Positive electrode active material for lithium secondary battery
The inventive concepts relate to a positive electrode active material for lithium secondary battery, and more particularly, relate to a positive electrode active material which includes a first concentration gradient portion, a second concentration gradient portion, and a first concentration maintained portion. The first and second concentration gradient portions have gradients of concentrations of nickel, manganese, and cobalt in the direction from the center to the surface, and the first concentration maintained portion has constant concentrations of nickel, manganese, and cobalt between the first concentration gradient portion and the second concentration gradient portion.
1. A positive electrode active material for lithium secondary battery, the positive electrode active material comprising a plurality of particles, each particle comprising:
a first concentration gradient portion having gradients of concentrations of nickel, manganese, and cobalt in a direction from a center of the particle to a surface of the particle, wherein the center of the particle is included in the first concentration gradient portion;
a first concentration maintained portion that is formed on the outside of the first concentration gradient portion and has concentrations of nickel, manganese, and cobalt which are maintained at a terminal of the first concentration gradient portion; and
a second concentration gradient portion that is formed on the outside of the first concentration maintained portion and has gradients of concentrations of nickel, manganese, and cobalt in the direction from the center of the particle to the surface of the particle,
wherein an average concentration of nickel in the first concentration gradient portion is higher than an average concentration of nickel in the first concentration maintained portion, and the average concentration of nickel in the first concentration maintained portion is higher than an average concentration of nickel in the second concentration gradient portion,
wherein an average concentration of cobalt and an average concentration of manganese in the first concentration gradient portion are lower than an average concentration of cobalt and an average concentration of manganese in the first concentration maintained portion, respectively, and the average concentration of cobalt and the average concentration of manganese in the first concentration maintained portion are lower than an average concentration of cobalt and an average concentration of manganese in the second concentration gradient portion, respectively, and
wherein the average concentration of nickel is higher than the average concentration of cobalt and the average concentration of manganese in the first concentration gradient portion.
2. The positive electrode active material of claim 1 , wherein the first concentration maintained portion is represented by the following Chemical Formula 1:
Li 1+a Ni x1 Co y1 Mn 1−x1−y1−d1 O 2+d1
wherein, in the chemical Formula 1, 0.6≤x1≤0.8, 0.05≤y1≤0.0.2, 0.1≤1−x≤1−y1−d1≤0.25, 0.01≤a≤0.1, and 0.01≤d1≤0.1.
3. The positive electrode active material of claim 1 , wherein the first concentration maintained portion has a thickness of from 0.1 μm to 0.6 μm.
4. The positive electrode active material of claim 1 , further comprising: a second concentration maintained portion that is formed on the outside of the second concentration gradient portion and has concentrations of nickel, manganese, and cobalt which are maintained.
5. The positive electrode active material of claim 4 , wherein the concentrations of nickel, manganese, and cobalt in the second concentration maintained portion are the same as the concentrations of nickel, manganese, and cobalt at a terminal of the second concentration gradient portion.
6. The positive electrode active material of claim 4 , wherein the concentrations of nickel, manganese, and cobalt in the second concentration maintained portion are discontinuous with respect to the concentrations of nickel, manganese, and cobalt of a terminal of the second concentration gradient portion.
7. The positive electrode active material of claim 4 , wherein the second concentration maintained portion has a thickness of from 0.1 μm to 0.6 μm.
8. The positive electrode active material of claim 4 , wherein the second concentration maintained portion is represented by the following Chemical Formula 2:
Li 1+a Ni x1 Co y1 Mn 1−x1−y1−d1 O 2+d1
wherein, in the chemical Formula 2, 0.5≤x2≤0.6, 0.15≤y2≤0.0.25, 0.2≤1−x2−y2−d2≤0.35, 0.01≤a≤0.1, and 0.01≤d2≤0.1.