X-Ray Absorption Spectroscopy of Solid-State Lithium Sulfur Batteries

Abstract

Lithium sulfur (Li-S) batteries have become a popular focus of energy storage research due to their high theoretical energy density and the economic viability. The primary issues that Li-S batteries face stem from the high solubility of lithium polysulfide reaction intermediates (e.g. Li2Sx, 2 ≤ x ≤ 8) formed during the charge and discharge processes. Dissolution of these species out of the cathode causes battery capacity to fade and leads to detrimental reactions that occur at the lithium anode, ultimately causing the battery to fail. The reaction pathways through which lithium polysulfides are formed have been studies for over four decades, but are still unclear. Complicating spectro-electrochemical approaches to study Li-S redox chemistry is the challenge of obtaining spectral standards for individual polysulfide species. Lithium polysulfide molecules cannot be isolated as individual components, and instead exist in solution as a distribution of molecules formed via disproportionation reactions (e.g. 2 Li2S6 ↔ Li2S4 + Li2S8). In this work, X-ray absorption spectroscopy (XAS) at the sulfur K-edge was used to probe solid-state Li-S battery redox pathways. Solid-state Li-S cells consisted of a lithium metal anode, a block copolymer electrolyte separator of polystyrene-poly(ethylene oxide) (SEO) containing LiClO4, and a cathode containing carbon black, elemental sulfur, and SEO/LiClO4. Cells were charged/discharged while X-ray photons probed the battery cathode in situ to provide spectroscopic information regarding the sulfur-containing species formed during the charge and discharge processes. If properly interpreted, sulfur K-edge XAS can provide power insight into the species formed during charge/discharge. To overcome the issues typically associated with spectroscopic studies of Li-S chemistry, ab initio molecular dynamics simulations were performed to probe lithium polysulfide species dissolved in an oligomer of the polymer electrolyte used in the battery, tetraethylene glycol dimethyl ether (TEGDME). Based on these simulations, theoretical X-ray absorption spectra for each lithium polysulfide were obtained using the eXcited Core Hold (XCH) approach. Theoretical spectra were used to interpret experimentally obtained spectra to reveal the distribution of species formed throughout the charge and discharge processes.