Structural analysis of layered Li2MnO3-LiMO2 (M=Ni1/3Mn1/3Co1/3, Ni1/2Mn1/2) cathode materials by Rietveld refinement and first-principles calculations

Layered LiNi1/3Mn1/3Co1/3O2, LiNi1/2Mn1/2O2, 0.35Li2MnO3·0.65LiNi1/3Mn1/3Co1/3O2 and 0.4Li2MnO3·0.6LiNi1/2Mn1/2O2 cathode materials were investigated by experimental and computational methods. The Rietveld refinement showed that the X-ray diffraction patterns of LiNi1/3Mn1/3Co1/3O2 and LiNi1/2Mn1/2O2 were well fitted with a model based on a single hexagonal R3¯m structure. The characteristic peaks of the 0.35Li2MnO3·0.65LiNi1/3Mn1/3Co1/3O2 and 0.4Li2MnO3·0.6LiNi1/2Mn1/2O2 samples between 20° and 25° could not be refined using this single hexagonal phase model. Refinement based on a model with two phases from the hexagonal R3¯m and monoclinic C2/m structures were well fitted, with Li/Mn disorder in the 2b, 4h and 4g sites as well as different Wyckoff positions and lattice parameters for the Li2MnO3 phase. Energies and the density of states (DOS) analysis calculated by First-principles further clarified the structural stability.