context: To examine the catalytic and capture property of 1T MoS2 NDs in a working Li–S cell, MoS2 ND/porous carbon/Li2S6 cathodes were prepared and subjected to synchrotron in situ XRD and in situ EIS characterizations. Fig. 4 shows the contour plot of the in situ XRD patterns collected during the first two cycles. Before discharging, no crystalline peaks were observed, confirming the high purity of the polysulfide catholyte and the low content of MoS2 NDs. When the cell was discharged to the plateau at 2.1 V, peaks referring to the (111) and (200) planes of cubic Li2S (PDF No. 023-0369, marked with a black dashed-line) appeared and reached their maximum intensity at the end of lithiation (bottom of Fig. 4). Upon charging, the intensity of Li2S decreased gradually, followed with no discernible XRD peaks and then the generation of monoclinic S8 (PDF No. 071-0137, marked with white dashed-lines), illustrating the solid (Li2S)–liquid (polysulfides)–solid (S8) reactions during the charging process. During the 2nd discharge/charge, reversible transitions between sulfur and Li2S were observed. When we compare the current in situ XRD results with peer studies, two findings can be extracted. First, MoS2 NDs propel the formation of Li2S crystals. Nelson et al. and Yang et al. both argued that the sluggish kinetics for solid (Li2S2)–solid (Li2S) conversion prevents Li2S formation upon the full discharge of sulfur/carbon electrodes, resulting in undetectable Li2S crystals during in situ XRD studies. The incomplete reduction accounted for the deficient sulfur utilization and low reversible capacities. Fortunately, the powerful MoS2 ND catalysts enabled the reversible formation of crystalline Li2S in our operando XRD studies. Second, Ye et al. reported that the poor catalytic capability of MoN led to occasioned residual S8 XRD peaks for the MoN/sulfur electrode after full discharging, which was also observed for conventional 2H MoS2 flakes-modified porous carbon/Li2S6 cathodes in this work (Fig. S9, ESI†). In contrast, no residual sulfur/Li2S crystals were observed for MoS2 ND/porous carbon/Li2S6 after full discharging/charging, respectively, again indicating the high catalytic property of the 1T MoS2 NDs.
answer: porous carbon/Li2S6