As-grown, amorphous LiCoO2 cathodes grown using a sputtering system were annealed in the temperature range of 650–900°C under an Ar and O2 ambients, respectively, in order to obtain crystalline LiCoO2 cathodes. The resulting cathodes were then systematically compared with respect to their charge–discharge properties. The post-annealing process under an O2 ambient resulted in superior cycling performance and thermal stability at high temperature than the Ar ambient. This effect is discussed in terms of grain boundary, surface roughness, and the outdiffusion of oxygen.