Assembly of battery components using thermal interface dispense bead path

Abstract

A method of bonding a stack of prismatic battery cells 1 to a cooling plate 6 within a vehicle battery module comprises applying a bead of a thermally conductive adhesive composition onto a bonding face of the cell stack or the cooling plate in a zigzag pattern. The cell stack comprises a plurality of prismatic cells stacked together to define a substantially rectangular cell stack bonding face, and the cooling plate comprises a substantially rectangular cooling plate bonding face which corresponds to the bonding face of the cell stack. The zigzag pattern comprises angled line segments extending between first and second edge regions of the bonding face, wherein each angle formed between adjacent line segments is between 10 and 30 degrees (figure 4). The bonding faces are compressed together before curing the adhesive layer. Preferably, the cell stack is enclosed in a thermoplastic polymer casing, such that its bonding face is formed from the thermoplastic polymer. Preferably, a two-component adhesive composition is used. A battery module comprising the cell stack, cooling plate and thermally conductive adhesive layer with an average height between 1-3 mm and having a zigzag pattern is also claimed.

Description

TECHNICAL FIELD The present invention relates generally to a method of bonding a stack of battery cells to a cooling plate. In particular, but not exclusively, the invention relates to methods of bonding a stack of prismatic battery cells to a cooling plate within a vehicle battery module. BACKGROUND Battery-powered vehicles now constitute the most rapidly expanding sector of the automotive industry, spurring a surge in innovation within vehicle battery technology. As manufacturers strive to meet the increasing demand for battery-powered vehicles, they are placing a strong emphasis on enhancing the efficiency and thermal management of battery cells within their modules. In electric vehicles, there are mainly two types of batteries commonly used: traction batteries and auxiliary batteries. Traction batteries are the primary power source for electric vehicles. These batteries are typically high-capacity and designed for sustained discharging and recharging cycles. Lithium-ion batteries are the most common type of traction battery used in electric vehicles. Electric vehicles also include auxiliary batteries, which serve various secondary functions, such as powering onboard electronics, lights, and accessories. Traction batteries are typically made up of individual lithium-ion battery cells. Several battery cells can be configured by connecting them either in series or in parallel, allowing for the creation of a larger battery assembly or a stack of battery cells capable of delivering high voltage and high current. The battery cells can be packaged together with various mechanical and electrical couplings between the battery cells to form the assembly. There are different types of battery cells, for instance flat, curved, rounded, and cylindrical variations. Furthermore, for space optimization and footprint reduction in the overall assembly, prismatic or pouchshaped batteries are often chosen. These can be strategically arranged in a stacked configuration, allowing for a more compact and efficient design. During the charging and discharging processes, the battery cells generate heat. The accumulation of heat within the battery assembly can significantly impact the performance of these battery cells. The need to manage heat generated during charging and discharging processes has led to the integration of cooling systems within battery modules. These cooling systems can take the form of heat sinks, fluid heat exchangers, which function by circulating air over the cells, or they may employ liquid cooling mechanisms. A specific example of a cooling system is the use of a cooling plate. The cooling plate, functioning as a heat sink, may typically have a large surface and is made of a thermally conductive material, such as metal, which allows it to efficiently absorb and distribute the heat. As the battery cells produce heat, this thermal energy flows from the cells into the cooling plate. The cooling plate then serves as a pathway for the heat to spread over a larger surface area. Therefore, a thermal connection must be established between the battery cells and the cooling plate. To facilitate this thermal connection, the bonding gap between the cooling plate and the battery stack is typically filled with a Thermal Interface Material (TIM). TIMs are designed to transfer heat from one surface to another. Thermal Interface Material (TIM) is a substance or compound that is applied between two surfaces to enhance the thermal coupling between them. Its primary purpose is to fill in the microscopic gaps and imperfections between the two surfaces, typically a heat-generating component (like a CPU or GPU) and a heatsink or heat spreader. Thermal Interface Materials (TIMs) are engineered with a primary focus on improving heat transfer rather than providing structural support between two components. As a result, they are generally designed with properties that prioritize thermal conductivity over mechanical strength. One alternative for attaching the cooling plates to the battery cell stack is the use of mechanical fasteners, such as screws, bolts, or clamps. These fasteners are tightened to create a strong and reliable connection between the cooling plate and the battery stack. However, using mechanical fasteners can lead to a more complex assembly and the assembly time can be longer compared to other attachment methods like adhesive bonding. The utilization of adhesives or adhesive compositions as an alternative to mechanical fasteners for attaching cooling plates to battery stacks necessitates the careful selection of an adhesive that is not only thermally conductive but also mechanically strong. While the primary role of the adhesives, such as TIM, is to facilitate effective heat dissipation from the battery to the cooling plate, it is equally important to choose an adhesive that can provide a robust mechanical bond. This is essential to withstand operational stresses and ensure the long-term integrity of the bond. Alongside selecting an adhesive with the right thermal and mechanical properties, it is desirable to manage how the adhesive is applied, ensuring the adhesive is dispensed precisely and cures effectively to avoid issues like air entrapment, which can compromise the overall efficacy and durability of the bond. In any adhesive bonding process, precise dispensing of adhesive composition presents challenges, including the need for accurate control of adhesive flow rates, dispensing patterns, the dispensing of the correct volume of the adhesive in an accurate and repeatable manner and enabling appropriate curing times to ensure reliable and durable bonds. Accurate control over these aspects of the adhesive application process is important because of the risks associated with air entrapment. Air entrapment may lead to adhesive failure. Air entrapment refers to the presence of air bubbles or air voids within the adhesive after it has been applied and cured. When adhesive is applied between two surfaces and then compressed, any air that is trapped and not able to escape forms these air voids. The process of compression reduces the routes available for the air to escape, leading to the formation of the air voids. This is problematic for several reasons. Firstly, the air voids reduce the mechanical strength of the bond, as the adhesive layer is not uniform and continuous due to the presence of these voids. Also, the air is a poor conductor of heat, leading to reduced thermal conductivity of the adhesive layer. Further, these air voids can create hot spots or uneven cooling across the battery stack, potentially leading to thermal runaway or reduced battery life. There is a need for a bonding method between a battery cell stack and a cooling plate that substantially eliminates, or at least reduces, the entrapment of air within the adhesive, ensuring or promoting a void-free bond. There is also a need for a bonding method to join the battery cell stack and the cooling plate that relatively quickly and evenly dispenses adhesive in a predefined pattern while achieving highly uniform and reproducible bonding results. There is a need for adhesive that can quickly establish a strong bond within a very short time after application whilst still meeting good heat transfer criteria. It is an object of the present invention to at least partially mitigate one or more of the above problems. SUMMARY OF THE INVENTION According to an aspect of the invention for which protection is sought, there is provided a method of bonding a cell stack to a cooling plate, wherein the cell stack is for a traction battery of an electric vehicle and comprises a plurality of prismatic cells stacked together to define a substantially rectangular cell stack bonding face; and the cooling plate comprises a substantially rectangular cooling plate bonding face which corresponds to the bonding face of the cell stack; the method comprising: applying a bead of thermally conductive adhesive to either the cell stack bonding face or the cooling plate bonding face in a zigzag pattern; arranging the cell stack and the cooling plate so that the bonding faces are facing each other; compressing the cell stack bonding face and the cooling plate bonding face together to form a compressed adhesive layer; and curing the compressed adhesive layer. The zigzag pattern of adhesive provides a bonding method substantially eliminates trapped air voids or air bubbles in the adhesive layer. The method of the present invention further provides substantially uniform and repeatable bonding of the cooling plate to the stack of prismatic battery cells. According to an aspect of the invention for which protection is sought, there is provided a method of bonding a cell stack to a cooling plate, wherein the cell stack is for a traction battery of an electric vehicle and comprises a plurality of prismatic cells stacked together to define a substantially rectangular cell stack bonding face; and the cooling plate comprises a substantially rectangular cooling plate bonding face which corresponds to the bonding face of the cell stack; the method comprising: applying a bead of thermally conductive adhesive to either the cell stack bonding face or the cooling plate bonding face in a zigzag pattern, wherein the zigzag pattern comprises angled line segments which extend from a first edge region of the bonding face to an opposing second edge region of the bonding face, wherein each angle formed between adjacent line segments is between 10 and 30 degrees; arranging the cell stack and the cooling plate so that the bonding faces are aligned and facing each other; compressing the cell stack bonding face and the cooling plate bonding face together to form a compressed adhesive layer; and curing the compressed adhesive layer. With the described dispensing pattern of the adhesive of the present invention, a bonding method is provided, that substantially eliminates trapped air voids or air bubbles in the adhesive layer. The method of the present invention further provides substantially uniform and repeatable bonding of the cooling plate to the stack of prismatic battery cells. Each angled line segment of the zigzag pattern may be aligned with a lower surface of a corresponding single prismatic cell of the cell stack when the cell stack and the cooling plate are arranged for bonding. Advantageously a secure bond is formed between the cooling plate at the prismatic cells. The cell stack bonding face can be formed by an underside of the stack of cells. Further, the stack of prismatic cells may be enclosed in a thermoplastic polymer casing, for example a polyethylene terephthalate casing, such that the cell stack bonding face is essentially formed from a thermoplastic polymer, for example polyethylene terephthalate. Advantageously the prismatic cells are secured with a mechanically robust wrapper with good heat transfer properties. Each cell may comprise a dilatation groove positioned on its lower surface to accommodate excess thermally conductive adhesive, for example, during the step of compressing the bonding face of the cell stack and the bonding face of the cooling plate together. In an embodiment, the cooling plate bonding face may be substantially composed of a metal or metal alloy, for example aluminium or an aluminium alloy. Advantageously, the present invention minimizes or at least reduces the formation of air voids or trapped air within the thermally conductive adhesive used for bonding a battery cell stack to a cooling plate. In an embodiment, the thermally conductive adhesive may be applied to the stack of battery cells. In an alternative embodiment, the thermally conductive adhesive may be applied to the cooling plate. In an embodiment, the layer of adhesive may be applied to the cooling plate or the stack of battery cells by one or more nozzles. Such a nozzle provides an efficient way to spread a layer of adhesive of constant thickness. In an embodiment, the adhesive may be a two-component adhesive, i.e., an adhesive formulated with separate resin and hardener. Once these two components are mixed, the cure reaction starts. Normally the formulations will cure at room temperature although cure rates can be increased by raising the temperature. Two-component adhesive types may include epoxies, acrylics, silicones and polyurethanes. Advantageously, the adhesive may be dispensed in zigzag pattern allowing any trapped air to escape and making the bond stronger and more reliable. According to a further aspect of the invention, there is provided a battery module comprising: a cell stack having a plurality of cells stacked together to define a substantially rectangular cell stack bonding face, a cooling plate having a substantially rectangular cooling plate bonding face which corresponds to the bonding face of the cell stack, a thermally conductive adhesive layer having an average height of from 1 to 3 mm, and positioned between the cell stack bonding face and the cooling plate bonding face, and formed by compressing a zigzag pattern disposed on one of the bonding faces prior to forming the thermally conductive adhesive layer by compression. Each cell of the cell stack, within the battery module, may be a prismatic cell having an lower surface. The thermally conductive adhesive layer positioned between the cell stack bonding face and the cooling plate bonding face in the battery module may be obtained by applying a bead of thermally conductive adhesive to either the cell stack bonding face or the cooling plate bonding face in a zigzag pattern. The zigzag pattern may comprise angled line segments which extend from a first edge region of the bonding face to an opposing second edge region of the bonding face. Each angle formed between adjacent line segments may be in the range of from 10 degrees to 30 degrees. The thermally conductive adhesive layer may have an average height of from 1 mm to 3 mm. The thermally conductive adhesive composition may be dispensed at a flow rate of between 4 cm3 / s and 8 cm3 / s, for example between 6.0 cm3 / s and 7.0 cm3 / s, for example 6.3 cm3 / s, at a constant deposition velocity in the range of from 150 mm / s to 190 mm / s. The thermally conductive adhesive layer may have a tensile strength that exceeds 0.5 MPa within 60 minutes of curing. Advantageously, the thermally conductive adhesive layer has a tensile strength that exceeds 0.5 MPa, for example 1 MPa, for example 1.3 MPa within 60 minutes of post-mixing the two components of the adhesive composition. According to a further aspect of the present invention, there is provided a battery pack comprising a plurality of battery modules as described above. According to a further aspect of the present invention, there is provided a vehicle comprising a battery pack or a battery module as described above. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. The above features and advantages of the present invention will be better understood with reference to the accompanying figures and detailed description of embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 shows an exploded view of state-of-the art prismatic battery pack. FIG. 2 shows an image of an assembled battery cell stack in accordance with embodiments of the invention. FIG. 3 illustrates an example of a plurality of prismatic battery cells bonded to a cooling plate in accordance with embodiments of the invention. FIG. 4 illustrates an embodiment in accordance with embodiments of the present invention, showing a bead of adhesive applied in a zigzag pattern. FIG. 5 illustrates is a schematic drawing of the zigzag pattern. In particular, it shows angled line segments forming the zigzag pattern. FIG. 6 shows the formed adhesive layer after compression. FIG. 7 is a graph that analyzes the response of a polyurethane adhesive composition to tensile stress over a period. FIG. 8 shows a schematic vehicle comprising a battery cell stack according to embodiments of the invention. FIG. 9 shows a schematic method of bonding a cell stack to a cooling plate according to embodiments of the invention. DETAILED DESCRIPTION In the battery manufacturing industry, the use of adhesive materials is common for bonding components such as battery cells to cooling plates to for a battery module. When these components, both typically characterized by flat surfaces, are joined together, the process can trap air or gaseous bubbles within the adhesive layer, leading to the formation of air voids. In the context of battery assemblies, if these air voids are not effectively eliminated, they can compromise the integrity of the adhesive bond. This, in turn, can lead to failures in the thermal management system of the battery, impacting overall performance and reliability. In light of these challenges, the present invention is aimed at addressing the issue of air entrapment in adhesive layers used in battery assembly processes. This ensures a more reliable and effective bonding between battery cells and cooling plates, important for optimizing both the thermal management and the overall performance of battery systems. By mitigating the risks associated with air void formation in the adhesive, the structural integrity and longevity of the battery assembly is enhanced. Definitions and Abbreviations As used herein: The term "room temperature" refers to a temperature of 22°C to 25°C. The terms “cure” or “curable” refer to joining polymers chains together by covalent chemical bonds , usually via crosslinking molecules or groups. The term “wetting” refers to the ability of the adhesive to spread out and intimately contact a solid surface. Good wetting occurs when a liquid spreads evenly across a surface. The term "in situ" means "on site" or "in place", for example, the adhesive composition being applied directly to the surface of the component that it will be bonding, without the need for any intermediate steps or processing. The term “thixotropic” refers to refers to a property of certain fluids or gels that are thick or viscous under static conditions but become less viscous and more fluid when agitated, shaken, or otherwise stressed. The terms “void”, "air void", “bubble”, “air bubble”, "air gaps", “air pockets” are used herein interchangeably. The terms "cell" and "battery cell," as well as "stack," "cell stack," and "battery cell stack," are used interchangeably herein. The term “vertex” herein refers the point where two or more lines or edges meet, forming an angle, such as in “V” shape. The term "work life" of an adhesive refers to the period after mixing (in the case of multi-component adhesives) or after opening (for single-component adhesives) during which the adhesive remains usable and retains its ability to bond effectively. During this time, the adhesive maintains a workable consistency and viscosity, and bonding properties after curing. The term “ adhesive” or “adhesive composition” refers to a substance used to bind materials together by surface attachment, typically providing a strong and lasting bond. The battery described in the embodiments of the present application may refer to a rechargeable battery. In the following, a lithium-ion battery will be taken as an example to describe the embodiments disclosed in the present application, for example a traction battery. It should be understood that the embodiments disclosed in the present application are applicable to any other suitable types of rechargeable batteries. The battery mentioned in the embodiments disclosed in the present application can be directly or indirectly applied to an appropriate device to power the device. The rechargeable batteries, particularly lithium-ion types exemplified in the application, may be designed to be implemented in electric vehicles, either directly or indirectly. In high-power applications like electric vehicles, the battery system includes three levels: battery cell, battery module and battery pack, as shown in FIG. 1 and described further below. The battery module is formed by electrically connecting a certain number of battery cells together by a busbar assembly and putting them into a frame to protect the battery cells from external impact, heat, vibration, etc. The battery modules may be connected in series to create a battery pack. The battery pack is the final state of the battery system mounted into the electric vehicle. The battery pack generally includes a case for packaging one or more battery cells. The case can prevent liquid or other foreign objects from affecting the charging or discharging of the battery cell. The case is generally composed of a cover body and a case shell. For example, FIG. 1 shows an exploded view of state-of-the art prismatic cell battery pack 104 comprising a plurality of prismatic cells 1, where each cell has a substantially rectangular shape, a bus-bar carrier 2, a bus-bar 3, a compressive plate 4, a side wall 5, a cooling plate 6, an interface 7 between the cells and the cooling plate. The battery cell 1, or simply referred herein as the cell, is the basic unit in the battery, and may be generally grouped, according to the way of packaging, into: cylindrical battery cell, prismatic battery cell and pouch battery cell. The following will mainly focus on the prismatic battery cell, as the cells shown in FIG. 1. It should be understood that the embodiments described below are also applicable to the cylindrical battery cell or the pouch battery cell in certain aspects. A prismatic cell stack 314 as shown in FIG. 3 refers to an assembly where multiple prismatic cells are aligned and integrated together, as shown in FIG. 2. These prismatic cells have rectangular shapes and are encased in a hard outer shell, typically made of steel or aluminium. This robust outer shell not only provides structural integrity but also aids in protecting the cell's internal components. The design of prismatic cells allows for efficient use of space within the battery pack 104, as their flat surfaces facilitate closer packing without significant wasted space. When these cells are stacked to form a prismatic cell stack, the arrangement may be done in a manner that optimizes both energy density and thermal management. The prismatic cells may have an insulation covering the outer shell, such as polyvinyl chloride, polyethylene terephthalate (PET), polypropylene, polyimide films, or for example polyethylene terephthalate (PET). FIG. 3 shows a plurality of prismatic battery cells 11 bonded to a cooling plate 21 to form a cell stack 314. The battery or battery module 104 may thus also include a thermal management component 21. The thermal management component 21 in a battery system, as described, is designed to regulate and control the temperature of the battery cells. The management and adjustment of the temperature may include heating or cooling the plurality of battery cells 11. For example, in the case of cooling or lowering the temperature of the battery cells, the thermal management component 21 is configured to dissipate heat generated by the battery cells 11 during operation, such as during charging and discharging processes. In this case, the thermal management component 21 may also be called a cooling component, a cooling system or a cooling plate. The cooling plate 21 typically may consist of a thermally conductive material, such as aluminium or copper, and is designed to have direct contact with the cells 11 or their housing. It may incorporate channels or pathways for a coolant fluid to flow through, which helps in transferring heat away from the cells. This setup ensures that the temperature of the cells remains within an optimal range, enhancing their performance and safety. The thermal management component 21, in particular the cooling plate may be attached to the cell stack 11 by means of a thermally conductive adhesive; for example, located between the lower bonding face 41 of the stack of cells 11 and the bonding face 51 of the cooling plate 21. The present invention relates to, as shown in FIG. 9, a method 900 of bonding a cell stack to a cooling plate 21, utilizing an adhesive as the bonding agent. The cell stack is for a traction battery of an electric vehicle and comprises a plurality of prismatic cells 11 stacked together to define a substantially rectangular cell stack bonding face 41. The cooling plate 21 comprises a substantially rectangular cooling plate bonding face 51 which corresponds to the bonding face of the cell stack 41. The approach of the present invention involves the application of adhesive in a specific pattern, i.e., a zigzag pattern which can be executed on either the cooling plate 21 (on the bonding face 51) or the battery cell stack 11 (on the bonding face 41). The prismatic cells 11, also may be arranged next to each other in a horizontal arrangement, side-by-side, creating a flat, wide assembly forming a cell stack, or in a vertical arrangement, where the cells are stacked one on top of the other, rather than being laid out side-by-side. This vertical arrangement may create a taller, narrower assembly compared to the flat, wide structure of the horizontal arrangement. As shown in FIG. 3, a cell stack 314 may have a top surface 31 and a bottom surface 41. In the prismatic cell stack with a side-by-side arrangement, a cooling plate 21 may be typically placed directly underneath the horizontally aligned cells 11 or on the bottom surface 41 of the prismatic cell stack. The bottom surface 41 is the surface through which heat is transferred to the cooling plate 21 for dissipation. The bottom surface 41 is formed by the collective alignment of individual prismatic cells 11, each with its own width and length dimensions. The bottom surface 41 of the prismatic battery cell stack is referred herein also as a cell stack bonding face. This terminology is used to specifically identify and refer to the lower side of the prismatic cell stack that interfaces with the cooling plate bonding face 51, as shown in FIG. 3. This bottom surface 41 of the cell stack 11 is in direct contact with the cooling plate 21. The cooling plate 21 is designed to match the dimensions of this bottom surface, ensuring that each cell’s lower surface area is in thermal contact with the plate 21 for effective heat dissipation. Each prismatic cell 11 in the stack has at least two key dimensions - width and length. These prismatic cells are flat, with a rectangular shape. Each prismatic battery cell 11 may also have a lower surface 41 and a central line running along the length of this lower surface 41. The lower surface of each cell is defined by the width (the shorter side of the rectangle) and length (the longer side) of each cell. The lower surface 41 of the cell is the surface where the adhesive 61 may be applied to be bonded with the cooling plate 21. The central line runs lengthwise along the prismatic cell, effectively bisecting its width, and creating symmetrical halves on either side. The prismatic battery cells 11 may be placed next to each other in a horizontal layout. This means their lengths are aligned parallel to each other, and their widths are positioned adjacent. The side-by-side placement creates a flat, wide assembly. When the cells 11 are aligned side-by-side, these central lines run parallel to each other across the entire bottom surface 41 of the prismatic battery cell stack. Each cell 11 may comprise a dilatation grove positioned on its lower surface 41 to accommodate excess thermally conductive adhesive. This dilatation groove may be located along the central line of each prismatic cell 11. Typically, in the assembly of prismatic cell stacks 11, a bead of adhesive may be applied along the central line of each prismatic cell, following a specific pattern. In an embodiment, the adhesive 61 may be a two-component adhesive with high thermal conductivity. A two-component adhesive, also known as a two-part adhesive, is a type of bonding agent that consists of two separate components which must be mixed together before use. These two components typically include a resin or base, i.e., the main component of the adhesive, providing the primary bonding properties, and hardener or activator, i.e., a component that reacts with the resin to cause the adhesive to set or cure. Once these two components are mixed, the cure reaction starts. This mixing process allows for precise control over the adhesive's working time, ensuring that it remains in a liquid state until it is applied to the bonding surfaces. Normally the formulationswill cure at room temperature (22 to 25 °C). Room temperature curing adhesives eliminate the need for elevated temperatures during the curing process, reducing the risk of subjecting the battery to damaging heat. Two-component adhesive types may include epoxies, acrylics, silicones and polyurethanes. In an embodiment, a two-component polyurethane adhesive composition may comprise a first component and a second component, with the first component comprising at least one polyol, selected from the group consisting of fatty alcohols, polyester polyols, polyether polyols, polyether-polyester polyols and polycarbonate polyols; The polyol may have a molecular weight Mw of 302 to 1,200 g / mol, in particular 400 to 1,000 g / mol. Examples of commercially available polyols are, for example, DESMOPHEN 1262 BD from Covestro, or VORANOL WD 2104 Polyol from Dow. The second component may comprise at least one polyisocyanate, a compound with reactive isocyanate groups (NCO). In some embodiments, the polyisocyanate-containing compound may be reaction products of hydroxylterminated polybutadiene with an excess amount of isocyanates. The isocyanates compound may be selected from the groups consisting of 1, 5-naphthylendiisocyanate (NDI) , 2, 4 ‘-or 4, 4‘-diphenylmethandiisocyanate (MDI) , isomers of toluylendiisocyanates (TDI) methylentriphenyltriisocyanate (MIT) , hydrated MDI (H12MDI) , tetramethylxylylendiisocyanate (TMXDI) , 1-isocyanatomethyl-3-isocyanato-1,5, 5-trimethylcyclohexane (IPDI), xylylendiisocyanate (XDI), hexan-1, 6-diisocyanate (HDI) , pentamethylendiisocyanate, dicyclohexylmethandiisocyanate as well as dimers, trimers, oligomers and polymers of the same. (Poly) isocyanate prepolymers can, for example, be obtained by reacting a polybutadiene diol with an excess amount of aromatic isocyanate. The hydroxyl-terminated polybutadiene can be a polybutadiene diol having terminal hydroxyl groups and a number-average molecular weight (Mn) in the range of 1,000 -5,000 g / mol, or 1,200 -4,500 g / mol, or 2,000 -3,000 g / mol, and / or an average hydroxyl (OH) functionality in the range of 1.7 -3.5, or 1.9 -2.6, or 2.2 -2.5. The first component may further comprise a crosslinker and thermal conductive fillers. Usually, crosslinkers are low molecular weight molecules used to link the polymer and form a network structure. The crosslinker may be selected from the group consisting of triol polyether polyol, triol polyester polyol and mixtures thereof. Upon mixing these components, a cross-linking reaction occurs, which results in the formation of the polyurethane polymer network. Further, the two-component adhesive composition may comprise thermal conductive fillers, pigments, rheology modifiers, drying agents, flame retardants, surface active agents and anti-foaming agents. The thermal conductive filler may be selected from the group consisting of metal oxides, nonmetal oxide, metal nitride, nonmetal nitride, metal hydroxides, metal silicates, metal sulfides and combinations thereof. Suitable metal oxides to be used as the thermal conductive fillers may be selected from the oxides of metals selected from the group consisting of tin, indium, antimony, aluminium, titanium, iron, magnesium, zinc, rare earth metals, alkaline metals, oxides of Mg, Ca, Sr and Ba, mixed metal oxides and mixtures thereof. The primary function of thermal conductive fillers is to enhance the heat dissipation capability of the adhesive. Thus, the two-component adhesive composition is also a thermally conductive adhesive composition. The performance of the bonds made from two-component adhesives depends on having the correct ratio of components intimately mixed and applied within the working life of the system. Mix ratios of adhesive components can vary enormously, for example the ratios may be in the range from 4:1 to 1:4, for example from 2:1 to 1:2, for example 1:1. It's important to mix these compounds with adequate shear forces to ensure a uniform mixture. Various mixing tools can be used for effective blending, including static mixers, magnetic stirrer setups, wire whisk devices, augers, batch mixers, planetary mixers, C.W. Brabender or Banbury® style mixers, and high-shear mixing equipment such as blade-style blenders and rotary impellers. The adhesive composition according to the invention may have a viscosity of no more than 1500 Pa s, for example no more than 1,000 Pa s, in particular 30 to 800 Pa s, or 220 to 700 Pa s, for example 220-290 Pa.s, for example of from 260 to 270 Pa.s, determined at 25°C (tested by Anton Paar Rheometer, using a PP25 spindle, at the shear rate of 1 s-1 according to ASTM D2196-2015). The two component curable compositions may exhibit an initial viscosity - determined immediately after mixing, for example, up to two minutes after mixing - of less than 400 Pa.s, for instance less than 300 Pa.s, at 25°C, for example equal to or less than 290 Pa.s, according to ASTM D2196-2015. The adhesive 61 may be dispensed from a moving dispensing nozzle, which is part of a robotic system, positioned above the surface to be bonded. The dispensing applicator may have a two-component mixing valve. In an embodiment, liquid thermally conductive adhesives may be favoured over solid adhesives. This arises because solid adhesives usually contain air pockets at the material interfaces, creating additional resistance to heat transfer. These air pockets can be randomly distributed, causing the cooling of the battery cells within the battery module to become highly non-uniform. In an embodiment, the thermally conductive adhesive is applied in a zigzag pattern 404 onto the bonding face of either the cooling plate orthe stack of cells, as illustrated in FIG. 4. The zigzag pattern 404 comprises angled line segments which extend from a first edge region of the bonding face to an opposing second edge region of the bonding face, wherein each angle formed between adjacent line segments is between 10 and 30 degrees, as shown in FIG. 5. The zigzag pattern 510 addresses challenges associated with adhesive bonding, such as the minimization of air entrapment by providing an air escape path. The alternating angles of the zigzag pattern create channels through which air can be pushed out as the adhesive is applied and the surfaces are brought together. This reduces the likelihood of air pockets or bubbles, which are poor conductors of heat and can significantly hinder heat transfer. As the cooling plate and cell stack are compressed together, the thermally conductive adhesive is displaced from the corners (vertex) of the zigzag pattern, effectively spreading and filling the spaces between the line segments to prevent the formation of air gaps or air voids. Herein the corner is defined as the point where two angled line segments of the zigzag pattern meet. The zigzag pattern may extend from one side of the bonding face to the other. Thus, applying a thermally conductive adhesive in a zigzag pattern that has a specific angle between the angled line segments 10 provides a more uniform adhesive layer without air gaps, that ensures that heat is distributed more evenly across the interface between the cell stack and the cooling plate, improving this way the heat transfer efficiency. The angle between adjacent line segments in the zigzag pattern, meeting at a vertex, ranges of from 10 to 30 degrees. Applicants have surprisingly found that when the angle between adjacent line segments of the zigzag pattern connected at a vertex is in the range of from 10 to 30 degrees, no air voids or air bubbles are formed in the adhesive layer 602, as it provides continuous paths for air to be pushed out, reducing the likelihood of air voids (see FIG. 6). The adhesive is pushed away from the corners of the zigzag pattern and spreads evenly between the line segments of the zigzag pattern. By doing so, it effectively fills the spaces between these line segments, thereby eliminating the possibility of voids forming. By filling the spaces between these line segments, the total contact area of the bonding surfaces is enhanced, which in turn improves the adhesion strength and the heat transfer, as it ensures more extensive and uniform contact between the surfaces. If the angle is greater than 30 degrees, the adhesive does not spread as evenly between the line segments and the effectiveness of this air escape mechanism is reduced, leading to the formation of air voids. If the angle is smaller than 10 degrees, thus an extremely narrow angle, it has been discovered that is also not as effective in facilitating air escape during the bonding process, as it may not provide sufficient channels for air to be expelled efficiently, which again leads to formation of air pockets or voids. In an embodiment, the angle between adjacent line segments in the zigzag pattern, meeting at a vertex can range of from 10 to 25 degrees, 10 to 20 degrees, or 10 to 15 degrees. In an embodiment, the angle between adjacent line segments in the zigzag pattern, meeting at a vertex can range of from 15 to 30 degrees, 15 to 25 degrees, or 15 to 20 degrees. In an embodiment, the angle between adjacent line segments in the zigzag pattern, meeting at a vertex can range offrom20to 30 degrees, or 25 to 30 degrees. The determination of the angle between adjacent line segments of the zigzag pattern depends on the width of each cell. The deviation of each line segment from a central reference line may be calculated in relation to the width of a given cell. The central reference line, in this context, is the line that bisects each cell's width into two equal parts, also defined herein as the central line of each cell. A deviation parameter (a linear measurement) is calculated as being in the range of from 5 to 15 % of the width of the cell. This percentage may be used to determine the starting point of each line segment in the zigzag pattern. Thus, the starting point for each line segment in the zigzag pattern is set at a distance in the range of from 5 to 15% of the cell's width, measured from the central line of each cell. The starting point and the opposite end of each line segment in the pattern are symmetrically positioned at a distance in the range of from 5 to 15% of the cell's width from the central line, on opposite sides of this central line. FIG. 5 illustrates an aspect of the subject matter according to one embodiment. It demonstrates an example of how the adhesive composition is applied in the form of angled line segments 510, which collectively create a zigzag pattern. In this configuration, an angled line segment intersects the Y-axis, forming symmetric angles on both sides. These angles, between the angled line segments and the Y-axis, are identical and fall within the range of 5 to 15 degrees. This means that the line diverges from the vertical Y-axis at an angle of 5 to 15 degrees on both the left and right sides. The vertex angle formed by the V-shape, i.e. the angle inside the V - is in the range of from 10 to 30 degrees. Thus, each angle formed between adjacent line segments of the zigzag pattern is in the range of from 10 to 30 degrees; Each angled line segment is offset from the central line 10, where the central line is defined as a line along the Y-axis bisecting the width of the cell into two equal halves. The zigzag pattern may be applied in a continuous manner 404, starting from a first edge region of the bonding face of the cooling plate to an opposing second edge region of the bonding face, as shown in FIG. 4. The cooling plate may have a substantially rectangular shape that closely matches the bottom surface of the cell stack, ensuring a complementary and precise fit. The line segments of the zigzag pattern may run parallel to the longer sides (length) of the rectangular cooling plate. When the cooling plate and the cell stack are pressed together, each angled line segment on the cooling plate aligns with and adheres to the lower surface of each individual prismatic cell in the stack. This ensures that there is one angled line segment corresponding to each cell. Each angled line segment may align with the central line of each cell. Instead of spreading a thin layer of adhesive across the entire surface, the method of the present invention may feature applying small "beads" or lines of adhesive. These beads are like tiny ridges or rows of the material. The adhesive may have a high thermal conductivity in the range of from 1.8 to 3.5 W / mK and further reduces reliance on the use of mechanical fasteners. The cooling plate 21 and the cell stack may be compressed with the adhesive composition disposed between and adhering the cooling plate bonding face to the cell stack bonding face. Generally the bond strength (e.g. peel strength, tensile strength, overlap shear strength or impact strength) of the adhesive continues to build well after the initial cure time. In an embodiment, the cooling plate bonding face may be formed of metal, such aluminium or aluminium alloy. In an embodiment, the cell stack bonding face may be formed of thermoplastic polymer, for example a polyethylene terephthalate. The thermally conductive adhesive layer formed between the cooling plate and the cell stack has a tensile strength that exceeds 0.5 MPa within 60 minutes of curing, as determined by the test procedure in Example 2, as shown in FIG. 7 showing a plot 702 of time in seconds on the horizontal axis against force in N on the vertical axis. This enables the cell stack together with the cooling plate to be lifted and inserted into the battery pack 104, all within a timeframe of 60 minutes following the mixing of the two-component adhesive composition. The indicators #1, #2, #3 and #4 relate to different tests performed by applying adhesive in a zigzag as discussed below. For achieving the desired level of tensile strength within the range of from 30 min to 120 min, for example 60 minutes post-mixing the two components of the adhesive composition, it is useful to determine an optimal flow rate for dispensing the adhesive. This flow rate is important to ensure uniform and precise application, directly influencing the curing process and the development of tensile strength. Simultaneously, selecting an adhesive composition with an appropriate viscosity is equally important. The right viscosity not only facilitates efficient application and spreading but also plays a vital role in achieving the necessary bonding characteristics within the specified timeframe. The synergy between the carefully selected flow rate and the viscosity of the adhesive composition is fundamental to meeting the requirement of significant tensile strength development within the 60-minute timeframe. Furthermore, precise control over the thickness of the adhesive bead is important, as an adhesive bead that is too thick may not cure properly within the allotted time, while one that is too thin might fail to provide sufficient bonding strength. For example, FIG. 7 presents a graph (Tensile strength vs cure time , 0.5 mm bond gap, 40 mm disc, tested at 15 mm / min rate) that analyses the response of a two-component thermally conductive polyurethane adhesive composition with viscosity of 290 Pa.s after mixing at room temperature to tensile stress over a period of 30 min to 2hr cure time (# 3 at 30 min cure time, # 4 at 45 min cure time, # 2 at 1 hr cure time, # at 2 hr cure time) all at a velocity rate of 15mm / min. The size of the aluminium disc was 40 mm the bond gap 0.5 mm. For example, series # 2 at 1 hr cure time and peak force of 3920 N, had a tensile strength (peak stress) of 3.12 MPa. Applicants have found that applying a bead of adhesive in a zigzag pattern at flow rate in the range of from 5.5 to 7.5 cm3 / s, for example 6.3 cm3 / s at 150-190 mm / s robot speed and having a viscosity in the range of from 220 to 290 Pa.s , as determined by Anton Paar MC301 Rheometer, with a gap of 500 pm, at 25°C, and a shear rate of 2.4 / second, results in adhesive that within 60 minutes of post-mixing the two component adhesive, achieves a tensile strength, as determined at 25°C, of at least 0.5 MPa, for example at least 0,9 MPa, for example y of at least 1.3 MPa. In an embodiment, the adhesive layer may have a thickness ranging from 1 0.5 to 3 mm. Generally, to maximize the heat conduction through the adhesive layer and to minimize the cost, the adhesive should be as thin as possible , while maintaining good contact with the cooling plate. In an embodiment, the two-component adhesive cures at room temperature, or at a temperature no greater than 30°C, for example no greater than 25°C or even no greater than 20°C. The two-component adhesive composition may be mixed and dispensed in situ. FIG. 8 shows a schematic vehicle 800 comprising a battery cell stack according to embodiments of the invention. FIG. 9 shows a schematic method 900 of bonding a cell stack to a cooling plate, wherein the cell stack is for a traction battery of an electric vehicle and comprises a plurality of prismatic cells stacked together to define a substantially rectangular cell stack bonding face; and the cooling plate comprises a substantially rectangular cooling plate bonding face which corresponds to the bonding face of the cell stack. The method 900 comprising: applying 902 a bead of thermally conductive adhesive to eitherthe cell stack bonding face or the cooling plate bonding face in a zigzag pattern, wherein the zigzag pattern comprises angled line segments which extend from a first edge region of the bonding face to an opposing second edge region of the bonding face, wherein each angle formed between adjacent line segments is between 10 and 30 degrees; arranging 904 the cell stack and the cooling plate so that the bonding faces are aligned and facing each other; compressing 906 the cell stack bonding face and the cooling plate bonding face together to form a compressed adhesive layer; and curing 908 the compressed adhesive layer. EXAMPLE 1 Two-component mixing valve applicator was used, namely static mixer ME 13-24T with a system temperature of 35 °C (mixing valve temperature 35 °C) and robot arm KUKA KR201. A two-component polyurethane adhesive composition was used. The first component was selected from polyether polyol and also comprising a crosslinker, and the second component was selected from isocyanate-containing compound and / or a (poly) isocyanate prepolymer. Further, one of the components comprised a thermal conductive filler in the range of from 82.5-85.0 wt. percent based on the total amount of the composition. The two components were mixed by a static mixing system at volume ratio of 1:1. The mixed material was applied in a form of a bead of adhesive by the robot equipped with a dispenser. The robot path was 2 mm away at both sides from the centre point to give more air evacuate path as shown in FIG. 4 and FIG. 5. The target volume (4+ 1 beads) was 42,4 ± 4 cm3. The bond gap was 1 mm, compression force 1730 N. The viscosity (2.4 s-1) of the adhesive after mixing was 220 Pa.s, as measured by Anton Paar MC301, (500 um, 25 °C , 2.4 s”1), thermal conductivity of 3.0 W / mK, as measured by ASTM D5470. The application parameters were: flow rate of 6.3 cm3 / s and middle point speed of 160 mm / s, triangle point speed of 170 mm / s, offset 18 mm. The width of each cell was approximately 20 mm, thus the deviation of each zigzag segment was equal to 10 % of the width of the cell. The beads were applied off-centre in a zigzag pattern, forming angled line segments with regard to the central line of the lower surface of the cell to ensure that the adhesive spreads evenly when pressure is applied. The (vertex) angle, formed at the point where two adjacent angled line segments converge, was approximately 20 degrees. It was visually observed that the process yielded a result with no voids present, as shown in FIG. 6. EXAMPLE 2 Tensile test procedure The tensile strength of an adhesive is a critical measure indicating the maximum load or stress it can withstand per unit area before experiencing failure, which can be calculated using the following formula: o=F / A, where o (sigma) represents the tensile strength, F is the force applied to the adhesive measured in Newtons (N), and A is the cross-sectional area over which the force is distributed, measured in square meters (m2). A bead of a two-component thermally conductive adhesive composition, with a main component polyurethane was applied onto a base plate via manual dispense. The viscosity of the mixed adhesive composition was 220-290 Pa.s, as measured by ASTM D2196-2015, (500 um, 25 °C , 2.4 s"1). An aluminium circular disk (40 mm diameter, 1256.637 mm2) was then gradually lowered onto the adhesive bead at a controlled rate of 15 mm / min until it reached a preliminary bond gap of 2 mm. This position was maintained for approximately 10 seconds, during which time the force exerted was continuously recorded. This step was important for ensuring complete wetting out of the adhesive across the joint. Following the initial 10-second period, the disk was further lowered to establish the final bond gap (1.5 mm, 1 mm, 0.5 mm). This final position was held steady for a minimum of another 10 seconds, and during this phase, the compressive force was again recorded, allowing the adhesive to stop its flow. After this stage, the specimen was either left to cure for a predetermined duration or subjected to a tensile test (pulled apart) at a rate of 15 mm / min. This process ensured a controlled and consistent application of the adhesive, critical for evaluating its bonding characteristics and mechanical strength. 5 The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing 10 embodiments, but also any embodiments which fall within the scope of the claims. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. 15

Claims

1. A method of bonding a cell stack to a cooling plate, wherein the cell stack is for a traction battery of an electric vehicle and comprises a plurality of prismatic cells stacked together to define a substantially rectangular cell stack bonding face; andthe cooling plate comprises a substantially rectangular cooling plate bonding face which corresponds to the bonding face of the cell stack;the method comprising:applying a bead of thermally conductive adhesive to either the cell stack bonding face or the cooling plate bonding face in a zigzag pattern, whereinthe zigzag pattern comprises angled line segments which extend from a first edge region of the bonding face to an opposing second edge region of the bonding face, wherein each angle formed between adjacent line segments is between 10 and 30 degrees;arranging the cell stack and the cooling plate so that the bonding faces are aligned and facing each other;compressing the cell stack bonding face and the cooling plate bonding face together to form a compressed adhesive layer; andcuring the compressed adhesive layer.

2. The method according to claim 1, wherein each angled line segment of the zigzag pattern is aligned with a lower surface of a corresponding single prismatic cell of the cell stack when the cell stack and the cooling plate are arranged for bonding.

3. The method according to claim 1 or claim 2, wherein the cell stack bonding face is formed by an underside of the stack of cells.

4. The method according to any of claims 1 to 3, wherein each cell comprises a dilatation grove positioned on its lower surface.

5. The method according to any of claims 1 to 4, wherein the stack of prismatic cells is enclosed in a thermoplastic polymer casing, such that the cell stack bonding face is essentially formed from a thermoplastic polymer.

6. The method according to any of claims 1 to 5, wherein the thermally conductive adhesive is a two-component adhesive composition.

7. The method according to any of claims 1 to 6, wherein the thermally conductive adhesive composition is dispensed at a flow rate of 6.3 cm3 / s.

8. The method according to any of claims 1 to 7, wherein the adhesive layer is a thermally conductive adhesive layer with a tensile strength exceeding 0.5 MPa within 60 minutes of curing.

9. A battery module comprising:I) a cell stack having a plurality of cells stacked together to define a substantially rectangular cell stack bonding face,ii) a cooling plate having a substantially rectangular cooling plate bonding face which corresponds to the bonding face of the cell stack,Hi) a thermally conductive adhesive layer having an average height of from 1 to 3 mm, and positioned between the cell stack bonding face and the cooling plate bonding face, andformed by compressing a zigzag pattern disposed on the bonding face prior to forming the thermally conductive adhesive layer by compression.

10. The battery module of claim 9, wherein the thermally conductive adhesive layer is obtained by applying a bead of thermally conductive adhesive to either the cell stack bonding face or the cooling plate bonding face in a zigzag pattern, wherein the zigzag pattern comprises angled line segments which extend from a first edge region of the bonding face to an opposing second edge region of the bonding face, wherein each angle formed between adjacent line segments is between 10 and 30 degrees;11. The battery module of claim 9 or claim 10, wherein the thermally conductive adhesive layer has a tensile strength that exceeds 0.5 MPa within 60 minutes of curing.

12. The battery module of any of claims 9 to 11, wherein each cell of the cell stack is a prismatic cell having an upper surface.

13. The battery module of any of claims 9 to 12, each cell comprises a dilatation grove positioned on its upper surface to accommodate excess thermally conductive adhesive.

14. A battery pack comprising a plurality of battery modules as claimed in any one of claims 9 to 12.

15. A vehicle comprising a battery pack as claimed in claim 14 or a battery module as claimed in any one of claims 9 to 13.

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