Enhanced Li bonds enable bidirectional sulfur catalysis by a molecular Co-N4 catalyst for lithium-sulfur batteries

Sulfur conversion catalysis is an effective strategy to tackle the inherent polysulfide shutting and fast capacity decay in lithium-sulfur batteries. Previous studies identified cobalt phthalocyanine as a unique molecular catalyst with Co-N₄ active sites to promote sulfur reduction reactions. Herein, enhancing the Li bonds with the pyridinic N atoms on the phthalocyanine ligand is demonstrated to endow further catalytic activity in sulfur oxidation reactions. This is enabled by peripheral substitution with electron-donating methoxy groups that strengthens the electronegativity of the pyridinic N atoms. DFT calculations and metadynamics simulations identify the key role of Li bonds in reducing the energy barrier of Li₂S dissociation. The highly lithiophilic methoxy groups also stabilize the dissociated Li cations in their solvation structure. The dissociated S anion can then be oxidized to higher-order polysulfides or radicals by electron donation to the Co-N₄ center, which activates Li₂S for subsequent oxidation. Meanwhile, the catalytic activity in sulfur reduction reactions is also improved by a more efficient electron transfer to adsorbed Li₂S₄, during which the strengthened Li bonds are beneficial in mediating the breakage of the bridging S-S bond. Significant performance improvements are achieved with the bidirectional catalyst under high areal sulfur loading and in Li-S pouch cells.