MOF-Derived CoS₂/N-Doped Carbon Composite to Induce Short-Chain Sulfur Molecule Generation for Enhanced Sodium–Sulfur Battery Performance

Abstract

Dissolution of intermediate sodium polysulfides (Na₂S_<i>x</i>; 4≤<i>x</i>≤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_2-4), 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 CoS₂ and appropriate pore size. Based on density functional theory calculations, we predict that CoS₂ can serve as a catalyst to weaken the S-S bond in the S₈ ring structure, facilitating the formation of short-chain sulfur molecules. By experimentally tuning the pore size of the CoS₂-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 CoS₂. 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 CoS₂/N-doped carbon/S electrode showed excellent electrochemical performance with high reversible specific capacities of 488 mA h g^-1 (962 mA h g_(s)^-1) after 100 cycles (0.1 A g^-1) and 403 mA h g^-1 after 1000 cycles (1 A g^-1) with a superior rate performance (262 mA h g^-1 at 5.0 A g^-1).