Extended Data Fig. 6: Analysis of the basic components that comprise the Se K-edge X-ray absorption spectra during operando discharge at 170 °C.

a, b, The XRD pattern (a) and SEM image (b) of the selenium composite used for XAFS measurement; the absence of diffraction peaks indicates the non-crystalline nature of Se. c, The magnified XANES spectra in a representative region that shows the isosbestic points shared by the first few but all not all spectra.d, e, The plots of logarithmic variance (d) and eigenvalue (e) of each component for the principal component analysis (PCA), showing that at least 3 components are required to capture > 99.99% of the XAFS data set. f, The plots of first five PCA-derived components (Components 3–5 magnified in the inset); while Components 1, 2, 3 show meaningful curvature, Component 4 and beyond start to lose significance. g, The k-space EXAFS k²χ(k) oscillations of three identified basic components using MCR-ALS method, with the input data being either energy (E) space or frequency (k) space data sets. Notably, the three identified components are the same, irrespective of the type of input data set, validating the utilized analysis procedure. h-j, Fitting results of the EXAFS data (top: magnitude and real-part of Fourier transforms of k³χ(k); bottom: the corresponding k³χ(k) oscillations) for the standard selenium Se⁰ measured at 170 °C (h) and crystalline Na₂Se₂ measured at 25 °C (i) and the summary of the fitting parameters (j). Na₂Se₂ was synthesized by reaction of Na-naphthalene with selenium (see Methods). The window in the FT panels is the fit range used and the window in the EXAFS panels is the k-space data range used for the FT. The paths used for fitting Na₂Se₂ are from the Se-Se and Se-Na coordinations calculated from the crystalline Na₂Se₂. The fitting of Se⁰ allows us to determine the amplitude reduction factor (S₀²: 0.973) which is a beam-related constant and is used for fitting all spectra collected in this study. The methodology of fitting the Na₂Se₂ standard validates taking multiple paths from crystalline data to fit the first (and second) Se shell of unknown spectra as will be shown below.