Structures and Electrochemical Properties of LiNi0.5-xCo2x}Mn0.5-xO2as Cathode Materials for Lithium-ion Batteries

Abstract

LiNi0.5-xCo2xMn0.5-xO2 (x = 0, 0.1, 1/6, 1.2, 0.3) were synthesized by the solid-state reaction method. The crystal structure was analyzed by X-ray powder diffraction and Rietveld refinement. LiNi0.5-xCo2xMn0.5-xO2 samples give single phases of hexagonal layered structures with a space group of R-3m for x = 0.1, 1/6, 0.2, and 0.3. The lattice constants of a and c-axis were decreased with the increase in Co contents in samples. The thickness of MO2 slab was decreased and inter-slab distance was increased with the increase in Co contents in LiNi0.5-xCo2xMn0.5-xO2. According to XPS analysis, the valence states of Mn, Co, and Ni in the sample are mainly +4, +3, and +3, respectively. The discharge capacity of 202 mAh/g at 0.1C-rate in the potential range of 4.7 - 3.0 V was obtained in LiNi0.3Co0.4 Mn0.3O2 sample, and LiNi0.4Co0.2Mn0.4O2 gives excellent cycle performance in the same potential range. In order to achieve improved energy densities of LiNi0.5-xCo2x Mn0.5-xO2 cathodes, it is important to enhance the structural stability of these layered compounds at higher charge potentials. In this research, the analysis of crystal structures and valence states of metal species for the samples prepared by solid-state reaction were systematically investigated, and the various electrochemical properties at different charge potentials are reported in this paper. Experimental Section LiNi0.5-xCo2xMn0.5-xO2 (x = 0, 0.1, 1/6, 0.2, 0.3) samples were synthesized by solid-state reactions. Li2CO3, NiO, Co3O4, and MnO2 as starting materials were milled with zirconia balls in ethyl alcohol for 24 hours. The ratio of Li : (Ni + Co + Mn) was adjusted to 1.05 : 1 to adjust lithium loss at high heating temperature. To remove ethyl alcohol, the mixtures of starting materials were dried in oven at 100