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Li-S batteries now a more practical proposition for electric vehicles

02 April 2013

Fraunhofer researchers have succeeded in significantly increasing the number of times a lithium-sulphur battery can be charged.

Roll-to-roll coating of lithium-sulphur battery electrodes at IWS (photo: Jürgen Leibmann)

Lithium-sulphur is a promising avenue of EV battery research. Lithium-sulphur batteries are significantly more powerful and less expensive than lithium-ion battery, but their short lifespan has so far precluded their use in electric vehicles. However, thanks to work by scientists at the Fraunhofer Institute for Material and Beam Technology (IWS) in Dresden, this may be about to change.

IWS has developed a new design that increases the number of charge cycles possible for lithium-sulphur batteries by a factor of seven," says Dr Holger Althues, head of the Chemical Surface Technology group at IWS. “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."

The anode of the prototype cell is not made from the usual metallic lithium, but from a silicon-carbon compound instead. This compound is significantly more stable, as it changes less during each charging process than does metallic lithium.

The more the structure of the anode changes, the more it interacts with the liquid electrolyte containing 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 interplay between anode and cathode is the critical factor determining the performance and lifespan of a battery. In the lithium-sulphur model, the cathode is composed of elemental sulphur. The advantage here 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 remains, however, 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.

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,” explains Althues. He and his colleagues have developed a method of manufacturing these special cathodes.
 
The experts at IWS measure the capacity of a battery in watt-hours per kilogram (Wh/kg). Over the long term, they expect lithium-sulphur batteries to reach an energy density of up to 600 Wh/kg. For comparison: the maximum energy density of the lithium-ion batteries currently in use is 250 Wh/kg.

“In the medium term, figures around the 500 Wh/kg mark are more realistic," adds Althues. "In practical terms, this means you can drive twice as far with the same battery weight.”

This of course implies that significantly lighter battery models are possible – an interesting prospect not only for the automotive sector, but for smartphone manufacturers, too.


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