Metal-doped cobalt precursor for preparing positive electrode active material for secondary battery

Abstract

Provided is a cobalt precursor for preparing a lithium cobalt oxide of a layered structure which is included in a positive electrode active material, wherein the cobalt precursor is cobalt oxyhydroxide (CoMOOH) doped with, as dopants, magnesium (Mg) and M different from the magnesium.

Claims

1. A cobalt precursor for preparing a lithium cobalt oxide of a layered structure which is included in a positive electrode active material, wherein the cobalt precursor is cobalt oxyhydroxide represented by the chemical formula (Co.sub.xMg.sub.yM.sub.z)OOH, wherein x, y, and z satisfy the following weight ratio conditions: a content of x is in the range of 99 wt % to 99.8 wt % and a total content of y and z is in the range of 0.2 wt % to 1 wt %, when the sum of x, y, and z is regarded as 100 wt %; M is one or more selected from the group consisting of Al, Ti, Mn, Zr, Ba, B, Ca, Ta, Mo, Nb, W, Sr and PO.sub.4, wherein the cobalt precursor is doped with, as dopants, magnesium (Mg) and M.

2. A method of preparing a lithium cobalt oxide using the cobalt precursor, the method comprising: (a) preparing the cobalt precursor of claim 1 which is doped with dopants provided from a doping precursor by co-precipitation reaction of a cobalt acid salt and the doping precursor; and (b) mixing the dopant-doped cobalt precursor and a lithium precursor to form a mixture; (c) heat treating the mixture.

3. The method of claim 2, wherein the doping precursor is mixed such that a content of the cobalt is in the range of 99 wt % to 99.8 wt %, and a total content of the dopants is in the range of 0.2 wt % to 1 wt %, when the sum of the cobalt and the dopants is regarded as 100 wt %.

4. The method of claim 2, wherein the heat treating is performed at 950 C. to 1100 C. for 8 hours to 15 hours.

5. The method of claim 2, wherein the cobalt acid salt is cobalt oxyhydroxide represented by the chemical formula CoOOH.

6. The method of claim 2, wherein the lithium precursor is one or more selected from the group consisting of Li.sub.2CO.sub.3, LiOH, LiNO.sub.3, CH.sub.3COOLi, and Li.sub.2(COO).sub.2.

7. The method of claim 2, wherein the doping precursor includes: one or more selected from the group consisting of a mixed metal of Mg, and one or more selected from the group consisting of Al, Ti, Mn, Zr, Ba, B, Ca, Ta, Mo, Nb, W, Sr, and P, a metal oxide thereof, and a metal salt thereof.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIGS. 1A and 1B show graphs of capacity retention ratios of Example 1 and Comparative Examples 1-4 according to the tests conducted in Experimental Example 1;

(2) FIGS. 2 and 3 show XRD graphs of peak intensities of Example 1 and Comparative Examples 1, 3 and 4 according to the tests conducted in Experimental Example 2; and

(3) FIG. 4 shows a graph of a discharge rate of Example 1 and Comparative Example 1 according to the tests conducted in Experimental Example 3.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) Hereinafter, the present invention will be described in more detail with reference to Examples. However, the scope of the present invention is not intended to be limited by these Examples.

(5) Preparation of Positive Electrode Active Material

Example 1

(6) Each of Mg(SO.sub.4), Al(SO.sub.4).sub.3, and Co(SO.sub.4) was added to an aqueous solution along with NaOH and NH.sub.4OH according to a stoichiometric ratio thereof such that Mg and Al were included in an amount of 3000 ppm and 2000 ppm, respectively, based on the total weight of Al lithium cobalt oxide, and co-precipitation was allowed to prepare a cobalt precursor (CoMgAl)OOH (a total content of dopants: 0.5 wt %) doped with Mg and Al.

(7) The cobalt precursor was mixed with Li.sub.2CO.sub.3 at a Li/M ratio of 1.02, and then sintered in a furnace at 1050 C. for 10 hours to prepare a lithium cobalt oxide Li.sub.1.02(CoMgAl)O.sub.2 (a total content of dopants: 0.5 wt %) doped with Mg and Al at the cobalt position thereof.

Example 2

(8) A cobalt precursor (CoMgAl)OOH (a total content of dopants: 0.2 wt %) doped with Mg and Al was prepared in the same manner as in Example 1, except that Mg and Al were included in an amount of 1000 ppm and 1000 ppm, respectively, based on the total weight of lithium cobalt oxide in Example 1. This cobalt precursor was used to prepare a lithium cobalt oxide.

Example 3

(9) A cobalt precursor (CoMgAl)OOH (a total content of dopants: 1 wt %) doped with Mg and Al was prepared in the same manner as in Example 1, except that Mg and Al were included in an amount of 4000 ppm and 6000 ppm, respectively, based on the total weight of lithium cobalt oxide in Example 1. This cobalt precursor was used to prepare a lithium cobalt oxide.

Comparative Example 1

(10) A lithium cobalt oxide Li.sub.1.02CoO.sub.2 was prepared using CoOOH and LiOH in the same amounts as in Example 1, except that Mg and Al were not included.

Comparative Example 2

(11) 200 g of the lithium cobalt oxide Li.sub.1.02CoO.sub.2 prepared in Comparative Example 1 was dry-mixed with 0.995 g of MgO and 1.55 g of Al.sub.2O.sub.3 such that Mg and Al were included in an amount of 3000 ppm and 2000 ppm, respectively, based on the total weight of lithium cobalt oxide. Then, the mixture was sintered in a furnace at 500 C. for 5 hours to prepare a lithium cobalt oxide coated with magnesium oxide and aluminum oxide on the surface thereof.

Comparative Example 3

(12) 0.995 g of MgO, 1.55 g of Al.sub.2O.sub.3, 200 g of CoOOH, and 79.5 g of Li.sub.2CO.sub.3 were dry-mixed with each other such that Mg and Al were included in an amount of 3000 ppm and 2000 ppm, respectively, based on the total weight of lithium cobalt oxide. Then, the mixture was sintered in a furnace at 1050 C. for 10 hours to prepare a lithium cobalt oxide Li.sub.1.02(CoMgAl)O.sub.2 (a total content of dopants: 0.5 wt %) doped with Mg and Al at the cobalt position thereof.

Comparative Example 4

(13) A lithium cobalt oxide Li.sub.1.02(CoMgAl)O.sub.2 (a total content of dopants: 0.13 wt %) was prepared in the same manner as in Example 1, except that Mg and Al were included in an amount of 500 ppm and 800 ppm, respectively, based on the total weight of lithium cobalt oxide.

(14) Manufacture of Secondary Battery

(15) Each of the positive electrode active materials prepared in Examples 1 to 3, and Comparative Examples 1 to 4, a PVdF binder, and a natural graphite conductive material were mixed well at a weight ratio of 96:2:2 (positive electrode active material:binder:conductive material) in NMP, and then applied to an Al foil having a thickness of 20 and dried at 130 C. to manufacture each positive electrode. As a negative electrode, a lithium foil was used, an electrolyte containing 1M LiPF.sub.6 in a solvent of EC:DMC:DEC=1:2:1 was used to manufacture each coin-type half cell.

Experimental Example 1

(16) Analysis of Capacity Retention Ratio

(17) From the coin-type half cells as manufactured above, the coin-type half cells including each of the positive electrode active materials of Examples 1 to 3 and Comparative Examples 1 to 4 were charged at 0.5 C to an upper voltage limit of 4.55 V at 25 C., and then discharged at 1.0 C to a lower voltage limit of 3 V. This procedure was regarded as 1 cycle, and capacity retention ratios after 50 cycles were measured. The results of measuring the capacity retention ratios of Example 1 and Comparative Examples 1 to 4 are shown in FIG. 1A, and the results of Examples 2 and 3 are shown in FIG. 1B.

(18) Referring to FIG. 1A, when Mg and Al were doped in an amount of 3000 ppm and 2000 ppmm, respectively as in Example 1, the coin-type half cell manufactured using the positive electrode active material of Example 1 showed a capacity retention ratio similar to the coin-type half cells manufactured using the positive electrode active materials of Comparative Examples 1 to 4, after less than about 50 cycles. However, over 50 cycles, Example 1 showed more excellent capacity retention ratio. Further, referring to FIG. 1B, in Examples 2 and 3, even though the doping amounts of Mg and Al were controlled within the range of the present invention, excellent capacity retention ratio equivalent to that of Example 1 was observed.

(19) Accordingly, when the contents of the dopants are controlled in the positive electrode active material according to the present invention, the average oxidation number of the dopants in the lithium cobalt oxide may be controlled within the desired range, thereby achieving improved lifespan characteristic at a high voltage of 4.5 V or more.

Experimental Example 2

(20) XRD Analysis

(21) In order to examine changes in the crystal structures of the lithium cobalt oxides of Example 1 and Comparative Examples 1, 3 and 4, coin-type half cells including the same were manufactured, and peak intensity was measured while increasing the upper limit voltage from 4.53 V to 4.55 V at 0.01 V intervals. The XRD graphs (2-theta-scale) thus measured are shown in FIGS. 2 and 3.

(22) Referring to FIGS. 2 and 3, the positive electrode active material of Example 1 showed a peak in the range of 23 to 24 degree even at 4.55 V, and a peak intensity of (003) plane at 4.55 V was 70% or more of a peak intensity of (003) plane at 4.53 V. Even at the high voltage, there was no phase transition of the lithium cobalt oxide or no collapse of the crystal structure, indicating improvement of structural stability. However, the positive electrode active materials of Comparative Examples 1, 3 and 4 showed remarkably low peak intensities of (003) plane at 4.55 V, indicating that the phase transition of the lithium cobalt oxides or collapse of the crystal structures occurred.

Experimental Example 3

(23) Discharge Rate Analysis

(24) From the coin-type half cells as manufactured above, the coin-type half cells including each of the positive electrode active materials of Example 1 and Comparative Example 1 were initially charged at 0.5 C to an upper voltage limit of 4.55 V at 25 C., and then initially discharged at 1.0 C to a lower voltage limit of 3 V. Thereafter, discharge rates were measured and the results are shown in FIG. 4.

(25) Referring to FIG. 4, when Mg and Al were doped in an amount of 3000 ppm and 2000 ppmm, respectively as in Example 1, the crystal structure of the lithium cobalt oxide was stably maintained, and the discharge rate was mostly constant, and therefore, a plateau, in which the voltage maintains constant, was not observed. However, Comparative Example 1 including no Mg and Al showed a rapid change in the discharge rate due to phase transition of the lithium cobalt oxide, and therefore, a plateau, in which the voltage maintains constant, was observed.

(26) Although the present invention has been described in connection with the exemplary embodiments of the present invention, it will be apparent to those skilled in the art to which the present invention pertains that various modifications and changes may be made thereto without departing from the scope of the invention.