Thermal runaway in EV batteries is a critical concern with doubtlessly catastrophic penalties. When a single cell overheats due a fault it could possibly set off a sequence response in adjoining cells, resulting in a violent exothermic occasion and doubtlessly critical fireplace incident. Whereas passenger EVs drive the best battery demand amongst e-mobility, electrical buses pose vital security issues attributable to 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 inconceivable to foretell. It might probably 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 fee, there may be nonetheless at the moment a non-zero threat, which necessitates strong security measures.
“Documented fireplace incidents have occurred throughout a number of OEM automobiles – from BYD to Tesla, Hyundai, and BMW, to call a number of,” 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 fireplace incidence in EVs in comparison with ICE automobiles, there are way more older ICE automobiles on the street, so the info might 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 fireside might doubtlessly enhance,” 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 subsequently shifting the goalposts for what they should think about.
“As batteries transfer in the direction of better ranges of integrations, they’re eradicating lots of inactive supplies, so the necessity for multi-function supplies is changing into essential to offer decrease price and manufacturing simplicity,” says Edmonson. “An rising variety of gamers are coming to this market because of the variety of completely 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 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 limitations which can 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 attributable to their low density and intensely low thermal conductivity. Different supplies embrace 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 need to exhibit extraordinarily low thermal conductivity and low density to keep up weight effectivity and vitality density. Nevertheless, with the demand for elevated vitality density comes better cell vitality, so their potential 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 would not have to sacrifice area or vitality density to implement it. With such a excessive floor space, encapsulant foam is ideally suited 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 Shield encapsulant platform attributable to its efficiency.
“We’re working in a quickly evolving market the place traits embrace elevated security, greater vitality density for longer vary, decrease manufacturing prices, stricter security laws, and a powerful deal with sustainability,” says Malene Valverde, enterprise improvement supervisor at H.B. Fuller. “After we first started the journey to determine on the precise formulation for encapsulant expertise we tried to determine on an important properties related to efficiency of EV batteries.”
One of many most important options is the flexibility of froth encapsulants to cease thermal propagation if there may be thermal runaway occurring. The fabric firstly should combat the fireplace, and secondly thermally shield the adjoining cells so in addition they don’t go into propagation themselves
“Now we have spent lots of time and analysis to make sure the encapsulant foam expertise 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 sizzling 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 fireplace.
To fulfill the necessity for longer vary, present and future cells are packing much more vitality. Subsequently, 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 changing into greater there may be much more vitality popping out of them, so we work with our clients 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 strong that has been round because the Nineteen Thirties however they’re changing into the following massive factor in battery cell insulation and fireplace 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 expertise, has seen its enterprise path change course due to the EV motion. Traditionally, it equipped its Aerogel expertise 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 bolstered submatrix Aspen was capable of make use of it for a lot of fascinating functions. Aspen first used it as insulation for subsea vitality pipelines and passive fireplace 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 limitations. 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, 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 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 whole 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 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 vitality is likely one of the largest challenges,” he says.
Insulating cells is, , it may be argued, a comparatively straightforward job. It may be achieved with the quite a few supplies available on the market from ceramics, mica, foams, encapsulants, coatings, part change supplies, and extra. The most important 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 is no such thing as a area 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 area inside a module or pack. So, one resolution that has been applied 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 limitations between the cells provides an analogous setup to bulkheads within the hull of an ocean liner. It seals every area 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 situation 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 robust 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 in opposition to cell areal vitality density we are able to obtain an infinite delay, so primarily zero-propagation.”
Future battery expertise fireplace safety
Rising battery applied sciences and chemistries, akin to sodium-ion and solid-state batteries, promise vital enhancements however nonetheless require strong fireplace safety. Sodium-ion, whereas cheaper and safer in preliminary assessments, nonetheless presents thermal runaway dangers. Stable-state batteries, regardless of their greater security profile, pose design challenges attributable to their selection and potential for high-temperature occasions.
“Sodium-ion remains to be at comparatively early improvement phases, attributable to a decrease vitality density than lithium-ion,” says Edmonson. “Nevertheless, sodium is extra ample and is cheaper than lithium. Like LFP expertise, preliminary penetration assessments on sodium-ion cells have efficiently yielded no ignition or flames.
“The opposite advantage of 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 threat of fireside risk. Nevertheless, thermal runaway and cell-to-cell propagation nonetheless exist with sodium-ion; it’s simply these occasions provoke at greater temperatures and are much less violent and slower to unfold. A low threat remains to be a threat, so fireplace safety supplies will nonetheless be required.”
Stable-state batteries additionally current distinctive challenges. They exchange 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 vitality density, posing a threat for inside quick circuit situations.
“Early research present the utmost temperatures reached in a thermal runaway state of affairs are proportional to the quantity of vitality saved,” says Edmonson. “So, by storing extra vitality in a smaller pack area, the temperatures could possibly be even greater than we see with lithium-ion methods.
“Stable-state batteries aren’t a single expertise. There are large 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 problem. Stable-state batteries are prone to be a lot safer however nonetheless not 100% secure, so the system design and fireplace safety supplies will nonetheless be essential,” he concludes.
New battery traits and challenges
The e-mobility business is shifting in the direction of greater vitality density batteries, which current new challenges for thermal administration. A decade in the past, most EV battery packs had been small, no matter their format (cylindrical, prismatic, or pouch). At this time, the most typical structure is module-to-pack (MTP), permitting for nice customization and scalability. Nevertheless, 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 provide better vitality density and diminished manufacturing prices. These new architectures require structural assist, which multi-functional materials applied sciences are aiming to offer, alongside thermal safety.
“Now we have pushed the boundaries of what PU foam can do when it comes to structural potential. Now we have developed merchandise starting from very tender 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 entire battery meeting and is agnostic to battery expertise. Nevertheless, even with developments in ‘safer’ cell chemistry, akin to solid-state or sodium-ion, they’re nonetheless in danger from a thermal occasion.
“Lots of efforts in cell chemistry enchancment purpose to make cells safer, lowering the variety of runaway occasions or releasing much less vitality at cooler temperatures. Nevertheless, all specialists agree that each one cell applied sciences and chemistries nonetheless require fireplace 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 greater challenges is cells which can be much more dynamic mechanically than what we now have 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 usually are not distinctive to aerogel, but additionally for foam limitations. These will pose design challenges.”
“As we have a look at extra built-in battery designs, it’s going to be essential to have supplies which have multiple operate. For instance, when you can have an aerogel that gives thermal insulation, fireplace safety, and compression you possibly can doubtlessly cut back the whole variety of supplies within the battery pack, which might decrease manufacturing complexity and value.”
