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_xMo₆S₈, Mg_xTi₂S₄, etc.), but the voltages of these materials are too low to prepare a high energy density Mg cell. Theoretical computations predicted that Mg_xCr₂S₄ operating with the high-voltage Cr^3+/4+ redox couple would serve as a suitable cathode candidate, but experimental attempts to extract Mg²⁺ from the lattice have been largely unsuccessful. We show that reversible electrochemical activity within a thiospinel framework (AB₂S₄) 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 MgCr₂S₄, the Cr^3+/4+ redox couple is inaccessible so that reversible (de)intercalation of Mg²⁺ 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 MgCr₂S₄ 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_xCrS₂ structure could serve as a reversible high-voltage Mg cathode.

中文翻译：

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使用硫化铬主体开发高压镁阴极

镁离子电池作为先进的电化学储能系统的发展依赖于高压正极（阴极）材料的设计和发现。迄今为止，已经报道了多种硫化物阴极（例如，Mg _x Mo ₆ S ₈、Mg _x Ti ₂ S ₄等），但是这些材料的电压太低而无法制备高能量密度的镁电池。理论计算预测 Mg _x Cr ₂ S ₄与高压 Cr ^3+/4+氧化还原对一起运行将作为合适的阴极候选物，但实验尝试提取 Mg ²⁺从格子已经基本上不成功。我们表明，硫尖晶石骨架 ( AB ₂ S ₄ )内的可逆电化学活性依赖于B位点中存在的氧化还原活性过渡金属；否则，阴离子氧化还原活性会触发尖晶石结构的分解。由于 Cr 和 S 态在 MgCr ₂ S ₄中高度耦合，Cr ^3+/4+氧化还原对是不可接近的，因此 Mg ^{2+ 的}可逆（脱）嵌入不能发生并且充电导致活性材料的溶解。这些发现表明之前用于识别 MgCr ₂ S ₄的筛选标准存在不足作为一种很有前途的镁阴极。因此，提出了一种基于放电材料的电子结构的可计算描述符来预测阳离子与阴离子氧化还原的普遍性，并改进未来对潜在候选物的调查。在尖晶石骨架的限制内，在硫存在的情况下，高压铬氧化还原对不太可能被氧化；然而，合适的层状 Mg _x CrS ₂结构可以用作可逆高压镁阴极。