Electrochemical Mg2+ Displacement Driven Reversible Copper Extrusion/Intrusion Reactions for High-Rate Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) based on metal Mg anodes have shown great potential owing to the abundant natural resources, high volumetric capacity, and low safety hazard. Nevertheless, the development of RMBs is hampered by the sluggish kinetics of Mg2+ diffusion and the limited cyclability of cathode materials. Herein, nonstoichiometric copper selenide (Cu2-xSe) are synthesized via a solution-based method and exploited as a durable cathode material based on ionic displacement mechanism for RMBs. The copper ions in the Se2- based sub-lattices are reversibly exchanged by Mg2+ ions without causing lattice collapse. Owing to the same face-centered cubic Se2- sub-lattices and similar unit cell size of Cu2-xSe and MgSe, the energy barrier for lattice reconstruction during cycling processes is very low, significantly improving the rate performance, structural stability, and cycle life of the Cu2-xSe cathode. Moreover, metal Cu is in situ generated during discharging, thus greatly facilitating electron transport. Comprehensive characterizations confirm that the Cu2-xSe cathode undergoes reversible copper ion extrusion/reinjection during the discharge-charge steps. This work suggests the great potential for exploring high-performance electrode materials based on ionic displacement mechanism for advanced multivalent-ion secondary batteries.