Solid electrolyte sheet, method for producing solid electrolyte sheet, and secondary battery

Abstract

Provided is a solid electrolyte sheet comprising a wet-laid nonwoven fabric containing synthetic fibers and a solid electrolyte. The wet-laid nonwoven fabric includes at least a flat first short fiber in which a ratio a / b of the length of a long axis a and the length of a short axis b of a cross section is 5 or more. In a cross section in the thickness direction of the solid electrolyte sheet, a ratio d / c of an area c of a solid part composed of fibers constituting the wet-laid nonwoven fabric and a solid electrolyte and an area d of a void part is 0 to 0.1. A ratio e / f of an area e of fibers constituting the nonwoven fabric in the solid part and an area f of the solid electrolyte f is 0.05 to 0.5.

Description

Solid electrolyte sheet, method for manufacturing solid electrolyte sheet, and secondary battery

[0001] The present invention relates to a solid electrolyte sheet, a method for manufacturing a solid electrolyte sheet, and a secondary battery.

[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), hybrid electric vehicles (HEVs), and the like, and in addition to larger batteries, further improvements in safety and reliability are desired.

[0003] In secondary batteries whose electrolyte layer is made of a liquid electrolyte, various separators are used to prevent short-circuiting between the positive and negative electrodes. Patent Document 1 describes a spunbond nonwoven fabric in which, when the length of the major axis of a cross-section of a fiber made of a polyolefin resin is a and the length of the minor axis is b, the flatness a / b of the fibers contained in a region A that is 30% from one surface of the cross-section of the spunbond nonwoven fabric in the thickness direction and the flatness a / b of the fibers contained in a region C that is 30% from the other surface are both 1.5 to 4.0, and the flatness a / b of the fibers contained in a region B between the regions A and C is 1.0 to less than 1.5. The spunbond nonwoven fabric of Patent Document 1 has excellent liquid retention and a small pore size, making it a separator with high filtering performance.

[0004] Meanwhile, all-solid-state secondary batteries are being investigated, which aim to improve safety and reliability by changing the electrolyte layer of secondary batteries from a liquid electrolyte containing a flammable organic solvent to a non-flammable solid electrolyte, thereby preventing electrolyte leakage, fire, etc. Furthermore, by changing from a liquid electrolyte to a solid electrolyte, it is possible to create a stacked structure in which the electrodes and electrolyte are directly arranged in series. As a result, a higher energy density is possible compared to secondary batteries using organic electrolytes, and applications in various electronic devices, electric vehicles, large-scale storage batteries, and more are expected.

[0005] Toward the practical application of such all-solid-state secondary batteries, active research is being conducted on all-solid-state secondary batteries and the components that constitute these batteries. Patent Document 2 describes a solid electrolyte sheet in which the thickness of the insulating porous substrate is 70% or more of the thickness of the solid electrolyte sheet. It describes that this solid electrolyte sheet has excellent shape retention, can be made large in area, and results in an all-solid-state battery with excellent discharge characteristics. Furthermore, Patent Document 3 describes a method for calculating the average fiber diameter of all fibers, L (μm), 1 m ２ Volume per unit (cm ３ ) is V, 0.25≦(L ２ The document describes a solid electrolyte sheet using a nonwoven fabric having a surface roughness (Sr / V) of 10 or less. It also describes that this solid electrolyte sheet can be made thin, and can achieve both good dusting properties of solid electrolyte particles and good electrical conductivity.

[0006] Japanese Patent Application Publication No. 2023-14436 International Publication No. 2020 / 054081 International Publication No. 2021 / 229981

[0007] In the method disclosed in the above-mentioned Patent Document 2, although the solid electrolyte can be made self-supporting and flexible, since the powdered solid electrolyte is integrated using an insulating polyvinyl acetal resin as a binder, the solid electrolyte is easily broken down by the molten resin, and there is a possibility that the ion conductivity will be poor. Furthermore, since the method involves mixing the polyvinyl acetal resin and the solid electrolyte, hot pressing, and stretching, the productivity tends to be poor.

[0008] On the other hand, the reinforcing sheet for solid electrolytes disclosed in Patent Document 3 can impart self-supporting properties to the solid electrolyte, but because it is a relatively thick sheet made of woven glass fiber, it has reduced flexibility and issues with ionic conductivity. Furthermore, because it is a woven fabric using multifilament warp and weft, steps are likely to occur where the warp and weft threads overlap, and solid electrolyte layers using this sheet tend to have poor surface smoothness in contact with the positive and negative electrodes, which also tends to result in poor ionic conductivity.

[0009] In view of the above, the present invention aims to provide a thin, self-supporting solid electrolyte sheet that has excellent ion conductivity.

[0010] In order to solve the above problems, the present invention provides a solid electrolyte sheet having the following configuration: (1) A solid electrolyte sheet having a wetlaid nonwoven fabric containing synthetic fibers and a solid electrolyte, wherein the wetlaid nonwoven fabric contains at least first flat short fibers having a ratio a / b of the lengths of the major axis a and the minor axis b of a cross section of 5 or more, and wherein in a cross section in the thickness direction of the solid electrolyte sheet, a ratio d / c of ​​an area c of a solid portion formed by the fibers constituting the wetlaid nonwoven fabric and the solid electrolyte to an area d of a void portion is 0 or more and 0.1 or less, and a ratio e / f of an area e of the fibers constituting the nonwoven fabric in the solid portion to an area f of the solid electrolyte is 0 or more and 0.05 or less and 0.5 or less. (2) The solid electrolyte sheet according to (1), wherein the solid electrolyte sheet contains at least second short fibers different from the first short fibers, and the second short fibers are core-sheath composite fibers. (3) The solid electrolyte sheet according to (2), characterized in that the ratio g / h of the number g of the first short fibers to the number h of the second short fibers included in the cross-sectional area in the thickness direction of the solid electrolyte sheet is 1 or more and 40 or less. (4) The solid electrolyte sheet according to any one of (1) to (3), characterized in that the first short fibers and the second short fibers are made of polyester fibers. (5) The solid electrolyte sheet according to any one of (1) to (4), characterized in that the solid electrolyte is a sulfide solid electrolyte. (6) The solid electrolyte sheet according to (5), characterized in that the sulfide solid electrolyte is an amorphous sulfide solid electrolyte, a crystalline sulfide solid electrolyte, or a sulfide glass ceramic solid electrolyte. (7) The solid electrolyte sheet according to (5) or (6), characterized in that the sulfide solid electrolyte contains lithium atoms, sulfur atoms, phosphorus atoms, and halogen atoms, and the halogen atoms are one or more of chlorine atoms, bromine atoms, and iodine atoms. (8) The solid electrolyte sheet according to (5) or (6), characterized in that the average particle diameter (D ５０(9) A method for producing the solid electrolyte sheet according to any one of (1) to (8). (10) A secondary battery having the solid electrolyte sheet according to any one of (1) to (8).

[0011] According to the present invention, it is possible to provide a thin, self-supporting solid electrolyte sheet having high ionic conductivity, and a secondary battery having excellent charge-discharge characteristics.

[0012] Fig. 1 is a schematic diagram of an example of the cross-sectional structure of a solid electrolyte sheet of the present invention. Fig. 2 is a schematic diagram of an example of the cross-sectional structure of a first short fiber of the present invention. Fig. 3 is a schematic diagram of an example of the cross-sectional structure of a core-sheath composite short fiber used as a second short fiber of the present invention. Fig. 4 is a cross-sectional view for explaining an example of a method for producing a solid electrolyte sheet. Fig. 5 is a schematic diagram of a cross section of an embodiment of a lithium ion battery using a solid electrolyte sheet of the present invention.

[0013] The solid electrolyte sheet of the present invention includes a wetlaid nonwoven fabric containing synthetic fibers and a solid electrolyte. The wetlaid nonwoven fabric includes at least flat first short fibers having a ratio a / b (hereinafter sometimes referred to as flatness) of 5 or greater between the length a of the major axis and the length b of the minor axis in a cross section. In a cross section in the thickness direction of the solid electrolyte sheet, a ratio d / c of ​​an area c of a solid portion formed by the fibers constituting the wetlaid nonwoven fabric and the solid electrolyte to an area d of a void portion is 0 or greater and 0.1 or less. In the solid portion, a ratio e / f of an area e of the fibers constituting the nonwoven fabric to an area f of the solid electrolyte is 0 or greater and 0.05 or less and 0.5 or less. Here, the solid portion refers to a region in the cross section of the solid electrolyte sheet where at least one of the fibers constituting the wetlaid nonwoven fabric and the solid electrolyte is present. The void portion refers to a region in the cross section of the solid electrolyte sheet where neither the fibers constituting the wetlaid nonwoven fabric nor the solid electrolyte is present. The void portion may be formed by the generation of cracks or the like. In this specification, the term "cross section" refers to a cross section perpendicular to the spinning direction of the fiber (fiber cross section).

[0014] The solid electrolyte sheet of the present invention contains at least flat first short fibers having a flatness of 5 or more, so that the solid electrolyte can be maintained in a dense state with no gaps between the solid electrolyte particles, thereby providing a solid electrolyte sheet with excellent ion conductivity. The details of the present invention will be described below in order.

[0015] <Solid Electrolyte Sheet> Fig. 1 is a schematic diagram of an example of the cross-sectional structure of a solid electrolyte sheet of the present invention. The solid electrolyte sheet 10 shown in Fig. 1 has at least a solid electrolyte 13 filled in the gaps between flat first short fibers 11 and second short fibers 12 different from the first short fibers that constitute a wetlaid nonwoven fabric. A binder may be present in the gaps between the first short fibers 11 and the second short fibers 12.

[0016] In the solid electrolyte sheet of the present invention, the wet-laid nonwoven fabric fixes the solid electrolyte. Therefore, the strength of the solid electrolyte sheet can be maintained and it can be handled as a free-standing membrane (a sheet that can maintain its shape without any other support). Furthermore, by disposing the solid electrolyte sheet between the positive electrode and the negative electrode of a secondary battery, it is possible to prevent short-circuiting between the positive electrode and the negative electrode while maintaining lithium ion conductivity between the positive electrode and the negative electrode.

[0017] (1) Ratio of Area c of Solid Portions to Area d of Void Portions In the solid electrolyte sheet of the present invention, the ratio d / c of ​​the area c of solid portions formed by the fibers constituting the wetlaid nonwoven fabric and the solid electrolyte to the area d of void portions is preferably 0 or more and 0.1 or less in order to ensure high ionic conductivity. The small number of void portions ensures a path (hereinafter sometimes referred to as an ion path) for lithium ions moving through the solid electrolyte, resulting in a solid electrolyte sheet with excellent ionic conductivity. If the ratio d / c of ​​the area c of solid portions to the area d of void portions is greater than 0.1, an ion path cannot be ensured within the solid electrolyte sheet, which can lead to the generation of lithium dendrites during charge and discharge, causing a short circuit.

[0018] (2) Ratio of Fiber Area to Solid Electrolyte Area in Solid Portion In the solid electrolyte sheet of the present invention, the ratio e / f of the area e of the fibers constituting the nonwoven fabric in the solid portion to the area f of the solid electrolyte is preferably 0.05 or more from the viewpoint of maintaining the strength of the solid electrolyte sheet. If it is less than 0.05, the amount of fibers constituting the nonwoven fabric will be too small relative to the solid electrolyte, and the solid electrolyte will not be fixed, resulting in chipping. On the other hand, from the viewpoint of maintaining high ionic conductivity, it is preferably 0.5 or less. If it is greater than 0.5, the amount of fibers constituting the nonwoven fabric will be excessive relative to the solid electrolyte, resulting in fewer ion paths in the solid electrolyte sheet and reduced ionic conductivity.

[0019] (3) Ratio of First Short Fibers to Second Short Fibers The solid electrolyte sheet of the present invention preferably contains second short fibers different from the first short fibers. The second short fibers are short fibers with a flatness of less than 5, preferably with a round cross section. The ratio of the number g of first short fibers to the number h of second short fibers contained in the cross-sectional area in the thickness direction of the solid electrolyte sheet (hereinafter sometimes referred to as the flat fiber ratio) is preferably 1 or more, more preferably 5 or more, and even more preferably 10 or more. If the ratio is less than 1, the influence of the second short fibers exceeds the influence of the flat first short fibers, making the solid electrolyte more susceptible to cracking. The flat fiber ratio is preferably 40 or less, more preferably 30 or less, and even more preferably 20 or less. If the flat fiber ratio is 40 or more, the overlap of the first short fibers increases, resulting in blockage of ion paths and reduced ionic conductivity.

[0020] (4) Thickness of Solid Electrolyte Sheet The thickness of the solid electrolyte sheet of the present invention depends on the thickness of the wet nonwoven fabric, and may be equal to or greater than the thickness of the wet nonwoven fabric. The presence of the solid electrolyte on the upper or lower surface of the wet nonwoven fabric without supporting the wet nonwoven fabric improves adhesion between the solid electrolyte sheet and the electrodes, resulting in a low-resistance battery. From the viewpoint of further reducing the battery resistance of the all-solid-state battery, the thickness of the solid electrolyte sheet is preferably 70 μm or less, more preferably 50 μm or less, and even more preferably 30 μm or less. The thickness of the solid electrolyte sheet is preferably 1 μm or more, more preferably 10 μm or more.

[0021] (5) Binder for Solid Electrolyte Sheet The solid electrolyte sheet of the present invention may contain a binder for the purpose of improving adhesion between solid electrolyte particles or between the fibers constituting the nonwoven fabric and the solid electrolyte. Examples of binders include rubber-based binders such as butadiene rubber, hydrogenated butadiene rubber, styrene butadiene rubber (SBR), hydrogenated styrene butadiene rubber, nitrile butadiene rubber, hydrogenated nitrile butadiene rubber, and ethylene propylene rubber; and fluoride-based binders such as polyvinylidene fluoride (PVDF), polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVDF-HFP), polytetrafluoroethylene, and fluororubber. The proportion of the binder in the solid electrolyte sheet is, for example, 0 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the solid electrolyte.

[0022] <Short Fibers> Next, short fibers, including flat first short fibers, constituting the wetlaid nonwoven fabric used in the solid electrolyte sheet of the present invention will be described. (1) Flatness of First Short Fibers The short fibers constituting the wetlaid nonwoven fabric in the solid electrolyte sheet of the present invention will be described. FIG. 2 is a schematic diagram of an example of the cross-sectional structure of a flat first short fiber of the present invention. The first short fiber 11 must have a ratio a / b (flatness) of 5 or more, where a is the length of the major axis of the fiber cross section and b is the length of the minor axis. First short fibers with a flatness of 5 or more have a smaller compression amount during pressing when producing a solid electrolyte sheet than round cross-section fibers, and therefore reduce springback of the fibers after pressing. This prevents cracks caused by stress on the solid electrolyte due to springback. Furthermore, by setting the flatness within the above-mentioned range, the contact area between single yarns increases compared to round cross-section fibers, tending to improve mechanical strength. This makes it possible to reduce the number of fibers constituting the wetlaid nonwoven fabric, resulting in a more sparse wetlaid nonwoven fabric and an improved filling rate of the solid electrolyte. From the above viewpoint, the flatness is more preferably 10 or more, and even more preferably 20 or more. The flatness can be calculated by dividing the length a (nm) of the major axis of the fiber cross section by the length b (nm) of the minor axis.

[0023] Here, a method for measuring the major axis length a and minor axis length b of the first staple fibers will be described. A fiber bundle consisting of flat first staple fibers or a wet-laid nonwoven fabric containing the first staple fibers 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 using a scanning electron microscope (SEM) or the like at a magnification that allows the cross section to be identified. 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 a of the single fiber, rounded to the nearest integer in nm. Next, the length at the midpoint of this maximum length where a line segment perpendicular to the line segment of the maximum length intersects with the fiber cross section is measured, and this value is expressed as the minor axis length b of the single fiber, rounded to the nearest integer in nm. Using these major axis length and minor axis length, the above-mentioned formula is used for calculation. This measurement is carried out for 100 fibers to calculate the flatness of each first short fiber, and the arithmetic average of these is defined as the flatness in the present invention.

[0024] (2) Specific Surface Area of ​​First Short Fibers As described above, the first short fibers have a high flatness and therefore a high specific surface area. Due to the high specific surface area of ​​the first short fibers, a solid electrolyte sheet having a wetlaid nonwoven fabric containing the first short fibers has an increased contact area between the first short fibers and the solid electrolyte, resulting in excellent adhesion, making it possible to suppress chipping, cracking, and the like of the solid electrolyte. From the above viewpoint, the specific surface area of ​​the first short fibers is set to 0.0010 nm －１ It is preferable that the thickness is 0.0040 nm or more. －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 equal to or greater than 0.0040 nm, the specific surface area is equivalent to that of ultrafine fibers with a fiber diameter of several μm. －１ If the specific surface area is 0.0080 nm or more, the specific surface area is equivalent to that of nanofibers with a fiber diameter of several hundred nm. －１ If the specific surface area is greater than this, a solid electrolyte sheet made of a wetlaid nonwoven fabric containing the first short fibers will have excellent adhesion to the solid electrolyte.

[0025] (3) Minor Axis Length of First Short Fibers The dispersibility of the first short fibers, which contributes to the uniformity of a wetlaid nonwoven fabric, is affected not only by the flatness of the fiber cross section but also by the cross-sectional shape of the fiber. To ensure sufficient improvement in dispersibility due to the cross-sectional shape under a wide range of stirring conditions, from low speed to high speed, the fiber diameter is an important factor. As an index of this fiber diameter, it is essential that the average minor axis length b of the first short fibers is 2,000 nm or less. The average minor axis length b here is determined by rounding off the arithmetic average of the minor axis lengths of 100 first short fibers measured when calculating the flatness in (1) above to an integer in nm.

[0026] When the average short axis length of the first short fibers is 2000 nm or less, the settling rate of the fibers in the liquid medium is sufficiently slow, and the fiber dispersion state is maintained uniformly. Based on this technical concept, the shorter the short axis length, the slower the settling rate of the fibers and the less likely the fibers are to settle over time. Therefore, it is preferable that the average short axis length of the first short fibers is 1000 nm or less. Within this range, the fibers do not settle and a uniform dispersion state can be maintained even when stirred with weak force. Furthermore, even when no stirring force is applied for a short period of time, such as during the infusion step in the papermaking process, the fibers do not settle if the average short axis length is 500 nm or less, and this range is a more preferable example. It is even more preferable that the average short axis length is 250 nm or less. Within this range, the fibers do not settle and a uniform dispersion state can be maintained even when the fiber dispersion is stored and no stirring force is applied for a long period of time.

[0027] Furthermore, by maintaining the dispersion state of the short fibers for a longer period of time, the occurrence of clumps of poor dispersion can be suppressed, and the wetlaid nonwoven fabric has a structure in which voids are uniformly dispersed, and further improves the surface smoothness. On the other hand, the lower limit of the minor axis length is preferably 20 nm or more. By setting the minor axis length to 20 nm or more, the nonwoven fabric is less likely to break when an external force is applied during a stirring process, etc.

[0028] The first short fibers constituting the nonwoven fabric contained in the solid electrolyte sheet of the present invention have an ultra-flat cross section in which the long axis is extremely long relative to the short axis in the fiber cross section, thereby restricting the bending direction of the short fibers themselves, and when dispersed in a liquid medium such as water, the range of conditions under which a uniform dispersion state can be maintained is significantly wider than in the prior art, such as allowing a wide range of stirring conditions from high shear to low shear. In this characteristic fiber cross section, from the viewpoint of suppressing entanglement and adhesion between adjacent fibers and preventing the fibers from crowding together, it is preferable that the shape of the fiber cross section has a distribution within a certain range, and it is also preferable that the length of the short axis varies.

[0029] The above-mentioned variation in minor axis length (CV value) is determined by calculating the arithmetic mean and standard deviation using the minor axis lengths of 100 fibers measured above, dividing the standard deviation by the arithmetic mean to obtain the coefficient of variation, and rounding off the decimal point to an integer in percentage. Having a moderate distribution of minor axis lengths not only makes joining difficult due to mismatching of fiber cross-sectional shapes, but also causes each short fiber to behave differently when an external force such as shear is applied, resulting in differences in buckling behavior when short fibers come into contact with each other and bend in a liquid medium, preventing entanglement between the short fibers and maintaining a homogeneous state over time. From the above viewpoints, it is preferable that the variation in minor axis length (CV value) of the first short fibers constituting the wetlaid nonwoven fabric of the present invention is 10% or more. By setting the CV value within this range, even in a liquid medium containing a high concentration of short fibers that tend to come into contact with each other, the bending behavior and other properties of the short fibers are different, so that entanglement and other problems do not occur, and a uniform dispersion state can be ensured. Furthermore, ensuring a uniform dispersion state further improves the smoothness of the surface of the wetlaid nonwoven fabric.

[0030] Furthermore, when the variation in the length of the minor axis (CV value) is 20% or more, the short fibers are less likely to entangle even in a clay-like liquid medium containing an extremely high concentration of the first short fibers relative to the liquid medium, and they exhibit excellent dispersibility again when diluted with liquid, etc., and this can be considered a more preferable range in the present invention.

[0031] Furthermore, it is particularly preferable that the first short fibers have a minor axis length variation (CV value) of 30% or more, and within this range, aggregation of the short fibers in the wetlaid nonwoven fabric can be prevented, thereby improving the ionic conductivity of the solid electrolyte sheet. From this perspective, the greater the minor axis length variation (CV value), the better the dispersibility in the liquid medium. However, when an external force is applied during a stirring process or the like, uneven dispersibility may occur, or the short fibers may break due to an excessively short minor axis length. Therefore, it is preferable that the minor axis length variation (CV value) be 50% or less.

[0032] (4) Irregularity of the First Short Fibers In addition to preventing entanglement due to differences in fiber cross-sections among the short fibers, the unevenness of the outer periphery of the cross-section of the first short fibers can prevent adhesion and entanglement between the short fibers due to steric hindrance. Therefore, it is preferable that the unevenness of the cross-section of the first short fibers (hereinafter sometimes referred to as "irregularity") be 20% or more. If the unevenness is 20% or more, the fibers constituting the nonwoven fabric are uniformly arranged and fiber aggregation is prevented, thereby ensuring ion paths in the solid electrolyte sheet and maintaining good ion conductivity. Furthermore, having minute irregularities on the surface of the short fibers improves adhesion between the wetlaid nonwoven fabric and the solid electrolyte, preventing chipping and cracking of the solid electrolyte. 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, loads are concentrated on parts of the short fiber cross-sections, preventing fiber cracking and maintaining the strength of the solid electrolyte sheet.

[0033] The irregularity is calculated as follows. Using an image of the fiber cross section, the maximum length of the cross section is divided into 10 equal parts, and the lengths at which line segments perpendicular to the maximum length intersect with the outer periphery of the fiber cross section are measured. The arithmetic mean and standard deviation of these 10 lengths are calculated, and the standard deviation is divided by the arithmetic mean and rounded to the nearest percent to calculate a value. Similar measurements are performed on 10 fiber cross sections, and the calculated arithmetic mean of the irregularities of the 10 fibers is the irregularity referred to here.

[0034] (5) Polymer Constituting the First Short Fibers The polymer constituting the first short fibers is preferably a polymer with excellent heat resistance and chemical resistance. That is, the polymer constituting the short fibers 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, allowing the above-mentioned short fibers to be produced by a highly productive melt spinning method. They are also suitable for adjusting mechanical properties, such as by highly oriented crystallization during the drawing process. In particular, from the perspective of ensuring uniformity in the distribution of the short fibers constituting the nonwoven fabric, it is more preferable that the first short fibers be composed of a polymer with a high elastic modulus, such as polyester or polyphenylene sulfide. Using such polymers can suppress bending of fibers when external forces are applied during papermaking, effectively preventing poor dispersion due to entanglement of fibers. Furthermore, selecting a polymer having functional groups that exert electrical repulsive forces, such as polyester, such as carboxyl end groups, can prevent aggregation due to repulsive forces between fibers, making it easier to achieve a uniform fiber distribution.

[0035] (6) Second Short Fibers The wetlaid nonwoven fabric contained in the solid electrolyte sheet of the present invention may contain second short fibers different from the first short fibers. The second short fibers are short fibers with an aspect ratio of less than 5. Examples of the second short fibers include sheath-core composite short fibers, or fibers with an average fiber diameter of 1 μm to 10 μm and a melting point of 200°C or higher. The second short fibers are preferably sheath-core composite short fibers. FIG. 3 is a schematic diagram of an example of the cross-sectional structure of a sheath-core composite short fiber used as the second short fiber of the present invention. The sheath-core composite short fiber has a structure in which a core 31 is covered by a sheath 32. When the second short fibers are sheath-core composite short fibers, the first short fibers serve as the fiber that forms the framework of the wetlaid nonwoven fabric, and the second short fibers serve as the binder fiber, thereby providing sufficient mechanical strength to the wetlaid nonwoven fabric and allowing the solid electrolyte sheet to be self-supporting. Furthermore, when the second short fibers are core-sheath composite short fibers, the sheath component melts during the composite process with the solid electrolyte, and the molten sheath component firmly adheres to the solid electrolyte due to an anchor effect, making it possible to suppress chipping, cracking, and the like of the solid electrolyte.

[0036] When core-sheath type composite staple fibers are used as the second staple fibers, the resin constituting the sheath is not particularly limited as long as it has a lower melting point than the resin constituting the core. Examples of the combination of resins constituting the core / sheath include polypropylene / polyethylene, polyester / polyethylene, and polyester / polyester copolymer.

[0037] <Wetlaid Nonwoven Fabric> Next, the wetlaid nonwoven fabric constituting the solid electrolyte sheet of the present invention will be described. This wetlaid nonwoven fabric contains the first short fibers described above and is produced by a papermaking process. The first short fibers described above, i.e., first short fibers made of polyester, polyamide, polyphenylene sulfide, polyolefin, etc., may be used singly or in combination of two or more. In addition to these first short fibers, second short fibers, such as round short fibers made of a single thermoplastic resin component such as polyester, polyamide, polyphenylene sulfide, or polyolefin, or composite short fibers made of two or more thermoplastic resins, may also be used. By appropriately designing the blending ratio of the first short fibers and the second short fibers, and adjusting the thickness, basis weight, porosity, surface smoothness, mechanical strength, etc., a solid electrolyte sheet can be obtained that is self-supporting despite being a thin film and has high ionic conductivity. Each of these components will be described below.

[0038] (1) Thickness The lower limit of the thickness of the wetlaid nonwoven fabric constituting the solid electrolyte sheet of the present invention is preferably 3 μm or more, more preferably 5 μm or more, from the viewpoint of mechanical strength. If the lower limit of the thickness is less than 3 μm, it becomes difficult to maintain the mechanical strength of the solid electrolyte sheet. On the other hand, from the viewpoint of improving the ionic conductivity of the solid electrolyte sheet, the upper limit of the thickness is preferably 50 μm or less, more preferably 30 μm or less, and even more preferably 15 μm or less. If the upper limit of the thickness exceeds 50 μm, the resistance of the solid electrolyte sheet increases, making it more likely to cause a short circuit when used in a battery.

[0039] (2) Fiber Composition The content of the first short fibers in the present wetlaid nonwoven fabric is preferably 10% by mass or more, more preferably 25% by mass or more. By setting the content of the first short fibers to 10% by mass or more, the wetlaid nonwoven fabric contained in the solid electrolyte sheet can be made thinner, making it possible to suppress a decrease in the ionic conductivity of the solid electrolyte sheet. This effect becomes more pronounced when the content of the first short fibers is 25% by mass or more. On the other hand, the upper limit of the content of the first short fibers is preferably 75% by mass or less, more preferably 50% by mass or less. By setting the content of the first short fibers with high flatness to 75% by mass or less, the solid electrolyte is more likely to fill the gaps between the fibers constituting the wetlaid nonwoven fabric. This effect becomes more pronounced when the content of the first short fibers is 50% by mass or less. Note that the first short fibers used in this embodiment serve as the fibers that form the framework of the wetlaid nonwoven fabric, so it is preferable to use drawn yarns that are highly oriented and have excellent mechanical properties.

[0040] Furthermore, when sheath-core composite staple fibers are used as second staple fibers different from flat staple fibers, the blending amount of the sheath-core composite staple fibers is not particularly limited, but from the viewpoint of functioning as binder fibers 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. On the other hand, from the viewpoint of obtaining a thinner wetlaid nonwoven fabric, the upper limit is preferably 90% by mass or less, more preferably 75% by mass or less. Since the fiber diameter of a typical sheath-core composite staple fiber is about 10 μm, a thinner wetlaid nonwoven fabric can be obtained by setting the blending amount of the sheath-core composite staple fibers to the above-mentioned upper limit.

[0041] In a second embodiment of the present invention, a wetlaid nonwoven fabric contains 10 to 75 mass% of first short fibers and further contains second short fibers different from the first short fibers, the second short fibers have an average fiber diameter of 1 μm or more and 10 μm or less, and the first short fibers and the second short fibers have melting points of 200° C. or more.

[0042] In the second embodiment, the second short fibers serve as the fibers that form the framework of the wetlaid nonwoven fabric, and the first short fibers serve as the binder fibers, thereby increasing the mechanical strength of the wetlaid nonwoven fabric and providing a solid electrolyte sheet with excellent self-supporting properties. Furthermore, when the wetlaid nonwoven fabric according to the second embodiment is composited with a solid electrolyte, the first short fibers have a high degree of flatness and a large specific surface area, resulting in excellent adhesion to the solid electrolyte. The second short fibers have a small fiber diameter and a large specific surface area, resulting in an increased contact area with the solid electrolyte and excellent adhesion. In other words, the effects of both the first short fibers and the second short fibers result in excellent adhesion to the solid electrolyte, making it possible to prevent chipping, cracking, and the like of the solid electrolyte.

[0043] The first short fibers used in the second embodiment have the same flatness, average minor axis length, CV value, and unevenness as the first short fibers used in the first embodiment, but it is preferable to use first short fibers with a low degree of crystallinity from the viewpoints of softening and flowing easily during heat treatment such as calendering and of excelling in function as binder fibers. Note that the first short fibers contained in the wetlaid nonwoven fabric after calendering have progressed in crystallization and become substantially the same as the first short fibers of the first embodiment.

[0044] In the second embodiment, the blending amount of the first short fibers in the wetlaid nonwoven fabric is preferably 10% by mass or more, and more preferably 25% by mass or more. By blending the first short fibers, which have a short minor axis length, i.e., a thin thickness, in an amount of 10% by mass or more, a thinner wetlaid nonwoven fabric can be obtained, and a decrease in the ionic conductivity of the solid electrolyte sheet can be suppressed. This effect becomes more pronounced when the blending amount of the first short fibers is 25% by mass or more. Note that the first short fibers in the second embodiment function as binder fibers, but because they have a high flatness and specific surface area, they are effective even when blended in a small amount.

[0045] On the other hand, the upper limit of the blending amount of the first short fibers is preferably 75% by mass or less, and more preferably 50% by mass or less. By setting the blending amount of the first short fibers having high flatness to 75% by mass or less, when the nonwoven fabric is made into a reinforcing sheet for a solid electrolyte, the solid electrolyte can be easily filled to the inside. This effect becomes more remarkable when the blending amount of the first short fibers is 50% by mass or less. Note that, since the first short fibers used in the second embodiment play the role of binder fibers, it is preferable to use low-oriented undrawn yarns.

[0046] In the second embodiment, the second short fibers preferably have an average fiber diameter of 1 μm or more and 10 μm or less. If the average fiber diameter is less than 1 μm, the specific surface area of ​​the fibers increases, but the porosity of the wetlaid nonwoven fabric tends to decrease. If the average fiber diameter is more than 10 μm, the thickness of the wetlaid nonwoven fabric tends to increase. The resin constituting the second short fibers is not particularly limited, but is preferably a thermoplastic resin such as polyester, polyamide, polyphenylene sulfide, or polyolefin.

[0047] In the second embodiment, the blending amount of the second short fibers is not particularly limited, but is preferably 25% by mass or more, and more preferably 50% by mass or more, from the viewpoint of serving as the skeleton of the wetlaid nonwoven fabric. On the other hand, the upper limit is preferably 90% by mass or less, and more preferably 75% by mass or less, from the viewpoint of ensuring a sufficient relative amount of the first short fibers, which serve as binder fibers, relative to the second short fibers, to obtain a wetlaid nonwoven fabric having excellent mechanical strength.

[0048] Furthermore, by setting the melting points of the first and second short fibers used in the second embodiment to 200°C or higher, when used as a reinforcing sheet for a solid electrolyte, the mechanical properties are less likely to be deteriorated by thermal energy applied during the manufacturing process. Furthermore, the first and second short fibers used in the first and second embodiments are preferably polyester fibers. By using fibers made of the same resin as the first and second short fibers used in each embodiment, the binder fiber is more easily compatible with the skeletal fibers, making it possible to obtain a stronger wetlaid nonwoven fabric.

[0049] (3) Basis Weight In order to improve the ionic conductivity of the solid electrolyte sheet, the upper limit of the basis weight of the wet-laid nonwoven fabric is 10 g / m ２ Preferably, it is 8 g / m or less. ２ More preferably, it is 5 g / m or less. ２ It is more preferable that the upper limit of the basis weight is 8 g / m or less. ２ Below, 5 g / m ２ On the other hand, there is no particular lower limit to the basis weight, but from the viewpoint of maintaining the mechanical strength required for the productivity of wetlaid nonwoven fabrics, it is preferable that the basis weight is 3 g / m or less. ２ It is preferable that this is equal to or greater than this.

[0050] (4) Porosity The porosity of the wetlaid nonwoven fabric of the present invention is preferably 65% ​​or more, more preferably 70% or more, from the viewpoint of improving the ionic conductivity of the solid electrolyte sheet. By setting the porosity within the above-mentioned range, a sufficient amount of solid electrolyte can be supported. From the viewpoint of mechanical strength, the porosity of the wetlaid nonwoven fabric is preferably 95% or less, more preferably 90% or less.

[0051] (5) Mechanical Strength By setting the thickness, basis weight, and porosity of the wetlaid nonwoven fabric of the present invention as described above, the wetlaid nonwoven fabric has mechanical strength that allows for good productivity, and a solid electrolyte sheet having the wetlaid nonwoven fabric also has mechanical strength that allows for good productivity. The mechanical strength of the wetlaid nonwoven fabric is preferably a tensile strength of 0.5 N / 15 mm or more. By setting the tensile strength to 0.5 N / 15 mm or more, the wetlaid nonwoven fabric has excellent mechanical strength and is less likely to break, thereby improving productivity.

[0052] (6) Air permeability The wetlaid nonwoven fabric of the present invention has an air permeability of 100 cm ３ / cm ２ / s or more, and 200 cm ３ / cm ２ / s or more, and more preferably 400 cm ３ / cm ２ / s or more is more preferable. Air permeability is an index of the ease of air passing 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 the more ion paths can be secured in the solid electrolyte sheet. Air permeability is 100 cm ３ / cm ２ By setting the air permeability to 200 cm / s or more, the nonwoven fabric can be more easily filled with the solid electrolyte, and when it is made into a solid electrolyte sheet, it has excellent ion conductivity. ３ / cm ２ By setting the value to 1 / s or more, the above-mentioned effect becomes more remarkable.

[0053] <Solid Electrolyte> The solid electrolyte used in the solid electrolyte sheet according to the present invention is not particularly limited, and examples thereof include sulfide solid electrolytes. The sulfide solid electrolyte is a solid electrolyte that contains at least sulfur atoms and exhibits ionic conductivity due to the contained metal ions. In addition to sulfur atoms, it preferably contains lithium atoms and phosphorus atoms, and more preferably contains lithium atoms, phosphorus atoms, and halogen atoms, and has ionic conductivity due to lithium atoms. The sulfide solid electrolyte may be an amorphous sulfide solid electrolyte or a crystalline sulfide solid electrolyte.

[0054] (1) Amorphous Sulfide Solid Electrolyte Any amorphous sulfide solid electrolyte can be used without any particular limitation as long as it contains at least sulfur atoms and exhibits ionic conductivity due to the contained metal ions. Representative examples include Li ２ S-P ２ S ５ a solid electrolyte containing sulfur atoms, lithium atoms, and phosphorus atoms, which is composed of lithium sulfide and phosphorus sulfide such as Li; ２ S-P ２ S ５ - LiI, Li ２ S-P ２ S ５ -LiCl, Li ２ S-P ２ S ５ - LiBr, Li ２ S-P ２ S ５-LiI-LiBr, etc., a solid electrolyte composed of lithium sulfide, phosphorus sulfide, and lithium halide; further containing other elements such as oxygen and silicon, for example, Li ２ S-P ２ S ５ -Li ２ O-LiI, Li ２ S-P ２ S ５ In order to obtain higher ionic conductivity, a solid electrolyte such as Li ２ S-P ２ S ５ - LiI, Li ２ S-P ２ S ５ -LiCl, Li ２ S-P ２ S ５ - LiBr, Li ２ S-P ２ S ５ A solid electrolyte composed of lithium sulfide, phosphorus sulfide, and lithium halide, such as LiI-LiBr, is preferred. The types of elements constituting the amorphous sulfide solid electrolyte can be confirmed, for example, by an ICP emission spectrometer.

[0055] The amorphous sulfide solid electrolyte contains at least Li ２ S-P ２ S ５ When Li ２ S and P ２ S ５ From the viewpoint of obtaining high chemical stability and higher ionic conductivity, the molar ratio of Li to Li is preferably 65 to 85:15 to 35, more preferably 70 to 80:20 to 30, and even more preferably 72 to 78:22 to 28. ２ S-P ２ S ５In the case of -LiI-LiBr, the total content of lithium sulfide and diphosphorus pentasulfide is preferably 60 to 95 mol%, more preferably 65 to 90 mol%, and even more preferably 70 to 85 mol%. The ratio of lithium bromide to the total of lithium bromide and lithium iodide is preferably 1 to 99 mol%, more preferably 20 to 90 mol%, even more preferably 40 to 80 mol%, and particularly preferably 50 to 70 mol%.

[0056] When the amorphous sulfide solid electrolyte contains lithium atoms, sulfur atoms, phosphorus atoms, and halogen atoms, the compounding ratio (molar ratio) of these atoms is preferably 1.0 to 1.8: 1.0 to 2.0: 0.1 to 0.8: 0.01 to 0.6, more preferably 1.1 to 1.7: 1.2 to 1.8: 0.2 to 0.6: 0.05 to 0.5, and even more preferably 1.2 to 1.6: 1.3 to 1.7: 0.25 to 0.5: 0.08 to 0.4. Furthermore, when bromine and iodine are used in combination as halogen atoms, the compounding ratio (molar ratio) of lithium atoms, sulfur atoms, phosphorus atoms, bromine atoms, and iodine atoms is preferably 1.0 to 1.8: 1.0 to 2.0: 0.1 to 0.8: 0.01 to 0.3: 0.01 to 0.3, more preferably 1.1 to 1.7: 1.2 to 1.8: 0.2 to 0.6: 0.02 to 0.25: 0.02 to 0.25, more preferably 1.2 to 1.6: 1.3 to 1.7: 0.25 to 0.5: 0.03 to 0.2: 0.03 to 0.2, and even more preferably 1.35 to 1.45: 1.4 to 1.7: 0.3 to 0.45: 0.04 to 0.18: 0.04 to 0.18. By setting the compounding ratio (molar ratio) of lithium atoms, sulfur atoms, phosphorus atoms, and halogen atoms within the above range, it becomes easier to obtain a solid electrolyte having a thiolisiconregion II type crystal structure described below and having higher ionic conductivity.

[0057] The shape of the amorphous sulfide solid electrolyte is not particularly limited, but may be, for example, particulate. The average particle diameter (D ５０ The average particle diameter (D ) of the amorphous sulfide solid electrolyte is preferably in the range of 0.01 μm to 500 μm, or 0.1 to 200 μm, for example. From the viewpoint of high ionic conductivity and easy filling of the pores of the nonwoven fabric, ５０) is preferably 0.01 to 15 μm, more preferably 0.1 to 10 μm, and even more preferably 0.3 to 5 μm.

[0058] In this specification, the average particle diameter (D ５０ ) is the particle size at which 50% of the total particle size is reached when the particle size distribution is accumulated in order from the smallest particle size when an accumulation curve of particle size distribution is drawn, and the volume distribution is an average particle size that can be measured using, for example, a laser diffraction / scattering particle size distribution measuring device.

[0059] (2) Crystalline sulfide solid electrolyte The crystalline sulfide solid electrolyte may be, for example, a so-called glass ceramic obtained by heating the amorphous sulfide solid electrolyte to a temperature equal to or higher than the crystallization temperature, and a sulfide solid electrolyte having the following crystal structure may be used. Examples of crystal structures that the crystalline sulfide solid electrolyte containing lithium atoms, sulfur atoms, and phosphorus atoms may have include Li ３ P.S. ４ Crystal structure, Li ４ P ２ S ６ Crystal structure, argyrodite-type crystal structure, Li ７ P ３ S １１ Examples of such structures include a crystal structure having peaks at 2θ=approximately 20.2° and 23.6° (for example, JP 2013-16423 A).

[0060] The crystalline sulfide solid electrolyte containing lithium atoms, sulfur atoms, phosphorus atoms, and halogen atoms may have a crystal structure such as Li ４－ｘ Ge １－ｘ P ｘ S ４ Thio-LISICON Region II crystal structure (Kanno et al., Journal of the Electrochemical Society, 148(7)A742-746(2001)), Li ４－ｘ Ge １－ｘ P ｘ S ４Examples of the thio-lisicon region II crystal structure include those having a crystal structure similar to the thio-lisicon region II type (see Solid State Ionics, 177 (2006), 2721-2725). ４－ｘ Ge １－ｘ P ｘ S ４ Thio-LISICON Region II crystal structure, Li ４－ｘ Ge １－ｘ P ｘ S ４ This indicates that the thio-LISICON region II type has a similar crystal structure.

[0061] In X-ray diffraction measurement using CuKα radiation, Li ３ P.S. ４ Diffraction peaks of the crystal structure appear, for example, at 2θ=17.5°, 18.3°, 26.1°, 27.3°, and 30.0°. ４ P ２ S ６ The diffraction peaks of the crystalline structure appear, for example, at 2θ=16.9°, 27.1°, and 32.5°, and the diffraction peaks of the argyrodite-type crystalline structure appear, for example, at 2θ=15.3°, 25.2°, 29.6°, and 31.0°. ７ P ３ S １１ Diffraction peaks of the crystal structure appear, for example, at 2θ=17.8°, 18.5°, 19.7°, 21.8°, 23.7°, 25.9°, 29.6°, and 30.0°, and Li ４－ｘ Ge １－ｘ P ｘ S ４ The diffraction peaks of the thio-LISICON Region II crystal structure appear, for example, at 2θ=20.1°, 23.9°, and 29.5°, and Li ４－ｘ Ge １－ｘ P ｘ S ４Diffraction peaks of a crystal structure similar to that of thio-LISICON Region II type appear, for example, at 2θ=20.2° and 23.6°. Note that these peak positions may vary within a range of ±0.5°.

[0062] As the argyrodite type crystal structure, for example, Li ７ P.S. ６ Crystal structure; Li ７ P.S. ６ The structural skeleton of the composition formula Li ７－ｘ P １－ｙ Si ｙ S ６ and Li ７＋ｘ P １－ｙ Si ｙ S ６ (x is -0.6 to 0.6, y is 0.1 to 0.6); Li ７－ｘ－２ｙ P.S. ６－ｘ－ｙ Cl ｘ (0.8≦x≦1.7, 0<y≦−0.25x+0.5); Li ７－ｘ P.S. ６－ｘ Ha ｘ (Ha is Cl or Br, and x is preferably 0.2 to 1.8).

[0063] Among the above crystal structures, the crystal structure of the crystalline sulfide solid electrolyte is Li ３ P.S. ４ The crystalline sulfide solid electrolyte preferably has a thiolicon region II type crystal structure, an argyrodite type crystal structure, or a thiolicon region II type crystal structure. The shape of the crystalline sulfide solid electrolyte is not particularly limited, but may be, for example, a particulate shape. The average particle diameter (D ５０ ) is the average particle diameter (D ５０ ), for example, the average particle diameter (D ５０ ) is preferably 0.01 to 15 μm, more preferably 0.1 to 10 μm, and even more preferably 0.3 to 5 μm.

[0064] (3) Oxide Solid Electrolyte The oxide solid electrolyte contains, for example, Li element, Z element (Z is at least one of Nb, B, Al, Si, P, Ti, Zr, Mo, W, and S), and O element. Examples of the oxide solid electrolyte include garnet-type solid electrolytes, perovskite-type solid electrolytes, Nasicon-type solid electrolytes, Li-P-O-based solid electrolytes, and Li-B-O-based solid electrolytes. Examples of the garnet-type solid electrolyte include Li ７ La ３ Zr ２ O １２ , Li ７－ｘ La ３ (Zr ２－ｘ Nb ｘ ) O １２ (0≦x≦2), Li ５ La ３ Nb ２ O １２ Examples of perovskite-type solid electrolytes include (Li, La)TiO ３ , (Li, La)NbO ３ , (Li, Sr) (Ta, Zr) O ３ Examples of Nasicon-type solid electrolytes include Li(Al,Ti)(PO ４ ) ３ , Li(Al,Ga)(PO ４ ) ３ Examples of Li-P-O based solid electrolytes include Li ３ P.O. ４ , LIPON (Li ３ P.O. ４ As the Li-B-O solid electrolyte, Li ３ BO ３ , Li ３ BO ３ and compounds in which part of the O in the above formula is replaced by C.

[0065] (4) Hydride Solid Electrolyte The hydride solid electrolyte contains, for example, Li and a complex anion containing hydrogen. Examples of the complex anion include (BH ４ ) － , (NH ２ ) － , (AlH４ ) － , (AlH ６ ) ３－ etc.

[0066] (5) Halogenated Solid Electrolyte The halogenated solid electrolyte is, for example, a compound containing Li, M, and X. Here, M is at least one selected from the group consisting of metal elements and metalloid elements other than Li. X is at least one selected from the group consisting of F, Cl, Br, and I. The "metalloid elements" are B, Si, Ge, As, Sb, and Te. The "metal elements" are all elements included in Groups 1 to 12 of the periodic table (excluding hydrogen) and all elements included in Groups 13 to 16 of the periodic table (excluding B, Si, Ge, As, Sb, Te, C, N, P, O, S, and Se). In order to improve the ionic conductivity of the halide solid electrolyte, M may contain Y. The halide solid electrolyte is, for example, Li, a Me b Y c The compound may be a compound represented by X6. Here, the formula: a + mb + 3c = 6, and c > 0 is satisfied. The value of m represents the valence of Me. In order to improve the ionic conductivity of the halide solid electrolyte, Me may be at least one selected from the group consisting of Mg, Ca, Sr, Ba, Zn, Sc, Al, Ga, Bi, Zr, Hf, Ti, Sn, Ta, and Nb. In order to improve the ionic conductivity of the halide solid electrolyte, X may include at least one selected from the group consisting of Cl and Br. The halide solid electrolyte may include, for example, at least one selected from the group consisting of Li3YCl6 and Li3YBr6.

[0067] (6) Nitride Solid Electrolyte The nitride solid electrolyte may be, for example, Li ３ Examples include N.

[0068] <Method of Manufacturing a Solid Electrolyte Sheet> FIG. 4 is a cross-sectional view illustrating an example of a method of manufacturing a solid electrolyte sheet. As shown in FIG. 4( a), a solid electrolyte-containing slurry is applied to a substrate 41 such as aluminum foil to form a solid electrolyte membrane 42. Then, as shown in FIG. 4( b), a nonwoven fabric 43 is placed on the solid electrolyte membrane 42, and as shown in FIG. 4( c), another slurry is applied to the nonwoven fabric 43 to form the solid electrolyte membrane 42. The solvent in the slurry is then volatilized, followed by heating and drying under reduced pressure to obtain a sheet-like product in which a solid electrolyte 45 is laminated on a nonwoven fabric 46 impregnated with the solid electrolyte. This sheet-like product is then pressure-molded to obtain a solid electrolyte sheet in which at least a portion of the solid electrolyte 45 is embedded within the nonwoven fabric 46. The pressure molding can be performed using a pressure molding method such as a flat press, a roll press, or an isostatic press. The pressure during pressure molding is preferably 10 to 700 MPa. Heating may also be applied during pressure molding. By applying pressure after heating, fusion and bonding of the solid electrolytes occur, the grain boundaries of the solid electrolyte are densified, and the ionic conductivity of the solid electrolyte sheet is improved. The temperature during pressure molding after the heating is preferably, for example, 40°C or higher and 500°C or lower. The method for producing the solid electrolyte sheet is not limited to the above method. The slurry may be prepared by dispersing the solid electrolyte together with a binder in a solvent. The slurry is preferably produced by a method including a step of wet-filling the pores of a wet-laid nonwoven fabric. Examples of methods for filling the pores of a wet-laid nonwoven fabric with a slurry containing the solid electrolyte and a binder include coating methods such as screen printing, doctor blade, and dipping.

[0069] The slurry is prepared by adding the solid electrolyte and the binder to a solvent and mixing them together, using a general mixing device such as a dissolver, homomixer, kneader, roll mill, sand mill, attritor, ball mill, vibrator mill, high-speed impeller mill, ultrasonic homogenizer, or shaker.

[0070] It is preferable to use a solvent for the slurry that is less likely to deteriorate the solid electrolyte. In particular, sulfide solid electrolytes and hydride solid electrolytes undergo chemical changes and composition changes due to trace amounts of water, so it is preferable to use nonpolar aprotic solvents represented by hydrocarbon solvents such as hexane, heptane, octane, nonane, decane, decalin, toluene, and xylene, esters such as butyl butyrate and ethyl acetate, ethers such as dibutyl ether, ketones such as diisobutyl ketone (DIBK), methyl ketone, and methyl propyl ketone, and amines such as tributylamine and allylamine. The proportion of the solvent in the slurry is, for example, 60 parts by mass or more and 300 parts by mass or less, assuming that the solid component of the slurry is 100 parts by mass.

[0071] <Secondary Battery> The solid electrolyte sheet of the present invention can be applied to electrical devices such as primary batteries, secondary batteries, and capacitors. The solid electrolyte sheet of the present invention also includes a state in which it is configured as a solid electrolyte sheet or solid electrolyte layer in these electrical devices. Hereinafter, the present invention will be described using an all-solid-state lithium ion battery, which is one form of secondary battery, as an example.

[0072] A lithium ion battery using the solid electrolyte sheet of the present invention includes the above-described solid electrolyte sheet of the present invention, and its structure is not particularly limited, but may have, for example, the laminated structure shown in Fig. 5. The lithium ion battery 50 of Fig. 5 includes a positive electrode layer 51, a negative electrode layer 52, and a solid electrolyte layer 53 disposed between the positive electrode layer 51 and the negative electrode layer 52, and the solid electrolyte layer 53 may be made of the above-described solid electrolyte sheet of the present invention. The solid electrolyte layer 53 is a layer that can move lithium ions by an externally applied electric field.

[0073] The positive electrode layer 51 is an electrode layer containing a positive electrode active material that releases lithium ions during charging and absorbs lithium ions during discharging. Examples of the positive electrode active material include oxides, sulfides, and phosphates containing at least one metal element selected from manganese (Mn), cobalt (Co), nickel (Ni), iron (Fe), molybdenum (Mo), vanadium (V), and tungsten (W). Specifically, MoO ｘ , W.O.ｘ , V.O. ｘ , Li ｘ CoO ｙ (LiCoO ２ etc.), Li ｘ MnO ｙ (LiMnO ２ , LiMn ２ O ４ etc.), Li ｘ NiO ｙ (LiNiO ２ etc.), Li ｘ VO ｙ (LiVO ２ etc.), Li ｘ Mn ｙ Ni ｚ Co ｗ O (LiNi １／３ Co １／３ Mn １／３ O ２ etc.), Li ｘ FeP ｘ O ｙ (LiFePO ４ etc.), Li ｘ MnP ｘ O ｙ (LiMnPO ４ etc.), Li ｘ NiP ｘ O ｙ (LiNiPO ４ etc.), Li ｘ CuP ｘ O ｙ (LiCuPO ４ etc.), MoS ｘ , CuS ｘ , TiS ｘ , W.S. ｘ , Li ｘ S ｙ , Li ｘ P ｙ S ｚ The positive electrode layer 51 may be a composite positive electrode layer further containing a solid electrolyte, a conductive additive, and the like.

[0074] The negative electrode layer 52 is an electrode layer containing a negative electrode active material that absorbs lithium ions during charging and releases lithium ions during discharging. Examples of the negative electrode active material include carbon materials; metals such as lithium (Li), indium (In), aluminum (Al), and silicon (Si), or alloys containing these metals; Sn ｘ O ｙ , MoO ｘ , W.O. ｘ , Li ｘ CoO ｙ (LiCoO ２ etc.), Li ｘ Mn ｙ Ni ｚ Co ｗ O (LiNi １／３ Co １／３ Mn １／３ O ２ etc.), Li ｘ CuP ｘ O ｙ (LiCuPO ４ The negative electrode layer 52 may be a composite negative electrode layer further including a solid electrolyte, a conductive additive, and the like.

[0075] The conductive additive may be a carbon material, a metal powder, a metal compound, or the like, and among these, a carbon material is preferably used. Examples of the carbon material include plate-like conductive materials such as graphene; linear conductive materials such as carbon nanotubes and carbon fibers; carbon blacks such as ketjen black, acetylene black, thermal black, and channel black; and granular conductive materials such as graphite.

[0076] 5 , the lithium ion battery of the present invention may further include a positive electrode current collector that collects current from the positive electrode layer 51 and a negative electrode current collector that collects current from the negative electrode layer 52 (not shown). The positive electrode current collector or the negative electrode current collector may be made of, for example, stainless steel, gold, platinum, copper, zinc, nickel, tin, aluminum, or an alloy thereof, and may have a plate, foil, mesh, or other similar shape.

[0077] (Electrode body) A positive electrode and a negative electrode can be used in a battery in the form of a laminated electrode body in which the positive electrode and the negative electrode are laminated with the solid electrolyte sheet of the present invention interposed therebetween, or in the form of a wound electrode body in which this laminated electrode body is wound. When forming the electrode body, it is preferable to pressure-form the positive electrode, the negative electrode, and the solid electrolyte sheet in a stacked state, from the viewpoint of increasing the mechanical strength of the electrode body.

[0078] (Battery Configuration) The configuration of the all-solid-state lithium secondary battery is not limited to those generally referred to as coin-type batteries or button-type batteries. For example, it may have an exterior body made of a resin film or a metal-resin laminate film, or an exterior body having a metal outer can with a bottomed tubular shape (cylindrical or rectangular tubular) and a sealing structure that seals the opening. (Restraint Member) The all-solid-state battery may be equipped with a restraint member. The restraint member applies a restraint pressure to the positive electrode, solid electrolyte layer, and negative electrode in the thickness direction. The restraint pressure is preferably 0.1 MPa or more, more preferably 1 MPa or more, and even more preferably 5 MPa or more. By setting the restraint pressure within the above range, it is possible to suppress the generation of gaps due to the expansion and contraction of the electrodes during charge and discharge, thereby maintaining battery performance. The restraint pressure is preferably 100 MPa or less, more preferably 50 MPa or less, and even more preferably 20 MPa or less. The restraint pressure is preferably 0.1 MPa or more and 100 MPa or less.

[0079] 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.

[0080] <Solid Electrolyte Sheet> A. Ratio (d / c) of Area c of Solid Portions to Area d of Void Portions (1) Measurement Method Ten random images of the cross section of a solid electrolyte sheet cut by ion milling were taken at an LEI magnification of 1500 times (acceleration voltage 1.5 kV). Next, the areas of the short fibers (first short fibers and short fibers different from the first short fibers), solid electrolyte, and voids shown in each image were measured using analysis software (Avizo manufactured by Thermo Fisher SCIENTIFIC), and the ratio (d / c) of the area c of the solid portions to the area d of the void portions was calculated using the following formula. (2) Measurement Formula d / c = Total Area of ​​Voids / (Total Area of ​​Short Fibers + Total Area of ​​Solid Electrolyte)

[0081] B. Ratio (e / f) of the area e of the fibers constituting the nonwoven fabric to the area f of the solid electrolyte (1) Measurement method Ten random images of the cross section of a solid electrolyte sheet cut by ion milling were taken at an LEI magnification of 1500 times (acceleration voltage 1.5 kV). Next, the areas of the short fibers (first short fibers and short fibers different from the first short fibers), solid electrolyte, and voids shown in each image were measured using analysis software (Avizo manufactured by Thermo Fisher Scientific), and the ratio (e / f) of the area e of the short fibers constituting the nonwoven fabric to the area f of the solid electrolyte was calculated using the following formula. (2) Measurement formula e / f = total area of ​​short fibers / total area of ​​solid electrolyte

[0082] C. Ratio (g / h) of the number g of first short fibers to the number h of second short fibers (1) Measurement method Ten random images of a cross section of a solid electrolyte sheet cut by ion milling were taken at an LEI magnification of 1500 times (acceleration voltage 1.5 kV). Next, the number of first short fibers A and the number of second short fibers different from the first short fibers A captured in each image were measured at a magnification of 15,000 times, and the first short fiber ratio was calculated using the following formula. (2) Measurement formula First short fiber ratio = Total number of first short fibers (numbers) / Total number of second short fibers different from the first short fibers (numbers)

[0083] D. Thickness A portion of the solid electrolyte sheet was cut into a circular shape with a diameter of 10 mm and pressed at room temperature with a press at a pressure of 400 MPa (holding time: 30 seconds). The thickness of this solid electrolyte sheet was measured at three points in its surface with a micrometer, and the average value was taken as the thickness of the solid electrolyte sheet.

[0084] E. Ionic Conductivity A portion of the solid electrolyte sheet was cut into a circular shape with a diameter of 10 mm and placed in a pressure / measurement unit (a zirconia cylinder with steel pins inserted from both sides). It was then pressed at room temperature with a press at 400 MPa (holding time: 30 seconds). The sample was pressurized to 100 MPa with steel pins, and its ionic conductivity (25°C) was measured using an AC impedance method. A SOLARTRON IMPEDANCE ANALYZER "S1260" (model name) was used for the measurement, with the measurement conditions being an amplitude voltage of 10 mV and a frequency range of 5 MHz to 0.1 Hz. F. Cracks The pressed solid electrolyte sheet was cut in the thickness direction by ion milling, and 10 random images of the exposed cross section of the solid electrolyte sheet were taken at an LEI magnification of 1500x (accelerating voltage: 1.5 kV). The number of cracks present in each of the 10 images was evaluated. The evaluation criteria were as follows: ◎: 0 to 5 cracks ○: 6 to 20 cracks ×: 21 or more cracks

[0085] <Wet-laid nonwoven fabric> A. Basis weight (1) Reference standard: JIS P8124:1998 (2) Measurement method: Measure the mass (g) and measure it by 1 m ２ Mass per unit (g / m ２ (3) Measurement conditions: Size: 100 mm x 100 mm Number of samples: 5

[0086] B. Thickness (1) Measurement method: Measured using a digital thickness gauge "SMD-565J-L (manufactured by Teclock Corporation)." (2) Measurement conditions: Probe: φ10 mm ceramic; Number of electrodes: 5

[0087] C. Porosity (1) Measurement method Mass (g), volume (cm ３ ) was measured and the apparent density (g / cm ３The true density of the polyester used in the examples was 1.38 g / cm ３ (2) Measurement formula: Apparent density = mass (g) / volume (cm ３ ) Porosity = (1 - apparent density / true density) x 100 (%)

[0088] D. Air permeability (1) Reference standard: JIS L1096:1999 (Fragile method) (2) Measurement method: The air permeability of the test specimen was measured using a "Air Permeability Tester FX3300 (manufactured by Textest Co., Ltd.)". (3) Measurement conditions: Size: 200 mm x 200 mm; Pressure: 125 Pa; Measurement area: φ38 mm; Number of samples: 5

[0089] <Solid electrolyte> Average particle size Measurement was performed using a laser diffraction / scattering particle size distribution analyzer ("Partica LA-950 (model number)" manufactured by Horiba, Ltd.). Specifically, a mixture of dehydrated toluene (manufactured by Wako Pure Chemical Industries, special grade) and tertiary butyl alcohol (manufactured by Wako Pure Chemical Industries, special grade) in a mass ratio of 93.8:6.2 was used as the dispersion medium. 50 mL of the dispersion medium was injected into the flow cell of the device and circulated, and solid electrolyte powder was added and subjected to ultrasonic treatment, after which the particle size distribution was measured. Average particle size (D ５０ ) was the particle size at which the particle size distribution reached 50% (volume basis) of the total when the particle size distribution curve was drawn and the particle size was accumulated in order from the smallest particle size.

[0090] <First Short Fibers> First Short Fibers A First short fibers A (polyethylene terephthalate) used in the following examples were produced by the method described in WO 2023 / 243396. The specific surface area of ​​the first short fibers A was 0.0042 nm －１ The flatness is 20, and the average length of the minor axis is 502 nm. The variation in the length of the minor axis of the cross section (CV value) is 37%, the irregularity in the cross section is 25%, and the average fiber length is 3.0 mm. First Short Fiber B: Produced using the method described in International Publication No. 2023 / 243396. The specific surface area of ​​the first short fiber B is 0.0044 nm －１The flatness is 10, and the average length of the minor axis is 498 nm. The variation in the length of the minor axis of the cross section (CV value) is 37%, the irregularity in the cross section is 26%, and the average fiber length is 3.0 mm. First Short Fiber C: Produced using the method described in International Publication No. 2023 / 243396. The specific surface area of ​​the first short fiber C is 0.0048 nm －１ The flatness is 5, and the average length of the minor axis is 505 nm. The variation in the length of the minor axis of the cross section (CV value) is 35%, the irregularity in the cross section is 25%, and the average fiber length is 3.0 mm. First Short Fiber D: Produced using the method described in International Publication No. 2023 / 243396. The specific surface area of ​​the first short fiber D is 0.0041 nm －１ The flatness is 40, and the average length of the minor axis is 503 nm. The variation in the length of the minor axis of the cross section (CV value) is 32%, the irregularity in the cross section is 25%, and the average fiber length is 3.0 mm. First Short Fiber E: Produced using the method described in International Publication No. 2023 / 243396. The specific surface area of ​​the first short fiber D is 0.0042 nm －１ The flatness is 20, the average length of the minor axis is 500 nm, the variation in the length of the minor axis of the cross section (CV value) is 38%, the irregularity in the cross section is 23%, and the average fiber length is 3.0 mm. <Second Staple Fibers> Second Staple Fiber A: Sheath-core composite staple fiber (average fineness: 1.1 dtex, fiber length: 5 mm) in which the resin constituting the core is polyester and the resin constituting the sheath is copolymerized PET (amorphous polyester, melting point 110°C). Second Staple Fiber B: Polyester fiber (round cross section, average fineness 0.3 dtex, average fiber length 3.0 mm). Second Staple Fiber C: Sheath-core composite staple fiber (average fineness 0.5 dtex, fiber length 5 mm) in which the resin constituting the core is polyester and the resin constituting the sheath is copolymerized PET (amorphous polyester, melting point 110°C). Second Staple Fiber D: Polyester fiber (round cross section, average fineness 1.1 dtex, average fiber length 3.0 mm). Second Staple Fiber E: PPS fiber (round cross section, average fineness 1 dtex, fiber length 3 mm). Second Staple Fiber F Amorphous PPS fiber (round cross section, average fineness 3 dtex, average fiber length 3.0 mm)

[0091] [Production Example 1] First staple fibers A: 10% by mass of polyester fibers (flatness 20, minor axis length 502 nm, average fiber length 3.0 mm) and 90% by mass of second staple fibers A: core-sheath composite staple fibers (average fiber diameter: 1.1 dtex, fiber length: 5 mm) in which the resin constituting the core was polyester and the resin constituting the sheath was copolymerized PET were stirred in water, and the mixture was made into paper. The paper was then calendered at a linear pressure of 80 kg and 80°C to obtain a thickness of 10 μm and a basis weight of 3.2 g / m. ２ A first wetlaid nonwoven fabric was obtained.

[0092] [Production Example 2] A sheet having a thickness of 10 μm and a basis weight of 3.2 g / m was produced in the same manner as in Production Example 1, except that the first short fibers A were used in an amount of 25% by mass and the second short fibers A in an amount of 75% by mass. ２ A second wetlaid nonwoven fabric was obtained.

[0093] [Production Example 3] A paper sheet having a thickness of 16 μm and a basis weight of 3.1 g / m was produced in the same manner as in Production Example 1, except that the first short fibers A were used in an amount of 50% by mass, the second short fibers A in an amount of 50% by mass, and no calendering treatment was performed after papermaking. ２ A third wetlaid nonwoven fabric was obtained.

[0094] [Production Example 4] The wetlaid nonwoven fabric 3 of Production Example 3 was pressed at a linear pressure of 80 kg at 80°C to a thickness of 10 μm and a basis weight of 3.0 g / m ２ A fourth wetlaid nonwoven fabric was obtained.

[0095] [Production Example 5] A spunbonded woven fabric was prepared in the same manner as in Production Example 1, except that the first short fibers A were used in an amount of 50% by mass and the second short fibers A in an amount of 50% by mass, and the thickness was 7 μm and the basis weight was 2.1 g / m. ２ [Production Example 6] A fifth wetlaid nonwoven fabric was obtained in the same manner as in Production Example 1, except that the first staple fiber B was blended in an amount of 10% by mass and the second staple fiber A in an amount of 90% by mass. ２ [Production Example 7] A sixth wetlaid nonwoven fabric was obtained in the same manner as in Production Example 1, except that the first short fiber C was blended in an amount of 10% by mass and the second short fiber A in an amount of 90% by mass. ２ [Production Example 8] A seventh wetlaid nonwoven fabric having a thickness of 3 μm and a basis weight of 1.8 g / m was obtained in the same manner as in Production Example 1, except that the first short fibers A and E were blended in an amount of 70% by mass and 30% by mass, respectively. ２[Production Example 9] A wetlaid nonwoven fabric having a thickness of 4 μm and a basis weight of 2.2 g / m was obtained in the same manner as in Production Example 1, except that the first short fibers B were blended in an amount of 70% by mass and the first short fibers E in an amount of 30% by mass. ２ [Production Example 10] A ninth wetlaid nonwoven fabric having a thickness of 5 μm and a basis weight of 3.0 g / m was obtained in the same manner as in Production Example 1, except that the first short fibers C were blended in an amount of 70% by mass and the first short fibers E in an amount of 30% by mass. ２ [Production Example 11] A tenth wetlaid nonwoven fabric was obtained in the same manner as in Production Example 1, except that the first short fibers D were blended in an amount of 10% by mass and the second short fibers A in an amount of 90% by mass. ２ An eleventh wetlaid nonwoven fabric was obtained.

[0096] [Production Example 12] A sheet having a thickness of 10 μm and a basis weight of 3.0 g / m was produced in the same manner as in Production Example 1, except that 50% by mass of the second short fibers A and 50% by mass of the second short fibers B were blended. ２ A twelfth wetlaid nonwoven fabric was obtained.

[0097] [Production Example 13] A sheet having a thickness of 10 μm and a basis weight of 3.0 g / m was produced in the same manner as in Production Example 1, except that 50% by mass of the second short fibers A and 50% by mass of the second short fibers D were blended. ２ A thirteenth wetlaid nonwoven fabric was obtained.

[0098] [Production Example 14] 50% by mass of second short fibers A and 50% by mass of second short fibers D were blended, wet-laid, and dried at 120°C with a Yankee dryer to obtain a sheet with a thickness of 28 μm and a basis weight of 18.5 g / m ２ A 14th wetlaid nonwoven fabric was obtained.

[0099] [Production Example 15] A sheet having a thickness of 14 μm and a basis weight of 8 g / m was produced in the same manner as in Production Example 1, except that 50% by mass of the second short fibers E and 50% by mass of the second short fibers F were blended. ２ A 15th wetlaid nonwoven fabric was obtained.

[0100] [Production Example 16] A sheet having a thickness of 31 μm and a basis weight of 8 g / m was produced in the same manner as in Production Example 1, except that 50% by mass of the second short fibers E and 50% by mass of the second short fibers F were blended. ２ A 16th wetlaid nonwoven fabric was obtained.

[0101] [Example 1] A solid electrolyte having an average particle diameter (D ５０ ) is 2.0 μm Li ２ S-P ２ S ５ A sulfide-based solid electrolyte, LiBr-LiI glass ceramic, was used. A butyl butyrate solution of SBR binder was added to 1.95 g of solid electrolyte so that the binder mass was 0.05 g. Butyl butyrate was added to this mixture so that the solid content concentration was 33%, and the mixture was stirred at 2000 rpm for 2 minutes using a planetary mixer to obtain a solid electrolyte slurry. The resulting slurry was uniformly coated onto a 30 μm-thick aluminum foil using an applicator. A first wet-laid nonwoven fabric was placed on the coating film, and the slurry was further coated on top of it. The sheet was heated at 50°C for 1 hour to volatilize the butyl butyrate, and then dried under reduced pressure at 50°C for 3 hours to obtain a solid electrolyte sheet 10. The solid electrolyte sheet was cut into a 10 mm diameter circle, pressed at a pressure of 400 MPa, and its ionic conductivity was measured using an AC impedance method.

[0102] Example 2 A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wet-laid nonwoven fabric was changed to the second wet-laid nonwoven fabric.

[0103] Example 3 A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wet-laid nonwoven fabric was changed to a third wet-laid nonwoven fabric.

[0104] Example 4 A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wet-laid nonwoven fabric was changed to the fourth wet-laid nonwoven fabric.

[0105] [Example 5] A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wetlaid nonwoven fabric was changed to the fifth wetlaid nonwoven fabric. [Example 6] A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wetlaid nonwoven fabric was changed to the sixth wetlaid nonwoven fabric. [Example 7] A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wetlaid nonwoven fabric was changed to the seventh wetlaid nonwoven fabric. [Example 8] A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wetlaid nonwoven fabric was changed to the eighth wetlaid nonwoven fabric. [Example 9] A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wetlaid nonwoven fabric was changed to the ninth wetlaid nonwoven fabric. [Example 10] A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wetlaid nonwoven fabric was changed to the tenth wetlaid nonwoven fabric. Example 11 A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wetlaid nonwoven fabric was changed to the eleventh wetlaid nonwoven fabric.

[0106] Comparative Example 1 A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wetlaid nonwoven fabric 1 was changed to the sixth wetlaid nonwoven fabric.

[0107] Comparative Example 2 A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wetlaid nonwoven fabric 1 was changed to the seventh wetlaid nonwoven fabric.

[0108] Comparative Example 3 A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wetlaid nonwoven fabric was changed to the eighth wetlaid nonwoven fabric.

[0109] Comparative Example 4 A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wetlaid nonwoven fabric was changed to the ninth wetlaid nonwoven fabric.

[0110] Comparative Example 5 A solid electrolyte sheet 10 was obtained in the same manner as in Example 1, except that the first wetlaid nonwoven fabric was changed to the tenth wetlaid nonwoven fabric.

[0111] The solid electrolyte sheets of Examples 1 to 11 and Comparative Examples 1 to 5 were evaluated for their respective properties using the above-described measurement methods, and the results are shown in Table 1.

[0112] A comparison between Example 4 and Comparative Example 2 shows that the solid electrolyte sheet fabricated using a wetlaid nonwoven fabric containing flat first short fibers is less susceptible to cracking within the solid electrolyte sheet. As a result, the ion paths within the solid electrolyte sheet are maintained, resulting in excellent ion conductivity. A comparison between Examples 1 to 3 shows that the smaller the ratio of the number of fibers present in the cross section of the solid electrolyte sheet (flat first short fibers g / second short fibers h), the higher the ion conductivity. A larger number of flat first short fibers reduces the likelihood of cracking, but also shortens the inter-fiber distance in the thickness direction, potentially making it more difficult to maintain the ion paths.

[0113]

[0114] The solid electrolyte sheet of the present invention is a thin film, has self-supporting properties, and has high ionic conductivity, and therefore can be suitably used in secondary batteries such as all-solid-state batteries.

[0115] REFERENCE SIGNS LIST 10 Solid electrolyte sheet 11 First short fiber 12 Second short fiber 13 Solid electrolyte 31 Core 32 Sheath 41 Substrate 42 Solid electrolyte membrane 43 Nonwoven fabric 45 Solid electrolyte 46 Nonwoven fabric impregnated with solid electrolyte 50 Lithium ion battery 51 Positive electrode layer 52 Negative electrode layer 53 Solid electrolyte layer

Claims

1. A solid electrolyte sheet having a wetlaid nonwoven fabric containing synthetic fibers and a solid electrolyte, wherein the wetlaid nonwoven fabric contains at least first flat short fibers having a ratio a / b of the length a of the major axis to the length b of the minor axis in a cross section of 5 or more, wherein in a cross section in the thickness direction of the solid electrolyte sheet, the ratio d / c of ​​the area c of solid portions formed by the fibers constituting the wetlaid nonwoven fabric and the solid electrolyte to the area d of void portions is 0 or more and 0.1 or less, and wherein the ratio e / f of the area e of the fibers constituting the nonwoven fabric in the solid portions to the area f of the solid electrolyte is 0.05 or more and 0.5 or less.

2. The solid electrolyte sheet according to claim 1, characterized in that it contains at least second short fibers different from the first short fibers, and the second short fibers are core-sheath composite fibers.

3. The solid electrolyte sheet according to claim 2, wherein the ratio g / h of the number g of the first short fibers to the number h of the second short fibers contained in a cross-sectional area of ​​the solid electrolyte sheet in the thickness direction is 1 or more and 40 or less.

4. The solid electrolyte sheet according to claim 2, wherein the first short fibers and the second short fibers are made of polyester fibers.

5. A solid electrolyte sheet according to claim 1 or 2, wherein the solid electrolyte is a sulfide solid electrolyte.

6. The solid electrolyte sheet according to claim 5, wherein the sulfide solid electrolyte is an amorphous sulfide solid electrolyte, a crystalline sulfide solid electrolyte, or a sulfide glass ceramic solid electrolyte.

7. The solid electrolyte sheet according to claim 5 or 6, wherein the sulfide solid electrolyte contains lithium atoms, sulfur atoms, phosphorus atoms, and halogen atoms, and the halogen atoms are one or more of chlorine atoms, bromine atoms, and iodine atoms.

8. The average particle diameter (D ５０ 7. The solid electrolyte sheet according to claim 5, wherein the thickness of the first electrode is 0.01 μm or more and 15 μm or less.

9. A method for producing the solid electrolyte sheet according to claim 1.

10. A secondary battery comprising the solid electrolyte sheet according to claim 1.

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