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

They did that by harnessing the lithium polysulphides themselves. The polysulphides stuck to the aramid nanofibres, and their negative charges repelled the other lithium polysulphide ions that continued to form at the sulphur electrode. Positively charged lithium ions, however, could pass freely. The team has designed a cell with a capacity and efficiency approaching the theoretical limits. It can also handle the temperature extremes of automotive designs and supports 1000 cycles of fast charging. But commercial LiS cells are already becoming available for testing by developers ahead of volume production. For example, the LytCell, from Lyten in the US, uses a form of graphene with an innate, 3D structure that can ‘cage’ the sulphur within highly reactive orthogonal graphene layers. This is far more effective in driving full conversions to Li 2 S during discharge and to S 8 during charging within the cathode. It also mitigates the polysulphide shuttle effect. Lyten also uses an engineered coating on the lithium metal anode to mitigate the formation of dendrites. This has achieved lifetimes of over 1400 cycles in prototypes of its designs that have an energy density of 900 Wh/kg, and it plans to start volume commercial production in 2025. The 3D graphene platform can be tuned at the molecular level to specific battery application requirements to improve energy density, provide better temperature performance and enable faster charging in 20 minutes with improved safety. Unlike lithium-ion batteries, it can operate from -30 C to +60 C, reducing the system’s cost by reducing the need for heat sinks and the amount of thermal management required. Testing LiS cells One of the major challenges however is testing LiS cells. In order to distinguish the electrochemical responses from the anode and cathode, a third reference electrode has to be introduced. This technique is widely used for lithium-ion batteries, but there are few reports of three-electrode LiS cells, and those that are used are mostly for electrochemical impedance spectroscopy to measure an electrode, rather than the performance of a cell at different states of charge. As a result, the resistance of the contribution of the lithium anode over extended cycling in the LiS system has not yet been well-investigated. A team of researchers in the Department of Chemistry at Uppsala University, in Sweden, has therefore looked into this. A major challenge in building three-electrode LiS cells arises from the sensitivity of the cell response to the volume of the electrolyte. As an example, the Uppsala team initially looked at the construction of three-electrode cells in a vacuum- sealed pouch cell and in a reusable three-electrode cell housing from a commercial supplier. Cells built in each format showed shorter cycle life and higher cell resistance than expected. This was a result of the vacuum sealing process of the pouch cells, which removes an uncontrolled amount of electrolyte, while the O-rings of the customised cells appeared not to provide a sufficient seal. So the team used a gold microwire coated with a polyimide material to create a three-electrode cell that behaves in a comparable manner to a two-electrode equivalent. The polyimide insulates the microwire throughout the cell, leaving only a small cross-section between the two main electrodes, and makes it easy to ensure good electrode alignment. Crucially for the LiS system, the microwire electrode provides a valuable function when lithiated as a conventional third reference electrode, or as an unlithiated pseudo-reference electrode with the same performance. This is the first effective three- electrode system for measuring the performance of LiS cells. Eliminating lithium Another project, led by Chalmers University, in Sweden, aims to eliminate the lithium metal in the anode for a LiS cell. The Lithium Sulphur Superbattery Exploiting Nanotechnology (LISSEN) project aims to create a cell with a lithiated silicon anode instead, and a nanostructured sulphur-carbon composite as the cathode. This would offer an energy density of at least three times that available from present lithium battery technology, a comparatively long cycle life, a much lower cost by replacing cobalt with sulphur and a high safety degree by eliminating the lithium metal. The researchers have previously developed a 3D sponge, or aerogel, made from graphene, for LiS batteries. This acts as a free-standing Researchers have used a gold microwire coated with a polyimide material to create a three-electrode cell with comparable behaviour to a two-electrode equivalent (Courtesy of Uppsala University) Summer 2022 | E-Mobility Engineering 61 Deep insight | Lithium-sulphur batteries