Role of a multivalent ion–solvent interaction on restricted Mg2+ diffusion in dimethoxyethane electrolytes

Abstract

The diffusion behavior of Mg²⁺ in electrolytes is not as readily accessible as that from Li⁺ or Na⁺ utilizing PFG NMR, due to the low sensitivity, poor resolution, and rapid relaxation encountered when attempting ²⁵Mg NMR. In MgTFSI₂/DME solutions, “bound” DME (coordinating to Mg²⁺) and “free” DME (bulk) are distinguishable from ¹H NMR. With the exchange rates between them obtained from 2D ¹H EXSY NMR, we can extract the self-diffusivities of free DME and bound DME (which are equal to that of Mg²⁺) before the exchange occurs using PFG diffusion NMR measurements coupled with analytical formulas describing diffusion under two-site exchange. The high activation enthalpy for exhange (65–70 kJ/mol) can be explained by the structural change of bound DME as evidenced by its reduced C–H bond length. Comparison of the diffusion behaviors of Mg²⁺, TFSI^-, DME, and Li⁺ reveals a relative restriction to Mg²⁺ diffusion that is caused by the long-range interaction between Mg²⁺ and solvent molecules, especially those with suppressed motions at high concentrations and low temperatures.

Rasha Atwi

PhD Candidate

My research interests include high-throughput scientific computing, quantum chemistry, and molecular dynamics.