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.