Faradaic impedance and discharge reactions in lithium sulfur battery with sparingly solvating electrolyte

Lithium-sulfur batteries promote next-generation secondary batteries. Elucidating their electrochemical reactions, such as sulfur reduction reactions, particularly their kinetics, is crucial for further improvement of lithium-sulfur batteries. Therefore, experimental knowledge of their reaction rates is required. In this study, we applied in situ electrochemical impedance spectroscopy to a lithium-sulfur battery using a sulfolane-based super-concentrated electrolyte solution to obtain insights into sulfur reduction reaction kinetics. The Faradaic impedance of a sulfur-positive electrode yielded inductive and capacitive semicircles at first and second plateau voltages, respectively, in a low-frequency range. This impedance behavior under a low-frequency range was derived from multistep reactions. The impedance behavior suggests that the reaction rate constant of the second step was greater than that of the first step at the first plateau, whereas at the second plateau, the reaction rate constant of the second step was smaller than that of the first step. A Faradaic impedance simulation was performed to estimate the reaction rate constants of the sulfur reduction reactions. The simulation well reproduced the experimental data. Furthermore, the simulation showed that the internal resistance of the battery decreased as the rate constant of each reaction changed, suggesting that Faradaic impedance analysis is useful not only for investigating reaction kinetics but also for improving lithium-sulfur battery performance.