UC Berkeley

In this study, we propose a modified mathematical model for Lithium/Sulfur (Li/S) cells and present a detailed numerical analysis showing the effects of (1) the presence of solid sulfur species (S8(s), Li2S2(s), Li2S(s)), (2) the solubility and diffusivity of polysulfides in the electrolyte, and (3) reaction rate constants of polysulfide reduction reactions at Li electrode on the electrochemical performance characteristics of Li/S cells including the cycling performance. The cell potential profiles predicted from numerical analysis were compared with experimental discharge and charge curves with emphasis on the possibility of the presence of the intermediate solid phase Li2S2(s). Numerical results suggest that the cell potential obtained without the consideration of the intermediate solid phase Li2S2(s) is in the best agreement with experimental results. Also, the polysulfide shuttling phenomenon has been numerically analyzed with electrolytes of different polysulfide solubility and compared with experimental cell discharge and charge curves. Our model clearly shows that the electrolyte of high polysulfide solubility and unprotected negative (anode) electrode can lead to unwanted reduction reactions of high-order polysulfides on the Li electrode, resulting in a significant overcharge problem. This agrees well with our experimental results for identical experimental conditions. Furthermore, the cycling performance of a Li/S cell was predicted, including the effect of the polysulfide shuttle on the subsequent discharge/charge curves. In the simulation, it was found that the high voltage plateau is reduced markedly in the discharge process after incomplete charge because sulfur (S8) could not be recovered when the polysulfide shuttle problem is significant. Also, it is suggested that cycling performance of Li/S cells would be improved by limiting both the polysulfide diffusivity and the reduction reaction rate of high-order polysulfides at Li electrode during charge.