Positive active material composition for lithium battery, method of preparing the same, and lithium battery including the same

1. A positive active material for a lithium battery comprising: an internal bulk part comprising at least two metals selected from the group consisting of nickel, manganese, and cobalt;

an external bulk part surrounding the internal bulk part and comprising the same at least two metals as the internal part; and

an interface between the internal bulk part and the external bulk part,

wherein the internal bulk part extends from a center of the positive active material to the interface, and the external bulk part extends from the interface to a surface of the positive active material,

wherein,

each of the at least two metals in the internal bulk part and the interface has a constant concentration,

each of the at least two metals in the external bulk part has a continuous concentration gradient starting from the same concentration as each of the at least two metals in the internal bulk part and the interface toward the surface of the positive active material,

a concentration change rate of each of the at least two metals from the interface to the surface of the positive active material is in a range of 0.1 mol %/0.1 μm to 50 mol %/0.1 μm, and

wherein a concentration of nickel decreases, and a concentration of each of manganese and cobalt increases from the interface to the surface of the positive active material.

2. The positive active material of claim 1 , wherein the concentration change rate of each of the at least two metals from the interface to the surface of the positive active material is in a range of 0.1 mol %/0.1 μm to 20 mol %/0.1 μm.

3. The positive active material of claim 1 ,

wherein

the internal bulk part comprises nickel and cobalt,

the concentration change rate of nickel in the external bulk part is in a range of 2 mol %/0.1 μm to 20 mol %/0.1 μm, and

the concentration change rate of cobalt in the external bulk part is in a range of 0.1 mol %/0.1 μm to 10 mol %/0.1 μm.

4. The positive active material of claim 3 , further comprising manganese in the internal bulk part, wherein the concentration change rate of nickel, the concentration change rate of cobalt, and a concentration change rate of manganese in the external bulk part satisfy the following relation formulas 1 and 2:

M Ni ≥M Co   [Relation formula 1]

M Ni ≥M Mn , and  [Relation formula 2]

wherein M Ni is the concentration change rate of nickel, M Co is the concentration change rate of cobalt, and M Mn is the concentration change rate of manganese.

5. The positive active material of claim 1 , wherein a concentration of a metal in a surface portion of the external bulk part is at least 5% more than a concentration of the metal in the internal bulk part.

6. The positive active material of claim 1 , wherein the internal bulk part comprises a high-capacity compound including nickel, cobalt, and manganese, and the external bulk part comprises a compound including nickel and manganese and having excellent thermal stability.

7. The positive active material of claim 1 , wherein the internal bulk part comprises a lithium-containing compound expressed by the following chemical formula 1,

wherein the external bulk part includes one selected from a group consisting of a material expressed by the following chemical formula 2, a material expressed by the following chemical formula 3, and a mixture thereof,

Li a Ni i-x-y-z Co x Mn y M z O 2-δ X δ   [Chemical formula 1]

wherein, in the chemical formula 1, 0.95≤a≤1.2, 0.01≤x≤0.5, 0.01≤y≤0.5, 0.005≤z≤0.3, 0.05≤x+y+z≤0.4, M is at least one element selected from a group consisting of Mg, Al, Cr, V, Ti, Cr, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, and combinations thereof, X is a halogen of F, Cl, Br, or I, and 0≤δ≤0.1,

Li a Ni i-x-y-z Co x Mn y M z O 2-δ X δ   [Chemical formula 2]

wherein, in the chemical formula 2, 0.95≤a≤1.2, 0.01≤x≤0.4, 0.01≤y≤0.5, 0.002≤z≤0.2, 0.4<x+y+z≤0.95, M is at least one element selected from a group consisting of Mg, Al, Cr, V, Ti, Cr, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, and combinations thereof, X is a halogen of F, Cl, Br, or I, and 0≤δ≤0.1, and

Li a Ni x Co 1-2x Mn x-y M y O 2-δ X δ   [Chemical formula 3]

wherein, in the chemical formula 3,

0.95≤a≤1.2, 0.01≤x≤0.5, 0≤y≤0.1, M is at least one element selected from the group consisting of Mg, Al, Cr, V, Ti, Cr, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, and combinations thereof, X is a halogen of F, Cl, Br, or I, and 0≤δ≤0.1.

8. The positive active material of claim 1 , wherein an average particle diameter of the internal bulk part is in a range of 4 μm 15 μm, and an average particle diameter of particles of the positive active material is in a range of 9 μm to 30 μm.