Transport and Electrochemical Properties of Three Quaternary Ammonium Ionic Liquids and Lithium Salts Doping Effects Studied by NMR Spectroscopy

Ionic liquids (IL) composed of a quaternary ammonium cation having an ether chain, <i>N</i>,<i>N</i>-diethyl-<i>N</i>-methyl-<i>N</i>-(2-methoxyethyl)­ammonium (DEME) are electrochemically stable and used in electric double layer capacitors and also one of important candidates to use lithium secondary batteries. In this study, three DEME-based ILs with anions BF₄, CF₃BF₃, and N­(SO₂CF₃)₂ were studied by measuring temperature dependences of ionic conductivity σ, viscosity η and density ρ. Also ion diffusion coefficients <i>D</i>_anion and <i>D</i>_cation were obtained by NMR spectroscopy in the wide temperature range. Using the classical Stokes–Einstein (SE) and Nernst–Einstein (NE) equations, the relationships between η and <i>D</i>, and σ and <i>D</i> were evaluated. The lithium salt doping effects were studied by ⁷Li NMR spectroscopy. The lithium ion diffusion was slower than other ion diffusion at every temperature. Arrhenius-type plots of ⁷Li <i>T</i>₁ showed minima in the three doped samples. Then one-jump distance of lithium ion was estimated in the temperature range between 273 and 373 K

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

Decoupling between the Temperature-Dependent Structural Relaxation and Shear Viscosity of Concentrated Lithium Electrolyte

The intermediate scattering functions of concentrated solutions of LiPF₆ in propylene carbonate (PC) were measured at various temperatures, two different wavenumbers, and three different concentrations using neutron spin echo (NSE) spectroscopy. The temperature dependence of the relaxation time was larger than that of the steady-state shear viscosity in all cases. The shear relaxation spectra were also determined at different temperatures. The normalized spectra reduced to a master curve when the frequency was multiplied by the steady-state shear viscosity, indicating that the temperature dependence of the steady-state shear viscosity can be explained by that of the relaxation time of the shear stress. It is thus suggested that the dynamics of the shear stress is decoupled from the structural dynamics on the molecular scale

Relationship between Structural Relaxation, Shear Viscosity, and Ionic Conduction of LiPF₆/Propylene Carbonate Solutions

The structure and dynamics of the solutions of LiPF₆ in propylene carbonate over a concentration range of 0–3 mol/kg are studied with neutron spin echo spectroscopy, alternating-current (AC) conductometry, and shear impedance spectroscopy. The neutron diffraction shows a prepeak at ≈10 nm^–1 in addition to the main peak at ≈14 nm^–1 when the concentration of the salt is no less than 2 mol/kg. Compared with the frequency-dependent shear viscosity and AC conductivity, the relaxation of the shear stress agrees with that expected from the structural relaxation of the main peak. On the other hand, the relaxation of the conductivity is slower than the shear relaxation at all the concentrations, and the former approximately matches with the relaxation of the prepeak at the highest concentration, 3 mol/kg, which is several times slower than that of the main peak. The possible contribution of the prepeak structure to the ionic conduction is discussed

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

Solvent Activity in Electrolyte Solutions Controls Electrochemical Reactions in Li-Ion and Li-Sulfur Batteries

Solvent–ion and ion–ion interactions have significant effects on the physicochemical properties of electrolyte solutions for lithium batteries. The solvation structure of Li⁺ and formation of ion pairs in electrolyte solutions composed of triglyme (G3) and a hydrofluoroether (HFE) containing 1 mol dm^–3 Li­[TFSA] (TFSA: bis­(trifluoromethanesulfonyl)­amide) were analyzed using pulsed-field gradient spin–echo (PGSE) NMR and Raman spectroscopy. It was found that Li⁺ is preferentially solvated by G3 and forms a [Li­(G3)]⁺ complex cation in the electrolytes. The HFE scarcely participates in the solvation because of low donor ability and relatively low permittivity. The dissociativity of Li­[TFSA] decreased as the molar ratio of G3/Li­[TFSA] in the solution decreased. The activity of G3 in the electrolyte diminishes negligibly as the molar ratio approaches unity because G3 is involved in 1:1 complexation with Li⁺ ions. The negligible activity of G3 in the electrolyte solutions has significant effects on the electrochemical reactions in lithium batteries. As the activity of G3 diminished, the oxidative stability of the electrolyte was enhanced. The corrosion rate of the Al current collector of the positive electrode was suppressed as the activity of G3 diminished. The high oxidative stability and low corrosion rate of Al in the G3/Li­[TFSA] = 1 electrolyte enabled the stable operation of 4-V-class lithium batteries. The activity of G3 also has a significant impact on the Li⁺ ion intercalation reaction of the graphite electrode. The desolvation of Li⁺ occurs at the interface of graphite and the electrolyte when the activity of G3 in the electrolyte is significantly low, while the cointercalation of Li⁺ and G3 takes place in an electrolyte containing excess G3. The activity of G3 influenced the electrochemical reaction process of elemental sulfur in a Li–S battery. The solubility of lithium polysulfides, which are reaction intermediates of the sulfur electrode, decreased as the activity of G3 in the electrolyte decreased. In the G3/Li­[TFSA] = 1 electrolyte, the solubility of Li₂S_<i>m</i> is very low, and highly efficient charge/discharge of the Li–S battery is possible without severe side reactions

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