Thermal runaway in EV batteries is a severe concern with probably 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 probably severe hearth incident. Whereas passenger EVs drive the best battery demand amongst e-mobility, electrical buses pose important security considerations as a consequence of 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 unimaginable to foretell. It will possibly 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’s nonetheless at the moment a non-zero threat, which necessitates sturdy 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 unbiased 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 way more older ICE autos on the street, so the info may be deceptive. “As EVs age and their batteries degrade, it’s unknown what have an effect on this may occasionally have on pack elements, akin to cell face integrity, busbars, or insulators. So, the danger of fireplace may probably 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 subsequently shifting the goalposts for what they should think about.
“As batteries transfer in the direction of larger ranges of integrations, they’re eradicating lots of inactive supplies, so the necessity for multi-function supplies is turning into crucial to offer decrease value 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 embody 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. Hearth safety strategies are carried out at each the cell stage and the pack stage.
On the pack stage, supplies akin 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 obstacles 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 as a consequence of their low density and intensely 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 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. Nonetheless, with the demand for elevated vitality density comes larger cell vitality, so their capacity to guard towards 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 do not need to sacrifice area or vitality 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 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 as a consequence of its efficiency.
“We’re working in a quickly evolving market the place traits embody elevated security, larger vitality 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. “After 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 important options is the power of froth encapsulants to cease thermal propagation if there’s thermal runaway occurring. The fabric firstly should battle the hearth, and secondly thermally shield the adjoining cells so additionally they don’t go into propagation themselves
“We’ve 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 hearth, successfully defending the neighboring cells and suppressing hearth.
To satisfy 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 regularly exploring methods for its supplies to fulfill the must be more practical in propagation.
“We’ve performed testing on cylindrical in addition to prismatic cells. With trendy cells turning into larger there’s 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 turning into the following huge 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 provide a really excessive floor space at a really low density.
Aspen Aerogels, a frontrunner within the expertise, has seen its enterprise course 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 kind, aerogels are fairly brittle, however by incorporating the aerogel into a versatile composite utilizing a fiber strengthened submatrix Aspen was capable of make use of it for a lot of attention-grabbing 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 obstacles. 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 circumstances, 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 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 may be lower than 10% of the whole exothermic potential of this occasion. The remainder goes by 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 without doubt one of the greatest challenges,” he says.
Insulating cells is, , it may be argued, a comparatively simple process. 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 largest 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 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 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 affords 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 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 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 vitality and vary.
“PU foam is main for its low density, however Pyrothin has the identical density and in addition robust thermal runaway propagation delay larger than 5 minutes,” says Gurian. “We carry out a number of testing, the place we’re C2C thermal conduction occasions. Utilizing sure compressed thickness towards cell areal vitality density we will obtain an infinite delay, so primarily zero-propagation.”
Future battery expertise hearth safety
Rising battery applied sciences and chemistries, akin to sodium-ion and solid-state batteries, promise important enhancements however nonetheless require sturdy hearth safety. Sodium-ion, whereas cheaper and safer in preliminary exams, nonetheless presents thermal runaway dangers. Stable-state batteries, regardless of their larger security profile, pose design challenges as a consequence of their selection and potential for high-temperature occasions.
“Sodium-ion remains to be at comparatively early improvement levels, as a consequence of a decrease vitality density than lithium-ion,” says Edmonson. “Nonetheless, sodium is extra plentiful and is cheaper than lithium. Like LFP expertise, preliminary penetration exams 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, that means it may be transported with no cost and fewer threat of fireplace menace. Nonetheless, 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.”
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. Nonetheless, they will nonetheless burn at excessive temperatures and have the next vitality density, posing a threat for inner quick circuit eventualities.
“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 might be even larger than we see with lithium-ion techniques.
“Stable-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 difficulty. Stable-state batteries are prone to be a lot safer however nonetheless not 100% protected, so the system design and hearth safety supplies will nonetheless be crucial,” he concludes.
New battery traits 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 have been small, no matter their format (cylindrical, prismatic, or pouch). At present, the commonest structure is module-to-pack (MTP), permitting for nice customization and scalability. Nonetheless, 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 larger 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’ve pushed the boundaries of what PU foam can do by way of structural capacity. We’ve 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 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 the whole battery meeting and is agnostic to battery expertise. Nonetheless, even with developments in ‘safer’ cell chemistry, akin to solid-state or sodium-ion, they’re nonetheless in danger from a thermal occasion.
“A whole lot 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. Nonetheless, 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 types 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 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, for those who can have an aerogel that gives thermal insulation, hearth safety, and compression you might probably scale back the whole variety of supplies within the battery pack, which might decrease manufacturing complexity and price.”
