Cathode active material and lithium secondary battery comprising the same

Abstract

Disclosed is a cathode active material for secondary batteries comprising one or more compounds having a layered-crystal structure, represented by the following Formula 1, wherein a transition metal layer contains Li, in an amount lower than 20%, based on a total amount of a transition metal site, and a ratio of Ni positioned in a lithium layer, that is, a cation mixing ratio is 1% to 4.5%, based on a total amount of a lithium site in the lithium layer to stably support the layered-crystal structure: (1-s-t)[Li(Li.sub.aMn.sub.(1-a-x-y)Ni.sub.xCo.sub.y)O.sub.2]*s[Li.sub.2CO.sub.3]*t[LiOH] (1), wherein 0<a<0.2; 0<x<0.9; 0<y<0.5; a+x+y<1; 0<s<0.03; and 0<t<0.03. The cathode active material exhibits long lifespan and superior stability at room temperature and high temperatures in spite of repeated charge and discharge at a high current.

Claims

1. A cathode active material for secondary batteries comprising one or more compounds having a layered-crystal structure, represented by the following Formula 1, wherein a transition metal layer contains Li, in an amount lower than 20%, based on a total amount of a transition metal site, and an amount of Ni positioned in a lithium layer is 1.5% to 4.0%, based on a total amount of lithium sites in the lithium layer to stably support the layered-crystal structure, and wherein a lithium secondary battery comprising the cathode active material exhibits a cycle capability of 92% to 97% after the 30.sup.th cycle compared to the 1.sup.st cycle:
(1-s-t)[Li(Li.sub.a(Mn.sub.(1-a-x-y)Ni.sub.xCo.sub.y).sub.1-a)O.sub.2]*s[Li.sub.2CO.sub.3]*t[LiOH]  (1) wherein 0<a<0.2; 0<x<0.9; 0<y<0.5; a+x+y<1; 0.0010≦s<0.0022, 0.0012≦t<0.0026, 0.0022≦s+t<0.0048, and a, x and y represent molar ratios, and s and t represent weight ratios.

2. The cathode active material according to claim 1, wherein a is 0.01 to 0.19.

3. The cathode active material according to claim 1, wherein x is not lower than 0.3 and is lower than 0.8.

4. The cathode active material according to claim 1, wherein y is greater than 0 and is not higher than 0.3.

5. The cathode active material according to claim 1, wherein the transition metal is substituted in a predetermined amount by a metal or non-metal element having a 6-coordination structure.

6. The cathode active material according to claim 5, wherein a substitution amount of the metal or non-metal element having a 6-coordination structure is 10 mol % or less, based on the total weight of the transition metal.

7. The cathode active material according to claim 1, wherein Ni positioned in the lithium layer is Ni.sup.2+ derived from the transition metal layer.

8. A cathode mix comprising the cathode active material according to claim 1.

9. A cathode for secondary batteries in which the cathode mix according to claim 8 is applied to a current collector.

10. A lithium secondary battery comprising the cathode for secondary batteries according to claim 9.

11. A middle- or large-sized battery pack comprising the secondary battery according to claim 10 as a unit battery.

12. An electric vehicle, a hybrid electric vehicle or a plug-in hybrid electric vehicle comprising the middle- or large-sized battery pack of claim 11.

Description

BEST MODE

(1) Now, the present invention will be described in more detail with reference to the following examples. These examples are provided only to illustrate the present invention and should not be construed as limiting the scope and spirit of the present invention.

Example 1

(2) A precursor was synthesized such that a molar ratio of Ni:Mn:Co was 53:27:20 and mixed with Li.sub.2CO.sub.3, and an active material of 0.9978Li(Li.sub.0.02(Ni.sub.0.53Mn.sub.0.27Co.sub.0.20).sub.0.98)O.sub.2*0.0012LiOH*0.0010Li.sub.2CO.sub.3 was prepared at a furnace temperature of 940° C. under a controlled cooling atmosphere.

(3) The amounts of LiOH and Li.sub.2CO.sub.3 in the product were measured by adding 10 g of the active material to 200 ml of water and measuring an amount of the base by titration with 0.1N HCl.

Comparative Example 1

(4) An active material of 0.9948Li(Li.sub.0.02(Ni.sub.0.78Mn.sub.0.12Co.sub.0.10).sub.0.98)O.sub.2*0.0052Li.sub.2CO.sub.3 was prepared in the same manner as in Example 1, except that an amount of carbonate was maximized by controlling the cooling atmosphere.

Comparative Example 2

(5) An active material was prepared in the same manner as in Example 1, except that an amount of Li in the transition metal layer was zero.

Comparative Example 3

(6) An active material was prepared in the same manner as in Example 1, except that Li was not present in the transition metal layer and a ratio of Li and transition metal was 1:0.99.

Comparative Example 4

(7) An active material was prepared in the same manner as in Example 1, except that Li was not present in the transition metal layer and a ratio of Li and the transition metal was 0.97.

Example 2

(8) An active material of 0.9972Li(Li.sub.0.02(Ni.sub.0.53Mn.sub.0.27Co.sub.0.20).sub.0.98)O.sub.2*0.0018LiOH*0.0010Li.sub.2CO.sub.3 was prepared in the same manner as in Example 1, except that an amount of OH was increased by controlling the cooling atmosphere.

Example 3

(9) An active material of 0.9972Li(Li.sub.0.02(Ni.sub.0.53Mn.sub.0.27Co.sub.0.20).sub.0.98)O.sub.2*0.0008LiOH*0.0020Li.sub.2CO.sub.3 was prepared in the same manner as in Example 1, except that an amount of OH was increased by controlling the cooling atmosphere.

Comparative Example 5

(10) The active material was washed with distilled water to remove the base of the active material prepared in Example 1 and dried in an oven at 130° C. for 24 hours to prepare Li(Li.sub.0.02(Ni.sub.0.53Mn.sub.0.27Co.sub.0.20).sub.0.98)O.sub.2.

Example 4

(11) A precursor was synthesized such that a molar ratio of Ni:Mn:Co was 78:12:10 and mixed with Li.sub.2CO.sub.3, and an active material of 0.9952Li(Li.sub.0.02(Ni.sub.0.78Mn.sub.0.12Co.sub.0.10).sub.0.98)O.sub.2*0.0026LiOH*0.0022Li.sub.2CO.sub.3 was prepared at a furnace temperature of 890° C. under a controlled cooling atmosphere.

(12) The amounts of LiOH and Li.sub.2CO.sub.3 in the product were measured by adding 10 g of the active material to 200 ml of water and measuring an amount of the base by titration with 0.1N HCl.

Comparative Example 6

(13) An active material of 0.9948Li(Li.sub.0.02(Ni.sub.0.78Mn.sub.0.12Co.sub.0.10).sub.0.98)O.sub.2*0.0052Li.sub.2CO.sub.3 was prepared by treating the precursors of Example 4 and Li.sub.2CO.sub.3 in the same manner as in Comparative Example 1.

Comparative Example 7

(14) An active material was prepared in the same manner as in Example 4, except that an amount of Li in the transition metal layer was zero.

Comparative Example 8

(15) An active material was prepared in the same manner as in Example 4, except that Li was not present in the transition metal layer and a ratio of Li and the transition metal was 1:0.99.

Comparative Example 9

(16) An active material was prepared in the same manner as in Example 4, except that Li was not present in the transition metal layer and a ratio of Li and the transition metal was 1:0.97.

Comparative Example 10

(17) An active material of Li(Li.sub.0.02(Ni.sub.0.78Mn.sub.0.12Co.sub.0.10).sub.0.98)O.sub.2 was prepared by treating the active material prepared in Example 4 in the same manner as in Comparative Example 5.

Example 5

(18) A precursor was synthesized such that a molar ratio of Ni, Mn and Co was 50:40:10 and mixed with Li.sub.2CO.sub.3, and an active material of 0.9967Li(Li.sub.0.1(Ni.sub.0.5Mn.sub.0.4Co.sub.0.1).sub.0.9)O.sub.2*0.0021LiOH*0.0012Li.sub.2CO.sub.3 was prepared at a furnace temperature of 950° C. under a controlled cooling atmosphere.

(19) The amounts of LiOH and Li.sub.2CO.sub.3 in the product were measured by adding 10 g of the active material to 200 ml of water and measuring an amount of the base by titration with 0.1N HCl.

Comparative Example 11

(20) An active material of 0.9966Li(Li.sub.0.1(Ni.sub.0.5Mn.sub.0.4Co.sub.0.1))O.sub.2*0.0034Li.sub.2CO.sub.3 was prepared by treating the precursors of Example 5 and Li.sub.2CO.sub.3 in the same manner as in Comparative Example 1.

Comparative Example 12

(21) An active material of Li(Li.sub.0.1(Ni.sub.0.5Mn.sub.0.4Co.sub.0.1))O.sub.2 was prepared by treating the active material prepared in Example 5 in the same manner as in Comparative Example 5.

Experimental Example 1

(22) The active materials synthesized in Examples 1 to 5 and Comparative Examples 1 to 12, a conductive material and a binder were mixed at a ratio of active material:conductive material:binder of 95:2.5:2.5 to prepare a slurry and the slurry was coated on an Al foil. The obtained electrode was pressed at a porosity of 23% and punched into a circular shape to fabricate a coin-type battery. At this time, Li metal was used for the anode, and a solution of 1M LiPF.sub.6 in a solvent mixed at a ratio of EC:DMC:DEC of 1:2:1 was used as an electrolyte. The fabricated batteries were subjected to a variety of tests under the conditions shown in Table 1 below.

(23) In addition, data of active materials synthesized in Examples 1 to 5 and Comparative Examples 1 to 12 were obtained by X-ray diffraction analysis and an amount of transition metal in the structure was determined by structural refinement.

(24) These results are shown in Table 1 below.

(25) TABLE-US-00001 TABLE 1 Electrochemical test results Rate Cycle Ni Occ. Discharge 1.sup.st cycle capability capability In Li capacity efficiency 2.0 C/0.1 C 30.sup.th cycle/1.sup.st site (%) (mAh/g) (%) (%) cycle (%) (%) Ex. 1 163 88 85 95 2.4 Comp. Ex. 1 158 85 78 89 3.0 Comp. Ex. 2 154 82 74 90 5.2 Comp. Ex. 3 152 80 75 88 6.0 Comp. Ex. 4 145 80 71 85 6.7 Ex. 2 164 89 86 94 2.6 Ex. 3 161 87 84 92 2.8 Comp. Ex. 5 165 89 85 82 2.5 Ex. 4 195 89 84 92 1.9 Comp. Ex. 6 189 86 79 87 2.2 Comp. Ex. 7 186 85 73 90 4.7 Comp. Ex. 8 178 82 68 89 7.3 Comp. Ex. 9 170 82 66 88 9.2 Comp. Ex. 196 90 85 80 2.1 10 Ex. 5 159 91 87 97 2.4 Comp. Ex. 149 87 78 89 2.6 11 Comp. Ex. 160 91 86 85 3.0 12

(26) As can be seen from Table 1 above, LiOH and Li.sub.2CO.sub.3 play an important role in the active material. In Comparative Examples 5, 10 and 12 in which LiOH and Li.sub.2CO.sub.3 are not present in respective active materials, rate and cycle characteristics are sharply decreased. Variation in these characteristics increases 10 to 15 fold, when actual battery cycles, i.e., 300 or 500 cycles, of the active material are repeated and, in particular, this increase becomes more severe when applied to batteries for electric vehicles. In addition, as can be seen from the results of Comparative Examples 1, 6 and 11, performance is deteriorated in a case in which Li.sub.2CO.sub.3 is present alone.

(27) Furthermore, comparing the results of Example 1 and Comparative Examples 2 to 4 with the results of Example 4 and Comparative Examples 7 to 9, depending on presence of Li in the transition metal layer, the examples according to the present invention exhibit considerably superior rate characteristics, as compared to Comparative Examples.

(28) Accordingly, the cathode active materials of the present invention exhibit superior lifespan and rate characteristics.

INDUSTRIAL APPLICABILITY

(29) As can be seen from above, the cathode active material having a crystal structure according to the present invention can secure stability of secondary batteries and improve lifespan under rapid high-current charge/discharge conditions and high temperature conditions.

(30) 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.