Lithium-air, or lithium-oxygen, batteries are often considered to be the 'ultimate' battery due to their theoretical energy density, which is ten times that of traditional lithium-ion units, making them comparable to gasoline.

Simply put, commercial lithium-air batteries could revolutionise the clean energy industry. They may enable electric cars to run on a battery that's a fifth of the cost and a fifth of the weight of batteries currently on the market, allowing you to travel from London to Edinburgh - just over 400 miles - on a single charge. Today, an electric car can drive between 50 and 80 miles per charge.

However, there are practical challenges impeding the development of lithium-air batteries, and previous attempts to develop a full-scale solution have resulted in low efficiencies and poor performance.

After only three year's work, scientists at the University of Cambridge have developed a working laboratory demonstrator of a lithium-oxygen battery which has a high energy density, is more than 90% efficient and can be recharged more than 2,000 times.

Heidi Vella-Starr speaks to university lead professor of research, Clare Grey to find out more.

Heidi Vella-Starr: Why are lithium-air batteries the so-called 'ultimate battery'?

Clare Grey: The current generation of lithium-ion batteries has probably got an energy density ten times less than gasoline. Whereas lithium-air batteries - so the reaction between lithium and oxygen - have an energy density that is almost the same as gasoline. It's the ultimate battery because it essentially has the highest energy density that is to-date theoretically possible.

HVS: What are the general technical challenges of developing a lithium-air battery?

CS: The reaction of lithium plus oxygen forms a solid product called lithium-peroxide on the cathode - or positive electrode of the battery. This reaction goes via a whole series of reactive intermediates that basically destroy the electrolyte, which is the liquid that transports the ions in the battery from the anode to the cathode and vice versa, and so it means that a [lithium-air] battery traditionally hasn't lasted very long. They die pretty quickly so there are a lot of problems associated with capacity fade - that is problem number one.

Problem number two is that theoretical energy density is calculated based on using lithium metal on the anode side. Lithium metal itself brings a whole set of challenges associated with plating and stripping of lithium, forming Li dendrites and reacting with the electrolytes, so these problems need to be solved.

Then the third problem is that we talk about lithium-air but actually most of the demonstration projects have been lithium and oxygen because lithium metal, in particular, reacts with nitrogen, carbon dioxide and water, ie., all the other components of air, so to get a practical system to work using air you would have to have a filtration system to get rid of most of the components.

"They die pretty quickly so there are a lot of problems associated with capacity fade - that is problem number one."

The fourth problem is the full energy density of the battery hadn't been realised because of the difficulty of getting the reaction to form the solid product, lithium peroxide, to work reversibly.

The fifth problem is that the efficiency was very bad - the difference between the discharge and charge voltage was very large so you lost a lot of energy by discharging and charging your battery. That isn't practical for a large-scale battery.

And the sixth problem is that the rate is not very high, you can't draw power very fast out of your battery. That is a particular problem for transport.

HVS: What is special about the lithium-air battery demonstrator that you and your team has developed?

CG: We published this recent paper where we showed that we could reduce the inefficiency of the battery. People have used things called redox mediators, molecules that go into a solution and help the reaction process. We found one that brings down the distance between the charge and discharge to its lowest value to date of only 0.2V. So we have made a battery that is 93% efficient. I know that's not 99.9% efficient, but it is the most efficient lithium-air battery to date.

The second thing we did was to get the battery to react reversibly, not by forming the solid product lithium peroxide but by forming lithium-hydroxide instead. It is a different chemical reaction and one that forms a more stable product that seems to be less susceptible to decomposition reactions, so our batteries last longer. We have had batteries that have been running for six months or more without any capacity fade.

The third thing we have managed to do, via this combination of lithium hydroxide and this redox mediator lithium iodide, is to get close to theoretical capacity of lithium-air batteries.

So, out of the multiple problems I described, we improved the efficiency of the battery, improved its longevity and its energy density and produced a battery that can tolerate water to a degree.

HVS: Could a commercial lithium battery work with a filtration system?

CG: It can work with a filtration system. I think the first demonstration will be in a closed system. I don't actually see for some applications that as a problem, it's just that when you say I am not going to use air I am going to use a closed vessel of oxygen, you then need to include the volume of the gas vessel into the (volumetric) energy density calculation; so it is just how you do the numbers game. But I think the first demonstration projects and the first reincarnation of these batteries will be actually in a closed vessel where you'll just get rid of the nitrogen and CO2 totally.

We are working with a company, Johnson Matthey, to try and see what we can do; but filtration systems increase cost and reduce the efficiency of the whole process.

HVS: However you've said commercialisation is still roughly ten years off?

CG: Well, I think we should recognise what that statement means. When you say things are ten years off it means there are a lot of problems you don't know how to solve at this point. If you think about something that just needs development that would be a two to seven-year project. However, we need to solve a multitude of problems to make it commercial.

I don't want to over-emphasise our work but I think what's important is that this is battery technology that people had slightly given up hope on. There was a lot of hype and over-investment in the technology and people had not really seen a way forward, and our work shows that actually there were some strategies and potential ways forward, so I think that is a significant result.

HVS: Lithium-air batteries are often talked about for use in electric cars, but could they also be used for mass energy storage?

CG: People have pushed lithium-air batteries for cars because of that need to find some battery that competes with gasoline and because it has the potential [to do this].

"I personally think that lithium-air would be better for grid-scale and better for static storage."

However, I personally think that lithium-air would be better for grid-scale and better for static storage because it competes with gasoline on a gravimetric [weight] basis, but it doesn't compete with it on a volumetric basis. Because the products are quite fluffy and light, it's volumetrically not great, but when you look at grid-scale storage you are not so bothered about the volumetric energy density. So I actually think lithium-air may play more of a role for the grid than in transport myself, though others may disagree with that.

HVS: What is the plan now in terms of building on your research?

CG: Our focus now is to understand the mechanisms by which these new reactions works. Second off, we want to work on improving the rate - in other words how fast you can get the electrons in or out of the battery. So we are factor ten or more down from a practical rate for a car battery. And we are working on more robust electrode structures, so systems that are less fragile and that also can be used to scale-up to make bigger batteries. At the moment our batteries are 1cm or 1.5cm in diameter, so we need to make batteries that are inherently bigger and more scalable.

HVS: What will a final, commercial lithium-air battery look like, do you think?

CG: Who knows what the ultimate battery will look like? I personally think it will look something like a battery and a fuel cell. We would flow in the lithium-ions and the oxygen when they need them, and the products would flow out, so your energy density would not be limited by the box you have to store your product in.

Grey's research is funded by the UK Government, EPSRC, Johnson Matthey and US Department of Energy.