JP2012222266A - Separator for electrochemical element and electrochemical element using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for electrochemical element exhibiting excellent absorbency of electrolytic solution, and to provide an electrochemical element using the separator and having excellent internal resistance characteristics and capacity retention rate.SOLUTION: The separator for electrochemical element is composed of a wet nonwoven fabric containing synthetic staple fibers and beaten solvent spinning cellulose fibers, as essential components, in which the freeness variant of the beaten solvent spinning cellulose fibers is 0-400 ml, and the length weighted average fiber length is 0.20-2.00 mm.

Description

  The present invention relates to a separator for an electrochemical element and an electrochemical element using the same.

  A capacitor, which is a kind of electrochemical element, has a large electric capacity and has high stability against repeated charge and discharge, and is therefore widely used in applications such as a power supply used in vehicles and electrical equipment. The capacitor has a built-in separator, and the separator separates the positive electrode and the negative electrode so that the positive electrode and the negative electrode are not in direct contact with each other in the capacitor, that is, so as not to cause an internal short circuit. In order to reduce the internal resistance of the capacitor, vacancies capable of efficiently transmitting electrolyte ions must be formed in the separator. Therefore, the separator needs to be porous.

  As a separator for a capacitor, conventionally, a paper separator (for example, see Patent Documents 1 to 3) mainly composed of solvent-spun cellulose fibers or regenerated cellulose fibers and a separator made of synthetic fibers (for example, see Patent Document 4). Is used.

  Capacitors with electrolytes composed of organic solvents and electrolytes will not have a predetermined voltage when water is mixed in even if a slight amount of water is mixed, because the voltage will fluctuate and internal resistance will increase. The capacitor is manufactured after drying for a long time at a high temperature to remove moisture contained in these members. However, when a paper separator is processed at a high temperature of 150 ° C. or higher, the mechanical strength is reduced due to carbonization or decomposition of the cellulose component. There was a problem that the separator was broken while repeating the above, and the internal short circuit defect rate increased.

  In addition, with the recent increase in functionality of electronic components, capacitors are also required to have low resistance. For this reason, the separator is required to have a thin thickness and a high electrolyte solution retention property. However, a separator made of synthetic fibers has insufficient insulation between the positive electrode and the negative electrode when the thickness is reduced. In other words, there is a problem that the internal short-circuit defect rate is high, and a liquid retention property of the electrolytic solution is low, so that ion conductivity is low and internal resistance is high.

  Electrochemical elements will be increasingly used in the future as power sources and auxiliary power sources for hybrid vehicles and electric vehicles. For this reason, the storage element is required to have a high capacity, and in particular, it is important to have a high capacity retention rate even when charging / discharging with a large current.

JP-A-5-267103 JP-A-11-168033 JP 2000-3834 A JP 2003457575 A

  The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a separator for an electrochemical element that is excellent in the absorbability of an electrolytic solution, and an electrochemical element that is excellent in internal resistance characteristics and discharge capacity retention rate.

  In the present invention, as a result of diligent research to solve this problem, the type of cellulose fiber and the degree of drainage are optimized, and further the fiber length distribution of the cellulose fiber is optimized to absorb the electrolyte. It has been found that an electrochemical device separator excellent in performance, an electrochemical device excellent in internal resistance characteristics and discharge capacity retention rate can be realized, and has led to the present invention described below.

(1) A synthetic short fiber, a wet nonwoven fabric containing solvent-spun cellulose fibers formed by beating as an essential component, and the modified freeness of solvent-spun cellulose fibers defined below is 0 to 400 ml. And the separator for electrochemical elements whose length weight average fiber length is 0.20-2.00 mm.

  Modified freeness: Freeness measured in accordance with JIS P8121, except that an 80-mesh wire mesh having a wire diameter of 0.14 mm and an aperture of 0.18 mm was used as the sieve plate, and the sample concentration was 0.1%.

(2) In the fiber length distribution histogram of solvent-spun cellulose fibers formed by beating, the ratio of fibers having a maximum frequency peak between 0.00 and 1.00 mm and having a fiber length of 1.00 mm or more is 10%. The separator for an electrochemical element according to (1) as described above.
(3) In the fiber length distribution histogram of solvent-spun cellulose fibers formed by beating, the slope of the ratio of fibers having a fiber length of 0.05 mm between 1.00 and 2.00 mm is −3.0 or more and −0 The separator for electrochemical devices according to (2), which is .5 or less.

(4) In a fiber length distribution histogram of solvent-spun cellulose fibers formed by beating, the ratio of fibers having a maximum frequency peak between 0.00 and 1.00 mm and a fiber length of 1.00 mm or more is 50%. The separator for an electrochemical element according to (1) as described above.
(5) The separator for an electrochemical element according to (4), which has a peak between 1.50 and 3.50 mm in addition to the maximum frequency peak in the fiber length distribution histogram of the solvent-spun cellulose fiber formed by beating.

(6) An electrochemical element comprising the electrochemical element separator according to any one of (1) to (5).

  The separator (1) for an electrochemical element of the present invention comprises a wet nonwoven fabric containing synthetic short fibers and beaten solvent-spun cellulose fibers as essential components, and modified freeness of solvent-spun cellulose fibers obtained by beating. Is 0 to 400 ml, and the length weighted average fiber length is 0.20 to 2.00 mm, so that the pores of the separator are uniformly formed, have an appropriate denseness, and absorbability of the electrolyte Excellent. Therefore, the electrochemical device comprising the separator has a low internal resistance and an excellent discharge capacity retention rate.

  Moreover, in the fiber length distribution histogram of the solvent-spun cellulose fiber formed by beating, the ratio of fibers having a maximum frequency peak between 0.00 and 1.00 mm and having a fiber length of 1.00 mm or more is 10% or more. In the electrochemical element separator (2), the liquid absorption is good, and the inclination of the ratio of the fibers having a fiber length of every 0.05 mm between 1.00 and 2.00 mm is −3.0 or more. In the electrochemical element separator (3) which is −0.5 or less, the liquid absorbency is more excellent.

  And in the fiber length distribution histogram of the solvent-spun cellulose fiber formed by beating, the ratio of fibers having a maximum frequency peak between 0.00 and 1.00 mm and having a fiber length of 1.00 mm or more is 50% or more. In the electrochemical element separator (4), the fiber gap does not become too narrow and too wide, and both papermaking properties and liquid absorption properties can be achieved. In addition to the maximum frequency peak, the separator for electrochemical devices (5) having a peak between 1.50 and 3.50 mm has better liquid absorbency than solvent-spun cellulose fibers that do not have such a peak. Become.

A graph showing the relationship between Canadian standard freeness and modified freeness (freeness measured according to JIS P8121 except that the sample concentration is 0.03%) of the solvent-spun cellulose fiber that is beaten It is. Modified drainage of solvent-spun cellulose fiber formed by beating (conforming to JIS P8121, except that 80-mesh wire mesh with a wire diameter of 0.14 mm and mesh size of 0.18 mm was used as the sieve plate and the sample concentration was 0.1%. It is an example of the graph showing the modified drainage degree measured in this way. Modified drainage of solvent-spun cellulose fibers used in the examples of the present invention (80-mesh wire mesh having a wire diameter of 0.14 mm and an aperture of 0.18 mm was used as a sieve plate, and the sample concentration was 0.1%. It is the graph showing the modified drainage degree measured based on JIS P8121 except having made it. It is a fiber length distribution histogram of solvent-spun cellulose fiber [I] beaten. It is a fiber length distribution histogram of solvent-spun cellulose fiber [II] obtained by beating. In the fiber length distribution histogram of the solvent-spun cellulose fibers [I] and [II] that are beaten, a graph of the ratio of fibers having a fiber length of every 0.05 mm between 1.00 and 2.00 mm and an approximate line FIG. It is an example of the fiber length distribution histogram of solvent-spun cellulose fiber [i] beaten having a maximum frequency peak between 0.00 and 1.00 mm. It is an example of the fiber length distribution histogram of solvent-spun cellulose fiber [ii] formed by beating having a peak between 1.50 and 3.50 mm in addition to the maximum frequency peak.

<Electrochemical element separator>
In the present invention, the expression “separator” means an electrochemical element separator.

  As the synthetic short fiber in the present invention, polypropylene, polyethylene, polymethylpentene, polyester, polyester derivative, acrylic polymer, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyvinyl chloride, polyvinylidene chloride, polyvinyl ether, Polyvinyl ketone, polyether, polyvinyl alcohol, aliphatic polyamide, aromatic polyamide, wholly aromatic polyamide, wholly aromatic polyester, polyamide imide, polyimide, polyether ether ketone, polyphenylene sulfide, polybenzimidazole, poly-p-phenylene benzo Bisthiazole, poly-p-phenylenebenzobisoxazole, polytetrafluoroethylene, single fibers made of these derivatives, and composite fibers made by combining two or more of these resins It is below.

  Examples of polyesters include polyethylene terephthalate, polybutylene terephthalate, and polyethylene isophthalate. Acrylic polymer is made of 100% acrylonitrile polymer, and acrylonitrile is copolymerized with (meth) acrylic acid derivatives such as acrylic acid, methacrylic acid, acrylic ester, methacrylic ester, vinyl acetate, etc. It refers to what you let me. Polyamide refers to aliphatic polyamide, semi-aromatic polyamide, and wholly aromatic polyamide. The aromatic polyamide refers to an aromatic polyamide having a fatty chain or the like in a part of the main chain.

  Examples of the composite fiber include a core-sheath type, an eccentric type, a side-by-side type, a sea-island type, an orange type, a multiple bimetal type, and a split type. Examples of the split type composite fiber include a fiber in which resins made of different components are adjacent to each other and a sea-island type fiber. The former is mechanically divided by a method of stirring with a pulper or a mixer or a method of applying a high-pressure water flow, and the latter is chemically divided by a method of eluting a resin as a sea component with a chemical to obtain ultrafine fibers. Examples of the cross-sectional shape of the former split type composite fiber include a radial type, a layered type, a comb type, and a grid type. The average fiber diameter of the split composite fibers is preferably 3.0 to 18.0 μm, more preferably 6.0 to 16.0 μm. If it is less than 3.0 μm, it may be difficult to divide, and if it is thicker than 18.0 μm, the long axis of the cross section of the ultrafine fiber after division becomes long, so that the void of the nonwoven fabric may be blocked.

  The average fiber diameter of each synthetic short fiber other than the split composite fiber is preferably 0.05 to 12.0 μm, more preferably 0.5 to 10.0 μm, and further preferably 1.0 to 5.0 μm. If it is thicker than 12.0 μm, it may be difficult to reduce the thickness of the wet nonwoven fabric. If it is less than 0.05 μm, stable production of the fiber becomes difficult. Furthermore, in the wet nonwoven fabric of the present invention, the average fiber diameter refers to a value obtained by converting the area of the fiber cross section to the diameter of the same area of a perfect circle. The fiber length of the synthetic short fiber is preferably 0.1 to 10 mm, and more preferably 0.3 to 6 mm. If the fiber length is less than 0.1 mm, the strength of the nonwoven fabric may be insufficient. If the fiber length is longer than 10 mm, the fibers may be twisted to form formation spots or thickness spots.

  In this invention, it is preferable to contain the synthetic short fiber whose theoretical flatness is 1.0-5.0. The theoretical flatness means a value obtained by dividing the maximum length of the major axis of the fiber cross section by the minor axis length. A synthetic short fiber having a theoretical flatness of 1.0 to 5.0 is a flat fiber obtained by dividing a flat fiber obtained by spinning from a flat spinneret or a split type composite fiber having a flat cross section after division. Means fiber. In the case of flat fibers obtained by direct spinning, the theoretical flatness can be calculated from the flatness of the spinneret. In the case of a split type composite fiber, the theoretical flatness can be calculated from the fiber diameter and the number of splits of the split type composite fiber before splitting. If the theoretical flatness is greater than 5.0, the voids of the wet nonwoven fabric may be blocked. The short axis length of the cross section of the synthetic short fiber having a theoretical flatness of 1.0 to 5.0 is preferably 1.0 to 5.0 μm, and more preferably 1.0 to 3.0 μm. If the thickness is less than 1.0 μm, the theoretical flatness of the cross section may become too large and the voids of the nonwoven fabric may be blocked. If the thickness exceeds 5.0 μm, the thickness of the nonwoven fabric may be difficult to reduce. The minor axis length means the maximum length in the minor axis direction of the ultrafine fiber cross section. The theoretical flatness of the ultrafine fiber is more preferably 1.5 to 3.0.

  20-70 mass% is preferable, as for the content rate of the synthetic short fiber in the wet nonwoven fabric in this invention, 30-70 mass% is more preferable, and 41-70 mass% is more preferable. If the amount is less than 20% by mass, the tensile strength and puncture strength may be insufficient. If the amount exceeds 70% by mass, a large through-hole may be formed or fluff may be likely to occur.

  The solvent-spun cellulose fiber in the present invention means a cellulose fiber obtained by directly dissolving and spinning in an organic solvent without passing through a cellulose derivative. In JIS L0204-2, the term “Lyocell” is used as the name of this fiber. In the present invention, the freeness of solvent-spun cellulose fibers formed by beating is expressed by modified freeness.

  The modified freeness in the present invention means that either the sample concentration or the sieve plate, or both the sample concentration and the sieve plate are changed with respect to the Canadian standard freeness measurement method defined in JIS P8121. Means the freeness measured. So far, the relationship between Canadian standard freeness and modified freeness of natural cellulose fibers such as conifer wood pulp, hardwood wood pulp, hemp pulp and esparto pulp has been reported. The relationship between Canadian standard freeness and modified freeness of spun cellulose fibers has not been clarified. In the present invention, the solvent-spun cellulose fiber was refined using a refiner, and the Canadian standard freeness and modified freeness were measured for each degree of refinement. It has been found that it is different from the drainage behavior of natural cellulose fibers disclosed in JP 2000-331663 A.

  FIG. 1 shows the relationship between Canadian standard freeness and modified freeness of solvent-spun cellulose fibers obtained by beating. In FIG. 1, the standard freeness means the Canadian standard freeness of JIS P8121. The modified freeness means the freeness measured in accordance with JIS P8121, except that the sample concentration is 0.03%. The horizontal axis in FIG. 1 indicates the length-weighted average fiber length, and the degree of miniaturization progresses toward the right. The Canadian standard freeness is 0.5 ml with a length-weighted average fiber length of 0.72 mm, but the shorter the length-weighted average fiber length is 0.55 mm or less, the greater the freeness. Yes. On the other hand, the modified freeness increases as the degree of refinement progresses. This drainage behavior is the drainage behavior of natural cellulose fibers disclosed in JP 2000-331663 A, that is, the Canadian standard freeness and modified freeness decrease as the degree of refinement progresses. The drainage behavior is quite different.

  The reason why the degree of freezing increases as the degree of refinement increases is that the length-weighted average fiber length of the solvent-spun cellulose fibers that are beaten as the refinement progresses, and the sample concentration is particularly low. In this case, since the entanglement between the fibers is reduced and the fiber network is hardly formed, the solvent-spun cellulose fiber itself that has been beaten passes through the holes in the sieve plate. In other words, in the case of solvent-spun cellulose fibers that have been refined, an accurate freeness cannot be measured by the measuring method of JIS P8121. In more detail, natural cellulose fibers are in a state where many fine fibrils are torn from the trunk of the fiber as the degree of miniaturization progresses, so fibers tend to get entangled through the fibrils and form a fiber network. On the other hand, solvent-spun cellulose fibers are easily finely divided in parallel to the long axis of the fiber by the refinement treatment, and the fiber diameter uniformity in each of the divided fibers is high. It is thought that it becomes difficult to form a fiber network because it becomes less entangled.

  In view of this, in the present invention, investigations were carried out to measure the exact freeness of solvent-spun cellulose fibers formed by beating. FIG. 2 shows an example of modified freeness measured by changing both the sample concentration and the sieve plate. That is, the modified freeness measured by using an 80-mesh wire net instead of the sieve plate specified in JIS P8121, and setting the sample concentration to 0.1%. The wire diameter of 80 mesh was 0.14 mm in diameter, and a wire mesh (manufactured by PULP AND PAPER RESEARCH INSTITUTE OF CANADA) having an opening of 0.18 mm was used. As is clear from FIG. 2, the degree of drainage decreased as the degree of refinement progressed, and the removal of the solvent-spun cellulose fibers formed by beating was suppressed, and the more precise degree of drainage could be measured. I understand. Hereinafter, the modified freeness in the present invention is based on JIS P8121, except that an 80 mesh wire net having a wire diameter of 0.14 mm and an aperture of 0.18 mm is used as a sieve plate, and the sample concentration is 0.1%. It means the measured modified freeness, and is simply expressed as “modified dryness” unless otherwise specified.

  The fiber length and the fiber length distribution histogram of the solvent-spun cellulose fiber that has been beaten can be obtained using the deflection characteristics obtained by applying laser light to the fiber, and measured using a commercially available fiber length measuring instrument. Can do. In the present invention, JAPAN TAPPI paper pulp test method no. It was measured using Kajaani Fiber Lab V3.5 (manufactured by Metso Automation) according to 52 “Fiber length test method for paper and pulp (automatic optical measurement method)”. The “fiber length”, “average fiber length” and “fiber length distribution” of the solvent-spun cellulose fibers beaten are the “length-weighted fiber length” and “length-weighted average fiber length” measured and calculated according to the above. ”And“ length-weighted fiber length distribution ”.

  In addition, the length-weighted average fiber length can be adjusted in any way within the range of 0 to 400 ml of modified freeness by changing the conditions for refinement. Even so, solvent-spun cellulose fibers formed by beating different length-weighted average fiber lengths can be produced. FIG. 3 shows the modified freeness of solvent-spun cellulose fibers used in Examples 11 to 24 of the present invention.

  Solvent-spun cellulose fibers having a modified freeness of 0 to 400 ml and a length-weighted average fiber length of 0.20 to 2.00 mm have a narrow fiber diameter and high uniformity. It is formed uniformly. The solvent-spun cellulose fiber of the present invention preferably has a modified freeness of 0 to 300 ml, more preferably 0 to 250 ml. If the modified freeness exceeds 400 ml, the refinement process is insufficient, the fiber division does not proceed sufficiently, and the proportion of the original thick fiber diameter remains large, so a large through-hole is opened in the separator. It becomes easy. The solvent-spun cellulose fiber of the present invention preferably has a length weighted average fiber length of 0.40 to 1.80 mm, and more preferably 0.50 to 1.50 mm. When the thickness is less than 0.20 mm, the drainage property is deteriorated, and the papermaking property is sometimes deteriorated. If it exceeds 2.00 mm, the fineness is insufficient, the ratio of the thick fiber diameter is increased, the large through-holes are easily opened in the separator, and the fibers are twisted, resulting in variations in formation and thickness. The length-weighted average fiber length of the solvent-spun cellulose fiber in the present invention can be measured using a commercially available fiber length measuring device that is obtained by utilizing the deflection characteristics obtained by applying laser light to the fiber.

  Furthermore, in the present invention, in a solvent-spun cellulose fiber having a modified freeness of 0 to 400 ml and a length-weighted average fiber length of 0.20 to 2.00 mm, a fiber length distribution histogram is obtained. As a result of detailed examination, when the first fiber length distribution described below is obtained, the effect of improving the liquid absorbency of the electrolytic solution is obtained, and when the second fiber length distribution is provided, the papermaking property and the liquid absorbency are obtained. It has been found that it is more preferable because the effect of satisfying both is obtained.

  In the solvent-spun cellulose fiber that is beaten, the preferred first fiber length distribution has a maximum frequency peak between 0.00 and 1.00 mm in the fiber length distribution histogram, and a fiber length of 1.00 mm or more. The ratio of the fiber having 10% or more. It has been found that the solvent-spun cellulose fiber that has been beaten in this way has good electrolyte absorbency. Moreover, when the inclination of the ratio of the fiber having a fiber length of 0.05 mm between 1.00 and 2.00 mm is −3.0 or more and −0.5 or less, the liquid absorbency of the electrolytic solution is more excellent And found that it is more preferable.

  FIG. 4 and FIG. 5 are fiber length distribution histograms of solvent-spun cellulose fibers formed by beating, fibers having a maximum frequency peak between 0.00 and 1.00 mm and having a fiber length of 1.00 mm or more. Is 10% or more. In terms of liquid absorbency, more preferably, in the fiber length distribution histogram, the ratio of fibers having a maximum frequency peak between 0.30 and 0.70 mm and having a fiber length of 1.00 mm or more is 12% or more. However, it is sufficient if it is about 50%.

  In the fiber length distribution histogram of solvent-spun cellulose fibers formed by beating, the slope of the ratio of fibers having a fiber length of 0.05 mm between 1.00 and 2.00 mm is −3.0 or more and −0.5 or less. -2.5 or more and -0.8 or less are more preferable, and -2.0 or more and -1.0 or less are more preferable. When the inclination is smaller than −3.0, the fiber gap is narrowed and the liquid absorbency may be lowered. On the other hand, if the inclination exceeds -0.5, the separator may be unsatisfactory. As shown in FIG. 4 and FIG. 5, “large inclination” means that the fiber length distribution of the solvent-spun cellulose fiber formed by beating is wide. “Slight inclination” is a state in which the fiber length distribution of the solvent-spun cellulose fibers formed by beating is narrow and the fiber lengths are more uniform. Note that the slope of the solvent-spun cellulose fiber [I] in FIG. 4 is -2.9, and the slope of the solvent-spun cellulose fiber [II] in FIG. 5 is -0.6. is there.

  In addition, "the inclination of the ratio of the fiber having a fiber length of every 0.05 mm between 1.00 and 2.00 mm" is 0. 0 between 1.00 and 2.00 mm as shown in FIG. For the value of the proportion of fibers having a fiber length of every 05 mm, an approximate straight line is calculated by the method of least squares, and the inclination of the obtained approximate straight line is meant.

  In the solvent-spun cellulose fiber formed by beating, the preferred second fiber length distribution has a maximum frequency peak between 0.00 and 1.00 mm in the fiber length distribution histogram of the solvent-spun cellulose fiber formed by beating. The ratio of fibers having a fiber length of 1.00 mm or more is 50% or more. The solvent-spun cellulose fiber thus beaten does not become too narrow and too wide, and can achieve both papermaking properties and liquid absorption. Moreover, when it has a peak between 1.50 and 3.50 mm other than a maximum frequency peak, since a liquid absorptivity improves rather than the solvent spinning cellulose fiber which does not have such a peak, it is preferable.

  FIG. 7 is a fiber length distribution histogram of the solvent-spun cellulose fiber formed by beating, and the ratio of fibers having a maximum frequency peak between 0.00 and 1.00 mm and having a fiber length of 1.00 mm or more. 50% or more. In terms of liquid absorbency, in the fiber length distribution histogram, the ratio of fibers having a maximum frequency peak between 0.30 and 0.70 mm and having a fiber length of 1.00 mm or more is 55% or more. More preferred. A higher proportion of fibers having a fiber length of 1.00 mm or more is desirable, but about 75% is sufficient.

  In the fiber length distribution histogram of the solvent-spun cellulose fiber that is beaten, it is more preferable to have a peak between 1.50 and 3.50 mm in addition to the above maximum frequency peak, as shown in FIG. It is more preferable to have a peak between .75 and 3.25 mm, and it is particularly preferable to have a peak between 1.90 and 3.00 mm. Having a peak in this range is preferable because the liquid absorbency is further improved. When the fiber length of the peak is shorter than 1.50 mm, the liquid absorbability may be deteriorated. On the other hand, if the thickness exceeds 3.50 mm, lumps may be generated, resulting in uneven thickness, and the separator may be unsatisfactory, or the fiber gap may become too wide, resulting in poor liquid absorbency.

  In order to obtain a solvent-spun cellulose fiber having a modified freeness of 0 to 400 ml and a length-weighted average fiber length of 0.20 to 2.00 mm, the solvent-spun cellulose short fibers are dispersed in water at an appropriate concentration. , A refiner, a beater, a mill, a grinding device, a rotary blade homogenizer that applies a shearing force by a high-speed rotating blade, a shearing force is generated between a cylindrical inner blade that rotates at high speed and a fixed outer blade It passes through a double-cylindrical high-speed homogenizer, an ultrasonic crusher that is miniaturized by impact by ultrasonic waves, a high-pressure homogenizer, etc., and adjusts the conditions such as blade shape, sample concentration, flow rate, number of treatments, treatment speed, etc. What is necessary is just to process.

  The content of the solvent-spun cellulose fiber having a modified freeness of 0 to 400 ml and a length-weighted average fiber length of 0.20 to 2.00 mm in the separator of the present invention is preferably 30 to 80% by mass, 30 -70 mass% is more preferable, and 30-59 mass% is further more preferable. If it is less than 30% by mass, a large through hole may be formed. When the amount is more than 80% by mass, the puncture strength may be insufficient or the papermaking property may be deteriorated.

  The separator of the present invention may contain fibers other than synthetic short fibers and solvent-spun cellulose fibers formed by beating. For example, natural cellulose fibers, pulped or fibrillated natural cellulose fibers, solvent-spun cellulose short fibers, fibrils, pulped or fibrillated synthetic resins, and inorganic fibers may also be included. The pulped or fibrillated natural cellulose fiber preferably has a Canadian standard freeness of 0 to 700 ml or a modified freeness of 0 to 400 ml. Examples of the inorganic fiber include glass, alumina, silica, ceramics, and rock wool. When inorganic fiber is contained, it is preferable because the heat resistant dimensional stability and puncture strength of the separator are improved.

  The separator of the present invention is a method of paper making using a circular paper machine, a long paper machine, a short paper machine, an inclined paper machine, a combination paper machine in which the same or different kinds of paper machines are combined from these, etc. Can be manufactured by. In addition to the fiber raw material, a dispersant, a thickener, an inorganic filler, an organic filler, an antifoaming agent, a release agent and the like are appropriately added to the raw material slurry as necessary, and a solid content of about 5 to 0.001% by mass Adjust slurry to concentration. This raw slurry is further diluted to a predetermined concentration to make paper. The separator for an electrochemical element obtained by papermaking is subjected to calendering, thermal calendering, heat treatment and the like as necessary.

  10-50 micrometers is preferable, as for the thickness of the separator of this invention, 10-45 micrometers is more preferable, and 10-40 micrometers is more preferable. If it is less than 10 μm, sufficient mechanical strength cannot be obtained, or the insulation between the positive electrode and the negative electrode is insufficient and the internal short circuit defect rate is increased. When it is thicker than 50 μm, the internal resistance of the electrochemical element increases.

Density of the separator in the present invention is not particularly limited but is preferably 0.250~0.700g / cm ^3, more preferably 0.250~0.650g / cm ^3, 0.300~0.600 / cm ³ is more preferable. If it is less than 0.250 g / cm ³ , sufficient mechanical strength may not be obtained, or the insulation between the positive electrode and the negative electrode may be insufficient and the internal short circuit failure rate may increase. If it exceeds 0.700 g / cm ³ , the internal resistance of the electrochemical device may increase. The density is adjusted by calendaring.

  The Gurley air permeability (JIS P8117) of the separator in the present invention is preferably 0.4 to 20.0 s / 100 ml, more preferably 1.0 to 15.0 s / 100 ml, and 1.0 to 10.0 s / 100 ml. Further preferred. If it is less than 0.4 s / 100 ml, the internal short circuit failure rate is high, and if it is greater than 20.0 s / 100 ml, the internal resistance is high.

  As the electrochemical element in the present invention, a capacitor is particularly preferable.

  The capacitor means an electric double layer capacitor, a lithium ion capacitor, a hybrid capacitor, or a redox capacitor. In the electric double layer capacitor, an electric double layer is formed at the interface between the electrode and the electrolytic solution to store electricity. As the electrode active material, carbon materials such as activated carbon, carbon black, carbon aerogel, carbon nanotube, and non-porous carbon are mainly used. As an electrolytic solution, an aqueous solution in which an ion dissociable salt is dissolved, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, acetonitrile, γ-butyrolactone, dimethylformamide, tetrahydrofuran, dimethoxyethane, dimethoxymethane, sulfolane, dimethyl sulfoxide, Examples include, but are not limited to, those obtained by dissolving an ion dissociable salt in an organic solvent such as ethylene glycol, propylene glycol, methyl cellosolve, and mixed solvents thereof, and ionic liquids (solid molten salts). is not.

In the lithium ion capacitor, the negative electrode active material is a material capable of reversibly supporting lithium ions, and the positive electrode active material is a material capable of reversibly supporting lithium ions and / or anions. This is a capacitor in which lithium ions are supported. Examples of the negative electrode active material include graphite, non-graphitizable carbon, polyacene organic semiconductor, and lithium titanate. Examples of the positive electrode active material include conductive polymers such as polypyrrole, polythiophene, polyaniline, and polyacetylene, activated carbon, and polyacene organic semiconductor. As the electrolytic solution, an aprotic organic solvent of a lithium salt is used. Examples of the lithium salt include LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiPF ₆ , and Li (C ₂ F ₅ SO ₂ ) N. Examples of the aprotic organic solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, acetonitrile, γ-butyrolactone, dimethylformamide, tetrahydrofuran, dimethoxyethane, dimethoxymethane, sulfolane, dimethyl sulfoxide, ethylene glycol, propylene glycol, and methyl. Cellsolve and mixed solvents thereof may be mentioned.

A hybrid capacitor is a capacitor in which the reaction mechanism or electrode material of the positive electrode and the negative electrode are different. For example, the negative electrode is an oxidation-reduction reaction, and the positive electrode is an electric double layer reaction. Examples of the negative electrode active material of the hybrid capacitor include activated carbon, graphite, hard carbon, polyacene, metal oxide such as Li ₄ Ti ₅ O ₁₂ , and n-type conductive polymer. Examples of the positive electrode active material include activated carbon, metal oxides such as MnO ₂ , LiCoO ₂ , ruthenium oxide, graphite, and p-type conductive polymer. Examples thereof include carbon black, carbon aerogel, carbon nanotube, and nonporous carbon.

A redox capacitor has a storage and discharge mechanism that uses all or part of oxidation / reduction of an electrode active material, adsorption / desorption of ions on the electrode surface, and charge / discharge in an electric double layer. Examples of electrode active materials of redox capacitors include metal oxides such as ruthenium oxide, iridium oxide, titanium oxide, zirconium oxide, nickel oxide, vanadium oxide, tungsten oxide, manganese oxide, and cobalt oxide, and composites of these metal oxides. Hydrates of these metal oxides, composites of these metal oxides and carbon materials, molybdenum nitride, composites of molybdenum nitride and metal oxides, graphite capable of intercalating lithium ions, and Li ₄ Ti ₅ O ₁₂ , lithium metal oxides such as LiFePO ₄ , polypyrrole, polyaniline, polythiophene, polyacene, derivatives thereof, polyfluorene derivatives, polyquinoxaline derivatives, polyindoles, cyclic indole polymers, 1,5-diaminoanthraquinone, 1, Examples include 4-benzoquinone, graphite and a complex of these quinone compounds, and metal complex polymers. As an electrolytic solution, an aqueous solution in which an ion dissociable salt is dissolved, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, acetonitrile, γ-butyrolactone, dimethylformamide, tetrahydrofuran, dimethoxyethane, dimethoxymethane, sulfolane, dimethyl sulfoxide, Examples include, but are not limited to, those obtained by dissolving an ion dissociable salt in an organic solvent such as ethylene glycol, propylene glycol, methyl cellosolve, and mixed solvents thereof, and ionic liquids (solid molten salts). is not.

  The capacitor electrode preferably contains a conductive agent. The conductive agent is not particularly limited, but carbon materials such as carbon black, ketjen black (registered trademark of ketjen black international), acetylene black, carbon whisker, carbon fiber, natural graphite, artificial graphite, carbon nanofiber, and carbon nanotube , Particles containing titanium dioxide, ruthenium oxide, aluminum, nickel, silver and the like and metals containing metal fibers. 0.1-20 mass parts is preferable with respect to 100 mass parts of electrode active materials, and, as for the compounding quantity of a electrically conductive agent, 2-10 mass parts is more preferable.

  The binder used for the capacitor electrode must be sufficiently bonded to the electrode active material and the conductive agent, and is selected from those having resistance to the electrolytic solution, voltage resistance, and resistance to the oxidation-reduction reaction. Examples include water-insoluble resins such as polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyethylene terephthalate, polybutylene terephthalate, polyamide, polyamideimide, polytetrafluoroethylene, polyimide, polyethylene naphthalate, polyphenylene sulfide, carboxy Cellulose derivatives such as methyl cellulose, starch and derivatives thereof, casein, sodium alginate, polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylamide, polyvinyl methyl ether, polyethylene glycol, and other water-soluble resins, and rubbers such as styrene butadiene rubber and acrylonitrile butadiene rubber Can be mentioned. As for the compounding quantity of a binder, 0.05-25 mass parts is preferable with respect to 100 mass parts of electrode active materials.

  The thickness of the capacitor electrode is preferably 10 to 300 μm. If the thickness is less than 10 μm, it may be difficult to obtain a sufficient electric capacity in the capacitor, and if the thickness exceeds 300 μm, the internal resistance may increase.

  In general, the electrode for a capacitor is produced by dry kneading or wet kneading an electrode active material, a conductive agent, and a binder, and then molding the sheet into a sheet or rod by a press molding method or an extrusion molding method, and punching or cutting. In addition, there are known a production method in which an electrode slurry containing an electrode active material, a conductive agent, and a binder is applied to the surface of the current collector and dried. Any method can be applied in the present invention. The electrode slurry coating method is not particularly limited, and a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure coating method, a screen printing method, and the like can be applied.

  The material for the current collector includes a conductive material, and examples thereof include gold, silver, copper, aluminum, nickel, and platinum. Examples of the shape of the current collector include, but are not limited to, a plate shape, a fiber shape, a sheet shape, a film shape, and a mesh shape.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in more detail, this invention is not limited to an Example.

<< Examples 1 to 10, Comparative Examples 1 to 3 >>
[Physical properties of solvent-spun cellulose fiber formed by beating]
About solvent-spun cellulose fiber that is beaten
(1) Modified freeness measured in accordance with JIS P8121, except that an 80-mesh wire mesh with a wire diameter of 0.14 mm and an aperture of 0.18 mm was used as the sieve plate, and the sample concentration was 0.1%. Law Freeness "
(2) Length-weighted average fiber length: “Average fiber length”
Is shown in Table 1. Solvent-spun cellulose fibers C1 to C11 that are beaten are produced by treating solvent-spun cellulose short fibers that are not beaten (fineness 1.7 dtex, fiber length 6 mm, manufactured by Coatles) using a double disc refiner. did.

  Table 1 shows fibers used in Examples and Comparative Examples of the present invention. C12 in Table 1 means that the linter fiber is fibrillated with a high-pressure homogenizer. The split type composite fiber F6 means a 16 split type composite fiber in which an ethylene-vinyl alcohol copolymer and polypropylene are adjacent to each other, the fiber diameter before splitting is 16 μm, and the short axis length of the cross section after splitting. Is 2.95 μm. The raw material symbols in Table 2 correspond to the fiber symbols in Table 1. According to the blending ratio shown in Table 2, a papermaking slurry was prepared.

Example 1
After F2 was dispersed in water with a pulper, C1 was added and stirred for a predetermined time to prepare slurry 1. Slurry 1 was fed to a circular paper machine and wet-made, and was dried at a Yankee dryer temperature of 110 ° C. to produce the separator of Example 1.

(Examples 2, 3, 5-7, 9, 10)
Similarly to the preparation of the slurry 1, after the synthetic short fibers are dispersed in water with a pulper, the solvent-spun cellulose fibers formed by beating are added and stirred for a predetermined time, and the slurries 2, 3, 5-7, 9, 10 are added. Was prepared. Wet papermaking was performed in the same manner as Example 1 using Slurries 2, 3, 5-7, 9, and 10, and separators of Examples 2, 3, 5-7, 9, and 10 were produced.

(Examples 4 and 8)
After dispersing C12 in water with a pulper, synthetic short fibers were added and stirred for a predetermined time, and solvent-spun cellulose fibers formed by beating were further added and stirred for a predetermined time to prepare slurries 4 and 8. Using the slurries 4 and 8, wet papermaking was performed in the same manner as in Example 1 to produce separators of Examples 4 and 8.

(Comparative Example 1)
After F1 and F2 were dispersed in water with a pulper, C9 was added and stirred for a predetermined time to prepare slurry 11. The slurry 11 was fed to a circular paper machine and wet-made, and dried at a Yankee dryer temperature of 120 ° C. to produce a separator of Comparative Example 1.

(Comparative Examples 2 and 3)
Similarly to the slurry 11, after the synthetic short fiber was dispersed in water with a pulper, the solvent-spun cellulose fiber formed by beating was added and stirred for a predetermined time to prepare slurry 12 and 13. Wet paper making was performed in the same manner as Comparative Example 1 using Slurries 12 and 13, and separators of Comparative Examples 2 and 3 were produced.

[Evaluation]
The separators prepared in Examples 1 to 10 and Comparative Examples 1 to 3 were evaluated as follows, and the results are shown in Table 3.

<Thickness>
The thickness was measured according to JIS P8118, and the average value was calculated.

<Density>
The basis weight of the separator was measured in accordance with JIS P8124, and the value obtained by dividing the basis weight by the thickness and multiplying by 100 was defined as the density.

<Making paper>
“◎” indicates that the separator can be stably wet-made at a papermaking speed of 40 m / min or more, and “◯” indicates that the paper can be stably formed at a papermaking speed of 20 m / min or more and less than 40 m / min. The case where stable wet papermaking was possible at a papermaking speed of not less than 20 m / min but not less than 20 m / min was indicated as “Δ”, and the case where wet papermaking was stable only at a papermaking speed of less than 10 m / min was indicated as “x”. Higher papermaking speed means better papermaking properties.

<Liquid absorption>
A sample in which the separator was cut to a width of 20 mm and a length of 150 mm was prepared. At this time, the direction cut to a length of 150 mm was the CD direction, that is, the direction perpendicular to the winding length direction of the separator winding (width direction). The lower end 10 mm of the separator sample was immersed and fixed in the electrolytic solution, and the suction height when it was allowed to stand for 10 minutes was measured. The higher the suction height, the better. As the electrolytic solution, a solution obtained by dissolving (C ₂ H ₅ ) ₃ (CH ₃ ) NBF ₄ in propylene carbonate so as to have a concentration of 1.5 mol / l was used.

<Electric double layer capacitor>
[Production of electrodes]
10 parts by mass of polyvinylidene fluoride is dissolved in 90 parts by mass of N-methyl-2-pyrrolidone, and 80 parts by mass of powdered activated carbon having an average particle size of 5.0 μm and a specific surface area of 2000 m ² / g starting from a phenol resin. Then, 10 parts by mass of acetylene black having an average particle diameter of 200 nm and 300 parts by mass of N-methyl-2-pyrrolidone were added and mixed sufficiently with a mixing stirrer to obtain an electrode slurry. After applying and drying the above electrode slurry to a 30 μm thick aluminum foil current collector whose surface has been etched with hydrochloric acid using an applicator, a press process is performed using a roll press device to produce a 150 μm thick electrode did.

[Production of electric double layer capacitor]
Two electrodes were cut into 30 mm × 50 mm squares, and laminated so that the separators of Examples 1 to 10 and Comparative Examples 1 to 3 were interposed between the electrodes. This was stored in an aluminum storage bag, and after vacuum heating at 150 ° C. for 10 hours, an electrolyte was injected into the aluminum storage bag, and the inlet was sealed, and Examples 1 to 10 and Comparative Examples 1 to 3 electric double layer capacitors were produced. As the electrolytic solution, a solution obtained by dissolving (C ₂ H ₅ ) ₃ (CH ₃ ) NBF ₄ in propylene carbonate so as to have a concentration of 1.5 mol / l was used.

[Evaluation]
The electric double layer capacitors produced in the examples and comparative examples were evaluated as follows, and the results are shown in Table 3.

[DC resistance]
Using the electric double layer capacitor of Example and Comparative Example, constant current charge / discharge was repeated 10 cycles at a charge / discharge voltage range of 0 to 2.7 V and a charge / discharge current of 200 mA. The resistance was calculated, and the average value of 100 electric double layer capacitors was defined as the DC resistance.

[Capacity maintenance rate]
Using the electric double layer capacitors of Examples and Comparative Examples, constant current charging was performed up to 2.7 V at a charging current of 200 mA, and after reaching 2.7 V, constant voltage charging was performed for 1 hour. The electrostatic capacity when discharged to 0 V at 25 ° C. and a discharge current of 200 mA after completion of charging was defined as the initial capacity. Next, charging / discharging was repeated for 1000 hours at 60 ° C. and a current of 1 A, the ratio of the capacitance after 1000 hours to the initial capacity was calculated, and the average value of 100 electric double layer capacitors was taken as the capacity retention rate.

  The separators of Examples 1 to 10 are made of wet nonwoven fabric containing synthetic short fibers and beating solvent-spun cellulose fibers as essential components, and the modified freeness of solvent-spun cellulose fibers being beaten is 0 to 400 ml. And since the length weighted average fiber length is 0.20-2.00 mm, it was excellent in the liquid absorption property of electrolyte solution. The electric double layer capacitors formed using the separators of Examples 1 to 10 were excellent in that the DC resistance was low and the discharge capacity retention rate was high.

  On the other hand, in the separator of Comparative Example 1, since the length-weighted average fiber length of the solvent-spun cellulose fiber formed by beating is less than 0.20 mm, the drainage is poor, the papermaking property is poor, and the fiber gap is narrow. The liquidity was slightly bad. The electric double layer capacitor using the separator had a high DC resistance and a poor discharge capacity retention rate.

  In the separator of Comparative Example 2, since the length-weighted average fiber length of the solvent-spun cellulose fiber formed by beating exceeded 2.00 mm, the paper-making property was good, but there were uneven spots and the liquid-absorbing property was bad. . The electric double layer capacitor using the separator had a high DC resistance and a poor discharge capacity retention rate.

  The separator of Comparative Example 3 had good papermaking properties because the modified freeness of solvent-spun cellulose fibers beaten up exceeded 400 ml and the length-weighted average fiber length exceeded 2.00 mm. There was uneven formation, and the liquid absorption was bad. The electric double layer capacitor using the separator had a high DC resistance and a poor discharge capacity retention rate.

<< Examples 11 to 24 >>
[Physical properties of solvent-spun cellulose fiber formed by beating]
About solvent-spun cellulose fiber that is beaten
(1) Fiber length of maximum frequency peak in fiber length distribution histogram: “Fiber length of maximum frequency peak”
(2) Ratio of fibers having a fiber length of 1.00 mm or more: “fiber ratio of 1.00 mm or more”
(3) In the fiber length distribution histogram, the slope of the proportion of fibers having a fiber length of every 0.05 mm between 1.00 and 2.00 mm: “ratio of the proportion”
(4) Length-weighted average fiber length: “Average fiber length”
(5) Modified freeness measured according to JIS P8121, except that an 80-mesh wire mesh with a wire diameter of 0.14 mm and an aperture of 0.18 mm was used as the sieve plate, and the sample concentration was 0.1%. Law Freeness "
Is shown in Table 4. Solvent-spun cellulose fibers C13 to C24 that have been beaten are produced by treating solvent-spun cellulose short fibers that have not been beaten (fineness 1.7 dtex, fiber length 6 mm, manufactured by Cortols) using a double disc refiner. did.

  The slurry shown in Table 5 was prepared, and the slurry corresponding to Examples 11 to 24 was wet-paper-made using a double-type circular net paper machine, and dried at a Yankee dryer temperature of 110 ° C. to produce a wet nonwoven fabric. did. Next, according to the heat treatment temperature and heat treatment time shown in Table 6, the front and back surfaces of the wet nonwoven fabric were brought into contact with the metal roll, heat treated, and further calendered to adjust the thickness, and separators of Examples 11 to 24 were produced. The separator was evaluated in the same manner as in Example 1, and the evaluation results are shown in Table 6.

  The raw material symbols in Table 5 correspond to the symbols in Tables 1 and 4.

[Evaluation]
The electric double layer capacitors produced using the separators produced in Examples 11 to 24 were evaluated in the same manner as in Example 1, and the results are shown in Table 6.

  The separators produced in Examples 11 to 22 are made of wet nonwoven fabric containing synthetic short fibers and beaten solvent-spun cellulose fibers as essential components, and the modified freeness of solvent-spun cellulose fibers obtained by beating is 0. In a fiber length distribution histogram of solvent-spun cellulose fibers having a length-weighted average fiber length of 0.20 to 2.00 mm and a beat-weighted average fiber length of .about.400 ml, the maximum frequency is between 0.00 and 1.00 mm. Since the ratio of fibers having a peak and a fiber length of 1.00 mm or more is 10% or more, the electrolyte solution was excellent in liquid absorbency. The electric double layer capacitor provided with the separator has a low DC resistance and a high capacity retention rate.

  The separator produced in Example 23 does not have a maximum frequency peak between 0.00 and 1.00 mm in the fiber length distribution histogram of the solvent-spun cellulose fiber formed by beating. The liquid property was inferior to Examples 11-22. The DC resistance of the electric double layer capacitor comprising the separator was inferior to that of Examples 11 to 21, and the capacity retention rate was inferior to that of Examples 11 to 22.

  In the separator produced in Example 24, the ratio of fibers having a fiber length of 1.00 mm or more is less than 10% in the fiber length distribution histogram of the solvent-spun cellulose fiber formed by beating, so that the fiber gap is narrow and the absorption is short. The liquid property was inferior to Examples 11-22. The DC resistance and capacity retention of the electric double layer capacitor comprising the separator were inferior to those of Examples 11-22.

  The separators produced in Examples 11 to 16 and 18 to 21 are fibers having a fiber length of every 0.05 mm between 1.00 and 2.00 mm in a fiber length distribution histogram of solvent-spun cellulose fibers formed by beating. Since the slope of the ratio of −3.0 or more and −0.5 or less, the liquid absorbency was more excellent.

  In the separator produced in Example 17, in the fiber length distribution histogram of solvent-spun cellulose fibers formed by beating, the slope of the ratio of fibers having a fiber length of every 0.05 mm between 1.00 and 2.00 mm is − Since it was a minus side from 3.0, the fiber gap was somewhat narrow, and the liquid absorbency was inferior to Examples 11-16 and 18-21. The DC resistance and capacity retention ratio of the electric double layer capacitor comprising the separator were inferior to those of Examples 11-16 and 18-21.

  In the separator produced in Example 22, in the fiber length distribution histogram of solvent-spun cellulose fibers formed by beating, the slope of the ratio of fibers having a fiber length of every 0.05 mm between 1.00 and 2.00 mm is − Since it was on the plus side of 0.5, the paper-making property was good, but the liquid absorbency was inferior to Examples 11-16 and 18-21. The DC resistance and capacity retention ratio of the electric double layer capacitor comprising the separator were inferior to those of Examples 11-16 and 18-21.

<< Examples 25-37 >>
[Physical properties of solvent-spun cellulose fiber formed by beating]
Regarding solvent-spun cellulose fiber formed by beating (1) Ratio of fiber having fiber length of 1.00 mm or more: “fiber ratio of 1.00 mm or more”
(2) Fiber length of maximum frequency peak in fiber length distribution histogram: “Fiber length of maximum frequency peak”
(3) Fiber length of peaks other than the maximum frequency peak: “fiber length of second peak”
(4) Length-weighted average fiber length: “Average fiber length”
(5) Freeness measured in accordance with JIS P8121, except that an 80-mesh wire mesh with a wire diameter of 0.14 mm and an aperture of 0.18 mm was used as the sieve plate, and the sample concentration was 0.1%. Water level "
Is shown in Table 7. Solvent-spun cellulose fibers C25 to C35 that have been beaten are produced by treating solvent-spun cellulose short fibers that have not been beaten (fineness 1.7 dtex, fiber length 6 mm, manufactured by Cortols) using a double disc refiner. did.

  In the same manner as slurry 1, slurries 28 to 40 shown in Table 8 were prepared. The fiber symbols in Table 8 correspond to the symbols in Table 1 and Table 7. Slurries corresponding to Examples 25 to 37 were fed to a circular paper machine and wet-made, and dried at a Yankee dryer temperature of 110 ° C. to produce separators of Examples 25 to 37. The separator was evaluated in the same manner as in Example 1, and the evaluation results are shown in Table 9.

  Using the separators produced in Examples 25 to 37, an electric double layer capacitor was produced in the same manner as in Example 1. The electric double layer capacitor was evaluated in the same manner as in Example 1, and the evaluation results were obtained. It is shown in Table 9.

  The separators produced in Examples 25 to 35 are composed of a wet nonwoven fabric containing synthetic short fibers and beaten solvent-spun cellulose fibers as essential components, and the modified freeness of solvent-spun cellulose fibers is 0. In a fiber length distribution histogram of solvent-spun cellulose fibers having a length-weighted average fiber length of 0.20 to 2.00 mm and a beat-weighted average fiber length of .about.400 ml, the maximum frequency is between 0.00 and 1.00 mm. Since the proportion of fibers having a peak and having a fiber length of 1.00 mm or more is 50% or more, the papermaking property and the liquid absorption property of the electrolytic solution were excellent.

  The separator produced in Example 36 does not have a maximum frequency peak between 0.00 and 1.00 mm in the fiber length distribution histogram of the solvent-spun cellulose fiber formed by beating, and the maximum frequency peak exceeds 1.00 mm. However, the paper absorbability was inferior to those of Examples 25 to 35. The DC resistance and capacity retention of the electric double layer capacitor comprising the separator were inferior to those of Examples 25-35.

  In the separator produced in Example 37, the ratio of the fibers having a fiber length of 1.00 mm or more is less than 10% in the fiber length distribution histogram of the solvent-spun cellulose fiber formed by beating, so that the fiber gap is narrow and The liquid absorbency of the liquid was inferior to Examples 25-35. The electric double layer capacitor comprising the separator was inferior to Examples 25 to 35 in DC resistance and capacity retention.

  The separators produced in Examples 25-29 and 31-34 have a peak between 1.50 and 3.50 mm in addition to the maximum frequency peak in the fiber length distribution histogram of solvent-spun cellulose fibers formed by beating, Excellent papermaking properties. The electric double layer capacitor provided with the separator has a low DC resistance and a high capacity retention rate.

  In the fiber length distribution histogram of the solvent-spun cellulose fiber formed by beating the separator produced in Example 30, since the peak other than the maximum frequency peak is shorter than 1.50 mm, the fiber gap is narrow and the liquid absorbency is implemented. It was inferior to Examples 25-29 and 31-34. The DC resistance and capacity retention of the electric double layer capacitor comprising the separator were inferior to those of Examples 25-29 and 31-34.

  The separator produced in Example 35 was good in papermaking properties because the peak present in addition to the maximum frequency peak was longer than 3.50 mm in the fiber length distribution histogram of the solvent-spun cellulose fibers formed by beating, but the liquid absorbency was good. However, it was inferior to Examples 25-29 and 31-34. The DC resistance and capacity retention of the electric double layer capacitor comprising the separator were inferior to those of Examples 25-29 and 31-34.

  As an application example of the present invention, a separator for an electrochemical element such as an electric double layer capacitor and a lithium ion capacitor, an electric double layer capacitor using the separator, and an electrochemical element such as a lithium ion capacitor are suitable.

Claims (6)

Synthetic short fibers, which are wet nonwoven fabrics containing, as an essential component, beaten solvent-spun cellulose fibers, have a modified freeness of 0 to 400 ml as defined below for the beaten solvent-spun cellulose fibers, and The separator for electrochemical elements whose length weighted average fiber length is 0.20-2.00 mm.
Modified freeness: Freeness measured in accordance with JIS P8121, except that an 80-mesh wire mesh having a wire diameter of 0.14 mm and an aperture of 0.18 mm was used as the sieve plate, and the sample concentration was 0.1%.   In the fiber length distribution histogram of solvent-spun cellulose fibers beaten, the maximum frequency peak is between 0.00 and 1.00 mm, and the ratio of fibers having a fiber length of 1.00 mm or more is 10% or more. The separator for an electrochemical element according to claim 1.   In the fiber length distribution histogram of solvent-spun cellulose fibers formed by beating, the slope of the ratio of fibers having a fiber length of 0.05 mm between 1.00 and 2.00 mm is −3.0 or more and −0.5 or less. The separator for an electrochemical element according to claim 2.   In the fiber length distribution histogram of the solvent-spun cellulose fiber that is beaten, the maximum frequency peak is between 0.00 and 1.00 mm, and the proportion of fibers having a fiber length of 1.00 mm or more is 50% or more. The separator for an electrochemical element according to claim 1.   The separator for an electrochemical element according to claim 4, wherein in the fiber length distribution histogram of the solvent-spun cellulose fiber formed by beating, there is a peak between 1.50 and 3.50 mm in addition to the maximum frequency peak.   The electrochemical element which comprises the separator for electrochemical elements in any one of Claims 1-5.