How electric car batteries work

03 March 2020

Interview with 

Dr David Hall, Cambridge University

ELECTRIC CAR

ELECTRIC CAR BEING CHARGED

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What’s actually powering the average electric car? Chemist David Hall works on battery technology at the University of Cambridge, and Chris Smith and Katie Haylor asked him for the lowdown on batteries...

David - Every battery, whether it's a lead acid in your petroleum based vehicle, whether it's a watch battery, or your cell phone's lithium-ion battery, it always has two electrodes. They're solid pieces of material, and they're separated by an electrolyte that's made up of a salt and some sort of a solvent. So lead-acid uses acid dissolved in water. Lithium-ion uses a lithium salt dissolved in an organic solvent, which is carbon based.

Chris - How does it actually make electricity?

David - Batteries use the principle of electrochemistry, where we can convert between chemical energy, and electricity. And we do this by passing current through a wire outside, and then there's a current inside the battery that flows through that electrolyte. That's the movement of those charged particles in the salt.

Chris - So when a battery is charged up versus discharged, what's the chemical difference?

David - So in the case of a lithium-ion battery, basically we take lithium from one material on the positive, and we move it into the layers of the material on the negative side. And that's a very unstable, high energy arrangement, and it wants to get back to how it was before.

Chris - When it's in that unstable state, it's charged?

David - Exactly.

Chris - And when it flows sort of, down the energy hill as it were, to get back to the other material where it's more comfortable, it's discharged?

David - Correct. Exactly.

Chris - So that's why it wants to push electricity around the circuit, because to make that journey, the lithium has got to surrender some charge around the circuit first?

David - Exactly. And it's a much more controlled way to release energy than say, burning fuel, which just releases heat uncontrollably basically.

Chris - And the cars that are being put on the roads that are electric vehicles, do they just use the same technology, the same lithium batteries effectively, that are in my mobile phone?

David - They're very similar in many ways. What's different is the exact components or composition of those materials, especially the positive electrode material. So a lot of phones will have cobalt, whereas modern batteries will have cobalt, nickel, manganese, aluminium in some combination.

Katie - David talking of phones, my mobile phone battery is, ugh, it's really in need of replacement. Why do batteries degrade over time?

David - As you're using your battery, ideally all of these changes, moving lithium from one side to the other is completely reversible, but unfortunately there are these side reactions. You can think of it like how your car rusts with time. It just slowly degrades those materials, and they break down. And you also slowly degrade that organic solvent that I mentioned, releasing gases and oxygen and that sort of thing.

Katie - Can you then tweak the chemistry to try and fix that problem?

David - Absolutely, and this is a huge area of research in the UK and around the world, to understand what exactly is causing those different degradation, parasitic reactions. How can you slow them, or control them, or even just predict them reliably.

Katie - What about habits though? Bad charging habits for batteries. I'm thinking of phones, but I guess with electric cars it must be a similar sort of thing, right?

David - Certainly for phones and laptops, the best thing you can do is make sure they don't get too hot. So don't leave it running full power in your shag carpeting, where it gets really, really hot. Heat is the main thing.

Chris - The guy who invented lithium batteries got the Nobel prize, didn't he? Why are we still married to lithium though, David? Why have we not found an even better chemical or a better way of doing this? Because people say to me, although lithium batteries have made a sort of revolution in storable energy, they're still not without their constraints. As Katie says, her phones clapping out, my laptop will only run for 10 minutes now compared to what used to be five hours.

David - The main concept of lithium ion batteries really hasn't changed. Take lithium out of one side, put it into the negative electrode on the other, but those materials are significantly different than the original ones that got that prize. So modern battery chemistry is actually very different, and you can hold something like two or three times as much energy in the same space. So there is a lot of progress, even within lithium-ion.

Katie - We mentioned charging habits. I'm just curious if you have any advice for charging habits when it comes to an electric vehicle, in terms of the battery?

David - that's a huge area of research as well, that I've spent many years thinking about, especially how can I charge a vehicle very quickly. And usually the faster you charge it, the more damaging and more destructive it is on it. So you try to make things more conductive, so that ions and electricity can flow more quickly and more easily. And that's really a design aspect for where we think these vehicles will be used, how we think they'll be used.

Chris - So it's just a question really then, of not abusing your battery. Is there technology built into batteries to stop us abusing them?

David - Absolutely, and it's a huge area in engineering to build what we call battery management systems, that regulate the temperature, that try not to pass too much current into one battery too quickly. It's another area of development that the auto makers are really getting better at.

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