Design and Production of Lithium-Sulfur Battery Cathodes based on Sulfur-Polyacrylonitrile

Today Lithium-ion batteries (LIBs) are an important technology for many applications, including consumer electronics, mobility, and grid stability. However, classic LIBs have limits to their energy storage capacity defined by the active materials used. This is why the scientific community is searching for next-generation materials and has identified sulfur as a possible future cathode active material. However, sulfur suffers from intrinsic drawbacks like severe capacity decay, low rate performance and intense side reactions. Different approaches are under consideration to tackle these drawbacks, with sulfur-polyacrylonitrile (SPAN) being one of them. This material exhibits stable cycling behaviour, good performance and tuneable structural characteristics due to covalent sulfur fixation. This thesis focuses on the processing behaviour of the 2002 discovered material, bridging a gap in the global research efforts. It investigates the SPAN material during classic LIB production steps using established pilot-scale production equipment, including slurry mixing, wet coating, drying and calendering. During the investigations, an extensive characterization of the process-structure-property relationships from the raw material to the slurry, the dry electrode, and finally, the lab cell is carried out. The target is to achieve a holistic view of the SPAN material's processing behaviour, benefits, drawbacks and capabilities during production on established equipment. The integrity and resilience of the conductive carbon additive network were identified as a key aspect here. Since the pristine SPAN material shows a limited electrical conductivity, the performance of SPAN electrodes with carbon black as the conductive additive is also limited electrically. A direct correlation between the electrical conductivity of the electrodes and their electrochemical performance is proven for lab cells regarding their rate performance, cycling stability and voltage hysteresis. It is shown that preserving the hierarchical structure and networking of the carbon black is of utmost importance for achieving this good electrical conductivity within the electrodes. This can be achieved by using appropriate process step designs. Regarding slurry mixing, short mixing times and low mixing intensities preserve larger carbon black particles and a higher agglomeration degree, benefiting the network formation and percolation. During coating and drying, the conductive network can be supported by using carbon-primered current collectors to reduce contact resistance and low drying intensities to limit segregation and improve electrode homogeneity. SPAN cathodes with a homogeneous, robust and sufficiently percolating conductive network show high rate performance up to 2C for relevant areal capacities of 3.5 mAh/cm², good cycling stability and a small voltage hysteresis. Investigations on the calendering behaviour were done as well, showing an intense spring-back behaviour of the electrodes dominated by the elasticity of the active material. This spring-back leads to a weakening of the conductive network and negatively affects the electrochemical performance. Further improvements in the calendering process are necessary to allow for a process step parameter definition.