Synthesis and electrochemical performance of Li3NbO4-based cation-disordered rock-salt cathode materials for Li-ion batteries

In this reported study, Li₃NbO₄-based, Li-excess cathode materials xLi₃NbO₄• (1-x)LiMnO₂ (0.2 ≤ x ≤ 0.5) were synthesized using a solid state reaction method. XRD results showed that the fabricated powders with x = 0.3–0.5 could be indexed to a typical rock-salt structure (Fm-3m space group). Rietveld refinements showed that the lattice parameters increased with the increase in Nb⁵⁺ content. SEM and TEM images of the product showed that the elements Nb, Mn and O were evenly distributed in the samples, which further confirmed the cubic rock-salt structure of the product. Electrochemical tests of the samples showed that the sample with x = 0.4 exhibited the best electrochemical performance (232 mAh/g in the first cycle and more than 175 mAh/g in 50 cycles, as well as relatively high discharge capacity at high discharge current density). The redox mechanism analysis of the experimental results indicated that there were two stages in charge process: one was the Mn^3+/4+ oxidation reaction below 4.3 V, and the other was related to the oxidation of O²⁻ to O⁻ or the release of O₂, which improved the reversible capacity of the material. The structural evolution analysis of the samples showed that a spinel phase structure was present during the cycling process and the shift in the XRD peaks indicated that the sample with x = 0.4 exhibited the best structural stability. The electrochemical impedance spectroscopy analysis showed that the sample with x = 0.4 had the smallest R_ct. The results for the kinetics of lithium ion diffusion indicated that the sample with x = 0.4 had largest D_Li+, which was consistent with the best electrochemical performance for this material, especially the rate capability.