JP2024065513A - Heat-insulating sheet and battery pack - Google Patents

Abstract

To provide a heat-insulating sheet that features superior heat insulation and winding processability, and a battery pack that includes the heat-insulating sheet.SOLUTION: A heat-insulating sheet includes inorganic fibers, diatomaceous earth, and sepiolite. The diatomaceous earth content is 30 mass% or more relative to the total amount of the inorganic fibers, diatomaceous earth and sepiolite. The total content of the diatomaceous earth and sepiolite is 50-90 mass% relative to the total amount of the inorganic fibers, diatomaceous earth and sepiolite.SELECTED DRAWING: None

Description

The present invention relates to a heat insulating sheet and a battery pack.

High-power, high-capacity rechargeable batteries such as lithium-ion batteries are widely used in mobile devices, tools, automobiles, trains, aircraft, etc. When a high-power, high-capacity rechargeable battery is damaged or short-circuited due to internal impurities, the internal energy is instantly released as heat. As a result, the battery deteriorates faster and may even catch fire.
For example, high voltage and high output are required for applications such as automobiles that require a large amount of stored electricity. For this reason, assembled batteries (sometimes called battery packs or assemblies) in which adjacent cells are packed so that a large number of cells are stacked are often used. In such assembled batteries, there is a concern that a malfunction of one cell may affect adjacent cells.
In order to prevent a defect in one cell from affecting adjacent cells, it has been proposed to place a non-flammable heat insulating sheet between the cells (for example, Patent Documents 1 and 2).

International Publication No. 2020/129274 JP 2006-310274 A

In addition to excellent thermal insulation properties, heat insulating sheets are also required to be able to be rolled up into the smallest possible diameter (winding processability) in consideration of ease of transportation and productivity.
The present invention provides a heat insulating sheet having excellent heat insulating properties and winding processability; and a battery pack including the heat insulating sheet.

The present inventors attempted to make paper from a slurry containing inorganic fibers, diatomaceous earth and sepiolite in order to obtain a flame-retardant insulating sheet that has excellent insulating properties and winding processability, and also has excellent shape conformability so that it can follow changes in shape due to ignition or heat generation of a single battery.
However, the slurries containing these have high film forming ability, and water does not escape between the fibers during papermaking, making it difficult to form a wet web. In addition, in the case of a heat insulating sheet obtained from a wet web made from the slurry, powder may fall off the surface of the sheet, and the surface strength and tensile strength are also insufficient. The powder falling off the sheet can cause a decrease in workability during transportation, and can also adhere to production equipment, causing breakdowns and malfunctions.
Therefore, the inventors carefully investigated the compounding ratio of inorganic fibers, diatomaceous earth and sepiolite, and discovered that it was possible to solve these problems and obtain an insulating sheet with excellent insulating properties and winding processability, thereby completing the present invention.

The present invention has the following aspects.
[1] A heat insulating sheet comprising inorganic fibers, diatomaceous earth, and sepiolite; a content of the diatomaceous earth is 30 mass% or more based on a total amount of the inorganic fibers, the diatomaceous earth, and the sepiolite; and a total content of the diatomaceous earth and the sepiolite is 50 to 90 mass% based on a total amount of the inorganic fibers, the diatomaceous earth, and the sepiolite.
[2] The heat insulating sheet according to [1], wherein the mass ratio of the diatomaceous earth to the sepiolite is 50/50 or more.
[3] The heat insulating sheet according to [1] or [2], wherein the inorganic fibers have a fiber diameter of 3 to 13 μm.
[4] The heat insulating sheet according to any one of [1] to [3], wherein the heat insulating sheet has a thermal mass reduction rate of 20 mass% or less when heated at 450°C for 2 hours.
[5] The heat insulating sheet according to any one of [1] to [4], wherein the basis weight of the heat insulating sheet is 20 to 500 g / m ² and the thickness of the heat insulating sheet is 0.05 to 3 mm.
[6] The heat insulating sheet according to any one of [1] to [5], wherein the heat insulating sheet has an air permeability of 50 cc/cm ² /sec or less.
[7] An assembled battery comprising: a plurality of stacked unit cells; and a heat insulating sheet inserted between the plurality of unit cells; and at least one of the heat insulating sheets is the heat insulating sheet according to any one of [1] to [6].

The present invention provides an insulating sheet having excellent heat insulation properties and winding processability; and a battery pack including the insulating sheet.

FIG. 2 is a diagram illustrating an example of a configuration of a battery pack.

In this specification, the use of "to" indicating a range of values means that the values before and after it are included as the lower and upper limits.
The lower and upper limits of the numerical ranges disclosed in this specification can be combined in any manner to create new numerical ranges.

<Thermal insulation sheet>
The heat insulating sheet of the present invention contains inorganic fibers, diatomaceous earth, and sepiolite.
The heat insulating sheet of the present invention may further contain components other than inorganic fibers, diatomaceous earth and sepiolite as optional components within a range that does not impair the effects of the invention.

(Inorganic fiber)
Examples of inorganic fibers include glass fibers, carbon fibers, fused rock fibers such as glass wool, rock wool, and basalt fibers, ceramic fibers such as alumina fibers, and silicon carbide fibers. Among these, glass fibers and ceramic fibers are preferred. Glass fibers are even more preferred because they are inexpensive, have no electrical conductivity, and cause less wear on the cutting blade when cutting the sheet.
The inorganic fibers may be used alone or in combination of two or more kinds.

As the glass fiber, in addition to the general E-glass, for example, high-strength S-glass and excellent acid resistance C-glass can be used. From the viewpoint of cost, inexpensive E-glass is preferable. The cross-sectional shape of the glass fiber is not particularly limited. For example, circular and flat shapes can be used.
When glass fibers are used as the inorganic fibers, one type of glass fiber may be used alone, or two or more types may be used in combination.

The fiber diameter of the inorganic fibers is preferably from 3 to 13 μm, more preferably from 4 to 10 μm, and even more preferably from 5 to 8 μm.
When the fiber diameter of the inorganic fiber is equal to or greater than the lower limit of the above-mentioned numerical range, it is easy to ensure the force to maintain the gaps between the fibers. Therefore, it is easy to avoid the gaps between the fibers being crushed by the capillary force of the adsorbed water due to moisture absorption and water absorption. In addition, it is easy to avoid the thickness of the entire insulation sheet being reduced by the compression force between the unit cells. Therefore, it is possible to suppress the reduction in insulation properties due to the reduction in thickness. In addition, it is easy to avoid the increase in the heat transfer coefficient due to the increase in the contact points between the fibers. The fiber diameter is a value calculated by measuring the fiber lengths of 100 fibers by microscopic observation and averaging the 100 fibers.

In addition, according to the World Health Organization (WHO), "WHO respirable fibers" are fibrous substances that are inhaled into the body and reach the lungs, and have a length of more than 5 μm, a diameter of less than 3 μm, and an aspect ratio of more than 3. There are concerns about the impact of using WHO respirable fibers on health, and there are restrictions on their use. For this reason, it is desirable for inorganic fibers to have a fiber diameter of 3 μm or more.

When the fiber diameter of the inorganic fibers is equal to or less than the upper limit of the above numerical range, the gaps between the inorganic fibers become narrow, making it difficult for convection and gas to pass through the gaps, and it is easy to obtain a heat insulating effect. In addition, since it is easy to secure contact points and intertwining points between the inorganic fibers, the tensile strength of the entire insulation sheet increases and it becomes easier to handle. In addition, it does not become too irritating to the skin, and it is easy to suppress fluffing and powder falling during cutting processing. In addition, since the gaps between the fibers do not become excessively large, it is easy to prevent the passage of heated air and the occurrence of convection in the gaps.

As the inorganic fibers, inorganic fibers having a fiber diameter of 3 to 13 μm and inorganic fibers having a fiber diameter of more than 13 μm may be used in combination.
By using a combination of inorganic fibers with different fiber diameters, it may be possible to obtain the effect of improving the insulation effect and smoothing the surface by narrowing the gaps between the inorganic fibers, and the effect of improving the tensile strength by ensuring contact points and intertwining points between the inorganic fibers, while ensuring the thickness of the insulation sheet and maintaining the gaps between the fibers.

The fiber length of the inorganic fibers is preferably from 1 to 25 mm, more preferably from 3 to 20 mm, and even more preferably from 5 to 15 mm.
When the fiber length of the inorganic fibers is equal to or greater than the lower limit of the above-mentioned range, strength during the sheet manufacturing process is easily ensured.When the fiber length of the inorganic fibers is equal to or less than the upper limit of the above-mentioned range, bundling due to fiber twisting does not occur, and good texture (uniformity of thickness and fiber density) is easily maintained.
The fiber length is a value obtained by measuring the fiber lengths of 100 fibers by microscopic observation and calculating the average value of the 100 fibers.

(Diatomaceous earth, sepiolite)
Diatomaceous earth is a porous natural material composed mainly of silica formed by the accumulation of diatoms. Diatomaceous earth from various places of origin, both domestic and foreign, can be used. In addition, one type of diatomaceous earth may be used alone, or two or more types may be used in combination.

Sepiolite is a type of naturally occurring clay mineral. It is a hydrated magnesium silicate with a unique chain-like particle structure. Sepiolite from a variety of sources, both domestic and overseas, can be used. Sepiolite may be used alone or in combination of two or more types.

Among sepiolite, it is preferable to select and use β-type sepiolite. Sepiolite is divided into α-type sepiolite and β-type sepiolite, depending on the origin of its formation. α-type sepiolite is formed by hydrothermal action under high temperature and pressure. α-type sepiolite has a relatively high degree of crystallinity and a clear fibrous form with long fibers. α-type sepiolite is also called mountain skin. β-type sepiolite is formed by depositional action on the bottom of shallow seas and lakes. β-type sepiolite has a relatively low degree of crystallinity. β-type sepiolite can take any of the following forms, for example: short fiber, clump, or clay. β-type sepiolite is more preferable because it contains a relatively small amount of crystalline silica (impurity content) that is unsafe for the human body.

Among β-type sepiolite, it is preferable to select and use sepiolite from a place with a low crystalline silica content. Crystalline silica accelerates wear of the blade when cutting the sheet, so a low crystalline silica content is preferable.

The content of crystalline silica can be quantified by X-ray powder diffraction method, by comparing the peak intensity of crystalline silica with the peak intensity of a standard sample.
The content of crystalline silica in the heat insulating sheet is preferably 1 mass % or less, more preferably 0.5 mass % or less, and even more preferably 0.2 mass % or less.

(Optional ingredients)
A heat insulating sheet further containing a binder as an optional component is also useful. The binder is for binding the inorganic fibers. The binder is preferably one that has binding properties, heat resistance, and low corrosiveness to batteries, electrodes, and wiring. The binder may be an inorganic binder (excluding sepiolite and diatomaceous earth) or an organic binder.
Examples of inorganic binders include various inorganic cements and various glasses. Examples of organic binders include heat-fusible resin powders, heat-fusible resin fibers, resin emulsions, resin solutions, thermosetting resins, and thermoplastic resins.
The inorganic binder and the organic binder may be used alone or in combination of two or more kinds.

Among these, polyvinyl alcohol fibers (PVA fibers) are preferably used because they have excellent binding properties and low heat generation relative to the amount added. Acrylic resin emulsions and the like are also preferred from the viewpoint of water resistance. Core-sheath type binder fibers in which the outside is a low melting point resin and the inside is a high melting point resin are also preferred. In the case of core-sheath type binder fibers, the binder content is calculated as the total amount of the resin in the core and the resin in the sheath.
In addition, para-aramid fibers, aramid pulp, crystalline polyester, liquid crystal polyester, wood-derived pulp, grass-derived pulp, and the like, which have no melt adhesiveness but have a high ability to be finely fibrillated and entangled, can also be used as organic binders.

Examples of other optional components include clay minerals other than sepiolite, fillers other than diatomaceous earth (excluding clay minerals), dispersants, thickeners, flocculants, paper strength improvers, and retention improvers.
The other components may be used alone or in combination of two or more.

Examples of clay minerals other than sepiolite include kaolinite, smectite, montmorillonite, sericite, illite, glauconite, chlorite, α-type sepiolite, and talc.
Examples of fillers other than diatomaceous earth include calcium silicate, calcium carbonate, silica aerosol, fumed silica, talc, plastic pigments, hollow glass beads, and shirasu balloons.

(Mixing ratio)
The content of diatomaceous earth is 30% by mass or more, preferably 30 to 85% by mass, more preferably 35 to 80% by mass, and even more preferably 40 to 70% by mass, based on the total amount of inorganic fibers, diatomaceous earth, and sepiolite (hereinafter referred to as "total amount (T)"). When the content of diatomaceous earth is equal to or more than the lower limit of the aforementioned numerical range, a heat insulating sheet having excellent heat insulating properties and winding processability is likely to be obtained. When the content of diatomaceous earth is equal to or less than the upper limit of the aforementioned numerical range, a heat insulating sheet having excellent surface strength and tensile strength is likely to be obtained.

The total content of diatomaceous earth and sepiolite is 50 to 90% by mass, preferably 60 to 90% by mass, more preferably 65 to 90% by mass, and even more preferably 70 to 85% by mass, based on the total amount (T). When the total content of diatomaceous earth and sepiolite is equal to or greater than the lower limit of the aforementioned numerical range, a heat insulating sheet with excellent heat insulating properties is easily obtained. When the total content of diatomaceous earth and sepiolite is equal to or less than the upper limit of the aforementioned numerical range, a wet web is easily formed during papermaking.

The mass ratio of diatomaceous earth to sepiolite is 50/50 or more, preferably 50/50 to 90/10, more preferably 55/45 to 90/10, and even more preferably 70/30 to 80/20. When the mass ratio is equal to or greater than the lower limit of the above numerical range, it is easy to obtain an insulating sheet with excellent heat insulation properties and winding processability. When the mass ratio is equal to or less than the upper limit of the above numerical range, it is easy to obtain an insulating sheet with excellent surface strength and tensile strength.

(Composition of the heat insulating sheet)
The proportion of inorganic fibers is preferably 5 to 50 mass %, more preferably 10 to 40 mass %, and even more preferably 10 to 35 mass %, based on the total mass of the heat insulating sheet. When the proportion of inorganic fibers is equal to or greater than the lower limit of the above numerical range, a wet web is easily formed during papermaking. When the proportion of inorganic fibers is equal to or less than the upper limit of the above numerical range, a heat insulating sheet having excellent heat insulating properties and winding processability is easily obtained.

The proportion of diatomaceous earth is preferably 35 to 85 mass% of the total mass of the insulating sheet, more preferably 40 to 80 mass%, and even more preferably 45 to 75 mass%. When the proportion of diatomaceous earth is equal to or greater than the lower limit of the above numerical range, an insulating sheet with excellent insulating properties and roll-up processability is likely to be obtained. When the proportion of diatomaceous earth is equal to or less than the upper limit of the above numerical range, an insulating sheet with excellent surface strength and tensile strength is likely to be obtained.

The proportion of sepiolite is preferably 5 to 45% by mass, more preferably 10 to 35% by mass, and even more preferably 15 to 25% by mass, based on the total mass of the insulating sheet. When the proportion of sepiolite is equal to or greater than the lower limit of the aforementioned numerical range, an insulating sheet having excellent insulation properties, surface strength, and tensile strength is likely to be obtained. When the proportion of sepiolite is equal to or less than the upper limit of the aforementioned numerical range, an insulating sheet having excellent winding processability is likely to be obtained, and a wet web is easily formed during papermaking.

The total amount (T) is preferably 90 to 100% by mass, more preferably 93 to 100% by mass, and even more preferably 95 to 100% by mass, based on the total mass of the heat insulating sheet. If the total amount (T) is equal to or greater than the lower limit of the aforementioned numerical range, a heat insulating sheet with excellent heat insulating properties and heat resistance is likely to be obtained. If the total amount (T) is equal to or less than the upper limit of the aforementioned numerical range, when the heat insulating sheet contains other components, the properties of the other components are likely to be expressed.

(Properties of the heat insulating sheet)
The thermal mass reduction rate when the heat insulating sheet is heated for 2 hours at 450° C. is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less. When the thermal mass reduction rate is equal to or less than the upper limit of the above-mentioned numerical range, the heat insulating sheet has excellent heat resistance, generates less smoke due to ignition of the unit cell, and undergoes less change in shape and properties due to thermal degradation.
From the viewpoint of heat resistance, the lower the thermal mass loss, the better. There is no particular limit to the lower limit, but considering that the insulating sheet is obtained by binding inorganic fibers, it is thought to be, for example, about 0.1 mass %.
The thermal mass reduction rate of the heat insulating sheet is a value determined by the method described in the examples below.

The basis weight of the heat insulating sheet is preferably 20 to 500 g/ ^m2 , more preferably 50 to 400 g/ ^m2 , and even more preferably 80 to 350 g/ ^m2 . When the basis weight of the heat insulating sheet is equal to or greater than the lower limit of the above-mentioned numerical range, the heat insulating properties of the heat insulating sheet tend to be improved. When the basis weight of the heat insulating sheet is equal to or less than the upper limit of the above-mentioned numerical range, the winding processability of the heat insulating sheet tends to be improved.
The basis weight of the heat insulating sheet can be determined by the method described in the Examples below.

The thickness of the heat insulating sheet is preferably 0.05 to 3.00 mm, more preferably 0.07 to 2.50 mm, and even more preferably 0.10 to 2.00 mm. When the thickness of the heat insulating sheet is equal to or greater than the lower limit of the above-mentioned numerical range, the heat insulating property of the heat insulating sheet is likely to be improved. When the thickness of the heat insulating sheet is equal to or less than the upper limit of the above-mentioned numerical range, it is easy to realize a thin battery pack to which the heat insulating sheet is applied.
The thickness of the heat insulating sheet can be determined by the method described in the Examples below.

The air permeability of the heat insulating sheet is preferably 50 cc/cm ² /sec or less, more preferably 40 cc/cm ² /sec or less, and even more preferably 20 cc/cm ² /sec or less. When the air permeability of the heat insulating sheet is equal to or less than the upper limit, the heat insulating sheet is likely to have improved heat insulating properties. From the viewpoint of heat insulating properties, the lower the air permeability of the heat insulating sheet, the better. The lower limit is not particularly limited, but considering the physical limit derived from the shape of the material and the practical thickness, it is considered to be, for example, about 0.01 cc/cm ² /sec.
The air permeability of the heat insulating sheet is a value determined by the method described in the Examples section below.

The insulation temperature T of the heat insulating sheet is preferably 400° C. or less, and more preferably 395° C. or less. When the insulation temperature T of the heat insulating sheet is equal to or less than the upper limit of the above numerical range, it can be said that the heat insulating sheet has excellent heat insulating properties. The lower the insulation temperature T of the heat insulating sheet, the better.
The insulation temperature T of the heat insulating sheet is a value (T ₁ , T ₂ ) determined by the method described in the Examples section below.

The tensile strength of the heat insulating sheet is preferably 10N/30mm or more, more preferably 15N/30mm or more, and even more preferably 20N/30mm or more. If the ash content of the heat insulating sheet is equal to or more than the lower limit, it can be said that the heat insulating sheet has excellent strength. The higher the tensile strength of the heat insulating sheet, the better. The upper limit is not particularly limited, but considering the physical limit derived from the material and the practical thickness, it is considered to be, for example, about 200N/30mm.
The tensile strength of the heat insulating sheet is a value determined by the method described in the Examples section below.

(Production method)
The heat insulating sheet can be manufactured, for example, by forming a wet web by papermaking a slurry containing at least inorganic fibers, diatomaceous earth, and sepiolite, and then drying the wet web. In the heat insulating sheet obtained by the wet method, the diatomaceous earth and sepiolite can be incorporated into the heat insulating sheet. Therefore, the diatomaceous earth and sepiolite can be uniformly dispersed in the gaps between the inorganic fibers and on the fiber surfaces. As a result, the heat insulating sheet can follow the shape changes caused by ignition or thermal expansion when applied to a battery pack, and a heat insulating sheet with excellent shape followability can be obtained.

The slurry may further contain optional components as required, such as a dispersant, a liquid retaining agent, a viscosity adjuster, a pH adjuster, and a filler.
When the slurry contains a heat-melting type binder, the inorganic fibers in the wet web can be bonded by a thermal bonding method.When the slurry does not contain a heat-melting type binder, the wet web can be formed by entangling the fibers by adding fibrillated fibers, adding fine fibers, needle punching, hydroentangling with a water jet, or the like.

When wet-processing the slurry into paper, various papermaking machines can be used. Examples of papermaking machines include cylinder papermaking machines, inclined papermaking machines, Fourdrinier papermaking machines, and short wire papermaking machines. Multi-layer papermaking can be performed by combining the same or different types of these papermaking machines.

(Mechanism of action)
In the heat insulating sheet of the present invention described above, the content of diatomaceous earth is 30 mass% or more relative to the total amount (T), which improves the winding processability of the heat insulating sheet.
In the heat insulating sheet of the present invention, the content of diatomaceous earth is 30% by mass or more with respect to the total amount (T), and the combined content of diatomaceous earth and sepiolite is 50% by mass or more with respect to the total amount (T), thereby improving the heat insulating properties of the heat insulating sheet.
The heat insulating sheet of the present invention contains sepiolite, which improves the surface strength and tensile strength of the heat insulating sheet.

<Battery pack>
The heat insulating sheet of the present invention can be applied to various industrial applications requiring thermal insulation between objects. For example, the heat insulating sheet of the present invention can be suitably applied to a battery pack and can be suitably used as a heat insulating sheet for a battery pack.
The battery pack includes a plurality of stacked unit cells and a heat insulating sheet inserted between the unit cells, at least one of which is the heat insulating sheet of the present invention described above.

Figure 1 is a diagram showing the configuration of a battery pack 50 according to one embodiment. As shown in Figure 1, in the battery pack 50, a heat insulating sheet 10 is inserted between each of a plurality of laminated type cells 20. Heat insulating sheets 10 are also arranged on the outside of the bottom and top laminated type cells 20 (the laminated type cells 20 stacked in the outermost layer). The battery pack 50 is housed in a metal exterior body or the like to form a battery pack.

The laminated type battery 20 may be any type of laminated type battery as long as an electrode group and an electrolyte are contained within a laminate film. As shown in Fig. 1, a positive electrode tab 21 and a negative electrode tab 22 are provided outside the laminated film.
The insulating sheet 10 is inserted between the laminated type cells 20 so as to insulate them from each other over the entire surface direction, but the positive electrode tab 21 and the negative electrode tab 22 are extended to the outside of the insulating sheet 10 .

In the battery pack 50, a heat insulating sheet 10 is inserted between each of the laminated type cells 20. Therefore, even if a malfunction such as heat generation occurs in one of the laminated type cells 20, it is possible to prevent or delay the malfunction from adversely affecting the adjacent laminated type cells 20.
Furthermore, even if a malfunction such as heat generation occurs in the lowermost or uppermost laminated cell 20, it is possible to prevent or delay the malfunction from adversely affecting other battery packs.

For ease of explanation, the battery pack 50 is described as having a laminated cell 20 as the cell, but the cell is not limited to a laminated cell. The cell may be, for example, a cell in which an electrode group and an electrolyte are housed in a metal case. However, laminated cells are susceptible to the effects of heat. Therefore, from the perspective of making the most of the effects of the present invention, laminated cells are preferred.

For ease of explanation, all of the multiple insulating sheets in the battery pack 50 are the insulating sheets of the present invention, but the form of the battery pack is not limited to that exemplified in Fig. 1. That is, all of the multiple insulating sheets may be the insulating sheets of the present invention as in the battery pack 50, or some of the multiple insulating sheets may be insulating sheets other than the insulating sheets of the present invention.
However, the use of the heat insulating sheet of the present invention has the advantages of being relatively inexpensive, easy to work with, and excellent in heat insulating properties.

A plurality of overlapping heat insulating sheets may be inserted between each of the laminated type cells 20. When a plurality of sheets are overlapped, only the heat insulating sheet of the present invention may be overlapped, only another heat insulating sheet may be overlapped, or the heat insulating sheet of the present invention and another heat insulating sheet may be overlapped.
From the viewpoint of reducing the overall thickness of the battery pack 50, it is preferable to insert only one heat insulating sheet, and when multiple heat insulating sheets are stacked, it is preferable to stack two heat insulating sheets.

The present invention will be specifically described below using examples, but the present invention is not limited to the following description.

<Ingredients>
The raw materials used in each example are shown below.
(Inorganic fiber)
Glass fiber: E-glass fiber with a fiber diameter of 6 μm and a fiber length of 6 mm Ceramic fiber: Ceramic fiber with a fiber diameter of 3 μm and a fiber length of 6 mm

(binder)
Core-sheath PET fiber: core-sheath heat-bonded polyester fiber with a fiber diameter of 14 μm and a fiber length of 5 mm (manufactured by Teijin Fibers Limited, melting point of the core 260° C., adhesion temperature of the sheath 110° C.)
PVA fiber: polyvinyl alcohol fiber with a fiber diameter of 11 μm and a fiber length of 3 mm (manufactured by Kuraray Co., Ltd., adhesion temperature 60° C.)

Example 1
A sample was prepared by adjusting the basis weight so that the thickness was 500 μm with the composition shown in Table 1. First, E-glass fiber, core-sheath PET fiber, and PVA fiber were mixed and dispersed in water to finally obtain a fiber slurry of 0.4 mass %.
Separately, a mixture of diatomaceous earth and sepiolite was dispersed in water, and 1.8 parts by mass (based on solids) of aluminum sulfate, an amphoteric polyacrylamide resin-based retention aid (manufactured by Arakawa Chemical Industries, Ltd.), and 0.6 parts by mass (based on solids) of an anionic polyacrylamide resin-based polymer flocculant (manufactured by MT Aqua Polymer Co., Ltd.) were successively added and stirred to finally obtain a 0.4% by mass filler slurry.
The raw material slurry obtained by mixing and stirring the above-mentioned fiber slurry and filler slurry was fed into a hand-made sheet machine, and wet papermaking was carried out to obtain a wet web. The wet web was then dehydrated by suction and dried in a hot air dryer at 180° C. to obtain a heat insulating sheet (25 cm×25 cm).

<Examples 2 to 7, Comparative Examples 1 to 3>
Except for changing the slurry composition and the type of inorganic fiber as shown in Tables 1 and 2, the heat insulating sheets of each example were produced by the same method and under the same conditions as in Example 1. However, in Example 3, ceramic fiber was used instead of glass fiber. Also, with the slurry composition of Comparative Example 3, no heat insulating sheet was obtained.

<Comparative Example 4>
A commercially available aerogel-impregnated glass paper (manufactured by Panasonic, thickness 1000 μm) was prepared.

<Measurement and evaluation methods>
(grammage)
The basis weight of each example of the heat insulating sheet was measured in accordance with the method specified in JIS P 8124:2011.

(thickness, density)
The thickness of each heat insulating sheet was measured in accordance with JIS P 8118: 1998, with the pressure between the pressure surfaces set to 50 kPa. The density was calculated by dividing the basis weight by the thickness.

(Breathability)
The air permeability of each example of the heat insulating sheet was measured according to Method A (Fragile type method) specified in JIS L 1096:2010.

(Winding processability)
The winding processability was evaluated based on the windable diameter. The windable diameter of each example of the heat insulating sheet was measured as follows. A sample of 200 mm in length and 30 mm in width was cut out from the sheet, and the sample was wound around cylinders of different diameters, starting from the largest diameter. The minimum winding diameter that could be wound without cracking, folding, or wrinkling on the sheet surface was determined as the windable diameter. The windable diameter was evaluated according to the following criteria.
A: The winding diameter is less than 65 mm.
B: The winding diameter is 65 mm or more and 110 mm or less.
C: The winding diameter is more than 110 mm.

(Surface strength)
The surface strength was evaluated based on the amount of powder that fell off. The amount of powder that fell off for each example of the heat insulating sheet was measured as follows. A 15 mm wide cellophane tape was cut out to a length of 50 mm, and the adhesive side was placed on the sheet surface. The index finger and middle finger were aligned and rubbed back and forth on the cellophane tape three times without applying force to make it adhere. The cellophane tape was peeled off from the sheet and pasted on black drawing paper, and the state of the powder and fibers attached to the cellophane tape was visually confirmed.

The surface strength was evaluated according to the following criteria.
A: Almost no adhesion to powder or fiber cellophane tape was observed.
B: A small amount of powder or fiber adhesion to the cellophane tape is observed, but is within the acceptable range.
C: A large amount of powder or fiber adhesion to the cellophane tape was observed.

(Tensile strength)
The tensile strength of each heat insulating sheet was measured as follows: A sample 240 mm long and 15 mm wide was cut out from the sheet, and this was set in a tensile tester (model name: RTC-1210A, manufactured by Orientec Co., Ltd.) at 23°C and 50% RH so that the test length was 180 mm, and the tensile strength was measured according to a method in accordance with JIS P8113.

The tensile strength was evaluated according to the following criteria.
A: Tensile strength is 15 N/30 mm or more.
B: The tensile strength is 10 N/30 mm or more and less than 15 N/30 mm.
C: Tensile strength is less than 10 N/30 mm.

(Thermal mass reduction rate)
The thermal mass loss rate of each example of the heat insulating sheet was measured as follows. The sample was dried at 105°C for 2 hours, cooled in a desiccator, and the bone dry weight was measured. The same sample was heated at 450°C for 2 hours, cooled in a desiccator, and the post-heating weight was measured. The thermal mass loss was measured from the difference between the post-heating weight and the bone dry weight.

(Temperature _T1 after 10 minutes at 500°C after heat compression, thickness of 500 μm, thickness of 1000 μm)
The heat insulation was evaluated based on the temperature T after 10 minutes at 500°C after thermal compression. For each example of the heat insulation sheet, the temperature T after 10 minutes at 500°C after thermal compression was measured as follows. A sample of 80 mm x 100 mm was cut out from the sheet to obtain a single-sheet, double-sheet, or triple-sheet sample. After measuring the thickness, the sample was compressed at 200°C and 2 MPa for 15 minutes, and then cooled while maintaining the compression. The compressed sample was placed on a hot plate at 500°C and maintained for 10 minutes while applying a pressure of 1 kPa, and the temperature of the surface not in contact with the hot plate was measured with a contact thermometer. Measurements were performed on the samples of single-sheet compression, double-sheet compression, and triple-sheet compression, respectively, to obtain a regression curve of the thickness before thermal compression and the temperature reached after 10 minutes. Using this regression curve, the temperature _T1 after 10 minutes of heating at 500°C corresponding to a thickness of 500 μm before thermal compression and the temperature _T2 after 10 minutes of heating at 500°C corresponding to a thickness of 1000 μm were obtained.

_T1 equivalent to 500 μm was evaluated according to the following criteria.
A: _T1 is less than 395°C.
B: _T1 is 395°C or higher and lower than 400°C.
C: _T1 is greater than 400°C.

_T2 equivalent to 1000 μm was evaluated according to the following criteria.
A: _T2 is less than 372°C.
B: _T2 is 372°C or higher and lower than 377°C.
C: _T2 is greater than 377°C.

<Results>
The measurement results and evaluation results of each example are shown in Tables 1 and 2. The evaluation results are indicated in parentheses in each column with either A, B, or C along with the measurement results.

In Examples 1 to 7, heat insulating sheets with excellent heat insulating properties and winding processability were obtained. In contrast, in Comparative Example 1, the winding processability was insufficient. In Comparative Example 2, the winding processability, surface strength, tensile strength, and heat insulating properties were all insufficient. In Comparative Example 3, the bonds between the fibers during papermaking were strong, and water could not escape from the wet web, so a heat insulating sheet could not be obtained.

The present invention provides an insulating sheet having excellent heat insulation properties and winding processability; and a battery pack including the insulating sheet.

10...insulating sheet, 20...laminated cell, 21...positive electrode tab, 22...negative electrode tab, 50...battery pack.

Claims (7)

Contains inorganic fibers, diatomaceous earth, and sepiolite;
The content of the diatomaceous earth is 30% by mass or more based on the total amount of the inorganic fibers, the diatomaceous earth, and the sepiolite;
The heat insulating sheet has a total content of the diatomaceous earth and the sepiolite of 50 to 90 mass% based on the total amount of the inorganic fibers, the diatomaceous earth and the sepiolite. The heat insulating sheet according to claim 1, wherein the mass ratio of the diatomaceous earth to the sepiolite is 50/50 or more. The heat insulating sheet according to claim 1, wherein the inorganic fibers have a fiber diameter of 3 to 13 μm. The heat insulating sheet according to claim 1, wherein the thermal mass reduction rate when the heat insulating sheet is heated at 450°C for 2 hours is 20% by mass or less. The heat insulating sheet according to claim 1, wherein the basis weight of the heat insulating sheet is 20 to 500 g/ ^m2 and the thickness of the heat insulating sheet is 0.05 to 3 mm. The heat insulating sheet according to claim 1, wherein the heat insulating sheet has an air permeability of 50 cc/cm ² /sec or less. A plurality of stacked unit cells;
A heat insulating sheet is inserted between the plurality of unit cells;
Equipped with
A battery pack, wherein at least one of the heat insulating sheets is the heat insulating sheet according to any one of claims 1 to 6.

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