Correlation of capacity fading processes and electrochemical impedance spectra in lithium/sulfur cells

The capacity fading of lithium/sulfur (Li/S) cells is one major challenge that has to be overcome for a successful commercialization of this electrochemical storage system. Therefore it is essential to detect the major fading mechanisms for further improvements of this system. In this work, the processes leading to fading are analyzed in terms of a linear four state model and correlated to the distribution of relaxation times calculated with a modified Levenberg-Marquardt algorithm. Additionally, the Warburg impedance and the solution resistance are also obtained by the same algorithm. The detailed analysis of intermediate states during the first cycle gives the distinction between relaxation processes at the sulfur cathode and at the lithium anode. The influence of the polysulfides on the impedance parameters was evaluated using symmetric cells; this yields a good correlation with the results obtained from the first discharge/charge experiment. A fast and a slow capacity fading process are observed for the charge and the discharge during 50 cycles. The fast fading process can be assigned to Faradaic reactions at the lithium anode.