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ARTICLE COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00626-2 Table 1 Surface area and pore volume measurements of CNFs before and after thermal treatment. Sample SBET (m2 g−1) Average pore diameter (nm) CNFs before thermal treatment 458 1.12 CNFs after thermal treatment 3.14 n/a Fig. 3 Phase and surface characterization of CNFs and γ-CNFs. a XRD pattern of CNFs, γS-CNFs, and calculated pattern of γS. b, c SEM image of γS-CNFs showing well-distributed sulfur deposition and EDS elemental mapping. d–f XPS spectra of S, O, and C on the γS-CNFs. been made towards the development and understanding of this structure30. Only a handful of reports (<5) in the past two centuries have even mentioned the presence of γ-monoclinic sulfur at room temperature with a short lifetime31,32. For example, Watanabe in 1974 confirmed the synthesis of γ- monoclinic sulfur (γ-S) by treating cuprous ethyl xanthate with pyridine, however, the γ-S crystals soon converted to the stable orthorhombic α sulfur at room temperature30. In our case, we have studied the stability of our (γ-S) phase for 2+ years at room temperature and it remains stable, showing no signatures of the phase change (Supplementary Fig. 1). However, why sulfur stabilized with a monoclinic gamma crystal structure after deposition in our samples is currently unclear. A recent DFT study on the stabilization of metastable sulfur shows that the carbon host can facilitate the stabilization of a monoclinic sulfur phase if the number of carbon atoms exceeds 0.3 per S8 unit crystal structure33. In addition, it was recently suggested by Moon et al. that carbon facilitates the formation and helps in retaining the monoclinic structure at room temperature for longer periods31. In our study, we hypothesize that the (γ-S) phase formed at elevated temperatures penetrates the porous carbon structures and retains its crystal structure even after cooling due to the local carbon density within the pores. This unique crystal structure once trapped within the pores possibly propagates throughout the sulfur blocks including those that are externally deposited in an “unconfined” state. Figure 3b, c shows the EDX mapping and corresponding low magnification SEM image exhibiting the uniform distribution of sulfur. To confirm the chemical composition and surface properties of pristine γ-monoclinic-sulfur-based CNF cathodes (γS-CNFs), XPS measurements were performed and the results are displayed in Fig. 3d–f. The survey spectra (Supplementary Fig. 2) show the existence of C1s, S2p, and O1s peaks in the composite. The peaks centered at 284.6, 531.0, and 533 eV spectra correspond to the C1s, O1s, and the adsorbed surface hydroxyl group (−OH), respectively34. Figure 3d displays the high- resolution S2p spectra of the composite. The S 2p3/2 peak at 163.7 eV and S 2p1/2 peak at 164.9 eV with an area ratio of 1:2 and ΔE of 1.18eV are the characteristics of solid sulfur in the composite35. Another broad peak centered at 168.8 eV can be attributed to the surface oxidation of sulfur during high- temperature sulfur deposition treatment. The smooth Lorentzian asymmetric peak of carbon further confirms that sulfur does not react with the bare carbon surface. Electrochemical characterization. Figure 4 shows the electro- chemical performance evaluation of γS-CNFs used as free- standing cathodes in CR 2032 type coin cells with reference/ counter as lithium. Both ether and carbonate-based electrolytes were employed to understand the electrochemical phenomenon in each system. Due to the insulating nature of sulfur, electro- chemical impedance spectroscopy (EIS) was first performed on 4 COMMUNICATIONS CHEMISTRY | (2022)5:17 | https://doi.org/10.1038/s42004-022-00626-2 | www.nature.com/commschem Content courtesy of Springer Nature, terms of use apply. Rights reservedPDF Image | Stabilization of gamma sulfur at room temperature
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