Thermal runaway in EV batteries is a critical concern with probably catastrophic penalties. When a single cell overheats due a fault it might probably set off a sequence response in adjoining cells, resulting in a violent exothermic occasion and probably critical hearth incident. Whereas passenger EVs drive the best battery demand amongst e-mobility, electrical buses pose vital security considerations 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 unattainable 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 price, there may be nonetheless at the moment a non-zero threat, 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 a number of,” 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 will be deceptive. “As EVs age and their batteries degrade, it’s unknown what have an effect on this will likely have on pack parts, reminiscent of cell face integrity, busbars, or insulators. So, the danger of fireplace may probably enhance,” says Edmonson. “Early EV fashions employed minimal thermal administration by 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 take into account.
“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 growing 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 defend cells embody ceramics, mica, aerogels, foams, encapsulants, coatings, and section change supplies. These will be utilized to the battery enclosure both externally, internally, or between the cells. Fireplace safety strategies are applied at each the cell stage and the pack stage.
On the pack stage, supplies reminiscent of 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 attributable to their low density and very low thermal conductivity. Different supplies embody powder coatings and intumescent coatings that develop with warmth, and compression pads with fire-protective components for pouch cells.
The objective of those supplies is that they need to exhibit extraordinarily low thermal conductivity and low density to take care of weight effectivity and vitality density. Nevertheless, with the demand for elevated vitality density comes better cell vitality, so their means to guard towards 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 should not have to sacrifice area 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 Shield encapsulant platform attributable to its efficiency.
“We’re working in a quickly evolving market the place developments embody elevated security, larger vitality density for longer vary, decrease manufacturing prices, stricter security laws, and a powerful give attention to sustainability,” says Malene Valverde, enterprise improvement supervisor at H.B. Fuller. “Once we first started the journey to determine on the correct formulation for encapsulant expertise we tried to determine on a very powerful properties related to efficiency of EV batteries.”
One of many fundamental options is the power of froth encapsulants to cease thermal propagation if there may be thermal runaway occurring. The fabric firstly should combat the fireplace, and secondly thermally defend the adjoining cells so in addition they don’t go into propagation themselves
“We have now spent lots of time and analysis to make sure the encapsulant foam expertise has a UL 94 V-0 flame ranking, the best flame safety ranking,” 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 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 should be more practical in propagation.
“We’ve performed testing on cylindrical in addition to prismatic cells. With trendy cells changing 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 – reminiscent of 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 Thirties however they’re changing into the subsequent 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 expertise, has seen its enterprise course change course due to the EV motion. Traditionally, it provided its Aerogel expertise 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 Vehicles, 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 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 immediately 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 massive cell face would be the first place the vitality will go. Conduction from cell-to-cell will be lower than 10% of the full exothermic potential of this occasion. The remainder goes by 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 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 in the marketplace from ceramics, mica, foams, encapsulants, coatings, section 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 isn’t a area in packs or modules to individually ‘plumb’ the cells to a vent. Nobody is individually exhausting piping to the cells as a result of we’re working with millimeters of area 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 presents 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 develop 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 problem 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 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 towards cell areal vitality density we are able to obtain an infinite delay, so primarily zero-propagation.”
Future battery expertise hearth safety
Rising battery applied sciences and chemistries, reminiscent of 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. Strong-state batteries, regardless of their larger security profile, pose design challenges attributable to their selection and potential for high-temperature occasions.
“Sodium-ion remains to be at comparatively early improvement levels, 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 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, that means it may be transported with no cost and fewer threat of fireplace risk. Nevertheless, 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 remains to be a threat, so hearth safety supplies will nonetheless be required.”
Strong-state batteries additionally current distinctive challenges. They exchange the flammable natural liquid electrolyte with a non-flammable solid-state electrolyte, probably spreading warmth extra effectively and working inside a wider temperature vary. Nevertheless, they’ll nonetheless burn at excessive temperatures and have the next 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 may very well be even larger than we see with lithium-ion methods.
“Strong-state batteries aren’t a single expertise. There are big varieties of various chemistries going into them. Many are utilizing a sulfide-based solid-state electrolyte, which may probably kind hydrogen-sulfides within the occasion of a problem. Strong-state batteries are more likely to be a lot safer however nonetheless not 100% protected, so the system design and hearth safety supplies will nonetheless be mandatory,” he concludes.
New battery developments and challenges
The e-mobility trade is shifting in the direction of larger 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 commonest 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, reminiscent of cell-to-pack (CTP) and cell-to-chassis (CTC), which supply better vitality density and lowered manufacturing prices. These new architectures require structural assist, which multi-functional materials applied sciences are aiming to offer, alongside thermal safety.
“We have now pushed the boundaries of what PU foam can do when it comes to structural means. We have now developed merchandise starting from very mushy 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 expertise. Nevertheless, even with developments in ‘safer’ cell chemistry, reminiscent of solid-state or sodium-ion, they’re nonetheless in danger from a thermal occasion.
“Loads of efforts in cell chemistry enchancment intention to make cells safer, lowering the variety of runaway occasions or releasing much less vitality 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 varieties 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’ve got 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 should not distinctive to aerogel, but in addition for foam boundaries. These will pose design challenges.”
“As we take a look at extra built-in battery designs, it’s going to be essential to have supplies which have a couple of operate. For instance, in case you can have an aerogel that gives thermal insulation, hearth safety, and compression you possibly can probably scale back the full variety of supplies within the battery pack, which might decrease manufacturing complexity and value.”
