The lithium–sulfur battery (Li–S battery) is a type of rechargeable battery, notable for its high specific energy. The low atomic weight of lithium and moderate atomic weight of sulfur means that Li–S batteries are relatively light (about the density of water). They were used on the longest and highest-altitude unmanned solar-powered aeroplane flight (at the time) by Zephyr 6 in August 2008.
Lithium–sulfur batteries may succeed lithium-ion cells because of their higher energy density and reduced cost due to the use of sulfur. Currently the best Li–S batteries offer specific energies on the order of 500 W·h/kg, significantly better than most lithium-ion batteries, which are in the range of 150–250 W·h/kg. Li–S batteries with up to 1,500 charge and discharge cycles have been demonstrated, but cycle life tests at commercial scale and with lean electrolyte are still needed. As of early 2014, none were commercially available. The key issue of Li–S battery is the polysulfide “shuttle” effect that is responsible for the progressive leakage of active material from the cathode resulting in low life cycle of the battery. Moreover, the extremely low electrical conductivity of sulfur cathode requires an extra mass for a conducting agent in order to exploit the whole contribution of active mass to the capacity. Large volume expansion of sulfur cathode from S to Li2S and the large amount of electrolyte needed are also issues to address.
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