MOF-Derived CoS/N-Doped Carbon Composite to Induce Short-Chain Sulfur Molecule Generation for Enhanced Sodium-Sulfur Battery Performance.

Dissolution of intermediate sodium polysulfides (NaS; 4≤≤8) is a crucial obstacle for the development of room-temperature sodium-sulfur (Na-S) batteries. One promising strategy to avoid this issue is to load short-chain sulfur (S), which could prohibit the generation of soluble polysulfides during the sodiation process. Herein, unlike in the previous reported cases where short-chain sulfur was stored by confinement within a small-pore-size (≤0.5 nm) carbon host, we report a new strategy to generate short-chain sulfur in larger pores (>0.5 nm) by the synergistic catalytic effect of CoS and appropriate pore size. Based on density functional theory calculations, we predict that CoS can serve as a catalyst to weaken the S-S bond in the S ring structure, facilitating the formation of short-chain sulfur molecules. By experimentally tuning the pore size of the CoS-based hosts and comparing their performances as cathodes in Na-S and Li-S batteries, we conclude that such a catalytic effect depends on the proximity of sulfur to CoS. This avoids the generation of soluble polysulfides and results in superior electrochemical properties of the composite materials introduced here for Na-S batteries. As a result, the optimized CoS/N-doped carbon/S electrode showed excellent electrochemical performance with high reversible specific capacities of 488 mA h g (962 mA h g) after 100 cycles (0.1 A g) and 403 mA h g after 1000 cycles (1 A g) with a superior rate performance (262 mA h g at 5.0 A g).