Multivalent batteries have been explored in response to the increasing demand for high-energy rechargeable batteries utilizing sustainable resources. Solvation structures of working cations have been recognized as a key component in the design of electrolytes. In this study, we use multidmodal NMR analysis, including multinuclear ¹H, ¹³C, ¹⁷O, ¹⁹F, and ²⁵Mg NMR, NMR relaxation, PFG diffusion, 2D EXSY, and lineshape analysis at varying temperatures, to investigate ion solvation structure, diffusion, and solvent exchange dynamics of magnesium electrolytes with MgTFSI₂ in dimethoxyethane (DME) and a series of mixed solvents including cyclic ethers, carbonates, sulfones, phosphates, and nitrogen-containing solvents. Since bound and free DME are distinguishable from ¹H NMR, we can estiamte the exchange rates between them directly and extract their self-diffusivities using PFG diffusion NMR coupled with analytical formulas describing diffusion under two-site exchange. 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. Furthermore, NMR studies of MgTFSI₂-DME-cosolvents demonstrate a variety of solvation structures with a fraction of DME, cosolvent or TFSI^- forming stable first solvation sheath with Mg²⁺. The estimated residence times of the coordinating solvents exhibit a wide time scale ranging from 1 μs to 10 ms. The coordination power and activation energy of solvent exchange dynamics can be correlated to the physicochemical properties of the solvents. Moreover, through carefully designed experiments with similar solvation structures, we found that the charge transfer efficiency at the electrolyte/electrode interface is closely related to the solvent exchang rate, suggesting that the desolvation process is the rate-governing step at the interface.

Jesse Sears

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