The Interplay of Al and Mg Speciation in Advanced Mg Battery Electrolyte Solutions

Mg batteries are an attractive alternative to Li-based energy storage due to the possibility of higher volumetric capacities with the added advantage of using sustainable materials. A promising emerging electrolyte for Mg batteries is the magnesium aluminum chloride complex (MACC) which shows high Mg electrodeposition and stripping efficiencies and relatively high anodic stabilities. As prepared, MACC is inactive with respect to Mg deposition; however, efficient Mg electrodeposition can be achieved following an electrolytic conditioning process. Through the use of Raman spectroscopy, surface enhanced Raman spectroscopy, ²⁷Al and ³⁵Cl nuclear magnetic resonance spectroscopy, and pair distribution function analysis, we explore the active vs inactive complexes in the MACC electrolyte and demonstrate the codependence of Al and Mg speciation. These techniques report on significant changes occurring in the bulk speciation of the conditioned electrolyte relative to the as-prepared solution. Analysis shows that the active Mg complex in conditioned MACC is very likely the [Mg₂(μ-Cl)₃·6THF]⁺ complex that is observed in the solid state structure. Additionally, conditioning creates free Cl^- in the electrolyte solution, and we suggest the free Cl^- adsorbs at the electrode surface to enhance Mg electrodeposition.