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ARTICLE COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00626-2 in ether electrolyte we performed the cycling test with and without the addition of LiNO3 since it plays a key role in passivation of lithium surface (Supplementary Fig. 6). The results show improved stability and gradual capacity fade by the addition of LiNO3. To further corroborate this unique electrochemical behavior of γS-CNFs in carbonate electrolytes and infer information about the reaction mechanism, we conducted differential capacity (dQ/ dV) analysis and EIS as a function of voltage. We see a consistent single peak in the dQ/dV plot for 2000 cycles further strengthening our finding of a single-phase conversion. The peaks minimally shift during cycling suggesting good material integrity and minimal increase in resistance during cycling. As a next step, the EIS measurements of the lithium half-cells with γS- CNFs as composite cathodes were carried out at various potentials during charge–discharge cycles. Although owing to the complexity of any battery assembly, a straightforward and quantitative interpretation of the EIS data is non-trivial. Never- theless, it can provide powerful information on qualitative trends. Figure 4f presents the typical Nyquist plots for our Li–S batteries illustrating their impedance trends as a function of voltage. As seen in this figure, a typical Nyquist plot consists of a semicircle in the high frequency to medium frequency range, which is attributed to the interfacial charge transfer resistance. The charge transfer resistance (Rct) and series resistance monotonically decrease as the cathodic curve progresses towards a lower potential for the entire discharge cycle. The trend is reversed when the battery is charged back to a higher potential. This observation contrasts with the literature, wherein the Rseries first decreases and then increases back again in the same discharge cycle due to the formation of soluble polysulfides at intermediate voltages36–38. These intermediate polysulfides significantly lower the Rs and Rct due to the disappearance of both of the solid insulating materials—the initial reactant, sulfur, and final product, Li2S. It is worth noting that in the literature, Rct of the final discharged cell still remains lower than the initial Rct (at OCV) due to the reduced resistance of Li2S compared to pure sulfur39. A monotonic decrease in Rs and Rct during discharge in our work provides further evidence that we are eliminating the formation of polysulfides. To evaluate the practical application of our carbonate-based Li–S system, we cycled our cells with γS-CNFs cathodes at various C rates and loadings. As shown earlier, these batteries demonstrate stable capacity at 0.5 C rate for over 4000 cycles. To demonstrate battery operation at harsh conditions, we tested the batteries for long-term cycling at 0.1 C (Fig. 5a). The batteries provided stable ~550 mAh·g−1 capacity for over 1000 cycles with a small 0.0015% decay and coulombic efficiency ≧99%. In addition, these batteries show excellent rate performance with a capacity of 1170, 1080, 980, 900, 750, 600, and 410 mAh·g−1 at 1, 2, 5, 10, 15, 30, and 40 C, respectively (Fig. 5b). It is interesting to see these cells exhibiting a capacity of 400 mAh/g even at 40 C corresponding to discharge and charge time of only ~30 s. The traditional ether-based batteries perform only up to 2 C at which the performance deteriorates significantly. Figure 5c shows that our cells exhibit a similar single plateau discharge at all C rates. Such rate capability suggests efficient nanoscale contact between γ-monoclinic sulfur and the host CNFs and good interfacial electrode–electrolyte contact owing to the 3D inter-fiber porous architecture. Furthermore, the binder-free freestanding format of the CNF host, we believe, provides uninterrupted electron pathways despite the presence of insulating sulfur. This is unique compared to traditional slurry-based cathodes where carbon and sulfur powders are mixed together with limited to no control over spatial morphology deteriorating overall composite conductivity. Figure 5d shows the cycling data for higher commercially relevant sulfur loadings. Cells with 5 mg·cm−2 of sulfur demonstrate stable cycling for 300 cycles at 0.1 C (2.35 mAh·cm−2). This finding demonstrates that unconfined sulfur deposition using γ-S-CNF can pave the path toward commercially relevant sulfur loadings in carbonate electrolytes. Exposed unconfined sulfur in the past has been associated with irreversible reactions with carbonate electrolyte and battery shut down after the first cycle as shown by Kim et al. On comparing our work with these previous unsuccessful sulfur studies in carbonate electrolytes, the striking difference is the crystal structure of the sulfur in our cathodes. Most Li–S literature, regardless of the electrolyte, uses α-orthorhombic sulfur, which is the most stable sulfur allotrope at room temperature. It is Fig. 5 Rate performance and high loading analysis of γS-CNFs. a Long cycling of γS-CNFs at a low current rate 0.1 C at 1.2 mg·cm−2 loading. b Rate performance of γS-CNFs in carbonate electrolyte at 0.5 mg·cm−2. c Charge–discharge profiles of γS-CNFs at various C rates. d Long cycling as a function of higher loading. The cells were assembled with variable S loading of 1.2–5.03 mg·cm−2 using ~50 wt% sulfur and an E/S ratio of 20. 6 COMMUNICATIONS CHEMISTRY | (2022)5:17 | https://doi.org/10.1038/s42004-022-00626-2 | www.nature.com/commschemPDF Image | Stabilization of gamma sulfur at room temperature to enable the use of carbonate
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