JP7680876B2 - Heat diffusion sheet - Google Patents

Description

The present invention relates to a thermal diffusion sheet.

Heat diffusion sheets and heat dissipation sheets for facilitating heat dissipation from heat-generating bodies are used in a variety of applications. For example, in vehicle-mounted or stationary power supply devices that have multiple stacked secondary battery cells to achieve high output and capacity, it is known that the secondary battery cells generate heat when they are charged and discharged. To improve the heat dissipation of such secondary battery cells, a heat diffusion sheet is attached to the exterior can of the battery cell. To improve heat dissipation with a heat diffusion sheet, it is necessary to increase the thermal conductivity.

On the other hand, when using a large number of high-capacity secondary battery cells such as lithium-ion secondary batteries, there is a concern that one secondary battery cell may become too hot for some reason, causing thermal runaway and adversely affecting other adjacent secondary battery cells. For this reason, it is necessary to thermally insulate adjacent secondary battery cells from each other in the event of thermal runaway. However, in order to provide thermal insulation performance, it is necessary to suppress thermal conductivity at high temperatures, which is a characteristic that is contradictory to improving heat dissipation at low temperatures, i.e., improving thermal conductivity.

To meet these conflicting demands, laminates equipped with a thermal diffusion layer and a heat insulating layer have been proposed (Patent Documents 1 to 4). However, these configurations have concerns about their stability in environments where temperatures can reach several hundred degrees due to thermal runaway, and there are doubts about whether they can effectively prevent a chain reaction of thermal runaway between secondary battery cells.

As a means of preventing a chain reaction of thermal runaway, a thermal runaway prevention sheet has been proposed that undergoes at least one of the following structural changes when the temperature rises: phase change, expansion, foaming, and hardening (see, for example, Patent Document 5). According to this, the sheet has a laminated structure with an intermediate layer that undergoes a structural change at a certain temperature, and at high temperatures the intermediate layer expands or foams, inhibiting heat transfer through the air layer and providing thermal insulation.

However, this configuration has the problem that the volume of the intermediate layer increases due to foaming, making the thermal diffusion sheet thicker. Secondary battery cells such as lithium-ion secondary batteries are configured by inserting a current collector into a hard aluminum exterior can, and it is known that the exterior can expands due to rapid charging and discharging. For this reason, the thermal diffusion sheet interposed between the exterior cans that expand during thermal runaway is pressed with high pressure, making it difficult to secure space to allow the expansion. In particular, there is a strong demand for miniaturization of power supply devices that include thermal diffusion sheets in order to secure cabin space in vehicles, and large power supply devices are often not allowed. For these reasons, it is practically difficult to adopt a thermal diffusion sheet that expands in volume at high temperatures, and a thermal diffusion sheet with little volume change is required.

Patent Publication No. 2017-159527 JP 2019-147357 A JP 2015-196332 A JP 2011-108617 A JP 2018-206605 A

The present invention was made in light of this background, and one of its objectives is to provide a thermal diffusion sheet that prevents fire from spreading at high temperatures while suppressing volumetric changes.

Means for solving the problem and effects of the invention

The thermal diffusion sheet according to the first embodiment of the present invention is a sheet-shaped thermal diffusion sheet comprising a first insulating layer and a first heat transfer layer disposed on each side of the first insulating layer, and the thermal conductivity of the first heat transfer layer in the surface direction is at least 10 times the thermal conductivity of the first insulating layer in the surface direction.

The thermal diffusion sheet according to the second embodiment of the present invention is a sheet-shaped thermal diffusion sheet comprising a first insulating layer and a first heat transfer layer disposed on each side of the first insulating layer, the thermal conductivity of the first insulating layer being 1.7 W/m·K or less in the surface direction and 0.14 W/m·K or less in the thickness direction, the thermal conductivity of the first heat transfer layer being 19 W/m·K or more in the surface direction and 1.7 W/m·K or less in the thickness direction, and the sheet is electrically insulating. The above configuration provides the advantage of diffusing heat from a heat source without transmitting it.

Furthermore, according to a third embodiment of the present invention, the thermal diffusion sheet is a sheet-shaped thermal diffusion sheet comprising a first insulating layer and a second heat transfer layer disposed on both sides of the first insulating layer, the thermal conductivity of the first insulating layer is 1.7 W/m·K or less in the surface direction and 0.14 W/m·K or less in the thickness direction, and the thermal conductivity of the second heat transfer layer is 120 W/m·K or more in the surface direction and 1.3 W/m·K or less in the thickness direction. The above configuration provides the advantage of diffusing heat from a heat source without transmitting it.

Furthermore, according to a fourth embodiment of the present invention, there is provided a thermal diffusion sheet in the form of a sheet, comprising a second heat transfer layer and a first heat insulating layer disposed on each side of the second heat transfer layer, wherein the thermal conductivity of the first heat insulating layer is 1.7 W/m·K or less in the surface direction and 0.14 W/m·K or less in the thickness direction, and the thermal conductivity of the second heat transfer layer is 120 W/m·K or more in the surface direction and 3 W/m·K or less in the thickness direction, and the sheet is electrically insulating. With the above configuration, it is possible to obtain the advantage of diffusing heat from a heat source without transmitting it.

Furthermore, according to a fifth embodiment of the present invention, in addition to any one of the above configurations, the thermal diffusion sheet contains at least one of inorganic fibers and inorganic fillers in an amount of 80% or more.

Furthermore, according to the thermal diffusion sheet of the sixth embodiment of the present invention, in addition to any of the above configurations, when heated from one side at 40°C for 15 minutes, the temperature difference per unit thickness from the back side is 3°C/mm or more.

Furthermore, according to the seventh embodiment of the thermal diffusion sheet of the present invention, in addition to any one of the above configurations, when heated for 10 minutes in accordance with the JIS L 1091 A-1 method (1999) test, the temperature difference per unit thickness between the flame contact part and the back surface is 200°C/mm or more.

Furthermore, according to the thermal diffusion sheet of the eighth embodiment of the present invention, in addition to any one of the above configurations, when heated for 10 minutes in accordance with the JIS L 1091 A-1 method (1999) test, the area of the back surface that is ashed is 500 ^mm2 or less.

Furthermore, according to the thermal diffusion sheet of the ninth embodiment of the present invention, in addition to any one of the above configurations, when heated for 10 minutes in accordance with the JIS L 1091 A-1 method (1999) test, the temperature difference between the back side and a distance 50 mm below the back side is 250° C. or less.

Furthermore, according to a thermal diffusion sheet according to a tenth embodiment of the present invention, in addition to any one of the configurations described above, the change in thickness in the thickness direction at 150° C. is 18% or less with respect to 15° C. as the reference temperature.

Furthermore, according to the thermal diffusion sheet of the eleventh embodiment of the present invention, in addition to any one of the above configurations, the volume resistivity of the entire laminate is 1 x ¹⁰¹² Ω·cm or more, the AC breakdown voltage is 2.5 kV or more, and the DC breakdown voltage is 4.5 kV or more.

Furthermore, according to a thermal diffusion sheet of a twelfth embodiment of the present invention, in addition to any one of the configurations described above, it is used by being sandwiched between objects which repeatedly expand and contract.

Furthermore, according to a thirteenth aspect of the present invention, in addition to any of the above configurations, there is provided a thermal diffusion sheet for insulating between a plurality of secondary battery cells that are stacked and connected in series and/or parallel to each other, the thermal diffusion sheet comprising an intermediate layer containing inorganic fibers and surface layers stacked on both sides of the intermediate layer so as to sandwich the intermediate layer, the thermal conductivity of the intermediate layer is 1.7 W/m·K or less in the surface direction and 0.14 W/m·K or less in the thickness direction, and the thermal conductivity of the surface layer is 120 W/m·K or more in the surface direction and 3 W/m·K or less in the thickness direction. The above configuration provides the advantage of diffusing heat from a heat source without transmitting it.

1 is an exploded perspective view showing a power supply device according to a first embodiment of the present invention. 1 is a schematic cross-sectional view of a thermal diffusion sheet according to a first embodiment. FIG. 6 is a schematic cross-sectional view of a thermal diffusion sheet according to a second embodiment. FIG. 11 is a schematic cross-sectional view of a thermal diffusion sheet according to a third embodiment. FIG. 5A is a perspective view showing a power supply device according to a fourth embodiment of the present invention, and FIG. 5B is a perspective view showing the power supply device with secondary battery cells in a horizontally placed position. FIG. 4 is a schematic cross-sectional view of a thermal diffusion sheet according to Comparative Example 1. FIG. 11 is a schematic cross-sectional view of a thermal diffusion sheet according to Comparative Example 2. FIG. 2 is a schematic diagram showing a combustion test of a sample. FIG. 13 is a schematic diagram showing a heating test using a heater. FIG. 10A is a photograph showing the ashed area of the sample after the heating test, and FIG. 10B is a photograph showing the carbonized area. FIG. 11A is a photograph of the burned surface of the sample according to Example 1, and FIG. 11B is a photograph of the back surface. FIG. 12A is a photograph of the burned surface of the sample according to Example 2, and FIG. 12B is a photograph of the back surface. FIG. 13A is a photograph of the burned surface of the sample according to Example 3, and FIG. 13B is a photograph of the back surface. FIG. 14A is a photograph of the burned surface of the sample according to Comparative Example 1, and FIG. 14B is a photograph of the back surface. FIG. 15A is a photograph of the burned surface of the sample according to Comparative Example 2, and FIG. 15B is a photograph of the back surface.

Hereinafter, the embodiments of the present invention will be described with reference to the drawings. However, the embodiments shown below are merely examples for embodying the technical idea of the present invention, and the present invention is not limited to the following. In addition, this specification does not specify the members shown in the claims as the members of the embodiments. In particular, the dimensions, materials, shapes, and relative positions of the components described in the embodiments are merely explanatory examples, and are not intended to limit the scope of the present invention, unless otherwise specified. Note that the size and positional relationship of the members shown in each drawing may be exaggerated to clarify the explanation. Furthermore, in the following explanation, the same name and symbol indicate the same or similar members, and detailed explanation will be omitted as appropriate. Furthermore, each element constituting the present invention may be configured as a form in which multiple elements are composed of the same member, and one member serves multiple elements, or conversely, the function of one member can be shared by multiple members.
[Embodiment 1]

The thermal diffusion sheet according to the embodiment of the present invention can be used appropriately in applications where heat insulation is required and where the risk of fire spreading is avoided. For example, it is suitable for applications such as lithium ion secondary batteries, which require heat dissipation during normal use, but require the prevention of fire spreading from a safety standpoint when high temperatures are reached during thermal runaway. Here, an example is described in which a thermal diffusion sheet is used as a spacer between adjacent secondary battery cells in a power supply device in which a large number of rectangular secondary battery cells are stacked and connected in series or parallel. Such a power supply device is used as a power source for driving electric vehicles such as electric cars, hybrid cars, electric buses, trains, and electric carts, as a backup power source for factories and base stations, and as a storage battery for home use. It can be preferably used as a thermal diffusion sheet that is sandwiched between objects that repeatedly expand and contract.

The power supply device according to the first embodiment is shown in an exploded perspective view in FIG. 1. The power supply device 100 shown in this figure includes a plurality of secondary battery cells 20 and a thermal diffusion sheet 10 interposed between the secondary battery cells 20. The secondary battery cells 20 have an outer can 21 in a rectangular shape with a bottom, and a plurality of cells are stacked with their main surfaces facing each other. For example, the stacking is performed by covering both end faces of a battery stack 25 in which the secondary battery cells 20 are stacked with end plates 30, and fastening the end plates 30 together with fastening members. In addition, the battery stack 25 is fixed onto a base plate 40 as necessary. The base plate 40 can function as a cooling plate, for example, by circulating a refrigerant inside.

Each secondary battery cell 20 has an electrode body housed inside an exterior can 21, and an open end is sealed with a sealing plate 22. A pair of electrodes 23 and an explosion-proof valve 24 are provided on the sealing plate 22 located on the top surface of the exterior can 21 in Fig. 1. The multiple secondary battery cells 20 are electrically connected to each other in series and/or parallel by connecting the electrodes 23 to each other with a bus bar. The explosion-proof valve 24 is a member that opens when it detects an increase in the internal pressure of the exterior can 21, and discharges high-pressure gas from inside the exterior can 21. Each explosion-proof valve 24 is connected to a gas duct for guiding high-pressure gas to the outside as necessary.
(Thermal diffusion sheet 10)

A thermal diffusion sheet 10 is interposed between adjacent secondary battery cells 20. The thermal diffusion sheet 10 is also called a spacer or separator, and insulates the exterior can 21 between adjacent secondary battery cells 20 to prevent short-circuiting.

A cross-sectional view of the thermal diffusion sheet 10 is shown in FIG. 2. The thermal diffusion sheet 10 shown in this figure is composed of an intermediate layer 11 and surface layers 12 laminated on both sides of the intermediate layer 11. Here, the intermediate layer constitutes the first insulating layer, and the surface layer constitutes the first heat transfer layer. The sheet-like thermal diffusion sheet is composed of a three-layer laminate with the surface layer/intermediate layer/surface layer being the first heat transfer layer/first insulating layer/first heat transfer layer.

The thermal conductivity of the first heat transfer layer in the surface direction is preferably 10 times or more the thermal conductivity of the first insulation layer in the surface direction. Specifically, the thermal conductivity of the first insulation layer is preferably 0.14 W/m·K or less in the surface direction and 1.7 W/m·K or less in the thickness direction. The thermal conductivity of the first heat transfer layer is preferably 19 W/m·K or more in the surface direction and 1.7 W/m·K or less in the thickness direction. This configuration provides the advantage that the back surface temperature does not change over time.

Furthermore, the thermal diffusion sheet has insulating properties. Specifically, the volume resistivity of the entire laminate is 1×10 ¹² or more, the dielectric breakdown voltage for AC is 2.5 kV or more, and the dielectric breakdown voltage for DC is 4.5 kV or more.

Each layer contains at least 80% of inorganic fibers and/or inorganic fillers. This suppresses volumetric changes at high temperatures. It also prevents the material from melting at high temperatures and losing its insulating properties. Resin fibers such as PET and aramid are added as needed.

Conventionally, in power supply devices in which multiple lithium-ion secondary battery cells are stacked, separators made of resins such as polyolefin resin, phenolic resin, and epoxy resin have been used to insulate the secondary battery cells from each other. However, such separators do not have sufficient heat resistance, and cannot cope with a situation in which if any secondary battery cell becomes too hot due to some abnormality, the high temperature propagates to the other secondary battery cells, causing them to catch fire.

On the other hand, even under normal use conditions, lithium-ion secondary batteries generate heat due to rapid charging and discharging, so they require heat dissipation properties to release the generated heat to the outside. Thus, separators placed between secondary battery cells are required to have the contradictory properties of being thermally conductive at normal temperatures and insulating at high temperatures, and it has been difficult to achieve these contradictory properties.

The thermal diffusion sheet according to this embodiment has the ability to dissipate heat at normal temperatures, but is also made of a material with excellent flame retardancy, which allows it to suppress the spread of fire at high temperatures. Specifically, the thermal diffusion sheet shown in FIG. 2 has a three-layer structure of a first heat transfer layer, a first insulation layer, and a first heat transfer layer, and is made of materials with excellent flame retardancy and fire resistance. Specifically, it is made of inorganic fillers such as silicate minerals, metal oxides, and graphite, inorganic fibers such as glass fibers, organic fibers with excellent flame retardancy such as aramid, and organic fibers containing flame retardants.

As an index of thermal diffusion of the thermal diffusion sheet, when the thermal diffusion sheet is heated at 40°C for 15 minutes, the temperature difference per unit thickness between the front and back sides is preferably 3°C or less. When the thermal diffusion sheet is heated for 10 minutes according to the JIS L 1091 A-1 method (1999) test, the temperature difference per unit thickness between the flame-contacting part and the back side is preferably 200°C/mm or more. Furthermore, when the thermal diffusion sheet is exposed to a flame from a burner for 10 minutes, the area incinerated on the back side is preferably 500 ^mm2 or less. Furthermore, when the thermal diffusion sheet is heated for 10 minutes according to the JIS L 1091 A-1 method (1999) test, the temperature difference between the back side and a point 50 mm below it is preferably 250°C or less.

On the other hand, the thermal conductivity in the plane direction of the first heat transfer layer is set to be 10 times or more the thermal conductivity in the plane direction of the first insulation layer. In this way, by placing a heat transfer layer that exhibits heat dissipation on the surface and creating a multi-layer structure with an insulation layer in the middle, it is possible to ensure heat dissipation performance from the first heat transfer layer on the surface at room temperature, while at abnormally high temperatures such as during thermal runaway, the first insulation layer provides insulation and increases safety.

In addition, the thermal diffusion sheet according to the embodiment preferably has a thickness change of 18% or less at 150°C, based on 15°C. This allows it to be applied to thermally expanding objects such as secondary battery cells. Among conventional thermal diffusion sheets, a laminated structure has been proposed in which an intermediate layer that undergoes a phase change, expansion, or foaming at high temperatures is interposed to prevent fire spreading, and the intermediate layer undergoes a phase change, expansion, or foaming at high temperatures to provide thermal insulation through an air layer, while acting as a thermal diffusion material at low temperatures. However, there was a problem in that the volume of the intermediate layer increases due to the phase change, expansion, or foaming, resulting in a thick thermal diffusion sheet. For this reason, in order to allow the thermal diffusion sheet interposed between the exterior cans that have expanded during thermal runaway to further expand, it is necessary to ensure a suitable space and flexibility of the bind bar that fastens the secondary battery cells in a stacked state, which has been extremely difficult to achieve. In contrast, the thermal diffusion sheet according to the present embodiment can avoid such problems by suppressing the change in volume even at high temperatures, making it possible to ensure highly practical heat dissipation and insulation performance.
[Embodiment 2]

The above-mentioned heat transfer layer is an example, and other heat transfer layers can be appropriately adopted. As an example, the thermal diffusion sheet according to the second embodiment is shown in the cross-sectional view of FIG. 3. The thermal diffusion sheet 10' shown in this figure has a three-layer structure in which the second heat transfer layer 12B is disposed on both sides of the first heat insulation layer 11B. The first heat insulation layer 11B can adopt the same material and thermal conductivity as the first embodiment. On the other hand, the thermal conductivity of the second heat transfer layer 12B is 120 W/m·K or more in the surface direction and 3 W/m·K or less in the thickness direction. Even with such a configuration, the advantage of diffusing heat from a heat source without transmitting it can be obtained.
[Embodiment 3]

In the above example, the multi-layered thermal diffusion sheet is laminated in the order of thermal conduction layer/insulation layer/thermal conduction layer, but the present invention is not limited to this configuration, and may be laminated in the order of insulation layer/thermal conduction layer/insulation layer. Such an example is shown in the cross-sectional view of FIG. 4 as a thermal diffusion sheet 10C according to embodiment 3. The thermal diffusion sheet 10C shown in this figure has a three-layer structure in which a first insulation layer 12C is arranged on both sides of a second heat transfer layer 11C. The second heat transfer layer 11C and the first insulation layer 12C can be selected from the same materials and thermal conductivities as those in embodiment 2 described above. With this configuration, it becomes easier to make the surface of the thermal diffusion sheet exhibit electrical insulation, and it can be suitably used for applications requiring insulation. For example, when a thermal diffusion sheet is applied to the power supply device shown in FIG. 1, since the surface layer is generally conductive when it is composed of a heat transfer layer, it was necessary to insulate the surface of the secondary battery cell in order to prevent a short circuit between the exterior cans of adjacent secondary battery cells. For example, when the exterior can of the secondary battery cell is made of aluminum, it was necessary to cover it with a shrink tube such as PET resin. In contrast, in the thermal diffusion sheet according to the third embodiment, the surface is made of a first insulating layer, which makes it easier to achieve insulation, and makes it unnecessary to insulate the exterior can of such a secondary battery cell. It goes without saying that if the exterior can of the secondary battery cell is insulating, for example in the case of a pouch battery, insulation is essentially unnecessary.

In the above example, a thermal diffusion sheet having a three-layer structure has been described, but it goes without saying that the present invention does not limit the number of layers of the thermal diffusion sheet constituted by the laminate to three, and it may have four or more layers.
(First insulation layer)

The first insulation layer of the thermal diffusion sheet 10 described above includes a fiber base material, a filler, and a binder. Preferably, natural pulp and inorganic fibers are used as the fiber base material, silicate mineral is used as the filler, and a rubber composition is used as the binder. Specifically, the first insulation layer in the first embodiment includes hemp pulp and microglass as the fiber base material, talc and sepiolite as the filler, and NBR as the binder.

The fiber substrate (also called substrate fiber) can be inorganic fibers such as glass fiber, carbon fiber, or ceramic fiber, or organic fibers such as aromatic polyamide fiber or PET fiber. Here, natural pulp, an organic fiber, is used as the fiber substrate. Hemp pulp is preferably used as the natural pulp. The blending ratio of hemp pulp is, for example, 5% to 20% by weight, and preferably 10% by weight.

The fiber base material may also contain inorganic fibers. The blending ratio of inorganic fibers is 5% to 20% by weight, preferably 8% to 15% by weight. In the first embodiment, 12% by weight of microglass is added as inorganic fibers.

Inorganic fillers can be used as the filler. Examples of inorganic fillers include silicate minerals such as sepiolite, talc, kaolin, mica, sericite, zeolite, and bentonite, magnesium carbonate, calcium carbonate, hard clay, calcined clay, barium sulfate, calcium silicate, wollastonite, sodium bicarbonate, synthetic silica such as white carbon and fused silica, natural silica such as diatomaceous earth, aluminum hydroxide, magnesium hydroxide, and glass beads, which can be used alone or in combination. The addition of these inorganic fillers shows effects such as maintaining the shape in a high-temperature atmosphere and improving heat insulation. In the first embodiment, talc, which has high flexibility, is used. The amount of filler to be mixed in the thermal diffusion sheet is preferably 5% to 65% by weight. In the first embodiment, magnesium silicate is used as the filler, and 58% by weight of talc and 14% by weight of sepiolite are added.

In addition to synthetic resins such as vinyl chloride resin, vinylidene chloride resin, acrylic acid resin, urethane resin, vinyl acetate resin, polyethylene resin, polystyrene resin, acrylobutadiene styrene resin, acrylonitrile styrene resin, fluororesin, silicone resin, epoxy resin, and phenol resin, the binder may also be acrylonitrile butadiene rubber, hydrogenated acrylonitrile butadiene rubber, acrylic rubber, acrylonitrile rubber, ethylene propylene rubber, styrene butadiene rubber, chloroprene rubber, butadiene rubber, butyl rubber, fluororubber, silicone rubber, fluorinated silicone rubber, chlorosulfonated rubber, ethylene vinyl acetate rubber, chlorinated polyethylene, chlorinated butyl rubber, epichlorohydrin rubber, nitrile isoprene rubber, natural rubber, and isoprene rubber. Among these, acrylonitrile butadiene rubber (NBR) is preferred because of its high water resistance and oil resistance. These rubbers may be used alone or in combination of two or more. In addition, for the purpose of higher water resistance and oil resistance, sizing agents such as alkyl ketene dimer and fluorine-based and silicone-based water repellents may also be used in combination. When a rubber composition is used as the binder, the amount of rubber in the thermal diffusion sheet is preferably 0 to 40% by weight, more preferably 3 to 10% by weight. Here, 6.0% by weight of Nipol 1562, which is NBR, is added.

The first heat insulating layer has a thickness of 0.03 mm to 5.5 mm, preferably 0.05 mm to 2 mm, and more preferably 0.07 mm to 1 mm. The first heat transfer layer may be formed of a single layer or may be formed by laminating a plurality of inorganic fiber layers, such as glass fiber layers or ceramic fiber layers, in a layered configuration.
(First heat transfer layer)

The first heat transfer layer has a thermal conductivity in the thickness direction of 1.00 W/m·K or more, preferably 1.30 W/m·K to 20.00 W/m·K, and more preferably 1.70 W/m·K to 15.00 W/m·K. Alternatively, the thermal conductivity in the thickness direction of the first insulating layer may be 3.00 W/m·K or less. The thermal conductivity in the surface direction of the first insulating layer is preferably 1000 W/m·K or less. With this configuration, when the power supply device 100 is provided with a gas exhaust device such as an explosion-proof valve, it is possible to ensure a time lag for the thermal decomposition gas of the electrolyte to be exhausted and the power supply device 100 to be cooled before the high heat is transmitted to another secondary battery cell through the outer can during the process of thermal runaway of any of the secondary battery cells 1.

In order to achieve sufficient thermal conductivity, the first heat transfer layer preferably contains organic fibers and thermally conductive fillers. The organic fibers can be any one or more of para-aramid fibers, para-aramid pulp, meta-aramid pulp, polyphenylene sulfide fibers, PET fibers, flame-retardant PET fibers, and flame-retardant rayon fibers. The thermally conductive filler can be magnesium oxide, aluminum oxide, boron nitride, aluminum nitride, or the like. The first heat transfer layer may also contain inorganic fibers. The inorganic fibers can be glass fibers, ceramic fibers, or the like. The intermediate sheet made of a paper sheet may be subjected to a heating and pressurizing process using a hot calendar roll or the like. This can densify the inside and increase the thermal conductivity.
(Second heat transfer layer)

The second heat transfer layer may contain aluminum, copper, graphite, carbon nanotubes, etc. as heat conductive fillers, and carbon fibers, etc. as inorganic fibers, in addition to the components of the first heat transfer layer described above.

The first heat transfer layer and the second heat transfer layer have a thickness of 0.02 mm to 0.8 mm, preferably 0.03 mm to 0.6 mm, and more preferably 0.03 mm to 0.3 mm.

In addition, the first heat insulating layer, the first heat transfer layer and the second heat transfer layer preferably have an air resistance of 3000 sec/ml or more when measured with a Gurley standard densometer conforming to the JIS P 8117 (2009) test. This provides a layer with little powder falling off, and has the advantage of making it unnecessary to seal the thermal diffusion sheet 10, making production easy and inexpensive.
(Adhesive layer)

The first heat transfer layer and the first heat insulation layer are bonded with an adhesive. An adhesive layer formed by hardening the adhesive is interposed between the first heat transfer layer and the first heat insulation layer. The adhesive is preferably made of a material with excellent heat resistance. Examples of such adhesives that can be used include acrylic adhesives, polyvinyl chloride adhesives, polyvinyl acetate adhesives, and hot melts. The adhesive can be in the form of a liquid or slurry, or a heat-sealed sheet in which the hot melt adhesive is molded into a nonwoven fabric or mesh.

The overall thickness of the thermal diffusion sheet 10 is 0.2 mm to 6.0 mm, preferably 0.2 mm to 4.0 mm, and more preferably 0.3 mm to 2.0 mm.

Furthermore, the thermal diffusion sheet 10 is flexible and pliable. As a result, when the secondary battery cell 1 expands, it follows the deformation of the secondary battery cell 1, maintaining a close contact state, and avoiding a situation in which thermal conductivity is reduced due to the formation of voids. In particular, many conventional thermal diffusion sheets are hard and have low followability to deformation, so that voids are formed on the contact surface and thermal conductivity is reduced due to the insulating effect of the air layer. If the thermal diffusion sheet is intended to exhibit insulating performance to prevent fire from spreading, a hard thermal diffusion sheet is actually preferable because the insulating performance is further improved by the air layer. In contrast, as in the thermal diffusion sheet 10 of this embodiment, in order to exhibit heat dissipation performance, the first insulating layer is made of a thermal diffusion sheet 10 that has flexibility and pliability rather than such a hard material, so that the heat conductivity can be maintained high and heat dissipation performance can be exhibited.

Also, by giving the thermal diffusion sheet 10 flexibility, it can be wound up on a roll of material such as a roll material, and can be stored and transported in roll form, improving handling. In order to achieve flexibility, for example, when a cylinder with an outer diameter of 110 mm is placed on one side of the thermal diffusion sheet 10 and it is bent 90 degrees, no wrinkles or cracks are generated.

Furthermore, it is desirable that the thermal diffusion sheet 10 has heat resistance and flame retardancy. By using a material that is difficult to deform or melt even when the secondary battery cell 1 becomes hot, it is possible to maintain the heat insulating performance. Preferably, the heat resistance temperature of the thermal diffusion sheet 10 is 300 to 600°C. In the thermal diffusion sheet according to the present embodiment, the surface layer is made of fibers, a filler, and a binder, so that it can exhibit a high heat resistance temperature and maintain insulation even in a high temperature environment. Furthermore, it is preferable that the ashing area of the back surface when heated for 10 minutes according to the JIS L 1091 A-1 method (1999) test is suppressed to 500 ^mm2 or less.

In addition, in the thermal diffusion sheet 10 according to this embodiment, it is preferable that the smoothness of the entire thermal diffusion sheet is 15 to 150 sec. This has the advantage that it is not necessary to seal the thermal diffusion sheet, making it easier and cheaper to manufacture.

The thermal diffusion sheet according to the present embodiment can be flexible enough to be deformed. Preferably, the sheet has flexibility enough not to break even when wrapped around a paper tube with a curvature radius of 55 mm. This allows the sheet to follow the deformation and maintain a tight contact state even if the object with which the sheet is in contact expands or deforms, thereby preventing the formation of voids and the resulting decrease in thermal conductivity.
(Method of manufacturing the thermal diffusion sheet 10)

The thermal diffusion sheet 10 can be manufactured in a roll-to-roll manner by sandwiching a heat-sealing sheet between the first heat transfer layer and the first heat insulating layer, which are in the form of rolls, and passing them between two heat-pressing rolls to bond them together. Alternatively, a liquid adhesive may be applied to one or both sides of the first heat transfer layer or the first heat insulating layer, and they may be bonded together. In Examples 1 to 3 and Comparative Examples 1 and 2 described below, a polyethylene heat-sealing sheet is sandwiched between the first heat transfer layer and the first heat insulating layer, and they are bonded together by applying pressure of 50 kPa for 20 seconds in a hot press at 150°C.

In the above example, the thermal diffusion sheet 10 has been described as having a three-layer structure in which both sides of the first heat transfer layer are covered with a single layer of the first heat insulating layer. However, the present invention is not limited to such a three-layer structure, and it is also possible to have a multi-layer structure of four or more layers, for example, by having multiple surface layers or multiple intermediate layers. Alternatively, depending on the application, it may be a two-layer structure in which a surface layer is provided on only one side of the intermediate layer.

In the example shown in Figure 1, the secondary battery cell 1 is placed vertically, but it goes without saying that the thermal diffusion sheet can also be used in a power supply device in which the secondary battery cell is placed horizontally.

Furthermore, the thermal diffusion sheet 10 can be used not only for thermal insulation between secondary battery cells, but also for thermal insulation between battery modules each composed of a plurality of secondary battery cells.
[Embodiment 4]

In the above example, the heat insulating material is applied to a secondary battery cell using a rectangular exterior can as the secondary battery cell. However, the present invention is not limited to a rectangular shape of the secondary battery cell, and can be applied to secondary battery cells of other shapes such as a cylindrical shape or a pouch shape. As an example, FIG. 5A shows an example of application to a cylindrical secondary battery cell as a power supply device according to embodiment 5. In the power supply device 500A shown in this figure, a thermal diffusion sheet 10 is interposed between adjacent secondary battery cells in a state where a plurality of cylindrical secondary battery cells 20B are arranged side by side. As a result, even if any of the secondary battery cells 20B becomes hot, the thermal diffusion sheet 10 can suppress heat propagation. In this example, in order to separate the secondary battery cells 20B, a cut is formed from one end of one thermal diffusion sheet 10A, and a cut is formed from the other end of another thermal diffusion sheet 10B, and these cuts are combined so that the thermal diffusion sheets cross each other. Furthermore, in the example of FIG. 5A, the secondary battery cell 20B is placed vertically, but it goes without saying that it may be placed horizontally as shown in FIG. 5B.
[Examples 1 to 3]

Next, thermal diffusion sheets according to Examples 1 to 3 and Comparative Examples 1 and 2 were produced, and the characteristics of each sample were examined. The thickness and thermal conductivity of the intermediate layer and surface layer used in each sample are shown in Table 1.

In Examples 1 to 3 and Comparative Example 1, the heat insulating layer was made of the same sheet made of natural pulp, micro glass, silicate mineral powder, and rubber-based resin as a binder. To create the heat insulating layer, first, disintegrated natural pulp was prepared, and micro glass and silicate mineral powder were uniformly dispersed. The rubber-based resin was added to the natural pulp to obtain a papermaking slurry, which was then made into a paper by a wet papermaking method to obtain a heat insulating layer base sheet having a thickness of about 0.70 mm. The heat insulating layer base sheet had a thermal conductivity (thickness direction) of 0.14 W/m·K, a thermal conductivity (plane direction) of 1.7 W/m·K, a smoothness of 46.8 sec, and an air permeability resistance of 30 sec/100 ml. Both sides of the heat insulating layer base sheet were covered with different heat transfer layers, respectively, to prepare the samples of Examples 1 and 2.
Example 1

A papermaking sheet containing 80% boron nitride powder was used as the heat transfer layer in Example 1. Specifically, a papermaking slurry was prepared by dispersing a mixture of boron nitride powder with an average particle size of 200 μm, scaly boron nitride with an average particle size of 40 μm, and organic fibers in water in a weight ratio of 80:20. The sheet obtained by the wet papermaking method was subjected to heat and pressure processing to obtain a surface layer base sheet. The thickness was 0.20 mm.

A surface layer base sheet was laminated on both sides of the aforementioned heat insulation layer base sheet, and a polyethylene heat-sealing sheet was sandwiched between the surface layer and the intermediate layer. The sheets were then bonded together using a hot press at 150°C and a pressure of 50 kPa for 20 seconds to obtain a thermal diffusion sheet according to Example 1.
Example 2

A papermaking sheet containing 90% graphite powder was used as the heat transfer layer of Example 2. Specifically, graphite powder and organic fiber were dispersed in water in a weight ratio of 90:10 to prepare a papermaking slurry, and the sheet obtained by the wet papermaking method was subjected to heat and pressure processing to obtain a surface layer substrate sheet. The thickness was 0.23 mm. Surface layer substrate sheets were attached to both sides of the above-mentioned heat insulating layer substrate sheet in the same manner as in Example 1 to obtain a thermal diffusion sheet according to Example 2.
Example 3

The intermediate layer of Example 3 was the same sheet as that used for the heat transfer layer of Example 2. The above-mentioned heat insulating layer base sheet was bonded to each side of this intermediate layer base sheet to obtain a thermal diffusion sheet according to Example 3.
(Comparative Example 1)

As the thermal diffusion sheet 10D according to Comparative Example 1, as shown in the cross-sectional view of FIG. 6, a three-layer structure of a heat insulating layer 12D/heat insulating layer 11D/heat insulating layer 12D was used. As the surface layer, a heat insulating papermaking sheet was used as the intermediate layer. Specifically, a papermaking slurry obtained by mixing the raw materials in the same weight ratio as in Example 1 was made into a paper by a wet papermaking method to obtain a surface layer base sheet with a thickness of about 0.3 mm. The obtained surface layer base sheet was bonded to both sides of the heat insulating layer base sheet in the same manner as in Example 1, etc., to obtain a thermal diffusion sheet according to Comparative Example 1.
(Comparative Example 2)

As shown in the cross-sectional view of FIG. 7, six heat dissipation layers 11E were laminated to form a thermal diffusion sheet 10E according to Comparative Example 2. For each heat dissipation layer, a papermaking slurry was obtained by mixing the raw materials in the same weight ratio as the heat transfer layer of Example 2, and a heat transfer layer base sheet with a thickness of approximately 0.3 mm was obtained by papermaking using a wet papermaking method. Six layers of the obtained heat transfer layer base sheet were laminated, and a heat fusion sheet similar to that of Example 1 was attached between each layer, followed by hot pressing to obtain a thermal diffusion sheet according to Comparative Example 2.

The layer structure, thickness of the surface layer, thickness of the intermediate layer, thickness of the entire thermal diffusion sheet, and thermal conductivity (thickness direction and surface direction) of the surface layer and intermediate layer of the thermal diffusion sheets of Examples 1 to 3 and Comparative Examples 1 and 2 are shown in Table 1.

The insulation properties of the thus obtained Examples 1 to 3 and Comparative Examples 1 to 2 were measured. The insulation volume resistivity was measured in accordance with JIS K6911 "General test method for thermosetting plastics" using a super insulation meter SM-10E manufactured by DKK-TOA under an environment of 23°C. The dielectric strength (AC voltage) was measured in accordance with JIS C2110-1 "Solid electrical insulating materials - Dielectric breakdown strength test method - Part 1: Test by application of commercial frequency AC voltage" using a Tokyo Seiden voltage tester TW-5110ADL under an environment of 23°C. The dielectric strength (DC voltage) was measured in accordance with JIS C2110-2 "Solid electrical insulating materials - Dielectric breakdown strength test method - Part 2: Test by application of DC voltage" using a Tokyo Seiden voltage tester TW-5110ADL under an environment of 23°C. The voltage increase rate was 100V/min for both AC and DC. The results are shown in Table 2.

(40°C heating test)
In order to confirm the performance of the thermal diffusion sheet at normal use temperature, a heating test using a heater was performed. Here, for the samples SM of Comparative Examples 1-2 and Examples 1-3, a 25 mm square micro ceramic heater CH was placed on the sample SM cut to 100 mm x 80 mm as shown in FIG. 9. Heat dissipation grease HG was applied to the contact surface. Thermocouples TC4 and TC5 were placed on the heater CH and on the back surface of the sample SM directly below the heater CH, and electricity was applied to the heater SH, and the output was adjusted so that the thermocouple TC4 was stable at about 40°C. The temperatures of the thermocouples TC4 and TC5 at this time were read, and the temperature difference between the front and back was divided by the sheet thickness to calculate the temperature difference per unit thickness. The above results are shown in Table 3.

All of Examples 1 to 3 show a temperature difference per unit thickness of 3°C/mm or more, suggesting that heat diffusion in the in-plane direction is possible while suppressing heat transmission to the back surface of the sheet. Comparative Example 1 has low thermal conductivity in both the thickness direction and the in-plane direction, and it is believed that the temperature difference is small as a result of heat transmission concentrating at one point. Comparative Example 2 has only a layer with high thermal conductivity, so the amount of heat transmitted in the thickness direction is also large, and it is believed that the temperature difference is small as a result.
(600°C heating test)

To confirm the heat insulating properties of the thermal diffusion sheets according to the examples and comparative examples prepared as described above, a heating test using a gas burner was carried out. Here, samples of comparative examples 1-2 and examples 1-3 were heated with a gas burner flame according to the JIS L 1091 A-1 method (1999) test (45° microburner method), and the temperature change 10 minutes after the start of heating the samples was measured.

As shown in Figure 8, each sample SM was attached to the jig at a 45° angle in the same manner as in the 45° microburner method. Thermocouples TC1 and TC2 were attached to the flame-contacting portion and backside of the gas burner GB, and TC3 was attached 50 mm below TC2. The flame of the gas burner GB was applied to the sample, and the temperatures of thermocouples TC1, TC2, and TC3 were measured when the temperature change stabilized 10 minutes after the start of heating. To evaluate the thermal barrier properties, the temperature difference between the front and back sides was calculated from the difference between TC1 and TC2 sheets, and the temperature difference per unit thickness was calculated by dividing the difference by the sheet thickness. To evaluate the thermal diffusivity within the surface, the temperature difference between TC2 and TC3 was calculated and evaluated according to the following criteria.
⊚: Temperature difference is 200° C. or less; ◯: Temperature difference is 300° C. or less; △: Temperature difference is 400° C. or more. The results are shown in Table 4.

In Comparative Example 1, the temperature difference was smaller than in the Examples, even though the structure was made by laminating only heat insulating sheets. This is presumably because the thermal diffusion in the in-plane direction was low, and heat was transmitted only intensively to the burner flame contact area. In Comparative Example 2, the thermal diffusion was high overall, and the temperature difference was the smallest. On the other hand, all of Examples 1 to 3 maintained a temperature difference of 200°C or more per mm of thickness, and the effect of suppressing heat transmission in the thickness direction while diffusing heat in the in-plane direction was confirmed.

Furthermore, without attaching a thermocouple, the flame of a gas burner GB was applied to the sample to observe whether or not it burned, and the state of both sides of the sample after the test was photographed. This state is shown in Figures 10A to 10B. In these figures, Figure 10A shows the ashing area of the sample, and Figure 14B shows the carbonization area. When the organic components were burned and turned white, the area was measured using the image processing software "leafareacounter_plus3_3". Using this image processing software, the ashing area was measured by measuring the area where the organic components had disappeared due to burning, leaving only the inorganic components, which had turned white, as shown by the dashed line in Figure 10A. Similarly, the carbonization area was measured by measuring the area that was blackened, including the ashing area, as shown by the red line in Figure 10B. In this way, the combustion test results of the samples of Examples 1 to 3 and Comparative Examples 1 and 2 are shown in Figures 11A to 15B. In these figures, Fig. 11A shows a photograph of the burned surface of a sample according to Example 1, Fig. 11B shows a photograph of the back side, Fig. 12A shows a photograph of the burned surface of a sample according to Example 2, Fig. 12B shows a photograph of the back side, Fig. 13A shows a photograph of the burned surface of a sample according to Example 3, Fig. 13B shows a photograph of the back side, Fig. 14A shows a photograph of the burned surface of a sample according to Comparative Example 1, Fig. 14B shows a photograph of the back side, Fig. 15A shows a photograph of the burned surface of a sample according to Comparative Example 2, and Fig. 15B shows a photograph of the back side.

In addition, the dimensional change rate in the thickness direction was measured when Examples 1 to 3 and Comparative Examples 1 and 2 were heated to 150°C. Specifically, each sample was cut to 10 cm x 10 cm and left to stand in a dryer set to 150°C for 1 hour, and the thickness was measured immediately after removal and before heating (15°C) in accordance with JIS P 8118 (2014), and the increase in thickness compared to the thickness before heating was calculated as a percentage. The results are shown in Table 5.

The thermal diffusion sheet of the present invention can be used as a thermal diffusion sheet sandwiched between objects that repeatedly expand and contract. For example, it can be used as an insulating spacer interposed between secondary battery cells or secondary battery cell modules, as a buffer sheet interposed between an explosion-proof valve and a gas duct, or as an insulating material to protect the drive circuitry of an ECU, etc.

100, 500A, 500B... Power supply unit 10, 10', 10A, 10B, 10C, 10D, 10E... Thermal diffusion sheet 11... Intermediate layer 12... Surface layer 11B... First insulating layer 12B... Second heat transfer layer 11C... Second heat transfer layer 12C... First insulating layer 11D... Insulating layer 12D... Insulating layer 11E... Heat dissipation layer 20, 20B... Secondary battery cell 21... Outer can 22... Sealing plate 23... Electrode 24... Explosion-proof valve 25... Battery stack 30... End plate 40... Base plate TC1, TC2, TC3, TC4, TC5... Thermocouple SM... Sample GB... Gas burner CH... Micro ceramic heater HG... Heat dissipation grease

Claims (10)

A second heat transfer layer;
A first heat insulating layer is disposed on each side of the second heat transfer layer;
A sheet-shaped thermal diffusion sheet comprising:
The thermal conductivity of the first insulation layer is 1.7 W/m K or less in the surface direction and 0.14 W/m K or less in the thickness direction;
The thermal conductivity of the second heat transfer layer is 120 W/m K or more in the surface direction and 3 W/m K or less in the thickness direction;
A thermal diffusion sheet with electrical insulation properties. The thermal diffusion sheet according to claim 1 ,
A thermal diffusion sheet containing at least 80% of at least one of inorganic fibers and inorganic fillers. The thermal diffusion sheet according to claim 1 or 2 ,
A thermal diffusion sheet that, when heated from one side at 40°C for 15 minutes, has a temperature difference per unit thickness between the reverse side and one side of the sheet of 3°C/mm or more. The thermal diffusion sheet according to any one of claims 1 to 3 ,
A thermal diffusion sheet in which the temperature difference per unit thickness between the flame-contacting part and the back surface is 200° C./mm or more when heated for 10 minutes in accordance with the JIS L 1091 A-1 (1999) test. The thermal diffusion sheet according to any one of claims 1 to 4 ,
A thermal diffusion sheet in which the area of the backside that is incinerated is 500 ^mm2 or less when heated for 10 minutes in accordance with the JIS L 1091 A-1 (1999) test. The thermal diffusion sheet according to any one of claims 1 to 5 ,
A thermal diffusion sheet in which the temperature difference between the back surface and a distance of 50 mm therebelow is 250° C. or less when heated for 10 minutes in accordance with the JIS L 1091 A-1 (1999) test. The thermal diffusion sheet according to any one of claims 1 to 6 ,
A thermal diffusion sheet in which the change in thickness in the thickness direction at 150°C is 18% or less with respect to 15°C as the standard. The thermal diffusion sheet according to any one of claims 1 to 7 ,
A thermal diffusion sheet having a volume resistivity of the entire laminate of 1×10 ¹² Ω·cm or more, an AC breakdown voltage of 2.5 kV or more, and a DC breakdown voltage of 4.5 kV or more. The thermal diffusion sheet according to any one of claims 1 to 8 ,
A thermal diffusion sheet is used by being sandwiched between objects that repeatedly expand and contract. A thermal diffusion sheet for insulating between a plurality of secondary battery cells that are stacked and connected in series and/or parallel to each other,
An intermediate layer including inorganic fibers;
surface layers laminated on both sides of the intermediate layer so as to sandwich the intermediate layer;
Equipped with
The thermal conductivity of the intermediate layer is 1.7 W/m K or less in the surface direction and 0.14 W/m K or less in the thickness direction,
A thermal diffusion sheet, wherein the thermal conductivity of the surface layer is 120 W/m·K or more in a plane direction and 3 W/m·K or less in a thickness direction.

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