JP2002319402A - 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, in the existence of 0.6-3.0 pts.wt. of α-methylstyrene dimer, 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.% that are capable of co-polymerization with these. The quantity of α-methylstyrene dimer that remains in the copolymer latex is 400-3000 ppm against the solid portion of the copolymer latex.

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 stores and releases 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. However, when forming an electrode film, there is a problem in that the conductivity is impaired and the adhesive strength between the current collector and the electrode film is insufficient. 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, and 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. Α-methylstyrene dimer 0.6 to 3.0 with respect to 100 parts by weight of a total of monomers consisting of 1 to 10% by weight of a monomer and 30 to 79% by weight of another monomer copolymerizable therewith.
A copolymer latex obtained by emulsion polymerization in the presence of parts by weight, and α remaining in the copolymer latex
The present invention provides a binder for a negative electrode of a secondary battery, wherein the amount of methylstyrene dimer is from 400 to 3000 ppm based on the solid content of the copolymer latex.

[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 one or more kinds can be used. In particular, 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 kind or two 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.

The α-methylstyrene dimer used in the present invention has 2,4-diphenyl-4-methyl-1-pentene, 2,4-diphenyl-4-methyl-2-pentene and 1, Although there is 1,3-trimethyl-3-phenylindane, α-methylstyrene dimer used in the present invention includes 2,4-diphenyl-
The content of 4-methyl-1-pentene is 60% by weight or more;
In particular, it is preferably at least 80% by weight.

The α-methylstyrene dimer to be present during the emulsion polymerization of the above-mentioned monomers is a total of 10
It is 0.6 to 3.0 parts by weight with respect to 0 parts by weight. 0.6
If the amount is less than 3 parts by weight, sufficient adhesion to the conductive carbonaceous material cannot be obtained. If the amount exceeds 3.0 parts by weight, the electrode composition of the present invention is applied to a current collector to produce a battery negative electrode. However, it is not preferable because a problem that the resistance to the electrolytic solution is lowered is observed. Further, the amount of α-methylstyrene dimer remaining in the copolymer latex of the present invention needs to be 400 to 3000 ppm based on the solid content of the latex.
Preferably it is 500-2500 ppm, More preferably, it is 600-2000 ppm. If the residual amount of α-methylstyrene dimer is 400 ppm or less, sufficient adhesion to the conductive carbonaceous material cannot be obtained, and the residual amount is 300 ppm.
At 0 ppm or more, there is a problem that the electrolytic solution resistance is reduced when the electrode composition of the present invention is applied to a current collector to produce a battery negative electrode. For the residual α-methylstyrene dimer, the amount used during the polymerization of the copolymer latex and its addition method, the monomer composition of the copolymer latex and its addition method, the polymerization temperature, and the aging temperature after polymerization It can be appropriately adjusted depending on the aging time and the like.

In the present invention, the following chain transfer agents can be used together with α-methylstyrene dimer. Examples of such a chain transfer agent include alkyl mercaptans such as n-hexyl mercaptan, n-octyl mercaptan, t-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, and n-stearyl mercaptan, dimethyl xanthogen disulfide, and diisopropyl xanthogen gene. Xanthogen compounds such as sulfide, terpinolene, thiuram-based compounds such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetramethylthiuram monosulfide, phenols such as 2,6-di-t-butyl-4-methylphenol and styrenated phenol Compound,
Allyl compounds such as allyl alcohol, dichloromethane,
Halogenated hydrocarbon compounds such as dibromomethane and carbon tetrabromide, vinyl ethers such as α-benzyloxystyrene, α-benzyloxyacrylonitrile and α-benzyloxyacrylamide, triphenylethane, pentaphenylethane, acrolein, methacrolein, and thiol Glycolic acid, thiomalic acid, 2-ethylhexyl thioglycolate and the like can be mentioned, and one or more of these can be used. Although the amount of the chain transfer agent is not particularly limited, it is generally used in an amount of 0 to 5 parts by weight based on 100 parts by weight of the monomer.

In the present invention, when the 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, as the initiator, a water-soluble initiator such as potassium persulfate, ammonium persulfate, and sodium persulfate, a redox-based initiator, or an oil-soluble initiator such as benzoyl peroxide can be used.

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, more preferably 70 to 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 one kind or a mixture of two or more kinds can be used.

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]Residual α-methylstyrene of copolymer latex
Lendimer measurement  Residual α-methylstyrene dimer in copolymer latex
ー is a gas chromatograph GC-14A manufactured by Shimadzu Corporation.
Was carried out by the following method. (1) Preparation of sample 1 g of a copolymer latex was precisely weighed, and N, N-dimethyl
Dissolve in 25 ml of Lumamide and leave for 24 hours, then measure
It was a constant sample. (2) Gas chromatograph measurement conditions Sample amount: 5 μl Detector: FID Inj / Det temperature: 190 ° C. Column temperature: 180 ° C. Column: glass column 1.6 mx 2.6 mm φ carrier: Chromosorb WAW DMCS (particle size 80/100) Liquid phase : Silicone OV-17 Carrier gas: nitrogen, 25 ml / min. Hydrogen: 0.7 kg / cm2 Air: 0.9 kg / cm2 (3) Determination method 2,4-diphenyl-4-methyl-1-pen of known concentration
Using the calibration curve created with
From the chromatogram to the solid content of the copolymer latex
Determination of 2,4-diphenyl-4-methyl-1-pentene
Values were determined as residual α-methylstyrene dimer. same
One sample was measured three times, and the average value was expressed in ppm.
It was shown to.

The method for preparing the copolymer latex will be described below. The α-methylstyrene dimer used in preparing the copolymer latex was manufactured by Goi Kasei Co., Ltd.
GRH (containing 97.0 to 98.0% of 2,4-diphenyl-4-methyl-1-pentene) was used.

[0035]Preparation of copolymer latex  125 parts of pure water and dodecylbenzene in a pressure-resistant polymerization reactor
0.8 parts of sodium sulfonate, 1.2 parts of potassium persulfate
Parts, and after sufficiently stirring, each unit of the first stage shown in Table 1 was added.
Monomer and 1.4 parts of α-methylstyrene dimer, t-
Add 0.2 part of dodecyl mercaptan and polymerize at 75 ° C
Started. After 1.5 hours, lower the polymerization temperature to 73 ° C and keep
Each monomer and potassium persulfate in the second stage shown in Table 1
Add 0.5 parts and 10 parts of pure water continuously for 2 hours
After that, 5 parts of each monomer and pure water in the third stage shown in Table 1 were added to 4 parts
Added continuously over time. 75 ℃
And maintained for 5 hours, after which the polymerization was terminated. Then,
Adjust the pH of the polymer latex to about 7 with aqueous sodium hydroxide solution.
After adjustment, unreacted monomers and other low
The boiling point compound is removed to obtain a copolymer latex (a)
Was. Also, except that the conditions shown in Table 1 were changed,
In the same manner as in Tex (a), copolymer latex
(B) to (g) were obtained. Incidentally, copolymer latex
(A), (b) and (g) are copolymers obtained by three-stage polymerization.
Tex (c), (d), (e) and (f) are double-tier
Created by the combination.

[0036]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.

[0037][Examples 1-3, Comparative Examples 1-4]  Each composition for a battery electrode was formed into a 20 μm-thick current collector.
Apply to both sides of copper foil, dry at 120 ° C for 5 hours,
Examples 1 to 3 and Comparative Examples 1 to 4
Negative electrodes were formed. About these evaluation contents
It is as follows. Table 1 shows the evaluation results.
Indicated.

[0038]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).

[0039]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.

[0040]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.

[0041]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.

[0042]Cycle  3. Using the cylindrical battery thus prepared, charge and discharge conditions; 1 or
2. 7V, 1mA / cm^Two And the second discharge volume
Measure the number of cycles when the volume reaches 90% of the volume.
Was.

[0043]

[Table 1]

[0044]

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.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H029 AJ05 AJ11 AK03 AL06 AL07 AM03 AM04 AM05 AM07 BJ02 CJ11 DJ08 EJ14 HJ01 5H050 AA07 AA14 BA17 CA08 CB07 CB08 DA03 DA11 DA18 EA28 GA11 HA01

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. 0.6 to 3.0 parts by weight of α-methylstyrene dimer is based on 100 parts by weight of a total of 1 to 10% by weight of a monomer and 30 to 79% by weight of another monomer copolymerizable therewith. Is a copolymer latex obtained by emulsion polymerization in the presence of, and the amount of α-methylstyrene dimer remaining in the copolymer latex is 400 to 3000 ppm based on the solid content of the copolymer latex.
A binder for a negative electrode of a secondary battery, comprising: 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.