Positive electrode material for lithium secondary battery, method of preparing the same, and positive electrode for lithium secondary battery and lithium secondary battery which includes the positive electrode material

1. A positive electrode material comprising:

a first positive electrode active material represented by Formula 1; and

a second positive electrode active material in a form of a single particle and represented by Formula 2,

wherein an amount of lithium impurities on a surface of the second positive electrode active material is 0.14 wt % or less based on a total weight of the second positive electrode active material, and

at least one of nickel (Ni), cobalt (Co), and manganese (Mn) included in the second positive electrode active material has a concentration gradient gradually changing from a center of the particle to a surface thereof:

LiCo 1-a M 1 a O 2   [Formula 1]

LiNi b Co c Mn d M 2 e O 2   [Formula 2]

wherein, in Formula 1, M 1 comprises at least one selected from the group consisting of aluminum (Al), titanium (Ti), magnesium (Mg), and zirconium (Zr), and 0≤a≤0.2, and

wherein, in Formula 2, M 2 comprises at least one selected from the group consisting of Al, Ti, Mg, Zr, yttrium (Y), strontium (Sr), and boron (B), and 0<b≤0.6, 0<c≤0.35, 0<d≤0.35, and 0≤e≤0.1,

wherein Mn included in the second positive electrode active material has a concentration gradient gradually decreasing from the center of the particle to the surface thereof.

2. The positive electrode material of claim 1 , wherein the first positive electrode active material and the second positive electrode active material are included in a weight ratio of 40:60 to 90:10.

3. The positive electrode material of claim 1 , wherein the first positive electrode active material has an average particle diameter (D 50 ) of 10 μm or more.

4. The positive electrode material of claim 1 , wherein the second positive electrode active material has an average particle diameter (D 50 ) of 5 μm to 10 μm.

5. The positive electrode material of claim 1 , wherein the second positive electrode active material has a grain size of 200 nm to 500 nm.

6. A positive electrode for a lithium secondary battery, the positive electrode comprising the positive electrode material of claim 1 .

7. A lithium secondary battery comprising the positive electrode of claim 6 .

8. A method of preparing a positive electrode material, the method comprising:

preparing a first positive electrode active material represented by Formula 1 by mixing and sintering a cobalt oxide, a lithium-containing raw material, and a doping element M 1 -containing raw material;

preparing a second positive electrode active material in a form of a single particle, which is represented by Formula 2, by sintering a nickel cobalt manganese hydroxide precursor having a core-shell structure and a lithium-containing raw material at 900° C. or more; and

mixing the prepared first positive electrode active material and the prepared second positive electrode active material,

wherein at least one of nickel, cobalt, and manganese in the second positive electrode active material has a concentration gradient gradually changing from a center of the particle to a surface thereof:

LiCo 1-a M 1 a O 2   [Formula 1]

LiNi b Co c Mn d M 2 e O 2   [Formula 2]

wherein, in Formula 1,

M 1 comprises at least one selected from the group consisting of aluminum (Al), titanium (Ti), magnesium (Mg), and zirconium (Zr), and 0≤a≤0.2, and

wherein, in Formula 2,

M 2 comprises at least one selected from the group consisting of Al, Ti, Mg, Zr, yttrium (Y), strontium (Sr), and boron (B), and 0<b≤0.6, 0<c≤0.35, 0<d≤0.35, and 0≤e≤0.1.

9. The method of claim 8 , wherein an average composition of the nickel cobalt manganese hydroxide precursor is represented by Formula 3:

Ni b1 Co c1 Mn d1 (OH) 2   [Formula 3]

wherein, in Formula 3,

0<b1≤0.6, 0<c1≤0.35, and 0<d1≤0.35.

10. The method of claim 8 , further comprising a step of preparing the nickel cobalt manganese hydroxide precursor including:

preparing a first transition metal-containing solution including nickel, cobalt, and manganese and a second transition metal-containing solution including nickel, cobalt, and manganese in a concentration different from that of the first transition metal-containing solution; and

mixing the first transition metal-containing solution and the second transition metal-containing solution such that a mixing ratio of the first transition metal-containing solution and the second transition metal-containing solution is gradually changed from 100 vol %:0 vol % to 0 vol %: 100 vol % and adding an ammonium ion-containing solution and a basic aqueous solution at a same time,

wherein at least one of the nickel, the cobalt, and the manganese has a concentration gradient gradually changing from the center of the particle to the surface thereof.

11. The method of claim 8 , wherein the first positive electrode active material and the second positive electrode active material are mixed in a weight ratio of 40:60 to 90:10.