Anionic Effects on Solvate Ionic Liquid Electrolytes in Rechargeable Lithium–Sulfur Batteries

A series of equimolar mixtures of Li salts (LiX) and glymes (triglyme (G3) and tetraglyme (G4)), Li(glyme)X with different anions (X: N(SO2C2F5)2 = BETI; N(SO2CF3)2 = TFSA; CF3SO3 = OTf; BF4; NO3), were used as electrolytes to study the anionic effects of Li(glyme)X on the performance of lithium–sulfur (Li–S) batteries. The dissolution of lithium polysulfides (Li2Sm), which are discharge products of elemental sulfur, was significantly suppressed in the solvate ionic liquid (IL) electrolytes, as seen in Li(G4)BETI and Li(glyme)TFSA, wherein all of the glymes participated in the formation of the complex cation Li(glyme)+. It was found that NO3 anions were irreversibly reduced at the composite cathode during discharge and BF4 anions formed unexpected byproducts through a chemical reaction with the polysulfide anions. Successful charge/discharge of Li–S cell could not be performed in Li(glyme)X in the presence of these anions because of the undesired side reactions. The solvate IL Li(G4)BETI was found to be electrochemically stable in the Li–S cell and allowed a stable operation with a capacity of 600–700 mAh·g–1 and a Coulombic efficiency of 98.5% over 100 cycles, similar to that achieved by Li(glyme)TFSA. In contrast, the Li–S cell with a concentrated electrolyte solution, Li(G3)OTf, showed a much lower capacity and Coulombic efficiency.