Extended Data Fig. 1: Healable interfaces in SSLSBs with a low-melting-point sulfur iodide, and the synthesis and characterization of sulfur iodide materials.

(a) Schematic of a solid state battery with elemental sulfur as the active material. Poor solid/solid contact develops during cycling due to volume changes of the active material. (b) Schematic of a solid state battery with sulfur iodide as the active material. Ideal active material/electrolyte interface is achieved through periodical heating to melt the cathode, thus healing the interface. (c) Illustration of the procedures to prepare S_9.3I. (d) The DSC curves of sulfur iodide with different S:I ratios in the temperature window of 20−150 °C. With S:I ratios decreasing from 1:0 to 9.3:1, the typical sulfur endothermic peak gradually disappears to transition to one phase melting behavior. Beyond 6:1, the exothermic peaks assigned to iodine are observed. (e) Spectra obtained during in situ heating XRD of S_9.3I from 30 to 130 °C with an 5 °C interval. (f) Raman spectra of sulfur iodide with different ratios, which features an iodine peak located at 423.8 cm⁻¹ when the S:I ratio is less than 9.3:1. The cryo-SEM image (g) and the corresponding EDX mapping images of I (h) and S (i) element distribution of S_9.3I after melting. (j) XRD of S, I₂, S_35.7I, S_15.9I, S_9.3I, S₆I. In the low angle range, the relative intensity of the broadened peak centered at ~1.5° gradually increases with the increase of iodine, but from 9.3:1 to 6:1, the iodine diffraction peaks also emerge. (k) Mass spectra of S and S_9.3I. Compared to S, S_9.3I shows two additional peaks centered at 127.0 and 253.8 m/z, which are attributed to I⁻ and I₂ species respectively. No peaks can be indexed to species containing S-I bonds. (l) The PDF of sulfur iodide materials after melting with different S and I ratios, elemental sulfur and iodine. When the ratio of S to I is below 9.3:1, bulk I₂ is present as indicated by the signals from the short and long range structures.