practical application. Therefore, we diluted SILs with molecular solvents in order to enhance their ionic conductivities. To determine the stabilities of the complex cations in diluted SILs, their conductivity and viscosity, the self-diffusion coefficients, and Raman spectra were measured. [Li­(G3)]⁺ and [Li­(G4)]⁺ were stable in nonpolar solvents, that is, toluene, diethyl carbonate, and a hydrofluoroether (HFE); however, ligand exchange took place between glyme and solvent when polar solvents, that is, water and propylene carbonate, were used. In acetonitrile (AN) mixed solvent complex cations [Li­(G3)­(AN)]⁺ and [Li­(G4)­(AN)]⁺ were formed. [Li­(G4)]­[TFSA] was more conductive than [Li­(G3)]­[TFSA] when diluted with nonpolar solvents due to the greater ionic dissociativity in [Li­(G4)]­[TFSA] mixtures. In view of the stability of the Li–glyme complex cations, the enhanced ionic conductivities, and the intrinsic electrochemical stabilities of the diluting solvents, [Li­(G4)]­[TFSA] diluted by toluene or HFE, can be a candidate for an alternative battery electrolyte

Local Structure of Li⁺ in Concentrated Ethylene Carbonate Solutions Studied by Low-Frequency Raman Scattering and Neutron Diffraction with ⁶Li/⁷Li Isotopic Substitution Methods

Isotropic Raman scattering and time-of-flight neutron diffraction measurements were carried out for concentrated LiTFSA-EC solutions to obtain structural insight on solvated Li⁺ as well as the structure of contact ion pair, Li⁺···TFSA^–, formed in highly concentrated EC solutions. Symmetrical stretching vibrational mode of solvated Li⁺ and solvated Li⁺···TFSA^– ion pair were observed at ν = 168–177 and 202–224 cm^–1, respectively. Detailed structural properties of solvated Li⁺ and Li⁺···TFSA^– contact ion pair were derived from the least-squares fitting analysis of first-order difference function, Δ_Li(<i>Q</i>), between neutron scattering cross sections observed for ⁶Li/⁷Li isotopically substituted 10 and 25 mol % *LiTFSA-EC<i>d</i>₄ solutions. It has been revealed that Li⁺ in the 10 mol % LiTFSA solution is fully solvated by ca. 4 EC molecules. The nearest neighbor Li⁺···O­(EC) distance and Li⁺···O­(EC)C­(EC) bond angle are determined to be 1.90 ± 0.01 Å and 141 ± 1°, respectively. In highly concentrated 25 mol % LiTFSA-EC solution, the average solvation number of Li⁺ decreases to ca. 3 and ca. 1.5. TFSA^– are directly contacted to Li⁺. These results agree well with the results of band decomposition analyses of isotropic Raman spectra for intramolecular vibrational modes of both EC and TFSA^–

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

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 <i>x</i>_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 [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 Δ<i>H</i>° and the temperature multiplied entropy <i>T</i>Δ<i>S</i>° 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 Δ<i>H</i>° and Δ<i>S</i>° 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

Li⁺ Local Structure in Li–Tetraglyme Solvate Ionic Liquid Revealed by Neutron Total Scattering Experiments with the ^6/7Li Isotopic Substitution Technique

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 ^6/7Li 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