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A highly reversible cuprous mediated cathode chemistry for magnesium batteries.

The sluggish kinetic and poor reversibility of cathode chemistry is the major challenge for magnesium (Mg) battery to achieve high volumetric capacity. Here we demonstrate that the introduction of the cuprous (Cu + ) as a charge carrier can decouple the magnesiation related energy storage from the cathode electrochemistry. Importantly, Cu + is generated from a fast equilibrium between copper selenide electrode and Mg electrolyte during standing time, rather than in the electrochemical process. A reversible chemical magnesiation/de-magnesiation can be driven by this solid/liquid equilibrium. During a typical discharge process, Cu + is reduced to Cu and drives the equilibrium to promote the magnesiation process. The reversible Cu to Cu + redox promotes the recharge process. This novel Cu + mediated cathode chemistry of Mg battery leads to a high reversible areal capacity of 12.5 mAh cm -2 with high mass loading (49.1 mg/cm 2 ) of electrode. 80% capacity retention can be achieved for 200 cycles after conditioning process.