![]() ![]() However, it is not clear how the smaller allotropes exhibited a capacity close to the theoretical capacity of S 8→Li 2S conversion. They showed stable capacity (with single discharge plateau) in carbonate electrolytes for up to 200 cycles. They proposed that the confinement within sub-nano pores prevented the formation of larger sulfur allotropes (S 5–8) and possibly resulted in small sulfur allotropes (S 2–4) only, which in turn converted to Li 2S without the intermediate polysulfides (Li 2S 8, Li 2S 6…). synthesized sulfur cathodes via confining sulfur molecules into 0.5 nm pores of microporous carbon host materials 25. A common feature in these works is the nano-confinement of sulfur. These papers propose a few different concepts/hypotheses that potentially enable successful battery operation in carbonate electrolytes. A handful of reports have recently demonstrated the use of Li–S batteries with carbonate-based electrolytes with stable and reversible capacity 22, 23, 24, 25, 26, 27. However, it is known that when carbonate electrolyte is used in Li–S batteries, an irreversible reaction between carbonate species and polysulfides takes place to form thiocarbonate and ethylene glycol, terminating further redox reactions and shutting down the battery 21. 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. In addition, flame retardant additives have been extensively researched, designed, and applied for carbonate-based electrolytes to enhance their reliability 20. 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 ☌) and wide operational window 18, 19, 20. 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 issues 17. Ether-based solvents are highly volatile and have low flash points posing a significant risk of operating such batteries beyond room temperatures 13, 14, 15 For example, dimethoxyethane (DME), an important ingredient used in present-day Li–S batteries has a boiling point of only 42 ☌ 16. A much less discussed, but debilitating drawback for the commercial viability of Li–S batteries is the use of the ether electrolyte itself. This series of challenges have been extensively studied in the past decade with most studies being in the ether electrolyte-based Li–S batteries 7, 8, 9, 10, 11, 12. Polysulfide shuttle results in an uncontrollable deposition of sulfide species on the lithium metal anode reducing coulombic efficiency and increasing capacity fade 6. A bigger challenge is the dissolution of the intermediate reaction products, lithium-polysulfides (LiPs), into the electrolyte causing the well-known “shuttle-effect” 4. The insulating nature of both sulfur and the final discharge product, Li 2S, results in low material utilization during the redox processes. However, the current Li-S system is plagued by numerous challenges 4, 5. In addition, sulfur is both environmentally friendly and naturally abundant in the earth’s crust. 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, 2, 3. We hope that this striking discovery of solid-to-solid reaction will trigger new fundamental and applied research in carbonate electrolyte Li-S batteries. To the best of our knowledge, this is the first study to report the synthesis of stable γ-sulfur and its application in Li-S batteries. Through electrochemical characterization and post-mortem spectroscopy/ microscopy studies on cycled cells, we demonstrate an altered redox mechanism in our cells that reversibly converts monoclinic sulfur to Li 2S without the formation of intermediate polysulfides for the entire range of 4000 cycles. Carbonates are known to adversely react with the intermediate polysulfides and shut down Li-S batteries in first discharge. Here, we stabilize a rare monoclinic γ-sulfur phase within carbon nanofibers that enables successful operation of Lithium-Sulfur (Li-S) batteries in carbonate electrolyte for 4000 cycles. However, these works utilize ether electrolytes that are highly volatile severely hindering their practicality. This past decade has seen extensive research in lithium-sulfur batteries with exemplary works mitigating the notorious polysulfide shuttling. ![]()
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