The effect of Ni oxidation state on the crystal structure and electrochemical properties of LiNi0.8Co0.1Mn0.1O2 cathode material for highly reversible lithium storage

Abstract

In this work, Ni-rich layered micron-sized LiNi0.8Co0.1Mn0.1O2 (SC-NCM) single crystals were prepared by wet ball-milling and molten-salt methods. Through X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectrometry (XPS), the differences between SC-NCM and a commercial polycrystalline micron-sized NCM material (M-NCM) were compared. An extended sintering process for SC-NCM plays a role in decreasing cationic mixing accompanied by the ordered growth of single crystals. The SC-NCM sample presents homogeneous micron-sized crystals. The Ni3+ content in SC-NCM constitutes 90.45% of the total Ni content, while the Ni3+ in M-NCM constitutes only 68.49%. The existence of Ni2+ in ternary cathode materials is harmful to electrochemical stability and cyclic stability. The initial specific discharge capacity of SC-NCM (168 mAh g−1) is lower than that of M-NCM (187 mAh g−1); however, M-NCM shows a rapid decline in capacities and voltage plateaus from the 1st to 180th cycle. After 300 cycles at a 5 C rate, M-NCM remains only 42 mAh g−1 with a capacity retention rate of only 25.6% in comparison with SC-NCM with a remaining capacity of 80.7 mAh g−1 and a capacity retention of 62.0%. The effect of micron-sized single crystals on electrochemical properties during redox reactions is negative in terms of conductivity but positive in terms of structural stability.