EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

For magnesium ion batteries to be used commercially, new cathodes must be developed that show stable reversible Mg intercalation. VS4 is one such promising material, with vanadium and disulfide anions [S2]2– forming one-dimensional linear chains, with a large interchain spacing enabling reversible Mg insertion. VS4 has shown uptake of 0.84 mol of Mg2+ during the first discharge (rate C/12), and long term cyclability restores to nearly 0.5 mol of Mg.1 X ray photoelectron spectroscopy has identified the surface oxidation of V4+ to V5+ and reduction of [S2]2- to S2- at the end of discharge. However, the questions remain, including how is Mg2+ intercalated in the VS4 layer? Is VS4 reforming on charging? Does any conversion take place? And, what would be the structure of insertion materials?
In this presentation, we will revisit the magnesium electrochemistry of VS4, unravelling the mechanism behind the electrochemical features of magnesiation and demagnetisation.2 Employing a suite of local structure characterization methods including XPS, V and S XANES, and 51V Hahn-echo and MATPASS NMR, we show that the reaction proceeds via internal electron transfer from V4+ to [S2]2–, resulting in the simultaneous and coupled oxidation of V4+ to V5+ and reduction of [S2]2– to S2–. We present the formation of a previously unknown intermediate in the Mg–V–S compositional space, Mg3V2S8, comprising [VS4]3– tetrahedral units, identified by using an evolutionary structure-predicting algorithm, and verified experimentally via XPDF analysis. The voltage associated with the conversion reaction to form MgS + V metal is similar to that of intermediate formation, resulting in two competing reaction pathways. Partial reversibility is seen to re-form the V5+ and S2– containing intermediate on charging instead of VS4.

1. Y. Wang et al., Adv. Mater. 2018, 30, 32, 1802563.
2. S. Dey et al., J. Am. Chem. Soc. 2020, 142, 46, 19588.