Electron diffraction and high-resolution electron microscopy studies on layered Li2−δ(Mn1−xCox)1+δO3

The structure of Li2MnO3–LiCoO2 solid solutions or Li2−δ(Mn1−xCox)1+δO3 (LMCO) which are promising lithium-battery cathodes was studied by high-resolution electron microscopy (HREM). X-ray diffraction (XRD) analyses indicate that Li1.6Mn0.2Co1.2O3 and Li1.7Mn0.4Co0.9O3 take a rhombohedral (R3̄m space group) structure, whereas Li1.85Mn0.7Co0.45O3 and Li1.95Mn0.9Co0.15O3 take a monoclinic (C2/m) superstructure. HREM studies on those samples reveal that all of the observed crystallites in sample Li1.95Mn0.9Co0.15O3 contain sharp stripe structures due to the planar defects along the ch-axis (stacking direction of the hexagonal close-packed planes) of parent rhombohedral R3̄m cell. Such stripe structures become faint with increasing x, Co content. The origin of the planar defects is mainly attributed not to the different structures but to the different local orientations of the monoclinic LMCO. Local disordering of Li and (Mn,Co) in (Li,Mn,Co) planes is developed with increasing x and the structures are transformed from C2/m to R3̄m. The alternative (Li,Mn,Co)-plane stacking structure, trigonal P3112 LMCO, was occasionally observed. However, only P3112 LMCO platelets with a thickness of a single unit cell were detected.