Heat insulation sheet and method for producing the same

Abstract

The purpose of the present invention is to provide a heat-insulating sheet that is placed between battery cells and can reduce the influence on the adjacent battery cell not only during normal use but also when one battery cell becomes hot. A heat-insulating sheet (11) with silica xerogel (13)-supported glass fiber sheets (12), the heat-insulating sheet (11) being configured so as to contain an infrared absorber (14) constituting 2-15% by weight. Thus, even when one battery cell becomes hot and emits infrared rays, it is possible to prevent the infrared rays from reaching the adjacent battery cell.

Description

Technical Field

[0001] The present disclosure relates to a heat insulating sheet used as a heat insulating measure and a method for manufacturing the same.

Background Art

[0002] In recent years, the demand for energy conservation has been increasing. As a method for realizing energy conservation, there is a method of improving energy efficiency by heat-insulating equipment. In addition, in a secondary battery or the like in which a plurality of battery cells are combined, there is also a desire to insulate between two adjacent battery cells so that when one battery cell becomes hot, it does not affect the adjacent battery cell. As one of these measures, a heat insulating sheet using silica xerogel having excellent heat insulating effect may be used between two adjacent battery cells.

[0003] Note that, as prior art document information related to the invention of this application, for example, Patent Document 1 is known.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

[0005] The heat insulation sheet using silica xerogel exhibits heat insulation performance by suppressing air convection. However, when this heat insulation sheet is used between two adjacent battery cells, if one battery cell becomes too hot, heat will be transferred not only by direct heat conduction from one battery cell to the other but also by infrared radiation. In contrast, silica xerogel easily transmits infrared rays. Therefore, when one of the two adjacent battery cells becomes hot, infrared rays are radiated from its surface. These infrared rays pass through the heat insulation sheet and irradiate the other battery cell, thereby transferring heat to the other battery cell. As a result, there arises a problem that the temperature of the other battery cell is increased.

[0006] In order to solve the above problems, the invention according to the present disclosure is a heat insulation sheet in which silica xerogel is carried on a fiber sheet, and this heat insulation sheet is configured to contain an infrared absorber in a weight ratio of 2% or more and 15% or less.

[0007] By configuring as described above, at normal temperatures, heat insulation performance is exhibited by silica xerogel, and even if one battery cell becomes hot and radiates infrared rays, they are absorbed by the infrared absorber contained in the heat insulation sheet, suppressing heat transfer to the adjacent battery cell.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0009] Hereinafter, a heat insulation sheet in one embodiment of the present disclosure will be described.

[0010] Note that the configuration, size, shape, etc. of the heat insulation sheet shown below are merely examples, and are not intended to be limited to such configuration etc. without particular notice.

[0011] FIG. 1 is a cross-sectional view of a heat insulation sheet 11 in an embodiment of the present disclosure. FIG. 2 is a plan view of the heat insulation sheet 11. FIG. 1 is a cross-sectional view of a portion cut along line segment I-I shown in FIG. 2 and perpendicular to the upper surface of the heat insulation sheet 11. The heat insulation sheet 11 is composed of a glass fiber sheet 12 having a space inside and silica xerogel 13 carried in the space of the glass fiber sheet 12, and has a rectangular shape with a length of about 150 mm and a width of about 100 mm, and a thickness of about 1 mm. The glass fiber sheet 12 is made of glass fibers with an average fiber thickness of about 10 μm, and the proportion of the space occupied in the glass fiber sheet 12 is about 90%. The space inside the glass fiber sheet 12 is filled with silica xerogel 13. Since this silica xerogel 13 has nano-sized spaces inside, the thermal conductivity of the portion filled with this silica xerogel 13 is about 30 mW / m·K. Note that this silica xerogel is a xerogel in a broad sense in a state where the gel is dried, and it may be obtained by methods such as supercritical drying and freeze drying, not just ordinary drying. Furthermore, water is dispersed and contained as an infrared absorber 14 throughout the heat insulation sheet 11, and the ratio of the weight of water to the weight of the entire heat insulation sheet 11 is about 6%.

[0012] It is desirable that the thermal conductivity of this heat insulation sheet at 25°C be 25 mW / m·K or more and 60 mW / m·K or less. By doing so, a sufficient heat insulation effect can be obtained both at normal temperature and when it becomes high temperature.

[0013] The heat insulation sheet 11 configured as described above is disposed between the battery cells. At normal operating temperatures, most heat transfer occurs through heat conduction. Therefore, this heat insulation sheet 11 can sufficiently suppress heat transfer between the battery cells. However, when one battery cell becomes hot, heat is transferred not only by normal heat conduction but also by infrared radiation. Since the glass fiber sheet 12 or the silica xerogel 13 allows most infrared rays to pass through, the heat due to infrared radiation is directly transferred to the adjacent battery cell, which may cause the adjacent battery cell to also become hot and be affected. For this reason, an aluminum (Al) layer is provided on the moisture-free heat insulation sheet 11. It has been attempted to use the aluminum layer to reflect infrared rays so that the heat insulation sheet can perform heat shielding (Patent Document 2). However, simply reflecting the infrared rays causes the infrared rays to return to the battery cell, making it easier for the temperature to rise further and not achieving a sufficient effect. In contrast, the heat insulation sheet 11 of this embodiment can suppress temperature rise because the moisture dispersed in the heat insulation sheet 11 absorbs infrared rays and takes away the latent heat by vaporizing when the temperature rises.

[0014] In the heat insulation sheet using the ordinary silica xerogel 13, since the silica xerogel 13 is hydrophobized, the moisture content is less than 0.2% and it can hardly absorb infrared rays. However, the silica xerogel 13 of this embodiment contains about 6% of moisture as the infrared absorber 14, so it can sufficiently absorb infrared rays. The water content is desirably 2% or more and 15% or less by weight of the heat insulation sheet 11. If the content rate is lower than 2%, it cannot sufficiently absorb infrared rays. If it exceeds 15%, the heat conductivity of the heat insulation sheet 11 itself becomes high due to the heat conduction of water.

[0015] Water has an absorption rate of approximately 96% per 10 μm thickness for a wavelength of 3 μm. On the other hand, the peak of infrared radiation when the surface of the battery cell reaches about 600 °C is approximately 3 μm. That is, water can sufficiently absorb the infrared rays when the battery cell reaches about 600 °C. By dispersing water in the heat insulation sheet 11 as described above, the infrared rays radiated from the heated cell can be absorbed, preventing heat transfer. Furthermore, since water takes away the latent heat when it vaporizes, it can further prevent the temperature from rising.

[0016] Also, when considering the heat insulation sheet 11, it is desirable to have a transmittance of 2% or less per 1 mm thickness for infrared rays with a wavelength of 3 μm. By doing so, it is possible to prevent adverse effects on other battery cells even when one battery cell becomes hot.

[0017] When all the water contained in the heat insulation sheet 11 disappears, the effect of infrared absorption is lost. However, since it takes about several minutes for the battery cell to become hot, if heat transfer can be blocked even only during this time, the influence on adjacent battery cells can be reduced. In this case, for a battery cell that has once become hot, it can be replaced with a new battery cell later. At that time, the heat insulation sheet 11 in contact with the hot battery cell can also be replaced with a new heat insulation sheet 11. By doing so, a battery composed of a combination of multiple battery cells can continue to be used. Examples of a battery composed of a combination of multiple battery cells include secondary batteries.

[0018] Also, the material contained in the heat insulation sheet 11 does not have to be water. As long as it is a material having an absorption rate of 90% or more per thickness 10μm for infrared rays with a wavelength of 3 μm, the same effect can be obtained. As an example, ethylene glycol can be used. When ethylene glycol is used as the infrared absorption material 14, since its boiling point is higher than that of water, it can be prevented from evaporating naturally.

[0019] Also, it is desirable that the infrared absorbing material be a material that is liquid at room temperature. Since it absorbs heat and takes away the latent heat when vaporizing from a liquid, it can prevent the temperature from rising.

[0020] Also, the heat insulating sheet 11 may contain moisture and may be added with finely cut aluminum foil, other finely cut metal layers, or alumina (Al2O3) particles of about several tens of μm so that the heat insulating sheet 11 reflects infrared rays.

[0021] Also, the surface of the heat insulating sheet 11 may be covered with a protective film. By doing so, evaporation of water can be prevented. Examples of the protective film include a film made of polyimide. Also, as the protective film, not only polyimide but also other organic films and inorganic films can be mentioned. Also, a coating film can be mentioned as the protective film.

[0022] Hereinafter, a method for manufacturing a heat insulating sheet according to an embodiment of the present disclosure will be described. The manufacturing method is performed in the order of the steps shown in the flowchart of FIG. 3.

[0023] First, a glass fiber sheet 12 having a space inside is prepared (preparation step).

[0024] Next, polyethylene carbonate is added as a catalyst to a high-molar silicic acid aqueous solution to prepare a silica sol solution. Here, the silica sol solution is impregnated into the internal space of a glass fiber sheet 12 made of glass fibers with a thickness of about 1 mm so that the weight ratio of the glass fiber sheet 12 after solvent removal to the silica xerogel 13 derived from the silica sol solution is about 1:1.1. Note that as an impregnation method, a method of impregnating the silica sol solution by dropping or printing or the like may be employed. Leave it for about 1 minute in a state impregnated with the silica sol solution and wait for gelation. Once gelation is confirmed, press it to make the thickness uniform. As a method of adjusting the thickness, a method such as roll pressing may be used. Place the one with the adjusted thickness in a container and grow the skeleton of the silica hydrogel present in the internal space of the fiber sheet under the conditions of about 90°C and a humidity of about 90%. In this way, the internal space of the fiber sheet is impregnated with the silica hydrogel (sol solution impregnation step).

[0025] The water present inside the silica hydrogel impregnated in the glass fiber sheet 12 is immersed in isopropyl alcohol (hereinafter referred to as IPA) for about 30 minutes to perform solvent substitution. This substitution time may be changed depending on the type of solvent and the thickness of the heat insulating material. The solvent at this time may be other alcohol solutions or the like as long as it is not only IPA but also a solvent having a surface tension of 30 mN / m or less. Thereby, the capillary force during drying can be reduced and the aggregation breakdown of the silica xerogel 13 can be prevented (curing step).

[0026] In order to promote the strength of the skeleton, heating is performed at a temperature of about 400 °C for about 2 hours to remove the solvent, obtaining silica xerogel 13 from the silica hydrogel inside the fiber sheet, and promoting the dehydration reaction of silanol groups from the surface of the gel to form siloxane bonds, thereby strengthening the skeleton of silica xerogel 13 (skeleton strengthening step). By doing so, even without hydrophobizing silica xerogel 13, it is possible to prevent the internal structure of silica xerogel 13 from being destroyed in subsequent processes. In the step of strengthening the skeleton of silica xerogel 13, it is desirable that the temperature be 300 °C or higher and 450 °C or lower. If this temperature is lower than 300 °C, it is difficult to cause the dehydration reaction of silanol groups. Conversely, if it exceeds 450 °C, the glass fiber softens, which is not desirable.

[0027] When the infrared absorber 14 is encapsulated in the heat insulation sheet 11, the internal structure of the silica xerogel 13 is not destroyed by the stress due to capillary force.

[0028] Next, the heat insulation sheet 11 is stored in an environment at 85 °C and a vapor pressure of 85% of the saturated vapor of the infrared absorber 14 for about 12 hours (infrared absorber impregnation step). Here, as an example, water is used as the infrared absorber 14 to adsorb water on the surface of the silica xerogel 13 in the heat insulation sheet, so that about 6% of the moisture can be encapsulated inside the heat insulation sheet 11. The water content is desirably 2% or more and 15% or less by weight of the heat insulation sheet 11. If this content is lower than 2%, it cannot sufficiently absorb infrared rays, and if it exceeds 15%, the thermal conductivity of the heat insulation sheet 11 itself increases due to the heat conduction of water.

[0029] In the heat insulation sheet 11 produced by the above method, the silica xerogel is likely to shed powder, and in order to prevent the infrared absorber 14 from detaching from the heat insulating material, it is desirable to cover it with a protective film that blocks the solvent. As the protective film, for example, a film made of polyimide can be considered. By doing so, the evaporation of water can be prevented, and even if one battery cell becomes hot over a long period of time, it is possible to make it less likely to affect the adjacent battery cell.

[0030] Note that as the protective film, for example, a film made of polyimide can be mentioned. Also, as the protective film, not only polyimide but also other organic films and inorganic films can be mentioned. Further, as the protective film, a coating film can also be mentioned.

[0031] Note that the manufacturing method of the heat insulation sheet 11 shown above is merely an example of an optimal manufacturing method, and it is also possible to obtain the heat insulation sheet 11 according to the present disclosure by applying other manufacturing methods.

Industrial Applicability

[0032] The heat insulation sheet and its manufacturing method according to the present disclosure exhibit heat insulation properties due to silica xerogel at normal temperatures. Even if one battery cell becomes hot and radiates infrared rays, they are absorbed by the infrared absorber contained in the heat insulation sheet, and the transfer of heat to the adjacent battery cell can be suppressed, which is industrially useful.

Explanation of Reference Numerals

[0033] 11 Heat insulation sheet 12 Glass fiber sheet 13 Silica xerogel 14 Infrared absorber

Claims

1. A step of preparing a glass fiber sheet having a space inside; A step of impregnating the internal space of the glass fiber sheet with a silica sol solution; A curing step of gelling the silica sol solution to support silica xerogel on the glass fiber; A strengthening step of softening the silica xerogel and strengthening the gel skeleton by holding the sheet formed with the silica xerogel at a predetermined temperature; A step of impregnating the infrared absorber by placing the sheet supporting the silica xerogel in an atmosphere of the infrared absorber, and comprising: A method for manufacturing a heat insulating sheet, wherein the sheet supporting the silica xerogel contains 2% or more and 15% or less of an infrared absorber by weight ratio.

2. The method for manufacturing a heat insulating sheet according to claim 1, wherein in the strengthening step, the temperature to be held is 300°C or more and 450°C or less.

Images