Positive active material for rechargeable lithium battery, preparing method thereof and rechargeable lithium battery comprising positive electrode including positive active material

1. A positive active material for a rechargeable lithium battery, the positive active material comprising:

a first positive active material in a form of secondary particles in which a plurality of primary particles are aggregated,

wherein at least a portion of the primary particles have a radially arranged structure; and

a second positive active material having a monolithic structure,

wherein both the first positive active material and the second positive active material are nickel-based positive active materials,

each of the first positive active material and the second positive active material is coated with cobalt, and

a maximum roughness (R max ; peak to peak height) of the second positive active material surface is greater than or equal to about 15 nm.

2. The positive active material of claim 1 , wherein:

an average roughness (R a ) of the surface of the second positive active material is greater than or equal to about 1.5 nm, and

a root mean square roughness (R q ) of the surface of the second positive active material is greater than or equal to about 2.0 nm.

3. The positive active material of claim 1 , wherein a BET specific surface area of the positive active material comprising the first positive active material and the second positive active material is about 0.3 m 2 /g to about 0.6 m 2 /g.

4. The positive active material of claim 1 , wherein the second positive active material has protrusions and depressions on the surface.

5. The positive active material of claim 1 , wherein:

an average particle diameter of the secondary particles of the first positive active material is about 7 μm to about 25 μm, and

an average particle diameter of the second positive active material is about 1 μm to about 10 μm.

6. The positive active material of claim 1 , wherein:

in the first positive active material, the primary particles have a plate shape, and at least some of the primary particles have a long axis arranged in a radial direction, and

an average length of the primary particles is about 0.01 μm to about 5 μm.

7. The positive active material of claim 1 , wherein:

the first positive active material is comprised in an amount of about 50 wt % to about 90 wt % based on a total amount of the first positive active material and the second positive active material, and

the second positive active material is comprised in an amount of about 10 wt % to about 50 wt % based on a total amount of the first positive active material and the second positive active material.

8. The positive active material of claim 1 , wherein:

the first positive active material comprises a compound represented by Chemical Formula 1, and

the second positive active material comprises a compound represented by Chemical Formula 11:

Li a1 Ni x1 M 1 y1 M 2 1−x1−y1 O 2   Chemical Formula 1

wherein, in Chemical Formula 1, 0.9≤a1≤1.8, 0.3≤x1≤1, 0≤y1≤0.7, and M 1 and M 2 are each independently selected from Al, B, Ce, Co, Cr, F, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and a combination thereof, and

Li a11 Ni x11 M 11 y11 M 12 1−x11−y11 O 2   Chemical Formula 11

wherein, in Chemical Formula 11, 0.9≤a11≤1.8, 0.3≤x11≤1, 0≤y11≤0.7, and M 11 and M 12 are each independently selected from Al, B, Ce, Co, Cr, F, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and combinations thereof.

9. A rechargeable lithium battery comprising:

a positive electrode comprising the positive active material of claim 1 , a negative electrode, and an electrolyte.