Article pubs.acs.org/JPCC
Solvation of Magnesium Ion in Triglyme-Based Electrolyte Solutions Toshiki Kimura, Kenta Fujii,* Yoshiki Sato, Masayuki Morita, and Nobuko Yoshimoto Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan S Supporting Information *
ABSTRACT: We report here structural study on solvation of Mg2+ 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 Mg2+ ion is solvated with two G3 molecules to form [Mg(G3)2]2+ complexes. No direct coordination of TFSA− anion to Mg2+ ion occurs in the solutions with the salt concentrations cMg = 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(triG3)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 cMg, which is opposite to conventional organic electrolyte solutions but is similar to the LiTFSA/glyme solutions.
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INTRODUCTION Magnesium (Mg), one of the s-block elements that can exist as a divalent cation Mg2+ 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(AlCl3R)2 or Mg(AlCl2RR′) 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 = ClO4, BF4, CF3SO3, 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 batteries.7,8 Furthermore, Orikasa et al. recently proposed a © XXXX American Chemical Society
novel Mg battery full cell using ion-exchanged MgFeSiO4, Mg metal, and Mg(TFSA)2 in triethylene glycol dimethyl ether (triglyme, G3) solutions as a cathode, anode, and electrolyte, respectively. The Mg battery system works at 100 °C to show reversible charge−discharge capacity of 166 mAh g−1.9 In development and design of Mg batteries based on glyme-type electrolytes,10,11 a major and fundamental problem is that the mechanism of electrode reaction is not clear yet. The knowledge of the solvation of Mg2+ ion in glyme-type electrolyte solutions, which is the primary for understanding the electrode reaction, is still limited at the present stage. Watanabe et al. reported a systematic study on Li salt/glyme electrolytes from electrochemical, physicochemical, and structural aspects.12−21 They showed that equimolar mixtures of glymes and certain Li salts are liquid state at room temperature to give solvate ionic liquids12−17 and can be used as electrolytes for 4 V class Li batteries.18−20 Particularly, in triglyme (G3) solvent, it has been established that Li ion is solvated by a G3 molecule to from [Li(G3)+] complex in the solutions and pointed out that the Li+ desolvation process at the electrode interface controls electrochemical reaction in Li ion batteries.16,17,20 To understand and control the electrochemical reaction in Mg batteries using glyme-based electrolytes, in this work we investigated the solvation structure of Mg2+ ions in G3-based electrolyte solutions as a first step. The Mg ion solvation at the molecular level is directly related to ion diffusion in the bulk and charge transfer kinetics at the electrode interface, which will play an essential role in controlling the Mg battery performance.22,23 Received: May 14, 2015 Revised: July 29, 2015
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DOI: 10.1021/acs.jpcc.5b04626 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
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EXPERIMENTAL SECTION Materials. Mg(TFSA)2 salt (
