Synthesis and Operando Sodiation Mechanistic Study of Nitrogen‐Doped Porous Carbon Coated Bimetallic Sulfide Hollow Nanocubes as Advanced Sodium Ion Battery Anode

Abstract Due to the obvious advantages of utilizing naturally abundant and low cost sodium resources, sodium ion batteries (SIBs) show great potential for large‐scale energy storage applications. And the high theoretical capacities of transition metal sulfides (TMSs) make them appealing anode materials for SIBs; however, structural collapse caused by the severe volume change during de/sodiation processes results in poor capacity retention and rate capabilities. Compared to the development of new materials and the improvement of their electrochemical performance, the studies on their reaction mechanisms are still rare, especially the operando characterizations. Herein, the synthesis, anode application, and the operando observation of the de/sodiation mechanism of a nitrogen‐doped porous carbon coated nickel cobalt bimetallic sulfide hollow nanocube ((Ni 0.5 Co 0.5 ) 9 S 8 @NC) composite are reported. Such a material is synthesized via facile sulfidation of phenol formaldehyde coated Ni 3 [Co(CN) 6 ] 2 metal–organic framework precursors with Na 2 S followed by calcination. The nanocomposite displays a remarkable specific capacity of 752 mAh g −1 at 100 mA g −1 after 100 cycles and outstanding rate capability due to the synergistic effect of several appealing features. Particularly, the pseudocapacitive effect appears to substantially contribute to the sodium storage capability. Operando X‐ray diffraction reveals the conversion reaction mechanism of (Ni 0.5 Co 0.5 ) 9 S 8 @NC, forming Ni, Co, Na 2 S, and Na 2 S 5 .