Uncovering the role of Nb modification in improving the structure stability and electrochemical performance of LiNi0.6Co0.2Mn0.2O2 cathode charged at higher voltage of 4.5 V

Ni-rich cathode materials attract ongoing interest due to their high specific capacity (similar to 200 mAh g(-1)). However, these materials suffer rapid capacity fading when charged to a high voltage and cycled at elevated temperature. In this study, we propose a facile method to reconstruct the surface structure of LiNi0.6Co0.2Mn0.2O2 via Nb modification, which integrates the merits of partial Nb5+ doping in the pristine structure and surface Li3NbO4 coating. The obtained results from Rietveld refinement and high resolution transmission electron microscopy confirm that Nb5+ is partially doped into Li+ sites within the surface lattice. Further ex-situ powder X-ray diffraction and kinetic analysis using electrochemical impedance spectroscopy reveal that Nb modification stabilizes the layered structure and facilitates the charge transfer process. Owing to the robust surface structure, 1 mol% Nb modified LiNi0.6Co0.2Mn0.2O2 delivers a discharge capacity of 160.9 mAh g(-1) with 91% capacity retention after 100 cycles at 3.0-4.5 V, whereas the discharge capacity of the pristine sample drops to 139.6 mAh g-1, corresponding to 78% of its initial value. The presence of Nb5+ in the Li layer exhibits positive effects on stability of layered structure, and the surface Li3NbO4 coating layer increases interfacial stability, which results in superior electrochemical performance.