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.