Fluffy carbon electrodes bring lithium-air batteries closer to reality

Production of batteries with a higher energy density than lithium-ion is still 10 years off

Ten times more power than lithium ion -- but still ten years off: A fluffy carbon electrode has brought scientists at Cambridge University a step closer to producing a workable lithium-air battery, but many technical challenges remain.

Today's lithium-ion batteries are light, but bulky for the charge they store. Other battery chemistries have a better energy density. For years, scientists have been looking for ways to make batteries with all the advantages of Li-ion, but that take up less space.

Lithium-air batteries, with a theoretical energy density ten times that of Li-ion, are seen as the way forward, but experimental models have so far proven unstable, with poor charge or discharge rates and low energy efficiency. Worse, they can only be operated in pure oxygen, making them impractical for use in a normal atmosphere.

The atmospheric problem is still unsolved, but in a paper published in the journal Science on Friday, researchers at Cambridge University describe how they have solved some of the stability and efficiency problems by adding lithium iodide and using a fluffy carbon electrode made of sheets of graphene.

The positive and negative electrodes in Li-ion batteries are made of a metal oxide and of graphite, respectively. A lithium salt dissolved in an organic solvent acts as an electrolyte, carrying lithium ions between the two electrodes.

In the lithium-air battery developed by Tao Liu, Clare P. Grey and colleagues at Cambridge, the carbon electrode is made of a porous form of graphene.

They chose to store charge by forming and removing crystalline lithium hydroxide (LiOH) rather than the lithium peroxide used in other lithium-air battery designs.

By adding lithium iodide they were able to avoid many of the unwanted chemical reactions that have slowly poisoned previous designs. That improved the cell's stability even after multiple charge and discharge cycles. So far, they have been able to recharge the cell 2000 times.

This and other tweaks to their design allowed them to keep the voltage gap between charge and discharge in line with that of Li-ion cells, around 0.2 volts, compared to 0.5-1V for other lithium air designs. That, they say, makes their cell 93 percent energy-efficient.

There are still a host of problems to be solved, though, before the battery could enter commercial production. Its capacity is highly dependent on the rate of charge and discharge, and it is still susceptible to the formation of dendrites, fibers of pure lithium that can short-circuit the battery's electrode and cause an explosion. There's also the problem of air, which contains nitrogen, carbon dioxide and water vapor in addition to the pure oxygen that the experimental cell requires.

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Peter Sayer

IDG News Service
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