Heat-absorbing body and secondary battery module comprising said heat-absorbing body

Abstract

The purpose of the present disclosure is to provide: a heat-absorbing body that has exceptional pressure resistance, heat absorption, and temperature-increase-suppressing effects, and that can change from having a heat absorption effect to having a heat insulation effect in a high temperature region; and a secondary battery module comprising said heat-absorbing body.　The present disclosure is a heat-absorbing body having: a bag body that can be filled with contents; and a hydrogel and an inorganic powder that are filled into the bag body as the contents.

Description

Heat absorber and secondary battery module equipped with the heat absorber

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

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

[0003] For example, Patent Documents 1 and 2 disclose techniques that are excellent in heat insulation and fire spread prevention. ２ / Na ２Patent Document 1 describes a laminated fire spread prevention material comprising a layer A containing sodium silicate with an O molar ratio of less than 3.1 and a layer B containing precipitated silica. Patent Document 1 describes that the fire spread prevention material is used in a battery pack having two or more cells, suppressing heat transfer between cells under normal conditions and suppressing heat spread to adjacent cells under abnormal conditions. Patent Document 2 describes an insulating sheet for a battery pack, which has an insulating layer composed of at least inorganic fibers or inorganic powder, and heat absorption layers composed of at least inorganic hydrate formed on both sides of the insulating layer. Patent Document 2 describes that when the inorganic hydrate in the outer heat absorption layer is heated by heat generated in the battery cell, the inorganic hydrate exerts an endothermic effect by absorbing the heat and releasing moisture, thereby effectively reducing the heat generation of the battery cell.

[0004] International Publication No. 2022 / 270359 Japanese Patent Application Laid-Open No. 2019-83150

[0005] However, in the technology of Patent Document 1, the water content is limited because the heat absorption occurs due to an endothermic reaction of water contained in Layer A (e.g., water molecules in sodium silicate) in the temperature range of 100 to 300°C, and therefore a sufficient heat absorption effect cannot be achieved. Furthermore, in the technology of Patent Document 2, the heat absorption layer is made of an inorganic hydrate, and therefore the heat absorption amount is approximately 1000 J / g or less. The content of the inorganic hydrate is limited due to constraints on the thickness or flexibility of the sheet, and therefore a sufficient heat absorption effect cannot be achieved. Furthermore, the technologies of Patent Documents 1 and 2 use a laminate having two or more layers, and therefore a thick laminate is required to achieve sufficient heat absorption and thermal insulation. Furthermore, when the battery temperature rises and separator meltdown begins, the battery cells expand. Therefore, cushioning or mechanical strength for the expanded battery cells is required. However, the technologies of Patent Documents 1 and 2 do not consider the mechanical properties of the laminate, such as shock absorption. Therefore, an object of the present disclosure is to provide a heat absorber that has excellent pressure resistance, heat absorption capacity, and temperature rise suppression effect, and that can change from a heat absorption effect to an insulating effect in a high temperature range, and a secondary battery module equipped with the heat absorber.

[0006] The present inventors have discovered that a heat absorber containing a hydrogel and inorganic powder within a bag has excellent cushioning properties, heat absorption capacity, and temperature rise suppression effect, and can change from an endothermic effect to an insulating effect in the high temperature range, and have completed the present invention as described below.

[0007] [1] The present disclosure provides a heat absorber having a bag that can be filled with a content, and a hydrogel and an inorganic powder that are filled into the bag as the content.

[0008] [2] The heat absorber according to [1] above, wherein the hydrogel is composed of a hydrogel body and an aqueous solvent.

[0009] [3] The heat absorber according to the above [1] or [2], further containing an antifreeze agent as the content.

[0010] [4] The water vapor permeability ([g / (m ２ ・24h)]) is 50g / (m ２ - The heat absorber according to any one of the above [1] to [3], wherein the heat absorption time is 24 h or less.

[0011] [5] The heat absorber according to [2] above, wherein the hydrogel body is made from at least a water-soluble organic monomer and a water-swellable clay mineral as reaction raw materials.

[0012] [6] A heat absorber exhibiting high cushioning properties, comprising a bag body, a hydrogel body formed from three-dimensional polymer chains and an aqueous solvent, and inorganic powder, contained as contents of the bag body.

[0013] [7] The following formula (I): [Equation 1] "Cushioning (%) = h ａ / h ｂ × 100 (in the above formula (I), h ａ is the height (mm) of the pressed part 5 minutes after pressing the surface of the heat absorber at 1 MPa for 60 seconds, and h ｂ The heat absorber according to any one of [1] to [6], which has a cushioning property of 90% or more, as expressed by the following formula: (where "height" represents the height (mm) before the surface of the heat absorber is pressed at 1 MPa for 60 seconds).

[0014] [8] The heat absorber according to any one of [1] to [7], wherein the bag body can be filled with a content, the aqueous solvent, and the hydrogel are filled into the content, and the pressure resistance during heating is excellent.

[0015] [9] The heat absorber according to any one of [1] to [8], wherein the hydrogel and the inorganic powder are filled into the content as a composite.

[0016]

[10] The following formula (II): [Mathematical Formula 2] "Thickness change rate (%) = (thickness of the heat absorber after pressing the surface of the heat absorber heated under the heating conditions below at 0.5 MPa for 60 seconds) / (thickness of the heat absorber before pressing the surface of the heat absorber heated under the heating conditions below at 0.5 MPa for 60 seconds) × 100" Heating conditions: "50 kW / m by radiant heat ２ The heat absorber according to any one of [1] to [9], wherein the thickness change rate (%) expressed as "" is 70% or more, and the heat absorber is 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 reaches a predetermined temperature, and then the heat absorber is allowed to dissipate heat at room temperature and naturally cooled until the temperature of the surface of the heat absorber reaches room temperature. The thickness change rate (%) before and after heating is calculated. The heated surface of the heat absorber is pressed at 0.5 MPa for 60 seconds.

[0017]

[11] A heat absorption starting temperature of 400°C or less and an endothermic peak temperature in the range of at least 80°C to 400°C, and is expressed by the following formula (II): [Mathematical Expression 3] "Thickness change rate (%) = (thickness of the heat absorber after pressing the surface of the heat absorber heated under the heating conditions below at 0.5 MPa for 60 seconds) / (thickness of the heat absorber before pressing the surface of the heat absorber heated under the heating conditions below at 0.5 MPa for 60 seconds) × 100" Heating conditions: "50 kW / m by radiant heat" 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 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. A heat absorber whose thickness change rate (%) represented by the following formula is 70% or more.

[0018]

[12] A secondary battery module comprising the heat absorber according to any one of [1] to

[11] .

[0019]

[13] A secondary battery module in which the heat absorber according to any one of [1] to

[11] is sandwiched between battery cells.

[0020]

[14] Any of the heat absorbers [8] to

[10] is a high pressure resistant heat absorber.

[0021] According to the heat absorber of the present disclosure, the pressure resistance, the heat absorption amount, and the temperature rise suppression effect are excellent, and the effect can change from the heat absorption effect to the heat insulating effect in a high temperature range. According to the present disclosure, by including a heat absorber that is excellent in the pressure resistance, the heat absorption amount, and the temperature rise suppression effect, and the effect can change from the heat absorption effect to the heat insulating effect in a high temperature range, it is possible to provide a highly safe secondary battery module.

[0022] FIG. 1 shows an example of a secondary battery module on which the heat absorber of this embodiment can be mounted. FIG. 2 is a perspective view showing the disassembled state of the secondary battery module of FIG. 1 . FIG. 3 shows a schematic diagram of the cone calorimeter test device used in the examples and comparative examples. FIG. 4 shows photographs of the heat absorber of Example 1 after the cone calorimeter test. FIG. 4(a) shows a photographic image (top) of the heat absorber of Example 1 after the cone calorimeter test, FIG. 4(b) shows a photographic image (perspective) of the heat absorber of Example 1 after the cone calorimeter test, and FIG. 4(c) shows an enlarged photographic image of (b). Cone calorimeter tests (test conditions: radiation intensity 50 kW / m) of the heat absorbers of the examples and the sheets of the comparative examples were carried out. ２ 1 is a graph showing the results of a test using a 100% ethanol-based sintered body (heating time: 20 minutes), where the vertical axis represents temperature and the horizontal axis represents elapsed time.

[0023] Hereinafter, embodiments of the present invention (hereinafter referred to as "present embodiments") will be described in detail. However, the present disclosure is not limited to the following description and can be practiced in various modifications within the scope of its gist. [Definitions] In this specification, "reaction raw materials" refers to compounds used to obtain a target compound through a chemical reaction such as synthesis or decomposition and that partially constitute the chemical structure of the target compound. Substances that serve as chemical reaction aids, such as solvents, catalysts, and polymerization initiators, are excluded. In this specification, "structural unit" refers to a (repeating) unit of a chemical structure formed during a reaction or polymerization. In other words, in a product compound formed by a reaction or polymerization, a partial structure other than the structure of the chemical bonds involved in the reaction or polymerization, i.e., a residue. In this specification, a "hydrogel" refers to a three-dimensional network of polymers containing an aqueous solvent such as water, and examples of such materials include jelly, diaper absorbents, konjac, and agar. The three-dimensional network polymer that forms the skeleton of the hydrogel is referred to as the hydrogel body, and the hydrogel body contains an aqueous solvent. Thus, the hydrogel has a hydrogel body and an aqueous solvent.

[0024] [Heat Absorber] The heat absorber of the present disclosure comprises a pouch that can be filled with contents, a hydrogel, and an inorganic powder. The hydrogel and the inorganic powder are filled into the pouch as the contents. The hydrogel contains a hydrogel body and an aqueous solvent. This provides excellent cushioning, heat absorption, and temperature rise suppression, and can change from an endothermic effect to an insulating effect at high temperatures. The hydrogel itself within the heat absorber exhibits a swollen state at room temperature (approximately 20°C to 27°C), including a hydrogel body composed of a three-dimensional network polymer that forms the matrix of the hydrogel and an aqueous solvent incorporated into the hydrogel body. Therefore, the temperature can be stabilized even at room temperature because the water-based solvent absorbs sensible heat. The heat absorber has excellent cushioning and shock absorption properties. When heat is applied from the outside to this state, the latent heat of vaporization of the water-based solvent incorporated into the hydrogel produces an endothermic effect. Furthermore, if the inorganic powder contained in the heat absorber has a heat absorption effect, the latent heat of vaporization of the aqueous solvent and the inorganic powder can provide a higher heat absorption effect (e.g., heat absorption effect in different temperature ranges). Next, when the heat absorber is exposed to high heat, such as by combustion, the so-called gel water in the hydrogel evaporates, and the contents of the bag form an inorganic foam, thereby exhibiting thermal insulation and fire protection effects. More specifically, the entire heat absorber can become porous due to air bubbles trapped in the hydrogel or pores in the hydrogel created by the evaporation of the aqueous solvent. As a result, it is believed that thermal insulation and fire protection effects are exhibited. Furthermore, when the heat absorber is heated to a higher temperature, an inorganic porous body can be formed by sintering the inorganic powder. Therefore, the inorganic porous body can exhibit fire resistance and thermal insulation effects as an inorganic insulating material.

[0025] In other words, the heat absorber of the present disclosure functions primarily as a heat absorber with excellent cushioning and shock absorption properties in a relatively low temperature range (e.g., from room temperature to around 100°C). On the other hand, at temperatures from around 100°C to the critical temperature (e.g., 150°C), the gel water evaporates, turning the entire heat absorber into a porous body, and the hydrogel body, which is the substantial content, can impart thermal insulation, cushioning, and shock resistance to the heat absorber. Furthermore, in a temperature range from the critical temperature (e.g., 150°C) to a thermal runaway temperature (e.g., around 1000°C), the gel water evaporates, turning the entire heat absorber into a porous body, so that the heat absorber also functions as a thermal insulator. In addition, the inorganic powder inside can be sintered using the porous body as a scaffold to form a sintered inorganic porous body, which is thought to further improve the strength (compression resistance and fire resistance) of the heat absorber as a whole. As a result, for example, when the heat absorber of this embodiment is disposed between cells of a battery stack (battery module) in which multiple battery cells (hereinafter sometimes referred to as battery cells or simply cells) are stacked, the heat absorber can exhibit both heat absorption and heat insulation effects throughout its entire thickness, and can become an inorganic porous body with excellent mechanical strength, thereby exhibiting an extremely excellent effect in blocking or suppressing thermal effects on adjacent cells. Furthermore, in a temperature range from the critical temperature (e.g., 150°C) to the thermal runaway temperature (e.g., around 1000°C), the heat absorber changes from a heat absorber to a heat insulator, thereby exhibiting both heat absorption and heat insulation effects throughout its entire thickness. This also results in excellent space-saving properties that allow for effective use of space.

[0026] Inclusion of a hydrogel body as the content of the heat absorber of the present disclosure can impart cushioning and impact resistance to the heat absorber. Furthermore, inclusion of an inorganic powder as the content can continuously absorb heat at a different endothermic temperature from that of the aqueous solvent. Furthermore, when the aqueous solvent evaporates, voids are created within the hydrogel, and the composite containing the hydrogel and inorganic powder is more likely to form a porous body. This changes the hydrogel from a heat absorber to an insulator, thereby enhancing both the endothermic and insulating effects throughout the entire thickness of the heat absorber. Furthermore, inclusion of an antifreeze as an optional component in the content can suppress freezing below freezing points. Furthermore, by utilizing the heat of solidification of the aqueous solvent, a decrease in battery temperature in cold environments can be suppressed. While water generates heat of solidification around 0°C, the use of an antifreeze can lower the temperature at which this heat of solidification occurs, thereby suppressing a decrease in battery temperature at lower temperatures. Furthermore, inclusion of a hydrogel body as the content provides superior shape retention when placed upright compared to a content that does not contain a hydrogel body, and also maintains the dispersion state of the inorganic powder. The heat absorber of the present disclosure not only exhibits excellent shape retention when placed vertically between cells, but also exhibits excellent shape retention when placed on, for example, the top plate of a housing (case) that houses battery cells. Specifically, when placed on the top plate of a housing that houses battery cells, uneven distribution of the hydrogel and dispersed inorganic powder within the bag due to its own weight can be suppressed, allowing the shape to be maintained. Furthermore, if the bag is damaged by heat or the like, the shape retention of the hydrogel makes it difficult for the hydrogel to leak from the damaged area of ​​the bag, so even if the bag is damaged, the functions of the hydrogel and inorganic powder are not easily lost. Note that the heat absorber of the present disclosure not only exhibits heat absorption effects but also thermal insulation effects, and therefore, strictly speaking, the heat absorber can be called a component that can absorb and insulate heat from the outside, i.e., a thermal control component that controls heat from the outside.

[0027] In addition to the hydrogel and inorganic powder, the heat absorber of this embodiment may optionally contain one or more components selected from the group consisting of inorganic fibers, antifreeze agents, and additives. Inclusion of inorganic fibers as a component facilitates the formation of a porous body using the inorganic powder as a foam nucleating agent. Furthermore, when the inorganic powder transforms into a porous body, a porous composite containing the inorganic fibers and the inorganic powder can be formed. This allows the inorganic powder to function as a composite material above the thermal runaway temperature, making it easier to form an insulating wall with the desired mechanical strength. Inclusion of an antifreeze as a component can suppress freezing below freezing. In this case, the heat of solidification of the aqueous solvent can be utilized to suppress a decrease in the temperature of the battery in a cold environment. While water generates heat of solidification at around 0°C, the use of an antifreeze can lower the temperature at which the heat of solidification of the antifreeze generates. Therefore, when the heat absorber of this embodiment is placed between the cells of a battery stack (battery module) containing multiple stacked cells, a decrease in the temperature of the battery stack (battery module) can be suppressed at lower temperatures.

[0028] <Characteristics of Heat Endogenous Material> The endothermic onset temperature of the heat endogenous material of this embodiment is preferably 400°C or lower, more preferably 160°C or lower, even more preferably 120°C or lower, more preferably 110°C or lower, and even more preferably 100°C or lower. The endothermic onset temperature range of the heat endogenous material of this embodiment is preferably 35°C or higher and 400°C or lower, more preferably 37°C or higher and 160°C or lower, and even more preferably 40°C or higher and 110°C or lower. The upper and lower limits of the endothermic onset temperature of the endogenous material can be adjusted as appropriate. In this specification, the endothermic onset temperature (°C) is defined as the temperature at the intersection of a straight line extending the low-temperature side baseline toward the high-temperature side in a DSC measurement curve obtained by measurement with a differential scanning calorimeter (DSC), and a tangent drawn at the point where the gradient of the low-temperature side curve of the endothermic peak associated with evaporation is maximum. However, when multiple endothermic peaks are observed, the intersection of a straight line extending the low-temperature baseline toward the high-temperature side with a tangent drawn to the curve of each of the multiple endothermic peaks at the point where the gradient is greatest is calculated for each of the multiple endothermic peaks, and the lowest temperature among the temperatures at the intersection is defined as the endothermic onset temperature. The endothermic peak temperature of the endothermic body of this embodiment is preferably in the range of at least 80°C to 400°C, more preferably in the range of 90°C to 350°C, even more preferably in the range of 102°C to 280°C, and even more preferably in the range of 110°C to 240°C. In another aspect, the endothermic peak temperature is more preferably in the range of 90°C to 160°C. The preferred range of the endothermic peak temperature can be determined by appropriately combining the upper and lower limits. 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 obtained by measurement with a differential scanning calorimeter (DSC). When multiple endothermic peaks are observed, at least one of the multiple endothermic peaks may be present in the range of 80° C. to 400° C. The endothermic amount of the heat absorber of this embodiment is not particularly limited, but is preferably 100 J / g or more and 3000 J / g or less, more preferably 200 J / g or more and 2500 J / g or less, even more preferably 300 J / g or more and 2000 J / g or less, and still more preferably 500 J / g or more and 1500 J / g or less at the endothermic peak temperature (in the range of 80° C. to 400° C.).The range of the endothermic amount of the heat endotherm of this embodiment is preferably 100 J / g or more and 3000 J / g or less, more preferably 200 J / g or more and 2500 J / g or less, and even more preferably 300 J / g or more and 2000 J / g or less. The preferred range of the endothermic amount can be determined by appropriately combining the above upper and lower limits. The endothermic onset temperature, endothermic peak temperature, and endothermic amount of the heat endotherm of this embodiment are values ​​determined using a differential scanning calorimeter (DSC) according to the method described in the Examples below.

[0029] <Preferred Shape of Heat Absorber> The shape or size of the heat absorber of this embodiment is not particularly limited and may be, for example, substantially spherical, substantially flat, or irregular, and is appropriately selected depending on the intended use. For example, a substantially flat heat absorber is preferable because it is easy to install between adjacent battery cells. When the heat absorber of this embodiment is substantially flat, the average thickness is not particularly limited, but may be, for example, in the range of 100 μm to 50,000 μm. The average thickness is more preferably 200 μm to 20,000 μm, even more preferably 500 μm to 10,000 μm, and particularly preferably 1,000 μm to 8,000 μm. The preferred range of the average thickness can be determined by appropriately combining the upper and lower limits described above.

[0030] Below, the essential components of the heat absorber of this embodiment, namely, the bag, the hydrogel (including the aqueous solvent contained in the hydrogel), and the inorganic powder, as well as optional components such as an antifreeze agent and additives that are blended as needed, will be described.

[0031] (Bag) The bag of this embodiment is not particularly limited as long as it can be filled with contents such as hydrogel or inorganic powder. A preferred example of the bag is a three-sided bag having an opening at the top end, a body portion with a closed bottom end, and a structure in which the opening is heat-sealed to close after the contents such as hydrogel or inorganic powder are completely contained. The three-sided bag is constructed by bonding the bottom ends and side portions of two sheets together, and then sealing the bag after filling the contents through the opening. This structure provides excellent airtightness, and its substantially flat shape makes it easy to insert between battery cells. The bag of this embodiment is preferably made of a sheet. A preferred embodiment of the bag of this embodiment is one in which two films of a desired size and shape (e.g., rectangular or (substantially) circular) are stacked on top of each other, and a predetermined heat-sealed region (e.g., an edge of the film) is heat-pressed to form an opening. This allows the production of a three-sided bag having an internal space area that can be filled with contents, an opening that allows the contents to be filled into the internal space area through the opening, and heat-sealed areas of the two sheets that are bonded together. After the contents are filled, the contents can be sealed by crimping the openings together and heat-sealing them. In this specification, "sealed" refers to a state in which the inside and outside of the bag are substantially isolated from each other.

[0032] The sheet used in the bag of this embodiment is not particularly limited as long as it exhibits water-blocking properties, and examples thereof include known resin films, resin films with a metal layer, and films with 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. 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 the bag as films, sheets, tubes, and the like. Furthermore, the resin film with a metal layer may be laminated with a metal such as aluminum, or a metal oxide such as silica or alumina, as a metal foil or vapor-deposited film. The use of a resin film with 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 the lamination method include dry lamination, extrusion lamination, thermal lamination, coextrusion, multilayer blow molding, laminate injection molding, coating, etc. 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 deposition layer) is integrated with a thermoplastic resin film (e.g., a polyethylene film, a PP film, or a PET film) laminated on at least one side thereof).

[0033] In this embodiment, an adhesive layer may be formed in the heat seal area and the closed opening for sealing purposes. 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 adhesives. For example, in the present invention, a bag-shaped product made from a laminate film is preferred, and a laminate film having a void is preferably a film made by laminating a metal foil and a resin film. An example of such a laminate film is a three-layer structure consisting of an outer resin film, a metal foil, and an inner resin film. Specifically, a bag formed by sealing a resin film having an aluminum-deposited layer on the outside with a polyurethane-based laminate adhesive layer; a bag formed by sealing 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 with a polyurethane-based laminate adhesive layer; or a bag formed by sealing a laminate film having a PET layer, an aluminum layer, and a polyethylene layer with a polyurethane-based laminate adhesive layer. Examples of suitable bags 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 (e.g., 120 to 140°C) of the adhesive used to close the opening of the bag or in the heat-sealed region of the bag, the stronger the strength and the better it tends to withstand internal pressure.

[0034] The water vapor permeability ([g / (m ２ ・24h)]) is 50g / (m ２ 24h) or less, and ２ 24h) or less, and more preferably 5g / (m ２ It is more preferable that the water vapor permeability of the sheet constituting the bag is 50 g / (m ２ When the water vapor permeability ([g / (m２ The temperature and humidity are measured in accordance with JIS K7129 at a temperature of 40°C and a relative humidity of 90%.

[0035] (Hydrogel) In the heat absorber of this embodiment, a hydrogel composed of a hydrogel main body and an aqueous solvent can be filled into the pouch. Furthermore, the hydrogel of this embodiment preferably includes an aqueous solvent and a three-dimensional polymer network structure (=hydrogel main body) that retains the aqueous solvent. The inclusion of a hydrogel main body as a content can impart pressure resistance and impact resistance to the heat absorber. Even if the pouch is ruptured, the aqueous solvent can be retained as a hydrogel, making it less likely for the aqueous solvent to leak to the outside. As a result, when the heat absorber is installed inside a battery, the interior is less likely to be wetted with the aqueous solvent. Furthermore, the heat absorber can be easily molded into a desired shape. Therefore, when a heat absorber containing a hydrogel as a content is placed vertically, it exhibits excellent shape retention when placed vertically, or the dispersion state of the inorganic powder in the content can be maintained. The hydrogel is not particularly limited and any known hydrogel can be used, but more preferably comprises an aqueous solvent and a hydrogel body that is a three-dimensional network structure of a polymer containing a water-swellable clay mineral and a water-soluble organic monomer structural unit, at least a portion of which is dissolved or dispersed in the aqueous solvent. In other words, the hydrogel of this embodiment is preferably, for example, a hydrogel that retains an aqueous solvent within a three-dimensional network structure (three-dimensional network) primarily composed of a polymer synthesized from a water-soluble organic monomer, etc. Even more preferably, the hydrogel of this embodiment is a hydrogel that retains an aqueous solvent within a three-dimensional network structure (three-dimensional network) primarily composed of a polymer synthesized from a water-soluble organic monomer in the presence of a water-swellable clay mineral. When the content of the aqueous solvent in the hydrogel of this embodiment is in the range of 5 to 99% by mass relative to the total hydrogel, it can exhibit a heat-absorbing effect sufficient to effectively prevent fires and damage due to thermal runaway in secondary batteries, making it suitable as a heat absorber. The content of the aqueous solvent in the hydrogel is preferably 5% by mass or more and 99% by mass or less, more preferably 10% by mass or more and 90% by mass or less, and even more preferably 20% by mass or more and 80% by mass or less, based on the total mass of the gel. The preferred range of the aqueous solvent content can be determined by appropriately adjusting the upper and lower limits.

[0036] Specific examples of such hydrogels include organic-inorganic composite hydrogels (NC gels), interpenetrating network hydrogels (DN gels), slide-ring gels (SR gels), and hydrogels known as aquamaterials. Among these, organic-inorganic composite hydrogels (also referred to as organic-inorganic hydrogels or NC gels (nanocomposite gels)) are preferred because of their excellent heat-absorbing properties due to the aqueous solvent contained therein, as well as their excellent cushioning and creep resistance.

[0037] The content of the hydrogel contained as the content of the heat absorber of this embodiment is preferably 10% by mass or more and 95% by mass or less, preferably 20% by mass or more and 90% by mass or less, preferably 30% by mass or more and 80% by mass or less, and preferably 40% by mass or more and 70% by mass or less, based on the total amount of the content of the heat absorber. The preferred range of the hydrogel content can be determined by appropriately combining the upper and lower limits. A hydrogel content within the above range can provide a heat absorber with excellent cushioning properties. As a result, the inclusion of the heat absorber exhibits its heat absorption properties, thereby suppressing the temperature impact on other battery cells and providing a highly safe secondary battery module. Furthermore, the heat absorber of the present disclosure has sufficient strength to be used as a standalone heat absorber. If necessary, an additive may be further filled into the pouch. The hydrogel of this embodiment preferably comprises an aqueous solvent and a three-dimensional polymer network structure that holds the aqueous solvent therein, and comprises an aqueous solvent and a hydrogel body that is a three-dimensional polymer network structure that includes a water-swellable clay mineral and a water-soluble organic monomer structural unit, at least a portion of which is dissolved or dispersed in the aqueous solvent.

[0038] Preferred hydrogels of this embodiment include interpenetrating network gels in which two types of acrylic polymers each form a three-dimensional network structure, slide-ring gels in which cyclodextrin forms the backbone of the three-dimensional network structure, and aquamaterials in which a hyperbranched dendrimer forms the main backbone of the three-dimensional network structure and a water-swellable clay mineral is added. However, this disclosure will be described below based on an organic-inorganic composite hydrogel, which is one aspect of this embodiment. The organic-inorganic composite hydrogel of this embodiment has a three-dimensional network structure containing water-soluble organic monomer structural units and a water-swellable clay mineral as the hydrogel body. More specifically, the hydrogel body of the organic-inorganic composite hydrogel (hereinafter also referred to as the organic-inorganic composite hydrogel body) is considered to be a polymer gel (i.e., a three-dimensional network structure) in which polymer chains composed of multiple water-soluble organic monomer structural units are crosslinked via water-swellable clay minerals that function as binding points. The organic-inorganic composite hydrogel can swell by incorporating an aqueous solvent such as water into the three-dimensional network structure of the polymer gel. As a result, the swollen organic-inorganic composite hydrogel not only has heat absorption properties and exhibits excellent cushioning properties that allow it to adapt to relatively short-term deformations such as expansion and contraction due to charging and discharging of the battery cell, but also exhibits excellent creep resistance that can alleviate internal pressure due to expansion of the battery cell over time. The organic-inorganic composite hydrogel preferably has a three-dimensional network structure and is made from a water-soluble organic monomer and a water-swellable clay mineral as reaction raw materials.

[0039] The organic-inorganic composite hydrogel according to one aspect of this embodiment preferably uses at least a water-soluble organic monomer structural unit and a water-swellable clay mineral as reaction raw materials. A preferred method for producing the organic-inorganic composite hydrogel according to one aspect of this embodiment involves polymerizing the water-soluble organic monomer in a dispersion (a) containing a water-soluble organic monomer, a water-swellable clay mineral, an aqueous solvent, and, if necessary, a polymerization initiator and additives, since this method allows for the easy production of an organic-inorganic composite hydrogel having a three-dimensional network structure. The resulting polymer of the water-soluble organic monomer forms a three-dimensional network structure together with the water-swellable clay mineral, becoming a component of the organic-inorganic composite hydrogel (the hydrogel main body).

[0040] The content of the hydrogel body according to this embodiment is preferably 1% by mass or more and 95% by mass or less, more preferably 10% by mass or more and 90% by mass or less, and even more preferably 20% by mass or more and 80% by mass or less, based on the total amount of the hydrogel. The upper limit of the content of the hydrogel body is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less. For example, when the hydrogel body is composed of the water-soluble organic monomer structural units described below, the content of the hydrogel body can be the total amount of the water-soluble organic monomer structural units and the water-swellable clay mineral. The preferred range of the content of the hydrogel body can be determined by appropriately combining the above upper and lower limits. The content of the hydrogel body is calculated by the mass change (%) before and after drying the hydrogel at 120°C for 2 hours.

[0041] <<Water-Soluble Organic Monomer>> The water-soluble organic monomer used in this embodiment constitutes the organic-inorganic hybrid hydrogel body as the water-soluble organic monomer structural unit. Furthermore, the content of the water-soluble organic monomer structural unit in the hydrogel according to this embodiment may be preferably 0.9% by mass or more and 50% by mass or less, more preferably 1% by mass or more and 40% by mass or less, and even more preferably 5% by mass or more and 30% by mass or less, relative to the total amount of the hydrogel. The preferred range of the content of the water-soluble organic monomer structural unit can be determined by appropriately adjusting the above-mentioned upper and lower limits. The type of water-soluble organic monomer used in this embodiment is not particularly limited, and examples thereof include a monomer having a (meth)acrylamide group, a monomer having a (meth)acryloyloxy group, and an acrylic monomer having a hydroxyl group. In this specification, "(meth)acrylamide" refers to either or both of acrylamide and methacrylamide, "(meth)acryloyloxy" refers to either or both of acryloyloxy and (meth)acryloyloxy, "(meth)acrylate" refers to either or both of acrylate and methacrylate, and "(meth)acrylic monomer" refers to either or both of acrylic monomer and methacrylic monomer.

[0042] Examples of the monomer having a (meth)acrylamide group include acrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N-methylacrylamide, N-ethylacrylamide, N-isopropylacrylamide, N-cyclopropylacrylamide, N,N-dimethylaminopropylacrylamide, N,N-diethylaminopropylacrylamide, acryloylmorpholine, methacrylamide, N,N-dimethylmethacrylamide, N,N-diethylmethacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, N-isopropylmethacrylamide, N-cyclopropylmethacrylamide, N,N-dimethylaminopropylmethacrylamide, and N,N-diethylaminopropylmethacrylamide. Preferred examples of the monomer having a (meth)acrylamide group include acrylamide, N,N-dimethylacrylamide, N-methylacrylamide, N-ethylacrylamide, acryloylmorpholine, methacrylamide, N,N-dimethylmethacrylamide, N-methylmethacrylamide, and N-ethylmethacrylamide.

[0043] Examples of the monomer having a (meth)acryloyloxy group include methoxyethyl acrylate, ethoxyethyl acrylate, methoxyethyl methacrylate, ethoxyethyl methacrylate, methoxymethyl acrylate, and ethoxymethyl acrylate.

[0044] Examples of the acrylic monomer having a hydroxyl group include hydroxyethyl acrylate, hydroxyethyl methacrylate, etc. As the water-soluble organic monomer used in this embodiment, the acrylic monomer having a hydroxyl group may be used alone or in combination of two or more thereof.

[0045] In this embodiment, when a homopolymer is obtained from a water-soluble organic monomer, the resulting homopolymer preferably has a lower critical solution temperature (°C) of 50°C or higher, more preferably 60°C or higher. It is preferable that the homopolymer does not have a lower critical solution temperature (°C), i.e., that the lower critical solution temperature (°C) cannot be observed in an aqueous solvent. The upper limit of the lower critical solution temperature (°C) of the homopolymer may be, for example, 100°C. This prevents the polymer chains of water-soluble organic monomer structural units constituting the organic-inorganic composite hydrogel from undergoing a phase transition at temperatures below 50°C, thereby achieving both superior cushioning properties and heat absorption capacity even at high temperatures. More specifically, the organic-inorganic composite hydrogel according to one aspect of this embodiment has, as a constituent component, a polymer chain composed of water-soluble organic monomer structural units obtained by polymerization or reaction of a water-soluble organic monomer. Therefore, when the temperature reaches or exceeds the lower critical solution temperature (°C) of the polymer chain composed of the water-soluble organic monomer structural units, the polymer chains undergo a phase transition due to desolvation, resulting in separation into a phase containing the polymer chains and a phase containing the aqueous solvent. As a result, the three-dimensional network structure of the organic-inorganic composite hydrogel encapsulating the aqueous solvent cannot be maintained, making it difficult to exhibit cushioning properties. Therefore, the lower critical solution temperature (°C) of the organic-inorganic composite hydrogel in an aqueous solvent is preferably 50°C or higher, more preferably 60°C or higher, and preferably does not have a lower critical solution temperature (°C), i.e., it is not possible to observe a lower critical solution temperature (°C) in an aqueous solvent. The upper limit of the lower critical solution temperature (°C) of the organic-inorganic composite hydrogel can be, for example, 100°C.

[0046] From the viewpoint of suppressing the phase transition of the organic-inorganic composite hydrogel, the preferred water-soluble organic monomer used in this embodiment is a water-soluble organic monomer having a homopolymer repeating unit that does not have a lower critical solution temperature in an aqueous solvent. Specifically, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, acryloylmorpholine, methoxyethyl acrylate, ethoxyethyl acrylate, methoxyethyl methacrylate, ethoxyethyl methacrylate, methoxymethyl acrylate, ethoxymethyl acrylate, hydroxyethyl acrylate, or hydroxyethyl methacrylate is preferred. When the organic-inorganic composite hydrogel is heated to or above the lower critical solution temperature (°C) of the organic-inorganic composite hydrogel or the polymer chains constituting the organic-inorganic composite hydrogel, the organic-inorganic composite hydrogel undergoes a phase transition and separates into a phase in which polymer chains containing water-soluble organic monomer structural units constituting the organic-inorganic composite hydrogel are aggregated and a phase of the aqueous solvent contained within the organic-inorganic composite hydrogel. Therefore, it is particularly preferred that the polymer chains constituting the organic-inorganic composite hydrogel do not have a lower critical solution temperature in an aqueous solvent. Therefore, a water-soluble organic monomer capable of forming a highly hydrophilic polymer chain is preferred. Furthermore, when the polymer chain constituting the organic-inorganic composite hydrogel is composed of two or more water-soluble organic monomers, from the viewpoint of reducing the decrease in cushioning property or liquid leakage due to phase transition, the lower critical solution temperature (°C) of the copolymer composed of the two or more water-soluble organic monomers is preferably 50°C or higher, more preferably 60°C or higher, and preferably does not have a lower critical solution temperature (°C), i.e., the lower critical solution temperature (°C) cannot be observed in an aqueous solvent. The upper limit of the lower critical solution temperature (°C) of the copolymer composed of the two or more water-soluble organic monomers can be, for example, 100°C.

[0047] Among the above water-soluble organic monomers, from the viewpoints of solubility and cushioning properties of the resulting organic-inorganic hybrid hydrogel, it is preferable to use a monomer having a (meth)acrylamide group, it is more preferable to use acrylamide, N,N-dimethylacrylamide, or acryloylmorpholine, it is even more preferable to use N,N-dimethylacrylamide or acryloylmorpholine, and from the viewpoint of ease of polymerization, N,N-dimethylacrylamide is particularly preferable. The above water-soluble organic monomers may be used alone or in combination of two or more.

[0048] <<Water-Swellable Clay Mineral>> The water-swellable clay mineral used in this embodiment forms a three-dimensional network structure together with the polymer chain having the water-soluble organic monomer structural unit (polymer having the water-soluble organic monomer structural unit), and serves as a component of the organic-inorganic composite hydrogel and the organic-inorganic composite hydrogel body. The water-swellable clay mineral used in this embodiment is not particularly limited, and examples thereof include water-swellable smectite and water-swellable mica. Examples of the water-swellable smectite include water-swellable hectorite, water-swellable montmorillonite, and water-swellable saponite. Examples of the water-swellable mica include water-swellable synthetic mica. Among these, from the viewpoint of dispersibility, it is preferable to use water-swellable hectorite and water-swellable montmorillonite, and it is more preferable to use water-swellable hectorite.

[0049] The water-swellable clay mineral used in this embodiment may be naturally occurring, synthetic, or surface-modified. Examples of surface-modified water-swellable clay minerals include phosphonic acid-modified hectorite and fluorine-modified hectorite. From the viewpoint of the heat absorption and cushioning properties of the resulting organic-inorganic composite hydrogel, it is preferable to use phosphonic acid-modified hectorite. The above-mentioned water-swellable clay minerals may be used alone or in combination of two or more.

[0050] The phosphonic acid-modified hectorite forms a three-dimensional network structure with the polymer of the water-soluble organic monomer, becoming a component of the organic-inorganic composite hydrogel. Examples of the phosphonic acid-modified hectorite include pyrophosphate-modified hectorite, etidronic acid-modified hectorite, alendronic acid-modified hectorite, methylenediphosphonic acid-modified hectorite, and phytic acid-modified hectorite. These phosphonic acid-modified hectorites may be used alone or in combination of two or more. The content of the water-swellable clay mineral of this embodiment is preferably 0.1% by mass or more to 50% by mass or less, more preferably 1% by mass or more to 30% by mass or less, even more preferably 2% by mass or more to 20% by mass or less, and particularly preferably 3% by mass or more to 10% by mass or less, based on the total amount of the polymer derived from the water-soluble organic monomer, the water-swellable clay mineral, the polymerization initiator, and the aqueous solvent contained in the organic-inorganic composite hydrogel. The upper and lower limits of the content of the water-swellable clay mineral in the organic-inorganic composite hydrogel can be adjusted as appropriate.

[0051] <Reaction Raw Materials for Hydrogel> In one aspect of this embodiment, the heat absorber preferably includes a bag, and as the contents of the bag, an aqueous solvent and an organic-inorganic composite hydrogel. The organic-inorganic composite hydrogel contains at least a water-swellable clay mineral and a water-soluble organic monomer as reaction raw materials. In this case, the organic-inorganic composite hydrogel may be filled into the bag as a gel, and the bag may then be sealed. Alternatively, the water-soluble organic monomer and the water-swellable clay mineral, which are reaction raw materials for the organic-inorganic composite hydrogel, may be filled into the bag together with the aqueous solvent (optionally, an organic solvent and a polymerization initiator may also be added), and the water-soluble organic monomer and the water-swellable clay mineral inside the bag may then be gelled while the bag is sealed.

[0052] A preferred method for producing an organic-inorganic composite hydrogel according to one aspect of this embodiment involves polymerizing the water-soluble organic monomer in a dispersion (a) containing the reaction raw materials (water-soluble organic monomer, water-swellable clay mineral, polymerization initiator, and aqueous solvent) because this method allows for easy production of the organic-inorganic composite hydrogel. More specifically, a preferred method involves filling a bag with the dispersion (a) (containing the reaction raw materials (water-soluble organic monomer, water-swellable clay mineral, polymerization initiator, and aqueous solvent) together with an inorganic powder, and then polymerizing the water-soluble organic monomer under predetermined polymerization conditions while the bag is sealed. The content of the water-soluble organic monomer in the dispersion (a) is, for example, in the range of 0.9 to 50% by mass, based on the total amount (mass) of the water-soluble organic monomer, water-swellable clay mineral, and aqueous solvent. The content of the water-soluble organic monomer is preferably 0.9% by mass or more and 50% by mass or less, more preferably 1% by mass or more and 40% by mass or less, and even more preferably 5% by mass or more and 30% by mass or less. The upper and lower limits of the content can be appropriately combined. A content of the water-soluble organic monomer of 0.9% by mass or more is preferred because a hydrogel with excellent mechanical properties can be obtained. On the other hand, a content of the water-soluble organic monomer of 50% by mass or less is preferred because the dispersion can be easily prepared. The content of the water-swellable clay mineral in the dispersion (a) is, for example, in the range of 0.1 to 50% by mass, based on the total amount (mass) of the water-soluble organic monomer, the water-swellable clay mineral, and the aqueous solvent. The lower limit of the content of the water-swellable clay mineral is preferably 0.1% by mass or more, more preferably 1% by mass or more, and even more preferably 3% by mass or more. Furthermore, the upper limit of the content of the water-swellable clay mineral is preferably 50% by mass or less, more preferably 30% by mass or less, and even more preferably 10% by mass or less. The upper and lower limits of the content can be appropriately combined. A content of the water-swellable clay mineral of 0.1% by mass or more is preferred because the mechanical properties of the resulting hydrogel are further improved. On the other hand, if the content of the water-swellable clay mineral is 50% by mass or less, an increase in the viscosity of the dispersion liquid (a) can be further suppressed, which is preferable.The use of phosphonic acid-modified hectorite as the water-swellable clay mineral in the dispersion (a) provides superior storage stability, but other water-swellable clay minerals may also be included as long as storage stability is not impaired. The content of the aqueous solvent in the dispersion (a) is, for example, in the range of 50 to 99% by mass, and more preferably 60% to 90% by mass, based on the total amount (mass) of the water-soluble organic monomer, the water-swellable clay mineral, and the aqueous solvent. The upper and lower limits of the above content can be appropriately combined. A content of the aqueous solvent of 90% by mass or less is preferred because a hydrogel with excellent mechanical properties can be obtained. On the other hand, a content of the aqueous solvent of 60% by mass or more is preferred because it facilitates the preparation of a dispersion (a) in which each component is uniformly dispersed. The upper and lower limits of the above content can be appropriately combined. In addition to the reaction raw materials, namely, the water-soluble organic monomer and the water-swellable clay mineral, the polymerization initiator, and the aqueous solvent, the dispersion liquid (a) may contain one or more selected from the group consisting of low-volatility solvents and additives described below.

[0053] The dispersion liquid (a) of this embodiment preferably contains a polymerization initiator. Examples of the polymerization initiator include, but are not limited to, water-soluble peroxides and water-soluble azo compounds. Examples of the water-soluble peroxides include potassium peroxodisulfate, ammonium peroxodisulfate, sodium peroxodisulfate, and t-butyl hydroperoxide. Examples of the water-soluble azo compounds include 2,2'-azobis(2-methylpropionamidine) dihydrochloride and 4,4'-azobis(4-cyanovaleric acid). Among these, in terms of interaction with the water-swellable clay mineral, it is preferable to use water-soluble peroxides, and it is more preferable to use potassium peroxodisulfate, ammonium peroxodisulfate, or sodium peroxodisulfate, with sodium peroxodisulfate and ammonium peroxodisulfate being even more preferable. The above-mentioned polymerization initiators may be used alone or in combination of two or more.

[0054] In one aspect of the present embodiment, the polymerization initiator is not essential. However, when used, the molar ratio of the polymerization initiator to the water-soluble organic monomer in the dispersion liquid (a) (polymerization initiator / water-soluble organic monomer) is preferably 0.01 or more, more preferably 0.02 to 0.1, and even more preferably 0.04 to 0.1.

[0055] The content of the polymerization initiator in the dispersion (a) is not essential, but when used, it is preferably 0.1 to 10 mass% and more preferably 0.2 to 5 mass% relative to the total amount (mass) of the water-soluble organic monomer, the water-swellable clay mineral, the aqueous solvent, and the polymerization initiator. A polymerization initiator content of 0.1 mass% or more is preferred because it enables polymerization of the water-soluble organic monomer even in an air atmosphere. On the other hand, a polymerization initiator content of 10 mass% or less is preferred because it allows the dispersion to be used without agglomeration before polymerization, improving handleability.

[0056] The dispersion liquid (a) contains a water-soluble organic monomer, a water-swellable clay mineral, and an aqueous solvent, and may further contain an organic solvent, a catalyst, an organic crosslinking agent, a preservative, a thickener, etc., as necessary. Examples of the organic solvent include alcohol compounds such as methanol, ethanol, propanol, isopropyl alcohol, and 1-butanol; ether compounds such as ethyl ether and ethylene glycol monoethyl ether; amide compounds such as dimethylformamide and N-methylpyrrolidone; and ketone compounds such as acetone and methyl ethyl ketone. Among these, from the viewpoint of the dispersibility of the water-swellable clay mineral, it is preferable to use an alcohol compound, and it is more preferable to use methanol, ethanol, n-propyl alcohol, and isopropyl alcohol, and it is even more preferable to use methanol and ethanol. These organic solvents may be used alone or in combination of two or more.

[0057] The catalyst has the function of increasing the polymerization rate when polymerizing a water-soluble organic monomer. Examples of the catalyst include, but are not limited to, tertiary amine compounds, thiosulfates, and ascorbic acids. Examples of the tertiary amine compounds include N,N,N',N'-tetramethylethylenediamine and 3-dimethylaminopropionitrile. Examples of the thiosulfates include sodium thiosulfate and ammonium thiosulfate. Examples of the ascorbic acids include L-ascorbic acid and sodium L-ascorbate. Among these, from the viewpoint of dispersion stability, it is preferable to use a tertiary amine compound, and it is more preferable to use N,N,N',N'-tetramethylethylenediamine. The above-mentioned catalysts may be used alone or in combination of two or more. Although a catalyst is not essential, when a catalyst is used, the content of the catalyst in the dispersion (a) is preferably 0.01 to 1 mass %, more preferably 0.05 to 0.5 mass %, relative to the total mass of the water-soluble organic monomer, the water-swellable clay mineral, the aqueous solvent, and the catalyst. A catalyst content of 0.01 mass % or more is preferred because it can efficiently promote the synthesis of a hydrogel obtained from the water-soluble organic monomer. On the other hand, a catalyst content of 1 mass % or less is preferred because it allows the dispersion to be used without flocculation before polymerization, improving handleability.

[0058] Examples of methods for preparing the dispersion liquid (a) include a method in which a water-soluble organic monomer, a water-swellable clay mineral, a polymerization initiator, an aqueous solvent such as water, and the like are mixed all at once; and a multi-liquid mixing method in which a dispersion liquid (a1) containing a water-soluble organic monomer and a solution (a2) containing a polymerization initiator are prepared as separate dispersion liquids or solutions and then mixed immediately before use. However, from the viewpoints of dispersibility, storage stability, viscosity control, and the like, the multi-liquid mixing method is preferred.

[0059] The dispersion (a1) containing the water-soluble organic monomer may be, for example, a dispersion in which a water-soluble organic monomer and a water-swellable clay mineral are mixed.

[0060] The solution (a2) containing the polymerization initiator may be, for example, an aqueous solution obtained by mixing a polymerization initiator with water.

[0061] The organic-inorganic composite hydrogel can be obtained by polymerizing a water-soluble organic monomer in the dispersion liquid (a), and the polymerization method is not particularly limited and can be performed by a known method. Specific examples include radical polymerization by heating or ultraviolet irradiation, and radical polymerization using a redox reaction.

[0062] The polymerization temperature for polymerizing the organic-inorganic composite hydrogel is preferably 10 to 80°C, more preferably 20 to 80°C. A polymerization temperature of 10°C or higher is preferred because the radical reaction can proceed in a chain reaction. On the other hand, a polymerization temperature of 80°C or lower is preferred because the water contained in the dispersion (a) can be polymerized without boiling. The polymerization time for the organic-inorganic composite hydrogel varies depending on the type of polymerization initiator and catalyst, but is typically between several tens of seconds and 24 hours. In particular, in the case of radical polymerization using heat or redox, a polymerization time of 1 to 24 hours is preferred, more preferably 5 to 24 hours. A polymerization time of 1 hour or more is preferred because the polymer of the water-swellable clay mineral and the water-soluble organic monomer can form a three-dimensional network structure. On the other hand, since the polymerization reaction is nearly complete within 24 hours, a polymerization time of 24 hours or less is preferred.

[0063] The heat absorber of this embodiment contains a hydrogel body and an aqueous solvent as its contents. When exposed to high temperatures such as combustion, the aqueous solvent evaporates, creating pores within the hydrogel, which can provide thermal insulation and fire protection.

[0064] <Aqueous Solvent> The heat absorber of this embodiment contains an aqueous solvent along with the hydrogel body. This allows heat absorption by the latent heat of vaporization of the aqueous solvent, particularly water, within the pouch. This allows for utilization of the latent heat of vaporization of water, which has a greater heat absorption capacity than typical hydrates. Furthermore, since the heat is absorbed as sensible heat of the aqueous solvent, temperature can be stabilized even at room temperature. The heat absorber contains a hydrogel (including at least the hydrogel body and the aqueous solvent). This allows for utilization of the latent heat of vaporization of the aqueous solvent, particularly water, within the hydrogel. This allows for utilization of the latent heat of vaporization of water, which has a greater heat absorption capacity than typical hydrates. Meanwhile, when exposed to high temperatures, such as during combustion, the aqueous solvent evaporates, creating voids within the hydrogel, potentially providing thermal insulation and fire protection. It is preferable that the aqueous solvent be present primarily within the hydrogel, e.g., an organic-inorganic composite hydrogel.

[0065] 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, or 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% or more by mass of water relative to the total amount of the aqueous solvent. Furthermore, purified water, pure water, ultrapure water, distilled water, and the like can be used as the water without any particular limitation. Examples of the salt include alkali metal halides such as sodium chloride or potassium chloride; alkaline earth metal halides such as magnesium chloride or calcium chloride; and salts with buffering capacity such as tris-hydrochloric acid, glycine hydrochloride, citric acid-sodium citrate, acetic acid-sodium acetate, citric acid-disodium hydrogen phosphate, sodium dihydrogen phosphate-disodium hydrogen phosphate, glycine-sodium hydroxide, and sodium carbonate-sodium bicarbonate. Furthermore, Good's buffers such as HEPES or MOPS may also be used as the aqueous solvent. Examples of the solvent other than water that constitutes the mixed solvent include organic solvents that are uniformly miscible with water (for example, lower alcohols, lower ketones, etc.), and low-volatility solvents that are used as antifreeze agents.

[0066] 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 of this embodiment can be set to 1% by mass to 94% by mass, more preferably 10% by mass to 90% by mass, even more preferably 20% by mass to 80% by mass, and particularly preferably 30% by mass to 70% by mass, based on the total amount of the contents of the heat endotherm (100% by mass). The preferred range of the aqueous solvent content can be determined by appropriately combining the above upper and lower limits. When the content of the aqueous solvent is within the above range, the resulting composition exhibits superior heat absorption and temperature rise suppression effects, and can transition from an endothermic effect to an insulating effect in the high-temperature range.

[0067] <Antifreezing Agent> In this embodiment, an antifreezing agent may be added to the aqueous solvent as needed, since this can improve the effect of suppressing a temperature drop below freezing. In particular, adding an antifreezing agent to the contents of the heat absorber can maintain high cushioning properties over a wide temperature range. The antifreezing agent of this embodiment may be an inorganic or organic antifreezing agent. The antifreezing agent may be in the form of a liquid, powder, solid, or the like. Preferred inorganic antifreezing agents are chlorides such as sodium chloride, calcium chloride, or magnesium chloride (including hydrates such as magnesium chloride hexahydrate). On the other hand, preferred organic antifreezing agents are salts of organic acids or low-volatility substances (low-volatility solvents or urea), with organic acid salts or low-volatility solvents being more preferred. As the salts (including hydrates) of the organic acids, it is preferable to use salts of sodium, potassium, magnesium, ammonium, etc. of formic acid, acetic acid, propionic acid, succinic acid, etc., such as disodium succinate (including hydrates such as disodium succinate hexahydrate) or sodium propionate. In addition, as the low-volatility substance, urea or a low-volatility solvent (for example, a polyhydric alcohol) can be used. Examples of the low-volatility solvent include ethylene glycol, diethylene glycol, glycerin, dipropylene glycol, propylene glycol, butyrolactone, N,N-dimethylformamide, glycerol, 1,3-propanediol, glycol ether, glycol ether, glycol monoether, ethylene glycol, diethylene glycol, propylene glycol, isopropanol, propylene glycol monomethyl ether, di- or tripropylene glycol monomethyl ether, cyclohexanol, glucose, mannose, fructose, galactose, sucrose, lactose, maltose, xylose, arabinose, sorbitol, mannitol, trehalose, and raffinose.

[0068] The low-volatility solvent of this embodiment has a volatility of 1 cm in an open system at 60°C and 1 atmosphere. ２ ・0.1g or less per hour (0.1g / cm ２More preferably, an organic solvent of 0.05 g or less, and even more preferably 0.01 g or less is used. Specifically, a solvent that is easily miscible with water is preferred, and therefore glycerin (0.001 g or less / cm ２ hr 60 ° C 1 atm), diglycerin (0.001 g or less / cm ２ hr 60°C 1 atm), ethylene glycol (0.01 g or less / cm ２ hr 60 ° C 1 atm), propylene glycol (0.001 g or less / cm ２ hr 60°C 1 atm), polyethylene glycol (0.001 g or less / cm ２ Polyhydric alcohols such as ethanol (water, ethanol, ethanol 3000 kJ / hr, 60°C, 1 atm) are preferred, with glycerin and diglycerin being more preferred. These low-volatile solvents may be used alone or in combination of two or more. It is also preferred that these low-volatile solvents are uniformly contained in the organic-inorganic composite hydrogel. By including a low-volatile solvent, particularly a polyhydric alcohol, as the content of the heat endotherm of this embodiment, volatilization of the aqueous solvent is suppressed or prevented, or a decrease in cushioning properties at low temperatures is suppressed (improved anti-freeze effect). When a low-volatile solvent is used as an optional component, the mass ratio of the aqueous solvent to the low-volatile solvent (aqueous solvent / low-volatile solvent) in the content of the heat endotherm of this embodiment is preferably 95 / 5 to 30 / 70, more preferably 90 / 10 to 50 / 50, and even more preferably 85 / 15 to 65 / 35. The content of the antifreeze agent in this embodiment can be 0% by mass or more and 70% by mass or less, preferably 5% by mass or more and 60% by mass or less, more preferably 10% by mass or more and 50% by mass or less, even more preferably 15% by mass or more and 35% by mass or less, and particularly preferably 20% by mass or more and 30% by mass or less, relative to the total amount (100% by mass) of the contents of the heat absorber. The preferred range of the content of the antifreeze agent can be determined by appropriately adjusting 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.

[0069] (Inorganic Powder) The heat absorber of this embodiment essentially contains an inorganic powder as its content. When the heat absorber is exposed to high temperatures, the aqueous solvent inside evaporates and the hydrogel and inorganic powder are calcined, forming an inorganic porous body from the inorganic powder, thereby exhibiting excellent thermal insulation and fire protection. More specifically, the inorganic powder exhibits a synergistic effect with the heat absorption of the aqueous solvent, allowing for continuous heat absorption at a different endothermic temperature than the aqueous solvent. Furthermore, even when exposed to high temperatures, the inorganic powder present as a content of the heat absorber acts as air bubbles trapped in the hydrogel body, allowing the inorganic powder and hydrogel as a whole to form a porous body as a type of composite (see, for example, the photograph in Figure 4 described below). As a result, excellent thermal insulation and fire protection are exhibited. Therefore, a heat absorber containing an inorganic powder primarily functions as a heat absorber in relatively low temperature ranges (e.g., above room temperature to around 100°C). On the other hand, in the temperature range from the critical temperature (e.g., 150°C) to the thermal runaway temperature (e.g., around 1000°C), the inorganic powder and hydrogel as a whole become porous and can also function as a thermal insulator. As a result, when the heat absorber of this embodiment is placed between cells in a battery stack consisting of multiple stacked cells, it can block or suppress thermal effects on adjacent cells. Furthermore, when the inorganic powder has a heat absorption effect (e.g., when it absorbs external heat and undergoes thermal decomposition), it can exhibit a synergistic effect with the heat absorption of the aqueous solvent. Furthermore, when thermal runaway occurs, the cells expand, compressing the heat absorber itself between the cells, reducing voids and preventing effective thermal insulation performance. When the heat absorber of this embodiment contains an inorganic powder, the inorganic powder itself sinters when heated to high temperatures due to thermal runaway, etc., thereby increasing the pressure resistance of the entire heat absorber. This maintains effective thermal insulation and effectively suppresses repeated explosions between cells. The inorganic powder of this embodiment is preferably an inorganic powder having a heat absorption effect, and more preferably one or more types selected from the group consisting of heat absorption materials, porous powders, hollow particles, and other inorganic powders. The heat absorption amount (= the heat absorption amount (J / g) when heated from room temperature (23°C) to 1000°C) of the inorganic powder 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 heat absorption capacity of the inorganic powder is not particularly limited, but is preferably 4000 J / g or less. The heat absorption capacity of the inorganic powder may preferably be 100 J / g or more and 4000 J / g or less. When the heat absorption capacity of the inorganic powder is within the above range, the heat absorption effect is improved, resulting in a synergistic effect with the heat absorption of the aqueous solvent, making it easier to suppress ignition. The upper and lower limits of the content can be combined as appropriate. The upper and lower limits of the heat absorption capacity of the inorganic powder can be rearranged as appropriate. The heat absorption capacity of the inorganic powder can be measured using a differential scanning calorimeter (DSC) as described in the Examples section.

[0070] The thermal decomposition onset temperature of the 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. By setting the thermal decomposition onset temperature of the inorganic powder to the above upper limit or lower, the inorganic powder itself quickly decomposes, making it easier to suppress ignition. The above upper and lower limits of the thermal decomposition onset temperature of the inorganic powder can be appropriately adjusted. The thermal decomposition onset temperature can be measured using a differential scanning calorimeter (DSC).

[0071] The shape of the inorganic powder of this embodiment is not particularly limited, and examples thereof include powder, particle, and plate shapes. Furthermore, the inorganic powder of this embodiment is preferably a water-soluble inorganic powder with a heat-absorbing effect. Preferred embodiments of the inorganic powder include porous powders, solid particles, and hollow particles. The average particle diameter of the inorganic powder is preferably 0.1 to 200 μm, more preferably 1 to 140 μm, and even more preferably 10 to 100 μm. By setting the average particle diameter within the above range, the inorganic powder is easily dispersed within the system. The average particle diameter may be the median diameter (D50) measured using a laser diffraction / scattering particle size distribution analyzer.

[0072] The inorganic powder material of this embodiment is not particularly limited, but is preferably one or more compounds consisting of a combination of inorganic cations and organic and inorganic anions. Examples of the inorganic cations include alkali metal ions, alkaline earth metal ions, aluminum ions, zinc ions, silver ions, copper(I) ions, and copper(II) ions. One or more of the inorganic cations are preferably selected from the group consisting of potassium ions, calcium ions, magnesium ions, and aluminum ions. The organic and inorganic anions are preferably one or more of the organic and inorganic anions selected from oxygen ions, sulfate ions, halogen ions (chloride ions, fluoride ions, bromine ions, etc.), nitrate ions, carbonate ions, acetate ions, and phosphate ions.

[0073] The inorganic powder of this embodiment is preferably one or more selected from the group consisting of hydrated metal compounds, inorganic materials other than the hydrated metal compounds, hollow materials, and porous powders, and is preferably a hydrated metal compound having a thermal decomposition onset temperature of 350° C. or less and an endothermic heat of 700 J / g or more. Furthermore, the hydrated metal compound is preferably one or more selected from the group consisting of aluminum hydroxide, magnesium hydroxide, sodium acetate (including anhydrous and trihydrate), calcium hydroxide, calcium sulfate, calcium sulfate 0.5-hydrate, calcium sulfate dihydrate, zinc borate, calcium carbonate, basic magnesium carbonate, magnesium oxide, sodium acetate (including hydrates such as sodium acetate trihydrate), potassium acetate (including hydrates), calcium acetate (including hydrates such as calcium acetate monohydrate), magnesium acetate (including hydrates such as magnesium acetate tetrahydrate), aluminum oxide, talc, karyon clay, dawsonite, boehmite, hydrotalcite, calcium aluminate, alum, and magnesium sulfate heptahydrate. Furthermore, from the viewpoints of fire resistance, pinhole resistance, and moldability, it is more preferable to use at least one selected from the group consisting of sodium acetate, aluminum hydroxide, magnesium hydroxide, calcium sulfate dihydrate, and magnesium sulfate heptahydrate. ３ R １ (SO ４ )２ ・12H ２ O. ３ represents a trivalent metal atom (e.g., Al, Fe, or Cr), and R １ is a monovalent cation (e.g., K, NH ４ or Na), and potassium alum (particularly, potassium aluminum sulfate dodecahydrate: AlK(SO ４ ) ２ ・12H ２ O) and ammonium alum (particularly ammonium aluminum sulfate dodecahydrate: AlNH ４ (SO ４ ) ２ ・12H ２O). Among the above, aluminum hydroxide or calcium sulfate dihydrate is particularly preferred as a hydrated metal compound. In this specification, the term "endothermic material" refers to a material that has the property of absorbing heat through physical change, elimination of water of crystallization, phase transition, dissolution, chemical reaction, or the like, and can be a material whose endothermic peak is measured in the range of approximately 80°C to 400°C by thermal analysis using a differential scanning calorimeter (DSC) or the like. "Porous powder" refers to a powder particle-like material having multiple continuous or independent pores within the powder particles. "Hollow particles" refers to a material having a single independent pore within the powder particles. Furthermore, "other inorganic powders" refer to materials that do not have the above characteristics (e.g., a thermal decomposition onset temperature of 350°C or less and an endothermic capacity of 500 J / g or more) but can reinforce the endothermic material by sintering. Examples of hollow materials or porous powders include hollow silica, shirasu balloons, hollow calcium carbonate particles, and glass balloons. Examples of inorganic materials other than the hydrated metal compounds 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. Silicon, titanium, zirconium, magnesium, aluminum, indium, tin, and single or composite oxides thereof, other than the above-mentioned examples of hydrated metal compounds, are preferred. Specific examples of the inorganic material include clay, ceramics, vermiculite, bentonite, perovskite compounds (strontium titanate), 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.In this embodiment, the content of the inorganic powder can be 1 to 90% by mass, preferably 3 to 80% by mass, more preferably 5 to 70% by mass, even more preferably 10 to 60% by mass, even more preferably 20 to 50% by mass, and even more preferably 25 to 35% by mass, relative to the total mass (100% by mass) of the contents of the heat absorber. The content of the inorganic powder can be set within the above range regardless of its type. However, when the inorganic powder is a water-soluble inorganic powder, the content of the water-soluble inorganic powder can be 1 to 90% by mass, preferably 3 to 80% by mass, and even more preferably 5 to 60% by mass, more preferably 10 to 50% by mass, and even more preferably 15 to 35% by mass. Note that when the inorganic powder is a water-soluble inorganic powder and a hydrate, as described below, the content of the inorganic powder does not include the water contained in the hydrated inorganic powder. In this embodiment, it is preferable that the contents of the heat absorber are converted into a porous body when heated to 120°C or higher. The temperature at which the heat absorber changes into a porous body is more preferably 150° C. or higher, even more preferably 180° C. or higher, still more preferably 210° C. or higher, even more preferably 240° C. or higher, and particularly preferably 250° C. or higher. When the heat absorber is exposed to high heat such as by combustion, the aqueous solvent evaporates, but the inorganic powder can be sintered to form a porous body, which can be physically isolated from adjacent components (e.g., battery cells), and can therefore exhibit thermal insulation and fire protection effects for the heat absorber and adjacent components (e.g., battery cells).

[0074] <<Preferred Inorganic Powder>> An example of a preferred inorganic powder in this embodiment is an inorganic powder that dissolves at least 1 g in 100 g of water at 20°C. In this specification, an inorganic powder that dissolves at least 1 g in 100 g of water at 20°C is referred to as a water-soluble inorganic powder. In other words, the water-soluble inorganic powder in this specification refers to the above-mentioned inorganic powders that dissolve at least 1 g in 100 g of water at 20°C. On the other hand, an inorganic powder that dissolves at less than 1 g in 100 g of water at 20°C is referred to as a poorly soluble inorganic powder. By using a water-soluble inorganic powder as the inorganic powder, the water-soluble inorganic powder exhibits hydrophilicity, making it easy to dissolve in the aqueous solvent in the hydrogel and facilitating the uniform distribution of the water-soluble inorganic powder within the contents. 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 is more likely to form a homogeneous porous body, and can more effectively function as a thermal insulator. Furthermore, when the water-soluble inorganic powder, aqueous solvent, and hydrogel main body 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 uniformly dispersed water-soluble inorganic powder can become a homogeneous porous body (see, for example, the photograph in Figure 4 below). As a result, superior thermal insulation and fire protection effects are exhibited. Therefore, a heat absorber containing a water-soluble inorganic powder primarily functions as a heat absorber in relatively low temperature ranges (e.g., above room temperature to around 100°C). On the other hand, in temperature ranges from the critical temperature (e.g., 150°C) to the thermal runaway temperature (e.g., around 1000°C), the entire water-soluble inorganic powder becomes a homogeneous porous body, and can also function as a thermal insulator. As a result, when the heat absorber of this embodiment is placed between cells in a battery stack consisting of multiple stacked cells, it is believed that thermal effects on adjacent cells can be better blocked or suppressed. For example, when thermal runaway occurs, the cells expand, compressing the heat absorber between the cells, and the distance between the cells becomes extremely small, making it difficult to exhibit effective heat insulating performance. 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 uniformly dispersed water-soluble inorganic powder itself sinters to form a porous body that exhibits a certain strength, thereby increasing the pressure resistance strength.This allows the distance between the cells to be kept constant, thereby maintaining effective heat insulation and more effectively suppressing consecutive explosions between the cells, etc. When emphasis is placed on the amount of heat absorption and delaying the time required to reach the meltdown temperature, it is preferable to use a water-soluble inorganic powder as the heat absorber of this embodiment.

[0075] The water-soluble inorganic powder of this embodiment dissolves in an aqueous solvent. This makes it easier to dissolve in an aqueous solvent, and the water-soluble inorganic powder is more likely to be uniformly distributed within the contents. In this specification, "water-soluble" means that 1 g or more dissolves in 100 g of water at 20°C. Therefore, the water-soluble inorganic powder of this embodiment can be an inorganic powder that dissolves 1 g or more in 100 g of water at 20°C.

[0076] The solubility of the water-soluble inorganic powder of this embodiment is preferably 1 g or more per 100 g of water at 20 ° C. From the viewpoints of the stability and dispersibility of the water-soluble inorganic powder in the heat absorber and sinterability by high-temperature heating, the solubility of the water-soluble inorganic powder (per 100 g of water at 20 ° C) is preferably 1 g or more and 100 g or less, more preferably 2 g or more and 90 g or less, even more preferably 3 g or more and 80 g or less, still more preferably 5 g or more and 70 g or less, even more preferably 15 g or more and 60 g or less, and particularly preferably 25 g or more and 50 g or less. The solubility of the water-soluble inorganic powder in 100 g of water at 20 ° C. can be appropriately adjusted by changing the above-mentioned upper and lower limits. When the solubility of the water-soluble inorganic powder at 20 ° C. is within the above range, solubility is ensured, and the water-soluble inorganic powder is uniformly dissolved or dispersed in the aqueous solvent, making it easier to form a homogeneous porous body during sintering.

[0077] The solubility of the water-soluble inorganic powder of this embodiment is preferably 10 g or more in 100 g of water at 60° C. The solubility of the water-soluble inorganic powder (in 100 g of water at 60° C.) is preferably 10 g or more and 150 g or less, more preferably 15 g or more and 120 g or less, even more preferably 20 g or more and 100 g or less, still more preferably 25 g or more and 80 g or less, still more preferably 30 g or more and 60 g or less, and particularly preferably 35 g or more. The solubility of the water-soluble inorganic powder in 100 g of water at 60° C. can be appropriately adjusted by changing the above-mentioned upper limit value and the above-mentioned lower limit value.

[0078] The solubility of the water-soluble inorganic powder of this embodiment is preferably 15 g or more in 100 g of water at 80° C. The solubility of the water-soluble inorganic powder (in 100 g of water at 80° C.) is preferably 15 g or more and 160 g or less, more preferably 20 g or more and 120 g or less, even more preferably 25 g or more and 100 g or less, still more preferably 30 g or more and 80 g or less, and particularly preferably 35 g or more and 60 g or less. The solubility of the water-soluble inorganic powder in 100 g of water at 80° C. can be determined by appropriately rearranging the above-mentioned upper and lower limits.

[0079] The solubility of the water-soluble inorganic powder of this embodiment is preferably 15 g or more in 100 g of water at 100° C. The solubility of the water-soluble inorganic powder (in 100 g of water at 100° C.) can be, for example, 15 g or more and 170 g or less, preferably 20 g or more and 130 g or less, more preferably 25 g or more and 100 g or less, even more preferably 30 g or more and 80 g or less, and particularly preferably 35 g or more and 60 g or less. The solubility of the water-soluble inorganic powder in 100 g of water at 100° C. can be determined by appropriately rearranging the above-mentioned upper and lower limits.

[0080] The preferred solubility of the water-soluble inorganic powder of this embodiment is 1 g or more and 90 g or less per 100 g of water at 20° C., more preferably 5 g or more and 90 g or less, 5 g or more and 100 g or less per 100 g of water at 40° C., 10 g or more and 150 g or less per 100 g of water at 60° C., 15 g or more and 160 g or less per 100 g of water at 80° C., and 15 g or more and 170 g or less per 100 g of water at 100° C. The solubility of the water-soluble inorganic powder in each temperature range is preferred from the viewpoint of exhibiting suitable endothermic action and pressure resistance. The solubility of the water-soluble inorganic powder in 100 g of water at 20° C. can be appropriately rearranged by appropriately rearranging the above-mentioned upper and lower limits.

[0081] The method for measuring solubility in this specification is as follows. A specified amount of inorganic powder to be measured is weighed into a glass bottle, and 100 g of pure water (pH = 7) is added to the glass bottle. The mixture is stirred at 1 atmosphere and at temperatures of 20°C, 40°C, 60°C, 80°C, and 100°C for 24 hours at a rotation speed of 80 rpm on a mix rotor to prepare a mixed solution. The transmittance of the mixed solution after 24 hours of stirring is then measured under the following conditions. In this case, the transmittance measurement is performed by changing the amount of dissolved water-soluble inorganic powder, and the upper limit amount (g) at which the transmittance becomes 99% is taken as the solubility of the water-soluble inorganic powder in water. <Conditions for Transmittance Measurement> Dynamic Light Scattering (DLS) Measurement Apparatus: DLS Measurement Apparatus DLS-8000 manufactured by Otsuka Electronics Laser Wavelength, Output: 488 nm / 100 mW Sample Cell: NMR Tube

[0082] The water-soluble inorganic powder of this embodiment is preferably solid at room temperature. Furthermore, in the heat absorber of this embodiment, it is preferable that an aqueous solution containing an aqueous solvent and a water-soluble inorganic powder is filled into a bag as the content of the heat absorber. By filling the content of the heat absorber with an aqueous solution containing an aqueous solvent and a water-soluble inorganic powder, the water-soluble inorganic powder is completely dissolved in the aqueous solvent, and therefore the water-soluble inorganic powder is uniformly present in the content, resulting in the formation of a homogeneous porous body. The transmittance of the aqueous solution is preferably 99% or more, more preferably 99.5% or more.

[0083] The heat absorption amount and thermal decomposition onset temperature of the water-soluble inorganic powder can be within the preferred ranges of the inorganic powder. The preferred shapes and modes of the water-soluble inorganic powder can also be applied to the preferred shapes and modes of the inorganic powder. Similarly, the preferred ranges of the inorganic powder can also be applied to the content of the water-soluble inorganic powder relative to the total amount (100% by mass) of the contents of the heat absorber.

[0084] The material of the water-soluble inorganic powder of this embodiment is preferably composed of a water-soluble inorganic salt. Similarly to the inorganic powder described above, the water-soluble inorganic salt is preferably one or more compounds composed of a combination of an inorganic cation and an organic / inorganic anion. Examples of the inorganic cation include alkali metal ions, alkaline earth metal ions, aluminum ions, zinc ions, silver ions, copper(I) ions, and copper(II) ions. Preferably, the inorganic cation is one or more selected from the group consisting of potassium ions, calcium ions, magnesium ions, and aluminum ions. The organic / inorganic anion is preferably one or more selected from oxygen ions, sulfate ions, halogen ions (chloride ions, fluoride ions, bromine ions, etc.), nitrate ions, carbonate ions, acetate ions, and phosphate ions. The water-soluble inorganic powder of this embodiment is preferably composed of one or more compounds selected from the group consisting of chlorides, sulfates, carbonates, nitrates, phosphates, acetates, alkali metal oxides, and alkaline earth metal oxides. This allows the powder to exhibit excellent solubility in aqueous solvents. The water-soluble inorganic powder before being mixed with the aqueous solvent may be in the form of an anhydrous or hydrated form, provided that it exhibits the desired solubility described above. In the bag, the hydrated form is usually present as an anhydrous form.

[0085] Specific examples of the water-soluble inorganic powder or water-soluble inorganic salt of this embodiment include chlorides such as sodium chloride, potassium chloride, and ammonium chloride; sulfates such as sodium sulfate, potassium sulfate, magnesium sulfate, aluminum sulfate, and alum; carbonates such as sodium bicarbonate, sodium sesquicarbonate, sodium carbonate, potassium carbonate, potassium sesquicarbonate, and ammonium carbonate; nitrates such as sodium nitrate, potassium nitrate, and calcium nitrate; phosphates such as sodium phosphate, sodium dihydrogen phosphate, dipotassium hydrogen phosphate, and sodium polyphosphate; acetates such as zinc acetate, sodium acetate, potassium acetate, copper(I) acetate, and copper(II) acetate; oxides such as chromium oxide, barium oxide, and boric oxide; and hydrates thereof. Among the above, magnesium sulfate or magnesium sulfate heptahydrate is particularly preferred. The water-soluble inorganic powder of this embodiment may be any of the above examples used alone or in combination.

[0086] (Inorganic Fibers) The contents of the heat absorber of this embodiment may contain inorganic fibers as needed. The inorganic fibers are a fiber aggregate in which fibers made of inorganic materials are entangled with each other, or a porous body made of inorganic materials, and have the effect of facilitating the formation of a porous body using inorganic powder as a foam nucleating agent. Furthermore, when the inorganic powder changes into a porous body, a porous composite containing the inorganic fibers and inorganic powder can be formed. This makes it easier to form an insulating wall with the desired mechanical strength when the temperature exceeds the thermal runaway temperature, for example. When prioritizing the amount of heat absorption and the delay in the time required to reach the meltdown temperature, inorganic fibers are preferably used as the heat absorber of this embodiment.

[0087] Specific examples of the inorganic fibers include woven or knitted fabrics (glass cloth or silica cloth), nonwoven fabrics (glass fiber or ceramic fiber), and cotton-like materials (including not only glass wool, rock wool, and ceramic wool, but also spongy materials (sponge materials)). The inorganic fibers of this embodiment preferably have a heat resistance of 300°C or higher, more preferably 700°C or higher, and even more preferably 1200°C or higher. The heat resistance refers to the temperature at which the rate of volume change (thickness direction) reaches -20% when a test specimen is heated from 200 to 700°C in 100°C increments and held at each temperature for 30 minutes. When the inorganic fibers of this embodiment are porous bodies having at least one of a specific airflow resistance, a specific porosity, a specific tortuosity, or a specific void ratio, the entire heat absorber tends to form a relatively stable porous body even in high temperature ranges (temperature ranges from a critical temperature (e.g., 150°C) to a thermal runaway temperature (e.g., near 1000°C)), and therefore tend to function as a heat insulator. In particular, when inorganic fibers have a specific porosity, they can become a better heat insulator when combined with inorganic powder.

[0088] <Porosity> The average porosity of the inorganic fibers of this embodiment is preferably 30% or more and 99.7% or less, more preferably 50% or more and 99.5% or less, even more preferably 70% or more and 99.3% or less, and particularly preferably 90% or more and 99% or less. In this specification, the average porosity of the inorganic fibers is a value calculated from the bulk density and true density described below, and is a density based on the volume occupied by the inorganic fibers. The bulk density is a density based on the volume including the voids contained in the inorganic fibers. In contrast, the true density is a density based on the volume occupied by the inorganic fiber material. The average porosity (%) can be calculated from the bulk density ρf and true density ρr described below using the following formula (1): Average porosity (%) = ((1 / ρf) - (1 / ρr)) / (1 / ρf) × 100 ... formula (1)

[0089] <Bulk density> The bulk density ρf of the inorganic fiber of this embodiment is 0.020 g / cm ３ 1g / cm or more ３ Preferably, it is 0.022 g / cm or less, more preferably 0.022 g / cm ３ 0.5g / cm or more３ More preferably, 0.024 g / cm or less ３ 0.1g / cm or more ３ Particularly preferably 0.026 g / cm ３ 0.07g / cm or more ３ The dimensions of the inorganic fibers are measured, and the bulk volume V of the inorganic fibers is calculated. Then, the mass M of the inorganic fibers is measured using a precision balance. From the obtained mass M and bulk volume V, the bulk density of the inorganic fibers can be calculated using the following formula (2). Bulk density ρf (g / cm ３ )=M / V...Formula (2)

[0090] <True Density> The true density ρr of the inorganic fiber of this embodiment is 0.5 g / cm ３ 10g / cm or more ３ Preferably, it is less than 1 g / cm ３ 7g / cm or more ３ More preferably, 1.5 g / cm or less ３ 5g / cm or more ３ Below 2 g / cm, particularly preferably ３ 3g / cm or more ３ The method for measuring the true density ρr of inorganic fibers is not particularly limited, but it can be calculated by the sink-float method using a mixed solution consisting of n-heptane, carbon tetrachloride, and ethylene dibromide. Specifically, first, an appropriate-sized sample piece of inorganic fiber is placed in a stoppered test tube. Next, a mixed solvent of three solvents is added to the test tube, and the tube is immersed in a constant temperature bath at 30°C. If the sample piece floats, n-heptane, which has a low density, is added. On the other hand, if the test piece sinks, ethylene dibromide, which has a high density, is added. This procedure is repeated until the test piece floats in the liquid. Finally, the density of the mixed solvent is measured using a Gay-Lussac pycnometer.

[0091] <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.

[0092] <Shape of Inorganic Material Fiber> The shape of the inorganic fiber in this embodiment may be selected from threads, fibers, fiber bundles, fiber aggregates, cotton, woven or knitted fabrics, nonwoven fabrics, etc. In this specification, "woven or knitted fabrics" refers to woven fabrics or knitted fabrics.

[0093] When the inorganic fiber according to the present embodiment is a woven fabric, known weaving methods such as plain weave, twill weave, satin weave, tatami weave, leno weave, and cord weave can be appropriately used as the weaving method for the woven fabric. Among these weaving methods, it is preferable to use a weaving method that provides a predetermined range of resistance to fluid passage between the communicating holes, i.e., between the spaces per mesh formed by the intersection of the warp and weft lines (for example, the airflow resistance described below). From these viewpoints, it is preferable to use a weaving method such as plain weave, twill weave, satin weave, leno weave, or tatami weave.

[0094] When the inorganic fiber according to the present embodiment is a knitted fabric, the knitting method of the knitted fabric may be warp knitting, which is knitted in the vertical direction, such as lace knitting, raschel knitting, tricot knitting, or vandyke knitting, or weft knitting, which is knitted in the horizontal direction, such as flat knitting, plain knitting, rib knitting, tubular knitting, jersey knitting, cannula knitting, rib knitting, or jacquard knitting, and any known knitting method may be appropriately adopted. Among these knitting methods, it is preferable to adopt a knitting method in which the passage resistance of a fluid passing through the communicating holes (for example, the airflow resistance described below) falls within a predetermined range. Furthermore, various knitting machines may be used, such as a warp knitting machine, a flat knitting machine, a circular knitting machine, or a raschel knitting machine.

[0095] When the inorganic fiber according to this embodiment is a woven or knitted fabric, the woven or knitted yarn used is not particularly limited, and the fineness is preferably 50 dtex or more and 8000 dtex or less, more preferably 100 dtex or more and 3000 dtex or less. The twisting method of the woven or knitted yarn is also not particularly limited, and the twisting method may be dry twisting, wet twisting performed by immersion in water, or a combination thereof. The twisting direction is also not particularly limited, and may be right-handed twisting, left-handed twisting, or a combination thereof. The woven or knitted yarn used in this embodiment may be false-twisted yarn, filament yarn, or yarn processed using the POY-DTY method or the PTY (Producers Textured Yarn) method. The conditions for the woven or knitted yarn used can be appropriately selected depending on the purpose of use, the type of aqueous solvent, etc. The material of the woven or knitted yarn may be the material constituting the inorganic fiber described above or the inorganic material contained in the inorganic fiber.

[0096] The BET specific surface area of ​​the inorganic fiber is 0.3 to 5000 m ２ / g, and 10 to 2000m ２ / g, and 30 to 1600m ２ The BET specific surface area of ​​the inorganic fiber may be measured using a specific surface area meter (Microtrac-Bell Corporation, BELSORP-mini), and the surface area per 1 g of the sample measured from the amount of nitrogen gas adsorbed by the BET method may be defined as the specific surface area (m ２ / g).

[0097] When the inorganic fiber of this embodiment is composed of a nonwoven fabric, the average fiber diameter of all fibers (fibers made from the inorganic raw material) constituting the nonwoven fabric is preferably 1 to 100 μm, more preferably 2 to 10 μm. It is preferable that the average fiber diameter of the fibers constituting the nonwoven fabric is within the above range, since it is easy to ensure the desired porosity. The average fiber diameter can be measured by microscope observation or image analysis using a fiber length measuring device (e.g., KAJAANI Fiber Lab.).

[0098] Furthermore, when the inorganic fiber of this embodiment is formed into a nonwoven fabric, the average fiber length of all fibers (raw fibers) constituting the nonwoven fabric is preferably 3 mm or more and 200 mm or less, more preferably 5 mm or more and 100 mm or less, and more preferably 10 mm or more and 50 mm or less. It is preferable that the average fiber length of all fibers constituting the nonwoven fabric is within the above range, since it is easier to ensure the desired porosity. The average fiber length can be measured by microscope observation or the average fiber diameter based on image analysis results using a fiber length measuring device (e.g., KAJAANI Fiber Lab.). When the inorganic fiber of this embodiment is formed into a cotton-like body, the average fiber length of all fibers (raw fibers) constituting the cotton-like body is preferably 0.5 μm or more and 50 μm or less, more preferably 0.8 μm or more and 32 μm or less, and more preferably 1 μm or more and 25 μm or less. It is preferable that the average fiber length of all fibers constituting the cotton-like material is within the above range, since it is easier to ensure the desired void ratio. The average fiber length can be measured by microscope observation or the average fiber diameter from image analysis results using a fiber length measuring device (e.g., KAJAANI Fiber Lab.). In this specification, cotton-like material is also a type of nonwoven fabric, but it is defined as a cotton-like material that is in the form of fibers and has a shape other than a cloth (or flat plate).

[0099] <Preferred Embodiments of Inorganic Fibers> Preferred embodiments of the inorganic fibers of the present embodiment are glass cloth, ceramic wool, rock wool, and glass wool.

[0100] The content of the inorganic fibers in this embodiment is preferably 0% by mass or more and 50% by mass or less, more preferably 1% by mass or more and 30% by mass or less, even more preferably 1% by mass or more and 10% by mass or less, and particularly preferably 1% by mass or more and 5% by mass or less, and particularly preferably 1% by mass or more and 3% by mass or less, relative to the total amount (100% by mass) of the contents of the heat absorber. The preferred range of the content of the inorganic fibers can be determined by appropriately combining the above upper and lower limits. When the content of the inorganic fibers is within the above range, the heat absorber has better heat absorption and pressure resistance, and can change from an endothermic effect to an insulating effect in the high temperature range.

[0101] (Additives) The heat absorber content or dispersion liquid (a) of this embodiment may contain various additives, such as ultraviolet absorbers, antioxidants, organic solvents, inorganic fillers other than the water-swellable clay minerals, viscosity modifiers such as thickeners, crosslinkers, and flame retardants, as needed. Although the various additives are optional components, when used, they are preferably used in proportions that do not impair the effects of the present disclosure and that correspond to the purpose of each additive. While the proportions cannot be determined in general, the content of the various additives is preferably 0% by mass or more and 50% by mass or less, and more preferably 10% by mass or more and 40% by mass or less, relative to the total amount (mass) of the aqueous solvent and various additives used in the present disclosure. The preferred range of the content of the various additives can be determined by appropriately adjusting the above upper and lower limits.

[0102] 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.

[0103] Examples of the antioxidant include "Sumilizer BBM-S" and "Sumilizer GA-80" manufactured by Sumitomo Chemical Co., Ltd. Examples of the organic solvent include aromatic hydrocarbons such as toluene and xylene; glycols such as ethylene glycol and propylene glycol; polyether glycols, which are polymers thereof; cellosolves; carbitols; and aliphatic alcohols such as methanol. The organic solvents can be used alone or in combination of two or more. Examples of the inorganic filler include fused silica, crystalline silica, alumina, silicon nitride, and aluminum hydroxide. Examples of the viscosity modifier such as the thickener include various tackifying resins such as rosin, polymerized rosin, polymerized rosin ester, rosin phenol, stabilized rosin ester, disproportionated rosin ester, terpene, terpene phenol, and petroleum resin. Examples of the crosslinking agent include known crosslinking agents such as isocyanate-based, epoxy-based, aziridine-based, polyvalent metal salt-based, metal chelate-based, ketohydrazide-based, oxazoline-based, carbodiimide-based, silane-based, and glycidyl (alkoxy) epoxy silane-based crosslinking agents.

[0104] Examples of the flame retardant include inorganic phosphorus compounds such as red phosphorus, ammonium phosphates such as monoammonium phosphate, diammonium phosphate, triammonium phosphate, and ammonium polyphosphate, and phosphoric acid amides; phosphoric acid ester compounds, phosphonic acid compounds, phosphinic acid compounds, phosphine oxide compounds, phosphorane compounds, organic nitrogen-containing phosphorus compounds, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 10-(2,5-dihydroxyphenyl)-10H-9-oxa-10-phosphaphenanthrene-10-oxy Examples of suitable flame retardants include organic phosphorus compounds such as cyclic organic phosphorus compounds such as 10-(2,7-dihydroxynaphthyl)-10H-9-oxa-10-phosphaphenanthrene-10-oxide and derivatives thereof obtained by reacting them with compounds such as epoxy resins and phenolic resins; nitrogen-based flame retardants such as triazine compounds, cyanuric acid compounds, isocyanuric acid compounds, and phenothiazine; silicone-based flame retardants such as silicone oil, silicone rubber, and silicone resin; and inorganic flame retardants such as metal hydroxides, metal oxides, metal carbonate compounds, metal powders, boron compounds, and low-melting-point glass. These flame retardants can be used alone or in combination of two or more. When these flame retardants are used, their amount is preferably in the range of 0.1 to 20% by mass based on the entire contents of the heat endotherm or the entire dispersion liquid (a).

[0105] (Method for Producing Heat Endotherm) As described above, one example of the method for producing the heat endotherm of this embodiment includes the steps of: (1) preparing a dispersion liquid (a) containing a water-soluble organic monomer and a water-swellable clay mineral, which are reaction raw materials for the hydrogel body, an aqueous solvent, and optionally a polymerization initiator, a catalyst, and / or additives; (2) filling a bag with the dispersion liquid (a) through its opening, and then sealing the opening; and (3) gelling the water-soluble organic monomer in the bag at a desired polymerization temperature. The inorganic powder of this embodiment can be added as a content during the steps (1) and (2). In one aspect of this embodiment, another method for producing a heat absorber includes the steps of: (4) preparing a dispersion (a) by mixing a water-soluble organic monomer and a water-swellable clay mineral, which are reaction raw materials for the hydrogel body, with a polymerization initiator and an aqueous or organic solvent; (5) heating the dispersion (a) to gel the dispersion (a) and thereby preparing an organic-inorganic composite hydrogel body; (6) immersing the organic-inorganic composite hydrogel body in the aqueous solvent for a desired time period to swell the organic-inorganic composite hydrogel in the aqueous solvent; and (7) filling a bag through its opening with the swollen organic-inorganic composite hydrogel or a mixture (1) containing the swollen organic-inorganic composite hydrogel, and then sealing the opening. The inorganic powder of this embodiment can be added in any of the steps (4) to (7). Furthermore, as another method for producing a heat absorber, after the above step (5), instead of the above steps (6) and (7), a step (8) may be added: filling the hydrogel bodies, aqueous solvent, and inorganic powder separately through the opening of a bag, and then sealing the opening of the bag. Furthermore, instead of step (6), a step (9) may be added: preparing a mixture (including a mixed liquid) (1) in advance by impregnating or dispersing the hydrogel bodies and / or inorganic powder in the aqueous solvent, filling the bag through the opening of the bag with the mixture (1), and then sealing the opening of the bag. The mixture (1) contains the hydrogel bodies and the aqueous solvent, and may optionally contain one or more selected from the group consisting of an antifreeze agent and an additive.The mixture (1) preferably contains 1 to 50% by mass (preferably 4 to 35% by mass) of the hydrogel main body, 1 to 98% by mass (preferably 2 to 91% by mass) of the aqueous solvent, 1 to 80% by mass (preferably 3 to 75% by mass) of the inorganic powder, 0 to 50% by mass of the antifreeze agent, and 0 to 10% by mass of the additives, relative to the total amount (100% by mass) of the mixture (1), and more preferably contains 5 to 30% by mass of the hydrogel main body, 10 to 80% by mass of the aqueous solvent, 5 to 70% by mass of the inorganic powder, 0 to 40% by mass of the antifreeze agent, and 0 to 10% by mass of the additives. In one aspect of this embodiment, the method for producing the heat absorber includes the steps of preparing a dispersion (a) containing reaction raw materials (a water-soluble organic monomer and a water-swellable clay mineral), an aqueous solvent, and optionally a polymerization initiator, a catalyst, a low-volatile solvent, and / or an additive, filling a bag with the dispersion (a) and the inorganic powder through an opening thereof, sealing the bag, and gelling the water-soluble organic monomer in the bag at a desired polymerization temperature, as described above. Alternatively, a dispersion (b) may be used in place of the dispersion (a), in which an antifreeze and / or an additive are mixed with the dispersion (a) as needed. In another aspect of this embodiment, another method for producing the heat absorber includes the steps of preparing a mixed solution (2) by mixing reaction raw materials (water-soluble organic monomer, water-swellable clay mineral, polymerization initiator, aqueous solvent or organic solvent, and inorganic powder); heating the mixed solution (2) to gel and prepare an organic-inorganic composite hydrogel body; impregnating the inorganic powder and the organic-inorganic composite hydrogel body in the aqueous solvent for a desired time to swell the organic-inorganic composite hydrogel in the aqueous solvent; and filling a bag with the swollen organic-inorganic composite hydrogel and the inorganic powder through an opening and then sealing the bag. This method for producing a heat absorber of this embodiment can be used to form a heat absorber even when inorganic powder is blended into the contents of the heat absorber. The mixed solution (2) preferably contains 1 to 80% by mass of inorganic powder and 20 to 99% by mass of dispersion liquid (a) relative to the total amount (100% by mass) of the mixed solution (2).

[0106] A suitable heat absorber of this embodiment may be a heat absorber including a bag and, as the contents of the bag, an aqueous solvent (e.g., 20 to 60% by mass of water relative to the total amount of the contents), a hydrogel body (e.g., 10 to 30% by mass of NC gel relative to the total amount of the contents), and an inorganic powder (e.g., 10 to 50% by mass of aluminum hydroxide relative to the total amount of the contents) relative to the total amount of the contents. This may provide a heat absorber that has a superior heat absorption capacity and temperature rise suppression effect, and that can switch from an endothermic effect to an insulating effect in a high temperature range. In this embodiment, the total content of the aqueous solvent, hydrogel body, and inorganic powder as the contents of the bag may be preferably 80 to 100% by mass, more preferably 92 to 99.5% by mass, and even more preferably 93 to 99% by mass, relative to the total amount (100% by mass) of the contents of the bag. In this embodiment, the total content of the aqueous solvent, hydrogel body, inorganic powder, antifreeze agent, and additives in the bag is preferably 83 to 100% by mass, more preferably 94 to 99.5% by mass, and even more preferably more than 95% by mass and 99% by mass or less, relative to the total amount (100% by mass) of the bag contents. The upper and lower limits of the total content can be adjusted as appropriate.

[0107] (Preferred Aspects of the Heat Absorber) The heat absorber of this embodiment preferably exhibits high cushioning properties and / or excellent pressure resistance during heating. Each preferred aspect is described in detail below. <High-Cushioning Heat Absorber> A preferred heat absorber of this embodiment exhibits high cushioning properties and includes a pouch and, as the contents of the pouch, a hydrogel and an inorganic powder. The hydrogel includes a hydrogel main body formed from three-dimensional polymer chains and an aqueous solvent. In this specification, a heat absorber exhibiting high cushioning properties is also referred to as a high-cushioning heat absorber. "High cushioning properties" refers to excellent cushioning properties, and when the pouch contains a hydrogel (hydrogel main body and aqueous solvent), high cushioning properties tend to be exhibited. Specifically, "high cushioning properties" refers to a cushioning property (%) expressed by the following formula (I) of 90% or more, and more preferably 93% or more. By exhibiting cushioning properties of 90% or more, the battery cell can more easily adapt to relatively short-term deformations such as expansion and contraction caused by charging and discharging. The upper limit of the high cushioning property may be 100%. [Equation 4] "Cushioning property (%) = h ａ / h ｂ × 100 Formula (I) (wherein, h ａ is the height (mm) of the pressed part 5 minutes after the surface of the high-cushion heat absorber is pressed at 1 MPa for 60 seconds and then released, and h ｂrepresents the height (mm) before the surface of the high-cushioning heat absorber is pressed at 1 MPa for 60 seconds. In the heat absorber of this embodiment, when cushioning is important, it is preferable that not only is the cushioning (%) represented by formula (I) 90% or more, but also that the content of the antifreeze agent be controlled to a predetermined value or less. That is, when cushioning is important, the content of the antifreeze agent is preferably 40% by mass or less, more preferably 30% by mass or less, even more preferably 20% by mass or less, even more preferably 12% by mass or less, even more preferably 9% by mass or less, even more preferably 6% by mass or less, and particularly preferably substantially absent (0.5% by mass or less) relative to the total amount of the contents. If the content of the antifreeze agent exceeds a predetermined amount relative to the total amount of the contents, the number of crosslinking points in the gel decreases, making the gel more flexible, and as a result, cushioning properties are thought to be reduced. On the other hand, under high-temperature heating conditions, the gel acts to smoothly flow the inorganic powder during evaporation, allowing the inorganic powder to be uniformly arranged, which is thought to result in increased strength. Furthermore, in the heat absorber of this embodiment, when cushioning properties are important, not only the content of the antifreeze agent but also the amount of hydrogel contained in the contents, preferably the total amount of the hydrogel (hydrogel main body and aqueous solvent) and inorganic powder, relative to the total amount of the contents, can be preferably more than 80% by mass, more preferably 83% by mass or more, even more preferably 87% by mass or more, still more preferably 91% by mass or more, even more preferably 94% by mass or more, still more preferably 98% by mass or more, and particularly preferably 100% by mass. It is thought that as the total amount of the hydrogel and inorganic powder increases relative to the total amount of the contents, the inorganic powder reinforces the hydrogel, which has a high recovery rate after deformation, thereby exhibiting high cushioning properties.

[0108] <High-Pressure-Resistant Heat Endogenous Body> A preferred heat endogenous body of this embodiment has excellent pressure resistance when heated, and is composed of a bag that can be filled with a content, an aqueous solvent, and an inorganic powder. In this specification, a heat endogenous body that has excellent pressure resistance when heated is also referred to as a high-pressure-resistant heat endogenous body. Such a high-pressure-resistant heat endogenous body tends to exhibit excellent pressure resistance when heated at high temperatures (e.g., 800°C or higher) due to thermal runaway of a battery or the like. "Exhibiting pressure resistance when heated" refers to the heat endogenous body exhibiting high pressure resistance when heated. When the bag further contains a hydrogel (hydrogel body and aqueous solvent), the pressure resistance is improved. Similarly, the bag contains an inorganic powder, which improves the pressure resistance. In particular, the bag contains both a hydrogel (hydrogel body and aqueous solvent) and an inorganic powder, which tends to exhibit excellent high pressure resistance, particularly when heated. Therefore, a preferred embodiment 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 contents, a hydrogel composed of a hydrogel body and an aqueous solvent, and an inorganic powder, wherein the hydrogel and the inorganic powder form a composite and are filled into the contents. A more preferred embodiment 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 contents, a hydrogel composed of a hydrogel body and an aqueous solvent, and an inorganic powder, wherein the hydrogel and the inorganic powder form a composite and are filled into the contents. In this case, it is preferable that the inorganic powder is uniformly dispersed in the contents. The coexistence of the hydrogel and the inorganic powder temporarily forms a foam film during thermal runaway, and it is believed that this foam film acts as a dispersion reinforcement. Furthermore, the coexistence of the hydrogel and the inorganic powder is thought to exhibit a synergistic effect of high pressure resistance because the hydrogel promotes the dispersibility of the inorganic powder. Here, the uniform dispersion means that, for example, when the composite content is taken out, the difference in concentration (mass %) of the inorganic powder at both ends present in a portion up to about 3 mm from the end of the composite is within ±15%.Furthermore, "exhibiting high pressure resistance upon heating" specifically means that after the heat absorber is heated until the surface opposite the heated surface thereof 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, the pressure resistance is excellent, and therefore, consecutive explosions between cells can be effectively suppressed and prevented. The upper limit of the thickness change rate can be 100%. [Mathematical Expression 5] Equation (II): "Thickness change rate (%) = (thickness of the heat absorber after pressurization by pressing the surface of the high pressure-resistant heat absorber after heating at 0.5 MPa for 60 seconds) / (thickness of the heat absorber before pressurization by pressing the surface of the high pressure-resistant heat absorber after heating at 0.5 MPa for 60 seconds) × 100" In the above equation (II), "the thickness of the heat absorber after pressurization by pressing the surface of the high pressure-resistant heat absorber after heating at 0.5 MPa for 60 seconds" refers to the average thickness (arithmetic mean of thicknesses at any five positions) of the heat absorber after heating to a predetermined temperature or higher (preferably, one surface (the heated surface that is the surface directly exposed to radiant heat) of the high pressure-resistant heat absorber after heating under the heating conditions described below) after pressing at 0.5 MPa for 60 seconds (within approximately 20 minutes from immediately after pressing). Similarly, in the above formula (II), "the thickness of the heat absorber before being pressed against 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 points) of the heat absorber after one surface of the high pressure resistant heat absorber is heated to a predetermined temperature or higher (preferably after being heated under the heating conditions described below) and before being pressed against one heated surface of the high pressure resistant heat absorber at 0.5 MPa for 60 seconds. Heating conditions: "50 kW / m by radiant heat. ２The heat absorber was heated with a heat quantity of 1000 kJ / cm2 until the temperature of the surface opposite the heated surface (back surface) reached a predetermined temperature, and then the heat absorber was allowed to dissipate heat at room temperature (22-28°C). The surface of the heat absorber was then naturally cooled until the temperature of the surface reached room temperature (22-28°C), and the thickness change rate (%) before and after heating was calculated. The heated surface of the heat absorber was pressed at 0.5 MPa for 60 seconds. The "predetermined temperature" in the heating conditions refers to the temperature at which the heat absorber becomes porous and can be set depending on the heat absorber's usage environment, the required explosion resistance, and the expected thermal runaway onset temperature. The "predetermined temperature" can be, for example, 240°C (240°C or higher), preferably 200°C (200°C or higher), more preferably 180°C (180°C or higher), even more preferably 160°C (160°C or higher), even more preferably 150°C or higher, and particularly preferably 120°C (120°C or higher). For example, if the heat absorber is heated until the temperature of the surface (back surface) opposite to the heated surface thereof reaches 150° C. and the thickness change rate (%) after heating is equal to or greater than a predetermined value, the heat absorber can achieve a thickness change rate equal to or greater than the predetermined value even in a temperature range of 150° C. When high pressure resistance is important, the heat absorber of this embodiment preferably contains a hydrogel (hydrogel main body and aqueous solvent) and an inorganic powder.

[0109] (Another Preferred Form of Heat Absorber) Another preferred form of the heat absorber of the present disclosure is one having an endothermic onset temperature of 400°C or less and an endothermic peak temperature in the range of at least 80°C to 400°C, and having a thickness change rate (%) of 50 kW / m 2 by radiant heat. [Mathematical Expression 6] "Thickness change rate (%) = (thickness of the heat absorber after pressing the surface of the heat absorber heated under the heating conditions below at 0.5 MPa for 60 seconds) / (thickness of the heat absorber before pressing the surface of the heat absorber heated under the heating conditions below at 0.5 MPa for 60 seconds) × 100" Heating conditions: "50 kW / m 2 by radiant heat" ２The heat absorber was heated with a heat quantity of 1000 kJ / cm2 until the temperature of the surface (back surface) opposite the heated surface reached a predetermined temperature, and then the heat absorber was allowed to dissipate heat at room temperature (22-28°C). The heat absorber was then naturally cooled until the surface temperature reached room temperature (22-28°C), and the thickness change rate (%) before and after heating was calculated. The high-pressure-resistant heat absorber was pressed against its heated surface at 0.5 MPa for 60 seconds. This heat absorber has a thickness change rate (%) of 70% or more. This provides excellent pressure resistance, heat absorption, and temperature rise suppression effect, and can change from an endothermic effect to an insulating effect in the high-temperature range. The "predetermined temperature" in the above heating conditions is as explained above.

[0110] <Particularly Preferred Form of Heat Absorber> In this embodiment, when pressure resistance is important, it is preferable that the heat absorber has a bag that can be filled with contents and a hydrogel and an inorganic powder that are filled in the bag as the contents, and the inorganic powder is a poorly soluble inorganic powder. As described above, the poorly soluble inorganic powder is an inorganic powder that dissolves in an amount of less than 1 g per 100 g of water at 20°C. Examples of the poorly soluble inorganic powder include aluminum hydroxide, calcium sulfate dihydrate, and other compounds exemplified in the above (Inorganic Powder) section, excluding water-soluble inorganic powders. The combination of a poorly soluble inorganic powder and a hydrogel can further improve the mechanical strength of the heat absorber after sintering. On the other hand, in this embodiment, when heat absorption and the temperature rise suppression effect are important, it is preferable that the heat absorber has a bag that can be filled with contents and a hydrogel and an inorganic powder that are filled in the bag as the contents, and the inorganic powder is a water-soluble inorganic powder. As described above, the poorly soluble inorganic powder is an inorganic powder that dissolves at least 1 g in 100 g of water at 20°C, and examples thereof include water-soluble inorganic powders such as magnesium sulfate heptahydrate. Regarding the water-soluble inorganic powder, the content of the "Preferred Inorganic Powders" section above is incorporated by reference. By using a water-soluble inorganic powder as the inorganic powder, the water-soluble inorganic powder is easily dissolved in the aqueous solvent in the hydrogel, making it easier for the water-soluble inorganic powder to be uniformly present within the contents. As a result, the heat absorption and temperature rise suppression effects are further improved, and in temperatures above the thermal runaway temperature (e.g., around 200°C), the water-soluble inorganic powder as a whole is more likely to form a homogeneous porous body, allowing it to function more effectively as a thermal insulator.

[0111] [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.

[0112] The secondary battery module capable of mounting the heat absorber of this embodiment is a secondary battery mounted on a mobile object such as a vehicle or an aircraft (particularly a drone), and includes multiple battery cells and a case for housing the multiple battery cells. The battery cells (hereinafter also referred to as battery cells) constituting the secondary battery module may be battery cells in which, for example, a battery exterior film is used as an exterior material and battery elements including at least a positive electrode material layer, a negative electrode material layer, a separator, a positive electrode current collector, and a negative electrode current collector are enclosed within the exterior material. A secondary battery module capable of mounting the heat absorber of this embodiment will be described below with reference to FIG. 1 . FIG. 1 shows a cross-sectional view of a stacked battery 20 as an example of a secondary battery. It should be noted that the secondary battery capable of mounting the heat absorber of this embodiment is not limited to the flat stacked battery 20 shown in FIG. 1 . The secondary battery capable of mounting the heat absorber of this embodiment may be cylindrical, such as a wound secondary battery, or may be a cylindrical secondary battery modified to have a flat rectangular shape. In this embodiment, the stacked battery 20 has a structure in which a flat, approximately rectangular battery element 10, in which charge and discharge reactions essentially proceed, is sealed inside battery exterior materials 18a, b. The battery element 10 has a structure in which a positive electrode, an electrolyte layer (or separator) 14, and a negative electrode are stacked. The positive electrode has a structure in which a positive electrode material layer 11 containing a positive electrode active material is disposed on both sides of a positive electrode current collector 12. The negative electrode has a structure in which a negative electrode material layer 16 containing a negative electrode active material is disposed on both sides of a negative electrode current collector 17. One positive electrode material layer 11 and the negative electrode material layer 16 adjacent to the positive electrode material layer 11 are arranged to face each other with the electrolyte layer 14 interposed therebetween, and the positive electrode, electrolyte layer 14, and negative electrode are stacked in sequence. As a result, adjacent positive electrodes, electrolyte layers 14, and negative electrodes form a single cell body. The stacked battery 20 shown in FIG. 1 has a structure in which a plurality of such cell bodies are stacked and electrically connected in parallel. In addition, an activated carbon layer 19 is provided to adsorb components derived from the positive electrode active material due to melting or sublimation of the positive electrode active material when the battery is exposed to high temperatures.As shown in FIG. 1 , the positive electrode current collector 12 and the negative electrode current collector 17 are respectively attached with a positive electrode terminal 13 and a negative electrode terminal 15, which electrically connect the positive and negative electrodes, and are structured to be sandwiched between the ends of the battery exterior materials 18a and 18b and extend outside the battery exterior materials 18a and 18b. The positive electrode terminal 13 and the negative electrode terminal 15 may be attached to the positive electrode current collector 12 and the negative electrode current collector 17 of each electrode by welding or the like via a positive electrode lead and a negative electrode lead (not shown), as necessary. The battery exterior materials 18a and 18b are made of laminated film, and typically, sealant layers formed on the surfaces of the battery exterior films 18a and 18b are heat-sealed to each other. The battery exterior materials 18a and 18b have a region at their peripheries where the sealant layers are in close contact with each other by heat sealing.

[0113] Next, a secondary battery module incorporating the heat absorber of this embodiment will be described with reference to FIG. 2 . FIG. 2 is a perspective view schematically illustrating the disassembled state of the secondary battery module of FIG. 1 . The battery element 10 shown in FIG. 2 has a configuration in which a positive electrode formed on a positive electrode current collector 12 (e.g., aluminum foil) having a positive electrode terminal 13 and a negative electrode disposed on a negative electrode current collector 17 (e.g., metal foil) having a negative electrode terminal 15 are stacked so as to face each other with a separator 14 containing an electrolyte interposed therebetween. A plurality of battery elements 10 are stacked and sealed with battery exterior materials 18 a, b (e.g., aluminum laminate exteriors). The heat absorber 1 of this embodiment is arranged so as to contact the negative electrode current collector 17. The heat absorber 1 may be arranged so as to contact not only the negative electrode current collector 17 but also the positive electrode current collector 12. Therefore, a secondary battery module having a heat absorber 1 mounted on a battery element 10 includes one or more laminates in which a positive electrode formed on a positive electrode current collector 12 (e.g., aluminum foil) having a positive electrode terminal 13, a separator 14 containing an electrolyte, and a negative electrode disposed on a negative electrode current collector 17 (e.g., metal foil) having a negative electrode terminal 15 are sequentially stacked, and the one or more heat absorbers 1 can be arranged so as to abut the positive electrode current collector 12 and / or the negative electrode current collector 17 but not the separator 14. When a solid electrolyte or a gel electrolyte is used as the electrolyte, the electrolyte can be interposed between the electrodes instead of the separator 14. Meanwhile, the aqueous solvent contained in the heat absorber of this embodiment does not come into direct contact with the battery element 10. Therefore, a suitable heat absorber for a secondary battery in this embodiment has a hydrogel, a bag filled with the aqueous solvent, and inorganic powder, but the contents of the bag do not include the hydrogel or the aqueous solvent in a manner that brings the battery element 10 into direct contact with the hydrogel or the aqueous solvent, and more preferably, the contents of the bag of the heat absorber for a secondary battery do not include the battery element 10.

[0114] The secondary battery module of the present disclosure may include a heat absorber 1 of the present embodiment sandwiched between adjacent battery elements 10 (also referred to as battery cells) housed in multiple cases (not shown) or multiple battery exterior films 18a, b. The cases may be formed, for example, from aluminum, iron, or a metal material containing these, or a resin material such as polyphenylene sulfide. Forming the cases from a resin material contributes to reducing the weight of the secondary battery module. The heat absorber 1 may be sandwiched between the multiple battery elements 10 using, for example, adhesives, fusion (ultrasonic fusion, high-frequency fusion, or heat fusion), or pressure-sensitive adhesives. With this configuration, the heat absorber 1 sandwiched between the battery elements 10 absorbs heat generated during charging of the secondary battery, thereby suppressing a sudden temperature rise in the battery elements 10 and preventing deterioration, fire, and the like of the battery elements 10. Furthermore, when the heat absorber 1 contains a hydrogel that has absorbed an aqueous solvent, if the heat absorber 1 is sandwiched between the battery elements 10, the thermal insulation properties can suppress the temperature influence between the battery elements 10, and further, the cushioning properties of the swollen hydrogel act as a buffer for volume changes due to expansion of the battery elements 10, which is thought to make it easier to alleviate the rise in internal pressure of the secondary battery module.

[0115] 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.

[0116] The present invention will be described in detail below with reference to examples and comparative examples. It should be noted that the present invention is not limited to the examples listed below. (1) Measurement of Endothermic Onset Temperature, Endothermic Peak Temperature, and Endothermic Amount The endothermic onset temperature and endothermic peak temperature of the endothermic bodies prepared in the present examples and comparative examples were measured as follows. Using a differential scanning calorimeter (DSC; DSC-7020, manufactured by Hitachi High-Tech Corporation), the temperature was raised from 20°C to 350°C at a rate of 1°C / min under a nitrogen atmosphere. The endothermic onset temperature (°C) was determined as the temperature at the intersection of a straight line extending the baseline on the low-temperature side of the DSC measurement curve toward the high-temperature side and a tangent drawn at the point where the gradient of the curve on the low-temperature side of the endothermic peak associated with evaporation is maximum. The point where the difference from the baseline on the DSC measurement curve is maximum was determined as the endothermic peak temperature (°C). The integral of the endothermic peak based on the baseline of the DSC measurement curve, divided by the mass of the inorganic powder used in the measurement, was determined as the endothermic amount (J / g or mJ / mg).

[0117] (2) Evaluation of cushioning properties The cushioning properties of the heat absorbers prepared in the present examples and comparative examples were evaluated using the following method. Specifically, at room temperature (23°C), the heat absorber was placed in a Tensilon universal testing machine ("RTE-1210" manufactured by Orientec Co., Ltd.) with a size of 100 mm length x 100 mm width x 4.8 mm height and equipped with a 7 mmφ indentation jig, and an indentation test was performed. The indentation test was performed by indenting the surface of the heat absorber at 1 MPa for 60 seconds, and then releasing the indentation and measuring the height (mm) h of the indented part on the surface 5 minutes later. ａ and the height (mm) h before pressing the surface of the heat absorber at 1 MPa for 60 seconds ｂ and was measured, and the cushioning (also called 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 (%) was calculated using the following formula (I), and the average value is shown in Table 1. Formula (I): Cushioning (%) = h ａ / h ｂ× 100 (Evaluation criteria for cushioning) "Returns to 90% or more of its original height" was marked as "◎". "Returns to 80% or more of its original height" was marked as "◯". "Returns to 70% or more of its original height" was marked as "△". "Returns to less than 70% of its original height" was marked as "X".

[0118] (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. 3 in accordance with JIS A 1316. More specifically, the cone calorimeter 30 is based on the principle that the amount of heat generated in combustion is 13.1 MJ per kg of oxygen, regardless of the type of organic material, and calculates the heat release rate, total heat release, etc., from the oxygen consumption method by measuring the oxygen concentration in the combustion exhaust gas and the exhaust gas flow rate. The heat absorber 1 was placed at the top of the holder 32 as a test specimen, and a 50 kW / m radiator was applied from the cone 31. ２ Then, the change in temperature was measured by the thermocouple 33 on the back surface of the heat absorber 1 until the back surface of the heat absorber 1 reached 200°C. In addition to the temperature change, the presence or absence of combustion was also observed.

[0119] (4) Evaluation of Pressure Resistance of Test Specimens After Heating Experiments After conducting heating experiments using the cone calorimeter described above, the heat absorbers prepared in the present examples and comparative examples were evaluated for pressure resistance using the following method. Specifically, an indentation test was performed by placing the heat absorbers at room temperature (23°C) in a Tensilon universal testing machine ("RTE-1210" manufactured by Orientec Co., Ltd.) equipped with a 7 mmφ indentation jig. The indentation test measured the indentation amount when pressed at 0.5 MPa for 60 seconds (= when pressurized). From the indentation amount, the thickness change rate when pressed was calculated according to the following formula (II) and evaluated according to the following criteria. Note that the indentation test was performed at two locations on the surface of the heat absorber after heating 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 values ​​are shown in Table 1. Equation (II): Thickness change rate (%) = (thickness of heat absorber after pressing the surface of the heated heat absorber at 0.5 MPa for 60 seconds) / (thickness of heat absorber before pressing the surface of the heated heat absorber at 0.5 MPa for 60 seconds) × 100 (Evaluation criteria for thickness change rate) A thickness change rate in the range of 70 to 100% was rated as "◎". A thickness change rate in the range of 40 to 69% was rated as "◯". A thickness change rate in the range of 0 to 39% was rated as "×". Note that a thickness change rate in the range of 70 to 100% has the best pressure resistance. A lower value for the amount of pressing (= amount crushed) indicates better pressure resistance.

[0120] <Inorganic Powders> The inorganic powders used in the examples and comparative examples are as follows: Aluminum hydroxide: (product name "Aluminum Hydroxide Grade 1", manufactured by Kanto Chemical Co., Inc., solubility in 100 g of water at 20°C (g / 100 g): less than 0.1 g) Calcium sulfate dihydrate: (product name "Calcium sulfate dihydrate, special grade", manufactured by Kanto Chemical Co., Inc., solubility in 100 g of water at 20°C (g / 100 g): 0.2 g) Magnesium sulfate heptahydrate: (product name "Magnesium sulfate heptahydrate, special grade", manufactured by Kanto Chemical Co., Inc., solubility in 100 g of water at 20°C (g / 100 g): 71 g) The solubilities of the above hydrates in the anhydrous form are as follows: - Solubility of anhydrous magnesium sulfate in 100 g of water at 20°C (g / 100 g): 30 g - Solubility of anhydrous calcium sulfate in 100 g of water at 20°C (g / 100 g): 0.2 g Within the endothermic bag, magnesium sulfate heptahydrate exists as an anhydrous form, and calcium sulfate dihydrate exists as a hydrate.

[0121] <Bag> The water vapor permeability ([g / (m ２ ・24h)]) is 50g / (m ２ - We confirmed that the time is less than 24 hours.

[0122] Example 1 20 parts by mass of N,N-dimethylacrylamide (hereinafter abbreviated as "DMAA"), 4.8 parts by mass of water-swellable synthetic hectorite (manufactured by BYK Japan K.K., "Laponite RD"), 0.5 parts by mass of sodium peroxodisulfate (hereinafter abbreviated as "NPS"), and 0.8 parts by mass of N,N,N',N'-tetramethylethylenediamine (hereinafter abbreviated as "TEMED") were mixed and stirred in 100 parts by mass of pure water to obtain a uniform dispersion (a-1) (see Table 1). An aluminum pouch ("Gas Barrier Bag" manufactured by Mitsubishi Gas Chemical Company, Inc., 0.094 mm thick, composed of a laminate of PET, aluminum foil, and polyethylene) was formed into a bag-like container measuring 116 mm long x 116 mm wide. 50 parts by mass of the dispersion (a-1) and 50 parts by mass of calcium sulfate dihydrate powder were injected into the bag-like aluminum pouch, the inlet was closed by heat sealing, and the aluminum pouch was then placed flat between 4.8 mm thick gap materials. A flat plate was placed on top of the aluminum pouch and allowed to stand at 20 ° C for 15 hours to produce a 4.8 mm thick sheet-like heat absorber (E). The obtained heat absorber (E) was then evaluated according to the procedures described in the evaluation section above. The results are shown in Table 1 and FIG. 5. Furthermore, when the heat absorber (E) produced in Example 1 was heated to 200 ° C using a cone calorimeter, it was confirmed that it changed into a porous body at 150 ° C or higher.

[0123] Example 2 30 parts by mass of the dispersion (a-1) described in Example 1 was further mixed with 70 parts by mass of aluminum hydroxide to prepare a dispersion (a-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-shaped container measuring 116 mm long x 116 mm wide. Next, 100 parts by mass of the dispersion (a-2) was poured into the aluminum pouch, the inlet was closed by heat sealing, and the aluminum pouch was placed flat between 4.8 mm thick gap materials. A flat plate was placed on top of it and allowed to stand at 20 ° C for 15 hours to produce a 4.8 mm thick sheet-shaped heat absorber (F). The obtained heat absorber (F) was then evaluated according to the procedures described in the evaluation section above. The results are shown in Table 1 and FIG. 5. Furthermore, when the heat absorber (F) prepared in Example 2 was heated to 200°C using a cone calorimeter, it was confirmed that the heat absorber (F) prepared in Example 2 also changed into a porous body at 150°C or higher, similar to the heat absorber of Example 1.

[0124] (Example 3) 54 parts by mass of magnesium sulfate heptahydrate was dissolved in 100 parts by mass of the dispersion liquid (a-1) described in the Example 1 column to prepare a water-soluble inorganic powder-containing aqueous solution (3). Then, 91 parts by mass of the water-soluble inorganic powder-containing aqueous solution (3) and 9 parts by mass of ceramic wool (average porosity 96.9%, true density 3, bulk density 0.093) were filled into a container in the form of an aluminum pouch with a length of 116 mm and a width of 116 mm, and the inlet was closed by heat sealing. The aluminum pouch bag was placed flat between 4.8 mm thick gap materials, and a flat plate was placed on top and left to stand at 20 ° C. for 15 hours to prepare a 4.8 mm thick sheet-like heat absorber (G). Then, the obtained heat absorber (G) was subjected to each evaluation according to the procedures described in the evaluation column above. The results are shown in Table 1 and FIG. 5. Furthermore, when the heat absorber ((G)) prepared in Example 3 was heated to 200°C using a cone calorimeter, it was confirmed that the heat absorber ((G)) prepared in Example 3 also changed into a porous body at 150°C or higher, similar to the heat absorbers of Examples 1 and 2.

[0125] Comparative Example 1: In Comparative Example 1, a commercially available material, "Xiaomei silica aerogel mat material, thickness 4.8 mm (measured), thermal conductivity: 0.012 to 0.018 W / m·K," was used as the comparative sheet (A). The silica aerogel mat material of Comparative Example 1 had a nominal thickness of 3 mm. The obtained comparative sheet (A) was then subjected to various evaluations according to the procedures described in the evaluation section above. The results are shown in Table 1 and FIG. 5. Because the material of Comparative Example 1, "Xiaomei silica aerogel mat material," does not substantially have heat absorption capacity, it is marked "substantially no heat absorption capacity" in the table for the items "endothermic onset temperature (°C), endothermic peak temperature (°C), and endothermic amount (J / g or mJ / mg) of the heat absorber."

[0126] Comparative Example 2: 100 parts by mass of resin 1 ("Boncoat 5400EF" (product name, water-dispersible acrylic resin emulsion, nonvolatile content 50%, manufactured by DIC Corporation)), 6 parts by mass of foam stabilizer 1 ("DICNAL M-40" (product name, sulfonic acid-type anionic surfactant, manufactured by DIC Corporation)), and 3 parts by mass of crosslinker 1 ("DICNAL GX" (product name, oxazoline group-containing polymer, manufactured by DIC Corporation)) were blended and stirred and mixed in a disper (2000 rpm, 3 minutes) to prepare a mechanical foaming binder. 39 parts by mass of the prepared acrylic resin binder were stirred and foamed to an expansion ratio of 2 times, and 61 parts by mass of calcium sulfate dihydrate as an inorganic powder were blended thereto. Stirring was continued for an additional 5 minutes to obtain a foamable mixture. The resulting foamable mixture was applied to a polyethylene terephthalate (PET) film with an applicator. The sheet was then pre-dried at 105°C for 5 minutes, then heated at 120°C for 3 minutes, and then turned over and further heat-treated at 120°C for 3 minutes to harden it, producing a comparative sheet (B) (=resin foam sheet) with a thickness of 4.8 mm. Furthermore, the comparative sheet (B) of Comparative Example 2 was found to have burned in the combustion evaluation in the heating experiment using the cone calorimeter, and therefore was found to be unable to function as a heat insulating material.

[0127] (Comparative Example 3) An aluminum pouch ("Gas Barrier Bag" manufactured by Mitsubishi Gas Chemical Company, Inc., 0.094 mm thick, composed of a laminate of PET, aluminum foil, and polyethylene) was formed into a bag-like container measuring 116 mm long x 116 mm wide, and 100 parts by mass of the dispersion (a-1) described in Example 1 was poured into the bag-like body of the aluminum pouch to fill it. The inlet was then 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 allowed to stand at 20 ° C. for 15 hours to produce a 4.8 mm thick sheet-like heat absorber (C). The obtained heat absorber (C) was then evaluated according to the procedures described in the evaluation section above. The results are shown in Table 1 and FIG. 5.

[0128] Comparative Example 4: An aluminum pouch (a 0.094 mm thick "gas barrier bag" manufactured by Mitsubishi Gas Chemical Company, Inc., composed of a laminate of PET, aluminum foil, and polyethylene) was prepared as a bag measuring 116 mm long and 116 mm wide. Next, 33 parts by mass of pure water and 67 parts by mass of calcium sulfate dihydrate as an inorganic powder were filled into the aluminum pouch, and the inlet was closed by heat sealing. The aluminum pouch was then placed flat between 4.8 mm thick gap materials, and a flat plate was placed on top of it. The resulting sheet-like heat absorber (D) was then evaluated according to the procedures described in the evaluation section above. The results are shown in Table 1 and FIG. 5. In the system of Comparative Example 4, since DSC of a liquid could not be measured, data on the endothermic capacity (J / g or mJ / mg) could not be obtained. Therefore, the value calculated based on 2000 J / g (100°C), which is an approximation of the literature value of 2257 J / g, is entered in the table as 1170 J / g (specifically, the approximate value of the heat absorption of water, 2000 J / g x 33% + the measured heat absorption of calcium sulfate, 762 J / g x 67% = 1170 J / g).

[0129] The inorganic powder in Example 3 is water-soluble. In Table 1, the "content of inorganic powder (% by mass)" in the "contents (amount blended at the time of preparation)" indicates the content in the hydrated state, the "content of inorganic powder (% by mass)" in the "contents (inside the 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 the aqueous solvent at the time of preparation, including the amount of water removed from the hydrate of the inorganic powder. The inorganic powders used in Example 1 and Comparative Examples 2 and 4 were water-insoluble, and the "content of inorganic powder (% by mass)" in the "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 the aqueous solvent at the time of preparation.

[0130]

[0131] The heat absorbers (E) to (G) prepared in Examples 1 to 3 were sandwiched between two plates with a gap of 4.8 mm and placed vertically with the side of the heat absorbers (E) to (G) facing the bottom. Even after overnight, the heat absorbers (E) to (G) maintained their original position and shape. On the other hand, the heat absorber (D) of Comparative Example 4 was similarly sandwiched between two plates with a gap of 4.8 mm and placed vertically with the side of the heat absorber (D) facing the bottom. After one hour, the heat absorber (D) appeared to be crushed by its own weight and was unable to maintain its original position and shape. Because the heat absorber of this embodiment, the entire contents and the pouch act as a porous body, sufficient heat absorption effect, flame retardancy, and transformation from a heat absorber to a heat insulator were confirmed, even when the battery temperature rose or over time.

[0132] [Explanation of symbols] 1 heat absorber 10 battery element 11 positive electrode material layer 12 positive electrode current collector 13 positive electrode terminal 14 electrolyte layer or separator 15 negative electrode terminal 16 negative electrode material layer 17 negative electrode current collector 18a, b battery exterior material 19 activated carbon layer 20 stacked battery 30 cone calorimeter 31 cone (heater) 32 stainless steel holder 33 aluminum foil cover 34 thermocouple 35 stainless steel holder 36 ceramic wool (large) 37 ceramic wool (small)

Claims

1. A heat absorber comprising: a bag capable of being filled with a content; and a hydrogel and an inorganic powder that are filled into the bag as the content.

2. The heat sink according to claim 1, wherein the hydrogel is composed of a hydrogel body and an aqueous solvent.

3. The heat absorber according to claim 1 or 2, further comprising an antifreeze agent as the content.

4. Water vapor permeability ([g / (m ２ ・24h)] is 50g / (m ２ The heat absorber according to claim 1 or 2, wherein the heat absorption time is 24 h or less.

5. The heat absorber according to claim 2, wherein the hydrogel body is made of at least a water-soluble organic monomer and a water-swellable clay mineral as reaction raw materials.

6. The endothermic start temperature is 400°C or less, and the endothermic peak temperature is at least in the range of 80°C to 400°C, and the thickness change rate (%) is calculated by the following formula (II): [Mathematical formula 1] "Thickness change rate (%) = (thickness of the heat absorber after pressing the surface of the heat absorber heated under the heating conditions below at 0.5 MPa for 60 seconds) / (thickness of the heat absorber before pressing the surface of the heat absorber heated under the heating conditions below at 0.5 MPa for 60 seconds) x 100" Heating conditions: "50 kW / m by radiant heat ２ The heat absorber was heated with a heat quantity of 10 ...

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

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