Positive electrode active matter and secondary battery using this

1. A cathode active material, comprising:

(a) a first oxide powder comprising a first oxide, the first oxide comprising (1) lithium (Li), nickel (Ni) and manganese, (2) a first element MI selected from the group consisting of elements in Group 2 to Group 14, and (3) oxygen, the composition of nickel, manganese and MI being as set forth in Chemical Formula 7

Ni 1-y-z Mn y MI z   (Chemical Formula 7)

wherein, 0.25≦y≦0.45 and 0.01≦z≦0.30; and

(b) a second oxide powder comprising a second oxide comprising (1) lithium, (2) a second element MII comprising nickel and cobalt, (3) manganese, (4) a third element comprising at least one of the elements of Group 2 to Group 14, except for nickel, cobalt and manganese, and (5) oxygen, the composition of the second element MII, manganese and the third element MIII being as set forth in Chemical Formula 8

MII 1-t-u Mn t MIII u   (Chemical Formula 8),

wherein, 0.05≦t≦0.20 and 0.01≦u≦0.10; and

wherein,

the cathode active material is the product of a process comprising:

adding the first oxide powder to a cathode mixture slurry;

adding the second oxide powder to the cathode mixture slurry; and

applying the slurry to a cathode current collector, and

a specific surface area of the first oxide powder is at least 0.40 m 2 /g to at most 2.0 m 2 /g, and a specific surface area of the second oxide powder is at least 0.10 m 2 /g to at most 2.0 m 2 /g.

2. A cathode active material according to claim 1 , wherein the first oxide powder is represented by Chemical Formula 9, and the second oxide powder is represented by Chemical Formula 10

Li x Ni 1-y-z Mn y MI z O 2   (Chemical Formula 9)

where MI represents the first element, the values of x, y and z are within a range of 0.9 ≦x≦1.1, 0.25≦y≦0.45 and 0.01≦z≦0.30, respectively

Li s MII 1-t-u Mn t MIII u O 2   (Chemical Formula 10)

where MII and MIII represent the second element and the third element, respectively, and the values of s, t and u are within a range of 0.9≦s<1.1, 0.05≦t≦0.20 and 0.01≦u≦0.10, respectively.

3. A cathode active material according to claim 1 , wherein a mixture ratio of the first oxide powder to the second oxide powder is from 85 wt % to 15 wt % to from 15 wt % to 85 wt %.

4. A cathode active material according to claim 1 , wherein each particle of the first oxide powder and the second oxide powder has the shape of a secondary particle formed through agglomerating primary particles with a particle diameter of less than 5 μm, and the average particle diameter of the secondary particle is 30 μm or less.