Binder for secondary battery exhibiting excellent adhesion force

Abstract

Provided is a binder for secondary battery electrodes comprising polymer particles obtained by polymerizing three or more kinds of monomers wherein the polymer particles have a mean particle diameter of 0.3 μm to 0.7 μm. The binder exhibits superior adhesion force to electrode current collectors and excellent support force to the active material and basically improves safety of electrodes, thus providing a secondary battery with superior cycle characteristics.

Claims

1. A binder for secondary battery electrodes comprising polymer particles, wherein the polymer of the polymer particles is a copolymer obtained by polymerizing three or more kinds of monomers, wherein the polymer particles have a mean particle diameter of 0.5 μm to 0.7 μm, and wherein the three or more kinds of monomers consist of a mixture of a (meth)acrylic acid ester monomer (a); at least one monomer selected from the group consisting of an acrylate monomer, a vinyl monomer and a nitrile monomer (b); and a unsaturated monocarbonic acid monomer (c), wherein the vinyl monomer is at least one selected from the group consisting of styrene, α-methylstyrene, β-methylstyrene, p-t-butylstyrene, and divinyl benzene.

2. The binder according to claim 1, wherein the (meth)acrylic acid ester monomer (a) is present in an amount of 15 to 99% by weight, the monomer (b) is present in an amount of 1 to 60% by weight, and the unsaturated monocarbonic acid monomer (c) is present in an amount of 1 to 25% by weight, based on the total weight of the binder.

3. The binder according to claim 1, wherein the (meth)acrylic acid ester monomer is at least one monomer selected from the group consisting of methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-amyl acrylate, isoamyl acrylate, n-ethyl hexyl acrylate, 2-ethyl hexyl acrylate, 2-hydroxy ethyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-amyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, n-ethyl hexyl methacrylate, 2-ethyl hexyl methacrylate, hydroxyethyl methacrylate and hydroxypropyl methacrylate.

4. The binder according to claim 1, wherein the acrylate monomer is selected from the group consisting of methacryloxy ethylethylene urea, β-carboxy ethylacrylate, aliphatic monoacrylate, dipropylene diacrylate, ditrimethylolpropane tetraacrylate, hydroxyethyl acrylate, dipentaerythritol hexaacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, lauryl acrylate, cetyl acrylate, stearyl acrylate, lauryl methacrylate, cetyl methacrylate and stearyl methacrylate.

5. The binder according to claim 1, wherein the nitrile monomer is at least one selected from the group consisting of succinonitrile, sebaconitrile, fluoronitrile, chloronitrile, acrylonitrile and methacrylonitrile.

6. The binder according to claim 1, wherein the unsaturated monocarbonic acid monomer is at least one selected from maleic acid, fumaric acid, methacrylic acid, acrylic acid, glutaconic acid, itaconic acid, tetrahydrophthalic acid, crotonic acid, isocrotonic acid and nadic acid.

7. A mix for secondary battery electrodes comprising: the binder for secondary battery electrodes according to claim 1; and an electrode active material capable of intercalating and de-intercalating lithium.

8. The mix according to claim 7, further comprising one or more selected from the group consisting of a viscosity controller and a filler.

9. The mix according to claim 7, wherein the electrode active material is a lithium transition metal oxide powder or a carbon powder.

10. An electrode for secondary batteries, in which the mix for electrodes according to claim 7 is applied to a current collector.

11. The electrode according to claim 10, wherein the current collector has a thickness of 3 to 500 μm and includes fine irregularities on the surface thereof.

12. A lithium secondary battery comprising the electrode for secondary batteries according to 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) Butyl acrylate (70 g), styrene (20 g) and acrylic acid (10 g) as monomers and polyoxyethylene glycol and sodium lauryl sulfate as emulsifying agents were added to water containing potassium persulfate as a polymerization initiator, and these ingredients were mixed and polymerized at 70° C. for about 10 hours. A binder for secondary battery electrodes containing polymer particles that are obtained by polymerizing the monomers and have a mean particle diameter of 0.3 μm was prepared through polymerization. The mean particle diameter of the polymer can be controlled by controlling amounts of two emulsifying agents. As an amount of the two emulsifying agents increases, particle diameter decreases and as an amount of the two emulsifying agents decreases, particle diameter increases.

Example 2

(3) A binder for secondary battery electrodes comprising polymer particles having a mean particle diameter of 0.4 μm was prepared in the same manner as Example 1, except that an amount of the emulsifying agent was decreased.

Example 3

(4) A binder for secondary battery electrodes comprising polymer particles having a mean particle diameter of 0.5 μm was prepared in the same manner as Example 1, except that an amount of the emulsifying agent was decreased.

Example 4

(5) A binder for secondary battery electrodes polymer particles having a mean particle diameter of 0.6 μm was prepared in the same manner as Example 1, except an amount of the emulsifying agent was decreased.

Example 5

(6) A binder for secondary battery electrodes comprising polymer particles having a mean particle diameter of 0.3 μm was prepared in the same manner as Example 1, except that acrylamide (1 g) was further used as a monomer.

Example 6

(7) A binder for secondary battery electrodes comprising polymer particles having a mean particle diameter of 0.3 μm was prepared in the same manner as Example 1, except that acrylonitrile was used as the monomer instead of styrene.

Comparative Example 1

(8) A binder for secondary battery electrodes comprising polymer particles having a mean particle diameter of 0.1 μm was prepared in the same manner as Example 1, except that an amount of the emulsifying agent was increased.

Comparative Example 2

(9) A binder for secondary battery electrodes comprising polymer particles having a mean particle diameter of 0.2 μm was prepared in the same manner as Example 1, except that an amount of the emulsifying agent was increased.

Comparative Example 3

(10) A binder for secondary battery electrodes comprising polymer particles having a mean particle diameter of 0.2 μm was prepared in the same manner as Example 6, except that an amount of the emulsifying agent was increased.

Comparative Example 4

(11) A binder for secondary battery electrodes comprising polymer particles having a mean particle diameter of 0.8 μm was prepared in the same manner as Example 1, except that an amount of the emulsifying agent was considerably decreased.

Experimental Example 1

Adhesion Force Test

(12) First, for the binders of Examples 1 to 6 and the binders of Comparative Examples 1 to 4, an anode active material, a conductive material, a thickener, and a binder were mixed in a ratio of 95:1:1:3 to prepare a slurry and the slurry was coated on an Al foil to fabricate an electrode. The electrode was dried at 90° C. and 120° C.

(13) The electrode thus fabricated was pressed to a predetermined thickness, cut into a predetermined size and fixed on a glass slide and 180 degree peel strength was measured, while the current collector was peeled off. The results thus obtained are shown in Table 1. Evaluation was based on an average of five or more peel strengths.

(14) TABLE-US-00001 TABLE 1 Adhesion force (gf/cm) Drying at 90° C. Drying at 120° C. Ex. 1 28 25 Ex. 2 32 27 Ex. 3 34 29 Ex. 4 30 25 Ex. 5 30 27 Ex. 6 31 26 Comp. Ex. 1 19 15 Comp. Ex. 2 23 17 Comp. Ex. 3 20 17 Comp. Ex. 4 21 20

(15) As can be seen from Table 1 above, electrodes employing the binders of Examples 1 to 6 according to the present invention exhibited considerably high adhesion force, as compared to electrodes employing the binders of Comparative Examples 1 to 4. In particular, it can be seen from comparison between the Comparative Example 2 and Example 1, and between Comparative Example 3 and Example 6, that when the mean particle diameter is 0.3 μm or more, an adhesion force is considerably increased. Also, variation in adhesion force according to particle diameter is great at 120° C., as compared to at 90° C. The reason for this is that when the mean particle diameter is higher than 0.3 μm, movement of the binder is considerably decreased during drying than when the mean particle diameter is lower than 0.3 μm, and the effect increases, as temperature increases. The reason is thought that a decrease in adhesion force caused by movement of the binder is greater than an increase in adhesion force caused by specific surface area of the binder.

(16) On the other hand, in Comparative Example 4 in which although the mean particle diameter is large, an adhesion force is low, when the mean particle diameter of the binder is 0.8 μm, movement of the binder is decreased, but a specific surface area of the binder is extremely decreased, an area where the binder contacts an active material is decreased and the adhesion force is thus greatly decreased.

(17) 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.