(Invited) Direct Observation of the Sulfur-Based Chemical Species in Li-S Batteries By Spatially-Resolved X-Ray Fluorescence Microscopy and X-Ray Absorption Spectroscopy

The demands on low cost and high energy density rechargeable batteries for both transportation and large-scale stationary energy storage are attracting more and more research attention toward new battery systems such as metal, metal-sulfur, metal-air, and multivalent batteries. Since sulfur is an earth-abundant material with low cost and has a high theoretical capacity, Li-S battery chemistry has attracted significant interest during the past decade. The Li-S battery utilizes electrochemical conversion of sulfur (S8) to lithium sulfide (Li2S), going through multiple electron transfer processes associated with long- and short-chain polysulfide (Li2Sx) intermediates. It is well known that the long-chain polysulfides can be dissolved into electrolyte with aprotic organic solvents and migrated to the Li anode side. This so-called “shuttle effect” is considered as the main reason for the capacity loss and low coulombic efficiency of the Li-S system. A lot of efforts have been made on suppressing the polysulfide dissolution through new sulfur-based material and electrolyte, as well as cell engineering. Although the Li-S battery performance has been improved over the past decades, the long‐term cycling stability is still one of the major barriers for the practical application of Li–S batteries. More in‐depth studies on the fundamental understanding of the sulfur reaction mechanism and interactions among the different polysulfide species, the electrolyte, and the electrodes are still greatly needed. In this study, we utilize the spatially-resolved X-ray fluorescence (XRF) microscopy combined with X-ray absorption spectroscopy (XAS) to directly probe the inhomogeneous electrochemical/chemical reactions of the sulfur cathode, and chemical reactions of sulfur contained chemical species. The morphology changes and the redistribution of sulfur and polysulfides in both the S cathode and lithium metal anode were monitored through the XRF images, while the oxidation state of sulfur and changes of the sulfur-containing interfacial layer (i.e., SEI) were characterized through the XAS spectra. Our characterization studies on the sulfur-based cathode materials in Li-S batteries using spatially resolved XRF/XAS techniques provides critical information enabling us to understand the reaction processes and their correlation to cell cycling behavior and failure mechanisms. More details of the result will be discussed at the meeting. Acknowledgement The work done at Brookhaven National Laboratory was supported by Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the U.S. DOE, through the Advanced Battery Materials Research (BMR) Program, including Battery500 Consortium under contract DE-SC0012704. This research used beamline 8-BM (TES) of the National Synchrotron Light Source II, U.S. DOE Office of Science User Facilities operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.