Discharge Behavior within Lithium–Sulfur Batteries Using Li–Glyme Solvate Ionic Liquids

Practical use of lithium–sulfur batteries can be realized by using "sparingly solvating electrolytes" such as [Li(G4)][TFSA] (G4, tetraglyme; TFSA, bis(trifluoromethanesulfonyl)amide) solvate ionic liquid, superconcentrated electrolyte solutions, and their hydrofluoroether-diluted electrolytes. On the other hand, the battery performance such as C-rate characteristics of lithium–sulfur batteries is different depending on the electrolyte used. In order to investigate the relationship between the discharge reaction and the battery performance for lithium–sulfur batteries with 1,1,2,2-tertafluoroethyl 2,2,3,3-tetrafluoropropyl ether-diluted [Li(G4)][TFSA], in situ electrochemical impedance spectroscopy has been conducted. During discharge at a high C-rate, a voltage drop was observed in the discharge curve at DOD = 20–35%. The charge transfer resistance of the lithium negative electrode remarkably increased when the voltage drop occurred. Moreover, this increase was only observed for in situ measurements. The charge transfer resistance of the negative electrode is related to the resistance of the Li+ ion dissolution/deposition reaction. During discharging at a high C-rate, the Li+ ion is abundant at the negative electrode interface owing to the changes in the Li+ ion solvation structure, suppressing the Li+ ion dissolution reaction.