Solid secondary battery

Abstract

The present invention addresses the problem of providing a solid secondary battery having low DC resistance and excellent cycle characteristics. In order to solve the problem, this solid secondary battery has a positive electrode layer, a negative electrode layer that includes at least a negative electrode current collector, a solid electrolyte layer that contains a solid electrolyte material, and an intermediate layer that is provided between the negative electrode layer and the solid electrolyte layer, and forms an electrode laminate, the positive electrode layer, the solid electrolyte layer, the intermediate layer, and the negative electrode layer are bonded together, and when the adhesion strength of the solid electrolyte layer is A1, the adhesion strength of the interface between the solid electrolyte layer and the intermediate layer is A2, and the adhesion strength of the intermediate layer is A3, A1, A2, and A3 are satisfied. And a prescribed relationship is satisfied.

Description

Technical Field

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

[0002] In recent years, research and development related to secondary batteries that contribute to energy efficiency have been carried out 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 are known to have a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer. In solid-state secondary batteries, to improve the adhesion strength of the solid electrolyte layer, a binder having functional groups such as carboxyl, carbonyl, or hydroxyl groups is sometimes used (see Patent Document 1). In addition, an intermediate layer is sometimes disposed between the negative electrode layer and the solid electrolyte layer (see Patent Document 2).

[0004] [Previous Technical Documents]

[0005] (Patent Documents)

[0006] Patent Document 1: Japanese Patent Application Publication No. 2018-129222

[0007] Patent Document 2: International Publication No. 2023 / 189892 Summary of the Invention

[0008] [The problem the invention aims to solve]

[0009] Furthermore, one of the challenges in solid-state secondary battery technology is reducing DC resistance. In particular, in solid-state secondary batteries with an intermediate layer between the negative electrode layer and the solid electrolyte layer, excessive increases in DC resistance not only degrade output characteristics but also may lead to internal short circuits during charging due to the deposition of charge-carrying media (e.g., lithium ions) between the solid electrolyte layer and the intermediate layer. Alternatively, uneven deposition can increase DC resistance and decrease cycle performance. Therefore, further reduction of DC resistance is required in solid-state secondary batteries with an intermediate layer.

[0010] The present invention was made in view of the above circumstances, and its object is to provide a solid-state secondary battery with low DC resistance and excellent cycle characteristics. Furthermore, it contributes to energy efficiency.

[0011] [Technical means to solve the problem]

[0012] The inventors have discovered that in a solid-state secondary battery with an intermediate layer disposed between the negative electrode layer and the solid electrolyte layer, the effective way to address the aforementioned problems is to ensure that the adhesion strength of the solid electrolyte layer, the adhesion strength at the interface between the solid electrolyte layer and the intermediate layer, and the adhesion strength of the intermediate layer are all within specified ranges, thereby completing the present invention. Therefore, the present invention provides the following invention.

[0013] (1) A solid secondary battery having a positive electrode layer, a negative electrode layer including at least a negative electrode current collector, a solid electrolyte layer containing a solid electrolyte material, and an intermediate layer disposed between the aforementioned negative electrode layer and the aforementioned solid electrolyte layer, and forming an electrode stack, wherein the electrode stack is formed by bonding the aforementioned positive electrode layer, the aforementioned solid electrolyte layer, the aforementioned intermediate layer and the aforementioned negative electrode layer respectively, and when the adhesion strength of the aforementioned solid electrolyte layer is set as A1, the adhesion strength of the interface between the aforementioned solid electrolyte layer and the aforementioned intermediate layer is set as A2, and the adhesion strength of the aforementioned intermediate layer is set as A3, the following relationships (I-1), (I-2) and (I-3) are satisfied;

[0014] A3<A2≦A1 (I-1)

[0015] 0.5 kN / m < A1 < 3 kN / m (I-2)

[0016] 0.2 kN / m<A3<2 kN / m (I-3).

[0017] According to the solid-state secondary battery of (1), since the adhesion strength A1 of the solid electrolyte layer, the adhesion strength A2 of the interface between the solid electrolyte layer and the intermediate layer, and the adhesion strength A3 of the intermediate layer satisfy the above-mentioned relationship, the charge-moving medium can easily move between the solid electrolyte layer and the negative electrode layer. In addition, since the solid electrolyte layer and the intermediate layer are not easily separated, the shape stability of the electrode stack is high. Therefore, the solid-state secondary battery of (1) has low DC resistance and excellent cycle characteristics.

[0018] (2) According to the solid secondary battery described in (1), when the adhesion strength of the interface between the aforementioned intermediate layer and the aforementioned negative electrode layer after the first charge and discharge is set as B, the following relationship (II-1) is satisfied.

[0019] 0.4 kN / m < B (II-1).

[0020] According to the solid secondary battery in (2), since the bonding strength B of the interface between the intermediate layer and the negative electrode layer after the first charge and discharge satisfies the above relationship, the intermediate layer and the negative electrode layer are less likely to peel off, and the shape stability of the electrode stack becomes higher.

[0021] (3) The solid secondary battery according to (1) or (2), wherein the aforementioned negative electrode layer has a metal layer stacked on one surface of the aforementioned negative electrode current collector, the aforementioned metal layer is disposed on the aforementioned intermediate layer side, and when the adhesion strength of the interface between the aforementioned intermediate layer and the aforementioned metal layer is set to A4, and the adhesion strength of the interface between the aforementioned metal layer and the aforementioned negative electrode current collector is set to A5, the following relationship of formula (I-4) and formula (I-5) is satisfied.

[0022] 0.4 kN / m < A4 (I-4)

[0023] 0.1 kN / m < A5 (I-5).

[0024] According to the solid secondary battery of (3), since the negative electrode layer has a metal layer disposed on the intermediate layer side of the negative electrode current collector, metal ions, which serve as charge transport media, are easily and stably deposited in the negative electrode layer during charging. In addition, since the adhesion strength A4 of the interface between the metal layer and the intermediate layer satisfies the above relationship after the electrode stack is assembled, and the adhesion strength A5 of the interface between the metal layer and the negative electrode current collector satisfies the above relationship, the intermediate layer, the metal layer, and the negative electrode current collector are less likely to peel off, and the shape stability of the electrode stack becomes higher.

[0025] (4) The solid secondary battery according to (1) or (2), wherein, after the aforementioned electrode stack is assembled, the aforementioned negative current collector is tightly bonded to the aforementioned intermediate layer.

[0026] According to the solid secondary battery in (4), after the electrode stack is assembled, the negative electrode current collector is tightly sealed with the intermediate layer, and the negative electrode layer is only composed of the negative electrode current collector. Therefore, it is possible to reduce the amount of rare metals used, reduce the manufacturing cost, and increase the energy density of the battery.

[0027] (5) The solid secondary battery according to (2), wherein the sealing strength B of the aforementioned formula (II-1) is the measured value when the state of charge (SOC) of the solid secondary battery is in the range of 10% or more and 100% or less.

[0028] According to the solid secondary battery in (5), since the adhesion strength B of the aforementioned formula (II-1) is the strength in the state where lithium is deposited on the negative electrode layer, the solid secondary battery that satisfies the aforementioned formula (II-1) has a low DC resistance and excellent cycle characteristics.

[0029] (6) The solid secondary battery according to any one of (1) to (5), wherein the composite elastic modulus of the aforementioned intermediate layer is less than 1 GPa.

[0030] According to the solid secondary battery in (6), since the composite elastic modulus of the intermediate layer is the value mentioned above, even if 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, and between the negative electrode layer and the intermediate layer, can be increased. Therefore, the DC resistance during charging and discharging at high current density can be reduced.

[0031] (7) The solid secondary battery according to any one of (1) to (6), wherein the relative density of the aforementioned intermediate layer is 30 to 60%.

[0032] According to the solid secondary battery of (7), since the relative density of the intermediate layer is within the above-mentioned 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 secondary battery can be improved.

[0033] (8) A solid secondary battery according to any one of (1) to (7), wherein the aforementioned intermediate layer comprises amorphous carbon.

[0034] According to the solid-state secondary battery of (8), the formation of dendrites caused by the precipitation of charge-moving media in the intermediate layer can be suppressed. Therefore, the cycle characteristics of the solid-state secondary battery can be further improved.

[0035] (The effect of the invention)

[0036] According to the present invention, a solid-state secondary battery with low DC resistance and excellent cycle characteristics can be provided, as well as an evaluation method for solid-state secondary batteries that is effective in predicting the DC resistance and cycle characteristics of solid-state secondary batteries. Attached Figure Description

[0037] Figure 1 This is a cross-sectional view of an example of a solid-state secondary battery according to the first embodiment of the present invention.

[0038] Figure 2 Yes Figure 1 A cross-sectional view illustrating an example of the state of a solid-state secondary battery after its initial charge.

[0039] Figure 3 Yes Figure 2 A cross-sectional view illustrating an example of the state of a solid-state secondary battery after its initial charge and discharge.

[0040] Figure 4 This is a schematic diagram illustrating the method for determining the adhesion strength based on the Surface and Interfacial Cutting Analysis System (SAICAS).

[0041] Figure 5This is a cross-sectional view illustrating an example of a solid-state secondary battery according to the second embodiment of the present invention.

[0042] Figure 6 Yes Figure 5 A cross-sectional view illustrating an example of the state of a solid-state secondary battery after its initial charge.

[0043] Figure 7 Yes Figure 6 A cross-sectional view illustrating an example of the state of a solid-state secondary battery after its initial charge and discharge.

[0044] Figure 8 This is a scanning electron microscope (SEM) image of a cross-section of the electrode stack taken from the solid secondary battery obtained in Example 1.

[0045] Figure 9 This is a SEM image of a cross-section of the electrode stack taken from the solid secondary battery obtained in Example 3.

[0046] Figure 10 This is a SEM image of a cross-section of the electrode stack taken from the solid secondary battery obtained in Comparative Example 1. Detailed Implementation

[0047] 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 configured as a lithium metal battery using lithium ions as the charge transfer medium.

[0048] [First Implementation Method]

[0049] 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 Yes Figure 1 The diagram shows a cross-sectional view of an example of a solid-state secondary battery after its initial charge. Figure 3 Yes Figure 2 A cross-sectional view illustrating an example of the state of a solid-state secondary battery after its initial charge and discharge.

[0050] The solid-state secondary battery 1a of this embodiment has a laminate formed by sequentially stacking a positive electrode layer 10, a solid electrolyte layer 20, an intermediate layer 30, and a negative electrode layer 40. The positive electrode layer 10, the solid electrolyte layer 20, the intermediate layer 30, and the negative electrode layer 40 are respectively bonded to each other.

[0051] 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 11.

[0052] Examples of the shapes of 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.

[0053] 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 retains lithium ions during discharging. Examples of lithium compounds that can be used include layered active materials, spinel-type active materials, and olivine-type active materials. 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), from Li 1+x Mn 2-x-y MO4 (x + y = 2, M is selected from at least one of Al, Mg, Co, Fe, Ni and Zn) represents heteroelement-substituted Li-Mn spinel, lithium titanate (containing oxides of Li and Ti), lithium metal phosphate (LiMPO4, M is selected from at least one of Fe, Mn, Co and Ni), etc. The positive electrode active material layer 12 may also contain conductive additives and binders.

[0054] The solid electrolyte layer 20 comprises 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 may also have a sulfide-germanium sulfide crystal structure. Examples of oxide solid electrolytes include sodium superionic conductor (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). 12Examples of perovskite 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.

[0055] The solid electrolyte layer 20 may also contain an adhesive. Examples of adhesives include resin-based adhesives, rubber-based adhesives, elastic adhesives, and cellulose-based adhesives. Examples of resins include polyvinylidene fluoride, polytetrafluoroethylene, polyimide, polyamide, and polyamide-imide. Examples of rubber-based adhesives include butadiene rubber, styrene-butadiene rubber, nitrile rubber, acrylic rubber, butyl rubber, and fluororubber. Examples of elastic adhesives include styrene-ethylene-butene-styrene block copolymers, styrene-isoprene-styrene block copolymers, and other styrene-based block copolymers. Examples of cellulose-based adhesives include carboxymethyl cellulose, methyl cellulose, and ethyl cellulose. The adhesive content of the solid electrolyte layer 20 may, for example, be in the range of 0.5% to 10% by mass.

[0056] An intermediate layer 30 is disposed between the solid electrolyte layer 20 and the negative electrode layer 40. The intermediate layer 30 has pores through which lithium metal, the charge transport medium of the solid secondary battery 1a, 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.

[0057] The composite elastic modulus of the intermediate layer 30 can also be less than 1 GPa. The composite elastic modulus of the intermediate layer 30 can also be in the range of 600–800 MPa. If the composite elastic modulus of the intermediate layer 30 is within the above-mentioned range, the flexibility of the intermediate layer 30 will be increased. Even if 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.

[0058] The relative density of the intermediate layer 30 can also 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. If the relative density of the intermediate layer 30 is as described above, 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 1a.

[0059] The intermediate layer 30 may also 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 also be particles. The metal particles can be contained in the intermediate layer 30 either as a mixture with amorphous carbon particles or as a substrate on amorphous carbon particles. Furthermore, the metal can exist as a film on the surface of the amorphous carbon particles or be impregnated within the amorphous carbon particles.

[0060] Amorphous carbon can be either easily graphitized carbon (soft carbon) or difficult-to-graphitize carbon (hard carbon). Amorphous carbon simply refers to any allotrope of carbon that does not exhibit a clearly defined crystalline state; it can also be an aggregate of finely crystalline graphite. Specific examples of amorphous carbon include acetylene black, furnace black, Ketjen black, coke, activated carbon, carbon nanotubes (CNTs), fullerenes, and graphene.

[0061] As the metal included in the intermediate layer 30, particles of a metal that can form an alloy with lithium can be used. Examples of metals that can form an alloy with lithium include Mg, Zn, Al, In, Si, Ge, Sn, Ag, Au, Pt, Pd, Pb, Sb, and Bi.

[0062] In addition to the substances mentioned above, the intermediate layer 30 may also contain an adhesive. The adhesive may be a substance used as the adhesive for the solid electrolyte layer 20. The adhesive used in the intermediate layer 30 may be the same as or different from the adhesive used in the solid electrolyte layer 20. The adhesive content of the intermediate layer 30 may, for example, be in the range of 0.5% to 10% by mass.

[0063] The negative electrode layer 40 has a negative electrode current collector 41 and a metal layer 42 disposed on the surface of the negative electrode current collector 41.

[0064] Examples of possible shapes for the negative current collector 41 include foil, plate, mesh, non-woven fabric, and foam. Examples of materials for the negative current collector 41 include copper, copper alloys, nickel, and stainless steel.

[0065] Lithium ions are deposited in the metal layer 42 during charging. Lithium and metals that can form alloys with lithium can be used as the material for the metal layer 42. Examples of metals that can form alloys with lithium include Mg, Zn, Al, In, Si, Ge, Sn, Ag, Au, Pt, Pd, Pb, Sb, and Bi.

[0066] There is no particular limitation on the manufacturing method of the solid secondary battery 1a. For example, the solid secondary battery 1a can be manufactured by sequentially stacking a positive electrode layer 10, a solid electrolyte layer 20, an intermediate layer 30, and a negative electrode layer 40, and then pressurizing the resulting stack. By pressing, an electrode stack is formed by bonding the positive electrode layer 10, the solid electrolyte layer 20, the intermediate layer 30, and the negative electrode layer 40 respectively.

[0067] The solid-state secondary battery 1a, after the electrode stack is assembled, is in a discharged state and therefore needs to be charged before use. During charging, lithium ions are released from the positive electrode active material layer 12 of the positive electrode layer 10, and these lithium ions are deposited on the surface of the metal layer 42 of the negative electrode layer 40. Thus, as... Figure 2 As shown, in the solid secondary battery 1b after initial charging, a lithium metal layer 43 is formed on the surface of the metal layer 42. Due to the formation of the lithium metal layer 43, the solid secondary battery 1b after charging is thicker than the solid secondary battery 1a assembled with the electrode stack.

[0068] When the solid-state secondary battery 1b is discharged after its initial charge, lithium ions are released from the lithium metal layer 43 and retained in the positive electrode active material layer 12. Thus, as... Figure 3 As shown, in the solid-state secondary battery 1c after the first charge and discharge, the thickness of the lithium metal layer 43 becomes thinner, or the lithium metal layer 43 disappears. Due to the thinning or disappearance of the lithium metal layer 43, the solid-state secondary battery 1c after the first charge and discharge is thinner than the solid-state secondary battery 1b after the first charge.

[0069] After the solid secondary battery 1a is assembled into an electrode stack, the bonding strength of the solid electrolyte layer 20 is set as A1, the bonding strength at the interface between the solid electrolyte layer 20 and the intermediate layer 30 is set as A2, and the bonding strength of the intermediate layer 30 is set as A3, satisfying the relationships of the following equations (I-1), (I-2), and (I-3). The term "after the electrode stack is assembled" refers to the state in which the layers of the solid secondary battery 1a are joined together to form a stack. The bonding strength A1 of the solid electrolyte layer 20 refers to the bonding strength of the composite material composed of the solid electrolyte material constituting the solid electrolyte layer 20 and the binder. The bonding strength A3 of the intermediate layer 30 refers to the bonding strength of the composite material composed of the material constituting the intermediate layer 30, amorphous carbon and metal nanoparticles, and the binder.

[0070] A3<A2≦A1 (I-1)

[0071] 0.5 kN / m < A1 < 3 kN / m (I-2)

[0072] 0.2 kN / m < A3 < 2 kN / m (I-3)

[0073] The solid secondary battery 1a can also satisfy the following relationships (I-4) and (I-5) when the adhesion strength of the interface between the intermediate layer 30 and the metal layer 42 is set to A4 and the adhesion strength of the interface between the metal layer 42 and the negative electrode current collector 41 is set to A5.

[0074] 0.4 kN / m < A4 (I-4)

[0075] 0.1 kN / m < A5 (I-5)

[0076] The adhesion strength A4 at the interface between the intermediate layer 30 and the metal layer 42 can also be less than 3 kN / m. The adhesion strength A5 at the interface between the metal layer 42 and the negative electrode current collector 41 can also be less than 3 kN / m.

[0077] The solid-state secondary battery 1c after its first charge and discharge can also satisfy the following relationship (II-1) when the adhesion strength of the interface between the intermediate layer 30 and the negative electrode layer 40 is set as B. The adhesion strength of each layer and the interface of each layer of the solid-state secondary battery 1c after its first charge and discharge can also be measured when the state of charge (SOC) is in the range of 10% or more and 100% or less, especially at 50%.

[0078] 0.4 kN / m < B (II-1)

[0079] B can also be below 3 kN / m.

[0080] The adhesion strengths A1 to A5 and adhesion strength B can be measured using the SAICAS (Surface and Interfacial Cutting Analysis System) method. By using the SAICAS method to measure adhesion strength, adhesion strengths A1 to A5 and adhesion strength B can be measured with high precision. However, in this embodiment, the method for measuring adhesion strength is not limited to the SAICAS method, as long as it can accurately evaluate the adhesion strength of a specified interface. For example, it can also be the micro-scratch method, nano-scratch method, peel test, tape peel test, centrifugation method, floating roller method, and other adhesion strength measurement methods.

[0081] Figure 4 This is a schematic diagram illustrating the method for determining the strength of a seal based on the SAICAS method. Figure 4 A method for measuring the adhesion strength of a test layer 101 disposed on a substrate 100 is shown. The adhesion strength is measured by measuring the vertical force F applied to the cutting edge 200 when the cutting edge 200 is moved in an inclined direction relative to the surface of the test layer 101. V With horizontal force F HThe depth of cut, d, of the cutting edge 200 was measured. Horizontal force F H It is the force received by the cutting edge 200 from a direction horizontal relative to the surface of the test layer 101. Vertical force F V It is the force received by the cutting edge 200 from a direction perpendicular to the surface of the test layer 101.

[0082] First, such as Figure 4 As shown in (a), the tip of the cutting edge 200 is brought into contact with the surface of the test layer 101. Then, the cutting edge 200 is moved in an inclined direction relative to the surface of the test layer 101. Thus, as... Figure 4 As shown in (b), the test layer 101 is cut. When the cutting edge 200 is further moved in an inclined direction relative to the surface of the test layer 101, as... Figure 4 As shown in (c), the tip of the cutting edge 200 reaches the surface of the substrate 100. Figure 4 As shown in the diagram, the cutting edge 200 is moved in a linearly increasing manner with a depth of cut d before it reaches the surface of the substrate 100. After the tip of the cutting edge 200 reaches the substrate 100, the cutting edge 200 is moved horizontally relative to the surface of the test layer 101.

[0083] The seal strength can be calculated using the following formula.

[0084] Adhesion strength (kN / m) = Horizontal force F H Average value (kN) ÷ Cutting edge width per unit area (m) of 200 mm

[0085] In calculating the sealing strength of test layer 101, the horizontal force F H The average value is the horizontal force F from the start of cutting the test layer 101 until the cutting edge 200 reaches the substrate 100. H The average value. When calculating the adhesion strength at the interface between the test layer 101 and the substrate 100, the horizontal force F... H The average value is the horizontal force F after the cutting edge 200 reaches the surface of the substrate 100. H The average value.

[0086] According to the first embodiment of the solid-state secondary battery 1a with the structure described above, since the adhesion strength A1 of the solid electrolyte layer 20, the adhesion strength A2 of the interface between the solid electrolyte layer 20 and the intermediate layer 30, and the adhesion strength A3 of the intermediate layer 30 satisfy the above-described relationships, lithium ions can easily move between the solid electrolyte layer 20 and the negative electrode layer 40. Furthermore, since the solid electrolyte layer 20 and the intermediate layer 30 are not easily peeled off, the shape stability of the electrode stack is high. Therefore, the solid-state secondary battery 1a of the first embodiment has low DC resistance and excellent cycle characteristics.

[0087] Furthermore, when the adhesion strength B at the interface between the intermediate layer 30 and the negative electrode layer 40 of the solid-state secondary battery 1c after the first charge and discharge satisfies the above-mentioned relationship, the intermediate layer 30 and the negative electrode layer 40 are less prone to peeling off, and the shape stability of the electrode stack becomes higher. The adhesion strength B is the measured value when the SOC of the solid-state secondary battery 1c after the first charge and discharge is in the range of 10% to 100%, and the adhesion strength B is the strength in the presence of the lithium metal layer 43 formed on the negative electrode layer 40. Therefore, the solid-state secondary battery 1c with an adhesion strength B satisfying the above-mentioned relationship indeed has low DC resistance and excellent cycle characteristics.

[0088] Furthermore, in the solid-state secondary battery 1a, since the negative electrode layer 40 has a metal layer 42 disposed on the side of the intermediate layer 30 of the negative electrode current collector 41, lithium ions can be easily and stably deposited in the negative electrode layer 40 during charging. In addition, since the adhesion strength A4 of the interface between the metal layer 42 and the intermediate layer 30 satisfies the above-mentioned relationship after the electrode stack is assembled, and the adhesion strength A5 of the interface between the metal layer 42 and the negative electrode current collector 41 satisfies the above-mentioned relationship, the intermediate layer 30, the metal layer 42 and the negative electrode current collector 41 are less likely to peel off, and the shape stability of the electrode stack becomes higher.

[0089] [Second Implementation]

[0090] Figure 5 This is a cross-sectional view illustrating an example of a solid-state secondary battery according to the second embodiment of the present invention. Figure 6 Yes Figure 5 The diagram shows a cross-sectional view of an example of a solid-state secondary battery after its initial charge. Figure 7 Yes Figure 6 A cross-sectional view illustrating an example of the state of a solid-state secondary battery after its initial charge and discharge.

[0091] The solid-state secondary battery 2a of this embodiment is identical to the solid-state secondary battery 1a of the first embodiment after the electrode stack is assembled, except that after the electrode stack is assembled, the negative electrode layer 40 is composed only of the negative electrode current collector 41, and the negative electrode current collector 41 is tightly bonded to the intermediate layer 30. Therefore, the same reference numerals are used for the parts that are common to the solid-state secondary battery 1a of the first embodiment, and their descriptions are omitted.

[0092] When the solid secondary battery 2a is charged, lithium ions released from the positive electrode active material layer 12 of the positive electrode layer 10 are deposited on the surface of the negative electrode current collector 41 of the negative electrode layer 40. Thus, as... Figure 6 As shown, in the solid secondary battery 2b after the first charge, a lithium metal layer 43 is formed on the surface of the negative electrode current collector 41. The charging conditions are the same as in the first embodiment.

[0093] When the solid-state secondary battery 2b is discharged after its initial charge, lithium ions are released from the lithium metal layer 43. The solid-state secondary battery 2c after its initial charge and discharge is as follows: Figure 7 As shown, the thickness of the lithium metal layer 43 becomes thinner, or the lithium metal layer 43 disappears. The discharge conditions are the same as in the first embodiment.

[0094] In the second embodiment of the solid-state secondary battery 2a, which has the structure described above, lithium ions can easily move between the solid electrolyte layer 20 and the negative electrode layer 40 because the adhesion strength A1 of the solid electrolyte layer 20, the adhesion strength A2 of the interface between the solid electrolyte layer 20 and the intermediate layer 30, and the adhesion strength A3 of the intermediate layer 30 satisfy the aforementioned relationships. Furthermore, since the solid electrolyte layer 20 and the intermediate layer 30 are not easily peeled off, the shape stability of the electrode stack is improved. Therefore, the second embodiment of the solid-state secondary battery 2a has low DC resistance and excellent cycle characteristics.

[0095] Furthermore, when the adhesion strength B of the interface between the intermediate layer 30 and the negative electrode layer 40 of the solid secondary battery 2c after the first charge and discharge satisfies the above-mentioned relationship, similarly to the solid secondary battery 1a of the first embodiment, the intermediate layer 30 and the negative electrode layer 40 are less likely to peel off, and the shape stability of the electrode stack becomes higher.

[0096] Furthermore, since the negative electrode current collector 41 is tightly bonded to the intermediate layer 30 after the electrode layer stack is assembled, the negative electrode layer 40 is composed only of the negative electrode current collector 41 and does not contain a metal layer, thus reducing manufacturing costs.

[0097] Evaluation methods for solid-state secondary batteries

[0098] The evaluation method for solid-state secondary batteries in this embodiment is an evaluation method for solid-state secondary batteries 1a and 2a having a positive electrode layer 10, a negative electrode layer 40 including at least a negative electrode current collector 41, a solid electrolyte layer 20 containing a solid electrolyte material 21, and an intermediate layer 30 disposed between the negative electrode layer 40 and the solid electrolyte layer 20. The method involves removing the laminate containing the solid electrolyte layer 20, the intermediate layer 30, and the negative electrode layer 40 from the solid-state secondary batteries 1a and 2a after the electrode laminate is assembled, and measuring the adhesion strength A1 of the solid electrolyte layer 20, the adhesion strength A2 of the interface between the solid electrolyte layer 20 and the intermediate layer 30, and the adhesion strength A3 of the intermediate layer 30 using the SAICAS method. The method for measuring the adhesion strengths A1 to A3 based on the SAICAS method is as described above.

[0099] In the evaluation method of the solid secondary battery of this embodiment, when the sealing strength A1 to A3 satisfies the relationship of the above-mentioned formulas (I-1), (I-2) and (I-3), it is evaluated as having low DC resistance and excellent cycle characteristics.

[0100] According to the evaluation method of the solid secondary battery of this embodiment, the bonding strength between the solid electrolyte layer 20 and the intermediate layer 30 between the positive electrode layer 10 and the negative electrode layer 40 of the solid secondary batteries 1a and 2a can be measured with high precision, so the DC resistance and cycle characteristics of the solid secondary batteries 1a and 2a can be predicted with high precision.

[0101] The embodiments of the present invention have been described above, but the present invention is not limited to the embodiments described above. For example, in this embodiment, the solid secondary batteries 1a and 2a are lithium metal batteries that use lithium ions as the charge transfer medium, but the charge transfer medium is not limited to this. The solid secondary batteries 1a and 2a of this embodiment can also be batteries in which the thickness of the negative electrode layer changes due to charging and discharging.

[0102] [Example]

[0103] The present invention will be described below with reference to embodiments, but the present invention is not limited to these embodiments.

[0104] [Example 1]

[0105] (1) Fabrication of the positive electrode layer

[0106] 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) as the positive electrode active material, 17 parts by mass of a sulfide solid electrolyte of silver sulfide type, 2 parts by mass of carbon black as a conductive additive, and 1 part by mass of SBR (styrene-butadiene rubber) adhesive as a binder were mixed in this proportion. The resulting mixture was dispersed in 43 parts by mass of butyl butyrate to prepare a positive electrode active material slurry. The slurry was dried to a weight per unit area of ​​27 mg / cm² using a rod coater. 2 The obtained positive electrode active material slurry was coated on both sides of the positive electrode current collector and dried to form a positive electrode active material layer with a thickness of 80 μm, thus creating the positive electrode layer.

[0107] (2) Fabrication of solid electrolyte layer transfer sheet

[0108] 97 parts by mass of a sulfide solid electrolyte (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 produce a solid electrolyte layer transfer sheet (solid electrolyte layer thickness: 100 μm).

[0109] (3) Fabrication of intermediate layer transfer sheet

[0110] A mixture of 95 parts by mass of Sn particles (average particle size: 0.07 μm) as metallic particles, acetylene black (average particle size: 0.05 μm) as amorphous carbon particles, and 5 parts by mass of polyvinylidene fluoride (PVDF) adhesive as a binder was prepared. The resulting mixture was dispersed in 1000 parts by mass of NMP (N-methyl-2-pyrrolidone) to prepare an intermediate layer slurry. The intermediate layer slurry was coated onto a support sheet and dried to produce an intermediate layer transfer sheet (intermediate layer thickness: 3.0 μm).

[0111] (4) Fabrication of the negative electrode layer

[0112] A 10 μm thick copper foil was prepared as the negative electrode current collector. A 40 μm thick lithium metal foil was rolled and stacked on the surface of the copper foil to create the negative electrode layer.

[0113] (5) Fabrication of solid-state secondary batteries

[0114] On the surface of the positive electrode active material layer of the positive electrode layer, a solid electrolyte layer transfer sheet is overlapped and bonded using a uniaxial forming press under bonding conditions of 90 MPa, 3 minutes, and room temperature. Afterward, the support sheet of the solid electrolyte layer transfer sheet is peeled off to obtain the positive electrode layer-solid electrolyte layer composite. Next, on the surface of the solid electrolyte layer of the positive electrode layer-solid electrolyte layer laminate, an intermediate layer transfer sheet is overlapped and bonded using a uniaxial forming press under bonding conditions of 290 MPa, 5 minutes, and room temperature. Afterward, the support sheet of the intermediate layer transfer sheet is peeled off to obtain the positive electrode layer-solid electrolyte layer-intermediate layer composite. Then, using an isostatic pressing press, the integral positive electrode layer-solid electrolyte layer-intermediate layer composite is subjected to high-density treatment at a bonding pressure of 980 MPa, a bonding time of 5 minutes, and a bonding temperature of 120°C. Subsequently, on the surface of the intermediate layer of the positive electrode layer-solid electrolyte layer-intermediate layer composite, a lithium foil of the negative electrode layer is overlapped, and bonding is performed using a uniaxial forming and pressing device under the conditions of bonding pressure: 180 MPa, bonding time: 2 minutes, and bonding temperature: room temperature. In this way, an electrode laminate is obtained.

[0115] A solid-state secondary battery was fabricated by housing the obtained electrode stack within an aluminum laminate outer casing. A buffer material was placed on the negative electrode layer side, and the battery was constrained at a pressure of 3 MPa.

[0116] [Example 2]

[0117] Except that the thickness of the lithium metal foil of the negative electrode layer was set to 6.5 μm, a solid secondary battery was fabricated in the same manner as in Example 1.

[0118] [Example 3]

[0119] As the negative electrode layer, a copper foil (negative electrode current collector) with a thickness of 8 μm was used. The negative electrode layer was superimposed on the surface of the intermediate layer of the positive electrode layer-solid electrolyte layer-intermediate layer junction to form an electrode stack. The constraint pressure when constraining the electrode stack was set to 3 MPa. Otherwise, a solid secondary battery was fabricated in the same manner as in Example 1.

[0120] [Example 4]

[0121] As the negative electrode layer, a silver-copper stack with a silver layer of 20 nm thickness was sputtered onto one surface of a copper foil (negative electrode current collector) with a thickness of 8 μm. The silver layer of the negative electrode layer was superimposed on the surface of the intermediate layer of the positive electrode layer-solid electrolyte layer-intermediate layer junction to form an electrode stack. The constraint pressure when constraining the electrode stack was set to 3 MPa. Otherwise, a solid secondary battery was fabricated in the same manner as in Example 1.

[0122] [Example 5]

[0123] As a negative electrode layer, it is used on one surface of an 8 μm thick copper foil (negative electrode current collector) at a concentration of 0.04 mg / cm². 2 A carbon-copper laminate with a carbon coating was formed by coating a certain amount of carbon. The carbon layer of the negative electrode layer was superimposed on the surface of the intermediate layer of the positive electrode layer-solid electrolyte layer-intermediate layer composite. The electrode laminate was fabricated by bonding under a bonding pressure of 600 MPa. Otherwise, a solid secondary battery was fabricated in the same manner as in Example 1.

[0124] [Comparative Example 1]

[0125] The thickness of the lithium metal foil of the negative electrode layer was set to 6.5 μm, the amount of SBR (styrene-butadiene rubber) adhesive of the solid electrolyte layer was set to 5.5 parts by mass, and the thickness of the solid electrolyte layer was set to 30 μm. A composite of positive electrode layer-solid electrolyte layer and a composite of thin film solid electrolyte layer-intermediate layer-negative electrode layer were fabricated. The two were then bonded under the conditions of bonding pressure: 500 MPa, bonding time: 15 seconds, and bonding temperature: room temperature. Otherwise, a solid secondary battery was fabricated in the same manner as in Example 1.

[0126] [Comparative Example 2]

[0127] The thickness of the lithium metal foil of the negative electrode layer was set to 6.5 μm, the amount of SBR (styrene-butadiene rubber) adhesive of the solid electrolyte layer was 10 parts by mass, and the thickness of the solid electrolyte layer was 30 μm. The integral of the positive electrode layer-solid electrolyte layer-intermediate layer-negative electrode layer was bonded under the conditions of bonding pressure of 800 MPa, bonding time of 15 seconds, and bonding temperature of room temperature. Otherwise, a solid secondary battery was fabricated in the same manner as in Example 1.

[0128] [Comparative Example 3]

[0129] Except that the amount of SBR (styrene-butadiene rubber) adhesive in the solid electrolyte layer was set to 10 parts by mass and the thickness of the solid electrolyte layer was set to 30 μm, a solid secondary battery was fabricated in the same manner as in Example 1.

[0130] [Comparative Example 4]

[0131] Except that the amount of PVDF-based adhesive in the intermediate layer was set to 3 parts by mass, a solid secondary battery was prepared in the same manner as in Example 1.

[0132] [evaluate]

[0133] The following evaluation was performed on the solid secondary batteries obtained in Examples 1-5 and Comparative Examples 1-4.

[0134] (1) Adhesion strength (adhesion strength from the solid electrolyte layer to the negative electrode layer in a solid secondary battery)

[0135] The solid secondary battery was disassembled and the electrode stack was removed. The adhesion strength from the solid electrolyte layer to the negative electrode layer of the removed electrode stack was determined using the SAICAS method. The adhesion strength was measured under the conditions described below. The results are shown in Table 1.

[0136] (Conditions for determining the strength of the seal)

[0137] The seal was measured using a SAICAS testing machine in a low dew point environment (room temperature, dew point below -40°C) under constant speed mode, with a diamond cutting edge material, a cutting edge width of 0.3 mm, a horizontal cutting edge speed of 2.0 μm / sec, and a number of evaluations (n=3). The cutting edge was used to cut from the surface of the negative electrode current collector foil in the negative electrode layer to the interface for evaluating the seal. After reaching the target interface, the cutting edge was moved horizontally, and the horizontal force per unit width of the cutting edge was calculated based on the average horizontal force at this point to evaluate the seal.

[0138] (2) Adhesion strength (adhesion strength of the interface between the intermediate layer and the negative electrode layer of the solid secondary battery after the first charge and discharge)

[0139] The solid-state secondary battery was charged under constant current / constant voltage (CCCV) conditions at 60°C, 0.1 C, and 4.3 V. Then, it was discharged to 2.65 V under constant current (CCCV) conditions at 60°C and 0.1 C for the first charge-discharge cycle. Following aging, the SOC was adjusted to 50% under constant current / constant voltage (CCCV) conditions at 60°C, 0.1 C, and 50% of the state of charge (SOC). The solid-state secondary battery was then disassembled, and the electrode stack was removed. The positive electrode layer and solid electrolyte layer were peeled off from the removed electrode stack to obtain the intermediate layer-negative electrode layer junction. The adhesion strength of the interface between the intermediate layer and the negative electrode layer (lithium metal layer) of the obtained junction was measured using the SAICAS method. The adhesion strength was measured under the same conditions as the adhesion strength measurement from the solid electrolyte layer to the negative electrode layer in the solid-state secondary battery. The results are shown in Table 1.

[0140] (3) Composite elastic modulus (composite elastic modulus of solid electrolyte layer, intermediate layer and negative electrode layer of solid secondary battery)

[0141] The solid secondary battery was disassembled and the electrode stack was removed after being left to stand in the same manner as described in (1) above. The positive electrode layer was peeled off from the removed electrode stack, and the composite elastic modulus of the solid electrolyte layer was measured. Subsequently, the solid electrolyte layer was peeled off, and the composite elastic modulus of the intermediate layer was measured. Finally, the intermediate layer was peeled off, and the composite elastic modulus of the negative electrode layer (metal layer) was measured. The composite elastic modulus was measured using the nanoindentation method. The results are shown in Table 1.

[0142] (4) DC resistance

[0143] (DC resistance at low current discharge at 60℃)

[0144] Based on a charging state at 60°C and 50% SOC, with a current density of 2.7 mA / cm² 2 Voltage drop ΔV (V), current I (A), and positive electrode area Ac (cm²) during discharge 2 The DC resistance (Ω·cm) can be calculated using the following formula. 2 The results are shown in Table 2.

[0145] DC resistance (Ω·cm) 2 = Voltage drop ΔV (V) / Current value I (A) × Positive electrode area Ac (cm²) 2 )

[0146] (DC resistance during high-current discharge at 25℃)

[0147] Based on a charging state at 25°C and 50% SOC, with 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 DC resistance (Ω·cm) can be calculated using the following formula. 2 The results are shown in Table 2.

[0148] DC resistance (Ω·cm) 2 = Voltage drop ΔV (V) / Current value I (A) × Positive electrode area Ac (cm²) 2 )

[0149] (5) Observation of the initial lithium deposition state

[0150] A portion of the electrode stack obtained in (2) Adhesion Strength (adhesion strength of the interlayer-anode layer interface of the solid secondary battery after initial charging) was cut off and cross-sectionally observed using SEM (scanning electron microscope). Furthermore, a condition was considered "good" if there was no short-circuit behavior during or after the initial charge / discharge process, the lithium deposition location was between the interlayer and the anode current collector foil, and the lithium deposition layer thickness was less than the average thickness ±2 μm. A condition where lithium deposition was located between the solid electrolyte layer and the interlayer, inside the solid electrolyte layer, or at their composite location was considered "NG (Not Good)". A condition where the lithium layer deposited on the anode layer had thin and thick portions with a thickness difference of more than the average thickness ±2 μm, or where there were cracks or voids of more than 3 μm inside the lithium deposition layer, was considered "non-uniform". A condition where the interlayer showed internal cracks or peeled off from the solid electrolyte layer or the anode layer was considered "interlayer structural collapse". The results are shown in Table 2. Figure 8 Example 1 is shown in the figure. Figure 9 Example 3 is shown in the figure. Figure 10 The image shown is a cross-sectional SEM image of the electrode stack taken from the solid secondary battery obtained in Comparative Example 1.

[0151] (6) Cyclic test (45℃ cyclic test)

[0152] Charge-discharge cycle tests were conducted at a temperature of 45°C, a current of 1 / 3 C, a voltage range of 4.3 V (upper limit) and 2.65 V (lower limit), with constant current / constant voltage (CCCV) for charging and constant current (CC) for discharging. The discharge capacity retention rate and short-circuit occurrence rate after 100 cycles are shown in Table 2.

[0153] [Table 1]

[0154]

[0155] [Table 2]

[0156]

[0157] As shown in Tables 1 and 2, the solid-state secondary batteries of Examples 1-5, whose solid electrolyte layer adhesion strength A1, solid electrolyte layer-intermediate layer interface adhesion strength A2, and intermediate layer adhesion strength A3 meet the conditions of the present invention, have low DC resistance and high discharge capacity retention after 100 cycles. Conversely, the solid-state secondary batteries of Comparative Example 1 (where the solid electrolyte layer adhesion strength A1 is lower than the range of the present invention), Comparative Examples 2-3 (where the solid electrolyte layer adhesion strength A1 is higher than the range of the present invention), and Comparative Example 4 (where the intermediate layer adhesion strength A3 is lower than the range of the present invention) all have high DC resistance and low discharge capacity after 100 cycles. Furthermore, compared to Examples 1-5, Comparative Examples 1-4 have a higher short-circuit incidence rate after 100 cycles at 45°C.

[0158] In addition, such as Figure 8 and Figure 9 As shown, in the solid secondary batteries of Examples 1 and 3, where the adhesion strength A1 of the solid electrolyte layer, the adhesion strength A2 of the solid electrolyte layer-intermediate layer interface, and the adhesion strength A3 of the intermediate layer satisfy the conditions of the present invention, no lithium deposition was found between the solid electrolyte layer 20 and the intermediate layer 30. In contrast, as... Figure 10 As shown, in the solid secondary battery of Comparative Example 1, lithium was deposited between the solid electrolyte layer 20 and the intermediate layer 30. In addition, obvious cracks or voids were generated in the deposited lithium layer, indicating an uneven deposition state.

[0159] Figure Labels

[0160] 1A and 2A solid-state secondary batteries

[0161] 1b and 2b Solid-state secondary batteries after initial charging

[0162] Solid-state rechargeable batteries after 1C and 2C initial charge and discharge

[0163] 10 Positive electrode layer

[0164] 11 Positive current collector

[0165] 12 Positive electrode active material layer

[0166] 20 Solid electrolyte layer

[0167] 21 Solid Electrolyte Materials

[0168] 30 Intermediate Layer

[0169] 40 Negative electrode layer

[0170] 41 Negative current collector

[0171] 42 Metal Layer

[0172] 43 Lithium metal layer

[0173] 100 substrates

[0174] 101 Test Layer

[0175] 200 cutting edge

Claims

1. A solid-state secondary battery, comprising a positive electrode layer, a negative electrode layer including at least a negative electrode current collector, a solid electrolyte layer comprising a solid electrolyte material, and an intermediate layer disposed between the aforementioned negative electrode layer and the aforementioned solid electrolyte layer. An electrode stack is formed by bonding the aforementioned positive electrode layer, the aforementioned solid electrolyte layer, the aforementioned intermediate layer, and the aforementioned negative electrode layer together. When the adhesion strength of the aforementioned solid electrolyte layer is set as A1, the adhesion strength of the interface between the aforementioned solid electrolyte layer and the aforementioned intermediate layer is set as A2, and the adhesion strength of the aforementioned intermediate layer is set as A3, the following relationships (I-1), (I-2) and (I-3) are satisfied. A3<A2≦A1 (I-1) 0.5 kN / m < A1 < 3 kN / m (I-2) 0.2 kN / m<A3<2 kN / m (I-3).

2. The solid-state secondary battery according to claim 1, wherein, When the adhesion strength of the interface between the aforementioned intermediate layer and the aforementioned negative electrode layer after the first charge and discharge is set as B, the following relationship (II-1) is satisfied. 0.4 kN / m < B (II-1).

3. The solid-state secondary battery according to claim 1 or 2, wherein, The aforementioned negative electrode layer has a metal layer stacked on one surface of the aforementioned negative electrode current collector. The aforementioned metal layer is disposed on the aforementioned intermediate layer side. When the adhesion strength of the interface between the aforementioned intermediate layer and the aforementioned metal layer is set to A4, and the adhesion strength of the interface between the aforementioned metal layer and the aforementioned negative electrode current collector is set to A5, the following relationship between equations (I-4) and (I-5) is satisfied. 0.4 kN / m < A4 (I-4) 0.1 kN / m < A5 (I-5).

4. The solid-state secondary battery according to claim 1 or 2, wherein, After the aforementioned electrode stack is assembled, the aforementioned negative current collector is tightly sealed with the aforementioned intermediate layer.

5. The solid-state secondary battery according to claim 2, wherein, The sealing strength B in the aforementioned formula (II-1) is the measured value when the charge rate of the solid secondary battery is in the range of 10% or more and 100% or less.

6. 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.

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

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

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