Understanding the Impedance Response of Lithium Polysulfide Symmetric Cells

Lithium-sulfur (Li-S) batteries are highly considered for next-generation energy storage due to their ultrahigh theoretical energy density of 2600 Wh kg^-1. The conversion reactions between lithium polysulfides (LiPSs) constitute the core process in working Li-S batteries. Electrochemical impedance spectroscopy (EIS) analysis of LiPS symmetric cells is an effective tool to provide detailed information on the LiPS conversion reactions and direct further kinetic promotion. However, reasonable interpretation of the EIS responses is so far insufficiently addressed without a well-defined equivalent circuit. Herein, a systematic analysis on the EIS responses of LiPS symmetric cells is conducted to provide a comprehensible equivalent circuit. Interfacial contact, surface reaction, and diffusion are decoupled according to their respective characteristic frequency using the distribution of relaxation time analysis method. A well-defined equivalent circuit is proposed to accurately fit the experimental EIS responses, unambiguously interpret key parameters, and be feasible with a wide range of experimental conditions. This work presents the methodology of understanding the EIS responses of LiPS symmetric cells and inspires analogous analysis on vital electrochemical processes.