JP2021057486A - Separator for electric double layer capacitor - Google Patents

Abstract

To provide a separator for electric double layer capacitor, capable of achieving excellent production efficiency in alternately stacking an electrode and a separator before winding.SOLUTION: The electric double layer separator for capacitor contains fibrillation solvent spinning cellulose fiber and regenerated cellulose fiber and has a difference in surface smoothness of front and rear faces, which is 20 sec/10 cc or more.SELECTED DRAWING: None

Description

The present invention relates to a separator for an electric double layer capacitor.

Electric double layer capacitors have a large electric capacity and are highly stable against repeated charging and discharging, so they are widely used in applications such as power supply and power supply used in vehicles and electrical equipment. The separator for electric double layer capacitors is separated so that the positive electrode and the negative electrode do not come into direct contact with each other.

As a separator for electric double layer capacitors (hereinafter, when simply referred to as "separator", it means a separator for electric double layer capacitors), a paper separator mainly composed of beaten material of solvent-spun cellulose fiber or regenerated cellulose fiber has been conventionally used. (See, for example, Patent Documents 1 to 3) and separators made of synthetic fibers (see, for example, Patent Document 4) are used. Further, a separator for an electrochemical element containing beaten solvent-spun cellulose fibers and rayon fibers having a fiber diameter of 9.5 μm or less is disclosed (see, for example, Patent Document 5).

In the manufacture of electric double layer capacitors, electrodes and separators are alternately laminated and wound, but when winding, the separators tend to shift from each other depending on the surface condition of the separator, which takes time to adjust and reduces production efficiency. There was a problem.

Japanese Unexamined Patent Publication No. 2002-231580 Japanese Unexamined Patent Publication No. 11-16803 Japanese Unexamined Patent Publication No. 2000-3834 Japanese Unexamined Patent Publication No. 2003-45752 Japanese Unexamined Patent Publication No. 2018-139283

An object of the present invention is to provide a separator for an electric double layer capacitor, which is excellent in production efficiency when electrodes and separators are alternately laminated and wound in view of the above circumstances.

An electric double layer capacitor separator comprising fibrillated solvent-spun cellulose fibers and regenerated cellulose fibers, wherein the difference in surface smoothness between the front and back surfaces is 20 sec / 10 cc or more.

The separator for an electric double layer capacitor of the present invention contains fibrillated solvent-spun cellulose fibers and regenerated cellulose fibers, and the difference in surface smoothness between the front and back surfaces is 20 sec / 10 cc or more, so that electrodes and separators are alternately laminated. When winding, the surface with high smoothness is hard to shift due to the increase in the adhesion points between the electrode and the separator, and even if it shifts, it is corrected by shifting only the electrode in contact with the surface with low smoothness. Since it is easy to remove, the effect of improving production efficiency can be obtained.

The separator for electric double layer capacitors of the present invention is a separator for electric double layer capacitors containing fibrillated solvent-spun cellulose fibers and regenerated cellulose fibers, and the difference in surface smoothness between the front and back surfaces of the separator is 20 sec / 10 cc or more. It is characterized by that.

In the separator for electric double layer capacitors of the present invention, the difference in surface smoothness between the front and back surfaces of the separator is 20 sec / 10 cc or more, more preferably 30 sec / 10 cc or more, and further preferably 40 sec / 10 cc or more. When the difference in surface smoothness is less than 20 sec / 10 cc, there arises a problem that the production efficiency is lowered due to a deviation when the electrodes and the separator are alternately laminated and wound. The upper limit of the difference in surface smoothness is not particularly limited, but is preferably 150 sec / 10 cc or less. The front side of the separator is the one with high smoothness, and the back side of the separator is the one with low smoothness.

In the present invention, the surface smoothness of the separator was measured using a Beck smoothness tester in accordance with JIS P 8119: 1998. It is a measurement result in the time when 10cc (10 ml) of air of the above-mentioned tester flows.

In the present invention, examples of the regenerated cellulose fiber include a single fiber and a composite fiber made of rayon, polynosic, cupra, acetate, triacetate and the like. These regenerated cellulose fibers may be used alone or in combination of two or more. Moreover, you may use the thing which divided various split type composite fibers. Of these, rayon and cupra are preferable. When rayon or cupra is used, it is easy to form a network structure by uniformly entwining with fibrillated solvent-spun cellulose fibers, so that a separator for electric double layer capacitors with higher surface smoothness, fineness and mechanical strength can be obtained. be able to.

The average fiber diameter of the regenerated cellulose fibers is preferably 0.1 to 13.0 μm, more preferably 2.5 to 11.5 μm, still more preferably 4.0 to 10.0 μm. If the average fiber diameter is less than 0.1 μm, the fibers may be too thin and fall off from the separator. If the average fiber diameter is thicker than 13.0 μm, it may be difficult to reduce the thickness of the separator or the denseness may be insufficient. Further, since the number of fibers is reduced, the mechanical strength of the separator may be lowered. Further, when the separator has a low thickness of less than 20 μm, the maximum pore diameter may increase and the internal short circuit defect rate may increase. The average fiber diameter is determined by measuring the area of the fibers forming the separator by observing the cross section of the separator and the surface with a scanning electron microscope, measuring the fiber diameter converted to a perfect circle, and selecting 100 fibers at random. It is an average value.

The fiber length of the regenerated cellulose fiber is preferably 0.3 to 10 mm, more preferably 0.5 to 7 mm, and even more preferably 1 to 5 mm. If the fiber length is shorter than 0.3 mm, it may fall off from the separator. If the fiber length is longer than 10 mm, the fibers may become entangled and become lumpy, resulting in uneven thickness.

In the present invention, the ratio of the regenerated cellulose fiber to the separator is preferably 5 to 40% by mass, more preferably 7.5 to 30% by mass, and further preferably 10 to 25% by mass. preferable. If the proportion of regenerated cellulose fibers is less than 5% by mass, the mechanical strength of the separator may be weakened. Further, when the thickness is reduced, the impedance indicating resistance may become too large. When the proportion of regenerated cellulose fibers exceeds 40% by mass, the liquid retention property of the electrolytic solution is insufficient and the internal resistance is high, or the denseness of the separator is insufficient when the basis weight is low. The internal short-circuit defect rate and the variation in discharge characteristics may increase.

In the present invention, fibrillation refers to a fiber that is not in the form of a film but mainly has a portion that is very finely divided in a direction parallel to the fiber axis, and at least a part of the fiber has a fiber diameter of 1 μm or less. Point to. The aspect ratio of the length and width of the fibrillated fibers is preferably in the range of about 20 to about 100,000.

In the present invention, the solvent-spun cellulose fiber is different from the so-called regenerated cellulose fiber in which cellulose is once chemically converted into a cellulose derivative and then returned to cellulose, as in the case of conventional biscous rayon and copper ammonia rayon. Refers to a fiber in which cellulose is precipitated by dry-wet spinning a spinning stock solution dissolved in amine oxide in water without chemically changing. Compared to natural cellulose fibers, bacterial cellulose fibers, and regenerated cellulose fibers, solvent-spun cellulose fibers have molecules that are highly arranged in the long axis direction of the fibers. It is easy to make finer and produces fine and long fibers. In order to firmly hold the electrolytic solution between these thin and long fibers, the fibrillated solvent-spun cellulose fibers that have been made finer are electrolyzed than the fibrillated (micronized) of natural cellulose fibers, bacterial cellulose fibers, and regenerated cellulose fibers. Excellent liquid retention.

As a method for producing fibrillated solvent-spun cellulose fibers, a refiner, a beater, a mill, a grinder, a rotary blade homogenizer that applies shearing force by a high-speed rotary blade, and a cylindrical inner blade that rotates at high speed are fixed. A double-cylindrical high-speed homogenizer that generates shearing force with the outer blade, an ultrasonic crusher that is miniaturized by the impact of ultrasonic waves, and a fiber suspension that is passed through a small-diameter orifice by applying a pressure difference of at least 20 MPa. A high-pressure homogenizer that applies shearing force and cutting force to fibers by making them collide with each other and rapidly decelerating them. Of these, the refiner is particularly preferable.

The modified drainage of the fibrillated solvent-spun cellulose fiber is preferably 75 to 220 ml, more preferably 90 to 175 ml, and even more preferably 90 to 120 ml. If the modified drainage degree is less than 75 ml, the fibers may become too fine, the strength of the separator cannot be maintained, and paper breakage may easily occur. If the modified drainage degree is greater than 220 ml, the separator may become insufficiently dense and the internal short-circuit defect rate may increase.

The modified drainage degree is measured according to JIS P8121: 2012 except that an 80-mesh wire mesh with a wire diameter of 0.14 mm and a mesh size of 0.18 mm is used as a sieving plate and the sample concentration is 0.1% by mass. It is the value that was set.

In the case of solvent-spun cellulose fibers, the fiber length becomes shorter as the micronization progresses, and especially when the sample concentration is low, the fibers are less entangled with each other and it becomes difficult to form a fiber network. Therefore, the solvent-spun cellulose fibers themselves. Slip through the holes in the sieving board. That is, in the case of fibrillated solvent-spun cellulose fibers, the degree of drainage cannot be accurately measured by the measurement method of JIS P8121-2: 2012. More specifically, as the degree of miniaturization of natural cellulose fibers increases, a large number of fine fibrils are torn from the trunk of the fibers, so that the fibers are easily entangled with each other through the fibrils and easily form a fiber network. On the other hand, solvent-spun cellulose fibers are easily divided into fine fibers parallel to the long axis of the fibers by the micronization treatment, and the uniformity of the fiber diameter in each of the divided fibers is high. Therefore, the shorter the average fiber length, the more. It is considered that the fibers are less likely to be entangled with each other and it is difficult to form a fiber network. Therefore, in the present invention, in order to measure the accurate drainage degree of the fibrillated solvent-spun cellulose fiber, an 80-mesh wire mesh having a wire diameter of 0.14 mm and a mesh size of 0.18 mm is used as a sieving plate, and the sample concentration is 0.1. The modified drainage degree measured in accordance with JIS P8121-2: 2012 was used except for the mass%.

The length-weighted average fiber length of the fibrillated solvent-spun cellulose fibers is preferably 0.1 to 3.0 mm, more preferably 0.2 to 2.0 mm, and even more preferably 0.3 to 1.1 mm. If the fiber length is shorter than 0.1 mm, it may fall off from the separator or the mechanical strength of the separator may decrease. If the fiber length is longer than 3.0 mm, the fiber fibrillation becomes insufficient and the internal short circuit failure rate increases. In some cases, the fibers may become entangled and become lumpy, resulting in uneven thickness. The length-weighted average fiber length of the fibrillated solvent-spun cellulose fibers was measured using Kajani FiberLab V3.5 (manufactured by Metso Automation) as an apparatus. The length-weighted fiber length (L (l), unit [mm]) in the projected fiber length (Proj) mode of the above device is the "length-weighted average fiber length".

In the present invention, the ratio of the fibrillated solvent-spun cellulose fiber to the separator is more preferably 50 to 90% by mass, further preferably 60 to 80% by mass. When the proportion of fibrillated solvent-spun cellulose fibers is less than 50% by mass, the liquid retention property of the electrolytic solution may be insufficient and the internal resistance may increase when the basis weight is low. In addition, the tightness of the separator may be insufficient, and the internal short-circuit defect rate may increase. In addition, the surface smoothness may decrease and winding deviation may easily occur. When the proportion of fibrillated solvent-spun cellulose fibers exceeds 90% by mass, the mechanical strength of the separator may be weakened. Further, when the thickness is reduced, the impedance indicating resistance may become too large.

The separator for an electric double layer capacitor of the present invention is a paper machine having one type of paper making method from among the paper making methods such as a circular net type, a long net type, a short net type, and an inclined short net type, and two of the same type or different types. It can be produced by a method of making paper using a combination paper machine or the like having a combination of more than one kind of paper making method. In addition to the fiber raw material, a dispersant, a thickener, an inorganic filler, an organic filler, an antifoaming agent and the like can be appropriately added to the raw material slurry, and the content is about 5 to 0.001% by mass. Prepare the raw material slurry to the solid content concentration. This raw material slurry is further diluted to a predetermined concentration, paper-made, and dried. The separator for an electric double layer capacitor obtained by papermaking is subjected to calendering, thermal calendering, heat treatment and the like, if necessary.

In the present invention, the ratio of fine fibers to the separator is the ratio of fine fibers of fibrillated solvent-spun cellulose fibers at the raw material stage, papermaking method, type of papermaking net, ratio of fibrillated solvent-spun cellulose fibers / regenerated cellulose fibers, at the time of papermaking. It can be adjusted by appropriately changing the slurry concentration, slurry temperature, slurry viscosity, dehydration strength, and the like.

The basis weight of the separator for an electric double layer capacitor is preferably 4~25g / ^{m 2,} more preferably 5~23g / ^{m 2,} more preferably 6~22g / ^{m 2.} If it is less than 4 g / m ² , sufficient mechanical strength may not be obtained, the insulating property between the positive electrode and the negative electrode may be insufficient, and the internal short-circuit defect rate and cycle characteristics may be lowered. If it exceeds 25 g / m ² , the internal resistance of the electric double layer capacitor may increase or the discharge characteristics may decrease. The basis weight of the separator of the present invention is a value measured in accordance with JIS P8124: 2011.

The thickness of the separator for an electric double layer capacitor is preferably 6 to 60 μm, more preferably 7 to 55 μm, and even more preferably 8 to 50 μm. If it is less than 6 μm, sufficient mechanical strength may not be obtained, the insulating property between the positive electrode and the negative electrode may be insufficient, the internal short circuit defect rate, and the cycle characteristics may be deteriorated. If it is thicker than 60 μm, the internal resistance of the electric double layer capacitor may increase or the discharge characteristics may decrease. The thickness of the separator of the present invention means a value under a 5N load measured by an outer micrometer specified in JIS B7502: 2016.

In the present invention, in order to make the difference in surface smoothness between the front and back surfaces 20 sec / 10 cc or more, in the drying step, one surface is attached to the surface of the Yankee dryer and dried by using a Yankee dryer type dryer. The method can be mentioned.

The electric double layer capacitor is a capacitor that stores electric charges in the electric double layer formed on the surfaces of the positive electrode and the negative electrode. By allowing more ions to be adsorbed on the surfaces of the positive electrode and the negative electrode, an electric double layer capacitor having a larger capacity can be obtained. In order to allow more ions to be adsorbed on the surfaces of the positive electrode and the negative electrode, the positive electrode and the negative electrode need to have a larger specific surface area. Further, it is necessary that the positive electrode and the negative electrode of the electric double layer capacitor do not cause an electrochemical reaction. Activated carbon; graphite; carbon nanofibers, nanocarbons such as graphene and the like are mainly used as materials satisfying these conditions for the positive electrode and the negative electrode of the electric double layer capacitor. As the electrolytic solution, an aqueous sulfuric acid solution, a solution in which a salt that does not cause an electrochemical reaction at the working potential is dissolved in a polar organic solvent, an ionic liquid, or the like can be used. Examples of salts that do not cause an electrochemical reaction at the working potential include salts of tetraethylammonium and tetrafluoroboric acid (TEA / BF ₄ ), salts of triethylmethylammonium and tetrafluoroboric acid (TEMA / BF ₄ ), 5-. Examples thereof include salts of azoniaspiro [4,4] nonane and tetrafluoroboric acid (SBP · BF ₄ ). Examples of the polar organic solvent include acetonitrile; γ-butyrolactone; carbonic acid esters such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), and ethylmethyl carbonate (EMC).

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the Examples. In the examples, the number of copies is based on mass.

[Example 1]
Fibrilized solvent-spun cellulose fiber (modified method) obtained by fibrilizing solvent-spun cellulose fiber having a fineness of 0.8 dtex, 15 parts of regenerated cellulose fiber having a fiber length of 3 mm, an average fiber diameter of 12.7 μm, and a fiber length of 4 mm using a refiner. Eighty-five parts of a drainage degree of 110 ml and a length-weighted average fiber length of 0.95 mm) are mixed together, disaggregated in water of pulper, and a uniform raw material slurry (0.5 mass% concentration) is prepared under stirring with an agitator. Prepared. This raw material slurry was dried at a Yankee dryer temperature of 100 ° C. using a tilted short net paper machine to obtain a wet sheet, and then subjected to calendering treatment to have a basis weight of 22.0 g / m ² and a thickness of 50. A separator for an electric double layer capacitor of 2 μm was obtained. The number of copies is based on mass.

[Example 2]
^{Basis weight 21.8 g / m 2 in the same} manner as in Example 1 except that 25 parts of regenerated cellulose fiber having a fineness of 0.8 dtex and a fiber length of 3 mm and 75 parts of fibrillated solvent-spun cellulose fiber similar to Example 1 were used. , A separator for an electric double layer capacitor having a thickness of 50.5 μm was obtained.

[Example 3]
^{Basis weight 22.3 g / m 2 in the same} manner as in Example 1 except that 5 parts of regenerated cellulose fiber having a fineness of 0.8 dtex and a fiber length of 3 mm and 95 parts of fibrillated solvent-spun cellulose fiber similar to Example 1 were used. , A separator for an electric double layer capacitor having a thickness of 48.8 μm was obtained.

[Example 4]
^{Basis weight 22.4 g / m 2 in the same} manner as in Example 1 except that 40 parts of regenerated cellulose fiber having a fineness of 0.8 dtex and a fiber length of 3 mm and 60 parts of fibrillated solvent-spun cellulose fiber similar to Example 1 were used. , A separator for an electric double layer capacitor having a thickness of 49.9 μm was obtained.

[Example 5]
15 parts of regenerated cellulose fiber with fineness of 0.8 dtex and fiber length of 3 mm, 80 parts of fibrilized solvent-spun cellulose fiber similar to Example 1, fibrillated natural cellulose fiber (modified drainage 250 ml, length-weighted average fiber length 0) ^{.20 mm) A separator for an electric double-layer capacitor having a basis weight of 21.8 g / m 2} and a thickness of 50.0 μm was obtained in the same manner as in Example 1 except that 5 parts were used.

[Example 6]
Examples except that 10 parts of regenerated cellulose fiber having a fineness of 0.8 dtex and a fiber length of 3 mm, 80 parts of fibrillated solvent-spun cellulose fiber similar to Example 1, and 10 parts of fibrillated natural cellulose fiber similar to Example 5 were used. A separator for an electric double layer capacitor having a basis weight of 21.5 g / m ² and a thickness of 48.0 μm was obtained in the same manner as in 1.

[Comparative Example 1]
^{Basis weight 22.1 g / m 2 in the same} manner as in Example 1 except that 60 parts of regenerated cellulose fiber having a fineness of 0.8 dtex and a fiber length of 3 mm and 40 parts of fibrillated solvent-spun cellulose fiber similar to Example 1 were used. , A separator for an electric double layer capacitor having a thickness of 53.1 μm was obtained.

[Comparative Example 2]
^{Basis weight 21.8 g / m 2 in the same} manner as in Example 1 except that 70 parts of regenerated cellulose fiber having a fineness of 0.8 dtex and a fiber length of 3 mm and 30 parts of fibrillated solvent-spun cellulose fiber similar to Example 1 were used. , A separator for an electric double layer capacitor having a thickness of 55.1 μm was obtained.

[Comparative Example 3]
Examples except that 65 parts of regenerated cellulose fiber having a fineness of 0.8 dtex and a fiber length of 3 mm, 30 parts of fibrillated solvent-spun cellulose fiber similar to Example 1, and 5 parts of fibrillated natural cellulose fiber similar to Example 5 were used. A separator for an electric double layer capacitor having a basis weight of 22.5 g / m ² and a thickness of 55.0 μm was obtained in the same manner as in 1.

[Comparative Example 4]
Example 1 except that 15 parts of regenerated cellulose fiber having a fineness of 0.8 dtex and a fiber length of 3 mm and 85 parts of fibrillated solvent-spun cellulose fiber similar to Example 1 were used and the drying method was changed from a Yankee dryer to a multi-cylinder dryer. A separator for an electric double layer capacitor having a basis weight of 22.1 g / m ² and a thickness of 54.0 μm was obtained in the same manner as in the above.

The raw materials and separators used in Examples and Comparative Examples were measured and evaluated as follows, and the results are shown in Table 1.

[Basis weight]
Basis weight was measured according to JIS P8124: 2011.

[thickness]
The thickness under 5N load measured by an outer micrometer specified in JIS B7502: 2016 was measured.

[Surface smoothness]
The smoothness of the front and back surfaces was measured by the method specified in JIS P8119: 1998, that is, the smoothness test method using a Beck smoothness tester, and the difference was calculated.

[Evaluation of winding deviation]
The separator was wound between the prepared separator and the electrode made of aluminum foil, and the wound shape was visually observed and evaluated according to the following criteria. Due to the occurrence of winding misalignment, the joint cannot be sealed in the process of adhering the joint during the manufacture of the electric double layer capacitor, which may cause problems such as a decrease in product yield.

⊚: No winding deviation was observed.
◯: Almost no winding deviation was observed.
Δ: Winding deviation of less than 0.5 mm was observed, but it is within the usable range.
X: A winding deviation of 0.5 mm or more is observed, which causes a defect when the battery is manufactured.

As shown in Table 1, since the difference in surface smoothness between the front and back surfaces of the separators for electric double layer capacitors of Examples 1 to 5 is 20 sec / cc or more, winding deviation is not seen or almost not seen. It was.

Since the difference in surface smoothness between the front and back surfaces of the separators for electric double layer capacitors of Comparative Examples 1 to 4 was less than 20 sec / cc, the winding deviation was larger than that of Examples 1 to 5.

By comparing the examples and the comparative examples, the difference in surface smoothness between the front and back surfaces of the separator for an electric double layer capacitor including the regenerated cellulose fiber and the fibrillated solvent-spun cellulose fiber is 20 sec / cc or more. It was found that the effect of preventing winding misalignment during fabrication of an electric double layer capacitor can be obtained. If the winding deviation does not occur, the defect rate of the electric double layer capacitor is reduced, and the effect of improving the production efficiency can be obtained.

As an example of utilization of the present invention, an electric double layer capacitor is suitable.

Claims (1)

An electric double layer capacitor separator comprising fibrillated solvent-spun cellulose fibers and regenerated cellulose fibers, wherein the difference in surface smoothness between the front and back surfaces is 20 sec / 10 cc or more.