Patent Publication Number: US-2023138031-A1

Title: Heat transfer suppression sheet and battery pack

Description:
TECHNICAL FIELD 
     The present invention relates to a heat transfer suppression sheet and a battery pack in which the heat transfer suppression sheet is interposed between battery cells. 
     BACKGROUND ART 
     In recent years, an electric vehicle, a hybrid vehicle, or the like driven by an electric motor has been actively developed from a viewpoint of environmental protection. A battery pack in which a plurality of battery cells is connected in series or in parallel is mounted on the electric vehicle, the hybrid vehicle, or the like to serve as a power source of a driving electric motor. 
     A lithium ion secondary battery capable of high capacity and high output as compared with a lead storage battery, a nickel-metal hydride battery, or the like is mainly used as the battery cell. Then, when a thermal runaway occurs in which a certain battery cell rapidly rises in temperature due to an internal short circuit, overcharge, or the like of the battery and then continues to generate heat, the heat from the battery cell in which the thermal runaway occurs may propagate to another adjacent battery cell to cause the thermal runaway of another battery cell. 
     As a technique for preventing the propagation of heat from the battery cell in which the thermal runaway occurs as described above, a heat transfer suppression sheet is interposed between the battery cells. For example, Patent Literature 1 proposes a heat transfer suppression sheet containing at least one of a mineral powder and a flame retardant, and a matrix resin selected from a thermosetting resin, a thermoplastic elastomer, and a rubber. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP-A-2018-206605 
     SUMMARY OF INVENTION 
     Technical Problem 
     In such a heat transfer suppression sheet, it is required to satisfactorily retain powder having a heat transfer prevention effect (that is, to prevent powder falling), and to retain a shape and continue to be present between battery cells even when the battery cells are heated to a high temperature due to thermal runaway. In the heat transfer suppression sheet described in Patent Literature 1, a matrix resin is used in order to retain a mineral powder and a flame retardant. However, such a matrix resin is melted at the high temperature. Therefore, in the heat transfer suppression sheet described in Patent Literature 1, the shape may not be retained when the thermal runaway occurs in the battery cell. 
     Accordingly, an object of the present invention is to provide a heat transfer suppression sheet capable of preventing falling of powder or the like having a heat transfer prevention effect and having excellent shape retainability at a high temperature, and a battery pack in which the heat transfer suppression sheet is interposed between battery cells. 
     Solution to Problem 
     The above object is achieved by the heat transfer suppression sheet of the following (1) according to the present invention. 
     (1) A heat transfer suppression sheet includes: inorganic particles; first inorganic fibers; and second inorganic fibers, in which 
     the first inorganic fibers are amorphous fibers, and 
     the second inorganic fibers contain at least one kind selected from amorphous fibers having a glass transition point higher than that of the first inorganic fibers and crystalline fibers. 
     The heat transfer suppression sheet according to the present invention is preferably (2) to (17) below. 
     (2) The heat transfer suppression sheet according to (1), in which the first inorganic fibers have a glass transition point of less than 700° C. 
     (3) The heat transfer suppression sheet according to (1) or (2), in which an average fiber diameter of any one of the first inorganic fibers and the second inorganic fibers is larger than an average fiber diameter of the other. 
     (4) The heat transfer suppression sheet according to (3), in which the average fiber diameter of the first inorganic fibers is larger than the average fiber diameter of the second inorganic fibers. 
     (5) The heat transfer suppression sheet to any one of (1) to (4), in which the first inorganic fibers are fibers containing SiO 2 , and 
     the second inorganic fibers are fibers containing at least one kind selected from silica fibers, alumina fibers, alumina silicate fibers, zirconia fibers, and mineral-based fibers. 
     (6) The heat transfer suppression sheet according to (5), in which the first inorganic fibers are glass fibers, and 
     the second inorganic fibers are the mineral-based fibers. 
     (7) The heat transfer suppression sheet according to any one of (1) to (6), in which the inorganic particles have an average secondary particle diameter of 0.01 μm or more and 200 μm or less. 
     (8) The heat transfer suppression sheet according to any one of (1) to (7), in which the inorganic particles contain at least one kind selected from oxide particles, carbide particles, nitride particles, and inorganic hydrate particles. 
     (9) The heat transfer suppression sheet according to (8), in which the inorganic particles contain the oxide particles. 
     (10) The heat transfer suppression sheet according to (9), in which the oxide particles have an average primary particle diameter of 0.001 μm or more and 50 μm or less. 
     (11) The heat transfer suppression sheet according to any one of (8) to (10), in which the oxide particles contain at least one kind selected from silica, titania, zirconia, zircon, barium titanate, zinc oxide, and alumina. 
     (12) The heat transfer suppression sheet according to any one of (8) to (11), in which the inorganic particles contain at least one kind selected from nanoparticles, hollow particles, and porous particles. 
     (13) The heat transfer suppression sheet according to (12), in which the inorganic particles contain the nanoparticles. 
     (14) The heat transfer suppression sheet according to (12) or (13), in which the nanoparticles are silica nanoparticles. 
     (15) The heat transfer suppression sheet according to any one of (12) to (14), in which the nanoparticles have an average primary particle diameter of 1 nm or more and 100 nm or less. 
     (16) The heat transfer suppression sheet according to any one of (11) to (15), in which the oxide particles contain titania. 
     (17) The heat transfer suppression sheet according to any one of (1) to (16), in which, with respect to a total mass of the heat transfer suppression sheet, 
     a content of the inorganic particles is 30 mass % or more and 94 mass % or less, 
     a content of the first inorganic fibers is 3 mass % or more and 30 mass % or less, and 
     a content of the second inorganic fibers is 3 mass % or more and 30 mass % or less. 
     The object of the present invention is achieved by the configuration of the following (18) related to a battery pack. 
     (18) A battery pack, in which a plurality of battery cells is connected in series or in parallel, and 
     the heat transfer suppression sheet according to any one of (1) to (17) is interposed between the battery cells. 
     Advantageous Effects of Invention 
     Since the heat transfer suppression sheet according to the present invention contains inorganic particles having an excellent heat transfer prevention effect and inorganic fibers, the heat transfer prevention effect is excellent. 
     In the heat transfer suppression sheet according to the present invention, since the first inorganic fibers and the second inorganic fibers are entangled to form a network, the inorganic particles can be satisfactorily retained in the network. 
     Further, when the heat transfer suppression sheet according to the present invention is exposed to a high temperature, a surface of the first inorganic fibers, which are amorphous fibers having a low glass transition point, is softened relatively early, and the inorganic particles and the second inorganic fibers are bound. Accordingly, the heat transfer suppression sheet according to the present invention can improve the mechanical strength at the high temperature. Further, the heat transfer suppression sheet may receive a pressing force due to expansion caused by thermal runaway of the battery cell or may receive a wind pressure caused by ignition of the battery cell. Since the heat transfer suppression sheet according to the present invention has excellent mechanical strength at the high temperature, the heat transfer suppression sheet can resist these external forces. 
     In the battery pack according to the present invention, the above-described heat transfer suppression sheet is interposed between the battery cells. Therefore, the battery pack according to the present invention can minimize a damage of the thermal runaway of the battery cell. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view schematically showing an embodiment of a heat transfer suppression sheet according to the present invention. 
         FIG.  2    is an SEM photograph showing a cross section of the heat transfer suppression sheet according to the embodiment of the present invention produced by a dry extrusion molding method. 
         FIG.  3    is a cross-sectional view schematically showing an embodiment of a battery pack according to the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of a heat transfer suppression sheet and a battery pack according to the present invention will be described in detail with reference to the drawings. The present invention is not limited to the embodiment described below. In the drawings, members and portions having the same functions may be denoted by the same reference numerals, and duplicate descriptions may be omitted or simplified. The embodiment described in the drawings are schematically illustrated for clear explanation of the present invention, and do not necessarily accurately represent actual sizes and scales. 
     [1. Heat Transfer Suppression Sheet] 
       FIG.  1    is a cross-sectional view schematically showing an embodiment of a heat transfer suppression sheet according to the present invention. An illustrated heat transfer suppression sheet  10  includes inorganic particles  20 , first inorganic fibers  30 , and second inorganic fibers  31 . The inorganic particles  20 , the first inorganic fibers  30 , and the second inorganic fibers  31  are all heat resistant materials, and furthermore, innumerable minute spaces are formed between the particles, between the particles and the fibers, and between the fibers, and a heat insulating effect by air is also exhibited, so that heat transfer prevention performance is excellent. 
     &lt;1-1. Inorganic Particles&gt; 
     Although a material of the inorganic particles is not particularly limited, from the viewpoint of the heat transfer prevention effect, the inorganic particles  20  preferably contain at least one selected from oxide particles, carbide particles, nitride particles, and inorganic hydrate particles, and more preferably contain oxide particles. 
     A shape and a size of the inorganic particles  20  are also not particularly limited. It is preferable that the inorganic particles  20  contain at least one selected from nanoparticles, hollow particles, and porous particles, and it is more preferable that the inorganic particles  20  contain nanoparticles. 
     As the inorganic particles  20 , a single kind of inorganic particles may be used, or two or more kinds of inorganic particles  20  may be used in combination. When two or more kinds of the inorganic particles  20  having different heat transfer prevention effects are used in combination, a heating element can be cooled in multiple stages, and an endothermic effect can be exhibited in a wider temperature range. It is also preferable to use a mixture of large-diameter particles and small-diameter particles as the inorganic particles  20 . When the small-diameter inorganic particles  20  enter gaps between the large diameter inorganic particles  20 , a structure becomes denser, and the heat transfer prevention effect can be improved. 
     When the average secondary particle diameter of the inorganic particles  20  is 0.01 μm or more, it makes the inorganic particles  20  easily available, and an increase in production cost can be prevented. In a case of 200 μm or less, a desired heat insulating effect can be obtained. Therefore, an average secondary particle diameter of the inorganic particles  20  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. 
     Next, an example of the material or the shape of particles that can be used as the inorganic particle  20  will be described in detail below. 
     (1-1-1. Oxide Particles) 
     Since oxide particles have a high refractive index and a strong effect of diffusely reflecting light, when the oxide particles are used as the inorganic particles, radiant heat transfer can be prevented particularly in a high temperature region such as abnormal heat generation. As the oxide particles, particles of at least one selected from silica, titania, zirconia, zircon, barium titanate, zinc oxide, and alumina can be used. That is, among the oxide particles that can be used as inorganic particles, only one kind may be used, or two or more kinds of oxide particles may be used. In particular, silica is a component having high heat insulating property, titania is a component having a refractive index higher than that of other metal oxides, and has a high effect of diffusely reflecting light and blocking radiant heat in a high temperature region of 500° C. or higher. Therefore, silica and titania are most preferably used as the oxide particles. 
     (Average Primary Particle Diameter of Oxide Particles: 0.001 μm or More and 50 μm or Less) 
     Since a particle diameter of the oxide particles may affect influence of reflecting the radiant heat, when an average primary particle diameter is limited to a predetermined range, higher heat insulating property can be obtained. 
     That is, when the average primary particle diameter of the oxide particles is 0.001 μm or more, the average primary particle diameter is sufficiently larger than a wavelength of the light contributing to heating, and the light is efficiently diffusely reflected. Therefore, the radiant heat transfer of heat in the heat transfer suppression sheet is prevented in the high temperature region of 500° C. or more, and the heat insulating property can be further improved. On the other hand, when the average primary particle diameter of the oxide particles is 50 μm or less, contact points and the number between the particles are not increased even when the oxide particles are compressed, and it is difficult to form a path of conductive heat transfer. Therefore, in particular, it is possible to reduce the influence on the heat insulating property in a normal temperature range in which the conductive heat transfer is dominant. 
     When two or more kinds of oxide particles are used, it is also preferable to use a mixture of the large-diameter particles and the small-diameter particles (nanoparticles). In this case, the average primary particle diameter of the large-diameter particles is more preferably 1 μm or more and 50 μm or less, still more preferably 5 μm or more and 30 μm or less, and most preferably 10 μm or less. 
     In the present invention, the average primary particle diameter can be determined by observing particles with a microscope, comparing with a standard scale, and averaging any of 10 particles. 
     (1-1-2. Nanoparticles) 
     In the present invention, the nanoparticles represent nanometer-order particles having an average primary particle diameter of less than 1 μm which are spherical particles or particles close to the spherical shape. Since the nanoparticles have a low density, conductive heat transfer is prevented, and when the nanoparticles are used as the inorganic particles, voids are further finely dispersed, and thus excellent heat insulating property of preventing convective heat transfer can be obtained. 
     Therefore, it is preferable to use the nanoparticles from the viewpoint that heat conduction between adjacent nanoparticles can be prevented when a battery in a normal ordinary temperature range is used. 
     In the present invention, at least one kind of the oxide particles, the carbide particles, the nitride particles, and the inorganic hydrate particles selected as the inorganic particles is preferably the nanoparticles. 
     Further, when the nanoparticles having a small average primary particle diameter are used as the oxide particles, even when the heat transfer suppression sheet is compressed due to expansion caused by thermal runaway of the battery cell and an internal density increases, an increase in conductive heat transfer of the heat transfer suppression sheet can be prevented. It is considered that this is because nanoparticles form fine voids easily between the particles due to a repulsive force caused by static electricity, and a bulk density is low, and thus the particles are filled so as to have cushioning properties. 
     In the present invention, when the nanoparticles are used as the inorganic particles, the material is not particularly limited as long as the material conforms to the definition of the nanoparticles as described above. For example, since the silica nanoparticles are a material having high heat insulating property and have small contact points between the particles, an amount of heat conducted by the silica nanoparticles is smaller than that in the case where silica particles having a large particle diameter are used. Since the generally available silica nanoparticles have a bulk density of about 0.1 g/cm 3 , for example, even when the battery cells disposed on both sides of a heat insulation sheet thermally expand and a large compressive stress is applied to the heat insulation sheet, a size (area) and the number of contact points between the silica nanoparticles do not significantly increase, and the heat insulating property can be maintained. Therefore, the silica nanoparticles are preferably used as the nanoparticles. As the silica nanoparticles, wet silica, dry silica, aerogel, and the like can be used. 
     As described above, titania has a high effect of blocking radiant heat, the silica nanoparticles have extremely small conductive heat transfer, and can maintain excellent heat insulating property even when the compressive stress is applied to the heat insulation sheet. Therefore, it is most preferable to use both titania and silica nanoparticles as the inorganic particles. 
     (Average Primary Particle Diameter of Nanoparticles: 1 nm or More and 100 nm or Less) 
     When the average primary particle diameter of the nanoparticles is limited to a predetermined range, even higher heat insulating property can be obtained. 
     That is, when the average primary particle diameter of the nanoparticles is 1 nm or more and 100 nm or less, the convection heat transfer and conductive heat transfer of heat in the heat transfer suppression sheet can be prevented particularly in a temperature range of less than 500° C., and the heat insulation property can be further improved. Even when the compressive stress is applied, the voids remaining between the nanoparticles and the contact points between many particles can prevent the conductive heat transfer, and the heat insulating property of the heat transfer suppression sheet can be maintained. 
     The average primary particle diameter of the nanoparticles is more preferably 2 nm or more, and still 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 still more preferably 10 nm or less. 
     (1-1-3. Inorganic Hydrate Particles) 
     The inorganic hydrate particles are thermally decomposed at equal to or higher than the thermal decomposition initiation temperature by receiving heat from a heating element, and release crystal water possessed by the inorganic hydrate particles to lower the temperature of the heating element and surroundings thereof, that is, exhibit a so-called “endothermic effect”. In addition, after the crystal water is released, a porous body is formed, and a heat insulating effect is exhibited by a large number of air holes. 
     Specific examples of the inorganic hydrate 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 ). 
     For example, aluminum hydroxide has about 35% of crystal water. As shown in the following formula, the aluminum hydroxide is thermally decomposed to release the crystal water, thereby exhibiting the endothermic effect. After the crystal water is discharged, alumina (Al 2 O 3 ), which is the porous body, functions as a heat insulator. 
       2Al(OH) 3 →Al 2 O 3 +3H 2 O
 
     As described later, in the battery pack according to the present invention, the heat transfer suppression sheet  10  is interposed between the battery cells. In the battery cell in which the thermal runaway occurs, the temperature rapidly rises to a temperature exceeding 200° C., and the temperature continues to rise to around 700° C. Therefore, the inorganic particles preferably contain an inorganic hydrate having a thermal decomposition initiation temperature of 200° C. or higher. 
     The thermal decomposition initiation temperature of the inorganic hydrate described above is about 200° C. for aluminum hydroxide, about 330° C. for magnesium hydroxide, about 580° C. for calcium hydroxide, about 200° C. for zinc hydroxide, about 350° C. for iron hydroxide, about 300° C. for manganese hydroxide, about 300° C. for zirconium hydroxide, and about 300° C. for gallium hydroxide. All of the hydrates are preferably inorganic hydrates since the temperature substantially overlaps with a temperature range of a rapid temperature rise of the battery cell in which the thermal runaway occurs and can efficiently prevent the temperature rise. 
     When the inorganic hydrate particles are used as the inorganic particles  20 , if the average particle diameter thereof is too large, a certain amount of time is required for the inorganic particles  20  (inorganic hydrate) in the vicinity of a center of the heat transfer suppression sheet  10  to reach the thermal decomposition temperature thereof, and thus the inorganic particles  20  in the vicinity of the center of the sheet may not be completely thermally decomposed. Therefore, 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. 
     &lt;1-2. First Inorganic Fiber&gt; 
     The first inorganic fibers  30  are amorphous fibers. The second inorganic fibers are fibers containing at least one selected from amorphous fibers having a glass transition point higher than that of the first inorganic fibers and crystalline fibers. A melting point of the crystalline inorganic fibers is usually higher than the glass transition point of the amorphous inorganic fibers. Therefore, when the first inorganic fibers  30  are exposed to a high temperature, surfaces of the first inorganic fibers  30  are softened before the second inorganic fibers  31  are softened to bind the inorganic particles  20  and the second inorganic fibers  31 , and thus a mechanical strength of the heat transfer suppression sheet  10  can be improved. 
     Specifically, the first inorganic fibers  30  are preferably inorganic fibers having the melting point of less than 700° C., and many amorphous inorganic fibers can be used. Among these, fibers containing SiO 2  are preferable, and glass fibers are more preferable since that the glass fibers are inexpensive, easily available, and excellent in handleability and the like. 
     &lt;1-3. Second Inorganic Fiber&gt; 
     As described above, the second inorganic fibers  31  are fibers containing at least one kind selected from amorphous fibers having a glass transition point higher than that of the first inorganic fibers and crystalline fibers. As the second inorganic fibers  31 , many crystalline inorganic fibers can be used. 
     When the second inorganic fibers  31  contain crystalline fibers or have the glass transition point higher than that of the first inorganic fibers  30 , the second inorganic fibers  31  are not melted or softened even if the first inorganic fibers  30  are softened when exposed to a high temperature. Therefore, the shape can be maintained even at the time of thermal runaway of the battery cell, and it can continue to exist between the battery cells. 
     When the second inorganic fibers  31  are not melted or softened, minute spaces are maintained between the inorganic particles  20 , between the inorganic particles  20  and the first inorganic fibers and the second inorganic fibers, and between the first inorganic fibers and the second inorganic fibers. Therefore, the heat insulating effect by air is exhibited, and excellent heat transfer prevention performance can be maintained. 
     When the second inorganic fibers  31  are crystalline, examples of the second inorganic fibers  31  include ceramic-based fibers such as silica fibers, alumina fibers, alumina silicate fibers, and zirconia fibers, and mineral-based fibers such as rock wool, alkaline earth silicate fibers, zirconia fibers, potassium titanate fibers, and wollastonite. 
     Among the fibers exemplified as the second inorganic fibers  31 , when the melting point is higher than 1000° C., the second inorganic fibers  31  do not melt or soften even when the thermal runaway of the battery cell occurs, and the shape thereof can be maintained. 
     Among the fibers exemplified as the second inorganic fibers  31 , for example, the ceramic-based fibers such as silica fibers, alumina fibers, and alumina silicate fibers, and the mineral-based fibers are more preferably used. Among these, fibers having a melting point of more than 1000° C. are further preferably used. 
     Even when the second inorganic fibers  31  are amorphous, fibers having the glass transition point higher than that of the first inorganic fibers  30  can be used. For example, the glass fiber having the glass transition point higher than that of the first inorganic fiber  30  may be used as the second inorganic fiber  31 . 
     As the second inorganic fibers  31 , the various inorganic fibers exemplified above may be used alone, or two or more kinds thereof may be used in combination. 
     As described above, the first inorganic fibers  30  have a lower glass transition point than the second inorganic fibers  31 . When exposed to the high temperature, the first inorganic fibers  30  soften first, so that the inorganic particles  20  and the second inorganic fibers  31  can be bound by the first inorganic fibers  30 . However, for example, when the second inorganic fibers  31  are amorphous and a fiber diameter thereof is smaller than a fiber diameter of the first inorganic fibers  30 , if the glass transition points of the first inorganic fibers  30  and the second inorganic fibers  31  are close to each other, the second inorganic fibers  31  may be softened first. 
     Therefore, when the second inorganic fibers  31  are amorphous fibers, the glass transition point of the second inorganic fibers  31  is preferably higher than the glass transition point of the first inorganic fibers  30  by 100° C. or more, and more preferably 300° C. or more. 
     &lt;1-4. Average Fiber Diameter of Inorganic Fibers&gt; 
     In the present invention, the inorganic fibers having a large average fiber diameter (large diameter) have an effect of improving the mechanical strength and shape retainability of the heat transfer suppression sheet  10 . In a case where any one of the first inorganic fibers  30  and the second inorganic fibers  31  have a large diameter, the above effect can be obtained. Since an impact from the outside may act on the heat transfer suppression sheet  10 , impact resistance is enhanced by including large-diameter inorganic fibers. The impact from the outside is, for example, a pressing force due to expansion of the battery cell, or a wind pressure due to ignition of the battery cell. 
     In order to improve the mechanical strength and the shape retainability of the heat transfer suppression sheet  10 , the large-diameter inorganic fibers have particularly preferably a linear shape or a needle shape. The fiber having the linear shape or the needle shape refers to a fiber having a crimp degree described later of, for example, less than 10%, preferably 5% or less. 
     More specifically, in order to improve the mechanical strength and the shape retainability of the heat transfer suppression sheet  10 , the average fiber diameter of the large-diameter inorganic fibers is preferably 1 μm or more, and more preferably 3 μm or more. If the large-diameter inorganic fiber is too thick, moldability and processability of the heat transfer suppression sheet  10  may be reduced. Therefore, the average fiber diameter is preferably 20 μm or less, and more preferably 15 μm or less. 
     If the large-diameter inorganic fibers are too long, the moldability and the processability may be reduced. Therefore, a fiber length is preferably 100 mm or less. Further, if the large-diameter inorganic fibers are too short, since the shape retainability and the mechanical strength are reduced, the fiber length is preferably 0.1 mm or more. 
     On the other hand, inorganic fibers having a small average fiber diameter (small diameter) have an effect of improving the retainability of the inorganic particles  20  and increasing flexibility of the heat transfer suppression sheet  10 . Therefore, in a case where the other of the first inorganic fibers  30  and the second inorganic fibers  31  have a small diameter, the above-described effect can be obtained. 
     More specifically, in order to improve the retainability of the inorganic particles  20 , it is preferable that the small-diameter inorganic fibers are easily deformed and have flexibility. Therefore, the average fiber diameter of the small-diameter inorganic fibers is preferably less than 1 μm, and more preferably 0.1 μm or less. However, if the small-diameter inorganic fibers are too thin, the fibers are easily broken, and an ability to retain the inorganic particles  20  is reduced. In addition, a proportion of the fibers present in the sheet in an entangled state without retaining the inorganic particles  20  increases, and in addition to a reduction in the ability to retain the inorganic particles  20 , the moldability and the shape retainability are also deteriorated. Therefore, the average fiber diameter of the small-diameter inorganic fibers is preferably 1 nm or more, and more preferably 10 nm or more. 
     When the small-diameter inorganic fibers are too long, since the moldability and the shape retainability are reduced, the fiber length is preferably 0.1 mm or less. 
     In addition, the small-diameter inorganic fibers preferably have a dendritic shape or a crimped shape. When the small-diameter inorganic fibers have such a shape, the small-diameter inorganic fibers are entangled with the large-diameter inorganic fibers and the inorganic particles  20  in the heat transfer suppression sheet  10 . Therefore, the ability to retain the inorganic particles  20  is improved. When the heat transfer suppression sheet receives the pressing force or the wind pressure, it is prevented that the small-diameter inorganic fibers slide and move, and thus the mechanical strength against the pressing force or the impact from the outside is improved. 
     The dendritic shape is a two-dimensionally or three-dimensionally branched structure, for example, a feather-like shape, a tetrapot shape, a radial shape, or a three-dimensional mesh-like shape. 
     When the small-diameter inorganic fibers  31  have dendritic shape, the average fiber diameter thereof can be obtained by measuring the diameters of a trunk portion and a branch portion at several points by SEM and calculating an average value thereof. 
     The crimped shape is a structure in which fibers are bent in various directions. As one of methods for quantifying the crimped form, it is known to calculate the crimp degree from an electron micrograph, and the crimp degree can be calculated from, for example, the following formula. 
       Crimp degree(%)=(fiber length−distance between the ends of the fiber)/(fiber length)×100
 
     Here, both the fiber length and the distance between the ends of the fiber are measurement values on the electron micrograph. That is, these are the fiber length and the distance between the ends of the fiber under projection onto a two-dimensional plane, and are shorter than actual values. Based on the formula, the crimp degree of the small-diameter inorganic fibers is preferably 10% or more, and more preferably 30% or more. When the crimp degree is small, the ability to retain the inorganic particles  20 , the entanglement (network) between the large-diameter inorganic fibers or with the large-diameter inorganic fibers is less likely to be formed. 
     As described above, the average fiber diameter of any one of the first inorganic fibers  30  and the second inorganic fibers  31  is preferably larger than the average fiber diameter of the other. However, in the present invention, the average fiber diameter of the first inorganic fibers  30  is more preferably larger than the average fiber diameter of the second inorganic fibers  31 . When the average fiber diameter of the first inorganic fibers  30  is large, the glass transition point of the first inorganic fibers  30  is low and the first inorganic fibers  30  soften quickly. Therefore, as the temperature rises, it becomes membranous and gets hard. On the other hand, when the average fiber diameter of the second inorganic fibers  31  is small, the small-diameter second inorganic fibers  31  remain in the form of fibers even if the temperature rises. Therefore, the structure of the heat transfer suppression sheet  10  can be maintained and powder falling can be prevented. 
     It is most preferable that both large-diameter inorganic fibers having the linear shape or the needled shape and small-diameter inorganic fibers having the dendritic shape or the crimped-shape are used as the first inorganic fiber  30 , and both large-diameter inorganic fibers having the linear shape or the needle shape and small-diameter inorganic fibers having the dendritic shape or the crimped shape are used as the second inorganic fiber  31  because the retaining effect of the inorganic particles, mechanical strength, and shape retainability can be further enhanced. 
     &lt;1-5. Contents of Inorganic Particles, First Inorganic Fibers, and Second Inorganic Fibers&gt; 
     A content of the inorganic particles  20  is preferably 30 mass % or more and 94 mass % or less with respect to the total mass of the heat transfer suppression sheet  10 , the content of the first inorganic fibers  30  is preferably 3 mass % or more and 30 mass % or less with respect to the total mass of the heat transfer suppression sheet  10 , and the content of the second inorganic fibers  31  is preferably 3 mass % or more and 30 mass % or less with respect to the total mass of the heat transfer suppression sheet  10 . 
     More preferably, with respect to the total mass of the heat transfer suppression sheet  10 , the content of the inorganic particles  20  is 60 mass % or more and 90 mass % or less, the content of the first inorganic fibers  30  is 5 mass % or more and 15 mass % or less, and the content of the second inorganic fibers  31  is 5 mass % or more and 15 mass % or less. By setting such a content, heat absorption and heat insulating effect by the inorganic particles  20 , the shape retainability, pressing force resistance, and wind pressure resistance by the first inorganic fibers  30 , and the ability of the second inorganic fibers  31  to retain the inorganic particles  20  are exhibited in a well-balanced manner. 
     &lt;1-6. Other Blending Materials&gt; 
     An organic fiber, an organic binder, or the like may be blended in the heat transfer suppression sheet  10  as necessary. All of these are useful for the purpose of reinforcing the heat transfer suppression sheet  10  and improving the moldability, and the total amount thereof is preferably 10 mass % or less with respect to the total amount of the sheet. 
     The kinds of the organic fibers, the organic binder, and the like to be used are not particularly limited, and examples of the organic fibers include pulp fibers and polyester fibers. As the organic binder, a preferable binder can be selected according to a production method. The organic binder that can be selected in the present invention will be described later. 
     &lt;1-7. Thickness of Heat Transfer Suppression Sheet&gt; 
     The thickness of the heat transfer suppression sheet  10  is not particularly limited, and is preferably 0.05 mm or more and 5 mm or less. When the thickness is less than 0.05 mm, sufficient mechanical strength cannot be imparted to the heat transfer suppression sheet  10 . On the other hand, when the thickness is more than 5 mm, the heat transfer suppression sheet  10  may be difficult to form. 
     [2. Production Method of Heat Transfer Suppression Sheet] 
     The heat transfer suppression sheet  10  is produced by molding a material including at least the inorganic particles  20 , the first inorganic fibers  30 , and the second inorganic fibers  31  by a dry molding method or a wet molding method. As the dry molding method, for example, a press molding method (dry press molding method) and an extrusion molding method (dry extrusion molding method) can be used. 
     &lt;2-1. Production Method Using Dry Press Molding Method&gt; 
     In the dry press molding method, the inorganic particles  20 , the first inorganic fibers  30 , the second inorganic fibers  31 , and if necessary, the organic fibers, the organic binder, and the like are put into a mixer such as a V-type mixer at a predetermined proportion. Then, after the materials put into the mixer are sufficiently mixed, a mixture is put into a predetermined mold and press-molded, so that the heat transfer suppression sheet  10  can be obtained. At the time of press forming, heating may be performed as necessary. 
     A press pressure during press forming is preferably in a range of 0.98 MPa or more and 9.80 MPa or less. When the press pressure is less than 0.98 MPa, the strength of the heat transfer suppression sheet  10  to be obtained may not be maintained, and the heat transfer suppression sheet  10  may collapse. On the other hand, when the press pressure is more than 9.80 MPa, the processability may be reduced due to excessive compression, or solid heat transfer may increase due to an increase in the bulk density, and the heat insulating property may be reduced. 
     When using the dry press molding method, polyvinyl alcohol (PVA) is preferably used as the organic binder, whereas the organic binder is not particularly limited as long as it is an organic binder generally used when using the dry press molding method. 
     &lt;2-2. Production Method Using Dry Extrusion Molding Method&gt; 
     In the dry extrusion molding method, water is added to the inorganic particles  20 , the first inorganic fibers  30 , the second inorganic fibers  31 , and if necessary, the organic fibers, the organic binder, and the like which are a binder, and the mixture is kneaded by a kneader to prepare a paste. Thereafter, the obtained paste is extruded from a slit-shaped nozzle using an extrusion molding machine, and further dried, so that the heat transfer suppression sheet  10  can be obtained. When using the dry extrusion molding method, methyl cellulose, water-soluble cellulose ether, and the like are preferably used as the organic binder, whereas the organic binder is not particularly limited as long as it is an organic binder generally used when using the dry extrusion molding method. 
       FIG.  2    is an SEM photograph showing a cross section of the heat transfer suppression sheet according to the embodiment of the present invention produced by the dry extrusion molding method. As shown in  FIG.  2   , the heat transfer suppression sheet includes the inorganic particles  20 , the large-diameter first inorganic fibers  30 , and the small-diameter second inorganic fibers  31 . Numerous minute spaces are formed between the inorganic particles  20 , between the inorganic particles  20  and the inorganic fibers  30  and  31 , and between the inorganic fibers  30  and  31 , and the heat insulating effect by air is also exhibited, so that the heat transfer prevention performance is excellent. 
     &lt;2-3. Production Method Using Wet Molding Method&gt; 
     In the wet molding method, the inorganic particles  20 , the first inorganic fibers  30 , the second inorganic fibers  31 , and if necessary, the organic binder as the binder are mixed in water, and the mixture is stirred with a stirrer to prepare a mixed liquid. Thereafter, the obtained mixed liquid is poured into a molding machine in which a filtration mesh is formed at a bottom surface, and the mixed liquid is dehydrated through the mesh to prepare a wet sheet. Thereafter, the obtained wet sheet is heated and pressurized, so that the heat transfer suppression sheet  10  can be obtained. 
     Prior to a heating and pressurizing step, a ventilation drying treatment in which hot air is passed through the wet sheet to dry the sheet may be performed, whereas the wet sheet may be heated and pressurized in a wet state without performing the ventilation drying treatment. When the wet molding method is used, cationic starch or an acrylic resin can be selected as the organic binder. 
     [3. Battery Pack] 
     In the battery pack according to the present invention, the heat transfer suppression sheet  10  is interposed between the battery cells. Specifically, as shown in  FIG.  3   , a battery pack  100  includes a plurality of battery cells  101  arranged side by side, connected in series or in parallel, and accommodated in a battery case  110 , and the heat transfer suppression sheet  10  is interposed between the battery cells  101 . 
     In such a battery pack  100 , even when one battery cell  101  is thermally runaway to reach a high temperature, and expands or ignites, the influence on adjacent other battery cell  101  is prevented by the heat transfer suppression sheet  10  excellent in the heat transfer prevention effect and the strength at the high temperature. Therefore, in the battery pack according to the present invention, a chain of the thermal runaway of the battery cells is prevented, and even if the thermal runaway occurs in one battery cell  101 , damage is reduced to the minimum. 
     Although various embodiments are described above with reference to the drawings, it goes without saying that the present invention is not limited to such examples. It is apparent to those skilled in the art that various changes and modifications can be conceived within the scope of the claims, and it is also understood that such variations and modifications belong to the technical scope of the present invention. In addition, constituent elements in the embodiments described above may be combined freely within a range not departing from the spirit of the present invention. 
     The present application is based on a Japanese patent application (No. 2020-119430) filed on Jul. 10, 2020, contents of which are incorporated herein by reference. 
     REFERENCE SIGNS LIST 
     
         
           10  heat transfer suppression sheet 
           20  inorganic particles 
           30  first inorganic fibers 
           31  second inorganic fibers 
           100  battery pack 
           110  battery case