Positive active material and positive electrode for rechargeable lithium battery and rechargeable lithium battery including the positive electrode

1. A positive active material for a rechargeable lithium battery, consisting of a first positive active material represented by Formula 1 and a second positive active material represented by Formula 2:

Li a Ni b Co c M d O 2   Formula 1

wherein 0.95≦a≦1.1, 0.5≦b≦0.9, 0≦c≦0.3, and 0≦d≦0.1, and M is selected from the group consisting of Al, B, Cr, Fe, Mg, Sr, and V;

Li e Ni f Co g Mn h O 2   Formula 2

wherein 0.95≦e≦1.1, 0.33≦f≦0.5, 0.15≦g≦0.33, and 0.3≦h≦0.35,

wherein the first positive active material comprises a plurality of secondary particles and the second positive active material comprises a plurality of secondary particles, and

wherein a weight ratio of the first positive active material to the second positive active material ranges from about 70:30 to about 50:50.

2. The positive active material of claim 1 , wherein a weight ratio of the first positive active material to the second positive active material ranges from about 70:50 to about 30:50.

3. The positive active material of claim 1 , wherein the plurality of secondary particles of the first positive active material have an average diameter ranging from about 5 μm to about 15 μm, and each of the secondary particles of the first positive active material comprises an agglomeration of primary particles.

4. The positive active material of claim 1 , wherein the first positive active material comprises a plurality of primary particles having an average primary particle diameter ranging from about 0.1 μm to about 0.5 μm.

5. The positive active material of claim 1 , wherein the plurality of secondary particles of the second positive active material have an average diameter ranging from about 3 μm to about 15 μm, and each of the secondary particles of the second positive active material comprises an agglomeration of primary particles.

6. The positive active material of claim 1 , wherein the positive active material has an active mass density ranging from about 2.0 to about 4.0 g/cc.

7. The positive active material of claim 1 , wherein the positive active material has a loading level ranging from about 10 to about 40 mg/cm 2 .

8. A positive electrode, comprising the positive active material of claim 1 , a conductive material, and a binder.

9. A rechargeable lithium battery comprising

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

a negative electrode, and

an electrolyte.

10. The positive active material of claim 1 , wherein M is Al, and d is greater than 0.

11. The positive active material of claim 10 , wherein c is greater than 0.

12. The positive active material of claim 1 , wherein the second positive active material comprises the plurality of secondary particles having the average secondary particle diameter ranging from about 3 μm to about 6 μm.

13. The positive active material of claim 1 , wherein a weight ratio of the first positive active material to the second positive active material is equal to or less than 70:50.

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

a weight ratio of the first positive active material to the second positive active material is equal to or less than 70:50,

M is Al,

d is greater than 0, and

c is greater than 0.

15. The positive active material of claim 14 , wherein the weight ratio of the first positive active material to the second positive active material is equal to or greater than 50:50.

16. The positive active material of claim 1 , wherein 0.95≦a≦1.1, 0.5≦b≦0.9, 0≦c≦0.3, 0≦d≦0.1, 0.95≦e≦1.1, 0.33≦f≦0.5, 0.15≦g≦0.33, and 0.3≦h≦0.35; and wherein M is Al.

17. The positive active material of claim 16 , wherein c is 0.15, and d is 0.05.

18. The positive active material of claim 17 , wherein a is 1, and b is 0.8.