All-solid-state secondary battery and method for manufacturing the same

Abstract

To provide an all-solid-state secondary battery suppressed in a short circuit and excellent in load characteristics and cycle characteristics, and a production method thereof, which relate to Goal 7, "Affordable and Clean Energy for All", and Goal 12, "Responsible Consumption and Production" of the Sustainable Development Goals.SOLUTION: According to the present invention, an all-solid type secondary battery includes a positive electrode 10, a negative electrode 20, and a solid electrolyte layer 30 containing a sulfide-based solid electrolyte. In the solid electrolyte layer 30, thickness is 15 μm or more and 120 μm or less, a void ratio calculated from a scanning electron microscope (SEM) image is 3% or less about a central part R2 of a cross section of the solid electrolyte layer 30, and heights of irregularities between the solid electrolyte layer 30, and the positive electrode 10 and the negative electrode 20 are each 7 μm or less. The solid electrolyte layer 30 can be acquired, for example, by a step of pressure-molding a layered aggregate of sulfide-based solid electrolyte particles charged into a mold without using a solvent with a surface pressure of 1000 MPa or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an all-solid-state secondary battery having excellent load characteristics and cycle characteristics and a method for manufacturing the same.

Background Art

[0002] In recent years, with the development of portable electronic devices such as mobile phones and notebook personal computers, and the practical application of electric vehicles, there has been an increasing need for secondary batteries that are small, lightweight, and have high capacity and high energy density.

[0003] Currently, in lithium secondary batteries, particularly lithium-ion secondary batteries, which can meet this requirement, lithium-containing composite oxides such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2) are used as the positive electrode active material, graphite or the like is used as the negative electrode active material, and an organic electrolyte solution containing an organic solvent and a lithium salt is used as the non-aqueous electrolyte.

[0004] With the further development of the applicable devices of lithium-ion secondary batteries, further improvement in long life, high capacity, and high energy density of lithium-ion secondary batteries is required. Furthermore, high reliability of lithium-ion secondary batteries with improved long life, high capacity, and high energy density is also highly demanded.

[0005] However, the organic electrolyte solution used in lithium-ion secondary batteries contains an organic solvent, which is a flammable substance. In addition, with the increasing energy density of recent lithium-ion secondary batteries and the tendency of the amount of organic solvent in the organic electrolyte solution to increase, higher reliability of lithium-ion secondary batteries is required.

[0006] Under the above circumstances, all-solid-state lithium secondary batteries (all-solid-state secondary batteries) that do not use an organic solvent have attracted attention. An all-solid-state secondary battery uses a molded body of a solid electrolyte that does not use an organic solvent instead of the conventional organic solvent-based electrolyte.

[0007] Various improvements have been attempted in the aforementioned all-solid-state secondary battery. For example, Patent Document 1 shows that short circuits between the positive and negative electrode layers can be suppressed by having a porosity of 10% or less in the solid electrolyte layer, and by satisfying the relationship Rz1 + Rz2 ≤ 25 between the surface roughness Rz1 of the positive electrode layer and the surface roughness Rz2 of the negative electrode layer.

[0008] Patent Document 1 states that the thickness of the solid electrolyte layer is preferably 20 μm or less from the viewpoint of improving the energy density of the battery. By reducing the irregularities on the surfaces of the positive electrode layer and the negative electrode layer, contact between the positive electrode layer and the negative electrode layer is prevented, and by lowering the porosity of the solid electrolyte layer, dendrite growth is suppressed, preventing short circuits between the positive electrode layer and the negative electrode layer even when the solid electrolyte layer is thinned.

[0009] In Patent Document 1, a solid electrolyte and a binder are mixed to prepare a composite powder, and then this composite powder is dispersed in a solvent to produce a paste for forming a solid electrolyte layer. The paste for forming the solid electrolyte layer is uniformly applied to the surface of a support substrate and dried to form an electrolyte green sheet on the surface of the support substrate. Furthermore, a positive electrode green sheet and a negative electrode green sheet are produced in the same manner as the electrolyte green sheet, except that a positive electrode paste and a negative electrode paste are used. The positive electrode green sheet and the negative electrode green sheet are laminated with the electrolyte green sheet, and the binder is burned off at a temperature of 300°C or higher to assemble a coin-type all-solid-state battery having a positive electrode layer, a negative electrode layer and a solid electrolyte layer.

[0010] Furthermore, Patent Document 2 discloses that in the process of preparing a composition for forming a solid electrolyte layer by dispersing a solid electrolyte in a solvent, a compound such as 1-hydroxyethyl-2-alkenylimidazoline is included in the solvent as a dispersant. This suppresses aggregation of the solid electrolyte and reduces surface irregularities of the solid electrolyte layer formed by applying and drying the composition for forming the solid electrolyte layer. As a result, an all-solid-state lithium secondary battery can be assembled with a surface roughness Ra of 1.0 μm or less at the interface between the positive electrode mixture layer and the solid electrolyte layer. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Patent No. 6729796 [Patent Document 2] Japanese Patent Publication No. 2020-161364 [Overview of the project] [Problems that the invention aims to solve]

[0012] However, in the all-solid-state battery described in Patent Document 1, although the porosity of the solid electrolyte layer can be reduced, in the case of materials that may degrade at high temperatures, such as sulfide-based solid electrolytes, the green sheet cannot be heated to a temperature that burns off the binder. As a result, the binder remains in the solid electrolyte layer, reducing its ionic conductivity and making it difficult to obtain sufficient load characteristics. Furthermore, as shown in Table 1, although the sum of the surface roughness Rz1 of the positive electrode layer and the surface roughness Rz2 of the negative electrode layer can be reduced, the larger of Rz1 and Rz2 is still relatively large, at 10 μm or more. Therefore, when the thickness of the solid electrolyte layer is reduced, the charge-discharge reaction may become non-uniform in some areas, potentially leading to a decrease in cycle characteristics.

[0013] Furthermore, in the all-solid-state battery described in Patent Document 2, the solid electrolyte needs to be made into a paint. In the case of materials that readily react with moisture, such as sulfide-based solid electrolytes, the moisture contained in the solvent used for paint formation reacts with the solid electrolyte, reducing the ionic conductivity of the solid electrolyte layer and making it difficult to obtain sufficient load characteristics.

[0014] The present invention has been made in view of the above circumstances, and its purpose is to improve the load characteristics of an all-solid-state secondary battery having a solid electrolyte layer containing a sulfide-based solid electrolyte. [Means for solving the problem]

[0015] The all-solid-state secondary battery in an embodiment of the present invention has a positive electrode, a negative electrode, and a solid electrolyte layer containing a sulfide-based solid electrolyte disposed between the positive electrode and the negative electrode. The solid electrolyte layer is characterized by having a thickness of 15 μm or more and 120 μm or less, a porosity of 3% or less calculated from a scanning electron microscope (SEM) image in the central part of the cross-section of the solid electrolyte layer, and the height of the irregularities at the interface between the solid electrolyte layer and the positive electrode and the negative electrode being 7 μm or less, respectively.

[0016] Furthermore, the present invention provides a method for manufacturing an all-solid-state secondary battery, characterized by comprising the steps of: filling a mold with sulfide-based solid electrolyte particles without using a solvent to form a layered aggregate of sulfide-based solid electrolyte particles; and pressurizing the layered aggregate of sulfide-based solid electrolyte particles with a surface pressure of 1000 MPa or more. [Effects of the Invention]

[0017] According to the present invention, the load characteristics can be improved in an all-solid-state secondary battery having a solid electrolyte layer containing a sulfide-based solid electrolyte. [Brief explanation of the drawing]

[0018] [Figure 1] This diagram illustrates the region for measuring the porosity of the solid electrolyte layer. [Figure 2] This is a histogram of the grayscale distribution of an SEM image used to explain binarization. [Figure 3] This diagram illustrates a method for measuring the thickness of the solid electrolyte layer and the surface irregularities of its interface. [Figure 4] This is a schematic cross-sectional view showing an example of the all-solid-state secondary battery of the present invention. [Modes for carrying out the invention]

[0019] The all-solid-state secondary battery in an embodiment of the present invention has a positive electrode, a negative electrode, and a solid electrolyte layer containing a sulfide-based solid electrolyte disposed between the positive electrode and the negative electrode. The solid electrolyte layer has a thickness of 15 μm or more and 120 μm or less, the porosity calculated from a scanning electron microscope (SEM) image of the central part of the cross-section of the solid electrolyte layer is 3% or less, and the height of the irregularities at the interface between the solid electrolyte layer and the positive electrode and the negative electrode is 7 μm or less, each.

[0020] The solid electrolyte layer containing the sulfide-based solid electrolyte in the above-mentioned all-solid-state secondary battery has a thickness of 120 μm or less. This configuration can reduce the internal resistance of the battery. On the other hand, since the solid electrolyte layer has a thickness of 15 μm or more, even if the height of the irregularities at the interface between the solid electrolyte layer and the positive electrode, and the interface between the solid electrolyte layer and the negative electrode, both become close to 7 μm, short circuits due to contact between the positive and negative electrodes can be prevented.

[0021] Furthermore, in the cross-section of the solid electrolyte layer, the porosity in the region including the center in both the width and thickness directions (central part) is 3% or less. By configuring the porosity of the central part of the solid electrolyte layer in this way, the packing efficiency of the solid electrolyte layer is increased, and the ion conductivity can be improved. In addition, the height of the irregularities at the interface between the solid electrolyte layer and the positive electrode (height in the thickness direction) and the height of the irregularities at the interface between the solid electrolyte layer and the negative electrode (height in the thickness direction) are both 7 μm or less. By configuring the interface of the solid electrolyte layer in this way, even if the thickness of the solid electrolyte layer is reduced, short circuits due to contact between the positive and negative electrodes can be prevented, and variations in the thickness of the solid electrolyte layer can be suppressed, making the battery reaction closer to uniform. As a result, the load characteristics of the all-solid-state secondary battery are improved.

[0022] As a result of the above, we can provide all-solid-state secondary batteries with excellent load characteristics, which can contribute, for example, to Goal 7 "Affordable and Clean Energy" and Goal 12 "Responsible Consumption and Production" of the United Nations' Sustainable Development Goals (SDGs).

[0023] Generally, the porosity near the edges in the width direction of the solid electrolyte layer tends to be larger than that of the central part. By making the porosity of the region including the center in the thickness direction (periphery), centered at 100 μm from the edge in the width direction of the cross-section of the solid electrolyte layer, close to that of the central part, the packing efficiency of the solid electrolyte layer as a whole can be increased, and ion conduction within the solid electrolyte layer can be made uniform. This is preferable because it allows the charge-discharge reaction to proceed uniformly throughout, further improving the load characteristics. The difference in porosity between the peripheral and central parts of the solid electrolyte layer is preferably 4% or less, and more preferably 3% or less.

[0024] For the purpose of calculating the porosity, the target area of ​​the SEM image for both the peripheral and central regions will be a rectangular region with a width of 25 μm and a thickness of 19 μm, as a rough guideline. That is, for the central region, the porosity will be calculated for a rectangular region of the aforementioned size that includes the center of the cross-section of the solid electrolyte layer in both the width and thickness directions. For the peripheral region, the porosity will be calculated for a rectangular region of the aforementioned size that is centered at a point 100 μm from the edge in the width direction of the cross-section of the solid electrolyte layer and includes the center in the thickness direction.

[0025] However, if the thickness of the solid electrolyte layer is approximately 24 μm or less, the length in the thickness direction of the rectangular area used for calculating the porosity should be reduced according to the thickness of the solid electrolyte layer. In this case, the length in the thickness direction of the rectangular area should be set to 80% of the thickness of the solid electrolyte layer so that the rectangular area does not include the areas near the positive electrode interface and the areas near the negative electrode interface of the solid electrolyte layer. This reduces measurement variability.

[0026] Even if the solid electrolyte layer of the above-mentioned all-solid-state secondary battery has a thickness of 15 μm or more, and both the height of the unevenness in the thickness direction at the interface with the positive electrode and the height of the unevenness in the thickness direction at the interface with the negative electrode are, for example, about 7 μm, short circuits due to contact between the positive and negative electrodes can be prevented. Therefore, the constraints on the conditions when pressure molding sulfide-based solid electrolyte particles are reduced, making it easier to manufacture all-solid-state secondary batteries with excellent load characteristics.

[0027] The positive electrode may have a molded body of a positive electrode mixture containing positive electrode active material powder, a conductive additive, and a solid electrolyte. The solid electrolyte of the positive electrode mixture may include, for example, a sulfide-based solid electrolyte. The positive electrode active material powder may be, for example, particles in which a lithium-ion conductive coating material is formed on the surface of the positive electrode active material. In this case, the positive electrode active material powder may, as an example, be primary particles in which a lithium-ion conductive coating material is formed on the surface of the positive electrode active material.

[0028] The negative electrode may have a molded body of a negative electrode mixture containing negative electrode active material powder, a conductive additive, and a solid electrolyte. The solid electrolyte of the negative electrode mixture may include, for example, a sulfide-based solid electrolyte.

[0029] The above-described method for manufacturing an all-solid-state secondary battery includes, for example, the steps of filling a mold with sulfide-based solid electrolyte particles without using a solvent to form a layered aggregate of the sulfide-based solid electrolyte particles, and pressurizing the layered aggregate of sulfide-based solid electrolyte particles with a surface pressure of 1000 MPa or more.

[0030] In the above manufacturing method, sulfide-based solid electrolyte particles are filled into a mold by a dry process that does not use a solvent. This prevents the reaction between the water contained in the solvent and the sulfide-based solid electrolyte particles. Furthermore, by filling the mold and applying pressure, a binder is not required. Therefore, it is not necessary to heat the sulfide-based solid electrolyte particles to the binder's burnout temperature. This suppresses the deformation of the sulfide-based solid electrolyte particles due to heating. The inventors have also found that by pressurizing a layered aggregate of sulfide-based solid electrolyte particles filled into a mold by a dry process at a surface pressure of 1000 MPa or more, a solid electrolyte layer with low porosity and interface irregularities between the positive and negative electrodes suppressed to 7 μm or less can be efficiently formed. By the above manufacturing method, an all-solid-state secondary battery having a solid electrolyte layer containing a sulfide-based solid electrolyte, an all-solid-state secondary battery with excellent load characteristics can be obtained.

[0031] The manufacturing method may further include a step of laminating the positive electrode, the negative electrode, and the solid electrolyte layer. In this case, the solid electrolyte layer is laminated so as to be positioned between the positive electrode and the negative electrode. The step of press-molding the layered aggregate of sulfide-based solid electrolyte particles at a surface pressure of 1000 MPa or more may be performed by press-molding the layered aggregate of sulfide-based solid electrolyte particles while it is laminated on at least one of the positive electrode and the negative electrode. Alternatively, the layered aggregate of sulfide-based solid electrolyte particles may be press-molded before it is laminated on the positive electrode and the negative electrode.

[0032] Furthermore, it is preferable from the viewpoint of manufacturing efficiency that the layered aggregate of sulfide-based solid electrolyte particles is pressure-molded before being laminated onto the positive electrode and the negative electrode. This is because it makes it easier to adjust the height of the irregularities at the interface between the solid electrolyte layer and the positive electrode, and the interface between the solid electrolyte layer and the negative electrode, to 7 μm or less.

[0033] For example, the manufacturing method may include a first pressure molding step of pressure molding a layered aggregate of sulfide-based solid electrolyte particles, a second pressure molding step of laminating a negative electrode material onto one side of the pressure-molded layered aggregate of sulfide-based solid electrolyte particles and pressure molding it, and a third pressure molding step of laminating a positive electrode material onto the other side of the pressure-molded layered aggregate of sulfide-based solid electrolyte particles and pressure molding it. In this case, pressure molding may be performed at a surface pressure of 1000 MPa or more in at least one of the first, second, and third pressure molding steps. This makes it easier to adjust the height of the irregularities at the interfaces between the solid electrolyte layer and the positive and negative electrodes to 7 μm or less.

[0034] The embodiments will be described in detail below. In the figures, identical and corresponding components are denoted by the same reference numerals, and the same explanation will not be repeated. In order to make the explanation easier to understand, in the drawings referred to below, components may be shown in a simplified or schematic form, or some components may be omitted.

[0035] (positive electrode) The positive electrode of the all-solid-state secondary battery has a molded body of a positive electrode mixture containing a positive electrode active material, a conductive aid, and a solid electrolyte, etc. For example, it can be composed only of the above-mentioned molded body, or it can also be integrally formed with a current collector.

[0036] The positive electrode active material is not particularly limited as long as it is a positive electrode active material conventionally used in lithium-ion secondary batteries, that is, an active material capable of occluding and releasing Li ions. Such a positive electrode active material may be used alone or in combination of two or more. For example, LiM y Mn 2-x O4 (where M is at least one element selected from the group consisting of Li, B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Co, Ni, Cu, Al, Sn, Sb, In, Nb, Mo, W, Y, Ru, and Rh, and 0.01 ≦ x ≦ 0.5), spinel-type lithium manganese-containing oxide represented by Li x Ni (1-y-z) Co y M z O (2-k) F l (where M is at least one element selected from the group consisting of Mn, Mg, Al, B, Ti, V, Cr, Fe, Cu, Zn, Zr, Mo, Sn, Ca, Sr, and W, 0.8 ≦ x ≦ 1.2, 0 ≦ y < 0.5, 0 ≦ z < 0.5, k + l < 1, -0.1 ≦ k ≦ 0.2, 0 ≦ l ≦ 0.1), layered-structured lithium nickel-containing oxide represented by Li x Co (1-y) M y O (2-k) F l (where M is at least one element selected from the group consisting of Ni, Mn, Mg, Al, B, Ti, V, Cr, Fe, Cu, Zn, Zr, Mo, Sn, Ca, Sr, and W, 0.8 ≦ x ≦ 1.2, 0 ≦ y ≦ ...... <x≦1.2, 0≦y≦0.2, k + l < 1, -0.1 ≦ k ≦ 0.2, 0 ≦ l ≦ 0.1), layered-structured lithium cobalt-containing oxide represented by LiM 1-x x 1-x Q xExamples include olivine-type lithium composite compounds represented as PO4 (where M is at least one element selected from the group consisting of Fe, Mn, and Co, and Q is at least one element selected from the group consisting of Al, Mg, Ti, Zr, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, and 0 ≤ x ≤ 0.5).

[0037] To accommodate the increased voltage of batteries, it is preferable to include a layered lithium cobalt-containing oxide as the positive electrode active material.

[0038] When a sulfide-based solid electrolyte is used as the solid electrolyte of the positive electrode, it is desirable to form a lithium-ion conductive coating on the surface of the positive electrode active material that is less likely to react with the sulfide-based solid electrolyte in order to prevent direct reaction between the solid electrolyte and the positive electrode active material. Lithium niobium-containing oxides such as LiNbO3 are preferably used as the lithium-ion conductive coating.

[0039] When forming a molded body by pressure molding a positive electrode mixture, or when forming a solid electrolyte layer by pressure molding a layered aggregate of sulfide-based solid electrolyte particles, it is desirable to use primary particle powder (positive electrode active material powder) for the positive electrode active material in order to prevent cracking or other damage from occurring in the molded body of the positive electrode mixture even when high pressure is applied.

[0040] Furthermore, by using primary particle powder for the positive electrode active material, the powder with a lithium-ion conductive coating formed on its surface can also be composed of primary particles.

[0041] Furthermore, if the average particle size of the positive electrode active material powder, which is composed of primary particles with a lithium-ion conductive coating on its surface, becomes too large, the particles may break when pressed under high pressure, potentially causing a reaction between the active material and the sulfide-based solid electrolyte. Therefore, the average particle size of the positive electrode active material powder should be 8 μm or less, and preferably 6 μm or less.

[0042] On the other hand, if the average particle size of the positive electrode active material powder becomes too small, it becomes difficult to pack it at high density, leading to a decrease in the capacity of the positive electrode. Therefore, it is preferable that the average particle size of the positive electrode active material powder be 1 μm or larger, and preferably 3 μm or larger.

[0043] By using a positive electrode active material powder composed of primary particles within the aforementioned average particle diameter range, the porosity of the molded positive electrode mixture can be reduced to, for example, 10% or less.

[0044] The average particle size of the positive electrode active material powder can be measured using a particle size distribution analyzer (such as the Microtrac-Bell particle size distribution analyzer (product name) "MT3300EXII"), and is determined as the 50th percentile diameter (d50) in the volume-based integrated fraction when calculating the integrated volume from small particles. The same applies to the negative electrode active material and solid electrolyte.

[0045] The positive electrode active material powder consisting of the primary particles can be a commercially available product. While it is preferable that the positive electrode active material powder consists only of primary particles, it may also contain some secondary particles. However, to prevent the aforementioned problems, it is preferable that the proportion of secondary particles in the positive electrode active material powder be 30% by mass or less, more preferably 15% by mass or less, and particularly preferably 10% by mass or less.

[0046] Furthermore, the lithium-ion conductive coating material can be formed on the surface of the positive electrode active material by methods such as the sol-gel method, mechanofusion method, CVD method, and PVD method.

[0047] The amount of lithium-ion conductive coating material formed is preferably such that the ratio of the coating material to the total positive electrode active material is 0.1% by mass or more, and more preferably 0.5% by mass or more, in order to increase the coverage rate of the positive electrode active material surface and facilitate its effect. On the other hand, if the amount of lithium-ion conductive coating material formed is too large, the volume per unit weight of the positive electrode active material powder will decrease, so it is preferable that the ratio of the coating material to the total positive electrode active material be 5% by mass or less, and more preferably 2% by mass or less.

[0048] The content of the positive electrode active material powder in the positive electrode mixture is preferably 65% ​​by mass or more, and more preferably 70% by mass or more, in order to increase the capacity of the positive electrode. On the other hand, in order to improve the ionic conductivity and electronic conductivity of the positive electrode mixture by keeping the content of the solid electrolyte and conductive additive in the positive electrode mixture above a certain level, the content of the positive electrode active material powder in the positive electrode mixture is preferably 85% by mass or less, and more preferably 80% by mass or less.

[0049] The conductive additives for the positive electrode can include carbon materials such as graphite (natural graphite, artificial graphite), graphene, carbon black, vapor-grown carbon fiber (VGCF), carbon nanofibers, and carbon nanotubes.

[0050] For the positive electrode solid electrolyte, it is desirable to use a sulfide-based solid electrolyte to enhance ionic conductivity. Suitable sulfide-based solid electrolytes include compounds composed of Li, S, and P, and optionally halogen elements and Si, such as Li2S-P2S5, Li2S-SiS2, Li2S-P2S5-GeS2, and Li2S-B2S3 glass; Li 10 GeP2S 12 (LGPS system); Li 7+x P 1-y Si y S6 (-0.6≦x≦0.6, 0.1≦y≦0.6), Li 7-x+y PS 6-x Cl x+y (0.05≦y≦0.9, -3x+1.8≦y≦-3x+5.7), Li7-x PS 6-x Cl y Br z (x = y + z, 1 < x ≤ 1.8, 0.1 ≤ z / y ≤ 10), etc. Among them, Li having higher ionic conductivity 7-x+y PS 6-x Cl x+y (0.05 ≤ y ≤ 0.9, -3x + 1.8 ≤ y ≤ -3x + 5.7), Li 7-x PS 6-x Cl y Br z (x = y + z, 1 < x ≤ 1.8, 0.1 ≤ z / y ≤ 10) compounds having an argyrodite-type crystal structure are more preferable, and Li with high stability 7-x+y PS 6-x Cl x+y (0.05 ≤ y ≤ 0.9, -3x + 1.8 ≤ y ≤ -3x + 5.7) is particularly preferably used.

[0051] The average particle diameter of the sulfide-based solid electrolyte is preferably 0.1 μm or more, more preferably 0.5 μm or more, and preferably 5 μm or less in order to increase the packing density and reduce the porosity of the molded body of the positive electrode mixture having the positive electrode active material powder of the above particle size.

[0052] The positive electrode mixture may contain another solid electrolyte together with or in place of the sulfide-based solid electrolyte. Examples of the other solid electrolyte of the sulfide-based solid electrolyte include hydride-based solid electrolytes and oxide-based solid electrolytes.

[0053] Examples of the hydride-based solid electrolyte include LiBH4, a solid solution of LIBH4 and the following alkali metal compounds (for example, those having a molar ratio of LiBH4 to the alkali metal compound of 1:1 to 20:1). Examples of the alkali metal compound in the solid solution include lithium halides (such as LiI, LiBr, LiF, LiCl), rubidium halides (such as RbI, RbBr, RbiF, RbCl), cesium halides (such as CsI, CsBr, CsF, CsCl), lithium amide, rubidium amide, cesium amide, etc.

[0054] Examples of oxide-based solid electrolytes include Li7La3Zr2O 12 Examples include LiTi(PO4)3, LiGe(PO4)3, and LiLaTiO3.

[0055] However, in order to prevent a decrease in the ionic conductivity of the positive electrode mixture, the proportion of solid electrolytes other than sulfide-based solid electrolytes in the total amount of solid electrolyte used in the positive electrode mixture is preferably 30% by mass or less, more preferably 10% by mass or less, and particularly preferably no other solid electrolytes are included.

[0056] The content of the solid electrolyte in the positive electrode mixture is preferably 15% by mass or more, and more preferably 20% by mass or more, in order to enhance ionic conductivity. On the other hand, in order to ensure sufficient content of the positive electrode active material powder, it is preferably 35% by mass or less, and more preferably 30% by mass or less.

[0057] The positive electrode mixture may contain a resin binder as needed. Examples of resin binders include fluororesins such as polyvinylidene fluoride (PVDF) and acrylic resins such as acrylic acid-acrylic acid ester copolymers. However, since the resin binder acts as a resistive component in the positive electrode mixture, it is desirable to minimize the amount of resin binder included, preferably to 0.5% by mass or less, more preferably to 0.3% by mass or less, and most preferably to 0% by mass (i.e., no resin binder is included).

[0058] When a current collector is used for the positive electrode, the current collector can be made of metal foil such as aluminum or stainless steel, perforated metal, mesh, expanded metal, foamed metal, carbon sheet, etc.

[0059] From the viewpoint of increasing the battery capacity, the thickness of the molded body of the positive electrode mixture is preferably 200 μm or more, and more preferably 500 μm or more. On the other hand, from the viewpoint of making it easier to form a molded body with a low porosity and improving the load characteristics, the thickness of the molded body of the positive electrode mixture is preferably 2 mm or less, and more preferably 1.2 mm or less.

[0060] In the case of a positive electrode with a current collector, the thickness of the molded positive electrode mixture on one side of the current collector should be within the aforementioned range.

[0061] (Negative electrode) The negative electrode of an all-solid-state secondary battery has a molded body of a negative electrode mixture containing a negative electrode active material, a conductive additive, and a solid electrolyte. For example, it can be composed of the molded body alone, or it can be composed of the molded body and a current collector integrated together.

[0062] As the negative electrode active material, one or more carbon-based materials capable of intercalating and releasing lithium, such as graphite, pyrolytic carbons, cokes, glassy carbons, calcined organic polymer compounds, mesocarbon microbeads (MCMBs), and carbon fibers, can be used. In addition, individual elements that can form alloys with lithium, such as Si, Sn, Ge, Bi, Sb, In, and Al, as well as alloys of such elements with elements that do not form alloys with lithium, and compounds such as oxides of such elements; lithium-containing nitrides; composite oxides such as lithium titanium oxide having a spinel structure; and lithium metal can also be used as negative electrode active materials.

[0063] In order to increase the capacity of the negative electrode, the content of the negative electrode active material in the negative electrode mixture is preferably 45% by mass or more, and more preferably 50% by mass or more. On the other hand, in order to improve the ionic conductivity and electronic conductivity of the negative electrode mixture by keeping the content of the solid electrolyte and conductive additive in the negative electrode mixture above a certain level, the content of the negative electrode active material in the negative electrode mixture is preferably 65% ​​by mass or less, and more preferably 60% by mass or less.

[0064] As the conductive aid for the negative electrode, carbon materials such as graphite (natural graphite, artificial graphite), graphene, carbon black, carbon nanofibers, carbon nanotubes, etc. can be used. From the viewpoint of improving the electron conductivity in the molded body of the negative electrode mixture, the content of the conductive aid in the negative electrode mixture is preferably 5% by mass or more, and more preferably 7% by mass or more. Also, if the amount of the conductive aid in the negative electrode mixture is too large, voids tend to increase in the molded body of the negative electrode mixture, making it difficult to increase its packing rate, and there is a risk that the volume energy density of the all-solid-state secondary battery will decrease. Therefore, from the viewpoint of reducing the porosity of the molded body of the negative electrode mixture, the content of the conductive aid in the negative electrode mixture is preferably 15% by mass or less, and more preferably 12% by mass or less.

[0065] For the solid electrolyte of the negative electrode as well, in order to enhance the ionic conductivity, it is desirable to use a sulfide-based solid electrolyte. As the sulfide-based solid electrolyte, the same solid electrolytes as those exemplified as the sulfide-based solid electrolytes that can be used for the positive electrode mixture can be used, but compounds having an argyrodite-type crystal structure with higher ionic conductivity are preferably used, and Li 7-x PS 6-x Cl y Br z (x = y + z, 1 < x ≤ 1.8, 0.1 ≤ z / y ≤ 10) is more preferably used. By using the solid electrolyte, the porosity of the negative electrode mixture during the subsequent pressure molding can be, for example, 20% or less, preferably 15% or less. '

[0066] The negative electrode mixture may contain another solid electrolyte together with the sulfide-based solid electrolyte or in place of the sulfide-based solid electrolyte. Examples of other solid electrolytes for the sulfide-based solid electrolyte include hydride-based solid electrolytes, oxide-based solid electrolytes, etc. For these solid electrolytes, the same solid electrolytes as those exemplified for the positive electrode mixture can be used.

[0067] However, in order to prevent a decrease in the ionic conductivity of the negative electrode mixture, the proportion of solid electrolytes other than sulfide-based solid electrolytes in the total amount of solid electrolyte used in the negative electrode mixture is preferably 30% by mass or less, more preferably 10% by mass or less, and particularly preferably no other solid electrolytes are included.

[0068] The content of the solid electrolyte in the negative electrode mixture is preferably 35% by mass or more, and more preferably 40% by mass or more, in order to enhance ionic conductivity. On the other hand, in order to ensure sufficient content of the negative electrode active material, it is preferably 55% by mass or less, and more preferably 50% by mass or less.

[0069] The negative electrode mixture may contain a resin binder as needed. Examples of resin binders include fluororesins such as polyvinylidene fluoride (PVDF) and acrylic resins such as acrylic acid-acrylic acid ester copolymers. However, since the resin binder acts as a resistive component in the negative electrode mixture, it is desirable to minimize the amount of resin binder included, preferably to 0.5% by mass or less, more preferably to 0.3% by mass or less, and most preferably to 0% by mass (i.e., no resin binder is included).

[0070] When a current collector is used for the negative electrode, the current collector can be made of copper or nickel foil, perforated metal, mesh, expanded metal, foamed metal, carbon sheet, etc.

[0071] From the viewpoint of increasing the battery capacity, the thickness of the molded negative electrode mixture is preferably 200 μm or more, and more preferably 500 μm or more. On the other hand, from the viewpoint of making it easier to form a molded body with a low porosity and improving load characteristics, the thickness of the molded negative electrode mixture is preferably 3 mm or less, and more preferably 2 mm or less. In the case of a negative electrode with a current collector, the thickness of the molded negative electrode mixture on one side of the current collector may be within the above range.

[0072] (Solid electrolyte layer) In the solid electrolyte of the solid electrolyte layer of the all-solid-state secondary battery, a sulfide-based solid electrolyte is used to enhance ion conductivity. As the sulfide-based solid electrolyte, the same solid electrolyte as exemplified as the sulfide-based solid electrolyte that can be used for the positive electrode mixture can be used, but a compound having an argyrodite-type crystal structure with higher ion conductivity is preferably used. Li 7-x PS 6-x Cl y Br z (x = y + z, 1 < x ≤ 1.8, 0.1 ≤ z / y ≤ 10) is more preferably used.

[0073] In order to reduce the porosity during pressure molding, the average particle diameter of the sulfide-based solid electrolyte is desirably 0.3 to 1.5 μm.

[0074] Note that the solid electrolyte layer may be composed of only one layer, but it can also have a laminated structure. The layer in contact with the positive electrode is composed of a highly stable sulfide-based solid electrolyte, for example, Li 7-x+y PS 6-x Cl x+y (0.05 ≤ y ≤ 0.9, -3x + 1.8 ≤ y ≤ -3x + 5.7), and the layer on the negative electrode side can be composed of a sulfide-based solid electrolyte having high ion conductivity, for example, Li 7-x PS 6-x Cl y Br z (x = y + z, 1 < x ≤ 1.8, 0.1 ≤ z / y ≤ 10).

[0075] The solid electrolyte layer may contain other solid electrolytes together with the sulfide-based solid electrolyte. In order to prevent the ion conductivity of the solid electrolyte layer from decreasing, the content ratio of the solid electrolyte other than the sulfide-based solid electrolyte in the solid electrolyte layer is preferably 30% by mass or less of the total solid electrolyte, more preferably 10% by mass or less, and particularly preferably not containing other solid electrolytes.

[0076] The thickness of the solid electrolyte layer is 15 μm or more, preferably 20 μm or more, more preferably 25 μm or more, and even more preferably 30 μm or more, in order to ensure insulation between the positive and negative electrodes. On the other hand, in order to reduce the internal resistance of the battery, the thickness of the solid electrolyte layer is 120 μm or less, preferably 100 μm or less, more preferably 80 μm or less, and particularly preferably 60 μm or less.

[0077] (Laminated electrode) The molded body of the positive electrode mixture is composed of a positive electrode mixture prepared by mixing, for example, a positive electrode active material powder, a conductive additive, a solid electrolyte, and a binder added as needed. The positive electrode mixture can be formed by pouring it into a mold to form a layer of a predetermined thickness, and then compressing it by pressure molding or the like. The molded body of the negative electrode mixture is composed of a negative electrode mixture prepared by mixing, for example, a negative electrode active material, a conductive additive, a solid electrolyte, and a binder added as needed. The negative electrode mixture can be formed by pouring it into a mold to form a layer of a predetermined thickness, and then compressing it by pressure molding or the like.

[0078] The molded bodies of the positive electrode mixture and the negative electrode mixture are used as electrodes (positive and negative electrodes) either as they are or bonded to a current collector by methods such as crimping.

[0079] A solid electrolyte layer can be formed, for example, by introducing solid electrolyte particles into a mold to form a layer of a predetermined thickness (a layered aggregate of solid electrolyte particles), and then compressing the layer in the thickness direction by pressure molding. The solid electrolyte particles are filled into the mold without any solvent or binder. That is, the solid electrolyte particles are filled into the mold as a layered aggregate of particles without any solvent or binder, and then pressurized. The solid electrolyte layer, in its molded state after being pressurized, is used in the assembly of the battery. The pressure applied during pressurization should be 1000 MPa or more in surface pressure. This makes it easy to reduce the porosity of the solid electrolyte layer and the height of the surface irregularities.

[0080] Furthermore, if pressurization is performed multiple times during the formation of the solid electrolyte layer, it is sufficient if the pressure applied during one of these pressurizations is 1000 MPa or more in terms of surface pressure. In addition, the pressure applied to the layered aggregate of solid electrolyte particles may be 1200 MPa or more.

[0081] The molded bodies of the positive electrode mixture and the negative electrode mixture are stacked and integrated via a solid electrolyte layer to form a laminated electrode body, which is used in the assembly of a battery.

[0082] The conditions for producing the molded body of the positive electrode mixture can be, for example, the following conditions.

[0083] A positive electrode mixture is prepared by mixing a positive electrode active material powder consisting of primary particles with a sulfide-based solid electrolyte, etc., to contain the positive electrode active material powder in a proportion of 65 to 85% by mass. This mixture is then placed in a mold to form a layer of a predetermined thickness, and pressurized to produce a molded body of the positive electrode mixture. The pressure applied during pressurization is 1000 MPa (10 tons / cm²) as a surface pressure. 2 It is preferable to set it to ) or higher. This makes it easy to set the porosity of the molded article to 10% or less.

[0084] Furthermore, the molded negative electrode mixture can be produced under the same conditions as the molded positive electrode mixture, for example, with the pressure applied as a surface pressure of 1000 MPa (10 ton / cm²). 2 ) This can also be used to increase the filling rate.

[0085] While the pressurization of the positive electrode mixture layer and the negative electrode mixture may be performed in a single step, it is also possible to first pressurize to a pressure of less than 1000 MPa to increase the filling density of the mixture to a certain extent, and then further pressurize to a pressure of 1000 MPa or higher to form the desired molded body.

[0086] The order in which each layer is stacked is not limited. For example, a layer of the negative electrode mixture may be formed first, followed by a layer of the solid electrolyte, and then a layer of the positive electrode mixture on top of that to form a laminate. Alternatively, a layer of the solid electrolyte may be prepared first, followed by a layer of either the negative electrode mixture or the positive electrode mixture on one side of the solid electrolyte layer, and then the other layer of the negative electrode mixture or the positive electrode mixture on the other side of the solid electrolyte layer to form a laminate.

[0087] Alternatively, the positive electrode mixture molded body, the solid electrolyte layer, and the negative electrode mixture molded body may be manufactured separately in advance and then laminated together at the end. For example, the positive electrode mixture molded body, the solid electrolyte layer, and the negative electrode mixture molded body may each be manufactured separately by pressure molding in advance.

[0088] Thus, from the viewpoint of manufacturing efficiency, it is preferable to first create a solid electrolyte layer by pressurizing a layered aggregate of solid electrolyte particles with a surface pressure of 1000 MPa, and then create layers of negative electrode and positive electrode mixtures, which are separately produced by pressure molding, on both sides of the solid electrolyte layer. This is because in this case, it is easier to reduce the height of the irregularities at the interface between the solid electrolyte layer and the positive and negative electrodes, and it is easier to form a homogeneous solid electrolyte layer.

[0089] The surface pressure applied to each layer during pressurization is typically 2000 MPa (20 tons / cm²), considering productivity limitations imposed by the manufacturing equipment. 2 It is thought that the pressure can be applied up to a certain extent, and the porosity of the molded bodies of the positive electrode mixture and the negative electrode mixture obtained is thought to have a lower limit of about 5%.

[0090] In this specification, the porosity of the molded bodies of the positive electrode mixture and the negative electrode mixture is a value calculated by determining the sum for each component i using the following formula (1) from the thickness of the molded body, the mass per unit area, and the density of the constituent components.

[0091] P = 100 - (Σai / ρi) × (m / t) (1)

[0092] Here, in equation (1) above, ai: ratio of component i expressed in mass%, ρi: density of component i (g / cm³) 3 ), m: Mass per unit area of ​​the molded body (g / cm²) 2 ), t: thickness of the molded body (cm).

[0093] Alternatively, the cross-sections of the molded bodies of the positive electrode mixture and the negative electrode mixture may be observed using a scanning electron microscope (SEM), and the images may be binarized by image processing to determine the proportion of voids, thereby calculating the porosity of the molded body of the positive electrode mixture. In this case, it is desirable to determine the image processing conditions so that the resulting value is consistent with the porosity obtained by the above method.

[0094] The porosity of the central and peripheral portions of the solid electrolyte layer is measured as follows: The solid electrolyte layer is cut along the longest width line, viewed from the thickness direction, using a plane parallel to the thickness direction. SEM images of the peripheral and central portions of the cut surface are then obtained.

[0095] Figure 1 is a diagram illustrating the regions (periphery and center) for measuring the porosity of the solid electrolyte layer. In the example shown in Figure 1, a cross-section of an all-solid-state secondary battery comprising a positive electrode 10, a negative electrode 20, and a solid electrolyte layer 30 interposed between the positive electrode 10 and the negative electrode 20 is shown. The solid electrolyte layer 30 is viewed from the thickness direction (z direction in Figure 1) and cut along the line in the width direction (x direction) of the longest width. In the example in Figure 1, the shape of the solid electrolyte layer 30 viewed from the thickness direction (top view shape) is circular. In this case, the solid electrolyte layer 30 is cut along the line passing through the center of the circle in the thickness direction. If the top view shape is elliptical, it is cut along the major axis of the ellipse. If the top view shape is rectangular, it is cut along the center line between the two long sides. In the example shown in Figure 1, R1 is the periphery and R2 is the center.

[0096] The peripheral region is a rectangular region with a width of 25 μm and a thickness of 19 μm, centered at a point s = 100 μm from the edge in the width direction of the solid electrolyte layer interposed between the positive and negative electrodes in the cross-section, and including the center in the thickness direction. The central region is a rectangular region with a width of 25 μm and a thickness of 19 μm, centered at the center of the cross-section of the solid electrolyte layer interposed between the positive and negative electrodes in the cross-section (center in both the thickness and width directions).

[0097] These SEM images of the peripheral and central regions are binarized using image analysis software (such as ImageJ). Bright contrast areas (high brightness) are identified as material, and dark contrast areas (low brightness) as voids. The porosity is then calculated by determining the area ratio of the void portions.

[0098] The equipment and conditions used can be, for example, as follows: Scanning electron microscope: Hitachi S4800 Detector: Backscattered electron-enhanced mode Acceleration voltage: 2kV Magnification: 5,000x Capture resolution: 2,560 x 1,920 Minimum dot length: 9.9nm (equivalent circle diameter: 11.2nm) Minimum dot area: 98nm 2 Analysis area: 4.81×10 8 nm 2

[0099] The obtained SEM images are converted to grayscale using Photoshop®. A Gaussian filter (0.5 pixels) is used to reduce noise. Next, the histogram of the SEM images is quantified using ImageJ. Figure 2 shows an example of an SEM image histogram. In the histogram, the luminance at the maximum frequency, a, is taken as the central value. On the higher luminance side than the central value, the luminance b at the frequency where the value is 3% of the maximum frequency is read, and further, on the lower luminance side than the central value, the luminance d (=a-c) at which the value is smaller than the central value by the difference between b and a, c (=ba), is found, and this luminance d is taken as the threshold.

[0100] Furthermore, using the image analysis software "Eizo-kun," the SEM image is binarized using the previously determined threshold d to separate it into low-luminance and high-luminance regions. The ratio of the area of ​​the low-luminance region in the SEM image is calculated as the porosity. In this case, the area is 900 nm. 2 (0.09 × 10 -2 μm 2 Low-luminance areas below the threshold (which are smaller than the threshold) are considered noise components and are therefore excluded when determining the void ratio.

[0101] The size of the rectangular region used to acquire the SEM image (backscattered electron image) can be changed according to the thickness of the solid electrolyte layer if it is thin. If the thickness of the solid electrolyte layer is approximately 24 μm or less, the length of the rectangular region in the thickness direction should be set to 80% of the thickness of the solid electrolyte layer so that the rectangular region does not include the region near the positive electrode interface and the region near the negative electrode interface of the solid electrolyte layer.

[0102] The porosity of the central part of the cross-section of the solid electrolyte layer, as measured as described above, is 3% or less, preferably 2% or less, and more preferably 1% or less, as the solid electrolyte layer is formed in such a way.

[0103] Furthermore, the porosity of the periphery of the solid electrolyte layer measured as described above is preferably 5% or less, more preferably 4% or less, and even more preferably 3% or less.

[0104] The thickness of the solid electrolyte layer and the height of the interface irregularities are measured using values ​​near the center, as described below. For the cross-section of the solid electrolyte layer set when measuring the porosity, an SEM image is acquired that includes regions of 100 μm on each side (a region with a total length of 200 μm in the width direction) from the center in the width direction of the cross-section, so that the state of the irregularities at the interface between the positive electrode and the solid electrolyte layer, and the state of the irregularities at the interface between the solid electrolyte layer and the negative electrode can be seen. The SEM image can be obtained by taking multiple images while shifting them in the width direction and stitching them together to get an image of the entire region. In the acquired SEM image of the entire region, as shown in Figure 3, at the interface between the positive electrode and the solid electrolyte layer, a line L1 perpendicular to the thickness direction is drawn passing through the point closest to the solid electrolyte layer, and a line L2 perpendicular to the thickness direction is drawn passing through the point closest to the positive electrode. Similarly, at the interface between the negative electrode and the solid electrolyte layer, a line L3 perpendicular to the thickness direction is drawn passing through the point closest to the solid electrolyte layer, and a line L4 perpendicular to the thickness direction is drawn passing through the point closest to the negative electrode. In this case, the distance T1 between L1 and L3 is defined as the thickness of the solid electrolyte layer. The distance H1 between lines L1 and L2 is defined as the height of the unevenness in the thickness direction at the interface between the positive electrode and the solid electrolyte layer. The distance H2 between lines L3 and L4 is defined as the height of the unevenness in the thickness direction at the interface between the negative electrode and the solid electrolyte layer.

[0105] The thickness of the solid electrolyte layer measured as described above is 15 μm or more. This prevents short circuits even if there is some variation in the thickness of the solid electrolyte layer formed by pressure molding of sulfide-based solid electrolyte particles, i.e., if there are some irregularities at the interface of the solid electrolyte layer.

[0106] The height of the surface irregularities at the interface between the solid electrolyte and the positive electrode, and the height of the surface irregularities at the interface between the solid electrolyte and the negative electrode, as measured as described above, should both be 7 μm or less, preferably 5 μm or less, and more preferably 3 μm or less, in order to ensure uniform charge-discharge reactions between the positive and negative electrodes.

[0107] At least one of the positive electrode mixture, the negative electrode mixture, and the solid electrolyte layer forming composition may be pressure-molded while heated. Molding the solid electrolyte material while it is softened by heating increases the contact area between the active material and the solid electrolyte material, or between the solid electrolyte materials themselves, thereby improving the filling properties of the molded body.

[0108] The heating temperature depends on the composition of the solid electrolyte material, but can be, for example, 60 to 200°C. From the viewpoint of suppressing the deformation of the sulfide-based solid electrolyte due to heating, the heating temperature in the solid electrolyte layer formation process is preferably 200°C or lower, more preferably 150°C or lower, and even more preferably 100°C or lower.

[0109] (Battery type) The all-solid-state secondary battery of the present invention can be configured, for example, as a coin-type battery. Figure 4 schematically shows a cross-sectional view of a coin-type battery. The battery 1 shown in Figure 4 has an outer casing formed by an outer casing 40, a sealing casing 50, and a resin gasket 60 interposed between them, within which a positive electrode 10, a negative electrode 20, and a solid electrolyte layer 30 interposed between the positive electrode 10 and the negative electrode 20 are sealed.

[0110] The sealing can 50 is fitted into the opening of the outer can 40 via a gasket 60, and the open end of the outer can 40 is tightened inward, causing the gasket 60 to come into contact with the sealing can 50, thereby sealing the opening of the outer can 40 and creating a sealed structure inside the element.

[0111] Stainless steel can be used for the outer casing and sealing can. For the gasket material, polypropylene and nylon can be used. Furthermore, if heat resistance is required due to the battery's application, heat-resistant resins with melting points exceeding 240°C, such as fluororesins like tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA), polyphenylene ether (PEE), polysulfone (PSF), polyarylate (PAR), polyethersulfone (PES), polyphenylene sulfide (PPS), and polyetheretherketone (PEEK), can also be used. Additionally, if the battery is used in applications requiring heat resistance, a glass hermetic seal can be used for sealing.

[0112] The form of an all-solid-state secondary battery is not limited to a flat shape having an outer casing composed of an outer casing, a sealing casing, and a gasket, as shown in Figure 4. For example, it may have an outer casing made of a resin film or a metal-resin laminate film, or an outer casing made of metal with a bottomed cylindrical shape (cylindrical or rectangular) and a sealing structure that seals its opening.

[0113] The all-solid-state secondary battery of the present invention can be applied to the same uses as conventionally known secondary batteries, but because it has a solid electrolyte instead of a liquid electrolyte, it has excellent heat resistance and can be preferably used in applications that are exposed to high temperatures. [Examples]

[0114] The present invention will be described in detail below based on the following examples. However, the following examples are not intended to limit the present invention.

[0115] (Example 1) <Fabrication of laminated electrode bodies> Sulfide-based solid electrolyte with an average particle size of 0.7 μm: Li 5.4 PS 4.4 Cl 0.8 Br 0.8 9.6 mg of the powder was placed in a powder molding die and pressed using a press machine at 70 MPa (0.7 tf / cm²). 2A temporary solid electrolyte layer was formed by pressure molding under the specified pressure.

[0116] Next, on the pre-molded layer of the solid electrolyte in the mold, lithium titanate (Li4Ti5O) with an average particle size of 2 μm is added. 12 (negative electrode active material) and a sulfide-based solid electrolyte with an average particle size of 0.7 μm: Li 5.4 PS 4.4 Cl 0.8 Br 0.8 A negative electrode mixture of 101 mg, prepared by mixing graphene (a conductive additive) in a mass ratio of 50:41:9, was added, and the pressure was increased to 300 MPa (3 tf / cm²). 2 A laminate was fabricated by pressurizing under the pressure of ), in which a preliminary molded layer of the negative electrode was formed on one side of the preliminary molded layer of the solid electrolyte.

[0117] Furthermore, after inverting the mold having the laminate, on top of the pre-formed layer of the solid electrolyte positioned on the upper side, LiCoO2 (positive electrode active material) with an average particle size of 5 μm and a LiNbO3 coating layer formed on its surface, and a sulfide-based solid electrolyte with an average particle size of 3 μm: Li 7.0 PS 5.4 Cl 1.2 Then, 73 mg of a cathode mixture prepared by mixing carbon black and vapor-grown carbon fiber (VGCF) in a mass ratio of 70:26.8:1.1:2.1 was added, and the pressure was increased to 1300 MPa (13 tf / cm²). 2 By performing pressure molding at the specified pressure, the pre-formed negative electrode layer, the pre-formed solid electrolyte layer, and the positive electrode mixture were compressed to obtain a laminated electrode body in which the negative electrode layer, a solid electrolyte layer with a thickness of 110 μm, and a positive electrode layer were integrated. The ratio of the weight of the coating layer to the total weight of the positive electrode active material powder including the coating layer was 2% by mass.

[0118] The porosity of the peripheral and central parts of the cross-section of the solid electrolyte layer was determined by performing the above image processing on SEM images obtained at a magnification of 5000x. The determined porosity was 2.9% at the peripheral and 0.8% at the central part. The height of the surface irregularities at the interface between the solid electrolyte layer and the positive and negative electrodes was 3.9 μm and 4.4 μm, respectively.

[0119] <Battery assembly>

[0120] A coin-type all-solid-state secondary battery was assembled by using a stainless steel sealing can and outer can as the outer casing, and sealing the battery by placing a porous carbon sheet with a thickness of 0.1 mm between the sealing can, outer can, and the laminated electrode body.

[0121] (Comparative Example 1) After forming the laminate of three layers, the pressure used for press molding with a press machine is set to 600 MPa (6 ton / cm²). 2 A stacked electrode body was fabricated in the same manner as in Example 1, except that the above was omitted, and a coin-type all-solid-state secondary battery was assembled.

[0122] The porosity of the cross-section of the solid electrolyte layer, determined by image processing of SEM images obtained at 5000x magnification, was 5.2% in the central part and 7.9% at the periphery. The heights of the surface irregularities at the interfaces between the solid electrolyte layer and the positive and negative electrodes were 4.2 μm and 4.8 μm, respectively. The thickness of the solid electrolyte layer was 115 μm.

[0123] (Comparative Example 2) 101 mg of the negative electrode mixture prepared in Example 1 was placed in a powder molding die and pressed using a press machine at 300 MPa (3 tf / cm²). 2 A temporary molded layer of the negative electrode was formed by pressure molding under the following pressure: Next, on the pre-formed layer of the negative electrode, a sulfide-based solid electrolyte with an average particle size of 0.7 μm is placed: Li 5.4 PS 4.4 Cl 0.8 Br 0.8 Add 9.6 mg of the powder and apply 70 MPa (0.7 tf / cm²). 2 A laminate was fabricated by pressurizing under the pressure of ), with a pre-formed layer of solid electrolyte formed on one of the pre-formed layers of the negative electrode. Furthermore, 73 mg of the positive electrode mixture prepared in Example 1 was added to the pre-formed layer of the solid electrolyte, and pressure was applied at 1300 MPa (13 tf / cm²). 2By performing pressure molding at the specified pressure, the temporary molded layer of the negative electrode, the temporary molded layer of the solid electrolyte, and the positive electrode mixture were compressed to obtain a laminated electrode body in which the negative electrode layer, the solid electrolyte layer, and the positive electrode layer were integrated.

[0124] The porosity of the cross-section of the solid electrolyte layer, determined by image processing of SEM images obtained at 5000x magnification, was 1.0% in the central part and 3.3% at the periphery. The height of the surface irregularities at the interfaces between the solid electrolyte layer and the positive and negative electrodes was 7.8 μm and 8.8 μm, respectively. The thickness of the solid electrolyte layer was 111 μm.

[0125] (Comparative Example 3) A coin-type all-solid-state secondary battery was assembled in the same manner as in Example 1, except that the amount of sulfide-based solid electrolyte powder added was adjusted to produce a laminated electrode body so that the thickness of the solid electrolyte layer after pressure molding was 10 μm.

[0126] The all-solid-state secondary batteries of Example 1 and Comparative Examples 1-2 were evaluated as described below. The all-solid-state secondary battery of Comparative Example 3 was not evaluated because it experienced a short circuit.

[0127] <Load Characteristics Evaluation> For the batteries of Example 1 and Comparative Examples 1-2 that were prepared, constant current charging was performed by combining constant current charging, where the battery voltage was 3.1V at a current of 0.2C, and constant voltage charging, where the current was 0.02C at a voltage of 3.1V. Furthermore, constant current discharge was performed, where the battery voltage was 1.2V at a current of 0.1C, and the discharge capacity at 0.1C was measured.

[0128] Next, constant current-constant voltage charging was performed in the same manner as described above, and then constant current discharge was performed at a current value of 0.5C until the battery voltage reached 1.2V, and the discharge capacity at 0.5C was measured. The value (%) obtained by dividing the discharge capacity at 0.5C by the discharge capacity at 0.1C was calculated, and the load characteristics of each battery were evaluated.

[0129] <Cycle Characteristics Evaluation> The batteries of the examples and comparative examples were subjected to 50 charge-discharge cycles under the following conditions in an environment of 100°C.

[0130] The charge-discharge cycle was repeated 50 times, with constant current charging performed at a current of 0.2C until the voltage reached 3.1V, followed by constant voltage charging at 3.1V until the current reached 0.02C, and then constant current discharge at a current of 0.2C until the voltage reached 1.2V. The cycle characteristics were evaluated by the rate of decrease in discharge capacity at the 50th cycle compared to the discharge capacity at the 2nd cycle (capacity degradation rate). The evaluation results are shown in Table 1.

[0131] [Table 1]

[0132] In the all-solid-state secondary battery of Example 1, a solid electrolyte layer with a thickness of 15 μm or more was formed by pressure molding sulfide-based solid electrolyte particles, the void ratio in the central part was set to 35% or less, and the height of the irregularities at the interface between the solid electrolyte layer and the positive and negative electrodes was set to 7 μm or less, resulting in a battery with excellent load characteristics and cycle characteristics.

[0133] On the other hand, in the battery of Comparative Example 1, the porosity in the central part of the solid electrolyte layer exceeded 5%, resulting in reduced load characteristics and cycle characteristics. In the battery of Comparative Example 2, the height of the unevenness at the interface between the solid electrolyte layer and the positive and negative electrodes exceeded 7 μm, resulting in reduced cycle characteristics. Furthermore, in the battery of Comparative Example 3, where the thickness of the solid electrolyte layer was less than 15 μm, a short circuit occurred. [Explanation of symbols]

[0134] 1 All-solid-state secondary battery 10 positive electrode 20 negative electrode 30 Solid electrolyte layer 40 outer cans 50 sealed cans 60 Gasket

Claims

1. A method for manufacturing an all-solid-state secondary battery having a positive electrode, a negative electrode, and a solid electrolyte layer containing a sulfide-based solid electrolyte disposed between the positive electrode and the negative electrode, The solid electrolyte layer has a thickness of 15 μm or more and 120 μm or less, the porosity calculated from scanning electron microscope (SEM) images of the central part of the cross-section of the solid electrolyte layer is 3% or less, and the height of the surface irregularities at the interface between the solid electrolyte layer and the positive electrode and the negative electrode is 7 μm or less, each. A step of filling a mold with sulfide-based solid electrolyte particles without using a solvent to form a layered aggregate of the sulfide-based solid electrolyte particles, A method for manufacturing an all-solid-state secondary battery, characterized by comprising a step of pressurizing a layered aggregate of sulfide-based solid electrolyte particles with a surface pressure of 1,000 MPa or more.

2. A method for manufacturing an all-solid-state secondary battery according to claim 1, A method for manufacturing an all-solid-state secondary battery, characterized in that the negative electrode comprises at least a negative electrode active material powder and a sulfide-based solid electrolyte.

3. A method for manufacturing an all-solid-state secondary battery according to claim 1 or 2, wherein in the step of forming a layered aggregate of sulfide-based solid electrolyte particles, the sulfide-based solid electrolyte particles are filled into a mold without using a solvent and a binder.

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