Understanding the Role of Minor Molybdenum Doping in LiNi_0.5Co_0.2Mn_0.3O₂ Electrodes: From Structural and Surface Analyses and Theoretical Modeling to Practical Electrochemical Cells

Doping LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) cathode material by small amount of Mo⁶⁺ ions, around 1 mol %, affects pronouncedly its structure, surface properties, and electronic and electrochemical behavior. Cathodes comprising Mo⁶⁺-doped NCM523 exhibited in Li cells higher specific capacities, higher rate capabilities, lower capacity fading, and lower charge-transfer resistance that relates to a more stable electrode/solution interface due to doping. This, in turn, is ascribed to the fact that the Mo⁶⁺ ions tend to concentrate more at the surface, as a result of a synthesis that always includes a necessary calcination, high-temperature stage. This phenomenon of the Mo dopant segregation at the surface in NCM523 material was discovered in the present work for the first time. It appears that Mo doping reduces the reactivity of the Ni-rich NCM cathode materials toward the standard electrolyte solutions of Li-ion batteries. Using density functional theory (DFT) calculations, we showed that Mo⁶⁺ ions are preferably incorporated at Ni sites and that the doping increases the amount of Ni²⁺ ions at the expense of Ni³⁺ ions, due to charge compensation, in accord with X-ray absorption fine structure (XAFS) spectroscopy measurements. Furthermore, DFT calculations predicted Ni-O bond length distributions in good agreement with the XAFS results, supporting a model of partial substitution of Ni sites by molybdenum.