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

1. 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;

Li a Ni 1-(b+c+d) Co b M1 c M2 d O 2   [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.

2. The positive electrode active material of claim 1 , wherein Ni in the primary particles present at the surface of the secondary particle exhibits a concentration gradient which increases toward the center of the secondary particle.

3. The positive electrode active material of claim 1 , wherein the diffusion path of lithium ions arranged toward the center of the secondary particle in the primary particles is formed.

4. The positive electrode active material of claim 1 , wherein the secondary particles comprise a first region and a second region, which have different M2 element concentrations, and the M2 element included in the primary particles present in the first region exhibits a concentration gradient which decreases toward the center of the secondary particle.

5. The positive electrode active material of claim 4 , wherein the concentration variation of the M2 element included in the primary particles present in the first region is 50% or more.

6. The positive electrode active material of claim 4 , wherein the concentration variation of the M2 element included in the primary particles present in the second region is 49% or less.

7. The positive electrode active material of claim 1 , wherein a concentration gradient of the M2 element included in the primary particles at the surface side of the secondary particle is greater than that of the center side of the secondary particle, on the basis of the center of the primary particle.

8. The positive electrode active material of claim 1 ,

wherein the lithium alloy oxide exhibits a concentration gradient which decreases from the surface to the center of the secondary particle.

9. The positive electrode active material of claim 1 , wherein the lithium alloy oxide is represented by Formula 2 below:

Li e M3 f O g   [Formula 2]

wherein, M3 is at least one selected from Ti, Zr, Mg, V, B, Mo, Zn, Nb, Ba, Ca, Ta, Fe, Cr, Sn, Hf, Ce, W, Nd and Gd, 0≤e≤6, 0<f≤6, and 0<g≤10.

10. The positive electrode active material of claim 1 , wherein the lithium alloy oxide comprises a primary lithium alloy oxide represented by Formula 3 below, and a secondary lithium alloy oxide represented by Formula 4 below:

Li h W i O j   [Formula 3]

wherein, 0≤h≤6, 0<i≤6, and 0<j≤10,

Li k M4 m O n   [Formula 4]

wherein, M4 is at least one selected from Ti, Zr, Mg, V, B, Mo, Zn, Nb, Ba, Ca, Ta, Fe, Cr, Sn, Hf, Ce, Nd and Gd, 0≤k≤6, 0<m≤6, and 0<n≤10.

11. A lithium secondary battery which uses a positive electrode comprising the positive electrode active material of claim 1 .