Solid-state secondary batteries

Abstract

To provide a solid-state rechargeable battery with low DC resistance and excellent cycle characteristics. [Solution] The solid-state secondary battery has a positive electrode layer, a negative electrode layer including at least a negative electrode current collector, a solid electrolyte layer including a solid electrolyte material, and an intermediate layer provided between the negative electrode layer and the solid electrolyte layer. The positive electrode layer, the solid electrolyte layer, the intermediate layer, and the negative electrode layer are joined together to form an electrode laminate, and a predetermined relationship is satisfied 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.

Description

[Technical Field]

[0001] This invention relates to a solid-state rechargeable battery. [Background technology]

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

[0003] As an example of such a secondary battery, a solid-state secondary battery is known in which a solid electrolyte layer is placed between the positive electrode layer and the negative electrode layer. In solid-state secondary batteries, in order to improve the adhesion strength of the solid electrolyte layer, a binder having functional groups such as carboxyl groups, carbonyl groups, or hydroxyl groups may be used for the solid electrolyte layer (see Patent Document 1). In addition, an intermediate layer may be placed between the negative electrode layer and the solid electrolyte layer (see Patent Document 2). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2018-129222 [Patent Document 2] International Publication No. 2023 / 189892 [Overview of the project] [Problems that the invention aims to solve]

[0005] Incidentally, in solid-state secondary battery technology, reducing DC resistance is one of the challenges. In particular, in solid-state secondary batteries in which an intermediate layer is placed between the negative electrode layer and the solid electrolyte layer, if the DC resistance increases excessively, not only will the output characteristics deteriorate, but during charging, the charge transfer medium (e.g., lithium ions) may deposit between the solid electrolyte layer and the intermediate layer, causing an internal short circuit, or uneven deposition may increase the DC resistance, potentially degrading the cycle characteristics. For this reason, further reduction of DC resistance is required in solid-state secondary batteries with an intermediate layer.

[0006] This invention has been made in view of the above circumstances, and aims to provide a solid-state secondary battery with low DC resistance and excellent cycle characteristics. Ultimately, it contributes to energy efficiency. [Means for solving the problem]

[0007] The inventors have found that, in a solid-state secondary battery in which an intermediate layer is disposed between the negative electrode layer and the solid electrolyte layer, setting the adhesion strength of the solid electrolyte layer, the adhesion strength of the interface between the solid electrolyte layer and the intermediate layer, and the adhesion strength of the intermediate layer to a predetermined range is effective in addressing the above-mentioned problems, and have completed the present invention. Accordingly, the present invention provides the following.

[0008] (1) A solid-state secondary battery having a positive electrode layer, a negative electrode layer including at least a negative electrode current collector, a solid electrolyte layer including a solid electrolyte material, and an intermediate layer provided between the negative electrode layer and the solid electrolyte layer, wherein the positive electrode layer, the solid electrolyte layer, the intermediate layer, and the negative electrode layer are joined together to form an electrode laminate, 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, the relationship between the following equations (I-1), (I-2), and (I-3) is satisfied. A3 <A2≦A1 (I-1) 0.5 kN / m <A1<3kN / m (I-2) 0.2 kN / m <A3<2kN / m (I-3)

[0009] According to the solid 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 relationship, the charge transfer medium can easily move between the solid electrolyte layer and the negative electrode layer. Further, since the solid electrolyte layer and the intermediate layer are less likely to peel off, the shape stability of the electrode laminate is increased. Therefore, the solid secondary battery of (1) has a low DC resistance and excellent cycle characteristics.

[0010] The solid secondary battery according to (1), wherein when the adhesion strength of the interface between the intermediate layer and the negative electrode layer after the first charge and discharge is B, the relationship of the following formula (II-1) is satisfied. 0.4 kN / m < B (II-1)

[0011] According to the solid secondary battery of (2), since the adhesion 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 laminate is further increased.

[0012] The solid secondary battery according to (1) or (2), wherein the negative electrode layer has a metal layer laminated on one surface of the negative electrode current collector, the metal layer is disposed on the intermediate layer side, and when the adhesion strength of the interface between the intermediate layer and the metal layer is A4 and the adhesion strength of the interface between the metal layer and the negative electrode current collector is A5, the relationships of the following formulas (I-4) and (I-5) are satisfied. 0.4 kN / m < A4 (I-4) <> 0.1 kN / m < A5 (I-5)

[0013] 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 are charge transfer media, are likely to be stably deposited on the negative electrode layer during charging. Further, after the assembly of the electrode laminate, since the adhesion strength A4 of the interface between the metal layer and the intermediate layer satisfies the above relationship 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 laminate is further increased.

[0014] (4) The solid secondary battery according to (1) or (2), wherein, after the assembly of the electrode stack, the negative electrode current collector is in close contact with the intermediate layer.

[0015] In the case of the solid-state secondary battery of (4), after the assembly of the electrode stack, the negative electrode current collector is in close contact with the intermediate layer 30, and the negative electrode layer is composed only of the negative electrode current collector, which makes it possible to reduce the amount of rare metals used, reduce manufacturing costs, and increase the energy density of the battery.

[0016] (5) The solid secondary battery described in (2), wherein the adhesion strength B in formula (II-1) is a measured value when the SOC of the solid secondary battery is in the range of 10% or more and 100% or less.

[0017] In the case of the solid-state secondary battery of (5), the adhesion strength B in equation (II-1) is the strength when lithium deposited in the negative electrode layer is present. Therefore, a solid-state secondary battery that satisfies equation (II-1) will definitely have low DC resistance and excellent cycle characteristics.

[0018] (6) A solid-state secondary battery according to any one of (1) to (5), wherein the composite elastic modulus of the intermediate layer is less than 1 GPa.

[0019] In the solid-state secondary battery of (6), since the composite elastic modulus of the intermediate layer is the value mentioned above, 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 even when the thickness of the negative electrode layer changes due to charging and discharging. For this reason, the DC resistance during charging and discharging at high current densities can be reduced.

[0020] (7) A solid-state secondary battery according to any one of (1) to (6), wherein the relative density of the intermediate layer is 30 to 60%.

[0021] In the solid-state secondary battery of (7), since the relative density of the intermediate layer is within the above range, the adhesion 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.

[0022] (8) The solid secondary battery according to any one of (1) to (7), wherein the intermediate layer contains amorphous carbon.

[0023] According to the solid-state secondary battery of (8), the formation of dendrites due to the deposition of charge transfer media in the intermediate layer can be suppressed. Therefore, the cycle characteristics of the solid-state secondary battery can be further improved. [Effects of the Invention]

[0024] According to the present invention, it is possible to provide a solid-state secondary battery with low DC resistance and excellent cycle characteristics, and a solid-state secondary battery evaluation method that is effective for predicting the DC resistance and cycle characteristics of the solid-state secondary battery. [Brief explanation of the drawing]

[0025] [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. [Figure 2] Figure 1 is a cross-sectional view showing an example of the state of a solid-state secondary battery after its initial charge. [Figure 3] Figure 2 is a cross-sectional view showing an example of the state of a solid-state secondary battery after its initial charge and discharge. [Figure 4] This is a schematic diagram illustrating the method for measuring adhesion strength using the SAICAS method. [Figure 5] This is a cross-sectional view showing an example of a solid-state secondary battery according to a second embodiment of the present invention. [Figure 6] Figure 5 is a cross-sectional view showing an example of the state of a solid-state secondary battery after its initial charge. [Figure 7] Figure 6 is a cross-sectional view showing an example of the state of a solid-state secondary battery after its initial charge and discharge. [Figure 8] This is an SEM image of a cross-section of the electrode stack removed from the solid-state secondary battery obtained in Example 1. [Figure 9] This is an SEM image of a cross-section of the electrode stack removed from the solid-state secondary battery obtained in Example 3. [Figure 10] This is an SEM image of a cross-section of the electrode stack extracted from the solid-state secondary battery obtained in Comparative Example 1. [Modes for carrying out the invention]

[0026] Embodiments of the present invention will be described below with reference to the drawings. In this embodiment, the solid-state secondary battery is a lithium metal battery using lithium ions as the charge transfer medium.

[0027] [First Embodiment] Figure 1 is a cross-sectional view showing an example of a solid-state secondary battery according to the first embodiment of the present invention. Figure 2 is a cross-sectional view showing an example of the state of the solid-state secondary battery shown in Figure 1 after its first charge, and Figure 3 is a cross-sectional view showing an example of the state of the solid-state secondary battery shown in Figure 2 after its first charge and discharge.

[0028] The solid-state secondary battery 1a of this embodiment has a laminate 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 are each bonded to one another.

[0029] The positive electrode layer 10 comprises a positive electrode current collector 11 and a positive electrode active material layer 12 disposed on the surface of the positive electrode current collector.

[0030] Examples of the shape of the positive electrode current collector 11 include foil, plate, mesh, nonwoven fabric, and foam forms. Examples of materials for the positive electrode current collector 11 include aluminum, aluminum alloy, stainless steel, nickel, iron, and titanium.

[0031] 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 discharge. As lithium compounds, for example, layered active materials, spinel-type active materials, and olivine-type active materials can be used. Specific examples of positive electrode active materials include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and lithium nickel manganese cobalt oxide (NMC:LiNi p Mn q Co r O2(p+q+r=1), LiNip Al q Co r O2 (p + q + r = 1), lithium manganate (LiMn2O4), Li 1+x Mn 2-x-y Hetero-element-substituted Li-Mn spinel represented by MO4 (x + y = 2, M = at least one selected from Al, Mg, Co, Fe, Ni, and Zn), lithium titanate (oxide containing Li and Ti), lithium metal phosphate (LiMPO4, M = at least one selected from Fe, Mn, Co, and Ni), etc. may be mentioned. The positive electrode active material layer 12 may further contain a conductive assistant and a binder.

[0032] The solid electrolyte layer 20 contains a solid electrolyte material 21. As an example of the solid electrolyte material 21, a sulfide solid electrolyte, an oxide solid electrolyte, a nitride solid electrolyte, or a halide solid electrolyte can be used. Examples of the sulfide solid electrolyte include Li2S-P2S5, Li2S-P2S5-LiI, etc. The sulfide solid electrolyte may have an argyrodite-type crystal structure. Examples of the oxide solid electrolyte include NASICON-type oxides, garnet-type oxides, and perovskite-type oxides. Examples of the NASICON-type oxide include oxides containing Li, Al, Ti, P, and O (for example, Li 1.5 Al 0.5 Ti 1.5 (PO4)3). Examples of the garnet-type oxide include oxides containing Li, La, Zr, and O (for example, Li7La3Zr2O 12 ). Examples of the perovskite-type oxide include oxides containing Li, La, Ti, and O (for example, LiLaTiO3). The thickness of the solid electrolyte layer 20 is, for example, within the range of 10 to 100 μm.

[0033] The solid electrolyte layer 20 may contain a binder. Examples of binders that can be used include resin-based binders, rubber-based binders, elastomer-based binders, and cellulose-based binders. Examples of resins include polyvinylidene fluoride, polytetrafluoroethylene, polyimide, polyamide, and polyamideimide. Examples of rubber-based binders include butadiene rubber, styrene-butadiene rubber, nitrile-butadiene rubber, acrylic rubber, butyl rubber, and fluororubber. Examples of elastomer-based binders include styrene-based block copolymers such as styrene-ethylene-butylene-styrene block copolymer and styrene-isoprene-styrene block copolymer. Examples of cellulose-based binders include carboxymethylcellulose, methylcellulose, and ethylcellulose. The binder content of the solid electrolyte layer 20 may be, for example, in the range of 0.5 to 10% by mass.

[0034] The intermediate layer 30 is placed between the solid electrolyte layer 20 and the negative electrode layer 40. The intermediate layer 30 has voids through which lithium metal, which is the charge transfer medium of the solid secondary battery 1a, can pass. By passing lithium metal through the intermediate layer 30, the 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.

[0035] The composite modulus of the intermediate layer 30 may be less than 1 GPa. The composite modulus of the intermediate layer 30 may be in the range of 600 to 800 MPa. When the composite modulus of the intermediate layer 30 is within the above values, the flexibility of the intermediate layer 30 increases, 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 at high current densities can be reduced.

[0036] The relative density of the intermediate layer 30 may be within the range of 30-60%. 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 within 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, and the cycle characteristics of the solid secondary battery 1a can be improved.

[0037] The intermediate layer 30 may contain a material having lithium metallic conductivity and a material having electronic conductivity. For example, amorphous carbon particles can be used as the material having lithium metallic conductivity. For example, a metal can be used as the material having electronic conductivity. The metal may be in the form of particles. The metal particles may be included in the intermediate layer 30 as a mixture with amorphous carbon particles, or they may be included in the intermediate layer 30 supported on amorphous carbon particles. Furthermore, the metal may be present as a film on the surface of the amorphous carbon particles, or it may be impregnated into the interior of the amorphous carbon particles.

[0038] Amorphous carbon may be either easily graphitizable carbon (soft carbon) or difficult-to-graphitize carbon (hard carbon). Amorphous carbon can be any allotrope of carbon that does not exhibit a clear crystalline state, and may even be an aggregate of fine graphite crystals. Specific examples of amorphous carbon include carbon blacks such as acetylene black, furnace black, and Ketjen black, as well as coke, activated carbon, carbon nanotubes (CNTs), fullerenes, and graphene.

[0039] As the metal contained in the intermediate layer 30, particles of a metal that forms an alloy with lithium can be used. Examples of metals that form alloys with lithium include Mg, Zn, Al, In, Si, Ge, Sn, Ag, Au, Pt, Pd, Pb, Sb, and Bi.

[0040] The intermediate layer 30 may contain a binder in addition to the above-mentioned substances. The binder used is the same as that used in the solid electrolyte layer 20. However, the binder used in the intermediate layer 30 and the binder used in the solid electrolyte layer 20 may be the same or different. The binder content of the intermediate layer 30 may be, for example, in the range of 0.5 to 10% by mass.

[0041] The negative electrode layer 40 comprises a negative electrode current collector 41 and a metal layer 42 disposed on the surface of the negative electrode current collector 41.

[0042] Examples of the negative electrode current collector 41's shape include foil, plate, mesh, nonwoven fabric, and foam. Examples of materials for the negative electrode current collector 41 include copper, copper alloy, nickel, and stainless steel.

[0043] Lithium ions are deposited in the metal layer 42 during charging. The material of the metal layer 42 can be lithium or a metal that forms an alloy with lithium. Examples of metals that form alloys with lithium include Mg, Zn, Al, In, Si, Ge, Sn, Ag, Au, Pt, Pd, Pb, Sb, and Bi.

[0044] There are no particular limitations on the method for manufacturing the solid-state secondary battery 1a. For example, the solid-state secondary battery 1a can be manufactured by 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 pressurizing, an electrode stack is formed in which the positive electrode layer 10, the solid electrolyte layer 20, the intermediate layer 30, and the negative electrode layer 40 are joined together.

[0045] Since the solid-state secondary battery 1a is in a discharged state after the electrode stack assembly is completed, it is 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. As a result, as shown in Figure 2, a lithium metal layer 43 is formed on the surface of the metal layer 42 in the solid-state secondary battery 1b after the initial charge. Due to the formation of the lithium metal layer 43, the solid-state secondary battery 1b after charging is thicker than the solid-state secondary battery 1a after the electrode stack assembly is completed.

[0046] When the solid-state secondary battery 1b is discharged after its initial charge, lithium ions are released from the lithium metal layer 43 and absorbed into the positive electrode active material layer 12. As a result, as shown in Figure 3, the lithium metal layer 43 of the solid-state secondary battery 1c after its initial charge and discharge 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 its initial charge and discharge becomes thinner than the solid-state secondary battery 1b after its initial charge.

[0047] The solid-state secondary battery 1a satisfies the relationships of equations (I-1), (I-2), and (I-3) after the electrode stack assembly, where A1 is the adhesion strength of the solid electrolyte layer 20, A2 is the adhesion strength of the interface between the solid electrolyte layer 20 and the intermediate layer 30, and A3 is the adhesion strength of the intermediate layer 30. "After the electrode stack assembly" refers to the state in which each layer of the solid-state secondary battery 1a is joined together and assembled as a stack. The adhesion strength A1 of the solid electrolyte layer 20 refers to the adhesion strength of the composite material consisting of the solid electrolyte material and binder that constitute the solid electrolyte layer 20. The adhesion strength A3 of the intermediate layer 30 refers to the adhesion strength of the composite material consisting of the materials that constitute the intermediate layer 30, amorphous carbon or metal nanoparticles, and binder. A3 <A2≦A1 (I-1) 0.5 kN / m <A1<3kN / m (I-2) 0.2 kN / m <A3<2kN / m (I-3)

[0048] The solid secondary battery 1a may satisfy the following relationships (I-4) and (I-5) when the adhesion strength at the interface between the intermediate layer 30 and the metal layer 42 is A4, and the adhesion strength at the interface between the metal layer 42 and the negative electrode current collector 41 is A5. 0.4 kN / m <A4 (I-4) 0.1 kN / m <A5 (I-5)

[0049] The adhesion strength A4 at the interface between the intermediate layer 30 and the metal layer 42 may be 3 kN / m or less. The adhesion strength A5 at the interface between the metal layer 42 and the negative electrode current collector 41 may be 3 kN / m or less.

[0050] The solid secondary battery 1c after its initial charge and discharge may satisfy the relationship shown in equation (II-1) below, where B is the adhesion strength at the interface between the intermediate layer 30 and the negative electrode layer 40. The adhesion strength of each layer and each layer interface of the solid secondary battery 1c after its initial charge and discharge may be measured when the State of Charging (SOC) is in the range of 10% to 100%, particularly at 50%. 0.4 kN / m

[0051] B may be 3 kN / m or less.

[0052] Adhesion strengths A1-A5 and B can be values ​​measured by the SAICAS (Surface And Interfacial Cutting Analysis System) method. By measuring adhesion strength using the SAICAS method, adhesion strengths A1-A5 and B can be measured accurately. However, in this embodiment, the method for measuring adhesion strength is not limited to the SAICAS method, as long as it is a method that can accurately evaluate the adhesion strength of a predetermined interface. For example, the micro-scratch method, nano-scratch method, peel test, tape peel test, centrifugal method, floating roller method, or other adhesion strength measurement methods may be used.

[0053] ​Figure 4 is a schematic diagram illustrating the method for measuring adhesion strength using the SAICAS method. Figure 4 shows a method for measuring the adhesion strength of a test layer 101 placed on a substrate 100. The adhesion strength is measured by the vertical force F acting on the cutting blade 200 when the cutting blade 200 is moved obliquely to the surface of the test layer 101. V and horizontal force F H This is done by measuring the cutting depth d of the cutting blade 200. Horizontal force F H This is the force that the cutting blade 200 receives from a direction horizontal to the surface of the test layer 101. (Normal force F) V This is the force that the cutting blade 200 receives from a direction perpendicular to the surface of the test layer 101.

[0054] First, as shown in Figure 4(a), the tip of the cutting blade 200 is brought into contact with the surface of the test layer 101. Next, the cutting blade 200 is moved obliquely to the surface of the test layer 101. As a result, the test layer 101 is cut, as shown in Figure 4(b). Further oblique movement of the cutting blade 200 to the surface of the test layer 101 causes the tip of the cutting blade 200 to reach the surface of the substrate 100, as shown in Figure 4(c). As shown in the graph in Figure 4, the cutting blade 200 is moved so that the cutting depth d increases linearly until the cutting blade 200 reaches the surface of the substrate 100. After the tip of the cutting blade 200 reaches the substrate 100, the cutting blade 200 is moved horizontally to the surface of the test layer 101.

[0055] The adhesion strength can be calculated using the following formula.

[0056] Adhesion strength (kN / m) = Horizontal force F H Average value (kN) ÷ Unit cutting width (m) of cutting blade 200

[0057] When calculating the adhesion 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 blade 200 reaches the substrate 100. H This is the average value. When calculating the adhesion strength of the interface between the test layer 101 and the substrate 100, the horizontal force F HThe average value is the horizontal force F after the cutting blade 200 reaches the surface of the substrate 100. H This is the average value.

[0058] In the solid-state secondary battery 1a of the first embodiment, which has the above configuration, 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 relationship, so lithium ions move easily between the solid electrolyte layer 20 and the negative electrode layer 40. In addition, the solid electrolyte layer 20 and the intermediate layer 30 are less likely to peel off, so the shape stability of the electrode stack is improved. For this reason, the solid-state secondary battery 1a of the first embodiment has low DC resistance and excellent cycle characteristics.

[0059] Furthermore, if the adhesion strength B at the interface between the intermediate layer 30 and the negative electrode layer 40 of the solid secondary battery 1c after the first charge and discharge satisfies the above relationship, the intermediate layer 30 and the negative electrode layer 40 become less likely to peel off, and the shape stability of the electrode stack becomes higher. If the adhesion strength B is a measurement taken when the SOC of the solid secondary battery 1c after the first charge and discharge is in the range of 10% to 100%, then the adhesion strength B is the strength when the lithium metal layer 43 formed on the negative electrode layer 40 is present. For this reason, a solid secondary battery 1c in which the adhesion strength B satisfies the above relationship has a reliably low DC resistance and excellent cycle characteristics.

[0060] Furthermore, in the solid-state secondary battery 1a, the negative electrode layer 40 has a metal layer 42 positioned on the intermediate layer 30 side of the negative electrode current collector 41, which makes it easier for lithium ions to stably deposit on the negative electrode layer 40 during charging. In addition, after the electrode stack assembly, the adhesion strength A4 at the interface between the metal layer 42 and the intermediate layer 30 satisfies the above relationship, and the adhesion strength A5 at the interface between the metal layer 42 and the negative electrode current collector 41 also satisfies the above relationship, making it even more difficult for the intermediate layer 30, the metal layer 42, and the negative electrode current collector 41 to peel off, and further improving the shape stability of the electrode stack.

[0061] [Second Embodiment] Figure 5 is a cross-sectional view showing an example of a solid-state secondary battery according to a second embodiment of the present invention. Figure 6 is a cross-sectional view showing an example of the state of the solid-state secondary battery shown in Figure 5 after its initial charge, and Figure 7 is a cross-sectional view showing an example of the state of the solid-state secondary battery shown in Figure 6 after its initial charge and discharge.

[0062] The solid-state secondary battery 2a of this embodiment is the same as the solid-state secondary battery 1a of the first embodiment after the electrode stack assembly, except that the negative electrode layer 40 after the electrode stack assembly consists only of the negative electrode current collector 41, and the negative electrode current collector 41 is in close contact with the intermediate layer 30. For this reason, parts that are common to the solid-state secondary battery 1a of the first embodiment are given the same reference numerals, and their descriptions are omitted.

[0063] When the solid-state 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. As a result, as shown in Figure 6, a lithium metal layer 43 is formed on the surface of the negative electrode current collector 41 of the solid-state secondary battery 2b after the initial charge. The charging conditions are the same as in the first embodiment.

[0064] When the solid-state secondary battery 2b is discharged after its initial charge, lithium ions are released from the lithium metal layer 43. As shown in Figure 7, after the initial charge and discharge, the solid-state secondary battery 2c shows a decrease in the thickness of the lithium metal layer 43, or the lithium metal layer 43 disappears. The discharge conditions are the same as in the first embodiment.

[0065] In the solid-state secondary battery 2a of the second embodiment, which has the configuration described above, 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 relationship. Therefore, similar to the solid-state secondary battery 1a of the first embodiment, lithium ions move easily between the solid electrolyte layer 20 and the negative electrode layer 40. In addition, the solid electrolyte layer 20 and the intermediate layer 30 are less likely to peel off, so the shape stability of the electrode stack is improved. For this reason, the solid-state secondary battery 2a of the second embodiment has low DC resistance and excellent cycle characteristics.

[0066] Furthermore, if the adhesion strength B at the interface between the intermediate layer 30 and the negative electrode layer 40 of the solid secondary battery 2c after the initial charge and discharge satisfies the above relationship, then, similar to the solid secondary battery 1a of the first embodiment, the intermediate layer 30 and the negative electrode layer 40 become less prone to peeling, and the shape stability of the electrode laminate becomes higher.

[0067] Furthermore, in the solid-state secondary battery 2a, after the electrode stack assembly, the negative electrode current collector 41 is in close contact with the intermediate layer 30, and the negative electrode layer 40 is composed solely of the negative electrode current collector 41, thus reducing manufacturing costs by not including a metal layer.

[0068] [Evaluation methods for solid-state rechargeable batteries] The evaluation method for solid-state secondary batteries of this embodiment is a method for evaluating solid-state secondary batteries 1a, 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 including a solid electrolyte material 21, and an intermediate layer 30 provided between the negative electrode layer 40 and the solid electrolyte layer 20. The evaluation method involves removing the laminate having the solid electrolyte layer 20, the intermediate layer 30, and the negative electrode layer 40 from the solid-state secondary batteries 1a, 2a after electrode laminate assembly, and measuring the adhesion strength A1 of the solid electrolyte layer 20 of the laminate, 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 by the SAICAS method. The method for measuring the adhesion strengths A1 to A3 by the SAICAS method is as described above.

[0069] In the evaluation method for solid-state secondary batteries of this embodiment, if the adhesion strengths A1 to A3 satisfy the relationships of equations (I-1), (I-2), and (I-3) described above, the DC resistance is low and the cycle characteristics are excellent.

[0070] According to the evaluation method for solid-state secondary batteries of this embodiment, the adhesion strength between the solid electrolyte layer 20 and the intermediate layer 30 located between the positive electrode layer 10 and the negative electrode layer 40 of solid-state secondary batteries 1a and 2a can be measured with high accuracy, thereby enabling accurate prediction of the DC resistance and cycle characteristics of solid-state secondary batteries 1a and 2a.

[0071] Although embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. For example, in this embodiment, the solid secondary batteries 1a and 2a are lithium metal batteries using 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 may have a negative electrode layer whose thickness changes with charging and discharging. [Examples]

[0072] The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0073] [Example 1] (1) Fabrication of the positive electrode layer 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 mixed as the positive electrode active material, 17 parts by mass of argyrodite-type sulfide solid electrolyte as the solid electrolyte material, 2 parts by mass of carbon black as a conductive additive, and 1 part by mass of SBR (styrene-butadiene rubber) binder as a binder. The resulting mixture was dispersed in 43 parts by mass of butyl butyrate to prepare a positive electrode active material layer slurry. The obtained positive electrode active material layer slurry was applied to both sides of the positive electrode current collector, with a dry basis weight of 27 mg / cm². 2 The material was coated using a bar coater and dried to form a positive electrode active material layer with a thickness of 80 μm, thereby creating the positive electrode layer.

[0074] (2) Preparation of a solid electrolyte layer transfer sheet 97 parts by mass of argyrodite-type sulfide solid electrolyte (median diameter 3.0 μm) and 3 parts by mass of SBR (styrene-butadiene rubber)-based binder were mixed. The resulting mixture was dispersed in a solvent to prepare a solid electrolyte slurry. The resulting solid electrolyte slurry was applied to a support sheet and dried to produce a solid electrolyte layer transfer sheet (solid electrolyte layer thickness: 100 μm).

[0075] (3) Preparation of the intermediate layer transfer sheet A total of 95 parts by mass of Sn particles (average particle size: 0.07 μm) as metal particles and acetylene black (average particle size: 0.05 μm) as amorphous carbon particles were mixed with 5 parts by mass of a PVDF-based binder. The resulting mixture was dispersed in 1000 parts by mass of NMP (N-methyl-2-pyrrolidone) to prepare an intermediate layer slurry. The resulting intermediate layer slurry was applied to a support sheet and dried to produce an intermediate layer transfer sheet (intermediate layer thickness: 3.0 μm).

[0076] (4) Fabrication of the negative electrode layer A 10 μm thick copper foil was prepared as the negative electrode current collector. A 40 μm thick metallic lithium foil was rolled and laminated onto the surface of the copper foil to create the negative electrode layer.

[0077] (5) Fabrication of solid-state secondary batteries The solid electrolyte layer of the solid electrolyte layer transfer sheet was superimposed on the surface of the positive electrode active material layer of the positive electrode layer, and bonded using a uniaxial forming press under the following conditions: bonding pressure: 90 MPa, bonding time: 3 minutes, bonding temperature: room temperature. After that, the support sheet of the solid electrolyte layer transfer sheet was peeled off to obtain a positive electrode layer-solid electrolyte layer laminate. Next, the intermediate layer of the intermediate layer transfer sheet was superimposed on the surface of the solid electrolyte layer of the positive electrode layer-solid electrolyte layer laminate, and bonded using a uniaxial forming press under the following conditions: bonding pressure: 290 MPa, bonding time: 5 minutes, bonding temperature: room temperature. After that, the support sheet of the intermediate layer transfer sheet was peeled off to obtain a positive electrode layer-solid electrolyte layer-intermediate layer laminate. Next, the integrated positive electrode layer-solid electrolyte layer-intermediate layer laminate was subjected to a densification treatment using an isotropic forming press at a bonding pressure: 980 MPa, bonding time: 5 minutes, bonding temperature: 120°C. Next, the metallic lithium foil of the negative electrode layer was superimposed on the surface of the intermediate layer of the positive electrode layer-solid electrolyte layer-intermediate layer assembly, and bonded using a uniaxial forming press under the following conditions: bonding pressure: 180 MPa, bonding time: 2 minutes, bonding temperature: room temperature. In this way, an electrode laminate was obtained. The resulting electrode stack was housed in an aluminum laminate film casing to fabricate a solid-state secondary battery. A buffer material was placed on the negative electrode layer side and restrained with a restraining pressure of 3 MPa.

[0078] [Example 2] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that the thickness of the metallic lithium foil in the negative electrode layer was set to 6.5 μm.

[0079] [Example 3] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that a copper foil with a thickness of 8 μm (negative electrode current collector) was used as the negative electrode layer, the negative electrode layer was placed on top of the surface of the intermediate layer of the positive electrode layer-solid electrolyte layer-intermediate layer junction, and the constraint pressure for constraining the electrode stack was set to 3 MPa.

[0080] [Example 4] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that a silver-copper laminate was used as the negative electrode layer, in which a 20 nm thick silver layer was deposited on one surface of an 8 μm thick copper foil (negative electrode current collector) by sputtering, and 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 create an electrode laminate, and the constraint pressure for constraining the electrode laminate was set to 3 MPa.

[0081] [Example 5] As the negative electrode layer, a copper foil (negative electrode current collector) with a thickness of 8 μm has a carbon layer of 0.04 mg / cm³ on one surface. 2 A solid-state secondary battery was fabricated in the same manner as in Example 1, except that a carbon-copper laminate was used to form a carbon coating layer by coating it with a certain amount, and 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 joint, and the electrode laminate was fabricated by joining them under the condition of a joining pressure of 600 MPa.

[0082] [Comparative Example 1] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that the thickness of the metallic lithium foil in the negative electrode layer was 6.5 μm, the amount of SBR (styrene-butadiene rubber) binder in the solid electrolyte layer was 5.5 parts by mass, the thickness of the solid electrolyte layer was 30 μm, a bond between the positive electrode layer and the solid electrolyte layer, and a bond between the thin film solid electrolyte layer, intermediate layer, and negative electrode layer were fabricated and joined together under the conditions of a bonding pressure of 500 MPa, a bonding time of 15 seconds, and a bonding temperature of room temperature.

[0083] [Comparative Example 2] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that the thickness of the metallic lithium foil in the negative electrode layer was 6.5 μm, the amount of lithium was 6.5 μm, the amount of SBR (styrene-butadiene rubber) binder in the solid electrolyte layer was 10 parts by mass, the thickness of the solid electrolyte layer was 30 μm, and the integrated positive electrode layer-solid electrolyte layer-intermediate layer-negative electrode layer was bonded under the conditions of bonding pressure: 800 MPa, bonding time: 15 seconds, and bonding temperature: room temperature.

[0084] [Comparative Example 3] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that the amount of SBR (styrene-butadiene rubber) binder 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.

[0085] [Comparative Example 4] A solid-state secondary battery was fabricated in the same manner as in Example 1, except that the amount of PVDF-based binder in the intermediate layer was set to 3 parts by mass.

[0086] [evaluation] The solid-state secondary batteries obtained in Examples 1-5 and Comparative Examples 1-4 were evaluated as follows.

[0087] (1) Adhesion strength (adhesion strength from the solid electrolyte layer to the negative electrode layer of a solid secondary battery) The solid-state 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 measured using the SAICAS method. The measurement conditions for adhesion strength were as follows. The results are shown in Table 1.

[0088] (Measurement conditions for adhesion strength) Measurements were taken using a SAICAS test machine under room temperature and a low dew point environment of -40°C or lower, with constant speed mode, cutting blade material: diamond, cutting blade width: 0.3 mm, cutting blade horizontal speed: 2.0 μm / sec, and evaluation number n=3. The cutting blade was used to cut from the surface of the negative electrode current collector foil in the negative electrode layer to the interface to be evaluated for adhesion. After reaching the target interface, the cutting blade was moved horizontally, and the horizontal force per unit width of the cutting blade was calculated from the average value of the horizontal force at that time to evaluate the adhesion.

[0089] (2) Adhesion strength (adhesion strength of the intermediate layer-negative electrode layer interface of a solid secondary battery after the first charge and discharge) A solid-state rechargeable battery was charged under constant current / constant voltage (CCCV) conditions at a temperature of 60°C, a current of 0.1C, and a voltage of 4.3V. The initial charge-discharge was then performed by discharging to 2.65V under constant current (CC) conditions at a temperature of 60°C and a voltage of 0.1C. After aging, the SOC was adjusted to 50% under constant current / constant voltage (CCCV) conditions at a temperature of 60°C, a current of 0.1C, and a voltage equivalent to 50% of the SOC. The solid-state rechargeable battery was then disassembled, and the electrode stack was removed. The positive electrode layer and solid electrolyte layer were peeled from the removed electrode stack to obtain an intermediate layer-negative electrode junction. The adhesion strength at 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 measurement conditions for adhesion strength were the same as those for measuring adhesion strength from the solid electrolyte layer to the negative electrode layer of the solid-state rechargeable battery. The results are shown in Table 1.

[0090] (3) Composite modulus (composite modulus of the solid electrolyte layer, intermediate layer, and negative electrode layer of a solid-state secondary battery) As in (1) above, the solid-state secondary battery was disassembled after being left to stand, and the electrode stack was removed. The positive electrode layer was peeled off from the removed electrode stack, and the composite elastic modulus of the solid electrolyte layer was measured. Next, 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 used for measurement. The composite elastic modulus was measured using the nanoindentation method. The results are shown in Table 1.

[0091] (4) DC resistance (DC resistance at 60°C with low current discharge) At a temperature of 60°C and a state of charge of 50%, the current density is 2.7 mA / cm². 2 Voltage drop ΔV (V), current value I (A), and positive electrode area Ac (cm²) during discharge. 2 From the following formula, the DC resistance (Ω·cm) can be calculated. 2 ) was calculated. DC resistance (Ω cm 2) = Voltage drop ΔV (V) / Current value I (A) × Positive electrode area Ac (cm²) 2 )

[0092] (DC resistance at 25°C high-current discharge) At a temperature of 25°C and a state of charge of 50%, the current density is 15.1 mA / cm². 2 Voltage drop ΔV (V), current value I (A), and positive electrode area Ac (cm²) during discharge. 2 From the following formula, the DC resistance (Ω·cm) can be calculated. 2 ) was calculated. DC resistance (Ω cm 2 ) = Voltage drop ΔV (V) / Current value I (A) × Positive electrode area Ac (cm²) 2 )

[0093] (5) Observation of the initial lithium deposition state (2) A portion of the electrode stack obtained by measuring the adhesion strength (adhesion strength of the intermediate layer-negative electrode layer interface of the solid secondary battery after the first charge) was cut out and observed in cross-section using an SEM (scanning electron microscope). A "○" was given if there was no short-circuit behavior during or after the first charge / discharge, the lithium deposition location was between the intermediate layer and the negative electrode current collector foil, and the lithium deposition layer thickness was less than the average thickness ±2 μm. A "× deposition location NG" was given if the lithium deposition location was between the solid electrolyte layer and the intermediate layer, inside the solid electrolyte layer, or in a combined state of these. A "× non-uniform" was given if the lithium deposition layer on the negative electrode layer had thin and thick parts, with a thickness difference of more than the average thickness ±2 μm, or if there were cracks or voids of 3 μm or more inside the lithium deposition layer. A "× intermediate layer structural collapse" was given if there were cracks inside the intermediate layer or if it peeled off from the solid electrolyte layer or negative electrode layer. Figure 8 shows an SEM image of a cross-section of the electrode stack removed from the solid-state secondary battery obtained in Example 1, Figure 9 shows an SEM image of Example 3, and Figure 10 shows an SEM image of a cross-section of the electrode stack removed from the solid-state secondary battery obtained in Comparative Example 1.

[0094] (6) Cycle Test (45°C cycle) Charge-discharge cycle tests were conducted at a temperature of 45°C, with a current of 1 / 3C, a voltage range of 4.3V upper limit and 2.65V lower limit, constant current / constant voltage (CCCV) charging, and constant current (CC) discharging. Table 2 shows the discharge capacity retention rate and short-circuit rate after 100 cycles.

[0095] [Table 1]

[0096] [Table 2]

[0097] From the results in Tables 1 and 2, it can be seen that the solid-state secondary batteries of Examples 1 to 5, in which 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, have low DC resistance and high discharge capacity retention rates after 100 cycle tests. In contrast, the solid-state secondary batteries of Comparative Example 1, in which the adhesion strength A1 of the solid electrolyte layer is lower than the range of the present invention, Comparative Examples 2 and 3, in which the adhesion strength A1 of the solid electrolyte layer is higher than the range of the present invention, and Comparative Example 4, in which the adhesion strength A3 of the intermediate layer is lower than the range of the present invention, all have high DC resistance and low discharge capacity after 100 cycle tests. Furthermore, it can be seen that Comparative Examples 1 to 4 have a higher short-circuit rate at 45°C for 100 cycles compared to Examples 1 to 5.

[0098] Furthermore, as shown in Figures 8 and 9, in the solid-state 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 all met the conditions of the present invention, no lithium deposition was observed between the solid electrolyte layer 20 and the intermediate layer 30. In contrast, as shown in Figure 10, in the solid-state secondary battery of Comparative Example 1, lithium deposition occurred between the solid electrolyte layer 20 and the intermediate layer 30, and significant cracks and voids were present within the deposited lithium layer, resulting in an uneven deposition state. [Explanation of Symbols]

[0099] 1a, 2a solid state secondary battery 1b, 2b Solid-state rechargeable battery after initial charging 1C, 2C Solid-state rechargeable battery after initial charge / discharge 10 Positive electrode layer 11 Positive electrode current collector 12 Cathode active material layer 20 Solid electrolyte layer 21 Solid electrolyte materials 30 Middle Class 40 Negative electrode layer 41 Negative electrode current collector 42 Metal layer 43 Lithium metal layer 100 circuit boards 101 Test Layer 200 cutting blade

Claims

1. It comprises a positive electrode layer, a negative electrode layer including at least a negative electrode current collector, a solid electrolyte layer including a solid electrolyte material, and an intermediate layer provided between the negative electrode layer and the solid electrolyte layer. An electrode laminate is formed by joining the positive electrode layer, the solid electrolyte layer, the intermediate layer, and the negative electrode layer, respectively. A solid-state secondary battery that satisfies the following relationships (I-1), (I-2), and (I-3), where A1 is the adhesion strength of the solid electrolyte layer, A2 is the adhesion strength of the interface between the solid electrolyte layer and the intermediate layer, and A3 is the adhesion strength of the intermediate layer. A3<A2≦A1 (I-1) 0.5kN / m<A1<3kN / m (I-2) 0.2kN / m<A3<2kN / m (I-3)

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

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

4. The solid-state secondary battery according to claim 1 or 2, wherein the negative electrode current collector is in close contact with the intermediate layer after the assembly of the electrode stack.

5. The solid secondary battery according to claim 2, wherein the adhesion strength B in formula (II-1) is a measured value when the SOC 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 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 intermediate layer is 30 to 60%.

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

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