Chemical Activation as a Pathway to Enhance Monoclinic Sulfur Loading on Carbon Fiber Matrices for Li-S Battery Cathodes

In recent decades, there has been a notable surge in demand for energy storage, driven by widespread adoption of electric vehicles, portable electronics, and the global shift towards energy generation from renewable sources. While lithium-ion batteries (LIBs) dominate the current energy storage market, alternatives to state-of-the-art LIBs are being sought as LIBs are expensive, rely on non-abundant materials, and offer a limited energy density of approximately 250 Wh/kg. Lithium-sulfur (Li-S) batteries present a promising alternative with a theoretical capacity ten times higher than that of LIBs (1675 mAh g⁻¹) and a theoretical energy density of 2500 Wh/kg. However, sulfur cathodes face challenges such as low cycle life, poor rate capability, polysulfide shuttling, and incompatibility with carbonate electrolytes. Various approaches have been explored to mitigate the phenomena of polysulfide formation and shuttling which includes nanoconfinement of sulfur, attaching sulfur to an organic backbone. In addition to these methods, recent studies indicate that the monoclinic phase of sulfur enables a single-step reaction from S₈ to Li₂S during cycling, avoiding intermediate polysulfides and resulting in stable cycling for over 1000 cycles. In this work, we report the loading of monoclinic sulfur onto electrospun polyacrylonitrile (PAN)-derived carbon fiber (CF) mats for use as cathodes in Li-S batteries. We demonstrate that KOH activation of these CF mats significantly enhances the loading of monoclinic sulfur, achieving a nearly four-fold increase. Consequently, Li-S cells fabricated with these activated and sulfur-loaded CF mats, paired with a low-cost carbonate electrolyte and lithium metal anode, achieved a specific capacity of 970 mAh g^-1 at 1C (areal capacity of 1 mAh cm^-2). The cells retained 91% of their initial capacity after 1000 charge-discharge cycles without the need for sulfur confinement. Additionally, the cells delivered a specific capacity of 400 mAh g^-1 at a C-rate as high as 40C. These findings suggest that activated CFs with non-confined monoclinic sulfur have significant potential for the commercialization of Li-S batteries using cost-effective carbonate electrolytes.