Synthesis of Iron Sulfide Nanocrystals Encapsulated in Highly Porous Carbon‐Coated CNT Microsphere as Anode Materials for Sodium‐Ion Batteries

Abstract Highly porous carbon materials with a rationally designed pore structure can be utilized as reservoirs for metal or nonmetal components. The use of small‐sized metal or metal compound nanoparticles, completely encapsulated by carbon materials, has attracted significant attention as an effective approach to enhancing sodium ion storage properties. These materials have the ability to mitigate structural collapse caused by volume expansion during the charging process, enable short ion transport length, and prevent polysulfide elution. In this study, a concept of highly porous carbon‐coated carbon nanotube (CNT) porous microspheres, which serve as excellent reservoir materials is suggested and a porous microsphere is developed by encapsulating iron sulfide nanocrystals within the highly porous carbon‐coated CNTs using a sulfidation process. Furthermore, various sulfidation processes to determine the optimal method for achieving complete encapsulation are investigated by comparing the morphologies of diverse iron sulfide‐carbon composites. The fully encapsulated structure, combined with the porous carbon, provides ample space to accommodate the significant volume changes during cycling. As a result, the porous iron sulfide‐carbon‐CNT composite microspheres exhibited outstanding cycling stability (293 mA h g⁻¹ over 600 cycles at 1 A g⁻¹) and remarkable rate capability (100 mA h g⁻¹ at 5 A g⁻¹).