In situ Fe K-edge X-ray absorption fine structure of a pyrite electrode in a Li/polyethylene oxide(LiClO₄)/FeS₂ battery environment

Electronic and structural properties of materials generated by the reduction and subsequent oxidation of pyrite (FeS₂) in a Li⁺-based solid polymer electrolyte (SPE) have been examined in situ by fluorescence Fe K-edge X-ray absorption fine structure (XAFS) in a Li/SPE/FeS₂ battery environment. The in situ X-ray absorption near edge structure (XANES) of fully, i.e., four-electron, discharged FeS₂ displayed features consistent with the presence of metallic iron, in agreement with information derived from in situ X-ray diffraction (XRD) and in situ ⁵⁷Fe Mossbauer effect spectroscopy (MES) reported by other groups. The two-electron reoxidation (half-recharge) of such fully reduced material yielded an in situ XANES very similar to that of genuine crystalline Li₂FeS₂ prepared and characterized in this laboratory. These spectroscopic findings support conclusions made purely on the basis of electrochemical information, which identified Li₂FeS₂ as an intermediate in the charge-discharge mechanism of FeS₂ in room-temperature Li⁺-containing electrolytes.