Cell energy density and electrolyte/sulfur ratio in Li–S cells

Li–S cells have high potential gravimetric cell energy densities between 200 and 600 Wh kg−1. To obtain a high cell energy density, the sulfur fraction in the electrode and the sulfur load per cm2 electrode should be as high as possible next to a good electrochemical sulfur utilization. The quantity of electrolyte added to a cell is crucial for the latter, and an excess of electrolyte is generally beneficial for the electrochemical results. Existing publications on Li–S cells therefore use an excess of electrolyte leading to high electrolyte/sulfur (E/S in ml g−1) ratios as these enable high cycle numbers and good sulfur utilization. However, these studies do not take account of the high passive weight of the electrolyte. The high E/S ratios involved mean that the obtained cell energy density is below commercial lithium-ion cell level. To emphasize the impact of the electrolyte on the cell energy density we calculated possible cell energy densities from material test cell experiments for various E/S ratios, sulfur and carbon loads. Furthermore small pouch cells with only a very small dead cell volume absorbing electrolyte are created to examine ideal E/S ratios for a specific electrode. •Li–S cells usually have an excess of electrolyte and Li to improve the performance.•The cell energy density drastically depends on the added electrolyte amount.•Only small electrolyte/sulfur (E/S) ratios ≤3:1 enable high energy density cells.•Pouch cells were created to examine electrode specific electrolyte/sulfur ratios.•Low electrolyte/sulfur ratios decrease the sulfur utilization.