Exploring the Peierls-Distorted Vanadium Sulphide as A Rechargeable Mg-Ion Cathod .pdf
2020
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.
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