Positive electrode active material for secondary battery, method of preparing the same, and lithium secondary battery including the positive electrode active material

1. A positive electrode active material for a secondary battery, the positive electrode active material comprising:

a first positive electrode active material and a second positive electrode active material,

wherein an average particle diameter (D 50 ) of the first positive electrode active material is twice or more an average particle diameter (D 50 ) of the second positive electrode active material, and

the second positive electrode active material has a crystallite size of 200 nm or more.

2. The positive electrode active material of claim 1 , wherein the second positive electrode active material is a secondary particle formed by agglomeration of primary particles, and an average particle diameter (D 50 ) of the primary particles of the second positive electrode active material is 1 μm or more.

3. The positive electrode active material of claim 2 , wherein the average particle diameter (D 50 ) of the primary particles of the second positive electrode active material is from 1 μm to 8 μm.

4. The positive electrode active material of claim 1 , wherein the second positive electrode active material has an average particle diameter (D 50 ) of 9 μm or less.

5. The positive electrode active material of claim 4 , wherein the average particle diameter (D 50 ) of the second positive electrode active material is from 1 μm to 9 μm.

6. The positive electrode active material of claim 1 , wherein the first positive electrode active material has an average particle diameter (D 50 ) of 8 μm to 30 μm.

7. The positive electrode active material of claim 1 , wherein the first positive electrode active material and the second positive electrode active material are lithium composite transition metal oxides having the same composition or are lithium composite transition metal oxides having different compositions from each other.

8. The positive electrode active material of claim 1 , wherein the first positive electrode active material and the second positive electrode active material are each independently represented by Formula 1:

Li p Ni 1−(x1+y1+z1) Co x1 M a y1 M b z M c q1 O 2   [Formula 1]

wherein, in Formula 1, M a is at least one element selected from the group consisting of manganese (Mn) and aluminum (Al), M b is at least one element selected from the group consisting of barium (Ba), calcium (Ca), zirconium (Zr), titanium (Ti), magnesium (Mg), tantalum (Ta), niobium (Nb), and molybdenum (Mo), M c is at least one element selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo, and chromium (Cr), and 0.9≤p≤1.5, 0<x1≤0.4, 0<y1≤0.4, 0≤z1≤0.1, 0≤q1≤0.1, and 0<x1+y1+z1≤0.4.

9. The positive electrode active material of claim 1 , wherein the first positive electrode active material and the second positive electrode active material are mixed in a weight ratio of 9:1 to 1:9.

10. The positive electrode active material of claim 1 , wherein the crystallite size of the second positive electrode active material is from 200 nm to 500 nm.

11. The positive electrode active material of claim 1 , wherein the first positive electrode active material is a secondary particle formed by agglomeration of primary particles, and an average particle diameter (D 50 ) of the primary particles of the first positive electrode active material is 100 nm to 3 μm.

12. A positive electrode for a secondary battery, the positive electrode comprising the positive electrode active material of claim 1 .

13. A lithium secondary battery comprising the positive electrode of claim 12 .

14. A method of preparing the positive electrode active material for a secondary battery of claim 1 , the method comprising:

mixing the first positive electrode active material and the second positive electrode active material to form a positive electrode active material having a bimodal particle size distribution,

wherein an average particle diameter (D 50 ) of the first positive electrode active material is twice or more an average particle diameter (D 50 ) of the second positive electrode active material.

15. The method of claim 14 , wherein the second positive electrode active material is a secondary particle formed by agglomeration of primary particles, and an average particle diameter (D 50 ) of the primary particles of the second positive electrode active material is 1 μm or more.

16. The method of claim 14 , wherein the second positive electrode active material has an average particle diameter (D 50 ) of 9 μm or less.

17. The method of claim 14 , wherein the first positive electrode active material has an average particle diameter (D 50 ) of 8 μm to 30 μm.

18. The method of claim 14 , wherein the first positive electrode active material and the second positive electrode active material are each independently represented by Formula 1:

Li p Ni 1−(x1+y1+z1) Co x1 M a y1 M b z1 M c q1 O 2   [Formula 1]

wherein, in the Formula 1, M a is at least one element selected from the group consisting of manganese (Mn) and aluminum (Al), M b is at least one element selected from the group consisting of barium (Ba), calcium (Ca), zirconium (Zr), titanium (Ti), magnesium (Mg), tantalum (Ta), niobium (Nb), and molybdenum (Mo), M c is at least one element selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo, and chromium (Cr), and 0.9≤p≤1.5, 0<x1≤0.4, 0<y1≤0.4, 0≤z1≤0.1, 0≤q1≤0.1, and 0<x1+y1+z1≤0.4.

19. The method of claim 14 , wherein the first positive electrode active material and the second positive electrode active material are mixed in a weight ratio of 9:1 to 1:9.

20. The method of claim 14 , wherein the second positive electrode active material is prepared by over-sintering such that the crystallite size becomes 200 nm or more.