JP2002319403A - Secondary battery anode binder and secondary battery electrode composition - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery anode binder that has good adhesion to the current collector and excellent electrolyte resistance, and that enables to obtain a battery having excellent cycle efficiency when used, particularly, for a lithium ion secondary battery using a conductive carbonaceous material as an anode active substance. SOLUTION: This is a secondary battery anode binder that uses a conductive carbonaceous material as an active substance and the binder is a copolymer latex that is obtained by emulsifying and polymerizing the total 100 pts.wt. of monomers consisting of an aliphatic group-conjugated diene system monomer 20-60 wt.%, ethylene system unsaturated carbonic acid monomer 1-10 wt.%, and the other monomers 30-79 wt.% capable of co-polymerization with these in the existence of a cyclic unsaturated hydrocarbon having one unsaturated bond in the ring that is selected from cyclopentane, cyclohexene, cycloheptane, 4-methylcyclohexene, 1-methylcyclohexene.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a binder for a negative electrode of a secondary battery using a conductive carbonaceous material as a negative electrode active material, and a composition for a secondary battery electrode containing the binder for a negative electrode of a secondary battery.

[0002]

2. Description of the Related Art Lithium-ion secondary batteries using conductive carbonaceous materials that occlude and release lithium ions for their electrodes are increasingly important as power sources for small electronic devices because of their light weight and high energy density. . An electrode mainly composed of a conductive carbonaceous material that inserts and extracts lithium ions generally uses a polymer binder as a binder. The polymer binder is required to have adhesiveness to an active material, resistance to a polar solvent used as an electrolytic solution, and stability under an electrochemical environment. Traditionally,
Fluorine-based polymers such as polyvinylidene fluoride are used in this field, but there are problems such as the inhibition of conductivity when forming the electrode film and the lack of adhesive strength between the current collector and the electrode film. is there. In addition, when a fluorine-based polymer is used for a negative electrode under reducing conditions, stability is not sufficient, and there are problems such as a decrease in cyclability of a secondary battery. Improvement of these problems is desired. . For this reason, non-fluorinated polymers have been developed. For example, JP-A-63-1
JP-A No. 21257 describes an acrylonitrile-based polymer, and JP-A-1-186557 describes a polyester-based polymer. However, this is not enough to solve the above-mentioned problems, and furthermore, the elution to the electrolytic solution and Problems such as swelling were also derived, and problems with battery performance such as cycleability and manufacturing suitability were significant.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a lithium ion secondary battery which has good adhesion to a current collector and has excellent resistance to an electrolytic solution, and particularly uses a conductive carbonaceous material as a negative electrode active material. An object of the present invention is to provide a binder for a secondary battery negative electrode capable of obtaining a battery having excellent cycle efficiency when used.

[0004]

Means for Solving the Problems As a result of intensive studies to solve such problems, as a result of using a specific copolymer latex as a binder for a secondary battery negative electrode using a conductive carbonaceous material as a negative electrode active material, The inventors have found that the problem is solved, and have completed the present invention. That is, the present invention relates to a binder for a secondary battery negative electrode using a conductive carbonaceous material as a negative electrode active material, wherein the binder comprises 20 to 60% by weight of an aliphatic conjugated diene monomer and an ethylenically unsaturated carboxylic acid. A total of 100 parts by weight of a monomer composed of 1 to 10% by weight of a monomer and 30 to 79% by weight of another monomer copolymerizable therewith is mixed with cyclopentene, cyclohexene, cycloheptene, 4-methylcyclohexene, 1-methyl Provided is a binder for a secondary battery negative electrode, which is a copolymerized latex obtained by emulsion polymerization in the presence of a cyclic unsaturated hydrocarbon having one unsaturated bond in a ring selected from cyclohexene. Is what you do.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail.

The copolymer latex in the present invention is obtained by emulsion polymerization of an aliphatic conjugated diene monomer, an ethylenically unsaturated carboxylic acid monomer and another monomer copolymerizable therewith. It is.

[0007] As the aliphatic conjugated diene-based monomer,
3-butadiene, 2-methyl-1,3-butadiene,
2,3-dimethyl-1,3 butadiene, 2-chloro-
Examples thereof include 1,3-butadiene, substituted linear conjugated pentadienes, substituted and side chain conjugated hexadienes, and the like, and one or more kinds can be used. Particularly, 1,3-butadiene is preferred.

Examples of the ethylenically unsaturated carboxylic acid monomer include mono- or dicarboxylic acids (anhydrides) such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid and itaconic acid. Two or more types can be used.

Other monomers copolymerizable therewith include aromatic vinyl monomers, vinyl cyanide monomers,
Examples thereof include an unsaturated carboxylic acid alkyl ester monomer, an unsaturated monomer containing a hydroxyalkyl group, and an unsaturated carboxylic acid amide monomer, and one or more of these can be used.

As the aromatic vinyl monomers, styrene, α-methylstyrene, methyl α-methylstyrene,
Vinyl toluene and divinyl benzene, and the like,
One or more kinds can be used. Particularly, styrene is preferred.

Examples of the vinyl cyanide-based monomer include acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, α-ethylacrylonitrile and the like, and one or more kinds can be used. Particularly, acrylonitrile and methacrylonitrile are preferred.

The unsaturated carboxylic acid alkyl ester monomers include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, glycidyl methacrylate, dimethyl fumarate, diethyl fumarate, dimethyl maleate, diethyl maleate, dimethyl dimethyl Examples include itaconate, monomethyl fumarate, monoethyl fumarate, 2-ethylhexyl acrylate, and the like, and one or more kinds can be used. Particularly, methyl methacrylate is preferred.

Examples of unsaturated monomers containing a hydroxyalkyl group include β-hydroxyethyl acrylate and β-hydroxyethyl acrylate.
-Hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate,
Hydroxybutyl acrylate, hydroxybutyl methacrylate, 3-chloro-2-hydroxypropyl methacrylate, di- (ethylene glycol) maleate,
Di- (ethylene glycol) itaconate, 2-hydroxyethyl maleate, bis (2-hydroxyethyl)
Maleate, 2-hydroxyethyl methyl fumarate and the like can be mentioned, and one kind or two or more kinds can be used. Particularly, β-hydroxyethyl acrylate is preferable.

The unsaturated carboxylic acid amide monomers include:
Acrylamide, methacrylamide, N-methylol acrylamide, N-methylol methacrylamide, N,
N-dimethylacrylamide and the like can be mentioned, and one or more kinds can be used. Acrylamide is particularly preferred.

Further, in addition to the above monomers, any of the monomers used in ordinary emulsion polymerization, such as ethylene, propylene, vinyl acetate, vinyl propionate, vinyl chloride, and vinylidene chloride, can be used.

The polymerizable monomer composition includes 20 to 60% by weight of an aliphatic conjugated diene monomer, 1 to 10% by weight of an ethylenically unsaturated carboxylic acid monomer, and another monomer copolymerizable therewith. 30-79% by weight.

If the amount of the aliphatic conjugated diene monomer is less than 20% by weight, sufficient adhesion to the conductive carbonaceous material cannot be obtained when the electrode composition containing the binder of the present invention is applied to a current collector. On the other hand, when the content exceeds 60% by weight, the problem that the resistance to an electrolytic solution is reduced when a battery negative electrode is produced by applying the electrode composition to a current collector is not preferable. Preferably 30 to 55
% By weight.

If the amount of the ethylenically unsaturated carboxylic acid monomer is less than 1% by weight, the stability of the copolymer latex itself and the electrode composition may be poor, and if it exceeds 10% by weight, the viscosity of the latex may increase. However, there is a possibility that a problem may occur in handling the copolymer latex itself, which is not preferable. Preferably it is 1 to 7% by weight.

When the amount of the other copolymerizable monomer is less than 30% by weight, the electrode composition containing the binder of the present invention is applied to a current collector to produce a negative electrode for a battery. If it exceeds 79% by weight, the adhesion to the conductive carbonaceous material when the electrode composition is applied to the current collector is poor, which is not preferable. Preferably it is 38 to 69% by weight.

In the present invention, when the above monomer is subjected to emulsion polymerization, a cyclic unsaturated compound having one unsaturated bond in a ring selected from cyclopentene, cyclohexene, cycloheptene, 4-methylcyclohexene and 1-methylcyclohexene is used. It is necessary to carry out the emulsion polymerization in the presence of a saturated hydrocarbon emulsion polymerization. The proportion of the unsaturated hydrocarbon used is not particularly limited, but is 0.1 to 50 parts by weight based on 100 parts by weight of the total amount of the monomers. If the amount is less than 0.1 part by weight, the effect of the present invention is insufficient, and if it exceeds 50 parts by weight, the amount of the compound remaining as an unreacted substance also relatively increases, and the energy required for the recovery is greatly increased. Is not preferred. Preferably it is 0.5 to 30 parts by weight.

In the present invention, if necessary, a conventionally known chain transfer agent, for example, n-hexyl mercaptan,
n-octyl mercaptan, t-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, alkyl mercaptans such as n-stearyl mercaptan, xanthogen compounds such as dimethyl xanthogen disulfide, diisopropyl xanthogen disulfide, α-methyl styrene dimer, terpinolene And thiuram-based compounds such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetramethylthiuram monosulfide, 2,6-di-t-butyl-4-methylphenol, phenolic compounds such as styrenated phenol, and allyl alcohol. Allyl compounds, dichloromethane, dibromomethane, carbon tetrachloride, halogenated hydrocarbon compounds such as carbon tetrabromide, α-benzyloxystyrene, α- Vinyl ethers such as benzyloxyacrylonitrile and α-benzyloxyacrylamide, and one kind of triphenylethane, pentaphenylethane, acrolein, methacrolein, α-benzyloxystyrene, thioglycolic acid, thiomalic acid, 2-ethylhexyl thioglycolate, etc. Alternatively, two or more kinds can be used. The use amount of these chain transfer agents is not limited at all, and can be appropriately adjusted according to the performance required for the copolymer latex, but is preferably 0.05 to 10 parts by weight based on 100 parts by weight of the monomer mixture. Parts by weight.

In the present invention, when a copolymer latex is obtained by emulsion polymerization, a usual emulsifier is used. Examples of the emulsifier include anionic surfactants such as sulfates of higher alcohols, alkylbenzene sulfonates, alkyldiphenyl ether sulfonates, aliphatic sulfonates, aliphatic carboxylates and sulfates of nonionic surfactants. Alternatively, one or more nonionic surfactants such as alkyl ester type, alkyl phenyl ether type, and alkyl ether type of polyethylene glycol are used.

In the present invention, water-soluble initiators such as potassium persulfate, ammonium persulfate and sodium persulfate, redox initiators, and oil-soluble initiators such as benzoyl peroxide can be used as the polymerization initiator.

In the polymerization of the copolymer latex,
The method for adding the monomer and other components is not particularly limited, and a batch addition method, a split addition method,
Any of the continuous addition methods can be employed, and in the present invention, any of single-stage polymerization, two-stage polymerization, and multi-stage polymerization can be employed.

The number average particle size of the copolymer latex is not particularly limited, but is preferably 50 to 250 n.
m, preferably 70-200 nm.

The conductive carbonaceous material used in the present invention is used as a negative electrode active material of a lithium ion secondary battery, and is not particularly limited. Examples thereof include graphite, carbon fiber, resin-fired carbon, and linear carbonaceous material. Graphite hybrid, coke, pyrolysis gas phase growth carbon, furfuryl alcohol resin fired carbon, polyacene organic semiconductor, mesocarbon microbeads, mesophase pitch carbon, graphite whisker, pseudo isotropic carbon, fired natural material, And pulverized products thereof, and these can be used alone or in combination of two or more.

The above-mentioned copolymer latex in the present invention is used as a binder for a negative electrode of a secondary battery using a conductive carbonaceous material as a negative electrode active material, and comprises particles of a conductive carbon material as a negative electrode active material. It acts as a binder between the conductive carbonaceous material and the current collector. At that time, the copolymer latex has a solid content of 0.1 to 10 parts by weight based on 100 parts by weight of the conductive carbonaceous material,
Preferably, the composition for an electrode can be prepared by containing 0.5 to 5 parts by weight. If the amount of the copolymer latex of the present invention is less than 0.1 part by weight,
Good adhesion to a current collector or the like cannot be obtained, and if it exceeds 10 parts by weight, overvoltage tends to increase remarkably when assembled as a battery, which tends to adversely affect battery characteristics.

Various additives such as a water-soluble thickener may be added to the electrode composition containing the binder for a secondary battery negative electrode of the present invention, if necessary. Examples include carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch, water-soluble thickeners such as casein, sodium hexametaphosphate, sodium tripolyphosphate, sodium pyrophosphate. And a dispersant such as sodium polyacrylate, and a nonionic or anionic surfactant as a latex stabilizer.

The composition for an electrode of the present invention is applied to a current collector,
It is dried and used as a negative electrode of a lithium ion secondary battery. Further, as a method of applying the electrode composition of the present invention to a current collector, a reverse roll method, a comma bar method, a gravure method, an arbitrary coater head such as an air knife method can be used, and the drying method is standing drying, An air dryer, a warm air dryer, an infrared heater, a far infrared heater, or the like can be used. The drying temperature is usually 100 ° C. or higher.

A positive electrode active material, a positive electrode binder, a current collector, a separator, and a non-aqueous electrolyte used in producing a lithium ion secondary battery using the negative electrode prepared by using the electrode composition of the present invention. Existing terminals, insulators, battery containers and the like can be used without any particular limitation. Also,
It is also possible to use the copolymer latex of the present invention as a binder for a positive electrode.

[0031]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples unless the gist of the present invention is changed. In the examples,
Parts and percentages indicating percentages are based on weight. The evaluation of various physical properties in the examples was based on the following methods.

Number average particle diameter : The number average particle diameter of the copolymer latex was measured by a dynamic light scattering method. In the measurement, LPA-3000 / 3100 (manufactured by Otsuka Electronics) was used.

[0033]Preparation of copolymer latex  In a pressure-resistant polymerization reactor, 125 parts of pure water and dodecylbenze
0.8 parts of sodium sulfonate, potassium persulfate 1.
After charging 2 parts and stirring thoroughly, each of the first stages shown in Table 1
Monomer and 5 parts of cyclopentene, α-methylstyrene
0.3 part of dimer, 0.2 part of t-dodecyl mercaptan
Was added to initiate polymerization at 75 ° C. Polymerization after 1.5 hours
Keep the temperature down to 73 ° C and keep each single unit in the second stage shown in Table 1.
Body and potassium persulfate 0.5 part, pure water 10 parts for 2 hours
After the continuous addition over the
Add monomer and 5 parts of pure water continuously over 4 hours
Was. After raising the temperature to 75 ° C and keeping it for 5 hours,
Finished. Next, the copolymer latex was added to caustic soda.
After adjusting the pH to about 7 with an aqueous solution, steam distillation
Removal of reactive monomers and other low-boiling compounds, copolymers
Latex (a) was obtained. Also changed the conditions shown in Table 1.
The procedure is the same as for the copolymer latex (a) except that
Thus, copolymer latexes (b) to (g) were obtained. In addition,
Polymer latex (a), (b) and (g) are three-stage polymerization
The copolymer latex (c), (d), (e) and
And (f) were prepared by two-stage polymerization.

[0034]Preparation of composition for battery electrode  Artificial black with a particle size of 1 to 35 nm as conductive carbonaceous material
Using lead, 100% by weight of artificial graphite slurry thickener
2 parts by weight of carboxymethylcellulose as copolymer
With 5 parts of body latex (a), the total solid content becomes 50%.
After adding an appropriate amount of water and kneading, the battery of Example 1 shown in Table 1 was added.
An electrode composition was prepared. Similarly, Table 1
Example 2 using copolymer latexes (b) to (c)
3. The composition for battery electrodes according to 3 above, and a copolymer latex
Compositions for battery electrodes of Comparative Examples 1 to 4 using (d) to (g)
Created things.

Examples 1 to 3 and Comparative Examples 1 to 4 Each composition for a battery electrode was applied to both surfaces of a copper foil having a thickness of 20 μm as a current collector, and dried at 120 ° C. for 5 hours. The negative electrodes of Examples 1 to 3 and Comparative Examples 1 to 4 were formed by compression molding with a press. The details of these evaluations are as follows. Table 1 shows the evaluation results.

[0036]Binding  Using a knife on the surface of the negative electrode, the current collector is separated from the active material layer.
Cut in depth up to the depth of 2mm at 2mm intervals
I made six cuts in the grid. This cut
Paste the adhesive tape on the
The degree of dropout was visually determined from 5 points (no dropout) to 1 point (complete).
(All dropped out).

[0037]Electrolyte resistance  In the same manner as above, insert the negative electrode
Ethylene carbonate having a quantitative ratio of 1: 1 and 1,2-diene
Lithium hexafluorophosphate in mixed solvent of methoxyethane
40 ° C, which was dissolved and adjusted to 1 mol / l
Immersed in the electrolyte solution for 12 hours and taken out.
From 5 points (no change) to 1 point (completely dropped) by visual judgment
Was evaluated.

[0038]Creating the positive electrode  100 parts of LiCoO 2 as a positive electrode active material and a conductive agent
And 5 parts of acetylene black
Kneading 6 parts of vinylidene fluoride in 2-methylpyrrolidone
Thus, a positive electrode active material slurry was prepared. The resulting slurry
On both sides of a 20 μm thick aluminum foil as a current collector
Apply, dry at room temperature, compression mold with hot press to form positive electrode
Created.

[0039]Battery manufacturing method  After welding the lead plate to each end of the positive and negative electrodes,
Positive electrode, 30 μm thick porous polypropylene separator
-, The negative electrode, and the separator are laminated in this order and spirally wound.
It was a wound body. This wound body is used as a cylindrical electrode
Stored in a pond can, lead the battery can and negative electrode, positive terminal and positive electrode
Terminals, and the capacity ratio is 1: 1 in the battery can.
Mixture of Tylene carbonate and 1,2-dimethoxyethane
1 mol / l lithium hexafluorophosphate in solvent
After filling with the adjusted electrolyte solution,
Battery cover with battery
Battery was created.

[0040]Cycle  3. Using the cylindrical battery thus prepared, charge and discharge conditions; 1 or
2. 7V, 1mA / cm^Two And the second discharge capacity
The number of cycles when 90% of the capacity was reached was measured.

[0041]

[Table 1]

[0042]

By using the binder for a secondary battery negative electrode according to the present invention, a lithium ion secondary battery having good adhesion to a current collector, excellent electrolytic solution resistance, and excellent cycle efficiency can be obtained. A secondary battery can be obtained.

Continued on the front page F term (reference) 4J100 AB02Q AB03Q AB04Q AB16Q AJ01R AJ02R AJ08R AJ09R AK31R AL03Q AL04Q AL09Q AL10Q AL34Q AL36Q AL41Q AM02Q AM03Q AM15Q AM21Q AR03S AR04S AR05S AS01P AS06QA03 BA03 A03 BA05 A03 BA05 A03 HA02

Claims (2)

[Claims] 1. A binder for a negative electrode of a secondary battery comprising a conductive carbonaceous material as a negative electrode active material, the binder comprising 20 to 60% by weight of an aliphatic conjugated diene monomer and an ethylenically unsaturated carboxylic acid. A total of 100 parts by weight of a monomer composed of 1 to 10% by weight of a monomer and 30 to 79% by weight of another monomer copolymerizable therewith is combined with cyclopentene, cyclohexene, cycloheptene, 4-methylcyclohexene,
A binder for a secondary battery negative electrode, which is a copolymer latex obtained by emulsion polymerization in the presence of a cyclic unsaturated hydrocarbon having one unsaturated bond in a ring selected from methylcyclohexene. 2. A composition for a secondary battery electrode comprising 0.1 to 10 parts by weight of the binder for a secondary battery negative electrode according to claim 1, based on 100 parts by weight of the conductive carbonaceous material.