Positive active material, positive electrode and lithium secondary battery containing the material, and method of preparing the material

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

a lithium transition metal composite oxide represented by Formula 1

Li a Ni b M1 c M2 d M3 e O 2   Formula 1

wherein, in Formula 1,

M1 includes Co, Mn, or a combination thereof,

M2 includes Mg and Ti,

M3 includes Al, B, Ca, Na, K, Cr, V, Fe, Cu, Zr, Zn, Sr, Sb, Y, Nb, Ga, Si, Sn, Mo, W, Ba, a rare earth element, or a combination thereof, 0.9≤a≤1.1, 0.7≤b<1.0, 0<c≤0.3, 0<d≤0.03, 0≤e≤0.05, and 0.95≤(b+c+d+e)≤1.05, and

a molar ratio of Ti:Mg in M2 is about 1:1 to about 3:1, and

wherein the lithium transition metal composite oxide has a grain size of about 50 nanometers to about 500 nanometers.

2. A positive active material for a lithium secondary battery, the positive active material comprising: a lithium transition metal composite oxide represented by Formula 2

Li a Ni b CO c′ Mn c″ Mg d′ Ti d″ M3 e O 2   Formula 2

wherein, in Formula 2,

M3 may include Al, B, Ca, Na, K, Cr, V, Fe, Cu, Zr, Zn, Sr, Sb, Y, Nb, Ga, Si, Sn, Mo, W, Ba, a rare earth element, or a combination thereof, 0.9≤a≤1.1, 0.7≤b<1.0, 0<c′<0.3, 0<c″<0.3, 0<(c′+c″)≤0.3, 0<d′<0.03, 0<d″<0.03, 0<(d′+d″)≤0.03, 0≤e≤0.05, and (b+c′+c″+d′+d″+e)=1, and

a molar ratio of Ti:Mg is about 1:1 to about 3:1, and

wherein the lithium transition metal composite oxide has a grain size of about 50 nanometers to about 1000 nanometers.

3. The positive active material of claim 2 , wherein a degree of crystallinity of the lithium transition metal composite oxide increases as an amount of Li in the lithium transition metal oxide increases.

4. A lithium secondary battery comprising the positive electrode of claim 3 .

5. A positive electrode comprising the positive active material of claim 2 .

6. The positive active material of claim 1 , wherein the lithium transition metal composite oxide comprises a layered crystalline phase belonging to an R-3m space group.

7. The positive active material of claim 1 , wherein the lithium transition metal composite oxide does not comprise a crystalline phase belonging to an Fm 3 m space group.

8. The positive active material of claim 1 , wherein a degree of crystallinity of the lithium transition metal composite oxide increases as an amount of Li in the lithium transition metal oxide increases.

9. The positive active material of claim 1 , wherein the lithium transition metal composite oxide is in the form of small-diameter particles having an average particle diameter of about 1 micrometer to about 5 micrometer, the small-diameter particles comprising an agglomeration of a plurality of grains of the lithium transition metal composite oxide.

10. The positive active material of claim 1 , wherein the lithium transition metal composite oxide is in the form of large-diameter particles having an average particle diameter of about 10 micrometers to about 20 micrometers, the large-diameter particles comprising an agglomeration of a plurality of grains of the lithium transition metal composite oxide.

11. The positive active material of claim 1 , wherein the positive active material comprises small-diameter particles having an average particle diameter of about 1 micrometer to about 5 micrometer and large-diameter particles having an average particle diameter of about 10 micrometers to about 20 micrometers, wherein the small-diameter particles and the large-diameter particles comprise an agglomeration of a plurality of grains of the lithium transition metal composite oxide.

12. The positive active material of claim 1 , wherein Mg and Ti are uniformly distributed in the lithium transition metal composite oxide.

13. The positive active material of claim 1 , further comprising a surface modifying layer on a surface of the positive active material.

14. The positive active material of claim 13 , wherein the surface modifying layer comprises a surface coating layer comprising a metal compound comprising a metal oxide, a metal sulfide, a metal chloride, a metal nitride, a metal fluoride, a metal phosphide, a metal alkoxide, or a combination thereof.

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

16. A lithium secondary battery comprising the positive electrode of claim 15 .

17. A method of preparing the positive active material of claim 1 , the method comprising:

providing a mixture comprising a transition metal precursor, a lithium source, a Mg-containing compound, and a Ti-containing compound; and

thermally treating the mixture to prepare the lithium transition metal composite oxide represented by Formula 1,

wherein an amount of the Mg-containing compound and an amount of the Ti-containing compound are such that a molar ratio of Ti:Mg is about 1:1 to about 3:1, and

wherein the lithium transition metal composite oxide has a grain size of about 50 nanometers to about 500 nanometers.

18. The method of claim 17 , wherein the providing of the mixture comprises preparing the mixture by dry mixing the lithium source, the Mg-containing compound, and the Ti-containing compound.

19. The method of claim 17 , wherein the thermal treatment of the mixture is performed at about 600° C. to about 1,000° C. in an atmosphere comprising oxygen.

20. The method of claim 17 , further comprising removing free lithium from a surface of the positive active material using a solvent.

21. The method of claim 17 , further comprising forming a surface coating layer on a surface of the positive active material.