Adhesion Matters - Thermal Runaway

Episode Date: August 6, 2025

Thank you for listening to the Adhesion Matters podcast.  The widespread adoption of Electric Vehicles (EVs) hinges on advancements in lithium-ion (Li-ion) battery technology, particularly concerning energy density and safety. Thermal runaway, an uncontrollable self-heating state in Li-ion cells, poses a significant safety risk, capable of escalating into widespread fires. Advanced adhesive solutions are crucial for mitigating this risk, serving as multi-functional components for thermal management, electrical insulation, structural integrity, and fire safety. These materials not only prevent thermal events but also contain their propagation. Both liquid adhesives (epoxies, silicones, polyurethanes, acrylics) and adhesive tapes contribute uniquely, tailored for specific applications within the battery pack. During this special episode, we discuss how the industry emphasizes meticulous material selection, collaborative partnerships, and continuous innovation for future EV battery safety and performance.

The global shift to electric vehicles, it's really speeding up, isn't it? And when you think about them, your mind probably go straight to range or charging time, maybe those sleek designs. Mm-hmm. The obvious things. Exactly. But there's this crucial kind of unseen element underpinning it all. Safety. And especially with EV battery safety, there's this unsung hero doing some incredibly heavy lifting. Absolutely. Today, we're going to do a deep dive into the surprise. rising multifunctional role of adhesives in electric vehicle batteries. I mean, these aren't just, you know, sticky tapes. They are critical enablers for mitigating something called thermal

runaway. That's right. And our mission here really is unpack how these specialized adhesive solutions have, well, they've evolved way beyond just being bonding agents. They've become completely indispensable components. We're talking comprehensive thermal management, robust electrical insulation, better structural integrity, and proactive fire safety. inside the battery pack. So they prevent and contain. Exactly. They work on both ends

stopping thermal events from starting. And if one does kick off, limiting how far it spreads. Okay. And for this deep dive, we're drawing from some really detailed reports on these adhesive technologies

and also looking at the role of companies like Bodomiller-Kamey in making it all work. Yeah, some fascinating stuff in there. So let's unpack this first bit. For anyone needing a refresher, what exactly is thermal runaway

in a lithium-ion battery? Why is it such a huge safety concern for EVs? Okay, so thermal runaway is essentially an uncontrollable self-heating state. It happens within a single lithium ion cell. Just one cell. Start to one cell, yeah. But once it kicks off, it can escalate incredibly fast. We're talking extreme temperatures, sometimes up to like a thousand degrees Celsius.

Wow. Yeah, and that leads to violent cell, venting gases, escaping forcefully smoke and critically fire. Now, an event in just one cell might be made. manageable. But the real danger is how quickly it can spread, propagate throughout the entire battery pack. And that's when it becomes a major, major safety issue for the vehicle and, of course, everyone inside. That's a pretty intense scenario. And it connects to something else. We all want more range faster charging in our EVs. That constant push for better performance does that actually make this thermal runaway risk higher? That's a really sharp question. There's definitely an inherent tension there. The drive for higher energy densities, which is exactly what gives us that longer range and quicker charging it, directly correlates with an increased intrinsic thermal runaway risk. Intrinsic, meaning it's just baked into the chemistry. Pretty much. Think about advanced cathode chemistries like these nickel-rich layered oxides.

They just pack more energy into the same space. And this sets up a kind of positive feedback loop. How does that work? Well, so you get an initial temperature increase could be from anything, a small defect, maybe a bit of overcharging. that extra heat speeds up the exothermic chemical reactions inside the cell. Exothermic meaning they produce heat. Exactly. So those reactions generate more heat, which then makes the temperature climb even faster,

which speeds up the reactions again. And you see how it runs away. A vicious cycle. Completely. And this is where adhesives play such a strategic role. They can be designed to help dissipate that heat, get it out and away from the cell, or they can provide thermal insulation to act like a barrier. Essentially breaking or at least dramatically slowing down that self-accelerating loop.

Okay. That makes sense. So if we're trying to prevent these things from even starting, what are the usual suspects? What typically kicks off thermal runaway in a cell? Well, there are generally four main root causes, and it's quite interesting how adhesives can play a role in mitigating pretty much all of them. All right, let's hear them. First up is mechanical abuse. I think crushing, puncturing, a big impact like in a crows.

crash, or even just constant heavy vibration. Any of that physical damage can compromise the cell's internal separator, that thin layer keeping things apart. If that fails, you get internal short circuits, and boom, rapid heating. Adhesives help here by providing real structural integrity, absorbing shock, acting as compression pads. They directly counter those physical forces. So they make the battery tougher, basically.

In a way, yes. Second is electrical stress. like overcharging, discharging too deeply, or an external short circuit somewhere. Right. These create uncontrolled electrical current, which, again, leads to heat. Now, adhesives don't stop the electrical fault itself. They're not a fuse. No, exactly.

But they provide really robust electrical insulation. They stop unintended current paths, prevent arc-in-between components, or secondary shorts that could make the heating much worse. What it? What's third? Third is thermal exposure. Or just being in extremely hot or cold environments for too long can mess with the cell stability. Makes sense. So here you need

adhesives that have a really wide and stable operating temperature range. It typically something like minus 40 Celsius up to 150 C. Some silicones can even go up to 200 C. They need to maintain their properties, not degrade, because adhesive degradation could potentially

lead to other faults. Okay. Stable across temperatures. And the last one. The fourth one is manufacturing defects and internal failures. These can be tiny microscopic things, in Purities in the materials, maybe uneven coatings during production, or slight damage to that separator layer we mentioned. Hard to catch sometimes, I imagine.

Very. And this is where things like conformable gab fillers, a type of adhesive, play a crucial role. They fill in tiny irregularities, make sure heat transfers evenly, and prevent those localized hotspots from forming, which could otherwise be the starting point for runaways. Okay, so adhesives are working against mechanical, electrical, thermal, and internal defect issues. It's quite a job description. It really is. But even if an event does start in one cell, you said the real danger is when it spreads. How does that happen?

How does this thermal propagation move through a densely packed battery? Right. Propagation is the key danger. It's a complex chain reaction. And it happens through several different mechanisms. The most direct one is just cell to cell conduction. Heat moving directly through contact. Exactly.

Heat transferring straight from the compromised cell to its immediate neighbors. Now, for pouch or prismatic cells, cells, that's usually two neighbors. But for cylindrical cells, like the ones you see in some EVs, it could be up to six neighbors touching. More paths for the heat. Precisely. And adhesives used as cell-to-cell barriers, sometimes called CTC barriers, act like critical firewalls. They're designed either to insulate and block the heat or sometimes to redirect it towards a cooling system. Okay, direct contact. What else? Then you have primary and secondary combustion and gas management. This is where it's really violent. Hot, fuel-rich gases get ejected

forcefully from the failing cell. Yes, but these gases are hot and flammable. They can spread and ignite adjacent cells directly. And this brings up a really important point. What about the risk of those toxic hot gases getting into the passenger compartment? Yeah, that's critical. So the adhesives used here need serious flame retardancy. They have to resist catching fire themselves and slow down the spread. They also have to be strong enough to withstand the pressure of these venting gases. And they contribute to sealing the whole battery pack enclosure. directing that gas flow safely away, perhaps through design vents. Think of things like Henkel's fire protective coatings.

Okay, so managing the fire and the gas, intense. Very. Then there's hot particulate ejection. We're talking tiny bits of molten metal, plastic, maybe copper, being shot out at high velocity from the failing cell. Like shrapnel. Essentially, yes. So adhesives or coatings applied with adhesives need to act as robust physical barriers here. They need to resist being punctured or abraded by these particles.

This is where you see things like ablative coatings. They basically sacrifice a layer to absorb the energy and trap those particles. Ablative, like on spacecraft reentry. Similar principle, yeah. Absorbing energy by sacrificing material. Now, here's where the engineering gets really, really tricky. Secondary conductive pathways. What does that mean?

Think about the other components inside the pack. Bus bars, connecting cells, cooling plates. These are often made of highly conductive metals like aluminum or copper. Right, to move electricity. or heat efficiently. Exactly. But that high conductivity means they can unintentionally act like heat tunnels, bypassing those primary C2C barriers we talked about, and carrying heat rapidly to other parts of the pack. Ah, so the solution becomes part of the problem. It can be if not managed. So adheses are strategically applied to insulate these pathways, or sometimes they're used

to bond materials together that deliberately break these thermal bridges. It really highlights this complex balancing act. You need electrical conductivity for performance, but you have to manage thermal conductivity for safety. That is nuanced. Any other ways it spreads. One more main one. Natural convection across air gaps. If there are air spaces within the pack, hot combustion products and gases can simply circulate via air currents spreading the heat around. So filling the gaps helps. Precisely. Adheses used for sealing the main enclosure and also gap fillers used within the pack help minimize these air pathways. Essentially, trapping hot gas isn't slowing their spread. It sounds incredibly fast and chaotic when it happens. You mentioned cooling

systems. Yeah. What about the vehicle's active liquid cooling system? Isn't that supposed to handle heat? Why isn't it enough to stop this kind of rapid escalation? That's a great point. Active cooling systems are absolutely vital for normal battery operation, keeping temps in the optimal range. Yeah. But during the initial super rapid phase of thermal runaway, the sheer amount of heat being generated is just overwhelming. Too much, too fast. Exactly. The cooling system simply can't remove heat fast enough to stop that escalating feedback loop. Plus, think about a crash scenario. The active cooling system pumps, hoses, radiator might be damaged, compromised, or simply lack power. Right. It might not even be working. Exactly. And that really underscores why passive safety solutions like

these specialized adhesives and barriers are so critical. They are the first line of defense. They provide intrinsic built-in protection that works immediately without needing external power or complex electronic controls. They're always on. Okay, so they're the immediate passive safety net. It's clear these are way more than just, you know, glue. Let's really break down their core functions. What are these adhesives doing in there to mitigate thermal runaway? This is where the unsung hero part really comes in, isn't it? Absolutely. This is where you see their incredible multifunctionality. First off, there's heat dissipation and thermal conductivity. We have thermally conductive adhesives, often called TCA's.

Their job is to create a really efficient thermal pathway between the individual cells and the cooling system components, like cold plates. They fill in microscopic air gaps, which are terrible heat conductors, and efficiently transfer heat away from the cells. And crucially, they do this while still providing electrical insulation. How conductive are we talking? It varies quite a bit, depending on the chemistry and fillers used. You might see values around 0.65 watts per meter, Kelvin, that's the unit WMK for some polyurethanes. But you can get up to, say, 7.9 WMK, or even higher for things like silver-filled epochsies, which are extremely conductive thermally. Okay, so moving heat is key. What else?

Second, as we touched on, is electrical insulation and short-circuit prevention. In a high-voltage battery pack, this is absolutely critical. Well, adhesives create dielectric barriers, meaning they don't conduct electricity well, providing accidental electrical contact between cells, modules, or the casing. They stop arcing. You need materials with high breakdown voltage and good dielectric strength here. Makes sense. High voltage needs good insulation.

Definitely. Third, they provide structural integrity and mechanical reinforcement. Instead of just spot welds or mechanical fasteners, adhesives create continuous bond lines. This distributes stress more evenly, making the whole pack much more durable, and improving his performance in a crash. So they actually make the battery structure stronger? Significantly. This even enables advanced designs like Silda Body or CTB, where the battery pack itself becomes part of the vehicle's structure, replacing heavier traditional frame components. Adhesives make that possible. And beyond just strength, they protect against chemical attack,

UV degradation, humidity, ensuring long-term reliability. Okay, strength and durability. That's huge. What's next? Fourth is fire resistance and flame retardancy. This is direct safety. Specialized flame retardant tapes or coatings applied with adhesives act like protective shields. They minimize fire spread if a cell does ignite, buying crucial time for occupants to egress the vehicle. Egress time, right. And some of these materials integrate clever chemistry. Yeah, intermessive materials they swell up when heated, corming a thick insulating char layer that blocks heat and flames. Or ablative materials, like we mentioned, that absorb heat and trap particles by sacrificing themselves. Wow, active firefighting almost. In a passive way, yes. Fifth, a really interesting one. Accommodation of cell expansion and contraction.

Lithium ion cells actually breathe slightly. They change volume a tiny bit during charge and discharge cycles. I didn't know that. Yeah, it's a well-known phenomenon. If the bonding is too rigid, this constant mechanical stress can cause failures over time, debonding, cracking. So you need flexible adhesive formulations, things like silicones, polyurethanes, certain acrylic foam tapes that can absorb this stress, move the cell while still maintaining a strong bond and performing their other functions. So they need to be strong and flexible.

Exactly that balance. And finally, number six is environmental protection. The adhesive itself and the bond it creates needs to resist moisture ingress, attack from chemicals like battery electrolytes or coolants, and degradation from UV radiation over the battery's entire lifespan, which could be 10, 15 years or more. This ensures the adhesive keeps doing all its other jobs. effectively. It really is like a tiny multi-tool inside the pack. Heat management, insulation, structure, fire safety, flexibility, environmental ceiling. It's a demanding list of requirements for one material system. No kidding. If you had to pick one function that seems the most, I don't know,

counterintuitive or surprising for an adhesive, what would it be? Hmm. That's a good question. Maybe the combination of providing significant structural reinforcement while simultaneously needing to be flexible enough to accommodate that cell breathing. Right. You think structural means rigid. Typically, yes. But here, the engineering is so sophisticated that these materials provide immense bond strength and stiffness in certain directions, contributing to the PAC's overall rigidity and crashworthiness, yet they have enough flexibility or give to handle those small cyclical cell movements without failing. That combination is pretty amazing, I think.

That really does highlight the advanced material science involved. Okay, so given all these critical jobs, where are these specialized adhesives actually put inside the battery pack? It must be a very carefully engineered layout. Oh, absolutely. The placement is highly strategic. You'll find them used as cell-to-cell barriers, like we discussed. Those crucial firewalls or insulators sitting directly between adjacent cells to stop or slow thermal propagation. Right, the first line of defense.

Definitely. They're also vital for cell-to-module and module to pack bonding. This is about securely holding the cells together within their modules, and then holding those modules securely within the main battery pack enclosure. This contributes massively to structural integrity and crash performance. Holding everything together tightly. Exactly. Then, for integration with cooling systems, you have those thermally conductive adhesives and gap fillers that Tim's thermal interface materials. They're essential for filling those microscopic air gaps between the cells and the cooling plates, or heat sinks, ensuring that heat can get out efficiently.

maximizing the cooling effect. Precisely. You also see adhesives used in or to bond compression pads in anti-swelling solutions. These are often flexible, pad-like materials placed between cells to manage the mechanical stress from that expansion contraction, and they can sometimes offer additional thermal runaway protection, too. The adhesive holds them reliably in place.

Managing the breathing. Yes. And finally, they're critical for enclosure ceiling and structural reinforcement. Adhesives create strong, durable, often moisture-resistant seals around the main battery pack casing. This protects the internals from the environment, helps contain internal gas pressure during a thermal event, and can even add to the overall structural rigidity of the pack itself. So they're really integrated everywhere, performing different roles in different spots. It's a system-level approach.

You need the right adhesive in the right place, performing the right combination of functions. This sounds like a field with some serious innovation happening. Who are the key players? Who's actually developing and making these advanced adhesive solutions? Yeah, there's a lot of R&D. The several major chemical companies are leaders here. For example, DuPont. They have products like Betamintest structural adhesives,

which are widely used for that structural integrity piece and enabling those cell-to-body designs. Okay, DuPont. They also offer Beta Tick-Tem-Tem's thermal interface materials. Interestingly, some of these are formulated to be diso-sionate and silicone-free, which can be important for EHS reasons. and they're designed with low pullout force. Meaning...

Meaning it's easier to disassemble the battery pack for repair or recycling later on. That connects directly to the whole circular economy push, which is a big deal now. And they also have things like Beta Force 2,800 TC, specifically aimed at handling the heat from fast charging. Ah, tackling specific challenges. Who else? Henkel is another major play. They have Loctite products you might recognize. For batteries, things like Loctite TLB-9300 APSI, it's an engineering. Injectible two-part polyurethane offers good thermal conductivity around 3WMK plus structural bonding and electrical insulation, and it cures at room temperature. Injectable sounds useful for manufacturing.

Exactly. It signals a focus on precision application, maybe robotics, automation and high-volume production. Henkel also has high-strength epoxy like Loctite EA 9497 and a whole range of fire protective coatings. Locktide EA 9400 is an intemescent one. Locktight FPC 5060 is a wire. water-based inorganic type, lots of options. Okay, Henkel. Any others? Dowell is definitely significant too. They offer Dow-Syle silicone products like TC 2035, another thermal conductor around 3.3 WMK. And they have their Voratron line of polyurethane gap fillers like the 1000 series, which are known for having a very low squeeze force.

Why is low squeeze force good? It makes them really easy to dispense accurately and quickly in automated manufacturing lines, reducing stress on the components and the dispensing equipment, it's all about optimizing for mass production. Right. efficiency matters. So we have these big manufacturers creating the chemistries. But you mentioned distributors playing a role, too, like Bodomuller-Chimmy. How do they fit in? Ah, yes. Companies like Boto-Mellor's Chimmy are absolutely crucial. They're much more than just distributors. Think of them as global full-line suppliers, specifically focused on future mobility solutions. They've been involved in like over 2,000 projects worldwide in this area.

Wow, that's a lot of experience. It really is. They act as a vital development partner for both the adhesive manufacturers and the automotive OEMs or battery makers. They provide deep technical expertise, R&D support. They could do customer-specific application testing in their labs, and they handle complex global logistics. So they bridge the gap between the chemical company and the car company.

Perfectly put. They essentially translate the complex. chemical properties of these adheses into practical, usable, technical design data that engineers can actually work with. They have strong partnerships with all the majors, Hankel, Huntsman Dow DuPont. And they're also involved in bringing new solutions to market, like innovative polyurethane structural phones for cell separation that also improve crash stability, reduce vibration, and add thermal insulation. And you mentioned something really interesting earlier, debonding. Yes. This is a really exciting emerging area.

Debonding on demand. They're working. with special primers that allow these super strong structural adhesive bonds to essentially be dissolved or released with minimal effort, maybe using heat or a specific trigger. Why is that important? Think about repair and end-of-life recycling. Right now, glued together battery packs can be really hard to take apart non-destructively. Debonding on demand is a massive step towards a true circular economy for batteries, making them easier to repair, refurbish, and recycle components from. It's about sustainable design right from the start. That sounds like a potential game

changer for the industry's footprint. It really could be. Well, this has been an absolutely incredible deep dive. It's just eye-opening how something we might think of as simple glue is actually this incredibly sophisticated multifunctional material that's totally central to the safety, the performance, and even the future sustainability of electric vehicles. You summarized it perfectly. These materials are working silently, constantly behind the scenes. They're dissipating heat. insulating high voltages, providing structural backbone, resisting fire, accommodating those tiny cell movements, protecting against the environment. They truly are the hidden heroes, making sure our electric future is a safe one. So next time you hear about the latest EV

innovation, faster charging, longer range, remember these silent, sticky heroes working deep inside the battery pack. It really makes you wonder, doesn't it? What other invisible materials like these are secretly revolutionizing industries all around us? And And you know, how will their roles continue to evolve as technology pushes forward?