Electrochemical Properties and Speciation in Mg(HMDS)2-Based Electrolytes for Magnesium Batteries as a Function of Ethereal Solvent Type and Temperature

Magnesium batteries are a promising alternative to lithium-ion batteries due to the widespread abundance of magnesium and its high specific volumetric energy capacity. Ethereal solvents such as tetrahydrofuran (THF) are commonly used for magnesium-ion electrolytes due to their chemical compatibility with magnesium metal, but the volatile nature of THF is a concern for practical application. Herein, we investigate magnesium bis(hexamethyldisilazide) plus aluminum chloride (Mg(HMDS)₂-AlCl₃) electrolytes in THF, diglyme, and tetraglyme at varying temperature. We find that, despite the higher thermal stability of the glyme-based electrolytes, THF-based electrolytes have better reversibility at room temperature. Deposition/stripping efficiency is found to be a strong function of temperature. Diglyme-based Mg(HMDS)₂-AlCl₃ electrolytes are found to not exchange as quickly as THF and tetraglyme, stabilizing AlCl₂⁺ and facilitating undesired aluminum deposition. Raman spectroscopy, ²⁷Al NMR, and mass spectrometry are used to identify solution speciation.