CATHODE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY

Abstract

Disclosed is a cathode active material for a lithium secondary battery including a core containing lithium composite metal oxide, and a coating layer disposed on the core and including an amorphous phase, wherein the amorphous phase contains lithium oxide and boron oxide in a form of mixture.

Claims

1. A cathode active material for a lithium secondary battery comprising: a core containing lithium composite metal oxide; and a coating layer disposed on the core, the coating layer including an amorphous phase, wherein the amorphous phase contains lithium oxide and boron oxide in a form of mixture.

2. The cathode active material according to claim 1, wherein the coating layer comprises a composition of the following Formula 2:
αB.sub.xO.sub.y−βLi.sub.2O  (2) wherein the conditions of α+β=1 and 0.35≤x/y≤0.75 are satisfied.

3. The cathode active material according to claim 2, wherein the coating layer comprises a composition of the following Formula 3:
αB.sub.2O.sub.3−βLi.sub.2O  (3) wherein the condition of α+β=1 is satisfied.

4. The cathode active material according to claim 1, wherein, in X-ray diffractometry, no peak is observed in the vicinity of any one of 2θ=32.05°, 26.003°, 28.051°, 14.971°, 33.646°, and 56.407°.

5. The cathode active material according to claim 4, wherein the X-ray diffractometry is performed under the following XRD diffraction measurement conditions: Target: Cu (Kα ray) graphite monochromator Slit: divergence slit=1 degree, receiving slit=0.1 mm, scatter slit=1 degree Measuring zone and step angle/measuring time: 10.0 degrees<2θ<80 degrees, 2 degrees/1 minute (=0.1 degrees/3 seconds), where 2θ (theta) represents a diffraction angle.

6. The cathode active material according to claim 1, wherein the core has an average particle diameter of 1 to 50 μm.

7. The cathode active material according to claim 1, wherein the lithium oxide is present in an amorphous phase in an amount of 0.01 to 2 parts by weight and the boron oxide is present in an amorphous phase in an amount of 0.01 to 2 parts by weight based on 100 parts by weight of the core.

8. The cathode active material according to claim 1, wherein the amorphous phase comprises only the lithium oxide and the boron oxide in the form of mixture.

9. The cathode active material according to claim 1, wherein the coating layer has a thickness of 0.01 to 1 μm.

10. The cathode active material according to claim 1, wherein the coating layer is coated on 40 to 100% of a surface area of the core.

11. A method of preparing the cathode active material according to claim 1, the method comprising: mixing (i) a boron-containing powder, or (ii) a boron-containing powder and a lithium-containing powder, with a lithium composite metal oxide powder for a core, followed by firing in an atmosphere containing oxygen in a temperature range within which an amorphous coating layer is formed.

12. The method according to claim 11, wherein the method comprises mixing the boron-containing powder with the lithium composite metal oxide powder, followed by firing, and the lithium oxide of the amorphous coating layer is derived from a lithium-containing component present on the surface of the lithium composite metal oxide powder.

13. The method according to claim 11, wherein the temperature range is 250 to 350° C.

14. A lithium secondary battery comprising the cathode active material according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0052] FIG. 1 is an X-ray diffraction graph of cathode active materials of Examples 1, 2, and 3 and Comparative Example 1; and

[0053] FIGS. 2A and 2B illustrate the results of FE-SEM for comparative surface analysis of cathode active materials of Example 1 and Comparative Example 1.

BEST MODE

[0054] Now, the present invention will be described in more detail with reference to the following examples. These examples should not be construed as limiting the scope of the present invention.

Example 11

(Preparation of Cathode Active Material)

[0055] H.sub.3BO.sub.3 was mixed in the amount shown in Table 1 below using a dry mixer with 100 parts by weight of lithium composite metal oxide (Li(Ni.sub.0.82Co.sub.0.11Mn.sub.0.07).sub.0.994Ti.sub.0.004Zr.sub.0.002O.sub.2) which was washed with distilled water and dried in an oven, followed by heat treatment in an O.sub.2 atmosphere at 300° C. for 12 hours, to prepare a cathode active material having a coating layer having an amorphous phase containing lithium oxide and boron oxide.

[0056] It was ascertained that lithium oxide (Li.sub.2O) was produced by oxidation of the byproduct remaining on the surface of the lithium composite metal oxide and the content of the lithium compound remaining on the surface of the lithium composite metal oxide prior to the heat treatment was about 0.3 to 0.6 parts by weight when measured using acid/base neutralization titration, and about 0.1 to 0.25 parts by weight of lithium oxide (Li.sub.2O) due to oxidation by heat treatment.

(Production of Cathode)

[0057] The cathode active material prepared above, Super-P as a conductive material, and PVdF as a binder were mixed at a weight ratio of 96.5:1.5:2 in the presence of N-methylpyrrolidone as a solvent to prepare a cathode active material paste. The cathode active material paste was applied onto an aluminum current collector, dried at 120° C., and then rolled to produce a cathode.

(Production of Lithium Secondary Battery)

[0058] A porous polyethylene film as a separator was interposed between the cathode produced above and a Li metal as an anode to produce an electrode assembly, the electrode assembly was placed in a battery case, and an electrolyte was injected into the battery case to produce a lithium secondary battery. The electrolyte used herein was prepared by dissolving 1.0M lithium hexafluorophosphate (LiPF.sub.6) in an organic solvent containing vinylene carbonate (VC: 2 wt %), in addition to ethylene carbonate/dimethyl carbonate/diethyl carbonate (mixed at a volume ratio of EC/DMC/DEC=1/2/1).

Example 2

[0059] A cathode active material, a cathode, and a lithium secondary battery were produced under the same conditions as in Example 1, except that the heat treatment temperature was 250° C.

Example 3

[0060] A cathode active material, a cathode, and a lithium secondary battery were produced under the same conditions as in Example 1, except that the heat treatment temperature was 350° C.

Comparative Example 1

[0061] A cathode active material, a cathode, and a lithium secondary battery were produced under the same conditions as in Example 1, except that heat-treatment was performed without mixing with H.sub.3BO.sub.3.

Comparative Example 2

[0062] A cathode active material, a cathode, and a lithium secondary battery were produced under the same conditions as in Example 1, except that the heat treatment temperature was 400° C.

Comparative Example 3

[0063] A cathode active material, a cathode, and a lithium secondary battery were produced under the same conditions as in Example 1, except that the heat treatment temperature was 500° C.

Comparative Example 4

[0064] A cathode active material, a cathode, and a lithium secondary battery were produced under the same conditions as in Example 1, except that the heat treatment temperature was 150° C.

TABLE-US-00001 TABLE 1 Heat treatment Core composition H.sub.3BO.sub.3 temperature Ni:Co:Mn: (parts by weight) (° C.) Example 1 82:11:7 0.46 300 Example 2 82:11:7 0.46 250 Example 3 82:11:7 0.46 350 Comparative 82:11:7 — — Example 1 Comparative 82:11:7 0.46 400 Example 2 Comparative 82:11:7 0.46 500 Example 3 Comparative 82:11:7 0.46 150 Example 4

Experimental Example 1

[0065] In order to identify the presence of crystalline phases of lithium oxide and boron oxide contained in the cathode active material according to an embodiment of the present invention, XRD diffraction of the cathode active materials prepared in Examples 1 and 2 was measured using Cu (Kα ray) and the results are shown in FIG. 1.

[0066] The XRD diffraction measurement conditions are as follows. [0067] Target: Cu (Kα ray) graphite monochromator [0068] Slit: divergence slit=1 degree, receiving slit=0.1 mm, scatter slit=1 degree [0069] Measuring zone and step angle/measuring time: 10.0 degrees<2θ<80 degrees, 2 degrees/1 minute (=0.1 degrees/3 seconds), where 2θ (theta) represents the diffraction angle.

[0070] As can be seen from FIG. 1, in the XRD diffraction measurement results, the cathode active materials of Examples 1 and 2 did not show peaks at 2θ of near 33.646° and 56.407° corresponding to crystalline Li.sub.2O, peaks at 2θ of near 32.05° and 26.003° corresponding to crystalline B.sub.2O.sub.3, and peaks at 2θ of near 28.051° and 14.971° corresponding to crystalline H.sub.3BO.sub.3.

[0071] In addition, as can be seen from FIGS. 2A and 2B together, compared to the cathode active material of Comparative Example 1 (FIG. 2A), the cathode active material of Example 1 (FIG. 2B) is uniformly coated in an amorphous form without crystalline grain growth on the surface of the active material.

[0072] Accordingly, in the cathode active material according to an embodiment of the present invention, it may be determined that the coating layer is formed of Li.sub.2O and B.sub.2O.sub.3 in an amorphous phase, rather than a crystalline phase.

Experimental Example 2

[0073] Each of the lithium secondary batteries produced in Examples 1 to 4 and Comparative Examples 1 to 5 was subjected to 0.1C charge and 0.1C discharge twice at room temperature (25° C.) wherein charging was performed at 4.3V and the discharge cutoff voltage was 2.5V. The results are shown in the following Table 2.

TABLE-US-00002 TABLE 2 Formation 0.1 C/0.1 C (4.3-2.5 V, 25° C.) Item CC DC Eff (%) Example 1 228.3 206.8 90.6 Example 2 228.0 205.7 90.2 Example 3 228.0 205.1 90.0 Comparative 225.0 202.3 89.9 Example 1 Comparative 225.8 203.9 90.3 Example 2 Comparative 225.2 202.9 90.1 Example 3 Comparative 224.9 201.2 89.5 Example 4

[0074] Then, the lithium secondary batteries were repeatedly subjected to 0.5C charge and 1.0C discharge wherein charging was performed at 4.3V and at 45° C. and the discharge cutoff voltage was 3.0V for evaluation of high-temperature lifespan characteristics. The discharge capacities at the 10.sup.th, 20.sup.th and 30.sup.th cycles compared to the discharge capacity at the 1.sup.st cycle are shown in Table 3 below.

TABLE-US-00003 TABLE 3 Cycle 0.5 C/1.0 C (4.3-3.0 V, 45° C.) Cycle capacity (mAh/g) Cycle retention (%) DCIR (%) Item 1CY 10CY 20CY 30CY 10CY/1CY 20CY/1CY 30CY/1CY 30CY/1CY Example 1 218.0 216.8 215.2 212.6 99.4 98.7 97.5 37.7% Example 2 217.4 215.7 213.6 211.2 99.2 98.3 97.1 41.9% Example 3 217.0 215.1 213.2 210.9 99.1 98.2 97.2 39.7% Comparative 213.4 210.6 207.8 205.0 98.7 97.4 96.1 114.0% Example 1 Comparative 215.6 214.3 210.9 208.6 99.4 97.8 96.8 66.9% Example 2 Comparative 214.8 212.8 209.2 206.9 99.1 97.4 96.3 96.9% Example 3 Comparative 211.6 207.4 205.1 202.6 98.0 96.9 95.7 127.2% Example 4

[0075] As can be seen from Tables 2 and 3, the lithium secondary batteries of Examples 1, 2, and 3 according to the present invention have high charge capacity and high discharge efficiency and exhibit remarkably excellent cycle characteristics at high temperatures, in particular, excellent resistance characteristics based on the reduction in the increase rate of direct current internal resistance (DCIR) related to the lifespan of the secondary battery.

[0076] The reason for this is that, in active materials of Examples according to the present invention, as the heat treatment for forming the coating layer is performed at a relatively low temperature, lithium oxide and boron oxide contained in the coating layer are uniformly formed in an amorphous phase on the surface of the active material and thus prevent side reactions with the electrolyte, facilitate the movement of lithium ions and improve electrical conductivity (lithium ion conductor).

[0077] In Comparative Example 2, the firing temperature was relatively high and thus there was no great influence on the increase in the charging capacity and the high-temperature lifespan compared to Comparative Example 1, but the discharge efficiency slightly increased due to the formation of an amorphous coating layer to some extent, and the lifespan and lifespan resistance characteristics were slightly improved.

[0078] Comparative Example 3 exhibited poor properties compared to Examples of the present invention although the coating layer contains boron oxide like Examples of the present invention. The reason for this is that, as the heat treatment to form the coating layer was performed at a relatively high temperature, a crystalline coating layer was formed on the surface of the core.

[0079] Comparative Example 4 exhibited poor properties compared to Examples of the present invention although the coating layer contains boron oxide like Examples of the present invention. The reason for this is that, as the heat treatment to form the coating layer was performed at a relatively low temperature, the melting point of H.sub.3BO.sub.3 was not reached and a coating layer was not formed uniformly on the surface of the core active material.

[0080] Although preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.