Impedance Spectroscopy Studies of Changes in the Properties of Lithium-Sulfur Cells in the Course of Cycling

Changes in the properties of lithium-sulfur cells during cycling were studied by impedance spectroscopy. The electric conductivity of the electrolyte changed during the charging and discharging of the lithium-sulfur cells as a result of the dissolution of lithium polysulfides formed in electrochemical reactions. The maximum resistance of the electrolyte and the surface layers on the sulfur and lithium electrodes was achieved in the region of the transition between the low- and high-voltage areas on the charge and discharge curves of the cells. This region corresponded to the highest concentration of lithium polysulfides in the electrolyte. For nearly charged or discharged lithium-sulfur cells, the impedance spectra contained linear segments which could be attributed to diffusion limitations at low frequencies. An analysis of the results of impedance studies suggested that the electrochemical processes in lithium-sulfur cells were controlled by diffusion in the surface layer on the sulfur electrode at high degrees of charge or discharge and by the transport properties of the electrolytic system at moderate degrees of charging.