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

Abstract

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).

Interface Engineering of Fe7S8/FeS2 Heterostructure in situ Encapsulated into Nitrogen-Doped Carbon Nanotubes for High Power Sodium-Ion Batteries

“…In addition, the Nyquist plots of the three electrodes after 50 cycles at 1.0 A g −1 manifest that the Fe 7 S 8 /FeS 2 /NCNT electrode displays a lower charge transfer resistance (93.5 Ω) compared with those of Fe 7 S 8 /NCNT (130.1 Ω) and FeS 2 /NCNT (134.4 Ω), indicating better electrochemical kinetics of the special designed Fe 7 S 8 /FeS 2 /NCNT heterostructure (Fig. 3 f) [ 21 , 23 ]. Therefore, the high capacitive contribution of the charge storage as well as the low energy barrier of charge transfer can facilitate the electrons/ions transport, thus dramatically decreasing the electrochemical polarization.…”

“…During the charging process, the characteristic peaks, corresponding to the Na 2 S and Fe, gradually weaken, combined with the appearance of Na x FeS 2 , suggesting the deintercalation of Na + . Finally, the formation of FeS 2 and Fe 7 S 8 suggests the reverse phase transition, highlighting its superior electrochemical reversibility [ 23 ]. Figure 3 h typically illustrates the working mechanism of Fe 7 S 8 /FeS 2 /NCNT electrode during sodiation/desodiation process.…”

“…Figure 3 h typically illustrates the working mechanism of Fe 7 S 8 /FeS 2 /NCNT electrode during sodiation/desodiation process. It undergoes a two-step reaction mechanism upon sodium storage: the Na + intercalation and phase transformation during the discharge process; the reverse conversion reaction and Na + extraction during the charging process [ 23 ].…”

“…As a typical TMSs, pyrite FeS 2 with a high theoretical capacity (894 mAh g −1 ), low cost, and environmental friendliness, is a promising and effective sodium storage electrode material [ 16 – 19 ]. However, similar to other metal sulfides, the iron sulfides-based electrodes suffer from poor electronic conductivity and huge volume change upon cycling, resulting in sluggish Na + insertion/extraction, large electrochemical polarization, electrode degradation, and hence poor rate capability and limited cycling life [ 20 – 23 ].…”

The Anode Materials for Lithium‐Ion and Sodium‐Ion Batteries Based on Conversion Reactions: a Review

“…It is useful to resist volume changes, shorten ion routes, improve electrical conductivity, and maintain structural stability thanks to the unique pomegranate‐like structure and thin carbon shell. At 1 A g −1 , FeS/NiS@NCS exhibits good cycling stability (414.6 mAh g −1 with 86.7 % capacity maintenance) after 500 cycles, as shown in Figure 10i [110] . Fang et al.…”