Cathode active material and lithium secondary battery containing them

1. A cathode active material for a lithium secondary battery, comprising a mixture of a lithium/manganese spinel oxide of Formula I and a lithium/nickel/cobalt/manganese oxide of Formula II:

Li 1+x Mn 2-y M y O 4   (I)

Li 1+z Ni b Mn c Co 1-(b+c) O 2   (II)

wherein,

M is a metal having an oxidation number of 2 to 3;

0≦x≦0.2;

0<y≦0.2;

−0.1≦z≦0.1;

0.4≦b≦0.7;

0.1≦c≦0.5; and

0.6≦b+c≦0.9,

wherein, in the lithium/nickel/cobalt/manganese composite oxide, lithium ions intercalate into and deintercalate from mixed transition metal oxide layers (“MO layers”) and some of MO layer-derived Ni ions are inserted into intercalation/deintercalation layers of lithium ions (“lithium layers”) to thereby interconnect the MO layers,

wherein the lithium/nickel/cobalt/manganese composite oxide may have preferably a structure wherein Ni 2+ and Ni 3+ coexist in the MO layers and some of Ni 2+ are inserted into the lithium layers,

wherein the mole fraction of Ni 2+ inserted and bound into the lithium layers is in a range of 0.03 to 0.07, based on the total content of Li sites present in the lithium layers,

wherein the mixing ratio of the lithium/manganese spinel oxide of Formula I: the lithium/nickel/cobalt/manganese composite oxide of Formula II is in the range of 10:90 to 90:10 (w/w); and

wherein the cathode active material exhibits the life characteristics that the capacity at 300 cycles is more than 80% relative to the initial capacity.

2. The cathode active material according to claim 1 , wherein M in Formula I is aluminum (Al), magnesium (Mg) or both of them.

3. The cathode active material according to claim 1 , wherein the value of y is in the range of 0.01 to 0.2.

4. The cathode active material according to claim 1 , wherein the element composition meets the requirements of 0.45≦b≦0.65, 0.2≦c≦0.4, and 0.65≦b+c≦0.85.

5. A lithium secondary battery comprising cathode active material of claim 1 .