JPH09320604A - Conductive binding composition for secondary battery electrode and method for producing the same - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a conductive binder composition for forming an electrode in a secondary battery, which exhibits performance with little change in characteristics during use or storage of the battery. [Structure] 100 parts by weight of a carboxy-modified styrene-butadiene copolymer latex having a butadiene bond content of 30 to 98% by weight and a gel content of 20 to 95% by weight,
Conductive carbon 10 to 200 parts by weight, aqueous dispersant 0.
A conductive binder composition for a secondary battery electrode, which comprises 1 to 100 parts by weight, and a method for producing the same.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention has a cycle property, a storage property,
The present invention relates to a conductive binder composition for secondary battery electrodes, which is excellent in safety.

2. Description of the Related Art In recent years, there has been a remarkable reduction in the size and weight of electronic devices, and there has been a great demand for smaller and lighter batteries that serve as power supplies. Various secondary batteries have been developed to satisfy such requirements, and nickel-hydrogen secondary batteries and lithium-ion secondary batteries have been put into practical use. However, although such a secondary battery is excellent in terms of performance such as high energy density and small size and light weight, it has a problem in charge / discharge recycling characteristics as compared with a conventional secondary battery typified by a lead battery, and thus has an inherent performance. I haven't pulled out enough.
As a conventional electrode molding method, a method of mixing an electrode active material and an organic polymer which is a binder, and forming by compression, and a method of dispersing the electrode active material in a solvent solution of the organic polymer and then coating and drying the same. A method of molding an electrode is known. In both methods, there is a problem in that the organic polymer, which is an insulating material, hinders the electrical conductivity between the electrode active material and the current collector, and the binding state changes during charge / discharge cycles to shorten its life. There has also been an attempt to improve the dispersibility by using a water-based binder (JP-A-5-7446).
1) had a problem that sufficient performance could not be obtained.

SUMMARY OF THE INVENTION Based on the above situation,
The present invention provides a conductive binder composition for forming an electrode in a secondary battery, which exhibits performance with little change in characteristics during use or storage of the battery.

[0002]

According to the present invention, the butadiene bond content is 30 to 98% by weight and the gel content is 20 to 9%.
5 parts by weight of styrene-butadiene copolymer latex 100 parts by weight, conductive carbon 10 to 200 parts by weight,
Provided is a conductive binder composition for a secondary battery electrode, which comprises 0.1 to 100 parts by weight of an aqueous dispersant.
Further, in the present invention, 50% by weight of the total aqueous dispersant (solid content)
The above and 50% by weight or more of the total conductive carbon are added and finely dispersed by a high shear mixing device, and then the remaining aqueous dispersant, the remaining conductive carbon and the carboxy-modified styrene butadiene copolymer latex are mixed. A method for producing a conductive binding composition for a secondary battery electrode is provided.

Hereinafter, the present invention will be described in detail. The carboxy-modified styrene-butadiene copolymer latex of the present invention is synthesized by a usual emulsion polymerization method and has a butadiene bond content of 30 to 98% by weight, preferably 30 to 70%.
% By weight, particularly preferably 30-40% by weight, styrene 3
.About.60% by weight, preferably 10 to 60% by weight, of other monomers copolymerizable with styrene and butadiene.
It is 1 to 50% by weight. If the butadiene bond content is less than 30% by weight, the electrode lacks adhesive strength and flexibility.
If it exceeds 98% by weight, the adhesive strength is insufficient. The ethylenically unsaturated carboxylic acid for carboxy modification is used in an amount of 0.1 to 8% by weight, preferably 0.5 to 6% by weight, more preferably 1 to 4% by weight based on the total amount of the monomers. Examples of the ethylenically unsaturated carboxylic acid include acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid and the like. In terms of the adhesive strength of the electrode, ethylenically unsaturated dicarboxylic acid such as itaconic acid, fumaric acid and maleic acid. Is preferably used. The amount of ethylenically unsaturated carboxylic acid is 0.1
When it is less than 8% by weight, the adhesive strength of the electrode is low, and when it exceeds 8% by weight, the surface of the conductive carbon is excessively covered and the overvoltage increases, and it cannot be used. Copolymerizable monomers other than styrene, butadiene, and unsaturated carboxylic acids can be used. For example, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile, hydroxyethyl ( Ethylenically unsaturated carboxylic acid esters such as (meth) acrylates can be used. Further, the gel content of the polymer is 20 to 95% by weight, preferably 40 to 9
5% by weight, particularly preferably 65 to 95% by weight. Here, the gel content is the solubility of the polymer in toluene. When the gel content of the polymer of the carboxy-modified styrene-butadiene copolymer latex exceeds 95% by weight, the binder performance of the polymer in this application is insufficient and the adhesive strength is poor, and if the amount of the binder is increased to cope with this, the overvoltage will increase. There is a problem of increasing the temperature, and it is not possible to obtain a battery with high characteristics. On the other hand, when the gel content is less than 20% by weight, a polymer flow occurs when the electrode is formed and dried by heating, and the active material and the conductive carbon are excessively covered, and the overvoltage also rises, making it unusable. Although it is not clear why the gel content has such a large influence on the battery characteristics, the binder is an essential material for binding the battery electrode active material, and the activity is improved by covering the surface of the active material particles. It is thought to be due to the action of reducing the activity of the substance. For adjusting the gel content, general methods such as adjusting the polymerization temperature, adjusting the amount of the polymerization initiator, adjusting the polymerization conversion rate, and adjusting the amount of the chain transfer agent are used. Although not particularly limited, preferably the particle size of the carboxy-modified styrene-butadiene copolymer latex is 0.01 to 0.5 μm,
More preferably, it is 0.01 to 0.3 μm. The particle size distribution of the carboxy-modified styrene butadiene copolymer latex has a weight average particle size (Dw) and a number average particle size (Dn).
Ratio (Dw / Dn) is 1.2 or less, preferably 1.1 or less. When Dw / Dn exceeds 1.2, the binder performance is insufficient and the adhesive strength is poor.

The conductive carbon used in the present invention may be carbon black, carbon fiber or graphite. Among these, oil furnace black (Ketjen black) and acetylene black are preferably used among the carbon blacks. These may be used alone or in plural as required. The conductive carbon used in the present invention is preferably porous. Further, the conductive carbon is preferably highly crystalline and has few impurities. The primary particle diameter of the conductive carbon is 0.1 μm or less, preferably 0.07 μm or less. If the primary particle diameter of the conductive carbon exceeds 0.1 μm, the conductive carbon is unlikely to form a chain structure in the conductive binder composition, resulting in a marked decrease in conductivity. The conductive carbon is 10 to 200 parts by weight, preferably 15 to 100 parts by weight, per 100 parts by weight of the carboxy-modified styrene-butadiene copolymer latex.
Use parts by weight. If the amount is less than 10 parts by weight, the conductivity is low and 2
If the amount exceeds 100 parts by weight, the binding performance becomes poor, which is not preferable. The aqueous dispersant used in the present invention is polyisoprene sulfonic acid, a sulfonated product of a styrene isoprene copolymer,
Polycarboxylic acid, hexametaphosphoric acid, tripolyphosphoric acid,
Ammonium salts, sodium salts, potassium salts and the like of pyrophosphoric acid can be used. Of these, sulfonated isoprene-styrene copolymers and ammonium salts of polycarboxylic acids are preferably used. In addition, these may be used alone or in plural as required. The amount of the aqueous dispersant used is preferably 0.1 to 5 parts by weight, more preferably 1 to 3 parts by weight, per 10 parts by weight of conductive carbon. If it is less than 0.1 parts by weight, the dispersion of the conductive carbon is poor, and if it exceeds 5 parts by weight, the internal resistance becomes high when used for an electrode, which is not preferable.

The conductive binder composition of the present invention is prepared by adding a styrene-butadiene copolymer latex to an aqueous dispersion prepared by previously adding an aqueous dispersant, conductive carbon and water if necessary, and further mixing It is adjusted by mixing with a dispersing device having a shearing force. Examples of the dispersing device having a high shearing force include a bead mill, a high pressure homogenizer, an ultrasonic homogenizer and the like. The conductive carbon and the latex can be mixed at a submicron level by using an apparatus having a high shear force. From the viewpoint of dispersion efficiency and production efficiency, 50% of the total amount of aqueous dispersant
The above and 50% or more of the total amount of conductive carbon are mixed in advance and finely dispersed by the device having a high shearing force, and then the remaining aqueous dispersant and the remaining conductive carbon and the styrene-butadiene copolymer latex are mixed. It is preferable. The degree of dispersion can be examined using a squeeze gauge, preferably 10 μm or less, more preferably 3 μm.
Or less, particularly preferably 1 μm or less.

The conductive binder composition of the present invention can be mixed with various additives as a binder of a negative electrode active material and applied to a current collector to form an electrode, or can be applied as a primer to the current collector for use. You can also do it. The amount of the binder used in the battery electrode is not particularly limited, but is usually 0.1 to 30 parts by weight, preferably 0.5 to 1 as the solid content of the composition with respect to 100 parts by weight of the negative electrode active material.
0 parts by weight. If it is less than 0.1 parts by weight, good adhesive strength cannot be obtained, and if it exceeds 30 parts by weight, the overvoltage is remarkably increased and the battery characteristics are adversely affected. When it is used as a primer for battery electrodes, the coating thickness after drying is 0.
The thickness is 5 μm to 100 μm, preferably 1 to 30 μm, and more preferably 2 to 10 μm. If it is less than 0.5 μm, the effect of improving the adhesion with the active material coating layer cannot be obtained, and if it is 100 μm,
If it exceeds m, the thickness becomes large when assembled as a battery, which is not preferable. The solid content concentration of the conductive binder composition of the present invention is not particularly limited, but is usually 10 to 65% by weight, preferably 20 to 65% by weight. Further, in the conductive binder composition of the present invention, a water-soluble thickener may be used as an additive in an amount of 2 to 60 parts by weight based on 100 parts by weight of the carboxy-modified styrene-butadiene copolymer latex solid content of the present invention. . As the water-soluble thickener, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol,
It includes polyacrylic acid (salt), oxidized starch, phosphorylated starch, casein and the like. The average particle size of the secondary battery electrode active material used in the present invention is 0.1 due to problems such as a decrease in current efficiency, a decrease in slurry stability, and an increase in interparticle resistance in the obtained electrode coating film. It is suitable that the range is ˜200 μm, preferably 3 to 100 μm, and more preferably 5 to 50 μm. The slurry is applied as a coating liquid on a substrate, heated and dried under the above conditions to form a battery negative electrode. At this time, if necessary, it may be molded together with the current collector material, or alternatively, a current collector such as an aluminum foil or a copper foil may be used as the base material. Further, as such a coating method, an arbitrary coater head such as a reverse roll method, a comma bar method, a gravure method or an air knife method can be used. The drying method is not particularly limited, and standing drying, blower dryer, warm air dryer, infrared heater, far infrared heater, etc. can be used.

The conductive binder composition of the present invention can be used in both aqueous batteries and non-aqueous batteries. Excellent performance can be obtained with a nickel-hydrogen battery negative electrode as a water-based battery and an alkaline secondary battery negative electrode, a lithium-ion battery negative electrode as a non-aqueous battery. Examples of the battery electrode active material used in the present invention include coke and graphite for a lithium ion battery negative electrode. When assembling a non-aqueous battery using a battery electrode using the conductive binding composition of the present invention, the electrolyte of the non-aqueous electrolyte is not particularly limited, but if an example of an alkaline secondary battery is shown, LiClO4, LiBF4, LiAsF6
, CF3 SO3 Li, LiPF6, LiI, LiAl
Cl4, NaClO4, NaBF4, NaI, (n-B
u) 4 NClO4, (n-Bu) 4 NBF4, KPF6
And the like. Examples of the organic solvent used in the electrolytic solution include ethers, ketones, lactones, nitriles, amines, amides, sulfur compounds, chlorinated hydrocarbons, esters, carbonates, nitro compounds, and phosphoric acid. Ester compounds, sulfolane compounds and the like can be used, and among them, ethers, ketones, nitriles, chlorinated hydrocarbons, carbonates, and sulfolane compounds are preferable. Typical examples thereof include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, anisole, monoglyme, acetonitrile, propionitrile, 4-methyl-2-pentanone, butyronitrile, valeronitrile, benzonitrile, 1,2- Dichloroethane, γ-butyrolactone, dimethoxyethane, methylformate, propylene carbonate, ethylene carbonate,
Examples thereof include, but are not limited to, dimethylformamide, dimethylsulfoxide, dimethylthioformamide, sulfolane, 3-methyl-sulfolane, trimethyl phosphate, triethyl phosphate, and a mixed solvent thereof. Further, the positive electrode material used is not particularly limited, but, for example,
MnO2, MoO3, V2 O5, V6 O13, Fe2 O3
, Fe3 O4, Li (1-x) CoO2, Li (1-x) .N
iO2, Lix Coy Snz O2, TiS2, TiS3
, MoS3, FeS2, CuF2, NiF2 and other inorganic compounds, fluorinated carbon, graphite, vapor grown carbon fiber and / or pulverized product thereof, PAN-based carbon fiber and / or pulverized product thereof, pitch-based carbon fiber and / or its produced product Examples thereof include carbon materials such as pulverized products, and conductive polymers such as polyacetylene and poly-p-phenylene.
In particular, Li (1-x) CoO2, Li (1-x) NiO2, Lix
When a lithium ion-containing composite oxide such as Coy Snz O2 or Li (1-X) Co (1-x) Niy O2 is used, both the positive and negative electrodes can be assembled in a discharged state, which is a preferable combination. Further, if necessary, a battery is configured using components such as a separator, a current collector, a terminal, and an insulating plate. Further, the structure of the battery is not particularly limited, but a positive electrode, a negative electrode, a paper type battery having a single-layer or multiple-layer separator as needed, or a positive electrode, a negative electrode, and a roll of the separator if necessary. An example is a form of a cylindrical battery wound in a shape.

[0008]

The present invention will be described in more detail with reference to the following examples. However, the present invention is not limited to these embodiments. In the following description, “part” means “part by weight”,
"%" Represents% by weight. Various evaluations in Examples and Comparative Examples were performed as follows. Method for measuring gel content: 0.5N aqueous ammonia and 0.
Latex adjusted to pH 8 with 5N hydrochloric acid at 120 ° C for 1 hour
After drying for a period of time to form a film, the film is dipped in 100 parts by weight of the polymer, toluene, shaken for 3 hours, filtered through a 200-mesh filter to collect insolubles, and heated at 120 ° C.
After drying for 1 hour, the weight of the insoluble matter was measured, and the gel content was determined by the following formula. Gel content = (toluene insoluble content weight / weight before immersion) x 1
00 (%) Dispersion degree: Measured by the crush gauge A method of JIS K5400. Conductivity measurement method: A conductive composition was applied to a PET film of 100 μm and dried at 120 ° C. for 10 minutes to give a film thickness of 20 μm.
m was obtained. The conductivity of the film was measured by the method of JIS K6911. Adhesion to metal foil: A conductive composition was applied to a copper foil and a nickel foil having a thickness of 50 μm and dried at 120 ° C. for 10 minutes to obtain a coating film having a thickness of 15 μm. Adhesion is a cross-cut test method JI
According to SK5400, the coating film was scratched in a grid pattern, a cellophane adhesive tape was attached on the scratch, and the number of squares remained after peeling off was evaluated. For example, if the adhesion is good, all the squares will remain on the coating film and 100/1
Shown as 00. <Example 1> 200 parts of distilled water in a high-speed stirring and mixing vessel, ammonium salt of sulfonated styrene-isoprene copolymer as an aqueous dispersant (weight average molecular weight = 40,000, sulfonation rate = 80%, solid content = 15%) ) 67 parts were added and stirred. Subsequently, as conductive carbon, Ketjen Black EC (manufactured by Japan EC, primary particle size = 0.03 μm) 3
0 part was added slowly. Next, a styrene-butadiene copolymer latex having a composition shown in Table 1 (solid content = 50%) 20
0 part was added to obtain an aqueous dispersion 1. Next, the mixture was stirred for 1 hour with a sand grinder (tabletop sand mill manufactured by Sekipe, media = 1 mm soda glass beads, mill base / media volume ratio = 1.5, disk rotation peripheral speed = 8 m / s). Table 1 shows the physical properties of the obtained conductive binding composition. <Examples 2 to 7> Conductive binder compositions were obtained in the same manner as in Example 1 except that the materials and conditions shown in Table 1 were changed. Table 1 shows the physical properties. Comparative Example 1 A binder composition A of Comparative Example 1 was obtained in exactly the same manner as in Example 1 except that the aqueous dispersant was not used. Table 1 shows the physical properties. <Comparative Examples 2 to 3> Binder compositions B to C of Comparative Examples were obtained in the same manner as in Example 1 except that the materials and conditions shown in Table 1 were changed. Table 1 shows the physical properties. <Test Examples 1 to 6 and Test Comparative Examples 1 to 3> 100 parts by weight of needle coke pulverized product (average particle size 12 μm) and 8 parts by weight of the conductive binder composition shown in Table 1 were used as a thickener. As an aqueous solution of carboxymethyl cellulose as a solid content of 1 part by weight,
0.5 parts by weight of 0.5N ammonia water was added, and the mixture was thoroughly mixed and decomposed to obtain a coating liquid. 120 g / m 2 of this coating liquid with a roll coater using a nickel metal foil with a thickness of 10 μm as a base material.
It was applied to a thickness of 2 and dried. This gives a thickness of 130 μm
A sheet-shaped negative electrode of was obtained. On the other hand, Li having an average particle size of 2 μm
1.03 Co0.95Sn0.042 O2 100 parts by weight, graphite powder 7.5 parts by weight, acetylene black 2.5 parts by weight are mixed, and 50 parts by weight of fluororubber methyl isobutyl ketone solution (concentration 4% by weight) is added and mixed and stirred. It was used as a coating liquid. This coating solution was applied and dried at 290 g / m @ 2 using a commercially available Al foil (thickness: 15 .mu.m) as a base material to obtain a positive electrode having a thickness of 110 .mu.m. This negative electrode and the positive electrode are 0.9 cm x 5.5
A lithium secondary battery was assembled by cutting out to cm. These are designated as Test Examples 1 to 3 and Test Comparative Examples 1 and 2. This battery was charged to 4.2 V and discharged at 10 mA to 2.5 V. The cycle was repeated. Table 2 shows the overvoltage in the charge / discharge cycle of these batteries, the capacity retention rate in the charge / discharge cycle, and the capacity change under accelerated storage conditions.

[0009]

[Table 1]

[0010]

[Table 2]

[0011]

The conductive binder composition of the present invention has a high performance of 2.
It is suitable for obtaining a secondary battery, particularly a secondary battery which has a large number of charge / discharge cycles and can withstand long-term use and storage.

Claims (2)

[Claims] 1. 100 parts by weight of a carboxy-modified styrene-butadiene copolymer latex having a butadiene bond content of 30 to 98% by weight and a gel content of 20 to 95% by weight, conductive carbon of 10 to 200 parts by weight, and an aqueous dispersant.
A conductive binder composition for a secondary battery electrode, comprising 0.1 to 100 parts by weight. 2. 50% by weight of the total amount of the aqueous dispersant (solid content)
50% by weight or more of the total amount of conductive carbon is added and finely dispersed by a high-shear mixing device, and then the remaining aqueous dispersant, the remaining conductive carbon, and the carboxy-modified styrene butadiene copolymer latex are mixed. The method for producing a conductive binder composition for a secondary battery electrode according to claim 1, wherein