First-principles and experimental investigation of the morphology of layer-structured LiNiO2 and LiCoO2

LiNiO2 (LNO)-based layered materials for use as cathodes in lithium ion batteries generally take the form of agglomerates composed of small particles. Such morphologies produce low electrode densities compared with LiCoO2 (LCO). In this study, the surface energies of various LNO and LCO crystal facets were calculated, and the morphological characteristics of the materials were interpreted based on these results. Crystal models were constructed and the energy of each facet was calculated using density functional theory methods. The chemical mechanisms underlying the atomic structure at each type of facet surface were analyzed using molecular orbital methods. All facet planes yielded surface energies for LNO that were lower than those for LCO. Atoms in the LNO crystal were found to be less ionized than in LCO, which provided weaker ionic bonding and a lower surface energy. The surface energy was proposed to be the main driving force underlying the morphological differences between LNO and LCO. LNO was expected to favor a high fraction of Li atoms at the facet planes. The interpretations based on the theoretical calculations were supported by the experimental results and showed excellent agreement.