Hybrid compression pad for a battery cell stack, manufacturing process for it, and a battery cell module built with it

Abstract

Compression pad (3) for a battery cell stack (1), comprising: a first material with an initial compressive strength; a second material (32) with a second compressive strength which is less than the first compressive strength; wherein the compression pad (3) has at least one volume (31) of the first material which is at least partially surrounded by the second material (32) with or without direct contact therewith such that, viewed in lateral cross-section, the second material (32) is arranged axially around the at least one volume (31) of the first material and wherein the at least one volume (31) of the first material extends laterally over the entire dimension of the compression pad (3); wherein the axial dimension of the compression pad (3) is larger than the lateral dimension of the compression pad (3).

Description

[0001] The present invention relates to hybrid compression pads for a battery cell pack and a manufacturing method for them. The invention further relates to a battery cell pack built on the basis of the hybrid compression pad.

[0002] The range of electric vehicles is largely determined by the traction battery installed within them. Modern electric vehicles are powered by appropriately sized high-voltage batteries, which are composed of battery cell modules (also called battery modules), each of which in turn contains a number of battery cells, each of which represents the smallest self-contained energy storage cell.

[0003] Battery modules are generally constructed using modules in which several parallel cells are arranged, with a compression pad (also called a compression insert or cell spacer) positioned between each pair of cells. These compression pads are used to clamp the cells in place, increasing cycle stability and longevity. This preload is achieved by pre-compressing the compression pads within the battery module. Furthermore, the compressible compression pads compensate for volume changes caused by swelling. The compression behavior of compression pads can generally be divided into three areas: the preload travel, which is necessary to apply a certain preload force to the cell; the working travel, which accommodates the cell's volume change; and the working travel, which accommodates the cell's volume change.As well as the residual block, which exhibits almost incompressible behavior after maximum compression.

[0004] Swelling refers to a change in the volume of a cell, particularly a lithium-ion cell, which can be observed during charging and discharging and also occurs on a slower timescale due to the aging of the battery cell. Swelling is caused by a structural change in the active layers within the battery cell, resulting from the rearrangement of lithium ions. Its extent is primarily determined by the cell chemistry. By placing compression pads between the battery cells in their stacking direction, as mentioned above, these pads can compensate for the volume changes within a battery module through compression.

[0005] Foamed elastomers (foam elastomers) are currently used to manufacture compression pads, and these can be further subdivided into open-cell and closed-cell types. Compared to solid elastomers, foam elastomers are characterized by high compressibility, which, however, means that they cannot withstand high preload forces. Solid elastomers offer an alternative, as they can withstand higher forces or stresses at the same compression rate. However, a disadvantage is that they lack pores and therefore cannot be compacted, meaning their compressibility is limited by their lateral contraction.

[0006] Publication EP3733511A1 discloses compression devices for removable batteries, and in particular those that can be installed in aircraft, such as aircraft wings, during transport and charging of the battery and removed before flight. The battery cells arranged in the battery housing are fixed therein by means of pre-tensioned spacers made of a foam or plastic material.

[0007] Document US2021257690A1 discloses a thermal barrier element which is designed in a sandwich-like manner and is arranged between two adjacent battery cells, wherein the two outer layers have high thermal conductivity and an intermediate porous layer arranged between them has low thermal conductivity.

[0008] Starting from the compression pads known from the prior art, the object of the present invention can be seen as providing compression pads for a battery cell module which eliminate or at least reduce the aforementioned problems regarding the conflict of objectives between high preload force with low compression at the beginning of the vehicle's life and high compressibility over the vehicle's life.

[0009] This problem is solved by means of the subject matter of the independent claims. Further preferred embodiments are found in the dependent claims.

[0010] The present invention solves this problem by means of a compression pad in which two materials with different mechanical properties are combined. The mechanical property can, in particular, be compressive strength, i.e., the different degree of deformation of the material under the influence of a compressive force. By selectively mixing (at least) two materials with different compressive strengths, compression pads can be provided which exhibit properties optimized for their application. Thus, at least a volume of a first material, e.g., an elastomer, is used with a specific surface area to set a preload of the compression pad within the desired range. A second material, e.g.,A foam, such as elastomeric foam, represents a material that at least partially surrounds the at least one volume of the first material, thus enabling transverse expansion of the elastomer. The second material can fill a cavity around the at least one volume of the first material and thus determine the resistance of the surrounding medium to the transverse expansion of the first material within this space filled with the second material. The overall stiffness of the compression pad according to the invention can be adjusted by a targeted selection of the material properties of the first and second materials.

[0011] The combination of the two materials is not a mixture of the materials at the molecular level, like an alloy, but rather a mixture of differently sized volumes or domains of a first material and a second material, with the volumes ranging in size from a few to several tens of centimeters. Each of these volumes represents a contiguous region of either the first or the second material.

[0012] According to the invention, a compression pad for a battery cell stack is provided, comprising a first material with a first compressive strength and a second material with a second compressive strength that differs from the first. For example, the second compressive strength can be lower than the first. The compression pad has at least one volume of the first material, which is at least partially surrounded by the second material, with or without direct contact. In other words, the second material can be directly adjacent to the at least one volume of the first material, or there can be a space between the at least one volume of the first material and the second material, which can, for example, be filled with air.Partial surrounding of the at least one volume of the first material by the second material can mean, among other things, that in the compression pad according to the invention, viewed in lateral cross-section, the second material is arranged axially around (at a distance from) or adjacent to (in direct contact with) the at least one volume of the first material. In such a configuration of the compression pad according to the invention, the at least one volume of the first material can absorb the force resulting from lateral pressure application, since the axially surrounding second material allows its lateral expansion. With regard to the intended use of the compression pad according to the invention in a battery cell stack, the lateral direction would correspond to the stacking direction of the battery cells. If required, the second material can also surround the at least one volume of the first material laterally, with or without direct contact with it.

[0013] According to further embodiments of the compression pad according to the invention, the second material can comprise a foamed elastomer. In principle, both soft-elastic (soft foams) and hard-tough (rigid foams) foamed elastomers can be used. A thermoplastic, a thermoset, or an elastomer can be used as the starting material, such as polystyrene (PS), polypropylene (PP), polyvinyl chloride (PVC), and polyurethane.

[0014] According to further embodiments of the compression pad according to the invention, the first material can comprise an elastomer, in particular a solid elastomer.

[0015] The first material, unlike the second material, is a non-foamed plastic, which, however, can be selected from the same group of raw materials as the second material.

[0016] According to further embodiments of the compression pad according to the invention, the compression pad can have several volumes of the first material distributed within the second material. The second material can surround the volumes of the first material with or without direct contact. This includes cases where the second material is in direct axial contact with the volumes of the first material and a space exists between the two materials in the lateral direction, or vice versa. The second material can serve as a support matrix in which bodies (volumes) of the first material are distributed.

[0017] According to further embodiments of the compression pad according to the invention, the distribution density of the volumes of the first material in the second material can be inhomogeneous. By adjusting the distribution density, which can decrease, for example, from the center of the compression pad towards its axial ends, the stiffness profile of the compression pad can be adjusted to a desired requirement profile. The distribution density can be varied by increasing or decreasing the number of constant volumes of the first material and / or by increasing or decreasing the volumes of the first material.

[0018] According to further embodiments of the compression pad according to the invention, the compression module can have a central region and adjacent side regions, wherein the distribution density of the volumes of the first material in the second material is greater in the central region than in the side regions. The side regions can correspond to axial regions of the compression pad.

[0019] According to further embodiments of the compression pad according to the invention, the volumes of the first material can have a straight shape. The volumes can, for example, be in the form of rods or cylinders.

[0020] According to further embodiments of the compression pad according to the invention, the volumes of the first material can have an arc shape. The volumes can, for example, be semicircular or have a half-elliptical shape.

[0021] According to further embodiments of the compression pad according to the invention, the first material can be arranged in at least one free space within the compression pad. The at least one free space can extend perpendicular to the axial direction of the compression pad and provide a space for the expansion of the first material when it expands due to compression by expanding adjacent battery cells.

[0022] According to the invention, a method for producing a compression pad for a battery cell stack is further provided. The method comprises providing at least one volume of the first material, which has a first compressive strength, and providing a second material, which has a second compressive strength that is lower than the first compressive strength. The method further comprises forming the compression pad by introducing the at least one volume of the first material into the second material such that the volume of the first material is at least partially surrounded by the second material, with or without direct contact therewith.

[0023] According to further embodiments, the manufacturing process can also include adjusting the stiffness of the compression pad by modifying the shape and / or the number and / or the distribution of the volumes of the first material within the second material. This step can be preceded by a planning phase in which the deformation behavior is calculated using a model of the compression pad based on known material properties. For this purpose, a finite element method (FEM) can be used, for example.

[0024] According to the invention, a battery cell stack is further provided, comprising an arrangement of individual battery cells, wherein a compression pad according to one of the preceding embodiments is arranged between each pair of battery cells.

[0025] In principle, the compression pad according to the invention can be used to construct a battery cell stack based on any type of battery cell, for example, pouch or prismatic battery cells. Pouch battery cells have a soft outer shell, while prismatic battery cells have a relatively rigid casing. Advantageously, the stiffness or compressive strength of the compression pad according to the invention as a whole can be adapted to the respective mechanical properties of the different battery cells by selecting the first and second materials and / or by the geometric arrangement of the materials within the compression pad.

[0026] It is understood that the features mentioned above and those to be explained below can be used not only in the combinations specified, but also in other combinations or on their own, without leaving the scope of the present invention.

[0027] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings. Fig. Figure 1 illustrates the swelling behavior of battery cells. Fig. 2A and Fig. Figure 2B shows diagrams illustrating the qualitative behavior of two materials with different compressive strengths. In Fig. 3A and Fig. Figure 3B shows the dynamic behavior of an embodiment of the compression pad according to the invention. In Fig. 4A and Fig. Figure 4B shows different basic structures of an exemplary compression pad according to the invention. Fig. Figure 5 illustrates another exemplary basic structure of the compression pad according to the invention. In Fig. 6A and Fig. Figure 6B shows further different basic structures of an exemplary compression pad according to the invention. Fig. Figure 7 shows further basic structures of an exemplary compression pad according to the invention.

[0028] In Fig. Figure 1 illustrates the swelling behavior using a highly simplified representation of a battery cell stack 1. The battery cell stack is shown in a side cross-sectional view and contains two battery cells 2. A compression pad 3 is attached to the side of each battery cell 2. On the left side, the battery cell stack 1 is not affected by swelling. Charging and aging of the battery cells 2 lead to swelling, which is illustrated on the right side of the battery cell stack 1. This causes the compression pads 3 located between the battery cells 2 to be compressed. The coordinate system in Fig. Figure 1, and in all subsequent figures where applicable, designates specific spatial directions. The z-axis denotes the direction subsequently designated as the axial direction. The stacking direction of the battery cells 2 in a battery cell stack 1 is in the x-direction.

[0029] In the Fig. 2A and Fig. Figure 2B shows compressive stress-strain diagrams illustrating the qualitative behavior of two materials with different compressive strengths. In both cases, the compression ε is plotted on the x-axis (22) and the compressive stress σ on the y-axis. The curves (24, 25) in both diagrams have a fundamentally similar shape, but differ in quantitative aspects.

[0030] Curve 24 in Fig. Figure 2A represents the stress distribution during compression deformation of a foam. A foam is easily compressible and can be compressed to up to 90% of its original dimensions, a value corresponding to the asymptotic limit 26 of curve 24. The pronounced plateau region of curve 24 typically begins for foams at values ​​27 in the range of 0.02–0.10 MPa. This value 27 can be interpreted as the prestress σ. vor This means that a foam is relatively soft and therefore, when shaped into a compression pad, cannot exert a large preload σ. vor to record.

[0031] Curve 25 in Fig. Figure 2B depicts the stress distribution during compression deformation of an elastomer. Compared to a foam, an elastomer exhibits significantly lower compressibility and can only be compressed to approximately 50% of its original dimensions, a value corresponding to the asymptotic limit 26 of curve 25. However, the less pronounced plateau region of curve 24 typically begins for elastomers at values ​​27 in the range of 1–2 MPa, which is an order of magnitude higher than the corresponding typical values ​​27 for foams. Therefore, an elastomer is relatively rigid and, when formed into a compression pad, can withstand higher forces or stresses. As mentioned earlier, however, the compressibility of elastomers is limited by their transverse contraction, as they lack pores and thus cannot be compacted.

[0032] In Fig. 3A and Fig. Figure 3B shows an embodiment of the compression pad 3 according to the invention in a cross-sectional view. This pad has an inner volume 31 made of the first material, an elastomer, and adjacent end regions 32 which comprise the second material, a foam. The second material is distributed axially around the first material or arranged axially adjacent to it. In its initial state, in which no forces act on the compression pad 3, it has a lateral dimension 33.

[0033] Since solid elastomers are not porous, their compressibility is limited by transverse contraction. The approach according to the invention consists of creating areas into which the elastomer can expand in order to achieve the desired behavior of the compression pad 3. For this purpose, the material properties of the two materials, such as the degree of filling or porosity of the foam elastomer and the Shore hardness of the solid elastomer, can be appropriately matched. The material properties also influence the overall stiffness of the compression pad 3.

[0034] As in Fig. As illustrated in Figure 3B, a lateral compression force K causes the compression pad 3 to compress overall, with the volume 31 of the first material expanding into the areas of the second material 32 due to its greater compressive strength. Simultaneously, the lateral dimension 33 of the compression pad 3 decreases. The compression force K is exerted by battery cells located on both sides of the compression pad 3. The stacking direction, i.e., the alternating arrangement of battery cell and compression pad, runs in the plane of the sheet, i.e., in the x-direction according to the coordinate system shown.

[0035] In Fig. 4A and Fig. Figure 4B shows different basic structures of an exemplary compression pad 3 according to the invention. A selection of conceivable basic structures (without claiming to completeness) is shown in a series and separated from each other by vertical dividing lines T. When using corresponding compression pads in a battery cell stack, the stacking direction would be in the x-direction, i.e., perpendicular to the yz-plane according to the coordinate system shown.

[0036] In Fig. 4A is the first material provided in the form of cylindrical or rod-shaped volumes 31 and embedded in the second material 32. The distribution density of the volumes 32 in the second material 32 increases from left to right. This can be adjusted as needed to achieve a desired overall stiffness of the compression pad 32. By adjusting the surface area of ​​the elastomer, a desired preload range of the compression pad 3 can be set. When the battery cells expand, pressure is exerted on the volumes 31 of the first material, causing them to be compressed along their longitudinal extension (into the plane of the sheet).

[0037] As in Fig. As shown in Figure 4B, the shape of the volumes 31 of the first material within the second material 32 can also be changed to adapt the dynamic behavior of the compression pad 3 according to the invention. In the basic structure shown on the left, the volumes 31 are circular or cylindrical (as in Figure 4B). Fig. 4A). In the basic structure shown in the center, the volumes 31 are plate-shaped (rod-shaped in the view shown). In the basic structure shown on the right, the volumes 31 of the first material are semi-tubular (semi-ring-shaped in the view shown). The shapes shown are only examples, and numerous other shapes are possible.

[0038] In principle, a wide variety of different arrangements of the second material within the first material are conceivable. In the Fig. 4A and Fig. Figure 4B shows only simple basic structures, although more complex geometries can of course also be used. When designing a compression pad according to the invention, the following five factors can be considered for its final structure: cell type (e.g., pouch, prismatic), swelling characteristic (degree of bulging of the cells used), cell or intercellular material dimensions, desired behavior under compression (e.g., compressive strength profile at the contact surface with the cell), and compression paths to be accommodated as a function of the force acting on the compression pad.

[0039] In Fig. Figure 5 shows another exemplary basic structure of the compression pad 3 according to the invention, which has an axially changing distribution density of the volumes 31 of the first material within the second material 32. As shown on the left side of Figure 5, the volume distribution density of the volumes 31 of the first material within the second material 32 varies. Fig. As illustrated in Figure 5, the swelling behavior of a battery cell 2 is not uniform, but rather more pronounced in the center of the battery cell 2 than at the edges. The compression pad 3 according to the invention, which is Fig. Figure 5 on the right, shown in a view rotated 90° about the z-axis, can be adapted to this behavior such that a first region 51 is provided centrally, in which the density of the first volumes 31 of the first material (i.e., the number of volumes 31, not the density of the first material itself) is greater than in the axially adjacent second regions 52. In further embodiments, the density can also decrease continuously towards the axial ends of the compression pad 3. Furthermore, the proportion of the first material can also be varied by changing the shape of the volumes 31 instead of, or in addition to, the number of volumes 31 (see Figure 5). Fig. 4B).

[0040] At the in Fig. In the embodiment shown in Figure 5, the first region 51, which corresponds to an inner region of the compression pad 3 according to the invention, is reinforced by the greater number per volume of the volumes 31 of the first material arranged therein compared to the second regions 52. In a further embodiment, however, the first region 51 can be made deliberately softer than the second regions 52, for example by a smaller number per volume of the volumes 31 of the first material arranged therein.

[0041] In Fig. 6A and Fig. Figure 6B shows further different basic structures of an exemplary compression pad 3 according to the invention in a top view, wherein the stacking direction is perpendicular to the coordinate system YZ in the X direction or perpendicular to the sheet plane. Fig. Figure 6A shows an embodiment of the compression pad 3 in which rod-shaped volumes of the first material 31 are embedded in the second material 32. Additionally, a free space 33 is provided around each volume of the first material 31, or each volume 31 is arranged within a corresponding free space 33. The free spaces 33 can be filled with air. When pressure is applied to the rod-shaped volumes 31, they are compressed and can expand freely into the surrounding free space 33. In the example shown, the free spaces 33 are arranged concentrically around the rod-shaped volumes 31, but this is not mandatory. The free spaces 33 can also have an elliptical or rectangular cross-section.

[0042] This is based on Fig. The principle illustrated in 6A can be applied to any other basic shape of the compression pad shown in the figures, such that a free space 33 providing a distance between the volumes of the first material 31 and the second material 32 can be provided.

[0043] In Fig. 6B is one of the ones in Fig. Figure 6A shows an embodiment of the compression pad 3 according to the invention, in which a free space 33 is arranged between the volume of the first material 31 and sections of the second material 32 arranged axially around the first volume 31. The compression pad 3 is shown here in a compressed form due to a compression force K acting laterally upon it. Because of the free space 33, there is no interaction between the two materials, or only interaction that occurs in the case of extreme lateral contraction, which allows the behavior of the compression pad 3 to be accurately predicted.

[0044] In Fig. Figure 7 shows two further basic structures of an exemplary compression pad 3 according to the invention in a top view, wherein these are separated by a vertical dividing line T. The stacking direction when installing the compression pads 3 would be perpendicular to the coordinate system YZ in the X direction or perpendicular to the sheet plane.

[0045] At the in Fig. In the embodiment shown on the left (7), the first material 31 is in the form of two plates (which appear rod-shaped in cross-section) arranged in the second material 32 in the upper and lower regions of the compression pad 3. The arrangement of the two plates can be rotated by 90° so that they are not positioned at the top and bottom but rather laterally within the compression pad 3. Preload forces can be applied selectively using the plates.

[0046] Another way to apply the desired preload force in a targeted manner is in Fig. Figure 7 on the right shows the first material 31 in the form of a frame embedded within, or framing a substantial part of, the second material. This means that the second material 32, located in the center of the compression pad, is only minimally stressed during the initial production process of a corresponding high-voltage battery and can therefore provide a large portion of its compressibility to compensate for swelling.

[0047] As already mentioned, especially the in Fig. The volumes of the first material 31 shown in 7 are not in direct contact with the second material 32 but are also embedded in a free space.

Claims

[1] Compression pad (3) for a battery cell stack (1), comprising: a first material with an initial compressive strength; a second material (32) with a second compressive strength which is less than the first compressive strength; wherein the compression pad (3) has at least one volume (31) of the first material which is at least partially surrounded by the second material (32) with or without direct contact therewith such that, viewed in lateral cross-section, the second material (32) is arranged axially around the at least one volume (31) of the first material and wherein the at least one volume (31) of the first material extends laterally over the entire dimension of the compression pad (3); wherein the axial dimension of the compression pad (3) is larger than the lateral dimension of the compression pad (3). [2] Compression pad (3) according to claim 1, wherein the first material comprises an elastomer. [3] Compression pad (3) according to claim 1 or 2, wherein the second material (32) comprises a foamed elastomer. [4] Compression pad (3) according to any one of claims 1 to 3, wherein the compression pad (3) has several volumes (31) of the first material distributed in the second material (32). [5] Compression pad (3) according to claim 4, wherein the distribution density of the volumes (31) of the first material in the second material (32) is inhomogeneous. [6] Compression pad (3) according to claim 5, wherein the compression pad (3) has a central region (51) and adjacent side regions (52), wherein in the central region (51) the distribution density of the volumes (31) of the first material in the second material (32) is greater than in the side regions. [7] Compression pad (3) according to any one of claims 4 to 6, wherein the first volumes (31) have a straight shape. [8] Compression pad (3) according to any one of claims 4 to 6, wherein the first volumes (31) have an arc shape. [9] Compression pad (3) according to any one of claims 1 to 8, wherein the first material is arranged in at least one free space (33) within the compression pad (3). [10] Method for manufacturing a compression pad (3) for a battery cell stack (1), comprising the steps: Providing at least one volume (31) of a first material which has a first compressive strength; Providing a second material (32) which has a second compressive strength which is less than the first compressive strength; Forming the compression pad (3) by introducing the at least one volume (31) of the first material into the second material (32) such that the volume (31) of the first material is at least partially surrounded by the second material (32) with or without direct contact therewith, such that, viewed in lateral cross-section, the second material (32) is arranged axially around the at least one volume (31) of the first material and wherein the at least one volume (31) of the first material extends laterally over the entire extent of the compression pad (3) wherein the axial dimension of the compression pad (3) is larger than the lateral dimension of the compression pad (3). [11] Method for manufacturing a compression pad (3) according to claim 11, further comprising: Adjusting the stiffness of the compression pad (3) by adjusting the shape and / or the number and / or the distribution of the volumes (31) of the first material in the second material (32). [12] Battery cell stack (1) comprising an arrangement of individual battery cells (2), preferably in pouch cell format, wherein a compression pad (3) according to the preceding claims is arranged between each pair of battery cells (2).

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