A positive electrode active material for a secondary battery according to an embodiment of the present invention comprises a lithium composite oxide represented by chemical formula 1 below and containing a layer-structured lithium excess oxide, wherein the lithium composite oxide comprises a secondary particle; the secondary particle comprises at least one primary particle; the primary particle comprises at least one crystallite; at least any one selected from the secondary particle, the primary particle, and the crystallite comprises a core and a shell occupying at least a portion of the surface of the core; and when the crystal structure belonging to space group C2/m is designated as [C2/m], the crystal structure belonging to space group R-3m is designated as [R-3m], and the ratio of the crystal structure belonging to space group C2/m versus the crystal structure belonging to space group R-3m is designated as [C2/m]/[R-3m], the [C2/m]/[R-3m] of the core differs from that of the shell in the secondary particle: [chemical formula 1] rLi 2 M1O 3 ·(1-r)Li a M2O 2