Method for manufacturing solid secondary battery

Abstract

To provide a method for manufacturing in an industrially advantageous manner a solid secondary battery the internal resistance of which is low, and which has a high charge / discharge capacity.SOLUTION: Provided is a method for manufacturing a solid secondary battery including: a first joining step for joining a cathode active material layer and a solid electrolyte layer under pressure to obtain a cathode layer-solid electrolyte layer joined body; a second joining step for joining the solid electrolyte layer of the cathode layer-solid electrolyte layer joined body and an intermediate layer under pressure to obtain a cathode layer-solid electrolyte layer-intermediate layer joined body; a densification step for pressure-molding the cathode layer-solid electrolyte layer-intermediate layer joined body in the thickness direction so that a porosity of the cathode active material layer and the solid electrolyte layer becomes 5% or less; and a third joining step for joining the intermediate layer of the cathode layer-solid electrolyte layer-intermediate layer joined body and an anode layer under pressure to obtain an electrode laminate.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a solid secondary battery. [Background technology]

[0002] In recent years, research and development into secondary batteries that contribute to energy efficiency has been conducted to ensure that more people have access to affordable, reliable, sustainable, and advanced energy. Among secondary batteries, solid-state secondary batteries, which have an electrode stack in which a solid electrolyte layer is disposed between a positive electrode layer and a negative electrode layer, have attracted particular attention due to their superior safety due to the non-flammable nature of the solid electrolyte and their higher energy density. To improve the performance of solid-state secondary batteries, the provision of an intermediate layer between the solid electrolyte layer and the negative electrode layer has been considered. For example, the provision of a protective layer, which is more stable with respect to reductive decomposition than a solid electrolyte and has a shear modulus of 2 GPa or more and a difference in shear modulus from the solid electrolyte layer of 50 GPa or less, has been considered as the intermediate layer (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Application No. 2022-055389 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in solid-state secondary batteries, increasing the charge / discharge capacity is a challenge. To achieve this, the use of lithium as the negative electrode active material and increasing the density of the positive electrode active material layer and solid electrolyte layer of the positive electrode layer of the electrode laminate have been considered. However, even if the positive electrode active material layer and solid electrolyte layer are densified, if the bonding strength between the positive electrode active material layer and the solid electrolyte layer of the positive electrode layer is low or the bonding area is small, the internal resistance of the electrode laminate may increase or a short circuit may occur due to current concentration, resulting in a decrease in charge / discharge capacity.

[0005] The present invention has been made in view of the above circumstances, and aims to provide a method for industrially advantageously producing a solid secondary battery having low internal resistance and high charge / discharge capacity, which will ultimately contribute to energy efficiency. [Means for solving the problem]

[0006] The present inventors have discovered that the above-mentioned problems can be solved by manufacturing an electrode laminate having an intermediate layer between an anode layer and a solid electrolyte layer, by pressure-bonding a cathode layer, an anode layer, and an intermediate layer to obtain an assembly, and then pressure-molding the assembly in the thickness direction to densify the cathode layer and the solid electrolyte layer, and then bonding the intermediate layer to the anode layer, and have completed the present invention.

[0007] (1) A method for manufacturing a solid secondary battery including an electrode stack having: a cathode layer having a cathode current collector and a cathode active material layer; an anode layer having an anode current collector facing the cathode active material layer; a solid electrolyte layer disposed between the cathode layer and the anode layer; and an intermediate layer disposed between the anode layer and the solid electrolyte layer, wherein the cathode layer, the solid electrolyte layer, the intermediate layer, and the anode layer are each bonded to an adjacent layer; a second bonding step of pressure-bonding the solid electrolyte layer and the intermediate layer of the cathode layer-solid electrolyte layer assembly to obtain a cathode layer-solid electrolyte layer-intermediate layer assembly; a densification step of pressure-molding the cathode layer-solid electrolyte layer-intermediate layer assembly in a thickness direction to densify it; and a third bonding step of pressure-bonding the intermediate layer and the anode layer of the cathode layer-solid electrolyte layer-intermediate layer assembly to obtain the electrode stack, wherein the porosity of the cathode active material layer and the solid electrolyte layer after the third bonding step is 5% or less.

[0008] In the method for producing a solid secondary battery (1), the cathode layer-solid electrolyte layer-intermediate layer assembly obtained in the second joining step is pressure-molded in the thickness direction in the densification step, thereby increasing the density and improving the adhesion of the joining interfaces of the cathode active material layer, solid electrolyte layer, and intermediate layer. Furthermore, since the cathode active material layer after the third joining step has a porosity within the above range and a high density, the electrical capacity is increased. Furthermore, since the solid electrolyte layer after the third joining step has a porosity within the above range and a high density, the conductivity of the charge transfer medium is improved. Therefore, the resulting solid secondary battery has a low internal resistance and a high charge / discharge capacity.

[0009] (2) The method for producing a solid secondary battery according to (1), wherein the pressure of the pressure molding in the densification step is higher than the pressure bonding pressure in any of the first bonding step, the second bonding step, and the third bonding step.

[0010] According to the method for producing a solid secondary battery of (2), the high pressure used in the compaction step reduces the porosity of each of the positive electrode active material layer, the solid electrolyte layer, and the intermediate layer, and improves the adhesion of the bonding interfaces between the layers, resulting in a lower internal resistance and a higher charge / discharge capacity.

[0011] (3) The method for producing a solid secondary battery according to (1) or (2), wherein the pressure applied to the pressure bonding in the second bonding step is higher than the pressure applied to the pressure bonding in the first bonding step.

[0012] According to the method for manufacturing a solid secondary battery (3), the pressure of the pressure bonding in the first bonding step is reduced to soften the positive electrode layer and solid electrolyte layer of the positive electrode layer-solid electrolyte layer assembly, leaving a crushing margin, and the pressure of the pressure bonding in the second bonding step is increased, thereby increasing the contact area between the solid electrolyte layer and the intermediate layer while keeping the intermediate layer soft and having an appropriate void, thereby improving adhesion. The softness and appropriate void of the intermediate layer makes it easier to uniformly transfer the charge transfer medium to the entire surface of the negative electrode during charging.

[0013] (4) The method for manufacturing a solid secondary battery according to any one of (1) to (3), wherein the pressure applied to the pressure bonding in the third bonding step is higher than the pressure applied to the pressure bonding in the first bonding step and lower than the pressure applied to the pressure bonding in the second bonding step.

[0014] According to the method for manufacturing a solid secondary battery of (4), since the pressure for pressure bonding in the third bonding step is within the above range, the intermediate layer and the negative electrode layer can be bonded without excessively pressurizing the intermediate layer, thereby maintaining the intermediate layer in a soft state with an appropriate amount of voids. Furthermore, structural collapse of the positive electrode layer and the solid electrolyte layer can be suppressed, thereby preventing short circuits and reducing internal resistance.

[0015] (5) The method for producing a solid secondary battery according to any one of (1) to (4), wherein the reduction rate of the porosity of the positive electrode active material layer due to pressure molding in the densification step is 77% or more, and the reduction rate of the porosity of the solid electrolyte layer due to pressure molding is 85% or more. The porosity reduction rate is a value calculated by the following formula (1). Reduction rate of void ratio (%) = (void ratio before densification process - void ratio after densification process) / void ratio before densification process × 100 (1)

[0016] According to the method for producing a solid secondary battery of (5), the reduction rates of the porosity of the positive electrode active material layer and the porosity of the solid electrolyte layer after the densification step are within the above-mentioned ranges, so that the positive electrode active material layer and the solid electrolyte layer have high densities. As a result, the effective reaction area in the positive electrode active material layer and the solid electrolyte layer is increased, and the adhesion at the interface between each layer is improved. Therefore, the resulting solid secondary battery has a lower internal resistance and a higher charge / discharge capacity.

[0017] (6) The method for producing a solid secondary battery according to any one of (1) to (5), wherein the temperature for pressure molding in the densification step is in the range of 60°C or higher and 200°C or lower.

[0018] According to the method for producing a solid secondary battery of (6), since the temperature of the pressure molding in the densification step is within the above range, the porosity of each of the positive electrode active material layer, the solid electrolyte layer, and the intermediate layer is further reduced in the densification step, and the adhesion of the bonding interfaces of each layer is more reliably improved, so that the obtained solid secondary battery has a further lower internal resistance and a higher charge / discharge capacity.

[0019] (7) The method for producing a solid secondary battery according to any one of (1) to (6), wherein the pressure molding in the densification step is carried out by an isostatic pressing method.

[0020] According to the method for producing a solid secondary battery of (7), the pressure molding in the densification step is performed by isostatic pressing, so that the porosity of each layer of the positive electrode active material layer, the solid electrolyte layer, and the intermediate layer is uniformly reduced in the densification step, and the adhesion of the bonding interfaces of each layer is uniformly improved, resulting in a solid secondary battery with a lower internal resistance and a higher charge / discharge capacity.

[0021] (8) The method for producing a solid secondary battery according to any one of (1) to (7), wherein the intermediate layer of the positive electrode layer-solid electrolyte layer-intermediate layer assembly after the densification step has a composite elastic modulus of less than 1 GPa.

[0022] According to the method for producing a solid secondary battery (8), the composite elastic modulus of the intermediate layer of the cathode layer-solid electrolyte layer-intermediate layer assembly after the densification step is within the above-mentioned range, and the intermediate layer is soft, so that the contact area at the interface between the solid electrolyte layer and the intermediate layer and between the intermediate layer and the anode layer is increased, thereby improving adhesion. Furthermore, the intermediate layer follows the expansion and contraction of the anode layer during charging and discharging, allowing the cathode layer and the anode layer to undergo uniform reactions in the plane and thickness direction. Therefore, the resulting solid secondary battery has a further lower internal resistance, and current concentration can be suppressed and short circuits can be prevented.

[0023] (9) The method for producing a solid secondary battery according to any one of (1) to (8), wherein the porosity of the intermediate layer of the positive electrode layer-solid electrolyte layer-intermediate layer assembly after the densification step is in the range of 40% or more and 70% or less.

[0024] According to the method for producing a solid secondary battery (9), the porosity of the intermediate layer of the positive electrode layer-solid electrolyte layer-intermediate layer assembly after the densification step is within the above range, so that an electrode laminate having voids in the intermediate layer can be obtained. The intermediate layer having voids is flexible, so it can follow the change in thickness of the negative electrode layer that occurs during charge and discharge. Therefore, the obtained solid secondary battery has a stably low internal resistance and a stably high charge and discharge capacity.

[0025] (10) The method for producing a solid secondary battery according to any one of (1) to (9), wherein the intermediate layer contains amorphous carbon particles.

[0026] According to the method for manufacturing a solid secondary battery of (10), the intermediate layer contains amorphous carbon particles, which improves the conductivity of the charge transfer medium in the intermediate layer, thereby further reducing the internal resistance of the resulting solid secondary battery and providing a more stable and high charge / discharge capacity. [Effects of the Invention]

[0027] According to the present invention, it is possible to industrially advantageously produce a solid secondary battery having a low internal resistance and a high charge / discharge capacity. [Brief explanation of the drawings]

[0028] [Figure 1] 1 is a cross-sectional view showing an example of a solid secondary battery obtained by a method for producing a solid secondary battery according to an embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view showing a step of a method for manufacturing a solid secondary battery according to an embodiment of the present invention, illustrating a state in which a positive electrode layer and a solid electrolyte layer are pressure-bonded together. [Figure 3] FIG. 2 is a cross-sectional view showing a step of a method for manufacturing a solid secondary battery according to an embodiment of the present invention, illustrating a state in which a solid electrolyte layer and an intermediate layer are pressure-bonded together. [Figure 4] FIG. 3 is a cross-sectional view showing a step of a method for manufacturing a solid secondary battery according to one embodiment of the present invention, illustrating a state in which an intermediate layer and a negative electrode layer are pressure-bonded together. DETAILED DESCRIPTION OF THE INVENTION

[0029] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the embodiments described below are merely examples of the present invention, and the present invention is not limited to the following.

[0030] 1 is a cross-sectional view showing an example of a solid secondary battery obtained by a method for manufacturing a solid secondary battery according to one embodiment of the present invention. As shown in FIG. 1, the solid secondary battery 1 includes an electrode stack 10 and an exterior body 60 that houses the electrode stack 10.

[0031] The electrode laminate 10 is a laminate including a positive electrode layer 20, a negative electrode layer 30, a solid electrolyte layer 40 disposed between the positive electrode layer 20 and the negative electrode layer 30, and an intermediate layer 50 disposed between the negative electrode layer 30 and the solid electrolyte layer 40. The positive electrode layer 20 includes a positive electrode current collector 21 and a positive electrode active material layer 22 laminated on one surface (the lower surface in FIG. 1 ) of the positive electrode current collector 21. The positive electrode current collector 21 is connected to a positive electrode terminal 26 via a positive electrode lead wire 25. The negative electrode layer 30 includes a negative electrode current collector 31 and a metal layer 32 laminated on the surface of the negative electrode current collector 31 facing the solid electrolyte layer 40. The negative electrode current collector 31 is connected to a negative electrode terminal 36 via a negative electrode lead wire 35. The negative electrode current collector 31 faces the positive electrode active material layer 22. The positive electrode layer 20, solid electrolyte layer 40, intermediate layer 50, and negative electrode layer 30 are each bonded to adjacent layers. Portions of the positive electrode terminal 26 and negative electrode terminal 36 are exposed from the exterior casing 60. The solid secondary battery 1 shown in FIG. 1 is in a discharged state. When the solid secondary battery 1 is charged, lithium ions, which serve as a charge transfer medium, are released from the positive electrode active material layer 22 and pass through the solid electrolyte layer 40 and intermediate layer 50. They are deposited on the surface of the metal layer 32 of the negative electrode layer 30, forming a lithium deposit layer, and the thickness of the negative electrode layer 30 increases. By passing through the intermediate layer 50, a lithium deposit layer can be uniformly formed on the surface of the metal layer 32. The lithium deposit layer acts as a negative electrode active material layer and releases lithium ions during discharge. Therefore, the thickness of the negative electrode layer 30 of the solid secondary battery 1 changes during charge and discharge.

[0032] The positive electrode active material layer 22, the solid electrolyte layer 40, and the intermediate layer 50 of the electrode stack 10 have voids. The porosity of the positive electrode active material layer 22 and the solid electrolyte layer 40 is set to 5% or less. The porosity of the positive electrode active material layer 22 and the solid electrolyte layer 40 may be 3% or less. The porosity of the intermediate layer 50 may be greater than that of the positive electrode active material layer 22 and the solid electrolyte layer 40. The porosity of the intermediate layer 50 may be, for example, in a range of 10 times or more and 40 times or less than that of the positive electrode active material layer 22 and the solid electrolyte layer 40. The porosity of the intermediate layer 50 may be, for example, in a range of 40% or more and 70% or less. The porosity may be a value calculated, for example, by measuring the weight, area, and film thickness of the materials constituting the intermediate layer 50 to determine the volume-based filling rate (%) of the intermediate layer 50 and using the following formula (2) from the obtained filling rate. The porosity is a value measured after the production of the electrode stack 10 and before charging and discharging the electrode stack 10. Porosity (%)=100-Filling rate (%) ···(2)

[0033] The method for calculating the "filling ratio" in formula (2) is not limited to the above method. The filling ratio may be expressed as a percentage of the density of the intermediate layer 50 after molding relative to the true density of the material constituting the intermediate layer 50. Furthermore, the filling ratio may be calculated from the pore volume measured by instrumental analysis such as the BET method, porosimetry, or gas diffusion method. The filling ratio may also be calculated by image analysis using a scanning electron microscope or the like.

[0034] The composite elastic modulus of each layer of the positive electrode active material layer 22, the solid electrolyte layer 40, the intermediate layer 50, and the metal layer 32 of the electrode laminate 10 may have a relationship of, for example, intermediate layer 50 < metal layer 32 < solid electrolyte layer 40 < positive electrode active material layer 22. The composite elastic modulus of the intermediate layer 50 may be, for example, in the range of 0.1 GPa to 2.0 GPa. The composite elastic modulus of the metal layer 32 may be, for example, in the range of 2 to 10 times the composite elastic modulus of the intermediate layer 50. The composite elastic modulus of the metal layer 32 may be, for example, in the range of 1.0 GPa to 4.0 GPa. The composite elastic modulus of the solid electrolyte layer 40 may be, for example, in the range of 5 to 20 times the composite elastic modulus of the metal layer 32. The composite elastic modulus of the solid electrolyte layer 40 may be, for example, in the range of 10 GPa to 50 GPa. The composite elastic modulus of the positive electrode active material layer 22 may be, for example, in the range of 2 to 5 times the composite elastic modulus of the solid electrolyte layer 40. The composite elastic modulus of the positive electrode active material layer 22 may be, for example, in the range of 50 to 200 GPa. The composite elastic modulus is a value measured by a nanoindentation method. The composite elastic modulus is a value measured after the production of the electrode stack 10 and before charging and discharging the electrode stack 10.

[0035] There are no particular limitations on the material or shape of the positive electrode current collector 21, as long as it has the function of collecting current from the positive electrode layer 20. Examples of materials for the positive electrode current collector 21 include aluminum, aluminum alloys, stainless steel, nickel, iron, and titanium, and among these, aluminum, aluminum alloys, and stainless steel are preferred. Examples of the shape of the positive electrode current collector 21 include a foil shape and a plate shape.

[0036] The positive electrode active material layer 22 contains at least one type of positive electrode active material. There are no particular limitations on the positive electrode active material, and any material used in the positive electrode layers of general solid secondary batteries can be used. As the positive electrode active material, for example, a layered active material containing lithium, a spinel-type active material, an olivine-type active material, etc. can be used. Specific examples of the positive electrode active material include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), LiNi p Mn q Co rO2(p+q+r=1), LiNi p Al q Co r O2 (p+q+r=1), lithium manganese oxide (LiMn2O4), Li 1+x Mn 2-x-y Examples include heteroelement-substituted Li-Mn spinel represented by MO4 (x+y=2, M=at least one selected from Al, Mg, Co, Fe, Ni, and Zn), lithium titanate (oxide containing Li and Ti), and lithium metal phosphate (LiMPO4, M=at least one selected from Fe, Mn, Co, and Ni).

[0037] The average particle size of the positive electrode active material may be, for example, in the range of 0.5 μm to 20 μm. In this embodiment, the average particle size is a value measured by a laser diffraction scattering method.

[0038] The positive electrode active material layer 22 may optionally contain a solid electrolyte from the viewpoint of improving lithium ion conductivity. It may also optionally contain a conductive additive to improve conductivity. Furthermore, it may also optionally contain a binder from the viewpoint of exhibiting flexibility, etc. There are no particular limitations on the solid electrolyte, conductive additive, and binder, and those used in the positive electrode layers of general solid secondary batteries may be used.

[0039] The material of the positive electrode lead wire 25 may be the same as or different from the material of the positive electrode current collector 21. The positive electrode lead wire 25 may be integrally connected to the positive electrode current collector 21. In this embodiment, the positive electrode lead wire 25 is formed by extending the positive electrode current collector 21 and is integrally connected to the positive electrode current collector 21. The material of the positive electrode terminal 26 may be the same as or different from the material of the positive electrode lead wire 25. The positive electrode terminal 26 may be integrally connected to the positive electrode lead wire 25. In this embodiment, the positive electrode terminal 26 and the positive electrode lead wire 25 are separate members and are electrically connected to each other.

[0040] The material and shape of the negative electrode current collector 31 are not particularly limited as long as it has the function of collecting current from the negative electrode layer 30. Examples of materials for the negative electrode current collector 31 include nickel, copper, and stainless steel. Examples of the shape of the negative electrode current collector 31 include a foil shape, a plate shape, and the like.

[0041] The metal layer 32 is not particularly limited in material or shape as long as it has the function of densely depositing lithium ions. A metallic lithium layer or a layer of a metal that forms an alloy with lithium can be used as the metal layer 32. Examples of metals that form an alloy with lithium include Mg, Si, Au, Ag, In, Ge, Sn, Pb, Al, and Zn. The metal that forms the metal layer 32 may be in the form of a powder or a thin film. By using the anode layer 30 having this metal layer 32, a uniform lithium deposit layer can be formed on the surface of the metal layer 32.

[0042] The material of the negative electrode lead wire 35 may be the same as or different from the material of the negative electrode current collector 31. The negative electrode lead wire 35 may be integrally connected to the negative electrode current collector 31. In this embodiment, the negative electrode lead wire 35 is formed by extending the negative electrode current collector 31 and is integrally connected to the negative electrode current collector 31. The material of the negative electrode terminal 36 may be the same as or different from the material of the negative electrode lead wire 35. The negative electrode terminal 36 may be integrally connected to the negative electrode lead wire 35. In this embodiment, the negative electrode terminal 36 and the negative electrode lead wire 35 are separate members and are electrically connected to each other.

[0043] The solid electrolyte layer 40 contains at least one type of solid electrolyte. The solid electrolyte layer 40 can conduct lithium ions between the positive electrode layer 20 and the negative electrode layer 30 via the solid electrolyte.

[0044] The solid electrolyte is not particularly limited as long as it has lithium ion conductivity, but for example, a sulfide solid electrolyte, an oxide solid electrolyte, a nitride solid electrolyte, a halide solid electrolyte, etc. can be used.

[0045] Examples of sulfide solid electrolytes include Li2S-P2S5, Li2S-P2S5-LiI, etc. The sulfide solid electrolyte may have an argyrodite-type crystal structure.

[0046] Examples of oxide solid electrolytes include NASICON-type oxides, garnet-type oxides, and perovskite-type oxides. Examples of NASICON-type oxides include oxides containing Li, Al, Ti, P, and O (e.g., Li 1.5 Al 0.5 Ti 1.5 Examples of garnet-type oxides include oxides containing Li, La, Zr, and O (e.g., LiLaZrO 12 Examples of perovskite oxides include oxides containing Li, La, Ti, and O (for example, LiLaTiO3).

[0047] The average particle diameter of the solid electrolyte contained in the solid electrolyte layer 40 may be equal to or smaller than the average particle diameter of the positive electrode active material contained in the positive electrode active material layer 22. SE and the average particle diameter D50 of the positive electrode active material Cathode Ratio of D50 SE / D50 Cathode may be in the range of greater than 0.1 and not greater than 1.0. The average particle size of the solid electrolyte may be, for example, in the range of 0.5 μm to 10 μm.

[0048] The solid electrolyte layer 40 may contain a binder. There are no particular restrictions on the binder, and any binder that is used in the solid electrolyte layer of a general solid secondary battery can be used.

[0049] The solid electrolyte layer 40 may have a porous substrate inside. The porous substrate may be, for example, a woven fabric or a nonwoven fabric. A solid electrolyte having a porous substrate inside has high strength.

[0050] The intermediate layer 50 is electronically conductive and has voids through which lithium metal can pass. The voids in the intermediate layer 50 give the intermediate layer 50 flexibility that allows it to follow changes in the thickness of the negative electrode layer 30 that occur during charge and discharge. Therefore, even when the solid secondary battery 1 is repeatedly charged and discharged, the interfacial adhesion between the layers of the electrode stack 10 can be maintained, and the durability of the solid secondary battery 1 can be improved.

[0051] The intermediate layer 50 may contain a material having lithium metal conductivity and a material having electronic conductivity. For example, amorphous carbon particles can be used as the material having lithium metal conductivity. For example, metal can be used as the material having electronic conductivity. The metal may be in the form of particles. The metal particles may be contained in the intermediate layer 50 in a state of being mixed with amorphous carbon particles, or may be contained in the intermediate layer 50 in a state of being supported on amorphous carbon particles. Furthermore, the metal may be present on the surface of the amorphous carbon particles as a film, or may be impregnated into the interior of the amorphous carbon particles.

[0052] Examples of amorphous carbon particles that can be used include graphitizable carbon (soft carbon) and non-graphitizable carbon (hard carbon). Specific examples of amorphous carbon particles include carbon blacks such as acetylene black, furnace black, and ketjen black, as well as coke, activated carbon, carbon nanotubes (CNTs), fullerenes, and graphene. Some of the carbon atoms in the amorphous carbon particles may be substituted with atoms such as B, P, S, O, and N by chemical treatment such as a liquid-phase method or a gas-phase method.

[0053] Particles of a metal that forms an alloy or an intermetallic compound with lithium ions can be used as the metal contained in the intermediate layer 50. For example, examples of metals that form an alloy or an intermetallic compound with lithium include Ag, Au, Pt, Pd, Si, Al, Bi, Sn, Zn, Ga, and In.

[0054] When the intermediate layer 50 contains a mixture of amorphous carbon particles and metal particles, the average particle size of the mixture may be smaller than the average particle size of the solid electrolyte contained in the solid electrolyte layer 40. This allows the intermediate layer 50 to penetrate into the gaps between the solid electrolyte particles present at the interface of the solid electrolyte layer 40, thereby increasing the developed area ratio of the contact interface between the solid electrolyte layer 40 and the intermediate layer 50 and improving the adhesion between the solid electrolyte layer 40 and the intermediate layer 50. The average particle size of the amorphous carbon particles may be, for example, in the range of 0.02 μm to 0.06 μm. The average particle size of the metal particles may be, for example, in the range of 0.06 μm to 0.1 μm.

[0055] The exterior body 60 is expandable and contractible in accordance with changes in the thickness of the negative electrode layer 30 due to charging and discharging. A laminate film can be used as the material for the exterior body 60. The laminate film can be a three-layer laminate film having an inner resin layer, a metal layer, and an outer resin layer stacked in this order from the inside. The outer resin layer can be, for example, a polyamide (nylon) layer or a polyethylene terephthalate (PET) layer, the metal layer can be, for example, an aluminum layer, and the inner resin layer can be, for example, a polyethylene layer or a polypropylene layer.

[0056] A method for manufacturing the solid secondary battery 1 of this embodiment will be described. Figures 2 to 4 are diagrams showing steps of the method for manufacturing the solid secondary battery according to one embodiment of the present invention. The method for manufacturing the solid secondary battery 1 of this embodiment includes a first bonding step, a second bonding step, a densification step, and a third bonding step.

[0057] In the first bonding step, as shown in FIG. 2 , the cathode layer 20 with the cathode lead wire 25 attached and the solid electrolyte layer 40 are pressure-bonded to obtain a cathode layer-solid electrolyte layer bonded assembly 11. The porosity of the cathode active material layer 22 of the cathode layer 20 before bonding may be, for example, in the range of 30% to 50%. The porosity of the solid electrolyte layer 40 before bonding may be, for example, in the range of 30% to 50%. The porosity of the cathode active material layer 22 before bonding may be the same as or different from the porosity of the solid electrolyte layer 40. The porosity of the solid electrolyte layer 40 may be, for example, in the range of 0.6 to 1.7 times the porosity of the cathode active material layer 22 before bonding.

[0058] The positive electrode layer 20 and the solid electrolyte layer 40 can be bonded, for example, by a pressing method. Examples of the press that can be used include a roll press and a uniaxial molding press. The bonding pressure during pressure bonding is, for example, in the range of 50 MPa to 300 MPa, the bonding time is, for example, in the range of 0.1 seconds to 5 minutes, and the bonding temperature is, for example, in the range of 20°C to 200°C. The reduction rate of the porosity of the positive electrode active material layer 22 due to pressure bonding in the first bonding step may be, for example, in the range of 10% to 60%. The reduction rate of the porosity of the solid electrolyte layer 40 may be, for example, in the range of 10% to 60%. The reduction rate of the porosity is a value calculated using the following formula (3): Porosity reduction rate (%) = (porosity before the first bonding process - porosity after the first bonding process) / porosity before the first bonding process × 100 (3)

[0059] In the second bonding step, as shown in FIG. 3 , the solid electrolyte layer 40 and the intermediate layer 50 of the cathode layer-solid electrolyte layer assembly 11 are pressure-bonded to obtain a cathode layer-solid electrolyte layer-intermediate layer assembly 12. The porosity of the solid electrolyte layer 40 before the second bonding may be, for example, in the range of 25% to 40%. The porosity of the intermediate layer 50 before the second bonding may be, for example, in the range of 70% to 90%. The porosity of the solid electrolyte layer 40 before the second bonding may be smaller than the porosity of the intermediate layer 50. The porosity of the solid electrolyte layer 40 may be, for example, in the range of 0.25 to 0.6 times the porosity of the intermediate layer 50 before bonding.

[0060] In the second bonding step, the solid electrolyte layer 40 and the intermediate layer 50 of the cathode layer-solid electrolyte layer assembly 11 can be bonded to each other by, for example, a pressing method. Examples of the press include a roll press and a uniaxial molding press. The bonding pressure during pressure bonding is, for example, 100 MPa to 600 MPa, the bonding time is, for example, 0.1 seconds to 5 minutes, and the bonding temperature is, for example, 20°C to 200°C. The bonding pressure in the second bonding step may be higher than the bonding pressure in the first bonding step. The bonding pressure in the second bonding step may be, for example, 1.5 times to 12 times the bonding pressure in the first bonding step. The reduction rate of the porosity of the cathode active material layer 22 due to pressure molding in the second bonding step may be, for example, 20% to 80%. The reduction rate of the porosity of the solid electrolyte layer 40 may be, for example, 20% to 80%. The reduction rate of the porosity of the intermediate layer 50 may be, for example, within a range of 20% to 60%. The reduction rate of the porosity is a value calculated by the following formula (4). Decrease rate of porosity (%) = (porosity before the second process - porosity before and after the second process) / porosity before the second process × 100 (4)

[0061] In the densification step, the cathode layer-solid electrolyte layer-intermediate layer assembly 12 obtained in the second bonding step is pressure-molded in the thickness direction to densify it. The cathode layer-solid electrolyte layer-intermediate layer assembly 12 can be bonded, for example, by isostatic pressing. A cold isostatic press (CIP) or a hot isostatic press (HIP) can be used as the press. The molding pressure during pressure molding is, for example, in the range of 600 MPa to 1200 MPa, the molding time is, for example, in the range of 0.1 seconds to 5 minutes, and the molding temperature is, for example, in the range of 60°C to 200°C. The molding pressure during the densification step may be higher than the bonding pressure during any of the pressure bonding steps in the first, second, and third bonding steps. The molding pressure during the densification step may be, for example, in the range of 2 to 24 times the bonding pressure during the first bonding step. The molding pressure in the densification step may be, for example, in the range of 1.1 to 12 times the bonding pressure in the second bonding step, and the molding pressure in the densification step may be, for example, in the range of 1.2 to 20 times the bonding pressure in the third bonding step.

[0062] The reduction rate of the porosity of the positive electrode active material layer 22 due to pressure molding in the densification step may be, for example, 77% or more, or may be in the range of 80% to 95%. The reduction rate of the porosity of the solid electrolyte layer 40 may be, for example, 85% or more, or may be in the range of 88% to 95%. The reduction rate of the porosity of the intermediate layer 50 may be in the range of 0% to 30%. The reduction rate of the porosity is a value calculated using the above formula (1).

[0063] The densification step increases the density of the positive electrode layer-solid electrolyte layer-intermediate layer assembly 12. The porosity of the positive electrode active material layer 22 and the solid electrolyte layer 40 of the positive electrode layer-solid electrolyte layer-intermediate layer assembly 12 after the densification step may be 5% or less. However, excessive densification of the intermediate layer 50 may result in a decrease in the electrical properties of the electrode stack 10. For this reason, the densification step may be performed so that the porosity of the intermediate layer 50 after the densification step is in the range of 40% to 70%. Furthermore, the densification step may be performed so that the composite elastic modulus of the intermediate layer 50 after the densification step is less than 1 GPa.

[0064] In the third joining step, as shown in FIG. 4, the intermediate layer 50 of the positive electrode layer-solid electrolyte layer-intermediate layer assembly 12 and the negative electrode layer 30 with the negative electrode lead wire 35 attached thereto are pressure-joined to obtain the electrode stack 10.

[0065] The intermediate layer 50 and the negative electrode layer 30 can be bonded, for example, by a press. Examples of the press include a roll press and a uniaxial molding press. The bonding pressure during pressure bonding is, for example, in the range of 60 MPa to 500 MPa, the bonding time is, for example, in the range of 0.1 seconds to 5 minutes, and the bonding temperature is, for example, in the range of 20°C to 200°C. The bonding pressure in the third bonding step may be higher than the bonding pressure in the first bonding step and lower than the bonding pressure in the second bonding step. The bonding pressure in the third bonding step may be, for example, in the range of 1.2 to 10 times the bonding pressure in the first bonding step. The bonding pressure in the third bonding step may be, for example, in the range of 0.1 to 0.9 times the bonding pressure in the second bonding step.

[0066] By the above manufacturing method, an electrode stack 10 is obtained which includes a positive electrode layer 20 having a positive electrode current collector 21 and a positive electrode active material layer 22, a negative electrode layer 30 having a negative electrode current collector 31 facing the positive electrode active material layer 22, a solid electrolyte layer 40 disposed between the positive electrode layer 20 and the negative electrode layer 30, and an intermediate layer 50 disposed between the negative electrode layer 30 and the solid electrolyte layer 40, and in which the positive electrode layer 20, the solid electrolyte layer 40, the intermediate layer 50, and the negative electrode layer 30 are each joined to an adjacent layer.

[0067] The solid secondary battery 1 can be manufactured as follows: The positive electrode lead wire 25 of the obtained electrode laminate 10 is connected to the positive electrode terminal 26, and the negative electrode lead wire 35 is connected to the negative electrode terminal 36. Next, the electrode laminate 10 is housed in an exterior body 60 so that the ends of the positive electrode terminal 26 and the negative electrode terminal 36 protrude, and the exterior body 60 is sealed.

[0068] In the method for manufacturing the solid secondary battery 1 of this embodiment configured as described above, the cathode layer-solid electrolyte layer-intermediate layer assembly 12 obtained in the second bonding step is pressure-molded in the thickness direction in the densification step to densify it, and the adhesion at the bonding interfaces of the cathode active material layer 22, the solid electrolyte layer 40, and the intermediate layer 50 is improved. Furthermore, since the cathode active material layer 22 after the third bonding step has a porosity within the above range and a high density, the electrical capacity is increased. Furthermore, since the solid electrolyte layer 40 after the third bonding step has a porosity within the above range and a high density, the conductivity of the charge transfer medium is improved. Therefore, the resulting solid secondary battery 1 has a low internal resistance and a high charge / discharge capacity.

[0069] In the method for manufacturing the solid secondary battery 1 of this embodiment, if the pressure during the densification step satisfies the above-described conditions, the porosity of each of the positive electrode active material layer 22, the solid electrolyte layer 40, and the intermediate layer 50 is further reduced during the densification step, and the adhesion of the bonding interfaces of each layer is further improved. Furthermore, if the reduction rates of the porosity of the positive electrode active material layer 22 and the porosity of the solid electrolyte layer 40 after the densification step are within the above-described ranges, the positive electrode active material layer 22 and the solid electrolyte layer 40 will have high densities. Therefore, the effective reaction area within the positive electrode active material layer 22 and the solid electrolyte layer 40 is increased, and the adhesion of the bonding interfaces of each layer is improved. Therefore, the resulting solid secondary battery 1 has a lower internal resistance and a higher charge / discharge capacity. Furthermore, if the temperature during the densification step is within the above-described range, the porosity of each of the positive electrode active material layer 22, the solid electrolyte layer 40, and the intermediate layer 50 is further reduced during the densification step, and the adhesion of the bonding interfaces of each layer is more reliably improved. Furthermore, when the pressure molding in the densification step is performed by an isostatic pressing method, the porosity of each of the positive electrode active material layer 22, the solid electrolyte layer 40, and the intermediate layer 50 is uniformly reduced in the densification step, and the adhesion of the bonding interfaces of each layer is uniformly improved.

[0070] In the method for manufacturing the solid secondary battery 1 of this embodiment, if the pressure applied during the pressure bonding in the second bonding step is higher than the pressure applied during the first bonding step, the pressure applied during the first bonding step is lowered to soften the positive electrode layer 20 and the solid electrolyte layer 40 of the positive electrode layer-solid electrolyte layer assembly 11 and leave a crushing margin, and the pressure applied during the pressure bonding in the second bonding step is increased, thereby increasing the contact area between the solid electrolyte layer 40 and the intermediate layer 50 and improving adhesion while keeping the intermediate layer 50 soft and having appropriate voids. The softness and appropriate voids of the intermediate layer 50 facilitate uniform transfer of the charge transfer medium toward the entire surface of the negative electrode during charging, and the resulting solid secondary battery 1 has a higher charge / discharge capacity. Furthermore, when the pressure applied during the pressure bonding in the third bonding step is higher than the pressure applied during the first bonding step and lower than the pressure applied during the second bonding step, the intermediate layer 50 and the negative electrode layer 30 can be bonded together without excessively pressurizing the intermediate layer 50, thereby maintaining the intermediate layer 50 in a soft state with an appropriate amount of voids.

[0071] In the method for manufacturing the solid secondary battery 1 of this embodiment, if the composite elastic modulus of the intermediate layer 50 of the cathode layer-solid electrolyte layer-intermediate layer assembly 12 after the densification step is within the above-mentioned range, the intermediate layer 50 is soft, thereby increasing the contact area at the interface between the solid electrolyte layer 40 and the intermediate layer 50 and between the intermediate layer 50 and the anode layer 30, thereby improving adhesion. Furthermore, since the intermediate layer 50 follows the expansion and contraction of the anode layer 30 during charge and discharge, the cathode layer 20 and the anode layer 30 can undergo uniform reactions in the plane and thickness directions. Therefore, the resulting solid secondary battery 1 has a lower internal resistance, and current concentration can be suppressed and short circuits can be prevented. In addition, in the method for manufacturing the solid secondary battery 1 of this embodiment, if the porosity of the intermediate layer 50 of the cathode layer-solid electrolyte layer-intermediate layer assembly 12 after the densification step is within the above-mentioned range, an electrode stack 10 having voids in the intermediate layer 50 can be obtained. The void-containing intermediate layer 50 is flexible and can follow changes in the thickness of the anode layer during charge and discharge. Therefore, the obtained solid secondary battery 1 has a stably low internal resistance and a stably high charge / discharge capacity.

[0072] In the method for manufacturing the solid secondary battery 1 of this embodiment, when the intermediate layer 50 contains amorphous carbon particles, the conductivity of the charge transfer medium in the intermediate layer is improved, and the obtained solid secondary battery 1 has a lower internal resistance and a more stable and high charge / discharge capacity.

[0073] Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. For example, although the negative electrode layer 30 includes the metal layer 32 in this embodiment, the metal layer 32 may be omitted, and lithium may be deposited on the surface of the negative electrode current collector 31. Alternatively, the metal layer 32 may be replaced with a layer containing a negative electrode active material capable of absorbing and releasing lithium ions. Examples of the negative electrode active material include lithium transition metal oxides such as lithium titanate, transition metal oxides such as TiO2, Nb2O3, and WOn, Si, SiO2, metal sulfides, metal nitrides, and carbon materials such as artificial graphite, natural graphite, graphite, soft carbon, and hard carbon. The negative electrode active material layer may optionally contain a solid electrolyte to improve lithium ion conductivity. It may also optionally contain a conductive additive to improve conductivity. Furthermore, it may optionally contain a binder to achieve flexibility. The solid electrolyte, conductive additive, and binder may be those commonly used in solid-state secondary batteries. [Example]

[0074] The present invention will be described in detail below using examples, but the present invention is not limited to these examples.

[0075] [Example 1] (Preparation of positive electrode layer) An aluminum foil with a thickness of 15.0 μm was prepared as a positive electrode current collector with a positive electrode lead wire. A mixture of 80 parts by mass of lithium nickel cobalt manganese composite oxide (NCM622) as the positive electrode active material, 17 parts by mass of an argyrodite-type sulfide solid electrolyte as the solid electrolyte, 2 parts by mass of carbon black as a conductive additive, and 1 part by mass of an SBR (styrene butadiene rubber) binder as the binder was prepared. The resulting mixture was dispersed in 43 parts by mass of butyl butyrate to prepare a positive electrode active material layer slurry. The resulting positive electrode active material layer slurry was applied to one surface of a positive electrode current collector to a weight per unit area of ​​27 mg / cm after drying. 2 The positive electrode active material layer was fabricated by applying the coating solution using a bar coater so that the thickness of the coating was 80.0 μm and drying the coating to form a positive electrode active material layer having a thickness of 80.0 μm. The positive electrode active material layer had a porosity of 45% and a composite elastic modulus of 0.1 GPa. The porosity of the positive electrode active material layer was calculated using the above formula (2). The packing ratio was defined as the percentage of the density of the positive electrode active material layer after molding relative to the true density of the positive electrode active material.

[0076] (Preparation of solid electrolyte layer transfer sheet) A dispersion of argyrodite-type sulfide solid electrolyte (average particle size: 3.0 μm) was applied to a support sheet and dried to form an argyrodite-type sulfide solid electrolyte layer with a thickness of 100 μm. The solid electrolyte layer had a porosity of 40% and a composite elastic modulus of 0.2 GPa.

[0077] (Preparation of intermediate layer transfer sheet) A total of 95 parts by mass of Sn particles (average particle diameter: 0.07 μm) as metal particles and acetylene black (average particle diameter: 0.05 μm) as amorphous carbon particles were mixed with 5 parts by mass of a PVDF-based binder as a binder. The resulting mixture was dispersed in 1,000 parts by mass of NMP (N-methyl-2-pyrrolidone) to prepare an intermediate layer slurry. The resulting intermediate layer slurry was applied to a support sheet and dried to produce an intermediate layer transfer sheet with a final thickness of 3.0 μm. The porosity of the intermediate layer was 82%, and the composite elastic modulus was 0.1 GPa.

[0078] (Production of negative electrode layer) A copper foil with a thickness of 10 μm was prepared as a negative electrode current collector with a negative electrode lead wire. A 40 μm thick lithium metal foil was rolled and laminated on the surface of the copper foil to form a negative electrode layer.

[0079] (Production of electrode stack) The solid electrolyte layer of the solid electrolyte layer transfer sheet was superimposed on the surface of the positive electrode active material layer of the positive electrode layer, and the two were bonded using a uniaxial molding press under conditions of a bonding pressure of 90 MPa, a bonding time of 3 minutes, and a bonding temperature of room temperature (first bonding step). The support sheet of the solid electrolyte layer transfer sheet was then peeled off to obtain a positive electrode layer-solid electrolyte layer assembly. Next, the intermediate layer of the intermediate layer transfer sheet was superimposed on the surface of the solid electrolyte layer of the positive electrode layer-solid electrolyte layer laminate, and the two were bonded using a uniaxial molding press under conditions of a bonding pressure of 290 MPa, a bonding time of 5 minutes, and a bonding temperature of room temperature (second bonding step). The support sheet of the intermediate layer transfer sheet was then peeled off to obtain a positive electrode layer-solid electrolyte layer-intermediate layer assembly. The resulting positive electrode layer-solid electrolyte layer-intermediate layer assembly was press-molded using an isostatic pressing machine under conditions of a molding pressure of 980 MPa, a molding time of 5 minutes, and a molding temperature of 120°C to densify it (densification step). Next, a metallic lithium foil for the negative electrode layer was placed on the surface of the intermediate layer of the densified positive electrode layer-solid electrolyte layer-intermediate layer assembly, and the assembly was bonded using a uniaxial pressing machine under the conditions of a bonding pressure of 180 MPa, a bonding time of 2 minutes, and a bonding temperature of room temperature (third bonding step). An electrode laminate was thus obtained. The bonding pressure for the first bonding step, the bonding pressure for the second bonding step, the molding pressure and molding temperature for the densification step, and the bonding pressure for the third bonding step are shown in Table 1 below.

[0080] [Examples 2 to 3] An electrode laminate was obtained in the same manner as in Example 1, except that the molding temperature in the densification step was changed to the temperatures shown in Table 1 below.

[0081] [Comparative Example 1] An electrode laminate was obtained in the same manner as in Example 1, except that the molding temperature in the densification step was room temperature.

[0082] [Comparative Examples 2 to 3] An electrode laminate was obtained in the same manner as in Example 1, except that the molding temperature in the densification step was room temperature and the molding pressure was a pressure shown in Table 1 below.

[0083] [Table 1]

[0084] [evaluation] (Porosity reduction rate) The reduction rates of the porosity of the positive electrode active material layer and the solid electrolyte layer in the first bonding step, the second bonding step, and the densification step, and the reduction rate of the porosity of the intermediate layer in the second bonding step and the densification step were measured for Examples 1 to 3 and Comparative Examples 1 to 3. The results are shown in Table 2.

[0085] (Physical properties of electrode laminate) The porosity of the positive electrode active material layer, solid electrolyte layer, and intermediate layer, and the composite elastic modulus of the intermediate layer were measured for the electrode laminates obtained in Examples 1 to 3 and Comparative Examples 1 to 3. The results are shown in Table 3 below.

[0086] (Battery characteristics) A positive electrode terminal was connected to the positive electrode lead wire of the electrode laminate obtained in Examples 1 to 3 and Comparative Examples 1 to 3, and a negative electrode terminal was connected to the negative electrode lead wire. Next, the electrode laminate was housed in an outer casing so that the ends of the positive electrode terminal and the negative electrode terminal protruded, and the outer casing was sealed to obtain a solid secondary battery. For the obtained solid secondary battery, the DC resistance, 1 / 3 C discharge capacity, and 1 / 3 C charge / discharge capability were measured using the following methods. Each measurement was performed at 25°C. The results are shown in Table 3 below.

[0087] (DC resistance) At a temperature of 25°C and a charging state of SOC of 50, the current density was 15.1 mA / cm 2 The voltage drop ΔV (V), current value I (A), and positive electrode area Ac (cm 2 ) and the DC resistance (Ω·cm 2 ) was calculated. DC resistance (Ω cm 2) = Voltage drop ΔV (V) / Current value I (A) × Positive electrode area Ac (cm 2 )

[0088] (1 / 3C discharge capacity) Temperature 25°C, upper limit charge voltage 4.3V, lower limit discharge voltage 2.65V, C rate 1 / 3C, current density 1.3mA / cm 2 The discharge capacity after the first charge was taken as the 1 / 3C discharge capacity.

[0089] (1 / 3C charging / discharging possible) When the design capacity is taken as 100%, if the charge / discharge capacity at 25°C and 1 / 3C charge / discharge is 95-105% and no short circuit behavior occurs during or after charge / discharge, it is marked as "Good." If short circuit behavior occurs during or after charge / discharge, the charge capacity is excessive compared to the design capacity, or the self-discharge amount after charge is large, it is marked as "Poor."

[0090] [Table 2]

[0091] [Table 3]

[0092] The results in Tables 1 to 3 show that the manufacturing methods of Examples 1 to 3 can increase the reduction rates of the porosity of the positive electrode active material layer and the solid electrolyte layer while keeping the reduction rate of the porosity of the intermediate layer low. The electrode laminates obtained in Examples 1 to 3 have low porosity of the positive electrode active material layer and the solid electrolyte layer, and maintain an appropriate porosity of the intermediate layer, resulting in a large developed area ratio of the contact interface between the layers. Therefore, solid secondary batteries using these electrode laminates have low internal resistance and high charge / discharge capacity. In contrast, the manufacturing methods of Comparative Examples 1 to 3 resulted in low reduction rates of the porosity of the positive electrode active material layer and the solid electrolyte layer. Therefore, solid secondary batteries using the electrode laminates obtained in Comparative Examples 1 to 3 had high DC resistance and low charge / discharge capacity. [Explanation of symbols]

[0093] 1 Solid state secondary battery 10 Electrode laminate 20 Positive electrode layer 21 Positive electrode current collector 22 Cathode active material layer 25 Positive lead wire 26 Positive terminal 30 negative electrode layer 31 Negative electrode current collector 32 metal layer 35 Negative lead wire 36 Negative terminal 40 Solid electrolyte layer 50 Middle Class 60 Exterior body

Claims

1. A method for manufacturing a solid secondary battery including an electrode stack having: a positive electrode layer having a positive electrode current collector and a positive electrode active material layer; a negative electrode layer having a negative electrode current collector facing the positive electrode active material layer; a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer; and an intermediate layer disposed between the negative electrode layer and the solid electrolyte layer, wherein the positive electrode layer, the solid electrolyte layer, the intermediate layer, and the negative electrode layer are each joined to an adjacent layer, a first bonding step of pressure-bonding the positive electrode active material layer and the solid electrolyte layer to obtain a positive electrode layer-solid electrolyte layer bonded body; a second bonding step of pressure-bonding the solid electrolyte layer and the intermediate layer of the cathode layer-solid electrolyte layer bonded body to obtain a cathode layer-solid electrolyte layer-intermediate layer bonded body; a densification step of densifying the positive electrode layer-solid electrolyte layer-intermediate layer assembly by pressure molding in the thickness direction; a third bonding step of pressure-bonding the intermediate layer and the negative electrode layer of the positive electrode layer-solid electrolyte layer-intermediate layer assembly to obtain the electrode stack; the positive electrode active material layer and the solid electrolyte layer have a porosity of 5% or less after the third bonding step.

2. 2 . The method for manufacturing a solid secondary battery according to claim 1 , wherein a pressure applied to the pressure molding in the densification step is higher than a pressure applied to any of the pressure bonding steps in the first bonding step, the second bonding step, and the third bonding step.

3. The method for manufacturing a solid secondary battery according to claim 1 , wherein a pressure applied to the pressure bonding in the second bonding step is higher than a pressure applied to the pressure bonding in the first bonding step.

4. 3 . The method for manufacturing a solid secondary battery according to claim 1 , wherein a pressure applied to the pressure bonding in the third bonding step is higher than a pressure applied to the pressure bonding in the first bonding step and lower than a pressure applied to the pressure bonding in the second bonding step.

5. 3. The method for producing a solid secondary battery according to claim 1, wherein the porosity of the positive electrode active material layer is reduced by 77% or more due to pressure molding in the densification step, and the porosity of the solid electrolyte layer is reduced by 85% or more due to pressure molding.

6. The method for producing a solid secondary battery according to claim 1 or 2, wherein the temperature of the pressure molding in the densification step is in the range of 60°C or higher and 200°C or lower.

7. The method for producing a solid secondary battery according to claim 1 or 2, wherein the pressure molding in the densification step is performed by an isostatic pressing method.

8. 3. The method for producing a solid secondary battery according to claim 1, wherein the intermediate layer of the positive electrode layer-solid electrolyte layer-intermediate layer assembly after the densification step has a composite elastic modulus of less than 1 GPa.

9. 3. The method for producing a solid secondary battery according to claim 1, wherein the porosity of the intermediate layer of the positive electrode layer-solid electrolyte layer-intermediate layer assembly after the densification step is in the range of 40% to 70%.

10. The method for producing a solid secondary battery according to claim 1 or 2, wherein the intermediate layer contains amorphous carbon particles.

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