Positive active material and positive electrode and lithium battery including positive active material

1. A positive active material comprising:

a first active material and a second active material,

wherein an average particle size of the first active material is larger than that of the second active material,

wherein the second active material comprises a Ni-based lithium-transition metal composite oxide and a coating layer comprising a Mn-containing compound on at least a portion of the surface thereof,

wherein the first active material comprises at least one selected from LiCoO 2 , LiNiO 2 , LiMnO 2 , LiMn 2 O 4 , Li(Ni a Co b Al c )O 2 , Li(Ni a Co b Mn c )O 2 where 0<a<1, 0<b<1, 0<c<1, and a+b+c=1, LiNi 1−Y Co Y O 2 , LiCo 1−Y Mn Y O 2 , LiNi 1−Y Mn Y O 2 where 0≦Y<1, Li(Ni a Co b Mn c )O 4 where 0<a<2, 0<b<2, 0<c<2, and a+b+c=2, LiMn 2−z Ni z O 4 , LiMn 2−z Co z O 4 where 0<Z<2, LiCoPO 4 , LiNiO 2 , LiFeO 2 , LiFePO 4 , V 2 O 5 , TiS, and MoS,

wherein an average particle diameter (D50) of the first active material is about 1 to about 50 μm,

wherein an average particle diameter (D50) of the second active material is about 0.05 to about 10 μm, and

wherein the Mn-containing compound is a lithium manganese oxide represented by Formula 1:

Li 1+a Mn 2−b M b O 4−c A c   <Formula 1>

wherein M is at least one element selected from Al, Mg, Ni, Co, Zn, Ca, Sr, Cu, Zr, P, Fe, Ga, In, Cr, Ge, and Sn,

A is at least one element selected from F, S, and P,

0≦a≦0.3, 0≦b≦0.2, and 0≦c≦0.02.

2. The positive active material of claim 1 , wherein the Mn-containing compound has a spinel crystal structure.

3. The positive active material of claim 1 , wherein an amount of Mn is about 0.01 to about 0.1 moles based on 1 mole of the second active material.

4. The positive active material of claim 1 , wherein the second active material comprises a diffusion portion into which at least a portion of the Mn-containing compound is diffused.

5. The positive active material of claim 4 , wherein the diffusion portion is formed to a predetermined thickness from the surface of the second active material.

6. The positive active material of claim 1 , wherein the amount of Ni is at least 0.5 moles based on 1 mole of the Ni-based lithium-transition metal composite oxide.

7. The positive active material of claim 1 , wherein the Ni-based lithium-transition metal composite oxide is represented by Formula 2:

Li x Ni 1−y M′ y O 2−z X z   <Formula 2>

wherein M′ is at least one metal selected from Co, Al, Mn, Mg, Cr, Fe, Ti, Zr, Mo, and alloys thereof,

X is an element selected from F, S, and P,

0.8≦x≦1.2, 0≦y≦0.5, and 0≦z≦2.

8. The positive active material of claim 1 , wherein the Ni-based lithium-transition metal composite oxide is represented by Formula 3:

Li x Ni 1−y′−y″ Co y′ Al y″ O 2   <Formula 3>

wherein 0.8≦x≦1.2, 0<y′+y″≦0.2, and 0<y″≦0.1.

9. The positive active material of claim 1 , wherein the first active material comprises a Ni-based lithium-transition metal composite oxide that is identical to or different from that of the second active material.

10. The positive active material of claim 1 , wherein an average particle diameter (D50) ratio of the first active material to the second active material is about 2:1 to about 100:1.

11. The positive active material of claim 1 , wherein the (D50) ratio of the first active material to the second active material may be about 3:1 to about 20:1.

12. The positive active material of claim 1 , wherein the weight ratio of the first active material to the second active material is from about 50:50 to about 99:1.

13. A positive electrode comprising the positive active material of claim 1 .

14. A lithium battery comprising:

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

a negative electrode facing the positive electrode; and

an electrolyte disposed between the positive electrode and the negative electrode.