Thermal runaway in EV batteries is a critical concern with doubtlessly catastrophic penalties. When a single cell overheats due a fault it may well set off a series response in adjoining cells, resulting in a violent exothermic occasion and doubtlessly critical hearth incident. Whereas passenger EVs drive the best battery demand amongst e-mobility, electrical buses pose vital 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 nearly unattainable to foretell. It will 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’s 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 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 information may be deceptive. “As EVs age and their batteries degrade, it’s unknown what have an effect on this will have on pack parts, akin to cell face integrity, busbars, or insulators. So, the chance of fireplace might doubtlessly improve,” 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 subsequently shifting the goalposts for what they should contemplate.
“As batteries transfer in direction of larger ranges of integrations, they’re eradicating a whole lot of inactive supplies, so the necessity for multi-function supplies is turning into crucial to offer 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 embrace ceramics, mica, aerogels, foams, encapsulants, coatings, and part 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 degree and the pack degree.
On the pack degree, supplies akin 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 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 usually used amongst cylindrical cells, as seen in Tesla’s 4680 pack. Aerogels are gaining traction as a consequence of their low density and very 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 objective of those supplies is that they have to exhibit extraordinarily low thermal conductivity and low density to keep up weight effectivity and power density. Nevertheless, 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 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 would not have 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 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 Shield encapsulant platform as a consequence of its efficiency.
“We’re working in a quickly evolving market the place tendencies embrace elevated security, greater power density for longer vary, decrease manufacturing prices, stricter security laws, and a robust deal with sustainability,” says Malene Valverde, enterprise improvement supervisor at H.B. Fuller. “After we first started the journey to determine on the suitable formulation for encapsulant expertise we tried to determine on a very powerful 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 additionally they don’t go into propagation themselves
“We’ve spent a whole lot 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 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 power. Subsequently, much more power is launched in a single occasion of catastrophe and sooner. As Valverde explains, the problem is evolving for H.B. Fuller, and its engineers are regularly exploring methods for its supplies to satisfy the must be simpler in propagation.
“We’ve performed testing on cylindrical in addition to prismatic cells. With trendy cells turning into greater there’s much more power 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 stable that has been round because the Nineteen Thirties however they’re turning into the subsequent huge factor in battery cell insulation and hearth 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 course change course due to the EV motion. Traditionally, it equipped 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 Vans, 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 fascinating functions. Aspen first used it as insulation for subsea power 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 flexibility of a typical thermal threshold of 900oC [1,652oF].
“We’re instantly 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 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 may be lower than 10% of the full exothermic potential of this occasion. The remaining goes via 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 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, 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 laborious 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 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 affords 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 keep the cell face stress for the mechanical retention, and life, of the cells.
One other challenge 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 plenty 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 hearth safety
Rising battery applied sciences and chemistries, akin to 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. Stable-state batteries, regardless of their greater security profile, pose design challenges as a consequence of their selection and potential for high-temperature occasions.
“Sodium-ion continues to be at comparatively early improvement phases, as a consequence 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 exams on sodium-ion cells have efficiently yielded no ignition or flames.
“The opposite advantage of 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 menace. Nevertheless, thermal runaway and cell-to-cell propagation nonetheless exist with sodium-ion; it’s simply these occasions provoke at greater temperatures and are much less violent and slower to unfold. A low threat continues to be a threat, so hearth 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. Nevertheless, they’ll nonetheless burn at excessive temperatures and have a better power density, posing a threat for inside brief circuit eventualities.
“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 area, the temperatures may very well be even greater 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 prone to be a lot safer however nonetheless not 100% protected, so the system design and hearth safety supplies will nonetheless be obligatory,” he concludes.
New battery tendencies and challenges
The e-mobility trade is transferring in direction of greater 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). At present, the commonest structure is module-to-pack (MTP), permitting for nice customization and scalability. Nevertheless, there’s a shift in direction of architectures with out intermediate modules, akin to cell-to-pack (CTP) and cell-to-chassis (CTC), which provide larger power density and decreased manufacturing prices. These new architectures require structural help, 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 capability. We’ve developed merchandise starting from very comfortable 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 help for the whole battery meeting and is agnostic to battery expertise. Nevertheless, even with developments in ‘safer’ cell chemistry, akin to solid-state or sodium-ion, they’re nonetheless in danger from a thermal occasion.
“A variety of efforts in cell chemistry enchancment intention 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 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 greater challenges is cells which are much more dynamic mechanically than what we now have 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 additionally 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 may doubtlessly scale back the full variety of supplies within the battery pack, which might decrease manufacturing complexity and value.”
