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MIT team advances understanding of lithium-air battery technology

13 May 2013

The lithium-air battery could store up to four times as much energy per weight as lithium-ion batteries, but the electrochemistry is poorly understood.

MIT graduate researchers Robert Mitchell and Betar Gallant connect a Li-air battery used to prepare the samples for in-situ TEM characterisation (photo: Jin Suntivich)
MIT graduate researchers Robert Mitchell and Betar Gallant connect a Li-air battery used to prepare the samples for in-situ TEM characterisation (photo: Jin Suntivich)

Researchers at MIT and Sandia National Laboratories have used transmission electron microscope (TEM) imaging to observe, at a molecular level, what goes on during a reaction called oxygen evolution as lithium-air batteries charge; this reaction is thought to be a bottleneck limiting further improvements to these batteries. The TEM technique could help in finding ways to make such batteries practical in the near future.

The new observations show, for the first time, the oxidation of lithium peroxide, the material formed during discharge in a lithium-air battery. At high charging rates, this oxidation occurs mostly at the boundary between the lithium peroxide and the carbon substrate on which it grows during discharge — in this case, multi-wall carbon nanotubes used in the battery electrode. 

The confinement to this interface shows that it is the resistance of lithium peroxide to a flow of electrons that limits the charging of such batteries under practical charging conditions.

An electrolyte-coated probe tip serves as the opposing electrode for removing lithium ions during charging, as electrons flow through the nanotube framework to the external circuit. During charging, the lithium peroxide particles shrink, beginning at the nanotube-peroxide interface, showing that oxidation occurs where it is easiest to remove electrons. 

According to the researchers, the lithium transport can keep up, which indicates that electron transport could be a critical limit on charging of batteries for electric vehicles.

In fact, the rate of lithium peroxide oxidation in these experiments was approximately 100 times faster than the charging time for laboratory-scale lithium-air batteries, and approaches what is needed for applications. This demonstrates that if these batteries’ electron-transfer characteristics can be improved, it could allow for much faster charging while minimising energy losses.

The researchers believe this is the first direct evidence that electron transport is limiting the charging. Lithium-air battery performance would improve if electrodes had a high-surface-area structure to maximise contact between lithium peroxide and the carbon required to transport electrons away during charging. 

The critical next step will be to measure actual currents during charging. 

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