Dual-Single-Atom Enables Bidirectional Catalytic Effect for SRR and SOR in Lithium-Sulfur Battery

Lithium-sulfur battery has been widely considered as one of the most potential candidates for traditional lithium-ion battery owing to its high theoretical capacity (1675 mAh g^-1 for sulfur), high energy density (2500 Wh kg^-1) and low cost. However, the shuttle effect of lithium polysulfides (Li₂S<i>_x</i> <i>x</i>=4, 6, 8), the low electrical conductivity of the final products (S₈ and Li₂S), and sluggish redox kinetics greatly impede the practical application of lithium-sulfur battery. Herein, we fabricated a novel hierarchical bidirectional electrocatalyst for sulfur cathode based on single atom engineering. In this structure, hollow carbon polyhedrons with a high specific surface area were modified with Ni-N₄ and Fe-N₄ single atomic sites simultaneously and served as the host for sulfur. From the experimental and DFT calculation results, Ni-N₄ sites could accelerate the liquid-solid conversion (Li₂S₄→Li₂S) while the Fe-N₄ center makes more contribution to the Li₂S oxidation process. Based on such bidirectional catalytic effect, the as-prepared catalyst with dual single atom sites exhibited outstanding rate performance (566 mAh g^-1 at 4 C) and remarkable capacity retention in the long-term cycling test (713 mAh g^-1 at 1 C after 600 cycles). More importantly, when the sulfur cathode was thickly coated with an areal sulfur loading of around 6 mg cm^-2, a high energy density over 4 mAh cm^-2 (commercial lithium-ion battery) can still be maintained during the cycles. We believe this work could pave a way for the next generation energy storage system investigation.