Ultra‐High Capacity and Cyclability of β‐phase Ca0.14V2O5as a Promising Cathode in Calcium‐Ion Batteries

Abstract

AbstractCrystalline water‐free β‐phase Ca0.14V2O5is reported for the first time as a viable cathode material for calcium‐ion batteries (CIBs). In contrast to layered α‐V2O5and δ‐CaxV2O5·nH2O, which have limited capacity, the β‐phase delivers a reversible capacity of ≈247 mAh g−1, which corresponds to the insertion/extraction of Ca2+between Ca0.14V2O5and Ca1.0V2O5. The process of Ca2+insertion process and the accompanying structural relaxation are theoretically and experimentally verified. The initial insertion of Ca2+into Ca0.14V2O5causes a slight shift of oxygen atoms surrounding hepta‐coordination sites, creating penta‐coordinated sites that are then partially filled up to Ca0.33V2O5. Further insertion occurs through the stepwise occupation of up to 50% of neighboring hexa‐ and tetra‐coordination sites to form Ca0.67V2O5and Ca1.0V2O5, respectively. The rearrangement of oxygen atoms in Ca0.14V2O5also minimizes dimensional changes, leading to high cyclic stability during repeated charge/discharge cycles. The remarkable electrochemical performance of full cells containing a Ca0.14V2O5cathode and a K metal anode in Ca2+/K+hybrid electrolytes, is also demonstrated, thanks to the inertness of K+insertion into Ca0.14V2O5and the absence of calcium plating/stripping. The cyclic stability and high capacity of Ca0.14V2O5is not compromised in hybrid electrolytes, making it a viable CIB cathode.