While lithium-air batteries are seen to be promising for use in electric cars and portable electronic devices, they waste energy and degrade relatively quickly. A further constraint is their open cell configuration, which requires extra components to pump oxygen in and out. Despite these drawbacks, they hold the prospect of delivering high energy output per weight.
However, a variation of the battery chemistry, developed by a team at MIT, could be used in a conventional sealed battery. The new approach relies on the creation of nanoscale particles – nanolithia – containing lithium and oxygen, confined tightly within a matrix of cobalt oxide, which stabilises the particles and acts as a catalyst.
Professor Ju Li says there is a mismatch between the charging and discharging voltages, with the output voltage more than 1.2V less than charging voltage. “You waste 30% of the electrical energy as heat in charging. It can actually burn if you charge it too quickly,” he says.
Conventional lithium-air batteries draw in oxygen from the air to drive a chemical reaction with lithium during the discharging cycle. Oxygen is then released to the atmosphere during the reverse reaction.
In the new variant, called a nanolithia cathode battery, the same kind of electrochemical reactions take place, but without the oxygen reverting to a gaseous form. Instead, says the team, the oxygen transforms directly between its three redox states, while bound in the form of three different solid chemical compounds – Li2O, Li2O2, and LiO2 – which are mixed together in the form of a glass. This reduces the voltage loss to 0.24V, so only 8% of the electrical energy is turned to heat. “This means faster charging for cars, as heat removal from the battery pack is less of a safety concern, as well as energy efficiency benefits,” Prof Li adds.
Because the ‘solid oxygen’ cathodes are lighter than conventional lithium-ion battery cathodes, the new design could store up to twice the amount of energy for a given cathode weight, the team says. With further refinement of the design, says Prof Li, the new batteries could double that capacity again.
The team expects to move from a lab scale proof of concept to a practical prototype within a year.
Pic courtesy MIT research team