Boosting anionic-cationic redox chemistry in anion-rich CuSe₂ cathode toward high-energy magnesium batteries

Electrochemical Mg-Cu displacement is recognized as the prominent capacity-contribution reaction in copper-based chalcogenide cathodes. However, such one-sided mechanism experiences low discharge voltage plateau and thus restricts the exploitation of high-energy magnesium batteries. Herein, a synergetic cationic-anionic redox chemistry mechanism is revealed in anion-rich copper selenide (CuSe₂) cathode for high-energy magnesium batteries. Ex-situ spectroscopic characterization and DFT calculations demonstrate the mechanism with high-voltage Se-Cl anionic redox chemistry and low-voltage cationic Mg-Cu replacement reaction. A series of copper selenides with controllable anion content are fabricated by phase engineering strategy via regulating the Se source concentration during selenization. The anion-rich CuSe₂ cathode shows both superior anionic and cationic redox reactions for Mg²⁺ storage kinetics with considerable capacity of 440.6 mAh g^–1 and high energy density of 439.4 Wh kg^–1. Based on the outstanding reaction kinetics, the CuSe₂ cathode also delivers remarkable rate capability with 169 mAh g^–1 at 2.0 A g^–1 and cycling life for 1500 cycles. Theoretical investigation suggests that the anion-rich phase can show the most effective adsorption of Mg²⁺ and Cl^– and the highest conductivity. This work unveils a brand-new anionic-cationic redox chemistry mechanism and provides a high-efficiency strategy for fabricating anion-rich copper selenides toward high-energy rechargeable magnesium batteries.