Lithium-sulfur (Li-S) batteries offer significantly higher theoretical energy density than conventional lithium-ion batteries (LIBs), making them attractive for electric vehicles and portable electronics. Sulfur is abundant and environmentally friendly. However, challenges include the insulating nature of sulfur and Li₂S (the discharge product), leading to low material utilization, and the polysulfide shuttle effect, where intermediate lithium polysulfides (LiPs) dissolve into the electrolyte, causing capacity fade and reduced coulombic efficiency. Most research has focused on ether-based electrolytes, which are highly volatile and pose safety risks due to their low flash points. In contrast, carbonate-based electrolytes, widely used in LIBs, offer better safety and wider operational windows. However, carbonates react irreversibly with polysulfides, typically shutting down Li-S batteries. Recent studies have shown some success using carbonate electrolytes with nano-confined sulfur, but these methods often require complex synthesis and limit sulfur loading. This study aims to address these limitations by exploring a novel approach: stabilizing the rare γ-monoclinic sulfur phase in Li-S batteries to enable their function in carbonate electrolytes without nano-confinement.

Extensive research has been conducted on mitigating the polysulfide shuttle effect in Li-S batteries, primarily using ether-based electrolytes. However, the volatility and safety concerns of ether electrolytes hinder their practical application. The use of carbonate electrolytes, which are safer and have a wider operational window, has been less explored due to their adverse reactions with polysulfides. Some studies have demonstrated stable capacity in carbonate electrolytes by nano-confining sulfur within sub-nanometer pores of carbon materials, proposing that this confinement prevents the formation of larger sulfur allotropes and promotes a direct conversion to Li₂S without intermediate polysulfides. However, these methods have limitations in terms of synthesis complexity and sulfur loading. This study offers a different approach, utilizing a novel sulfur phase without the need for stringent nano-confinement.

Free-standing carbon nanofibers (CNFs) were synthesized via electrospinning of polyacrylonitrile, followed by stabilization, carbonization, and activation. γ-monoclinic sulfur was deposited onto the CNFs using an in-house autoclave at 180 °C for 24 h, followed by slow cooling. The resulting γS-CNFs were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area analysis, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). Electrochemical performance was evaluated using CR2032 coin cells with lithium metal anodes and either ether (DME:DOL) or carbonate (EC:DEC) electrolytes. Galvanostatic charge-discharge cycling, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and post-mortem analysis (SEM, TEM, XRD, XPS) were performed to investigate the electrochemical behavior and redox mechanism. Different C-rates and sulfur loadings were also investigated to assess rate capability and practical feasibility.

The study successfully synthesized and stabilized the rare γ-monoclinic sulfur phase at room temperature on the surface of carbon nanofibers. XRD analysis confirmed the presence of the γ-phase. SEM images revealed uniform sulfur deposition on the CNF surface. BET analysis showed a significant reduction in surface area after sulfur deposition, suggesting pore filling. TGA indicated a sulfur content of ~50 wt%. Electrochemical testing demonstrated high reversible capacity in carbonate electrolyte: stabilizing at 800 mAh·g⁻¹ after initial cycles and maintaining a capacity of 650 mAh·g⁻¹ after 4000 cycles with a 0.0375% decay rate. The cells exhibited a single plateau in the charge-discharge profiles, both initially and throughout the 4000 cycles in the carbonate electrolyte, contrasted with the typical two-plateau behavior seen in ether electrolytes. Post-mortem XRD and XPS analysis after discharge revealed the formation of Li₂S, and after charge the conversion back to a different monoclinic sulfur phase. EIS analysis showed a monotonic decrease in charge transfer resistance during discharge, suggesting the absence of intermediate polysulfides. The γS-CNF cathode also demonstrated excellent rate capability, retaining significant capacity even at high C-rates (up to 40C), and high sulfur loadings (5 mg cm⁻²) showed stable cycling for 300 cycles.

The results demonstrate that the γ-monoclinic sulfur phase significantly alters the redox mechanism in Li-S batteries, enabling their successful operation in carbonate electrolytes. The single-plateau discharge profile and absence of intermediate polysulfides in the carbonate electrolyte indicate a direct conversion from sulfur to Li₂S, avoiding the detrimental reactions between polysulfides and carbonates. The high capacity retention over 4000 cycles and excellent rate performance highlight the potential of this approach for practical Li-S batteries. The different monoclinic phase observed after charging suggests a unique and stable redox cycle not previously observed in Li-S batteries.

This study presents the first successful stabilization of γ-monoclinic sulfur at room temperature and its application in high-performance Li-S batteries using carbonate electrolytes. The unique redox mechanism involving a direct conversion between γ-sulfur and Li₂S eliminates the polysulfide shuttle effect and enables stable cycling for over 4000 cycles. Future research should focus on optimizing the synthesis of CNFs, exploring different carbon materials, and investigating electrolyte additives to further enhance performance and enable commercialization of Li-S batteries based on this novel approach. Computational studies can further elucidate the underlying mechanisms of γ-sulfur stability and its unique redox behavior.

While this study demonstrates significant progress, limitations include the use of excess lithium metal as the counter electrode. Future work should investigate strategies for anode stabilization to enhance overall battery performance and enable the use of thinner lithium anodes. Further investigation is needed to fully understand the reasons for the stability of the γ-monoclinic sulfur and the specific crystal structure of the reformed sulfur after charging. The current synthesis method for the γ-monoclinic sulfur might need scaling-up and optimization for large-scale production.