Solvate Ionic Liquid Electrolytes for Mg Batteries

Abstract

We report here the structures and physicochemical properties of Mg[N(SO2CF3)2]2 (MgTFSA2)/glyme solvate ionic liquid electrolytes for Mg batteries. MgTFSA2 and certain glymes, such as triglyme (G3), tetraglyme (G4), and pentaglyme (G5), form equimolar complexes.1 The melting points of [Mg(G3)]TFSA2, [Mg(G4)]TFSA2, and [Mg(G5)]TFSA2 complexes are 70, 138, and 141 °C, respectively. X-ray crystallography revealed that each glyme wraparound a Mg2+ ion and forms a [Mg(glyme)]2+ complex cation in the crystalline solvate. To decrease the melting points of (MgTFSA2)/glyme complexes, asymmetric structure was introduced to the ligand (glyme). One of the terminal methyl groups of G3 molecule was substituted with other alkyl groups having various chain lengths. In this study, we used triethylene glycol ethyl methyl ether (G3Et) triethylene glycol butyl methyl ether (G3Bu) as ligands. As we expected, the melting point of [Mg(G3Et)]TFSA2 is 57.3 °C, which is lower than that of [Mg(G3)]TFSA2.2 The entropic term of melting increased with increasing asymmetric alkyl chain length, indicating that conformational flexibility successfully decreased the melting point. [Mg(G3Bu)]TFSA2 became a glass forming liquid and maintained liquid state at room temperature. Raman spectra for the [Mg(G3Bu)]TFSA2 indicated that the 1:1 complex structure of [Mg(G3Bu)]2+ is kept even in the liquid state. [Mg(G3Bu)]TFSA2 is thermally stable and does not decompose up to 251 °C. The oxidative stability of [Mg(G3Bu)]TFSA2 was analyzed by linear sweep voltammetry. [Mg(G3Bu)]TFSA2 possesses a higher oxidative stability compared with uncoordinated glymes. This is because the HOMO energy level of glyme is lowered by the complexation with Mg2+ ion.1,3 Mg metal deposition and dissolution are also possible in this electrolyte. Physicochemical properties of [Mg(glyme)]TFSA2 complexes will be compared with those of other glyme-based solvate ionic liquids such as [Li(glyme)]TFSA and [Na(glyme)]TFSA. Acknowledgements This study was supported in part by JSPS KAKENHI (Grant Nos. 18H03926) from the Japan Society for the Promotion of Science (JSPS), the MEXT program “Elements Strategy Initiative to Form Core Research Center” of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan and the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST). References S. Tsuzuki, T. Mandai, S. Suzuki, W. Shinoda, T. Nakamura, K. Ueno, S. Seki, Y. Umebaysashi, K. Dokko, M. Watanabe, Phys. Chem. Chem. Phys., 2017, 19, 18262-18272. K. Hashimoto, S. Suzuki, Morgan L. Thomas, T. Mandai, S. Tsuzuki, K. Dokko, Phys. Chem. Chem. Phys., 2018, 20, 7998-8007. T. Mandai, K. Dokko, M. Watanabe, Chem. Rec. 2019, 19, 708-722.