Separator for electrochemical element and electrochemical element

Abstract

Provided is a separator for an electrochemical element that maintains characteristics such as low resistance, short-circuit resistance, and acid resistance, and that is superior in electrolyte impregnation compared to conventional separators. A separator for an electrochemical element that is sandwiched between a pair of electrodes and that is capable of retaining an electrolyte containing an electrolyte is formed in the following configuration: the separator is a single-layer separator formed from solvent-spun cellulose fibers that have been beaten and synthetic fibers, the thickness of the separator is 10 to 70 μm, the density of the separator is 0.25 to 0.70 g / cm 3 , and the Bekk smoothness of both surfaces is 20 to 400 seconds.

Description

Technical Field

[0001] This invention relates to separators for electrochemical devices and electrochemical devices using the separators. Furthermore, this invention is applicable to, for example, double-layer capacitors, lithium-ion capacitors, and lithium-ion secondary batteries. Background Technology

[0002] Electrochemical components, especially high-capacity electrochemical components such as double-layer capacitors, lithium-ion capacitors, and lithium-ion secondary batteries, have been used in many fields in recent years, including power supplies for automotive-related equipment, backup equipment, renewable energy-related equipment such as wind power and solar power, and communication equipment such as smart meters. Their applications are expected to continue to expand in the future.

[0003] With the expansion of applications and the increasing performance of the equipment used in these electrochemical components, there is a growing demand for higher capacity and greater reliability to withstand long-term use under harsh conditions such as high voltage.

[0004] In recent years, the structures of high-capacity electrochemical elements, whose applications have gradually expanded, are mainly of two types: wound and laminated. The wound type involves winding a pair of electrodes with a separator between them, allowing the electrolyte to permeate, and then housing them in a metal casing and sealing them. The laminated type involves alternately stacking electrodes and separators, housing them in a metal casing or laminated film, injecting electrolyte, and then sealing them.

[0005] The main function of the separator in an electrochemical element is to isolate a pair of electrodes and retain the electrolyte.

[0006] In conventional separators for electrochemical devices, high density is required to suppress short-circuit defects. However, the denser the separator, the fewer voids it contains, leading to issues such as reduced electrolyte permeability and decreased productivity, capacity, and reliability of the electrochemical device.

[0007] To achieve high productivity, high capacity, and high reliability in electrochemical components, it is necessary to have separators that maintain characteristics such as low resistance and short-circuit resistance, while also exhibiting superior electrolyte permeability compared to previous designs.

[0008] As separators for electrochemical elements, various configurations have been proposed to improve characteristics such as short-circuit withstand capability (see, for example, Patent Documents 1-4).

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent Application Publication No. 2000-3834

[0012] Patent Document 2: Japanese Patent Application Publication No. 2017-117590

[0013] Patent Document 3: Japanese Patent Application Publication No. 2019-96681

[0014] Patent Document 4: Japanese Patent Application Publication No. 2013-171905 Summary of the Invention

[0015] The problem the invention aims to solve

[0016] In Patent Document 1, a method using solvent-spun regenerated cellulose fibers that have undergone pulping was proposed to improve the density of the separator and reduce its electrical resistance. Solvent-spun regenerated cellulose fibers capable of pulping are processed to obtain fine fibrils smaller than 1 μm. Therefore, the separator made of pulped solvent-spun regenerated cellulose fibers becomes a highly dense, microporous sheet.

[0017] However, with the increasing voltage of electrochemical components in recent years, there are further requirements for the reliability of separators, such as improved chemical stability.

[0018] Electrolytes containing fluorine compounds are widely used in the electrolytes of electrochemical devices. These electrolytes decompose within the electrochemical device system due to trace amounts of moisture, producing hydrofluoric acid. Electrode materials and separators are dried before use, but complete removal of moisture is difficult. Under high voltage conditions, more hydrofluoric acid is generated than at normal voltages, raising concerns about increased acidity and cellulose decomposition.

[0019] Patent document 2 discloses the following technology: by making thermoplastic synthetic fibers and regenerated cellulose fibers predominate in the thickness direction of the separator, the density, mechanical strength and chemical stability of the separator are improved.

[0020] By separating the roles of regenerated cellulose fibers, which ensure strength, from thermoplastic synthetic fibers, which ensure density and acid resistance, the strength, density, and acid resistance can be improved simultaneously.

[0021] Patent document 3 discloses a separator that is made by mixing pulped cellulose fibers with thermoplastic synthetic fibers to form a sheet, and then adjusting the thickness by soft calendering. As a result, the separator has a low short-circuit failure rate despite its thinness.

[0022] However, in order to achieve improved productivity, high capacity, and reliability of electrochemical components under harsh conditions such as high voltage, the permeability of the separators in Patent Documents 2 and 3 is sometimes insufficient.

[0023] Patent document 4 proposes a separator that improves density and mechanical strength by manufacturing a multilayer nonwoven fabric consisting of two or more nonwoven fabric layers having a high-density layer and a low-density layer.

[0024] In Patent Document 4, after wet papermaking of a separator containing solvent-spun cellulose and synthetic fibers with two or more layers, calendering is performed, thereby suppressing the problem of interlayer delamination in conventional multilayer separators. However, multilayer separators are difficult to make thinner than single-layer separators.

[0025] The above describes separators made of nonwoven fabric, used as separators in energy storage devices such as lithium-ion secondary batteries, and microporous membranes made of synthetic resin. Typically, films formed from polyolefin resins have fine pores.

[0026] Compared to separators formed from nonwoven fabric, such polyolefin resin films have low electrolyte permeability, which sometimes makes it difficult to improve the productivity and capacity of electrochemical elements. The separators also have high resistance, resulting in high initial resistance of the electrochemical elements.

[0027] The present invention was made in view of the above-mentioned problems, and its object is to provide a separator that maintains the characteristics of separators for electrochemical elements such as low resistance, short-circuit resistance, and acid resistance, while having superior electrolyte permeability compared to the past.

[0028] Furthermore, the present invention aims to provide an electrochemical element that, by applying the separator, achieves high production capacity, high capacity, and reliability for long-term use under harsh conditions such as high voltage.

[0029] Solution for solving the problem

[0030] The separator for electrochemical elements of the present invention is characterized in that it is a separator sandwiched between a pair of electrodes and capable of maintaining an electrolyte containing electrolyte. The separator is a layer formed from pulped solvent-spun cellulose fibers and synthetic fibers, wherein the separator has a thickness of 10-70 μm and a density of 0.25-0.70 g / cm³. 3 The smoothness of both sides of the Buick is 20~400 seconds.

[0031] The preferred feature is that the airtightness is 2 to 50 seconds.

[0032] In addition, the fiber diameter difference between the solvent-spun cellulose fiber and the synthetic fiber after pulping is preferably -1.0μm to 3.0μm.

[0033] Furthermore, the aforementioned synthetic fiber is preferably selected from one or more fibers chosen from polyester fibers, polyolefin fibers, and acrylic fibers.

[0034] The electrochemical element of the present invention is configured using the separator of the electrochemical element of the present invention described above.

[0035] In addition, the electrochemical element of the present invention can be selected from, for example, an electric double-layer capacitor, a lithium-ion capacitor, or a lithium-ion secondary battery.

[0036] The effects of the invention

[0037] According to the present invention, the permeability of the electrolyte can be improved without impairing the resistance, short-circuit withstand capability, and acid resistance of the separator.

[0038] Furthermore, by using the separator of the present invention, it is possible to improve the productivity of electrochemical elements, increase their capacity, and enhance their reliability to withstand long-term use under harsh conditions such as high voltage. Detailed Implementation

[0039] The separator for electrochemical elements of the present invention is a separator formed from solvent-spun cellulose fibers and synthetic fibers.

[0040] If the fiber is solvent-spun cellulose fiber, the desired fiber diameter can be achieved by pulping the fiber, and the desired fibril production rate can be controlled. This is also preferred from the viewpoint of low resistance and short-circuit resistance of the separator.

[0041] In addition, from the viewpoints of acid resistance, heat resistance, and papermaking adaptability, polyester fibers such as polyethylene terephthalate; polyolefin fibers such as polyethylene and polypropylene; and acrylic fibers such as polyacrylonitrile are preferred among synthetic fibers.

[0042] The separator for the electrochemical element of the present invention has a thickness of 10~70 μm and a density of 0.25~0.70 g / cm³. 3 The smoothness of both sides of the separator is 20-400 seconds, thereby enabling the separator to achieve excellent electrolyte permeability without compromising its resistance and short-circuit withstand capability. Furthermore, the air tightness of the separator is preferably 2-50 seconds. More preferably, the smoothness of both sides of the separator is 30-300 seconds, and the air tightness is 3-30 seconds.

[0043] The permeability of electrolytes in electrochemical components can be mainly summarized by two aspects: electrolyte liquid retention and permeation rate.

[0044] The electrolyte penetration portion consists of two parts: the part where the electrolyte penetrates into the interior of the separator (hereinafter referred to as part A) and the part where the electrolyte penetrates from the interface between the electrode and the separator (hereinafter referred to as part B).

[0045] Part A was improved by specifying the material, density, and air tightness of the previous separator components, but Part B was not adequately verified.

[0046] The inventors of this invention also focused on part B and discovered that it affected the smoothness of the Buick partition.

[0047] Buick smoothness refers to the time required for air to pass between the glass surface and the partition when a specified pressure is applied, and it is an evaluation method for the surface roughness of the partition.

[0048] In this invention, the impregnation of part B is improved by controlling the Buick smoothness. It should be noted that the lower the Buick smoothness value, the rougher the surface of the sheet.

[0049] Regarding the smoothness of the separator of the present invention, it is 20 to 400 seconds on both sides of the separator, more preferably 30 to 300 seconds.

[0050] When the smoothness of the Buick exceeds 400 seconds, the density of the separator in the planar direction is very high. Therefore, when manufacturing electrochemical elements, the electrodes are tightly sealed to the separator, hindering the penetration of the electrolyte in part B, thus deteriorating the permeability. For example, the separator in Patent Document 3 is equivalent to this.

[0051] When the Buick smoothness is less than 20 seconds, the surface of the separator is rough, thus improving the penetration of the electrolyte in part B. However, as it becomes a substance containing solvent-spun cellulose fibers with large diameters and / or fineness of synthetic fibers, the density decreases. Furthermore, the solvent-spun cellulose fibers have less interweaving, leading to the shedding of synthetic fibers.

[0052] The thickness of the separator in this invention is 10~70μm.

[0053] When the thickness of the separator is less than 10 μm, even a well-dense separator may not be able to suppress short-circuit failures in electrochemical elements.

[0054] On the other hand, when the thickness of the separator exceeds 70 μm, the resistance of the separator increases.

[0055] The density of the separator in this invention is 0.25~0.70 g / cm³. 3 .

[0056] The density of the separator is less than 0.25 g / cm³. 3 Sometimes, insufficient density can lead to short circuits.

[0057] On the other hand, the density exceeds 0.70 g / cm³. 3At this time, the fibers constituting the separator are pressed and fused together, thus increasing the resistance of the separator. Furthermore, due to the reduced permeability of part A, it becomes an electrochemical element with high resistance.

[0058] The separator of the present invention uses pulped solvent-spun cellulose (e.g., lyocell) and synthetic fibers.

[0059] Solvent-spun cellulose fibers are micronized through pulping (mechanical treatment in water), which maximizes the compactness of the separators and maintains low resistance.

[0060] Furthermore, improving the chemical stability of the separator through synthetic fibers can also enhance the stability of the separator under harsh environments such as high voltage, which has been a requirement in recent years.

[0061] The content of solvent-spun cellulose fiber is preferably in the range of 70-95% by mass, and the content of synthetic fiber is preferably in the range of 5-30% by mass.

[0062] When the content of solvent-spun cellulose fibers is less than 70% by mass and the content of synthetic fibers exceeds 30% by mass, there is less interweaving of fibers within the solvent-spun cellulose sheets, thus sometimes increasing the shedding of synthetic fibers from the separators. When the content of solvent-spun cellulose fibers exceeds 95% by mass and the content of synthetic fibers is less than 5% by mass, sometimes the chemical stability of the separators decreases.

[0063] The airtightness of the separator of the present invention is 2 to 50 seconds, more preferably 3 to 30 seconds.

[0064] When the air tightness is less than 2 seconds, the density is sometimes insufficient, resulting in short circuit failure.

[0065] On the other hand, when the air tightness exceeds 50 seconds, the resistance of the separator increases, and the permeability of the electrolyte in part A also deteriorates.

[0066] The value obtained by subtracting the diameter of the synthetic fiber from the diameter of the solvent-spun cellulose fiber after pulping (fiber diameter difference) is preferably in the range of -1.0 to 3.0 μm.

[0067] When the fiber diameter difference is less than -1.0 μm, it sometimes hinders the interweaving of fibers in solvent-spun cellulose, and increases the shedding of synthetic fibers from the separator.

[0068] On the other hand, when the fiber diameter difference is greater than 3.0 μm, the fiber bias occurs as in Patent Document 2, thus improving the smoothness of the surface with more synthetic fibers and making it a separator with low permeability.

[0069] Hereinafter, various specific embodiments and comparative examples of the separator for electrochemical elements of the present invention and electrochemical elements having the separator for electrochemical elements will be described in detail.

[0070] It should be noted that in the following embodiments, all are wet nonwoven fabrics in which the separators are formed by papermaking. However, in this invention, as long as the thickness, density, and smoothness of the separators are satisfied, the method of forming the separators is not limited to papermaking. For example, methods such as casting to form sheets from fiber dispersions are also acceptable.

[0071] [Evaluation methods for separators and electrochemical components]

[0072] The specific characteristics of the separators and electrochemical elements are determined and evaluated under the following conditions and methods.

[0073] [Fiber diameter of solvent-spun cellulose after pulping]

[0074] Use SEM (scanning electron microscope) to observe the pulped raw material or separator, and calculate the average value when only the core part (fiber with a diameter of 1 μm or more) is measured n=50 times.

[0075] It should be noted that for flat fibers, the length of the longer straight-line distance is measured.

[0076] In cases where it is difficult to distinguish from synthetic fibers, observe only the solvent-spun cellulose after taking it out, in accordance with JIS L1030-2 "Test Method for Blending Rate of Fiber Products".

[0077] [Diameter of synthetic fibers]

[0078] Use SEM to observe the raw materials or separators and calculate the average value when measuring n=50 lengths.

[0079] It should be noted that for flat fibers, the length of the longer straight-line distance is measured.

[0080] In cases where it is difficult to distinguish from solvent-spun cellulose, only synthetic fibers are taken out for observation in accordance with JIS L1030-2 "Test Method for Blending Rate of Fiber Products".

[0081] [Fiber diameter difference]

[0082] It is calculated by subtracting the diameter of the synthetic fiber from the diameter of the solvent-spun cellulose fiber after pulping.

[0083] [thickness]

[0084] Using the micrometer specified in "JIS C 2300-2 Electrical Cellulose Paper - Part 2: Test Methods" 5.1 Thickness, under "5.1.1 Measuring Instrument and Method a. Using an External Micrometer", the thickness of the separator is measured by folding the paper into 10 sheets as described in "5.1.3 Determining Thickness by Folding the Paper".

[0085] [density]

[0086] The density of the separator is calculated by determining the basis weight in an absolutely dry state using the method specified in Method B of "JIS C 2300-2 Electrical Cellulose Paper - Part 2: Test Methods" 7.0A Density.

[0087] [Air tightness]

[0088] The air impermeability of the separator was determined using a Type B testing machine according to the method specified in "JIS P 8117 Paper and paperboard - Test methods for air permeability and air impermeability (intermediate zone) - Gurley method".

[0089] [Smoothness of the separator]

[0090] The Buick smoothness of the separator was determined according to "JIS P 8119 Paper and paperboard - Smoothness test method using Buick smoothness tester".

[0091] It should be noted that the surface with a higher Buick smoothness value is designated as the "smooth surface", and the surface with a lower value is designated as the "rough surface".

[0092] [Methods for fabricating electrochemical components]

[0093] A separator is sandwiched between two electrode materials and then wound to create an element roll.

[0094] Each electrochemical element is obtained by immersing the element in an electrolyte and sealing it in a casing. Specifically, the electrochemical elements of double-layer capacitors, lithium-ion capacitors, and lithium-ion secondary batteries are fabricated using the following methods.

[0095] [How to make an electric double-layer capacitor]

[0096] Activated carbon electrodes and separators are wound together to obtain a double-layer capacitor element roll. The element roll is immersed in an electrolyte solution containing tetraethylammonium tetrafluoroborate as an electrolyte and vacuum impregnated for 1 hour. It is then placed in a bottomed cylindrical aluminum shell and sealed with sealing rubber to produce a double-layer capacitor with a rated voltage of 3.0V and a capacitance of 3000F.

[0097] [How to manufacture lithium-ion capacitors]

[0098] Activated carbon electrodes, typically used in lithium-ion capacitors, were used as the positive electrode material, and graphite electrodes were used as the negative electrode material. The separator and electrode materials were alternately folded to obtain a lithium-ion capacitor element. This element, along with lithium pre-doped foil, was placed in a multilayer laminated film, injected with electrolyte, and vacuum impregnated for 1 hour before sealing to produce a lithium-ion capacitor with a rated voltage of 4.0V and a capacitance of 2000F. The electrolyte used was a solution of lithium hexafluorophosphate dissolved in propylene carbonate solvent.

[0099] [Method for manufacturing lithium-ion secondary batteries]

[0100] Using lithium cobalt oxide electrodes (used in lithium-ion secondary batteries) as the positive electrode material and graphite electrodes as the negative electrode material, these electrodes are wound together with separators to obtain a lithium-ion secondary battery element roll. This element roll is then immersed in an electrolyte solution containing lithium hexafluorophosphate (as the electrolyte) dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate for one hour under vacuum. Afterward, it is placed in a bottomed cylindrical casing and sealed using a press to produce a lithium-ion secondary battery with a rated voltage of 4.2V and a discharge capacity of 3000mAh.

[0101] The characteristics of each fabricated electrochemical element were measured using the following methods. It should be noted that 1000 units of each electrochemical element were fabricated in each example for the following characteristic evaluation.

[0102] [Permeability]

[0103] The electrostatic capacity or discharge capacity of the fabricated electrochemical element is measured to evaluate its permeability.

[0104] Electrochemical elements with a capacitance or discharge capacity of 99% or higher are marked as “〇”, those less than 99% but more than 95% are marked as “△”, and those less than 95% are marked as “×”.

[0105] [Capacitance / Discharge Capacitance]

[0106] The capacitance of double-layer capacitors and lithium-ion capacitors is determined by the constant current discharge method specified in "5.5 Method 1 for determination of capacitance and internal resistance" of "JIS C 5160-1 Fixed double-layer capacitors for electrical and electronic equipment - Part 1: General rules for different types".

[0107] The discharge capacity of lithium-ion secondary batteries shall be determined in accordance with the "6.3 Discharge Performance" of "JIS C 8715-1 Single Cells and Battery Systems for Industrial Lithium Secondary Batteries - Part 1: Performance Requirements".

[0108] [Internal resistance]

[0109] The internal resistance of double-layer capacitors and lithium-ion capacitors is determined by the constant current discharge method specified in "5.5 Method 1 for determination of capacitance and internal resistance" of "JIS C 5160-1 Fixed double-layer capacitors for electrical and electronic equipment - Part 1: General rules for different types".

[0110] The internal resistance of lithium-ion secondary batteries shall be measured in accordance with the "6.5 internal resistance" specified in "JIS C 8715-1 Industrial Lithium Secondary Batteries - Single Cells and Battery Systems - Part 1: Performance Requirements".

[0111] [Short circuit failure rate]

[0112] Regarding the short-circuit failure rate of electrochemical elements, the case where the charging voltage does not rise to the rated voltage is considered a short-circuit failure. The number of these electrochemical elements exhibiting short-circuit failures is divided by the number of electrochemical elements manufactured, and the percentage is used as the short-circuit failure rate.

[0113] [Capacity Maintenance Rate]

[0114] A long-term reliability test was conducted on the electrochemical element under constant temperature of 65°C, applying the rated voltage for 1500 hours. It should be noted that electrochemical elements with short-circuit defects were excluded during the long-term reliability test. The capacity retention rate of the electrochemical element was calculated using the following formula (1).

[0115] Capacity retention rate (%) = 100 - (Ca - Cb) / Ca × 100 (1)

[0116] (Ca: Capacity before long-term reliability testing, Cb: Capacity before long-term reliability testing)

[0117] Example

[0118] Hereinafter, various specific embodiments, comparative examples, and existing examples of the separator for electrochemical elements of the present invention and electrochemical elements having the separator for electrochemical elements will be described in detail.

[0119] Regarding the separators in each embodiment, the separators are obtained by using the paper-making method of a paper machine.

[0120] (Example 1)

[0121] 95% by mass of lyocell fiber (average fiber diameter 2.1 μm), used as a solvent-spun cellulose fiber after pulping, was mixed with 5% by mass of polyethylene terephthalate fiber (hereinafter referred to as PET fiber) (average fiber diameter 3.0 μm), used as a synthetic fiber. The mixture was then subjected to wire-wire papermaking to obtain a thickness of 10.3 μm and a density of 0.69 g / cm³. 3 The separator.

[0122] The diameter difference between the Lyocell and PET fibers in this separator is -0.9 μm. Additionally, regarding Buick's smoothness, the time is 397.1 seconds on a smooth surface and 395.0 seconds on a rough surface, with an air tightness of 49.6 seconds / 100ml.

[0123] Using this separator, the electrochemical elements of Example 1 were fabricated.

[0124] (Example 2)

[0125] 95% by mass of lyocell fibers (average fiber diameter 3.0 μm), used as solvent-spun cellulose fibers after pulping, and 5% by mass of PET fibers (average fiber diameter 2.2 μm), used as synthetic fibers, were mixed and subjected to wire rod papermaking to obtain a paper with a thickness of 15.0 μm and a density of 0.64 g / cm³. 3 The separator.

[0126] The difference in fiber diameter between the Lyocell and PET fibers in this separator is 0.8 μm. Additionally, regarding Buick's smoothness, the time is 298.2 seconds on a smooth surface and 286.3 seconds on a rough surface, with an air tightness of 30.0 seconds / 100ml.

[0127] Using this separator, the electrochemical elements of Example 2 were fabricated.

[0128] (Example 3)

[0129] 70% by mass of lyocell fiber (average fiber diameter 10.8 μm), used as a solvent-spun cellulose fiber after pulping, was mixed with 30% by mass of polypropylene fiber (hereinafter referred to as PP fiber) (average fiber diameter 7.9 μm), used as a synthetic fiber, and the mixture was subjected to wire-wire papermaking to obtain a thickness of 68.7 μm and a density of 0.26 g / cm³. 3 The separator.

[0130] The difference in fiber diameter between the lyocell and PP fibers in this separator is 2.9 μm. Additionally, regarding Buick's smoothness, the time is 22.9 seconds on a smooth surface and 21.8 seconds on a rough surface, with an air tightness of 2.2 seconds / 100ml.

[0131] Using this separator, the electrochemical elements of Example 3 were fabricated.

[0132] (Example 4)

[0133] 70% by mass of lyocell fibers (average fiber diameter 9.9 μm), used as solvent-spun cellulose fibers after pulping, and 30% by mass of PP fibers (average fiber diameter 7.7 μm), used as synthetic fibers, were mixed and subjected to wire-wire papermaking to obtain a thickness of 64.5 μm and a density of 0.31 g / cm³. 3 The separator.

[0134] The difference in fiber diameter between the lyocell and PP fibers in this separator is 2.2 μm. Additionally, regarding Buick's smoothness, the time is 36.3 seconds on a smooth surface and 31.8 seconds on a rough surface, with an air tightness of 3.1 seconds / 100ml.

[0135] Using this separator, the electrochemical elements of Example 4 were fabricated.

[0136] (Example 5)

[0137] 85% by mass of lyocell fibers (average fiber diameter 5.1 μm), used as solvent-spun cellulose fibers after pulping, and 15% by mass of acrylic fibers (average fiber diameter 3.2 μm), used as synthetic fibers, were mixed and subjected to wire-wire papermaking to obtain a thickness of 19.6 μm and a density of 0.55 g / cm³. 3 The separator.

[0138] The difference in fiber diameter between the lyocell and acrylic fibers in this separator is 1.9 μm. Additionally, regarding Buick's smoothness, the time is 199.9 seconds on a smooth surface and 187.0 seconds on a rough surface, with an air tightness of 19.8 seconds / 100ml.

[0139] Using this separator, the electrochemical elements of Example 5 were fabricated.

[0140] (Example 6)

[0141] 85% by mass of lyocell fibers (average fiber diameter 6.3 μm, used as solvent-spun cellulose fibers after pulping) and 15% by mass of acrylic fibers (average fiber diameter 3.5 μm, used as synthetic fibers) were mixed and subjected to wire rod papermaking to obtain a paper with a thickness of 30.2 μm and a density of 0.49 g / cm³. 3 The separator.

[0142] The difference in fiber diameter between the lyocell and acrylic fibers in this separator is 2.8 μm. Additionally, regarding Buick's smoothness, the time is 155.5 seconds on a smooth surface and 150.3 seconds on a rough surface, with an air tightness of 14.6 seconds / 100ml.

[0143] Using this separator, the electrochemical elements of Example 6 were fabricated.

[0144] (Example 7)

[0145] 85% by mass of lyocell fibers (average fiber diameter 7.5 μm, used as solvent-spun cellulose fibers after pulping) and 15% by mass of PET fibers (average fiber diameter 4.9 μm, used as synthetic fibers) were mixed and subjected to two-wire papermaking to obtain a paper with a thickness of 35.4 μm and a density of 0.46 g / cm³. 3 The separator.

[0146] The difference in fiber diameter between the Lyocell and PET fibers in this separator is 2.6 μm. Additionally, regarding Buick's smoothness, it achieves 97.3 seconds on a smooth surface and 96.4 seconds on a rough surface, with an air tightness of 10.9 seconds / 100ml.

[0147] Using this separator, the electrochemical elements of Example 7 were fabricated.

[0148] (Example 8)

[0149] 80% by mass of lyocell fiber (average fiber diameter 6.9 μm), used as a solvent-spun cellulose fiber after pulping, and 20% by mass of PET fiber (average fiber diameter 5.4 μm), used as a synthetic fiber, were mixed and subjected to wire rod papermaking to obtain a thickness of 29.8 μm and a density of 0.41 g / cm³. 3 The separator.

[0150] The difference in fiber diameter between the Lyocell and PET fibers in this separator is 1.5 μm. Additionally, regarding Buick's smoothness, the time is 53.2 seconds on a smooth surface and 48.1 seconds on a rough surface, with an air tightness of 5.3 seconds / 100ml.

[0151] Using this separator, the electrochemical elements of Example 8 were fabricated.

[0152] (Comparative Example 1)

[0153] 95% by mass of lyocell fiber (average fiber diameter 1.6 μm), used as a solvent-spun cellulose fiber after pulping, and 5% by mass of PET fiber (average fiber diameter 1.7 μm), used as a synthetic fiber, were mixed and subjected to wire rod papermaking to obtain a paper with a thickness of 8.1 μm and a density of 0.75 g / cm³. 3 The separator.

[0154] The difference in fiber diameter between the Lyocell and PET fibers in this separator is -0.1 μm. Additionally, regarding Buick's smoothness, the time is 458.0 seconds on a smooth surface and 430.6 seconds on a rough surface, with an air tightness of 52.4 seconds / 100ml.

[0155] Using this separator, the electrochemical elements of Comparative Example 1 were fabricated.

[0156] (Comparative Example 2)

[0157] 70% by mass of lyocell fiber (average fiber diameter 11.3 μm), used as a solvent-spun cellulose fiber after pulping, was mixed with 30% by mass of PET fiber (average fiber diameter 8.4 μm), used as a synthetic fiber, and the mixture was then used for wire papermaking to obtain a paper with a thickness of 74.7 μm and a density of 0.23 g / cm³. 3 The separator.

[0158] The difference in fiber diameter between the Lyocell and PET fibers in this separator is 2.9 μm. Additionally, regarding Buick's smoothness, it takes 11.1 seconds on a smooth surface and 8.1 seconds on a rough surface, with an air tightness of 1.5 seconds / 100ml.

[0159] Using this separator, the electrochemical elements of Comparative Example 2 were fabricated.

[0160] (Comparative Example 3)

[0161] 65% by mass of lyocell fiber (average fiber diameter 10.7 μm), used as a solvent-spun cellulose fiber after pulping, was mixed with 35% by mass of PET fiber (average fiber diameter 7.8 μm), used as a synthetic fiber, and the mixture was then used for wire papermaking to obtain a paper with a thickness of 75.1 μm and a density of 0.20 g / cm³. 3 The separator.

[0162] The difference in fiber diameter between the Lyocell and PET fibers in this separator is 2.9 μm. Additionally, regarding Buick's smoothness, the time is 9.7 seconds on a smooth surface and 8.4 seconds on a rough surface, with an air tightness of 1.2 seconds / 100ml.

[0163] Using this separator, the electrochemical elements of Comparative Example 3 were fabricated.

[0164] (Comparative Example 4)

[0165] 85% by mass of lyocell fibers (average fiber diameter 6.3 μm, used as solvent-spun cellulose fibers after pulping) and 15% by mass of PET fibers (average fiber diameter 7.5 μm, used as synthetic fibers) were mixed and subjected to wire rod papermaking to obtain a paper with a thickness of 40.5 μm and a density of 0.24 g / cm³. 3 The separator.

[0166] The difference in fiber diameter between the Lyocell and PET fibers in this separator is -1.2 μm. Additionally, regarding Buick's smoothness, the time is 18.8 seconds on a smooth surface and 15.1 seconds on a rough surface, with an air tightness of 1.8 seconds / 100ml.

[0167] Using this separator, the electrochemical elements of Comparative Example 4 were fabricated.

[0168] (Existing Example 1)

[0169] Using 100% by mass of lyocell fibers (average fiber diameter 8.0 μm) spun from pulped solvent-spun cellulose fibers, a long-wire paper with a thickness of 30.1 μm and a density of 0.41 g / cm³ was obtained. 3 The separator.

[0170] Regarding the smoothness of the separator, it is 102.5 seconds on a smooth surface and 91.2 seconds on a rough surface, and the air tightness is 10.3 seconds / 100ml.

[0171] Using this separator, the electrochemical elements of Existing Example 1 are fabricated.

[0172] (Existing Example 2)

[0173] 75% by mass of lyocell fibers (average fiber diameter 6.9 μm, used as solvent-spun cellulose fibers after pulping) and 25% by mass of PET fibers (average fiber diameter 3.7 μm, used as synthetic fibers) were mixed and subjected to wire rod papermaking to obtain a paper with a thickness of 30.4 μm and a density of 0.45 g / cm³. 3 The separator.

[0174] The difference in fiber diameter between the Lyocell and PET fibers in this separator is 3.2 μm. Additionally, regarding Buick's smoothness, the time is 516.0 seconds on a smooth surface and 247.7 seconds on a rough surface, with an air tightness of 14.7 seconds / 100ml.

[0175] Using this separator, the electrochemical elements of Existing Example 2 are fabricated.

[0176] (Existing Example 3)

[0177] The separator of Example 6 was subjected to pressure adjustment by soft calendering to obtain a thickness of 25.1 μm and a density of 0.59 g / cm³. 3 The separator.

[0178] Regarding the smoothness of the separator, it is 633.4 seconds on a smooth surface and 625.5 seconds on a rough surface, with an air tightness of 20.0 seconds / 100ml.

[0179] Using this separator, the electrochemical elements of Existing Example 3 are fabricated.

[0180] (Existing Example 4)

[0181] A material with a thickness of 29.9 μm and a density of 0.53 g / cm³ was used. 3 A polyethylene microporous membrane is used as a separator.

[0182] The separator is not made of fiber, therefore the fiber diameter cannot be measured. The air impermeability of the separator is 301.0 seconds / 100ml, and the Buick smoothness is over 10,000 seconds on both smooth and rough surfaces.

[0183] The evaluation results of the separators in each embodiment, comparative example, and existing example are shown in Table 1.

[0184] In addition, the evaluation results of the electrochemical devices fabricated using the separators in each example are shown in Table 2.

[0185] [Table 1]

[0186]

[0187] [Table 2]

[0188]

[0189] As shown in Table 2, no short circuit failures occurred in the electrochemical elements of each embodiment.

[0190] Furthermore, all permeability tests were "○", and the electrochemical element achieved the target capacitance and discharge capacity. Moreover, the capacity retention rate in long-term reliability testing was excellent, exceeding 70%.

[0191] The thickness of the separator in Comparative Example 1 was 8.1 μm. Furthermore, short-circuit failure occurred in the electrochemical element using this separator.

[0192] In addition, it is composed of fine fibers with an average fiber diameter of 1.6 μm for lyocell fibers and an average fiber diameter of 1.7 μm for PET fibers, with a density of 0.75 g / cm³. 3 The air tightness is 52.4 seconds / 100ml, which is too high, resulting in high internal resistance of the electrochemical element.

[0193] Furthermore, regarding Buick's smoothness, the time is 458.0 seconds on smooth surfaces and 430.6 seconds on rough surfaces, indicating high density in the surface direction. Therefore, the permeability is "△", indicating deterioration of the internal resistance of the electrochemical element, and a decrease in electrostatic capacitance, discharge capacity, and capacity retention rate in long-term reliability tests.

[0194] The separator in Comparative Example 2 has a thickness of 74.7 μm. Due to its thickness, the electrochemical element has a high internal resistance.

[0195] The Buick separator exhibits a low smoothness (11.1 seconds on smooth surfaces and 8.1 seconds on rough surfaces), resulting in excellent impregnation. However, it is also composed of coarse fibers (11.3 μm average fiber diameter for Lyocell and 8.4 μm average fiber diameter for PET), resulting in a low density of only 0.23 g / cm³ despite its thickness. 3 The air tightness is as low as 1.5 seconds / 100ml, which can cause short circuits.

[0196] As can be seen from the comparison between Comparative Examples 1 and 2 and the various embodiments, the thickness of the separator is preferably 10-70 μm. Furthermore, the density of the separator is preferably 0.25-0.70 g / cm³. 3 The air tightness is preferably 2~50 seconds / 100ml, and the smoothness of both sides of the separator is preferably 20~400 seconds.

[0197] When a shorter impregnation time is desired to improve the productivity of electrochemical elements, a lower Buick smoothness value is preferable. Comparing Example 1 and Example 2, Example 2 has a smooth surface impregnation time of 397.1 seconds and a rough surface impregnation time of 395.0 seconds, while Example 1 has a smooth surface impregnation time of 298.2 seconds and a rough surface impregnation time of 286.3 seconds, indicating that Example 2 is lower. Furthermore, regarding air impermeability, Example 2 has an impermeability time of 30.0 seconds / 100ml, compared to 49.6 seconds / 100ml in Example 1, but without short-circuit defects. The electrochemical element of Example 2 exhibits low internal resistance and good capacity retention after long-term reliability testing.

[0198] Furthermore, in comparing Examples 3 and 4, the Buick smoothness of Example 3 was 22.9 seconds on a smooth surface and 21.8 seconds on a rough surface, while the Buick smoothness of Example 4 was 36.3 seconds on a smooth surface and 31.8 seconds on a rough surface. Example 3 is advantageous in terms of impregnation, but to obtain a separator with low Buick smoothness, the fiber diameter of the Lyocell and PET fibers needs to be increased. Therefore, Example 3 results in a thicker separator, and thus, Example 4 is more preferable in terms of the internal resistance of the electrochemical element.

[0199] By comparing Example 1 with Example 2, and Example 3 with Example 4, the smoothness of both sides of the separator is more preferably 30~300 seconds, and the air tightness is more preferably 3~30 seconds / 100ml.

[0200] The separator in Comparative Example 3 is a separator made by blending 65% by mass of lyocell fiber and 35% by mass of PET fiber. Compared with the separator in Comparative Example 2, it has a lower density and air impermeability, and therefore, despite its greater thickness, the short-circuit failure rate of the electrochemical element is also higher.

[0201] The separator in Example 1 is a separator formed solely of Lyocell fiber.

[0202] As can be seen from the comparison between the existing Example 1 and the various embodiments, by using the separator of the present invention containing synthetic fibers, the capacity retention rate after long-term reliability testing can be improved. This can be attributed to the fact that the chemical stability of the separator is improved by using synthetic fibers.

[0203] Therefore, the preferred content of solvent-spun cellulose fiber is 70-95% by mass, and the preferred content of synthetic fiber is 5-30% by mass.

[0204] Regarding the separator in Comparative Example 4, the fiber diameter difference between Lyocell and PET fibers is -1.2 μm, and the density is as low as 0.24 g / cm³. 3The air impermeability is as low as 1.8 μm. This is because PET fibers hinder the interweaving of lyocell fibers, causing PET fibers to detach from the separator. In electrochemical elements using the separator of Comparative Example 4, the short-circuit failure rate is high, and the capacity retention rate after long-term reliability testing is low.

[0205] Furthermore, the smoothness of the separator in Example 2 is 516.0 seconds on a smooth surface. This low permeability prevents the achievement of the target capacity for the electrochemical element.

[0206] This is because: the difference in fiber diameter between the Lyocell fiber and the PET fiber in the separator of Example 2 is 3.2 μm, so there is an imbalance in fiber weight, which makes the smoothness value of the smooth surface higher.

[0207] As can be seen from the comparison of Comparative Example 4 and Existing Example 2 with the various embodiments, the fiber diameter difference between solvent-spun cellulose fibers and synthetic fibers is preferably -1.0 μm to 3.0 μm.

[0208] Regarding the separator of Existing Example 3, the thickness of the separator of Example 6 was adjusted by soft calendering, but the smoothness was 633.4 seconds on the smooth surface and 625.5 seconds on the rough surface. Compared with the separator of Existing Example 2, the permeability was lower and the capacity of the electrochemical element was also lower.

[0209] In Example 4, the separator is a thin film, resulting in poor impregnation of the raw material itself. Consequently, the smoothness of the Buick is over 10,000 seconds on both sides, the impregnation time is 1 hour, and the capacity of the electrochemical element is too low compared to the target, thus making it impossible to conduct other evaluations.

[0210] As described above, the separator of the present invention is characterized in that it is a separator for an electrochemical element sandwiched between a pair of electrodes and capable of maintaining an electrolyte containing electrolyte. The separator is a layer formed from pulped solvent-spun cellulose fibers and synthetic fibers, wherein the separator has a thickness of 10-70 μm and a density of 0.25-0.70 g / cm³. 3 The smoothness of both sides of the Buick is 20~400 seconds.

[0211] By using the separator of the present invention, the permeability of the electrolyte can be improved without compromising the separator's resistance, short-circuit withstand capability, or acid resistance.

[0212] Furthermore, by using the separator of the present invention, it is possible to improve the productivity of electrochemical elements, increase their capacity, and enhance their reliability to withstand long-term use under harsh conditions.

[0213] The above describes examples of using the separator of this embodiment in electric double-layer capacitors, lithium-ion capacitors, and lithium-ion secondary batteries.

[0214] In the electrochemical element of the present invention, there are no special limitations on the electrode material, electrolyte material, and other components, and various materials can be used.

[0215] In addition, the separator for electrochemical elements 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, magnesium-sulfur secondary batteries, and the like.

Claims

1. A separator for an electrochemical element, characterized in that, It is sandwiched between a pair of electrodes and is able to maintain an electrolyte solution containing electrolytes. The separator is a layer formed from pulped solvent-spun cellulose fibers and synthetic fibers. The separator has a thickness of 10~70μm and a density of 0.25~0.70g / cm³. 3 The smoothness of both sides of the Buick is 20~400 seconds.

2. The separator for electrochemical elements according to claim 1, characterized in that, The airtightness of the separator is 2 to 50 seconds.

3. The separator for electrochemical elements according to claim 1, characterized in that, The diameter difference between the pulped solvent-spun cellulose fibers and the synthetic fibers in the separator is -1.0 to 3.0 μm.

4. The separator for electrochemical elements according to claim 1, characterized in that, The synthetic fiber is selected from one or more of polyester fibers, acrylic fibers, and polyolefin fibers.

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

6. The electrochemical element according to claim 5, characterized in that, It can be any one of the following: electric double layer capacitor, lithium-ion capacitor, or lithium-ion secondary battery.

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