Thermal runaway in EV batteries is a critical concern with probably catastrophic penalties. When a single cell overheats due a fault it might set off a sequence response in adjoining cells, resulting in a violent exothermic occasion and probably critical fireplace incident. Whereas passenger EVs drive the very best battery demand amongst e-mobility, electrical buses pose vital security considerations as a result of their capability to hold quite a few passengers. Efficiently managing or eliminating thermal runaway propagation is essential to security. Sadly, thermal runaway is nearly 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 fee, there may be nonetheless presently 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 impartial 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 information might be deceptive. “As EVs age and their batteries degrade, it’s unknown what have an effect on this may occasionally have on pack elements, comparable to cell face integrity, busbars, or insulators. So, the chance of fireplace might probably enhance,” says Edmonson. “Early EV fashions employed minimal thermal administration by way of air-cooled battery techniques, whereas newer fashions use simpler energetic 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 variety of inactive supplies, so the necessity for multi-function supplies is changing into crucial to supply decrease value 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 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 carried out at each the cell degree and the pack degree.
On the pack degree, supplies comparable to 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 limitations 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 usually used amongst cylindrical cells, as seen in Tesla’s 4680 pack. Aerogels are gaining traction as a result of their low density and very 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 objective of those supplies is that they need to exhibit extraordinarily low thermal conductivity and low density to take care of 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 the way lengthy are going to be a decisive issue for OEMs, plus their market value 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 suits 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 fitted to cylindrical cell techniques.
Encapsulant foams can usually be present in epoxy or silicone foam type, 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 result 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 laws, and a robust deal with sustainability,” says Malene Valverde, enterprise growth supervisor at H.B. Fuller. “After we first started the journey to determine on the appropriate formulation for encapsulant expertise we tried to determine on an important properties related to efficiency of EV batteries.”
One of many principal options is the power of froth encapsulants to cease thermal propagation if there may be thermal runaway occurring. The fabric firstly should struggle the fireplace, and secondly thermally shield the adjoining cells so additionally they don’t go into propagation themselves
“We now have spent a variety of time and analysis to make sure the encapsulant foam expertise 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 fireplace, successfully defending the neighboring cells and suppressing fireplace.
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 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 simpler in propagation.
“We’ve performed testing on cylindrical in addition to prismatic cells. With trendy cells changing into greater there may be much more power popping out of them, so we work with our clients to find out how a lot materials – comparable 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 fireplace 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 type, aerogels are fairly brittle, however by incorporating the aerogel into a versatile composite utilizing a fiber bolstered submatrix Aspen was in a position to make use of it for a lot of fascinating purposes. Aspen first used it as insulation for subsea power 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 power of a typical thermal threshold of 900oC [1,652oF].
“We’re instantly 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 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 big cell face would be the first place the power 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 power is among 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 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 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 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 limitations 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 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 power 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 taking a look at C2C thermal conduction occasions. Utilizing sure compressed thickness in opposition to cell areal power density we are able to obtain an infinite delay, so primarily zero-propagation.”
Future battery expertise fireplace safety
Rising battery applied sciences and chemistries, comparable to sodium-ion and solid-state batteries, promise vital enhancements however nonetheless require strong fireplace safety. Sodium-ion, whereas cheaper and safer in preliminary checks, nonetheless presents thermal runaway dangers. Stable-state batteries, regardless of their increased security profile, pose design challenges as a result of their selection and potential for high-temperature occasions.
“Sodium-ion remains to be at comparatively early growth levels, as a result of a decrease power density than lithium-ion,” says Edmonson. “Nevertheless, sodium is extra ample and is cheaper than lithium. Like LFP expertise, 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, which 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 increased 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 substitute 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 power density, posing a threat for inside 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 may very well be even increased than we see with lithium-ion techniques.
“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 probably type hydrogen-sulfides within the occasion of a difficulty. Stable-state batteries are more likely to be a lot safer however nonetheless not 100% protected, so the system design and fireplace safety supplies will nonetheless be crucial,” he concludes.
New battery tendencies and challenges
The e-mobility trade is shifting 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). Right now, 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, comparable 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 now have pushed the boundaries of what PU foam can do by way of structural means. We now 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 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 the whole battery meeting and is agnostic to battery expertise. Nevertheless, even with developments in ‘safer’ cell chemistry, comparable to solid-state or sodium-ion, they’re nonetheless in danger from a thermal occasion.
“Plenty of efforts in cell chemistry enchancment goal to make cells safer, decreasing the variety of runaway occasions or releasing much less power 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 types and chemistries face additional elevated temperatures for cell faces in thermal runaway.
“One of many greater challenges is cells which are 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 will not be distinctive to aerogel, but in addition for foam limitations. 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, if you happen to can have an aerogel that gives thermal insulation, fireplace safety, and compression you might probably scale back the full variety of supplies within the battery pack, which might decrease manufacturing complexity and price.”
