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Research on lithium-sulphur batteries at ZBT

In the IGF project 15 EWN, ZBT developed novel electrodes for lithium-sulphur batteries based on lithium sulphide (Li2S) as cathode material together with project partners of the University of Cologne, Chair of Inorganic and Materials Chemistry and the University of Duisburg-Essen (UDE), Chair of Energy Technology.

Figure 2: Pan filled with lithium sulfate within a tube furnace

Figure 1: Comparison of capacity retention and Coulomb efficiency of commercial and modified Li2S.

Figure 3: Schematic representation of in-situ UV-VIS spectroscopy to clarify the influence of different metal oxides on the electrochemical properties of Li2S cathodes.

The functionality and challenges of lithium-sulphur batteries are briefly described here.

The University of Cologne synthesized a composite material containing Li2S, TiO2 and carbon by electrospinning. This material is the basis for the development of novel cathodes at ZBT (see Figure 1). The introduced carbon improves the electrical conductivity of the electrode and the metal oxide contributes to the reduction of the polysulfide shuttle mechanism. The polysulfides which are formed during cell operation are adsorbed on the surface of TiO2, which counteracts the flushing of the cathode active material. This leads to higher Coulomb efficiency and to a better capacity retention within the first cycles.   

The developed cathodes still show significant potential for improvements, e.g., in the achievable high rate capability and the avoidance of metallic lithium in full cells for safety reasons. The polysulfide shuttle mechanism could be reduced by modifying Li2S, but complete suppression was not feasible. This is shown by the fact that cells with an anode developed by the University of Duisburg-Essen could only be charged and discharged to a limited extent.

Based on these results, various approaches to improve lithium-sulphur batteries with Li2S cathodes are currently being researched in several projects.

In the "ILISKO" project (IGF project 21006 BG), the focus is on increasing the high rate capability and life time of lithium-sulphur batteries. At the ZBT, the synthesis of carbon-modified Li2S (see Figure 2) is carried out by carbothermal reduction. These materials are further modified by the Leibnitz Institute for Plasma Research in Greifswald. Magnetron sputtering is used to apply a nanographite layer. Another approach to improve the high rate capability is the synthesis of Li2S/carbon nanotube composites. To enhance the life time and the electrical particle connection these materials are additionally modified by a layer of cross-linked and partially electrically conductive polymers via plasma-assisted CVD processes

First results show that the high rate capability of Li2S cathodes is significantly improved by the modification of Li2S by a nanographite layer.

Within the "EIS-LIS" project (IGF project 21119 N) the cell and electrode design is being adapted in order to reduce the polysulphide shuttle mechanism. Thus, lithium-sulphur cells with a lithium metal-free anode can be realized. At ZBT, different metal oxides are added to the cathode to hold the polysulphides in the electrode. Residual polysulphides that are still in the electrolyte should also be retained by modified separators. Therefore, thin layers of metal oxides are sputtered on the surface of the separators at fem in Schwäbisch Gmünd. To investigate the influence of different metal oxides on the transport processes of the polysulphides, ZBT has already developed an in-situ UV-VIS spectroscopy setup in this project (Figure 3).

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