Two species redox reaction in sulfur-incorporated magnesium vanadate composite as a cathode for aqueous zinc-ion batteries

The vanadium (V)- based materials are treated as potential cathodes for aqueous zinc-ion batteries (ZIBs) owing to their excellent safety feature. Although the layer structure of the V-based cathodes is unstable, it results in drastic capacity decay. Sulfur incorporation of V-based materials is an effective strategy to protect the layer structure as well as boost the redox reaction. Herein, we designed a sulfur-incorporation of magnesium vanadate (MgV₂O₄) to form the magnesium vanadate and vanadium sulfide (V₃S₄) composite (i.e., MgV₂O₄@V₃S₄) using a microwave-assisted solvothermal method and utilized it as a cathode for aqueous ZIBs. For the comparison, using the same synthesis procedures, the MgV₂O₄@V₂O₃ sample also prepared without any sulfur incorporation. The MgV₂O₄@V₃S₄ cathode composite demonstrates excellent electrochemical performance due to the positive valence conversion of sulfur. Impressively, the MgV₂O₄@V₃S₄ cathode composite obtains a high discharge capacity of 340 mA h g⁻¹ over 200 cycles at 500 mA g⁻¹ (capacity retention: 93.4 %). The Zn//MgV₂O₄@V₃S₄ cell exhibits a very impressive energy density of 442 Wh kg⁻¹ at 733 W kg⁻¹, and 2690 W kg⁻¹ at 132.6 Wh kg⁻¹, revealing the good power features. Ex-situ XPS study reveals that the zinc-ion storage mechanism in MgV₂O₄@V₃S₄ occurs through V⁴⁺/V⁵⁺ and S²⁻/S⁴⁺ redox reaction.