Analysis of 3.4 Ah lithium-sulfur pouch cells by electrochemical impedance spectroscopy

This work is a study of Li-S pouch cell degradation by EIS at different DODs and temperatures; comparison of electrochemical processes during charging/discharging and analysis of the influence of superimposed AC current during dynamic EIS measurements. Despite great progress in lithium-sulfur (Li-S) batteries, the electrochemical reactions in the cell are not yet fully understood. Electrode processes, complex interfaces and internal resistance may be characterized by electrochemical impedance spectroscopy (EIS). EIS is a non-destructive technique and easy to apply, though there are challenges in ensuring the reproducibility of measurements and the interpretation of impedance data. Here, we present the impedance behavior of a 3.4 Ah Li-S pouch cell characterized by EIS. The impedance changes were analyzed over the entire depth-of-discharge, depth-of-charge, and at various temperatures. Based on the formation of intermediates during (dis)charging, the changes of resistances are observed. Overall, the increase in temperature causes a decrease in electrolyte viscosity, lowering the surface energy which can improve the penetration of the electrolyte into the electrode pores. Moreover, the effect of superimposed AC current during EIS measurement was analyzed, and the results show the dependence of the charge transfer resistance on superimposed AC current which was lower compared to steady-state conditions and consents with theory.