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Boffins build 'intimate contact' cathode for e-car 'super' batteries

Lithium-sulphur tech a step closer to commercialisation?

The smart choice: opportunity from uncertainty

Scientists from Canada's University of Waterloo have developed what they claim is a crucial step forward in the creation of lithium-sulphur rechargeable batteries.

A team led by Professor Linda Nazar have created a cathode capable of sustaining a reversible electrochemical reaction at high current rates.

Lithium-sulphur is seen as a strong contender for the next generation of rechargeable batteries thanks to an energy density way higher than lithium-ion. The upshot: batteries can be made smaller and still support a higher capacity than li-ion equivalents. They're also made from cheaper and less toxic materials.

The car industry is keen on the technology, hoping it with dramatically extend the range of plug-in e-cars. Not to mention the fact, that lithium-sulphur batteries are less, how shall we say, volatile than li-ions, as some notebook owners have found to their cost.

Li-S batteries have an energy density of around 2600Wh/kg. That's more than ten times the 250Wh/kg density of today's Li-ion e-car batteries.

There are problems with the technology, however. Lithium-sulphur designs currently support far fewer recharge cycles than Li-ion. Nazar's contribution helps out with another issue: getting the cathode-end reaction to store and release efficiently during the charge-recharge operations.

Lithium-sulphur battery cathode designs have, to date, been made using a variety of carbon-sulphur materials, none of which has brought the two substances close enough together to ensure smooth electron flow.

Nazar's approach used a form of carbon called 'mesporous carbon' to create an array of 6.5nm-thick carbon rods, each 3-4nm apart. The rods contain tiny pores of a consistent depth and diameter.

Liquid sulphur was poured onto the array and drawn onto the rods' pores by capillary action. The result, Nazar said, was a "most intimate" contact between the electrically active sulphur and the carbon cathode - essentially a much bigger surface area for electron exchange than has been achieved before.

"This composite material can supply up to nearly 80 per cent of the theoretical capacity of sulphur, which is three times the energy density of lithium transition metal oxide cathodes, at reasonable rates with good cycling stability," claimed Nazar.

The next step: refine the process to improve its charge-recharge performance before commercialising the technique. ®

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