Uniform growth of MoS2 nanosheets on carbon nanofibers with enhanced electrochemical utilization for Li-ion batteries

Accepted Manuscript Title: Uniform growth of MoS2 nanosheets on carbon nanofibers with enhanced electrochemical utilization for Li-ion batteries Authors: Jian-Gan Wang, Rui Zhou, Dandan Jin, Keyu Xie, Bingqing Wei PII: DOI: Reference:

S0013-4686(17)30145-7 http://dx.doi.org/doi:10.1016/j.electacta.2017.01.108 EA 28775

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

21-9-2016 21-11-2016 18-1-2017

Please cite this article as: Jian-Gan Wang, Rui Zhou, Dandan Jin, Keyu Xie, Bingqing Wei, Uniform growth of MoS2 nanosheets on carbon nanofibers with enhanced electrochemical utilization for Li-ion batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.01.108 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Uniform growth of MoS2 nanosheets on carbon nanofibers with enhanced electrochemical utilization for Li-ion batteries

Jian-Gan Wang a,*, Rui Zhou a, Dandan Jin a, Keyu Xie a, Bingqing Wei a, b * a

State Key Laboratory of Solidification Processing, Center for Nano Energy

Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Lab of Graphene (NPU), Xi’an 710072, China. b

Department of Mechanical Engineering, University of Delaware, Newark, DE19716,

USA E-mail: [email protected] (B. Wei); [email protected] (J.-G. Wang) Tel./Fax: 029 88460204

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Abstract A facile route for the uniform growth of MoS2 nanosheets on carbon nanofibers (CNFs) via a simple hydrothermal method is reported. The CNF network provides a porous and three-dimensional scaffold for the deposition of ultrathin MoS2 nanosheets. Benefiting from the highly conductive and porous CNFs as well as the nanostructured MoS2, the coaxial MoS2@CNF nanocomposites, when served as a binder-free and flexible anode for Li-ion batteries, could generate a synergistic effect to gain a high electrochemical utilization of MoS2. As a result, the nanocomposites exhibit a high reversible capacity of 846 mAh g-1 at 100 mA g-1, excellent high-rate capability (455 mAh g-1 even at 2 A g-1), and good cycling retention with only 7.4% capacity degradation after 100 cycles. The superior Li-ion storage properties enable the nanocomposites to hold great potential application in high-performance Li-ion batteries. Keywords: MoS2; carbon nanofiber; nanocomposite; anode; Li-ion battery

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1. Introduction The global concern about energy crisis and environmental pollution has triggered intensive research interests in developing sustainable and renewable energy storage devices [1, 2]. Among them, Li-ion batteries (LIBs) have attracted great attention as power sources for thriving electric vehicles and portable electronics owing to their high specific energy, long cycle life and no memory effect [3-5]. As the most widely used anode material in commercial LIBs, graphite suffers from a low theoretical capacity of 372 mAh g-1, which cannot meet the increasing requirements of energy density. It is, therefore, urgent to develop new anode alternatives with high reversible specific capacity, long cyclic stability and low cost for LIBs [6-8]. Recently, layered transition metal dichalcogenide compounds, MX2 (M = Mo, Ti, V, and W, X = S), have been considered as one class of promising anode materials for LIBs [9, 10]. As a typical member of layered material family, MoS2 shows great potential as an anode material for LIBs owing to its high theoretical specific capacity of 670 mAh g-1 [11-13]. There are a few reports on the rational synthesis of different nanostructured MoS2, such as nanoflowers, nanosheets, and nanotubes, which showed high specific capacity when used as anodes for LIB [14-16]. Nevertheless, these pure MoS2 nanostructures suffer from poor cycling stability and inferior rate capability due to the severe restacking and significant volume change of MoS2 during charging/discharging processes, as well as its low electrical conductivity. To address these obstacles, one of the most effective methods is to combine MoS 2 with highly conductive and porous carbon substrates, such as graphene, carbon nanotubes, carbon

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nanofibers

(CNF),

carbon

spheres,

etc.[17-20]

The

carbon-MoS2

hybrid

nanostructures can benefit from a synergistic effect between each component, leading to a great improvement in the Li-ion storage performance [25]. For example, a simple hydrothermal method was employed to

synthesize MoS2/graphene hybrid

nanoflowers, which delivered a specific capacity of 1150 mAh g-1 after 60 cycles [21]. The decoration of MoS2 on CNF surfaces results in a good capacity retention of 688 mAh g-1 after 300 cycles [22]. Zhang et al. have synthesized C@MoS2 composite, and it showed a high specific capacity of 750 mAh g-1 at 100 mA g-1 and excellent rate performance (500 mAh g-1 at 1000 mA g-1) [20]. It should be noted that these anode materials are in the form of powders, which need the assistance of conductive additives and polymer binders to prepare an electrode. This not only requires a complex fabrication process but also decreases the overall specific capacity when taking the 20-30 wt% additive mass into account [23]. To overcome the aforementioned limitation, a feasible strategy is to construct a freestanding electrode material without the use of any additives. Electrospinning is a straightforward method to fabricate freestanding one-dimensional (1D) nanostructures with porous network structure and robust mechanical stability [24-26]. MoS2 nanosheets have been incorporated into CNF fabrics to improve their electrochemical utilization [27, 28]. Liu et al. have reported a synthesis of freestanding membranes of porous MoS2@CNF hybrid nanofibers with an improved LIB performance [27]. However, the synthesis method not only employed expensive raw materials of (NH4)2MoS4 but also generated a large number of non-uniform MoS2 aggregations on

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the CNF surface. Uniform distribution of MoS2 nanosheets on the 1D CNF is found to be efficient to build up a hierarchical three-dimensional (3D) architecture for better electrochemical performance. Therefore, it is still a challenge to explore a low cost yet more effective method to obtain uniform MoS2 nanosheets with enhanced electrochemical utilization. In this work, we develop a simple and low-cost method to uniformly grow MoS2 nanosheets on the CNF surface through a combination of electrospinning and hydrothermal treatment, as schematically illustrated in Fig. 1. The electrospun CNF monolith not only provides a 3D open structure and large specific surface area for homogeneous MoS2 deposition, but also offers highly electrical conductive channels to improve the electrochemical reaction of MoS2. Meanwhile, the MoS2 nanosheets could shorten the solid ion/electron transport pathways to boost the reaction kinetics. Benefiting from the favorable synergistic effect between each component, the as-prepared coaxial MoS2@CNF hybrid nanofibers, when served as a binder-free and flexible anode for LIBs, exhibit excellent electrochemical performance, including a high reversible specific capacity of 803 mAh g-1 at 200 mA g-1 with good cycling capacity retention and high rate capability.

2. Experimental Section 2.1 Materials Synthesis All reagents were of analytical grade and used as received without further purification. In a typical procedure, 3.0 g of polyacrylonitrile (PAN) powder was dissolved in 22 mL of dimethylformamide (DMF) followed by dwelling at 60 °C for 6 h to obtain a

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homogeneous solution. Then the orange solution was transferred into a syringe with a needle-to-collector distance of 10 cm. The electrospinning process was performed under an applied voltage of 18 kV with a feeding speed of 1 mL h−1. The as-received nanofiber mat was heated to 250 °C in air at a rate of 2 °C min-1 and maintained 2 h for pre-oxidation. The carbonization was carried out at 850 °C for 2 h (heating rate: 5 °C min-1) under an inert N2 atmosphere. Subsequently, MoS2@CNF nanocomposites were synthesized using the one-step hydrothermal method. Typically, 0.121 g of sodium molybdate (Na2MoO4·2H2O) and 0.242 g of L-cysteine was dissolved in 35 ml deionized water under magnetic stirring for 30 min. The CNF fabric was pre-treated to be hydrophilic by soaking in a 6.0 M HNO3 solution for 1 h. Then the modified CNF fabric was immersed into the above Mo-containing solution for subsequent growth of MoS2. The solution was transferred into a 50 ml Teflon-lined stainless steel autoclave and kept at 180 °C for 24 h. After cooling down to room temperature, the black fabric was washed with deionized water and ethanol several times and finally dried at 60 °C for 12 h. For comparison, MoS2 powder was synthesized via the same hydrothermal procedure. 2.2 Material Characterization The phase structure of the samples was characterized by X-ray diffraction (XRD, X’Pert PRO MPD, Philips) with a Cu Kα radiation (λ=0.15418 nm). Raman spectra analysis was carried out on a Renishaw inVia Raman Spectrometer using an excitation wavelength of 532 nm. X-ray photoelectron spectrometry (XPS) spectra were

recorded

on

ESCALAB

250Xi

6

X-ray

photoelectron

spectroscopy.

Thermalgravimetric analysis (TGA) was performed on Mettler Toledo TGA/DSC 3+. The morphology was examined using a field emission scanning electron microscopy (FE-SEM, FEI Nano SEM 450) and transmission electron microscopy (TEM, FEI Tecnai F30G2). The Brunauer-Emmett-Teller (BET) specific surface area was determined using N2 adsorption/desorption measurement at a liquid nitrogen temperature (Micromeritics ASAP 2020). 2.3 Electrochemical measurements The electrochemical performance was evaluated using CR 2016 coin cells assembled in an argon-filled glove box. The as-prepared MoS2@CNF fabric was directly used as the anode, while Li foil, a Celgard 2400 microporous polypropylene membrane and a solution of 1M LiPF6 in ethylene carbonate (EC), dimethyl carbonate (DMC) and ethylmethyl carbonate (EMC) (1:1:1, volume ratio) were served as the counter electrode, the separator, and the electrolyte, respectively. Solartron electrochemical workstation (1260 + 1287, England) was employed to obtain the cyclic voltammetry (CV) in the potential range of 0.01–3.0 V (vs. Li/Li+) at a rate of 0.1 mV s−1, and the electrochemical impedance spectra (EIS) in the frequency range from 1 MHz to 50 mHz. Galvanostatic charge/discharge cycling was conducted on a Land battery system (LAND, BT2013A, China) under different current densities.

3. Results and discussion The as-prepared samples were first characterized by XRD, as shown in Fig. 2(a). The three distinct diffraction peaks of 2θ around 14°, 33°, and 59° represent the (002), (100), and (110) reflections of MoS2, respectively [29]. The broad diffraction peak

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centered at around 25° can be ascribed to the amorphous structure of CNF [30]. No other impurities were detected, indicating a high purity of the MoS2@CNF composite. Raman spectroscopy was further carried out to understand the composition and structure of the MoS2@CNF sample. As shown in Fig. 2(b), the two broad peaks at 1329 and 1576 cm-1 can be assigned to the D and G bands of carbon structure [31, 32]. In addition, the peaks at 375 and 406 cm-1 correspond to the E2g and A1g modes of MoS2, respectively [33]. These results confirm the presence of MoS2 and carbon in the nanocomposites. The mass content of MoS2 is determined using the TGA method. As shown in Fig. S1, the MoS2 loading ratio in the composite is estimated to be approximately 82.2 wt.% (calculation details see Supporting Information). XPS was performed to investigate the chemical states of Mo and S in the MoS2@CNF nanocomposite. The core level spectrum of Mo 3d (Fig. 2(c)) is composed of a Mo 3d3/2 peak at 231.7 eV and a Mo 3d5/2 peak at 228.5 eV, which agrees well with the characteristics of Mo4+ in MoS2. The peak at 235.8 eV is related to the 3d3/2 of Mo6+, suggesting the presence of MoO3 being formed during the hydrothermal growth [20]. The peaks with binding energies at 163.2 and 162 eV in the S 2p spectrum (Fig. 2(d)) belong to the S 2p1/2 and S 2p3/2 components, respectively, which can be readily attributed to the S2- of MoS2 [36]. These results strongly support the successful growth of MoS2 nanostructures on CNF during the hydrothermal treatment. The morphology of the pristine CNF and MoS2@CNF fabric was observed using FE-SEM and TEM imaging. As shown in Fig. 3(a-b), the pristine CNF fabric possesses a porous network structure with nanofibers having a smooth surface and an

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average diameter of ~300 nm. Fig. 3(c) shows the typical morphology of the MoS2@CNF hybrid nanofiber. The interconnected network structure is well preserved without any agglomerations, indicating the uniform distribution of MoS 2 nanostructures. The hybrid nanofiber fabric can be bent repetitively without any cracks (inset in Fig. 3(c)), suggesting its robust flexibility as a binder-free anode for LIB. A close SEM observation in Fig. 3(d) clearly demonstrates the ultrathin MoS 2 nanosheets vertically anchoring around the surface of individual CNF, which is further confirmed by the TEM imaging (Fig. 3(e)). In addition, the sonication treatment during the TEM sample preparation does not destruct the coaxial morphology of MoS2@CNF, indicating a strong connection of MoS2 to the CNF. This characteristic is rather favorable for accelerating the solid charge-transfer and maintaining the electrode integrity during cycling operation. The HRTEM image (Fig. 3(f)) demonstrates that the MoS2 nanosheets are 4-10 layers in thickness, and the interlayer spacing is approximately 0.65 nm, which is consistent with the d-spacing of (002) plane of hexagonal MoS2 [34]. Moreover, the vertical growth of ultrathin MoS2 nanosheets confined by the CNF scaffold is capable of increasing the exposure of edge sites for better electrochemical utilization [35]. For comparison, pure MoS 2 nanoflowers with a diameter of 500 nm were prepared as a controlled sample (Fig. S2). The specific surface area was examined by N2 adsorption/desorption measurements. As shown in Fig. S3, the MoS2@CNF nanocomposites show high specific surface area of 21.3 m2 g-1, which is much higher than that of CNF (10.2 m2 g-1) and MoS2 (9.7 m2 g-1).

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The electrochemical performance of MoS2@CNF hybrid was measured using coin-type half cells. The as-prepared MoS2@CNF fabric was directly employed as an anode without conductive additives and polymeric binders. CV measurement was firstly conducted to understand the electrochemical behavior. Fig. 4(a) shows the initial five CV curves of the pure MoS2 electrode. In the first cathodic sweep, two reduction peaks appeared at 1.5 and 0.46 V correspond to the insertion of Li + into the interlayer space of MoS2 to form LixMoS2 (Reaction (1)) and a subsequent conversion reaction between LixMoS2 and Li+ to form Li2S and metallic Mo (Reaction (2)), respectively. [37] During the reverse anodic scan, the dominant peak at about 2.38 V is attributed to the oxidation of Li2S to sulfur, while the relatively small peak in the potential range of 1.5-2.0 V is associated with the multistep oxidation of metallic Mo to a higher state, such as Mo4+ and Mo6+ [38, 39]. In the following cycles, the cathodic peaks at 1.25 V and 1.9 V are ascribed to the reduction of Mo6+/Mo4+ and the formation of Li2S, respectively [17, 40]. Similarly, the MoS2@CNF @CNF hybrid electrode exhibits identical electrochemical behavior to the MoS2 electrode (Fig. 4(b)), indicating the Li-ion storage is primarily from the active MoS2 component. In addition, the well overlapped CV curves after the first cycle suggest high electrochemical reversibility and good cycling stability of MoS2, which will be further confirmed by the charge/discharge measurements. MoS2 + xLi+ + xe- → LixMoS2

(1)

LixMoS2 + (4-x)Li+ + (4-x)e- → Mo + 2Li2S

(2)

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Fig. 4(c) displays the galvanostatic charge/discharge profiles of the MoS2@CNF at a current density of 100 mA g-1 in the voltage range of 0.01–3 V. The voltage plateaus are in good consistent with the CV analysis. The MoS2@CNF hybrid electrode delivers an initial reversible capacity of 846 mAh g-1 along with a high Coulombic efficiency of 71%, which are much higher than those of pure MoS2 (i.e., 689 mAh g-1 and 60%) and pure CNF electrodes (i.e., 248 mAh g-1 and 44%), suggesting a strong synergistic interaction between the components. Additionally, the efficiency of the hybrid electrode rapidly increases more than 98% from the second cycle, indicating the excellent electrochemical reversibility. More encouragingly, the hybrid electrode can maintain a capacity retention of 92.6% after 100 cycles (Fig. 4(d)). The performance is superior to the previous reported MoS2 aggregates on CNF (736 mAh g-1 (~76%) after 50 cycles at 50 mA g-1) [27], indicating the importance of uniformity of MoS2 nanosheets on CNF. The excellent cycling performance of MoS2@CNF anode is further validated by another cell tested at a high current density of 500 mA g-1. As shown in Fig. 4(e), a high and stable specific capacity of 670 mAh g-1 is retained for the hybrid electrode after 200 cycles. The morphology of the MoS2@CNF electrode after 200 cycles was characterized by SEM. As shown in Fig. S4, the MoS2 nanostructures are observed to stand robustly on the CNF, indicating the strong electrode integrity for long cycling stability. For comparison, the pure MoS2 electrode suffered from severe capacity degradation with only 290 mAh g-1 being retained after 100 cycles (Fig. 4(c)). In addition, as summarized in Table S1, the excellent Li-ion storage performance of MoS2@CNF is superior or comparable to

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most reported MoS2@C electrodes, especially when taking the whole electrode mass into account. This comparison suggests that the presence of CNF not only improves the electrochemical utilization of MoS2 but also provides a solid backbone to prevent MoS2 nanosheets from restacking and agglomeration during the charge/discharge processes, thereby resulting in high specific capacity and excellent long-term cyclic performance. Fig. 5(a) shows the rate capabilities of the MoS2@CNF nanocomposite and pure MoS2 powder. Notably, the hybrid anode delivers a high specific capacity of 998 mAh g-1 at a low current density of 50 mA g-1. The specific capacity normalized to the MoS2 mass is determined to be as high as 1162 mAh g-1, indicating a substantial enhancement in the electrochemical utilization of MoS2. The capacity contribution of MoS2 reaches approximately 95.7%, suggesting Li-ion storage mainly comes from the active MoS2 component. As the current density increases in a stepwise manner to a high rate of 2000 mA g-1, a high reversible capacity of 455 mAh g-1 is achieved, which is still higher than the theoretical value of graphite, suggesting superior rate performance of the MoS2@CNF anode. When the current density is returned to 50 mA g-1, the reversible capacity of the MoS2@CNF is recovered to be 986 mAh g-1, again indicating the outstanding cycling stability and electrochemical reversibility. In sharp contrast, the MoS2 powder anode almost loses its Li-ion storage capability at this high current rate (43 mAh g−1). In order to better understand the rate capability of the electrodes, EIS analysis was carried out. Fig. 5(b) shows the resulting Nyquist plots, both of which are composed of a compressed semicircle in the high-medium

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frequency region and a straight line at low frequencies. It is obviously observed that the diameter of the semicircle in the hybrid electrode is much smaller than that of the controlled electrode (150 vs. 270 ), suggesting a greatly reduced charge -transfer resistance (Rct) at the electrode/electrode interface. The decreased Rct reveals that the combination of these two components provides improved electronic/ionic conductivity for fast electrochemical reactions (1) and (2). It is believed that the excellent electrochemical performance of MoS2@CNF can be ascribed to its unique coaxial structure. First, the nanoscaled size and uniform distribution of the MoS2 nanosheets in the hybrid hierarchical architecture enlarge the contact area between MoS2 and electrolyte, thereby offering more active sites for Li-ion storage. Second, the MoS2 nanostructures shorten the solid transport pathways to accelerate the Li+ transport, while the conductive CNF cores afford fast charge collection and transfer, thus resulting in enhanced reaction kinetics. Third, the strong connection of MoS2 nanosheets to the CNF enables to maintain the electrode integrity for long-term cycling endurance. In summary, all these characteristics of the MoS2@CNF electrode collectively ensure the high specific capacity and excellent rate/cycling performance.

4. Conclusions MoS2 nanosheets are uniformly grown on the CNF surface through a simple and low-cost hydrothermal method. The well-defined 2D/1D hierarchical architecture affords fast electron/ion transport rate for enhanced reaction kinetics as well as prevent the aggregation of MoS2 nanosheets during the charge-discharge processes,

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which is rather favorable for high-efficient energy storage. The synergistic interaction leads to improved electrochemical performance, including a high specific capacity of 803 mAh g-1 based on the total electrode mass, a high-rate capacity of 455 mAh g-1 even at 2 A g-1, and a capacity retention of 92.6% over 100 cycles. The remarkable lithium storage performance makes it a potential candidate as advanced electrode materials for energy storage applications.

Acknowledgements The authors acknowledge the financial supports of this work by the National Natural Science Foundation of China (51402236, 51472204, 51302219), the Natural Science Foundation of Shannxi Province (2015JM5180), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No.: 123-QZ-2015), the Key Laboratory of New Ceramic and Fine Processing (Tsinghua University, KF201607), the Seed Foundation of Innovation and Creation for Graduate Students in Northwestern Polytechnical University (Z2016067) and the Fundamental Research Funds for the Central Universities (3102015BJ(II)MYZ02).

References [1] K. Chen, S. Song, F. Liu, D. Xue, Chem. Soc. Rev. 44 (2015) 6230-6257. [2] J.-G. Wang, F. Kang, B. Wei, Prog. Mater. Sci. 74 (2015) 51-124. [3] P.P.R.M.L. Harks, F.M. Mulder, P.H.L. Notten, J. Power Sources 288 (2015) 92-105. [4] J.-G. Wang, C. Zhang, D. Jin, K. Xie, B. Wei, J. Mater. Chem. A 3 (2015)

14

13699-13705. [5] J.-G. Wang, D. Jin, H. Liu, C. Zhang, R. Zhou, C. Shen, K. Xie, B. Wei, Nano Energy 22 (2016) 524-532. [6] N. Nitta, F. Wu, J.T. Lee, G. Yushin, Mater. Today 18 (2015) 252-264. [7] J. Wang, C. Zhang, F. Kang, ACS Appl. Mater. Interfaces 7 (2015) 9185-9194. [8] Y. Zhao, X. Li, B. Yan, D. Li, S. Lawes, X. Sun, J. Power Sources 274 (2015) 869-884. [9] X. Cao, C. Tan, X. Zhang, W. Zhao, H. Zhang, Adv. Mater. (2016) doi:10.1002/adma.201504833. [10] M. Chhowalla, H.S. Shin, G. Eda, L.J. Li, K.P. Loh, H. Zhang, Nature Chem. 5 (2013) 263-275. [11] U.K. Sen, S. Mitra, ACS Appl. Mater. Interfaces 5 (2013) 1240-1247. [12] Y. Zhang, Y. Li, H. Li, F. Yin, Y. Zhao, Z. Bakenov, J. Nanopart. Res. 18 (2016) 1-9. [13] Z. Wang, T. Chen, W. Chen, K. Chang, L. Ma, G. Huang, D. Chen, J.Y. Lee, J. Mater. Chem. A 1 (2013) 2202-2210. [14] H. Hwang, H. Kim, J. Cho, Nano Lett. 11 (2011) 4826-4830. [15] S. Zhuo, Y. Xu, W. Zhao, J. Zhang, B. Zhang, Angew. Chem. Int. Ed. 52 (2013) 8602-8606. [16] G. Ma, H. Peng, J. Mu, H. Huang, X. Zhou, Z. Lei, J. Power Sources 229 (2013) 72-78. [17] L. Jiang, B. Lin, X. Li, X. Song, H. Xia, L. Li, H. Zeng, ACS Appl. Mater.

15

Interfaces 8 (2016) 2680-2687. [18] J.-Z. Wang, L. Lu, M. Lotya, J.N. Coleman, S.-L. Chou, H.-K. Liu, A.I. Minett, J. Chen, Adv. Energy Mater. 3 (2013) 798-805. [19] Q. Qu, F. Qian, S. Yang, T. Gao, W. Liu, J. Shao, H. Zheng, ACS Appl. Mater. Interfaces 8 (2016) 1398-1405. [20] Y. Teng, H. Zhao, Z. Zhang, Z. Li, Q. Xia, Y. Zhang, L. Zhao, X. Du, Z. Du, P. Lv, K. Świerczek, ACS Nano 10 (2016) 8526–8535. [21] H. Li, K. Yu, H. Fu, B. Guo, X. Lei, Z. Zhu, J. Phys. Chem. C 119 (2015) 7959-7968. [22] F. Zhou, S. Xin, H.W. Liang, L.T. Song, S.H. Yu, Angew. Chem. Int. Ed. 53 (2014) 11552-11556. [23] J.G. Wang, D. Jin, R. Zhou, X. Li, X.R. Liu, C. Shen, K. Xie, B. Li, F. Kang, B. Wei, ACS Nano 10 (2016) 6227–6234. [24] G. Wang, C. Pan, L. Wang, Q. Dong, C. Yu, Z. Zhao, J. Qiu, Electrochim. Acta 69 (2012) 65-70. [25] S. Wang, L. Xia, L. Yu, L. Zhang, H. Wang, X.W.D. Lou, Adv. Energy Mater. 6 (2016) 1502217. [26] J. Wang, Y. Yang, F. Kang, Electrochim. Acta 168 (2015) 271-276. [27] Y.E. Miao, Y. Huang, L. Zhang, W. Fan, F. Lai, T. Liu, Nanoscale 7 (2015) 11093-11101. [28] J. Kong, C. Zhao, Y. Wei, X. Lu, ACS Appl. Mater. Interfaces 7 (2015) 24279-24287.

16

[29] K. Chang, W. Chen, ACS Nano 5 (2011) 4720-4728. [30] J.-G. Wang, Y. Yang, Z.-H. Huang, F. Kang, Electrochim. Acta 170 (2015) 164-170. [31] W. Luo, X. Hu, Y. Sun, Y. Huang, ACS Appl. Mater. Interfaces 5 (2013) 1997-2003. [32] C. Zhang, J.-G. Wang, D. Jin, K. Xie, B. Wei, Electrochim. Acta 180 (2015) 990-997. [33] K.-J. Huang, L. Wang, Y.-J. Liu, Y.-M. Liu, H.-B. Wang, T. Gan, L.-L. Wang, Int. J. Hydrogen Energy 38 (2013) 14027-14034. [34] Y. Wang, L. Yu, X.W. Lou, Angew. Chem. Int. Ed. 55 (2016) 5990–5993. [35] H. Zhu, F. Lyu, M. Du, M. Zhang, Q. Wang, J. Yao, B. Guo, ACS Appl. Mater. Interfaces 6 (2014) 22126-22137. [36] P.P. Wang, H. Sun, Y. Ji, W. Li, X. Wang, Adv. Mater. 26 (2014) 964-969. [37] Y. Chen, B. Song, X. Tang, L. Lu, J. Xue, Small 10 (2014) 1536-1543. [38] D. Xie, D.H. Wang, W.J. Tang, X.H. Xia, Y.J. Zhang, X.L. Wang, C.D. Gu, J.P. Tu, J. Power Sources 307 (2016) 510-518. [39] C. Zhu, X. Mu, P.A. van Aken, J. Maier, Y. Yu, Adv. Energy Mater. 5 (2015) 1401170. [40] X. Xu, Z. Fan, X. Yu, S. Ding, D. Yu, X.W.D. Lou, Adv. Energy Mater. 4 (2014) 1400902.

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Figure captions:

Fig. 1 Schematic illustration of the fabrication process of MoS2@CNF hybrid nanofibers. Fig. 2 (a) XRD pattern of the samples. (b) Raman spectrum and (c) XPS spectrum of the as-prepared MoS2@CNF nanocomposite: (c) Mo 3d and (d) S 2p. Fig. 3 FESEM images of (a-b) CNF and (c-d) MoS2@CNF, (e) TEM and (f) HRTEM images of the MoS2@CNF. Fig.4 CV curves of (a) MoS2 and (b) MoS2@CNF in the first five cycles (0.1 mV s−1); (c) Charge/discharge curves of MoS2@CNF at 100 mA g−1. (d) Cycling performance of MoS2@CNF, MoS2 powder and CNF electrodes at 100 m A g−1. (e) Cycling performance of MoS2@CNF hybrid electrode at a high current density of 500 mA g-1. Fig. 5 (a) Rate performance and (b) Nyquist plots of the MoS2 and MoS2@CNF electrodes.

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Fig. 1

19

(100)MoS

2

(110)MoS

2

(002)CNF

40

2θ (degree)

Intensity (a.u)

(c)

240

Mo4+

60

80

400

Mo6+

S 2s

232

1200

(d)

Mo 3d5/2

Mo 3d3/2

236

800

1583 1600

2000

Raman shift ( cm-1)

228

S 2p3/2 S 2p1/2

Intensity (a.u.)

20

406

2

1350

(b)

375

(002)MoS

MoS2@CNF CNF MoS2

Intensity (a.u.)

Intensity (a.u.)

(a)

224

166

Binding Energy (eV)

164

162

160

Binding energy (eV)

Fig. 2

20

158

Fig. 3

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(b)

(a) 0.25

Current (mA)

0.00

-0.25

1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle

1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle

-0.50

-0.50

-0.75

0.5

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(c)

1400

Voltage (V)

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2.0 1.5 1.0 0.5

0

200

400

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Specific capacity (mAh g-1)

3.0

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3.0

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discharge

MoS2

charge

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discharge

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(e) 1000 Specific capacity (mAh g-1)

2.5

Potential (V)

Coulombic efficiency (%)

-0.25

0.00

100 80

800 600

60

400

40

Charge capacity Discharge capacity Coulombic efficiency

200 0 0

20

40

60

80

100

120

Cycle number

Fig.4

22

140

160

180

20 0 200

Coulombic efficiency (%)

Current (mA)

0.25

1200

MoS2

discharge

charge

MoS2@CNF

discharge

charge mA g-1 50

50

1000

100 250

800

500 1000

600

2000

400 200 0

0

20

40

60

Cycle number 200

(b) MoS2 MoS2@CNF

150

-Z'' (ohm)

Specific capacity (mAh g-1)

(a)

1400

100

50

0

0

100

200

Z' (ohm) Fig. 5

23

300