Lithium-Sulfur-Batteries (LSBs) have been discussed as one of the most promising post-lithium-ion-battery technologies in literature for the last decade due to their high theoretical specific energy of 2600 Wh/kg. However, in case of liquid LSB-concepts, the actual achievable specific energy is limited to about 400 Wh/kg as high access of electrolyte and other passive materials are required to run the cells. The conversion mechanism involves the dissolution of the active material, sulfur, as polysulfides in the liquid electrolyte, which leads to the so-called polysulfide shuttle. Polysulfides as well as electrolyte components are getting consumed as part of decomposition reactions at the lithium anode surface during cycling. As a result minimizing the electrolyte content leads to low cycle life of the LSB cells. Several electrolyte approaches in order to limit polysulfide dissolution have been developed over the last 5-10 years, however, those electrolyte formulations usually lead to a strong lithium metal corrosion. Electrolytes that enable stable cycling of lithium metal batteries have been successfully developed, however, they are usually not compatible with the polysulfides of Li-S batteries. Hence, decoupling the anolyte from the catholyte has been much desirable. Recently introduced solid-state lithium sulfur batteries based on sulfidic solid electrolytes might overcome some challenges of conventional liquid LSBs since no formation or diffusion of polysulfide species can take place, and very high sulfur utilization have been already demonstrated. Cycling solid-state LSBs is still challenging due to pressure induced lithium dendrite formation resulting in short circuits, when a thin layer of solid electrolyte and lithium metal anode (without indium) were employed, especially in pouch cells.

Herein, a new battery design, a “semi-solid”-LSB, is presented. The concept combines the advantages of liquid and solid state LSBs. On the cathode side, an electrode comprising carbon, sulfur and sulfidic solid electrolyte (argyrodite) enables a solid to solid conversion of sulfur resulting in a complete suppression of the polysulfide shuttle and yet guarantees high sulfur utilization.
On the anode side, a tailored liquid electrolyte enabling stable cycling of a pure lithium metal anode is employed. A careful design of the liquid electrolyte is crucial in order to obtain high stability of the liquid electrolyte against the lithium metal anode as well as to avoid uncontrolled side reaction between the solid electrolyte of the cathode and the liquid electrolyte. Different ether-based electrolytes are discussed for the new cell design and proof of concept was successfully reached in coin cells leading to 100 cycles with a stable coulombic efficiency of over 99 % which outperforms the majority of previously published Li-S cycling data. A specific energy exceeding 600 Wh/kg is projected for this cell design. In addition, the concept was also transferred into first semi-solid lithium sulfur-pouch cells – an important step in the direction of practical applications.