Structural Stability of LiCoO2 at 400°C

The relative stability of the lithiated-spinel structure, Li₂[Co₂]O₄, at 400°C to the layered LiCoO₂ structure has been investigated. “Low-temperature” LT-LiCoO₂ samples were synthesized at 400°C by the solid-state reaction of Li₂CO₃ with CoCO₃ (or Co₃O₄) for various times between 10 min and 232 days. Least-squares refinements of X-ray powder diffraction patterns were used to determine the fractions of lithiated-spinel Li₂[Co₂]O₄ and layered LiCoO₂ in the samples. X-ray powder diffraction and transmission electron microscope data show that Li₂[Co₂]O₄ nucleates from an intermediate Li_xCo_1−x[Co₂]O₄ spinel product before transforming very slowly to layered LiCoO₂. The experimental data confirm the theoretical prediction that layered LiCoO₂ is thermodynamically more stable than the lithiated-spinel structure at 400°C and support the arguments that a non-ideal cation distribution in Li₂[Co₂]O₄, non-stoichiometry and kinetic factors restrict the transformation of the lithiated-spinel structure to layered LiCoO₂ at this temperature.