Exploring Cation–Anion Redox Processes in One-Dimensional Linear Chain Vanadium Tetrasulfide Rechargeable Magnesium Ion Cathodes

Abstract

For magnesium ion batteries (MIBs) to be used commercially, new cathodes must be developed that show stable reversible Mg intercalation. VS 4 is one such promising material, with vanadium and disulfide anions [S 2 ] 2− forming onedimensional linear chains, with a large interchain spacing (5.83 Å) enabling reversible Mg insertion. However, little is known about the details of the redox processes and structural transformations that occur upon Mg intercalation and deintercalation. Here, employing a suite of local structure characterization methods including X-ray photoelectron spectroscopy (XPS), V and S X-ray absorption nearedge spectroscopy (XANES), and 51 V Hahn echo and magic-angle turning with phase-adjusted sideband separation (MATPASS) NMR, we show that the reaction proceeds via internal electron transfer from V 4+ to [S 2 ] 2− , resulting in the simultaneous and coupled oxidation of V 4+ to V 5+ and reduction of [S 2 ] 2− to S 2− . We report the formation of a previously unknown intermediate in the Mg−V−S compositional space, Mg 3 V 2 S 8 , comprising [VS 4 ] 3− tetrahedral units, identified by using density functional theory coupled with an evolutionary structure-predicting algorithm. The structure is verified experimentally via X-ray pair distribution function analysis. The voltage associated with the competing conversion reaction to form MgS plus V metal directly is similar to that of intermediate formation, resulting in two competing reaction pathways. Partial reversibility is seen to re-form the V 5+ and S 2− containing intermediate on charging instead of VS 4 . This work showcases the possibility of developing a family of transition metal polychalcogenides functioning via coupled cationic−anionic redox processes as a potential way of achieving higher capacities for MIBs.

Synergy Strategy of Electrical Conductivity Enhancement and Vacancy Introduction for Improving the Performance of VS4 Magnesium-Ion Battery Cathode

“…Such results indicate that Mo-VS 4 /N-TG shows a high reversible electrochemical insertion/extraction of Mg 2+ and excellent tolerance to current mutation. In addition, the Mo-VS 4 /N-TG electrode also shows a higher rate performance compared to the other reported cathode materials, and the intuitive comparison diagram is shown in Figure f. ,,− Figure g shows the cyclic performances of three electrodes evaluated at 50 mA g –1 . Mo-VS 4 /N-TG delivers a stable specific capacity of about 140 mAh g –1 after 600 cycles.…”

Structural Evolution of Layered Manganese Oxysulfides during Reversible Electrochemical Lithium Insertion and Copper Extrusion

“…23 , 25 , 26 Recent work has also shown that anion redox involving sulfur can be important in controlling reaction pathways. For example, sulfides such as VS 4 27 , 28 and Li 2 FeS 2 29 undergo reversible ion intercalation/deintercalation reactions involving coupled S 2– /S 2 2– beside transition-metal redox couples.…”

“…Co and Ni compounds contain many fewer vacancies in the Cu 2 S 2 layers than the analogues with the more readily oxidized Mn cations, indicating that the composition of the oxide layer and/or the presence of vacancies in the copper sulfide slabs affects the lithiation processes. ,, Recent work has also shown that anion redox involving sulfur can be important in controlling reaction pathways. For example, sulfides such as VS 4 , and Li 2 FeS 2 undergo reversible ion intercalation/deintercalation reactions involving coupled S 2– /S 2 2– beside transition-metal redox couples.…”

Structural Evolution of Layered Manganese Oxysulfides during Reversible Electrochemical Lithium Insertion and Copper Extrusion

“…23,25,26 Recent work has also show that anion redox involving sulfur can be important in controlling reaction pathways. For example, sulphides such as VS4 27,28 and FeS2 29 undergo reversible ion intercalation-deintercalation reactions involving coupled S 2− /S 2 2− beside transition-metal redox couples.…”