Solvation of Magnesium Ion in Triglyme-Based Electrolyte Solutions

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 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.