E-Mobility Engineering 014 l InoBat Auto dossier l In Conversation: Brandon Fisher l Battery monitoring focus l Supercapacitor applications insight l Green-G ecarry digest l Lithium-sulphur batteries insight l Cell-to-pack batteries focus

Lithium-sulphur batteries su er from short lifetimes owing to the polysulphide shuttle e ect. Nick Flaherty reports on attempts to reduce this Shuttle systems T he most significant advantage that lithium-sulphur (LiS) batteries offer over rival battery technologies is their substantially higher energy density per unit weight. LiS cells with lithium metal anodes have a theoretical energy density of 2700 Wh/kg, dramatically higher than lithium-ion cells that with silicon anodes are reaching 470 Wh/kg. This is naturally of interest to vehicle makers to reduce the weight of battery packs while boosting the range. It also avoids the use of nickel and cobalt, which have jumped in price and come from unsustainable sources. LiS cells in comparison can use low- cost sulphur and carbon materials yet still tap into the lithium supply chain catering to the industry, producing cells with the same energy density as lithium-ion for half the cost. They also offer major safety benefits over other battery types thanks to their operating mechanism. The conversion reaction, which forms new materials during charge and discharge, eliminates the need to host lithium ions in materials, and reduces the risk of catastrophic failure of batteries. Alongside that, the highly reactive lithium metal anode is passivated with sulphide materials during operation, further reducing the risk of a dangerous failure. As the cells still use lithium, thermal runaway remains a possibility, but research has shown that the magnitude of this failure is far lower than with lithium-ion cells. LiS technology is also a prime candidate for solid-state batteries. A protective solid-state layer at the anode mitigates the risk of short-circuiting of cells, and the conversion mechanism also enables LiS cells to be stored safely for extended durations and shipped in a fully discharged state, enabling transportation via air freight as the cell is not unstable at low discharge states, unlike lithium-ion batteries. The high energy density means the technology is also of vital interest to electric aircraft designers. Studies by NASA for example see LiS cells as a candidate for large single-aisle passenger aircraft that can carry up to 150 people. While the energy density falls as a result of the practicalities of producing a battery cell, prototype LiS technology has already been demonstrated at 470 Wh/kg, the same as the leading edge lithium-ion cells. Projects around the world are finding ingenious ways to boost the capacity and performance of LiS cells. Development projects In an LiS cell, elemental sulphur (S 8 ) is converted to Li 2 S, with each sulphur atom reacting with two lithium ions during the conversion reaction. In comparison, traditional lithium-ion battery cathodes accommodate only an average of 0.5-0.7 lithium ions per host atom. Most LiS batteries use lithium metal anodes and a cheaper plating/stripping chemistry compared to the graphite This lithium-sulphur technology exploits a method of stabilising a rare form of sulphur in a cathode (Courtesy of Drexel University) 56 Summer 2022 | E-Mobility Engineering