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COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00626-2 ARTICLE presence of LiF and LiFxNy peaks from salt decomposition and salt species at 685.5 and 688 eV, respectively in charged samples. Figure 7a–c shows the XRD patterns of the pristine, discharged, and charged γS-CNFs cathodes. The diffractogram of the pristine cathode as discussed earlier shows peaks of γ-S. During initial discharge, we observe a plateau at 2.0 V and the plateau continues to progress towards complete sulfur reduction at 1.0 V at a rate of 0.05 C. After complete reduction of cathodes, the diffraction pattern shows the presence of Li2S peaks (JCPDS 00-023-0369) at 2θ = 26.9, 31.2, 44.8, and 53.08 which correspond to reflections (111), (200), (220), and (311), respectively. It confirms the finding from XPS and TEM that the single plateau observed in the discharge cycle is associated with the reduction of γ-sulfur to lithium sulfide (Li2S). After the charge, interestingly, a completely different sulfur XRD pattern is observed. Such pattern has not been reported yet in the Li–S literature. These peaks are attributed to cyclo-deca-cyclo-hexa sulfur, also belonging to a monoclinic crystal structure family. No overlapping gamma monoclinic sulfur peaks were observed. This post mortem study demonstrates the complete conversion of γ-monoclinic sulfur to Li2S and back to a new sulfur monoclinic crystal phase. This is the first-ever study to report stability of such sulfur crystal structures in Li–S batteries and their operation in carbonate electrolytes. It has been previously reported in ether-based Li–S batteries that α-orthorhombic sulfur allotrope (the most stable sulfur allotrope at room temperature) indeed converts to the β-monoclinic phase and that phase dominates after the first charge cycle39. Using this analogy, we hypothesize that the monoclinic phase is thermodynamically more stable in the Li–S electrolyte medium and is therefore retained in our system even after the charging cycle. Nevertheless, we observe a unique monoclinic phase in these charged samples, which is different from the β-monoclinic phase seen in ether electrolyte, and this possibly plays a role in retaining single plateau behavior in charge- discharge profiles for over 4000 cycles. Further studies, particularly computational modeling and simulations, are necessary to under- stand the origin of this phenomenon. Conclusions In this work, we synthesize and study a novel phase of sulfur (γ- monoclinic phase) in carbonate electrolyte-based lithium-sulfur batteries. Carbonate electrolytes, despite their tremendous com- mercial success in Li-ion batteries for the past three decades, are known to cause unfavorable and irreversible side reactions with intermediate sulfur reduction products (polysulfides) in Li–S batteries resulting in a complete cell shutdown. In our work, we demonstrate that despite an exposed “un-confined” deposition of the γ-monoclinic sulfur on the host carbon material, the carbonate-based battery exhibits high reversible capacity, which stabilizes to 800 mAh/g in the first few cycles and then it remains stable with a small 0.0375% decay rate over 4000 cycles. Funda- mental electrochemical characterization and post-mortem XRD, XPS, SEM, and TEM studies on cycled cells reveal an altered redox mechanism that reversibly converts γ-monoclinic sulfur to Li2S without the formation of intermediate polysulfides elim- inating irreversible side reactions for the entire range of 4000 cycles. Nevertheless, practical applications require far more aggressive optimizations with the large-scale continuous fabrica- tion of CNFs, tuning its surface porosity, and finally additives in the electrolyte to stabilize the system to achieve commercial-grade performance. To the best of our knowledge, we are the first to report both the stabilization of γ-monoclinic sulfur at room temperature and its utilization in Li–S batteries. We believe, this work will trigger new fundamental research, especially to understand the sulfur phase-performance correlations in various electrolytes coupled with in situ/operando characterization to elucidate information on structure evolution, redox mechanisms, changes in the system environment contributing towards phase stability and ion transport properties. This will enable a deeper understanding of the system facilitating the commercialization of Li–S batteries. Methods Materials. Polyacrylonitrile (PAN, Mw 150 000 g mol−1), N, N-Dimethylformamide (DMF, purity 99.8%), Sulfur (S, purity 99.998% trace metals basis), ethylene carbonate (EC, purity ≥ 99%, acid < 10 ppm, H2O < 10 ppm), diethyl carbonate (DEC, pur- ity ≥ 99%, acid < 10 ppm, H2O < 10 ppm),lithium nitrate (Sigma Aldrich) 1,2-dime- thoxyethane (DME) (Sigma Aldrich), and lithium hexafluorophosphate (LiPF6, purity ≥ 99.99% trace metals basis, battery grade) were purchased from Sigma Aldrich. 1,3-dioxolane (DOL) (99.8%, anhydrous, stabilized with 75 ppm BHT) and lithium trifluoromethanesulfonate were purchased from Acros Organics. All chemicals were used without further processing. Material synthesis Synthesis of CNFs. The free-standing CNFs were made by electrospinning42. Typi- cally, 10 wt% polyacrylonitrile, was added to DMF and stirred overnight to form a polymeric solution. This solution was then loaded into a Becton Dickinson 5 mL syringe with a Luer lock tip and an 18-gauge stainless steel needle (Hamilton Cor- poration). The syringe with the needle was connected to a NE-400 model syringe pump (New Era Pump Systems, Inc.) to control the feeding rate of the solution. The grounded aluminum collector was placed 6 in. from the tip of the needle. Electro- spinning was performed at room temperature with a relative humidity below 15%. A potential difference of 7–8 KV (Series ES -30 KV, Gamma High Voltage Research, Inc.) was applied between the collector and the tip of the needle. The flow rate of the solution was kept constant at 0.2 mL h−1. The as-spun nanofibers were collected and stabilized in a convection oven at 280 °C for 6 h in air atmosphere. The stabilized nanofiber mats were then placed in alumina plates and carbonized in a nitrogen environment up till 900 °C at a ramp rate of 2.5 °C min−1 and then activated under CO2 flow for 1 h in a horizontal tube furnace (MTI. Corp). The furnace was then cooled at 2 °C min−1 until it reached room temperature. Monoclinic γ-sulfur deposition on CNFs. The free-standing CNF mats were pun- ched with stainless steel die (φ = 11 mm) and dried at 150 °C overnight under vacuum. The CNF discs were then weighed and placed in an in-house developed autoclave (Stainless steel 316) and subjected to 180 °C for 24 h in an oven. The autoclave consisted of a sulfur reservoir at the bottom and a perforated disk for placing electrodes at the top. After 24 h the autoclave was cooled to room tem- perature slowly in a span of 6–8 h. The electrodes were weighed and transferred in an Argon-filled glove box via overnight room temperature vacuum drying in the antechamber for battery fabrication. Characterization Material characterization. Morphological and elemental characterization of the nanofibers was conducted using an SEM (Zeiss Supra 50 VP, Germany) equipped with energy-dispersive X-ray spectroscopy with an in-lens detector and 30 μm aperture. XRD patterns were acquired on a diffractometer (Rigaku Smartlab, Tokyo, Japan) using Cu Kα radiation (40 kV and 44 mA) with a step size of 0.02° in the 2θ range of 10°–70°. The surface chemistry of the samples was analyzed using XPS spectra (Physical Electronics Versa Probe 5000 spectrometer with mono- chromatic Al Kα as an excitation source) with a spot size of 200 μm and pass energy of 23.5 eV. A step size of 0.5 eV was used to gather the high-resolution spectra. CasaXPS Version 2.3.19PR1.0 software was used for spectra analysis. The XPS spectra were calibrated by setting the valence edge to zero, which was cal- culated by fitting the valence edge with a step-down function and setting the intersection to 0 eV. The background was determined using the Shirley algorithm, which is a built-in function in the CasaXPS software. TGA data were collected on TA Instruments 2950 (TA Instruments, New Castle, DE) under steady argon flow at a heating ramp rate of 5 °C min−1. Nitrogen adsorption–desorption analysis of the freestanding nanofiber mats was performed at −196.15 °C on an automated gas sorption analyzer (AutoSorb iQ2, Quantachrome Instruments). The sample was degassed overnight at 150 °C under N2 flow prior to this analysis. Electrochemical characterization. Electrochemical measurements were conducted by assembling 2032—type coin cells (MTI and Xiamen TMAX battery equipment) in an argon-filled glove box (MBraun Labstar pro, MB 10 G, H2O, and O2 < 1 ppm). As-transferred electrodes were used as working electrodes and 13 mm lithium discs punched from Lithium foil (Alfa Aesar, 0.75 mm thick) were used as counter/reference electrodes. To improve the mass loading, cathodes were stacked onto each other. A typical sulfur weight loading of around 45–50% was used with a mass loading of around 0.5–5 mg cm−2 for electrochemical testing. The ether electrolyte was prepared by dissolving 1.0 M Lithium bis(trifluoromethanesulfonyl) COMMUNICATIONS CHEMISTRY | (2022)5:17 | https://doi.org/10.1038/s42004-022-00626-2 | www.nature.com/commschem 9 Content courtesy of Springer Nature, terms of use apply. Rights reservedPDF Image | Stabilization of gamma sulfur at room temperature
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Sulfur Deposition on Carbon Nanofibers using Supercritical CO2 Sulfur Deposition on Carbon Nanofibers using Supercritical CO2. Gamma sulfur also known as mother of pearl sulfur and nacreous sulfur... More Info
CO2 Organic Rankine Cycle Experimenter Platform The supercritical CO2 phase change system is both a heat pump and organic rankine cycle which can be used for those purposes and as a supercritical extractor for advanced subcritical and supercritical extraction technology. Uses include producing nanoparticles, precious metal CO2 extraction, lithium battery recycling, and other applications... More Info
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