Advances in lithium-air technology indicate high-capacity batteries in a decade

Scientists at the university say they have developed a working laboratory demonstrator of a lithium-air battery which has very high energy density, is more than 90% efficient, and can be recharged over 2,000 times.

Lithium-oxygen, or lithium-air, batteries are seen as the ‘ultimate’ battery due to their theoretical energy density, which is ten times that of a lithium-ion battery. Such a high energy density would be comparable to that of petrol, and would enable an electric car with a battery that is a fifth the cost and weight of those currently on the market.

Several practical challenges still need to be addressed before lithium-air batteries become a viable alternative to petrol, but the Cambridge team believes it has overcome some of them with its lab-based demonstrator.

Their demonstrator relies on a highly porous, carbon electrode made from graphene, and additives that alter the chemical reactions at work in the battery, making it more stable and more efficient. While the results, reported in Science, are promising, the researchers warn that a practical lithium-air battery still remains at least a decade away.

“What we’ve achieved is a significant advance for this technology and suggests whole new areas for research – we haven’t solved all the problems inherent to this chemistry, but our results do show routes forward towards a practical device,” said Prof Clare Grey of Cambridge’s Department of Chemistry, the paper’s senior author.

According to the university, previous attempts at working demonstrators have had low efficiency, poor rate performance, unwanted chemical reactions, and can only be cycled in pure oxygen.

The team at Cambridge, including  Dr Tao Liu from the Department of Chemistry, have developed a battery that uses a very different chemistry than earlier attempts at a non-aqueous lithium-air battery, relying on lithium hydroxide (LiOH) instead of lithium peroxide (Li₂O₂). With the addition of water and the use of lithium iodide as a ‘mediator’, their battery showed far less of the chemical reactions which can cause cells to die, making it far more stable after multiple charge and discharge cycles.

By precisely engineering the structure of the electrode, changing it to a highly porous form of graphene, adding lithium iodide, and changing the chemical makeup of the electrolyte, the researchers were able to reduce the voltage gap between charge and discharge to 0.2 volts.

The university says that a small voltage gap equals a more efficient battery – previous versions of a lithium-air battery have only managed to get the gap down to 0.5 – 1.0 volts, whereas 0.2 volts is closer to that of a Li-ion battery, and equates to an energy efficiency of 93%.

The highly porous graphene electrode also greatly increases the capacity of the demonstrator, although only at certain rates of charge and discharge. Other issues that still have to be addressed include finding a way to protect the metal electrode so that it doesn’t form dendrites. Additionally, the demonstrator can only be cycled in pure oxygen.

The technology has been patented and is being commercialised through Cambridge Enterprise, the University’s commercialisation arm.