Positive electrode active material and lithium secondary battery comprising the same

1 . A positive electrode active material comprising a lithium composite oxide particle comprising at least nickel and cobalt,

wherein, based on a cross-section of the lithium composite oxide particle, the lithium composite oxide particle comprises:

a first section, in which cobalt concentration decreases from a surface portion towards a central portion of the lithium composite oxide particle, and

a second section located inward of the first section in the direction toward the central portion of the lithium composite oxide particle, in which cobalt concentration is maintained within a predetermined range

wherein d1/d is 0.08 to 0.27, where d1 is a thickness of the first section and d is an average radius of the lithium composite oxide particle, and wherein a ratio of an average Co concentration (c1) in the first section and an average Co concentration (c) measured based on ICP analysis for the lithium composite oxide particle is 1.70 to 2.60.

2 . The positive electrode active material of claim 1 , wherein an average molar ratio of Co/Ni in the first section is 0.25 to 0.39.

3 . The positive electrode active material of claim 1 , wherein d2/d is more than 0.73 and less than 0.92, where d2 is a thickness of the second section.

4 . The positive electrode active material of claim 1 , wherein a change rate of the cobalt concentration in the second section is 10 mol % or less.

5 . The positive electrode active material of claim 1 , wherein the lithium composite oxide particle is represented by Formula 1 below,

Li w Ni 1−(x+y+z) Co x M1 y M2 z O 2   [Formula 1]

Wherein,

M1 is at least one selected from Mn and Al,

M2 is at least one selected from P, Sr, Ba, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd and Cu,

M1 and M2 are different,

0.5≤w≤1.5, 0≤x≤0.50, 0≤y≤0.20, and 0≤z≤0.20.

6 . The positive electrode active material of claim 1 , wherein in a cross-sectional SEM image obtained by photographing a cross-section of the lithium composite oxide particle using a scanning electron microscope (SEM) after cross-sectioning the lithium composite oxide particle, a grain boundary density calculated by Equation 1 below for crystallites lying on an imaginary straight line crossing a center of the lithium composite oxide particle in the minor axis direction is 0.50 or less,

Grain boundary density=(Number of grain boundaries between crystallites lying on the imaginary straight line/Number of crystallites lying on the imaginary straight line).  [Equation 1]

7 . The positive electrode active material of claim 1 , wherein the positive electrode active material is an aggregate of a plurality of lithium composite oxide particles, and

in a cross-sectional SEM image obtained by photographing a cross-section of the lithium composite oxide particle using a scanning electron microscope (SEM) after cross-sectioning the lithium composite oxide particle of the positive electrode active material, a proportion of the lithium composite oxide particle with a grain boundary density of 0.50 or less calculated by Equation 1 for crystallites lying on an imaginary straight line crossing the center of the lithium composite oxide particle in the minor axis direction is 30% or more,

Grain boundary density=(Number of grain boundaries between crystallites lying on the imaginary straight line/Number of crystallites lying on the imaginary straight line).  [Equation 1]

8 . The positive electrode active material of claim 1 , wherein the lithium composite oxide particle is a non-aggregated single particle consisting of a single crystallite.

9 . The positive electrode active material of claim 1 , further comprising a coating layer that covers at least a part of the surface of the lithium composite oxide particle, and

wherein there is at least one type of metal oxide represented by Formula 2 below in the coating layer,

Li a M3 b O c   [Formula 2]

Wherein,

M3 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, V, Ba, Ta, Sn, Hf, Ce, Gd and Nd,

0≤a≤10, 0≤b≤8, and 2≤c≤13.

10 . A positive electrode comprising the positive electrode active material of claim 1 .

11 . A lithium secondary battery using the positive electrode of claim 10 .