Separator for electrochemical device

Abstract

To provide a separator for electrochemical elements capable of realizing an electrochemical element in which short circuit is hardly generated.SOLUTION: A separator for electrochemical elements has a fiber assembly containing composite fiber with an elongation of 30% or less, thereby preventing short circuit caused by insufficient strength of the fiber assembly due to the high strength of the composite fiber itself. Furthermore, the fiber assembly contains modified cross-section fiber having cross-sectional shape whose flatness degree is less than 0.34 as a constituent fiber, the separator for electrochemical elements can realize an electrochemical element in which short circuit is hardly generated.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a separator for an electrochemical element.

Background Art

[0002] Conventionally, an electrochemical element separator has been provided and used between a positive electrode and a negative electrode of an electrochemical element (for example, a capacitor, a primary battery or a secondary battery such as a lithium battery, a nickel-hydrogen battery, a fuel cell, a solid battery, an air battery, etc.) to separate the positive electrode and the negative electrode to prevent short circuit and to smoothly perform an electromotive reaction. And, it is required that the separator for an electrochemical element can prevent the occurrence of short circuit of the electrochemical element. In order to meet such a demand, a separator for an electrochemical element including a fiber aggregate containing composite fibers such as core-sheath type adhesive fibers has been studied.

[0003] For example, Japanese Patent Application Laid-Open No. 2002-180330 (Patent Document 1) discloses that, based on the finding that composite fibers are used to increase the strength of a nonwoven fabric, as composite fibers optimal for applications such as wet nonwoven fabrics for separators for electrochemical elements, drawn composite fibers having an elongation of 30% or less are employed.

[0004] Further, in Japanese Patent Application Laid-Open No. 2012-216427 (Patent Document 2), based on the finding that "the high-strength, low-elongation, high Young's modulus drawn composite fibers proposed in Japanese Patent Application Laid-Open No. 2002-180330, which can be used as a battery separator material, (omitted) are composite fibers with extremely low elongation. When such fibers are used as a separator material, although the strength of the drawn composite fibers themselves is high, (omitted) the separator material using them is also difficult to deform and lacks flexibility. In the case of battery separator applications, there is a risk of being easily broken from parts other than the drawn composite fibers (for example, the thermally bonded parts between constituent fibers) by needle-like foreign matters such as dendrites", a separator material containing high-strength composite fibers with an elongation greater than 30% and 60% or less is disclosed. And it is disclosed that the short-circuit resistance can be enhanced by the separator material containing high-strength composite fibers having such a configuration.

[0005] In addition, Patent Document 2 discloses that the constituent fibers of the separator material may further include profiled cross-section fibers formed by splitting split fibers. And as a specific example, in the examples of Patent Document 1, a separator material containing high-strength composite fibers and profiled cross-section fibers formed by splitting fibers having a hollow composite split orange-shaped cross-section (composite ratio: 5 / 5 (volume ratio), hollow ratio: 12%, number of splits: 16 splits, fineness: 1.1 dtex) was prepared.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] The applicant of the present application has continued to study a separator for an electrochemical element according to the prior art as disclosed in Patent Document 1, that is, a separator for an electrochemical element including a fiber aggregate containing composite fibers having an elongation of 30% or less. As a result of the study, in an electrochemical element using a separator for an electrochemical element satisfying the above configuration, as disclosed in Patent Document 2 (the finding that a separator for an electrochemical element including a fiber aggregate containing composite fibers having an elongation of 30% or less is inferior in short-circuit resistance), short circuits are likely to occur due to burrs and other foreign matters in addition to needle-like foreign matters such as dendrites present on the electrodes.

[0008] Therefore, as long as a separator for an electrochemical element according to the prior art is used, it has been difficult to realize an electrochemical element in which short circuits are unlikely to occur.

Means for Solving the Problems

[0009] A first aspect of the present invention is a separator for an electrochemical element including a fiber aggregate containing profiled cross-section fibers and composite fibers having an elongation of 30% or less, wherein in a cross-section obtained by cutting the profiled cross-section fibers in a direction perpendicular to the fiber length direction, the flatness calculated by the following method is less than 0.34. Note (1) Photograph the fiber aggregate using an electron microscope, and ensure that the cross-section of the profiled cross-section fibers to be measured appears in the electron micrograph obtained by the photographing. (2) Draw a plurality of line segments having both ends on the outer periphery of the cross-section on the electron micrograph. (3) Select the longest line segment among the drawn line segments, and let the length of the line segment be a. (4) Draw a straight line passing through the midpoint of the selected line segment and perpendicular to the line segment on the electron micrograph. (5) Draw a line segment having both ends on the outer periphery of the cross-section on the straight line, and let the length of the line segment be b. (6) Divide the length b by the length a to calculate a value V. (7) The above-mentioned steps (2) to (6) are repeated, and the value V is calculated for each of the cross sections of the 10 modified cross-section fibers shown in the electron microscope photograph. (8) The average of the calculated values ​​V is the flatness.

[0010] The second invention is "the separator for electrochemical elements according to claim 1, wherein the percentage of the mass of the modified cross-section fibers having a flatness of less than 0.34 in the mass of the fibers constituting the fiber assembly is greater than 9.3 mass%." Effect of the Invention

[0011] As a result of continued investigations, the applicant of the present application has found that it is possible to realize an electrochemical element that is less susceptible to short circuits by using a separator for an electrochemical element that satisfies the configuration of the present invention.

[0012] The separator for electrochemical elements according to the present invention "has a fiber aggregate containing composite fibers having an elongation of 30% or less," and as disclosed in Patent Document 2, the composite fibers themselves have high strength, making it difficult for short circuits caused by insufficient strength of the fiber aggregate (for example, a short circuit caused by damage to the fiber aggregate during the manufacturing process) to occur.

[0013] Furthermore, the separator for electrochemical elements according to the present invention further includes modified cross-section fibers as constituent fibers of the fiber aggregate, and by adopting modified cross-section fibers having an optimal cross-sectional shape as the modified cross-section fibers, specifically, by adopting modified cross-section fibers having a cross-sectional shape with a flatness calculated by the above-mentioned method of less than 0.34, it is possible to provide a separator for electrochemical elements that can realize an electrochemical element that is even less susceptible to short circuits.

[0014] Furthermore, the more the separator for an electrochemical element includes a fiber aggregate having a large proportion of profiled cross-section fibers with a flatness of less than 0.34, the more feasible it is to realize an electrochemical element in which a short circuit is less likely to occur. Specifically, by having the percentage of the mass of the profiled cross-section fibers with a flatness of less than 0.34 in the mass of the fibers constituting the fiber aggregate be more than 9.3% by mass, it is possible to provide a separator for an electrochemical element that can realize an electrochemical element in which a short circuit is less likely to occur.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0016] In the present invention, various configurations such as the following can be appropriately selected. Note that various measurements described in the present invention were performed under atmospheric pressure unless otherwise specified. Also, the measurements were performed under a temperature condition of 25°C. And, unless otherwise specified, various measurement results described in the present invention were measured up to a value one digit smaller than the required value, and the required value was calculated by rounding the said value. As a specific example, when the value up to the first decimal place is the required value, the value up to the second decimal place was obtained by measurement, and the value up to the first decimal place was calculated by rounding the obtained value of the second decimal place, and this value was taken as the required value. And, each upper limit value and each lower limit value exemplified in the present invention can be arbitrarily combined.

[0017] In the separator for an electrochemical element according to the present invention, the fiber aggregate plays a role of forming the main skeleton of the separator for an electrochemical element. The fiber aggregate can be, for example, a fibrous web, a non-woven fabric, or a sheet-like fabric such as a woven fabric or a knitted fabric. In particular, when the fiber aggregate is formed by randomly entangled fibers such as a fibrous web or a non-woven fabric, it is possible to realize an electrochemical element in which a short circuit is less likely to occur, and by having a uniform porosity and pore diameter, it is possible to provide a separator for an electrochemical element that can realize an electrochemical element having more excellent electrical output characteristics, which is preferable.

[0018] The present invention is characterized in that, in addition to composite fibers having an elongation of 30% or less as the fibers constituting the fiber aggregate, it includes profiled cross-section fibers.

[0019] The profiled cross-section fibers referred to here mean fibers having a cross-sectional shape (hereinafter sometimes referred to as a fiber cross-section) other than a substantially circular or elliptical shape when cut in a direction perpendicular to the fiber length direction. As specific examples, fibers having a substantially square shape such as a substantially triangular shape or a substantially trapezoidal shape can be exemplified. Although details will be described later, by subjecting a solid composite fiber having a plurality of types of resins and having the resins distributed in a cross-sectional shape as shown in FIG. 1 to a splitting process, profiled cross-section fibers having a substantially triangular shape can be obtained. Also, by subjecting a hollow composite fiber having a plurality of types of resins and having the resins distributed in a cross-sectional shape as shown in FIG. 2 to a splitting process, profiled cross-section fibers having a substantially trapezoidal shape can be obtained.

[0020] The resin constituting the profiled cross-section fiber can be appropriately selected. For example, polyolefin resins (such as polyethylene, polypropylene, polymethylpentene, polyolefin resins with a structure in which a part of hydrocarbons is substituted with a nitrile group or a halogen such as fluorine or chlorine, etc.), styrene resins, polyvinyl alcohol resins, polyether resins (such as polyether ether ketone, polyacetal, modified polyphenylene ether, aromatic polyether ketone, etc.), polyester resins (such as polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, polycarbonate, polyarylate, wholly aromatic polyester resin, etc.), polyimide resins, polyamideimide resins, polyamide resins (such as aromatic polyamide resins, aromatic polyetheramide resins, nylon resins, etc.), resins having a nitrile group (such as polyacrylonitrile, etc.), urethane resins, epoxy resins, polysulfone resins (such as polysulfone, polyether sulfone, etc.), fluorine resins (such as polytetrafluoroethylene, polyvinylidene fluoride, etc.), cellulose resins, polybenzimidazole resins, acrylic resins (such as polyacrylonitrile-based resins copolymerized with acrylic acid ester or methacrylic acid ester, modacrylic resins copolymerized with acrylonitrile and vinyl chloride or vinylidene chloride, etc.), etc. can be used to be constituted. Note that these resins may be composed of either linear polymers or branched polymers, and the resin may be a block copolymer or a random copolymer, and the three-dimensional structure or the presence or absence of crystallinity of the resin is not particularly limited. Furthermore, a mixture of multi-component resins may also be used.

[0021] The profiled cross-section fiber may be composed of two types of resins or may be composed of three or more types of resins.

[0022] The fiber length of the profiled cross-section fiber can be appropriately selected. However, due to the uniform and dense aperture diameter, it is possible to realize an electrochemical element in which short circuits are unlikely to occur. Also, due to the uniformity of the porosity and aperture diameter, it is possible to realize a separator for an electrochemical element that can realize an electrochemical element with more excellent electrical output characteristics. Therefore, the fiber length is preferably 20 mm or less, more preferably 15 mm or less, and still more preferably 10 mm or less. On the other hand, the lower limit value of the fiber length can be appropriately selected, but it is practical that it is 0.5 mm or more. Note that the "fiber length" refers to the value measured in accordance with JIS L1015 (2010), 8.4.1 c) direct method (method C).

[0023] The average fiber diameter of the profiled cross-section fiber can be appropriately selected. However, in order to realize a separator for an electrochemical element with a uniform and dense aperture diameter, the average fiber diameter is preferably 10 μm or less, more preferably 5 μm or less, still more preferably 4.5 μm or less, and even more preferably 4 μm or less. On the other hand, the lower limit value of the average fiber diameter can be appropriately selected, but it is practical that it is 1 μm or more, and it is more practical that it is 2 μm or more.

[0024] Note that the "average fiber diameter" as used in the present invention refers to the arithmetic average value of the diameters of the outer shapes of the cross-sections of 50 fibers to be measured, which appear in an electron micrograph at 5000 times magnification of the cross-section or surface of the fiber aggregate. When the outer shape is non-circular, the diameter of a circle having the same area as the area that entirely fills the inside of the outer shape is regarded as the diameter of the outer shape of the fiber cross-section. When the fiber is too thin to be measured, the measurement can be based on an electron micrograph at a higher magnification.

[0025] The profiled cross-section fiber according to the present invention is characterized in that the flatness of the cross-section cut in a direction perpendicular to the fiber length direction is less than 0.34. The applicant of the present application has found that by adopting a profiled cross-section fiber having a fiber cross-section with a flatness of less than 0.34, it is possible to provide a separator for an electrochemical element in which a short circuit is unlikely to occur.

[0026] The reason why this effect is exerted has not been fully clarified, but it is considered that this is because the following effects are exerted.

[0027] The cross-sectional shape of the profiled fiber according to the present invention has a flatness of less than 0.34, and the cross-sectional shape has a flat shape in which the ratio of the lengths in the longitudinal direction and the transverse direction is extremely biased. Therefore, in the fiber aggregate according to the present invention, each profiled fiber is likely to be deposited in a state where its long side direction (the direction of the line segment having the length a described later) is parallel to the main surface of the fiber aggregate. As a result, in the separator for an electrochemical element including the fiber aggregate according to the present invention, even when the porosity is high and there may be voids linearly connecting between both main surfaces, it is considered that the voids are covered by the above-described portion having the long side direction of the profiled fiber according to the present invention (the portion having a large area in the profiled fiber). Therefore, it is considered that the separator for an electrochemical element according to the present invention has a structure in which burrs and other foreign matters are difficult to penetrate in addition to needle-like foreign matters such as dendrites present on the electrode.

[0028] The flatness of the profiled fiber can be calculated by subjecting it to the following measurement. (Calculation method of flatness of profiled fiber) (1) Photograph the surface and / or cross-section of the fiber aggregate using an electron microscope, or the constituent fibers of the fiber aggregate extracted from the fiber aggregate. In addition, by adjusting the photographing so that the fiber cross-section of the fiber constituting the fiber aggregate appears in the electron micrograph obtained by the photographing, the fiber cross-section of the profiled fiber to be measured can be confirmed. (2) Draw a plurality of line segments having both ends on the outer periphery of the cross-section in the electron micrograph. (3) Select the longest line segment among the drawn line segments, and let the length of the line segment be a. (4) Draw a straight line passing through the midpoint of the selected line segment and perpendicular to the line segment in the electron micrograph. (5) Draw a line segment having both ends on the outer periphery of the cross-section on the straight line, and let the length of the line segment be b. (6) Divide the length b by the length a to calculate the value V. (7) Repeat the above steps (1) to (6), and calculate the value V for each of the cross-sections of the 10 deformed cross-section fibers shown in the electron micrograph. (8) Take the average value of the calculated values V as the flatness.

[0029] If the flatness of the deformed cross-section fibers contained in the fiber aggregate is less than 0.34, it can be adjusted as appropriate. However, in order to provide a separator for an electrochemical element that can realize an electrochemical element in which short circuits are less likely to occur, the flatness is preferably 0.30 or less, more preferably 0.25 or less, and even more preferably 0.21 or less. On the other hand, the lower limit value of the flatness is selected as appropriate. However, the flatness of the deformed cross-section fibers contained in the fiber aggregate is greater than 0, and it is realistic to be 0.10 or more, and more realistic to be 0.15 or more.

[0030] When the manufacturing process of the fiber aggregate is known, measure the flatness of the deformed cross-section fibers (or the part that becomes the deformed cross-section fibers, which constitutes the split-type fibers capable of generating the deformed cross-section fibers) used in the manufacturing process of the fiber aggregate. If the flatness is satisfied, the fiber aggregate prepared using the deformed cross-section fibers (or the split-type fibers capable of generating the deformed cross-section fibers) can be regarded as satisfying the configuration of the present invention.

[0031] The fiber aggregate according to the present invention contains composite fibers having an elongation of 30% or less.

[0032] The resin constituting the composite fiber can be appropriately selected, but the resins exemplified as being capable of constituting the above-described deformed cross-section fibers can be adopted. Note that these resins may be composed of either linear polymers or branched polymers, and the resin may be a block copolymer or a random copolymer. Moreover, the three-dimensional structure and the presence or absence of crystallinity of the resin are not particularly limited. Furthermore, a mixture of multi-component resins may also be used.

[0033] The composite fiber is composed of multiple types of resins and is generally referred to as a composite fiber. For example, it can be in the form of a core-sheath type, a sea-island type, a side-by-side type, an orange type, a bimetal type, etc.

[0034] When the composite fiber is a heat-sealable fiber, the strength and morphological stability can be improved by heat-sealing the constituent fibers to each other. It may be a fully heat-sealed type of heat-sealable fiber or a partially heat-sealed type of heat-sealable fiber. As the component (resin) that exhibits heat-sealing properties, for example, heat-sealable fibers containing a low-melting-point polyolefin resin or a low-melting-point polyester resin can be appropriately selected and used.

[0035] The fiber length of the composite fiber can be appropriately selected, but in order to realize a separator for an electrochemical element with a uniform and dense pore diameter, the fiber length is preferably 20 mm or less, more preferably 15 mm or less, and still more preferably 10 mm or less. On the other hand, the lower limit value of the fiber length can be appropriately selected, but it is realistically 0.5 mm or more. The average fiber diameter of the composite fiber can be appropriately selected, but it is preferably 5 to 32 μm, and more preferably 8 to 17 μm.

[0036] The composite fiber employed in the present invention is characterized in that its elongation is 30% or less. The elongation referred to in the present invention is determined by performing a tensile test using a tensile tester with a grip interval of 20 mm for the sample in accordance with JIS L1015 "Test Method for Chemical Fiber Staple", and the elongation rate (%) at the time of breakage is taken as the elongation of the fiber. The elongation of the composite fiber can be appropriately adjusted, but it can be 0% or more and 30% or less, 25% or less, 20% or less, 15% or less, 10% or less, or 5% or less.

[0037] In addition to the profiled cross-section fibers and composite fibers with an elongation of 30% or less, the fiber assembly may contain, as other fibers, ultra-fine fibers with a finer average fiber diameter. The average fiber diameter of such ultra-fine fibers is preferably 2 μm or less, and the lower limit of the average fiber diameter is not particularly limited, but can be 0.001 μm or more, can be 0.01 μm or more, and can be 0.1 μm or more. The form of the other fibers can be appropriately selected and adopted from the resin compositions and fiber structures described above.

[0038] The constituent fibers of the fiber assembly as described above can be obtained by known methods such as, for example, the melt spinning method, dry spinning method, wet spinning method, direct spinning method (melt blowing method, spunbond method, electrospinning method, etc.), a method of extracting fibers with a fine fiber diameter by removing one or more resin components from composite fibers, a method of obtaining fibers by beating and splitting fibers.

[0039] Fiber webs and non-woven fabrics can be prepared, for example, by a dry method of entangling fibers by supplying the above-described constituent fibers to a carding device or an air laying device, a wet method of dispersing fibers in a dispersion medium and forming a sheet to entangle the fibers, a direct spinning method (melt blowing method, spunbond method, electrospinning method, a method of spinning by discharging a spinning dope and a gas stream in parallel (for example, the method disclosed in JP-A-2009-287138)), etc., and collecting the spun fibers.

[0040] The constituent fibers of the prepared fiber web can be entangled and / or integrated to prepare a nonwoven fabric. As methods for entangling and / or integrating the constituent fibers with each other, for example, methods of entangling by needles or water flow, methods of adhesively integrating or melt-integrating the constituent fibers with each other by an adhesive or adhesive fibers such as by subjecting the fiber web to a heat treatment can be mentioned. Note that the method of heat treatment can be appropriately selected. For example, methods of heating or heating and pressing by rolls, methods of heating by supplying to a heating machine such as an oven dryer, a far-infrared heater, a hot air dryer, a heat pump dryer, methods of irradiating infrared rays under no pressure to heat the contained resin, etc. can be used. However, it is preferable that the fiber aggregate is one in which the constituent fibers are adhered to each other by the composite fiber according to the present invention as the adhesive fiber so as to prevent the apertures of the fiber aggregate from being unintentionally blocked by the adhesive. Further, it is preferable that the constituent fibers are adhered by the composite fiber according to the present invention, so that a separator for an electrochemical element with improved strength and difficult to short-circuit can be provided.

[0041] When the fabric is a woven fabric or a knitted fabric, the woven fabric or the knitted fabric can be prepared by weaving or knitting the fibers prepared as described above.

[0042] In addition to the fiber web, nonwoven fabrics or fabrics such as woven fabrics and knitted fabrics may be subjected to the method of entangling and / or integrating the above-described constituent fibers with each other.

[0043] The percentage by mass (unit: mass%) of the mass of the profiled cross-section fibers in the mass of the fibers constituting the fiber aggregate can be appropriately adjusted so as to be more than 0 mass%, but in order to provide a separator for an electrochemical element capable of realizing an electrochemical element in which short-circuiting hardly occurs, it is preferably 5 to 70 mass%, preferably 10 to 60 mass%, and preferably 20 to 50 mass%.

[0044] In particular, an electrochemical element separator including a fiber aggregate having a large proportion of profiled cross-section fibers with a flatness of less than 0.34 makes it possible to realize an electrochemical element in which a short circuit is less likely to occur. Therefore, the percentage of the mass of the profiled cross-section fibers with a flatness of less than 0.34 in the mass of the fibers constituting the fiber aggregate is preferably 9.3% by mass or more, more preferably more than 9.3% by mass, preferably 10% by mass or more, and preferably 11% by mass or more. The upper limit is appropriately adjusted but less than 100% by mass, and it is realistic to be 90% by mass or less, can be 50% by mass or less, and can be 40% by mass or less.

[0045] Incidentally, the method for calculating the percentage of the mass of the profiled cross-section fibers with a flatness of less than 0.34 can be calculated using the method described below. (Method for calculating the percentage of the mass of the profiled cross-section fibers with a flatness of less than 0.34) (1) Randomly extract 100 fibers from the measurement object such as the fiber aggregate constituting the separator for the electrochemical element, and measure the total mass Y thereof. (2) Measure the mass X of the profiled cross-section fibers with a flatness of less than 0.34 among the extracted fibers. Whether the extracted fiber is the profiled cross-section fiber can be confirmed by the method described in (Method for calculating the flatness of the profiled cross-section fibers) above. (3) Substitute the mass Y and the mass X obtained as described above into the following formula, and the calculated value is taken as the percentage of the mass of the profiled cross-section fibers with a flatness of less than 0.34 (unit: %). Percentage of the mass of the profiled cross-section fibers with a flatness of less than 0.34 (unit: mass %) = 100 × (X / Y)

[0046] Also, the percentage of the mass of the composite fibers with an elongation of 30% or less (unit: mass %) in the mass of the fibers constituting the fiber aggregate can be appropriately adjusted to be more than 0% by mass. However, in order to provide a separator for an electrochemical element that can realize an electrochemical element in which a short circuit is less likely to occur, it is preferably 95 to 30% by mass, preferably 90 to 40% by mass, and preferably 80 to 50% by mass.

[0047] The calculation method for the percentage of the mass of composite fibers with an elongation of 30% or less can be calculated using the method described below. (Calculation method for the percentage of the mass of composite fibers with an elongation of 30% or less) (1) Randomly extract 100 fibers from the measurement object such as the fiber assembly constituting the separator for the electrochemical element, and measure the total mass Y. (2) Measure the elongation of each of the extracted fibers, and measure the mass X of the composite fibers with an elongation of 30% or less. Whether the extracted fiber is a composite fiber or not can be determined by visually checking an electron micrograph of the fiber cross-section of the fiber to be measured. At this time, together with visual confirmation, it may be determined by combining the results of subjecting the fiber to be measured to various analyzers such as an infrared spectroscopic analyzer (FT-IR), an elemental analyzer, and a nuclear magnetic resonance apparatus (NMR). (3) Substitute the mass Y and the mass X obtained as described above into the following formula, and use the calculated value as the percentage (unit: %) of the mass of the composite fibers with an elongation of 30% or less. Percentage of the mass of composite fibers with an elongation of 30% or less (unit: mass %) = 100 × (X / Y)

[0048] In addition, when the manufacturing process of the fiber assembly or the separator for the electrochemical element is known, based on the specific gravity and blending ratio of the resin constituting each fiber blended in the manufacturing process (for example, split-type fibers that can generate profiled cross-section fibers with a flatness of less than 0.34 by subjecting them to a splitting process, or composite fibers with an elongation of 30% or less) and the mass ratio of each fiber blended, the percentage of the mass of profiled cross-section fibers with a flatness of less than 0.34 and the percentage of the mass of composite fibers with an elongation of 30% or less in the mass of the fibers constituting the fiber assembly may be calculated.

[0049] The basis weight of the fiber assembly is appropriately selected, but the lower limit can be 1 g / m 2 or more, and can be 2 g / m 2 or more, and can be 3 g / m 2 or more. On the other hand, the upper limit can be 80 g / m 2 or less, and can be 70 g / m 2It can be as follows, 65 g / m 2 It can be as follows, 55 g / m 2 It can be as follows, 40 g / m 2 It can be as follows, 30 g / m 2 It can be as follows.

[0050] The thickness of the fiber assembly is appropriately selected. However, in order to easily prevent short circuits due to burrs and other foreign matters in addition to needle-like foreign matters such as dendrites existing on the electrodes, it preferably has a certain thickness. More specifically, it is preferably 160 μm or more, more preferably 170 μm or more, and even more preferably 180 μm or more. On the other hand, in order to realize a separator for an electrochemical element with low electrical resistance and fabricate a high-capacity battery, it is preferably 250 μm or less.

[0051] In addition, the "thickness" in the present invention refers to the arithmetic mean value obtained by measuring at 5 N load using an external micrometer (0 to 25 mm) specified in JIS B7502:1994 for 10 randomly selected points.

[0052] The above-described fiber assembly may be used as it is as a separator for an electrochemical element, but it may be subjected to secondary processes such as carrying inorganic particles such as silica and alumina or a binder on the surface, smoothing the surface, and adjusting the thickness, and then used as a separator for an electrochemical element. Further, in order to improve the retention of the electrolytic solution, it may be subjected to secondary processes such as a hydrophilization treatment process. Examples of this hydrophilization treatment process include sulfonation treatment, fluorine gas treatment, graft polymerization treatment of vinyl monomers, surfactant treatment, discharge treatment, or hydrophilic resin imparting treatment.

[0053] Various configurations of the separator for an electrochemical element, such as basis weight, thickness, and porosity, are appropriately adjusted so as to be able to realize an electrochemical element having excellent electrical output characteristics such as excellent high-rate discharge. The lower limit value of the basis weight can be 5 g / m 2 or more, can be 8 g / m 2 or more, can be 10 g / m 2It can be the above. On the other hand, the upper limit value can be 80 g / m 2 or less, and can be 70 g / m 2 or less, and can be 65 g / m 2 or less, and can be 55 g / m 2 or less, and can be 45 g / m 2 or less, and can be 40 g / m 2 or less.

[0054] Also, the thickness is preferably 160 μm or more, more preferably 170 μm or more, and even more preferably 180 μm or more. On the other hand, in order to realize a separator for an electrochemical element with low electrical resistance and fabricate a high-capacity battery, it is preferably 250 μm or less.

[0055] Also, the porosity can be 20% or more, can be 30% or more, and can be 40% or more.

[0056] Note that the "porosity (P)" (unit: %) referred to in the present invention means a value obtained from the following formula. P = 100 - (Fr 1 + Fr 2 + ··· + Fr n ) Here, Fr n represents the filling rate (unit: %) of various components n constituting the separator for the electrochemical element, and means a value obtained from the following formula. Fr n = {(M × Pr n ) / (T × SG n )} × 100 Here, M is the basis weight (unit: g / cm 2 ) of the separator for the electrochemical element, T is the thickness (unit: cm) of the separator for the electrochemical element, Pr n is the mass ratio of the presence of various components n of the separator for the electrochemical element, and SG n is the specific gravity (unit: g / cm 3 ) of various components n, respectively.

[0057] The separator for an electrochemical element prepared as described above may be subjected to various secondary processes, such as punching the shape according to the shape of the electrochemical element to be used, or processing it to obtain a wound shape.

[0058] Next, a manufacturing example of the method for manufacturing a separator for an electrochemical element according to the present invention will be given and described. Note that the description of the same items and configurations as those described above will be omitted.

[0059] The method for manufacturing a separator for an electrochemical element can be appropriately selected. As an example, (1) A step of preparing a composite fiber having an elongation of 30% or less and a split-type fiber capable of generating a deformed cross-section fiber having a flatness of less than 0.34 by subjecting it to a splitting process, (2) A step of preparing a papermaking liquid obtained by mixing the prepared fibers into a dispersion medium, (3) A step of lifting the papermaking liquid to prepare a wet fiber web, (4) A step of subjecting the wet fiber web to a water flow complexing process to split the split-type fiber, which is a constituent fiber of the wet fiber web, to generate a deformed cross-section fiber having a flatness of less than 0.34, (5) A step of removing the dispersion medium of the papermaking liquid from the wet fiber web after being subjected to the water flow complexing process, The manufacturing method of the separator for an electrochemical element including these steps can be used.

[0060] In step (1), if deformed cross-section fibers having a flatness of less than 0.34 can be generated, the type of split-type fiber to be used can be appropriately selected.

[0061] As the split-type fiber, a split-type fiber having a solid cross-sectional shape made of a plurality of types of resins can be adopted. For example, when a split-type fiber having an orange-type cross-sectional shape made of two types of resins (resin A and resin B) as shown in FIG. 1 is adopted, by splitting, deformed cross-section fibers having a cross-sectional shape substantially triangular made of resin A and deformed cross-section fibers having a cross-sectional shape substantially triangular made of resin B can be obtained.

[0062] In addition, as the split fiber, a split fiber having a hollow portion made of a plurality of types of resins can be adopted. For example, when a split fiber having an orange-shaped cross-sectional shape with a hollow portion made of two types of resins (resin A and resin B) as shown in FIG. 2 is adopted, by splitting, a deformed cross-sectional fiber having a substantially trapezoidal cross-sectional shape made of resin A and a deformed cross-sectional fiber having a substantially trapezoidal cross-sectional shape made of resin B can be obtained.

[0063] Note that as the split fiber, a split fiber having a cross-sectional shape of a multi-bimetal type made of a plurality of types of resins can also be adopted, and by splitting the split fiber, a deformed cross-sectional fiber having a substantially rectangular shape can be obtained. However, the deformed cross-sectional fiber obtained from the split fiber has a cross-sectional shape in which the opposing long sides are parallel or substantially parallel compared to the deformed cross-sectional fiber having a substantially triangular shape or a substantially trapezoidal shape. As a result, the deformed cross-sectional fibers constituting the fiber aggregate tend to form a state in which the deformed cross-sectional fibers are densely deposited with the long sides facing each other. As a result, there is a risk that the separator for an electrochemical element has poor ion permeability because it lacks even the minimum voids required for ions to pass as intended.

[0064] Therefore, it is preferable to adopt a split fiber having an orange-shaped cross-sectional shape made of a plurality of types of resins or a split fiber having an orange-shaped cross-sectional shape with a hollow portion as the split fiber.

[0065] The number of deformed cross-sectional fibers obtained by splitting the split fiber, in other words, the number of splits of the split fiber can be appropriately selected as long as it can generate a deformed cross-sectional fiber having a flatness of less than 0.34. In order to easily obtain a deformed cross-sectional fiber having a flatness of less than 0.34, the number of splits of the split fiber is preferably 16 or more, the number of splits of the split fiber is preferably 18 or more, the number of splits of the split fiber is preferably 20 or more, the number of splits of the split fiber is preferably 22 or more, and the number of splits of the split fiber is preferably 24 or more.

[0066] In addition, the ratio of the areas of the plurality of types of resins in the fiber cross-section of the segmented fiber can be appropriately selected as long as it can generate a deformed cross-section fiber with a flatness of less than 0.34. For example, as shown in FIGS. 1 to 2, when two types of resins, resin A (shown in white sandwiched by black in the figure) and resin B (shown in black in the figure), alternately exist in the fiber cross-section of the segmented fiber, the ratio of the area of one resin A to the area of one resin B can be adjusted as appropriate. As a specific example, it can be a 1:1 mode (the mode in FIG. 1), or a 6:4 mode (not shown) or a 1:3 mode (the mode in FIG. 2).

[0067] The fineness and fiber length of the segmented fiber, which can be used for manufacturing the separator for the electrochemical element of the present invention, can be appropriately adjusted. The fineness can be 0.1 to 4 dtex, can be 0.5 to 3.3 dtex, and can be 0.8 to 2.2 dtex. Also, the fiber length is preferably 20 mm or less, preferably 15 mm or less, and preferably 10 mm or less. On the other hand, the lower limit value of the fiber length is appropriately selected, but it is practical to be 0.5 mm or more.

[0068] The types of each resin (for example, resin A and resin B) constituting the segmented fiber can be appropriately selected, but it is preferably a combination of resins with different resin types so that they can be easily separated. As a specific example, in the case of a segmented fiber composed of two types of resins, it can be a combination of polypropylene and polymethylpentene, a combination of polypropylene and polyethylene, a combination of polypropylene and ethylene-vinyl alcohol copolymer, etc.

[0069] In steps (2) and (3), the dispersion medium of the papermaking liquid can be appropriately selected, and a dispersion medium containing a dispersant and / or an activator can be used. When lifting the papermaking liquid, an unnecessary dispersion medium may be removed by suction using a suction device. In this example, the ratio of the split-type fibers and the composite fibers with an elongation of 30% or less blended in the papermaking liquid can be appropriately adjusted, but the blending ratio can be 20% by mass: 80% by mass to 80% by mass: 20% by mass, can be 30% by mass: 70% by mass to 70% by mass: 30% by mass, and can be 40% by mass: 60% by mass to 60% by mass: 40% by mass. In addition to the split-type fibers, other fibers (for example, ultra-fine fibers with a small fineness), inorganic particles, a binder, etc. may be blended in the papermaking liquid.

[0070] In step (4), the strength of the water flow acting on the wet fiber web in the water flow complexing treatment can be appropriately selected as long as it can split the split-type fibers and generate deformed cross-section fibers with a flatness of less than 0.34. Although not particularly limited, for example, a water flow with a pressure of 1 MPa to 30 MPa may be ejected from a nozzle plate with nozzles arranged in one row or two or more rows with a diameter of 0.05 to 0.3 mm and a pitch of 0.2 to 3 mm onto the wet fiber web.

[0071] In this step, the water flow complexing treatment may be performed only on one main surface of the wet fiber web, or may be performed on both main surfaces of the wet fiber web. Also, the number of times of the water flow complexing treatment may be once or multiple times.

[0072] In order to facilitate splitting of the split-type fibers in the water flow complexing treatment, it is preferable to fiber-bond the constituent fibers including the split-type fibers with the adhesive component of the adhesive fibers before the water flow complexing treatment and then supply it to the water flow complexing treatment.

[0073] In step (5), the method for removing the dispersion medium is appropriately selected. For example, methods such as removing the dispersion medium by suction or blowing it away, removing the dispersion medium by subjecting it to a heating machine, removing the dispersion medium by leaving it in a room temperature environment or a reduced pressure environment, and removing the dispersion medium by absorbing the dispersion medium with a water-absorbent fabric such as felt can be used. In addition, in this step, by subjecting it to a heat treatment, composite fibers with an elongation of 30% or less or fibers may be adhered to each other with a binder.

[0074] The heat treatment method can be appropriately selected, and the methods described above can be used. The heating temperature in this step can be appropriately adjusted, but it can be 80°C or higher. In addition, the upper limit value of the heating temperature can be appropriately adjusted, but it is preferably 150°C or lower so as to prevent the constituent fibers from melting or softening unintentionally.

[0075] An electrochemical element can be prepared using the separator for an electrochemical element thus prepared as it is. However, if necessary, inorganic particles such as silica and alumina can be supported on the surface, or another member can be provided as a separate reinforcing layer to prepare a separator for an electrochemical element.

[0076] In addition, in order to impart or improve the retention of the electrolyte solution, it may be subjected to a hydrophilization treatment step. Examples of this hydrophilization treatment step include sulfonation treatment, fluorine gas treatment, graft polymerization treatment of vinyl monomers, surfactant treatment, discharge treatment, or hydrophilizing resin imparting treatment.

[0077] Furthermore, the separator for an electrochemical element may be prepared by subjecting it to various secondary processes such as punching it according to the shape of the electrochemical element to be used or processing it so as to obtain a wound shape.

Examples

[0078] Hereinafter, the present invention will be specifically described by way of examples, but these do not limit the scope of the present invention.

[0079] The "average diameter of the hollow part" in the split fiber refers to the arithmetic mean value of the diameters of the hollow parts existing in each cross-section of the split fiber among 50 fibers to be measured that appear in an electron micrograph at 5000 times magnification of the cross-section or surface of the fiber aggregate. When the hollow part is non-circular, the diameter of a circle with the same area as the area filled by the hollow part is regarded as the diameter of the hollow part. When the fiber is too thin to be measured, the measurement can be based on an electron micrograph at a higher magnification.

[0080] (Preparation of Split Fibers) Split fibers 1 to 3 with the following configuration were prepared, which have a substantially circular hollow part at the center and an orange-shaped cross-sectional shape in which polypropylene (hereinafter sometimes abbreviated as PP, melting point: 168°C, specific gravity: 0.91) and polymethylpentene (hereinafter sometimes abbreviated as PMP, melting point: 235°C, specific gravity: 0.83) alternately exist. Split Fiber 1 · Fiber length: 5 mm · Average fiber diameter: 15 μm (fineness: 1.7 dtex) · Ratio of the area of one PP part to the area of one PMP part in the fiber cross-section: 6 to 4 · Number of splits: 16 splits · Average diameter of the hollow part: 4 μm · Anisotropic cross-section fibers produced by splitting: PP anisotropic cross-section fibers with a flatness of 0.42 and PMP anisotropic cross-section fibers with a flatness of 0.34. Split Fiber 2 · Fiber length: 5 mm · Average fiber diameter: 15 μm (fineness: 1.7 dtex) · Ratio of the area of one PP part to the area of one PMP part in the fiber cross-section: 3 to 1 · Number of splits: 16 splits · Average diameter of the hollow part: 4 μm · Anisotropic cross-section fibers produced by splitting: PP anisotropic cross-section fibers with a flatness of 0.51 and PMP anisotropic cross-section fibers with a flatness of 0.21. Split Fiber 3 · Fiber length: 5 mm · Average fiber diameter: 15 μm (fineness: 1.7 dtex) - The ratio of the area of ​​one PP part to the area of ​​one PMP part in the fiber cross section: 1:1; ·Number of divisions: 24 divisions, - Average diameter of hollow part: 2μm, - Non-circular fibers produced by splitting: PP non-circular fiber with a flatness of 0.18, PMP non-circular fiber with a flatness of 0.18.

[0081] In addition, when the hollow composite split type fiber having an orange-shaped cross section (composite ratio: 5 / 5 (volume ratio), hollowness: 12%, number of divisions: 16 divisions, fineness: 1.1 dtex) used in the examples of Patent Document 1 is divided, a substantially trapezoidal irregular cross section fiber having an arithmetic flatness of 0.40 is produced.

[0082] (Preparation of composite fibers with elongation of 30% or less) A core-sheath type adhesive fiber (fiber diameter: 10 μm, fiber length: 5 mm, Young's modulus: 45 cN / dtex, tensile strength: 6.5 cN / dtex, heat shrinkage rate: 12%, elongation: 20%) was prepared, with a core of polypropylene (melting point: 168°C) and a sheath of high-density polyethylene (melting point: 130°C).

[0083] (Comparative Examples 1 to 2, Examples 1 to 4) Each papermaking solution was prepared by mixing splittable fibers and composite fibers with an elongation of 30% or less in a dispersion medium so as to obtain the fiber composition summarized in Table 1. Then, after papermaking using each papermaking solution, the splittable fibers were split by subjecting to a hydroentanglement treatment, and wet-laid fiber webs were prepared in which the modified cross-section fibers and composite fibers with an elongation of 30% or less were entangled. The prepared wet fiber webs were heated to 140° C. without pressure to remove the dispersion medium and melt the sheath component of the composite fiber having an elongation of 30% or less. Then, the webs were allowed to cool to produce nonwoven fabrics in which the modified cross-section fibers were fiber-bonded by the sheath component of the composite fiber having an elongation of 30% or less.

[0084] Various components and physical properties of each of the nonwoven fabrics prepared in the Comparative Examples and Examples were measured and are summarized in Table 1. Components that were not present are marked with "-" in the table.

[0085] The "maximum pore diameter (unit: μm)", "minimum pore diameter (unit: μm)", and "average pore diameter (unit: μm)" refer to the values obtained by subjecting the nonwoven fabric to be measured to the bubble point method using a porometer [Polometer, manufactured by Coulter] and performing measurements. That is, the measured maximum flow pore diameter is defined as the "maximum pore diameter", the measured minimum flow pore diameter is defined as the "minimum pore diameter", and the measured average flow pore diameter is defined as the "average pore diameter".

[0086] The "short - circuit rate (unit: %)" refers to the result of the following measurements when the nonwoven fabric to be measured is used alone as a separator for an electrochemical element. 1. First, 0.005 g of iron powder with an average particle diameter of 150 μm was uniformly applied within a circular range with a diameter of 14 mm to the central part of one main surface of a separator for an electrochemical element cut out in a 5 cm × 5 cm square shape, and a sample was prepared. Similarly, 10 samples were prepared. 2. A Ni plate was laminated on the main surface of the sample where the iron powder was applied, and another Ni plate was laminated on the other main surface of the sample. 3. While applying a pressure of 3 MPa between the two Ni plates, with the Ni plate laminated on the main surface side where the iron powder was applied to the sample as the positive electrode, a DC voltage of 0.2 kV was applied between the Ni plates for 5 seconds. 4. The above - mentioned measurement was also performed in the same manner for the remaining 9 samples. 5. Among the 10 measurements thus performed, the percentage of the number of samples in which a short - circuit occurred between the two electrodes was calculated, and this was defined as the short - circuit rate of the separator for an electrochemical element. Note that the lower the short - circuit rate, the more it means that the separator for an electrochemical element can realize an electrochemical element in which a short - circuit is less likely to occur.

[0087]

Table 1

[0088] Generally, the larger the "maximum pore diameter", "minimum pore diameter", or "average pore diameter" of a separator for an electrochemical device, the more it means that the separator for an electrochemical device has large voids. Therefore, a separator for an electrochemical device with large values of various pore diameters has large-diameter paths (voids) connecting between both main surfaces, and becomes a separator for an electrochemical device that easily allows burrs and other foreign matters to pass through in addition to needle-shaped foreign matters such as dendrites present on the electrodes. Therefore, considering based on common general knowledge, since in Example 1 compared with Comparative Example 1, and also in Example 3 compared with Comparative Example 2, any of the values of "maximum pore diameter", "minimum pore diameter", and "average pore diameter" were large, it was considered that Example 1 compared with Comparative Example 1, and also Example 3 compared with Comparative Example 2, would become separators for electrochemical devices that are prone to short circuit.

[0089] However, contrary to the assumption based on the above-mentioned common general knowledge, Example 1 compared with Comparative Example 1, and also Example 3 compared with Comparative Example 2, were separators for electrochemical devices with a low short-circuit rate and difficult to cause short circuit. It was considered that the reason for such a reversal phenomenon was that the separator for an electrochemical device contained non-circular cross-section fibers with a flatness of less than 0.34 (preferably 0.21 or less).

[0090] Also, since Example 2 also contained non-circular cross-section fibers with a flatness of less than 0.34 (flatness of 0.18), its short-circuit rate was lower than that of Comparative Example 1, and it was a separator for an electrochemical device that was difficult to cause short circuit. Also, since Example 4 also contained non-circular cross-section fibers with a flatness of less than 0.34 (flatness of 0.18), its short-circuit rate was lower than that of Comparative Example 2, and it was a separator for an electrochemical device that was difficult to cause short circuit. From the above, it was found that according to the present invention, a separator for an electrochemical device capable of realizing an electrochemical device that is difficult to cause short circuit can be provided.

[0091] In addition, from the results of comparing Example 1 and Example 2, and also from the results of comparing Example 3 and Example 4, it was found that by containing non-circular cross-section fibers with a lower flatness (preferably less than 0.21), a separator for an electrochemical device that is more difficult to cause short circuit can be provided.

[0092] Further, it has been found that by providing a fiber aggregate having a proportion of profiled cross-section fibers with a flatness of less than 0.34 of 9.3 mass% or more, a separator for an electrochemical element in which short circuits are less likely to occur can be provided. In particular, from the results of comparing Example 1 and Example 3, it has been found that by providing a fiber aggregate having a proportion of profiled cross-section fibers with a flatness of less than 0.34 greater than 9.3 mass%, a separator for an electrochemical element in which short circuits are even less likely to occur can be provided.

Industrial Applicability

[0093] The separator for an electrochemical element of the present invention can be used regardless of whether it is aqueous or non-aqueous as a separator for an electrochemical element that separates between electrodes in, for example, a primary battery (such as a lithium battery, manganese battery, magnesium battery, etc.) or a secondary battery (such as a lithium ion battery, nickel metal hydride battery, nickel cadmium battery, zinc battery, redox flow battery, etc.), a capacitor, a fuel cell, etc.

Explanation of Symbols

[0094] 10... Divided fiber A... Resin A B... Resin B

Claims

1. A separator for an electrochemical element, comprising a fiber aggregate containing a fiber having an irregular cross-section and a composite fiber having an elongation of 30% or less, wherein, in a cross-section obtained by cutting the fiber having an irregular cross-section in a direction perpendicular to the fiber length direction, the flatness calculated by the following method is less than 0.

34. A separator for an electrochemical element. Record (1) Photograph the fiber aggregate using an electron microscope. In the electron micrograph obtained by the photographing, ensure that the cross-section of the fiber having an irregular cross-section to be measured is shown. (2) Draw a plurality of line segments having both ends on the outer periphery of the cross-section in the electron micrograph. (3) Select the longest line segment among the drawn line segments, and let the length of the line segment be a. (4) Draw a straight line passing through the midpoint of the selected line segment and perpendicular to the line segment in the electron micrograph. (5) Draw a line segment having both ends on the outer periphery of the cross-section on the straight line, and let the length of the line segment be b. (6) Divide the length b by the length a to calculate the value V. (7) Repeat the steps (2) to (6) described above, and calculate the value V for each of the cross-sections of the 10 fibers having an irregular cross-section shown in the electron micrograph. (8) Take the average value of the calculated values V as the flatness.

2. The separator for an electrochemical element according to claim 1, wherein the percentage of the mass of the fiber having an irregular cross-section with a flatness of less than 0.34 in the mass of the fibers constituting the fiber aggregate is more than 9.3% by mass.

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