Robust VS4@rGO nanocomposite as a high-capacity and long-life cathode material for aqueous zinc-ion batteries

Although vanadium (V)-based sulfides have been investigated as cathodes for aqueous zinc-ion batteries (ZIBs), the performance improvement and the intrinsic zinc-ion (Zn²⁺) storage mechanism revelation is still challenging. Here, VS₄@rGO composite with optimized morphology is designed and exhibits ultrahigh specific capacity (450 mA h g^-1 at 0.5 A g^-1) and high-rate capability (313.8 mA h g^-1 at 10 A g^-1) when applied as cathode material for aqueous ZIBs. Furthermore, the VS₄@rGO cathode presents long-life cycling stability with capacity retention of ∼82% after 3500 cycles at 10 A g^-1. The structural evolution, redox, and degradation mechanisms of VS₄ during (dis)charge processes are further probed by in situ XRD/Raman techniques and TEM analysis. Our results indicate that the main energy storage mechanism is derived from the intercalation/deintercalation reactions in the open channels of VS₄. Notably, an irreversible phase transition of VS₄ into Zn₃(OH)₂V₂O₇·2H₂O (ZVO) during the charging process and the further transition from ZVO to ZnV₃O₈ during long-term cycles are also observed, which might be the main reason leading to the capacity degradation of VS₄@rGO. Our study further improves the electrochemical performance of VS₄ in aqueous ZIBs through morphology design and provides new insights into the energy storage and performance degradation mechanisms of Zn²⁺ storage in VS₄, and thus may endow the large-scale application of V-based sulfides for energy storage systems.