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

so on). As the cell cycles, some lithium and sulphur is lost to polysulphide molecules that fail to complete the conversion, eventually causing the battery to fail. Various attempts have been made to mitigate the polysulphide shuttle effect. Most efforts focus on adding separators or cathode coatings that keep the polysulphides localised on or near the cathode. Using a solid layer to protect the lithium anode and adding materials to trap polysulphides and reduce the shuttle effect are among the most promising ongoing research areas. The sulphur used at the cathode also undergoes significant expansion as it reacts during discharge. This can result in fracturing of the cathode and a loss of electrical connectivity in it, again reducing the number of charge- discharge cycles. An expansion-tolerant electrode avoids these problems and increases the amount of sulphur in the cathode, giving a performance closer to the theoretical maximum. Further challenges comes from the low solubility of polysulphides in the electrolyte and the rate at which the polysulphide products react during cell discharge. To overcome these issues, materials that increase the rate of the reactions can be incorporated into the cathode to promote improved performance, while extensive research is underway to improve the electrolytes used. The challenge in the development of the technology comes from delivering enough power from the cell to take advantage of the energy density. Historically, most LiS research has focused on developing new materials to improve the sulphur cathode. However, in recent years it has been recognised that developments are needed across the cell to improve the performance of LiS batteries. The Faraday Institution’s Lithium- Sulphur Technology Accelerator (LiSTAR) project, led by University College London with six other universities and seven leading industrial partners, is researching the development of LiS batteries. The project has four work packages focused on improving the critical areas to accelerate the commercialisation of these devices. The first is to design cathodes that maximise the sulphur content and accommodate swelling during operation while including catalytic components to improve the performance of cells. The second is to develop safer electrolytes and incorporate new materials that can improve the efficiency of the battery and mitigate the polysulphide shuttling effect. Advanced computational techniques are being developed in the third work package to speed up the design of the materials and understand the key degradation pathways in LiS cells. The fourth work package is concerned with the design of the lithium metal anode in commercial cell formats. The project sees the possibility of producing a cell with an energy density of 600 Wh/kg and a 500-cycle life at a charging rate of 1 C in the near future. Elsewhere, researchers at Drexel University in the US have developed anodes and intercalation chemistry of traditional lithium-ion batteries. Aside from these differences, LiS batteries’ other components (current collectors, electrolyte, separator) are similar to those used in traditional lithium-ion batteries, although their chemistries are often tuned to interact more optimally with sulphur. A pure sulphur cathode does not conduct electricity, so carbon is typically added in relatively high quantities to improve the performance, and there is a focus on different types of carbon used. This varies from highly processed nanotube carbon for high performance to carbon generated from waste fruit or plastic for low cost. One of the main challenges of LiS cells is the lifetime. A phenomenon called the polysulphide shuttle effect causes a loss of active sulphur from the cathode. Polysulphides formed during cell operation shuttle back and forth between the electrodes, reducing the lifetime of the cell to less than 100 cycles. While the simplified chemical conversion during discharge is 16 Li + 8 S -> 8 Li 2 S, the actual conversion is more complex, and includes intermediate conversions to various polysulphides (Li 2 S 8 , Li 2 S 6 and Carbon nanotubes can help to increase the lifetime of lithium- sulphur cells (Courtesy of Drexel University) Summer 2022 | E-Mobility Engineering 57 Deep insight | Lithium-sulphur batteries