New insight into the discharge process of sulfur cathode by electrochemical impedance spectroscopy

In this paper, the electrochemical reactions of sulfur cathode during discharge–charge process were investigated by EIS technique combining with XRD, SEM and EDS methods. The discharge process of the sulfur cathode could be divided into two discharge regions. These are the first discharge region (2.5–2.05 V) where the reduction of elemental sulfur to form soluble polysulfides and further reduction of the soluble polysulfides occurs, and the second discharge region (2.05–1.5 V) where the soluble polysulfides are reduced to form a Li₂S solid film covered over the carbon matrix. It was found that the EIS can distinguish the individual contributions of charge transfer resistances, ion diffusion impedance and properties originating from Li₂S film in the frequency domain of 100 kHz to 100 mHz. During the upper voltage plateau, the impedance of interfacial charge transfer dominates the reduction reaction, while during the lower voltage plateau, the mass transport in the cathode is a control step. It was also proved that the solid Li₂S appeared at the beginning of the lower voltage plateau region and became denser during the following discharge process.