Cathode active material, method for preparing the same, and lithium secondary batteries including the same
The present invention relates to a cathode active material for a lithium secondary battery, a method for preparing the same, and a lithium secondary battery including the same, and provides a cathode active material including: a lithium manganese-excess layered structure composite oxide represented by Formula Li[Li x-z (Ni a Co b Mn c ) 1-x ]O 2-y F y (here, a+b+c=1, 0.05≦x≦0.33, 0≦y≦0.08, and 0<z≦0.05); a metal fluoride coating layer coated on a surface of the composite oxide; and a metal phosphate coating layer coated on the metal fluoride coating layer.
1. A cathode active material comprising:
a lithium manganese-excess layered structure composite oxide represented by Formula Li[Li x-z (Ni a Co b Mn c ) 1-x ]O 2-y F y (here, a+b+c=1, 0.05<×<0.33, 0<y<0.08, and 0<z<0.05);
a metal fluoride coating layer coated on a surface of the composite oxide; and
a metal phosphate coating layer coated on the metal fluoride coating layer.
2. The cathode active material of claim 1 , further comprising:
a lithium manganese oxide (LiMn 2 O 4 ) having a cubic spinel structure, which is formed by a change in a part of a monoclinic Li 2 MnO 3 in the composite oxide in a vicinity of a surface of the composite oxide.
3. The cathode active material of claim 2 , wherein the lithium manganese oxide having a cubic spinel structure performs a reduction reaction at 2.8 V to 3.0 V.
4. The cathode active material of claim 1 , wherein the composite oxide comprises a rhombohedral LiMO 2 (here, M is Ni, Co, and Mn) and a monoclinic Li 2 MnO 3 .
5. The cathode active material of claim 1 , wherein a total content of the metal fluoride coating layer and the metal phosphate coating layer is 0.5 wt% to 2.0 wt% based on a content of the cathode active material.
6. The cathode active material of claim 5 , wherein a weight ratio of the metal fluoride coating layer and the metal phosphate coating layer is within a range of 1.0 <the metal fluoride coating layer/the metal phosphate coating layer <5.0.
7. The cathode active material of claim 1 , wherein the metal fluoride is one selected from the group consisting of AlF 3 , MgF 2 , CoF 3 , NaF, and a mixture thereof.
8. The cathode active material of claim 1 , wherein the metal phosphate is one selected from the group consisting of AlPO 4 , Mg 3 (PO 4 ) 2 , Co 3 (PO 4 ) 2 , and a mixture thereof.
9. The cathode active material of claim 1 , wherein a content of the metal phosphate coating layer is 1.0 wt % or less.
10. The cathode active material of claim 9 , wherein a content of the metal phosphate coating layer is 0.2 wt % to 0.4 wt %.
11. A method for preparing a cathode active material comprising:
preparing a lithium manganese-excess layered structure composite oxide represented by Formula Li[Li x-z (Ni a Co b Mn c ) 1-x ]O 2-y F y (here, a+b+c=1, 0.05 <×<0.33, 0<y<0.08, and 0<z<0.05);
performing a primary coating on a surface of the composite oxide with a metal fluoride; and
performing a secondary coating on the metal fluoride coating layer with a metal phosphide.
12. The method of claim 11 , further comprising:
forming a lithium manganese oxide having a cubic spinel structure between the composite oxide and the metal fluoride coating layer.
13. The method of claim 12 , wherein the forming comprises:
forming a lithium-metal-fluoride (Li-M-F) composite by reacting lithium in a monoclinic Li 2 MnO 3 region in the composite oxide with the metal fluoride coating layer; and
changing a part of Li 2 MnO 3 in the monoclinic Li 2 MnO 3 region into LiMn 2 O 4 having a cubic spinel structure.
14. The method of claim 11 , further comprising:
heat-treating the cathode active material under an inert atmosphere.
15. The method of claim 11 , wherein the primary coating and the secondary coating are continuously performed.
16. The method of claim 11 , wherein a total content of the coating layer coated through the primary coating and the secondary coating is 0.5 wt % to 2.0 wt % based on the content of the cathode active material.
17. The method of claim 11 , wherein a weight ratio of the metal fluoride coating layer coated through the primary coating and the metal phosphate coating layer coated through the secondary coating is within a range of 1.0<the metal fluoride coating layer/the metal phosphate coating layer <5.0.
18. The method of claim 11 , wherein the metal fluoride is one selected from the group consisting of AlF 3 , MgF 2 , CoF 3 , NaF, and a mixture thereof.
19. The method of claim 11 , wherein the metal phosphate is one selected from the group consisting of AlPO 4 , Mg 3 (PO 4 ) 2 , Co 3 (PO 4 ) 2 , and a mixture thereof.
20. A lithium secondary battery comprising:
a cathode comprising the cathode active material of claim 1 ;
an anode comprising an anode active material; and
an electrolyte present between the cathode and the anode.
21. The lithium secondary battery of claim 20 , wherein a nominal voltage of a discharge curve (1 C and 4.6 V to 2.5 V) is 3.6 V or more.