Thermal runaway in EV batteries is a critical concern with probably catastrophic penalties. When a single cell overheats due a fault it could 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 important 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 nearly unimaginable to foretell. It could happen in any state of affairs, from regular driving operation, after a crash, throughout charging, and even when the car is parked. Regardless of the low incidence charge, there may be nonetheless at the moment a non-zero threat, which necessitates sturdy security measures.
“Documented fireplace 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 fireplace incidence in EVs in comparison with ICE automobiles, there are much more older ICE automobiles on the highway, 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, corresponding to cell face integrity, busbars, or insulators. So, the danger of fireside might probably enhance,” says Edmonson. “Early EV fashions employed minimal thermal administration via 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 due to this fact shifting the goalposts for what they should take into account.
“As batteries transfer in the direction of larger ranges of integrations, they’re eradicating quite a lot of inactive supplies, so the necessity for multi-function supplies is turning into essential to supply decrease value and manufacturing simplicity,” says Edmonson. “An growing 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 embody 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 corresponding 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 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 usually 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 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. Nonetheless, with the demand for elevated power density comes larger cell power, so their capacity 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 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 do not need to sacrifice area or power density to implement it. With such a excessive floor space, encapsulant foam is ideally suited to cylindrical cell techniques.
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 attributable to its efficiency.
“We’re working in a quickly evolving market the place tendencies embody 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 resolve on the suitable formulation for encapsulant expertise we tried to resolve on crucial properties related to efficiency of EV batteries.”
One of many foremost 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
“Now we have spent quite a lot of time and analysis to make sure the encapsulant foam expertise has a UL 94 V-0 flame ranking, the very 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 fireplace.
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 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 fashionable cells turning into larger there may be much more power popping out of them, so we work with our clients to find out how a lot materials – corresponding 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 turning into the subsequent 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 pacesetter within the expertise, has seen its enterprise route change course due to the EV motion. Traditionally, it provided its Aerogel expertise 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 Common 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 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 immediately 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 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 remainder goes via 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 among 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 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’t be retrofitted after design,” he says. “The issue is there isn’t any 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 resolution 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 limitations between the cells presents an identical 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 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 stress 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 likewise sturdy thermal runaway propagation delay larger 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 expertise fireplace safety
Rising battery applied sciences and chemistries, corresponding to sodium-ion and solid-state batteries, promise important enhancements however nonetheless require sturdy fireplace safety. Sodium-ion, whereas cheaper and safer in preliminary checks, nonetheless presents thermal runaway dangers. Strong-state batteries, regardless of their increased security profile, pose design challenges attributable to their selection and potential for high-temperature occasions.
“Sodium-ion remains to be at comparatively early growth levels, attributable to a decrease power density than lithium-ion,” says Edmonson. “Nonetheless, 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, 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 menace. Nonetheless, 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.”
Strong-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. Nonetheless, they will nonetheless burn at excessive temperatures and have the next power density, posing a threat for inside brief circuit situations.
“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 area, the temperatures might be even increased than we see with lithium-ion techniques.
“Strong-state batteries aren’t a single expertise. There are enormous varieties of various chemistries going into them. Many are utilizing a sulfide-based solid-state electrolyte, which might probably kind hydrogen-sulfides within the occasion of a difficulty. Strong-state batteries are prone to be a lot safer however nonetheless not 100% protected, so the system design and fireplace safety supplies will nonetheless be needed,” 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 had been small, no matter their format (cylindrical, prismatic, or pouch). In the present day, the most typical structure is module-to-pack (MTP), permitting for excellent customization and scalability. Nonetheless, there’s a shift in the direction of architectures with out intermediate modules, corresponding to cell-to-pack (CTP) and cell-to-chassis (CTC), which provide larger 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.
“Now we have pushed the boundaries of what PU foam can do by way of structural capacity. Now we have developed merchandise starting from very smooth 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. Nonetheless, even with developments in ‘safer’ cell chemistry, corresponding to solid-state or sodium-ion, they’re nonetheless in danger from a thermal occasion.
“Lots 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. Nonetheless, all consultants agree that every 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 larger challenges is cells which can be 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 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 necessary to have supplies which have multiple operate. For instance, for those who can have an aerogel that gives thermal insulation, fireplace safety, and compression you would probably cut back the entire variety of supplies within the battery pack, which might decrease manufacturing complexity and value.”
