Steel plate for top cover of battery pack and method of manufacturing the same

Abstract

The top cover of the battery pack includes a metal-coated steel plate covered with an organic coating on both sides, and the organic coating is thinner on the inner side of the battery pack than on the outer side of the battery pack.

Description

Technology Field

[0001] The present invention relates to housing elements of batteries in the automotive industry. More specifically, the present invention relates to a top cover of a battery pack for an electric or hybrid vehicle having good resistance to fire exposure. Background Technology

[0002] An electric or hybrid vehicle must include at least one heavy and bulky battery pack. Such a battery pack is composed of multiple battery modules, each module containing battery cells. The battery pack must be very well protected against thermal loads that may occur in the event of an accident, fire, or any exposure to high temperatures, and must be so during assembly or throughout the vehicle's extended lifespan.

[0003] The current trend is to have increasingly larger modules, and even to store all battery cells within the battery pack housing while leaving intermediate compartments for the modules. The internal architecture of the battery pack can consist of cells grouped into modules, or it can be manufactured as a container that directly encloses the battery cells and is closed by a lid. Regardless of the internal architecture, the top surface of the battery pack is closed by an upper cover.

[0004] As illustrated in FIG. 1, the battery pack comprises the following from the bottom to the top:

[0005] ·Shielding element (1);

[0006] · An external frame (2) including an internal architecture of a battery pack including battery cells, reinforcing parts, and optionally battery modules;

[0007] · An upper cover named top cover (3).

[0008] The top cover may be bonded adhesively and / or screw-fastened to other components of the battery pack. It may also be connected to the internal architecture by any assembly method, such as welding.

[0009] The top cover can be manufactured from aluminum sheets, for example, from 6000-series aluminum alloy and possibly from a specific AL 6016 alloy.

[0010] The top cover can also be manufactured from galvanized steel.

[0011] Fire risks associated with batteries are a major aspect of safety in electric or hybrid vehicles. In particular, thermal runaway, once initiated in a single battery cell, generates enough heat to cause adjacent cells to also enter the thermal runaway state. This creates a fire that spreads repeatedly as each battery cell heats up, breaks, and can explode, releasing its contents. The chemicals inside the battery heat up, causing further degradation of any enclosure, whether it be the cell enclosure, the module enclosure, or the enclosure of the entire battery pack. Flammable electrolytes can ignite or even explode when exposed to oxygen in the air.

[0012] The battery pack's top cover serves as the first separation point between the battery cells and the passenger compartment, making it critical for the battery pack's fire resistance. The top cover must ensure a safe separation between the battery pack and the passenger compartment even at high temperatures. For this reason, the top cover must maintain its physical integrity throughout the vehicle's lifespan. Consequently, the protection of the top cover must also prevent corrosion of the battery pack from the external atmosphere.

[0013] The top cover must also emit little to no gas when subjected to high temperatures. In particular, gases such as CO, CO2, or other vapor combustion products can be released from within the battery pack and, when heated by fire, can drastically increase the pressure inside the battery pack. This can lead to the opening of the pack, cracking through the housing, and explosion.

[0014] Patent application US 2019131602 discloses a housing for a battery pack having a top cover. The cover plate is constructed as a sandwich comprising at least a metal part and a plastic part, and the metal part is made of at least one of steel and aluminum. The problem to be solved

[0015] The objective of the present invention is to provide a top cover with excellent resistance to fire exposure, including the risk of explosion, throughout the life of the vehicle. means of solving the problem

[0016] This objective is achieved by providing a top cover according to claim 1. The top cover may also include any or all features according to claims 2 through 11. Another objective of the present invention is a battery pack comprising a top cover according to the present invention.

[0017] Other features and advantages of the present invention will become apparent from the detailed description of the present invention that follows. Brief explanation of the drawing

[0018] To explain the present invention, various embodiments and non-limiting examples will be described, in particular with reference to the drawings below. Figure 1 shows a battery pack and its top cover in an electric battery vehicle. FIG. 2 shows a top cover according to the present invention after being exposed to fire for 130 seconds at a temperature of 1300°C. FIG. 3 shows a top cover not according to the present invention after being exposed to fire at 1300°C for 130 seconds. Specific details for implementing the invention

[0019] The present invention relates to a top cover for a battery pack comprising a metal-coated steel plate, wherein the metal coating is covered by an organic coating.

[0020] For this purpose, any steel may be used within the scope of the present invention. Preferably, steels with excellent formability are highly suitable. For example, the top cover may be made of mild steel for deep drawing, such as IF steel (Interstitial Free 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%.

[0021] For example, the top cover may be manufactured from 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%.

[0022] A steel plate can be obtained by hot rolling a steel slab and then cold rolling the obtained steel coil, according to a desired thickness which may be, for example, 0.6 to 1.4 mm, preferably 0.7 to 1.2 mm.

[0023] Subsequently, the steel sheet is coated with a metal coating by any coating process. For example, the steel sheet is coated by hot-dip plating in a molten bath and then wiped by air knives. Advantageously, the molten bath is zinc-based and contains unavoidable impurities.

[0024] In a preferred embodiment, the metal coating comprises 4.0 to 5.0 weight percent aluminum, 0.2 to 0.6 weight percent magnesium, and the remainder being zinc and unavoidable impurities from the manufacturing process, up to 0.2 weight percent. This coating, comprising aluminum and magnesium, increases corrosion resistance.

[0025] In another preferred embodiment, the metal coating comprises, by weight, 4.4 to 5.6% aluminum, 0.3 to 0.56% magnesium, and optionally up to 0.2% tin, and the remainder of the metal coating comprises exclusively zinc, inevitable impurities generated from the process, and optionally one or more additional elements selected from the group consisting of Si, Ti, Ca, Mn, La, Ce, and Bi, wherein the weight content of each additional element in the metal coating is less than 0.3%, and the presence of nickel is excluded.

[0026] Optionally, the coating contains, on a weight basis, up to 0.2% tin, preferably up to 0.1%, and advantageously up to 0.035% tin.

[0027] Optionally, when the coating is applied by hot-dip galvanizing, the coating contains up to 3.0 weight percent iron. The iron comes from the melting of the steel sheet in the hot-dip galvanizing coating bath and may vary during production.

[0028] The weight of the metal coating on both sides may be 60 to 120 g / m² or less. Preferably, the coating weight is 80 to 100 g / m². For example, the metal coating thickness of the top cover is 5 to 15 μm per side.

[0029] After metal coating, the steel sheet is covered with an organic coating, for example, on an organic coating line. The surface can be manufactured by a subsequent conversion treatment applied by a degreasing step and a roll coat to ensure the grip of the organic coating layer.

[0030] The metal-coated steel plate according to the present invention is coated with one or two layers of organic coating on both sides.

[0031] The organic coating can be applied by roll coating. Subsequently, the organic coating is baked in an oven. In the following, the thickness of the organic layer refers to the thickness after baking.

[0032] 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. A person skilled in the art will be able to determine a suitable organic coating thickness according to the specific architecture of the battery pack and the vehicle.

[0033] The internal atmosphere of the battery pack is sealed and non-corrosive. This does not change throughout the vehicle's lifespan. There is no risk of atmospheric corrosion on the top cover of the battery pack's inner surface.

[0034] When a battery pack is manufactured from aluminum, a top cover made of steel may come into contact with the aluminum, causing galvanic coupling. Due to this galvanic coupling, corrosion of the aluminum occurs. Contact must be prevented by a layer of organic coating.

[0035] In the event of fire or high temperature, the coating will not increase the pressure inside the battery pack. The combustion of the organic coating may release gas depending on its composition and thickness.

[0036] 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 found that if the organic coating is thinner than 4 μm, it is too thin to prevent contact with aluminum and galvanic coupling. If the organic coating is thicker than 20 μm, in the event of a fire, it may release fumes and gases due to combustion, thereby increasing the pressure inside the battery pack.

[0037] 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.

[0038] 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.

[0039] In another preferred embodiment, the organic coating covering the top cover on the inner side of the battery pack is deposited in two layers, having 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.

[0040] The outer surface of a battery pack, particularly the outer surface of the top cover, may be exposed to external air, moisture, dust, and possibly other elements coming from the road. The coating according to the present invention also provides corrosion protection of the top cover against the atmosphere on the outer surface of the battery pack throughout the vehicle's life.

[0041] 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.

[0042] In a preferred embodiment, the organic coating covering the top cover on the outer surface 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.

[0043] In another embodiment, the organic coating on the outer surface of the battery pack has a first layer of 10 to 25 μm in contact with a metal coating and a second layer of 15 to 30 μm in contact with the first layer.

[0044] Next, the metal and organic coated steel sheets can be cut into blanks. The blanks can be formed by press stamping into a specific shape of the top cover.

[0045] The present invention will now be described by non-limiting embodiments.

[0046] Examples

[0047] To determine the fire resistance of the top cover, several tests were performed. All tests were conducted on the same test device.

[0048] The test device was modified from the test device described in standard ISO 2685:1998. All of the following modifications were performed: First, the sample was thermally isolated from the structure of the test device by a 10 mm thick plate of calcium silicate. Second, the flame-generating gas burner was calibrated to achieve a target temperature on the surface of the sample exposed to the flame.

[0049] For all tests, the samples have the same dimensions of 150 x 150 mm². Each sample is placed in front of a gas burner to face the flame. The plate between the sample and the burner has an opening area of ​​90 x 90 mm².

[0050] The flame simulated a fire inside the battery pack. According to the present invention, the side having the thinnest organic coating is exposed to the flame.

[0051] I tested three materials:

[0052] - Material 1 is a steel plate with a thickness of 0.7 mm. It is coated with a hot-dip galvanized metal. The metal coating contains 5.0 wt% aluminum and 0.5 wt% magnesium, with the remainder being zinc. The total weight of the metal coating on both sides is 120 g / m². Material 1 is also coated with an organic coating. On the flame-exposed surface, a single 5 µm layer is coated in contact with the metal coating. On the flame-unexposed surface, the organic coating consists of: a first layer with a thickness of 5 µm in contact with the metal coating and a second layer with a thickness of 20 µm in contact with the first layer.

[0053] - Material 2 is a 1.0 mm thick aluminum sheet of the 6016 series.

[0054] - Material 3 is a 0.8 mm galvanized steel sheet coated with an e-coat. The hot-dip galvanizing coating contains 0.2 wt% aluminum, with the remainder being zinc. The metal coating weight is 140 g / m². After the phosphorylation step, the sample was immersed in an e-coat bath. The e-coat tested is Powercron® 6200 HE from supplier PPG. After baking, the dry thickness of the organic coating is 25 µm on each side.

[0055] - Material 4 is a steel plate with a thickness of 0.7 mm. It is hot-dip galvanized. The metal coating contains up to 0.2 wt% aluminum, with the remainder being zinc. The total weight of the metal coating on both sides is 275 g / m². On the flame-exposed surface, the organic coating consists of: a first layer with a thickness of 4 µm in contact with the metal coating and a second layer with a thickness of 8 µm in contact with the first layer. On the flame-exposed surface, the organic coating consists of: a first layer with a thickness of 5 µm in contact with the metal coating and a second layer with a thickness of 20 µm in contact with the first layer.

[0056] In the following, Sample 1 is composed of Material 1, Sample 2 is composed of Material 2, and Sample 3 is composed of Material 3.

[0057] All samples were exposed to the same fire test: the flame temperature was 1300°C and the exposure time was 130 seconds.

[0058] Several criteria are considered for test analysis. The integrity of the sheet, namely whether the flame penetrated the sheet, and the temperature of the side not exposed to the flame at the end of the test.

[0059] There are bubbles in the coating after the test. The presence of bubbles indicates the release of gas.

[0060]

[0061] After exposure to 1300°C for 130 seconds, the backs of samples 1 and 3, made of steel, remained at a temperature below 750°C and showed no signs of melting. Conversely, the flame penetrated material 2, which was made of thicker aluminum.

[0062] Samples 1 and 4 according to the present invention do not show any bubbles as can be seen in FIG. 2, and only cracks caused by different thermal expansion between the steel plate and the organic coating layer are visible.

[0063] Sample 3, having the same organic coating thickness on the inner and outer surfaces, shows black dots as can be seen in Fig. 3. These bubbles released combustion products of the organic coating in gaseous form.

Claims

Claim 1 A top cover for a battery pack comprising a metal-coated steel plate covered by an organic coating on both sides, wherein the organic coating is thinner on the inner side of the battery pack than on the outer side of the battery pack. Claim 2 A top cover of a battery pack according to claim 1, wherein the metal coating comprises 4.0 to 5.0 weight percent aluminum, and the remainder is zinc and up to 0.2 percent unavoidable impurities resulting from the manufacturing process. Claim 3 In claim 1, the top cover of the battery pack, wherein the organic coating on the inner surface of the battery pack has a thickness of 4 to 20 μm. Claim 4 In claim 1, the top cover of the battery pack, wherein the organic coating on the outer surface of the battery pack has a thickness of 22 to 55 μm. Claim 5 A top cover of a battery pack, wherein, in any one of claims 1 to 4, the organic coating on the outer surface of the battery pack has two layers. Claim 6 A top cover of a battery pack according to any one of claims 1 to 4, wherein the organic coating on the inner surface of the battery pack has a single layer. Claim 7 A top cover of a battery pack, wherein, in any one of claims 1 to 4, the organic coating on the inner surface of the battery pack has two layers. Claim 8 A battery pack including a top cover according to any one of paragraphs 1 to 4.

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