Tuning primary shell nitrogen in Fe–N–C for enhanced sulfur redox electrocatalysis

Lithium-sulfur (Li–S) batteries promise high energy density but are limited by sluggish sulfur redox kinetics and polysulfide shuttling effect, which lead to poor rate performance and rapid capacity fading. Electrocatalysis has emerged as an effective strategy to accelerate the conversion of polysulfides and improve battery performance. Among various electrocatalysts, metal–nitrogen–carbon catalysts are considered particularly appealing. It is well-recognized that coordination environment and the type of nitrogen species (e.g., pyridinic or pyrrolic nitrogen) in metal–nitrogen–carbon materials can profoundly influence their catalytic activity. However, the underlying relationship between these different ligand structures and the sulfur redox mechanism remains elusive. Herein, we report a systematic study establishing the relationship between sulfur redox activity and the Fe–N–C sites with precisely regulated primary-shell coordination environments, specifically pyridinic-N (Npd) and pyrrolic-N (Npl). Comprehensive theoretical simulations and experimental analyses reveal that pyridinic-N coordination exhibits a lower free energy for Li adsorption and positions the d-band center of the Fe-Npd-SAC catalyst closer to the Fermi level compared to pyrrolic-N. This electronic structure modulation enhances the sulfur redox kinetics. As a result, the optimized pyridinic Fe-Npd-SAC electrode delivers favorable electrochemical performance, achieving a specific capacity of 1265 mAh g–1 with a capacity fading of <0.05% per cycle. Furthermore, such an electrode enables Li–S full cells with an areal-capacity of 7.1 mAh cm–2 at a sulfur loading of 7.2 mg cm–2. These findings elucidate the underlying role of first-shell nitrogen coordination in accelerating sulfur redox kinetics, offering insights into the rational construction of practical Li–S batteries.