Positive electrode active material for lithium secondary battery including lithium cobalt oxide having core-shell structure, method for producing the same, and positive electrode and secondary battery including the positive electrode active material

Abstract

The present invention relates to a positive electrode active material for a lithium secondary battery including a lithium cobalt oxide having a core-shell structure, wherein the lithium cobalt-doped oxide of the core and the lithium cobalt-doped oxide of the shell include each independently three kinds of dopants and satisfy specific conditions, a method for producing the same, and a positive electrode and a secondary battery containing the positive electrode active material.

Claims

1. A positive electrode active material for a lithium secondary battery comprising a lithium cobalt-doped oxide having a core-shell structure, wherein the lithium cobalt-doped oxide of a core has a composition of the following Chemical Formula (1) and the lithium cobalt-doped oxide of a shell has a composition of the following Chemical Formula (2):
Li.sub.aCo.sub.1-x-y-zM1.sub.xM2.sub.yM3.sub.zO.sub.2  (1) wherein in Chemical Formula 1, M1, M2 and M3 are each independently one element selected from the group consisting of Ti, Mg, Al, Zr, Ba, Ca, Ta, Nb, Mo, Zn, Si, and V; 0.95≤a≤1.05; 0<x≤0.04, 0<y≤0.04, and 0<z≤0.04
Li.sub.bCo.sub.1-s-t-wM1′.sub.sM2′.sub.tM3′.sub.wO.sub.2  (2) wherein in Chemical Formula 2, M1′, M2′ and M3′ are each independently one element selected from the group consisting of Ti, Mg, Al, Zr, Ba, Ca, Ta, Nb, Mo, Zn, Si, and V; 0.95≤b≤1.05; 0<s≤0.04, 0<t≤0.04, and 0<w≤0.04, wherein the lithium cobalt-doped oxide of the core and the lithium cobalt-doped oxide of the shell include each independently three kinds of dopants and satisfy the following (a) or (b): (a) a ratio between the average oxidation number of the dopants present in the core and the average oxidation number of the dopants present in the shell satisfies the following condition (1);
0.7≤t(molar ratio)=OC/OS<0.95  (1) wherein, OC is the average oxidation number of the dopants present in the core, and OS is the average oxidation number of the dopants present in the shell, or (b) the dopants of the core are a metal (M1) having an oxidation number of +2, a metal (M2) having an oxidation number of 1-3 and a metal (M3) having a oxidation number of +4, and a content of M1 M2 and M3 satisfy the following condition (2) based on the molar ratio; the dopants of the shell are a metal(M1′) having an oxidation number of +2, a metal(M2′) having an oxidation number of +3 and a metal(M3′) having a oxidation number of +4, and a content of M1′, M2′ and M3′ satisfy the following condition (3) based on the molar ratio:
2≤r(molar ratio)=CM1/(CM2+CM3)≤3  (2)
0.5≤r′(molar ratio)=CM1′(CM2′+CM3′)<2  (3) wherein CM1 is the content of M1, CM2 is the content of M2, CM3 is the content of M3, CM1′ is the content of M1′, CM2′ is the content of M2′, and CM3′ is the content of M3′.

2. The positive electrode active material according to claim 1, wherein in (a), t (molar ratio) satisfies the condition of 0.8≤t<0.95.

3. The positive electrode active material according to claim 1, wherein in (b), r(molar ratio) satisfies the condition of 2≤r≤2.5, and r′ (molar ratio) satisfies the condition of 0.5≤r′≤1.5.

4. The positive electrode active material according to claim 1, wherein the lithium cobalt-doped oxide having a core-shell structure maintains the crystal structure without phase change in a range where the positive electrode potential during full charge is higher than 4.5 V on the basis of a Li potential.

5. The positive electrode active material according to claim 1, wherein the M1 and M1′ are each independently one element selected from the group consisting of Mg, Ca, and Ba; the M2 and M2′ are each independently one element selected from the group consisting of Ti, Al, Ta and Nb; and the M3 and M3′ are each independently selected from the group consisting of Ta, Nb, and Mo and are elements different from M2 and M2′.

6. The positive electrode active material according to claim 1, wherein the thickness of the shell is 50 to 2000 nm.

7. The positive electrode active material according to claim 1, wherein Al.sub.2O.sub.3 having a thickness of 50 nm to 100 nm is coated onto the surface of the shell.

8. A method for producing a lithium cobalt-doped oxide having a core-shell structure of the positive electrode active material according to claim 1, the method comprising: (i) preparing a doped cobalt precursor containing three kinds of dopants by co-precipitation; (ii) mixing the doped cobalt precursor and a lithium precursor, and subjecting them to a primary calcination to prepare core particles; and (iii) mixing the core particles, the cobalt precursor, the lithium precursor, and the three kinds of dopant precursors, and subjecting them to a secondary calcination to form a shell on the core particle surface, thereby preparing a lithium cobalt-doped oxide having a core-shell structure.

9. The method according to claim 8, wherein in the preparing a doped cobalt precursor, dopant element-containing salts and cobalt salts are dissolved in water to obtain a solution, and then the solution is converted to a basic atmosphere and subjected to a co-precipitation to prepare a doped cobalt oxide as the doped cobalt precursor.

10. A method for producing a lithium cobalt-doped oxide having a core-shell structure of the positive electrode active material according to claim 1, the method comprising: (i) mixing a cobalt precursor, a lithium precursor, and three kinds of dopant precursors and subjecting them to a primary calcination to prepare core particles; and (ii) mixing the core particles, the cobalt precursor, the lithium precursor, and the three kinds of dopant precursors independently of the primary calcination, and subjecting them to a secondary calcination to form a shell on core particle surface, thereby preparing a lithium cobalt-doped oxide having a core-shell structure.

11. The method according to claim 8, wherein the dopants of the three kinds of dopant precursors have different oxidation numbers.

12. The method according to claim 8, wherein the lithium cobalt-doped oxide having a core-shell structure satisfies the following conditions (a) and (b): (a) the ratio between the average oxidation number of the dopants present in the core and the average oxidation number of the dopants present in the shell satisfies the following condition (1);
0.7≤t(molar ratio)=OC/OS<0.95  (1) wherein, OC is the average oxidation number of the dopants present in the core, and OS is the average oxidation number of the dopants present in the shell, or (b) the dopants of the core are a metal (M1) having an oxidation number of +2, a metal (M2) having an oxidation number of +3 and a metal (M3) having a oxidation number of +4, the contents of M1, M2, and M3 satisfy the following condition (2) based on the molar ratio; the dopants of the shell are a metal(M1′) having an oxidation number of +2, a metal(M2′) having an oxidation number of +3 and a metal(M3′) having a oxidation number of +4, and the contents of M1′, M2′ and M3′ satisfy the following condition (3) based on the molar ratio:
2≤r(molar ratio)=CM1/(CM2+CM3)≤3  (2)
0.5≤r′(molar ratio)=CM1′/(CM2′+CM3′)<2  (3) wherein CM1 is the content of M1, CM2 is the content of M2, CM3 is the content of M3, CM1′ is the content of M1′, CM2′ is the content of M2′, and CM3′ is the content of M3′.

13. The method according to claim 8, wherein the primary calcination is performed at a temperature of 850° C. to 1100° C. for 8 to 12 hours, and the secondary calcination is performed at a temperature of 700° C. to 1100° C. for 5 to 12 hours.

14. A positive electrode in which a positive electrode mixture containing the positive electrode active material according to any one of claim 1, a conductive material, and a binder is applied to a current collector.

15. A secondary battery comprising the positive electrode according to claim 14.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIGS. 1a and 1b are a graph showing the capacity retention when charged at 25° C. at an upper limit voltage of 4.55 V according to Experimental Example 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(2) Hereinafter, the present invention is described with reference to Examples, but these examples are provided for better understanding of the invention, and are not intended to limit the scope of the present invention thereto.

Preparation of Core

Preparation Example 1

(3) Co.sub.3O.sub.4 and Li.sub.2CO.sub.3 were dry-mixed so as to have a composition of 4 mol of MgO, 1 mol of Al.sub.2O.sub.3, and 1 mol of TiO.sub.2, and then calcined at 1,050° C. for 10 hours in a furnace to obtain a lithium cobalt-doped oxide Li.sub.1.02Co.sub.0.94Mg.sub.0.04Al.sub.0.01Ti.sub.0.01O.sub.2 doped with Mg, Al and Ti.

Preparation Example 2

(4) Co.sub.3O.sub.4 and Li.sub.2CO.sub.3 were dry-mixed so as to have a composition of 0.6 mol of MgO, 4 mol of Al.sub.2O.sub.3, and 1 mol of TiO.sub.2, and then calcined at 1,050° C. for 10 hours in a furnace to obtain a lithium cobalt-doped oxide Li.sub.1.02Co.sub.0.944Mg.sub.0.006Al.sub.0.04Ti.sub.0.01O.sub.2 doped with Mg, Al and Ti.

Preparation Example 3

(5) A precursor particle of (Co.sub.0.94Mg.sub.0.04Al.sub.0.01Ti.sub.0.01)(OH).sub.2 was obtained by dispersing in a mixed aqueous solution in which Co.sub.3(SO.sub.4).sub.4, sodium sulfate (MgSO.sub.4), aluminum sulfate (Al.sub.2(SO.sub.4).sub.3) and titanium sulfate (Ti(SO.sub.4).sub.2) were mixed at a ratio of Co Mg:Al:Ti=0.94:0.04:0.01:0.01, and co-precipitating the result using sodium hydroxide.

(6) 41 g of LiOH.H.sub.2O was added to 100 g of the precursor so that the molar ratio of the total elements in the particle became the molar ratio of Li:M (Co, Mg, Al, Ti)=1.02:1, and then mixed with a zirconia ball using a ball mill. The mixture was primarily calcined at 1010° C. for 12 hours in an air atmosphere to prepare a lithium cobalt-doped oxide Li.sub.1.02Co.sub.0.94Mg.sub.0.04Al.sub.0.01Ti.sub.0.01O.sub.2 doped with Mg, Al and Ti.

Preparation Example 4

(7) 3 mol of MgO, 0.4 mol of Al.sub.2O.sub.3, 0.2 mol of TiO.sub.2, Co.sub.3O.sub.4, and Li.sub.2CO.sub.3 were dry-mixed, and then calcined at 1,050° C. for 10 hours in a furnace to obtain a lithium cobalt-doped oxide Li.sub.1.02Co.sub.0.964Mg.sub.0.03Al.sub.0.004Ti.sub.0.002O.sub.2 doped with Mg, Al and Ti.

Preparation Example 5

(8) 3 mol of MgO, 0.5 mol of Al.sub.2O.sub.3, 0.5 mol of TiO.sub.2, Co.sub.3O.sub.4 and Li.sub.2CO.sub.3 were dry-mixed, and then calcined at 1,050° C. for 10 hours in a furnace to obtain a lithium cobalt-doped oxide Li.sub.1.02Co.sub.0.96Mg.sub.0.03Al.sub.0.005Ti.sub.0.005O.sub.2 doped with Mg, Al and Ti.

Preparation Example 6

(9) 0.5 mol of MgO, 1 mol of Al.sub.2O.sub.3, 0.5 mol of TiO.sub.2, Co.sub.3O.sub.4 and Li.sub.2CO.sub.3 were dry-mixed, and then calcined at 1,050° C. for 10 hours in a furnace to obtain a lithium cobalt-doped oxide Li.sub.1.02Co.sub.0.98Mg.sub.0.005Al.sub.0.01Ti.sub.0.05O.sub.2 doped with Mg, Al and Ti.

Preparation Example 7

(10) A precursor particle of (Co.sub.0.96Mg.sub.0.03Al.sub.0.005Ti.sub.0.005) (OH).sub.2 was obtained by dispersing in a mixed aqueous solution in which Co.sub.3(SO.sub.4).sub.4, sodium sulfate (MgSO.sub.4), aluminum sulfate (Al.sub.2(SO.sub.4).sub.3) and titanium sulfate (Ti(SO.sub.4).sub.2) were mixed at a ratio of Co:Mg:Al:Ti=0.96:0.03:0.005:0.005, and co-precipitating the result using sodium hydroxide.

(11) 41 g of LiOH.H.sub.2O was added to 100 g of the precursor so that the molar ratio of the total elements in the particle became the molar ratio of Li:M (Co, Mg, Al, Ti)=1.02:1, and then mixed with a zirconia ball using a ball mill. The mixture was primarily calcined at 1010° C. for 12 hours in an air atmosphere to prepare a lithium cobalt-doped oxide Li.sub.1.02Co.sub.0.96Mg.sub.0.03Al.sub.0.005Ti.sub.0.005O.sub.2 doped with Mg, Al and Ti.

Preparation Example 8

(12) 1.3 mol of MgO, 0.1 mol of Al.sub.2O.sub.3, 0.2 mol of TiO.sub.2, Co.sub.3O.sub.4 and Li.sub.2CO.sub.3 were dry-mixed, and then calcined at 1,050° C. for 10 hours in a furnace to obtain a lithium cobalt-doped oxide Li.sub.1.02Co.sub.0.984Mg.sub.0.013Al.sub.0.001Ti.sub.0.002O.sub.2 doped with Mg, Al and Ti.

Example 1

(13) 200 g of the lithium cobalt-doped oxide prepared in Preparation Example 1, 0.6 mol of MgO, 1 mol of Al.sub.2O.sub.3, 1 mol of TiO.sub.2, Co.sub.3O.sub.4 and Li.sub.2CO.sub.3 were dry-mixed and then calcined at 950° C. for 10 hours in a furnace to prepare a positive electrode active material having a core-shell structure in which a lithium cobalt-doped oxide Li.sub.1.02Co.sub.0.944Mg.sub.0.006Al.sub.0.04Ti.sub.0.01O.sub.2 doped with Mg, Al and Ti was formed in the core of Li.sub.1.02Co.sub.0.94Mg.sub.0.04Al.sub.0.01Ti.sub.0.01O.sub.2.

Example 2

(14) Al.sub.2O.sub.3 having an average particle size of 50 nm was further added to the lithium cobalt-doped oxide prepared in Example 1 in an amount of 0.05% by weight based on the total mass of the positive electrode active material, and then secondarily calcinated at 570° C. for 6 hours to form a coating layer of aluminum (500 ppm). At this time, the aluminum coating layer was formed to have an average thickness of approximately 50 nm.

Example 3

(15) 200 g of the lithium cobalt-doped oxide prepared in Preparation Example 5, 3 mol of MgO, 0.5 mol of Al.sub.2O.sub.3, 0.5 mol of TiO.sub.2, Co.sub.3O.sub.4 and Li.sub.2CO.sub.3 were dry-mixed and then calcined at 950° C. for 10 hours in a furnace to prepare a positive electrode active material having a core-shell structure in which a lithium cobalt-doped oxide Li.sub.1.02Co.sub.0.977Mg.sub.0.008Al.sub.0.01Ti.sub.0.005O.sub.2 doped with Mg, Al and Ti was formed in the core of Li.sub.1.02Co.sub.0.96Mg.sub.0.03Al.sub.0.005Ti.sub.0.005O.sub.2.

Example 4

(16) Al.sub.2O.sub.3 having an average particle size of 50 nm was further added to the lithium cobalt-doped oxide prepared in Example 3 in an amount of 0.05% by weight based on the total mass of the positive electrode active material, and then secondarily calcinated at 570° C. for 6 hours to form a coating layer of aluminum (500 ppm). At this time, the aluminum coating layer was formed to have an average thickness of approximately 50 nm.

Example 5

(17) 200 g of the lithium cobalt-doped oxide prepared in Preparation Example 1, 0.4 mol of MgO, 1 mol of Al.sub.2O.sub.3, 2 mol of TiO.sub.2, Co.sub.3O.sub.4 and Li.sub.2CO.sub.3 were dry-mixed and then calcined at 950° C. for 10 hours in a furnace to prepare a positive electrode active material having a core-shell structure in which a lithium cobalt-doped oxide Li.sub.1.02Co.sub.0.944Mg.sub.0.004Al.sub.0.01Ti.sub.0.02O.sub.2 doped with Mg, Al and Ti was formed in the core of Li.sub.1.02Co.sub.0.94Mg.sub.0.04Al.sub.0.01Ti.sub.0.01O.sub.2.

Comparative Example 1

(18) 200 g of the lithium cobalt-doped oxide prepared in Preparation Example 2, 0.076 g of MgO, 0.267 g of Al.sub.2O.sub.3, 0.43 g of TiO.sub.2, 50 g of Co.sub.3O.sub.4 and 20.475 g of Li.sub.2CO.sub.3 were dry-mixed and then calcined at 950° C. for 10 hours in a furnace to prepare a positive electrode active material having a core-shell structure in which a lithium cobalt-doped oxide Li.sub.1.02Co.sub.0.957Mg.sub.0.013Al.sub.0.02Al.sub.0.02Ti.sub.0.01O.sub.2 doped with Mg, Al and Ti was formed in the core of Li.sub.1.02Co.sub.0.944Mg.sub.0.006Al.sub.0.04Ti.sub.0.01O.sub.2.

Comparative Example 2

(19) 200 g of the lithium cobalt-doped oxide prepared in Preparation Example 4, 0.07 g of MgO, 0.53 g of Al.sub.2O.sub.3, 1.73 g of TiO.sub.2, 50 g of Co.sub.3O.sub.4 and 20.475 g of Li.sub.2CO.sub.3 were dry-mixed and then calcined at 950° C. for 10 hours in a furnace to prepare a positive electrode active material having a core-shell structure in which a lithium cobalt-doped oxide Li.sub.1.02Co.sub.0.908Mg.sub.0.012Al.sub.0.04Ti.sub.0.04O.sub.2 doped with Al and Ti was formed in the core of Li.sub.1.02Co.sub.0.964Mg.sub.0.03Al.sub.0.004Ti.sub.0.002O.sub.2.

Comparative Example 3

(20) 200 g of the lithium cobalt-doped oxide prepared in Preparation Example 6, 0.48 g of MgO, 0.13 g of Al.sub.2O.sub.3, 0.216 g of TiO.sub.2, 50 g of Co.sub.3O.sub.4 and 20.475 g of Li.sub.2CO.sub.3 were dry-mixed and then calcined at 950° C. for 10 hours in a furnace to prepare a positive electrode active material having a core-shell structure in which a lithium cobalt-doped oxide Li.sub.1.02Co.sub.0.97Mg.sub.0.02Al.sub.0.005Ti.sub.0.005O.sub.2 doped with Mg, Al and Ti was formed in the core of Li.sub.1.02Co.sub.0.98Mg.sub.0.005Al.sub.0.01Ti.sub.0.005O.sub.2.

(21) Table 1 below shows the average oxidation number (up to the first decimal point) of the doping elements of Examples 1 to 5 and Comparative Examples 1 to 3 and the ratio thereof.

(22) TABLE-US-00001 TABLE 1 OC OS t Example 1 2.5 3.1 0.81 Example 2 2.5 3.1 0.81 Example 3 2.4 2.9 0.83 Example 4 2.4 2.9 0.83 Example 5 2.5 3.5 0.72 Comparative Example 1 3.1 2.9 1.07 Comparative Example 2 2.2 3.3 0.67 Comparative Example 3 3 2.5 1.2

(23) Tables 2 and 3 below show the content of the doping elements of Examples 1 to 5 and Comparative Examples 1 to 3 and the content ratio thereof.

(24) TABLE-US-00002 TABLE 2 CM1 CM2 CM3 r Example 1 4 1 1 2 Example 2 4 1 1 2 Example 3 3 0.5 0.5 3 Example 4 3 0.5 0.5 3 Example 5 4 1 1 2 Comparative Example 1 0.6 4 1 0.12 Comparative Example 2 3 0.4 0.2 5 Comparative Example 3 0.5 1 0.5 0.33

(25) TABLE-US-00003 TABLE 3 CM1′ CM2′ CM3′ r′ Example 1 0.6 4 1 0.12 Example 2 0.6 4 1 0.12 Example 3 0.8 1 0.5 0.53 Example 4 0.8 1 0.5 0.53 Example 5 0.4 1 1 0.2 Comparative Example 1 1.3 2 1 2.3 Comparative Example 2 1.2 4 4 0.15 Comparative Example 3 2 0.5 0.2 2.85

Experimental Example 1

(26) The oxide particles prepared in Examples 1 and 3 and Comparative Examples 1 to 3 were used as a positive electrode active material, PVdF as a binder, and a natural graphite as a conductive material. The positive electrode active material: binder: conductive material were thoroughly mixed with NMP so that the weight ratio became 96:2:2, then applied to an Al foil having a thickness of 20 μm, and then dried at 130° C. to prepare a positive electrode. Lithium foil was used as a negative electrode, and an electrolyte containing IM of LiPF.sub.6 in a solvent of EC:DMC:DEC=1:2:1 was used to prepare half-coin cells.

(27) The half coin cells thus prepared were charged at 25° C. with 0.5 C at an upper limit voltage of 4.55 V and again discharged with 1.0 C at a lower limit voltage of 3 V, the procedures of which were set as one cycle. The charge/discharge were repeated 50 times. The capacity retention at the fifth cycle was measured and the results are shown in FIG. 1.

(28) Referring to FIG. 1, the battery using the positive electrode active materials of Examples according to the present invention showed a capacity retention of 90% or more, whereas the battery using the positive electrode active material of Comparative Examples not satisfying either condition showed the capacity retention of about 85% or less, which was not good in lifetime characteristic. Thus, it can be seen that Examples satisfying the conditions of the present invention had higher high-voltage and high-temperature lifetime characteristics, from which can be expected that as the cycle progresses, the difference will be further accelerated.

(29) While the present invention has been described with reference to exemplary embodiments, it will be apparent to those having ordinary knowledge in the relevant field that various applications and modifications can be made within the scope of the present invention based on the above contents.

INDUSTRIAL APPLICABILITY

(30) As set forth above, in the positive electrode active material according to the present invention, three kinds of dopants are doped each independently onto the lithium cobalt doped oxide of the core and the lithium cobalt-doped oxide of the shell, and the average oxidation number ratio of the doped dopants satisfies the condition (1) of claim 1. Thereby, the structural stability of the crystal structure is improved even in the operation voltage range exceeding 4.5 V and the crystal structure is maintained. In addition, the structural stability is maintained even at a high temperature, and the lifetime characteristics are improved.