Chalcogenides have been viewed as important conversion-type Mg 2+ -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 Mg 2+ -storage mechanism has yet to be clarified. Herein, we illustrate that the charge storage mechanism of Cu 2-x Se cathode is a reversible displacement reaction along with 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 Mo 6 S 8 into 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 inspirations for the modification of Mg 2+ -storage cathode, which is a milestone for high performance Mg batteries.