Revealing the Electrochemical Charging Mechanism of Nanosized Li2S by in Situ and Operando X-ray Absorption Spectroscopy.

Lithium sulfide (Li2S) is a promising cathode material for lithium-sulfur (Li/S) cells due to its high theoretical specific capacity (1166 mAh g-1) and ability to pair with nonmetallic lithium anodes to avoid potential safety issues. However, when used as the cathode, a high charging voltage (∼4 V versus Li+/Li) is always necessary to activate Li2S in the first charge process, and the voltage profile becomes similar to that of a common sulfur electrode in the following charge processes. In this report, we have prepared an electrode of nanosphere Li2S particles and investigated its charging mechanism of the initial two charge processes by in situ and operando X-ray absorption spectroscopy. The results indicate that Li2S is directly converted to elemental sulfur through a two-phase transformation in the first charge process, while it is oxidized first to polysulfides and then to sulfur in the second charge process. The origin of the different charging mechanisms and corresponding charge-voltage profiles of the first and second charge processes is found to be related to the remaining polysulfides at the end of the first discharge process: they can not only facilitate the charge-transfer process at the Li2S/electrolyte interface but also chemically react with Li2S and act as the polysulfide facilitator for the electrochemical oxidation of Li2S in the following charge processes. Our present study provides a new fundamental understanding of the charging mechanism of the Li2S electrode, which should be of help for the further development of high-performance Li/S cells.