Electric vehicles: Floating on air
Lithium-air batteries could one day offer much higher energy density than today's technology. But researchers need to improve the components to reach the durability required
James Scoltock in Focus.
- Published in Focus.
Obvious limitations mean lithium-ion batteries can’t meet all of the carmakers' demands
Lithium-air battery technology is being researched as a solution to the problem of the limited range of today's electric vehicles. The technology works by oxidising lithium at the anode and reducing oxygen at the cathode to induce a current flow.
Lithium-ion batteries are widely used now for alternative powertrains, but the chemistry has limited energy density.
The maximum possible is thought to be 300Wh/kg. Current batteries hold
100Wh/kg, giving electric vehicles a typical range of 160km. A typical combustion engine vehicle can travel 700km on a single tank of fuel.
Lithium-air battery packs could increase energy density to 1,000Wh/kg, giving electric cars the same range as combustion engine vehicles. There are challenges to overcome first, however.
Professor Peter Bruce from the Edinburgh and St Andrews Research School of Chemistry in the UK is investigating catalysts for lithium-air batteries. He says: “It's come a long way in the last few years but there are quite a number of hurdles that need to be addressed before it could be a viable battery technology.”
An automotive battery needs to be able to be charged and discharged up to 3,000 times in its life. Lithium-air batteries being tested in the laboratory manage only a fraction of that.
“One of the main challenges is the rate at which you can cycle a lithium-air battery,” says Bruce. “And the voltages that you charge the battery at are higher than the voltage you discharge. That means the energy efficiency isn't optimised.” Initial research showed that the difference in voltage was almost 1V, but that has now been reduced to 0.5V.
Most battery manufacturers have focused their research on lithium-ion, but its obvious limitations mean it can’t meet all of the carmakers' demands.
Dr Allan Paterson, a senior electrochemical engineer at Axeon, has worked with both technologies. He says: “Lithium-air is still at the lab stage but the theory behind it is sound and the theoretical energy densities that might be achievable are well understood. Those gains are significant and bring you into the realms of competing with gasoline.”
Much of the development work is focused on making lithium-air batteries rechargeable. Metal-air batteries, such as zinc-air, used in hearing aids are energy-dense but not rechargeable. Developing rechargeable versions will require research into new catalysts, membranes and extra components within the cells.
Paterson says: “The catalysts catalyse the reverse reactions that allow you to recharge the system. There are aqueous and non-aqueous catalysts. Traditional lithium-ion cells use non-aqueous solvents with mobile lithium ions within them. The aqueous systems have to use extra components to stabilise the technology – ceramic or protective layers to keep moisture out of the system. But it becomes more complex.”
