All-solid-state secondary battery and production method thereof

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 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 using 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 with 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 are attracting 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, Patent Document 1 shows that a short circuit between the positive electrode layer and the negative electrode layer can be suppressed by setting the porosity of the solid electrolyte layer to 10% or less and the surface roughness Rz1 of the positive electrode layer and the surface roughness Rz2 of the negative electrode layer to satisfy the relationship Rz1 + Rz2 ≦ 25.

[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 roughness of the surfaces of the positive electrode layer and the negative electrode layer, the positive electrode layer and the negative electrode layer are prevented from contacting each other, and by reducing the porosity of the solid electrolyte layer, the growth of dendrites is suppressed, thereby preventing a short circuit 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 mixture powder, and then this mixture powder is dispersed in a solvent to prepare a paste for forming a solid electrolyte layer. The paste for forming a 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 prepared 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 stacked on the electrolyte green sheet, and the binder is burned off at a temperature of 300°C or higher to assemble a coin-shaped 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 in which a solid electrolyte is dispersed in a solvent, a compound such as 1-hydroxyethyl-2-alkenylimidazoline is added as a dispersant to the solvent. This suppresses aggregation of the solid electrolyte and reduces the surface irregularities of the solid electrolyte layer formed by applying and drying the composition for forming a solid electrolyte layer. As a result, an all-solid-state lithium secondary battery can be assembled in which the surface roughness Ra at the interface between the positive electrode mixture layer and the solid electrolyte layer is 1.0 μm or less. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Patent No. 6729796 [Patent Document 2] Japanese Patent Application Publication No. 2020-161364 Summary of the Invention [Problem to be solved by the invention]

[0012] However, in the all-solid-state battery of Patent Document 1, although the porosity of the solid electrolyte layer can be reduced, in the case of materials that may be denatured at high temperatures, such as sulfide-based solid electrolytes, the green sheet cannot be heated to a temperature that burns off the binder. This causes the binder to remain in the solid electrolyte layer, reducing the ionic conductivity of the solid electrolyte layer and resulting in insufficient 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 only relatively large, at 10 μm or more. Therefore, if the thickness of the solid electrolyte layer is reduced, the charge / discharge reaction may become uneven depending on the location, potentially degrading cycle characteristics.

[0013] Furthermore, in the all-solid-state battery of Patent Document 2, the solid electrolyte needs to be made into a paint. In the case of a material that is easily reactive with moisture, such as a sulfide-based solid electrolyte, the moisture contained in the solvent used in making the paint reacts with the solid electrolyte, reducing the ionic conductivity of the solid electrolyte layer, which can easily lead to the problem of insufficient load characteristics being obtained.

[0014] The present invention has been made in view of the above circumstances, and an object of the present invention 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] 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 to 120 μm, a porosity of 3% or less at the center of a cross section of the solid electrolyte layer calculated from a scanning electron microscope (SEM) image, and heights of irregularities at the interfaces between the solid electrolyte layer and the positive electrode and the negative electrode are each 7 μm or less.

[0016] The method for producing an all-solid-state secondary battery of the present invention is characterized by comprising the steps of filling a mold with sulfide-based solid electrolyte particles without using a solvent to form a layered assembly of the sulfide-based solid electrolyte particles, and press-molding the layered assembly of the sulfide-based solid electrolyte particles at a surface pressure of 1000 MPa or more. [Effects of the Invention]

[0017] According to the present invention, it is possible to improve the load characteristics of an all-solid-state secondary battery having a solid electrolyte layer containing a sulfide-based solid electrolyte. [Brief explanation of the drawings]

[0018] [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

[0019] 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 to 120 μm, a porosity of 3% or less at the center of a cross section of the solid electrolyte layer calculated from a scanning electron microscope (SEM) image, and heights of irregularities at the interfaces between the solid electrolyte layer and the positive electrode and the negative electrode are each 7 μm or less.

[0020] The solid electrolyte layer containing a sulfide-based solid electrolyte of the all-solid-state secondary battery has a thickness of 120 μm or less. This configuration can reduce the internal resistance of the battery. Meanwhile, because the solid electrolyte layer has a thickness of 15 μm or more, it is possible to prevent short circuits due to contact between the positive electrode and the negative electrode even if 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 both approaches 7 μm.

[0021] Furthermore, in the cross section of the solid electrolyte layer, the porosity of a region (central portion) including the center in the width direction and the center in the thickness direction is 3% or less. By configuring the porosity of the central portion of the solid electrolyte layer in this manner, the packing rate of the solid electrolyte layer is increased, and ionic conductivity can be improved. Furthermore, the heights (height in the thickness direction) of the unevenness at the interface between the solid electrolyte layer and the positive electrode and the heights (height in the thickness direction) of the unevenness at the interface between the solid electrolyte layer and the negative electrode are each 7 μm or less. By configuring the interface of the solid electrolyte layer in this manner, even if the thickness of the solid electrolyte layer is thin, it is possible to prevent short circuits due to contact between the positive electrode and the negative electrode, and it is also possible to suppress variations in the thickness of the solid electrolyte layer, thereby making the battery reaction more uniform. As a result, the load characteristics of the all-solid-state secondary battery are improved.

[0022] As a result of the above, it is possible to provide all-solid-state secondary batteries with excellent load characteristics, which can contribute, for example, to the achievement of Goal 7 "Affordable and clean energy" and Goal 12 "Responsible consumption and production" of the Sustainable Development Goals (SDGs) advocated by the United Nations.

[0023] In general, the porosity of the solid electrolyte layer near the edges in the width direction tends to be greater than the porosity of the central portion. By setting the porosity of the region (periphery) 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 to a value close to the porosity of the central portion, the packing rate of the solid electrolyte layer as a whole can be increased and ionic 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 portion and the central portion of the solid electrolyte layer is preferably 4% or less, and more preferably 3% or less.

[0024] 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 central portion is calculated for a rectangular region of the aforementioned size that includes the width and thickness centers of the cross section of the solid electrolyte layer. The porosity of the peripheral portion is calculated for a rectangular region of the aforementioned size that is centered 100 μm from the edge in the width direction of the cross section of the solid electrolyte layer and includes the thickness center.

[0025] 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.

[0026] 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 when 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 both, for example, about 7 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.

[0027] 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.

[0028] 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.

[0029] The method for producing the all-solid-state secondary battery includes, for example, a step of filling a mold with sulfide-based solid electrolyte particles without using a solvent to form a layered assembly of the sulfide-based solid electrolyte particles, and a step of press-molding the layered assembly of the sulfide-based solid electrolyte particles at a surface pressure of 1000 MPa or more.

[0030] In the above manufacturing method, sulfide-based solid electrolyte particles are filled into a mold using a dry process that does not use a solvent. This prevents the reaction between the moisture 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, the sulfide-based solid electrolyte particles do not need to be heated to the binder's burnout temperature. This prevents the sulfide-based solid electrolyte particles from deteriorating due to heating. Furthermore, the inventors discovered that by dry-pressing a layered assembly of sulfide-based solid electrolyte particles filled into a mold 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 of 7 μm or less can be efficiently formed. The above manufacturing method provides an all-solid-state secondary battery having a solid electrolyte layer containing a sulfide-based solid electrolyte and excellent load characteristics.

[0031] 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 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.

[0032] From the viewpoint of manufacturing efficiency, it is preferable to pressure-form the layered aggregate of sulfide-based solid electrolyte particles before laminating them on the positive electrode and the negative electrode, because this makes it easier to adjust the height of the irregularities at the interface between the solid electrolyte layer and the positive electrode and at 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 assembly of sulfide-based solid electrolyte particles, a second pressure-molding step of stacking a negative electrode material on one side of the pressure-molded layered assembly of sulfide-based solid electrolyte particles and pressure-molding the resulting product, and a third pressure-molding step of stacking a positive electrode material on the other side of the pressure-molded layered assembly of sulfide-based solid electrolyte particles and pressure-molding the resulting product. In this case, pressure molding may be performed at a surface pressure of 1000 MPa or more in at least one of the first pressure-molding step, the second pressure-molding step, and the third pressure-molding step. This makes it easier to adjust the height of the irregularities at the interfaces between the solid electrolyte layer and the positive electrode and the negative electrode to 7 μm or less.

[0034] 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.

[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 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.

[0036] 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).

[0037] 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.

[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 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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 with 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 alluaudite-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 also 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.

[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, oxide-based solid electrolytes, etc.

[0053] 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.

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

[0055] 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.

[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, 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.

[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 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).

[0058] 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.

[0059] 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.

[0060] 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.

[0061] (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.

[0062] 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.

[0063] 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.

[0064] As the conductive assistant 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 assistant 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 assistant 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 assistant 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 pressure molding described later 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 the other solid electrolytes in addition to the sulfide-based solid electrolyte include hydride-based solid electrolytes, oxide-based solid electrolytes, etc. The same solid electrolytes as those exemplified for the positive electrode mixture can be used for these solid electrolytes.

[0067] 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.

[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, 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.

[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 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).

[0070] 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.

[0071] 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.

[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 ionic 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 ionic 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 size 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 ionic 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 ionic 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 to ensure insulation between the positive electrode and the negative electrode, while 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 to reduce the internal resistance of the battery.

[0077] (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.

[0078] 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.

[0079] The solid electrolyte layer can be formed, for example, by placing solid electrolyte particles in 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 particles are filled into a mold in the absence of either a solvent or a binder. That is, the solid electrolyte particles are filled into a mold in the form of a layered aggregate of particles without any solvent or binder, and then pressurized. The solid electrolyte layer is used in the battery assembly in the pressed compact state. The pressure during pressing may be 1000 MPa or more in terms of surface pressure. This makes it easy to reduce the porosity of the solid electrolyte layer and the height of the interfacial irregularities.

[0080] 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.

[0081] 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.

[0082] The conditions for producing the positive electrode mixture compact can be, for example, as follows.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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 formed thereon, and a layer of cathode mixture formed thereon to form a laminate. Alternatively, a layer of solid electrolyte may be formed first, and one of a layer of anode mixture and a layer of cathode mixture may be formed on one side of the layer of solid electrolyte, and the other of a layer of anode mixture and a layer of cathode mixture may be formed on the other side of the layer of solid electrolyte to form a laminate.

[0087] 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.

[0088] From the viewpoint of manufacturing efficiency, it is preferable to first prepare a solid electrolyte layer by pressing a layered assembly of solid electrolyte particles at a surface pressure of 1000 MPa, and then separately prepare a layer of anode material mixture and a layer of cathode material mixture by pressure molding on both sides of the solid electrolyte layer. In this case, the height of the unevenness at the interfaces between the solid electrolyte layer and the cathode and anode can be easily reduced, and a homogeneous solid electrolyte layer can be easily formed.

[0089] 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%.

[0090] 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.

[0091] P = 100-(Σai / ρi)×(m / t) (1)

[0092] 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).

[0093] 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.

[0094] 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.

[0095] FIG. 1 is a diagram illustrating the regions (peripheral and central regions) for measuring the porosity of the solid electrolyte layer. 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.

[0096] 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).

[0097] 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.

[0098] 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

[0099] 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 - c) where the numerical value is smaller than the center value by the difference between b and a: c (= b a ), is found, and the brightness d is set as the threshold.

[0100] 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.

[0101] 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.

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

[0103] The porosity of the peripheral portion 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 interfacial irregularities are measured as follows, using values ​​near the center. For the cross section of the solid electrolyte layer set when measuring the porosity, SEM images are taken of 100 μm regions on both sides of the center of the cross section (a region 200 μm long in the width direction as a whole) so that the irregularities at the interface between the positive electrode and the solid electrolyte layer and the interface between the solid electrolyte layer and the negative electrode can be seen. Multiple SEM images are acquired while shifting the image in the width direction, and then stitched together to form an image of the entire region. In the SEM image of the entire region, as shown in FIG. 3, a line L1 is drawn perpendicular to the thickness direction 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 is drawn perpendicular to the thickness direction at the point closest to the positive electrode. Similarly, a line L3 is drawn perpendicular to the thickness direction at the interface between the negative electrode and the solid electrolyte layer, passing through the point closest to the solid electrolyte layer, and a line L4 is drawn perpendicular to the thickness direction at the point closest to the negative electrode. In this case, the distance T1 between L1 and L3 is the thickness of the solid electrolyte layer. The distance H1 between lines L1 and L2 is the height of the unevenness in the thickness direction of the interface between the positive electrode and the solid electrolyte layer. The distance H2 between lines L3 and L4 is the height of the unevenness in the thickness direction of 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, which makes it possible to prevent 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., even if there is some unevenness in the interface of the solid electrolyte layer.

[0106] The height of the unevenness at the interface between the solid electrolyte and the positive electrode and the height of the unevenness at the interface between the solid electrolyte and the negative electrode measured as described above are both set to 7 μm or less, preferably 5 μm or less, and more preferably 3 μm or less, in order to make the charge / discharge reactions of the positive electrode and the negative electrode uniform.

[0107] 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.

[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 denaturation of the sulfide-based solid electrolyte due to heating, the heating temperature in the solid electrolyte layer formation step is preferably 200° C. or less, more preferably 150° C. or less, and even more preferably 100° C. or less.

[0109] (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.

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

[0111] 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.

[0112] 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.

[0113] 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]

[0114] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples.

[0115] Example 1 <Fabrication of laminated electrode body> 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 powder was placed in a powder molding die and pressed under pressure of 70 MPa (0.7 tf / cm 2) to form a preformed layer of the solid electrolyte.

[0116] Next, 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 0.8 101 mg of a negative electrode mixture prepared by mixing graphene (conductive additive) in a mass ratio of 50:41:9 was added, and the pressure was 300 MPa (3 tf / cm 2 ) to prepare a laminate in which a pre-molded layer of the negative electrode was formed on one side of a pre-molded layer of the solid electrolyte.

[0117] Furthermore, after the mold having the laminated body is turned upside down, LiCoO2 (positive electrode active material) having an average particle size of 5 μm and having a LiNbO3 coating layer formed on the surface of the preformed layer of the solid electrolyte placed on the upper side, and a sulfide-based solid electrolyte: Li 7.0 PS 5.4 Cl 1.2 73 mg of a positive electrode 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 battery was subjected to a pressure of 1300 MPa (13 tf / cm 2 The negative electrode preformed layer, the solid electrolyte preformed layer, and the positive electrode mixture were compressed by pressure molding at a pressure of 1000 kJ / cm 2 , thereby obtaining a laminated electrode body in which the negative electrode layer, the solid electrolyte layer having a thickness of 110 μm, and the positive electrode layer were integrated. The weight of the coating layer accounted for 2% by mass of the total weight of the positive electrode active material powder including the coating layer.

[0118] The porosity of the peripheral portion and the central portion of the solid electrolyte layer were determined by image processing of SEM images taken at 5000x magnification. The porosity of the peripheral portion was 2.9%, and the porosity of the central portion was 0.8%. The heights of the irregularities at the interfaces between the solid electrolyte layer and the positive and negative electrodes were 3.9 μm and 4.4 μm, respectively.

[0119] <Battery assembly>

[0120] 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.

[0121] (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.

[0122] The porosity of the cross section of the solid electrolyte layer, determined by image processing of SEM images taken at 5000x magnification, was 5.2% in the center and 7.9% in the peripheral area. The heights of the 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 at 300 MPa (3 tf / cm) using a press. 2 ) to form a preformed layer for the negative electrode. Next, a sulfide-based solid electrolyte having an average particle size of 0.7 μm: Li 5.4 PS 4.4 Cl 0.8 Br 0.8 9.6 mg of powder was added and the pressure was 70 MPa (0.7 tf / cm 2 ) to prepare a laminate in which a preformed layer of solid electrolyte was formed on one of the preformed layers of negative electrode. Furthermore, 73 mg of the positive electrode mixture prepared in Example 1 was placed on the preformed layer of the solid electrolyte, and the pressure was increased to 1300 MPa (13 tf / cm 2The provisionally formed layer of the negative electrode, the provisionally formed layer of the solid electrolyte, and the positive electrode mixture were compressed by performing pressure molding at a pressure of 1000 kJ / cm 2 , thereby obtaining 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 an SEM image taken at 5000x magnification, was 1.0% in the center and 3.3% in the periphery. The heights of the irregularities at the interfaces between the solid electrolyte layer and the positive and negative electrodes were 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 prepare a laminated electrode body so that the thickness of the solid electrolyte layer after pressure molding would be 10 μm.

[0126] The following evaluations were carried out on the all solid state secondary batteries of Example 1 and Comparative Examples 1 and 2. The all solid state secondary battery of Comparative Example 3 was not evaluated because a short circuit had occurred.

[0127] <Load characteristic evaluation> The batteries prepared in Example 1 and Comparative Examples 1 and 2 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.

[0128] 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.

[0129] <Cycle 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.

[0130] 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.

[0131] [Table 1]

[0132] 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, the porosity of the central portion was set to 35% or less, and the heights of the irregularities at the interfaces between the solid electrolyte layer and the positive electrode and the negative electrode were each 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 of the central part of the solid electrolyte layer exceeded 5%, resulting in a decrease in load characteristics and cycle characteristics. In the battery of Comparative Example 2, the height of the unevenness at the interfaces between the solid electrolyte layer and the positive and negative electrodes exceeded 7 μm, resulting in a decrease in cycle characteristics. In addition, in the battery of Comparative Example 3, in which 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 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 solid electrolyte layer has a thickness of 15 μm or more and 120 μm or less, a porosity calculated from a scanning electron microscope (SEM) image at a central portion of a cross section of the solid electrolyte layer is 3% or less, and heights of irregularities at interfaces between the solid electrolyte layer and the positive electrode and between the solid electrolyte layer and the negative electrode are each 7 μm or less; a step of filling a mold with sulfide-based solid electrolyte particles without using a solvent to form a layered assembly of the sulfide-based solid electrolyte particles; A method for producing an all-solid-state secondary battery, comprising the step of press-molding the layered aggregate of sulfide-based solid electrolyte particles at a surface pressure of 1000 MPa or more.

Images