JP2024065516A - Heat insulating sheets, battery packs - Google Patents

Abstract

[Problem] The present invention provides an insulating sheet having excellent heat insulation properties and winding processability; and a battery pack including said insulating sheet. [Solution] The insulating sheet of the present invention contains wollastonite, sepiolite, inorganic fibers, and a binder. The content of inorganic fibers in the insulating sheet is 10 mass% or more relative to 100 mass% of the insulating sheet. The total content of wollastonite and sepiolite is 48 mass% or more relative to 100 mass% of the insulating sheet. The mass ratio of sepiolite to wollastonite in the insulating sheet is 0.1 or more and 5.0 or less. [Selected Figure] 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 a paper from a slurry containing inorganic fibers, a binder, wollastonite and β-type 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 conform to changes in shape due to ignition or heat generation of a single battery.
However, in the case of a heat insulating sheet obtained from a wet web made from a slurry containing these, powder sometimes falls off the surface of the sheet, resulting in insufficient surface strength and insufficient tensile strength. The powder that falls 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 have thoroughly investigated the compounding ratio of inorganic fibers, binder, wollastonite and β-type sepiolite, and have found that it is possible to obtain a heat insulating sheet having excellent heat insulating properties and winding processability while solving these problems, thereby completing the present invention.

The present invention has the following aspects.
[1] A heat insulating sheet comprising: wollastonite, sepiolite, and inorganic fibers; a content of the inorganic fibers is 10 mass% or more, based on 100 mass% of the heat insulating sheet; a total content of the wollastonite and the sepiolite is 48 mass% or more, based on 100 mass% of the heat insulating sheet; and a mass ratio of the sepiolite to the wollastonite is 0.1 or more and 5.0 or less.
[2] The heat insulating sheet according to [1], wherein the wollastonite has an average length of 5 to 100 μm, an average diameter of 1 to 30 μm, and an aspect ratio of 3 to 100.
[3] The heat insulating sheet according to [1] or [2], wherein the content of organic components in the heat insulating sheet is 8 mass% or less based on the total mass of the heat insulating sheet.
[4] The heat insulating sheet according to any one of [1] to [3], wherein the basis weight of the heat insulating sheet is 20 to 500 g / ^m2 .
[5] The heat insulating sheet according to any one of [1] to [4], wherein the thickness of the heat insulating sheet is 0.1 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].
[8] The battery pack according to [7], wherein a heat insulating sheet is also disposed on the outside of an outermost stacked cell among the plurality of cells.

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 wollastonite, sepiolite, inorganic fibers, and a binder.
The heat insulating sheet of the present invention may further contain components other than the inorganic fibers, wollastonite, sepiolite and binder 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 12 μm, and even more preferably from 5 to 10 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 absorbed water due to the sheet absorbing moisture and water. It is also easy to avoid the thickness of the entire insulating sheet being reduced by the compression force between the unit cells. Therefore, it is possible to suppress the reduction in the insulating property due to the reduction in the sheet thickness. It is also easy to avoid the increase in the thermal conductivity due to the increase in the contact points between the 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. The use of WHO respirable fibers raises concerns about their impact 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, so that convection and gas passage in the gaps are less likely to occur, and it is easy to obtain a heat insulating effect. Since it is easy to secure contact points and intertwining points between the inorganic fibers, the tensile strength of the entire heat insulating sheet is increased. This improves handling properties and also makes it easy to suppress fluffing and powder falling during cutting processing. At the same time, this also prevents skin irritation from increasing. 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. The fiber length of the inorganic fibers is a value calculated by measuring the fiber lengths of 100 fibers by microscopic observation and averaging the lengths of the 100 fibers.

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 fiber is equal to or greater than the lower limit of the above-mentioned numerical range, the mechanical strength during the sheet manufacturing process is easily ensured. When the fiber length of the inorganic fiber is equal to or less than the upper limit of the above-mentioned numerical range, the fibers are not twisted to cause bundling, and good texture (uniformity of thickness and fiber density) is easily maintained. The fiber diameter of the inorganic fiber is a value calculated as the average value of 100 fibers.

The heat insulating sheet may contain organic fibers as necessary. Examples of the organic fibers include chemical fibers such as polyolefin fibers, nylon fibers, rayon fibers, polyvinyl chloride fibers, acrylic fibers, polyester fibers, polyurethane fibers, polyparaphenylene benzobisoxazole fibers, polyamideimide fibers, polyimide fibers, polyarylate fibers, polyetherimide fibers, vinylon fibers, polycarbonate fibers, ethylene-vinyl acetate fibers, polyphenylene sulfide fibers, polyethylene terephthalate fibers, polybutylene terephthalate fibers, polyethylene naphthalate fibers, and aramid fibers.
By supporting a filler containing calcium silicate hydrate and magnesium silicate hydrate on a heat insulating sheet, the heat shielding properties, handling properties, toughness, and flexibility can be improved.

Calcium silicate hydrate crystals have a plate-like or needle-like microstructure, with many voids formed inside the crystals. These voids suppress the conduction of heat, providing heat retention and insulation. In addition, when heated, the crystalline hydrate evaporates, slowing the rise in temperature due to the latent heat of vaporization.

Calcium silicate board is one of the most representative non-flammable building materials. Calcium silicate boards are produced by mixing siliceous and lime raw materials with water and reinforcing materials such as fibers, and then hydrothermally synthesizing calcium silicate hydrate in an autoclave, producing boards with thicknesses of approximately 5 to 15 mm. Prior to the hydrothermal synthesis process, the board is formed into a flat plate shape, which is generally produced by cutting a sheet obtained by papermaking, or by pouring the sheet into a mold in a fluid state and forming it. After autoclaving, although some products are given a certain degree of flexibility, the board is basically a rigid plate. Therefore, production must be done in batches, and continuous production is difficult.

By supporting fine particles of crystalline calcium silicate hydrate during the nonwoven fabric manufacturing process, it is possible to obtain a more flexible and rollable calcium silicate heat-resistant material, which contributes to improved productivity.

Examples of crystalline calcium silicate hydrates include foshagite, gyrolite, hillebrandite, tobermorite, truscotite, wollastonite, zonolite, etc., or mixtures thereof.

Of these, wollastonite is preferred. Wollastonite is mainly produced as a natural mineral, and the crystals have a high aspect ratio, such as needle-like or long columnar shape, so that when bound to fibers, etc. within the supported paper web, it can achieve both reinforcing effects and flexibility. Wollastonite from a variety of places of origin, both domestic and overseas, can be used. Furthermore, one type of wollastonite may be used alone, or two or more types may be used in combination.

The average length of the wollastonite is preferably 5 to 100 μm, more preferably 7 to 90 μm, and even more preferably 10 to 80 μm. If the average length of the wollastonite is equal to or greater than the lower limit of the above numerical range, sufficient reinforcing effect can be achieved when bonded to fibers. If the average length of the wollastonite is equal to or less than the upper limit of the above numerical range, sufficient flexibility can be achieved. The average length of the wollastonite is the average value of the lengths of 100 wollastonite particles in a planar photograph taken at a magnification of 200 times using an electron microscope. The wollastonite used for measurement is the wollastonite located on the diagonal of the observation range of the electron microscope image.

The average diameter of the wollastonite is preferably 1 to 30 μm, more preferably 2 to 28 μm, and even more preferably 3 to 25 μm. When the average diameter of the wollastonite is equal to or greater than the lower limit of the above-mentioned numerical range, a sufficient number of contact points are formed between the fibers and the wollastonite, making it easier to obtain strength. When the average diameter of the wollastonite is equal to or less than the upper limit of the above-mentioned numerical range, it is easier to balance flexibility and strength. The average diameter of the wollastonite is the average value obtained by treating the width as the diameter for 100 pieces of wollastonite in a planar photograph taken at a magnification of 200 times using an electron microscope. The wollastonite used for measurement is the wollastonite located on the diagonal of the observation range of the electron microscope image.

The aspect ratio (average length/average diameter) of wollastonite is preferably 3 to 100, more preferably 4 to 90, and even more preferably 5 to 80. When the aspect ratio of wollastonite is equal to or greater than the lower limit of the above numerical range, it is easy to obtain strength through entanglement. When the aspect ratio of wollastonite is equal to or less than the upper limit of the above numerical range, it is easy to achieve a balance between flexibility and strength.

By including hydrous magnesium silicate in the filler in a specified ratio in addition to wollastonite, the necessary heat shielding properties, handling properties, toughness, and flexibility can be achieved. The binding strength between the fine needle-shaped particles of hydrous magnesium silicate is stronger than that of wollastonite, so it fills the supported paper web at a high density. As a result, the sheet-shaped heat-resistant material exhibits high shielding properties against gases while at the same time increasing its mechanical strength. This not only reduces the contribution of gas heat transfer to improve heat shielding performance, but also improves handling properties, and the improved flexibility improves winding properties during papermaking.

When a heat insulating sheet-like heat-resistant material is placed between unit cells to form a battery pack, the heat insulating property of the sheet-like heat-resistant material is deteriorated due to the compressive force between the unit cells, but the thickness reduction is suppressed by adding a filler, and the effect of preventing the increase in thermal conductivity is improved. This effect is further enhanced by the mechanical strength enhancing effect of hydrated magnesium silicate.
However, as the amount of the filler increases, the effect of increasing the thermal conductivity due to the solid heat transfer of the filler itself cannot be ignored, and the thermal conductivity of hydrated magnesium silicate is slightly higher than that of wollastonite. Therefore, depending on the addition rate, the heat insulation performance may decrease.

Examples of the hydrous magnesium silicate include sepiolite, attapulgite, kaolinite, smectite, montmorillonite, sericite, illite, glauconite, chlorite, talc, and mixtures thereof.
Among them, the hydrous magnesium silicate preferably contains at least one selected from the group consisting of sepiolite and attapulgite. Such hydrous magnesium silicate has a particularly high binding strength and is easy to obtain mechanical strength.

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.

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 shows a clear fibrous morphology with long fibers. α-type sepiolite is sometimes called mountain skin. β-type sepiolite is formed by sedimentation on the bottom of shallow seas and lakes. β-type sepiolite has a relatively low degree of crystallinity.

Wollastonite and β-type sepiolite are preferred because they contain a relatively small amount of crystalline silica such as quartz (impurity content) which is undesirable from the standpoint of safety for the human body.
Among them, it is preferable to select and use one produced in a place with a low content of crystalline silica. Also, the content of crystalline silica is preferably low because crystalline silica accelerates wear of a blade when cutting a sheet (sheet-shaped heat-resistant material).

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.

The filler may further contain an organic compound. Specifically, the filler preferably contains, for example, a fluorine-based resin, a silicone-based resin, or a silane coupling agent for the purpose of imparting water resistance. Among them, the silane coupling agent can connect the particles of hydrated magnesium silicate to each other, and therefore can further increase the mechanical strength (film strength) of the coating. As a result, it is easy to suppress powder falling when cutting the sheet-shaped heat-resistant material. In addition, the effect of increasing the water resistance of the coating can be expected by connecting the particles of hydrated magnesium silicate to each other via the silane coupling agent.
The silane coupling agent may be present only on the outermost surface of the sheet, or may be present inside the sheet. For example, a silane compound having a vinyl group, an epoxy group, a methacryloxy group, or an amino group as a functional group can be used as the silane coupling agent. These silane coupling agents have high hydrophobicity. Therefore, the water resistance of the sheet can be further improved, and it is easy to prevent the sheet from absorbing moisture or water and blocking the gaps between inorganic fibers.

(binder)
A binder may be included to facilitate the papermaking process and to increase the mechanical strength of the paper web by binding the inorganic or organic 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 wollastonite) or an organic binder.
Examples of the inorganic binder include various inorganic cements, various glasses, as well as hydrated magnesium silicate, colloidal silica, silica sol, alumina sol, and the like.
On the other hand, examples of the organic binder include various thermosetting resins, various thermoplastic resins, etc. Examples of the form of the organic binder include powder, particles, fibers, emulsion, solution, varnish, etc.
Specific examples of the organic binder include polyethylene resin, vinyl chloride resin, (meth)acrylic acid ester resin, styrene-acrylic acid ester copolymer, vinyl acetate resin, vinyl acetate-(meth)acrylic acid ester copolymer, ethylene-vinyl acetate copolymer, polyester resin, polyvinyl alcohol (PVA), ethylene-vinyl alcohol copolymer, styrene-butadiene rubber (SBR), nitrile rubber (NBR), phenol resin, epoxy resin, melamine resin, urea resin, unsaturated polyester resin, polyurethane resin, and thermosetting polyimide resin.
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.

The more the amount of organic binder added, the more the mechanical strength of the sheet (sheet-shaped heat-resistant material) improves. However, if a large amount of organic binder is included, the thermal shrinkage rate of the sheet increases, and the heat resistance is likely to decrease. In addition, when the sheet-shaped heat-resistant material is heated, the organic binder may be oxidized to generate heat or generate decomposition gas.
Therefore, it is preferable that the content of the organic binder in the sheet is as small as possible.

In addition to the organic binder component added to the raw material slurry for manufacturing the nonwoven fabric, a liquid containing the organic binder component as described above may be applied (external application) to the nonwoven fabric carrying the filler obtained in the previous process by a method such as spray application, curtain application, impregnation application, bar application, roll application, blade application, etc. The nonwoven fabric to be externally applied may be a dry nonwoven fabric after drying, or a wet web before drying. By externally applying the organic binder, the flexibility of the sheet can be increased and the winding property can be further improved. In addition, the falling off of the filler and fibers from the sheet surface can be further prevented. However, like the organic binder added to the raw material slurry, when the sheet-shaped heat-resistant material is heated, the organic binder may be oxidized to generate heat or generate decomposition gas.

(Optional ingredients)
The heat insulating sheet may contain, as necessary, auxiliary agents such as crosslinking agents, silane coupling agents, antioxidants, light stabilizers, ultraviolet absorbers, thickeners, nucleating agents, neutralizing agents, lubricants, antiblocking agents, dispersants, flow improvers, release agents, flame retardants, foaming agents, colorants, wetting agents, tackifiers, retention improvers, paper strength improvers, drainage agents, pH adjusters, defoamers, preservatives, and pitch control agents, and fillers such as calcium silicate, calcium carbonate, silica aerosol, fumed silica, kaolinite, smectite, montmorillonite, sericite, illite, glauconite, chlorite, talc, plastic pigments, hollow glass beads, and shirasu balloons.

Examples of clay minerals other than β-sepiolite include kaolinite, smectite, montmorillonite, sericite, illite, glauconite, chlorite, α-sepiolite, and talc.
Examples of the filler include calcium silicate and calcium carbonate.

(Composition of the heat insulating sheet)
The proportion of inorganic fibers is 10% by mass or more, preferably 10 to 52% by mass, more preferably 13 to 51% by mass, and even more preferably 15 to 50% by 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 surface strength, tensile strength, and heat insulating properties is easily obtained.

The combined content of wollastonite and β-type sepiolite is 48% by mass or more, preferably 48 to 90% by mass, more preferably 49 to 87% by mass, and even more preferably 50 to 85% by mass, based on the total mass of the heat insulating sheet. When the proportion of wollastonite is equal to or greater than the lower limit of the aforementioned numerical range, it is easy to obtain a heat insulating sheet with excellent heat insulating properties and winding processability. When the proportion of wollastonite is equal to or less than the upper limit of the aforementioned numerical range, it is easy to avoid a decrease in heat insulating properties due to an increase in solid heat transfer.

The mass ratio of wollastonite to β-sepiolite in the heat insulating sheet is 0.1 to 5.0, preferably 0.15 to 4.5, more preferably 0.2 to 4.0, and even more preferably 0.25 to 3.5. When the mass ratio is equal to or greater than the lower limit of the numerical range, a heat insulating sheet with excellent tensile strength is likely to be obtained. When the mass ratio is equal to or less than the upper limit of the numerical range, a heat insulating sheet with excellent heat shielding properties is likely to be obtained.

The total amount (T) is preferably 80 to 100% by mass, more preferably 90 to 100% by mass, even more preferably 95 to 100% by mass, and most preferably 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 winding processability 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 content of the organic component in the heat insulating sheet is preferably 8% by mass or less, more preferably 7% by mass or less, and even more preferably 5% by mass or less, based on the total mass of the heat insulating sheet. When the content of the organic component 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 is less susceptible to changes in shape and properties due to thermal degradation.
From the viewpoint of heat resistance, the lower the content of the organic component, the better. There is no particular limit to the lower limit of the organic component content. However, taking into consideration that the insulating sheet is obtained by binding inorganic fibers, the lower limit is thought to be, for example, about 0.1 mass %.

The basis weight of the heat insulating sheet is preferably 20 to 500 g/ ^m2 , more preferably 30 to 400 g/ ^m2 , and even more preferably 40 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 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 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 0.1 to 3 mm, more preferably 0.15 to 2.5 mm, and even more preferably 0.2 to 2 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 using the heat insulating sheet.
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/ ^cm2 /sec or less, more preferably 30 cc/ ^cm2 /sec or less, and even more preferably 20 cc/ ^cm2 /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. The lower the air permeability of the heat insulating sheet, the better from the viewpoint of heat insulating properties. 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/ ^cm2 /sec.
The air permeability of the heat insulating sheet is a value determined by the method described in the Examples section below.

The tensile strength of the heat insulating sheet is preferably 12N/15mm or more, more preferably 14N/15mm or more, and even more preferably 15N/15mm or more. If the ash content of the heat insulating sheet is equal to or more than the lower limit, the heat insulating sheet can be said to have excellent strength. The higher the tensile strength of the heat insulating sheet, the better, and 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 about 100N/15mm, for example.
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, wollastonite, β-sepiolite, and a binder, and then drying the wet web. In the heat insulating sheet obtained by the wet method in this way, the wollastonite and β-sepiolite can be incorporated into the heat insulating sheet. Therefore, the wollastonite and β-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 necessary, 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 inorganic fibers is 10% by mass or more relative to the heat insulating sheet, the total content of wollastonite and β-sepiolite is 48% by mass or more relative to the heat insulating sheet, and the mass ratio of β-sepiolite to wollastonite in the heat insulating sheet is 0.1 to 5.0. Therefore, the heat insulating sheet has excellent heat insulating properties and winding processability, and also has good surface strength and tensile strength.

<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 heat insulating sheet of the present invention has the advantage that it has high resistance to pressure destruction due to its flexibility, and therefore is more likely to maintain its heat insulating properties when the battery is damaged.

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

(Organic Fiber)
PET fiber: PET fiber with a fiber diameter of 12 μm and a fiber length of 5 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, melting point of the core 260° C., adhesion temperature of the sheath 110° C.)
PVA binder fiber: polyvinyl alcohol binder with a fiber diameter of 11 μm and a fiber length of 3 mm (manufactured by Kuraray, dissolution temperature in water of 60° C.)

Example 1
A mixture was obtained by mixing glass fibers, core-sheath PET fibers, and PVA binder fibers in the composition ratio shown in Table 1. The mixture was then dispersed in water to finally obtain a fiber slurry with a concentration of 0.4% by mass.
Separately, wollastonite and β-type sepiolite were mixed in the composition ratio shown in Table 1 to obtain a mixture. The mixture was then dispersed in water to obtain a filler slurry. To this filler slurry, 3 parts by weight of aluminum sulfate based on solid content, 0.5 parts by weight of an amphoteric polyacrylamide resin-based retention aid (manufactured by Arakawa Chemical Industries Co., Ltd.) based on solid content, and 0.2 parts by weight of an anionic polyacrylamide resin-based polymer flocculant (manufactured by MT Aqua Polymer Co., Ltd.) based on solid content were added in this order and stirred to form a filler floc. The final concentration was adjusted to 0.4% by weight.
The fiber slurry was mixed with the filler slurry forming the flocks in a mass ratio (fiber slurry:filler slurry=41.6:58.4) to obtain a raw material slurry having a concentration of 0.4 mass %.
This raw material slurry was wet-formed using a hand-made sheet machine to obtain a wet-formed web in which the E-glass fibers were randomly arranged and a filler consisting of 41.6 fibers (including 40 E-glass fibers), 49.4 wollastonite, and 9 β-type sepiolite was added to the paper by mass ratio. This wet-formed web was dehydrated by suction and then dried in a hot air dryer at 180°C to obtain a heat insulating sheet (25 cm x 25 cm) with a basis weight of 186.3 g/ ^m2 and a thickness of 52 ^m3 .

<Examples 2 to 7, Comparative Examples 1 to 3>
The ratios of glass fiber, PET fiber, core-sheath PET fiber, and PVA fiber, as well as the ratios of wollastonite and β-sepiolite, were changed as shown in Table 1, and insulating sheets of each example having the basis weight, thickness, and density shown in Table 1 were manufactured in the same manner as in Example 1.

<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)
The thickness of the heat insulating sheet of each example was measured in accordance with JIS P 8118:1998, with the pressure between the pressure surfaces set at 50 kPa.

(density)
For each heat insulating sheet example, the density was calculated by dividing the basis weight obtained in the previous section by the thickness.

(Tensile strength)
The tensile strength of each example of the 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 a test length of 180 mm in an environment of 23°C and 50% RH, and the tensile strength was measured according to a method in accordance with JIS P8113. The obtained tensile strength was evaluated according to the following criteria.

A: Tensile strength is 15 N/15 mm or more.
B: The tensile strength is 12 N/15 mm or more and less than 15 N/15 mm.
C: Tensile strength is less than 12 N/15 mm.

(Surface strength)
The surface strength of each heat insulating sheet was evaluated as follows. A 15 mm wide cellophane tape was cut into a 50 mm long piece and placed with the adhesive side facing the sheet surface. The index finger and middle finger were placed together 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 stuck to 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.

(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 diameter that could be wound without cracking, folding, or wrinkling on the sheet surface was determined as the windable diameter.

The possible winding diameter was evaluated according to the following criteria.
A: The winding diameter is less than 60 mm.
B: The winding diameter is 60 mm or more and less than 80 mm.
C: The winding diameter is 80 mm or more.

(Breathability)
The air permeability of each heat insulating sheet was measured according to Method A (Fragile type method) specified in JIS L 1096:2010, and evaluated according to the following criteria.
A: Air permeability is 20 cc/cm ² /sec or less.
B: Air permeability is more than 20 cc/cm ² /sec and 50 cc/cm ² /sec or less.
C: Air permeability is more than 50 cc/cm ² /sec.

(Heat insulation)
The heat shielding properties of each example of the heat insulating sheet were evaluated by the following procedure. A sample of 80 mm x 100 mm was cut out from the sheet, the thickness was measured as in the previous section, and the sheet was compressed at 200°C and 2 MPa for 15 minutes, and then cooled while still compressed. The thickness of the samples cut out in the same manner was measured in the same manner, and the samples were heated and compressed after being stacked in two and three sheets.
The single compressed sample was placed on a hot plate at 500°C and held there for 10 minutes while applying a pressure of 1 kPa. The temperature of the top surface of the sample, i.e., the temperature of the surface opposite to the surface in contact with the hot plate, was then measured using a contact thermometer.
The same procedure was used to measure the two-ply and three-ply compressed samples, and a regression curve was obtained between the thickness before heat compression and the temperature reached on the opposite side after heating at 500°C for 10 minutes. Using this regression curve, the temperatures reached on the opposite side after heating at 500°C for 10 minutes, which correspond to thicknesses of 500 μm, 1000 μm, and 1500 μm before heat compression, were obtained. These values were evaluated according to the following evaluation criteria.

Before heat compression, the thickness is 500 μm. After heating at 500 ° C for 10 minutes, the temperature of the opposite side to the heating _T1
A: _T1 is less than 400°C.
B: _T1 is 400°C or higher and lower than 405°C.
C: _T1 is 405°C or higher.

Before heat compression, the thickness is 1000 μm. After heating at 500 ° C for 10 minutes, the temperature reached on the opposite side of the heated surface is _T2.
A: _T2 is less than 365°C.
B: _T2 is 365°C or more and less than 370°C.
C: _T2 is 370°C or higher.

Before heat compression, the thickness is 1500 μm. After heating at 500 ° C for 10 minutes, the temperature reached on the opposite side of the heated surface is _T3.
A: _T3 is less than 345°C.
B: _T3 is 345°C or higher and lower than 350°C.
C: _T3 is 350°C or higher.

<Results>
The measurement results and evaluation results of each example are shown in Table 1.

In Examples 1 to 7, insulating sheets with excellent heat insulation properties and winding processability were obtained, and the surface strength and tensile strength of the insulating sheets were also good. In contrast, in Comparative Example 1, the tensile strength was insufficient. In Comparative Example 2, the surface strength, tensile strength, and insulating properties were all insufficient. In Comparative Example 3, the tensile strength, winding processability, and insulating properties were all insufficient.

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 (8)

A heat insulating sheet,
Contains wollastonite, sepiolite, and inorganic fibers;
The content of the inorganic fibers is 10% by mass or more relative to 100% by mass of the heat insulating sheet;
The total content of the wollastonite and the sepiolite is 48% by mass or more relative to 100% by mass of the heat insulating sheet;
A heat insulating sheet, wherein the mass ratio of the sepiolite to the wollastonite is 0.1 or more and 5.0 or less. The heat insulating sheet according to claim 1, wherein the wollastonite has an average length of 5 to 100 μm, an average diameter of 1 to 30 μm, and an aspect ratio of 3 to 100. The heat insulating sheet according to claim 1, wherein the content of organic components in the heat insulating sheet is 8% by mass or less based on the total mass of the heat insulating sheet. The heat insulating sheet according to claim 1, wherein the basis weight of the heat insulating sheet is 20 to 500 g/ ^m2 . The heat insulating sheet according to claim 1, wherein the thickness of the heat insulating sheet is 0.1 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. The battery pack according to claim 7, wherein a heat insulating sheet is also disposed on the outside of the outermost stacked cell among the plurality of cells.

Images