Wet nonwoven fabric and reinforcement sheet for solid electrolyte

Abstract

To provide a wet nonwoven fabric that has low basis weight, is a thin film, in addition to these, can suppress formation of pores in an inside of a sheet-like matter loaded with a granular matter on a wet nonwoven fabric when loading the granular matter such as a solid electrolyte on the wet nonwoven fabric by having high porosity, and moreover, has high mechanical characteristics (tensile strength) and has excellent productivity, and to provide a reinforcement sheet for a solid electrolyte.SOLUTION: A wet nonwoven fabric includes at least a staple fiber A and a staple fiber B, wherein the staple fiber A is an irregular shape cross-sectional fiber having a flatness, that is a value obtained by dividing a major axis length of a fiber cross section by a minor axis length, of 5 or more and having an average length of the minor axis of 2000 nm or shorter, the staple fiber B is a sheath core type composite staple fiber, and the wet nonwoven fabric has a content of the staple fiber A to the whole of 5-25 mass%, a basis weight of 1.0-10.0 g / m2, a thickness of 1-15 μm, and a porosity of 70-95%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a wetlaid nonwoven fabric and a reinforcing sheet for a solid electrolyte. [Background technology]

[0002] Lithium-ion secondary batteries, which have high energy density and long life, have traditionally been used as power sources for home appliances such as personal computers (PCs) and digital cameras, as well as portable electronic devices such as smartphones and tablets. In recent years, lithium-ion secondary batteries have also been installed in electric vehicles (EVs) and hybrid electric vehicles (HEVs), and as a result, not only are batteries becoming larger, but further improvements in safety and reliability are also desired.

[0003] In light of the above-mentioned background, all-solid-state batteries are being studied to improve safety and reliability by changing the electrolyte layer of lithium-ion secondary batteries from a liquid electrolyte containing a flammable organic solvent to a non-flammable solid electrolyte, thereby preventing electrolyte leakage, fire, etc. All-solid-state batteries are broadly classified into sulfide-based, oxide-based, and polymer-based batteries depending on the type of electrolyte, and active research is being conducted toward the practical application of sulfide-based all-solid-state batteries, which have excellent electrical conductivity (ionic conductivity).

[0004] Sulfide-based all-solid-state batteries are constructed by sandwiching a powdered sulfide-based electrolyte (hereinafter sometimes referred to as solid electrolyte) between different active materials for the positive electrode (+) and negative electrode (-). Therefore, one of the challenges for practical application, i.e., for larger batteries, is to give the particulate solid electrolyte self-supporting properties and flexibility and turn it into a sheet.

[0005] To address the above-mentioned challenges, namely, to provide particulate solid electrolytes with self-supporting properties and flexibility, attempts have been made to integrate solid electrolytes into woven or nonwoven sheets of inorganic or organic fibers (hereinafter referred to as "solid electrolyte reinforcing sheets"). These solid electrolyte reinforcing sheets must be low in basis weight, thin, and highly porous to ensure excellent ionic conductivity of the solid electrolyte layer. Compared to dry-laid nonwoven fabrics and long-fiber nonwoven fabrics, wet-laid nonwoven fabrics, which can be made thinner and have a lower basis weight, are preferred. Furthermore, from the perspective of productivity when forming wet-laid nonwoven fabrics and solid electrolyte layers, high mechanical properties (e.g., tensile strength) are required. Therefore, the fiber types and manufacturing methods of the wet-laid nonwoven fabrics described above are limited to specific ranges to improve mechanical properties.

[0006] Here, Patent Document 1 proposes a wet-laid nonwoven fabric made of a fiber assembly using polyolefin resin fibers, and a reinforcing sheet for a solid electrolyte. Also, Patent Document 2 proposes a wet-laid nonwoven fabric suitable for a reinforcing sheet for a solid electrolyte, and a manufacturing method thereof. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent Publication No. 2021-28869 [Patent Document 2] Japanese Patent Application Publication No. 2023-177489 Summary of the Invention [Problem to be solved by the invention]

[0008] In the method disclosed in the above-mentioned Patent Document 1, a polyolefin fiber aggregate (wet-laid nonwoven fabric) with a tensile strength per basis weight of 0.5 N / 50 mm or more is proposed to obtain a reinforcing sheet for a solid electrolyte from which solid electrolyte particles are less likely to fall off. However, this method requires a basis weight of 5 g / m 2The above disclosures only disclose thin-film / low-porosity or thick-film / high-porosity wetlaid nonwoven fabrics. Therefore, when solid electrolyte particles are supported to form a solid electrolyte layer, the ionic conductivity of the solid electrolyte layer tends to be poor because the low porosity makes it difficult to support a sufficient number of solid electrolyte particles, or because the thick film inhibits ionic conduction due to fibers present in the thickness direction.

[0009] On the other hand, the wetlaid nonwoven fabric disclosed in Patent Document 2 has a low basis weight and high porosity, allowing it to support sufficient solid electrolyte particles, but its thickness tends to make ion conduction easily inhibited by fibers present in the thickness direction. Furthermore, while the wetlaid nonwoven fabric (tensile strength in this document) has sufficient performance, its performance is ensured by its thickness (thickness in the examples is 17 to 30 μm), and improvements are needed from the perspective of achieving both high porosity and thin film thickness. Furthermore, since the wetlaid nonwoven fabric is made only of core-sheath adhesive fibers with a circular cross section, its wet web strength during production is low, and productivity also tends to be poor because the wetlaid nonwoven fabric is obtained by peeling off the auxiliary substrate after papermaking on it.

[0010] In view of the above, the present invention aims to provide a wetlaid nonwoven fabric and a reinforcing sheet for a solid electrolyte that are suitable for use as a reinforcing sheet for a solid electrolyte, imparting self-supporting properties and flexibility to the solid electrolyte, which has a low basis weight and a thin film, and in addition has a high porosity, so that when particulate matter such as a solid electrolyte is supported on the wetlaid nonwoven fabric, the formation of voids within the sheet-like material formed by supporting the particulate matter on the wetlaid nonwoven fabric can be suppressed, and further has high mechanical properties (tensile strength) and excellent productivity. [Means for solving the problem]

[0011] In order to solve the above problems, the present invention provides a wetlaid nonwoven fabric and a reinforcing sheet for a solid electrolyte, which have the following configurations: (1) A wetlaid nonwoven fabric containing at least staple fibers A and B, wherein the staple fibers A are modified cross-section fibers having a flatness of 5 or more, which is the value obtained by dividing the length of the major axis of the fiber cross section by the length of the minor axis, and an average minor axis length of 2000 nm or less, and the staple fibers B are core-sheath composite staple fibers, the content of the staple fibers A relative to the entire wetlaid nonwoven fabric is 5 to 25 mass %, and the basis weight is 1.0 to 10.0 g / m 2 a wet-laid nonwoven fabric having a thickness of 1 to 25 μm and a porosity of 70 to 95%; (2) The wetlaid nonwoven fabric according to (1), wherein the thickness of the wetlaid nonwoven fabric is 1 to 15 μm. (3) A wetlaid nonwoven fabric according to (1) or (2), in which the ratio of the tensile strength in the MD direction to the tensile strength in the TD direction of the wetlaid nonwoven fabric is 3 or more, and the tensile strength in the MD direction is 0.15 N / μm (15 mm width) or more. (4) The air permeability of the wet-laid nonwoven fabric is 300 cm 3 / cm 2 / s or more and a water retention rate of 1000 mass% or more. (5) The wetlaid nonwoven fabric according to any one of (1) to (4), wherein the staple fibers A and the staple fibers B are both polyester fibers. (6) A reinforcing sheet for a solid electrolyte, which uses the wetlaid nonwoven fabric according to any one of (1) to (5). [Effects of the Invention]

[0012] According to the present invention, there is provided a wetlaid nonwoven fabric suitable for a reinforcing sheet for a solid electrolyte that imparts self-supporting properties and flexibility to the solid electrolyte, which has a low basis weight and a thin film, and in addition has a high porosity, so that when particulate matter such as a solid electrolyte is supported on the wetlaid nonwoven fabric, the generation of voids inside the sheet-like material formed by supporting the particulate matter on the wetlaid nonwoven fabric can be suppressed, and further, the wetlaid nonwoven fabric and reinforcing sheet for a solid electrolyte are provided with high mechanical properties (tensile strength) and excellent productivity. [Brief explanation of the drawings]

[0013] [Figure 1]1 is a schematic diagram showing an example of the cross-sectional structure of short fiber A contained in the wetlaid nonwoven fabric of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] The wetlaid nonwoven fabric of the present invention is a wetlaid nonwoven fabric containing at least staple fibers A and B, wherein the staple fibers A are modified cross-section fibers having a flatness of 5 or more, which is the value obtained by dividing the length of the major axis of the fiber cross section by the length of the minor axis, and an average minor axis length of 2000 nm or less, and the staple fibers B are core-sheath type composite staple fibers, the content of the staple fibers A relative to the entire wetlaid nonwoven fabric is 5 to 25 mass %, and the basis weight is 1.0 to 10.0 g / m 2 The thickness is 1 to 25 μm and the porosity is 70 to 95%. The reinforcing sheet for a solid electrolyte of the present invention is a reinforcing sheet for a solid electrolyte that supports a solid electrolyte, and is preferably one to which the above-mentioned wetlaid nonwoven fabric is applied.

[0015] The wetlaid nonwoven fabric of the present invention contains 5 to 25 mass% of modified cross-section fibers (short fiber A) having a flatness of 5 or more (the length of the major axis of the fiber cross section divided by the length of the minor axis) and an average minor axis length of 2000 nm or less, and short core-sheath composite short fibers (short fiber B). This results in a low basis weight, thin film, high porosity, high tensile strength, and excellent productivity. Therefore, a reinforcing sheet for a solid electrolyte using the wetlaid nonwoven fabric of the present invention also has a low basis weight, thin film, high porosity, and excellent tensile strength, resulting in a solid electrolyte layer using the reinforcing sheet for a solid electrolyte of the present invention that is highly productive. Furthermore, a reinforcing sheet for a solid electrolyte using the wetlaid nonwoven fabric of the present invention has a low basis weight, thin film, high porosity, and excellent tensile strength, resulting in a solid electrolyte layer using the reinforcing sheet for a solid electrolyte of the present invention that is expected to have excellent ionic conductivity.

[0016] The present invention will be described in detail below.

[0017] <Short Fiber A> The short fibers A contained in the wetlaid nonwoven fabric of the present invention will be described below. In the present invention, the short fibers A play the role of fibers that form the framework of the wetlaid nonwoven fabric of the present invention, and have specific flatness, average minor axis length, etc. Below, they will be described in order.

[0018] (1) Flatness The staple fiber A in the wetlaid nonwoven fabric of the present invention must have a flatness of 5 or more, which is the value obtained by dividing the length of the major axis of the fiber cross section by the length of the minor axis. A flatness of 5 or more results in a higher specific surface area compared to round cross-section yarns (details will be described later), which increases the contact area between single fibers and improves wet web strength during production. This enables papermaking without the use of an auxiliary substrate, thereby improving the productivity of the wetlaid nonwoven fabric of the present invention. Furthermore, a flatness of 5 or more for staple fiber A increases the specific surface area per single fiber, i.e., per staple fiber A. This makes it possible to reduce the number of fibers constituting the wetlaid nonwoven fabric while maintaining the tensile strength of the wetlaid nonwoven fabric, thereby enabling the production of a thin wetlaid nonwoven fabric with a low basis weight. Furthermore, a flatness of 5 or more for staple fiber A results in a significant difference in bending rigidity between the minor axis and the major axis due to the shape anisotropy of the cross section, which restricts the bending direction of staple fiber A to the minor axis direction when dispersed in a liquid medium such as water. This prevents the short fibers A from being bent and intricately entangled, resulting in clumps of poor dispersion, and makes it possible to obtain a uniform wetlaid nonwoven fabric, i.e., a wetlaid nonwoven fabric having many voids and in which these voids are uniformly formed.

[0019] The above-mentioned effect becomes more pronounced as the flatness increases. For example, if the flatness is 15 or more, the bending rigidity in the minor axis direction of the cross section differs by 200 times or more from that in the major axis direction. Since the bending direction is strongly restricted to the minor axis direction, the short fibers are less likely to entangle with each other in the liquid medium even when stirred at high speed, and excellent dispersibility is exhibited.

[0020] On the other hand, due to the anisotropy of the cross-sectional shape of flat fibers, when they are made into a wetlaid nonwoven fabric, the long axis of the cross section is oriented parallel to the surface direction of the wetlaid nonwoven fabric, and therefore the porosity of the wetlaid nonwoven fabric tends to decrease as the flatness increases. From the viewpoint of achieving both the dispersibility of the staple fibers A and the porosity of the wetlaid nonwoven fabric, the upper limit of the flatness of the staple fibers A is preferably 30 or less, and more preferably 20 or less.

[0021] The flatness can be determined by dividing the major axis length (nm) by the minor axis length (nm) in the fiber cross section (see Figure 1). Figure 1 shows a schematic diagram of an example of the cross-sectional structure of staple fiber A contained in the wetlaid nonwoven fabric of the present invention. The cross section A of staple fiber A has a flat shape, and the length of the major axis is greater than the length of the minor axis.

[0022] Here, the method for measuring the major axis length and minor axis length will be described. A fiber bundle consisting of short fiber A or a wet-laid nonwoven fabric containing short fiber A is embedded in an embedding agent such as epoxy resin, and the fiber cross section is cut and scraped using a microtome equipped with a diamond knife. This cross section is then photographed at a magnification that allows the cross section to be identified using a scanning electron microscope (SEM) or the like. For the cross section of a single fiber present in the photographed image, the maximum length of the cross section is measured using image analysis software, and this value is expressed as the major axis length of a single fiber, rounded to an integer in nm. Next, the length where a line segment perpendicular to the line segment of the maximum length intersects with the fiber cross section at the midpoint of this maximum length is measured, and this value is expressed as the minor axis length of a single fiber, rounded to an integer in nm. The major axis length and minor axis length are used to calculate the flatness using the above-mentioned formula. This measurement is performed on 100 fibers to calculate the flatness of each fiber, and the arithmetic average of these values ​​is the flatness in this invention.

[0023] (2) Specific surface area As described above, the short fibers A have a high degree of flatness and therefore a high specific surface area, which increases the contact area between the single fibers and improves the wet web strength during production. Furthermore, when the wetlaid nonwoven fabric is used as a reinforcing sheet for a solid electrolyte, the contact area with the solid electrolyte particles increases, resulting in excellent adhesion between the solid electrolyte particles and the reinforcing sheet for a solid electrolyte, making it possible to suppress chipping, cracking, etc. of the solid electrolyte layer.

[0024] From the above viewpoint, the specific surface area of ​​short fiber A is 0.0010 nm -1 It is preferable that the value is equal to or greater than 0.0040 nm. -1 More preferably, it is 0.0080 nm or more. -1 It is more preferable that the specific surface area is 0.0010 nm -1 If the specific surface area is greater than this, the fiber diameter will be equivalent to that of ultrafine fibers with a diameter of several μm, and the specific surface area will be 0.0040 nm -1 If the fiber diameter is more than this, the specific surface area is equivalent to that of nanofibers with a fiber diameter of several hundred nm, and the specific surface area is 0.0080 nm -1 If so, the specific surface area will be greater than this, and therefore a reinforcing sheet for a solid electrolyte made of a wet-laid nonwoven fabric containing the short fibers A will have excellent adhesion to the solid electrolyte.

[0025] As described above, the short fiber A of the present invention has a high cross-sectional shape anisotropy (flatness), which limits the bending direction in a liquid medium from all directions of 360° to only the minor axis direction, compared to round cross-section fibers. This suppresses the occurrence of poor dispersion and results in a wetlaid nonwoven fabric with uniformly formed voids. Furthermore, the high flatness of the short fiber A also increases the specific surface area, increasing the contact area between the single fibers and improving the wet web strength during production. Therefore, a reinforcing sheet for a solid electrolyte using this wetlaid nonwoven fabric can uniformly support solid electrolyte particles and has excellent adhesion to the solid electrolyte particles.

[0026] (3) Average length of the minor axis As described above, when staple fiber A is made into a wetlaid nonwoven fabric, the cross-sectional major axis plane of staple fiber A is oriented horizontally to the surface direction of the wetlaid nonwoven fabric, and therefore the minor axis plane of staple fiber A, i.e., the average length of the minor axis, is oriented horizontally to the thickness direction. Therefore, the shorter the average length of the minor axis, the thinner the wetlaid nonwoven fabric can be. Furthermore, the shorter the average length of the minor axis, the slower the sedimentation rate in the liquid medium, and the fiber dispersion state is maintained, resulting in a wetlaid nonwoven fabric with uniformly formed voids.

[0027] From the above viewpoint, it is essential that the average short axis length of the short fibers A of the present invention is 2000 nm or less. By setting the average short axis length of the short fibers A to 2000 nm or less, not only is the short fibers A thin, but the settling rate of the short fibers A in the liquid medium is sufficiently slow under a wide range of stirring conditions, from low to high speeds, and dispersibility is maintained, making it possible to obtain a wetlaid nonwoven fabric with uniformly formed voids. Furthermore, the average short axis length of the short fibers A is preferably 1000 nm or less, more preferably 500 nm or less, and even more preferably 250 nm or less. By setting the short axis length of the short fibers A within the above range, a wetlaid nonwoven fabric with a thin film and uniformly formed voids can be obtained. Therefore, a reinforcing sheet for a solid electrolyte made of this wetlaid nonwoven fabric also becomes thin, and further, the uniformly formed voids in this wetlaid nonwoven fabric make it possible to uniformly support solid electrolyte particles, which is expected to have the effect of suppressing a decrease in ionic conductivity.

[0028] On the other hand, the lower limit of the average minor axis length of the short fibers A is preferably 20 nm or more. By setting the average minor axis length to 20 nm or more, the fibers are less likely to break when an external force is applied during the stirring step in wetlaid papermaking, and the tensile strength of the wetlaid nonwoven fabric is improved.

[0029] The average minor axis length in the present invention is determined by measuring the minor axis lengths of 100 fibers using the above-mentioned measurement method, and rounding off the arithmetic average to an integer in nm.

[0030] Furthermore, as described above, the staple fibers A of the present invention have a flat cross section in which the major axis is longer than the minor axis, which limits the bending direction of the staple fibers themselves, and thus makes it possible to set a wide range of stirring conditions, from high shear to low shear, in the stirring step of wetlaid papermaking, etc., thereby maintaining a uniformly dispersed state over a much wider range of conditions than in conventional techniques. On the other hand, because this characteristic shape makes the fibers prone to overlapping with each other, it is preferable that the staple fibers A of the present invention have a variation in the length of the minor axis.

[0031] In the present invention, the variation (CV value) is used as an index of the variation in minor axis length, and can be determined by calculating the arithmetic mean and standard deviation from the minor axis lengths of 100 staple fibers A measured by the above-mentioned method, and then dividing the standard deviation by the arithmetic mean. The variation (CV value) of the staple fibers A of the present invention is preferably 10% or more, more preferably 20% or more, and even more preferably 30% or more. By setting the variation (CV value) to 10% or more, fibers are less likely to overlap with each other, making it possible to obtain a uniform wetlaid nonwoven fabric. By setting the variation (CV value) to 20% or more or 30% or more, this effect becomes significant. On the other hand, the upper limit of the variation (CV value) is preferably 50% or less. By setting the variation (CV value) to 50% or less, the staple fibers A are less likely to break when external force is applied during a stirring process, etc.

[0032] (4) Unevenness In the staple fibers A used in the wetlaid nonwoven fabric of the present invention, the irregularities on the outer periphery of the fiber cross section can prevent overlapping and entanglement between the staple fibers A due to steric hindrance. In the present invention, the irregularity of the cross section of the staple fibers A (hereinafter sometimes referred to as "irregularity") is preferably 20% or more.

[0033] The irregularity referred to in the present invention is determined by measuring the length of each line segment perpendicular to the maximum length line segment intersecting the fiber cross section at each of 10 equal points on the maximum length of the cross section using a photographed image of the fiber cross section, calculating the arithmetic mean and standard deviation of these 10 lengths, and then dividing the standard deviation by the arithmetic mean and rounding to the nearest percent to obtain the irregularity of a single fiber. Similar measurements are performed on 10 fiber cross sections, and the arithmetic mean of the irregularities calculated for each of the 10 single fibers is defined as the irregularity referred to herein. A irregularity of 20% or more facilitates dispersion of short fibers via minute voids between fibers, enabling uniform dispersion in a short time. Achieving uniform dispersion in a short time enhances the productivity of wetlaid nonwoven fabrics.

[0034] Furthermore, by providing the short fibers with minute irregularities on their surfaces, when the wetlaid nonwoven fabric is used as a reinforcing sheet for a solid electrolyte, it has excellent adhesiveness to the solid electrolyte.

[0035] On the other hand, the upper limit of the unevenness is preferably 50% or less. By setting the upper limit of the unevenness to 50% or less, the load is concentrated on a part of the short fiber cross section, and fiber breakage can be suppressed.

[0036] (5) Fiber length When the wetlaid nonwoven fabric of the present invention is produced by wetlaid papermaking, the short fibers A of the present invention have excellent dispersibility in a liquid medium such as water, and are fibers cut to a desired length along the longitudinal direction of the fibers, with the fiber length preferably being 100 mm or less, more preferably 50 mm or less, and even more preferably 10 mm or less. On the other hand, the lower limit of the fiber length is preferably 0.5 mm or more, more preferably 1 mm or more. By setting the fiber length within the above-mentioned range, the dispersibility of the short fibers A is improved, and a wetlaid nonwoven fabric with uniformly formed voids can be obtained.

[0037] (5) Polymer Considering the intended effects of the present invention and the practical utility of the wetlaid nonwoven fabric and the reinforcing sheet for solid electrolytes, the polymer constituting the staple fibers A contained in the wetlaid nonwoven fabric of the present invention is preferably a polymer with excellent heat resistance and chemical resistance. That is, the polymer constituting the staple fibers A is preferably at least one polymer selected from the group consisting of polyester, polyamide, polyphenylene sulfide, and polyolefin. In addition to the above-mentioned advantages, these polymers are thermoplastic, which not only allows the staple fibers of the present invention to be produced by a highly productive melt spinning method, but also makes them suitable from the viewpoint of adjusting mechanical properties, such as by achieving highly oriented crystallization during the drawing process.

[0038] In particular, from the viewpoint of ensuring the dispersibility of the short fibers A, it is more preferable that the short fibers A of the present invention be made of a polymer with a high elastic modulus, such as polyester or polyphenylene sulfide, which can suppress bending of the fibers when an external force is applied and can effectively suppress the occurrence of poor dispersion due to entanglement of the fibers during the dispersion process of the short fibers A. Furthermore, by selecting a polymer having a functional group that exerts an electrical repulsive force, such as a carboxyl terminal group, such as polyester, a uniformly dispersed state can be easily achieved without the repulsive force acting between the fibers causing aggregation.

[0039] <Short Fiber B> The short fiber B contained in the wetlaid nonwoven fabric of the present invention will now be described. In the present invention, the short fiber B is a core-sheath composite short fiber. This short fiber B serves both as a binder fiber and as a fiber that forms the framework of the wetlaid nonwoven fabric, thereby improving the mechanical strength of the wetlaid nonwoven fabric and also improving the productivity of the wetlaid nonwoven fabric. The resin in the sheath, which has a low melting point, melts when exposed to thermal energy from hot air or a heat press, and acts as a binder fiber, while the resin in the core does not melt and acts as a fiber that forms the framework. Furthermore, because only the sheath melts, voids between the fibers in the wetlaid nonwoven fabric are easily maintained, resulting in a wetlaid nonwoven fabric with a high porosity.

[0040] The short fiber B, which is the core-sheath type composite short fiber described above, is not particularly limited as long as the resin constituting the sheath has a lower melting point than the resin constituting the core. Examples of the resin combination constituting the core / sheath include polypropylene / polyethylene, polyester / polyethylene, and polyester / polyester copolymer.

[0041] From the above viewpoint, it is preferable that both the staple fibers A and the staple fibers B are polyester fibers.

[0042] <Wet-processed nonwoven fabric> Next, the wetlaid nonwoven fabric of the present invention will be described. This wetlaid nonwoven fabric contains the above-mentioned staple fibers A and staple fibers B, and is produced by a papermaking process. In addition to staple fibers A and B, fibers C may also be included, such as circular cross-section fibers made of a single thermoplastic resin component, such as polyester, polyamide, polyphenylene sulfide, or polyolefin. By appropriately designing the blending ratio of staple fibers A, B, and circular cross-section fibers of the present invention, it is possible to produce a wetlaid nonwoven fabric and a reinforcing sheet for solid electrolytes that have a low basis weight, thin film, high porosity, high tensile strength, and excellent productivity. Each of these will be described in turn below.

[0043] (1) Fiber composition The wetlaid nonwoven fabric of the present invention contains 5 to 25 mass% of the above-mentioned staple fiber A and further contains staple fiber B, which is a core-sheath composite staple fiber. As described above, staple fiber A serves as the skeletal fiber of the wetlaid nonwoven fabric, and staple fiber B serves as both the binder fiber and the skeletal fiber, thereby providing the wetlaid nonwoven fabric with sufficient tensile strength and excellent productivity. Furthermore, when this wetlaid nonwoven fabric is composited with a solid electrolyte to form a reinforcing sheet for the solid electrolyte, staple fiber A has a high degree of flatness and a large specific surface area, resulting in excellent adhesion to the solid electrolyte. Furthermore, the sheath component of staple fiber B, which is a core-sheath composite staple fiber, melts during the composite process with the solid electrolyte, and the molten sheath component firmly adheres to the solid electrolyte due to the anchor effect. In other words, the above-described properties of both the staple fiber A and the core-sheath composite staple fiber B result in excellent adhesion between the reinforcing sheet for solid electrolytes and the solid electrolyte. This makes it possible to suppress chipping and cracking of the solid electrolyte in a sheet-like product in which the reinforcing sheet for solid electrolytes supports the solid electrolyte. Furthermore, by including 5 to 25 mass% of the staple fiber A, a low-basis-weight, thin-film, and highly porous wetlaid nonwoven fabric can be obtained. When used as a reinforcing sheet for solid electrolytes, the solid electrolyte layer is expected to have excellent ionic conductivity. Here, because the wetlaid nonwoven fabric of the present invention is thin, has a low basis weight, and is highly porous, it is believed that a solid electrolyte using this wetlaid nonwoven fabric as a reinforcing sheet for solid electrolytes will have a small amount of fibers that inhibit ionic conductivity in the thickness direction. As a result, the solid electrolyte layer will have excellent ionic conductivity, as described above.

[0044] In the wetlaid nonwoven fabric of the present invention, the content of staple fiber A must be 5% by mass or more, and preferably 10% by mass or more. Staple fiber A is characterized by a short average length of its minor axis, i.e., a thin thickness, and therefore the higher the content of staple fiber A, the thinner the wetlaid nonwoven fabric will be.

[0045] Furthermore, staple fiber A has a high degree of flatness and is therefore characterized by a high specific surface area, so that when it is used in combination with staple fiber B, which is a sheath-core conjugate staple fiber, the contact area with staple fiber B is higher than that of a round cross section yarn. This reduces the proportion of staple fiber B, which is a sheath-core conjugate staple fiber, exposed on the surface of the wetlaid nonwoven fabric, making it possible to prevent staple fiber B, which is a binder fiber and also a sheath-core conjugate staple fiber, from sticking to the dryer during production of the wetlaid nonwoven fabric, thereby preventing paper breakage and the like, and improving the productivity of the wetlaid nonwoven fabric of the present invention.

[0046] On the other hand, the upper limit of the content of short fiber A must be 25% by mass or less, and preferably 15% by mass or less. By setting the content of short fiber A, which has high flatness, to 25% by mass or less, a wetlaid nonwoven fabric with high porosity is obtained, and when this wetlaid nonwoven fabric is used as a reinforcing sheet for a solid electrolyte, the wetlaid nonwoven fabric becomes one in which the solid electrolyte can easily fill deep into the interior. This effect becomes more pronounced when the blending ratio of short fiber A is 15% by mass or less. Note that, since the short fiber A contained in the wetlaid nonwoven fabric of the present invention plays the role of a fiber that forms the skeleton of the wetlaid nonwoven fabric, it is preferable to use a drawn yarn that is highly oriented and has excellent mechanical properties.

[0047] The content of staple fiber B, which is a core-sheath composite staple fiber, contained in the wetlaid nonwoven fabric of the present invention is not particularly limited, but from the viewpoint of functioning as a binder fiber and increasing the mechanical strength of a thin wetlaid nonwoven fabric, it is preferably 25% by mass or more, more preferably 50% by mass or more, and even more preferably 75% by mass or more, while the upper limit is preferably 90% by mass or less, more preferably 80% by mass or less.

[0048] (2) Thickness The lower limit of the thickness of the wetlaid nonwoven fabric of the present invention must be 1 μm or more, and preferably 3 μm or more. If the lower limit of the thickness is below 1 μm, the mechanical strength (details will be described later) required for producing the wetlaid nonwoven fabric and the solid electrolyte layer tends to be insufficient, resulting in a wetlaid nonwoven fabric and a reinforcement sheet for a solid electrolyte that are prone to paper breakage and have poor productivity. By setting the thickness to 3 μm or more, productivity is further improved. On the other hand, the upper limit of the thickness must be 25 μm or less, preferably 15 μm or less, and more preferably 10 μm or less. If the upper limit of the thickness exceeds 25 μm, the resistance when used as a solid electrolyte layer tends to increase and the ionic conductivity tends to decrease. This effect is remarkable when the upper limit of the thickness is 15 μm or less or 10 μm or less.

[0049] (3)Basic weight The upper limit of the basis weight of this wet-laid nonwoven fabric is 10.0 g / m 2 It must be less than 8.0 g / m 2 Preferably, it is 5.0 g / m or less. 2 More preferably, it is 3.5 g / m or less. 2 It is more preferable that the basis weight is 10.0 g / m or less. 2 By setting the upper limit of the basis weight at 8.0 g / m, it becomes possible to reduce the weight of batteries incorporating a reinforcing sheet for solid electrolytes using this wet-laid nonwoven fabric. Furthermore, when fibers of the same specific gravity are used, the thickness also becomes thinner due to the lower basis weight. 2 Below that, an additional 5.0g / m 2 Below, especially 3.5g / m 2 This effect becomes significant when the basis weight is set to 1.0 g / m or less. 2 It must be at least 1.5g / m 2 It is preferable that the content is 3.0 g / m or more. 2 The basis weight is more preferably 1.0 g / m or more. 2 By setting the lower limit of the basis weight to 1.5 g / m or more, it is possible to maintain the mechanical strength required for the productivity of wetlaid nonwoven fabrics. 2 Above that, an additional 3.0g / m 2 By doing so, this effect becomes significant.

[0050] (4) Porosity The porosity of the wetlaid nonwoven fabric of the present invention indicates the ratio of pores that support the solid electrolyte when used as a reinforcing sheet for a solid electrolyte, and from the viewpoint of supporting an amount of solid electrolyte that provides good ionic conductivity and maintaining mechanical strength, it is essential that the porosity be 70% or more and 95% or less, and more preferably 75% or more and 90% or less. By setting the porosity within the above range, a sufficient amount of solid electrolyte can be supported, and the wetlaid nonwoven fabric will have excellent mechanical strength.

[0051] (5) Breathability The breathability of the wet-laid nonwoven fabric of the present invention is 300 cm 3 / cm 2 / s or more is preferable, and 500 cm 3 / cm 2 / s or more is more preferable. Air permeability is an index of the ease with which air passes through, and is a value that correlates with the continuous voids in the wetlaid nonwoven fabric. In other words, the higher the air permeability of the wetlaid nonwoven fabric, the more continuous voids there are, and when used as a reinforcing sheet for a solid electrolyte, the wetlaid nonwoven fabric is easily filled with a solid electrolyte. 3 / cm 2 By setting the air permeability to 500 cm / s or more, the number of continuous voids in the wetlaid nonwoven fabric increases, resulting in a wetlaid nonwoven fabric that is more easily filled with a solid electrolyte. When such a wetlaid nonwoven fabric is used as a reinforcing sheet for a solid electrolyte, the sheet-like material obtained by supporting a solid electrolyte on the reinforcing sheet for a solid electrolyte has excellent ionic conductivity. 3 / cm 2 By setting the value to / s or more, the above-mentioned effect becomes more pronounced.

[0052] (6) Water retention rate The water retention of the wetlaid nonwoven fabric of the present invention is preferably 1000% by mass or more. Water retention is an index indicating the amount of water retained. The solid electrolyte particles are sufficiently diluted with a liquid medium or the like to form a slurry, which is then applied to and impregnated into a reinforcing sheet for a solid electrolyte, which is made of a wetlaid nonwoven fabric. Therefore, the amount of solid electrolyte particles that the wetlaid nonwoven fabric can support is correlated with water retention. In other words, the higher the water retention, the more easily the nonwoven fabric can be filled with solid electrolyte when used as a reinforcing sheet for a solid electrolyte. Therefore, in the present invention, water retention, together with air permeability, can be used as an index of the amount of solid electrolyte particle loading. By achieving a water retention of 1000% by mass or more, the wetlaid nonwoven fabric can be more easily filled with solid electrolyte. When used as a reinforcing sheet for a solid electrolyte, the sheet-like material obtained by supporting the solid electrolyte on the reinforcing sheet for a solid electrolyte exhibits excellent ionic conductivity. By achieving a water retention of 2000% by mass or more, the above-mentioned effects become more pronounced.

[0053] That is, the wetlaid nonwoven fabric of the present invention is more easily filled with a solid electrolyte, and when such a wetlaid nonwoven fabric is used as a reinforcing sheet for a solid electrolyte, the sheet-like product in which the solid electrolyte is supported on the reinforcing sheet for a solid electrolyte has excellent ionic conductivity. From this viewpoint, the air permeability of the wetlaid nonwoven fabric of the present invention is set to 300 cm 3 / cm 2 / s or more, and the water retention rate is preferably 1000 mass % or more.

[0054] (7) Tensile strength The wetlaid nonwoven fabric preferably has a ratio of the tensile strength in the MD (unwinding direction) to the tensile strength in the TD (width direction) (MD / TD ratio) of 3 or more, and a tensile strength per thickness in the MD of 0.15 N / μm (15 mm width) or more. By setting the MD / TD ratio to 3 or more and the tensile strength in the MD per μm of thickness to 0.15 N or more, the wetlaid nonwoven fabric will have sufficient tensile strength even if it is a thin film. By increasing the tensile strength in the MD, where tension is applied during the production of the wetlaid nonwoven fabric and the solid electrolyte layer, paper breaks and the like are less likely to occur, resulting in excellent productivity.

[0055] The tensile strength ratio in the MD / TD directions of a wetlaid nonwoven fabric is roughly determined by the number of fibers oriented in each direction, and if the number of fibers oriented in the MD / TD directions is 1:1, the ratio of the tensile strength in the MD direction to the tensile strength in the TD direction will be roughly 1. Therefore, the wetlaid nonwoven fabric of the present invention has a configuration in which the number of fibers oriented in the MD direction is greater than that in the TD direction.

[0056] <Wet-laid nonwoven fabric manufacturing method> The method for producing the wetlaid nonwoven fabric will now be described. In the method for producing the wetlaid nonwoven fabric, staple fibers A and staple fibers B (including other fibers, dispersants, etc. as necessary) dispersed in water are made into paper using a cylinder paper machine, a short wire paper machine, or a fourdrinier paper machine.

[0057] The papermaking method used in the present invention makes it possible to produce nonwoven fabrics that combine fibers of different compositions or fibers of different diameters, and the nonwoven fabrics have little in-plane performance variation and are of excellent quality.

[0058] In addition, in order to obtain a thinner wet-laid nonwoven fabric, it is preferable to consolidate it using a calender or press. In this case, heat is applied to fuse the short fibers or other thermoplastic fibers, which is preferable because it improves the mechanical strength. [Example]

[0059] The present invention will be described in more detail below with reference to examples. The property values ​​shown in the examples were measured by the following methods.

[0060] <Short fibers> A. Flatness A fiber bundle consisting of short fiber A was embedded in an embedding medium such as epoxy resin, frozen using a Reichert FC-4E cryosectioning system, and sectioned using a Reichert-Nissei Ultracut N (ultramicrotome) equipped with a diamond knife. Images of the sectioned surface were then taken using a Hitachi H-7100FA transmission electron microscope (TEM) at a magnification sufficient to visualize the cross section. Using image analysis software (WINROOF), the maximum length of the cross section of the single fiber was measured, and this value was calculated as the long axis length of the single fiber, expressed as an integer in nm (rounded to the nearest integer). Next, the length of the line segment perpendicular to the longest line at the midpoint of the maximum length intersecting the fiber cross section was measured, and this value was calculated as the short axis length of the single fiber, expressed as an integer in nm (rounded to the nearest integer). The flatness of the single fiber was calculated using the long axis length and short axis length using the following formula: Flatness = length in the major axis direction (nm) / length in the minor axis direction (nm) The above measurement was performed on 100 fibers to calculate the flatness of each fiber, and the arithmetic average of these was rounded off to the nearest integer to calculate the flatness.

[0061] B. Specific surface area Using image analysis software (WINROOF) for the fiber cross-sectional image taken above, an arbitrary position on the outer periphery of the cross section was set as the measurement start point, and the length from the measurement start point to the outer periphery was measured using a series of images, and the length was measured until the measurement start point was reached again. This value was taken as the outer periphery of one fiber, and expressed as an integer in nm (rounded to the nearest integer). The area of ​​the inner part surrounded by this outer periphery was also measured using image analysis software, and this value was taken as the cross-sectional area of ​​one fiber, and expressed as nm 2 The specific surface area of ​​a single fiber was calculated using the perimeter and cross-sectional area, rounded to the nearest five decimal places, using the following formula: -1 )=Perimeter length (nm) / Cross-sectional area (nm 2 The above measurement was carried out for 100 fibers to calculate the specific surface area of ​​each fiber, and the arithmetic average of these was taken as the specific surface area of ​​the present invention.

[0062] C. Average length of minor axis The average minor axis length was calculated as the arithmetic mean of the lengths of the minor axes of the 100 fibers measured above, expressed as an integer in nm (rounded to the nearest integer).

[0063] D. Variation in minor axis length (CV value) The arithmetic mean and standard deviation were calculated using the short axis lengths of the 100 fibers measured above, and the coefficient of variation was obtained by dividing the standard deviation by the arithmetic mean, and the variation in the short axis lengths was calculated by rounding off the decimal point to an integer in percentage units.

[0064] E. Irregularity in the cross section of short fibers Using the image of the fiber cross section taken above, the maximum length of the cross section was divided into 10 equal parts, and the lengths at which line segments perpendicular to the maximum length intersected the fiber cross section were measured. The arithmetic mean and standard deviation of these 10 lengths were calculated, and the standard deviation was divided by the arithmetic mean and rounded to the nearest percent to calculate the irregularity of the single fiber. Similar measurements were made on the fiber cross sections of 10 single fibers, and the arithmetic mean of the irregularities calculated for each of the 10 single fibers was calculated as the irregularity.

[0065] <Short Fiber A> Component A: polyethylene terephthalate (PET) and component B: polyethylene terephthalate copolymerized with 8.0 mol% 5-sodium sulfoisophthalic acid and 9 wt% polyethylene glycol (SSIA-PEG copolymerized PET) were melted separately, and then a multilayer laminated fiber was obtained with 32 alternating layers of components A and B.

[0066] The above-mentioned multilayer laminated fiber was cut to a fiber length of 3 mm, and then immersed in a 1% by mass sodium hydroxide solution (bath ratio 1 / 100) heated to 90°C for 30 minutes to melt and remove the easily soluble polymer component B, thereby obtaining short fiber A with a flat cross section.

[0067] The flatness of short fiber A is 20, the average length of the short axis is 502 nm, the variation in the length of the short axis (CV value) is 37%, the irregularity is 25%, and the specific surface area is 0.0040 nm -1 It was.

[0068] <Short Fiber B> Short fiber B was a core-sheath composite short fiber (average fiber diameter: 1.1 dtex, fiber length: 5 mm) in which the resin constituting the core was polyethylene terephthalate (PET) and the resin constituting the sheath was isophthalic acid copolymerized PET.

[0069] <Wet-processed nonwoven fabric> A.Basic weight (1) Referenced Standards JIS P8124:1998 (2)Measurement method Measure the mass (g) and measure it over 1m. 2 Mass per unit (g / m 2 ) was converted. (3) Measurement conditions Size: 100mm x 100mm Number of items: 5

[0070] B. Thickness (1)Measurement method Measurements were taken using a digital thickness gauge SMD-565J-L (manufactured by Teclock Corporation). (2) Measurement conditions Probe: φ10mm ceramic Number of items: 5

[0071] C. Porosity (1)Measurement method Mass (g), volume (cm 3 ) and the apparent density (g / cm 3 ), and then the porosity (%) was calculated. The true density of the polyester used in the examples was 1.38 g / cm 3 was used. (2) Measurement formula Apparent density = mass (g) / volume (cm 3 ) Porosity = (1 - apparent density / true density) x 100 (%) D. Tensile strength (1) Referenced Standards JIS P8113:2006 (2)Measurement method Using an Autograph AGS-J (Shimadzu Corporation), the tensile strength in the MD direction (unwinding method) and TD direction (width direction) was measured, and then divided by the thickness (μm) to calculate the tensile strength. (3) Measurement conditions Size: 150mm x 15mm Grip spacing: 50mm Pulling speed: 20mm / min Number of rows: 5 vertically and 5 horizontally.

[0072] E. Air permeability (1) Referenced Standards JIS L1096:1999 (Fragile method) (2)Measurement method The breathability of the test specimen was measured using the "Breathability Tester FX3300 (manufactured by Textest Co., Ltd.)" The measurements were carried out. (3) Measurement conditions Size: 200mm x 200mm Pressure: 125Pa ·Measurement area: φ38mm Number of items: 5

[0073] F. Water retention rate (1)Measurement method The mass (mg) of the test specimen was measured and recorded as the pre-test mass (m1). After immersion in distilled water for 10 minutes, the specimen was removed, hung by clips at both ends, and left to stand for 1 minute. The mass (mg) was then measured and recorded as the post-test mass (m2). The water retention rate (%) was calculated using the following formula: (2) Measurement formula Water retention rate (%)=((m2-m1) / m1)×100 G. Productivity A papermaking test of approximately 300 m was carried out on each specimen, and the specimens were evaluated on a two-level scale: A and B.

[0074] A: Continuous paper making possible without paper breaks B: Paper breaks occur frequently and continuous papermaking is not possible [Example 1] 25% by mass of short fibers A and 75% by mass of short fibers B were mixed in water and made into paper using a cylinder paper machine. The thickness was 10 μm and the basis weight was 2.8 g / m. 2 The papermaking conditions were set at a papermaking speed of 30 rpm and a dryer temperature of 110°C.

[0075] [Example 2] A thickness of 11 μm and a basis weight of 2.7 g / m were obtained in the same manner as in Example 1, except that the short fibers A and B were changed to 10 mass % and 90 mass %, respectively. 2 A wet-laid nonwoven fabric of this size was obtained.

[0076] [Example 3] A thickness of 13 μm and a basis weight of 2.9 g / m were obtained in the same manner as in Example 1, except that the amount of staple fiber A was changed to 5% by mass and the amount of staple fiber B was changed to 95% by mass. 2 A wet-laid nonwoven fabric of this size was obtained.

[0077] [Example 4] The thickness and basis weight were adjusted in the same manner as in Example 2, with a thickness of 8 μm and a basis weight of 1.9 g / m 2 A wet-laid nonwoven fabric of this size was obtained.

[0078] [Example 5] After papermaking in the same manner as in Example 2, calendering was carried out at room temperature and 80 kg / cm to obtain a thickness of 9 μm and a basis weight of 2.6 g / m. 2 A wet-laid nonwoven fabric of this size was obtained.

[0079] [Example 6] The thickness and basis weight were adjusted in the same manner as in Example 2, with a thickness of 15 μm and a basis weight of 5.0 g / m 2 A wet-laid nonwoven fabric of this size was obtained.

[0080] [Example 7] The thickness and basis weight were adjusted in the same manner as in Example 2, with a thickness of 20 μm and a basis weight of 7.2 g / m 2 A wet-laid nonwoven fabric of this size was obtained.

[0081] [Comparative Example 1] A thickness of 8 μm and a basis weight of 3.2 g / m were obtained in the same manner as in Example 1, except that the short fibers A were changed to 75 mass % and the short fibers B were changed to 25 mass %. 2 A wet-laid nonwoven fabric of this size was obtained.

[0082] Comparative Example 2 A thickness of 9 μm and a basis weight of 3.0 g / m were obtained in the same manner as in Example 1, except that the short fibers A and B were changed to 50% by mass and 50% by mass, respectively. 2 A wet-laid nonwoven fabric of this size was obtained.

[0083] Comparative Example 3 A thickness of 16 μm and a basis weight of 3.3 g / m were obtained in the same manner as in Example 1, except that 100% by mass of short fiber B was used. 2 A wet-laid nonwoven fabric of this size was obtained.

[0084] The wetlaid nonwoven fabrics of Examples 1 to 5 and Comparative Examples 1 to 3 were evaluated for their respective properties using the above-mentioned measurement methods, and the results are shown in Table 1. The wetlaid nonwoven fabrics of Examples 6 and 7 were evaluated for their respective properties using the above-mentioned measurement methods, and the results are shown in Table 2.

[0085] In Example 1, the ratio of staple fiber A to staple fiber B was 25% by mass:75% by mass, in Example 2, 10% by mass:90% by mass, and in Example 3, 5% by mass:95% by mass, the basis weight was 3 g / m 2 The higher the blending ratio of short fiber A, the thinner the wetlaid nonwoven fabric. All of them had a high porosity of 80% or more, and were excellent in both breathability and water retention. Next, the basis weight was 1.9 g / m 2 Although the wetlaid nonwoven fabrics of Example 4, in which the basis weight was 5.0, 7.2, and 9.3 g / m², tended to be thinner and have lower tensile strength than those of Example 2, they had a high porosity of 80% or more and were excellent in breathability and water retention. 2 Although Examples 6 and 7, in which the thickness was thicker and the porosity and breathability tended to be lower than those of Example 2, the wetlaid nonwoven fabrics had high tensile strength.

[0086] Furthermore, in Example 5, which was subjected to calendering, the porosity tended to be lower than in Example 2, but the wetlaid nonwoven fabric was thin and had excellent tensile strength. All Examples had high tensile strength in the MD direction, with a tensile strength ratio (MD / TD) of 3 or more and a tensile strength per unit thickness of 0.15 N / μm (15 mm width). Regarding productivity, the higher the blending ratio of short fiber A, the higher the wet web strength tended to be, and all were rated A. In all Examples, the dispersibility of short fiber A in water was good.

[0087] On the other hand, in Comparative Example 1, in which the ratio of staple fiber A to staple fiber B was 75% by mass:25% by mass, and Comparative Example 2, in which the ratio was 50% by mass:50% by mass, the wetlaid nonwoven fabrics had a thickness of less than 10 μm due to the high blending ratio of staple fiber A, but had low porosity, air permeability, and water retention. Furthermore, in Comparative Example 3, which was composed only of staple fiber B, i.e., the content of staple fiber B was 100% by mass, the lack of staple fiber A meant that the wet web strength was low and there was significant sticking to the dryer, making continuous papermaking difficult, and the productivity was ranked B.

[0088] As described above, Examples 1 to 5, which were composed of staple fiber A and staple fiber B and contained 5 to 25 mass% of staple fiber A, were low in basis weight, thin, and had high porosity, resulting in high air permeability and water retention, and were wetlaid nonwoven fabrics suitable for reinforcing sheets for solid electrolytes.

[0089] The short fibers A used in the examples had a specific flatness, variation in the length of the short axis (CV value), and unevenness in the cross section, and therefore had good dispersibility without entanglement, overlapping, or breakage of the fibers during the stirring process, and all of the obtained wetlaid nonwoven fabrics were uniform.

[0090] [Table 1]

[0091] [Table 2] [Industrial Applicability]

[0092] The wetlaid nonwoven fabric of the present invention has a low basis weight, is thin, and has a high porosity, thereby providing high air permeability and water retention, as well as high tensile strength and excellent productivity. Therefore, it can be used in applications where granular materials such as solid electrolytes are to be supported, and is suitably used as a reinforcing sheet for solid electrolytes. [Explanation of symbols]

[0093] A: Short fiber

Claims

1. A wetlaid nonwoven fabric containing at least staple fibers A and staple fibers B, The short fibers A are modified cross-section fibers having a flatness of 5 or more, which is the value obtained by dividing the length of the major axis of the fiber cross section by the length of the minor axis, and an average length of the minor axis of 2000 nm or less, The short fiber B is a core-sheath type composite short fiber, The content of the short fibers A in the entire wetlaid nonwoven fabric is 5 to 25% by mass, Basis weight: 1.0 to 10.0 g / m ２ and The thickness is 1 to 25 μm, A wet-laid nonwoven fabric having a porosity of 70 to 95%.

2. 2. The wetlaid nonwoven fabric according to claim 1, wherein the thickness of the wetlaid nonwoven fabric is 1 to 15 μm.

3. 2. The wetlaid nonwoven fabric according to claim 1, wherein the ratio of the tensile strength in the MD direction to the tensile strength in the TD direction of the wetlaid nonwoven fabric is 3 or more, and the tensile strength in the MD direction is 0.15 N / μm (15 mm width) or more.

4. The wet-laid nonwoven fabric has an air permeability of 300 cm ３ / cm ２ 2. The wetlaid nonwoven fabric according to claim 1, having a water retention rate of 1000 mass % or more and a viscosity of 1000 s. / s or more.

5. The wetlaid nonwoven fabric according to claim 1, wherein the staple fibers A and the staple fibers B are both polyester fibers.

6. A reinforcing sheet for a solid electrolyte, which uses the wetlaid nonwoven fabric according to claim 1.

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