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 cm(2) 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.