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

Polyanionic cathode materials with robust structural stability and large Ca²⁺ 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_xNaV_1.5Cr_0.5(PO₄)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_xNaV_1.5Cr_0.5(PO₄)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_xNaV_1.5Cr_0.5(PO₄)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_xNaV_1.5Cr_0.5(PO₄)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.