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 (<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/Cu1.8S and CoS2 -> Cu7S4 -> Cu2S, which are associated with the redox reactions of Cu2+/Cu+& Vert;Co2+/Co and Cu2+/Cu+& Vert;S-2(2-)/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. (c) 2025 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.