Fingerprinting Lithium-Sulfur Battery Reaction Products by X-ray Absorption Spectroscopy

Lithium-sulfur batteries have a theoretical specific energy that is a factor of five greater than that of current lithium-ion batteries, but suffer from consequences of the solubility of lithium polysulfide reaction intermediates that form as the batteries are charged and discharged. These species can react with each other and diffuse out of the cathode, causing battery capacity to fade and ultimately, cell failure. In spite of work that has spanned four decades, "fingerprints" of polysulfides have not yet been established, precluding a systematic study of lithium-sulfur chemistry. Herein we demonstrate the use of principal component analysis of X-ray absorption spectroscopy (XAS) to obtain fingerprints of lithium polysulfides. This approach enables interpretation of spectral data without any assumptions regarding the origin of the observed spectral features or knowledge of the stability of the polysulfide species of interest. We show that in poly(ethylene oxide)-based solid electrolytes containing polysulfides made by chemically reacting Li2S and elemental sulfur, Li2S2 and Li2S6 spontaneously disproportionate to give binary Li2S/Li2S4 and Li2S4/Li2S8 mixtures, respectively, while Li2S4 and Li2S8 exist as single molecular species. XAS fingerprints of Li2S4 and Li2S8 are thus presented.