Method of preparing positive electrode active material for lithium secondary battery and positive electrode active material for lithium secondary battery prepared thereby

Abstract

The present invention relates to a method of preparing a positive electrode active material for a lithium secondary battery and the positive electrode active material for the lithium secondary battery prepared thereby, and more specifically, to a method of preparing a positive electrode active material for a lithium secondary battery, the method comprising doping or coating the positive electrode active material for the lithium secondary battery with a predetermined metal oxide, and the positive electrode active material for the lithium secondary battery which is prepared thereby and has a reduced amount of residual lithium.

Claims

1. A method of producing a lithium nickel composite oxide represented by following chemical formula 2, the method comprising the steps of: i) producing a nickel composite hydroxide represented by following chemical formula 1;
Ni.sub.1−x−yM1.sub.xM2.sub.y(OH).sub.2;   [Chemical Formula 1] wherein M1 is one or more elements selected from the group consisting of Co and Mn, M2 is one or more elements selected from the group consisting of Al, Mn, Mg, Si, P, and Ga, and 0<x≤0.03, and 0<y≤0.03; ii) washing the compound obtained in the step i) by adding the compound to a washing solution; iii) drying the compound washed in the step ii); iv) mixing the compound dried in the step iii) with LiOH, Al.sub.2O.sub.3, Mg(OH).sub.2, and CeO.sub.2; and v) heating the compound mixed in the step iv); whereby a lithium nickel composite oxide represented by [Chemical Formula 2] Li.sub.1+aNi.sub.1-x-yM1.sub.xM2.sub.yM3.sub.zO.sub.2 is produced; wherein in Chemical Formula 2, M1 is one or more elements selected from the group consisting of Co and Mn, M2 is one or more elements selected from the group consisting of Al, Mn, Mg, Si, P, and Ga; M3 represents individually Al, Mg, and Ce such that Al, Mg, and Ce are all present in the composite oxide; and wherein 0≤a≤0.3, 0<x≤0.03, 0<y≤0.03, and 0<z≤0.03.

2. The method of claim 1, wherein the washing solution of the step ii) includes one or more selected from the group consisting of distilled water, methanol, ethanol, 2-propanol, 1-butanol, ethylene glycol, polyvinyl alcohol (PVA), acetone, acetyl acetone, benzophenone, NaOH, NH.sub.4OH, LiOH, KOH, Mg(OH).sub.2, and Ba(OH).sub.2.

3. The method of claim 1, wherein the step iii) of drying the compound washed in the step ii) includes drying the washed compound at 50 to 300° C. in a depressurized atmosphere.

4. The method of claim 1, wherein the CeO.sub.2 has a particle diameter of 5 μm or smaller than 5 μm in the step iv).

5. The method of claim 1, in the step iv), wherein 0.001 to 10 parts by weight of all the Al.sub.2O.sub.3, Mg(OH).sub.2, and CeO.sub.2 is mixed with 100 parts by weight of the compound dried in the step iii).

6. The method of claim 1, further comprising a step (vi) of washing the compound heated in the step v) by adding to a washing solution.

7. The method of claim 6, wherein the washing solution in the step vi) includes one or more selected from the group consisting of distilled water, methanol, ethanol, 2-propanol, 1-butanol, ethylene glycol, polyvinyl alcohol (PVA), acetone, acetyl acetone, benzophenone, NaOH, NH.sub.4OH, LiOH, KOH, Mg(OH).sub.2, and Ba(OH).sub.2.

8. A lithium nickel composite oxide produced by claim 1.

9. The method of claim 1, further comprising: a step of mixing the lithium nickel composite oxide represented by the chemical formula 2 with a surface coating metal oxide including M4, wherein M4 is one or more elements selected from the group consisting of Al, Ba, Mg, Ce, Cr, Li, Mo, Sr, Ti, and Zr; and a step of heating the mixed lithium nickel composite oxide represented by the Chemical Formula 2 and surface coating metal oxide.

10. The method of claim 9, wherein the surface coating metal oxide including M4 has a particle diameter of 5 μm or less.

11. The method of claim 9, wherein the surface coating metal oxide including M4 is CeO.sub.2.

12. A lithium nickel composite oxide produced by the method of claim 9.

13. The lithium nickel composite oxide of claim 12, wherein the lithium nickel composite oxide has peaks which are detected within 20 ranges of 28° to 29°, 45° to 50° and 55° to 60° in an X-ray diffraction (XRD) measurement.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1(a) to 1(e) and FIGS. 2(a) to 2(b) show results of measuring scanning electron microscope (SEM) photographs of active materials prepared in Examples and Comparative Examples of the present invention.

(2) FIG. 3 and FIG. 4 show results of measuring X-ray diffractions (XRD) of active materials prepared in Examples and Comparative Example of the present invention.

(3) FIG. 5 shows results of measuring charge and discharge characteristics of batteries including active materials prepared in Examples and Comparative Examples of the present invention.

(4) FIG. 6 shows results of measuring C-rates of batteries including active materials prepared in Examples and Comparative Examples of the present invention.

(5) FIG. 7 shows results of measuring lifetime characteristics of batteries including active materials prepared in Examples and Comparative Examples of the present invention.

(6) FIGS. 8(a) to 8(b) show results of measuring impedance characteristics before and after high temperature storage of batteries including active materials prepared in Examples and Comparative Examples of the present invention.

(7) FIG. 9 shows results of measuring thermal stabilities of batteries including active materials prepared in Examples and Comparative Examples of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

(8) Hereinafter, the present invention will be described more in detail by embodiments. However, the present invention is not limited to the following embodiments.

Example 1

Preparation of a Positive Electrode Active Material

(9) In order to prepare an NCA positive electrode active material, a precursor represented by NiCoAl(OH).sub.2 was produced in advance by a coprecipitation reaction.

(10) A positive electrode active material for a lithium secondary battery was prepared by adding LiOH as a lithium compound and 1.4 mole of Al.sub.2O.sub.3 and 0.125 mole of Mg(OH).sub.2 as compounds including a dopant M3 to the produced precursor, and heat-treating a mixture of the lithium compound, the compounds including the dopant M3 and the precursor.

(11) After preparing distilled water and constantly maintaining temperature of the distilled water, the prepared positive electrode active material for the lithium secondary battery was injected into the distilled water to wash the positive electrode active material for the lithium secondary battery with the distilled water while maintaining temperature of the distilled water.

(12) After mixing the washed positive electrode active material with 0.005 mole of CeO.sub.2 as a M4-containing compound for coating to obtain a mixture, the mixture was heat-treated at a second temperature.

Examples 2 to 4

(13) Positive electrode active materials of Examples 2 to 4 were prepared in the same manner as in Example 1 except that CeO.sub.2 as the M4-containing compound for coating was added at a mixing ratio as shown in the following Table 1.

(14) TABLE-US-00001 TABLE 1 M3, mol % M4, mol % Classification Al Mg Ce Ti Ce Ti Example-1 1.4 0.125 0.05 Example-2 1.4 0.125 0.1 Example-3 1.4 0.125 0.25 Example-4 1.4 0.125 0.5 Example-5 1.4 0.125 0.05 Example-6 1.4 0.125 0.05 0.05 Example-7 1.4 0.125 0.05 0.05 Comparative 1.4 0.125 Example-1 Comparative 1.4 0.125 0.1 Example-2

Example 5

(15) A precursor represented by NiCoAl(OH).sub.2 was produced by the coprecipitation reaction.

(16) A positive electrode active material for a lithium secondary battery was prepared by adding LiOH as the lithium compound and 1.4 mole of Al.sub.2O.sub.3, 0.125 mole of Mg(OH).sub.2 and 0.05 mole of CeO.sub.2 as the compounds including the dopant M3 to the produced precursor, and heat-treating a mixture of the lithium compound, the compounds including the dopant M3 and the precursor.

(17) After preparing distilled water and constantly maintaining temperature of the distilled water, the prepared positive electrode active material for the lithium secondary battery was injected into the distilled water to wash the positive electrode active material for the lithium secondary battery with the distilled water while maintaining temperature of the distilled water.

(18) Thereafter, a positive electrode active material of Example 5 was prepared by heat-treating the washed positive electrode active material at a second temperature.

Example 6

(19) A precursor represented by NiCoAl(OH).sub.2 was produced by the coprecipitation reaction.

(20) A positive electrode active material for a lithium secondary battery was prepared by adding LiOH as the lithium compound and 1.4 mole of Al.sub.2O.sub.3, 0.125 mole of Mg(OH).sub.2 and 0.05 mole of CeO.sub.2 as the compounds including the dopant M3 to the produced precursor, and heat-treating a mixture of the lithium compound, the compounds including the dopant M3 and the precursor.

(21) After preparing distilled water and constantly maintaining temperature of the distilled water, the prepared positive electrode active material for the lithium secondary battery was injected into the distilled water to wash the positive electrode active material for the lithium secondary battery with the distilled water while maintaining temperature of the distilled water.

(22) After mixing the washed positive electrode active material with 0.005 mole of TiO.sub.2 as the M4-containing compound for coating to obtain a mixture, the mixture was heat-treated at a second temperature.

Example 7

(23) A precursor represented by NiCoAl(OH).sub.2 was produced by the coprecipitation reaction.

(24) A positive electrode active material for a lithium secondary battery was prepared by adding LiOH as the lithium compound and 1.4 mole of Al.sub.2O.sub.3, 0.125 mole of Mg(OH).sub.2 and 0.05 mole of CeO.sub.2 as the compounds including the dopant M3 to the produced precursor, and heat-treating a mixture of the lithium compound, the compounds including the dopant M3 and the precursor.

(25) After preparing distilled water and constantly maintaining temperature of the distilled water, the prepared positive electrode active material for the lithium secondary battery was injected into the distilled water to wash the positive electrode active material for the lithium secondary battery with the distilled water while maintaining temperature of the distilled water.

(26) After mixing the washed positive electrode active material with 0.005 mole of CeO.sub.2 as the M4-containing compound for coating to obtain a mixture, the mixture was heat-treated at a second temperature.

Comparative Example 1

(27) A precursor represented by NiCoAl(OH).sub.2 was produced by the coprecipitation reaction. A positive electrode active material for a lithium secondary battery was prepared by adding LiOH as a lithium compound and 1.4 mole of Al.sub.2O.sub.3 and 0.125 mole of Mg(OH).sub.2 as the compounds including the dopant M3 to the produced precursor, and heat-treating a mixture of the lithium compound, the compounds including the dopant M3 and the precursor. After preparing distilled water and constantly maintaining temperature of the distilled water, the prepared positive electrode active material for the lithium secondary battery was injected into the distilled water to wash the positive electrode active material for the lithium secondary battery with the distilled water while maintaining temperature of the distilled water. Thereafter, a positive electrode active material of Comparative Example 1 was prepared by heat-treating the washed positive electrode active material at a second temperature.

Comparative Example 2

(28) A precursor represented by NiCoAl(OH).sub.2 was produced by the coprecipitation reaction. A positive electrode active material for a lithium secondary battery was prepared by adding LiOH as the lithium compound and 1.4 mole of Al.sub.2O.sub.3, 0.125 mole of Mg(OH).sub.2 and 0.1 mole of TiO.sub.2 as the compounds including the dopant M3 to the produced precursor, and heat-treating a mixture of the lithium compound, the compounds including the dopant M3 and the precursor. After preparing distilled water and constantly maintaining temperature of the distilled water, the prepared positive electrode active material for the lithium secondary battery was injected into the distilled water to wash the positive electrode active material for the lithium secondary battery with the distilled water while maintaining temperature of the distilled water. Thereafter, a positive electrode active material of Comparative Example 2 was prepared by heat-treating the washed positive electrode active material at a second temperature.

Experimental Example

Measurement of SEM Photographs

(29) SEM photographs of the positive electrode active materials prepared in Examples 1 to 5 and Comparative Examples 1 and 2 are measured and shown in FIGS. 1(a) to 1(e) and FIGS. 2(a) to 2(b).

Experimental Example

Measurement of XRD

(30) X-ray diffractions (XRD) of the positive electrode active materials prepared in Examples 1 to 5 and Comparative Examples 1 and 2 are measured and shown in FIG. 3 and FIG. 4.

(31) In case of the positive electrode active materials prepared by Examples of the present invention in FIG. 3 and FIG. 4, it can be seen that distinguishing peaks are detected in 2θ ranges of 28° to 29°, 45° to 50° and 55° to 60°, and higher intensities of peaks detected are measured when injecting CeO.sub.2 after firing than when simultaneously injecting a lithium source and CeO2 particularly in the 2θ range of 28° to 29°.

Experimental Example

Measurement of Residual Lithium

(32) After measuring residual lithium amounts of lithium-nickel composite oxides produced in Examples and Comparative Examples, results of measuring the residual lithium amounts are shown in Table 2.

(33) To measure residual lithium amounts, after immersing 1 g of the active materials in 5 g of distilled water, stirring the active materials in the distilled water for 5 minutes to obtain a mixture, and filtering the mixture to produce a filtrate, the filtrate was taken and titrated with 0.1 M HCl. The residual lithium amounts of the active materials were analyzed by measuring volumes of HCl injected until pH values of the filtrates became 4.

(34) It can be confirmed that the residual lithium amounts are greatly reduced when cerium is doped by Examples of the present invention compared to Comparative Example-1 of doping Mg only.

(35) TABLE-US-00002 TABLE 2 Dis- Residual charge Lifetime DSC After lithium, ppm capacity retention peak, storage No LiOH Li.sub.2CO.sub.3 (0.1 C) rate, % ′ C. lmp., ohm Example-1 782 1,123 215 76.6 236.8 13.9 Example-2 927 1,183 215 77.7 236.2 17.2 Example-3 1,401 1,885 215 64.4 — 26.3 Example-4 1,502 2,006 215 63.0 — 33.9 Example-5 1,444 1,494 215.6 70.9 243.5 11.4 Comparative 1,592 2,025 219 45.6 232.8 35.1 Example-1 Comparative 819 1,348 219 60.9 236.5 16.2 Example-2

Manufacturing Example

Manufacturing of Cells

(36) Slurries were prepared by mixing the positive electrode active materials for the lithium secondary batteries prepared according to Examples and Comparative Examples respectively, artificial graphite as a conductive material, and polyvinylidene fluoride (PVdF) as a binding material at a weight ratio of 85:10:5. Positive electrodes for the lithium secondary batteries were produced by uniformly applying the slurries onto an aluminum foil with a thickness of 15 μm and vacuum drying the slurries applied onto the aluminum foil at 135° C.

(37) Coin cells were manufactured in an ordinary manner using the positive electrodes and a lithium foil as a counter electrode, a porous polypropylene membrane with a thickness of 20 μm as a separator, and an electrolyte obtained by dissolving LiPF.sub.6 at a concentration of 1.15 M in a solvent in which ethylene carbonate, diethyl carbonate and ethyl methyl carbonate were mixed at a volume ratio of 3:1:6.

Experimental Example

Evaluation of Charge and Discharge Characteristics

(38) Charge and discharge characteristics of the cells manufactured in Manufacturing Example were measured, and measurement results are shown in Table 2 and FIG. 5.

Experimental Example

Measurement Results of C-Rates

(39) C-rates of coin cells including the positive electrode active materials of Examples and Comparative Examples were measured, and measurement results are shown in Table 2 and FIG. 6.

(40) It can be confirmed that the C-rates are greatly improved in Examples of the present invention of doping cerium compared to Comparative Example-1 of doping Mg only.

Experimental Example

Measurement of Lifetime Characteristics

(41) Lifetime characteristics of the coin cells including the positive electrode active materials of Examples and Comparative Examples were measured, and measurement results are shown in Table 2 and FIG. 7.

(42) It can be confirmed that the lifetime characteristics are greatly improved in Example 1 in which the heat-treated cerium oxide was introduced during coating after heat-treating a cerium oxide compared to Comparative Example-1 in which cerium was not introduced.

Experimental Example

Measurement Results of Storage Stability-Impedance

(43) After storing the cells including the active materials of Examples and Comparative Examples at a high temperature of 60° C. for 7 days, impedance variations of the cells before and after storage were measured, and measurement results are shown in Table 2 and FIGS. 8(a) and 8(b).

(44) It can be seen from Table 2 and FIGS. 8(a) to 8(b) that the cell including the positive electrode active material of Example 5 of doping cerium along with a lithium source by the present invention not only has a small impedance value measured before storage, but also has the smallest impedance increment after storage.

Experimental Example

Measurement of Thermal Stabilities

(45) In order to evaluate thermal stabilities of the cells including the active materials of Examples and Comparative Examples, differential scanning calorimetry (DSC) peak temperatures were measured, and measurement results are shown in Table 2 and FIG. 9.

(46) It can be confirmed in FIG. 9 that the cell including the positive electrode active material of Example 5 of doping a cerium oxide along with the lithium source has a higher peak temperature than the positive electrode active material of Comparative Example 1 which is not doped or coated with a cerium oxide, thereby exhibiting excellent thermal stability. It was confirmed that the positive electrode active material of Example 1 of introducing the heat-treated cerium oxide during coating after heat-treating the cerium oxide shows excellent thermal stability.

(47) A method of preparing a positive electrode active material for a lithium secondary battery of the present invention comprises making a lithium compound to react with a precursor to obtain a reaction product, washing the surface of the reaction product, additionally doping the washed reaction product with a metal oxide, and heat-treating the reaction product doped with the metal oxide such that the reaction product is doped with the metal oxide along with a lithium source, or an active material is prepared to enable the metal oxide to be coated on the surface of the active material. Therefore, the positive electrode active material for the lithium secondary battery prepared by the method of preparing the positive electrode active material for the lithium secondary battery of the present invention shows high capacity characteristics while reducing the amount of an unreacted lithium in the surface of the positive electrode active material.