Heat transfer suppression sheet and battery pack

Abstract

Provided are: a heat transfer suppression sheet which exhibits high holding ability of a particle material and also excellent compression characteristics, is capable of preventing increase in convective heat transfer, and exhibits excellent thermal insulation performance; and a battery pack which comprises said heat transfer suppression sheet and is excellent in safety. A heat transfer suppression sheet (10) includes inorganic particles (30) and first organic fibers (20) having wide stem parts (21) in which the dimension equivalent to the width thereof is 1-100 μm, and branch parts (22) which branch from the stem parts (21). The stem parts (21) of the first organic fibers (20) are multilayered and aligned along a main surface (10a) of the heat transfer suppression sheet (10). In addition, this battery pack has: a plurality of battery cells; and the heat transfer suppression sheet (10). The plurality of battery cells are connected in series or in parallel.

Description

Heat transfer suppression sheet and assembled battery

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

[0002] In recent years, in order to protect the environment, lithium-ion secondary batteries have been used in electric vehicles, etc. However, because lithium-ion secondary batteries use an organic electrolyte, there is a risk of fire if they ignite during thermal runaway, causing damage to the battery pack.

[0003] To address this issue, heat insulating materials containing inorganic fibers or inorganic particles, or heat transfer suppression sheets that have been laminated with multiple layers to enhance their heat insulating and flame retardant effects, have been used. For example, Patent Document 1 proposes a heat insulating material that includes a composite layer between the cells of a battery pack, containing a fiber sheet and nanosilica, and that is configured by folding the fiber sheet, to provide high insulation and suppress heat transfer to adjacent battery cells when abnormal heat generation occurs.

[0004] Japanese Patent Application Publication No. 2018-204708

[0005] To improve thermal insulation performance, it is necessary to increase the number of nanosilica particles and to maintain a large number of nanosilica particles in a good condition. However, in the thermal insulation material of Patent Document 1, the fiber sheet is composed of entangled linear fibers, and the nanosilica is maintained between the fibers, which does not necessarily provide sufficient nanosilica retention. Furthermore, although the fibers tend to overlap, these spaces are easily collapsed by external pressure, resulting in poor compression characteristics for the thermal insulation material as a whole and potentially increasing internal convective heat transfer. Furthermore, in battery packs, the thermal insulation material is disposed between battery cells. However, because the battery cells expand even during normal charging, the thermal insulation material is constantly subjected to pressure. In the event of a battery abnormality, the thermal insulation material may be subjected to significantly greater pressure instantaneously from a battery cell experiencing thermal runaway. Therefore, a decrease in compression characteristics must be avoided.

[0006] Therefore, an object of the present invention is to provide a heat transfer suppression sheet that has a high capacity for retaining particulate material, excellent compression characteristics, can prevent an increase in convective heat transfer, and has excellent insulating performance, as well as a battery pack that is equipped with the heat transfer suppression sheet and has excellent safety.

[0007] The above object of the present invention is achieved by the heat transfer-suppressing sheet having the following configuration [1].

[0008] [1] A heat-transfer-suppressing sheet comprising: inorganic particles; and first organic fibers having a wide trunk portion with a width-equivalent dimension of 1 μm to 100 μm and branches branching from the trunk portion; wherein the trunk portions of the first organic fibers are oriented in multiple layers along a main surface of the heat-transfer-suppressing sheet.

[0009] Further, preferred embodiments of the present invention relating to the heat transfer-suppressing sheet relate to the following [2] to [9].

[0010] [2] The heat-transfer-suppressing sheet according to [1], wherein the branch portions of the first organic fibers hold the inorganic particles. [3] The heat-transfer-suppressing sheet according to [1] or [2], wherein the branch portions are crimped. [4] The heat-transfer-suppressing sheet according to any one of [1] to [3], wherein the inorganic particles comprise silica particles in an amount of 50 mass% or more of the total amount of the heat-transfer-suppressing sheet. [5] The heat-transfer-suppressing sheet according to any one of [1] to [4], wherein the first organic fibers are beaten fibers. [6] The heat-transfer-suppressing sheet according to any one of [1] to [5], wherein the inorganic particles comprise at least one particle selected from titania, zircon, zirconia, silicon carbide, zinc oxide, and alumina. [7] The heat-transfer-suppressing sheet according to any one of [1] to [6], further comprising at least one of inorganic fibers, second organic fibers, and an organic binder. [8] The heat-transfer-suppressing sheet according to any one of [1] to [7], further comprising an organic binder, wherein the difference between the glass transition point of the organic binder and the glass transition point of the first organic fibers is 10° C. or more. [9] The heat-transfer-suppressing sheet according to any one of [1] to [8], further comprising second organic fibers that are linear fibers having neither the trunk nor the branches.

[0011] The above object of the present invention is achieved by the following configuration

[10] relating to a battery pack.

[0012]

[10] A battery pack comprising a plurality of battery cells and the heat transfer-suppressing sheet according to any one of [1] to [9], the plurality of battery cells being connected in series or in parallel.

[0013] The heat-transfer-suppressing sheet of the present invention comprises inorganic particles and first organic fibers having wide trunks and branches branching from the trunks, the trunks of the first organic fibers being oriented in multiple layers along the main surface of the heat-transfer-suppressing sheet. Because the trunks of the first organic fibers are oriented along the main surface of the heat-transfer-suppressing sheet, there are few voids between the fibers, making the heat-transfer-suppressing sheet less likely to be crushed. For example, when the heat-transfer-suppressing sheet is disposed between battery cells, the heat-transfer-suppressing sheet has high resistance to pressure from the battery cells. Furthermore, the branches of the first organic fibers effectively hold the inorganic particles, further improving the heat-insulating performance and preventing the inorganic particles from detaching even when a larger amount of inorganic particles is incorporated, resulting in a heat-transfer-suppressing sheet with superior heat-insulating performance.

[0014] Furthermore, because the heat transfer suppression sheet of the present invention is disposed on the inner wall of the metal case and between the battery cells, the assembled battery of the present invention has excellent heat insulating performance and is highly resistant to pressure from the battery cells. Even if a battery cell experiences thermal runaway, the spread of fire to the outside or to other battery cells can be more reliably prevented, resulting in a high level of safety.

[0015] FIG. 1 is a schematic enlarged view of the internal structure of an example of a heat-transfer-suppressing sheet of the present invention. FIG. 2 is a schematic view of a first organic fiber. FIG. 3 is a schematic view illustrating gaps formed between fibers when linear organic fibers are used. FIG. 4 is a cross-sectional view of an example of a battery pack of the present invention. FIG. 5 is a photograph, in place of a drawing, of the surface of a heat-transfer-suppressing sheet produced in an example. FIG. 6 is a partial enlarged view of FIG. 5.

[0016] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments described below, and can be implemented with any modifications within the scope of the gist of the present invention.

[0017] [Heat Transfer-Suppressing Sheet] <First Organic Fiber> Figure 1 is a schematic diagram showing an enlarged view of the inside of an example of a heat transfer-suppressing sheet 10 of the present invention, and as shown in the figure, the heat transfer-suppressing sheet has first organic fibers 20 having a wide trunk 21 and branches 22 branching from the trunk 21. Note that, although the trunk 21 is shown as a flat plate in Figure 1 for the sake of convenience, the cross-sectional shape thereof is not limited as long as it is wide, and it may, for example, have an oblong cross section or an irregular shape in which the thickness (T: see Figure 2) varies depending on the location.

[0018] 2, the width-equivalent dimension W of the stem portions 21 of the first organic fibers 20 is not particularly limited, but is preferably 1 μm to 100 μm, more preferably 3 μm to 50 μm, and even more preferably 5 μm to 30 μm, on average. The heat-transfer-suppressing sheet 10 is produced by a papermaking method as described below, and during dehydration, the stem portions 21 of the first organic fibers 20 are oriented in multiple layers along the main surface 10a of the heat-transfer-suppressing sheet 10, as shown in FIG. 1. It is preferable that the width-equivalent dimension W of the stem portions 21 of the first organic fibers 20 be within the above-mentioned range, as this facilitates orientation during dehydration.

[0019] In FIG. 1, the first organic fibers 20 are shown with their trunks 21 oriented in multiple layers along one direction, i.e., the left-right direction of the paper, but some of the first organic fibers 20 may cross each other.

[0020] There is no limitation on the thickness T of the trunk 21 of the first organic fiber 20, but if it is too thin, the strength will be insufficient, and if it is too thick, the flexibility will be lost. Therefore, the average thickness is preferably 0.2 μm to 10 μm, more preferably 0.5 μm to 5 μm, and even more preferably 1 μm to 3 μm.

[0021] The length of the trunk portion 21 of the first organic fiber 20 is not particularly limited, but from the viewpoint of ensuring moldability and processability, it is preferably 20 mm or less, more preferably 10 mm or less, and even more preferably 5 mm or less, on average. The lower limit is preferably 0.5 mm or more, more preferably 1 mm or more, and even more preferably 2 mm or more, on average, from the viewpoint of allowing the first organic fiber 20 to function as a skeleton and ensuring the compressive strength of the heat transfer-suppressing sheet 10.

[0022] As shown in Figure 1, the branches 22 of the first organic fibers 20 hold the inorganic particles 30. Therefore, it is preferable that the branches 22 have a small fiber diameter and a long fiber length, as this increases the ability to hold the inorganic particles 30. There are no limitations on the fiber diameter or length, but the average fiber diameter is preferably 0.5 to 400 nm, more preferably 1 to 200 nm, and even more preferably 3 to 100 nm. The fiber length is preferably 0.5 to 50 µm, more preferably 1 to 20 µm, and even more preferably 2 to 10 µm.

[0023] The branches 22 are preferably curled. When the branches 22 have such a shape, they are well entangled with the inorganic particles 30, improving the retention ability.

[0024] The crimped structure refers to a structure in which fibers are bent in various directions. One known method for quantifying this crimped structure is to calculate the degree of crimp from an electron microscope photograph, which can be calculated, for example, using the following formula: Crimp (%) = (fiber length - distance between fiber ends) / (fiber length) × 100. Here, both the fiber length and the distance between fiber ends are measured values ​​on an electron microscope photograph. In other words, these are the fiber length and the distance between fiber ends projected onto a two-dimensional plane, and are shorter than their actual values. Based on this formula, the degree of crimp of the branch portions 22 is preferably 10% or more, and more preferably 30% or more. If the degree of crimp is low, the ability to retain the inorganic particles 30 is likely to decrease.

[0025] The first organic fibers 20 having such trunks 21 and branches 22 are obtained by beating base organic fibers. There are no limitations on the type of the first organic fibers 20, and examples thereof include chemical fibers such as polyvinyl alcohol (PVA) fibers, polyethylene fibers, nylon fibers, polyurethane fibers, and ethylene-vinyl alcohol copolymer fibers, as well as plant fibers such as cellulose.

[0026] The heat transfer-suppressing sheet 10 can also be manufactured by a papermaking method. However, since it is difficult to raise the heating temperature in this case above 250°C, the glass transition point of the first organic fiber 20 is preferably 250°C or lower, and more preferably 200°C or lower.

[0027] Although the lower limit of the glass transition point is not particularly limited, when an organic binder (described later) is used in combination, if the difference between the glass transition point of the organic binder and the glass transition point of the first organic fiber 20 is 10° C. or more, the organic binder solidifies after the first organic fiber 20, which was in a semi-molten state, has completely solidified in the cooling step during production, and therefore the skeleton reinforcing effect of the organic binder can be sufficiently obtained. Therefore, the difference between the glass transition point of the organic binder and the glass transition point of the first organic fiber 20 is preferably 10° C. or more, and more preferably 30° C. or more.

[0028] On the other hand, if the difference in glass transition point between them is 130°C or less, the time from when the first organic fiber 20 completely solidifies until the organic binder starts to solidify can be appropriately adjusted, and the organic binder solidifies while remaining in a well-dispersed state, thereby achieving an even greater skeleton reinforcement effect. Therefore, the difference in glass transition point between them is preferably 130°C or less, more preferably 120°C or less, even more preferably 100°C or less, even more preferably 80°C or less, and particularly preferably 70°C or less.

[0029] 1 , in the heat-transfer-suppressing sheet 10, inorganic particles 30 are held on the branches 22 of the first organic fibers 20. The inorganic particles 30 are a compounding component that greatly contributes to the heat insulating performance of the heat-transfer-suppressing sheet 10, and preferred inorganic particles 30 are shown below.

[0030] As the inorganic particles, a single inorganic particle may be used, or two or more types of inorganic particles may be used in combination. From the viewpoint of the heat transfer suppression effect, it is preferable to use particles made of at least one inorganic material selected from oxide particles, carbide particles, nitride particles, and inorganic hydrate particles, and it is more preferable to use oxide particles. Furthermore, the shape is not particularly limited, but it is preferable to include at least one type selected from nanoparticles, hollow particles, and porous particles. Specifically, 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. may also be used.

[0031] 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.

[0032] In addition, when two or more inorganic particles with 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. When large-diameter particles and small-diameter particles are used, the small-diameter particles enter the gaps between the large-diameter particles, resulting in a denser structure and improving the heat transfer suppression effect. Hereinafter, the inorganic particles will be described in more detail, with the small-diameter inorganic particles referred to as the first inorganic particles and the large-diameter inorganic particles referred to as the second inorganic particles.

[0033] (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 those caused by abnormal heat generation. As the oxide particles, at least one type of particles 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.

[0034] Since the particle size of the oxide particles can affect the effect of reflecting radiant heat, even higher thermal insulation can be obtained by limiting the average primary particle size to a predetermined range. That is, when the average primary particle size 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 in high-temperature regions of 500°C or higher, thereby further improving thermal insulation. On the other hand, when the average primary particle size of the oxide particles is 50 μm or less, the number and number of contact points between particles do not increase even when compressed, making it difficult to form a path for conductive heat transfer, thereby reducing the impact on thermal insulation, especially in normal temperature regions where conductive heat transfer is dominant.

[0035] In the present invention, the average primary particle size can be determined by observing particles under a microscope, comparing with a standard scale, and taking the average of any 10 particles.

[0036] In the present invention, nanoparticles refer to particles on the nanometer order that are spherical or nearly spherical and have an average primary particle diameter of less than 1 μm. Nanoparticles have a low density, which suppresses conductive heat transfer. Using nanoparticles as the first inorganic particles further refines the three-dimensionally connected pores, thereby achieving excellent thermal insulation that suppresses convective heat transfer. Therefore, nanoparticles are preferred because they can suppress heat conduction between adjacent nanoparticles during normal battery operation at room temperature. Furthermore, using nanoparticles with a small average primary particle diameter as oxide particles can suppress an increase in conductive heat transfer through the heat-transfer-suppressing sheet, even when the heat-transfer-suppressing sheet 10 is compressed by expansion due to thermal runaway in the battery cell, increasing its internal density. This is thought to be because nanoparticles are prone to forming small voids between particles due to electrostatic repulsion, and their low bulk density allows the particles to be packed together to provide cushioning.

[0037] In the present invention, when nanoparticles are used as the first inorganic particles, there are no particular limitations on the material as long as they comply with the definition of nanoparticles. For example, silica nanoparticles are a material with high heat insulating properties, and the contact points between particles are small, so the amount of heat conducted by silica nanoparticles is smaller than when silica particles with a large particle diameter are used. Furthermore, commonly available silica nanoparticles have a bulk density of 0.1 (g / cm ３ ), for example, even if the battery cells arranged on both sides of the heat-transfer-suppressing sheet thermally expand and a large compressive stress is applied to the heat-transfer-suppressing sheet, the size (area) and number of contact points between the silica nanoparticles do not increase significantly, and 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.

[0038] Generally, wet silica particles are aggregated, whereas dry silica particles can be dispersed. Since heat conduction is predominant in the temperature range of 300°C or less, dry silica particles can be dispersed, which allows for better heat insulation performance than wet silica.

[0039] Limiting the average primary particle diameter of the nanoparticles to a predetermined range can achieve even higher thermal insulation. That is, when the average primary particle diameter of the nanoparticles is 1 nm or more and 100 nm or less, convective heat transfer and conductive heat transfer in 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. Meanwhile, the average primary particle diameter of the nanoparticles is more preferably 50 nm or less, and even more preferably 10 nm or less.

[0040] 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. A specific example of an inorganic hydrate is 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.

[0041] 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

[0042] As will be described later, the heat transfer-suppressing sheet 10 is disposed between the battery cells 110 of the battery pack 100 shown in FIG. 4 , for example. In a battery cell 110 that experiences thermal runaway, the temperature rises rapidly to over 200°C and continues to rise to around 700°C. Therefore, it is preferable that the inorganic particles 30 be made of an inorganic hydrate whose thermal decomposition onset temperature is 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 110 that experiences thermal runaway, and can efficiently suppress the temperature rise. Therefore, these inorganic hydrates are preferable.

[0043] Furthermore, when inorganic hydrate particles are used as the first inorganic particles, if the average particle size is too large, it takes some time for the first inorganic particles (inorganic hydrate) near the center of the heat transfer-suppressing sheet 10 to reach their thermal decomposition temperature, and the first inorganic particles near the center of the sheet may not be completely thermally decomposed. For this reason, the average secondary particle size of the inorganic hydrate particles is preferably 0.01 μm or more and 200 μm or less, and more preferably 0.05 μm or more and 100 μm or less.

[0044] Particles Made of Thermally Expandable Inorganic Material Examples of thermally expandable inorganic materials include vermiculite, bentonite, mica, and perlite.

[0045] 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.

[0046] Inorganic Balloons When inorganic balloons are included, convective heat transfer or conductive heat transfer in the heat-transfer-suppressing sheet 10 can be suppressed in a temperature range of less than 500° C., thereby further improving the heat insulation properties. As the inorganic balloons, at least one type selected from shirasu balloons, silica balloons, fly ash balloons, barite balloons, and glass balloons can be used.

[0047] (Second inorganic particles) When two types of inorganic particles are used, the second inorganic particles are not particularly limited as long as they are different from the first inorganic particles in material, particle size, etc. That is, as the type of the second inorganic particles, the above-mentioned particles that are the first inorganic particles can be used, and the details thereof are as described above.

[0048] Preferably, silica particles, particularly nanosilica particles, are used as the first inorganic particles, and at least one particle selected from titania, zircon, zirconia, silicon carbide, zinc oxide, and alumina is used as the second inorganic particles, and the two are mixed together.

[0049] Furthermore, from the viewpoint of heat insulating performance, it is preferable that silica particles, preferably silica nanoparticles, account for 50 mass % or more of the total amount of the heat transfer-suppressing sheet.

[0050] <Other Blending Materials> The heat transfer-suppressing sheet 10 may be blended with at least one of inorganic fibers, second organic fibers, and an organic binder.

[0051] (Inorganic Fibers) The inorganic fibers may be those typically used in heat insulating materials, etc., and can improve the mechanical strength of the heat transfer-suppressing sheet 10 and the ability to retain the first inorganic particles and the second inorganic particles.

[0052] Specific examples of the inorganic fibers that can be used include ceramic fibers such as silica fibers, alumina fibers, alumina silicate fibers, zirconia fibers, carbon fibers, soluble fibers, refractory ceramic fibers, aerogel composites, magnesium silicate fibers, alkaline earth silicate fibers, and potassium titanate fibers; glass fibers such as glass fibers and glass wool; artificial mineral fibers such as rock wool and basalt fiber; and natural mineral fibers such as wollastonite.

[0053] The inorganic fibers may be used alone or in combination of two or more of the various inorganic fibers exemplified above. When using a combination of two or more inorganic fibers, it is preferable to use fibers with melting points exceeding 1000°C and fibers with melting points of 700°C or less. When using fibers with melting points exceeding 1000°C, inorganic fibers with melting points exceeding 1000°C do not melt or soften and can maintain their shape even if thermal runaway occurs in the battery cell 110, making them suitable for use. The average fiber diameter of the inorganic fibers is preferably 1 nm or more and 20 μm or less, and more preferably 1 μm or more and 15 μm or less. The average fiber diameter can be obtained by measuring 20 points using a SEM and calculating the average value. The fiber length of the inorganic fibers is preferably 100 mm or less, and more preferably 0.1 mm or more.

[0054] (Second Organic Fiber) The second organic fiber is a linear fiber that does not have the trunk 21 and branches 22 like the first organic fiber 20. The type of fiber may be the same as or different from the first organic fiber 20. Two or more types may be mixed and used. Specific examples of the second organic fiber include chemical fibers such as polyvinyl alcohol (PVA) fiber, polyethylene fiber, nylon fiber, polyurethane fiber, and ethylene-vinyl alcohol copolymer fiber, as well as plant fibers such as cellulose. In particular, when it is desired to improve the strength of the heat transfer-suppressing sheet 10, it is preferable to use a fiber with heat fusion properties, such as polyvinyl alcohol (PVA) fiber or polyethylene fiber.

[0055] (Organic Binder) The heat transfer-suppressing sheet 10 may also contain an organic binder. The organic binder is not particularly limited as long as it has a glass transition point lower than that of the first organic fibers 20. For example, an organic binder containing at least one selected from a styrene-butadiene resin, an acrylic resin, a silicone-acrylic resin, and a styrene resin can be used.

[0056] The lower limit of the glass transition point of the organic binder is preferably −10° C. or higher. If the glass transition point of the organic binder is room temperature or higher, the strength of the heat-transfer-suppressing sheet 10 can be further improved when the heat-transfer-suppressing sheet 10 is used at room temperature. Therefore, the glass transition point of the organic binder is, for example, more preferably 20° C. or higher, even more preferably 30° C. or higher, even more preferably 50° C. or higher, and particularly preferably 60° C. or higher.

[0057] (Others) In addition to the above, materials conventionally used as heat insulating materials can also be added to the heat-transfer-suppressing sheet 10 as needed.

[0058] [Method for Manufacturing Heat Transfer-Suppressing Sheet] A papermaking method is used to manufacture the heat transfer-suppressing sheet 10. That is, the heat transfer-suppressing sheet 10 is manufactured by dispersing inorganic fibers, second organic fibers, an organic binder, and the like in water together with the first organic fibers 20 and the inorganic particles 30, and then dehydrating, molding, and drying the resulting dispersion. When using the papermaking method, a dispersant, a flocculating agent, and the like may be added.

[0059] In the heat-transfer-suppressing sheet 10 described above, trunk portions 21 of the first organic fibers 20 are oriented in multiple layers along the main surface 10a, as shown in Fig. 1. This reduces the number of voids between the fibers of the first organic fibers 20, suppressing heat transfer between the fibers. Furthermore, the heat-transfer-suppressing sheet 10 is less likely to be crushed. For example, when the heat-transfer-suppressing sheet 10 is disposed between battery cells 110 of a battery pack 100 shown in Fig. 4, the sheet has high resistance to pressure from the battery cells 110.

[0060] 3 is a schematic diagram showing the gaps formed between fibers when linear organic fibers 50 without trunks or branches are used. As shown in the figure, the linear organic fibers 50 are randomly oriented, with many contact points between the fibers, facilitating heat transfer between the fibers. Furthermore, there are many gaps between the fibers, making the entire sheet prone to collapse. Furthermore, the inorganic particles 30 are contained in the gaps between the fibers, and their retention force is not sufficient. It is presumed that increasing the amount of inorganic particles to improve thermal insulation performance would make it difficult to prevent powder shedding.

[0061] In contrast, when first organic fibers 20 having trunk portions 21 and branch portions 22 are used as in the present invention, the trunk portions 21 are oriented in multiple layers along the main surface 10a of the heat-transfer-suppressing sheet 10, as shown in Fig. 1, with few contact points between fibers and few gaps. Furthermore, because the inorganic particles 30 are well held by the branch portions 22, powder falling can be suppressed even if the number of inorganic particles 30 is increased. In other words, the heat-transfer-suppressing sheet 10 of the present invention has excellent heat insulation properties and compression characteristics, and powder falling can be suppressed even if the number of inorganic particles 30 is increased to further improve heat insulation performance.

[0062] 4, the battery pack 100 includes a plurality of battery cells 110 housed in a battery case 120, and the battery cells 110 are connected in series or parallel. In this embodiment, the heat transfer-suppressing sheet 10 is disposed between the battery cells 110. Although not shown, the heat transfer-suppressing sheet 10 may be disposed on at least one of the top, side wall, and bottom wall of the battery case 120.

[0063] Beaten cellulose nanofibers were used as the first organic fibers, nanosilica particles and titania particles were used as the inorganic particles, glass fibers were used as the inorganic fibers, an acrylic resin was used as the organic binder, and a dispersant and a flocculant were added to and mixed with water, and a heat transfer-suppressing sheet was produced by a papermaking method.

[0064] The specifications of the cellulose nanofibers are as follows: Average fiber diameter of the trunk: 20 μm Average fiber length of the trunk: 3 mm Average fiber diameter of the branches: 3 nm Average fiber length of the branches: 3 μm

[0065] The amounts of these ingredients are shown in Table 1.

[0066]

[0067] Fig. 5 shows an SEM photograph of the surface of the produced heat-transfer-suppressing sheet, and Fig. 6 shows an enlarged view of a portion of the photograph, which shows that trunk portions 21 of first organic fibers 20 are stacked in multiple layers and oriented in multiple layers along the surface (main surface) of the heat-transfer-suppressing sheet. It can also be seen that numerous thin branches 22 branch out from trunk portions 21 and support the inorganic fibers.

[0068] 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.

[0069] This application is based on a Japanese patent application (Patent Application No. 2024-046469) filed on March 22, 2024, the contents of which are incorporated herein by reference.

[0070] REFERENCE SIGNS LIST 10 heat transfer suppression sheet 10a principal surface 20 first organic fiber 21 trunk portion 22 branch portion 30 inorganic particle 50 linear organic fiber 100 assembled battery 110 battery cell 120 battery case

Claims

1. A heat-transfer-suppressing sheet comprising: inorganic particles; and first organic fibers having a wide trunk portion with a width-equivalent dimension of 1 μm to 100 μm and branches branching from the trunk portion; wherein the trunk portions of the first organic fibers are oriented in multiple layers along the main surface of the heat-transfer-suppressing sheet.

2. The heat transfer suppressing sheet according to claim 1, wherein the branch portions of the first organic fibers hold the inorganic particles.

3. The heat transfer suppression sheet according to claim 1, wherein the branch portions are curled.

4. The heat-transfer-suppressing sheet according to claim 1, wherein the inorganic particles comprise silica particles in an amount of 50 mass % or more of the total amount of the heat-transfer-suppressing sheet.

5. The heat transfer suppression sheet according to claim 1, wherein the first organic fibers are beaten fibers.

6. The heat transfer-suppressing sheet according to claim 1, wherein the inorganic particles include at least one particle selected from the group consisting of titania, zircon, zirconia, silicon carbide, zinc oxide, and alumina.

7. The heat transfer suppression sheet according to claim 1, further comprising at least one of inorganic fibers, second organic fibers, and an organic binder.

8. The heat transfer-suppressing sheet according to claim 1, further comprising an organic binder, wherein the difference between the glass transition point of the organic binder and the glass transition point of the first organic fiber is 10°C or more.

9. The heat transfer suppression sheet according to claim 1, further comprising second organic fibers that are linear fibers having neither the trunk nor the branches.

10. 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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