Understanding the Role of Minor Molybdenum Doping in LiNi 0.5 Co 0.2 Mn 0.3 O 2 Electrodes: from Structural and Surface Analyses and Theoretical Modeling to Practical Electrochemical Cells.

Doping Ni-rich LiNi0.5Co0.2Mn0.3O2 (NCM523) cathode material by small amount of Mo6+ ions, around 1 mol.%, affects pronouncedly its structure, surface properties, 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 towards 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. Further, 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.