Thermal runaway in EV batteries is a critical concern with probably catastrophic penalties. When a single cell overheats due a fault it will possibly set off a sequence response in adjoining cells, resulting in a violent exothermic occasion and probably critical hearth incident. Whereas passenger EVs drive the very best battery demand amongst e-mobility, electrical buses pose important security issues as a consequence of their capability to hold quite a few passengers. Efficiently managing or eliminating thermal runaway propagation is vital to security. Sadly, thermal runaway is nearly not possible to foretell. It might happen in any situation, from regular driving operation, after a crash, throughout charging, and even when the automobile is parked. Regardless of the low incidence price, there may be nonetheless presently a non-zero threat, which necessitates sturdy security measures.
“Documented hearth 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 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 automobiles, there are way more older ICE automobiles on the highway, so the information may be deceptive. “As EVs age and their batteries degrade, it’s unknown what have an effect on this may occasionally have on pack parts, resembling cell face integrity, busbars, or insulators. So, the danger of fireside might probably improve,” says Edmonson. “Early EV fashions employed minimal thermal administration by means of air-cooled battery programs, whereas newer fashions use simpler lively 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 take into account.
“As batteries transfer in direction of larger ranges of integrations, they’re eradicating a variety of inactive supplies, so the necessity for multi-function supplies is turning into essential to supply decrease price and manufacturing simplicity,” says Edmonson. “An rising 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 defend 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. Hearth safety strategies are applied at each the cell stage and the pack stage.
On the pack stage, supplies resembling 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 intensely 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 purpose of those supplies is that they have to exhibit extraordinarily low thermal conductivity and low density to keep up weight effectivity and vitality density. Nevertheless, with the demand for elevated vitality density comes larger cell vitality, 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 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 don’t have to sacrifice area or vitality density to implement it. With such a excessive floor space, encapsulant foam is ideally suited to cylindrical cell programs.
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 consequence of its efficiency.
“We’re working in a quickly evolving market the place traits embrace elevated security, increased vitality density for longer vary, decrease manufacturing prices, stricter security rules, 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 proper formulation for encapsulant know-how we tried to determine on crucial properties related to efficiency of EV batteries.”
One of many major options is the power of froth encapsulants to cease thermal propagation if there may be thermal runaway occurring. The fabric firstly should battle the hearth, and secondly thermally defend the adjoining cells so additionally they don’t go into propagation themselves
“We’ve spent a variety of time and analysis to make sure the encapsulant foam know-how 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 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. Due to this fact, 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 should be simpler in propagation.
“We’ve carried out testing on cylindrical in addition to prismatic cells. With trendy cells turning into greater there may be much more vitality popping out of them, so we work with our clients to find out how a lot materials – resembling 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 for the reason that Thirties however they’re turning into the subsequent massive 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 know-how, has seen its enterprise route change course due to the EV motion. Traditionally, it provided its Aerogel know-how 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 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 in a position to make use of it for a lot of attention-grabbing purposes. 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 boundaries. 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 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 overall exothermic potential of this occasion. The remaining goes by means of secondary conduction paths, the busbars, the cooling plate.
“The remaining 70-90% of the overall 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 likely one of the largest challenges,” he says.
Insulating cells is, , it may be argued, a comparatively simple job. 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 most important 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 isn’t any area in packs or modules to individually ‘plumb’ the cells to a vent. Nobody is individually arduous 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 applied 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 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 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 strain 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 likewise sturdy thermal runaway propagation delay larger than 5 minutes,” says Gurian. “We carry out a number of testing, the place we’re taking a look at C2C thermal conduction occasions. Utilizing sure compressed thickness in opposition to cell areal vitality density we are able to obtain an infinite delay, so primarily zero-propagation.”
Future battery know-how hearth safety
Rising battery applied sciences and chemistries, resembling sodium-ion and solid-state batteries, promise important enhancements however nonetheless require sturdy hearth safety. Sodium-ion, whereas cheaper and safer in preliminary assessments, nonetheless presents thermal runaway dangers. Strong-state batteries, regardless of their increased 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 levels, as a consequence of a decrease vitality density than lithium-ion,” says Edmonson. “Nevertheless, sodium is extra considerable and is cheaper than lithium. Like LFP know-how, preliminary penetration assessments 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, which means it may be transported with no cost and fewer threat of fireside menace. 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 continues to be a threat, so hearth safety supplies will nonetheless be required.”
Strong-state batteries additionally current distinctive challenges. They exchange 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 will nonetheless burn at excessive temperatures and have the next vitality 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 vitality saved,” says Edmonson. “So, by storing extra vitality in a smaller pack area, the temperatures could possibly be even increased than we see with lithium-ion programs.
“Strong-state batteries aren’t a single know-how. There are big 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. Strong-state batteries are more likely to be a lot safer however nonetheless not 100% protected, so the system design and hearth safety supplies will nonetheless be mandatory,” he concludes.
New battery traits and challenges
The e-mobility trade is shifting in direction of increased vitality 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). As we speak, 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, resembling cell-to-pack (CTP) and cell-to-chassis (CTC), which provide larger vitality density and decreased manufacturing prices. These new architectures require structural help, which multi-functional materials applied sciences are aiming to supply, alongside thermal safety.
“We’ve pushed the boundaries of what PU foam can do when it comes to structural capacity. 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 all the battery meeting and is agnostic to battery know-how. Nevertheless, even with developments in ‘safer’ cell chemistry, resembling 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. Nevertheless, all consultants 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 varieties 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 have now 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 boundaries. These will pose design challenges.”
“As we have a look at extra built-in battery designs, it’s going to be essential to have supplies which have multiple perform. For instance, when you can have an aerogel that gives thermal insulation, hearth safety, and compression you possibly can probably cut back the overall variety of supplies within the battery pack, which might decrease manufacturing complexity and value.”
