Towards Enhanced Cycling Stability of Lithium-Sulfur Batteries: Lithium Sulfide Cathodes and Coated Separators

Batteries are an important factor in the standard of living of people in western industrialized countries. They are used in mobile applications such as electric cars, laptops and smartphones, but they are also increasingly being used in stationary applications, e.g. as intermediate storage for electricity generated from renewable sources. Lithium-ion batteries are the most widespread batteries here. They have a high energy density and long service life. However, this technology is based on the use of heavy metals, which are used in the cathode. These metals can cause environmental pollution and pose a health risk to humans and animals when they are mined, processed and at the end of the battery's service life. Lithium-sulfur batteries could be an alternative that is widely discussed in research. Sulfur as an active material is neither environmentally harmful nor dangerous. It is also significantly lighter than the metals used in lithium-ion batteries, which leads to a much higher theoretical energy density. However, batteries of this type currently still have a much too short lifetime, meaning that they are only used in niche markets. The reason for the lack of stability is the polysulfide shuttle mechanism. It describes the leaching of liquid intermediate products (polysulfides) of the active material that are soluble in the electrolyte during the cell reaction and further unwanted side reactions. This thesis describes three approaches for suppressing the PSM. In the first part, chromium oxide and iron oxide were investigated as additives in the active material coating for lithium-sulfur cathodes. The oxides are intended to act as anchor points for the liquid polysulfides to prevent them from being leached out. First, the general adsorption behavior was examined with the help of adsorption isotherms of the polysulfide Li2S6 on the oxides. In half-cell tests, the capacity loss over 200 cycles was reduced by 59% with iron oxide and by 48 % with chromium oxide. In the second part, molybdenum oxysulfide, chromium oxide and manganese oxide were applied to the separator surface as a coating using magnetron sputtering. This layer serves as a barrier layer to prevent polysulfides leached from the cathode from diffusing through the cell to the anode, thus suppressing the shuttle mechanism. By this approach the capacity of the cells could be improved. The third part of this work deals with the direct coating of the active material. In a multi-stage process, lithium sulfide is first synthesized and then coated. Second, a nanoscale carbon layer was applied to support the electrical connection of the active material to the current collector. This was followed by a coating of plasma-synthesized polythiophene. The polymer layer was intended to serve as a physical barrier for liquid polysulfides, with the sulfur groups of the polymer reinforcing this effect through polar interactions. This method reduced the charging voltage of the cells and improved the utilization of the active material. These effects were attributed to an enhanced electronic contact between the active material and the current collector.