Heat insulation sheet, method for manufacturing same, and battery pack

Abstract

The present invention provides: a heat insulation sheet excellent in heat insulation performance and capable of ensuring excellent heat insulation performance even when cracks occur; a method for manufacturing the same; and a battery pack having the heat insulation sheet. The heat insulation sheet contains inorganic particles and glass fibers. In a fracture test in which a plate-shaped cutting jig is pressed against a test piece (20), which is sampled from the heat insulation sheet and has a length of 50 mm on one side and another side of a main surface and a thickness of 5 mm, in a state of supporting a pair of a first end face (71) and a second end face (72), which face each other, such that the plate-shaped cutting jig is orthogonal to the main surface in a direction parallel to the first end face (71) and the second end face (72), and a load is applied until the test piece (20) fractures, selecting the cross section for which the difference between a first distance X and a second distance Y is a maximum value Z results in the maximum value Z being 4 mm or more.

Description

Heat insulating sheet, manufacturing method thereof, and battery pack

[0001] The present invention relates to a heat insulating sheet, a method for manufacturing the same, and a battery pack having the heat insulating sheet.

[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. However, if a battery cell experiences thermal runaway, where it suddenly rises in temperature and continues to generate heat 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.

[0005] For example, Patent Document 1 proposes a battery insulating material that includes an insulating section disposed opposite the surface of the battery and a buffer section that is more susceptible to compressive deformation than the insulating section, with at least a portion of the buffer section disposed closer to the battery surface than the insulating section. In this battery insulating material, when a battery cell expands and its surface protrudes toward the insulating material, the buffer section deforms in response to the deformation of the battery cell surface. Patent Document 1 describes that this insulating material can effectively prevent problems caused by excessive suppression of battery cell expansion, even if the insulating material is not susceptible to compressive deformation.

[0006] Japanese Patent Application Publication No. 2021-140968

[0007] In a battery pack installed in a vehicle, the expansion of battery cells due to charging presses against the insulating material (insulating sheet). Meanwhile, when the vehicle is running, the battery cells contract due to discharge, releasing the pressure on the insulating sheet. In this way, the battery cells repeatedly expand and contract, repeatedly pressing against and releasing the insulating sheet.

[0008] Cracks may occur in the insulation sheet due to fatigue caused by the repeated stresses mentioned above. When cracks occur, thermal conductivity increases, and if the temperature of a battery cell rises suddenly, the heat may spread to other adjacent battery cells, causing thermal runaway in those cells.

[0009] The present invention has been made in consideration of these problems, and aims to provide a heat insulating sheet that has excellent heat insulating performance and can ensure excellent heat insulating performance even when cracks occur, a method for manufacturing the same, and a battery pack including the heat insulating sheet.

[0010] The above object of the present invention is achieved by the following configuration [1] relating to a heat insulating sheet.

[0011] [1] A heat insulating sheet comprising inorganic particles and glass fibers, wherein a test piece having a thickness of 5 mm and a pair of main surfaces and end surfaces connecting the pair of main surfaces is taken from the heat insulating sheet, and a pair of opposing first and second end surfaces of the end surfaces of the test piece is supported, and a plate-shaped cutting tool is pressed against the test piece in a direction parallel to the first and second end surfaces and perpendicular to the main surfaces, and a load is applied to the test piece until the test piece breaks, wherein, in a cross-sectional view perpendicular to the first and second end surfaces, a distance from the first end surface to a fracture surface on one of the main surfaces is defined as a first distance, and a distance from the first end surface to the fracture surface on the other main surface is defined as a second distance, and when a cross-section at which the difference between the first distance and the second distance is maximum is selected, the maximum value is 4 mm or more.

[0012] Furthermore, preferred embodiments of the present invention relating to the heat insulating sheet relate to the following [2] to [6].

[0013] [2] The heat insulating sheet according to [1], characterized in that the aspect ratio calculated by dividing the average fiber length of the glass fibers by the average fiber diameter is 300 or more.

[0014] [3] The heat insulating sheet according to [1] or [2], characterized in that the content of the glass fiber is 3 mass % or more and 20 mass % or less with respect to the total mass of the heat insulating sheet.

[0015] [4] The heat insulating sheet according to any one of [1] to [3], characterized in that the inorganic particles are particles made of at least one inorganic material selected from oxide particles, carbide particles, nitride particles, and inorganic hydrate particles.

[0016] [5] The heat insulating sheet according to [4], characterized in that the inorganic particles include at least one type of particles selected from dry silica particles and silica aerogel.

[0017] [6] The heat insulating sheet according to [5], wherein the inorganic particles further include at least one type of particles selected from titania, zircon, zirconia, silicon carbide, zinc oxide, and alumina.

[0018] The above object of the present invention is achieved by the following configuration [7] relating to a method for producing a heat insulating sheet.

[0019] [7] A method for manufacturing a heat insulating sheet according to any one of [1] to [6], comprising: a mixing step of mixing the heat insulating sheet material containing the inorganic particles and the glass fibers in a mixer; and a molding step of pressing the mixed heat insulating sheet material and processing it into a sheet, wherein the mixing step involves mixing the heat insulating sheet material until a plurality of lump-shaped lumps are formed in the heat insulating sheet material.

[0020] The above object of the present invention is achieved by the following configuration [8] relating to a battery pack.

[0021] [8] A battery pack comprising a plurality of battery cells and the heat insulating sheet according to any one of [1] to [6], wherein the plurality of battery cells are connected in series or in parallel.

[0022] The heat insulating sheet of the present invention has a large maximum value Z of the fracture surface in a fracture test, so when the heat insulating sheet is broken, the heat conduction path becomes longer and the heat insulating properties can be maintained.

[0023] Furthermore, the method for producing a heat insulating sheet of the present invention includes a molding step in which the heat insulating sheet material is pressed after a stirring step in which the material is stirred until multiple lump-shaped lumps are formed, thereby forming lumpy portions in the heat insulating sheet. This makes it possible to produce a heat insulating sheet in which significant unevenness is formed along the lumpy portions when cracks occur.

[0024] Furthermore, as described above, the battery pack of the present invention has a heat insulating sheet that can maintain high heat insulating properties even when cracks occur, so that thermal runaway of the battery cells in the battery pack can be suppressed.

[0025] FIG. 1 is a schematic cross-sectional view showing a heat insulating sheet according to an embodiment of the present invention. FIG. 2 is a cross-sectional view showing a battery pack including a heat insulating sheet according to an embodiment of the present invention. FIG. 3A is a side view showing a test method for a break test. FIG. 3B is a plan view showing a test method for a break test. FIG. 4 is a cross-sectional view showing an example of a test piece after a break test. FIG. 5A is a photograph, in place of a drawing, of a test piece after a break test taken from an oblique direction. FIG. 5B is a photograph, in place of a drawing, of a test piece after a break test taken from a top direction. FIG. 6 is a cross-sectional view showing an example of a test piece after a break test in the case where no lumpy portion is present in the heat insulating sheet. FIG. 7A is a photograph, in place of a drawing, of a test piece after a break test taken from an oblique direction. FIG. 7B is a photograph, in place of a drawing, of a test piece after a break test taken from a top direction. FIG. 8A is a photograph, in place of a drawing, showing a material after a stirring step in a method for producing a heat insulating sheet according to the present embodiment. Figure 8B is a photograph showing an insulating sheet manufactured by the manufacturing method of this embodiment after a molding process. Figure 8C is a photograph showing the appearance of an insulating sheet manufactured by the manufacturing method of this embodiment after a breaking test. Figure 9A is a photograph showing an insulating sheet material after a stirring process when manufacturing an insulating sheet of a comparative example. Figure 9B is a photograph showing an insulating sheet of a comparative example. Figure 9C is a photograph showing the appearance of an insulating sheet of a comparative example after a breaking test.

[0026] The present inventors have conducted extensive research into a heat insulating sheet that can solve the above-mentioned problems. As a result, they have found that if a block is formed inside the heat insulating sheet, the sheet will break along the block when a crack occurs, thereby maintaining excellent heat insulating properties. In other words, if significant unevenness is observed when a crack occurs, the unevenness will block the crack, making it difficult for a space to form in the crack, and making it possible to maintain heat insulating properties even when a crack occurs.

[0027] The present invention has been made based on the above findings. Hereinafter, a heat insulating sheet according to an embodiment of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiment described below, and can be implemented with any modifications within the scope of the gist of the present invention.

[0028] [Thermal Insulation Sheet] Figure 1 is a schematic cross-sectional view showing a thermal insulation sheet according to an embodiment of the present invention. The thermal insulation sheet 10 according to this embodiment contains inorganic particles and glass fibers (not shown), and has aggregate portions 11 formed by entanglement of the inorganic particles and the glass fibers. Specific materials contained in the thermal insulation sheet 10 will be described in detail below. The thermal insulation sheet 10 has a pair of opposing first and second main surfaces 21 and 22, and end surfaces connecting the first and second main surfaces 21 and 22. The end surfaces are composed of a pair of opposing first and second end surfaces 31 and 32, and a pair of opposing end surfaces (not shown) perpendicular to the first and second end surfaces 31 and 32.

[0029] The structure and effects of the heat insulating sheet 10 configured as described above when applied to a battery pack will be specifically described below. Fig. 2 is a cross-sectional view schematically showing a battery pack having a heat insulating sheet according to an embodiment of the present invention. Note that part of the structure of the heat insulating sheet 10 shown in Fig. 1 is simplified in Fig. 2.

[0030] The battery pack 100 includes a battery case 30, a plurality of battery cells 20a, 20b, and 20c housed inside the battery case 30, and a heat insulating sheet 10 interposed between the battery cells 20a and 20b, and between the battery cells 20b and 20c. That is, the first main surface 21 and the second main surface 22 of the heat insulating sheet 10 are arranged to face different battery cells. The plurality of battery cells 20a, 20b, and 20c are connected in series or in parallel by bus bars (not shown) or the like. 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.

[0031] In the battery pack 100 configured in this manner, the insulating sheet 10 contains inorganic particles and has high thermal insulation properties, thereby suppressing the transfer of heat from a battery cell experiencing thermal runaway to adjacent battery cells. Furthermore, when a battery cell expands and a load is applied to the insulating sheet 10, the insulating sheet 10 may break. In this embodiment, the insulating sheet 10 has chunks 11, which cause complex-shaped cracks to form along the chunks 11, resulting in a fracture surface with significant irregularities. As a result, compared to an insulating sheet without chunks 11, the irregularities seal the cracks, making it less likely for spaces to form in the cracks, and allowing the sheet to maintain excellent thermal insulation properties.

[0032] In this embodiment, since it is difficult to actually confirm the presence of the lump portion 11 in the cross section of the heat insulating sheet, the presence of the lump portion 11 is confirmed by the following breaking test.

[0033] [Break Test] FIG. 3A is a side view showing the test method for the break test, and FIG. 3B is a plan view thereof. The break test will be described with reference to FIGS. 3A and 3B. First, a test specimen 20 for the break test is taken from the heat insulating sheet 10. The test specimen 20 has a pair of opposing first and second main surfaces 61 and 62, and a first end surface 71 and a second end surface 72 connecting the pair of main surfaces. The size of the test specimen 20 is 50 mm on one side of the main surface, 50 mm on the other side, and 5 mm thick. Furthermore, as shown in FIGS. 3A and 3B, rectangular parallelepiped first and second pedestals 41 and 42 are prepared to support the test specimen 20. Then, the pedestals are placed at an arbitrary distance apart, with the main surfaces of the first and second pedestals 41 and 42 facing each other.

[0034] Next, the second main surface 62 of the test piece 20 is placed on the first pedestal 41 and the second pedestal 42 with the second main surface 62 facing downward. At this time, the distance between the first pedestal 41 and the second pedestal 42 is adjusted to 30 mm. The dimensions of the pedestals are not particularly limited. However, it is sufficient that the first pedestal 41 and the second pedestal 42 have the same dimensions and that the first end surface 71 and the second end surface 72 of the test piece 20 are supported by the pedestals.

[0035] A cutting jig 80 is used for the fracture test. The cutting jig 80 is plate-shaped, with a thickness that decreases toward the tip, forming a tapered shape in cross-section. The thickness of the tip of the cutting jig 80 is 0.25 mm, and the tip angle in cross-section is 32°. The length of one side of the cutting jig 80 is longer than the lengths of the first end face 71 and the second end face 72 of the test piece 20 in a plan view. Then, the tip of the cutting jig 80 is pressed parallel to the first end face 71 and the second end face 72 so as to bisect the first main surface 61 of the test piece 20, and a load is applied from the first main surface 61 side toward the second main surface 62 side until the test piece 20 fractures.

[0036] Figure 4 is a cross-sectional view showing an example of a test piece after a fracture test. Figure 5A is a photograph of the test piece 20 taken from an oblique angle after the fracture test, and Figure 5B is a photograph of the test piece taken from above after the fracture test. The fracture test splits the test piece into two, forming a pair of fracture surfaces 81 and 82. As shown in Figure 4, the fracture surfaces 81 and 82 of the test piece 20 show significant irregularities, indicating that the cracks have complex shapes. This is thought to be because, as shown in Figure 1, the heat insulating sheet 10 has a block portion 11, and therefore breaks along the block portion when it breaks.

[0037] In this embodiment, the magnitude of the irregularities on the fracture surfaces 81 and 82 is measured in a cross-sectional view perpendicular to the first end face and the second end face. To determine whether excellent thermal insulation can be maintained in the event of a crack, it is preferable to define the maximum value Z as the difference between the most recessed and most protruding portions on the fracture surfaces. However, after an actual fracture test, a fracture surface that is inclined from the first main surface to the second main surface is often formed. Therefore, in this embodiment, the distance from the first end face 71 to the fracture surface 81 on the first main surface 61 of the test specimen is defined as the first distance X, and the distance from the first end face 71 to the fracture surface 81 on the second main surface 62 is defined as the second distance Y. A cross section where the absolute value of the difference between the first distance X and the second distance Y is largest is selected. The maximum value Z on that cross section is then evaluated as the magnitude of the irregularities.

[0038] A maximum value Z of less than 4 mm means that there is little unevenness on the fracture surface of the test piece. When there is little unevenness and the distance of the heat conduction path is shortened, heat conduction becomes more likely and the thermal insulation becomes poor. Therefore, the maximum value Z of the difference between the first distance X and the second distance Y in the selected cross section is preferably 4 mm or more, more preferably 4.5 mm or more, and even more preferably 5 mm or more. On the other hand, the larger the maximum value Z, the less likely it is that a space will be generated in the crack portion, improving the thermal insulation, but if the unevenness when a crack occurs becomes too large, chipping or the like will easily occur in the convex portion, which may reduce the strength of the thermal insulation sheet. Therefore, the maximum value Z of the difference between the first distance X and the second distance Y in the selected cross section is preferably 15 mm or less, more preferably 10 mm or less.

[0039] In the above-mentioned breaking test, the thickness of the test piece 20 for the breaking test is 5 mm, but if the thickness of the test piece that can be taken from the insulation sheet to be measured is less than 5 mm, excellent heat insulation properties can be maintained when cracks occur as long as the ratio calculated by dividing the maximum value Z by the thickness of the test piece is 0.8 or more. Note that if the thickness of the test piece 20 exceeds 5 mm, the test piece should be taken so that the thickness is 5 mm.

[0040] Figure 6 is a cross-sectional view showing an example of a test specimen after a fracture test when no lumps are present in the thermal insulation sheet. Figure 7A is a photograph of the test specimen 20 taken from an oblique angle after the fracture test, and Figure 7B is a photograph of the test specimen taken from above after the fracture test. As with the case where lumps are present in the thermal insulation sheet, the fracture test splits the test specimen into two, forming a pair of fracture surfaces 81 and 82. As shown in Figures 6, 7A, and 7B, the fracture surfaces 81 and 82 of the test specimen 20 have fine irregularities, but the crack shape is linear from the first main surface 61 to the second main surface 62 of the test specimen 20. This is thought to be because the test specimen 20 does not have a lumpy portion 11 and therefore fractures along the position where stress is applied by the cutting tool. Therefore, compared to the case where lumps are present in the thermal insulation sheet, spaces are more likely to form in the cracked area. As a result, the thermal insulation properties are reduced when a crack occurs.

[0041] The materials of the heat insulating sheet 10 according to this embodiment will be described in detail below.

[0042] <Inorganic Particles> The heat insulating sheet contains inorganic particles. A single inorganic particle may be used as the inorganic particle, or two or more types of inorganic particles may be used in combination. 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. Particles containing multiple metal oxides as the main component may also be used. The shape of the inorganic particles is not particularly limited, but it is preferable to include at least one type selected from nanoparticles, hollow particles, porous particles, and scale-like particles. Specific examples of inorganic particles that can be used 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. Furthermore, particles made of mica may also be used as the inorganic particles.

[0043] 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. 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.

[0044] In addition, when two or more inorganic particles having different heat transfer suppression effects are used in combination, the heat generating body can be cooled 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 being referred to as the first inorganic particles and the large-diameter inorganic particles being referred to as the second inorganic particles.

[0045] <First Inorganic Particles> (Oxide Particles) Oxide particles have a high refractive index and a strong effect of diffusely reflecting light. Therefore, when oxide particles are used as the first inorganic particles, radiant heat transfer can be suppressed, particularly in high-temperature regions such as abnormal heat generation. As the oxide particles, at least one type of particle selected from silica, titania, zirconia, zircon, barium titanate, zinc oxide, and alumina can be used. 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.

[0046] (Average primary particle diameter of oxide particles: 0.001 μm or more and 50 μm or less) The particle diameter of the oxide particles can affect the effect of reflecting radiant heat, so limiting the average primary particle diameter to a predetermined range can achieve even higher thermal insulation. That is, when the average primary particle diameter of the oxide particles is 0.001 μm or more, the particles are sufficiently larger than the wavelength of light that contributes to heating, and efficiently diffusely reflect light, thereby suppressing radiant heat transfer within the thermal insulation sheet in high temperature ranges of 500°C or more, thereby further improving thermal insulation. On the other hand, when the average primary particle diameter of the oxide particles is 50 μm or less, the number and number of contact points between particles do not increase even when compressed, making it difficult to form a path for conductive heat transfer, thereby reducing the impact on thermal insulation, especially in normal temperature ranges where conductive heat transfer is dominant.

[0047] 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.

[0048] (Nanoparticles) In the present invention, nanoparticles refer to particles on the nanometer order that are spherical or nearly spherical and have an average primary particle diameter of less than 1 μm. Nanoparticles have a low density, which suppresses conductive heat transfer. Using nanoparticles as the first inorganic particles results in the dispersion of even finer voids, providing excellent thermal insulation by suppressing convective heat transfer. Therefore, nanoparticles are preferred because they can suppress heat conduction between adjacent nanoparticles during normal battery operation at room temperature. Furthermore, using nanoparticles with a small average primary particle diameter as oxide particles can suppress an increase in conductive heat transfer through the insulating sheet, even when the insulating sheet is compressed by expansion due to thermal runaway in the battery cell, increasing its internal density. This is thought to be because nanoparticles are prone to forming fine voids between particles due to electrostatic repulsion, and their low bulk density allows the particles to be packed together to provide cushioning.

[0049] In the present invention, when nanoparticles are used as the first inorganic particles, there are no particular limitations on the material as long as they comply with the definition of nanoparticles. For example, silica nanoparticles are a material with high heat insulating properties, and the contact points between particles are small, so the amount of heat conducted by silica nanoparticles is smaller than when silica particles with a large particle diameter are used. Furthermore, commonly available silica nanoparticles have a bulk density of 0.1 (g / cm ３ ), for example, even if the battery cells arranged on both sides of the heat insulating sheet thermally expand and a large compressive stress is applied to the heat insulating sheet, the size (area) and number of contact points between the silica nanoparticles do not increase significantly, and heat insulating 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 silica nanoparticles that are particularly suitable for this embodiment will be described below.

[0050] Generally, wet silica particles are aggregated, whereas dry silica particles can be dispersed. Since conductive heat transfer is dominant in the temperature range of 300°C or less, dry silica, which allows particles to be dispersed, can provide superior heat insulation performance compared to wet silica. It is preferable to use a manufacturing method in which a mixture containing materials is processed into a sheet by a dry process for the heat insulation sheet according to this embodiment. Therefore, it is preferable to use dry silica, silica aerogel, or the like, which has low thermal conductivity, as the inorganic particles.

[0051] (Average primary particle diameter of nanoparticles: 1 nm or more and 100 nm or less) Limiting the average primary particle diameter of nanoparticles to a predetermined range can achieve even higher thermal insulation. That is, when the average primary particle diameter of nanoparticles is 1 nm or more and 100 nm or less, convective heat transfer and conductive heat transfer within the heat insulating sheet can be suppressed, particularly in the temperature range below 500°C, thereby further improving the thermal insulation. Furthermore, even when compressive stress is applied, the voids remaining between the nanoparticles and the contact points between many particles suppress conductive heat transfer, thereby maintaining the thermal insulation properties of the heat insulating sheet. The average primary particle diameter of the nanoparticles is more preferably 2 nm or more, and even more preferably 3 nm or more. On the other hand, the average primary particle diameter of the nanoparticles is more preferably 50 nm or less, and even more preferably 10 nm or less.

[0052] (Inorganic hydrate particles) When inorganic hydrate particles receive heat from a heating element and reach a temperature above the thermal decomposition initiation temperature, they undergo thermal decomposition, releasing their own water of crystallization to lower the temperature of the heating element and its surroundings, thereby exhibiting the so-called "endothermic effect." After releasing the water of crystallization, they become porous, and exhibit heat insulating properties due to the countless air holes. 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 ), gallium hydroxide (Ga(OH) ３ ) etc.

[0053] For example, aluminum hydroxide contains 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 (Al ２ O ３ ) and functions as a heat insulating sheet. ３ →Al ２ O ３ +3H２ O

[0054] As described above, the heat insulating sheet 10 according to this embodiment is preferably interposed between battery cells, for example. In a battery cell that has experienced thermal runaway, the temperature rises rapidly to over 200°C and continues to rise to around 700°C. Therefore, the inorganic particles contained in the heat insulating sheet 10 are preferably inorganic hydrates with a thermal decomposition onset temperature of 200°C or higher. The thermal decomposition onset 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. These inorganic hydrates are preferred because they can efficiently suppress temperature rises and are within the temperature range of the rapid temperature rise of a battery cell that has experienced thermal runaway.

[0055] (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 the average particle diameter is too large, it will take some time for the first inorganic particles (inorganic hydrate) near the center of the heat insulating sheet 10 to reach their thermal decomposition temperature, and the first inorganic particles near the center of the heat insulating sheet 10 may not be completely thermally decomposed. For this reason, the average secondary particle diameter 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.

[0056] (Particles Made of Thermally Expandable Inorganic Material) Examples of thermally expandable inorganic materials include vermiculite, bentonite, mica, and perlite.

[0057] (Particles Made of Hydrous Porous Material) Specific examples of hydrous porous materials include zeolite, kaolinite, montmorillonite, acid clay, diatomaceous earth, wet silica, dry silica, aerogel, mica, and vermiculite.

[0058] (Inorganic Balloons) The heat insulating sheet used in the present invention may contain inorganic balloons as the first inorganic particles. When inorganic balloons are contained, convective or conductive heat transfer within the heat insulating sheet can be suppressed in a temperature range of less than 500°C, thereby further improving the heat insulating properties of the heat insulating sheet. As the inorganic balloons, at least one selected from shirasu balloons, silica balloons, fly ash balloons, barite balloons, and glass balloons can be used.

[0059] (Inorganic Balloon Content: 60% by Mass or Less, Based on the Total Mass of the Heat Insulating Sheet) The inorganic balloon content is preferably 60% by mass or less, based on the total mass of the heat insulating sheet.

[0060] (Average Particle Diameter of Inorganic Balloons: 1 μm or More and 100 μm or Less) The average particle diameter of the inorganic balloons is preferably 1 μm or more and 100 μm or less.

[0061] <Second inorganic particles> When two types of inorganic particles are contained in the heat insulating sheet, 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.

[0062] Nanoparticles have extremely low conductive heat transfer and can maintain excellent heat insulation even when compressive stress is applied to the heat insulating sheet. Metal oxide particles such as titania are highly effective at blocking radiant heat. Furthermore, when large-sized inorganic particles and small-sized inorganic particles are used, the small-sized inorganic particles penetrate into the gaps between the large-sized 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 larger in diameter than the first inorganic particles in the heat insulating sheet. Examples of metal oxides include silicon oxide, titanium oxide, aluminum oxide, barium titanate, zinc oxide, zirconium oxide, and the like. Titanium oxide (titania) has a higher refractive index than other metal oxides and is highly effective at scattering light and blocking radiant heat at high temperatures above 500°C, making titania the most preferable choice.

[0063] When at least one type of particles selected from dry silica particles and silica aerogel is used as the first inorganic particles and at least one type of particles selected from titania, zircon, zirconia, silicon carbide, zinc oxide, and alumina is used as the second inorganic particles, in order to obtain excellent heat insulating performance within a temperature range of 300° C. or less, the content of the first inorganic particles 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 inorganic particles. Furthermore, the content of the first inorganic particles 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 inorganic particles.

[0064] 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, and 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.

[0065] (Average primary particle diameter of second inorganic particles) When second inorganic particles made of a metal oxide are contained in a heat insulating sheet, if the average primary particle diameter 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 diameter of the second inorganic particles is more preferably 5 μm or more and 30 μm or less, and most preferably 10 μm or less.

[0066] (Inorganic Particle Content) In this embodiment, if the total content of inorganic particles in the heat insulating sheet 10 is appropriately controlled, sufficient heat insulating properties can be ensured. The total content of inorganic particles is preferably 60% by mass or more, and more preferably 70% by mass or more, relative to the total mass of the heat insulating sheet 10. Here, if the content of glass fiber, which will be described later, is low, the glass fiber will not form clumps when stirred with the inorganic particles, and heat insulating properties cannot be maintained when cracks occur. Therefore, in order to sufficiently obtain the skeleton reinforcing effect and the inorganic particle retention effect, the total content of inorganic particles is preferably 95% by mass or less, and more preferably 90% by mass or less, relative to the total mass of the heat insulating sheet 10.

[0067] <Glass Fiber> Glass fiber is a material with excellent mechanical strength, heat resistance, chemical resistance, and electrical insulation. Due to these properties, using glass fiber in a heat insulating sheet improves the heat insulating properties of the heat insulating sheet, extends its service life, and enables its use in a wide range of environments, including areas subject to heavy loads and high temperatures. In other words, even if the heat insulating sheet is exposed to high heat in an emergency, it is resistant to melting, allowing the shape of the heat insulating sheet to be maintained. Furthermore, by incorporating glass fibers having a desired aspect ratio into the heat insulating sheet, the glass fibers and inorganic particles are easily entangled to form clumps, improving the heat insulating properties in the event of cracking. Furthermore, glass fibers also have the cost advantage of being inexpensive and easy to obtain and handle.

[0068] If the average fiber length of the glass fibers is less than 1 mm, the glass fibers are less likely to entangle with each other, which may result in a decrease in the insulating properties and mechanical strength of the insulating sheet 10 when cracks occur. Therefore, the average fiber length of the glass fibers is preferably 2 mm or more, and more preferably 3 mm or more. On the other hand, if the average fiber length of the glass fibers exceeds 10 mm, the aggregate portions may become too large, making it difficult to obtain the desired uneven shape. Therefore, the average fiber length of the glass fibers is preferably 10 mm or less.

[0069] If the average fiber diameter of the glass fibers exceeds 15 μm, entanglement of the glass fibers becomes difficult. Furthermore, solid heat transfer through the glass fibers may increase, resulting in a decrease in thermal insulation properties, and the moldability and strength of the heat insulating sheet may be impaired. Therefore, the average fiber diameter of the glass fibers is preferably 10 μm or less, and more preferably 8 μm or less. On the other hand, if the average fiber diameter of the glass fibers is less than 5 μm, the mechanical strength of the glass fibers themselves may be reduced. Furthermore, from the viewpoint of the impact on human health, the average fiber diameter of the glass fibers is preferably 5 μm or more.

[0070] In this embodiment, if the aspect ratio of the glass fiber is appropriately controlled, sufficient heat insulation can be achieved when cracks occur. The aspect ratio of glass fibers is a value calculated by dividing the average fiber length of the glass fibers by the average fiber diameter. If the length of the glass fibers is the same and the diameter is narrowed, the aspect ratio increases, and the number of fibers increases even when the amount added is the same. Therefore, when mixed with inorganic particles in the manufacturing process of the heat insulating sheet, the glass fibers tend to form clumps, which leads to improved heat insulation when cracks occur. Therefore, the aspect ratio of the glass fibers is preferably 300 or more, more preferably 350 or more, and even more preferably 400 or more.

[0071] (Glass Fiber Content) In this embodiment, the glass fiber content in the heat insulating sheet 10 is preferably 3 mass % or more and 20 mass % or less relative to the total mass of the heat insulating sheet 10 .

[0072] The glass fiber content is more preferably 5% by mass or more and 15% by mass or less of the total mass of the heat insulating sheet 10. By setting the content at this level, the shape retention, pressure resistance, wind pressure resistance, and inorganic particle retention ability of the glass fiber are well balanced. Furthermore, the glass fiber tends to form clumps when mixed with the inorganic particles, which also leads to improved insulation in the event of cracks.

[0073] <Other Compounding Materials> (Organic Fibers) When the heat insulating sheet material contains organic fibers, the organic fibers melt during the production of the heat insulating sheet, fusing the surrounding inorganic particles and glass fibers, forming a single block of heat insulating sheet. When this block is broken by applying a load, a linear fracture surface is formed, and spaces are likely to form in the cracked portions, making it impossible to maintain heat insulating properties when cracks occur. Therefore, the content of organic fibers is preferably less than 2% by mass, and it is more preferable that the block does not contain any organic fibers.

[0074] (Hot Melt Powder) In this embodiment, in addition to the inorganic particles, glass fibers, and organic fibers, a hot melt powder may be contained in the material mixture. Hot melt powder is a powder that melts 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 inorganic particles and glass fibers. Hot melt powders with various melting points can be used, and a hot melt powder with an appropriate melting point can be selected. Components that make up the hot melt powder include polyethylene, polyester, polyamide, ethylene vinyl acetate, etc.

[0075] (Hot Melt Powder Content) When hot melt powder is added to the insulating sheet material to suppress shedding (dusting), even a small amount can achieve the effect of suppressing dusting. However, as with the case of containing organic fibers, if the hot melt powder content in the insulating sheet material increases, the hot melt powder melts during the manufacturing process of the insulating sheet, welding the surrounding inorganic particles and glass fibers, and the insulating sheet becomes a single mass. As a result, a linear fracture surface is formed upon fracture, and the insulating properties cannot be maintained even when cracks occur. Therefore, when hot melt powder is added to the insulating sheet material, the hot melt powder content is preferably less than 2% by mass of the total mass of the insulating sheet material, and it is more preferable to not include hot melt powder. Furthermore, when both organic fibers and hot melt powder are added to the insulating sheet material, the total content of the hot melt powder and the organic fibers is preferably less than 2% by mass of the total mass of the insulating sheet material.

[0076] The heat insulating sheet 10 may further contain other binders, colorants, etc. as needed. These are all useful for reinforcing the heat insulating sheet 10 and improving its formability, and the total amount of these additives relative to the total mass of the heat insulating sheet 10 is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 5% by mass or less.

[0077] Thermal conductivity can be used as an index of thermal insulation effect, and in this embodiment, the thermal conductivity of the heat insulating sheet is preferably less than 1 (W / m·K), more preferably less than 0.5 (W / m·K), and even more preferably less than 0.2 (W / m·K). Furthermore, the thermal conductivity of the heat insulating sheet is more preferably less than 0.1 (W / m·K), more preferably less than 0.05 (W / m·K), and particularly preferably less than 0.02 (W / m·K). The thermal conductivity of the heat insulating sheet can be measured in accordance with the "Test method for thermal conductivity of refractories" described in JIS R 2251.

[0078] (Size of Heat Insulating Sheet) The size of the heat insulating sheet can be designed arbitrarily depending on the size of the battery cells 20a, 20b, and 20c shown in FIG.

[0079] [Method for manufacturing a heat insulating sheet] An example of a method for manufacturing a heat insulating sheet according to an embodiment of the present invention will be described below.

[0080] <Insulating Sheet Manufacturing Process> The insulating sheet manufacturing process includes the stirring process and molding process described below. These processes will be described in more detail. Fig. 8A is a photograph showing the material after the stirring process in the insulating sheet manufacturing method according to this embodiment. Fig. 8B is a photograph showing the insulating sheet after the molding process manufactured by the manufacturing method according to this embodiment.

[0081] 8A, the heat insulating sheet material 40 containing glass fibers and inorganic particles is charged into a mixer such as a V-type mixer at a predetermined ratio and stirred until the glass fibers and inorganic particles are entangled and a plurality of lump-shaped lumps 91 are formed in the heat insulating sheet material 40.

[0082] (Molding step) The mixed heat insulating sheet material is then pressed and processed into a sheet, thereby obtaining the heat insulating sheet 10.

[0083] In this embodiment, it is preferable to manufacture the heat insulating sheet 10 by a dry method. When using the dry method, inorganic particles suitable for the dry method are used, and the heat insulating sheet material does not contain solvents such as water, which are necessary when forming the heat insulating sheet by a wet method. However, to prevent powder such as inorganic particles from flying around during the manufacture of the heat insulating 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 heat insulating sheet material can suppress the scattering of inorganic particles during production.

[0084] As shown in Figure 8B, a mesh-like pattern is observed on the main surface of the heat insulating sheet 10 obtained after the molding process. This is thought to be because the glass fibers and inorganic particles that became lump-like lumps 91 after stirring became lumpy portions 11 inside the heat insulating sheet 10 as a result of the molding process, and these lumpy portions 11 then emerged as patterns on the first main surface 21 and the second main surface 22.

[0085] 8C is a photograph showing the state of the heat insulating sheet manufactured by the manufacturing method of this embodiment after a breaking test. Significant irregularities are observed on the fracture surfaces 81 and 82. In this embodiment, the heat insulating sheet is manufactured by pressing the heat insulating sheet material 40 having the lump-shaped mass 91, so when the heat insulating sheet breaks, it breaks along the lump portion 11.

[0086] [Battery Assembly] An example of a battery assembly, which is an example of a power storage device, to which the heat insulating sheet 10 according to an embodiment of the present invention is applied is as illustrated in Fig. 2 above. The configuration and effects of the battery assembly are also as described above. That is, the heat insulating sheet 10 has excellent heat insulating performance and can maintain excellent heat insulating properties even when cracks occur, making it possible to obtain a highly safe battery assembly.

[0087] The battery pack 100 of this embodiment is not limited to the battery pack illustrated in Fig. 2. For example, the heat insulating 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.

[0088] In the battery pack 100 configured in this manner, if a battery cell ignites, the flame can be prevented from spreading outside the battery case 30. Furthermore, even if the battery cells 20a, 20b, and 20c expand and crack the insulating sheet 10, the high insulating properties of the insulating sheet 10 can be maintained. For example, the battery pack 10 according to this embodiment may be used in an electric vehicle (EV) or the like and placed under the passenger floor. In this case, even if a battery cell ignites, the safety of the passengers can be ensured. Furthermore, since the insulating sheet 10 can be placed not only between the battery cells but also between the battery cells 20a, 20b, and 20c and the battery case 30, there is no need to fabricate additional flame retardant materials, and a safe battery pack 100 can be easily constructed at low cost.

[0089] EXAMPLES The heat insulating sheet of the present invention will be further described below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0090] <Production of Heat Insulating Sheet> Glass fibers having the fiber diameters and fiber lengths shown in Table 1 below, and silica and titania inorganic particles were prepared, and these heat insulating sheet materials were placed in a mixer and stirred. The stirred heat insulating sheet material was then pressed in a press and processed into a sheet to produce a heat insulating sheet. The heat insulating sheet material contained 59.5% by mass of silica, 25.5% by mass of titania, and 15% by mass of glass fiber.

[0091] <Fracture Test> Five test pieces for the fracture test were taken from two types of insulation sheets manufactured using glass fibers with different fiber diameters. Each test piece had a main surface with a side length of 50 mm, a side length of 50 mm, and a thickness of 5 mm. Then, as shown in FIGS. 3A and 3B , the test piece 20 was placed on the first base 41 and the second base 42, and the test piece 20 was fractured using a plate-shaped cutting jig 80. As shown in FIG. 4 , the distance from the first end face 71 on the first main surface 61 of the test piece to the fracture surface 81 was defined as the first distance X, and the distance from the first end face 71 on the second main surface 62 to the fracture surface 81 was defined as the second distance Y. The cross section with the largest absolute value of the difference between the first distance X and the second distance Y was selected. In this example, the cross section with the largest absolute value was selected visually. However, the cross section with the largest difference between the first distance X and the second distance Y may also be selected by photographing the shape of the fracture surface using a coordinate measuring machine or the like. Thereafter, in the selected cross section, the maximum value Z of the difference between the first distance X and the second distance Y was calculated. The aspect ratios of the glass fibers used and the calculation results of the maximum value Z are also shown in Table 1 below.

[0092]

[0093] As shown in Table 1 above, the heat insulating sheets of the invention examples were manufactured by stirring the material to form lump-shaped lumps, which were then pressed, resulting in large irregularities on the fracture surface and the maximum value Z being 5 mm to 8 mm.

[0094] 9A, 9B, and 9C are photographs showing the insulating sheet material after the stirring step in the production of the insulating sheet of the comparative example.

[0095] As shown in Figure 9A, the insulating sheet of the comparative example did not have sufficient intertwining of glass fibers and inorganic particles even after the stirring process. Therefore, compared to the inventive example, the number of lump-shaped lumps formed during the stirring process was significantly reduced. As a result, as shown in Figure 9B, no clear pattern was observed on the main surface of the insulating sheet 10 of the comparative example. Furthermore, as shown in Figure 9C, no significant irregularities were observed on the fracture surfaces 81 and 82 after the fracture test. From these results, it was found that the insulating sheet of the example fractured along the significant irregularities observed in the insulating sheet when a crack occurred, making it less likely for spaces to form in the cracked area and allowing for high thermal insulation properties to be maintained.

[0096] Although various embodiments have been described above with reference to the drawings, it goes without saying that the present invention is not limited to such examples. It is clear that a person skilled in the art can conceive of various modifications or alterations within the scope of the claims, and it is understood that these also naturally fall within the technical scope of the present invention. Furthermore, the components of the above-described embodiments may be combined in any manner without departing from the spirit of the invention.

[0097] This application is based on a Japanese patent application (Patent Application No. 2024-154221) filed on September 6, 2024, the contents of which are incorporated herein by reference.

[0098] REFERENCE SIGNS LIST 10 Heat insulating sheet 11 Lump-shaped portion 20 Test piece 20a, 20b, 20c Battery cell 21, 61 First main surface 22, 62 Second main surface 30 Battery case 31, 71 First end surface 32, 72 Second end surface 40 Heat insulating sheet material 41 First base 42 Second base 80 Cutting jig 81, 82 Fracture surface 91 Lump-shaped mass 100 Battery pack X First distance Y Second distance Z Maximum value

Claims

1. A heat insulating sheet containing inorganic particles and glass fibers, wherein a test piece having a thickness of 5 mm and a pair of main surfaces and end surfaces connecting the pair of main surfaces is taken from the heat insulating sheet, and a pair of opposing first and second end surfaces of the end surfaces of the test piece is supported, and a plate-shaped cutting tool is pressed against the test piece in a direction parallel to the first and second end surfaces and perpendicular to the main surfaces, and a load is applied until the test piece breaks, wherein, in a cross-sectional view perpendicular to the first and second end surfaces, the distance from the first end surface to the fracture surface on one main surface is defined as a first distance, and the distance from the first end surface to the fracture surface on the other main surface is defined as a second distance, and when a cross-section at which the difference between the first distance and the second distance is maximum is selected, the maximum value is 4 mm or more.

2. The heat insulating sheet according to claim 1, wherein the aspect ratio calculated by dividing the average fiber length of the glass fibers by the average fiber diameter is 300 or more.

3. The heat insulating sheet according to claim 2, characterized in that the content of the glass fibers is 3% by mass or more and 20% by mass or less relative to the total mass of the heat insulating sheet.

4. The heat insulating sheet according to claim 1, characterized in that 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.

5. The heat insulating sheet according to claim 4, wherein the inorganic particles include at least one type of particles selected from the group consisting of dry silica particles and silica aerogel.

6. The heat insulating sheet according to claim 5, 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.

7. A method for manufacturing a heat insulating sheet according to any one of claims 1 to 6, comprising: a mixing step of mixing the heat insulating sheet material containing the inorganic particles and the glass fibers in a mixer; and a molding step of pressing the mixed heat insulating sheet material and processing it into a sheet, wherein the mixing step involves mixing the heat insulating sheet material until a plurality of lump-shaped lumps are formed in the heat insulating sheet material.

8. A battery pack comprising a plurality of battery cells and the heat insulating sheet according to any one of claims 1 to 6, the plurality of battery cells being connected in series or in parallel.

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