Structural analysis and electrochemical investigation of dual-doped NMC622 cathode material: Effect of sodium and neodymium on the performance in Li-ion batteries

Ni-rich layered oxides are the most promising cathode materials for high-energy-density Li-ion batteries. Full utilization of their potential resulting from high nickel content, mainly high capacity, is impaired due to the rapid performance degradation. Mitigating the main limitations of Ni-rich materials is possible through element doping. In this work, a series of LiNi0.6Mn0.2Co0.2O2 (NMC622) cathode materials doped with sodium and/or neodymium was synthesized and systematically studied (XRD, SEM, TEM, STEM-EDX, XPS, galvanostatic charge/discharge tests, cyclic voltammetry). The strategies of single-element and dual-element doping were applied to study the effect of Na and Nd doping on the electrochemical performance of the modified NMC622 materials. The Na-doped NMC622 material exhibited improved capacity retention of 80.5% after 100 cycles, which is superior to the undoped one (62.7%), possibly owing to enlarged Li layer spacing and decreased Li ion migration activation energy. For the Nd-doped NMC622 cathode material, both the initial discharge capacity and capacity retention were much improved compared to the undoped NMC622. This can be related to the enhanced structural stability brought by the formation of strong bonds between neodymium and oxygen atoms. In contrast, the dual-doping of Na and Nd in NMC622 material resulted in much poorer electrochemical and cycling performance, but the reason for this is unclear. The experimental data suggest that the combination of Na and Nd dopants caused the deterioration of crystal structure, possibly due to the introduced impurities. This consequently affected the structural stability of the dual-doped material, leading to the lowest discharge capacity and capacity retention among the studied NMC622 materials.