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, NaFeVNi(PO), shows excellent electrochemical performances including high specific capacity (102.2 mAh g at 0.1C), excellent rate capability (79.3 mAh g at 20C), cycling stability (84.6% of capacity retention over 1400 cycles at 20C), low-temperature performance (89.7 mAh g 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 for full batteries based on cathode and anode materials is also confirmed.