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Lithium Batteries Get a Sulphur Makeover

Thu, 4th Apr 2013

Laurie Winkless

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German scientists have designed a high-efficiency, low-cost lithium-sulphur Smart carbattery, which they believe will increase the use of electric vehicles in Germany and beyond.

Earlier this year, the UK government announced an £11M investment in public and private charging points for electric vehicles, which, its hoped will help increase the popularity of electric cars. At present though, the numbers are still small. According to Germany’s Federal Ministry of Transport, there are over 40 million cars on the country’s roads, but only a tiny fraction of them – around 6,400 – are powered by electrical energy.

The high price of both batteries and vehicles may be a factor in this low uptake, but a more real concern is the comparatively short range of electric cars – their lithium-ion batteries need to be recharged after just 100 km of driving. But a team at the Fraunhofer Institute in Dresden have developed a new lithium-sulphur battery, which is significantly more powerful and less expensive than the standard Li-ion batteries.

Previously, lithium-sulphur batteries have had such short lifetimes (just 200 charge-discharge cycles) that they’ve been unsuitable for use in cars, but the team’s new design increases the lifetime of lithium-sulphur batteries by a factor of seven. They have done this by altering the standard combination of anode and cathode material in a lithium battery. The interaction between these electrodes and the liquid electrolyte between them is the critical factor in determining a battery’s performance.

The team, led by Dr. Holger Althues, replaced the usual lithium anode with one made from a silicon- carbon compound, which is much more stable during the charge-discharge cycle. This improved stability reduces the risk of unwanted interactions between the anode and the electrolyte, which, in extreme conditions can lead to a fatal short circuit within the battery. In lithium-ion batteries, the main cathode material is cobalt, a rare, and therefore expensive, material. The Fraunhofer team have replaced the cobalt with elemental sulphur, greatly reducing the cost of the battery. And they have introduced porous carbon into the electrode, which traps the sulphur within the pores, slowing down interactions between the sulphur cathode and the electrolyte.

Suitable large-scale manufacturing methods for these Li-S batteries are now under review, but Althues has also shown that, with further optimisation, the energy density of their lithium-sulphur battery can reach 500 Wh/kg – more than double that of the lithium-ion batteries currently in use in electric cars – meaning that for the same weight of battery, you could drive double the distance.



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But does this new design suffer the thermal runaway that has dogged early Lithium-ion batteries? They sometimes caught fire inside laptops, cellphones and aeroplanes.

When temperatures got high (eg during rapid charging), the electrode decomposed, releasing the stored chemical energy as heat, causing the temperature to rise further.... evan_au, Fri, 5th Apr 2013

evan_au Yep, you're right, that has been a problem with Li-ion batteries for a while. When lithium-ion batteries are recharged, ions move through the electrolyte that separates the battery’s two electrodes. This charge flow also draws material from the electrolyte and results in the formation of lithium dendrites on the positive electrode (anode) of the battery. These dendrites can grow so large that they span the distance between the electrodes; the moment the dendrite reaches the cathode, the battery fails. In this new Li-Sulphur battery, the anode (that would normally be lithium) has been replaced with a silicon-carbon compound which is much more stable at these higher temperatures. The cathode (which is cobalt in a Li-ion battery) is made with sulphur, surrounded by porous carbon - the sulphur will eventually leak into the electrolyte at high temperatures, but it'll do so much more slowly, and more predictably, than the cobalt in Li-ion batteries. Hope that helps :) Laurie , Fri, 5th Apr 2013

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