A LiCoO2-containing powder. and a method for preparing a LiCoO2-containing powder, includes LiCoO2 having a stoichiometric composition via heat treatment of a lithium cobalt oxide and a lithium buffer material to make an equilibrium of a lithium chemical potential therebetween; the lithium buffer material which acts as a Li acceptor or a Li donor to remove or supplement a Li-excess or a Li-deficiency, the lithium buffer material coexisting with the stoichiometric lithium metal oxide. Also an electrode includes the LiCoO2-containing powder as an active material, and a rechargeable battery includes the electrode.