Separator for electrochemical element

Abstract

The present invention relates to a separator for an electrochemical element comprising synthetic fibers and beaten cellulose fibers, wherein the beaten cellulose fibers have a Canadian Standard Freeness of 50 ml or more and 500 ml or less as measured according to JIS P 8121, in a fiber diameter distribution histogram of the beaten cellulose fibers, (1) the fibers have a maximum frequency peak in a 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.

Description

TECHNICAL FIELD

[0001] This patent application claims priority based on Japanese Patent Application No. 2019-234225 (Filing Date: December 25, 2019) under the Paris Convention, the entire contents of which are hereby incorporated by reference into the present specification.

[0002] The present application relates to an electrochemical element separator for an electrochemical element suitable for an electrochemical element, and an electrochemical element comprising the same. BACKGROUND

[0003] An electrochemical element such as a capacitor can perform charge and discharge of a large current, and has less performance deterioration caused by repeated charge and discharge, and a long life, and thus, in addition to uses such as a backup power source for small products such as mobile phones, smart phones, and the like, in recent years, uses such as storage and stabilization of electric power, power assistance, backup power sources, energy regeneration, and the like for large products such as automobiles, digital multifunction machines, vending machines, and the like have been put into practical use and researched.

[0004] In such an electrochemical element, a separator is used in order to separate the positive electrode and the negative electrode. For example, in Patent Literature 1, an electrochemical element separator made of a wet nonwoven fabric containing synthetic short fibers and solvent-spun cellulose fibers having a specific modified water filtration degree and a specific length-weighted average fiber length as essential components is disclosed.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT LITERATURE

[0007] Patent Literature 1: Japanese Patent Application Publication No. 2012-222266 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] In recent years, for the purpose of further improvement of the performance of an electrochemical element, and miniaturization and / or light weight of the electrochemical element, it is required to thin (sheeting) a separator in an electrochemical element, and reduce the capacity of the separator. However, according to the research by the present inventors and the like, it has been found that the existing separator as described in the above-described literature is difficult to sheet without reducing the weight per unit area, and if the separator is sheeted, the weight per unit area is reduced, and thus, there are problems that the strength of the separator is reduced, and the strength, durability, and the like of the electrochemical element are also reduced.

[0010] Therefore, an object of the present application is to provide an electrochemical element separator capable of achieving sheeting while maintaining the strength of the separator.

[0011] MEANS FOR SOLVING THE PROBLEMS

[0012] The present inventors have conducted intensive studies in order to solve the above problems, and as a result, the present application has been completed. That is, the present application provides the following preferred modes.

[0013] [1] A separator for an electrochemical element, comprising synthetic fibers and beaten cellulose fibers, wherein

[0014] The beaten cellulose fibers have a Canadian Standard Freeness of 50 ml or more and 500 ml or less as measured according to JIS P 8121,

[0015] In a fiber diameter distribution histogram of the beaten cellulose fibers,

[0016] (1) the fibers have a maximum frequency peak in a range of 50 μm or less,

[0017] (2) the proportion of the fibers having a fiber diameter of 20 μm or less is 55% or more.

[0018] [2] The separator for an electrochemical element according to [1], further comprising a binder.

[0019] [3] The separator for an electrochemical element according to [1] or [2], wherein

[0020] 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.

[0021] [4] The separator for an electrochemical element according to any one of [1] to [3], wherein

[0022] The synthetic fibers are polyvinyl alcohol-based fibers and / or polyester-based fibers.

[0023] [5] The separator for an electrochemical element according to any one of [1] to [4], wherein

[0024] The beaten cellulose fibers are natural cellulose fibers that have been beaten.

[0025] [6] The separator for an electrochemical element according to any one of [2] to [5], wherein

[0026] The binder is a polyvinyl alcohol-based binder.

[0027] [7] The separator for an electrochemical element according to any one of [1] to [6], wherein

[0028] The content of the synthetic fibers is 1% by mass or more and 55% by mass or less with respect to the total mass of the separator.

[0029] [8] The separator for an electrochemical element according to any one of [1] to [7], wherein

[0030] The content of the beaten cellulose fiber is 30 mass% or more and 95 mass% or less relative to the total mass of the separator.

[0031] [9] The separator for an electrochemical element according to any one of [2] to [8], wherein

[0032] The content of the binder is 0.5 mass% or more and 20 mass% or less relative to the total mass of the separator.

[0033]

[10] The separator for an electrochemical element according to any one of [1] to [9], wherein

[0034] The thickness is 10 μm or more and less than 70 μm.

[0035]

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

[10] .

[0036] Effects of the Invention

[0037] According to the present application, it is possible to provide a separator for an electrochemical element that can achieve thinning while maintaining the strength of the separator. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 is a histogram of the fiber diameter distribution of natural cellulose fiber having a water filtration degree of 25 ml.

[0039] Figure 2 is a histogram of the fiber diameter distribution of natural cellulose fiber having a water filtration degree of 150 ml.

[0040] Figure 3 is a histogram of the fiber diameter distribution of natural cellulose fiber having a water filtration degree of 250 ml.

[0041] Figure 4 is a histogram of the fiber diameter distribution of natural cellulose fiber having a water filtration degree of 350 ml.

[0042] Figure 5 is a histogram of the fiber diameter distribution of natural cellulose fiber having a water filtration degree of 550 ml.

[0043] Figure 6 is a histogram of the fiber diameter distribution of organic solvent-based cellulose fiber having a water filtration degree of 50 ml.

[0044] Figure 7 is a histogram of the fiber diameter distribution of organic solvent-based cellulose fiber having a water filtration degree of 250 ml. DETAILED DESCRIPTION

[0045] Separator for an Electrochemical Element

[0046] The separator for an electrochemical element (hereinafter, also simply referred to as "separator") of the present application contains synthetic fibers and beaten cellulose fibers. The beaten cellulose fibers have a Canadian Standard Freeness of 50 ml or more and 500 ml or less, and in a fiber diameter distribution histogram of the beaten cellulose fibers, (1) the fibers have a maximum frequency peak in a 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.

[0047] <SYNTHETIC FIBER>

[0048] The separator of the present application has high strength by containing synthetic fibers.

[0049] As the synthetic fibers, for example, polyvinyl alcohol-based fibers, ethylene-vinyl alcohol-based copolymer fibers, polyester-based fibers, polypropylene fibers, polyethylene fibers, polypropylene-polyethylene composite fibers, polyamide fibers, polyamide-modified polyamide composite fibers, and the like can be given. These synthetic fibers can be one alone or a combination of two or more.

[0050] Among these, from the viewpoint of easily improving the strength of the separator, it is preferable that the synthetic fibers be polyvinyl alcohol-based fibers and polyester-based fibers, and from the viewpoint of easily thinning the separator and easily obtaining a separator having low resistance, it is more preferable that the synthetic fibers be polyvinyl alcohol-based fibers.

[0051] The vinyl alcohol-based polymer constituting the polyvinyl alcohol-based fibers is not particularly limited, and for example, a vinyl alcohol-based polymer having an average degree of polymerization of 1000 to 5000 and a saponification degree of 95 mol% or more can be given. The above vinyl alcohol-based polymer can be a homopolymer of vinyl alcohol or a copolymer of vinyl alcohol and another copolymer component. In the case where the vinyl alcohol-based polymer contains another copolymer component, the proportion of the copolymer component in the vinyl alcohol-based polymer is preferably 20 mol% or less and more preferably 10 mol% or less from the viewpoint of water resistance and the like. In addition, the above vinyl alcohol-based polymer can be subjected to a treatment such as acetalization as necessary.

[0052] In the present application, the polyvinyl alcohol-based fibers as the synthetic fibers can be composed only of a vinyl alcohol-based polymer, or can be composite spun fibers, mixed spun fibers (island-in-sea fibers), and the like of a vinyl alcohol-based polymer and another polymer. From the viewpoint of liquid absorption into the electrolyte and strength, the proportion of the vinyl alcohol-based polymer in the above polyvinyl alcohol-based fibers is preferably 30% by mass or more, more preferably 50% by mass or more, and further preferably 80% by mass or more, based on the total mass of the polyvinyl alcohol-based fibers. In addition, the upper limit of the above proportion is not particularly limited, and for example, can be 100% by mass or less.

[0053] From the viewpoint of heat resistance of the separator, the polyvinyl alcohol-based fiber as the synthetic fiber is preferably heat-treated. The heat treatment temperature is not particularly limited, and can be, for example, 60°C or higher and 210°C or lower. In addition, the heat treatment time is not particularly limited, and can be, for example, 1 second or longer and 30 minutes or shorter.

[0054] In one embodiment of the present application, from the viewpoint of shielding property of the separator, the number average fiber diameter of the synthetic fiber is preferably 0.1 μm or more, more preferably 0.5 μm or more, and further preferably 1.0 μm or more. In addition, from the viewpoint of easy sheeting of the separator, it is preferably 20 μm or less, more preferably 15 μm or less, and further preferably 10 μm or less. The number average fiber diameter of the synthetic fiber can be measured by "Fiber Tester" manufactured by Lorentzen & Wettre Co.

[0055] In one embodiment of the present application, from the viewpoint of shielding property (air permeability) of the separator, the denier of the synthetic fiber is preferably 0.1 dtex or more, and more preferably 0.2 dtex or more. In addition, from the viewpoint of easy sheeting (easy production of a thin separator) of the separator, it is preferably 1.0 dtex or less, more preferably 0.8 dtex or less, and further preferably 0.6 dtex or less. The denier of the synthetic fiber can be measured by an optical microscope or an electron microscope (SEM).

[0056] As the cross-sectional shape of the synthetic fiber, for example, a circular shape, an elliptical shape, a cocoon type, a flat type, and the like can be given, and from the viewpoint of easy sheeting of the separator, the cocoon type is preferred.

[0057] In a preferred embodiment of the present application, from the viewpoint of easy improvement of the strength of the separator, the content of the synthetic fiber is 1 mass% or more and 55 mass% or less with respect to the total mass of the separator. In addition, from the viewpoint of easy further improvement of the strength of the separator, the above content of the synthetic fiber is more preferably 5 mass% or more, further preferably 10 mass% or more, and particularly preferably 15 mass% or more with respect to the total mass of the separator. In addition, from the viewpoint of easy sheeting of the separator, it is more preferably 50 mass% or less, further preferably 40 mass% or less, and particularly preferably 30 mass% or less with respect to the total mass of the separator.

[0058] <Beating cellulose fiber>

[0059] The separator of the present application is easily sheeted without reducing the weight per unit area by containing a beating cellulose fiber having a specific water filtration degree and a specific fiber diameter distribution, and thus, high strength maintenance and sheeting can be balanced. Note that the beating cellulose fiber refers to a cellulose fiber subjected to beating.

[0060] The beaten cellulose fiber in the present application has a Canadian Standard Freeness of 50 ml or more and 500 ml or less as measured according to JIS P 8121. When the above-mentioned Freeness is outside the above-mentioned range, it is difficult to ensure the function as a separator. Specifically, if the above-mentioned Freeness is less than the above-mentioned lower limit value, the air permeability of the separator excessively decreases, the electric resistance value easily increases, and, in addition, if the above-mentioned Freeness exceeds the above-mentioned upper limit value, the air permeability excessively increases, the shielding property easily decreases. In addition, if the above-mentioned Freeness exceeds the above-mentioned upper limit value, it is difficult to thin the separator. In addition, from the viewpoint of easily improving the strength and the shielding property of the separator, the above-mentioned Freeness is preferably 100 ml or more, more preferably 150 ml or more, and further preferably 200 ml or more. In addition, from the viewpoint of easily thinning the separator and easily decreasing the electric resistance value, the above-mentioned Freeness is preferably 400 ml or less, more preferably 350 ml or less, and further preferably 300 ml or less. The above-mentioned Freeness can be adjusted by the beating degree of the beaten cellulose fiber and the beating treatment method of the cellulose fiber.

[0061] For the beaten cellulose fiber in the present application, in the fiber diameter distribution histogram of the beaten cellulose fiber, there is a maximum frequency peak in the range of 50 μm or less. If the range having the above-mentioned maximum frequency peak exceeds the above-mentioned upper limit value, it is difficult to balance the maintenance of the strength of the separator and the thinning of the separator, and it is difficult to obtain a thin separator having high strength. In addition, in one embodiment of the present application, from the viewpoint of easily obtaining a thinner separator, the upper limit value of the range having the above-mentioned maximum frequency peak is preferably 40 μm or less, more preferably 30 μm or less, and further preferably 20 μm or less. From the viewpoint of easily thinning the separator, the lower limit value of the above-mentioned range is not particularly limited, and can generally exceed 0 μm, and from the viewpoint of easily decreasing the electric resistance of the separator, it is preferably 6 μm or more, more preferably 10 μm or more, and further preferably 14 μm or more.

[0062] The fiber diameter distribution histogram of the beaten cellulose fiber is made using the fiber diameter data of the beaten cellulose fiber.

[0063] The fiber diameter data used for making the above-mentioned histogram is obtained as follows: First, 100 g of the beaten cellulose fiber is dispersed in 10 L of water to prepare a slurry, and then, using the prepared slurry, the fiber diameter of the beaten cellulose fiber is measured by, for example, the method described in the examples, using a "Fiber Tester" manufactured by Lorentzen & Wettre Co.

[0064] In the present application, the width of the class in the fiber diameter histogram of the beaten cellulose fiber is appropriately set to be able to confirm whether it is a fiber having a desired fiber diameter, and is preferably 10 or less, more preferably 5 or less, further preferably 3 or less, and particularly preferably 2 or less, from an analytical device or the like.

[0065] In the present application, the maximum frequency peak in the fiber diameter distribution histogram refers to the class (or data region) having the highest frequency in the fiber diameter distribution histogram. Note that, in the case where the class having the highest frequency in the histogram is 2 or more, the class having a larger fiber diameter in the class having the highest frequency is regarded as the maximum frequency peak.

[0066] The above maximum frequency peak can be adjusted depending on the drainage degree of the beaten cellulose fiber and the kind thereof, and the like.

[0067] Further, in the beaten cellulose fiber in the present application, the proportion of the fiber having a fiber diameter of 20 μm or less in the fiber diameter distribution histogram of the beaten cellulose fiber is 55% or more. If the above proportion is less than the lower limit value, it is difficult to achieve both the maintenance of the barrier strength and the thinning of the barrier. In addition, in one embodiment of the present application, the above proportion is preferably 60% or more, more preferably 63% or more, and further preferably 65% or more, from the viewpoint of easily obtaining a thinner barrier. In the present application, the proportion of the fiber having a fiber diameter of 20 μm or less in the fiber diameter distribution histogram refers to the proportion of the sum of the frequencies from the class including the smallest fiber diameter to the class including the fiber diameter of 20 μm (the sum of the frequencies of 20 μm or less) to the sum of the frequencies of all the classes (all the data regions) of the above fiber diameter distribution histogram (the total frequency) ((the sum of the frequencies of 20 μm or less / total frequency) x 100). For example, in the case where the width of the class of the above fiber diameter distribution histogram is 2, the above proportion becomes the proportion of 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 (the sum of the frequencies of 20 μm or less) to the sum of the frequencies of all the classes of the above fiber diameter distribution histogram (the total frequency) ((the sum of the frequencies of 20 μm or less / total frequency) x 100).

[0068] In one embodiment of the present application, for the pulp cellulose fiber, the proportion of the fiber having a fiber diameter of 30 μm or less in the fiber diameter distribution histogram of the pulp cellulose fiber is preferably 90% or more, more preferably 92% or more, and further preferably 95% or more, from the viewpoint of easily achieving a balance between the maintenance of the strength of the separator and the thinning of the separator. In the present application, the proportion of the fiber having a fiber diameter of 30 μm or less in the fiber diameter distribution histogram refers to the proportion of the sum of the frequencies from the class including the minimum fiber diameter to the class including the fiber diameter of 30 μm ((sum of frequencies of 30 μm or less / total frequencies) x 100) with respect to the sum of the frequencies of all classes (total frequencies) of the above fiber diameter distribution histogram. For example, in the case where the class width of the above fiber diameter distribution histogram is 2, the above proportion is the proportion of the sum of the frequencies from the class including the minimum fiber diameter to the class of 30 μm or more and less than 32 μm ((sum of frequencies of 30 μm or less / total frequencies) x 100) with respect to the sum of the frequencies of all classes (total frequencies).

[0069] The higher the proportion of the above fiber of 20 μm or less and the proportion of the above fiber of 30 μm or less, the more there is a tendency to easily thin the separator, and thus the upper limit value is not particularly limited and can be 100% or less.

[0070] In a preferred embodiment of the present application, from the viewpoint of easily achieving a balance between the maintenance of the strength of the separator and the thinning of the separator, the proportion of the fiber having a fiber diameter of more than 30 μm in the fiber diameter distribution histogram of the pulp cellulose fiber in the present application is 10% or less, more preferably 8% or less, and further preferably 5% or less. The proportion of the fiber having a fiber diameter of more than 30 μm in the fiber diameter distribution histogram refers to the proportion of the sum of the frequencies from the next class including the fiber diameter of 30 μm to the class including the maximum fiber diameter ((sum of frequencies of more than 30 μm / total frequencies) x 100) with respect to the sum of the frequencies of all classes (total frequencies) of the above fiber diameter distribution histogram. For example, in the case where the class width of the above fiber diameter distribution histogram is 2, the above proportion refers to the proportion of the sum of the frequencies from the class of 32 μm or more and less than 34 μm to the class including the maximum fiber diameter ((sum of frequencies of 32 μm or more / total frequencies) x 100) with respect to the sum of the frequencies of all classes (total frequencies). The lower the proportion of the above fiber having a fiber diameter of more than 30 μm, the more the separator is easily thinned, and thus the lower limit value is not particularly limited and can be 0%.

[0071] The respective proportions of the beaten cellulose fibers having fiber diameters of 20 μm or less, 30 μm or less, and more than 30 μm in the fiber diameter distribution histogram of the beaten cellulose fibers can be adjusted by the drainage of the beaten cellulose fibers and the kind thereof.

[0072] The number average fiber diameter of the beaten cellulose fibers is preferably 20 μm or less, more preferably 19 μm or less, and further 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 further preferably 15 μm or more from the viewpoint of easily reducing the resistance value of the separator.

[0073] The maximum fiber diameter of the beaten cellulose fibers is preferably 70 μm or less, more preferably 69 μm or less, and further 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 further preferably 55 μm or more from the viewpoint of easily reducing the resistance value of the separator.

[0074] The minimum fiber diameter of the beaten cellulose fibers is preferably 1 μm or more, more preferably 3 μm or more, and further 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 further preferably 12 μm or less from the viewpoint of easily thinning the separator.

[0075] The number average fiber diameter, the maximum fiber diameter, and the minimum fiber diameter of the beaten cellulose fibers can be adjusted by the drainage of the beaten cellulose fibers and the kind thereof. In addition, these fiber diameters can be calculated from the fiber diameter measured using "Fiber Tester" manufactured by Lorentzen & Wettre Co.

[0076] In the present application, the beaten cellulose fibers are not particularly limited as long as they satisfy the following condition: the drainage is 50 ml or more and 500 ml or less, in the fiber diameter histogram of the fibers, there is a maximum frequency peak in the range of 50 μm or less, and the proportion of the fibers having a fiber diameter of 20 μm or less is 55% or more. The beaten cellulose fibers can be, for example, natural cellulose fibers that have been beaten, organic solvent-based cellulose fibers that have been beaten, or a mixture thereof. Note that the organic solvent-based cellulose fibers refer to cellulose fibers (Lyocell) obtained by an organic solvent spinning method in which cellulose is dissolved directly without using a derivative and is spun.

[0077] In the preferred embodiment of the present application, the beaten cellulose fiber is preferably a natural cellulose fiber that is beaten from the viewpoint of easily thinning the separator. The organic solvent-based cellulose fiber becomes a fiber having a thick portion as a trunk and a thin portion as a branch extending from the thick portion as a trunk by beating, and the thick portion as a trunk has a fiber diameter that is the same as that of the fiber before beating even after beating. In contrast, the natural cellulose fiber is not easily beaten to produce the thick portion as a trunk that is produced in the case of beating the organic solvent-based cellulose fiber, and thus the fiber diameter is easily reduced by beating. Therefore, in the histogram of the beaten cellulose fiber, the upper limit value of the range having the largest frequency peak is easily further reduced (for example, to 50 μm or less) and the proportion of the fiber having a fiber diameter of 20 μm or less is easily further increased (for example, to 55% or more) for the natural cellulose fiber, and thus a thinner separator is easily obtained by using the natural cellulose fiber.

[0078] If the natural cellulose fiber is used as the beaten cellulose fiber, the content of the natural cellulose fiber is preferably 60% by mass or more, more preferably 80% by mass or more, and further preferably 90% by mass or more with respect to the total mass of the beaten cellulose fiber from the viewpoint of easily thinning the separator. The upper limit of the above content is not particularly limited and can be, for example, 100% by mass or less.

[0079] As the natural cellulose fiber, for example, there are needle tree pulp, broad-leaved tree pulp, cotton linter pulp, hemp pulp, and the like, and the wood pulp is preferred. These natural cellulose fibers can be one alone or a combination of two or more. The natural cellulose fiber is preferably mercerized from the viewpoint of the stability of the form of the separator.

[0080] As the cross-sectional shape of the beaten cellulose fiber, for example, there are a circular shape, an elliptical shape, a cocoon type, a flat type, and the like, and the flat type is preferred from the viewpoint of easily thinning the separator.

[0081] The beaten cellulose fiber can be produced by beating treatment of a cellulose fiber such as a natural cellulose fiber, an organic solvent-based cellulose fiber, or the like. The beating treatment method of the cellulose fiber is not particularly limited and can be performed in accordance with JIS P-8221-1-98, for example.

[0082] In a preferred embodiment of the present application, the content of the beaten cellulose fiber is 30% by mass or more and 95% by mass or less, relative to the total mass of the separator, from the viewpoint of easily thinning the separator. Further, the content of the beaten cellulose fiber is more preferably 50% by mass or more, further preferably 60% by mass or more, and particularly preferably 70% by mass or more, from the viewpoint of easily further thinning the separator. Further, the content is more preferably 90% by mass or less, further preferably 85% by mass or less, and particularly preferably 80% by mass or less, from the viewpoint of easily increasing the strength of the separator.

[0083] <Other fiber>

[0084] The separator of the present application can contain other fibers, in addition to the above-mentioned synthetic fiber and beaten cellulose fiber, as needed. The other fiber is not particularly limited as long as the effect of the present application is not impaired, and cellulose fibers other than the beaten cellulose fiber in the present application, for example, can be given. In the case where the separator contains the other fiber, the content of the other fiber is not limited as long as the effect of the present application is not impaired, and can be 0.1% by mass or more and 20% by mass or less, for example, relative to the total mass of the separator. The content of the other fiber is preferably 15% by mass or less, more preferably 10% by mass or less, and further preferably 5% by mass or less, from the viewpoint of easily thinning the separator.

[0085] <Binder>

[0086] In a preferred embodiment of the present application, the separator of the present application further contains a binder, from the viewpoint of easily increasing the strength of the separator. The binder is not particularly limited, and a binder capable of bonding the fibers contained in the separator to each other is preferred. As examples of the binder, polyvinyl alcohol-based binders, ethylene-vinyl alcohol-based binders, and the like can be given. Among them, a polyvinyl alcohol-based binder is preferred from the viewpoint of easily increasing the strength of the separator.

[0087] The ethylene-vinyl alcohol polymer constituting the polyvinyl alcohol-based binder can be the same as the ethylene-vinyl alcohol polymer constituting the polyvinyl alcohol-based fiber as the above-mentioned synthetic fiber.

[0088] The form of the raw material of the binder contained in the separator is not particularly limited, and can be, for example, a fibrous form, a powdery form, a solution form, or the like, and a fibrous form is preferred from the viewpoint of easily increasing the adhesiveness of the fibers contained in the separator to each other.

[0089] In the case where a polyvinyl alcohol-based fiber is used as the raw material of the polyvinyl alcohol-based binder, the above-mentioned polyvinyl alcohol-based fiber as the above-mentioned synthetic fiber can be used as the polyvinyl alcohol-based fiber. Among them, the polyvinyl alcohol-based fiber as the raw material of the binder is preferably not subjected to heat treatment from the viewpoint of easily increasing the adhesiveness of the synthetic fiber to the beaten cellulose fiber.

[0090] In the preferred embodiment of the present application, in the case where the separator contains a binder, 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 from the viewpoint of easily improving the strength of the separator. In addition, the above content of the binder is more preferably 1% by mass or more, further preferably 2% by mass or more, and particularly preferably 3% by mass or more with respect to the total mass of the separator from the viewpoint of easily improving the strength of the separator and from the viewpoint of easily thinning the separator. In addition, the above content of the binder is more preferably 15% by mass or less, further preferably 10% by mass or less, and particularly preferably 8% by mass or less with respect to the total mass of the separator from the viewpoint of easily reducing the electric resistance value of the separator.

[0091] <Separator for Electrochemical Element>

[0092] In the present application, since the separator contains synthetic fibers and beaten cellulose fibers having a specific water drainage degree and a specific fiber diameter distribution, a separator having high strength and being thin can be obtained. The thickness of the separator of the present application can be appropriately selected depending on the kind of the electrochemical element using the separator, and can be, for example, 10 μm or more and less than 70 μm. In addition, the thickness of the separator of the present application is preferably 60 μm or less, more preferably 55 μm or less, and further preferably 53 μm or less from the viewpoint of easily obtaining a separator capable of improving the performance of the electrochemical element and achieving miniaturization and weight reduction. In addition, the thickness of the separator of the present application is preferably 20 μm or more, more preferably 30 μm or more, and further preferably 35 μm or more from the viewpoint of easily improving the strength of the separator. The thickness of the separator can be adjusted by the water drainage degree and the fiber diameter distribution of the beaten cellulose fibers and the weight per unit area of the separator, and the like. Note that the thickness of the separator can be measured in accordance with JIS P 8118.

[0093] The weight per unit area of the separator of the present application is preferably 10 g / m 2 or more, more preferably 15 g / m 2 or more, and further preferably 18 g / m 2 or more. In addition, the separator of the present application is preferably 30 g / m 2 or more, more preferably 25 g / m 2 or more, and further preferably 23 g / m 2 or more from the viewpoint of easily thinning the separator. The above weight per unit area can be measured in accordance with JIS P 8124.

[0094] The separator of the present application has high strength even if it is thin, because it contains synthetic fibers and beaten cellulose fibers having a specific water permeability and a specific fiber diameter distribution. The strength of the separator of the present application is preferably 0.3 kg / 15 mm or more, more preferably 0.35 kg / 15 mm or more, and further preferably 0.4 kg / 15 mm or more, from the viewpoint of easily improving the durability of the separator and the electrochemical element containing the same. In addition, the higher the above strength, the more the tendency to improve the durability is present, and thus the upper limit value is not particularly limited and can be 1.0 kg / 15 mm or less. The above strength can be adjusted by the content of synthetic fibers, the weight per unit area, and the like in the separator. Note that the above strength can be measured using a tensile testing machine and in accordance with JIS P-8113.

[0095] The air permeability of the separator of the present application is preferably 5.5 cc / cm 2 / sec or more, more preferably 6.0 cc / cm 2 / sec or more, and further preferably 6.5 cc / cm 2 / sec or more, and from the viewpoint of easily improving the shielding property of the separator, is preferably 20 cc / cm 2 / sec or less, more preferably 18 cc / cm 2 / sec or less, and further preferably 15 cc / cm 2 / sec or less. The air permeability of the separator can be adjusted depending on the water permeability of the beaten cellulose fibers, the fiber composition of the separator (for example, the content ratio of synthetic fibers and beaten cellulose fibers in the separator), and the like. The air permeability can be measured in accordance with JIS L 1096 6.27.

[0096] The resistance of the separator of the present application is preferably 3.0 Ω or less, more preferably 2.8 Ω or less, and further preferably 2.5 Ω or less, from the viewpoint of the practicality of the separator. The resistance value of the separator can be adjusted by the water permeability of the beaten cellulose fibers, the fiber composition contained in the separator, and the like. The resistance value of the separator can be measured by a resistance measuring device in accordance with, for example, the method described in the examples.

[0097] Method for manufacturing a separator for an electrochemical element

[0098] The method for manufacturing the separator of the present application is not particularly limited and can be manufactured by a publicly known papermaking method. For example, it can be manufactured by mixing synthetic fibers and beaten cellulose fibers, and other fibers and / or binders used as necessary, then preparing a slurry by dispersing in water, and performing papermaking using a general wet papermaking machine.

[0099] As the wire mesh used in a paper machine, for example, there are a cylinder wire, a short wire, a long wire, and the like, and these wire meshes can be used alone and provided as a single layer, or can be provided as a multi-layered mixed paper based on a combination of wire meshes. From the viewpoint of obtaining a paper having no unevenness in texture and excellent electrical properties, it is preferable to provide a multi-layered mixed paper, and among them, a two-layered mixed paper produced by a short wire-cylinder wire paper machine is preferable. After paper production by a wet paper machine, drying is performed by a Yankee-type dryer or the like, whereby the target separator can be obtained. In addition, heat press processing or the like can be performed as needed. In addition, from the viewpoint of improving the electrolyte absorbability, a hydrophilic treatment such as a surfactant treatment can be performed.

[0100]

[0101] The present application also includes a chemical element including the separator of the present application. The separator of the present application can achieve improvement in performance, miniaturization, and light weight of the electrochemical element without reducing the durability of the electrochemical element because it can be thinned while maintaining high strength.

[0102] As the electrochemical element, for example, there are an electric dipole layer capacitor, a lithium ion capacitor, an aluminum electrolytic capacitor, a lithium ion secondary battery, a sodium ion secondary battery, a sodium-sulfur secondary battery, and the like. Among these electrochemical elements, the separator of the present application is suitable for a capacitor such as an electric dipole layer capacitor and a lithium ion capacitor.

[0103] In the electrochemical element of the present application, in addition to the separator of the present application, a positive electrode and a negative electrode, and an electrolyte are included. The positive electrode and the negative electrode included in the electrochemical element are not particularly limited, and for example, known positive electrodes and negative electrodes for electrochemical elements can be used. In addition, the electrolyte is also not particularly limited, and for example, an organic electrolyte (non-aqueous electrolyte) can be used. As the organic electrolyte, for example, an electrolyte obtained by dissolving a salt in which a tetraalkylammonium cation and a BF4 - , PF6 - , SO3CF3 - anion are combined in an organic solvent such as propylene carbonate or ethylene carbonate can be used.

[0104] The shape of the separator in the electrochemical element is not particularly limited, and for example, a cross belt (a bottomed cylindrical shape having a cross structure), a circular belt (a cylindrical shape wound in a circular shape), a spiral shape (a spiral wound structure), and the like can be used.

[0105] The method for producing the electrochemical element is not particularly limited, and can be produced according to a known method.

[0106] Example

[0107] Hereinafter, the present application will be specifically described by examples and comparative examples, but the present application is not limited to the following examples. ​

[0108] Each property of the beaten cellulose fiber can be measured and evaluated by the following method shown below.

[0109] 〔Freeness of beaten cellulose fiber〕

[0110] The Canadian Standard Freeness was measured in accordance with JIS P-8121 (Test method of Freeness of pulp) and using a Canadian Freeness Tester (manufactured by Kumagai Riki Kogyo Co., Ltd., "Canadian Freeness Tester").

[0111] 〔Fiber diameter and fiber diameter distribution of beaten cellulose fiber〕

[0112] (1) Number average fiber diameter and maximum fiber diameter

[0113] A pulp was prepared by dispersing 100 g of the beaten cellulose fiber in 10 L of water. Using the obtained pulp, the fiber diameter of the beaten cellulose fiber was measured by a "Fiber Tester" manufactured by Lorentzen & Wettre Co. under the following conditions.

[0114] Measurement conditions:

[0115] Mode: Automatic mode

[0116] Data area (range of fiber diameter): 0 to 100 μm

[0117] Class width: 2

[0118] From the fiber diameter data of the beaten cellulose fiber obtained by the measurement, the number average fiber diameter and the maximum fiber diameter of the beaten cellulose fiber were calculated, respectively. The results are shown in Table 1.

[0119] (2) Maximum frequency peak in fiber diameter distribution histogram

[0120] Using the fiber diameter data of the beaten cellulose fiber obtained by the measurement of the above (1), a fiber diameter distribution histogram (class width: 2) of the beaten cellulose fiber was prepared. The fiber diameter distribution histograms of the beaten cellulose fiber of each freeness are shown in Figures 1-7 . Note that the fiber diameter of the horizontal axis in the figure indicates the lower limit fiber diameter in each class (for example, 20 μm of the horizontal axis indicates the class of the fiber diameter of 20 μm or more and less than 22 μm).

[0121] In the fiber diameter distribution histogram of the produced beaten cellulose fiber, the highest frequency class is adopted as the maximum frequency peak. The fiber diameter under the maximum frequency peak is shown in Table 1. Note that the fiber diameter under the maximum frequency peak refers to the fiber diameter of the lower limit of the class in the highest frequency class. For example, in the case where the highest frequency class is 18 μm or more and less than 20 μm, the fiber diameter under the maximum frequency peak is set to 18 μm.

[0122] (3) Proportion of fibers having a certain fiber diameter in the fiber diameter distribution histogram

[0123] In the beaten cellulose fiber, each proportion 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 is calculated as follows: the total of the frequencies of all classes (total frequency), the total of the frequencies from the class including the minimum fiber diameter to the class of 20 μm or more and less than 22 μm (total of the frequencies of 20 μm or less), the total of the frequencies from the class including the minimum fiber diameter to the class of 30 μm or more and less than 32 μm (total of the frequencies of 30 μm or less), and the total of the frequencies from the class of 32 μm or more and less than 34 μm to the class including the maximum fiber diameter (total of the frequencies of more than 30 μm) are calculated from the histogram by the following formula.

[0124] Proportion of fibers having a fiber diameter of 20 μm or less = (total of the frequencies of 20 μm or less / total frequency) x 100

[0125] Proportion of fibers having a fiber diameter of 30 μm or less = (total of the frequencies of 30 μm or less / total frequency) x 100

[0126] Proportion of fibers having a fiber diameter of more than 30 μm = (total of the frequencies of more than 30 μm / total frequency) x 100

[0127] Each property of the separators obtained in the examples and comparative examples was measured by the following method. The measurement results are shown in Table 1.

[0128] [Weight per unit area]

[0129] The measurement was performed in accordance with JIS P 8124 (Method of measuring weight per square meter of paper).

[0130] [Thickness]

[0131] The measurement was performed in accordance with JIS P 8118 (Test method for thickness and density of paper and paperboard).

[0132] [Strength]

[0133] The measurement was performed in accordance with JIS P-8113 (Paper and paperboard - Test methods for tensile properties) using a tensile tester (manufactured by Instron Corporation, "5543").

[0134] 〔Air permeability〕

[0135] The measurement was performed in accordance with JIS L 1096 6.27 (General test methods for textile fabrics - Air permeability) using an air permeability tester (manufactured by KATO TECH Co., Ltd., "KES-F8-AP1").

[0136] 〔Resistance value〕

[0137] The resistance value of the separators obtained in the examples and comparative examples was measured using a resistance measuring device (manufactured by Kokusan Electric Machine Industry Co., Ltd., "KC-547 LCR METER") in an automatic mode. Specifically, five pieces of the separators obtained in the examples and comparative examples were overlapped to prepare a sample for resistance measurement, the sample for resistance measurement was immersed in an electrolyte (a reagent for capacitors "Electrolyte CAPASTE" manufactured by Toyama Kagaku Kogyo Co., Ltd.) for 1 hour, the sample for resistance measurement was taken out of the electrolyte, and the resistance was measured using the above device in a state where the sample for resistance measurement was sandwiched with platinum from above and below.

[0138] (Example 1)

[0139] A natural cellulose fiber (mercerized wood pulp) (minimum fiber diameter: 5 μm, maximum fiber diameter: 70 μm) was subjected to a beating treatment using a refiner (manufactured by Kumagaya Rikagaku Kogyo Co., Ltd., "Test Niagara beater") in accordance with JIS P-8221-1-98 (Pulp - Beating methods - Part 1 : Beating machine method) to obtain a beaten cellulose fiber having a drainage degree of 150 ml. The obtained natural cellulose fiber having a drainage degree of 150 ml (75 mass%), 0.3 dtex x 3 mm polyvinyl alcohol-based fiber (manufactured by Kuraray Co., Ltd., vinylon, VN30300) (20 mass%), and 1.1 dtex x 3 mm polyvinyl alcohol-based binder (manufactured by Kuraray Co., Ltd., vinylon binder: VPB107-1 x 3) (5 mass%) were dispersed in water to prepare a slurry (total mass of synthetic fiber, beaten cellulose fiber, and binder : water = 100 g : 10 L), and papermaking was performed by a papermaking machine (manufactured by Kumagaya Rikagaku Kogyo Co., Ltd., "square sheet machine (25 cm square)") in accordance with JIS P-8222 (Pulp - Preparation of handsheets for testing) by 2-layer mixed beating. After drying at 120°C for 1 minute with a moisture content of 70% using a rotary dryer (manufactured by Kumagaya Rikagaku Kogyo Co., Ltd., "rotary dryer DR-200"), a paper sheet having a weight per unit area of 20 g / m 2, a separator for an electric dipole layer capacitor (EDLC) having a thickness of 48 μm.

[0140] (Example 2)

[0141] The drainage of the beaten cellulose fibers was adjusted to 250 ml, and otherwise, a separator for an EDLC having a thickness of 51 μm was obtained in the same manner as in Example 1. 2 , a separator for an EDLC having a thickness of 50 μm.

[0142] (Example 3)

[0143] The drainage of the beaten cellulose fibers was adjusted to 350 ml, and otherwise, a separator for an EDLC having a thickness of 52 μm was obtained in the same manner as in Example 1. 2 , a separator for an EDLC having a thickness of 52 μm.

[0144] (Example 4)

[0145] The drainage of the beaten cellulose fibers was adjusted to 250 ml, the compounding amount of the beaten cellulose fibers was changed to 77.5 mass%, and the compounding amount of the polyvinyl alcohol-based adhesive was changed to 2.5 mass%, and otherwise, a separator for an EDLC having a thickness of 51 μm was obtained in the same manner as in Example 1. 2 , a separator for an EDLC having a thickness of 51 μm.

[0146] (Example 5)

[0147] The drainage of the beaten cellulose fibers was adjusted to 250 ml, the compounding amount of the beaten cellulose fibers was changed to 79.5 mass%, and the compounding amount of the polyvinyl alcohol-based adhesive was changed to 0.5 mass%, and otherwise, a separator for an EDLC having a thickness of 52 μm was obtained in the same manner as in Example 1. 2 , a separator for an EDLC having a thickness of 52 μm.

[0148] (Example 6)

[0149] The unit area weight was adjusted to 18 g / m 2 , and otherwise, a separator for an EDLC having a thickness of 45 μm was obtained in the same manner as in Example 2. 2 , a separator for an EDLC having a thickness of 45 μm.

[0150] (Example 7)

[0151] The unit area weight was adjusted to 18 g / m 2 , and otherwise, a separator for an EDLC having a thickness of 46 μm was obtained in the same manner as in Example 3. 2 , a separator for an EDLC having a thickness of 46 μm.

[0152] (Example 8)

[0153] A 0.4 dtex x 3 mm polyester fiber (Toray Industries, Inc., polyester, EP043) was used instead of the polyvinyl alcohol fiber as the synthetic fiber, and otherwise the same as in Example 1, an EDLC separator having a unit area weight of 20 g / m 2 and a thickness of 50 μm was obtained.

[0154] (Example 9)

[0155] The blending amount of the beaten cellulose fiber was changed to 40 mass%, the blending amount of the polyvinyl alcohol fiber was changed to 55 mass%, and otherwise the same as in Example 1, an EDLC separator having a unit area weight of 20 g / m 2 and a thickness of 53 μm was obtained.

[0156] (Example 10)

[0157] The blending amount of the beaten cellulose fiber was changed to 90 mass%, the blending amount of the polyvinyl alcohol fiber was changed to 5 mass%, and otherwise the same as in Example 1, an EDLC separator having a unit area weight of 20 g / m 2 and a thickness of 46 μm was obtained.

[0158] (Example 11)

[0159] The unit area weight was adjusted to 15 g / m 2 and otherwise the same as in Example 1, an EDLC separator having a unit area weight of 15 g / m 2 and a thickness of 42 μm was obtained.

[0160] (Comparative Example 1)

[0161] The drainage of the beaten cellulose fiber was adjusted to 25 ml, and otherwise the same as in Example 1, an EDLC separator having a unit area weight of 20 g / m 2 and a thickness of 41 μm was obtained.

[0162] (Comparative Example 2)

[0163] The drainage of the beaten cellulose fiber was adjusted to 550 ml, and otherwise the same as in Example 1, an EDLC separator having a unit area weight of 20 g / m 2 and a thickness of 55 μm was obtained.

[0164] (Comparative Example 3)

[0165] The organic solvent type cellulose fiber "Lyocell" (TENCEL® "Lyocell" manufactured by Lenzing Corporation) was subjected to beating treatment in the same manner as in Example 1, and the drainage degree was adjusted to 50 ml, and 100 mass% of the beaten cellulose fiber was dispersed in water to prepare a slurry (beaten cellulose fiber : water = 100 g : 10 L). Papermaking and drying were performed in the same manner as in Example 1 to obtain an EDLC separator having a basis weight of 20 g / m2and a thickness of 55 μm. 2 , a thickness of 55 μm.

[0166] (Comparative Example 4)

[0167] The basis weight was adjusted to 17 g / m2, and otherwise, an EDLC separator having a basis weight of 17 g / m2and a thickness of 50 μm was obtained in the same manner as in Comparative Example 3. 2 , a thickness of 50 μm. 2

[0168] (Comparative Example 5)

[0169] The basis weight was adjusted to 15 g / m2, and otherwise, an EDLC separator having a basis weight of 15 g / m2and a thickness of 45 μm was obtained in the same manner as in Comparative Example 3. 2 , a thickness of 45 μm. 2

[0170] (Comparative Example 6)

[0171] The drainage degree of the organic solvent type cellulose fiber was adjusted to 250 ml, and otherwise, an EDLC separator having a basis weight of 20 g / m2and a thickness of 59 μm was obtained in the same manner as in Comparative Example 3. 2 , a thickness of 59 μm.

[0172]

[0173] As shown in Table 1, it was confirmed that the separators obtained in Examples 1 to 11 had high strength even if they were thin. Furthermore, the separators obtained in Examples 1 to 11 exhibited low resistance values. On the other hand, in Comparative Examples 2 to 6, thin separators having high strength could not be obtained. Furthermore, the separators obtained in Comparative Example 1 and Comparative Example 3 had high resistance values, and were not suitable as separators for electrochemical elements. In addition, the separator obtained in Comparative Example 2 had high air permeability, and was insufficient in shielding properties.​​

Claims

1. A separator for an electrochemical element, comprising synthetic fibers, pulped cellulose fibers, and an adhesive, wherein the adhesive is a polyvinyl alcohol-based adhesive, wherein... According to JIS P 8121, the Canadian standard filtration efficiency of this pulped cellulose fiber is above 50 ml and below 500 ml. In the histogram of fiber diameter distribution of the pulped cellulose fibers, (1) The fiber has the largest frequency peak in the range below 50μm. (2) The proportion of this fiber with a fiber diameter of less than 20 μm is more than 55%. The content of the pulped cellulose fiber is 60% or more by mass relative to the total mass of the partition. The thickness of the separator used in the electrochemical element is less than 60 μm.

2. The separator for electrochemical elements according to claim 1, wherein, In the histogram of fiber diameter distribution of pulped cellulose fibers, the proportion of fibers with a diameter exceeding 30 μm is less than 10%.

3. The separator for an electrochemical element according to claim 1 or 2, wherein, The synthetic fibers are polyvinyl alcohol fibers and / or polyester fibers.

4. The separator for an electrochemical element according to any one of claims 1 to 3, wherein, Pulped cellulose fiber is a natural cellulose fiber made by pulping.

5. The separator for an electrochemical element according to any one of claims 1 to 4, wherein, The content of synthetic fibers is between 1% and 55% by mass relative to the total mass of the partition.

6. The separator for an electrochemical element according to any one of claims 1 to 5, wherein, The content of pulped cellulose fiber relative to the total mass of the partition is more than 60% by mass and less than 95% by mass.

7. The separator for an electrochemical element according to any one of claims 1 to 6, wherein, The adhesive content is 0.5% by mass or more and 20% by mass or less relative to the total mass of the partition.

8. The separator for an electrochemical element according to any one of claims 1 to 7, wherein, The adhesive content is between 0.5% and 8% by mass relative to the total mass of the partition.

9. The separator for an electrochemical element according to any one of claims 1 to 8, wherein the thickness is 10 μm or more and 60 μm or less.

10. An electrochemical element comprising a separator for an electrochemical element according to any one of claims 1 to 9.

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