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(G(4))][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 600700 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.