Flame shield for a battery of an electric vehicle and battery housing comprising it

Abstract

A flame shield for in particular a battery electric vehicle, comprising at least one flame-resistant layer comprising reinforcing fibers with a melting temperature above 500°C fully embedded in a thermoset matrix material, characterized in that the matrix material comprises at least one of a polyisocyanurate, or a polyisocyanurate-urethane.

Description

DescriptionFlame shield for a battery of an electric vehicle and battery housing comprising itTechnical Field

[0001] The invention relates to a flame shield for an electric vehicle battery and to a method for manufacturing it. Further, the invention relates to a battery housing comprising such a flame shield.Background Art

[0002] An electric vehicle is generally powered by a rechargeable battery, which provides the electric power needed to drive the electric motor or the electric motors used to propel the vehicle itself. An electric battery generally consists of a number of electrically connected battery cells, possibly organized in battery modules.

[0003] Among the different types of rechargeable batteries, lithium-ion batteries, in particular, are widely used in electric vehicles because of their high energy density which, for a given battery weight, may guarantee a longer driving range.

[0004] In an electric vehicle, the battery is normally installed in the region underneath the vehicle body. Typically, it is enclosed within a battery case consisting of a lower shell, hereafter referred to as battery tray, on which the battery sits and an upper shell, hereafter referred to as battery cover or battery lid, which encases the battery from above. Typically, the battery case, in particular the battery cover, is made with a metallic material. Aluminium is very commonly used for this purpose because of its advantageous stiffness / weight ratio.

[0005] In some cases, the battery may be enclosed between the vehicle body floor and a battery tray that is directly fixed to the vehicle body and on which the battery sits. In these cases, it is the vehicle body floor, typically made with aluminium or steel, which acts as battery cover.

[0006] Rechargeable batteries for traction of electric vehicles, in particular lithium-ion batteries, may raise important safety concerns. In case of mechanical abuse (e.g. impact, piercing) and / or thermal abuse (e.g.over-heating) and / or electrical abuse (e.g. over-charging), short- circuits can occur inside one or more battery cells, which may lead to exothermic decomposition of their chemical constituents. This leads to a strong increase in their temperature, which may propagate to nearby battery cells, in turn over-heating them and generating a cascade effect known as "thermal runaway”.

[0007] During thermal runaway, very high temperatures may be generated in a very short time inside the battery and the chemical constituents of the battery cells may catch fire, ultimately leading, in extreme cases, also to explosions. In particular, thermal runaway in lithium-ion batteries can generate flames with temperatures up to about 900°C. These temperatures are well-above the melting temperature of typical battery covers and / or vehicle body floor panels made, e.g., with aluminium.

[0008] Furthermore, as the battery ages, its sensitivity to thermal and electrical abuse increases and, with that, the risk of thermal runaway.

[0009] In the event of a thermal runaway, in order not to endanger the safety of the occupants of the passenger compartment, it is first of all of paramount importance that they are warned early enough to leave the passenger compartment before unbearable temperatures and / or flames reach it. For this purpose, Chinese standard GB 38031-2020 requires that the battery management system provides an alarm signal at least 5 minutes (the so called “time-to-escape”) before the thermal propagation caused by a thermal runaway event (even of a single battery cell) occurs. In addition to this, in order to further protect the passenger compartment from high temperatures and / or fire during the above-mentioned “time-to-escape”, a protective flame shield is typically applied on the battery cover and / or on the vehicle floor body panel, generally on the side facing the battery.

[0010] State of the art flame shields typically consist of a thin mica layer, which is adhered to the battery cover and / or to the vehicle body floor panel, on the side facing the battery. In fact, mica is a very effective flame shielding material, being its melting temperature around 1300°C. However, flame shields based on mica layer pose a number of problems.

[0011] Mica is a rather heavy material, having a density of between 2700kg / m3and 3000kg / m3. Thus, flame shields comprising a layer of mica may be very heavy, with area weights ranging between 2.5kg / m2and 6kg / m2, depending on the thickness of the mica layer. Such an area weight may be even higher than the area weight of the battery cover the flame shield is applied to. This is very detrimental to the driving range of the electric car, a feature that is very relevant for end-consumers interested in this kind of vehicles.

[0012] Furthermore, the thin mica shields are very brittle and the risk of breakage increases during aging of the part.

[0013] Finally mica is a natural sourced material, however the sourcing or mining of the material is not without discussion, and therefore carmakers are asking for alternative solutions free from mica with the aim of banning the material in the future.

[0014] It is the object of the present invention to provide a flame shield for in particular a battery electric vehicles that represents an alternative solution to mica based flame shields, in particularly that are lighter and less prone to breakage.Summary of invention

[0015] The object of the invention is achieved by a flame shield for in particularly a battery electric vehicle according to claim 1, a method for producing such a flame shield according to claim 14, and a battery housing comprising such a flame shield according to claim 16.

[0016] In its main aspect, the invention concerns a flame shield for a battery electric vehicle, comprising at least one flame-resistant layer comprising reinforcing fibers with a melting temperature above 500° C fully embedded in a thermoset matrix material, whereby the matrix material comprises at least one of a polyisocyanurate, or a polyisocyan urate- urethane.

[0017] Preferably the polyisocyanurate (PI R) based matrix material embeds and surrounds the fibers in such a way that the flame-resistant layer formed is impervious to air-flow. Reducing or eliminating voids in the layer prevents leakage of fumes during a thermal runaway event, while also the flame may find an easier path through the flame resistantlayer. Hence a non foamed PIR is preferred for similar reasons. Voids for instance formed by foam bubbles would not only create weak spots in the flame resistant layer it would also provide trapped air pockets, further fuelling the fire. Higher density material means less trapped air within the material used.

[0018] The flame resistant layer according to the invention is able to withstand high temperatures generated with an open flame, as well as the negative effects of the open flame itself. It was found that with a flame-resistant layer with PIR according to the invention, it is possible to prolong the “time to escape” as well as to increase the delta temperature between the surface subjected to an open flame and the opposite surface.

[0019] The polyisocyanurate according to the invention is typically obtained from the reaction of a polyurethane-polyisocyanurate forming mixture comprising polyol and isocyanate, which may be premixed just before application. The polyisocyanurate forming mixture generally used in the process of the present invention will include: at least one polyol component with an average OH number of 300 to 900, the individual polyols having a functionality of 2 to 6; at least one isocyanate, preferably at least one of an isomer blend of methylene diphenyl diisocyanate (MDI), polymeric MDI, or prepolymers thereof; one or more catalysts including those known in the state of the art to promote polyisocyanurate groups such as potassium octoate or potassium acetate; surfactants or stabilizers; mould release agent and additives such as fillers, colorants, or rheology modifiers. Suitable polyols can be polyester or polyether based.

[0020] Standard fillers that do not impact the flame resistance of the flame shield negatively might be used in the PIR based matrix material, for instance inorganic materials like barium sulphate or calcium carbonate, chalk, talc, or other nonreactive fillers.

[0021] Polyisocyanurate, as is known in the state of the art, comprises a reaction product of a polyol and an isocyanate. Polyisocyanurate is characterized and differentiated from a polyurethane by the formation of high heat-stability isocyanurate groups during the reaction.Preferably the formation of isocyanurate groups is increased, for instance by adapting the ratio of the amount of reactive isocyanate (NCO) groups to the amount of reactive hydroxyl (OH) groups.Preferably the amount of reactive isocyanate (NCO) group exceeds the amount of reactive hydroxyl (OH) group preferably in a ratio of at least 1.8 to 1. Alternatively, the Isocyanate / polyol ratio, also called its index (defined as the ratio of NCO groups to OH groups multiplied by 100), higher than 180.

[0022] Surprisingly, by increasing the isocyanurate groups in the PIR based matrix material, it is possible to create a more heat stable layer, able to withstand the direct effect of the flame over a prolonged time period before a hole is burned through the material.

[0023]

[0024] Preferably the flame-resistant layer is able to withstand the open flame test without burn-through for at least 10 minutes, preferably at least 15 minutes, even more preferred at least 20 minutes. Surprisingly, the burn through is not dependent on the thickness of the layer within the range claimed.

[0025] Furthermore the final temperature reached after a 10 minute open flame test is a third of the flame temperature. Lowering the temperature reached on the opposite side of the flame shield has the advantage that when used between cells adjacent cells are less heated and will start degassing later, reducing the speed of the overall thermal runaway event.

[0026]

[0027] Preferably the weight of the reinforcing fibers is between 20% and 95% of the total weight of the flame-resistant layer, preferably between 40% and 85% of the total weight of the flame resistant layer, even more preferable between 50 and 70% reinforcing fibers based on the total weight of the flame resistant layer.

[0028] Thanks to the use of a high volume of fibers vs PIR based matrix material the overall levels of PIR based matrix material in the product can be low and the maximal amount of fumes during flame tests minimised.

[0029]

[0030] The flame resistant layer comprises reinforcing fibers fully embedded in a PIR based material according to the invention. To maintain stability of the flame shield in case of a runaway event the fibers need to have melting temperature of at least 500° C. Preferably the reinforcing fibers comprises at least one of ceramic fibers, glass fibers, carbon fibers, mineral-based fibers, oxidized polyacrylonitrile fibers, quartz fibers or a mixture of such fibers.

[0031] The reinforcing fibers can be short or long staple fibers or endless filaments or in any combination of staple fibers and filaments. The fibers may be provided in any known structure, like for instance a textile or nonwoven type structure. The fibres may be lightly consolidated using mechanical or chemical means like for instance needling, stitching, air laying or carding structures. Or they may be lightly bound using a polymer, or resin binder, or sol-gel binders as are known in the art. Binders may include polymers, such as polyvinyl alcohol, urea-formaldehyde resins, phenol-formaldehyde resins, epoxy resins, silica or ceramic sols, or the like. A thermoset binder is preferred.

[0032] Various sizing agents may also be included on the high temperature fibers, as are known in the state of the art. These sizing agents provide a thin surface binding layer on for instance a glass or mineral fiber for adhesion and flowability. The sizing agent may comprise flame retardants.

[0033] Providing a fibres layer that may be consolidated lightly has the advantage that the material can be stored and handled without losing its structure. Material mats or unidirectional tapes, preferably lightly bound, may be used. Preferably the fiber material is already subjected to a pre-shaping of the material into or directed to the final shape of the part, before the impregnation with the polyisocyanurate-forming mixture. Combinations of sandwich material with a staple fiber core ad unidirectional tapes may be used, whereby at least one or all of the layers may be embedded in the PIR based matrix material. Optionally layers facing away from the open flame source may be embedded in adifferent material, like a thermoset material or a polyurethane material, or may be used without matrix material but with just a consolidation of the layer.

[0034]

[0035] Flame shields may comprise one or multiple flame resistant layers according to the invention, whereby the layers may comprise the same material or differ in fiber mixture and or amount. They may also vary over the surface in density and or thickness. Flame shields may be used partially or fully overlapping. Overlapping areas may be designed, with staggered joints, ship lapped or tongue & groove joints to enable a more seamless cover for instance on larger areas. Areas overlapping may be sealed or bound using adhesive tape to secure the flame shields.

[0036] Other appliances like for instance mounting means may be integrated into the flame shield.

[0037]

[0038] The thickness of the flame-resistant layer is at least 0.25mm.Depending on the shape and design requirements as well as on the temperature difference between the side of the flame resistant layer required the thickness can be a high a 10mm. Preferably to reduce overall weight and to minimise the space necessary in most cases the preferred thickness may be between 0.5 and 4mm, more preferred between 1 and 3 mm.

[0039] The flame resistant layer is already effective at a very low thickness to prevent flame-burn-through. This has the advantage that the flame shield comprising the flame resistant layer can be used already in areas with not much space, for instance between the battery cells and the surface of the battery casing. Thicker shield might be more beneficial for instance in situation with a high bending stiffness need.

[0040] A flame shield according to any of the preceding claims, wherein the density of the flame-resistant layer is between 1000kg / m3and 2400kg / m3, preferably between 1200kg / m3and 1800kg / m3.

[0041]

[0042] The flame resistant layer according to the invention may further comprise flame retarding additives as known in the art.

[0043] Flame retardant additives that may be used in the flame resistant layer of the invention include, but are not limited to, chlorinated organic compounds and polymers, brominated organic compounds and polymers, phosphate-functionalized organic compounds and polymers such as phosphate esters, chloroalkyl phosphate esters, phosphonate esters, and ammonium salts of phosphoric acid, polyphosphoric acid, sulfuric acid, and hydrochloric acid or mixtures thereof.

[0044]

[0045] The PIR matrix material may further comprise flame retardant synergists such as antimony trioxide and other inorganic compounds such as calcium borate, zinc borate, zinc stannate, and zinc hydroxy stannate.

[0046]

[0047] The flame retardant additives and / or the flame retardant synergist may be added in the PIR based matrix material as a solid or liquid additive.

[0048] Preferably the flame retardant additives and or synergists or the amount added does not cause an expansion of the layer more than 10% in thickness. The amount of flame retardant and or synergist depending on the type and requirements, may comprise up to 25% of the polyisocyanurate material.

[0049]

[0050] Alternative flame retardant and or flame retardant synergist, preferably as mentioned above, may be applied as a coating on the flame shield, as a coating on the fibers.

[0051] The PIR based matrix material may further comprise non fibrous noncombustible fillers to enhance the flame-resistant properties and or to reduce smoke generation further, preferably one of mica, aluminium or magnesium hydroxides.

[0052]

[0053] In case the flame shield is comprising of more than one flame resistant layer of the invention, the layers may vary in the availability and or typeand or amount of flame retardant additives and or flame retardant synergists.

[0054]

[0055] In its simplest embodiment, the flame shield according to the invention may consist of one single flame-resistant layer according to the invention.

[0056] Depending on the design of the area in need of a flame shield, the flame shield may consist of two or more flame-resistant layers according to the invention, partially or totally overlapping each other.

[0057] In case of multiple flame resistant layers partially of totally overlapping each other or at least touching each other might be bound to each other by a simple adhesive layer, or by mechanical means, like clipping for instance. The overlap may be in the form of a recess, to create a closed seal when installed in the car.

[0058] The flame shield can be moulded in any form or design depending on the need, for instance it can be formed as a simple substantially plain shield to put on top or underneath the battery cells, or it may be formed in a box or cylinder type of shape to enclose a cell or a group of cells.

[0059] Flame shield may be used in all areas that might be prone directly or indirectly to shield against a potential fire risk. This might be related to the risk of the main traction battery but might also be used for other potential fire hazards. The flame shield might also be used in other vehicles, or areas in need for a similar flame resistance.

[0060] This might be within the battery casing and or outside of the battery casing for instance between the battery casing and the main floor panel of the car or within the dash panel or flooring part inside the car.

[0061] In more complex embodiments, the flame shield according to the invention may comprise also complementary layers, i.e. layers different from a flame-resistant layer according to the invention. Such layers may be comprised in the flame shield according to the invention to further enhance its flame shielding properties and / or for other purposes e.g. to enhance mechanical properties and / or improve workability during production or installation and / or improve appearance.

[0062]

[0063] Surprisingly, sufficient flame shielding properties as required by the car makers are obtained with the flame resistant layer according to the invention thanks to the synergy between the reinforcing fibers having a melting temperature of above 500° C and the matrix material formed by the PI R. On one side the fibers, besides giving dimensional stability to the flame-resistant layer according to the invention, may conduct the heat distributing it throughout the surface of the layer and act as an effective heat storage medium. On the other side, the polyisocyanurate, by fully embedding the fiber material in such a way to make the flameresistant layer impervious to air-flow, avoids the propagation of a flame through the layer. Furthermore, by sticking to reinforcing fibers, the polyisocyanurate does not drip or crumble, when subjected to an open flame, and quickly carbonizes, creating a char deposit around the fibers and on the surface of the layer that acts as a further flame barrier.

[0064] In order to obtain this effect, it is preferably that the polyisocyanurate fully embeds the fibers, in such a way to make it impervious to air-flow, i.e. that the fiber layer is soaked in the polyisocyanurate, in such a way that the polyisocyanurate blocks all the pores between the fibers and tightly surrounds all the fibers comprised in it.

[0065]

[0066] Flame resistance test

[0067] To establish the flame resistant properties of the flame shield according to the invention comprising such a layer or the flame resistant layer comprised in such a flame shield, a flame resistance test was used, whereby one surface of the material is subjected to an open flame in a controlled experiment. Flat square specimen of the flameresistant layer are held into an open flame such that the surface is exposed to the direct flame at 1100° ± 100° C.

[0068] The specimen is exposed to the flame for a pre-defined time from one side (normally referred to as the “flame side”). During the test the temperature of the specimen at the centre of the side opposite to the one exposed to the flame (normally referred to as the “cover side”) is measured and recorded over time by means of any of the above-mentioned measurement devices (e.g. a digital thermometer or a thermocouple).

[0069] The difference in temperature between the temperature in the middle of the flame on the flame side and the temperature at the centre of the specimen on the cover side was calculated. This temperature difference indicates the insulation properties of the flame resistant layer upon exposure to an open flame. (cover side)

[0070] Figure shows a graph with such the temperature on the cover side over time when exposed to a flame of 1100° C ± 100° .

[0071] The part fails when the flame burns through the material before the time required, for instance at least not before 10 minutes, preferably at least not before 15 minutes, preferably not before 20 minutes. A sample would also fail when the material sags, drips or deforms excessively during this test.

[0072] The test can also be considered failed if the gases released on the cover side during the test, self-ignite. Self-ignition on the cover side would be evident visually but would also show a temperature spike on the cover side temperature probe.

[0073] The samples shown in figure 5 did not burn through, but the experiment was stopped after 10 minutes.

[0074]

[0075] The hot particle impact test

[0076] In a second test to measure durability and flame resistance, the flame shield mechanical resistance during burning is measured using hot particle impact testing.

[0077] During this test, a flame is directed at a vertically mounted sample until the temperature of the flame reaches 1200° C. Once 1200° C is reached, silicon carbide particles of size 200-500 microns are fired onto the sample at a pressure of 2 bar in 8 bursts of 5 s each adding up to a total of 300 mL of particles per cycle. Material is then held at 1200° C for 2 minutes before a further round of particles is fired. This is repeated until failure. The test is stopped once there is a clear sign of failure. Temperatures at the front and back side of the material are measured throughout the duration.

[0078] Surprisingly, already a flame resistant layer comprising short cut fibers fully embedded in PIR according to the invention was able to pass this test, showing low particle wear, and only showed a failure after 5 minutes at 1200° C with particles, which is comparable with the state of the art mica plates currently used. By using longer fibers or endless filaments the overall time to failure may be increased even further.

[0079]

[0080] In order to enhance the protection guaranteed by the flame shield according to the invention during the time-to-escape, the temperature drop across a flame-resistant layer according to the invention measured in the open flame test without particles is preferably at least 500° C, more preferably at least 600° C and even more preferably at least 700° C. Preferably the temperature at the cover side is not reaching more than the melting temperature of metallic materials like aluminium used in the battery casing and or car body.

[0081]

[0082] Preferably, the mechanical properties exceed a flexural modulus of 5000 MPa in a bending test ISO 178 with a test speed of lOmm / min and an e-modulus evaluation range of 0.05-0.25%.

[0083] Also preferably, the flexural strain at flexural strength in the same test exceeds 2%. This percentage is small compared to polymer layers, but for thermoset composites it represents some ability to be deformed before it shatters, which would be advantageous compared to more brittle flame shields such as mica.

[0084] The tensile strength of the flame shield preferably exceeds 100 MPa at break with a tensile strain preferably exceeding 1.5% at break.

[0085]

[0086] In a preferred embodiment, the reinforcing fibers are in the form of a non-woven comprising one or more layers comprising E-glass fibers, which may partially or totally overlap each other. The E-glass fibers may resist thermal expansion, which may provide dimensional stability to the layer in presence of the temperature fluctuations that may take place over the life-time of the battery. The E-glass fibers may also provide high mechanical strength and stiffness at low weight and theymay exhibit at the same time low values for dielectric constant, dielectric loss, or both. The E-glass fibers may include any of silica, alumina, calcium oxide and boron oxide.

[0087] In a further preferred embodiment, the E-glass fibers may be free of boron oxide. Such boron-free material is referred to as E-CR glass. The E-CR glass may provide acid and / or chemical resistance. The E-CR glass may also provide enhanced temperature resistance.

[0088] Layers comprising E-glass and / or E-CR glass fibers may be formed by any suitable process known in the art, including -but not limited to- needle punching, air-laying, wet-laying.

[0089] In a particularly preferred embodiment, the fibrous non-woven comprises one or more layers of E-glass fibers, wherein each layer is in the form of a chopped-strand-mat consisting of E-glass staple fibers bound together by minor amount of a thermoset binder, preferably less than 10% by weight, more preferably less than 5% by weight of thermoset binder. The thermoset binder is preferably a phenolic-based or an epoxy-based polyester resin. In this embodiment, the E-glass staple fibers have preferably a length between 20mm and 80mm, more preferably between 30mm and 70mm, even more preferably between 40mm and 60mm.

[0090] In another preferred embodiment, the fibrous non-woven according to the invention may include one or more layers essentially consisting of a blanket of ceramic fibers (hereafter referred to as “ceramic blanket”). The ceramic blanket layer may provide improved handling strength, enhanced thermal properties, or both. The ceramic blanket layer may have excellent thermal stability, good flexibility, easiness to cut and shape. The ceramic blanket may have also good resistance to tearing. The ceramic blanket layer may be formed by any suitable process, including but not limited to needle punching.

[0091] The ceramic blanket may be substantially free of binder (e.g., about 1% by weight or less of the layer) or even entirely free of binder. The ceramic blanket may be formed of inorganic materials in an amount up to and including about 100% by weight of the layer. For example, theceramic blanket may comprise silicon dioxide, calcium oxide, and magnesium oxide.

[0092] The fibrous non-woven according to the invention may comprise one or more layers of fiberglass, which may impart to the flame-resistant layer according to the invention high temperature resistance and thermal stability, resistance to abrasion, tearing and chemical solvents.

[0093] The fibrous non-woven may also comprise one or more layers comprising organic fibers. A layer comprising organic fibers may comprise preferably at least 50% by weight of organic fibers, more preferably at least 75% by weight of organic fibers. This may be combined with glass fibers. In a preferred embodiment, the fibers may be formed of or include an organic synthetic thermoplastic polymer resin. For example, the fibers may be polyacrylonitrile fibers. The polyacrylonitrile fibers may be oxidized polyacrylonitrile fibers, such as Ox-PAN, OPAN, or PANOX.

[0094]

[0095] The fibers comprised in the fibrous non-woven according to the invention may be virgin and / or they may originate from recycling processes, which may be post-industrial or post-consumer recycling processes.

[0096] For example, glass fibers comprised in the fibrous non-woven according to the invention may originate from the end-of-life treatment of GFRP (Glass Fiber Reinforced Polymer) products such as aircraft and boats body panels, thanks to thermal (e.g. fluidised bed processing) and / or chemical processes that allow the glass-fibers to be separated from the polymeric matrix and to be recycled. In another example, glass fibers comprised in the fibrous non-woven according to the invention may be produced by recycling post-industrial glass waste.

[0097] The flame shield according to the invention may be produced with methods known in the art. A preferred production process may comprise at least following steps1. Provide a reinforcement fibers with a melting point of at least 500° C preferably as a mat or felt layer; The fibers may optionallybe consolidated by mechanical or chemical binding. Preferably the fibers are already formed in a pre-shape to aid the forming of the product.2. Apply a polyisocyanurate-forming mixture on the reinforcing fibers. The polyisocyanurate-forming mixture, following the mixing of the polyol and the isocyanate components, is typically in the form of a viscous fluid, that may be easily poured or sprayed onto the reinforcing fibers , e.g. by means of a mixing head. The polyisocyanurate-forming mixture may be mixed and dosed immediately before spreading it on the fibers. Additives might be added to the polyol or isocyanate components before mixing or during the mixing phase;3. Thermally moulding the thus prepared reinforcing fibers and or layers in the shape required, preferably by compression moulding. The heat of the mould will (further) activate the polyisocyanurate mixture creating a pressure that will make the polyisocyanurate impregnate the fiber material, by spreading around the fibers and through the pores of the fibrous non-woven and embedding the fibers within the matrix material;4. De-mould and optionally trim the edges.

[0098] In a variant of the above-described process, the spreading of the polyisocyanurate-forming mixture onto the fibrous non-woven may take place directly inside the heated mould. This may be advantageous, since it may reduce the space and the time needed to produce the flame-shield according to the invention.

[0099] As already mentioned, this process is remarkably simple, when compared to the process needed to produce prior art flame shields based on mica layers. Thanks to the above-described process, a flame shield according to the invention and consisting of only one flameresistant layer may be obtained. Quite obviously, this process may be extended to the case in which the flame shield according to the invention comprises more than one flame-resistant layer.

[0100] Remarkably, by means of the above-described process it is possible to obtain flame-resistant layers having a three-dimensionalshape, i.e. a shape that substantially deviates from a planar one. This design flexibility is a consequence of the flame-resistant layer constituents, i.e. the fibrous non-woven and the polyisocyanurate, both having high formability properties. This is advantageous compared to prior art flame shields based on mica layers, with which it may be not possible to follow the shape of battery covers having strong curvatures, due to the brittleness of mica.

[0101]

[0102] A further aspect of the invention concerns a battery housing or battery case for a battery of an electric vehicle, in particular a lithium- ion battery, which comprises a battery cover and a battery tray, wherein a flame shield according to the invention is applied on the battery cover and / or on the battery tray on the side facing the battery.

[0103] The design flexibility of the flame shield according to the invention allows following the shape of intricated 3 Dimensional shapes, like for instance, the battery cover and / or of the battery tray, even in areas where this shape is far from being planar, e.g. it presents strong curvatures and / or features such as beads, embossments, etc. This may be advantageous in order to maximize the degree of coverage of the flame shield according to the invention and thus its effectiveness.

[0104] The degree of coverage of the flame shield is the ratio between the area of the battery cover / battery tray side facing the battery that is covered by the flame shield and the total area of this same side. This ratio is hereafter expressed in percentage.

[0105] Furthermore, in the battery housing according to the invention, the flame shield according to the invention may be applied on the battery cover and / or on the battery tray in one single piece or in patches, depending on design needs.

[0106] Preferably, in order to enhance the flame-shielding properties of the flame shield according to the invention, the degree of coverage of the flame shield according to the invention is at least 50%, more preferably at least 60%, even more preferably at least 70%.

[0107]

[0108] Any range given throughout this description should include the starting and end points as well as normal expected deviations in the measurements. Start and end-point values of different ranges may be combined.

[0109] Further embodiments of the invention may be derived from the description also by combining the different embodiments and examples of the invention and may be also derived from the description of the embodiments shown in the figures. The figures are schematic and not necessarily in scale.

[0110] In this disclosure, an “battery electric vehicle” or “electric vehicle” is any vehicle powered, even only partially, by an electric battery including, but not limited to, fully electric vehicles, hybrid vehicles, plug-in hybrid vehicles and vehicles equipped with a range extender.

[0111] A “layer” is a body consisting of one or more materials and filling the space between two closely spaced surfaces, wherein the distance between the surfaces is substantially smaller than their size. The two surfaces are indicated as the "sides" of the layer and they are opposite to each other. The distance between the two surfaces is referred to as the thickness of the layer, which may be variable. A layer may comprise other layers. A layer impermeable to air-flow is meant to be a layer having an air permeance equal to or less than 0.02 l / sm2at 75 Pa pressure differential when tested according to ASTM E 2178 or E 283.

[0112] A “fibrous layer” is an ordered or randomly oriented network of fibers bonded together by chemical, mechanical, thermal or solvent treatment.Brief description of drawings

[0113] Figure 1 show a flame resistant layer according to the invention.

[0114] Figure 2 displays a battery case according to the invention.

[0115] Figure 3 shows a comparison of 2 samples according to the invention and a reference in a flame test

[0116] Figures 1 show a flame shield (1) formed by one flame resistant layer according to the invention in its simplest embodiment, which consists in a substantially flat flame-resistant layer consisting of a reinforcing fibers (3) fully embedded in the PIR based matrix material(2). As it is visible in the drawing, the polyisocyanurate fully surrounds the reinforcing fibers and blocks its pores in such a way to make the flame-resistant layer impervious to air-flow. Preferably no fiber ends are sticking out of at least the surface facing the potential fire source.

[0117] Figure 2 shows a housing for a battery for an electric vehicle (30) according to the invention, together with the battery comprised in it. The battery housing comprises a battery cover (or battery lid) (31) and battery tray (32). The battery consists of a number of battery cells (33). A flame shield according to the invention (1) is applied on the battery cover on the side facing the battery and it covers it over almost all its entire extent, even in areas having strong curvature and in areas with beads. The possibility of such a good and safe coverage is a consequence of the fact that both basic constituents of the flame shield according to the invention have an excellent design flexibility. Such a good and safe coverage would be difficult, if not impossible, with prior art flame shields based on mica plates, due to their tendency to cleave and / or break.

[0118] In the embodiment shown in Figure 2, the flame shield according to the invention (1) covers only the battery cover (31). In other embodiments not shown here the flame shield according to the invention may be applied only on the battery tray (32) or both on the battery cover (31) and on the battery tray (32).

[0119] In an comparison 2 flame resistant layers according to the invention and one reference sample were subjected to the flame resistance test as described before.

[0120] Sample R is the reference sample comprising a mica layer consisting of 90% mica and 10%silicon binder. Having a thickness of 0.8mm.

[0121] Sample A is a first sample according to the invention comprising 60% of glass fibers embedded in PIR matrix material and a thickness of around 1mm.

[0122] Sample B is a second sample according to the invention comprising 60% of glass fibers embedded in PIR matrix material and a thickness of around 2mm.

[0123] Surprisingly, the samples according to the invention perform as good as the mica sample, showing no hole after 10 minutes of exposure to an open flame. And even more surprising, the temperature measured at the surface facing away from the flame was the highest for the mica with 499° C, while the temperature for the samples according to the invention was lower for both (493° C for sample A and 395° C for the 2 mm sample)

Claims

ClaimsClaim 1. A flame shield for a battery electric vehicle, comprising at least one flame-resistant layer comprising reinforcing fibers with a melting temperature above 500° C fully embedded in a thermoset matrix material, characterized in that the matrix material comprises at least one of a polyisocyanurate, or a polyisocyanurate-urethane.Claim 2. A flame shield according claim 1, wherein the density of the flame-resistant layer is between 1000kg / m3and 2400kg / m3, preferably between 1200kg / m3and 1800kg / m3.Claim 3. A flame shield according to any of the preceding claims, wherein the weight of the reinforcing fibers is between 20% and 95% of the total weight of the flame-resistant layer, preferably between 40% and 85% of the total weight of the flame resistant layer, more preferably between 50 and 70% of the total weight of the flame resistant layer.Claim 4. A flame shield according to any of the preceding claims, wherein the thickness of the at least one flame-resistant layer is at least 0.25 mm, preferably at least between 0.25 and 10 mm, more preferably between 0.25 and 4 mm, more preferably between 1 and 3mm.Claim 5. A flame shield according to any of the preceding claims, wherein the reinforcing fibers comprise at least one of ceramic fibers, glass fibers, carbon fibers, mineral-based fibers, oxidized polyacrylonitrile fibers.Claim 6. A flame shield according to one of the preceding claims, whereby the flame shield has a flexural strain at flexural strength according to ISO 178 of above 1%, preferably above 2%.Claim 7. A flame shield according to one of the preceding claims, whereby the flame shield has a bending modulus according to ISO 178 above 5000 MPa, preferably above 7000 MPa.Claim 8. A flame shield according to any of the preceding claims, whereby the flame-resistant layer further comprises a flame retardant, preferably at least one of chlorinated organic compounds and polymers, brominated organic compounds and polymers, phosphate-functionalized organic compounds and polymers such as phosphate esters, chloroalkyl phosphate esters, phosphonate esters, and ammonium salts of phosphoric acid, polyphosphoric acid, sulfuric acid, and hydrochloric acid and or flameretardant synergists such as antimony trioxide and or inorganic compounds such as calcium borate, zinc borate, zinc stannate, and zinc hydroxy stannateClaim 9. A flame shield according to one of the preceding claims, whereby the flame shield is able to withstand an flame resistance test with a temperature of 1100° C for at least 10 minutes without the formation of a hole or burn-through hole in the flame shield.Claim 10. A flame shield according to any of the preceding claims, wherein the reinforcing fibers comprises endless filaments, or short staple fibers, or long staple fibers, or any combination of such fibers.Claim 11. A flame shield according to one of the preceding claims, whereby the reinforcing fibers are provided in a nonwoven or textile structure, preferably a knit, a needled felt, or a weave, a tape, or a consolidated nonwoven structure maintaining the structure in the final flame resistant layer.Claim 12. A flame shield according to one of the preceding claims, wherein the flame resistant layer further comprises a binder to bind the reinforcing fibers before embedding with the polyurethane matrix material, preferably a thermoset binder, like epoxy or urethane, or a ceramic thermoset binder such as a silica or ceramic sol-gel.Claim 13. A flame shield according to one of the preceding claims comprising 2 or more flame resistant layers according to one of the preceding claims.Claim 14. A flame shield according to one of the preceding claims comprising at least one other layer preferably at least one of a metal layer, aluminium layer, intumescent layer, fibrous layer comprising fibers with a melting temperature above 500° C, melamine foam, preferably on the surface facing away from the possible fire hazard.Claim 15. Housing for a battery electric vehicle comprising a battery cover and a battery tray wherein the battery cover and / or the battery tray comprises at least one flame shield according to any of claims 1 to 12.

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