Elastic sheet for all-solid-state secondary battery and all-solid-state secondary battery module

Abstract

To provide an elastic sheet that exhibits sufficient flame retardancy and can repeatedly absorb volume changes due to expansion and contraction of an all-solid-state secondary battery.SOLUTION: In an elastic sheet for an all-solid-state secondary battery, which is used together with an all-solid-state secondary battery to buffer volume changes due to expansion and contraction of the all-solid-state secondary battery, the elastic sheet is made of a closed-cell foam formed from a fluorine-based elastomer, and the porosity of the foam is 40% or more and 70% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an elastic sheet for an all-solid-state secondary battery and an all-solid-state secondary battery module including the elastic sheet for an all-solid-state secondary battery. [Background technology]

[0002] All-solid-state secondary batteries are considered to be relatively safe because they do not contain an organic electrolyte. To further improve the safety of these all-solid-state secondary batteries, risk countermeasures against external factors, such as when moisture enters from the outside air due to cracking or the like and exothermically reacts with lithium in the all-solid-state secondary battery, are being considered. One example of such risk countermeasures is to place a flame-retardant resin layer around the all-solid-state secondary battery.

[0003] Furthermore, in all-solid-state batteries, the volume expands and contracts due to charge and discharge, temperature changes, etc. Therefore, in order to prevent damage to the restraining member due to such expansion and contraction, for example, it is conceivable to interpose a resin layer between all-solid-state secondary batteries.

[0004] As a conventional example of disposing a resin layer between all-solid-state secondary batteries in this way, there are those disclosed in Patent Documents 1 to 3, in which a resin layer is disposed inside an all-solid-state secondary battery module including a plurality of all-solid-state secondary batteries, and flame retardancy is imparted by including a filler or the like in the resin layer. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2022-527885 [Patent Document 2] Patent No. 6906698 [Patent Document 3] Patent No. 6966058 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the resin layers described in the above-mentioned Patent Documents 1 to 3 have a problem in that they lose elasticity because they are made flame-retardant by adding fillers or the like, and the resin layers cannot repeatedly absorb volume changes due to expansion and contraction of the all-solid-state secondary battery.

[0007] The present invention has been made in view of the above problems, and aims to provide an elastic sheet that exhibits sufficient flame retardancy and can repeatedly absorb and sufficiently cushion volume changes due to expansion and contraction of all-solid-state secondary batteries, for example, by being disposed in an all-solid-state secondary battery module together with all-solid-state secondary batteries. [Means for solving the problem]

[0008] That is, the elastic sheet according to the present invention and the all-solid-state secondary battery module including this elastic sheet are as follows. [1] An elastic sheet used together with an all-solid-state secondary battery to buffer volume changes due to expansion and contraction of the all-solid-state secondary battery, It is made of a closed-cell foam formed from a fluorine-based elastomer, The foam has a porosity of 40% or more and 70% or less. [2] The elastic sheet for an all-solid-state secondary battery according to [1], wherein the fluorine content of the fluorine-based elastomer is 60% by mass or more and 80% by mass or less. [3] The elastic sheet for an all-solid-state secondary battery according to [1] or [2], which has a strain of 10% or less when an external pressure of 0.1 MPa is applied. [4] The stress retention rate is 80% or more; The elastic sheet for an all-solid-state secondary battery according to any one of [1] to [3]. [5] The flexible sheet for an all-solid-state secondary battery according to any one of [1] to [4], which has a thickness of 10 μm or more and 1000 μm or less. [6] The flexible sheet for an all-solid-state secondary battery according to any one of [1] to [5], which has a flame retardancy rating of V-0 according to the UL94 standard. [7] The flexible sheet for an all-solid-state secondary battery according to any one of [1] to [6], which shows no change in infrared absorption spectrum before and after contact with metallic lithium for 4 days. [8] The flexible sheet for an all-solid-state secondary battery according to any one of [1] to [7], which has a mass change rate of less than 1% before and after contact with a sulfide-based solid electrolyte for 30 days. [9] The elastic sheet for an all-solid-state secondary battery according to any one of [1] to [8], which is electrically insulating.

[10] An all-solid-state secondary battery module comprising an all-solid-state secondary battery, the elastic sheet for an all-solid-state secondary battery according to any one of [1] to [9], and a housing for accommodating these therein. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide an elastic sheet that has sufficient flame retardancy and is capable of repeatedly absorbing volume changes due to expansion and contraction of an all-solid-state secondary battery. For example, by forming an all-solid-state secondary battery module using such an elastic sheet between a plurality of all-solid-state secondary batteries, it is possible to suppress the occurrence of fires within the all-solid-state secondary battery module and improve safety. The elastic sheet according to the present invention can also repeatedly absorb volume changes due to expansion and contraction of each all-solid-state secondary battery. As a result, it is possible to sufficiently suppress non-uniform pressure within the all-solid-state secondary battery module, suppress current concentration in a portion of the all-solid-state secondary batteries, and maintain the cycle characteristics of the all-solid-state secondary battery module longer than conventional. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a schematic diagram showing a schematic configuration of an all-solid-state secondary battery module according to an embodiment of the present invention. [Figure 2] 1 is a cross-sectional view schematically illustrating a configuration of an all-solid-state secondary battery module according to an embodiment of the present invention. [Figure 3] 2 is a cross-sectional view schematically illustrating the structure of an all-solid-state secondary battery and an elastic sheet included in the all-solid-state secondary battery module according to the embodiment. FIG. [Figure 4] FIG. 2 is a cross-sectional view schematically illustrating the structure of an all-solid-state secondary battery according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In this specification and the drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant description will be omitted. Furthermore, the components in the drawings have been enlarged or reduced as appropriate for ease of explanation, and the size and proportions of the components in the drawings may differ from those in reality.

[0012] <1. Configuration of the all-solid-state secondary battery module according to this embodiment> As shown in FIG. 1 or 2, the all-solid-state secondary battery module 100 according to this embodiment includes a housing The battery pack includes a housing C, one or more all-solid-state secondary batteries A housed inside the housing C, and one or more elastic sheets B. Each of these configurations will be described below.

[0013] <1-1. Cabinet> As shown in FIG. 1 or 2, the housing C accommodates a plurality (for example, 13) of all-solid-state secondary batteries A and a plurality (here, 14) of elastic sheets B stacked on top of each other. The casing C may be made of a metal such as aluminum having a rectangular shape (rectangular parallelepiped), or may be made of something like an aluminum laminate film. The casing C has, for example, three pairs of opposing surfaces with different areas, one of which has the smallest area as a bottom surface S, and houses the all-solid-state secondary batteries A and the elastic sheets B in a height direction (a direction perpendicular to the bottom surface S) such that the stacking directions of the above-mentioned multiple all-solid-state secondary batteries A and multiple elastic sheets B coincide with each other, and no gaps are formed inside in this height direction. If the casing C is made of a soft material such as an aluminum laminate film, it is possible to house the all-solid-state secondary batteries A and the multiple elastic sheets B, and then seal the casing C under reduced pressure to prevent gaps from being formed inside.

[0014] <1-2. All-solid-state secondary battery> The all-solid-state secondary battery A according to this embodiment includes a positive electrode layer 10, a negative electrode layer 20, and a solid electrolyte layer 30, and is, for example, as shown in Fig. 3, an all-solid-state lithium secondary battery including a positive electrode layer 10, solid electrolyte layers 30 laminated on both sides of the positive electrode layer 10, and negative electrode layers 20 laminated on the surfaces of the solid electrolyte layers 30 opposite to the positive electrode layer 10. The stacking direction of the layers constituting the all-solid-state secondary battery A coincides with the stacking direction when the all-solid-state secondary battery A and the elastic sheet B are stacked together, as described above.

[0015] (1-2-1. Positive electrode layer) As shown in FIG. 3, the positive electrode layer 10 includes a positive electrode current collector 11 and a positive electrode active material layer 12. Examples of the positive electrode current collector 11 include a plate or foil made of stainless steel, titanium (Ti), nickel (Ni), aluminum (Al), or an alloy thereof. The thickness of the positive electrode current collector 11 is, for example, 1 μm or more and 50 μm or less, more preferably 5 μm or more and 30 μm or less. 3, the positive electrode active material layers 12 are disposed on both sides of the positive electrode current collector 11. The positive electrode active material layers 12 contain at least a positive electrode active material and a solid electrolyte. The solid electrolyte contained in the positive electrode active material layer 12 may or may not be the same type as the solid electrolyte contained in the solid electrolyte layer 30. Details of the solid electrolyte will be described later in the section on the solid electrolyte layer 30.

[0016] The positive electrode active material may be any positive electrode active material that can reversibly store and release lithium ions.

[0017] For example, the positive electrode active material may be in the form of a powder or granules, and may be formed using lithium salts such as lithium cobalt oxide (hereinafter referred to as LCO), lithium nickel oxide, lithium nickel cobalt oxide, lithium nickel aluminum oxide (hereinafter referred to as NCA), lithium nickel cobalt manganese oxide (hereinafter referred to as NCM), lithium manganate, and lithium iron phosphate, nickel sulfide, copper sulfide, sulfur, iron oxide, or vanadium oxide. These positive electrode active materials may be used alone or in combination of two or more.

[0018] The positive electrode active material is preferably formed by including a lithium salt of a transition metal oxide having a layered rock salt structure among the above-mentioned lithium salts. Here, "layered" refers to a thin sheet shape. The "rock salt structure" refers to a sodium chloride structure, which is a type of crystalline structure, and specifically refers to a structure in which face-centered cubic lattices formed by cations and anions are shifted from each other by half the edge of the unit lattice.

[0019] Examples of lithium salts of transition metal oxides having such a layered rock salt structure include LiNi x Co y Al z O2(NCA), or LiNi x Co y Mn zExamples of lithium salts of ternary transition metal oxides include O2(NCM) (where 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1).

[0020] When the positive electrode active material contains a lithium salt of a ternary transition metal oxide having the above-described layered rock salt structure, the energy density of the all-solid-state secondary battery A can be improved.

[0021] The surface of the positive electrode active material may be covered with a coating layer. Here, the coating layer of the present embodiment may be any known coating layer for the positive electrode active material of the all-solid-state secondary battery A. Examples of the coating layer include, for example, LiNBO3, Li2WO4, Li2O-ZrO2, etc.

[0022] Further, when the positive electrode active material is formed of a lithium salt of a ternary transition metal oxide such as NCA or NCM and contains nickel (Ni) as the positive electrode active material, the capacity density of the all-solid-state secondary battery A can be increased.

[0023] Here, examples of the shape of the positive electrode active material include particle shapes such as true spherical and elliptical spherical. Also, the particle size of the positive electrode active material is not particularly limited as long as it is within a range applicable to the positive electrode active material of a conventional all-solid-state secondary battery. In addition, the content of the positive electrode active material in the positive electrode layer 10 is not particularly limited as long as it is within a range applicable to the positive electrode layer of a conventional all-solid-state secondary battery.

[0024] In addition to the above-described positive electrode active material and solid electrolyte, additives such as a conductive aid, a binder, a filler, a dispersant, and an ion conduction aid may be appropriately blended in the positive electrode active material layer 12.

[0025] Examples of conductive additives that can be incorporated into the positive electrode active material layer 12 include graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanotubes, graphene, and metal powder. Examples of binders that can be incorporated into the positive electrode active material layer 12 include styrene butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, a copolymer of vinylidene fluoride and 2,3,3,3-tetrafluoropropylene, polyethylene, and modified products thereof. Examples of fillers, dispersants, ion-conducting additives, and the like that can be incorporated into the positive electrode active material layer 12 include known materials generally used in electrodes of the all-solid-state secondary battery A.

[0026] The thickness of the positive electrode active material layer 12 in the completed battery is not particularly limited, but is preferably, for example, 20 μm or more and 1000 μm or less, more preferably 50 μm or more and 500 μm or less, and particularly preferably 100 μm or more and 300 μm or less.

[0027] (1-2-2. Negative electrode layer) The negative electrode layer 20 includes, for example, a plate-shaped or foil-shaped negative electrode current collector 21 and a negative electrode active material layer 22 formed on the negative electrode current collector 21, as shown in FIG. In this embodiment, the negative electrode current collector 21 forms the outermost layer of the all-solid-state secondary battery A.

[0028] The negative electrode current collector 21 is preferably made of a material that does not react with lithium, that is, that does not form any alloy or compound with lithium. In addition to stainless steel, examples of materials that can be used to form the negative electrode current collector 21 include copper (Cu), titanium (Ti), iron (Fe), cobalt (Co), and nickel (Ni). The negative electrode current collector 21 may be made of any one of these metals, or may be made of an alloy or clad material of two or more metals. The thickness of the negative electrode current collector 21 is, for example, 1 μm or more and 50 μm or less, and more preferably 5 μm or more and 30 μm or less.

[0029] The negative electrode active material layer 22 contains, for example, at least one of a negative electrode active material that forms an alloy with lithium and a negative electrode active material that forms a compound with lithium. The negative electrode active material layer 22 may be configured so that metallic lithium can be deposited on one or both surfaces of the negative electrode active material layer 22 by containing such a negative electrode active material, as described below.

[0030] Examples of the negative electrode active material include graphite, amorphous carbon, gold, platinum, palladium (Pd), silicon (Si), silver, aluminum (Al), bismuth (Bi), tin, antimony, and zinc. Examples of the amorphous carbon include carbon black such as acetylene black, furnace black, and ketjen black, and graphene.

[0031] The shape of the negative electrode active material is not particularly limited, and may be granular or may be a uniform layer such as a plating layer. The lithium ions or lithium may be entirely occluded inside the granular negative electrode active material, or inside the negative electrode active material, or inside the plating layer, or may pass through gaps between the negative electrode active materials to form a metal layer mainly composed of lithium between the negative electrode active material layer 22 and the negative electrode current collector 21, with some of the lithium being present in the negative electrode active material layer 22 by forming an alloy with a metal element in the negative electrode active material.

[0032] Among the above, the negative electrode active material layer 22 is made of amorphous carbon having a specific surface area of ​​10 m2 measured by nitrogen gas adsorption method. 2 / g or more 100m 2 / g or less is preferable.

[0033] The negative electrode active material layer 22 may contain only one of these negative electrode active materials, or may contain two or more negative electrode active materials. For example, the negative electrode active material layer 22 may contain only amorphous carbon as the negative electrode active material, or may contain one or more selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, antimony, and zinc. Alternatively, the negative electrode active material layer 22 may contain a mixture of amorphous carbon and one or more selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, antimony, and zinc.

[0034] The mixing ratio (mass ratio) of the mixture of amorphous carbon and the above-mentioned metal such as gold is preferably about 1:2 to 4:1. By using these materials as the negative electrode active material, the characteristics of the all-solid-state secondary battery A are further improved.

[0035] When the negative electrode active material is one or more selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, antimony, and zinc together with amorphous carbon, the particle size of the negative electrode active material is preferably 4 μm or less, which further improves the characteristics of the all-solid-state secondary battery A.

[0036] When the negative electrode active material is a material capable of forming an alloy with lithium, such as at least one selected from the group consisting of gold, platinum, palladium, silicon, silver, aluminum, bismuth, tin, antimony, and zinc, the negative electrode active material layer 22 may be a layer made of such a metal. For example, the metal layer may be a plating layer.

[0037] The negative electrode active material layer 22 may further contain a binder, if necessary. Examples of such binders include styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyvinylidene fluoride, polyethylene oxide, carboxymethyl cellulose and its alkali metal salts, ammonium salts, and modified products thereof. The binder may be composed of one or more of these. By including a binder in the negative electrode active material layer 22, particularly when the negative electrode active material is granular, it is possible to suppress the separation of the negative electrode active material. The content of the binder in the negative electrode active material layer 22 is, for example, 0.3% by mass to 20.0% by mass, preferably 1.0% by mass to 15.0% by mass, and more preferably 3.0% by mass to 15.0% by mass, relative to the total mass of the negative electrode active material layer 22.

[0038] The negative electrode active material layer 22 may further contain a solid electrolyte, a negative electrode layer conductive aid, and other additives contained in the conventional all-solid-state secondary battery A.

[0039] The conductive additive for the negative electrode layer and the solid electrolyte can be the same compounds as the conductive agent and the solid electrolyte contained in the positive electrode active material layer 12. Therefore, a description of their configurations will be omitted here.

[0040] The thickness of the anode active material layer 22 is not particularly limited, but when the anode active material is granular, the thickness in the completed battery state is, for example, 1 μm or more and 1000 μm or less. When lithium ions or lithium pass through the gaps between the anode active material layers to form a metal layer mainly composed of lithium between the anode active material layer 22 and the anode current collector 21, and some lithium exists in the anode active material layer 22 by forming an alloy with a metal element in the anode active material, the thickness is preferably 2 μm or more and 20 μm or less. By setting the thickness in this range, the resistance value of the anode active material layer 22 can be sufficiently reduced, and the characteristics of the all-solid-state secondary battery A can be improved.

[0041] (1-2-3.Solid electrolyte layer) The solid electrolyte layer 30 is, for example, as shown in FIG. 3, a layer formed between the positive electrode layer 10 and the negative electrode layer 20, and contains a solid electrolyte. The thickness of the solid electrolyte layer 30 in the completed battery state may be 5 μm or more and 100 μm or less, preferably 8 μm or more and 80 μm or less, and more preferably 10 μm or more and 50 μm or less.

[0042] The solid electrolyte is, for example, in powder form, and is made of, for example, a sulfide-based solid electrolyte material. Examples of the sulfide-based solid electrolyte material include Li2S-P2S5, Li2S-P2S5-LiX (X is a halogen element, for example, I, Br, or Cl), Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, and Li2 S -SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-Z m S n (m and n are positive numbers, Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li p MO q (p and q are positive numbers, and M is any of P, Si, Ge, B, Al, Ga, and In). Here, the sulfide-based solid electrolyte material is produced by treating a starting material (e.g., Li2S, P2S5, etc.) by a melt quenching method, a mechanical milling method, or the like. Furthermore, these treatments may be followed by a heat treatment. The solid electrolyte may be amorphous, crystalline, or a mixture of both.

[0043] Among these, Li6PS5Cl, a solid electrolyte material with an Argyrodite-type crystal structure (Argyrodite-based), is preferred because its particle size can be easily controlled and refined to enable high packing density and the formation of a favorable interface with the positive electrode active material. It is also a desirable material because it has a wide potential window, excellent voltage resistance, and a mass production process is being established, making it a promising material from a cost perspective. In addition to the solid electrolyte materials mentioned above, Argyrodite-based Li6PS5BrxCl1-x and glass ceramic-based Li2S-P2S5-LiX (X = Br or I) may also be used.

[0044] The solid electrolyte layer 30 may further contain a binder. Examples of binders contained in the solid electrolyte layer 30 include styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), a copolymer of vinylidene fluoride and hexafluoropropylene, a copolymer of vinylidene fluoride and 2,3,3,3-tetrafluoropropylene, polyethylene (PE), polyacrylic acid (PAA), a copolymer of acrylic ester, and modified products thereof. The binder contained in the solid electrolyte layer 30 may be the same type as the binder in the positive electrode active material layer 12 and the negative electrode active material layer 22, or may be a different type.

[0045] (1-2-4. Current collecting part) 1 and 2, the positive electrode current collector 11 has a positive electrode current collecting portion 111 that protrudes laterally, and is connected to external wiring via this positive electrode current collecting portion 111. Similarly, the negative electrode current collector 21 has a negative electrode current collecting portion 211 that protrudes laterally, and is connected to external wiring via this negative electrode current collecting portion 211. In this specification, the term "lateral" refers to, for example, a direction extending from the outer peripheral edge of the positive electrode current collector along its surface toward the outside, and more specifically, refers to a direction perpendicular to the stacking direction of the layers constituting the all-solid-state secondary battery A.

[0046] <1-3. Elastic sheet> The elastic sheet B according to this embodiment is a film, sheet, or plate having a rectangular shape in a plan view, and is arranged so that its thickness direction coincides with the stacking direction of the all-solid-state secondary battery A, thereby absorbing volume changes due to expansion and contraction of the all-solid-state secondary battery A. In this embodiment, the elastic sheets B are provided between adjacent all-solid-state secondary batteries A and between the all-solid-state secondary batteries A and the housing C at both ends in the stacking direction, as shown in FIG. 1 or 2, for example.

[0047] As described above, the all-solid-state secondary battery A according to this embodiment may be one in which metallic lithium is precipitated on the negative electrode during charging. In this case, lithium metal is precipitated and eluted during charging and discharging, resulting in a relatively large change in volume in the stacking direction. The elastic sheet B according to this embodiment is arranged as described above, and contracts in accordance with the expansion of the all-solid-state secondary battery A when the battery A is charged, and uniformly pushes the battery A back to its original volume by the restoring force (elasticity) of the elastic sheet B when the battery A is discharged, thereby maintaining a state in which the charge / discharge reaction occurs uniformly throughout each of the all-solid-state secondary batteries A in the casing C.

[0048] In order for this elastic sheet B to perform the function described above, it is preferable that the area of ​​each elastic sheet B is formed to be equal to or larger than the area of ​​each all solid state secondary battery A when viewed from the stacking direction of the all solid state secondary battery A (i.e., when viewed from above), and that the elastic sheet B is provided so as to cover the parts of the all solid state secondary battery A excluding the positive electrode current collecting part 111 and the negative electrode current collecting part 211.

[0049] Furthermore, it is preferable that the total thickness of each elastic sheet B is set to be greater than the total amount of change in thickness along the stacking direction of each all-solid-state secondary battery A due to charge and discharge. Although the elastic sheet B is a component included in the all-solid-state secondary battery module 100, it is a component that does not contribute to energy storage as a battery, and therefore it is preferable that the thickness of the elastic sheet B is small in order to maximize the energy density per volume of the all-solid-state secondary battery module 100. The thickness of the elastic sheet B is preferably 10 μm or more and 1000 μm or less, more preferably 50 μm or more and 800 μm or less, and even more preferably 100 μm or more and 600 μm or less. The thickness of this elastic sheet B can be measured, for example, using a constant pressure thickness gauge manufactured by Teclock Corporation, Model J Type PG-01 (measuring probe diameter 5 mm), by attaching a 5.6 g stainless steel weight to the top of the measuring probe at room temperature, while applying a pressure of approximately 15 kPa to elastic sheet B.

[0050] The hardness of elastic sheet B, as measured by durometer A, is preferably in the range of 5 to 70, more preferably 10 to 60, and even more preferably 20 to 50. The hardness of type C, as measured by durometer C, is preferably in the range of 10 to 80, more preferably 20 to 75, and even more preferably 30 to 70.

[0051] As described above, all-solid-state secondary battery A has the property of repeatedly expanding and contracting due to charging and discharging, and therefore elastic sheet B needs to repeatedly absorb the volume change caused by this expansion and contraction. From this perspective, it is preferable that the stress retention rate of elastic sheet B is 80% or more and 100% or less. As will be explained in the Examples section, the stress retention rate here is calculated from the rate of change in pressure between the first release and the release after a predetermined number of times when compression and release are repeatedly performed at a constant thickness ((pressure at release after a predetermined number of times / pressure at first release) × 100).

[0052] The all-solid-state secondary battery module 100 according to this embodiment may include a process of reducing the pressure inside the housing C during manufacture, such as by enclosing the all-solid-state secondary battery A in a vacuum state. Because the elastic sheet B is a closed-cell porous sheet, a pressure difference occurs between the inside of the cells and the outside of the elastic sheet B during pressure reduction, which may cause distortion (volume change) in the elastic sheet B, making it difficult to accurately stack the all-solid-state secondary battery A and the elastic sheet B. It is preferable that the distortion (volume change rate) of the elastic sheet B when an external pressure of 0.1 MPa is applied to the elastic sheet B is 10% or less, since this reduces the distortion (volume change rate) caused by the pressure difference between the inside of the cells and the outside of the elastic sheet B during pressure reduction. Note that the distortion in the elastic sheet B according to this embodiment is caused by the difference between the external pressure and the pressure inside the closed cells. Therefore, it is considered that the distortion (volume change due to contraction) when an external pressure of 0.1 MPa is applied and the distortion (volume change due to expansion) when a pressure reduction of 0.1 MPa is performed are the same.

[0053] It is preferable that the elastic sheet B is flame retardant from the viewpoint of sufficiently suppressing the occurrence of a fire in the event of an emergency in the all-solid-state secondary battery module 100. The flame retardancy can be evaluated by a vertical combustion test according to the UL94 standard, and is preferably rated as V-0.

[0054] It is preferable that elastic sheet B does not react with the components constituting all-solid-state secondary battery A so that its properties do not change during operation of all-solid-state secondary battery A. The all-solid-state secondary battery A according to this embodiment may be one in which metallic lithium is deposited on the negative electrode during charging. In this case, there is a possibility of contact with metallic lithium, so it is preferable that the reactivity upon contact with metallic lithium is low. Regarding reactivity upon contact with metallic lithium, it is preferable that there is no change in the infrared absorption spectrum of the surface of elastic sheet B before and after contact (e.g., the appearance of a new peak of a size that can be visually confirmed) when metallic lithium is contacted with elastic sheet B for four days. Furthermore, since sulfide-based solid electrolytes are generally highly reactive materials, it is preferable that the reactivity with sulfide-based solid electrolytes is low. The reactivity with sulfide-based solid electrolytes can be determined by contacting Li6PS5Cl for 30 days and measuring the mass change rate between before and after contact. This mass change rate is preferably zero or greater than zero and less than 1%.

[0055] The elastic sheet B is preferably electrically insulating in order to reduce the risk of an internal short circuit due to electrical conduction between the positive electrode layer 10 and the negative electrode layer 20 via the elastic sheet B.

[0056] Specifically, the elastic sheet B according to this embodiment is made of a closed-cell foam formed from a fluorine-based elastomer. In a closed-cell foam, the cells are not connected, which allows for small cell size and cell size distribution, making the microscopic physical properties of the elastic sheet B uniform and allowing for the stress retention rate to fall within a favorable range in order to achieve the object of this embodiment. The porosity of the elastic sheet B is preferably 40% or more and 70% or less, more preferably 41% or more and 69% or less, and even more preferably 42% or more and 68% or less.

[0057] A fluorine-based elastomer is an elastomer containing fluorine as a constituent element, and the fluorine content, when the entire elastomer is taken as 100% by mass, is preferably 60% by mass to 80% by mass, and more preferably 65% ​​by mass to 75% by mass. Specific examples of such fluorine elastomers include fluoroelastomers (FKM) and perfluoroelastomers (FFKM). The fluorine content, when the entire elastomer is taken as 100% by mass, is a theoretical value calculated from the molecular structural formula of the elastomer used, but it can also be measured by analytical methods such as combustion ion chromatography.

[0058] Examples of such elastic sheet B include those obtained by heating a composition in which a fluorine-based elastomer is mixed with a foaming agent and a crosslinking agent to crosslink and foam. Examples of the fluorine-based elastomer include the Dai-el series manufactured by Daikin Industries, Ltd. Examples of the foaming agent include the Uniform series azodicarbonamide foaming agent manufactured by Otsuka Chemical Co., Ltd. Examples of the crosslinking agent include the Perhexa series organic peroxide manufactured by NOF Corporation.

[0059] <2. Effects of this embodiment> According to the elastic sheet B of this embodiment, the evaluation in the vertical combustion test of the UL94 standard is V-0, and therefore, when used in the all-solid-state secondary battery module 100, it can exhibit sufficient flame retardancy to ensure safety. Moreover, since the stress retention rate is 80% or more, the volume change due to the expansion and contraction of the all-solid-state secondary battery A can be repeatedly absorbed. Furthermore, according to the elastic sheet B of this embodiment, the strain (amount of change in the thickness direction) when an external pressure of 0.1 MPa is applied is 10% or less, so even when the pressure is reduced during the production of the all-solid-state secondary battery module 100, the strain is sufficiently small, and it is possible to precisely stack the all-solid-state secondary batteries A and the elastic sheet B. As a result, it is possible to buffer the volume change of the all-solid-state secondary batteries A in the all-solid-state secondary battery module 100 as uniformly as possible. Note that being able to precisely stack means that it is possible to prevent the elastic sheet B from shifting in a direction perpendicular to the stacking direction (plane direction) due to the occurrence of strain in the elastic sheet B, which has been dried under reduced pressure in advance, during the step of stacking the all-solid-state secondary batteries A and the elastic sheet B, resulting in a partial reduction in its thickness, or the occurrence of strain in the elastic sheet B during the reduction in pressure during the sealing inside an exterior body, resulting in a partial increase in its thickness. Since the properties are unlikely to change due to reaction with metallic lithium or the solid electrolyte, the function of elastic sheet B can be maintained for as long as possible, and the cycle characteristics of the all-solid-state secondary battery module 100 can be maintained for as long as possible. Since the elastic sheet B according to this embodiment is made of a fluorine-based elastomer, it can exhibit sufficiently high flame retardancy without the need for additional flame retardant fillers, etc. As a result, the addition of a large amount of fillers does not affect the stress retention rate, etc., and therefore the desired elasticity (hardness and stress retention rate) as described above can be exhibited even when the thickness of the elastic sheet B is reduced.

[0060] 3. Other embodiments of the present invention The all-solid-state secondary battery according to the present invention is not limited to the one described above. For example, in the above-described embodiment, elastic sheets are provided between all adjacent all-solid-state secondary batteries and between the all-solid-state secondary batteries at both ends along the stacking direction and the casing, but this is not limiting. Elastic sheets may be provided so as to sandwich an all-solid-state secondary battery group consisting of a plurality of all-solid-state secondary batteries from above and below along the stacking direction. In this case, insulating layers such as insulating films may be provided in the areas between the all-solid-state secondary batteries where no elastic sheets are provided.

[0061] In the all-solid-state secondary battery module of the above embodiment, the all-solid-state secondary battery and the elastic sheet are housed such that the facing surface of the casing having the smallest area is the bottom surface, and the height direction of the all-solid-state secondary battery and the elastic sheet coincide with the stacking direction. However, this is not limited to this. In other embodiments, the all-solid-state secondary battery and the elastic sheet may be housed such that the facing surface of the casing other than the facing surface having the smallest area is the bottom surface, and the height direction of the all-solid-state secondary battery and the elastic sheet coincide with the stacking direction.

[0062] For example, as shown in FIG. 4, an insulating member may be further provided around the positive electrode layer or the negative electrode layer of the all-solid-state secondary battery, covering the positive electrode layer or the negative electrode layer in a direction perpendicular to the stacking direction (circumferential direction) to suppress short-circuiting between the positive electrode layer and the negative electrode layer.

[0063] In the above-described embodiment, an all-solid-state secondary battery having a sandwich structure in which single cells are stacked on both sides of a positive electrode current collector at the center has been described as the all-solid-state secondary battery. However, the present invention is not limited to this, and an all-solid-state secondary battery in which a solid electrolyte and a negative electrode layer are stacked only on one side of a positive electrode current collector may also be used.

[0064] In the above-described embodiment, an all-solid-state secondary battery was described in which a metallic lithium layer was deposited on the negative electrode layer, but a metallic lithium layer does not have to be deposited on the negative electrode layer. For example, if a material capable of reversibly absorbing and releasing more lithium ions, such as silicon, is used, the volume of the negative electrode active material will change several times. In this embodiment, the total thickness of the positive electrode layer, solid electrolyte layer, and negative electrode layer of each battery unit is configured to be larger in a fully charged state than in a discharged state. Furthermore, the elastic sheet according to the present invention is not limited to all-solid-state secondary batteries including all-solid-state lithium ion secondary batteries, but can be widely applied to all-solid-state secondary batteries that undergo volume changes during charging and discharging, for example. [Example]

[0065] The present invention will be explained in more detail below with reference to specific examples, but it goes without saying that the present invention is not limited to these. In the present examples, the porous sheets formed from various resins as shown in Table 1 were evaluated by the following tests. The porous sheets used in Examples 1 and 2 and Comparative Examples 1 to 3 were of the closed-cell type, while the porous sheet used in Comparative Example 4 was not a resin made porous by foaming, but a porous stretched sheet that did not contain closed cells and was made porous by stretching.

[0066] <Porosity evaluation> The porosity of the porous sheet was calculated as follows. First, the mass per volume of the porous sheet is measured and the apparent density is calculated. The porosity was calculated from this apparent density and the true density of the resin that constitutes the porous sheet. The results are shown in Table 1.

[0067] <Flame retardancy evaluation> The flame retardancy of the porous sheets was evaluated according to the UL94 standard UL94 50W (20 mm) vertical flame test protocol: V-0, V-1, V-2. The results are shown in Table 1. The thickness of each porous sheet used in the test was as shown in Table 1, and each porous sheet was tested five or more times.

[0068] <Strain evaluation> Each porous sheet listed in Table 1 was cut to the same thickness and volume, and the amount of deformation in the thickness direction when a pressure of 0.1 MPa was applied to this porous sheet was measured. The value calculated as a percentage of the thickness before pressure application was taken as the strain value. The results are shown in Table 1.

[0069] <Evaluation of stress maintenance rate> Each porous sheet (0.5 mm thick) listed in Table 1 was subjected to a pressure of 2 MPa, with the initial state being the state in which the porous sheet was compressed by 50 μm from the initial state, and then the pressure was reduced until the porous sheet expanded by 50 μm from the initial state. This cycle was repeated 10 times. The pressure at the 10th expansion divided by the pressure at the first expansion multiplied by 100 was used as the stress retention rate for each porous sheet. The results are shown in Table 1.

[0070] <Evaluation of reaction with solid electrolyte> A sheet (solid electrolyte sheet) made of Li6PS5Cl, a sulfide-based solid electrolyte, cut to a size larger than the porous sheet was placed on a SUS foil, and the porous sheet shown in Table 1 was laminated on the solid electrolyte sheet so that the entire surface of the sheet was in direct contact with the sheet. This laminate was vacuum-packed and left at 60°C for 30 days. After 30 days, the vacuum pack was opened, and the surface of the porous sheet was washed with ethanol to remove the solid electrolyte attached to the surface of the porous sheet. After the porous sheet was thoroughly dried, the change in mass of the porous sheet before and after contact with the solid electrolyte was measured, and the rate of change (%) of the mass of the porous sheet before and after the test was calculated. As a result of the test, the rate of change in mass of the porous sheet was less than 1% for all of the porous sheets of Example 1, Comparative Example 2, and Comparative Example 4. It can be assumed that the same results would be obtained for Comparative Examples 1, 3, and Example 2 because the porous sheets had similar compositions.

[0071] <Reaction evaluation with metallic lithium> A piece of lithium metal foil cut to a size larger than the porous sheet was placed on a SUS foil, and a porous sheet shown in Table 1 was laminated on the lithium metal foil so that the entire surface of the foil was in direct contact with the lithium metal foil. The SUS foil was then laminated on top of the porous sheet. The laminate was vacuum-packed and allowed to stand at 45°C for 4 days. After 4 days, the vacuum pack was opened, and the visual change in appearance and infrared absorption spectrum of the porous sheet surface that had been in contact with the lithium metal foil were compared with those of the porous sheet that had not been in contact with the lithium metal foil. The infrared absorption spectrum was measured using a Thero Scientific FT-IR Nicolet iS5 (with iD7 ATR accessory). The measurements were performed at room temperature, a dew point of -40°C, and a wavenumber range of 4000 to 650 cm. -1 The ATR method was used with 16 accumulations in the range of 16. After correction was performed using the spectrum measured without a sample as the background, the infrared absorption spectrum of the porous sheet that had been in contact with the metallic lithium foil was compared with the infrared absorption spectrum of the porous sheet that had not been in contact with the metallic lithium foil to evaluate whether there were any changes in the peak heights or positions. As a result of the test, the porous sheets of Comparative Examples 2 and 3 showed no change in appearance, and no change in the infrared absorption spectrum was observed. Since the porous sheets of Examples 1 and 2 and Comparative Example 1 have similar compositions to those of Comparative Examples 2 and 3, it can be assumed that the same results would be obtained. On the other hand, in Comparative Example 4, a clear change in appearance was observed, and a change also appeared in the infrared absorption spectrum.

[0072] [Table 1]

[0073] The results in Table 1 show that the closed-cell porous sheets made of the elastomers shown in Examples 1 and 2 can realize elastic sheets that combine sufficient flame retardancy and elasticity (appropriate hardness and stress retention rate). [Explanation of symbols]

[0074] 100 All-solid-state secondary battery module A All-solid-state secondary battery 10 Positive electrode layer 11 Positive electrode current collector 12 Cathode active material layer 20 negative electrode layer 21 Negative electrode current collector 22 Negative electrode active material layer 30 Solid electrolyte layer B Elastic sheet C chassis

Claims

1. An elastic sheet used together with an all-solid-state secondary battery to buffer volume changes due to expansion and contraction of the all-solid-state secondary battery, It is made of a closed-cell foam formed from a fluorine-based elastomer, The foam has a porosity of 40% or more and 70% or less.

2. 2. The elastic sheet for an all-solid-state secondary battery according to claim 1, wherein the fluorine element content is 60% by mass or more and 80% by mass or less, when the entire fluorine-based elastomer is taken as 100% by mass.

3. 2. The elastic sheet for an all-solid-state secondary battery according to claim 1, wherein the strain when an external pressure of 0.1 MPa is applied is 10% or less.

4. The elastic sheet for an all-solid-state secondary battery according to claim 1 , wherein the stress retention rate is 80% or more.

5. The elastic sheet for an all-solid-state secondary battery according to claim 1, having a thickness of 10 μm or more and 1000 μm or less.

6. The elastic sheet for an all-solid-state secondary battery according to claim 1, wherein the flame retardancy according to the UL94 standard is evaluated as V-0.

7. 2. The elastic sheet for an all-solid-state secondary battery according to claim 1, which shows no change in infrared absorption spectrum before and after contact with metallic lithium for four days.

8. 2. The elastic sheet for an all-solid-state secondary battery according to claim 1, wherein the mass change rate before and after contact with a sulfide-based solid electrolyte for 30 days is less than 1%.

9. The elastic sheet for an all-solid-state secondary battery according to claim 1 , which is electrically insulating.

10. An all-solid-state secondary battery module comprising: an all-solid-state secondary battery; the elastic sheet for an all-solid-state secondary battery according to any one of claims 1 to 9; and a housing that houses these therein.

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