Lithium-sulfur (Li-S) batteries have garnered significant attention recently due to their theoretical gravimetric energy densities substantially surpassing those of lithium-ion batteries. However, practical implementation faces considerable challenges...
Lithium-sulfur (Li-S) batteries have garnered significant attention recently due to their theoretical gravimetric energy densities substantially surpassing those of lithium-ion batteries. However, practical implementation faces considerable challenges, including limited cycle life and low Coulombic efficiency. This study introduces an innovative dual-layer sulfur cathode design, comprising a sulfur-impregnated composite of insulating mesoporous carbon (p-MC) with polar functional groups (hydroxyl and carbonyl) and a conductive multi-walled carbon nanotube (CNT) film devoid of active materials. In this configuration, solid sulfur is completely isolated from the conducting carbon networks, with charge transfer reactions occurring exclusively on the CNT surfaces. Upon initial discharge, solid sulfur embedded in nonconductive p-MC particles undergoes conversion to polysulfides in the CNT bottom layer through a three-step process: (1) sulfur dissolution in the electrolyte, (2) migration of dissolved molecular sulfur to the CNT bottom layer, and (3) electrochemical reduction to generate polysulfides on the CNT surfaces. Compared to a sulfur-layer CNT cathode, the dual-layer cathode exhibited superior longevity, maintaining a high discharge capacity of 803 mAh g-1 over 300 cycles with 80.4% capacity retention. This performance significantly outpaces its counterpart, which delivered only 462 mAh g-1 with 56.4% retention over the same period. The exceptional cycling stability of the dual-layer cathode is attributed to the unique surface and morphological properties of the p-MC. While the highly porous structure of the CNT film facilitates accommodation of a substantial amount of generated polysulfides, its inert surface limits secure confinement of polysulfides within its pores, leading to their continuous migration. EDS elemental analyses of separators from cycled cells strongly support the effectiveness of the p-MC layer in inhibiting soluble polysulfide diffusion into the electrolyte. Furthermore, XPS studies on the cycled cathode reveal the formation of thiosulfate groups through reactions between polysulfides and hydroxyl groups on the p-MC. Both thiosulfate and carbonyl groups interact reversibly with lithium polysulfides, effectively constraining active materials within the cathode region. These findings underscore the efficacy of the dual-layer cathode architecture in addressing critical issues in Li-S batteries, paving the way for reliable, long-lasting energy storage solutions. We anticipate that significantly higher energy densities could be achieved by incorporating a sulfur-infused CNT layer as the bottom layer in future iterations of this dual-layer cathode design.