Thermal runaway in EV batteries is a critical concern with doubtlessly catastrophic penalties. When a single cell overheats due a fault it could actually set off a sequence response in adjoining cells, resulting in a violent exothermic occasion and doubtlessly critical hearth incident. Whereas passenger EVs drive the best battery demand amongst e-mobility, electrical buses pose vital security considerations resulting from their capability to hold quite a few passengers. Efficiently managing or eliminating thermal runaway propagation is vital to security. Sadly, thermal runaway is sort of unimaginable to foretell. It could possibly 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 price, there may be nonetheless presently a non-zero threat, which necessitates strong security measures.
“Documented hearth incidents have occurred throughout a number of OEM automobiles – from BYD to Tesla, Hyundai, and BMW, to call just a few,” says James Edmondson, analysis director at impartial 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 automobiles, there are much more older ICE automobiles on the highway, so the information may be deceptive. “As EVs age and their batteries degrade, it’s unknown what have an effect on this will likely have on pack parts, akin to cell face integrity, busbars, or insulators. So, the chance of fireplace might doubtlessly improve,” says Edmonson. “Early EV fashions employed minimal thermal administration by means 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 plenty of inactive supplies, so the necessity for multi-function supplies is changing into essential to supply decrease price and manufacturing simplicity,” says Edmonson. “An rising variety of gamers are coming to this market as a result of variety of completely different battery designs and applied sciences.”
Superior supplies for battery security
Present supplies used to insulate and shield cells embody ceramics, mica, aerogels, foams, encapsulants, coatings, and section change supplies. These may be utilized to the battery enclosure both externally, internally, or between the cells. Hearth safety strategies are carried out at each the cell degree and the pack degree.
On the pack degree, supplies akin to coatings, mica sheets (significantly 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 usually used amongst cylindrical cells, as seen in Tesla’s 4680 pack. Aerogels are gaining traction resulting from 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 purpose of those supplies is that they have to exhibit extraordinarily low thermal conductivity and low density to take care of weight effectivity and power density. Nonetheless, with the demand for elevated power density comes better cell power, so their skill to guard in opposition to 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 should not have to sacrifice house or power density to implement it. With such a excessive floor space, encapsulant foam is ideally fitted to cylindrical cell methods.
Encapsulant foams can usually 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 resulting from its efficiency.
“We’re working in a quickly evolving market the place developments embody elevated security, larger power density for longer vary, decrease manufacturing prices, stricter security laws, and a robust give attention to sustainability,” says Malene Valverde, enterprise growth supervisor at H.B. Fuller. “Once we first started the journey to determine on the best formulation for encapsulant know-how we tried to determine on an important properties related to efficiency of EV batteries.”
One of many major options is the flexibility of froth encapsulants to cease thermal propagation if there may be thermal runaway occurring. The fabric firstly should battle the fireplace, and secondly thermally shield the adjoining cells so in addition they don’t go into propagation themselves
“Now we have spent plenty of time and analysis to make sure the encapsulant foam know-how has a UL 94 V-0 flame score, the 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 fireplace, successfully defending the neighboring cells and suppressing hearth.
To fulfill the necessity for longer vary, present and future cells are packing much more power. Due to this fact, much more power is launched in a single occasion of catastrophe and sooner. As Valverde explains, the problem is evolving for H.B. Fuller, and its engineers are regularly exploring methods for its supplies to fulfill the must be more practical in propagation.
“We’ve performed testing on cylindrical in addition to prismatic cells. With fashionable cells changing into greater there may be much more power popping out of them, so we work with our prospects to find out how a lot materials – akin to 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 for the reason that Nineteen Thirties however they’re changing into the following large factor in battery cell insulation and hearth safety. They’re characterised by a posh morphology that consists of a community of pores and tunnels at a nanoscale, which supply a really excessive floor space at a really low density.
Aspen Aerogels, a pacesetter within the know-how, has seen its enterprise path change course due to the EV motion. Traditionally, it equipped its Aerogel know-how to the oil and gasoline 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 in a position to make use of it for a lot of fascinating functions. Aspen first used it as insulation for subsea power 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 flexibility of a typical thermal threshold of 900oC [1,652oF].
“We’re straight 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 power would be the lynchpin in electrifying our mobility options and power infrastructure.”
When a cell experiences a fault that results in an exothermic occasion, there are a number of of power pathways the thermal runaway takes, Gurian explains.
“Firstly, cell-to-cell conduction path. In a prismatic system, the massive cell face would be the first place the power will go. Conduction from cell-to-cell may be lower than 10% of the whole exothermic potential of this occasion. The remainder goes by means of secondary conduction paths, the busbars, the cooling plate.
“The remaining 70-90% of the whole 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 power is among the largest challenges,” he says.
Insulating cells is, , it may be argued, a comparatively straightforward 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 not be retrofitted after design,” he says. “The issue is there isn’t any house in packs or modules to individually ‘plumb’ the cells to a vent. Nobody is individually onerous piping to the cells as a result of we’re working with millimeters of house inside a module or pack. So, one answer that has been carried out is, as a substitute 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 affords an identical 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 preserve the cell face strain for the mechanical retention, and life, of the cells.
One other subject many battery producers face is that with insulation and thermal safety comes at a trade-off of both mass weight or cells, which reduces power and vary.
“PU foam is main for its low density, however Pyrothin has the identical density and in addition robust thermal runaway propagation delay better than 5 minutes,” says Gurian. “We carry out a lot of testing, the place we’re C2C thermal conduction occasions. Utilizing sure compressed thickness in opposition to cell areal power density we are able to obtain an infinite delay, so basically zero-propagation.”
Future battery know-how hearth safety
Rising battery applied sciences and chemistries, akin to sodium-ion and solid-state batteries, promise vital enhancements however nonetheless require strong hearth safety. Sodium-ion, whereas cheaper and safer in preliminary checks, nonetheless presents thermal runaway dangers. Stable-state batteries, regardless of their larger security profile, pose design challenges resulting from their selection and potential for high-temperature occasions.
“Sodium-ion continues to be at comparatively early growth phases, resulting from a decrease power density than lithium-ion,” says Edmonson. “Nonetheless, sodium is extra considerable and is cheaper than lithium. Like LFP know-how, preliminary penetration checks on sodium-ion cells have efficiently yielded no ignition or flames.
“The opposite good thing about sodium-ion is, not like lithium-ion, it may be transported at 0V with out damaging its efficiency, that means it may be transported with no cost and fewer threat of fireplace risk. 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 threat continues to be a threat, 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’ll nonetheless burn at excessive temperatures and have a better power density, posing a threat for inner quick circuit situations.
“Early research present the utmost temperatures reached in a thermal runaway situation are proportional to the quantity of power saved,” says Edmonson. “So, by storing extra power 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 enormous 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 crucial,” he concludes.
New battery developments and challenges
The e-mobility trade is shifting in the direction of larger power 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 nice customization and scalability. Nonetheless, there’s a shift in the direction of architectures with out intermediate modules, akin to cell-to-pack (CTP) and cell-to-chassis (CTC), which supply better power density and decreased manufacturing prices. These new architectures require structural assist, which multi-functional materials applied sciences are aiming to supply, alongside thermal safety.
“Now we have pushed the boundaries of what PU foam can do by way of structural skill. Now we have developed merchandise starting from very delicate to very inflexible, all retaining their toughness and integrity. Importantly, all these options are achievable at a really ultralight weight, able to a particularly 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 entire battery meeting and is agnostic to battery know-how. Nonetheless, even with developments in ‘safer’ cell chemistry, akin to solid-state or sodium-ion, they’re nonetheless in danger from a thermal occasion.
“Plenty of efforts in cell chemistry enchancment purpose to make cells safer, lowering the variety of runaway occasions or releasing much less power at cooler temperatures. Nonetheless, all consultants agree that every one cell applied sciences and chemistries nonetheless require hearth safety,” states Valverde.
Equally, Aspen Aerogel’s Gurian believes newer battery applied sciences with new types and chemistries face additional elevated temperatures for cell faces in thermal runaway.
“One of many greater challenges is cells which might be much more dynamic mechanically than what now we have 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 usually are not distinctive to aerogel, but in addition for foam obstacles. These will pose design challenges.”
“As we take a look at extra built-in battery designs, it’s going to be vital to have supplies which have multiple operate. For instance, should you can have an aerogel that gives thermal insulation, hearth safety, and compression you could possibly doubtlessly scale back the whole variety of supplies within the battery pack, which might decrease manufacturing complexity and value.”
