Low-Coordinated Zn-N 2 Sites as Bidirectional Atomic Catalysis for Room-Temperature Na-S Batteries.

The rational design of advanced catalysts for sodium-sulfur (Na-S) batteries is important but remains challenging due to the limited understanding of sulfur catalytic mechanisms. Here, we propose an efficient sulfur host consisting of atomic low-coordinated Zn-N 2 sites dispersed on N-rich microporous graphene (Zn-N 2 @NG), which realizes state-of-the-art sodium-storage performance with a high sulfur content of 66 wt %, high-rate capability (467 mA h g -1 at 5 A g -1 ), and long cycling stability for 6500 cycles with an ultralow capacity decay rate of 0.0062% per cycle. Ex situ methods combined with theoretical calculations demonstrate the superior bidirectional catalysis of Zn-N 2 sites on sulfur conversion (S 8 ↔ Na 2 S). Furthermore, in situ transmission electron microscopy was applied to visualize the microscopic S redox evolution under the catalysis of Zn-N 2 sites without liquid electrolytes. During the sodiation process, both surface S nanoparticles and S molecules in the mircopores of Zn-N 2 @NG quickly convert into Na 2 S nanograins. During the following desodiation process, only a small part of the above Na 2 S can be oxidized into Na 2 S x . These results reveal that, without liquid electrolytes, Na 2 S is difficult to be decomposed even with the assistance of Zn-N 2 sites. This conclusion emphasizes the critical role of liquid electrolytes in the catalytic oxidation of Na 2 S, which was usually ignored by previous works.