Study of the discharge/charge process of lithium–sulfur batteries by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) was used to study the initial discharge/charge process in a sulfur cathode with different potentials. In the second discharge region (2.00–1.70 V), where soluble polysulfides are reduced to Li2S, the EIS spectra exhibit three semicircles/arcs as the frequency decreased. An appropriate equivalent circuit is proposed to fit the experimental EIS data. Based on detailed analysis of the change in kinetic parameters obtained from simulating the experimental EIS data as functions of potential, the high-frequency, middle-frequency and low-frequency semicircles/arcs can be attributed to the Schottky contact reflecting the electronic properties of materials, the charge transfer step and the formation of Li2S respectively. The inclined line arises from the diffusion process in the detectable potentials and frequency range. Several important electrochemical reactions also have been verified by cyclic voltammetry curves.