68 May/June 2024 | E-Mobility Engineering through the external electric circuit. This prevents further heat generation within the cell. Mechanical shutdown mechanisms respond to excessive pressure within a cell arising from gas generation during an incipient thermal runaway by deforming to block ion flow. Essentially, the same job can be done chemically through the inclusion of additives within the separator that respond to abnormal conditions, such as excessive temperatures or charge states, by undergoing chemical reactions that lead to increased resistance or ion-flow blockage. It is important for the cell that has entered thermal runaway to be vented in a controlled manner to relieve pressure. Bottom-venting cells are a useful advance in design. They can discharge their exhaust gases down through a perforated cooling plate, making the gas management problem much easier to solve. Venting is just as critical in other areas of the pack. Pressure relief valves protect housings against mechanical damage that may occur in the event of pressure differences between the interior of the housing and the environment. They also seal against external liquids and dirt. One company we consulted has developed a new pressure relief valve for venting battery housings in case of sudden overpressure due to thermal runaway, which helps to prevent thermal propagation. Protection of surrounding cells from the effects of heat and particle bombardment from a failing cell requires heat-resistant materials with low thermal conductivity and high mechanical strength. Propagation barrier materials Typically, there are five main challenges involved in preventing propagation during thermal runaway, an expert says, and the selection of materials has an impact on all of them. The first is conductive heat transfer from cell to cell, while heat transfer through secondary components such as module housings is the second. The third is the ejection of hot gas and/or flame from the failing cell, which heats adjacent cells. Particles ejected from the cell, causing failures in adjacent cells or modules through short-circuiting, for example, pose the fourth. Finally, electrical arcing can damage nearby cells and other components, and ignite flammable gasses, an expert says. “Typical solutions for cell-to-cell heat transfer include aerogels and silicone foams, with aerogels signi cantly increasing in market share due to their strong performance. Other solutions are being developed, depending on cell format, such as potting resins and silicones. “The other issues are best addressed using robust thermal and electrical insulation, varying depending on the position of use and the properties required, such as processability and dielectric properties,” the previous expert says. “A large suite of solutions is available, such as mica composites, carbon-fibre composites, meta-aramids, polyimides or moulded plastics, all of which depend on the requirements at part level as to which is most suited.” The two most commonly used containment and mitigation measures are thermal propagation protection between modules and cells, and the other is fire containment within the pack, notes another expert. In the first solution, aerogel, mica or fire-resistant foams are used between cells or modules to minimise heat transfer. For the second measure, fire-protection pads or coatings are applied on the casing area, such as the battery lid, to minimise the heating of the substrate and prevent burn-through. In pouch or prismatic packs, ensuring a thermal runaway is contained to a single cell or a group of cells requires a high-temperature thermal insulation, such as an aerogel, to act as a firewall. A cell-to-cell barrier acts as the first line of defence. Any measure that could immediately change the temperature, cut access to oxygen within the cell, and immediately drain or dry the flammable electrolyte could help in theory,” another expert says. “But in practice these measures are not really widely available; hence the focus should be on containment.” Another option is rapid cooling, which prevents further propagation and can sometimes even stop the runaway. Rapid cooling can be practical in certain applications, such as stationary storage or commercial vehicles with segmented energy storage, according to an expert. “The main barrier is ensuring the cooling system is sufficiently isolated Explosive failure of a lithium-ion cell under test, with ejection of hot gas and flame (Image courtesy of US CDC)
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