Method for manufacturing sulfide solid electrolyte sheet

Abstract

The present disclosure provides a sulfide solid electrolyte sheet having sufficient self-supporting properties and uniformity. Provided is a method for manufacturing a sulfide solid electrolyte sheet, the method comprising: preparing a slurry by mixing a sulfide solid electrolyte, a non-polar solvent, and at least one polar solvent selected from ethers and alcohols; impregnating a support having voids with the slurry; and removing the non-polar solvent and the polar solvent from the support after impregnation with the slurry.

Description

Method for manufacturing a sulfide solid electrolyte sheet

[0001] This invention relates to a method for producing a sulfide solid electrolyte sheet.

[0002] In recent years, research and development on lithium secondary batteries using oxide-based or sulfide-based solid electrolytes has been actively pursued. Solid electrolytes are materials mainly composed of ion conductors capable of ion conduction in a solid state. Therefore, all-solid-state lithium secondary batteries have the advantage that, in principle, various problems caused by flammable organic electrolytes, as seen in conventional liquid-based lithium secondary batteries, do not occur. In addition, generally, using high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials can significantly improve the power density and energy density of the battery.

[0003] In a method for manufacturing lithium secondary batteries using sulfide-based solid electrolytes, a slurry containing a sulfide solid electrolyte and a solvent is generally applied to a substrate, and then the solvent is dried and removed to form a solid electrolyte layer and an electrode active material layer. For example, Japanese Patent Application Publication No. 2021-99950 discloses a technique in which a mixed solvent containing a first solvent and a second solvent is used as the solvent in a slurry containing a sulfide solid electrolyte and a solvent, and the ratio (V1 / V2) of the evaporation rate V1 of the first solvent to the evaporation rate V2 of the second solvent is set to 10 or more. According to this document, a slurry having such a configuration can provide an all-solid-state battery with good peel strength.

[0004] To improve the energy density of a battery, it is desirable to make the solid electrolyte layer thinner. On the other hand, in order to prevent short circuits between electrodes, it is necessary to maintain the mechanical strength of the solid electrolyte layer. As a means of achieving both thinning the solid electrolyte layer and maintaining its mechanical strength, a solid electrolyte sheet is known in which the solid electrolyte is arranged in the voids of a support having voids.

[0005] However, when the inventors manufactured sulfide solid electrolyte sheets using the slurry described in the above-mentioned literature, it was found that in some cases they were unable to obtain sulfide solid electrolyte sheets with sufficient self-supporting properties and uniformity.

[0006] Therefore, the object of the present invention is to provide a sulfide solid electrolyte sheet having sufficient self-supporting properties and uniformity.

[0007] The inventors of the present invention conducted thorough research in view of the above problems. They found that the above problems could be solved by preparing a slurry containing a sulfide solid electrolyte, a nonpolar solvent, and an ether and / or alcohol, impregnating a support having voids with the slurry, and then removing the solvent to produce a sulfide solid electrolyte sheet, and thus completed the present invention.

[0008] In other words, one embodiment of the present invention is a method for producing a sulfide solid electrolyte sheet, comprising: preparing a slurry by mixing a sulfide solid electrolyte, a nonpolar solvent, and at least one polar solvent selected from ether and alcohol; impregnating a void-filled support with the slurry; and removing the nonpolar solvent and the polar solvent from the support after impregnation with the slurry.

[0009] The embodiments of the present invention will be described below, but the technical scope of the present invention should be determined based on the claims and is not limited to the following forms.

[0010] One embodiment of the present invention is a method for producing a sulfide solid electrolyte sheet, comprising: preparing a slurry by mixing a sulfide solid electrolyte, a nonpolar solvent, and at least one polar solvent selected from ether and alcohol (hereinafter also referred to as the "slurry preparation step"); impregnating a support having voids with the slurry (hereinafter also referred to as the "slurry impregnation step"); and removing the nonpolar solvent and the polar solvent from the support after impregnation with the slurry (hereinafter also referred to as the "solvent removal step"). According to the production method of this embodiment, a sulfide solid electrolyte sheet having sufficient self-supporting properties and uniformity can be provided.

[0011] The mechanism by which the manufacturing method of this embodiment produces the effects described above is not fully understood and is not bound by any theory, but the following mechanism is hypothesized. In the manufacturing method of this embodiment, a slurry is prepared by using a nonpolar solvent and at least one polar solvent selected from ether and alcohol as the solvent (dispersion medium) for the sulfide solid electrolyte. Sulfide solid electrolytes have low solubility in nonpolar solvents but some solubility in polar solvents. Therefore, by using a nonpolar solvent and a polar solvent in combination, a portion of the sulfide solid electrolyte dissolves in the solvent in the slurry, while the remainder remains dispersed without dissolving. By impregnating a support with such a slurry, the undissolved sulfide solid electrolyte fills the large voids in the form of particles, and the dissolved sulfide solid electrolyte enters the fine voids on the order of nanometers in the form of solution. Then, by removing the nonpolar solvent and polar solvent from the support after impregnation with the slurry, the dissolved sulfide solid electrolyte reprecipitates, and the sulfide solid electrolyte fills the fine voids as well. This ensures that the sulfide solid electrolyte is uniformly filled into the voids of the support, improving the adhesion between the sulfide solid electrolyte and the support. As a result, it is presumed that a sulfide solid electrolyte sheet with sufficient self-supporting properties and uniformity can be obtained. However, this mechanism is based solely on speculation, and its accuracy does not affect the technical scope of the present invention.

[0012] The following describes each step in the manufacturing method related to this embodiment.

[0013] [Slurry Preparation Step] In this step, a slurry is prepared by mixing a sulfide solid electrolyte, a nonpolar solvent, and at least one polar solvent selected from ethers and alcohols.

[0014] (Sulfide Solid Electrolytes) Sulfide solid electrolytes refer to solid electrolytes containing the element S. In this specification, solid electrolytes refer to materials mainly composed of ion conductors capable of ion conduction in a solid state, and in particular, materials with a lithium ion conductivity of 1 × 10 at room temperature (25°C) －５A material with a lithium ion conductivity of 1×10 －４ S / cm or more. [Here, the value of the lithium ion conductivity can be measured by the AC impedance method.] The sulfide solid electrolyte essentially contains the S element, preferably contains the S element, the Li element, and the M element (where M is at least one selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl, and I), and more preferably contains the S element, the Li element, and the P element.

[0015] Examples of the sulfide solid electrolyte include, for example, LiI-Li ２ S-SiS ２ 、LiI-Li ２ S-P ２ O ５ 、LiI-Li ３ PO ４ -P ２ S ５ 、Li ２ S-P ２ S ５ 、LiI-Li ３ PS ４ 、LiI-LiBr-Li ３ PS ４ 、Li ３ PS ４ [[ID=4l]]、Li ２ S-P ２ S ５ -LiI、Li ２ S-P ２ S ５ -Li ２ O、Li ２ S-P ２ S ５ -Li ２ O-LiI、Li ２ S-SiS ２ 、Li ２ S-SiS ２ -LiI、Li ２ S-SiS ２ -LiBr、Li ２ S-SiS ２ -LiCl、Li ２ S-SiS ２ -B ２ S ３ -LiI、Li ２ S-SiS２ -P ２ S ５ -LiI, Li ２ S-B ２ S ３ , Li ２ S-P ２ S ５ -Z ｍ S ｎ (where m, n are positive numbers, and Z is any of Ge, Zn, Ga), Li ２ S-GeS ２ , Li ２ S-SiS ２ -Li ３ PO ４ , Li ２ S-SiS ２ -Li ｘ MO ｙ (where x, y are positive numbers, and M is any of P, Si, Ge, B, Al, Ga, In), etc. are included. Note that the description of "Li ２ S-P ２ S ５ " means a sulfide solid electrolyte formed using a raw material composition containing Li ２ S and P ２ S ５ and the same applies to other descriptions.

[0016] The sulfide solid electrolyte may, for example, have a Li ３ PS ４ skeleton, may have a Li ４ P ２ S ７ skeleton, and may have a Li ４ P ２ S ６ skeleton. Examples of the sulfide solid electrolyte having a Li ３ PS ４ skeleton include, for example, LiI-Li ３ PS ４ , LiI-LiBr-Li<​​​​​​​​​​​​Examples of the sulfide solid electrolyte having a skeleton include, for example, a Li-P-S-based solid electrolyte called LPS (e.g., Li ７ P ３ S １１ ). Further, as the sulfide solid electrolyte, for example, LGPS represented by Li （４－ｘ） Ge （１－ｘ） P ｘ S ４ (where x satisfies 0 < x < 1) may be used. Among them, a sulfide solid electrolyte containing P element is preferable. Furthermore, the sulfide solid electrolyte may contain a halogen (F, Cl, Br, I), and an example thereof is Li ６ PS ５ X (where X is Cl, Br or I, preferably Cl). Note that the sulfide solid electrolyte may be used alone or in combination of two or more kinds.

[0017] (Non-polar solvent) The non-polar solvent functions as a solvent (dispersion medium) for the sulfide solid electrolyte together with the polar solvent described later. In this specification, the non-polar solvent refers to a hydrocarbon (a compound composed only of carbon atoms and hydrogen atoms) that is liquid at 25°C. The non-polar solvent is not particularly limited, and examples thereof include hexane, heptane, octane, nonane, decane, cyclohexane, tetralin, decalin, benzene, toluene, xylene, mesitylene, ethylbenzene, propylbenzene, isopropylbenzene, etc. The non-polar solvent may be used alone or in combination of two or more kinds. Among them, the non-polar solvent preferably contains at least one selected from toluene, xylene, mesitylene, ethylbenzene, isopropylbenzene, more preferably contains mesitylene, and even more preferably is mesitylene.

[0018] (Polar Solvent) In addition to the nonpolar solvent mentioned above, the slurry contains at least one polar solvent selected from ethers and alcohols. Examples of ethers are not particularly limited, but include diethyl ether, diisopropyl ether, dimethoxyethane, diglyme, tetrahydrofuran, dioxane, anisole, and dimethoxybenzene. Examples of alcohols are not particularly limited, but include methanol, ethanol, 1-propanol, 2-propanol, n-butanol, 2-butanol, tert-butanol, 1-pentanol, 2-pentanol, 3-methyl-1-butanol, 2-methyl-2-butanol, 2,2-dimethyl-1-propanol, 1-hexanol, 1-heptanol, and 1-octanol. The polar solvent may be used alone or in combination of two or more. In particular, it is preferable that the polar solvent contains at least one alcohol. The inclusion of an alcohol in the polar solvent increases the solubility of the sulfide solid electrolyte, making it easier for the sulfide solid electrolyte to fill the fine voids. Furthermore, it is more preferable that the polar solvent contains at least one lower alcohol having 3 to 5 carbon atoms. By using a lower alcohol having 3 to 5 carbon atoms, the sulfide solid electrolyte is not over-dissolved, and the solvent can be easily evaporated in the solvent removal step described later. Among the lower alcohols having 3 to 5 carbon atoms, it is even more preferable to include at least one selected from 1-propanol, 2-propanol, n-butanol, 2-butanol, and tert-butanol, particularly preferably 2-propanol, and most preferably 2-propanol.

[0019] The mixing ratio of the nonpolar solvent and the polar solvent is not particularly limited, but when the polar solvent contains alcohol, the alcohol content in the slurry is preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 30% by mass or less, and most preferably 25% by mass or less, relative to the total mass of the nonpolar solvent and the polar solvent. By keeping the alcohol content within the above range, the decrease in the ionic conductivity of the sulfide solid electrolyte can be suppressed. This is presumed to be due to the suppression of nucleophilic attack on the sulfide solid electrolyte by the alcohol. From the viewpoint of further improving the self-supporting and / or uniformity of the sulfide solid electrolyte sheet, the alcohol content in the slurry is preferably 3% by mass or more, and more preferably 5% by mass or more. According to one preferred embodiment, the alcohol content in the slurry is preferably 3 to 50% by mass, more preferably 5 to 50% by mass, even more preferably 5 to 40% by mass, particularly preferably 5 to 30% by mass, and most preferably 5 to 25% by mass.

[0020] In the manufacturing method according to this embodiment, the amount of solvents other than the nonpolar solvent and the polar solvent (ether and / or alcohol) contained in the slurry (hereinafter also referred to as "other solvents") is preferably small from the viewpoint of further improving the effects of the present invention. The amount of other solvents is preferably 3% by mass or less, more preferably 1% by mass or less, even more preferably 0.1% by mass or less, and particularly preferably 0% by mass (not included), based on the total mass of the solvents contained in the slurry.

[0021] The above nonpolar solvent and polar solvent preferably have a low water content, specifically, a water content of less than 0.2% by mass. More preferably, the water content in these solvents is 0.1% by mass or less, even more preferably 0.05% by mass or less, even more preferably 0.02% by mass or less, even more preferably 0.01% by mass or less, even more preferably 0.005% by mass or less, and particularly preferably 0.002% by mass or less. The water content in the solvent can be measured, for example, by Karl Fischer coulometric titration.

[0022] The order in which the sulfide solid electrolyte, nonpolar solvent, and polar solvent are mixed is not particularly limited, but it is preferable to prepare the slurry by first mixing the nonpolar solvent and polar solvent to obtain a mixed solvent, and then mixing the mixed solvent with the sulfide solid electrolyte. That is, the slurry preparation step preferably includes: mixing the nonpolar solvent and the polar solvent to obtain a mixed solvent; and mixing the mixed solvent with the sulfide solid electrolyte. By mixing each component in this order, the self-supporting and / or uniformity of the sulfide solid electrolyte sheet can be further improved.

[0023] (Binder) In this step, if necessary, a binder may be mixed in addition to the sulfide solid electrolyte, nonpolar solvent and polar solvent described above to prepare a slurry. The mechanical strength of the sulfide solid electrolyte sheet can be improved by using a binder. When a binder is used, the slurry preparation step preferably includes: mixing the nonpolar solvent and the polar solvent to obtain a mixed solvent; mixing the mixed solvent and the binder to obtain a binder solution; and mixing the binder solution and the sulfide solid electrolyte. Mixing each component in this order makes it easier to obtain a uniform slurry, and the self-supporting and / or uniformity of the sulfide solid electrolyte sheet can be further improved.

[0024] The type of binder is not particularly limited, and any known binder in the art can be used as appropriate. Examples include styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), and carboxymethylcellulose (CMC). Among these, styrene-butadiene rubber, polytetrafluoroethylene, and polyvinylidene fluoride are preferred, and styrene-butadiene rubber is more preferred. One type of binder may be used alone, or two or more types may be used in combination.

[0025] The binder is preferably soluble in the mixed solvent. Using such a binder results in a more uniform slurry. As a result, the self-supporting and / or uniformity of the sulfide solid electrolyte sheet can be further improved. In this specification, whether or not a binder is "soluble in the mixed solvent" is determined by the following method: Place 900 mg of the mixed solvent and 12 mg of the binder in a glass container and stir at 300 rpm for 20 minutes at room temperature (25°C) using a stirrer. Visually check whether any solid matter remains in the container, and if no solid matter remains, it is determined that "the binder is soluble in the mixed solvent."

[0026] The binder content in the slurry is not particularly limited, but is preferably 1 to 10% by mass, and more preferably 3 to 8% by mass, based on 100% by mass of the total solid content in the slurry. When the binder content is within the above range, the self-supporting and / or uniformity of the sulfide solid electrolyte sheet can be further improved.

[0027] The solid content concentration of the slurry is not particularly limited, but is preferably 40% by mass or more, more preferably 45% by mass or more, even more preferably 50% by mass or more, and particularly preferably 50 to 70% by mass. When the solid content concentration is within the above range, the occurrence of pinholes in the sulfide solid electrolyte sheet is suppressed, and uniformity can be further improved.

[0028] [Slurry Impregnation Process] In this process, the slurry is impregnated into a support having voids.

[0029] The support having voids (also simply referred to as "support") is not particularly limited, but examples include fibrous structures such as nonwoven fabrics and woven fabrics, and integral porous bodies such as foams in which multiple spaces are formed inside an integral structure. Among these, the support is preferably a fibrous structure, and more preferably a nonwoven fabric.

[0030] The porosity of the support is not particularly limited, but is preferably 70 to 98%, and more preferably 75 to 90%. The thickness of the support depends on the thickness of the target sulfide solid electrolyte sheet, but is preferably 1 to 50 μm, and more preferably 5 to 10 μm. When the support is a fibrous structure, the fiber diameter of the fibers constituting the fibrous structure is preferably 0.02 to 5 μm, and more preferably 0.1 to 1 μm.

[0031] The material of the support is not particularly limited, but an insulator (with an electronic conductivity of 10 at 25°C) is preferable. －６ Preferably, the ratio is S / m or less, and specifically, examples include organic materials such as polyester (e.g., polyethylene terephthalate), polypropylene, polyethylene, cellulose, aramid, and polytetrafluoroethylene, as well as inorganic materials such as glass and alumina.

[0032] The method for impregnating the support with slurry is not particularly limited. Examples include immersing a nonwoven fabric in slurry in a container; dropping the slurry onto the surface of the support; and applying it using a screen printer, bar coater, die coater, reverse coater, comma coater, gravure coater, spray coater, doctor knife, etc.

[0033] [Solvent Removal Step] In this step, the non-polar solvent and the polar solvent are removed from the support after it has been impregnated with the slurry. The conditions for removing the solvent are not particularly limited, but for example, at atmospheric pressure or reduced pressure, the temperature is, for example, 25 to 130°C, preferably 25 to 100°C, more preferably 25 to 70°C, and even more preferably 30 to 50°C.

[0034] The sulfide solid electrolyte sheet produced by the above process is suitable for use as a solid electrolyte layer in a lithium secondary battery because it exhibits excellent self-supporting properties and uniformity. Therefore, according to the present invention, a lithium secondary battery is also provided that comprises a battery element in which a positive electrode, the above-mentioned sulfide solid electrolyte sheet, and a negative electrode are laminated.

[0035] The following embodiments are also included in the scope of the present invention: 1. A method for producing a sulfide solid electrolyte sheet, comprising: preparing a slurry by mixing a sulfide solid electrolyte, a nonpolar solvent, and at least one polar solvent selected from ether and alcohol; impregnating a void-filled support with the slurry; and removing the nonpolar solvent and the polar solvent from the support after impregnation with the slurry; 2. The method for producing a sulfide solid electrolyte sheet according to 1, wherein the polar solvent contains at least one alcohol; 3. The method for producing a sulfide solid electrolyte sheet according to 1 or 2, wherein the polar solvent contains at least one lower alcohol having 3 to 5 carbon atoms; 4. The method for producing a sulfide solid electrolyte sheet according to 2 or 3, wherein the alcohol content in the slurry is 50% by mass or less relative to the total mass of the nonpolar solvent and the polar solvent; Item 5: The method for producing a sulfide solid electrolyte sheet according to any one of items 2 to 4, wherein the content of the alcohol in the slurry is 25% by mass or less relative to the total mass of the nonpolar solvent and the polar solvent; Item 6: The method for producing a sulfide solid electrolyte sheet according to any one of items 2 to 5, wherein the content of the alcohol in the slurry is 5% by mass or more relative to the total mass of the nonpolar solvent and the polar solvent; Item 7: The method for producing a sulfide solid electrolyte sheet according to any one of items 1 to 6, wherein the solid content concentration of the slurry is 50% by mass or more; Item 8: The method for producing a sulfide solid electrolyte sheet according to any one of items 1 to 7, wherein preparing the slurry comprises: mixing the nonpolar solvent and the polar solvent to obtain a mixed solvent; and mixing the mixed solvent and the sulfide solid electrolyte; Item 9: The method for producing the slurry comprises: mixing the nonpolar solvent and the polar solvent to obtain a mixed solvent; and mixing the mixed solvent and a binder to obtain a binder solution; A method for producing a sulfide solid electrolyte sheet according to any one of claims 1 to 8, comprising mixing the binder liquid and the sulfide solid electrolyte;Item 10: The method for producing a sulfide solid electrolyte sheet according to item 9, wherein the binder comprises at least one selected from the group consisting of styrene-butadiene rubber, polytetrafluoroethylene, polyvinylidene fluoride, and carboxymethylcellulose; Item 11: The method for producing a sulfide solid electrolyte sheet according to item 9 or 10, wherein the binder is soluble in the mixed solvent; Item 12: The method for producing a sulfide solid electrolyte sheet according to any one of items 1 to 11, wherein the polar solvent comprises at least one selected from the group consisting of diethyl ether, diisopropyl ether, dimethoxyethane, diglyme, tetrahydrofuran, dioxane, anisole, dimethoxybenzene, methanol, ethanol, 1-propanol, 2-propanol, n-butanol, 2-butanol, tert-butanol, 1-pentanol, 2-pentanol, 3-methyl-1-butanol, 2-methyl-2-butanol, 2,2-dimethyl-1-propanol, 1-hexanol, 1-heptanol, and 1-octanol; Item 13: The method for producing a sulfide solid electrolyte sheet according to any one of items 1 to 12, wherein the nonpolar solvent comprises at least one selected from the group consisting of hexane, heptane, octane, nonane, decane, cyclohexane, tetralin, decalin, benzene, toluene, xylene, mesitylene, ethylbenzene, propylbenzene, and isopropylbenzene. ;

[0036] The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. In the following, the preparation and evaluation of sulfide solid electrolyte sheets were carried out in a glove box with an argon atmosphere and a dew point of -68°C or lower. Furthermore, the instruments and equipment used in the glove box were thoroughly dried beforehand.

[0037] <Examples of preparing sulfide solid electrolyte sheets> [Comparative Example 1] (Preparation of slurry) 60 parts by mass of mesitylene as a nonpolar solvent was dissolved in 2 parts by mass of styrene-butadiene rubber (SBR) as a binder. To the obtained binder solution, an argyrodite-type sulfide solid electrolyte (Li) was added as a sulfide-based solid electrolyte. ６ PS５ 38 parts by mass of Cl were added, and the mixture was stirred using a stirrer at room temperature (25°C) for 20 minutes to obtain a slurry (total mass 1.5 g).

[0038] (Preparation of sulfide solid electrolyte sheet) A polyester nonwoven fabric (cut to a size of 3 cm x 5 cm, thickness 10 μm, porosity 78%) was placed on a PET film as a support. The slurry was then applied to the nonwoven fabric using a bar coater, with a basis weight of sulfide solid electrolyte of 0.013 g / cm³. ２ The material was coated to achieve the desired result. The nonwoven fabric coated with the slurry was placed on a metal plate along with the PET film. By heating this metal plate on a hot plate at 50°C for 20 minutes, the solvent was removed, and the sulfide solid electrolyte sheet of this comparative example was obtained.

[0039] [Comparative Example 2] The above (preparation of slurry) was carried out by the following method: 60 parts by mass of 2-propanol as a polar solvent, and an argyrodite-type sulfide solid electrolyte (Li as a sulfide-based solid electrolyte) ６ PS ５ 40 parts by mass of Cl) were added and stirred using a stirrer at room temperature for 20 minutes to obtain a slurry (total mass 1.5 g); the sulfide solid electrolyte sheet of this comparative example was obtained by the same method as in Comparative Example 1 described above.

[0040] [Example 1] The above (preparation of slurry) was carried out by the following method: A mixed solvent was prepared by mixing 95 parts by mass of mesitylene as a nonpolar solvent and 5 parts by mass of 2-propanol as a polar solvent. 2 parts by mass of styrene-butadiene rubber (SBR) as a binder was added to 50 parts by mass of this mixed solvent and dissolved. To the obtained binder solution, an argyrodite-type sulfide solid electrolyte (Li) was added as a sulfide-based solid electrolyte. ６ PS ５ 48 parts by mass of Cl) were added and stirred with a stirrer at room temperature for 20 minutes to obtain a slurry (total mass 1.5 g); the sulfide solid electrolyte sheet of this example was obtained by the same method as in Comparative Example 1 described above.

[0041] [Example 2] The above (preparation of slurry) was carried out by the following method: A mixed solvent was prepared by mixing 75 parts by mass of mesitylene as a nonpolar solvent and 25 parts by mass of 2-propanol as a polar solvent. 2 parts by mass of styrene-butadiene rubber (SBR) as a binder was added to 50 parts by mass of this mixed solvent and dissolved. To the obtained binder solution, an argyrodite-type sulfide solid electrolyte (Li) was added as a sulfide-based solid electrolyte. ６ PS ５ 48 parts by mass of Cl) were added and stirred with a stirrer at room temperature for 20 minutes to obtain a slurry (total mass 1.5 g); the sulfide solid electrolyte sheet of this example was obtained by the same method as in Comparative Example 1 described above.

[0042] [Example 3] The above (preparation of slurry) was carried out by the following method: A mixed solvent was prepared by mixing 75 parts by mass of mesitylene as a nonpolar solvent and 25 parts by mass of 2-propanol as a polar solvent. 2 parts by mass of styrene-butadiene rubber (SBR) as a binder was added to 60 parts by mass of this mixed solvent and dissolved. To the obtained binder solution, an argyrodite-type sulfide solid electrolyte (Li) was added as a sulfide-based solid electrolyte. ６ PS ５ 38 parts by mass of Cl) were added and stirred using a stirrer at room temperature for 20 minutes to obtain a slurry (total mass 1.5 g); the sulfide solid electrolyte sheet of this example was obtained by the same method as in Comparative Example 1 described above.

[0043] [Example 4] The above (preparation of slurry) was carried out by the following method: A mixed solvent was prepared by mixing 70 parts by mass of mesitylene as a nonpolar solvent and 30 parts by mass of 2-propanol as a polar solvent. 2 parts by mass of styrene-butadiene rubber (SBR) as a binder was added to 60 parts by mass of this mixed solvent and dissolved. To the obtained binder solution, an argyrodite-type sulfide solid electrolyte (Li) was added as a sulfide-based solid electrolyte. ６ PS ５ 38 parts by mass of Cl) were added and stirred using a stirrer at room temperature for 20 minutes to obtain a slurry (total mass 1.5 g); the sulfide solid electrolyte sheet of this example was obtained by the same method as in Comparative Example 1 described above.

[0044] [Example 5] The above (preparation of slurry) was carried out by the following method: A mixed solvent was prepared by mixing 60 parts by mass of mesitylene as a nonpolar solvent and 40 parts by mass of 2-propanol as a polar solvent. 1 part by mass of styrene-butadiene rubber (SBR) as a binder was added to 60 parts by mass of this mixed solvent and dissolved. To the obtained binder solution, an argyrodite-type sulfide solid electrolyte (Li) was added as a sulfide-based solid electrolyte. ６ PS ５ 39 parts by mass of Cl) were added and stirred using a stirrer at room temperature for 20 minutes to obtain a slurry (total mass 1.5 g); the sulfide solid electrolyte sheet of this example was obtained by the same method as in Comparative Example 1 described above.

[0045] [Example 6] The above (preparation of slurry) was carried out by the following method; a mixed solvent was prepared by mixing 50 parts by mass of mesitylene as a nonpolar solvent and 50 parts by mass of 2-propanol as a polar solvent. 60 parts by mass of this mixed solvent was mixed with an argyrodite-type sulfide solid electrolyte (Li) as a sulfide-based solid electrolyte. ６ PS ５ 40 parts by mass of Cl were added and stirred using a stirrer at room temperature for 20 minutes to obtain a slurry (total mass 1.5 g); the sulfide solid electrolyte sheet of this example was obtained by the same method as in Comparative Example 1 described above.

[0046] [Example 7] (Preparation of slurry) A mixed solvent was prepared by mixing 50 parts by mass of mesitylene as a nonpolar solvent and 50 parts by mass of ethanol as a polar solvent. 60 parts by mass of this mixed solvent was mixed with an argyrodite-type sulfide solid electrolyte (Li) as a sulfide-based solid electrolyte. ６ PS ５ 40 parts by mass of Cl were added, and the mixture was stirred using a stirrer at room temperature for 20 minutes to obtain a slurry (total mass 1.5 g).

[0047] (Preparation of sulfide solid electrolyte sheet) A polyester nonwoven fabric (cut to a size of 3 cm x 5 cm, thickness 10 μm, porosity 78%) was placed on a PET film as a support. The slurry was coated onto the nonwoven fabric using a bar coater. The nonwoven fabric coated with the slurry, along with the PET film, was placed on a metal plate. The solvent was removed by heating this metal plate on a hot plate at 130°C for 20 minutes, thereby obtaining the sulfide solid electrolyte sheet of this example.

[0048] [Example 8] The above (preparation of slurry) was carried out by the following method; 50 parts by mass of mesitylene as a nonpolar solvent and 50 parts by mass of tetrahydrofuran (THF) as a polar solvent were mixed to prepare a mixed solvent. 60 parts by mass of this mixed solvent was mixed with an argyrodite-type sulfide solid electrolyte (Li) as a sulfide-based solid electrolyte. ６ PS ５ 40 parts by mass of Cl were added and stirred using a stirrer at room temperature for 20 minutes to obtain a slurry (total mass 1.5 g); the sulfide solid electrolyte sheet of this example was obtained by the same method as in Comparative Example 1 described above.

[0049] [Example 9] The above (preparation of slurry) was carried out by the following method; a mixed solvent was prepared by mixing 50 parts by mass of mesitylene as a nonpolar solvent and 50 parts by mass of anisole as a polar solvent. 60 parts by mass of this mixed solvent was mixed with an argyrodite-type sulfide solid electrolyte (Li) as a sulfide-based solid electrolyte. ６ PS ５ 40 parts by mass of Cl were added and stirred using a stirrer at room temperature for 20 minutes to obtain a slurry (total mass 1.5 g); the sulfide solid electrolyte sheet of this example was obtained by the same method as in Comparative Example 1 described above.

[0050] <Evaluation of Sulfide Solid Electrolyte Sheets> (Self-Standing Ability) The sulfide solid electrolyte sheets prepared in the examples and comparative examples were peeled from the PET film. At this time, sheets in which the support and sulfide solid electrolyte were integrated were marked with "○", and sheets in which only the support peeled off and a sulfide solid electrolyte sheet in which the support and sulfide solid electrolyte were not integrated were marked with "×", and the self-standing ability of the sheets was evaluated.

[0051] (Uniformity) The surface of the sulfide solid electrolyte sheets (after being peeled from the PET film) that received a "○" in the above (self-supporting) evaluation was visually observed, and sheets with 10 or fewer pinholes were marked "○", while those with more than 10 pinholes were marked "×", to evaluate the uniformity of the sheets.

[0052] (Powder shedding) When the sulfide solid electrolyte sheets prepared in the examples and comparative examples were peeled from the PET film, those from which no sulfide solid electrolyte powder fell were evaluated as "○", and those from which powder fell were evaluated as "△".

[0053] (Punching Test) The sulfide solid electrolyte sheets (after being peeled from the PET film) that received a "○" in the above (self-supporting) evaluation were punched out into circles with a diameter (φ) of 10 mm. The resulting circular sheets were visually inspected and evaluated as follows: those without chipping at the edges or peeling of the sulfide solid electrolyte were given a "○", and those with such chipping or peeling were given a "△".

[0054] (Ionic Conductivity) The slurries prepared in the examples and comparative examples were coated onto PET films using a bar coater. The PET films coated with the slurry were placed on a metal plate. The metal plate was heated on a hot plate at a predetermined temperature (150°C for the slurry in Example 7, and 50°C for the other slurries) for 20 minutes to remove the solvent and obtain a sulfide solid electrolyte powder. 100 mg of the powder was placed in a φ10 mm polycarbonate tube and pressurized uniaxially at a pressure of 300 MPa at room temperature for 1 minute to create a sulfide solid electrolyte layer. Stainless steel current collector pins were placed at the top and bottom of the tube to create an evaluation cell consisting of a laminate of current collector pins / sulfide solid electrolyte layer / current collector pins, with a constraining pressure of 100 MPa applied. Impedance measurements were performed on this evaluation cell in the range of 7 MHz to 0.1 Hz at room temperature. After the impedance measurement, the cell was disassembled and the thickness of the sulfide solid electrolyte layer was measured. Ionic conductivity was calculated from the resistance value and thickness obtained by impedance measurement.

[0055] These results are shown in Table 1 below.

[0056]

[0057] The results shown in Table 1 demonstrate that the present invention provides a sulfide solid electrolyte sheet with sufficient self-supporting properties and uniformity. Comparative Example 1 is an example using a slurry containing a nonpolar solvent and not containing ether and / or alcohol, but it was not possible to create a self-supporting film. This is presumed to be because the sulfide solid electrolyte did not fill the fine voids, resulting in insufficient adhesion between the sulfide solid electrolyte and the support. Comparative Example 2 is an example containing ether and / or alcohol and not containing a nonpolar solvent, but the sulfide solid electrolyte sheet had many pinholes on its surface and lacked uniformity. This is presumed to be because the amount of dissolved sulfide solid electrolyte in the slurry was large, and reprecipitation of the dissolved sulfide solid electrolyte occurred on the surface of the support, which led to the generation of pinholes.

[0058] A comparison of Examples 1-7 with Examples 8-9 shows that including alcohol as a polar solvent reduces powder shedding. Furthermore, a comparison of Examples 1-3 with Examples 4-6 shows that setting the alcohol percentage to 25% by mass or less relative to the total mass of the non-polar and polar solvents results in favorable punching test results.

[0059] Furthermore, Examples 1 to 6 show that the lower the alcohol content relative to the total mass of the nonpolar and polar solvents, the more the decrease in ionic conductivity in the sulfide solid electrolyte can be suppressed.

[0060] This application is based on Japanese Patent Application No. 2024-168486, filed on 27 September 2024, the disclosures of which are referenced and incorporated in whole.

Claims

1. A method for producing a sulfide solid electrolyte sheet, comprising: preparing a slurry by mixing a sulfide solid electrolyte, a nonpolar solvent, and at least one polar solvent selected from ether and alcohol; impregnating a void-filled support with the slurry; and removing the nonpolar solvent and the polar solvent from the support after impregnation with the slurry.

2. The method for producing a sulfide solid electrolyte sheet according to claim 1, wherein the polar solvent contains at least one alcohol.

3. The method for producing a sulfide solid electrolyte sheet according to claim 2, wherein the polar solvent contains at least one lower alcohol having 3 to 5 carbon atoms.

4. The method for producing a sulfide solid electrolyte sheet according to claim 2, wherein the alcohol content in the slurry is 50% by mass or less relative to the total mass of the non-polar solvent and the polar solvent.

5. The method for producing a sulfide solid electrolyte sheet according to claim 2, wherein the alcohol content in the slurry is 25% by mass or less relative to the total mass of the non-polar solvent and the polar solvent.

6. The method for producing a sulfide solid electrolyte sheet according to claim 2, wherein the alcohol content in the slurry is 5% by mass or more relative to the total mass of the non-polar solvent and the polar solvent.

7. The method for producing a sulfide solid electrolyte sheet according to claim 1, wherein the solid content concentration of the slurry is 50% by mass or more.

8. The method for producing a sulfide solid electrolyte sheet according to claim 1, comprising: preparing the slurry by mixing the non-polar solvent and the polar solvent to obtain a mixed solvent; and mixing the mixed solvent and the sulfide solid electrolyte.

9. The method for producing a sulfide solid electrolyte sheet according to claim 1, comprising: preparing the slurry by mixing the nonpolar solvent and the polar solvent to obtain a mixed solvent; mixing the mixed solvent and a binder to obtain a binder solution; and mixing the binder solution and the sulfide solid electrolyte.

10. The method for producing a sulfide solid electrolyte sheet according to claim 9, wherein the binder is soluble in the mixed solvent.

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