Steel sheet for battery pack top cover and manufacturing method thereof

Abstract

A top cover for a battery pack comprising a metal-coated steel plate covered on both sides with an organic coating, the organic coating being thinner on an inner side of the battery pack than on an outer side of the battery pack.

Description

[Technical Field]

[0001] The present invention deals with a housing element for batteries in the automotive industry. More specifically, the present invention relates to a top cover for a battery pack of an electric or hybrid vehicle that has good resistance to fire exposure. [Background technology]

[0002] Electric or hybrid vehicles must incorporate at least one heavy and bulky battery pack, which is made up of multiple battery modules, each containing a battery cell. The battery pack must be highly protected from thermal loads that may occur in the event of an accident, fire, or exposure to high temperatures, whether during assembly or during the vehicle's further life.

[0003] The current trend is to have larger and larger modules and even to house all battery cells in the battery pack housing while leaving intermediate storage in the modules. The internal architecture of the battery pack can consist of cells grouped into modules or can be made of a container that directly contains the battery cells and is closed by a lid. Whatever the internal architecture of the battery pack, its top surface is closed by a top cover.

[0004] As shown in Figure 1, the battery pack is arranged as follows from bottom to top: 1 shield element, an external frame 2 containing the internal architecture of the battery pack including battery cells and reinforcement components, and optionally battery modules; Top cover, also called top cover 3 Equipped with.

[0005] The top cover may be adhesively bonded and / or screwed to other components of the battery pack, and it may also be connected to the internal architecture by any assembly method, such as welding.

[0006] The top cover may be made of aluminum sheet, for example from a 6000 series aluminum alloy, and in some cases from the specific AL 6016 alloy.

[0007] The top cover can also be made of galvanized steel sheet.

[0008] Battery-related fire hazards are a major safety issue in electric or hybrid vehicles. In particular, thermal runaway, initiated in one battery cell, generates enough heat to cause adjacent cells to thermally runaway as well. This can lead to repeated fires as each battery cell heats up, ruptures, and can explode, releasing its contents. Chemicals within the battery heat up, which causes further degradation of any enclosure, whether it be a cell, module, or entire battery pack enclosure. Flammable electrolytes can ignite or even explode when exposed to oxygen in the air.

[0009] The battery pack's top cover, which is the primary separation between the battery cells and the vehicle interior, is crucial to the battery pack's fire resistance. The top cover must ensure safe separation between the battery pack and the vehicle interior, even at high temperatures. Therefore, the top cover must maintain its physical integrity throughout the life of the vehicle. Therefore, the protection of the top cover must also prevent corrosion from the atmosphere outside the battery pack.

[0010] The top cover must also emit little or no gases when subjected to high temperatures. In particular, gases such as CO or CO2 or other vapor combustion products, when released into the pack and heated by a fire, can significantly increase the pressure within the battery pack. This can cause the pack to open, crack through the housing, and explode.

[0011] U.S. Patent Application Publication No. 2019131602 discloses a housing for a battery pack having a top cover plate configured as a sandwich including at least a metal portion and a plastic portion, the metal portion being made from at least one of steel and aluminum. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] US Patent Application Publication No. 2019 / 131602 Summary of the Invention [Problem to be solved by the invention]

[0013] It is an object of the present invention to provide a top cover that has outstanding resistance to fire exposure, including explosion hazards, over the life of the vehicle. [Means for solving the problem]

[0014] This object is achieved by providing a top cover as claimed in claim 1. The top cover may also include any or all of the features of claims 2 to 11. Another object of the present invention is a battery pack including a top cover according to the present invention.

[0015] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0016] To illustrate the invention, various embodiments and implementations will now be described by way of non-limiting examples, with particular reference to the following drawings: [Brief explanation of the drawings]

[0017] [Figure 1] 1 shows a battery pack and its top cover in an electric battery vehicle. [Figure 2]1 shows a top cover according to the invention after fire exposure at a temperature of 1300° C. for 130 seconds. [Figure 3] 1 shows a top cover not according to the invention after fire exposure at 1300° C. for 130 seconds. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention relates to a top cover for a battery pack comprising a metal-coated steel sheet, with an organic coating over the metal coating.

[0019] For this purpose, any steel can be used for the frame of the present invention. Preferably, steel with good formability is well suited. For example, the top cover can be made of a mild steel for deep drawing, such as IF steel, having the following weight composition: C≦0.01%; Si≦0.3%; Mn≦1.0%; P≦0.1%; S≦0.025%; Al≧0.01%; Ti≦0.12%; Nb≦0.08%; Cu≦0.2%.

[0020] For example, the top cover can be made of high-strength low-alloy (HSLA) steel having the following composition by weight: C≦0.1%; Si≦0.5%; Mn≦1.4%; P≦0.04%; S≦0.025%; Al≧0.01%; Ti≦0.15%; Nb≦0.09%; Cu≦0.2%.

[0021] The steel sheet can be obtained by hot rolling of a steel slab and subsequent cold rolling of the resulting steel coil, depending on the desired thickness, which can be, for example, 0.6 to 1.4 mm, preferably 0.7 to 1.2 mm.

[0022] The steel sheet is then coated with a metallic coating by any coating process, for example, by hot-dip coating in a molten bath and then wiped off with an air knife, preferably the molten bath being zinc-based and containing inevitable impurities.

[0023] In a preferred embodiment, the metallic coating contains 4.0-5.0% by weight aluminum, 0.2-0.6% by weight magnesium, with the remainder being zinc and up to 0.2% unavoidable impurities resulting from the manufacturing process. Such coatings containing aluminum and magnesium enhance corrosion resistance.

[0024] In another preferred embodiment, the metal coating comprises 4.4-5.6% by weight of aluminum, 0.3-0.56% by weight of magnesium, optionally up to 0.2% of tin, the remainder of the metal coating being exclusively zinc, unavoidable impurities arising from the process, and optionally one or more further elements selected from the group consisting of Si, Ti, Ca, Mn, La, Ce and Bi, the weight content of each further element in the metal coating being less than 0.3%, excluding the presence of nickel.

[0025] Optionally, the coating comprises at most 0.2% by weight, preferably at most 0.1% by weight, advantageously at most 0.035% by weight of tin.

[0026] Optionally, the coating comprises up to 3.0 wt. % iron if the coating is applied by hot dip coating. The iron comes from dissolution of the steel sheet in the hot dip coating bath and can vary during manufacturing.

[0027] Metal coating weight is 60-120g / m on both sides 2 Preferably, the coating weight is 80 to 100 g / m 2 For example, the metal coating thickness of the top cover is 5 to 15 μm per side.

[0028] After metallization, the steel sheet is covered with an organic coating, for example on an organic coating line. The surface can be prepared by a degreasing step and a subsequent chemical conversion treatment applied by roll coating to ensure adhesion of the organic coating layer.

[0029] The metal-coated steel sheet according to the invention is coated on both sides with one or two layers of an organic coating.

[0030] The organic coating can be applied by roll coating. The organic coating is then baked in an oven. The thickness of the organic layer below refers to the thickness after baking.

[0031] The organic coating covering the top cover according to the present invention is thinner on the inner side of the battery pack than on the outer side of the battery pack. Those skilled in the art can determine the appropriate organic coating thickness depending on the particular architecture of the battery pack and vehicle.

[0032] The internal atmosphere of the battery pack is sealed and non-corrosive. This will remain unchanged for the life of the vehicle. There is no risk of atmospheric corrosion of the top cover on the inner side of the battery pack.

[0033] If the battery pack is made of aluminum, galvanic coupling can occur when the steel top cover comes into contact with the aluminum, causing corrosion of the aluminum, which must be prevented by a layer of organic coating.

[0034] In case of fire or high temperature, this coating must not increase the pressure inside the battery pack. Combustion of the organic coating can release gases depending on its composition and thickness.

[0035] Preferably, the organic coating covering the top cover on the inner side of the battery pack has a thickness of 4 to 20 μm. The inventors have found that if the organic coating is thinner than 4 μm, it may be too thin to prevent contact between the aluminum and the galvanic coupling. If the organic coating is thicker than 20 μm, its combustion in the event of a fire may release fumes and gases, which may increase the pressure inside the battery pack.

[0036] More preferably, the organic coating covering the top cover on the inner side of the battery pack has a thickness of 4 to 15 μm, or even 4 to 10 μm.

[0037] In a preferred embodiment, the organic coating covering the top cover on the inner side of the battery pack has a single layer of 4 to 6 μm in contact with the metal coating.

[0038] In another preferred embodiment, the organic coating covering the top cover on the inner side of the battery pack is deposited in two layers, with a first layer of 3 to 5 μm in contact with the metal coating and a second layer of 6 to 15 μm in contact with the first layer.

[0039] The exterior sides of the battery pack, particularly the exterior side of the top cover, may be exposed to outside air, moisture, dust, and possibly other road-borne elements. The coating according to the present invention also provides corrosion protection of the top cover from the atmosphere outside the battery pack for the life of the vehicle.

[0040] Preferably, the organic coating covering the top cover on the outer side of the battery is deposited in two layers and has a total thickness of 22 to 55 μm.

[0041] In a preferred embodiment, the organic coating covering the top cover on the outer side of the battery pack has a first layer of 4 to 6 μm in contact with the metal coating and a second layer of 18 to 20 μm in contact with the first layer.

[0042] In another embodiment, the organic coating on the exterior side of the battery pack has a first layer of 10 to 25 μm in contact with the metal coating and a second layer of 15 to 30 μm in contact with the first layer.

[0043] The metal and organic coated steel sheets can then be cut into blanks, which can be formed by press stamping into the specific shape of the top cover.

[0044] The present invention will now be illustrated by examples, which are not limiting. [Example]

[0045] To determine the fire resistance of the top cover, several tests were carried out, all performed on the same test equipment.

[0046] The test apparatus was adapted to that described in standard ISO 2685:1998. Both of the following adaptations were made: first, the sample was thermally isolated from the structure of the test apparatus by a 10 mm thick calcium silicate plate, and second, the gas burner generating the flame was calibrated to achieve a target temperature on the face of the sample exposed to the flame.

[0047] For all tests, the samples were 150 x 150 mm 2 Each sample was placed in front of a gas burner so that it was exposed to the flame. The plate between the sample and the burner was 90 x 90 mm. 2 The opening area has a dimension of

[0048] The flame simulates a fire inside the battery pack, and in accordance with the present invention, the side with the thinnest organic coating is exposed to the flame.

[0049] Three materials were tested.

[0050] Material 1 is a steel sheet with a thickness of 0.7 mm. It is coated with a hot-dip metal coating. The metal coating contains 5.0% by weight of aluminum and 0.5% by weight of magnesium, with the remainder being zinc. The metal coating weight is 120 g / m² on both sides. 2 Material 1 is also organically coated. On the side exposed to the flame, it is coated with one 5 μm layer in contact with the metal coating. On the side not exposed to the flame, the organic coating is as follows: a first layer 5 μm thick in contact with the metal coating, and a second layer 20 μm thick in contact with the first layer.

[0051] Material 2 is a 6016 series aluminum plate with a thickness of 1.0 mm.

[0052] Material 3 is a 0.8 mm galvanized steel sheet coated with an e-coat. The hot-dip coating contains 0.2% aluminum by weight, the remainder zinc. The metal coating weight is 140 g / m 2After the phosphating step, the samples were immersed in an e-coat bath. The e-coat tested is Powercron® 6200 HE from the supplier PPG. The dry thickness of the organic coating after firing is 25 μm on each side.

[0053] Material 4 is a 0.7 mm thick steel sheet. It is hot-dip galvanized. The metallic coating contains a maximum of 0.2% aluminum by weight, the remainder being zinc. The metallic coating weight is 275 g / m2 on both sides. On the side exposed to the flame, the organic coating is as follows: a first layer 4 μm thick in contact with the metallic coating, and a second layer 8 μm thick in contact with the first layer. On the side not exposed to the flame, the organic coating is as follows: a first layer 5 μm thick in contact with the metallic coating, and a second layer 20 μm thick in contact with the first layer.

[0054] In the following, Sample 1 is made from Material 1, Sample 2 is made from Material 2, and Sample 3 is made from Material 3.

[0055] All samples were subjected to the same fire test: flame temperature 1300°C, exposure time 130 seconds.

[0056] Several criteria are taken into account in the analysis of the test: the integrity of the plate, i.e. whether the flame penetrated the plate or not, and the temperature of the side not exposed to the flame at the end of the test.

[0057] Presence of bubbles in the coating after testing. The presence of bubbles indicates gas evolution. [Table 1]

[0058] After 130 seconds of exposure at 1300°C, the backsides of steel samples 1 and 3 remain below 750°C and show no signs of melting. In contrast, the flame penetrated the thicker aluminum material 2.

[0059] Samples 1 and 4 according to the invention do not show bubbles as can be seen in FIG. 2, but only cracks resulting from the differential thermal expansion between the steel sheet and the organic coating layer.

[0060] As can be seen in Figure 3, Sample 3, which has the same organic coating thickness on the inner and outer sides, exhibits black dots, which are bubbles that release the combustion products of the organic coating in the form of gas.

Claims

1. A top cover for a battery pack comprising a metal-coated steel plate coated on both sides with an organic coating, the organic coating being thinner on an inner side of the battery pack than on an outer side of the battery pack.

2. 2. The top cover of the battery pack according to claim 1, wherein the metal coating comprises 4.0 to 5.0% by weight of aluminum, the remainder being zinc and a maximum of 0.2% of unavoidable impurities resulting from the manufacturing process.

3. 3. The top cover of a battery pack according to claim 1, wherein the organic coating on the inner side surface of the battery pack has a thickness of 4 to 20 μm.

4. The top cover of a battery pack according to any one of claims 1 to 3, wherein the organic coating on the outer side surface of the battery pack has a thickness of 22 to 55 µm.

5. The top cover of a battery pack according to any one of claims 1 to 4, wherein the organic coating on the outer side of the battery pack has two layers.

6. The top cover of a battery pack according to any one of claims 1 to 4, wherein the organic coating on the inner side surface of the battery pack has a single layer.

7. The top cover of a battery pack according to any one of claims 1 to 4, wherein the organic coating on the inner side surface of the battery pack has two layers.

8. A battery pack comprising the top cover according to any one of claims 1 to 7.

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