Enhanced monoclinic sulfur loading on carbon fiber matrices for Li-S battery cathodes via a chemical activation pathway

Although lithium-sulfur (Li-S) batteries present a promising alternative to conventional Li-ion batteries, their adoption is severely limited by low cycle life, poor rate capability, polysulfide shuttling, and incompatibility with carbonate electrolytes. 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 on the loading of monoclinic sulfur onto electrospun polyacrylonitrile (PAN)-derived carbon fiber (CF) mats for their 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. 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 1 C (areal capacity of 1 mAh cm⁻²). The cells retain 91 % of their initial capacity even after 1000 charge-discharge cycles without the need for sulfur confinement. Additionally, the cells deliver a specific capacity of 400 mAh g⁻¹ at a C-rate as high as 40 C. 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.