Sulfur-Doped Carbon-Coated Fe0.95S1.05 Nanospheres as Anodes for High-Performance Sodium Storage

Sodium-ion batteries (SIBs), featuring with adequate sodium resources, relatively high safety, and similar chemical properties between sodium and lithium, have been considered one of the most potential candidates to lithium-ion batteries (LIBs). However, the larger radii of sodium ions (vs. lithium ions) lead to sluggish diffusion kinetics of sodium ions, low storage capacity, and adverse volume variation during sodiation and desodiation. In particular, anode materials work well in LIBs have been proved ineffective in SIBs. Therefore, the development of cheap anode materials with remarkable performance is critical to the commercialization of SIBs. Despite the good conductivity and robust stability of carbon materials, they usually showcase moderate discharge capacity and poor rate performance in SIBs. Iron sulfides are considered promising anode materials for SIBs due to their high theoretical capacity. Nevertheless, iron sulfides exhibit severe volumetric expansion during charge and discharge, resulting in low rate performance and poor stability. In this regard, hybridizing carbon materials with iron sulfides to configure composite materials is an important way to improve the electrochemical performance of SIBs. Here, three-dimensional clusterstructured sulfur-doped carbon-coated Fe_0.95S_1.05 nanospheres (Fe_0.95S_1.05@SC) are crafted by one-step annealing of ferrocene and sulfur powder, of which the implementation as anode of sodium ion batteries is reported. Scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results confirm the successful synthesis of the Fe_0.95S_1.05@SC composite. The coated sulfur-doped carbon layer can improve the conductivity of the Fe_0.95S_1.05 material and alleviate corresponding volume expansion during the reaction process, thus delivering a robust electrochemical stability. The interconnected cluster structures of Fe_0.95S_1.05@SC provide channels for the transport of electrons and ions, enabling the material excellent rate performance. Thanks to the unique structures of as-made Fe_0.95S_1.0@SC, when acting as anodes of SIBs, it demonstrates stable cycling performance and high rate performance. The electrochemical reaction process on Fe_0.95S_1.05@SC electrode is studied by cyclic voltammetry, validating that this electrode has good electrochemical reversibility. During the first few cycles of charging and discharging process, stable solid electrolyte interphase (SEI) layer forms on the surface of the carbon layer, which helps to avoid the direct exposure of Fe_0.95S_1.05 to the electrolyte and prevent the material from inactivation by the dissolution or escape of the sulfur element within Fe_0.95S_1.05 In the half-battery system, after 100 cycles at 0.1 A·g⁻¹, the high specific capacity of 614.7 mAh·g⁻¹ for Fe_0.95S_1.05@SC is retained, and the specific capacity at 10 A·g⁻¹ can still reach 235.7 mAh·g⁻¹. In the full battery system, the reversible capacity at 0.1 and 10 A·g⁻¹ is 482.8 and 288.3 mAh·g⁻¹, respectively. The as-made Fe_0.95S_1.05@SC with excellent electrochemical properties holds promise as anodes for sodium-ion batteries.

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