Elsevier

Journal of Alloys and Compounds

Volume 727, 15 December 2017, Pages 1280-1287
Journal of Alloys and Compounds

Boosting sodium-ion storage performance of MoSe2@C electrospinning nanofibers by embedding graphene nanosheets

https://doi.org/10.1016/j.jallcom.2017.08.258Get rights and content

Highlights

  • MoSe2@C@GR nanofibers was first successfully fabricated by electrospinning.

  • The graphene facilitates reaction kinetics as the nano-network in the nanofibers.

  • Graphene efficiently inhibits the growth of MoSe2 nanosheets on the fiber surface.

Abstract

In this work, a facile strategy for embedding graphene (GR) nanosheets into carbon-coated MoSe2 (MoSe2@C@GR) hybrid nanofibers has been developed by a simple electrospinning route. One dimensional nanofibers have excellent structural stability resulting from strong tolerance to stress change. The highly conductive graphene nanosheets play an excellent coating effect on MoSe2 to inhibit the growth of MoSe2 nanosheets on the nanofiber surface and enhance the conductivity of hybrid nanofibers, which substantially improves the rate performance and cycle stability of carbon-coated MoSe2 (MoSe2@C) nanofibers. In particular, the MoSe2@C@GR electrode delivers a high discharge capacity of 366.9 mAh g−1 after 200 cycles at 0.2 A g−1, meanwhile exhibits superior rate performance. The analysis of Nyquist plots reveals that the addition of graphene nanosheets can greatly strengthen the kinetics of electrode reaction and the electrochemical activity.

Introduction

Recently, with severe stress from global warming and energy crisis, tremendous attention has been attracted in developing rechargeable batteries as energy storage devices for the replacement of fossil fuels, like Li-ion batteries (LIBs) and Na-ion batteries (NIBs) [1], [2], [3]. Although LIBs have been industrialization[4], the scarcity of Li is difficult to meet the demand of the development for the future. Owing to the high content of sodium in the crust, low cost, and similar working mechanism of NIBs and LIBs, NIBs are considered as the next generation of energy storage batteries [5], [6]. However, the larger ionic radius of Na+ compared with Li+ (1.02 vs 0.76 Å) can cause some obstacles, like the large volume expansions and poor kinetics during the charge and discharge process, which lead the poor cycle stability and slow rate performance [7], [8]. Therefore, it is highly desired but challenged to find one kind of electrode materials that can commendably accommodate insertion and extraction of sodium ions.

Layered transition metal dichalcogenides (TMDs) have attracted considerable attention owing to unique electronic and physical properties [9]. TMDs, such as WS2[10], [11], MoS2[12], [13], and MoSe2[14], [15], [16], have been investigated as the most promising NIBs anode materials due to remarkable electrochemical properties. Sun et al.[13] successfully synthesized MoS2 transition metal dichalcogenide nanodots with a narrow size distribution ranging from 2.2 to 5.3 nm. The MoS2/GO hybrid composites exhibits long cycling life with a capacity of 220 mAh g−1 at 20 A g−1 after 10000 cycles. As one of the typical TMDs, MoSe2 has the analogous structure of MoS2, which is consists of covalently bonded Se-Mo-Se atom layers held together by van der Waals forces [17], [18]. Compared to MoS2, MoSe2 has better conductivity to contribute rate performance. The particular open 2D channel crystal structures facilitate fast intercalation and exfoliation of Na ions, resulting in the higher reversible capacity of MoSe2. Based on the sodium storage mechanism, the calculated theoretical capacity is 422.28 mAh g−1 [19]. Tang et al. [20] have reported carbon-stabilized interlayer-expanded few-layer MoSe2 nanosheets that show a capacity of about 445 mAh g−1 after 100 cycles at 1 A g−1 for sodium-ion storage. Yang et al.[21] have synthesized coaxial-cable MoSe2/hollow carbon nanofiber composites that exhibit a reversible capacity of about 423 mAh g−1 and 395 mAh g−1 after 100 cycles at 0.5 A g−1 and 1 A g−1 for Na-ion batteries, respectively. Despite the high sodium storage ability, the MoSe2 material still suffers from poor cycling stability and rate capability, which are attributed to the large volume change during the cycling and poor conductivity [15]. To overcome these issues, many strategies have been developed to improve the electrochemical performance of MoSe2 by using carbonaceous materials as the conductive matrix, such as graphene[15], [16], [22], carbon spheres [23], [24], [25], carbon nanofiber[21], [26] and carbon nanotube [27]. Among them, one-dimensional (1D) carbon nanoarchitectures with uniform structure, orientated electronic and ionic transport path as well as strong tolerance to stress change could enable efficient transport of both Na-ions and electrons. Electrospinning represents a relatively simple, low-priced and effective technique to fabricate nanofibers with diameters ranging from a few nanometers up to micrometers [28], [29]. Based on this strategy, lots of carbon-coated metal oxides hybrid nanofibers were prepared towards high-performance lithium/sodium ion batteries. As expected, the metal oxide nanoparticles are uniformly dispersed in the carbon nanofibers, resulting in improved rate performance and long cycle lifetime [30], [31]. In our previous work, Sn quantum dots/carbon hybrid nanofibers prepared by electrospinning were found to exhibit excellent cycle stability; A high reversible capacity of 887 mAh g−1 was remained even after 200 cycles at a current density of 0.1 A g−1 (about 75% retention of the initial capacity) [32]. Besides, the electrochemical performance of carbon-coated metal oxide nanofibers is significantly dependent on the electronic conductivity. As a result of carbon nanofibers hybrid materials synthesized by pyrolysis of organic precursors with low electronic conductivity, their conductivity is still needed to further improve considering the further improvement of overall electrochemical performance.

Herein, a facile strategy for further optimizing the electrochemical performance of MoSe2@C hybrid nanofibers was developed by embedding graphene nanosheets with low content. Graphene commendably inhibits the growth of MoSe2 nanosheets on the fiber surface by excellent coating effect on MoSe2. The highly conductive graphene enhanced the electrochemical properties of the MoSe2@C@GR electrode including rate performance and cycle stability. When used as an anode for sodium ion batteries, the MoSe2@C@GR electrode exhibits a high discharge capacity of 366.9 mAhg−1 after 200 cycles at 0.2 A g−1 and the nanofibers maintain their original morphologies. Through the analysis of Nyquist plots, we can conclude that graphene nanosheets greatly boosted the kinetics of electrode reaction and the electrochemical activity. This work provides an effective route for further enhance the electrochemical performance of Na+ storage electrode materials by electrospinning hybrid nanofibers.

Section snippets

Sample preparation

0.5297 g Ammonium molybdate tetrahydrate (H24Mo7N6O24▪4H2O, AR, Mw = 1235.86, Aladdin Industrial Corporation, China) was dissolved into 1 mL deionized water and 1 mL ethylene glycol (AR, Sinopharm Chemical Reagent Co., Ltd, China) to form solution A; 0.6114 g polyvinyl pyrrolidone (PVP, Mw = 1300000, Aladdin Industrial Corporation, China) powder was dissolved and stirred for 30 min at 60 °C in 2 mL deionized water to form solution B. Then solution A and 2 mL graphene solution which contained

Results and discussion

The morphologies of the nanofibers are characterized via field emission scanning electron microscopy (FESEM), indicating a typical image of the electrospinning nanofibers (Fig. 1(a) and (b)). The uniform nanofibers with 200 nm in diameter are not porous. There is no obvious pore structure on the nanofibers, moreover there is no spindle-like beads and aggregated large particles of graphene after calcination, indicating that the graphene is well dispersed in nanofibers. Pure carbon nanofibers

Conclusions

In summary, we introduced a new approach for incorporation of graphene nanosheets into MoSe2@C nanofibers by a simple one-pot electrospinning route. The addition of the highly conductive graphene nanosheets can optimize coating effect on MoSe2 and heighten electrical conductivity to substantially enhance electrochemical properties of the MoSe2@C@GR hybrid nanofibers. Therefore, improved cycle stability and rate performance were achieved. The MoSe2@C@GR electrode stably shows a discharge

Acknowledgement

This research work has been financially supported by the National Natural Science Foundation of China (51771236, 21373081), Innovation-Driven Project of Central South University (No. 2017CX002), Program for Shenghua Overseas Talents from Central South University, and Self-established Project of State Key Laboratory of Powder Metallurgy.

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