Positive electrode active material for lithium secondary battery, method of preparing the same and lithium secondary battery including the same
The present invention provides a positive electrode active material for a lithium secondary battery including a core including first lithium cobalt oxide, and a surface modifying layer positioned on a surface of the core. The surface modifying layer includes a lithium compound discontinuously distributed on the surface of the core, and second lithium cobalt oxide distributed while making a contact with or adjacent to the lithium compound, with a Li/Co molar ratio of less than 1. The positive electrode active material according to the present invention forms a lithium deficient structure in the positive electrode active material of lithium cobalt oxide and changes two-dimensional lithium transport path into three-dimensional path. The transport rate of lithium ions may increase when applied to a battery, thereby illustrating improved capacity and rate characteristic without decreasing initial capacity.
1. A positive electrode active material for a lithium secondary battery, comprising:
a core comprising a first lithium cobalt oxide; and
a surface modifying layer positioned on a surface of the core,
wherein the surface modifying layer comprises:
a lithium compound discontinuously distributed on the surface of the core; and
a second lithium cobalt oxide distributed while making a contact with or adjacent to the lithium compound, the second lithium cobalt oxide having a LiCo molar ratio of less than 1,
wherein the lithium compound comprises at least one lithium reactive element selected from the group consisting of Ti, W, Zr, Mn, Mg, P, Ni, Al, Sn, V, Cr, and Mo.
2. The positive electrode active material for a lithium secondary battery of claim 1 , wherein the lithium reactive element is included in an amount ratio from 50 ppm to 50,000 ppm relative to a total weight of the positive electrode active material.
3. The positive electrode active material for a lithium secondary battery of claim 1 , wherein the lithium compound comprises at least one selected from the group consisting of Li 2 TiO 3 , Li 4 Ti 5 O 12 , Li 3 PO 4 , Li 2 MnO 3 , LiMn 2 O 4 , and LiAlO 2 , or a mixture of two or more thereof.
4. The positive electrode active material for a lithium secondary battery of claim 1 , wherein the lithium compound is distributed in an island shape on the surface of the core.
5. The positive electrode active material for a lithium secondary battery of claim 1 , wherein the first lithium cobalt oxide comprises a compound of the following Formula 1, and the second lithium cobalt oxide comprises a compound of the following Formula 2:
Li a CoM x O 2 [Formula 1]
Li 1-b CoM′ y O 2 [Formula 2]
(in Formula 1 and 2, M and M′ each independently comprises at least one metal element selected from the group consisting of W, Mo, Zr, Ti, Mg, Ta, Al, Fe, V, Cr, and Nb, and a, b, x, and y satisfy the following relations of 1≦a≦1.2, 0≦b≦0.05, 0≦x≦0.02 and 0≦y ≦0.02).
6. The positive electrode active material for a lithium secondary battery of claim 1 , wherein the first lithium cobalt oxide has a layered crystal structure, and the second lithium cobalt oxide belongs to an Fd-3m space group and has a cubic crystal structure.
7. The positive electrode active material for a lithium secondary battery of claim 5 , wherein
a concentration of lithium in the core is higher than a concentration of lithium included in the surface modifying layer,
the lithium is distributed with a gradually increasing concentration gradient from an interface of the core and the surface modifying layer to a center of the core, and
in Formula 1, a increases toward the center of the core within a range of 1≦a≦1.2.
8. The positive electrode active material for a lithium secondary battery of claim 1 , wherein
a concentration of lithium in the core is higher than a concentration of lithium included in the surface modifying layer, and
the lithium has one concentration value in each region of the core and the surface modifying layer.
9. The positive electrode active material for a lithium secondary battery of claim 1 , wherein the core and the surface modifying layer have a thickness ratio from 1:0.01 to 1:0.1.
10. The positive electrode active material for a lithium secondary battery of claim 1 , comprising a monolith structure having an average particle diameter (D50) from 3 to 50μm.
11. The positive electrode active material for a lithium secondary battery of claim 1 , having an inflection point is within a voltage range from 4.0 V to 4.2 V when measuring a voltage profile when charging and discharging.
12. A positive electrode for a lithium secondary battery, the positive electrode comprising the positive electrode active material according to claim 1 .
13. A lithium secondary battery comprising the positive electrode of claim 12 .
14. A battery module comprising the lithium secondary battery of claim 13 as a unit cell.
15. A battery pack comprising the battery module of claim 14 .
16. The battery pack of claim 15 , wherein the battery pack is used as a power source of a medium and large size device.
17. The battery pack of claim 16 , wherein the medium and large size device is selected from the group consisting of an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and an energy storage system.