The present disclosure relates to a method for making a cathode active material coating liquid including steps: forming a phosphate ester solution by adding a phosphate ester in an alcoholic solvent; and introducing an aluminum salt to the phosphate ester solution, the aluminum salt being soluble to the alcoholic solvent, and the aluminum salt reacting with the phosphate ester to form a homogeneous clear solution. The present disclosure also relates to a cathode active material coating liquid and a method for coating the cathode active material.