Extended Data Fig. 6: Ex situ synchrotron sulfur K-edge XANES profiles of the sulfur cathode fitted with LBPSI electrolyte and ex situ Raman and XPS studies of the cells prepared with LBPSI as the active material in the cathode (configured as In/InLi|LBPSI|LBPSI–C).

a, Sulfur K-edge XANES spectra of the sulfur cathode fitted with LBPSI electrolyte performed ex situ at different discharge/charge states, along with the sulfur standards for references. b, Ex situ Raman spectra of the pristine cathode and the cathode charged to 3.2 V of the cell using LBPSI as the active material (containing no sulfur) in the cathode. The Raman spectra of LiI, I₂ and milled I₂/LiI ((I₂+LiI)-BM) are measured as references. c,d, The I 3d (c) and S 2p (d) XPS data of the pristine LBPSI material, the pristine cathode, the electrode charged to 3.2 V and the electrode subsequently discharged to 1.4 V in the cell configured as In/InLi|LBPSI|LBPSI–C (dotted lines, raw data; black lines, fitted total; coloured lines, fitted components). e, The I 3d spectra of the pristine electrode and the electrode charged to 3.2 V in the cell for better visibility of the signal shift. We performed ex situ sulfur K-edge XANES measurements and observe that the sulfur cathode using LBPSI electrolyte discharges with the formation of Li₂S_x intermediates and that Li₂S/Li₂S_x is completely converted to sulfur on charge. The S/KB and Li₂S/KB samples prepared by mixing the respective sulfur species with KB were used as references. The S/KB shows two features at 2,472.6 eV (the white line) and 2,480.0 eV; the Li₂S/KB shows three features at 2,473.6, 2,476.2 and 2,484.0 eV. The spectrum of the pristine cathode shows the features of S⁰ at 2,472.6 and 2,480.0 eV. When discharged to 1.4 V, the features of Li₂S at 2,473.6, 2,476.2 and 2,484.0 eV as well as that for Li₂S_x at 2,471.8 eV appear^5,58. After the subsequent charging, the features corresponding to Li₂S or Li₂S_x disappear and S⁰ features at 2,472.6 and 2,480.0 eV emerge again. This indicates that the charging has fully converted Li₂S (and Li₂S_x) back to sulfur. When discharged to 1.4 V again, the features of S⁰ disappear and the features of Li₂S and Li₂S_x appear. This demonstrates the reversibility of the SSSRR during charging and discharging. In the Raman spectra of the LBPSI–C electrodes (b), compared with the pristine electrode, for the electrode charged to 3.2 V, a broad band appears at 100–200 cm⁻¹. The peak at around 181 cm⁻¹ is attributed to I₂ and the peaks around 110 cm⁻¹ and 167 cm⁻¹ are attributed to I₃⁻ (ref. ⁵⁹). For a clear reference of the assignment of these peaks, we measured the Raman spectra of crystalline LiI, crystalline I₂ and a milled mixture of I₂/LiI (which forms LiI₃). We can see that the peak around 181 cm⁻¹ is clearly attributed to the vibrations of I₂ and the peaks around 110 cm⁻¹ and 167 cm⁻¹ are attributed to the vibrations of I₃⁻. From the I 3d XPS spectra (c,e), we can clearly observe the signal shift of the I 3d spectra to higher binding energy on charging the cell to 3.2 V (e), confirming the I on the surface of the SE being present with a higher valence state than that in LBPSI (I⁻). By a closer observation of the fitted I 3d spectra, the pristine LBPSI electrolyte and LBPSI–C electrode show anionic I⁻, as evidenced by the I 3d doublet signals at 618.8 eV (3d_5/2 energy)⁴¹ (c). On charging to 3.2 V, the oxidized iodine species are clearly observed (tentatively assigned to I₂/I₃⁻)⁴², which locates at around 619.9 eV (3d_5/2 energy). Furthermore, after subsequent discharging to 1.4 V, the signals of oxidized iodine species disappear, demonstrating the reversible oxidation/reduction of iodine species. For the S 2p spectra (d), the pristine LBPSI electrolyte and electrode contain [PS₄] units (and [BS₄], that is, non-bridging sulfur) and P–S–P (and B–S–B, that is, bridging sulfur) signals, with characteristic S 2p signals located at 161.5 eV and 162.5 eV (2p_3/2 energy)^43,60. The LBPSI electrode charged to 3.2 V contains oxidized sulfur (S⁰), which is evident by the characteristic S 2p doublet signals at around 163.6 eV (2p_3/2 energy)⁶¹. After discharge to 1.4 V, the signal of oxidized sulfur (S⁰) disappears and signals of bridging sulfur and non-bridging sulfur appear, which demonstrate the reversible oxidation/reduction of sulfur species.