Cathode active material and lithium secondary battery for controlling impurity or swelling including the same and method of preparing cathode active material with enhanced productivity

A cathode active material represented by Formula 1 below and comprising a metal cation having a greater ionic radius than a Ni cation and represented by M of Formula 1 at a Li cation site or in an empty space within a crystal lattice so as to prevent mixing of Ni cations into a Li layer,

• wherein the cathode active material further comprises a Li-containing compound,
• wherein an amount of the Li-containing compound is greater than 0 and less than or equal to 0.161 wt % based on a total weight of the cathode active material and the Li-containing compound is Li2CO3 and/or LiOH:
• the cathode active material is prepared by a method comprising the steps of sintering a mixture of a transition metal source and a lithium source in air, wherein the mixture comprises an alkaline earth metal source having an oxidation number of +2 so that the sintering is performed at 890° C. to 930° C., and
• the alkaline earth metal is Sr or Ba:
• LiaNixMnyCozMwO2−tAt (Formula 1)
• wherein
• 0≤a≤1.2, 0<x≤0.9, 0<y≤0.9, 0<z≤0.9, 0<w≤0.3, 2≤a+x+y+z+w≤2.3, x>y, x>z, and 0≤t<0.2;
• M is Sr2+or Ba2+;
• and
• A is at least one monovalent or divalent anion.