|An early demonstration model of the St Andrews air cell. Air enters and leaves via the porous circular membrane in the centre (Image: Peter Bruce/EPSRC)|
The lithium ion batteries used in laptops and cellphones, and tipped for future use in electric cars, are approaching their technological limits. But chemists in the UK say that there's a way to break through the looming energy capacity barrier – let the batteries "breathe" oxygen from the air.
A standard lithium ion battery contains a negative electrode of graphite, a positive electrode of lithium cobalt oxide, and a lithium salt-containing electrolyte. Lithium ions shuttle between the two electrodes during charging and discharging, sending electrons around the external circuit to power a gadget in the process.
The problem with that design, says Peter Bruce at the University of St Andrews, is that the lithium cobalt oxide is bulky and heavy. "The major barrier to increasing the energy density of these batteries is the positive electrode," he says. "Everyone wants to find a way to push up the amount of lithium stored there, which would raise the capacity."
The answer, he thinks, is to borrow an idea from the zinc-air batteries used in hearing aids, which get their power reacting zinc with oxygen from air. So, working with colleagues at the Universities of Strathclyde and Newcastle, Bruce has begun designing a lithium-air battery.
The new battery has a higher energy density than existing lithium ion batteries because it no longer contains dense lithium cobalt oxide. Instead, the positive electrode is made from lightweight porous carbon, and the lithium ions are packed into the electrolyte which floods into the spongy material.
When the battery is discharged, oxygen from the air also floods through a membrane (see image, top) into the porous carbon, where it reacts with lithium ions in the electrolyte and electrons from the external circuit to form a solid lithium oxide.
The solid lithium oxide gradually fills the pore spaces inside the carbon electrode as the battery discharges. But when the battery is recharged the lithium oxide decomposes again, releasing lithium ions again and freeing up pore space in the carbon. The oxygen is released back to the atmosphere.
Most batteries have all the chemicals they need built in from the start. "By using oxygen from the environment instead you save weight and volume because you don't have to carry the reagents around inside the battery – you just need the carbon scaffold," says Bruce.
The new design is like a battery-fuel cell hybrid, says Bruce. Like a fuel cell it uses reactants from outside the system, while like a battery it also has internal reactants.
The team's prototype device has a capacity-to-weight ratio of 4000 milliamp hours per gram – eight times that of a cellphone battery. Even a 10-fold improvement is possible, but tweaking conventional lithium-ion designs will likely offer only a doubling in capacity, Bruce estimates.
Chemist Saiful Islam researches batteries at the University of Bath, and was not involved in the new design. "My understanding is that the lithium-air battery indeed has the potential to deliver an eight-to-10-fold increase in energy density," he told New Scientist.
However, work is still needed to fully understand the processes taking place in the novel battery, he adds. That should help optimise the technology so it can become a commercially viable product.
Bruce and colleagues are now working to transform their proof-of-principle version into a small working battery like those used in mobile electronic devices. "But the technology could be just as important for electric and hybrid vehicles in future," Bruce points out.