Rationally Designed Sodium Chromium Vanadium Phosphate Cathodes with Multi-Electron Reaction for Fast-Charging Sodium-Ion Batteries

Sodium super-ionic conductor (NASICON)-structured phosphates are emerging as rising stars as cathodes for sodium-ion batteries. However, they usually suffer from a relatively low capacity due to the limited activated redox couples and low intrinsic electronic conductivity. Herein, a reduced graphene oxide supported NASICON Na₃Cr_0.5V_1.5(PO₄)₃ cathode (VC/C-G) is designed, which displays ultrafast (up to 50 C) and ultrastable (1 000 cycles at 20 C) Na⁺ storage properties. The VC/C-G can reach a high energy density of ≈470 W h kg⁻¹ at 0.2 C with a specific capacity of 176 mAh g⁻¹ (equivalent to the theoretical value); this corresponds to a three-electron transfer reaction based on fully activated V⁵⁺/V⁴⁺, V⁴⁺/V³⁺, V³⁺/V²⁺ couples. In situ X-ray diffraction (XRD) results disclose a combination of solid-solution reaction and biphasic reaction mechanisms upon cycling. Density functional theory calculations reveal a narrow forbidden-band gap of 1.41 eV and a low Na⁺ diffusion energy barrier of 0.194 eV. Furthermore, VC/C-G shows excellent fast-charging performance by only taking ≈11 min to reach 80% state of charge. The work provides a widely applicable strategy for realizing multi-electron cathode design for high-performance SIBs.