The electrochemical behavior of lithium-sulfur (Li-S) batteries was investigated by means of electrochemical impedance spectroscopy (EIS). Measurements were performed in equidistant charge intervals at different depths of discharge and charge during the first cycle. Additionally, the degradation of the cells was analyzed for up to 50 cycles. An equivalent electrical circuit is proposed to simulate the electrochemical processes and to quantify the impedance contributions of Li-S batteries. EIS as a function of the cycle number shows an increased capacity for fading correlated with a decrease in the charge transfer resistance of the cathode. Atomic force microscopy (AFM) was also used to provide information about changes in the electrical conductivity of the cathode surface as they are related to the building of isolating films.(C) 2013 Elsevier Ltd. All rights reserved.