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 <i>c</i>_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 <i>c</i>_Mg, which is opposite to conventional organic electrolyte solutions but is similar to the LiTFSA/glyme solutions