COMPOSITE POSITIVE ELECTRODE ACTIVE MATERIAL, METHOD OF PREPARING THE SAME, POSITIVE ELECTRODE INCLUDING THE SAME, AND SECONDARY BATTERY INCLUDING THE SAME

A positive electrode active material includes a lithium-transition metal composite phosphate including a first crystalline phase having a composition represented by Formula 1 and having an olivine structure, and a second crystalline phase having a composition represented by Formula 2 and having a pyrophosphate-containing structure, wherein the second crystalline phase is in an amount of greater than 0 mole percent and not greater than 50 mole percent with respect to a total number of moles of the first crystalline phase and the second crystalline phase, a positive electrode, a secondary battery: LiM1PO Formula 1 LiM2(PO) Formula 2