Alysha Liebscher and Gary Gayman of Morgan Advanced Materials discuss the phenomenon of thermal runaway in electric vehicles and the options available to manufacturers.
Electric and Hybrid vehicle production is going through high levels of growth as consumers become more aware of global sustainability issues. People are exploring alternative choices in transport and the cars they buy.
Despite this underlying willingness to move towards electric transport, there is still range anxiety and consumers are expecting greater charge capacity, faster charging rates and longer range.
To this end, automotive battery manufacturers are pouring huge effort into developing lithium-ion battery packs that can carry cars further and further. The current state-of-the-art is reflected by a Japanese car manufacturer that is likely to produce extended range versions of its vehicles this year with active thermal management.
Thermal management is key and while range is important for use, thermal management is vital to the actual safety of the battery, vehicle and its occupants. This is due to the phenomenon of thermal runaway, a dangerous reaction that can occur in lithium-ion batteries.
What is Thermal Runaway?
Increasing the range of an electric vehicle can be done in multiple ways. This includes having larger battery packs with more modules and cells, through to putting in higher energy density cells with higher capacity. However, all systems are susceptible to thermal runaway, some more so than others.
Each cell in a lithium-ion battery contains a flammable liquid electrolyte. If the cell short-circuits, the electrolyte can combust and the pressure within the cell will rapidly increase until the cell vents the flammable electrolyte. Temperatures at the ruptured cell can increase to above 1,000°C. This rapid and extreme rise in temperature is termed thermal runaway and when it initiates the same reaction in adjacent cells, it is known as thermal runaway propagation.
When thermal runaway happens, it can produce smoke, fire and even explosions. Occupants need to have as much time as possible to escape the vehicle if it does occur.
Although thermal runaway is life-threatening, to date there is yet to be global regulation in place. Whereas China has implemented the GB/T 31485 standard, the UN has only proposed legislation. This leaves automotive manufacturers with the choice of whether they want to design their battery packs with systems designed to deal with thermal runaway incidents. It’s up to their own risk assessment programmes to determine how likely thermal runaway is to occur.
Putting any protection in is likely to hinder the range capacity of the vehicle though – naturally, more protective materials equals less space for cells in a finite space.
Beyond the middle ground
Seemingly, there is no middle ground between range and thermal stability. However, it does not need to be the case that battery manufacturers compromise safety for range, or vice versa.
Morgan Advanced Materials has been significantly 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 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. Namely, these are cell-to-cell, module-to-module, and battery pack level.
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 due to 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.
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.
With module-to-module protection, 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 of protection. 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 is a common form of pack level protection, such as Superwool Plus Paper.
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 Management, and Passive Management.
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 of protection 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.
The materials tested included foam, insulation materials, Intumescent that expand with temperature and endothermic materials that absorb energy upon exposure to heat.
The test results showed that endothermic materials are the best performing from those tested, whether that is with or without active cooling management. Foam materials do not perform well in thermal runaway situations.