MULTILAYER ELECTRODE AND LITHIUM SECONDARY BATTERY INCLUDING THE SAME

Abstract

Disclosed herein are a multilayer electrode and a lithium secondary battery including the same. The multilayer electrode includes an electrode current collector for transmitting electrons between an external wire and an electrode active material and three or more electrode mixture layers sequentially applied to the electrode current collector, wherein each of the electrode mixture layers includes an electrode active material and a conducting agent, and wherein the content of the conducting agent of one of adjacent electrode mixture layers that is relatively close to the current collector in the direction in which the electrode mixture layers are formed is higher than that of the conducting agent of the other of the adjacent electrode mixture layers that is relatively distant from the current collector.

Claims

1. A multilayer electrode comprising an electrode current collector for transmitting electrons between an external wire and an electrode active material and three or more electrode mixture layers sequentially applied to the electrode current collector, wherein each of the electrode mixture layers comprises an electrode active material and a conducting agent, and wherein a content of the conducting agent of one of adjacent electrode mixture layers that is relatively close to the current collector in a direction in which the electrode mixture layers are formed is higher than a content of the conducting agent of the other of the adjacent electrode mixture layers that is relatively distant from the current collector.

2. The multilayer electrode according to claim 1, wherein a difference in the content of the conducting agent between the adjacent electrode mixture layers is 0.5 weight % to 10 weight %.

3. The multilayer electrode according to claim 2, wherein the difference in the content of the conducting agent between the adjacent electrode mixture layers is 2 weight % to 5 weight %.

4. The multilayer electrode according to claim 1, wherein the content of the conducting agent of an innermost one of the electrode mixture layers, which directly contacts the electrode current collector, is 3 weight % to 40 weight % based on a total weight of the innermost electrode mixture layer within a range in which the content of the conducting agent of the innermost electrode mixture layer is higher than the content of the conducting agent of an electrode mixture layer that is adjacent to the innermost electrode mixture layer.

5. The multilayer electrode according to claim 1, wherein the content of the conducting agent of an outermost one of the electrode mixture layers, which is most distant from the electrode current collector, is 1 weight % to 10 weight % based on a total weight of the outermost electrode mixture layer within a range in which the content of the conducting agent of the outermost electrode mixture layer is lower than the content of the conducting agent of an electrode mixture layer that is adjacent to the outermost electrode mixture layer.

6. The multilayer electrode according to claim 1, wherein the conducting agents of the adjacent electrode mixture layers are not mixed with each other but adjoin each other at an interface between the adjacent electrode mixture layers.

7. The multilayer electrode according to claim 1, wherein the conducting agents of the adjacent electrode mixture layers are mixed with each other at an interface between the adjacent electrode mixture layers so as to have a concentration gradient.

8. The multilayer electrode according to claim 7, wherein the conducting agents of the adjacent electrode mixture layers have a concentration gradient in which the contents of the conducting agents are sequentially reduced in a direction that becomes distant from the electrode current collector.

9. The multilayer electrode according to claim 1, wherein the three or more electrode mixture layers have a same thickness.

10. The multilayer electrode according to claim 1, wherein two or more of the three or more electrode mixture layers have different thicknesses.

11. The multilayer electrode according to claim 1, wherein the electrode active materials in the three or more electrode mixture layers are of a same kind.

12. The multilayer electrode according to claim 1, wherein the electrode active materials in two or more of the three or more electrode mixture layers are of different kinds.

13. The multilayer electrode according to claim 1, wherein the conducting agents in the three or more electrode mixture layers are of a same kind.

14. The multilayer electrode according to claim 1, wherein the conducting agents in two or more of the three or more electrode mixture layers are of different kinds.

15. The multilayer electrode according to claim 1, wherein each of the three or more electrode mixture layers further comprises a binder.

16. The multilayer electrode according to claim 1, wherein the multilayer electrode is a positive electrode.

17. A method of manufacturing the electrode according to claim 1, the method comprising: (a) preparing three or more electrode slurries having different contents of conducting agents; and (b) sequentially applying the electrode slurries to a surface of an electrode current collector, with one of the electrode slurries having a highest content of the conducting agent being applied first, and drying the electrode slurries to form electrode mixture layers.

18. The method according to claim 17, wherein step (b) comprises individually drying each of the electrode slurries after application of each of the electrode slurries.

19. A lithium secondary battery comprising the multilayer electrode according to claim 1.

20. A battery module comprising the lithium secondary battery according to claim 19 as a unit cell.

21. A device comprising the battery module according to claim 20 as a power source.

22. The device according to claim 21, wherein the device is an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or a power storage system.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0055] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0056] FIG. 1 is a schematic view showing the distribution of content of a conducting agent in a conventional single-layer electrode;

[0057] FIG. 2 is a schematic view showing the distribution of content of a conducting agent in an electrode according to an embodiment of the present invention; and

[0058] FIG. 3 is a schematic view showing the distribution of content of a conducting agent in an electrode according to another embodiment of the present invention.

BEST MODE

[0059] Now, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted, however, that the scope of the present invention is not limited by the illustrated embodiments.

[0060] FIG. 2 is a schematic view showing the distribution of content of a conducting agent in an electrode according to an embodiment of the present invention for easier understanding of the construction of the electrode according to the present invention.

[0061] Referring to FIG. 2, an electrode 100 is configured to have a three layer structure including a first electrode mixture layer 110 applied to a current collector 140, a second electrode mixture layer 120 applied to the first electrode mixture layer 110, and a third electrode mixture layer 130 applied to the second electrode mixture layer 120.

[0062] The content of a conducting agent 111 included in the first electrode mixture layer 110 is higher than that of a conducting agent 121 included in the second electrode mixture layer 120, and the content of the conducting agent 121 included in the second electrode mixture layer 120 is higher than that of a conducting agent 131 included in the third electrode mixture layer 130. The conducting agents 111, 121, and 131 are not mixed with each other but adjoin each other at the interface between the first electrode mixture layer 110 and the second electrode mixture layer 120 and at the interface between the second electrode mixture layer 120 and the third electrode mixture layer 130.

[0063] FIG. 3 is a schematic view showing the distribution of content of a conducting agent in an electrode according to another embodiment of the present invention.

[0064] Referring to FIG. 3, an electrode 200 is configured to have a three layer structure including a first electrode mixture layer 210 applied to a current collector 240, a second electrode mixture layer 220 applied to the first electrode mixture layer 210, and a third electrode mixture layer 230 applied to the second electrode mixture layer 220, in the same manner as the electrode 100 of FIG. 2. In addition, the content of a conducting agent 211 included in the first electrode mixture layer 210 is higher than that of a conducting agent 221 included in the second electrode mixture layer 220, and the content of the conducting agent 221 included in the second electrode mixture layer 220 is higher than that of a conducting agent 231 included in the third electrode mixture layer 230.

[0065] Unlike the electrode 100 of FIG. 2, however, the conducting agents 111, 121, and 131 are mixed with each other at the interface between the first electrode mixture layer 210 and the second electrode mixture layer 220 and at the interface between the second electrode mixture layer 220 and the third electrode mixture layer 230. As a result, the electrode 200 has a concentration gradient in which the contents of the conducting agents are sequentially reduced from the first electrode mixture layer 210 to the second electrode mixture layer 220 and from the second electrode mixture layer 220 to the third electrode mixture layer 230.

[0066] In the electrodes 100 and 200 of FIGS. 2 and 3, the content of the conducting agent included in the first electrode mixture layer 110 is 0.5 weight % to 10 weight % higher than that of the conducting agent included in the second electrode mixture layer 120, and the content of the conducting agent included in the second electrode mixture layer 120 is 0.5 weight % to 10 weight % higher than that of the conducting agent included in the third electrode mixture layer 130, although the electrodes are slightly different in structure from each other. As a result, the contents of the conducting agents 111 and 211 in the vicinity of the current collectors 140 and 240 are the highest, whereby it is possible to prevent an increase in resistance due to lack of the conducting agents, thereby improving the performance of a battery.

[0067] Meanwhile, in FIGS. 2 and 3, only the conducting agents are shown as being included in the electrode mixture layers in order to effectively describe the structure of the electrode according to the present invention. However, it is a matter of course that other compounds, such as electrode active materials and binders, are included.

[0068] Hereinafter, the present invention will be described in more detail with reference to the following example. This example is provided only for illustration of the present invention and should not be construed as limiting the scope of the present invention.

EXAMPLE 1

[0069] 88 weight % of Li.sub.1.2Ni.sub.0.2Mn.sub.0.5Co.sub.0.1O.sub.2 as a positive electrode active material, 7 weight % of natural graphite as a conductive agent, and 5 weight % of PVdF as a binder were mixed with NMP as a solvent to manufacture a first positive electrode slurry.

[0070] 91 weight % of Li.sub.1.2Ni.sub.0.2Mn.sub.0.5Co.sub.0.1O.sub.2 as a positive electrode active material, 5 weight % of natural graphite as a conductive agent, and 4 weight % of PVdF as a binder were mixed with NMP as a solvent to manufacture a second positive electrode slurry.

[0071] 94 weight % of Li.sub.1.2Ni.sub.0.2Mn.sub.0.5Co.sub.0.1O.sub.2 as a positive electrode active material, 3 weight % of natural graphite as a conductive agent, and 3 weight % of PVdF as a binder were mixed with NMP as a solvent to manufacture a third positive electrode slurry.

[0072] The first positive electrode slurry was applied to aluminum foil having a thickness of 20 μm such that the first positive electrode slurry had a thickness of 40 μm and was then pressed and dried, the second positive electrode slurry was applied to the first positive electrode slurry such that the second positive electrode slurry had a thickness of 40 μm and was then pressed and dried, and the third positive electrode slurry was applied to the second positive electrode slurry such that the third positive electrode slurry had a thickness of 40 μm and was then pressed and dried to manufacture a positive electrode.

[0073] 84.15 weight % of natural graphite as a negative electrode active material, 9.35 weight % of SiO, 2 weight % of a conductive agent (Super-P), 3 weight % of a binder (SBR), and 1.5 weight % of a thickening agent (CMC) were mixed with H.sub.2O as a solvent to manufacture a negative electrode mixture. The negative electrode mixture was applied to copper foil having a thickness of 20 μm such that the negative electrode mixture had a thickness of 120 μm and was then pressed and dried to manufacture a negative electrode.

[0074] A porous polyethylene separator was disposed between the positive electrode and the negative electrode, and then the positive electrode, the porous polyethylene separator, and the negative electrode were impregnated with an electrolytic solution having 1 weight % of an additive (VC), 1.5 weight % of PS, and 1M of LiPF.sub.6 dissolved in a carbonate solvent of EC:EMC=1:2 to manufacture a sheet type lithium secondary battery having a size of 3 cm×4 cm.

COMPARATIVE EXAMPLE 1

[0075] A positive electrode and a lithium secondary battery were manufactured in the same manner as in Example 1 except that only a first positive electrode slurry was applied to a thickness of 120 μm.

COMPARATIVE EXAMPLE 2

[0076] A positive electrode and a lithium secondary battery were manufactured in the same manner as in Example 1 except that only a second positive electrode slurry was applied to a thickness of 120 μm.

COMPARATIVE EXAMPLE 3

[0077] A lithium secondary battery were manufactured in the same manner as in Example 1 except that a first positive electrode slurry was applied to aluminum foil having a thickness of 20 μm such that the first positive electrode slurry had a thickness of 60 μm and was then pressed and dried, and a second positive electrode slurry was applied to the first positive electrode slurry such that the second positive electrode slurry had a thickness of 60 μm and was then pressed and dried to manufacture a positive electrode.

EXPERIMENTAL EXAMPLE 1

[0078] Rate tests were carried out on the secondary batteries manufactured according to Example 1 and Comparative Examples 1 to 3 in a voltage range of 2.5 V to 4.4 V. The results are shown in Table 1 below.

TABLE-US-00001 TABLE 1 0.1 C/0.1 C 0.5 C/0.5 C 1 C/1 C 2 C/2 C vs. vs. vs. vs. 0.1 C/0.1 C 0.1 C/0.1 C 0.1 C/0.1 C 0.1 C/0.1 C Example 1 100%, 54.9 92.0% 83.8% 72.2% mAh Comparative 100%, 51.2 89.7% 77.4% 58.2% Example 1 mAh Comparative 100%, 54.1 88.3% 74.9% 51.6% Example 2 mAh Comparative 100%, 53.1 91.1% 80.3% 65.7% Example 3 mAh

[0079] Referring to Table 1 above, it can be seen that the secondary battery of Example 1 having the electrode structure according to the present invention exhibits higher rate characteristics than the secondary batteries of Comparative Examples 1 and 2 having the single-layer structure and the secondary battery of Comparative Example 3 having the two layer structure.

[0080] In particular, it can be seen that the secondary batteries of Comparative Examples 1 and 3 exhibit lower rate characteristics than the secondary battery of Example 1, although the content of the conducting agent in the electrode of each of the secondary batteries of Comparative Examples 1 and 3 is higher than that of the conducting agent in the electrode of the secondary battery of Example 1.

[0081] Although the exemplary 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.

INDUSTRIAL APPLICABILITY

[0082] As is apparent from the above description, an electrode according to the present invention is configured to have a multilayer structure, e.g. a structure including three or more layers, in which the content of a conducting agent of one of adjacent electrode mixture layers that is relatively close to a current collector in the direction in which a plurality of electrode mixture layers is formed is higher than that of a conducting agent of the other of the adjacent electrode mixture layers that is relatively distant from the current collector, whereby it is possible to prevent an increase in resistance due to lack of the conducting agent in the vicinity of the current collector, thereby improving electrical conductivity. Consequently, it is possible to improve the capacity and output characteristics of a secondary battery including the electrode according to the present invention.