In situ sulfur K-edge X-ray absorption near edge structure of an embedded pyrite particle electrode in a non-aqueous Li⁺-based electrolyte solution

Changes in the composition of embedded pyrite (FeS₂) particle electrodes in 1 M LiClO₄ propylene carbonate solutions as a function of the applied potential have been examined in situ by S K-edge fluorescence X-ray absorption near edge structure (XANES), using a specially designed cell that minimizes attenuation of low energy X-rays. Pyrite electrodes that had been scanned from 3.0 V versus Li/Li⁺, i.e. close to the open circuit voltage, down to 1.0 V (fully discharged state, i.e. 4e^--reduction) and then half recharged (2e^--reoxidation) by scanning the potential in the positive direction up to 2.2 V versus Li/Li⁺, revealed features consistent with the presence of Li₂FeS₂, in agreement with in situ Fe K-edge results reported earlier by this research group. Moreover, only subtle changes were discerned between the in situ S K-edge XANES of the half-, and fully-recharged electrodes. This close resemblance may reflect similarities between the spectral signatures of Li₂FeS₂ and Fe_1-xS (pyrrhotite), which is the main product of the discharge reaction. Evidence for the formation of elemental sulfur and Li₂S, which are believed to be minor products of the reaction, was obtained from analysis of the first differential S XANES and selected difference spectra. The compositional variations of the embedded pyrite particles throughout the course of the electrochemical processes occur in the presence of a persistent sulfate coating.