runaway avoidance Morgan Advanced Materials has been
researching and developing a range of thermal management protection materials and methods over many years. These can provide more time for occupants to exit a vehicle, while the dissipation of heat lessens the chance of thermal runaway spreading uncontrollably. It is not a one-size-fits-all approach, though. Every battery design is different, and so the protection method must therefore be unique. There are three levels of protection that engineers
can design into their systems to significantly reduce the impact of thermal runaway in electric vehicles: cell-to-cell, module-to-module, and battery pack level.
❱❱ Increased EV range doesn’t necessarily mean that thermal stability of battery cells is compromised, above; standard insulating paper is a common form of pack level protection for occupant safety, left; and insulation form depends upon the cell or module geometry, below and inset
CELL-TO-CELL Cell-to-cell protection involves designing a material to go between individual cells. It is the highest level of protection, but also the most challenging because of space constraints. If an individual cell experiences thermal runaway, the absorption of heat and deflection of flame from the protective materials minimise the thermal effects to adjacent cells. One of the most effective methods of protection at
cell level is by using phase change materials (PCMs), such as Morgan’s thermal insulation EST (energy storage technology) Superwool Block, a material that can be used for certain cell formats. PCMs absorb the heat of ruptured cells, as when the temperature of the cells gets too high, they turn the insulation material from either solid to liquid, or liquid to gas. During the phase change, the heat can be
dissipated throughout the body of the material. If the phase change is from solid to gas, this offers additional protection as the gas from the insulation material pushes the cell’s gases out through vents in the module, helping to lower the temperature more quickly. It is important to consider the cell’s shape when
specifying cell protection, as different cells have different insulation needs. Cells are split into three main types, cylindrical, prismatic and pouch. With cylindrical batteries, the insulation material can be solid shapes, but with pouch cells, they expand and contract, so you cannot use a rigid insulation to protect them. Prismatic cells can use either solid or flexible insulation materials.
MODULE-TO-MODULE There are several materials designed to go between modules, depending on the module size and design. Thermal runaway within the module can occur but can be contained to stop spread to adjacent modules.
In the module-to-module option, protection can
come in a paper format. Notably, module-to-module protection offers significant weight savings compared to cell-to-cell protection. Lighter batteries in turn increase the range and allow the battery to be more easily accommodated in the vehicle’s design.
PACK LEVEL PROTECTION Pack level protection is the simplest and most affordable type. This is aimed at improving safety for the vehicle’s occupants by giving them additional time to exit the vehicle, but provides little protection for the battery pack itself. That said, it is still a far better option than no protection at all. Standard insulating paper, such as Superwool
Plus Paper, is a common form of pack level protection.
PASSIVE THERMAL MANAGEMENT Automotive manufacturers have plenty of choice when specifying what level of thermal protection they want to use. These methods of thermal management are split between two categories: active and passive. Active thermal management denotes cooling
technologies that must introduce or remove energy using a substance to augment the heat transfer process. In electric vehicles, this includes air cooling, liquid cooling and refrigerant cooling, and involves an external device that helps with heat dissipation. These are generally more expensive and complex in comparison to passive thermal management techniques. Passive thermal management techniques on the
other hand, are technologies that rely upon thermo- dynamics of conduction, convection and radiation to transfer heat. Passive battery cooling technology includes metal heat sinks, phase change materials (PCMs) and specialised heat shields. These are typically cheaper than active thermal management technologies and are easier to implement. As cell-to-cell protection is generally the highest
level to achieve, Morgan’s Thermal Ceramics business has been testing and experimenting with different passive thermal management materials to learn how each one reacts in a thermal runaway situation. Those tested included foam insulation materials
that are intumescent which expand with temperature and endothermic materials that absorb energy upon exposure to heat. The test results showed that of those tested
endothermic materials are the best performing, whether that is with or without active cooling management. Foam materials do not perform well in thermal runaway situations. T&TH
March 2019 /// Testing & Test Houses /// 27
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