Solid electrolyte layer for all-solid-state batteries, method for producing same, and all-solid-state battery using said solid electrolyte layer for all-solid-state batteries

Abstract

The purpose of the present invention is to provide a means which is capable of improving the mechanical strength of a solid electrolyte layer in an all-solid-state battery. One embodiment of the present invention provides a method for producing a solid electrolyte layer for all-solid-state batteries, the method comprising: obtaining a first sheet by spreading a powder composition in one direction, the powder composition containing a solid electrolyte and a fibrous binder; and obtaining a second sheet by stacking two or more of the first sheets in such a manner that the angle A between the spreading direction Xa of one first sheet and the spreading direction Xb of another first sheet is 30° to 90° inclusive.

Description

Solid electrolyte layer for all-solid-state battery, method for producing the same, and all-solid-state battery using the solid electrolyte layer for all-solid-state battery

[0001] The present invention relates to a solid electrolyte layer for an all-solid-state battery, a method for producing the same, and an all-solid-state battery using the solid electrolyte layer for the all-solid-state battery.

[0002] In recent years, research and development into all-solid-state lithium secondary batteries using oxide- or sulfide-based solid electrolytes has been actively pursued. Solid electrolytes are materials primarily composed of ionic conductors capable of ion conduction in a solid state. Therefore, all-solid-state lithium secondary batteries, in principle, do not encounter the various problems associated with flammable organic electrolytes that occur in conventional liquid-based lithium secondary batteries. In addition, the use of high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials generally leads to significant improvements in the battery's power density and energy density.

[0003] In general all-solid-state lithium secondary batteries, the solid electrolyte layer typically has a structure in which the solid electrolyte is bound by a binder. International Publication No. 2022 / 050252 (corresponding to U.S. Patent Application Publication No. 2023 / 0231182) discloses a technique for using a polytetrafluoroethylene resin having a fibrous structure with a fibril diameter (median) of 70 nm or less as the binder in an all-solid-state secondary battery mixture containing a solid electrolyte and a binder. According to the above document, this configuration results in less degradation of the solid electrolyte and an all-solid-state secondary battery mixture with good properties. The above document also describes the use of a sheet-shaped all-solid-state secondary battery mixture as the solid electrolyte layer of the all-solid-state secondary battery.

[0004] However, according to the investigations of the present inventors, the solid electrolyte layer described in the above document may have insufficient strength, and has a problem in that when pressure is applied in the stacking direction after stacking the electrode active material layers, cracks occur at the contact portions with the ends of the electrode active material layers.

[0005] Therefore, an object of the present invention is to provide a means for improving the mechanical strength of a solid electrolyte layer in an all-solid-state battery.

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that the above-mentioned problems can be solved by, when producing a solid electrolyte layer, stretching a powder composition containing a solid electrolyte and a fibrous binder in one direction to prepare a sheet, and then laminating two or more of the sheets so that the stretching directions differ from each other by 30° or more, thereby completing the present invention.

[0007] That is, one aspect of the present invention is a method for producing a solid electrolyte layer for an all-solid-state battery, the method comprising: stretching a powder composition containing a solid electrolyte and a fibrous binder in one direction to obtain a first sheet; and stacking two or more of the first sheets such that an angle A formed between the stretching direction Xa of one of the first sheets and the stretching direction Xb of another of the first sheets is 30° or more and 90° or less to obtain a second sheet.

[0008] Another aspect of the present invention is a solid electrolyte layer for an all-solid-state battery, comprising a solid electrolyte and a fibrous binder, wherein a histogram based on a frequency distribution of an orientation angle of the fibrous binder when one direction on a main surface of the solid electrolyte layer for an all-solid-state battery is referenced is bimodal, and when the orientation angle at the top of the first peak is Ya and the orientation angle at the top of the second peak is Yb (where Ya<Yb), the value of (Yb-Ya) is 30° or more and 90° or less.

[0009] FIG. 1 is a perspective view showing the appearance of a flat-layered all-solid-state lithium secondary battery according to one embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line 2-2 in FIG. 1. FIG. 3 is a schematic diagram showing an example of a branched-chain fibrous binder. FIG. 4 is a diagram illustrating an example of a second sheet preparation step. FIG. 5 is an explanatory diagram illustrating a method for measuring the angle θ formed by a line segment connecting the start point and end point of a fibrous binder with a reference direction in an observation image during orientation angle measurement. FIG. 6 is an observation image of the fibrous binder in the solid electrolyte layer prepared in Example 1, observed using a scanning electron microscope (SEM). FIG. 7 is an observation image of the fibrous binder in the solid electrolyte layer prepared in Comparative Example 1, observed using a scanning electron microscope (SEM).

[0010] One aspect of the present invention is a method for producing a solid electrolyte layer for an all-solid-state battery, the method comprising: stretching a powder composition containing a solid electrolyte and a fibrous binder in one direction to obtain a first sheet; and stacking two or more of the first sheets such that an angle A formed between the stretching direction Xa of one of the first sheets and the stretching direction Xb of another of the first sheets is 30° or more and 90° or less to obtain a second sheet.

[0011] Another aspect of the present invention is a solid electrolyte layer for an all-solid-state battery, comprising a solid electrolyte and a fibrous binder, wherein a histogram based on a frequency distribution of an orientation angle of the fibrous binder when one direction on a main surface of the solid electrolyte layer for an all-solid-state battery is referenced is bimodal, and when the orientation angle at the top of the first peak is Ya and the orientation angle at the top of the second peak is Yb (where Ya<Yb), the value of (Yb-Ya) is 30° or more and 90° or less.

[0012] According to the present invention, the mechanical strength of the solid electrolyte layer in an all-solid-state battery can be improved.

[0013] Hereinafter, an embodiment of an all-solid-state battery employing the solid electrolyte layer for an all-solid-state battery according to the present embodiment will be described with reference to the drawings. However, the technical scope of the present invention should be defined based on the claims and is not limited to the following embodiment. Note that the dimensional ratios in the drawings are exaggerated for convenience of explanation and may differ from the actual ratios. Below, this embodiment will be described using as an example a flat stacked (internal parallel connection) all-solid-state lithium secondary battery (hereinafter simply referred to as a "stacked battery"), which is one form of secondary battery. However, in terms of the internal electrical connection form (electrode structure) of the secondary battery according to this embodiment, it can be applied to both non-bipolar (internal parallel connection) batteries and bipolar (internal series connection) batteries.

[0014] Fig. 1 is a perspective view showing the appearance of a flat laminated type all-solid-state lithium secondary battery according to one embodiment of the present invention, and Fig. 2 is a cross-sectional view taken along line 2-2 shown in Fig. 1.

[0015] As shown in Fig. 1, the stacked battery 10a has a flat rectangular shape, with a negative current collector 25 and a positive current collector 27 extending from both sides for extracting power. The power generating element 21 is wrapped in the battery exterior material (laminate film 29) of the stacked battery 10a, and the periphery is heat-sealed, sealing the power generating element 21 with the negative current collector 25 and the positive current collector 27 extending to the outside. Note that the current collectors (25, 27) shown in Fig. 1 may be configured so that the negative current collector 25 and the positive current collector 27 extend from the same side, or the negative current collector 25 and the positive current collector 27 may each be divided into multiple plates and extended from each side.

[0016] 2, the stacked battery 10a of this embodiment has a structure in which a power generating element 21, in which the charge / discharge reaction actually proceeds, is sealed inside a laminate film 29, which is a battery exterior material. Here, the power generating element 21 has a configuration in which a positive electrode, a solid electrolyte layer 17, and a negative electrode are stacked. The positive electrode has a structure in which positive electrode active material layers 15 containing a positive electrode active material are disposed on both sides of a positive electrode current collector 11". The negative electrode has a structure in which negative electrode active material layers 13 containing a negative electrode active material are disposed on both sides of a negative electrode current collector 11'. Specifically, the positive electrode, solid electrolyte layer, and negative electrode are stacked in this order such that one positive electrode active material layer 15 faces an adjacent negative electrode active material layer 13 with a solid electrolyte layer 17 interposed therebetween. As a result, the adjacent positive electrode, solid electrolyte layer, and negative electrode constitute one unit cell layer 19. Therefore, the stacked battery 10a shown in FIG. 1 can also be said to have a configuration in which a plurality of unit cell layers 19 are stacked and electrically connected in parallel. Furthermore, a restraining pressure is applied to the stacked battery 10a in the stacking direction of the power generation element 21 by a restraining member (pressure member) (not shown). Therefore, the volume of the power generation element 21 is kept constant.

[0017] A negative electrode current collector (tab) 25 and a positive electrode current collector (tab) 27 that are electrically connected to the respective electrodes (negative and positive electrodes) are attached to the negative electrode current collector 11′ and the positive electrode current collector 11″, respectively, and are configured to be sandwiched between the ends of a laminate film 29 that is a battery outer casing material and extended to the outside of the laminate film 29. The negative electrode current collector 25 and the positive electrode current collector 27 may be attached to the negative electrode current collector 11′ and the positive electrode current collector 11″ of the respective electrodes by ultrasonic welding, resistance welding, or the like, via a negative electrode lead and a positive electrode lead (not shown) as necessary.

[0018] The stacked battery 10a according to this embodiment is preferably sandwiched between two plate-shaped members and further fastened using fastening members. As a result, the plate-shaped members and fastening members function as pressure members that pressurize (restrain) the power-generating element 21 in the stacking direction. Examples of the plate-shaped members include metal plates and resin plates. Examples of the fastening members include bolts and nuts. However, the pressure members are not particularly limited as long as they can pressurize the power-generating element 21 in the stacking direction. A typical pressure member is a combination of a plate made of a rigid material, such as the plate-shaped members, and the fastening members described above. Furthermore, the fastening members may be not only bolts and nuts, but also tension plates that secure the ends of the plate-shaped members so as to restrain the power-generating element 21 in the stacking direction. The lower limit of the load applied to the power-generating element 21 (the restraining pressure in the stacking direction of the power-generating element) is, for example, 0.1 MPa or more, preferably 1 MPa or more, more preferably 3 MPa or more, and even more preferably 5 MPa or more. The upper limit of the restraining pressure in the stacking direction of the power generating element is, for example, 100 MPa or less, preferably 70 MPa or less, more preferably 40 MPa or less, and even more preferably 10 MPa or less.

[0019] The main components of the stacked battery 10a will now be described.

[0020] [Current Collectors] The current collectors (negative electrode current collector 11′ and positive electrode current collector 11″) have the function of mediating the transfer of electrons from the electrode active material layer. There are no particular restrictions on the material that constitutes the current collectors. Examples of materials that can be used to constitute the current collectors include metals such as aluminum, nickel, iron, stainless steel, titanium, and copper, as well as conductive resins. There are also no particular restrictions on the thickness of the current collectors, but one example is 10 to 100 μm.

[0021] [Negative Electrode Active Material Layer] The negative electrode active material layer 13 contains a negative electrode active material. The type of negative electrode active material is not particularly limited, but includes carbon materials, metal oxides, and metal active materials. A lithium-containing active material may also be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is a lithium-containing active material, and examples thereof include lithium metal and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li with at least one of In, Al, Si, Sn, Mg, Au, Ag, and Zn. The negative electrode active material preferably contains lithium metal or a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and more preferably contains lithium metal or a lithium-containing alloy. When lithium metal or a lithium-containing alloy is used as the negative electrode active material, the all-solid-state battery according to this embodiment may be a so-called lithium deposition type battery in which lithium metal as the negative electrode active material is deposited on the negative electrode current collector during charging. In this configuration, the thickness of the negative electrode active material layer increases as the charging process progresses, and decreases as the discharging process progresses. The negative electrode active material layer does not need to be present during full discharge, but in some cases, a negative electrode active material layer made of a certain amount of lithium metal may be present during full discharge. The thickness of the negative electrode active material layer (lithium metal layer) during full charge is not particularly limited, but is typically 0.1 to 1000 μm.

[0022] [Solid Electrolyte Layer] The solid electrolyte layer 17 includes a solid electrolyte and a fibrous binder.

[0023] Examples of solid electrolytes include sulfide solid electrolytes and oxide solid electrolytes. In this specification, the term "solid electrolyte" refers to a material mainly composed of an ion conductor capable of conducting ions in a solid state, and in particular, a material having a lithium ion conductivity of 1×10 at room temperature (25° C.). －５ S / cm or more, and this lithium ion conductivity is preferably 1×10 －４ The lithium ion conductivity is 2.5 S / cm or more. The lithium ion conductivity can be measured by an AC impedance method.

[0024] From the viewpoint of exhibiting excellent lithium ion conductivity and being able to better follow the volume change of the electrode active material that accompanies charge and discharge, the solid electrolyte is preferably a sulfide solid electrolyte containing an S element, more preferably a sulfide solid electrolyte containing a Li element, an M element, and an S element, wherein the M element contains at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl, and I, and even more preferably a sulfide solid electrolyte containing an S element, a Li element, and a P element.

[0025] The sulfide solid electrolyte is Li ３ P.S. ４ It may have a Li framework. ４ P ２ S ７ It may have a Li framework. ４ P ２ S ６ It may have a Li skeleton. ３ P.S. ４ Examples of sulfide solid electrolytes having a skeleton include LiI-Li ３ P.S. ４ , LiI-LiBr-Li ３ P.S. ４ , Li ３ P.S. ４ In addition, Li ４ P ２ S ７ Examples of sulfide solid electrolytes having a skeleton include Li-P-S solid electrolytes called LPS. （４－ｘ） Ge （１－ｘ） Pｘ S ４ (x satisfies 0<x<1) or the like. More specifically, for example, LPS (Li ２ S-P ２ S ５ ), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S., Li. ３．２５ Ge ０．２５ P ０．７５ S ４ , Li １０ GeP ２ S １２ , or Li ６ P.S. ５ X (wherein X is Cl, Br or I). ２ S-P ２ S ５ " is written by Li ２ S and P ２ S ５ The same applies to other descriptions. Among them, the sulfide solid electrolyte is preferably LPS (Li ２ S-P ２ S ５ ), Li ６ P.S. ５ X (wherein X is Cl, Br or I), Li ７ P ３ S １１ , Li ３．２ P ０．９６ S and Li ３ P.S. ４ is selected from the group consisting of:

[0026] The solid electrolyte preferably has a particulate shape such as a spherical shape, an oval sphere shape, etc. When the solid electrolyte is particulate, its average particle diameter (D50) is not particularly limited, but is preferably 0.01 μm or more and 40 μm or less, more preferably 0.1 μm or more and 20 μm or less, and even more preferably 0.5 μm or more and 10 μm or less.

[0027] The content of the solid electrolyte in the solid electrolyte layer is, for example, preferably in the range of 50 to 99 mass %, more preferably in the range of 70 to 98 mass %, and even more preferably in the range of 80 to 95 mass %, relative to the total mass of the solid electrolyte layer.

[0028] The solid electrolyte layer contains a fibrous binder as a binder. In this specification, the term "fibrous binder" refers to a binder mainly composed of fibers having an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 μm or less in an observation image of a cross section of a solid electrolyte observed using a scanning electron microscope (SEM). Here, the aspect ratio is calculated by dividing the maximum Feret diameter of the binder by the minimum Feret diameter. The maximum Feret diameter is the maximum distance between two parallel lines when the outline of the binder is sandwiched between the lines, and the minimum Feret diameter is the minimum distance between the lines when the outline of the binder is sandwiched between the lines. When a binder is "mainly composed" of the above fibers, it means that the area ratio of the above fiber portion to the total area of ​​the binder in the SEM observation image is 50% or more. One fibrous binder may include a portion other than the fiber having an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 μm or less (a portion having an aspect ratio of less than 10 or a portion having a minimum Feret diameter of more than 0.2 μm). However, the area ratio of the portion other than the fiber to the total area of ​​the fibrous binder in the SEM observation image must be less than 50%, preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less (the lower limit is 0%).

[0029] Fibrous binders include not only those composed of a single fiber but also those composed of two or more fibers connected to each other. Specific shapes of binders composed of two or more fibers connected to each other include branched, radial, and mesh shapes, as well as combinations of these. Here, we will explain how to determine the maximum and minimum Feret diameters for binders composed of two or more fibers connected to each other. Figure 3 is a schematic diagram showing an example of a branched fibrous binder. The fibrous binder 30 shown in Figure 3 has a structure in which fiber X, fiber Y, and fiber Z are connected to each other. Each dashed line represents a line connecting the centers (half width) of the fiber widths, and points A, B, and C represent the ends of each dashed line. The ends of each dashed line coincide with the ends of the fibers. Point D represents the intersection of the three dashed lines. That is, the binder 30 shown in FIG. 3 can be said to have a shape in which fiber X from point A to point D, fiber Y from point B to point D, and fiber Z from point C to point D are bonded at point D. The maximum Feret diameter of fiber X in the binder 30 shown in FIG. 3 is defined as the distance from point A to point D. Similarly, the maximum Feret diameter of fiber Y is the distance from point B to point D, and the maximum Feret diameter of fiber Z is the distance from point C to point D. The minimum Feret diameter of fiber X is the minimum distance between two parallel lines when the outline of the binder (fiber) from point A to point D is sandwiched between the lines. The same applies to the minimum Feret diameters of fiber Y and fiber Z. In the binder 30 shown in FIG. 3, fiber Y and fiber Z have an aspect ratio of 10 or more and a minimum Feret diameter of 0.2 μm or less, while fiber X has an aspect ratio of less than 10. However, since the area of ​​the fiber X portion occupies less than 50% of the total area of ​​the binder 30, the binder shown in FIG. 3 can be said to be a fibrous binder.

[0030] The type of fibrous binder is not particularly limited as long as it has the above-described shape in the solid electrolyte layer. However, binders that fibrillate upon application of shear force are preferably used. Examples of such fibrillizable binders include polytetrafluoroethylene (PTFE), carboxymethyl cellulose, polyvinyl alcohol, and polyethylene, with PTFE being more preferred. By using such a fibrous binder, a solid electrolyte layer that combines high ionic conductivity and high mechanical strength can be obtained. A single type of fibrous binder may be used alone, or two or more types may be used in combination. That is, in the solid electrolyte layer for an all-solid-state battery or the method for manufacturing the same according to one embodiment of the present invention, the fibrous binder preferably contains polytetrafluoroethylene, and more preferably polytetrafluoroethylene. Note that, in this specification, the compound name of the binder refers not only to the compound indicated by the compound name, but also to forms in which the terminals or part of the side chains are substituted (modified) with other substituents. In cases where the terminals or part of the side chains are substituted (modified) with other substituents, the proportion of structural units whose terminals or side chains have been substituted (modified) with other substituents in 100 mol % of all structural units is preferably 10 mol % or less, and more preferably 5 mol % or less.

[0031] When the fibrous binder contains PTFE, it is preferable that the solid electrolyte layer further contains another binder. The combined use of PTFE and another binder can further improve the mechanical strength of the solid electrolyte layer. Here, the other binder may be either a fibrous binder or a non-fibrous binder (a binder other than a fibrous binder) as long as it is other than PTFE. Here, the other binder is not particularly limited, but polyvinylidene fluoride, polypropylene, polyethylene, polystyrene, polyethylene terephthalate, and polyethylene oxide are preferred. Only one type of other binder may be used alone, or two or more types may be used in combination.

[0032] The binder content in the solid electrolyte layer is, for example, preferably in the range of 1 to 50 mass%, more preferably in the range of 2 to 30 mass%, and even more preferably in the range of 5 to 20 mass%, relative to the total mass of the solid electrolyte layer. When the binder content is within the above range, a good balance between the mechanical strength and ionic conductivity of the solid electrolyte layer can be achieved. However, the mass proportion of the fibrous binder relative to the total binder in the solid electrolyte layer is preferably 50 mass% or more, more preferably 80 mass% or more, even more preferably 90 mass% or more, even more preferably 95 mass% or more, particularly preferably 98 mass% or more, and most preferably 100 mass%. When the mass proportion of the fibrous binder is within the above range, the ionic conductivity of the solid electrolyte layer can be further improved.

[0033] As described below, the solid electrolyte layer according to this embodiment has excellent mechanical strength, and therefore can have sufficient durability even when its thickness is reduced (thinned) from the viewpoint of energy density. In other words, when the thickness of the solid electrolyte layer is small (thinned), the effects of the present invention can be more significantly exhibited. The thickness of the solid electrolyte layer is not particularly limited, but from the above viewpoint, it is preferably 100 μm or less, more preferably 0.1 μm or more and 100 μm or less, even more preferably 1 μm or more and 100 μm or less, and particularly preferably 10 μm or more and 80 μm or less.

[0034] (Method for Producing a Solid Electrolyte Layer) The solid electrolyte layer according to this embodiment is produced by spreading a powder composition containing a solid electrolyte and a fibrous binder in one direction to produce a sheet, and then stacking two or more of the sheets so that the spreading directions differ by 30° or more. That is, a method for producing a solid electrolyte layer for an all-solid-state battery according to one embodiment of the present invention includes the steps of: spreading a powder composition containing a solid electrolyte and a fibrous binder in one direction to obtain a first sheet (first sheet production step); and stacking two or more of the first sheets so that the angle A between the spreading direction Xa of one of the first sheets and the spreading direction Xb of another of the first sheets is 30° or more and 90° or less to obtain a second sheet (second sheet production step). Each step is described in detail below.

[0035] (1) First Sheet Preparation Step In this step, a powder composition containing a solid electrolyte, a fibrous binder, and other components that may be added as needed is stretched in one direction to obtain a first sheet (S1). Details of each component are as described above, and therefore will not be described here. In this step, by stretching the powder composition in one direction, the orientation of the fibrous binder is aligned from a random direction to one direction (stretching direction) (the fibrous binder is oriented in one direction (stretching direction)).

[0036] There are no particular limitations on the method for obtaining the powder composition; the components constituting the powder composition may be added and mixed in any order. The mixing means used in this process are also not particularly limited; methods using conventionally known mixing means such as a mortar, mixer, or mill may be appropriately employed. Here, there are no particular limitations on the method for blending the fibrous binder into the powder composition; a fibrous binder may be prepared separately and mixed with other components. Furthermore, when a fibrillizable binder, which is a precursor of the fibrous binder, is mixed with other components, a shear force may be applied to the fibrillizable binder to fibrillate it, thereby producing the fibrous binder in the powder composition.

[0037] The means for spreading the powder composition in one direction is not particularly limited, and methods using conventionally known sheet forming means (hand rollers or roll presses) can be appropriately employed. As used herein, "spreading in one direction" refers to the fact that the spreading force (stretching force) applied to the powder composition is in only one direction. For example, when using a sheet forming means using a hand roller or roll press, the powder composition is spread one or more times so that the roller moves in only one direction (or the opposite direction) relative to the powder composition until a sheet of the desired thickness is obtained. Note that, as used herein, "one direction" may include an error of about ±5°. The thickness of the first sheet can be appropriately set depending on the desired thickness of the solid electrolyte layer, but is preferably 10 to 500 μm, more preferably 50 to 200 μm. The number of times the first sheet is spread (the number of times a force is applied in the spreading direction) may be one or more times, preferably two or more times, more preferably three or more times, and even more preferably five or more times. The more times the stretching is performed, the more easily the fibrous binder is oriented in the stretching direction, which is preferable. The upper limit of the number of times the stretching is performed is not particularly limited, but is usually 20 times or less, and in consideration of work efficiency, it is preferably 10 times or less.

[0038] In this step, the powder composition may be spread at room temperature (e.g., 20 to 30°C), but it is also preferable to spread the powder composition while heating it. This can further promote fibrillation of the fibrous binder and fiber orientation. The heating temperature when heating the powder composition can be set appropriately depending on the type of fibrous binder, but is preferably 40 to 90°C, and more preferably 60 to 80°C.

[0039] Furthermore, when using a sheet forming method using a hand roller or a roll press, it is preferable to rotate the roller so that the force applied in the stretching direction (stretching force) is equal to or greater than the force applied perpendicular to the stretching direction. Specifically, when using a hand roller, it is preferable to rotate the roller while applying the hand roller to the powder composition at a shallow angle so that the angle between the surface on which the powder composition is placed and the axis of the hand roller is 45° or less (more preferably less than 45°, even more preferably 40° or less). Furthermore, when using a roll press, it is preferable to set the peripheral speed ratio of the two rolls sandwiching the powder composition to 1:1.05 or more (more preferably 1:1.10 or more, even more preferably 1:1.15 or more). This applies shear force to the powder composition, thereby further promoting fibrillation of the fibrous binder and fiber orientation.

[0040] (2) Second Sheet Preparation Step In this step, two or more of the first sheets are laminated so that the angle A between the stretching direction Xa of one first sheet and the stretching direction Xb of the other first sheet is 30° or more and 90° or less to obtain a second sheet. Here, the two or more first sheets used in this step are not particularly limited as long as they are obtained by the first sheet preparation step, but it is preferable that they are prepared under the same conditions. In addition, the number of first sheets to be laminated is not particularly limited as long as it is two or more, but is preferably 2 to 8, more preferably 2 to 6, even more preferably 2 to 4, and particularly preferably 2.

[0041] FIG. 4 is a diagram illustrating an example of the second-sheet manufacturing process. FIG. 4 illustrates an example in which two first sheets are stacked. FIG. 4( a) shows two first sheets (S1a, S1b) obtained by the first-sheet manufacturing process performed under the same conditions. The first sheet (S1a) is manufactured by stretching the sheet in the stretching direction Xa in the first-sheet manufacturing process, and the fibrous binder is oriented in the stretching direction Xa. Similarly, the first sheet (S1b) is manufactured by stretching the sheet in the stretching direction Xb in the first-sheet manufacturing process, and the fibrous binder is oriented in the stretching direction Xb. These two first sheets (S1a, S1b) are stacked so that the angle A between the stretching direction Xa and the stretching direction Xb is 90°, as shown in FIG. 4( b). As a result, in the second sheet (S2), the angle formed by the fibrous binder derived from the first sheet (S1a) and the fibrous binder derived from the first sheet (S1b) can be 90°.

[0042] In this step, when three or more first sheets are stacked, each first sheet is oriented toward either one first sheet (S1a) or another first sheet (S1b). This can further improve the mechanical strength of the solid electrolyte layer. On the other hand, if the first sheets are stacked so that they are oriented in a direction different from that of the one first sheet (S1a) and another first sheet (S1b), the orientation of the fibrous binder in the resulting solid electrolyte layer becomes closer to random, which may reduce the mechanical strength of the solid electrolyte layer.

[0043] The angle A between the extension direction Xa and the extension direction Xb is essentially 30° or more and 90° or less, preferably more than 45° and 90° or less, more preferably 60° or more and 90° or less, even more preferably 80° or more and 90° or less, particularly preferably 85° or more and 90° or less, and most preferably 90°. The effect of the present invention (the effect of improving the mechanical strength of the solid electrolyte layer) can be more pronounced as the angle A approaches 90°. If the angle A is less than 30°, the orientation of the fibrous binder in the obtained solid electrolyte layer approaches a state where it is oriented in only one direction, which may reduce the mechanical strength of the solid electrolyte layer.

[0044] (3) Re-expanding Step The manufacturing method according to this embodiment may include a step of further expanding the obtained second sheet (re-expanding step) after the second sheet preparation step. This allows the thickness of the second sheet to be adjusted to the desired thickness of the solid electrolyte layer. This also has the effect of improving the adhesion between the stacked first sheets, further improving the ionic conductivity of the solid electrolyte layer.

[0045] The stretching direction of the second sheet in this step is not particularly limited. However, from the viewpoint of further improving the mechanical strength of the solid electrolyte layer, it is preferable to stretch the second sheet in at least one of the stretching direction Xa of one of the first sheets and the stretching direction Xb of the other first sheet, and it is more preferable to stretch the second sheet in both the stretching direction Xa of one of the first sheets and the stretching direction Xb of the other first sheet. Furthermore, when stretching in both the stretching direction Xa and the stretching direction Xb, it is even more preferable to stretch the second sheet alternately in the stretching direction Xa and the stretching direction Xb. Such a stretching treatment can maintain the orientation of the fibrous binder.

[0046] (Orientation State of Fibrous Binder in Solid Electrolyte Layer) The solid electrolyte layer obtained by the above-mentioned manufacturing method includes a fibrous binder derived from one first sheet oriented in the extension direction Xa and a fibrous binder derived from another first sheet oriented in the extension direction Xb. Therefore, when focusing on the orientation state of the fibrous binder, the solid electrolyte layer obtained by the above-mentioned manufacturing method can have the following characteristics. That is, a solid electrolyte layer for an all-solid-state battery according to another aspect of the present invention contains a solid electrolyte and a fibrous binder. The solid electrolyte layer for an all-solid-state battery is characterized in that a histogram based on the frequency distribution of the orientation angle of the fibrous binder relative to one direction on a main surface of the solid electrolyte layer for an all-solid-state battery is bimodal, and when the orientation angle at the top of the first peak is Ya and the orientation angle at the top of the second peak is Yb (where Ya<Yb), the value of (Yb-Ya) is 30° or more and 90° or less.

[0047] In this embodiment, the orientation angle frequency distribution of the fibrous binder is determined by the following method. First, a cross section parallel to the main surface of the solid electrolyte layer is observed using a scanning electron microscope (SEM) to obtain an observation image. Next, the presence of the fibrous binder (PTFE) is detected by mapping elements specific to the fibrous binder (e.g., fluorine (F) when the binder is PTFE) using energy dispersive X-ray analysis (EDX). The size of the field of view (photography magnification) when obtaining the SEM observation image is adjusted so that a statistically significant number (e.g., 100 or more) of fibrous binders are included in one field of view. While the EDX method is used to detect the presence of the binder here, other methods, such as Auger electron spectroscopy (AES) and electron probe microanalysis (EPMA), may be used to perform mapping and detect the presence of the binder, as long as they provide the same results. The presence of the binder can also be confirmed by treating the solid electrolyte layer with water to remove the solid electrolyte.

[0048] The mapping data obtained as described above is input into an image processing device, and for the observed fibrous binders, one direction on the main surface of the solid electrolyte layer is set as a reference direction, and the angle θ [°] formed with this reference direction is measured. Here, in measuring θ for each fibrous binder, as shown in Figure 5, the absolute value of the angle formed with the reference direction by a line segment connecting the start point and end point where the fibrous binder can be observed in the observation image (fibrous binder 31 in Figure 5) is measured. Then, a frequency distribution of the angle θ and its frequency (number of fibrous binders) for all the observed fibrous binders is created. In this case, the classes are set in 1° increments, starting with "greater than or equal to 0° but less than 1°."

[0049] In the solid electrolyte layer according to the present embodiment, the histogram based on the orientation angle frequency distribution is bimodal (having two peaks). According to the manufacturing method, the number (frequency) of fibrous binders oriented in the extension direction Xa and its vicinity (within a range of ±5°) and the number (frequency) of fibrous binders oriented in the extension direction Xb and its vicinity (within a range of ±5°) are relatively large, while the number of fibrous binders oriented in a direction intermediate between the extension directions Xa and Xb is relatively small, resulting in a bimodal histogram.

[0050] The solid electrolyte layer according to the present embodiment is also characterized in that, when the orientation angle at the top of the first peak in the histogram is Ya and the orientation angle at the top of the second peak is Yb (where Ya<Yb), the value of (Yb-Ya) is 30° or more and 90° or less. This is because the value of (Yb-Ya) corresponds to the value of the angle A between the stretching direction Xa and the stretching direction Xb in the manufacturing method. Therefore, like angle A, the value of (Yb-Ya) is essentially 30° or more and 90° or less, preferably more than 45° and 90° or less, more preferably 60° or more and 90° or less, even more preferably 80° or more and 90° or less, particularly preferably 85° or more and 90° or less, and most preferably 90°. The effect of the present invention (the effect of improving the mechanical strength of the solid electrolyte layer) can be more significantly exhibited as (Yb-Ya) approaches 90°. If (Yb-Ya) is less than 30°, the orientation of the fibrous binder in the obtained solid electrolyte layer approaches a state where it is oriented in only one direction, which may result in a decrease in the mechanical strength of the solid electrolyte layer.

[0051] [Positive Electrode Active Material Layer] The positive electrode active material layer 15 contains a positive electrode active material, and may contain a solid electrolyte, a binder, and a conductive additive as needed.

[0052] The type of the positive electrode active material is not particularly limited, but LiCoO ２ , LiMnO ２ , LiNiO ２ , LiVO ２ , Li(Ni-Mn-Co)O ２ Layered rock salt active materials such as LiMn ２ O ４ , LiNi０．５ Mn １．５ O ４ Spinel-type active materials such as LiFePO ４ , LiMnPO ４ Olivine type active materials such as Li ２ FeSiO ４ , Li ２ MnSiO ４ Examples of oxide active materials other than those mentioned above include Si-containing active materials such as Li ４ Ti ５ O １２ Among them, Li(Ni-Mn-Co)O ２ Also, those in which part of these transition metals has been replaced with other elements (hereinafter also simply referred to as "NMC composite oxides") are preferably used as the positive electrode active material.

[0053] In addition, a sulfur-based positive electrode active material is also one of the preferred embodiments. Examples of the sulfur-based positive electrode active material include particles or thin films of organic sulfur compounds or inorganic sulfur compounds, and any material can be used as long as it is capable of releasing lithium ions during charging and absorbing lithium ions during discharging by utilizing the oxidation-reduction reaction of sulfur.

[0054] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably, for example, 30 to 99 mass %, more preferably 40 to 90 mass %, and even more preferably 45 to 80 mass %.

[0055] The positive electrode active material layer preferably further contains a solid electrolyte. The specific form of the solid electrolyte contained in the positive electrode active material layer may be the same as that described in the solid electrolyte layer section. A sulfide solid electrolyte is preferably used because it has excellent lithium ion conductivity and a low bulk modulus, allowing it to follow the volumetric changes of the positive electrode active material that occur during charging and discharging. The content of the solid electrolyte in the positive electrode active material layer is not particularly limited, but is, for example, 1 to 70% by mass, preferably 10 to 60% by mass, and more preferably 20 to 55% by mass.

[0056] The positive electrode active material layer preferably further contains a binder. The binder contained in the positive electrode active material layer is not particularly limited, and known binders can be used as appropriate. In particular, the fibrous binders described in the solid electrolyte layer section can be similarly used. The content of the binder in the positive electrode active material layer is not particularly limited, but is, for example, 1 to 10 mass %.

[0057] The conductive additive used in the positive electrode active material layer is not particularly limited, and examples thereof include carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.), carbon fiber (specifically, vapor-grown carbon fiber (VGCF), polyacrylonitrile-based carbon fiber, pitch-based carbon fiber, rayon-based carbon fiber, activated carbon fiber, etc.), graphene, and fibrous carbon such as carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes). The content of the conductive additive in the positive electrode active material layer is not particularly limited, but is, for example, 10 to 30% by mass.

[0058] The thickness of the positive electrode active material layer varies depending on the intended configuration of the secondary battery, but is usually 0.1 to 1000 μm, preferably 10 to 300 μm.

[0059] [Positive electrode current collector plate and negative electrode current collector plate] The material constituting the current collector plate (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for lithium secondary batteries can be used. Metal materials such as aluminum, carbon-coated aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as the constituent material of the current collector plate. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The negative electrode current collector plate 25 and the positive electrode current collector plate 27 may be made of the same material or different materials.

[0060] [Positive Electrode Lead and Negative Electrode Lead] Although not shown in the drawings, the current collectors (11", 11') and the current collector plates (27, 25) may be electrically connected via a positive electrode lead or a negative electrode lead. As the constituent materials of the positive electrode and the negative electrode lead, materials used in known lithium secondary batteries can be similarly adopted. Note that the portion removed from the exterior is preferably covered with a heat-resistant, insulating heat-shrinkable tube or the like so as to prevent contact with peripheral devices or wiring, etc., causing electrical leakage and affecting products (e.g., automobile parts, particularly electronic devices, etc.).

[0061] [Battery Exterior Body] As shown in FIG. 1 , a bag-shaped case using an aluminum-containing laminate film 29 capable of covering the power generating element can be used as the battery exterior body. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is desirable from the viewpoint of its high output and excellent cooling performance, making it suitable for use in batteries for large devices such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable as the exterior body because it allows for easy adjustment of the collective pressure applied to the power generating element from the outside.

[0062] Although the present embodiment has been described above using an all-solid-state lithium secondary battery as an example, the lithium secondary battery according to the present embodiment does not have to be all-solid-state. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolytic solution). There is no particular limitation on the amount of liquid electrolyte (electrolytic solution) that can be contained in the solid electrolyte layer, but it is preferable that the amount be such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and leakage of the liquid electrolyte (electrolytic solution) does not occur. Note that the liquid electrolyte (electrolytic solution) is a solution in the form of a conventionally known lithium salt dissolved in a conventionally known organic solvent. The liquid electrolyte (electrolytic solution) may further contain additives other than the organic solvent and the lithium salt. These additives may be used alone or in combination of two or more. Furthermore, when an additive is used in the electrolyte solution, the amount used can be adjusted appropriately.

[0063] The following embodiments are also included within the scope of the present invention: a manufacturing method according to claim 1 having the features of claim 2; a manufacturing method according to claim 1 or 2 having the features of claim 3; a manufacturing method according to claim 3 having the features of claim 4; a manufacturing method according to any one of claims 1 to 4 having the features of claim 5; a manufacturing method according to any one of claims 1 to 5 having the features of claim 6; a solid electrolyte layer for an all-solid-state battery according to claim 7 having the features of claim 8; and a solid electrolyte layer for an all-solid-state battery according to claim 7 or 8 having the features of claim 9.

[0064] 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 instruments and devices used in the glove box were thoroughly dried beforehand.

[0065] <Example of Preparation of Solid Electrolyte Layer> [Example 1] (1) First Sheet Preparation Step In a glove box with an argon atmosphere having a dew point of −68° C. or less, an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ 90 parts by weight of ethylenediaminetetraacetic acid (Ethylenediaminetetraacetic acid), average particle size (D50) 3 μm, manufactured by Ampcera, and 10 parts by weight of polytetrafluoroethylene (PTFE) (Polyflon F106, manufactured by Daikin Industries, Ltd.) as a binder were mixed in an agate mortar, and then kneaded for 2 minutes while applying shear force in the mortar. After confirming that the binder had fibrillated, the resulting powder composition (kneaded product) was sandwiched between two SUS304 foils (thickness 50 μm) and heated on a hot plate at 80 ° C. while being formed into a sheet with a thickness of 200 μm using a hand roller, thereby obtaining a first sheet (S1). At this time, the hand roller was applied to the powder composition at a shallow angle so that the angle formed between the hot plate and the axis of the hand roller was 45 ° or less, and the powder composition was spread (stretched) in one direction.

[0066] (2) Second Sheet Production Step Two first sheets (S1) were prepared. The two first sheets were laminated such that the angle A formed by the extension direction Xa of the first first sheet (S1a) and the extension direction Xb of the second first sheet (S1b) was 90°, thereby obtaining a second sheet (S2).

[0067] (3) Re-extension step The second sheet (S2) was further stretched to a thickness of 200 μm using a hand roller while being heated on a hot plate at 80° C., thereby obtaining a sheet (S2′) (re-extension 1). At this time, the hand roller was applied to the sheet at a shallow angle so that the angle A formed between the hot plate and the axis of the hand roller was 45° or less, and the sheet (S2) was stretched alternately in the stretching direction Xa of the first first sheet (S1a) and the stretching direction Xb of the second first sheet (S1b). The obtained sheet (S2') was sandwiched between two pieces of SUS304 foil (thickness: 50 μm) and further stretched using a tabletop roll press (manufactured by Hohsen Co., Ltd.) equipped with a heating mechanism, with the roll surface temperature set to 80°C, while reducing the gap by 10 μm at a time, until the final thickness reached 100 μm, thereby obtaining a sheet (S2") (re-stretching 2). At this time, the sheet (S2') was stretched alternately in the stretching direction Xa of the first first sheet (S1a) and the stretching direction Xb of the second first sheet (S1b). The obtained sheet (S2") was punched out into a square measuring 25 × 25 mm, thereby obtaining the solid electrolyte layer of this example. At this time, the sheet (S2") was punched out so that the stretching direction Xa of the first first sheet (S1a) was the same direction as one side of the square.

[0068] [Example 2] In the above "(2) second sheet preparation step", the second sheet (S2) was obtained by laminating two first sheets (S1a, S1b) such that the angle A formed by the extension direction Xa of the first first sheet (S1a) and the extension direction Xb of the second first sheet (S1b) was 45°. Except for this, the solid electrolyte layer of this example was obtained by the same method as in Example 1.

[0069] [Example 3] In "(Re-extension 2)" of the above "(3) Re-extension step", further extension was performed until the final thickness reached 80 µm, thereby obtaining a sheet (S2") The solid electrolyte layer of this example was obtained by the same method as in Example 1 except for this.

[0070] Comparative Example 1 In a glove box with an argon atmosphere having a dew point of −68° C. or less, an argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ 90 parts by mass of ethylenediaminetetraacetic acid (Ethylenediaminetetraacetic acid), average particle size (D50) 3 μm, manufactured by Ampcera, and 10 parts by mass of polytetrafluoroethylene (PTFE) (Polyflon F106, manufactured by Daikin Industries, Ltd.) as a binder were mixed using an agate mortar, and then kneaded for 2 minutes while applying shear force in the mortar. After confirming that the binder had fibrillated, the resulting powder composition (kneaded product) was sandwiched between two SUS304 foils (thickness 50 μm) and heated on a hot plate at 80 ° C. while being formed into a sheet with a thickness of 200 μm using a hand roller to obtain a first sheet (S1a). At this time, the hand roller was applied to the powder composition at a shallow angle so that the angle between the hot plate and the axis of the hand roller was 45 ° or less, and the powder composition was spread (stretched) in one direction.

[0071] The obtained first sheet (S1a) was sandwiched between two SUS304 foils (thickness: 50 μm) and further stretched to a thickness of 100 μm using a tabletop roll press (manufactured by Hosen Co., Ltd.) equipped with a heating mechanism, with the roll surface temperature set to 80°C, while reducing the gap by 10 μm at a time, to obtain a sheet (S1a'). At this time, the sheet (S1a) was stretched in the stretching direction Xa of the first sheet. The obtained sheet (S1a') was punched into a square measuring 25 × 25 mm, to obtain the solid electrolyte layer of this comparative example. At this time, the sheet (S1a') was punched so that the stretching direction Xa of the first sheet was the same direction as one side of the square.

[0072] Observation of Fibrous Binder in Solid Electrolyte Layer To observe the fibrous binder in the solid electrolyte layers prepared in Example 1 and Comparative Example 1, the solid electrolyte layers were treated with pure water to remove the solid electrolyte. Specifically, the solid electrolyte layers were immersed in pure water at 25°C for 4 hours. The samples were then rinsed three times with pure water to remove reactants, air-dried, and then vacuum-dried at 45°C for 4 hours. The resulting pure water-treated samples were then platinum-deposited to impart conductivity and subjected to SEM observation. Observations were performed using a scanning electron microscope (S-4700, manufactured by Hitachi High-Technologies Corporation). An example of an SEM image for Example 1 is shown in FIG. 6 , and an example of an SEM image for Comparative Example 1 is shown in FIG. 7 . Similar to Example 1 and Comparative Example 1, SEM observations were also performed for Examples 2 and 3. Next, a histogram based on the orientation angle frequency distribution of the fibrous binder was created from the obtained SEM images. It was confirmed that the histograms of Examples 1 to 3 were bimodal (having two distribution peaks), and the histogram of Comparative Example 1 was unimodal (having one distribution peak). Furthermore, for the histograms of Examples 1 to 3, the value of (Yb-Ya) was calculated, where Ya is the orientation angle at the top of the first peak and Yb is the orientation angle at the top of the second peak (where Ya<Yb). In Examples 1 and 3, the value of (Yb-Ya) was 90°, and in Example 2, the value of (Yb-Ya) was 45°.

[0073] <Tensile Test> Tensile tests were performed on each solid electrolyte layer prepared in the Examples and Comparative Examples in accordance with JIS K 6251:2017 to measure the breaking strength. First, a dumbbell-shaped test piece was prepared by punching out the solid electrolyte layer. Two types of test pieces were prepared: Test Piece 1, which was punched out so that the longitudinal direction of the dumbbell was the same as the stretching direction Xa, and Test Piece 2, which was punched out so that the width direction of the dumbbell was the same as the stretching direction Xa (the longitudinal direction of the dumbbell was the same as the direction perpendicular to the stretching direction Xa). A tensile test was performed on each of Test Piece 1 and Test Piece 2 using a universal testing machine (manufactured by Imada Co., Ltd.) at a tension rate of 10 mm / min at room temperature (25°C). The width and thickness of the dumbbell portion of the test piece were measured, and the breaking strength of the solid electrolyte layer was determined. The results are shown in Table 1 below.

[0074] <Checking for cracks> (1) Preparation of a test positive electrode active material layer In a glove box with an argon atmosphere at a dew point of −68° C. or less, LiNi as a positive electrode active material was ０．８ Mn ０．１ Co ０．１ O ２ (manufactured by NEI Co., Ltd.), 78 parts by mass of acetylene black (manufactured by Denka Co., Ltd.) as a conductive additive, 2 parts by mass of argyrodite-type sulfide solid electrolyte (Li ６ P.S. ５ 20 parts by mass of PO4 (Pyrolyzed Polytetrafluoroethylene (PTFE) with an average particle size (D50) of 1 μm, manufactured by Ampcera) were mixed using an agate mortar. 2 parts by mass of polytetrafluoroethylene (PTFE) was added as a binder to 100 parts by mass of the resulting mixed powder, and the mixture was mixed using an agate mortar. The mixture was then kneaded for 2 minutes while applying shear force in the mortar. After confirming that the binder had fibrillated, the resulting powder composition (kneaded product) was stretched to a thickness of 200 μm using a hand roller, and then further stretched to a thickness of 120 μm using a tabletop roll press (manufactured by Hosen Co., Ltd.) while reducing the gap by 10 μm at a time. The resulting sheet-like molded product was punched into a square measuring 19 × 19 mm to obtain a test positive electrode active material layer.

[0075] (2) Preparation of Test Laminates Test laminates were prepared using the solid electrolyte layers prepared in Examples 1 to 3 and Comparative Example 1 using the following method. First, a carbon-coated aluminum foil (30 × 30 mm square), a test cathode active material layer (19 × 19 mm square), and a solid electrolyte layer (25 × 25 mm square) were laminated in this order on a SUS foil (50 μm thick). This laminate was placed in a laminate bag and sealed using a vacuum sealer. The sealed laminate was sandwiched between an aluminum plate and a rubber plate, placed in a resin bag, and sealed using a vacuum sealer to obtain a test laminate. At this time, the aluminum plate, the sealed laminate, and the rubber plate were stacked so that the main surface of the solid electrolyte layer not in contact with the cathode active material layer faced the main surface of the rubber plate. To densify the cathode active material layer and the solid electrolyte layer, the test laminate was subjected to cold isostatic pressing (CIP) at 25°C, 700 MPa, and 1 minute. The pressed laminate was disassembled, and the solid electrolyte layer was removed. The presence or absence of cracks was confirmed using an X-ray microscope (ZEISS Xradia 620 Versa, manufactured by Carl Zeiss K.K.). A rating of ◯ was given for no cracks, △ for cracks occurring only on a portion of the edge, and × for cracks occurring to the opposite side. The results are shown in Table 1 below.

[0076] <Ionic Conduction Resistance> The ionic conduction resistance of each solid electrolyte layer prepared in the Examples and Comparative Examples was measured using the following method. In a glove box containing an argon atmosphere with a dew point of −70°C or lower, the solid electrolyte layer was punched out to a diameter of 10 mm. A SUS cylindrical convex punch (φ10.2 mm) was inserted into one side of a ceramic cylindrical tube jig (φ10.2 mm), and the solid electrolyte layer (φ10 mm) was inserted into the other side of the cylindrical tube jig. Then, another SUS cylindrical convex punch was inserted to sandwich the solid electrolyte layer, and the resulting mixture was uniaxially press-molded at 25°C and 100 MPa to prepare a SUS / solid electrolyte layer / SUS blocking cell. The impedance of the blocking cell was measured at an amplitude of 10 mV and from 1 MHz to 0.1 Hz using electrochemical impedance spectroscopy (EIS) (measuring device: Solartron 1260). The resistance was calculated from the intercept of the X-axis in the Cole-Cole plot and normalized based on the cross-sectional area and thickness of the solid electrolyte layer to determine the ionic conduction resistance. The relative values ​​of each ionic conduction resistance, when the ionic conduction resistance of Comparative Example 1 was set to 1.0, are shown in Table 1 below.

[0077]

[0078] As shown in Table 1, according to the present invention, the mechanical strength of the solid electrolyte layer used in the all-solid-state battery can be improved. Comparing Example 1 with Example 2 shows that the mechanical strength is further improved when the angle A is greater than 45° and equal to or less than 90°. Furthermore, comparing Example 1 with Example 3 shows that the solid electrolyte layer according to the present invention has sufficient mechanical strength to suppress the occurrence of cracks even when it is thin.

[0079] This application is based on Japanese Patent Application No. 2023-141515, filed on August 31, 2023, the disclosure of which is hereby incorporated by reference in its entirety.

[0080] REFERENCE SIGNS LIST 10a: laminated battery, 11': negative electrode current collector, 11": positive electrode current collector, 13: negative electrode active material layer, 15: positive electrode active material layer, 17: solid electrolyte layer, 19: single cell layer, 21: power generating element, 25: negative electrode current collector, 27: positive electrode current collector, 29: laminate film, 30, 31: fibrous binder, S1a, S1b: first sheet, S2: second sheet, X, Y, Z: fiber, Xa, Xb: extension direction.

Claims

1. A method for manufacturing a solid electrolyte layer for an all-solid-state battery, comprising: stretching a powder composition containing a solid electrolyte and a fibrous binder in one direction to obtain a first sheet; and stacking two or more of the first sheets such that an angle A between the stretching direction Xa of one of the first sheets and the stretching direction Xb of another of the first sheets is 30° or more and 90° or less to obtain a second sheet.

2. The manufacturing method according to claim 1, wherein said angle A is greater than 45° and not greater than 90°.

3. The manufacturing method according to claim 1, further comprising stretching the second sheet in the stretching direction Xa of one of the first sheets and in the stretching direction Xb of the other of the first sheets.

4. A manufacturing method according to claim 3, wherein the second sheet is stretched alternately in the stretching direction Xa of one of the first sheets and the stretching direction Xb of the other of the first sheets.

5. The method of claim 1, wherein the fibrous binder comprises polytetrafluoroethylene.

6. The method of claim 5, wherein the powder composition further comprises one or more selected from the group consisting of polyvinylidene fluoride, polypropylene, polyethylene, polystyrene, polyethylene terephthalate, and polyethylene oxide.

7. A solid electrolyte layer for an all-solid-state battery comprising a solid electrolyte and a fibrous binder, wherein a histogram based on a frequency distribution of an orientation angle of the fibrous binder when one direction on a main surface of the solid electrolyte layer for an all-solid-state battery is based on a bimodal distribution, and when the orientation angle at the top of the first peak is Ya and the orientation angle at the top of the second peak is Yb (where Ya<Yb), the value of (Yb-Ya) is 30° or more and 90° or less.

8. The solid electrolyte layer for an all-solid-state battery according to claim 7, wherein the fibrous binder contains polytetrafluoroethylene.

9. The solid electrolyte layer for an all-solid-state battery according to claim 7, which has a thickness of 100 μm or less.

10. An all-solid-state battery comprising the solid electrolyte layer for an all-solid-state battery according to any one of claims 7 to 9.

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