Heat absorber and secondary battery module equipped with the 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

[Technical Field]

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

[0002] Secondary batteries, which can control the time difference between energy storage and demand, are used in a variety of applications, including automobiles and mobile devices, and are becoming increasingly important as they are needed to expand the introduction of renewable energy from the perspective of building a low-carbon society and ensuring energy security. However, secondary batteries, such as lithium-ion batteries, are at risk of thermal runaway damage when their temperature rises due to heat generation during high-speed charging or high-power discharge. Furthermore, as ultra-high-speed charging advances, it is predicted that the amount of heat generated will increase, creating a need for methods to suppress temperature rise in order to improve battery safety. Secondary batteries can also experience thermal runaway due to internal short circuits, resulting in problems such as fire or smoke. Therefore, in order to minimize the damage caused by such malfunctions, there is a need for technology that can suppress, prevent, or delay multiple explosions by absorbing the heat from abnormally high battery temperatures to extinguish the fire, or by absorbing heat and insulating it to suppress the heat transfer to other battery cells (hereinafter referred to as battery cells, or simply cells).

[0003] For example, Patent Documents 1 and 2 are cited as examples of technologies that have excellent heat insulation and fire spread prevention properties. Patent Document 1 describes a laminated fire spread prevention material that includes a layer A containing sodium silicate with an SiO2 / Na2O molar ratio of less than 3.1 and a layer B containing precipitated silica. Patent Document 1 also describes that the fire spread prevention material is used in a battery pack having two or more cells, and therefore suppresses heat transfer between cells under normal conditions and suppresses the spread of heat to adjacent cells under abnormal conditions. Patent Document 2 also describes a partition member that includes a liquid, a heat insulating material, and an exterior body that houses the liquid and the heat 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-shaped member that contacts the heat insulating material, the partition member maintains its cooling function during long-term use and has excellent stability in the release temperature of the cooling liquid inside the sheet-shaped member. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2022 / 270359 [Patent Document 2] Japanese Patent Application Publication No. 2020-161290 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the technology of Patent Document 1, the water contained in layer A (for example, water molecules in sodium silicate) undergoes an endothermic reaction in the temperature range of 100 to 300°C, so the water content is limited and a sufficient endothermic effect cannot be obtained. Furthermore, the technology of Patent Document 2 merely lists various porous bodies, fibers, or particles as heat insulating materials, and does not consider at all whether the partition member itself exhibits an insulating effect and suppresses heat transfer to other cells to suppress, prevent, or delay multiple 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 when the cells themselves expand and contract during charging and discharging, or when a cell suddenly expands during thermal runaway. Therefore, the separator member is required to have mechanical properties such as a certain pressure resistance. However, because Patent Document 2 does not consider mechanical properties such as pressure resistance, problems arise, particularly when the cells expand, as the distance between the cells becomes shorter, making heat transfer between adjacent cells easier. 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. [Means for solving the problem]

[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 be transformed into a heat insulator in a high temperature range (e.g., 150°C or higher), and have completed the present invention as described below. [1] The present disclosure relates to a heat absorber having a bag that can be filled with a content, and a water-soluble inorganic powder that dissolves in an amount of 1 g or more in an aqueous solvent and 100 g of water at 20°C, the water-soluble inorganic powder being filled in the bag as the content.

[0007] [2] The heat endothermic material 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], wherein the contents further contain one or more selected from the group consisting of antifreeze agents and inorganic fibers.

[0009] [4] The heat absorber according to any one of [1] to [3], wherein 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.

[0010] [5] The heat absorber according to any one of [1] to [4], wherein the content is filled with an aqueous solution containing the water-based 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 content is transformed into a porous body when heated to 120°C or higher.

[0012] [7] Formula (I): [Number 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: "50kW / m by 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 reached a predetermined temperature, and then the heat absorber was allowed to dissipate heat at room temperature and naturally cooled until the temperature of the surface of the heat absorber reached room temperature, and the thickness change rate (%) before and after heating was calculated. Note that the heat absorber was pressed against the heated surface at 0.5 MPa for 60 seconds. The heat absorber according to any one of [1] to [6], wherein the rate of change in thickness expressed by the following formula 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. [Effects of the Invention]

[0015] The heat absorber of the present disclosure has excellent heat absorption properties and can be transformed into a heat insulator in a high temperature range, thereby providing a heat absorber with 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 that has excellent heat insulation properties and pressure resistance by virtue of being able to change into a heat insulator in a high temperature range and having excellent heat absorption properties. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 shows an example of a secondary battery module in which the heat absorber of this embodiment can be mounted. [Figure 2]FIG. 2 is a perspective view schematically showing the secondary battery module of FIG. 1 in an exploded state. [Figure 3] FIG. 3 is a graph showing the results of a cone calorimeter test (test conditions: radiation intensity 50 kw / m 2 , heating time 20 minutes) on the heat absorber of the example and the sheet of the comparative example, where the vertical axis represents temperature and the horizontal axis represents elapsed time. [Figure 4] FIG. 4 shows a schematic diagram of the cone calorimeter test device used in the examples and comparative examples. [Figure 5] 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 Fig. 5(a) and (b) are images showing the porous body with the edge cut. DETAILED DESCRIPTION OF THE INVENTION

[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 includes a bag that can be filled with a content, and a water-soluble inorganic powder that dissolves in an amount of 1 g or more in 100 g of water at 20°C and an aqueous solvent, which is filled in the bag as the content. This provides excellent heat absorption, heat insulation and pressure resistance, and can change into a heat insulator in a high temperature range. The heat can be absorbed by the latent heat of vaporization of the aqueous solvent or the water-soluble inorganic powder within the pouch. 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. When exposed to high heat, such as from combustion, the aqueous solvent evaporates, but the presence of the water-soluble inorganic powder provides thermal insulation and fire protection. More specifically, the pouch primarily functions as a heat absorber in relatively low temperatures (e.g., above room temperature to around 100°C). Meanwhile, in temperatures ranging from the critical temperature (e.g., 150°C) to the thermal runaway temperature (e.g., around 1000°C), the water-soluble inorganic powder within the heat absorber transforms into a porous material, allowing it to also function as a thermal insulator. As a result, when the heat absorber of this embodiment is placed 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 contained in the contents of this embodiment as a hydrogel composed of a hydrogel body and an aqueous solvent, if necessary. When the hydrogel body is contained as the content, cushioning properties and impact resistance can be imparted to the heat absorber. In addition, since the heat absorber of the present disclosure exhibits not only a heat absorption effect but also effects such as heat insulation, the heat absorber can be strictly referred to as a component that can absorb and insulate heat from the outside, that is, a thermal control component that controls heat from the outside.

[0021] The heat absorber of this embodiment may further contain one or more materials selected from the group consisting of inorganic fibers, antifreezing agents, and additives. If inorganic fibers are further contained as the filler, the heat absorber can be further endowed with cushioning properties and pressure resistance, or the inorganic fibers act as a foam nucleating agent, making it easier for the water-soluble inorganic powder to form a foam (i.e., a porous body). Furthermore, the inorganic fibers contained as the filler retain water, making it easier to mold the heat absorber into a desired shape. 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, which changes the heat absorber into a thermal insulator, thereby enabling the heat absorber to exhibit both heat absorption and thermal insulation effects throughout its thickness. Furthermore, if an antifreeze is added as a content, it is possible to prevent freezing at sub-zero temperatures. Also, by utilizing the heat of solidification of the water-based solvent, it is possible to prevent the battery temperature from dropping in cold environments. In the case of water, heat of solidification occurs at around 0°C, but by using an antifreeze in this case, it is possible to lower the temperature at which this heat of solidification occurs, thereby preventing the battery temperature from dropping in even colder temperatures.

[0022] <Characteristics of heat absorbers> The endothermic onset temperature of the heat absorber 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 starting temperature range of the heat absorber 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 starting temperature of the heat absorber can be suitably changed. In this specification, the endothermic onset temperature (°C) is defined as the temperature at the intersection of a line extending the low-temperature 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 is maximum on the low-temperature side of the endothermic peak associated with evaporation. However, when multiple endothermic peaks are observed, the intersection of a line extending the low-temperature baseline toward the high-temperature side with a tangent drawn at the point where the gradient is maximum on the low-temperature side of the curve for each of the multiple endothermic peaks is calculated for each of the multiple endothermic peaks, and the lowest temperature among the temperatures at the intersections is defined as the endothermic onset temperature. The endothermic peak temperature of the heat absorber of this embodiment is preferably in the range of at least 80°C to 400°C, and more preferably in the range of 90°C to 160°C. The endothermic peak temperature in this specification refers to the temperature (°C) at the maximum value of the endothermic peak due to evaporation in a DSC measurement curve, which is the measurement result obtained with a differential scanning calorimeter (DSC). When multiple endothermic peaks are observed, it is sufficient that at least one of the multiple endothermic peaks is present in the range of 80°C to 160°C. The endothermic amount of the heat absorber of this embodiment is not particularly limited, but 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 still more preferably 500 J / g to 1500 J / g at the endothermic peak temperature (range of 80°C to 160°C). The preferred range of the endothermic amount can be determined by appropriately adjusting the upper and lower limits. The endothermic onset temperature, endothermic peak temperature and endothermic amount of the endothermic body 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 selected appropriately 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 adjusting the 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 body) 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 in an amount of 1 g or more in 100 g of water at 20° C. As the bag, for example, a three-sided bag having an opening at the upper end, a body portion with a closed lower end, and a structure that can be heat-sealed so that the opening is closed after the contents such as the aqueous solvent and the water-soluble inorganic powder have all been placed inside is preferred. The three-sided bag is constructed by gluing together the bottom ends and side portions of two sheets, and then sealing the bag after filling it with contents through the opening. This gives the bag excellent airtightness, and its roughly 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 involves stacking two sheets of film of a desired size and shape (e.g., rectangular or (approximately) circular) depending on the intended use, and then heat-sealing a predetermined heat-sealed area (e.g., the edge of the film) to form an opening, thereby bonding the heat-sealed area. This creates an internal space area that can be filled with contents, and a three-sided bag can be produced, having an opening so that the contents can be filled into the internal space area through the opening, and in which the heat-sealed areas of the two sheets are bonded together. After filling the bag with contents, the openings can be heat-sealed by crimping them together. In this specification, the term "sealed" refers to a state in which the inside and outside of the bag are substantially isolated from each other. The sheet used in the bag body of this embodiment is not particularly limited as long as it is water-proof, and examples thereof include known resin films, resin films having a metal layer, and films having a metal layer. The average thickness of the sheet 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, for example. 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. Furthermore, the resin film having the metal layer may be formed by laminating a metal such as aluminum or a metal oxide such as silica or alumina as a metal foil, vapor-deposited film, or the like to the resin film. 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 the selection, 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 in which aluminum foil (including an aluminum vapor-deposited layer) is integrated 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 on the heat seal area and the closed opening for the purpose of sealing. Suitable adhesive layers include laminate adhesives such as polyester adhesives, polyether adhesives, and polyurethane adhesives. The adhesive may be of any type, including solvent-based, solventless, and aqueous types.

[0025] For example, in the present invention, a bag-shaped product made from a laminate film is preferred. The preferred laminate film is a laminate of a metal foil and a resin film. Examples include a three-layer laminate film consisting of an outer resin film, a metal foil, and an inner resin film. Specifically, a bag made from a resin film having an aluminum-deposited layer on the outside is sealed with a polyurethane-based laminating 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 is sealed with a polyurethane-based laminating adhesive layer; or a laminate film having a PET layer, an aluminum layer, and a polyethylene layer is sealed with a polyurethane-based laminating adhesive layer. 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 of this embodiment or applied to the heat-sealed area, the higher the strength and the more likely it is to withstand internal pressure.

[0026] The water vapor permeability ([g / (m 2 24h)]) is 50g / (m 2 24h) or less, and 10g / (m 2 24h) or less is more preferable, and 5g / (m 2 It is more preferable that the time is 24 hours or less. The water vapor permeability of the sheet that makes up the bag is 50g / (m 2 If the storage time is within the range of 24 hours or less, the moisture inside the bag can be prevented from leaking to the outside, which is preferable from the viewpoint of preventing deterioration of heat absorption performance over time. The water vapor permeability ([g / (m 2 24h)]) is measured in accordance with JIS K7129 standards at a temperature of 40°C and a relative humidity of 90%.

[0027] (Water-soluble inorganic powder that dissolves 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 in an amount of 1 g or more in 100 g of water at 20°C. Because the water-soluble inorganic powder is hydrophilic, it is easily dissolved in aqueous solvents, making it easier for the water-soluble inorganic powder to be uniformly distributed within the content. As a result, in a temperature range 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 also effectively function as a heat insulator. Furthermore, when the water-soluble inorganic powder and the aqueous solvent are present in the heat absorber, a synergistic effect with the heat absorption of the aqueous solvent can be exhibited, allowing for continuous heat absorption at a different endothermic temperature from that of the aqueous solvent. Furthermore, even when the heat absorber is exposed to high temperatures, the water-soluble inorganic powder can become porous (see, for example, the photographs in Figures 5 and 6 described below). As a result, excellent thermal insulation and fire prevention effects are exhibited. Therefore, a heat absorber containing a water-soluble inorganic powder primarily functions as a heat absorber in a relatively low temperature range (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 a thermal runaway temperature (e.g., around 1000°C), the entire water-soluble inorganic powder becomes porous, and thus can also function as a thermal insulator. As a result, when the heat absorber of this embodiment is disposed between cells in 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, resulting in an extremely small distance between the cells, 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 a high temperature due to thermal runaway or the like, the water-soluble inorganic powder itself sinters to form a porous body that exhibits a certain strength, thereby increasing pressure resistance. This allows the distance between the cells to be kept constant, maintaining effective thermal insulation and effectively suppressing repeated explosions between the cells.

[0028] The water-soluble inorganic powder of this embodiment dissolves in an aqueous solvent. This allows the powder to be easily dissolved in an aqueous solvent, making it easier for the water-soluble inorganic powder to be uniformly distributed within the contents. In this specification, "water-soluble" refers to dissolving at least 1 g in 100 g of water at 20°C. Therefore, the water-soluble inorganic powder of this embodiment can be an inorganic powder that dissolves at least 1 g 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 in 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 the sinterability by high-temperature heating, the solubility of the water-soluble inorganic powder (in 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, still 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 adjusted appropriately by changing the upper limit value and the lower limit value. When the solubility of the water-soluble inorganic powder at 20°C is within the above range, the solubility is ensured, and the water-soluble inorganic powder is dissolved or dispersed uniformly 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 adjusted appropriately by changing the upper and lower limits mentioned above.

[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 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 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. It is preferable that the solubility of the water-soluble inorganic powder at each temperature is within the above range, from the viewpoint of exhibiting a suitable endothermic effect and pressure resistance. The solubility of the water-soluble inorganic powder in 100 g of water at 20° C. can be determined by appropriately rearranging the above upper and lower limits.

[0034] The method for measuring the 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 20°C, 40°C, 60°C, 80°C, and 100°C under 1 atmosphere at a rotation speed of 80 rpm on a mix rotor for 24 hours to prepare a mixed solution. The transmittance of the mixed solution after 24 hours of stirring is then measured under the following conditions. The transmittance is measured by changing the amount of dissolved water-soluble inorganic powder, and the upper limit (g) at which the transmittance becomes 99% is defined as the solubility of the water-soluble inorganic powder in water. <Transmittance measurement conditions> Dynamic light scattering (DLS) measurements Equipment: DLS measurement equipment DLS-8000 manufactured by Otsuka Electronics Laser wavelength, output: 488nm / 100mW Sample cell: NMR tube

[0035] The water-soluble inorganic powder of the present embodiment is preferably solid at room temperature. In addition, in the heat absorber of the present embodiment, it is preferable that an aqueous solution containing an aqueous solvent and the water-soluble inorganic powder is filled in a bag as the content of the heat absorber. Since the heat absorber is filled with an aqueous solution containing an aqueous solvent and water-soluble inorganic powder as its contents, the water-soluble inorganic powder is completely dissolved in the aqueous solvent, and therefore the water-soluble inorganic powder is uniformly present in the contents, resulting in the formation of a homogeneous porous body. The transmittance of the aqueous solution is preferably 99% or more, and 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 amount of the water-soluble inorganic powder is 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 amount of the water-soluble inorganic powder can be appropriately adjusted. When the heat absorption amount of the water-soluble inorganic powder is within the above range, the heat absorption effect is improved, and a synergistic effect with the heat absorption of the aqueous solvent is exhibited, making it easier to suppress ignition. The upper and lower limits of the content can be appropriately combined. The endothermic heat of the water-soluble inorganic powder can be measured by 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 above thermal decomposition onset temperature can be adjusted as appropriate. The thermal decomposition starting temperature can be measured by 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, particle, crystalline, and plate-like shapes. 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, and when the water-soluble inorganic powder is in powder or particulate form, the average particle size 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 size within the above range, the water-soluble inorganic powder can be easily dispersed in the system. The average particle size may be the value of the median diameter (D50) measured by a laser diffraction / scattering particle size distribution analyzer.

[0039] The water-soluble inorganic powder material of this embodiment is preferably composed of a water-soluble inorganic salt, and 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, and it is preferable that the inorganic cation is one or more ions selected from the group consisting of potassium ions, calcium ions, magnesium ions, and aluminum ions. The organic or inorganic anion is preferably one or more selected from oxygen ions, sulfate ions, halogen ions (chlorine ions, fluorine ions, bromine ions, etc.), nitrate ions, carbonate ions, acetate ions, and phosphate ions. The water-soluble inorganic powder of the present 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, thereby exhibiting 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] The water-soluble inorganic powder or the water-soluble inorganic salt of this embodiment is preferably selected from the following: 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 acid 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 one of the above examples, or a combination of two or more thereof.

[0041] The content of the water-soluble inorganic powder in this embodiment can be 1 to 90 mass % relative to the total amount (100 mass %) of the contents of the heat absorber, or may be 3 to 80 mass %, preferably 5 to 60 mass %, more preferably 10 to 50 mass %, more preferably 10 to 35 mass %, and even more preferably 15 to 30 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 that is the water-soluble inorganic powder. The upper and lower limits of the content of the water-soluble inorganic powder can be appropriately adjusted.

[0042] In this embodiment, it is preferable that the content of the heat absorber changes to a porous body when the content is heated to 120° C. or higher. The temperature at which the content changes to 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 temperatures such as during 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 to 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, so it is believed that excellent thermal insulation and fire prevention effects are exhibited. Therefore, when the heat absorber of this embodiment is placed in a secondary battery module, such as a stacked battery described below, heat transfer to other cells is suppressed, thereby suppressing, preventing, or delaying multiple explosions.

[0043] (aqueous solvent) The heat absorber of this embodiment contains an aqueous solvent as its content. This allows the heat to be absorbed 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 greater 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 during 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.

[0044] The aqueous solvent of this embodiment may contain water as the main component, and refers to water or a solvent containing water as the main 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 the main component" refers to a solvent containing 45% by mass or more of water based on the total mass of the aqueous solvent. Purified water, pure water, ultrapure water, distilled water, and the like can be used as the water, without any particular limitations. Examples of such salts include alkali metal halides such as sodium chloride or potassium chloride, alkaline earth metal halides such as magnesium chloride or calcium chloride, and salts having 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, sodium carbonate-sodium hydrogen carbonate, etc. Furthermore, Good's buffers such as HEPES or MOPS may also be used as aqueous solvents. 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 water content 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 in this embodiment is preferably 10% by mass or more and 95% by mass or less, more preferably 20% by mass or more and 90% by mass or less, even more preferably 30% by mass or more and 80% by mass or less, and particularly preferably 50% by mass or more and 70% by mass or less, relative to 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 changing the upper and lower limits. When the content of the aqueous solvent main body is within the above range, the heat absorption property, heat insulation property and pressure resistance are excellent, and the composition can be changed into a heat insulator in a high temperature range.

[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 agents, and additives.

[0047] The contents of the heat absorber of this embodiment may contain one or more kinds selected from the group consisting of inorganic fibers and antifreezing agents. (inorganic fiber) 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 the water-soluble inorganic powder acts as a foam nucleating agent to facilitate the formation of porous bodies. Furthermore, when the water-soluble inorganic powder transforms into a porous body, a porous composite containing the inorganic fibers and the water-soluble inorganic powder can be formed. This facilitates the formation of 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 and 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 is measured by changing the temperature of a test specimen from 200 to 700°C in 100°C increments and holding it at each temperature for 30 minutes, and the temperature at which the rate of change in volume (thickness direction) reaches -20%. When the inorganic fiber of this embodiment is a porous body having at least one of a specific airflow resistance, a specific porosity, a specific tortuosity, and a specific porosity, the entire heat absorber tends to form a relatively stable porous body even in a high temperature range (a temperature range from a critical temperature (e.g., 150°C) to a thermal runaway temperature (e.g., around 1000°C)), and therefore tends to function as a thermal insulator. In particular, when the inorganic fiber has a specific porosity, it can become an even better thermal insulator when combined with a water-soluble inorganic powder.

[0049] <Porosity> The average porosity of the inorganic fibers of this embodiment is preferably 30% to 99.7%, more preferably 50% to 99.5%, even more preferably 70% to 99.3%, and particularly preferably 90% to 99%. In this specification, the average porosity of 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 shown below using the following formula (1). Average porosity (%)=((1 / ρf)-(1 / ρr)) / (1 / ρf)×100...Equation (1)

[0050] <Bulk density> The bulk density ρf of the inorganic fiber of this embodiment is 0.020 g / cm 3 More than 1g / cm 3 Preferably, it is less than 0.022 g / cm 3 More than 0.5g / cm 3 or less, more preferably 0.024 g / cm 3 More than 0.1g / cm 3 Below 0.026 g / cm, particularly preferably 3 More than 0.07g / cm 3 The following is the result. 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 More than 10g / cm 3 Preferably, it is equal to or less than 1 g / cm 3 More than 7g / cm 3or less, more preferably 1.5 g / cm 3 More than 5g / cm 3 Below 2 g / cm, particularly preferably 3 More than 3g / cm 3 The following is the result. There are no particular limitations on the method for measuring the true density ρr of inorganic fibers, but it can be calculated by the sink-float method using a mixture of n-heptane, carbon tetrachloride, and ethylene dibromide. Specifically, first, an appropriate-sized inorganic fiber sample is placed in a stoppered test tube. Next, a mixed solvent of three solvents is added to the test tube, which is then immersed in a constant-temperature bath at 30°C. If the sample 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 fibers> The shape of the inorganic fiber of 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 this 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 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 for 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. Of these knitting methods, it is preferable to adopt a knitting method that provides a predetermined range of resistance to passage of a fluid passing through the communicating holes (for example, the airflow resistance described below). Various knitting machines may be used, such as warp knitting machines, flat knitting machines, circular knitting machines, or raschel knitting machines.

[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 of these. The twisting direction is also not particularly limited, and may be right twisting, left twisting, or a combination of these. The woven or knitted yarn used in this embodiment may be false twist textured yarn, filament yarn, or yarn processed using the POY-DTY method or PTY (Producers Textured Yarn) method. The conditions for the woven or knitted yarn to be used can be appropriately selected depending on the purpose of use, the type of aqueous solvent, etc. The material for the woven or knitted yarn is the material constituting the inorganic fiber described above or the inorganic material contained in the inorganic fiber.

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

[0059] Furthermore, when the inorganic fiber of this embodiment is composed of a nonwoven fabric, the average fiber length of all fibers (raw fiber) 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, because it is easy to ensure the desired porosity. The average fiber length can be measured by microscope observation or the average fiber diameter from the results of image analysis using a fiber length measuring device (e.g., KAAJAANI Fiber Lab.). When the inorganic fiber of this embodiment is formed from a cotton-like material, the average fiber length of all fibers (raw fiber) constituting the cotton-like material is preferably 0.5 μm to 50 μm, more preferably 0.8 μm to 32 μm, and more preferably 1 μm to 25 μm. It is preferable that the average fiber length of all fibers constituting the cotton-like material is within the above range, since it is easy 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., KAAJAANI Fiber Lab.). In this specification, a flocculent material is also a type of nonwoven fabric, but the flocculent material refers to a 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 in this embodiment include 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, 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 preferable range of the content of the inorganic fibers can be determined by appropriately changing the upper and lower limits. When the content of inorganic fibers is within the above range, the heat absorption capacity and pressure resistance are superior, and the heat absorption effect can be changed to a heat insulating effect in the high temperature range.

[0062] <Antifreezing agent> In this embodiment, an antifreeze agent may be added to the aqueous solvent or the contents as needed to improve the effect of suppressing a temperature drop below the freezing point. In particular, by adding an antifreeze agent to the contents of the heat absorber, high cushioning properties can be maintained over a wide temperature range. The antifreezing agent of this embodiment may be an inorganic antifreezing agent or an organic antifreezing agent, and may be in the form of a liquid, powder, solid, or the like. The inorganic antifreeze 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, organic antifreeze agents are preferably salts of organic acids or low-volatility substances (low-volatility solvents or urea), and more preferably salts of organic acids or low-volatility solvents. As the salts (including hydrates) of the organic acids, it is preferable to use salts of sodium, potassium, magnesium, ammonium, or the like of formic acid, propionic acid, succinic acid, or the like, and examples thereof include disodium succinate (including hydrates such as disodium succinate hexahydrate) and sodium propionate. Examples of the low-volatility substance include urea and low-volatility solvents (e.g., polyhydric alcohols). 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 Less than 0.1g per hour (0.1g / cm 2 1000g / cm 3 10 ... 2 ·hr·60℃·1atm), diglycerin (0.001g or less / cm 2 ·hr·60℃·1atm), ethylene glycol (0.01g or less / cm 2 hr 60℃ 1atm), propylene glycol (0.001g or less / cm 2 ·hr·60℃·1atm), polyethylene glycol (0.001g or less / cm 2 Preferred are polyhydric alcohols such as glycerin and diglycerin. These low-volatility solvents may be used alone or in combination of two or more. The heat absorber of this embodiment contains a low-volatility solvent, particularly a polyhydric alcohol, which suppresses or prevents the evaporation of the aqueous solvent, or suppresses a decrease in cushioning properties at low temperatures (improving the anti-freeze effect). When a low-volatility solvent is used as an optional component, the mass ratio of the aqueous solvent to the low-volatility solvent (aqueous solvent / low-volatility solvent) in the content of the heat absorber 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 optionally 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. While 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, of 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, aliphatic alcohols such as methanol, etc. 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 viscosity modifiers such as the thickeners include various tackifying resins such as rosin-based, polymerized rosin-based, polymerized rosin ester-based, rosin phenol-based, stabilized rosin ester-based, disproportionated rosin ester-based, terpene-based, terpene phenol-based oils, and petroleum resin-based. 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 total content of the heat absorber or the total dispersion liquid (a).

[0068] (Method of manufacturing heat absorber) A preferred example of a method for producing the heat absorber of this embodiment includes the steps of filling an aqueous solvent, a water-soluble inorganic powder, and one or more components selected from the group consisting of inorganic fibers and additives, which are blended as needed, into a bag through an opening thereof, and sealing the opening of the bag to seal the bag. The aqueous solvent, the water-soluble inorganic powder, and one or more components selected from the group consisting of inorganic fibers and additives, which are optionally blended, may be filled separately from 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 the inorganic fibers and additives, which are optionally blended, in the aqueous solvent, and then the mixed solution may be filled from 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 one or more kinds selected from the group consisting of inorganic fibers and additives, which are blended as necessary. The mixed solution (1) contains, with respect to the total amount (100 mass%) of the mixed solution (1), The composition 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 hydrates), and 0 to 10 mass% additives; the composition 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 hydrates), 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 hydrates), 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 (for example, 4 to 9 mass% rock wool with respect to the total amount of the contents), an aqueous solvent (for example, 20 to 60 mass% water with respect to the total amount of the contents), and a water-soluble inorganic powder (for example, 10 to 50 mass% magnesium sulfate with respect to the total amount of the contents). This makes it possible to provide a heat absorber with superior heat absorption and pressure resistance, and capable of changing 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 mass%, more preferably 91 to 99.5 mass%, even more preferably 94 to 99 mass%, and even more preferably 96 to 99.4 mass%, relative to the total amount (100 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 mass%, more preferably 92 to 99.5 mass%, even more preferably 93 to 99.4 mass%, and even more preferably 96 to 99.1 mass%, relative to the total amount (100 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 is preferably 83 to 100 mass%, more preferably 94 to 99.8 mass%, even more preferably more than 95 mass% and not more than 99.6 mass%, and even more preferably more than 96 mass% and not more than 99.5 mass%, relative to the total amount (100 mass%) of the contents of the bag. The upper and lower limits of the total content can be appropriately changed.

[0070] (Preferred embodiment of heat absorber) The heat absorber of this embodiment preferably exhibits high cushioning properties and / or is excellent in pressure resistance during heating. Each of these preferred aspects will be described in detail below. <Highly cushioned heat absorber> A preferred heat absorber of this embodiment has a pouch and a content filled in the pouch, 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 has a pouch and the content filled in the pouch contains an aqueous solvent (preferably water), a water-soluble inorganic powder (preferably magnesium sulfate), and inorganic fibers (preferably ceramic wool), exhibiting high cushioning properties. In this specification, a heat absorber that exhibits high cushioning properties is also referred to as a high cushioning heat absorber. The expression "exhibiting high cushioning properties" means that the cushioning properties are excellent, and when the contents of the bag contain inorganic fibers, there is a tendency for the bag to exhibit high cushioning properties. Furthermore, "exhibiting high cushioning properties" specifically means that the cushioning property (%) represented by the following formula (I) is preferably 90% or more, more preferably 93% or more. By exhibiting the above-mentioned cushioning property 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 property can be 100%. [Number 2] Cushioning (%) = h a / h b ×100 Formula (I) (In the above formula (I), h a is the height (mm) of the pressed part after 5 minutes have passed since the surface of the high-cushion heat absorber was pressed under 1 MPa for 60 seconds and then released. b indicates the height (mm) before the surface of the high-cushion heat absorber is pressed at 1 MPa for 60 seconds.) In the heat absorber of this embodiment, when cushioning property is important, it is preferable that not only is the cushioning property (%) represented by formula (I) 90% or more, but also that the content of the antifreeze agent be controlled to a predetermined value or less. That is, when cushioning property 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, still more preferably 6% by mass or less, and particularly preferably substantially none (0.5% by mass or less) relative to the total amount of the contents. Furthermore, in the case where cushioning properties are important in the heat absorber of this embodiment, the lower limit of the total amount of the aqueous solvent, inorganic fibers, water-soluble inorganic powder, and antifreeze agent contained in the contents may be preferably more than 30% by mass, more preferably 50% by mass or more, even more preferably 70% by mass or more, still more preferably 80% by mass or more, even more preferably 90% by mass or more, still 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 fiber, water-soluble inorganic powder, and antifreeze agent contained in the contents may be 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 when the total amount of 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 absorber> A preferred heat absorber of this embodiment is a heat absorber having excellent pressure resistance when heated, which has 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 absorber 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, which has excellent pressure resistance when heated. In this specification, a heat absorber having excellent pressure resistance during heating is also referred to as a high-pressure-resistant heat absorber. Such a high-pressure-resistant heat absorber tends to exhibit excellent pressure resistance when heated to high temperatures (e.g., 800°C or higher) due to thermal runaway of a battery or the like. The expression "exhibiting pressure resistance when heated" means that the heat absorber exhibits high pressure resistance when heated. Therefore, a preferred aspect of the high-pressure-resistant heat absorber of this embodiment may be a high-pressure-resistant heat absorber having a bag that can be filled with a content, an aqueous solvent, a water-soluble inorganic powder, and inorganic fibers, and the inorganic fibers and the water-soluble inorganic powder are filled as a composite into the content. In this case, since the water-soluble inorganic powder has 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 powders temporarily forms a foam film during thermal runaway, and this foam film acts as a dispersion medium for reinforcement. Furthermore, the coexistence of inorganic fibers and water-soluble inorganic powders allows the inorganic fibers to support the water-soluble inorganic powders and promotes the dispersibility of the water-soluble inorganic powders, which is thought to result in a synergistic effect of high pressure resistance. More specifically, when the high-pressure-resistant heat absorber of this embodiment is heated, the aqueous solvent (water component) within the heat absorber exhibits a heat-absorbing effect and evaporates while forming a foamed film. As the aqueous solvent decreases, voids form between the inorganic fibers, improving the heat insulating performance. Furthermore, during the evaporation of the aqueous solvent, the water-soluble inorganic powder precipitates and is supported by the inorganic fibers. If the water-soluble inorganic powder is a hydrate, the water component contained in the hydrate also evaporates while exhibiting a heat-absorbing effect as heating progresses. Since the water-soluble inorganic powder can be uniformly dispersed or dissolved in the aqueous solvent, uneven distribution and support of the water-soluble inorganic powder on the inorganic fibers can be prevented. 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 of thermal insulation and high pressure resistance. Note that this uniform dispersion may mean, for example, that when the composite, which is the content, is removed, the difference in concentration (% by 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 when heated" specifically means that after the heat absorber is heated until the surface opposite to 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. By exhibiting the above-mentioned thickness change rate of 70% or more, pressure resistance is excellent, and consecutive explosions between cells can be effectively suppressed and prevented. The upper limit of the thickness change rate can be 100%. [Number 3] Formula (II): "Thickness change rate (%) = (thickness of the heat absorber after 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 pressing the surface of the high pressure resistant heat absorber after heating at 0.5 MPa for 60 seconds) × 100" In the above formula (II), "the thickness of the heat absorber after applying pressure at 0.5 MPa to the surface of the high pressure resistant heat absorber after heating" refers to the average thickness (arithmetic mean of thicknesses at any five positions) of the heat absorber after applying pressure at 0.5 MPa for 60 seconds to one surface of the high pressure resistant heat absorber after heating to a predetermined temperature or higher, preferably one surface of the high pressure resistant heat absorber after heating under the heating conditions described below (within approximately 20 minutes from immediately after applying pressure). Similarly, in the above formula (II), "the thickness of the heat absorber before applying pressure to 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 one surface of the high pressure resistant heat absorber to a predetermined temperature or higher, preferably after heating under the heating conditions described below, and before applying pressure to the surface of the high pressure resistant heat absorber at 0.5 MPa for 60 seconds. Heating conditions: "50kW / m by radiant heat 2 The heat absorber was heated with a heat quantity of 1000 kJ / cm2 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 (22 to 28°C) and naturally cooled until the temperature of the surface of the heat absorber reached room temperature (22 to 28°C), and the thickness change rate (%) before and after heating was calculated. The heat absorber was pressed against the heating surface at 0.5 MPa for 60 seconds. In the heating conditions, the "predetermined temperature" refers to the temperature at which the heat absorber becomes porous, and can be set depending on the environment in which the heat absorber is used, 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 the thickness change rate (%) of a heat absorber heated until the temperature of the surface (back surface) opposite to the heated surface (surface directly exposed to radiant heat) of the heat absorber reaches 150°C 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 or higher. When high pressure resistance is important, the heat absorber of this embodiment preferably contains inorganic fibers, an aqueous solvent, and water-soluble inorganic powder as its components.

[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 moving object such as a vehicle or an aircraft (particularly a drone), and has a plurality of battery cells and a case for housing the plurality of battery cells. The battery cells (also referred to as battery cells) constituting the secondary battery module can be battery cells in which, for example, a battery exterior film is used as an exterior material and a battery element 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 is 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. Note 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 / discharge reactions substantially 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 this order. As a result, the adjacent positive electrode, electrolyte layer 14, and negative electrode 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. Also provided is an activated carbon layer 19 that adsorbs 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. 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 conduct the positive and negative electrodes, and are structured to be sandwiched between the ends of the battery outer casing materials 18a and 18b and led out of the battery outer casing materials 18a and 18b. The positive electrode terminal 13 and the negative electrode terminal 15 can 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 films, and the sealant layers formed on the surfaces of the battery exterior materials 18a and 18b are usually heat-sealed together. The battery exterior materials 18a and 18b also have regions around their peripheries where the sealant layers are in close contact with each other by heat sealing.

[0074] Next, a secondary battery module equipped with the heat absorber of this embodiment will be described with reference to FIG. 2. FIG. 2 is a perspective view schematically illustrating an exploded 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 18a, b (e.g., aluminum laminate exterior). The heat absorber 1 of this embodiment is arranged so as to be in contact with the negative electrode current collector 17. The heat absorber 1 may be arranged so as to be in contact not only with the negative electrode current collector 17 but also with the positive electrode current collector 12. Therefore, a secondary battery module having a heat absorber 1 mounted on a battery element 10 has one or more laminates in which a positive electrode formed on a positive electrode current collector 12 (aluminum foil or the like) having a positive electrode terminal 13, a separator 14 containing an electrolyte, and a negative electrode placed on a negative electrode current collector 17 (metal foil or the like) having a negative electrode terminal 15 are sequentially stacked, and the one or more heat absorbers 1 can be arranged so as to abut against the positive electrode current collector 12 and / or the negative electrode current collector 17 but not against the separator 14. When a solid electrolyte or a gel electrolyte is used as the electrolyte, the separator 14 can be replaced by the electrolyte interposed between the electrodes. On the other hand, the aqueous solvent, which is the content of the heat absorber of this embodiment, does not come into direct contact with the battery element 10. Therefore, a suitable heat absorber for a secondary battery of 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 content of the bag does not include the aqueous solvent and the battery element 10 so as to bring them into direct contact with each other, and more preferably, the content of the bag of the heat absorber for a secondary battery does not include the battery element 10.

[0075] The secondary battery module of the present disclosure may have the heat absorber 1 of this embodiment sandwiched between adjacent battery elements 10 (also referred to as battery cells) housed in a plurality of cases (not shown) or a plurality of battery exterior films 18 a, b. The cases can be made of, for example, aluminum, iron, or a metal material containing these, or a resin material such as polyphenylene sulfide, and using a resin material can contribute to reducing the weight of the secondary battery module. The heat absorber 1 can be sandwiched between the plurality of battery elements 10 by using, for example, an adhesive, fusion (ultrasonic fusion, high frequency fusion, heat fusion), pressure sensitive adhesive, or the like. With this configuration, the heat absorber 1 sandwiched between the battery elements 10 absorbs heat generated during charging, etc., thereby suppressing sudden temperature increases in the battery elements 10 and preventing deterioration and fire of the battery elements 10. The heat absorber 1 sandwiched between the battery elements 10 is thought to suppress temperature effects between the battery elements 10 through its insulating properties and also act as a buffer against volume changes due to expansion of the battery elements 10, thereby mitigating increases in internal pressure of the secondary battery module. On the other hand, when thermal runaway occurs due to excessive heat generation in the battery elements 10, the water-soluble inorganic powder in the heat absorber 1 sinters and transforms into a hard, plate-like porous material, thereby increasing pressure resistance and decreasing thermal conductivity. As a result, the heat absorber 1 maintains a constant distance between the cells, maintaining effective insulating properties and controlling thermal conductivity, effectively suppressing repeated explosions of the battery elements 10.

[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. [Example]

[0077] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to the examples given 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, 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 temperature at the intersection of a straight line extending the low-temperature baseline of the DSC measurement curve toward the high-temperature side 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 was defined as the endothermic onset temperature (°C), and the point where the difference from the baseline of the DSC measurement curve is maximum was defined as the endothermic peak temperature (°C). The integral value 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 defined as the endothermic amount (J / g or mJ / mg).

[0078] (2) Evaluation of cushioning 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 dimensions 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 under 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 (%) = h a / h b ×100 (Cushioning evaluation criteria) "Returning to more than 90% of its original height" was marked as "◎". "Returning to 80% or more of its original height" was marked as "Good." "Returning to 70% or more of its original height" was marked as "△". "Returning to less than 70% of the original height or being unable to measure" was marked as "X".

[0079] (3) Heating experiment using a cone calorimeter The heat absorbers produced in the present examples 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 the JIS A 1316 standard. 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 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 the pressure resistance of the test specimen after the heating experiment The heat absorbers prepared in the present examples and comparative examples were subjected to a heating experiment using the cone calorimeter described above, and then their pressure resistance was evaluated using the following method. Specifically, an indentation test was performed at room temperature (23°C) in a Tensilon universal testing machine ("RTE-1210" manufactured by Orientec Co., Ltd.) equipped with a 7 mm diameter indentation jig. The indentation test measured the indentation depth when pressed at 0.5 MPa for 60 seconds (i.e., when pressurized). The thickness change rate when pressed was calculated from the indentation depth according to the following formula (II) and evaluated according to the following criteria. Note that the indentation test was performed at two locations on the surface of the heat absorber after the heating experiment using the cone calorimeter described above. The height (or thickness) was measured at each location, and the thickness change rate was calculated using the following formula (II). The average value is 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 marked as "Excellent." A thickness change rate in the range of 40 to 69% was rated as "good." The thickness change rate was marked "x" when it was in the range of 0 to 39% or was not measurable. The most excellent pressure resistance is achieved when the thickness change rate is in the range of 70 to 100%. The lower the amount of compression (= amount of crushing), the better the 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 surface of the heat absorber was allowed to cool naturally until it reached 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) Measurement methods for average porosity, true density, and bulk density The average porosity of the 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...Equation (1) <True density measurement> 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. The dried inorganic fibers were then placed in a stoppered test tube, and a mixed solvent of three solvents was added to the stoppered test tube, which was then immersed in a thermostatic bath at 30°C. If the inorganic fibers floated, n-heptane, which has a low density, was added. On the other hand, if the inorganic fibers sank, ethylene dibromide, which has a high density, was added. This procedure was repeated until the inorganic fibers were floating 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. Then, 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 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 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 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, Grade 1," manufactured by Kanto Chemical Co., Ltd., solubility in 100g of water at 20°C (g / 100g): 0.2g) The solubility of the above hydrates in the anhydrous form is as follows: Solubility of anhydrous magnesium sulfate in 100g of water at 20°C (g / 100g): 30g Solubility of calcium sulfate anhydrous in 100g of water at 20°C (g / 100g): 0.2g In the heat absorber pouch, magnesium sulfate heptahydrate exists as an anhydrous form, and calcium sulfate dihydrate exists as a hydrate. <Bag body> The water vapor permeability ([g / (m 2 24h)]) is 50g / (m 2-24 hours or less.

[0084] (3) Examples and Comparative Examples Example 1 A water-soluble inorganic powder-containing aqueous solution (1) was prepared by mixing 32 parts by mass of magnesium sulfate heptahydrate (the amount including the water in the hydrate) and 60 parts by mass of pure water. 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 then inserted into a 116 mm long x 116 mm wide aluminum pouch (Mitsubishi Gas Chemical Company, Inc., "Gas Barrier Bag," 0.094 mm thick, composed of a laminate of PET, aluminum foil, and polyethylene). Next, the aluminum pouch bag was filled with the water-soluble inorganic powder-containing aqueous solution (1), the inlet was closed by heat sealing, and the aluminum pouch bag was placed flat between 4.8 mm thick gap materials, a flat plate was placed on top, and the mixture was 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) prepared in Example 1 was heated to a back surface temperature of 200°C using a cone calorimeter, it was confirmed that the heat absorber (1) prepared in Example 1 changed to a porous body at temperatures above 150°C. Figure 5(a) shows a cross-sectional photograph of the heat absorber (1) that had changed to 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 placed in 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 (2), the inlet was closed by heat sealing, and the aluminum pouch bag was placed flat between 4.8 mm thick gap materials, a flat plate was placed on top, and the mixture was left to stand at 20°C for 10 minutes to produce a 4.8 mm thick sheet-like heat absorber (2). The obtained heat absorber (2) 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) 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 to a porous body at temperatures above 180°C. Figure 5(b) shows a cross-sectional photograph of the heat absorber (2) that had changed to a porous body.

[0086] Example 3 A water-soluble inorganic powder-containing aqueous solution (3) was prepared by mixing 35 parts by mass of magnesium sulfate heptahydrate (amount including water in the hydrate) and 65 parts by mass of pure water. 100 parts by mass of the water-soluble inorganic powder-containing aqueous solution (3) was then placed in 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., 0.094 mm thick, laminated with PET, aluminum foil, and polyethylene). Next, the aluminum pouch bag was filled with the water-soluble inorganic powder-containing aqueous solution (3), the inlet was closed by heat sealing, and the aluminum pouch bag was placed flat between 4.8 mm thick gap materials, a flat plate was placed on top, and the mixture was left 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. Furthermore, 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 (amount including 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). 100 parts by mass of the water-soluble inorganic powder-containing aqueous solution (4) was then placed in 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., 0.094 mm thick, laminated with PET, aluminum foil, and polyethylene). Next, the water-soluble inorganic powder-containing aqueous solution (4) was filled into the aluminum pouch bag, the inlet was closed by heat sealing, and the aluminum pouch bag was placed flat between 4.8 mm thick gap materials. A flat plate was placed on top of the aluminum pouch bag and allowed to stand at 20°C for 10 minutes to produce a 4.8 mm thick sheet-like 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 for endotherm amount, meltdown temperature, endotherm onset temperature, endotherm peak temperature, pressure resistance, and cushioning property 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: 0.012-0.018 W / m K," was used as a 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 evaluated according to the procedures described in the evaluation section above. The results are shown in Table 1 and Figure 3. The material of Comparative Example 1, "Xiaomei silica aerogel mat material," does not substantially have heat absorption capacity, and therefore, in the table, "Heat absorption onset temperature (°C), endothermic peak temperature (°C), and endothermic amount (J / g or mJ / mg)" are marked with "substantially no heat absorption capacity." Furthermore, although the bulk density of the material used in Comparative Example 1 could be measured, the true density could not be measured, and therefore, "Cannot be measured" is marked in the table for true density and average porosity.

[0089] <Comparative Example 2> An aluminum pouch (Mitsubishi Gas Chemical Company, Inc., "Gas Barrier Bag" 0.094 mm thick, composed of a laminate of PET, aluminum foil, and polyethylene) was prepared as a bag measuring 116 mm long and 116 mm wide. Next, 33 parts by weight of pure water and 67 parts by weight 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. The aluminum pouch was then placed flat between 4.8 mm thick gap fillers, a flat plate was placed on top, and the bag was left to stand at 20°C for 10 minutes to produce a 4.8 mm thick sheet for comparison (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. In addition, since DSC of the liquid could not be measured in the system of Comparative Example 2, data on the endothermic heat (J / g or mJ / mg) could not be obtained. Therefore, the value of 1170 J / g calculated based on 2000 J / g (100°C), an approximation of the literature value of 2257 J / g, is entered in the table. (Specifically, the approximate heat absorption of water is 2000 J / g x 33% + the heat absorption of calcium sulfate (measured value) is 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 made of an aluminum pouch ("Gas Barrier Bag" manufactured by Mitsubishi Gas Chemical Company, Inc., thickness 0.094 mm, composed of a laminate of PET, aluminum foil, and polyethylene) in the shape of a bag measuring 116 mm in length and 116 mm in width. Next, the aluminum pouch was 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 it and left to stand at 20°C for 10 minutes to produce a 4.8 mm thick comparative sheet (C3). 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 Figure 3.

[0091] <Comparative Example 4> Dispersion (1) was prepared by mixing 27 parts by mass of aluminum hydroxide (solubility 0.0001 g in 100 g of water at 20°C) and 64 parts by mass of pure water. 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 placed in 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 dispersion liquid (1), the inlet was closed by heat sealing, and the aluminum pouch bag was placed flat between 4.8 mm thick gap materials, a flat plate was placed on top, and the mixture was left at 20 ° C for 10 minutes to produce a 4.8 mm thick sheet-like comparative sheet (C4). The obtained comparative sheet (C4) was then evaluated according to the procedures described in the evaluation column above. The results are shown in Table 1 and Figure 3.

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

[0093] [Table 1]

[0094] [Table 2]

[0095] From the above experimental results, it was confirmed that the heat absorber of this embodiment, because the entire contents and pouch act as a porous body, maintains sufficient heat absorption effect, pressure resistance, and changes from a heat absorber to a heat insulator even when the battery temperature rises or over time. The heat absorbers using water-insoluble inorganic powder (Comparative Examples 2 and 4) were particularly inferior in pressure resistance compared to heat absorbers containing water-soluble inorganic powder. Note that the content values ​​of each content composition in Table 1 are rounded to the nearest whole number. 3 shows a graph of the results of a cone calorimeter test on the heat absorbers (1) to (3) produced in Examples 1 to 4 and the comparative sheets (C1), (C3), and (C4) produced in Comparative Examples 1, 3, and 4. The vertical axis of FIG. 3 represents temperature (°C), and the horizontal axis represents elapsed time (seconds). Therefore, (1) to (3) and (C1), (C3), and (C4) in FIG. 3 correspond to the heat absorbers (1) to (3) and comparative sheets (C1), (C3), and (C4) of the Examples and Comparative Examples. From the experimental results in FIG. 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 the time it takes for the temperature of the heat absorber to reach 200°C is delayed.

[0096] [Explanation of symbols] 1. Heat absorber 10 Battery element 11 Cathode material layer 12 Positive electrode current collector 13 Positive terminal 14 Electrolyte layer or separator 15 Negative 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 heat absorber having a bag that can be filled with a content and the content that is filled in the bag, the contents are composed only of an aqueous solution containing an aqueous solvent and a water-soluble inorganic powder that dissolves in 100 g of water at 20°C in an amount of 5 g or more, or are composed only of one or more components selected from the group consisting of inorganic fibers, antifreeze agents, and additives, and the aqueous solution; The inorganic fiber is a fiber aggregate in which fibers made of an inorganic material are entangled with each other or a porous body made of an inorganic material, The additive is one or more selected from the group consisting of an ultraviolet absorber, an antioxidant, an organic solvent, an inorganic filler other than a water-swellable clay mineral, a viscosity modifier, a crosslinking agent, and a flame retardant, and the inorganic filler other than a water-swellable clay mineral is fused silica, crystalline silica, alumina, silicon nitride, or aluminum hydroxide.

2. The heat absorber according to claim 1, wherein the content further contains one or more kinds selected from the group consisting of an antifreeze agent and inorganic fibers.

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. 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% with respect to the total amount (100 mass%) of the contents of the bag. The heat absorber according to claim 1 or 2.

5. 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% by mass with respect to the total amount (100% by mass) of the contents of the bag. The heat absorber according to claim 1 or 2.

6. The heat absorber according to claim 1 or 2, wherein the endothermic onset temperature ranges from 35°C to 400°C.

7. The heat absorber according to claim 1 or 2, wherein the heat absorption amount 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 transformed into a porous body when heated to 120°C or higher.

10. 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) × 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 temperature of the surface 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 against the surface at 0.5 MPa for 60 seconds. The heat absorber according to claim 1 or 2, wherein the thickness change rate expressed by the following formula is 70% or more.

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

12. The secondary battery module according to claim 1 or 2, wherein the heat absorber is sandwiched between battery cells.

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