Solid-state secondary battery

Abstract

Provided is a solid-state secondary battery having a low direct current resistance in a charged state and improved cycle characteristics. The solid-state secondary battery of the present application has an electrode laminate in which a positive electrode layer, a negative electrode layer, a solid-state electrolyte layer provided between the positive electrode layer and the negative electrode layer, and an intermediate layer provided between the negative electrode layer and the solid-state electrolyte layer are laminated, and the negative electrode layer contains a negative electrode current collector having an arithmetic mean roughness Ra of less than 2.5 μm, and the intermediate layer has a complex elastic modulus smaller than that of the negative electrode current collector.

Description

Technical Field

[0001] This invention relates to a solid-state secondary battery. Background Technology

[0002] In recent years, research and development has been carried out on rechargeable batteries that can help improve energy efficiency in order to ensure that more people have access to affordable, reliable, sustainable and advanced energy.

[0003] As such secondary batteries, solid-state secondary batteries with a solid electrolyte layer disposed between a positive electrode layer and a negative electrode layer are known. As solid-state secondary batteries, secondary batteries combining a positive electrode and a negative electrode are known, wherein the positive electrode has a positive electrode active material layer containing a positive electrode active material capable of absorbing and releasing lithium ions, and the negative electrode, during charging, deposits lithium metal on the negative electrode current collector to form a lithium metal layer (e.g., Patent Document 1). Patent Document 1 discloses a negative electrode current collector, wherein the surface of the solid electrolyte layer side of the negative electrode current collector has a ten-point average roughness (Rz). JIS The thickness ranges from 3 to 50 μm.

[0004] [Existing Technical Documents]

[0005] (Patent Documents)

[0006] Patent Document 1: Japanese Patent Application Publication No. 2022-114216 Summary of the Invention

[0007] [The problem the invention aims to solve]

[0008] However, improving cycle performance is one of the challenges in solid-state secondary battery technologies. The inventors' research indicates that in solid-state secondary batteries with a lithium metal layer formed on the negative electrode current collector during charging, short circuits caused by current concentration, uneven lithium deposition, and the formation of local voids lead to an increase in DC resistance during charging, further deteriorating cycle performance.

[0009] The present invention was made in view of the above-mentioned circumstances, and its object is to provide a solid-state secondary battery with low DC resistance and improved cycle characteristics in the charging state. Furthermore, it further contributes to improving energy efficiency.

[0010] [Technical means to solve the problem]

[0011] The inventors have discovered that using a flat surface as the negative electrode current collector, and placing an intermediate layer with a smaller composite elastic modulus and greater deformability between the solid electrolyte layer and the negative electrode current collector, effectively solves the aforementioned problems, thus completing this invention. Therefore, this invention provides the following solution.

[0012] (1) A solid-state secondary battery having an electrode stack having a positive electrode layer, a negative electrode layer, a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer, and an intermediate layer disposed between the negative electrode layer and the solid electrolyte layer, wherein the negative electrode layer includes a negative electrode current collector, the arithmetic mean roughness Ra of the negative electrode current collector is less than 2.5 μm, and the composite elastic modulus of the intermediate layer is smaller than that of the negative electrode current collector.

[0013] According to the solid-state secondary battery of (1), since the surface of the negative electrode current collector has an arithmetic mean roughness Ra of less than 2.5 μm and is flat, it is not easy for short circuits or uneven lithium deposition caused by current concentration to occur during charging, thus making it difficult to form local voids in the lithium metal layer. In addition, since the composite elastic modulus of the intermediate layer is smaller than that of the negative electrode current collector, the deformation of the intermediate layer makes it easier for the negative electrode current collector and the intermediate layer to form a tight bond, so lithium becomes easier to move towards the negative electrode current collector during charging. Therefore, the solid-state secondary battery of (1) has low DC resistance in the charging state and improved cycle characteristics.

[0014] (2) According to the solid secondary battery described in (1), further, the maximum height roughness Rz of the aforementioned negative electrode current collector is less than 10 μm.

[0015] According to the solid-state secondary battery in (2), since the surface of the negative electrode current collector has a maximum height roughness Rz of less than 10 μm and the surface is finely textured, it is less likely to cause short circuits or uneven lithium deposition due to current concentration during charging, and thus it is more difficult to form local voids in the lithium metal layer.

[0016] (3) According to the solid secondary battery described in (1) or (2), before charging, the aforementioned negative electrode current collector is connected to the aforementioned intermediate layer.

[0017] According to the solid-state secondary battery in (3), since the negative electrode current collector is in contact with the intermediate layer, lithium becomes easier to move through the negative electrode current collector during charging.

[0018] (4) In any one of (1) to (3) the solid secondary battery, the arithmetic mean roughness Ra of the aforementioned intermediate layer on the side opposite to the aforementioned negative electrode current collector is less than 1 μm, and the maximum height roughness Rz is less than 4 μm.

[0019] According to the solid-state secondary battery in (4), since the surface of the intermediate layer on the side opposite to the negative electrode current collector has an arithmetic mean roughness Ra of less than 1 μm and is flat, and the maximum height roughness Rz of less than 4 μm and the surface has fine bumps, it is less likely to cause short circuits or uneven lithium deposition due to current concentration during charging, and thus it is more difficult to form local voids in the lithium metal layer.

[0020] (5) In any one of (1) to (4) the composite elastic modulus of the aforementioned intermediate layer of the solid secondary battery is less than 1 GPa.

[0021] According to the solid-state secondary battery in (5), since the composite elastic modulus of the intermediate layer is as described above, even when the thickness of the negative electrode layer changes due to charging and discharging, the contact area between the solid electrolyte layer and the intermediate layer, as well as between the negative electrode layer and the intermediate layer, can be increased. Therefore, the DC resistance during charging and discharging at high current densities can be reduced.

[0022] (6) In any one of (1) to (5) the solid secondary battery, the relative density of the aforementioned intermediate layer is 30 to 60%.

[0023] According to the solid-state secondary battery of (6), since the relative density of the intermediate layer is within the above range, the tightness between the solid electrolyte layer and the intermediate layer, and between the negative electrode layer and the intermediate layer, can be maintained. Therefore, the cycle characteristics of the solid-state secondary battery can be improved.

[0024] (7) The solid-state secondary battery according to any one of (1) to (6), wherein the aforementioned intermediate layer comprises amorphous carbon.

[0025] According to the solid-state secondary battery of (7), dendrite formation caused by the precipitation of charge-moving medium in the intermediate layer can be suppressed. Therefore, the cycle characteristics of the solid-state secondary battery can be further improved.

[0026] (8) In any one of (1) to (7) the composite elastic modulus of the aforementioned intermediate layer is smaller than that of the aforementioned solid electrolyte layer.

[0027] According to the solid-state secondary battery in (8), since the composite elastic modulus of the intermediate layer is smaller than that of the solid electrolyte layer, the solid electrolyte layer and the intermediate layer are more easily connected by the deformation of the intermediate layer, so lithium becomes easier to move to the intermediate layer during charging.

[0028] (The effect of the invention)

[0029] According to the present invention, a solid-state secondary battery is provided, which has low DC resistance and improved cycle characteristics in the charging state. Attached Figure Description

[0030] Figure 1 This is a cross-sectional view of a solid-state secondary battery according to an embodiment of the present invention.

[0031] Figure 2 It is a drawing Figure 1 The image shows a cross-sectional view of the solid-state secondary battery after charging.

[0032] Figure 3This is a SEM image of a cross-section of an electrode stack at 50% SOC, taken from the solid-state secondary battery obtained in Example 2.

[0033] Figure 4A This is a SEM image of a cross-section of an electrode stack at 50% SOC, taken from a solid-state secondary battery obtained in Comparative Example 1.

[0034] Figure 4B yes Figure 4A Enlarged SEM images. Detailed Implementation

[0035] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Furthermore, in this embodiment, the solid-state secondary battery is a lithium metal battery that uses lithium ions as the charge transport medium.

[0036] Figure 1 This is a cross-sectional view illustrating an example of a solid-state secondary battery according to the first embodiment of the present invention. Figure 2 It is a drawing Figure 1 A cross-sectional view of an example of a solid-state secondary battery after charging.

[0037] The solid-state secondary battery 1 of this embodiment has an electrode stack in which a positive electrode layer 10, a solid electrolyte layer 20, an intermediate layer 30, and a negative electrode layer 40 are stacked in this order. The positive electrode layer 10, the solid electrolyte layer 20, the intermediate layer 30, and the negative electrode layer 40 can be connected in a manner that allows lithium ions or lithium metal to move, and each layer can be bonded together. To facilitate the movement of lithium ions or lithium metal between layers, the solid-state secondary battery 1 can be constrained in the stacking direction by a subsequent component (not shown). Furthermore, in the following description, unless otherwise specified, the composite elastic modulus of the solid electrolyte layer 20, the intermediate layer 30, and the negative electrode layer 40 (negative electrode current collector 41) refers to the value after the electrode stack is assembled. Unless otherwise specified, the arithmetic mean roughness Ra and the maximum height roughness Rz of the intermediate layer 30 refer to the values ​​after the electrode stack is assembled. Unless otherwise specified, the arithmetic mean roughness Ra and maximum height roughness Rz of the negative electrode layer 40 (negative electrode current collector 41) refer to the values ​​after the electrode stack is assembled. "After the electrode stack is assembled" refers to the immediate state after the layers of the solid-state secondary battery 1 are stacked and the electrode stack is assembled. Furthermore, the arithmetic mean roughness Ra and maximum height roughness Rz of the negative electrode layer 40 (negative electrode current collector 41) will show the same values ​​before and after the assembly of the electrode stack, depending on their material properties and the assembly conditions of the electrode stack.

[0038] The positive electrode layer 10 has a positive current collector 11 and a positive active material layer 12 disposed on the surface of the positive current collector.

[0039] Examples of possible shapes for the positive current collector 11 include foil, plate, mesh, non-woven fabric, and foam. Examples of materials for the positive current collector 11 include aluminum, aluminum alloy, stainless steel, nickel, iron, and titanium.

[0040] The positive electrode active material layer 12 contains a positive electrode active material. The positive electrode active material is a lithium compound that releases lithium ions during charging and absorbs lithium ions during discharging. As the lithium compound, layered active materials, spinel-type active materials, and olivine-type active materials can be used, for example. Specific examples of positive electrode active materials include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and lithium nickel manganese cobalt oxide (NMC:LiNiO2). p Mn q Co r O2(p+q+r=1)), LiNi p Al q Co r O2(p+q+r=1), lithium manganese oxide (LiMn2O4), Li 1+x Mn 2-x-y M y O4 (x+y=2, M=at least one selected from Al, Mg, Co, Fe, Ni and Zn) represents heteroelement-substituted Li-Mn spinel, lithium titanate (an oxide containing Li and Ti), lithium metal phosphate (LiMPO4, M=at least one selected from Fe, Mn, Co and Ni), etc. The positive electrode active material layer 12 may further include conductive additives and binders.

[0041] The solid electrolyte layer 20 includes a solid electrolyte material 21. Examples of the solid electrolyte material 21 include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and halide solid electrolytes. Examples of sulfide solid electrolytes include Li₂S-P₂S₅ and Li₂S-P₂S₅-LiI. Sulfide solid electrolytes can have an argyroite-type crystal structure. Examples of oxide solid electrolytes include NASICON-type oxides, garnet-type oxides, and perovskite-type oxides. Examples of NASICON-type oxides include oxides containing Li, Al, Ti, P, and O (e.g., Li). 1.5 Al 0.5 Ti 1.5 (PO4)3). Examples of garnet-type oxides include oxides containing Li, La, Zr, and O (e.g., Li7La3Zr2O). 12 Examples of perovskite-type oxides include oxides containing Li, La, Ti, and O (e.g., LiLaTiO3). The thickness of the solid electrolyte layer 20 is, for example, in the range of 10–100 μm.

[0042] The solid electrolyte layer 20 may include a binder. As a binder, for example, resin-based binders, rubber-based binders, elastic binders, and cellulose-based binders can be used. Examples of resins include polyvinylidene fluoride, polytetrafluoroethylene, polyimide, polyamide, and polyamide-imide. Examples of rubber-based binders include butadiene rubber, styrene-butadiene rubber, nitrile rubber, acrylic rubber, butyl rubber, and fluororubber. Examples of elastic binders include styrene-ethylene-butene-styrene block copolymers, styrene-isoprene-styrene block copolymers, and other styrene-based block copolymers. Examples of cellulose-based binders include carboxymethyl cellulose, methyl cellulose, and ethyl cellulose. The binder content of the solid electrolyte layer 20 can be, for example, in the range of 0.5% to 10% by mass.

[0043] The composite elastic modulus of the solid electrolyte layer 20 can be in the range of 10 to 100 GPa.

[0044] An intermediate layer 30 is disposed between the solid electrolyte layer 20 and the negative electrode layer 40. The intermediate layer 30 has voids through which lithium metal, the charge transport medium of the solid-state secondary battery 1, can pass. By allowing lithium metal to pass through the intermediate layer 30, lithium metal can be uniformly deposited on the surface of the negative electrode layer 40. The thickness of the intermediate layer 30 is, for example, in the range of 0.3 to 5 μm.

[0045] The composite elastic modulus of the intermediate layer 30 can be less than 1 GPa. Alternatively, the composite elastic modulus of the intermediate layer 30 can be in the range of 600–800 MPa. When the composite elastic modulus of the intermediate layer 30 is within the above range, the flexibility of the intermediate layer 30 is improved, and even when the thickness of the negative electrode layer 40 changes due to charging and discharging, the contact area between the solid electrolyte layer 20 and the intermediate layer 30, and between the negative electrode layer 40 and the intermediate layer 30, can be increased. Therefore, the DC resistance under high current density can be reduced.

[0046] The composite elastic modulus of the intermediate layer 30 can be smaller than that of the solid electrolyte layer 20 and the negative electrode current collector 41. The composite elastic modulus of the intermediate layer 30 can be 0.1 to 10 GPa smaller than that of the solid electrolyte layer 20. The ratio of the composite elastic modulus of the intermediate layer 30 to that of the solid electrolyte layer 20 can be in the range of 1 / 1000 to 99 / 100. The composite elastic modulus of the intermediate layer 30 can be 0.1 to 10 GPa smaller than that of the negative electrode current collector 41. The ratio of the composite elastic modulus of the intermediate layer 30 to that of the negative electrode current collector 41 can be in the range of 4 / 10000 to 6 / 10.

[0047] The relative density of the intermediate layer 30 can be in the range of 30% to 60%. Relative density refers to the percentage of the density of the intermediate layer after molding relative to the true density. When the relative density of the intermediate layer 30 is at the above value, the adhesion between the solid electrolyte layer 20 and the intermediate layer 30, and between the negative electrode layer 40 and the intermediate layer 30 can be maintained, thereby improving the cycle characteristics of the solid secondary battery 1.

[0048] The intermediate layer 30 may contain a material with lithium metal conductivity and a material with electronic conductivity. For example, amorphous carbon particles can be used as the material with lithium metal conductivity. For example, a metal can be used as the material with electronic conductivity. The metal can be particles. The metal particles can be contained in the intermediate layer 30 in a mixture with amorphous carbon particles, or they can be contained in the intermediate layer 30 loaded onto amorphous carbon particles. Furthermore, the metal can exist as a film on the surface of the amorphous carbon particles, or it can permeate into the interior of the amorphous carbon particles. By containing amorphous carbon in the intermediate layer 30, dendrite formation caused by lithium deposition within the intermediate layer 30 can be suppressed. Therefore, the cycle characteristics of the solid-state secondary battery 1 are further improved.

[0049] Amorphous carbon can be either easily graphitized carbon (soft carbon) or difficult-to-graphitize carbon (hard carbon). Amorphous carbon simply refers to any substance that does not exhibit a clearly crystalline state among carbon allotropes; it can be an aggregate of fine graphite crystals. Specific examples of amorphous carbon include acetylene black, furnace black, Ketjen black, coke, activated carbon, CNTs (carbon nanotubes), fullerenes, and graphene.

[0050] The metal contained in the intermediate layer 30 can be metal particles that form an alloy with lithium. Examples of metals that form an alloy with lithium include Mg, Zn, Al, In, Si, Ge, Sn, Ag, Au, Pt, Pd, Pb, Sb, and Bi.

[0051] In addition to the substances described above, the intermediate layer 30 may also contain a binder. The same binder used in the solid electrolyte layer 20 can be used as the binder. However, the binder used in the intermediate layer 30 may be the same as or different from the binder used in the solid electrolyte layer 20. The binder content in the intermediate layer 30 may, for example, be in the range of 0.5% to 10% by mass.

[0052] The surface of the intermediate layer 30 facing the negative current collector 41 has an arithmetic mean roughness Ra of less than 1 μm. Furthermore, the surface of the intermediate layer 30 facing the negative current collector 41 has a maximum height roughness Rz of less than 4 μm. The surface of the intermediate layer 30 facing the negative current collector 41 also has an arithmetic mean roughness Ra of less than 0.3 μm and a maximum height roughness Rz of less than 1.5 μm.

[0053] The negative electrode layer 40 has a negative electrode current collector 41. The negative electrode current collector 41 is, for example, in the form of a foil or a plate. Lithium or a lithium-containing alloy is deposited on the surface of the negative electrode current collector 41. The material of the negative electrode current collector 41 can be a material that does not form an alloy with lithium or a material that forms an alloy with lithium. As a material that does not form an alloy with lithium, Cu, Cu alloys, Ni, Fe, and stainless steel can be used. As a material that can form an alloy with lithium, Mg, Zn, Al, In, Si, Ge, Sn, Ag, Au, Pt, Pd, Pb, Sb, and Bi can be used. The negative electrode current collector 41 can be covered with a metal thin film or a carbon-coated thin film. The metal thin film can be a metal that forms an alloy with lithium.

[0054] The surface of the negative electrode current collector 41 has an arithmetic mean roughness Ra of less than 2.5 μm and is flat. The arithmetic mean roughness Ra of the negative electrode current collector 41 can be less than 2.0 μm or less than 1.0 μm.

[0055] The surface of the intermediate layer 30 side of the negative current collector 41 can be configured to have a maximum height roughness Rz of less than 10 μm and a fine surface texture. The maximum height roughness Rz of the negative current collector 41 can be less than 8.0 μm or less than 5.0 μm. The negative current collector 41 can have an arithmetic mean roughness Ra of less than 2.5 μm and a maximum height roughness Rz of less than 10 μm, an arithmetic mean roughness Ra of less than 2.0 μm and a maximum height roughness Rz of less than 8.0 μm, or an arithmetic mean roughness Ra of less than 1.0 μm and a maximum height roughness Rz of less than 5.0 μm. The arithmetic mean roughness Ra and the maximum height roughness Rz of the negative current collector 41 are values ​​measured, for example, using a laser microscope. However, the measurement method is not limited to a laser microscope; other contact or non-contact surface roughness measurement methods can also be used.

[0056] The composite elastic modulus of the negative electrode current collector 41 can be 1 to 220 GPa.

[0057] When the solid-state secondary battery 1 is charged, lithium ions are released from the positive electrode active material layer 12 of the positive electrode layer 10 and deposited on the surface of the negative electrode current collector 41 of the negative electrode layer 40. Thus, as... Figure 2 As shown, a lithium metal layer 42 is formed on the surface of the negative electrode current collector 41. By forming the lithium metal layer 42, the solid-state secondary battery 1 becomes thicker after charging than before charging. When the charged solid-state secondary battery 1 is discharged, lithium ions are released from the lithium metal layer 42 and are absorbed by the positive electrode active material layer 12. In the discharged solid-state secondary battery 1, as... Figure 2 As shown, the negative current collector 41 is in close contact with the intermediate layer 30.

[0058] There are no particular limitations on the manufacturing method of the solid-state secondary battery 1. The solid-state secondary battery 1 can be manufactured by, for example, stacking a positive electrode layer 10, a solid electrolyte layer 20, an intermediate layer 30, and a negative electrode layer 40 in this order, and then pressurizing the resulting stack. By pressing, an electrode stack in which the positive electrode layer 10, the solid electrolyte layer 20, the intermediate layer 30, and the negative electrode layer 40 are respectively bonded is formed.

[0059] According to the solid-state secondary battery 1 of this embodiment with the above-described structure, since the surface of the negative electrode current collector 41 has an arithmetic mean roughness Ra of less than 2.5 μm and is flat, short circuits or uneven lithium deposition due to current concentration are less likely to occur during charging, thus making it difficult to form local voids in the lithium metal layer. Furthermore, since the composite elastic modulus of the intermediate layer 30 is smaller than that of the negative electrode current collector 41, deformation of the intermediate layer 30 facilitates close contact between the negative electrode current collector 41 and the intermediate layer 30, making it easier for lithium to move towards the negative electrode current collector during charging. Therefore, the solid-state secondary battery 1 has low DC resistance in the charging state and improved cycle characteristics.

[0060] In the solid-state secondary battery 1 of this embodiment, since the surface of the negative electrode current collector 41 has fine unevenness when the maximum height roughness Rz of the negative electrode current collector 41 is less than 10 μm, it is less likely to cause short circuits or uneven lithium deposition due to current concentration during charging, and thus it is more difficult to form local voids in the lithium metal layer.

[0061] In the solid-state secondary battery 1 of this embodiment, if the negative electrode current collector 41 is already in close contact with the intermediate layer 30 before charging, lithium is more likely to move towards the negative electrode current collector 41 during charging.

[0062] In the solid-state secondary battery 1 of this embodiment, when the arithmetic mean roughness Ra and the maximum height roughness Rz of the surface of the intermediate layer 30 on the side opposite to the negative electrode current collector 41 are within the above range, the surface of the intermediate layer 30 is flat and has fine unevenness. Therefore, it is less likely to cause short circuits or uneven lithium deposition due to current concentration during charging, and it is more difficult to form local voids in the lithium metal layer.

[0063] In the solid-state secondary battery 1 of this embodiment, when the composite elastic modulus of the intermediate layer 30 is smaller than that of the solid electrolyte layer 20, the solid electrolyte layer 20 and the intermediate layer 30 are easily connected by deformation of the intermediate layer 30, so lithium becomes easier to move to the intermediate layer 30 during charging.

[0064] The embodiments of the present invention have been described above, but the present invention is not limited to the above embodiments. For example, in this embodiment, although the solid-state secondary battery 1 uses a lithium metal battery that uses lithium ions as the charge transfer medium, the charge transfer medium is not limited to this. The solid-state secondary battery 1 of this embodiment can be a solid-state secondary battery in which the thickness of its negative electrode layer changes with charging and discharging.

[0065]

Example

[0066] The present invention will be described below with reference to embodiments, but the present invention is not limited to these embodiments.

[0067] [Example 1]

[0068] (1) Fabrication of the positive electrode layer

[0069] A 15 μm thick aluminum foil was prepared as the positive electrode current collector. 80 parts by mass of lithium nickel cobalt manganese composite oxide (NCM622) was used as the positive electrode active material, 17 parts by mass of sulfide-germanium sulfide solid electrolyte was used as the solid electrolyte material, 2 parts by mass of carbon black was used as a conductive additive, and 1 part by mass of SBR (styrene-butadiene rubber) binder was used as a binder. The resulting mixture was dispersed in 43 parts by mass of butyl butyrate to prepare a positive electrode active material slurry. The obtained positive electrode active material slurry was coated onto both sides of the positive electrode current collector using a doctor blade, achieving a dry area weight of 27 mg / cm². 2 After drying, a positive electrode active material layer with a thickness of 80 μm is formed, thus obtaining the positive electrode layer.

[0070] (2) Fabrication of solid electrolyte layer transfer sheet

[0071] 97 parts by mass of a sulfide solid electrolyte of silver-germanium sulfide type (median particle size 3.0 μm) and 3 parts by mass of an SBR (styrene-butadiene rubber) binder were mixed in this ratio. The resulting mixture was dispersed in a solvent to prepare a solid electrolyte slurry. The obtained solid electrolyte slurry was coated onto a support sheet and dried to obtain a solid electrolyte layer transfer sheet (solid electrolyte layer thickness: 100 μm).

[0072] (3) Fabrication of intermediate layer transfer sheet

[0073] Sn particles (average particle size: 0.07 μm) were used as metal particles, and acetylene black (average particle size: 0.05 μm) were used as amorphous carbon particles. A total of 95 parts by mass of both were mixed with 5 parts by mass of PVDF-based binder as a binder. The resulting mixture was dispersed in 1000 parts by mass of NMP (N-methyl-2-pyrrolidone) to prepare an intermediate layer slurry. The obtained intermediate layer slurry was coated onto a support sheet and dried to obtain an intermediate layer transfer sheet (intermediate layer thickness: 3.0 μm).

[0074] (4) Negative electrode layer

[0075] Electrolytic nickel foil is prepared as the negative electrode current collector. The thickness T, arithmetic mean roughness Ra, maximum height roughness Rz, and the ratio of maximum height roughness Rz to thickness T Rz / T of the electrolytic nickel foil are shown in Table 1 below.

[0076] (5) Fabrication of solid-state secondary batteries

[0077] On the surface of the positive electrode active material layer of the positive electrode layer, a solid electrolyte layer transfer sheet is laminated, and the solid electrolyte layer is bonded using a uniaxial forming press under bonding conditions of 90 MPa, 3 minutes, and room temperature. Subsequently, the support sheet of the solid electrolyte layer transfer sheet is peeled off to obtain a positive electrode layer-solid electrolyte layer composite. Next, on the surface of the solid electrolyte layer of the positive electrode layer-solid electrolyte layer composite, an intermediate layer transfer sheet is laminated, and the intermediate layer is bonded using a uniaxial forming press under bonding conditions of 290 MPa, 5 minutes, and room temperature. Subsequently, the support sheet of the intermediate layer transfer sheet is peeled off to obtain a positive electrode layer-solid electrolyte layer-intermediate layer composite. Then, using an isotropic pressing press, the entire positive electrode layer-solid electrolyte layer-intermediate layer composite is densified under bonding conditions of 980 MPa, 5 minutes, and 120°C. Next, a negative electrode current collector is stacked on the surface of the intermediate layer of the positive electrode layer-solid electrolyte layer-intermediate layer junction to obtain an electrode stack. The obtained electrode stack is housed in an aluminum-plastic composite film casing to form a solid-state secondary battery. A buffer material is placed on the negative electrode layer side and constrained with a 3 MPa constraint pressure.

[0078] [Examples 2-5, Comparative Example 1]

[0079] Except for using electrolytic copper foil (Example 2), electrolytic nickel foil (Example 3), silver-plated electrolytic copper foil (Example 4), carbon-plated electrolytic copper foil (Example 5), and rough-surfaced nickel-plated electrolytic copper foil (Comparative Example 1) as negative electrode current collectors, the solid-state secondary battery was fabricated in the same manner as in Example 1. The thickness T, arithmetic mean roughness Ra, maximum height roughness Rz, and the ratio of maximum height roughness Rz to thickness T Rz / T for each negative electrode current collector are shown in Table 1 below.

[0080] [Comparative Example 2]

[0081] Except for the fact that the intermediate layer is not bonded to the surface of the solid electrolyte layer in the positive electrode layer-solid electrolyte layer stack, but instead the negative electrode current collector of the negative electrode layer is stacked on the surface of the solid electrolyte layer to serve as the electrode layer stack, the solid secondary battery is fabricated in the same way as in Example 1.

[0082] [Table 1]

[0083]

[0084] [evaluate]

[0085] For the solid-state secondary batteries fabricated in Examples 1-5 and Comparative Examples 1 and 2, the composite elastic modulus of the solid electrolyte layer, intermediate layer, and negative electrode current collector, the relative density of the intermediate layer, the arithmetic mean roughness Ra, the maximum height roughness Rz, and the initial internal resistance of the battery were measured using the following methods. Cross-sectional observation of the lithium metal layer at 50% SOC and cycle tests were also performed. Furthermore, the thickness T, arithmetic mean roughness Ra, and maximum height roughness Rz of the negative electrode current collector in the solid-state secondary batteries fabricated in Examples 1-5 and Comparative Examples 1 and 2 were the same as before the fabrication of the solid-state secondary batteries.

[0086] (Determination of the composite elastic modulus of the solid electrolyte layer, intermediate layer and negative electrode current collector)

[0087] The newly assembled solid-state secondary battery was disassembled, and the electrode stack was removed. The composite elastic modulus of the solid electrolyte layer, intermediate layer, and negative electrode current collector of the obtained electrode stack was measured by nanoindentation.

[0088] (Relative density of the intermediate layer, arithmetic mean roughness Ra, maximum height roughness Rz)

[0089] The newly assembled solid-state secondary battery was disassembled, the electrode stack was removed, and the relative density, arithmetic mean roughness Ra, and maximum height roughness Rz of the intermediate layer were measured.

[0090] The relative density is calculated using the following formula (1). Furthermore, the "filling density" in formula (1) can be calculated by multiplying the area density of the intermediate layer by its thickness. The area density of the intermediate layer can be calculated based on the area and weight of the intermediate layer before transfer. The thickness of the intermediate layer can be determined by observing the cross-section of the electrode laminate using SEM (scanning electron microscope) after cross-sectioning the electrode laminate.

[0091] Relative density (%) = Filled density (g / cc) / True density (g / cc) × 100……(1)

[0092] In addition, the method for calculating relative density is not limited to the above methods. It can also be calculated by BET method, porosity meter, gas diffusion and other instrument analysis or SEM image analysis.

[0093] The arithmetic mean roughness Ra and maximum height roughness Rz of the intermediate layer were measured using a laser microscope. The intermediate layer was exposed after the negative current collector was removed from the electrode stack.

[0094] (Measurement of initial DC resistance value)

[0095] The solid-state secondary battery was charged under constant current and constant voltage (CCCV) conditions at 25°C, 0.1 C, and 4.3 V. Then, it was discharged to 2.65 V under constant current (CCCV) conditions at 60°C, 0.1 C, and 60°C. Following this initial charge-discharge cycle, after aging, the SOC was adjusted to 50% under constant current and constant voltage (CCCV) conditions at 60°C, 0.1 C, and the voltage corresponding to 50% SOC. The solid-state secondary battery with the adjusted SOC was then tested at 25°C and a current density of 15.1 mA / cm². 2 Voltage drop ΔV (V), current I (A), and positive electrode area Ac (cm²) during discharge 2 The initial internal resistance (Ω·cm) can be calculated using the following formula. 2 The results are shown in Table 2 below.

[0096] Initial DC resistance (Ω·cm) 2 = Voltage drop ΔV (V) / Current value I (A) × Positive electrode area Ac (cm²) 2 )

[0097] (Cross-sectional observation of lithium metal layer at 50% SOC)

[0098] Similar to the initial DC resistance measurement described above, after the first charge and discharge, the solid-state secondary battery with a SOC adjusted to 50% was disassembled, and the electrode stack was removed. Using SEM (scanning electron microscopy), the cross-section of the lithium metal layer deposited on the surface of the negative electrode current collector of the removed electrode stack was observed. Ten observations were made on the lithium metal layer cross-section. Cases where no voids were observed in the lithium metal layer at any of the ten locations were recorded as "no voids," and cases where voids were observed in the lithium metal layer at at least one of the ten locations were recorded as "voids present." The results are shown in Table 2 below. Figure 3 A cross-sectional SEM image of the electrode stack of the solid-state secondary battery fabricated in Example 2 is shown. Figure 4A and Figure 4B A cross-sectional SEM image of the electrode stack of the solid-state secondary battery fabricated in Comparative Example 1 is shown.

[0099] (Cyclic test)

[0100] Under conditions of 45℃, 1 / 3 C current, upper voltage limit of 4.3 V, and lower voltage limit of 2.65 V, charge-discharge cycle tests were conducted using constant current constant voltage (CCCV) charging and constant current (CC) discharging. The discharge capacity retention rate after the 50th cycle is shown in Table 2.

[0101] [Table 2]

[0102]

[0103] From the results in Table 2 and Figure 3 The cross-sectional SEM images show that the arithmetic mean roughness Ra of the negative electrode current collector is within the scope of this invention. The solid-state secondary batteries of Examples 1-5, which have an intermediate layer between the solid electrolyte layer and the negative electrode current collector with a composite elastic modulus smaller than that of the negative electrode current collector, exhibit low initial DC resistance and no voids in the lithium metal layer of the negative electrode layer. Furthermore, they show high discharge capacity retention after 50 cycles, demonstrating good cycle characteristics. In contrast, the solid-state secondary battery of Comparative Example 1, whose arithmetic mean roughness Ra of the negative electrode current collector exceeds the scope of this invention, has a high initial DC resistance, such as... Figure 4A and Figure 4B As shown by the black circle, a void was created in the lithium metal layer 42 between the negative electrode current collector 41 and the intermediate layer 30, resulting in a lower discharge capacity retention rate after the 50th cycle. Furthermore, in Comparative Example 2, the solid-state secondary battery without an intermediate layer between the solid electrolyte layer and the negative electrode current collector, the initial DC resistance value increased significantly, and short-circuit behavior occurred during charging at 45°C and a current of 1 / 3 C, making further cycle testing impossible.

[0104] Figure Labels

[0105] 1: Solid-state rechargeable battery

[0106] 10: Positive electrode layer

[0107] 11: Positive current collector

[0108] 12: Positive electrode active material layer

[0109] 20: Solid electrolyte layer

[0110] 21: Solid electrolyte materials

[0111] 30: Intermediate layer

[0112] 40: Negative electrode layer

[0113] 41: Negative current collector

[0114] 42: Lithium metal layer.

Claims

1. A solid-state secondary battery comprising an electrode stack having a positive electrode layer, a negative electrode layer, a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer, and an intermediate layer disposed between the negative electrode layer and the solid electrolyte layer, wherein... The aforementioned negative electrode layer includes a negative electrode current collector. The arithmetic mean roughness Ra of the aforementioned negative electrode current collector is less than 2.5 μm. The composite elastic modulus of the aforementioned intermediate layer is smaller than that of the aforementioned negative electrode current collector.

2. The solid-state secondary battery according to claim 1, wherein, Furthermore, the maximum height roughness Rz of the aforementioned negative electrode current collector is less than 10 μm.

3. The solid-state secondary battery according to claim 1 or 2, wherein, Before charging, the aforementioned negative current collector is connected to the aforementioned intermediate layer.

4. The solid-state secondary battery according to claim 1 or 2, wherein, The arithmetic mean roughness Ra of the aforementioned intermediate layer on the side opposite to the aforementioned negative electrode current collector is less than 1 μm, and the maximum height roughness Rz is less than 4 μm.

5. The solid-state secondary battery according to claim 1 or 2, wherein, The composite elastic modulus of the aforementioned intermediate layer is less than 1 GPa.

6. The solid-state secondary battery according to claim 1 or 2, wherein, The relative density of the aforementioned intermediate layer is 30-60%.

7. The solid-state secondary battery according to claim 1 or 2, wherein, The aforementioned intermediate layer contains amorphous carbon.

8. The solid-state secondary battery according to claim 1 or 2, wherein, The composite elastic modulus of the aforementioned intermediate layer is smaller than that of the aforementioned solid electrolyte layer.

Images