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

Abstract

To provide an all-solid secondary battery which is arranged so that a short circuit can be suppressed, and which is superior in load and cycle characteristics, and a manufacturing method thereof; the present invention relates to the goal 7, "Affordable and clean energy" and the goal 12, "Responsible consumption and production" of Sustainable Development Goals.SOLUTION: An all-solid secondary battery of the invention comprises a positive electrode 10, a negative electrode 20, and a solid electrolyte layer 30 containing a sulfide-based solid electrolyte. The solid electrolyte layer 30 has a thickness of 15 μm or more. A peripheral edge R1 of the solid electrolyte layer 30 has a porosity of 5% or less, which is larger than that of a center portion R2. The solid electrolyte layer 30 can be obtained by e.g. the step of compression forming a layered collection of sulfide-based solid electrolyte particles in a die 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 technology]

[0002] In recent years, with the development of portable electronic devices such as mobile phones and laptop personal computers, and the practical application of electric vehicles, there has been a growing demand for small, lightweight secondary batteries with high capacity and high energy density.

[0003] Currently, lithium secondary batteries, particularly lithium ion secondary batteries, that can meet this demand use lithium-containing composite oxides such as lithium cobalt oxide (LiCoO) and lithium nickel oxide (LiNiO) as the positive electrode active material, graphite or the like as the negative electrode active material, and an organic electrolyte solution containing an organic solvent and a lithium salt as the non-aqueous electrolyte.

[0004] As devices that use lithium-ion secondary batteries continue to develop, there is a demand for longer life, higher capacity, and higher energy density for lithium-ion secondary batteries. Furthermore, there is also a strong demand for the reliability of lithium-ion secondary batteries that have longer life, higher capacity, and higher energy density.

[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 recent trend toward higher energy density of lithium-ion secondary batteries and an increasing amount of organic solvent in the organic electrolyte solution, there is a demand for even higher reliability in lithium-ion secondary batteries.

[0006] In light of the above, all-solid-state lithium secondary batteries (all-solid-state secondary batteries) that do not use organic solvents have attracted attention. All-solid-state secondary batteries use a molded body of a solid electrolyte that does not use organic solvents instead of the conventional organic solvent-based electrolytes.

[0007] Various improvements have been attempted for the all-solid-state secondary battery. For example, Japanese Patent Application Laid-Open No. 2013-16280 (Patent Document 1) proposes using a sulfide-based solid electrolyte and increasing the porosity of the peripheral region, which is a ring-shaped portion including the outer edge of the solid electrolyte layer, to be greater than the porosity of the central region, thereby suppressing the decrease in discharge capacity associated with the charge and discharge of the battery. Japanese Patent Application Laid-Open No. 2013-16280 states that the porosity of the peripheral region is preferably 5% or less, and that a difference of 2 to 4% is preferably set between the porosity of the peripheral region and the porosity of the central region.

[0008] In order to achieve this configuration, JP 2013-16280 A discloses forming a positive electrode active material layer and a negative electrode active material layer at a pressure of 100 to 400 MPa, then forming a solid electrolyte layer of 5 μm thickness on the surface of each active material layer using a vapor phase method, and then pressing the solid electrolyte layers together while heating them in an opposing relationship to form a laminate in which the positive electrode active material layer, the solid electrolyte layer, and the negative electrode active material layer are integrated. In this process, the pressure applied to the central region is set higher (10 to 20 MPa), while the pressure applied to the peripheral region is set lower (10 to 20 MPa) than that of the central region. This allows the porosity of the peripheral region of the solid electrolyte layer to be greater than that of the central region. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-16280 Summary of the Invention [Problem to be solved by the invention]

[0010] However, the method of changing the pressure conditions between the central region and the peripheral region complicates the process and reduces productivity.

[0011] Furthermore, when a solid electrolyte layer is formed by a method other than the gas-phase method, for example, by compression molding solid electrolyte particles directly in a mold, if the solid electrolyte layer is formed to a thickness of about 10 μm, unevenness in the thickness is likely to occur, making it more likely to cause a short circuit due to contact between the positive electrode and the negative electrode. On the other hand, if the thickness of the solid electrolyte layer is increased to prevent the short circuit, even if the central region and the peripheral region are pressed with the same pressure, the resistance from the mold wall surface will make the porosity of the peripheral region of the solid electrolyte layer too large compared to the porosity of the central region, which is likely to result in a deterioration in load characteristics and cycle characteristics.

[0012] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an all-solid-state secondary battery that suppresses short circuits due to uneven thickness of a solid electrolyte layer and has excellent load characteristics and cycle characteristics, and a manufacturing method thereof. [Means for solving the problem]

[0013] An all-solid-state secondary battery according to an embodiment of the present invention includes 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 the porosity of a cross section of the solid electrolyte layer calculated from a scanning electron microscope (SEM) image is 5% or less at the periphery, and the porosity of the periphery is greater than the porosity of a central portion.

[0014] The method for producing an all-solid-state secondary battery of the present invention is characterized by comprising a step of press-molding a layered assembly of sulfide-based solid electrolyte particles in a mold at a surface pressure of 1000 MPa or more. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide an all-solid-state secondary battery that is less likely to cause a short circuit and has excellent load characteristics and cycle characteristics, and a method for manufacturing the same. [Brief explanation of the drawings]

[0016] [Figure 1]FIG. 2 is a diagram for explaining a region where the porosity of a solid electrolyte layer is measured. [Figure 2] 10 is a gradation distribution histogram of an SEM image for explaining binarization. [Figure 3] 1A and 1B are diagrams for explaining a method for measuring the thickness of a solid electrolyte layer and the unevenness of an interface. [Figure 4] FIG. 1 is a cross-sectional view schematically illustrating an example of an all-solid-state secondary battery of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] An all-solid-state secondary battery according to an embodiment of the present invention includes 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 the porosity of a cross section of the solid electrolyte layer calculated from a scanning electron microscope (SEM) image is 5% or less at the periphery, and the porosity of the periphery is greater than the porosity of a central portion.

[0018] In the cross section of the solid electrolyte layer of the all-solid-state secondary battery, the peripheral portion is a region having a center 100 μm from the edge in the width direction of the cross section of the solid electrolyte layer and including the center in the thickness direction, and the central portion is a region including the center in the width direction and the center in the thickness direction of the cross section of the solid electrolyte layer. The solid electrolyte layer of the all-solid-state secondary battery has a thickness of 15 μm or more. This configuration can suppress unevenness in the thickness of the solid electrolyte layer. Even if the thickness unevenness becomes large to a certain extent, short circuits due to contact between the positive and negative electrodes can be prevented. Furthermore, in the cross section of the solid electrolyte layer, the porosity of the peripheral portion calculated from an SEM image is 5% or less, which is higher than the porosity of the central portion. On the other hand, in conventional technologies, when a solid electrolyte layer is formed to a thickness of 15 μm or more, it is not easy to achieve a porosity of 5% or less in the peripheral portion. For example, under the pressure conditions described in Patent Document 1, the solid electrolyte particles are difficult to pack in the peripheral portion due to friction with the mold wall. As a result, the porosity of the peripheral portion of the solid electrolyte layer becomes high, which tends to cause a large difference with the porosity of the central portion. In contrast, the solid electrolyte layer of the all-solid-state secondary battery includes a sulfide-based solid electrolyte and has a thickness of 15 μm or more, and a porosity of 5% or less in the peripheral portion, which is higher than the porosity of the central portion. This configuration increases the packing density of the solid electrolyte layer as a whole and enables uniform ion conduction within the solid electrolyte layer. This allows the charge / discharge reaction to proceed uniformly throughout, resulting in an all-solid-state secondary battery with excellent load and cycle characteristics. As a result, short circuits due to contact between the positive and negative electrodes are prevented, and an all-solid-state secondary battery with excellent load and cycle characteristics is provided, even while allowing for some unevenness in the thickness of the solid electrolyte layer. This effect can contribute, for example, to the achievement of Goal 7 "Affordable and Clean Energy" and Goal 12 "Responsible Consumption and Production" of the United Nations' Sustainable Development Goals (SDGs).

[0019] As a rough guide, the target regions of the SEM image for calculating the porosity are rectangular regions with a width of 25 μm and a thickness of 19 μm for both the peripheral and central portions. That is, the porosity of the peripheral portion is calculated for a rectangular region of the above dimensions, centered 100 μm from the edge in the width direction of the cross section of the solid electrolyte layer and including the center in the thickness direction. The porosity of the central portion is calculated for a rectangular region of the above dimensions, including the center in the width direction and the center in the thickness direction of the cross section of the solid electrolyte layer.

[0020] However, when the thickness of the solid electrolyte layer is approximately 24 μm or less, the length in the thickness direction of the rectangular region for calculating the porosity is reduced according to the thickness of the solid electrolyte layer. In this case, the length in the thickness direction of the rectangular region is 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 side interface and the region near the negative electrode side interface of the solid electrolyte layer. This reduces the variation in the measurement.

[0021] The solid electrolyte layer of the all-solid-state secondary battery has a thickness of 15 μm or more, and can prevent short circuits due to contact between the positive electrode and the negative electrode even if the unevenness in the thickness direction of either the interface with the positive electrode or the interface with the negative electrode is, for example, about 5 μm. Therefore, there are fewer restrictions on the conditions for pressure molding the sulfide-based solid electrolyte particles, making it easier to produce an all-solid-state secondary battery with excellent load characteristics and cycle characteristics.

[0022] The positive electrode may have a compact of a positive electrode mixture containing a 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 of a positive electrode active material on the surface of which a lithium ion conductive coating material is formed. In this case, the positive electrode active material powder may, for example, be primary particles of a positive electrode active material on the surface of which a lithium ion conductive coating material is formed.

[0023] The negative electrode may have a molded body of a negative electrode mixture containing a 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.

[0024] The method for producing the all-solid-state secondary battery includes, for example, a step of press-molding a layered assembly of sulfide-based solid electrolyte particles in a mold at a surface pressure of 1000 MPa or more, thereby efficiently producing an all-solid-state secondary battery in which the porosity of the peripheral portion of the cross section of the solid electrolyte layer is 5% or less and is larger than that of the central portion.

[0025] That is, by pressure-molding sulfide-based solid electrolyte particles in a mold at a surface pressure of 1000 MPa or more, it is possible to easily form a solid electrolyte layer in which the porosity is 5% or less even in the peripheral portion, where the porosity is higher than in the central portion, and the difference in porosity between the peripheral portion and the central portion is small.

[0026] The manufacturing method may include a step of forming the solid electrolyte layer by pressing the entire layered assembly of sulfide-based solid electrolyte particles in a mold at a surface pressure of 1000 MPa or more. That is, both the central portion and the peripheral portion around the central portion of the layered assembly of sulfide-based solid electrolyte particles may be pressed at the same surface pressure of 1000 MPa or more. This allows the porosity of the peripheral portion of the solid electrolyte layer to be 5% or less and higher than the porosity of the central portion, even without applying different surface pressures to the central portion and the peripheral portion.

[0027] The manufacturing method may further include a step of stacking the positive electrode, the negative electrode, and the solid electrolyte layer. In this case, the solid electrolyte layer is stacked so as to be disposed between the positive electrode and the negative electrode. The step of press-molding the layered assembly of sulfide-based solid electrolyte particles in a mold at a surface pressure of 1000 MPa or more may be a step of press-molding the layered assembly of sulfide-based solid electrolyte particles in a state where the layered assembly is stacked on at least one of the positive electrode and the negative electrode. Alternatively, the layered assembly of sulfide-based solid electrolyte particles may be press-molded before being stacked on the positive electrode and the negative electrode.

[0028] The following describes the embodiments in detail. The same or corresponding components in the drawings are designated by the same reference numerals, and the same description will not be repeated. For ease of understanding, the drawings referred to below may show simplified or schematic configurations, or may omit some components.

[0029] (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 additive, a solid electrolyte, and the like, and can be formed, for example, from the molded body alone, or can be formed by integrating the molded body with a current collector.

[0030] The positive electrode active material is not particularly limited as long as it is a positive electrode active material used in conventionally known lithium ion secondary batteries, that is, an active material capable of absorbing and releasing Li ions, and such positive electrode active materials may be used alone or in combination of two or more. For example, LiM x Mn 2-x Lithium manganese-containing spinel oxide represented by LiO4 (wherein 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), x Ni (1-y-z) Co y M z O (2-k) F l (wherein 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, and 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), lithium nickel-containing oxide having a layered structure represented by Li x Co (1-y) M y O (2-k) F l (wherein 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, and 0.8≦x≦1.2, 0≦y≦0.2, k+l<1, −0.1≦k≦0.2, 0≦l≦0.1), lithium cobalt-containing oxide having a layered structure represented by LiM 1-x Q xPO4 (wherein 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).

[0031] In order to cope with the increasing voltage of the battery, it is preferable that the positive electrode active material contains a layered lithium-cobalt-containing oxide.

[0032] 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 material on the surface of the positive electrode active material, which 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. As the lithium ion conductive coating material, a lithium niobium-containing oxide such as LiNbO3 is preferably used.

[0033] When the positive electrode mixture is pressure-molded into a molded body, or when a layered aggregate of sulfide-based solid electrolyte particles is pressure-molded to form a solid electrolyte layer, the positive electrode active material is preferably in the form of a powder of primary particles (positive electrode active material powder) so that cracks and the like are less likely to occur in the molded body of the positive electrode mixture even when a high pressure is applied.

[0034] By using a powder of primary particles as the positive electrode active material, the powder having a lithium ion conductive coating material formed on its surface can also be composed of primary particles.

[0035] If the average particle size of the positive electrode active material powder, which is composed of primary particles and has a lithium ion conductive coating material formed on the surface thereof, becomes too large, the particles may crack when pressed under high pressure, which may cause a reaction between the active material and the sulfide-based solid electrolyte. Therefore, the average particle size of the positive electrode active material powder is set to 8 μm or less, and preferably 6 μm or less.

[0036] On the other hand, if the average particle size of the positive electrode active material powder is too small, it becomes difficult to pack the particles at a high density, which leads to a decrease in the capacity of the positive electrode. Therefore, the average particle size of the positive electrode active material powder is set to 1 μm or more, and preferably 3 μm or more.

[0037] By using a positive electrode active material powder composed of primary particles within the above average particle size range, the porosity of the positive electrode mixture compact can be set to, for example, 10% or less.

[0038] The average particle size of the positive electrode active material powder can be measured using a particle size distribution analyzer (e.g., a particle size distribution analyzer manufactured by Microtrac-Bell, Inc., under the trade name Microtrac "MT3300EXII"), and is determined as the 50% diameter (d50) in the volume-based integrated fraction when determining the integrated volume from particles with small particle sizes. The same applies to the negative electrode active material and the solid electrolyte.

[0039] The positive electrode active material powder composed of the primary particles may be a commercially available product. The positive electrode active material powder is preferably composed of only primary particles, but may also contain some secondary particles. To prevent the above-mentioned problems, however, the proportion of secondary particles in the positive electrode active material powder is preferably 30% by mass or less, more preferably 15% by mass or less, and particularly preferably 10% by mass or less.

[0040] The lithium ion conductive coating material can be formed on the surface of the positive electrode active material by a sol-gel method, a mechanofusion method, a CVD method, a PVD method, or the like.

[0041] The amount of lithium ion conductive coating material formed is preferably an amount such that the ratio of the coating material to the entire positive electrode active material including the coating material is 0.1% by mass or more, more preferably 0.5% by mass or more, in order to increase the surface coverage of the positive electrode active material and facilitate its action. On the other hand, if the amount of lithium ion conductive coating material formed is too large, the capacity per unit weight of the positive electrode active material powder decreases, so the ratio of the coating material to the entire positive electrode active material is preferably an amount such that the ratio is 5% by mass or less, more preferably 2% by mass or less.

[0042] The content of the positive electrode active material powder in the positive electrode mixture is set to 65% by mass or more, and preferably 70% by mass or more, in order to increase the capacity of the positive electrode. On the other hand, the content of the positive electrode active material powder in the positive electrode mixture is set to 85% by mass or less, and preferably 80% by mass or less, in order to make the ionic conductivity and electronic conductivity of the positive electrode mixture excellent by setting the content of the solid electrolyte and the conductive additive in the positive electrode mixture to a certain level or more.

[0043] As the conductive additive for the positive electrode, carbon materials such as graphite (natural graphite, artificial graphite), graphene, carbon black, vapor grown carbon fiber (VGCF), carbon nanofiber, and carbon nanotube can be used.

[0044] For the positive electrode solid electrolyte, it is desirable to use a sulfide-based solid electrolyte in order to improve ionic conductivity. Examples of sulfide-based solid electrolytes include compounds composed of Li, S, and P, and optionally halogen elements or Si, such as Li2S-P2S5, Li2S-SiS2, Li2S-P2S5-GeS2, and Li2S-B2S3-based glasses; 10 GeP2S 12 (LGPS series); 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) and other compounds having an alligatorite-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.

[0045] 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 property of the molded body of the positive electrode mixture having the positive electrode active material powder of the above particle size and reduce the porosity.

[0046] 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, oxide-based solid electrolytes, etc.

[0047] Examples of the hydride-based solid electrolyte include, for example, 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 (LiI, LiBr, LiF, LiCl, etc.), rubidium halides (RbI, RbBr, RbiF, RbCl, etc.), cesium halides (CsI, CsBr, CsF, CsCl, etc.), lithium amide, rubidium amide, cesium amide, etc.

[0048] Examples of oxide-based solid electrolytes include Li7La3Zr2O 12 , LiTi(PO4)3, LiGe(PO4)3, LiLaTiO3, etc.

[0049] However, in order to prevent a decrease in the ionic conductivity of the positive electrode mixture, the proportion of solid electrolytes other than the sulfide-based solid electrolyte in the total amount of solid electrolytes used in the positive electrode mixture is preferably 30 mass% or less, more preferably 10 mass% or less, and particularly preferably contains no other solid electrolytes.

[0050] 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, while it is preferably 35% by mass or less, and more preferably 30% by mass or less, in order to ensure the content of the positive electrode active material powder.

[0051] 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 resistance component in the positive electrode mixture, it is desirable to minimize the amount of the resin binder contained therein. The content is preferably 0.5% by mass or less, more preferably 0.3% by mass or less, and most preferably 0% by mass (i.e., no resin binder is contained).

[0052] When a current collector is used for the positive electrode, the current collector may be made of a metal foil such as aluminum or stainless steel, a punched metal, a mesh, an expanded metal, a foamed metal, a carbon sheet, or the like.

[0053] The thickness of the positive electrode mixture compact is preferably 200 μm or more, more preferably 500 μm or more, from the viewpoint of increasing the capacity of the battery, while the thickness of the positive electrode mixture compact is preferably 2 mm or less, more preferably 1.2 mm or less, from the viewpoint of facilitating the formation of a compact with low porosity and improving load characteristics.

[0054] In the case of a positive electrode having a current collector, the thickness of the positive electrode mixture compact per one surface of the current collector may be set within the above range.

[0055] (Negative electrode) The negative electrode of the all-solid-state secondary battery has a molded body of a negative electrode mixture containing a negative electrode active material, a conductive additive, a solid electrolyte, and the like, and can be formed, for example, from the molded body alone, or can be formed by integrating the molded body with a current collector.

[0056] The negative electrode active material may be one or a mixture of two or more carbonaceous materials capable of absorbing and releasing lithium, such as graphite, pyrolytic carbons, cokes, glassy carbons, fired organic polymer compounds, mesocarbon microbeads (MCMB), and carbon fiber. Other examples of the negative electrode active material include: elements capable of forming alloys with lithium, such as Si, Sn, Ge, Bi, Sb, In, and Al; alloys of these elements with elements that do not form alloys with lithium; oxides and other compounds of these elements; lithium-containing nitrides; composite oxides such as lithium-titanium oxide having a spinel structure; and lithium metal.

[0057] The content of the negative electrode active material in the negative electrode mixture is preferably 45% by mass or more, more preferably 50% by mass or more, in order to increase the capacity of the negative electrode. On the other hand, in order to make the content of the solid electrolyte and the conductive additive in the negative electrode mixture at a certain level or more and to improve the ionic conductivity and electronic conductivity of the negative electrode mixture, the content of the negative electrode active material in the negative electrode mixture is preferably 65% by mass or less, more preferably 60% by mass or less.

[0058] As the conductive additive 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 additive 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 additive 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 additive in the negative electrode mixture is preferably 15% by mass or less, and more preferably 12% by mass or less.

[0059] 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 made, for example, 20% or less, preferably 15% or less.

[0060] 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 the other solid electrolytes in addition to 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.

[0061] However, in order to prevent a decrease in the ionic conductivity of the negative electrode mixture, the proportion of solid electrolytes other than the sulfide-based solid electrolyte in the total amount of solid electrolytes used in the negative electrode mixture is preferably 30 mass% or less, more preferably 10 mass% or less, and particularly preferably contains no other solid electrolytes.

[0062] 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, while it is preferably 55% by mass or less, and more preferably 50% by mass or less, in order to ensure the content of the negative electrode active material.

[0063] 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 resistance component in the negative electrode mixture, it is desirable to minimize the amount of the resin binder when it is contained. The content is preferably 0.5% by mass or less, more preferably 0.3% by mass or less, and most preferably 0% by mass (i.e., no resin binder is contained).

[0064] When a current collector is used for the negative electrode, the current collector may be made of copper or nickel foil, punched metal, mesh, expanded metal, foamed metal; carbon sheet; or the like.

[0065] From the viewpoint of increasing the capacity of the battery, the thickness of the negative electrode mixture compact is preferably 200 μm or more, and more preferably 500 μm or more. On the other hand, from the viewpoint of facilitating the formation of a compact with low porosity and improving load characteristics, the thickness of the negative electrode mixture compact is preferably 3 mm or less, and more preferably 2 mm or less. In the case of a negative electrode having a current collector, the thickness of the negative electrode mixture compact per one side of the current collector may be within the above range.

[0066] (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.

[0067] In order to reduce the porosity during pressure molding, the average particle diameter of the sulfide-based solid electrolyte is preferably 0.3 to 1.5 μm.

[0068] 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 is 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).

[0069] Furthermore, the solid electrolyte layer may have a porous body such as a resin non-woven fabric as a support.

[0070] 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.

[0071] 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 to ensure insulation between the positive electrode and the negative electrode, while the thickness of the solid electrolyte layer is 300 μm or less, more preferably 200 μm or less, and even more preferably 100 μm or less to reduce the internal resistance of the battery.

[0072] (Laminated electrode body) The compact of the positive electrode mixture is composed of, for example, a positive electrode mixture prepared by mixing a positive electrode active material powder, a conductive additive, a solid electrolyte, and a binder added as needed, and can be formed by pouring the positive electrode mixture into a mold to form a layer of a predetermined thickness, and then compressing it by pressure molding, etc. The compact of the negative electrode mixture is composed of, for example, a negative electrode mixture prepared by mixing a negative electrode active material, a conductive additive, a solid electrolyte, and a binder added as needed, and can be formed by pouring the negative electrode mixture into a mold to form a layer of a predetermined thickness, and then compressing it by pressure molding, etc.

[0073] The positive electrode mixture compact and the negative electrode mixture compact are used as electrodes (positive electrode and negative electrode) as they are or are attached to current collectors by pressure bonding or the like.

[0074] The 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 or the like. The solid electrolyte layer is used in the formed state for assembling a battery. The pressure applied during pressing can be 1000 MPa or more in terms of surface pressure. This allows for the formation of a solid electrolyte layer in which the porosity is higher in the peripheral region than in the central region, and the porosity of the peripheral region is 5% or less, without changing the pressing conditions between the central region and the peripheral region. In the above process, the amount of solid electrolyte particles introduced into the mold can be an amount that results in a thickness of 15 μm or more after pressure molding. When the solid electrolyte layer is pressure molded using a mold, the solid electrolyte particles are less likely to fill the region near the periphery of the mold due to friction with the mold wall. Therefore, the difference in porosity between the peripheral region and the central region of the solid electrolyte layer after pressure molding becomes larger. If the amount of solid electrolyte particles introduced into the mold increases and the thickness of the solid electrolyte layer increases, this difference between the peripheral region and the central region becomes more pronounced. Therefore, by setting the surface pressure inside the mold during pressure molding to 1000 MPa or more, it becomes possible to prevent the porosity of the peripheral part of the solid electrolyte layer after pressure molding from increasing, and to easily keep the difference between the porosity of the peripheral part and the porosity of the central part below a certain level (for example, 4% or less).

[0075] When the solid electrolyte layer is formed by applying pressure multiple times, the pressure applied in one of the multiple times may be 1000 MPa or more in terms of surface pressure. The pressure applied to the layered assembly of solid electrolyte particles may be 1200 MPa or more.

[0076] Alternatively, the solid electrolyte particles may be dispersed in a solvent and used in the form of a paint to form the solid electrolyte layer. The paint may be applied to a substrate, a positive electrode mixture, or a negative electrode mixture, dried to form a layered aggregate having a predetermined thickness, and then pressure-molded.

[0077] The solvent used to prepare the solid electrolyte coating is preferably one that is less likely to deteriorate the solid electrolyte. In particular, sulfide-based and hydride-based solid electrolytes undergo chemical reactions with trace amounts of water, so nonpolar aprotic solvents, such as hydrocarbon solvents like hexane, heptane, octane, nonane, decane, decalin, toluene, and xylene, are preferred. Ultra-dehydrated solvents with a water content of 0.001% by mass (10 ppm) or less are particularly preferred. Fluorine-based solvents such as "Vertrel®" from DuPont-Mitsui Fluorochemicals, "Zeorolla®" from Nippon Zeon, and "Novec®" from Sumitomo 3M can also be used, as well as nonaqueous organic solvents such as dichloromethane and diethyl ether.

[0078] The positive electrode mixture compact and the negative electrode mixture compact are stacked and integrated with a solid electrolyte layer interposed therebetween to form a laminated electrode body, which is used for assembling a battery.

[0079] The conditions for producing the positive electrode mixture compact can be, for example, as follows.

[0080] A cathode active material powder consisting of primary particles is mixed with a sulfide-based solid electrolyte or the like to prepare a cathode mixture containing the cathode active material powder in a proportion of 65 to 85 mass %. This mixture is then placed in a mold to form a layer of a predetermined thickness, and pressurized to produce a molded cathode mixture. The pressure during pressing is 1000 MPa (10 ton / cm) in terms of surface pressure. 2 ) or more. This makes it easy to make the porosity of the molded body 10% or less.

[0081] The negative electrode mixture compact can also be produced under the same conditions as those for producing the positive electrode mixture compact, and the pressure during pressing is set to 1000 MPa (10 ton / cm) as the surface pressure. 2 ) or more, the filling rate can be increased.

[0082] The positive electrode mixture layer and the negative electrode mixture layer may be pressed at once, or they may be pressed at a pressure of less than 1000 MPa to increase the filling rate of the mixture to a certain extent, and then further pressed at a pressure of 1000 MPa or more to form the desired molded body.

[0083] The order in which the layers are stacked is not limited. For example, a layer of anode mixture may be formed first, a layer of solid electrolyte powder may be formed thereon, and a layer of cathode mixture may be further formed thereon to form a laminate. Alternatively, a layer of solid electrolyte powder may be formed first, and one of anode mixture and cathode mixture layers may be formed on one side of the layer of solid electrolyte powder, and the other of anode mixture and cathode mixture layers may be formed on the other side of the layer of solid electrolyte powder to form a laminate.

[0084] Alternatively, the positive electrode mixture compact, the solid electrolyte layer, and the negative electrode mixture compact may be separately prepared in advance and then laminated together to form an integrated body. For example, the positive electrode mixture compact, the solid electrolyte layer, and the negative electrode mixture compact may each be separately prepared in advance by pressure molding.

[0085] The surface pressure when pressing each layer is usually set to 2000 MPa (20 ton / cm) considering the constraints of the manufacturing equipment and productivity. 2 ) can be applied, and the lower limit of the porosity of the actually obtained positive electrode mixture compact and negative electrode mixture compact is thought to be about 5%.

[0086] The porosity of the positive electrode mixture and negative electrode mixture compacts referred to in this specification is a value calculated by finding the sum for each component i using the following formula (1) from the thickness of the compact, the mass per area, and the density of the constituent components.

[0087] P = 100-(Σai / ρi)×(m / t) (1)

[0088] In the formula (1), ai: the ratio of component i expressed in mass%, ρi: the density of component i (g / cm 3 ), m: mass per unit area of molded body (g / cm 2), t: thickness of the molded body (cm).

[0089] Alternatively, the cross sections of the positive electrode mixture and negative electrode mixture compacts may be observed with a scanning electron microscope (SEM), and the image may be binarized by image processing to determine the proportion of voids, and the porosity of the positive electrode mixture compact may be calculated. In this case, it is desirable to determine the conditions for image processing so that the value obtained is consistent with the porosity determined by the above method.

[0090] The porosity of the central and peripheral parts of the solid electrolyte layer is measured as follows: The solid electrolyte layer is cut along a plane that passes through the longest line and is parallel to the thickness direction when viewed from the thickness direction. SEM images of the peripheral and central parts of the cut cross section are obtained.

[0091] FIG. 1 is a diagram illustrating the regions (peripheral and central regions) where the porosity of the solid electrolyte layer is measured. The example shown in FIG. 1 illustrates a cross section of an all-solid-state secondary battery including 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. When viewed in the thickness direction (z direction in FIG. 1), the solid electrolyte layer 30 is cut in the thickness direction along a line in the width direction (x direction) of the longest width. In the example shown in FIG. 1, the shape of the solid electrolyte layer 30 viewed in the thickness direction (top view shape) is circular. In this case, the solid electrolyte layer 30 is cut in the thickness direction along a line passing through the center of the circle. When the top view shape is elliptical, the cut is made along the major axis of the ellipse. When the top view shape is rectangular, the cut is made along the center line between the two long sides. In the example shown in FIG. 1, R1 is the peripheral region, and R2 is the central region.

[0092] The peripheral portion is a rectangular region having a width of 25 μm and a thickness of 19 μm, centered at a position s=100 μm from the edge of the cross section 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 portion is a rectangular region having 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 (the center in the thickness direction and the width direction), and including the center in the cross section of the layer (the center in the thickness direction and the width direction).

[0093] These SEM images of the peripheral and central areas are binarized using image analysis software (ImageJ, etc.), and the bright contrast (high brightness) is considered to represent the substance and the dark contrast (low brightness) is considered to represent the voids. The porosity is calculated by determining the area ratio of the voids.

[0094] The equipment and conditions used can be, for example, as follows: Scanning electron microscope: Hitachi S4800 Detector: Backscattered electron enhancement mode Accelerating voltage: 2 kV Magnification: 5,000x Capture resolution: 2,560 x 1,920 Minimum dot length: 9.9 nm (equivalent circle diameter: 11.2 nm) Minimum dot area: 98nm 2 Analysis area: 4.81×10 8 nm 2

[0095] The obtained SEM image is converted to grayscale using Photoshop (registered trademark). At this time, a Gaussian filter (0.5 pixels) is used to reduce noise. Next, the histogram of the SEM image is quantified using ImageJ. Figure 2 shows an example of a histogram of an SEM image. In the histogram, the brightness: a at the maximum frequency is set as the center value. On the higher brightness side than the center value, brightness: b is read at a frequency where the numerical value is 3% of the maximum frequency. Furthermore, on the lower brightness side than the center value, brightness: d (= a - b) where the numerical value is smaller than the center value by the difference between b and a: c (= b a) is found, and this brightness d is set as the threshold.

[0096] Furthermore, the image analysis software "Eizou-kun" binarizes the SEM image using the previously determined threshold value d to separate it into low-brightness and high-brightness areas. The ratio of the area of the low-brightness area in the SEM image is calculated as the porosity. 2 (0.09×10 -2 μm 2) are judged to be noise components, so they are excluded from the low-brightness areas when calculating the porosity.

[0097] The size of the rectangular region for acquiring an SEM image (backscattered electron image) can be changed depending on the thickness of the solid electrolyte layer if the solid electrolyte layer is thin. When the thickness of the solid electrolyte layer is approximately 24 μm or less, the length of the rectangular region in the thickness direction can 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 side interface and the region near the negative electrode side interface of the solid electrolyte layer.

[0098] The solid electrolyte layer is formed so that the porosity of the peripheral portion of the solid electrolyte layer measured as described above is 5% or less and is larger than the porosity of the central portion, thereby making the reaction of the all-solid-state battery more uniform and improving the load characteristics and cycle characteristics.

[0099] The porosity of the peripheral portion of the solid electrolyte layer measured as described above is preferably 4% or less, more preferably 3.5% or less, and even more preferably 3% or less. The porosity of the central portion of the solid electrolyte layer measured as described above is preferably 3% or less, more preferably 2% or less, and even more preferably 1% or less.

[0100] The thickness of the solid electrolyte layer and the thickness-wise roughness of the interface are measured as follows, and the values near the center are used. For the cross section of the solid electrolyte layer set when measuring the porosity, an SEM image is obtained, including a 100 μm region on each side from the center of the width direction of the cross section (a region with a total width of 200 μm), within a range that allows the roughness of the interface between the positive electrode and the solid electrolyte layer and the interface between the solid electrolyte layer and the negative electrode to be seen. Multiple SEM images are acquired while shifting the image in the width direction, and stitched together to form an image of the entire region. If necessary, multiple images can also be stitched together in the thickness direction. In the SEM image of the entire region obtained, as shown in FIG. 3, a line L1 perpendicular to the thickness direction is drawn at the interface between the positive electrode and the solid electrolyte layer, passing through the point closest to the solid electrolyte layer, and a line L2 perpendicular to the thickness direction is drawn at the point closest to the positive electrode. Similarly, at the interface between the negative electrode and the solid electrolyte layer, a line L3 is drawn perpendicular to the thickness direction and passes through the point closest to the solid electrolyte layer, and a line L4 is drawn perpendicular to the thickness direction and passes 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.

[0101] The thickness of the solid electrolyte layer measured as described above is 15 μm or more, which makes it possible to prevent short circuits even if the thickness of the solid electrolyte layer formed by pressure molding of sulfide-based solid electrolyte particles is uneven, i.e., even if some unevenness occurs at the interface of the solid electrolyte layer.

[0102] The height of the unevenness at the interface between the solid electrolyte layer and the positive electrode and the interface between the solid electrolyte layer and the negative electrode measured as described above may be 0.3 μm or more, thereby increasing the contact area between the positive electrode and the solid electrolyte layer and the negative electrode, and reducing the resistance at the interface.

[0103] At least one of the positive electrode mixture, the negative electrode mixture, and the composition for forming a solid electrolyte layer may be pressure-molded in a heated state. By molding the solid electrolyte material in a heated and softened state, the contact area between the active material and the solid electrolyte material or between the solid electrolyte materials increases, thereby improving the filling property of the molded body.

[0104] The heating temperature may be, for example, 60 to 200°C, although it depends on the composition of the solid electrolyte material.

[0105] (Battery type) The all-solid-state secondary battery of the present invention can be configured as, for example, a coin-type battery. Fig. 4 is a schematic cross-sectional view of a coin-type battery. The battery 1 shown in Fig. 4 has 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 enclosed in an exterior body formed by an exterior can 40, a sealing can 50, and a resin gasket 60 interposed between them.

[0106] 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 abut against the sealing can 50, thereby sealing the opening of the outer can 40 and creating an airtight structure inside the element.

[0107] The outer can and the sealing can can be made of stainless steel or other materials. The gasket can be made of polypropylene, nylon, or other materials. If heat resistance is required for the battery's intended use, heat-resistant resins with melting points exceeding 240°C, such as fluororesins such as tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA), polyphenylene ether (PEE), polysulfone (PSF), polyarylate (PAR), polyethersulfone (PES), polyphenylene sulfide (PPS), and polyetheretherketone (PEEK), can also be used. If the battery is intended for use in applications requiring heat resistance, a glass hermetic seal can be used for sealing.

[0108] The form of the all-solid-state secondary battery is not limited to a flat one having an exterior body composed of an exterior can, a sealing can, and a gasket as shown in FIG. 4, but may also be one having an exterior body composed of a resin film or a metal-resin laminate film, or one having an exterior body made of metal and having a tubular (cylindrical or rectangular) exterior can with a bottom and a sealing structure that seals the opening of the can.

[0109] The all-solid-state secondary battery of the present invention can be used in the same applications as conventionally known secondary batteries, but since it has a solid electrolyte instead of an electrolytic solution, it has excellent heat resistance and can be preferably used in applications where it is exposed to high temperatures. [Example]

[0110] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples.

[0111] Example 1 <Fabrication of laminated electrode body> The positive electrode active material powder (average particle size: 5 μm) consists of primary particles of LiCoO2 with a LiNbO3 coating layer formed on the surface, and a sulfide-based solid electrolyte (Li) with an average particle size of 3 μm. 7.0 PS 5.4 Cl 1.2 The powder was mixed with carbon black and vapor-grown carbon fiber (VGCF) in a mass ratio of 70:26.8:1.1:2.1 and thoroughly kneaded to prepare a positive electrode mixture. The weight of the coating layer accounted for 2 mass% of the total weight of the positive electrode active material powder including the coating layer.

[0112] In addition, lithium titanate (Li4Ti5O 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 and graphene (conductive additive) in a mass ratio of 50:41:9, and the mixture was thoroughly kneaded to prepare a negative electrode mixture.

[0113] Next, 92 mg of the positive electrode mixture was placed in a powder molding die and pressed at 100 MPa (1 ton / cm) using a press. 2 The positive electrode mixture was pressed under a pressure of 0.7 μm. A sulfide-based solid electrolyte (Li) with an average particle size of 0.7 μm was then placed on the positive electrode mixture. 5.4 PS 4.4 Cl 0.8 Br 0.8 16 mg of powder was added and the mixture was pressed at 100 MPa (1 ton / cm 2 Further, 129 mg of the negative electrode mixture was placed on the layered aggregate of the formed sulfide-based solid electrolyte particles to form a three-layer laminate, and then the laminate was pressed at a pressure of 1200 MPa (12 ton / cm) using a press. 2 ) to produce a laminated electrode body having a solid electrolyte layer with a thickness of 0.19 mm.

[0114] In the laminated electrode body, the porosity of the positive electrode mixture compact was 8%, and the porosity of the negative electrode mixture compact was 10%. Furthermore, the porosity of the peripheral portion and the porosity of the central portion were determined by performing the image processing on an SEM image of a cross section of the solid electrolyte layer taken at a magnification of 5000 times. The determined porosity was 3.4% for the peripheral portion and 1.0% for the central portion.

[0115] <Battery assembly>

[0116] A stainless steel sealing can and an outer can were used as exterior bodies, and porous carbon sheets each having a thickness of 0.1 mm were placed between the sealing can and the outer can and the laminated electrode body, respectively, to seal the battery, thereby assembling a coin-type all-solid-state secondary battery.

[0117] (Comparative Example 1) After forming the three-layer laminate, the pressure used for press molding was 600 MPa (6 ton / cm 2 ) was used, a laminated electrode body was produced in the same manner as in Example 1, and a coin-type all-solid-state secondary battery was assembled.

[0118] In the laminated electrode body, the porosity of the positive electrode mixture compact was 15%, and the porosity of the negative electrode mixture compact was 16%. Furthermore, the porosity of the cross section of the solid electrolyte layer, determined by image processing of an SEM image taken at 5000x magnification, was 5.8% in the center and 8.8% in the peripheral region.

[0119] (Comparative Example 2) 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 prepare a laminated electrode body so that the thickness of the solid electrolyte layer after pressure molding would be 10 μm.

[0120] The following evaluations were carried out for the all solid state secondary batteries of Example 1 and Comparative Example 1. The all solid state secondary battery of Comparative Example 2 was not evaluated because a short circuit had occurred.

[0121] <Load characteristic evaluation> The produced batteries of the examples and comparative examples were subjected to constant current-constant voltage charging, which combined constant current charging at a current value of 0.2 C until the battery voltage reached 3.1 V and constant voltage charging at a voltage of 3.1 V until the current value reached 0.02 C. Furthermore, constant current discharging was performed at a current value of 0.1 C until the battery voltage reached 1.2 V, and the discharge capacity at 0.1 C was measured.

[0122] Next, the same constant current-constant voltage charging as above was performed, and then constant current discharging was performed at a current value of 0.5 C until the battery voltage reached 1.2 V, and the discharge capacity at 0.5 C was measured. The discharge capacity at 0.5 C was divided by the discharge capacity at 0.1 C to obtain a value (%), and the load characteristics of each battery were evaluated.

[0123] <High temperature characteristic evaluation> The batteries of the examples and comparative examples were subjected to 50 charge / discharge cycles in an environment of 100° C. under the following conditions.

[0124] The battery was charged at a constant current of 0.2 C until the voltage reached 3.1 V, then charged at a constant voltage of 3.1 V until the current reached 0.02 C, and then discharged at a constant current of 0.2 C until the voltage reached 1.2 V. This charge-discharge cycle was repeated 50 times, and the cycle characteristics were evaluated based on the ratio of the decrease in discharge capacity at the 50th cycle to the discharge capacity at the 2nd cycle (capacity decrease rate). The evaluation results are shown in Table 1.

[0125] [Table 1]

[0126] In the all-solid-state secondary battery of Example 1, a solid electrolyte layer having a thickness of 15 μm or more was formed by pressure-molding sulfide-based solid electrolyte particles, and the porosity of the peripheral portion was set to 5% or less and larger than that of the central portion, resulting in a battery with excellent load characteristics and cycle characteristics.

[0127] On the other hand, in the battery of Comparative Example 1, the porosity of the solid electrolyte layer was larger in the peripheral part than in the center, but the porosity of the peripheral part exceeded 5%, resulting in a deterioration in load characteristics and cycle characteristics. Also, in the battery of Comparative Example 2, in which the thickness of the solid electrolyte layer was less than 15 μm, a short circuit occurred. [Explanation of symbols]

[0128] 1 All-solid-state secondary battery 10 positive electrode 20 negative electrode 30 Solid electrolyte layer 40 outer can 50 sealed cans 60 gaskets

Claims

[Claim 1] A method for producing 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, comprising: the negative electrode has a molded body of a negative electrode mixture containing a negative electrode active material powder, a conductive additive, and a solid electrolyte, the solid electrolyte layer has a thickness of 15 μm or more, and a porosity calculated from a scanning electron microscope (SEM) image of a cross section of the solid electrolyte layer is 5% or less at a peripheral portion, and the porosity of the peripheral portion is larger than the porosity of a central portion; The manufacturing method for an all-solid-state secondary battery includes a step of pressurizing a layered aggregate of sulfide-based solid electrolyte particles in a mold at a surface pressure of 1000 MPa or more.

Images