Steel sheet for battery pack top cover and manufacturing method thereof

Abstract

The present invention deals with a top cover of a battery pack comprising a metal coated steel sheet, said metal coating being covered with an organic coating, said organic coating having two layers, a first layer of the organic coating in contact with the metal coating having a thickness of 2-25 μm and a second layer of the organic coating being based on polyester or polyurethane.

Description

[Technical Field]

[0001] The present invention deals with housing elements 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 having good fire resistance. [Background technology]

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

[0003] The current trend is to have larger modules and even store all battery cells in the battery pack housing while leaving intermediate containment in the module. The internal architecture of the battery pack can consist of cells grouped into modules, or it 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 an upper cover.

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

[0005] The top cover may be adhesively bonded and / or screwed together with other components of the battery pack. 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] Battery-related fire hazards are a major safety aspect in electric or hybrid vehicles. In particular, thermal runaway, initiated in one battery cell, can generate enough heat to cause adjacent cells to also experience thermal runaway. This can result in recurring fires as each battery cell heats up, ruptures, and may explode, releasing its contents. Chemicals within the battery generate heat, which causes further degradation of any enclosure, whether that be the cell, module, or entire battery pack enclosure. Flammable electrolytes can ignite or even explode when exposed to oxygen in the air.

[0008] The battery pack's top cover, which is the first separation between the battery cells and the vehicle compartment, is critical to the battery pack's fire resistance. The top cover must ensure safe separation between the battery pack and the vehicle compartment, even at high temperatures. The top cover must also emit little or no gases when exposed to high temperatures. In particular, gases such as CO2 or other vapor combustion products, if released inside the pack and heated by a fire, can significantly increase the pressure inside the battery pack. This can lead to the pack opening, cracking through the housing, and explosion.

[0009] 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]

[0010] [Patent Document 1] US Patent Application Publication No. 2019 / 131602 Summary of the Invention [Problem to be solved by the invention]

[0011] It is an object of the present invention to provide a top cover that has outstanding resistance to flame exposure, including the risk of explosion.

[0012] 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 4. Another object of the present invention is a battery pack including a top cover according to the present invention. [Means for solving the problem]

[0013] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0014] To illustrate the invention, various embodiments and implementations will now be described by way of non-limiting examples, with particular reference to the following figures: [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a diagram showing a battery pack and its top cover in an electric battery vehicle. FIG. [Figure 2] FIG. 1 shows a top cover according to the present invention after flame exposure at a temperature of 1300° C. for 130 seconds. [Figure 3] FIG. 1 shows a top cover not according to the invention after flame exposure at 1000° C. for 130 seconds. [Figure 4] FIG. 1 shows a top cover according to the present invention after flame exposure at a temperature of 1000° C. for 130 seconds. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention relates to a top cover for a battery pack comprising a steel plate coated with a metallic coating, the metallic coating being covered with an organic coating.

[0017] 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 deep-drawable mild steel such as IF steel having the following composition by weight: 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%.

[0018] For example, the top cover can be made of high strength low alloy (HSLA) steel having the following weight composition: 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%.

[0019] Steel sheets can be obtained by hot rolling of steel slabs and subsequent cold rolling of the resulting steel coils, depending on the desired thickness, which can be, for example, 0.6-1.0 mm.

[0020] The steel sheet is then coated with a metal coating by any coating process, for example, by hot-dipping the steel sheet in a molten bath followed by wiping with an air knife.

[0021] The molten bath is zinc-based and may contain unavoidable impurities.

[0022] In a preferred embodiment, the bath is zinc-based and optionally contains 2% by weight aluminum.

[0023] Metallic coating weight is 50-200g / m2 total on both sides 2 For example, the coating thickness on the inside of the battery pack is 10 to 40 μm.

[0024] After the hot-dip metal coating, the steel sheet is painted, for example on an organic paint line. The surface can be prepared by a degreasing process and a subsequent chemical conversion treatment, which is applied by roll coating to ensure adhesion of the first paint layer.

[0025] The first layer of paint, also known as the primer, can have a thickness of 2 to 25 μm. Primers can be based on different resins such as polyester, polyurethane or epoxy.

[0026] The second layer of paint is also applied by roll coating and is based on polyester or polyurethane. In a preferred embodiment, its thickness is between 2 and 40 μm, preferably between 5 and 25 μm.

[0027] The metal-coated steel sheet used in this invention is coated with an organic paint. The organic coating used in this invention consists of two layers. The first layer of the organic coating, which contacts the metal coating, has a thickness of 2 to 25 μm, and the second organic layer is based on polyester or polyurethane. The organic coating is then baked in an oven.

[0028] Such a coating releases very little gas when exposed to flame temperatures, and in the event of a fire or high temperature, it does not increase pressure inside the battery pack.

[0029] The metal and organic coated steel sheets can then be cut into blanks. The blanks can be formed by stamping into the specific shape of the top cover. This specific shape is design related. The top cover is a large horizontal section and can be subject to vibrations. To reduce these vibrations and subsequent noise, stiffeners are typically punched into the top cover during the stamping operation. Finally, the top cover is attached to the pack by any removable or non-removable means, such as screwing, welding, or gluing. [Example]

[0030] To determine the fire resistance of the top cover, several tests were performed, all performed on the same test rig.

[0031] The test apparatus was adapted from that described in standard ISO 2685:1998. Both of the following adaptations were made: first, the specimen 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 specimen surface exposed to the flame.

[0032] For all tests, the specimens were 150 x 150 mm 2 Each specimen is placed in front of a gas burner so that it is exposed to the flame. The plate between the specimen and the burner is 90 x 90 mm. 2 The opening area has a dimension of

[0033] Three materials were tested: -Material 1 is a 0.7 mm thick steel plate. It is hot-dip galvanized. The metal coating weight is 275 g / m 2 It is also organically coated on the flame-exposed side with the following layers: a first layer 4 μm thick in contact with the metal coating and a second layer 8 μm thick based on polyester.

[0034] -Material 2 is a 6016 series aluminum sheet, 1.0 mm thick.

[0035] - Material 3 is a 0.8 mm galvanized steel sheet coated with an epoxy-based e-coat. The hot-dip coating contains 0.2% by weight aluminum and the balance zinc. The metal coating weight is 140 g / m 2 After the phosphating step, the samples were immersed in an e-coating bath. The e-coat tested was Powercron® 6200 HE from the supplier PPG. The dry thickness of the paint after baking is 25 μm on each side.

[0036] -Material 4 is a 0.7 mm thick steel plate. It is hot-dip galvanized. The metal coating weight is 275 g / m 2 It is also organically coated on the flame-exposed side with the following layers: a first layer 5 μm thick in contact with the metal coating and a second layer 20 μm thick based on polyester.

[0037] In the following, sample 1 is made of material 1, sample 2 is made of material 2, sample 3 is made of material 3, and sample 4 is made of material 4.

[0038] Two scenarios of flame exposure were tested: in Scenario A, the flame temperature was 1300°C and the exposure time was 130 seconds, and in the less severe Scenario B, the flame temperature was 1000°C and the exposure time was 130 seconds.

[0039] Several criteria are taken into account in the analysis of the test: the integrity of the sheet, i.e. whether the flame penetrated the sheet, the temperature of the side not exposed to the flame (backside) at the end of the test, and the presence of bubbles in the coating after the test, which indicates gas release.

[0040] Table 1 - Fire Exposure Scenarios

[0041] [Table 1]

[0042] Table 2 - Scenario A: 1300°C for 130 seconds

[0043] [Table 2] *According to the present invention

[0044] After 130 seconds of exposure at 1300°C, the backsides of specimens 1 and 4, made of steel, remain at a temperature below 700°C and do not show any signs of melting. In contrast, the flame has penetrated material 2, made of thicker aluminum.

[0045] Furthermore, Sample 1 does not exhibit any bubbles, as can be seen in Figure 2, and only shows cracks resulting from the differential thermal expansion between the steel sheet and the organic coating layer. Sample 4 has a similar appearance to Sample 1, and neither exhibits any bubbles.

[0046] Table 3 - Scenario B: 1000°C for 130 seconds

[0047] [Table 3] *According to the present invention

[0048] After 130 seconds of exposure at 1000°C, the backside of Sample 3 clearly shows bubbles as can be seen in Figure 3. These open bubbles released combustion products of the paint in the form of gases.

[0049] Sample 4 does not show any bubbles as can be seen in Figure 4. Sample 1 has a similar appearance to Sample 4, neither of which show any bubbles.

Claims

1. 1. A top cover for a battery pack for an electric or hybrid vehicle, comprising a press-stamped metal-coated steel sheet, the metal coating being covered with an organic coating and having a thickness of 10-40 μm inside the battery pack, the organic coating having two layers, a first layer of the organic coating in contact with the metal coating having a thickness of 2-25 μm, and the second organic layer being polyester or polyurethane based.

2. 10. The battery pack top cover of claim 1, wherein the metallic coating is zinc-based and optionally contains up to 2% by weight of aluminum and unavoidable impurities.

3. The metal coating has a total thickness of 50 to 200 g / m on both sides. ２ 10. The battery pack top cover of claim 1, having a coating weight of

4. 10. The battery pack top cover of claim 1, wherein the first layer of the organic coating in contact with the metallic coating is polyester-based.

5. 10. The battery pack top cover of claim 1, wherein the first layer of the organic coating in contact with the metallic coating is polyurethane-based.

6. 10. The battery pack top cover of claim 1, wherein the first layer of the organic coating in contact with the metallic coating is epoxy-based.

7. A battery pack comprising the top cover according to any one of claims 1 to 6.

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