Although widely used, current Lithium-ion batteries still rely on cathode materials such as LiCoO2 or LiFePO4 with relatively low theoretical specific capacities of 272 mAh g–1 and 170 mAh g–1 respectively. However, in real systems, the achievable capacities can even be considerably lower.
In order to circumvent this problem and to meet the demands set for future energy storage systems, sulfur is considered a good alternative in comparison to the above mentioned state-of-the-art cathode materials in lithium-containing batteries. This instance can be mainly attributed to the high theoretical specific capacity of elemental sulfur in such systems of 1672 mAh g–1. In addition, elemental sulfur is produced annually on a multimillion ton scale worldwide and is thus a highly inexpensive material. These circumstances and the fact that sulfur is non-toxic make it very desirable for said application.
However, some challenges will have to be overcome before the lithium-sulfur will be able to replace the current systems in use. On mayor issue in that respect is the formation of soluble polysulfides during the lithiation of sulfur. These compounds can diffuse through the separator, a process that is called “Polysulfide shuttle effect”. This phenomenon consequently leads to a loss of active material on the cathode side of the cell, to damages of the anode, to a reduction of coulombic efficiency and therefore ultimately to a notably reduced cycle life of the battery. Furthermore sulfur and its related lithiated species are electrically non-conducting.
These issues are mainly addressed by employing porous carbon materials as hosts for the sulfur. By employing carbons with a specific, tailored porosity, sulfur is retained in the electrode, which suppresses the Polysulfide shuttle effect. In addition sulfur is electrically well-contacted, which leads to an overall improved utilization of active material and increased specific power.
We are currently working on a broad range of different carbon materials for this application, whereas special attention is given to synthesis pathways that allow for an easy upscaling while requiring only a small number of process steps.
Dr. Lars Giebeler
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M. Klose, K. Pinkert, M. Zier, M. Uhlemann, F. Wolke, T. Jaumann, P. Jehnichen, D. Wadewitz, S. Oswald, J. Eckert, L. Giebeler, Hollow carbon nano-onions with hierarchical porosity derived from commercial metal organic framework, Carbon 79 (2014), 302-309. URL