Heat transfer suppression sheet, method for producing same, and battery pack

Abstract

Provided are: a heat transfer suppression sheet that has excellent sheet strength and makes it possible to inhibit inorganic particles from falling off, maintain excellent thermal insulation performance, and mitigate stresses from impacts, pressing force, and the like; and a battery pack having the heat transfer suppression sheet. The heat transfer suppression sheet (10) includes inorganic particles (4) and organic fibers (11) and has, on at least one of the front surface and the back surface thereof, a fiber aggregate section (2) formed by intertwining a plurality of the organic fibers (11), and a recess (3) recessed from the fiber aggregate section (2). In addition, the battery pack includes a plurality of battery cells and the heat transfer suppression sheet (10), and the plurality of battery cells are connected in series or in parallel.

Description

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

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

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

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

[0004] A common method for suppressing heat propagation from a battery cell experiencing thermal runaway is to interpose a heat insulating sheet between the battery cells. For example, Patent Document 1 describes a thermal runaway suppression fire-resistant sheet having a substrate containing glass fiber, a heat-and-moisture adhesive binder fiber, and a fibrillated heat-resistant fiber, an inorganic particle layer containing inorganic particles and an inorganic binder, the inorganic particle layer having a coating layer that covers the surface of the fiber contained in the substrate, and a heat insulating layer present on at least one surface of the substrate.

[0005] Patent Document 2 discloses a heat insulating sheet for a battery pack, which includes first particles made of silica nanoparticles and second particles made of a metal oxide. Patent Document 1 also discloses that the heat insulating sheet may include a binding material made of at least one material selected from fibers, binders, and heat-resistant resins.

[0006] Japanese Patent Publication No. 2021-96935 Japanese Patent Publication No. 2021-34278

[0007] Incidentally, thermal runaway suppressing fireproof sheets and heat insulating sheets (heat transfer suppressing sheets) are also used as spacers (heat transfer suppressing sheets 10 in FIG. 7) inserted between battery cells 110a, 110b, and 110c in a battery pack 100 as shown in FIG. 7. The battery cells 110a, 110b, and 110c expand and contract during normal charging and discharging, so the heat transfer suppressing sheets must be flexible to accommodate this expansion and contraction, and there is a strong demand for improvements in this flexibility.

[0008] The heat-transfer-suppressing sheet may also be used as an inner wall material for the battery case (battery case 30 in FIG. 7 ) of the battery pack 100, in which case the heat-insulating sheet is adhered to the battery case 30. Therefore, there is a strong demand for the heat-transfer-suppressing sheet to have high bonding strength with other components such as the battery cells and the battery case.

[0009] However, in the thermal runaway suppression fire-resistant sheet described in Patent Document 1, the blended materials in each portion of the sheet are uniform throughout, so there is no portion that relieves stress when subjected to impact, pressure, or other stress, and the sheet strength is not necessarily sufficient. In recent battery packs, the capacity of battery cells has been further improved, resulting in an increased expansion rate during charging and discharging. Therefore, for example, when placed between battery cells of a battery pack, if the sheet strength is low, the expansion of the battery cells during charging and discharging will compress the insulating sheet, causing the inorganic particles to fall off (powder), resulting in a decrease in insulating performance. Furthermore, if the battery cells experience thermal runaway, they will be subjected to significantly greater stress, and a sheet with high strength will be insufficient to withstand this.

[0010] Furthermore, in the thermal runaway suppressing fire-resistant sheet described in Patent Document 1, glass fibers are an essential compounding material, and therefore heat transfer occurs between the glass fibers, which may reduce the heat insulating performance.

[0011] The heat insulating sheet described in Patent Document 2 is also required to have greater flexibility and improved bonding strength with other members.

[0012] The present invention has been made in view of the above-mentioned problems, and has an object to provide a heat-transfer-suppressing sheet and an assembled battery including the heat-transfer-suppressing sheet that can suppress powder falling off of inorganic particles, maintain excellent heat insulating performance, have excellent flexibility that can absorb stress such as impact and pressure, have high sheet strength, and also have excellent bonding strength when stacked with other components such as battery cells and battery cases.

[0013] The above object of the present invention is achieved by the heat transfer-suppressing sheet having the following configuration [1].

[0014] [1] A heat-transfer-suppressing sheet comprising inorganic particles and organic fibers, the heat-transfer-suppressing sheet having, on at least one of its front and back surfaces, fiber aggregation portions formed by entanglement of a plurality of the organic fibers, and recesses recessed from the fiber aggregation portions.

[0015] Further, preferred embodiments of the present invention relating to the heat transfer-suppressing sheet relate to the following [2] to [6].

[0016] [2] The heat-transfer-suppressing sheet according to [1], wherein the recesses form grooves. [3] The heat-transfer-suppressing sheet according to [1] or [2], wherein the recesses have a lower fiber density than the fiber aggregation portions. [4] The heat-transfer-suppressing sheet according to any one of [1] to [3], wherein the fiber aggregation portions are continuous streaks. [5] The heat-transfer-suppressing sheet according to any one of [1] to [4], wherein at least some of the recesses are surrounded by the fiber aggregation portions. [6] The heat-transfer-suppressing sheet according to any one of [1] to [5], wherein at least one of the front and back surfaces includes a polished surface or a machined surface, and wherein the fiber aggregation portions and the recesses are located on the polished surface or the machined surface.

[0017] The above object of the present invention is achieved by the heat transfer-suppressing sheet according to the following configuration [7].

[0018] [7] A heat-transfer-suppressing sheet comprising inorganic particles and organic fibers, and at least one of the front and back surfaces of which includes a polished or machined surface.

[0019] Further, preferred embodiments of the heat-transfer-suppressing sheet of the present invention relate to the following [8] to

[10] . [8] The heat-transfer-suppressing sheet according to [7], characterized in that organic fibers are exposed and fluffed on the polished or machined surface. [9] The heat-transfer-suppressing sheet according to [7] or [8], characterized in that another member is laminated on the polished or machined surface.

[10] The heat-transfer-suppressing sheet according to any one of [1] to [9], 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.

[0020] The above object of the present invention is achieved by the following configuration

[11] relating to a method for producing a heat transfer-suppressing sheet.

[0021]

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

[0022] Furthermore, preferred embodiments of the present invention relating to a method for producing a heat transfer-suppressing sheet relate to the following

[12] to

[14] .

[0023]

[12] The method for producing a heat transfer-suppressing sheet according to

[11] , wherein binder fibers having a core-sheath structure are used as the organic fibers.

[0024]

[13] The method for producing a heat-transfer-suppressing sheet according to

[11] or

[12] , wherein the precursor-producing step produces the heat-transfer-suppressing sheet precursor by a dry method.

[14] The method for producing a heat-transfer-suppressing sheet according to any one of

[11] to

[13] , wherein the method further comprises a re-pressing step of pressing the sheet surface after the surface processing step.

[0025] The above object of the present invention is achieved by the following configuration

[15] relating to a battery pack.

[0026]

[15] A battery pack comprising a plurality of battery cells and the heat transfer-suppressing sheet according to any one of [1] to

[10] , the plurality of battery cells being connected in series or in parallel.

[0027] The heat-transfer-suppressing sheet of the present invention has, on at least one of its front and back surfaces, fiber agglomerations formed by entanglement of a plurality of organic fibers and recesses recessed below the fiber agglomerations, so that the fiber agglomerations increase the strength of the sheet, and the recesses relieve stress from external impacts, pressure, etc. As a result, powder fall of the inorganic particles is suppressed, a decrease in the heat insulating effect due to sheet deformation is prevented, and a heat-transfer-suppressing sheet that maintains excellent heat insulating performance and has excellent sheet strength can be provided.

[0028] The heat-transfer-suppressing sheet of the present invention contains inorganic particles and organic fibers, and the inorganic particles, which have excellent heat insulating properties, are well held by the organic fibers. At least one of the front and back surfaces includes a polished or machined surface, which enhances flexibility. Furthermore, when other components, such as a battery cell or a battery case, are stacked on the polished or machined surface, the surface anchor effect effectively prevents misalignment between the heat-transfer-suppressing sheet and the other components.

[0029] Furthermore, in a preferred form, when the organic fibers are exposed and fluffy on the polished or cut surface, the bonding strength is further increased when other components such as a battery cell or a battery case are laminated thereon.

[0030] According to the method for producing a heat-transfer-suppressing sheet of the present invention, it is possible to produce a heat-transfer-suppressing sheet that has high sheet strength and heat insulating properties as well as excellent flexibility as described above.

[0031] Furthermore, the battery pack of the present invention includes a heat-transfer-suppressing sheet that has the above-described high sheet strength and heat insulating performance and is highly flexible, and therefore can effectively suppress thermal runaway of the battery cells in the battery pack and the spread of flames to the outside of the battery case.

[0032] FIG. 1 is a photograph, used as a substitute for a drawing, showing the appearance of the surface of a heat-transfer-inhibiting sheet according to an embodiment of the present invention. FIG. 2 is a photograph, used as a substitute for a drawing, showing the appearance of a cross section of a heat-transfer-inhibiting sheet according to an embodiment of the present invention. FIG. 3 is a photograph, used as a substitute for a drawing, showing an enlarged region of a recessed portion on the surface (machined surface) of a heat-transfer-inhibiting sheet. FIG. 4A is a diagram illustrating a method for manufacturing a heat-transfer-inhibiting sheet, and is a schematic diagram showing the side of a heat-transfer-inhibiting sheet precursor. FIG. 4B is a diagram illustrating a method for manufacturing a heat-transfer-inhibiting sheet, and is a schematic diagram showing a surface processing step in which the surface of the heat-transfer-inhibiting sheet precursor is machined or polished. FIG. 4C is a diagram illustrating a method for manufacturing a heat-transfer-inhibiting sheet, and is a schematic diagram showing the side of the resulting heat-transfer-inhibiting sheet. FIG. 5A is a schematic diagram showing a surface processing step for explaining why fiber aggregation portions and recessed portions are formed on the surface of a heat-transfer-inhibiting sheet. FIG. 5B is a side view of a heat-transfer-inhibiting sheet and waste material obtained by the surface processing step of FIG. 5A. FIG. 6A is a photograph, used as a substitute for a drawing, showing an enlarged portion of the heat-transfer-inhibiting sheet precursor. Fig. 6B is a photograph, substituted for a drawing, showing the cross section of Fig. 6A. Fig. 7 is a schematic diagram showing a battery pack having a heat transfer-suppressing sheet according to an embodiment of the present invention. Fig. 8 is a graph showing the results of Example Test 1. Fig. 9A is a photograph, substituted for a drawing, showing the results of Example Test 2, which is a diagram showing the surface of the machined surface before re-pressing after the surface processing step. Fig. 9B is a photograph, substituted for a drawing, showing the surface of the machined surface after re-pressing after the surface processing step.

[0033] The present inventors have conducted extensive research into heat-transfer-suppressing sheets that can solve the above-mentioned problems, and as a result have found that by forming fiber aggregation portions formed by entanglement of multiple organic fibers and recesses recessed below the fiber aggregation portions on the surface of the heat-transfer-suppressing sheet, the sheet strength of the heat-transfer-suppressing sheet can be improved and high heat insulating performance can be maintained.The present inventors have also found that polishing or cutting at least one of the front and back surfaces of a heat-transfer-suppressing sheet precursor to expose and fluff the organic fibers that are a compounding material is also effective in solving the problems.

[0034] A heat transfer-suppressing sheet, a method for manufacturing the same, and a battery pack according to an embodiment of the present invention (hereinafter referred to as the "present embodiment") will be described in detail below. Note that the present invention is not limited to the embodiment described below, and can be modified as desired without departing from the spirit and scope of the present invention.

[0035] 1 is a photograph showing the appearance of the front surface of a heat-transfer-suppressing sheet according to this embodiment. In this specification, the front and back surfaces of a heat-transfer-suppressing sheet refer to a pair of main surfaces that are perpendicular to the thickness direction of the heat-transfer-suppressing sheet.

[0036] The heat-transfer-suppressing sheet 10 according to this embodiment includes inorganic particles 4 and organic fibers 11. As shown in Fig. 1 , the surface 10a of the heat-transfer-suppressing sheet 10 is formed with fiber aggregation portions 2 each having streak-like fiber bundles 7 made of a plurality of organic fibers 11, and recesses 3 each having no fiber bundles 7 and recessed below the fiber aggregation portions 2. The recesses 3 recessed below the fiber aggregation portions 2 preferably form continuous grooves.

[0037] The fiber bundles 7 are formed by entangling a plurality of organic fibers 11 and extend in stripes in a direction substantially parallel to the surface of the heat transfer-suppressing sheet 10. There are no restrictions on the height difference between the fiber aggregation portions 2 and the depressions 3, but a height difference of more than 0 mm to 0.5 mm is preferable, and a height difference of more than 0 mm to 0.2 mm is more preferable.

[0038] In the recesses 3, although a few organic fibers 11 are observed in some places, fiber bundles 7 in which a plurality of organic fibers 11 are entangled are not observed. Therefore, the recesses 3 have a lower fiber density than the fiber aggregation portions 2.

[0039] The fiber aggregation portions 2 and the recessed portions 3 form a sea-island structure, with the recessed portions 3 corresponding to the island portions being formed so as to be surrounded by the fiber aggregation portions 2 corresponding to the sea portion.

[0040] In the heat transfer-suppressing sheet 10 configured in this manner, the fiber bundles 7 formed by entanglement of organic fibers 11 in the fiber aggregation portions 2 are present in stripes on the sheet surface, thereby improving the sheet strength. Furthermore, the stripes of fiber bundles 7 also improve the flexibility of the sheet. Note that, because the fiber aggregation portions 2 are formed in continuous stripes by the fiber bundles 7 formed by entanglement of organic fibers 11, the sheet strength can be further improved and the flexibility of the sheet can also be further improved.

[0041] Furthermore, it is preferable that the continuous streak-like fiber aggregation portions 2 have a curved shape. Furthermore, by connecting a plurality of curved streak-like fiber aggregation portions 2 with different shapes, the streak-like fiber aggregation portions 2 form a mesh-like shape, which further increases the strength and flexibility of the sheet.

[0042] On the other hand, the recessed portion 3 is recessed further than the fiber aggregation portion 2, and therefore acts to release stress such as external impact or pressure.

[0043] Furthermore, since at least some of the recesses 3 are surrounded by strong fiber agglomerations 2 formed by fiber bundles 7 in which organic fibers 11 are entangled, the shape of the recesses 3 is more easily maintained, and the function of releasing stress from external impacts, pressure, etc. becomes more effective.

[0044] To form such fiber aggregation portions 2 and recesses 3 on the surface of the heat-transfer-suppressing sheet 10, it is advisable to perform cutting or polishing on the surface 1a of the prepared heat-transfer-suppressing sheet precursor 1, as explained in the manufacturing method shown in FIGS. 4A to 4C, which will be described later.

[0045] FIG. 2 is a photograph showing a cross section of a heat-transfer-suppressing sheet according to an embodiment of the present invention. The heat-transfer-suppressing sheet 10 shown in FIG. 2 has fiber aggregation portions 2 and recesses 3 on both the front surface 10a and the back surface 10b. As shown in FIG. 2, organic fibers 11 and fiber bundles 7 formed by entanglement of organic fibers 11 are present not only on the front surface 10a and the back surface 10b of the heat-transfer-suppressing sheet 10 but also inside the sheet. This allows for even greater sheet strength and flexibility. Thus, the fiber aggregation portions 2 and recesses 3 may be formed on at least one of the front surface 10a and the back surface 10b of the heat-transfer-suppressing sheet 10, and are preferably formed on both the front surface 10a and the back surface 10b.

[0046] In the heat-transfer-suppressing sheet 10, it is preferable that the length of the fiber bundles 7 is relatively long. One method for determining the length of the fiber bundles 7 is to simply measure the length of the fiber bundles 7 extending in a stripe shape. For example, a string or the like can be placed on the surface of the heat-transfer-suppressing sheet 10 along the fiber bundles 7, and then the length of the string can be measured. When the length of continuous fiber bundles 7 is measured, if there are fiber bundles 7 with a length of 20 mm or more, the effect of improving the strength of the heat-transfer-suppressing sheet 10 can be sufficiently obtained.

[0047] Furthermore, if the fiber bundles 7 are connected in a mesh pattern on the surface of the heat transfer-suppressing sheet 10, the sheet strength can be further improved.

[0048] 2 has an uneven surface having fiber aggregation portions 2 and recesses 3 on the front surface 10a and back surface 10b, but a heat-transfer-suppressing sheet 10 manufactured by a surface processing step in which cutting or polishing is performed may be re-pressed for the purpose of flattening the surface, increasing the density of the sheet, etc. After re-pressing, the pattern formed by the fiber aggregation portions 2 and recesses 3 on the surface remains intact, and the respective functions of the fiber aggregation portions 2 and recesses 3 described above are maintained.

[0049] As described above, the heat-transfer-suppressing sheet 10 may be prepared by cutting or polishing the front surface 1a (and back surface 1b) of the heat-transfer-suppressing sheet precursor 1. That is, it is preferable that at least one of the front surface and back surface of the heat-transfer-suppressing sheet 10 includes a polished or cut surface, and that this polished or cut surface has fiber aggregation portions 2 and recesses 3.

[0050] Figure 3 is a photograph showing an enlarged view of the recessed portion 3 on the surface 10a (machined surface 12) of the heat-transfer-suppressing sheet 10. As shown in Figures 2 and 3, the heat-transfer-suppressing sheet 10 has no looped or horizontally lying organic fibers 11 on the machined surface 12, but rather has many upright organic fibers 11, with the organic fibers 11 being fluffy over the entire surface. Therefore, when another member is laminated on the machined surface 12, the exposed organic fibers 11 have an anchoring effect, which increases the bonding strength to the other member. Bonding to other members will be described in detail later.

[0051] [Method of Manufacturing Heat-Transfer-Suppressing Sheet] Figures 4A to 4C are diagrams illustrating a method of manufacturing a heat-transfer-suppressing sheet. Figure 4A is a schematic diagram showing the side of a heat-transfer-suppressing sheet precursor. Figure 4B is a schematic diagram showing a surface processing step in which the surface of the heat-transfer-suppressing sheet precursor is cut or polished. Figure 4C is a schematic diagram showing the side of the resulting heat-transfer-suppressing sheet.

[0052] 4A to 4C, the method for producing the heat-transfer-suppressing sheet 10 includes a precursor-preparing step of producing a heat-transfer-suppressing sheet precursor 1 using a mixture containing inorganic particles 4 and organic fibers 11, and a surface-processing step of cutting or polishing at least one of the front and back surfaces of the heat-transfer-suppressing sheet precursor 1 produced in the precursor-preparing step. If necessary, a re-pressing step of pressing the sheet surface after the surface-processing step may also be included. Each step will be described in detail below.

[0053] (Precursor Preparation Step) As shown in FIG. 4A , in the precursor preparation step, inorganic particles 4, organic fibers 11, and other compounding materials are added to a mixer such as a V-type mixer in a predetermined ratio to prepare a mixture. The heat transfer-suppressing sheet precursor 1 can be prepared by either a wet method or a dry method, but a dry method is preferred. Mixing the materials using a dry method facilitates three-dimensional entanglement of the binder fibers, facilitating the formation of fiber aggregation portions 2 and recesses 3. As described below, it is preferable to use, as the organic fibers 11, binder fibers having a core-sheath structure with a core made of a first organic material and a sheath made of a second organic material. In this case, the melting point of the first organic material is higher than that of the second organic material.

[0054] The mixture is then placed in a mold and pressed with a press or the like, and the resulting molded body is heated to melt the sheath of the binder fiber. The heated molded body is then cooled, whereby the molten sheath on the surface and inside of the molded body solidifies, and the cores (organic fibers 11) are fused together.

[0055] Inside the heat-transfer-suppressing sheet precursor 1, the organic material constituting the molten sheath and the inorganic particles 4 present around the binder fibers are fused to the core and are also fused to each other in the regions where the binder fibers were in contact with each other, and a matrix containing inorganic particles is formed between the multiple organic fibers. This makes it possible to obtain a heat-transfer-suppressing sheet precursor 1 processed into a sheet shape. Such a heat-transfer-suppressing sheet precursor 1 has a high ability to retain the inorganic particles 4, and powdering of the inorganic particles 4 is suppressed.

[0056] An adhesive such as hot melt powder may be contained in the mixture as a compounding material of the heat-transfer-suppressing sheet 10. By appropriately adjusting the type and content of the adhesive contained in the mixture, the holding power of the inorganic particles 4 can be improved and powder falling off of the inorganic particles 4 can be further suppressed. Furthermore, the heat-transfer-suppressing sheet 10 has even higher strength, and can maintain its shape even when a strong external stress is applied, thereby maintaining excellent heat insulating performance.

[0057] In the precursor preparation step, when the heat transfer-suppressing sheet precursor is prepared by a dry method, a small amount of solvent such as water can be added within the range of the dry method to prevent powder such as the inorganic particles 4 from flying about during mixing, making it difficult to handle the raw materials. For example, by adding a small amount of solvent such as water to the mixture, scattering of powder during production can be suppressed.

[0058] The process of processing the mixture into a sheet includes a process of pressing the mixture and a process of heating the mixture. When a binder fiber having a core-sheath structure is used as the material for the heat transfer-suppressing sheet 10, the heating temperature in the heating process is preferably higher than the melting point of the second organic material constituting the sheath and lower than the melting point of the first organic material constituting the core. By setting the heating temperature in this manner, as described above, the strength of the sheet can be ensured by the core on both the surface side and the center side of the sheet, and the inorganic particles 4 can be held by the fused sheath.

[0059] Specifically, the heating temperature in the heating step is preferably set to be 10° C. or more higher, more preferably 20° C. or more higher, than the melting point of the second organic material constituting the sheath. On the other hand, the heating temperature is preferably set to be 10° C. or more lower, more preferably 20° C. or more lower, than the melting point of the first organic material constituting the core.

[0060] The heating time is not particularly limited, but is preferably set to a time sufficient to melt the sheath, for example, from 3 minutes to 15 minutes.

[0061] Furthermore, when a hot-melt powder is included, the heating temperature in the heating step is preferably set to be at least 10°C higher, and more preferably 20°C higher, than the higher of the melting point of the second organic material constituting the sheath and the melting point of the third organic material constituting the hot-melt powder. On the other hand, the heating temperature is preferably set to be at least 10°C lower, and more preferably 20°C lower, than the melting point of the first organic material constituting the core. Setting the heating temperature in this range makes it possible to form a strong skeleton in the heat-transfer-suppressing sheet precursor 1, further improving the sheet strength and preventing the inorganic particles 4 from falling off.

[0062] (Surface Processing Step) Next, as shown in FIG. 4B , at least one of the front surface 1a and the back surface 1b of the heat transfer-suppressing sheet precursor 1 (here, the upper surface in FIG. 4B , i.e., the front surface 1a) is subjected to cutting or polishing.

[0063] 4B shows a case where cutting is performed using a cutting tool 5 such as a knife in the surface processing step, but polishing can also be performed using, for example, a grindstone or a brush. The cutting allowance or polishing allowance indicated by thickness t in the figure is not particularly limited as long as the fiber agglomeration portions 2 and recesses 3 are clearly visible on the cut or polished surface formed by processing, and / or the organic fibers 11, which are a compounding material, are exposed and fluffy. For example, a thickness t in the range of 0.05 mm to 1.0 mm is preferred from the viewpoint of workability in surface processing.

[0064] 4C , a heat-transfer-suppressing sheet 10 of the present embodiment is obtained in which a pattern that becomes the fiber aggregation portions 2 and depressions 3 appears on the surface 10a, which is the machined surface 12. Like the heat-transfer-suppressing sheet precursor 1, the heat-transfer-suppressing sheet 10 configured in this manner also has a high ability to retain the inorganic particles 4, and powdering of the inorganic particles 4 is suppressed.

[0065] (Re-pressing step) If necessary, the surface 10a (machined surface 12 or polished surface) of the heat-transfer-suppressing sheet 10 obtained above may be re-pressed. By re-pressing after the surface processing step, the surface 10a of the heat-transfer-suppressing sheet 10 is made more flat, but the pattern consisting of the fiber aggregation portions 2 and the recesses 3 remains even after the re-pressing step, and the same effect as before the re-pressing can be obtained.

[0066] The method for manufacturing the heat transfer-suppressing sheet has been described above.

[0067] The following explains why cutting or polishing the surface 1a of the heat-transfer-suppressing sheet precursor 1 results in fiber aggregation portions 2 formed by a plurality of entangled organic fibers 11 and recesses 3 recessed from the fiber aggregation portions 2 on the surface 10a (cut surface 12 or polished surface) of the heat-transfer-suppressing sheet 10. As shown in Fig. 5A, when cutting or polishing the surface 1a of the heat-transfer-suppressing sheet precursor 1, the fiber aggregation portions 2 formed by a plurality of entangled organic fibers 11 present inside the heat-transfer-suppressing sheet precursor 1 are strengthened by this entanglement and are therefore unlikely to be cut parallel to the surface 1a of the heat-transfer-suppressing sheet precursor 1 by cutting or polishing.

[0068] 5B , the cutting tool 5 moves along the surface of the fiber aggregation portions 2 without cutting the fiber aggregation portions 2 themselves, and as a result, fiber aggregation portions 2 and recesses 3 are formed on the surface 10a of the heat-transfer-suppressing sheet 10 and the surface 20a of the waste material 20, which are formed after polishing or cutting. Therefore, the surface 10a (cut or polished surface) of the heat-transfer-suppressing sheet 10 obtained has fiber aggregation portions 2 formed by entangled organic fibers 11, and recesses 3 recessed below the fiber aggregation portions 2.

[0069] Here, the characteristics of the heat-transfer-suppressing sheet 10 according to the embodiment of the present invention will be further described in comparison with the surface properties of the heat-transfer-suppressing sheet precursor. Fig. 6A is a photograph showing an enlarged view of a portion of the surface 1a of the heat-transfer-suppressing sheet precursor 1, and Fig. 6B is a photograph showing a cross section of the enlarged view.

[0070] As shown in Figures 6A and 6B, the front surface 1a of the heat-transfer-suppressing sheet precursor 1 contains many looped organic fibers 11 and organic fibers 11 that lie horizontally. In contrast, as shown in Figures 2 and 3, the front surface 1a and back surface 1b of the heat-transfer-suppressing sheet 10 are machined surfaces 12, and the machined surface 12 does not contain any looped or horizontally laid organic fibers 11, but contains many upright organic fibers 11, with the entire surface being fluffy. Therefore, when another member is laminated on the machined surface 12, the exposed organic fibers 11 have an anchoring effect, thereby increasing the bonding strength to the other member. In Figures 2, 3, 6A, and 6B, the white line segments exposed from the surface represent "organic fibers 11."

[0071] 7 , for example, if the heat-transfer-reducing sheet 10 is used as a spacer inserted between the battery cells 110 a, 110 b, and 110 c, the battery cells 110 a, 110 b, and 110 c would be considered other components. Also, if the heat-transfer-reducing sheet 10 is used as an inner wall material attached to the inner wall of the battery case 30 in the battery pack 100, the battery case 30 would be considered other components.

[0072] 2, the precursor preparation step is preferably carried out by a dry method in order to expose and fluff the organic fibers 11 on the machined surface 12 of the heat-transfer-suppressing sheet 10. The heat-transfer-suppressing sheet precursor 1 may be produced by either a wet method or a dry method, but is preferably produced by a dry method.

[0073] In the wet method, the organic fibers 11 tend to be oriented along the surface 1 a of the heat-transfer-suppressing sheet precursor 1, and during cutting, the cutting edge of the cutting tool 5 slides along the surface of the organic fibers 11, making it difficult to cut the organic fibers 11. In contrast, in the dry method, the heat-transfer-suppressing sheet precursor 1 contains many organic fibers 11 distributed in directions oblique or perpendicular to the surface 1 a, and therefore, during cutting, the cutting edge of the cutting tool 5 frequently comes into contact with the side surfaces (long portions) of the organic fibers 11, making it easy for the organic fibers 11 to be exposed.

[0074] There are no limitations on the materials that can be used to form the heat transfer-suppressing sheet 10, as long as the heat transfer-suppressing sheet 10 contains organic fibers and inorganic particles. However, the following is an example of a preferable material that provides excellent heat insulating performance.

[0075] <Organic Fibers> The organic fibers 11 have the effect of imparting flexibility to the heat transfer-suppressing sheet 10 and improving the strength and shape of the sheet. The organic fibers 11 can be at least one selected from polyethylene, polypropylene, polystyrene, vinyl chloride, nylon, acrylic, epoxy resin, polyurethane, polyether ether ketone, polyetherimide, polyethylene terephthalate, polytetrafluoroethylene (PTFE), polyphenyl sulfide, polycarbonate, and aramid.

[0076] Although single-component organic fibers can be used as the material for the organic fibers 11 in the heat transfer-suppressing sheet 10, it is preferable to use binder fibers with a core-sheath structure. The binder fibers with a core-sheath structure have a core extending in the longitudinal direction of the fiber and a sheath formed to cover the outer surface of the core. The core is made of a first organic material, and the sheath is made of a second organic material, with the melting point of the first organic material being higher than the melting point of the second organic material.

[0077] Whether single-component organic fibers or core-sheath binder fibers are used as the material for the organic fibers 11, heating during press molding melts part of the fiber surface during production of the heat transfer-suppressing sheet precursor 1. Then, by subsequent cooling, welded parts (not shown) are formed around the organic fibers 11. The welded parts fuse the inorganic particles 4 to the surfaces of the organic fibers 11 and fuse the organic fibers 11 together, so that the formation of the welded parts allows for excellent sheet strength to be obtained.

[0078] When binder fibers with a core-sheath structure are used as the organic fibers 11, the core of the heat-transfer-suppressing sheet 10 corresponds to the organic fibers 11. If binder fibers with a core-sheath structure are used when producing the heat-transfer-suppressing sheet precursor 1, adjacent binder fibers are fused to each other, making it easier to form fiber bundles 7 and further increasing the sheet strength. Furthermore, when binder fibers with a core-sheath structure are used, the second organic material that constitutes the sheath melts and then solidifies again in a state that includes the inorganic particles 4 present around them, thereby improving the retention of the inorganic particles 4.

[0079] (First Organic Material) When a binder fiber having a core-sheath structure is used as the material for the organic fibers 11, the first organic material constituting the core, i.e., the organic fibers 11, is not particularly limited as long as it has a melting point higher than that of the sheath, i.e., the second organic material, which is present on the outer surface of the organic fibers 11. Examples of the first organic material include at least one selected from polyethylene terephthalate, polypropylene, and nylon.

[0080] (Second Organic Material) When binder fibers having a core-sheath structure are used as the material for the organic fibers 11, the second organic material constituting the sheath portion is not particularly limited as long as it has a melting point lower than that of the first organic material constituting the organic fibers 11. Examples of the second organic material include at least one selected from polyethylene terephthalate, polyethylene, polypropylene, and nylon. The melting point of the second organic material is preferably 90°C or higher, and more preferably 100°C or higher. The melting point of the second organic material is preferably 150°C or lower, and more preferably 130°C or lower.

[0081] (Organic Fiber Content) When the content of organic fibers 11 in the heat-transfer-suppressing sheet 10 is appropriately controlled, the effect of improving the strength of the heat-transfer-suppressing sheet 10 can be sufficiently obtained. The content of organic fibers 11 is preferably 2 mass % or more, more preferably 4 mass % or more, and even more preferably 5 mass % or more, relative to the total mass of the heat-transfer-suppressing sheet 10. Furthermore, if the content of organic fibers 11 is too high, the content of inorganic particles 4 will relatively decrease. Therefore, in order to obtain the desired heat insulating performance, the content of organic fibers 11 is preferably 10 mass % or less, more preferably 8 mass % or less, and even more preferably 6 mass % or less, relative to the total mass of the heat-transfer-suppressing sheet 10.

[0082] (Fiber Length of Organic Fibers) While there are no particular limitations on the fiber length of the organic fibers 11, from the viewpoint of ensuring moldability and processability, the average fiber length of the organic fibers 11 is preferably 10 mm or less. Note that, from the viewpoint of improving the strength of the heat transfer-suppressing sheet 10, the average fiber length of the organic fibers 11 is preferably 0.5 mm or more.

[0083] Note that binder fibers having a core-sheath structure such as the one described above are generally commercially available, and the materials constituting the core and sheath may be the same or different. Examples of binder fibers in which the core and sheath are made of the same material but have different melting points include those in which the core and sheath are made of polyethylene terephthalate, polypropylene, or nylon. Examples of binder fibers in which the core and sheath are made of different materials include those in which the core is made of polyethylene terephthalate and the sheath is made of polyethylene, and those in which the core is made of polypropylene and the sheath is made of polyethylene.

[0084] <Inorganic particles> As the inorganic particles, a single inorganic particle may be used, or two or more types of inorganic particles may be used in combination. As the type of inorganic particles, from the viewpoint of the heat transfer suppression effect, it is preferable to use particles made of at least one inorganic material selected from oxide particles, carbide particles, nitride particles and inorganic hydrate particles, and it is more preferable to use oxide particles. In addition, the shape is not particularly limited, but it is preferable to include at least one selected from nanoparticles, hollow particles and porous particles. Specifically, it is also possible to use inorganic balloons such as silica nanoparticles, metal oxide particles, microporous particles and hollow silica particles, particles made of thermally expandable inorganic materials, particles made of hydrous porous bodies, etc.

[0085] 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.

[0086] 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.

[0087] <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.

[0088] (Average Primary Particle Diameter of Oxide Particles) 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 heat-transfer-suppressing sheet 10 in high-temperature regions of 500°C or higher, 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. This can reduce the impact on thermal insulation, particularly in normal temperature regions where conductive heat transfer is dominant.

[0089] In this embodiment, the average primary particle size can be determined by observing particles under a microscope, comparing with a standard scale, and taking the average of any 10 particles.

[0090] (Nanoparticles) In this embodiment, 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. Furthermore, when nanoparticles are used as the first inorganic particles, the finely dispersed voids further suppress convective heat transfer, providing excellent thermal insulation. Therefore, nanoparticles are preferred because they can suppress heat transfer between adjacent nanoparticles during normal battery operation at room temperature. Furthermore, when nanoparticles with a small average primary particle diameter are used as oxide particles, an increase in conductive heat transfer of the heat-transfer-suppressing sheet 10 can be suppressed even when the heat-transfer-suppressing sheet 10 is compressed due to expansion associated with thermal runaway of the battery cell, increasing its internal density. This is thought to be because nanoparticles are prone to forming small voids between particles due to electrostatic repulsion, and their low bulk density allows the particles to be packed together to provide cushioning.

[0091] In this embodiment, when nanoparticles are used as the first inorganic particles, there are no particular limitations on the material as long as they comply with the definition of nanoparticles. For example, silica nanoparticles are a material with high heat insulating properties, and the contact points between particles are small, so the amount of heat conducted by silica nanoparticles is smaller than when silica particles with a large particle diameter are used. Furthermore, commonly available silica nanoparticles have a bulk density of 0.1 (g / cm ３ ), for example, in a battery pack, even if the battery cells arranged on both sides of the heat-transfer-suppressing sheet 10 thermally expand and a large compressive stress is applied to the heat-transfer-suppressing sheet 10, the size (area) and number of contact points between the silica nanoparticles do not increase significantly, and heat insulation properties can be maintained. Therefore, it is preferable to use silica nanoparticles as the nanoparticles. Examples of silica nanoparticles include wet silica, dry silica, and aerogel, but particularly suitable silica nanoparticles will be described below.

[0092] Generally, wet silica particles are aggregated, whereas dry silica particles can be dispersed. In the temperature range of 300°C or less, heat conduction is dominated by conductive heat transfer, so dry silica, which allows particles to be dispersed, can provide superior heat insulating performance compared to wet silica. As will be described later, the heat transfer-suppressing sheet 10 is preferably produced by processing a mixture containing compounding materials into a sheet using a dry method, and therefore, dry silica, silica aerogel, or the like, which has low thermal conductivity, is preferably used as the inorganic particles.

[0093] (Average Primary Particle Diameter of Nanoparticles) 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-transfer-suppressing sheet 10 can be suppressed, particularly in a temperature range below 500°C, thereby further improving 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-transfer-suppressing sheet 10. 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.

[0094] (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.

[0095] 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 material. 2Al(OH) ３ →Al ２ O ３ +3H ２ O

[0096] As will be described later, the heat transfer-suppressing sheet 10 is preferably interposed between battery cells, for example. In a battery cell that has experienced thermal runaway, the temperature rises sharply to over 200°C and continues to rise to around 700°C. Therefore, the inorganic particles are preferably made of inorganic hydrates 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. All of these temperatures roughly overlap the temperature range of the rapid temperature rise in a battery cell that has experienced thermal runaway, and can efficiently suppress the temperature rise. Therefore, these inorganic hydrates are preferred.

[0097] (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 takes some time for the first inorganic particles (inorganic hydrate) near the center of the heat transfer-suppressing sheet 10 to reach their thermal decomposition temperature, and the first inorganic particles near the center of the sheet may not be completely thermally decomposed. For this reason, the average secondary particle 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.

[0098] (Particles Made of Thermally Expandable Inorganic Material) Examples of thermally expandable inorganic materials include vermiculite, bentonite, mica, and perlite.

[0099] (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.

[0100] (Inorganic Balloons) The heat-transfer-suppressing sheet 10 according to this embodiment may contain inorganic balloons as the first inorganic particles. The inclusion of inorganic balloons can suppress convective or conductive heat transfer within the heat-transfer-suppressing sheet 10 in a temperature range of less than 500°C, further improving the heat insulating properties of the heat-transfer-suppressing sheet 10. The inorganic balloons can be at least one selected from shirasu balloons, silica balloons, fly ash balloons, barite balloons, and glass balloons.

[0101] (Inorganic Balloon Content) The inorganic balloon content is preferably 60 mass % or less relative to the total mass of the heat transfer-suppressing sheet 10 .

[0102] (Average Particle Diameter of Inorganic Balloons) The average particle diameter of the inorganic balloons is preferably 1 μm or more and 100 μm or less.

[0103] <Second Inorganic Particles> When two types of inorganic particles are contained in the heat transfer-suppressing sheet 10, 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.

[0104] Nanoparticles have extremely low conductive heat transfer and can maintain excellent heat insulation even when compressive stress is applied to the heat transfer-suppressing sheet 10. Metal oxide particles such as titania are highly effective at blocking radiant heat. Furthermore, when large-diameter inorganic particles and small-diameter inorganic particles are used, the small-diameter inorganic particles penetrate into the gaps between the large-diameter inorganic particles, resulting in a denser structure and improved heat transfer suppression. Therefore, when nanoparticles are used as the first inorganic particles, it is preferable to further include second inorganic particles made of a metal oxide larger in diameter than the first inorganic particles in the heat transfer-suppressing sheet 10. Examples of metal oxides include silicon oxide, titanium oxide, aluminum oxide, barium titanate, zinc oxide, zircon, and zirconium oxide. Titanium oxide (titania), in particular, has a higher refractive index than other metal oxides and is highly effective at scattering light and blocking radiant heat in high-temperature regions of 500°C or higher. Therefore, titania is most preferably used.

[0105] When at least one type of particle selected from dry silica particles and silica aerogel is used as the first inorganic particles and at least one type of particle 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.

[0106] 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.

[0107] (Average Primary Particle Diameter of Second Inorganic Particles) When second inorganic particles made of a metal oxide are contained in the heat transfer-suppressing sheet, if the average primary particle 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.

[0108] (Inorganic Particle Content) When the total content of the inorganic particles 4 in the heat-transfer-inhibiting sheet 10 is appropriately controlled, the heat insulation properties of the heat-transfer-inhibiting sheet 10 can be sufficiently ensured. The total content of the inorganic particles 4 is preferably 60 mass % or more, more preferably 65 mass % or more, and even more preferably 70 mass % or more, relative to the total mass of the heat-transfer-inhibiting sheet 10. Furthermore, the total content of the inorganic particles 4 is even more preferably 75 mass % or more, even more preferably 80 mass % or more, and particularly preferably 85 mass % or more, relative to the total mass of the heat-transfer-inhibiting sheet 10. On the other hand, if the total content of the inorganic particles 4 is too high, the content of the organic fibers 11 will relatively decrease. Therefore, in order to fully obtain the sheet strength-improving effect of the organic fibers 11, the total content of the inorganic particles 4 is preferably 95 mass % or less, and more preferably 90 mass % or less, relative to the total mass of the heat-transfer-inhibiting sheet 10.

[0109] The content of inorganic particles 4 in heat-transfer-suppressing sheet 10 can be calculated, for example, by heating heat-transfer-suppressing sheet 10 at 800° C. to decompose the organic components, and then measuring the mass of the remaining portion.

[0110] In this embodiment, the total content of the inorganic particles 4 and the organic fibers 11 is preferably 65% ​​by mass or more, more preferably 70% by mass or more, and even more preferably 75% by mass or more, relative to the total mass of the heat-transfer-suppressing sheet 10. The total content of the inorganic particles 4 and the organic fibers 11 is even more preferably 80% by mass or more, even more preferably 85% by mass or more, and particularly preferably 90% by mass or more, relative to the total mass of the heat-transfer-suppressing sheet 10. The total content of the inorganic particles 4 and the organic fibers 11 may be 100% by mass, relative to the total mass of the heat-transfer-suppressing sheet 10.

[0111] Furthermore, the heat-transfer-suppressing sheet 10 may contain, in addition to the organic fibers 11 and inorganic particles 4, organic fibers made of an organic material different from the organic fibers 11, inorganic fibers, etc. When the heat-transfer-suppressing sheet 10 contains inorganic fibers, the inorganic fibers that are preferably contained in the present embodiment will be described below.

[0112] <Inorganic Fibers> As the inorganic fibers, a single inorganic fiber may be used, or two or more inorganic fibers may be used in combination. Examples of inorganic fibers include ceramic fibers such as silica fiber, alumina fiber, alumina silicate fiber, zirconia fiber, carbon fiber, soluble fiber, refractory ceramic fiber, aerogel composite material, magnesium silicate fiber, alkaline earth silicate fiber, potassium titanate fiber, silicon carbide fiber, and potassium titanate whisker fiber; glass fibers such as glass fiber, glass wool, and slag wool; artificial mineral fibers such as rock wool and basalt fiber; and natural mineral fibers such as wollastonite and mullite fiber. These inorganic fibers are preferred in terms of heat resistance, strength, and availability. Among the inorganic fibers, silica-alumina fiber, alumina fiber, silica fiber, rock wool, alkaline earth silicate fiber, and glass fiber are particularly preferred from the viewpoint of handleability.

[0113] The cross-sectional shape of the inorganic fiber is not particularly limited, and examples thereof include a circular cross section, a flat cross section, a hollow cross section, a polygonal cross section, a core cross section, etc. Among these, modified cross section fibers having a hollow cross section, a flat cross section, or a polygonal cross section are preferably used because they have slightly improved heat insulation properties.

[0114] The preferred lower limit of the average fiber length of the inorganic fibers is 0.1 mm, more preferably 0.5 mm. Meanwhile, the preferred upper limit of the average fiber length of the inorganic fibers is 50 mm, more preferably 10 mm. If the average fiber length of the inorganic fibers is less than 0.1 mm, the inorganic fibers are less likely to be entangled with each other, which may reduce the mechanical strength of the heat transfer-suppressing sheet 10. Meanwhile, if the average fiber length exceeds 50 mm, although a reinforcing effect can be obtained, the inorganic fibers may not be able to be tightly entangled with each other, or may be curled up by a single inorganic fiber, which may result in continuous voids and reduce the heat insulating properties.

[0115] The preferred lower limit of the average fiber diameter of the inorganic fibers is 1 μm, more preferably 2 μm, and even more preferably 3 μm. On the other hand, the preferred upper limit of the average fiber diameter of the inorganic fibers is 15 μm, and more preferably 10 μm. If the average fiber diameter of the inorganic fibers is less than 1 μm, the mechanical strength of the inorganic fibers themselves may be reduced. Furthermore, from the viewpoint of the effects on human health, the average fiber diameter of the inorganic fibers is preferably 3 μm or more. On the other hand, if the average fiber diameter of the inorganic fibers is greater than 15 μm, solid-state heat transfer via the inorganic fibers may increase, resulting in a decrease in thermal insulation, and the formability and strength of the heat transfer-suppressing sheet may be impaired.

[0116] (Inorganic Fiber Content) When heat-transfer-suppressing sheet 10 contains inorganic fibers, the inorganic fiber content is preferably 3 mass % or more and 15 mass % or less relative to the total mass of heat-transfer-suppressing sheet 10 .

[0117] Furthermore, the content of the inorganic fibers is more preferably 5% by mass or more and 10% by mass or less, relative to the total mass of the heat transfer-suppressing sheet 10. By setting the content in this range, the shape retention, pressing force resistance, wind pressure resistance, and inorganic particle retention ability of the inorganic fibers are exhibited in a balanced manner. Furthermore, by appropriately controlling the content of the inorganic fibers, the organic fibers 11 and the inorganic fibers become entangled with each other to form a three-dimensional network, which further improves the effect of retaining the inorganic particles 4 and other compounded materials described below.

[0118] <Other Compounding Materials> If necessary, the heat-transfer-suppressing sheet 10 may contain a binder, a colorant, and the like. All of these are useful for the purposes of reinforcing the heat-transfer-suppressing sheet 10 and improving its formability, and the total amount of these materials relative to the total mass of the heat-transfer-suppressing sheet 10 is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 5% by mass or less.

[0119] The heat-transfer-suppressing sheet 10 is configured as described above, but may be covered with a film or the like to further suppress powder falling off of the inorganic particles 4. Examples of films that can be used include polyimide, polycarbonate, PET, p-phenylene sulfide, polyetherimide, cross-linked polyethylene, flame-retardant chloroprene rubber, polyvinyl denfluoride, rigid polyvinyl chloride, polybutylene terephthalate, PTFE, PFA, FEP, ETFE, rigid PVC, flame-retardant PET, polystyrene, polyethersulfone, polyamideimide, polyacrylonitrile, polyethylene, polypropylene, and polyamide. The method for covering the heat-transfer-suppressing sheet 10 with a film is not particularly limited, and examples include a method of attaching the sheet with an adhesive or the like, a method of wrapping the heat-transfer-suppressing sheet 10 in a film, and a method of housing the heat-transfer-suppressing sheet 10 in a bag-shaped film.

[0120] 7 is a schematic diagram showing a battery pack including a heat-transfer-suppressing sheet 10 according to this embodiment. As shown in the figure, the battery pack 100 includes a plurality of battery cells 110a, 110b, and 110c connected in series or parallel and a heat-transfer-suppressing sheet 10. The heat-transfer-suppressing sheet 10 is interposed between the battery cells 110a, 110b, and 110c.

[0121] In the battery pack 100 configured in this manner, even if a certain battery cell (e.g., 110a) experiences thermal runaway, the heat transfer suppression sheet 10, which has a heat transfer suppression effect, is present between the adjacent battery cell 110b, so that the spread of fire to the battery cell 110b can be prevented.

[0122] Although not shown, the heat transfer-suppressing sheet 10 may be disposed between the battery cells 110 a , 110 b , and 110 c and the battery case 30 , or may be attached to the inner surface of the battery case 30 .

[0123] (Test 1) Here, the flexibility of the heat-transfer-suppressing sheet precursor and the heat-transfer-suppressing sheet was evaluated. The heat-transfer-suppressing sheet precursor contained dry silica and titania as inorganic particles and core-sheath binder fibers as organic fibers. The specific contents and names of each component are shown below.・Inorganic particles: "Dry silica" (57.7% by mass, average primary particle diameter: 0.012 μm) ・Inorganic particles: "Titania" (24.7% by mass, average primary particle diameter: 8 μm) ・Organic fiber: "PET / low melting point PET fiber" (TJ04CN: manufactured by Teijin Frontier Co., Ltd., 15% by mass, average fiber length: 5 mm) which is a binder fiber with a core-sheath structure Core: "Polyethylene terephthalate" (melting point: 240°C) Sheath: "Low melting point polyethylene terephthalate" (melting point: 110°C) ・Hot melt powder adhesive: "Powder resin" (PR D60C-Z: manufactured by Tokyo Ink Co., Ltd., 2.6% by mass, melting point: 100°C)

[0124] As a comparative example, a density of about 3.0 g / cm ３ and a high density product (Comparative Example 1) with a density of about 2.0 g / cm ３A low-density product (Comparative Example 2) was prepared. In both cases, the heat-transfer-suppressing sheet precursor was used as is, without being subjected to cutting processing as in the samples of the Examples described later. The blending materials of the heat-transfer-suppressing sheet precursor used as the sample of the Comparative Example were the same as those in the Examples.

[0125] As an example sample, a density of about 2.0 g / cm ３ 4B , a machined surface was formed on the heat-transfer-suppressing sheet precursor. In Example 1, only the front surface 1a of the heat-transfer-suppressing sheet precursor was machined, producing a heat-transfer-suppressing sheet 10 in which only the front surface 10a was machined surface 12. In Example 2, both the front surface 1a and the back surface 1b of the heat-transfer-suppressing sheet precursor were machined, producing a heat-transfer-suppressing sheet 10 in which both surfaces were machined surfaces 12.

[0126] The cutting allowance was adjusted so that the heat-transfer-suppressing sheet precursors of the comparative examples (Comparative Examples 1 and 2) and the heat-transfer-suppressing sheets of the examples (Examples 1 and 2) had a sheet thickness of approximately 2 mm.

[0127] The compression force was then gradually increased from no pressure (i.e., 0 MPa) to 1 MPa for each sample, and the change in compression ratio (%) was determined. The results are shown in Figure 8. A comparison of Comparative Example 1 (high-density product) and Comparative Example 2 (low-density product) reveals that the higher the density, the steeper the slope of the "compression ratio - compression force" curve, i.e., the lower the flexibility. Furthermore, compared to Comparative Example 2, Examples 1 and 2, which had machined surfaces, tended to exhibit a slower rise in pressure relative to compression ratio. While the compression ratio range typically used for heat transfer-suppressing sheets for battery cells is 20% or less, it was shown that Example 1 (machined surface on only the front surface) and Example 2 (machined surfaces on both sides) exhibited greater flexibility than Comparative Examples 1 and 2, which did not have machined surfaces. Furthermore, it was shown that Example 2, which had machined surfaces on both sides, exhibited greater flexibility than Example 1, which had machined surfaces on only the front surface.

[0128] (Test 2) Here, the presence or absence of re-pressing was evaluated.

[0129] Figure 9A is a photograph of the machined surface of the sample of Example 1 before re-pressing, and clearly shows a pattern of streaky fiber aggregates surrounding the recesses. Even after re-pressing the machined surface of the same sample, the pattern before re-pressing still remains, as shown in Figure 9B.

[0130] Thus, even after re-pressing, the fiber bundles remain extending along the sheet surface, allowing the heat transfer-suppressing sheet to maintain its strength and flexibility. Furthermore, the presence of recesses that are recessed deeper than the fiber aggregation portions allows the sheet to maintain its ability to release stress from external impacts, pressure, and the like.

[0131] 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.

[0132] This application is based on a Japanese patent application filed on March 29, 2024 (Patent Application No. 2024-056364) and a Japanese patent application filed on March 29, 2024 (Patent Application No. 2024-056365), the contents of which are incorporated by reference into this application.

[0133] REFERENCE SIGNS LIST 1 heat transfer-suppressing sheet precursor 2 fiber aggregation portion 3 recess 4 inorganic particle 5 cutting tool 7 fiber bundle 10 heat transfer-suppressing sheet 11 organic fiber 12 cutting surface 20 waste material 30 battery case 100 battery pack 110a, 110b, 110c battery cell

Claims

1. A heat-transfer-suppressing sheet comprising inorganic particles and organic fibers, characterized in that at least one of the front and back surfaces has fiber aggregation areas formed by entanglement of a plurality of the organic fibers, and recesses recessed below the fiber aggregation areas.

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

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

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

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

6. The heat transfer suppressing sheet according to claim 1, wherein at least one of the front and back surfaces includes a polished or machined surface, and the fiber aggregation portion and the recessed portion are provided on the polished or machined surface.

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

8. The heat transfer suppressing sheet according to claim 7, wherein organic fibers are exposed and fluffed on the polished or machined surface.

9. The heat transfer suppressing sheet according to claim 7, wherein another member is laminated on the polished or machined surface.

10. The heat transfer suppression sheet according to claim 1 or 7, 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.

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

12. The method for producing a heat transfer-suppressing sheet according to claim 11, wherein binder fibers having a core-sheath structure are used as the organic fibers.

13. The method for producing a heat-transfer-suppressing sheet according to claim 12, wherein the precursor-producing step produces the heat-transfer-suppressing sheet precursor by a dry method.

14. The method for producing a heat transfer-suppressing sheet according to any one of claims 11 to 13, further comprising a re-pressing step of pressing the sheet surface after the surface processing step.

15. A battery pack comprising a plurality of battery cells and the heat transfer suppression sheet according to any one of claims 1 to 9, the plurality of battery cells being connected in series or parallel.

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