Thermal runaway in EV batteries is a critical concern with doubtlessly catastrophic penalties. When a single cell overheats due a fault it might set off a series response in adjoining cells, resulting in a violent exothermic occasion and doubtlessly critical hearth incident. Whereas passenger EVs drive the very best battery demand amongst e-mobility, electrical buses pose vital security issues as a consequence of their capability to hold quite a few passengers. Efficiently managing or eliminating thermal runaway propagation is essential to security. Sadly, thermal runaway is sort of unattainable to foretell. It could actually happen in any situation, from regular driving operation, after a crash, throughout charging, and even when the car is parked. Regardless of the low incidence fee, there may be nonetheless at present a non-zero danger, which necessitates strong security measures.
“Documented hearth incidents have occurred throughout a number of OEM autos – from BYD to Tesla, Hyundai, and BMW, to call just a few,” says James Edmondson, analysis director at unbiased researcher IDTechEx. “Early in 2024, three electrical bus fires in London triggered security checks on 400 buses in service.”
Regardless of the decrease hearth incidence in EVs in comparison with ICE autos, there are much more older ICE autos on the highway, so the information might be deceptive. “As EVs age and their batteries degrade, it’s unknown what have an effect on this will have on pack parts, resembling cell face integrity, busbars, or insulators. So, the danger of fireplace might doubtlessly enhance,” says Edmonson. “Early EV fashions employed minimal thermal administration by way of air-cooled battery methods, whereas newer fashions use more practical lively liquid cooling.”
Whereas engineers are tasked with stopping or delaying thermal runaway from spreading from a defective cell to adjoining cells, battery designs are evolving and due to this fact shifting the goalposts for what they should contemplate.
“As batteries transfer in the direction of better ranges of integrations, they’re eradicating quite a lot of inactive supplies, so the necessity for multi-function supplies is turning into essential to supply decrease price and manufacturing simplicity,” says Edmonson. “An growing variety of gamers are coming to this market as a result of variety of totally different battery designs and applied sciences.”
Superior supplies for battery security
Present supplies used to insulate and defend cells embody ceramics, mica, aerogels, foams, encapsulants, coatings, and section change supplies. These might be utilized to the battery enclosure both externally, internally, or between the cells. Fireplace safety strategies are carried out at each the cell degree and the pack degree.
On the pack degree, supplies resembling coatings, mica sheets (notably efficient for prismatic cell codecs, as utilized by SAIC), ceramic papers, and blankets are employed between cells and the enclosure lid. These supplies function obstacles which might be designed to inhibit the switch of warmth from one cell to a different, thus stopping the unfold of thermal runaway all through the pack.
For cell-level safety, encapsulating foams are sometimes used amongst cylindrical cells, as seen in Tesla’s 4680 pack. Aerogels are gaining traction as a consequence of their low density and very low thermal conductivity. Different supplies embody powder coatings and intumescent coatings that broaden with warmth, and compression pads with fire-protective components for pouch cells.
The aim of those supplies is that they have to exhibit extraordinarily low thermal conductivity and low density to take care of weight effectivity and vitality density. Nonetheless, with the demand for elevated vitality density comes better cell vitality, so their capability to guard towards excessive excessive heats and for the way lengthy are going to be a decisive issue for OEMs, plus their market price at scale.
Cylindrical Cells
Foam encapsulants are injected as a liquid that expands and turns into semi-rigid across the cells and into every a part of the battery pack. As the froth materials matches into very tight areas, battery designers wouldn’t have to sacrifice house or vitality density to implement it. With such a excessive floor space, encapsulant foam is ideally fitted to cylindrical cell methods.
Encapsulant foams can sometimes be present in epoxy or silicone foam kind, however for H.B. Fuller, an organization that has been specializing in speciality chemical merchandise throughout a number of industries for 130 years, it has opted for polyurethane (PU) foam for its EV Defend encapsulant platform as a consequence of its efficiency.
“We’re working in a quickly evolving market the place traits embody elevated security, larger vitality density for longer vary, decrease manufacturing prices, stricter security rules, and a robust concentrate on sustainability,” says Malene Valverde, enterprise improvement supervisor at H.B. Fuller. “Once we first started the journey to resolve on the suitable formulation for encapsulant know-how we tried to resolve on an important properties related to efficiency of EV batteries.”
One of many essential options is the power of froth encapsulants to cease thermal propagation if there may be thermal runaway occurring. The fabric firstly should combat the hearth, and secondly thermally defend the adjoining cells so in addition they don’t go into propagation themselves
“We have now spent quite a lot of time and analysis to make sure the encapsulant foam know-how has a UL 94 V-0 flame score, the very best flame safety score,” says Valverde. “We additionally consider how the fabric will act in an explosion and the way it handles the quantity of venting scorching fumes.”
The PU foam materials, when uncovered to open flame, undergoes a chemical change that actively fights the hearth, successfully defending the neighboring cells and suppressing hearth.
To satisfy the necessity for longer vary, present and future cells are packing much more vitality. Due to this fact, much more vitality is launched in a single occasion of catastrophe and quicker. As Valverde explains, the problem is evolving for H.B. Fuller, and its engineers are frequently exploring methods for its supplies to satisfy the have to be more practical in propagation.
“We’ve performed testing on cylindrical in addition to prismatic cells. With trendy cells turning into larger there may be much more vitality popping out of them, so we work with our prospects to find out how a lot materials – resembling foam thickness – must be within the neighborhood of the cells to assist cease propagation.”
Defending prismatic and pouch cells
Aerogels are a chemically manufactured stable that has been round because the Nineteen Thirties however they’re turning into the subsequent large factor in battery cell insulation and hearth safety. They’re characterised by a fancy morphology that consists of a community of pores and tunnels at a nanoscale, which provide a really excessive floor space at a really low density.
Aspen Aerogels, a frontrunner within the know-how, has seen its enterprise course change course due to the EV motion. Traditionally, it provided its Aerogel know-how to the oil and fuel trade and is now working in main quantity manufacturing with automotive OEMs throughout the globe together with Toyota, Audi, Scania Vans, and Basic Motors as a part of its Ultium platform. At its native kind, aerogels are fairly brittle, however by incorporating the aerogel into a versatile composite utilizing a fiber strengthened submatrix Aspen was capable of make use of it for a lot of attention-grabbing purposes. Aspen first used it as insulation for subsea vitality pipelines and passive hearth safety merchandise designed for extended excessive temperature publicity.
In 2020, Aspen heard accounts of its merchandise being utilized in EV battery pack and module housings, in addition to cell-to-cell obstacles. As such, Aspen launched its Pyrothin product – a thermal barrier, particularly designed to suit between cells in a battery pack and has the power of a typical thermal threshold of 900oC [1,652oF].
“We’re instantly working with OEMs as they attempt to delay, or in some circumstances, totally cease, the propagation of thermal runaway in a pack,” says Tyler Gurian, senior program engineer at Aspen Aerogels. “The power to efficiently handle this switch of vitality would be the lynchpin in electrifying our mobility options and vitality infrastructure.”
When a cell experiences a fault that results in an exothermic occasion, there are a number of of vitality pathways the thermal runaway takes, Gurian explains.
“Firstly, cell-to-cell conduction path. In a prismatic system, the big cell face would be the first place the vitality will go. Conduction from cell-to-cell might be lower than 10% of the full exothermic potential of this occasion. The remaining goes by way of secondary conduction paths, the busbars, the cooling plate.
“The remaining 70-90% of the full warmth launch is popping out of the cell within the type of solid-liquid-gas plasmatic ejecta that spews out of the cell. It’s an aggressive occasion when these cells expertise thermal runaway. Managing this vitality is likely one of the greatest challenges,” he says.
Insulating cells is, , it may be argued, a comparatively simple activity. It may be achieved with the quite a few supplies available on the market from ceramics, mica, foams, encapsulants, coatings, section change supplies, and extra. The largest problem to a non-propagating cell design is managing the cell vent gases, Gurian believes.
“That is one thing that can’t be retrofitted after design,” he says. “The issue is there is no such thing as a house in packs or modules to individually ‘plumb’ the cells to a vent. Nobody is individually arduous piping to the cells as a result of we’re working with millimeters of house inside a module or pack. So, one resolution that has been carried out is, as an alternative of plumbing, compartmentalize the cells, particularly for pouch cells which aren’t as courteous as prismatic cells as they don’t have devoted failure factors.”
In a cell-to-cell system, extending the obstacles between the cells provides the same setup to bulkheads within the hull of an ocean liner. It seals every house off within the occasion of propagation.
One other standards Aerogels should face is how lithium-ion cells broaden and contract as they warmth up throughout charging or discharge throughout use. The C2C barrier compression pad is being squeezed and should keep the cell face stress for the mechanical retention, and life, of the cells.
One other difficulty many battery producers face is that with insulation and thermal safety comes at a trade-off of both mass weight or cells, which reduces vitality and vary.
“PU foam is main for its low density, however Pyrothin has the identical density and likewise sturdy thermal runaway propagation delay better than 5 minutes,” says Gurian. “We carry out plenty of testing, the place we’re C2C thermal conduction occasions. Utilizing sure compressed thickness towards cell areal vitality density we are able to obtain an infinite delay, so primarily zero-propagation.”
Future battery know-how hearth safety
Rising battery applied sciences and chemistries, resembling sodium-ion and solid-state batteries, promise vital enhancements however nonetheless require strong hearth safety. Sodium-ion, whereas cheaper and safer in preliminary exams, nonetheless presents thermal runaway dangers. Stable-state batteries, regardless of their larger security profile, pose design challenges as a consequence of their selection and potential for high-temperature occasions.
“Sodium-ion continues to be at comparatively early improvement phases, as a consequence of a decrease vitality density than lithium-ion,” says Edmonson. “Nonetheless, sodium is extra considerable and is cheaper than lithium. Like LFP know-how, preliminary penetration exams on sodium-ion cells have efficiently yielded no ignition or flames.
“The opposite good thing about sodium-ion is, in contrast to lithium-ion, it may be transported at 0V with out damaging its efficiency, which means it may be transported with no cost and fewer danger of fireplace menace. Nonetheless, thermal runaway and cell-to-cell propagation nonetheless exist with sodium-ion; it’s simply these occasions provoke at larger temperatures and are much less violent and slower to unfold. A low danger continues to be a danger, so hearth safety supplies will nonetheless be required.”
Stable-state batteries additionally current distinctive challenges. They substitute the flammable natural liquid electrolyte with a non-flammable solid-state electrolyte, doubtlessly spreading warmth extra effectively and working inside a wider temperature vary. Nonetheless, they will nonetheless burn at excessive temperatures and have the next vitality density, posing a danger for inner brief circuit situations.
“Early research present the utmost temperatures reached in a thermal runaway situation are proportional to the quantity of vitality saved,” says Edmonson. “So, by storing extra vitality in a smaller pack house, the temperatures could possibly be even larger than we see with lithium-ion methods.
“Stable-state batteries aren’t a single know-how. There are big varieties of various chemistries going into them. Many are utilizing a sulfide-based solid-state electrolyte, which might doubtlessly kind hydrogen-sulfides within the occasion of a difficulty. Stable-state batteries are more likely to be a lot safer however nonetheless not 100% secure, so the system design and hearth safety supplies will nonetheless be obligatory,” he concludes.
New battery traits and challenges
The e-mobility trade is transferring in the direction of larger vitality density batteries, which current new challenges for thermal administration. A decade in the past, most EV battery packs have been small, no matter their format (cylindrical, prismatic, or pouch). Right now, the commonest structure is module-to-pack (MTP), permitting for excellent customization and scalability. Nonetheless, there’s a shift in the direction of architectures with out intermediate modules, resembling cell-to-pack (CTP) and cell-to-chassis (CTC), which provide better vitality density and diminished manufacturing prices. These new architectures require structural assist, which multi-functional materials applied sciences are aiming to supply, alongside thermal safety.
“We have now pushed the boundaries of what PU foam can do when it comes to structural capability. We have now developed merchandise starting from very gentle to very inflexible, all retaining their toughness and integrity. Importantly, all these options are achievable at a really ultralight weight, able to an especially excessive compression modulus at a really low density of 0.4g/cm3 or much less, which is exclusive,” H.B Fuller’s Malene Valverde explains.
Encapsulant foam acts as a unifying assist for your complete battery meeting and is agnostic to battery know-how. Nonetheless, even with developments in ‘safer’ cell chemistry, resembling solid-state or sodium-ion, they’re nonetheless in danger from a thermal occasion.
“A whole lot of efforts in cell chemistry enchancment intention to make cells safer, decreasing the variety of runaway occasions or releasing much less vitality at cooler temperatures. Nonetheless, all consultants agree that each one cell applied sciences and chemistries nonetheless require hearth safety,” states Valverde.
Equally, Aspen Aerogel’s Gurian believes newer battery applied sciences with new varieties and chemistries face additional elevated temperatures for cell faces in thermal runaway.
“One of many larger challenges is cells which might be much more dynamic mechanically than what we’ve now in quantity manufacturing. These cells are respiration within the order of 10% of their full thickness, i.e. a 30mm cell respiration 3mm for every cycle. These mechanical challenges aren’t distinctive to aerogel, but in addition for foam obstacles. These will pose design challenges.”
“As we have a look at extra built-in battery designs, it’s going to be necessary to have supplies which have a couple of operate. For instance, when you can have an aerogel that gives thermal insulation, hearth safety, and compression you could possibly doubtlessly scale back the full variety of supplies within the battery pack, which might decrease manufacturing complexity and price.”
