Towards the understanding of (dis)charging mechanism of VS₄ cathode for magnesium batteries

Rechargeable magnesium batteries are promising energy storage technology which could eventually power electric cars. However, the double charge of Mg-ion results in sluggish kinetics in most cathode materials. Due to that, exotic materials, with more complex discharging mechanisms than what we are used for conventional Li-ion batteries, have been explored. In particular, vanadium tetrasulfide (VS₄), a quasi-1D material, was recently shown to be a good candidate for magnesium storage, providing good theoretical capacity and excellent kinetics for magnesium intercalation. Here we present a DFT-based analysis of the complex magnesation process of VS₄. The results indicate a mixed hetero- and homogeneous process with Mg_0.75VS₄ formed at the initial stages of the cathode discharge. At higher magnesiation levels (i.e., Mg_xVS₄ with x > 1) we observed a possible degradation mechanism related to the V[sbnd]S bond breaking, which leads to the formation of magnesium sulfur clusters inside the structure. All of that enables us to identify the origin of the superior properties of VS₄ as cathode material, enabling the design of strategies to further improve its performance.