Positive electrode active material and lithium secondary battery including the same

A bimodal-type positive electrode active material comprising a first lithium composite oxide as a small particle and a second lithium composite oxide as a large particle,

• wherein the first lithium composite oxide and the second lithium composite oxide include at least nickel and cobalt, and the cobalt in each of the first lithium composite oxide and the second lithium composite oxide has a concentration gradient that decreases from a surface portion toward a central portion, and
• when s1 represents an absolute value of a slope of the concentration gradient of the cobalt decreasing from the surface portion toward the central portion of the first lithium composite oxide and s2 represents an absolute value of a slope of the concentration gradient of the cobalt decreasing from the surface portion toward the central portion of the second lithium composite oxide, s1 and s2 satisfy the following Equation 1 to Equation 3:
• s1<s2, [Equation 1]
• 0.5<s1<2.0, and [Equation 2]
• 1.5<s2<3.0. [Equation 3]