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The ionic structure of organic electrolyte solutions has been investigated, by means of Raman spectroscopy for mixed aprotic solvents that dissolve lithium salts. The solutions consisted of binary solvent systems of a high permittivity solvent (ethylene carbonate, EC) mixed with low viscosity alkyl carbonates (dimethyl carbonate, DMC, and diethyl carbonate, DEC) or a linear alkyl ether (1,2-dimethoxyethane, DME) and of LiCF 3 SO 3 , and as the solute. The Raman shifts based on the solvents varied with the sort of) 2 dissolved lithium salts and their concentration. The ion solvation was estimated from the side-bands of the Raman scattering for the CwO stretch of single bonds of the carbonate groups and for the symmetric ring deformation of EC. The number of EC molecules interacting with the lithium cation (Li`) was higher than that of DMC in a mixed EC ] DMC (50 : 50 by volume) system. The apparent solvation number of Li`in 1.5 mol dm~3 solution was about 2.8 in EC ] DMC, whereas that in 1.5 M solution was LiCF 3 SO 3 LiPF 6 about 3.9. SpeciÐc solvation of Li`was distinguished in EC ] DME (50 : 50), where DME predominantly coordinates to Li`. However, it was also conÐrmed that EC, which has a lower donicity, interacts with Liè ven in the EC ] DME system. That is, the solvation number of EC in remained LiCF 3 SO 3 /(EC ] DME) ca. 1.0 even in high salt concentrations.

[1] A series of triaxial compression tests were conducted in order to investigate the mechanical behavior of gas-saturated methane hydrate-bearing sediments, and a comparison was made between gas-saturated and water-saturated specimens. Measurements on gas-saturated specimens indicate that (1) the larger the methane hydrate saturation, the larger the failure strength and the more apparent the shear dilation behavior; (2) failure strength and stiffness increase with increasing effective confining stress and pore pressure applied during compression, though the specimen becomes less dilative under higher effective confining stress; (3) lower temperatures lead to an increase of the stiffness and failure strength; (4) stiffness of specimens formed under lower pore pressure is higher than that of specimens formed under higher pore pressure but at the same effective stress; (5) stiffness and failure strength of gas-saturated specimens are higher than those of watersaturated specimens; (6) gas-saturated specimens show more apparent strain-softening behavior and larger volumetric strain than that of water-saturated specimens.

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We report here structural study on solvation of Mg 2+ ion in triglyme (G3)-based solutions applying as a novel electrolyte for rechargeable Mg batteries. In Mg(TFSA) 2 /G3 electrolyte solutions (TFSA = bis(trifluoromethanesulfonyl)amide), we found from Raman spectroscopy that Mg 2+ ion is solvated with two G3 molecules to form [Mg(G3) 2 ] 2+ complexes. No direct coordination of TFSA − anion to Mg 2+ ion occurs in the solutions with the salt concentrations c Mg = 0−1.60 M. The geometries and interaction energies for the [Mg(G3) 2 ] 2+ were evaluated by DFT calculations and indicated that G3 molecules in the most stable complex act as a tridentate ligand, i.e., octahedral [Mg(tri-G3) 2 ] 2+ . However, the Raman spectra implies that [Mg(tri-G3) 2 ] 2+ coexists with [Mg(tetra-G3) (bi-G3)] 2+ in the solutions where tetra-G3 and bi-G3 are G3 molecules acting as tetra-and bidentate ligands, respectively, in the solvation sphere. The Walden plots indicated that the dissociativity (or ionicity) of Mg(TFSA) 2 in G3 solutions increases with increasing c Mg , which is opposite to conventional organic electrolyte solutions but is similar to the LiTFSA/glyme solutions. ■ INTRODUCTIONMagnesium (Mg), one of the s-block elements that can exist as a divalent cation Mg 2+ has attracted attention in battery chemistry because of its high volumetric capacity, high negative reduction potential, and low cost relative to lithium-based batteries. To establish a practical rechargeable Mg battery system, it is well-recognized that the development of electrode materials and electrolytes using nonaqueous solvents play a key role and thus are now required to improve battery performance. In the latter viewpoints, the design of electrolyte is important to control ionic conductivity, ion diffusion, and solvation/ desolvation of metal ion at the electrode/electrolyte interface. The solvation/desolvation process of Mg ion is related to the charge transfer kinetics, i.e., dissolution/deposition behavior of Mg metal at the electrode, which controls current density directly. It is well-known that using Grignard reagents (R− Mg−X; R = alkyl groups and X = halides such as Br and Cl) Mg can be reversibly deposited in ethereal solvents such as tetrahydrofuran (THF) to give a rechargeable Mg battery system. 1 Aurbach et al. also proposed organo-haloaluminate salts such as Mg(AlCl 3 R) 2 or Mg(AlCl 2 RR′) in THF as promising electrolytes showing reversible dissolution/deposition of Mg. 2−5 Except for such electrolytes containing organoMg complexes, there have been no reports on reversible dissolution/deposition system using electrolyte solutions of simple MgX salts (X = ClO 4 , BF 4 , CF 3 SO 3 , etc.) in conventional nonaqueous solvents (propylene carbonate, acetonitrile, N,N-dimethylformamide, etc.) similar to electrolytes for Li ion batteries. 6 Recently, a system using simple Mg salt with bis(trifluoromethanesulfonyl)amide (TFSA) anion, Mg(TFSA) 2 , in dimethoxy ethylene (glyme)-type solvents was reported as a promising electrolyte for rechargeable Mg b...

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We report the relation between the structural and electrochemical properties of N,N-dimethyl­formamide (DMF)-based electrolytes containing lithium bis­(trifluoro­methane­sulfonyl)­amide (LiTFSA) in the concentration range c Li = 0–3.2 mol dm–3. Raman spectroscopy and DFT calculations indicate that Li+ ions are solvated by DMF molecules in the form of [Li­(DMF)4]+ complexes at low c Li (<2.0 mol dm–3, LiTFSA:DMF = 1:4 by mol), whereas the coordination of TFSA– anions to Li+ ions occurs and gradually increases as c Li increases above 2.0 mol dm–3. Quantitative Raman data analysis reveals that TFSA– anions coordinate with Li+ ions in a monodentate manner (mono-TFSA) in the c Li range of 2.0–2.5 mol dm–3, and mono-TFSA coexists with TFSAs as a bidentate manner (bi-TFSA) in solutions with c Li > 2.5 mol dm–3. The high c Li solutions, in which all the DMF molecules solvate to Li+ ions (i.e., no DMF remains in the bulk), make the electrochemical window wider; the oxidative stability is enhanced owing to lower HOMO energy levels of solvated DMF molecules relative to those in the bulk. The salt concentration also controls the reductive stability; coordinated TFSA– anions within the Li-ion complexes formed in concentrated solutions affect the LUMO energy levels of the electrolyte. The LUMOs located on the TFSA– anions lead to a preferential reduction of the TFSA component rather than DMF to form a solid electrolyte interphase on graphite negative electrodes, resulting in the Li-ion insertion/desertion into/from graphite in the concentrated solutions.

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