Mg2+ Solvation Structures and Diffusion Properties in Dimethoxyethane Solutions Using Multimodal NMR Analysis

Abstract

Mg-based secondary battery systems represent a promising advancement over current technology, however a detailed molecular view of the battery components are lacking which prevents the implementation of such technologies. Diffusion behaviors of Mg2+ in electrolytes are usually not as readily accessible as Li+ and Na+ through PFG NMR, due to the low sensitivity and resolution of 25Mg NMR. Herein, we report the evolution of solvation structures and diffusion behaviors in the traditional MgTFSI2/DME based electrolyte using quantitative multinuclear (1H, 19F, 17O) NMR, NMR relaxation, 2D 1H EXSY NMR, as well as PFG diffusion measurements. High resolution 1H NMR reveals bound DME (DME molecules coordinating to Mg2+) and bulk DME (“free” diffusing DME) within solution structure, where the diffusion of bound DME provides informatin about Mg2+ diffusion through a two-site exchange-diffusion model. We concluded that 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 between 0.06 M 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. Furthermore, the comparison between diffusion behaviros of Mg2+ and TFSI- reveals the formation of supercluster structures that highly constrain Mg2+ diffusion at high concentrations and low temperatures but present little restricticiton to TFSI- diffusion, properbly due to the long-distance effect of multivalent Mg2+ on DME with suppressed motion under these conditions.