A method for preparing a cathode active material includes mixing a phosphorus source material, a lithium source material, and a dispersing agent together to form a first liquid mixture. An iron powder is added into the first liquid mixture. The first liquid mixture with the iron powder therein is dried to achieve a precursor. The precursor is calcined in a protective gas at a temperature of about 600° C. to about 800° C. for more than about 2 hours.