Porous sheet, separator for electrochemical element, and electrochemical element

Abstract

To provide a porous sheet that is excellent in performance as a separator for an electrochemical element, in that it is thin but does not cause short circuits. The porous sheet is made of fibers and has a thickness of 4.8 to 30.0 μm, an air resistance of 1 to 1000 seconds / 100 ml, and a valley area of ​​1.0 to 38.0 mm as measured using a three-dimensional white light interference microscope (manufactured by BRUKER). 2 The present invention also provides an electrochemical element including this porous sheet as a separator for the electrochemical element.

Description

[Technical Field]

[0001] The present invention relates to a porous sheet made of fibers, and more particularly to a porous sheet suitable for a separator for an electrochemical element. [Background technology]

[0002] In recent years, the use of electricity as an energy source has been increasing in order to address environmental issues such as the depletion of fossil fuels and the reduction of CO2 emissions. For example, the automotive industry has been actively developing electric vehicles that use secondary batteries. Secondary batteries are also attracting attention from the perspective of effective utilization of natural energy sources such as solar and wind power.

[0003] Currently, lithium-ion secondary batteries are generally used as the secondary batteries for driving electric vehicles due to their relationship between output and energy density. However, companies are focusing their efforts on developing next-generation batteries that offer higher energy density, output, and safety, and this is an area where a large market is expected.

[0004] Meanwhile, separators made of paper, nonwoven fabric, microporous film, etc. are used not only in lithium-ion secondary batteries but also in other devices such as secondary batteries, primary batteries, and capacitors. The performance required of separators generally includes prevention of short circuits between positive and negative electrodes, chemical stability against electrolytes, low internal resistance, etc. While the degree of these required performances varies depending on the type of device, they are also common characteristics required of separators regardless of the type.

[0005] Most lithium-ion secondary batteries use microporous membranes made of polymer organic compounds such as polypropylene and polyethylene as separators. These microporous membranes have several characteristics that make them suitable for lithium-ion secondary batteries. For example, 1) It is chemically stable against the electrolyte and does not cause fatal defects due to the separator. 2) The thickness of the separator can be freely designed, making it possible to provide separators that meet a variety of requirements. 3) The pore size can be designed to be small, resulting in excellent lithium blocking properties and reducing the risk of short circuits caused by lithium dendrites. 4) When a lithium-ion secondary battery experiences thermal runaway, the polypropylene or polyethylene melts, narrowing the pores and making it possible to suppress the initial thermal runaway. These are some of the points:

[0006] However, research into lithium-ion secondary batteries to date has not elucidated the fundamental causes of thermal runaway, and currently, each company is using empirical methods to study and propose measures to avoid the risk of thermal runaway for the various materials used in secondary batteries. By clarifying the principles of thermal runaway and establishing a unified evaluation method, it is expected that the development of materials suitable for safer automotive applications will progress, and safety issues will also be resolved.

[0007] The second issue facing secondary batteries for automotive applications is cost. The separator accounts for 20% of the battery cost, and further cost reductions are currently required.

[0008] Incidentally, for example, in the field of rechargeable transportation equipment such as electric vehicles, and in the field of portable electronic terminals such as mobile phones, there is a demand for electric energy storage devices that can store a large amount of electric energy per unit volume so that they can operate for a long time even in a small volume. One example of such an electric energy storage device is an electric double layer capacitor, in which an electrolyte dissolved in an electrolytic solution is adsorbed onto an electrode, and electric energy is stored in the interface (electric double layer) formed between the electrolyte and the electrode.

[0009] The main roles of the separator in an electric double layer capacitor are to prevent short circuits between electrodes (separability) and not to hinder the movement of ions in the electrolyte (low internal resistance). However, the high density of the microporous membranes mentioned above tends to result in high internal resistance. While it is known to use nonwoven fabrics as capacitor separators, reducing the fiber diameter or increasing the fiber density to maintain separation properties raises the problem of high internal resistance. Therefore, the development of a separator with low internal resistance is desirable. Furthermore, increasing the capacity of an electric double layer capacitor requires thicker electrode layers, which requires a correspondingly thinner separator.

[0010] Manufacturing methods for polymeric microporous membranes, such as polypropylene and polyethylene, can be broadly divided into wet and dry processes. Each manufacturing method has its own characteristics. The wet process involves adding a plasticizer to a polymer such as polyethylene, forming it into a film, then biaxially stretching it, and washing the plasticizer out with a solvent to create micropores. This method offers excellent control over pore size and film thickness, making it possible to meet the diverse requirements of each battery type. However, the complex manufacturing process results in high costs. In contrast, the dry process involves dissolving a polymer such as polyolefin, extruding it onto a film, annealing it, and stretching it at a low temperature to form initial pores, followed by high-temperature stretching to create pores. This method has the advantages of being able to laminate polymers with different melting points and being inexpensive due to its simple process, but it suffers from a lack of precision in controlling pore size and film thickness.

[0011] In addition to polymer microporous films, separators made from nonwoven fabrics composed of synthetic fibers, inorganic fibers, etc. have also been proposed. Conventionally, nonwoven fabrics, both dry-laid and wet-laid, have been used as separators. However, dry-laid nonwoven fabrics, lacking uniform fiber distribution, are considered unsuitable for lithium-ion secondary batteries due to their poor electrode isolation effect. On the other hand, wet-laid nonwoven fabrics feature more uniform fiber distribution than dry-laid nonwoven fabrics. Due to their manufacturing process, wet-laid nonwoven fabrics can be manufactured with a higher porosity than microporous films, enabling the creation of sheets with low impedance. However, their use in batteries with graphite anodes, which are currently widely used in lithium-ion secondary batteries, is practically difficult. This is due to the tendency of lithium-ion secondary batteries to form lithium dendrites on the anode side, which tend to form on the surface of the anode where lithium ions pass through. For this reason, in nonwoven fabrics, where the sheet itself can vary in density on the order of several tens of micrometers, the areas where lithium dendrites are likely to form are rough, and so the interruption characteristics that prevent short circuits when lithium dendrites form are considered to be lower than those of film types. Also, as mentioned above, there is a demand for thinner separators, but separators made of nonwoven fabric in particular lose strength as they become thinner, which can result in safety issues such as short circuits and the risk of the separator being difficult to process when manufacturing double-layer battery capacitors.

[0012] To solve these problems, it is common to specify the pore size (pore diameter) of the separator within a certain range, as in Patent Document 1 (JP Patent Publication No. 11-040130). However, because the pore size depends on the fiber diameter, controlling the pore size requires reducing the fiber diameter, but using thin fibers generally results in a decrease in strength.

[0013] As a solution to the problem of reduced strength, Patent Document 2 (JP 2017-123279 A) claims that a low internal short-circuit rate and strength can both be achieved by blending fibrillated solvent-spun cellulose fibers, synthetic fibers, and fibrillated natural cellulose fibers in specific ratios. However, when synthetic fibers with a larger fiber diameter than the fibrillated (fine) fibers are blended in this way, the apparent thickness is determined by the overlapping synthetic fibers, and areas where no synthetic fibers are present form valleys that are thinner than the apparent thickness, which can lead to short circuits when used as a separator.

[0014] As a means of reducing the thickness and thickness of separators, i.e., their surface roughness, Patent Document 3 (JP 2019-96681 A) describes a method of producing a sheet by mixing finely divided solvent-spun cellulose fibers with thermoplastic synthetic fibers, and then calendering the membrane. This method is said to improve the density of the separator, providing a separator with excellent shielding properties and reduced deterioration in mechanical strength. A smoothness of 500 to 2000 seconds is considered preferable as an indicator of separator density. However, it is difficult to determine the degree of density and shielding properties based on the smoothness specification alone, and calendering the membrane may cause blockages between fibers, increasing battery resistance. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Japanese Patent Application Publication No. 11-040130 [Patent Document 2] JP 2017-123279 A [Patent Document 3] Japanese Patent Application Publication No. 2019-96681 Summary of the Invention [Problem to be solved by the invention]

[0016] The present invention has been made in view of the above circumstances, and has an object to provide a porous sheet which is thin but does not cause short circuits and has excellent performance as a separator for an electrochemical element. [Means for solving the problem]

[0017] As a result of extensive research to solve the above problems, the present inventors have found that a porous sheet having specific physical properties has excellent performance as a separator for an electrochemical device, and have completed the present invention.

[0018] That is, the present invention provides a porous sheet made of fibers, having a thickness of 4.8 to 30.0 μm, an air resistance of 1 to 1000 seconds / 100 ml, and a valley area of ​​1.0 to 38.0 mm as measured using a three-dimensional white light interference microscope (manufactured by BRUKER). 2 is.

[0019] The porous sheet preferably contains cut fibers having an average fiber diameter of 0.5 to 7.0 μm in an amount of 5 to 50% by weight based on the total weight of the fibers.

[0020] Preferably, the fibers are natural fibers.

[0021] The present invention also relates to a separator for an electrochemical element, which includes the porous sheet, and further to an electrochemical element, such as a battery or a capacitor, which includes the separator for an electrochemical element. [Effects of the Invention]

[0022] Although the porous sheet of the present invention is thin, it is resistant to short circuits and has excellent performance as a separator for electrochemical elements.

[0023] The porous sheet of the present invention is composed of fine fibers, and the fibers are uniformly distributed, so that it has excellent lithium dendrite blocking properties. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 10 is a diagram showing a measurement example of a valley area test. DETAILED DESCRIPTION OF THE INVENTION

[0025] [Porous sheet composition] The porous sheet of the present invention is a porous sheet made of fibers, and has a thickness of 4.8 to 30.0 μm, an air resistance of 1 to 1000 seconds / 100 ml, and a valley area of ​​1.0 to 38.0 mm as measured using a three-dimensional white light interference microscope (manufactured by BRUKER). 2 is.

[0026] The fibers used in the porous sheet of the present invention are not particularly limited, and any fiber that can be formed into a sheet can be used. For example, natural fibers such as cellulose fibers, modified pulps such as cationized pulp and mercerized pulp, synthetic fibers such as rayon, vinylon, nylon, acrylic, polyester, polyolefin, and aramid, chemical fibers, or microfibrillated pulps thereof, and inorganic fibers such as glass fibers, carbon fibers, and alumina fibers can be used alone or in combination. Fibers containing fibrils or fibers that can be fibrillated are particularly preferred.

[0027] The fibers used in the porous sheet of the present invention are preferably natural fibers, and more preferably cellulose fibers. There are no particular limitations on the cellulose fibers, and known fibers can be used. For example, bleached wood chemical pulps such as softwood bleached kraft pulp (NBKP), hardwood bleached kraft pulp (LBKP), and softwood bleached sulfite pulp (NBSP); mechanical pulps such as groundwood pulp (GP), thermomechanical pulp (TMP), and chemical thermomechanical pulp (BCTMP); non-wood pulps such as hemp, bamboo, straw, kenaf, mitsumata, kozo, and cotton; and recycled paper pulp can be used. These cellulose fibers can be used alone or in combination of two or more. The type of cellulose, such as type I or type II, is not particularly limited in the present invention, but natural fibers of type I cellulose, such as cotton, cotton linters, and wood pulp, are preferred. More preferred is NBKP, a bleached wood pulp derived from conifers, which is more likely to fibrillate than cut during fiber defibration. Cellulose type II fibers, typified by regenerated cellulose, have a lower crystallinity than cellulose type I fibers and are more likely to be shortened during fibrillation treatment, which may reduce the strength improvement effect when composited with a resin, making them undesirable. For example, solvent-spun cellulose fibers are undesirable.

[0028] The cellulose fibers used in the porous sheet of the present invention are preferably obtained by a pulverization treatment of cellulose fibers. The equipment used to pulverize cellulose fibers is not particularly limited, but examples include high-pressure homogenizer treatment (high-pressure dispersion treatment using a Manton-Gaulin type disperser), a Lanier-type pressure homogenizer, ultra-high-pressure homogenizer treatment (Ultimizer™ (manufactured by Sugino Machine Co., Ltd.)), dispersion devices such as bead mills and Ryusei mills, and homogenizers such as the Mass Colloider (manufactured by Masuko Sangyo Co., Ltd.). The degree of beating is not particularly limited, but a beater used in papermaking, such as a double-disc refiner or beater, can be used for pretreatment before pulverization. In particular, in the case of pulverization treatment devices that pass the material through an orifice, such as a high-pressure homogenizer, if the raw material is not beaten in the pretreatment, there is a risk that the raw material will clog and prevent stable pulverization. Furthermore, because the production efficiency of such pulverization treatment devices is generally low, pretreatment using a beater is preferable because it increases production efficiency. Furthermore, cellulose nanofibers (TOCN) that have been converted into nanofibers by chemical treatment using a TEMPO (2,2,6,6-tetramethylpiperidine-1-oxy radical) oxidation catalyst can also be used.

[0029] Cellulose fibers can be uniformly dispersed in water due to the hydroxyl groups in the cellulose molecules, but the viscosity of the resulting slurry depends on the fiber length and surface area of ​​the cellulose fibers. As the cellulose fibers become thinner, the surface area of ​​the cellulose increases, which inevitably increases the viscosity of the slurry. Furthermore, as the fiber length increases, the interactions between the fibers increase, which is also thought to be a factor leading to an increase in viscosity. The increase in viscosity due to these interactions is a factor that inhibits sheet formation at high concentrations, so the general method of handling nanocellulose is to reduce the concentration.

[0030] Furthermore, cellulose fibers possess the ability to form hydrogen bonds with each other during the dehydration process due to their hydroxyl groups, a feature not found in nonwoven fabrics made from synthetic fibers other than regenerated cellulose. While this hydrogen bond formation process contributes to the development of strength, the interaction between fibers also results in greater shrinkage during the drying process than nonwoven fabrics made from synthetic fibers. This shrinkage is particularly pronounced as the fiber diameter decreases, as the stiffness of the fibers decreases. Furthermore, sheets made from highly fibrillated fibers are known to become transparent due to the complete adhesion between the fibers. Therefore, narrowing the fiber diameter alone is not enough to control pore size, let alone create porous sheets. Therefore, to produce porous sheets, it is necessary to suppress shrinkage during drying and inhibit hydrogen bonding between fibers. Specific methods proposed so far include replacing raw materials formed into sheets by papermaking or casting methods with a hydrophilic solvent such as acetone, followed by further replacement with a more hydrophobic solvent such as a mixture of toluene and acetone, followed by drying. However, this method has two problems. The first is the process of replacing the dispersion solvent (water) with acetone. Because cellulose fibers have higher water retention as the fiber diameter decreases, replacing water with a solvent is a time-consuming process, which reduces productivity in actual production. As described below, in the porous sheet of the present invention, it is preferable to apply a slurry containing a hydrophilic pore-opening agent to a substrate and dry it, which can significantly improve production efficiency. Another problem is that the pore size depends on the fiber diameter, so the pore size is ultimately controlled by the fiber diameter. Therefore, the desired pore size cannot be obtained unless uniform fibers are used, and the cellulose fiber processing process also requires time and cost.

[0031] The cellulose fibers preferably contain 50 wt % or more, more preferably 70 wt % or more, and particularly preferably 90 wt % or more, of cellulose fibers with a fiber diameter of 1000 nm or less, preferably 500 nm or less, more preferably 300 nm or less, based on the total weight of the cellulose fibers. Therefore, the cellulose fibers are preferably composed primarily of relatively fine cellulose fibers with a fiber diameter of 300 nm or less. The smaller the fiber diameter, the higher the thickness uniformity of the separator and the smaller the variations in air resistance and puncture strength. The fiber diameter here refers to the fiber diameter obtained by magnifying and observing multiple locations on the surface of the porous sheet using an electron microscope, randomly selecting a predetermined number of fibers from each electron microscope image, measuring the fiber diameters of the selected fibers, and arithmetically averaging them. The number of fibers selected is 60 or more, preferably 80 or more, and more preferably 100 or more. The fiber diameter of the cellulose fibers can be adjusted as described above by microfibrillation using a homogenizer or beater, or nanofiberization using a TEMPO oxidation catalyst.

[0032] The thickness of the porous sheet of the present invention is 4.8 to 30.0 μm. If the thickness of the porous sheet is less than 4.8 μm, it is difficult to stably prevent short circuits, and there is a risk of short circuits occurring. If the thickness of the porous sheet exceeds 30.0 μm, it is difficult to increase the capacity of the electrochemical element, and the internal resistance of the electrochemical element also increases, which may result in a decrease in output characteristics. The thickness of the porous sheet is preferably 5.0 to 25.0 μm, more preferably 5.0 to 20.0 μm, even more preferably 5.0 to 15.0 μm, particularly preferably 5.0 to 13.0 μm, more preferably 6.0 to 13.0 μm, and even more preferably 7.0 to 13.0 μm.

[0033] The air resistance of the porous sheet of the present invention is 1 to 1000 seconds / 100 ml. If the air resistance of the porous sheet is less than 1 second / 100 ml, the lithium blocking properties will be reduced when used in a lithium ion secondary battery, and short circuits due to lithium dendrites may occur. If the air resistance of the porous sheet exceeds 1000 seconds / 100 ml, the internal resistance of the electrochemical element will increase, and output characteristics may be reduced. The air resistance of the porous sheet is preferably 3 to 800 seconds / 100 ml, more preferably 5 to 200 seconds / 100 ml, and even more preferably 15 to 100 seconds / 100 ml. The air resistance of the porous sheet can be measured in accordance with JIS P8117.

[0034] The valley area of ​​the porous sheet of the present invention measured using a three-dimensional white light interference microscope (manufactured by BRUKER) is 1.0 to 38.0 mm 2 The valley area of ​​the porous sheet is 1.0 mm 2 If the valley area of ​​the porous sheet is less than 38.0 mm, the impregnation and retention of the electrolyte may be reduced. 2 If the valley area exceeds 2.0 to 35.0 mm, it becomes difficult to stably prevent short circuits, and there is a risk of short circuits occurring. 2 It is preferable that the thickness is 3.0 to 30.0 mm. 2 More preferably, it is 6.0 to 30.0 mm. 2 In order to achieve the above range of valley area, it is advisable to appropriately adjust the type of fiber, fiber length, fiber diameter, type of cut fiber, length of cut fiber, fiber diameter of cut fiber, amount of cut fiber, type of hydrophilic binder, amount of hydrophilic binder, type of hydrophilic pore-opening agent, amount of hydrophilic pore-opening agent, sheet density, sheet thickness, paint concentration, paint viscosity, coating speed, coating gap, coating film thickness (wet film thickness), drying temperature, drying air volume, etc.

[0035] The surface roughness of the porous sheet of the present invention is preferably 0.1 to 1.4 μm, and more preferably 0.2 to 1.1 μm. A three-dimensional white light interference microscope can be suitably used to measure the surface roughness of the porous sheet. If the surface roughness is less than 0.1 μm, the impregnation and retention of the electrolyte may be reduced. If the surface roughness exceeds 1.4 μm, it becomes difficult to stably prevent short circuits, increasing the risk of short circuits.

[0036] The porosity of the porous sheet of the present invention is preferably 20 to 70%.

[0037] The porous sheet of the present invention may contain cut fibers in addition to fibers. Cut fibers are non-fibrillated fibers. The use of cut fibers can improve the strength of the porous sheet and improve its handleability. The porous sheet preferably contains cut fibers with an average fiber diameter of 0.5 to 7.0 μm in an amount of 5 to 50 wt % based on the total weight of the fibers. If the average fiber diameter of the cut fibers exceeds 7.0 μm and the cut fibers account for more than 50% of the total weight of the fibers, it becomes difficult to adjust the thickness of the porous sheet, and the valley area and surface roughness of the porous sheet increase, which may lead to short circuits. The average fiber diameter of the cut fibers is preferably 1.0 to 6.0 μm, more preferably 2.0 to 5.5 μm, and even more preferably 2.5 to 5.0 μm. Furthermore, the cut fibers are preferably contained in an amount of 10 to 45 wt % based on the total weight of the fibers, and even more preferably 20 to 40 wt %.

[0038] The cut fibers can be made of a polymer selected from polyvinyl alcohol, aliphatic polyamide, semi-aromatic polyamide, aromatic polyamide, polysulfone, cellulose acetate, cellulose, polyethylene, polypropylene, polyester, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyethylene oxide, polymethylpentene, polyacrylonitrile polyphenylene sulfide, polyacetyl, polyurethane, polyacrylonitrile, polymethyl methacrylate, polystyrene, polytetrafluoroethylene, ethylene-tetrafluoroethylene, PPS resin, regenerated cellulose, and combinations thereof.

[0039] The polymer constituting the cut fibers is preferably polyester, more preferably polyethylene terephthalate, polypropylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, or a combination thereof, and even more preferably polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, or a combination thereof.

[0040] The porous sheet of the present invention may contain one or more organic or inorganic materials. Examples of organic materials include cellulose derivatives (such as carboxymethyl cellulose), polyvinyl alcohol, aliphatic polyamides, semi-aromatic polyamides, aromatic polyamides, polysulfone, cellulose acetate, cellulose, polyethylene, polypropylene, polyester, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyethylene oxide, polymethylpentene, polyacrylonitrile polyphenylene sulfide, polyacetyl, polyurethane, polyacrylonitrile, polymethyl methacrylate, polystyrene, polytetrafluoroethylene, ethylene-tetrafluoroethylene, PPS resin, and polymers selected from combinations thereof. Examples of inorganic materials include alumina, silica (silicon oxide), Oxide ceramics such as titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; Examples of suitable materials include ceramics such as silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand. In addition to the above-mentioned materials, the constituent components may contain various known additives (e.g., antioxidants when the material is a resin) as needed. From the viewpoint of the heat resistance of the separator and the suitability for winding during battery production, a coating layer containing one or more of the above-mentioned organic and inorganic materials may be formed.

[0041] [Method for manufacturing porous sheet] The method for producing the porous sheet of the present invention is not particularly limited, and known techniques can be used. For example, the porous sheet can be produced by known papermaking methods using papermaking machines such as a Fourdrinier paper machine or a cylinder paper machine. Another example of a production method involves applying a slurry containing cellulose fibers having an average fiber diameter of 20 to 500 nm to a substrate and drying the applied slurry. The fiber concentration in the slurry is not limited, but can be, for example, 0.1 to 10 wt %, preferably 0.5 to 5 wt %, and even more preferably 1 to 3 wt %. The porous sheet can be obtained by a production method including at least the steps of applying the slurry to a substrate, drying the slurry to form a sheet on the substrate, and peeling the sheet from the substrate to obtain a porous sheet.

[0042] The slurry preferably contains a hydrophilic pore-opening agent. By applying a slurry containing a hydrophilic pore-opening agent to a substrate and drying it as a means of making a fiber sheet porous, production efficiency can be significantly improved. Furthermore, in the present invention, the pore size of the sheet can be controlled by adjusting the solubility of the hydrophilic pore-opening agent in water. Furthermore, in the present invention, the porosity can be freely controlled by adjusting the amount of hydrophilic pore-opening agent added. For example, in the present invention, the hydrophilic pore-opening agent can be used in a ratio of preferably 10 to 800 parts by weight, more preferably 30 to 600 parts by weight, and even more preferably 100 to 500 parts by weight per 100 parts by weight (mass) of the solid content of the cellulose fiber.

[0043] The hydrophilic pore-opening agent used in the present invention is not particularly limited as long as it is a hydrophilic substance capable of forming fine pores in a sheet made of fibers. However, it is preferable that the boiling point of the hydrophilic pore-opening agent be 180°C or higher. When the fibers are capable of hydrogen bonding, such as cellulose fibers, it is known that hydrogen bonds between fibers are formed when the sheet moisture content during drying is 10 to 20% by weight. When these hydrogen bonds are formed, the hydrophilic pore-opening agent is present in the sheet and inhibits the hydrogen bonds between the fibers, thereby making the sheet porous. If a pore-opening agent with a boiling point of less than 180°C is used, even if a large amount is added, the pore-opening agent may volatilize during the drying process, preventing sufficient porosity. Therefore, a pore-opening agent with a boiling point of 180°C or higher is preferred, but a boiling point of 200°C or higher is more preferable. For example, primary alcohols with molecular weights smaller than hexanol are both water-soluble and hydrophobic, but they volatilize more easily than water during the drying process and therefore cannot inhibit hydrogen bonds sufficiently, and therefore cannot be used in the present invention. However, if a drying method different from the usual drying conditions is used, such as drying using air filled with vapor of a hydrophilic pore-opening agent or multi-stage drying using a solvent with a lower vapor pressure than water, the boiling point does not necessarily need to be 180°C or higher.

[0044] The hydrophilic pore-opening agent used in the present invention preferably has a water solubility of 10% by weight or more, more preferably 20% by weight or more, and even more preferably 30% by weight or more. When a pore-opening agent with a water solubility of less than 10% by weight is used, the amount of pore-opening agent added is limited, making it difficult to control the desired porosity solely by the amount of pore-opening agent added. Furthermore, as drying progresses, the amount of solvent decreases, causing the insoluble pore-opening agent to separate, making it difficult to uniformly create pores in the plane and thickness directions of the sheet. While such hydrophobic pore-opening agents can be emulsified with an emulsifier or the like to create uniform pores to a certain extent, controlling the pore size is difficult. On the other hand, when a pore-opening agent with a water solubility of 10% by weight or more is used, it can be uniformly dispersed in the slurry, and due to its high solubility in water, it does not separate during the drying process, thereby uniformly inhibiting hydrogen bonds during the drying process, thereby creating uniform pores.

[0045] The hydrophilic pore-opening agent used in the present invention preferably has a vapor pressure of less than 0.1 kPa, more preferably less than 0.09 kPa, and even more preferably less than 0.08 kPa at 25° C. Hydrophilic pore-opening agents with a vapor pressure of 0.1 kPa or more are highly volatile and therefore tend to volatilize before contributing to the formation of pores in the membrane, which may result in difficulty in obtaining a microporous membrane.

[0046] The hydrophilic pore-forming agent used in the present invention preferably has a water / octanol partition coefficient (Log Pow) in the range of -1.2 to 0.8, more preferably in the range of -1.1 to 0.8, and even more preferably in the range of -0.7 to 0.4. If a hydrophilic pore-forming agent with a partition coefficient of less than -1.2 is used, the impedance value of the resulting porous sheet may increase.

[0047] Specific examples of hydrophilic pore-forming agents that can be used in the present invention include the following. Examples of the alkyl ether include higher alcohols such as 1,5-pentanediol and 1-methylamino-2,3-propanediol, lactones such as iprosin caprolactone and α-acetyl-γ-butyrolactone, glycols such as diethylene glycol, 1,3-butylene glycol and propylene glycol, glycols such as triethylene glycol dimethyl ether, tripropylene glycol dimethyl ether, diethylene glycol monobutyl ether, triethylene glycol monomethyl ether, triethylene glycol butyl methyl ether, tetraethylene glycol dimethyl ether, diethylene glycol monoethyl ether acetate, diethylene glycol monoethyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monobutyl ether, dipropylene glycol monomethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoisopropyl ether, ethylene glycol monoisobutyl ether, tripropylene glycol monomethyl ether, diethylene glycol methyl ethyl ether and diethylene glycol diethyl ether, carbonates such as propylene carbonate and ethylene carbonate, and others, but are not limited to glycerin and N-methylpyrrolidone. Among these, glycol ethers and carbonates have low vapor pressure and are therefore most suitable for the production method of the present invention.

[0048] In addition to the fibers, cut fibers, and hydrophilic pore-opening agent, the slurry used in the present invention preferably contains 3 to 80 parts by weight, preferably 5 to 50 parts by weight, and more preferably 10 to 30 parts by weight of a hydrophilic polymer binder as an adhesive for connecting the fibers, per 100 parts by weight of the fibers and cut fibers combined. In addition to its adhesive function, the hydrophilic polymer binder can also improve the dispersibility of the fibers. To achieve a uniform pore distribution, the fibers must be uniformly dispersed in the slurry. The hydrophilic polymer binder acts similarly to a protective colloid by adhering to the fiber surface, thereby improving dispersibility. Adding less than 3 parts by weight of the binder may reduce the strength of the finished sheet, and the dispersibility of the fibers may be impaired, making it difficult to obtain uniform pores. On the other hand, adding more than 80 parts by weight is undesirable because the binder fills the pores, resulting in a structure that increases the volume resistivity of the porous sheet.

[0049] Examples of the hydrophilic polymer binder that can be used include cellulose derivatives such as methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropylmethyl cellulose, and hydroxyalkyl cellulose; polysaccharide derivatives such as phosphated starch, cationic starch, and corn starch; and binders such as styrene-butadiene copolymer emulsions and polyvinylidene fluoride, which are known as binders for electrodes.

[0050] The substrate used in the present invention is not particularly limited, but polymer films, glass plates, metal plates, release papers, etc. can be used. The substrate material is preferably a material that does not allow the hydrophilic pore-opening agent in the slurry to bleed through to the back, such as wire, filter cloth, or filter paper. When using a hydrophilic pore-opening agent to create pores, if the hydrophilic pore-opening agent bleeds through to the back of the substrate before drying, the sheet cannot be sufficiently porous. Furthermore, since the dried sheet has the property of transferring the surface properties of the substrate, it is preferable that the surface of the substrate is as smooth as possible. Considering these factors, biaxially stretched polyethylene terephthalate film is preferred as a substrate because it is flexible and has a relatively high melting temperature, so it is less affected by elongation and shrinkage during drying. In addition, since it has a higher polarity than polypropylene film, it is easy to coat even in aqueous slurry formulations and can be used preferably.

[0051] In the production method usable in the present invention, the method of applying the slurry containing fibers made of cellulose fibers and cut fibers, and preferably a hydrophilic pore-opening agent, to the substrate can be any coating method that can apply the coating uniformly so that the coating layer has a thickness within a certain range. For example, coating can be performed using a pre-metering type coater such as a slot die coater or a curtain coater, or a post-metering type coater such as an MB coater, MB reverse coater, or a comma coater.

[0052] In the present invention, if necessary, a surfactant can be added to the slurry as an additive. Nonionic surfactants, such as acetylene glycol, can be used as antifoaming agents or leveling agents to the extent that they do not affect the performance of the electrochemical device. Ionic surfactants are preferably not used because they may affect the performance of the electrochemical device.

[0053] In addition to the hydrophilic polymer binder and the surfactant, the slurry can also contain fillers. For example, inorganic fillers such as silica particles and alumina particles, and organic fillers such as silicone powder can be used. These particles can be added to a degree that does not affect the pores of the porous sheet, but it is preferable to use particles with an average particle size of less than 2 μm. An average particle size of 2 μm or more is undesirable because the gaps between the particles create large pores. These fillers have the effect of reducing the viscosity of the coating slurry, allowing for an increase in coating concentration, which is ideal for improving production efficiency. However, adding too much filler reduces strength, so adding more than 100 parts by weight per 100 parts by weight of cellulose fibers is not recommended.

[0054] Although water is basically required as the solvent for the slurry used in the present invention, it is possible to add up to 50% by weight of a solvent with a higher vapor pressure than water, such as alcohols (e.g., methanol, ethanol, t-butyl alcohol), ketones (e.g., acetone, methyl ethyl ketone), or ethers (e.g., diethyl ether, ethyl methyl ether), in order to improve drying efficiency. Adding more than 50% by weight of these solvents is undesirable because it reduces the dispersibility of the cellulose fibers and reduces the uniformity of the pore distribution.

[0055] In the method for producing a porous sheet of the present invention, the slurry applied to the substrate is dried to obtain a sheet. The drying method is not particularly limited, but can be carried out using commonly used drying techniques such as hot air drying and far-infrared drying, either alone or in combination. For example, the hot air temperature can be set to 30 to 150°C, preferably 60 to 135°C. However, it is necessary to adjust the hot air temperature, hot air volume, far-infrared irradiation conditions, etc. so that the structure of the sheet is dried as uniformly as possible in the thickness direction. Microwave heating can also be used to improve drying efficiency.

[0056] In the method for producing a porous sheet of the present invention, the sheet formed on the substrate in this manner can be peeled off to obtain the porous sheet. The method for peeling the porous sheet from the substrate is not particularly limited.

[0057] [Separators for electrochemical elements, electrochemical elements] The porous sheet of the present invention can be used as a separator for an electrochemical element. The electrochemical element is an electrochemical element comprising a positive electrode, a negative electrode, and a separator, and can be used for, for example, various secondary batteries such as lithium ion secondary batteries and polymer lithium batteries, and various capacitors such as aluminum electrolytic capacitors, electric double layer capacitors, and lithium ion capacitors.

[0058] The electrochemical element can have the same structure as a conventional electrochemical element, except that the porous sheet of the present invention is used as a separator for the electrochemical element. The cell structure of the electrochemical element is not particularly limited, and examples thereof include a stacked type, a cylindrical type, a prismatic type, and a coin type.

[0059] For example, a lithium ion secondary battery as an electrochemical element equipped with the separator of the present invention has a positive electrode and a negative electrode, with the separator for electrochemical elements of the present invention disposed between them, and this separator for electrochemical elements is impregnated with an electrolytic solution.

[0060] The positive electrode and negative electrode contain electrode active materials. Positive electrode active materials include conventionally known materials, such as lithium transition metal oxides (e.g., LiCoO2, LiNiO2, and LiMn2O4) and lithium metal phosphates (e.g., LiFePO4). Negative electrode active materials include conventionally known materials, such as carbon materials (e.g., graphite) and lithium alloys. Furthermore, conventionally known conductive additives and binders may be added to the electrodes as needed.

[0061] To manufacture a lithium-ion secondary battery, a positive electrode mixture and a negative electrode mixture containing a positive electrode active material, a negative electrode active material, and, if necessary, a conventionally known conductive additive and binder are first applied to a conventionally known current collector. For example, aluminum is used as the current collector for the positive electrode, and copper, nickel, etc. is used for the negative electrode. After the positive electrode mixture and the negative electrode mixture are applied to the current collector, they are dried and pressure-molded to obtain a positive electrode and a negative electrode, each with an active material layer formed on the current collector.

[0062] Next, the obtained positive electrode and negative electrode and the separator for electrochemical devices of the present invention are laminated or wound together in the order of positive electrode, separator for electrochemical devices, and negative electrode to form an element. Next, the element is housed in a housing material, the current collector is connected to an external electrode, and the element is impregnated with a conventionally known electrolyte solution, and then the housing material is sealed to obtain a lithium ion secondary battery.

[0063] Furthermore, for example, an electric double layer capacitor as an electrochemical element comprising the separator of the present invention has a positive electrode and a negative electrode, with the separator for electrochemical elements of the present invention disposed between them, and this separator for electrochemical elements is impregnated with an electrolytic solution.

[0064] The positive and negative electrodes can be obtained, for example, by applying an electrode mixture containing activated carbon powder and a conventionally known conductive additive and binder to a conventionally known current collector, drying the mixture, and then press-molding the resulting material. The current collector can be made of, for example, aluminum.

[0065] An electric double layer capacitor is obtained by stacking or winding a positive electrode, a negative electrode, and the separator for an electrochemical element of the present invention in this order, and then housing the element in an outer packaging material, connecting the current collector to an external electrode, impregnating the element with a conventionally known electrolyte solution, and then sealing the outer packaging material. [Example]

[0066] The present invention will be described in more detail below using examples and comparative examples, but the scope of the present invention is not limited to the examples. Hereinafter, the term "sample" refers to a porous sheet.

[0067] (Measurement items and test methods in the examples) [Measurement of average fiber diameter] The sample surface was observed with a scanning electron microscope (Hitachi High-Tech Corporation, Model: SU-8020), and lines were drawn horizontally and vertically on the obtained image. The fiber diameters of at least 20 or more fibers intersecting the two lines were then measured from the enlarged image, and the number-average fiber diameter was calculated from the measurement results. The number-average fiber diameters were calculated for a total of five locations, and the average value of the number-average fiber diameters was used as the average fiber diameter.

[0068] [Thickness measurement] Eight samples cut to a size of 50 mm x 50 mm were stacked and conditioned for one day in an atmosphere of 23°C and 50% relative humidity. After that, the thickness was measured using a Digisickness Tester 201 (manufactured by Toyo Seiki Seisakusho). The total thickness of the eight samples was divided by 8 to obtain the thickness.

[0069] [Basis weight measurement] A sample cut to a size of 50 mm x 50 mm was weighed using a five-digit balance.

[0070] [Air resistance measurement] Measurement was carried out according to JIS P8117.

[0071] [Valley area measurement] A 50mm x 50mm sample was cut and measured using a 3D white light interference microscope (Contour GT-I, BRUKER). The sample was placed on the instrument stage and suction was applied to adhere it to the stage. The measurement mode was VSI mode, with a 5x objective lens and a 0.55x internal lens. The measurement parameters were set to 1x speed, 15μm backscan (representing the measurement range in the Z-axis), and 15μm length. If the measurement data acquisition rate was less than 99%, appropriate values ​​were set. The threshold, which represents the camera sensitivity, was set to 0.01%, but this varies depending on the sample's reflectivity and shape, so adjustments are necessary depending on the sample. The light intensity was then automatically adjusted to generate interference fringes, and a single measurement was performed to confirm the settings were correct. The sample tilt was corrected as needed. Next, stitching was used, with the measurement type set to rectangular and the measurement size set to 10mm x 10mm. The data resolution was adjusted to full size for each individual data point before stitching, and half size for the stitched data, allowing for data thinning to a level that would not affect the measurement results. Measurements were performed with Trim Pixels During Fitting set to 0, Overlap Region Fit Type set to Flat Data, Lowest Slope Data % set to 80, and Overlap Area set to 5%. Vision64 was used to analyze the measurement data. Sample tilt was corrected by selecting Cylinder and Tilt in Terms Removal, and the Zero Level was set to Rp% 10, with the average height of the top 10% of all data taken as the reference plane. Filtering was performed by selecting Median in the Statistical Filter and setting the Window Size to 3. Then, Long Wavelength Pass was selected in the Gaussian Regression Filter, with the Long Wavelength Cutoff (L-Filter) set to 0.02 mm, Order set to 0, Type set to Regular, and Filter Cutoff Ratio set to 1. If the measurement data acquisition rate was less than 99%, remeasurement was performed.For the Multiple Region, the By Threshold(s) was set to 4 μm, the Minimum Region Size to 10 pix, the Region Level to Valleys, the Zero Level to Zero, and Term Removal to None (no change), and the total area of ​​the parts 4 μm or more below the reference plane was calculated. The value calculated using the following formula was then used as the valley area. A total of five measurements were taken, and the arithmetic mean value was used. For sheets manufactured by applying the slurry to a substrate, the measurement was performed on the side opposite the surface that came into contact with the substrate, but for sheets manufactured by other methods, the measurement surface is not particularly limited. Figure 1 shows an example of a valley area test. The total value of the parts 4 μm or more below the reference plane (filled in areas) was used to calculate the valley area. Valley area (mm 2 ) = Total area of ​​parts 4 μm or more below the reference plane (mm 2 )×(8 2 / Sheet Thickness 2 )

[0072] [Surface roughness measurement] As with the measurement of the valley area, the arithmetic mean roughness Sa was calculated from the data measured using a three-dimensional white light interference microscope (Contour GT-I, manufactured by BRUKER). The value calculated using the following formula was then used as the surface roughness, and a total of five measurements were taken, and the arithmetic mean value was used. Note that for sheets produced by applying the slurry to a substrate, the measurement was performed on the surface opposite to the surface in contact with the substrate, but the measurement surface for sheets produced by other methods is not particularly limited. Surface roughness (μm) = arithmetic mean roughness Sa (μm) × (8 / sheet thickness)

[0073] [Beck smoothness] Measurement was carried out according to the method of JIS P8119.

[0074] [Short-circuit evaluation] The battery electrode material, sample, electrode material, sample, and electrode material were stacked in this order and dried at 105°C for at least 4 hours. The stack was then sandwiched between two rubber plates, with cardboard placed on the outside of each rubber plate. This state was then sandwiched between the pressure plates of a benchtop test press (SA-302, manufactured by Tester Sangyo). An electrical resistance meter was connected to the center electrode material and the lower rubber plate, and electrical resistance was measured under pressure. The rubber plates, cardboard, and benchtop test press were heated to 60°C. A short circuit was defined as an electrical resistance of 20 MΩ or less when a pressure of 30 MPa was applied using the benchtop test press. A total of 10 or more measurements were performed and evaluated according to the following criteria, with an "X" indicating a failure. ○: 100% of the total did not short circuit △: More than 80% of the total did not short circuit ×: Less than 80% of the total did not short circuit

[0075] [Example 1] Bleached softwood kraft pulp was dispersed in ion-exchanged water to a concentration of 3% by weight, beaten using a double-disc refiner, and then treated 15 times using a high-pressure homogenizer (manufactured by SMT, LAB1000) at a pressure of 750 bar to obtain cellulose fibers.

[0076] The solid content of the cellulose fiber was 100 parts by weight, and to this was added 300 parts by weight of ethylene carbonate as a hydrophilic pore-opening agent, and 20 parts by weight of carboxymethyl cellulose (trade name: 2200, manufactured by Daicel) dissolved in ion-exchanged water at a concentration of 1.5% by weight as a water-soluble polymer binder.Ion-exchanged water was added to the paint so that the final solid content concentration was 2.0% by weight, and the paint was dispersed until uniformly mixed using a homomixer (manufactured by AS ONE) to prepare Slurry 1.

[0077] The prepared slurry 1 was used as a coating material and applied to a substrate using a die coater to create a wet film. Generally, when applying with a die coater, it is said that the coating gap should be close enough to avoid scratching the paint, as this will prevent air bubbles from being mixed in and stabilize the coating film. Scraping The film was coated onto a 100 μm thick PET film substrate using a die coater to a wet film thickness of 200 μm, and then dried using hot air at 120 °C and an infrared heater. The resulting coating film was peeled off from the PET film to obtain a porous sheet with a thickness of 8 μm. The average fiber diameter of the cellulose fibers in this porous sheet was measured and found to be 500 nm or less.

[0078] [Example 2] A porous sheet having a thickness of 12 μm was obtained in the same manner as in Example 1, except that the wet film thickness was changed to 300 μm.

[0079] [Example 3] A porous sheet having a thickness of 10 μm was obtained in the same manner as in Example 2, except that 200 parts by weight of ethylene carbonate was used as the hydrophilic pore-forming agent.

[0080] [Example 4] A porous sheet having a thickness of 5 μm was obtained in the same manner as in Example 1, except that 100 parts by weight of ethylene carbonate was used as the hydrophilic pore-forming agent and the wet film thickness was changed to 180 μm.

[0081] [Example 5] 100 parts by weight of polyethylene terephthalate fibers (product name: TA04PN, manufactured by Teijin Frontier) with an average fiber diameter of 3.5 μm (0.1 dtex) and a fiber length of 3 mm were added as cut fibers, and 20 parts by weight of carboxymethyl cellulose (product name: 2200, manufactured by Daicel) dissolved in ion-exchange water at a concentration of 1.5 wt% as a hydrophilic polymer binder was added. Water was added to a final solids concentration of 2 wt%, and the mixture was dispersed until uniformly mixed using a homomixer to produce Slurry 2. Next, Slurry 1 and Slurry 2 described in Example 1 were mixed in a weight ratio of 67.5:32.5 (fiber basis) so that the cut fiber content was 32.5 wt% relative to the total amount of cellulose fiber and cut fiber. Furthermore, 375 parts by weight of ethylene carbonate was added as a hydrophilic pore-opening agent per 100 parts by weight of the cellulose fiber solids, and the mixture was stirred at 180 rpm with a stirrer until uniform. This prepared the coating. The prepared coating material was applied in the same manner as in Example 1, except that the wet film thickness was set to 270 μm, to obtain a porous sheet having a thickness of 12 μm.

[0082] [Example 6] The coating material was prepared in the same manner as in Example 5, except that 430 parts by weight of ethylene carbonate was used as the hydrophilic pore-opening agent, and polyethylene terephthalate fibers (product name: TA04PN, manufactured by Teijin Frontier) with an average fiber diameter of 5 μm (0.3 dtex) and a fiber length of 3 mm were used as the cut fibers, and the blending ratio of the cut fibers was 50% by weight. The coating was then applied in the same manner as in Example 1, except that the wet film thickness was 210 μm and the temperature was 135°C, to obtain a porous sheet with a thickness of 14 μm.

[0083] [Example 7] A porous sheet having a thickness of 12 μm was obtained in the same manner as in Example 5, except that the coating gap was set to 400 μm.

[0084] [Comparative Example 1] A porous sheet having a thickness of 8 μm was obtained in the same manner as in Example 5, except that the wet film thickness was 180 μm and the coating gap was 700 μm.

[0085] Comparative Example 2 A porous sheet having a thickness of 8 μm was obtained in the same manner as in Comparative Example 1, except that 350 parts by weight of ethylene carbonate was added as a hydrophilic pore-opening agent, polyethylene terephthalate fibers (product name: TA04PN, manufactured by Teijin Frontier) having an average fiber diameter of 2 μm (0.06 dtex) and a fiber length of 3 mm were used as cut fibers, the cut fiber blending ratio was 20%, and the wet film thickness was 150 μm.

[0086] Comparative Example 3 A porous sheet having a thickness of 10 μm was obtained in the same manner as in Comparative Example 2, except that the wet film thickness was changed to 190 μm.

[0087] Comparative Example 4 A porous sheet having a thickness of 8 μm was obtained in the same manner as in Comparative Example 2, except that the blending ratio of cut fiber was 10% and the wet film thickness was 150 μm.

[0088] Table 1 shows the physical properties of the porous sheets produced in Examples 1 to 7 and Comparative Examples 1 to 4.

[0089] [Table 1]

[0090] As is clear from the results in Table 1, the porous sheet of the present invention has a valley area of ​​1.0 to 38.0 mm 2 By doing so, it can be seen that short circuits do not occur despite the thin thickness. Specifically, in Examples 1 to 7, the valley area is 38.0 mm 2 It was shown that the current was smaller and did not short circuit.

[0091] Comparative Example 1 had the same thickness, basis weight, and density as Example 1, but the valley area was large, and it was shown that short circuits occurred.

[0092] In Comparative Examples 2 and 4, the smoothness falls within the range of 500 to 2000 seconds, which is the range specified as an index of denseness in Patent Document 3, but the valley area is large and short-circuiting occurs. On the other hand, in Examples 3 and 5, the smoothness is less than 500 seconds, but the valley area is 38.0 mm 2 It was shown that the short circuit did not occur because the

[0093] Comparative Example 3 has the same basis weight as Example 6 and has a smaller surface roughness than Example 6, but the valley area is 38.0 mm 2 Although Example 6 had the greatest surface roughness among all Examples and Comparative Examples, it did not short-circuit, which indicates that the valley area is more important than the surface roughness in order to solve the problem of the present invention.

[0094] In other words, the valley area is important in producing a thin, non-short-circuiting separator, such as the porous sheet of the present invention, and by keeping the valley area small and within a certain range, it becomes possible to produce a porous sheet with a low defect rate without impairing battery characteristics, and an electrochemical element equipped with such a porous sheet as a separator.

Claims

1. A porous sheet made of fibers, having a thickness of 4.8 to 30.0 μm, an air resistance of 1 to 1000 seconds / 100 ml, a surface roughness of 0.1 to 1.4 μm, and a valley area of ​​10 mm x 10 mm measured using a three-dimensional white light interference microscope (manufactured by BRUKER) of 1.0 to 38.0 mm ２ A separator for an electrochemical element, comprising a porous sheet comprising:

2. 2. The separator for an electrochemical element according to claim 1, wherein the cut fibers having an average fiber diameter of 0.5 to 7.0 μm are contained in an amount of 5 to 50% by weight based on the total weight of the fibers.

3. 2. The separator for an electrochemical element according to claim 1, wherein the fibers are natural fibers.

4. An electrochemical device comprising the separator for an electrochemical device according to any one of claims 1 to 3.

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