In-Operando Imaging Studies on Dynamics of Sulfur/Polysulfides in Li-S Batteries

Abstract

Lithium-sulfur (Li-S) battery is a promising rechargeable battery system that has both high theoretical capacity and energy density. Moreover, sulfur (S) is inexpensive and nontoxic, making Li-S suitable for large-scale energy storage applications. Sulfur (S8) lithiation during the operation of Li-S battery is a multi-step electrochemical process that involves different lithium polysulfide (Li2Sn) intermediates. The long-chain polysulfides (PSs) are highly soluble in the aprotic organic electrolyte. There has been a broad consensus that polysulfide dissolution is strongly linked to the key performance indexes, such as columbic efficiency, self-discharge, capacity fading and etc., of Li-S batteries. Re-deposition of S/PS takes place during de-lithaition, which has never been investigated in the literature but is believed to play equally important part as the dissolution process in affecting the cycle stability of the S electrode. We carry out in this work operando observations at two different scales on the dynamics of dissolution and re-deposition processes of polysulfide/S particles during the operation of Li-S batteries. In-operando synchrotron transmission X-ray microscopy (TXM) has been used to monitor in real-time the dynamics of the S/PS dissolution and re-deposition based on the dimensional variation of the micron-sized S-containing active particles. On the other hand, in-operando optical microscopy has been used to reveal the large-scale motion of the polysulfides within a model cell. The combined information from these two in-operando studies provides valuable evidences that unravel the existence of unprecedented chemical-reaction driven charge-transfer mechanisms for S lithiation/delithiation. Figure 1