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 degrees C and 650 degrees 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, similar to 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. (C) 2014 Elsevier B.V. All rights reserved.