German scientists have dramatically increased the lifespan of lithium-sulphur batteries by using a new design.
Lithium-sulphur batteries are significantly more powerful and less expensive than the better-known lithium-ion battery, but their short lifespan has made them unsuitable for use in cars before now.
But scientists at the Fraunhofer Institute for Material and Beam Technology IWS in Dresden believe they may have solved the problem, increasing the charge cycles of the batteries by a factor of seven.
“During previous tests, the batteries scarcely crossed the 200-cycle mark. By means of a special combination of anode and cathode material, we have now managed to extend the lifespan of lithium-sulphur button cells to 1,400 cycles,” says Dr Holger Althues, head of the Chemical Surface Technology group at IWS.
The anode of the team’s prototype is not made from the usual metallic lithium, but from a silicon-carbon compound instead, which is significantly more stable as it changes less during each charging process than metallic lithium.
The more the structure of the anode changes, the more it interacts with the liquid electrolyte, which is situated between the anode and the cathode and carries the lithium-ions.
This process causes the liquid to break down into gas and solids and the battery to dry out.
“In extreme cases, the anode ‘grows’ to reach the cathode, creating a short circuit and causing the battery to stop working altogether,” explains Althues.
The advantages of a lithium-sulphur battery is that unlike cobalt – the main cathode material used in lithium-ion batteries – sulphur is available in almost unlimited quantities and is therefore cheaper.
The problem is that sulphur also interacts with the liquid electrolyte, which impairs the performance of batteries and, in the worst case, causes them to lose capacity entirely.
But now the IWS researchers are using porous carbons to slow down this process.
“We have precisely altered the pores to allow the sulphur to lodge there, slowing down the rate at which it combines with the electrolyte,” says Althues.
Over the long term, the experts at IWS expect lithium-sulphur batteries to reach an energy density of up to 600 Wh/kg compared to the maximum energy density of the lithium-ion batteries currently in use of around 250 Wh/kg.
“In the medium term, figures around the 500 Wh/kg mark are more realistic. In practical terms, this means you can drive twice as far with the same battery weight,” says Althues.
This of course implies that significantly lighter battery models are possible – an interesting prospect not only for automakers but for smartphone manufacturers too.
“Lithium-sulphur technology might even make electric flying a realistic possibility. Although such progress is still a long way off,” adds Althues.