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Magnesium Bis(trifluoromethanesulfonyl)amide Complexes with Triglyme and Asymmetric Homologues: Phase Behavior, Coordination Structures and Melting Point Reduction

Abstract

The phase behavior of binary mixtures of triglyme (G3) and Mg[TFSA]2 (TFSA: bis(trifluoromethanesulfonyl)amide) was investigated, towards development of a Mg2+-based room-temperature solvate ionic liquid (SIL) electrolyte. In a 1:1 molar ratio, G3 and Mg[TFSA]2 form a thermally stable complex (decomposition temperature, Td: 240 °C) with a melting point (Tm) of 70 °C, which is considerably lower than that of the analogous tetraglyme (G4) system (137 °C). X-ray crystallography of a single crystal of [Mg(G3)][TFSA]2 revealed that a single Mg2+ cation is coordinated by a single, distorted, tetradentate G3 molecule from one side, and two monodentate [TFSA]− anions, with transoid conformation, from the reverse side to form an ion pair. Raman spectra of [Mg(G3)][TFSA]2 in the molten state revealed the presence of different coordination structures, as the liquid exhibits changes in the vibrational modes corresponding to G3 and the [TFSA]− anion compared to those observed for the solid. Investigation of the ion pair stabilization energies by DFT calculation suggests that higher stability cation complexes and ion pairs co-exist in the molten state than those observed in the crystalline state. These results imply that the coordination structures of the ion pairs play a key role in providing SILs with low Tm. To decrease the Tm further, several asymmetric homologues of G3, which have higher conformational flexibility than G3, were investigated. Notably, the 1:1 mixture of Mg[TFSA]2 with G3Bu (where one of the terminal methyl groups of G3 is substituted for a butyl group) formed a thermally stable complex (Td: 251 °C) without any distinct Tm and showed reasonable ionic conductivity at room-temperature, indicating partial dissociation of ions. In this electrolyte, which showed high oxidative stability, quasi-reversible Mg deposition/dissolution was achieved, indicating that Mg2+-based room-temperature SILs can be utilized as a new class of Mg electrolyte.

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Publication details

The article was received on 14 Dec 2017, accepted on 13 Feb 2018 and first published on 13 Feb 2018


Article type: Paper
DOI: 10.1039/C7CP08367J
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    Magnesium Bis(trifluoromethanesulfonyl)amide Complexes with Triglyme and Asymmetric Homologues: Phase Behavior, Coordination Structures and Melting Point Reduction

    K. Hashimoto, S. Suzuki, M. L. Thomas, T. MANDAI, S. Tsuzuki, K. Dokko and M. Watanabe, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C7CP08367J

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