Thermal runaway in EV batteries is a severe concern with doubtlessly catastrophic penalties. When a single cell overheats due a fault it may well set off a sequence response in adjoining cells, resulting in a violent exothermic occasion and doubtlessly severe fireplace incident. Whereas passenger EVs drive the very best battery demand amongst e-mobility, electrical buses pose important security issues resulting from their capability to hold quite a few passengers. Efficiently managing or eliminating thermal runaway propagation is vital to security. Sadly, thermal runaway is nearly unattainable to foretell. It could possibly 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 price, there’s nonetheless at present a non-zero danger, 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 a couple 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 information may be deceptive. “As EVs age and their batteries degrade, it’s unknown what have an effect on this will likely have on pack parts, similar to cell face integrity, busbars, or insulators. So, the chance of fireplace might doubtlessly improve,” says Edmonson. “Early EV fashions employed minimal thermal administration by air-cooled battery techniques, whereas newer fashions use more practical 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 think about.
“As batteries transfer in direction of larger ranges of integrations, they’re eradicating a number of inactive supplies, so the necessity for multi-function supplies is changing into crucial to supply decrease price and manufacturing simplicity,” says Edmonson. “An rising variety of gamers are coming to this market because of the 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 section change supplies. These may be utilized to the battery enclosure both externally, internally, or between the cells. Fireplace safety strategies are carried out 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 obstacles which might 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 resulting from their low density and intensely low thermal conductivity. Different supplies embrace powder coatings and intumescent coatings that develop with warmth, and compression pads with fire-protective components for pouch cells.
The purpose of those supplies is that they have 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 capability 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 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 shouldn’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 sometimes 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 resulting from 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 powerful concentrate on sustainability,” says Malene Valverde, enterprise improvement supervisor at H.B. Fuller. “Once we first started the journey to resolve on the precise formulation for encapsulant expertise we tried to resolve on an important properties related to efficiency of EV batteries.”
One of many predominant options is the flexibility of froth encapsulants to cease thermal propagation if there’s thermal runaway occurring. The fabric firstly should battle the fireplace, and secondly thermally shield the adjoining cells so in addition they don’t go into propagation themselves
“We’ve spent a number 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 regularly exploring methods for its supplies to fulfill the should be more practical in propagation.
“We’ve carried out testing on cylindrical in addition to prismatic cells. With trendy cells changing into greater there’s much more power popping out of them, so we work with our prospects 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 strong that has been round because the Nineteen Thirties however they’re changing into the following large 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 path change course due to the EV motion. Traditionally, it equipped 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 Vehicles, and Basic 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 capable of make use of it for a lot of attention-grabbing 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 obstacles. 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 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 may be lower than 10% of the full exothermic potential of this occasion. The remaining 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 power is likely one of the largest 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, 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’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 obstacles between the cells presents 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 develop 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 likewise robust thermal runaway propagation delay larger than 5 minutes,” says Gurian. “We carry out a lot 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 basically zero-propagation.”
Future battery expertise fireplace safety
Rising battery applied sciences and chemistries, similar to sodium-ion and solid-state batteries, promise important enhancements however nonetheless require sturdy fireplace 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 resulting from their selection and potential for high-temperature occasions.
“Sodium-ion continues to be at comparatively early improvement levels, resulting from a decrease power density than lithium-ion,” says Edmonson. “Nonetheless, 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 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 danger of fireplace risk. 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 danger continues to be a danger, 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. Nonetheless, they’ll nonetheless burn at excessive temperatures and have a better power density, posing a danger 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 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 enormous varieties of various chemistries going into them. Many are utilizing a sulfide-based solid-state electrolyte, which might doubtlessly type hydrogen-sulfides within the occasion of a problem. 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 transferring in 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). Right this moment, the commonest structure is module-to-pack (MTP), permitting for excellent customization and scalability. Nonetheless, there’s a shift in direction of architectures with out intermediate modules, similar 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.
“We’ve pushed the boundaries of what PU foam can do by way of structural capability. We’ve 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 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. Nonetheless, even with developments in ‘safer’ cell chemistry, similar to solid-state or sodium-ion, they’re nonetheless in danger from a thermal occasion.
“Numerous efforts in cell chemistry enchancment purpose 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 varieties and chemistries face additional elevated temperatures for cell faces in thermal runaway.
“One of many greater challenges is cells which might 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 will not be distinctive to aerogel, but additionally for foam obstacles. 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 multiple perform. For instance, should you can have an aerogel that gives thermal insulation, fireplace safety, and compression you would doubtlessly cut back the full variety of supplies within the battery pack, which might decrease manufacturing complexity and value.”
