Effect of imidazolium cation on cycle life characteristics of secondary lithium-sulfur cells using liquid electrolytes

The cycle life performance of rechargeable lithium-sulfur (Li-S) cells was examined by using DME/DOX (4/1) solvent systems, where supporting electrolyte systems used were classified into the two groups: (1) conventional lithium salts and (2) mixed salts composed of lithium salt and imidazolium salt. To analyze the effect of the imidazolium cation on the cycle life characteristics of secondary Li-S cells, we applied the following three kinds of immidazolium salts, with the same anion, to the mixed-salt systems: 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI); 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide (BMITFSI); 1,2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfony)imide (DMPITFSI). Even after 100 cycles, the EMI- and BMI-added Li-S cells showed much larger capacities, 721 and 642 mAh g^-1-sulfur, respectively, when compared to that (534 mAh g^-1-sulfur) of the LiTFSI-based Li-S cells. By contrast, the DMPI-added Li-S cells showed poor cycle life performance in that their capacity decreased to below 200 mAh g^-1-sulfur even before 10 cycles. It is probable that this phenomenon is due mainly to the deactivation of the carbon cathode caused by coating of non-conductive and insoluble materials, which might result from side reactions between DMPI and lithium polysulfides.