Heat transfer suppression sheet, manufacturing method thereof, and battery pack

Abstract

To provide a heat transfer suppression sheet that not only has excellent heat insulating properties but also has excellent flexibility and excellent bonding strength when laminated with other members, a manufacturing method thereof, and a battery pack having the heat transfer suppression sheet.SOLUTION: A heat transfer suppression sheet 10 includes inorganic particles and organic fibers, and at least one of its front surface 1a and back surface has a polished or machined surface 10a. The method for manufacturing the heat transfer suppression sheet includes a precursor-preparing step of preparing a heat-transfer-suppressing sheet precursor containing inorganic particles and organic fibers, and a surface-processing step of polishing or machining at least one of its front surface 1a and back surface of the heat-transfer-suppressing sheet precursor prepared in the precursor-preparing step. A battery pack includes a plurality of battery cells and the heat transfer suppression sheet 10, with the plurality of battery cells connected in series or parallel.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a heat-transfer-suppressing sheet, a method for producing the same, and a battery pack including the heat-transfer-suppressing sheet. [Background technology]

[0002] In recent years, from the viewpoint of environmental protection, active development has been made of electric vehicles, hybrid vehicles, and the like that are driven by electric motors. These electric vehicles, hybrid vehicles, and the like are equipped with assembled batteries in which multiple battery cells are connected in series or parallel to serve as the power source for the driving electric motor.

[0003] Furthermore, these battery cells are mainly lithium-ion secondary batteries, which have higher capacity and higher output than lead-acid batteries, nickel-metal hydride batteries, etc. If a battery cell experiences thermal runaway, where the temperature rises suddenly and continues to rise due to an internal short circuit or overcharging, the heat from the battery cell experiencing thermal runaway may be transmitted to other adjacent battery cells, potentially causing thermal runaway in those cells.

[0004] A common method for suppressing the propagation of heat from a battery cell that has experienced thermal runaway as described above is to place a heat insulating sheet between the battery cells. For example, Patent Document 1 discloses a heat insulating sheet for a battery pack that includes first particles made of silica nanoparticles and second particles made of a metal oxide. Patent Document 1 also describes that the heat insulating sheet may include a binding material made of at least one material selected from fibers, binders, and heat-resistant resins. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-34278 Summary of the Invention [Problem to be solved by the invention]

[0006] For example, in a battery pack 100 as shown in FIG. 5, a heat insulating sheet is used as a spacer (indicated by the reference numeral 10 in the figure) that is inserted between the battery cells 20a to 20c. However, since the battery cells 20a to 20c expand and contract even during normal charging and discharging, the heat insulating sheet needs to be flexible to accommodate this expansion and contraction, and there is a strong demand for improvements in this area.

[0007] The heat insulating sheet may also be used as an inner wall material of the battery case (reference numeral 30 in the drawing) of the battery pack 100, in which case the heat insulating sheet is adhered to the battery case 30. Therefore, there is a strong demand for heat insulating sheets to have high bonding strength with other components such as battery cells and battery cases.

[0008] The present invention has been made in view of these problems, and has an object to provide a heat-transfer-suppressing sheet that not only has excellent heat insulating performance but also has excellent flexibility and excellent bonding strength when laminated with other components such as battery cells and battery cases, a method for manufacturing the same, and a battery pack including the heat-transfer-suppressing sheet. [Means for solving the problem]

[0009] The above object of the present invention is achieved by the heat transfer-suppressing sheet having the following configuration [1].

[0010] [1] A composition comprising inorganic particles and organic fibers, A heat transfer-suppressing sheet, wherein at least one of the front and back surfaces includes a polished or machined surface.

[0011] Furthermore, preferred embodiments of the present invention relating to the heat transfer-suppressing sheet relate to the following [2] and [3]. [2] The heat transfer-suppressing sheet according to [1], wherein organic fibers are exposed and fluffed on the polished or machined surface. [3] The heat-transfer-suppressing sheet according to [1] or [2], wherein another member is laminated on the surface including the polished surface or the machined surface.

[0012] The above object of the present invention is achieved by the following configuration [4] relating to a method for producing a heat transfer-suppressing sheet.

[0013] [4] a precursor preparation step of preparing a heat transfer-suppressing sheet precursor having inorganic particles and organic fibers; a surface processing step of polishing or cutting at least one of the front and back surfaces of the heat-transfer-suppressing sheet precursor prepared in the precursor preparation step.

[0014] A preferred embodiment of the present invention relating to a method for producing a heat transfer-suppressing sheet relates to the following [5].

[0015] [5] The method for producing a heat-transfer-suppressing sheet according to [4], wherein the precursor-producing step produces the heat-transfer-suppressing sheet precursor by a dry method.

[0016] The above object of the present invention is achieved by the following configuration [6] relating to the battery pack.

[0017] [6] A battery pack comprising a plurality of battery cells and the heat transfer-suppressing sheet according to any one of [1] to [3], wherein the plurality of battery cells are connected in series or in parallel. [Effects of the Invention]

[0018] The heat-transfer-suppressing sheet of the present invention contains inorganic particles and organic fibers, and the inorganic particles, which have excellent heat insulating properties, are well held by the organic fibers. At least one of the front and back surfaces includes a polished or machined surface, which enhances flexibility. Furthermore, when other components, such as battery cells or battery cases, are stacked on the polished or machined surface, the surface anchor effect effectively prevents misalignment between the heat-transfer-suppressing sheet and the other components.

[0019] Furthermore, in a preferred form, the polished or cut surface has exposed organic fibers in a fluffy state, which further increases the bonding strength when other components such as a battery cell or a battery case are laminated thereon.

[0020] The battery pack of the present invention has a heat-transfer-suppressing sheet with high thermal insulation performance and excellent flexibility as described above, and therefore can suppress thermal runaway of the battery cells in the battery pack and the spread of flames to the outside of the battery case. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a schematic diagram illustrating a heat-transfer-suppressing sheet according to an embodiment of the present invention and a manufacturing process thereof, in which (A) is a side view of a heat-transfer-suppressing sheet precursor, (B) is a diagram illustrating a surface processing step in which the surface of the heat-transfer-suppressing sheet precursor is cut, and (C) is a side view of the resulting heat-transfer-suppressing sheet. [Figure 2] FIG. 2 is a photograph substituting for a drawing showing an enlarged portion of the heat-transfer-suppressing sheet precursor, where (A) is a surface view and (B) is a cross-sectional view. [Figure 3] FIG. 3 is a photographic representation of an enlarged portion of a heat-transfer-suppressing sheet having ground surfaces on both sides, where (A) is a surface view and (B) is a cross-sectional view. [Figure 4] FIG. 4 is a graph showing the results of evaluating the flexibility of the heat-transfer-suppressing sheet precursor and the heat-transfer-suppressing sheet. [Figure 5] FIG. 5 is a schematic diagram showing a battery pack having a heat transfer-suppressing sheet according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present inventors have conducted extensive research into a heat-transfer-suppressing sheet that can solve the above-mentioned problems, and have found that it is effective to polish or machine at least one of the front and back surfaces of a heat-transfer-suppressing sheet precursor, thereby exposing and fluffing the organic fibers that are a compounding material.

[0023] The present invention is based on this finding and will be described in detail below. Note that the present invention is not limited to the embodiments described below, and can be implemented with any modifications within the scope of the gist of the present invention.

[0024] [Heat transfer suppression sheet and method for producing the same] FIG. 1 is a schematic diagram illustrating a heat-transfer-suppressing sheet according to an embodiment of the present invention and a manufacturing process thereof. (A) is a side view of a heat-transfer-suppressing sheet precursor 1, (B) is a surface processing process for the heat-transfer-suppressing sheet precursor 1, and (C) is a side view of the resulting heat-transfer-suppressing sheet (invention product) 10.

[0025] 1(A), a heat transfer-suppressing sheet precursor 1 containing inorganic particles and organic fibers is prepared. This step is the "precursor preparation step."

[0026] An example of a method for producing the heat transfer-suppressing sheet precursor 1 will now be described. For example, binder fibers having a core-sheath structure and inorganic particles are put into a mixer such as a V-type mixer at a predetermined ratio to prepare a mixture. As described below, it is preferable to use a core-sheath fiber as the binder fiber, which has a core made of a first organic material and a sheath made of a second organic material, where the melting point of the first organic material is higher than that of the second organic material. The resulting mixture is then placed in a mold and pressurized with a press or the like. The resulting molded body is heated to melt the sheath of the binder fiber. The heated molded body is then cooled, and the second organic material constituting the molten sheath and the inorganic particles present around the binder fiber are fused to the core (organic fiber) and are also fused to each other in the areas where the binder fibers were in contact with each other. A matrix containing inorganic particles is formed between the multiple organic fibers. This allows the heat-transfer-suppressing sheet precursor 1 to be processed into a sheet shape.

[0027] 1(B), at least one of the front and back surfaces (front surface 1a in this case) of the heat transfer-suppressing sheet precursor 1 is polished or cut. This step is the "surface processing step."

[0028] In the surface processing step, cutting is performed using a cutting tool 5 such as a knife in Fig. 1(B), but polishing can also be performed using a grindstone, brush, etc. There are no restrictions on the cutting and polishing allowances indicated by the symbol t in the figure, as long as the organic fibers, which are the compounding materials, are exposed and in a fluffy state.

[0029] As a result, as shown in FIG. 1(C), heat-transfer-suppressing sheet 10 of the present invention is obtained, one surface of which is a machined surface 10a.

[0030] Fig. 2 is a photograph substituting a drawing showing an enlarged portion of the heat-transfer-suppressing sheet precursor 1, where (A) is a surface view and (B) is a cross-sectional view. Fig. 3 is a photograph substituting a drawing showing an enlarged portion of the heat-transfer-suppressing sheet 10, both of whose front and back surfaces are machined surfaces 10a, where (A) is a surface view and (B) is a cross-sectional view.

[0031] As shown in FIG. 2(B), the surface of the heat-transfer-suppressing sheet precursor 1 contains many looped organic fibers 11 and organic fibers 11 that lie horizontally. In contrast, as shown in FIG. 3(B), the surface of the heat-transfer-suppressing sheet 10 has no looped or horizontally laid organic fibers 11, but many upright organic fibers 11, and the entire surface is fluffy. Therefore, when another member is laminated on the machined surface 10a, the exposed organic fibers 11 have an anchoring effect, which increases the bonding strength to the other member. In FIGS. 2 and 3, the white lines exposed from the surface represent the "organic fibers 11."

[0032] 5, for example, if the heat-transfer-suppressing sheet 10 is used as a spacer (indicated by the reference numeral 10 in the figure) inserted between the battery cells 20a to 20c, the battery cells 20a to 20c would be the other component. If the heat-transfer-suppressing sheet 10 is used as an inner wall material of the battery pack 100, the battery case 30 would be the other component.

[0033] 3(B), the precursor production step is preferably carried out by a dry method in order to expose and fluff the organic fibers 11 on the machined surface 10a of the heat-transfer-suppressing sheet 10. The heat-transfer-suppressing sheet precursor 1 may be produced by either a wet method or a dry method, but the dry method is preferred.

[0034] In the wet method, the organic fibers 11 tend to be oriented along the surface 1a of the heat-transfer-suppressing sheet precursor 1, and the cutting edge of the cutting tool 5 slides along the surface of the organic fibers 11 during cutting, making it difficult to cut the organic fibers 11. In contrast, in the dry method, the heat-transfer-suppressing sheet precursor 1 contains many organic fibers 11 that are distributed in directions oblique or perpendicular to the surface 1a, and therefore the cutting edge of the cutting tool 5 frequently hits the side surfaces (long portions) of the organic fibers 11 during cutting, making it easy for the organic fibers 11 to become exposed.

[0035] There are no restrictions on the materials that can be used to form heat transfer-suppressing sheet 10, as long as it contains organic fibers and inorganic particles. However, the following are examples of preferred materials that provide excellent heat insulating properties.

[0036] <Organic fiber> The organic fibers impart flexibility to the heat transfer-suppressing sheet 10 and improve the strength and shape of the sheet. The organic fibers can be at least one selected from polyethylene, polypropylene, polystyrene, vinyl chloride, nylon, acrylic, epoxy resin, polyurethane, polyether ether ketone, polyetherimide, polyethylene terephthalate, polytetrafluoroethylene (PTFE), polyphenyl sulfide, polycarbonate, and aramid. While single-component organic fibers can be used, it is preferable to use binder fibers with a sheath-core structure. The binder fibers with a sheath-core structure have a core extending in the longitudinal direction of the fiber and a sheath formed to cover the outer periphery of the core. The core is made of a first organic material, and the sheath is made of a second organic material, with the melting point of the first organic material being higher than that of the second organic material.

[0037] Whether single-component organic fibers or core-sheath binder fibers are used, when the heat transfer-suppressing sheet precursor 1 is produced, heating during press molding melts part of the fiber surface, and subsequent cooling forms welded parts (not shown) around the organic fibers. The welded parts fuse the inorganic particles to the surfaces of the organic fibers and also fuse the organic fibers together, so the formation of the welded parts allows for excellent sheet strength to be obtained.

[0038] When sheath-core binder fibers are used as the material, the core corresponds to the organic fiber. The use of sheath-core binder fibers allows adjacent binder fibers to fuse together, making it easier to form fiber bundles and further increasing sheet strength. Furthermore, when sheath-core binder fibers are used, the second organic material that makes up the sheath melts and then solidifies again, incorporating the surrounding inorganic particles, improving the retention of the inorganic particles.

[0039] (First organic material) The first organic material constituting the core is not particularly limited as long as it has a melting point higher than that of the sheath, i.e., the second organic material, present on the outer peripheral surface of the organic fiber 11. The first organic material may be at least one selected from polyethylene terephthalate, polypropylene, and nylon.

[0040] (Second organic material) The second organic material constituting the sheath is not particularly limited as long as it has a melting point lower than that of the first organic material. Examples of the second organic material include at least one selected from polyethylene terephthalate, polyethylene, polypropylene, and nylon. The melting point of the second organic material is preferably 90°C or higher, more preferably 100°C or higher. The melting point of the second organic material is preferably 150°C or lower, more preferably 130°C or lower.

[0041] (organic fiber content) Appropriately controlling the content of organic fibers in heat-transfer-suppressing sheet 10 can adequately improve the strength of heat-transfer-suppressing sheet 10. The content of organic fibers 11 is preferably 2 mass % or more, more preferably 4 mass % or more, and even more preferably 5 mass % or more, relative to the total mass of heat-transfer-suppressing sheet 10. However, if the content of organic fibers is too high, the content of inorganic particles will relatively decrease. Therefore, in order to obtain the desired heat insulation performance, the content of organic fibers is preferably 10 mass % or less, more preferably 8 mass % or less, and even more preferably 6 mass % or less, relative to the total mass of heat-transfer-suppressing sheet 10.

[0042] (fiber length of organic fiber) Although there are no particular limitations on the fiber length of the organic fibers, from the viewpoint of ensuring moldability and processability, the average fiber length of the organic fibers is preferably 10 mm or less. Furthermore, from the viewpoint of improving the strength of the heat-transfer-suppressing sheet 10, the average fiber length of the organic fibers is preferably 0.5 mm or more.

[0043] <Inorganic particles> The inorganic particles may be a single inorganic particle or a combination of two or more inorganic particles. From the viewpoint of the heat transfer suppression effect, it is preferable to use particles made of at least one inorganic material selected from oxide particles, carbide particles, nitride particles, and inorganic hydrate particles, and it is more preferable to use oxide particles. The shape of the inorganic particles is not particularly limited, but it is preferable to use at least one selected from nanoparticles, hollow particles, and porous particles. Specific examples include silica nanoparticles, metal oxide particles, inorganic balloons such as microporous particles and hollow silica particles, particles made of thermally expandable inorganic materials, and particles made of hydrous porous bodies.

[0044] When the average secondary particle diameter of the inorganic particles is 0.01 μm or more, they are easily available and the increase in production costs can be suppressed. Furthermore, when the average secondary particle diameter is 200 μm or less, the desired heat insulating effect can be obtained. Therefore, the average secondary particle diameter of the inorganic particles is preferably 0.01 μm or more and 200 μm or less, and more preferably 0.05 μm or more and 100 μm or less.

[0045] In addition, by using two or more inorganic particles with different heat transfer suppression effects in combination, it is possible to cool a heat generating body in multiple stages, and the heat absorption effect can be exerted over a wider temperature range. Specifically, it is preferable to use a mixture of large-diameter particles and small-diameter particles. For example, when nanoparticles are used as one of the inorganic particles, it is preferable to include inorganic particles made of a metal oxide as the other inorganic particle. Hereinafter, the inorganic particles will be described in more detail, with the small-diameter inorganic particles referred to as the first inorganic particles and the large-diameter inorganic particles referred to as the second inorganic particles.

[0046] <First inorganic particle> (oxide particles) Oxide particles have a high refractive index and a strong effect of diffusely reflecting light. Therefore, using oxide particles as the first inorganic particles can suppress radiant heat transfer, particularly in high-temperature regions such as those caused by abnormal heat generation. The oxide particles can be at least one type of particle selected from silica, titania, zirconia, zircon, barium titanate, zinc oxide, and alumina. That is, among the above oxide particles that can be used as inorganic particles, only one type or two or more types of oxide particles can be used. In particular, silica is a component with high heat insulating properties, and titania is a component with a higher refractive index than other metal oxides, and is highly effective in diffusely reflecting light and blocking radiant heat in high-temperature regions of 500°C or higher. Therefore, it is most preferable to use silica and titania as the oxide particles.

[0047] (Average primary particle size of oxide particles: 0.001 μm to 50 μm) The particle size of the oxide particles can affect the effect of reflecting radiant heat, so if the average primary particle size is limited to a predetermined range, even higher heat insulating properties can be obtained. That is, when the average primary particle size of the oxide particles is 0.001 μm or more, the average primary particle size is sufficiently larger than the wavelength of light that contributes to heating, and light is efficiently diffused, thereby suppressing the radiation of heat transfer within heat-transfer-suppressing sheet 10 in the high-temperature range of 500°C or higher, thereby further improving the heat insulation properties. On the other hand, if the average primary particle size of the oxide particles is 50 μm or less, the number and number of contact points between the particles do not increase even when compressed, making it difficult to form paths for conductive heat transfer. This reduces the impact on thermal insulation, particularly in the normal temperature range where conductive heat transfer is dominant.

[0048] In the present invention, the average primary particle size can be determined by observing particles under a microscope, comparing with a standard scale, and taking the average of any 10 particles.

[0049] (nanoparticles) Nanoparticles refer to particles on the order of nanometers that are spherical or nearly spherical and have an average primary particle diameter of less than 1 μm. Nanoparticles have low density, which suppresses conductive heat transfer. Furthermore, when nanoparticles are used as the first inorganic particles, the finely dispersed voids provide excellent heat insulation, suppressing convective heat transfer. Therefore, the use of nanoparticles is preferable because they can suppress heat transfer between adjacent nanoparticles during normal use at room temperature. Furthermore, if nanoparticles with a small average primary particle size are used as the oxide particles, even if the heat-transfer-suppressing sheet is compressed due to expansion caused by thermal runaway in the battery cell, increasing the internal density, it is possible to suppress an increase in conductive heat transfer in the heat-transfer-suppressing sheet 10. This is thought to be because nanoparticles are prone to forming fine voids between particles due to electrostatic repulsion, and because their bulk density is low, the particles are packed together in a way that provides cushioning.

[0050] When nanoparticles are used as the first inorganic particles, there are no particular limitations on the material as long as they comply with the definition of nanoparticles. For example, silica nanoparticles are a material with high heat insulating properties, and the contact points between particles are small, so the amount of heat conducted by silica nanoparticles is smaller than when silica particles with a large particle diameter are used. Furthermore, commonly available silica nanoparticles have a bulk density of 0.1 (g / cm 3 ), for example, even if battery cells arranged on both sides of the heat-transfer-suppressing sheet 10 thermally expand and apply a large compressive stress to the heat-transfer-suppressing sheet 10, the size (area) and number of contact points between the silica nanoparticles do not increase significantly, and heat insulation properties can be maintained. Therefore, it is preferable to use silica nanoparticles as the nanoparticles. Examples of silica nanoparticles include wet silica, dry silica, and aerogel, but particularly suitable silica nanoparticles will be described below.

[0051] Generally, wet silica particles are agglomerated, while dry silica particles can be dispersed. In the temperature range below 300°C, heat conduction is dominated by conduction, so dry silica, which can disperse particles and has a low thermal conductivity, can achieve better insulation performance than wet silica.

[0052] (Average primary particle diameter of nanoparticles: 1 nm to 100 nm) If the average primary particle size of the nanoparticles is limited to a predetermined range, even higher heat insulating properties can be obtained. That is, when the average primary particle diameter of the nanoparticles is 1 nm or more and 100 nm or less, convective heat transfer and conductive heat transfer within heat-transfer-suppressing sheet 10 can be suppressed, particularly in the temperature range below 500°C, and the thermal insulation properties can be further improved. Furthermore, even when compressive stress is applied, the voids remaining between the nanoparticles and the contact points between many particles suppress conductive heat transfer, allowing the thermal insulation properties of heat-transfer-suppressing sheet 10 to be maintained. The average primary particle size of the nanoparticles is more preferably 2 nm or more, and even more preferably 3 nm or more, while the average primary particle size of the nanoparticles is more preferably 50 nm or less, and even more preferably 10 nm or less.

[0053] (Inorganic hydrate particles) When inorganic hydrate particles receive heat from a heating element and reach a temperature above their thermal decomposition initiation temperature, they undergo thermal decomposition and release their own water of crystallization, lowering the temperature of the heating element and its surroundings, thereby exhibiting a so-called "endothermic effect." After releasing the water of crystallization, the particles become porous, and the numerous air holes provide thermal insulation. Specific examples of inorganic hydrates include aluminum hydroxide (Al(OH)3), magnesium hydroxide (Mg(OH)2), calcium hydroxide (Ca(OH)2), zinc hydroxide (Zn(OH)2), iron hydroxide (Fe(OH)2), manganese hydroxide (Mn(OH)2), zirconium hydroxide (Zr(OH)2), and gallium hydroxide (Ga(OH)3).

[0054] For example, aluminum hydroxide has about 35% water of crystallization, and as shown in the following formula, it thermally decomposes, releasing the water of crystallization and exhibiting an endothermic effect. After releasing the water of crystallization, it becomes a porous alumina (Al2O3) and functions as a heat insulating material. 2Al(OH)3 → Al2O3 + 3H2O

[0055] In a battery pack 100 shown in Fig. 9, for example, heat transfer-suppressing sheet 10 is inserted between battery cells 20a to 20c, and in a battery cell that experiences thermal runaway, the temperature rises sharply to over 200°C and continues to rise to around 700°C. Therefore, it is preferable that the inorganic particles be made of an inorganic hydrate whose thermal decomposition temperature is 200°C or higher. The thermal decomposition starting temperatures of the inorganic hydrates listed above are approximately 200°C for aluminum hydroxide, approximately 330°C for magnesium hydroxide, approximately 580°C for calcium hydroxide, approximately 200°C for zinc hydroxide, approximately 350°C for iron hydroxide, approximately 300°C for manganese hydroxide, approximately 300°C for zirconium hydroxide, and approximately 300°C for gallium hydroxide.All of these temperatures roughly overlap with the temperature range in which a battery cell experiencing thermal runaway experiences a sudden rise in temperature, and can efficiently suppress temperature rise, making these inorganic hydrates preferable.

[0056] (Average secondary particle diameter of inorganic hydrate particles: 0.01 μm or more and 200 μm or less) Furthermore, when inorganic hydrate particles are used as the first inorganic particles, if their average particle size is too large, it takes some time for the first inorganic particles (inorganic hydrate) near the center of the heat transfer-suppressing sheet 10 to reach their thermal decomposition temperature, and the first inorganic particles near the center of the sheet may not be completely thermally decomposed. For this reason, the average secondary particle size of the inorganic hydrate particles is preferably 0.01 μm or more and 200 μm or less, and more preferably 0.05 μm or more and 100 μm or less.

[0057] (Particles made of thermally expandable inorganic material) Examples of thermally expandable inorganic materials include vermiculite, bentonite, mica, and perlite.

[0058] (Particles made of hydrous porous material) Specific examples of the hydrous porous material include zeolite, kaolinite, montmorillonite, acid clay, diatomaceous earth, wet silica, dry silica, aerogel, mica, and vermiculite.

[0059] (inorganic balloons) The heat transfer-suppressing sheet 10 may contain inorganic balloons as the first inorganic particles. When inorganic balloons are included, convective or conductive heat transfer within heat-transfer-suppressing sheet 10 can be suppressed in the temperature range below 500°C, and the heat insulating properties of heat-transfer-suppressing sheet 10 can be further improved. As the inorganic balloons, at least one selected from shirasu balloons, silica balloons, fly ash balloons, barite balloons, and glass balloons can be used.

[0060] (Inorganic balloon content: 60% by mass or less of the total mass of the insulation material) The content of the inorganic balloons is preferably 60 mass % or less based on the total mass of the heat insulating material.

[0061] (Average particle size of inorganic balloons: 1 μm to 100 μm) The average particle size of the inorganic balloons is preferably 1 μm or more and 100 μm or less.

[0062] <Second inorganic particles> The second inorganic particles are not particularly limited as long as they are different from the first inorganic particles in terms of material, particle size, etc. Examples of the second inorganic particles that can be used include oxide particles, carbide particles, nitride particles, inorganic hydrate particles, silica nanoparticles, metal oxide particles, inorganic balloons such as microporous particles and hollow silica particles, particles made of a thermally expandable inorganic material, and particles made of a hydrous porous body, the details of which are as described above.

[0063] Nanoparticles have extremely low conductive heat transfer and can maintain excellent heat insulation even when compressive stress is applied to the heat-transfer-suppressing sheet 10. Metal oxide particles such as titania are also effective at blocking radiant heat. Furthermore, when large-diameter inorganic particles and small-diameter inorganic particles are used, the small-diameter inorganic particles fill the gaps between the large-diameter inorganic particles, resulting in a denser structure and improved heat-transfer suppression. Therefore, when nanoparticles are used as the first inorganic particles, it is preferable to further include second inorganic particles made of a metal oxide that are larger in diameter than the first inorganic particles in the heat-transfer-suppressing sheet 10. Examples of metal oxides include silicon oxide, titanium oxide, aluminum oxide, barium titanate, zinc oxide, zircon, zirconium oxide, etc. In particular, titanium oxide (titania) is a component with a higher refractive index than other metal oxides, and is highly effective in scattering light and blocking radiant heat in a high temperature range of 500°C or higher, so it is most preferable to use titania.

[0064] When the first inorganic particles are at least one type of particles selected from dry silica particles and silica aerogel, and the second inorganic particles are at least one type of particles selected from titania, zircon, zirconia, silicon carbide, zinc oxide, and alumina, the first inorganic particles preferably account for 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more of the total mass of the inorganic particles, in order to obtain excellent heat insulating performance within a temperature range of 300° C. or less. Furthermore, the first inorganic particles preferably account for 95% by mass or less, more preferably 90% by mass or less, and even more preferably 80% by mass or less of the total mass of the inorganic particles.

[0065] On the other hand, in order to obtain excellent heat insulating performance in a temperature range exceeding 300° C., the content of the second inorganic particles is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 20% by mass or more, based on the total mass of the inorganic particles. Also, the content of the second inorganic particles is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less, based on the total mass of the inorganic particles.

[0066] (Average primary particle size of second inorganic particles) When second inorganic particles made of a metal oxide are contained, if the average primary particle size of the second inorganic particles is 1 μm or more and 50 μm or less, radiation heat transfer can be efficiently suppressed in a high temperature range of 500° C. or more. The average primary particle size of the second inorganic particles is more preferably 5 μm or more and 30 μm or less, and most preferably 10 μm or less.

[0067] (Inorganic particle content) When the total content of inorganic particles in heat-transfer-suppressing sheet 10 is appropriately controlled, the heat insulating properties of heat-transfer-suppressing sheet 10 can be sufficiently ensured. The total content of inorganic particles is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 75% by mass or more, relative to the total mass of heat-transfer-suppressing sheet 10. Furthermore, if the total content of inorganic particles is too high, the content of organic fibers will relatively decrease. Therefore, in order to fully obtain the sheet strength-enhancing effect of the organic fibers, the total content of inorganic particles is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less, relative to the total mass of heat-transfer-suppressing sheet 10.

[0068] The content of inorganic particles in heat-transfer-suppressing sheet 10 can be calculated, for example, by heating heat-transfer-suppressing sheet 10 at 800° C. to decompose the organic components, and then measuring the mass of the remainder.

[0069] In addition to the organic fibers and inorganic particles, the heat-transfer-suppressing sheet 10 may contain other materials, such as organic fibers and inorganic fibers made of an organic material different from the first organic material. However, the organic fibers and inorganic particles are the main components of the heat-transfer-suppressing sheet 10, and their combined amount is the largest compared to the content of each of the other materials. The combined amount of the organic fibers and inorganic particles is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, and most preferably 95% by mass or more, of the total mass of the heat-transfer-suppressing sheet 10.

[0070] <Inorganic fibers> The inorganic fibers may be a single inorganic fiber or a combination of two or more inorganic fibers. Examples of the inorganic fibers include ceramic fibers such as silica fiber, alumina fiber, alumina silicate fiber, zirconia fiber, carbon fiber, soluble fiber, refractory ceramic fiber, aerogel composite material, magnesium silicate fiber, alkaline earth silicate fiber, potassium titanate fiber, silicon carbide fiber, and potassium titanate whisker fiber; glass fibers such as glass fiber, glass wool, and slag wool; and mineral fibers such as rock wool, basalt fiber, wollastonite, and mullite fiber. These inorganic fibers are preferred in terms of heat resistance, strength, availability, etc. Among the inorganic fibers, silica-alumina fibers, alumina fibers, silica fibers, rock wool, alkaline earth silicate fibers, and glass fibers are particularly preferred in terms of ease of handling.

[0071] The cross-sectional shape of the inorganic fiber is not particularly limited, and examples thereof include a circular cross section, a flat cross section, a hollow cross section, a polygonal cross section, a core cross section, etc. Among these, modified cross section fibers having a hollow cross section, a flat cross section, or a polygonal cross section are preferably used because they have slightly improved heat insulation properties.

[0072] The preferred lower limit of the average fiber length of the inorganic fibers is 0.1 mm, more preferably 0.5 mm. Meanwhile, the preferred upper limit of the average fiber length of the inorganic fibers is 50 mm, more preferably 10 mm. If the average fiber length of the inorganic fibers is less than 0.1 mm, the inorganic fibers are less likely to intertwine with each other, which may reduce the mechanical strength of the heat-transfer-suppressing sheet 10. On the other hand, if the average fiber length exceeds 50 mm, although a reinforcing effect is obtained, the inorganic fibers may not be able to intertwine closely with each other or may curl up individually, which may result in continuous voids and reduce the heat insulating properties.

[0073] The preferred lower limit of the average fiber diameter of the inorganic fibers is 1 μm, more preferably 2 μm, and even more preferably 3 μm. Meanwhile, the preferred upper limit of the average fiber diameter of the inorganic fibers is 15 μm, and more preferably 10 μm. If the average fiber diameter of the inorganic fibers is less than 1 μm, the mechanical strength of the inorganic fibers themselves may be reduced. Furthermore, from the viewpoint of the effects on human health, the average fiber diameter of the inorganic fibers is preferably 3 μm or more. Meanwhile, if the average fiber diameter of the inorganic fibers is greater than 15 μm, solid-state heat transfer via the inorganic fibers may increase, resulting in reduced thermal insulation, and the formability and strength of the heat transfer-suppressing sheet may be impaired.

[0074] (Inorganic fiber content) The inorganic fiber content is preferably 3% by mass or more and 15% by mass or less, based on the total mass of heat-transfer-suppressing sheet 10. The inorganic fiber content is more preferably 5% by mass or more and 10% by mass or less, based on the total mass of heat-transfer-suppressing sheet 10. This content ensures a good balance between the shape retention, compression force resistance, wind pressure resistance, and inorganic particle retention ability of the inorganic fibers. Furthermore, by appropriately controlling the inorganic fiber content, the organic and inorganic fibers become entangled with each other to form a three-dimensional network, further improving the retention of inorganic particles and other compounded materials described below.

[0075] <Other compounding materials> If necessary, binders, colorants, and the like can be added. These are all useful for purposes such as reinforcing heat-transfer-suppressing sheet 10 and improving its formability, and the total content of these ingredients relative to the total mass of heat-transfer-suppressing sheet 10 is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 5% by mass or less.

[0076] [Battery pack] 5 is a schematic diagram showing a battery pack 100 including a heat-transfer-suppressing sheet 10 according to an embodiment of the present invention. As shown in the figure, the battery pack 100 contains multiple battery cells 20a, 20b, and 20c housed in a battery case 30, with the heat-transfer-suppressing sheet 10 interposed between the cells. The battery cells 20a, 20b, and 20c are connected in series or in parallel.

[0077] Although not shown, the heat transfer-suppressing sheet 10 can also be used as an inner wall material for the battery case 30.

[0078] In the battery pack 100 configured in this manner, even if, for example, battery cell 20a experiences thermal runaway, the heat transfer to battery cell 20b can be suppressed because the heat transfer suppression sheet 10, which has a heat transfer suppression effect, is present between battery cell 20a and battery cell 20b. [Example]

[0079] The flexibility of the heat-transfer-suppressing sheet precursor and the heat-transfer-suppressing sheet was evaluated. The heat-transfer-suppressing sheet precursor, "Sample A," was prepared, containing fumed silica and titania as inorganic particles and core-sheath binder fibers as organic fibers. The specific contents and names of each component are shown below. Inorganic particles: "Dry silica" (57.7% by mass, average primary particle size: 0.012 μm) ·Inorganic particles: "Titania" (24.7% by mass, average primary particle diameter: 8μm) Organic fiber: "PET / low melting point PET fiber" (TJ04CN: Teijin Frontier Co., Ltd.: 15% by mass, average fiber length: 5 mm), a binder fiber with a core-sheath structure Core: "Polyethylene terephthalate" (melting point: 240°C) Sheath: "Low melting point polyethylene terephthalate" (melting point: 110°C) Hot melt adhesive powder: "Powder Resin" (PR D60C-Z: manufactured by Tokyo Ink Co., Ltd., 2.6% by mass, melting point: 100°C)

[0080] Furthermore, heat-transfer-suppressing sheets were prepared with the same composition and density as the heat-transfer-suppressing sheet precursor, with "Sample B" having a machined surface on only one side and "Sample C" having machined surfaces on both the front and back sides. The amount of machining was adjusted for Samples B and C so that all samples had the same sheet thickness. For evaluation, pressures were applied sequentially at room temperature from no pressure (0 MPa) to 1 MPa, and the "pressure-compression ratio" was measured.

[0081] The measurement results are shown in a graph in Figure 4. Compared to "Sample A," the heat-transfer-suppressing sheet precursor, "Sample B" and "Sample C," which are heat-transfer-suppressing sheets with machined surfaces, tended to have a slower rise in pressure relative to the compression ratio. Furthermore, while the compression ratio range typically used for heat-transfer-suppressing sheets in battery cells is 20% or less, it was found that they soften in the low-pressure range that includes compression ratios of 20% or less. Furthermore, compared to "Sample B," which has machined surfaces on only one side, "Sample C," which has machined surfaces on both sides, tended to be softer. [Explanation of symbols]

[0082] 1. Heat transfer suppression sheet precursor 1a Surface of the heat transfer suppression sheet precursor 5 Cutting tools 10 Heat transfer suppression sheet 10a Machined surface of heat transfer suppression sheet 11 Organic Fibers 20a, 20b, 20c battery cells 30 Battery case 100 battery packs

Claims

1. Contains inorganic particles and organic fibers, A heat transfer-suppressing sheet, wherein at least one of the front and back surfaces includes a polished or machined surface.

2. 2. The heat transfer-suppressing sheet according to claim 1, wherein organic fibers are exposed and fluffed on the polished or machined surface.

3. 2. The heat transfer suppressing sheet according to claim 1, wherein another member is laminated on the polished surface or the machined surface.

4. A method for producing a heat-transfer-suppressing sheet, comprising: a precursor-preparing step of preparing a heat-transfer-suppressing sheet precursor having inorganic particles and organic fibers; and a surface-processing step of polishing or cutting at least one of a front surface and a back surface of the heat-transfer-suppressing sheet precursor prepared in the precursor-preparing step.

5. The method for producing a heat-transfer-suppressing sheet according to claim 4 , wherein the precursor-producing step produces the heat-transfer-suppressing sheet precursor by a dry method.

6. 4. A battery pack comprising a plurality of battery cells and the heat transfer-suppressing sheet according to claim 1, wherein the plurality of battery cells are connected in series or in parallel.

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