Bimetal Substitution Enabled Energetic Polyanion Cathode for Sodium-Ion Batteries

The practical application of Na-superionic conductor structured materials is hindered by limited energy density and structure damage upon activating the third Na⁺. We propose a bimetal substitution strategy with cheaper Fe and Ni elements for costive vanadium in the polyanion to improve both ionic and electronic conductivities, and a single two-phase reaction during Na⁺ intercalation/deintercalation and much reduced Na⁺ diffusion barrier are uncovered by ex-situ X-ray diffraction and density functional theory calculations. Thus, the obtained cathode, Na₃Fe_0.8VNi_0.2(PO₄)₃, shows excellent electrochemical performances including high specific capacity (102.2 mAh g^-1 at 0.1C), excellent rate capability (79.3 mAh g^-1 at 20C), cycling stability (84.6% of capacity retention over 1400 cycles at 20C), low-temperature performance (89.7 mAh g^-1 at 2C and -10 °C), and structure stability in an extended voltage window for the third Na⁺ utilization. A competitive energy density of ≈287 Wh kg^-1 for full batteries based on cathode and anode materials is also confirmed.