The present disclosure provides a method for making a cathode active material coating liquid. A phosphate ester solution is formed by adding a phosphate ester in an alcoholic solvent. An aluminum nitrate is introduced in the phosphate ester solution. The aluminum nitrate is soluble to the alcoholic solvent, and reacts with the phosphate ester to form a homogeneous clear solution. A pH value of the homogeneous clear solution is regulated to a range from about 6 to about 7 by adding an acidity regulator. The acidity regulator contains ammonium cation. The ammonium nitrate is removed from the clear solution after regulating the pH value. A method for coating the cathode active material is also provided.