Extended Data Fig. 1: Structural characterizations and physicochemical properties of the LBPSI GSEs (the ratio x is defined as n(P₂S₅)/n(P₂S₅ + B₂S₃)).

a, XRD patterns for the LBPSI electrolytes with x of 0, 0.11, 0.17, 0.29, 0.50, 0.67, 0.75 and 0.80. b, Raman spectra of LBPSI electrolytes with x from 0 to 0.80. c, Enlarged view focused on the [P_mS_n] clusters. d, Schematic drawing of the clusters with their characteristic Raman shift. e, ³¹P MAS NMR spectra of LBPSI electrolytes with x of 0.17 and 0.29. f–h, DC polarization curves with an ion-blocking cell configuration of Ti|electrolyte|Ti for the three LBPSI electrolytes with x of 0 (f), 0.17 (g) and 0.29 (h). The thicknesses of the pellets during measurement for the three electrolytes are 0.028 cm, 0.022 cm and 0.023 cm, respectively. The measurements were conducted at 25 °C. i, The trend of electron conductivities obtained from DC polarization curves and the corresponding E_a values. The electronic conductivities for the three LBPSI electrolytes with x of 0, 0.17 and 0.29 are 6.4 × 10⁻¹⁰ S cm⁻¹, 2.8 × 10⁻¹¹ S cm⁻¹ and 2.8 × 10⁻¹⁰ S cm⁻¹, respectively. It is clear that, by fine-tuning the P/B ratio in the sulfide glass, the electronic conductivity of the electrolytes can be reduced. The values are much lower than the typical electron conductivity of argyrodite Li₆PS₅Cl and Li_5.5PS_4.5Cl_1.5 (typically measured to be 10⁻⁸–10⁻⁹ S cm⁻¹)^51,52, which may lead to low self-discharge in a cell. Discussions on the evolution of local clustering structures: on the basis of the XRD patterns (a), for the P₂S₅-free electrolyte (x = 0), a crystalline LiI phase is present, indicating that the LBSI glass framework cannot fully include the amount of LiI added, which leads to the precipitation of crystalline-phase LiI on quenching. With the addition of P₂S₅, the fraction of the crystalline phase of LiI markedly decreases and is barely observed at x = 0.17, indicating that, with a small amount of P₂S₅, the glass structure can better dissolve and integrate the LiI. The generated I⁻ in the glass structure can therefore contribute to a weaker coulombic attraction for Li⁺. However, at high P₂S₅/(P₂S₅ + B₂S₃) ratios of x = 0.75 and 0.80, the peak of crystalline LiI gradually appears again, suggesting that excessive P₂S₅ counteracts on the integration of LiI and would impede the Li⁺ diffusion. The Raman spectrum of P₂S₅-free electrolyte (x = 0) (b,c) shows the presence of only the [B_mS_n] group: the bands around 394 cm⁻¹ and 433 cm⁻¹ correspond to the B–S breathing mode of [BS₃] groups³⁴; the bands around 312 cm⁻¹, 497 cm⁻¹ and 763 cm⁻¹ correspond to the vibrations of metathioborate [B₃S₆], thiopyroborate [B₂S₅] and [BS₄] groups, respectively^34,53. With x increased to 0.11 and 0.17, a strong Raman peak corresponding to the [PS₄] group appears at 420 cm⁻¹ (ref. ³⁵), along with decrease of the [B_mS_n] peaks. Notably, at x = 0.29, two more peaks appear at 386 cm⁻¹ and 407 cm⁻¹, corresponding to the polyanionic groups of [P₂S₆]_hypo and [P₂S₇], respectively^34,35. Further, when x = 0.50, the Raman peak corresponding to [PS₄] greatly fades, whereas those of [P₂S₆]_hypo and [P₂S₇] groups are more pronounced, along with the appearance of a new peak at 582 cm⁻¹ ascribed to [P₂S₆]_hypo (refs. ^54,55). Here the two peaks at 386 cm⁻¹ and 582 cm⁻¹ represent the symmetric and asymmetric stretching modes of the [P₂S₆]_hypo group, respectively (d)⁵⁴. As x further increases to 0.67, the [PS₄] peak is no longer observed and a new peak appears at 425 cm⁻¹, along with two subtle peaks at 315 cm⁻¹ and 368 cm⁻¹, corresponding to the [P₂S₆]_meta group^54,55. Further increasing the fraction of P₂S₅ to x = 0.75 and 0.80 leads to a much higher intensity of the [P₂S₇] peak, along with the peaks of [P₂S₆]_meta and [P₂S₆]_hypo. On the basis of these results, we can clearly observe two transitions of the local structures in the LBPSI electrolytes as x increases: (1) the appearance of the [PS₄] group at x = 0.11; (2) the gradual disappearance of the [PS₄] group, along with the dominance of the [P₂S₆] and [P₂S₇] groups at x = 0.50. By associating the local structural changes with the evolution of ionic conductivity (Fig. 1b), we can draw the following conclusions: at x = 0.11, the appearance of the [PS₄] group substantially enhances the ionic conductivity, accounting for the high conductivity of more than 1 mS cm⁻¹ at x = 0.11, 0.17 and 0.29. We believe that the [PS₄] group promotes disruption of the large B–S network and formation of island-like structures, thereby enhancing the Li⁺ ion transport. The fragmentation is probably because of the different coordination preference of P (fourfold coordinated) and B (threefold coordinated), that is, the incorporation of P thermodynamically breaks the threefold coordinated B–S network. As x increases to 0.50, the appearance and dominance of [P₂S₆] and [P₂S₇] polyanions decreases the ionic conductivity, which ultimately reduces to 10⁻³ mS cm⁻¹ at x = 0.80, which is probably because of the highly charged polyanions exhibiting higher coulombic attraction for Li⁺ ions than the [PS₄] group³⁵. Further, as shown in ³¹P MAS NMR spectra (e), the resonances around 83 ppm, 99 ppm and 108 ppm can be ascribed to the [PS₄], [P₂S₇] and [P₂S₆] groups, respectively³⁴. It is clear that the resonance of [PS₄] appears in the electrolytes with x = 0.17 and 0.29. Further, the resonance of polyanionic [P₂S₇] and [P₂S₆] groups appear in the electrolyte with x = 0.29 (and not in the one with 0.17), which is in accordance with the Raman results. Altogether, in LBPSI, we find evidence for a more fragmented network (which should provide more free volume) and effective inclusion of I⁻, which results in higher ionic conductivity and easier breaking of the bond between Li⁺ and the anionic framework ligands at the electrode–electrolyte interface.