Vanadium‐Based Cathode Materials for Calcium‐Ion Batteries

Calcium‐ion batteries (CIBs) are a promising complementary technology to lithium‐ion batteries due to the high abundance and low cost of calcium, as well as potentially higher achievable capacity due to the divalent nature of calcium ions. Yet the development of CIBs is largely limited by sluggish Ca 2+ kinetics and the scarcity of suitable cathode materials. In recent years, vanadium‐based compounds have attracted increasing interest as CIB cathodes owing to their diverse crystal structures, flexible valence states, and tuneable electrochemical properties. This review summarises recent progress in vanadium‐based cathode materials for CIBs through categorising them into two‐dimensional and three‐dimensional structures, including vanadium oxides, vanadyl phosphates and oxygen‐free vanadium compounds as the representative examples for the former, and polyanionic phosphates and fluorides for the latter. Modification strategies for vanadium‐based CIB cathodes are discussed, with emphasis on the relationships between structure and performance/calcium storage mechanisms. Also, outlooks are outlined, covering hybrid ion cell systems, electrolyte engineering, and solvent‐Ca 2+ co‐intercalation in conjunction with anode development.