Nanostructures in Batteries

Sarah: -  Now, lithium ion batteries are an essential part of most of our everyday lives.  They're in our mobile phones, our iPods, and our laptops, and they can be used for hybrid or electric cars, and for storage of electricity from renewables like wind turbines.  For each different job, you're looking at different properties whether it's fast-charging time, efficiency, safety, or cost.  At the moment, there's a trade-off between these properties, but nanotechnology could offer a solution.  Professor Clare Grey from the University of Cambridge is looking at how...

Clare: -   A battery comprises three main components.  You have an anode and a cathode, otherwise known as a positive or a negative electrode, and they're separated by an electrolyte which in most lithium ion batteries is an organic solvent with a lithium salt that allows the lithium to shuttle backwards and forwards.  Now if I want to charge a battery very quickly, that means I have to get the lithium and the electrons in and out of my particles in the electrodes, and so your commercial standard battery material in the cathode is lithium cobalt oxide, and that's a material that's many microns in size.  And so, when I charge the battery, I've got to pull the lithium out of this micron-size material and that takes a long time.  And that's why it takes you a long time to charge your battery for your cell phone or your laptop.  But if I could make the particles smaller, then I would be able to do this much more quickly and get a much higher rate material.

Sarah: -   Nano structures would seem to be the ideal candidates of this task as their small size and large surface area would allow for quicker reactions, leading to quicker charging times.  But it's not all plain sailing.

Clare: -   There are some very good things about nano, but there are also some disadvantages, and one of the major disadvantages is the surface areas.  And so, if you have high surface area, you have a higher potential for side reactions with the electrolyte, and most nano materials and battery materials are not actually stable in the organic electrolyte you use in your batteries.  Now what happens is, as you're cycling material, they form a passivating layer, a coating of inorganics and organics on the surfaces of the materials that protect the material from further attack by the electrolyte.  But now if you imagine it's nano-size, you have a massive surface area.  And so, that whole decomposition to form this passivating layer eats up lithium, and you have a finite amount of lithium in your cell.  So the more you eat up, the less you have to use in your battery application.  Another disadvantage with nano particles, because you can have so many side reactions, is if you have too many side reactions, the system can heat up. 

Once the system starts heating up, many meta-stable materials can release oxygen.  It's the oxygen that then reacts with the organic electrolyte and at high temperatures, that electrolyte can catch fire, and it's that electrolyte burning that you see in many photos on the web of batteries exploding.  And so, if you examine the pros and cons of nano, it's clear that nano materials and nano structured materials allow you to get your lithium and electrons in very rapidly, and so, that's going to be a massive advantage when you want high rate systems.  But it's going to be a disadvantage if you're looking at safety.  I think there are some solutions that will allow us to use nano structures, but it's a very, very important design criteria that we need to build safety into our design.

Sarah: -   And Professor Grey's work is helping to examine exactly what those solutions to the problems with the use of nano structures in batteries might be.

Clare: -   We're very interested in working out the fundamentals by which battery materials function.  So how do the lithiums come in and out of the materials. We do that because if we understand how they function, we can use that to design better materials, and we can also use that to understand why sometimes they don't work.  So what we've done recently that we are very excited by is we've developed a setup, whereby, we can make little batteries that are about the same size as hearing aid batteries. And we connect them up to a potentiostat and that potentiostat is just like a battery charger you might have at home, except it's a little bit smaller, and a bit more accurate.  And then we make use of the fact that lithium has nuclear spin that you can see in an NMR spectrometer.  So that allows us to see where the lithiums are going as we charge the batteries. 

The exciting thing for us is that we can see functioning in real time, all the different components, and we can work out what each component is doing and how each component is influenced by things like charging fast.  So when you're wanting to charge your battery for transportation applications, or if you're using a hybrid electric vehicle and you put your foot on the brake, that requires an extremely rapid charge, and that puts tremendous demands on your batteries, and that often in itself encourages the formation of side products that then may have negative consequences on how the battery then functions, and the bottom line is we can see this in real time, and we can try and device strategies to prevent that happening.  So we're looking at function and structure, and then trying to use that to then design newer systems or improve systems that both last longer, and more safe.

Sarah: -   And if you've ever seen a battery self-ignite, I'm sure you'll agree that safety is paramount.  That was Clare Grey from the Department of Chemistry at Cambridge University.