Method for inspecting solid electrolytes and method for manufacturing lithium secondary batteries

Abstract

This invention provides a means to more accurately evaluate the ionic conductivity of solid electrolytes. [Solution] A method for inspecting a solid electrolyte, comprising: preparing a laminate by placing a current collector foil substantially made of a metal with a thickness of at least 7 times the D90 of the material with the largest average particle size among the materials contained in the solid electrolyte-containing layer, and a Young's modulus smaller than that of the solid electrolyte-containing layer, on both sides of the solid electrolyte-containing layer which contains at least a solid electrolyte; applying a restraining pressure in the lamination direction of the laminate; and measuring the resistance of the solid electrolyte-containing layer while the restraining pressure is applied.

Description

[Technical Field]

[0001] This invention relates to a method for testing solid electrolytes and a method for manufacturing lithium secondary batteries. [Background technology]

[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 ionic conductors capable of ion conduction in a solid state. Therefore, lithium secondary batteries using solid electrolytes 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] Since solid electrolytes are particulate, when measuring the ionic conductivity of solid electrolytes, a method is used in which pressure is applied to a material containing the solid electrolyte to form pellets, which are then used as a sample and measured by AC impedance under pressure. Patent Document 1 discloses that, in the inspection of battery short circuits, an elastic electrically conductive metal resin body (electrically conductive metal rubber mat) is attached to the tips of the metal terminals used for current conduction that sandwich the sample to be measured from both poles. By doing so, contact between the terminals and the sample to be measured under pressure is improved, and current conduction can be ensured, thus potentially improving inspection performance. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Utility Model Publication No. 60-70082 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Lithium-ion batteries using solid electrolytes can be used at higher temperatures than liquid-based lithium-ion batteries. Therefore, to accommodate such high-temperature use, it is necessary to confirm the ionic conductivity of the solid electrolyte at temperatures up to approximately 100°C. However, the electrically conductive metal resin body described in Patent Document 1 does not have sufficient heat resistance, making it difficult to use the technology described in Patent Document 1 to evaluate the ionic conductivity of solid electrolytes. Therefore, the present invention aims to provide a means for more accurately evaluating the ionic conductivity of solid electrolytes. [Means for solving the problem]

[0006] The inventors of the present invention conducted thorough research in view of the above problems. They discovered that the above problems can be solved by measuring the resistance of a solid electrolyte-containing layer containing a solid electrolyte, by placing current collector foils made of a metal with a lower Young's modulus than the solid electrolyte-containing layer and having a predetermined thickness on both sides of the solid electrolyte-containing layer, and applying a restraining pressure to perform the measurement. Thus, the present invention was completed.

[0007] In other words, one embodiment of the present invention is a method for inspecting a solid electrolyte, comprising: preparing a laminate by arranging a current collector foil substantially made of a metal with a thickness of at least 7 times the D90 of the material with the largest average particle size among the materials contained in the solid electrolyte-containing layer, and having a Young's modulus smaller than that of the solid electrolyte-containing layer, on both sides of the solid electrolyte-containing layer, applying a restraining pressure in the lamination direction of the laminate, and measuring the resistance of the solid electrolyte-containing layer while the restraining pressure is applied. [Effects of the Invention]

[0008] According to the present invention, the ionic conductivity of solid electrolytes can be evaluated more accurately. [Brief explanation of the drawing]

[0009] [Figure 1]Figure 1(a) is a schematic cross-sectional view showing the state when a restraining pressure is applied to a laminate including a solid electrolyte-containing layer and current collector foils arranged on both sides of the solid electrolyte-containing layer, which is the subject of resistance measurement by the method of this embodiment. Figures 1(b) and (c) are schematic cross-sectional views showing the state when a restraining pressure is applied to the laminate in the comparative example method. [Modes for carrying out the invention]

[0010] 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.

[0011] One embodiment of the present invention is a method for inspecting a solid electrolyte (resistance measurement method), comprising the steps of: preparing a laminate by arranging current collector foils substantially made of a metal with a thickness of at least 7 times the D90 of the material with the largest average particle size among the materials contained in the solid electrolyte-containing layer, and having a Young's modulus smaller than that of the solid electrolyte-containing layer, on both sides of the solid electrolyte-containing layer (laminated structure preparation step); applying a restraining pressure in the lamination direction of the laminate (restraining pressure application step); and measuring the resistance of the solid electrolyte-containing layer while the restraining pressure is applied (resistance measurement step). According to the method of this embodiment, the ionic conductivity of the solid electrolyte can be evaluated more accurately.

[0012] In lithium secondary batteries using a solid electrolyte, the power generation element, consisting of a positive electrode, a solid electrolyte layer, and a negative electrode stacked in that order, is sealed inside the battery casing. The solid electrolyte can be used, for example, as the main component in the solid electrolyte layer. When manufacturing lithium secondary batteries using a solid electrolyte, it is extremely important to understand the ionic conductivity of the solid electrolyte used as a raw material.

[0013] When measuring the ionic conductivity of a solid electrolyte, a method is used in which a solid electrolyte-containing layer, obtained by pressure molding a material containing the solid electrolyte into pellets, is sandwiched on both sides with metal current-collecting pins, such as stainless steel, and the resistance of the solid electrolyte-containing layer is measured while applying pressure with a pressing member in the stacking direction of the laminate of current-collecting pins / solid electrolyte-containing layer / current-collecting pins. However, our studies have shown that when resistance is measured using this method, variations occur in the ionic conductivity value of the solid electrolyte-containing layer. Furthermore, it has been found that variations in ionic conductivity values ​​can lead to variations in cell evaluation when designing and manufacturing batteries.

[0014] Because the solid electrolyte is a powdered solid, sufficient contact may not be achieved at the interface with the current collector foil. Figure 1 shows a schematic cross-sectional view illustrating the state when a restraining pressure is applied to a laminate containing a solid electrolyte-containing layer to be measured for resistance and current collector foils arranged on both sides of the solid electrolyte-containing layer. In conventional methods for resistance measurement, as shown in Figure 1(b), current collector foils 3' made of a hard metal such as stainless steel are placed on both sides of a solid electrolyte-containing layer 1 having solid electrolyte particles 2 to create a laminate 5, and pressure is applied in the lamination direction of the laminate 5 as indicated by the arrows. However, when the solid electrolyte-containing layer 1 is made by compacting the solid electrolyte particles 2, there is a large variation in the surface roughness of the solid electrolyte-containing layer 1 after pressing, resulting in variations in contact with the current collector foil 3'. As a result, the formation of the interface between the solid electrolyte-containing layer 1 and the current collector foil 3' becomes insufficient, leaving a gap 4 between the solid electrolyte-containing layer 1 and the current collector foil 3', which can increase resistance. Therefore, when resistance measurements are performed using such laminates, variations in the measured values ​​may occur. In addition, the measured values ​​may not reflect the true state of the solid electrolyte, making it impossible to accurately understand the electrochemical properties of the solid electrolyte, such as its ionic conductivity.

[0015] In contrast, in the method according to this embodiment, as shown in Fig. 1(a), pressure is applied in the stacking direction as indicated by the arrow to measure the resistance of a laminate 5 in which current collector foils 3 substantially made of a metal having a Young's modulus smaller than that of the solid electrolyte-containing layer 1 are disposed on both surfaces of the solid electrolyte-containing layer 1. By making the Young's modulus of the metal constituting the current collector foil 3 smaller than that of the solid electrolyte-containing layer 1, when pressure is applied, the current collector foil 3 plastically deforms in accordance with the shape of the solid electrolyte-containing layer 1 to fill the voids. That is, the current collector foil 3 follows the shape of the solid electrolyte particles 2, making it difficult for voids to occur between the solid electrolyte-containing layer 1 and the current collector foil 3. As a result, the variation in the value obtained by the resistance measurement can be reduced.

[0016] Here, as shown in Fig. 1(c), in the laminate 5, if the thickness of the current collector foil 3 substantially made of a metal having a Young's modulus smaller than that of the solid electrolyte-containing layer 1 is smaller than a predetermined thickness, the volume of the current collector foil 3 is insufficient, so the current collector foil 3 cannot deform sufficiently to fill the voids 4 between it and the solid electrolyte-containing layer 1. Therefore, the measured value in the resistance measurement tends to be high, making it difficult to accurately grasp the electrochemical properties such as the ionic conductivity of the solid electrolyte.

[0017] Hereinafter, each step in the method according to this embodiment will be described.

[0018] [Laminate Fabrication Step] In this step, a laminate is fabricated by disposing current collector foils, which are made substantially of a metal having a Young's modulus smaller than that of the solid electrolyte-containing layer and have a thickness of 7 times or more the D90 of the material having the largest average particle diameter among the materials contained in the solid electrolyte-containing layer, on both surfaces of the solid electrolyte-containing layer containing the solid electrolyte.

[0019] (Solid Electrolyte-Containing Layer) The solid electrolyte-containing layer contains a solid electrolyte. In this specification, the solid electrolyte refers to a material mainly composed of an ion conductor capable of ionic conduction in a solid state. In particular, the lithium ion conductivity at room temperature (25 °C) is 1×10 -5This refers to a material with a lithium ion conductivity of S / cm or higher, and this lithium ion conductivity is preferably 1 × 10⁻⁶. -4 The conductivity is greater than or equal to S / cm. Here, the lithium-ion conductivity can be measured by the AC impedance method.

[0020] The solid electrolyte is not particularly limited, and sulfide solid electrolytes, oxide solid electrolytes, halide solid electrolytes, etc., can be used. However, it is preferable to include a sulfide solid electrolyte or a halide solid electrolyte because it exhibits excellent lithium-ion conductivity and is suitable for use in lithium secondary batteries, and it is more preferable to include a sulfide solid electrolyte because it is a material that is easily plastically deformable.

[0021] A sulfide solid electrolyte refers to a solid electrolyte containing the element S. A sulfide solid electrolyte must contain the element S, preferably S, Li and M (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 S, Li and P.

[0022] Examples of sulfide solid electrolytes include LiI-Li2S-SiS2, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li2S-P2S5, LiI-Li3PS4, LiI-LiBr-Li3PS4, Li3PS4, Li2S-P2S5-LiI, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, and Li2S-P2S5-Z. m S n (where m and n are positive numbers, and Z is one of Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li x MO y(However, x and y are positive numbers, and M is any one of P, Si, Ge, B, Al, Ga, In, etc.). Note that the description of "Li2S-P2S5" means a sulfide solid electrolyte formed using a raw material composition containing Li2S and P2S5, and the same applies to other descriptions.

[0023] The sulfide solid electrolyte may, for example, have a Li3PS4 skeleton, a Li4P2S7 skeleton, or a Li4P2S6 skeleton. Examples of the sulfide solid electrolyte having a Li3PS4 skeleton include LiI-Li3PS4, LiI-LiBr-Li3PS4, and Li3PS4. Further, examples of the sulfide solid electrolyte having a Li4P2S7 skeleton include a Li-P-S-based solid electrolyte called LPS (e.g., Li7P3S 11 ). Also, as the sulfide solid electrolyte, for example, LGPS represented by Li (4-x) Ge (1-x) P x S4 (where x satisfies 0 < x < 1) may be used. Among them, a sulfide solid electrolyte containing the P element is preferable. Furthermore, the sulfide solid electrolyte may contain a halogen (F, Cl, Br, I), and an example thereof is a sulfide solid electrolyte having an argyrodite-type crystal structure such as Li6PS5X (where X is Cl, Br, or I, preferably Cl). Note that the sulfide solid electrolyte may be used alone or two or more kinds may be used in combination.

[0024] In this specification, the "halide solid electrolyte" means a solid electrolyte containing a halogen element and not containing a sulfur element. Examples of the halide solid electrolyte include Li p M q X rA solid electrolyte represented by the following compositional formula is given: (where M is a metallic element, X is a halogen element, p is 1, 2, or 3, q ​​is 0 or 1, and r is one integer from 1 to 6. In this case, p, q, and r are appropriately selected so that the total charge of the compound represented by the above compositional formula is 0). The metallic element M is at least one selected from the group Al, Mg, Fe, Ga, Y, Zr, and In, but is not limited to these. The halogen element X is at least one selected from the group F, Cl, Br, and I, but is not limited to these.

[0025] Examples of halogen solid electrolytes include those having the compositions of LiX, LiMX4, Li2MX4, Li2MX6, or Li3MX6. Among these, Li3MX6 is preferred as the halogen solid electrolyte because it allows for the creation of higher-performance lithium secondary batteries. M and X are the same as the above-mentioned Li. p M q X r The definitions of M and X are similar to those in the following context.

[0026] Examples of Li3MX6 include, but are not limited to, Li3AlF6, Li3AlCl6, Li3AlBr6, Li3AlI6, Li3GaF6, Li3GaCl6, Li3GaBr6, Li3GaI6, Li3YF6, Li3YCl6, Li3YBr6, Li3YI6, Li3InF6, Li3InCl6, Li3InBr6, and Li3InI6.

[0027] The solid electrolyte can be in particulate form, and examples of particulate shapes include spherical and ellipsoidal shapes. The average particle diameter (D50) of the solid electrolyte is not particularly limited, but is, for example, 0.1 to 20 μm, preferably 0.5 to 10 μm, and more preferably 0.5 to 5 μm. The D90 of the solid electrolyte is also not particularly limited, but is, for example, 0.1 to 20 μm, preferably 0.7 to 14 μm, and more preferably 0.7 to 10 μm. The effects of the present invention can be obtained even more significantly when the D90 is within the above range.

[0028] In this specification, the particle size of solid electrolyte particles is determined by spreading the dry solid electrolyte powder onto a glass slide using a blower, photographing the particles with a 50x SEM, and performing image analysis of the particles. Specifically, a circle with an area equivalent to the projected area of ​​a single particle is considered, and the diameter of this circle is defined as the particle size. The particle sizes are arranged by volume frequency, and the average particle size (D50) and D90 are calculated.

[0029] The solid electrolyte-containing layer may further contain other components, such as a binder, in addition to the solid electrolyte. The binder is not particularly limited, and any known in the art can be used as appropriate.

[0030] The solid electrolyte content in the solid electrolyte-containing layer is, for example, more than 50% by mass and 100% by mass or less, preferably 80-100% by mass, more preferably 90-100% by mass, even more preferably 95-100% by mass, even more preferably 98-100% by mass, and most preferably 100% by mass.

[0031] The method for preparing the solid electrolyte-containing layer is not particularly limited. Preferably, the solid electrolyte-containing layer can be prepared by a method that includes a pressure molding step in which a material containing a solid electrolyte is pressure-molded to obtain the solid electrolyte-containing layer.

[0032] The means for press-molding the above material are not particularly limited. Press-molding may be uniaxial or biaxial. The press-molding apparatus is also not particularly limited. For example, a press-molding apparatus can be used that consists of an electronically insulated frame for holding the above material and press members positioned above and below to sandwich the above material. The press members can be made of, for example, stainless steel. A hydraulic press, a mechanical press, or the like can be used as the molding machine.

[0033] The pressure used during the above-mentioned pressure molding is not particularly limited, but is preferably 75 to 450 MPa, more preferably 200 to 400 MPa, and even more preferably 250 to 350 MPa. A pressure of 75 MPa or higher allows for sufficiently high density of the solid electrolyte-containing layer, enabling more accurate resistance measurement. A pressure of 450 MPa or lower is preferable because it suppresses density non-uniformity due to particle breakage, etc., and reduces the likelihood of measurement variations. The time used during the above-mentioned pressure molding is also not particularly limited, but is, for example, 0.1 to 10 minutes. The temperature used during the above-mentioned pressure molding is also not particularly limited and can be, for example, at room temperature (20 to 25°C).

[0034] The thickness of the solid electrolyte-containing layer is not particularly limited, but is, for example, 20 to 1000 μm, preferably 100 to 1000 μm, and more preferably 500 to 1000 μm.

[0035] The Young's modulus of the solid electrolyte-containing layer is not particularly limited as long as it is greater than the Young's modulus of the metal constituting the current collector foil, as described later. For example, it is 12 to 180 GPa, preferably 12 to 100 GPa, more preferably 14 to 50 GPa, and even more preferably 16 to 50 GPa. The Young's modulus of the solid electrolyte-containing layer can be measured by the following method: First, a nanoindenter with an indenter tip radius of 100 μm is used to apply an indentation load of 5000 mN or more. Next, an indentation load is applied using a nanoindenter with an indenter tip radius of 1 μm, targeting the central part of this 100 μm radius area, and the Young's modulus is determined from the indentation depth and indentation load.

[0036] (Current collector foil) The current collector foils are arranged one on each side of the solid electrolyte-containing layer and are electrically connected to the resistance measuring device. The current collector foils may also be connected to the resistance measuring device via, for example, current collector pins made of SUS (stainless steel). These current collector foils may be identical or different, but it is preferable that they be identical, as long as they are substantially made of a metal with a Young's modulus lower than that of the solid electrolyte-containing layer, and each has a thickness of at least seven times the D90 of the material with the largest average particle size among the materials contained in the solid electrolyte-containing layer.

[0037] The current collector foil is substantially made of a metal with a Young's modulus lower than that of the solid electrolyte-containing layer. "Substantially made of a metal with a Young's modulus lower than that of the solid electrolyte-containing layer" means that the inclusion of impurities of approximately 2-3% by mass or less is permissible. In a preferred embodiment, the current collector foil is made of a metal with a Young's modulus lower than that of the solid electrolyte-containing layer.

[0038] Examples of metals with a Young's modulus lower than that of the solid electrolyte-containing layer include indium (In, Young's modulus: 11 GPa). Indium is preferable because it is easily plastically deformed to conform to the shape of the solid electrolyte-containing layer, further reducing voids and enabling more accurate resistance measurements. In a preferred embodiment, the current collector foil is substantially made of indium. In a preferred embodiment, the current collector foil consists solely of indium. The Young's modulus values ​​of the metals can be found in the Iwanami Dictionary of Physics and Chemistry, 5th Edition, Iwanami Shoten, published February 20, 1998. The Young's modulus of the above metals is not particularly limited, but may be, for example, 17 GPa or less. The difference between the Young's modulus of the solid electrolyte-containing layer and the Young's modulus of the above metal is not particularly limited, but may be, for example, (Young's modulus of the solid electrolyte-containing layer) - (Young's modulus of the metal) ≥ 5 GPa.

[0039] The thickness of the current collector foil is not particularly limited as long as it is at least 7 times the D90 of the material with the largest average particle size among the materials contained in the solid electrolyte-containing layer. Here, the material with the largest average particle size among the materials contained in the solid electrolyte-containing layer refers to the solid electrolyte if the solid electrolyte-containing layer consists of only one type of solid electrolyte, and refers to the solid electrolyte with the largest average particle size if the solid electrolyte-containing layer consists of two types of solid electrolytes. Furthermore, if other components such as a binder are included, it refers to the material with the largest average particle size among all materials, including the solid electrolyte and other components. There is no particular upper limit to the thickness of the current collector foil, but from the viewpoint of reducing costs, it is preferable to be 500 μm or less. In a preferred embodiment, the thickness of the current collector foil is 21 to 500 μm, more preferably 50 to 400 μm, and even more preferably 100 to 300 μm.

[0040] The ratio of the thickness of the current collector foil to the D90 of the material with the largest average particle size among the materials contained in the solid electrolyte layer (thickness of current collector foil / D90) should be 7 or greater, but the effects of the present invention can be more significantly obtained if it is 30 or greater. Furthermore, a ratio of 180 or less is preferable from the viewpoint of reducing costs.

[0041] There are no particular restrictions on the source of the current collector foil or the means of arranging the current collector foil on both sides of the solid electrolyte-containing layer.

[0042] In a preferred embodiment, during the fabrication of the laminate, the current collector foil is positioned such that its outer edge, when viewed from above, is located inside the outer edge of the solid electrolyte-containing layer. Then, in the restraining pressure application step described later, restraining pressure is applied in the lamination direction of the laminate to deform the current collector foil so that its outer edge and the outer edge of the solid electrolyte-containing layer substantially coincide. This is preferable because it prevents short circuits caused by the current collector foil sinking into the solid electrolyte-containing layer, compared to using a current collector foil of the same size as the solid electrolyte-containing layer from the beginning. For example, by appropriately adjusting the material, size, and restraining pressure of the current collector foil, the area of ​​the current collector foil after applying the restraining pressure can be made substantially the same as the area of ​​the solid electrolyte-containing layer.

[0043] [Constraining pressure application process] In this process, a restraining pressure is applied in the stacking direction of the laminate.

[0044] The means for applying restraining pressure in the stacking direction of the laminate are not particularly limited. For example, a pressure molding apparatus used for pressure molding the material containing the solid electrolyte described above can be used.

[0045] The restraining pressure is not particularly limited, but from the viewpoint of effectively plastically deforming the current collector foil, it is preferably 4 MPa or more, more preferably 10 to 400 MPa, even more preferably 20 to 200 MPa, and even more preferably 50 to 150 MPa.

[0046] Here, it is preferable to keep the constraining pressure applied to the laminate from the start of applying the constraining pressure until the resistance measurement in the resistance measurement step described later is completed. In this case, it is preferable that the constraining pressure applied to the laminate remains constant from the start of applying the constraining pressure until the resistance measurement. Here, "constant constraining pressure" means that unintentional pressure fluctuations may exist. By doing so, the current collector foil is more effectively plastically deformed to match the shape of the solid electrolyte-containing layer, so that voids are further reduced and the effects of the present invention can be obtained even more significantly.

[0047] By applying restraining pressure to the laminate, the thickness of the solid electrolyte-containing layer may change. However, in a preferred embodiment, the resistance measurement in the resistance measurement step described later is performed only after the displacement of the thickness of the solid electrolyte-containing layer (the difference between the thickness of the solid electrolyte-containing layer before restraining and the thickness of the solid electrolyte-containing layer after restraining) has become constant. Here, the displacement of the thickness of the solid electrolyte-containing layer can be determined by removing the pressurizing member and measuring it with a caliper in the state of the laminate including the solid electrolyte-containing layer. The thickness displacement is considered to have become constant when there is no longer any measurable change in thickness over time. By doing so, the current collector foil more effectively plastically deforms to match the shape of the solid electrolyte-containing layer, so that the voids are further reduced and the effects of the present invention can be obtained even more significantly.

[0048] The duration for which the restraining pressure is applied is not particularly limited and can be adjusted as appropriate, but it is preferable to adjust the measurement so that the resistance of the solid electrolyte-containing layer is measured after holding it under the applied restraining pressure for 20 hours or more. Under pressurized conditions, the voids decrease sufficiently over time and contact improves, so the effects of the present invention can be obtained even more significantly. There is no particular upper limit to the duration for which the restraining pressure is applied and held, but from the viewpoint of inspection efficiency, it is, for example, 100 hours or less. In a preferred embodiment, the above holding time is 30 to 50 hours.

[0049] The temperature at which restraining pressure is applied in the stacking direction of the laminate is not particularly limited, but it can be done, for example, at room temperature (20-25°C).

[0050] [Resistance measurement process] In this process, the resistance of the solid electrolyte-containing layer is measured while the restraining pressure is applied to the laminate.

[0051] The means of resistance measurement are not particularly limited, and conventionally known knowledge may be referred to as appropriate, but for example, it can be performed by the AC impedance method. For example, current collector pins, which also serve as pressurizing members, can be placed on current collector foils arranged on both sides of the solid electrolyte-containing layer, and wires can be connected to each pin to a potentiostat equipped with a frequency response analyzer, and the resistance can be measured by the AC impedance method. The specific conditions for resistance measurement by the AC impedance method are not particularly limited and can be set as appropriate. Furthermore, the ionic conductivity of the solid electrolyte-containing layer can be calculated based on the resistance value obtained by the above resistance measurement.

[0052] The above process allows for more accurate evaluation of the electrochemical properties of the solid electrolyte, such as its ionic conductivity. Therefore, this method is suitably applicable to evaluation during acceptance inspections when receiving solid electrolytes as raw materials for the manufacture of lithium secondary batteries.

[0053] Furthermore, this method allows for a more accurate determination of the electrochemical properties of the solid electrolyte, such as its ionic conductivity. By evaluating the solid electrolyte as a raw material using this method, the design and manufacture of lithium secondary batteries using it can be made more efficient. In addition, variations in the performance of the manufactured lithium secondary batteries can be reduced. That is, one embodiment of the present invention is a method for manufacturing a lithium secondary battery, which includes evaluating the ionic conductivity of a solid electrolyte by the method described above.

[0054] The following embodiments are also included within the scope of the present invention: Item 1: A laminate is fabricated by placing a current collector foil substantially made of a metal with a thickness of at least 7 times the D90 of the material with the largest average particle size among the materials contained in the solid electrolyte-containing layer, and with a Young's modulus smaller than that of the solid electrolyte-containing layer, on both sides of the solid electrolyte-containing layer, A restraining pressure is applied in the stacking direction of the laminated body, A method for inspecting a solid electrolyte, comprising measuring the resistance of the solid electrolyte-containing layer while the aforementioned restraining pressure is applied; Item 2: The method according to Item 1, wherein the restraining pressure remains constant from the time the application of the restraining pressure is started until the resistance of the solid electrolyte-containing layer is measured; Item 3: The method according to item 1 or 2, wherein the resistance of the solid electrolyte-containing layer is measured after the thickness displacement of the solid electrolyte-containing layer has become constant; Item 4: The method according to any one of items 1 to 3, wherein the resistance of the solid electrolyte-containing layer is measured after holding the restraining pressure in place for 20 hours or more; Item 5: When manufacturing the laminate, the current collector foil is positioned such that, when the laminate is viewed from above, the outer edge of the current collector foil is located inside the outer edge of the solid electrolyte-containing layer. The method according to any one of items 1 to 4, wherein the current collector foil is deformed by applying a restraining pressure in the lamination direction of the laminate so that the outer edge of the current collector foil and the outer edge of the solid electrolyte-containing layer substantially coincide; Item 6: The method according to any one of items 1 to 5, wherein the thickness of the current collector foil is 500 μm or less; Item 7: The current collector foil comprises indium, as described in any of items 1 to 6; Item 8: A method for manufacturing a lithium secondary battery, comprising evaluating the ionic conductivity of the solid electrolyte by the method described in any of Items 1 to 7. [Examples]

[0055] 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 the solid electrolyte-containing layer were carried out in a glove box with an argon atmosphere with a dew point of -68°C or lower. Furthermore, the instruments and equipment used in the glove box were thoroughly dried beforehand.

[0056] [Example 1] (Preparation of a solid electrolyte-containing layer) A Macol tube with an inner diameter of 10.5 mm and current-collecting pins with an outer diameter of 10 mm to sandwich both sides of the solid electrolyte-containing layer were prepared. In a glove box with an argon atmosphere with a dew point of -68°C or lower, at 25°C, one of the current-collecting pins was placed on the underside of the Macol tube, and 100 mg of argyrodite-type sulfide solid electrolyte (average particle size: 1.0 μm, D90: 2.9 μm) was weighed and poured into the Macol tube from the top. Next, the solid electrolyte powder was leveled with the other current-collecting pin, and the solid electrolyte powder, along with the upper and lower current-collecting pins, was compacted at 300 MPa using a uniaxial molding machine to obtain a solid electrolyte-containing layer with a thickness of 750 μm.

[0057] Separately, the Young's modulus of the obtained solid electrolyte-containing layer was measured using the method described above, and it was found to be 17.4 GPa.

[0058] (Fabrication of laminates and application of restraining pressure) Subsequently, the current collector pins on both sides were removed, and 300 μm thick In foil (Young's modulus: 11 GPa), punched out to a diameter of φ9 mm, was placed on both sides of the solid electrolyte-containing layer as current collector foil. Then, the current collector pins were repositioned in the Macol tube. An evaluation cell was prepared by restraining the pair of current collector pins and the Macol tube together at 100 MPa using a sealed restraint jig, and it was held in this 100 MPa state for 48 hours.

[0059] Furthermore, when the thickness of the solid electrolyte-containing layer was measured separately using calipers along with the laminate containing the solid electrolyte-containing layer, the displacement of the solid electrolyte-containing layer's thickness (the difference between the thickness of the solid electrolyte-containing layer before and after restraint) became constant within a few seconds of starting the 100 MPa restraint, and remained constant without changing for 48 hours.

[0060] (Measurement of ionic conductivity) An impedance analyzer (manufactured by Toyo Technica Corporation) was connected to a current collection pin, and impedance measurements were performed on the evaluation cell at a frequency range of 7 MHz to 100 mHz, an amplitude of 10 mV, and at 25°C. The resistance value at the point where the Nyquist plot intersected the real axis was then determined. After impedance measurement, the cell was disassembled, and the thickness of the solid electrolyte-containing layer was measured with calipers. Ionic conductivity was calculated using the formula: (thickness of solid electrolyte-containing layer) / ((resistance value) × (cross-sectional area of ​​solid electrolyte-containing layer)). Using the above method, the ionic conductivity was determined for five evaluation cells, and the variance and standard deviation were calculated using all of these values.

[0061] Furthermore, after measuring the impedance, the cell was disassembled and the size of the current collector foil was checked, and it was found that the outer edge of the current collector foil coincided with the outer edge of the solid electrolyte-containing layer.

[0062] [Example 2] In Example 1, the ionic conductivity of the solid electrolyte-containing layer was measured in the same manner as in Example 1, except that the holding time under restraint at 100 MPa was changed from 48 hours to 24 hours. After the impedance measurement, the cell was disassembled and the size of the current collector foil was confirmed to be the same as the outer edge of the current collector foil.

[0063] [Example 3] In Example 1, the ionic conductivity of the solid electrolyte-containing layer was measured in the same manner as in Example 1, except that the holding time under restraint at 100 MPa was changed from 48 hours to 1 hour. After the impedance measurement, the cell was disassembled and the size of the current collector foil was confirmed to be the same as the outer edge of the current collector foil.

[0064] [Example 4] In Example 1, the ionic conductivity of the solid electrolyte-containing layer was measured in the same manner as in Example 1, except that 100 μm thick In foils punched out with a diameter of φ9 mm were placed on both sides of the solid electrolyte-containing layer, and the holding time under constraint at 100 MPa was changed from 48 hours to 1 hour. After the impedance measurement, the cell was disassembled and the size of the current collector foil was confirmed to be the same as the outer edge of the current collector foil and the outer edge of the solid electrolyte-containing layer.

[0065] [Comparative Example 1] In Example 1, the ionic conductivity of the solid electrolyte-containing layer was measured in the same manner as in Example 1, except that 10 μm thick SUS430 foil (Young's modulus: 200 GPa) punched out with a diameter of φ9 mm was placed on both sides of the solid electrolyte-containing layer, and the holding time under restraint at 100 MPa was changed from 48 hours to 1 hour.

[0066] [Comparative Example 2] In Example 1, the ionic conductivity of the solid electrolyte-containing layer was measured in the same manner as in Example 1, except that 20 μm thick Al foil (Young's modulus: 68.6 GPa) punched out with a diameter of φ9 mm was placed on both sides of the solid electrolyte-containing layer, and the holding time under restraint at 100 MPa was changed from 48 hours to 1 hour.

[0067] [Comparative Example 3] In Example 1, the ionic conductivity of the solid electrolyte-containing layer was measured in the same manner as in Example 1, except that 20 μm thick In foils punched out with a diameter of φ9 mm were placed on both sides of the solid electrolyte-containing layer, and the holding time under restraint at 100 MPa was changed from 48 hours to 1 hour.

[0068] These results are shown in Table 1 below.

[0069] [Table 1]

[0070] As shown in Table 1, in Examples 1 to 4, where a solid electrolyte-containing layer was sandwiched between current collector foils made of a metal with a Young's modulus lower than that of the solid electrolyte-containing layer, and the ratio of the thickness of the current collector foil to the D90 of the solid electrolyte (thickness of current collector foil / D90) was 7 or more, a restraining pressure was applied to the laminate and resistance measurements were performed, resulting in large ionic conductivity values ​​with small variability. In contrast, in Comparative Examples 1 and 2, where the Young's modulus of the metal constituting the current collector foil was greater than that of the solid electrolyte-containing layer, the variability of the measured ionic conductivity values ​​was large. Furthermore, in Comparative Example 2, where the value of (thickness of current collector foil / D90) was less than 7, the measured ionic conductivity values ​​were small, indicating that they did not reflect the intrinsic ionic conductivity of the material.

[0071] Furthermore, a comparison of Examples 1 to 4 shows that by limiting the holding time after restoring the laminate to 20 hours or more, and especially 30 hours or more, the measured ionic conductivity becomes larger, and the ionic conductivity can be measured more accurately. [Explanation of symbols]

[0072] 1 solid electrolyte-containing layer, 2. Solid electrolyte particles, 3, 3' Current collector foil, 4 void, 5. Laminated structure.

Claims

1. A laminate is fabricated by placing a current collector foil substantially made of a metal with a thickness of at least seven times the D90 of the material with the largest average particle size among the materials contained in the solid electrolyte-containing layer, and with a Young's modulus smaller than that of the solid electrolyte-containing layer, on both sides of the solid electrolyte-containing layer, A restraining pressure is applied in the stacking direction of the laminated body, A method for inspecting a solid electrolyte, comprising measuring the resistance of the solid electrolyte-containing layer while the aforementioned restraining pressure is applied.

2. The method according to claim 1, wherein the restraining pressure remains constant from the time the application of the restraining pressure is started until the resistance of the solid electrolyte-containing layer is measured.

3. The method according to claim 1, wherein the resistance of the solid electrolyte-containing layer is measured after the displacement of the thickness of the solid electrolyte-containing layer becomes constant.

4. The method according to claim 1, wherein the resistance of the solid electrolyte-containing layer is measured after holding the layer in the state of applying the restraining pressure for 20 hours or more.

5. When manufacturing the laminate, the current collector foil is positioned such that, when the laminate is viewed from above, the outer edge of the current collector foil is located inside the outer edge of the solid electrolyte-containing layer. The method according to claim 1, wherein a restraining pressure is applied in the lamination direction of the laminate to deform the current collector foil so that the outer edge of the current collector foil and the outer edge of the solid electrolyte-containing layer substantially coincide.

6. The method according to claim 1, wherein the thickness of the current collector foil is 500 μm or less.

7. The method according to claim 1, wherein the metal includes indium.

8. A method for manufacturing a lithium secondary battery, comprising evaluating the ionic conductivity of the solid electrolyte by the method described in claim 1.

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