Speciation of Haloaluminate Electrolytes in Mg-Ion Batteries: A Combined Study of ab Initio Molecular Dynamics and Simulated X-Ray Absorption Spectroscopy

Since the development of prototype Mg-ion battery system [Aurbach et al., Nature 407, 724 (2000)] that employs organohaloaluminate (Mg(AlCl₂BuEt)₂) in tetrahydrofuran (THF) as the electrolyte, the exact solvation structure of Mg²⁺ in solution is still under debate. For further development of divalent Mg-ion batteries, a molecular-scale understanding of how Mg ion is solvated in such complex solution is vital to understand and improve ion transport properties in the electrolyte and dissolution/deposition processes at the electrolyte/electrode interfaces.

In this work, we aim to resolve the controversy of the exact solvation structure of Mg²⁺ by performing ab initio molecular dynamics (AIMD) simulations. To validate our AIMD predicated Mg solvation structures, we also simulate the corresponding Mg and Cl K-edge X-ray absorption spectra and compare with existing literature. We conclude that in the dilute limit of salt concentration, Mg ion is always tetracoordinated. This corrects a long-standing bias that Mg ions should be hexacoordinated in THF solution. By comparing with various experimental measurements, our results also imply that the solvation structure of Mg ion in haloaluminate solution is very sensitive to local salt concentration, relative Mg/Al ratio and bias-dependent electric filed during cycles.

This work is supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.