Urchin-like FeS₂ hierarchitectures wrapped with N-doped multi-wall carbon nanotubes@rGO as high-rate anode for sodium ion batteries

Highlights

• Urchin-like FeS₂@N-CNTs@rGO spheres are prepared by a simple hydrothermal method.

• N-CNTs and rGO endow FeS₂ high conductivity and buffer the volume expansion.

• Sodium ion battery with FeS₂@N-CNTs@rGO delivers high capacity and long lifespan.

Abstract

Transition metal chalcogenides have been investigated as underlying anodes for sodium ion batteries (SIBs). Nevertheless, the unsatisfactory property, which is determined by huge volume variation and low conductivity, restricts their practical applications. The design of micro-nano hierarchitectures is a resultful approach to strengthen structural stability and reaction kinetics during the discharge-charge process. Herein, urchin-like FeS₂ hierarchitectures wrapped with reduced graphene oxide and N-doped multi-wall carbon nanotubes (FeS₂@N-CNTs@rGO) are designed and synthesized by hydrothermal route. FeS₂@N-CNTs@rGO shows high capacity with 578 mAh g⁻¹ at 0.1 A g⁻¹, splendid rate property with 419 mAh g⁻¹ at 5 A g⁻¹ and preeminent capacity retention (≈100%) over 750 cycles at 2 A g⁻¹. The reversible phase transformation, capacitive Na⁺ storage behaviour and micro-nano hierarchitectures are responsible for the outstanding rate performance.

Introduction

The enhancive demands for sustainable energy have promoted the development of new-type batteries beyond lithium ion batteries (LIBs). Compared with LIBs, sodium-ion batteries (SIBs) have a greater development prospect due to high abundance, widespreading and inexpensive of sodium resources [[1], [2], [3], [4], [5]]. Nonetheless, the large radius and heavy molar weight of Na⁺ bring about sluggish kinetics and severe structural collapse during sodiated/desodiated process, which leading to poor cycling and rate performance [[6], [7], [8], [9]]. Common approaches such as constructing nanostructure and fabricating composites with conductive layers have been implemented to enhance electrochemical reaction kinetics and relieve volume expansion, obtaining good cycling and rate performance [10,11]. Therefore, exploring suitable high capacity materials with considerable rate and cycling stability is essential for developing SIBs.

Heretofore, various materials involving carbon-based materials, metal oxides, alloy materials and metal sulfides have served as anodes for SIBs [[12], [13], [14], [15], [16]]. Among them, FeS₂ has been deemed as latent anode due to its inexpensive, natural abundance, high theoretical capacity (894 mAh g⁻¹) and eco-friendliness [[17], [18], [19]]. Nevertheless, the weak electrical conductivity and notable volume dilatation give rise to inferior electrochemical performance [[20], [21], [22]]. To solve these problems, some main approaches have been suggested to boost the electrochemical property of FeS₂. The first approach is to regulate the voltage ranges to suppress the adverse electrochemical reactions [[23], [24], [25]]. Chen et al. proposed that FeS₂ exhibits a long lifetime of 20000 cycles with ~90% capacity retention adjusting the redox potential window (0.8–3.0 V) [25]. Combining FeS₂ with other electrically conductive compounds to augment the electrical conductivity is the second strategy [[26], [27], [28]]. Wang's team stated that reduced graphene oxide (rGO)-wrapped FeS₂ complex remains a discharge capacity of 610 mAh g⁻¹ for 100th cycle at 0.1 A g⁻¹ [26]. The third strategy is to devise multifarious nanostructures of electrode materials to shorten ion/electron transport paths [29,30]. Kovalenko et al. concluded that nano-sized FeS₂ delivers 500 mA h g⁻¹ over 400 cycles at 1 A g⁻¹ [29].

Herein, hierarchical urchin-like FeS₂ wrapped with rGO and N-doped multi-wall carbon nanotubes (N-CNTs) was produced through a hydrothermal method. The micro-nano hierarchitectures can heighten stability of electrode structure and electrochemical reaction kinetics upon repeated sodiated/desodiated process. Therefore, FeS₂@N-CNTs@rGO exhibits splendid electrochemical performance, including a high capacity of 558 mAh g⁻¹ at 0.1 A g⁻¹ and superior rate capacity of 419 mAh g⁻¹ at 5 A g⁻¹. Furthermore, a capacity of 513 mAh g⁻¹ at 2 A g⁻¹ after 750 cycles can be noted. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses reveal that the reaction mechanism in FeS₂ is ascribed as the reversible formation of Fe and Na₂S. The reaction kinetics of capacitive storage behaviour is studied by cyclic voltammetry (CV) curves.