Dual Active Sites along with Hydrophobic Structure Modulation of Vanadium Hexacyanoferrate for Aqueous Zn-Ion Batteries

Prussian blue analogs (PBAs) have attracted significant attention for use in aqueous zinc-ion batteries (AZIBs) because of their open framework, tunability, and ease of preparation. However, PBAs are still faced with low specific capacity or poor cycling performance as cathode materials for AZIBs, which is attributed to the insufficient number of active sites and structural instability due to water molecules. In this study, vanadium with multivalent properties has been introduced to form a dual active site with Fe, providing multiple electron transfers and possessing a higher specific capacity. Meanwhile, a coprecipitation method is used to form a β-cyclodextrin (β-CD) surface layer with an excluded-volume effect and rich hydroxyl side groups on the surface of vanadium hexacyanoferrate (VOHCF). The surface layer effectively prevents the direct interaction of VOHCF with active water molecules in the electrolyte while also regulating the desolvation structure of Zn2+, enhancing the long-cycle stability of electrode materials. The prepared β-cyclodextrin-vanadium hexacyanoferrate (β-CD-VOHCF) achieves a high reversible capacity (204.1 mAh·g-1 at 0.2 A·g-1), and the capacity retention ratio improves by 65% compared with VOHCF after 3200 cycles at 5 A·g-1. This study offers new ideas to inhibit vanadium dissolution and establish a foundation for the development of VOHCF.