Bilayered Ca_0.28V₂O₅·H₂O: High-Capacity Cathode Material for Rechargeable Ca-Ion Batteries and Its Charge Storage Mechanism

Despite the attractive theoretical benefits of calcium-ion batteries (CIBs) as post-lithium-ion batteries, only a limited number of host materials are known to reversibly intercalate calcium ions to date, and their intercalation mechanism is barely understood. Herein, we report bilayered Ca_0.28V₂O₅·H₂O as a high-capacity CIB cathode material. It exhibits a capacity of 142 mA h g^-1 at ∼3.0 V vs Ca/Ca²⁺ and excellent cyclability. Ca_0.28V₂O₅·H₂O undergoes irreversible structural transformation to a two-fold superstructure during the first charge, which triggers its electrochemical activity from the subsequent cycling. Its intercalation mechanism is unique; upon charging, complete calcium extraction occurs from every two interlayers, maintaining only a fraction of calcium ions in the other interlayers; on discharge, calcium ions are irregularly inserted into the interlayers, resulting in stacking faults. This charge-discharge cycle is highly reversible. This work would be the first report that experimentally unveils the electrochemical calcium storage mechanism of an intercalation host material, providing valuable insights for developing high-performance CIB cathodes.