Exploration of the zinc storage mechanism and kinetics of vanadium sulfides/reduced graphene oxide composites for aqueous zinc-ion battery cathodes

Aqueous zinc-ion batteries (AZIBs) are considered suitable devices for large-scale energy storage systems. Vanadium sulfides have gained wide attention as AZIB cathode materials owing to their low cost, high specific capacity, and fast Zn-ion insertion/extraction ability. However, a thorough examination of their actual operation as AZIB cathodes remains lacking. In this study, we synthesized three types of vanadium sulfides/reduced graphene oxide (V_xS₈/rGO, x = 2, 5, and 6), fabricated electrodes from these materials, and systemically explored their Zn-ion storage mechanisms and kinetics. All three V_xS₈/rGO electrodes required an electrochemical activation step, which involved charging over 1.8 V (vs. Zn/Zn²⁺), to obtain high reversible discharging–charging capacity. The V₅S₈/rGO and V₆S₈/rGO electrodes exhibited structural and morphological evolution during electrochemical activation and maintained 70% of their capacities for 700 cycles at a current density of 5 A g⁻¹. The V₂S₈/rGO electrode maintained its initial state during repeated discharge–charge cycling and, thus, exhibited exceptional long-term cycling stability with 99% capacity retention for 700 cycles at the same current density. These findings highlight the importance of an in-depth study of vanadium sulfide materials requiring electrochemical activation to achieve high-power- and energy–density AZIBs.