Nitrogen-Doped MOF-Derived Micropores Carbon as Immobilizer for Small Sulfur Molecules as a Cathode for Lithium Sulfur Batteries with Excellent Electrochemical Performance

Abstract

Nitrogen-doped carbon (NDC) spheres with abundant 22 nm mesopores and 0.5 nm micropores are obtained by directly carbonization of nitrogen-contained metal organic framework (MOF) nanocrystals. Large S₈ and small S_2–4 molecules are successfully infiltrated into 22 nm mesopores and 0.5 nm micropores, respectively. We successfully investigate the effect of sulfur immobilization in mesopores and micropores on the electrochemical performance of lithium–sulfur (Li–S) battery based on NDC–sulfur hybrid cathodes. The large S₈ molecules in 22 nm mesopores can be removed by a prolonged heat treatment, with only small molecules of S_2–4 immobilized in micropores of NDC matrices. The NDC/S_2–4 hybrid exhibits excellent cycling performance, high Coulombic efficiency, and good rate capability as cathode for Li–S batteries. The confinement of smaller S_2–4 molecules in the micropores of NDS efficiently avoids the loss of active sulfur and formation of soluble high-order Li polysulfides. The porous carbon can buffer the volume expansion and contraction changes, promising a stable structure for cathode. Furthermore, N doping in MOF-derived carbon not only facilitates the fast charge transfer but also is helpful in building a stronger interaction between carbon and sulfur, strengthening immobilization ability of S_2–4 in micropores. The NDS–sulfur hybrid cathode exhibits a reversible capacity of 936.5 mAh g^–1 at 100th cycle with a Coulombic efficiency of 100% under a current density of 335 mA g^–1. It displays a superior rate capability performance, delivering a capacity of 632 mAh g^–1 at a high rate of 5 A g^–1. This uniquely porous NDC derived from MOF nanocrystals could be applied in related high-energy storage devices