[CROSS-REFERENCE TO RELATED APPLICATIONS] [0001] This application claims the benefits of priority from Korean Patent Application No. 2014-0133428 and No. 2014- 0133429, filed on October 2, 2014, and Korean Patent Application No. 2015-0138746, filed on October 1, 2015, the entire contents described in the disclosure of corresponding Korean patent applications are hereby incorporated as a part of the present specification. [TECHNICAL FIELD] [0002] The present invention relates to a positive electrode active material for a lithium secondary battery, a method of preparing the same, and a lithium secondary battery including the same. [BACKGROUND ART] [0003] As technical developments and demands on mobile devices are increasing, demands on secondary batteries as an 1 WO2016/053053 PCT/KR2015/010447 energy source is being rapidly increasing. Among the secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle life, and low selfdischarge rate are commercialized and widely used. [0004] However, the lithium secondary battery has a limitation that the life thereof decreases rapidly via repeating charge and discharge. Particularly, the limitation is more serious at high temperatures. The reason is that an electrolyte may be decomposed due to moisture in the battery or other factors, an active material may be deteriorated, or the internal resistance of the battery may increase. [0005] A positive electrode active material for a lithium secondary battery, which is being actively researched, developed and used, is LiCo02 with a layered structure. LiCo02 may be easily synthesized and has good electrochemical properties including life property, and is the most widely used material. However LiCo02 has low structural stability, and the application thereof to a battery with high capacity is limited. [0006] As the substituents of the positive electrode active material, various lithium transition metal oxides such as LiNi02, LiMn02, LiMn204, or LiFePCu have been developed. LiNi02 has merits of providing the battery properties of high discharge capacity, however is hardly synthesized by a simple solid phase reaction and has low thermal stability and cycle property. In addition, lithium manganese oxides such as LiMn02 or LiMn204 have merits of good thermal stability and low cost, however have limitations of a small capacity and inferior properties at high temperatures. Particularly, for LiMn204, some products are commercialized at low cost; however the life property thereof is not good due to Jahn- Teller distortion owing to Mn3+. Since LiFePCu is inexpensive and safe, a lot of research is being conducted for the use in a hybrid electric vehicle (HEV), however the application thereof to another fields is hard due to low conductivity. [0007] Due to such circumstances, a lithium nickel manganese cobalt oxide, Li (NixCoyMnz) O2 (where x, y, and z are atomic fraction of each independent oxide component elements and satisfy 0