BiVO4/V2O5 heterostructures for durable and highly reversible calcium- and zinc-ion batteries†

The potential of BiVO₄/V₂O₅ (BVO/VO) heterostructures for Ca²⁺ and Zn²⁺ ion storage is demonstrated. BVO micro-clusters and VO micro-platelets, characterized by large vacant voids and wide inter-layer spacings, enable facile Zn²⁺ ion intercalation via a diffusion mechanism, owing to its small size and ease of de-solvation at the electrolyte/BVO/VO interface. A Zn-ion battery (ZIB) fabricated with the following architecture, BVO/VO/carbon nanotubes (CNTs)/Zn²⁺/Zn-activated carbon (AC), delivers an initial discharge capacity of ∼162 mA h g⁻¹ and retains nearly 100% of its original capacity after 100 cycles at 30 mA g⁻¹. Accelerated cycling at 2 A g⁻¹ showed this ZIB to retain ∼82% of its initial capacity after 2500 cycles. The highly stable and reversible response is attributed to the formation of robust interphases at the cathode and anode that allow facile Zn²⁺ ion diffusion and prevent any Zn-ion consuming decomposition reactions, as both the electrodes retain their structural integrity with cycling. In a similar vein, a Ca-ion battery (CIB) with a BVO/VO/CNTs/Ca²⁺/AC configuration provides an initial capacity of 120 mA h g⁻¹, with 100% retention after 100 cycles. The large size of Ca²⁺ ions and their large solvation shell inhibit direct intercalation into BVO/VO, allowing only surface faradaic reactions at the cathode/electrolyte interface and anion adsorption/desorption at the AC anode. The consistent storage capacity retained by the cell with cycling is attributed to the stability of the BVO/VO heterostructures and AC, which are largely unaffected by the back-and-forth movement of Ca²⁺ ions during charge–discharge.