Deciphering the multi-electron redox chemistry of metal-sulfide electrode toward advanced aqueous Cu ion storage

While neutral aqueous metal batteries, featuring cost-effectiveness and non-flammability, hold significant potential for large-scale energy storage, their practical application is hampered by the limited specific capacity of cathode materials (less than 500 mAh g⁻¹). Herein, capacity-oriented CoS₂ and rate-optimized Co₉S₈ cathodes are developed based on the aqueous copper ion system. The charge-storage mechanism is systematically investigated through a series of ex-situ tests and density functional theory calculations, focusing on the reversible transitions of Co₉S₈→Cu₇S₄→Cu₉S₅/Cu_1.8S and CoS₂→Cu₇S₄→Cu₂S, which are associated with the redox reactions of Cu²⁺/Cu⁺‖Co²⁺/Co and Cu²⁺/Cu⁺‖S₂²⁻/S²⁻, respectively. The electrochemical results show that CoS₂ can exhibit a superior capacity of 619 mAh g⁻¹ at 1 A g⁻¹ after 400 cycles, while Co₉S₈ maintains an outstanding rate performance of 497 mAh g⁻¹ at 10 A g⁻¹ (the retention rate is 95% compared to 521 mAh g⁻¹ at 1 A g⁻¹). As a proof of concept, an advanced CoS₂//Zn hybrid aqueous battery demonstrates a working voltage of 1.20 V and a specific energy of 663 Wh kg_cathode⁻¹. This work provides an alternative direction for developing sulfide cathodes in energetic aqueous metal batteries.