JP4318337B2 - Electrode for non-aqueous electrochemical device and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a reduction in the capacity of an electrochemical element, the expansion of the capacity of the element, a deformation of the element and the like, by a method wherein the solvent residual quantity in a disc-shaped electrode material is set in a weight lighter than a specified weight including a polyvinylidene fluoride polymer and a poor solvent, which consists of a powder carbon material and contains a polyvinylidene fluoride of a quantity detectable by a gas chromatography. SOLUTION: Active carbon powder and a carbon blacks are mixed with an N-methyl-2- pyrrolidone and a polyvinylidene fluoride and an electrode flux slurry is obtained. This electrode flux slurry is applied on the one side of an aluminum foil and is dried by heating to obtain a sheetlike electrode material. This sheetlike electrode material is dipped in a dichloromethane consisting of a poor solvent to clean and is dried by heating. The sheetlike electrode material subsequent to the cleaning and the drying is punched out into a circular form and, after the circular electrode material is pressed, a disc-shaped electrode material is obtained and the disc-shaped electrode material is formed into polarized electrodes 1 and 2. The obtained disc-shaped electrode material contains lower than 10 wt.% of a solvent, preferably 2 wt.% of a solvent, as a residual solvent quantity measured by a thermobalance. At this time, the electrode material contains the solvent in a quantity detectable the dichloromethane by a gas chromatography.

Description

【００３９】
表１によれば本発明の実施例にかかる電極は、比較例に比べ、容量劣化が小さく信頼性が高いことがわかる。
【００４０】
【発明の効果】
上述したように、本発明によれば、一旦フッ化ビニリデン系重合体の良溶媒を用いて形成したフッ化ビニリデン系重合体系電極構造体を、更に低沸点のフッ化ビニリデン系重合体の貧溶媒で洗浄後に乾燥することにより、従来に比べて、著しく溶剤残留量を低減した非水系電気二重層キャパシタ用電極が提供され、これにより残留溶剤により惹き起されていた素子容量の低下、素子ケースの膨張変形などの従来素子に見られた問題が著しく低減した信頼性の高い電気二重層キャパシタ、が得られる。
【図面の簡単な説明】
【図１】 本発明により構成される電気二重層キャパシタの一例の構造の断面図。
【符号の説明】
１、２ 分極性電極
３ セパレータ
４ キャップ
５ 缶
６ 電解液
７ パッキング[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a non-aqueous-based electric double layer Capacity vinylidene polymer system of electrodes is preferably used to configure the data. In particular, it relates to an improvement in the reliability of a non-aqueous electric double layer capacitor .
[0002]
[Prior art]
In recent years, there have been remarkable developments including reductions in size and weight of electronic or electrical equipment. Along with this, there is a strong demand for increasing the capacity and miniaturization of electrochemical elements such as batteries or electric double layer capacitors that serve as power sources for these devices. In this aspect, non-aqueous electrochemical elements that use organic solvent electrolytes Is remarkably suitable. In particular, the electric double layer capacitor has a large capacity, long life, can be rapidly filled, is easily charged / discharged, and has excellent cycle characteristics compared to the secondary battery. Since it has a feature that it is cheaper than a -Cd battery, functional applications are expected in many fields as a new energy device. In addition, electric double layer capacitors are being studied for applications in low power applications such as backup power supplies for electronic devices, and in high power applications such as auxiliary power supplies for electric vehicles and hybrid cars. Along with this, high performance such as a large capacity is also required for polarizable electrodes.
[0003]
An electric double layer capacitor is a capacitor that utilizes a capacity accumulated in an electric double layer generated at an interface between a polarizable electrode and an electrolyte. Electrolytic solutions used for electric double layer capacitors are roughly classified into organic solvent systems and aqueous solution systems. Organic solvent systems are advantageous as high-capacitance capacitors because of their high withstand voltage and large capacity. A polarizable electrode is required to have a large specific surface area and bulk density, be electrochemically inactive, and have a low electrical resistance. A polarizable electrode structure for an electric double layer capacitor is composed of a powdered activated carbon material, a conductive material imparting electrical conductivity, a binder as a binder, and a metal current collector. The electrode structure is manufactured by adding a solvent to a mixture of a powdered activated carbon material, a conductive material, and a binder to form a mixed slurry, which is applied to or immersed in a current collector and dried (for example, a special material). No. 10-64765), powdered activated carbon material, conductive material, a mixture of a binder insoluble in a solvent, a solvent is added, kneaded and dried, and a sheet obtained by conductive bonding to the surface of the current collector There are methods (for example, Japanese Patent Laid-Open No. 9-275041) for producing by pressing and heat treatment drying after joining via an agent, and the former production method is particularly preferred in view of the cost of the production process.
[0004]
Polyvinylidene fluoride has attracted attention as a binder in terms of electrochemical stability. JP-A-8-55761 discloses fluorine-containing polymers such as polyvinylidene fluoride and N-methyl 2-pyrrolidone, toluene, ethyl acetate, dimethyl. A step of coating a current collector with a slurry comprising an organic solvent such as phthalate, activated carbon powder, and a conductivity-imparting agent as necessary, followed by drying to remove the organic solvent to form a polarizable electrode. A method of manufacturing an electric double layer capacitor is disclosed.
[0005]
An electrode formed by a similar method, but using mainly graphite-based or non-graphite-based carbon powder instead of activated carbon powder, is widely used as a negative electrode for non-aqueous secondary batteries.
[0006]
[Problems to be solved by the invention]
However, in the electrochemical element including the electrode formed as described above, particularly the electric double layer capacitor, there is often a problem that the element capacity is reduced or the element case is expanded or deformed with continued use. It was found.
[0007]
Therefore, the main object of the present invention is to reduce the element capacity, expand and deform the element case, etc. under continuous use of the non-aqueous electric double layer capacitor including the electrode formed using the vinylidene fluoride polymer binder. It is an object of the present invention to provide a method for producing an electrode of a vinylidene fluoride polymer system suitable for reducing the above-mentioned problem and constituting a reliable device.
[0008]
According to the study by the present inventors, it is extremely effective to adopt an electrode manufacturing method that effectively reduces the solvent of the vinylidene fluoride polymer used in the manufacturing process in order to achieve the above-mentioned object. It was found that there was.
[0009]
That is, the method for producing an electrode for a non-aqueous electric double layer capacitor according to the present invention comprises an electrode assembly comprising 0.5 to 10 % by weight of a vinylidene fluoride polymer and an activated carbon powder having a specific surface area of 500 to 3000 m ² / g. An electrode structure including an electrode mixture layer containing a good solvent for vinylidene fluoride polymer in addition to the agent has a boiling point of 100 ° C. or lower that is compatible with the good solvent and lower than the boiling point of the good solvent A non-aqueous electrical system having a solvent residual amount of 2% by weight or less measured by a thermobalance (TGA) method by washing with a poor solvent of a vinylidene fluoride polymer selected from alcohols and chlorinated hydrocarbons and then drying. An electrode for a double layer capacitor is manufactured .
[0010]
The background that the present inventors have studied for the above-mentioned purpose and arrived at the present invention will be added.
[0011]
In the non-aqueous electrochemical element as described above, the cause of the inconvenience such as reduction in element capacity and expansion / deformation of the element case is that part of the solvent that should be removed from the electrode after electrode formation is This is caused by electrochemical decomposition of the residual solvent during continuous use of the device. In particular, the above problem can be more apparent in an electric double layer capacitor using activated carbon powder having a large specific surface area and a rich pore structure (in fact, 10 wt% in the electrode depending on the combination of activated carbon powder and an organic solvent). An amount exceeding percent can remain). It is understood that these were at least within the scope of speculation by some of the related engineers who faced the above problem.
[0012]
Therefore, some attempts have been made to selectively use an organic solvent having a relatively low boiling point and a good solubility in a vinylidene fluoride polymer in order to efficiently remove the solvent after forming the electrode. However, a good solvent for a vinylidene fluoride polymer generally has a boiling point exceeding 100 ° C., and it is difficult to effectively reduce the residual amount of the solvent in the electrode by selecting and using such a good solvent. In particular, a vinylidene fluoride polymer that should form an electrode mixture layer in combination with a powdered carbon material is an inactive substance in the electrode functionally, and in order to increase the electrode capacity, the electrode mixture The amount used in the layer should be reduced as much as possible, for example 10% by weight or less. In order to effectively hold the powdered carbon material with such a small amount of vinylidene fluoride polymer and form a robust electrode mixture layer, the vinylidene fluoride polymer is efficiently dissolved. Therefore, in order to be able to hold the powdered carbon material uniformly in a thin film and in a reticulated state, it is indispensable to use a good solvent for the vinylidene fluoride polymer, and at the expense of its dissolving ability, Reducing the amount is not a good idea in terms of forming electrodes with good characteristics. As a result of further researches from such a viewpoint, the present inventors have found that a vinylidene fluoride polymer and a powdered carbon material , in particular, activated carbon powder, and a good solvent for the vinylidene fluoride polymer, vinylidene fluoride. What is a good solvent for the vinylidene fluoride polymer at a relatively low boiling point after forming an electrode structure having an electrode mixture layer that retains the powdered carbon material in a state where the polymer is well thinned and reticulated? Residual amount of solvent in the electrode mixture layer formed by adopting the process combination of washing the electrode structure with an organic solvent which is a poor solvent for vinylidene fluoride polymer and then drying it. Since the organic solvent itself is a poor solvent for the vinylidene fluoride polymer, the binding function of the powdered carbon material by the thin film of the vinylidene fluoride polymer once formed is essentially Spoiled Is it and completed the knowledge to the present invention that there is no.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
A typical embodiment of the method for producing a non-aqueous electrode of the present invention will be described sequentially.
[0014]
Examples of the vinylidene fluoride polymer constituting the electrode binder include a homopolymer of vinylidene fluoride or a copolymer of 50 weight percent or more of vinylidene fluoride and 50 weight percent or less of a monomer copolymerizable therewith. It is appropriately selected and used.
[0015]
Examples of the monomer copolymerizable with the vinylidene fluoride monomer include hydrocarbon monomers such as ethylene and propylene, vinyl fluoride, ethylene trifluoride, ethylene trifluoride, tetrafluoroethylene. , Fluorine-containing monomers such as propylene hexafluoride and fluoroalkyl vinyl ether, carboxyl group-containing monomers such as monomethyl maleate and monomethyl citraconic acid, or epoxy groups such as allyl glycidyl ether and crotonic acid glycidyl ester Examples of the vinyl monomer include, but are not necessarily limited to these. Of these, a vinylidene fluoride copolymer containing propylene hexafluoride or ethylene trifluoride chloride is preferably used.
[0016]
The inherent viscosity of the vinylidene fluoride polymer as a binder is not particularly limited and can be selected according to the purpose. However, the inherent viscosity is 0.5 dl / g to 5.0 dl / g. From the viewpoints of properties, mechanical strength, etc. Here, the inherent viscosity is used as a measure of the molecular weight of the polymer, and refers to the logarithmic viscosity at 30 ° C. of a solution in which 4 g of resin is dissolved in 1 liter of N, N-dimethylformamide.
[0017]
Generally, a vinylidene fluoride polymer as described above is once dissolved in a good solvent to form a binder solution in advance, and then a powder carbon material is dispersed in the binder solution to form a mixture slurry for electrode formation. To do.
[0018]
In the present invention, the good solvent for the vinylidene fluoride polymer can form a binder solution having a concentration of 1% by weight or more, preferably 5% by weight or more of the vinylidene fluoride polymer at room temperature (25 ° C.). Examples of the organic solvent having a dissolving ability include N-methyl-2-pyrrolidone (boiling point 202 ° C.) dimethylformamide (boiling point 163 ° C.), N, N-dimethylacetamide (boiling point 165 ° C.), N, N-dimethyl sulfoxide (boiling point 189 ° C.), hexamethylphosphoamide (boiling point 235 ° C.), tetramethylurea (boiling point 178 ° C.), triethyl phosphate (boiling point 215 ° C.), etc. What was used for formation of this is mentioned. These organic solvents may be used alone or in combination of two or more to form a binder solution having a vinylidene fluoride polymer concentration of generally 0.5 to 30% by weight, preferably about 2 to 15% by weight.
[0019]
The electrode mixture slurry is an optional additive such as a binder solution obtained as described above held under heating at 40 ° C. or higher as necessary, a powdered carbon material, and a conductive material added later if necessary. Are mixed so that the amount of the vinylidene fluoride polymer binder in the slurry solid content is about 0.5 to 10 % by weight, more preferably 2 to 10 % by weight. However, in terms of operation, an electrode mixture slurry can be formed by mixing a vinylidene fluoride polymer, an organic solvent, a powdered carbon material, and the like all at once without preparing a binder solution.
[0020]
In the case of an electrode mixture for forming a polarizable electrode for an electric double layer capacitor, the activated carbon powder having a specific surface area of 500 to 3000 m ² / g can be suitably used. activated carbon, phenol-based activated carbon, petroleum coke, pitch-based activated carbon, polyvinylidene chloride-based active carbon, and the like.
[0021]
An electrode mixture for producing a polarizable electrode is composed of the above-mentioned solvent, a vinylidene fluoride polymer, and activated carbon powder . In order to impart electrical conductivity, it is usual to add a conductive material. Specific examples of the conductive material include carbon black, natural graphite, artificial graphite, metal oxides such as titanium oxide and ruthenium oxide, and metal fibers. Can be used. Among them, ketjen black and acetylene black which are a kind of carbon black are preferably used. The amount of the conductive material added can be selected according to the required degree of conductivity.
[0022]
The mixture slurry prepared as described above has good coatability to the current collector. The application method may be a known method, among which the doctor blade method is preferably used. The current collector (electrode structure) coated with the mixture is once dried at 20 to 200 ° C., if necessary, and then subjected to a washing step with a low-boiling poor solvent according to the present invention.
[0023]
Examples of the low-boiling poor solvent used as the cleaning solvent include N-methyl-2-pyrrolidone (boiling point 202 ° C.) and N, N-dimethylformamide (boiling point 163 ° C.), which are good solvents for the above-mentioned vinylidene fluoride polymer. It is not particularly limited as long as it is a poor solvent for the vinylidene fluoride polymer having compatibility and having a boiling point lower than that of the good solvent used. More specifically, as the above-mentioned washing solvent, at room temperature (25 ° C.), it exhibits a solubility of not more than 0.5% by weight, preferably not more than 0.1% by weight, based on the vinylidene fluoride polymer, And what has compatibility (preferably infinite solubility) with the good solvent of the vinylidene fluoride polymer used at an electrode structure manufacturing process is used preferably. For example, alcohols include methyl alcohol (boiling point 65 ° C.), isopropyl alcohol (boiling point 82 ° C.), and chlorinated hydrocarbons include methylene chloride (boiling point 40 ° C.), 1,1,1-trichloroethane (boiling point 74 ° C.), etc. Examples of these solvents are methylene chloride, which is nonflammable and easy to dry. When a good solvent is used as the cleaning solvent, the vinylidene fluoride polymer as a binder flows out of the electrode structure during cleaning, and the function as the binder is lost.
[0024]
The poor solvent for the vinylidene fluoride polymer as described above has not been used in the past to form at least a vinylidene fluoride polymer binder solution for electrode formation of a non-aqueous electrochemical element.
[0025]
As a cleaning method, there is a cleaning effect just by leaving a current collector coated with a mixture after drying in a cleaning solvent at a temperature of 5 to 100 ° C., preferably 10 to 50 ° C., but if necessary, continuous In particular, the cleaning effect can be enhanced by moving through the solvent or applying vibrations such as ultrasonic waves. The electrode structure after cleaning is heat-treated at 40 to 170 ° C. for drying. The electrode structure after drying is provided as an electrode structure for a non-aqueous electric double layer capacitor through a pressing process as necessary.
[0026]
The electrode mixture layer in the electrode structure for a non-aqueous electric double layer capacitor of the present invention thus obtained has a residual solvent amount of 2 % by weight or less and is extremely effectively reduced. As a result of the production method, it is characteristic that a poor solvent for a vinylidene fluoride polymer not found in an ordinary vinylidene fluoride polymer electrode is contained in an amount detectable by gas chromatography.
[0027]
The electric double layer capacitor composed of the onset bright, can be exemplified by the structure shown in FIG. That is, FIG. 1 is a cross-sectional view of an example of a single cell electric double layer capacitor. In this electric double layer capacitor, a separator 3 is sandwiched between two polarizable electrodes 1 and 2, and these are further sandwiched between a stainless steel cap 4 and a stainless steel can 5 containing an electrolyte 6 through a packing 7. Is enclosed. As a result, the electrolytic solution 6 is impregnated in the separator 3 and disposed between the pair of polarizable electrodes 1 and 2. Propylene carbonate is generally used as the solvent for the electrolytic solution, and quaternary phosphonium salts and quaternary ammonium salts are commonly used as the electrolyte. For example, a propylene carbonate solution of (C ₂ H ₅ ) ₄ NBF ₄ The organic electrolyte solution can be used. The concentration of the electrolyte in the electrolytic solution can be appropriately selected within a range of 5 to 95% by weight.
[0028]
In the above, the electric double layer capacitor of the present invention, the polarizable electrode contained therein, and the electrode mixture for forming the electrode have been mainly described .
[0029]
【Example】
Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples.
[0030]
Example 1
To 80 parts by weight of activated carbon powder (specific surface area 1200 m ² / g) and 12 parts by weight of carbon black (“DENKA BLACK” manufactured by Denki Kagaku Kogyo Co., Ltd.), N-methyl-2-pyrrolidone and polyvinylidene fluoride (“KF # 1700”) Kureha Chemical Industry Co., Ltd.) dissolved in N-methyl-2-pyrrolidone was added to 8 parts by weight of polyvinylidene fluoride and 331 parts by weight of N-methyl-2-pyrrolidone and mixed at 30 ° C. An electrode mixture slurry was obtained.
[0031]
This electrode mixture was applied onto one surface of an aluminum foil by a doctor blade method and dried by heating (130 ° C., 30 minutes) to obtain a sheet-like electrode material. The sheet-like electrode was washed by being immersed in methylene chloride at 25 ° C. for 30 minutes, and further dried by heating at 130 ° C. for 10 minutes. The sheet-like electrode after washing and drying was punched into a circle having a diameter of 17 mm, pressed (102 MPa, 1 minute), and then punched into a circle having a diameter of 15 mm to obtain a total of three disc-shaped electrode materials. Of these, two were used as polarizable electrodes 1 and 2 as shown in FIG. 1, and the remaining one was used as a residual solvent test sample. The polarizable electrodes 1 and 2 were accommodated in a container composed of a stainless steel cap 4 and a stainless steel can 5 with a glass fiber nonwoven fabric separator 3 interposed therebetween. Next, a predetermined electrolytic solution 6 ((C2H5) 4NBF4 propylene carbonate solution: 1 mol / l) is placed in a stainless steel container, and the polarizable electrodes 1 and 2 and the separator 3 are sufficiently impregnated, and then passed through a polypropylene packing 7. The ends of the cap 4 and the can 5 were caulked and integrated.
[0032]
The reliability of the electric double layer capacitor constructed as shown in FIG. 1 was evaluated by performing an acceleration test in a constant temperature bath at 60 ° C. as follows (200 hours at 60 ° C. Is considered to be equivalent to about 4 months at 20 ° C). That is, the capacitor is held in a constant temperature bath at 60 ° C., charged to 2.5 V at a current density of 1.6 mA / cm ² , held in a charged state for 3 hours, and then discharged to 0 V at the same current density. The amount of energy was determined and this value was taken as the initial energy capacity. Immediately after that, the battery was recharged under the same conditions, held in a charged state for 200 hours, discharged under the same conditions, and the discharge capacity was measured. The capacity was calculated as the energy capacity (mWh / g) per unit weight of activated carbon in the electrode. The electrode after the test was taken out and visually observed, but no change in shape was observed.
[0033]
Separately, a part of the disk-shaped electrode material obtained above was placed on a thermobalance (“TGA” manufactured by Mettler) and heated to 600 ° C. at 20 ° C./min in a nitrogen stream. On the way, the reduced weight observed at about 40 ° C. to about 350 ° C. was calculated as a residual solvent amount, which was 0.2% by weight based on the weight of the electrode mixture excluding the aluminum foil.
[0034]
The results are shown in Table 1 below together with those of the following examples and comparative examples.
[0035]
Separately, a part of the disk-shaped electrode material was dispersed in a solvent carbon disulfide, and after filtration, the mother liquor was analyzed by gas chromatography (“GC-9A” manufactured by Shimadzu Corporation). -It was confirmed that 1.5 ppm of methylene chloride was contained with respect to 4.6 ppm of methyl-2-pyrrolidone.
[0036]
(Example 2)
A polarizable electrode and an electric double layer capacitor were prepared in the same manner as in Example 1 except that the mixture slurry obtained in the same manner as in Example 1 was used and the washing solvent was changed from methylene chloride to methyl alcohol. ,evaluated.
[0037]
(Comparative Example 1)
A polarizable electrode and an electric double layer capacitor were prepared and evaluated in the same manner as in Example 1 except that the mixture slurry obtained in the same manner as in Example 1 was used and methylene chloride was not washed.
[0038]
[Table 1]

[0039]
According to Table 1, it can be seen that the electrode according to the example of the present invention has less capacity deterioration and higher reliability than the comparative example.
[0040]
【The invention's effect】
As described above, according to the present invention, a vinylidene fluoride polymer-based electrode structure once formed using a good solvent of a vinylidene fluoride polymer is a poor solvent for a vinylidene fluoride polymer having a lower boiling point. By drying after washing, a non-aqueous electric double layer capacitor electrode is provided in which the residual amount of solvent is remarkably reduced as compared with the prior art, thereby reducing the device capacity caused by the residual solvent, expansion deformation problems seen in the conventional device, such as a significantly reduced high reliability have electric double layer capacitors, is obtained.
[Brief description of the drawings]
Cross-sectional view of an example of the structure of the electric double layer capacitor constituted by the present invention; FIG.
[Explanation of symbols]
1, 2 Polarized electrode 3 Separator 4 Cap 5 Can 6 Electrolyte 7 Packing

Claims (2)

In addition to an electrode mixture containing 0.5 to 10 % by weight of a vinylidene fluoride polymer and an activated carbon powder having a specific surface area of 500 to 3000 m ² / g, an electrode mixture containing a good solvent for the vinylidene fluoride polymer An electrode structure including a layer is formed of a vinylidene fluoride polymer selected from alcohols having a boiling point of 100 ° C. or lower that is compatible with the good solvent and lower than the boiling point of the good solvent , and a chlorinated hydrocarbon . A method for producing an electrode for a non-aqueous electric double layer capacitor having a solvent residual amount of 2% by weight or less measured by a thermobalance (TGA) method, wherein the electrode is washed with a poor solvent and then dried. The production method according to claim 1, wherein the good solvent is N-methyl-2-pyrrolidone and the poor solvent is methylene chloride.

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