Separators for electrochemical elements

Abstract

The present invention relates to a separator for electrochemical elements, which comprises a synthetic fiber and a beaten cellulose fiber, wherein the Canadian standard type freeness of the beaten cellulose fiber as measured in accordance with JIS P 8121 is 50 ml to 500 ml inclusive, and, in a fiber diameter distribution histogram of the beaten cellulose fiber, (1) the fiber has a maximum frequency peak in the range of 50 μm or less and (2) the percentage of the amount of the fiber having a fiber diameter of 20 μm or less is 55% or more.

Description

[Technical Field]

[0001] This patent application claims priority under the Paris Convention to Japanese Patent Application No. 2019-234225 (filing date: December 25, 2019), the entire contents of which are incorporated herein by reference. The present invention relates to a separator for an electrochemical element suitable for an electrochemical element, and an electrochemical element including the separator. [Background technology]

[0002] Electrochemical elements such as capacitors are capable of charging and discharging large currents, and have a long lifespan with little performance degradation due to repeated charging and discharging. In addition to applications such as backup power sources for small products such as mobile phones and smartphones, in recent years they have also been put to practical use and are being considered for applications such as power storage and stabilization, power assist, backup power sources, and energy regeneration for large products such as automobiles, digital multifunction peripherals, and vending machines.

[0003] In such electrochemical elements, a separator is used to separate the positive and negative electrodes. For example, Patent Document 1 discloses a separator for electrochemical elements made of a wet-laid nonwoven fabric containing, as essential components, synthetic short fibers and solvent-spun cellulose fibers having a specific modified freeness and a specific length-weighted average fiber length. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-222266 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, there has been a demand for thinner separators in electrochemical elements to reduce their capacity, with the aim of further improving the performance of electrochemical elements and reducing the size and / or weight of electrochemical elements. However, according to the studies of the present inventors, it has been found that it is difficult to thin conventional separators such as those described in the above-mentioned documents without reducing the basis weight, and that thinning the separator reduces the basis weight, which in turn reduces the strength of the separator and can cause problems such as reduced strength and durability of the electrochemical element.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a separator for an electrochemical element that can be made thinner while maintaining the strength of the separator. [Means for solving the problem]

[0007] The present inventors have conducted extensive research to solve the above problems and have completed the present invention. That is, the present invention provides the following preferred embodiments.

[0008] [1] A separator for an electrochemical element comprising synthetic fibers and beaten cellulose fibers, The Canadian standard freeness of the beaten cellulose fiber measured in accordance with JIS P 8121 is 50 ml or more and 500 ml or less, In the fiber diameter distribution histogram of the beaten cellulose fiber, (1) the fiber has a maximum frequency peak in the range of 50 μm or less, (2) The proportion of the fibers having a fiber diameter of 20 μm or less is 55% or more. Separators for electrochemical elements. [2] The separator for an electrochemical element according to [1], further comprising a binder. [3] The separator for an electrochemical element according to [1] or [2], wherein in a fiber diameter distribution histogram of the beaten cellulose fibers, the proportion of the fibers having a fiber diameter of more than 30 μm is 10% or less. [4] The separator for an electrochemical element according to any one of [1] to [3], wherein the synthetic fiber is a polyvinyl alcohol-based fiber and / or a polyester-based fiber. [5] The separator for an electrochemical element according to any one of [1] to [4], wherein the beaten cellulose fibers are natural cellulose fibers that have been beaten. [6] The separator for an electrochemical element according to any one of [2] to [5], wherein the binder is a polyvinyl alcohol-based binder. [7] The separator for an electrochemical element according to any one of [1] to [6], wherein the content of the synthetic fiber is 1% by mass or more and 55% by mass or less with respect to the total mass of the separator. [8] The separator for an electrochemical element according to any one of [1] to [7], wherein the content of the beaten cellulose fibers is 30% by mass or more and 95% by mass or less with respect to the total mass of the separator. [9] The separator for an electrochemical element according to any one of [2] to [8], wherein the content of the binder is 0.5% by mass or more and 20% by mass or less with respect to the total mass of the separator.

[10] The separator for an electrochemical element according to any one of [1] to [9], which has a thickness of 10 μm or more and less than 70 μm.

[11] An electrochemical element comprising the separator for an electrochemical element according to any one of [1] to

[10] . [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a separator for an electrochemical element that can be thinned while maintaining the strength of the separator. [Brief explanation of the drawings]

[0010] [Figure 1] Figure 1 shows a histogram of the fiber diameter distribution of natural cellulose fibers with a freeness of 25 ml. [Figure 2] Figure 2 shows a histogram of the fiber diameter distribution of natural cellulose fibers with a freeness of 150 ml. [Figure 3] Figure 3 shows a histogram of the fiber diameter distribution of natural cellulose fibers with a freeness of 250 ml. [Figure 4] Figure 4 shows a histogram of the fiber diameter distribution of natural cellulose fibers with a freeness of 350 ml. [Figure 5] Figure 5 shows a histogram of the fiber diameter distribution of natural cellulose fibers with a freeness of 550 ml. [Figure 6] Figure 6 shows a histogram of the fiber diameter distribution of organic solvent-based cellulose fibers with a freeness of 50 ml. [Figure 7] Figure 7 shows a histogram of the fiber diameter distribution of organic solvent-based cellulose fibers with a freeness of 250 ml. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Separators for electrochemical elements] The separator for an electrochemical element (hereinafter simply referred to as "separator") of the present invention comprises synthetic fibers and beaten cellulose fibers. The beaten cellulose fibers have a Canadian Standard Freeness of 50 ml to 500 ml, and in a fiber diameter distribution histogram of the beaten cellulose fibers, (1) the fibers have a maximum frequency peak in the range of 50 μm or less, and (2) the proportion of the fibers having a fiber diameter of 20 μm or less is 55% or more.

[0012] <Synthetic fiber> The separator of the present invention has high strength due to the inclusion of synthetic fibers.

[0013] Examples of synthetic fibers include polyvinyl alcohol fibers, ethylene-vinyl alcohol copolymer fibers, polyester fibers, polypropylene fibers, polyethylene fibers, polypropylene-polyethylene composite fibers, polyamide fibers, polyamide-modified polyamide composite fibers, etc. These may be used alone or in combination of two or more. Among these, polyvinyl alcohol-based fibers and polyester-based fibers are preferred synthetic fibers from the viewpoint of easily increasing the strength of the separator, and polyvinyl alcohol-based fibers are more preferred from the viewpoint of easily making the separator thinner and easily obtaining a separator with low resistance.

[0014] The vinyl alcohol polymer constituting the polyvinyl alcohol fiber is not particularly limited and may be, for example, a vinyl alcohol polymer having an average degree of polymerization of 1,000 to 5,000 and a degree of saponification of 95 mol % or more. The vinyl alcohol polymer may be a homopolymer of vinyl alcohol or a copolymer of vinyl alcohol and another copolymer component. When the vinyl alcohol polymer contains another copolymer component, the proportion of the copolymer component in the vinyl alcohol polymer is preferably 20 mol % or less, more preferably 10 mol % or less, from the viewpoint of water resistance, etc. Furthermore, the vinyl alcohol polymer may be subjected to a treatment such as acetalization, if necessary.

[0015] In the present invention, the polyvinyl alcohol-based fiber as a synthetic fiber may be composed solely of a vinyl alcohol-based polymer, or may be a conjugate spun fiber of a vinyl alcohol-based polymer and another polymer, a mixed spun fiber (sea-island fiber), or the like. From the viewpoints of electrolyte absorption and strength, the proportion of the polyvinyl alcohol-based polymer in the polyvinyl alcohol-based fiber is preferably 30% by weight or more, more preferably 50% by weight or more, and even more preferably 80% by weight or more, based on the total weight of the polyvinyl alcohol-based fiber. The upper limit of this proportion is not particularly limited and may be, for example, 100% by weight or less.

[0016] The polyvinyl alcohol-based synthetic fibers are preferably heat-treated from the viewpoint of the heat resistance of the separator. The heat treatment temperature is not particularly limited and may be, for example, 60° C. or higher and 210° C. or lower. The heat treatment time is also not particularly limited and may be, for example, 1 second or higher and 30 minutes or lower.

[0017] In one embodiment of the present invention, the number average fiber diameter of the synthetic fibers is preferably 0.1 μm or more, more preferably 0.5 μm or more, and even more preferably 1.0 μm or more, from the viewpoint of the shielding properties of the separator. Furthermore, from the viewpoint of easily thinning the separator, the number average fiber diameter is preferably 20 μm or less, more preferably 15 μm or less, and even more preferably 10 μm or less. The number average fiber diameter of the synthetic fibers can be measured using a "Fiber Tester" manufactured by Lorentzen & Wettre.

[0018] In one embodiment of the present invention, the fineness of the synthetic fibers is preferably 0.1 dtex or more, more preferably 0.2 dtex or more, from the viewpoint of the shielding properties (air permeability) of the separator. Furthermore, from the viewpoint of easily thinning the separator (easiness to obtain a thin separator), the fineness is preferably 1.0 dtex or less, more preferably 0.8 dtex or less, and even more preferably 0.6 dtex or less. The fineness of the synthetic fibers can be measured using an optical microscope or a scanning electron microscope (SEM).

[0019] The cross-sectional shape of the synthetic fibers may be, for example, circular, elliptical, cocoon-shaped, flat, etc., and the cocoon-shaped shape is preferred from the viewpoint of making the separator thinner.

[0020] In a preferred embodiment of the present invention, the synthetic fiber content is 1% by mass or more and 55% by mass or less, based on the total mass of the separator, from the viewpoint of easily increasing the strength of the separator. Furthermore, from the viewpoint of easily increasing the strength of the separator, the synthetic fiber content is more preferably 5% by mass or more, even more preferably 10% by mass or more, and particularly preferably 15% by mass or more, based on the total mass of the separator. Furthermore, from the viewpoint of easily thinning the separator, the synthetic fiber content is more preferably 50% by mass or less, even more preferably 40% by mass or less, and particularly preferably 30% by mass or less, based on the total mass of the separator.

[0021] <Beaten cellulose fiber> The separator of the present invention contains beaten cellulose fibers having a specific freeness and a specific fiber diameter distribution, which makes it easy to thin the separator without reducing the basis weight, thereby achieving both high strength and thinning. Note that beaten cellulose fibers refer to cellulose fibers that have been beaten.

[0022] The Canadian Standard Freeness of the beaten cellulose fiber of the present invention, measured according to JIS P 8121, is 50 ml or more and 500 ml or less. If the freeness is outside this range, it becomes difficult to ensure the function as a separator. Specifically, if the freeness is below the lower limit, the air permeability of the separator decreases excessively, making it easier to increase the resistance. If the freeness exceeds the upper limit, the air permeability increases excessively, making it easier to decrease the shielding property. Furthermore, if the freeness exceeds the upper limit, it becomes difficult to thin the separator. Furthermore, from the viewpoint of easily increasing the strength and shielding property of the separator, the freeness is preferably 100 ml or more, more preferably 150 ml or more, and even more preferably 200 ml or more. Furthermore, from the viewpoint of easily thinning the separator and easily reducing the resistance, the freeness is preferably 400 ml or less, more preferably 350 ml or less, and even more preferably 300 ml or less. The freeness can be adjusted by the degree of beating of the beaten cellulose fibers and the method of beating the cellulose fibers.

[0023] The beaten cellulose fiber of the present invention has a maximum frequency peak in the range of 50 μm or less in a fiber diameter distribution histogram of the beaten cellulose fiber. If the range having the maximum frequency peak exceeds the upper limit, it becomes difficult to maintain the separator strength while thinning the separator, making it difficult to obtain a thin separator with high strength. Furthermore, in one embodiment of the present invention, from the viewpoint of easily obtaining a thinner separator, the upper limit of the range having the maximum frequency peak is preferably 40 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. The lower limit of the range is not particularly limited from the viewpoint of easily thinning the separator, and may usually be greater than 0 μm. From the viewpoint of easily reducing the separator resistance, the lower limit is preferably 6 μm or more, more preferably 10 μm or more, and even more preferably 14 μm or more.

[0024] The fiber diameter distribution histogram of the beaten cellulose fibers is created using the fiber diameter data of the beaten cellulose fibers. The fiber diameter data used to create the histogram is obtained by first dispersing 100 g of beaten cellulose fiber in 10 L of water to prepare a slurry, and then measuring the fiber diameter of the beaten cellulose fiber using the prepared slurry with a "Fiber Tester" manufactured by Lorentzen & Wettre, for example, by the method described in the Examples.

[0025] In the present invention, the class width of the fiber diameter histogram of beaten cellulose fibers may be appropriately set depending on the analytical device, etc., so as to be able to confirm whether the fibers have the desired fiber diameter, and is preferably 10 or less, more preferably 5 or less, even more preferably 3 or less, and particularly preferably 2 or less.

[0026] In the present invention, the maximum frequency peak in a fiber diameter distribution histogram refers to the class (or data interval) with the highest frequency in the fiber diameter distribution histogram. When there are two or more classes with the highest frequency in the histogram, the class with the largest fiber diameter among the classes with the highest frequency is taken as the maximum frequency peak. The maximum frequency peak can be adjusted by the freeness and type of beaten cellulose fibers.

[0027] The beaten cellulose fibers of the present invention further have a fiber diameter distribution histogram in which the proportion of fibers having a fiber diameter of 20 μm or less is 55% or more. If this proportion is below the lower limit, it is difficult to maintain the separator strength while thinning the separator. In one embodiment of the present invention, from the viewpoint of easily obtaining a thinner separator, this proportion is preferably 60% or more, more preferably 63% or more, and even more preferably 65% ​​or more. In the present invention, the proportion of fibers having a fiber diameter of 20 μm or less in the fiber diameter distribution histogram means the ratio of the sum of frequencies from the class including the smallest fiber diameter to the class including a fiber diameter of 20 μm (total frequency of fibers having a diameter of 20 μm or less) to the sum of frequencies of all classes (all data sections) in the fiber diameter distribution histogram (total frequency) ((total frequency of fibers having a diameter of 20 μm or less / total frequency)×100). For example, when the class width of the fiber diameter distribution histogram is 2, the ratio is the ratio of the sum of the frequencies from the class containing the smallest fiber diameter to the class of 20 μm or more and less than 22 μm (total frequencies of 20 μm or less) to the sum of the frequencies of all classes (total frequencies) of the fiber diameter distribution histogram ((total frequencies of 20 μm or less / total frequencies)×100).

[0028] In one embodiment of the present invention, from the viewpoint of easily achieving both maintaining the strength of the separator and thinning the separator, the beaten cellulose fiber preferably has a ratio of fibers having a fiber diameter of 30 μm or less in a fiber diameter distribution histogram of the beaten cellulose fiber of 90% or more, more preferably 92% or more, and even more preferably 95% or more. In the present invention, the ratio of fibers having a fiber diameter of 30 μm or less in a fiber diameter distribution histogram means the ratio of the sum of frequencies from the class including the smallest fiber diameter to the class including a fiber diameter of 30 μm (total frequencies of 30 μm or less) to the sum of frequencies of all classes (all data sections) in the fiber diameter distribution histogram (total frequency) ((total frequencies of 30 μm or less / total frequency)×100). For example, when the class width of the fiber diameter distribution histogram is 2, the ratio is the ratio of the sum of the frequencies from the class containing the smallest fiber diameter to the class of 30 μm or more and less than 32 μm (total frequencies of 30 μm or less) to the sum of the frequencies of all classes (total frequency) ((total frequencies of 30 μm or less / total frequency)×100).

[0029] The higher the ratio of the fibers of 20 μm or less and the ratio of the fibers of 30 μm or less, the easier it is to thin the separator, so the upper limit is not particularly limited and may be 100% or less.

[0030] In a preferred embodiment of the present invention, from the viewpoint of easily achieving both maintaining the separator strength and thinning the separator, the proportion of fibers having a fiber diameter of more than 30 μm in a fiber diameter distribution histogram of the beaten cellulose fiber of the present invention is 10% or less, more preferably 8% or less, and even more preferably 5% or less. The proportion of fibers having a fiber diameter of more than 30 μm in a fiber diameter distribution histogram means the ratio ((total frequency of fibers having a fiber diameter of more than 30 μm) of the sum of frequencies of all classes (all data intervals) in the fiber diameter distribution histogram (total frequency) of the fibers having a fiber diameter of more than 30 μm to the class having the largest fiber diameter (total frequency of fibers having a fiber diameter of more than 30 μm) to the sum of frequencies of all classes (all data intervals) in the fiber diameter distribution histogram (total frequency) ((total frequency of fibers having a fiber diameter of more than 30 μm / total frequency) × 100). For example, when the class width of the fiber diameter distribution histogram is 2, the ratio means the ratio of the sum of frequencies from the class of 32 μm or more and less than 34 μm to the class containing the largest fiber diameter (total frequencies of 32 μm or more) to the sum of frequencies of all classes (total frequency) ((total frequencies of 32 μm or more / total frequency)×100). The lower the ratio of fibers having a fiber diameter of more than 30 μm, the easier it is to thin the separator, so the lower limit is not particularly limited and may be 0%.

[0031] The proportions of fibers having diameters of 20 μm or less, 30 μm or less, and more than 30 μm in the fiber diameter distribution histogram of beaten cellulose fibers can be adjusted by the freeness and type of the beaten cellulose fibers.

[0032] The number average fiber diameter of the beaten cellulose fibers is preferably 20 μm or less, more preferably 19 μm or less, and even more preferably 18.5 μm or less, from the viewpoint of easily thinning the separator, and is preferably 10 μm or more, more preferably 13 μm or more, and even more preferably 15 μm or more, from the viewpoint of easily reducing the resistance value of the separator.

[0033] The maximum fiber diameter of the beaten cellulose fiber is preferably 70 μm or less, more preferably 69 μm or less, and even more preferably 67 μm or less, from the viewpoint of easily thinning the separator, and is preferably 40 μm or more, more preferably 50 μm or more, and even more preferably 55 μm or more, from the viewpoint of easily reducing the resistance value of the separator.

[0034] The minimum fiber diameter of the beaten cellulose fibers is preferably 1 μm or more, more preferably 3 μm or more, and even more preferably 6 μm or more, from the viewpoint of easily reducing the resistance value of the separator, and is preferably 15 μm or less, more preferably 14 μm or less, and even more preferably 12 μm or less, from the viewpoint of easily thinning the separator.

[0035] The number-average fiber diameter, maximum fiber diameter, and minimum fiber diameter of the beaten cellulose fibers can be adjusted by the freeness and type of the beaten cellulose fibers, etc. These fiber diameters can be calculated from the fiber diameter measured using a "Fiber Tester" manufactured by Lorentzen & Wettre.

[0036] In the present invention, the beaten cellulose fibers are not particularly limited as long as they have a freeness of 50 ml to 500 ml, a fiber diameter histogram of the fibers has a maximum frequency peak in the range of 50 μm or less, and the proportion of fibers having a fiber diameter of 20 μm or less is 55% or more. For example, the beaten cellulose fibers may be natural cellulose fibers obtained by beating, organic solvent-based cellulose fibers obtained by beating, or a mixture thereof. Note that organic solvent-based cellulose fibers are cellulose fibers (lyocell) obtained by an organic solvent spinning method in which cellulose is directly dissolved and spun without going through a derivative.

[0037] In a preferred embodiment of the present invention, the beaten cellulose fibers are natural cellulose fibers that have been beaten to facilitate thinning of the separator. Organic solvent-based cellulose fibers are beaten to form fibers having a thick trunk and thin branches extending from the thick trunk, and the thick trunk retains a fiber diameter similar to that of the original fiber before beating. In contrast, natural cellulose fibers are less likely to produce a thick trunk, as occurs when organic solvent-based cellulose fibers are beaten, and therefore the fiber diameter is easily reduced by beating. Therefore, with natural cellulose fibers, the upper limit of the range having the highest frequency peak in a histogram of beaten cellulose fibers is more likely to be smaller (e.g., 50 μm or less), and the proportion of fibers having a fiber diameter of 20 μm or less is more likely to be higher (e.g., 55% or more). Therefore, the use of natural cellulose fibers facilitates the production of thinner separators.

[0038] Since the use of natural cellulose fibers as the beaten cellulose fibers makes it easier to make the separator thinner, the content of the natural cellulose fibers is preferably 60% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, based on the total mass of the beaten cellulose fibers. The upper limit of the content is not particularly limited, and may be, for example, 100% by mass or less.

[0039] Examples of natural cellulose fibers include wood pulp such as softwood pulp and hardwood pulp, cotton linter pulp, and hemp pulp, with wood pulp being preferred. These natural cellulose fibers may be used alone or in combination of two or more. Furthermore, from the viewpoint of dimensional stability of the separator, it is preferable that the natural cellulose fibers are mercerized.

[0040] The cross-sectional shape of the beaten cellulose fiber may be, for example, a circular shape, an oval shape, a cocoon shape, a flat shape, or the like, and the flat shape is preferred from the viewpoint of facilitating thinning of the separator.

[0041] Beaten cellulose fibers can be prepared by beating cellulose fibers such as natural cellulose fibers, organic solvent-based cellulose fibers, etc. The method for beating cellulose fibers is not particularly limited, and may be performed, for example, according to JIS P-8221-1-98.

[0042] In a preferred embodiment of the present invention, the content of beaten cellulose fibers is 30% by mass or more and 95% by mass or less, based on the total mass of the separator, from the viewpoint of easily thinning the separator. Furthermore, from the viewpoint of easily thinning the separator, the content of beaten cellulose fibers is more preferably 50% by mass or more, even more preferably 60% by mass or more, and particularly preferably 70% by mass or more. Furthermore, from the viewpoint of easily increasing the strength of the separator, the content is more preferably 90% by mass or less, even more preferably 85% by mass or less, and particularly preferably 80% by mass or less.

[0043] <Other fibers> The separator of the present invention may contain other fibers, if necessary, in addition to the above-described synthetic fibers and beaten cellulose fibers. The other fibers are not particularly limited as long as they do not impair the effects of the present invention, and examples include cellulose fibers other than the beaten cellulose fibers of the present invention. When the separator contains other fibers, the content of the other fibers is not limited as long as the effects of the present invention are not impaired, and may be, for example, 0.1% by mass or more and 20% by mass or less, relative to the total mass of the separator. From the viewpoint of easily thinning the separator, the content of the other fibers is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less.

[0044] <Binder> In a preferred embodiment of the present invention, the separator of the present invention further contains a binder from the viewpoint of easily increasing the strength of the separator. The binder is not particularly limited, but it is preferable that it is capable of bonding the fibers contained in the separator to each other. Examples of binders include polyvinyl alcohol-based binders and ethylene-vinyl alcohol-based binders. Among them, polyvinyl alcohol-based binders are preferred from the viewpoint of easily increasing the strength of the separator.

[0045] The vinyl alcohol polymer constituting the polyvinyl alcohol binder may be the same as the vinyl alcohol polymer constituting the polyvinyl alcohol fiber as the synthetic fiber.

[0046] The raw material form of the binder contained in the separator is not particularly limited and may be, for example, fibrous, powdery, or in solution form. However, from the viewpoint of easily improving the adhesiveness of the fibers contained in the separator, fibrous form is preferred.

[0047] When polyvinyl alcohol-based fibers are used as the polyvinyl alcohol-based binder raw material, the polyvinyl alcohol-based fibers may be the same as those described above as the synthetic fibers. However, from the viewpoint of easily improving the adhesiveness between the synthetic fibers and the beaten cellulose fibers, it is preferable that the polyvinyl alcohol-based fibers used as the binder raw material are not heat-treated.

[0048] In a preferred embodiment of the present invention, when the separator contains a binder, the content of the binder is 0.5% by mass or more and 20% by mass or less, relative to the total mass of the separator, from the viewpoint of easily increasing the strength of the separator. Furthermore, from the viewpoint of easily increasing the strength of the separator and easily thinning the separator, the content of the binder is more preferably 1% by mass or more, even more preferably 2% by mass or more, and particularly preferably 3% by mass or more, relative to the total mass of the separator. Furthermore, from the viewpoint of easily reducing the resistance value of the separator, the content of the binder is more preferably 15% by mass or less, even more preferably 10% by mass or less, and particularly preferably 8% by mass or less, relative to the total mass of the separator.

[0049] <Separators for electrochemical elements> In the present invention, the separator contains synthetic fibers and beaten cellulose fibers with a specific freeness and specific fiber diameter distribution, enabling the production of a separator that is both strong and thin. The thickness of the separator of the present invention can be appropriately selected depending on the type of electrochemical device in which the separator is used, and may be, for example, 10 μm or more and less than 70 μm. Furthermore, from the viewpoint of easily obtaining a separator that can improve the performance, reduce the size, and reduce the weight of the electrochemical device, the thickness of the separator of the present invention is preferably 60 μm or less, more preferably 55 μm or less, and even more preferably 53 μm or less. Furthermore, from the viewpoint of easily increasing the strength of the separator, the thickness is preferably 20 μm or more, more preferably 30 μm or more, and even more preferably 35 μm or more. The thickness of the separator can be adjusted by the freeness and fiber diameter distribution of the beaten cellulose fibers, the basis weight of the separator, and other factors. The thickness of the separator can be measured according to JIS P 8118.

[0050] The basis weight of the separator of the present invention is preferably 10 g / m from the viewpoint of easily increasing the strength of the separator. 2 More preferably, 15 g / m 2 More preferably, 18 g / m 2 From the viewpoint of making it easier to thin the separator, it is preferably 30 g / m 2 Less than 25 g / m 2or less, more preferably 23 g / m 2 The basis weight can be measured in accordance with JIS P 8124.

[0051] The separator of the present invention contains synthetic fibers and beaten cellulose fibers having a specific freeness and a specific fiber diameter distribution, and therefore has high strength despite its thinness. The strength of the separator of the present invention is preferably 0.3 kg / 15 mm or more, more preferably 0.35 kg / 15 mm or more, and even more preferably 0.4 kg / 15 mm or more, from the viewpoint of easily improving the durability of the separator and electrochemical devices containing the separator. Furthermore, since higher strength tends to increase durability, the upper limit is not particularly limited and may be 1.0 kg / 15 mm or less. The strength can be adjusted by the content and basis weight of the synthetic fibers in the separator. The strength can be measured using a tensile tester in accordance with JIS P-8113.

[0052] The air permeability of the separator of the present invention is preferably 5.5 cc / cm from the viewpoint of easily reducing the resistance value of an electrochemical element including the separator. 2 / sec or more, preferably 6.0cc / cm 2 / sec or more, more preferably 6.5cc / cm 2 / sec or more, and from the viewpoint of easily increasing the shielding property of the separator, it is preferably 20 cc / cm 2 / sec or less, preferably 18cc / cm 2 / sec or less, more preferably 15cc / cm 2 / sec or less. The air permeability of the separator can be adjusted by the freeness of the beaten cellulose fiber, the fiber composition of the separator (for example, the content ratio of synthetic fiber and beaten cellulose fiber in the separator), etc. The air permeability can be measured in accordance with JIS L 1096 6.27.

[0053] From the viewpoint of practicality of the separator, the resistance of the separator of the present invention is preferably 3.0 Ω or less, more preferably 2.8 Ω or less, and even more preferably 2.5 Ω or less. The resistance of the separator can be adjusted by the freeness of the beaten cellulose fiber, the fiber composition contained in the separator, etc. The resistance of the separator can be measured using a resistance measuring device, for example, by the method described in the Examples.

[0054] [Method for manufacturing separator for electrochemical element] The separator of the present invention can be produced by any known papermaking method, for example, by mixing synthetic fibers and beaten cellulose fibers, and optionally other fibers and / or binders, dispersing the mixture in water to prepare a slurry, and then making paper using a general wet papermaking machine. Examples of wires used in papermaking machines include cylinder wire, short wire, and fourdrinier wire. These wires may be used alone to form a single layer, or multiple layers may be formed by combining wires. To obtain uniform paper with no formation irregularities and excellent electrical properties, multiple layers are preferably formed, and two-layer paper made using a short wire-cylinder papermaking machine is particularly preferred. After papermaking using a wet papermaking machine, the desired separator can be obtained by drying in a Yankee dryer or the like. Furthermore, heat pressing or other processing may be performed as necessary. Furthermore, to improve electrolyte absorption, hydrophilization treatment such as surfactant treatment may be performed.

[0055] [Electrochemical element] The present invention also encompasses an electrochemical device comprising the separator of the present invention. The separator of the present invention can be thinned while maintaining high strength, and therefore can improve the performance and reduce the size and weight of the electrochemical device without reducing the durability of the electrochemical device.

[0056] Examples of electrochemical elements include electric double layer capacitors, lithium ion capacitors, aluminum electrolytic capacitors, lithium ion secondary batteries, sodium ion secondary batteries, sodium sulfur secondary batteries, etc. Among these electrochemical elements, the separator of the present invention is suitable for capacitors such as electric double layer capacitors and lithium ion capacitors.

[0057] The electrochemical element of the present invention includes a positive electrode, a negative electrode, and an electrolyte solution in addition to the separator of the present invention. The positive electrode and negative electrode included in the electrochemical element are not particularly limited and may be, for example, known positive electrodes and negative electrodes used in electrochemical elements. The electrolyte solution is also not particularly limited and may be, for example, an organic electrolyte solution (non-aqueous electrolyte solution). Examples of organic electrolyte solutions include a mixture of tetraalkylammonium cations and BF4 - , PF6 - , SO3CF3 - Examples of such an electrolyte include a salt with an anion such as the above dissolved in an organic solvent such as propylene carbonate or ethylene carbonate.

[0058] The shape of the separator in the electrochemical element is not particularly limited, and examples thereof include cross strip (a cross-shaped cylindrical shape with a bottom), round strip (a cylindrically wound shape), and spiral (a spirally wound structure).

[0059] The method for producing the electrochemical element is not particularly limited, and the element can be produced according to a conventionally known method. [Example]

[0060] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0061] The physical properties of the beaten cellulose fibers were measured and evaluated by the following methods. [Freeness of beaten cellulose fiber] The Canadian standard freeness was measured according to JIS P-8121 (Testing method for freeness of pulp) using a Canadian standard freeness tester (manufactured by Kumagai Riki Kogyo Co., Ltd., "Canadian Freeness Tester").

[0062] [Fiber diameter and fiber diameter distribution of beaten cellulose fibers] (1) Number average fiber diameter and maximum fiber diameter 100 g of beaten cellulose fiber was dispersed in 10 L of water to prepare a slurry. Using the obtained slurry, the fiber diameter of the beaten cellulose fiber was measured using a "Fiber Tester" manufactured by Lorentzen & Wettre under the following conditions. Measurement conditions: Mode: Auto mode Data range (fiber diameter range): 0 to 100 μm Class width: 2 The number-average fiber diameter and maximum fiber diameter of the beaten cellulose fibers were calculated from the fiber diameter data obtained by the measurement. The results are shown in Table 1. (2) The maximum frequency peak in the fiber diameter distribution histogram Using the fiber diameter data of the beaten cellulose fibers obtained by the measurement in (1) above, fiber diameter distribution histograms (class width: 2) of the beaten cellulose fibers were created. The fiber diameter distribution histograms of the beaten cellulose fibers with each freeness are shown in Figures 1 to 7. Note that the fiber diameter on the horizontal axis in the figures represents the lower limit fiber diameter in each class (for example, 20 μm on the horizontal axis represents a class of fiber diameters of 20 μm or more and less than 22 μm). In the prepared fiber diameter distribution histogram of the beaten cellulose fibers, the most frequent class was taken as the maximum frequency peak. Table 1 shows the fiber diameters at the maximum frequency peak. The fiber diameter at the maximum frequency peak was the lower limit fiber diameter of the most frequent class. For example, if the most frequent class was 18 μm or more and less than 20 μm, the fiber diameter at the maximum frequency peak was taken as 18 μm. (3) The proportion of fibers with a specific diameter in the fiber diameter distribution histogram In the beaten cellulose fibers, the proportions of fibers having a fiber diameter of 20 μm or less, a fiber diameter of 30 μm or less, and a fiber diameter of more than 30 μm were calculated from the histogram by determining the sum of the frequencies of all classes (all data intervals) (total frequency), the sum of the frequencies from the class including the smallest fiber diameter to the class of 20 μm or more and less than 22 μm (total frequencies of 20 μm or less), the sum of the frequencies from the class including the smallest fiber diameter to the class of 30 μm or more and less than 32 μm (total frequencies of 30 μm or less), and the sum of the frequencies from the class of 32 μm or more and less than 34 μm to the class including the largest fiber diameter (total frequencies of more than 30 μm), and then using the following formula. Percentage of fibers with a diameter of 20 μm or less = (total frequency of fibers with a diameter of 20 μm or less / total frequency) × 100 Percentage of fibers with a diameter of 30 μm or less = (total frequency of fibers with a diameter of 30 μm or less / total frequency) × 100 Percentage of fibers with a diameter of over 30 μm = (total frequency of fibers over 30 μm / total frequency) × 100

[0063] The properties of the separators obtained in the examples and comparative examples were measured by the methods described below. The measurement results are shown in Table 1.

[0064] [Metsuke] Measurement was carried out in accordance with JIS P 8124 (method for measuring metric basis weight of paper).

[0065] [Thickness] Measurement was carried out in accordance with JIS P 8118 (Testing method for thickness and density of paper and paperboard).

[0066] [Strong] Measurement was carried out using a tensile tester (manufactured by Instron, "5543") in accordance with JIS P-8113 (Paper and paperboard - Testing methods for tensile properties).

[0067] [Breathability] Measurement was carried out using an air permeability tester (manufactured by Kato Tech Co., Ltd., "KES-F8-AP1") in accordance with JIS L 1096 6.27 (general fabric testing method, air permeability).

[0068] [Resistance value] The resistance values ​​of the separators obtained in the Examples and Comparative Examples were measured in auto mode using a resistance measurement device (KC-547 LCR METER, manufactured by Kokuyo Electric Industry Co., Ltd.) Specifically, five separators obtained in the Examples and Comparative Examples were stacked to prepare a resistance measurement sample, and the resistance measurement sample was immersed in an electrolyte (CAPASTE, a capacitor reagent manufactured by Toyama Pharmaceutical Co., Ltd.) for 1 hour. The resistance measurement sample was then removed from the electrolyte, and the resistance was measured using the device with the resistance measurement sample sandwiched between platinum plates.

[0069] Example 1 Natural cellulose fibers (mercerized wood pulp) (minimum fiber diameter: 5 μm, maximum fiber diameter: 70 μm) were beaten using a refiner (Kumagaya Riki Kogyo Co., Ltd., "Niagara Beater for Testing") according to JIS P-8221-1-98 (Pulp - Beating Method - Part 1: Beater Method) to obtain beaten cellulose fibers with a freeness of 150 ml. The obtained natural cellulose fiber (75% by mass) with a freeness of 150 ml, 20% by mass of polyvinyl alcohol-based fiber (manufactured by Kuraray Co., Ltd., Vinylon, VN30300) with a size of 0.3 dtex × 3 mm, and 5% by mass of polyvinyl alcohol-based binder (manufactured by Kuraray Co., Ltd., Vinylon binder: VPB107-1 × 3) with a size of 1.1 dtex × 3 mm were dispersed in water to produce a slurry (total mass of synthetic fiber, beaten cellulose fiber, and binder: water = 100 g: 10 L). This slurry was made into a two-layer paper using a papermaking machine (manufactured by Kumagaya Riki Kogyo Co., Ltd., "Square sheet machine (25 cm square)") according to JIS P-8222 (Pulp - Preparation method for test handsheets). The paper was dried at 120 °C for 1 minute with an initial moisture content of 70% using a rotary dryer (manufactured by Kumagaya Riki Kogyo Co., Ltd., "Rotary Dryer DR-200"), and the resulting paper had a basis weight of 20 g / m. 2 A separator for an electric double layer capacitor (EDLC) having a thickness of 48 μm was obtained.

[0070] Example 2 The same procedure as in Example 1 was repeated except that the freeness of the beaten cellulose fiber was adjusted to 250 ml, and the basis weight was 20 g / m. 2Thus, a separator for EDLC having a thickness of 50 μm was obtained.

[0071] Example 3 The same procedure as in Example 1 was repeated except that the freeness of the beaten cellulose fiber was adjusted to 350 ml, and the basis weight was 20 g / m. 2 Thus, a separator for EDLC having a thickness of 52 μm was obtained.

[0072] Example 4 A woven fabric with a basis weight of 20 g / m was prepared in the same manner as in Example 1, except that the freeness of the beaten cellulose fiber was adjusted to 250 ml, the blending amount of the beaten cellulose fiber was changed to 77.5% by mass, and the blending amount of the polyvinyl alcohol binder was changed to 2.5% by mass. 2 Thus, a separator for EDLC having a thickness of 51 μm was obtained.

[0073] Example 5 A woven fabric having a basis weight of 20 g / m was prepared in the same manner as in Example 1, except that the freeness of the beaten cellulose fiber was adjusted to 250 ml, the blending amount of the beaten cellulose fiber was changed to 79.5% by mass, and the blending amount of the polyvinyl alcohol binder was changed to 0.5% by mass. 2 Thus, a separator for EDLC having a thickness of 52 μm was obtained.

[0074] Example 6 Weight: 18g / m 2 The same procedure as in Example 2 was repeated except that the weight was adjusted to 18 g / m 2 Thus, a separator for EDLC having a thickness of 45 μm was obtained.

[0075] Example 7 Weight: 18g / m 2 The same procedure as in Example 3 was repeated except that the weight was adjusted to 18 g / m 2 Thus, a separator for EDLC having a thickness of 46 μm was obtained.

[0076] Example 8 A spun woven fabric with a basis weight of 20 g / m was prepared in the same manner as in Example 1, except that a 0.4 dtex x 3 mm polyester fiber (manufactured by Kuraray Co., Ltd., polyester, EP043) was used as the synthetic fiber instead of the polyvinyl alcohol fiber. 2Thus, a separator for EDLC having a thickness of 50 μm was obtained.

[0077] Example 9 A fabric having a basis weight of 20 g / m was prepared in the same manner as in Example 1, except that the blending amount of beaten cellulose fiber was changed to 40% by mass and the blending amount of polyvinyl alcohol-based fiber was changed to 55% by mass. 2 Thus, a separator for EDLC having a thickness of 53 μm was obtained.

[0078] Example 10 A fabric having a basis weight of 20 g / m was prepared in the same manner as in Example 1, except that the blending amount of beaten cellulose fiber was changed to 90 mass % and the blending amount of polyvinyl alcohol-based fiber was changed to 5 mass %. 2 Thus, a separator for EDLC having a thickness of 46 μm was obtained.

[0079] Example 11 Weight: 15g / m 2 The same procedure as in Example 1 was repeated except that the weight was adjusted to 15 g / m 2 Thus, a separator for EDLC having a thickness of 42 μm was obtained.

[0080] (Comparative Example 1) The same procedure as in Example 1 was repeated except that the freeness of the beaten cellulose fiber was adjusted to 25 ml, and the basis weight was 20 g / m. 2 Thus, a separator for EDLC having a thickness of 41 μm was obtained.

[0081] (Comparative Example 2) A 20 g / m2 weight spun cellulose fiber was prepared in the same manner as in Example 1, except that the freeness of the beaten cellulose fiber was adjusted to 550 ml. 2 Thus, a separator for EDLC having a thickness of 55 μm was obtained.

[0082] (Comparative Example 3) Organic solvent-based cellulose fiber "Lyocell" (Lenzing's "Lyocell") was beaten in the same manner as in Example 1, and 100% by mass of the beaten cellulose fiber adjusted to a freeness of 50 ml was dispersed in water to produce a slurry (beaten cellulose fiber:water = 100 g: 10 L). Papermaking and drying were carried out in the same manner as in Example 1, and a basis weight of 20 g / m was obtained. 2 Thus, a separator for EDLC having a thickness of 55 μm was obtained.

[0083] Comparative Example 4 Weight: 17g / m 2 The same procedure as in Comparative Example 3 was repeated except that the weight was adjusted to 17 g / m 2 Thus, a separator for EDLC having a thickness of 50 μm was obtained.

[0084] (Comparative Example 5) Weight: 15g / m 2 The same procedure as in Comparative Example 3 was repeated except that the weight was adjusted to 15 g / m 2 Thus, a separator for EDLC having a thickness of 45 μm was obtained.

[0085] (Comparative Example 6) The same procedure as in Comparative Example 3 was repeated except that the freeness of the organic solvent-based cellulose fiber was adjusted to 250 ml, and the basis weight was 20 g / m 2 Thus, a separator for EDLC having a thickness of 59 μm was obtained.

[0086] [Table 1]

[0087] As shown in Table 1, it was confirmed that the separators obtained in Examples 1 to 11 had high strength despite their thinness. Furthermore, the separators obtained in Examples 1 to 11 exhibited low resistance values. In contrast, separators that were thin and had high strength were not obtained in Comparative Examples 2 to 6. Furthermore, the separators obtained in Comparative Examples 1 and 3 had high resistance values ​​and were not suitable as separators for electrochemical elements. Furthermore, the separator obtained in Comparative Example 2 had high air permeability and insufficient shielding properties.

Claims

1. A separator for an electrochemical element comprising synthetic fibers and beaten cellulose fibers, the Canadian Standard Freeness of the beaten cellulose fiber measured in accordance with JIS P 8121 is 50 ml or more and 500 ml or less; In the fiber diameter distribution histogram of the beaten cellulose fiber, (1) The fiber has a maximum frequency peak in the range of 10 μm or more and 50 μm or less, (2) The proportion of the fibers having a fiber diameter of 20 μm or less is 55% or more, The thickness is 55 μm or less, The basis weight is 10 to 25 g / m 2 , Separators for electrochemical elements.

2. 2. The separator for an electrochemical element according to claim 1, further comprising a binder and not comprising a polyamine epichlorohydrin resin.

3. 3. The separator for an electrochemical element according to claim 1, wherein in a fiber diameter distribution histogram of the beaten cellulose fibers, the proportion of the fibers having a fiber diameter of more than 30 μm is 10% or less.

4. 4. The separator for an electrochemical element according to claim 1, wherein the synthetic fiber is a polyvinyl alcohol fiber and / or a polyester fiber.

5. 5. The separator for an electrochemical element according to claim 1, wherein the beaten cellulose fibers are natural cellulose fibers that have been beaten.

6. The separator for an electrochemical element according to any one of claims 2 to 5, wherein the binder is a polyvinyl alcohol-based binder.

7. 7. The separator for an electrochemical element according to claim 1, wherein the content of the synthetic fiber is 1% by mass or more and 55% by mass or less with respect to the total mass of the separator.

8. 8. The separator for an electrochemical element according to claim 1, wherein the content of the beaten cellulose fibers is 30% by mass or more and 95% by mass or less with respect to the total mass of the separator.

9. 9. The separator for an electrochemical element according to claim 2, wherein the content of the binder is 0.5% by mass or more and 20% by mass or less with respect to the total mass of the separator.

10. The separator for an electrochemical element according to any one of claims 1 to 9, which has a thickness of 10 µm or more and 55 µm or less.

11. An electrochemical element comprising the separator for an electrochemical element according to any one of claims 1 to 10.

Images