JP2000216065A - Electrode for non-aqueous electrochemical element and manufacture thereof - 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

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vinylidene fluoride polymer-based electrode suitably used for forming a non-aqueous electrochemical device such as a non-aqueous battery or an electric double layer capacitor, particularly an electric double layer capacitor. And its manufacturing method. In particular, it relates to improvement in reliability of these devices.

[0002]

2. Description of the Related Art In recent years, there have been remarkable developments including reduction in size and weight of electronic or electric equipment. Along with this,
There is also a strong demand for large-capacity and miniaturized electrochemical devices such as batteries or electric double-layer capacitors, which are the power sources for these devices. In this respect, non-aqueous electrochemical devices using organic solvent-based electrolytes are extremely suitable. I have. In particular, electric double layer capacitors have a large capacity and long life, and can be filled quickly.
It is easy to charge and discharge, has excellent cycle characteristics compared to secondary batteries, and is inexpensive compared to the most reliable Ni-Cd batteries among secondary batteries. Functional applications are expected in many fields. Further, the application of the electric double-layer capacitor to a small electric power such as a backup power supply of an electronic device and a large electric power field such as an auxiliary power supply of an electric vehicle or a hybrid car is being studied. Along with this, there is a demand for higher performance such as a larger capacity of the polarizable electrode.

[0003] An electric double layer capacitor is a capacitor that utilizes capacitance accumulated in an electric double layer generated at an interface between a polarizable electrode and an electrolyte. Electrolyte 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 capacity capacitors because they have high withstand voltage and can increase the capacity. The polarizable electrode is required to have a large specific surface area and bulk density, be electrochemically inert, and have a low electric resistance. The polarizable electrode structure for an electric double layer capacitor is composed of a powdered activated carbon material, a conductive material for providing electric conductivity, a binder as a binder, and a metal current collector. As a method of manufacturing the electrode structure,
A method in which a mixture of a powdered activated carbon material, a conductive material, and a binder to which a solvent is added to form a mixed slurry is applied or immersed in a current collector and then dried to produce a mixed slurry (for example, Japanese Patent Laid-Open No.
0-64765), powdered activated carbon material, conductive material,
A sheet is obtained by adding a solvent to a mixture of a binder insoluble in a solvent, kneading and forming, drying and joining the obtained sheet to a current collector surface via a conductive adhesive or the like, and then pressing and drying the sheet by heat treatment. Method (for example, see JP-A-9-27504).
No. 1), and considering the cost of the manufacturing process,
In particular, the former method is preferable.

Polyvinylidene fluoride has attracted attention as a binder in terms of electrochemical stability and the like.
No. 5761 discloses a fluorine-containing polymer such as polyvinylidene fluoride and N-methyl 2-pyrrolidone, toluene,
A slurry comprising an organic solvent such as ethyl acetate, dimethyl phthalate, etc., activated carbon powder, and a conductivity imparting agent is coated on the current collector, and then dried to remove the organic solvent to form a polarizable electrode. There is disclosed a method for manufacturing an electric double layer capacitor having the steps of:

[0005] An electrode formed by a similar method, but mainly using a graphite-based or non-graphite-based carbon powder instead of the activated carbon powder, is widely used as a negative electrode of a non-aqueous secondary battery.

[0006]

However, with continued use of an electrochemical device including the electrode formed as described above, particularly an electric double layer capacitor, the device capacity is reduced or the device case is reduced. Problems such as expansion and deformation were often found.

Accordingly, a main object of the present invention is to provide a non-aqueous electrochemical device including an electrode formed by using a vinylidene fluoride-based polymer binder, and to reduce the device capacity, expand the device case, An object of the present invention is to provide a vinylidene fluoride polymer-based electrode suitable for reducing problems such as deformation and forming a reliable device, and a method for producing the same.

According to the study of the present inventors, in order to achieve the above-mentioned object, it is necessary to adopt a method for producing an electrode which effectively reduces the solvent of the vinylidene fluoride polymer used in the production process. It has been found to be very effective.

That is, the method for producing an electrode for a non-aqueous electrochemical device of the present invention comprises the steps of: forming an electrode structure containing a vinylidene fluoride polymer good solvent in addition to a vinylidene fluoride polymer and a powdered carbon material; The method is characterized by being washed with a poor solvent of a vinylidene fluoride-based polymer having a boiling point lower than that of the good solvent and compatible with the good solvent, followed by drying.

The electrode for a non-aqueous electrochemical element of the present invention comprises a vinylidene fluoride-based polymer and a powdered carbon material as a result of the above manufacturing method, and has an amount of vinylidene fluoride detectable by gas chromatography. Residual amount of solvent including the poor solvent of the polymer (when the electrode is composed of a current collector such as a metal and an electrode mixture layer obtained by coating and drying on the current collector, the electrode mixture 10% by weight)
It is characterized by the following.

[0011] The inventors of the present invention have studied for the above-mentioned purpose, and will add to the background of reaching the present invention.

As described above, in nonaqueous electrochemical devices, the causes of inconveniences such as reduction in device capacity and expansion / deformation of the device case are caused by one of the solvents which should be removed from the electrodes after the electrodes are formed. This is due to the fact that parts remain in the electrode and the residual solvent is electrochemically decomposed 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 large pore structure (in fact, depending on the combination of activated carbon powder and an organic solvent, 10 wt. More than a percent may remain). It is understood that these facts were at least within the scope of speculation by some of the relevant engineers who faced the above problems.

Therefore, some attempts have been made to select and 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. I have. However, good solvents for vinylidene fluoride polymers generally have a boiling point of 10
When the temperature exceeds 0 ° C., it is difficult to effectively reduce the residual amount of the solvent in the electrode by selectively using such a good solvent. In particular, the vinylidene fluoride polymer that should form the electrode mixture layer in combination with the powdered carbon material is functionally an inert substance in the electrode, and in order to increase the electrode capacity,
The amount used in the electrode mixture layer should be reduced as much as possible, for example to 10% by weight or less. In order to effectively hold the powdered carbon material with such a small amount of vinylidene fluoride-based polymer and form a robust electrode mixture layer, the vinylidene fluoride-based polymer must be dissolved efficiently. Therefore, the use of a good solvent for the vinylidene fluoride polymer is indispensable in order to be able to hold the powdered carbon material in a uniformly thin film and reticulated state. Reducing the amount is not advisable in terms of forming an electrode with good characteristics. The present inventors have further studied from such a viewpoint, and as a result, the vinylidene fluoride polymer and the powdered carbon material, and a good solvent of the vinylidene fluoride polymer, the vinylidene fluoride polymer, After forming an electrode structure having an electrode mixture layer that holds the powdered carbon material in a well-thinned and reticulated state, it has a relatively low boiling point and is compatible with a good solvent of the vinylidene fluoride polymer. However, by itself, the electrode structure is washed with an organic solvent, which is a poor solvent for the vinylidene fluoride-based polymer, and then dried. Since the organic solvent itself is a poor solvent for the vinylidene fluoride-based polymer, the binding function of the powdered carbon material by the thin film of the vinylidene fluoride-based polymer once formed may be essentially impaired. Absent And finding the door in which the present invention has been completed.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Typical embodiments of the method for producing a non-aqueous electrode of the present invention will be described one by one.

The vinylidene fluoride polymer constituting the electrode binder may be a homopolymer of vinylidene fluoride or a copolymer of vinylidene fluoride of 50% by weight or more and a copolymerizable monomer of 50% by weight or less. A polymer is appropriately selected and used.

The monomers copolymerizable with the vinylidene fluoride monomer include, for example, hydrocarbon monomers such as ethylene and propylene, vinyl fluoride, ethylene trifluoride, ethylene trifluoride chloride, Fluorinated monomers such as fluorinated ethylene, hexafluoropropylene, and fluoroalkyl vinyl ethers; carboxyl group-containing monomers such as monomethyl maleate and monomethyl citrate; or allyl glycidyl ether and glycidyl crotonate; Examples include, but are not necessarily limited to, epoxy group-containing vinyl monomers. Among them, vinylidene fluoride copolymers containing propylene hexafluoride and ethylene trifluoride chloride are preferably used.

The inherent viscosity of the vinylidene fluoride polymer as the binder is not particularly limited and can be selected according to the purpose.
It is more preferably from dl / g to 5.0 dl / g from the viewpoint of electrode adhesion, mechanical strength and the like. 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 obtained by dissolving 4 g of a resin in 1 liter of N, N-dimethylformamide.

In general, the above-mentioned vinylidene fluoride-based polymer is once dissolved in a good solvent to form a binder solution in advance, and then the powdered carbon material is dispersed in the binder solution to form an electrode-forming mixture. Form a slurry.

In the present invention, the good solvent for the vinylidene fluoride polymer is 1% by weight or more, preferably 5% by weight of the vinylidene fluoride polymer at room temperature (25 ° C.).
An organic solvent having a dissolving ability capable of forming a binder solution having the above concentration is mentioned, and preferable examples thereof include N-methyl-2-pyrrolidone (boiling point: 202 ° C.), dimethylformamide (boiling point: 163 ° C.), N, N- Dimethylacetamide (boiling point 165 ° C), N, N-dimethylsulfoxide (boiling point 189 ° C), hexamethylphosphamide (boiling point 235)
C.), tetramethylurea (boiling point: 178 ° C.), triethyl phosphate (boiling point: 215 ° C.), etc., which have been conventionally used for forming a binder solution for forming an electrode of a non-aqueous electrochemical element. These organic solvents may be used alone or as a mixture of two or more, and generally have a vinylidene fluoride polymer concentration of 0.5 to 30% by weight, preferably 2 to 15% by weight.
A degree of binder solution is formed.

The electrode mixture slurry is heated at 40 ° C. if necessary.
The binder solution obtained above kept under the above-mentioned heating, the powdered carbon material and optional additives such as a conductive material to be described later added as needed, a vinylidene fluoride-based It is obtained by mixing such that the amount of the polymer binder is about 0.5 to 15% by weight, more preferably 2 to 10% by weight. However, in operation, the electrode mixture slurry can also be formed by mixing the vinylidene fluoride-based polymer, the organic solvent, the powdered carbon material, and the like at once without preparing a binder solution.

In the case of an electrode mixture for forming a polarizable electrode of an electric double layer capacitor as a preferred embodiment, a powdered carbon material having a specific surface area of 500 to 3000 m ² / g can be preferably used. Specific examples include coconut-based activated carbon, phenol-based activated carbon, petroleum coke-based
Examples include pitch-based activated carbon, polyvinylidene chloride-based activated carbon, and polyacene.

The electrode mixture for preparing the polarizable electrode is composed of the above-mentioned solvent, vinylidene fluoride polymer and powdered carbon material. It is usual to add a conductive material to impart electric conductivity, and 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.
Above all, Ketjen black and acetylene black, which are a kind of carbon black, are preferably used. The amount of the conductive material to be added can be selected according to the required degree of conductivity.

The mixture slurry prepared as described above has good coatability on the current collector. The coating method may be a known method, and among them, the doctor blade method is preferably used. The current collector (electrode structure) coated with the mixture is
If necessary, once dried at 20 to 200 ° C., it is subjected to a washing step with a low boiling point poor solvent according to the present invention.

Examples of the low boiling point poor solvent used as the washing solvent include N-methyl-2-pyrrolidone (boiling point: 202 ° C.), which is a good solvent for the vinylidene fluoride polymer described above, and N, N
The solvent is not particularly limited as long as it is compatible with dimethylformamide (boiling point: 163 ° C.) and the like and is a poor solvent for the vinylidene fluoride polymer having a boiling point lower than that of the good solvent used. More specifically, the above-mentioned washing solvent has a solubility of 0.5% by weight or less, preferably 0.1% by weight or less in a vinylidene fluoride-based polymer at room temperature (25 ° C.), Further, those having compatibility (preferably, infinite solubility) with a good solvent of the vinylidene fluoride polymer used in the electrode structure manufacturing process are preferably used. For example, alcohols include methyl alcohol (boiling point 65 ° C.), isopropyl alcohol (boiling point 82
℃), such as methylene chloride (boiling point 4
0 ° C), 1,1,1-trichloroethane (boiling point 74 ° C)
Examples of the solvent include, but methylene chloride, which is nonflammable and easy to dry, is particularly preferable. When a good solvent is used as the cleaning solvent, the vinylidene fluoride-based polymer as a binder flows out of the electrode structure during the cleaning, and loses the function as a binder, which is not preferable.

The above-mentioned poor solvent for the vinylidene fluoride polymer has not been used at least in forming a binder solution of a vinylidene fluoride polymer for forming an electrode of a non-aqueous electrochemical element.

As a cleaning method, a cleaning effect can be obtained simply by leaving the current collector coated with the dried mixture in a cleaning solvent at a temperature of 5 to 100 ° C., preferably 10 to 50 ° C.
The cleaning effect can be enhanced by continuously moving in the solvent or by applying vibrations such as ultrasonic waves as needed. The washed electrode structure 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 electrochemical element through a pressing step as necessary.

The thus obtained electrode mixture layer in the electrode structure for a non-aqueous electrochemical element of the present invention has a solvent residual amount of 10% by weight or less, preferably 5% by weight or less, particularly preferably In addition to being extremely effectively reduced to 2% by weight or less, as a result of the production method, a poor solvent for the vinylidene fluoride-based polymer which is not found in a normal vinylidene fluoride-based polymer electrode is removed. It is characteristic that it is contained in an amount detectable by gas chromatography.

As the electric double layer capacitor including the electrodes according to the preferred embodiment of the present invention, one having the structure shown in FIG. 1 can be exemplified. 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 interposed between a stainless steel cap 4 and a stainless steel can 5 containing an electrolyte 6 via a packing 7. It is sealed. As a result, the electrolytic solution 6 is impregnated in the separator 3 and disposed between the pair of polarizations 1 and 2. Propylene carbonate is generally used as the solvent of the electrolytic solution, and quaternary phosphonium salts and quaternary ammonium salts are generally used as the electrolyte. For example, (C ₂ H ₅ ) ₄ N
It can be used organic electrolyte such as propylene carbonate solution of BF _4. The concentration of the electrolyte in the electrolyte is 5
It can be appropriately selected in the range of up to 95% by weight.

In the above description, the electric double layer capacitor as a preferred embodiment of the electrochemical device of the present invention, the polarizable electrode contained therein, and the electrode mixture for forming the same have been mainly described. However, the electrode of the present invention uses a graphite-based or non-graphite-based carbon material having a relatively low specific surface area in place of activated carbon powder as a powdered carbon material, so that a negative electrode of a non-aqueous secondary battery such as a lithium ion battery can be used. Also used as

[0030]

The present invention will be described below more specifically with reference to examples and comparative examples.

Example 1 Activated carbon powder (specific surface area: 120
0 m ² / g) and 12 parts by weight of carbon black (“DENKA BLACK” manufactured by Denki Kagaku Kogyo Co., Ltd.) to N-methyl-2-pyrrolidone and polyvinylidene fluoride (“KF # 1700” by Kureha Chemical Industry Co., Ltd.) Made)
Was dissolved in N-methyl-2-pyrrolidone so as to be 8 parts by weight of polyvinylidene fluoride and 331 parts by weight of N-methyl-2-pyrrolidone, and mixed at 30 ° C. to obtain an electrode mixture slurry. .

This electrode mixture is applied on one side of an aluminum foil by a doctor blade method and dried by heating (130 ° C., 30 minutes).
Thus, a sheet electrode material was obtained. This sheet electrode is 25
The substrate was immersed in methylene chloride at 30 ° C. for 30 minutes for washing, and further dried by heating at 130 ° C. for 10 minutes. The sheet electrode after washing and drying is punched into a circular shape having a diameter of 17 mm, and pressed (10 mm).
(2 MPa, 1 minute), and punched out into a circle having a diameter of 15 mm to obtain a total of three disk-shaped electrode materials. Of these, two were used as polarizable electrodes 1 and 2 as shown in FIG. 1, and the other was used as a sample for a residual solvent test. The polarizable electrodes 1 and 2 were housed in a container comprising 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 ((C ₂
H ₅ ) ₄ NBF ₄ in propylene carbonate solution: 1 m
ol / l) and polarizable electrodes 1 and 2 and separator 3
After fully impregnating the polypropylene packing 7
And the ends of the cap 4 and the can 5 were caulked and integrated.

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 described below (60 ° C.). 200 hours is considered to be more than 4 months at 20 ° C). That is, the capacitor is 6
It was kept in a thermostat at 0 ° C., and the current density was 1.6 mA / cm ²
After charging the battery to 2.5 V and maintaining the charged state for 3 hours, the battery was discharged to 0 V at the same current density to determine the amount of energy. Immediately thereafter, the battery was recharged under the same conditions, kept in the charged state for 200 hours, and then discharged under the same conditions to measure the discharge capacity. The capacity was calculated as the energy capacity per unit weight of activated carbon in the electrode (mWh / g). The electrodes after the above test were taken out and visually observed,
No shape change was observed.

Separately, a part of the disk-shaped electrode material obtained above is placed on a thermobalance (“TGA” manufactured by Mettler), and
The temperature was raised to 600 ° C. at a rate of 20 ° C./min in a nitrogen stream. On the way, the weight loss observed at about 40 ° C. to about 350 ° C. was calculated as the amount of residual solvent by weight% based on the electrode mixture excluding the aluminum foil, and was 0.2% by weight.

The results are shown in Table 1 below together with those of the following Examples and Comparative Examples.

Separately, a part of the disc-shaped electrode material was dispersed in carbon disulfide as a solvent, filtered, and the mother liquor was analyzed by gas chromatography ("GC-9A" manufactured by Shimadzu Corporation). 3. N-methyl-2-pyrrolidone in ratio
It was confirmed that 1.5 ppm of methylene chloride was contained with respect to 6 ppm.

Example 2 A polarizable electrode was 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. And electric double layer capacitor,
evaluated.

Comparative Example 1 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 Example 1 was used and methylene chloride was not washed. Fabricated and evaluated.

[0039]

[Table 1]

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.

[0041]

As described above, according to the present invention, a vinylidene fluoride polymer electrode structure once formed using a good solvent of a vinylidene fluoride polymer can be used to further reduce the vinylidene fluoride polymer electrode structure having a lower boiling point. By drying after washing with a poor solvent for the polymer, an electrode for a non-aqueous electrochemical element is provided, in which the amount of residual solvent is significantly reduced as compared with the conventional method, thereby reducing the element capacity caused by the residual solvent. Thus, it is possible to obtain a highly reliable electrochemical device, particularly an electric double layer capacitor, in which problems such as expansion and deformation of the device case, which are observed in the conventional device, are significantly reduced.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of an example of a structure of an electric double layer capacitor including a preferred embodiment of an electrode of the present invention.

[Explanation of symbols]

 1, 2 polar electrodes 3 Separator 4 Cap 5 Can 6 Electrolyte 7 Packing

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4J002 BD141 DA036 FD110 GQ00 GQ02 4J100 AA02Q AA03Q AC23Q AC24P AC25Q AC26Q AC27Q AC30Q AE09Q AE18Q AL36Q BB17Q BC54Q CA01 CA04 DA40 GC35 GD02 JA45

Claims (5)

[Claims] 1. A solvent comprising a vinylidene fluoride-based polymer and a powdered carbon material, the residual amount of which is not more than 10% by weight including a poor solvent for the vinylidene fluoride-based polymer in an amount detectable by gas chromatography. An electrode for a non-aqueous electrochemical element, comprising: 2. The electrode according to claim 1, wherein the residual amount of the solvent is 2% by weight or less. 3. The electrode according to claim 1, which is an electrode for an electric double layer capacitor. 4. An electrode structure containing a vinylidene fluoride polymer and a good solvent of a vinylidene fluoride polymer in addition to a powdered carbon material, the electrode structure being compatible with the good solvent and having a boiling point higher than that of the good solvent. A method for producing an electrode for a non-aqueous electrochemical element, comprising washing with a poor solvent of a vinylidene fluoride polymer having a low boiling point and drying. 5. The production method according to claim 4, wherein the poor solvent for the vinylidene fluoride-based polymer has a boiling point of 100 ° C. or less.