JPH08250122A - Forming method for battery electrode - Google Patents

Abstract

PURPOSE: To manufacture a battery electrode excellent in a cycle property, a retaining characteristic, and safety, by applying the mixture of styrene- butadiene copolymer latex, containing the specific rate of butadiene bond, and carbonaceous material, on a substrate to be dried at specific temperature. CONSTITUTION: Styrene-butadiene copolymer latex, having a butadiene bond containing quantity of 10-40wt.%, is mixed by about 0.1-20wt. parts in solid part to a carbonaceous material of 100 wt. parts. Moreover as required, water soluble thickener (carboxymethylcellulose etc.) can be added by about 2-60wt. parts to a latex solid parts of 100 wt. parts. This mixture is applied onto a base plate to be dried at a temperature of 50 deg.C or higher, preferably at about 90-200 deg.C. Consequently, a battery electrode can be obtained which is suitable for manufacturing a secondary battery many in the number of 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 method for forming a battery electrode having excellent safety.

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 latex 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 latex having a high butadiene bond content has a soft particle, so that it adheres to the surface of the electrode active material. It is considered that this is due to deformation and movement during use of the battery, but there was 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]

DISCLOSURE OF THE INVENTION The inventors of the present invention diligently studied various water-based polymer latices and found that styrene / butadiene latices having a low butadiene bond content, which were conventionally considered unusable from the viewpoints of current efficiency and characteristic changes. The present invention was surprisingly found by heating and drying under constant conditions to form a battery electrode with little change in characteristics, and arrived at the present invention. That is, the present invention applies a mixture of a styrene-butadiene copolymer latex having a butadiene bond content of 10 to 40% by weight and a carbonaceous material onto a substrate,
The present invention provides a method for forming a battery electrode, which comprises drying at a temperature of 50 ° C or higher.

The present invention will be described in detail below. The styrene-butadiene copolymer latex used in the present invention is
The butadiene bond content is 10 to 40% by weight, preferably 2
0 to 40% by weight, styrene content of 40 to 90% by weight, preferably 30 to 80% by weight, and 0 to 50% by weight, preferably 0 to 40% by weight of a monomer copolymerizable therewith. Obtained by emulsion polymerization. In addition, butadiene of the present invention,
Examples of copolymerizable monomers other than styrene include ethylenically unsaturated carboxylic acids such as methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile, and hydroxyethyl (meth) acrylate. Esters and also ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid and the like 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. When the butadiene bond content exceeds 40% by weight, the battery electrode obtained has a large change in battery characteristics during use or storage. When the content of butadiene bond is less than 10% by weight, the flexibility is poor and the adhesive strength of the electrode is inferior even if the drying temperature for drying the electrode is increased. Furthermore, the gel content of the polymer obtained by drying the styrene-butadiene copolymer latex is 10 to 100% by weight, preferably 5
It is 0 to 100% by weight. Here, the gel content means the insoluble content of the polymer in toluene. If the gel content in the latex is less than 10% by weight, polymer flow occurs when the electrode is formed and dried by heating, and the active material is excessively covered, and the overvoltage increases and it cannot be used. To adjust the gel content,
General methods such as adjustment of the polymerization temperature, adjustment of the amount of polymerization initiator, adjustment of the polymerization conversion rate, adjustment of the amount of chain transfer agent, etc. are used. Although not particularly limited, the particle size of the latex is preferably 0.01 to 0.5 µ, more preferably 0.01 to 0.3 µ. Carbonaceous Material The average particle size of the carbonaceous material used in the present invention is 0 because of problems such as a decrease in current efficiency, a decrease in stability of the coating liquid, and an increase in interparticle resistance in the resulting electrode coating film. .1 to 50 μ
m, preferably 3 to 25 μm, more preferably 5 to 1
The range is preferably 5 μm.

The amount of the latex compounded is not particularly limited, but is usually 0.1 to 20 parts by weight, preferably 0.5 to 10 parts by weight, based on 100 parts by weight of the carbonaceous material as a solid content. If the content of the latex is less than 0.1 part by weight, good adhesive strength cannot be obtained, and if it exceeds 20 parts by weight, the overvoltage is remarkably increased and the battery characteristics are adversely affected. The solid content concentration of the mixture of latex and carbonaceous material (hereinafter referred to as "coating liquid") is not particularly limited, but is usually 30 to 65% by weight, preferably 40 to 65% by weight. Furthermore, in the present invention, the coating liquid contains a water-soluble thickener as an additive in an amount of 2 per 100 parts by weight of the latex solid content.
-60 parts by weight may be used. Examples of the water-soluble thickener include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch and casein. 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. . In the present invention, the drying temperature at which the coating liquid is applied to the substrate and dried is 50 ° C or higher, preferably 70 to 250 ° C, more preferably 90 to 200 ° C. If the heating is not performed in the drying or the drying temperature is lower than 50 ° C., the film-forming property of the binder is insufficient and the adhesive strength of the electrode is poor. In addition, at a drying temperature higher than 250 ° C, the binder deteriorates and the adhesive strength also deteriorates. The drying temperature in the present invention means the maximum temperature of the base material in the drying step. The coating liquid is applied 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 as long as it can heat the substrate and the coated product to the above temperature, but a warm air dryer, an infrared heater, a far infrared heater and the like can be used.

The battery electrode formed by the present invention can be used in both aqueous and non-aqueous batteries, but when used as the negative electrode of a non-aqueous battery, particularly excellent battery performance can be obtained. When assembling a non-aqueous battery using the battery electrode formed according to the present invention, the electrolyte of the non-aqueous electrolyte is not particularly limited, but examples of alkaline secondary batteries include 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 of the electrolytic solution used include ethers, ketones, lactones, nitriles, amines, amides, sulfur compounds, chlorinated hydrocarbons, esters, carbonates, nitro compounds, and phosphoric acid. Although ester compounds, sulfolane compounds, etc. can be used, among these, ethers, ketones, nitriles, chlorinated hydrocarbons, carbonates, sulfolane 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,
Examples thereof include, but are not limited to, dimethylformamide, dimethylsulfoxide, dimethylthioformamide, sulfolane, 3-methyl-sulfolane, trimethyl phosphate, triethyl phosphate, and mixed solvents 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. Furthermore, 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 a positive electrode, a negative electrode, if necessary, a paper-type battery having a separator in a single layer or a multilayer, or a positive electrode, a negative electrode, if necessary, a separator roll. An example is a form of a cylindrical battery that is wound into a shape.

[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 measuring gel content; 0.5N aqueous ammonia 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 5, Comparative Examples 1 to 5 100 parts by weight of needle coke pulverized product (average particle size 12 μm) and 6 parts by weight of latex solid content obtained by emulsion polymerization with the polymer composition shown in Table 1. 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 liquid 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 immediately put in a hot air dryer having an internal temperature of 150.degree. C. for 15 minutes,
It was dried. The base material is 147 ℃ at maximum during this drying
Rose to. As a result, a sheet-shaped negative electrode having a thickness of 130 μm was obtained. On the other hand, Li1.03Co0.95 with an average particle size of 2μ
100 parts by weight of Sn0.042 O2, 7.5 parts by weight of graphite powder and 2.5 parts by weight of acetylene black are mixed, and a solution of fluororubber in methyl isobutyl ketone (concentration 4% by weight).
Was added to 50 parts by weight and mixed and stirred to obtain a coating liquid. 290 g / m 2 of this coating liquid using a commercially available Al foil (thickness 15μ) as a base material
It was applied and dried in 2 to obtain a positive electrode having a thickness of 110 μm. The negative electrode and the positive electrode were cut out into 0.9 cm × 5.5 cm to assemble a lithium secondary battery. This is described in Examples 1 to 5,
It is set as Comparative Examples 1 to 3. Further, Comparative Examples 4 and 5 were the same as Example 1 except that the drying conditions of the dry coating liquid were 25 ° C. × 16 hours and 45 ° C. × 4 hours. This battery is 4.2
The cycle of charging to V and discharging to 2.5 V at 10 mA 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 butadiene bond content of 10 to 4 on a substrate.
A method for forming a battery electrode, which comprises applying a mixture of 0% by weight of a styrene-butadiene copolymer latex and a carbonaceous material, and drying the mixture at a temperature of 50 ° C. or higher.