Thermal runaway in EV batteries is a critical concern with doubtlessly catastrophic penalties. When a single cell overheats due a fault it may possibly 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 important security considerations 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 may happen in any state of affairs, from regular driving operation, after a crash, throughout charging, and even when the automobile is parked. Regardless of the low incidence price, there’s nonetheless at the moment 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 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 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 may increasingly have on pack parts, similar to cell face integrity, busbars, or insulators. So, the chance of fireplace may doubtlessly enhance,” says Edmonson. “Early EV fashions employed minimal thermal administration by means of air-cooled battery techniques, 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 subsequently shifting the goalposts for what they should think about.
“As batteries transfer in the direction of better ranges of integrations, they’re eradicating a number 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 shield cells embrace ceramics, mica, aerogels, foams, encapsulants, coatings, and part change supplies. These might be utilized to the battery enclosure both externally, internally, or between the cells. Hearth safety strategies are applied at each the cell degree and the pack degree.
On the pack degree, supplies similar 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 boundaries which are 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 intensely low thermal conductivity. Different supplies embrace powder coatings and intumescent coatings that increase with warmth, and compression pads with fire-protective components for pouch cells.
The purpose of those supplies is that they need to exhibit extraordinarily low thermal conductivity and low density to keep up weight effectivity and power density. Nevertheless, with the demand for elevated power density comes better cell power, so their means to guard in opposition to excessive excessive heats and for a 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 power density to implement it. With such a excessive floor space, encapsulant foam is ideally suited to cylindrical cell techniques.
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 tendencies embrace elevated security, increased power 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. “After we first started the journey to determine on the proper formulation for encapsulant know-how we tried to determine on a very powerful properties related to efficiency of EV batteries.”
One of many foremost options is the power of froth encapsulants to cease thermal propagation if there’s thermal runaway occurring. The fabric firstly should struggle the hearth, and secondly thermally shield the adjoining cells so in addition they don’t go into propagation themselves
“We’ve got spent a number 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 sizzling 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 fulfill the necessity for longer vary, present and future cells are packing much more power. Subsequently, much more power 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 regularly 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 fashionable cells turning into larger there’s much more power popping out of them, so we work with our clients to find out how a lot materials – similar 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 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 supply a really excessive floor space at a really low density.
Aspen Aerogels, a pacesetter within the know-how, has seen its enterprise route change course due to the EV motion. Traditionally, it provided its Aerogel know-how to the oil and fuel business and is now working in main quantity manufacturing with automotive OEMs throughout the globe together with Toyota, Audi, Scania Vans, and Normal 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 boundaries. 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 straight working with OEMs as they attempt to delay, or in some instances, absolutely cease, the propagation of thermal runaway in a pack,” says Tyler Gurian, senior program engineer at Aspen Aerogels. “The flexibility 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 might be lower than 10% of the entire exothermic potential of this occasion. The remaining goes by means of secondary conduction paths, the busbars, the cooling plate.
“The remaining 70-90% of the entire 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 without doubt one of the greatest challenges,” he says.
Insulating cells is, , it may be argued, a comparatively straightforward activity. It may be achieved with the quite a few supplies in the marketplace from ceramics, mica, foams, encapsulants, coatings, part 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 isn’t a house in packs or modules to individually ‘plumb’ the cells to a vent. Nobody is individually laborious 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 applied 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 boundaries 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 increase 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 strain 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 power and vary.
“PU foam is main for its low density, however Pyrothin has the identical density and in addition sturdy thermal runaway propagation delay better than 5 minutes,” says Gurian. “We carry out numerous testing, the place we’re taking a look at C2C thermal conduction occasions. Utilizing sure compressed thickness in opposition to cell areal power density we will obtain an infinite delay, so primarily zero-propagation.”
Future battery know-how hearth safety
Rising battery applied sciences and chemistries, similar to sodium-ion and solid-state batteries, promise important enhancements however nonetheless require strong hearth safety. Sodium-ion, whereas cheaper and safer in preliminary assessments, nonetheless presents thermal runaway dangers. Stable-state batteries, regardless of their increased security profile, pose design challenges as a consequence of their selection and potential for high-temperature occasions.
“Sodium-ion remains to be at comparatively early improvement phases, as a consequence of a decrease power density than lithium-ion,” says Edmonson. “Nevertheless, sodium is extra considerable and is cheaper than lithium. Like LFP know-how, preliminary penetration assessments on sodium-ion cells have efficiently yielded no ignition or flames.
“The opposite advantage of sodium-ion is, not like 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 risk. Nevertheless, thermal runaway and cell-to-cell propagation nonetheless exist with sodium-ion; it’s simply these occasions provoke at increased temperatures and are much less violent and slower to unfold. A low danger remains 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. Nevertheless, they will nonetheless burn at excessive temperatures and have a better power density, posing a danger for inside brief circuit eventualities.
“Early research present the utmost temperatures reached in a thermal runaway state of affairs are proportional to the quantity of power saved,” says Edmonson. “So, by storing extra power in a smaller pack house, the temperatures might be even increased than we see with lithium-ion techniques.
“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 may doubtlessly kind hydrogen-sulfides within the occasion of a difficulty. Stable-state batteries are prone to be a lot safer however nonetheless not 100% secure, so the system design and hearth safety supplies will nonetheless be essential,” he concludes.
New battery tendencies and challenges
The e-mobility business is transferring in the direction of increased 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). As we speak, the commonest structure is module-to-pack (MTP), permitting for excellent customization and scalability. Nevertheless, there’s a shift in the direction of architectures with out intermediate modules, similar to cell-to-pack (CTP) and cell-to-chassis (CTC), which supply better power 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’ve got pushed the boundaries of what PU foam can do by way of structural means. We’ve got developed merchandise starting from very comfortable 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 all the battery meeting and is agnostic to battery know-how. Nevertheless, even with developments in ‘safer’ cell chemistry, similar to solid-state or sodium-ion, they’re nonetheless in danger from a thermal occasion.
“A number 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. Nevertheless, all specialists 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 kinds and chemistries face additional elevated temperatures for cell faces in thermal runaway.
“One of many larger challenges is cells which are much more dynamic mechanically than what we have now now in quantity manufacturing. These cells are respiratory within the order of 10% of their full thickness, i.e. a 30mm cell respiratory 3mm for every cycle. These mechanical challenges will not be distinctive to aerogel, but additionally for foam boundaries. These will pose design challenges.”
“As we have a look at extra built-in battery designs, it’s going to be vital to have supplies which have multiple operate. For instance, when you can have an aerogel that gives thermal insulation, hearth safety, and compression you possibly can doubtlessly cut back the entire variety of supplies within the battery pack, which might decrease manufacturing complexity and price.”
