Pomegranate-Inspired Nitrogen-Doped Carbon-Coated Bimetallic Sulfides as a High-Performance Anode of Sodium-Ion Batteries and Their Structural Evolution Analysis

Metal sulfides are regarded as the most promising candidates for sodium-ion battery (SIB) anodes because of their merits of high theoretical capacity, stable redox reactions, and lowionic diffusion, and unstable reaction interfaces have largely limited their practical applications. To tackle these problems, a special pomegranate structure, composed of many nitrogen-doped carboncoated bimetallic sulfide nanoparticles (FeS/NiS@NCS), is deliberated designed. When used as the anode of SIBs, FeS/ NiS@NCS has exhibited a high reversible capacity (668.7 mA h g-1 at 0.1 A g-1), a superior cycling stability (414.6 mA h g-1 with 86.7% capacity retention after 500 cycles at 1 A g-1), and a high rate capability (251 mA h g-1 at 5 A g-1). Moreover, when paired with the cathode material of carbon-coated Na3V2(PO4)2F3 (CNVPF), the full cell delivers good cycle performances (65.07 mA h g-1 with 75.8% capacity retention after 100 cycles at 1 A g-1). Besides, the in situ X-ray diffraction technique was performed to analyze its structural evolution, confirming that the FeS/NiS@NCS anode undergoes a two-step reaction mechanism (first Na+ insertion process and then phase conversion reaction during the discharging process, conversion reaction, and ionic extraction during the charging process).