Heat transfer suppression sheet and battery module

Abstract

To provide a heat transfer suppression sheet exhibiting excellent thermal insulation performance and capable of retaining its shape when subjected to high heat or flame from a heat source such as a battery cell that has experienced thermal runaway, and to provide a battery module equipped with the heat transfer suppression sheet and exhibiting excellent safety.SOLUTION: A heat transfer suppression sheet 10 comprises silica particles containing at least one softening-point-lowering component selected from Li2CO3, Li2SO4, Na2CO3, Na2SO4, K2CO3, K2SO4, Li2O, Na2O, K2O, P2O5 and B2O3, the softening-point-lowering component being 0.2 to 10.0 mass% of an entire amount of the heat transfer suppression sheet 10, the heat transfer suppression sheet 10 being employed in a battery module 100. The battery module 100 comprises a plurality of battery cells 110 and the heat transfer suppression sheet 10.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] In recent years, 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, and 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 fiber sheet and nanosilica between the cells of a battery pack, and provides a composite layer made of folded fiber sheets to provide high insulation and suppress heat transfer to adjacent battery cells when abnormal heat generation occurs. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-204708 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the insulating material of Patent Document 1, examples of fiber materials for the fiber sheet include PET (polyethylene terephthalate), flame-retardant oxidized acrylic, and glass wool. However, when exposed to high heat or flames during thermal runaway, these fibers decompose, scattering nanosilica and making it impossible to maintain the sheet shape.

[0006] Therefore, an object of the present invention is to provide a heat-transfer-suppressing sheet that not only has excellent thermal insulation performance but also can maintain its shape even when exposed to high heat or flames from a heat source such as a battery cell that has experienced thermal runaway, and to provide a battery pack that is equipped with the heat-transfer-suppressing sheet and has excellent safety. [Means for solving the problem]

[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 for use in a battery pack formed by housing a plurality of battery cells in a battery case, comprising: Li2CO3, Li2SO4, Na2CO3, Na2SO4, K2CO3, K2SO4, Li2O, Na2O, K2O, and silica particles containing at least one softening point reducing component selected from P2O5 and B2O3; and A heat-transfer-suppressing sheet, wherein the softening point lowering component accounts for 0.2 to 10.0 mass % of the total amount of the heat-transfer-suppressing sheet.

[0009] Furthermore, preferred embodiments of the present invention relating to the heat transfer-suppressing sheet relate to the following [2] to [6].

[0010] [2] The heat-transfer-suppressing sheet according to [1], further comprising particles of at least one material selected from the group consisting of titania, zircon, zirconia, silicon carbide, zinc oxide, and alumina. [3] The heat-transfer-suppressing sheet according to [1] or [2], wherein the content of the silica particles containing the softening-point-decreasing component is 40 mass % or more of the total amount of the heat-transfer-suppressing sheet. [4] The heat-transfer-suppressing sheet according to any one of [1] to [3], further comprising at least one of inorganic fibers, organic fibers, and organic binders. [5] The heat-transfer-suppressing sheet according to any one of [1] to [4], further comprising titania containing the softening-point-decreasing component. [6] The heat-transfer-suppressing sheet according to [5], wherein the softening-point lowering component contained in the titania is at least one of K2O and P2O5.

[0011] The above object of the present invention is achieved by the following configuration [7] relating to a battery pack.

[0012] [7] A battery pack comprising a plurality of battery cells and the heat-transfer-suppressing sheet according to any one of [1] to [6], the plurality of battery cells being connected in series or in parallel. [Effects of the Invention]

[0013] The heat-transfer-suppressing sheet of the present invention has first inorganic particles containing a softening-point-lowering component and an SiO2 component (silica component) dispersed in an aggregate, and is used in contact with or facing a heat source such as a battery cell. When exposed to high heat or flame from the heat source, the first inorganic particles in the surface layer facing the heat source soften and bond together to spread into a film, improving heat dissipation in the plane direction and suppressing heat transfer in the thickness (depth) direction of the heat-transfer-suppressing sheet, thereby maintaining the sheet shape.

[0014] Furthermore, in the battery pack of the present invention, the heat transfer suppression sheet of the present invention is disposed on the inner wall of the metal case and between the battery cells, so that 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. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a cross-sectional view schematically illustrating an example of the heat transfer-suppressing sheet of the present invention. [Figure 2] FIG. 2 is a cross-sectional view that schematically shows the cross section of a heat-transfer-suppressing sheet when subjected to high heat or flame. [Figure 3] FIG. 3 is a cross-sectional view schematically showing an example of a battery pack of the present invention. [Figure 4] FIG. 4 is a photograph, used as a substitute for a drawing, showing the surface of the heat-transfer-suppressing sheet after heating. DETAILED DESCRIPTION OF THE INVENTION

[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 suppression sheet] 1 is a cross-sectional view schematically showing a heat-transfer-suppressing sheet 10 of the present invention, and as shown in the figure, first inorganic particles 20 and second inorganic particles 21 are dispersed in the aggregate of the heat-transfer-suppressing sheet 10. The aggregate is composed of inorganic fibers 30, organic fibers 31, an organic binder (not shown), etc.

[0018] <First inorganic particle> The first inorganic particles 20 are silica particles containing at least one softening point lowering component selected from Li2CO3, Li2SO4, Na2CO3, Na2SO4, K2CO3, K2SO4, Li2O, Na2O, K2O, P2O5, and B2O3. Among the softening point lowering components, adding one to silica particles changes the covalent bond length within the glass, thereby lowering the melting point. Therefore, it is more preferable to use at least one of Li2CO3, Li2SO4, Na2CO3, Na2SO4, K2CO3, K2SO4, Li2O, Na2O, and K2O. Furthermore, it is even more preferable to use at least one of Li2O, Na2O, and K2O.

[0019] Furthermore, it is more preferable that the first inorganic particles 20 are nanoparticles. Because nanoparticles have a low density, they suppress heat transfer through contact points between the first inorganic particles 20 and between the first inorganic particles 20 and the second inorganic particles 21, inorganic fibers 30, and organic fibers 31. Furthermore, the first inorganic particles 20 penetrate into and disperse in gaps formed by the second inorganic particles 21, inorganic fibers 30, and organic fibers 31, thereby giving the heat transfer-suppressing sheet 10 a denser structure and improving its strength. In the present invention, nanoparticles refer to particles of the order of nanometers that are spherical or nearly spherical and have an average primary particle diameter of less than 1 μm.

[0020] Furthermore, when nanoparticles with a smaller average primary particle diameter are used, heat transfer can be suppressed even when the internal density of heat-transfer-suppressing sheet 10 increases due to expansion caused by thermal runaway of the battery cell. This is thought to be because nanoparticles are prone to forming fine voids between particles due to electrostatic repulsion, and because their bulk density is low, the particles are packed together to provide cushioning properties.

[0021] Furthermore, even higher thermal insulation can be achieved by limiting the average primary particle diameter of the nanoparticles to a predetermined range. For example, by setting the average primary particle diameter of the nanoparticles to 1 nm or more and 100 nm or less, the thermal insulation of the heat-transfer-suppressing sheet 10 can be further improved, particularly in the temperature range below 500°C. Furthermore, even when compressive stress is applied to the heat-transfer-suppressing sheet 10, the voids remaining between the nanoparticles and the contact points between the many particles suppress heat transfer, thereby maintaining thermal insulation. 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.

[0022] Thus, as the first inorganic particles 20, silica nanoparticles containing the softening point lowering component are particularly preferred.

[0023] Fig. 2 is a schematic diagram (see Fig. 3) showing a heat-transfer-suppressing sheet 10 disposed between multiple battery cells 110A and 110B in a battery pack. In Fig. 2, when battery cell 110A experiences thermal runaway and the resulting high heat and flames act on heat-transfer-suppressing sheet 10, the first inorganic particles 20 in surface layer 10a of heat-transfer-suppressing sheet 10 on the side that contacts battery cell 110A soften and even melt due to the softening-point-lowering component, bonding the particles together and spreading out into a film, as shown by the hatching in Fig. 2. This allows heat from battery cell 110A to be transferred and dissipated in a planar manner, suppressing heat transfer to the compounded material located deeper than surface layer 10a. Furthermore, because a film of first inorganic particles 20 is formed on surface layer 10a and the compounded material located deeper than surface layer 10a remains intact, the shape of heat-transfer-suppressing sheet 10 is maintained.

[0024] To obtain such an effect of the softening-point lowering component, the content of the softening-point lowering component is 0.2 to 10.0 mass%, more preferably 0.5 to 5.0 mass%, and even more preferably 0.5 to 3.0 mass% of the total amount of the heat-transfer-suppressing sheet 10. If the content of the softening-point lowering component is less than 0.2 mass%, the effect of softening and even melting the first inorganic particles 20 is not sufficiently obtained. On the other hand, if the content of the softening-point lowering component is less than the impurity level (i.e., 0.1 mass%), the softening-point lowering component's effect of lowering the glass transition temperature is not fully exerted.

[0025] On the other hand, if the content of the softening point lowering component exceeds 10.0 mass %, heat transfer-suppressing sheet 10 becomes too soft and is unable to maintain its shape when exposed to high heat or flames from battery cell 110A that has experienced thermal runaway.

[0026] The content of the first inorganic particles 20 must satisfy the above-mentioned content range of the softening point lowering component, but in order to obtain sufficient heat insulating performance from the silica base material, the content is preferably 40 mass % or more of the total amount of the heat transfer-suppressing sheet 10, and more preferably 50 mass % or more.

[0027] The first inorganic particles 20 can be produced by a precipitation method. That is, sulfuric acid is added to an aqueous solution of sodium silicate as a base material and a softening point lowering component to cause a reaction, and the reaction product is precipitated, followed by filtering, washing with water, drying, pulverization, etc.

[0028] <Second inorganic particles> There are no restrictions on the second inorganic particles 21, but from the viewpoint of the heat transfer suppression effect, it is preferable that they are made of at least one inorganic material selected from oxide particles, carbide particles, nitride particles, and inorganic hydrate particles, and it is more preferable that they contain oxide particles or inorganic hydrate particles.

[0029] (oxide particles) Oxide particles have a high refractive index and a strong effect of diffusely reflecting light, so they can suppress radiant heat transfer, particularly in high-temperature regions. As the oxide, it is preferable to use at least one selected from titania, zirconia, zircon, barium titanate, zinc oxide, and alumina. In particular, titania is the most preferable because it has 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.

[0030] Furthermore, titania may contain softening-point depressing components such as K2O and P2O5. By selecting titania containing a softening-point depressing component and incorporating it into heat-transfer-inhibiting sheet 10, it is expected that titania will soften and even melt and spread into a film-like shape together with first inorganic particles 20 containing the softening-point depressing component in surface layer 10a of heat-transfer-inhibiting sheet 10, thereby making it possible to more effectively exert the above-mentioned effects.

[0031] Regarding the particle size of the oxide particles, if the average primary particle size is limited to a predetermined range, even higher thermal insulation properties can be obtained. That is, if 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 radiative heat transfer within the heat-transfer-suppressing sheet in high-temperature ranges of 500°C or higher, thereby further improving thermal insulation properties. On the other hand, if 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 paths for conductive heat transfer. This reduces the impact on thermal insulation, particularly in normal temperature ranges where conductive heat transfer is dominant.

[0032] (Inorganic hydrate particles) When inorganic hydrate particles receive heat from a heating element and reach a temperature above their thermal decomposition initiation temperature, they undergo thermal decomposition and release their own water of crystallization, lowering the temperature of the heating element and its surroundings, thereby exhibiting a so-called "endothermic effect." After releasing the water of crystallization, the particles become porous, and the numerous air holes provide thermal insulation.

[0033] Specific examples of inorganic hydrates include aluminum hydroxide (Al(OH)3), magnesium hydroxide (Mg(OH)2), calcium hydroxide (Ca(OH)2), zinc hydroxide (Zn(OH)2), iron hydroxide (Fe(OH)2), manganese hydroxide (Mn(OH)2), zirconium hydroxide (Zr(OH)2), and gallium hydroxide (Ga(OH)3).

[0034] For example, aluminum hydroxide has about 35% water of crystallization, and as shown in the following formula, it thermally decomposes, releasing the water of crystallization and exhibiting an endothermic effect. After releasing the water of crystallization, it becomes a porous alumina (Al2O3) and functions as a heat insulating material. 2Al(OH)3 → Al2O3 + 3H2O

[0035] The heat-transfer-suppressing sheet 10 is also used in a battery pack 100 (see FIG. 3 ), and 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, inorganic hydrates with a thermal decomposition onset temperature of 200°C or higher are preferred. 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 experiences thermal runaway, and can efficiently suppress temperature rise, making these inorganic hydrates preferred.

[0036] If the average particle size of the inorganic hydrate particles is too large, it will take some time for the inorganic hydrate particles near the center of heat-transfer-suppressing sheet 10 to reach their thermal decomposition temperature, and the inorganic hydrate particles near the center of heat-transfer-suppressing sheet 10 may not be completely thermally decomposed. For this reason, the average 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.

[0037] The second inorganic particles 21 may be any of the above particles or a combination of two or more types. The combined use of two or more inorganic particles with different heat transfer suppression effects allows for multi-stage cooling, enabling the endothermic effect to be exerted over a wider temperature range. It is also preferable to use a mixture of large and small diameter particles. When large and small diameter particles are used, the small diameter particles fill the gaps between the large diameter particles, resulting in a denser structure and improved heat transfer suppression effect.

[0038] Other examples that can be used include particles made of thermally expandable inorganic materials such as vermiculite, bentonite, mica, and perlite; particles made of hydrous porous materials such as zeolite, kaolinite, montmorillonite, acid clay, diatomaceous earth, aerogel, mica, and vermiculite; and inorganic balloons such as shirasu balloons, silica balloons, fly ash balloons, barite balloons, and glass balloons.

[0039] Furthermore, the heat transfer-suppressing sheet 10 contains inorganic fibers 30, organic fibers 31, an organic binder, and the like to hold the first inorganic particles 20 and second inorganic particles 21.

[0040] <Inorganic fibers> The inorganic fibers 30 may be inorganic fibers typically used in heat insulating materials, but preferably have first and second inorganic fibers that differ from each other in at least one property selected from the group consisting of average fiber diameter, shape, and glass transition point. By including two types of inorganic fibers that differ from each other in properties, the mechanical strength of the heat transfer-suppressing sheet 10 and its ability to retain the first inorganic particles 20 and the second inorganic particles 21 can be improved.

[0041] When two types of inorganic fibers are contained, it is preferable that the average fiber diameter of the first inorganic fibers be larger than that of the second inorganic fibers, that the first inorganic fibers be linear or needle-like, and that the second inorganic fibers be dendritic or crimped. First inorganic fibers with a larger average fiber diameter (larger diameter) have the effect of improving the mechanical strength and shape retention of the heat-transfer-inhibiting sheet 10. This effect can be achieved by making one of the two types of inorganic fibers, for example, the first inorganic fibers, larger in diameter than the second inorganic fibers. Because the heat-transfer-inhibiting sheet 10 may be subjected to external impacts, including the first inorganic fibers, the heat-transfer-inhibiting sheet 10 has improved impact resistance. Examples of external impacts include pressure due to the expansion of the battery cells 110 and wind pressure due to the ignition of the battery cells 110.

[0042] Furthermore, it is particularly preferable that the first inorganic fibers be linear or needle-shaped in order to improve the mechanical strength and shape retention of the heat transfer-suppressing sheet 10. Note that linear or needle-shaped fibers refer to fibers having a crimp degree, as described below, of less than 10%, preferably 5% or less.

[0043] More specifically, the average fiber diameter of the first inorganic fibers is preferably 1 μm or more, more preferably 3 μm or more. If the first inorganic fibers are too thick, moldability and processability may be reduced, so the average fiber diameter of the first inorganic fibers is preferably 20 μm or less, more preferably 15 μm or less.

[0044] If the first inorganic fibers are too long, moldability and processability may decrease, so the fiber length is preferably 100 mm or less.Furthermore, if the first inorganic fibers are too short, shape retention and mechanical strength may decrease, so the fiber length is preferably 0.1 mm or more.

[0045] On the other hand, the second inorganic fibers having a small average fiber diameter (thin diameter) have the effect of improving the retention property and increasing the flexibility of the heat insulating material, and therefore it is preferable that the second inorganic fibers have a smaller diameter than the first inorganic fibers.

[0046] More specifically, to improve retention, the second inorganic fibers are preferably flexible and easily deformable. Therefore, the second inorganic fibers, which are thin, preferably have an average fiber diameter of less than 1 μm, more preferably 0.1 μm or less. However, if they are too thin, they are prone to breakage and their retention ability decreases. Furthermore, the proportion of entangled fibers present in the insulation increases, which not only reduces retention ability but also leads to poor moldability and shape retention. Therefore, the average fiber diameter of the second inorganic fibers is preferably 1 nm or more, more preferably 10 nm or more.

[0047] If the second inorganic fibers are too long, moldability and shape retention will decrease, so the fiber length of the second inorganic fibers is preferably 0.1 mm or less.

[0048] The second inorganic fibers are preferably dendritic or crimped. Such a shape allows the second inorganic fibers to be well entangled with the inorganic particles, improving retention. Furthermore, when the thermal insulation material is subjected to pressure or wind pressure, the second inorganic fibers are prevented from sliding and moving, thereby improving mechanical strength, particularly against external pressure and impact.

[0049] The term "dendritic" refers to a two-dimensionally or three-dimensionally branched structure, such as feather-like, tetrapod-like, radial, or three-dimensional mesh-like.

[0050] When the second inorganic fibers are dendritic, the average fiber diameter can be obtained by measuring the diameters of the trunk and branches at several points using an SEM and calculating the average value of these.

[0051] The crimped structure refers to a structure in which fibers are bent in various directions. One method for quantifying the crimped structure is to calculate the crimp degree from an electron microscope photograph, which can be calculated, for example, using the following formula: Crimp degree (%) = (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, they are the fiber length and the distance between fiber ends projected onto a two-dimensional plane, and are shorter than the actual values. Based on this formula, the crimp degree of the second inorganic fiber is preferably 10% or more, and more preferably 30% or more. If the crimp degree is low, the holding ability decreases, and it becomes difficult to form entanglements (networks) between the second inorganic fibers and between the first inorganic fibers and the second inorganic fibers.

[0052] When two or more types of inorganic fibers are contained, it is preferable that the first inorganic fiber is an amorphous fiber, and the second inorganic fiber is at least one type of fiber selected from an amorphous fiber having a glass transition point higher than that of the first inorganic fiber and a crystalline fiber.

[0053] The melting point of crystalline inorganic fibers is usually higher than the glass transition point of amorphous inorganic fibers. Therefore, when exposed to high heat, the surface of the first inorganic fiber softens before the second inorganic fiber, binding the inorganic particles. Therefore, by including the first inorganic fiber, the mechanical strength of the thermal insulation material can be improved.

[0054] Specifically, the first inorganic fiber is preferably an inorganic fiber having a melting point of less than 700°C, and many amorphous inorganic fibers can be used. Among them, a fiber containing SiO2 is preferable, and glass fiber is more preferable because it is inexpensive, easily available, and has excellent handleability.

[0055] As described above, the second inorganic fibers are at least one type of fibers selected from amorphous fibers and crystalline fibers having a glass transition temperature higher than that of the first inorganic fibers. Many crystalline inorganic fibers can be used as the second inorganic fibers.

[0056] If the second inorganic fiber is made of crystalline fiber or has a higher glass transition temperature than the first inorganic fiber, the second inorganic fiber will not melt or soften even if the first inorganic fiber softens when exposed to high heat. Therefore, when applied to a battery pack, the second inorganic fiber will maintain its shape even if a battery cell experiences thermal runaway.

[0057] Furthermore, if the second inorganic fibers do not melt or soften, tiny spaces are maintained between the first inorganic particles 20 and the second inorganic particles 21, between each particle and the inorganic fibers 30 or the organic fibers 31, and between each fiber, thereby providing an insulating effect through air.

[0058] When the second inorganic fiber is crystalline, specific examples that can be used include ceramic fibers such as silica fiber, alumina fiber, alumina silicate fiber, zirconia fiber, carbon fiber, soluble fiber, refractory ceramic fiber, aerogel composite, magnesium silicate fiber, alkaline earth silicate fiber, and potassium titanate fiber; glass fibers such as glass fiber and glass wool; and natural mineral fibers such as rock wool, basalt fiber, and wollastonite.

[0059] Furthermore, if the melting point exceeds 1000°C, the second inorganic fiber will not melt or soften and will be able to maintain its shape even if thermal runaway occurs in the battery cell, and therefore can be used favorably. Of the fibers listed as the second inorganic fiber, it is more preferable to use ceramic fibers such as silica fibers, alumina fibers, and alumina silicate fibers, as well as mineral fibers, and it is even more preferable to use fibers with a melting point exceeding 1000°C.

[0060] Furthermore, even if the second inorganic fibers are amorphous, they can be used as long as they have a higher glass transition temperature than the first inorganic fibers. For example, glass fibers having a higher glass transition temperature than the first inorganic fibers may be used as the second inorganic fibers.

[0061] As the second inorganic fiber, the various inorganic fibers exemplified above may be used alone or in combination of two or more kinds.

[0062] As described above, the first inorganic fiber has a lower glass transition point than the second inorganic fiber, and when exposed to high heat, the first inorganic fiber softens first, allowing the first inorganic fiber to bind inorganic particles. However, for example, if the second inorganic fiber is amorphous and has a smaller fiber diameter than the first inorganic fiber, and the glass transition points of the first and second inorganic fibers are close, the second inorganic fiber may soften first. Therefore, when the second inorganic fiber is amorphous, the glass transition point of the second inorganic fiber is preferably at least 100°C higher than the glass transition point of the first inorganic fiber, and more preferably at least 300°C higher.

[0063] The fiber length of the first inorganic fibers is preferably 100 mm or less, and more preferably 0.1 mm or more, and the fiber length of the second inorganic fibers is preferably 0.1 mm or less, for the reasons described above.

[0064] When two types of inorganic fibers are contained, it is preferable that the first inorganic fibers are amorphous fibers, the second inorganic fibers are at least one type of fiber selected from amorphous fibers having a glass transition point higher than that of the first inorganic fibers and crystalline fibers, and the average fiber diameter of the first inorganic fibers is larger than the average fiber diameter of the second inorganic fibers.

[0065] As described above, it is preferable that the average fiber diameter of the first inorganic fibers be larger than that of the second inorganic fibers. It is also preferable that the thick first inorganic fibers be amorphous fibers, and the thin second inorganic fibers be at least one type of fiber selected from amorphous fibers and crystalline fibers having a higher glass transition point than the first inorganic fibers. This allows the first inorganic fibers to have a low glass transition point and soften quickly, forming a film and hardening as the temperature rises. On the other hand, if the thin second inorganic fibers are at least one type of fiber selected from amorphous fibers and crystalline fibers having a higher glass transition point than the first inorganic fibers, the thin second inorganic fibers remain in their fibrous form even when the temperature rises, thereby maintaining the structure of the thermal insulating material and preventing powder shedding.

[0066] Even in this case, the fiber length of the first inorganic fibers is preferably 100 mm or less, and more preferably 0.1 mm or more. The fiber length of the second inorganic fibers is preferably 0.1 mm or less. The reasons for this are as described above.

[0067] (organic fiber) The heat transfer-suppressing sheet 10 may also contain organic fibers 31. As the organic fibers 31, for example, at least one type selected from cellulose fibers, PET fibers, polyvinyl alcohol (PVA) fibers, polyethylene fibers, nylon fibers, polyurethane fibers, and ethylene-vinyl alcohol copolymer fibers can be used.

[0068] The heat transfer-suppressing sheet 10 can also be manufactured by a papermaking method. However, since it is difficult to raise the heating temperature above 250°C in this case, the glass transition point of the organic fibers 31 is preferably 250°C or lower, and more preferably 200°C or lower.

[0069] Although the lower limit of the glass transition point of the organic fibers 31 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 fibers 31 and the glass transition point of the organic binder is 10°C or more, the organic binder will solidify after the organic fibers 31, which were in a semi-molten state, have completely solidified in the cooling step during production, and 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 organic fibers is preferably 10°C or more, and more preferably 30°C or more.

[0070] On the other hand, if the difference in glass transition point between them is 130°C or less, the time from when the organic fibers 31 completely solidify 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 between the glass transition point of the organic binder and the glass transition point of the organic fibers 31 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.

[0071] Two or more types of organic fibers can also be used, in which case at least one type of organic fiber should act as the skeleton, i.e., should have a glass transition point higher than that of the organic binder. As described above, the difference between the glass transition point of the organic binder and that of the at least one type of organic fiber is preferably 10°C or more, more preferably 30°C or more, and 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.

[0072] When two or more types of organic fibers are contained, it is sufficient that at least one of the organic fibers has a glass transition point higher than that of the organic binder. However, it is more preferable that the other organic fibers contain crystalline organic fibers that do not have a glass transition point.

[0073] Crystalline organic fibers that do not have a glass transition point can also be included, but because these crystalline organic fibers have no softening point, the overall strength of heat-transfer-suppressing sheet 10 can be maintained even when exposed to high temperatures that would soften the organic fibers that form the framework. Furthermore, by including crystalline organic fibers, these organic fibers also function as the framework of heat-transfer-suppressing sheet 10 at room temperature. This improves the flexibility and handleability of heat-transfer-suppressing sheet 10.

[0074] An example of the crystalline organic fiber is polyester (PET) fiber.

[0075] The fiber length of organic fibers 31 is not particularly limited, but from the viewpoint of ensuring moldability and processability, the average fiber length is preferably 10 mm or less. On the other hand, from the viewpoint of allowing organic fibers 31 to function as a skeleton and ensuring the compressive strength of heat transfer-suppressing sheet 10, the average fiber length is preferably 0.5 mm or more.

[0076] (organic binder) The heat transfer-suppressing sheet 10 may also contain an organic binder. There are no particular limitations on the organic binder as long as it has a glass transition point lower than that of the organic fibers 31. For example, an organic binder containing at least one resin selected from styrene-butadiene resin, acrylic resin, silicon-acrylic resin, and styrene resin can be used.

[0077] Although the glass transition point of the organic binder is not particularly limited, it 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 is used at room temperature. Therefore, the glass transition point of the organic binder is 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.

[0078] (Other compounding materials) In addition to the above, materials conventionally used as heat insulating materials can also be added to heat-transfer-suppressing sheet 10 as needed.

[0079] The heat transfer-suppressing sheet 10 can be manufactured by a papermaking method. That is, the first inorganic particles 20, the second inorganic particles 21, the inorganic fibers 30, the organic fibers 31, the organic binder, etc. are dispersed in water, and the resulting dispersion is dehydrated, molded, and dried to produce the sheet. When the papermaking method is used, a dispersant, a flocculant, etc. may be added.

[0080] Alternatively, the porous material can be produced by a dry method, in which the first inorganic particles 20, the second inorganic particles 21, the inorganic fibers 30, the organic fibers 31, the organic binder, and the like are placed in an appropriate mixer, thoroughly dispersed, and pre-molded.

[0081] [Battery pack] 3, the battery pack 100 has 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. [Example]

[0082] A heat-transfer-suppressing sheet was produced by a papermaking method using precipitated nanosilica particles containing a softening-point-decreasing component (hereinafter, including Table 2, abbreviated as "nanosilica particles") as the first inorganic particles, titania particles as the second inorganic particles, glass fibers as the inorganic fibers, cellulose fibers as the organic fibers, and acrylic resin as the organic binder, with a dispersant and a flocculant further added.

[0083] Furthermore, component analysis of the nanosilica particles (values ​​at 800°C) revealed that they contained 0.24 mass% Na2O, 99.1 mass% SiO2, and 0.47 mass% Al2O3, which are softening point lowering components.

[0084] The amounts of the above ingredients are as shown in Table 1.

[0085] [Table 1]

[0086] A flame of approximately 1000°C was applied from a burner to the surface of the heat transfer-suppressing sheet that had been produced, and then an SEM photograph was taken of the area of ​​the surface that had been exposed to the flame. As shown in Figure 4, it can be seen that some of the nanosilica particles have softened and melted. The melting point of silica is over 1500°C, so it does not soften or melt when exposed to a flame of 1000°C. However, the softening point is lowered by the inclusion of Na2O, a softening point lowering component, and the sheet softens and melts even at 1000°C. [Explanation of symbols]

[0087] 10 Heat transfer suppression sheet 10a Surface layer 20 First inorganic particles 21 Secondary inorganic particles 30 Inorganic Fibers 31 Organic Fibers 100 battery packs 110,110A battery cell 120 Battery Case

Claims

1. A heat transfer-suppressing sheet for use in a battery pack formed by housing a plurality of battery cells in a battery case, Li ２ CO ３ , Li ２ SO ４ , Na ２ CO ３ , Na ２ SO ４ , K. ２ CO ３ , K. ２ SO ４ , Li ２ O, Na ２ O.K. ２ O, P ２ O ５ and B ２ O ３ and comprising silica particles containing at least one softening point reducing component selected from A heat-transfer-suppressing sheet, wherein the softening point lowering component is present in an amount of 0.2 to 10.0 mass % of the total amount of the heat-transfer-suppressing sheet.

2. 2. The heat transfer-suppressing sheet according to claim 1, further comprising particles of at least one material selected from the group consisting of titania, zircon, zirconia, silicon carbide, zinc oxide, and alumina.

3. 2. The heat-transfer-suppressing sheet according to claim 1, wherein the content of the silica particles containing the softening point lowering component is 40 mass % or more of the total amount of the heat-transfer-suppressing sheet.

4. The heat transfer-suppressing sheet according to claim 1 , further comprising at least one of inorganic fibers, organic fibers, and an organic binder.

5. The heat transfer-suppressing sheet according to claim 1 , further comprising titania containing the softening point-decreasing component.

6. The softening point lowering component contained in the titania is K ２ O and P ２ O ５ The heat transfer suppressing sheet according to claim 5 , wherein the heat transfer suppressing sheet is at least one of the following:

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

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