Heat absorber and secondary battery module

Abstract

The present disclosure addresses the problem of providing a heat absorber that can absorb the expansion of a battery cell by being sufficiently deformed in accordance with the expansion of the battery cell and has a restoring force for restoring the heat absorber to a shape before deformation. The solution is a heat absorber (1) comprising: a content (4) containing an aqueous solvent; a bag body (2) having a filling section (5) filled with the content (4) and an accommodation section (6) capable of accommodating the content (4) deformed by the application of an external force to the filling section (5); and a restoration member (3) that returns the content (4) accommodated in the accommodation section (6) to the filling section (5), wherein a deformed state in which the content (4) is accommodated in the accommodation section (6) due to the deformation of the filling section (5) caused by the external force, and a normal state in which the content (4) accommodated in the accommodation section (6) is returned into the filling section (5) by the restoration member (3) are reversible.

Description

Heat absorber and secondary battery module 【0001】 This disclosure relates to a heat absorber and a secondary battery module. This application claims priority to Japanese Patent Application No. 2024-217115, filed in Japan on December 11, 2024, and the entire disclosure of the earlier application is incorporated herein by reference. 【0002】 Rechargeable batteries, which allow for control of the time difference between charging and discharging, are used in a variety of applications, including automobiles and mobile devices. However, rechargeable batteries, such as lithium-ion batteries, are susceptible to thermal runaway and battery damage if their temperature rises due to heat generated during high-speed charging or high-power discharging. Therefore, technologies to improve the safety of rechargeable batteries are being developed. 【0003】 For example, Patent Document 1 discloses a partition member having a thickness direction and a surface direction perpendicular to the thickness direction, which separates the individual cells constituting the battery pack, or members other than the individual cells constituting the battery pack, in the thickness direction, and which includes a liquid, an insulating material, and an outer casing that contains the liquid and the insulating material. 【0004】 Japanese Patent Publication No. 2020-161290 【0005】 In secondary batteries, battery cells repeatedly expand and contract with charging and discharging. Therefore, components provided around battery cells, particularly components provided between battery cells such as partition members as described in Patent Document 1, are required to not only enhance the safety of secondary batteries but also to deform in accordance with the expansion of adjacent battery cells and absorb the volume of the expanded battery cells. On the other hand, when the expanded battery cells contract, the components are required to return to their original shape. However, with conventional components, the amount of deformation due to the expansion of adjacent battery cells is insufficient, resulting in inadequate absorption of the volume of the expanded adjacent battery cells. Furthermore, when the expanded battery cells contract, the shape of the components sometimes did not return to their original shape. 【0006】Therefore, the object of this disclosure is to solve the problems of the above-mentioned prior art and to provide a heat absorber that can deform sufficiently in response to the expansion of an adjacent battery cell to absorb the volume of the expansion of the battery cell, and that has a restoring force that restores it to its original shape, and a secondary battery module equipped with the heat absorber. 【0007】 The present inventors, in order to solve the above problems, conducted diligent studies and found that the above problems can be solved by a heat-absorbing body equipped with a bag body having a filling section into which contents are filled, and a storage section capable of accommodating the contents that have been deformed by the application of an external force to the filling section, and a restoring member, so that the contents are contained in the storage section as the filling section is deformed by an external force, and the contents contained in the storage section are returned to the filling section by the restoring member, thereby completing the present invention. The gist of the present disclosure that solves the above problems is as follows. 【0008】 [1] A heat absorber comprising: contents containing an aqueous solvent; a bag body having a filling section into which the contents are filled, and a storage section capable of accommodating the contents after they have been deformed by the application of an external force to the filling section; and a restoring member that returns the contents stored in the storage section back to the filling section, wherein the deformed state in which the contents are stored in the storage section due to the deformation of the filling section by the external force, and the normal state in which the contents stored in the storage section are returned to the filling section by the restoring member, are reversible. 【0009】 [2] The heat-absorbing body according to [1], wherein the housing expands in conjunction with the storage of the contents, and the expanded housing contracts due to the restoration member. 【0010】 [3] The heat-absorbing element according to [1] or [2], wherein the maximum capacity of the housing portion is 5% by volume or more and 100% by volume or less of the maximum capacity of the filling portion. 【0011】 [4] The heat-absorbing material according to any one of [1] to [3], wherein the maximum capacity of the filling portion is 50% by volume or more and 95% by volume or less of the maximum capacity of the bag body. 【0012】[5] The heat-absorbing body according to any one of [1] to [4], wherein the restoring member is an elastic member or a pair of magnets that contract the expanded housing to control the volume of the housing. 【0013】 [6] The heat-absorbing body according to [5], wherein the bag body is in the form of a sheet, and the elastic member or the pair of magnets are attached in contact with the surface of the bag body so as to extend in the in-plane direction of the surface of the bag body. 【0014】 [7] The heat-absorbing body according to [5] or [6], wherein the elastic member is a leaf spring. 【0015】 [8] The endotherm according to [1], wherein the contents are a hydrogel containing the aqueous solvent having voids, and the hydrogel is the restorative member. 【0016】 [9] The deformed state in which the deformed portion due to the deformation of the hydrogel is contained in the void which is the containment portion is reversible to the normal state by the restoration of the hydrogel, as described in [8]. 【0017】

[10] The heat-absorbing body according to [8] or [9], wherein the void is an open void that is open to the surface of the hydrogel. 【0018】

[11] The heat-absorbing body according to [8] or [9], wherein the void is an independent void that is closed off inside the hydrogel. 【0019】

[12] The heat-absorbing body according to any one of [1] to

[11] , wherein the contents further comprise an inorganic powder. 【0020】

[13] The heat-absorbing body according to any one of [1] to

[12] , wherein the contents further contain one or more selected from the group consisting of antifreeze agents and inorganic fibers. 【0021】

[14] The heat-absorbing body according to any one of [1] to

[13] , wherein the contents change into a porous body when heated to 120°C or higher. 【0022】

[15] A secondary battery module equipped with a heat-absorbing element as described in any of [1] to

[14] . 【0023】A secondary battery module in which the heat absorber according to any one of [1] to

[14] is sandwiched between battery cells. 【0024】 According to the present disclosure, it is possible to provide a heat absorber that can be sufficiently deformed as the adjacent battery cells expand to absorb the volume of the expanded battery cells and has a restoring force that restores to the shape before deformation, and a secondary battery module including the heat absorber. 【0025】Figure 1A is a schematic front view of a heat absorber in its normal state according to the first embodiment of this disclosure. Figure 1B is a schematic front view of a heat absorber in its deformed state according to the first embodiment of this disclosure, showing the state in which an external force F is applied to the heat absorber shown in Figure 1A. Figure 2A is a schematic cross-sectional view taken along line A-A of Figure 1A. Figure 2B is a schematic cross-sectional view taken along line A-A of Figure 1B. Figure 3A is a schematic front view of a heat absorber in its normal state according to the second embodiment of this disclosure. Figure 3B is a schematic front view of a heat absorber in its deformed state according to the second embodiment of this disclosure, showing the state in which an external force F is applied to the heat absorber shown in Figure 3A. Figure 4A is a schematic cross-sectional view taken along line B-B of Figure 3A. Figure 4B is a schematic cross-sectional view taken along line B-B of Figure 3B. Figure 5A is a schematic front view showing one modified example of a heat absorber in its normal state according to the first embodiment of this disclosure. Figure 5B is a schematic front view of a heat absorber in a deformed state according to the first embodiment of this disclosure, showing the state in which an external force F is applied to the heat absorber shown in Figure 5A. Figure 6A is a schematic cross-sectional view taken along line C-C of Figure 5A. Figure 6B is a schematic cross-sectional view taken along line C-C of Figure 5B. Figure 7 is a schematic top view of a heat absorber according to the first embodiment of this disclosure. Figure 8 is a schematic cross-sectional view showing one modified example of a heat absorber according to the first embodiment of this disclosure. Figure 9A is a schematic cross-sectional view showing one modified example of a heat absorber according to the second embodiment of this disclosure. Figure 9B is a schematic cross-sectional view showing one modified example of a heat absorber according to the second embodiment of this disclosure. Figure 10A is a schematic perspective view showing one modified example of a hydrogel filled inside a heat absorber according to the second embodiment of this disclosure. Figure 10B is a schematic perspective view showing one modified example of a hydrogel filled inside a heat absorber according to the second embodiment of this disclosure. Figure 11A is a schematic perspective view showing one modified example of a hydrogel filled inside a heat absorber according to the second embodiment of this disclosure. Figure 11B is a schematic perspective view showing one modified example of the hydrogel filled in the endothermic body according to the second embodiment of this disclosure. Figure 12 is a schematic example of a secondary battery module on which the endothermic body of this embodiment can be mounted. Figure 13 is a schematic perspective view showing the secondary battery module of Figure 12 disassembled. 【0026】 The heat-absorbing element and secondary battery module of this disclosure will be described in detail below based on their embodiments. In each figure, identical components are denoted by the same reference numerals. 【0027】<Definition> In this specification, "normal state" refers to the state of the heat absorber when no external force is applied to the heat absorber. In this specification, "deformed state" refers to the deformed state of the heat absorber when an external force is applied to the heat absorber. "Hydrogel" in this specification is a material in which a three-dimensional network of polymers contains an aqueous solvent such as water. For example, jelly, a diaper absorber, konjac, agar, etc. fall under this category. The three-dimensional network polymer that is the backbone of the hydrogel is referred to as the hydrogel body, and the aqueous solvent is present inside the hydrogel body. Therefore, a hydrogel has a hydrogel body and an aqueous solvent. The z-axis direction appropriately shown in each figure is the vertical direction with the positive side being the "upper side" and the negative side being the "lower side", and the positive direction of the z-axis may be referred to as the upward direction and the negative direction of the z-axis may be referred to as the downward direction. Also, these may be collectively referred to as the vertical direction. The x-axis direction appropriately shown in each figure is the thickness direction (sometimes also referred to as the front-back direction) with the positive side being the "front side" and the negative side being the "rear side", and the positive direction of the x-axis may be referred to as the front direction and the negative direction of the x-axis may be referred to as the rear direction. Also, these may be collectively referred to as the front-back direction. The y-axis direction appropriately shown in each figure is the lateral direction with the positive side being the "right side" and the negative side being the "left side", and the positive direction of the y-axis may be referred to as the right direction and the negative direction of the y-axis may be referred to as the left direction. 【0028】 <Heat Absorber> The heat absorber of the present disclosure essentially has a bag body main body, a restoring member, and a content containing an aqueous solvent, and is mainly classified into an aspect in which the restoring member is provided on the bag body main body and an aspect in which the restoring member is provided on the content itself (that is, the content can be the content and also the restoring member). For convenience of explanation, the aspect in which the restoring member is provided on the bag body main body is taken as the first embodiment, and the aspect in which the restoring member is provided on the content is taken as the second embodiment, and the heat absorber of the present disclosure will be described below. 【0029】 <First Embodiment> Hereinafter, the configuration of the heat absorber according to the first embodiment of the present disclosure will be described while referring to FIGS. 1 and 2. 【0030】Figure 1 shows an example of a schematic front view of the heat absorber, and Figure 2 shows an example of a cross-sectional view of the heat absorber in Figure 1, cut along line A-A. Figures 1A and 2B show an example of the normal state of the heat absorber 1 when no external force F is applied to it, and Figures 1B and 2B show an example of the deformed state when an external force F is applied to the heat absorber 1 and the heat absorber 1 is deformed. In other words, the heat absorber 1 according to the first embodiment comprises a contents 4 containing an aqueous solvent, a bag body 2 having a filling section 5 into which the contents 4 are filled, and a storage section 6 capable of accommodating the contents 4 deformed by the application of an external force F to the filling section 5, and a restoring member 3 that returns the contents 4 contained in the storage section 6 back to the filling section 5, characterized in that the deformed state in which the contents 4 are contained in the storage section 6 due to the deformation of the filling section 5 by the external force F, and the normal state in which the contents 4 contained in the storage section 6 are returned to the filling section 5 by the restoring member 3 are reversible. As a result, it can deform sufficiently in response to the expansion of the battery cell to absorb the volume of the battery cell's expansion, and a restoring force acts to restore it to its pre-deformation shape. Note that "the volume of the battery cell's expansion" may refer to the entire volume of the battery cell's expansion, or to a part of the volume of the battery cell's expansion. Also, "restoring force" refers to the action of returning to the pre-deformation shape or normal state. 【0031】As shown in Figures 1 and 2, the heat absorber 1 comprises a bag body 2, a restoration member 3, and contents 4 (not shown in Figure 1). More specifically, (omitted) the contents 4 (hereinafter also simply referred to as "contents 4") containing an aqueous solvent are filled and / or contained inside the rectangular bag body 2 (filling section 5 and / or storage section 6), and the restoration member 3 is attached to one end of the bag body 2. As shown in Figures 1A and 2A, when no external force F is applied to the heat absorber 1 (normal state of the heat absorber 1), the contents 4 are mainly filled inside the filling section 5 of the bag body 2. Also, in the normal state of the heat absorber 1, the storage section 6 capable of containing the contents 4 is closed by the restoration member 3 and does not have much internal space capable of containing the contents 4. In Figures 1A and 2A, for the sake of explanation, an example of the containment section 6 is shown in which a portion of the opposing inner surfaces of the bag body 2 are brought into contact with each other by the restoring member 3 at the end of the bag body 2 (or the bag body 2 is held by the restoring member 3 so that the opposing inner surfaces of the bag body 2 are in contact), and there is substantially no internal space (space) in which the contents 4 can be contained. However, the volume of the internal space (space) of the containment section 6 in the normal state of the heat absorber 1 may be less than the volume of the filling section 5 in the normal state of the heat absorber 1. Therefore, the opposing inner surfaces of the bag body 2 may be spaced apart. Furthermore, although the restoring member 3 will be described later, the restoring member 3 refers to a mechanism or member that has the effect of returning to the shape before deformation or to the normal state. For example, by using a leaf spring or a pair of sheet-shaped magnets as the restoring member 3, a portion of the opposing inner surfaces of the bag body 2 can be brought into contact. 【0032】Next, as shown in Figures 1B and 2B, when an external force F is applied to the heat absorber 1 in its normal state to deform it (deformed state of the heat absorber 1), the volume of the internal space of the storage section 6 expands due to the pressure of the contents 4 deformed by the application of the external force F to the filling section 5 (Figure 2B shows that the storage section 6 expands in the direction of the arrow (outward direction of the bag body 2)). In other words, as the filling section 5 deforms, at least a portion of the contents 4 containing the aqueous solvent in the filling section 5 flows into the storage section 6, and the internal space of the storage section 6 expands in proportion to the amount of at least a portion of the contents 4 in the filling section 5 that flows into the storage section 6. Therefore, it is considered that, for example, it can deform sufficiently in response to the expansion of an adjacent battery cell to form a space that can accommodate the volume of the expanded battery cell (i.e., it can absorb the volume). In this case, the volume of the expanded internal space of the storage section 6 may depend on the magnitude of the external force F and the contact force between opposing inner surfaces of the bag body 2 by the restoring member 3 (for example, the attractive force of a pair of magnets or the elastic force of a leaf spring). 【0033】 When the external force F applied to the heat absorber 1 in its normal state, as shown in Figures 1B and 2B, is reduced or removed, the internal space of the housing 6 is reduced by the restoring member 3 (for example, the attractive force between the north and south poles of a pair of magnets or the elastic force of a leaf spring), and consequently the contents 4 housed in the housing 6 move (return) to the filling section 5. Therefore, when the applied external force F is reduced or removed, the heat absorber 1 returns to its normal state as shown in Figures 1A and 2A. Furthermore, when the external force F is applied to this restored heat absorber 1 in its normal state, it returns to the deformed state of the heat absorber 1 as described above, as shown in Figures 1B and 2B. Therefore, the normal state heat absorber 1 and the deformed state heat absorber 1 are reversible due to the external force F and the restoring member 3. Therefore, when the heat-absorbing body 1 of this embodiment is placed between the battery cells of a stack of battery cells, it deforms sufficiently in response to the expansion of the battery cells adjacent to the heat-absorbing body 1, absorbing the volume of the expanded battery cells, and a restoring force acts to restore it to its original state. 【0034】[Bag Body] The bag body 2 of this disclosure is filled with and / or contains contents 4. The bag body 2 has an internal space that is closed off from the outside, and the contents 4 are filled with and / or contained within this internal space. This prevents the contents 4 from flowing out to the outside. As shown in Figures 2A and 2B, the bag body 2 has a filling section 5 into which the contents 4 are filled, and a storage section 6 that can accommodate the contents 4 that have been deformed by the application of an external force F to the filling section 5 (so-called overflowing contents 4). In addition, a restoration member 3 is attached to the outer surface of the bag body 2, and the contents 4 are filled with and / or contained on the inner surface side (or inside) of the bag body 2. In its normal state, the bag body 2 contains the contents 4 in the filling section 5, and in its deformed state, it contains the contents 4 in the filling section 5 and the storage section 6, or in the storage section 6. In other words, inside the bag body 2, there exists a filling section 5, which is an internal space filled with contents 4 (when no external force F is applied), and a storage section 6, which can form an internal space capable of accommodating contents 4 deformed by the application of external force F to the filling section 5 (so-called overflowing contents 4). Under normal conditions, the internal space of the storage section 6 exists inside the bag body 2 in a contracted state. On the other hand, in a deformed state, the internal space of the storage section 6 expands to accommodate contents 4 deformed by the application of external force F to the filling section 5 (so-called overflowing contents 4). The bag body 2 shown in Figures 1 and 2 is an example of an embodiment, and is a three-sided bag having an opening at one end and a body with a closed lower end, and having a structure in which the opening is heat-sealed so that it is sealed after the contents 4 are contained. However, the form of the bag body 2 is not limited to this. The three-sided bag is constructed by joining the lower ends and sides of two sheets together, and then sealing it after filling the contents 4 through the opening. As such, it has excellent airtightness and, due to its sheet-like shape, is easy to insert between battery cells. 【0035】The bag body 2 is a sheet-like bag. "Sheet-like" refers to a shape with a thin thickness relative to its planar area. Furthermore, the restoration member 3 is attached to the surface of the bag body 2, in contact with it, so as to extend in the in-plane direction of the bag body 2's surface. More specifically, as shown in Figure 1, the restoration member 3 is attached to at least one end of the bag body 2, in contact with it, so that its longitudinal direction extends along a direction perpendicular to the longitudinal direction of the bag body 2 (approximately). Therefore, the restoration member 3 may be attached to both ends of the sheet-like bag body 2 in the longitudinal direction. Further details regarding the material of the bag body 2 will be described later. 【0036】 [[Filling Section]] The filling section 5 of this disclosure is the part (or internal space) where the contents 4 containing an aqueous solvent are mainly filled when no external force F is applied. As shown in Figure 2A, under normal conditions, the filling section 5 is located inside the bag body 2 and is a part (or internal space) that is closed off from the outside of the bag body 2 by sealing the bag body 2 or the like. On the other hand, as shown in Figure 2B, in a deformed state, the filling section 5 is in communication with the containment section 6, which is an expanded internal space. For example, in Figure 2B, an example of the filling section 5 is shown, which is the internal space inside the bag body 2 and is below the dotted line. More specifically, the filling section 5 in Figure 2B is the area surrounded by the plane including the dotted line and the inner surface of the bag body 2 below the dotted line. 【0037】The maximum capacity (= volume) of the filling section 5 is preferably 50% to 95% of the maximum capacity of the bag body 2. When the maximum capacity of the filling section 5 is within the above range, it can sufficiently absorb the expansion of the battery cells. From the viewpoint of the heat absorption capacity of the heat absorber, the maximum capacity of the filling section 5 is preferably 55% or more, more preferably 60% or more, and even more preferably 70% or more, of the maximum capacity of the bag body 2. Furthermore, from the viewpoint of absorbing the expansion of the battery cells, the maximum capacity of the filling section 5 is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less, of the maximum capacity of the bag body 2. These upper and lower limits can be combined as appropriate. For example, the maximum capacity of the filling section 5 is preferably 55% to 90% of the maximum capacity of the bag body 2, more preferably 60% to 85%, and even more preferably 70% to 80%. 【0038】 [Storage Section] The storage section 6 of this disclosure is a portion (or internal space) capable of accommodating contents 4 containing an aqueous solvent when the storage section 6 is deformed by the application of an external force F to the heat absorber 1 or the filling section 5. In one example of the embodiment shown in Figures 1A and 2A, in the normal state of the heat absorber 1, the storage section 6 is a contracted region in which the bag body 2 is held between opposing inner surfaces of the bag body 2 by the restoration member 3 described later, and there is virtually no internal space. On the other hand, in the deformed state of the heat absorber 1, as shown in Figures 1B and 2B, the bag body 2 expands in the direction of the solid black arrow due to the movement of the contents 4 deformed by the application of an external force to the heat absorber 1 or the filling section 5, and an internal space as the storage section 6 is formed. At this time, the expanded internal space, the storage section 6, and the filling section 5 are in communication, and at least a portion of the contents 4 in the filling section 5 are accommodated in the internal space formed as the storage section 6. For example, in Figure 2B, an example of the containment section 6 is shown, which is the internal space inside the bag body 2 and the area above the dotted line. More specifically, the filling section 5 in Figure 2B is the area enclosed by the plane containing the dotted line and the inner surface of the bag body 2 above the dotted line. 【0039】As shown in Figures 2A and 2B, the storage section 6 expands (or forms an internal space) as the contents 4 flow in (or move), and the expanded storage section 4 contracts due to the restoration member 3, which will be described later. The detailed mechanism of the restoration member 3 will be described later. 【0040】 The maximum capacity (= volume) of the storage section 6 is preferably 5% to 100% of the maximum capacity of the filling section 5. When the maximum capacity of the storage section 6 is within the above range, it can sufficiently absorb the expansion of the battery cells. From the viewpoint of sufficiently absorbing the expansion of the battery cells, the maximum capacity of the storage section 6 is preferably 10% or more, more preferably 15% or more, and even more preferably 20% or more, of the maximum capacity of the filling section 5. Furthermore, from the viewpoint of the heat absorption capacity of the heat absorber, the maximum capacity of the storage section 6 is preferably 95% or less, more preferably 90% or less, and even more preferably 80% or less, of the maximum capacity of the filling section 5. The above upper and lower limits can be combined as appropriate. For example, the maximum capacity of the storage section 6 is preferably 10% to 95% of the maximum capacity of the filling section 5, more preferably 15% to 90%, and even more preferably 20% to 80%. 【0041】 [Contents] The contents 4 of this disclosure must contain an aqueous solvent. As will be described later, the contents 4 may also contain components other than the aqueous solvent (any additive components). As shown in Figure 2A, in the normal state of the heat absorber 1, the contents 4 are filled into the filling section 5 in the bag body 2. On the other hand, as shown in Figure 2B, when an external force is applied to the heat absorber 1 or the filling section 5, at least a portion of the contents 4 moves along with the deformation of the filling section 5, so the contents 4 are contained in the filling section 5 and the storage section 6, or in the storage section 6. Further details regarding the substances that may be contained in the contents 4 will be described later. 【0042】[Restoration Member] The restoration member 3 of this disclosure is not particularly limited as long as it is a mechanism, member, means, or element that restores the deformed heat absorber 1, which has been subjected to an external force F, to its normal state as a heat absorber 1 without the external force F being applied. In other words, the restoration member 3 may be a mechanism, member, means, or element that returns the contents 4 containing an aqueous solvent (for example, 50% by mass or more of the contents 4 contained in the containment section 6) to the filling section 5. Figures 1 and 2 show, as an example of the restoration member 3, a member that has the function of closing (or contracting) the internal space inside the bag body 2, particularly the internal space that can become the containment section 6, in the case of a normal heat absorber 1. Examples of such members include a leaf spring, an elastic member such as (wound) rubber, or a pair of plate-shaped magnets. More specifically, when an external force F is applied to the heat-absorbing body 1 in its normal state, the pressure of the contents 4 (for example, the inflow of contents 4 containing an aqueous solvent into the containment section 6 due to the deformation of the filling section 5) becomes greater than the contraction force of the restoring member 3, causing an internal space to be formed or the internal space to expand. When returning from the deformed state to the normal state of the heat-absorbing body 1, for example, in the examples of Figures 1 and 2, the restoring force of the restoring member 3 acts in the inward direction of the bag body 2, causing the bag body 2, which has expanded or widened in the outward direction, to contract. As a result, the restoring member 3 has the effect of returning the contents 4 contained in the containment section 6 back to the filling section 5. For example, in the examples of Figures 1 and 2, the restoring member 3 is attached to the upper end of the bag body 2 in contact with the bag body 2 such that the longitudinal direction of the restoring member 3 extends in the short direction of the bag body 2. 【0043】 In a preferred embodiment where the restoring member 3 is provided on the bag body, it is preferable that the restoring member 3 is an elastic member. The elastic member may be a member that can reduce the volume of the housing 6 by contracting the volume of the housing 6 which has expanded or expanded by elastic force. The elastic member is preferably, for example, a leaf spring. 【0044】(Effects) As shown in Figures 1A and 2A, when no external force is applied to the heat absorber 1 (normal state), the containment section 6 is held in place by the restoration member 3. Therefore, the internal space for containing the contents 4 as the containment section 6 is closed in a sense, and there is virtually no internal space for containing the contents 4 as the containment section 6. For this reason, in the normal state of the heat absorber 1, the contents 4 are not substantially contained in the containment section 6, but are mainly filled in the filling section 5. The containment section 6 and the filling section 5 are fluidly connected. Therefore, the internal space of the filling section 5 and the internal space for containing the contents 4 as the containment section 6 are in communication. When an external force F is applied to the filling section 5 or the heat absorber 1, the filling section 5 itself may deform due to the external force F. When the filling section 5 deforms, the volume of the filling section 5 during deformation becomes smaller than the volume of the filling section 5 in the normal state. As a result, the contents 4 containing the aqueous solvent corresponding to the decrease in the volume of the filling section 5 flow out into the inside of the bag body 2. As a result, pressure is applied in the direction in which the contents 4 flow out. Furthermore, when this pressure exceeds the contraction force of the restoring member 3 acting on the containment section 6, which is in communication with the filling section 5, the contents 4 move into the containment section 6, which is also a so-called evacuation area for the flowing contents 4. Under normal conditions, the containment section 6 is closed by the restoring member 3, but as the contents 4 move (or flow in), a force acts outward (in the direction of the solid black arrow in Figure 2B) that is greater than the force exerted inward on the bag body 2 by, for example, the elastic force of the restoring member 3, causing the containment section 6 to expand or widen. As a result, the contents 4 are contained in the containment section 6 while an external force is applied to the filling section 5 (in a deformed state). 【0045】 Thus, according to the first embodiment of the heat absorber of this disclosure, when a force is applied to the filling portion 5, the contents 4 can be moved by an amount corresponding to the deformation of the filling portion 5, so that the heat absorber can be sufficiently deformed. In other words, the heat absorber of the first embodiment of this disclosure can be sufficiently deformed in response to the external force on the heat absorber due to the expansion of the battery cell and absorb the expansion of the battery cell. 【0046】Furthermore, after an external force F is applied to the filling section 5, when the external force F is reduced or removed, the restoring force (e.g., elastic force) of the restoring member 3 acts inward on the bag body 2, causing the expanded or enlarged storage section 6 to contract and the contents 4 stored in the storage section 6 to return to the filling section 5. 【0047】 Thus, when an external force is applied, the filling section 5 deforms, and after the contents 4 are contained in the storage section 6, a restoring force acts to return to the pre-deformation state (normal state) when the external force F is reduced or removed. In other words, according to the heat absorber of the first embodiment of this disclosure, a restoring force acts to restore it to its pre-deformation shape. Therefore, according to the heat absorber of the first embodiment of this disclosure, it can deform sufficiently in response to the expansion of the battery cell to absorb the expansion of the battery cell, and a restoring force acts to restore it to its pre-deformation shape. 【0048】 (Second Embodiment) Hereinafter, a different configuration of the heat absorber according to the second embodiment of the present disclosure will be described with reference to Figures 3 and 4. Matters common to the heat absorber according to the first embodiment will not be explained. Figure 3 shows an example of a schematic front view of the heat absorber 1, and Figure 4 shows an example of a cross-sectional view obtained by cutting the heat absorber 1 of Figure 3 along the line B-B. Figures 3A and 4A show an example of the normal state of the heat absorber 1 when no external force is applied to it, and Figures 3B and 4B show an example of the deformed state of the heat absorber 1 when an external force F is applied to it and the heat absorber 1 is deformed. 【0049】 In other words, the heat absorber 1 according to the second embodiment has a hydrogel (hereinafter sometimes simply referred to as "hydrogel") containing an aqueous solvent and a hydrogel body, which has voids, as its contents 4, and the hydrogel is a restorative member. As a result, it can deform sufficiently in response to the expansion of the battery cell to absorb the volume of the expanded battery cell, and a restorative force acts to restore it to its original shape. In the heat absorber 1 according to the second embodiment, the hydrogel is both the contents 4 and also functions as a restorative member (that is, the contents can be both contents and a restorative member). 【0050】As shown in Figures 3 and 4, the heat absorber 1 comprises a bag body 2, contents 4 (not shown in Figure 3) containing an aqueous solvent, and a restoration member. The contents 4 containing the aqueous solvent have voids 7 and contain a hydrogel body and a hydrogel containing the aqueous solvent. Furthermore, the restoration member is a hydrogel. More specifically, (omitted) the contents 4 containing the aqueous solvent, consisting of a hydrogel body and a hydrogel containing the aqueous solvent, are filled and / or housed inside a rectangular bag body 2. The hydrogel also has voids 7 and 7b as notches. In Figures 3 and 4, hexahedral voids 7 and 7b are shown as an example of voids 7, but the shape of the voids 7 and 7b is not particularly limited. 【0051】 As shown in Figures 3A and 4A, when no external force F is applied to the heat absorber 1 (the normal state of the heat absorber 1), the contents 4, consisting of a hydrogel with voids 7 and 7b, is filled into the filling section 5 of the bag body 2. In Figure 3A, the approximate location and shape of the voids 7 are shown by dotted lines. In the case of the heat absorber 1 according to the second embodiment, in which a restoring member is provided in the contents 4, as shown in Figures 3 and 4, the filling section 5 corresponds to the region within the bag body 2 that excludes the hydrogel from the hydrogel containing the aqueous solvent, the three-dimensional network structure (so-called cross-linked polymer chains) which is the hydrogel body, and the optional components described later. In other words, the region in the hydrogel within the bag body 2 where the aqueous solvent and the optional components described later exist corresponds to the filling section 5. The voids 7 and 7b, which are also notches in the hydrogel, correspond to the containment section 6. The restoring member is the hydrogel, and more specifically, the restoring member mainly corresponds to the hydrogel body. Because the hydrogel body has a three-dimensional network structure, it is not only deformable but also possesses a restorative force. As a result, it can deform sufficiently in response to the expansion of adjacent battery cells, absorbing the volume of the expanded battery cells, and a restorative force acts to restore it to its original shape. Therefore, in the case of the heat absorber 1 according to the second embodiment, as shown in Figures 3 and 4, in which a restorative member is provided in the contents 4, even in the normal state of the heat absorber 1, the housing portion 6 capable of accommodating the contents 4 has internal space as voids 7 and 7b. 【0052】Next, as shown in Figures 3B and 4B, when an external force F is applied to the normal state of the heat absorber 1 to deform it (deformed state of the heat absorber 1), the hydrogel deforms due to the application of the external force F, and the deformed hydrogel or the aqueous solvent absorbed within the hydrogel moves into the voids 7, 7b which are the containment parts 6. Then, when the external force F applied to the normal state of the heat absorber 1 shown in Figures 3B and 4B is reduced or removed, it returns to its original shape due to the elastic force or restoring force (= restoring member) of the hydrogel itself. Furthermore, when an external force F is applied to this restored normal state of the heat absorber 1, it returns to the deformed state of the heat absorber 1 shown in Figures 3B and 4B as described above. Therefore, the normal state of the heat absorber 1 and the deformed state of the heat absorber 1 are reversible due to the external force F and the restoring force of the hydrogel itself. Therefore, when the heat-absorbing body 1 of this embodiment is placed between the battery cells of a stack of battery cells, it deforms sufficiently in response to the expansion of the battery cells adjacent to the heat-absorbing body 1, absorbing the volume of the expanded battery cells, and a restoring force acts to restore it to its original state. 【0053】 [Bag body] Bag body 2 contains a hydrogel containing an aqueous solvent, a hydrogel body, and an optional additive component. 【0054】 As shown in Figure 3, the bag body 2 is a sheet-like bag. Note that "sheet-like" refers to a shape where the thickness is thin relative to the planar area. 【0055】 [[Filling Section]] In this second embodiment, the filling section 5 is the portion of the bag body 2 in its normal state where the hydrogel exists. In the heat-absorbing body 1 according to the second embodiment, as shown in Figures 3 and 4, the filling section 5 coincides with the portion where the contents 4 exist. That is, the shape of the filling section 5 and the shape of the contents 4 are the same. 【0056】[[Storage Section]] In this second embodiment, the storage section 6 is a space capable of accommodating the hydrogel (or the aqueous solvent absorbed by the hydrogel and any optional components described later) when the endothermic body 1 is deformed or the hydrogel, which is the contents 4, is deformed. In the endothermic body 1 according to this second embodiment, the storage section 6 corresponds to the voids 7, 7b of the hydrogel, which is the contents 4. More specifically, in Figure 4, the storage section 6 is a space surrounded by the hydrogel and the inner surface of the bag body 2. In the endothermic body according to this second embodiment, the shape of the storage section 6 is not particularly limited; for example, in Figure 4, it is a roughly rectangular parallelepiped shape that follows the shape of the notch in the hydrogel. 【0057】[Contents] In the heat absorber 1 according to this second embodiment, the contents 4 essentially include a hydrogel containing a hydrogel body and an aqueous solvent having voids 7, 7b, and may further contain optional components that are added as needed. In the heat absorber 1 according to the second embodiment, the hydrogel is the contents 4, but also serves as a restorative member. More specifically, the hydrogel body contained in the hydrogel is a polymer having a three-dimensional network structure, and therefore has the role of an elastic body. For this reason, when the heat absorber 1 (or hydrogel) is deformed by applying an external force F to the heat absorber 1 in its normal state and returns to its normal state, the restorative force of the hydrogel (i.e., the elastic force of the hydrogel body) restores the hydrogel from its deformed shape to its normal shape before deformation. Furthermore, because the hydrogel (or the hydrogel body) has voids 7, 7b that serve as the containment portion 6, a portion of the deformed hydrogel (or a portion of the aqueous solvent and / or any additive components contained within the hydrogel) can enter the voids 7, 7b when an external force F is applied (or occupy at least a portion of the voids 7, 7b). In this disclosure, the voids 7, 7b that are open to the surface of the hydrogel in the hydrogel shown in Figure 4 are referred to as open voids. In Figure 4, an example is shown in which the void 7b is formed on the surface of the hydrogel as a so-called notch, but similar to the closed-cell porous body shown in Figure 9 described later, the voids 7, 7b may exist as closed voids inside the hydrogel. Such closed voids inside the hydrogel, i.e., voids that exist closed within the hydrogel, are referred to as independent voids. The area enclosed by the dashed line in Figure 3 corresponds to the recessed void 7. The substances that may be included in hydrogels will be described later. 【0058】(Effects) When an external force F is applied to the heat absorber according to this second embodiment in the normal state as shown in Figure 4A, a portion of the deformed hydrogel (or the aqueous solvent and / or any additive components contained in the hydrogel) enters the containment portion 6, which is the void portion 7b, as shown in Figure 4B. Therefore, when an external force F is applied, the heat absorber 1 can have space to be occupied by the deformed portion. As a result, the heat absorber 1 according to this second embodiment can deform sufficiently in response to the expansion of the battery cell and absorb the expansion of the battery cell. Furthermore, after the heat absorber 1 has been deformed by the applied external force F (or the hydrogel is in a deformed state), when the external force F is reduced or removed, the hydrogel itself attempts to return from its deformed shape to its normal shape due to its own restorative force (elastic force of the hydrogel body), thus the hydrogel acts as a restorative member. Therefore, the normal state of the heat absorber 1 (or the hydrogel in its normal state) and the deformed state of the heat absorber 1 (or the hydrogel in its deformed state) are reversible due to the external force F and the restoring member, the hydrogel (more specifically, the hydrogel body). Accordingly, the heat absorber according to the second embodiment of this disclosure deforms sufficiently in response to the expansion of the battery cell to absorb the expansion of the battery cell, and a restoring force acts to restore it to its shape before deformation. 【0059】 The heat-absorbing body 1 according to the first embodiment and the heat-absorbing body 1 according to the second embodiment of this disclosure have been described above, but this disclosure is not limited thereto, and can be modified as appropriate without departing from the technical spirit of the present invention. 【0060】(Modification of the First Embodiment) In the heat absorption body 1 according to this first embodiment, one example of the restorative member 3 is shown in Figures 1 and 2, but the heat absorption body 1 may have two or more restorative members 3. For example, Figures 5 and 6 show an example in which multiple restorative members 3 are provided. As shown in Figures 5 and 6, as a first modification of this first embodiment, an example is shown in which the heat absorption body 1 has two restorative members 3. Note that the number of restorative members 3 is not limited to this and may be three or more. By controlling the number of restorative members 3 in this way, the amount of deformation of the heat absorption body 1 can be controlled, making it easier to control the amount of absorption of the expansion of the battery cell. In addition, the position in which the restorative members 3 are provided is not limited to the position shown in Figures 5 and 6, but can be any position. For example, the positions in which the two restorative members 3 are provided may be the upper end and the lower end of the bag body 2. In Figures 5 and 6, (omitted) the contents 4 (hereinafter also simply referred to as "contents 4") containing an aqueous solvent are filled and / or contained inside the rectangular bag body 2 (filling section 5 and / or storage section 6), and two restoring members 3 are attached to two locations between the center and both ends in the longitudinal direction of the bag body 2. As shown in Figures 5A and 6A, when no external force F is applied to the heat absorber 1 (normal state of the heat absorber 1), the contents 4 are filled inside the filling section 5 of the bag body 2. In the normal state of the heat absorber 1, it has two storage sections 6 capable of containing the contents 4, and the two storage sections 6 are closed by the restoring members 3, and therefore do not have much internal space (space) capable of containing the contents 4. Similarly, in Figures 5A and 6A, at two locations between the center and both ends of the bag body 2, a portion of the opposing inner surfaces of the bag body 2 are in contact with each other by the restoring member 3 (or the bag body 2 is held in place by the restoring member 3 so that the opposing inner surfaces of the bag body 2 are in contact), and the bag body 2 does not substantially have an internal space (space) capable of accommodating the contents 4. However, the volume of the internal space (space) of the containment portion 6 in the normal state of the heat absorber 1 may be less than the volume of the filling portion 5 in the normal state of the heat absorber 1. Therefore, at the two locations where the restoring member 3 is attached, the opposing inner surfaces of the bag body 2 may be spaced apart.Furthermore, although the restoration member 3 will be described later, for example, by using a leaf spring or a pair of sheet-shaped magnets as the restoration member 3, parts of the opposing inner surfaces of the bag body 2 can be brought into contact with each other. 【0061】 Next, as shown in Figures 5B and 6B, when an external force F is applied to the heat-absorbing body 1 in its normal state to deform it (deformed state of the heat-absorbing body 1), the volume of the internal space (space) of the storage section 6 expands due to the pressure of the contents 4 deformed by the application of the external force F to the filling section 5 (Figure 6B shows that the storage section 6 expands in the direction of the solid black arrow (outward direction of the bag body 2)). In other words, as the filling section 5 deforms, at least a portion of the contents 4 containing the aqueous solvent in the filling section 5 flows into the storage section 6, so at least a portion of the contents 4 in the filling section 5 is stored in the storage section 6, and the internal space (space) of the storage section 6 expands or widens. 【0062】 As shown in Figures 5B and 6B, when the external force F applied to the normal state heat absorber 1 is reduced or removed, the internal space of the housing 6 is reduced by the respective restoring members 3 (for example, the attractive force between the N and S poles of a pair of magnets or the elastic force of a leaf spring), and consequently the contents 4 housed in the housing 6 move into the filling section 5 (return to the filling section 5). Therefore, when the applied external force F is reduced or removed, the heat absorber 1 is restored to the normal state shown in Figures 5A and 6A. Furthermore, when the external force F is applied to this restored normal state heat absorber 1, it enters the deformed state of the heat absorber 1 shown in Figures 5B and 6B as described above. Therefore, the normal state heat absorber 1 and the deformed state heat absorber 1 are reversible due to the external force F and the restoring members 3. 【0063】 In the heat-absorbing body according to this first embodiment, the restoring member 3 was a leaf spring, but it is not limited to this, and can be, for example, rubber, a magnet, etc. Also, in cases where the heat-absorbing body 1 has two or more restoring members 3, as in the first modified example of the first embodiment described above, the two or more restoring members 3 may each be different, and may be used in combination, for example, a leaf spring and a magnet, a leaf spring and rubber, or rubber and a magnet. 【0064】 Here, when the restoring member 3 is a leaf spring, the structure of the leaf spring will be described with reference to Figure 7. Figure 7 is a cross-sectional view of the upper surface of the heat-absorbing body according to this first embodiment. As shown in Figure 7, the leaf spring surrounds the bag body 2 and closes it by sandwiching it. That is, the bag body 2 is closed by a leaf spring that is square-shaped (square frame-shaped) when viewed from above. Note that the shape of the leaf spring is not limited to this, and it may be a V-shaped, C-shaped, U-shaped, I-shaped, or U-shaped (angular U-shaped) leaf spring. When using these leaf springs, a part of the leaf spring may be joined together with fasteners or adhesive. It is also possible to use two leaf springs in combination, and in such cases, the two leaf springs may be joined together by fasteners or adhesive. An example of using two leaf springs in combination is to join together two I-shaped leaf springs. For the fasteners mentioned above, stainless steel tape or the like can be used. The leaf spring may also be attached to the bag body 2 with adhesive or the like. 【0065】 As described above, magnets may be used as the restoration member 3. When magnets are used as the restoration member 3, the bag body 2 may be closed using a pair of magnets (N pole and S pole) connected at one point. Alternatively, two independent magnets may be attached to the front and back outer surfaces of the bag body 2 via the bag body 2. 【0066】As described above, rubber may be used as the restoration member 3. The case in which rubber is used as the restoration member 3 will be explained with reference to Figure 8. The heat absorber shown in Figure 8 is an example in which a donut-shaped rubber is wound around the bag body 2 as the restoration member 3 shown in Figure 6, and is an example in which rigid members (rod-shaped body 9a, plate-shaped body 9b) are provided on the inside and outside of the bag body 2 along the extending direction of the restoration member 3. As shown in Figure 8, rigid members (rod-shaped body 9a, plate-shaped body 9b) may be provided inside or outside the heat absorber 1 along the restoration member 3. For example, if a rod-shaped body 9a is provided as a rigid member along the extending direction of the restoration member 3, the member can act as a so-called tension rod, making it difficult for the rod-shaped body 9a to contract in the longitudinal direction, and thus the contraction direction of the restoration member 3 (for example, a donut-shaped rubber band) can be controlled. For this reason, the rigid members (rod-shaped body 9a, plate-shaped body 9b) can function as contraction direction control members. Therefore, by providing such a component, it is possible to prevent the bag body 2 from being closed laterally by the rubber. In Figure 8, the rod-shaped body 9a is provided inside the bag body 2, but it may also be provided outside the bag body 2. Similarly, the plate-shaped body 9b is provided outside the bag body 2, but it may also be provided inside. Furthermore, in Figure 8, both the rod-shaped body 9a and the plate-shaped body 9b are provided, but only the rod-shaped body 9a or only the plate-shaped body 9b may be provided. The material of the rod-shaped body 9a and the plate-shaped body 9b is not limited, but examples include resin and metal. 【0067】(Modification of the Second Embodiment) A first modification of the heat-absorbing body 1 according to the second embodiment will be described with reference to Figure 9A. In Figure 9A, the hydrogel or hydrogel body, which is the contents 4, has independent voids 7a (so-called closed-cell type pores) that exist closed inside the hydrogel or hydrogel body. In this specification, "independent void" refers to a void 7 that exists closed inside the hydrogel or hydrogel body, and two or more "independent voids" may be in communication with each other. On the other hand, "open void" refers to a void 7 that exists in the hydrogel or hydrogel body and is a pore that communicates with the outside, and is not a closed space. Also, in this specification, "void" is a general term that includes "independent voids" and "open voids". In Figure 9A, the shape of the independent void 7a is spherical, but is not limited to this. Examples of the shape of the independent void 7a include a roughly spherical shape, a roughly rectangular parallelepiped shape, an elliptical shape, etc. Furthermore, the independent voids 7a may be interconnected, forming a continuous void. The first modified example of the heat absorber 1 according to the second embodiment shown in Figure 9A shows a form in which independent voids 7a are located inside, instead of the hydrogel having voids 7 on its surface as shown in Figures 3 and 4. Similar to the hydrogel having voids 7 on its surface, when an external force F is applied to the heat absorber 1 in its normal state as shown in Figure 9A to deform it (deformed state of the heat absorber 1), the hydrogel deforms due to the application of the external force F, and the deformed hydrogel or the aqueous solvent absorbed within the hydrogel moves to the independent voids 7a, which are the containment part 6. When the applied external force F is reduced or removed, the hydrogel returns to its original shape due to the elastic force or restorative force of the hydrogel, or more specifically, the elastic force or restorative force of the hydrogel itself (= restorative member). Therefore, even if the voids 7 are independent voids 7a, the same effects as in Figures 3 and 4 are achieved. 【0068】 A second modification of the heat-absorbing body 1 according to this second embodiment will be described with reference to Figure 9B. As shown in Figure 9B, the shape of the independent void portion 7a may be (omitted) a rectangular parallelepiped. Also, although the void portions are regularly arranged in Figure 9B, they may be irregularly arranged. 【0069】Furthermore, although not shown in Figures 9A and 9B, the hydrogel or hydrogel body may have open voids 7b in addition to the independent voids 7a. That is, the hydrogel or hydrogel body may have both independent voids 7a and open voids 7b as voids 7. 【0070】 -Preferred Shape of Hydrogel or Hydrogel Body- In the heat absorber 1 according to this second embodiment, the shape of the hydrogel or hydrogel body can be various shapes. The shape of the hydrogel or hydrogel body in this embodiment is not particularly limited as long as there are voids (parts) on the surface or inside, but an example of the shape of the hydrogel or hydrogel body will be described with reference to Figures 10 and 11. The shape of the hydrogel or hydrogel body in Figures 10 and 11 is the shape of a plate-shaped chocolate with irregularities on the surface. In this specification, "approximately frustum-shaped" includes not only a frustum shape but also a shape in which at least one face of the frustum is curved. 【0071】 Figure 10A shows the shape of a hydrogel or hydrogel body in which roughly frustum-shaped protrusions are regularly and alternately arranged in two directions: one direction approximately perpendicular to the plane and the opposite direction. In Figure 10A, the shape of the protrusions is roughly frustum-shaped, but is not limited to this and can be various shapes. The shape of the protrusions may be, for example, cylindrical, conical, etc. Also, the size of the protrusions can be changed as appropriate. 【0072】 Figure 10B shows a hydrogel or hydrogel body in which approximately frustum-shaped protrusions are regularly arranged in one direction perpendicular to a plane. The shape of the protrusions is not limited to this; for example, they may be cylindrical, conical, pyramidal, etc. The size of the protrusions can also be changed as appropriate. 【0073】Figure 11A shows the shape of a hydrogel or hydrogel body in which multiple cylindrical protrusions of hydrogel pieces are arranged on the surface of a flat hydrogel substrate. In Figure 11A, the shape of the protrusions is cylindrical, but is not limited to this, and may be rectangular, conical, hemispherical, pyramidal, truncated pyramidal, etc. Also, the size of the protrusions can be changed as appropriate. 【0074】 Figure 11B shows the shape of a hydrogel in which roughly frustum-shaped protrusions are arranged in one direction (approximately) perpendicular to the plane, and a portion of the roughly frustum-shaped protrusions has a recess. With such a hydrogel, deformation of each protrusion is likely to occur. Furthermore, the shape of the protrusions is not limited to this, and may be a shape with a recess in a portion of a frustum-shaped protrusion, a shape with a recess in a portion of a cylindrical protrusion, a shape with a recess in a portion of a conical protrusion, etc. Also, the size of the protrusions can be changed as appropriate. 【0075】 Up to this point, the structure and effects of the heat absorber 1 of this embodiment have been mainly described. Below, the details of the material and other aspects of the heat absorber 1 according to this embodiment will be described. 【0076】 <Heat Absorbing Element> -Shape- The shape of the heat absorbing element 1 according to the first and second embodiments is sheet-like (approximately flat), but the shape or size of the heat absorbing element in this embodiment is not particularly limited. The shape or size of the heat absorbing element in this embodiment may be, for example, approximately spherical or irregular in shape, and can be appropriately selected depending on the application. A sheet-like (approximately flat) shape for the heat absorbing element is preferable because it is easy to install between adjacent battery cells. In this specification, sheet-like and approximately flat refer to the same shape. The average thickness of the heat absorbing element in this embodiment when it is sheet-like (approximately flat) is not particularly limited, but for example, it can be in the range of 100 μm or more and 50,000 μm or less. The average thickness is preferably 100 μm or more, more preferably 200 μm or more and 20,000 μm or less, even more preferably 500 μm or more and 10,000 μm or less, and particularly preferably 1,000 μm or more and 8,000 μm or less. The preferred range of the average thickness described above can be determined by appropriately rearranging the above-mentioned upper and lower limits. 【0077】 -Characteristics of the heat-absorbing element- The endothermic start temperature of the heat-absorbing element 1 of this embodiment is preferably 400°C or lower, more preferably 160°C or lower, even more preferably 120°C or lower, even more preferably 110°C or lower, and even more preferably 100°C or lower. The endothermic start temperature (°C) in this specification is the temperature at the intersection of a straight line drawn by extending the low-temperature baseline to the high-temperature side in the DSC measurement curve, which is the measurement result of a differential scanning calorimetry (DSC), and a tangent line drawn at the point where the slope is maximum on the low-temperature side curve of the endothermic peak associated with evaporation. However, if multiple endothermic peaks are observed, the intersection point of the straight line drawn by extending the low-temperature baseline to the high-temperature side and a tangent line drawn at the point where the slope is maximum on the low-temperature side curve of each of the multiple endothermic peaks is calculated for each of the multiple endothermic peaks, and the lowest temperature among the temperatures at these multiple intersection points is taken as the endothermic start temperature. The endothermic peak temperature of the endothermic element 1 in this embodiment is preferably in the range of at least 80°C to 400°C, and more preferably in the range of 90°C to 160°C. In this specification, the endothermic peak temperature refers to the temperature (°C) at the maximum value of the endothermic peak due to evaporation in the DSC measurement curve, which is the measurement result of a differential scanning calorimetry (DSC). If multiple endothermic peaks are observed, it is sufficient that at least one of the multiple endothermic peaks is in the range of 80°C to 160°C. The amount of heat absorbed by the endothermic element 1 in this embodiment is not particularly limited, but at the endothermic peak temperature (in the range of 80°C to 160°C), it is preferably in the range of 100 J / g to 3000 J / g, more preferably 200 J / g to 2500 J / g, even more preferably 300 J / g to 2000 J / g, and even more preferably 500 J / g to 1500 J / g. The preferred range of the heat absorption amount can be appropriately determined by rearranging the above upper and lower limits. The heat absorption start temperature, heat absorption peak temperature, and heat absorption amount of the heat absorption element 1 in this embodiment were determined using a differential scanning calorimetry (DSC). 【0078】In the heat absorber of this embodiment, the compression set (%) expressed by the following formula (I) is preferably 40% or less, more preferably more than 0% and 38% or less, and even more preferably 0.1% or more and 37% or less. Formula (I): Compression set (%) = {(t0 - t1) / (t0 - ts)} × 100 (In the above formula (I), t0 represents the average thickness of the heat absorber (mm) before the compression test, t1 represents the average thickness of the heat absorber (mm) after the compression test, and ts represents the average thickness of the spacer (mm). The average thickness (mm) was the arithmetic mean of the values ​​measured a total of three times at any three locations using a micrometer.) The compression test is performed under the conditions and procedures described in the Examples section below. 【0079】[Bag Body] The bag body 2 of this embodiment is preferably made of a sheet. In addition, a preferred form of the bag body 2 of this embodiment is to overlap two films of a desired size and shape (for example, rectangular or (approximately) circular) depending on the purpose of use, and then heat-seal a predetermined heat-seal area (for example, the edge of the film) is heat-pressed to form an opening, thereby bonding the heat-seal area. This makes it possible to create a three-sided bag that has an opening so that the contents can be filled into the internal space through the opening, and the heat-seal areas of the two sheets are bonded together. After filling with contents, the contents can be sealed by heat-sealing the openings together. In this specification, "sealed" means a state in which the inside and outside of the bag are substantially separated. The sheet used in the bag body 2 of this embodiment is not particularly limited as long as it exhibits water impermeability, and examples include known resin films, resin films with a metal layer, or films with a metal layer. The average thickness of the sheet used in the bag body 2 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 resin film materials include one or more resins such as polyester resin, nylon resin, polycarbonate resin, polypropylene resin, polyethylene resin, cyclic polyolefin resin, polystyrene resin, fluororesin, or elastomer. These plastics can be used in bags as films, sheets, tubes, etc. Furthermore, as the resin film having a metal layer, metals such as aluminum, silica, or alumina may be laminated onto the resin film as metal foil, a vapor-deposited film, etc. Using a resin film with a metal layer can lower the water vapor permeability of the resin film. The water vapor permeability of the sheet can be adjusted by selecting materials, thickness, combinations, etc. Lamination methods include dry lamination, extrusion lamination, heat lamination, co-extrusion, multilayer blow molding, laminated injection molding, and coating.A preferred form of the resin film having a metal layer is an aluminum laminate film (a film in which an aluminum foil (including an aluminum vapor-deposited layer) and a thermoplastic resin film (e.g., polyethylene film, PP film, PET film) laminated on at least one side thereof are integrated). In this embodiment, an adhesive layer may be formed in the heat-sealed area and the opening to be closed for sealing purposes. Suitable adhesive layers include, for example, polyester adhesives, polyether adhesives, or polyurethane adhesives, which are laminate adhesives. Furthermore, the properties of the adhesive are not particularly limited, and any of the following can be used: solvent-based, solvent-free, or aqueous. 【0080】 For example, in this disclosure, the bag body 2 is preferably a bag-shaped body made of a laminate film, and the laminate film is preferably a film made by laminating a metal foil and a resin film, and a laminated film with a three-layer structure consisting of an outer resin film / metal foil / inner resin film is exemplified. Specifically, a bag body can be suitably used that is made of a resin-based film having an aluminum vapor-deposited layer on the outside sealed via a polyurethane laminate adhesive layer, a bag body made of a three-layer laminate film having a nylon film on the outside, an aluminum foil in the center, and an adhesive layer such as modified polypropylene on the inside sealed via a polyurethane laminate adhesive layer, or a bag body made of a laminate film having a PET layer, an aluminum layer, and a polyethylene layer sealed via a polyurethane laminate adhesive layer. Examples of commercially available products 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 laminate film may be a uniform film formed by laminating a metal foil and a resin film. The higher the melting temperature (for example, 120 to 140°C) of the adhesive used to close the opening of the bag body in this embodiment or to provide the heat-sealed area, the higher the strength and the greater the ability to withstand internal pressure. 【0081】 The water vapor permeability of the sheet constituting the bag body 2 of this embodiment is ([g / (m²)). ２・ The upper limit of [g / (m ２ ・ 24 h)] is preferably 50 g / (m ２ ・ 24 h) or less, more preferably 10 g / (m ２ ・ 24 h) or less, and even more preferably 5 g / (m ２ ・ 24 h) or less. Further, the lower limit of the water vapor permeability of the sheet ([g / (m ２ ・ 24 h)] is preferably more than 0 g / (m ２ ・ 24 h). When the water vapor permeability of the sheet constituting the bag body 2 is in the range of 50 g / (m ２ ・ 24 h) or less, it is preferable from the viewpoint of preventing the moisture inside the bag body from leaking to the outside and preventing the deterioration of the heat absorption performance over time. Also, since the water vapor permeability of the sheet is more than 0 g / (m ２ ・ 24 h), it is possible to prevent the sheet from bursting due to rapid expansion. The water vapor permeability ([g / (m 【0082】 ・ 24 h)] in this specification is measured in an environment of a temperature of 40 °C and a relative humidity of 90% in accordance with the standard of JIS K7129. 【0082】 [Contents] In the heat absorber of the present embodiment, the content 4 essentially contains an aqueous solvent. Heat can be absorbed by the latent heat of vaporization of the aqueous solvent. Therefore, it is possible to utilize the latent heat of vaporization of water, which has a larger heat absorption amount than general hydrates. Also, since it is absorbed as the sensible heat of the aqueous solvent, the temperature can be stabilized even at room temperature. The content 4 of the present disclosure preferably contains a hydrogel body and / or any additive component in addition to the aqueous solvent. In one embodiment, the content 4 preferably contains a hydrogel containing an aqueous solvent and a hydrogel body, and / or any additive component. Since the hydrogel body has a role of an elastic body that maintains its shape while absorbing the aqueous solvent, it can be used as a restoring member. By the additive component, the heat absorption amount of the heat absorber 1 of the present embodiment can be further improved, or the heat absorber can be changed to a heat insulator in a high temperature range. 【0083】-Hydrogel- In this embodiment, the contents 4 may contain a hydrogel. In the endothermic body 1 of this disclosure, if the contents 4 is a hydrogel containing the aqueous solvent having voids 7, the hydrogel includes a hydrogel body and an aqueous solvent. The hydrogel is not particularly limited, and known hydrogels can be used. It is more preferable that the hydrogel has an aqueous solvent and a hydrogel body which is a three-dimensional network structure of a polymer containing a water-swellable clay mineral and water-soluble organic monomer structural units, at least a portion of which is dissolved or dispersed in the aqueous solvent. In other words, as the hydrogel of this embodiment, for example, it is preferable that the aqueous solvent is held within a three-dimensional network structure (three-dimensional network) mainly composed of a polymer synthesized from a water-soluble organic monomer, and it is even more preferable that the aqueous solvent is held within a three-dimensional network structure (three-dimensional network) mainly composed of a polymer synthesized from a water-soluble organic monomer in the presence of a water-swellable clay mineral. When the heat absorber 1 of this embodiment contains a hydrogel, if the content of the aqueous solvent in the hydrogel is in the range of 5% by mass or more and 99% by mass or less relative to the entire hydrogel, it can exhibit a heat absorbent effect at a level that can effectively prevent ignition and damage due to thermal runaway of secondary batteries, and is suitable as a heat absorber. The upper limit of the content of the aqueous solvent in the hydrogel is preferably 99% by mass or less, more preferably 90% by mass or less, and even more preferably 80% by mass or less, relative to the entire gel. The lower limit of the content of the aqueous solvent is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 20% by mass or more. The preferred range of the content of the aqueous solvent can be appropriately combined from the upper and lower limits above. That is, 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, relative to the entire gel. 【0084】Specific examples of such hydrogels include organic-inorganic composite hydrogels (NC gels), interpenetrating network structure hydrogels (DN gels), cyclic gels (SR gels), and hydrogels called aquamaterials. Of these, organic-inorganic composite hydrogels (also referred to as organic-inorganic hydrogels or NC gels (nanocomposite gels)) are preferred because they have excellent endothermic properties due to the aqueous solvent they contain, as well as excellent cushioning and creep resistance. Furthermore, the lower critical eutectic temperature (°C) of the hydrogel according to this embodiment in an aqueous solvent (e.g., pure water) is preferably 50°C or higher, more preferably 60°C or higher, and preferably does not have a lower critical eutectic temperature (°C), that is, it is preferable that the lower critical eutectic temperature (°C) cannot be observed in an aqueous solvent (e.g., pure water). 【0085】Preferred hydrogels in this embodiment include organic-inorganic composite hydrogels (NC gels), interpenetrating network structure gels in which two types of acrylic polymers each separately form a three-dimensional network structure, cyclic gels in which cyclodextrin forms the backbone of a three-dimensional network structure, and aquamaterials in which multi-branched dendrimers form the main backbone of a three-dimensional network structure and water-swellable clay minerals are added. In this disclosure, the following description will be based on the 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 water-swellable clay minerals 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 (= three-dimensional network structure) in which polymer chains composed of a plurality of water-soluble organic monomer structural units are crosslinked via water-swellable clay minerals that function as binding points, and the organic-inorganic composite hydrogel can swell when an aqueous solvent such as water is incorporated into the three-dimensional network structure of the polymer gel. As a result, the swollen organic-inorganic composite hydrogel not only exhibits endothermic properties and excellent cushioning that can follow relatively short-term deformations such as expansion and contraction due to charging and discharging of battery cells, but also exhibits excellent creep resistance that can alleviate internal pressure caused by the expansion of battery cells over time. The organic-inorganic composite hydrogel preferably has a three-dimensional network structure and uses water-soluble organic monomers and water-swellable clay minerals as reaction raw materials. 【0086】 In one aspect of this embodiment, the organic-inorganic composite hydrogel preferably uses at least water-soluble organic monomer structural units and water-swellable clay minerals as reaction raw materials. As a method for producing the organic-inorganic composite hydrogel in one aspect of this embodiment, a method is preferred in which the water-soluble organic monomer is polymerized in a dispersion containing the water-soluble organic monomer, water-swellable clay minerals, an aqueous solvent, and optionally a polymerization initiator and additives, since an organic-inorganic composite hydrogel having a three-dimensional network structure can be easily obtained. The resulting polymer of water-soluble organic monomer forms a three-dimensional network structure together with the water-swellable clay minerals and becomes a component of the organic-inorganic composite hydrogel (hydrogel body). 【0087】 -Additive Components- In this embodiment, the additive components that may be contained in the contents 4 include one or more components selected from the group consisting of inorganic powder, inorganic fiber, antifreeze, and additives. The inorganic powder, aqueous solvent, inorganic fiber, antifreeze, and additives that may be contained in the contents 4 of this embodiment will be described in detail below. 【0088】 -Inorganic Powder- The contents 4 in the heat absorber of this disclosure may further contain inorganic powder. When the contents contain inorganic powder, the aqueous solvent evaporates when exposed to high temperatures due to combustion, etc., but the presence of inorganic powder provides heat insulation and fire prevention effects. More specifically, it mainly acts as a heat absorber in the relatively low temperature range (for example, above room temperature and around 100°C). On the other hand, in the temperature range from the critical temperature (for example, 150°C) to the runaway thermal temperature (for example, around 1000°C), the inorganic powder in the heat absorber changes into a porous material, and therefore can also act as a heat insulator. As a result, when the heat absorber of this embodiment is placed between the battery cells of a battery stack (battery module) in which multiple battery cells are stacked, the thermal influence on adjacent cells can be blocked or suppressed. 【0089】The inorganic powder that may be included in the contents is preferably water-soluble. That is, the inorganic powder is preferably water-soluble. Furthermore, it is preferable that the water-soluble inorganic powder dissolves at a concentration of 1 g or more per 100 g of water at 20°C. Since the water-soluble inorganic powder exhibits hydrophilicity, it readily dissolves in aqueous solvents, making it easier for the water-soluble inorganic powder to be uniformly distributed within the contents. As a result, at temperatures exceeding the thermal runaway temperature (e.g., around 1000°C), the entire water-soluble inorganic powder tends to form a homogeneous porous body, which can effectively act as an insulator. Moreover, when water-soluble inorganic powder and an aqueous solvent are present in the contents of the endothermic body, a synergistic effect with the endothermic effect of the aqueous solvent is observed, allowing for continuous heat absorption at a different endothermic temperature than that of the aqueous solvent. Furthermore, even when the endothermic body is exposed to high temperatures, the water-soluble inorganic powder can form a porous body. As a result, it exhibits excellent heat insulation and fire prevention effects. Therefore, an endothermic body containing water-soluble inorganic powder mainly functions as an endothermic body at relatively low temperatures (e.g., above room temperature and 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 entire water-soluble inorganic powder becomes porous, and can therefore act as an insulator. As a result, when the heat absorber of this embodiment is placed between cells in a battery stack in which multiple cells are stacked, the thermal influence on adjacent cells can be blocked or suppressed. For example, when thermal runaway occurs, the cells expand, compressing the heat absorber between cells, and the distance between cells becomes extremely small, making it difficult to exhibit effective heat insulation performance. However, when the heat absorber of this embodiment contains water-soluble inorganic powder, when heated to high temperatures due to thermal runaway, the water-soluble inorganic powder itself sinters to form a porous body with a certain strength, thereby increasing the pressure resistance. This allows the distance between cells to be kept constant, maintaining effective heat insulation and effectively suppressing chain explosions between cells. 【0090】Water-soluble inorganic powders dissolve in aqueous solvents. This makes them easily soluble in aqueous solvents, thus facilitating a uniform distribution of water-soluble inorganic powder within the contents. In this specification, "water-soluble" means dissolving 1 g or more in 100 g of water at 20°C. Therefore, the water-soluble inorganic powder of this embodiment may be an inorganic powder that dissolves 1 g or more in 100 g of water at 20°C. 【0091】 The solubility of the water-soluble inorganic powder in this embodiment is 1 g or more per 100 g of water at 20°C. From the viewpoint of the stability, dispersibility, and sinterability by high-temperature heating of the water-soluble inorganic powder in the endothermic body, 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, even 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 combined with the above upper and lower limits. Since solubility is ensured when the solubility of the water-soluble inorganic powder at 20°C is within the above range, 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. 【0092】 The solubility of the water-soluble inorganic powder in this embodiment is preferably 10 g or more per 100 g of water at 60°C. The solubility of the water-soluble inorganic powder (per 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, even more preferably 25 g or more and 80 g or less, even more preferably 30 g or more and 60 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 60°C can be appropriately adjusted by changing the above upper and lower limits. 【0093】The solubility of the water-soluble inorganic powder in this embodiment is preferably 15 g or more per 100 g of water at 80°C. The solubility of the water-soluble inorganic powder (per 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, 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 80°C can be appropriately adjusted by changing the above upper and lower limits. 【0094】 The solubility of the water-soluble inorganic powder in this embodiment is preferably 15 g or more per 100 g of water at 100°C. The solubility of the water-soluble inorganic powder (per 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 appropriately adjusted by changing the above upper and lower limits. 【0095】 The preferred solubility of the water-soluble inorganic powder in this embodiment is preferably 1 g to 100 g per 100 g of water at 20°C, more preferably 2 g to 90 g, preferably 5 g to 100 g per 100 g of water at 40°C, preferably 10 g to 150 g per 100 g of water at 60°C, preferably 15 g to 160 g per 100 g of water at 80°C, and preferably 15 g to 170 g per 100 g of water at 100°C. Having the solubility of the water-soluble inorganic powder within the above ranges at each temperature is preferable from the viewpoint of exhibiting suitable endothermic effect and pressure resistance. The solubility of water-soluble inorganic powders in 100g of water at 20°C, 40°C, 60°C, 80°C, and 100°C can be appropriately adjusted by changing the above upper and lower limits. 【0096】The method for measuring solubility in this specification is as follows: A specified amount of the water-soluble inorganic powder to be measured is weighed into a glass bottle. Then, 100 g of pure water (pH = 7) is added to the glass bottle, and the mixture is stirred at a rotation speed of 80 rpm on a mix rotor for 24 hours at 1 atm and temperatures of 20°C, 40°C, 60°C, 80°C, and 100°C 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 water-soluble inorganic powder dissolved, and the upper limit amount (g) at which the transmittance becomes 99% is defined as the solubility of the water-soluble inorganic powder in water. <Transmittance Measurement Conditions> Dynamic Light Scattering (DLS) measuring device: DLS-8000 manufactured by Otsuka Electronics Laser wavelength, output: 488 nm / 100 mW Sample cell: NMR tube 【0097】 The water-soluble inorganic powder in this embodiment is preferably solid at room temperature. Furthermore, in the endotherm of this embodiment, it is preferable that an aqueous solution containing an aqueous solvent and water-soluble inorganic powder is filled into the bag body as the contents of the endotherm. Because the aqueous solution containing an aqueous solvent and water-soluble inorganic powder is filled into the contents of the endotherm, the water-soluble inorganic powder is completely dissolved in the aqueous solvent, so that the water-soluble inorganic powder is uniformly present in the contents, and as a result, a homogeneous porous body can be formed. The permeability of the aqueous solution is preferably 99% or higher, and more preferably 99.5% or higher. 【0098】 The amount of heat absorbed by the water-soluble inorganic powder (the amount of heat absorbed when heated from room temperature (23°C) to 1000°C (J / g)) 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 amount of heat absorbed by the water-soluble inorganic powder is not particularly limited, but is preferably 4000 J / g or less. The above upper and lower limits can be combined as appropriate. When the amount of heat absorbed by the water-soluble inorganic powder is within the above range, the endothermic effect is improved, resulting in a synergistic effect with the endothermic effect of the aqueous solvent, making it easier to suppress ignition. The above upper and lower limits of the content can be combined as appropriate. The amount of heat absorbed by the water-soluble inorganic powder is a value measured by a differential scanning calorimeter (DSC). 【0099】 The thermal decomposition initiation temperature of the water-soluble inorganic powder in this embodiment is preferably 80°C to 800°C, more preferably 90°C to 500°C, even more preferably 100°C to 350°C, and still more preferably 110°C to 150°C. Having the thermal decomposition initiation temperature of the water-soluble inorganic powder within the above range allows the water-soluble inorganic powder itself to decompose rapidly, making it easier to suppress ignition. The upper and lower limits of the thermal decomposition initiation temperature can be adjusted as appropriate. The thermal decomposition initiation temperature can be measured using a differential scanning calorimeter (DSC). 【0100】 The shape of the water-soluble inorganic powder in this embodiment is not particularly limited and can be, for example, in powder, particulate, crystalline, or plate form. Furthermore, a water-soluble inorganic powder with an endothermic effect is preferred as the water-soluble inorganic powder in this embodiment. Preferred forms of the water-soluble inorganic powder include porous powder, solid particles, or hollow particles. The water-soluble inorganic powder may be in any shape or form as long as it is dissolved in an aqueous solvent. When the water-soluble inorganic powder is in powder or particulate form, the average particle diameter of the water-soluble inorganic powder is, for example, preferably 0.01 μm or more and 200 μm or less, more preferably 0.1 μm or more and 140 μm or less, and even more preferably 10 μm or more and 100 μm or less. By keeping the average particle diameter within the above range, the water-soluble inorganic powder is easily dispersed in the system. The average particle diameter may be the median diameter (D50) value measured by a laser diffraction / scattering particle size distribution analyzer. 【0101】The water-soluble inorganic powder material of this embodiment is preferably composed of a water-soluble inorganic salt. The water-soluble inorganic salt is preferably one or more compounds consisting of an inorganic cation and a combination of organic and inorganic anions. Examples of the inorganic cation include alkali metal ions, alkaline earth metal ions, aluminum ions, zinc ions, silver ions, copper(I) ions, and copper(II) ions, and it is preferable that it be one or more 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 selected from oxygen ions, sulfate ions, halogen ions (chloride ions, fluoride ions, bromide 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 for excellent solubility in aqueous solvents. The water-soluble inorganic powder before mixing with the aqueous solvent may be anhydrous or hydrated, as long as it exhibits the desired solubility described above. Note that within the bag, the hydrate of the water-soluble inorganic powder usually exists as an anhydrous form. 【0102】The inorganic powder or water-soluble inorganic salt of this embodiment is preferably a chloride such as sodium chloride, potassium chloride, or ammonium chloride; a sulfate such as sodium sulfate, potassium sulfate, magnesium sulfate, aluminum sulfate, or alum; a carbonate such as sodium bicarbonate, sodium sesquicarbonate, sodium carbonate, potassium carbonate, potassium sesquicarbonate, or ammonium carbonate; a nitrate such as sodium nitrate, potassium nitrate, or calcium nitrate; a phosphate such as sodium phosphate, sodium dihydrogen phosphate, dipotassium hydrogen phosphate, or sodium polyphosphate; an acetate such as zinc acetate, sodium acetate, potassium acetate, copper(I) acetate, or copper(II) acetate; an oxide such as chromium oxide, barium oxide, or boric acid oxide; or a hydrate thereof. Among these, magnesium sulfate or magnesium sulfate heptahydrate is particularly preferred. The inorganic powder of this embodiment may be used individually or in combination with the above examples. 【0103】 In the heat-absorbing body of this embodiment, if the contents include inorganic powder, the inorganic powder content can be 1% to 90% by mass, 3% to 80% by mass, preferably 5% to 60% by mass, more preferably 10% to 50% by mass, more preferably 10% to 35% by mass, and even more preferably 15% to 30% by mass, based on the total amount of contents of the heat-absorbing body (100% by mass). When the inorganic powder content is within the above range, the heat-absorbing body readily sintersects to form a homogeneous porous body when exposed to high heat. Furthermore, when the water-soluble inorganic powder is a hydrate, the water-soluble inorganic powder content does not include the water content contained in the hydrate that is the water-soluble inorganic powder. The above upper and lower limits for the water-soluble inorganic powder content can be rearranged as appropriate. 【0104】In this embodiment, it is preferable that the contents 4 of the heat absorber 1 change into a porous body when heated to 120°C or higher. The temperature at which it changes into a porous body is preferably 150°C or higher, preferably 180°C or higher, preferably 200°C or higher, preferably 210°C or higher, preferably 240°C or higher, and preferably 250°C or higher. When the heat absorber is exposed to high temperatures due to combustion or the like, the aqueous solvent evaporates, but the water-soluble inorganic powder can form a porous body through sintering, which can provide heat insulation and fire protection to the heat absorber and adjacent components (e.g., battery cells). In particular, when a component (e.g., a battery cell) is sandwiched or surrounded by two or more heat absorbers 1 of this embodiment, it is considered that heat from the component is less likely to leak to the outside. For example, when the component is exposed to high temperatures, the water-soluble inorganic powder inside the heat absorber forms a porous body through sintering, and the porous body acts as a kind of fire barrier, thus providing excellent heat insulation and fire protection. Therefore, if the heat-absorbing body 1 of this embodiment is placed, for example, in a secondary battery module such as a stacked battery described later, it is possible to suppress, prevent, or delay chain explosions by suppressing heat transfer to other cells. 【0105】 -Aqueous Solvent- The contents 4 of the heat-absorbing body 1 in this embodiment contain an aqueous solvent. As a result, heat can be absorbed by the aqueous solvent in the bag body 2, particularly the latent heat of vaporization of water, which has a larger heat absorption capacity than typical hydrates, and thus the latent heat of vaporization of water can be utilized. Furthermore, since the heat is absorbed as the sensible heat of the aqueous solvent, the temperature can be stabilized even at room temperature. 【0106】In this embodiment, the aqueous solvent only needs to contain water as its main component, and means water or a solvent with water as its main component. Therefore, the aqueous solvent includes mixed solvents with solvents other than water, and aqueous solutions containing salts (e.g., buffer solutions, electrolyte solutions). In this specification, "containing water as its main component" means containing 45% by mass or more of water relative to the total aqueous solvent. Furthermore, the water can be purified water, pure water, ultrapure water, or distilled water, etc., without any particular restrictions. 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 buffering salts such as Tris-hydrochloric acid, glycine hydrochloride, citrate-sodium citrate, acetate-sodium acetate, citrate-disodium hydrogen phosphate, sodium dihydrogen phosphate-disodium hydrogen phosphate, glycine-sodium hydroxide, and sodium carbonate-sodium bicarbonate. In addition, Good Buffer such as HEPES or MOPS may be used as the aqueous solvent. Other solvents that make up the mixed solvent include organic solvents that can be uniformly mixed with water (e.g., lower alcohols, lower ketones, etc.), or low-volatility solvents used as antifreeze agents. 【0107】In this embodiment, the water content in the aqueous solvent is preferably 50% to 100% by mass, more preferably 80% to 100% by mass, even more preferably 90% to 100% by mass, and particularly preferably 95% to 100% by mass, relative to the total aqueous solvent. The preferred range for the water content in the aqueous solvent can be adjusted by appropriately rearranging the upper and lower limits. In this embodiment, the water content is preferably 10% to 95% by mass, more preferably 20% to 90% by mass, even more preferably 30% to 80% by mass, and particularly preferably 50% to 70% by mass, relative to the total amount (100% by mass) of the contents of the endothermic body. The preferred range for the water content can be adjusted by appropriately rearranging the upper and lower limits. When the water content is within the above range, the endothermic, heat insulating, and pressure resistant properties are superior, and the material can transform into a heat insulating material in the high-temperature range. 【0108】 - Inorganic Fibers and Antifreeze - The contents of the heat absorber of this embodiment may contain one or more selected from the group consisting of antifreeze and inorganic fibers. 【0109】--Antifreezing Agent-- In this embodiment, an antifreezing agent may be added to the contents as needed, as it is possible to improve the effect of suppressing temperature drop below freezing point. In particular, by adding an antifreezing agent to the contents of the heat absorber, high cushioning properties can be maintained over a wide temperature range. The antifreezing agent in this embodiment may be an inorganic antifreezing agent or an organic antifreezing agent. The antifreezing agent may be in liquid, powder, or solid form. The inorganic antifreezing agent is preferably a chloride such as sodium chloride, calcium chloride, or magnesium chloride (including hydrates such as magnesium chloride hexahydrate). On the other hand, the organic antifreezing agent is preferably a salt of an organic acid or a low-volatility substance (low-volatility solvent or urea), and more preferably a salt of an organic acid or a low-volatility solvent. The salt (including hydrate) of the organic acid is preferably a salt of sodium, potassium, magnesium, or ammonia of formic acid, propionic acid, or succinic acid, and examples include disodium succinate (including hydrates such as disodium succinate hexahydrate) or sodium propionate. The low volatile substance is urea or a low volatile solvent (for example, a polyhydric alcohol). Examples of such low volatile solvents include ethylene glycol, diethylene glycol, glycerin, dipropylene glycol, propylene glycol, butyrolactone, N,N-dimethylformamide, 1,3-propanediol, glycol ether, glycol ether, glycol monoether, isopropanol, propylene glycol monomethyl ether, di- or tripropylene glycol monomethyl ether, cyclohexanol, glucose, mannose, fructose, galactose, sucrose, lactose, maltose, xylose, arabinose, sorbitol, mannitol, trehalose, or raffinose. 【0110】 The low-volatility solvent in this embodiment is one that has a volatility of 1 cm in an open system at 60°C and 1 atm. ２ - Less than 0.1 g per hour (0.1 g / cm³) ２An organic solvent with a frequency of hr, 60°C, and 1 atm or less is more preferable, even more preferably 0.05 g or less, and even more preferably 0.01 g or less. Specifically, since a solvent that is easily miscible with water is preferred, glycerin (0.001 g or less / cm³) is used. ２ (hr, 60℃, 1atm), diglycerin (0.001 g or less / cm³) ２ hr, 60℃, 1atm), ethylene glycol (0.01 g or less / cm³) ２ hr, 60℃, 1atm), propylene glycol (0.001 g or less / cm³) ２ (hr, 60℃, 1atm), polyethylene glycol (0.001 g or less / cm³) ２ Polyhydric alcohols such as (hr, 60°C, 1 atm) are preferred, and glycerin and diglycerin are more preferred. These low-volatility solvents may be used alone or in combination of two or more. By including a low-volatility solvent, particularly a polyhydric alcohol, as the contents of the endothermic body of this embodiment, the volatilization of the aqueous solvent is suppressed or prevented, or the decrease in cushioning properties at low temperatures is suppressed (improvement of freeze-prevention effect). When a low-volatility solvent is used as an optional component, the mass ratio of the aqueous solvent to the low-volatility solvent in the contents of the endothermic body of this embodiment (aqueous solvent / low-volatility solvent) is preferably 95 / 5 or more and 30 / 70 or less, more preferably 90 / 10 or more and 50 / 50 or less, and even more preferably 85 / 15 or more and 65 / 35 or less. If the contents contain an antifreeze, the amount of antifreeze can be 0% to 70% by mass relative to the total amount (100% by mass) of the contents of the heat absorber, preferably 5% to 60% by mass, more preferably 10% to 50% by mass, even more preferably 15% to 35% by mass, and particularly preferably 20% to 30% by mass. The preferred range for the amount of antifreeze can be adjusted by appropriately combining the above upper and lower limits. Furthermore, if the antifreeze is contained within the above range, it will not freeze easily even at -20°C, allowing the battery to be used over a wide temperature range. 【0111】--Inorganic Fibers-- The inorganic fibers of this embodiment are a fiber aggregate in which fibers made of inorganic material are intertwined with each other, or a porous body made of inorganic material, and the inorganic powder acts as a foaming nucleating agent, facilitating the formation of the porous body. Furthermore, when the inorganic powder transforms 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 desired mechanical strength when the temperature exceeds the thermal runaway temperature, for example. 【0112】 Specifically, the inorganic fibers include woven 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 sponge-like 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. This heat resistance refers to the temperature at which the rate of change in volume (in the thickness direction) becomes -20% when the test specimen is heated in 100°C increments from 200°C to 700°C and held at each temperature for 30 minutes. The inorganic fibers of this embodiment are porous materials having at least one of a specific air permeability resistance, a specific porosity, a specific tortuousity, or a specific void ratio. As a result, even in high-temperature ranges (temperature ranges from critical temperature (e.g., 150°C) to thermal runaway temperature (e.g., around 1000°C)), the entire heat-absorbing material tends to form a relatively stable porous material, and thus tends to act as an insulator. In particular, if the inorganic fibers have a specific porosity, they can be combined with inorganic powders to create a more superior insulating material. 【0113】[Porrosion] The average porosity of the inorganic fibers in this embodiment is preferably 30% to 99.7%, more preferably 50% to 99.5%, even more preferably 70% to 99.3%, and particularly preferably 90% to 99%. In this specification, the average porosity of the inorganic fibers is a value obtained 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 material of the inorganic fibers. The average porosity (%) can be calculated from the bulk density ρf and true density ρr below using the following formula (1). Average porosity (%) = ((1 / ρf) - (1 / ρr)) / (1 / ρf) × 100 ... Formula (1) 【0114】 [Bulk Density] The bulk density ρf of the inorganic fiber in this embodiment is 0.020 g / cm³. ３ 1g / cm or more ３ The following is preferred, and more preferably, 0.022 g / cm³. ３ 0.5g / cm or more ３ More preferably, 0.024 g / cm³ ３ 0.1g / cm or more ３ The following is particularly preferred: 0.026 g / cm³ ３ 0.07g / cm or more ３ The following procedure is followed: The dimensions of the inorganic fiber are measured, and the bulk volume V of the inorganic fiber is calculated. Then, the mass M of the inorganic fiber is measured using a precision balance. From the obtained mass M and bulk volume V, the bulk density of the inorganic fiber can be determined using the following formula (2): Bulk density ρf (g / cm³) ３ )=M / V...Formula (2) 【0115】 [True density] The true density ρr of the inorganic fiber in this embodiment is 0.5 g / cm³. ３ 10g / cm or more ３ The following is preferable, and more preferably, 1 g / cm³ ３ 7g / cm or more ３ More preferably, 1.5 g / cm³ ３ 5g / cm or more ３ The following is particularly preferable: 2 g / cm³ ３3g / 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 buoyancy method using a mixed solution consisting of n-heptane, carbon tetrachloride, and ethylene dibromide. Specifically, first, a sample piece of inorganic fiber of an appropriate size is placed in a stoppered test tube. Next, a mixed solvent, which is a mixture of the three solvents in appropriate proportions, is added to the test tube and it is immersed in a 30°C constant temperature bath. 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 operation is repeated until the test piece floats in the liquid. Finally, the density of the mixed solvent is measured using a Gay-Lussac gravity bottle. 【0116】 [Composition of Inorganic Fibers] Examples of materials constituting the inorganic fibers of this embodiment or inorganic materials contained in said 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 or composite oxides of said elements, single sulfides or composite sulfides of said elements, and single phosphate compounds or composite phosphate compounds of said elements, with silicon, titanium, zirconium, magnesium, aluminum, indium, tin, and their single or composite oxides being preferred. Specifically, the 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, aerogel, and the like. 【0117】 [Shape of Inorganic Fibers] The shape of the inorganic fibers in this embodiment can be selected from yarn-like, fibrous, fiber bundle-like, fiber aggregate-like, cotton-like, woven / knitted, nonwoven fabric-like, etc. In this specification, "woven / knitted" refers to a woven or knitted fabric. 【0118】If the inorganic fibers according to this embodiment are woven fabric, then known weaving methods such as plain weave, twill weave, satin weave, leno weave, and blind weave can be appropriately adopted as the weaving method of the fabric. From among these weaving methods, it is preferable to adopt a weaving method in which the resistance of fluid passage through the spaces between the connecting holes, that is, the spaces for each individual (eye) formed by the intersection of the warp and weft lines (for example, the air permeability resistance described later) is within a predetermined range. From these viewpoints, it is preferable to adopt weaving methods such as plain weave, twill weave, satin weave, leno weave, and leno weave. 【0119】 If the inorganic fiber according to this embodiment is a knitted fabric, the knitting method can be warp knitting, which knits vertically, such as lace knitting, raschel knitting, tricot knitting, and van dyke knitting, or weft knitting, which knits horizontally, such as weft knitting, plain knitting, rib knitting, tubular knitting, jersey knitting, kanako knitting, rib knitting, and jacquard knitting, and known knitting methods can be appropriately adopted. From among these knitting methods, it is preferable to adopt a knitting method in which the resistance of fluid passage through the communication holes (for example, the air permeability resistance described later) is within a predetermined range. Furthermore, various knitting machines can be used, such as warp knitting machines, weft knitting machines, circular knitting machines, and raschel knitting machines. 【0120】 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 its fineness is preferably 50 dtex or more and 8000 dtex or less, more preferably 100 dtex or more and 3000 dtex or less. Furthermore, the twisting method of the woven or knitted yarn is not limited, and the twisting method may be dry twisting, wet twisting by immersion in water, or a combination thereof. Furthermore, the direction of twisting is not particularly limited, and may be right-hand twist, left-hand twist, or a combination thereof. The woven or knitted yarn used in this embodiment may be false-twisted yarn, filament yarn, or yarn processed by the POY / DTY method or the PTY (Producers Textured Yarn) method. The conditions for the woven or knitted yarn used can be appropriately selected according to the purpose of use or the type of aqueous solvent. 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. 【0121】The BET specific surface area of ​​inorganic fibers is 0.3 m². ２ / g or more 5000m ２ It may be less than / g, and 10m ２ / g or more 2000m ２ It may be less than / g, and 30m ２ / g or more 1600m ２ It may be less than / g. The BET specific surface area of ​​the inorganic fiber described above is measured using a specific surface area meter (BELSORP-mini, manufactured by Microtrac-Bell Co., Ltd.), and the surface area per gram of sample measured from the amount of nitrogen gas adsorbed by the BET method is used as the specific surface area (m²). ２ This value was calculated as ( / g). 【0122】 When the inorganic fibers of this embodiment are composed of a nonwoven fabric, the average fiber diameter of all the fibers constituting the nonwoven fabric (fibers made from the inorganic raw material) 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 be within the above range because it is easier to secure the desired porosity. The average fiber diameter can be measured by microscopic observation or by image analysis results using a fiber length measuring device (for example, KAJAANI Fiber Lab.). 【0123】Furthermore, when the inorganic fibers of this embodiment are composed of a nonwoven fabric, the average fiber length of all the fibers (raw material fibers) constituting the nonwoven fabric is preferably 3 mm to 200 mm, more preferably 5 mm to 100 mm, and more preferably 10 mm to 50 mm. It is preferable that the average fiber length of all the fibers constituting the nonwoven fabric is within the above range, as this makes it easier to secure the desired porosity. The average fiber length can be determined by measuring the average fiber diameter by microscopic observation or by image analysis results using a fiber length measuring device (for example, KAJAANI Fiber Lab.). When the inorganic fibers of this embodiment are formed from a cotton-like material, the average fiber length of all the fibers (raw material fibers) constituting the cotton-like material is preferably 0.5 μm to 50 μm, more preferably 0.8 μm to 32 μm, and more preferably 1 μm to 25 μm. It is preferable that the average fiber length of all fibers constituting the cotton-like material is within the above range, and that the average fiber system of the fibers constituting the cotton-like material is within the above range, as this makes it easier to secure the desired porosity. The average fiber length can be determined by measuring the average fiber diameter by microscopic observation or by image analysis results using a fiber length measuring device (e.g., KAJAANI Fiber Lab.). In this specification, although a cotton-like material is a type of nonwoven fabric, it refers to a material that is in the form of fibers and has a shape other than cloth (or flat plate). 【0124】 [Preferred Embodiments of Inorganic Fibers] Preferred embodiments of inorganic fibers in this embodiment are glass cloth, ceramic wool, rock wool, and glass wool. 【0125】When the contents contain inorganic fibers, the inorganic fiber content is preferably 0% to 50% by mass, more preferably 1% to 30% by mass, even more preferably 1% to 10% by mass, even more preferably 1% to 5% by mass, and particularly preferably 1% to 3% by mass, based on the total amount of contents (100% by mass). The preferred range for the inorganic fiber content can be appropriately rearranged by changing each of the upper and lower limits above. When the inorganic fiber content is within the above range, it has better heat absorption and pressure resistance, and can change from an endothermic effect to an insulating effect in the high-temperature range. 【0126】 - Additives - The contents 4 of the heat absorber 1 of this embodiment may optionally contain various additives such as ultraviolet absorbers, antioxidants, organic solvents, inorganic fillers other than the water-swellable clay minerals, viscosity modifiers such as thickeners, crosslinking agents, and flame retardants. The various additives are optional components, but when using them, it is preferable to use them in proportions that do not impair the effects of this disclosure and that are appropriate to the purpose of each additive. The proportions cannot be determined in general terms, but 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 this disclosure. 【0127】 Examples of the above-mentioned UV absorbers 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'-xanthen carboxy-5'-methylphenyl)benzotriazole, 2-(2'-o-nitrobenzyloxy-5'-methylphenyl)benzotriazole, 2-xanthen carboxy-4-dodecyloxybenzophenone, and 2-o-nitrobenzyloxy-4-dodecyloxybenzophenone. These UV absorbers can be used alone or in combination of two or more. 【0128】 Examples of the above antioxidants include "Sumiriser BBM-S" and "Sumiriser GA-80" manufactured by Sumitomo Chemical Co., Ltd. Examples of the above organic solvents 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 above organic solvents can be used alone or in combination of two or more. Examples of the above inorganic fillers include fused silica, crystalline silica, alumina, silicon nitride, and aluminum hydroxide. Examples of viscosity modifiers such as the above thickeners include various tackifying resins such as rosin-based, polymerized rosin-based, polymerized rosin ester-based, rosin phenol-based, stabilized rosin ester-based, disproportionated rosin ester-based, terpene-based, terpene phenol-based, and petroleum resin-based resins. Examples of known crosslinking agents include isocyanates, epoxys, aziridines, polyvalent metal salts, metal chelates, ketohydrazides, oxazolines, carbodiimides, silanes, and glycidyl(alkoxy)epoxysilanes. 【0129】Examples of the above flame retardants include inorganic phosphorus compounds such as red phosphorus, monoammonium phosphate, diammonium phosphate, triammonium phosphate, polyammonium phosphate, and other ammonium phosphates and phosphate amides; phosphate ester compounds, phosphonic acid compounds, phosphinic acid compounds, phosphine oxide compounds, phospholane compounds, organic nitrogen-containing phosphorus compounds, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, and 10-(2,5-dihydrooxyphenyl)-10H-9-oxa-10-phosphaphenanthrene-10-oxy Examples of flame retardants include cyclic organophosphorus compounds such as 10-(2,7-dihydrooxynaphthyl)-10H-9-oxa-10-phosphaphenanthrene-10-oxide, and derivatives 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 phenothiazines; silicone-based flame retardants such as silicone oil, silicone rubber, and silicone resins; 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 individually or in combination of two or more. When using these flame retardants, it is preferable that the amount is in the range of 0.1 to 20% by mass relative to the total contents of the heat absorber. 【0130】(Method for manufacturing the heat absorber 1) As an example of the method for manufacturing the heat absorber 1 of this embodiment, it is preferable to have the steps of filling the contents 4 into the opening of the bag body 2 and sealing the bag body 2 by heat sealing the opening of the bag body 2. Furthermore, if the heat absorber 1 has a restoration member on the outside of the bag body, the method includes, in addition to the above steps, a step of attaching the restoration member. The aqueous solvent and one or more selected from the group consisting of inorganic powder, inorganic fiber, and additives, which are added as needed, may be filled separately into the opening of the bag, or a mixed solution (1) may be prepared in advance by impregnating or dispersing water-soluble inorganic powder, and inorganic fiber and additives, which are added as needed, in the aqueous solvent, and then filling the mixed solution into the opening of the bag. The mixed solution (1) preferably contains the aqueous solvent, and in particular preferably contains the aqueous solvent, inorganic powder, and one or more selected from the group consisting of inorganic fiber and additives, which are added as needed. The mixed solution (1) preferably contains, based on the total amount (100% by mass) of the mixed solution (1), 0 to 50% by mass of inorganic fiber, 10% to 95% by mass of aqueous solvent, 0% to 70% by mass of antifreeze, 1% to 90% by mass of inorganic powder (including water in the case of hydrates), and 0% to 10% by mass of additives, or 0% to 30% by mass of inorganic fiber, 10% to 95% by mass of aqueous solvent, and 5% by mass of antifreeze. Preferably, the mixture contains 60% by mass or less of inorganic powder (including water in the case of hydrates), 5% by mass or more and 60% by mass or less of inorganic powder (including water in the case of hydrates), and 0% by mass or more and 10% by mass or less of additives. More preferably, it contains 1% by mass or more and 12% by mass or less of inorganic fiber, 20% by mass or more and 94% by mass or less of aqueous solvent, 10% by mass or more and 50% by mass or less of antifreeze, 10% by mass or more and 50% by mass or less of inorganic powder (including water in the case of hydrates), and 0% by mass or more and 50% by mass or less of additives. The amount of "inorganic powder (including water in the case of hydrates)" relative to the total amount (100% by mass) of the above mixed solution (1) means the amount blended. Therefore, if the inorganic powder is a hydrate, it includes the amount of water contained in the inorganic powder that is a hydrate. 【0131】The heat-absorbing body 1 of this embodiment has been described above, but this disclosure is not limited thereto and can be modified as appropriate without departing from the technical spirit of the invention. 【0132】 <Secondary Battery Module> The secondary battery module of this embodiment is characterized by comprising the heat-absorbing body 1 of this embodiment. Furthermore, it is preferable that the heat-absorbing body 1 of this embodiment is sandwiched between battery cells in the secondary battery module. 【0133】 The type of secondary battery on which the heat-absorbing element 1 of this embodiment can be mounted is not particularly limited, and examples include lithium-ion batteries, lithium-ion polymer batteries, lead-acid batteries, nickel-metal hydride batteries, nickel-cadmium batteries, nickel-iron batteries, nickel-zinc batteries, silver oxide-zinc batteries, metal-air batteries, polyvalent cation batteries, capacitors, and the like. Among these, lithium-ion batteries are a particularly suitable application. 【0134】The secondary battery module capable of mounting the heat-absorbing element 1 of this embodiment is a secondary battery mounted on a mobile device such as a vehicle or an aircraft (especially a drone), and has a plurality of battery cells and a case that houses such plurality of battery cells. The battery cells (or battery cells) constituting the secondary battery module can be, for example, battery outer film used as the outer material, and a battery element comprising 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 sealed inside the outer material. The secondary battery module capable of mounting the heat-absorbing element 1 of this embodiment will be described below with reference to Figure 12. Figure 12 shows a cross-sectional view of a stacked battery 20 as an example of a secondary battery. Note that the secondary battery capable of mounting the heat-absorbing element of this embodiment is not limited to a flat stacked battery 20 as shown in Figure 12. The secondary battery capable of mounting the heat-absorbing element 1 of this embodiment may be cylindrical in shape, such as a wound secondary battery, or a cylindrical secondary battery may be deformed into a rectangular flat shape. In this embodiment, the stacked battery 20 has a structure in which a flattened, roughly rectangular battery element 10, on which the charge-discharge reaction substantially proceeds, is sealed inside the battery casing materials 18a and 18b. The battery element 10 has a configuration 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 positive electrode active material is arranged 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 negative electrode active material is arranged on both sides of a negative electrode current collector 17. One positive electrode material layer 11 and a negative electrode material layer 16 adjacent to the positive electrode material layer 11 are arranged to face each other via the electrolyte layer 14, and the positive electrode, electrolyte layer 14, and negative electrode are stacked sequentially. As a result, adjacent positive electrodes, electrolyte layers 14, and negative electrodes form one single cell body. The stacked battery 20 shown in Figure 12 has a configuration in which multiple such single cell bodies are stacked and electrically connected in parallel. Furthermore, an activated carbon layer 19 is installed to adsorb components derived from the positive electrode active material that may melt or sublimate when the battery is exposed to high temperatures.Furthermore, as shown in Figure 12, the positive electrode current collector 12 and the negative electrode current collector 17 are each attached to a positive electrode terminal 13 and a negative electrode terminal 15, to which the positive and negative electrodes are electrically connected, and have a structure that leads out to the outside of the battery casing materials 18a and 18b so as to be sandwiched between the ends of the battery casing materials 18a and 18b. The positive electrode terminal 13 and the negative electrode terminal 15 can be attached to the positive electrode current collector 12 and the negative electrode current collector 17 of each electrode by welding or other means via positive electrode leads and negative electrode leads (not shown) as needed. Laminate film is used for the battery casing materials 18a and 18b, and normally the sealant layers formed on the surfaces of the battery casing films 18a and 18b are heat-sealed together. In addition, there is a region around the periphery of the battery casing materials 18a and 18b where the sealant layers are in close contact with each other by heat sealing. 【0135】Next, a secondary battery module equipped with the heat absorber of this embodiment will be described using Figure 13. Figure 13 is a schematic perspective view showing the secondary battery module of Figure 12 disassembled. The battery element 10 shown in Figure 13 has a configuration in which a positive electrode formed on a positive electrode current collector 12 (aluminum foil, etc.) having a positive electrode terminal 13 and a negative electrode placed on a negative electrode current collector 17 (metal foil, etc.) having a negative electrode terminal 15 are stacked facing each other via a separator 14 containing an electrolyte. Multiple battery elements 10 are stacked and sealed with battery casing materials 18a, 18b (for example, aluminum laminate casings, etc.). The heat absorber 1 of this embodiment is positioned to be in contact with the negative electrode current collector 17. The heat absorber 1 may be positioned to be in contact not only with the negative electrode current collector 17 but also with the positive electrode current collector 12. Therefore, a secondary battery module equipped with a heat absorber 1 on a battery element 10 has one or more laminates in which a positive electrode formed on a positive electrode current collector 12 (aluminum foil, etc.) having a positive electrode terminal 13, a separator 14 containing an electrolyte, and a negative electrode placed on a negative electrode current collector 17 (metal foil, etc.) having a negative electrode terminal 15 are sequentially stacked, and one or more heat absorbers 1 can be arranged so that they come into contact with the positive electrode current collector 12 and / or the negative electrode current collector 17, but not with the separator 14. When a solid electrolyte or gel electrolyte is used as the electrolyte, these electrolytes can be interposed between the electrodes instead of the separator 14. On the other hand, the aqueous solvent, which is the contents of the heat absorber in this embodiment, does not come into direct contact with the battery element 10. Therefore, a preferred heat absorber for secondary batteries in this embodiment is a bag filled with an aqueous solvent, a water-soluble inorganic powder, and inorganic fibers, but the contents of the bag are not such that the aqueous solvent and the battery element 10 are in direct contact, and more preferably the contents of the heat absorber bag for secondary batteries do not include the battery element 10. 【0136】The secondary battery module of this disclosure may have the heat-absorbing body 1 of this embodiment sandwiched between multiple cases (not shown) or multiple adjacent battery elements 10 (also referred to as battery cells) housed in battery outer films 18a, 18b. The cases can be made of, for example, aluminum, iron, or a metal material containing these, or a resin material such as polyphenylene sulfide. If made of a resin material, it can contribute to reducing the weight of the secondary battery module. The heat-absorbing body 1 can be sandwiched between multiple battery elements 10 by, for example, adhesive, fusion (ultrasonic fusion, high-frequency fusion, heat fusion), or adhesive. With this configuration, the heat-absorbing body 1 sandwiched between the battery elements 10 absorbs the heat generated during charging of the secondary battery, thereby suppressing a rapid rise in the temperature of the battery elements 10 and preventing deterioration, ignition, etc. of the battery elements 10. When the heat-absorbing body 1 is sandwiched between the battery elements 10, the temperature influence between the battery elements 10 can be suppressed by its insulating properties. Furthermore, it acts as a buffer against volume changes due to the expansion of the battery elements 10, making it easier to mitigate the rise in internal pressure of the secondary battery module. On the other hand, when the battery elements 10 experience thermal runaway due to excessive heat generation, the water-soluble inorganic powder in the heat-absorbing body 1 sintersects and changes into a hard, plate-like porous body, thereby increasing its pressure resistance and decreasing its thermal conductivity. As a result, the heat-absorbing body 1 can maintain a constant distance between cells, maintain effective insulating properties, control thermal conductivity, and effectively suppress chain explosions of the battery elements 10. 【0137】 Furthermore, in a secondary battery module, the heat-absorbing element 1 of this embodiment may be placed in a case or battery casing film that houses multiple battery elements (battery cells). 【0138】 The present disclosure will be further described below with reference to examples, but the present disclosure is not limited in any way to the examples described below. 【0139】(1) Evaluation of Resilience The resilience of the heat absorbers prepared in this embodiment and comparative example was evaluated using the following method. Specifically, in a constant temperature and humidity chamber (20°C, 60%), the heat absorbers whose thickness had been measured in advance were subjected to a compression set test using a 4.72 mm spacer in a compression set tester (manufactured by Polymer Instruments Co., Ltd.). In this compression set test, the heat absorber and spacer were sandwiched between the metal plates of the compression set tester, the heat absorber was compressed to the thickness of the spacer with a nut, and then left to stand for 24 hours. After that, the heat absorber was removed from the compression set tester and left to stand in the constant temperature and humidity chamber for a sufficient amount of time, and then the thickness of the heat absorber after the test was measured. The compression set was calculated from the following formula (I) using the heat absorber thickness t0 (mm) before compression, the heat absorber thickness t1 (mm) after compression, and the spacer thickness ts (mm). The resilience (%) (also referred to as the degree of recovery) was calculated by subtracting the compression set from 100%. The thickness of the heat absorber was measured three times at three arbitrary locations, and the compression set (%) was calculated at each location using the following formula (I). From the compression set (%), the recovery (%) was calculated. The average values ​​are shown in Table 1. Formula (I): Compression set (%) = {(t0 - t1) / (t0 - ts)} × 100 (Criteria for evaluating recovery) Return to 80% or more of the original height... A Return to 70% or more but less than 80% of the original height... B Return to 60% or more but less than 70% of the original height... C Return to less than 60% of the original height... D 【0140】 <Preparation of dispersion (a)> Dispersion (a) (aqueous aqueous solution containing water-soluble inorganic powder) was prepared by mixing 32 parts by mass of magnesium sulfate heptahydrate (amount including water in the hydrate) with 60 parts by mass of pure water. 【0141】 <Preparation of Dispersion (b)> Dispersion (b) (a precursor of hydrogel or NC gel) was prepared by mixing and stirring 20 parts by mass of N,N-dimethylacrylamide, 4.8 parts by mass of water-swellable synthetic hectorite (Vic Chemie Japan Co., Ltd., "Laponite RD"), 0.5 parts by mass of sodium peroxodisulfate, and 0.8 parts by mass of N,N,N',N'-tetramethylethylenediamine in 100 parts by mass of pure water. 【0142】<Fabrication of Heat Absorbing Element> (Example 1) A steel leaf spring with a width of 12.1 mm, a length of 60.2 mm, and a height of 7.7 mm was obtained, along with a SUS301 stainless steel plate with a thickness of 0.3 mm, cut to the same width and length as the steel leaf spring. The stainless steel plate was joined to the steel leaf spring with 1 mm of stainless steel tape at both ends to form a restoring member (for example, an elastic member) to be attached later, so that the stainless steel plate would push back against the steel leaf spring. Separately, ceramic wool (average porosity 96.9%, true density 3, bulk density 0.093) was cut to 70 mm in length and 3.5 mm in width, and the ceramic wool was inserted into an aluminum pouch (Mitsubishi Gas Chemical Co., Ltd.'s "Gas Barrier Bag", thickness 0.094 mm, constructed by laminating PET, aluminum foil, and polyethylene) with inner dimensions of 90 mm in length and 40 mm in width. Next, the dispersion (a) was filled into the bag-like body of the aluminum pouch so that the thickness of the ceramic wool portion was 6.29 mm, and the inlet was closed with a heat seal. One of the restoration members was attached to the top 2 mm of the aluminum pouch containing the dispersion (a) and ceramic wool to create a heat absorber (1a) for restorative performance evaluation. Then, the restorative performance of the obtained heat absorber (1a) was evaluated. The evaluation results are shown in Table 1. 【0143】 (Example 2) Width 19.1 mm, length 60.2 mm, thickness 1.5 mm, suction force 0.931 N [gf / cm] ２ Two rubber-type magnetic sheets were prepared. Double-sided tape was attached to one side of the magnetic sheets, and a heat-absorbing body (1b) for restorative performance evaluation was created by attaching it to one location such that the top 2 mm of an aluminum pouch containing the same contents as in Example 1 was sandwiched between the magnetic sheets. The restorative performance evaluation was then performed on the obtained heat-absorbing body (1b). The evaluation results are shown in Table 1. 【0144】(Example 3) Dispersion (b) was poured into a container on which a 15 mm square grid made from a 1 mm wide rod-shaped material was placed in an area of ​​100 mm vertically x 60 mm horizontally, to a height of 9 mm. Then, it was left to stand overnight at room temperature (23°C) to completely harden dispersion (b). The grid-shaped material was removed from dispersion (b) to obtain a hydrogel, which is the contents having the uneven structure shown in Figure 10B. The hydrogel was cut to 70 mm vertically x 40 mm horizontally, inserted into the bag-like body of an aluminum pouch, and the opening was closed with a heat seal to create a heat-absorbing body (2a) for evaluating resilience. 【0145】 (Example 4) Dispersion (b) was poured into a mold in which hemispherical depressions with a diameter of 10 mm were aligned at 1 mm intervals to a height of 9 mm. Then, it was left to stand overnight at room temperature (23°C) to completely harden the dispersion (b). The mold was removed from the dispersion (b) to obtain a hydrogel, which was the contents having the uneven structure shown in Figure 11A. The hydrogel was cut to 70 mm in length and 40 mm in width, inserted into the bag-like body of an aluminum pouch, and the opening was closed with a heat seal to prepare a heat-absorbing body (2b) for evaluating resilience. 【0146】 (Comparative Example 1) As Comparative Example 1, a heat absorber was prepared using the same procedure as in Example 1, except that a restoring member was not attached, and a heat absorber (1c) without an elastic member was prepared. Then, the restorability of the obtained heat absorber (1c) was evaluated. The evaluation results are shown in Table 1. 【0147】 (Comparative Example 2) In Comparative Example 2, the hydrogel (plate-shaped gel) containing the contents was prepared using the same procedure as in Example 2, except that the dispersion (b) was cured without using a grid-like member to create a plate-shaped hydrogel without an uneven structure. A heat-absorbing body (2c) was then prepared by sealing the plate-shaped hydrogel in the aluminum pouch. 【0148】 【0149】 Examples 1 to 4 and Comparative Examples 1 and 2 confirm that the heat-absorbing material of this embodiment deforms sufficiently in response to the expansion of the battery cell, thereby absorbing the expansion of the battery cell, and that a restoring force acts to restore it to its original shape. 【0150】To evaluate the endothermic properties of the contents of the heat-absorbing material, the following heating experiment was conducted using a cone calorimeter. 【0151】 (2) Heating experiment using a cone calorimeter (Preparation of heat-absorbing body) [Heat-absorbing body (3)] 92 parts by mass of the dispersion (a) (water-soluble inorganic powder-containing aqueous solution) prepared by the above method was filled into an aluminum pouch-like body measuring 100 mm in length and 100 mm in width together with 8 parts by mass of ceramic wool, the opening was closed with a heat seal, the aluminum pouch-like body was laid flat between 4.8 mm thick gap materials, a flat plate was placed on top of it and left to stand at 20°C for 10 minutes to prepare the heat-absorbing body (3) for the heating experiment. 【0152】 [Heat-absorbing body (4)] In addition, 94 parts by mass of the dispersion (b) and 6 parts by mass of ceramic wool were filled into the aluminum pouch bag, the aluminum pouch bag was laid flat between 4.8 mm thick gap materials, a flat plate was placed on top of it, and it was left to stand at room temperature for 24 hours to fully harden, after which the injection port was closed with a heat seal to produce a heat-absorbing body (4) for heating experiments. 【0153】 [Heat-absorbing material (5)] 10 parts by mass of ceramic wool (average porosity 97%, true density 3, bulk density 0.092) was inserted into a container made of an aluminum pouch (Mitsubishi Gas Chemical Co., Ltd.'s "Gas Barrier Bag," 0.094 mm thick, constructed by laminating PET, aluminum foil, and polyethylene) measuring 116 mm in length and 116 mm in width. Next, 100 parts by mass of pure water were injected to fill the inside of the aluminum pouch, and the injection port was closed with a heat seal. Then, the aluminum pouch was laid flat between 4.8 mm thick gap materials, a flat plate was placed on top of it, and it was left to stand at 20°C for 10 minutes to produce a 4.8 mm thick sheet-like heat-absorbing material (5) for heating experiments. 【0154】 [Heat absorbent (6)] An aluminum pouch similar to that of the heat absorbent (5) above was made into a bag-like container measuring 116 mm in length and 116 mm in width. 100 parts by mass of the dispersion (b) were poured into the container to fill it, and the opening was sealed with a heat seal. The aluminum pouch bag-like container was then laid flat between 4.8 mm thick gap materials, a flat plate was placed on top, and it was left to stand at 20°C for 15 hours to produce a 4.8 mm thick sheet-like heat absorbent (6) for heating experiments. 【0155】(Method of Heating Experiment) Each heat-absorbing element used for the heating experiment was directly heated by radiant heat using a cone calorimeter (manufactured by Toyo Seiki Co., Ltd.) in accordance with the JIS A 1316 standard. More specifically, the cone calorimeter calculates the heat generation rate, total heat generation amount, etc., from the oxygen consumption method by measuring the oxygen concentration in the combustion exhaust gas and the exhaust gas flow rate, 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. Each heat-absorbing element was placed as a test specimen at the top of the holder, and the temperature was set to 50 kW / m from the cone. ２ The heat was applied to the respective heat absorbers. A thermocouple was then attached to the back surface of each heat absorber, and the temperature change until it reached 160°C was measured using the thermocouple. The evaluation results for each heat absorber are shown in Table 2. 【0156】 【0157】 Table 2 confirms that the heat absorber maintains a sufficient heat absorption effect even when the battery temperature rises or as time passes. 【0158】 According to this disclosure, it is possible to provide a heat-absorbing material that can deform sufficiently in response to the expansion of a battery cell to absorb the expansion of the battery cell, and that has a restoring force that restores it to its original shape. 【0159】 1: Heat absorber 2: Bag body 3: Restoration member 4: Contents 5: Filling section 6: Storage section 7: Recess 7a: Independent void section 7b: Open void section 8: Leaf spring 9a: Rod-shaped body 9b: Plate-shaped body 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, 18b: Battery casing material 19: Activated carbon layer 20: Stacked battery

Claims

1. A heat-absorbing body comprising: contents containing an aqueous solvent; a bag body having a filling section into which the contents are filled, and a storage section capable of accommodating the contents after they have been deformed by the application of an external force to the filling section; and a restoring member that returns the contents stored in the storage section back to the filling section, wherein the deformed state in which the contents are stored in the storage section due to the deformation of the filling section by the external force, and the normal state in which the contents stored in the storage section are returned to the filling section by the restoring member, are reversible.

2. The heat-absorbing body according to claim 1, wherein the housing expands in conjunction with the housing of the contents, and the expanded housing contracts due to the restoration member.

3. The heat-absorbing element according to claim 1 or 2, wherein the maximum capacity of the containment section is 5% by volume or more and 100% by volume or less of the maximum capacity of the filling section.

4. The heat-absorbing material according to claim 1 or 2, wherein the maximum volume of the filling portion is 50% by volume or more and 95% by volume or less of the maximum volume of the bag body.

5. The heat-absorbing body according to claim 1 or 2, wherein the restoring member is an elastic member or a pair of magnets that contract the expanded housing to control the volume of the housing.

6. The heat-absorbing body according to claim 5, wherein the bag body is in the form of a sheet, and the elastic member or the pair of magnets are attached in contact with the surface of the bag body so as to extend in the in-plane direction of the surface of the bag body.

7. The heat-absorbing body according to claim 6, wherein the elastic member is a leaf spring.

8. The heat-absorbing body according to claim 1, wherein the contents are a hydrogel having voids and comprising the aqueous solvent and the hydrogel body, and the hydrogel is the restorative member.

9. The heat-absorbing body according to claim 8, wherein the deformed state in which the deformed portion due to the deformation of the hydrogel is contained in the void which is the housing portion is reversible to the normal state by the restoration of the hydrogel.

10. The heat-absorbing body according to claim 8 or 9, wherein the void is an open void that opens onto the surface of the hydrogel.

11. The heat-absorbing body according to claim 8 or 9, wherein the void is an independent void that exists closed within the hydrogel.

12. The heat-absorbing body according to claim 1 or 2, wherein the contents further comprise an inorganic powder.

13. The heat-absorbing body according to claim 1 or 2, wherein the contents further contain one or more selected from the group consisting of antifreeze agents and inorganic fibers.

14. The heat-absorbing body according to claim 12, wherein the contents change into a porous body when heated to 120°C or higher.

15. A secondary battery module comprising the heat-absorbing element according to claim 1 or 2.

16. A secondary battery module in which the heat-absorbing material according to claim 1 or 2 is sandwiched between battery cells.

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