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 A 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 LixCo1-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 A distribution in Li2[Co2]O4, non-stoichiometry and kinetic factors restrict the transformation of the lithiated-spinel structure to layered LiCoO2 at this temperature.