Electrochemical Properties and Speciation in Mg(HMDS)-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)-AlCl) 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)-AlCl electrolytes are found to not exchange as quickly as THF and tetraglyme, stabilizing AlCl and facilitating undesired aluminum deposition. Raman spectroscopy, Al NMR, and mass spectrometry are used to identify solution speciation.