Nickel-based active material for lithium secondary battery, preparing method thereof, and lithium secondary battery including positive electrode including the same

1. A nickel-based active material for a lithium secondary battery, the nickel-based active material comprising a porous inner portion having closed pores and an outer portion,

wherein the porous inner portion has a density less than that of the outer portion,

the nickel-based active material has a net density of 4.7 g/cc or less, and

wherein a volume of the porous inner portion is 20 to 35% based on a total volume of the nickel-based active material.

2. The nickel-based active material of claim 1 , wherein a pore size of the closed pores is in a range of 150 nm to 1 μm in the porous inner portion.

3. The nickel-based active material of claim 1 , wherein

the porous inner portion has a porosity of 3 to 30 %, and

the porosity of the porous inner portion is 1.2 times or more greater than that of the outer portion.

4. The nickel-based active material of claim 1 , wherein the outer portion has a radially arranged structure.

5. The nickel-based active material of claim 1 , wherein the outer portion has a pore size of less than 150 nm.

6. The nickel-based active material of claim 1 , wherein

the nickel-based active material comprises plate particles, and

major axes of the plate particles are radially arranged.

7. The nickel-based active material of claim 6 , wherein the plate particles have an average thickness of 100 nm to 200 nm, and an average length of 150 nm to 500 nm.

8. The nickel-based active material of claim 6 , wherein the plate particles have a ratio of an average thickness to an average length of 1:2 to 1:10.

9. The nickel-based active material of claim 1 , wherein the nickel-based active material comprises a compound represented by Formula 1 below:

Li a (Ni 1-x-y-z Co x Mn y M z )O 2   Formula 1

wherein in Formula 1, M is an element selected from the group consisting of boron (B), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zirconium (Zr), and aluminum (Al), and

0.95≤a≤1.3, x≤(1-x-y-z), y≤(1-x-y-z), 0<x<1, 0≤y<1, and 0≤z<1.

10. The nickel-based active material of claim 9 , wherein in Formula 1, 1.0≤a≤1.3, 0<x≤⅓, 0≤y≤0.5, 0≤z≤0.05, and ⅓(1-x-y-z)≤0.95.

11. The nickel-based active material of claim 9 , wherein the nickel-based active material comprises LiNi 0.6 Co 0.2 Mn 0.2 O 2 , LiNi 0.5 Co 0.2 Mn 0.3 O 2 , LiNi 1/3 Co 1/3 Mn 1/3 O 2 , LiNi 0.8 Co 0.1 Mn 0.1 O 2 , and/or LiNi 0.85 Co 0.1 Al 0.05 O 2 .

12. A method of preparing a nickel-based active material for a lithium secondary battery, the method comprising:

mixing a lithium precursor and a metal hydroxide to form a mixture, and

conducting a primary heat treatment on the mixture under an oxidizing gas atmosphere at 600° C. to 800° C.,

wherein the nickel-based active material comprises a porous inner portion having closed pores and an outer portion,

the porous inner portion has a density less than that of the outer portion, and

the nickel-based active material has a net density of 4.7 g/cc or less, and

wherein a volume of the porous inner portion is 20 to 35% based on a total volume of the nickel-based active material.

13. The method of claim 12 , further comprising conducting a secondary heat treatment on the mixture under an oxidizing gas atmosphere at 700° C. to 900° C. after the conducting of the primary heat treatment,

wherein the secondary heat treatment is conducted at a higher temperature than the primary heat treatment.

14. The method of claim 12 , further comprising pretreating the mixture under an oxidizing gas atmosphere at 400° C. to 700° C. for 1 to 3 hours prior to the conducting of the primary heat treatment.

15. The method of claim 12 , wherein the metal hydroxide comprises porous secondary particles comprising radially oriented primary particles.

16. The method of claim 12 , wherein the metal hydroxide comprises a compound represented by Formula 2 below:

(Ni 1-x-y-z Co x Mn y M z )(OH) 2   Formula 2

wherein in Formula 2, M is an element selected from the group consisting of boron (B), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zirconium (Zr), and aluminum, and

x≤(1-x-y-z), y≤(1-x-y-z), 0<x<1, 0≤y<1, and 0≤z<1.

17. A lithium secondary battery comprising a positive electrode comprising the nickel-based active material of claim 1 , a negative electrode, and an electrolyte between the positive electrode and the negative electrode.

18. A nickel-based active material for a lithium secondary battery, the nickel-based active material comprising a porous inner portion having closed pores and an outer portion,

wherein the porous inner portion has a density less than that of the outer portion, and

the nickel-based active material has a net density of 4.7 g/cc or less,

wherein the nickel-based active material has a net density of 4.50 to 4.7 g/cc.