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

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.