Positive electrode active material for lithium secondary battery, preparation method therefor, and lithium secondary battery comprising same

1 . A cathode active material for a secondary battery comprising:

lithium composite oxide particles;

overlithiated oxide represented by Formula 1 below and having a layered crystal structure inside the lithium composite oxide particles; and

lithium manganese oxide represented by Formula 2 below outside the lithium composite oxide particles,

wherein, when a ratio of a number of moles of lithium (Li) to a total number of moles of metal (M) excluding lithium is defined as Li/M, the overlithiated oxide comprised inside the lithium composite oxide particles and the lithium manganese oxide comprised outside the lithium composite oxide particles have different values of Li/M,

rLi 2 MnO 3 ·(1- r )Li a Ni x Co y Mn z M1 1-(x+y+z) O2  [Formula 1]

wherein r, a, x, y, and z satisfy 0<r≤0.6, 0<a≤1, 0≤x≤1, 0<y<1, 0≤z<1, and 0<x+y+z<1, and

M1 comprises at least one selected from Ba, Sr, P, Y, Zr, Nb, Mo, Ta, or W,

Li b Mn p O q   [Formula 2]

wherein b, p and q satisfy 0.1≤b/p≤2.5, and 0<q≤15,

wherein a Li/M ratio inside the lithium composite oxide particles is 1.1 to 1.6, and

wherein:

a) a Li/M ratio outside the lithium composite oxide particles is from 0.1 to 0.9, and the outside of the lithium composite oxide particles has a spinel crystal structure; or

b) a Li/M ratio outside the lithium composite oxide particles is from 1.8 to 2.5, and the outside of the lithium composite oxide particles has a layered crystal structure.

2 . The cathode active material for a secondary battery according to claim 1 , wherein the lithium composite oxide particles have a lithium concentration gradient from an inside to an outside of the lithium composite oxide particles.

3 . The cathode active material for a secondary battery according to claim 1 , wherein a ratio (Mn/Ni) of a number of moles of manganese (Mn) to a total number of moles of nickel (Ni) inside the lithium composite oxide particles is 1 to 4.5.

4 . The cathode active material for a secondary battery according to claim 1 , wherein the lithium composite oxide particles have a manganese concentration gradient from an inside to an outside of the lithium composite oxide particles.

5 . The cathode active material according to claim 1 , wherein the lithium composite oxide particles comprise secondary particles formed by agglomeration of primary particles, and

M1 of Formula 1 is a dopant acting as a flux for growing the primary particles.

6 . The cathode active material according to claim 1 , wherein the lithium composite oxide particles comprise secondary particles formed by agglomeration of primary particles, and

primary particles having a size of 300 nm to 10 μm are present in an amount of 50 to 100 vol % based on a total amount of the primary particles comprised in the secondary particles.

7 . A method of preparing the cathode active material for a secondary battery according to claim 1 , the method comprising:

forming precursor particles for forming an inside of the cathode active material;

mixing the formed precursor particles with a lithium compound and performing primary heat treatment;

dispersing the primary heat-treated particles in distilled water or an alkaline aqueous solution and then coating the particles with a compound containing manganese to form an outside of the cathode active material; and

mixing the coated particles with a lithium compound and performing secondary heat treatment.

8 . The method according to claim 7 , wherein, in the primary heat treatment, a compound containing M1 of Formula 1 is further mixed and heat-treated.

9 . A secondary battery comprising the cathode active material according to claim 1 .