Modulating the Coordination Environment of Single-Atom Catalysts—Impacts on Lithium-Sulfur Battery Performance

Single-atom catalysts (SACs), known for upper catalytic efficiency, has recently been used in lithium-sulfur batteries to modulate the sulfur conversion reaction. Their maximum atomic utilization enables comparable catalytic effects to nano-sized catalyst particles with significantly reduced mass proportions, mitigating the energy density loss caused by introducing such non-active substance in batteries. To further enhance electrocatalytic activity of SACs, altering the coordination environment of central mentals is a viable strategy, which avoids the excessive use of rare and expensive metals as observed in strategies involving increased mass loading of SACs. The most common type of transition metal-nitrogen (TM-N)4 SACs were constituted by four TM-N bonds with a highly symmetric structure. And the electronic structure of TM–N₄ could be altered by substituting the N atom with other atom of weaker electronegativity (e.g O, B, S, P.). Additionally, the different length of TM–N bond to TM-other atom will certainly cause the change of geometric structure of SACs, which could give rise to a redistribution of electron cloud density and thus promises to optimize the adsorption/desorption of intermediates for lithium-sulfur battery redox reaction. Herein, single atom Co-P₂N₂ decorated hierarchical porous carbon were designed to compare with that of classical single atom Co-N₄. When used in lithium-sulfur batteries, both of SAC Co-P₂N₂ and Co-N₄ act as catalysts to adsorb polysulfide intermediates and accelerate the conversion reaction of sulfur species. DFT calculation result revealed that although both of the geometric structure of SAC Co-P₂N₂ and Co-N₄ are symmetric, they have distinct electron configurations. Specifically, the adsorption energy of SAC Co-P₂N₂ with Li₂S₆is stronger than that of Co-N₄. And after adsorption, the bonding-state of the adjacent S-S bond of lithium polysulfide will be weaker, which means it can more effectively promote the breakage of S-S bonds and make the redox reaction easier. Moreover, the Li₂S decomposition energy barrier under the influence of Co-P₂N₂ is lower than that under Co-N₄, emphasizing the superior catalytic effect of SAC Co-P₂N₂ on lithium-sulfur battery reactions. Using SAC Co-P₂N₂ decorated hierarchical porous carbon as an interlayer assembly for lithium-sulfur batteries achieved better performance than SAC Co-N₄, consisting with theoretical calculations. This research contributes to a better understanding of how the structure of SACs influences lithium-sulfur battery reactions.