Role of Solvent Rearrangement on Mg²⁺ Solvation Structures in Dimethoxyethane Solutions using Multimodal NMR Analysis

One of the main impediments faced for predicting emergent properties of a multivalent electrolyte (such as conductivity and electrochemical stability) is the lack of quantitative analysis of ion-ion and ion-solvent interactions, which manifest in solvation structures and dynamics. In particular, the role of ion-solvent interactions is still unclear in cases where the strong electric field from multivalent cations can influence intramolecular rotations and conformal structural evolution (i.e., solvent rearrangement process) of low permittivity organic solvent molecules on solvation structure. Using quantitative ¹H, ¹⁹F, and ¹⁷O NMR together with ¹⁹F nuclear spin relaxation and diffusion measurments, we find an unusual correlation between ion concentration and solvation structure of Mg(TFSI)₂ salt in dimethoxyethane (DME) solution. The dominant solvation structure evolves from contact ion pairs (i.e., [Mg(TFSI)(DME)_1-2]⁺) to fully solvated clusters (i.e., [Mg(DME)₃]²⁺) as salt concentration increases or as temperature decreases. This transition is coupled to a phase separation, which we study here between 0.06 and 0.36 M. Subsequent analysis is based on an explanation of the solvent rearrangement process and the competition between solvent molecules and TFSI anions for cation coordination.