Positive active material for rechargeable lithium battery, preparing method thereof and rechargeable lithium battery including the same
Disclosed are a positive active material for a rechargeable lithium battery, a method of preparing the same, and a rechargeable lithium battery including the same. The positive active material includes a first positive active material in a form of secondary particles including a plurality of primary particles that are aggregated together, and a second positive active material having a single crystal form, wherein both of the first positive active material and the second positive active material are nickel-based positive active materials, each of the first positive active material and the second positive active material is coated with cobalt, and a maximum roughness of a surface of the second positive active material is greater than or equal to about 15 nm.
1. A positive active material for a rechargeable lithium battery, comprising:
a first positive active material in a form of secondary particles comprising a plurality of primary particles that are aggregated together, and
a second positive active material having a single crystal form,
wherein both of the first positive active material and the second positive active material are nickel-based positive active materials,
each of the first positive active material and the second positive active material is coated with cobalt, and
a maximum roughness (R max ; peak to peak height) of a surface of the second positive active material is greater than or equal to about 15 nm.
2. The positive active material of claim 1 , wherein:
an average roughness (R a ) of the surface of the second positive active material is greater than or equal to about 1.5 nm, and
a root mean square roughness (R q ) of the surface of the second positive active material is greater than or equal to about 2.0 nm.
3. The positive active material of claim 1 , wherein a BET specific surface area of the positive active material including the first positive active material and the second positive active material is about 0.3 m 2 /g to about 0.6 m 2 /g.
4. The positive active material of claim 1 , wherein the second positive active material has protrusions and depressions on the surface of the second positive active material.
5. The positive active material of claim 1 , wherein:
an average particle diameter of the first positive active material is about 7 μm to about 25 μm, and
an average particle diameter of the second positive active material is about 1 μm to about 10 μm.
6. The positive active material of claim 1 , wherein the first positive active material is included in an amount of about 50 wt % to about 90 wt %, and the second positive active material is included in an amount of about 10 wt % to about 50 wt % based on the total amount of the first positive active material and the second positive active material.
7. The positive active material of claim 1 , wherein:
the first positive active material comprises a compound represented by Chemical Formula 1, and
the second positive active material comprises a compound represented by Chemical Formula 11:
Li a1 Ni x1 M 1 y1 M 2 1−x1−y1 O 2 Chemical Formula 1
wherein, Chemical Formula 1, 0.9≤a1≤1.8, 0.3≤x1≤1, 0≤y1≤0.7, and M 1 and M 2 are each independently selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and a combination thereof,
Li a11 Ni x11 M 11 y11 M 12 1−x11−y11 O 2 Chemical Formula 11
wherein, in Chemical Formula 11, 0.9≤a11≤1.8, 0.3≤x11≤1, 0≤y11≤0.7, and M 11 and M 12 are each independently selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and a combination thereof.
8. A method of preparing a positive active material for a rechargeable lithium battery as claimed in claim 1 , the method comprising:
mixing a first nickel-based hydroxide and a lithium raw material together and performing a first heat-treatment to prepare a first nickel-based oxide in a form of secondary particles comprising a plurality of primary particles that are aggregated together;
mixing a second nickel-based hydroxide together with a lithium raw material and performing a second heat-treatment to prepare a second nickel-based oxide in a single crystal form, and
mixing the first nickel-based oxide, the second nickel-based oxide, and a cobalt compound together and performing a third heat-treatment to coat the first nickel-based oxide and the second nickel-based oxide with cobalt, and obtaining a final positive active material including the first positive active material and the second positive active material.
9. The method of claim 8 , wherein the first nickel-based hydroxide and the second nickel-based hydroxide are each independently represented by Chemical Formula 21:
Ni x21 M 21 y21 M 22 1−x21−y21 (OH) 2 Chemical Formula 21
wherein, in Chemical Formula 21, 0.3≤x21≤1, 0≤y21≤0.7, and M 21 and M 22 are each independently selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, F, Fe, Mg, Mn, Mo, Nb, P, S, Si, Sr, Ti, V, W, Zr, and a combination thereof.
10. The method of claim 8 , wherein:
the mixing of the first nickel-based hydroxide together with the lithium raw material is performed in a ratio of a mole number of lithium in the lithium raw material relative to a mole number of elements excluding hydrogen and oxygen included in the first nickel-based hydroxide of greater than or equal to about 0.9 and less than or equal to about 1.2, and
the mixing of the second nickel-based hydroxide together with the lithium raw material is performed in a ratio of a mole number of lithium in the lithium raw material relative a mole number of elements excluding hydrogen and oxygen included in the second nickel-based hydroxide of greater than or equal to about 0.9 and less than or equal to about 1.2.
11. The method of claim 8 , wherein the first heat-treatment is performed at about 600° C. to about 900° C.
12. The method of claim 8 , wherein the first heat-treatment is performed for about 5 hours to about 20 hours.
13. The method of claim 8 , wherein the second heat-treatment is performed at about 800° C. to about 1100° C.
14. The method of claim 8 , wherein the second heat-treatment is performed for about 5 hours to about 20 hours.
15. The method of claim 8 , wherein:
the preparing of the second nickel-based oxide comprises obtaining the second nickel-based oxide in the single crystal form by mixing the second nickel-based hydroxide together with the lithium raw material, secondarily heat-treating the resultant mixture, and pulverizing a resultant therefrom.
16. The method of claim 8 , wherein:
the mixing of the first nickel-based oxide and the second nickel-based oxide is performed by mixing the first nickel-based oxide and the second nickel-based oxide together in a weight ratio of about 9:1 to about 5:5.
17. The method of claim 8 , wherein:
the mixing of the first nickel-based oxide, the second nickel-based oxide, and the cobalt compound together is performed so that cobalt included in the cobalt compound is included in an amount of about 0.01 parts by mole to about 7 parts by mole, based on 100 parts by mole of the total amount of elements excluding lithium and oxygen in the first nickel-based oxide and the second nickel-based oxide.
18. The method of claim 8 , wherein the third heat-treatment is performed at about 650° C. to about 900° C.
19. The method of claim 8 , wherein the third heat-treatment is performed for about 5 hours to about 30 hours.
20. A rechargeable lithium battery comprising:
a positive electrode including the positive active material of claim 1 , a negative electrode, and an electrolyte.