Lithium composite oxide, positive electrode active material and lithium secondary battery comprising the same

A positive electrode active material for a lithium secondary battery, comprising:

• a lithium composite oxide including primary particles represented by Formula 1 below and secondary particles formed by agglomerating the primary particles;
• LiaNi1-(b+c+d)CobM1cM2dO2 [Formula 1]
• wherein, M1 is at least one selected from Mn and Al, M2 is at least one selected from Ti, Zr, Mg, V, B, Mo, Zn, Nb, Ba, Ta, Fe, Cr, Sn, Hf, Ce, W, Nd and Gd, 0.5≤a≤1.5, 0.01≤b≤0.50, 0.01≤c≤0.20, and 0.001≤d≤0.20, and
• a lithium alloy oxide at least a part of a) the interface between primary particles and b) the surface of secondary particles,
• wherein the M2 element included in the primary particle present at the surface of the secondary particle exhibits a concentration gradient which decreases toward the center of the secondary particle, and
• wherein the M2 content (at %) at the surface of the positive electrode active material be larger than the Ni content (at %) at the same position.