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

Abstract

To provide a heat transfer suppression sheet that can suppress powder falling off of inorganic particles, maintain excellent heat insulating performance, and alleviate stress from impacts, pressure, etc., while having excellent sheet strength, and a battery pack that includes the heat transfer suppression sheet.SOLUTION: A heat transfer suppression sheet 10 includes inorganic particles 4 and organic fibers 1, and has, on its surface, fiber aggregation portions 2 formed by a plurality of entangled organic fibers 1, and recesses 3 recessed from the fiber aggregation portions 2. A battery pack includes a plurality of battery cells and the heat transfer suppression sheet 10, and the plurality of battery cells are 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 heat propagation from a battery cell experiencing thermal runaway is to interpose a heat insulating sheet between the battery cells. For example, Patent Document 1 describes a thermal runaway suppression fire-resistant sheet having a substrate containing glass fiber, a heat-and-moisture adhesive binder fiber, and a fibrillated heat-resistant fiber, an inorganic particle layer containing inorganic particles and an inorganic binder, the inorganic particle layer having a coating layer that covers the surface of the fiber contained in the substrate, and a heat insulating layer present on at least one surface of the substrate. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-96935 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the thermal runaway suppressing fire-resistant sheet described in Patent Document 1, glass fibers are an essential compounding material, and therefore heat transfer occurs between the glass fibers, which may reduce the heat insulating performance.

[0007] Furthermore, because the compounded materials in each part of the sheet are uniform throughout, there is no part to cushion stress when subjected to impact, pressure, or other stress, and the sheet strength is not necessarily sufficient. In recent years, the capacity of battery cells in assembled batteries has been further improved, resulting in an increased expansion rate during charging and discharging. Therefore, for example, if the sheet strength is low when placed between battery cells in an assembled battery, the expansion of the battery cells during charging and discharging will compress the insulating sheet, causing the inorganic particles to fall off (powder), resulting in a decrease in insulating performance. Furthermore, if the battery cells experience thermal runaway, they will be subjected to significantly greater stress, and a sheet with high strength will be unable to withstand this.

[0008] The present invention has been made in view of the above-mentioned problems, and has an object to provide a heat-transfer-suppressing sheet that can suppress powder falling off of inorganic particles, maintain excellent heat insulating performance, and alleviate stresses such as those caused by impacts and pressure, and has excellent sheet strength, as well as a battery pack including this 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 heat transfer-suppressing sheet having inorganic particles and organic fibers, A heat transfer-suppressing sheet, characterized in that the sheet has, on its surface, fiber aggregation portions formed by a plurality of entangled organic fibers, and recesses recessed from the fiber aggregation portions.

[0011] Furthermore, preferred embodiments of the present invention relating to the heat transfer-suppressing sheet relate to the following [2] to [8].

[0012] [2] The heat-transfer-suppressing sheet according to [1], wherein the recessed portions form grooves. [3] The heat-transfer-suppressing sheet according to [1] or [2], wherein the recesses have a lower fiber density than the fiber aggregation portions. [4] The heat-transfer-suppressing sheet according to any one of [1] to [3], wherein the fiber aggregation portions are continuous stripes. [5] The heat-transfer-suppressing sheet according to any one of [1] to [4], wherein at least some of the recesses are surrounded by the fiber aggregation portions. [6] The heat-transfer-suppressing sheet according to any one of [1] to [5], wherein the inorganic particles are particles made of at least one inorganic material selected from the group consisting of oxide particles, carbide particles, nitride particles, and inorganic hydrate particles. [7] The heat-transfer-suppressing sheet according to [6], wherein the inorganic particles include at least one type of particles selected from dry silica particles and silica aerogel. [8] The heat-transfer-suppressing sheet according to [6] or [7], wherein the inorganic particles further include at least one type of particles selected from titania, zircon, zirconia, silicon carbide, zinc oxide, and alumina.

[0013] The above object of the present invention is achieved by the following configuration [9] relating to a method for producing a heat transfer-suppressing sheet.

[0014] [9] a precursor preparation step of preparing a heat transfer-suppressing sheet precursor using a mixture containing inorganic particles and binder fibers having a core-sheath structure; a surface processing step of cutting or polishing at least one of the front and back surfaces of the heat-transfer-suppressing sheet precursor prepared in the precursor preparation step.

[0015] Furthermore, preferred embodiments of the present invention relating to a method for producing a heat transfer-suppressing sheet relate to the following

[10] to

[11] .

[0016]

[10] The method for producing a heat-transfer-suppressing sheet according to [9], wherein the precursor-producing step produces the heat-transfer-suppressing sheet precursor by a dry method.

[11] The method for producing a heat transfer-suppressing sheet according to [9] or

[10] , further comprising a re-pressing step of pressing the sheet surface after the surface processing step.

[0017] The above object of the present invention is achieved by the following configuration

[12] relating to a battery pack.

[0018]

[12] A battery pack comprising a plurality of battery cells and the heat-transfer-suppressing sheet according to any one of [1] to [8], the plurality of battery cells being connected in series or in parallel. [Effects of the Invention]

[0019] The heat-transfer-suppressing sheet of the present invention has, on its surface, fiber agglomerations formed by entanglement of a plurality of organic fibers and recesses recessed below the fiber agglomerations, so that the fiber agglomerations increase the strength of the sheet, and the recesses relieve stress from external impacts, pressure, etc. As a result, powder fall-off of the inorganic particles is suppressed, a decrease in the heat insulating effect due to sheet deformation is prevented, and a heat-transfer-suppressing sheet that maintains excellent heat insulating performance and has excellent sheet strength can be provided.

[0020] Furthermore, according to the method for producing a heat-transfer-suppressing sheet of the present invention, it is possible to produce a heat-transfer-suppressing sheet that has high sheet strength and a high retention effect of heat insulating performance as described above.

[0021] Furthermore, the battery pack of the present invention includes a heat transfer-suppressing sheet that has the above-described high sheet strength and high thermal insulation performance retention effect, and therefore can effectively 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]

[0022] [Figure 1] FIG. 1 is a photograph, substituted for a drawing, showing the surface of a heat-transfer-suppressing sheet according to an embodiment of the present invention. [Figure 2] FIG. 2 is a photograph, substituted for a drawing, showing a cross section of a heat-transfer-suppressing sheet according to an embodiment of the present invention. [Figure 3] FIG. 3 is a schematic diagram illustrating the manufacturing process of a heat-transfer-suppressing sheet, 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 or polished, and (C) is a side view of the resulting heat-transfer-suppressing sheet. [Figure 4] FIG. 4 is a schematic diagram illustrating why fiber aggregation portions and recesses are obtained on the surface of a heat-transfer-inhibiting sheet by cutting or polishing the surface of a heat-transfer-inhibiting sheet precursor. FIG. 4A shows the surface processing step of cutting or polishing the surface of the heat-transfer-inhibiting sheet precursor, and FIG. 4B is a side view of the resulting heat-transfer-inhibiting sheet and waste material. [Figure 5] FIG. 5 is a schematic diagram showing a battery pack having a heat transfer-suppressing sheet according to this embodiment. [Figure 6] FIG. 6 is a graph showing the results of Test 1 of the Example. [Figure 7] FIG. 7 is a photograph showing the results of Test 2 of the embodiment, in which (A) shows the surface of the machined surface before re-pressing after the surface processing process, and (B) shows the surface of the machined surface after re-pressing after the surface processing process. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present inventors have conducted extensive research into heat-transfer-suppressing sheets that can solve the above-mentioned problems, and as a result have found that by forming fiber aggregation portions formed by entanglement of a plurality of organic fibers and recesses recessed below the fiber aggregation portions on the surface of the heat-transfer-suppressing sheet, it is possible to improve the sheet strength of the heat-transfer-suppressing sheet and maintain high heat insulating performance.

[0024] A heat transfer-suppressing sheet, a method for manufacturing the same, and a battery pack according to an embodiment of the present invention (hereinafter referred to as the "present embodiment") will be described in detail below. Note that the present invention is not limited to the embodiment described below, and can be modified as desired without departing from the spirit and scope of the present invention.

[0025] [Heat transfer suppression sheet] FIG. 1 is a photograph, substituted for a drawing, showing the surface of a heat-transfer-suppressing sheet according to this embodiment.

[0026] The heat-transfer-suppressing sheet 10 according to this embodiment includes inorganic particles 4 (not shown) and organic fibers 1 (not shown). As shown in Fig. 1, the surface of the heat-transfer-suppressing sheet 10 is formed with fiber aggregation portions 2 each having streaky fiber bundles 7 made of a plurality of organic fibers 1, and recesses 3 each having no fiber bundles 7 and recessed below the fiber aggregation portions 2. Preferably, the recesses 3 recessed below the fiber aggregation portions 2 form continuous grooves.

[0027] The fiber bundles 7 are formed by entangling a plurality of organic fibers 1 with one another, and extend in stripes in a direction substantially parallel to the surface of the heat transfer-suppressing sheet 10 . There is no limitation on the height difference between the fiber aggregation portions 2 and the recesses 3, but it is preferably greater than 0 and 0.5 mm, and more preferably greater than 0 and 0.2 mm.

[0028] In the recesses 3, although a few organic fibers 1 are observed in some places, fiber bundles 7 in which a plurality of organic fibers 1 are entangled are not observed. Therefore, the recesses 3 have a lower fiber density than the fiber aggregation portions 2.

[0029] The fiber aggregation portions 2 and the recessed portions 3 form a sea-island structure, with the recessed portions 3 corresponding to the island portions being formed so as to be surrounded by the fiber aggregation portions 2 corresponding to the sea portion.

[0030] In the heat transfer-suppressing sheet 10 configured in this manner, the fiber bundles 7 formed by entanglement of the organic fibers 1 in the fiber aggregation portions 2 are present in stripes on the sheet surface, improving the sheet strength. Furthermore, the fiber bundles 7 extending in stripes also improves the flexibility of the sheet. Note that, because the fiber aggregation portions 2 are formed in continuous stripes by the fiber bundles 7 formed by entanglement of the organic fibers 1, the sheet strength can be further improved, and the flexibility of the sheet also improves.

[0031] The continuous streak-like fiber aggregation portions 2 may also have a curved shape. Furthermore, by connecting a plurality of curved streak-like fiber aggregation portions 2 with different shapes, the streak-like fiber aggregation portions 2 form a mesh-like shape, further increasing the strength and flexibility of the sheet.

[0032] On the other hand, the recessed portions 3 are recessed further than the fiber aggregation portions 2, and therefore act to release stress such as external impacts and pressure.

[0033] Furthermore, since at least some of the recesses 3 are surrounded by strong fiber aggregation portions 2 formed by fiber bundles 7 in which organic fibers 1 are entangled, the shape of the recesses 3 is more easily maintained, and the function of dissipating stress from external impacts, pressure, etc. becomes more effective.

[0034] To form such fiber aggregation portions 2 and recesses 3 on the surface of the heat-transfer-suppressing sheet 10, it is advisable to perform cutting or polishing on the surface (reference numeral 30a in FIG. 3) of the prepared heat-transfer-suppressing sheet precursor (reference numeral 30 in FIG. 3), as will be explained in the manufacturing method described below.

[0035] 2, the organic fibers 1 and fiber bundles 7 formed by entanglement of the organic fibers 1 are present not only on the surface of the heat-transfer-suppressing sheet 10 but also inside the sheet, thereby achieving even greater sheet strength and flexibility.

[0036] In the heat-transfer-suppressing sheet 10, it is preferable that the length of the fiber bundles 7 is relatively long. One method for determining the length of the fiber bundles 7 is to simply measure the length of the fiber bundles 7 extending in a stripe shape. For example, a string or the like can be placed on the surface of the heat-transfer-suppressing sheet 10 along the fiber bundles 7, and then the length of the string can be measured. When the length of continuous fiber bundles 7 is measured, if there are fiber bundles 7 with a length of 20 mm or more, the effect of improving the strength of the heat-transfer-suppressing sheet 10 can be sufficiently obtained.

[0037] Furthermore, if the fiber bundles 7 are connected in a mesh pattern on the surface of the heat transfer-suppressing sheet 10, the sheet strength can be further improved.

[0038] Furthermore, the heat-transfer-suppressing sheet 10 has an uneven surface 10a with fiber aggregation portions 2 and recesses 3, but for the purposes of flattening the surface, increasing the density of the sheet, etc., it is possible to re-press the heat-transfer-suppressing sheet 10 that has been manufactured by a surface processing step in which cutting or polishing is performed. Even after re-pressing, the pattern formed on the surface by the fiber aggregation portions 2 and recesses 3 remains intact, and the respective functions of the fiber aggregation portions 2 and recesses 3 described above are maintained.

[0039] The preferred materials constituting the heat transfer-suppressing sheet 10 will now be described.

[0040] <Organic fiber> The organic fiber 1 provides flexibility to the heat-transfer-suppressing sheet 10 and also has the effect of improving the strength and shape of the sheet. While single-component organic fibers can be used as the material for the organic fiber 1 in the heat-transfer-suppressing sheet 10, it is preferable to use binder fibers with a core-sheath structure. Binder fibers with a core-sheath structure have a core extending in the longitudinal direction of the fiber and a sheath formed to cover the outer surface 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.

[0041] Whether single-component organic fibers or binder fibers with a core-sheath structure are used as the material for the organic fibers 1, heating during production of the heat transfer-suppressing sheet 10 melts part of the surface of the fibers, and subsequent cooling forms welded parts (not shown) around the organic fibers 1. The welded parts fuse the inorganic particles 4 to the surfaces of the organic fibers 1 and fuse the organic fibers 1 together, so the formation of the welded parts provides excellent sheet strength.

[0042] When binder fibers with a sheath-core structure are used as the material, the core of the heat-transfer-suppressing sheet 10 corresponds to the organic fiber 1. If binder fibers with a sheath-core structure are used during the production of the heat-transfer-suppressing sheet 10, adjacent binder fibers are fused to each other, making it easier to form fiber bundles 7 and further increasing the sheet strength. Furthermore, when binder fibers with a sheath-core structure are used, the second organic material that makes up the sheath melts and then solidifies again in a state that includes the surrounding inorganic particles 4, thereby improving the retention of the inorganic particles 4.

[0043] (First organic material) When a binder fiber having a core-sheath structure is used as the material for the organic fiber 1, the first organic material constituting the core, i.e., the organic fiber 1, 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 surface of the organic fiber 1. The first organic material may be at least one selected from polyethylene terephthalate, polypropylene, and nylon.

[0044] (Second organic material) When a binder fiber having a core-sheath structure is used as the material for the organic fiber 1, the second organic material constituting the sheath portion is not particularly limited as long as it has a melting point lower than that of the first organic material constituting the organic fiber 1. 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.

[0045] (organic fiber content) Appropriately controlling the content of organic fiber 1 in heat-transfer-inhibiting sheet 10 makes it possible to sufficiently improve the strength of heat-transfer-inhibiting sheet 10. The content of organic fiber 1 is preferably 2 mass % or more, and more preferably 4 mass % or more, relative to the total mass of heat-transfer-inhibiting sheet 10. Furthermore, if the content of organic fiber 1 is too high, the content of inorganic particles 4 will relatively decrease. Therefore, in order to obtain the desired heat insulating performance, the content of organic fiber is preferably 10 mass % or less, and more preferably 8 mass % or less, relative to the total mass of heat-transfer-inhibiting sheet 10.

[0046] (fiber length of organic fiber) The fiber length of the organic fibers 1 is not particularly limited, but from the viewpoint of ensuring moldability and processability, it is preferable that the average fiber length of the organic fibers be 10 mm or less. From the viewpoint of improving the strength of heat-transfer-suppressing sheet 10, the average fiber length of organic fibers 1 is preferably 0.5 mm or more.

[0047] Note that binder fibers having a core-sheath structure such as the one described above are generally commercially available, and the materials constituting the core and sheath may be the same or different. Examples of binder fibers in which the core and sheath are made of the same material but have different melting points include those in which the core and sheath are made of polyethylene terephthalate, polypropylene, or nylon. Examples of binder fibers in which the core and sheath are made of different materials include those in which the core is made of polyethylene terephthalate and the sheath is made of polyethylene, and those in which the core is made of polypropylene and the sheath is made of polyethylene.

[0048] <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.

[0049] 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.

[0050] 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.

[0051] <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 may be used, or two or more types of oxide particles may 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.

[0052] (Average primary particle size of oxide particles) 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 inside the 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.

[0053] In this embodiment, 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.

[0054] (nanoparticles) In this embodiment, 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 voids are finely dispersed, resulting in excellent heat insulation that suppresses convective heat transfer. Therefore, it is preferable to use nanoparticles because they can suppress heat transfer between adjacent nanoparticles during normal use of the battery at room temperature. Furthermore, if nanoparticles with a small average primary particle size are used as the oxide particles, even if heat-transfer-suppressing sheet 10 is compressed due to expansion caused by thermal runaway of the battery cell, increasing the internal density, it is possible to suppress an increase in conductive heat transfer in 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 to provide cushioning.

[0055] In this embodiment, 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, in a battery pack, even if the battery cells arranged on both sides of the heat-transfer-suppressing sheet 10 thermally expand and a large compressive stress is applied 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.

[0056] Generally, wet silica particles are agglomerated, while dry silica particles can be dispersed. Because heat conduction is predominant in the temperature range below 300°C, dry silica, which can disperse particles, can achieve superior heat insulation performance compared to wet silica. As will be described later, the heat transfer-suppressing sheet 10 is preferably produced by processing a mixture containing compounding materials into a sheet using a dry method, and therefore, it is preferable to use dry silica, silica aerogel, or the like, which have low thermal conductivity, as the inorganic particles.

[0057] (average primary particle size of nanoparticles) 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 heat 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 heat 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.

[0058] (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).

[0059] 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

[0060] As will be described later, heat transfer-suppressing sheet 10 is preferably interposed between battery cells, for example, but in a battery cell that has experienced thermal runaway, the temperature rises sharply to over 200°C and continues to rise to around 700°C. Therefore, the inorganic particles are preferably made of inorganic hydrates whose thermal decomposition temperature begins at 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.

[0061] (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.

[0062] (Particles made of thermally expandable inorganic material) Examples of thermally expandable inorganic materials include vermiculite, bentonite, mica, and perlite.

[0063] (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.

[0064] (inorganic balloons) The heat transfer-suppressing sheet 10 according to this embodiment 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, thereby further improving the insulating properties of heat-transfer-suppressing sheet 10. As the inorganic balloons, at least one selected from shirasu balloons, silica balloons, fly ash balloons, barite balloons, and glass balloons can be used.

[0065] (Inorganic balloon content) The content of inorganic balloons is preferably 60 mass % or less relative to the total mass of heat transfer-suppressing sheet 10 .

[0066] (average particle size of inorganic balloons) The average particle size of the inorganic balloons is preferably 1 μm or more and 100 μm or less.

[0067] <Second inorganic particles> When heat transfer-suppressing sheet 10 contains two types of 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] (Average primary particle size of second inorganic particles) When second inorganic particles made of a metal oxide are contained in the heat transfer-suppressing sheet, 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.

[0072] (Inorganic particle content) When the total content of inorganic particles 4 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 the inorganic particles 4 is preferably 60% by mass or more, more preferably 65% ​​by mass or more, and even more preferably 70% by mass or more, relative to the total mass of the heat-transfer-suppressing sheet 10. Furthermore, the total content of the inorganic particles 4 is even more preferably 75% by mass or more, even more preferably 80% by mass or more, and particularly preferably 85% by mass or more, relative to the total mass of the heat-transfer-suppressing sheet 10. On the other hand, if the total content of inorganic particles 4 becomes too high, the content of organic fibers 1 will decrease relatively. Therefore, in order to fully obtain the sheet strength-improving effect of organic fibers 1, the total content of inorganic particles 4 is preferably 95 mass % or less, and more preferably 90 mass % or less, of the total mass of heat transfer-suppressing sheet 10.

[0073] The content of inorganic particles 4 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 remaining portion.

[0074] In this embodiment, the combined content of the inorganic particles 4 and the organic fibers 1 is preferably 65% ​​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 the heat-transfer-suppressing sheet 10. The combined content of the inorganic particles 4 and the organic fibers 1 is even more preferably 80% by mass or more, even more preferably 85% by mass or more, and particularly preferably 90% by mass or more, relative to the total mass of the heat-transfer-suppressing sheet 10. The combined content of the inorganic particles 4 and the organic fibers 1 may be 100% by mass, relative to the total mass of the heat-transfer-suppressing sheet 10.

[0075] In addition to the organic fibers 1 and inorganic particles 4, the heat-transfer-suppressing sheet 10 may also contain organic fibers made of an organic material different from the organic fibers 1, inorganic fibers, etc. When the heat-transfer-suppressing sheet 10 contains inorganic fibers, the inorganic fibers that are preferably contained in this embodiment will be described below.

[0076] <Inorganic fibers> As the inorganic fiber, a single inorganic fiber may be used, or two or more inorganic fibers may be used in combination. Examples of 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; artificial mineral fibers such as rock wool and basalt fiber; and natural mineral fibers such as wollastonite and mullite fiber. These inorganic fibers are preferred in terms of heat resistance, strength, and availability. Among the inorganic fibers, silica-alumina fiber, alumina fiber, silica fiber, rock wool, alkaline earth silicate fiber, and glass fiber are particularly preferred from the standpoint of handleability.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] (Inorganic fiber content) When heat-transfer-suppressing sheet contains inorganic fibers, the content of inorganic fibers is preferably 3 mass % or more and 15 mass % or less relative to the total mass of heat-transfer-suppressing sheet .

[0081] The inorganic fiber content is more preferably 5% by mass or more and 10% by mass or less, based on the total mass of the heat transfer-suppressing sheet 10. This content allows the inorganic fibers to exhibit a good balance of shape retention, pressure resistance, wind pressure resistance, and inorganic particle retention. By appropriately controlling the inorganic fiber content, the organic fibers 1 and the inorganic fibers become entangled with each other to form a three-dimensional network, further improving the retention of the inorganic particles 4 and other blended materials described below.

[0082] <Other compounding materials> If necessary, heat-transfer-suppressing sheet 10 may contain binders, colorants, and the like. These are all useful for reinforcing heat-transfer-suppressing sheet 10 and improving its formability, and the total amount of these additives relative to the total mass of heat-transfer-suppressing sheet 10 is preferably 10 mass % or less.

[0083] Although heat-transfer-suppressing sheet 10 is configured as described above, it may be coated with a film or the like to further suppress powder falling off of inorganic particles 4. Examples of films include polyimide, polycarbonate, PET, p-phenylene sulfide, polyetherimide, cross-linked polyethylene, flame-retardant chloroprene rubber, polyvinyl fluoride, rigid polyvinyl chloride, polybutylene terephthalate, PTFE, PFA, FEP, ETFE, rigid PVC, flame-retardant PET, polystyrene, polyethersulfone, polyamideimide, polyacrylonitrile, polyethylene, polypropylene, and polyamide. The method of coating with the film is not particularly limited, and examples include a method of attaching heat-transfer-suppressing sheet 10 with an adhesive or the like, a method of wrapping heat-transfer-suppressing sheet 10 in a film, and a method of housing heat-transfer-suppressing sheet 10 in a bag-shaped film.

[0084] [Method of manufacturing heat transfer suppression sheet] 3, the method for producing the heat-transfer-suppressing sheet 10 includes a precursor-preparing step of producing a heat-transfer-suppressing sheet precursor 30 using a mixture containing inorganic particles 4 (not shown) and binder fibers with a core-sheath structure that will become organic fibers 1 (not shown), and a surface-processing step of cutting or polishing at least one of the front and back surfaces of the heat-transfer-suppressing sheet precursor 30 produced in the precursor-preparing step. Furthermore, if necessary, a re-pressing step of pressing the sheet surface can be included after the surface-processing step.

[0085] (Precursor production process) As shown in Figure 3(A), in the precursor preparation process, inorganic particles 4, binder fibers with a core-sheath structure that will become organic fibers 1, and other compounding materials are added in a predetermined ratio to a dry mixer such as a V-type mixer to prepare a mixture. Dry mixing facilitates three-dimensional entanglement of the binder fibers. In wet mixing, the binder fibers do not form flocks, and fiber agglomerations 2 and recesses 3 are not formed.

[0086] The resulting mixture is then placed in a predetermined mold and pressed with a press or the like, and the resulting molded body is heated to melt the sheaths of the binder fibers. The heated molded body is then cooled, whereby the molten sheaths on the surface and inside of the molded body solidify, and the cores (organic fibers 1) are welded together, thereby obtaining the heat-transfer-suppressing sheet precursor 30.

[0087] Inside the heat transfer-suppressing sheet precursor 30, the organic material constituting the molten sheath and the inorganic particles 4 present around the binder fibers are fused to the core, and are also fused to each other in the areas where the binder fibers were in contact with each other. This increases the ability to retain the inorganic particles 4, and prevents the inorganic particles 4 from falling off.

[0088] An adhesive such as hot melt powder may be contained in the mixture as a compounding material of the heat-transfer-suppressing sheet 10. By appropriately adjusting the type and content of the adhesive contained in the mixture, the holding power of the inorganic particles 4 can be improved and powder falling off of the inorganic particles 4 can be further suppressed. Furthermore, the heat-transfer-suppressing sheet 10 has even greater strength, and can maintain its shape even when a strong external stress is applied, thereby maintaining excellent heat insulating performance.

[0089] In the above process, to prevent powder such as the inorganic particles 4 from flying about during mixing and making it difficult to handle the raw materials, a small amount of solvent such as water may be added within the range considered to be a dry method. For example, by adding a small amount of solvent such as water to the mixture, scattering of powder during production can be suppressed.

[0090] The process of processing the mixture into a sheet includes a process of pressing the mixture and a process of heating the mixture. When a binder fiber having a core-sheath structure is used as the material for the heat transfer-suppressing sheet 10, the heating temperature in the heating process is preferably higher than the melting point of the second organic material constituting the sheath and lower than the melting point of the first organic material constituting the core. By setting the heating temperature in this manner, as described above, the strength of the sheet can be ensured by the core on both the surface side and the center side of the sheet, and the inorganic particles 4 can be held by the fused sheath.

[0091] Specifically, the heating temperature in the heating step is preferably set to be 10°C or more higher, more preferably 20°C or more higher, than the melting point of the second organic material constituting the sheath, while the heating temperature is preferably set to be 10°C or more lower, more preferably 20°C or more lower, than the melting point of the first organic material constituting the core.

[0092] The heating time is not particularly limited, but it is preferable to set the heating time so that the sheath can be sufficiently melted, for example, from 3 minutes to 15 minutes.

[0093] Furthermore, when a hot melt powder is included, the heating temperature in the heating step is preferably set to be at least 10°C higher, and more preferably at least 20°C higher, than the higher of the melting point of the second organic material constituting the shell and the melting point of the third organic material constituting the hot melt powder. On the other hand, the heating temperature is preferably set to be at least 10°C lower, and more preferably at least 20°C lower, than the melting point of the first organic material constituting the core. Setting the heating temperature in this range makes it possible to form a strong skeleton, further improving the sheet strength and preventing the inorganic particles 4 from falling off.

[0094] (Surface processing process) Next, as shown in FIG. 3(B), at least one of the front and back surfaces of the heat transfer-suppressing sheet precursor 30 (here, the upper surface in the figure, ie, the front surface 30a) is subjected to cutting or polishing.

[0095] 3(B) shows a case where cutting is performed using a cutting tool 35 such as a knife in the surface processing step, but polishing can also be performed using, for example, a grindstone or a brush. The cutting and polishing allowances indicated by the symbol t in the figure are not particularly limited as long as the fiber agglomeration portions 2 and recesses 3 are clearly visible, but from the viewpoint of workability in surface processing, it is preferable to set the allowance in the range of 0.05 mm to 1.0 mm, for example.

[0096] As a result, as shown in FIG. 3(C), a heat-transfer-suppressing sheet 10 of this embodiment is obtained, in which a pattern that becomes fiber aggregation portions 2 and depressions 3 appears on the surface 10a (the machined or polished surface).

[0097] (Re-pressing process) If necessary, the surface 10a (machined or polished surface) of the heat-transfer-suppressing sheet 10 obtained above may be re-pressed. By re-pressing after the surface processing step, the surface 10a of the heat-transfer-suppressing sheet 10 becomes more flat, but the pattern consisting of the fiber aggregation portions 2 and depressions 3 still remains.

[0098] The method for manufacturing the heat transfer-suppressing sheet has been described above.

[0099] The following explains why cutting or polishing the surface 30a of the heat-transfer-suppressing sheet precursor 30 results in fiber aggregation sections 2 formed by a plurality of entangled organic fibers 1 and recesses 3 recessed below the fiber aggregation sections 2 on the surface 10a (cut or polished surface) of the heat-transfer-suppressing sheet 10. As shown in FIG. 4(A), when cutting or polishing the surface 30a of the heat-transfer-suppressing sheet precursor 30, the fiber aggregation sections 2 formed by a plurality of entangled organic fibers 1 present inside the heat-transfer-suppressing sheet precursor 30 are strengthened by this entanglement and are therefore unlikely to be cut parallel to the surface 30a of the heat-transfer-suppressing sheet precursor 30 by the cutting or polishing process.

[0100] 4(B), the cutting tool 35 moves along the surface of the fiber aggregation portions 2 without cutting the fiber aggregation portions 2 themselves, resulting in the formation of fiber aggregation portions 2 and recesses 3 on the surface 10a of the heat-transfer-suppressing sheet 10 and the surface 20a of the waste material 20, which are formed after polishing or cutting. As a result, the surface 10a (cut or polished surface) of the heat-transfer-suppressing sheet 10 has fiber aggregation portions 2 formed by entangled organic fibers 1, and recesses 3 recessed below the fiber aggregation portions 2.

[0101] [Battery pack] 5 is a schematic diagram showing a battery pack including a heat-transfer-suppressing sheet 10 of this embodiment. As shown in the figure, the battery pack 100 includes a plurality of battery cells 110a, 110b, and 110c, and a heat-transfer-suppressing sheet 10. The plurality of battery cells 110a, 110b, and 110c are connected in series or parallel. The heat-transfer-suppressing sheet 10 is interposed between the battery cells 110a, 110b, and 110c.

[0102] In the battery pack 100 configured in this manner, even if a certain battery cell (e.g., 110a) experiences thermal runaway, the heat transfer suppression sheet 10, which has a heat transfer suppression effect, is present between the battery cell 110b and the adjacent battery cell 110b, thereby preventing the fire from spreading to the battery cell 110b.

[0103] Although not shown, the heat transfer-suppressing sheet 10 may be disposed between the battery cells 110a, 110b, and 110c and the battery case 120, or may be attached to the inner surface of the battery case 120. [Example]

[0104] (Test 1) Here, the flexibility of the heat-transfer-suppressing sheet precursor and the heat-transfer-suppressing sheet was evaluated. The heat transfer-suppressing sheet precursor prepared contained dry silica and titania as inorganic particles and binder fibers with a core-sheath structure 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)

[0105] A comparative sample with a density of approximately 3.0 g / cm 3 and a high density product (Comparative Example 1) with a density of approximately 2.0 g / cm 3 A low-density product (Comparative Example 2) was prepared. In both cases, the heat-transfer-suppressing sheet precursor was used as is, without being subjected to cutting processing as in the samples of the Examples described below. The blending materials of the heat-transfer-suppressing sheet precursor used as the sample of the Comparative Example were the same as those in the Examples.

[0106] As an example sample, the density is about 2.0 g / cm 3A machined surface was formed on the heat-transfer-suppressing sheet precursor by cutting as shown in Fig. 3(B) In Example 1, only one surface of the heat-transfer-suppressing sheet precursor was machined, while in Example 2, both surfaces of the heat-transfer-suppressing sheet precursor were machined.

[0107] In addition, the thickness of the samples was set to about 2 mm in both the comparative examples (Comparative Examples 1 and 2) and the examples (Examples 1 and 2).

[0108] Then, for each sample, the pressure was gradually increased from no pressure (i.e., 0 MPa) to 1 MPa, and the change in compression ratio (%) was determined. The results are shown in Figure 6. From a comparison between Comparative Example 1 (high-density product) and Comparative Example 2 (low-density product), it can be seen that the higher the density, the steeper the slope of the "compression ratio - pressure" curve, i.e., the lower the flexibility. Furthermore, as in Example 1 (machined surface on only one side) and Example 2 (machined surfaces on both sides), it can be seen that forming a machined surface increases flexibility, and that a machined surface on both sides is more flexible than a machined surface on only one side.

[0109] (Test 2) Here, we evaluated whether or not the item had been re-pressed.

[0110] Figure 7(A) is a photograph of the machined surface of the sample of Example 1 before re-pressing, and clearly shows a pattern of streaky fiber aggregates surrounding the recesses. Even after re-pressing the machined surface of the same sample, the pattern before re-pressing still remains, as shown in Figure 7(B). [Explanation of symbols]

[0111] 1. Organic Fiber 2 Fiber aggregation area 3 recess 4 Inorganic particles 7 Fiber bundles 10 Heat transfer suppression sheet 10a,20a,30a surface 20 Waste materials 21 Virtual Frame 30 Heat transfer suppression sheet precursor 35 Cutting tools 100 battery packs 110a, 110b, 110c battery cells 120 Battery Case

Claims

1. A heat transfer-suppressing sheet including inorganic particles and organic fibers, A heat transfer-suppressing sheet, characterized in that the sheet has, on its surface, fiber aggregation portions formed by a plurality of entangled organic fibers, and recesses recessed from the fiber aggregation portions.

2. The heat transfer suppressing sheet according to claim 1 , wherein the recessed portion forms a groove.

3. The heat transfer suppressing sheet according to claim 1 , wherein the recessed portion has a lower fiber density than the fiber aggregation portion.

4. The heat transfer suppressing sheet according to claim 1 , wherein the fiber aggregation portions are continuous stripes.

5. The heat transfer suppressing sheet according to claim 1 , wherein at least a portion of the recess is surrounded by the fiber aggregation portion.

6. 2. The heat transfer-suppressing sheet according to claim 1, wherein the inorganic particles are particles made of at least one inorganic material selected from the group consisting of oxide particles, carbide particles, nitride particles, and inorganic hydrate particles.

7. The heat transfer-suppressing sheet according to claim 6, wherein the inorganic particles include at least one type of particles selected from dry silica particles and silica aerogel.

8. 8. The heat transfer-suppressing sheet according to claim 7, wherein the inorganic particles further include at least one type of particles selected from the group consisting of titania, zircon, zirconia, silicon carbide, zinc oxide, and alumina.

9. a precursor preparation step of preparing a heat transfer-suppressing sheet precursor using a mixture containing inorganic particles and binder fibers having a core-sheath structure; a surface processing step of cutting or polishing at least one of the front and back surfaces of the heat-transfer-suppressing sheet precursor prepared in the precursor preparation step.

10. The method for producing a heat-transfer-suppressing sheet according to claim 9 , wherein the precursor-producing step produces the heat-transfer-suppressing sheet precursor by a dry process.

11. The method for producing a heat transfer-suppressing sheet according to claim 9 or 10, further comprising a re-pressing step of pressing the sheet surface after the surface processing step.

12. 9. 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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