Heat absorber and secondary battery module provided with heat absorber

Abstract

The purpose of the present disclosure is to provide: a heat absorber that has an excellent heat absorption property, heat insulation property, and pressure resistance and can change to a heat insulator in a high temperature range; and a secondary battery module provided with the heat absorber.　The present disclosure comprises: a heat absorber having a bag body that can be filled with a content, a water-based solvent and a water-soluble inorganic powder that fill the bag body as the content; and a secondary battery module provided with the heat absorber.

Description

Heat absorber and secondary battery module equipped with the heat absorber

[0001] The present disclosure relates to a heat absorber and a secondary battery module including the heat absorber.

[0002] Secondary batteries, which can control the time lag between energy storage and demand, are used in a variety of applications, including automobiles and mobile devices. Their importance is currently increasing due to their need for the widespread adoption of renewable energy sources, which contribute to the creation of a low-carbon society and energy security. However, secondary batteries, such as lithium-ion batteries, are at risk of thermal runaway when their temperature rises due to heat generation during fast charging or high-output discharging. Furthermore, as ultra-fast charging becomes more common, even greater heat generation is expected, necessitating the development of methods to suppress temperature rise and improve battery safety. Secondary batteries can also experience thermal runaway due to internal short circuits, resulting in problems such as fire or smoke. Therefore, to minimize damage caused by such problems, technologies are needed to suppress, prevent, or delay battery explosions by absorbing heat from excessively high battery temperatures or by suppressing heat transfer to other battery cells (hereinafter referred to as battery cells or simply cells) through heat absorption and insulation.

[0003] For example, Patent Documents 1 and 2 disclose techniques that are excellent in heat insulation and fire spread prevention. ２ / Na ２Patent Document 1 describes a laminated fire spread prevention material including a layer A containing sodium silicate with an O molar ratio of less than 3.1 and a layer B containing precipitated silica. Patent Document 1 describes that the fire spread prevention material is used in a battery pack including two or more cells, thereby suppressing heat transfer between cells under normal conditions and suppressing heat spread to adjacent cells under abnormal conditions. Patent Document 2 describes a partition member including a liquid, a thermal insulating material, and an exterior body that contains the liquid and the thermal insulating material. Patent Document 2 also describes that by appropriately setting the peel strength and crystalline melting characteristics of the sealant resin layer of the sheet-like member that contacts the thermal insulating material, the partition member maintains its cooling function during long-term use and exhibits excellent stability in the release temperature of the cooling liquid inside the sheet-like member.

[0004] International Publication No. 2022 / 270359 Japanese Patent Application Laid-Open No. 2020-161290

[0005] However, in the technology of Patent Document 1, the heat absorption occurs when water contained in Layer A (e.g., water molecules in sodium silicate) undergoes an endothermic reaction at temperatures between 100 and 300°C, limiting the water content and preventing a sufficient endothermic effect. Furthermore, the technology of Patent Document 2 merely lists various porous, fibrous, or particulate insulating materials, and does not consider at all whether the partition member itself exhibits an insulating effect to suppress heat transfer to other cells and thereby suppress, prevent, or delay consecutive explosions in a battery that reaches abnormally high temperatures. Furthermore, when a separator member such as the partition member of Patent Document 2 is used in a stacked secondary battery, the thickness between adjacent cells changes due to the expansion and contraction of the cells themselves during charging and discharging, or the sudden expansion of cells during thermal runaway. Therefore, the separator member is required to have certain mechanical properties, such as pressure resistance. However, because Patent Document 2 does not consider mechanical properties such as pressure resistance, problems arise, particularly when cells expand, as the distance between cells shortens, making heat transfer between adjacent cells more likely. Therefore, an object of the present disclosure is to provide a heat absorber that has excellent heat absorption properties and can be transformed into an insulator in high temperature ranges, thereby having excellent insulation properties and pressure resistance, and a secondary battery module equipped with the heat absorber.

[0006] The present inventors have discovered that a heat absorber containing an aqueous solvent and a water-soluble inorganic powder in a bag exhibits excellent heat absorption, heat insulation, and pressure resistance, and can change into a heat insulator in a high temperature range (for example, 150°C or higher), and have completed the following invention: [1] The present disclosure relates to a heat absorber having a bag that can be filled with a content, and an aqueous solvent and a water-soluble inorganic powder that dissolves in an amount of 1 g or more in 100 g of water at 20°C, the water-soluble inorganic powder being filled in the bag as the content.

[0007] [2] The heat absorber according to [1], wherein the water-soluble inorganic powder has a solubility (g) in water at 20°C of 5 g / 100 g or more.

[0008] [3] The heat absorber according to [1] or [2], further comprising one or more selected from the group consisting of an antifreeze agent and an inorganic fiber in the content.

[0009] [4] The water-soluble inorganic powder is one or more selected from chlorides, sulfates, carbonates, nitrates, phosphates, acetates, alkali metal oxides, and alkaline earth metal oxides. [1] The heat absorber according to any one of [3] to [4].

[0010] [5] The heat absorber according to any one of [1] to [4], wherein the content is filled with an aqueous solution containing the aqueous solvent and the water-soluble inorganic powder, and the content of the water-soluble inorganic powder in the aqueous solution is 5 to 80 mass% with respect to the total amount of the aqueous solution.

[0011] [6] The heat absorber according to any one of [1] to [5], wherein the contents are transformed into a porous body when heated to 120°C or higher.

[0012] [7] The following formula (I): [Mathematical formula 1] "Thickness change rate (%) = (thickness of the heat absorber after pressing the surface of the heat absorber heated under the heating conditions below at 0.5 MPa for 60 seconds) / (thickness of the heat absorber before pressing the surface of the heat absorber heated under the heating conditions below at 0.5 MPa for 60 seconds) × 100" Heating conditions: "50 kW / m by radiant heat ２The heat absorber according to any one of [1] to [6] is heated with a heat quantity of 1000 kJ / cm until the temperature of the surface (back surface) opposite to the heated surface of the heat absorber reaches a predetermined temperature, and then the heat absorber is allowed to dissipate heat at room temperature and naturally cooled until the temperature of the surface of the heat absorber reaches room temperature, and the thickness change rate (%) before and after heating is calculated. Note that the heated surface of the heat absorber was pressed at 0.5 MPa for 60 seconds. The thickness change rate, expressed as ", is 70% or more.

[0013] [8] A secondary battery module comprising the heat absorber according to any one of [1] to [7].

[0014] [9] A secondary battery module in which the heat absorber according to any one of [1] to [7] is sandwiched between battery cells.

[0015] According to the heat absorber of the present disclosure, a heat absorber having excellent heat absorption properties and being able to change into a heat insulator in a high temperature range, thereby providing a heat absorber having excellent heat insulation properties and pressure resistance. According to the present disclosure, a highly safe secondary battery module can be provided by including a heat absorber having excellent heat absorption properties and being able to change into a heat insulator in a high temperature range, thereby providing excellent heat insulation properties and pressure resistance.

[0016] FIG. 1 shows an example of a secondary battery module on which the heat absorber of this embodiment can be mounted. FIG. 2 is a perspective view showing a schematic exploded view of the secondary battery module of FIG. 1. FIG. 3 shows the results of a cone calorimeter test (test conditions: radiation intensity 50 kW / m) of the heat absorber of the example and the sheet of the comparative example. ２ 5(a) and 5(b) are graphs showing the results of a test using a cone calorimeter (heating time: 20 minutes), where the vertical axis represents temperature and the horizontal axis represents elapsed time. FIG. 4 shows a schematic diagram of the cone calorimeter test device used in the examples and comparative examples. FIG. 5(a) is an image showing that the heat absorber (1) produced in Example 1 has been transformed into a porous body. FIG. 5(b) is an image showing that the heat absorber (2) produced in Example 2 has been transformed into a porous body. Both FIGS. 5(a) and (b) are images showing the state in which the edges of the porous body have been cut.

[0017] The following describes in detail an embodiment of the present invention (hereinafter referred to as the "present embodiment"); however, the present disclosure is not limited to the following description and can be implemented in various modifications within the scope of its gist.

[0018] [Heat Absorber] The heat absorber of the present disclosure comprises a bag that can be filled with contents, and an aqueous solvent and a water-soluble inorganic powder that dissolves at least 1 g per 100 g of water at 20°C, which are filled into the bag as the contents. This provides excellent heat absorption, heat insulation, and pressure resistance, and can transform into a heat insulator at high temperatures. Heat can be absorbed by the latent heat of vaporization of the aqueous solvent within the bag or the water-soluble inorganic powder. Therefore, the latent heat of vaporization of water, which has a larger heat absorption capacity than general hydrates, can be utilized. Furthermore, since the heat is absorbed as sensible heat of the aqueous solvent, the temperature can be stabilized even at room temperature. Meanwhile, when exposed to high temperatures such as combustion, the aqueous solvent evaporates, but the presence of the water-soluble inorganic powder provides thermal insulation and fire protection. More specifically, the heat absorber primarily functions as a heat absorber at relatively low temperatures (e.g., above room temperature to around 100°C). On the other hand, in a temperature range from the critical temperature (e.g., 150°C) to the thermal runaway temperature (e.g., around 1000°C), the water-soluble inorganic powder in the heat absorber changes to a porous material, so that the heat absorber can also function as a heat insulator. As a result, when the heat absorber of this embodiment is disposed between battery cells in a battery stack (battery module) in which multiple battery cells are stacked, it can block or suppress the thermal effects on adjacent cells.

[0019] Furthermore, the heat absorber of the present disclosure has fluidity of the contents at relatively low temperatures, allowing it to easily conform to and adhere to the cells, and exhibits cushioning properties that absorb stress even when the cells expand and contract due to heat. Meanwhile, at temperatures above the thermal runaway temperature, the water-soluble inorganic powder in the heat absorber sinters and transforms into a hard, plate-like porous body, increasing pressure resistance. This allows the distance between cells to be kept constant, maintaining effective insulation and controlling thermal conductivity, effectively preventing repeated cell explosions.

[0020] In the heat absorber of the present disclosure, the aqueous solvent may be included in the content of the present embodiment as a hydrogel composed of a hydrogel body and an aqueous solvent, if necessary. Including the hydrogel body as the content can provide the heat absorber with cushioning properties and impact resistance. Since the heat absorber of the present disclosure not only exhibits a heat absorption effect but also an insulating effect, the heat absorber can be strictly referred to as a component capable of absorbing and insulating heat from the outside, i.e., a thermal control component that controls heat from the outside.

[0021] The heat absorber of this embodiment may further contain one or more components selected from the group consisting of inorganic fibers, antifreeze agents, and additives. The addition of inorganic fibers as a component can further impart cushioning and pressure resistance to the heat absorber, or the inorganic fibers act as a foam nucleating agent, facilitating the formation of a foam (i.e., porous body) from the water-soluble inorganic powder. Furthermore, the water retention of the inorganic fibers facilitates the formation of a desired shape for the heat absorber. Furthermore, when the aqueous solvent evaporates, voids are formed within the inorganic fibers, and the composite containing the inorganic fibers and the water-soluble inorganic powder is more likely to form a porous body. This changes the heat absorber into a thermal insulator, thereby enhancing both the heat absorption and thermal insulation effects throughout the entire thickness of the heat absorber. Furthermore, the addition of an antifreeze as a component can suppress freezing at sub-zero temperatures. Furthermore, by utilizing the heat of solidification of the aqueous solvent, a decrease in battery temperature in cold environments can be suppressed. In the case of water, heat of solidification occurs at around 0°C, but by using an antifreeze, the temperature at which this heat of solidification occurs can be lowered, making it possible to prevent the battery from losing temperature at lower temperatures.

[0022] <Characteristics of Heat Endogenous Material> The endothermic onset temperature of the heat endogenous material of this embodiment is preferably 400°C or lower, more preferably 160°C or lower, even more preferably 120°C or lower, more preferably 110°C or lower, and even more preferably 100°C or lower. The endothermic onset temperature range of the heat endogenous material of this embodiment is preferably 35°C or higher and 400°C or lower, more preferably 37°C or higher and 160°C or lower, and even more preferably 40°C or higher and 110°C or lower. The upper and lower limits of the endothermic onset temperature of the endogenous material can be adjusted as appropriate. In this specification, the endothermic onset temperature (°C) is defined as the temperature at the intersection of a straight line extending the low-temperature side baseline toward the high-temperature side in a DSC measurement curve obtained by measurement with a differential scanning calorimeter (DSC), and a tangent drawn at the point where the gradient of the low-temperature side curve of the endothermic peak associated with evaporation is maximum. However, when multiple endothermic peaks are observed, the intersection of a straight line extending the low-temperature baseline toward the high-temperature side with a tangent drawn at the point where the gradient of the low-temperature curve of each of the multiple endothermic peaks is maximum is calculated for each of the multiple endothermic peaks, and the lowest temperature among the temperatures at the intersection is taken as the endothermic onset temperature. The endothermic peak temperature of the endothermic body of this embodiment is preferably at least in the range of 80°C to 400°C, more preferably in the range of 90°C to 160°C. In this specification, the endothermic peak temperature refers to the temperature (°C) at the maximum value of the endothermic peak due to evaporation in the DSC measurement curve obtained by measurement with a differential scanning calorimeter (DSC). Note that when multiple endothermic peaks are observed, at least one of the multiple endothermic peaks may be present in the range of 80°C to 160°C. The endothermic amount of the heat endotherm of this embodiment is not particularly limited, but at the endothermic peak temperature (in the range of 80 ° C to 160 ° C), it is preferably in the range of 100 J / g to 3000 J / g, more preferably 200 J / g to 2500 J / g, even more preferably 300 J / g to 2000 J / g, and even more preferably 500 J / g to 1500 J / g. The preferred range of the endothermic amount can be obtained by appropriately combining the above upper and lower limits. The endothermic onset temperature, endothermic peak temperature, and endothermic amount of the heat endotherm of this embodiment are values ​​determined using a differential scanning calorimeter (DSC) according to the method described in the Examples below.

[0023] <Preferred Shape of Heat Absorber> The shape or size of the heat absorber of this embodiment is not particularly limited and may be, for example, substantially spherical, substantially flat, or irregular, and is appropriately selected depending on the intended use. For example, a substantially flat heat absorber is preferable because it is easy to install between adjacent battery cells. When the heat absorber of this embodiment is substantially flat, the average thickness is not particularly limited, but may be, for example, in the range of 100 μm to 50,000 μm. The average thickness is preferably 100 μm or more, more preferably 200 μm to 20,000 μm, even more preferably 500 μm to 10,000 μm, and particularly preferably 1,000 μm to 8,000 μm. The preferred range of the average thickness can be determined by appropriately combining the above upper and lower limits. The following describes the essential components of the heat absorber of this embodiment, namely, the bag, the water-soluble inorganic powder that dissolves in an amount of 1 g or more in 100 g of water at 20°C, and the aqueous solvent, as well as optional components such as inorganic fibers, antifreeze agents, and additives that may be blended as needed.

[0024] (Bag) The bag of this embodiment is not particularly limited as long as it can be filled with contents such as an aqueous solvent and a water-soluble inorganic powder that dissolves at least 1 g in 100 g of water at 20°C. A preferred example of the bag is a three-sided bag having an opening at the top end, a body portion with a closed bottom end, and a structure in which the opening is heat-sealed to close after the contents such as the aqueous solvent and the water-soluble inorganic powder are completely contained. The three-sided bag is constructed by bonding the bottom ends and side portions of two sheets together on three sides, and then filling the contents through the opening and then sealing. This structure provides excellent airtightness, and its substantially flat shape makes it easy to insert between battery cells. The bag of this embodiment is preferably made of a sheet. A preferred embodiment of the bag of this embodiment is one in which two films of a desired size and shape (e.g., rectangular or (substantially) circular) are stacked on top of each other, and a predetermined heat-sealed region (e.g., the edge of the film) is heat-pressed to form an opening. This allows the production of a three-sided bag having an internal space region capable of being filled with contents, an opening for allowing the contents to be filled into the internal space region through the opening, and a heat-sealed region between the two sheets. After filling the contents, the contents can be sealed by crimping the openings together and heat-sealing them. Note that "sealed" in this specification refers to a state in which the interior and exterior of the bag are substantially isolated from each other. The sheets used in the bag of this embodiment are not particularly limited as long as they are water-proof, and examples include known resin films, resin films with metal layers, and films with metal layers. The average thickness of the sheets used in the bag of this embodiment is not particularly limited, but is preferably 30 μm to 200 μm, and more preferably 60 μm to 150 μm. Examples of materials for the resin film include one or more resins such as polyester resin, nylon resin, polycarbonate resin, polypropylene resin, polyethylene resin, cyclic polyolefin resin, polystyrene resin, fluororesin, and elastomer.These plastics can be used for bags in the form of films, sheets, tubes, etc. In addition, the resin film having the metal layer may be laminated to the resin film with a metal such as aluminum or a metal oxide such as silica or alumina as a metal foil, vapor-deposited film, etc. The use of a resin film having a metal layer can reduce the water vapor permeability of the resin film. The water vapor permeability of the sheet can be adjusted by selecting, thickness, and combination of materials. Examples of lamination methods include dry lamination, extrusion lamination, thermal lamination, coextrusion, multilayer blow molding, laminate injection molding, and coating. A preferred form of the resin film having a metal layer is an aluminum laminate film (a film formed by integrating aluminum foil (including an aluminum vapor-deposited layer) with a thermoplastic resin film (e.g., polyethylene film, PP film, PET film) laminated on at least one side thereof). In this embodiment, an adhesive layer may be formed in the heat-sealed area and the closed opening for sealing purposes. Examples of the adhesive layer that can be used include laminate adhesives such as polyester-based adhesives, polyether-based adhesives, and polyurethane-based adhesives. Furthermore, the type of the adhesive is not particularly limited, and for example, any of solvent-based, solventless, and aqueous types can be used.

[0025] For example, in the present invention, a bag-shaped product made of a laminate film is preferred. The laminate film is preferably a film made of a metal foil and a resin film, and an example of such a laminate film is a three-layer laminate film consisting of an outer resin film, a metal foil, and an inner resin film. Specifically, a bag made of a resin film having an aluminum-deposited layer on the outside sealed with a polyurethane-based laminate adhesive layer, a three-layer laminate film having a nylon film on the outside, an aluminum foil in the center, and an adhesive layer such as modified polypropylene on the inside sealed with a polyurethane-based laminate adhesive layer, or a laminate film having a PET layer, an aluminum layer, and a polyethylene layer sealed with a polyurethane-based laminate adhesive layer can be used. Examples include the Gas Barrier Aluminum Bag AB Series (manufactured by Mitsubishi Gas Chemical Company, Inc.) and the Lamizip AL Type (manufactured by Seisan Nippon Co., Ltd.). The higher the melting temperature (for example, 120 to 140° C.) of the adhesive used to close the opening of the bag body of this embodiment or applied to the heat-sealed area, the stronger the bag body will be and the more it will be able to withstand internal pressure.

[0026] The water vapor permeability ([g / (m ２ ・24h)]) is 50g / (m ２ 24h) or less, and ２ 24h) or less, and more preferably 5g / (m ２ It is more preferable that the water vapor permeability of the sheet constituting the bag is 50 g / (m ２ When the water vapor permeability ([g / (m ２ The temperature and humidity are measured in accordance with JIS K7129 at a temperature of 40°C and a relative humidity of 90%.

[0027] (Water-soluble inorganic powder dissolving at least 1 g in 100 g of water at 20°C (hereinafter also referred to as water-soluble inorganic powder)) The heat absorber of this embodiment contains a water-soluble inorganic powder as its content. The water-soluble inorganic powder dissolves at least 1 g in 100 g of water at 20°C. Because the water-soluble inorganic powder is hydrophilic, it is easily dissolved in an aqueous solvent, making it easier for the water-soluble inorganic powder to be uniformly distributed within the content. As a result, in temperatures exceeding the thermal runaway temperature (e.g., around 1000°C), the water-soluble inorganic powder as a whole tends to form a homogeneous porous body, which can effectively function as a heat insulator. Furthermore, when the water-soluble inorganic powder and the aqueous solvent are present in the content of the heat absorber, a synergistic effect with the endothermic effect of the aqueous solvent is exhibited, allowing for continuous heat absorption at a different endothermic temperature from that of the aqueous solvent. Even when the heat absorber is exposed to high temperatures, the water-soluble inorganic powder can become porous (see, for example, the photographs of Figures 5 and 6 described below). As a result, excellent heat insulation and fire prevention effects are exhibited. Therefore, a heat absorber containing a water-soluble inorganic powder primarily functions as a heat absorber in relatively low temperature ranges (e.g., above room temperature to around 100°C). On the other hand, in temperature ranges from the critical temperature (e.g., 150°C) to the thermal runaway temperature (e.g., around 1000°C), the water-soluble inorganic powder as a whole becomes porous and can also function as a thermal insulator. As a result, when the heat absorber of this embodiment is placed between cells of a battery stack in which multiple cells are stacked, it can block or suppress the thermal effects on adjacent cells. For example, when thermal runaway occurs, the cells expand, compressing the heat absorber between the cells, significantly reducing the distance between the cells and making it difficult to achieve effective thermal insulation. However, when the heat absorber of this embodiment contains a water-soluble inorganic powder, when heated to high temperatures due to thermal runaway or other reasons, the water-soluble inorganic powder itself sinters to form a porous body exhibiting a certain strength, thereby increasing pressure resistance. This allows the distance between cells to be maintained constant, thereby maintaining effective thermal insulation and effectively suppressing repeated explosions between cells.

[0028] The water-soluble inorganic powder of this embodiment dissolves in an aqueous solvent. This makes it easier to dissolve in an aqueous solvent, and the water-soluble inorganic powder is more likely to be uniformly distributed within the contents. In this specification, "water-soluble" means that 1 g or more dissolves in 100 g of water at 20°C. Therefore, the water-soluble inorganic powder of this embodiment can be an inorganic powder that dissolves 1 g or more in 100 g of water at 20°C.

[0029] The solubility of the water-soluble inorganic powder of this embodiment is 1 g or more per 100 g of water at 20°C. From the viewpoints of the stability and dispersibility of the water-soluble inorganic powder in the heat absorber and sinterability upon high-temperature heating, the solubility of the water-soluble inorganic powder (per 100 g of water at 20°C) is preferably 1 g or more and 100 g or less, more preferably 2 g or more and 90 g or less, even more preferably 3 g or more and 80 g or less, still more preferably 5 g or more and 70 g or less, even more preferably 15 g or more and 60 g or less, and particularly preferably 25 g or more and 50 g or less. The solubility of the water-soluble inorganic powder in 100 g of water at 20°C can be appropriately adjusted by changing the above-mentioned upper and lower limits. When the solubility of the water-soluble inorganic powder at 20°C is within the above range, solubility is ensured, and the water-soluble inorganic powder is uniformly dissolved or dispersed in the aqueous solvent, making it easier to form a homogeneous porous body during sintering.

[0030] The solubility of the water-soluble inorganic powder of this embodiment is preferably 10 g or more in 100 g of water at 60° C. The solubility of the water-soluble inorganic powder (in 100 g of water at 60° C.) is preferably 10 g or more and 150 g or less, more preferably 15 g or more and 120 g or less, even more preferably 20 g or more and 100 g or less, still more preferably 25 g or more and 80 g or less, still more preferably 30 g or more and 60 g or less, and particularly preferably 35 g or more. The solubility of the water-soluble inorganic powder in 100 g of water at 60° C. can be appropriately adjusted by changing the above-mentioned upper limit value and the above-mentioned lower limit value.

[0031] The solubility of the water-soluble inorganic powder of this embodiment is preferably 15 g or more in 100 g of water at 80° C. The solubility of the water-soluble inorganic powder (in 100 g of water at 80° C.) is preferably 15 g or more and 160 g or less, more preferably 20 g or more and 120 g or less, even more preferably 25 g or more and 100 g or less, still more preferably 30 g or more and 80 g or less, and particularly preferably 35 g or more and 60 g or less. The solubility of the water-soluble inorganic powder in 100 g of water at 80° C. can be determined by appropriately rearranging the above-mentioned upper and lower limits.

[0032] The solubility of the water-soluble inorganic powder of this embodiment is preferably 15 g or more in 100 g of water at 100° C. The solubility of the water-soluble inorganic powder (in 100 g of water at 100° C.) can be, for example, 15 g or more and 170 g or less, preferably 20 g or more and 130 g or less, more preferably 25 g or more and 100 g or less, even more preferably 30 g or more and 80 g or less, and particularly preferably 35 g or more and 60 g or less. The solubility of the water-soluble inorganic powder in 100 g of water at 100° C. can be determined by appropriately rearranging the above-mentioned upper and lower limits.

[0033] The preferred solubility of the water-soluble inorganic powder of this embodiment is 1 g or more and 90 g or less per 100 g of water at 20° C., more preferably 5 g or more and 90 g or less, 5 g or more and 100 g or less per 100 g of water at 40° C., 10 g or more and 150 g or less per 100 g of water at 60° C., 15 g or more and 160 g or less per 100 g of water at 80° C., and 15 g or more and 170 g or less per 100 g of water at 100° C. The solubility of the water-soluble inorganic powder in each temperature range is preferred from the viewpoint of exhibiting suitable endothermic action and pressure resistance. The solubility of the water-soluble inorganic powder in 100 g of water at 20° C. can be appropriately rearranged by appropriately rearranging the above-mentioned upper and lower limits.

[0034] The method for measuring solubility in this specification is as follows. A specified amount of water-soluble inorganic powder to be measured is weighed into a glass bottle, and 100 g of pure water (pH = 7) is added to the glass bottle. The mixture is stirred at 1 atmosphere and at temperatures of 20°C, 40°C, 60°C, 80°C, and 100°C for 24 hours at a rotation speed of 80 rpm on a mix rotor to prepare a mixed solution. The transmittance of the mixed solution after 24 hours of stirring is then measured under the following conditions. In this case, the transmittance is measured by changing the amount of dissolved water-soluble inorganic powder, and the upper limit amount (g) at which the transmittance becomes 99% is taken as the solubility of the water-soluble inorganic powder in water. <Conditions for Transmittance Measurement> Dynamic Light Scattering (DLS) Measurement Apparatus: DLS Measurement Apparatus DLS-8000 manufactured by Otsuka Electronics Laser Wavelength, Output: 488 nm / 100 mW Sample Cell: NMR Tube

[0035] The water-soluble inorganic powder of this embodiment is preferably solid at room temperature. Furthermore, in the heat absorber of this embodiment, it is preferable that an aqueous solution containing an aqueous solvent and a water-soluble inorganic powder is filled into a bag as the content of the heat absorber. By filling the content of the heat absorber with an aqueous solution containing an aqueous solvent and a water-soluble inorganic powder, the water-soluble inorganic powder is completely dissolved in the aqueous solvent, and therefore the water-soluble inorganic powder is uniformly present in the content, resulting in the formation of a homogeneous porous body. The transmittance of the aqueous solution is preferably 99% or more, more preferably 99.5% or more.

[0036] The endothermic capacity of the water-soluble inorganic powder (= the endothermic capacity (J / g) when heated from room temperature (23°C) to 1000°C)) is preferably 100 J / g or more, more preferably 500 J / g or more, and even more preferably 700 J / g or more. On the other hand, the upper limit of the endothermic capacity of the water-soluble inorganic powder is not particularly limited, but is preferably 4000 J / g or less. The endothermic capacity of the water-soluble inorganic powder may be preferably 100 J / g or more and 4000 J / g or less, more preferably 500 J / g or more and 4000 J / g or less. The upper and lower limits of the endothermic capacity of the water-soluble inorganic powder can be appropriately combined. When the endothermic capacity of the water-soluble inorganic powder is within the above range, the endothermic effect is improved, thereby exhibiting a synergistic effect with the endothermic capacity of the aqueous solvent, making it easier to suppress ignition. The upper and lower limits of the content can be appropriately combined. The endothermic capacity of the water-soluble inorganic powder can be measured using a differential scanning calorimeter (DSC) as described in the Examples section.

[0037] The thermal decomposition onset temperature of the water-soluble inorganic powder of this embodiment is preferably 80°C or higher and 800°C or lower, more preferably 90°C or higher and 500°C or lower, even more preferably 100°C or higher and 350°C or lower, and even more preferably 110°C or higher and 150°C or lower. When the thermal decomposition onset temperature of the water-soluble inorganic powder is within the above range, the water-soluble inorganic powder itself decomposes quickly, making it easier to suppress ignition. The upper and lower limits of the thermal decomposition onset temperature can be adjusted as appropriate. The thermal decomposition onset temperature can be measured using a differential scanning calorimeter (DSC).

[0038] The shape of the water-soluble inorganic powder of this embodiment is not particularly limited, and examples thereof include powder, particulate, crystalline, and plate-like. Furthermore, the water-soluble inorganic powder of this embodiment is preferably a water-soluble inorganic powder having a heat-absorbing effect, and preferred embodiments of the water-soluble inorganic powder include porous powders, solid particles, and hollow particles. The water-soluble inorganic powder may be in any shape or form as long as it dissolves in an aqueous solvent. When the water-soluble inorganic powder is in powder or particulate form, the average particle diameter of the water-soluble inorganic powder is, for example, preferably 0.01 to 200 μm, more preferably 0.1 to 140 μm, and even more preferably 10 to 100 μm. By setting the average particle diameter within the above range, the water-soluble inorganic powder is easily dispersed in the system. The average particle diameter may be the median diameter (D50) measured using a laser diffraction / scattering particle size distribution analyzer.

[0039] The material of the water-soluble inorganic powder of this embodiment is preferably composed of a water-soluble inorganic salt. The water-soluble inorganic salt is preferably one or more compounds composed of a combination of an inorganic cation and an organic / inorganic anion. Examples of the inorganic cation include alkali metal ions, alkaline earth metal ions, aluminum ions, zinc ions, silver ions, copper(I) ions, and copper(II) ions. Preferably, the inorganic cation is one or more selected from the group consisting of potassium ions, calcium ions, magnesium ions, and aluminum ions. The organic / inorganic anion is preferably one or more selected from oxygen ions, sulfate ions, halogen ions (chloride ions, fluoride ions, bromine ions, etc.), nitrate ions, carbonate ions, acetate ions, and phosphate ions. The water-soluble inorganic powder of this embodiment is preferably composed of one or more compounds selected from the group consisting of chlorides, sulfates, carbonates, nitrates, phosphates, acetates, alkali metal oxides, and alkaline earth metal oxides. This allows the powder to exhibit excellent solubility in aqueous solvents. The water-soluble inorganic powder before being mixed with the aqueous solvent may be in the form of an anhydrous or hydrated form, provided that it exhibits the desired solubility described above. In addition, the hydrated form of the water-soluble inorganic powder is usually present in the form of an anhydrous form in the bag.

[0040] Specific examples of the water-soluble inorganic powder or water-soluble inorganic salt of this embodiment include chlorides such as sodium chloride, potassium chloride, and ammonium chloride; sulfates such as sodium sulfate, potassium sulfate, magnesium sulfate, aluminum sulfate, and alum; carbonates such as sodium bicarbonate, sodium sesquicarbonate, sodium carbonate, potassium carbonate, potassium sesquicarbonate, and ammonium carbonate; nitrates such as sodium nitrate, potassium nitrate, and calcium nitrate; phosphates such as sodium phosphate, sodium dihydrogen phosphate, dipotassium hydrogen phosphate, and sodium polyphosphate; acetates such as zinc acetate, sodium acetate, potassium acetate, copper(I) acetate, and copper(II) acetate; oxides such as chromium oxide, barium oxide, and boric oxide; and hydrates thereof. Among the above, magnesium sulfate or magnesium sulfate heptahydrate is particularly preferred. The water-soluble inorganic powder of this embodiment may be any of the above examples used alone or in combination.

[0041] The content of the water-soluble inorganic powder in this embodiment can be 1 to 90% by mass, or alternatively 3 to 80% by mass, and preferably 5 to 60% by mass, based on the total amount (100% by mass) of the contents of the heat absorber. It is particularly preferably 10 to 50% by mass, more preferably 10 to 35% by mass, and even more preferably 15 to 30% by mass. When the content of the water-soluble inorganic powder is within the above range, the heat absorber is easily sintered to form a homogeneous porous body when exposed to high heat. Furthermore, when the water-soluble inorganic powder is a hydrate, the content of the water-soluble inorganic powder does not include the content of water contained in the hydrate. The upper and lower limits of the content of the water-soluble inorganic powder can be adjusted as appropriate.

[0042] In this embodiment, the contents of the heat absorber preferably transform into a porous body when heated to 120°C or higher. The temperature at which the contents transform into a porous body is preferably 150°C or higher, preferably 180°C or higher, preferably 200°C or higher, preferably 210°C or higher, preferably 240°C or higher, and preferably 250°C or higher. When the heat absorber is exposed to high heat such as by combustion, the aqueous solvent evaporates, but the water-soluble inorganic powder can form a sintered porous body, which can provide thermal insulation and fire protection for components adjacent to the heat absorber (e.g., battery cells). In particular, when two or more heat absorbers of this embodiment sandwich or surround a component (e.g., a battery cell), it is believed that heat from the component is less likely to leak to the outside. For example, when the component is exposed to high heat, the water-soluble inorganic powder inside the heat absorber sinters to form a porous body, and the porous body acts as a fire wall, thereby providing excellent thermal insulation and fire protection. Therefore, when the heat absorber of this embodiment is arranged in a secondary battery module such as a stacked battery described below, the heat transfer to other cells is suppressed, thereby suppressing, preventing or delaying the explosion.

[0043] (Aqueous Solvent) The heat absorber of this embodiment contains an aqueous solvent as its content. This allows heat absorption by the latent heat of vaporization of the aqueous solvent, particularly water, within the bag, making it possible to utilize the latent heat of vaporization of water, which has a larger heat absorption capacity than general hydrates. Furthermore, since the heat is absorbed as sensible heat of the aqueous solvent, the temperature can be stabilized even at room temperature. Meanwhile, when the heat absorber is exposed to high temperatures such as combustion, the aqueous solvent evaporates, but the water-soluble inorganic powder can be sintered to form a porous body, which can provide thermal insulation and fire protection to members adjacent to the heat absorber.

[0044] The aqueous solvent of this embodiment may contain water as a primary component, and refers to water or a solvent containing water as a primary component. Therefore, aqueous solvents encompass mixed solvents with solvents other than water, and aqueous solutions containing salts (e.g., buffer solutions, electrolyte solutions). In this specification, "containing water as a primary component" refers to a solvent containing 45% or more by mass of water relative to the total amount of the aqueous solvent. Furthermore, purified water, pure water, ultrapure water, distilled water, and the like can be used as the water without any particular limitation. Examples of the salt include alkali metal halides such as sodium chloride or potassium chloride; alkaline earth metal halides such as magnesium chloride or calcium chloride; and salts with buffering capacity such as tris-hydrochloric acid, glycine hydrochloride, citric acid-sodium citrate, acetic acid-sodium acetate, citric acid-disodium hydrogen phosphate, sodium dihydrogen phosphate-disodium hydrogen phosphate, glycine-sodium hydroxide, and sodium carbonate-sodium bicarbonate. Furthermore, Good's buffers such as HEPES or MOPS may also be used as the aqueous solvent. Examples of the solvent other than water that constitutes the mixed solvent include organic solvents that are uniformly miscible with water (for example, lower alcohols, lower ketones, etc.), and low-volatility solvents that are used as antifreeze agents.

[0045] The content of water in the aqueous solvent of this embodiment is preferably 50% by mass to 100% by mass, more preferably 80% by mass to 100% by mass, more preferably 90% by mass to 100% by mass, and particularly preferably 95% by mass to 100% by mass, based on the total amount of the aqueous solvent. The preferred range of the water content in the aqueous solvent can be determined by appropriately combining the above upper and lower limits. The content of the aqueous solvent of this embodiment is preferably 10% by mass to 95% by mass, more preferably 20% by mass to 90% by mass, even more preferably 30% by mass to 80% by mass, and particularly preferably 50% by mass to 70% by mass, based on the total amount (100% by mass) of the contents of the heat endotherm. The preferred range of the content of the aqueous solvent can be determined by appropriately combining the above upper and lower limits. When the content of the aqueous solvent as a whole is within the above range, the heat endotherm, heat insulation, and pressure resistance are excellent, and the aqueous solvent can be transformed into a heat insulator at high temperatures.

[0046] The above is a description of the bag, water-soluble inorganic powder, and aqueous solvent, which are essential components of the present disclosure. Below, we will explain the optional components of the present disclosure, namely, inorganic fibers, antifreeze, and additives.

[0047] The contents of the heat absorber of this embodiment may contain one or more selected from the group consisting of inorganic fibers and antifreeze agents. (Inorganic Fibers) The inorganic fibers of this embodiment are fiber aggregates in which fibers made of inorganic materials are entangled with each other, or porous bodies made of inorganic materials, and have the effect of facilitating the formation of porous bodies with water-soluble inorganic powders as foam nucleating agents. Furthermore, when the water-soluble inorganic powders are transformed into porous bodies, a porous composite containing the inorganic fibers and the water-soluble inorganic powders can be formed. This makes it easier to form insulating walls with the desired mechanical strength when the temperature exceeds the thermal runaway temperature, for example.

[0048] Specific examples of the inorganic fibers include woven or knitted fabrics (glass cloth or silica cloth), nonwoven fabrics (glass fiber or ceramic fiber), and cotton-like materials (including not only glass wool, rock wool, and ceramic wool, but also spongy materials (sponge materials)). The inorganic fibers of this embodiment preferably have a heat resistance of 300°C or higher, more preferably 700°C or higher, and even more preferably 1200°C or higher. The heat resistance refers to the temperature at which the rate of volume change (thickness direction) reaches -20% when a test specimen is heated from 200 to 700°C in 100°C increments and held at each temperature for 30 minutes. When the inorganic fibers of this embodiment are porous bodies having at least one of a specific airflow resistance, a specific porosity, a specific tortuosity, or a specific void ratio, the entire heat absorber tends to form a relatively stable porous body even in high temperature ranges (temperature ranges from a critical temperature (e.g., 150°C) to a thermal runaway temperature (e.g., near 1000°C)), and therefore tend to function as a heat insulator. In particular, when inorganic fibers have a specific porosity, they can become a better heat insulator when combined with water-soluble inorganic powder.

[0049] <Porosity> The average porosity of the inorganic fibers of this embodiment is preferably 30% or more and 99.7% or less, more preferably 50% or more and 99.5% or less, even more preferably 70% or more and 99.3% or less, and particularly preferably 90% or more and 99% or less. In this specification, the average porosity of the inorganic fibers is a value calculated from the bulk density and true density described below, and is a density based on the volume occupied by the inorganic fibers. The bulk density is a density based on the volume including the voids contained in the inorganic fibers. In contrast, the true density is a density based on the volume occupied by the inorganic fiber material. The average porosity (%) can be calculated from the bulk density ρf and true density ρr described below using the following formula (1): Average porosity (%) = ((1 / ρf) - (1 / ρr)) / (1 / ρf) × 100 ... formula (1)

[0050] <Bulk density> The bulk density ρf of the inorganic fiber of this embodiment is 0.020 g / cm ３ 1g / cm or more ３ Preferably, it is 0.022 g / cm or less, more preferably 0.022 g / cm ３ 0.5g / cm or more ３ More preferably, 0.024 g / cm or less ３ 0.1g / cm or more ３ Particularly preferably 0.026 g / cm ３ 0.07g / cm or more ３ The dimensions of the inorganic fibers are measured, and the bulk volume V of the inorganic fibers is calculated. Then, the mass M of the inorganic fibers is measured using a precision balance. From the obtained mass M and bulk volume V, the bulk density of the inorganic fibers can be calculated using the following formula (2). Bulk density ρf (g / cm ３ )=M / V...Formula (2)

[0051] <True Density> The true density ρr of the inorganic fiber of this embodiment is 0.5 g / cm ３ 10g / cm or more ３ Preferably, it is less than 1 g / cm ３ 7g / cm or more ３ More preferably, 1.5 g / cm or less ３ 5g / cm or more ３ Below 2 g / cm, particularly preferably ３ 3g / cm or more３ The method for measuring the true density ρr of inorganic fibers is not particularly limited, but it can be calculated by the sink-float method using a mixed solution consisting of n-heptane, carbon tetrachloride, and ethylene dibromide. Specifically, first, an appropriate-sized sample piece of inorganic fiber is placed in a stoppered test tube. Next, a mixed solvent of three solvents is added to the test tube, and the tube is immersed in a constant temperature bath at 30°C. If the sample piece floats, n-heptane, which has a low density, is added. On the other hand, if the test piece sinks, ethylene dibromide, which has a high density, is added. This procedure is repeated until the test piece floats in the liquid. Finally, the density of the mixed solvent is measured using a Gay-Lussac pycnometer.

[0052] <Composition of inorganic fibers> Examples of materials constituting the inorganic fibers of this embodiment or inorganic materials contained in the inorganic fibers include elements selected from the group consisting of silicon, titanium, barium, zirconium, zinc, calcium, magnesium, cerium, aluminum, indium, tin, and lanthanum, single oxides of the elements or composite oxides of the elements, single sulfides of the elements or composite sulfides of the elements, and single phosphate compounds of the elements or composite phosphate compounds of the elements, and silicon, titanium, zirconium, magnesium, aluminum, indium, tin, and single or composite oxides thereof are preferred. Specific examples of inorganic materials constituting the inorganic fibers include glass, shirasu, silica, silica gel, alumina, clay, ceramics, vermiculite, bentonite, perovskite compounds (strontium titanate), talc, mica, wollastonite, potassium titanate, calcium oxide, basic magnesium sulfate, sepiolite, xonotlite, perlite, zeolite, apatite, hydroxyapatite, kaolinite, montmorillonite, acid clay, diatomaceous earth, basalt, wet silica, dry silica, aerogel, mica, and vermiculite.

[0053] <Shape of Inorganic Material Fiber> The shape of the inorganic fiber in this embodiment may be selected from threads, fibers, fiber bundles, fiber aggregates, cotton, woven or knitted fabrics, nonwoven fabrics, etc. In this specification, "woven or knitted fabrics" refers to woven fabrics or knitted fabrics.

[0054] When the inorganic fiber according to the present embodiment is a woven fabric, known weaving methods such as plain weave, twill weave, satin weave, tatami weave, leno weave, and cord weave can be appropriately used as the weaving method for the woven fabric. Among these weaving methods, it is preferable to use a weaving method that provides a predetermined range of resistance to fluid passage between the communicating holes, i.e., between the spaces per mesh formed by the intersection of the warp and weft lines (for example, the airflow resistance described below). From these viewpoints, it is preferable to use a weaving method such as plain weave, twill weave, satin weave, leno weave, or tatami weave.

[0055] When the inorganic fiber according to the present embodiment is a knitted fabric, the knitting method of the knitted fabric may be warp knitting, which is knitted in the vertical direction, such as lace knitting, raschel knitting, tricot knitting, or vandyke knitting, or weft knitting, which is knitted in the horizontal direction, such as flat knitting, plain knitting, rib knitting, tubular knitting, jersey knitting, cannula knitting, rib knitting, or jacquard knitting, and any known knitting method may be appropriately adopted. Among these knitting methods, it is preferable to adopt a knitting method in which the passage resistance of a fluid passing through the communicating holes (for example, the airflow resistance described below) falls within a predetermined range. Furthermore, various knitting machines may be used, such as a warp knitting machine, a flat knitting machine, a circular knitting machine, or a raschel knitting machine.

[0056] When the inorganic fiber according to this embodiment is a woven or knitted fabric, the woven or knitted yarn used is not particularly limited, and the fineness is preferably 50 dtex or more and 8000 dtex or less, more preferably 100 dtex or more and 3000 dtex or less. The twisting method of the woven or knitted yarn is also not particularly limited, and the twisting method may be dry twisting, wet twisting performed by immersion in water, or a combination thereof. The twisting direction is also not particularly limited, and may be right-handed twisting, left-handed twisting, or a combination thereof. The woven or knitted yarn used in this embodiment may be false-twisted yarn, filament yarn, or yarn processed using the POY-DTY method or the PTY (Producers Textured Yarn) method. The conditions for the woven or knitted yarn used can be appropriately selected depending on the purpose of use, the type of aqueous solvent, etc. The material of the woven or knitted yarn may be the material constituting the inorganic fiber described above or the inorganic material contained in the inorganic fiber.

[0057] The BET specific surface area of ​​the inorganic fiber is 0.3 to 5000 m ２ / g, and 10 to 2000m ２ / g, and 30 to 1600m ２ The BET specific surface area of ​​the inorganic fiber may be measured using a specific surface area meter (Microtrac-Bell Corporation, BELSORP-mini), and the surface area per 1 g of the sample measured from the amount of nitrogen gas adsorbed by the BET method may be defined as the specific surface area (m ２ / g).

[0058] When the inorganic fiber of this embodiment is composed of a nonwoven fabric, the average fiber diameter of all fibers (fibers made from the inorganic raw material) constituting the nonwoven fabric is preferably 1 to 100 μm, more preferably 2 to 10 μm. It is preferable that the average fiber diameter of the fibers constituting the nonwoven fabric is within the above range, since it is easy to ensure the desired porosity. The average fiber diameter can be measured by microscope observation or image analysis using a fiber length measuring device (e.g., KAJAANI Fiber Lab.).

[0059] Furthermore, when the inorganic fiber of this embodiment is formed into a nonwoven fabric, the average fiber length of all fibers (raw fibers) constituting the nonwoven fabric is preferably 3 mm or more and 200 mm or less, more preferably 5 mm or more and 100 mm or less, and more preferably 10 mm or more and 50 mm or less. It is preferable that the average fiber length of all fibers constituting the nonwoven fabric is within the above range, since it is easier to ensure the desired porosity. The average fiber length can be measured by microscope observation or the average fiber diameter based on image analysis results using a fiber length measuring device (e.g., KAJAANI Fiber Lab.). When the inorganic fiber of this embodiment is formed into a cotton-like body, the average fiber length of all fibers (raw fibers) constituting the cotton-like body is preferably 0.5 μm or more and 50 μm or less, more preferably 0.8 μm or more and 32 μm or less, and more preferably 1 μm or more and 25 μm or less. It is preferable that the average fiber length of all fibers constituting the cotton-like material is within the above range, since it is easier to ensure the desired void ratio. The average fiber length can be measured by microscope observation or the average fiber diameter from image analysis results using a fiber length measuring device (e.g., KAJAANI Fiber Lab.). In this specification, cotton-like material is also a type of nonwoven fabric, but it is defined as a cotton-like material that is in the form of fibers and has a shape other than a cloth (or flat plate).

[0060] <Preferred Embodiments of Inorganic Fibers> Preferred embodiments of the inorganic fibers of the present embodiment are glass cloth, ceramic wool, rock wool, and glass wool.

[0061] The content of the inorganic fibers in this embodiment is preferably 0% by mass or more and 50% by mass or less, more preferably 1% by mass or more and 30% by mass or less, even more preferably 1% by mass or more and 10% by mass or less, and particularly preferably 1% by mass or more and 5% by mass or less, and particularly preferably 1% by mass or more and 3% by mass or less, relative to the total amount (100% by mass) of the contents of the heat absorber. The preferred range of the content of the inorganic fibers can be determined by appropriately combining the above upper and lower limits. When the content of the inorganic fibers is within the above range, the heat absorber has better heat absorption and pressure resistance, and can change from an endothermic effect to an insulating effect in the high temperature range.

[0062] <Antifreezing Agent> In this embodiment, an antifreezing agent may be blended into the aqueous solvent or contents as needed, since this can improve the effect of suppressing a temperature drop below freezing. In particular, adding an antifreezing agent to the contents of the heat absorber can maintain high cushioning properties over a wide temperature range. The antifreezing agent of this embodiment may be an inorganic or organic antifreezing agent. The antifreezing agent may be in the form of a liquid, powder, solid, or the like. The inorganic antifreezing agent is preferably a chloride such as sodium chloride, calcium chloride, or magnesium chloride (including hydrates such as magnesium chloride hexahydrate). On the other hand, the organic antifreezing agent is preferably a salt of an organic acid or a low-volatility substance (a low-volatility solvent or urea), and more preferably a salt of an organic acid or a low-volatility solvent. As the salts of organic acids (including hydrates), it is preferable to use salts of sodium, potassium, magnesium, ammonium, etc. of formic acid, propionic acid, succinic acid, etc., such as disodium succinate (including hydrates such as disodium succinate hexahydrate) or sodium propionate. Furthermore, as the low-volatility substance, urea or a low-volatility solvent (for example, a polyhydric alcohol) can be used. Examples of the low-volatility solvent include ethylene glycol, diethylene glycol, glycerin, dipropylene glycol, propylene glycol, butyrolactone, N,N-dimethylformamide, glycerol, 1,3-propanediol, glycol ether, glycol ether, glycol monoether, ethylene glycol, diethylene glycol, propylene glycol, isopropanol, propylene glycol monomethyl ether, di- or tripropylene glycol monomethyl ether, cyclohexanol, glucose, mannose, fructose, galactose, sucrose, lactose, maltose, xylose, arabinose, sorbitol, mannitol, trehalose, and raffinose.

[0063] The low-volatility solvent of this embodiment has a volatility of 1 cm in an open system at 60°C and 1 atmosphere. ２ ・0.1g or less per hour (0.1g / cm ２More preferably, an organic solvent of 0.05 g or less, and even more preferably 0.01 g or less is used. Specifically, a solvent that is easily miscible with water is preferred, and therefore glycerin (0.001 g or less / cm ２ hr 60 ° C 1 atm), diglycerin (0.001 g or less / cm ２ hr 60°C 1 atm), ethylene glycol (0.01 g or less / cm ２ hr 60 ° C 1 atm), propylene glycol (0.001 g or less / cm ２ hr 60°C 1 atm), polyethylene glycol (0.001 g or less / cm ２ Polyhydric alcohols such as (aqueous solvents) (water, 2000kJ / hr, 60°C, 1 atm) are preferred, with glycerin and diglycerin being more preferred. These low-volatile solvents may be used alone or in combination of two or more. By including a low-volatile solvent, particularly a polyhydric alcohol, as the content of the heat endotherm of this embodiment, volatilization of the aqueous solvent is suppressed or prevented, or a decrease in cushioning properties at low temperatures is suppressed (improved anti-freeze effect). When a low-volatile solvent is used as an optional component, the mass ratio of the aqueous solvent to the low-volatile solvent (aqueous solvent / low-volatile solvent) in the content of the heat endotherm of this embodiment is preferably 95 / 5 to 30 / 70, more preferably 90 / 10 to 50 / 50, and even more preferably 85 / 15 to 65 / 35. The content of the antifreeze agent in this embodiment can be 0% by mass or more and 70% by mass or less, preferably 5% by mass or more and 60% by mass or less, more preferably 10% by mass or more and 50% by mass or less, even more preferably 15% by mass or more and 35% by mass or less, and particularly preferably 20% by mass or more and 30% by mass or less, relative to the total amount (100% by mass) of the contents of the heat absorber. The preferred range of the content of the antifreeze agent can be determined by appropriately combining the upper and lower limits. Furthermore, when the antifreeze agent is contained within the above range, the battery is less likely to freeze even at -20°C, allowing it to be used over a wide temperature range.

[0064] (Additives) The heat absorber content or dispersion liquid (a) of this embodiment may contain various additives, such as ultraviolet absorbers, antioxidants, organic solvents, inorganic fillers other than the water-swellable clay minerals, viscosity modifiers such as thickeners, crosslinking agents, and flame retardants, as needed. Although the various additives are optional components, when used, they are preferably used in proportions that do not impair the effects of the present disclosure and that correspond to the purpose of each additive. While the proportions cannot be determined in general, the content of the various additives is preferably 0% by mass or more and 50% by mass or less, and more preferably 10% by mass or more and 40% by mass or less, relative to the total amount (mass) of the aqueous solvent and various additives used in the present disclosure.

[0065] Examples of the ultraviolet absorber include triazine derivatives such as 2-[4-{(2-hydroxy-3-dodecyloxypropyl)oxy}-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine and 2-[4-{(2-hydroxy-3-tridecyloxypropyl)oxy}-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine, 2-(2'-xanthenecarboxy-5'-methylphenyl)benzotriazole, 2-(2'-o-nitrobenzyloxy-5'-methylphenyl)benzotriazole, 2-xanthenecarboxy-4-dodecyloxybenzophenone, 2-o-nitrobenzyloxy-4-dodecyloxybenzophenone, etc. These ultraviolet absorbers can be used alone or in combination of two or more.

[0066] Examples of the antioxidant include "Sumilizer BBM-S" and "Sumilizer GA-80" manufactured by Sumitomo Chemical Co., Ltd. Examples of the organic solvent include aromatic hydrocarbons such as toluene and xylene; glycols such as ethylene glycol and propylene glycol; polyether glycols, which are polymers thereof; cellosolves; carbitols; and aliphatic alcohols such as methanol. The organic solvents can be used alone or in combination of two or more. Examples of the inorganic filler include fused silica, crystalline silica, alumina, silicon nitride, and aluminum hydroxide. Examples of the viscosity modifier such as the thickener include various tackifying resins such as rosin, polymerized rosin, polymerized rosin ester, rosin phenol, stabilized rosin ester, disproportionated rosin ester, terpene, terpene phenol, and petroleum resin. Examples of the crosslinking agent include known crosslinking agents such as isocyanate-based, epoxy-based, aziridine-based, polyvalent metal salt-based, metal chelate-based, ketohydrazide-based, oxazoline-based, carbodiimide-based, silane-based, and glycidyl (alkoxy) epoxy silane-based crosslinking agents.

[0067] Examples of the flame retardant include inorganic phosphorus compounds such as red phosphorus, ammonium phosphates such as monoammonium phosphate, diammonium phosphate, triammonium phosphate, and ammonium polyphosphate, and phosphoric acid amides; phosphoric acid ester compounds, phosphonic acid compounds, phosphinic acid compounds, phosphine oxide compounds, phosphorane compounds, organic nitrogen-containing phosphorus compounds, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 10-(2,5-dihydroxyphenyl)-10H-9-oxa-10-phosphaphenanthrene-10-oxy Examples of suitable flame retardants include organic phosphorus compounds such as cyclic organic phosphorus compounds such as 10-(2,7-dihydroxynaphthyl)-10H-9-oxa-10-phosphaphenanthrene-10-oxide and derivatives thereof obtained by reacting them with compounds such as epoxy resins and phenolic resins; nitrogen-based flame retardants such as triazine compounds, cyanuric acid compounds, isocyanuric acid compounds, and phenothiazine; silicone-based flame retardants such as silicone oil, silicone rubber, and silicone resin; and inorganic flame retardants such as metal hydroxides, metal oxides, metal carbonate compounds, metal powders, boron compounds, and low-melting-point glass. These flame retardants can be used alone or in combination of two or more. When these flame retardants are used, their amount is preferably in the range of 0.1 to 20% by mass based on the entire contents of the heat endotherm or the entire dispersion liquid (a).

[0068] (Method for Manufacturing Heat Absorber) An example of a method for manufacturing a heat absorber according to this embodiment preferably includes the steps of filling an opening of a bag with an aqueous solvent, a water-soluble inorganic powder, and optionally one or more selected from the group consisting of inorganic fibers and additives, and sealing the opening of the bag to seal the bag. The aqueous solvent, the water-soluble inorganic powder, and optionally one or more selected from the group consisting of inorganic fibers and additives may be filled separately through the opening of the bag. Alternatively, a mixed solution (1) may be prepared in advance by impregnating or dispersing the water-soluble inorganic powder and optionally the inorganic fibers and additives in the aqueous solvent, and then the mixed solution may be filled through the opening of the bag. The mixed solution (1) preferably contains the aqueous solvent and the water-soluble inorganic powder, and more preferably contains the aqueous solvent, the water-soluble inorganic powder, and optionally one or more selected from the group consisting of inorganic fibers and additives. The mixed solution (1) preferably contains 0 to 50 mass% inorganic fibers, 10 to 95 mass% aqueous solvent, 0 to 50 mass% antifreeze, 5 to 90 mass% water-soluble inorganic powder (including water in the case of a hydrate), and 0 to 10 mass% additives, relative to the total amount (100 mass%) of the mixed solution (1); preferably contains 0 to 30 mass% inorganic fibers, 10 to 95 mass% aqueous solvent, 0 to 50 mass% antifreeze, 5 to 70 mass% water-soluble inorganic powder (including water in the case of a hydrate), and 0 to 10 mass% additives; and more preferably contains 1 to 12 mass% inorganic fibers, 20 to 94 mass% aqueous solvent, 0 to 25 mass% antifreeze, 5 to 50 mass% water-soluble inorganic powder (including water in the case of a hydrate), and 0 to 50 mass% additives. The amount of "water-soluble inorganic powder (including water in the case of a hydrate)" relative to the total amount (100% by mass) of the mixed solution (1) means the blending amount. Therefore, when the water-soluble inorganic powder is a hydrate, the amount of "water-soluble inorganic powder (including water in the case of a hydrate)" includes the amount of water contained in the water-soluble inorganic powder that is a hydrate.

[0069] A suitable heat absorber of this embodiment may be a heat absorber including a bag and, as the contents of the bag, inorganic fibers (e.g., 4 to 9% by mass of rock wool relative to the total amount of the contents), an aqueous solvent (e.g., 20 to 60% by mass of water relative to the total amount of the contents), and a water-soluble inorganic powder (e.g., 10 to 50% by mass of magnesium sulfate relative to the total amount of the contents). This may provide a heat absorber that has superior heat absorption and pressure resistance and can switch from an endothermic effect to an insulating effect in a high temperature range. In this embodiment, the total content of the aqueous solvent and the water-soluble inorganic powder contained as the contents of the bag may be preferably 79 to 100% by mass, more preferably 91 to 99.5% by mass, even more preferably 94 to 99% by mass, and even more preferably 96 to 99.4% by mass, relative to the total amount (100% by mass) of the contents of the bag. In this embodiment, the total content of the aqueous solvent, inorganic fibers, and water-soluble inorganic powder contained as the contents of the bag may be preferably 80 to 100% by mass, more preferably 92 to 99.5% by mass, even more preferably 93 to 99.4% by mass, and even more preferably 96 to 99.1% by mass, based on the total amount (100% by mass) of the contents of the bag. In this embodiment, the total content of the aqueous solvent, inorganic fibers, water-soluble inorganic powder, antifreeze agent, and additives contained as the contents of the bag may be preferably 83 to 100% by mass, more preferably 94 to 99.8% by mass, even more preferably more than 95% by mass and not more than 99.6% by mass, and even more preferably more than 96% by mass and not more than 99.5% by mass, based on the total amount (100% by mass) of the contents of the bag. The upper and lower limits of the above total content can be adjusted as appropriate.

[0070] (Preferred Aspects of Heat Absorber) The heat absorber of this embodiment preferably exhibits high cushioning properties and / or excellent pressure resistance during heating. Each preferred aspect will be described in detail below. <Highly Cushioning Heat Absorber> A preferred heat absorber of this embodiment has a bag and a content filled in the bag, the content containing an aqueous solvent (preferably water) and a water-soluble inorganic powder (preferably magnesium sulfate), and exhibits high cushioning properties. More preferably, the heat absorber exhibits high cushioning properties and has a bag and the content filled in the bag includes an aqueous solvent (preferably water), a water-soluble inorganic powder (preferably magnesium sulfate), and inorganic fibers (preferably ceramic wool). In this specification, a heat absorber exhibiting high cushioning properties is also referred to as a highly cushioning heat absorber. "Exhibiting high cushioning properties" means having excellent cushioning properties, and when the content of the bag contains inorganic fibers, high cushioning properties tend to be exhibited. Furthermore, "exhibiting high cushioning" specifically means that the cushioning (%) expressed by the following formula (I) is preferably 90% or more, more preferably 93% or more. By exhibiting the above-mentioned cushioning of 90% or more, it becomes easier to follow deformations that occur over a relatively short period of time, such as expansion and contraction due to charging and discharging of the battery cell. The upper limit of the high cushioning can be 100%. [Mathematical formula 2] "Cushioning (%) = h ａ / h ｂ × 100 Formula (I) (wherein, h ａ is the height (mm) of the pressed part 5 minutes after the surface of the high-cushion heat absorber is pressed at 1 MPa for 60 seconds and then released, and h ｂrepresents the height (mm) before the surface of the high-cushioning heat absorber is pressed at 1 MPa for 60 seconds.) In the heat absorber of this embodiment, when cushioning is important, it is preferable that not only is the cushioning (%) represented by formula (I) 90% or more, but also that the content of the antifreeze agent is controlled to a predetermined value or less. That is, when cushioning is important, the content of the antifreeze agent is preferably 40% by mass or less, more preferably 30% by mass or less, even more preferably 20% by mass or less, still more preferably 12% by mass or less, even more preferably 9% by mass or less, even more preferably 6% by mass or less, and particularly preferably substantially absent (0.5% by mass or less) relative to the total amount of the contents. Furthermore, in the heat absorber of this embodiment, when cushioning is important, the lower limit of the total amount of the aqueous solvent, inorganic fibers, water-soluble inorganic powder, and antifreeze agent contained in the contents is preferably more than 30% by mass, more preferably 50% by mass or more, even more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 100% by mass, relative to the total amount (100% by mass) of the contents of the heat absorber. On the other hand, the upper limit of the total amount of the aqueous solvent, inorganic fibers, water-soluble inorganic powder, and antifreeze agent contained in the contents is preferably 100% by mass or less, more preferably 50% by mass or less, relative to the total amount (100% by mass) of the contents of the heat absorber. It is believed that as the total amount of the inorganic fibers, aqueous solvent, and water-soluble inorganic powder increases relative to the total amount of the contents, the inorganic fibers and water-soluble inorganic powder reinforce the elasticity of the contents, thereby exhibiting high cushioning properties.

[0071] <High-Pressure-Resistant Heat Endogenous Material> A preferred heat endogenous material of this embodiment is a heat endogenous material having excellent pressure resistance when heated, the heat endogenous material comprising a bag and a content filled in the bag, the content including an aqueous solvent (preferably water) and a water-soluble inorganic powder (preferably magnesium sulfate). More preferably, the heat endogenous material has a bag and a content filled in the bag, the content including an aqueous solvent (preferably water), a water-soluble inorganic powder, and inorganic fibers. In this specification, a heat endogenous material having excellent pressure resistance when heated is also referred to as a high-pressure-resistant heat endogenous material. Such a high-pressure-resistant heat endogenous material tends to exhibit excellent pressure resistance when heated at high temperatures (e.g., 800°C or higher) due to thermal runaway of a battery, etc. "Exhibiting pressure resistance when heated" means that the heat endogenous material exhibits high pressure resistance when heated. Therefore, a preferred embodiment of the high-pressure-resistant heat absorber of this embodiment may include a bag capable of being filled with a content, an aqueous solvent, a water-soluble inorganic powder, and inorganic fibers, with the inorganic fibers and the water-soluble inorganic powder forming a composite and filling the content. In this case, since the water-soluble inorganic powder exhibits excellent solubility in water, the water-soluble inorganic powder can be uniformly dispersed or dissolved in the content. The coexistence of inorganic fibers and water-soluble inorganic powder temporarily forms a foamed film during thermal runaway, and it is believed that this foamed film acts as a reinforcing dispersion agent. Furthermore, the coexistence of inorganic fibers and water-soluble inorganic powder is believed to result in a synergistic effect of high pressure resistance, as the inorganic fibers support the water-soluble inorganic powder and promote the dispersibility of the water-soluble inorganic powder. More specifically, when the high-pressure-resistant heat absorber of this embodiment is heated, the aqueous solvent (water component) within the heat absorber exhibits an endothermic effect and evaporates while forming a foamed film. At this time, the reduction in the aqueous solvent creates voids between the inorganic fibers, improving the insulating performance. Furthermore, during the evaporation of the aqueous solvent, water-soluble inorganic powder precipitates and is supported by the inorganic fibers. If the water-soluble inorganic powder is a hydrate, as heating progresses, the water component contained in the hydrate also evaporates while exhibiting an endothermic effect. At this time, the water-soluble inorganic powder can be uniformly dispersed or dissolved in the aqueous solvent, preventing the water-soluble inorganic powder from being unevenly distributed and supported on the inorganic fibers. As heating progresses, the water-soluble inorganic powder supported on the inorganic fibers sinters, firmly reinforcing the network structure of the inorganic fibers.Furthermore, the inorganic powder forms a porous body within the inorganic fibers, further improving thermal insulation. As described above, the dispersed presence of the water-soluble inorganic powder enables the reinforcement of the network structure and the formation of a porous body throughout the inorganic fibers. This allows the high-pressure-resistant heat absorber of this embodiment to exhibit a synergistic effect between thermal insulation and high pressure resistance. The uniform dispersion may mean, for example, that when the composite content is removed, the difference in concentration (mass%) of the water-soluble inorganic powder at both ends of the composite within approximately 3 mm from the end of the composite is within ±15%. Furthermore, "exhibiting high pressure resistance upon heating" specifically means that after the heat absorber is heated until the surface opposite the heating surface of the heat absorber reaches a predetermined temperature, the thickness change rate (%) represented by the following formula (II) is preferably 70% or more, more preferably 75% or more, even more preferably 85% or more, and particularly preferably 90% or more. A thickness change rate of 70% or more provides excellent pressure resistance, effectively suppressing and preventing inter-cell explosions. The upper limit of the thickness change rate may be 100%. [Mathematical Expression 3] Equation (II): "Thickness change rate (%) = (thickness of the heat absorber after pressurization by pressing the surface of the high pressure-resistant heat absorber after heating at 0.5 MPa for 60 seconds) / (thickness of the heat absorber before pressurization by pressing the surface of the high pressure-resistant heat absorber after heating at 0.5 MPa for 60 seconds) × 100" In the above equation (II), "the thickness of the heat absorber after pressurization by pressing the surface of the high pressure-resistant heat absorber after heating at 0.5 MPa for 60 seconds" refers to the average thickness (arithmetic mean of thicknesses at any five positions) of the heat absorber after heating to a predetermined temperature or higher, preferably after heating under the following heating conditions, by pressing one surface of the high pressure-resistant heat absorber for 60 seconds (within approximately 20 minutes from immediately after pressing). Similarly, in the above formula (II), "the thickness of the heat absorber before pressing the surface of the high pressure resistant heat absorber after heating at 0.5 MPa for 60 seconds" refers to the average thickness of the heat absorber (arithmetic mean of thicknesses at any five points) after one surface of the high pressure resistant heat absorber is heated to a predetermined temperature or higher, preferably after heating under the heating conditions described below, and before pressing the surface of the high pressure resistant heat absorber at 0.5 MPa for 60 seconds. Heating conditions: "50 kW / m by radiant heat.２ The heat absorber was heated with a heat quantity of 1000 kJ / cm2 until the temperature of the surface opposite the heated surface (back surface) reached a predetermined temperature, and then the heat absorber was allowed to dissipate heat at room temperature (22-28°C). The surface of the heat absorber was then naturally cooled until the temperature of the surface reached room temperature (22-28°C), and the thickness change rate (%) before and after heating was calculated. The heated surface of the heat absorber was pressed at 0.5 MPa for 60 seconds. The "predetermined temperature" in the heating conditions refers to the temperature at which the heat absorber becomes porous and can be set depending on the heat absorber's usage environment, the required explosion resistance, and the expected thermal runaway onset temperature. The "predetermined temperature" can be, for example, 240°C (240°C or higher), preferably 200°C (200°C or higher), more preferably 180°C (180°C or higher), even more preferably 160°C (160°C or higher), even more preferably 150°C or higher, and particularly preferably 120°C (120°C or higher). For example, if a heat absorber is heated until the temperature of the surface (back surface) opposite to the heating surface (surface directly exposed to radiant heat) of the heat absorber reaches 150° C., and the thickness change rate (%) after heating is equal to or greater than a predetermined value, the heat absorber can achieve a thickness change rate equal to or greater than the predetermined value even in a temperature range of 150° C. When high pressure resistance is important, the heat absorber of this embodiment preferably contains inorganic fibers, an aqueous solvent, and a water-soluble inorganic powder as its contents.

[0072] [Secondary Battery Module] The type of secondary battery that can be equipped with the heat absorber of this embodiment is not particularly limited, and examples thereof include lithium ion batteries, lithium ion polymer batteries, lead storage batteries, nickel-metal hydride storage batteries, nickel-cadmium storage batteries, nickel-iron storage batteries, nickel-zinc storage batteries, silver oxide-zinc storage batteries, metal-air batteries, polyvalent cation batteries, condensers, and capacitors, etc. Among these, lithium ion batteries are preferred.

[0073] The secondary battery module capable of mounting the heat absorber of this embodiment is a secondary battery mounted on a mobile object such as a vehicle or an aircraft (particularly a drone), and includes multiple battery cells and a case for housing the multiple battery cells. The battery cells (also referred to as battery cells) constituting the secondary battery module may be battery cells in which, for example, a battery exterior film is used as an exterior material and battery elements including at least a positive electrode material layer, a negative electrode material layer, a separator, a positive electrode current collector, and a negative electrode current collector are enclosed within the exterior material. A secondary battery module capable of mounting the heat absorber of this embodiment will be described below with reference to FIG. 1 . FIG. 1 shows a cross-sectional view of a stacked battery 20 as an example of a secondary battery. It should be noted that the secondary battery capable of mounting the heat absorber of this embodiment is not limited to the flat stacked battery 20 shown in FIG. 1 . The secondary battery capable of mounting the heat absorber of this embodiment may be cylindrical, such as a wound secondary battery, or may be a cylindrical secondary battery modified to have a flat rectangular shape. In this embodiment, the stacked battery 20 has a structure in which a flat, approximately rectangular battery element 10, in which charge and discharge reactions essentially proceed, is sealed inside battery exterior materials 18a, b. The battery element 10 has a structure in which a positive electrode, an electrolyte layer (or separator) 14, and a negative electrode are stacked. The positive electrode has a structure in which a positive electrode material layer 11 containing a positive electrode active material is disposed on both sides of a positive electrode current collector 12. The negative electrode has a structure in which a negative electrode material layer 16 containing a negative electrode active material is disposed on both sides of a negative electrode current collector 17. One positive electrode material layer 11 and the negative electrode material layer 16 adjacent to the positive electrode material layer 11 are arranged to face each other with the electrolyte layer 14 interposed therebetween, and the positive electrode, electrolyte layer 14, and negative electrode are stacked in sequence. As a result, adjacent positive electrodes, electrolyte layers 14, and negative electrodes form a single cell body. The stacked battery 20 shown in FIG. 1 has a structure in which a plurality of such cell bodies are stacked and electrically connected in parallel. In addition, an activated carbon layer 19 is provided to adsorb components derived from the positive electrode active material due to melting or sublimation of the positive electrode active material when the battery is exposed to high temperatures.As shown in FIG. 1 , the positive electrode current collector 12 and the negative electrode current collector 17 are respectively attached with a positive electrode terminal 13 and a negative electrode terminal 15, which electrically connect the positive and negative electrodes, and are structured to be sandwiched between the ends of the battery exterior materials 18a and 18b and extend outside the battery exterior materials 18a and 18b. The positive electrode terminal 13 and the negative electrode terminal 15 may be attached to the positive electrode current collector 12 and the negative electrode current collector 17 of each electrode by welding or the like via a positive electrode lead and a negative electrode lead (not shown), as necessary. The battery exterior materials 18a and 18b are made of laminated film, and typically, sealant layers formed on the surfaces of the battery exterior films 18a and 18b are heat-sealed to each other. The battery exterior materials 18a and 18b have a region at their peripheries where the sealant layers are in close contact with each other by heat sealing.

[0074] Next, a secondary battery module incorporating the heat absorber of this embodiment will be described with reference to FIG. 2 . FIG. 2 is a perspective view schematically illustrating the disassembled state of the secondary battery module of FIG. 1 . The battery element 10 shown in FIG. 2 has a configuration in which a positive electrode formed on a positive electrode current collector 12 (e.g., aluminum foil) having a positive electrode terminal 13 and a negative electrode disposed on a negative electrode current collector 17 (e.g., metal foil) having a negative electrode terminal 15 are stacked so as to face each other with a separator 14 containing an electrolyte interposed therebetween. A plurality of battery elements 10 are stacked and sealed with battery exterior materials 18 a, b (e.g., aluminum laminate exteriors). The heat absorber 1 of this embodiment is arranged so as to contact the negative electrode current collector 17. The heat absorber 1 may be arranged so as to contact not only the negative electrode current collector 17 but also the positive electrode current collector 12. Therefore, a secondary battery module having a heat absorber 1 mounted on a battery element 10 includes one or more laminates in which a positive electrode formed on a positive electrode current collector 12 (e.g., aluminum foil) having a positive electrode terminal 13, a separator 14 containing an electrolyte, and a negative electrode disposed on a negative electrode current collector 17 (e.g., metal foil) having a negative electrode terminal 15 are sequentially stacked, and the one or more heat absorbers 1 can be arranged so as to abut the positive electrode current collector 12 and / or the negative electrode current collector 17 but not the separator 14. When a solid electrolyte or a gel electrolyte is used as the electrolyte, the electrolyte can be interposed between the electrodes instead of the separator 14. Meanwhile, the aqueous solvent contained in the heat absorber of this embodiment does not come into direct contact with the battery element 10. Therefore, a preferred heat absorber for a secondary battery in this embodiment is in a form in which an aqueous solvent, a water-soluble inorganic powder, and inorganic fibers are filled in a bag, but the contents of the bag do not include the aqueous solvent and the battery element 10 in direct contact with each other, and more preferably, the contents of the bag of the heat absorber for a secondary battery do not include the above-mentioned battery element 10.

[0075] The secondary battery module of the present disclosure may include a heat absorber 1 of the present embodiment sandwiched between adjacent battery elements 10 (also referred to as battery cells) housed in multiple cases (not shown) or multiple battery exterior films 18a, b. The cases may be formed, for example, from aluminum, iron, or a metal material containing these, or a resin material such as polyphenylene sulfide. Forming the cases from a resin material contributes to reducing the weight of the secondary battery module. The heat absorber 1 may be sandwiched between the multiple battery elements 10 using, for example, adhesives, fusion (ultrasonic fusion, high-frequency fusion, or heat fusion), or pressure-sensitive adhesives. With this configuration, the heat absorber 1 sandwiched between the battery elements 10 absorbs heat generated during charging of the secondary battery, thereby suppressing a sudden temperature rise in the battery elements 10 and preventing deterioration, fire, and the like of the battery elements 10. When the heat absorber 1 is sandwiched between the battery elements 10, the thermal insulation properties can suppress the temperature effects between the battery elements 10. Furthermore, it acts as a buffer against volume changes due to the expansion of the battery elements 10, which is thought to help mitigate the increase in internal pressure of the secondary battery module. On the other hand, when the battery elements 10 experience thermal runaway due to excessive heat generation, the water-soluble inorganic powder in the heat absorber 1 sinters and transforms into a hard, plate-like porous material, increasing pressure resistance and decreasing thermal conductivity. This allows the heat absorber 1 to maintain a constant distance between the cells, maintaining effective insulation and controlling thermal conductivity, effectively suppressing the battery elements 10 from exploding.

[0076] In addition, in a secondary battery module, the heat absorber of this embodiment may be disposed in a case that houses a plurality of battery elements (battery cells) or in a battery exterior film.

[0077] The present invention will be described in detail below with reference to examples and comparative examples. It should be noted that the present invention is not limited to the examples listed below. (1) Measurement of Endothermic Onset Temperature, Endothermic Peak Temperature, and Endothermic Amount The endothermic onset temperature and endothermic peak temperature of the endothermic bodies prepared in the present examples and comparative examples were measured as follows. Using a differential scanning calorimeter (DSC; DSC-7020, manufactured by Hitachi High-Tech Corporation), the temperature was raised from 20°C to 350°C at a rate of 1°C / min under a nitrogen atmosphere. The endothermic onset temperature (°C) was determined as the temperature at the intersection of a straight line extending the baseline on the low-temperature side of the DSC measurement curve toward the high-temperature side and a tangent drawn at the point where the gradient of the curve on the low-temperature side of the endothermic peak associated with evaporation is maximum. The point where the difference from the baseline on the DSC measurement curve is maximum was determined as the endothermic peak temperature (°C). The integral of the endothermic peak based on the baseline of the DSC measurement curve, divided by the mass of the water-soluble inorganic powder used in the measurement, was determined as the endothermic amount (J / g or mJ / mg).

[0078] (2) Evaluation of cushioning properties The cushioning properties of the heat absorbers prepared in the present examples and comparative examples were evaluated using the following method. Specifically, at room temperature (23°C), the heat absorber was placed in a Tensilon universal testing machine ("RTE-1210" manufactured by Orientec Co., Ltd.) with a size of 100 mm length x 100 mm width x 4.8 mm height and equipped with a 7 mmφ indentation jig, and an indentation test was performed. The indentation test was performed by indenting the surface of the heat absorber at 1 MPa for 60 seconds, and then releasing the indentation and measuring the height (mm) h of the indented part on the surface 5 minutes later. ａ and the height (mm) h before pressing the surface of the heat absorber at 1 MPa for 60 seconds ｂ The cushioning property (also referred to as the degree of return) was observed and evaluated according to the following criteria using the following formula (I). The indentation test was performed at two points on the surface of the heat absorber, and the height (or thickness) was measured at each point. The cushioning property (%) was calculated using the following formula (I), and the average value is shown in Table 1. Formula (I): Cushioning property (%) = h ａ / h ｂ× 100 (Evaluation criteria for cushioning) "Returns to 90% or more of its original height" was marked as "◎". "Returns to 80% or more of its original height" was marked as "◯". "Returns to 70% or more of its original height" was marked as "△". "Returns to less than 70% of its original height or cannot be measured" was marked as "×".

[0079] (3) Heating Experiment Using a Cone Calorimeter The heat absorbers produced in the present example and comparative examples were directly heated by radiant heat using a cone calorimeter 30 (manufactured by Toyo Seiki Co., Ltd.) shown in Fig. 4 in accordance with JIS A 1316. More specifically, the cone calorimeter 30 is based on the principle that the amount of heat generated in combustion is 13.1 MJ per kg of oxygen, regardless of the type of organic material, and calculates the heat release rate, total heat release, etc. from the oxygen consumption method by measuring the oxygen concentration in the combustion exhaust gas and the exhaust gas flow rate. The heat absorber 1 was placed at the top of the holder 32 as a test specimen, and a 50 kW / m radiator was applied from the cone 31. ２ Then, the change in temperature was measured by the thermocouple 33 on the back surface of the heat absorber 1 until the back surface of the heat absorber 1 reached 200°C. In addition to the temperature change, the presence or absence of combustion was also observed.

[0080] (4) Evaluation of Pressure Resistance of Test Specimens After Heating Experiments After the heating experiments using the cone calorimeter described above were performed on the heat absorbers produced in the present examples and comparative examples, their pressure resistance was evaluated using the following method. Specifically, an indentation test was performed at room temperature (23°C) by placing the heat absorbers in a Tensilon universal testing machine ("RTE-1210" manufactured by Orientec Co., Ltd.) equipped with a 7 mmφ indentation jig. The indentation test measured the indentation amount when pressed at 0.5 MPa for 60 seconds (= when pressurized). From the indentation amount, the thickness change rate when pressed was calculated according to the following formula (II) and evaluated according to the following criteria. Note that the indentation test was performed at two indentation points on the surface of the heat absorber after the heating experiments using the cone calorimeter described above. The height (or thickness) was measured at each point, and the thickness change rate was calculated using the following formula (II). The average values ​​are shown in Table 1. Equation (II): Thickness change rate (%) = (thickness of the heat absorber after pressing the surface of the heat absorber after heating at 0.5 MPa for 60 seconds) / (thickness of the heat absorber before pressing the surface of the heat absorber after heating at 0.5 MPa for 60 seconds) × 100 (Evaluation criteria for thickness change rate) A thickness change rate in the range of 70 to 100% was rated as "◎". A thickness change rate in the range of 40 to 69% was rated as "◯". A thickness change rate in the range of 0 to 39% or immeasurable was rated as "×". Note that a thickness change rate in the range of 70 to 100% has the best pressure resistance. A lower value of the pressing amount (= crushed amount) indicates better pressure resistance.

[0081] For Example 1, in the heating experiment using the cone calorimeter, when the back surface of the heat absorber 1 reached temperatures of 150°C, 160°C, 170°C, 180°C, and 240°C, the heat absorber was naturally cooled until the surface temperature returned to room temperature, and the thickness change rate (%) before and after heating at each temperature was calculated. The results are shown in Table 2 below.

[0082] (5) Method for Measuring Average Porosity, True Density, and Bulk Density The average porosity of inorganic fibers was calculated using the following formula (1) from the bulk density ρf and true density ρr measured by the following method: Average porosity (%) = ((1 / ρf) - (1 / ρr)) / (1 / ρf) × 100 Formula (1) <Measurement of True Density> The inorganic fibers removed from the contents of the heat absorber or the inorganic fibers before being sealed in a bag were thoroughly washed with distilled water and dried overnight. The dried inorganic fibers were then placed in a stoppered test tube, and a mixed solvent prepared by appropriately mixing three solvents was added to the stoppered test tube and immersed in a thermostatic bath at 30°C. If the inorganic fibers float, n-heptane, which has a low density, was added. On the other hand, if the inorganic fibers sink, ethylene dibromide, which has a high density, was added. This procedure was repeated until the inorganic fibers were suspended in the liquid, and the density of the mixed solvent was measured using a Gay-Lussac pycnometer. <Measurement of bulk density> The inorganic fibers removed from the contents of the heat absorber or the inorganic fibers before being sealed in the bag were thoroughly washed with distilled water and dried overnight, and then the dimensions of the dried inorganic fibers were measured to calculate the bulk volume V of the inorganic fibers. Thereafter, the mass M of the inorganic fibers was measured using a precision balance. From the obtained mass M and bulk volume V, the bulk density of the inorganic fibers was calculated using the following formula (2). Bulk density ρf (g / cm ３ )=M / V...Formula (2)

[0083] (2) Raw Materials Used <Water-soluble Inorganic Powder> The water-soluble inorganic powders used in the Examples and Comparative Examples are as follows. Magnesium sulfate heptahydrate: (product name "Magnesium sulfate heptahydrate Shika Grade 1" manufactured by Kanto Chemical Co., Ltd., solubility in 100g of water at 20°C (g / 100g): 71g) Aluminum hydroxide: (product name "Aluminum hydroxide Shika Grade 1" manufactured by Kanto Chemical Co., Ltd., solubility in 100g of water at 20°C (g / 100g): less than 0.1g) Sodium acetate: (product name "Sodium acetate Shika Grade 1" manufactured by Kanto Chemical Co., Ltd., solubility in 100g of water at 20°C (g / 100g): 46.5g) Calcium sulfate dihydrate: (product name "Calcium sulfate dihydrate Shika Grade 1" manufactured by Kanto Chemical Co., Ltd., solubility in 100g of water at 20°C (g / 100g): 0.2g) The solubilities of the above hydrates in anhydrous form are as follows. Solubility of anhydrous magnesium sulfate in 100 g of water at 20°C (g / 100 g): 30 g Solubility of anhydrous calcium sulfate in 100 g of water at 20°C (g / 100 g): 0.2 g In the endothermic pouch, magnesium sulfate heptahydrate exists as anhydrous, and calcium sulfate dihydrate exists as hydrate. <Bag> The water vapor permeability ([g / (m ２ ・24h)]) is 50g / (m ２ - We confirmed that the time is less than 24 hours.

[0084] (3) Examples and Comparative Examples Example 1 32 parts by mass of magnesium sulfate heptahydrate (the amount including water in the hydrate) and 60 parts by mass of pure water were mixed to prepare a water-soluble inorganic powder-containing aqueous solution (1). Then, 92 parts by mass of the water-soluble inorganic powder-containing aqueous solution (1) and 8 parts by mass of ceramic wool (average porosity 96.9%, true density 3, bulk density 0.093) were inserted into a bag-shaped container (116 mm long x 116 mm wide) made of an aluminum pouch ("Gas Barrier Bag" manufactured by Mitsubishi Gas Chemical Company, Inc., thickness 0.094 mm, laminated with PET, aluminum foil, and polyethylene). Next, the aluminum pouch bag was filled with the water-soluble inorganic powder-containing aqueous solution (1), and the inlet was closed by heat sealing. The aluminum pouch bag was then placed flat between 4.8 mm thick gap materials, and a flat plate was placed on top of it and left to stand at 20 ° C for 10 minutes to produce a 4.8 mm thick sheet-like heat absorber (1). The obtained heat absorber (1) was then evaluated according to the procedures described in the evaluation section above. The results are shown in Table 1 and Figure 3. Furthermore, when the heat absorber (1) produced in Example 1 was heated to 200 ° C at the back surface temperature of the test specimen using a cone calorimeter, it was confirmed that the heat absorber (1) produced in Example 1 was transformed into a porous body at temperatures above 150 ° C. Figure 5(a) shows a cross-sectional photograph of the heat absorber (1) transformed into a porous body.

[0085] Example 2: 27 parts by mass of sodium acetate and 64 parts by mass of pure water were mixed to prepare a water-soluble inorganic powder-containing aqueous solution (2). Then, 91 parts by mass of the water-soluble inorganic powder-containing aqueous solution (2) and 9 parts by mass of ceramic wool (average porosity 96.8%, true density 3, bulk density 0.096) were inserted into a container made of an aluminum pouch (a "gas barrier bag" manufactured by Mitsubishi Gas Chemical Company, Inc., 0.094 mm thick, laminated with PET, aluminum foil, and polyethylene) in a bag shape measuring 116 mm long x 116 mm wide. The water-soluble inorganic powder-containing aqueous solution (2) was then filled into the aluminum pouch, the inlet was closed by heat sealing, and the aluminum pouch was placed flat between 4.8 mm thick gap members. A flat plate was placed on top of it and allowed to stand at 20 ° C. for 10 minutes to produce a 4.8 mm thick sheet-shaped heat absorber (2). The obtained heat absorber (2) was then subjected to various evaluations according to the procedures described in the evaluation section above. The results are shown in Table 1 and Figure 3. Furthermore, when the heat absorber (1) prepared in Example 2 was heated to 200°C using a cone calorimeter, it was confirmed that the heat absorber (2) prepared in Example 2 changed into a porous body at temperatures above 180°C. Figure 5(b) shows a cross-sectional photograph of the heat absorber (2) that had changed into a porous body.

[0086] Example 3: 35 parts by mass of magnesium sulfate heptahydrate (including the amount of water in the hydrate) and 65 parts by mass of pure water were mixed to prepare a water-soluble inorganic powder-containing aqueous solution (3). Then, 100 parts by mass of the water-soluble inorganic powder-containing aqueous solution (3) was inserted into a container in the form of a bag made of an aluminum pouch (a "gas barrier bag" manufactured by Mitsubishi Gas Chemical Company, Inc., 0.094 mm thick, laminated with PET, aluminum foil, and polyethylene) measuring 116 mm in length and 116 mm in width. The water-soluble inorganic powder-containing aqueous solution (3) was then filled into the aluminum pouch, the inlet was closed by heat sealing, and the aluminum pouch was placed flat between 4.8 mm thick gap members. A flat plate was placed on top of it and allowed to stand at 20 ° C. for 10 minutes to produce a 4.8 mm thick sheet-like heat absorber (3). The obtained heat absorber (3) was then evaluated according to the procedures described in the evaluation section above. The results are shown in Table 1 and Figure 3. In addition, when the heat absorber (3) prepared in Example 3 was heated to 200°C using a cone calorimeter, it was confirmed that the heat absorber (3) prepared in Example 3 also changed into a porous body at 180°C or higher, similar to the heat absorbers of Examples 1 and 2.

[0087] Example 4: 25 parts by mass of anhydrous magnesium sulfate (including the amount of water in the hydrate) and 75 parts by mass of pure water were mixed to prepare a water-soluble inorganic powder-containing aqueous solution (4). Then, 100 parts by mass of the water-soluble inorganic powder-containing aqueous solution (4) was inserted into a container in the form of a bag made of an aluminum pouch (a "gas barrier bag" manufactured by Mitsubishi Gas Chemical Company, Inc., 0.094 mm thick, laminated with PET, aluminum foil, and polyethylene) measuring 116 mm in length and 116 mm in width. The water-soluble inorganic powder-containing aqueous solution (4) was then filled into the aluminum pouch, the inlet was closed by heat sealing, and the aluminum pouch was placed flat between 4.8 mm thick gap members. A flat plate was placed on top of it and allowed to stand at 20 ° C. for 10 minutes to produce a 4.8 mm thick sheet-shaped heat absorber (4). The obtained heat absorber (4) was then evaluated according to the procedures described in the evaluation section above. As a result, the evaluation results of the endotherm amount, meltdown temperature, endotherm onset temperature, endotherm peak temperature, pressure resistance and cushioning properties were all similar to those of Example 2.

[0088] Comparative Example 1: In Comparative Example 1, a commercially available material, "Xiaomei silica aerogel mat material, thickness 4.8 mm (measured), thermal conductivity: 0.012 to 0.018 W / m·K," was used as the comparative sheet (C1). The silica aerogel mat material of Comparative Example 1 had a nominal thickness of 3 mm. The obtained comparative sheet (C1) was then subjected to various evaluations according to the procedures described in the evaluation section above. The results are shown in Table 1 and FIG. 3. Because the material of Comparative Example 1, "Xiaomei silica aerogel mat material," does not substantially possess heat absorption, the "Heat absorption onset temperature (°C), endothermic peak temperature (°C), and endothermic amount (J / g or mJ / mg)" columns in the table are marked with "Substantially no heat absorption." Furthermore, although the bulk density of the material used in Comparative Example 1 could be measured, the true density could not be measured. Therefore, the true density and average porosity columns in the table are marked with "Not measurable."

[0089] Comparative Example 2: An aluminum pouch (a 0.094 mm thick "Gas Barrier Bag" manufactured by Mitsubishi Gas Chemical Company, Inc., composed of a laminate of PET, aluminum foil, and polyethylene) was prepared in a 116 mm x 116 mm bag shape. Next, 33 parts by mass of pure water and 67 parts by mass of calcium sulfate dihydrate (including the amount of water in the hydrate) as an inorganic powder were filled into the aluminum pouch. The inlet was then heat-sealed, and the aluminum pouch was placed flat between 4.8 mm thick gap fillers. A flat plate was placed on top and allowed to stand at 20°C for 10 minutes to produce a 4.8 mm thick sheet-like comparative sheet (C2). The resulting comparative sheet (C2) was then evaluated according to the procedures described in the evaluation section above. The results are shown in Table 1 and Figure 3. Note that in the system of Comparative Example 2, because DSC of a liquid could not be measured, data on the endothermic energy (J / g or mJ / mg) could not be obtained. Therefore, the value calculated based on 2000 J / g (100°C), which is an approximation of the literature value of 2257 J / g, is entered in the table as 1170 J / g (specifically, the approximate value of the heat absorption of water, 2000 J / g x 33% + the measured heat absorption of calcium sulfate, 762 J / g x 67% = 1170 J / g).

[0090] Comparative Example 3: 91 parts by mass of pure water and 9 parts by mass of ceramic wool (average porosity 96.9%, true density 3, bulk density 0.092) were inserted into a container in the form of a bag made of an aluminum pouch (a "gas barrier bag" manufactured by Mitsubishi Gas Chemical Company, Inc., 0.094 mm thick, laminated with PET, aluminum foil, and polyethylene) measuring 116 mm long and 116 mm wide. The aluminum pouch was then filled with the pure water and the ceramic wool, the inlet was closed by heat sealing, and the aluminum pouch was placed flat between 4.8 mm thick gap materials. A flat plate was placed on top of the aluminum pouch and allowed to stand at 20°C for 10 minutes to produce a comparative sheet (C3) with a thickness of 4.8 mm. The obtained comparative sheet (C3) was then evaluated according to the procedures described in the evaluation section above. The results are shown in Table 1 and FIG. 3.

[0091] Comparative Example 4: 27 parts by mass of aluminum hydroxide (solubility in 100 g of water at 20 °C: 0.0001 g) and 64 parts by mass of pure water were mixed to prepare a dispersion (1). Then, 91 parts by mass of the dispersion (1) and 9 parts by mass of ceramic wool (average porosity 96.8%, true density 3, bulk density 0.096) were inserted into a container made of an aluminum pouch (a "gas barrier bag" manufactured by Mitsubishi Gas Chemical Company, Inc., 0.094 mm thick, laminated with PET, aluminum foil, and polyethylene) in a bag shape measuring 116 mm long x 116 mm wide. The aluminum pouch was then filled with the dispersion (1), the inlet was closed by heat sealing, and the aluminum pouch was placed flat between 4.8 mm thick gap materials. A flat plate was placed on top of it and allowed to stand at 20 °C for 10 minutes to produce a 4.8 mm thick sheet for comparison (C4). The comparative sheet (C4) was then subjected to various evaluations according to the procedures described in the evaluation section above, and the results are shown in Table 1 and FIG.

[0092] For Examples 1 and 3, the "content (% by mass) of water-soluble inorganic powder" in "contents (amount blended at preparation)" in Table 1 indicates the content in the hydrate state, and the "content (% by mass) of water-soluble inorganic powder" in "contents (inside bag)" indicates the content in the state where water has been removed from the hydrate (anhydrous form). The "content (% by mass) of aqueous solvent" indicates the content of the aqueous solvent at the time of preparation, including the amount of water removed from the hydrate of the water-soluble inorganic powder. For Comparative Examples 2 and 4, the "content (% by mass) of water-soluble inorganic powder" in "contents (amount blended at preparation)" in Table 1 and the "content (% by mass) of water-soluble inorganic powder" in "contents (inside bag)" indicate the content (% by mass) of water-insoluble inorganic powder. In Comparative Example 2, the "content (% by mass) of water-soluble inorganic powder" in "contents (inside bag)" indicates the content including the amount of water in the hydrate, and the "content (% by mass) of aqueous solvent" is the same as the content of the aqueous solvent at the time of preparation.

[0093]

[0094]

[0095] The above experimental results confirmed that the heat absorber of this embodiment, with the entire contents and pouch acting as a porous body, maintained sufficient heat absorption, pressure resistance, and transitioned from a heat absorber to a thermal insulator even when the battery temperature rose or over time. The heat absorbers using water-insoluble inorganic powders (Comparative Examples 2 and 4) exhibited particularly poor pressure resistance compared to heat absorbers containing water-soluble inorganic powders. The content values ​​for each content component in Table 1 are rounded to the nearest whole number. Figure 3 shows a graph of the results of a cone calorimeter test on the heat absorbers (1) to (3) prepared in Examples 1 to 4 and the comparative sheets (C1), (C3), and (C4) prepared in Comparative Examples 1, 3, and 4. The vertical axis of Figure 3 represents temperature (°C), and the horizontal axis represents elapsed time (seconds). Therefore, (1) to (3) and (C1), (C3), and (C4) in Figure 3 correspond to the heat absorbers (1) to (3) of the Examples and Comparative Examples and the comparative sheets (C1), (C3), and (C4). From the experimental results in Figure 3, it was confirmed that the heat absorbers (1) to (3) produced in Examples 1 to 3 have higher heat absorption capacity than the heat absorbers of the Comparative Examples, and therefore delay the time until the temperature of the heat absorber reaches 200°C.

[0096] [Explanation of symbols] 1 heat absorber 10 battery element 11 positive electrode material layer 12 positive electrode current collector 13 positive electrode terminal 14 electrolyte layer or separator 15 negative electrode terminal 16 negative electrode material layer 17 negative electrode current collector 18a, b battery exterior material 19 activated carbon layer 20 stacked battery 30 cone calorimeter 31 cone (heater) 32 stainless steel holder 33 aluminum foil cover 34 thermocouple 35 stainless steel holder 36 ceramic wool (large) 37 ceramic wool (small)

[0097] This application claims the benefit of priority to three applications: a Japanese patent application filed on July 3, 2023 (Patent Application No. 2023-109644), a Japanese patent application filed on December 25, 2023 (Patent Application No. 2023-218613), and a Japanese patent application filed on December 25, 2023 (Patent Application No. 2023-218614), the contents of which are incorporated herein by reference.

Claims

1. A bag capable of being filled with contents; an aqueous solution containing an aqueous solvent and a water-soluble inorganic powder that dissolves in the bag in an amount of 5 g or more per 100 g of water at 20° C.; A heat absorber having the above structure.

2. The heat absorber according to claim 1, further comprising one or more components selected from the group consisting of antifreeze agents and inorganic fibers in the content.

3. The heat absorber according to claim 1 or 2, wherein the water-soluble inorganic powder is one or more selected from the group consisting of chlorides, sulfates, carbonates, nitrates, phosphates, acetates, alkali metal oxides, and alkaline earth metal oxides.

4. A heat absorber as described in claim 1 or 2, wherein the total content of the aqueous solvent and the water-soluble inorganic powder contained as the contents of the bag is 79 to 100 mass% relative to the total amount (100 mass%) of the contents of the bag.

5. A heat absorber as described in claim 1 or 2, wherein the total content of the aqueous solvent, inorganic fibers, water-soluble inorganic powder, antifreeze agent, and additives contained as the contents of the bag is 83 to 100 mass% relative to the total amount (100 mass%) of the contents of the bag.

6. A heat absorber as described in claim 1 or 2, wherein the heat absorption onset temperature range is 35°C or higher and 400°C or lower.

7. A heat absorber as described in claim 1 or 2, wherein the amount of heat absorbed at the endothermic peak temperature (in the range of 80°C to 160°C) is 100 J / g or more and 3000 J / g or less.

8. A heat absorber as described in claim 1 or 2, wherein the content of the water-soluble inorganic powder in the aqueous solution is 5 to 80 mass % relative to the total amount of the aqueous solution.

9. 3. The heat absorber according to claim 1, wherein the content is changed into a porous body when the content is heated to 120° C. or higher.

10. The following formula (I): [Equation 1] "Thickness change rate (%) = (thickness of the heat absorber after pressing the surface of the heat absorber heated under the heating conditions below at 0.5 MPa for 60 seconds) / (thickness of the heat absorber before pressing the surface of the heat absorber heated under the heating conditions below at 0.5 MPa for 60 seconds) x 100" Heating conditions: "50kW / m due to radiant heat ２ The heat absorber was heated with a heat quantity of 0.5 MPa until the temperature of the surface (back surface) opposite to the heated surface of the heat absorber reached a predetermined temperature, and then the heat absorber was allowed to dissipate heat at room temperature and naturally cooled until the surface temperature of the heat absorber reached room temperature, and the thickness change rate (%) before and after heating was calculated. The heated surface of the heat absorber was pressed at 0.5 MPa for 60 seconds. The heat sink according to claim 1 or 2, wherein the thickness change rate represented by the formula (1) is 70% or more.

11. A secondary battery module comprising the heat sink according to claim 1 or 2.

12. The secondary battery module according to claim 1 , wherein the heat sink is sandwiched between battery cells.