Composite cathode active materials comprising a composite oxide and an acid treated with an organic solvent are provided. The composite cathode active materials are prepared by treating mixtures of nickel-based composite oxides and organic acids with organic solvents. The active materials suppress gelation of the electrode slurries for a long period of time, even when the active materials are mixed with fluorine-based polymers, by decreasing the basicity of the slurries and the amount of lithium present on the surfaces of the active materials. As a result, electrode slurries having high stability can be prepared. Cathodes and lithium batteries comprising the slurries have excellent charge-discharge characteristics, including high capacity and excellent high rate discharge characteristics.