Elastic member for battery pack and manufacturing method thereof

Abstract

To provide an elastic member for a battery pack, capable of both fixing battery cells and absorbing deformation during expansion, and a method for manufacturing the same.SOLUTION: An elastic member 4 for a battery pack is disposed in a battery pack 1, in which a plurality of battery cells 2 are lined up in a prescribed direction, in a state of being in elastic contact with the battery cells 2. The elastic member 4 for a battery pack comprises a foamed body that satisfies the following conditions (a) to (d). (a) The Asker C hardness is 20 or more and 40 or less. (b) The foaming ratio is 3.0 times or more and 5.0 times or less. (c) The closed-cell ratio is 50% or more and 95% or less. (d) The pore structure in a cross section in a thickness-direction has elliptical pores that are elongated in the thickness direction, when the thickness direction is defined as a direction in which the elastic member is in elastic contact with the battery cells 2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to an elastic member disposed between adjacent battery cells in a battery pack housing a plurality of battery cells, and a method for manufacturing the elastic member. [Background technology]

[0002] Hybrid vehicles and electric vehicles are equipped with battery packs containing multiple battery cells. In the battery pack, a battery module consisting of multiple stacked battery cells is housed in a housing and secured by fastening members on both sides in the stacking direction. To prevent the battery cells from shifting position due to vibrations while the vehicle is running and to absorb deformation (expansion and contraction) of the battery cells due to charging and discharging, elastic members that can elastically deform to follow the deformation of the battery cells are arranged between adjacent battery cells.

[0003] For example, Patent Document 1 describes a multilayer sheet placed between adjacent battery cells. As shown in FIG. 3 of Patent Document 1, the multilayer sheet is formed by stacking a first thermally conductive sheet / insulating sheet / rubber sheet / insulating sheet / first thermally conductive sheet in the stacking direction of the battery cells. The rubber sheet has cushioning properties, which improves adhesion between the battery cell and the first thermally conductive sheet and prevents damage to the first thermally conductive sheet and the insulating sheet due to load. An example of the rubber sheet is a silicone rubber foam sheet. Furthermore, Patent Document 2 describes a sheet-like pressing member that is placed between adjacent battery cells and presses the battery cells against the wall of the cell storage space. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-99940 [Patent Document 2] Japanese Patent Application Publication No. 2020-64795 Summary of the Invention [Problem to be solved by the invention]

[0005] The elastic member is required to fully exhibit battery performance by elastically deforming in response to the expansion and contraction of the battery cell that accompanies charging and discharging. That is, the elastic member must be able to fix the battery cell with its biasing force when the battery cell is discharging (contracting), and absorb the deformation of the battery cell by compressively deforming when the battery cell is charging (expanding).

[0006] For example, paragraph

[0049] of the above-mentioned Patent Document 2 describes that the pressing member includes a rubber or resin foam, and paragraph

[0050] describes that the foam's expansion ratio can be adjusted to adjust the pressing force against the battery cell and the degree to which the foam absorbs the battery cell's expansion force. However, for example, decreasing the expansion ratio reduces the proportion of air bubbles contained and makes the foam harder. As a result, the stress (repulsion force) during compression increases, making the elastic member (foam) less susceptible to compression deformation. Conversely, increasing the expansion ratio increases the proportion of air bubbles contained and reduces hardness. This reduces the stress during compression, making the elastic member more compressible, allowing it to absorb deformation due to battery cell expansion. However, the repulsion force during low compression decreases, resulting in insufficient biasing force against the battery cell. Thus, simply adjusting the expansion ratio makes it difficult to both secure the battery cell during contraction and absorb deformation during expansion.

[0007] The present disclosure has been made in consideration of the above-described circumstances, and aims to provide an elastic member for a battery pack that can both fix a battery cell and absorb deformation during expansion, and a method for manufacturing the same. [Means for solving the problem]

[0008] (1) In order to solve the above-mentioned problems, the elastic member for a battery pack of the present disclosure is an elastic member for a battery pack that is arranged in elastic contact with a plurality of battery cells in a battery pack in which the battery cells are arranged side by side in a predetermined direction, and is characterized in that it is made of a foam that satisfies the following (a) to (d): (a) Asker C hardness is 20 or more and 40 or less. (b) The expansion ratio is 3.0 times or more and 5.0 times or less. (c) The closed bubble rate is 50% or more and 95% or less. (d) The thickness direction is the direction in which the battery cell elastically contacts the cell, and the cell structure in a cross section in the thickness direction has elliptical cells that are elongated in the thickness direction.

[0009] In the elastic member for a battery pack (hereinafter sometimes simply referred to as the "elastic member") disclosed herein, the hardness and cell structure of the foam material are optimized to achieve both a restraining force when the battery cell (hereinafter sometimes simply referred to as the "cell") contracts and deformation absorption when the battery cell expands.

[0010] First, by setting the Asker C hardness within the range (a), the required hardness of the elastic member is ensured. This prevents the biasing force on the battery cell from decreasing even when the battery cell shrinks, and allows the cell to be fixed in place. Second, by setting the expansion ratio within the range (b), the closed cell ratio (closed cell ratio) within the range (c), and the cell shape within the range (d), the stress in the elastic member, mainly during high compression, is reduced. This makes it easier for the elastic member to compress when the battery cell expands, and allows it to absorb deformation due to cell expansion.

[0011] (2) In the above configuration, the ratio of the major axis to the minor axis of the elliptical cells (major axis / minor axis) may be 1.4 or more and 3.0 or less. This configuration allows the shape of the elliptical cells observed in a cross section in the thickness direction to be a desired ellipse. This configuration is suitable for achieving both hardness of the elastic member and a reduction in stress during high compression.

[0012] (3) In any of the above configurations, the average diameter of the cells in the cross section of the foam in the thickness direction may be 250 μm or more and 600 μm or less. This configuration allows the size of the cells in the foam to be adjusted to a desired size. This configuration is suitable for achieving both hardness of the elastic member and a reduction in stress during high compression.

[0013] (4) In any of the above configurations, the thickness of the elastic member may be 3 mm or more and 5 mm or less. According to this configuration, the elastic member is thin, making it suitable for placement in a limited space within a battery pack. Furthermore, according to this configuration, even if the battery cells expand significantly, the spacing between adjacent battery cells can be secured via the elastic member, thereby maintaining insulation between the cells. Note that it is more effective to combine all of the above configurations (1) to (4).

[0014] (5) A method for producing an elastic member for a battery pack according to the present disclosure, which is one embodiment of the method for producing an elastic member for a battery pack according to the present disclosure, is characterized by comprising a raw material composition placing step of placing a raw material composition of the foam in a molding die, a sheet member placing step of placing a sheet member on the surface of the raw material composition, and a foam molding step of foam-molding the raw material composition with the surface covered with the sheet member.

[0015] In the manufacturing method of the present disclosure, foam molding is performed with a sheet member placed on the surface of the raw material composition. This restricts the foaming reaction in the planar direction (the direction perpendicular to the thickness direction) of the raw material composition. As a result, bubbles tend to grow in the thickness direction during the foaming reaction, forming ellipsoidal bubbles with their major axes (longest lengths) oriented in the thickness direction. Furthermore, sheet-like foams can be molded even at high expansion ratios. [Effects of the Invention]

[0016] The elastic member for a battery pack of the present disclosure can both fix the battery cell and absorb deformation during expansion. The manufacturing method of the present disclosure can control the growth direction of the bubbles during the foaming reaction and increase the foaming ratio, so the elastic member for a battery pack of the present disclosure can be manufactured relatively easily and at low cost. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a cross-sectional schematic diagram of a battery pack in which an elastic member of the present disclosure is disposed. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, embodiments of the elastic member for a battery pack and the manufacturing method thereof according to the present disclosure will be described. Note that the embodiments are not limited to the following embodiments, and various modifications and improvements that can be made by those skilled in the art can be made.

[0019] <Elastic material for battery packs> The elastic member of the present disclosure is arranged in elastic contact with a battery cell in a battery pack in which a plurality of battery cells are arranged side by side in a predetermined direction. In a battery pack to which the elastic member of the present disclosure is applied, the type of battery cell is not particularly limited. Examples include lithium-ion batteries and nickel-metal hydride batteries. In the battery pack, components other than the battery cells and the elastic member are not particularly limited. For example, a battery module can be formed by stacking a plurality of cells and the elastic member, and the battery module can be housed in a housing together with fastening members that fasten the battery module from both sides in the stacking direction to form a battery pack.

[0020] The arrangement of the elastic member of the present disclosure and its size relative to the battery cells and the like are not particularly limited. The elastic member may be arranged between adjacent cells in the stacking direction, or between the cells at both ends of the stacking direction and fastening members or the like. The "state of elastic contact with the battery cells" of the elastic member of the present disclosure may be direct or indirect. That is, the elastic member of the present disclosure may be in direct contact with the battery cells, or may be in indirect contact via, for example, an insulating sheet or the like. The elastic member may or may not be fixed to adjacent members. When fixed, it may be attached using an adhesive or double-sided tape, or may be pressure-bonded.

[0021] The thickness of the elastic member may be determined appropriately taking into consideration the placement space, ensuring insulation between cells, etc. For example, from the perspective of achieving a thinner design, it is desirable to set the thickness to 6 mm or less, or even 5 mm or less. On the other hand, from the perspective of ensuring sufficient spacing between adjacent cells via the elastic member when the cells expand, it is desirable to set the thickness to 2 mm or more, or even 3 mm or more. The thickness and other dimensions of the elastic member may be changed depending on the placement location in the battery pack.

[0022] The elastic member of the present disclosure is made of a foam that satisfies the following (a) to (d). (a) Asker C hardness is 20 or more and 40 or less. If the foam's Asker C hardness is less than 20, the repulsive force at low compression is small, which may result in an insufficient biasing force on the battery cell, making it difficult to prevent displacement. A more suitable Asker C hardness is 25 or higher. Conversely, if the foam's Asker C hardness is greater than 40, the compressive stress of the foam (elastic member) increases, making it difficult for the foam to deform in response to the expansion of the battery cell. A more suitable Asker C hardness is 38 or lower.

[0023] In this specification, the Asker C hardness of a foam is a value obtained by the following measurement method using a Type C hardness tester (for example, the "Asker Rubber Hardness Tester Type C" manufactured by Kobunshi Keiki Co., Ltd.) Measurement method: A test piece having a thickness of 10 mm is prepared, and the indenter of the Type C hardness tester is pressed against the center of the piece with a load of 1 kgf (9.81 N), and the value after 3 seconds is measured.

[0024] (b) The expansion ratio is 3.0 times or more and 5.0 times or less. If the foam's expansion ratio is less than 3.0, the foam will have fewer air bubbles and will be closer to a solid, resulting in increased compressive stress and making it difficult for the foam to deform in response to the expansion of the battery cell. A more suitable expansion ratio is 3.1 or more. Conversely, if the foam's expansion ratio is greater than 5.0, the foam will have more air bubbles, reducing hardness and reducing resilience at low compression. As a result, the urging force applied to the battery cell may be insufficient, potentially making it difficult to adequately prevent displacement. A more suitable expansion ratio is 4.9 or less.

[0025] The expansion ratio of the foam may be calculated by measuring the density before and after foam molding in the process of producing the foam, and then using the following formula (I). Expansion ratio = density of foam raw material composition / density of foam (I)

[0026] (c) The closed bubble rate is 50% or more and 95% or less. If the closed cell ratio of a foam is less than 50%, the number of open cells increases, resulting in increased air ingress and egress into the foam, resulting in reduced hardness and reduced resilience at low compression. A more preferable closed cell ratio is 60% or more, and even 65% or more. Conversely, if the closed cell ratio of a foam is greater than 95%, the compressive stress of the foam may become too great. A more preferable closed cell ratio is less than 90%, and even 88% or less.

[0027] In this specification, the closed cell ratio of a foam is a value measured by the following method. In the measurement method, "foams" and "solids" are both polymers, and the only difference is whether or not they contain a blowing agent. (i) Cut out cube-shaped test pieces with sides of 1 to 2 cm from both the foam and the solid body. These test pieces are used for the following measurements. First, measure the density of the foam, the density of the solid body, and the dry mass of the foam. The mass and density measurements are performed at 23°C (same below). (ii) Submerge the foam in water, reduce the pressure to approximately 4 kPa (30 mmHg), and leave it for 3 minutes. Return the pressure to normal and leave it for another 3 minutes, then remove it, wipe off the surface moisture, and measure the mass of the foam after absorbing water. (iii) The open cell ratio is calculated using the following formula, and the closed cell ratio is calculated from this value. The dry mass of the foam is W dry (g), the mass of the foam after absorbing water is W wet (g), the density of the foam is D f (g / cm 3 ), the density of the solid is D s (g / cm 3 ), then the total volume of the bubbles is A (cm 3 ) is [A=(W dry / D f )-(W dry / D s )]. Also, if the density of water is 1g / cm 3 If so, the volume of the open cells is B (cm 3 ) is [B=W wet -W dry ]. The open cell ratio is the volume ratio of open cells to the total volume of cells, so the open cell ratio (%) is calculated as [open cell ratio = 100 x (B / A)]. The closed cell ratio (%) is calculated as [closed cell ratio = 100 - open cell ratio].

[0028] (d) The thickness direction is the direction in which the battery cell elastically contacts the cell, and the cell structure in a cross section in the thickness direction has elliptical cells that are elongated in the thickness direction. The cell shape of a foam can be identified by observing a cross section in the thickness direction under a microscope. Elliptical cells are thought to be cells that have been formed into an elliptical sphere by stretching spherical cells due to stress acting in the thickness direction during foam molding, or by compressing them due to stress acting in a direction perpendicular to the thickness direction. When elliptical cells are present in the cross section in the thickness direction, in other words, when a foam has elliptical spherical cells with their major axes oriented in the thickness direction, the compressive stress in the thickness direction of the foam can be reduced compared to when such cells are absent or are few in number. This allows the foam to be significantly compressed when a battery cell expands, absorbing deformation due to cell expansion.

[0029] The degree of flattening of the elliptical cells is not particularly limited. For example, the ratio of the major axis to the minor axis of the elliptical cells in a cross section in the thickness direction (major axis / minor axis, hereinafter sometimes referred to as the "aspect ratio") is preferably 1.4 or more and 3.0 or less. The aspect ratio of the elliptical cells can be calculated by taking a micrograph of the cross section in the thickness direction of the foam, taking the maximum length of the observed elliptical cells as the major axis, and the length of the perpendicular bisector of the major axis as the minor axis, and dividing the major axis by the minor axis. In the elastic member of the present disclosure, the micrograph obtained is subjected to image analysis to perform binarization and noise removal, and then the average aspect ratio of the cells obtained using a convexity of 0.84 or more as a threshold is preferably 1.4 or more.

[0030] The cells observed in the cross section of the foam in the thickness direction may all be elliptical, or may be a mixture of elliptical cells, circular cells, and other irregularly shaped cells. From the viewpoint of reducing compressive stress, the average diameter of the cells observed in the cross section of the thickness direction is preferably 250 μm or more, and more preferably 320 μm or more. On the other hand, from the viewpoint of ensuring the desired hardness, the average diameter of the cells is preferably 600 μm or less, and more preferably 550 μm or less. The average diameter of the cells is determined by taking a micrograph of the cross section of the foam in the thickness direction at 100x magnification, measuring the maximum length of each cell observed, and calculating the average value.

[0031] The elastic member of the present disclosure preferably has the following physical properties (e) and (f) in addition to the above-mentioned properties (a) to (d).

[0032] (e) Density is 250 kg / m 3 More than 500kg / m 3 The following is the result. The density of the foam is 250 kg / m 3 (0.25g / cm 3 ), the hardness decreases and the repulsive force at low compression decreases. As a result, the biasing force on the battery cell may be insufficient, and displacement may not be sufficiently suppressed. On the other hand, if the density of the foam is less than 500 kg / m 3 (0.5g / cm 3 ), the compressive stress of the foam increases, making it difficult for the foam to deform in response to the expansion of the battery cell.

[0033] (f) The 25% compressive stress is 0.235 MPa or more and 1.015 MPa or less. The 25% compressive stress is the force required to compress the foam by 25% (until the thickness is reduced to 3 / 4) in the thickness direction (the direction in which it elastically contacts the battery cell). If the 25% compressive stress of the foam is less than 0.235 MPa, the repulsive force at low compression is small, which may result in insufficient biasing force against the battery cell and insufficient prevention of displacement. Conversely, if the 25% compressive stress of the foam is greater than 1.015 MPa, it may be difficult for the foam to deform in response to the expansion of the battery cell. In this specification, the 25% compressive stress is a value measured in accordance with Method C specified in JIS K6254:2016.

[0034] An embodiment of the elastic member of the present disclosure is shown below. FIG. 1 shows a cross-sectional schematic diagram of a battery pack in which the elastic member of the present disclosure is arranged. Regarding the orientation in the figure, the arrangement direction of the battery cells (thickness direction of each component, stacking direction) is defined as the X direction, and of the two directions perpendicular to the X direction, the longitudinal direction of the battery cells is defined as the Y direction, and the lateral direction (the direction corresponding to the direction of gravity) is defined as the Z direction. First, the configuration of the battery pack of this embodiment will be described. As shown in FIG. 1, the battery pack 1 has a housing 10, multiple battery cells 2, a heat insulating material 3, and an elastic member 4.

[0035] The housing 10 is made of metal and has a box shape. The battery cells 2 are composed of lithium ion batteries and are housed in the housing 10. The battery cells 2 each have a rectangular thin plate shape and are stacked in the thickness direction (X direction). The heat insulating material 3 has a rectangular sheet shape with a thickness of 2 mm. The heat insulating material 3 has an insulating layer containing silica aerogel. Between adjacent battery cells 2, the heat insulating material 3 contacts one side of the battery cell 2, i.e., one of the YZ faces. The elastic member 4 has a rectangular sheet shape with a thickness of 4.5 mm. The elastic member 4 is made of a silicone rubber foam that satisfies the above-mentioned (a) to (d). Between adjacent battery cells 2, the elastic member 4 is in elastic contact with one side (YZ face) of the battery cell 2. The heat insulating material 3 and the elastic member 4 are stacked in the thickness direction (X direction) and are arranged between adjacent battery cells 2 and between the battery cells 2 at both ends in the X direction and the housing 10.

[0036] Next, the effects of the elastic member and battery pack of this embodiment will be described. The elastic member 4 elastically contacts the battery cells 2 and repeatedly compresses and restores in response to the expansion and contraction of the battery cells 2 during charging and discharging. This prevents the battery cells 2 from shifting position while the vehicle is running, and absorbs deformation due to the expansion of the battery cells 2. As a result, the performance of the battery cells 2 can be fully realized. Furthermore, if the temperature of one battery cell 2 rises for some reason, this heat may be transferred to an adjacent battery cell 2, potentially causing a serious incident due to a thermal chain reaction. In this regard, in this embodiment, by disposing the insulating material 3 together with the elastic member 4, the insulating effect of the insulating material 3 is exerted, and thermal chain reaction can be avoided.

[0037] <Method of manufacturing elastic member for battery pack> The method for producing the elastic member of the present disclosure is not particularly limited. As a preferred embodiment of the production method, the method for producing the elastic member of the present disclosure includes a raw material composition arrangement step, a sheet member arrangement step, and a foam molding step. Each step will be described below.

[0038] [Raw material composition placement process] This step involves placing the foam raw material composition in a mold. The raw material composition contains the foam base polymer and, if necessary, a crosslinking agent, catalyst, blowing agent, foam stabilizer, etc. These raw materials are mechanically stirred in advance using a propeller or the like to form a raw material composition, which is then poured into the mold. The type of base polymer is not particularly limited, but rubbers such as silicone rubber, ethylene-propylene rubber (EP), ethylene-propylene-diene rubber (EPDM), isoprene rubber (IR), and acrylonitrile butadiene rubber (NBR) are desirable from the viewpoint of low temperature dependence. The amounts of the crosslinking agent, catalyst, blowing agent, etc. may be appropriately adjusted so that the hardness, expansion ratio, etc., fall within the desired range.

[0039] [Sheet component placement process] This step involves placing a sheet member on the surface of the raw material composition placed in the mold in the previous step. Examples of the sheet member that can be used include polyesters such as polyethylene terephthalate (PET), resin films such as polyimides, and cloths made of resin fibers. Among these, a material that adheres closely to the raw material composition during the foaming and curing reaction of the raw material composition but does not easily adhere to the foam produced after the reaction is desirable. A polyimide film is a suitable example of such a material. The sheet member may be coated with a release agent or the like. Considering ease of handling, the thickness of the sheet member is preferably 10 μm or more. Furthermore, considering the adhesion to the raw material composition and the effect on the foaming and curing reactions, a thinner sheet member is desirable, for example, 200 μm or less.

[0040] The sheet member may be placed on the surface of the raw material composition from the viewpoint of regulating the direction of the foaming reaction, but may also be placed on the bottom of the mold from the viewpoint of increasing the regulating force. In this case, the sheet member may be placed on the bottom of the mold in advance, and the raw material composition may be poured therein.

[0041] [Foam molding process] This process involves foam-molding the raw material composition while its surface is covered with a sheet material. Covering the surface of the raw material composition with a sheet material restricts the foaming reaction in the planar direction (the direction perpendicular to the thickness direction), facilitating the growth of bubbles in the thickness direction. The temperature, pressure, and time used during foam molding can be determined appropriately depending on the type of base polymer, taking into account the progress of the foaming and curing reactions, the expansion ratio, and other factors. Foam molding can be performed in one stage, or it can be divided into two or three stages by changing the conditions. For example, in the first stage, the foaming and curing reactions can be partially carried out at a temperature lower than the specified molding temperature, and then in the second stage, the foaming and curing reactions can be completed at the specified molding temperature. In this case, if the first stage is performed for a short time, the curing reaction will not proceed smoothly, resulting in the foaming reaction proceeding. Conversely, if the first stage is performed for a long time, the curing reaction will proceed smoothly, resulting in the foaming reaction proceeding slowly. Therefore, the expansion ratio and other factors can be adjusted by changing the reaction time in the first stage. After foam molding is completed, the sheet material is peeled off to obtain a foam. Thereafter, if necessary, a heat treatment may be carried out in which the foam is kept at a predetermined temperature. [Example]

[0042] Next, the present disclosure will be described more specifically with reference to examples.

[0043] <Manufacturing elastic member samples> [Example 1] An elastic member sample of Example 1 was produced as follows: First, 100 parts by mass of silicone rubber a ("TSE221-5U" manufactured by Momentive Performance Materials Japan, LLC) as the base polymer of the foam, 0.2 parts by mass of di-(4-methylbenzoyl) peroxide ("TC-12" manufactured by Momentive Performance Materials Japan, LLC) as the crosslinking agent a, 1.4 parts by mass of 2,5-dimethyl-2,5-di(t-butylperoxy)hexane ("TC-8" manufactured by Momentive Performance Materials Japan, LLC) as the crosslinking agent b, and 3 parts by mass of 1,1'-azobis(1-acetoxy-1-phenylethane) ("OT AZO-15" manufactured by Otsuka Chemical Co., Ltd.) as the blowing agent were stirred at room temperature to prepare a raw material composition, which was then poured into a molding die (150 ton press) (raw material composition placement step). A polyimide film ("Kapton (registered trademark) 50H" manufactured by DuPont-Toray Co., Ltd., thickness 12.5 μm) was placed on the bottom of the mold beforehand, and the raw material composition was poured onto it. Next, the same polyimide film as that placed on the bottom was placed so as to cover the entire surface of the raw material composition (sheet member placement step). The polyimide film is included in the concept of the sheet member in the manufacturing method of the present disclosure.

[0044] The foam molding process was carried out in two stages. In the first stage, foam molding was carried out for 3 minutes at a pressure of 150 t, a target thickness of 1 mm, and a temperature of 100°C. Subsequently, in the second stage, foam molding was carried out for 10 minutes at a pressure of 150 t, a target thickness of 4.5 mm, and a temperature of 170°C. After demolding, the polyimide films were peeled off from both the front and back of the resulting silicone rubber foam, and the silicone rubber foam was heat-treated by holding it in an oven at 200°C for 4 hours. In this way, a 4.5 mm-thick sheet-shaped silicone rubber foam (elastic member sample of Example 1) was produced.

[0045] [Example 2] The elastic member sample of Example 2 was manufactured in the same manner as in Example 1, except that in the manufacturing method of the elastic member sample of Example 1, the base polymer of the foam was changed to silicone rubber b ("TSE221-7U" manufactured by Momentive Performance Materials Japan, LLC) and the time for the first stage of the foam molding process was shortened to 1 minute.

[0046] [Example 3] The elastic member sample of Example 3 was manufactured in the same manner as in Example 1, except that in the manufacturing method of the elastic member sample of Example 1, the base polymer of the foam was changed to silicone rubber B (same as above) and the time for the first stage of the foam molding process was shortened to 2 minutes.

[0047] [Example 4] An elastic member sample of Example 4 was produced in the same manner as in Example 1, except that the base polymer of the foam was changed to silicone rubber b (same as above).

[0048] [Example 5] The elastic member sample of Example 5 was manufactured in the same manner as in Example 1, except that in the manufacturing method of the elastic member sample of Example 1, the base polymer of the foam was changed to silicone rubber B (same as above) and the time for the first stage of the foam molding process was extended to 4 minutes.

[0049] [Comparative Example 1] A highly foamed silicone rubber sheet "E15" manufactured by Tigers Polymer Co., Ltd. was prepared as an elastic member sample of Comparative Example 1.

[0050] Comparative Example 2 A low-foaming silicone rubber sheet "SPO-35R1" manufactured by Tigers Polymer Co., Ltd. was prepared as an elastic member sample of Comparative Example 2.

[0051] Comparative Example 3 A highly foamed silicone rubber sheet "HT-800" manufactured by Inoac Corporation was prepared as an elastic member sample of Comparative Example 3.

[0052] <Evaluation method> [density] The density of each sample was measured using a hydrometer "DSG-1" manufactured by Toyo Seiki Seisakusho Co., Ltd.

[0053] [Asker C hardness] The Asker C hardness of each sample was measured using an "Asker Rubber Hardness Tester Type C" manufactured by Kobunshi Keiki Co., Ltd. For the measurement, a 10 mm thick test piece, which was separately manufactured for hardness measurement using the same manufacturing method, was used. An indenter was pressed against the center of the test piece with a load of 1 kgf (9.81 N), and the value was measured after 3 seconds.

[0054] [Expansion ratio] For the samples of the examples, the expansion ratio was calculated by dividing the density of the raw material composition by the density of each sample according to the above-mentioned formula (I). The density of the raw material composition was also measured using a hydrometer (same as above) manufactured by Toyo Seiki Seisaku-sho, Ltd. For the samples of the comparative examples, the catalog value was used for the density of the sample, and the density of the raw material composition was calculated using an estimated value obtained by estimating the density of the solid body.

[0055] [Closed foam rate] The closed cell ratio was calculated using the above-mentioned measurement methods (i) to (iii). Note that the density of the solid body in the comparative sample was an estimated value.

[0056] [Bubble structure] Cross-sectional photographs of each sample were taken at 30x magnification using a microscope to examine the bubble structure. The cross-sectional photographs were then binarized and denoised using image analysis with the No-Local Means Noising method. The binarization process was performed with h: 13, Template: 21, Search: 35, Block Size: 87, and C: 2, and noise removal was performed with Noise: 500. Here, h is the filter strength, Template is the size of the search area, Search is the size of the search area, Block Size is the size of the area referenced for calculating the threshold, and C is the threshold correction. The aspect ratio (major axis (maximum length) / minor axis) of the remaining shapes was then measured using a convexity (solidity) of 0.84 as the threshold to identify the bubble shape. In this example, an aspect ratio of 1.4 or greater was considered "elliptical," and an aspect ratio of less than 1.4 was considered "circular." In each sample, the bubbles recognized as "elliptical" all had an elliptical shape that was long in the thickness direction. On the other hand, the bubbles recognized as "circular" did not have a consistent direction of the long diameter, and no orientation in the thickness direction was observed. In addition, cross-sectional photographs of the thickness direction of each sample were taken at 100x magnification using a microscope, and the maximum length of each observed bubble was measured. The average value was calculated to determine the average diameter.

[0057] [25% compressive stress] The force required to compress each sample in the thickness direction by 25% (until the thickness reached 3.4 mm) was measured according to Method C specified in JIS K6254:2016. For the measurement, a rectangular parallelepiped test piece measuring 80 mm square and 4.5 mm thick, which had been separately manufactured using the same manufacturing method for compressive stress measurement, was used.

[0058] <Measurement results> The composition of the raw material composition of each sample and the measurement results of its physical properties are summarized in Table 1. In the overall evaluation in Table 1, cases where all of the above-mentioned (a) to (d) are satisfied and the 25% compressive stress is 0.235 MPa or more and 1.015 MPa or less are indicated by a circle, and cases where any of these conditions are not satisfied are indicated by an x. [Table 1]

[0059] As shown in Table 1, the samples of Examples 1 to 5 satisfied all of (a) to (d) and had a 25% compressive stress of 0.235 MPa or more and 1.015 MPa or less. These confirmed that the samples of Examples 1 to 5 are suitable as elastic members for battery packs. In contrast, the sample of Comparative Example 1 satisfied (a) to (c), but did not satisfy (d) because it did not have elliptical cells elongated in the thickness direction. As a result, the 25% compressive stress was lower than that of the samples of the Examples, resulting in insufficient resilience at low compression. Furthermore, the low 25% compressive stress resulted in a large amount of deformation at high compression, making it difficult to ensure sufficient spacing between adjacent battery cells. Furthermore, the sample of Comparative Example 2 had a high Asker C hardness and a low expansion ratio, failing to satisfy (a) and (b). As a result, the 25% compressive stress was higher than that of the samples of the Examples. Furthermore, the sample of Comparative Example 3 had a low Asker C hardness and did not satisfy (a). As a result, the 25% compressive stress was lower than that of the samples of the Examples. From the above, it was confirmed that the samples of Comparative Examples 1 to 3 are not suitable for use as elastic members for battery packs. [Explanation of symbols]

[0060] 1: Battery pack, 10: Housing, 2: Battery cell, 3: Heat insulating material, 4: Elastic member.

Claims

1. In a battery pack in which a plurality of battery cells are arranged side by side in a predetermined direction, an elastic member for a battery pack is arranged in elastic contact with the battery cells, An elastic member for a battery pack, comprising a foam that satisfies the following (a) to (d): (a) Asker C hardness is 20 or more and 40 or less. (b) The expansion ratio is 3.0 times or more and 5.0 times or less. (c) The closed cell rate is 50% or more and 95% or less. (d) The thickness direction is the direction in which the battery cell elastically contacts the cell, and the cell structure in a cross section in the thickness direction has elliptical cells that are elongated in the thickness direction.

2. The elastic member for a battery pack according to claim 1 , wherein the ratio of the major axis to the minor axis of the elliptical cells (major axis / minor axis) is 1.4 or more and 3.0 or less.

3. 2. The elastic member for a battery pack according to claim 1, wherein an average diameter of the bubbles in the cross section in the thickness direction of the foam is 250 μm or more and 600 μm or less.

4. 2. The elastic member for a battery pack according to claim 1, wherein the thickness is 3 mm or more and 5 mm or less.

5. 2. The method for manufacturing an elastic member for a battery pack according to claim 1, a raw material composition placing step of placing a raw material composition of the foam in a molding die; a sheet member arranging step of arranging a sheet member on the surface of the raw material composition; a foam molding step of foam molding the raw material composition in a state where the surface is covered with the sheet member; 1. A method for manufacturing an elastic member for a battery pack, comprising:

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