Regular Article
Synthesis and characterization of three-dimensional MoS2@carbon fibers hierarchical architecture with high capacity and high mass loading for Li-ion batteries

https://doi.org/10.1016/j.jcis.2017.09.078Get rights and content

Abstract

Three-dimensional (3D) MoS2@carbon fibers (CFs) hierarchical architectures are successfully synthesized via a simple hydrothermal method and subsequent annealing. MoS2 nanoflakes are grown on the twine carbon fibers of the carbonized waste cotton cloth. The twine CFs can provide a short diffusion path for ions in electrolyte, enhance the specific surface area, and improve the conductivity of the 3D MoS2@CFs hierarchical architectures with high mass loading of 4.4 mg cm−2. The 3D MoS2@CFs hierarchical architectures as the electrode material can achieve a high reversible areal capacity (5.2 mAh cm−2 at 2.5 mA cm−2) and exhibit an excellent rate performance. In addition, CFs are prepared by simply carbonizing the waste cotton and then used as carbon source, which is low-cost and eco-friendly. We also found that the Mo nanoparticles produced during the charge/discharge process exist in the hierarchical architectures during cycling and can improve the conductivity of the entire system as well as the cycling stability. Therefore, MoS2@CFs nanocomposites as electrode materials manifest a significant application potential for high-performance Li-ion batteries.

Introduction

The development of the energy storage devices has become one of the top priority tasks as solar energy and wind energy being widely applied in modern life and industry. Lithium-ion batteries (LIBs), as main energy storage devices, have attracted widespread attention from scientific and industrial fields for their high-energy density, low self-discharge, and environmental friendliness. LIBs currently dominate the marketplace of energy storage systems for portable electronic devices including cell phones, laptops, and gradually expanding to electric vehicles (EVs) [1]. However, they are subject to a bottleneck of low power density in applications for EVs. It is well known that electrode materials of LIBs play an important role in their electrochemical performance. The energy density is generally related to both capacity and voltage of the LIBs [2]. So how to develop new electrode materials with low cost, ultrahigh capacity and good cycling stability for LIBs is a crucial challenge to satisfy their application in the field of EVs. In the past few years, lots of attention has been paid to the two-dimensional (2D) crystal materials [3], [4] with exceptional electronic, optical and mechanical properties, such as 2D graphenes [5], carbides (MXenes) [6], [7], and layered transition metal dichalcogenides (TMDs) [8], [9], [10]. MoS2, as a typical layered transition metal sulfide, has sandwiched structure which allows a fast diffusion for the movement of Li-ions [11], [12], [13]. But a low conductivity and the significant volume change after its cycle processes make MoS2-based electrode materials hard to meet the demand of commercial application. In addition, like other 2D materials, the MoS2 nanoflakes are easy to stack/restack into bulk materials, resulting in a relatively short life time of the MoS2 electrode materials. To solve these problems, researchers did lots of works on compositing MoS2 with electronically conductive and chemically stable materials.

Recently, most of the MoS2@carbon composite nanostructures were synthesized by hydrothermal method. These carbon-based materials include graphenes [14], carbon fibers [15], carbon nanotubes (CNTs) [16], [17], carbon papers [18], MOF [19] or amorphous carbon [20]. These MoS2@carbon composite nanostructures can improve the conductivity of MoS2 as electrode and achieve a high reversible capacity as well as good cycling stability [21], [22]. Among them, most of the composite nanostructures used as working electrode need to be prepared by a coating procedure. The prepared slurry is then smeared on a foil, which increases the cost and the total weight of LIBs. Furthermore, the weak binding force between the electrode materials and the foil will inevitably make the electrode materials fall off from the foil during continual charge-discharge process, resulting in low initial coulombic efficiency (ICE < 60%) [23] and serious capacity fading [24]. Herein, we rationally designed a three-dimension (3D) MoS2@carbon fibers (CFs) hierarchical architecture free-standing film with flexibility, high energy storage capacity and environmental friendliness by synthesizing MoS2 nanoflakes on the carbonized waste cotton cloth (CWCC) via simple hydrothermal method. The MoS2 nanoflakes are vertically grown on the surface of twine CFs. The key advantage is to combine the 2D framework of MoS2 and 1D structure of CFs, leading to the formation of a 3D structure with an extremely high discharge capacity of 5.3 mAh cm−2 (1196.8 mAh g−1) at 0.5 mA cm−2 and 5.2 mAh cm−2 (1174.2 mAh g−1) at a high current density of 2.5 mA cm−2. The mass loading in this work is on the order of 4.4 mg cm−2. Another advantage is by simply carbonizing the CWCC, high conductive carbon fiber is obtained and the cost of the batteries is decreased by recycling waste materials.

Section snippets

Synthesis of CFs

Firstly, a piece of waste cotton cloth was immersed into 1 M NaF homogeneous solution. Subsequently, the “wet” cloth was dried in oven at 80 °C for 12 h. And then the dried cloth was carbonized in a horizontal tube furnace at 1000 °C for 1 h with continuous flow of nitrogen gas. After cooling, the CWCCs were washed with DI water to remove the NaF and dried in an oven at 80 °C for 12 h.

Synthesis of MoS2@CFs composites

To synthesize MoS2@CFs composite, 154 mg of ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O, AMT) and 2 g of thiourea

Results and discussion

XRD pattern acquired from the as-synthesized products after the hydrothermal treatment and subsequent annealing shows the typical XRD features of MoS2 (Fig. 1a). All the diffraction peaks are indexed to the 2H MoS2 phase (JCPDS no. 37-1492). In addition, no other phases are detected except a broad peak centered at about 24° from the carbon species. Raman spectrum of the as-synthesized MoS2@CFs was measured. Fig. 1b is a typical Raman spectrum of the as-synthesized MoS2@CFs. As can be seen from

Conclusions

The 3D MoS2@CFs hierarchical architecture is rationally designed as an eco-friendly electrode for LIBs, and synthesized by a facile hydrothermal method followed by annealing. Carbon fibers produced from recycled; second-hand cotton cloth by simple carbonization proves to be an efficient carbon substrate for MoS2. Without adding binders or conductive additives into the preparation of electrode, the free-standing 3D MoS2@CFs composite electrode has large mass loading and low cost. An extremely

Acknowledgements

This work was partially supported by the Natural Science Foundation of China (No. 51472066).

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