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

a way to stabilise a rare form of sulphur in a cathode. This would allow it to be used in the carbonate electrolyte used in commercial lithium-ion batteries. Previous attempts to use a sulphur cathode in a battery with a carbonate electrolyte solution resulted in nearly immediate shutdown and a complete failure of the battery after only one cycle. The researchers say this could provide three times the capacity of lithium-ion batteries and last more than 4000 recharges – the equivalent of 10 years of use. “Sulphur has been highly desirable for use in batteries for a number of years, because it is abundant and can be collected in a way that is safe and environmentally friendly,” says Dr Vibha Kalra, a professor in Drexel’s Department of Chemical and Biological Engineering, who led the research. “And as we have now demonstrated, it also has the potential to improve the performance of batteries in EVs and mobile devices in a commercially viable way. “Most LiS systems have adopted ether electrolytes to avoid the adverse reactions with carbonate,” he adds. “Then, over the years, the researchers deep-dived into enhancing performances in ether-based sulphur batteries by mitigating polysulphide shuttle. But the field completely overlooked the fact that the ether electrolyte itself is a problem. “In our work, the primary objective was to replace the ether with carbonate, but in doing so we also eliminated the polysulphides. “That meant no shuttling, so the battery could perform exceptionally well through thousands of cycles.” The team had previously developed a cathode using carbon nanofibres that slowed the shuttle effect in ether- based cells by reducing the movement of intermediate polysulphides. But to improve the commercial path of the cathodes, the group realised it needed to make them function with a commercially viable carbonate electrolyte. “Having a cathode that works with the carbonate electrolyte they are already using is the path of least resistance for commercial manufacturers,” says Dr Kalra. “So rather than pushing for industry adoption of a new electrolyte, our goal was to make a cathode that could work in the pre-existing lithium-ion electrolyte system.” A new form of sulphur The team tried to confine the sulphur in the carbon nanofibre cathode substrate using a vapour deposition technique. While this process did not succeed in embedding the sulphur within the nanofibre mesh, the sulphur crystallised in an unexpected way, forming a slight variation of it called monoclinic gamma- phase sulphur. This chemical phase of sulphur, which is not reactive with the carbonate electrolyte, had previously only been created at high temperatures in labs, and has only been observed in nature in the extreme environment of oil wells. “As we began the test, it started running beautifully – something we did not expect,” says Dr Kalra. “In fact, we tested it over and over again – more than 100 times – to ensure we were really seeing what we thought we were seeing. The sulphur cathode, which we suspected would cause the reaction to grind to a halt, actually performed amazingly well, and it did so again and again without causing shuttling.” Rahul Pai, a doctoral student in the department and co-author of the research, says, “At first it was hard to believe that this was what we were detecting, because in all previous research monoclinic sulphur has been unstable under 95 C. “In the past century there have been only a handful of studies that produced monoclinic gamma sulphur, and it has only been stable for 20 to 30 minutes at most. “But we had created it in a cathode that was undergoing thousands of charge-discharge cycles without diminished performance – and a year later, our examination of it shows The design of the separator membrane is key to the performance of lithium-sulphur cells (Courtesy of University of Michigan) 58 Summer 2022 | E-Mobility Engineering Deep insight | Lithium-sulphur batteries