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 CoS 2 and rate-optimized Co 9 S 8 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 Co 9 S 8 →Cu 7 S 4 →Cu 9 S 5 /Cu 1.8 S and CoS 2 →Cu 7 S 4 →Cu 2 S, which are associated with the redox reactions of Cu 2+ /Cu + ‖Co 2+ /Co and Cu 2+ /Cu + ‖S 2 2− /S 2− , respectively. The electrochemical results show that CoS 2 can exhibit a superior capacity of 619 mAh g −1 at 1 A g −1 after 400 cycles, while Co 9 S 8 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 CoS 2 //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.