Long-range and local structure in the layered oxide Li_1.2Co _0.4Mn_0.4O₂

The layered oxides being considered as intercalation compounds for lithium batteries display significant differences between the long-range crystal structure and local arrangements around individual atoms. These differences are important, because the local atomic environments affect Li-ion transport and, hence, the oxide's rate capability, by determining activation barrier energies, by blocking or opening Li-diffusion pathways, etc. Traditional diffraction methods provide key information on the average crystal structure. However, no single experimental technique can unequivocally determine the average long-range crystal structure and the distribution of local environments over crystallographic distances while retaining atomic-scale resolution. Therefore, in this study, we have employed a combination of diffraction, microscopy, and spectroscopy techniques to investigate the long-range (∼1 μm) and local structure (≥1 nm) of Li_1.2Co_0.4Mn_0.4O ₂, which is a model compound for layered oxides being considered for transportation applications. We find that Li_1.2Co _0.4Mn_0.4O₂ contains mostly Mn⁴⁺ in Li₂MnO₃-like atomic environments and Co³⁺ in LiCoO₂-like atomic environments, which are intimately mixed over length scales of ≥2-3 nm, resulting in a Li_1.2Co _0.4Mn_0.4O₂ crystallite composition that appears homogeneous over the long-range. In addition, we observed a quasi-random distribution of locally monoclinic structures, topotaxially integrated within a rhombohedral-NaFeO₂ framework. Based on these observations, we propose a dendritic microstructure model for Li_1.2Co _0.4Mn_0.4O₂ consisting of well integrated LiCoO₂- and Li₂MnO₃-like structures.