Insight into sulfur reactions in Li-S batteries

Understanding and controlling the sulfur reduction species (Li₂S_x, 1 ≤ x ≤ 8) under realistic battery conditions are essential for the development of advanced practical Li-S cells that can reach their full theoretical capacity. However, it has been a great challenge to probe the sulfur reduction intermediates and products because of the lack of methods. This work employed various ex situ and in situ methods to study the mechanism of the Li-S redox reactions and the properties of Li₂S_x and Li₂S. Synchrotron high-energy X-ray diffraction analysis used to characterize dry powder deposits from lithium polysulfide solution suggests that the new crystallite phase may be lithium polysulfides. The formation of Li₂S crystallites with a polyhedral structure was observed in cells with both the conventional (LiTFSI) electrolyte and polysulfide-based electrolyte. In addition, an in situ transmission electron microscopy experiment observed that the lithium diffusion to sulfur during discharge preferentially occurred at the sulfur surface and formed a solid Li₂S crust. This may be the reason for the capacity fade in Li-S cells (as also suggested by EIS experiment in Supporting Information). The results can be a guide for future studies and control of the sulfur species and meanwhile a baseline for approaching the theoretical capacity of the Li-S battery.