Low-Coordinated Zn-N2 Sites as Bidirectional Atomic Catalysis for Room-Temperature Na-S Batteries

Therational design of advanced catalysts for sodium-sulfur(Na-S) batteries is important but remains challenging due tothe limited understanding of sulfur catalytic mechanisms. Here, wepropose an efficient sulfur host consisting of atomic low-coordinatedZn-N-2 sites dispersed on N-rich microporous graphene(Zn-N-2@NG), which realizes state-of-the-art sodium-storageperformance with a high sulfur content of 66 wt %, high-rate capability(467 mA h g(-1) at 5 A g(-1)), andlong cycling stability for 6500 cycles with an ultralow capacity decayrate of 0.0062% per cycle. Ex situ methods combined with theoreticalcalculations demonstrate the superior bidirectional catalysis of Zn-N-2 sites on sulfur conversion (S-8 <-> Na2S). Furthermore, in situ transmission electron microscopywas applied to visualize the microscopic S redox evolution under thecatalysis of Zn-N-2 sites without liquid electrolytes.During the sodiation process, both surface S nanoparticles and S moleculesin the mircopores of Zn-N-2@NG quickly convert intoNa(2)S nanograins. During the following desodiation process,only a small part of the above Na2S can be oxidized intoNa(2)S (x) . These results revealthat, without liquid electrolytes, Na2S is difficult tobe decomposed even with the assistance of Zn-N-2 sites.This conclusion emphasizes the critical role of liquid electrolytesin the catalytic oxidation of Na2S, which was usually ignoredby previous works.