Investigation on the formation of Mg metal anode/electrolyte interfaces in Mg/S batteries with electrolyte additives

Abstract

The combination of sulfur (S) as cathode with a Mg anode is a promising approach for batteries because of the high theoretical capacity (1672 mA h g⁻¹) as well as the abundance and relatively low cost of these elements. However, up to now Mg/S batteries face serious drawbacks like a large overpotential between charge/discharge cycles, rapid capacity fading, poor cycling efficiency and slow kinetics, etc. Most of these problems are strongly correlated with polysulfide dissolution in the electrolyte and their subsequent shuttling to the Mg anode side. In this work, we investigate the Mg²⁺ deposition/dissolution process on a Mg metal electrode using a non-nucleophilic magnesium electrolyte (0.4 M Mg[B(hfip)₄]₂/DME) in the presence or absence of polysulfides. The large overpotential increase of the Mg metal anode in the presence of polysulfide species is reduced by the addition of optimum concentration of iodine additives to the Mg electrolyte, which help in the formation of a favorable, less corrosive, uniform and stable interfacial layer. In addition to the electrochemical studies, this work examines the chemical composition, thickness and morphology of the interphase layer formed on Mg metal by using X-ray photoelectron spectroscopy (XPS) with depth profiling and focused ion beam-scanning electron microscopy (FIB-SEM) techniques. Furthermore, the stability of different MgX phases (X = S, I₂ and F₂) formed between the Mg metal anode and different additive containing electrolytes and Mg diffusion through these interphases are investigated by density functional theory (DFT) calculations.