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

Abstract

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. Therefore, our results indicate that maintaining layer-like features and regulating V atoms will be important directions to improve the performance of Na_xV₂(PO₄)₂FO₂.