Exploring the Peierls-Distorted Vanadium Sulphide as A Rechargeable Mg-Ion Cathod .pdf

For magnesium ion batteries (MIB) to be used commercially, new cathodes must be developed that show stable reversible Mg intercalation. VS4 is one such promising material, with vanadium and disulphide anions [S2]2- forming one dimensional linear chains, with a large interlayer spacing (5.83 A) enabling Mg insertion. However, little is known about the details of the redox processes and structural transformations that occur upon Mg intercalation and deintercalation of VS4. Here we use a suite of local structure characterization methods including XPS, V and S X-ray Absorption Near Edge Spectroscopy and 51V Hahn-Echo and Magic Angle Turning with Phase Adjusted Sideband Separation NMR to elucidate the complex electrochemical reaction pathways. 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 report the formation of a previously unknown intermediate in the Mg-V-S compositional space, Mg3V2S8, which is made of [VS4]3- tetrahedral units and identified using an evolutionary structure predicting algorithm and verified experimentally via X-ray Pair Distribution Function analysis. Subsequent magnesiation gives rise to the reduction of V5+ towards V4+. Further magnesiation sees conversion to MgS plus V metal; this reaction potential is close to the conversion potential of VS4 to Mg3V2S8, leading to competing reaction pathways. Demagnesiation results in the reformation of the V5+, S2- containing intermediate instead of VS4. This work showcases the possibility of developing a family of transition metal polychalcogenides functioning via anionic as well as combined cationic-anionic redox processes, as a potential way of achieving higher capacities for MIBs.