The present invention provides a LiCoO2-containing powder comprising LiCoO2 having a stoichiometric composition via heat treatment of a lithium cobalt oxide and a lithium buffer material to make equilibrium of a lithium chemical potential therebetween; a lithium buffer material which acts as a Li acceptor or a Li donor to remove or supplement Li-excess or Li-deficiency, coexisting with a stoichiometric lithium metal oxide; and a method for preparing a LiCoO2-containing powder. Further, provided is an electrode comprising the above-mentioned LiCoO2-containing powder as an active material, and a rechargeable battery comprising the same electrode. The present invention enables production of a LiCoO2electrode active material which has improved high-temperature storage properties and high-voltage cycling properties, and is robust in composition fluctuation in the production process. Therefore, the present invention provides advantages such as reduction of time and labor required for quality control and process management in the mass-production of the electrode active material, and decreased production costs of LiCoO2.