Separator for electrochemical element and electrochemical element

Abstract

To provide a separator for an electrochemical element which has superior electrolyte impregnation properties compared to conventional separators while maintaining properties such as low resistance, short-circuit resistance, and acid resistance.SOLUTION: A separator for an electrochemical element, which is interposed between a pair of electrodes and capable of retaining an electrolyte-containing electrolytic solution, is a single layer separator made of beaten solvent-spun cellulose fibers and synthetic fibers, and has a thickness of 10 to 70 μm, a density of 0.25 to 0.70 g / cm3, and a Beck smoothness of 20 to 400 seconds on both sides.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a separator for an electrochemical element and an electrochemical element using the separator. The present invention is suitable for application to, for example, an electric double layer capacitor, a lithium ion capacitor, and a lithium ion secondary battery.

Background Art

[0002] Electrochemical elements, particularly those characterized by high capacitance such as electric double layer capacitors, lithium ion capacitors, and lithium ion secondary batteries, have been adopted in many fields in recent years, such as power sources and backup applications for automotive-related equipment, renewable energy-related equipment such as wind power generation and solar power generation, and communication equipment such as smart meters, and are expected to continue to expand their applications in the future. In these electrochemical elements, with the expansion of applications and the improvement of the performance of the equipment used, there is a demand for higher capacitance and reliability that can withstand long-term use under severe conditions such as higher voltage.

[0003] In recent years, the structures of high-capacitance electrochemical elements whose applications have been expanding mainly include a wound type and a laminated type. The wound type is formed by winding a pair of electrodes with a separator interposed therebetween, impregnating with an electrolytic solution, and then housing and sealing in a metal case. The laminated type is formed by alternately laminating electrodes and separators, housing in a metal case or a laminated film, injecting an electrolytic solution, and then sealing.

[0004] The main roles of the separator in an electrochemical element are to isolate a pair of electrodes and hold the electrolytic solution. In conventional separators for electrochemical elements, high tightness has been required to suppress short-circuit failures of electrochemical elements. On the other hand, as the separator becomes denser, the voids inside the separator decrease, resulting in a decrease in the impregnation property of the electrolytic solution, and problems such as a decrease in the productivity, capacitance, and reliability of the electrochemical element.

[0005] In order to realize an electrochemical element with excellent productivity, high capacity, and high reliability, a separator that maintains characteristics such as low resistance and short-circuit resistance while having better electrolyte impregnation properties than conventional ones is required.

[0006] As separators for electrochemical elements, various configurations have been proposed for the purpose of improving characteristics such as short-circuit resistance (for example, refer to Patent Documents 1 to 4).

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0008] In Patent Document 1, in order to improve the density of the separator and reduce the resistance, a method of using beaten solvent-spun regenerated cellulose fibers has been proposed. Beatable solvent-spun regenerated cellulose fibers can obtain fine fibrils less than 1 μm by subjecting them to a beating treatment. Therefore, a separator composed of beaten solvent-spun regenerated cellulose fibers becomes a microporous sheet with high density.

[0009] However, with the increase in the voltage of electrochemical elements in recent years, the separator is required to further improve reliability such as chemical stability. In the electrolytic solution of an electrochemical device, an electrolyte containing a fluorine compound is widely used. This electrolyte is decomposed by trace amounts of moisture in the system of the electrochemical device to produce hydrofluoric acid. Although the electrode material and separator are dried before use, it is difficult to completely remove the moisture. Under high-voltage conditions, more hydrofluoric acid is generated than under normal voltage, so there is a concern that the acidity will increase and the cellulose will decompose.

[0010] In Patent Document 2, a technique is disclosed in which a thermoplastic synthetic fiber and a regenerated cellulose fiber are unevenly distributed in the thickness direction of a separator to enhance the denseness, mechanical strength, and chemical stability of the separator. The respective roles are separated, such as a part with a large amount of regenerated cellulose fiber that ensures strength and a part with a large amount of thermoplastic synthetic fiber that ensures denseness and acid resistance, thereby enhancing both strength and denseness, and acid resistance.

[0011] In Patent Document 3, a separator with a low short-circuit defect rate despite its thin thickness is disclosed by mixing beaten cellulose fiber and thermoplastic synthetic fiber to form a sheet and then adjusting the thickness by soft calendering.

[0012] However, in order to obtain reliability that can withstand long-term use under severe conditions such as improving the productivity, increasing the capacity, and increasing the voltage of an electrochemical device, the impregnation properties of the separators in Patent Document 2 and Patent Document 3 may not be sufficient.

[0013] In Patent Document 4, a separator with enhanced denseness and mechanical strength is proposed by using a multilayer nonwoven fabric composed of two or more nonwoven fabric layers having a high-density layer and a low-density layer. In Patent Document 4, after wet papermaking a separator with two or more layers containing solvent-spun cellulose and synthetic fiber and then performing calendering treatment, delamination between layers, which was a problem of conventional multilayer separators, is suppressed. However, it is more difficult to reduce the thickness of a multilayer separator than that of a single-layer separator.

[0014] The above description was about the separator made of nonwoven fabric. As a separator for power storage devices such as lithium-ion secondary batteries, there is a microporous membrane made of synthetic resin. Generally, it is a film made of polyolefin resin and has fine pores. Compared with the separator made of nonwoven fabric, such a film made of polyolefin resin has low impregnation property of the electrolyte solution. Therefore, it is difficult to improve the productivity and increase the capacity of the electrochemical element, or the initial resistance of the electrochemical element is high because the resistance value of the separator is large.

[0015] The present invention has been made in view of the above-described problems, and an object thereof is to provide a separator that is excellent in impregnation property of an electrolyte solution as compared with the conventional one while maintaining characteristics such as low resistance, short-circuit resistance, and acid resistance of the separator for an electrochemical element. Another object of the present invention is to provide an electrochemical element that has high production capacity, high capacity, and reliability capable of withstanding long-term use under severe conditions such as high voltage by applying the separator.

Means for Solving the Problems

[0016] The separator for an electrochemical element of the present invention is a separator for an electrochemical element that is interposed between a pair of electrodes and can hold an electrolyte solution containing an electrolyte. The separator is a single-layer separator composed of beaten solvent-spun cellulose fibers and synthetic fibers, has a thickness of 10 to 70 μm, and a density of 0.25 to 0.70 g / cm 3 , and the Bek smoothness on both sides is 20 to 400 seconds.

[0017] Preferably, the air permeability resistance is 2 to 50 seconds. Also, the fiber diameter difference between the beaten solvent-spun cellulose fibers and the synthetic fibers is preferably -1.0 μm to 3.0 μm. Furthermore, it is preferable that the synthetic fiber is one or more fibers selected from polyester-based fibers, polyolefin-based fibers, and acrylic-based fibers.

[0018] The electrochemical device of the present invention has a configuration in which the separator for the electrochemical device of the present invention is used.

[0019] In addition, the electrochemical device of the present invention can be selected from, for example, an electric double layer capacitor, a lithium ion capacitor, and a lithium ion secondary battery.

Advantages of the Invention

[0020] According to the present invention, it is possible to improve the impregnation property of the electrolytic solution without impairing the resistance, short-circuit resistance, and acid resistance of the separator. In addition, by using the separator of the present invention, it is possible to contribute to the improvement of the productivity of the electrochemical device, and the reliability that can withstand long-term use under severe conditions such as high capacity and high voltage.

Embodiments for Carrying Out the Invention

[0021] The separator for the electrochemical device of the present invention is a separator composed of solvent-spun cellulose fibers and synthetic fibers. In the case of solvent-spun cellulose fibers, it is possible to beat the fibers to obtain a desired fiber diameter or control the fibrillation generation rate to a desired value, which is also preferable from the viewpoints of reducing the resistance of the separator and short-circuit resistance. Among synthetic fibers, polyester fibers such as polyethylene terephthalate, polyolefin fibers such as polyethylene and polypropylene, and acrylic fibers such as polyacrylonitrile are preferable from the viewpoints of acid resistance, heat resistance, and papermaking suitability.

[0022] The separator for the electrochemical device of the present invention has a separator thickness of 10 to 70 μm, a density of 0.25 to 0.70 g / cm 3 , and a Bekk smoothness of 20 to 400 seconds on both sides of the separator, so that it is possible to realize a separator excellent in the impregnation property of the electrolytic solution without impairing the resistance and short-circuit resistance of the separator. Further, the air permeability resistance of the separator is preferably 2 to 50 seconds. More preferably, the Bekk smoothness on both sides of the separator is 30 to 300 seconds, and the air permeability resistance is 3 to 30 seconds.

[0023] The impregnation property of the electrolytic solution in an electrochemical device mainly includes two aspects: the liquid retention property of the electrolytic solution and the impregnation rate. Among these, the parts where the electrolytic solution impregnates include two parts: the part where the electrolytic solution penetrates into the separator (hereinafter referred to as part A) and the part where the electrolytic solution penetrates from the interface between the electrode and the separator (hereinafter referred to as part B). For part A, improvement has been made by specifying the material, density, and air permeability resistance of the separator that constitutes it. However, for part B, sufficient verification has not been carried out. When the inventor of the present invention focused on part B, it was found that it affects the Bek smoothness of the separator.

[0024] The Bek smoothness is the time required for air to pass between a glass surface and the separator pressed against each other under a certain pressure, and it is an evaluation method indicating the surface roughness of the separator. In the present invention, by controlling the Bek smoothness, the impregnation property of part B is improved. It should be noted that the lower the numerical value of the Bek smoothness, the rougher the surface of the sheet is indicated.

[0025] The Bek smoothness of the separator of the present invention is 20 to 400 seconds on both sides of the separator, and more preferably 30 to 300 seconds. When the Bek smoothness exceeds 400 seconds, since the density in the plane direction of the separator is very high, when an electrochemical device is manufactured, the electrode and the separator are in close contact, inhibiting the penetration of the electrolytic solution in part B, resulting in deterioration of the impregnation property. For example, the separator in Patent Document 3 corresponds to this. When the Bek smoothness is less than 20 seconds, since the surface of the separator is rough, the impregnation property of the electrolytic solution in part B is improved. However, it contains solvent-spun cellulose fibers with a large fiber diameter and / or synthetic fibers with a large fineness, resulting in low density. In addition, there is less fiber entanglement between solvent-spun cellulose fibers, and shedding of synthetic fibers occurs.

[0026] The thickness of the separator of the present invention is 10 to 70 μm. If the thickness of the separator is less than 10 μm, even if the separator has good tightness, it may not be possible to suppress short - circuit failures in the electrochemical device. On the other hand, when the thickness of the separator exceeds 70 μm, the resistance of the separator increases.

[0027] The density of the separator of the present invention is 0.25 - 0.70 g / cm 3 is. If the density of the separator is less than 0.25 g / cm 3 there is insufficient tightness, and short - circuit failures may occur. On the other hand, when the density exceeds 0.70 g / cm 3 since the fibers constituting the separator are crimped or fused together, the resistance of the separator increases. Furthermore, since the impregnation property of part A decreases, an electrochemical device with high resistance is obtained.

[0028] The separator of the present invention uses beaten solvent - spun cellulose (for example, lyocell) and synthetic fibers. The solvent - spun cellulose fibers are refined by beating (mechanical treatment in water), and while maximizing the tightness of the separator, low resistance can be maintained. And the synthetic fibers improve the chemical stability of the separator, so that the stability under severe environments such as the increasing of high voltage required for separators in recent years can also be improved. The content of the solvent - spun cellulose fibers is preferably in the range of 70 - 95% by mass, and the content of the synthetic fibers is preferably in the range of 5 - 30% by mass. When the content of the solvent - spun cellulose fibers is less than 70% by mass and the content of the synthetic fibers exceeds 30% by mass, since there are few fiber entanglements inside the sheet of the solvent - spun cellulose, the dropout of the synthetic fibers from the separator may increase. When the content of the solvent - spun cellulose fibers exceeds 95% by mass and the content of the synthetic fibers is less than 5% by mass, the chemical stability of the separator may decrease.

[0029] The air - permeability resistance of the separator of the present invention is 2 - 50 seconds, more preferably 3 - 30 seconds. If the air permeability resistance is less than 2 seconds, the tightness is insufficient and short - circuit defects may occur. On the other hand, when the air permeability resistance exceeds 50 seconds, the resistance of the separator increases, and the impregnation property of the electrolyte in part A also deteriorates.

[0030] The value obtained by subtracting the fiber diameter of the synthetic fiber from the fiber diameter of the solvent - spun cellulose fiber after beating (fiber diameter difference) is preferably in the range of - 1.0 to 3.0 μm. When the fiber diameter difference is less than - 1.0 μm, the fiber entanglement between the solvent - spun celluloses is inhibited, and the dropout of the synthetic fiber from the separator may increase. On the other hand, when the fiber diameter difference is greater than 3.0 μm, fiber uneven distribution occurs as in Patent Document 2, so the Bekk smoothness of the surface with more synthetic fibers increases, resulting in a separator with low impregnation property.

[0031] Hereinafter, specific examples, comparative examples, etc. of the separator for an electrochemical element according to the present invention and the electrochemical element equipped with the separator for an electrochemical element will be described in detail.

[0032] In the following examples, all separators were wet non - woven fabrics formed by the papermaking method. However, in the present invention, as long as the thickness, density, and Bekk smoothness of the separator are satisfied, the formation method of the separator is not limited to the papermaking method. For example, a method of forming a fiber dispersion liquid into a sheet by casting or the like is also acceptable.

[0033] 〔Evaluation method for separator and electrochemical element〕 Specific characteristics of the separator and the electrochemical element were measured under the following conditions and by the following methods.

[0034] 〔Fiber diameter of solvent - spun cellulose after beating〕 The raw material after beating or the separator was observed with a SEM (scanning electron microscope), and the average value was obtained when measuring the length of only the core part (fibers with a fiber diameter of 1 μm or more) for n = 50. For flat - shaped fibers, the part with the longer straight - line distance was measured. When it is difficult to distinguish from synthetic fibers, observe after extracting only the solvent-spun cellulose in accordance with JIS L1030-2 "Test Method for Blending Ratio of Textile Products".

[0035] 〔Fiber Diameter of Synthetic Fiber〕 The raw material or separator was observed by SEM, and the average value was obtained when measuring the length of n = 50 pieces. For fibers with a flat shape, measure the part with the longer straight-line distance. When it is difficult to distinguish from solvent-spun cellulose, observe after extracting only the synthetic fibers in accordance with JIS L1030-2 "Test Method for Blending Ratio of Textile Products".

[0036] 〔Fiber Diameter Difference〕 It was calculated by subtracting the fiber diameter of the synthetic fiber from the fiber diameter of the solvent-spun cellulose fiber after beating.

[0037] 〔Thickness〕 Using the micrometer specified in "5.1.1 Measuring Instrument and Measuring Method a When Using an External Micrometer" of "JIS C 2300-2 'Cellulose Paper for Electrical Use - Part 2: Test Methods' 5.1 Thickness" and folding the separator 10 times as specified in "5.1.3 Measuring Thickness by Folding the Paper", the thickness of the separator was measured.

[0038] 〔Density〕 The density of the separator was calculated by measuring the basis weight in the absolutely dry state by the method specified in Method B of "JIS C 2300-2 'Cellulose Paper for Electrical Use - Part 2: Test Methods' 7.0A Density".

[0039] 〔Air Permeance Resistance〕 The air permeance resistance of the separator was measured using a Type B tester by the method specified in "JIS P 8117 'Paper and Paperboard - Test Methods for Air Permeability and Air Permeance Resistance (Intermediate Region) - Gurley Method'".

[0040] 〔Smoothness of Separator〕 The Bekk smoothness of the separator was measured in accordance with "JIS P 8119, Paper and Paperboard - Test Method for Smoothness by Bekk Smoothness Tester". Note that the surface with a larger Bekk smoothness value is defined as the "smooth surface", and the surface with a smaller value is defined as the "rough surface".

[0041] 〔Method for manufacturing an electrochemical element〕 An element roll was produced by interposing a separator between the two electrode materials and winding them. Each electrochemical element was obtained by immersing this element roll in an electrolytic solution and encapsulating it in a case. Specifically, the electrochemical elements of an electric double layer capacitor, a lithium ion capacitor, and a lithium ion secondary battery were manufactured by the following methods.

[0042] 〔Method for manufacturing an electric double layer capacitor〕 An activated carbon electrode and a separator were wound to obtain an electric double layer capacitor element roll. The element roll was immersed in an electrolytic solution in which tetraethylammonium tetrafluoroborate was dissolved as an electrolyte, and after performing vacuum impregnation for 1 hour, it was housed in a bottomed cylindrical aluminum case and sealed with a sealing rubber to manufacture an electric double layer capacitor with a rated voltage of 3.0 V and a capacitance equivalent to 3000 F.

[0043] 〔Method for manufacturing a lithium ion capacitor〕 As the positive electrode material, an activated carbon electrode for a lithium ion capacitor was used, and as the negative electrode material, a graphite electrode was used. The separator and the electrode material were alternately folded to obtain a lithium ion capacitor element. The element was housed in a multilayer laminate film together with a lithium pre-doping foil, an electrolytic solution was injected, and after performing vacuum impregnation for 1 hour, it was sealed to manufacture a lithium ion capacitor with a rated voltage of 4.0 V and a capacitance equivalent to 2000 F. As the electrolytic solution, a solution in which lithium hexafluorophosphate was dissolved as an electrolyte in a propylene carbonate solvent was used.

[0044] 〔Method for manufacturing a lithium ion secondary battery〕 As the positive electrode material, a lithium cobalt oxide electrode for a lithium-ion secondary battery was used, and a graphite electrode was used as the negative electrode material. They were wound together with a separator to obtain a lithium-ion secondary battery element roll. The element roll was immersed in a mixed solvent of ethylene carbonate and diethyl carbonate and an electrolyte solution in which lithium hexafluorophosphate was dissolved as an electrolyte, and vacuum impregnation was performed for 1 hour. Then, it was housed in a bottomed cylindrical case and sealed with a press machine to fabricate a lithium-ion secondary battery with a rated voltage of 4.2 V and a discharge capacity equivalent to 3000 mAh.

[0045] The characteristics of each fabricated electrochemical element were measured by the following method. For each example, 1000 electrochemical elements were fabricated and used for the following characteristic evaluations.

[0046] 〔Impregnability〕 The capacitance or discharge capacity of the fabricated electrochemical element was measured to evaluate the impregnability. When the capacitance or discharge capacity of the electrochemical element was 99% or more, it was rated as "〇"; when it was less than 99% and 95% or more, it was rated as "△"; when it was less than 95%, it was rated as "×".

[0047] 〔Capacitance·Discharge Capacity〕 The capacitance of the electric double layer capacitor and the lithium-ion capacitor was determined by the constant current discharge method of "5.5 Capacitance and Internal Resistance Measurement Method 1" specified in "JIS C 5160-1 Fixed Electric Double Layer Capacitors for Electrical and Electronic Equipment - Part 1: General Rules by Item". The discharge capacity of the lithium-ion secondary battery was measured in accordance with "6.3 Discharge Performance" specified in "JIS C 8715-1 Single Cells and Battery Systems of Industrial Lithium Secondary Batteries - Part 1: Performance Requirements".

[0048] 〔Internal Resistance〕 The internal resistance of the electric double layer capacitor and the lithium-ion capacitor was determined by the constant current discharge method of "5.5 Capacitance and Internal Resistance Measurement Method 1" specified in "JIS C 5160-1 Fixed Electric Double Layer Capacitors for Electrical and Electronic Equipment - Part 1: General Rules by Item". The internal resistance of the lithium-ion secondary battery was measured in accordance with "6.5 Internal Resistance" specified in "JIS C 8715-1 'Single Cells and Battery Systems for Industrial Lithium Secondary Batteries - Part 1: Performance Requirements'".

[0049] [Short circuit failure rate] For the short circuit failure rate of the electrochemical element, the case where the charging voltage did not rise to the rated voltage was regarded as a short circuit failure. The number of electrochemical elements that became these short circuit failures was divided by the number of produced electrochemical elements, and the percentage was taken as the short circuit failure rate.

[0050] [Capacity retention rate] A long-term reliability test was carried out in which the rated voltage was applied to the electrochemical element for 1500 hours under the condition of constant temperature at 65°C. In the long-term reliability test, the electrochemical elements with short circuit failures were excluded. The capacity retention rate of the electrochemical element was obtained using the following formula (1). Capacity retention rate (%) = 100 - (Ca - Cb) / Ca × 100 (1) (Ca: Capacity before the long-term reliability test, Cb: Capacity before the long-term reliability test) [Examples]

[0051] Hereinafter, various specific examples, comparative examples, and conventional examples of the separator for an electrochemical element according to the present invention and the electrochemical element provided with the separator for an electrochemical element will be described in detail. The separator of each example was obtained by a papermaking method using a paper machine.

[0052] (Example 1) As beaten solvent-spun cellulose fibers, 95% by mass of lyocell fibers with an average fiber diameter of 2.1 μm and 5% by mass of polyethylene terephthalate fibers (hereinafter referred to as PET fibers) with an average fiber diameter of 3.0 μm as synthetic fibers were mixed and formed into a long web paper, with a thickness of 10.3 μm and a density of 0.69 g / cm 3 of the separator was obtained. The fiber diameter difference between the lyocell fibers and the PET fibers of this separator was -0.9 μm. Also, the Bekk smoothness was 397.1 seconds for the smooth surface and 395.0 seconds for the rough surface, and the air permeability resistance was 49.6 seconds / 100 ml. Using this separator, each of the electrochemical elements of Example 1 was fabricated.

[0053] (Example 2) As the beaten solvent-spun cellulose fibers, 95% by mass of lyocell fibers with an average fiber diameter of 3.0 μm and 5% by mass of PET fibers with an average fiber diameter of 2.2 μm as synthetic fibers were mixed and made into a cylinder mold paper, resulting in a separator with a thickness of 15.0 μm and a density of 0.64 g / cm 3 . The fiber diameter difference between the lyocell fibers and the PET fibers of this separator was 0.8 μm. Also, the Bekk smoothness was 298.2 seconds for the smooth surface and 286.3 seconds for the rough surface, and the air permeability resistance was 30.0 seconds / 100 ml. Using this separator, each of the electrochemical elements of Example 2 was fabricated.

[0054] (Example 3) As the beaten solvent-spun cellulose fibers, 70% by mass of lyocell fibers with an average fiber diameter of 10.8 μm and 30% by mass of polypropylene fibers (hereinafter referred to as PP fibers) with an average fiber diameter of 7.9 μm as synthetic fibers were mixed and made into a cylinder mold paper, resulting in a separator with a thickness of 68.7 μm and a density of 0.26 g / cm 3 . The fiber diameter difference between the lyocell fibers and the PP fibers of this separator was 2.9 μm. Also, the Bekk smoothness was 22.9 seconds for the smooth surface and 21.8 seconds for the rough surface, and the air permeability resistance was 2.2 seconds / 100 ml. Using this separator, each of the electrochemical elements of Example 3 was fabricated.

[0055] (Example 4) As the beaten solvent-spun cellulose fibers, 70% by mass of lyocell fibers with an average fiber diameter of 9.9 μm and 30% by mass of PP fibers with an average fiber diameter of 7.7 μm as synthetic fibers were mixed and made into a cylinder mold paper, resulting in a separator with a thickness of 64.5 μm and a density of 0.31 g / cm 3 . The fiber diameter difference between the lyocell fiber and the PP fiber of this separator was 2.2 μm. Also, the Bekk smoothness was 36.3 seconds for the smooth surface and 31.8 seconds for the rough surface, and the air permeability resistance was 3.1 seconds / 100 ml. Using this separator, each electrochemical element of Example 4 was fabricated.

[0056] (Example 5) As beaten solvent-spun cellulose fibers, 85% by mass of lyocell fibers with an average fiber diameter of 5.1 μm and 15% by mass of acrylic fibers with an average fiber diameter of 3.2 μm as synthetic fibers were mixed and formed into a wet web, resulting in a separator with a thickness of 19.6 μm and a density of 0.55 g / cm 3 . The fiber diameter difference between the lyocell fiber and the acrylic fiber of this separator was 1.9 μm. Also, the Bekk smoothness was 199.9 seconds for the smooth surface and 187.0 seconds for the rough surface, and the air permeability resistance was 19.8 seconds / 100 ml. Using this separator, each electrochemical element of Example 5 was fabricated.

[0057] (Example 6) As beaten solvent-spun cellulose fibers, 85% by mass of lyocell fibers with an average fiber diameter of 6.3 μm and 15% by mass of acrylic fibers with an average fiber diameter of 3.5 μm as synthetic fibers were mixed and formed into a wet web, resulting in a separator with a thickness of 30.2 μm and a density of 0.49 g / cm 3 . The fiber diameter difference between the lyocell fiber and the acrylic fiber of this separator was 2.8 μm. Also, the Bekk smoothness was 155.5 seconds for the smooth surface and 150.3 seconds for the rough surface, and the air permeability resistance was 14.6 seconds / 100 ml. Using this separator, each electrochemical element of Example 6 was fabricated.

[0058] (Example 7) As beaten solvent-spun cellulose fibers, 85% by mass of lyocell fibers with an average fiber diameter of 7.5 μm and 15% by mass of PET fibers with an average fiber diameter of 4.9 μm as synthetic fibers were mixed and formed into a wet web, resulting in a separator with a thickness of 35.4 μm and a density of 0.46 g / cm 3 . The fiber diameter difference between the lyocell fiber and the PET fiber of this separator was 2.6 μm. Also, the Bekk smoothness was 97.3 seconds for the smooth surface and 96.4 seconds for the rough surface, and the air permeability resistance was 10.9 seconds / 100 ml. Using this separator, each electrochemical element of Example 7 was fabricated.

[0059] (Example 8) As the beaten solvent-spun cellulose fiber, 80% by mass of lyocell fiber with an average fiber diameter of 6.9 μm and 20% by mass of PET fiber with an average fiber diameter of 5.4 μm as a synthetic fiber were mixed and made into a cylinder mold paper, obtaining a separator with a thickness of 29.8 μm and a density of 0.41 g / cm 3 . The fiber diameter difference between the lyocell fiber and the PET fiber of this separator was 1.5 μm. Also, the Bekk smoothness was 53.2 seconds for the smooth surface and 48.1 seconds for the rough surface, and the air permeability resistance was 5.3 seconds / 100 ml. Using this separator, each electrochemical element of Example 8 was fabricated.

[0060] (Comparative Example 1) As the beaten solvent-spun cellulose fiber, 95% by mass of lyocell fiber with an average fiber diameter of 1.6 μm and 5% by mass of PET fiber with an average fiber diameter of 1.7 μm as a synthetic fiber were mixed and made into a cylinder mold paper, obtaining a separator with a thickness of 8.1 μm and a density of 0.75 g / cm 3 . The fiber diameter difference between the lyocell fiber and the PET fiber of this separator was -0.1 μm. Also, the Bekk smoothness was 458.0 seconds for the smooth surface and 430.6 seconds for the rough surface, and the air permeability resistance was 52.4 seconds / 100 ml. Using this separator, each electrochemical element of Comparative Example 1 was fabricated.

[0061] (Comparative Example 2) As the beaten solvent-spun cellulose fiber, 70% by mass of lyocell fiber with an average fiber diameter of 11.3 μm and 30% by mass of PET fiber with an average fiber diameter of 8.4 μm as a synthetic fiber were mixed and made into a cylinder mold paper, obtaining a separator with a thickness of 74.7 μm and a density of 0.23 g / cm 3 . The fiber diameter difference between the lyocell fiber and the PET fiber of this separator was 2.9 μm. Also, the Bekk smoothness was 11.1 seconds for the smooth surface and 8.1 seconds for the rough surface, and the air permeability resistance was 1.5 seconds / 100 ml. Using this separator, each electrochemical element of Comparative Example 2 was fabricated.

[0062] (Comparative Example 3) As the beaten solvent-spun cellulose fiber, 65% by mass of lyocell fiber with an average fiber diameter of 10.7 μm and 35% by mass of PET fiber with an average fiber diameter of 7.8 μm as a synthetic fiber were mixed and made into a cylinder mold paper, resulting in a separator with a thickness of 75.1 μm and a density of 0.20 g / cm 3 . The fiber diameter difference between the lyocell fiber and the PET fiber of this separator was 2.9 μm. Also, the Bekk smoothness was 9.7 seconds for the smooth surface and 8.4 seconds for the rough surface, and the air permeability resistance was 1.2 seconds / 100 ml. Using this separator, each electrochemical element of Comparative Example 3 was fabricated.

[0063] (Comparative Example 4) As the beaten solvent-spun cellulose fiber, 85% by mass of lyocell fiber with an average fiber diameter of 6.3 μm and 15% by mass of PET fiber with an average fiber diameter of 7.5 μm as a synthetic fiber were mixed and made into a cylinder mold paper, resulting in a separator with a thickness of 40.5 μm and a density of 0.24 g / cm 3 . The fiber diameter difference between the lyocell fiber and the PET fiber of this separator was -1.2 μm. Also, the Bekk smoothness was 18.8 seconds for the smooth surface and 15.1 seconds for the rough surface, and the air permeability resistance was 1.8 seconds / 100 ml. Using this separator, each electrochemical element of Comparative Example 4 was fabricated.

[0064] (Conventional Example 1) As the beaten solvent-spun cellulose fiber, 100% by mass of lyocell fiber with an average fiber diameter of 8.0 μm was made into a cylinder mold paper, resulting in a separator with a thickness of 30.1 μm and a density of 0.41 g / cm 3 . The Bek smoothness of this separator was 102.5 seconds for the smooth surface and 91.2 seconds for the rough surface, and the air permeability resistance was 10.3 seconds / 100 ml. Using this separator, each electrochemical element of Conventional Example 1 was fabricated.

[0065] (Conventional Example 2) As beaten solvent-spun cellulose fibers, 75% by mass of lyocell fibers with an average fiber diameter of 6.9 μm and 25% by mass of PET fibers with an average fiber diameter of 3.7 μm as synthetic fibers were mixed and formed into a wet laid sheet, resulting in a separator with a thickness of 30.4 μm and a density of 0.45 g / cm 3 was obtained. The fiber diameter difference between the lyocell fibers and the PET fibers of this separator was 3.2 μm. Also, the Bek smoothness was 516.0 seconds for the smooth surface and 247.7 seconds for the rough surface, and the air permeability resistance was 14.7 seconds / 100 ml. Using this separator, each electrochemical element of Conventional Example 2 was fabricated.

[0066] (Conventional Example 3) The separator of Example 6 was pressure-adjusted by soft calendering to obtain a separator with a thickness of 25.1 μm and a density of 0.59 g / cm 3 was obtained. The Bek smoothness of this separator was 633.4 seconds for the smooth surface and 625.5 seconds for the rough surface, and the air permeability resistance was 20.0 seconds / 100 ml. Using this separator, each electrochemical element of Conventional Example 3 was fabricated.

[0067] (Conventional Example 4) A polyethylene microporous membrane with a thickness of 29.9 μm and a density of 0.53 g / cm 3 was used as the separator. Since this separator is not made of fibers, the fiber diameter cannot be measured. The air permeability resistance of the separator was 301.0 seconds / 100 ml, and the Bek smoothness was more than 10,000 seconds for both the smooth surface and the rough surface.

[0068] The evaluation results of the separators of each example, comparative example, and conventional example are shown in Table 1. In addition, the evaluation results of the electrochemical elements produced using the separators of each example are shown in Table 2.

[0069]

Table 1

[0070]

Table 2

[0071] As can be seen from Table 2, no short-circuit failure has occurred in the electrochemical elements of each example. In addition, all the impregnation properties are "○", and the capacitance and discharge capacity of the electrochemical elements have achieved the target capacitance. Also, the capacitance retention rate in the long-term reliability test is 70% or more, which is good.

[0072] The separator of Comparative Example 1 has a thickness of 8.1 μm. And in the electrochemical element using this separator, a short-circuit failure has occurred. Moreover, it is composed of fine fibers with an average fiber diameter of 1.6 μm for lyocell fibers and 1.7 μm for PET fibers, and the density is 0.75 g / cm 3 , and due to the air permeability resistance being 52.4 seconds / 100 ml and the tightness being too high, the internal resistance of the electrochemical element is high. Furthermore, the Bekk smoothness of the separator is 458.0 seconds for the smooth surface and 430.6 seconds for the rough surface, and the tightness in the plane direction is also high. Therefore, the impregnation property is "△", the internal resistance of the electrochemical element deteriorates, and the capacitance and discharge capacity and the capacitance retention rate in the long-term reliability test decrease.

[0073] The separator of Comparative Example 2 has a thickness of 74.7 μm. Due to the thickness being thick, the internal resistance of the electrochemical element is high. Since the Bekk smoothness of the separator is low at 11.1 seconds for the smooth surface and 8.1 seconds for the rough surface, the impregnation property is good. On the other hand, since it is composed of thick fibers with an average fiber diameter of 11.3 μm for lyocell fibers and 8.4 μm for PET fibers, despite the thick thickness, the density is 0.23 g / cm 3The air permeability resistance is as low as 1.5 seconds / 100 ml, and short circuit defects occur.

[0074] From the comparison between Comparative Example 1 and Comparative Example 2 and each Example, it can be seen that the thickness of the separator is preferably 10 to 70 μm. Also, the density of the separator is 0.25 to 0.70 g / cm 3 It can be seen that the air permeability resistance is preferably 2 to 50 seconds / 100 ml, and the Bek smoothness on both sides of the separator is preferably 20 to 400 seconds.

[0075] For improving the productivity of the electrochemical element, when it is desired to shorten the impregnation time, a lower value of Bek smoothness is more preferable. In the comparison between Example 1 and Example 2, for Example 1, the smooth surface is 397.1 seconds and the rough surface is 395.0 seconds, while for Example 2, the smooth surface is 298.2 seconds and the rough surface is 286.3 seconds. Example 2 has a lower value. Also, the air permeability resistance of Example 1 is 49.6 seconds / 100 ml, while that of Example 2 is 30.0 seconds / 100 ml. There are no short circuit defects, and it can be seen that Example 2 has a lower internal resistance of the electrochemical element and a better capacity retention rate after the long-term reliability test.

[0076] Also, in the comparison between Example 3 and Example 4, for Example 3, the smooth surface of the Bek smoothness is 22.9 seconds and the rough surface is 21.8 seconds, while for Example 4, the smooth surface of the Bek smoothness is 36.3 seconds and the rough surface is 31.8 seconds. The impregnation property is more advantageous for Example 3. However, in order to obtain a separator with a low Bek smoothness, it is necessary to increase the fiber diameters of lyocell and PET fibers. Therefore, since Example 3 has a thicker separator, the internal resistance of the electrochemical element is more preferable for Example 4.

[0077] From the comparison between Example 1 and Example 2, and between Example 3 and Example 4, the Bek smoothness on both sides of the separator is more preferably 30 to 300 seconds, and the air permeability resistance is more preferably 3 to 30 seconds / 100 ml.

[0078] The separator of Comparative Example 3 is a separator in which 65% by mass of lyocell fibers and 35% by mass of PET fibers are mixed. Since the density and air permeability resistance are even lower than those of the separator of Comparative Example 2, although the thickness is thick, the short circuit failure rate of the electrochemical device is high.

[0079] The separator of Conventional Example 1 is a separator made of only lyocell fibers. From the comparison between Conventional Example 1 and each Example, it can be seen that by using the separator of the present invention containing synthetic fibers, the capacity retention rate after the long-term reliability test can be improved. This is presumably because the chemical stability of the separator is improved by the synthetic fibers. From this, it can be seen that the content of the solvent-spun cellulose fiber is preferably 70 to 95% by mass, and the content of the synthetic fiber is preferably 5 to 30% by mass.

[0080] The separator of Comparative Example 4 has a fiber diameter difference between lyocell fibers and PET fibers of -1.2 μm, and a density of 0.24 g / cm 3 , and the air permeability resistance is as low as 1.8 μm. This is because the PET fibers inhibit the fiber entanglement between the lyocell fibers, resulting in the dropout of the PET fibers from the separator. In the electrochemical device using the separator of Comparative Example 4, the short circuit failure rate is high and the capacity retention rate after the long-term reliability test is low. Also, the separator of Conventional Example 2 has a Bek smoothness of 516.0 seconds on the smooth surface. The impregnation property is low, and the target capacity of the electrochemical device has not been obtained. This is because the fiber diameter difference between the lyocell fibers and the PET fibers of the separator of Conventional Example 2 is 3.2 μm, resulting in uneven distribution of the fibers, which increases the value of the Bek smoothness on the smooth surface. From the comparison between Comparative Example 4 and Conventional Example 2 and each Example, it can be seen that the fiber diameter difference between the solvent-spun cellulose fiber and the synthetic fiber is preferably -1.0 μm to 3.0 μm.

[0081] The separator of Comparative Example 3 has the separator of Example 6 adjusted in thickness by a soft calender, but the Bek smoothness is 633.4 seconds for the smooth surface and 625.5 seconds for the rough surface, and the impregnation property is even lower than that of the separator of Comparative Example 2, and the capacitance of the electrochemical element is also low.

[0082] Since the separator of Comparative Example 4 is a film, the impregnation property of the material itself is poor. Furthermore, the Bek smoothness is more than 10,000 seconds for both sides. When the impregnation time was 1 hour, the capacitance of the electrochemical element was too low compared to the target value, so other evaluations could not be carried out.

[0083] As described above, the separator of the present invention is a separator for an electrochemical element that is interposed between a pair of electrodes and can hold an electrolyte-containing electrolytic solution. The separator is a single-layer separator composed of beaten solvent-spun cellulose fibers and synthetic fibers, with a thickness of 10 to 70 μm and a density of 0.25 to 0.70 g / cm 3 , and is a separator for an electrochemical element characterized in that the Bek smoothness of both sides is 20 to 400 seconds. By using the separator of the present invention, it is possible to improve the impregnation property of the electrolytic solution without impairing the resistance, short-circuit resistance, and acid resistance of the separator. In addition, by using the separator of the present invention, it is possible to contribute to improving the productivity of the electrochemical element, increasing the capacitance, and reliability that can withstand long-term use under harsh conditions.

[0084] As described above, an example of using the separator of the present embodiment for an electric double layer capacitor, a lithium ion capacitor, and a lithium ion secondary battery has been described. In the electrochemical element according to the present invention, there is no need for special limitation on the electrode material, electrolytic solution material, and other members, and various materials can be used. In addition, the separator for an electrochemical element of the present invention can also be applied to electrochemical elements other than those described in the present embodiment example, such as lithium primary batteries, sodium ion secondary batteries, sodium sulfur secondary batteries, magnesium ion secondary batteries, and magnesium sulfur secondary batteries.

Claims

1. A separator for an electrochemical device that is interposed between a pair of electrodes and can hold an electrolyte-containing electrolytic solution, wherein the separator is a single-layer separator composed of beaten solvent-spun cellulose fibers and synthetic fibers, with a thickness of 10 to 70 μm, a density of 0.25 to 0.70 g / cm ３ , and a Bekk smoothness on both sides of 20 to 400 seconds, a separator for an electrochemical element, characterized by the above features.

2. The separator for an electrochemical device according to Claim 1, wherein the air permeability resistance is 2 to 50 seconds.

3. The separator for an electrochemical device according to Claim 1, wherein the fiber diameter difference between the beaten solvent-spun cellulose fibers and the synthetic fibers is -1.0 to 3.0 μm.

4. The separator for an electrochemical device according to Claim 1, wherein the synthetic fiber is one or more fibers selected from polyester-based fibers, acrylic-based fibers, and polyolefin-based fibers.

5. An electrochemical device characterized by using the separator for an electrochemical device according to any one of Claims 1 to 4.

6. The electrochemical device according to Claim 5, which is any one of an electric double layer capacitor, a lithium ion capacitor, and a lithium ion secondary battery.

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