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COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00626-2 ARTICLE therefore likely that the single plateau behavior seen reversibly and consistently for 4000 cycles in our work is directly linked to the role of γ-monoclinic phase. Recently, Kaskel and coworkers demonstrated the effect on cathode electrolyte interphase (CEI) utilization of porous carbon structures in Li-S batteries for their use in carbonate-based electrolytes27. They utilized commercial micro/mesoporous carbon as well as in-house developed carbide-derived micropor- ous carbon. They have shown the development of SEI on the cathode (CEI) results in a high irreversible capacity loss in the first cycle, wherein the discharge capacities obtained are beyond the theoretical capacity of sulfur. This increase in capacity in the first cycle is attributed to the decomposition of the organic electrolyte as the potential is swept below 1 V wrt Li/Li+ for the formation of CEI in the first cycle. In addition, Aurbach et al., have demonstrated decomposition of ionic liquids (ILs) and organic electrolytes for the development of CEI on cathode interphase and concluded that the formation of SEI plays a dominant role in enabling sulfur utilization and not the confinement of sulfur in pores40. However, in our case, we see stable performance in cyclic voltammetry and charge–discharge cycles without subjecting our cathode below 1 V in the organic electrolyte from the initial cycling period. Although we observe the loss of capacity during cycling, the cathodes never achieved capacities beyond theoretical values. This striking difference in electrochemical response differentiates our behavior compared to CEI-based work demonstrated in the literature. To understand the importance of CNF based substrate for the deposition of γ-monoclinic sulfur contributing towards its stability we utilized a commercial microporous/mesoporous carbon substrate (C Novel, MH-00, Toyo Tanso, Japan). Upon the use of the same thermal treatments utilized for deposition of γ-sulfur, XRD results reveal a broad amorphous peak corre- sponding to carbon with no signature of sulfur. In addition, electrochemical results shown in Supplementary Fig. 7 demon- strate an extremely low capacity (1–3 V range) with triangular charge-discharge profiles, further suggesting the importance of CNFs for γ-monoclinic sulfur deposition and its subsequent utilization in Li–S batteries. Furthermore, to state the importance of monoclinic crystal structure on performance, we utilized the scraped residual material deposited on the top wall of our autoclave for cathode fabrication. The brownish shiny material has an XRD pattern showing mixed phase with few peaks corresponding to γ- monoclinic sulfur (Supplementary Fig. 8). Nevertheless, it is worth noting that this mixed-phase cathode yielded a single plateau charge-discharge similar to pure γS-CNFs in carbonate electrolyte and functions stably in carbonate electrolyte for close to 500 cycles (Supplementary Fig. 8c). The specific capacity is relatively lower possibly due to the absence of pure gamma phase resulting in lower sulfur utilization. Nevertheless, we still achieve 400mAh/g capacity in carbonate electrolyte despite a non-activated carbon substrate confirming the role of gamma sulfur in the observed electrochemical behavior. A possible reason for such a staggering effect of sulfur crystal structure could be the difference in phase density. While there are discrepancies in the reports on densities of various sulfur allotropes as synthesizing a metastable allotrope is non-trivial, Meyer et al. did groundbreaking work on sulfur allotropes in the early 1960s. He reported a density of γ-S to be higher than its α- counterpart (2.19g·cm−3 vs. 2.069g·cm−3)28,29. The close compactness within the γ-monoclinic crystal structure possibly provides greater stability and easy lithiation into gamma monoclinic crystal structure in the carbonate electrolyte. In the ether electrolyte, we believe that γ-sulfur converts to a more favorable phase to yield a two-plateau discharge. A study on the stability of this unique sulfur crystal structure in various electrolytes is underway. While providing experimental evidence for why the γ- monoclinic phase alters the discharge mechanism is non-trivial and will require future computational studies, we conducted post mortem studies using XRD and XPS to understand the redox products after charge and discharge cycles and to provide evidence that the stable capacity is indeed largely a result of the desired sulfur to Li2S reactions (and not any unwanted degradation reactions). This is also particularly important as most papers reporting single plateau discharge profile in Li–S batteries do not provide reactant and/or product characterization for a deeper understanding of the charge storage mechanism and to evaluate electrolyte decomposition (if any). Below we discuss both post mortem spectroscopy and microscopy data. Postmortem SEM and TEM analysis. To understand chemistry and surface morphology after cycling, we conducted postmortem microscopy of cycled cells. The surface morphology γS-CNFs after 20 charge and discharge cycles at 0.05 C is shown in Fig. 6a, b. Compared to pristine samples, the charged and discharged samples still retain their freestanding architecture. However, the surface deposited γ-sulfur redistributes itself on the surface possibly due to volume expansion–contraction during discharge–charge cycles. Nevertheless, γ sulfur still remains exposed and unconfined on the surface of CNFs. Figure 6c shows a TEM image taken after lithiation in a completely discharged sample post-1000 cycles. Despite ultra- sonication for TEM sample preparation, the sulfur particles appear to be well-adhered to the CNFs. An HRTEM image (Fig. 6d) taken from the deposited structures of discharged γS- CNFs confirms the formation of Li2S as its lattice fringe width was found to be 3.30 Å corresponding to the (111) orientation of the Li2S cubic phase. Postmortem XPS and XRD analysis. Figure 7 provides the postmortem XPS and XRD data both after discharge and after charge. In the center of the figure, we show a typical charge- discharge voltage profile that we obtain for our samples and the specific points where spectroscopy data was collected after fifth discharge and after fifth charge cycles at 0.05 C. Prior to XPS analysis, the cycled samples were thoroughly rinsed with the EC:DEC solvent and let to dry out under Ar atmosphere and later under dynamic vacuum for 48 h. The samples were then loaded in an XPS transfer assembly in the glove box and transferred to the XPS vacuum chamber avoiding any contact with the ambient atmosphere. As discussed earlier, in the pristine sample with vapor- deposited γS-CNFs, we see the presence of adventitious carbon, C at 284.6 eV from the CNF surface. The S 2p spectra show the presence of sulfur doublet peaks (S 2p3/2 and S 2p1/2) positioned at 163.7 and 164.9 eV with a peak separation of 1.18 eV. In addition, we see a peak at higher binding energy (168.94 eV) associated with the formation of surface oxides (S–O) during high-temperature deposition. A similar bond can be seen in O1s spectra, wherein the peak at 531.86 eV is attributed to the surface oxides. After complete discharge, the S2p spectra show the appearance of a new strong peak at a lower binding energy of 161.8eV associated with Lithium sulfide (Li2S) deposition. Interestingly, we also note the presence of a new peak at 685.5 eV attributed to LiF in F1s spectra (Supplementary Fig. 9). The signature of LiF species was not seen in the postmortem XRD spectrum (to be discussed below) denoting its extremely low contribution/amorphous nature. Furthermore, postmortem SEM or TEM images of charged and discharged samples shown above COMMUNICATIONS CHEMISTRY | (2022)5:17 | https://doi.org/10.1038/s42004-022-00626-2 | www.nature.com/commschem 7PDF Image | Stabilization of gamma sulfur at room temperature to enable the use of carbonate
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