Conditioning-Free Mg Electrolyte by the Minor Addition of Mg(HMDS)₂

Mg-based batteries are an attractive next-generation energy storage chemistry due to the high natural abundance and inexpensive cost of Mg, along with the high theoretical energy density compared to that of conventional Li-ion chemistry. The greater energy density is predicated on a Mg metal anode, and pathways to achieving reversible Mg electrodeposition and stripping are reliant on the development of Mg electrolytes. Although Mg electrolyte chemistry has advanced significantly from the reactive Grignards of the 1920s to the carboranes of this decade, there remains significant challenges in correlating the Mg metal anode electrochemistry with the composition of the electrolyte salts as a result of the complicated interface of Mg metal and the electrolyte. To probe the effect of the interface on Mg electrodeposition, we turn to an electrolyte with a known solution-phase composition: the magnesium aluminum chloride complex (MACC) electrolyte. The MACC electrolyte requires electrolytic conditioning to support reversible Mg electrodeposition and stripping. Here, we show that a small concentration (2-5 mM) of Mg(HMDS)₂ with respect to the MACC electrolyte salts suppresses Al³⁺ deposition and promotes reversible Mg electrodeposition and stripping in the first cycle. The significant effect of a small concentration of additive is attributed to changes to the electrode interface. The impact of the Mg interface on the observed electrochemical performance is discussed.