Identical cut-off voltage versus equivalent capacity: an objective evaluation of the impact of dopants in layered oxide cathodes

The structural and cycling instability of high-nickel layered oxide cathodes is highly related to the removal content of Li ions from the material, which is responsible for the limited lifetime of the corresponding batteries. Elemental doping is generally utilized as an efficient approach to solve this issue. However, nearly all the studies in the literature have evaluated the impact of the dopants on the structural and cycling stability basis at the same charge cut-off voltage rather than the equivalent extraction amount of Li ions in the cathode materials. To objectively evaluate the impact of the dopants on the cathodes, we compared the structural and cycling stability as well as the interphase of the Al- and Co-doped LiNiO₂ cathodes upon charging to the same cut-off voltage and equivalent capacity. Based on the same charge cut-off voltage, the doped Al improves the structural and interphasial stability and cycling life. However, based on an identical charge capacity, the electrochemical properties, interphase, and phase transition are deteriorated by Al substitution due to the greater lattice shrinkage of 6.4% than that of Co-doped cathodes (4.6%) and LiNiO₂ (1.63%). Plus, more phase transition from H2 to H3 was found to occur in Al-doped cathodes, contrary to the findings of most studies. The Co-doped cathode shows more remarkable performance in terms of cycling, phase transition, and interphase. This work presents a comprehensive assessment by which to systematically evaluate the impact of dopants from different perspectives and establishes fundamental guidelines for clarifying the nature of dopants.