Cathode active material and lithium secondary battery containing the same

1. A cathode active material for secondary batteries, the cathode active material being a mixture comprising at least one type of lithium manganese oxide (A) having a spinel crystal structure and at least two types of lithium nickel-manganese-cobalt composite oxides (B) containing Ni, Mn, and Co as transition metals,

wherein the lithium nickel-manganese-cobalt composite oxides (B) are different from each other in terms of at least one selected from the group consisting of particle diameter, and density,

wherein the lithium manganese oxide has a spinel crystal structure and has an elemental composition represented by Formula (1) and the lithium nickel-manganese-cobalt composite oxides are selected from compounds having an elemental composition represented by Formula (2):

Li 1+x Mn 2−y M y O 4   (1)

Li 1+z Ni a Mn b Co 1−(ab) O 2   (2)

where 0≦x≦0.2 and 0≦y≦1,

where M is at least one element selected from the group consisting of Al, Mg, Ni, Co, Fe, Cr, V, Ti, Cu, B, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W and Bi, and

where 0≦z≦0.1, 0.2≦a≦0.7, 0.2≦b≦0.7 and a+b<1,

wherein the lithium nickel-manganese-cobalt composite oxides (B) comprise two types of oxides having the same elemental composition and different particle diameters,

wherein the lithium nickel-manganese-cobalt composite oxides (B) comprising a first oxide (B1) and a second oxide (B2) have an average particle diameter in a range from 3 to 10 μm and an average particle diameter of the first oxide (B1) is 10 to 70% of an average particle diameter of the second oxide (B2), and

wherein the mixture ratio of the first oxide (B1) and the second oxide (B2) is 50:50 by weight.

2. The cathode active material according to claim 1 , wherein the mixture ratio of the lithium manganese oxide (A) to the lithium nickel-manganese-cobalt composite oxides (B) ranges from 30:70 to 90:10 by weight.

3. The cathode active material according to claim 1 , wherein the lithium nickel-manganese-cobalt composite oxides (B) have an elemental composition of Li 1+z Ni 1/3 Mn 1/3 Co 1/3 O 2 (where 0≦z≦0.1) or Li 1+z1 Ni a Mn b Co 1−(a+b) O 2 (where 0≦z≦0.1, 0.4≦a≦0.7, 0.2≦b≦0.4 and a+b<1).

4. The cathode active material according to claim 1 , wherein the active material has an energy density of 150 to 220 Wh/kg.

5. A cathode mix comprising the cathode active material according to claim 1 .

6. A cathode for secondary batteries, wherein the cathode mix according to claim 5 is applied to a current collector.

7. A lithium secondary battery comprising the cathode according to claim 6 .

8. The lithium secondary battery according to claim 7 , wherein the lithium secondary battery is used as a unit cell of a battery module that is a power source of a middle or large-scale device.

9. The lithium secondary battery according to claim 8 , wherein the middle or large-scale device is an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle or a power storage system.