Molecular Adsorption Versus Particulate Loading: Structure–Activity Relationship of Sulfonated Cobalt Phthalocyanine in Sulfur Cathodes

The dispersion state of molecular catalysts critically determines sulfur utilization efficiency and redox kinetics in lithium–sulfur cells. Cobalt phthalocyanine (CoPc) exhibits intrinsic catalytic activity in sulfur redox reactions, owing to its planar π-conjugated framework and highly active Co-N4 centers. However, its poor solubility in solvents confines active sites to particle surfaces, thereby restricting catalytic utilization. The high flexibility of phthalocyanines allows for the introduction of substituents to modulate solubility. This study aims to utilize the differing solubility of sulfonated cobalt phthalocyanine (CoPcS) in various solvents to achieve distinct loading morphologies on carbon host, investigating the structure–activity relationship induced by catalyst dispersion. In the molecular adsorption configuration, the Co-N4 active sites exhibit enhanced accessibility to Li2S4, where the sulfur atoms engage in stronger electron-transfer interactions with the Co centers. This strengthened orbital coupling weakens the bridging S-S bond and facilitates the liquid–solid conversion. Compared to particle-loaded cathodes, molecularly adsorbed cathodes exhibit a charge transfer impedance approximately 84.6% lower and a high reversible capacity of nearly 800 mAh g−1 at a 3C rate. Particularly at a 0.5C rate, they achieve a high initial specific capacity of nearly 1300 mAh g−1 and maintain over 80% capacity retention after 200 cycles. This study demonstrates that molecular-level dispersion, with effective exposure of active sites, is essential for activating the catalytic potential of molecular catalysts and offers a general molecular-engineering strategy for high-performance lithium–sulfur batteries.