Understanding the Initial Stages of Reversible Mg Deposition and Stripping in Inorganic Nonaqueous Electrolytes

Abstract

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