Cathode active material for lithium secondary battery

1. A cathode active material for a lithium secondary battery containing a lithium transition metal oxide capable of lithium ion intercalation/deintercalation, which further comprises a lithium nickel oxide represented by the following formula 2 in which nickel is partially substituted with other elements, as an additive in an amount of 0.1 to 2 parts by weight based on 100 parts by weight of the total cathode active material:

Li 2+x Ni 1−y M y O 2+a   [formula 2]

wherein, x is a number satisfying −0.5≦x≦0.5, y is a number satisfying 0<y<1, a is a number satisfying 0≦a<0.3, and M is at least one element selected from the group consisting of B and Al wherein the lithium transition metal oxide is at least one selected from the group consisting of LiCoO 2 , LiNiO 2 , LiMnO 2 , LiMn 2 O 4 , Li(Ni a Co b Mn c )O 2 , LiNi 1−d O 2 , LiCo 1−d Mn d O 2 , LiNi 1−d Mn d O 2 , Li(Ni x Co y Mn z )O 4 , LiMn 2−n Ni n O 4 , LiMn 2-n Co n O 4 , LiCoPO 4 and LiFePO 4 , wherein 0<a<1, 0<b<1, 0<c<1, 0<d<0, a+b+c=1, 0<x<2, 0<y<2, 0<z<2, x+y+z=2, and 0<n<2.

2. The cathode active material according to claim 1 , wherein the lithium nickel oxide represented by formula 2 belongs to a space group Immm.

3. The cathode active material according to claim 2 , wherein the lithium nickel oxide represented by formula 2 forms a tetra-coordinated planar structure, (Ni,M)O 4 and two tetra-coordinated planar structures facing to each other share one side formed by O—O so as to form a primary chain.

4. The cathode active material according to claim 2 , wherein the compound represented by formula 2 has the following lattice constants: a=3.7±0.5 Å, b=2.8±0.5 Å, c=9.2±0.5 Å, α=90°, β=90° and γ=90.

5. The cathode active material according to claim 1 , wherein in the lithium nickel oxide represented by formula 2, a is a number satisfying 0<a<0.3.

6. The cathode active material according to claim 1 , wherein in the lithium nickel oxide represented by formula 2, M is B.

7. A lithium secondary battery comprising a cathode, an anode, a separator and a non-aqueous electrolyte containing a lithium salt and an electrolyte compound,

wherein the cathode comprises the cathode active material containing a lithium transition metal oxide capable of lithium ion intercalation/deintercalation, which further comprises a lithium nickel oxide represented by the following formula 2 in which nickel is partially substituted with other elements, as an additive in an amount of 0.1 to 2 parts by weight based on 100 parts by weight of the total cathode active material:

Li 2+x Ni 1−y M y O 2+a   [formula 2]

wherein, x is a number satisfying −0.5≦x≦0.5, y is a number satisfying 0<y<1, a is a number satisfying 0≦a<0.3, and M is at least one element selected from the group consisting of B and Al wherein the lithium transition metal oxide is at least one selected from the group consisting of LiCoO 2 , LiNiO 2 , LiMnO 2 , LiMn 2 O 4 , Li(Ni a Co b Mn c )O 2 , LiNi 1−d O 2 , LiCo 1−d Mn d O 2 , LiNi 1−d Mn d O 2 , Li(Ni x Co y Mn z )O 4 , LiMn 2−n Ni n O 4 , LiMn 2−n Co n O 4 , LiCoPO 4 and LiFePO 4 , wherein 0<a<1, 0<b<1, 0<c<1, 0<d<0, a+b+c=1, 0<x<2, 0<y<2, 0<z<2, x+y+z=2, and 0<n<2.

8. The lithium secondary battery according to claim 7 , wherein the lithium nickel oxide represented by formula 2 belongs to a space group Immm.

9. The lithium secondary battery according to claim 8 , wherein the lithium nickel oxide represented by formula 2 forms a tetra-coordinated planar structure, (Ni,M)O 4 and two tetra-coordinated planar structures facing to each other share one side formed by O—O so as to form a primary chain.

10. The lithium secondary battery according to claim 8 , wherein the compound represented by formula 2 has the following lattice constants: a=3.7±0.5 Å, b=2.8±0.5 Å, c=9.2±0.5 Å, a=90°, β=90° and γ=90.

11. The lithium secondary battery according to claim 7 , wherein the lithium salt is at least one selected from the group consisting of LiClO 4 , LiCF 3 SO 3 , LiPF 6 , LiBF 4 , LiAsF 6 and LiN(CF 3 SO 2 ) 2 , and the electrolyte compound is at least one carbonate selected from the group consisting of ethylene carbonate, propylene carbonate, gamma-butyrolactone, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate and methylpropyl carbonate.

12. The lithium secondary battery according to claim 7 , wherein in the lithium nickel oxide represented by formula 2, a is a number satisfying 0<a<0.3.

13. The lithium secondary battery according to claim 7 , wherein in the lithium nickel oxide represented by formula 2, M B.