Thermal runaway in EV batteries is a serious concern with potentially catastrophic consequences. When a single cell overheats due a fault it can trigger a chain reaction in adjacent cells, leading to a violent exothermic event and potentially serious fire incident. While passenger EVs drive the highest battery demand among e-mobility, electric buses pose significant safety concerns due to their capacity to carry numerous passengers. Successfully managing or eliminating thermal runaway propagation is key to safety. Unfortunately, thermal runaway is almost impossible to predict. It can occur in any scenario, from normal driving operation, after a crash, during charging, or even when the vehicle is parked. Despite the low incidence rate, there is still currently a non-zero risk, which necessitates robust safety measures.
“Documented fire incidents have occurred across multiple OEM vehicles – from BYD to Tesla, Hyundai, and BMW, to name a few,” says James Edmondson, research director at independent researcher IDTechEx. “Early in 2024, three electric bus fires in London triggered safety checks on 400 buses in service.”
Despite the lower fire incidence in EVs compared to ICE vehicles, there are far more older ICE vehicles on the road, so the data can be misleading. “As EVs age and their batteries degrade, it’s unknown what affect this may have on pack components, such as cell face integrity, busbars, or insulators. So, the risk of fire could potentially increase,” says Edmonson. “Early EV models employed minimal thermal management through air-cooled battery systems, whereas newer models use more effective active liquid cooling.”
While engineers are tasked with preventing or delaying thermal runaway from spreading from a faulty cell to adjacent cells, battery designs are evolving and therefore shifting the goalposts for what they need to consider.
“As batteries move towards greater levels of integrations, they are removing a lot of inactive materials, so the need for multi-function materials is becoming very important to provide lower cost and manufacturing simplicity,” says Edmonson. “An increasing number of players are coming to this market due to the number of different battery designs and technologies.”
Advanced materials for battery safety
Current materials used to insulate and protect cells include ceramics, mica, aerogels, foams, encapsulants, coatings, and phase change materials. These can be applied to the battery enclosure either externally, internally, or between the cells. Fire protection methods are implemented at both the cell level and the pack level.
At the pack level, materials such as coatings, mica sheets (particularly effective for prismatic cell formats, as used by SAIC), ceramic papers, and blankets are employed between cells and the enclosure lid. These materials serve as barriers that are designed to inhibit the transfer of heat from one cell to another, thus preventing the spread of thermal runaway throughout the pack.
For cell-level protection, encapsulating foams are typically used among cylindrical cells, as seen in Tesla’s 4680 pack. Aerogels are gaining traction due to their low density and extremely low thermal conductivity. Other materials include powder coatings and intumescent coatings that expand with heat, and compression pads with fire-protective additives for pouch cells.
The goal of these materials is that they must exhibit extremely low thermal conductivity and low density to maintain weight efficiency and energy density. However, with the demand for increased energy density comes greater cell energy, so their ability to protect against extreme high heats and for how long are going to be a decisive factor for OEMs, plus their market cost at scale.
Cylindrical Cells
Foam encapsulants are injected as a liquid that expands and becomes semi-rigid around the cells and into each part of the battery pack. As the foam material fits into very tight spaces, battery designers do not have to sacrifice space or energy density to implement it. With such a high surface area, encapsulant foam is ideally suited for cylindrical cell systems.
Encapsulant foams can typically be found in epoxy or silicone foam form, but for H.B. Fuller, a company that has been specializing in speciality chemical products across multiple industries for 130 years, it has opted for polyurethane (PU) foam for its EV Protect encapsulant platform due to its performance.
“We are working in a rapidly evolving market where trends include increased safety, higher energy density for longer range, lower manufacturing costs, stricter safety regulations, and a strong focus on sustainability,” says Malene Valverde, business development manager at H.B. Fuller. “When we first began the journey to decide on the right formulation for encapsulant technology we tried to decide on the most important properties associated to performance of EV batteries.”
One of the main features is the ability of foam encapsulants to stop thermal propagation if there is thermal runaway occurring. The material firstly must fight the fire, and secondly thermally protect the adjacent cells so they also don’t go into propagation themselves
“We have spent a lot of time and research to ensure the encapsulant foam technology has a UL 94 V-0 flame rating, the highest flame protection rating,” says Valverde. “We also evaluate how the material will act in an explosion and how it handles the amount of venting hot fumes.”
The PU foam material, when exposed to open flame, undergoes a chemical change that actively fights the fire, effectively protecting the neighboring cells and suppressing fire.
To meet the need for longer range, current and future cells are packing a lot more energy. Therefore, a lot more energy is released in a single event of disaster and faster. As Valverde explains, the challenge is evolving for H.B. Fuller, and its engineers are continually exploring ways for its materials to meet the need to be more effective in propagation.
“We’ve conducted testing on cylindrical as well as prismatic cells. With modern cells becoming bigger there is a lot more energy coming out of them, so we work with our customers to determine how much material – such as foam thickness – needs to be in the vicinity of the cells to help stop propagation.”
Protecting prismatic and pouch cells
Aerogels are a chemically manufactured solid that has been around since the 1930s but they are becoming the next big thing in battery cell insulation and fire protection. They are characterized by a complex morphology that consists of a network of pores and tunnels at a nanoscale, which offer a very high surface area at a very low density.
Aspen Aerogels, a leader in the technology, has seen its business direction change course because of the EV movement. Historically, it supplied its Aerogel technology to the oil and gas industry and is now working in major volume production with automotive OEMs across the globe including Toyota, Audi, Scania Trucks, and General Motors as part of its Ultium platform. At its native form, aerogels are quite brittle, but by incorporating the aerogel into a flexible composite using a fiber reinforced submatrix Aspen was able to employ it for many interesting applications. Aspen first used it as insulation for subsea energy pipelines and passive fire protection products designed for prolonged high temperature exposure.
In 2020, Aspen heard accounts of its products being applied in EV battery pack and module housings, as well as cell-to-cell barriers. As such, Aspen introduced its Pyrothin product – a thermal barrier, specifically designed to fit between cells in a battery pack and has the ability of a typical thermal threshold of 900oC [1,652oF].
“We are directly working with OEMs as they try to delay, or in some cases, fully stop, the propagation of thermal runaway in a pack,” says Tyler Gurian, senior program engineer at Aspen Aerogels. “The ability to successfully manage this transfer of energy will be the lynchpin in electrifying our mobility solutions and energy infrastructure.”
When a cell experiences a fault that leads to an exothermic event, there are several of energy pathways the thermal runaway takes, Gurian explains.
“Firstly, cell-to-cell conduction path. In a prismatic system, the large cell face will be the first place the energy will go. Conduction from cell-to-cell can be less than 10% of the total exothermic potential of this event. The rest goes through secondary conduction paths, the busbars, the cooling plate.
“The remaining 70-90% of the total heat release is coming out of the cell in the form of solid-liquid-gas plasmatic ejecta that spews out of the cell. It is an aggressive event when these cells experience thermal runaway. Managing this energy is one of the biggest challenges,” he says.
Insulating cells is, , it can be argued, a relatively easy task. It can be achieved with the numerous materials on the market from ceramics, mica, foams, encapsulants, coatings, phase change materials, and more. The biggest challenge to a non-propagating cell design is managing the cell vent gases, Gurian believes.
“This is something that cannot be retrofitted after design,” he says. “The problem is there is no space in packs or modules to individually ‘plumb’ the cells to a vent. No one is individually hard piping to the cells because we are working with millimeters of space within a module or pack. So, one solution that has been implemented is, instead of plumbing, compartmentalize the cells, especially for pouch cells which are not as courteous as prismatic cells as they do not have dedicated failure points.”
In a cell-to-cell system, extending the barriers between the cells offers a similar setup to bulkheads in the hull of an ocean liner. It seals each space off in the event of propagation.
Another criteria Aerogels must face is how lithium-ion cells expand and contract as they heat up during charging or discharge during use. The C2C barrier compression pad is being squeezed and must maintain the cell face pressure for the mechanical retention, and life, of the cells.
Another issue many battery manufacturers face is that with insulation and thermal protection comes at a trade-off of either mass weight or cells, which reduces energy and range.
“PU foam is leading for its low density, but Pyrothin has the same density and also strong thermal runaway propagation delay greater than five minutes,” says Gurian. “We perform lots of testing, where we’re looking at C2C thermal conduction events. Using certain compressed thickness against cell areal energy density we can achieve an infinite delay, so essentially zero-propagation.”
Future battery technology fire protection
Emerging battery technologies and chemistries, such as sodium-ion and solid-state batteries, promise significant improvements but still require robust fire protection. Sodium-ion, while cheaper and safer in initial tests, still presents thermal runaway risks. Solid-state batteries, despite their higher safety profile, pose design challenges due to their variety and potential for high-temperature events.
“Sodium-ion is still at relatively early development stages, due to a lower energy density than lithium-ion,” says Edmonson. “However, sodium is more abundant and is cheaper than lithium. Like LFP technology, initial penetration tests on sodium-ion cells have successfully yielded no ignition or flames.
“The other benefit of sodium-ion is, unlike lithium-ion, it can be transported at 0V without damaging its performance, meaning it can be transported with no charge and less risk of fire threat. However, thermal runaway and cell-to-cell propagation still exist with sodium-ion; it’s just these events initiate at higher temperatures and are less violent and slower to spread. A low risk is still a risk, so fire protection materials will still be required.”
Solid-state batteries also present unique challenges. They replace the flammable organic liquid electrolyte with a non-flammable solid-state electrolyte, potentially spreading heat more efficiently and operating within a wider temperature range. However, they can still burn at high temperatures and have a higher energy density, posing a risk for internal short circuit scenarios.
“Early studies show the maximum temperatures reached in a thermal runaway scenario are proportional to the amount of energy stored,” says Edmonson. “So, by storing more energy in a smaller pack space, the temperatures could be even higher than we see with lithium-ion systems.
“Solid-state batteries aren’t a single technology. There are huge varieties of different chemistries going into them. Many are using a sulfide-based solid-state electrolyte, which could potentially form hydrogen-sulfides in the event of an issue. Solid-state batteries are likely to be much safer but still not 100% safe, so the system design and fire protection materials will still be necessary,” he concludes.
New battery trends and challenges
The e-mobility industry is moving towards higher energy density batteries, which present new challenges for thermal management. A decade ago, most EV battery packs were small, regardless of their format (cylindrical, prismatic, or pouch). Today, the most common architecture is module-to-pack (MTP), allowing for great customization and scalability. However, there’s a shift towards architectures without intermediate modules, such as cell-to-pack (CTP) and cell-to-chassis (CTC), which offer greater energy density and reduced manufacturing costs. These new architectures require structural support, which multi-functional material technologies are aiming to provide, alongside thermal protection.
“We have pushed the boundaries of what PU foam can do in terms of structural ability. We have developed products ranging from very soft to very rigid, all retaining their toughness and integrity. Importantly, all these features are achievable at a very ultralight weight, capable of an extremely high compression modulus at a very low density of 0.4g/cm3 or less, which is unique,” H.B Fuller’s Malene Valverde explains.
Encapsulant foam acts as a unifying support for the entire battery assembly and is agnostic to battery technology. However, even with advancements in ‘safer’ cell chemistry, such as solid-state or sodium-ion, they are still at risk from a thermal event.
“A lot of efforts in cell chemistry improvement aim to make cells safer, reducing the number of runaway events or releasing less energy at cooler temperatures. However, all experts agree that all cell technologies and chemistries still require fire protection,” states Valverde.
Similarly, Aspen Aerogel’s Gurian believes newer battery technologies with new forms and chemistries face further elevated temperatures for cell faces in thermal runaway.
“One of the bigger challenges is cells that are even more dynamic mechanically than what we have now in volume production. These cells are breathing in the order of 10% of their full thickness, i.e. a 30mm cell breathing 3mm for each cycle. These mechanical challenges are not unique to aerogel, but also for foam barriers. These will pose design challenges.”
“As we look at more integrated battery designs, it’s going to be important to have materials that have more than one function. For example, if you can have an aerogel that provides thermal insulation, fire protection, and compression you could potentially reduce the total number of materials in the battery pack, which would lower manufacturing complexity and cost.”
