Role of Cu in Mo₆S₈ and Cu mixture cathodes for magnesium ion batteries

The reversible capacity of Chevrel Mo₆S₈ cathode can be increased by the simple addition of the Cu metal to Mo₆S₈ electrodes. However, the exact reaction mechanism of the additional reversible capacity for the Mo₆S₈ and Cu mixture cathode has not been clearly understood yet. To clarify this unusual behavior, we synthesize a novel Cu nanoparticle/graphene composite for the preparation of the mixture electrode. We thoroughly investigate the electrochemical behaviors of the Mo₆S₈ and Cu mixture cathode with the relevant structural verifications during Mg²⁺ insertion and extraction. The in situ formation of Cu_xMo₆S₈ is observed, indicating the spontaneous electrochemical insertion of Cu to the Mo₆S₈ host from the Cu nanoparticle/graphene composite. The reversible electrochemical replacement reaction of Cu in the Mo₆S₈ structure is clarified with the direct evidence for the solid state Cu deposition/dissolution at the surface of Mo₆S₈ particles. Moreover, the Mo₆S₈ and Cu mixture cathode improves the rate capability compared to the pristine. We believe that our finding will contribute to understanding the origin of the additional capacity of the Mo₆S₈ cathode arising from Cu addition and improve the electrochemical performance of the Mo₆S₈ cathode for rechargeable Mg batteries.