Separator for electrochemical elements, and electrochemical element

Abstract

Provided is a separator for electrochemical elements, which has superior electrolyte impregnation properties compared to prior ones while maintaining characteristics such as low resistance, short circuit resistance and acid resistance. The separator for electrochemical elements is interposed between a pair of electrodes, and can hold an electrolyte solution that contains an electrolyte. The separator is configured as a layer that is composed of a beaten solvent-spun cellulose fiber and a synthetic fiber. The separator has a thickness of 10-70 μm, a density of 0.25-0.70 g / cm3, and a Bekk smoothness of 20-400 seconds on both surfaces.

Description

Separator for electrochemical element and electrochemical element

[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, electric double layer capacitors, lithium ion capacitors, and lithium ion secondary batteries.

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

[0003] The structures of high-capacity electrochemical elements, which have seen an expansion in use in recent years, are mainly wound and stacked. Wound types are made by winding a pair of electrodes with a separator between them, impregnating them with an electrolyte, and then encasing them in a metal case and sealing them. Stacked types are made by alternately stacking electrodes and separators, encasing them in a metal case or laminate film, injecting the electrolyte, and then sealing them.

[0004] The main role of a separator in an electrochemical element is to separate a pair of electrodes and retain the electrolyte. Conventional separators for electrochemical elements require high density to prevent short circuits in the electrochemical element. However, the denser the separator, the fewer voids there are inside the separator, which reduces the electrolyte impregnation, resulting in reduced productivity, capacity, and reliability of the electrochemical element.

[0005] To realize electrochemical elements that are highly manufacturable, high-capacity, and highly reliable, separators that have superior electrolyte impregnation properties while maintaining low resistance and short-circuit resistance are required.

[0006] Various configurations have been proposed for separators for electrochemical elements with the aim of improving properties such as short-circuit resistance (see, for example, Patent Documents 1 to 4).

[0007] JP 2000-3834 A JP 2017-117590 A JP 2019-96681 A JP 2013-171905 A

[0008] Patent Document 1 proposes a method of using beaten solvent-spun regenerated cellulose fibers to improve the density of separators and reduce resistance. Beaten solvent-spun regenerated cellulose fibers can be subjected to a beating process to obtain fine fibrils of less than 1 μm. Therefore, separators made of beaten solvent-spun regenerated cellulose fibers become highly dense, microporous sheets.

[0009] However, with the recent trend toward higher voltages in electrochemical elements, separators are required to have even greater reliability, such as chemical stability. Electrolytes containing fluorine compounds are widely used as the electrolyte solution in electrochemical elements. This electrolyte decomposes in the presence of trace amounts of water in the electrochemical element system, producing hydrofluoric acid. Although electrode materials and separators are dried before use, it is difficult to completely remove the water. Under high-voltage conditions, more hydrofluoric acid is produced than under normal voltages, raising concerns that this increases acidity and may lead to cellulose decomposition.

[0010] Patent Document 2 discloses a technology for increasing the density, mechanical strength, and chemical stability of a separator by unevenly distributing thermoplastic synthetic fibers and regenerated cellulose fibers in the thickness direction of the separator. By separating the roles of each fiber, such as a portion with a high content of regenerated cellulose fibers that ensures strength and a portion with a high content of thermoplastic synthetic fibers that ensures density and acid resistance, strength, density, and acid resistance are all increased.

[0011] Patent Document 3 discloses a separator that has a low short-circuit defect rate despite its thin thickness, which is produced by mixing beaten cellulose fibers and thermoplastic synthetic fibers to produce a sheet, and then adjusting the thickness using a soft calender.

[0012] However, in order to improve the productivity of electrochemical elements and obtain reliability that can withstand long-term use under harsh conditions such as higher capacities and higher voltages, the impregnation properties of the separators disclosed in Patent Documents 2 and 3 may not be sufficient.

[0013] Patent Document 4 proposes a separator with increased density and mechanical strength by using a multilayer nonwoven fabric composed of two or more nonwoven fabric layers, each having a high-density layer and a low-density layer. In Patent Document 4, a separator of two or more layers containing solvent-spun cellulose and synthetic fibers is wet-laid and then calendered to suppress delamination, which has been a problem with conventional multilayer separators. However, it is more difficult to make a multilayer separator thinner than a single-layer separator.

[0014] While the above has described separators made of nonwoven fabric, separators for power storage devices such as lithium-ion secondary batteries also include microporous membranes made of synthetic resins. These are generally films made of polyolefin resins and have fine pores. Compared to separators made of nonwoven fabric, such polyolefin resin films have poor electrolyte impregnation, making it difficult to improve the productivity and capacity of electrochemical elements. Furthermore, the high resistance of the separator results in high initial resistance of the electrochemical elements.

[0015] The present invention has been made in view of the above-mentioned problems, and aims to provide a separator for electrochemical elements that has better electrolyte impregnation properties than conventional separators while maintaining the properties of the separator, such as low resistance, short-circuit resistance, and acid resistance. Another aim of the present invention is to provide an electrochemical element that, by using the separator, has high productivity, high capacity, and reliability sufficient to withstand long-term use under harsh conditions such as high voltage.

[0016] The separator for an electrochemical element of the present invention is interposed between a pair of electrodes and is capable of retaining an electrolytic solution containing an electrolyte, the separator being a single layer separator made of beaten solvent-spun cellulose fiber and synthetic fiber, and having a thickness of 10 to 70 μm and a density of 0.25 to 0.70 g / cm ３The film is characterized in that the Beck smoothness on both sides is 20 to 400 seconds.

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

[0018] The electrochemical element of the present invention is configured to use the separator for an electrochemical element of the present invention.

[0019] 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.

[0020] According to the present invention, it is possible to improve the impregnation of the electrolyte without impairing the separator's resistance, short-circuit resistance, and acid resistance. Furthermore, use of the separator of the present invention can contribute to improving the productivity of electrochemical elements and increasing the reliability of the elements so that they can withstand long-term use under harsh conditions such as increasing capacity and voltage.

[0021] The separator for electrochemical elements of the present invention is a separator made of solvent-spun cellulose fibers and synthetic fibers. Solvent-spun cellulose fibers can be adjusted to a desired fiber diameter and controlled to a desired fibril generation rate by beating the fibers, and are therefore preferred from the viewpoints of reducing the separator's resistance 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 preferred from the viewpoints of acid resistance, heat resistance, and papermaking suitability.

[0022] The separator for an electrochemical element of the present invention has a thickness of 10 to 70 μm and a density of 0.25 to 0.70 g / cm ３By setting the Beck smoothness on both sides of the separator to 20 to 400 seconds, a separator with excellent electrolyte impregnation can be realized without impairing the separator's resistance and short-circuit resistance. Furthermore, the separator's air resistance is preferably 2 to 50 seconds. It is even more preferable that the Beck smoothness on both sides of the separator be 30 to 300 seconds and the air resistance be 3 to 30 seconds.

[0023] The electrolyte impregnation in an electrochemical element mainly refers to two aspects: the electrolyte retention capacity and the impregnation speed. There are two parts into which the electrolyte is impregnated: a part into which the electrolyte penetrates the separator (hereinafter referred to as part A), and a part into which the electrolyte penetrates from the interface between the electrode and the separator (hereinafter referred to as part B). While part A has traditionally been improved by specifying the separator's material, density, and air resistance, part B has not been sufficiently verified. The inventors of the present invention also focused on part B and found that it affects the separator's Beck smoothness.

[0024] Beck smoothness is the time required for air to pass between a glass surface and a separator when the two surfaces are pressed together with a constant pressure, and is an evaluation method for indicating the surface roughness of the separator. In the present invention, the impregnation property of portion B is improved by controlling the Beck smoothness. Note that the lower the Beck smoothness value, the rougher the sheet surface.

[0025] The Beck 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. If the Beck smoothness exceeds 400 seconds, the separator will have very high density in the plane direction, resulting in close contact between the electrodes and the separator when an electrochemical element is fabricated, inhibiting the penetration of the electrolyte solution in part B and resulting in poor impregnation. For example, the separator described in Patent Document 3 corresponds to this case. If the Beck smoothness is less than 20 seconds, the separator surface will be rough, improving the impregnation of part B with the electrolyte solution. However, the separator will contain solvent-spun cellulose fibers with a large fiber diameter and / or synthetic fibers with a large fineness, resulting in poor density. Furthermore, there will be little fiber entanglement between the solvent-spun cellulose fibers, resulting in the shedding of synthetic fibers.

[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 density, short-circuit defects in the electrochemical element may not be suppressed. On the other hand, if 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 to 0.70 g / cm ３ The density of the separator is 0.25 g / cm ３ If the density is less than 0.70 g / cm, the density is insufficient and short circuit defects may occur. ３ If the temperature exceeds this value, the fibers constituting the separator will be compressed or fused together, increasing the resistance of the separator. Furthermore, the impregnation of part A will decrease, resulting in an electrochemical element with high resistance.

[0028] The separator of the present invention uses beaten solvent-spun cellulose (e.g., lyocell) and synthetic fibers. The solvent-spun cellulose fibers are finely divided by beating (mechanical treatment in water), maximizing the separator's density while maintaining low resistance. Furthermore, the synthetic fibers improve the separator's chemical stability, thereby improving its stability in harsh environments, such as the high voltages required of separators in recent years. The solvent-spun cellulose fiber content is preferably 70 to 95% by mass, and the synthetic fiber content is preferably 5 to 30% by mass. If the solvent-spun cellulose fiber content is less than 70% by mass and the synthetic fiber content exceeds 30% by mass, the solvent-spun cellulose fibers within the sheet are less entangled, which may increase the risk of synthetic fiber shedding from the separator. If the solvent-spun cellulose fiber content exceeds 95% by mass and the synthetic fiber content is less than 5% by mass, the separator's chemical stability may be reduced.

[0029] The separator of the present invention has an air resistance of 2 to 50 seconds, more preferably 3 to 30 seconds. If the air resistance is less than 2 seconds, the density is insufficient and short-circuiting may occur. On the other hand, if the air resistance exceeds 50 seconds, the resistance of the separator increases and the impregnation of the electrolyte solution in part A deteriorates.

[0030] The difference in fiber diameter (fiber diameter difference) between the solvent-spun cellulose fibers and the synthetic fibers after beating is preferably in the range of −1.0 to 3.0 μm. If the difference in fiber diameter is less than −1.0 μm, entanglement of the solvent-spun cellulose fibers may be hindered, and the shedding of synthetic fibers from the separator may increase. On the other hand, if the difference in fiber diameter is greater than 3.0 μm, uneven distribution of fibers occurs, as in Patent Document 2, resulting in an increase in the Beck smoothness of the surface with a large amount of synthetic fibers and a separator with low impregnation ability.

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

[0032] In the following examples, the separators are all wet-laid nonwoven fabrics formed by a papermaking method, but in the present invention, the method for forming the separator is not limited to the papermaking method as long as the separator satisfies the thickness, density, and Beck smoothness requirements. For example, a method in which a fiber dispersion is formed into a sheet by casting or the like is also acceptable.

[0033] [Methods for Evaluating Separators and Electrochemical Devices] Measurement and evaluation of specific properties of the separators and electrochemical devices were carried out under the following conditions and methods.

[0034] [Fiber diameter of solvent-spun cellulose after beating] The raw material or separator after beating was observed with a scanning electron microscope (SEM), and the length of only the core portion (fibers with a fiber diameter of 1 μm or more) was measured for n=50 pieces, and the average value was calculated. For flat-shaped fibers, the length of the longest linear distance was measured. When it was difficult to distinguish from synthetic fibers, the solvent-spun cellulose alone was removed and observed according to JIS L1030-2 "Test method for blend ratio in textile products."

[0035] [Fiber diameter of synthetic fibers] The raw material or separator was observed under SEM, and the average value was calculated from n = 50 measurements. For flat fibers, the length of the longest linear distance was measured. When it was difficult to distinguish from solvent-spun cellulose, the synthetic fibers were taken out and observed according to JIS L1030-2 "Test method for blend ratio in textile products."

[0036] [Fiber Diameter Difference] The fiber diameter 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] The thickness of the separator was measured using a micrometer as specified in "JIS C 2300-2 'Cellulose paper for electrical purposes - Part 2: Test methods' 5.1 Thickness" in "5.1.1 Measuring instruments and measurement methods a. When an outside micrometer is used" and by the method of folding the paper into 10 sheets as specified in "5.1.3 When measuring thickness by folding the paper".

[0038] [Density] The density of the separator was calculated by measuring the basis weight in an absolute dry state according to 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 Resistance] The air resistance of the separator was measured using a B-type tester according to the method specified in "JIS P 8117 'Paper and paperboard - Test method for air permeability and air resistance (intermediate range) - Gurley method'".

[0040] [Smoothness of separator] The Beck smoothness of the separator was measured according to "JIS P 8119 'Paper and paperboard - Test method for smoothness using a Beck smoothness tester.'" The surface with a larger Beck smoothness value is called the "smooth surface," and the surface with a smaller Beck smoothness value is called the "rough surface."

[0041] [Method of manufacturing electrochemical elements] A separator was interposed between the two electrode materials and wound up to produce an element roll. Each electrochemical element was obtained by immersing this element roll in an electrolyte solution and sealing it in a case. Specifically, each electrochemical element, an electric double layer capacitor, a lithium ion capacitor, and a lithium ion secondary battery, was manufactured by the following method.

[0042] [Method for Producing an Electric Double Layer Capacitor] An activated carbon electrode and a separator were wound together to obtain an electric double layer capacitor element wound. The element wound was immersed in an electrolyte solution containing tetraethylammonium tetrafluoroborate dissolved therein as an electrolyte for 1 hour, and then vacuum impregnation was carried out. The element wound was then housed in a cylindrical aluminum case with a bottom and sealed with a rubber seal to produce an electric double layer capacitor with a rated voltage of 3.0 V and a capacitance of 3000 F.

[0043] [Method of Manufacturing a Lithium-Ion Capacitor] An activated carbon electrode for a lithium-ion capacitor was used as the positive electrode material, and a graphite electrode was used as the negative electrode material. The separator and electrode material were alternately folded to obtain a lithium-ion capacitor element. The element was then housed in a multilayer laminate film together with a lithium pre-doping foil, and the film was vacuum-impregnated with an electrolyte for 1 hour. The film was then sealed to produce a lithium-ion capacitor with a rated voltage of 4.0 V and a capacitance of 2000 F. The electrolyte solution used was a propylene carbonate solvent containing lithium hexafluorophosphate dissolved as an electrolyte.

[0044] [Method for Fabricating a Lithium-Ion Secondary Battery] A lithium cobalt oxide electrode for a lithium-ion secondary battery was used as the positive electrode material, and a graphite electrode was used as the negative electrode material. These were wound together with a separator to obtain a lithium-ion secondary battery element. The element was immersed in an electrolyte solution prepared by dissolving lithium hexafluorophosphate as an electrolyte in a mixed solvent of ethylene carbonate and diethyl carbonate for 1 hour, and then housed in a bottomed cylindrical case and sealed with a press to produce a lithium-ion secondary battery with a rated voltage of 4.2 V and a discharge capacity of 3000 mAh.

[0045] The properties of each of the electrochemical devices thus fabricated were measured by the following methods: 1000 electrochemical devices were fabricated for each example and used in the following property evaluations.

[0046] The capacitance or discharge capacity of the prepared electrochemical element was measured to evaluate the impregnation ability. An electrochemical element with a capacitance or discharge capacity of 99% or more was rated as "Good", a capacitance or discharge capacity of less than 99% to 95% or more was rated as "Average", and a capacitance or discharge capacity of less than 95% was rated as "Poor".

[0047] [Capacitance and 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 Measurement method 1 of capacitance and internal resistance" specified in "JIS C 5160-1 'Fixed electric double layer capacitors for use in electrical and electronic equipment - Part 1: General specifications 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 'Industrial lithium secondary battery cells and battery systems - 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 Measurement method 1 of capacitance and internal resistance" specified in "JIS C 5160-1 'Fixed electric double layer capacitors for use in electrical and electronic equipment - Part 1: General specifications 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 'Industrial lithium secondary battery cells and battery systems - Part 1: Performance requirements'".

[0049] [Short-circuit defect rate] The short-circuit defect rate of an electrochemical element was determined as a short-circuit defect when the charging voltage did not increase to the rated voltage, and the number of electrochemical elements that were short-circuited was divided by the number of electrochemical elements produced to obtain the short-circuit defect rate as a percentage.

[0050] [Capacity Retention Rate] A long-term reliability test was conducted in which a rated voltage was applied to the electrochemical element for 1,500 hours at a constant temperature of 65°C. Electrochemical elements that had short-circuited were excluded from the long-term reliability test. The capacity retention rate of the electrochemical element was calculated using the following formula (1): Capacity Retention Rate (%) = 100 - (Ca - Cb) / Ca x 100 (1) (Ca: capacity before the long-term reliability test, Cb: capacity before the long-term reliability test)

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

[0052] (Example 1) 95% by mass of lyocell fiber having an average fiber diameter of 2.1 μm as beaten solvent-spun cellulose fiber and 5% by mass of polyethylene terephthalate fiber (hereinafter referred to as PET fiber) having an average fiber diameter of 3.0 μm as synthetic fiber were mixed and Fourdrinier paper was made to a thickness of 10.3 μm and a density of 0.69 g / cm ３ A separator of Example 1 was obtained. The difference in fiber diameter between the Lyocell fiber and the PET fiber of this separator was -0.9 μm. The Beck smoothness was 397.1 seconds for the smooth surface and 395.0 seconds for the rough surface, and the air resistance was 49.6 seconds / 100 ml. Each electrochemical element of Example 1 was produced using this separator.

[0053] (Example 2) 95% by mass of lyocell fibers having an average fiber diameter of 3.0 μm as beaten solvent-spun cellulose fibers and 5% by mass of PET fibers having an average fiber diameter of 2.2 μm as synthetic fibers were mixed and Fourdrinier paper was made to a thickness of 15.0 μm and a density of 0.64 g / cm ３ A separator of Example 2 was obtained. The difference in fiber diameter between the Lyocell fiber and the PET fiber of this separator was 0.8 μm. The Beck smoothness was 298.2 seconds for the smooth surface and 286.3 seconds for the rough surface, and the air resistance was 30.0 seconds / 100 ml. Each electrochemical element of Example 2 was fabricated using this separator.

[0054] (Example 3) 70% by mass of lyocell fibers having an average fiber diameter of 10.8 μm as beaten solvent-spun cellulose fibers and 30% by mass of polypropylene fibers (hereinafter referred to as PP fibers) having an average fiber diameter of 7.9 μm as synthetic fibers were mixed and Fourdrinier paper was made to a thickness of 68.7 μm and a density of 0.26 g / cm ３ A separator of Example 3 was obtained. The difference in fiber diameter between the Lyocell fiber and the PP fiber of this separator was 2.9 μm. The Beck smoothness was 22.9 seconds for the smooth surface and 21.8 seconds for the rough surface, and the air resistance was 2.2 seconds / 100 ml. Each electrochemical element of Example 3 was fabricated using this separator.

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

[0056] (Example 5) A mixture of 85% by mass of lyocell fibers having an average fiber diameter of 5.1 μm as beaten solvent-spun cellulose fibers and 15% by mass of acrylic fibers having an average fiber diameter of 3.2 μm as synthetic fibers was made into a fourdrinier paper having a thickness of 19.6 μm and a density of 0.55 g / cm ３ A separator of Example 5 was obtained. The difference in fiber diameter between the lyocell fiber and the acrylic fiber of this separator was 1.9 μm. The Beck smoothness was 199.9 seconds for the smooth surface and 187.0 seconds for the rough surface, and the air resistance was 19.8 seconds / 100 ml. Each electrochemical element of Example 5 was fabricated using this separator.

[0057] (Example 6) A mixture of 85% by mass of lyocell fibers having an average fiber diameter of 6.3 μm as beaten solvent-spun cellulose fibers and 15% by mass of acrylic fibers having an average fiber diameter of 3.5 μm as synthetic fibers was made into a fourdrinier paper having a thickness of 30.2 μm and a density of 0.49 g / cm ３ A separator of Example 6 was obtained. The difference in fiber diameter between the lyocell fiber and the acrylic fiber of this separator was 2.8 μm. The Beck smoothness was 155.5 seconds for the smooth surface and 150.3 seconds for the rough surface, and the air resistance was 14.6 seconds / 100 ml. Each electrochemical element of Example 6 was fabricated using this separator.

[0058] (Example 7) 85% by mass of lyocell fibers having an average fiber diameter of 7.5 μm as beaten solvent-spun cellulose fibers and 15% by mass of PET fibers having an average fiber diameter of 4.9 μm as synthetic fibers were mixed and Fourdrinier paper was made to a thickness of 35.4 μm and a density of 0.46 g / cm ３A separator of Example 7 was obtained. The difference in fiber diameter between the Lyocell fiber and the PET fiber of this separator was 2.6 μm. The Beck smoothness was 97.3 seconds for the smooth surface and 96.4 seconds for the rough surface, and the air resistance was 10.9 seconds / 100 ml. Each electrochemical element of Example 7 was fabricated using this separator.

[0059] (Example 8) 80% by mass of lyocell fibers having an average fiber diameter of 6.9 μm as beaten solvent-spun cellulose fibers and 20% by mass of PET fibers having an average fiber diameter of 5.4 μm as synthetic fibers were mixed and Fourdrinier paper was made to a thickness of 29.8 μm and a density of 0.41 g / cm ３ A separator of Example 8 was obtained. The difference in fiber diameter between the Lyocell fiber and the PET fiber of this separator was 1.5 μm. The Beck smoothness was 53.2 seconds for the smooth surface and 48.1 seconds for the rough surface, and the air resistance was 5.3 seconds / 100 ml. Each electrochemical element of Example 8 was fabricated using this separator.

[0060] (Comparative Example 1) 95% by mass of lyocell fibers having an average fiber diameter of 1.6 μm as beaten solvent-spun cellulose fibers and 5% by mass of PET fibers having an average fiber diameter of 1.7 μm as synthetic fibers were mixed and Fourdrinier paper was made to a thickness of 8.1 μm and a density of 0.75 g / cm ３ A separator of Comparative Example 1 was obtained. The difference in fiber diameter between the Lyocell fiber and the PET fiber of this separator was -0.1 μm. The Beck smoothness was 458.0 seconds for the smooth surface and 430.6 seconds for the rough surface, and the air resistance was 52.4 seconds / 100 ml. Each electrochemical element of Comparative Example 1 was produced using this separator.

[0061] (Comparative Example 2) 70% by mass of lyocell fibers having an average fiber diameter of 11.3 μm as beaten solvent-spun cellulose fibers and 30% by mass of PET fibers having an average fiber diameter of 8.4 μm as synthetic fibers were mixed and Fourdrinier paper was made to a thickness of 74.7 μm and a density of 0.23 g / cm ３ A separator of Comparative Example 2 was obtained. The difference in fiber diameter between the Lyocell fiber and the PET fiber of this separator was 2.9 μm. The Beck smoothness was 11.1 seconds for the smooth surface and 8.1 seconds for the rough surface, and the air resistance was 1.5 seconds / 100 ml. Each electrochemical element of Comparative Example 2 was produced using this separator.

[0062] (Comparative Example 3) A mixture of 65% by mass of lyocell fibers having an average fiber diameter of 10.7 μm as beaten solvent-spun cellulose fibers and 35% by mass of PET fibers having an average fiber diameter of 7.8 μm as synthetic fibers was made into a fourdrinier paper having a thickness of 75.1 μm and a density of 0.20 g / cm ３ A separator of Comparative Example 3 was obtained. The difference in fiber diameter between the Lyocell fiber and the PET fiber of this separator was 2.9 μm. The Beck smoothness was 9.7 seconds for the smooth surface and 8.4 seconds for the rough surface, and the air resistance was 1.2 seconds / 100 ml. Each electrochemical element of Comparative Example 3 was fabricated using this separator.

[0063] (Comparative Example 4) A mixture of 85% by mass of lyocell fibers having an average fiber diameter of 6.3 μm as beaten solvent-spun cellulose fibers and 15% by mass of PET fibers having an average fiber diameter of 7.5 μm as synthetic fibers was made into a fourdrinier paper having a thickness of 40.5 μm and a density of 0.24 g / cm ３ A separator of Comparative Example 4 was obtained. The difference in fiber diameter between the Lyocell fiber and the PET fiber of this separator was -1.2 μm. The Beck smoothness was 18.8 seconds for the smooth surface and 15.1 seconds for the rough surface, and the air resistance was 1.8 seconds / 100 ml. Each electrochemical element of Comparative Example 4 was produced using this separator.

[0064] Conventional Example 1: 100% by mass of beaten solvent-spun cellulose fibers with an average fiber diameter of 8.0 μm were Fourdrinier-made paper having a thickness of 30.1 μm and a density of 0.41 g / cm ３ The separator had a Beck smoothness of 102.5 seconds on the smooth surface and 91.2 seconds on the rough surface, and an air resistance of 10.3 seconds / 100 ml. Each electrochemical element of Conventional Example 1 was fabricated using this separator.

[0065] (Conventional Example 2) 75% by mass of lyocell fiber having an average fiber diameter of 6.9 μm as beaten solvent-spun cellulose fiber and 25% by mass of PET fiber having an average fiber diameter of 3.7 μm as synthetic fiber were mixed and Fourdrinier paper was made to a thickness of 30.4 μm and a density of 0.45 g / cm ３A separator of Conventional Example 2 was obtained. The difference in fiber diameter between the Lyocell fiber and the PET fiber of this separator was 3.2 μm. The Beck smoothness was 516.0 seconds for the smooth surface and 247.7 seconds for the rough surface, and the air resistance was 14.7 seconds / 100 ml. Each electrochemical element of Conventional Example 2 was fabricated using this separator.

[0066] (Conventional Example 3) The separator of Example 6 was adjusted by a soft calender to a thickness of 25.1 μm and a density of 0.59 g / cm ３ The separator had a Beck smoothness of 633.4 seconds on the smooth surface and 625.5 seconds on the rough surface, and an air resistance of 20.0 seconds / 100 ml. Each electrochemical element of Conventional Example 3 was fabricated using this separator.

[0067] (Conventional Example 4) Thickness: 29.9 μm, density: 0.53 g / cm ３ The separator was a polyethylene microporous membrane. Because this separator was not made of fibers, the fiber diameter could not be measured. The separator's air resistance was 301.0 seconds / 100 ml, and the Beck smoothness was 10,000 seconds or more on both the smooth and rough surfaces.

[0068] The evaluation results of the separators of each Example, Comparative Example, and Conventional Example are shown in Table 1. Furthermore, the evaluation results of the electrochemical devices fabricated using the separators of each Example are shown in Table 2.

[0069]

[0070]

[0071] As can be seen from Table 2, no short-circuit defects occurred in the electrochemical elements of each example. The impregnation was also rated "Good" for all examples, and the electrostatic capacitance and discharge capacity of the electrochemical elements were as targeted. Furthermore, the capacity retention rate in the long-term reliability test was also good, at 70% or more.

[0072] The separator of Comparative Example 1 has a thickness of 8.1 μm. An electrochemical element using this separator experienced short-circuiting. The separator was made of thin fibers, with an average fiber diameter of 1.6 μm for the lyocell fiber and 1.7 μm for the PET fiber, and had a density of 0.75 g / cm. ３ The air resistance was 52.4 seconds / 100 ml, which was too dense, resulting in a high internal resistance of the electrochemical element. Furthermore, the Beck smoothness was 458.0 seconds for the smooth surface and 430.6 seconds for the rough surface, indicating high density in the surface direction. As a result, the impregnation was rated "Fair," the internal resistance of the electrochemical element deteriorated, and the electrostatic capacitance, discharge capacity, and capacity retention rate in long-term reliability tests were reduced.

[0073] The separator of Comparative Example 2 has a thickness of 74.7 μm. Because of its large thickness, the internal resistance of the electrochemical element is high. The Beck smoothness of the separator is low, at 11.1 seconds for the smooth surface and 8.1 seconds for the rough surface, so impregnation is good. However, because the separator is made of thick fibers, with an average fiber diameter of 11.3 μm for the Lyocell fiber and an average fiber diameter of 8.4 μm for the PET fiber, the density is 0.23 g / cm despite its large thickness. ３ The air resistance was low at 1.5 seconds / 100 ml, and short circuit defects occurred.

[0074] Comparison of Comparative Examples 1 and 2 with each Example shows that the thickness of the separator is preferably 10 to 70 μm. The density of the separator is preferably 0.25 to 0.70 g / cm. ３ It is found that the air resistance is preferably 2 to 50 seconds / 100 ml, and the Beck smoothness of both surfaces of the separator is preferably 20 to 400 seconds.

[0075] When shortening the impregnation time to improve the productivity of electrochemical elements, a lower Beck smoothness value is more preferable. Comparing Example 1 and Example 2, the Beck smoothness was 397.1 seconds for the smooth surface and 395.0 seconds for the rough surface in Example 1, while the Beck smoothness was 298.2 seconds for the smooth surface and 286.3 seconds for the rough surface in Example 2, making Example 2 lower. Furthermore, the air permeation resistance was 49.6 seconds / 100 ml in Example 1 and 30.0 seconds / 100 ml in Example 2, but there were no short-circuit defects, and it can be seen that Example 2 had a lower internal resistance for the electrochemical element and a better capacity retention rate after a long-term reliability test.

[0076] Furthermore, in a comparison between Example 3 and Example 4, the Beck smoothness of Example 3 is 22.9 seconds for the smooth surface and 21.8 seconds for the rough surface, while the Beck smoothness of Example 4 is 36.3 seconds for the smooth surface and 31.8 seconds for the rough surface. Example 3 is more advantageous in terms of impregnation, but in order to obtain a separator with low Beck smoothness, it is necessary to increase the fiber diameters of the Lyocell and PET fibers, resulting in a thicker separator in Example 3. Therefore, the internal resistance of the electrochemical element in Example 4 is more preferable.

[0077] Comparing Example 1 with Example 2 and Example 3 with Example 4, it is more preferable that the Beck smoothness on both sides of the separator is 30 to 300 seconds, and the air resistance is more preferably 3 to 30 seconds / 100 ml.

[0078] The separator of Comparative Example 3 is a separator made by mixing 65% by mass of Lyocell fiber and 35% by mass of PET fiber. Because the density and air resistance are even lower than those of the separator of Comparative Example 2, the short-circuit defect rate of the electrochemical element is high despite the separator's large thickness.

[0079] The separator of Conventional Example 1 is a separator made only of lyocell fiber. Comparing Conventional Example 1 with each of the Examples reveals that the use of the separator of the present invention containing synthetic fiber improves the capacity retention rate after long-term reliability testing. This is believed to be due to the improved chemical stability of the separator due to the synthetic fiber. This suggests that the solvent-spun cellulose fiber content is preferably 70 to 95% by mass, and the synthetic fiber content is preferably 5 to 30% by mass.

[0080] The separator of Comparative Example 4 has a fiber diameter difference between the lyocell fiber and the PET fiber of -1.2 μm and a density of 0.24 g / cm ３The air resistance was low at 1.8 μm. This was because the PET fibers prevented the lyocell fibers from intertwining, causing the PET fibers to fall off from the separator. The electrochemical element using the separator of Comparative Example 4 had a high short-circuit defect rate and a low capacity retention rate after long-term reliability testing. Furthermore, the separator of Conventional Example 2 had a Beck smoothness of 516.0 seconds on the smooth surface. The impregnation was poor, preventing the electrochemical element from achieving the desired capacity. This was because the fiber diameter difference between the lyocell fiber and the PET fiber in the separator of Conventional Example 2 was 3.2 μm, resulting in uneven fiber distribution and a high Beck smoothness value on the smooth surface. Comparison of Comparative Example 4 and Conventional Example 2 with each Example reveals 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 Conventional Example 3 was prepared by adjusting the thickness of the separator of Example 6 using a soft calender, but the Beck smoothness was 633.4 seconds on the smooth side and 625.5 seconds on the rough side, and the impregnation was even lower than that of the separator of Conventional Example 2, and the capacity of the electrochemical element was also lower.

[0082] The separator of Conventional Example 4 is a film, and therefore the material itself has poor impregnation properties. Furthermore, the Beck smoothness was 10,000 seconds or more on both sides, and the capacity of the electrochemical element was lower than the target when the impregnation time was 1 hour, so other evaluations could not be performed.

[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 is capable of retaining an electrolytic solution containing an electrolyte, and is a single-layer separator made of beaten solvent-spun cellulose fiber and synthetic fiber, and has a thickness of 10 to 70 μm and a density of 0.25 to 0.70 g / cm ３ and a Beck smoothness of 20 to 400 seconds on both sides. Use of the separator of the present invention makes it possible to improve the impregnation of the electrolyte without impairing the separator's resistance, short-circuit resistance, and acid resistance. Furthermore, use of the separator of the present invention contributes to improved productivity of electrochemical elements, higher capacity, and reliability capable of withstanding long-term use under harsh conditions.

[0084] The above describes examples in which the separator of this embodiment is used in an electric double layer capacitor, a lithium ion capacitor, and a lithium ion secondary battery. In the electrochemical element according to the present invention, the electrode material, the electrolyte material, and other components are not particularly limited, and various materials can be used. Furthermore, the separator for an electrochemical element of the present invention can also be applied to electrochemical elements other than those described in this embodiment, 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 element that is interposed between a pair of electrodes and is capable of retaining an electrolyte solution containing an electrolyte, the separator being a single layer separator made of beaten solvent-spun cellulose fiber and synthetic fiber, having a thickness of 10 to 70 μm and a density of 0.25 to 0.70 g / cm ３ A separator for an electrochemical element, characterized in that the Beck smoothness of both surfaces is 20 to 400 seconds.

2. The separator for an electrochemical element according to claim 1, characterized in that the separator has an air resistance of 2 to 50 seconds.

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

4. The separator for an electrochemical element according to claim 1, wherein the synthetic fibers are one or more types of fibers selected from the group consisting of polyester fibers, acrylic fibers, and polyolefin fibers.

5. An electrochemical element, characterized in that the separator for electrochemical elements according to any one of claims 1 to 4 is used.

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

Images