Thermal insulation sheet and method for manufacturing the same, and battery pack

Abstract

The present invention provides an insulating sheet that has excellent thermal insulation performance and can maintain excellent thermal insulation performance even when cracks occur, a method for manufacturing the same, and a battery pack having this insulating sheet. [Solution] The heat insulating sheet contains inorganic particles and glass fibers. A test piece 20 taken from the heat insulating sheet has a main surface with a length of 50 mm on one side and a thickness of 5 mm. In a fracture test in which a plate-shaped cutting jig is pressed against the main surface in a direction parallel to the first end surface 71 and the second end surface 72, while supporting a pair of opposing first end surfaces 71 and second end surfaces 72, and a load is applied until the test piece 20 breaks, when a cross section is selected in which the difference between the first distance X and the second distance Y is the maximum value Z, the maximum value Z is 4 mm or more.

Description

Technical Field

[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.

Background Art

[0002] In recent years, from the perspective of environmental protection, the development of electric vehicles or hybrid vehicles driven by electric motors has been actively promoted. Such electric vehicles or hybrid vehicles are equipped with a battery pack in which a plurality of battery cells are connected in series or in parallel to serve as a power source for the drive electric motor.

[0003] In addition, for this battery cell, a lithium-ion secondary battery that can achieve high capacity and high output compared to lead-acid batteries, nickel-metal hydride batteries, etc. is mainly used. However, when a certain battery cell causes thermal runaway due to internal short circuit or overcharging of the battery, and the temperature rises rapidly and then continues to generate heat, the heat from the battery cell that has caused thermal runaway may be transmitted to other adjacent battery cells, causing thermal runaway of other battery cells.

[0004] As a method for suppressing the propagation of heat from a battery cell that has caused thermal runaway as described above, a method of interposing a heat insulating sheet between battery cells is generally used.

[0005] [[ID= / / ]] For example, Patent Document 1 proposes a heat insulating material for a battery that includes a heat insulating portion disposed opposite to the surface of the battery and a buffer portion that is more easily compressed and deformed than the heat insulating portion, and at least a part of the buffer portion is disposed closer to the surface of the battery than the heat insulating portion. In the above heat insulating material for a battery, when the battery cell expands and its surface protrudes toward the heat insulating material, the buffer portion follows the deformation of the surface of the battery cell and deforms. Patent Document 1 describes that according to the above heat insulating material, even when the heat insulating material is difficult to be compressed and deformed, it is possible to effectively prevent the occurrence of problems due to excessive suppression of the expansion of the battery cell.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2021-140968 [Overview of the project] [Problems that the invention aims to solve]

[0007] Incidentally, in battery packs installed in automobiles and other vehicles, the expansion of battery cells during charging presses down on the insulating material (insulating sheet). On the other hand, while the vehicle is running, the battery cells contract due to discharge, releasing the pressure on the insulating sheet. In this way, the repeated expansion and contraction of the battery cells causes repeated pressing and releasing of pressure on the insulating sheet.

[0008] As described above, fatigue due to repeated stress can cause cracks in the insulation sheet. When cracks occur, thermal conductivity increases, and if the battery cell heats up rapidly, the cracks can propagate to adjacent battery cells, potentially causing thermal runaway in those cells.

[0009] This invention has been made in view of the above problems, and aims to provide an insulating sheet that has excellent heat insulation performance and can ensure excellent heat insulation performance even when cracks occur, a method for manufacturing the same, and a battery pack having this insulating sheet. [Means for solving the problem]

[0010] The above objective of the present invention is achieved by the configuration of the heat insulating sheet described below [1].

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

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

[0013] [2] The thermal insulation 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 fibers is 3% by mass or more and 20% by mass or less with respect to the total mass of the heat insulating sheet.

[0015] [4] The thermal insulation 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 thermal insulation sheet according to [4], characterized in that the inorganic particles include at least one particle selected from dry silica particles and silica aerogel.

[0017] [6] The thermal insulation sheet according to [5], characterized in that the inorganic particles further comprise at least one particle 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] related to the method for manufacturing a heat insulation sheet.

[0019] [7] A stirring step of stirring a heat insulation sheet material containing the inorganic particles and the glass fiber with a mixer; A molding step of pressing the stirred heat insulation sheet material to form it into a sheet shape, and having, In the stirring step, stirring is performed until a plurality of lump-shaped masses are formed in the heat insulation sheet material, and the method for manufacturing a heat insulation sheet according to any one of [1] to [6].

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

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

Advantages of the Invention

[0022] The heat insulation sheet of the present invention has a large maximum value Z of the fracture surface in the fracture test. Therefore, when the heat insulation sheet breaks, the heat conduction path becomes long and the heat insulation property can be maintained.

[0023] Moreover, according to the method for manufacturing a heat insulation sheet of the present invention, after the stirring step of stirring until a plurality of lump-shaped masses are formed, since there is a molding step of pressing this heat insulation sheet material, a lump-shaped portion is formed in the heat insulation sheet. Therefore, it is possible to manufacture a heat insulation sheet in which significant unevenness is formed along the lump-shaped portion when cracks occur.

[0024] Furthermore, according to the battery pack of the present invention, as described above, since it has a heat insulation sheet that can maintain high heat insulation even when cracks occur, it is possible to suppress thermal runaway of the battery cells in the battery pack.

Brief Description of the Drawings

[0025] [Figure 1] Figure 1 is a schematic cross-sectional view showing a thermal insulation sheet according to an embodiment of the present invention. [Figure 2] Figure 2 is a schematic cross-sectional view showing a battery pack having a heat insulating sheet according to an embodiment of the present invention. [Figure 3A] Figure 3A is a side view showing the test method for fracture testing. [Figure 3B] Figure 3B is a plan view showing the test method for the fracture test. [Figure 4] Figure 4 is a cross-sectional view showing an example of a test specimen after a fracture test. [Figure 5A] Figure 5A is a photograph used as a substitute for a drawing, showing the test specimen after the fracture test, taken from an oblique angle. [Figure 5B] Figure 5B is a photograph used as a substitute for a drawing, showing the test specimen from above after the fracture test. [Figure 6] Figure 6 is a cross-sectional view showing an example of a test specimen after a fracture test when no lumps are present within the insulation sheet. [Figure 7A] Figure 7A is a photograph used as a substitute for a drawing, showing the test specimen after the fracture test, taken from an oblique angle. [Figure 7B] Figure 7B is a photograph used as a substitute for a drawing, showing the test specimen from above after the fracture test. [Figure 8A] Figure 8A is a photographic representation of the material after the stirring step in the manufacturing method of the heat-insulating sheet according to this embodiment. [Figure 8B] Figure 8B is a photograph used as a substitute for a drawing, showing a heat insulating sheet after the molding process, manufactured by the manufacturing method according to this embodiment. [Figure 8C] Figure 8C is a photograph used as a substitute for a drawing, showing the state of the thermal insulation sheet after a rupture test, manufactured by the manufacturing method according to this embodiment. [Figure 9A] Figure 9A is a photograph used as a substitute for a drawing, showing the insulation sheet material after the stirring process during the manufacturing of the comparative example insulation sheet. [Figure 9B] Figure 9B is a photograph used as a substitute for a drawing, showing an example of an insulating sheet. [Figure 9C]Figure 9C is a photograph used as a substitute for a drawing, showing the condition of the comparative example's insulation sheet after the rupture test. [Modes for carrying out the invention]

[0026] The inventors of the present invention have diligently studied thermal insulation sheets that can solve the above problems. As a result, they have found that if a lump-like portion is formed inside the thermal insulation sheet, when a crack occurs, it will break along the lump-like portion, and excellent thermal insulation properties can be maintained. In other words, if a significant unevenness is observed when a crack occurs, the unevenness will seal the crack, making it difficult for a space to form in the crack, and thus enabling the thermal insulation properties to be maintained when a crack occurs.

[0027] The present invention is based on the above findings. Hereinafter, an embodiment of the present invention, specifically a heat-insulating sheet, will be described in detail with reference to the drawings. However, the present invention is not limited to the embodiments described below, and can be modified and implemented as desired without departing from the spirit of the invention.

[0028] [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 (not shown) and glass fibers, and a clump-like portion 11 is formed in which the inorganic particles and glass fibers are intertwined with each other. The specific materials included in the thermal insulation sheet 10 will be described in detail later. The thermal insulation sheet 10 has a pair of opposing first main surfaces 21 and second main surfaces 22, and an end surface connecting the first main surface 21 and the second main surface 22. The end surface is composed of a pair of opposing first end surfaces 31 and second end surfaces 32, and a pair of opposing end surfaces (not shown) perpendicular to these first end surfaces 31 and second end surfaces 32.

[0029] The configuration and effects of applying the heat insulating sheet 10, configured as described above, to a battery pack will be explained in detail below. Figure 2 is a schematic cross-sectional view showing a battery pack having a heat insulating sheet according to an embodiment of the present invention. Note that some of the structure of the heat insulating sheet 10 shown in Figure 1 is simplified in Figure 2.

[0030] The battery pack 100 comprises a battery case 30, a plurality of battery cells 20a, 20b, and 20c housed inside the battery case 30, and an insulating sheet 10 interposed between battery cells 20a and 20b, and between battery cells 20b and 20c. That is, the first main surface 21 and the second main surface 22 of the 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 busbars or the like (not shown). The battery cells 20a, 20b, and 20c are preferably lithium-ion secondary batteries, for example, but are not limited to this and can be applied to other secondary batteries as well.

[0031] In the battery pack 100 configured in this way, the heat insulating sheet 10 contains inorganic particles and has high heat insulating properties, so it can suppress the transfer of heat from a battery cell that has experienced thermal runaway to an adjacent battery cell. Furthermore, when a battery cell expands and a load is applied to the heat insulating sheet 10, the heat insulating sheet 10 may rupture. In this embodiment, since the heat insulating sheet 10 has a block-like portion 11, a complex-shaped crack occurs along this block-like portion 11, forming a fracture surface with significant irregularities. As a result, compared to a heat insulating sheet without a block-like portion 11, the irregularities fill the crack portion, making it less likely for a space to form in the crack portion, and thus it is possible to maintain excellent heat insulating properties.

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

[0033] [Fracture Test] Figure 3A is a side view showing the fracture test method, and Figure 3B is a plan view thereof. The fracture test will be explained with reference to Figures 3A and 3B. First, a test specimen 20 for fracture testing is taken from the insulation sheet 10. The test specimen 20 has a pair of opposing first main surfaces 61 and second main surfaces 62, and a first end surface 71 and a second end surface 72 connecting these two main surfaces. The size of the test specimen 20 is such that one side of the main surface is 50 mm long, the other side is 50 mm long, and the thickness is 5 mm. Also, as shown in Figures 3A and 3B, a rectangular prism first base 41 and a second base 42 are prepared to support the test specimen 20. Then, with the main surfaces of the first base 41 and the second base 42 facing each other, the bases are set up at an arbitrary distance apart.

[0034] Next, the second main surface 62 of the test specimen 20 is placed on the first base 41 and the second base 42 with its side facing downwards. At this time, the distance between the first base 41 and the second base 42 is adjusted to 30 mm. The dimensions of the bases are not particularly limited. However, the first base 41 and the second base 42 should be the same size, and the first end face 71 and the second end face 72 of the test specimen 20 should be supported by the bases.

[0035] A cutting jig 80 is used for the fracture test. The cutting jig 80 is a plate-like structure that becomes thinner towards the tip, and has a tapered shape in cross-section. The thickness of the tip of the cutting jig 80 is 0.25 mm, and the angle of the tip in cross-section is 32°. In addition, the length of one side of the cutting jig 80 is longer than the length of the first end face 71 and the second end face 72 of the test piece 20 in a plan view. Subsequently, 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 breaks.

[0036] Figure 4 is a cross-sectional view showing an example of a specimen after a fracture test. Figure 5A is a substitute photograph of the specimen 20 after the fracture test, taken from an oblique direction, and Figure 5B is a substitute photograph of the specimen after the fracture test, taken from a top direction. As a result of the fracture test, the specimen is divided into two parts, and a pair of fracture surfaces 81 and 82 are formed. As shown in Figure 4, significant irregularities are observed on the fracture surfaces 81 and 82 of the test specimen 20, indicating that the crack shape is complex. This is thought to be because, as shown in Figure 1, the heat insulating sheet 10 has a lumpy portion 11, and when it fractures, it fractures along the lumpy portion.

[0037] In this embodiment, the size of the irregularities on the fracture surfaces 81 and 82 is measured in a cross-sectional view perpendicular to the first and second end surfaces. To determine whether excellent thermal insulation can be maintained when cracks occur, it is preferable to use the difference between the largest indentation and the largest protrusion on the fracture surface as the maximum value Z. However, after actual fracture tests, a fracture surface that slopes from the first main surface to the second main surface is often formed. Therefore, in this embodiment, the distance from the first end surface 71 to the fracture surface 81 on the first main surface 61 of the test piece is defined as the first distance X, and the distance from the first end surface 71 to the fracture surface 81 on the second main surface 62 is defined as the second distance Y. The cross-section that maximizes the absolute value of the difference between the first distance X and the second distance Y is selected. The maximum value Z on that cross-section is then evaluated as the size 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 specimen. When there is little unevenness and the distance of the heat conduction path is short, heat conduction occurs more easily and the thermal insulation performance is lower. 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, as the maximum value Z increases, spaces are less likely to form in the crack area, and the thermal insulation performance improves. However, if the unevenness at the time of crack formation becomes too large, chipping and other damage are more likely to occur in the protruding parts, 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, and more preferably 10 mm or less.

[0039] In the fracture test described above, the thickness of the test specimen 20 used for the fracture test is set to 5 mm. However, if the thickness of the test specimen that can be taken from the insulation sheet being measured is less than 5 mm, then if the ratio calculated by dividing the maximum value Z by the thickness of the test specimen is 0.8 or higher, then excellent insulation performance can be maintained when cracks occur. If the thickness of the test specimen 20 exceeds 5 mm, then a test specimen 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 substitute photograph of the test specimen 20 after the fracture test, taken from an oblique direction, and Figure 7B is a substitute photograph of the test specimen after the fracture test, taken from a top direction. Similar to the case where lumps are present in the thermal insulation sheet, the test specimen is divided into two by the above fracture test, forming a pair of fracture surfaces 81 and 82. As shown in Figures 6, 7A, and 7B, in the test specimen 20, fine irregularities are present on the fracture surfaces 81 and 82, 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 lump 11, and therefore fractures along the position where stress is applied by the cutting jig. Consequently, compared to cases where a lump is present in the insulation sheet, spaces are more likely to form in the crack area. As a result, the insulation performance at the time of crack occurrence is reduced.

[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, or a combination of two or more inorganic particles may be used. From the viewpoint of heat transfer suppression, 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 more preferably oxide particles. Particles mainly composed of multiple metal oxides can also be used. The shape of the inorganic particles is not particularly limited, but it is preferable to include at least one selected from nanoparticles, hollow particles, porous particles, and flake-like particles. Specifically, inorganic particles can 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 water-containing porous materials. Furthermore, particles made of mica can also be used as inorganic particles.

[0043] If the average secondary particle diameter of the inorganic particles is 0.01 μm or more, they are readily available and manufacturing costs can be kept down. Furthermore, if it 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] Furthermore, by using two or more inorganic particles with different heat transfer suppression effects, the heat-generating element can be cooled in multiple stages, and the endothermic effect can be exhibited over a wider temperature range. Specifically, it is preferable to use a mixture of large-diameter and small-diameter particles. For example, when nanoparticles are used as one type of inorganic particle, it is preferable to include inorganic particles made of metal oxides as the other type of inorganic particle. Below, the inorganic particles will be described in more detail, with small-diameter inorganic particles referred to as the first inorganic particles and large-diameter inorganic particles as the second inorganic particles.

[0045] <First inorganic particle> (Oxide particles) Oxide particles have a high refractive index and a strong effect of diffusely reflecting light. Therefore, using oxide particles as the first inorganic particles can suppress radiative heat transfer, especially in high-temperature regions such as abnormal heat generation. As oxide particles, at least one particle selected from silica, titania, zirconia, zircon, barium titanate, zinc oxide, and alumina can be used. That is, only one of the above oxide particles that can be used as inorganic particles may be used, or two or more oxide particles may be used. In particular, silica is a component with high thermal insulation properties, and titania is a component with a high refractive index compared to other metal oxides. Since they have a high effect of diffusely reflecting light and blocking radiant heat in high-temperature regions of 500°C or higher, it is most preferable to use silica and titania as oxide particles.

[0046] (Average primary particle size of oxide particles: 0.001 μm or more and 50 μm or less) Since the particle size of oxide particles can affect the effect of reflecting radiant heat, limiting the average primary particle size to a predetermined range can result in even higher thermal insulation. In other words, if the average primary particle diameter of the oxide particles is 0.001 μm or larger, it is sufficiently larger than the wavelength of light that contributes to heating, and efficiently diffusely reflects the light. As a result, radiative heat transfer within the insulating sheet is suppressed in the high-temperature region of 500°C or higher, further improving the insulating properties. On the other hand, if the average primary particle diameter of oxide particles is 50 μm or less, the number of contact points between particles does not increase even when compressed, making it difficult to form conductive heat transfer paths. This reduces the impact on thermal insulation, especially in the normal temperature range where conductive heat transfer is dominant.

[0047] In this invention, the average primary particle diameter can be determined by observing the particles under a microscope, comparing them to a standard scale, and taking the average of 10 arbitrary particles.

[0048] (Nanoparticles) In this invention, nanoparticles refer to particles that are spherical or nearly spherical, with an average primary particle diameter of less than 1 μm on the order of nanometers. Because nanoparticles have low density, they suppress conductive heat transfer, and when nanoparticles are used as the first inorganic particles, even finer voids are dispersed, resulting in excellent heat insulation that suppresses convective heat transfer. For this reason, it is preferable to use nanoparticles when using batteries in the normal room temperature range, as it can suppress heat conduction between adjacent nanoparticles. Furthermore, by using nanoparticles with a small average primary particle diameter as oxide particles, even if the insulating sheet is compressed due to expansion caused by thermal runaway of the battery cell, and the internal density increases, the increase in conductive heat transfer of the insulating sheet can be suppressed. This is thought to be because nanoparticles easily form fine voids between particles due to electrostatic repulsion, and because their bulk density is low, the particles are packed in a way that provides cushioning.

[0049] In this invention, when nanoparticles are used as the first inorganic particles, the material is not particularly limited as long as it conforms to the above definition of nanoparticles. For example, 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 compared to when silica particles with a larger particle size are used. Also, commonly available silica nanoparticles have a bulk density of 0.1 g / cm³. 3 Because of this, for example, even if battery cells arranged on both sides of the heat insulating sheet undergo thermal expansion and a large compressive stress is applied to the heat insulating sheet, the size (area) and number of contact points between silica nanoparticles will not increase significantly, and the 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 has aggregated particles, while dry silica allows for particle dispersion. In the temperature range below 300°C, conduction is the dominant method of heat transfer, so dry silica, which allows for particle dispersion, can provide superior thermal insulation compared to wet silica. In this embodiment, it is preferable to use a manufacturing method in which a mixture containing the materials is processed into a sheet by a dry process. Therefore, it is preferable to use dry silica, silica aerogel, or the like as the inorganic particles, as they have low thermal conductivity.

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

[0052] (Inorganic hydrate particles) Inorganic hydrate particles, when exposed to heat from a heat source and exceeding their decomposition start temperature, undergo thermal decomposition, releasing their crystalline water and lowering the temperature of the heat source and its surroundings—a phenomenon known as "endothermic action." After releasing the crystalline water, they become porous, exhibiting insulating properties through their numerous air pores. 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).

[0053] For example, aluminum hydroxide contains approximately 35% crystal water, and as shown in the formula below, it undergoes thermal decomposition to release crystal water, exhibiting an endothermic effect. After releasing the crystal water, it becomes a porous alumina (Al2O3) and functions as an insulating sheet. 2Al(OH)3 → Al2O3 + 3H2O

[0054] As described above, the heat insulating sheet 10 according to this embodiment is preferably interposed between battery cells, for example. However, in a battery cell that has experienced thermal runaway, the temperature rapidly rises to over 200°C and continues to rise to around 700°C. Therefore, it is preferable that the inorganic particles contained in the heat insulating sheet 10 consist of inorganic hydrates whose thermal decomposition initiation temperature is 200°C or higher. The thermal decomposition initiation temperatures for the inorganic hydrates listed above are approximately 200°C for aluminum hydroxide, 330°C for magnesium hydroxide, 580°C for calcium hydroxide, 200°C for zinc hydroxide, 350°C for iron hydroxide, 300°C for manganese hydroxide, 300°C for zirconium hydroxide, and 300°C for gallium hydroxide. These temperatures largely overlap with the temperature range of rapid temperature increases in battery cells experiencing thermal runaway, and can effectively suppress temperature rise, making them desirable inorganic hydrates.

[0055] (Average secondary particle diameter of inorganic hydrate particles: 0.01 μm or more and 200 μm or less) Furthermore, if inorganic hydrate particles are used as the first inorganic particles, and their average particle size is too large, it may take a certain amount of time for the first inorganic particles (inorganic hydrates) near the center of the heat insulating sheet 10 to reach their thermal decomposition temperature, resulting in the first inorganic particles near the center of the heat insulating sheet 10 not being completely decomposed. For this reason, the average secondary particle size of the inorganic hydrate particles is preferably 0.01 μm or more and 200 μm or less, and more preferably 0.05 μm or more and 100 μm or less.

[0056] (Particles made of thermally expandable inorganic material) Examples of thermally expandable inorganic materials include vermiculite, bentonite, mica, and perlite.

[0057] (Particles made of a water-containing porous material) Specific examples of water-containing porous materials include zeolite, kaolinite, montmorillonite, acid clay, diatomaceous earth, wet silica, dry silica, aerogel, mica, and vermiculite.

[0058] (Inorganic balloon) The heat insulating sheet used in the present invention may contain inorganic balloons as the first inorganic particles. The inclusion of inorganic balloons can suppress convective or conductive heat transfer within the insulation sheet at temperatures below 500°C, thereby further improving the insulation performance of the insulation sheet. As the inorganic balloon, at least one selected from shirasu balloons, silica balloons, fly ash balloons, barlite balloons, and glass balloons can be used.

[0059] (Inorganic balloon content: 60% or less by mass relative to the total mass of the insulation sheet) The inorganic balloon content is preferably 60% by mass or less relative to the total mass of the heat insulating sheet.

[0060] (Average particle size of inorganic balloons: 1 μm to 100 μm) The average particle size of the inorganic balloons is preferably between 1 μm and 100 μm.

[0061] <Second inorganic particle> When the heat insulating sheet contains two types of inorganic particles, the second inorganic particle is not particularly limited as long as it differs from the first inorganic particle in terms of material, particle size, etc. The second inorganic particle can be 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 thermally expandable inorganic materials, particles made of water-containing porous materials, etc. Details of these are as described above.

[0062] Furthermore, nanoparticles exhibit extremely low conductive heat transfer and can maintain excellent thermal insulation even when compressive stress is applied to the thermal insulation sheet. In addition, metal oxide particles such as titania have a high effect in blocking radiant heat. Moreover, by using both large-diameter and small-diameter inorganic particles, the small-diameter inorganic particles can fill the gaps between the large-diameter inorganic particles, resulting in a denser structure and improving the heat transfer suppression effect. Therefore, when nanoparticles are used as the first inorganic particles, it is preferable to further include particles made of metal oxides, which are larger in diameter than the first inorganic particles, as second inorganic particles in the thermal insulation sheet. Examples of metal oxides include silicon dioxide, titanium dioxide, aluminum oxide, barium titanate, zinc oxide, zircon, and zirconium oxide. In particular, titanium dioxide (titania) has a higher refractive index compared to 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 using titania is most preferable.

[0063] When using at least one particle selected from dry silica particles and silica aerogel as the first inorganic particle, and at least one particle selected from titania, zircon, zirconia, silicon carbide, zinc oxide, and alumina as the second inorganic particle, in order to obtain excellent thermal insulation performance within a temperature range of 300°C or less, the first inorganic particle 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 first inorganic particle 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 insulation performance in a temperature range exceeding 300°C, the amount 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. Furthermore, the amount of the second inorganic particles is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less, based on the total mass of the inorganic particles.

[0065] (Average primary particle diameter of the second inorganic particle) When a second inorganic particle consisting of a metal oxide is incorporated into the heat insulating sheet, if the average primary particle diameter of the second inorganic particle is 1 μm or more and 50 μm or less, radiant heat transfer can be efficiently suppressed in the high-temperature region of 500°C or higher. It is more preferable that the average primary particle diameter of the second inorganic particle is 5 μm or more and 30 μm or less, and most preferably 10 μm or less.

[0066] (Content of inorganic particles) In this embodiment, sufficient heat insulation can be ensured if the total content of inorganic particles in the heat insulating sheet 10 is appropriately controlled. 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. If the content of glass fibers, which will be described later, is too low, the inorganic particles will not form a lump when mixed, and the heat insulating properties when cracks occur cannot be maintained. Therefore, in order to obtain sufficient structural reinforcement and inorganic particle retention effects, 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 properties. Due to these characteristics, using glass fiber in insulation sheets improves the insulation properties and extends the lifespan of the sheet, and allows for use in a wide range of environments, including areas under heavy load or in high-temperature environments. Specifically, even if the insulation sheet is exposed to high temperatures under abnormal conditions, it is less likely to melt, thus maintaining its shape. Furthermore, by incorporating glass fiber with a desired aspect ratio into the insulation sheet, the glass fiber and inorganic particles tend to intertwine and form clumps, improving insulation performance in the event of cracking. In addition, glass fiber offers cost advantages, being inexpensive and easy to obtain and handle.

[0068] If the average fiber length of the glass fibers is less than 1 mm, entanglement between the glass fibers is unlikely to occur, which may reduce the thermal insulation performance and mechanical strength of the thermal insulation sheet 10 when cracks occur. Therefore, it is preferable that the average fiber length of the glass fibers be 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 clumpy parts may become too large, and the desired uneven shape may not be obtained. Therefore, it is preferable that the average fiber length of the glass fibers be 10 mm or less.

[0069] When the average fiber diameter of glass fibers exceeds 15 μm, entanglement between glass fibers becomes less likely. Furthermore, solid heat transfer through the glass fibers may increase, potentially leading to a decrease in thermal insulation performance, and the moldability and strength of the thermal insulation sheet may also deteriorate. Therefore, the average fiber diameter of 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 glass fibers is less than 5 μm, the mechanical strength of the glass fibers themselves may decrease. Also, from the perspective of the impact on human health, the average fiber diameter of glass fibers is preferably 5 μm or more.

[0070] In this embodiment, if the aspect ratio of the glass fibers is appropriately controlled, sufficient thermal insulation can be obtained 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 lengths of the glass fibers are made the same and the diameter is made smaller, the aspect ratio increases, and the number of fibers increases even if the amount added is the same. Therefore, when mixed with inorganic particles in the manufacturing process of the thermal insulation sheet, it is more likely to form clumps, which leads to improved thermal insulation when cracks occur. For this reason, 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% by mass or more and 20% by mass or less of the total mass of the heat insulating sheet 10.

[0072] Furthermore, it is more preferable that the glass fiber content be between 5% and 15% by mass relative to the total mass of the insulation sheet 10. This content allows for a good balance of shape retention, compressive force resistance, wind pressure resistance, and inorganic particle retention capacity provided by the glass fibers. In addition, it makes it easier to form clumps when mixed with inorganic particles, which also leads to improved insulation performance in the event of cracking.

[0073] <Other ingredients> (Organic fibers) When an insulating sheet material contains organic fibers, the organic fibers melt during the manufacturing process, fusing them with surrounding inorganic particles and glass fibers, causing the insulating sheet to form a single mass. When this mass is then subjected to stress and fractured, a linear fracture surface is formed, making it easy for gaps to form in the crack, thus preventing the insulating properties from being maintained at the time of crack formation. Therefore, it is preferable that the organic fiber content be less than 2% by mass, and even more preferable that the material contains no organic fibers at all.

[0074] (Hot melt powder) In this embodiment, in addition to the inorganic particles, glass fibers, and organic fibers mentioned above, hot melt powder may be included in the material mixture. Hot melt powder is a powder that melts when heated. By including hot melt powder in the mixture and heating it, the hot melt powder melts, and then, upon cooling, it hardens while containing the surrounding inorganic particles and glass fibers. Various types of hot melt powders have different melting points, but it is sufficient to select a hot melt powder with an appropriate melting point. Examples of components that make up hot melt powder include polyethylene, polyester, polyamide, and ethylene vinyl acetate.

[0075] (Hot melt powder content) When hot melt powder is included in the material of an insulating sheet for purposes such as suppressing the shedding (powder falloff) of inorganic particles, even a small amount of hot melt powder can be used to suppress the shedding. However, as the amount of hot melt powder in the insulating sheet material increases, similar to when organic fibers are included, the hot melt powder melts during the manufacturing of the insulating sheet, fusing it with the surrounding inorganic particles and glass fibers, causing the insulating sheet to form a single mass. As a result, a linear fracture surface is formed when the sheet breaks, and the insulating properties at the time of cracking cannot be maintained. Therefore, when hot melt powder is included in the material of an insulating sheet, the amount of hot melt powder is preferably less than 2% by mass of the total mass of the insulating sheet material, and it is even more preferable not to include hot melt powder. Furthermore, when both organic fibers and hot melt powder are included in the material of an insulating sheet, the total amount of hot melt powder and organic fibers is preferably less than 2% by mass of the total mass of the insulating sheet material.

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

[0077] Thermal conductivity can be cited as an indicator of the heat insulation effect. In this embodiment, the thermal conductivity of the heat insulation 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 insulation sheet is more preferably less than 0.1 (W / m·K), even more preferably less than 0.05 (W / m·K), and particularly preferably less than 0.02 (W / m·K). The thermal conductivity of the insulation sheet can be measured in accordance with the "Test Method for Thermal Conductivity of Refractory Materials" described in JIS R 2251.

[0078] (Size of the insulation sheet) The size of the insulation sheet can be arbitrarily designed according to the sizes of the battery cells 20a, 20b, and 20c shown in Figure 2.

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

[0080] <Insulation sheet manufacturing process> The process for manufacturing the heat-insulating sheet includes the stirring step and the molding step described below. These steps will be explained in more detail. Figure 8A is a photographic substitute for a drawing showing the material after the stirring step in the manufacturing method of the heat insulating sheet according to this embodiment. Figure 8B is a photographic substitute for a drawing showing the heat insulating sheet after the molding step, manufactured by the manufacturing method according to this embodiment.

[0081] (Agitation process) As shown in Figure 8A, a heat insulating sheet material 40 containing glass fibers and inorganic particles is put into a mixer such as a V-type mixer in a predetermined ratio and stirred. In this stirring step, the glass fibers and inorganic particles are intertwined and stirred until multiple clumps 91 are formed in the heat insulating sheet material 40.

[0082] (molding process) Subsequently, the stirred heat-insulating sheet material is pressed and processed into a sheet. This allows us to obtain 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 no solvents such as water required for molding by the wet method are added to the heat insulating sheet material. However, in order to prevent the handling of raw materials from becoming difficult due to the scattering of inorganic particles and other powders during the manufacture of the heat insulating sheet 10, a small amount of solvent such as water may be added within the scope of the dry method. For example, by adding a small amount of solvent such as water to the heat insulating sheet material, the scattering of inorganic particles during manufacture can be suppressed.

[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 formed clumps 91 after stirring became clumps 11 inside the heat-insulating sheet 10 after the molding process, and these clumps then rose to the surface of the first main surface 21 and the second main surface 22 as a pattern.

[0085] Figure 8C is a photograph used as a substitute for a drawing, showing the state of the thermal insulation sheet after a fracture test of the manufacturing method according to this embodiment. Significant irregularities are observed in the fracture surfaces 81 and 82. In this embodiment, since the thermal insulation sheet is manufactured by pressing a thermal insulation sheet material 40 having a lump-like mass 91, the fracture occurs along the lump-like portion 11.

[0086] [Battery pack] An example of a battery pack, which is an example of an energy storage device to which the heat insulating sheet 10 according to an embodiment of the present invention is applied, is illustrated in Figure 2 above. The configuration and effects of the battery pack are also as described above. In other words, 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 pack.

[0087] It should be noted that the battery pack 100 in this embodiment is not limited to the battery pack illustrated in Figure 2. For example, the heat insulating sheet 10 may be placed 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 it may be attached to the inner surface of the battery case 30.

[0088] In the battery pack 100 configured in this way, if a battery cell ignites, it is possible to suppress the spread of flames outside the battery case 30. Furthermore, even if the battery cells 20a, 20b, and 20c expand and cracks occur in the insulation sheet 10, the high insulation performance of the insulation sheet 10 can be maintained. For example, the battery pack 100 according to this embodiment may be used in electric vehicles (EVs) and placed under the passenger floor. In this case, even if the battery cells catch fire, the safety of the passengers can be ensured. Furthermore, since the heat insulating sheet 10 can be placed not only between each battery cell, but also between the battery cells 20a, 20b, and 20c and the battery case 30, there is no need to newly manufacture fire-retardant materials, and a safe battery pack 100 can be easily constructed at low cost. [Examples]

[0089] The thermal insulation 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] <Manufacturing of insulation sheets> Glass fibers having the fiber diameter and fiber length listed in Table 1 below, along with silica and titania as inorganic particles, were prepared, and these insulation sheet materials were put into a mixer and stirred. Next, the stirred insulation sheet material was pressed using a press to process it into a sheet, thereby producing an insulation sheet. The silica content in the insulation sheet material was 59.5% by mass, the titania content was 25.5% by mass, and the glass fiber content was 15% by mass.

[0091] <Fracture Test> Five test specimens for fracture testing were taken from each of two types of thermal insulation sheets manufactured using glass fibers with different fiber diameters. Each specimen had a main surface with one side length of 50 mm, the other side length of 50 mm, and a thickness of 5 mm. Then, as shown in Figures 3A and 3B, the test specimens 20 were placed on the first base 41 and the second base 42, and fractured using a plate-shaped cutting jig 80. As shown in Figure 4, the distance from the first end face 71 on the first main surface 61 of the test specimen 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 that maximized the absolute difference between the first distance X and the second distance Y was selected. In this embodiment, the cross-section that maximized the absolute value was selected by visual inspection. However, the cross-section that maximized the 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 three-dimensional measuring machine or the like. Subsequently, the maximum value Z, the difference between the first distance X and the second distance Y, was calculated for the selected cross-section. The aspect ratio of the glass fiber used and the calculation results of the maximum value Z are shown in Table 1 below.

[0092] [Table 1]

[0093] As shown in Table 1 above, the heat insulating sheet of the invention was manufactured by forming lumpy masses by stirring the material and then pressing them. As a result, the uneven shape of the fracture surface became large, and the maximum value Z reached 5 mm to 8 mm.

[0094] Figure 9A is a photograph used as a substitute for a drawing, showing the insulation sheet material after the stirring process during the manufacturing of the comparative example's insulation sheet. Figure 9B is a photograph used as a substitute for a drawing, showing the comparative example's insulation sheet. Figure 9C is a photograph used as a substitute for a drawing, showing the comparative example's insulation sheet after the fracture test.

[0095] As shown in Figure 9A, the comparative example's heat-insulating sheet did not show sufficient entanglement of glass fibers and inorganic particles even after the stirring process. Therefore, compared to the inventive example, the amount of clumpy material 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 comparative example's heat-insulating sheet 10. Furthermore, as shown in Figure 9C, no significant irregularities were observed in the fracture surfaces 81 and 82 after the fracture test. From these findings, it was found that the thermal insulation sheet of the example fractures along the significant irregularities observed in the thermal insulation sheet when cracks occur, making it difficult for spaces to form in the cracked area and thus maintaining high thermal insulation performance. [Explanation of Symbols]

[0096] 10 Insulation Sheets 11 Massive area 20 test specimens 20a, 20b, 20c battery cells 21,61 First main surface 22,62 Second main surface 30 Battery Cases 31,71 1st end face 32,72 2nd end face 40. Insulation sheet materials 41. First Pedestal 42. Second Pedestal 80 Cutting jigs 81,82 Fracture surface 91. Lump-like mass 100 battery packs X 1st distance Y 2nd distance Maximum value of Z

Claims

1. An insulating sheet containing inorganic particles and glass fibers, A test piece having a pair of main surfaces and an end surface connecting the pair of main surfaces, and with a thickness of 5 mm, is taken from the aforementioned heat insulating sheet. In a fracture test in which a plate-shaped cutting jig is pressed against the end face of the test specimen in a direction parallel to the first and second end faces and perpendicular to the main surface, while supporting a pair of opposing first and second end faces, a load is applied until the test specimen breaks, In a cross-sectional view perpendicular to the first and second end faces, the distance from the first end face to the fracture surface on one main surface is defined as the first distance, and the distance from the first end face to the fracture surface on the other main surface is defined as the second distance. When a cross-section is selected in which the difference between the first distance and the second distance is the maximum value, A heat insulating sheet characterized in that the aforementioned maximum value is 4 mm or more.

2. The heat insulating sheet according to claim 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.

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 with respect to the total mass of the heat insulating sheet.

4. The thermal insulation sheet according to claim 1, 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.

5. The thermal insulation sheet according to claim 4, characterized in that the inorganic particles include at least one particle selected from dry silica particles and silica aerogel.

6. The thermal insulation sheet according to claim 5, characterized in that the inorganic particles further comprise at least one particle selected from titania, zircon, zirconia, silicon carbide, zinc oxide, and alumina.

7. A stirring step in which the heat insulating sheet material containing the inorganic particles and the glass fibers is stirred using a mixer, The process includes a molding step in which the stirred heat-insulating sheet material is pressed and processed into a sheet, A method for manufacturing an insulating sheet according to any one of claims 1 to 6, characterized in that the stirring step is performed until a plurality of lumpy masses are formed in the insulating sheet material.

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

Images