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ARTICLE COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-022-00626-2 State of the art lithium–sulfur (Li–S) batteries are attractive candidates for use in electric vehicles (EVs) and advanced portable electronic devices owing to an order of magnitude higher theoretical energy density than the conventional lithium- ion batteries (LIB)1–3. In addition, sulfur is both environmentally friendly and naturally abundant in the earth’s crust. However, the current Li-S system is plagued by numerous challenges4,5. The insulating nature of both sulfur and the final discharge product, Li2S, results in low material utilization during the redox processes. A bigger challenge is the dissolution of the intermediate reaction products, lithium-polysulfides (LiPs), into the electrolyte causing the well-known “shuttle-effect”4. Polysulfide shuttle results in an uncontrollable deposition of sulfide species on the lithium metal anode reducing coulombic efficiency and increasing capacity fade6. This series of challenges have been extensively studied in the past decade with most studies being in the ether electrolyte- based Li–S batteries7–12. A much less discussed, but debilitating drawback for the commercial viability of Li–S batteries is the use of the ether electrolyte itself. Ether-based solvents are highly volatile and have low flash points posing a significant risk of operating such batteries beyond room temperatures13–15 For example, dimethoxyethane (DME), an important ingredient used in present-day Li–S batteries has a boiling point of only 42 °C16. Therefore, despite tremendous research in overcoming Li–S battery challenges, the practicality of such battery chemistries is severely hindered due to severe safety concerns and transport issues17. LIB have been dominant in the commercial market for the past 30 years with the use of carbonate-based electrolytes, well known for their reasonably safe behavior beyond room temperature (typical boiling points of >200°C) and wide operational window18–20. In addition, flame retardant additives have been extensively researched, designed, and applied for carbonate-based electrolytes to enhance their reliability20. Hence, the tremendous knowledge gained on carbonate electrolytes in the Li-ion battery field over the past three decades can potentially be applied for the future development of Li–S batteries. However, it is known that when carbonate electrolyte is used in Li–S batteries, an irrever- sible reaction between carbonate species and polysulfides takes place to form thiocarbonate and ethylene glycol, terminating further redox reactions and shutting down the battery21. A handful of reports have recently demonstrated the use of Li–S batteries with carbonate-based electrolytes with stable and reversible capacity22–27. These papers propose a few different concepts/hypotheses that potentially enable successful battery operation in carbonate electrolytes. A common feature in these works is the nano-confinement of sulfur. For example, Xin et al. synthesized sulfur cathodes via confining sulfur molecules into 0.5nm pores of microporous carbon host materials25. They proposed that the confinement within sub-nano pores prevented the formation of larger sulfur allotropes (S5–8) and possibly resulted in small sulfur allotropes (S2–4) only, which in turn converted to Li2S without the intermediate polysulfides (Li2S8, Li2S6...). They showed stable capacity (with single discharge plateau) in carbonate electrolytes for up to 200 cycles. However, it is not clear how the smaller allotropes exhibited a capacity close to the theoretical capacity of S8→Li2S conversion. In another work, Fu et al. also synthesized carbon/sulfur cathodes with sulfur confined in sub-nanometer carbon pores (0.4–1nm)26. Their material also exhibited single plateau discharge and stable reversible capacity for 100 cycles in carbonate electrolyte. They proposed that the small pore size forced the de-solvation of lithium ions and resulted in solid-state lithiation and de-lithiation of confined S8 molecules. Overall, these works propose stringent pore size requirements (<0.5 nm) for the host carbon requiring complex synthesis procedures limiting broad deployment, while also theoretically limiting the possible sulfur loading (due to limited available volume of precisely sized micropores). More- over, none of these reports attempt to characterize the initial sulfur allotropes (reactants) nor the discharge or charge products formed, and therefore the source of energy storage/capacity is unclear. Moreover, to the best of our knowledge, there are no reports employing sulfur via a non-confinement approach in carbonate-based electrolytes in the Li–S system. In this study, we synthesize and study a novel phase of sulfur (γ-monoclinic phase) in carbonate-based Li–S batteries. We demonstrate that despite an exposed “un-confined” deposition of this sulfur phase on the host carbon material, the carbonate-based battery exhibits high reversible capacity, which stabilizes to 800 mAh·g−1 in the first few cycles and then it remains stable with a small 0.0375% decay rate over 4000 cycles. The cells exhibit a high capacity of 650 mAh·g−1 even after the end of 4000 cycles. The host electrode consists of freestanding, binder, and current collector-free carbon nanofibers (CNFs). After sulfur deposition and slow cooling at room temperature in an autoclave developed in-house, sulfur adopts the rare monoclinic γ-phase rather than the typical orthorhombic α-phase on the surface of CNFs. This phase remains stable at room temperature for over a year with no apparent evidence for phase change even beyond this timeframe. Electrochemical characterization and post-mortem spectroscopy/ microscopy studies on cycled cells reveal an altered redox mechanism that reversibly converts monoclinic sulfur to Li2S without the formation of intermediate polysulfides for the entire range of 4000 cycles. The development of unconfined high loading sulfur cathodes in Li–S batteries employing carbonate- based electrolytes can revolutionize the field of high energy density practical batteries. Results and discussion Material characterization. Figure 1 provides a schematic outline of a Li–S cell with the monoclinic gamma-sulfur-based cathode in carbonate electrolyte. The scanning electron microscopy (SEM) images in Fig. 2a show a smooth CNF surface with an average diameter of ~150 nm. After sulfur deposition in the autoclave, SEM images reveal a consistently rough fiber morphology sug- gesting a uniform and conformal coating of sulfur (Fig. 2b). Few regions display blocks of sulfur deposited within the inter-fiber spacing. Overall these images provide clear evidence that the sulfur is largely on the outer CNF surface. To further understand the effect of sulfur deposition on surface area and pore sizes of CNFs, a BET surface area analysis was conducted. Figure 2d, e shows the N2 absorption/desorption isotherm plots and pore size distribution of CNFs before and after sulfur deposition. For CNFs, the gas uptake increases to a high value at low relative pressure (P/P0 < 0.05), and the adsorption isotherm exhibits a plateau at middle and high relative pressures. The hysteresis loop at P/P0 = 0.2–1.0 represents mesoporosity. The adsorption iso- therm is a combination of IUPAC types I and IV isotherms which confirms the presence of both micro and mesopores on CNFs23. However, after the sulfur deposition in the autoclave, the iso- therm shows significantly lower gas uptake suggesting a decrease in surface pores. Pore size distribution in Fig. 2e shows that the CNFs portray a multi-modal pore structure in the nanoscale regime. After the sulfur deposition, the CNFs display an enor- mous reduction in surface area (3.14 m2 g−1) suggesting pore filling by sulfur. Pore structural parameters of all the materials are summarized in Table 1. The BET and SEM data suggest that sulfur is partially confined within the carbon nanopores. Never- theless, there is clear evidence of exposed unconfined sulfur on the external carbon surface. Figure 2f shows the thermogravi- metric analysis on sulfur-deposited CNFs conducted in an inert 2 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. 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