Initiating cationic-anionic chemistry with stepwise surface-to-inner conversion in copper selenide superstructures for high-energy rechargeable magnesium batteries

Copper selenides are viewed as the most capable cathode materials for rechargeable magnesium batteries, yet suffer from unsatisfactory energy density due to their low operating voltage plateau (∼0.9 V vs. Mg/Mg²⁺) and insufficient reversible capacity. Herein, a stepwise conversion from surface cationic-anionic redox to inside electrochemical displacement reaction is realized in copper selenide (Cu_2-xSe) superstructure cathodes. A highly reversible high-voltage platform at ∼1.6 V is discovered during discharge process. Ex-situ spectroscopy and microscopy results demonstrate that the high-voltage plateau is contributed by surface Cu²⁺ charge-carrier transformation and anionic Se^2– redox reaction while sufficient inner Cu-Mg replacement conversion at low-voltage region. Following the favorable mechanism, the Cu_2-xSe superstructure cathodes can present high specific capacity of 385.4 mAh g^–1 at 0.1 A g^–1 and outstanding energy density of 426.3 Wh kg^–1. Moreover, the Cu_2-xSe superstructure cathodes also maintain a reversible capacity of 223.6 mAh g^–1 over 700 cycles with 0.0147 % capacity degradation per cycle at 1.0 A g^–1 and long-term charge-discharge lifetime for 3000 cycles at 5.0 A g^–1. This research reports a new Mg²⁺ storage mechanism for copper selenide cathodes and provides novel route to develop high-energy-density rechargeable magnesium batteries.