Lithium metal battery

Abstract

The present invention addresses the problem of providing a lithium metal battery capable of obtaining preferable battery characteristics by having an intermediate layer disposed between a solid electrolyte layer and a negative electrode layer. One embodiment that solves said problem is a lithium metal battery having a structure in which a negative electrode layer containing metallic lithium, an intermediate layer, a solid electrolyte layer, and a positive electrode layer are laminated in the stated order. The yield contact pressure of the intermediate layer is 5.0 MPa or less.

Description

Lithium metal battery

[0001] The present invention relates to a lithium metal solid state battery.

[0002] In recent years, research and development into secondary batteries that contribute to energy efficiency has been conducted to ensure that more people have access to affordable, reliable, sustainable and advanced energy.

[0003] One known example of such a secondary battery is a lithium metal battery, which has a solid electrolyte layer disposed between a positive electrode layer and a lithium negative electrode layer. In a lithium metal battery, lithium metal precipitates between the solid electrolyte layer and the negative electrode layer during charging and dissolves during discharging. Therefore, a technique for providing another layer between the solid electrolyte layer and the negative electrode layer to suppress shrinkage in the thickness direction due to lithium metal has been disclosed (for example, Patent Document 1).

[0004] Japanese Patent Application Laid-Open No. 2023-24145

[0005] The technology disclosed in Patent Document 1 includes a resin layer disposed between a solid electrolyte layer and a negative electrode current collector, the resin layer having a first region and a second region in a planar direction, the first region bonding the solid electrolyte layer and the negative electrode current collector together so that a pressure higher than the yield stress of metallic lithium is applied to the metallic lithium, and the second region having electronic conductivity and a Young's modulus lower than that of metallic lithium. However, there is room for improvement in the physical properties of the layer disposed between the solid electrolyte layer and the negative electrode layer.

[0006] The present invention has been made in view of the above, and has an object to provide a lithium metal battery that can obtain favorable battery characteristics by using an intermediate layer disposed between a solid electrolyte layer and a negative electrode layer.

[0007] (1) A lithium metal battery having a structure in which a negative electrode layer containing lithium metal, an intermediate layer, a solid electrolyte layer, and a positive electrode layer are stacked in this order, wherein the yield contact pressure of the intermediate layer is 5.0 MPa or less.

[0008] According to the invention of (1), it is possible to provide a lithium metal battery that can obtain preferable battery characteristics by using an intermediate layer disposed between the solid electrolyte layer and the negative electrode layer.

[0009] (2) The lithium metal battery according to (1), wherein the intermediate layer has a yield contact pressure of 4.0 MPa or less.

[0010] According to the invention (2), a lithium metal battery can be provided that can obtain more preferable battery characteristics.

[0011] (3) The lithium metal battery according to (1) or (2), wherein the intermediate layer has a composite elastic modulus of 200 MPa or less.

[0012] According to the invention (3), a lithium metal battery with a high discharge capacity can be provided.

[0013] (4) The lithium metal battery according to any one of (1) to (3), wherein the intermediate layer has a composite elastic modulus of 150 MPa or less.

[0014] According to the invention (4), a lithium metal battery with a higher discharge capacity can be provided.

[0015] (5) The density of the intermediate layer is 1.7 g / cm ３ The lithium metal battery according to any one of (1) to (4), which is as follows:

[0016] According to the fifth aspect of the present invention, the composite elastic modulus of the intermediate layer can be controlled to 200 MPa or less, and the yield contact pressure can be controlled to 5 MPa or less, so that a lithium metal battery can be provided that can obtain favorable battery characteristics.

[0017] (6) The density of the intermediate layer is 1.5 g / cm ３ The lithium metal battery according to any one of (1) to (5), which is:

[0018] According to the sixth aspect of the present invention, the composite elastic modulus of the intermediate layer can be controlled to 200 MPa or less, and the yield contact pressure can be controlled to 5 MPa or less, so that a lithium metal battery can be provided that can obtain favorable battery characteristics.

[0019] (7) The density of the intermediate layer is 1.4 g / cm ３ The lithium metal battery according to any one of (1) to (6), which is as follows:

[0020] According to the seventh aspect of the present invention, the composite elastic modulus of the intermediate layer can be controlled to 200 MPa or less, and the yield contact pressure can be controlled to 5 MPa or less, so that a lithium metal battery can be provided that can obtain favorable battery characteristics.

[0021] (8) The lithium metal battery according to any one of (1) to (7), wherein the intermediate layer has a composite elastic modulus of 100 MPa or more.

[0022] According to the invention (8), a lithium metal battery with a higher discharge capacity can be provided.

[0023] (9) The intermediate layer contains a conductive additive, and the specific surface area of ​​the conductive additive is 40 m ２ / g or more.

[0024] According to the invention of (9), even when the intermediate layer is pressed under high pressure during the manufacturing process, an increase in the density of the intermediate layer can be suppressed, and therefore, a lithium metal battery with favorable battery characteristics can be provided.

[0025] (10) The lithium metal battery according to any one of (1) to (9), wherein the intermediate layer contains Sn particles, and the Sn particles have an average particle size (D50) of 100 nm or more.

[0026] According to the invention of (10), the density of the intermediate layer can be reduced, and the composite elastic modulus of the intermediate layer can be controlled to 200 MPa or less, and the yield contact pressure can be controlled to 5 MPa or less, so that a lithium metal battery with favorable battery characteristics can be provided.

[0027] FIG. 1 is a diagram showing the configuration of a lithium metal battery according to an embodiment of the present invention; FIG. 2 is a diagram showing the configuration of a test battery used in the examples of the present invention; FIG. 3 is a graph showing the relationship between the composite elastic modulus of an intermediate layer according to an example and the discharge capacity of a lithium metal battery; FIG. 4 is a graph showing the relationship between the yield contact pressure of an intermediate layer according to an example and the discharge capacity of a lithium metal battery; FIG. 5 is a graph showing the relationship between the compressed density of an intermediate layer according to an example and the composite elastic modulus of an intermediate layer; FIG. 6 is a graph showing the relationship between the compressed density of an intermediate layer according to an example and the yield contact pressure of an intermediate layer; and FIG. 7 is a graph showing the relationship between the compressed density of an intermediate layer according to an example and the discharge capacity of a lithium metal battery.

[0028] [Lithium Metal Battery] As shown in Fig. 1, the lithium metal battery 1 has an electrode stack in which an anode layer 2, an intermediate layer 5, and a solid electrolyte layer 4 are stacked in this order. In this embodiment, the structure in which the anode layer 2, intermediate layer 5, solid electrolyte layer 4, cathode layer 3, solid electrolyte layer 4, intermediate layer 5, and anode layer 2 are stacked in this order as shown in Fig. 1 will be described as the stacked structure of the lithium metal battery 1. However, the structure of the lithium metal battery 1 is not limited to the above, and it is sufficient that the lithium metal battery 1 has a structure in which the intermediate layer 5 is stacked between the anode layer 2 and the solid electrolyte layer 4.

[0029] (Negative Electrode Layer) The negative electrode layer 2 includes an anode active material layer 21 and an anode current collector layer 22. The negative electrode active material layer 21 contains lithium metal as the negative electrode active material. The lithium metal includes elemental lithium metal, lithium alloys, and mixtures thereof. In the lithium alloy, elements that can form an alloy with lithium metal include, for example, Al, Mg, K, Na, Ca, Sr, Ba, Si, Ge, Sb, Pb, Sn, In, and Zn.

[0030] The lithium metal battery 1 may be an anode-free battery in which the negative electrode active material layer 21 is not present during the initial charge. In this case, a lithium metal layer is formed as the negative electrode active material layer 21 after the initial charge / discharge.

[0031] The negative electrode active material layer 21 may contain other materials that can be contained in a negative electrode active material layer of a solid-state battery. Examples of such materials include a solid electrolyte, a conductive additive, and a binder. Examples of the solid electrolyte include the same solid electrolyte contained in the solid electrolyte layer 4 described below. Examples of the conductive additive include carbon black, natural graphite, carbon fiber, and carbon nanotubes. Examples of the binder include a nitrile polymer, a polyester polymer, an acrylic acid polymer, a cellulose polymer, a styrene polymer, a styrene-butadiene polymer, a vinyl acetate polymer, a urethane polymer, a vinylidene fluoride polymer, and a fluoroethylene polymer.

[0032] The negative electrode current collector layer 22 is not particularly limited, but may be made of copper, nickel, stainless steel, etc. Examples of the shape of the negative electrode current collector layer 22 include foil, plate, mesh, nonwoven fabric, and foam. A portion of the negative electrode current collector layer 22 extends in a predetermined direction to form a negative electrode current collector tab 22a.

[0033] (Intermediate Layer) The intermediate layer 5 is disposed between the negative electrode layer 2 and the solid electrolyte layer 4. The intermediate layer 5 has the function of uniformly depositing lithium metal, thereby stabilizing the interface between the intermediate layer 5 and the solid electrolyte layer 4.

[0034] The material constituting the intermediate layer 5 is not particularly limited, but examples thereof include metals capable of alloying with lithium and conductive additives. Examples of metals capable of alloying with lithium include tin (Sn), silicon (Si), zinc (Zn), magnesium (Mg), gold (Au), platinum (Pt), palladium (Pd), silver (Ag), aluminum (Al), bismuth (Bi), and antimony (Sb). The metal capable of alloying with lithium may be nanoparticles. Amorphous carbon can be used as the conductive additive. Examples of amorphous carbon include carbon blacks such as acetylene black, furnace black, and ketjen black, as well as coke and activated carbon. The amorphous carbon may be graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), CNT (carbon nanotube), fullerene, or graphene. The intermediate layer may contain a binder in addition to the above materials. The binder may be the same as the binder that can be contained in the negative electrode active material layer 21.

[0035] The intermediate layer 5 preferably contains tin (Sn) as a metal capable of alloying with lithium. The tin (Sn) is preferably contained as Sn particles. The average particle size (D50) of the Sn particles can be 50 nm or more, preferably 100 nm or more, more preferably 130 nm or more, and may be 150 nm or more. It is believed that by having the average particle size (D50) of the Sn particles within the above range, the density of the intermediate layer 5 can be reduced, and the yield contact pressure and composite elastic modulus can be easily controlled within preferred ranges. The upper limit of the average particle size (D50) of the Sn particles is not particularly limited, but may be 500 nm or less.

[0036] The intermediate layer 5 has a yield contact pressure of 5.0 MPa or less, which can improve the discharge capacity of the lithium metal battery 1. The yield contact pressure is preferably 4.0 MPa or less. The yield contact pressure is measured by a nanoindentation test.

[0037] The intermediate layer 5 preferably has a composite elastic modulus of 200 MPa or less. This can improve the discharge capacity of the lithium metal battery 1. The composite elastic modulus is more preferably 150 MPa or less. The composite elastic modulus may be 100 MPa or more. The composite elastic modulus is measured by a nanoindentation test.

[0038] The reason why the discharge capacity of the lithium metal battery 1 is improved when the intermediate layer 5 has a yield contact pressure of 5.0 MPa or less or a composite elastic modulus of 200 MPa or less is thought to be as follows. The above conditions for the intermediate layer 5 improve the bonding strength at the interface between the intermediate layer 5 and the negative electrode active material layer 21 and at the interface between the intermediate layer 5 and the solid electrolyte layer 4. This is because the negative electrode active material layer 21 and the solid electrolyte layer 4 are layers with relatively high composite elastic modulus and yield contact pressure. It is presumed that the improved bonding strength at the interfaces allows the reaction to proceed uniformly, improving the discharge capacity of the lithium metal battery 1.

[0039] The density (compressed density) of the intermediate layer 5 is 1.7 g / cm ３It is preferable that the density of the intermediate layer 5 is 1.5 g / cm or less. This makes it possible to control the composite elastic modulus of the intermediate layer 5 to 200 MPa or less and the yield contact pressure to 5 MPa or less. Therefore, the discharge capacity of the lithium metal battery 1 can be improved. The density of the intermediate layer 5 is 1.5 g / cm or less. ３ More preferably, it is 1.4 g / cm or less. ３ It is more preferable that:

[0040] The specific surface area of ​​the material constituting the intermediate layer 5 is 20 m ２ / g or more 140m ２ / g or less, and ２ / g or more 65m ２ It is more preferable that the SiO2 content is 1 / g or less.

[0041] The intermediate layer 5 preferably contains a conductive additive, and the conductive additive has a specific surface area of ​​40 m ２ The conductive additive satisfying the above conditions has a complex shape, and therefore it is difficult to increase the density of the intermediate layer 5 even when the intermediate layer 5 is pressed under high pressure during production. Therefore, it is preferable to set the density of the intermediate layer 5 to 1.7 g / cm or more. ３ This makes it easier to control the discharge capacity of the lithium metal battery 1, resulting in an improvement in the discharge capacity.

[0042] (Solid Electrolyte Layer) The solid electrolyte layer 4 is formed between the anode layer 2 and the cathode layer 3. In this embodiment, the solid electrolyte layer 4 is a single layer, but the solid electrolyte layer 4 may be made up of multiple layers.

[0043] The solid electrolyte material constituting the solid electrolyte layer 4 is not particularly limited, and may be any material that can be used as an electrolyte in a solid-state battery. Examples include inorganic solid electrolytes such as sulfide solid electrolyte materials, oxide solid electrolyte materials, halide solid electrolytes, and lithium-containing salts, and polymer solid electrolytes such as polyethylene oxide. The above solid electrolytes may be used alone or in combination of two or more.

[0044] In addition to the solid electrolyte material, the solid electrolyte layer 4 may contain materials that can be used in solid electrolyte layers of solid-state batteries, such as a binder or a substrate. The binder may be the same as the binder that can be contained in the negative electrode active material layer 21. Examples of the substrate include three-dimensional structures such as mesh, woven fabric, nonwoven fabric, embossed body, punched body, expanded body, and foam body.

[0045] (Positive Electrode Layer) The positive electrode layer 3 has a positive electrode active material layer 31 and a positive electrode current collector layer 32. In this embodiment, the positive electrode layer 3 has a configuration in which two positive electrode active material layers 31 are stacked on both sides of one positive electrode current collector layer 32. However, the configuration of the positive electrode layer 3 is not limited to the above, and the positive electrode layer 3 may have a configuration in which one positive electrode active material layer 31 is stacked on one side of one positive electrode current collector layer 32.

[0046] The positive electrode active material layer 31 is not particularly limited and can be made of a material that can be used as a positive electrode active material for a solid-state battery. Examples of the positive electrode active material that can be used to make the positive electrode active material layer 31 include LiCoO ２ , LiNiO ２ , LiCo ｘ Ni ｙ Mn ｚ O ２ (x+y+z=1), LiVO ２ , LiCrO ２ Layered positive electrode active material particles such as LiMn ２ O ４ , Li(Ni ０．２５ Mn ０．７５ ) ２ O ４ , LiCoMnO ４ , Li ２ NiMn ３ O ８ Spinel-type positive electrode active materials such as LiCoPO ４ , LiMnPO ４ , LiFePO ４ Olivine-type positive electrode active materials such as solid solution oxides (Li ２ MnO ３ -LiMO ２ (M=Co, Ni, etc.), conductive polymers such as polyaniline and polypyrrole, Li ２ S, CuS, Li-Cu-S compound, TiS２ , FeS, MoS ２ , sulfides such as Li—Mo—S compounds, mixtures of sulfur and carbon, etc. The positive electrode active material may be one of the above materials, or may be composed of two or more of the above materials.

[0047] An insulating frame 6 may be provided on the outer periphery of the positive electrode active material layer 31. The insulating frame 6 can prevent short circuits in the lithium metal battery 1 and improve its strength. In this embodiment, the insulating frame 6 is arranged to cover the side surfaces of the two positive electrode active material layers 31 formed on both sides of the positive electrode current collector layer 32. The insulating frame 6 also abuts against a portion of the stacking surface of the positive electrode current collector layer 32, and has a gap through which a positive electrode current collector tab 32a (described later) extends. The material constituting the insulating frame 6 is not particularly limited, but examples thereof include insulating oxides such as alumina, resins such as polyvinylidene fluoride (PVDF), and rubbers such as styrene-butadiene rubber (SBR).

[0048] The positive electrode current collector layer 32 is not particularly limited, and can be made of, for example, aluminum, stainless steel, conductive carbon (graphite, carbon nanotubes, etc.), etc. Examples of the shape of the positive electrode current collector layer 32 include foil, plate, mesh, nonwoven fabric, and foam. A portion of the positive electrode current collector layer 32 extends in a predetermined direction to form a positive electrode current collector tab 32a.

[0049] [Method for Manufacturing Lithium Metal Battery] The method for manufacturing the lithium metal battery 1 is not particularly limited, and may include, for example, a step of press-bonding the solid electrolyte layer 4 and the intermediate layer 5 to obtain an intermediate layer-solid electrolyte layer laminate, and an integration press step of arranging and integrating the layers as shown in FIG. 1. Specific examples of a method for arranging the intermediate layer 5 on the laminate surface of the solid electrolyte layer 4 include a method of transferring the intermediate layer using an intermediate layer transfer sheet. The intermediate layer transfer sheet can be obtained, for example, by dispersing the material constituting the intermediate layer 5 in a solvent to obtain a slurry, which is then applied to a support sheet and dried.

[0050] In the step of press-bonding the solid electrolyte layer 4 and the intermediate layer 5 to obtain an intermediate layer-solid electrolyte layer laminate, the pressing pressure can be, for example, 600 MPa or more, and may be 900 MPa or more. This improves the bonding strength between the solid electrolyte layer 4 and the intermediate layer 5. In addition, when the intermediate layer 5 has a specific surface area of ​​40 m, ２ When the conductive additive is contained in the intermediate layer 5, the density of the intermediate layer 5 is 1.7 g / cm 3 even under the above pressing pressure. ３ It is easy to control below.

[0051] Although the preferred embodiment of the present invention has been described above, the present invention is not limited to the above embodiment. The lithium metal battery 1 may have a configuration that can be used in a solid-state battery, such as an exterior body, in addition to the electrode stack shown in FIG.

[0052] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the contents of the following examples.

[0053] [Fabrication of a Test Lithium Metal Battery Cell] A test lithium metal cell 1a was fabricated by stacking the layers as shown in Figure 2. The reference symbols in Figure 2 correspond to the reference symbols in Figure 1, and therefore detailed explanations will be omitted. Copper foil was used as the negative electrode current collector layer 22 in Figure 2, a lithium metal layer (40 μm thick) was used as the negative electrode active material layer 21, and an argyrodite-type sulfide solid electrolyte was used as the solid electrolyte layer 4.

[0054] The intermediate layer 5 of the test lithium metal cell 1a was made of 23.75% by mass of tin (Sn), 71.25% by mass of a conductive additive (carbon black), and 5.0% by mass of a binder (polyvinylidene fluoride, PVDF). Intermediate layer-forming compositions #1 to #5, with the types and combinations of tin and conductive additives shown in Table 1 below, were prepared, and test lithium metal cells 1a were fabricated for each composition and the following tests were performed.

[0055]

[0056] Nanoindentation Test The composite elastic modulus (MPa) and yield contact pressure (MPa) of the intermediate layer were measured for each of the lithium metal test cells 1a #1 to #5 using a TriboIndenter (spherical indenter, tip radius: 4 μm) manufactured by Hysitron.

[0057] <Measurement of Density (Compressed Density) of Intermediate Layer> The density of the intermediate layer in each of the test lithium metal cells 1a #1 to #5 was measured. Specifically, the weight of an electrode punched to a diameter of 10 mm was measured using an electronic balance (HR202i, manufactured by A&D Co., Ltd.), and the electrode was pressed at 900 MPa using a hydraulic press (cylinder: MS1-150, pump: P-1B, manufactured by Riken Seiki Co., Ltd.). The electrode thickness was then measured using a micrometer (C112RXB, manufactured by Mitutoyo Co., Ltd.), and the density was calculated.

[0058] <Measurement of Discharge Capacity> Using the test lithium metal cells 1a #1 to #5, the discharge capacity (mAh) of each cell was measured. The discharge capacity (mAh) was measured using Toyo Systems charge / discharge testers, TOSCAT-3100 and TOSCAT-3200, and Espec thermostatic chambers, LU-123 and LU-124. The discharge capacity data for each cell was obtained by a discharge test at 25°C after aging at 60°C. For aging at 60°C, a constant current test (CC test) was performed with a current value of 0.1C, in which the cell was charged to 3.3 mAh and discharged to 3.0 mAh once, and then charged to 3.0 mAh and discharged to 3.0 mAh once. Note that 1C is 3.8 mA / cm ２ The test at 25°C was a constant current test in which the cells were charged to 3 mAh at a current value of 0.05 C, and then discharged to a lower limit voltage of -0.2 V at a current value of 1 C, and the discharge capacity at this time was compared for each cell.

[0059] The relationships between the above measured data are shown in Figures 3 to 7. In each figure, there are multiple data points for #1 to #5, but these are measurement results obtained by fabricating multiple cells under the same conditions.

[0060] [Relationship between the composite elastic modulus of the intermediate layer and the discharge capacity] The relationship between the composite elastic modulus (MPa) of the intermediate layer and the discharge capacity (mAh) is shown in Figure 3. As shown in Figure 3, a significant correlation was observed between the composite elastic modulus (MPa) of the intermediate layer and the discharge capacity (mAh), and when the composite elastic modulus of the intermediate layer was 200 MPa or less, a preferable discharge capacity (mAh) of the lithium metal cell 1a was obtained.

[0061] [Relationship between yield contact pressure of intermediate layer and discharge capacity] The relationship between the yield contact pressure (MPa) of the intermediate layer and the discharge capacity (mAh) is shown in Figure 4. As shown in Figure 4, a significant correlation was observed between the yield contact pressure (MPa) of the intermediate layer and the discharge capacity (mAh), and a preferable discharge capacity (mAh) of the lithium metal cell 1a was obtained when the yield contact pressure of the intermediate layer was 5 MPa or less.

[0062] [Relationship between Compressed Density of Intermediate Layer and Composite Elastic Modulus] The compressed density of the intermediate layer (g / cm ３ The relationship between the compressed density (g / cm) of the intermediate layer and the composite elastic modulus (MPa) is shown in Figure 5. As shown in Figure 5, ３ ) and the composite elastic modulus (MPa), and the compressed density of the intermediate layer was 1.7 g / cm ３ When the thickness is less than 200 MPa, the composite elastic modulus of the intermediate layer can be easily adjusted to 200 MPa or less.

[0063] [Relationship between Compressed Density of Intermediate Layer and Yield Contact Pressure] The compressed density of the intermediate layer (g / cm ３ The relationship between the compressed density (g / cm ) of the intermediate layer and the yield contact pressure (MPa) is shown in Figure 6. As shown in Figure 6, ３ ) and the yield contact pressure (MPa), a significant correlation was observed. ３ When the temperature is below 5.0 MPa, the yield contact pressure of the intermediate layer can be easily adjusted to 5.0 MPa or less.

[0064] [Relationship between Compressed Density of Intermediate Layer and Discharge Capacity] The compressed density (g / cm ３ 7 shows the relationship between the compressed density (g / cm 3 ) of the intermediate layer and the discharge capacity (mAh). ３ ) and discharge capacity (mAh), and the compressed density of the intermediate layer was 1.7 g / cm３ A preferable discharge capacity (mAh) of the lithium metal cell 1a was obtained when:

[0065] 1 Lithium metal battery 2 Anode layer 4 Solid electrolyte layer 5 Intermediate layer

Claims

1. A lithium metal battery having a structure in which a negative electrode layer containing lithium metal, an intermediate layer, and a solid electrolyte layer are laminated in this order, wherein the yield contact pressure of the intermediate layer is 5.0 MPa or less.

2. The lithium metal battery of claim 1, wherein the intermediate layer has a yield contact pressure of 4.0 MPa or less.

3. The lithium metal battery of claim 1, wherein the intermediate layer has a composite elastic modulus of 200 MPa or less.

4. The lithium metal battery of claim 1, wherein the intermediate layer has a composite elastic modulus of 150 MPa or less.

5. The density of the intermediate layer is 1.7 g / cm ３ 2. The lithium metal battery of claim 1, wherein:

6. The density of the intermediate layer is 1.5 g / cm ３ 2. The lithium metal battery of claim 1, wherein:

7. The density of the intermediate layer is 1.4 g / cm ３ 2. The lithium metal battery of claim 1, wherein:

8. The lithium metal battery of claim 1, wherein the intermediate layer has a composite elastic modulus of 100 MPa or greater.

9. The intermediate layer contains a conductive additive, and the specific surface area of ​​the conductive additive is 40 m ２ The lithium metal battery according to any one of claims 1 to 8, wherein the Cr content is 1 / g or more.

10. The lithium metal battery according to any one of claims 1 to 8, wherein the intermediate layer contains Sn particles, and the average particle size (D50) of the Sn particles is 100 nm or more.

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