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 low-cost raw materials. However, low electronic conductivity, sluggish ionic diffusion, and unstable reaction interfaces have largely limited their practical applications. To tackle these problems, a special pomegranate structure, composed of many nitrogen-doped carbon-coated 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 (C-NVPF), 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).