Toward the Development of a High-Voltage Mg Cathode Using a Chromium Sulfide Host

Development of Mg-ion batteries as advanced electrochemical energy storage systems relies on the design and discovery of high-voltage positive electrode (cathode) materials. To date, a variety of sulfide cathodes have been reported (e.g., Mg<sub>x</sub>Mo<sub>6</sub>S<sub>8</sub>, Mg<sub>x</sub>Ti<sub>2</sub>S4, etc.), but the voltages of these materials are too low to prepare a high energy density Mg cell. Theoretical computations predicted that Mg<sub>x</sub>Cr<sub>2</sub>S<sub>4</sub> operating with the high-voltage Cr<sup>3+/4+</sup> redox couple would serve as a suitable cathode candidate, but experimental attempts to extract Mg<sup>2+</sup> from the lattice have been largely unsuccessful. Here, we show that reversible electrochemical activity within a thiospinel framework (AB<sub>2</sub>S<sub>4</sub>) relies on a redox-active transition metal present in the B site; otherwise, anionic redox activity triggers decomposition of the spinel structure. Since Cr and S states are highly coupled in MgCr<sub>2</sub>S<sub>4</sub>, the Cr<sup>3+/4+</sup> redox couple is inaccessible so that reversible (de)intercalation of Mg<sup>2+</sup> cannot occur and charging leads to dissolution of the active material. These findings point to an insufficiency in the screening criteria previously used to identify MgCr<sub>2</sub>S<sub>4</sub> as a promising Mg cathode. Thus, a computable descriptor based on the electronic structure of the discharged material is proposed to predict the prevalence of cation vs anion redox and improve future surveys of potential candidates. It is unlikely that the high-voltage Cr redox couple will be accessible to oxidation in the presence of sulfur within the restrictions of a spinel framework; however, it is possible that a suitable layered Mg<sub>x</sub>CrS<sub>2</sub> structure could serve as a reversible high-voltage Mg cathode.