CATHODE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY

Abstract

Disclosed are 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, containing a mixture of lithium oxide, tungsten oxide, boron oxide and phosphorus oxide, and having an amorphous phase, and a lithium secondary battery including the same.

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, containing a mixture of lithium oxide, tungsten oxide, boron oxide and phosphorus oxide, and having an amorphous phase.

2. The cathode active material according to claim 1, wherein the lithium composite metal oxide has a layered crystal structure including one or more transition metals.

3. The cathode active material according to claim 1, wherein the lithium composite metal oxide is a substance represented by the following Formula 1:
Li[Li.sub.xM.sub.1−x−yD.sub.y]O.sub.2−aQ.sub.a  (1) wherein M includes at least one transition metal element that is stable in a 4- or 6-coordination structure; D includes at least one element selected from an alkaline earth metal, a transition metal, and a non-metal as a dopant; Q includes at least one anion; and 0≤x≤0.1, 0≤y≤0.1, 0≤a≤0.2.

4. The cathode active material according to claim 3, wherein M includes at least two elements selected from the group consisting of Ni, Co, and Mn; D includes at least one element selected from the group consisting of Al, W, Si, V, B, Ba, Ca, Zr, Ti, Mg, Ta, Nb and Mo; and Q includes at least one element selected from F, S and P.

5. The cathode active material according to claim 1, wherein the core has an average particle diameter (D50) of 1 to 50 μm.

6. The cathode active material according to claim 1, wherein the coating layer has a composition of the following Formula 2:
αW.sub.xO.sub.y-βB.sub.mO.sub.n-γP.sub.vO.sub.w-δLi.sub.2O  (2) wherein requirements of 0<α≤2, 0<β≤2, 0<γ≤2, 0<δ≤2, and 1≤x≤2, 2≤y≤4, 1≤m≤2, 3≤n≤5, 1≤v≤3, 4≤w≤8 are satisfied.

7. The cathode active material according to claim 1, wherein, based on 100 parts by weight of the core, a content of the lithium oxide in the coating layer is 0.01 to 2 parts by weight, a content of the tungsten oxide in the coating layer is 0.1 to 2 parts by weight, a content of the boron oxide in the coating layer is 0.2 to 2 parts by weight, and a content of the phosphorus oxide in the coating layer is 0.2 to 2 parts by weight.

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

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

10. A method of preparing the cathode active material according to claim 1, the method comprising: (i) mixing a tungsten-containing powder, a boron-containing powder and a phosphorus-containing powder with a lithium composite metal oxide powder for a core, or (ii) mixing a tungsten-containing powder, a boron-containing powder, a phosphorus-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.

11. The method according to claim 10, wherein the method comprises mixing the tungsten-containing powder, the boron-containing powder, and the phosphorus-containing material with the lithium composite metal oxide powder, followed by firing, wherein 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.

12. The method according to claim 10, wherein the temperature range is higher than 150° C. and is not higher than 500° C.

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

Description

BEST MODE

[0055] 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 1

(Preparation of Cathode Active Material)

[0056] B.sub.2O.sub.3, WO.sub.3 and NH.sub.4H.sub.2PO.sub.4 were mixed in the amounts shown in Table 1 below with 100 parts by weight of lithium composite metal oxide (Li(Ni.sub.0.82Co.sub.0.11Mn.sub.0.7).sub.0.994Ti.sub.0.004Zr.sub.0.002O.sub.2) using a dry mixer, followed by heat treatment in an air atmosphere at 400° C. for 7 hours, to prepare a cathode active material having a surface treatment layer (coating layer) containing glassy oxide shown in Table 1 below.

[0057] Lithium oxide was produced by oxidation of lithium byproducts remaining on the surface of the lithium composite metal oxide, and it was ascertained that about 0.20 parts by weight to about 0.25 parts by weight of lithium oxide (Li.sub.2O) was formed by oxidation during heat treatment.

(Production of Anode)

[0058] The cathode active material prepared above, Super-P as a conducting material, and PVdF as a binder were mixed at a weight ratio of 95:2:3 in the presence of N-methylpyrrolidone as a solvent to prepare a paste for forming a cathode. The paste for forming a cathode was applied onto an aluminum current collector, dried at 130° C., and then rolled to produce a cathode.

(Production of Lithium Secondary Battery)

[0059] Porous polyethylene as a separator was interposed between the cathode prepared above and an anode as a Li metal to produce an electrode assembly, the electrode assembly was placed in a cell case, and an electrolyte was injected into the cell case to produce a lithium secondary battery. At this time, the electrolyte was prepared by dissolving 1.0M lithium hexafluorophosphate (LiPF.sub.6) in an organic solvent consisting of ethylene carbonate/dimethyl carbonate (mixed at a volume ratio of EC/DMC=1/1).

Example 2

[0060] An electrode was produced and a lithium secondary battery was produced under the same conditions as in Example 1, except that the heat-treatment temperature was 500° C.

Example 3

[0061] An electrode was produced and a lithium secondary battery was produced under the same conditions as in Example 1, except that the heat-treatment temperature was 350° C.

Example 4

[0062] An electrode was produced and a lithium secondary battery was produced under the same conditions as in Example 1, except that (Li(Ni.sub.0.35Co.sub.0.37Mn.sub.0.28).sub.0.994Ti.sub.0.004Zr.sub.0.002O.sub.2 was used as the lithium composite metal oxide.

Example 5

[0063] An electrode was produced and a lithium secondary battery was produced under the same conditions as in Example 1, except that (Li(Ni.sub.0.50Co.sub.0.20Mn.sub.0.30).sub.0.994Ti.sub.0.004Zr.sub.0.002O.sub.2 was used as the lithium composite metal oxide.

Example 6

[0064] An electrode was produced and a lithium secondary battery was produced under the same conditions as in Example 1, except that (Li(Ni.sub.0.60Co.sub.0.20Mn.sub.0.20).sub.0.994Ti.sub.0.004Zr.sub.0.002O.sub.2 was used as the lithium composite metal oxide.

Example 7

[0065] An electrode was produced and a lithium secondary battery was produced under the same conditions as in Example 1, except that (Li(Ni.sub.0.70Co.sub.0.15Mn.sub.0.15).sub.0.994Ti.sub.0.004Zr.sub.0.002O.sub.2 was used as the lithium composite metal oxide.

Comparative Example 1

[0066] An electrode was produced and a lithium secondary battery was produced under the same conditions as in Example 1, except that B.sub.2O.sub.3, WO.sub.3 and NH.sub.4H.sub.2PO.sub.4 were not mixed.

Comparative Example 2

[0067] An electrode was produced and a lithium secondary battery was produced under the same conditions as in Example 1, except that the firing temperature was 550° C.

Comparative Example 3

[0068] An electrode was produced and a lithium secondary battery was produced under the same conditions as in Example 1, except that the firing temperature was 150° C.

Comparative Example 4

[0069] An electrode was produced and a lithium secondary battery was produced under the same conditions as in Example 4, except that B.sub.2O.sub.3, WO.sub.3 and NH.sub.4H.sub.2PO.sub.4 were not mixed.

Comparative Example 5

[0070] An electrode was produced and a lithium secondary battery was produced under the same conditions as in Example 5, except that B.sub.2O.sub.3, WO.sub.3 and NH.sub.4H.sub.2PO.sub.4 were not mixed.

Comparative Example 6

[0071] An electrode was produced and a lithium secondary battery was produced under the same conditions as in Example 6, except that B.sub.2O.sub.3, WO.sub.3 and NH.sub.4H.sub.2PO.sub.4 were not mixed.

Comparative Example 7

[0072] An electrode was produced and a lithium secondary battery was produced under the same conditions as in Example 7, except that B.sub.2O.sub.3, WO.sub.3 and NH.sub.4H.sub.2PO.sub.4 were not mixed.

Experimental Example 1

[0073] For the cathode active materials prepared in Examples 1 to 7 and Comparative Examples 1 to 7, the amount of lithium byproducts remaining on the surface of the core was measured using CS analysis (model name: ELTRA CS 2000) and HCl titration, and the result is shown in Table 1 below. For reference, a detailed description of the HCl titration is given below.

[0074] The amount of lithium remaining in the prepared cathode active material is a value (TTL, total lithium) obtained by separately calculating the total amount of Li alone after measuring remaining amounts of Li-containing compounds (for example, LiOH or Li.sub.2CO.sub.3) by potential difference neutralization titration. The calculation is performed in accordance with Equation 1 below.

TTL (Total Li)=LiOH assay value (%)*Li/LiOH+Li.sub.2CO.sub.3 assay value (%)*2Li/Li.sub.2CO.sub.3=LiOH assay value (%)*0.29+Li.sub.2CO.sub.3 assay value (%)*0.188  [Equation 1]

Experimental Example 2

[0075] For the lithium secondary batteries each prepared in Examples 1 to 7 and Comparative Examples 1 to 7, 2 cycles of 0.1C charging and 0.1C discharging were performed while cutting off 3.0V upon discharging after charging 4.3 V at room temperature for electrode stabilization. Then, charging at 0.2C and discharging at 0.2C and 2.0C were performed at −25° C. in order to evaluate the low-temperature output characteristics. Based on the output measured at 0.2C at −25° C., rate retention (%) of the output measured at 2.0C was calculated and is shown in Table 1 below.

TABLE-US-00001 TABLE 1 Heat- Residual Core WO.sub.3 B.sub.2O.sub.3 NH.sub.4H.sub.2PO.sub.4 treatment lithium Rate composition (parts by (parts by (parts by temperature (TTL) (parts retention Item (Ni:Co:Mn) weight) weight) weight) (° C.) by weight) (%, −25° C.)) Ex. 1 82:11:7 0.375 0.15 0.75 400 0.216 78.8 Ex. 2 82:11:7 0.375 0.15 0.75 500 0.180 76.9 Ex. 3 82:11:7 0.375 0.15 0.75 350 0.109 76.2 Ex. 4 35:37:28 0.375 0.15 0.75 400 0.007 77.8 Ex. 5 50:20:30 0.375 0.15 0.75 400 0.038 76.9 Ex. 6 60:20:20 0.375 0.15 0.75 400 0.062 76.8 Ex. 7 70:15:15 0.375 0.15 0.75 400 0.082 76.1 Comp. 82:11:7 — — — — 0.330 64.0 Ex. 1 Comp. 82:11:7 0.375 0.15 0.75 550 0.189 71.2 Ex. 2 Comp. 82:11:7 0.375 0.15 0.75 150 0.253 70.1 Ex. 3 Comp. 35:37:28 — — — 400 0.231 64.0 Ex. 4 Comp. 50:20:30 — — — 400 0.322 63.9 Ex. 5 Comp. 60:20:20 — — — 400 0.351 65.2 Ex. 6 Comp. 70:15:15 — — — 400 0.323 64.6 Ex. 7

[0076] As can be seen from Table 1 above, the cathode active materials of Examples 1 to 7 according to the present invention exhibit remarkably reduced amounts of lithium remaining on the surface compared to the cathode active materials of Comparative Examples 1 to 7. As a result, the cathode active materials of Examples 1 to 7 can remarkably improve the lifespan characteristics and high-voltage characteristics of the lithium secondary batteries.

[0077] In addition, the lithium secondary batteries of Examples 1 to 7 according to the present invention exhibit excellent output characteristics under low-temperature conditions and particularly excellent output characteristics under high-rate discharge conditions (2.0C discharge) compared to the lithium secondary batteries of Comparative Examples 1 to 7.

[0078] The reason for this is considered to be that the specific coating layer according to the present invention secures a conductive path of lithium ions to promote movement of the lithium ions and at the same time increases an electrical conductivity (lithium ion conductor).

[0079] Although the 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.