What makes EV fires so onerous to place out? [Video]

EVs do not catch hearth usually, however once they do, issues get spicy. How do these fires begin? And why are they so onerous to extinguish? There are scientists making an attempt to reply these questions, however there are additionally scientists who’re nonetheless making an attempt to know what really occurs (chemically) when a lithium-ion battery catches hearth. Can we resolve this drawback with out totally understanding what’s going on?

Video transcript:

EVs do not catch hearth usually, however once they do, issues get spicy.

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(journalist talking in a overseas language)

This hearth required 6,000 liters of water to place out. And that took practically 20,000.

So how do these fires begin and why are they so onerous to place out?

To determine this out, we are able to begin by wanting inside this nine-volt battery.

It seems {that a} 9 volt battery isn’t one. It is really six batteries. These particular person batteries are known as cells, and an EV battery is identical.

It is made up of a whole bunch or 1000’s of cells like this or this, and even larger ones that I could not purchase on-line for some purpose.

A battery hearth like this begins in a single cell by way of a chemical course of known as thermal runaway. We even have X-ray footage of this occurring shot at 1000’s of frames per second.

That is the highest of a cell that appears like this. The very best factor. Outer casing in metal. The sunshine strains are steel oxides and copper. The darkish components are fabricated from plastic and aluminum.

Okay, a close-up of the battery firing even slower.

So, gasoline is build up right here, which you’ll’t see immediately, but it surely’s displacing inner parts.

This factor proper right here, this can be a stress launch valve, and, proper now, it is breaking, which ought to stop an explosion by relieving the stress, however watch out.

Gasoline buildup pushes inner battery parts upward, blocking the valve.

With nowhere to vent, increasingly more gasoline retains build up and all the pieces will get hotter and warmer, which stretches the outer metal casing to its breaking level till it cracks and an explosion happens.

That is the after. Full devastation.

These white globes, that is molten copper. Copper melts at 1,085 levels

” data-gt-translate-attributes=”[{” attribute=””>Celsius, so, inside this battery, youve got temperatures about as hot as a steel forge.

Now, if one cell thermally runs away, it can heat the surrounding cells to the point that they do too, and you got a chain reaction and it spreads to even more surrounding cells, and, pretty soon, youve got a massive battery fire.

There are groups of scientists and engineers trying to solve this problem, but there are also scientists just trying to figure out whats actually happening chemically when a lithium battery catches fire. Because even after a few decades, we dont fully understand it yet.

So, what do we know, and can we solve this problem without fully understanding whats going on?

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Here on Reactions, were gonna find out, after this word from our sponsor.

Do not do this at home. Im a professional.

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This is called the jelly roll.

This is the anode and it releases electrons.

This is the cathode and it absorbs them.

If you put these two in direct contact with each other, youd get a spark, electrons jumping directly from here to here. But if you separate these layers with an electrically insulating layer and provide a path for the electrons via a couple of conducting layers, you can pull electrons out of the battery and make them do some work for you before they return to the battery again.

Now, the electrically insulating layer is called the separator, and it is key to understanding thermal runaway.

Its electrically insulating, but chemically conductive, meaning electrons cannot pass through it, but lithium ions can.

Let that sink in for a second.

Electrons are point-like particles with zero volume. We made a whole video about this. You should definitely check it out. And a piece of plastic with lots of holes blocks them.

But ions are much larger, do have volume, as you can see from this highly accurate diagram I drew, and they can pass right through.

Whats going on?

Well, it actually has nothing to do with size.

Electrons are way too reactive to travel through materials on their own. They need a path and conducting materials like metals have lots of accessible empty orbitals that form exactly that path.

Plastics dont.

So, even though the holes in a piece of plastic are way larger than an electron, theres no conductive path around or through those holes.

Ions, on the other hand, they dont need a path. They can just saunter right through.

So, anyway, the separator is made of plastic.

Now, lets say that this little cell is humming along in an EV, and, for some reason, the cell starts to overheat.

Once the temperature in the cell reaches about 130 degrees Celsius, the separator melts, which means the anode and the cathode make direct contact and you get a spark.

This is called an internal short circuit or an ISC.

Quick side note here. Lots of things can cause an internal short circuit.

If you hammer a nail into the battery, for example, or if a charger malfunctions and overcharges the battery, or if you overheat the battery, or manufacturing defects.

What? I gotta put this back together if Im gonna return it.

Okay, so if you wanted to design something that generates a complete chemical cluster (beep), a battery thats just experienced an ISC is an excellent way to do it.

Youve got a bunch of metals and metal oxides that are prone to give up or accept electrons. Thats why theyre in the battery to begin with. Youve got solids in there, but youve also got liquids. Youve got high temperatures, which means the liquids are prone to change into gases.

So now we have every phase of matter except

By the way, this is why EV fires are so hard to put out. Even though batteries are way less energy dense than gasoline, they generate their own oxygen when they burn.

Gasoline does not.

So, as long as the temperature is hot enough, the batteries can just keep reigniting unless you use thousands of gallons of water to bring the temperature down to the point where that cant happen.

This whole situation is happening in an electrically-conductive environment, which makes it extra hard to keep track of the electrons and by extension the reactions.

So, basically, a complete and utter chemical cluster, to the point where there are entire journal articles that list the chemical reactions that are happening that weve discovered so far.

So this is the crux of the problem. Its just really hard to untangle all of the chemistry that drives thermal runaway. So its hard to figure out a chemistry-centric way to stop it.

Also, battery tech is changing so fast that, even if you could totally untangle everything thats happening in, for example, this type of battery, by the time you do, the industrys moved on to a battery with a different cathode or separator material or whatever.

Despite all of that, though, we can say some really interesting things about thermal runaway, things that help us engineer against it.

Okay, look at this graph, which I have enlarged for your convenience, of temperature of a battery versus time.

As you can see, it happens slowly and then all at once. Now, this all at once bit right here, this is the main thermal runaway event.

Look at how fast that is. The temperature shoots up to its peak in about one second.

Now, the speed here tells us something really important about these reactions, and to understand what that is, we need to mix baking soda with room-temperature vinegar, and also with vinegar at 57 degrees Celsius.

The hot vinegar reacted much faster, and if youve ever cooked something, this makes sense. The hotter your pan, the faster the thing cooks.

Svante Arrhenius, a Swedish chemist living in the 1800s, developed a model to predict exactly how much faster a reaction would go at higher temperatures, and this is it, just one equation.

The important things are K, which you can think of as the speed of the reaction, and T up here in the exponent, that is temperature in Kelvin.

Now, suppose we wanted to compare the speed of a reaction at 298 Kelvin, which is about the temperature of a room, with the speed at 330 Kelvin, which is about the temperature of the hot vinegar.

What we do is we divide one rate constant by the other, and when you sub in this equation, you get this mess, and when you clean this up and rearrange some terms, you get this.

Now, lets say the activation energy, which is just the energy required to get the reaction to go, is 150 kilojoules per mole. Pretty typical. Throw that in, crunch these numbers, and you get 350. The ratio of speeds is 350.

In other words, weve only increased the temperature here by about 30 Kelvin, roughly 10%, but the reaction happens 350 times faster, and thats because of the temperature in the exponent.

When you increase temperature linearly, the speed tends to increase exponentially.

Now, here is a crazy wrinkle.

What happens when you have a reaction that gives off? Damn it.

What happens when you have a reaction that gives off?

When you have a reaction that gives off?

Is this just empty?

No.

Plenty.

What happens when you have a reaction that gives off?

(beep)

A bunch of heat?

You get what I like to call the Arrhenius ouroboros.

The reaction gives off heat, which increases the temperature. The increase in temperature means the reaction goes faster, which means it gives off heat faster, which means the temperature increases faster, which means it gets faster, which means it gives off heat even faster, which means the temperature increases even faster. which means it gets even faster.

This is a positive feedback loop, and, in particular, its an exponential positive feedback loop, which can drive the temperature in the battery from 200 to over a thousand degrees in half a second, and you get

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(reporter speaking in a foreign language)

By the way, this heat, it does dissipate, and it actually dissipates faster with increasing temperature, but that relationship is linear.

The exponential increase in reaction rate vastly overwhelms the linear increase in heat dissipation.

In the early days of lithium-ion, scientists knew a lot less than they know now, and yet they still manage to engineer solutions to problems they didnt fully understand.

How?

Well, sometimes, solutions are obvious, dont depend on the underlying chemistry, and are easy to do.

Batteries are exploding due to high pressure. Well, lets build in a pressure-release valve.

Sometimes solutions are obvious in theory, but the chemistry is hard.

Separators melting?

Lets engineer one with a higher melting point.

Easier said than done. Changing the melting point without changing other properties requires lots of trial and error, but we have been making progress. Separators today melt at much higher temperatures than they used to.

Sometimes the easiest, most obvious solution doesnt solve the problem all the time.

Remember at the beginning when we talked about this X-ray footage?

Despite this pressure release valve functioning exactly as it should, the battery still exploded.

Sometimes the solution involves a layer of tech on top of the chemistry.

Software battery controllers monitor things like charge state and battery temperature, and, for example, shut down the battery when things are getting a little too hot.

And sometimes the solution involves throwing out the whole battery and designing a whole new one with a completely different ion, say, sodium.

One cathode material being tested for sodium-ion batteries is sodium chromite, which releases far less oxygen when heated than similar cathodes in lithium-ion batteries.

Less oxygen means less combustion, which means less heat, which means less potential for thermal runaway.

And for reasons we dont really understand, sodium-ion batteries undergoing thermal runaway tend to release their energy a lot more slowly.

One study measured the rate of temperature increase for a typical lithium battery versus a sodium battery, and found that the lithium battery heated up almost four times faster than the sodium one.

That would give other safety features in the battery much more time to kick in and prevent an explosion.

Most of these solutions, pressure release valves, separators, different cathode materials, they dont require full knowledge of every single chemical reaction happening during thermal runaway.

All they require is understanding, oh, the separators melting? Well, we should design one that doesnt. The cathode is releasing oxygen? We should design one that doesnt.

We can solve these problems before we finish understanding them, like this video.

I dont really understand how were gonna end it, but we can solve that problem if I just keep talking and leave it up to Andrew to just end the credits whenever he sees fit, which could be now, or it could be now, or it could.