Ultra‐High Capacity and Cyclability of β‐phase Ca0.14V2O5 as a Promising Cathode in Calcium‐Ion Batteries

Abstract Crystalline water‐free β‐phase Ca 0.14 V 2 O 5 is reported for the first time as a viable cathode material for calcium‐ion batteries (CIBs). In contrast to layered α‐V 2 O 5 and δ‐Ca x V 2 O 5 ·nH 2 O, which have limited capacity, the β‐phase delivers a reversible capacity of ≈247 mAh g −1 , which corresponds to the insertion/extraction of Ca 2+ between Ca 0.14 V 2 O 5 and Ca 1.0 V 2 O 5 . The process of Ca 2+ insertion process and the accompanying structural relaxation are theoretically and experimentally verified. The initial insertion of Ca 2+ into Ca 0.14 V 2 O 5 causes a slight shift of oxygen atoms surrounding hepta‐coordination sites, creating penta‐coordinated sites that are then partially filled up to Ca 0.33 V 2 O 5 . Further insertion occurs through the stepwise occupation of up to 50% of neighboring hexa‐ and tetra‐coordination sites to form Ca 0.67 V 2 O 5 and Ca 1.0 V 2 O 5 , respectively. The rearrangement of oxygen atoms in Ca 0.14 V 2 O 5 also minimizes dimensional changes, leading to high cyclic stability during repeated charge/discharge cycles. The remarkable electrochemical performance of full cells containing a Ca 0.14 V 2 O 5 cathode and a K metal anode in Ca 2+ /K + hybrid electrolytes, is also demonstrated, thanks to the inertness of K + insertion into Ca 0.14 V 2 O 5 and the absence of calcium plating/stripping. The cyclic stability and high capacity of Ca 0.14 V 2 O 5 is not compromised in hybrid electrolytes, making it a viable CIB cathode.