Energy storage plays a central role in the energy transition. Efficient energy storage systems are needed to increase the range of vehicles, to meet the growing energy demand of portable electronic devices or to store energy for stationary applications. The lithium-ion batteries that are currently mainly used are reaching their limits in terms of energy densities and have an insufficient environmental balance and high material costs. An interesting alternative that is the focus of research is lithium-sulphur batteries, whose theoretical energy density is about three times higher than that of lithium-ion batteries. Compared to the cathode materials used in Li-ion batteries, sulphur is neither toxic nor environmentally hazardous and is also significantly cheaper due to its higher global availability.

Despite intensive research, lithium-sulphur batteries are not yet economically viable due to their low cycle stability and Coulomb efficiency. One reason for the low cycle stability is the so-called polysulphide shuttle mechanism. The cycle stability of lithium-sulphur batteries is largely determined by whether the polysulphides formed during the cell reaction can be kept in the cathode or prevented from passivating the anode. 

The aim of this project was, on the one hand, to develop separators with customised transition metal compounds applied by plasma coating. These compounds adsorb polysulphides and thus prevent their transfer to the anode. As a further solution, different transition metal compounds were to be introduced into the cathode. The development of the separator coating took place at the fem. Commercially available separators were used as the base material and different transition metal oxides or sulphides were applied by plasma gas phase deposition (PVD). The layers obtained were characterised in terms of morphology and chemical composition and the properties relevant for Li-S cells, such as Li ion conductivity and polysulphide diffusion, were investigated.

The electrochemical behaviour was investigated in full cells at the ZBT. It was found that some materials can suppress the polysulphide shuttle. Cathodes with different transition metal compounds were developed and electrochemically investigated at the ZBT. Likewise, the polysulphides were investigated using an insitu UV/Vis cell during cycling. Overall, it was possible to identify materials that suppress the polysulphide shuttle and thus contribute to extending the service life of Li-S cells.

Acknowledgements: The IGF project 21119 N of the Research Association for the Research Institute for Precious Metals and Metal Chemistry (fem) was funded by the Federal Ministry of Economic Affairs and Climate Action through the AIF within the framework of the Programme for the Promotion of Industrial Cooperative Research (IGF) based on a resolution of the German Bundestag.