Scientists demonstrate the usefulness of efficient lithium-oxygen batteries

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Researchers at the English University of Cambridge have overcome several obstacles to create lithium-oxygen batteries. However, that does not mean that Li-O2 batteries will be used in various devices in the foreseeable future.

A ‘traditional’ lithium-oxygen battery is built around an anode made of lithium and a cathode of porous carbon where oxygen is taken from the air and a conductive solution or electrolyte connects the anode and the cathode. Oxygen forms the oxidizer and this is constantly replenished from the air, so that the batteries are lighter than sealed batteries with a fixed amount of oxidizer. The ions are transferred between the cathode and the anode via the electrolyte. The oxidation process of the lithium anode releases energy that can be used as electrical energy. Charging provides the reverse process.

One of the problems that arises in such a lithium-oxygen battery is that when the electricity discharges, lithium peroxide binds to the cathode leading to clogging of the porous cathode. In addition, unwanted chemical reactions occur that cause the electrolyte to become dirty and thus reduce its efficiency.

The researchers were able to overcome these problems by forming lithium hydroxide at the electrodes of the cathode instead of lithium peroxide and by adding lithium iodide as a conductive substance. This prevents the main unwanted chemical reactions that plague lithium-oxygen batteries.

In addition, the researchers did not use porous graphite cathodes, but graphene cathodes to increase the internal surface area for oxygen. To prevent the lithium hydroxide crystals from clogging the porous graphene, the scientists increased the size of the pores from 10 to 100 nanometers to pores from 10 to 100 micrometers. By adding a solvent, the crystals formed are removed during charging and discharging.

However, a way has yet to be found to ensure that no dendrites or tree-shaped structures are formed on the lithium that can grow to such an extent that they can cause short circuits and even explode.

The battery demonstrated is said to be highly efficient, with an efficiency of up to 93 percent. The researchers use the difference between the charging voltage and the supplied voltage as an indication for this. In the prototype, that difference is very small, down to 0.2 volts. Until now, the batteries from Cambridge can be charged and discharged more than 2000 times. Lithium-oxygen batteries have a much higher energy density than conventional lithium-ion batteries. Measured by weight, the energy density is about ten times higher.

The researchers’ findings are published this week in the journal Science.

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