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 Mg0.75VS4 formed at the initial stages of the cathode discharge. At higher magnesiation levels (i.e., MgxVS4 with x > 1) we observed a possible degradation mechanism related to the VS 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.