Structural Study on Magnesium-Ion Solvation in Diglyme-Based

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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

Structural Study on Magnesium-Ion Solvation in DiglymeBased Electrolytes: IR Spectroscopy and DFT Calculations Kenta Fujii, Michiru Sogawa, Nobuko Yoshimoto, and Masayuki Morita J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b05586 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 8, 2018

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The Journal of Physical Chemistry

Structural Study on Magnesium-Ion Solvation in Diglyme-Based Electrolytes: IR Spectroscopy and DFT Calculations Kenta Fujii,* Michiru Sogawa, Nobuko Yoshimoto, and Masayuki Morita Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan. *Corresponding Author. Email: [email protected] (K.F.)

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ABSTRACT

We investigated the solvation structure of Mg ions in a diglyme (G2)-based electrolyte solution for Mg-ion batteries. The Walden plots based on ionic conductivity and viscosity of the Mg(TFSA)2/G2 [TFSA: bis(trifluoromethanesulfonyl)amide] solutions indicated that the dissociativity of Mg(TFSA)2 gradually increased, even with increasing salt concentration (cMg). This behavior is similar to that of the analogous triglyme (G3)-based solutions. Infrared (IR) spectroscopy revealed that Mg ions were coordinated by two G2 molecules to form an octahedral [Mg(G2)2]2+ complex in the cMg range examined herein (≤0.92 M). The detailed coordination geometry of the [Mg(G2)2]2+ complex was evaluated using density functional theory calculations. We found that G2 molecules coordinated in a tridentate ligand fashion to form an octahedral [Mg(tri-G2)2]2+ complex. This result was different from that of the G3 system, i.e., G3 molecules acted in three ligand modes (bidentate, tridentate, and tetradentate) that multiple solvation complexes such as [Mg(tri-G3)2]2+ and [Mg(bi-G3)(tetra-G3)]2+ complexes were formed. This difference between the G2 and G3 systems might be related to an entropy contribution in the liquid state, i.e., only one coordination structure exists for [Mg(tri-G2)2]2+ in the G2 system, whereas more coordination complex structures can be formed in the G3 system.


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INTRODUCTION Glymes [oligoethers: CH3O(CH2CH2O)nCH3 (Gn)] have been attracting attention as promising battery electrolyte organic solvents. Such battery types include rechargeable Li-ion,1-2 Li-sulfur,34

Li-O2,5 and Mg batteries and so on.6-8 Glymes have multiple oxygen atoms and thus, can act as

a multidentate ligand to a single metal ion,9-10 resulting in high salt-concentrated electrolyte solutions.11-13 Watanabe et al. 14-19 reported equimolar mixtures of glymes and LiX salts (where X is a weakly coordinating anion), i.e., solvate ionic liquids, and demonstrated that they exhibited unique physicochemical and electrochemical properties, similar to those of roomtemperature ionic liquids (ILs), and that LiX/glyme mixtures could be applied to practical 4 V class Li-ion batteries as electrolytes. From a structural standpoint, Li ions are coordinated through O atoms in a glyme molecule to yield a crown ether-like coordination structure. Hence, the [Li(glyme)]+ complex behaves like a bulky cation, resulting in solvate ILs composed of [Li(glyme)]+ and X−. Recently, a glyme-based electrolyte containing dissolved Mg bis(trifluoromethanesulfonyl)amide salt, i.e., [Mg(TFSA)2], was reported to function well as an electrolyte for the reversible deposition/dissolution of Mg metal.6-7 Orikasa et al. proposed a novel Mg battery cell based on Mg(TFSA)2 in a triglyme (G3) electrolyte solution in which Mg metal acts as the anode and ion-exchanged MgFeSiO4 acts as the cathode. This system worked well (at 100 °C) and demonstrated a reversible charge–discharge capacity of 166 mA h g−1.8 Mandai et al.20 demonstrated that ternary G3-based electrolytes, i.e., Mg(TFSA)2/G3, including a chloride-based Mg salt (non-Grignard Mg salt), exhibited enhanced thermal stability and excellent battery performance at 100 °C. To design high-performance Mg batteries using such a glyme-based electrolyte for operation at room temperature, we need to understand the electrode reaction mechanism in the electrolyte solutions; however, knowledge of the Mg2+ ion solvation


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structure in glymes, which plays a crucial role in the electrode reaction, is still limited. In a previous study, we reported the structural investigation of Mg2+ ion solvation in a Mg(TFSA)2/G3 electrolyte system at the molecular level.12-13 Using Raman spectroscopy and density functional theory (DFT) calculations, we found that: (1) Mg2+ ions were solvated by two G3 molecules to form [Mg(G3)2]2+ complexes in solution. (2) G3 molecules, which contain four O atoms in each molecule, mainly acted as tridentate ligands to yield a [Mg(tri-G3)2]2+ complex; however, tetra-dentate and bi-dentate G3 complexes were also present as minor species. (3) At higher Mg salt concentrations (cMg = 1.6 M, corresponding to Mg salt: G3 = 1:2 by mol), the concentrated electrolyte showed the highest ionicity and thus, might be classified as a solvate IL, [Mg(G3)22+][TFSA−]2. Hashimoto et al.21 recently reported that the highly concentrated Mg(TFSA)2/G3 electrolyte, in a 1:1 molar ratio, yields a thermally stable [Mg(G3)2+][TFSA−]2 complex. They proposed it as a Mg2+-based solvate IL. A theoretical study based on classical molecular dynamics simulations was applied to the Mg(TFSA)2/glyme electrolyte system to discuss the salt concentration (cMg) dependence of ion-pair formation between Mg2+ and TFSA− ions in solution,22-24 i.e., in diglyme (G2). The study found that (1) Mg2+ ions interacted with TFSA− ions directly to form contact ion pairs, even at low cMg (~0.4 M) and (2) the Mg2+ ion complexes existed as ion pairs ([Mg(TFSA)2] and [Mg(TFSA)]+) and solvated ions (i.e., ionpair-free [Mg(G2)n]2+) in a 1:1 ratio, approximately. However, direct experimental evidence for such an ion-pair formation in Mg/glyme electrolytes at a lower cMg has not yet been reported. To obtain more insights into the coordination of glymes to Mg ions in solution, we extended our structural study to Mg salts in a G2-based electrolyte system. G2 has three O atoms in each molecule and can be regarded as an analogous molecule to G3. Herein, we demonstrated that the solvation structure of Mg2+ in a G2 system is different from that in a G3 system at the molecular


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level using infrared (IR) spectroscopy assisted by DFT calculations. We also performed ionic conductivity and viscosity measurements for the G2-based electrolyte solutions to discuss dissociativity (or ionicity), which is an important parameter for designing higher ion-conducting battery electrolytes.

EXPERIMENTAL Materials. Mg(TFSA)2 (

Structural Study on Magnesium-Ion Solvation in Diglyme-Based

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