Structural Study on Magnesium Ion Solvation in Diglyme-Based Electrolytes: IR Spectroscopy and DFT Calculations

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) such 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