Lithium-Sulfur Cells: State of the Art, Electrolyte/Sulfur Ratio, Silicon Anodes and Safety Tests

Lithium-sulfur cells are potential high gravimetric energy density cells with 200-500 Wh/kg. Unfortunately typical used ether based electrolytes are not stable towards Li and degrade during cycling next to a decreasing active Li mass. Typical Li-S publications therefore work with an excess of electrolyte and Li combined with relatively low sulfur loads allowing high cycle numbers at the cost of low practical relevance.

High cell energy densities can only be obtained with a low electrolyte/sulfur ratio (ml/g) next to high sulfur utilization, fraction and load. Unfortunately low amounts of electrolyte typically decrease the sulfur utilization negatively and most sulfur cathodes need electrolyte/sulfur ratios between 5:1 and 10:1 to deliver maximum sulfur utilization. Unfortunately these ratios are too high to surpass the energy density of commercialized Li-Ion technology. We therefore suggest that the reserach focus should be switched away from maximizing only the sulfur utilization to optimization of the whole electrode capacity incl. the electrolytes weight. But even with such electrodes the obtainable cycle number will be low because of the electrolyte degradation. Since no superior, stable electrolytes exist up to now a good strategy might be the use of (lithiated) Si anodes instead of Li metal anodes. Si anodes face severe volume changes during cycling but are at least not completely converted like Li metal anodes. Thus, the electrolyte degradation might be slowlier with Si anodes and allow higher cycle numbers and also higher safety compared to Li metal anodes.

In the talk we would like to give a short overview about state of the art Li-S research, discuss the impact of the electrolyte/sulfur ratio on cell energy density, show some S₈-Li_xSi_y full cell results with very high active material loads (Si > 2mg/cm²!) and conlude with safety test results of small Li-S multi layer pouch cells.