JPH08250123A - Aqueous binder for forming battery electrode - Google Patents

Abstract

PURPOSE: To manufacture an aqueous binder for forming a battery electrode less in a characteristic change, by preparing the aqueous binder having the main component of styrene-butadiene copolymer latex wherein butadiene bond containing quantity and gel containing quantity are specified. CONSTITUTION: Styrene-butadiene copolymer latex (a particle diameter of 0.01-0.5μm degree), having butadiene bond containing quantity of 40-98wt.% and a gel containing quantity (polymer solubility to toluene) is 20-74wt.%, is prepared to manufacture an aqueous binder for forming a battery electrode. This latex solution is mixed with carbonaceous material, an active material, (a mean particle diameter of 0.1-50μm degree) to be applied onto a base material to be dried, thereby obtaining a battery electrode for a secondary battery. Consequently, a secondary battery can be obtained which is many in the number of a charge/discharge frequency and bearable in use and preservation for a long time.

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 water-based binder having excellent safety for forming a battery electrode.

2. Description of the Related Art In recent years, electronic devices have been remarkably reduced in size and weight, and accordingly, there has been a great demand for reduction in size and weight of batteries as power sources. Non-aqueous batteries have been attracting attention because it is impossible to meet the above requirements with conventional batteries using general aqueous electrolytes. Such a non-aqueous battery has excellent performances in terms of size and weight reduction.
Titanium disulfide secondary batteries and the like have been proposed, and some of them have already been put to practical use. However, although such a non-aqueous battery is excellent in terms of performance such as high energy density and small size and light weight, it has a drawback in output characteristics as compared with a water battery represented by a lead battery, and has not yet been widely used. In particular, in the field of secondary batteries where output characteristics are required, this defect is one of the factors hindering practical use. The reason why non-aqueous batteries are inferior in output characteristics is high ionic conductivity, usually 10 -1 Ω -1 C in the case of aqueous electrolytes.
This is because the non-aqueous system usually has a low ionic conductivity of 10 −2 to 10 −4 Ω −1 CM −1 while having a value on the order of M −1. As one method for solving such a problem, it is possible to increase the electrode area, that is, to use a thin film or a large area electrode. As a conventional method for forming an electrode, an electrode active material and an organic polymer are mixed,
The method of compression forming is common. With such a method,
While it has the advantages that the binder, which is an insulating material, has a relatively small effect on the electrode active material and that the type and shape of the binder used are also limited, it is extremely difficult to manufacture a thin film / large area electrode. On the other hand, as a method for producing a thin film / large area electrode, a method is known in which an electrode active material is dispersed in a solvent solution of an organic polymer and then the electrode is formed by coating and drying. According to this method, a thin film / large area electrode can be easily obtained, which is very convenient, but on the other hand, the influence of the binder, which is an insulating material, on the electrode active material is significantly large, and when the electrode is assembled into a battery,
For example, a remarkable increase in overvoltage was observed, which was not a practical method. Also known as water-based binders are water-soluble polymers such as carboxymethyl cellulose, polyvinyl alcohol, polyacrylic acid salts and starch, or styrene-butadiene copolymer latexes having a high butadiene bond content (JP-A-5-74461). However, the water-soluble polymer is inferior in binder performance because it is uniformly attached to the surface of the electrode active material, and the styrene-butadiene copolymer latex having a high butadiene bond content is attached to the surface of the electrode active material particle because the particle is soft. It is considered that this is because the battery later deforms and moves during use of the battery, but there is a problem that characteristics change during use of the battery.

On the basis of the above situation,
The present invention provides a water-based binder for forming an electrode exhibiting performance with little characteristic change during use or storage of a battery, mainly in a secondary battery, which uses a carbonaceous material as an electrode active material.

[0002]

[Means for Solving the Problems] The present inventors diligently studied various water-based polymer latices, and a styrene-butadiene copolymer having a low gel content, which has hitherto been considered unusable in terms of current efficiency and characteristic changes. The present invention has been reached by finding that a battery electrode having a small change in characteristics can be formed even with latex. That is, according to the present invention, the content of butadiene bond used in forming a battery electrode is 40 to 9 after mixing with a carbonaceous material as an active material, coating it on a substrate, and then drying it.
A water-based binder for forming an electrode, which comprises 8% by weight and a styrene-butadiene copolymer latex having a gel content of 20 to 74% by weight as a main component.

The present invention will be described in detail below. The styrene-butadiene copolymer latex of the present invention is synthesized by a usual emulsion polymerization method and has a butadiene bond content of 4
0 to 97% by weight, preferably 40 to 90% by weight, 3 to 60% by weight of styrene, preferably 10 to 60% by weight, and 0 to 50% by weight of other polymers copolymerizable with styrene and butadiene. If the butadiene bond content is less than 40% by weight, the electrode lacks adhesive strength and flexibility.
If it exceeds 98% by weight, the adhesive strength is insufficient. Copolymerizable monomers other than styrene and butadiene can be used, for example methyl (meth) acrylate,
Ethylenically unsaturated carboxylic acid esters such as ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile, hydroxyethyl (meth) acrylate, and further acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, etc. Ethylenically unsaturated carboxylic acids can be used. In particular, as the ethylenically unsaturated carboxylic acid, it is preferable to use a dicarboxylic acid such as itaconic acid, fumaric acid and maleic acid from the viewpoint of the adhesive strength of the electrode. Further, the gel content of the polymer is 20 to
It is 74% by weight, preferably 30 to 70% by weight. Here, the gel content is the solubility of the polymer in toluene. If the polymer gel content of the styrene-butadiene copolymer latex exceeds 74% by weight, the binder performance of the polymer in this application will be insufficient and the adhesive strength will be poor, and if the amount of binder is increased to cope with this, the overvoltage will increase. However, a battery with high characteristics cannot be obtained. On the other hand, when the gel content is less than 20% by weight, a polymer flow occurs when the electrode is formed and heated and dried to transiently cover the active material, and the overvoltage also rises, making it unusable.
Therefore, the gel content of the polymer of the styrene-butadiene copolymer latex of the present invention is 20 to 74% by weight,
Preferably 30-70% by weight, more preferably 40-
It is 60% by weight. 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, the particle size of the styrene-butadiene copolymer latex is preferably 0.01 to 0.5 μm, more preferably 0.01 to 0.
It is 3 μm.

The amount of the latex to be blended is not particularly limited, but is usually 0.1% with respect to 100 parts by weight of the negative electrode active material.
It is 1 to 20 parts by weight, preferably 0.5 to 10 parts by weight. If the amount is less than 0.1 parts by weight, good adhesion cannot be obtained, and 2
If it exceeds 0 parts by weight, the overvoltage will remarkably increase and the battery characteristics will be adversely affected. The solid content concentration of the coating liquid is not particularly limited, but is usually 30 to 65% by weight, preferably 40 to 65% by weight. Further, in the battery electrode 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 solid content of the styrene-butadiene copolymer latex of the present invention. As a water-soluble thickener,
Carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch,
Includes phosphorylated starch, casein and the like. The battery electrode of the present invention comprises an active material, a styrene-butadiene copolymer latex and, if necessary, a water-soluble thickener, but does not necessarily exclude other components. For example, dispersants such as sodium hexametaphosphate, sodium tripolyphosphate, sodium pyrophosphate, sodium polyacrylate, etc., and those to which additives such as nonionic and anionic surfactants as a latex stabilizer are added are also included. . The average particle size of the carbonaceous material used in the present invention is a decrease in current efficiency,
Due to problems such as a decrease in the stability of the slurry and an increase in interparticle resistance in the coating film of the obtained electrode, 0.1 to 50 μm,
The range is preferably 3 to 25 μ, and more preferably 5 to 15 μ. 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 battery electrode of the present invention can be used for both water-based batteries and non-aqueous batteries, but when used as the negative electrode of non-aqueous batteries, particularly excellent battery performance can be obtained. When assembling a non-aqueous battery using the battery electrode of the present invention, the electrolyte of the non-aqueous electrolytic solution is not particularly limited, but if an example of an alkaline secondary battery is shown, LiClO4,
LiBF4, LiAsF6, CF3 SO3 Li, LiP
F6, LiI, LiAlCl4, NaClO4, NaB
F4, NaI, (n-Bu) 4 NClO4, (n-B
u) 4 NBF4, KPF6 and the like. Further, as the organic solvent of the electrolytic solution used, for example, ethers,
Ketones, lactones, nitriles, amines, amides, sulfur compounds, chlorinated hydrocarbons, esters, carbonates, nitro compounds, phosphate ester compounds,
Although a sulfolane-based compound or the like can be used, among these, ethers, ketones, nitriles, chlorinated hydrocarbons, carbonates, and sulfolane-based compounds are preferable. As typical examples of these, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, anisole, monoglyme, acetonitrile, propionitrile, 4-methyl-2-pentanone, butyronitrile, valeronitrile, benzonitrile, 1,2- Dichloroethane, γ-butyrolactone, dimethoxyethane, methyl formate, propylene carbonate, ethylene carbonate, dimethylformamide, dimethylsulfoxide, dimethylthioformamide, sulfolane, 3-methyl-sulfolane, trimethyl phosphate, triethyl phosphate and mixed solvents thereof. However, the present invention is not limited to these. The positive electrode material used is not particularly limited, but for example, MnO2, MoO3, V2O
5, V6 O13, Fe2 O3, Fe3 O4, Li (1-x) C
oO2, Li (1-x) .NiO2, Lix Coy Snz O
2, TiS2, TiS3, MoS3, FeS2, CuF
2, inorganic compounds such as NiF2, fluorinated carbon, graphite, vapor grown carbon fibers and / or pulverized products thereof, carbon such as PAN-based carbon fibers and / or pulverized products thereof, pitch-based carbon fibers and / or pulverized products thereof Examples thereof include materials, and conductive polymers such as polyacetylene and poly-p-phenylene. Especially Li (1-x) CoO2, L
i (1-x) NiO2, Lix Coy Snz O2, Li (1-
When a lithium ion-containing composite oxide such as X) Co (1-x) Niy O2 is used, both positive and negative electrodes can be assembled in a discharged state, which is a preferable combination. Further, if necessary, a battery is constructed by 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 the positive electrode, the negative electrode,
Furthermore, if necessary, a paper-type battery having a single-layer or multi-layer separator, a positive electrode and a negative electrode, and if necessary, a cylindrical battery in which the separator is wound in a roll shape may be mentioned as an example.

[0006]

The present invention will be described in more detail with reference to the following examples. In the present invention, the gel content is determined as follows. Method for determining gel content; 0.5N antenia water and 0.5
A latex adjusted to pH 8 with N hydrochloric acid was dried at 120 ° C. for 1 hour to form a film, then dipped in 100 parts by weight of polymer, toluene, shaken for 3 hours, filtered through a 200-mesh filter, and insoluble. Was collected, dried at 120 ° C. 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 (%) Examples 1 to 6 and Comparative Examples 1 to 3 100 parts by weight of needle coke pulverized product (average particle size 12 μm) and styrene-butadiene copolymer latex solid content obtained by emulsion polymerization with the polymer composition of Table 1. 6 parts by weight,
As a thickener, 1 part by weight of a carboxymethylcellulose aqueous solution as a solid content and 0.5 part by weight of 0.5N ammonia water were added, and they were thoroughly mixed and decomposed to obtain a coating solution. This coating solution was applied to a thickness of 120 g / m @ 2 with a roll coater using a nickel metal foil having a thickness of 10 .mu.m as a base material and dried. As a result, a sheet-shaped negative electrode having a thickness of 130 μm was obtained. On the other hand, 100 parts by weight of Li1.03Co0.95Sn0.042O2 having an average particle size of 2μ, 7.5 parts by weight of graphite powder and 2.5 parts by weight of acetylene black were mixed, and a fluororubber solution in methylisobutylketone (concentration 4% by weight). Was added to 50 parts by weight and mixed and stirred to obtain a coating liquid. A commercially available Al foil (thickness: 15 μm) was used as a base material, and this coating solution was applied at 290 g / m 2 and dried to give a thickness of 11
A 0 μ positive electrode was obtained. 0.9c of this negative electrode and positive electrode
A lithium secondary battery was assembled by cutting out into m × 5.5 cm. These are Examples 1 to 6 and Comparative Examples 1 to 3. 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.

[0007]

[Table 1]

[0008]

[Table 2]

[0009]

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

Claims (1)

[Claims] 1. A water-based binder for forming a battery electrode, which is mainly composed of a styrene-butadiene copolymer latex having a butadiene bond content of 40 to 98% by weight and a gel content of 20 to 74% by weight.