Positive electrode active material for lithium secondary battery including core containing lithium cobalt oxide and shell being deficient in lithium, and method of preparing the same
Provided is a positive electrode active material particle for a secondary battery, including a core part containing lithium cobalt oxide; and a shell part that is located on the surface of the core part and contains lithium-deficient cobalt oxide which is deficient in lithium because a molar ratio of lithium to cobalt is 0.9 or less.
1. A positive electrode active material for a secondary battery, comprising:
a core part including lithium cobalt oxide; and
a shell part that is located on a surface of the core part and includes lithium-deficient cobalt oxide which is deficient in lithium because a molar ratio of lithium to cobalt is 0.9 or less,
wherein a molar ratio of lithium:cobalt in the lithium-deficient cobalt oxide of the shell part is in the range of 0.7:1 to 0.9:1,
wherein the lithium cobalt oxide of the core part has a composition of the following Chemical Formula 1:
Li a Co 1-b M b O 2 (1)
wherein, in Chemical Formula 1, M is each independently one or more selected from the gro up consisting of Ti, Mg, Al, Zr, Ba, Ca, Ti, Ta, Nb, and Mo;
0.95≤a≤1.02; and
0≤b≤0.2, and
wherein the lithium-deficient cobalt oxide of the shell part has a composition of the following Chemical Formula 2:
Li a Co 1-b M b O 2 (2)
Wherein, in Chemical Formula 2, M is each independently one or more selected from the group consisting of Ti, Mg, Al, Zr, Ba, Ca, Ti, Ta, Nb, and Mo;
0.7≤a≤0.9; and
0≤b≤0.2.
2. The positive electrode active material for the secondary battery of claim 1 , wherein the shell part including the lithium-deficient cobalt oxide generates a relatively small amount of tetravalent cobalt ions (Co 4+ ) in a charge/discharge range of 3.0 V to 4.6 V, compared to lithium cobalt oxide having a stoichiometric ratio of lithium:metal of 1:1, thereby providing stability of the particle surface of lithium cobalt oxide of the shell part.
3. The positive electrode active material for the secondary battery of claim 1 , wherein the lithium cobalt oxide of the core part is LiCoO 2 .
4. The positive electrode active material for the secondary battery of claim 1 , wherein the lithium-deficient cobalt oxide of the shell part is Li 0.8 CoO 2 .
5. The positive electrode active material for the secondary battery of claim 1 , wherein the shell part includes a relatively low concentration of lithium, compared to the core part, and lithium of the core part is distributed at a uniform concentration in the core part, and lithium of the shell part is distributed with a gradually increasing concentration gradient from the interface of the core and shell parts to the outer surface of the shell part.
6. The positive electrode active material for the secondary battery of claim 1 , wherein a thickness of the shell part is 30 nm to 500 nm.
7. The positive electrode active material for the secondary battery of claim 1 , wherein an average particle size (D50) of the positive electrode active material is 15 μm to 25 μm.
8. The positive electrode active material for the secondary battery of claim 1 , wherein the surface of the shell part is coated with Al 2 O 3 .
9. The positive electrode active material for the secondary battery of claim 8 , wherein a coating thickness of Al 2 O 3 is 50 nm to 100 nm.
10. A method of preparing the positive electrode active material for the secondary battery of claim 1 , the method comprising:
(a) forming a spherical core part including lithium cobalt oxide by mixing a cobalt source material and a lithium source material and then by primary sintering of the mixture;
(b) uniformly coating the outer surface of the core part with the cobalt source material, and then further injecting the lithium source material to mix them; and
(c) forming a shell part by secondary sintering of the mixture of the process (b), wherein in the process (b), the amounts of the cobalt source material and the lithium source material are determined such that the molar ratio of lithium:cobalt in the shell part formed by the secondary sintering of the process (c) is within the range of 0.7:1 to 0.9:1.
11. The method of preparing the positive electrode active material for the secondary battery of claim 10 , wherein in the process (a) and/or the process (b), sintering is carried out by further adding a doping precursor containing an element A, wherein the element A is one or more selected from the group consisting of Ti, Mg, Al, Zr, Ba, Ca, Ti, Ta, Nb and Mo.
12. The method of preparing the positive electrode active material for the secondary battery of claim 10 , wherein in the process (b), the amounts of the cobalt source material and the lithium source material are determined such that the molar ratio of lithium:cobalt in the shell part formed by the secondary sintering of the process (c) is within the range of 0.8:1.
13. The method of preparing the positive electrode active material for the secondary battery of claim 10 , wherein in the process (a), a temperature of the primary sintering is 950° C. to 1070° C., and a time of the primary sintering is 8 hours to 12 hours.
14. The method of preparing the positive electrode active material for the secondary battery of claim 10 , wherein in the process (c), a temperature of the secondary sintering is 900° C. to 1050° C., and a time of the secondary sintering is 5 hours to 12 hours.
15. A positive electrode which is manufactured by applying a slurry including the positive electrode active material for the secondary battery of claim 1 , a conductive material, and a binder onto a collector.
16. A lithium secondary battery comprising the positive electrode of claim 15 , a negative electrode, and an electrolyte.