Making lithium Ion batteries better
Clare Grey at the University of Cambridge is investigating how we can make those sorts of batteries even better...
Clare - We're looking at the diversity of different approaches of which one of them is lithium and I think the reason why it's lithium is that lithium is the lightest element. It's one of the most reactive elements and so that, in principle gives you the highest voltage from the material. There's also a massive industry associated with lithium and so, it's quite important to actually work on the technology we have and improve what we have got. Why am I doing it? So, if you take the material that's in your laptop, most of the laptops contain this material lithium cobalt oxide. It turns out that you can only pull out 50% of the lithium out of lithium cobalt oxide before 2 things happen. First of all, if you put all of the lithiums out, you get structural rearrangements. And so, what happens is the layers slip over each other and then it becomes very difficult to put those lithium back in again. And so, that means that you actually have to keep some of the lithium in those structure. You can't pull them all out. And so, we're carrying around 50% of dead weight in this lithium cobalt oxide that we can't use. And so, if we can figure out how we can make other layered materials cycle. So charge and discharge, pulling out all of the lithium, we would have a battery that would almost be twice as effective. So, that's reason number one. Reason number two is that lithium cobalt oxide, when you pull all the lithiums out, forms all cobalt 4 plus. Cobalt 4 plus is not a very stable oxidation state and it tends to lose oxygen. And so, it's this loss of oxygen, very rapidly associated also with heating and self-discharging that results in the safety instance and the fires that you may have seen on the web. And so, for safety reasons again, we only pull out 50% of the lithium. So, you've got this idea of the fact that you've got a fact of two just sitting in the materials we used today. If we could get that to work, that's the difference between a car which has a range of 100 kilometres going twice as far.
Kate - If you're sticking with lithium because that's what we got. That's the business model that we got laid out, what other aspects of a battery could you possibly change to improve its performance?
Clare - Well, just going back to this idea of approving the electrodes themselves and we talked about lithium cobalt oxide. So there are materials out there where people replace the cobalt with nickel and manganese. And those, instead of having 140 milliampers per gram of charge, so that's just how many electrons can you get per unit of weight, they can allow you to cycle up to 200 and 220 milliampers per gram. So, that's a significant improvement. Then on the anode side, a few could find materials that stored more lithium and the anode that would help. One of the materials for example that we're looking at is silicon. So, silicon allows you to store 10 times more lithium per amount of weight. And so, that's very exciting. At the same time going back to the cathode, you can increase the capacity, but you can also go up in voltage to increase the overall energy density. Now, the trade off with that is safety, The higher voltage, the more oxidising things become and increased risks there. But people are looking at trying to coat electromaterials and protect them in the same way that you might - you think about metal in the environment. You have a passivating coating on the copper on a church. And similar things, we're trying to coat the materials to protect them from these very harsh oxidising environments.
Kate - You've just mentioned a lot of different options. How do you go about testing the effectiveness of all the different options? Do you have to build this battery and how do you decide which materials to test out first?
Clare - So, there's been a lot of work using theoretical methodologies. You can take a structure now at this point and you can use first principle methods to actually calculate the voltage of the material. The challenge is actually to calculate the difficulty of pulling the lithiums out of the structure because the lithium has got to move through different holes. As they jump from site to site, there might be large activation barriers associated with that. in other words, you've got to supply additional energy to get it out. And we would supply that additional energy in a battery in the form of what's known as an over potential. So, a little bit of extra voltage to kick it out and that's inefficient fuel battery. So, those are some of the challenges associated using computations. So, what we would do as chemists, there are people who'd go into the lab and make new materials. So, one of the things that we're going for was instead of just taking cobalt 3 plus to 4 plus, we want to try and find elements where we can change the oxidisation state by more than one. So, we could nickel 2 plus to nickel 4 plus. At the same time, we need to be able to pull the lithiums out. So simply, if we're going to do a 2 plus to 4 plus, we need 2 lithiums. So, we can go in to the lab with those sort of design criteria and try and make materials that might fit that.
Kate - What are the limits on how much you're able to improve these batteries?
Clare - So, I think one thing that's important to remember is that if you have a material that's made up of atoms and ions, and there are only certain number of electrons that you can pull out per ion that you have in your material. So, if you have nickel, the chances are, you're only going to be able to oxidise between 2 plus and 4 plus. And so, the point is, for a unit mass of material, there were only so many numbers of electrons and that puts a fundamental limit in where we're going to go. And so, we can play games and we can find lighter materials, we can use a wider range of oxidation state, but there's a fundamental limit to what we're going to be able to do and I think people do need to recognise that in terms of how we move forward in terms of developing strategies for electrification or for designing of new devices.
Kat - Kate Lamble talking to Clare Grey from the University of Cambridge about her work.