Positive Electrode Active Material, Positive Electrode, and Lithium Secondary Battery

A positive electrode active material comprising a secondary particle in which a plurality of primary particles are aggregated, wherein:

• the plurality of primary particles have an average particle size measured from a scanning electron microscope (SEM) image of from 1.5 μm to 5.0 μm;
• a particle size of a primary particle is a particle size with respect to a long diameter of the primary particle;
• the plurality of primary particles include a disk-type primary particle;
• and
• in a primary particle observed from an SEM image of a surface or a cross-section of the secondary particle, when two imaginary tangent line with the most contact points are drawn, one for each of two boundary lines of a primary particle present within an angle of 45° or less with respect to a long diameter direction of the primary particle, and one imaginary line crossing the two imaginary tangent lines is drawn, the disk-type primary particle has interior angles of same side in a range of from 150° to 210°, a short diameter of 0.3 μm or more, and a ratio of a long diameter to the short diameter of 1.5 or more.