All-solid-state secondary batteries

Abstract

The present invention provides an all-solid-state secondary battery which is characterized by being provided with a sintered body that comprises a positive electrode layer, a negative electrode layer, and a solid electrolyte layer that is interposed between the positive electrode layer and the negative electrode layer. This all-solid-state secondary battery is also characterized in that: the solid electrolyte layer comprises a main phase that is composed of an electrolyte represented by formula (1), and a secondary phase that is composed of a boron-containing compound which contains lithium, boron and oxygen; and at least a part of the secondary phase is in contact with a part of the interface between the negative electrode layer and the solid electrolyte layer.　Formula (1): Li3+xSixP1-xO4 (In formula (1), x satisfies 0 < x < 1.)

Description

[Technical Field]

[0001] The present invention relates to an all-solid-state secondary battery. This application claims priority based on Japanese Patent Application No. 2022-058652, filed on March 31, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, electronics technology has made remarkable progress, leading to efforts to make portable electronic devices smaller, lighter, thinner, and more multifunctional. Accordingly, there is a strong demand for batteries, which serve as the power source for electronic devices, to be smaller, lighter, thinner, and more reliable. Therefore, all-solid-state secondary batteries using solid electrolytes have attracted attention. Conventionally, sulfide-based solid electrolytes and oxide-based solid electrolytes have mainly been used as solid electrolytes for all-solid-state secondary batteries.

[0003] Sulfide-based solid electrolytes have excellent plasticity, making it possible to form an interface between the solid electrolyte and the active material by compacting them. However, sulfide-based solid electrolytes have safety issues because they generate hydrogen sulfide when they react with water.

[0004] On the other hand, oxide-based solid electrolytes are safe because they do not react with water to generate hydrogen sulfide. However, in order to obtain high ionic conductivity in oxide-based solid electrolytes, they must be densified, which requires sintering at high temperatures. However, during sintering to form a dense solid electrolyte layer, the oxide-based solid electrolyte and the active material may react, resulting in the formation of a reaction layer with low ionic conductivity at the interface between the oxide-based solid electrolyte and the active material.

[0005] Even when sintered at high temperatures to densify, LSPO (Li 3+x Si x P 1-x However, LSPO has an ionic conductivity of 1×10 -6However, there is a problem in that the optical density is low, on the order of S / cm (see Non-Patent Document 2). One known technique for improving the ionic conductivity of LSPO is to add boron as a sintering aid. For example, Non-Patent Document 1 describes that adding lithium borate to LSPO promotes sintering and improves ionic conductivity.

[0006] Furthermore, Patent Document 1 describes an all-solid-state lithium ion secondary battery having low internal resistance and large capacity, in which a compound consisting of lithium and boron is contained in the positive electrode layer, the negative electrode layer, and the solid electrolyte layer. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent No. 6455807 (B) [Non-patent literature]

[0008] [Non-Patent Document 1] Solid State Ionics 283(2015)109-114 [Non-patent document 2] Ionics 7(2001)469-473 Summary of the Invention [Problem to be solved by the invention]

[0009] Conventionally, there has been a demand for all-solid-state secondary batteries to have a larger capacity. The present invention has been made in view of the above problems, and an object of the present invention is to provide an all-solid-state secondary battery with a large capacity. [Means for solving the problem]

[0010] In order to solve the above problems, the following means are provided.

[0011] [1] A sintered body including a positive electrode layer, a negative electrode layer, and a solid electrolyte layer sandwiched between the positive electrode layer and the negative electrode layer, wherein the solid electrolyte layer includes a main phase composed of an electrolyte represented by the following formula (1), and a sub-phase composed of a boron-containing compound containing lithium, boron, and oxygen, and at least a part of the sub-phase is in contact with a part of the interface between the negative electrode layer and the solid electrolyte layer. A all-solid-state secondary battery characterized by this. Li 3+x Si x P 1-x O4(1) (Formula (1) satisfies 0 < x < 1.) [2] The all-solid-state secondary battery according to [1], wherein the volume ratio of the sub-phase in the solid electrolyte layer is 1% by volume or more and 10% by volume or less. [3] The all-solid-state secondary battery according to [1] or [2], wherein the sub-phase is formed of particles having a particle size of 0.1 μm to 3 μm. [[ID=​​​​​​​​​​​​​​​​​[Figure 1] It is a schematic cross-sectional view showing an example of the all-solid-state secondary battery of the present invention. [Figure 2] It is a schematic cross-sectional view showing an enlarged part of the all-solid-state secondary battery 10 shown in FIG. 1. [Figure 3] It is a schematic cross-sectional view showing another example of the all-solid-state secondary battery of the present invention.

Embodiments for Carrying Out the Invention

[0014] In order to solve the above problems and provide a high-capacity all-solid-state secondary battery, the present inventor focused on the solid electrolyte layer and conducted intensive studies as follows. First, the present inventor considered using an electrolyte represented by the following formula (1) and a boron-containing compound as materials for the solid electrolyte layer. Li 3+x Si x P 1-x O4(1) (Formula (1) satisfies 0 < x < 1.)

[0015] Non-Patent Document 1 describes an all-solid-state lithium-ion secondary battery using an electrolyte represented by formula (1), a lithium compound Li2CO3, and a boron compound H3BO4 as materials for the solid electrolyte layer. However, with the technology described in Non-Patent Document 1, an all-solid-state secondary battery with a sufficiently large capacity could not be obtained.

[0016] Therefore, the present inventors focused on the chemical composition and distribution of the boron-containing compound contained in the solid electrolyte layer and conducted further studies. As a result, it was found that the solid electrolyte layer may include a main phase containing an electrolyte represented by formula (1) and a sub-phase composed of a boron-containing compound containing lithium, boron, and oxygen, and the sub-phase may be present in contact with a part of the interface between the negative electrode layer and the solid electrolyte layer, leading to the present invention.

[0017] In an all-solid-state secondary battery having such a solid electrolyte layer, the subphase present at a part of the interface between the negative electrode layer and the solid electrolyte layer serves as the starting point for the reaction of lithium ions, which is presumably the result of this all-solid-state secondary battery having a large capacity.

[0018] Incidentally, Non-Patent Document 1 does not describe an all-solid-state secondary battery manufactured using a boron-containing compound containing lithium, boron, and oxygen, such as lithium borate, as a material for the solid electrolyte layer, nor does it describe the capacity of the all-solid-state secondary battery. Thus, the capacity improvement effect of an all-solid-state secondary battery using a boron-containing compound containing lithium, boron, and oxygen as a material for the solid electrolyte layer has not previously been confirmed.

[0019] The all-solid-state secondary battery of this embodiment will be described in detail below with reference to the drawings as appropriate. The drawings used in the following description may show characteristic portions enlarged for the sake of clarity of the features of the present invention. Therefore, the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate modifications can be made within the scope of the present invention.

[0020] [All-solid-state secondary battery] Fig. 1 is a schematic cross-sectional view showing an example of an all-solid-state secondary battery of the present invention, and Fig. 2 is a schematic cross-sectional view showing an enlarged portion of the all-solid-state secondary battery shown in Fig. 1. In Fig. 1, reference numeral 10 denotes the all-solid-state secondary battery of this embodiment. The all-solid-state secondary battery 10 shown in Fig. 1 has a laminate 4, a first external terminal 5, and a second external terminal 6. The first external terminal 5 and the second external terminal 6 are formed of a conductive material. The first external terminal 5 and the second external terminal 6 are in contact with opposing surfaces of the laminate 4, respectively. The first external terminal 5 and the second external terminal 6 extend in a direction intersecting (orthogonal to) the laminate surface of the laminate 4.

[0021] The laminate 4 has a positive electrode layer 1, a negative electrode layer 2, and a solid electrolyte layer 3 sandwiched between the positive electrode layer 1 and the negative electrode layer 2. The laminate 4 is a sintered body formed by laminating and sintering the positive electrode layer 1 and the negative electrode layer 2 via the solid electrolyte layer 3. The number of layers of the positive electrode layer 1 and the negative electrode layer 2 included in the laminate 4 may be one layer each, or may be two or more layers. As shown in FIG. 1, the solid electrolyte layer 3 is not only between the positive electrode layer 1 and the negative electrode layer 2, but also between the positive electrode layer 1 and the second external terminal 6, and between the negative electrode layer 2 and the first external terminal 5. Further, as shown in FIG. 1, one end of the positive electrode layer 1 is connected to the first external terminal 5. One end of the negative electrode layer 2 is connected to the second external terminal 6.

[0022] The all-solid-state secondary battery 10 is charged or discharged by the transfer of ions through the solid electrolyte layer 3 between the positive electrode layer 1 and the negative electrode layer 2. In the present embodiment, the laminated all-solid-state secondary battery 10 shown in FIG. 1 will be described as an example.

[0023] "Solid electrolyte layer" The solid electrolyte layer 3 can move ions by an externally applied electric field. The solid electrolyte layer 3, for example, conducts lithium ions and inhibits the movement of electrons. The solid electrolyte layer 3 includes a main phase containing a solid electrolyte represented by the following formula (1) and a sub-phase composed of a boron-containing compound containing lithium, boron, and oxygen. Li 3+x Si x P 1-x O4(1) (Formula (1) satisfies 0 < x < 1.)

[0024] The main phase contains a solid electrolyte represented by formula (1). The solid electrolyte represented by formula (1) is a relatively stable compound with respect to the active material even when sintered at a high temperature for densification. Therefore, the all-solid-state battery 10 of the present embodiment is less likely to deteriorate and has excellent reliability. The solid electrolyte represented by formula (1) only needs to satisfy 0 < x < 1, and in order to become a more stable solid electrolyte with respect to the active material, it is more preferable to satisfy 0.3 ≦ x ≦ 0.8. The solid electrolyte represented by formula (1) forming the main phase may be only one type, or may be two or more types of solid electrolytes with different x values in formula (1), and can be appropriately determined according to requirements such as the characteristics required for the all-solid-state battery 10.

[0025] The main phase may consist only of the solid electrolyte represented by formula (1). The main phase may contain a compound other than the electrolyte represented by formula (1) within a range that does not impair the effects of the present invention. Examples of the compound other than the electrolyte represented by formula (1) include known compounds used as solid electrolytes. Specifically, a part of P in formula (1) may be substituted by at least one element selected from the group consisting of Ti, Co, Mn, and Ni.

[0026] The main phase may contain, for example, a sintering aid, a compound and / or an element derived from an active material material forming the positive electrode layer 1 and / or the negative electrode layer 2.

[0027] The subphase is composed of a boron-containing compound containing lithium, boron, and oxygen. The boron-containing compound forming the subphase may be any compound containing lithium, boron, and oxygen, and may include elements such as phosphorus and carbon in addition to lithium, boron, and oxygen. The boron-containing compound is preferably a sintered body, such as Li3BO3, LiBO2, B4Li2O7, or LiBPO4. The boron-containing compound may be a glass containing lithium, boron, phosphorus, and oxygen. The boron-containing compound forming the subphase is preferably Li3BO3, as this results in an all-solid-state secondary battery 10 with a larger capacity. The boron-containing compound forming the subphase may be one type or two or more types, and can be appropriately determined depending on the characteristics required of the all-solid-state secondary battery 10.

[0028] In this embodiment, for manufacturing reasons such as improving productivity and yield when forming the solid electrolyte layer 3, a sintering aid may be used in addition to the electrolyte represented by formula (1) and the compound that becomes the boron-containing compound containing lithium, boron, and oxygen as the material for the solid electrolyte layer 3. In this case, the main phase of the solid electrolyte layer 3 may contain compounds and / or elements derived from the sintering aid that are not the compound containing lithium, boron, and oxygen, which remain unremoved during a firing process or the like to produce the laminate 4. In this case, the subphase may contain the boron-containing compound containing lithium, boron, and oxygen, and compounds and / or elements derived from the sintering aid that are not the compound containing lithium, boron, and oxygen, which remain unremoved during a firing process or the like to produce the laminate 4.

[0029] Any known sintering aid can be used as long as it has the effect of improving sinterability. Specific examples of the sintering aid include compounds containing zinc (Zn), bismuth (Bi), etc.

[0030] As shown in FIG. 2, the solid electrolyte layer 3 in the all-solid-state secondary battery 10 of this embodiment is composed of a main phase 31 formed in a layer shape over the entire area between the positive electrode layer 1 and the negative electrode layer 2, and a plurality of particulate subphases 32. In the solid electrolyte layer 3 of the all solid state secondary battery 10 of this embodiment, the subphase 32 is present in contact with part of the interface between the anode layer 2 and the solid electrolyte layer 3. As a result, the all solid state secondary battery 10 of this embodiment has a larger capacity than, for example, an all solid state secondary battery in which the subphase is present over the entire surface of the interface with the anode layer 2 in the solid electrolyte layer 3, and an all solid state secondary battery in which the subphase is present approximately uniformly over the entire solid electrolyte layer 3. The reason why it becomes a large-capacity all-solid-state secondary battery is not clear, but it is presumed that, for example, the subphase 32 in the solid electrolyte layer 3 functions as a starting point for the reaction in the transfer of lithium ions.

[0031] In the all-solid-state secondary battery 10 of this embodiment, it is preferable that all of the subphases 32 contained in the solid electrolyte layer 3 are present in contact with the interface between the anode layer 2 and the solid electrolyte layer 3. In this case, all of the subphases 32 function as starting points for reactions in the transfer of lithium ions in the solid electrolyte layer 3, resulting in an all-solid-state secondary battery 10 with a larger capacity. In the all-solid-state secondary battery 10 of this embodiment, subphases 32 that are not in contact with the interface between the anode layer 2 and the solid electrolyte layer 3 may be present within a range in which the effects of the present invention can be obtained. The subphase 32 may be composed of a plurality of particles (lumps). A portion of the group of the plurality of particles may be in contact with the interface between the solid electrolyte layer 3 and the positive electrode layer 1. In this case, the proportion (volume proportion) of the subphase particles in contact with a portion of the interface between the negative electrode layer and the solid electrolyte layer may be greater than the proportion of the subphase particles in contact with a portion of the interface between the positive electrode layer and the solid electrolyte layer. Of the subphase particles contained in the solid electrolyte layer, the ratio of the proportion (volume proportion) of subphase particles in contact with a portion of the interface between the negative electrode layer and the solid electrolyte layer to the proportion (volume proportion) of subphase particles in contact with a portion of the interface between the positive electrode layer and the solid electrolyte layer may be 1 or more, or may be 1.5 or more.

[0032] Furthermore, in the solid electrolyte layer 3 of the all solid state secondary battery 10 of this embodiment, the main phase 31 is formed in a layer shape over the entire region between the positive electrode layer 1 and the negative electrode layer 2, and therefore the subphase 32 is not in contact with the positive electrode layer 1. As a result, the subphase 32 functions more effectively as a starting point for a reaction in the transfer of lithium ions in the solid electrolyte layer 3, resulting in an all solid state secondary battery 10 with a larger capacity.

[0033] The shape and size of the subphase 32 contained in the solid electrolyte layer 3 are not particularly limited, and may be, for example, flaky or particulate as shown in Fig. 2. When multiple subphases 32 are present in the solid electrolyte layer 3 as shown in Fig. 2, the shapes and sizes of the multiple subphases 32 may be substantially uniform or non-uniform. As shown in FIG. 2, when a plurality of particulate subphases 32 are scattered along the interface between the solid electrolyte 3 and the anode layer 2, the subphases 32 function more effectively as starting points for reactions in the transfer of lithium ions in the solid electrolyte layer 3, resulting in an all-solid-state secondary battery 10 with a larger capacity.

[0034] The subphase 32 present in the solid electrolyte 3 is preferably particles with an average particle size of 0.1 μm to 3 μm, and more preferably particles with an average particle size of 0.1 μm to 1 μm. When the subphase 32 is particles with an average particle size of 0.1 μm to 3 μm, the specific surface area of ​​the subphase 32 is sufficiently large. Therefore, the subphase 32 functions more effectively as a starting point for the reaction in the transfer of lithium ions. Furthermore, when the subphase 32 is particles with an average particle size of 0.1 μm to 3 μm, the subphase 32 is less likely to inhibit ion conduction in the main phase 31. For these reasons, when the subphase 32 is particles with an average particle size of 0.1 μm to 3 μm, the all-solid-state secondary battery 10 has an even greater capacity.

[0035] The volume fraction of the subphase 32 contained in the solid electrolyte layer 3 is preferably 1% by volume or more and 10% by volume or less, more preferably 1% by volume or more and 5% by volume or less, and even more preferably 1% by volume or more and 3% by volume or less. When the volume fraction of the subphase 32 is 1% by volume or more, the subphase 32 is sufficiently present in the solid electrolyte layer 3, and the subphase 32 functions more effectively as a starting point for the reaction in the transfer of lithium ions. This results in an all-solid-state secondary battery 10 with a larger capacity. Furthermore, when the volume fraction of the subphase 32 is 1% by volume or more, the subphase 32, which is formed by sintering and bonding compounds that become a boron-containing compound containing lithium, boron, and oxygen, is likely to form particles with a particle size of 0.1 μm or more, which is preferable. Furthermore, when the volume fraction of the subphase 32 is 10% by volume or less, the compounds that become a boron-containing compound containing lithium, boron, and oxygen are sintered and bonded, and thus the subphase 32 that is separated in the plane direction is likely to be formed. As a result, a solid electrolyte layer 3 is easily formed in which a plurality of particulate subphases 32 are scattered along the interface between the solid electrolyte 3 and the negative electrode layer 2, and the main phase 31 is disposed between adjacent subphases 32 and between the subphases 32 and the positive electrode active material layer 1B, resulting in a larger capacity all-solid-state secondary battery 10. Furthermore, when the volume ratio of the subphases 32 is 10% by volume or less, the subphases 32 formed by bonding by sintering compounds that become boron-containing compounds containing lithium, boron, and oxygen tend to form particles with a particle size of 3 μm or less, which is preferable. In the solid electrolyte layer 3, the sum of the volume of the main phase 31 and the volume of the subphase 32 may be 90% by volume or more, or the sum may be 100% by volume or less.

[0036] "Positive electrode layer" 1, the positive electrode layer 1 includes, for example, a positive electrode current collector 1A and a positive electrode active material layer 1B. As shown in FIG. 1, the positive electrode active material layer 1B may be formed on both sides of the positive electrode current collector 1A, or may be formed on only one side.

[0037] (Positive electrode current collector) The positive electrode current collector 1A has high electrical conductivity. The positive electrode current collector 1A contains a conductive material such as a metal, such as silver, palladium, gold, platinum, aluminum, copper, nickel, stainless steel, or iron, or an alloy thereof, or a conductive resin. The positive electrode current collector 1A may contain a positive electrode active material such as lithium cobalt oxide (LiCoO) or a lithium vanadium compound (LiVO, LiVO(PO), or LiVOPO).

[0038] (Cathode active material layer) The positive electrode active material layer 1B contains a positive electrode active material and may contain a conductive additive and a solid electrolyte.

[0039] (Cathode active material) The positive electrode active material is not particularly limited as long as it can reversibly release and store lithium ions and desorb and insert lithium ions. For example, a positive electrode active material used in a known lithium ion secondary battery can be used.

[0040] The positive electrode active material is preferably one or more selected from, for example, composite transition metal oxides and transition metal composite oxides.

[0041] Examples of the positive electrode active material include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese spinel (LiMn2O4), and lithium manganese oxide (LiMnO4) represented by the general formula: LiNi x Co y Mn z M a Lithium titanate (Li4Ti5O 12 ) etc.

[0042] A lithium-free positive electrode active material may be used as the positive electrode active material. The lithium-free positive electrode active material can be used by disposing metallic lithium and / or a negative electrode active material doped with lithium ions in the negative electrode layer 2 in advance, and starting the all-solid-state secondary battery 10 with discharge. Examples of lithium-free positive electrode active materials include metal oxides (MnO2, VO5, etc.).

[0043] (Conductive additive) The conductive additive is not particularly limited as long as it improves the electronic conductivity in the positive electrode active material layer 1B, and known conductive additives can be used. Examples of the conductive additive include carbon-based materials such as graphite, carbon black, graphene, and carbon nanotubes; metals such as gold, platinum, silver, palladium, aluminum, copper, nickel, stainless steel, and iron; conductive oxides such as ITO (indium tin oxide); and mixtures thereof. The conductive additive may be in the form of powder or fiber.

[0044] (solid electrolyte) The solid electrolyte contained in the positive electrode active material layer 1B improves the ionic conductivity in the positive electrode active material layer 1B. As the solid electrolyte, one or a mixture of two or more known solid electrolytes can be used. The solid electrolyte may contain the same compound as the compound forming the solid electrolyte layer 3 described above.

[0045] "Negative electrode layer" The negative electrode layer 2 shown in FIG. 1 serves as both a negative electrode current collector and a negative electrode active material layer. The negative electrode layer 2 exhibits a lower potential than the positive electrode active material. The negative electrode layer 2 preferably contains a metal selected from the group consisting of silver (Ag), palladium (Pd), gold (Au), and platinum (Pt). This is because Ag, Pd, Au, and Pt do not melt and are resistant to oxidation even when the sintering step for producing the laminate 4 is carried out in an air atmosphere. The negative electrode layer 2 may be made of any of the above metals, or may be made of an alloy containing any of the above metals.

[0046] The negative electrode layer 2 particularly preferably contains an AgPd alloy. This is because the subphase 32 contained in the solid electrolyte layer 3 in contact with the negative electrode layer 2 functions more effectively as a starting point for the reaction in the exchange of lithium ions, resulting in a larger capacity. Furthermore, when the negative electrode layer 2 contains an AgPd alloy, the resulting all-solid-state secondary battery 10 preferably has a high energy density. As the AgPd alloy, for example, one containing Ag and Pd in ​​a molar ratio of 8:2 (Ag:Pd) can be used.

[0047] The negative electrode layer 2 may contain metal particles made of any of the above metals and / or alloy particles made of an alloy containing any of the above metals, and one or more selected from a conductive additive and a solid electrolyte. (Conductive additive) The conductive additive improves the electronic conductivity of the negative electrode layer 2. As the conductive additive, the same materials as those used for the positive electrode active material layer 1B can be used.

[0048] (solid electrolyte) The solid electrolyte contained in the anode layer 2 improves ion conduction within the anode layer 2. As the solid electrolyte, one or a mixture of two or more known solid electrolytes can be used. The solid electrolyte may contain the same compound as the compound forming the solid electrolyte layer 3 described above.

[0049] [Manufacturing method of all-solid-state secondary battery] Next, a method for manufacturing the all-solid-state secondary battery 10 of this embodiment will be described. First, the laminate 4 is produced. The laminate 4 can be produced using, for example, a co-firing method or a sequential firing method, and is preferably produced using the co-firing method. The co-firing method is a method in which the materials for forming each layer are stacked and then fired all at once to produce the laminate 4. The sequential firing method is a method in which firing is performed after each layer is formed. The co-firing method can produce the laminate 4 with fewer steps than the sequential firing method. Furthermore, the laminate 4 produced by the co-firing method is denser than the laminate 4 produced by the sequential firing method. Below, a method for producing the laminate 4 will be described using an example in which the laminate 4 is produced using the co-firing method.

[0050] First, the materials for the positive electrode current collector 1A, the positive electrode active material layer 1B, the solid electrolyte layer 3, and the negative electrode layer 2 that constitute the laminate 4 are made into a paste, to produce a paste corresponding to the material for each layer. In this embodiment, a first paste containing an electrolyte represented by formula (1) and a second paste containing a compound that becomes a boron-containing compound containing lithium, boron, and oxygen are produced as pastes for the solid electrolyte layer 3. The first paste is used to form the main phase 31. The second paste is used to form the subphase 32.

[0051] The first paste can be obtained, for example, by forming particles of the electrolyte represented by formula (1) and a sintering aid, if necessary, into a paste. The particles of the electrolyte represented by formula (1) can be produced by a known production method. The second paste can be obtained by, for example, forming particles of a boron-containing compound containing lithium, boron, and oxygen, and optionally adding a sintering aid, into a paste. The second paste may further contain particles of an electrolyte represented by formula (1) as needed.

[0052] The particles of the boron-containing compound containing lithium, boron, and oxygen used as the material for the second paste may have an average particle size of, for example, 0.01 μm to 10 μm, preferably 0.1 μm to 3 μm. An average particle size of 0.01 μm or more is preferable because, when the particles of the boron-containing compound containing lithium, boron, and oxygen are made into a paste, the particles of the boron-containing compound containing lithium, boron, and oxygen are less likely to aggregate in the paste. An average particle size of 10 μm or less facilitates the formation of a large number of subphases 32 with sufficiently small average particle size and large specific surface areas. As a result, the subphases 32 can more effectively function as starting points for the lithium ion exchange reaction. The average particle size can be determined appropriately depending on the composition and particle size of the subphases 32, the heating temperature and firing time in the firing process, and other factors.

[0053] The method for forming the materials used in manufacturing the laminate 4 into a paste is not particularly limited, and for example, a method of mixing powders of the materials into a vehicle to obtain a paste can be used. Here, the term "vehicle" is a general term for a medium in a liquid phase. In this embodiment, the vehicle includes a solvent, a binding material (binder), and a plasticizer. For example, dihydroterpineol can be used as the solvent. For example, ethyl cellulose can be used as the binder.

[0054] Next, a green sheet is prepared. The green sheet is obtained by applying a paste prepared for each material onto a substrate such as a PET (polyethylene terephthalate) film, drying it as necessary, and then peeling off the substrate. The method for applying the paste is not particularly limited, and known methods such as screen printing, coating, transfer, and doctor blade can be used.

[0055] Next, the green sheets prepared for each material are stacked in the desired order and number of layers to prepare a laminate sheet. When stacking the green sheets, alignment and cutting are performed as necessary. For example, when preparing a parallel or series-parallel battery, it is preferable to align the green sheets so that the end face of the positive electrode current collector 1A does not coincide with the end face of the negative electrode current collector 2A, and then stack the green sheets.

[0056] The green sheets to be laminated may be green sheets to be a solid electrolyte layer unit, a positive electrode unit, and a negative electrode layer 2 that have been fabricated in advance.

[0057] The procedure for producing a positive electrode unit is as follows: First, a paste for the positive electrode active material layer 1B is printed by screen printing on a substrate such as a PET film and dried to form the positive electrode active material layer 1B. Next, a paste for the positive electrode current collector 1A is printed by screen printing on the positive electrode active material 1B and dried to form the positive electrode current collector 1A. Next, a paste for the positive electrode active material layer 1B is printed by screen printing on the positive electrode current collector 1A and dried to form the positive electrode active material layer 1B. The PET film is then peeled off to obtain the positive electrode unit. The positive electrode unit is a green sheet in which the positive electrode active material layer 1B, positive electrode current collector 1A, and positive electrode active material layer 1B are laminated in this order.

[0058] The solid electrolyte layer unit is formed by applying a first paste to a substrate such as a PET film by doctor blade coating and drying it to form a sheet. Next, the second paste is applied to the sheet made using the first paste by doctor blade coating and drying it to form a sheet. The solid electrolyte layer unit is a green sheet in which a sheet made using the first paste and a sheet made using the second paste are laminated.

[0059] Next, the prepared positive electrode unit and a green sheet that will become the negative electrode layer 2 are laminated with the solid electrolyte layer unit interposed between them. At this time, the sheet prepared using the second paste in the solid electrolyte layer unit is arranged to face the green sheet that will become the negative electrode layer 2. This produces a laminated sheet in which the positive electrode active material layer 1B, positive electrode current collector 1A, positive electrode active material layer 1B, a sheet prepared using the first paste, a sheet prepared using the second paste, a green sheet that will become the negative electrode layer 2, a sheet prepared using the second paste, and a sheet prepared using the first paste are laminated in this order.

[0060] When stacking the positive electrode unit, the solid electrolyte layer unit, and the green sheet that will become the negative electrode layer 2, they are stacked while being alternately shifted so that the positive electrode unit extends only on one end surface of the laminated sheet, and the green sheet that will become the negative electrode layer 2 extends only on the opposite end surface of the laminated sheet.

[0061] Next, the prepared lamination sheets are pressed together to increase the adhesion of each layer and form a laminate. Pressurization can be performed, for example, by a mold press, hot isostatic pressing (WIP), cold isostatic pressing (CIP), isostatic pressing, etc. Pressurization is preferably performed while heating. The heating temperature during pressurization can be, for example, 40 to 95°C. Next, the laminated substrate obtained after pressure application is cut using a dicing device to form laminated chips. The obtained laminated chips are then subjected to binder removal and firing processes. This results in a laminated body 4 made of a sintered body.

[0062] The binder removal and firing step can be carried out, for example, by placing the laminate on a ceramic stand. The binder removal and firing step can be, for example, a step of heating to 550°C to 1100°C in an air atmosphere. The heating time (firing time) can be, for example, 0.1 to 6 hours. The heating temperature and firing time in the binder removal and firing step can be appropriately determined depending on the composition of each layer constituting the laminate 4, etc.

[0063] In this embodiment, the binder removal and firing processes bond and integrate particles of the boron-containing compound containing lithium, boron, and oxygen contained in the second paste. As a result, the sheet (coated film formed on the sheet made using the first paste) made using the second paste and arranged facing the negative electrode layer 2 shrinks and splits in the planar direction, generating particulate subphases 32. A main phase 31 is formed between adjacent subphases 32. The boron-containing compound containing lithium, boron, and oxygen that forms the subphase 32 is unlikely to dissolve in the electrolyte represented by formula (1) that forms the main phase 31, and also functions as a sintering aid for the solid electrolyte layer 3. As a result, a solid electrolyte layer 3 is formed in which multiple particulate subphases 32 are scattered along the interface between the solid electrolyte 3 and the negative electrode layer 2, and a main phase 31 is arranged between adjacent subphases 32 and between the subphase 32 and the positive electrode active material layer 1B.

[0064] The heating temperature in the binder removal and firing steps is preferably 550°C to 1100°C, more preferably 600°C to 1000°C. A heating temperature of 550°C or higher in the binder removal and firing steps promotes bonding between particles of the boron-containing compound containing lithium, boron, and oxygen contained in the second paste. This promotes the sheet formed using the second paste arranged to face the negative electrode layer 2 to shrink in the planar direction and become easily separated. As a result, a solid electrolyte layer 3 is formed in which multiple particulate subphases 32 are scattered along the interface between the solid electrolyte 3 and the negative electrode layer 2, and the main phase 31 is disposed between adjacent subphases 32 and between the subphases 32 and the positive electrode active material layer 1B. Furthermore, a heating temperature of 1100°C or lower in the firing step prevents the boron-containing compound containing lithium, boron, and oxygen (subphase 32) and the electrolyte represented by formula (1) (main phase 31) from forming a solid solution.

[0065] The sintered laminate 4 (sintered body) may be placed in a cylindrical container together with an abrasive such as alumina and polished by barrel polishing. This allows the corners of the laminate 4 to be chamfered. The laminate 4 may also be polished by sandblasting. Sandblasting is preferred because it allows only specific portions of the surface of the laminate 4 to be polished.

[0066] Next, a first external terminal 5 and a second external terminal 6 are formed on opposing side surfaces of the fabricated laminate 4. The first external terminal 5 and the second external terminal 6 can be formed using a method such as sputtering, dipping, screen printing, or spray coating. Through the above steps, the all-solid-state secondary battery 10 can be manufactured.

[0067] In this embodiment, one or more of the following conditions (1) to (7) are appropriately changed in the manufacturing method of the all-solid-state secondary battery 10. This makes it possible to obtain an all-solid-state secondary battery 10 having a solid electrolyte layer 3 in which the volume ratio of the subphase 32 and the particle size of the subphase 32 are within desired ranges.

[0068] (1) Composition of particles of a boron-containing compound containing lithium, boron, and oxygen contained in the second paste. (2) The average particle size of the particles of the boron-containing compound containing lithium, boron, and oxygen contained in the second paste. (3) When the second paste contains particles made of the electrolyte represented by formula (1), the composition of the particles made of the electrolyte represented by formula (1). (4) When the second paste contains particles of the electrolyte represented by formula (1), the ratio of the particles of the electrolyte represented by formula (1) contained in the second paste to the particles of the boron-containing compound containing lithium, boron, and oxygen is 1. (5) The ratio of the thickness of the sheet produced using the first paste to the thickness of the sheet produced using the second paste. (6) Composition of particles made of an electrolyte represented by formula (1) contained in the first paste. (7) Heating temperature and firing time in the debinding and firing process.

[0069] In this embodiment, instead of the particles of the boron-containing compound containing lithium, boron, and oxygen contained in the second paste, or together with the particles of the boron-containing compound containing lithium, boron, and oxygen, one or more compounds that become a boron-containing compound containing lithium, boron, and oxygen when fired, and / or particles made of one or more elements that become a boron-containing compound containing lithium, boron, and oxygen when fired, may be used.

[0070] Examples of compounds and / or elements that become boron-containing compounds containing lithium, boron, and oxygen, which are used as materials for the second paste, include boron oxide, boric acid, boron hydroxide, lithium borate, lithium oxide, lithium carbonate, lithium phosphate, etc. Particles (powders) of compounds and / or particles (powders) of elements that become these boron-containing compounds can be produced by known production methods, and one or more types can be used in combination depending on the composition of the desired boron-containing compound containing lithium, boron, and oxygen.

[0071] The particles of the compound and / or element that form the boron-containing compound containing lithium, boron, and oxygen may have an average particle size of, for example, 0.01 μm to 10 μm, preferably 0.1 μm to 3 μm. An average particle size of 0.01 μm or more is preferable because, when the particles of the compound and / or element are formed into a paste, the particles of the compound and / or element are less likely to aggregate in the paste. An average particle size of 10 μm or less is preferable because the specific surface area of ​​the particles is sufficiently large, which facilitates the reaction to produce the boron-containing compound containing lithium, boron, and oxygen during the firing process. Furthermore, an average particle size of 10 μm or less facilitates the formation of a large number of subphases 32 with sufficiently small average particle size and large specific surface area. As a result, the subphases 32 can more effectively function as starting points for the reaction of donating and receiving lithium ions. The average particle size can be determined appropriately depending on the composition and particle size of the subphases 32, the heating temperature and firing time during the firing process, and other factors.

[0072] The all-solid-state secondary battery 10 of this embodiment shown in FIGS. 1 and 2 includes a sintered body having a positive electrode layer 1, a negative electrode layer 2, and a solid electrolyte layer 3 sandwiched between the positive electrode layer 1 and the negative electrode layer 2. The solid electrolyte layer 3 includes a main phase 31 containing an electrolyte represented by formula (1) and a subphase 32 made of a boron-containing compound containing lithium, boron, and oxygen. The subphase 32 is present in contact with a portion of the interface between the negative electrode layer 2 and the solid electrolyte layer 3. Therefore, in the all-solid-state secondary battery 10 of this embodiment, the subphase 32 in the solid electrolyte layer 3 serves as the starting point for a reaction in the transfer of lithium ions. As a result, the all-solid-state secondary battery 10 has a large capacity.

[0073] The above describes the embodiments of the present invention in detail with reference to the drawings. However, each configuration and combination thereof in each embodiment is an example, and additions, omissions, substitutions, and other modifications of the configurations are possible within the scope that does not deviate from the spirit of the present invention.

[0074] For example, in the all-solid-state secondary battery 10 of the above-described embodiment, as shown in FIG. 1, the anode layer 2 serves as both the anode current collector and the anode active material layer, but the anode layer in the all-solid-state secondary battery of the present invention may have both the anode current collector and the anode active material layer. Fig. 3 is a cross-sectional schematic diagram showing another example of the all-solid-state secondary battery of the present invention. In the all-solid-state secondary battery 20 shown in Fig. 3, the same components as those in the all-solid-state secondary battery 10 shown in Fig. 1 described above are denoted by the same reference numerals, and description thereof will be omitted. The all-solid-state secondary battery 20 shown in Fig. 3 differs from the all-solid-state secondary battery 10 shown in Fig. 1 only in the negative electrode layer 21.

[0075] The negative electrode layer 21 in the all-solid-state secondary battery 20 shown in Fig. 3 includes, for example, a negative electrode current collector 2A and a negative electrode active material layer 2B. As shown in Fig. 3, the negative electrode active material layer 2B may be formed on both sides of the negative electrode current collector 2A, or may be formed on only one side.

[0076] (Negative electrode current collector) The negative electrode current collector 2A is the same as the positive electrode current collector 1A. Specifically, the negative electrode current collector 2A can be made of a conductive material such as a metal, such as silver, palladium, gold, platinum, aluminum, copper, nickel, stainless steel, or iron, or an alloy thereof, or a conductive resin.

[0077] (Negative electrode active material layer) The negative electrode active material layer 2B contains a negative electrode active material. The negative electrode active material layer 2B may contain a conductive additive and a solid electrolyte. The conductive additive and solid electrolyte that may be contained in the negative electrode active material layer 2B may be the same as those that may be contained in the negative electrode layer 2 of the all-solid-state secondary battery 10 shown in FIG. 1.

[0078] (Negative electrode active material) The negative electrode active material is a compound capable of absorbing and releasing ions. The negative electrode active material is a compound that exhibits a lower potential than the positive electrode active material. The negative electrode active material may be the same material as the positive electrode active material. The negative electrode active material and the positive electrode active material to be used in the all-solid-state secondary battery 10 are determined in consideration of the potential of the negative electrode active material and the potential of the positive electrode active material.

[0079] The all-solid-state secondary battery 20 shown in FIG. 3 can be manufactured, for example, by the method described below. That is, the all-solid-state secondary battery 10 can be manufactured by the same method as the manufacturing method of the all-solid-state secondary battery 10 shown in FIGS. 1 and 2, except that the negative electrode unit shown below is used instead of the green sheet that becomes the negative electrode layer 2 manufactured in the above-described embodiment.

[0080] The negative electrode unit can be fabricated using the same procedure as the positive electrode unit fabricated in the above-described embodiment. Specifically, first, a paste for the negative electrode active material layer 2B is printed by screen printing on a substrate such as a PET film and dried to form the negative electrode active material layer 2B. Next, a paste for the negative electrode current collector 2A is printed by screen printing on the negative electrode active material layer 2B and dried to form the negative electrode current collector 2A. Next, a paste for the negative electrode active material layer 2B is printed by screen printing on the negative electrode current collector 2A and dried to form the negative electrode active material layer 2B. Thereafter, the PET film is peeled off to obtain the negative electrode unit. The negative electrode unit is a green sheet in which the negative electrode active material layer 2B, the negative electrode current collector 2A, and the negative electrode active material layer 2B are laminated in this order.

[0081] 1 and 2, the all solid state secondary battery 20 shown in Fig. 3 also has a solid electrolyte layer 3 including a main phase 31 containing the electrolyte represented by formula (1) and a subphase 32 made of a boron-containing compound containing lithium, boron, and oxygen, and the subphase 32 is present in contact with a part of the interface between the anode layer 2 and the solid electrolyte layer 3. Therefore, like the all solid state secondary battery 10 shown in Fig. 1 and 2, the all solid state secondary battery 20 shown in Fig. 3 also has a large capacity because the subphase 32 in the solid electrolyte layer 3 serves as the starting point for the reaction in the transfer of lithium ions. [Example]

[0082] "Example 1" (Preparation of solid electrolyte layer paste) As pastes for the solid electrolyte layer, a first paste and a second paste were produced by the following method.

[0083] (First paste) Li 3+x Si x P 1-x Particles made of O4 (x in formula (1) is 0.5), ethyl cellulose, and dihydroterpineol are added to a ball mill and wet mixed to form Li 3+x Si x P 1-x A first paste of Example 1 containing 15% by volume of particles made of O4 (x in formula (1) is 0.5) was prepared.

[0084] (Second paste) Particles of a boron-containing compound containing lithium, boron, and oxygen, consisting of Li3BO3 (lithium triborate) with an average particle size of 0.5 μm, were added to a ball mill and wet mixed with ethyl cellulose and dihydroterpineol to produce a second paste of Example 1 containing 2 volume % of particles of a boron-containing compound containing lithium, boron, and oxygen.

[0085] (Preparation of positive electrode current collector paste) A powder of an Ag-Pd alloy containing Ag and Pd in ​​a molar ratio of 8:2 (Ag:Pd), ethyl cellulose, and dihydroterpineol were added to a ball mill and wet mixed to prepare a positive electrode current collector paste of Example 1. (Preparation of Positive Electrode Active Material Layer Paste) Lithium cobalt oxide (LiCoO2), ethyl cellulose, and dihydroterpineol were added to a ball mill and wet mixed to prepare a positive electrode active material layer paste of Example 1 containing 15% by volume of lithium cobalt oxide (LiCoO2) particles.

[0086] (Preparation of negative electrode layer paste) Powder consisting of an Ag-Pd alloy containing Ag and Pd in ​​a molar ratio of 8:2 (Ag:Pd), ethyl cellulose, and dihydroterpineol were added to a ball mill and wet mixed to prepare a negative electrode layer paste of Example 1 containing 15% by volume of AgPd powder. The ethyl cellulose used in each of the above pastes is a binder, and dihydroterpineol is a solvent.

[0087] (Production of positive electrode unit) A positive electrode active material layer paste was applied to a thickness of 5 μm on a substrate made of a PET (polyethylene terephthalate) film using screen printing, and dried at 80°C for 5 minutes. A positive electrode current collector paste was applied to a thickness of 5 μm on the dried positive electrode active material layer paste using screen printing, and dried at 80°C for 5 minutes. A positive electrode active material layer paste was applied to a thickness of 5 μm on the dried positive electrode current collector paste using screen printing, and dried at 80°C for 5 minutes, and then peeled off from the substrate. This resulted in a positive electrode unit in which a positive electrode active material layer, a positive electrode current collector, and a positive electrode active material layer were stacked in this order.

[0088] (Fabrication of solid electrolyte layer unit) The first paste was applied to a substrate made of a PET (polyethylene terephthalate) film using screen printing to a thickness of 20 μm and dried at 80° C. for 10 minutes. The second paste was applied to a thickness of 1 μm using screen printing onto the dried first paste, and after drying at 80° C. for 5 minutes, it was peeled off from the substrate. In this way, a solid electrolyte layer unit was obtained in which a sheet made using the first paste and a sheet made using the second paste were stacked.

[0089] (Preparation of green sheet to be used as negative electrode layer) The negative electrode layer paste was applied to a substrate made of a PET (polyethylene terephthalate) film by screen printing to a thickness of 5 μm, dried at 80° C. for 5 minutes, and then peeled off from the substrate, thereby obtaining a green sheet that would become the negative electrode layer.

[0090] (Fabrication of all-solid-state secondary batteries) Four positive electrode units and four green sheets for the negative electrode layer were alternately stacked with a solid electrolyte layer unit sandwiched between them to obtain a laminate sheet for laminate 4. At this time, the sheet fabricated using the second paste for the solid electrolyte layer unit in the laminate sheet was positioned facing the green sheet for the negative electrode layer. The laminate sheets were stacked so that the top and bottom surfaces formed solid electrolyte layer units. The sheets were also stacked with a shifted offset so that the positive electrode unit extended only on one end surface of the laminate sheet and the green sheet for the negative electrode layer extended only on the opposite end surface.

[0091] Next, the produced laminated sheets were pressed together and cut using a dicing device to produce laminated chips. The laminated chips were then debindered and fired to obtain the laminated body of Example 1, which consisted of a sintered body. The binder removal process involved heating the laminated chips in an air atmosphere at 500°C for 10 hours, thereby removing the binder from the laminated chips. Firing was performed in an air atmosphere at 900°C for 1 hour. Next, a first external terminal and a second external terminal were formed on the opposing side surfaces of the fabricated laminate by Au sputtering. Through the above steps, the all-solid-state secondary battery 10 of Example 1 shown in FIG. 1 was obtained.

[0092] "Example 2" An all-solid-state secondary battery 10 of Example 2 was obtained in the same manner as in Example 1, except that the volume fraction (vol %) of particles of the boron-containing compound containing lithium, boron, and oxygen in the second paste was set to three times that of Example 1 to prepare a solid electrolyte layer unit.

[0093] "Example 3" An all-solid-state secondary battery 10 of Example 3 was obtained in the same manner as in Example 1, except that the volume fraction (vol %) of particles of the boron-containing compound containing lithium, boron, and oxygen in the second paste was set to 15 times that of Example 1 to prepare a solid electrolyte layer unit.

[0094] Example 4 An all-solid-state secondary battery 10 of Example 4 was obtained in the same manner as in Example 1, except that the volume fraction (vol %) of the particles of the boron-containing compound containing lithium, boron, and oxygen in the second paste was set to 0.5 times that of Example 1 to prepare a solid electrolyte layer unit.

[0095] "Example 5" An all-solid-state secondary battery 10 of Example 5 was obtained in the same manner as in Example 1, except that the second paste was prepared using particles made of Li3BO3 (lithium triborate) having an average particle size of 0.2 μm as particles of a boron-containing compound containing lithium, boron, and oxygen.

[0096] "Example 6" An all-solid-state secondary battery 10 of Example 6 was obtained in the same manner as in Example 1, except that the second paste was prepared using particles made of Li3BO3 (lithium triborate) having an average particle size of 5 μm as the particles of the boron-containing compound containing lithium, boron, and oxygen, the volume ratio (volume %) of the particles of the boron-containing compound containing lithium, boron, and oxygen in the second paste was 0.1 times that of Example 1, and the second paste was applied to a thickness of 10 μm on the dried first paste to prepare a solid electrolyte layer unit.

[0097] "Example 7" An all-solid-state secondary battery 10 of Example 7 was obtained in the same manner as in Example 1, except that the second paste was prepared using particles made of Li3BO3 (lithium triborate) having an average particle size of 0.05 μm as particles of a boron-containing compound containing lithium, boron, and oxygen.

[0098] "Example 8" The negative electrode layer is Cu / lithium titanate (Li4Ti5O 12 The all-solid-state secondary battery 10 of Example 8 was obtained in the same manner as in Example 1, except that the composition was changed to consist of 10 ... First, lithium titanate powder, ethyl cellulose, and dihydroterpineol were added to a ball mill and wet mixed to prepare a second negative electrode paste containing 15% by volume of lithium titanate particles. Next, Cu powder, ethyl cellulose, and dihydroterpineol were added to a ball mill and wet mixed to prepare a negative electrode current collector paste containing 15% by volume of Cu particles. Next, the second negative electrode layer paste was applied to a substrate made of a PET (polyethylene terephthalate) film using screen printing to a thickness of 5 μm, dried for 5 minutes at 80 ° C., and then the negative electrode current collector paste was applied and dried for 5 minutes, and then the second negative electrode layer paste was applied to a thickness of 5 μm, dried for 5 minutes at 80 ° C., and then peeled off from the substrate. This resulted in a negative electrode layer in which the second negative electrode layer / negative electrode current collector layer / second negative electrode layer were stacked in this order.

[0099] "Example 9" As the paste for the solid electrolyte layer, the first paste and the second paste of Example 1 were used instead of the first paste, and Li 3+x Si x P 1-x An all-solid-state secondary battery 10 of Example 9 was obtained in the same manner as in Example 1, except that a paste was used in which particles made of O4 (x in formula (1) is 0.5) and particles of a boron-containing compound containing lithium, boron, and oxygen were mixed at a volume ratio of 100:1.

[0100] "Example 10" An all-solid-state secondary battery 10 of Example 10 was obtained in the same manner as in Example 1, except that the second paste was prepared using particles made of Li3BO3 (lithium triborate) having an average particle size of 1 μm as particles of a boron-containing compound containing lithium, boron, and oxygen.

[0101] "Example 11" An all-solid-state secondary battery 10 of Example 11 was obtained in the same manner as in Example 6, except that the second paste was prepared using particles made of Li3BO3 (lithium triborate) having an average particle size of 3.0 μm as particles of a boron-containing compound containing lithium, boron, and oxygen.

[0102] "Example 12" An all-solid-state secondary battery 12 of Example 13 was obtained in the same manner as in Example 6, except that the second paste was prepared using particles made of Li3BO3 (lithium triborate) having an average particle size of 3.2 μm as particles of a boron-containing compound containing lithium, boron, and oxygen.

[0103] "Example 13" An all-solid-state secondary battery 10 of Example 13 was obtained in the same manner as in Example 1, except that the volume fraction (vol %) of particles of the boron-containing compound containing lithium, boron, and oxygen in the second paste was set to 10 times that of Example 1 to prepare a solid electrolyte layer unit.

[0104] "Example 14" An all-solid-state secondary battery 10 of Example 14 was obtained in the same manner as in Example 1, except that the volume fraction (vol %) of particles of the boron-containing compound containing lithium, boron, and oxygen in the second paste was set to 11 times that of Example 1 to prepare a solid electrolyte layer unit.

[0105] "Example 15" A second paste was prepared in the same manner as in Example 1, except that particles made of Li3BO3 (lithium triborate) with an average particle size of 0.05 μm were used as particles of a boron-containing compound containing lithium, boron, and oxygen. Next, in the production of the solid electrolyte layer unit, the first paste was applied to a substrate made of a PET (polyethylene terephthalate) film using screen printing to a thickness of 10 μm, and dried at 80°C for 10 minutes. After drying, the second paste was applied to a thickness of 1 μm using screen printing on the first paste, and after drying at 80°C for 5 minutes, it was peeled off from the substrate to produce the first solid electrolyte layer unit. Next, the first paste was applied to a thickness of 10 μm and dried at 80°C for 10 minutes. After drying, the second paste was applied to a thickness of 1.25 μm on the dried first paste using screen printing. After drying at 80°C for 5 minutes, the paste was peeled off from the substrate to produce a second solid electrolyte layer unit. The first solid electrolytic layer unit and the second solid electrolytic layer unit were attached so that the solid electrolyte layers faced each other, thereby obtaining a third solid electrolytic layer unit in which a sheet made using the second paste with a thickness of 1 μm, a sheet made using the first paste with a thickness of 20 μm, and a sheet made using the second paste with a thickness of 1.25 μm were laminated in that order. Furthermore, the all-solid-state secondary battery 10 of Example 15 was obtained in the same manner as in Example 1, except that the third solid electrolyte unit was arranged so that the first solid electrolyte unit faced the green sheet that would become the negative electrode layer and the second solid electrolyte unit faced the green sheet that would become the positive electrode layer.

[0106] "Example 16" An all-solid-state secondary battery 10 of Example 16 was obtained in the same manner as in Example 15, except that the second paste of the second solid electrolytic layer unit was set to a thickness of 1 μm.

[0107] "Example 17" An all-solid-state secondary battery 10 of Example 17 was obtained in the same manner as in Example 15, except that the second paste of the second solid electrolytic layer unit was set to a thickness of 0.8 μm.

[0108] "Example 18" An all-solid-state secondary battery 10 of Example 18 was obtained in the same manner as in Example 15, except that the second paste of the second solid electrolytic layer unit was set to a thickness of 0.7 μm.

[0109] "Comparative Example 1" An all-solid-state secondary battery 10 of Comparative Example 1 was obtained in the same manner as in Example 1, except that a lamination sheet to become a solid electrolyte layer produced by the method described below was used instead of the solid electrolyte layer unit. The first paste of Example 1 was applied to a substrate made of a PET (polyethylene terephthalate) film by screen printing to a thickness of 20 μm, dried at 80° C. for 10 minutes, and then peeled off from the substrate, thereby obtaining a lamination sheet that would become a solid electrolyte layer.

[0110] "Comparative Example 2" An all-solid-state secondary battery 10 of Comparative Example 2 was obtained in the same manner as in Example 1, except that a green sheet to become a solid electrolyte layer produced by the method described below was used instead of the solid electrolyte layer unit. The first paste and the second paste of Example 1 were mixed with Li 3+x Si x P 1-x Particles of O4 (x in formula (1) is 0.5) and particles of a boron-containing compound containing lithium, boron, and oxygen were mixed at a volume ratio of 100:1 to prepare a paste for the solid electrolyte layer of Comparative Example 2. The paste for the solid electrolyte layer was then applied to a substrate made of a PET (polyethylene terephthalate) film by screen printing to a thickness of 20 μm, dried at 80° C. for 10 minutes, and then peeled off from the substrate, thereby obtaining a green sheet for the solid electrolyte layer of Comparative Example 2.

[0111] "Comparative Example 3" As a solid electrolyte, Li 1.3 Al 0.3 Ti 1.7 An all-solid-state secondary battery 10 of Comparative Example 3 was obtained in the same manner as in Example 1, except that (PO4)3 was used.

[0112] Tables 1 to 3 show the configurations of the negative electrode layers of the all-solid-state secondary batteries 10 of Examples 1 to 18 and Comparative Examples 1 to 3 obtained in this manner.

[0113] [Table 1]

[0114] [Table 2]

[0115] [Table 3]

[0116] Furthermore, the locations of the subphases, the volume ratios (vol %) of the subphases in the solid electrolyte layer, and the particle sizes (μm) of the subphases were determined by the following methods for the all-solid-state secondary batteries 10 of Examples 1 to 18 and Comparative Examples 1 to 3. The results are shown in Tables 1 to 3.

[0117] The all-solid-state secondary battery 10 was cut to expose a cross section, and a polished cross section was obtained using a cross-section polisher (CP). The all-solid-state secondary battery 10 was cut so as to divide the laminate at approximately the center between the first external terminal and the second external terminal when viewed from the stacking surface. The obtained cross section was then observed at 5000x magnification using a scanning electron microscope (SEM), and backscattered electron images of 10 fields of view were obtained.

[0118] The main and subphases in the positive electrode layer, negative electrode layer, and solid electrolyte layer were identified based on the contrast of the backscattered electron images obtained from the 10 fields of view. The presence or absence of a subphase in the solid electrolyte layer was then confirmed. If a subphase was present in the solid electrolyte layer, it was further examined whether the subphase was in contact with a part of the interface between the solid electrolyte layer and the negative electrode layer.

[0119] Furthermore, for all-solid-state secondary batteries 10 in which a subphase exists in the solid electrolyte layer, each backscattered electron image was converted into a monochrome image and binarized. The number of pixels corresponding to the subphase in each field of view was then measured and added up. The volume fraction of the subphase was then calculated for each field of view using the following formula, and the average value of the 10 fields of view was taken as the volume fraction (volume %) of the subphase. Volume fraction of subphase (volume %) = (number of pixels of subphase in the field of view / total number of pixels in the field of view) × 100

[0120] In addition, in the all-solid-state secondary battery 10 in which a subphase exists in the solid electrolyte layer, the subphases identified by contrast were extracted from each of the backscattered electron images of the 10 fields of view, and the average value of their particle diameters was calculated to be the particle diameter (μm) of the subphase. When the cross-sectional shape of the subphase was not circular (irregular shape in cross section), the diameter of the subphase in the major axis direction was taken as the particle diameter.

[0121] Furthermore, the initial capacity (μA) was determined by the method described below for the all-solid-state secondary batteries 10 of Examples 1 to 18 and Comparative Examples 1 to 3. The results are shown in Tables 1 to 3. (Measurement of initial capacitance (μA)) The capacity is the discharge capacity, and one cycle consists of charging at a constant current of 10 μA (CC charging) in an environment of 60°C until the battery voltage reaches 3.9 V, and then discharging at a constant current of 10 μA (CC discharging) until the battery voltage reaches 0 V. After repeating this process 10 times, the discharge capacity (μAh) at the 10th cycle was measured.

[0122] As shown in Tables 1 to 3, it was confirmed that the all-solid-state secondary battery 10 of Example 118 contained a subphase in the solid electrolyte layer, and that the subphase was present in contact with part of the interface between the negative electrode layer and the solid electrolyte layer. Furthermore, the all-solid-state secondary batteries 10 of Examples 1 to 18 had larger initial capacities than the all-solid-state secondary battery of Comparative Example 1 in which no subphase existed in the solid electrolyte layer, and the all-solid-state secondary battery of Comparative Example 2 in which the subphase existed substantially uniformly throughout the solid electrolyte layer. This is presumably because, in the all-solid-state secondary batteries 10 of Examples 1 to 18, a subphase that serves as a starting point for the reaction in the transfer of lithium ions was present at the interface with the negative electrode layer in the solid electrolyte layer.

[0123] In particular, Example 1, in which the volume ratio of the subphase in the solid electrolyte layer was 1 vol%, and Example 2, in which the volume ratio of the subphase was 3 vol%, had larger initial capacities than Example 4, in which the volume ratio of the subphase was 0.5 vol%, and Example 3, in which the volume ratio of the subphase was 15 vol%.

[0124] Furthermore, Example 1, in which the particle size of the subphase in the solid electrolyte layer was 0.5 μm, had a larger initial capacity than Example 5, in which the particle size of the subphase was 0.2 μm, and Example 7, in which the particle size of the subphase was 0.05 μm. Furthermore, Example 1, in which the particle size of the subphase in the solid electrolyte layer was 0.5 μm, had a larger initial capacity than Example 6, in which the particle size of the subphase was 5 μm, and Example 10, in which the particle size of the subphase was 1 μm. [Industrial Applicability]

[0125] It is possible to provide an all-solid-state battery that is safer and has a larger capacity than conventional batteries. [Explanation of symbols]

[0126] 1 Positive electrode layer 1A positive electrode current collector 1B Cathode active material layer 2, 21 Negative electrode layer 2A negative electrode current collector 2B Negative electrode active material layer 3 Solid electrolyte layer 4 Laminate 5 First external terminal 6 Second external terminal 10, 20 All-solid-state secondary battery 31 Main phase 32 Deputy Minister

Claims

1. a sintered body having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer sandwiched between the positive electrode layer and the negative electrode layer; The solid electrolyte layer includes a main phase made of an electrolyte represented by the following formula (1) and a subphase made of a boron-containing compound containing lithium, boron, and oxygen: an all-solid-state secondary battery, wherein at least a portion of the subphase is in contact with a portion of the interface between the negative electrode layer and the solid electrolyte layer; Li ３＋ｘ Si ｘ P １－ｘ O ４ (1) (Equation (1) satisfies 0<x<1.)

2. 2. The all-solid-state secondary battery according to claim 1, wherein the volume ratio of the subphase in the solid electrolyte layer is 1% by volume or more and 10% by volume or less.

3. 3. The all-solid-state secondary battery according to claim 1, wherein the subphase is formed from particles having a particle size of 0.1 μm to 3 μm.

4. 2. The all-solid-state secondary battery according to claim 1, wherein a proportion of a subphase present in contact with a portion of the interface between the negative electrode layer and the solid electrolyte layer is greater than a proportion of a subphase present in contact with a portion of the interface between the positive electrode layer and the solid electrolyte layer.

5. 2. The all-solid-state secondary battery according to claim 1, wherein the sum of the volume of the main phase and the volume of the subphase in the solid electrolyte layer is 90% by volume or more.

6. 3. The all-solid-state secondary battery according to claim 1, wherein the negative electrode layer contains any metal selected from the group consisting of Ag, Pd, Au, and Pt.

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