Enabling Multielectron Reaction of Polyanionic Cathodes Toward High‐Energy Calcium Rechargeable Batteries

Abstract Polyanionic cathode materials with robust structural stability and large Ca 2+ diffusion channels have aroused great interest in propelling the development of calcium‐ion batteries (CIBs). However, polyanionic cathodes usually exhibit single‐electron transfer per unit, rendering limited specific capacity and energy densities. Herein, a new polyanionic Ca x NaV 1.5 Cr 0.5 (PO 4 ) 3 (0 ≤ x ≤ 1.4) cathode is proposed for high‐capacity and ultra‐stable CIBs by unlocking 1.87‐electron transfer per vanadium redox center during Ca ion insertion. The Ca x NaV 1.5 Cr 0.5 (PO 4 ) 3 cathode delivers a reversible calcium storage capacity of 162 mAh g −1 at an average voltage of ≈2.5 V at 10 mA g −1 , featuring a record‐high energy density of ≈400 Wh kg −1 . The low volume changes ( ∆V = 1.8%) and fast diffusion kinetics indicate excellent cycling stability of Ca x NaV 1.5 Cr 0.5 (PO 4 ) 3 with capacity retentions of 98.2% and 80.8% over 600 and 5000 cycles, respectively. In Ca metal full cells made from a Ca metal anode and a compatible electrolyte, the Ca x NaV 1.5 Cr 0.5 (PO 4 ) 3 presents a high energy density of 318 Wh kg −1 over 50 cycles, which rivals the state‐of‐the‐art CIB performance. This work sheds new light on the electrochemically activated multielectron redox reactions of polyanionic cathode materials for sustainable CIBs.