Support for secondary battery, solid electrolyte sheet, and secondary battery

Abstract

A support for a secondary battery that is for holding a solid electrolyte of a secondary battery, the support being substantially free of non-fibrillated fibers and comprising at least one selected from paper and non-woven fabric, and the transmittance of a perfluoropolyether in the thickness direction being 1-15%.

Description

Secondary battery support, solid electrolyte sheet, and secondary battery

[0001] The present invention relates to a support for a secondary battery that holds a solid electrolyte, a solid electrolyte sheet, and a secondary battery that includes this support.

[0002] Lithium-ion secondary batteries using a liquid electrolyte (hereinafter referred to as "electrolyte") have been used as secondary batteries with high energy density. Lithium-ion secondary batteries using an electrolyte have a structure in which a separator is interposed between a positive electrode and a negative electrode, and the separator holds the electrolyte.

[0003] Lithium-ion secondary batteries mainly use organic electrolytes. Organic electrolytes pose concerns about leakage due to their liquid nature and flammability. Therefore, to improve the safety of lithium-ion secondary batteries, secondary batteries using solid electrolytes (hereinafter referred to as all-solid-state batteries) instead of liquid electrolytes have been developed. All-solid-state batteries, naturally, do not leak because the electrolyte is solid, and are more flame-retardant and heat-resistant than liquid electrolytes, making them attractive as safe secondary batteries. Because of their high safety, small all-solid-state batteries are being mass-produced for use in wearable devices that come into direct contact with the skin.

[0004] Furthermore, unlike lithium-ion secondary batteries that use electrolytes, all-solid-state batteries are batteries that experience minimal deterioration in their characteristics at high temperatures, eliminating the need for cooling devices and making them advantageous for improving the energy density per unit volume of the battery pack. Because of their advantages as secondary batteries with high volumetric energy density, all-solid-state batteries are expected to be further enlarged for use in electric vehicles, etc.

[0005] Unlike secondary batteries that use a liquid electrolyte, all-solid-state batteries have a solid electrolyte layer between the positive and negative electrodes, rather than a separator containing a liquid electrolyte. For example, in the case of a lithium-ion all-solid-state battery, during charging, lithium ions pass from the positive electrode through the solid electrolyte layer to the negative electrode. On the other hand, during discharging, lithium ions pass from the negative electrode through the solid electrolyte layer to the positive electrode. Thus, in the case of all-solid-state batteries, various ion species, such as lithium ions and sodium ions, have been considered as the ion species conducting between the positive and negative electrodes (hereinafter referred to as carrier ions), from the perspective of avoiding issues such as stable resource supply. In order for these carrier ions to travel between the positive and negative electrodes through the solid electrolyte layer, a carrier ion path must be formed in the thickness direction of the solid electrolyte layer.

[0006] In other words, the solid electrolyte layer interposed between the positive and negative electrodes of an all-solid-state battery is required to have the functions of ion conduction of carrier ions between the positive and negative electrodes and preventing short-circuiting between the positive and negative electrode active materials. In addition, the thickness of the solid electrolyte layer is required to be thin in order to achieve excellent volumetric energy density and low internal resistance.

[0007] Methods for forming the solid electrolyte layer include a method in which a solid electrolyte and a binder are mixed and rolled under heat to form a sheet, and a method in which a solid electrolyte slurry is applied to an electrode and dried.

[0008] However, when forming a solid electrolyte layer for an all-solid-state battery used in a large battery, such as an electric vehicle, a solid electrolyte layer obtained by, for example, rolling under heat to form a sheet tends to break or crack during handling. Furthermore, when a method is used in which a slurry containing a solid electrolyte is applied to an electrode and then dried, distortion and cracking occur in the solid electrolyte layer during drying. Therefore, it is difficult to form a stable, thin, and uniform solid electrolyte layer. Failure to form a stable, thin, and uniform solid electrolyte layer can lead to poor ionic conduction and even short circuits. On the other hand, while the thickness of the solid electrolyte layer can be increased to prevent short circuits, a large thickness leads to a decrease in volumetric energy density, a long interelectrode distance, and high internal resistance.

[0009] To solve the above problems, it has been proposed to use a solid electrolyte sheet in an all-solid-state battery, in which a solid electrolyte is impregnated in a thin film sheet (hereinafter referred to as a support) and the solid electrolyte and the support are integrated. Various configurations have also been proposed for the support for the all-solid-state battery and the nonwoven fabric substrate for the lithium secondary battery separator.

[0010] For example, a technology has been disclosed relating to a solid electrolyte sheet having a plurality of through-holes formed by etching a support film (see, for example, Patent Document 1). In this technology, the through-holes formed by the etching process are filled with a solid electrolyte to form an all-solid-state battery having excellent energy density and output characteristics.

[0011] Also, a technology has been disclosed relating to a solid electrolyte sheet using a support having a porosity of 60% or more and 95% or less and a thickness of 5 μm or more and less than 20 μm (see, for example, Patent Document 2). It is disclosed that this solid electrolyte sheet is self-supporting despite its thin thickness.

[0012] Furthermore, a technology has been disclosed relating to a nonwoven fabric substrate for a lithium secondary battery separator, which contains unstretched polyester fibers and moist heat adhesive fibers as binder fibers (see, for example, Patent Document 3). This technology discloses that the unstretched polyester fibers soften or melt by heat and pressure treatment such as calendaring, thereby firmly adhering to other fibers, and that the moist heat adhesive fibers flow or easily deform in a wet state, thereby exhibiting adhesive properties. By incorporating these binders into the nonwoven fabric substrate, a nonwoven fabric substrate for a lithium secondary battery separator is constructed, which has high tensile strength and high productivity.

[0013] JP 2017-103146 ​​A JP 2020-77488 A JP 2020-161243 A

[0014] However, in the technology described in Patent Document 1, when a solid electrolyte sheet is fabricated, the through-holes are filled with the solid electrolyte, so that the solid electrolyte is filled only inside the formed through-holes. As a result, the insulating film portion remains in areas other than the through-holes, and an interface that does not allow carrier ions to pass between the positive electrode and the negative electrode and the film portion is formed. As a result, the interfacial resistance between the solid electrolyte sheet and the positive electrode and the negative electrode tends to be high, and the resistance of an all-solid-state battery using this support is high.

[0015] Furthermore, in the technology described in Patent Document 2, although the support has sufficient voids, if the support material contains thin fibers, the support will have a dense structure. As a result, it becomes difficult for the solid electrolyte slurry to penetrate into the support or to pass through in the thickness direction. This results in insufficient formation of carrier ion path lines in the thickness direction of the solid electrolyte sheet, resulting in a solid electrolyte sheet with high internal resistance. Furthermore, in the technology described in Patent Document 2, if the support has a small number of fibers, which are the support material, the permeability of the solid electrolyte slurry in the thickness direction of the support becomes too high, raising concerns about the retention of the solid electrolyte slurry. As a result, the solid electrolyte slurry cannot be retained on the support, the support's solid electrolyte retention effect cannot be fully achieved, and resistance increases.

[0016] Furthermore, in the technology described in Patent Document 3, as described above, the heat-and-moisture adhesive fibers contained in the nonwoven fabric substrate flow or deform when exhibiting their adhesive function, and as a result, they may not maintain their fibrous state and may end up filling voids within the nonwoven fabric substrate. Furthermore, if a large amount of binder fibers that cannot maintain their fibrous shape are included, the density increases. As a result, the penetration of the solid electrolyte slurry into the nonwoven fabric substrate and the permeability in the thickness direction of the nonwoven fabric substrate become insufficient. This reduces the amount of carrier ion pathlines formed within the solid electrolyte sheet, resulting in increased resistance of the all-solid-state battery.

[0017] In order to solve the above-mentioned problems, the present invention provides a support for a secondary battery capable of reducing the internal resistance of a solid electrolyte layer, a solid electrolyte sheet using this support, and a secondary battery using this support.

[0018] The support for a secondary battery of the present invention is a support for holding a solid electrolyte of a secondary battery, and is made of at least one material selected from paper and nonwoven fabric, which is substantially free of non-fibrillated fibers, and has a perfluoropolyether permeability in the thickness direction of 1 to 15%.

[0019] In addition, the solid electrolyte sheet of the present invention has a solid electrolyte supported on a thin film support made of at least one material selected from paper and nonwoven fabric, which is substantially free of non-fibrillated fibers, and has a permeability to perfluoropolyether of 1 to 15% in the thickness direction of the support.

[0020] The secondary battery of the present invention includes a positive electrode layer, a negative electrode layer, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer. In this secondary battery, the solid electrolyte layer is configured as a solid electrolyte sheet in which a solid electrolyte is supported on a support. The solid electrolyte sheet includes a support that is made of at least one material selected from paper and nonwoven fabric and that is substantially free of non-fibrillated fibers and has a permeability to perfluoropolyether of 1 to 15% in the thickness direction, and a solid electrolyte supported on the support.

[0021] According to the present invention, it is possible to provide a support for a secondary battery capable of reducing the internal resistance of a solid electrolyte layer, a solid electrolyte sheet using this support, and a secondary battery using this support for a secondary battery.

[0022] Hereinafter, examples of embodiments for carrying out the present invention will be described, but the present invention is not limited to the following examples. The description will be given in the following order: 1. Embodiment of a secondary battery support (first embodiment) 2. Embodiment of a secondary battery (second embodiment)

[0023] 1. Embodiments of a Secondary Battery Support (First Embodiment) Specific embodiments of a secondary battery support are described below. The secondary battery support of this embodiment (hereinafter also referred to simply as the support) is used to form a solid electrolyte layer interposed between a positive electrode and a negative electrode in a secondary battery. The support is for holding the solid electrolyte of the secondary battery, and is made of at least one material selected from paper and nonwoven fabric, and has a perfluoropolyether transmittance in the thickness direction of 1 to 15%. Note that in this disclosure, numerical ranges indicated by "to" include the numerical values ​​indicated as the upper and lower limits.

[0024] The solid electrolyte layer interposed between the positive electrode and negative electrode is required to conduct carrier ions between the positive electrode and negative electrode during charge and discharge. To achieve this, carrier ion path lines must be formed between the positive electrode and solid electrolyte layer, inside the solid electrolyte layer, and between the solid electrolyte layer and negative electrode. In other words, if the interfacial resistance between the positive electrode and solid electrolyte layer and between the solid electrolyte layer and negative electrode can be reduced, and the resistance inside the solid electrolyte layer can be reduced, the resistance of the all-solid-state battery can be lowered.

[0025] The present inventors have found that one of the factors inhibiting further reduction of the internal resistance of the solid electrolyte sheet is the influence of the permeability of the liquid in the thickness direction of the support. Therefore, unlike conventional gas permeability, the support of this embodiment uses a liquid, similar to that used in actual coating, to evaluate the diffusion of the solid electrolyte slurry into the support and the retention of the liquid by the support.

[0026] If the liquid permeability in the thickness direction of the support is too low, fewer carrier ion path lines will be formed inside the solid electrolyte sheet. On the other hand, if the liquid permeability in the thickness direction of the support is too high, the support will not retain the solid electrolyte slurry well, and less solid electrolyte slurry will remain on the support, resulting in fewer carrier ion path lines formed inside the solid electrolyte sheet. As a result, in both of the above cases, the resistance of the resulting solid electrolyte sheet will be high. Therefore, by optimizing the liquid permeability in the thickness direction of the support, it is possible to increase the number of carrier ion path lines formed inside the solid electrolyte sheet. As a result, it is possible to reduce the resistance of the solid electrolyte layer.

[0027] In this embodiment, the transmittance of perfluoropolyether in the thickness direction was used as an index for measuring the liquid permeability in the thickness direction of the support. Perfluoropolyether is a chemically inert substance with low surface tension that is liquid at room temperature, so it is a liquid that can be used to evaluate the permeability in the thickness direction due to the internal structure of the support.

[0028] For example, when a similar evaluation is performed using water, if the support has high hydrophilicity, the fibers constituting the support will swell due to the influence of water, causing changes in the internal structure of the support. Furthermore, if the support has low hydrophilicity, the wettability of the support will be low, resulting in reduced penetration into the support, making it impossible to accurately grasp the permeability and permeability due to the structure of the support. In addition, water has a high surface tension, which inhibits water penetration and permeation into the support, making it impossible to accurately evaluate the influence of the internal structure of the support. Furthermore, when a similar evaluation is performed using a common organic solvent, for example, the organic solvent evaporates during the test due to its low vapor pressure, making it impossible to accurately grasp the amount of permeation.

[0029] In other words, compared to other liquids, perfluoropolyether has a smaller interaction with the support, and therefore the influence of the test liquid on the fibers that make up the support is minimal, making it possible to accurately evaluate the inherent characteristics of the support for supports made of different fiber types.In this application, Propene, 1,1,2,3,3-Hexafluoro, oxidized, polymerized (surface tension 16 mN / m) is used as the perfluoropolyether.

[0030] The amount of perfluoropolyether permeated in the thickness direction is the mass of perfluoropolyether that is dropped onto the surface of the support in a predetermined amount, penetrates and diffuses through the support, and reaches the back surface of the support. The transmittance of perfluoropolyether in the thickness direction is calculated by dividing the amount of perfluoropolyether permeated in the thickness direction by the mass of the dropped perfluoropolyether, and converting the result into a percentage. In other words, by determining the transmittance of perfluoropolyether, the permeability of the solid electrolyte slurry in the thickness direction of the support can be determined.

[0031] A low permeability of the perfluoropolyether in the thickness direction indicates low permeability of the solid electrolyte slurry in the thickness direction of the support, whereas a high permeability of the perfluoropolyether in the thickness direction indicates too high permeability of the solid electrolyte slurry into the support, resulting in poor retention of the solid electrolyte slurry on the support.

[0032] The support has a perfluoropolyether permeability in the thickness direction controlled within a range of 1 to 15%. A support having a perfluoropolyether permeability in the thickness direction within the above range has excellent permeability and retention of solid electrolyte slurry in the thickness direction of the support, allowing the necessary amount of solid electrolyte slurry to be uniformly impregnated into the support. As a result, a necessary amount of carrier ion path lines can be formed within the obtained solid electrolyte sheet. These effects reduce the internal resistance of the solid electrolyte layer. In other words, the use of this support can reduce the resistance of the all-solid-state battery. Furthermore, from the viewpoint of the permeability and retention of solid electrolyte slurry through the support, it is more preferable that the perfluoropolyether permeability in the thickness direction of the support be within a range of 3 to 13%.

[0033] If the permeability of the perfluoropolyether in the thickness direction of the support is less than the above range, the amount of permeation of the solid electrolyte slurry in the thickness direction of the support is small, and therefore the number of carrier ion path lines formed in the thickness direction inside the solid electrolyte sheet is reduced.

[0034] On the other hand, if the permeability of the perfluoropolyether in the thickness direction of the support exceeds the above range, the amount of solid electrolyte slurry that can be retained by the support is small, resulting in fewer carrier ion path lines formed in the thickness direction inside the solid electrolyte sheet, and an increase in the internal resistance of the solid electrolyte sheet.

[0035] As long as the perfluoropolyether permeability is within the above range, other configurations of the support, such as thickness, basis weight, density, porosity, and tensile strength, are not particularly limited. As long as the perfluoropolyether permeability is within the above range, the support is not limited to its configuration, and it is possible to evaluate whether carrier ion pathlines are sufficiently formed within the solid electrolyte sheet, thereby reducing the internal resistance of the solid electrolyte sheet and the resistance of the all-solid-state battery. From the viewpoints of the permeability, retention, and tensile strength of the support to the solid electrolyte slurry, the basis weight of the support is 1.0 to 15.0 g / m ２ It is more preferable to set the range to:

[0036] The support is made of at least one material selected from paper and nonwoven fabric. Preferably, the support is made of at least one of paper and nonwoven fabric. This is for the following reasons: Paper refers to a material made by agglutinating plant fibers and other fibers. Nonwoven fabric refers to a sheet-like material made without the use of a loom by processing various fiber webs, such as natural, recycled, and synthetic fibers, mechanically, chemically, thermally, or a combination thereof, and bonding the constituent fibers to each other using adhesives or the adhesive strength of the fibers themselves. In other words, paper and nonwoven fabric are constructed with randomly arranged fibers, and therefore contain numerous voids and through-holes of various sizes within them. Therefore, the coated solid electrolyte slurry can spread not only in the thickness direction but also in the plane direction. In other words, the coated solid electrolyte may remain on the surface of the support, remain inside the support, or pass through the through-holes from the surface side to the back side.

[0037] Therefore, in a solid electrolyte sheet manufactured using at least one of paper and nonwoven fabric as a support, the solid electrolyte is filled not only on the surface of the support but also inside the support, forming a good path line for carrier ions. As a result, the internal resistance of the solid electrolyte sheet can be reduced, and the interfacial resistance between the solid electrolyte sheet and the positive electrode or negative electrode can be reduced. As a result, the resistance of the all-solid-state battery can be reduced.

[0038] Materials that can be used to form the support are not particularly limited as long as they do not repel the solid electrolyte slurry and do not adversely affect the solid electrolyte physically or chemically. Examples include organic fibers such as cellulose fibers, polyamide fibers, polyester fibers, polypropylene fibers, and acrylic fibers, and inorganic fibers such as glass fibers and alumina fibers. One or more fibers selected from these fibers can also be used. By using these fibers, a support with excellent solid electrolyte filling properties can be obtained.

[0039] Furthermore, from the viewpoint of maintaining the shape of the support and tensile strength, it is desirable for the support to contain adhesive fibers, such as fibers having fibrils on the surface thereof (hereinafter referred to as fibrillated fibers) and synthetic resin binder fibers.

[0040] For example, the adhesive strength of fibrillated cellulose fibers is due to physical bonds formed by the entanglement of cellulose fibers and chemical bonds formed by hydrogen bonds between hydroxyl groups in cellulose. Furthermore, the adhesive strength of fibrillated polyamide fibers and fibrillated acrylic fibers is due to physical bonds formed by the entanglement of fibers. Both types of fiber bonding are preferred because they contribute to maintaining the shape of the support and developing tensile strength. It is preferable that the support is substantially free of fibers that do not have fibrils on their surface (hereinafter referred to as non-fibrillated fibers). "Substantially free of non-fibrillated fibers" means that the content of non-fibrillated fibers in all fibers constituting the support is less than 1% by mass.

[0041] Synthetic resin binder fibers can be classified into two types: those that maintain their fibrous state when the support is formed, and those that cannot maintain their fibrous state and become, for example, film-like. Binder fibers that maintain their fibrous state when the support is formed are preferred because they are less likely to impair permeability and permeability and can improve the tensile strength of the support. Binder fibers that maintain their fibrous state when the support is formed exhibit adhesive strength by thermally bonding the fiber entanglement points. Therefore, binder fibers that maintain their fibrous state as a constituent material of the support can reduce breakage due to physical impact and are bonded only at fiber contact points, so they are less likely to impede the penetration and permeation of solid electrolyte slurry into the support when a solid electrolyte sheet is formed. On the other hand, synthetic resin binder fibers that cannot maintain their fibrous state when the support is formed undergo heat transformation into a film-like state during the support production process. When heat is applied near the melting point or softening point of the resin that constitutes the fiber, the resin melts and fuses at the fiber entanglement points. In other words, if a binder that is not in a fibrous state is used in a formed support, the binder component will form a film layer in the gaps between the fibers of the support and fill the gaps when the binder function is exerted, which may result in impeding the penetration and permeation of the solid electrolyte into the support, so care must be taken when using the binder.

[0042] Materials that can be used as synthetic resin binder fibers that maintain a fiber shape and have adhesive strength are not particularly limited as long as they do not repel solid electrolyte slurry and do not adversely affect the solid electrolyte physically or chemically, and examples thereof include fibrillated fibers such as beaten cellulose fibers, beaten polyamide fibers, and beaten acrylic fibers, polyamide binder fibers, polyester binder fibers, polyethylene binder fibers, polypropylene-polyethylene core-sheath binder fibers, etc. One or more types of fibers selected from these fibers can also be used.

[0043] In order to achieve a thickness-wise perfluoropolyether transmittance of 1 to 15% in the support, for example, the fiber length of fibrillated fibers may be controlled to 0.1 to 2 mm. To obtain fibrillated fibers with a fiber length of 0.1 to 2 mm, for example, the fibers may be beaten to a CSF (Canadian Standard freeness) value of 0 to 400 ml. However, this is not a limitation as long as the thickness-wise perfluoropolyether transmittance can be maintained within the range of 1 to 15%.

[0044] Fibrillated fibers have branched fibrils on the fiber surface, allowing a large number of fibers to support a solid electrolyte. Therefore, when the solid electrolyte is impregnated into the support, the support can retain the solid electrolyte by the fibrils. In other words, because the support can support a large amount of solid electrolyte, the occurrence of cracks inside the resulting solid electrolyte sheet can be suppressed. As a result, the resistance of the resulting solid electrolyte sheet can be reduced.

[0045] The method for producing the support is not particularly limited, and can be a dry method or a wet method. However, a papermaking method in which fibers dispersed in water are deposited on a wire, dehydrated, dried, and then papered is preferred from the viewpoint of uniformity of the support's formation, etc. The papermaking method for the support is not particularly limited as long as it satisfies the perfluoropolyether transmittance in the thickness direction, and papermaking methods such as Fourdrinier papermaking, Short Drain papermaking, and Cylinder papermaking can be used, and multiple layers formed by these papermaking methods can also be combined. In addition, additives such as dispersants, antifoaming agents, and paper strength agents can be added during papermaking, and post-processing such as paper strength strengthening, lyophilicity processing, calendering, hot calendering, and embossing can be performed after the paper layer formation.

[0046] 2. Secondary Battery Embodiment (Second Embodiment) Next, an embodiment of a secondary battery using the support described above will be described. The secondary battery is, for example, an all-solid-state battery including a positive electrode, a negative electrode, and a solid electrolyte layer. In this case, the solid electrolyte layer is disposed so as to be interposed between the positive electrode and the negative electrode. The solid electrolyte layer is configured as a solid electrolyte sheet in which a solid electrolyte is held by a support. The solid electrolyte sheet includes a support for a secondary battery and a solid electrolyte, and the solid electrolyte is held by the support, and the solid electrolyte and the support are configured to be integrated.

[0047] The types of positive and negative electrodes used in the all-solid-state battery are not particularly limited. For example, the all-solid-state battery may be configured with known positive and negative electrodes, but is not limited thereto. Furthermore, the type of all-solid-state battery is not particularly limited, and depending on the selection of materials constituting the all-solid-state battery, it can be formed into, for example, a lithium-ion secondary battery, a sodium-ion secondary battery, etc. The all-solid-state battery can be used, for example, as a storage battery for mobile communication devices, portable electronic devices, electric bicycles, electric motorcycles, electric vehicles, small-sized home power storage devices, etc.

[0048] [Positive Electrode, Negative Electrode] In the secondary battery, the positive electrode active material used in the positive electrode layer and the negative electrode active material used in the negative electrode layer are not particularly limited as long as they function as the positive electrode and negative electrode of the all-solid-state battery in accordance with various carrier ions.

[0049] The positive electrode is formed by including a positive electrode active material-containing layer and a positive electrode current collector. For example, if the all-solid-state battery is a lithium ion secondary battery, the positive electrode needs to be made of a material capable of absorbing and releasing lithium ions.

[0050] The positive electrode current collector may be made of, for example, aluminum. The positive electrode active material may be, for example, a sulfide-based material such as titanium sulfide (TiS ２ ), molybdenum sulfide (MoS ２ ), iron sulfide (FeS, FeS ２ ), copper sulfide (CuS) and nickel sulfide (Ni ３ S ２ ) and the like. In addition, examples of oxides include bismuth oxide (Bi２ O ３ ), bismuth lead oxide (Bi ２ Pb ２ O ５ ), copper oxide (CuO), vanadium oxide (V ６ O １３ ), lithium cobalt oxide (LiCoO ２ ), lithium nickel oxide (LiNiO ２ ), lithium manganese oxide (LiMnO ２ ), Li(NiCoMn)O ２ , Li(NiCoAl)O ２ , Li(NiCo)O ２ These may also be used in combination.

[0051] The negative electrode is formed by including a negative electrode current collector and a layer containing a negative electrode active material. For example, if the all-solid-state battery is a lithium-ion secondary battery, metallic lithium, metallic indium, or a material capable of absorbing and releasing lithium ions can be used as the negative electrode active material.

[0052] For example, copper can be used as the negative electrode current collector. Examples of the negative electrode active material include carbon materials, specifically, artificial graphite, graphite carbon fiber, resin-baked carbon, pyrolytic vapor-grown carbon, coke, mesocarbon microbeads (MCMB), furfuryl alcohol resin-baked carbon, polyacene, pitch-based carbon fiber, vapor-grown carbon fiber, natural graphite, and non-graphitizable carbon. Alternatively, a mixture of these may be used. Examples of the negative electrode active material include metals such as lithium metal, indium metal, aluminum metal, and silicon metal, or alloys of these metals with other elements or compounds.

[0053] For the positive electrode and negative electrode constituting the all-solid-state battery, two types are selected from materials that can be used to form electrodes, and the charge / discharge potentials of the two types of compounds are compared. The one exhibiting a more noble potential is used as the positive electrode, and the one exhibiting a more base potential is used as the negative electrode, to form an arbitrary battery.

[0054] [Solid Electrolyte Layer] In an all-solid-state battery, the solid electrolyte layer is configured as a solid electrolyte sheet in which a solid electrolyte is supported on a support. The type of solid electrolyte constituting the solid electrolyte sheet is not particularly limited, and for example, a known material that can be used as a solid electrolyte in an all-solid-state battery can be used. The solid electrolyte is not particularly limited, and any material that can conduct carrier ions between the positive electrode and the negative electrode can be used. Examples of the solid electrolyte include oxide-based solid electrolytes and sulfide-based solid electrolytes. If necessary, other components such as a binder may be added.

[0055] For example, sulfide-based solid electrolytes capable of conducting lithium ions include sulfide-based amorphous solid electrolytes and sulfide-based crystalline solid electrolytes. Specific examples of sulfide-based amorphous solid electrolytes include Li ２ S-SiS ２ , Li ２ S-GeS ２ , Li ２ , S-P ２ S ５ , Li ２ S-B ２ S ３ , Li ２ S-SiS ２ -Li ３ P.O. ４ , Li ２ S-S, iS ２ -Li ２ SO ４ , Li ２ S-P ２ S ５ - LiI, Li ２ S-P ２ S ５ -P ２ O ５ -, LiI, Li ２ S-B ２ S ３ - LiI, Li ２ S-P ２ S ５ -Li ２ O-LiI, Li ２ , S-SiS ２ -B ２ S ３-LiI, etc. The sulfide-based amorphous solid electrolyte may contain other elements. Specific examples of sulfide-based crystalline solid electrolytes include Li ３．２５ Ge ０．２５ P ０．７５ S ４ , Li １０ GeP ２ S １２ , Li ６ P.S. ５ However, the sulfide-based crystalline solid electrolyte is not limited to these element compositions.

[0056] The solid electrolyte may be other than a sulfide-based solid electrolyte or an oxide-based solid electrolyte, and other examples include semi-solid polymer electrolytes containing carrier ions, such as polyethylene oxide, polypropylene oxide, polyvinylidene fluoride, or polyacrylonitrile. The solid electrolyte may be a so-called gel-type electrolyte in which an electrolytic solution is held in a polymer solid electrolyte, such as a polyethylene oxide-based polymer, or a polymer containing at least one selected from a polyorganosiloxane chain and a polyoxyalkylene chain.

[0057] [Method for manufacturing a solid electrolyte sheet] The method for manufacturing a solid electrolyte sheet is not particularly limited, and a method commonly used in the art can be applied. For example, a method can be used in which a solid electrolyte is dispersed in a solvent to prepare a slurry, and the prepared slurry is applied to a support and dried. The solvent used for preparing the solid electrolyte slurry is not particularly limited as long as it does not adversely affect the performance of the solid electrolyte. For example, a non-aqueous solvent can be used.

[0058] The coating method for coating one or both sides of the support with the solid electrolyte-containing slurry is not particularly limited, and examples thereof include slide die coating, comma die coating, comma reverse coating, gravure coating, gravure reverse coating, etc. Drying performed after coating the solid electrolyte-containing slurry can be performed, for example, with a drying device using hot air, a heater, high frequency waves, etc.

[0059] The solid electrolyte sheet may be used as a dried sheet as is, or may be further pressed to increase its mechanical strength and density. Examples of the pressing method include sheet pressing and roll pressing.

[0060] [Method for manufacturing an all-solid-state battery] An all-solid-state battery can be manufactured by placing a solid electrolyte layer including a solid electrolyte sheet between the above-mentioned positive electrode layer and negative electrode layer, and bonding them together. The bonding method is not particularly limited, but examples include a method of stacking the sheets and applying pressure and crimping, and a method of applying pressure between two rolls (roll to roll). In addition, in order to improve the adhesion between the solid electrolyte layer and the positive electrode layer or the negative electrode layer, an active material having ion conductivity or an adhesive material that does not inhibit ion conductivity may be placed at the bonding interface.

[0061] Specific examples of the support according to the embodiment of the present invention will be described below. First, the support of Examples 1 to 5 and Comparative Examples 1 to 6 were prepared by the following method. The support was made of paper formed using a papermaking method or a wet-laid nonwoven fabric, except for Comparative Example 3.

[0062] Example 1 Fourdrinier paper was made using cellulose fibers with a CSF value of 10 ml and an average fiber length of 1.0 mm. The resulting paper was calendered to a thickness of 5 μm and a basis weight of 4.0 g / m. ２ , density 0.80g / cm ３ A support of 1000 .mu.m was obtained.

[0063] [Example 2] Fourdrinier paper was made using polyamide fibers with a CSF value of 0 ml and an average fiber length of 0.2 mm, and the thickness was 22 μm and the basis weight was 15.0 g / m ２ , density 0.69g / cm ３ A support of 1000 .mu.m was obtained.

[0064] [Example 3] A raw material was mixed with 50% by mass of cellulose fibers having a CSF value of 390 ml and an average fiber length of 2 mm, and 50% by mass of polyamide fibers having a CSF value of 5 ml and an average fiber length of 0.5 mm, and the mixture was subjected to cylinder papermaking to produce a paper having a thickness of 8 μm and a basis weight of 1.4 g / m. ２ , density 0.18g / cm ３ A support of 1000 .mu.m was obtained.

[0065] [Example 4] Cellulose fibers having a CSF value of 100 ml and an average fiber length of 1.1 mm were used to make short-wire paper, which had a thickness of 15 μm and a basis weight of 3.0 g / m ２ , density 0.20g / cm ３ A support of 1000 .mu.m was obtained.

[0066] [Example 5] A raw material was mixed with 70% by mass of cellulose fibers having a CSF value of 3 ml and an average fiber length of 0.5 mm and 30% by mass of polyamide fibers having a CSF value of 120 ml and an average fiber length of 1.5 mm, and short-wire paper was made into a sheet having a thickness of 40 μm and a basis weight of 10.0 g / m. ２ , density 0.25g / cm ３ A support of 1000 .mu.m was obtained.

[0067] Comparative Example 1: A raw material was prepared by mixing 15% by mass of polyester fibers having an average fiber diameter of 16 μm and an average fiber length of 5 mm and 85% by mass of polyester binder fibers having an average fiber diameter of 4 μm and an average fiber length of 3 mm. This was subjected to cylinder papermaking in accordance with the support manufacturing method described in Example 1 of Patent Document 2, and a thickness of 19 μm and a basis weight of 3.8 g / m was obtained. ２ , density 0.20g / cm ３ A support of 1000 .mu.m was obtained.

[0068] Comparative Example 2: A raw material was prepared by mixing 40% by mass of polyester fibers having an average fiber diameter of 2.3 μm and an average fiber length of 3 mm, 50% by mass of polyester binder fibers having an average fiber diameter of 4.2 μm and an average fiber length of 3 mm, and 10% by mass of ethylene-vinyl alcohol fibers having an average fiber diameter of 7.2 μm and an average fiber length of 5 mm. This raw material was subjected to short-wire papermaking and heat calendering in accordance with the method for producing a support described in Example 1 of Patent Document 3, to obtain a sheet having a thickness of 12 μm and a basis weight of 7.0 g / m. ２ , density 0.58g / cm ３ A support of 1000 .mu.m was obtained.

[0069] [Comparative Example 3] A support was produced in the same manner as in Example 2 of Patent Document 1, to obtain a support of Comparative Example 3. In Comparative Example 3, a polyimide film was etched to form 200 μm square holes, and a thickness of 30 μm and a basis weight of 8.8 g / m were obtained. ２ , density 0.29g / cm ３ A support of 1000 .mu.m was obtained.

[0070] [Comparative Example 4] Fourdrinier paper was made using polyamide fibers with a CSF value of 0 ml and an average fiber length of 0.1 mm, and the thickness was 18 μm and the basis weight was 16.0 g / m ２ , density 0.89g / cm ３ A support of 1000 .mu.m was obtained.

[0071] Comparative Example 5: A raw material was mixed with 70% by mass of cellulose fibers having a CSF value of 410 ml and an average fiber length of 2.5 mm and 30% by mass of polyamide fibers having a CSF value of 100 ml and an average fiber length of 0.7 mm, and the mixture was subjected to short-wire papermaking to produce a paper having a thickness of 15 μm and a basis weight of 2.3 g / m. ２ , density 0.15g / cm ３ A support of 1000 .mu.m was obtained.

[0072] Comparative Example 6 Short wire paper was made using cellulose fibers with a CSF value of 0 ml and an average fiber length of 0.04 mm, but a paper layer could not be formed and a support could not be obtained.

[0073] [Fabrication of All-Solid-State Battery] Next, all-solid-state batteries were fabricated using the supports of the above-mentioned Examples and Comparative Examples. The specific fabrication method is as follows. (Positive Electrode Structure) LiNiCoAlO was used as the positive electrode active material. ２ The ternary powder was prepared by adding Li as a sulfide-based solid electrolyte. ２ S-P ２ S ５ The amorphous powder was mixed with carbon fiber as a conductive additive. This mixed powder was mixed with a dehydrated xylene solution containing SBR (styrene butadiene rubber) as a binder to prepare a positive electrode coating solution. The positive electrode coating solution was applied to an aluminum foil current collector, which served as a positive electrode current collector, dried, and then rolled to obtain a positive electrode structure.

[0074] (Negative electrode structure) Graphite is used as the negative electrode active material, and Li is used as the sulfide-based solid electrolyte. ２ S-P ２ S ５ The amorphous powder was mixed with PVdF (polyvinylidene fluoride) as a binder and NMP (N-methyl-2-pyrrolidone) as a solvent to prepare a negative electrode coating solution. The negative electrode coating solution was applied to a copper foil current collector serving as a negative electrode current collector, dried, and then rolled to obtain a negative electrode structure.

[0075] (Solid electrolyte sheet) Li as a sulfide-based solid electrolyte ２ S-P ２ S ５ The amorphous powder was mixed with SBR as a binder and xylene as a solvent to prepare a solid electrolyte slurry. The solid electrolyte slurry was applied to the support of each of the above examples and comparative examples, and then dried to obtain a solid electrolyte sheet.

[0076] [Production of All-Solid-State Battery] A negative electrode structure measuring 88 mm × 58 mm, a solid electrolyte sheet measuring 92 mm × 62 mm, and a positive electrode structure measuring 87 mm × 57 mm were stacked, dry laminated, and bonded together to obtain a single cell of an all-solid-state battery. The resulting single cell was placed in an aluminum laminate film with terminals attached, degassed, heat-sealed, and packed.

[0077] [Methods for measuring characteristics of support and all-solid-state battery] The characteristics of the prepared support and all-solid-state battery were measured under the following conditions and methods. [CSF value] The CSF value was measured according to "JIS P8121-2 'Pulps - Determination of drainability - Part 2: 'Canadian Standard' freeness method' (ISO5267-2 'Pulps - Determination of drainability - Part 2: 'Canadian Standard' freeness method')".

[0078] [Average Fiber Length] [Fiber Length] Measurement was performed using a device described in "JIS P 8226-2 'Pulps - Determination of Fiber Length by Automated Optical Analysis - Part 2: Unpolarized Light Method'" (ISO 16065-2 'Pulps - Determination of Fiber Length by Automated Optical Analysis - Part 2: Unpolarized Light Method'), here Fiber Tester PLUS (manufactured by Lorentzen & Wettre), and the length-weighted average fiber length was taken as the fiber length of the fibers.

[0079] [Fiber Length of Polyester Fiber and Polyester Binder Fiber] Because polyester fiber and polyester binder fiber are optically transparent, the fibers cannot be accurately recognized in an image, making it impossible to accurately measure the fiber length using the above-mentioned optical automatic analysis method. Therefore, the fiber length of polyester fiber and polyester binder fiber was measured using the following method. A slide was prepared with fibers randomly dispersed. The fiber length of the fibers on the slide was measured directly using a scale.

[0080] [Thickness] The thickness of the support was measured by the method of folding paper into 10 sheets as described in "5.1.3 Measuring thickness by folding paper" in "JIS C 2300-2 'Cellulose paper for electrical purposes - Part 2: Test methods' 5.1 Thickness", except that the measuring force of the micrometer described in "5.1.1 Measuring instrument and measuring method a When an outside micrometer is used" was changed to 1.5 N and the diameter of the pressure surface was changed to 14.3 mmφ.

[0081] [Basis Weight] The basis weight of the support in an absolute dry state was measured according to the method specified in "JIS C 2300-2 'Cellulose paper for electrical purposes - Part 2: Test methods' 6 Basis Weight".

[0082] [Density] The density of the support was calculated using the following formula: Density (g / cm ３ )=W / T W: Basis weight (g / m ２ ), T: thickness (μm)

[0083] [Porosity] The porosity of the support was calculated using the following formula. When a plurality of materials constituting the support were mixed, the average specific gravity of the constituent fibers was calculated in proportion to the mixing ratio, and then the porosity was calculated. Porosity (%) = (1 - (D / S)) x 100 D: Support density (g / cm ３ ), S: specific gravity of constituent fibers (g / cm ３ )

[0084] [Permeability of perfluoropolyether in the thickness direction] A test piece cut to 50 mm x 50 mm was fixed to the bottom of a cylinder with an inner diameter of 12 mm and an outer diameter of 15 mm. The cylinder with the test piece attached was placed so that the test piece was in contact with a glass plate. 0.1 ml of perfluoropolyether was dropped into the cylinder, and the mass of the dropped perfluoropolyether was measured. After dropping the perfluoropolyether, it was left for 1 minute, and the cylinder with the test piece attached was removed. The mass of perfluoropolyether that penetrated and permeated the support from the inside of the cylinder and adhered to the glass plate was measured. The permeability of perfluoropolyether in the thickness direction of the support was calculated using the following formula:

[0085] Transmittance (%) = w2 / w1 x 100, where w1: mass (g) of dropped perfluoropolyether, and w2: mass (g) of permeated perfluoropolyether.

[0086] [Tensile Strength] The maximum tensile load in the longitudinal direction (manufacturing direction) of the support was measured using a test width of 15 mm according to the method specified in "JIS P 8113 'Paper and board-Determination of tensile properties-Part 2: Constant rate of elongation method'" (ISO 1924-2 'Paper and board-Determination of tensile properties-Part 2: Constant rate of elongation method'), and this was taken as the tensile strength of the support.

[0087] [Evaluation of Self-Supporting Ability] The self-supporting ability of each solid electrolyte sheet was evaluated. When a solid electrolyte sheet measuring 92 mm × 62 mm was lifted up while holding the edge of the short side, if no cracks or breaks were found in the solid electrolyte sheet by visual inspection, it was evaluated as ◯, and if cracks or breaks were found in the solid electrolyte sheet by visual inspection when lifted up, it was evaluated as ×.

[0088] [Internal Resistance of Solid Electrolyte Sheet] The all-solid-state battery was charged to 4.0 V at a current density of 0.1 C in an environment of 25° C., and the impedance was measured in the frequency range of 0.1 Hz to 1 MHz using an LCR meter. The arc portion of the obtained Cole-Cole plot was fitted to a semicircle with the x-axis as the base, and the numerical value at the point where the right end of the semicircle intersects with the x-axis was taken as the resistance value.

[0089] [Discharge Capacity of Solid Electrolyte Sheet] The all-solid-state battery was charged to 4.0 V at a current density of 0.1 C in an environment of 25° C., and then discharged to 2.5 V at a current density of 0.1 C, and the discharge capacity at this time was measured.

[0090] Table 1 shows the names and blending ratios of the fibers blended in each of the supports of Examples 1 to 5 and Comparative Examples 1 to 6 described above.

[0091]

[0092] Table 2 shows the evaluation results of the characteristics of each support, the self-supporting ability of the solid electrolyte sheet, and the battery characteristics of each of the above-described Examples and Comparative Examples.

[0093]

[0094] The evaluation results of the all-solid-state batteries using the supports of each Example and each Comparative Example are described in detail below. The solid electrolyte sheets using the supports of each Example were able to form self-supporting solid electrolyte sheets. Furthermore, the all-solid-state batteries using the supports of each Example had lower resistance and higher discharge capacity than the all-solid-state batteries using the supports of Comparative Examples 1 to 5.

[0095] A support could not be obtained in Comparative Example 6. The fibers used for the support in Comparative Example 6 had a short fiber length of 0.04 mm, which is thought to be why they did not have enough strength to withstand the manufacturing process. That is, a comparison between each Example and Comparative Example 6 shows that the fiber length of the fibers used for the support is preferably 0.1 mm or more.

[0096] The all-solid-state batteries using the supports of Comparative Example 1 and Comparative Example 5 had higher resistance and lower discharge capacity than the all-solid-state batteries using the supports of each Example. Furthermore, the supports of Comparative Example 1 and Comparative Example 5 had higher perfluoropolyether permeability in the thickness direction compared to the supports of each Example. Because the supports of Comparative Example 1 and Comparative Example 5 had high perfluoropolyether permeability in the thickness direction of 46.0% and 15.6%, respectively, the amount of solid electrolyte slurry that the supports could hold was small. As a result, it is believed that fewer carrier ion pathlines were formed in the thickness direction within the solid electrolyte sheet. The support of Comparative Example 5 used fibers with a high CSF value of 410 ml and a long fiber length of 2.5 mm compared to the other Examples. As a result, the space inside the resulting support was larger, which increased the perfluoropolyether permeability in the thickness direction and reduced the amount of perfluoropolyether that the support could hold. In other words, comparison of each Example with Comparative Example 1 and Comparative Example 5 reveals that a perfluoropolyether permeability in the thickness direction of the support is preferably 15% or less. Furthermore, a comparison between each Example and Comparative Example 5 reveals that the CSF value of the fibers used in the support is preferably 400 ml or less and the fiber length is preferably 2 mm or less.

[0097] The all-solid-state batteries using the supports of Comparative Example 2 and Comparative Example 4 have higher resistance and lower discharge capacity than the all-solid-state batteries using the supports of each Example. The support of Comparative Example 2 contains 10 mass % ethylene-vinyl alcohol fibers, which undergo shape changes due to moist heat. Therefore, when the support is formed, instead of remaining in a fibrous state, a film layer is formed inside the support, filling the gaps between the fibers. This is thought to have hindered the penetration of the solid electrolyte slurry into the support. Furthermore, the support of Comparative Example 4 has a basis weight of 16.0 g / m ２ Since the density is high, the number of fibers inside the support is large, and it is thought that when a predetermined amount of perfluoropolyether is dropped, the penetration and permeation of the perfluoropolyether in the thickness direction of the support is hindered.

[0098] The supports of Comparative Examples 2 and 4 had a low permeability of perfluoropolyether in the thickness direction, both at 0.4%, compared to the supports of each Example, and therefore the amount of solid electrolyte slurry permeating through the support in the thickness direction was low. As a result, it is believed that fewer carrier ion pathlines were formed in the thickness direction inside the solid electrolyte sheet. In other words, a comparison of each Example with Comparative Example 2 shows that the blending amount of synthetic resin binder fiber, which cannot maintain the fibrous state, is preferably less than 10 mass%. Furthermore, a comparison of each Example with Comparative Example 4 shows that the basis weight was 15.0 g / m ２ In addition, from a comparison of each Example with Comparative Example 2 and Comparative Example 4, it is clear that the transmittance of perfluoropolyether in the thickness direction of the support is preferably 1% or more.

[0099] The all-solid-state battery using the support of Example 3 has higher resistance and lower discharge capacity than the all-solid-state battery using the support of Example 4. Furthermore, the support of Example 3 has higher perfluoropolyether transmittance in the thickness direction than the support of Example 4. The perfluoropolyether transmittance in the thickness direction for the support of Example 3 is 14.8%, which is higher than 12.6% for the support of Example 4. Furthermore, the all-solid-state battery using the support of Example 2 has higher resistance and lower discharge capacity than the all-solid-state battery using the support of Example 5. Furthermore, the support of Example 2 has lower perfluoropolyether transmittance in the thickness direction than the support of Example 5. The perfluoropolyether transmittance in the thickness direction for the support of Example 2 is 1.2%, which is lower than 2.3% for the support of Example 5. In other words, a comparison of Examples 2 and 3 with Examples 4 and 5 reveals that a perfluoropolyether transmittance in the thickness direction of the support is more preferably 2 to 13%.

[0100] Unlike the paper or nonwoven fabric supports of each Example, the support of Comparative Example 3 is a support in which through-holes are formed in a film. The through-holes of the support of Comparative Example 3 can be filled with a solid electrolyte, but the solid electrolyte can only be filled inside the formed through-holes. Furthermore, in the solid electrolyte sheet made of the support of Comparative Example 3, it is believed that an interface between the insulating film and the positive electrode or negative electrode exists at the interface between the positive electrode or negative electrode and the solid electrolyte sheet. As a result, it is believed that the all-solid-state battery using the support of Comparative Example 3 had higher resistance and lower discharge capacity than the all-solid-state batteries using the supports of each Example. Comparison of each Example with Comparative Example 3 reveals that paper and nonwoven fabric are suitable as supports for reducing the resistance of all-solid-state batteries.

[0101] The above-described embodiment is merely an example, and those skilled in the art can appropriately modify the compositions of the carrier ions, solid electrolyte, positive electrode, and negative electrode, for example. As explained above, by using paper and nonwoven fabric with a perfluoropolyether permeability of 1 to 15% in the thickness direction, a support with excellent permeability of the solid electrolyte slurry to the support can be obtained. As a result, a required number of carrier ion path lines can be formed in the thickness direction of the support. By using this support, a low-resistance all-solid-state battery can be obtained.

[0102] The present invention is not limited to the configurations described in the above-described embodiments, and various modifications and changes are possible without departing from the scope of the present invention.

Claims

1. A support for holding a solid electrolyte of a secondary battery, comprising at least one selected from paper and non-woven fabric that substantially does not contain fibrillated fibers, and having a perfluoropolyether transmittance in the thickness direction of 1 to 15%, the support for a secondary battery.

2. The support for a secondary battery according to claim 1, wherein the paper and the non-woven fabric contain fibrillated fibers.

3. The support for a secondary battery according to claim 2, wherein the CSF value of the fibrillated fibers is 0 to 400 ml.

4. The support for a secondary battery according to claim 2, wherein the fiber length of the fibrillated fibers is 0.1 to 2 mm.

5. A solid electrolyte sheet in which a solid electrolyte is held on a thin film support made of at least one selected from paper and non-woven fabric that substantially does not contain fibrillated fibers, and having a perfluoropolyether transmittance in the thickness direction of the support of 1 to 15%, the solid electrolyte sheet.

6. A secondary battery comprising a positive electrode layer, a negative electrode layer, and a solid electrolyte sheet disposed between the positive electrode layer and the negative electrode layer, wherein the solid electrolyte sheet comprises a support made of at least one selected from paper and non-woven fabric that substantially does not contain fibrillated fibers and has a perfluoropolyether transmittance in the thickness direction of 1 to 15%, and a solid electrolyte held on the support, the secondary battery.

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