More hurdles jumped on path to a practical lithium-air battery

False-colour microscopic view of a reduced graphene oxide electrode (black, center), which hosts the large (on the order of 20 micrometers) lithium hydroxide particles (pink) that form when a lithium-oxygen battery discharges(Credit: Reprinted with permission from T Liu et al., Science 350: 530 (2015))

With theoretical energy densities as much as 10 times that of current lithium-ion batteries, lithium-air (or lithium-oxygen) batteries hold tremendous potential for storage of renewable energy and use in mobile devices and electric cars. Although a practical lithium-air battery is still some years from becoming a commercial reality, researchers at the University of Cambridge have developed a working laboratory demonstrator that shows how many of the problems holding back the development of lithium-air batteries could be overcome.

Because they "breathe" oxygen from the air to power the chemical reactions that release electricity rather than storing an oxidizer internally like lithium-ion batteries do, lithium-air batteries boast an energy density comparable to gasoline. This would allow an electric car with a battery one-fifth the cost and weight of those presently on the market to drive 400 miles (650 km) on a single charge.

That's the theory anyway. The problem is, practical attempts so far to develop the lithium-air battery have not yielded great results, despite efforts from the likes of IBM and MIT. Until now, these attempts have resulted in low efficiency and poor rate performance, instability and unwanted chemical reactions. In an attempt to overcome these problems, researchers from the University of Cambridge created a battery that uses a different chemistry to previous designs.

The team engineered the negative electrode from a highly porous form of graphene, added water and used lithium iodide for use as a "mediator." They also altered the chemical makeup of the electrolyte so that the battery creates lithium hydroxide (LiOH) at the cathode instead of relying on the lithium peroxide (Li2O2) used in earlier lithium-air battery designs. The products of the chemical reaction build up in the porous cathode as the battery discharges, and dissolve as the battery is recharged.

Combining the new chemical makeup of the electrolyte with the new, "fluffy" carbon electrode resulted in a battery that is far more stable, with an energy efficiency of 93 percent – which means the amount of energy lost as heat is close to that of a lithium-ion battery. The researchers claim that their demonstrator battery can be cycled more than 2,000 times with little impact.

This new approach is a significant advancement, but the researchers stress that the development of the lithium-air battery still has a long way to go, estimating a practical device is still a decade away. The main issue is that, unlike previous experimental lithium-air batteries that are able to take their oxygen from the air, the University of Cambridge demonstrator can only be cycled in pure oxygen. This is because other gases found in air, such as carbon dioxide, nitrogen and moisture, can damage the metal electrode.

"There's still a lot of work to do," said Dr Tao Liu, a researcher from the Department of Chemistry at the University of Cambridge. "But what we've seen here suggests that there are ways to solve these problems – maybe we've just got to look at things a little differently."

The team's research is detailed in a paper published in Science.

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