Specific line shape of the lowest frequency Raman scattering modes of triethylene glycol.

For both dielectric spectroscopy and light scattering spectra, the relaxation modes in the microwave region have been characterized by the Debye relaxation model, which is determined by the peak frequency, or by an empirically extended model (e.g., Cole-Davidson and Kohlrausch-Williams-Watts), which has the appropriate line shape.

Speciation and dipole reorientation dynamics of glass-forming liquid electrolytes: Li[N(SOCF)] mixtures of 1,3-propane sultone or tetrahydrothiophene-1,1-dioxide.

Recently new ionic fluids such as super-concentrated electrolyte solutions, solvate ionic liquids and deep eutectic solvents have attracted much attention in the field of liquid electrolytes for next-generation electrochemical devices and processes. The basic composition of these new ionic fluids is similar among them; a solvent and a large/excess amount of salt mixtures, though the solvent is sometimes a solid at ambient temperatures. Here, we found and demonstrated that LiTFSA (TFSA = (CFSO)N) mixtures with 1,3-propane sultone (PS) or tetrahydrothiophene-1,1-dioxide (SL) yield a homogeneous liquid at room temperature within a wide range of compositions.

Toward New Ion Conductive Liquids via Ionic Liquids.

In the current era that it is strongly expected the SDGs would be achieved, electrolyte solutions in electrochemical devices and processes are being studied from dilute and concentrated solutions, through inorganic molten salts, deep eutectic solvents, and ionic liquids, to super-concentrated solutions. Although concepts based on empirical laws such as the Walden rule and hydrodynamics such as the Stokes rule are still useful for ionic conduction in solution, it is expected that superionic conduction-like mechanisms that are scarcely found in conventional electrolytes. Here, the authors' recent results are described based on the local structure and speciation of ionic species in solution, focusing on protons and lithium ions.

Analytical chemistry toward on-site diagnostics.

Analytical Chemistry, through quantitative and/or qualitative analysis (identification), is a discipline that involves the development of methodologies and the exploration of new principles to obtain answers to given problems. In situ analysis techniques have attracted attention for its ability to elucidate phenomena occurring and to evaluate amount of a certain component in substances at real time and biological samples as applications of such analysis technology. Lots of techniques have been performed to understand the fundamental phenomena in varied fields such as X-ray, vibrational, and electrochemical impedance spectroscopies and also analytical reagents that enable to semi-quantitative analysis just observation.

Tools for studying ion solvation and ion pair formation in ionic liquids: isotopic substitution Raman spectroscopy.

Isotopic H/D or Li substitution Raman spectroscopy was applied to new kinds of ionic liquids; N-methylimidazole (CIm) and acetic acid (CHCOOH) as the pseudo-protic ionic liquid (pPIL), and both of the neat and the 2,2,3,3-tetrafluoropropyl ether (HFE) diluted Li-glyme solvate ionic liquids (SIL) [Li(Gn)][TFSA] (Gn, glyme n = 3 or 4); TFSA, bis(trifluoromethanesulfonyl)amide) to clarify the proton transfer or the Li solvation/ion pair formation. The isotopic substitution Raman (ISR) spectra were obtained as the difference between the samples containing the same composition except the substituted isotope. The calculated and theoretical ISR spectra were also evaluated for comparison.

Local Structure of Li in Superconcentrated Aqueous LiTFSA Solutions.

It has been reported that aqueous lithium ion batteries (ALIBs) can operate beyond the electrochemical window of water by using a superconcentrated electrolyte aqueous solution. The liquid structure, particularly the local structure of the Li, which is rather different from conventional dilute solution, plays a crucial role in realizing the ALIB. To reveal the local structure around Li, the superconcentrated LiTFSA (TFSA: bis(trifluoromethylsulfonil)amide) aqueous solutions were investigated by means of Raman spectroscopic experiments, high-energy X-ray total scattering measurements, and the neutron diffraction technique with different isotopic composition ratios of Li/Li and H/D.

Thermodynamic aspect of sulfur, polysulfide anion and lithium polysulfide: plausible reaction path during discharge of lithium-sulfur battery.

The elucidation of elemental redox reactions of sulfur is important for improving the performance of lithium-sulfur batteries. The energies of stable structures of S, S˙, S, [LiS] and LiS (n = 1-8) were calculated at the CCSD(T)/cc-pVTZ//MP3/cc-pVDZ level. The heats of reduction reactions of S and LiS with Li in the solid phase were estimated from the calculated energies and sublimation energies.

Effect of Brønsted Acidity on Ion Conduction in Fluorinated Acetic Acid and -Methylimidazole Equimolar Mixtures as Pseudo-protic Ionic Liquids.

To clarify proton conduction mechanism in protic ionic liquids (PILs) and -PILs (PILs), equimolar mixtures of -methylimidazole (CIm) with fluorinated acetic acids were investigated by Raman spectroscopy, X-ray scattering, and dielectric relaxation spectroscopy (DRS). Only the ionic species exist in the equimolar mixture of CIm and HTFA (HTFA: trifluoroacetic acid). On the other hand, the equimolar mixture of CIm and HDFA (HDFA: difluoroacetic acid) consists of both ionic and electrically neutral species.

Solvation Structure of Li in Concentrated Acetonitrile and ,-Dimethylformamide Solutions Studied by Neutron Diffraction with Li/Li Isotopic Substitution Methods.

Neutron diffraction measurements on Li/Li isotopically substituted 10 and 33 mol % *LiTFSA (lithium bis(trifluoromethylsulfonyl)amide)-AN- (acetonitrile-) and 10 and 33 mol % *LiTFSA-DMF-(-dimethylformamide-) solutions have been carried out in order to obtain structural insights on the first solvation shell of Li in highly concentrated organic solutions. Structural parameters concerning the local structure around Li have been determined from the least squares fitting analysis of the first-order difference function derived from the difference between carefully normalized scattering cross sections observed for Li-enriched and natural abundance solutions. In 10 mol % LiTFSA-AN- solution, 3.

Transport Properties of Ionic Liquid and Sodium Salt Mixtures for Sodium-Ion Battery Electrolytes from Molecular Dynamics Simulation with a Self-Consistent Atomic Charge Determination.

Ionic liquid (IL) has been considered as a potential electrolyte for developing next-generation sodium-ion batteries. A highly concentrated ionic system such as IL is characterized by the significant influence of intramolecular polarization and intermolecular charge transfer that vary with the combination of cations and anions in the system. In this work, a self-consistent atomic charge determination using the combination of classical molecular dynamics (MD) simulation and density functional theory (DFT) calculation is employed to investigate the transport properties of three mixtures of ILs with sodium salt relevant to the electrolyte for a sodium-ion battery: [1-ethyl-3-methylimidazolium, Na][bis(fluorosulfonyl)amide] ([CCim, Na][FSA]), [-methyl--propylpyrrolidinium, Na][FSA] ([CCpyrr, Na][FSA]), and [K, Na][FSA].

Speciation Analysis and Thermodynamic Criteria of Solvated Ionic Liquids: Ionic Liquids or Superconcentrated Solutions?

Lithium-glyme solvated ionic liquids (Li-G SILs) and superconcentrated electrolyte solutions (SCESs) are expected to be promising electrolytes for next-generation lithium secondary batteries. The former consists of only the oligoether glyme solvated lithium ion and its counteranion, and the latter contains no full solvated Li ion by the solvents due to the extremely high Li salt concentration. Although both of them are similar to each other, it is still unclear that both should be room-temperature ionic liquids.

Physicochemical compatibility of highly-concentrated solvate ionic liquids and a low-viscosity solvent.

High ionic carrier mobilities are important for the electrolyte solutions used in high-performance batteries. Based on the functional sharing concept, we fabricated mixed electrolytes consisting of solvate ionic liquids (SIL), which are highly concentrated solution electrolyte, and the non-coordinating low-viscosity dilution solvent 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE). We investigated the thermal, transport, and static properties of electrolytes with different ratios of SIL to HFE.

Possible Proton Conduction Mechanism in Pseudo-Protic Ionic Liquids: A Concept of Specific Proton Conduction.

In a previous work, we have found that the -protic ionic liquid -methylimidazolium acetate, [CHIm][OAc] or [Hmim][OAc], mainly consists of the electrically neutral molecular species -methylimidazole, CIm, and acetic acid, AcOH, even though the mixture has significant ionic conductivity. This system was revisited by employing isotopic substitution Raman spectroscopy (ISRS) and pulsed field gradient (PFG) NMR self-diffusion measurements. The ISRS and PFG-NMR results obtained fully confirm our earlier findings.

Solvation Structure of Li in Methanol and 2-Propanol Solutions Studied by ATR-IR and Neutron Diffraction with Li/Li Isotopic Substitution Methods.

Neutron diffraction measurements have been carried out on 10 mol % LiTFSA (TFSA: bis(trifluoromethylsulfonil)amide) solutions in methanol- d and 2-propanol- d to obtain information on the solvation structure of Li. The detailed coordination structure of solvent molecules within the first solvation shell of Li was determined through the least-squares fitting analysis of the difference function between normalized scattering cross sections observed for Li/Li isotopically substituted sample solutions. The nearest-neighbor Li···O distance and coordination number determined for the 10 mol % LiTFSA-methanol- d solution are r = 1.

Direct Evidence for Li Ion Hopping Conduction in Highly Concentrated Sulfolane-Based Liquid Electrolytes.

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 and sulfolane (SL).

Anion Coordination Characteristics of Ion-pair Complexes in Highly Concentrated Aqueous Lithium Bis(trifluoromethanesulfonyl)amide Electrolytes.

We report on the structures of Li-ion complexes in salt-concentrated aqueous electrolytes based on lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), particularly focusing on the anion coordination behavior of the ion-pair complexes in the high concentration region c > 3.0 mol dm. Quantitative data analysis of the Raman spectra revealed the following.

Enhanced Electrochemical Stability of Molten Li Salt Hydrate Electrolytes by the Addition of Divalent Cations.

Water can be an attractive solvent for Li-ion battery electrolytes owing to numerous advantages such as high polarity, nonflammability, environmental benignity, and abundance, provided that its narrow electrochemical potential window can be enhanced to a similar level to that of typical nonaqueous electrolytes. In recent years, significant improvements in the electrochemical stability of aqueous electrolytes have been achieved with molten salt hydrate electrolytes containing extremely high concentrations of Li salt. In this study, we investigated the effect of divalent salt additives (magnesium and calcium bis(trifluoromethanesulfonyl)amides) in a molten salt hydrate electrolyte (21 mol kg lithium bis(trifluoromethanesulfonyl)amide) on the electrochemical stability and aqueous lithium secondary battery performance.

Local Structure of Li in Concentrated Ethylene Carbonate Solutions Studied by Low-Frequency Raman Scattering and Neutron Diffraction with Li/Li 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, 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(Q), between neutron scattering cross sections observed for Li/Li isotopically substituted 10 and 25 mol % *LiTFSA-ECd solutions.

Effect of the cation on the stability of cation-glyme complexes and their interactions with the [TFSA] anion.

The interactions of glymes with alkali or alkaline earth metal cations depend strongly on the metal cations. For example, the stabilization energies (E) calculated for the formation of cation-triglyme (G3) complexes with Li, Na, K, Mg, and Ca at the MP2/6-311G** level were -95.6, -66.

Li(+) Local Structure in Li-Tetraglyme Solvate Ionic Liquid Revealed by Neutron Total Scattering Experiments with the (6/7)Li 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 (6/7)Li isotopic substitution to reveal the Li(+) solvation/local structure in [Li(G4)][TFSA] SILs.

A pH Scale for the Protic Ionic Liquid Ethylammonium Nitrate.

To quantify the properties of protic ionic liquids (PILs) as acid-base reaction media, potentiometric titrations were carried out in a neat PIL, ethylammonium nitrate (EAN). A linear relationship was found between the 14 pKa  values of 12 compounds in EAN and in water. In other words, the pKa  value in EAN was found to be roughly one unit greater than that in water regardless of the charge and hydrophobicity of the compounds.

Li(+) Local Structure in Hydrofluoroether Diluted Li-Glyme Solvate Ionic Liquid.

Hydrofluoroethers have recently been used as the diluent to a lithium battery electrolyte solution to increase and decrease the ionic conductivity and the solution viscosity, respectively. In order to clarify the Li(+) local structure in the 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) diluted [Li(G4)][TFSA] (G4, tetraglyme; TFSA, bis(trifluoromethanesulfonyl)amide) solvate ionic liquid, Raman spectroscopic study has been done with the DFT calculations. It has turned out that the HFE never coordinates to the Li(+) directly, and that the solvent (G4) shared ion pair of Li(+) with TFSA anion (SSIP) and the contact ion pair between Li(+) and TFSA anion (CIP) are found in the neat and HFE diluted [Li(G4)][TFSA] solvate ionic liquid.

Structural and aggregate analyses of (Li salt + glyme) mixtures: the complex nature of solvate ionic liquids.

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).

Relationship between low-Q peak and long-range ordering of ionic liquids revealed by high-energy X-ray total scattering.

The structures of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([CnmIm(+)][TFSA(-)]) ionic liquids (alkyl-chain length n = 4, 8, 10, and 12) have been studied by high-energy X-ray total scattering at T = 298-453 K. The low-Q peaks observed in the X-ray structure factors S(Q)s at 0.2 < Q < 0.