Oxygen Functional Groups Regulating Sulfur Distribution in Carbon Micropores to Enhance Solid-Phase Conversion Reactions for Lithium–Sulfur Batteries

The performance of lithium-sulfur (Li-S) batteries is determined by the cathode, which is to a large extent affected by the low electrical conductivity of S and the dissolution of lithium polysulfides (Li₂S_<i>x</i>). The confinement of S within microporous C is a promising method to address these challenges. The introduction of O-containing functional groups inside the C micropores improves the capacity for solid-phase conversion in Li-S batteries. However, the mechanism behind this enhanced performance, particularly the role of the O-containing functional groups on S inside the pores, remains unclear. In this study, we investigate the effect of these functional groups on S and/or the Li₂S_<i>x</i> inside the C micropores, focusing on their impact on the electrochemical efficiency and the suppression of polysulfide migration. Electrochemical impedance spectroscopy measurements show that these O-containing functional groups accelerate charge transfer reactions and Li⁺ ion diffusion. Cross-sectional scanning transmission electron microscopy-electron energy loss spectroscopy of the S-C composites reveals that, without O-containing functional groups, S and/or Li₂S_<i>x</i> migrate and localize to the inner edge of the carbon host during cycling. In contrast, the presence of O-containing functional groups inside the pores of the microporous C host maintains a uniform distribution of S and/or Li₂S_<i>x</i> within the C micropores, explaining the improved solid-phase conversion performance in Li-S batteries. In conclusion, this paper proposes a new design for the cathode of high-performance Li-S batteries. For the first time, experimental evidence is provided to confirm the mechanism whereby the introduction of O-containing functional groups into microporous C enhances the performance of Li-S batteries by lowering the resistance and preventing Li₂S_<i>x</i> migration. These modifications improve the electrochemical efficiency and offer insights for developing more effective cathodes to advance the commercialization of Li-S batteries.