JP7525321B2 - Heat insulating sheet for battery pack and battery pack - Google Patents

Description

The present invention relates to an insulating sheet for a battery pack that is interposed between battery cells of the battery pack, and to a battery pack in which an insulating sheet for a battery pack is interposed between battery cells.

Conventionally, heat insulating sheets have been used in close proximity to a heating element or with at least a portion in contact with the heating element to suppress heat transfer from the heating element to other objects.

In recent years, from the perspective of environmental protection, there has been active development of electric vehicles and hybrid vehicles that are driven by electric motors. These electric vehicles and hybrid vehicles are equipped with a battery pack in which multiple battery cells are connected in series or parallel to serve as the power source for the driving electric motor.

These battery cells are mainly lithium-ion secondary batteries, which are capable of higher capacity and power output than lead-acid batteries or nickel-metal hydride batteries. In batteries capable of high capacity and power output, if a battery cell experiences thermal runaway, where the temperature rises suddenly and continues to generate heat due to an internal short circuit or overcharging, the heat from the battery cell experiencing thermal runaway may propagate to other adjacent battery cells, causing thermal runaway in the other battery cells.

In the field of battery packs as described above, various insulating sheets have been proposed to be interposed between battery cells in order to suppress the transfer of heat from a battery cell that has experienced thermal runaway to adjacent battery cells and prevent a chain reaction of thermal runaway. Patent Document 1 discloses a method for manufacturing an insulating sheet that solves the problem that when an insulating sheet is sandwiched between battery cells, etc., the aerogel is compressed and crushed, especially under high load, resulting in a significant decrease in the insulating effect compared to when the load is low. The manufacturing method includes a complex formation step in which a basic sol produced by adding a carbonate ester to a water glass composition is impregnated into nonwoven fabric fibers to form a hydrogel-nonwoven fabric fiber complex, a surface modification step in which the complex is mixed with a silylation agent to modify the surface, and a drying step in which the liquid contained in the complex is removed by drying below the critical temperature and critical pressure of the liquid. The compressibility of the insulating sheet at 0.30 to 5.0 MPa is specified to be 40% or less.

JP 2019-99984 A

However, the heat insulating sheet obtained by the manufacturing method described in Patent Document 1 combines aerogel, which is inherently easily crushed, with nonwoven fabric fibers and adjusts the compression rate under a specified surface pressure to 40% or less, so the resilience of the sheet is mainly due to the nonwoven fabric fibers. Therefore, when strong pressure is applied, the thickness becomes thin and the heat insulating performance decreases. In addition, distortion occurs in the aerogel component, which is particularly easily compressed, causing countless cracks to occur, which causes particles to fall off. Once cracks occur, the aerogel component decreases due to repeated expansion and contraction, and the heat insulating performance decreases.

The present invention was made in consideration of the above-mentioned circumstances, and aims to provide an insulating sheet for a battery assembly that is resistant to deformation even when a strong compressive force is applied, has a structure that is less susceptible to particle shedding, and can provide stable insulating performance for a long period of time, and an assembled battery in which the insulating sheet for a battery assembly is interposed between battery cells.

The above object is achieved by the heat insulating sheet according to the present invention (1).

(1) A heat insulating sheet for a battery assembly, which is interposed between a plurality of battery cells in a battery assembly having the battery cells connected in series or parallel, comprising:
The composition includes first particles made of silica nanoparticles, second particles made of a metal oxide dispersed between the first particles, and a binder that binds the first particles and the second particles,
A content of the first particles is 30 mass% or more and 80 mass% or less with respect to a total mass of the insulating sheet for a battery assembly,
A heat insulating sheet for a battery pack, having a compression rate k of 5% or more and 40% or less when the surface pressure p is 5 MPa.

The heat insulating sheet of the present invention is preferably one of the following (2) to (12).

(2) The insulating sheet for a battery pack according to (1), wherein the first particles have an average particle diameter of 1 nm or more and 100 nm or less.
(3) The heat insulating sheet for a battery pack according to (1) or (2), wherein the second particles are at least one type selected from the group consisting of titania, zirconia, zircon, barium titanate, zinc oxide, and alumina.
(4) The insulating sheet for a battery pack according to (3), wherein the second particles are titania.
(5) The insulating sheet for a battery pack according to any one of (1) to (4), wherein the second particles have an average particle diameter of 1 μm or more and 50 μm or less.
(6) The insulating sheet for a battery pack according to any one of (1) to (5), wherein the content of the binder is 3 mass % or more and 30 mass % or less based on the total mass of the insulating sheet for a battery pack.
(7) The insulating sheet for a battery pack according to any one of (1) to (6), wherein the binder is at least one selected from the group consisting of methyl cellulose, water-soluble cellulose ether, hydroxypropyl methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxyethyl methyl cellulose, and derivatives thereof.

(8) The insulating sheet for a battery assembly according to any one of (1) to (7), wherein, when a differential value obtained by differentiating the surface pressure p with respect to the compression rate k is defined as dp/dk, the dp/dk with respect to the compression rate k has a maximum value.
(9) The insulating sheet for a battery assembly according to (8), wherein a first maximum value at which the dp/dk first becomes maximum appears when the compression rate k is in the range of 5% or more and 40% or less.
(10) The insulating sheet for a battery pack according to (9), wherein the first maximum value is 50 MPa or more and 100 MPa or less.
(11) The insulating sheet for a battery pack according to any one of (1) to (10), further comprising fibers dispersed between the first particles and the second particles.
(12) The insulating sheet for a battery pack according to (11), wherein the fibers are inorganic fibers or organic fibers.

The above object is achieved by the battery pack according to the present invention as set forth below in (13).

(13) A battery pack in which multiple battery cells are arranged via a heat insulating sheet for a battery pack described in any one of (1) to (12), and the multiple battery cells are connected in series or parallel.

According to the insulating sheet for battery packs of the present invention, the first particles and the second particles are bound by a binder, so that the elasticity of the binder provides a repulsive force to the insulating sheet. Therefore, even if the insulating sheet for battery packs is compressed, a certain thickness can be ensured and the insulating properties can be maintained. In addition, because the binder holds the first particles and the second particles, the first particles and the second particles can be prevented from falling off even if the insulating sheet is compressed.

In addition, the battery pack insulating sheet of the present invention has an appropriately adjusted content of the first particles, ensuring thermal insulation and allowing the desired amounts of the second particles and binder to be contained. Therefore, the second particles can block radiative heat transfer, and the binder can ensure a relationship between a predetermined surface pressure p and compression rate k, preventing particle falloff.

Furthermore, according to the insulating sheet for battery packs of the present invention, the compression rate k is 40% or less when the surface pressure p is 5 MPa, and it has a high resilience, so even if compression occurs due to battery expansion, it is difficult to collapse, a constant thickness can be ensured, and thermal insulation properties can be ensured. Also, since the compression rate k is 5% or more when the surface pressure p is 5 MPa, even if the battery cells expand or an external force is applied and distortion occurs, the insulating sheet for battery packs is unlikely to be subjected to large stress, and cracks can be prevented.

The battery pack of the present invention uses the insulating sheet for the battery pack, so even if the battery expands, the insulating sheet maintains a constant thickness and the insulating properties can be continuously ensured. Therefore, it is possible to prevent thermal runaway problems in individual batteries from spreading to adjacent batteries.

FIG. 1 is a schematic diagram showing the configuration of a heat insulating sheet for a battery pack according to an embodiment of the present invention. FIG. 2 is a cross-sectional view that illustrates a battery pack according to a first embodiment that uses the heat insulating sheet for a battery pack shown in FIG. FIG. 3 is a cross-sectional view that illustrates a battery pack according to a second embodiment that uses a thermal insulating sheet for a battery pack that includes fibers. FIG. 4 is a graph showing the change in surface pressure p versus compression ratio k, where the vertical axis represents surface pressure p and the horizontal axis represents compression ratio k. FIG. 5 is a graph showing the change in the differential value dp/dk with respect to the compression ratio k, where the vertical axis represents the differential value dp/dk and the horizontal axis represents the compression ratio k.

The inventors conducted extensive research in order to provide an insulating sheet for a battery pack that has a structure that is resistant to deformation even when a strong compressive force is applied, is less susceptible to particle shedding, and can provide stable insulating performance for a long period of time. As a result, they discovered that it is important that first particles made of silica nanoparticles and second particles made of a metal oxide that are dispersed between the first particles are bound together by a binder.
In other words, by providing an insulating sheet for an assembled battery with the above-described structure and by appropriately adjusting the content of the first particles and the compression rate k of the insulating sheet under a specified surface pressure, it is possible to provide an insulating sheet for an assembled battery that is resistant to deformation even when a strong compressive force is applied, has a structure that is resistant to particle fall-off, and can exhibit stable insulating performance for a long period of time.

<Basic structure of heat insulating sheet for battery pack>
Fig. 1 is a schematic diagram showing the configuration of a heat insulating sheet 10 for a battery pack according to an embodiment of the present invention. Fig. 2 is a cross-sectional view showing a battery pack 100 according to a first embodiment using the heat insulating sheet 10 for a battery pack shown in Fig. 1. The heat insulating sheet 10 includes first particles 21 made of silica nanoparticles, second particles 22 made of metal oxide, and a binder that binds the first particles 21 and the second particles 22. Although the binder is not shown, it exists between the particles to fix the first particles 21 and the second particles 22 to each other.
The silica nanoparticles preferably have an average particle size of 1 nm to 100 nm. The content of the first particles 21 is 30% by mass to 80% by mass based on the total mass of the insulating sheet for a battery pack. The insulating sheet for a battery pack 10 has a compressibility k of 5% to 40% when the surface pressure p is 5 MPa.

As a specific example of how the insulating sheet 10 for a battery pack is used, as shown in FIG. 2, the insulating sheet 10 for a battery pack is interposed between a number of battery cells 20, and the battery cells 20 are stored in a battery case 30 in a state in which they are connected in series or parallel (the connected state is not shown) to form the battery pack 100. Note that, for example, lithium ion secondary batteries are preferably used as the battery cells 20, but the present invention is not limited to this and can also be applied to other secondary batteries.

The following description assumes that a heated battery cell 20 is present on one side 10a of the insulating sheet 10. In the insulating sheet 10 configured in this manner, when the battery cell 20 generates heat, a portion of the heat incident from one side 10a of the insulating sheet 10 is conducted (solid conduction) toward the other side 10b of the insulating sheet 10 via the first particles 21 that are in contact with each other, as shown by arrow 15a in FIG. 1. At this time, because silica nanoparticles having insulating properties are used as the first particles 21, the amount of heat transfer decreases as it approaches the other side 10b of the insulating sheet 10.

Furthermore, when the battery cell 20 generates heat and some of the heat reaches the second particles 22 by radiation, as shown by arrow 15b, the heat is diffusely reflected by the second particles 22, which are metal oxides, and the presence of the second particles 22 can prevent the heat from being transmitted to the other surface 10b of the heat insulating sheet 10. Furthermore, in this embodiment, silica nanoparticles are used as the first particles 21, and since the contact points between the particles are small, the amount of heat conducted by the silica nanoparticles is smaller than when silica particles with a large particle diameter are used.
Furthermore, in the battery pack 100, even if the battery cells 20 generate gas inside and expand as they are repeatedly used, the binder binding the first particles 21 and the second particles 22 inside the insulating sheet 10 repels the first particles 21 and the second particles 22, resisting the expansion of the battery cells 20. Therefore, the repulsive force of the binder provides the insulating sheet 10 with a restoring force, and the insulating sheet 10 can ensure a constant thickness. Therefore, even if the battery cells 20 experience thermal runaway, the insulating properties of the insulating sheet can be maintained.

In addition, according to the insulating sheet 10 for battery packs of this embodiment, the second particles 22 are dispersed in the first particles 21 and are further fixed with a binder, so there is no variation in strength overall, and there are no brittle parts where particles may fall off even when compressed. This makes it possible to prevent dust generation, and the sheet is less likely to become thin due to particle fall-off with repeated use, preventing a decrease in insulating properties.

As described above, according to this embodiment, even if a battery cell 20 expands, particle falling off can be prevented and excellent insulation properties can be maintained.

In this embodiment, silica nanoparticles are used as the first particles 21, and since the contact points between the particles are small, the amount of heat conducted by the silica nanoparticles is smaller than that when silica particles with a large particle diameter are used. In addition, since commonly available silica nanoparticles contain many voids and have a bulk density of about 0.5 g/cm ³ , even if the battery cells 20 arranged on both sides of the heat insulating sheet 10 thermally expand and the heat insulating sheet 10 is slightly deformed by the pressure, the voids remain due to the repulsive force between the particles and the binder, and the size (area) of the contact points between the silica particles does not increase significantly, and the heat insulating properties can be maintained.

Furthermore, in this embodiment, the gaps formed between the silica nanoparticles are only a few tens of nanometers in size, so that only small convection currents occur in the gaps as shown by arrows 15c, preventing air movement and suppressing convective heat transfer. Therefore, by using silica nanoparticles as the first particles 21, heat transfer that penetrates from the front to the back of the insulating sheet 10 is less likely to occur, and the insulating properties of the insulating sheet 10 can be further improved.

<Details of the heat insulating sheet for battery packs>
Next, the first particles 21, the second particles 22, and the binder that constitute the battery pack insulating sheet 10 will be described in detail.

(Type of First Particle)
In this embodiment, silica nanoparticles are used as the first particles 21. As the silica nanoparticles, wet silica, dry silica, aerogel, or the like can be used.
In the present embodiment, the silica nanoparticles are silica particles that are spherical or nearly spherical and have an average particle size of less than 1 μm on the nanometer order.

(Content of first particles: 30% by mass or more and 80% by mass or less with respect to the total mass of the heat insulating sheet)
The content of the first particles 21 in this embodiment is 30 mass % or more and 80 mass % or less with respect to the total mass of the insulating sheet for a battery pack.
The silica nanoparticles contained as the first particles 21 in the insulating sheet 10 have a low density, which suppresses conductive heat transfer, and furthermore, because the voids are finely dispersed, they have excellent insulating properties that make it difficult for air to move, suppressing convective heat transfer.
In the battery pack insulating sheet 10 of this embodiment, the content of the first particles 21 is 30 mass% or more relative to the total mass of the battery pack insulating sheet, thereby ensuring thermal insulation, and the content of the first particles 21 is 80 mass% or less, thereby ensuring sufficient space for containing the second particles and binder, blocking radiative heat transfer by the second particles 22, and ensuring a relationship between a predetermined surface pressure p and compression rate k by the binder.
The content of the first particles 21 is preferably 45% by mass or more and 75% by mass or less based on the total mass of the insulating sheet for a battery pack. The content of the first particles 21 is more preferably 55% by mass or more and 70% by mass or less based on the total mass of the insulating sheet for a battery pack.

(Average particle size of first particles: 1 nm or more and 100 nm or less)
As described above, the particle size of the first particles 21 can affect the insulating properties of the insulating sheet 10, so by limiting the average particle size of the first particles 21 to a predetermined range, even higher insulating properties can be obtained.
In other words, by setting the average particle diameter of the first particles 21 to be 1 nm or more and 100 nm or less, convective heat transfer and conductive heat transfer within the insulating sheet 10 can be suppressed, particularly in the temperature range below 500°C, thereby further improving the insulation properties.
The average particle diameter of the first particles 21 is more preferably 2 nm or more, and even more preferably 3 nm or more. The average particle diameter of the first particles 21 is more preferably 50 nm or less, and even more preferably 10 nm or less.

(Type of second particle)
In this embodiment, titania, zirconia, zircon, barium titanate, zinc oxide, alumina, etc. can be used as the metal oxide constituting the second particles 22. In particular, titania is a component with a higher refractive index than other metal oxides, and has a high effect of diffusely reflecting heat in a high temperature range of 500° C. or more, so it is most preferable to use titania.

(Average particle size of second particles: 1 μm or more and 50 μm or less)
Since the particle size of the second particles 22 can affect the heat reflecting effect, by limiting the average particle size of the second particles 22 to a predetermined range, even higher heat insulation can be obtained.
That is, when the average particle diameter of the second particles 22 is 1 μm or more, it is sufficiently larger than the wavelength of light that contributes to heating, and efficiently diffuses light, and in the presence range (mass ratio) of the second particles 22 in this embodiment, the radiative heat transfer in the heat insulating sheet 10 is suppressed in the high temperature region of 500° C. or more, and the heat insulating property can be further improved. When the average particle diameter of the second particles 22 is 50 μm or less, the number and number of contact points between the particles do not increase, making it difficult to form a path for conductive heat transfer, and the effect on the heat insulating property in the normal temperature range of less than 500° C. where conductive heat transfer is dominant can be reduced.

The average particle diameter of the second particles 22 is more preferably 3 μm or more, and even more preferably 5 μm or more. The average particle diameter of the second particles 22 is more preferably 30 μm or less, and even more preferably 10 μm or less.
In this embodiment, the average particle size of the first particles 21 and the second particles 22 can be obtained by observing the particles under a microscope, comparing with a standard scale, and taking the average of any 10 particles.

(Content of second particles 22: 5% by mass or more and 40% by mass or less with respect to the total mass of the heat insulating sheet)
In this embodiment, the heat insulating sheet 10 contains the second particles 22 in order to improve the heat insulating property in the high temperature region of 500°C or more, but even if the amount of the second particles 22 added is small, the effect of suppressing the radiation conduction of heat can be obtained. Also, in order to obtain the effect of suppressing the convection heat transfer and the conduction heat transfer by the first particles 21, it is preferable to increase the amount of the first particles 21 added. As described above, since the mass ratio of the second particles 22 affects the heat insulating property in the region from normal temperature to high temperature of 500°C or more, in this embodiment, it is preferable to appropriately adjust the mass ratio of the second particles 22.

In the heat insulating sheet 10 of the present embodiment, the mass ratio of the second particles 22 is preferably 5 mass% or more relative to the total mass of the heat insulating sheet. When the content of the second particles 22 is 5 mass% or more relative to the total mass of the heat insulating sheet, it is considered that radiation heat transfer can be suppressed, particularly in a temperature range of 500°C or more where the effect of radiation is large, and high heat insulation can be obtained.
On the other hand, the mass ratio of the second particles 22 in the heat insulating sheet 10 of this embodiment is preferably 40 mass% or less with respect to the total mass of the heat insulating sheet. If the content of the second particles 22 exceeds 40 mass% with respect to the total mass of the heat insulating sheet, the first particles 21 and the second particles 22 may not provide sufficient effects, and it may become difficult to suppress the convective conduction or solid conduction of heat in the heat insulating sheet 10 in a temperature range below 500°C, which may result in a decrease in heat insulation.

(Type of binder)
As described above, in order to ensure a good relationship between the surface pressure p and the compressibility k, it is preferable to appropriately select the type of binder in the insulating sheet 10 for a battery pack according to this embodiment. As the binder, an organic binder, an inorganic binder, or the like can be used. In this embodiment, it is particularly preferable to use methyl cellulose, water-soluble cellulose ether, hydroxypropyl methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxyethyl methyl cellulose, acrylic emulsion, and derivatives thereof as the organic binder. There is no particular restriction on the type of inorganic binder, but examples of the inorganic binder that can be used include alumina sol and silica sol.

(Binder content: 3% by mass or more and 30% by mass or less)
In this embodiment, the insulating sheet 10 includes first particles 21 made of silica nanoparticles and second particles 22 made of metal oxide, and further includes a binder that bonds the first particles 21 and the second particles 22, but it is preferable that the content of the binder is appropriately adjusted in order to maintain the shape of the insulating sheet.
When the binder content is 3 mass% or more relative to the total mass of the heat insulating sheet 10, it is possible to reduce the falling off of silica nanoparticles from the heat insulating sheet 10. In addition, the binder content relative to the total mass of the heat insulating sheet 10 is preferably 5 mass% or more.

On the other hand, if the binder content exceeds 30% by mass relative to the total mass of the heat insulating sheet 10, the number of contact points between the first particles 21 and the second particles 22 increases too much, which weakens the effect of suppressing solid conduction and may result in reduced heat insulation. Therefore, the binder content relative to the total mass of the heat insulating sheet is preferably 30% by mass or less, and more preferably 20% by mass or less.
In this embodiment, as shown below, the compression rate k of the heat insulating sheet when the surface pressure p is 5 MPa is limited to 5% or more and 40% or less, so it is preferable to adjust the amount of binder appropriately so that the compression rate k is within this range.

(Compressibility k of the heat insulating sheet when the surface pressure p is 5 MPa: 5% or more and 40% or less)
The insulating sheet 10 for a battery pack in this embodiment has a compression rate k of 40% or less when the surface pressure p is 5 MPa, and has a high resilience, so that even if compression occurs due to battery expansion, the insulating sheet is not easily crushed and can maintain a constant thickness, thereby ensuring thermal insulation. On the other hand, by setting the compression rate k of the insulating sheet 10 at a surface pressure p of 5 MPa to 5% or more, the insulating sheet 10 for a battery pack is less likely to experience large stress and can be prevented from cracking, even if the insulating sheet 10 for a battery pack expands and becomes distorted, or if the insulating sheet 10 for a battery pack is distorted due to vibration, impact, or external deformation.
Therefore, when the surface pressure p is set to 5 MPa, the compression rate k of the heat insulating sheet 10 is set to be 5% or more and 40% or less.

The compression ratio k of the insulating sheet when the surface pressure p is 5 MPa can be measured, for example, by using a universal material testing machine (Model 5567 testing machine: manufactured by Instron Japan Company Limited).

(dp/dk has a maximum value with respect to the compression ratio k)
When the differential value obtained by differentiating the surface pressure p applied to the insulating sheet 10 with respect to the compression rate k is dp/dk, if dp/dk with respect to the compression rate k has a maximum value, the repulsive force of the insulating sheet 10 gradually increases as the surface pressure p increases, and the insulating sheet 10 is compressed while maintaining a constant repulsive force. Therefore, when dp/dk with respect to the compression rate k has a maximum value, the insulating sheet 10 for a battery pack sandwiched between the battery cells 20 is not completely crushed by compression, and is able to maintain its own holding force.

(The first maximum value of dp/dk with respect to the compression ratio k (first maximum value): the compression ratio k is in the range of 5% to 40%).
As described above, when dp/dk has a maximum value with respect to the compression rate k, the repulsive force of the insulating sheet 10 gradually increases with an increase in the surface pressure p. When the surface pressure p and the compression rate k are 0, dp/dk is also 0. When the point where dp/dk first reaches a maximum value (first maximum value) appears within the range of the compression rate k of 5% to 40%, the insulating sheet 10 can exert a sufficient repulsive force before being completely crushed. Therefore, strong elasticity can be obtained while maintaining the internal pores of the insulating sheet 10 for a battery pack, and the insulating properties can be ensured and particle falling off can be prevented.

(First maximum value of dp / dk: 50 MPa or more and 100 MPa or less)
When the first maximum value is 50 MPa or more and 100 MPa or less, a sufficient holding force is ensured, and even if deformation or the like is applied from the outside, a large force is unlikely to be applied, and particle falling off and cracks can be prevented.

(fiber)
The insulating sheet for a battery assembly of this embodiment preferably further contains fibers. A battery assembly according to a second embodiment using the insulating sheet for a battery assembly containing fibers will be described with reference to Fig. 3. In Fig. 3, the same components as those in Fig. 2 are designated by the same reference numerals, and detailed description thereof will be omitted.
As shown in Fig. 3, the heat insulating sheet 40 includes the first particles 21 and the second particles 22, and has fibers 23 dispersed therebetween. When the heat insulating sheet 40 includes the fibers 23, even if a crack occurs due to an external force or the like, the fibers 23 can maintain the overall shape, preventing collapse and allowing the sheet to be held in a predetermined position within the battery case 30. As the fibers 23, inorganic fibers or organic fibers can be used. Since inorganic fibers or organic fibers have low thermal conductivity, even if the inorganic fibers or organic fibers are dispersed between the first particles 21 and the second particles 22, the heat insulating properties are not reduced, and the heat insulating sheet 40 can be suitably used.

(Fiber content: 5% by mass or more and 30% by mass or less based on the total mass of the heat insulating sheet)
As shown in the second embodiment, when the insulating sheet 40 for a battery pack contains the fibers 23, the content of the fibers 23 is preferably 5% by mass or more and 30% by mass or less with respect to the total mass of the insulating sheet for a battery pack.
When the content of the fibers 23 is 5% by mass or more, the shape can be sufficiently maintained even if cracks occur. When the content of the fibers 23 is 30% by mass or less, a sufficient space can be secured to fill the first particles 21 having high thermal insulation properties, and thermal insulation can be ensured.

(Average fiber diameter: 1 μm or more and 20 μm or less)
The fibers 23 are linear or needle-like fibers with a large diameter, which contribute to improving the mechanical strength and shape retention of the heat insulating sheet 40 against pressure from the battery cells 20. To achieve this effect, the average fiber diameter is preferably 1 μm or more, and more preferably 2 μm or more. However, if the fibers 23 are too thick, there is a risk of reduced moldability and processability into the heat insulating sheet 40, so the average fiber diameter is preferably 20 μm or less, and more preferably 15 μm or less.

(Average fiber length: 0.1 mm or more and 300 mm or less)
When the fibers 23 are contained in the insulating sheet 40 for a battery pack, the fibers 23 are preferably entangled with each other when the insulating sheet 40 is formed, and sufficient surface pressure can be obtained. In order to obtain such an effect, when the fibers 23 are contained, the average fiber length is preferably 0.1 mm or more (100 μm or more), and more preferably 1 mm or more. However, when adopting a process in which the fibers 23, the first particles 21, and the second particles 22 are mixed once and then formed, if the average fiber length of the fibers 23 is too long, the fibers 23 may be too strongly entangled with each other during preparation of the raw material, and the fibers 23 may be easily accumulated non-uniformly after being formed into a sheet. Therefore, the average fiber length of the fibers 23 is preferably 300 mm or less, more preferably 60 mm or less, and even more preferably 20 mm or less.
The fiber diameter and fiber length of the fibers 23 can be measured by removing the binder component in the insulating sheet for a battery pack 40 using a solvent, then using tweezers to remove the fibers 23 from the molded sheet without breaking them, and comparing them with a standard scale. If necessary, the fiber diameter and fiber length can be measured by observing with an optical microscope.

(Method of manufacturing a heat insulating sheet for a battery pack)
Next, a method for producing the insulating sheet for a battery assembly according to this embodiment will be described in detail.
The heat insulating sheets 10 and 40 according to the present embodiment can be manufactured by using the first particles 21, the second particles 22, and a binder, and a material for heat insulating sheets containing fibers as necessary, and molding the material by an extrusion molding method, a wet papermaking method, a press molding method, etc. The manufacturing methods for obtaining the heat insulating sheets 10 and 40 by each molding method will be described below.

[Method of manufacturing heat insulating sheet by extrusion molding]
In the extrusion molding method, first, the first particles 21, the second particles 22, and the binder are mixed with fibers and auxiliary agents as necessary, and then a solvent such as water is added to prepare a paste-like raw material. The obtained paste-like raw material is then extruded from the nozzle of an extruder to obtain a green sheet. The obtained green sheet is then dried and cut to an appropriate size to obtain the insulating sheet for battery packs 10, 40. As described above, it is preferable to use methyl cellulose, water-soluble cellulose ether, etc. as the binder, but any organic binder that is generally used when using the extrusion molding method can be used without any particular limitation.

In addition, in the manufacturing method using the extrusion molding method, a lubricant can be used as an auxiliary agent to improve the releasability from the metal surface that becomes the mold during extrusion. Examples of lubricants that can be used include stearin, butyl stearate, octyl stearate, and glycerin monostearate.

[Method of manufacturing heat insulating sheet by wet papermaking method]
In the wet papermaking method, first, the first particles 21, the second particles, and the binder are mixed in water with fibers and auxiliary agents to be added as necessary, and the mixture is prepared by stirring with a stirrer. The mixture is then poured into a former having a filtering mesh formed on the bottom, and the mixture is dehydrated through the mesh to produce a wet sheet. Since most of the binder contained in the mixture flows out, the binder amount may be adjusted by impregnating the mixture after papermaking.
In addition, before the heating and pressurizing step, a through-drying treatment may be performed in which hot air is passed through the wet sheet to dry the sheet, but the sheet may also be heated and pressed in a wet state without this through-drying treatment.

[Method of manufacturing heat insulating sheet by press molding]
In the press molding method, first, the first particles 21, the second particles 22, a binder, a solvent, and, if necessary, fibers, auxiliary agents, etc. are stirred with a stirrer to prepare a slurry-like raw material. The obtained raw material is press molded to obtain the insulating sheet for a battery pack 10, 40 of the present embodiment.

The insulating sheets 10, 40 for battery packs of this embodiment may be manufactured by any method, but the amount of binder can be appropriately adjusted so that the compression rate k when the surface pressure p is 5 MPa is 5% or more and 40% or less.
Furthermore, the manufacturing method of the heat insulating sheets 10, 40 for battery packs of the present embodiment is not limited to the above, and any manufacturing method can be used to manufacture the sheets.

<Battery pack>
As illustrated in FIG. 2 , the battery pack 100 according to the first embodiment has the insulating sheet for a battery pack 10 interposed between a plurality of battery cells 20, and is stored in a battery case 30 with the plurality of battery cells 20 connected in series or in parallel.
The heat insulating sheet 10 is held in a compressed state between the battery cells 20. Therefore, the heat insulating sheet 10 is sandwiched between the battery cells 20 and is less likely to shift position.

As illustrated in FIG. 3 , the battery pack 110 according to the second embodiment has a plurality of battery cells 20 arranged via the insulating sheet 40 for the battery pack, and is stored in a metal battery case 30 with the plurality of battery cells 20 connected in series or in parallel.
The insulation sheet 40 has its overall shape maintained by the fibers 23 and is held in a compressed state between the battery cells. This makes it difficult for the insulation sheet 40 to shift position when sandwiched between the battery cells 20, and also prevents it from collapsing even at high temperatures.

Below, examples of the insulating sheet for a battery pack according to this embodiment are described, but the present invention is not limited to these examples.

The first particles, second particles, binder, fibers, and auxiliary agents shown in Table 1 below were prepared, and these materials were thoroughly stirred and mixed to prepare raw materials.
Next, water was added to the raw materials, which were then extruded into a paste and dried to obtain heat insulating sheet No. 1. Water was also added to the raw materials to prepare a slurry, which was then molded into a sheet and dried to obtain heat insulating sheet No. 5. The slurry was then molded into a sheet, which was then pressed to adjust the density, and the sheet was dried to obtain heat insulating sheets No. 2 to 4. However, the pressing pressure of heat insulating sheets No. 2 to 4 was adjusted so that the density was 0.4 to 0.65 g/cm ^3. Heat insulating sheets No. 1 to 5 were all dried at 110° C., and the size of the obtained heat insulating sheets was 80 mm wide, 80 mm long, and 1 mm thick. The densities of the heat insulating sheets are also shown in Table 1 below.

The first particles were silica nanoparticles (average particle diameter 5 nm), the second particles were titania (average particle diameter 8 μm), the binder was thermoplastic resin, and the fibers were inorganic fibers (average fiber diameter 5 μm, average fiber length 100 μm = 0.1 mm). In addition, in the insulation sheet No. 1 made by extrusion molding, a lubricant was used as an auxiliary agent, and in the insulation sheets No. 2 to 5 whose density was changed by pressing, a flocculant was used as an auxiliary agent.

The surface pressure of each heat insulating sheet was measured while compressing it. The surface pressure was measured using a universal material testing machine (5567 model testing machine: manufactured by Instron Japan Co., Ltd.). The sample size was 25 mm x 25 mm, and the compression speed was 0.5 mm/min.
FIG. 4 shows the change in surface pressure p versus compression ratio k, with the vertical axis representing surface pressure p and the horizontal axis representing compression ratio k. FIG. 5 shows the change in differential value dp/dk versus compression ratio k, with the vertical axis representing differential value dp/dk and the horizontal axis representing compression ratio k.

In addition, the compression ratio k was calculated when the surface pressure p was set to 5 MPa, and when the differential value dp/dk had a maximum value, the compression ratio k and the maximum value were observed when the first maximum value (first maximum value) appeared. Furthermore, when a surface pressure of 5 MPa was applied, the presence or absence of particle dropout was confirmed, and the thickness of each heat insulating sheet was measured after the surface pressure of 5 MPa was released, and the ratio to the original thickness (restoration rate) was calculated. It was determined that a restoration rate of 70% or more would prevent deformation and suppress deterioration of heat insulating properties.
The measurement results are shown in Table 2 below.

Since heat insulation sheet No. 1 was molded while pressure was applied by extrusion molding, it was as dense as heat insulation sheets No. 2 to 5, which were pressed to various densities after being molded into a sheet. In addition, since heat insulation sheets No. 1 to 5 contain a binder in the raw materials, no particle loss was observed even when compressed. Furthermore, heat insulation sheets No. 1 to 4 are examples that contain a specified first particle, a second particle, and a binder, the content of the first particle is within a specified range, and the compression ratio k when the surface pressure p is 5 MPa is 5% to 40%. Therefore, a recovery rate of 70% or more can be ensured, and no particle loss was observed. In other words, it was confirmed that heat insulation sheets No. 1 to 4 have a large force of recovery to the original even when compressed with a surface pressure of 5 MPa, so that a decrease in heat insulation can be suppressed.

In addition, among the heat insulating sheets of the examples, especially heat insulating sheets No. 1 and No. 2, the maximum value of the differential value dp/dk was confirmed within the range (0 to 10 MPa) in which the surface pressure was measured. In addition, it was confirmed that the first maximum value, which is the first maximum, is in the range of the compression ratio k of 5% to 40%, and the magnitude is in the range of 50 MPa to 100 MPa. Therefore, the recovery rate exceeds 90%, and the effect of suppressing the deterioration of the heat insulating property can be further obtained compared to other examples.
For heat insulating sheets No. 3 to 5, no maximum value of the differential value dp/dk was confirmed, and therefore they are indicated with "-" in Table 2.

On the other hand, insulation sheet No. 5 is a comparative example in which the compression ratio k exceeds 40% when the surface pressure p is 5 MPa. Since it contains a binder, no particle loss was observed, but the recovery rate was extremely low at 61.7%, and it was not possible to suppress the decrease in insulation properties.

Reference Signs List 10, 40: Heat insulating sheets 10a, 10b; Surface 20: Battery cell 21: First particle 22: Second particle 23: Fiber 30: Battery case 100, 110: Assembled battery

Claims (13)

A heat insulating sheet for a battery assembly, which is interposed between a plurality of battery cells in a battery assembly in which the battery cells are connected in series or in parallel,
The composition includes first particles made of silica nanoparticles, second particles made of a metal oxide dispersed between the first particles, and a binder that binds the first particles and the second particles,
A content of the first particles is 30 mass% or more and 80 mass% or less with respect to a total mass of the insulating sheet for a battery assembly,
A heat insulating sheet for a battery pack, having a compression rate k of 5% or more and 40% or less when the surface pressure p is 5 MPa. The insulating sheet for a battery pack according to claim 1, wherein the first particles have an average particle diameter of 1 nm or more and 100 nm or less. The insulating sheet for a battery pack according to claim 1 or 2, wherein the second particles are at least one selected from the group consisting of titania, zirconia, zircon, barium titanate, zinc oxide, and alumina. The insulating sheet for a battery pack according to claim 3, wherein the second particles are titania. The insulating sheet for a battery pack according to any one of claims 1 to 4, wherein the second particles have an average particle diameter of 1 μm or more and 50 μm or less. The insulating sheet for a battery pack according to any one of claims 1 to 5, wherein the content of the binder is 3% by mass or more and 30% by mass or less with respect to the total mass of the insulating sheet for a battery pack. The insulating sheet for a battery pack according to any one of claims 1 to 6, wherein the binder is at least one selected from methyl cellulose, water-soluble cellulose ether, hydroxypropyl methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxyethyl methyl cellulose, and derivatives thereof. The insulating sheet for a battery pack according to any one of claims 1 to 7, wherein when the differential value obtained by differentiating the surface pressure p with respect to the compression rate k is dp/dk, the dp/dk has a maximum value with respect to the compression rate k. The insulating sheet for a battery pack according to claim 8, wherein the first maximum value at which the dp/dk first becomes maximum appears when the compression rate k is in the range of 5% to 40%. The insulating sheet for a battery pack according to claim 9, wherein the first maximum value is 50 MPa or more and 100 MPa or less. The insulating sheet for a battery pack according to any one of claims 1 to 10, further comprising fibers dispersed between the first particles and the second particles. The insulating sheet for a battery pack according to claim 11, wherein the fibers are inorganic fibers or organic fibers. A battery pack in which multiple battery cells are arranged via the insulating sheet for a battery pack described in any one of claims 1 to 12, and the multiple battery cells are connected in series or parallel.

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