Positive electrode active material for lithium secondary battery, comprising lithium cobalt oxide for high voltage, and method for preparing same

Abstract

The present invention provides a positive active material for a rechargeable lithium battery, the active material including a dopant and having a crystalline structure in which metal oxide layers (MO layers) including metals and oxygen and reversible lithium layers are repeatedly stacked, wherein in a lattice configured by oxygen atoms of the MO layers adjacent to each other, the dopant time of charge, thereby forming a lithium trap and/or lithium dumbbell structure.

Claims

1. A positive active material for a rechargeable lithium battery comprising lithium cobalt oxide particles, wherein the lithium cobalt oxide particles having dopants, wherein the dopants are Mg and Zr, wherein Zr is primarily included in the outer bulk, and Mg is mainly included in the inner bulk, and the Zr concentration of the outer bulk of a particle surface to 0.9*r is relatively higher than the Mg concentration of the inner bulk of 0.9*r to a particle center; the lithium cobalt oxide particles have a crystalline structure in which metal oxide layers (MO layers) including metals and oxygen and reversible lithium layers in which lithium ions move reversibly at the time of charge and discharge are repeatedly stacked, and the dopant and/or lithium ions move from octahedral sites to tetrahedral sites at the time of charge in a lattice configured by oxygen atoms of the MO layers adjacent to each other, thereby forming a lithium trap and/or a lithium dumbbell structure and providing structural stability at a high voltage of 4.5 V or greater.

2. The positive active material of claim 1, wherein the dopant is included in an amount of 0.001 to 1 wt % based on a total weight of the lithium cobalt oxide particles.

3. The positive active material of claim 1, wherein the lithium trap structure is a structure in which the lithium ions are disposed in tetrahedral sites at the time of charge in a lattice configured by oxygen atoms of a first MO layer and a second MO layer adjacent to each other.

4. The positive active material of claim 1, wherein the lithium dumbbell structure is a structure in which lithium ions are disposed in tetrahedral sites between a first MO layer and a second MO layer, and the dopants are disposed in tetrahedral sites between the second MO layer and a third MO layer at the time of charge in a lattice configured by oxygen atoms of the first MO layer, the second MO, and the third MO layer, so the lithium ions and the dopants are symmetrically disposed in a center of the second MO layer.

5. The positive active material of claim 1, wherein the positive active material comprises the lithium trap and the lithium dumbbell structure.

6. The positive active material of claim 1, wherein the positive active material comprises the lithium trap structure.

7. The positive active material of claim 1, wherein the lithium trap or the lithium dumbbell structure suppresses a phenomenon of relatively sliding of the MO layers by a repulsive force generated among metals of the MO layer, lithium of the tetrahedral site, and the dopants which are each cations, thus suppressing a structural change.

8. The positive active material of claim 1, wherein the lithium cobalt oxide particles further comprise at least one selected from the group consisting of Ca, Al, and Sb as a dopant.

9. The positive active material of claim 1, wherein the lithium cobalt oxide particles are further coated with protective chemicals, and the protective chemicals are at least one of a metal, an oxide, a phosphate salt, and a fluoride.

10. The positive active material of claim 9, wherein the protective chemicals are included at 0.02 wt % to 0.8 wt %.

11. The positive active material of claim 9, wherein the protective chemicals have a thickness of 30 nm to 250 nm.

12. The positive active material of claim 1, wherein the lithium cobalt oxide particles have a medium particle size (D50) of 5 micrometers to 25 micrometers.

13. A method of fabricating lithium cobalt oxide particles of a positive active material of claim 1, comprising: (a) mixing a cobalt precursor, a lithium precursor, and a Mg doping precursor and then firing the same to synthesize first doping particles; (b) coating a Zr doping precursor on a surface of the Mg doping particles; and (c) heat treating the coated Mg doping particles to synthesize Zr doping particles which are lithium cobalt oxide particles, wherein an amount of the dopant doped from the Zr doping precursor is greater than or equal to an amount of the dopant doped from the Mg doping precursor.

14. The method of claim 13, wherein the Mg doping precursor and the Zr doping precursor are independently a metal, a metal oxide, or a metal salt including Mg or Zr.

15. The method of claim 13, wherein the firing of the process (a) is performed at 900° C. to 1100° C. for 8 hours to 12 hours.

16. The method of claim 13, wherein the heat treatment of the process (c) is performed at 700° C. to 900° C. for 1 hour to 6 hours.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view showing an octahedral site or a tetrahedral site where a dopant or a lithium ion is disposed.

(2) FIG. 2 is a graph showing generated energy of a lithium dumbbell structure according to Experimental Example 1 of the present invention.

(3) FIG. 3 is a graph showing generated energy of a lithium trap structure according to Experimental Example 1 of the present invention.

(4) FIG. 4 is a schematic view showing a lithium dumbbell and a lithium trap structure according to an embodiment of the present invention.

(5) FIG. 5 is a side-sectional view of a dopant or a lithium ion in an octahedral site or a tetrahedral site according to an embodiment of the present invention.

(6) FIG. 6 is a graph showing capacity retention of each battery of Examples 1 and 2 according to experimental examples of the present invention and a comparative example depending upon a cycle number.

MODE FOR INVENTION

(7) Hereinafter, exemplary embodiments of the present invention will be described referring to drawings, but this is for easier understanding of the present invention, and the scope of the present invention is not limited thereto.

Preparation Example 1

(8) 8.19 g of Co.sub.3O.sub.4 and 3.74 g of Li.sub.2CO.sub.3 were mixed and then fired in a furnace at 1000° C. for 10 hours to provide a lithium cobalt oxide.

Preparation Example 2

(9) 8.19 g of Co.sub.3O.sub.4, 3.74 g of Li.sub.2CO.sub.3, and 1000 ppm of Al (Al source: Al.sub.2O.sub.3) were mixed and then fired in a furnace at 1000° C. for 10 hours to provide a lithium cobalt oxide in which Al of the lithium cobalt oxide was doped.

Preparation Example 3

(10) A lithium cobalt oxide was obtained in accordance with the same procedure as in Preparation Example 2, except that Mg (Mg source: MgO) was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 4

(11) A lithium cobalt oxide was obtained in accordance with the same procedure as in Preparation Example 2, except that Ti (Ti source: TiO.sub.2) was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 5

(12) A lithium cobalt oxide was obtained in accordance with the same procedure as in Preparation Example 2, except that Zr (Zr source: ZrO.sub.2) was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 6

(13) A lithium cobalt oxide was obtained in accordance with the same procedure as in Preparation Example 2, except that Nb (Nb source: Nb.sub.2O.sub.5) was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 7

(14) A lithium cobalt oxide was obtained in accordance with the same procedure as in Preparation Example 2, except that Ta (Ta source: Ta.sub.2O.sub.5) was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 8

(15) A lithium cobalt oxide was obtained in accordance with the same procedure as in Preparation Example 2, except that Mo (Mo source: MoO.sub.3) was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 9

(16) A lithium cobalt oxide was obtained in accordance with the same procedure as in Preparation Example 2, except that W (W source: WO.sub.3) was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 10

(17) A lithium cobalt oxide was obtained in accordance with the same procedure as in Preparation Example 2, except that V (V source: V.sub.2O.sub.5) was used as a dopant instead of the Al in Preparation Example 2.

Preparation Example 11

(18) A lithium cobalt oxide was obtained in accordance with the same procedure as in Preparation Example 2, except that Mn (Mn source: MnO.sub.2) was used as a dopant instead of the Al in Preparation Example 2.

Experimental Example 1

(19) The lithium cobalt oxides obtained from Preparation Examples 1 to 11 were used as a positive active material, PVdF was used as a binder, and natural graphite was used as a conductive material. The positive active material:the binder:the conductive material were mixed at a weight ratio of 96:2:2 into NMP and then coated on an Al foil having a thickness of 20 μm and dried at 130° C. to provide a positive electrode. Using a lithium foil as a negative electrode and an electrolyte solution in which 1M of LiPF.sub.6 was dissolved in a solvent of EC:DMC:DEC=1:2:1, a half coin cell was fabricated.

(20) While the obtained coin cells were charged at 1.0 C to 4.48 V, generated energy producing a lithium trap structure or dumbbell structure was measured and is shown in FIGS. 2 and 3, and while the obtained coin cells were charged at 1.0 C to 4.60 V, generated energy producing a lithium trap structure or dumbbell structure was measured and is shown in FIGS. 2 and 3.

(21) Referring to FIG. 2, it is understood that when the lithium cobalt oxide particle includes Mg, Zr, Nb, or V as a dopant, the generated energy is a negative value, so the lithium dumbbell structure is spontaneously formed under a high voltage of 4.6 V.

(22) In addition, referring to FIG. 3, when the lithium cobalt oxide particle includes Mg, Ti, Zr, Nb, Mo, or V as a dopant, the generated energy is a negative value, so it is understood that the lithium trap structure is spontaneously formed under a high voltage of 4.6 V.

(23) Herein, referring to FIGS. 2 and 3, it is more preferable that Mg, Zr, Nb, or V among dopants have a negative value of the generated energy of the lithium trap structure and the dumbbell structure under the high voltage of 4.6 V. Particularly, Mg and Zr have the greatest negative value in the lithium dumbbell structure and the lithium trap structure, respectively, so it is understood that they are the most preferable elements for providing the structures.

(24) The structure generation energy is calculated using VASP (Vienna Ab initio Simulation Program), the DFT functional uses PBE, and the Pseudo-potential uses PAW-PBE. In addition, the cut-off energy is calculated at 500 eV.

Example 1

(25) 8.19 g of Co.sub.3O.sub.4, 3.74 g of Li.sub.2CO.sub.3, and 400 ppm of Mg (Mg source: MgO) were mixed to provide 0.04 wt % of Mg based on a total weight of the lithium cobalt oxide particles, and then primarily fired in a furnace at 1000° C. for 10 hours to provide a lithium cobalt oxide in which Mg was doped in the inner bulk of the lithium cobalt oxide at a concentration of 400 ppm. Then, in order to provide a coating layer on the obtained lithium cobalt oxide, a salt including 600 ppm of Zr, which was 15 times (0.06 wt %) the doped Mg amount, was dry-mixed with lithium cobalt oxide particles and coated, and secondarily fired in a furnace at 800° C. for 4 hours to provide a positive active material in which Zr was doped on the outer bulk at a concentration of 600 ppm.

Example 2

(26) 8.19 g of Co.sub.3O.sub.4, 3.74 g of Li.sub.2CO.sub.3, and 600 ppm of Mg (Mg source: MgO) were mixed to provide 0.06 wt % of Mg based on a total weight of the lithium cobalt oxide particles, and then primarily fired in a furnace at 1000° C. for 10 hours to provide a lithium cobalt oxide in which Mg was doped in the inner bulk of the lithium cobalt oxide at a concentration of 600 ppm. In order to provide a coating layer on the obtained lithium cobalt oxide, a salt including 400 ppm of Mg, which was 0.66 times (0.04 wt %) the doped Mg amount, was dry-mixed with lithium cobalt oxide particles and coated, and then subjected to secondary firing in a furnace at 800° C. for 4 hours to provide a positive active material in which Mg was doped in the outer bulk at a concentration of 400 ppm.

Example 3

(27) 8.19 g of Co.sub.3O.sub.4, 3.74 g of Li.sub.2CO.sub.3, and 600 ppm of Mg were mixed to provide 0.06 wt % of Mg based on a total weight of the lithium cobalt oxide particle, and subjected to primary firing in a furnace at 1000° C. for 10 hours to provide a lithium cobalt oxide in which Mg was doped in the inner bulk of the lithium cobalt oxide at a concentration of 600 ppm. In order to provide a coating layer on the obtained lithium cobalt oxide, a salt including 400 ppm of Zr, which was 0.66 times (0.04 wt %) the doped Mg amount, was dry-mixed with lithium cobalt oxide particles and coated, and then subjected to secondary firing in a furnace at 800° C. for 4 hours to provide a positive active material in which Zr was doped in the outer bulk at a concentration of 400 ppm.

Example 4

(28) 8.19 g of Co.sub.3O.sub.4, 3.74 g of Li.sub.2CO.sub.3, and 400 ppm of Mg were mixed to provide Mg at 0.04 wt % based on a total weight of the lithium cobalt oxide particles, and then subjected to primary firing in a furnace at 1000° C. for 10 hours to provide a lithium cobalt oxide in which Mg was doped in the inner bulk of the lithium cobalt oxide at a concentration of 400 ppm. In order to provide a coating layer on the obtained lithium cobalt oxide, a salt including 600 ppm of Mg, which was 1.5 times (0.06 wt %) the doped Mg amount, was dry-mixed with lithium cobalt oxide particles and coated, and then subjected to secondary firing in a furnace at 800° C. for 4 hours to provide a positive active material in which Mg was doped in the outer bulk at a concentration of 400 ppm.

Comparative Example 1

(29) The lithium cobalt oxide obtained from Preparation Example 3 was used as a positive active material.

Experimental Example 2

(30) Each positive active material particle obtained from Examples 1 and 2 and Comparative Example 1, a binder of PVdF, and a conductive material of natural graphite were used. The positive active material:the binder:the conductive material were well mixed into NMP to provide a weight ratio of 96:2:2 and coated on an Al foil having a thickness of 20 μm and then dried at 130° C. to provide a positive electrode. As a negative electrode, a lithium foil was used, and an electrolyte solution in which 1M of LiPF.sub.6 was dissolved in a solvent of EC:DMC:DEC=1:2:1 was used to provide a half coin cell.

(31) The obtained half coin cell was subjected to 30 cycles at 45° C. with an upper limit voltage of 4.5 V, and capacity retention was measured. The results are shown in the following Table 1 and FIG. 6.

(32) TABLE-US-00001 TABLE 1 Example Example Example Example Comparative 1 2 3 4 Example 1 Capacity 97.8% 91.3% 90.5% 96.7% 78.6% retention (%)

(33) Referring to Table 1, it is confirmed that Examples 1 to 4 maintained high performance with capacity retention of greater than or equal to 90% even after 30 cycles and even under a high voltage condition of 4.5 V, compared to the case of using the undoped lithium cobalt oxide according to Comparative Example 1.

(34) This is because the dopants included in the outer bulk of the lithium cobalt oxide particle suppressed the crystal structure collapse from the external surface, and the dopants included in the inner bulk suppressed a side reaction between the electrolyte solution and Co.sup.4+ ions present on the particle surface in a state of discharging lithium ions, so as to stably maintain the crystal structure.

(35) Meanwhile, as the concentration of dopants included in the outer bulk is higher than the concentration of the dopants included in the inner bulk in the cases of Examples 1 and 4, the surface stability may be further enhanced under the charge condition of greater than or equal to 4.5 V, compared to the cases of Examples 2 and 3, so it is confirmed that it has much better capacity retention of greater than or equal to 95% after the 30 cycles.

(36) In addition, compared to Examples 1 with 4, it is confirmed that the case of doping the same elements in both the inner bulk and the outer bulk may have much better effects than the case of doping different elements.

(37) Although description has been given with reference to an exemplary embodiment of the present invention, a person skilled in the art pertaining to the present invention may variously apply and modify the same within a scope of the present invention on the basis of the description.

(38) As described above, the positive active material particle according to the present invention includes a predetermined element as a dopant, and has a crystalline structure in which metal oxide layers (MO layers) including metals and oxygen and reversible lithium layers are repeatedly stacked, and the dopant and/or lithium ions move from octahedral sites to tetrahedral sites at the time of charge, thereby forming a lithium trap and/or a lithium dumbbell structure, thereby providing effects of suppressing the structural change on the particle surface to improve the cycle-life characteristics at a high temperature and also of stably maintaining the crystalline structure even when emitting a large amount of lithium ions.