Charge-Compensation in a Displacement Mg²⁺ Storage Cathode through Polyselenide-Mediated Anion Redox

Chalcogenides have been viewed as important conversion-type Mg²⁺ -storage cathodes to fulfill the high volumetric energy density promise of magnesium (Mg) batteries. However, the low initial Columbic efficiency and the rapid capacity degradation remain challenges for the chalcogenide cathodes, as the clear Mg²⁺ -storage mechanism has yet to be clarified. Herein, we illustrate that the charge storage mechanism of the Cu_2-x Se cathode is a reversible displacement reaction along with a polyselenide (PSe) mediated solution process of anion-compensation. The unique anion redox improves charge storage, while the dissolution of PSe also leads to performance degradation. To address this issue, we introduce Mo₆ S₈ into the Cu_2-x Se cathode to immobilize PSe, which significantly improves performance, especially the reversible capacity (from 140 mAh g^-1 to 220 mAh g^-1 ). This work provides inspiration for the modification of the Mg²⁺ -storage cathode, which is a milestone for high-performance Mg batteries.