Positive electrode active material for secondary battery, method of preparing the same, and secondary battery including the positive electrode active material

1. A positive electrode active material for a secondary battery, the positive electrode active material comprising:

a core;

a shell disposed to surround the core; and

a buffer layer which is disposed between the core and the shell and includes pores and a three-dimensional network structure connecting the core and the shell,

wherein the core, the shell, and the three-dimensional network structure of the buffer layer each independently comprise a lithium nickel manganese cobalt-based composite metal oxide, and

at least one metallic element of the nickel, the manganese, and the cobalt has a concentration gradient that gradually changes in any one region of the core, the shell, and the entire positive electrode active material.

2. The positive electrode active material for a secondary battery of claim 1 , wherein the core comprises the nickel in an amount of 60 mol % or more to less than 100 mol % based on a total mole of the metallic elements (except lithium) included in the core, and

the shell comprises the nickel in an amount of 30 mol % or more to less than 60 mol % based on a total mole of the metallic elements (except lithium) included in the shell.

3. The positive electrode active material for a secondary battery of claim 1 , wherein the nickel, the manganese, and the cobalt each independently have a gradually changing concentration gradient across an entire positive electrode active material particle,

a concentration of the nickel is decreased while the nickel has a gradual concentration gradient from a center of the positive electrode active material particle in a surface direction, and

concentrations of the cobalt and the manganese are increased while the cobalt and the manganese each independently have a gradual concentration gradient from the center of the positive electrode active material particle in the surface direction.

4. The positive electrode active material for a secondary battery of claim 1 , wherein the nickel, the manganese, and the cobalt each independently have a gradually changing concentration gradient in the core and the shell,

a concentration of the nickel is decreased while the nickel has a gradual concentration gradient from a center of the core to an interface between the core and the buffer layer and from an interface between the buffer layer and the shell to a surface of the shell, and

concentrations of the cobalt and the manganese are increased while the cobalt and the manganese each independently have a gradual concentration gradient from the center of the core to the interface between the core and the buffer layer and from the interface between the buffer layer and the shell to the surface of the shell.

5. The positive electrode active material for a secondary battery of claim 1 , wherein the core is a secondary particle in which primary particles of the lithium nickel manganese cobalt-based composite metal oxide are agglomerated.

6. The positive electrode active material for a secondary battery of claim 1 , wherein porosity of the buffer layer is 30 vol % or less based on a total volume of the positive electrode active material.

7. The positive electrode active material for a secondary battery of claim 1 , wherein the shell comprises crystal oriented particles of the lithium nickel manganese cobalt-based composite metal oxide which are radially grown from a center of the positive electrode active material in a surface direction.

8. The positive electrode active material for a secondary battery of claim 1 , wherein the lithium nickel manganese cobalt-based composite metal oxides of the core, the shell, and the buffer layer each independently comprise a compound of Formula 1:

Li a Ni1- x - y Co x Mn y M1 z M2 w O2  [Formula 1]

wherein, in Formula 1, M1 comprises at least one element selected from the group consisting of tungsten (W), molybdenum (Mo), and chromium (Cr), M2 comprises at least one element selected from the group consisting of aluminum (Al), zirconium (Zr), titanium (Ti), magnesium (Mg), tantalum (Ta), and niobium (Nb), 1.0≤a≤1.5, 0<x≤0.5, 0<y≤0.5, 0≤z≤0.03, 0≤w≤0.02, and 0<x+y<1.

9. The positive electrode active material for a secondary battery of claim 1 , wherein a ratio of a radius of the core to a radius of the positive electrode active material is greater than 0 to less than 0.4, and

a ratio of a length from a center of the positive electrode active material to an interface between the buffer layer and the shell to the radius of the positive electrode active material is greater than 0 to less than 0.7.

10. The positive electrode active material for a secondary battery of claim 1 , wherein a shell region, as a ratio of a thickness of the shell to a radius of the positive electrode active material, determined according to Equation 1 is in a range of 0.2 to 1:

shell region=(radius of positive electrode active material-core radius-buffer layer thickness)/radius of positive electrode active material.  [Equation 1]

11. A positive electrode for a secondary battery, the positive electrode comprising the positive electrode active material of claim 1 .

12. A lithium secondary battery comprising the positive electrode of claim 11 .