Understanding
the Role of Minor Molybdenum Doping
in LiNi0.5Co0.2Mn0.3O2 Electrodes: from Structural and Surface Analyses and Theoretical
Modeling to Practical Electrochemical Cells
posted on 2018-08-10, 00:00authored byOrtal Breuer, Arup Chakraborty, Jing Liu, Tatyana Kravchuk, Larisa Burstein, Judith Grinblat, Yaron Kauffman, Alexandr Gladkih, Prasant Nayak, Merav Tsubery, Anatoly I. Frenkel, Michael Talianker, Dan T. Major, Boris Markovsky, Doron Aurbach
Doping LiNi0.5Co0.2Mn0.3O2 (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.