Electronic Conductivity as a Rate‑Limiting Factor for Calcium‑Ion Storage in Layered Vanadium Oxides

Rechargeable calcium batteries are promising for sustainable energy storage but lack cathode materials that reversibly intercalate Ca²⁺ at practical rates. While considerable efforts have focused on improving ionic conductivity, this work investigates how electronic conductivity governs Ca²⁺ storage performance by employing bilayer δ‑V₂O₅·nH₂O (VOH) as a model cathode. Electronic conductivity is first enhanced by pre-intercalating 1‑butyl‑1‑methylpyrrolidinium cations (Pyr⁺) into the molecular layers. These cations attract the electron cloud of lattice oxygen, increasing the density of states at the Fermi level and shortening the electron transport distance between adjacent oxygen atoms, thereby resulting in higher electronic conductivity. Further improvements are achieved by coating the active particles with conductive polyaniline (PANI) and incorporating carbon nanotubes (CNTs) into the cathode formulation. Electrochemical measurement reveals a linear correlation between storage capacity and electronic conductivity. The resulting P‑PyrV@CNT cathode delivers a high capacity of 320 mAh g^-1 (eight‑fold increase over pristine VOH) and excellent cycling stability over 2000 cycles (80% retention). Structural and elemental analysis confirms reversible Ca²⁺ intercalation into the cathode bulk, accompanied by a coupled electron-ion motion. Overall, these findings identify electronic conductivity as a rate-limiting factor for Ca²⁺ intercalation in layered oxides, offering new design principles for high-performance cathodes for Ca batteries.