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 ( c_Mg). 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 c_Mg 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.