Activating Magnesium Electrolytes through Chemical Generation of Free Chloride and Removal of Trace Water

Mg batteries are attractive next-generation energy storage systems due to their high natural abundance, inexpensive cost, and high theoretical capacity compared to conventional Li-ion based systems. The high energy density is achieved by electrodeposition and stripping of a Mg metal anode and requires the development of effective electrolytes enabled by a mechanistic understanding of the charge-transfer mechanism. The magnesium aluminum chloride complex (MACC) electrolyte is a good model system to study the mechanism as the solution phase speciation is known. Previously, we reported that minor addition of Mg­(HMDS)₂ to the MACC electrolyte causes significant improvement in the Mg deposition and stripping voltammetry resulting in good Coulombic efficiency on cycle one and, therefore, negating the need for electrochemical conditioning. To determine the cause of the improved electrochemistry, here we probe the speciation of the electrolyte after Mg­(HMDS)₂ addition using Raman spectroscopy, ²⁷Al nuclear magnetic resonance spectroscopy, and ¹H–²⁹Si heteronuclear multiple bond correlation spectroscopy on MACC + Mg­(HMDS)₂ at various Mg­(HMDS)₂ concentrations. Mg­(HMDS)₂ scavenges trace H₂O, but it also reacts with MACC complexes, namely, AlCl₄^–, to form free Cl^–. We suggest that although both the removal of H₂O and the formation of free Cl^– improve electrochemistry by altering the speciation at the interface, the latter has a profound effect on electrodeposition and stripping of Mg.