36 Initial computational models omit many estimations, meaning that a lot of backand-forth interaction with the adhesive developer is required in formulating, manufacturing and testing materials. The target performance is also generally not well defined because there can be considerable customisation, especially for structural encapsulation. Replacing cell holders with adhesive means that the design is almost always customised. These structural adhesives typically provide a bonding strength of over 15 MPa that is sufficient for most designs, but bonding to a chassis comprising more aluminium – for improved heat removal – means a greater mismatch of metals with joints that are more complicated than those of steel. This leads to the use of polyamide plastics, glass-filled nylons and materials similar to PET, although this monomer is slightly trickier to bond. Weight reduction is a major focus of the C2C approach, and considerable innovation is geared toward developing flame-retardant plastics to eliminate the use of metal parts. Plastics can provide substantial weight reduction and are becoming cheaper to form in certain shapes compared with the extrusion processes required for metal parts. However, these plastics must be capable of absorbing vibrations and impacts and have flame-retardant properties to avoid thermal runaway; all of which make these materials tricky to bond. This leads to more custom requirements for encapsulation. The materials need a certain amount of structural thermal management, but the encapsulation, or potting, materials might also need to be electrically and thermally isolating. In the encapsulant, the responsibility of thermal management is inherent in the design of the battery, meaning that heat is conducted away vertically to a hot plate or laterally via a cooling system. This means that the encapsulant around the cell is less relevant for thermal management. As long as the performance of the encapsulant is comparable to that of air, there will be minimal impact on the thermal management system. Additionally, the encapsulant must act as a thermal barrier to avoid heating of the surrounding cells in the event of thermal runaway. However, some car makers require the encapsulant to be thermally conductive and some versions therefore do conduct heat. Manufacturing flow The flowability of the encapsulant is important in C2C architecture because it must fill a much larger area compared with a module. Many companies want to achieve this in the shortest time possible, for example, in under two minutes, meaning that the material must travel a large distance across a complex geometry with consequent impacts on the flow characteristics. The viscosity must be low enough to allow injection into certain areas and efficient self-levelling before starting to rise, whilst still achieving the desired cycle times; this represents an important challenge. Once the car maker has the material, further optimisation is possible because they design the best way of incorporating the material into the vehicle, for example, May/June 2025 | E-Mobility Engineering Prismatic cells from StoreDot support a cell-to-chassis architecture (Image courtesy of StoreDot)
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