Conditioning-Free MgCl2/AlCl3 Electrolyte for Magnesium Batteries By Chemical Formation of Solvated Chloride

Abstract

Mg metal is a promising alternative anode material that offers higher theoretical specific capacity than conventional Li-ion batteries (LIBs), and the high natural abundance of Mg makes it more economically appealing than other post-LIB anode materials. The greater energy density hinges on a Mg-metal anode, and reversible Mg electrodeposition and stripping require an effective Mg electrolyte. Various electrolyte systems that support reversible have been introduced, but many of them contain drawbacks, such as narrow electrochemical window, reactivity with electrode materials, or poor Coulombic efficiency. In addition, challenges remain in understanding the relationship between electrochemistry at the Mg anode and the electrolyte composition due to the complicated interface of Mg metal and the electrolyte.To probe the effect of the interface on Mg electrodeposition, an electrolyte with a known solution phase composition, the magnesium aluminum chloride complex (MACC) electrolyte, is explored. The MACC electrolyte undergoes electrolytic conditioning to support reversible Mg electrodeposition and stripping. During conditioning, free Cl- is released electrolytically while Al3+ is irreversibly deposited, and the concentration of the active species [Mg2(µ−Cl)3·6(THF)]+ increases, activating the MACC electrolyte.Here, we show that a small concentration of Mg(HMDS)2 to the MACC electrolyte suppresses Al3+ deposition and promotes reversible Mg electrodeposition and stripping on the first cycle. Such a drastic change from a small concentration suggests that changes are localized at the electrode-electrolyte interface. We show spectroscopically that Mg(HMDS)2 scavenges trace water in the solution. Spectroscopic results also suggest that Mg(HMDS)2 reacts with AlCl4 - in the MACC electrolyte to form free Cl-. Based on the spectroscopic and electrochemical results, we suggest that the formation of Cl- is the primary cause for improved behavior on cycle one, highlighting the role of Cl- in Mg deposition and stripping.