Extended Data Fig. 7: The experimental evidence of diffusion of I₂ along the SE particle surface and the occurrence of the reaction between I₂ as redox mediator and Li₂S.

a–c, Evidence of diffusion of I₂ along the SE particle surface. a, The LBPSI electrolyte pellet on direct contact with I₂ for 5 min. b, The LBPSI electrolyte pellet placed at the I₂ bottle cap for indirect contact with I₂ for 3 days. c, EDX mapping of the vertical cross-section of Li_5.5PS_4.5Cl_1.5 electrolyte pellet after indirect contact with I₂ for 3 days. d, Pictures showing the process of mixing Li₂S and I₂ in anhydrous ethanol. e,f, XRD pattern (e) and Raman spectrum (f) of the precipitated product. g,h, Characterization of the reaction of Li₂S and I₂ by a gentle ball-milling at 100 rpm for 2 h, with the XRD pattern (g) and Raman spectrum (h) of the product. i,j, Characterization of the reaction of Li₂S and I₂ by simple hand grinding in the mortar (5 min), with the XRD pattern (i) and Raman spectrum (j) of the product. The insets of j are visual pictures of the mixture of Li₂S and I₂ before and after hand grinding in the mortar. The diffusion and permeation of I₂ into the electrolyte pellet is shown in a,b. On direct contact of the LBPSI SE pellet with the iodine (brown), I₂ sublimes and diffuses very quickly over the LBPSI pellet surface within 5 min; the surface of the electrolyte is covered with brown iodine (a). The LBPSI pellet was placed at the cap of a bottle that contains I₂ and the bottle was sealed with grease to prevent escape of iodine vapour through the gap (that is, indirect contact; b). After 3 days without direct contact with I₂, the outer/top surface of the electrolyte pellet turns brown, indicating penetration of the iodine vapour across the SE pellet (b), which further proves the diffusion of I₂ along the grain boundaries and throughout the SE pellet. Note that these experiments were conducted in an Ar-filled glovebox and not in vacuum. Further, we performed the same indirect contact experiment with the iodine-free Li_5.5PS_4.5Cl_1.5 electrolyte. The cross-section of the pellet was subjected to scanning electron microscopy imaging and EDX analysis (c). We can see that the element I is uniformly distributed, along with the P, S and Cl, over the imaged area (several hundred µm; c). To mimic the reactions between Li₂S and I₂, equal molar amounts of Li₂S and I₂ were first added separately into anhydrous ethanol, the solutions of which are, respectively, colourless and purple (step 1; d) and, subsequently, the two solutions of Li₂S and I₂ were mixed. Along with the change of colour (pale yellow, step 2), we observed precipitation, which is confirmed to consist of crystallized LiI along with S (as shown by the XRD pattern in e). The Raman spectrum further shows a strong signal corresponding to S (f). Therefore, the products of Li₂S reacting with I₂ are confirmed to be S and LiI. With an effort to further mimic the condition of the reaction occurring within the all-solid-state Li–S cells, the reaction was performed in solid state. Specifically, equimolar amounts of Li₂S and I₂ were mixed by a gentle ball-milling at 100 rpm for 2 h (to prevent overheating of the jar) and LiI and S are formed on solid–solid mixing of the two materials (based on XRD pattern; g). As shown in h, the Raman spectrum exhibits strong S signals (154 cm⁻¹, 219 cm⁻¹ and 473 cm⁻¹), I₂ (signal around 181 cm⁻¹) and I₃⁻ (110 cm⁻¹ and 167 cm⁻¹)⁵⁹. In fact, even by simple hand grinding in the mortar, the two materials react very quickly (j); during 5 min of grinding, the two materials react to form LiI (based on XRD pattern; i), S and I₃⁻ (Raman spectrum; j). The feasibility of the chemical reaction between I₂ and Li₂S that we experimentally confirmed above is strong evidence for us to conclude that the occurrence of redox mediation in the cell at high charging rates are not cascade reactions (that is, occurring sequentially without interplay). Further, the 0.3–0.4 V higher potential for iodine redox over that of sulfur redox (Fig. 3a) serves as the theoretical basis for a chemical reaction between the two couples. This is distinct from the behaviour of sulfur cells using Li_5.5PS_4.5Cl_1.5, in which the SE redox is expected to occur simultaneously with sulfur redox owing to the lack of potential difference (Fig. 3b).