Positive electrode active material for secondary battery, and secondary battery comprising the same

1. A positive electrode active material for a secondary battery, the positive electrode active material being a primary particle having a monolithic structure that includes a lithium composite metal oxide of Formula 1 below,

wherein the primary particle has an average particle size (D 50 ) of 2 μm to 20 μm and a Brunauer-Emmett-Teller (BET) specific surface area of 0.15 m 2 /g to 0.5 m 2 /g,

wherein the lithium composite metal oxide is formed by sintering at 700° C. to 900° C. in the presence of a boron-based sintering additive:

Li a Ni 1-x-y Co x M1 y M3 z M2 w O 2   [Formula 1]

wherein, in Formula 1, M1 is Mn or combination of Mn and Al, M2 is any one or two elements selected from the group consisting of Zr and Mg, M3 is W, and 1.0≤a≤1.5, 0<x0.5, 0<y≤0.5, 0.005≤z<0.01, 0≤w≤0.04, 0<x+00.7.

2. The positive electrode active material of claim 1 , wherein in Formula 1, 0.4<x+y<0.7.

3. The positive electrode active material of claim 1 , wherein at least one metal element of nickel, M1, and cobalt exhibits a concentration gradient that changes in the active material.

4. The positive electrode active material of claim 1 , wherein:

nickel, M1, and cobalt independently exhibit a concentration gradient that changes throughout the active material;

the concentration of nickel decreases with a concentration gradient in a direction from a center of the active material to a surface thereof; and

the concentrations of cobalt and M1 independently increases with a concentration gradient in the direction from the center of the active material to the surface thereof.

5. The positive electrode active material of claim 1 , wherein the M1 is manganese (Mn).

6. The positive electrode active material of claim 1 , wherein the positive electrode active material has a polyhedral shape.

7. The positive electrode active material of claim 1 , wherein the positive electrode active material has a particle size distribution value (Dcnt), which is defined by Equation 1 below, of 0.5 to 1.0

Dcnt=[Dn90-Dn101]/Dn50   [Equation 1]

wherein, in Equation 1, Dn90, Dn10, and Dn50 are number average particle sizes measured under 90%, 10%, and 50%, respectively.

8. The positive electrode active material of claim 1 , wherein the positive electrode active material has a rolling density of 3.0 g/cc or higher under a pressure of 2 ton·f.

9. The positive electrode active material of claim 1 , wherein the average particle size (D 50 ) of the primary particle is 2 μm to 8 μm.

10. The positive electrode active material of claim 3 , wherein the concentration gradient that changes throughout the active material of at least one of nickel, M1, and cobalt independently changes in a direction from a center of the active material to a surface thereof at a constant slope.

11. A positive electrode for a secondary battery, the positive electrode comprising the positive electrode active material of claim 1 .

12. A lithium secondary battery comprising the positive electrode of claim 11 .

13. A method of fabricating the positive electrode active material for a secondary battery of claim 1 , the method comprising:

a step of preparing a precursor by mixing a nickel raw material, a cobalt raw material, and an M1 raw material, wherein M1 is Mn or the combination of Al and Mn, and then performing a reaction;

a step of mixing the precursor with a lithium raw material and a W raw material such that a molar ratio of Li/Me, wherein Me=the sum of metal elements in the precursor and W, is 2.0 or higher and then sintering at 700° C. to 900° C. in the presence of the boron-based sintering additive; and

a step of washing a product obtained by a result of the sintering such that a molar ratio of Li/Me′, wherein Me′=the sum of metal elements, excluding lithium, in the positive electrode active material in the finally fabricated positive electrode active material is from 1.0 to 1.5 and then drying at 150° C. to 400° C.

14. The method of claim 13 , wherein an M2 raw material, wherein M2 is any one or two elements selected from the group consisting of Zr and Mg, is further added in the preparing of the precursor or the sintering.

15. The method of claim 13 , wherein the precursor is fabricated by adding an ammonium cation-containing complexing agent and a basic compound to a metal-containing solution, which is produced by mixing the nickel raw material, the cobalt raw material, and the M1 raw material and performing a coprecipitation reaction.

16. The method of claim 15 , wherein a second metal-containing solution including the nickel raw material, the cobalt raw material, and the M1 raw material in different concentrations from the metal-containing solution is further added to the metal-containing solution.

17. The method of claim 13 , wherein the boron-based sintering additive includes any one or two or more selected from the group consisting of boric acid, lithium tetraborate, boron oxide, and ammonium borate.