Performance of Blended TiS2/Sulfur/Carbon Cathodes in Lithium-Sulfur Cells

A theoretical specific capacity of 1675 mAh/g and a theoretical specific energy of 2600 Wh/kg could be achieved with rechargeable lithium-sulfur cells based on the sulfur active material. This high capacity can be obtained according to the reaction 16 Li + S8 ↔ 8 Li2S [1]. The reaction is usually not complete because there are several problems due to insulating nature of sulfur and the reactions products such as Li2S. In addition, the reaction does not proceed in one step and numerous polysulfide intermediates such as Li2S8, Li2S6 and Li2S4 are formed. The discharge process occurs at a relatively low voltage of 2.1 to 2.3 V. Nevertheless, the lithium-sulfur battery system has attracted considerable attention over the last years due to its high potential for automotive applications. The system has several advantages as it is relatively cheap and environmentally benign [2]. TiS2 is known for a long time as cathode material used in the first generation of lithium cells [3]. It reversibly intercalates/deintercalates lithium ions at potentials between 1.7 and 2.5 V, which is very similar to the voltage range of a lithium-sulfur cell. In addition, TiS2 was found to be a semi-metal having the lowest weight among all the layered dichalcogenides. These properties make it an interesting candidate as second active component in sulfur-carbon cathodes. We have investigated various ratios of TiS2 and sulfur in combination with carbon for sulfur-carbon cathodes. TiS2, sulfur, carbon black, water and binder material were mixed for making electrode slurries. The resulting slurries were coated on aluminium current collectors. Lithium-sulfur cells were assembled using lithium metal foil, polyolefine separator and prepared TiS2/sulfur/carbon cathodes. A mixture of DOL/DME (1:1) and 1M LiTFSI was used as electrolyte. All cells were investigated using a Maccor battery test system. The cells were tested at a charge/discharge rates between 0.1C and 2C. Figure 1 shows the cycle performance of various lithium-sulfur cells employing different TiS2/sulfur/carbon cathodes. The maximum capacity of corresponding cathodes depends on the amount of TiS2 used. It can be seen from Figure 1 that even cells without any additional carbon could be cycled well for more than 40 cycles. Nevertheless, the capacity could be increased when carbon was added to the cathode blends. It was observed that TiS2 improves the pulse power capability of corresponding lithium-sulfur cells. A scanning electron micrograph of a TiS2/sulfur/carbon cathode used in corresponding lithium-sulfur cells is given in Figure 2. The TiS2 is homogeneously blended into the sulfur/carbon matrix. References