JP5301753B2 - Secondary battery negative electrode binder and secondary battery electrode composition - Google Patents

Description

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

  Lithium ion secondary batteries using conductive carbonaceous materials that occlude and release lithium ions as electrodes are becoming more important as power sources for small electronic devices because of their light weight and high energy density. In an electrode mainly composed of a conductive carbonaceous material that absorbs and releases lithium ions, a polymer binder is usually used as a binder. The polymer binder is required to have adhesiveness with an active material, resistance to a polar solvent used as an electrolytic solution, and stability in an electrochemical environment. Conventionally, fluorine-based polymers such as polyvinylidene fluoride have been used in this field. However, when the electrode film is formed, the conductivity is hindered, and the adhesive strength between the current collector and the electrode film is insufficient. There is a problem. In addition, when a fluorine-based polymer is used for the negative electrode, which is a reducing condition, there are problems such as insufficient stability and a decrease in cycle performance of the secondary battery, and improvement of these problems is desired. . For this reason, development of non-fluorine polymers has been carried out. For example, Japanese Patent Laid-Open No. 63-121257 discloses an acrylonitrile-based polymer, and Japanese Patent Laid-Open No. 1-186557 discloses a polyester-based polymer. However, this is not sufficient for solving the above problems. Problems such as elution and swelling in the electrolytic solution were also derived, and problems with battery performance such as cycle performance and manufacturing suitability were large.

Problems to be solved by the invention

The problem of the present invention is that the adherence to the current collector is good and the electrolytic solution resistance is excellent. In particular, when used in a lithium ion secondary battery using a conductive carbonaceous material as a negative electrode active material, the cycle efficiency is excellent. It is providing the binder for secondary battery negative electrodes which can obtain a battery.

Means for solving the problem

Means for Solving the Problems As a result of diligent studies to solve the above problems, the above-described use of a specific copolymer latex as a binder for a negative electrode of a secondary battery using a conductive carbonaceous material as a negative electrode active material results in the above. The inventors have found that the problems can be solved and have completed the present invention. That is, the present invention is a secondary battery negative electrode binder using a conductive carbonaceous material as a negative electrode active material, the binder comprising an aliphatic conjugated diene monomer 20 to 60% by weight, an ethylenically unsaturated carboxylic acid 2,4-diphenyl-4-methyl-1-pentene with respect to 100 parts by weight of the total monomer comprising 1 to 10% by weight of monomer and 30 to 79% by weight of other monomers copolymerizable therewith Is a copolymer latex obtained by emulsion polymerization in the presence of 0.6 to 1.5 parts by weight of α-methylstyrene dimer having a content of 80% by weight or more, and in the copolymer latex A secondary battery negative electrode binder is provided, wherein the amount of 2,4-diphenyl-4-methyl-1-pentene remaining is 400 to 2000 ppm based on the solid content of the copolymer latex. With things That.

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 other monomers copolymerizable therewith.

Examples of the aliphatic conjugated diene monomer include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, Examples thereof include chain conjugated pentadienes, substituted and side chain conjugated hexadienes and the like, and one kind or two or more kinds can be used. 1,3-butadiene is particularly preferable.

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. Can be used.

Other monomers copolymerizable with these include aromatic vinyl monomers, vinyl cyanide monomers, unsaturated carboxylic acid alkyl ester monomers, and unsaturated monomers containing hydroxyalkyl groups. Body, unsaturated carboxylic acid amide monomer, etc., and these can be used alone or in combination.

Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, methyl α-methylstyrene, vinyltoluene, divinylbenzene, and the like, and one or more can be used. Styrene is particularly preferable.

Examples of the vinyl cyanide monomer include acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, α-ethylacrylonitrile and the like, and one or more can be used. In particular, acrylonitrile and methacrylonitrile are preferable.

Examples of 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 itaconate, Monomethyl fumarate, monoethyl fumarate, 2-ethylhexyl acrylate and the like can be mentioned, and one or more can be used. Particularly preferred is methyl methacrylate.

Examples of unsaturated monomers containing a hydroxyalkyl group include β-hydroxyethyl acrylate, β-hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, and 3-chloro-2-hydroxypropyl. Methacrylate, di- (ethylene glycol) maleate, di- (ethylene glycol) itaconate, 2-hydroxyethyl maleate, bis (2-hydroxyethyl) maleate, 2-hydroxyethyl methyl fumarate, etc. More than one species can be used. In particular, β-hydroxyethyl acrylate is preferable.

Examples of the unsaturated carboxylic acid amide monomer include acrylamide, methacrylamide, N-methylol acrylamide, N-methylol methacrylamide, N, N-dimethylacrylamide, and the like, and one or more can be used. Particularly preferred is acrylamide.

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, vinylidene chloride can be used.

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

When 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 the current collector, and 60% by weight. If the ratio exceeds 50%, the electrode composition is applied to a current collector to produce a battery negative electrode, which causes a problem that the resistance to electrolytic solution is lowered, which is not preferable. Preferably it is 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 inferior, and if it exceeds 10% by weight, the viscosity of the latex increases, This is not preferable because it may cause a problem in handling the combined latex itself. Preferably it is 1 to 7 weight%.

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

The α-methylstyrene dimer used in the present invention includes 2,4-diphenyl-4-methyl-1-pentene, 2,4-diphenyl-4-methyl-2-pentene and 1,1,3 as isomers. -Trimethyl-3-phenylindane, but the α-methylstyrene dimer used in the present invention has a 2,4-diphenyl-4-methyl-1-pentene content of 60% by weight or more, particularly 80 It is preferable that it is weight% or more.

Further, the α-methylstyrene dimer to be present in the emulsion polymerization of the monomer is 0.6 to 3.0 parts by weight with respect to 100 parts by weight of all the monomers used. If the amount is less than 0.6 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 form a battery negative electrode. Since the problem that an electrolyte solution resistance falls at the time of manufacture is seen, it is not preferable.
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. When the residual amount of α-methylstyrene dimer is 400 ppm or less, sufficient adhesion to the conductive carbonaceous material cannot be obtained, and when the residual amount is 3000 ppm or more, the electrode composition of the present invention is applied to a current collector to form a battery. There is a problem that the electrolytic solution resistance is lowered when the negative electrode is produced.
Regarding the residual α-methylstyrene dimer, the amount used during the polymerization of the copolymer latex and the addition method thereof, the monomer composition of the copolymer latex and the addition method thereof, 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 agent can be used together with α-methylstyrene dimer.
These chain transfer agents include alkyl mercaptans such as n-hexyl mercaptan, n-octyl mercaptan, t-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, n-stearyl mercaptan, dimethylxanthogen disulfide, diisopropylxanthogendi Xanthogen compounds such as sulfide, terpinolene, thiuram compounds such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetramethylthiuram monosulfide, phenols such as 2,6-di-t-butyl-4-methylphenol, styrenated phenol Compounds, allyl compounds such as allyl alcohol, halogenated hydrocarbon compounds such as dichloromethane, dibromomethane, and carbon tetrabromide, α-benzyloxys Vinyl ethers such as len, α-benzyloxyacrylonitrile, α-benzyloxyacrylamide, triphenylethane, pentaphenylethane, acrolein, methacrolein, thioglycolic acid, thiomalic acid, 2-ethylhexylthioglycolate, etc. 1 type (s) or 2 or more types can be used.
The amount of these chain transfer agents is not particularly limited, but is usually 0 to 5 parts by weight with respect to 100 parts by weight of the monomer.

In the present invention, a normal emulsifier is used when the copolymer latex is obtained by emulsion polymerization. As an emulsifier, anionic surfactants such as sulfate esters of higher alcohols, alkylbenzene sulfonates, alkyl diphenyl ether sulfonates, aliphatic sulfonates, aliphatic carboxylates, sulfate esters of nonionic surfactants, etc. Alternatively, nonionic surfactants such as polyethylene glycol alkyl ester type, alkyl phenyl ether type, alkyl ether type and the like may be used alone or in combination.
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 initiator.

In the polymerization of the copolymer latex, the method for adding the monomer and other components is not particularly limited, and any of a batch addition method, a divided addition method, and a continuous addition method can be employed. In the present invention, any one 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 nm, and 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, linear graphite hybrid. , Coke, pyrolytic gas-layer-grown carbon, furfuryl alcohol resin calcined carbon, polyacene organic semiconductor, mesocarbon microbeads, mesophase pitch carbon, graphite whiskers, pseudo isotropic carbon, natural carbon fired body, and These pulverized materials can be used, and one kind or a mixture of two or more kinds can be used.

The copolymer latex in the present invention is used as a secondary battery negative electrode binder having a conductive carbonaceous material as a negative electrode active material, and particles of conductive carbon material that is a negative electrode active material, and It acts as a binder between the conductive carbonaceous material and the current collector. In this case, the copolymer latex is used for an electrode by containing 0.1 to 10 parts by weight, preferably 0.5 to 5 parts by weight as a solid content with respect to 100 parts by weight of the conductive carbonaceous material. It can be prepared as a composition. If the amount of the copolymer latex of the present invention is less than 0.1 parts by weight, good adhesion to a current collector or the like cannot be obtained, and if it exceeds 10 parts by weight, the overvoltage significantly increases when assembled as a battery. It 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 water-soluble thickeners such as carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch, casein, hexametaphosphate soda, tripolyphosphate soda, pyrophosphate soda , Dispersants such as sodium polyacrylate, and nonionic and anionic surfactants as latex stabilizers.

The electrode composition of the present invention is applied to a current collector, dried and used as a negative electrode of a lithium ion secondary battery. Further, as a method for applying the electrode composition of the present invention to a current collector, any coater head such as a reverse roll method, a comma bar method, a gravure method, an air knife method can be used, and a drying method is allowed to stand, A blower dryer, a hot 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, a non-aqueous electrolyte, a terminal, used when producing a lithium ion secondary battery using a negative electrode made using the electrode composition of the present invention, Existing insulators, battery containers, etc. can be used without any particular limitation. The copolymer latex of the present invention can also be used as a positive electrode binder.

EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated more concretely, this invention is not limited to these Examples, unless the summary is changed. In the examples, parts and percentages indicating percentages are based on weight. In addition, various physical properties in the examples were evaluated by 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.

Measurement of Residual α-Methylstyrene Dimer in Copolymer Latex Residual α-methylstyrene dimer in the copolymer latex was performed by the following method using a gas chromatograph GC-14A manufactured by Shimadzu Corporation.
(1) Preparation of sample 1 g of copolymer latex was precisely weighed, dissolved in 25 ml of N, N-dimethylformamide, allowed to stand for 24 hours, and used as a measurement sample.
(2) Gas chromatograph measurement conditions Sample volume: 5 μl
Detector: FID
Inj / Det temperature: 190 ° C
Column temperature: 180 ° C
Column: Glass column 1.6 m × 2.6 mmφ
Carrier: Chromosorb WADM DMCS
(Granularity 80/100)
Liquid phase: Silicone OV-17
Carrier gas: nitrogen, 25 ml / min.
Hydrogen: 0.7kg / cm2
Air: 0.9kg / cm2
(3) Quantitative method Using a calibration curve prepared using 2,4-diphenyl-4-methyl-1-pentene having a known concentration, 2,4-diphenyl relative to the copolymer latex solids was obtained from the obtained gas chromatograph. The quantitative value of -4-methyl-1-pentene was determined as residual α-methylstyrene dimer. The same sample was measured three times, and the average value is shown in Table 1 in ppm.

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

Preparation of copolymer latex A pressure resistant polymerization reactor was charged with 125 parts of pure water, 0.8 part of sodium dodecylbenzenesulfonate, and 1.2 parts of potassium persulfate, and after sufficient stirring, the first stage shown in Table 1 was prepared. Each monomer of the eye, 1.4 parts of α-methylstyrene dimer and 0.2 part of t-dodecyl mercaptan were added and polymerization was started at 75 ° C. After 1.5 hours, the polymerization temperature was lowered to 73 ° C., and each monomer in the second stage shown in Table 1, 0.5 part of potassium persulfate and 10 parts of pure water were continuously added over 2 hours. Each monomer in the third stage shown in Table 1 and 5 parts of pure water were continuously added over 4 hours. The temperature was further raised to 75 ° C. and maintained for 5 hours, and then the polymerization was terminated.
Next, after adjusting the pH of the copolymer latex to about 7 with an aqueous caustic soda solution, unreacted monomers and other low-boiling compounds were removed by steam distillation to obtain a copolymer latex (a).
Moreover, copolymer latex (b)-(g) was obtained by the method similar to copolymer latex (a) except changing the conditions shown in Table 1. The copolymer latexes (a), (b) and (g) were prepared by three-stage polymerization, and the copolymer latexes (c), (d), (e) and (f) were prepared by two-stage polymerization. .

Preparation of composition for battery electrode As the conductive carbonaceous material, artificial graphite having a particle diameter of 1 to 35 nm is used, artificial graphite is used as 100 parts by weight, and carboxymethyl cellulose as a slurry thickener is used as 2 parts by weight. An appropriate amount of water was added and kneaded with 5 parts of copolymer latex (a) so that the total solid content was 50% to prepare a battery electrode composition of Example 1 shown in Table 1. Similarly, using the copolymer latexes (b) to (c) described in Table 1, the battery electrode compositions of Examples 2 to 3 and the copolymer latexes (d) to (g) were used. Thus, compositions for battery electrodes of Comparative Examples 1 to 4 were prepared.

[Examples 1 to 3, Comparative Examples 1 to 4]
Each battery electrode composition was applied to both sides of a 20 μm thick copper foil serving as a current collector, dried at 120 ° C. for 5 hours, and compression-molded by hot pressing to Examples 1 to 3 and Comparative Example 1. Each of -4 negative electrodes was prepared. The contents of these evaluations are as follows. The evaluation results are shown in Table 1.

Binding property Using a knife on the surface of the negative electrode, six notches from the active material layer to the depth reaching the current collector were cut at 2 mm intervals in the vertical and horizontal directions to make a grid cut. An adhesive tape was affixed to the cut and immediately peeled off, and the degree of dropout of the active material was evaluated from 5 points (no dropout) to 1 point (complete dropout) by visual judgment.

Electrolytic solution resistance A negative electrode having grid cuts in the same manner as described above was prepared by mixing 1 mol / l of lithium hexafluorophosphate with a mixed solvent of ethylene carbonate and 1,2-dimethoxyethane having a volume ratio of 1: 1. The sample was soaked in a 40 ° C electrolyte solution that was dissolved and adjusted for 12 hours, and the negative electrode surface state was evaluated from 5 points (no change) to 1 point (completely dropped) by visual judgment.

100 parts of LiCoO 2 as create <br/> positive electrode active material of the positive electrode, 5 parts of acetylene black as a conductive agent, polyvinylidene fluoride 6 parts as a binder were kneaded in a 2-methylpyrrolidone a positive active material slurry It was adjusted. The obtained slurry was applied as a current collector on both sides of an aluminum foil having a thickness of 20 μm, dried at room temperature, and compression molded by hot pressing to prepare a positive electrode.

Battery Manufacturing Method After a lead plate was welded to each end of the positive electrode and the negative electrode, the positive electrode, a porous polypropylene separator having a thickness of 30 μm, a negative electrode, and a separator were laminated in this order and wound into a spiral to obtain a wound body. The wound body is housed in a cylindrical battery can that also serves as a negative electrode terminal, the battery can and the negative electrode, the positive electrode terminal and the positive electrode are connected by a lead terminal, and ethylene carbonate having a capacity ratio of 1: 1 in the battery can and 1 , 2-dimethoxyethane in a mixed solvent of lithium hexafluorophosphate dissolved in 1 mol / l and filled with an adjusted electrolyte, and then the battery lid provided with a positive electrode terminal is caulked through a gasket to form a cylindrical battery It was created.

Cycle property Using the produced cylindrical battery, charge and discharge conditions, 4. 1 to 2. The number of cycles when the capacity reached 90% of the second discharge capacity was set to 7V, 1 mA / cm ² .

Effect of the invention

  By using the secondary battery negative electrode binder in the present invention, a secondary battery such as a lithium ion secondary battery having good adhesion to the current collector, excellent electrolyte resistance, and excellent cycle efficiency is obtained. It is something that can be done.

Claims (2)

A binder for a secondary battery negative electrode using a conductive carbonaceous material as a negative electrode active material, the binder comprising 20 to 60% by weight of an aliphatic conjugated diene monomer, 1 to 1 of an ethylenically unsaturated carboxylic acid monomer The content of 2,4-diphenyl-4-methyl-1-pentene is 80 with respect to 100 parts by weight of the total of 10% by weight and 30 to 79% by weight of other monomers copolymerizable therewith. A copolymer latex obtained by emulsion polymerization in the presence of 0.6 to 1.5 parts by weight of α-methylstyrene dimer that is not less than wt%, and remains in the copolymer latex, 2, The binder for secondary battery negative electrodes, wherein the amount of 4-diphenyl-4-methyl-1-pentene is 400 to 2000 ppm based on the solid content of the copolymer latex.
The composition for secondary battery electrodes formed by containing 0.1 to 10 parts by weight of the binder for secondary battery negative electrode according to claim 1 with respect to 100 parts by weight of the conductive carbonaceous material.