Sluggish kinetics and poor reversibility of cathode chemistry is the major challenge for magnesium batteries to achieve high volumetric capacity. Introduction of the cuprous ion (Cu+ ) as a charge carrier can decouple the magnesiation related energy storage from the cathode electrochemistry. 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 the electrode. 80 % capacity retention can be achieved for 200 cycles after a conditioning process.
Keywords: cathodes; cuprous ions; energy storage mechanism; magnesium batteries; solution electrochemistry.
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