Positive electrode active material for secondary battery, method for preparing the same, and lithium secondary battery including the same
A positive electrode active material for a secondary battery is provided. The positive electrode active material being a lithium cobalt-based oxide includes a doping element M. A lithium cobalt-based oxide particle containing the doping element M in an amount of 3,000 ppm or more, wherein in a bulk portion corresponding to 90% of a core side among the radius from a core of the particle to a surface thereof, the doping element M in the lithium cobalt-based oxide particle is contained at a constant concentration, and in a surface portion from the surface of the particle to 100 nm in a core direction, the doping element M is contained at a concentration equal to or higher than that in the bulk portion and has a concentration in which the concentration thereof is gradient gradually decreased in the core direction from the surface of the particle.
1. A positive electrode active material for a secondary battery, comprising a lithium cobalt-based oxide particle contains a doping element M in an amount of 3,000 ppm or more,
wherein the particle has a bulk portion extending to 90% of a radius of the particle from a core of the particle to a surface thereof, wherein the doping element M is contained at a constant concentration in the bulk portion, and
wherein the particle has a surface portion extending from the bulk portion to the surface of the particle, wherein the surface portion has a thickness of 100 nm, wherein the doping element M in the surface portion is contained at a concentration equal to or higher than that in the bulk portion, and wherein the surface portion has a concentration gradient of the doping element M in which the concentration thereof decreases towards the bulk portion, and
wherein the doping element M is at least one selected from the group consisting of Al, Ti, Zr, Mg, Nb, Ba, Ca and Ta.
2. The positive electrode active material for a secondary battery of claim 1 , wherein the lithium cobalt-based oxide particle contains the doping element M in the amount of 5,000-8,000 ppm.
3. The positive electrode active material for a secondary battery of claim 1 , wherein the lithium cobalt-based oxide particle contains 30% or more of a total amount of the doping element M in the surface portion from the surface of the lithium cobalt-oxide particle to 100 nm in the core direction.
4. The positive electrode active material for a secondary battery of claim 1 , wherein the surface portion has the concentration gradient in which the concentration of the doping element M is gradually decreased from the surface of the lithium cobalt-based oxide particle to 10 to 50 nm in the core direction of the lithium cobalt-based oxide particle.
5. The positive electrode active material for a secondary battery of claim 1 , wherein the doping element M is Al, Ti or Mg.
6. The positive electrode active material for a secondary battery of claim 1 , wherein the lithium cobalt-based oxide particle is represented by Formula 1 below,
Li a CO (1-x) M x O 2 [Formula 1]
wherein, 0.95≤a≤1.05, 0<x≤0.2, and M is at least one selected from the group consisting of Al, Ti, Zr, Mg, Nb, Ba, Ca and Ta.
7. The positive electrode active material for a secondary battery of claim 1 , further comprising a coating layer on the surface of the lithium cobalt-based oxide particle,
wherein the coating layer comprises at least one oxide selected from the group consisting of Mg, Ti, Fe, Cu, Ca, Ba, Sn, Sb, Na, Zn, Si, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Sc, Ce, Pr, Nd, Gd, Dy, Yb, Er, Co, Al, Ga and B.
8. The positive electrode active material for a secondary battery of claim 1 , wherein the lithium cobalt-based oxide has a molar ratio of lithium to metals in a range of 0.98 to 1.1, where the metals are the remaining metal elements present in the lithium-cobalt based oxide excluding lithium.
9. A method for preparing a positive electrode active material for a secondary battery, comprising:
preparing a Co 3 O 4 or CoOOH precursor doped with a doping element M in an amount of 1,000 ppm or more;
mixing the doped Co 3 O 4 or CoOOH precursor with a lithium raw material and performing a first heat treatment to prepare a lithium cobalt-based oxide including the doping element M; and
mixing the lithium cobalt-based oxide with a raw material of the doping element M and performing a second heat treatment to prepare a lithium cobalt-based oxide particle having a surface portion extending from the surface of the particle towards the interior thereof, and having a concentration gradient of the doping element M decreasing from the surface of the particle towards the interior thereof,
wherein the doping element M is at least one selected from the group consisting of Al, Ti, Zr, Mg, Nb, Ba, Ca and Ta.
10. The method of claim 9 , wherein the preparing the doped Co 3 O 4 or CoOOH precursor doped with the doping element M in an amount of 1,000 ppm or more comprises:
preparing a precursor forming solution including a cobalt-containing starting material and the raw material of the doping element M; and
subjecting the precursor forming solution to coprecipitation reaction.
11. The method of claim 9 , wherein the precursor is doped with the doping element M in an amount of 3,000 to 6,000 ppm.
12. The method of claim 9 , wherein the lithium cobalt-based oxide including the doping element M prepared after the first heat treatment comprises the doping element M at a constant concentration in the lithium cobalt-based oxide particle.
13. The method of claim 9 , wherein the raw material of the doping element M is mixed in an amount of 0.05 to 0.5 parts by weight, with respect to 100 parts by weight of the lithium cobalt-based oxide.
14. The method of claim 9 , wherein the second heat treatment is performed at 800 to 950° C.
15. The method of claim 9 , wherein the lithium cobalt-based oxide particle comprises the doping element M in an amount of 3,000 ppm or more,
wherein the particle has a bulk portion extending to 90% of a radius of the particle from a core of the particle to the surface thereof, wherein the doping element M is contained at a constant concentration in the bulk portion, and
wherein the particle has the surface portion extending from the bulk portion to the surface of the particle, wherein the surface portion has a thickness of 100 nm, wherein the doping element M in the surface portion is contained at a concentration equal to or higher than that in the bulk portion and has a concentration gradient of the doping element M in which the concentration thereof decreases toward the bulk portion.
16. The method of claim 9 , wherein the performing a heat treatment to prepare the lithium cobalt oxide the surface of the lithium cobalt-based oxide is additionally doped with the doping element M comprises further mixing a cobalt-containing starting material when the lithium cobalt-based oxide is mixed with the raw material of the doping element M.
17. A positive electrode for a secondary battery comprising the positive electrode active material according to claim 1 .
18. A lithium secondary battery comprising the positive electrode according to claim 17 .