Patent Application
20080011986
The present invention will be described by way of the following examples. However, the present invention is not limited thereto. In the examples, “part(s)” and “%” represent “part(s) by weight” and “% by weight”, respectively, unless otherwise specified.
The following were charged into a reaction vessel with a stirrer; 100 parts of a monomer mixture having a composition shown in Table 1, 0.7 part of sodium dodecylbenzenesulfonate, 250 parts of ion-exchanged water, and 1.5 parts of ammonium persulfate. The solution was sufficiently stirred. Thereafter, the given components were polymerized at 80° C. for 5 hours to yield a latex having a solid content of about 30%. The polymerization conversion degree thereof was 95%, and the composition ratio of the polymer was consistent with the ratio between the charged monomers. Ammonia water was added to this latex to set the pH to 7. Thereafter, the latex was concentrated under reduced pressure to remove the remaining monomers, thereby yielding a binder composition as a latex (a binder dispersion in water) having a solid content of 40%. The amount of alkali metal ions measured by inductively coupled plasma emission spectrometry (ICP) was 0.1% by weight of the copolymer (the ratio of the weight of the copolymer being 100%).
Next, 100 parts of high-purity activated carbon powder (specific surface area: 2000 m2/g, and average particle diameter: 8 μm) as an active material for an electrode, and 1.5 parts of KETJENBLACK and 3 parts of acetylene black as electroconductivity additive were mixed, and the mixture was added to 12.5 parts of the binder composition. Thereto were further added 2 parts of an ammonium salt of carboxymethylcellulose (CMC Daicel DN-10L, manufactured by Daicel Chemical Industries, Ltd.) as a thickener. Water was added thereto so as to set the concentration of all solids to 43%. The components were mixed by means of a planetary mixer for 60 minutes. Thereafter, the mixture was diluted with water to set the solid concentration to 41%, and further the components were mixed for 10 minutes to yield a slurry for an electrode. This slurry was applied onto an aluminum foil of 20 μm thickness with a doctor blade. The resultant was dried with a blast drier at 80° C. for 30 minutes. Thereafter, the dried product was pressed with a roll press machine to yield an electrode of 80 μm in thickness, having an electrode layer of 0.6 g/cm3 density.
The electrode produced through the above-mentioned steps was cut out into two circles of 15 mm diameter. The circles were dried at 200° C. for 72 hours. The electrode layer faces of the two electrodes were made opposite to each other, and then a circular separator, 18 mm in diameter and 40 μm in thickness, made of cellulose was sandwiched therebetween. This was put into a coin-shaped outer packaging container (diameter: 20 mm, height: 1.8 mm, and stainless steel thickness: 0.25 mm) made of stainless steel, to which a packing made of polypropylene was fitted. An electrolytic solution was poured into this container without leaving air. A cap of 0.2 mm in thickness, made of stainless steel was put and fixed onto the outer packaging container through the polypropylene packing. The container was then sealed to produce a coin-shaped electric double layer capacitor of 20 mm in diameter and about 2 mm in thickness. The used electrolytic solution was a solution in which tetraethylammonium tetrafluoroborate was dissolved in propylene carbonate at a concentration of 1 mol/L.
Each binder composition, each slurry for an electrode, each electrode, and each electric double layer capacitor were produced in the same way as in Example 1 except that each monomer mixture shown in Table 1 was used as a monomer mixture.
(1) Physical Properties of the Binders
Physical properties of the polymers used in Examples 1 to 6 and Comparative Examples 1 to 2, which were each used as the binder, were measured by the following methods.
The content by percentage (composition ratio) of each of the monomer units in each of the polymers was obtained by 1H—and 13C-NMR measurements. The results are shown in Table 1.
Each of the polymers was cast onto a polytetrafluoroethylene plate, and dried for 2 days. Thereafter, the polymer was further dried at 120° C. for 15 minutes to produce a polymer film. The film was used, and the temperature thereof was raised at 5° C. per minute to measure the Tg with a differential scanning calorimeter (DSC.). The results are shown in Table 1.
The particle diameter of each of the polymers was gained as the number-average particle diameter thereof, which was obtained by measuring the diameters of 100 particles of the polymer selected at random in transmission electron microscopic photographs thereof and then calculating the arithmetic average of the diameters. The results are shown in Table 1.
Each of the binder compositions was applied onto a glass plate to give a polymer film of about 0.1 mm thickness. Thereafter, the resultant was naturally dried at room temperature for 24 hours. Furthermore, the resultant was vacuum-dried at 120° C. for 2 hours to form a cast film. This cast film was cut out into a piece of about 2 cm square. The piece was weighed and then immersed into an electrolytic solution of 60° C. temperature. The immersed film was pulled up after 72 hours, and wiped with towel paper. Immediately, the weight of the film was measured. The value of (the weight after the immersion)/(the weight before the immersion) was defined as the electrolytic solution swelling ratio. The used electrolytic solution was a solution in which tetraethylammonium tetrafluoroborate was dissolved in propylene carbonate at a concentration of 1 mol/L. As the electrolytic solution swelling ratio is smaller, the electrolytic solution resistance of the binder polymer is higher. The results are shown in Table 2.
(2) Performances of the Electrodes and the Electric Double Layer Capacitors
Regarding performances of the electrodes and the electric double layer capacitors produced in Examples 1 to 6 and Comparative Examples 1 to 2, the following evaluations were made. The results are shown in Table 2.
In accordance with JIS B0601, the arithmetic average roughness (Ra) of a surface 20 μm square of each of the electrode layers was observed with an atomic force microscope. (The electrode before the roll pressing was measured.)
Each of the electrodes was cut out into a rectangle of 2.5 cm width and 10 cm length. The rectangular piece was fixed to direct its electrode layer face upwards. A cellophane tape was stuck onto the electrode layer face. The tape was peeled at a speed of 50 mm/minute in the direction of an angle of 180°. The stress (N/cm) in this case was measured 10 times. The average thereof was defined as the peel strength. As this value is larger, the binding strength is higher and the active material for the electrode is less peeled from the current collector.
Each of the electrodes was cut into rectangles of 3 cm width and 9 cm length. The rectangular pieces were used as test pieces. One out of the test pieces was put on a desk to direct the current collector side face of the test piece downwards. A stainless steel bar of 3 mm diameter was set onto the current collector side surface thereof in the state that the bar was positioned at the center of the piece in the longitudinal direction (at a place 4.5 cm apart from ends thereof) and was laid along the short direction. The test piece was bent at an angle of 180° around this stainless steel rod so as to direct the electrode layer outwards. This test was made on the test pieces, the number of which was ten. It was observed whether or not the portion where the electrode layer of each of the test pieces was bent was cracked or peeled. The case that no cracking or peeling was generated in any one of the 10 pieces was judged to be good (o), and the case that one or more spots were cracked or peeled in one or more of the pieces was judged to be poor (X). When the electrode layer is not cracked or peeled, the electrode is excellent in flexibility.
At 25° C., each of the electric double layer capacitors was charged up to 2.7 V from 0 V at a constant current of 10 mA/cm2 over 10 minutes, and then discharged up to 0 V at a constant current of 1 mA/cm2. The internal resistance was calculated from the resultant charging and discharging curve in accordance with a calculating method of the standard RC-2377 prescribed by Japan Electronics and Information Technology Industries Association.
As is evident from Table 2, the electrodes of the present invention were excellent in cracking resistance and peel strength. Furthermore, the electric double layer capacitors produced by use of the electrodes also had a small internal resistance, and excellent performances for electric double layer capacitors. On the other hand, the electrodes using the binder having an excessively high glass transition temperature (Comparative Example 1) and using the binder having no monomer units derived from the compound (b) (Comparative Example 2) were poor in both of crack resistance and peel strength, and the performances of the electric double layer capacitors obtained therefrom were also poor.
The above has described the present invention in connection with embodiments which appear to be most preferable and most practical at present. However, the present invention is not limited to the embodiments disclosed in the present specification, and can be appropriately modified within the scope which does not depart from the subject matter or the conception of the present invention which can be understood from the claims and the whole of the specification. It should be understood that binders for electric double layer capacitor electrodes, binder compositions, slurry compositions, electrodes, and electric double layer capacitors with such modification are also included in the technical scope of the present invention.
The electrode, for an electric double layer capacitor, produced by use of a slurry composition, for an electric double layer capacitor electrode, which contains the binder composition of the present invention for an electric double layer capacitor electrode is excellent in smoothness, cracking resistance, and binding force. The use of this electrode for an electric double layer capacitor makes it possible to produce a superior electric double layer capacitor.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-286176 | Aug 2003 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP04/11503 | 8/4/2004 | WO | 00 | 1/18/2007 |
