How can we improve the performance of batteries?

Rechargeable lithium-ion batteries have revolutionised the modern electrical area, unlocking the door to powerful devices that we rely on each day, such as phones and electric cars. But where they fall down is in the time it takes to charge them, and how well they perform across a range of temperatures and especially in the cold. But now scientists in China have come up with a new recipe for the electrolyte that enables the lithium ions, which store the energy in these batteries, to move around. This can hugely increase the rate at which the battery is able to charge and discharge, and it does it even in the very cold. Rhodri Jervis works on ways to build better batteries at UCL.

Rhodri - All batteries, all electrochemical devices have to have three main things. Those are electrodes whereby the electron transfer happens, the electricity is transported, and they will have a positive and negative electrode. They will have an electrolyte, which is some sort of salt dissolved in some sort of liquid, which we call the solvent. And they will have a separator to keep those two electrodes apart and to allow the electrolyte to transfer between them. So if you opened up, and we do this in the lab in a very safe way obviously, but if you were to open up a lithium ion battery, you would see a couple of very thin foils, copper and aluminium foils, onto which these electrodes are actually printed. So the electrodes in lithium ion batteries are composed of particles of what we call active material, this is the material that can charge and discharge and there'd be a positive and negative electrode of these sort of coated foils. They look just like black inks, basically like dried black inks. And then in between those electrodes you would have this liquid solution of salt dissolved in the solvent. And that's the key thing that they've changed in this paper to allow for the charging of the battery to be much faster.

Chris - There are some constraints with the present generation of lithium batteries. I mean, they're brilliant and they've transformed industry as well as life in general, haven't they? But those constraints are - they don't work very well in the cold as my electric car keeps telling me, and they tend to, as my mobile phone is increasingly telling me, degrade with time, they don't hold as much charge, they don't work as well. Why does that happen? Before we get into what the new study shows, why do I see those changes in battery performance?

Rhodri - Yeah, it's a very good question. It's one that we're trying to solve all the time. Just to sort of take it back to the usefulness of lithium ion batteries, you're absolutely right. They have been transformative over the past 30 or 40 years since they've been around. And one of the key innovations in the commercialisation of these batteries by Sony was to use graphite as the negative electrode instead of lithium metal, which is what people used previously. This lithium metal wouldn't charge and discharge, wouldn't cycle very repeatedly, and also caused some safety issues as well. So the use of graphite was really key in making reliable safe batteries. However, that is what actually limits the rate at which the batteries can charge. When the graphite accepts lithium charged particles during that charging process, it can actually degrade a little bit of the electrolyte, that liquid that sits within the battery. And it could end up causing this very thin coating on the, on the surface of the graphite. We call this an SEI, it doesn't really matter what that means, but it's basically a very thin coating. This actually helps to protect the graphite causing any further breakdown of that electrolyte. So this SEI is a really key thing to the stable operation of these lithium ion batteries. However, that last journey for the lithium charge particle has to go through that thin SEI layer and that sort of dictates how quickly those lithium particles can move through the battery, which of course then dictates how quickly you can charge your car. So at the moment you'd be limited to maybe half an hour, an hour probably of charging over time. One of the degradation modes is for that SEI layer to thicken and therefore increase the resistance that the lithium particles feel when they travel through that. The materials can crack, we can have gas evolution. There's all sorts of ways in which lithium ion batteries slowly degrade. They are pretty remarkable in how long they last currently, but one of the key things we're trying to unlock is this fast charging ability. If you could charge your car in 10 minutes, that very much changes how you approach a long journey, how you approach range anxiety, and also probably the size of the battery pack that you want in your car. So currently people have very large battery packs so that they don't need to charge as much, because it takes at least half an hour to charge. So if you cut that down to five or 10 minutes, perhaps it's less of an issue.

Chris - Which of the problems have they addressed in this paper or both?

Rhodri - They've addressed a couple of different problems here. So the fundamental problem of the rate at which these charged lithium particles move through the electrolyte has improved. So when these lithium ions, these lithium particles, are dissolved in the electrolyte, they're surrounded by the other constituents of that electrolyte. And these serve to sort of slow down the movement of this lithium particle through the electrolyte. You can kind of think of it as someone trying to get through a crowded room at a party. If that person's very popular and everyone wants to stop and talk to them for a few minutes, that's going to take a long time for them to get from one side of the room to the other. But there are other mechanisms by which this lithium can travel through the electrolyte and one is by sort of hopping across a linked region of lots and lots of lithiums. So if you have a high concentration of these things, it's almost like passing a note amongst other people. And that's sort of what they've done here. They've created a network via which these lithium particles can travel very quickly, but they've also made a new type of solvent that allows for a formation of this SEI layer that I mentioned, this protective layer that allows for very rapid transport of lithium through that last little bit of its journey. So through the work in this paper, they've managed to even at very low temperatures of sort of minus 70 degrees, about as low as you'd ever <laugh> want to drive around in really, they've managed to allow for charging rates that would be the equivalent of charging your car at 10 minutes or less, which is really quite remarkable.

Chris - Is it practical though? Because there are things we can do which would work wonderfully, but they would be completely economically viable or toxic as you like. Are these materials that they're talking about invoking to do this environmentally friendly and sustainable and more moreover cheap?

Rhodri - That is a key issue. So currently lithium ion batteries employ what we would call organic solvents instead of for example, water, which would be very sustainable. And a lot of these things are not sustainable and they're actually quite flammable. This solvent is different to the organic solvents that are used currently, but it is still an organic material. So I'm not entirely sure of how sustainable this would be compared to the current materials, but one thing is that it would allow a much better performance and potentially than with the fast charging smaller battery packs. And one of the best things you can do for sustainability with batteries is use fewer batteries. In that respect, it should be very positive.