from the thermal runaway event to remain active, and ensuring there is enough cooling power from the system available to arrest the event and extract heat driving the process,” he adds. “This is often only practicable in an automotive application using immersion cooling, which remains an engineering challenge to produce efficiently at large scale with an acceptable cost and weight for mainstream applications.” In addition to the density of the cells and modules in the pack, and the insulation between them and on the lid, the material of the lid itself is important to containment, the expert says. One common choice is electro-coated steel with a melting point above 1,500 C, while other substrates such as aluminium and some polymer composites have much lower melting temperatures. Also important are interface materials, including adhesives, with high thermal conductivity to ensure good heat transfer from the cells to heat sinks or cooling plates. Preventing unwanted heat transfer with materials, such as thermally insulating materials and flame-retardant barriers placed between the cells, is also critical. Such materials include silicone potting, and foam sheets placed between the battery cover and the cells. Silicones do not generate toxic smoke, so they help provide extra time for passengers to exit the vehicle once a runaway has started, notes an expert in these materials. Further, electrical insulation between cells using coating or other barriers is vital to prevent electrical short circuits or arcing between cells and other electrical components. Material advances Progress in the materials used in cells and packs has been substantial in recent years, but there is more to come. As the industry develops new and better materials to include in battery packs, they must be thin enough to fit into the tight gaps between cells, or between the cell and cover, to maximise pack-level energy density without compromising their runaway mitigation performance. Because thermal runaway presents both fire and electrical hazards, multifunction materials that can withstand high temperatures and achieve strong dielectric performance are becoming increasingly important. Furthermore, the material should be easily applicable at industrial scale through automation. Our expert’s company is active in passive fire-protection coatings that are only activated during a thermal runaway event, so they can maintain a low thickness at ambient conditions. The material is mostly applied in areas where the substrate needs to be insulated, such as the battery cover/lid or frames, or the module cover, he explains. One such coating can be triggered by heat exposure. It absorbs heat for phase-changing and forms a foam-like structure with air pockets to insulate the substrate from fire. The expert says it contains thermal insulation/ heat-refractive pigments to minimise thermal conductivity, which minimises heat transfer from the cell fire to the substrate. He says the material can be sprayed via airless application or flat stream/ dispensing, and it has already been industrialised for years on customers’ automated lines. More recently, the company has developed a new coating with similar insulating and protective capabilities, but it expands much less during a thermal runaway so that it fits into compact battery designs better. It also offers much better dielectric insulation performance and durability against ageing, the expert adds. Besides aerogels, a wide range of materials – from mica, CFRP and metals as barriers to foams as cell-level insulation and propagation mitigation – are either proposed or in active use today, an expert says, adding: “Foam has been one of the most promising approaches and, although the idea originated in the 2010s, it has just recently been put to use by Tesla.” Silicone is well positioned for the compromise between density, fire protection, thermal resistance and mechanical properties. Foams formed from silicone are also very light, contributing to weight savings in the battery pack. Further, their application is easily automated, our expert says, as their two liquid components are mixed and injected into the pack, flowing and expanding to fill the gaps between the cells, and curing at room temperature if required by the manufacturing process. “Aerogel is the answer for stopping thermal propagation or mitigating Embedding AI into a BMS can give longer warnings of thermal runaways (Image courtesy of Eatron) Thermal runaway prevention | Focus 69 E-Mobility Engineering | May/June 2024
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