Determining the origin of poor electronic conductivity and ultrafast ionic conductivity in Na₃V₂(PO₄)₂FO₂ based on first principles and ab initio molecular dynamics methods

Sodium ion technology is increasingly investigated as a low-cost solution for grid storage applications. Among the reported cathode materials for sodium-ion batteries, Na₃V₂(PO₄)₂FO₂ is considered as one of the most promising materials due to its high operation voltage and good cyclability. Here, the de-sodiumization process of Na₃V₂(PO₄)₂FO₂ has been systematically examined using first-principles calculations to uncover the fundamental questions at the atomic level. Four stable intermediate products during the de-sodiumization process are firstly determined based on the convex hull, and three voltage platforms are then predicted. Except for two voltage platforms (3.37 V and 3.75 V) close to the experimental values, the platform up to 5.28 V exceeds the stability window (4.8 V) of a typical electrolyte, which was not observed experimentally. Excitingly, the change of volume is only 2% during the sodiumization process, which should be the reason for the good cycling stability of this material. Electronic structure analysis also reveals that the valence states of V ions will be changed from V⁵⁺ to V⁴⁺ during the sodiumization process, resulting in a weak Jahn-Teller distortion in VO₅F octahedra, and then making the lattice-constants asymmetrically change. More seriously, combined with a bandgap of 2.0 eV, the conduction band minimum mainly composed of V-t_2g non-bonding orbitals has strong localized characteristics, which should be the intrinsic origin of poor electron transport properties for Na_xV₂(PO₄)₂FO₂. Nonetheless, benefiting from the layer-like structure features with F-segmentation, this material has an ultrafast sodium ionic conductivity comparable to that of NASICON, with an activation energy of only 82 meV.