In situ high-energy X-ray diffraction was carried out to investigate the structural transformation of lithium cobalt oxides during the solid-state synthesis. Two allotropic phases were observed during the synthesis process; Li2Co2O4 with a spinel structure was formed within the temperature window between 450 °C and 650 °C, beyond which Li2Co2O4 was converted to its hexagonal counterpart, layered LiCoO2, through a cation exchange between Li and Co. In electrochemical tests, the Li2Co2O4 was estimated to have a very low reversible capacity, ∼20 mAh g−1, and a high initial irreversible capacity loss of about 80 mAh g−1. An interfacial phase between layered LiCoO2 domain and spinel Li2Co2O4 domain was also identified by ex situ high-resolution X-ray diffraction.