Essential Features of Electrolyte Solvents that Enable High-Performance Li-S Batteries

In a lithium-ion battery, the organic solvent in the electrolyte simply acts as a medium that facilitates ionic charge transfer, whereas for the lithium-sulfur (Li-S) battery, the choice of the electrolyte is critical as it influences the chemical nature of the soluble polysulfides and the cycling stability of the lithium metal anode. Li-S batteries currently use ether-based electrolytes with high polysulfide solubility. Until now, however, the necessary requirements and the roles of the electrolytes have been inconclusive. This work elucidates the essential features and functions of the electrolyte based on an intensive investigation into 17 variants of solvents. Furthermore, examples of electrolytes for high performance, rechargeable Li-S batteries are suggested. The three major study procedures are (1) analyzing the chemical stability of reactive polysulfide species and lithium metal in the solvents of choice, (2) inspecting the chemical reactivity of lithium metal in a polysulfide solution, and (3) evaluating the influences of the solvent on the performance of Li-S cells. Based on the chemical and electrochemical test results, we suggest that the stability of the active materials (i.e., lithium metal and lithium polysulfides) should be ensured in an electrolyte for the proper operation of Li-S cells, that chemical reactions between the polysulfide species and lithium metal should be negligible during the charge-discharge process. In addition, we have confirmed that the high polysulfide solubility is essential for high rate performance.