  Multi-valent (MV) battery architectures based on pairing a Mg metal anode
with a high-voltage ($\sim$ 3 V) intercalation cathode offer a realistic design
pathway toward significantly surpassing the energy storage performance of
traditional Li-ion based batteries, but there are currently only few
electrolyte systems that support reversible Mg deposition. Using both static
first-principles calculations and $ab\; initio$ molecular dynamics, we perform
a comprehensive adsorption study of several salt and solvent species at the
interface of Mg metal with an electrolyte of Mg$^{2+}$ and Cl$^-$ dissolved in
liquid tetrahydrofuran (THF). Our findings not only provide a picture of the
stable species at the interface, but also explain how this system can support
reversible Mg deposition and as such we provide insights in how to design other
electrolytes for Mg plating and stripping. The active depositing species are
identified to be (MgCl)$^+$ monomers coordinated by THF, which exhibit
preferential adsorption on Mg compared to possible passivating species (such as
THF solvent or neutral MgCl$_2$ complexes). Upon deposition, the energy to
desolvate these adsorbed complexes and facilitate charge-transfer is shown to
be small ($\sim$ 61 $-$ 46.2 kJ mol$^{-1}$ to remove 3 THF from the strongest
adsorbing complex), and the stable orientations of the adsorbed but desolvated
(MgCl)$^+$ complexes appear favorable for charge-transfer. Finally,
observations of Mg-Cl dissociation at the Mg surface at very low THF
coordinations (0 and 1) suggest that deleterious Cl incorporation in the anode
may occur upon plating. In the stripping process, this is beneficial by further
facilitating the Mg removal reaction.
