Positive electrode active material and lithium secondary battery including the same

A positive electrode active material comprising a lithium composite oxide containing at least nickel and cobalt, wherein:

• the cobalt in the lithium composite oxide has a concentration gradient that decreases from a surface portion of the lithium composite oxide toward a central portion thereof;
• and
• the concentration gradient of the cobalt has at least different slopes, and signs of the different slopes are the same,
• wherein, in Energy Profiling-Energy Dispersive X-Ray Spectroscopy (EP-EDS) analysis which measures a cumulative concentration of the cobalt from a surface of the lithium composite oxide to a depth to which an electron beam penetrates using the electron beam radiated with an acceleration voltage increasing from 1 kV to 30 kV with respect to the surface of the lithium composite oxide, an inflection point at which the slope of the concentration gradient of the cobalt in the lithium composite oxide changes in a region where the acceleration voltage is 7.5 kV to 12.5 kV is present, and
• wherein, when s1 represents the slope of the concentration gradient of the cobalt in the lithium composite oxide in a region where the acceleration voltage is 1 kV to 10 kV, s1 satisfies the following Equation 1,
• 2.0≤s1≤3.6. [Equation 1]