Improved performance of rechargeable lithium-air batteries brings them a step closer to powering cars, claim scientists in Japan. 

Lithium-air batteries are a viable option for use in vehicles due to their high energy-to-weight ratios, or energy density giving them a capacity for energy storage that could be five to 10 times greater than that of Lithium ion batteries. A lithium anode is electrochemically coupled to atmospheric oxygen through an air cathode. During discharge, lithium cations flow from the anode through a solid or gel electrolyte and combine with oxygen at the cathode. But instability of the lithium anode due to moisture and accumulation of charge at the anode and cathode, known as charge-discharge polarisation, results in reduced performance. 

Researchers led by Nobuyuki Imanishi, Mie University, Tsu, have used an aqueous acetic acid-water-lithium acetate electrolyte to attempt to minimise the energy loss. In their design, the Li-metal anode is protected from the aqueous layer by a polymer layer, which improves its stability and the aqueous electrolyte reduces the charge-discharge polarisation. Imanishi estimates the battery to have an energy density of 779 W h/kg, which is more than twice that of a typical lithium-ion battery and higher than previous lithium air batteries reported to be 250-350 W h/kg. 

'Using a water stable lithium-air reactor, the calculated energy density is up to three times higher than that of conventional lithium ion batteries,' says Imanishi. This drastic improvement in the energy density makes the lithium-air battery a much better contender for use in electric vehicles.

Khalil Amine, an expert in lithium ion batteries at Argonne National Laboratory, US, comments 'the polarisation [of lithium-air batteries] is usually very high, over one volt. Imanishi drops that to 0.75 V, but there is still more work to be done.' 

Imanishi agrees there is still further potential to optimise the battery performance. The team are currently studying a number of components of the battery, including improving the energy-to-weight ratio, decreasing the electrode resistance and minimising side product formation after repeated cycling of the battery, he says. 

Patricia Pantos 

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