Dependence of property, crystal structure and electrode characteristics on Li content for LixNi0.8Co0.2O2 as a cathode active material for Li secondary battery

We investigated the dependence of the properties, crystal and electronic structures and electrode characteristics of LixNi0.8Co0.2O2 as a cathode active material for Li secondary batteries. LixNi0.8Co0.2O2 was prepared by a solid-state method and solution method. The crystal structure was determined by neutron and X-ray diffractions using the Rietveld analysis. All the samples were obtained as the α-NaFeO2 type with the space group R-3m. From the charge–discharge test, the cycle performance was improved with the decreasing Li content (x ≦ 1.066) although the discharge capacity decreased. Samples made by the solid-state method showed a better electrode performance than those made by the solution method. We measured the chemical diffusion coefficient of Li (DLi+˜) by the GITT method. The DLi+˜ in the stable cycle region was much improved in the sample prepared by the solid-state method than by the solution method. From the neutron powder diffraction, it was confirmed that Li2CO3 was formed by increasing the Li content (0.994 < x ≦ 1.066) as a secondary phase. Cation mixing was improved with the decreasing Li content. The bond length of the 3b site–6c site decreased with decreasing Li content. From the electron density images on the (1 1 0) plane for LixNi0.8Co0.2O2, the covalent bond of the 3b site–6c site increased with the decreasing Li content. This may be one of the reasons why the cycle performance improved with the decreasing Li content.