Heat transfer suppression sheet and assembled battery

Abstract

To provide a heat transfer suppression sheet that has excellent heat insulating properties and can absorb pressure even when pressure is applied to the heat transfer suppression sheet, thereby suppressing powder falling and maintaining excellent heat insulating properties, and to provide a battery pack having the heat transfer suppression sheet.SOLUTION: A heat transfer suppression sheet 10 contains heat insulating particles 5 and organic fibers 3. The heat insulating particles 5 include large-diameter inorganic particles 1 having an average primary particle diameter of 0.1 μm or more and silica nanoparticles 2. The heat transfer suppression sheet 10 has first regions 11 in which the large-diameter inorganic particles 1 are unevenly distributed in a portion of its main surface 10a perpendicular to its thickness direction, and second regions 12 in which the large-diameter inorganic particles 1, silica nanoparticles 2, and organic fibers 3 are dispersed in another portion of the main surface 10a.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a heat transfer-suppressing sheet 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 proposes a thermal runaway-suppressing fire-resistant sheet containing a substrate and an inorganic particle layer. In the thermal runaway-suppressing fire-resistant sheet, the substrate contains glass fibers, moist heat-adhesive binder fibers, and fibrillated heat-resistant fibers, and the inorganic particle layer contains inorganic particles and an inorganic binder. The inorganic particle layer also has a coating layer that covers the surface of the fibers contained in the substrate and a heat-insulating layer present on at least one surface of the substrate.

[0005] Patent Document 1 describes that the thermal runaway suppressing fire-resistant sheet has an inorganic particle layer as a coating layer and an inorganic particle layer as a shielding layer, and therefore the sheet has fire resistance and heat insulation effects. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent Publication No. 2021-96935 Summary of the Invention [Problem to be solved by the invention]

[0007] The thermal runaway suppressing fire-resistant sheet described in Patent Document 1 contains a moisture-heat adhesive binder fiber, but the moisture-heat adhesive binder fiber needs to be in a wet state during production in order to exhibit its adhesiveness. Therefore, when a moisture-heat adhesive binder fiber is used, the fire-resistant sheet needs to be produced by a wet papermaking method.

[0008] However, when dry silica or silica aerogel, which have low thermal conductivity, are used to further improve thermal insulation performance, there is a problem that fireproof sheets cannot be produced by wet papermaking. This is because when a material containing dry silica is formed into a sheet by wet papermaking, the dry silica aggregates in the presence of water, increasing the thermal conductivity. Furthermore, since silica aerogel is generally difficult to disperse in water, when a material containing silica aerogel is formed by wet papermaking, a sheet with uniformly dispersed material cannot be obtained, resulting in a decrease in quality.

[0009] On the other hand, when inorganic particles such as dry silica or silica aerogel are used to manufacture a heat insulating sheet by a dry molding method, the inorganic particles may fall off (hereinafter also referred to as powder falling) due to pressure, impact, etc. In particular, in recent battery packs, the capacity of the battery cells has been further improved, resulting in an increased expansion rate during charging and discharging. Therefore, when a heat insulating sheet is placed between battery cells of a battery pack, if the strength of the heat insulating sheet as a whole is low, the expansion of the battery cells during charging and discharging, etc., can compress the heat insulating sheet, causing powder falling and reducing the heat insulating performance.

[0010] Thus, there is a need for research into heat-transfer-suppressing sheets that have even better thermal insulation properties and can suppress the deterioration of battery cell performance and the occurrence of powder falling due to external compression, thereby suppressing the deterioration of thermal insulation properties.

[0011] The present invention has been made in consideration of the above-mentioned problems, and has an object to provide a heat-transfer-inhibiting sheet that has excellent thermal insulation properties and can absorb pressure even when pressure is applied to the heat-transfer-inhibiting sheet, thereby suppressing powder falling and maintaining excellent thermal insulation properties, and a battery pack that includes this heat-transfer-inhibiting sheet. [Means for solving the problem]

[0012] The above object of the present invention is achieved by the heat transfer-suppressing sheet having the following configuration [1].

[0013] [1] A heat-transfer-suppressing sheet containing heat-insulating particles and organic fibers, The heat-insulating particles include large-diameter inorganic particles having an average primary particle diameter of 0.1 μm or more and silica nanoparticles, a first region in which the large-diameter inorganic particles are unevenly distributed in a part of a main surface perpendicular to the thickness direction; A heat transfer-suppressing sheet, comprising a second region in another portion of the main surface, in which the large-diameter inorganic particles, the silica nanoparticles, and the organic fibers are dispersed.

[0014] Furthermore, preferred embodiments of the present invention relating to the heat transfer-suppressing sheet relate to the following [2] to [7].

[0015] [2] The heat-transfer-suppressing sheet according to [1], wherein the second region has a surface shape that is raised relative to the surface that constitutes the first region.

[0016] [3] The heat-transfer-suppressing sheet according to [1] or [2], wherein the large-diameter inorganic particles have an average primary particle diameter of 50 μm or less.

[0017] [4] The heat-transfer-suppressing sheet according to any one of [1] to [3], wherein the large-diameter inorganic particles include at least one type of particles selected from metal oxides and metal carbides.

[0018] [5] The heat-transfer-suppressing sheet according to any one of [1] to [3], wherein the large-diameter inorganic particles include at least one type of particles selected from titania, zirconia, zircon, barium titanate, zinc oxide, alumina, and silicon carbide.

[0019] [6] The heat-transfer-suppressing sheet according to any one of [1] to [3], wherein the large-diameter inorganic particles contain titania.

[0020] [7] The heat-transfer-suppressing sheet according to any one of [1] to [6], wherein the silica nanoparticles have an average primary particle size of 1 nm or more and less than 100 nm.

[0021] The above object of the present invention is also achieved by the following configuration [8] relating to the battery pack.

[0022] [8] A battery pack comprising a plurality of battery cells and the heat-transfer-suppressing sheet according to any one of [1] to [7], the plurality of battery cells being connected in series or in parallel. [Effects of the Invention]

[0023] The heat-transfer-suppressing sheet of the present invention contains large-sized inorganic particles and silica nanoparticles, thereby achieving excellent heat insulation, and has a first region in which the large-sized inorganic particles are unevenly distributed, thereby further suppressing heat transfer in the thickness direction of the sheet. Furthermore, the heat-transfer-suppressing sheet has a second region in which the large-sized inorganic particles, silica nanoparticles, and organic fibers are dispersed, thereby reducing pressure on the surface of the heat-transfer-suppressing sheet and suppressing deterioration of heat-insulating performance due to powder falling off.

[0024] The battery pack of the present invention has a heat transfer-suppressing sheet that has excellent thermal insulation properties and the effect of retaining thermal insulation performance 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]

[0025] [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 showing an enlarged view of a portion of the surface of a heat transfer-suppressing sheet according to an embodiment of the present invention. [Figure 3] FIG. 3 is a schematic diagram showing a cross section of a heat-transfer-suppressing sheet according to an embodiment of the present invention. [Figure 4] FIG. 4 is a schematic diagram showing how infrared rays are reflected by large-diameter inorganic particles having a relatively small average primary particle size. [Figure 5] FIG. 5 is a schematic diagram showing how infrared rays are reflected by large-diameter inorganic particles having a relatively large average primary particle diameter. [Figure 6] FIG. 6 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

[0026] As a result of extensive research, the inventors have found that the above-mentioned problems can be solved by providing a heat-transfer-suppressing sheet having a surface including a first region in which large-sized inorganic particles having an average primary particle diameter equal to or greater than a predetermined value are unevenly distributed, and a second region in which large-sized inorganic particles, silica nanoparticles, and organic fibers are dispersed. Specifically, the first region on the surface of the heat-transfer-suppressing sheet improves the probability of reflecting radiant heat, which accounts for the majority of heat transfer, thereby achieving the effect of suppressing heat transfer. Furthermore, the presence of the second region in which large-sized inorganic particles, silica nanoparticles, and organic fibers are dispersed provides cushioning to the surface of the heat-transfer-suppressing sheet, thereby reducing pressure on the surface of the heat-transfer-suppressing sheet and also achieving the effect of retaining the large-sized inorganic particles, silica nanoparticles, etc., resulting in excellent thermal insulation.

[0027] The heat transfer-suppressing sheet, its manufacturing method, and battery pack according to embodiments of the present invention will be described in detail below. Note that the present invention is not limited to the embodiments described below, and can be modified as desired without departing from the spirit and scope of the present invention.

[0028] [Heat transfer suppression sheet] Fig. 1 is a photograph showing the surface of a heat-transfer-suppressing sheet according to an embodiment of the present invention, Fig. 2 is a photograph showing an enlarged portion of the surface, and Fig. 3 is a schematic diagram showing a cross section of a heat-transfer-suppressing sheet according to an embodiment of the present invention.

[0029] 1 to 3, a heat-transfer-suppressing sheet 10 according to this embodiment contains heat-insulating particles 5, which include large-diameter inorganic particles 1 and silica nanoparticles 2, and organic fibers 3. The heat-transfer-suppressing sheet 10 has, in a part of its main surface 10a perpendicular to its thickness direction, first regions 11 in which the large-diameter inorganic particles 1 are unevenly distributed. The heat-transfer-suppressing sheet 10 also has, in the main surface 10a thereof, second regions 12 in which the large-diameter inorganic particles 1, silica nanoparticles 2, and organic fibers are dispersed, in the remaining area excluding the first regions 11. In FIG. 1, the darker regions are the first regions 11, and the lighter regions are the second regions 12.

[0030] As shown in the enlarged photograph of FIG. 2, the main surface 10a of the heat-transfer-suppressing sheet 10 is formed with a sea portion consisting of the first regions 11 and island portions consisting of the second regions 12. The island portions are surrounded by other sea portions, forming a sea-island structure. The first regions 11 on the main surface 10a of the heat-transfer-suppressing sheet 10 are connected in a mesh-like shape. The island portions consisting of the second regions 12 vary in size and shape, with most of them being curved in one or more places, and some island portions having a constricted shape. Depending on the shape characteristics of the second regions 12, the mesh-like sea portion of the first regions 11 also has various shapes, such as being curved or partially constricted. These shape characteristics of the first regions 11 and the second regions 12 extend across the entire surface of the heat-transfer-suppressing sheet 10, thereby further enhancing the effects of reflecting radiant heat and mitigating pressure through cushioning properties.

[0031] 3, second regions 12 have a surface shape that is raised relative to the surface that constitutes first regions 11. The large-diameter inorganic particles 1 are not only unevenly distributed on the surface of heat-transfer-inhibiting sheet 10, but also unevenly distributed toward main surface 10a in the thickness direction of heat-transfer-inhibiting sheet 10 in a cross-sectional view.

[0032] One example of how the heat-transfer-suppressing sheet 10 can be used is by placing the heat-transfer-suppressing sheet 10 between a plurality of battery cells. Specific examples of how the heat-transfer-suppressing sheet 10 can be used are described below.

[0033] The effects of the heat transfer-suppressing sheet 10 configured as described above will be described below. Fig. 4 is a schematic diagram showing how infrared rays are reflected by large-diameter inorganic particles with a relatively small average primary particle diameter. Fig. 5 is a schematic diagram showing how infrared rays are reflected by large-diameter inorganic particles with a relatively large average primary particle diameter. Organic fibers are not shown in Figs. 4 and 5.

[0034] Large-sized inorganic particles have the effect of reflecting radiant heat, which accounts for most of the heat transfer, but the manner in which they reflect is greatly affected by the wavelength of the infrared rays and the particle size of the inorganic particles. For example, if the particle size of the large-sized inorganic particles 1 is approximately 1.5 μm, which is smaller than the wavelength of the infrared rays whose transfer is to be suppressed, the infrared rays 4 will penetrate the large-sized inorganic particles 1 and then penetrate the heat-transfer-suppressing sheet 10, as shown in FIG. 4. However, in this embodiment, because there are first regions 11 in which the large-sized inorganic particles 1 are unevenly distributed, these first regions 11 behave like inorganic particles larger than the original large-sized inorganic particles 1. In other words, the heat-transfer-suppressing sheet 10 has inorganic particles that appear larger on at least a portion of the main surface 10a, which increases the probability of reflecting the infrared rays 4. This allows heat transfer within the heat-transfer-suppressing sheet 10 to be suppressed, further improving its thermal insulation.

[0035] 5, when the particle size of the large-sized inorganic particles 1 is, for example, about 5 μm, infrared rays 4 having a wavelength similar to this particle size are reflected by the large-sized inorganic particles 1, thereby achieving a heat transfer suppression effect. On the other hand, when the wavelength of the infrared rays 4 exceeds 5 μm or when the infrared rays 4 pass through dispersed large-sized inorganic particles 1, the infrared rays 4 are reflected by the first region where the large-sized inorganic particles 1 are unevenly distributed. Therefore, as in the case shown in FIG. 4, heat transfer within the heat transfer-suppressing sheet 10 can be suppressed, further improving the heat insulation. That is, in this embodiment, heat transfer by infrared rays 4 of a wide range of wavelengths can be suppressed regardless of the particle size of the large-sized inorganic particles 1.

[0036] Regardless of which main surface of the heat-transfer-suppressing sheet 10 the heat (infrared rays 4) is generated from, the heat can be reflected as shown in Figures 4 and 5. Therefore, when the heat-transfer-suppressing sheet 10 is interposed between multiple battery cells, for example, the transfer of heat from one battery cell to another adjacent battery cell via the heat-transfer-suppressing sheet 10 can be suppressed, preventing thermal runaway.

[0037] Furthermore, in this embodiment, the island portions formed by the second regions 12 have a surface shape that is raised relative to the surface forming the sea portion formed by the first regions 11, and the raised regions (second regions 12) contain a higher amount of silica nanoparticles 2 and organic fibers 3 than the first regions 11. As a result, the second regions 12 are softer and have better cushioning properties than the first regions. Therefore, when an impact or pressure is applied to the heat-transfer-suppressing sheet 10, the raised second regions 12 absorb and mitigate the impact or pressure, and the pressure on the first regions 11 is lower than when the second regions 12 are not raised. As a result, it is possible to prevent the large-diameter inorganic particles 1 and the silica nanoparticles 2 from falling off from the main surface 10a of the heat-transfer-suppressing sheet 10, and to suppress a decrease in heat insulation properties.

[0038] As described above, in this embodiment, the organic fibers 3 are also present inside the heat transfer-suppressing sheet 10, and therefore the effect of retaining the large-diameter inorganic particles 1 and the silica nanoparticles 2 can be sufficiently obtained, and excellent sheet strength can be obtained.

[0039] The materials constituting the heat transfer-suppressing sheet according to this embodiment will be described in detail below.

[0040] <Organic fiber> The organic fibers 3 impart flexibility to the heat-transfer-suppressing sheet 10 and also have 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 fibers 3 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 the melting point 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 3, heating during production of the heat-transfer-suppressing sheet 10 melts part of the fiber surface, and subsequent cooling forms welded parts (not shown) around the organic fibers 3. The welded parts fuse the heat-insulating particles 5, including the large-diameter inorganic particles 1 and silica nanoparticles 2, to the surfaces of the organic fibers 3 and also fuse the organic fibers 3 together, so the formation of the welded parts allows for excellent sheet strength to be obtained.

[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 3. When binder fibers with a sheath-core structure are used in the production of the heat-transfer-suppressing sheet 10, adjacent binder fibers are more likely to be fused together to form fiber bundles, further increasing the sheet strength. Furthermore, when binder fibers with a sheath-core structure are used, the second organic material that constitutes the sheath melts and then solidifies again in a state that includes the surrounding heat-insulating particles 5, thereby improving the retention of the heat-insulating particles 5.

[0043] (First organic material) When binder fibers having a core-sheath structure are used as the material for the organic fibers 3, the first organic material constituting the core, i.e., the organic fibers 3, is not particularly limited as long as it has a melting point higher than that of the sheath, i.e., the second organic material, which is present on the outer surface of the organic fibers 3. The first organic material may be at least one selected from polyethylene terephthalate, polypropylene, and nylon.

[0044] (Second organic material) When binder fibers having a core-sheath structure are used as the material for the organic fibers 3, 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 fibers 3. 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, and more preferably 100° C. or higher. The melting point of the second organic material is preferably 150° C. or lower, and more preferably 130° C. or lower.

[0045] (organic fiber content) In this embodiment, if the content of the organic fibers 3 in the heat-transfer-inhibiting sheet 10 is appropriately controlled, the effects of improving the strength of the heat-transfer-inhibiting sheet 10, retaining the heat-insulating particles 5, and providing cushioning properties to the heat-transfer-inhibiting sheet 10 can be fully obtained. The content of organic fibers 3 is preferably 2% by mass or more, and more preferably 4% by mass or more, relative to the total mass of heat-transfer-suppressing sheet 10. Furthermore, if the content of organic fibers 3 is too high, the contents of large-diameter inorganic particles 1 and silica nanoparticles 2 will relatively decrease. Therefore, in order to obtain the desired heat insulating performance, the content of organic fibers is preferably 10% by mass or less, and more preferably 8% by mass or less, relative to the total mass of heat-transfer-suppressing sheet 10.

[0046] (fiber length of organic fiber) The fiber length of the organic fibers 3 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. On the other hand, from the viewpoint of improving the strength of the heat-transfer-suppressing sheet, the average fiber length of the organic fibers 3 is preferably 0.5 mm or more.

[0047] <Thermal insulating particles> The heat-transfer-suppressing sheet 10 according to this embodiment contains, as the heat-insulating particles 5, large-sized inorganic particles having an average primary particle size of 0.1 μm or more and silica nanoparticles. The combined use of inorganic particles with different heat-transfer-suppressing effects, such as large-sized inorganic particles 1 and silica nanoparticles 2, allows for multistage cooling of a heat-generating body, enabling the endothermic effect to be exerted over a wider temperature range. The silica nanoparticles and large-sized inorganic particles are described in more detail below.

[0048] <Silica nanoparticles> Silica is a highly insulating component, and nanoparticles have a low density, which has the effect of suppressing conductive heat transfer. Nanoparticles refer to particles on the order of nanometers, with a spherical or nearly spherical shape and an average primary particle diameter of less than 1 μm. When silica nanoparticles 2 are contained in the heat transfer-suppressing sheet 10, fine voids are dispersed within the sheet, resulting in excellent insulating properties that suppress convective heat transfer. Therefore, when the battery is used at normal room temperature, heat transfer between adjacent nanoparticles can be suppressed. Furthermore, when silica nanoparticles 2 are contained in heat-transfer-suppressing sheet 10, even if the heat-transfer-suppressing sheet is compressed due to expansion caused by thermal runaway of the battery cell, increasing the internal density, an increase in conductive heat transfer through the heat-transfer-suppressing sheet can be suppressed. This is thought to be because the silica nanoparticles are prone to forming tiny voids between particles due to electrostatic repulsion, and because their bulk density is low, the particles are packed together to provide cushioning.

[0049] Silica nanoparticles are a material with high thermal insulation properties, and because the contact points between particles are small, the amount of heat conducted by silica nanoparticles is smaller than when silica particles with a larger particle size are used. In addition, commonly available silica nanoparticles have a bulk density of 0.1 (g / cm 3 ), for example, even if the battery cells arranged on both sides of the heat-transfer-suppressing sheet thermally expand and a large compressive stress is applied to the heat-transfer-suppressing sheet, the size (area) and number of contact points between the silica nanoparticles do not increase significantly, and excellent heat insulation properties can be maintained. Examples of silica nanoparticles include wet silica, dry silica, and aerogel, but silica nanoparticles that are particularly suitable for this embodiment will be described below.

[0050] 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. The heat transfer-suppressing sheet according to this embodiment is preferably produced by a dry process in which a mixture containing the materials is processed into a sheet, and therefore, it is preferable to use dry silica, silica aerogel, or the like, which have low thermal conductivity, as the silica nanoparticles.

[0051] (Average primary particle size of silica nanoparticles: 1 nm or more and less than 100 nm) Limiting the average primary particle size of the silica nanoparticles to a predetermined range can achieve even higher thermal insulation. That is, when the average primary particle size of the silica nanoparticles is 1 nm or more but less than 100 nm, convective heat transfer and conductive heat transfer within the heat-transfer-suppressing sheet can be suppressed, particularly in the temperature range below 500°C, and thermal insulation can be further improved. Furthermore, even when compressive stress is applied, the voids remaining between the silica nanoparticles and the contact points between many particles suppress conductive heat transfer, allowing the thermal insulation of the heat-transfer-suppressing sheet to be maintained. The average primary particle size of the silica nanoparticles is more preferably 2 nm or more, and even more preferably 3 nm or more, while the average primary particle size of the silica nanoparticles is more preferably 50 nm or less, and even more preferably 20 nm or less.

[0052] 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.

[0053] <Large-sized inorganic particles> The heat-transfer-suppressing sheet 10 contains the silica nanoparticles 2 and large-sized inorganic particles 1 having an average primary particle size of 0.1 μm or more. The large-sized inorganic particles 1 have a high refractive index and a strong effect of diffusely reflecting light. Therefore, when the heat-transfer-suppressing sheet 10 contains the large-sized inorganic particles 1, it can suppress radiant heat transfer, particularly in high-temperature regions such as those caused by abnormal heat generation. The large-sized inorganic particles preferably contain at least one type of particle selected from metal oxides and metal carbides. Other examples of the large-sized inorganic particles 1 that can be contained in the heat-transfer-suppressing sheet 10 include inorganic hydrate particles, inorganic balloons such as microporous particles and hollow silica particles, particles made of a thermally expandable inorganic material, and particles made of a hydrated porous material. The metal oxides, metal carbides, inorganic hydrate particles, particles made of a thermally expandable inorganic material, particles made of a hydrated porous material, and inorganic balloons are described in detail below.

[0054] (metal oxide particles) Specific examples of metal oxide particles include titanium oxide (titania), zirconia, zircon, barium titanate, zinc oxide, and alumina.

[0055] (metal carbide particles) Metal carbide particles include silicon carbide.

[0056] (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).

[0057] 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

[0058] As will be described later, heat-transfer-suppressing sheet 10 according to this embodiment 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, heat-transfer-suppressing sheet 10 preferably contains an inorganic hydrate whose thermal decomposition starting 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.

[0059] (Particles made of thermally expandable inorganic material) Examples of thermally expandable inorganic materials include vermiculite, bentonite, mica, and perlite.

[0060] (Particles made of hydrous porous material) Specific examples of the hydrous porous material include zeolite, kaolinite, montmorillonite, acid clay, diatomaceous earth, mica, and vermiculite.

[0061] (inorganic balloons) The heat-transfer-suppressing sheet 10 according to this embodiment may contain inorganic balloons such as microporous particles or hollow silica particles as the heat-insulating particles 5. 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, 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.

[0062] It is preferable to use at least one type of particle selected from metal oxide particles and metal carbide particles among these large inorganic particles 1. Specifically, it is preferable to use at least one type of particle selected from titania, zirconia, zircon, barium titanate, zinc oxide, alumina, and silicon carbide as the large inorganic particles 1.

[0063] In particular, 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 the high-temperature range of 500°C or higher, so it is particularly preferable to use titania as the large-diameter inorganic particles. In this way, when large-diameter inorganic particles 1 and small-diameter silica nanoparticles 2 are contained in the heat-transfer-inhibiting sheet 10, the small-diameter silica nanoparticles 2 penetrate into the gaps between the large-diameter inorganic particles 1, resulting in a denser structure and improving the heat-transfer-inhibiting effect.

[0064] (Average primary particle size of large inorganic particles) By using large-sized inorganic particles 1 having an average primary particle diameter of 0.1 μm or more, radiant heat transfer can be efficiently suppressed in a desired high-temperature range. Furthermore, by using large-sized inorganic particles 1 having an average primary particle diameter of 1 μm or more, radiant heat transfer can be efficiently suppressed in a high-temperature range, for example, of 500°C or higher. Therefore, the average primary particle diameter of the large-sized inorganic particles 1 is set to 0.1 μm or more, preferably 1 μm or more, and more preferably 5 μm or more. Meanwhile, the average primary particle diameter of the large-sized inorganic particles 1 is preferably 50 μm or less, more preferably 30 μm or less, and even more preferably 10 μm or less.

[0065] In this embodiment, the heat transfer-suppressing sheet 10 contains large-sized inorganic particles 1 and silica nanoparticles 2. To obtain excellent heat insulating performance within a predetermined temperature range, it is preferable to appropriately adjust the contents of the large-sized inorganic particles 1 and silica nanoparticles 2. For example, to obtain excellent heat insulating performance within a temperature range of 300°C or less, the content of the silica nanoparticles 2 is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, based on the total mass of the heat insulating particles. Furthermore, the content of the silica nanoparticles 2 is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 80% by mass or less, based on the total mass of the heat insulating particles.

[0066] On the other hand, in order to obtain excellent heat insulating performance in a temperature range exceeding 300° C., the content of large inorganic particles 1 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 heat insulating particles. Moreover, the content of large inorganic particles 1 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 heat insulating particles.

[0067] (Thermal insulating particle content) In this embodiment, if the total content of the heat-insulating particles 5 in the heat-transfer-suppressing sheet 10 is appropriately controlled, the heat-insulating properties of the heat-transfer-suppressing sheet 10 can be sufficiently ensured. The total content of the heat-insulating particles 5 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 heat-insulating particles 5 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 the heat-insulating particles 5 is too high, the content of the organic fibers 3 will relatively decrease. Therefore, in order to fully obtain the sheet strength-enhancing effect of the organic fibers 3, the total content of the heat-insulating particles 5 is preferably 95% by mass or less, and more preferably 90% by mass or less, relative to the total mass of the heat-transfer-suppressing sheet 10. The content of the heat-insulating particles 5 in the heat-transfer-suppressing sheet 10 can be calculated, for example, by heating the heat-transfer-suppressing sheet 10 at 800°C, decomposing the organic components, and then measuring the mass of the remaining portion.

[0068] In this embodiment, the total content of the heat-insulating particles 5 and the organic fibers 3 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 total content of the heat-insulating particles 5 and the organic fibers 3 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 total content of the heat-insulating particles 5 and the organic fibers 3 may be 100% by mass, relative to the total mass of the heat-transfer-suppressing sheet 10.

[0069] The heat-transfer-suppressing sheet 10 according to this embodiment may contain, in addition to the organic fibers 3 and the heat-insulating particles 5, organic fibers made of an organic material different from the first organic material, 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.

[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) In this embodiment, when the heat-transfer-suppressing sheet contains inorganic fibers, the content of the inorganic fibers is preferably 3% by mass or more and 15% by mass or less relative to the total mass of the heat-transfer-suppressing sheet .

[0075] The inorganic fiber content is more preferably 5% by mass or more and 10% by mass or less of the total mass of the heat-transfer-suppressing sheet 10. This content allows the inorganic fibers to exhibit a good balance of shape retention, resistance to pressing force, resistance to wind pressure, and the ability to retain the heat-insulating particles. By appropriately controlling the inorganic fiber content, the organic fibers 3 and the inorganic fibers become entangled with each other to form a three-dimensional network, further improving the effect of retaining the heat-insulating particles and other compounded materials described below.

[0076] <Other compounding materials> The heat-transfer-suppressing sheet according to this embodiment may further contain, as needed, a binder, a colorant, etc. These are all useful for reinforcing the heat-transfer-suppressing sheet and improving its formability, and the total amount of these is preferably 10 mass % or less based on the total mass of the heat-transfer-suppressing sheet. When a core-sheath binder fiber is used as the material for producing the heat-transfer-suppressing sheet, the sheath melts during production, and only the core remains in the heat-transfer-suppressing sheet in the form of organic fiber. In this case, the content of the organic welded portion formed by the melting of the sheath is preferably 5% by mass or more and 15% by mass or less of the total mass of the heat-transfer-suppressing sheet 10.

[0077] [Method of manufacturing heat transfer suppression sheet] An example of a method for manufacturing the heat transfer-suppressing sheet 10 according to this embodiment will be described below. For example, organic fibers and insulating particles, such as silica nanoparticles and large-diameter inorganic particles, are mixed in a V-type mixer or other mixer at a predetermined ratio to produce a mixture. Preferably, the organic fibers are binder fibers with a core-sheath structure, with a core made of a first organic material and a sheath made of a second organic material. In this case, the melting point of the first organic material is higher than that of the second organic material.

[0078] The resulting mixture is then poured into a predetermined mold, and the mold is vibrated to smooth the surface, causing some of the large-diameter inorganic particles 1, which have a large mass, to move downward. The resulting molded body is then heated by applying pressure using a press or the like, thereby melting the sheaths of the binder fibers. The heated molded body is then cooled, solidifying the molten sheaths on the surface of the molded body, fusing the organic fibers 3 together and the organic fibers 3 to the heat-insulating particles 5. Depending on the type of large-diameter inorganic particles 1 selected, the second regions 12 may have a raised shape relative to the surface constituting the first regions 11 due to differences in shrinkage rates caused by heating and cooling. In this manner, a heat-transfer-suppressing sheet 10 having, on a portion of its main surface, first regions in which the large-diameter inorganic particles 1 are unevenly distributed can be produced.

[0079] In this embodiment, it is preferable to manufacture the heat-transfer-suppressing sheet 10 by a dry method. When using the dry method, dry silica nanoparticles suitable for the dry method are used as the silica nanoparticles, and a solvent such as water, which is required when forming by a wet method, is not added to the mixture. However, to prevent powder such as dry silica nanoparticles from flying around during the manufacture of the heat-transfer-suppressing sheet 10 and making the raw materials difficult to handle, a small amount of solvent such as water can be added within the range required for the dry method. For example, adding a small amount of solvent such as water to the mixture can suppress the scattering of heat-insulating particles during production.

[0080] According to the production method of this embodiment, first regions in which large-sized inorganic particles 1 are unevenly distributed are formed in parts of the main surface 10a of the heat-transfer-inhibiting sheet 10. This allows the large-sized inorganic particles 1 and the first regions 11 to reflect infrared rays 4 over a wide wavelength range, resulting in a heat-transfer-inhibiting sheet 10 with excellent heat-insulating properties. Furthermore, second regions in which large-sized inorganic particles 1, silica nanoparticles 2, and organic fibers 3 are dispersed are formed in other parts of the main surface, resulting in a heat-transfer-inhibiting sheet 10 with excellent strength and heat-insulating particle retention. Furthermore, if the second regions 12 are formed in a shape raised relative to the surface constituting the first regions 11, the pressing force on the first regions 11 is further reduced, making it possible to produce a heat-transfer-inhibiting sheet 10 that maintains excellent heat-insulating properties.

[0081] Even when organic fibers 3 that do not have a core-sheath structure are used, it is possible to obtain the effect of holding the heat-insulating particles 5. In this case, appropriate temperature control may be performed to melt only the surfaces of the organic fibers and adhere the heat-insulating particles to the surfaces, or to weld the organic fibers together. Regardless of the type of organic fiber used, when the organic fibers 3 are welded together inside or on the surface of the heat-transfer-suppressing sheet, this forms a framework that improves the strength of the sheet, thereby producing a heat-transfer-suppressing sheet 10 with excellent strength.

[0082] As will be described in detail later, an adhesive such as hot melt powder may be contained in the mixture as a raw material for the heat-transfer-suppressing sheet 10. By appropriately adjusting the type and amount of adhesive contained in the mixture, the holding power of the heat-insulating particles 5 can be improved, and powder fall-off can be further suppressed.

[0083] To further suppress powder falling, the surface of heat-transfer-suppressing sheet 10 may be coated with a film or the like. Examples of polymer films include films made of 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, polyamide, etc. The method for covering the surface of heat-transfer-suppressing sheet 10 with a film is not particularly limited, and examples include a method of attaching the film with an adhesive, 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] Next, organic fibers and heating conditions that are preferably used in the method for producing a heat transfer-suppressing sheet according to this embodiment will be described.

[0085] <Organic fiber> In this embodiment, when using organic fibers with a core-sheath structure, there are no particular limitations as long as the melting point of the first organic material constituting the core is higher than the melting point of the second organic material constituting the sheath. The first organic material constituting the core can be at least one selected from polyethylene terephthalate, polypropylene, and nylon. The second organic material constituting the sheath can be at least one selected from polyethylene terephthalate, polyethylene, polypropylene, and nylon.

[0086] If the melting point of the first organic material constituting the core is sufficiently higher than that of the second organic material constituting the sheath, the heating temperature setting margin in the heating step can be expanded, making it easier to set the temperature to obtain the desired structure. For example, the melting point of the first organic material is preferably 60°C or more higher than that of the second organic material, more preferably 70°C or more higher, and even more preferably 80°C or more higher.

[0087] Organic fibers having the above-described core-sheath structure are generally commercially available, and the materials constituting the core and sheath may be the same or different. Examples of organic 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, nylon, etc. Examples of organic 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.

[0088] In this embodiment, the melting point of the second organic material constituting the sheath of the organic fiber refers to the melting temperature at which the second organic material begins to melt and deform, but softening accompanied by a change in shape is also considered to be a type of melting deformation. The melting point of the sheath of the binder fiber can be measured, for example, by the following method. The organic fiber to be measured is placed in contact with a glass fiber having a higher melting point, and is heated from room temperature to, for example, 200°C at a temperature increase rate of 5°C / min, and then cooled to room temperature. If the surface of the organic fiber melts and deforms, and the area in contact with the glass fiber is fused, or if the cross-sectional shape of the organic fiber changes, it can be determined that the melting point of the second organic material constituting the sheath is 200°C or lower. In this embodiment, the heating temperature is varied and the state of fusion between the organic fiber and the glass fiber or the cross-sectional shape of the organic fiber after cooling is observed using the above method, thereby identifying the melting point of the second organic material constituting the sheath.

[0089] (organic fiber content) In this embodiment, when organic fibers with a core-sheath structure are used, if the content of organic fibers in the mixture is appropriately controlled, excellent heat insulating performance and heat insulating particle retention can be obtained, and excellent sheet strength can also be obtained. The content of organic fibers is preferably 5% by mass or more, more preferably 10% by mass or more, based on the total mass of the mixture. If the content of organic fibers is too high, the content of heat insulating particles will relatively decrease, so in order to obtain the desired heat insulating performance, the content of organic fibers is preferably 25% by mass or less, more preferably 20% by mass or less, based on the total mass of the mixture.

[0090] <Hot melt powder> In this embodiment, in addition to the organic fibers and heat-insulating particles, the mixture may contain a hot-melt powder. The hot-melt powder is a powder that contains, for example, a third organic material different from the first and second organic materials and has the property of melting when heated. When the hot-melt powder is contained in the mixture and heated, the hot-melt powder melts, and when cooled, it hardens in a state that includes the surrounding heat-insulating particles. This further prevents the heat-insulating particles from falling off from the heat-transfer-suppressing sheet 10.

[0091] Hot melt powders with various melting points can be used. A hot melt powder with an appropriate melting point can be selected based on the melting points of the core and sheath of the binder fiber used. Specifically, if the third organic material constituting the hot melt powder has a melting point lower than that of the first organic material constituting the organic fiber, the heating temperature can be set to melt the sheath and hot melt powder while leaving the core. For example, if the melting point of the hot melt powder is lower than that of the sheath, the heating temperature during production can be set between the melting points of the core and the sheath, making it even easier to set the heating temperature.

[0092] Alternatively, the type of hot melt powder used can be selected so that its melting point is between the melting points of the core and sheath. When a hot melt powder with such a melting point is used, the sheath and hot melt powder melt together, and then when they cool and harden, the organic fibers (core) 3, the molten sheath around them, and the hot melt powder present in the gaps between the insulating particles harden first. As a result, the position of the organic fibers 3 can be fixed, and then the molten sheath will fuse to the organic fibers, facilitating the formation of a three-dimensional skeleton. This further improves the strength of the entire sheet.

[0093] If the melting point of the third organic material constituting the hot melt powder is sufficiently lower than that of the first organic material constituting the core, the heating temperature setting latitude in the heating step can be expanded, making it easier to set the temperature to obtain the desired structure. For example, the melting point of the first organic material is preferably 60°C or more higher than that of the third organic material, more preferably 70°C or more higher, and even more preferably 80°C or more higher.

[0094] The melting point of the hot melt powder (third organic material) is preferably 80° C. or higher, and more preferably 90° C. or higher. The melting point of the hot melt powder (third organic material) is preferably 180° C. or lower, and more preferably 150° C. or lower. Components constituting the hot melt powder include polyethylene, polyester, polyamide, and ethylene vinyl acetate.

[0095] (Hot melt powder content) When hot melt powder is added to the mixture to prevent the heat insulating particles 5 from falling off, even a small amount of hot melt powder can be effective in preventing powder falling off. Therefore, the hot melt powder content is preferably 0.5% by mass or more, and more preferably 1% by mass or more, of the total mass of the mixture. On the other hand, if the content of the hot melt powder is increased, the content of the insulating particles 5 and the like will relatively decrease, so in order to obtain the desired insulating performance, the content of the hot melt powder is preferably 5 mass% or less, and more preferably 4 mass% or less, relative to the total mass of the mixture.

[0096] <Heating conditions> The process of processing the mixture into a sheet includes a process of pressing the mixture and a process of heating the mixture. When an organic 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 heat-insulating particles 5 can be held by the fused sheath.

[0097] 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.

[0098] 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.

[0099] When the heat-transfer-suppressing sheet contains a hot-melt powder as its material, the heating temperature in the heating step is preferably set at 10°C or more, and more preferably 20°C or more higher than the higher of the melting point of the second organic material constituting the sheath and the melting point of the third organic material constituting the hot-melt powder. Meanwhile, the heating temperature is preferably set at 10°C or more, and more preferably 20°C or more lower than the melting point of the first organic material constituting the core. Setting the heating temperature in this range allows for the formation of a strong skeleton, further improving the strength of the sheet and preventing the heat-insulating particles 5 from falling off.

[0100] <Thickness of heat transfer suppression sheet> The thickness of the heat-transfer-suppressing sheet 10 according to this embodiment is not particularly limited, but is preferably 0.05 mm or more and 10 mm or less. A thickness of 0.05 mm or more ensures sufficient compressive strength. On the other hand, a thickness of 10 mm or less ensures good heat insulation of the heat-transfer-suppressing sheet 10.

[0101] [Battery pack] Fig. 6 is a schematic diagram showing a battery pack including a heat-transfer-suppressing sheet according to an embodiment of the present invention. As shown in Fig. 6, a battery pack 100 includes a plurality of battery cells 20a, 20b, and 20c and a heat-transfer-suppressing sheet 10 according to this embodiment, and these plurality of battery cells 20a, 20b, and 20c are connected in series or in parallel. Specifically, the heat transfer-suppressing sheet 10 is interposed between the battery cell 20a and the battery cell 20b, and between the battery cell 20b and the battery cell 20c. The battery cells 20a, 20b, and 20c are connected in series or parallel (the connected state is not shown) and housed in the battery case 30 to form the battery pack 100. The battery cells 20a, 20b, and 20c are preferably, for example, lithium-ion secondary batteries, but are not limited thereto and may also be other secondary batteries. The heat-transfer-suppressing sheet 10 is as described above.

[0102] In the battery pack 100 configured in this manner, even if a certain battery cell 20a becomes hot, the heat transfer to the battery cell 20b can be suppressed because the heat transfer suppression sheet 10, which has an excellent heat transfer suppression effect, is present between the battery cell 20a and the battery cell 20b. Furthermore, the heat-transfer-suppressing sheet 10 according to this embodiment has high strength, shock absorption, and resistance to pressure, thereby suppressing thermal expansion of the battery cells 20a, 20b, and 20c even during charging and discharging. This ensures sufficient distance between the battery cells, maintaining excellent thermal insulation performance and preventing thermal runaway of the battery cells. Furthermore, the sheet is easy to handle because it suppresses powder shedding.

[0103] The battery pack 100 of this embodiment is not limited to the battery pack illustrated in Fig. 6. For example, the heat transfer-suppressing sheet 10 may be disposed not only between the battery cells 20a and 20b and between the battery cells 20b and 20c, but also between the battery cells 20a, 20b, and 20c and the battery case 30, or may be attached to the inner surface of the battery case 30.

[0104] In the battery pack 100 configured in this manner, if a battery cell catches fire, the flames can be prevented from spreading outside the battery case 30. For example, the battery pack 100 according to this embodiment may be used in an electric vehicle (EV) or the like and placed under the floor of a passenger area. In this case, even if a battery cell were to catch fire, the safety of the passengers can be ensured. Furthermore, the heat transfer suppression sheet 10 can be placed not only between each battery cell, but also between the battery cells 20a, 20b, 20c and the battery case 30, eliminating the need to fabricate new flame retardant materials, etc., and allowing for the easy, low-cost, and safe construction of the assembled battery 100.

[0105] In the battery pack of this embodiment, the heat-transfer-suppressing sheet 10, which is disposed between the battery cells 20a, 20b, and 20c and the battery case 30, may be in contact with the battery cells or may have gaps between them. For example, if the heat-transfer-suppressing sheet 10 is in contact with the battery cells 20a, 20b, and 20c, the heat-transfer-suppressing sheet 10 is fixed between the battery cells, preventing performance degradation due to misalignment. Furthermore, if there are gaps between the heat-transfer-suppressing sheet 10 and the battery cells 20a, 20b, and 20c, deformation of the battery cells can be tolerated even if the temperature of one of the multiple battery cells rises and the volume expands.

[0106] The heat transfer-suppressing sheet 10 according to this embodiment can be manufactured into various shapes depending on the manufacturing method. Therefore, it can be adapted to any shape, regardless of the shapes of the battery cells 20a, 20b, 20c and the battery case 30. Specifically, it can be applied to cylindrical batteries, flat batteries, etc., in addition to prismatic batteries. [Explanation of symbols]

[0107] 1. Large inorganic particles 2. Silica nanoparticles 3. Organic Fibers 4. Infrared 5. Heat-insulating particles 10 Heat transfer suppression sheet 11 First area 12 Second area 20a, 20b, 20c battery cells 30 Battery case 100 battery packs

Claims

1. A heat transfer-suppressing sheet containing heat insulating particles and organic fibers, The heat-insulating particles include large-diameter inorganic particles having an average primary particle diameter of 0.1 μm or more and silica nanoparticles, a first region in which the large-diameter inorganic particles are unevenly distributed in a part of a main surface perpendicular to the thickness direction; a heat transfer-suppressing sheet having, in another portion of the main surface, a second region in which the large-diameter inorganic particles, the silica nanoparticles, and the organic fibers are dispersed;

2. The heat transfer suppressing sheet according to claim 1 , wherein the second region has a surface shape that is raised relative to a surface that constitutes the first region.

3. 2. The heat transfer-suppressing sheet according to claim 1, wherein the large-diameter inorganic particles have an average primary particle size of 50 [mu]m or less.

4. The heat transfer-suppressing sheet according to claim 1 , wherein the large-diameter inorganic particles include at least one type of particles selected from the group consisting of metal oxides and metal carbides.

5. 2. The heat transfer-suppressing sheet according to claim 1, wherein the large-diameter inorganic particles include particles of at least one type selected from the group consisting of titania, zirconia, zircon, barium titanate, zinc oxide, alumina, and silicon carbide.

6. The heat transfer-suppressing sheet according to claim 1 , wherein the large-diameter inorganic particles contain titania.

7. The heat transfer-suppressing sheet according to claim 1 , wherein the silica nanoparticles have an average primary particle size of 1 nm or more and less than 100 nm.

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

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