Positive active material for rechargeable lithium battery, method of preparing the same and rechargeable lithium battery including the same

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

a lithium nickel-based composite oxide comprising a secondary particle in which a plurality of plate-shaped primary particles are agglomerated; and

a lithium manganese composite oxide on the outermost surface of the lithium nickel-based composite oxide and having at least two crystal lattice structures,

wherein the secondary particle has a regular array structure in which (003) planes of the primary particles are oriented in a vertical direction with respect to an outermost surface of the secondary particle.

2. The positive active material of claim 1 ,

wherein an amount of the lithium manganese composite oxide is about 0.1 wt % to about 5 wt % based on a total weight of the positive active material.

3. The positive active material of claim 1 , wherein the lithium manganese composite oxide has an average particle diameter of less than or equal to about 10 μm.

4. The positive active material of claim 1 , wherein the lithium manganese composite oxide is represented by Chemical Formula 1:

x LiMnO 2 .y Li 4 Mn 5 O 12 .z LiMn 2 O 4 .(1 −x−y−z )Li 2 MnO 3   Chemical Formula 1

(0≤x<1, 0≤y<1, 0≤z<1, 0<y+z<1, and 0<x+y+z<1).

5. The positive active material of claim 1 , wherein the lithium manganese composite oxide comprises a cubic crystal lattice structure and a monoclinic crystal lattice structure, and optionally an orthorhombic crystal lattice structure.

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

the cubic crystal lattice structure corresponds to at least one selected from LiMn 2 O 4 and Li 4 Mn 5 O 12 ,

the monoclinic crystal lattice structure corresponds to Li 2 MnO 3 , and

the orthorhombic crystal lattice structure corresponds to LiMnO 2 .

7. The positive active material of claim 1 , wherein the lithium nickel-based composite oxide has a porosity of about 1% to about 8%.

8. The positive active material of claim 1 , wherein the lithium nickel-based composite oxide has a specific surface area of about 0.3 m 2 /g to about 0.8 m 2 /g.

9. The positive active material of claim 1 , wherein the secondary particle is in a single-centered radial arrangement structure having one center or a multi-centered radial arrangement structure having a plurality of centers.

10. The positive active material of claim 1 , wherein an unreacted residual lithium content on the outermost surface of the positive active material is less than or equal to about 1,300 ppm.

11. The positive active material of claim 10 , wherein the unreacted residual lithium content on the outermost surface of the positive active material is less than or equal to about 1,000 ppm.

12. A rechargeable lithium battery comprising:

a positive electrode comprising the positive active material of claim 1 ;

a negative electrode comprising a negative active material; and

an electrolyte between the positive electrode and the negative electrode.

13. A method of preparing a positive active material for a rechargeable lithium battery, the method comprising:

mixing a metal hydroxide precursor and a lithium source to prepare a first mixture;

first heat-treating the first mixture at about 750° C. to about 950° C. to prepare a first fired product comprising residual lithium;

mixing the first fired product with a manganese-based oxide to prepare a second mixture; and

second heat-treating the second mixture,

wherein the positive active material comprises:

a lithium nickel-based composite oxide comprising a secondary particle in which a plurality of plate-shaped primary particles are agglomerated; and

a lithium manganese composite oxide having at least two crystal lattice structures,

wherein the secondary particle has a regular array structure in which (003) planes of the primary particles are oriented in a vertical direction with respect to an outermost surface of the secondary particle.

14. The method of claim 13 , wherein the manganese-based oxide is mixed in an amount of about 0.1 to about 5 parts by weight based on 100 parts by weight of the first fired product.

15. The method of claim 13 , wherein the second mixture further comprises a second lithium source.