Molybdenum disulfide nanosheet embedded three-dimensional vertically aligned carbon nanotube arrays for extremely-excellent cycling stability lithium-ion anodes

Abstract

Molybdenum disulfide (MoS₂) nanosheets embedded in three-dimensional (3D) vertically aligned carbon nanotube arrays (VACNTs) have been fabricated via a simple nebulization-assisted hydrothermal method. The MoS₂/VACNTs possess a highly ordered and uniformly oriented 3D structure with MoS₂ nanosheets adhering strictly to the surface of VACNTs. When evaluated as lithium-ion anode materials, so-obtained MoS₂/VACNTs composites containing 52 wt% MoS₂ exhibit superb electrochemical performances, including high capacity (1078 mA h g⁻¹ at 100 mA g⁻¹ after 1st cycle), good rate capability (789 mA h g⁻¹ at 2000 mA g⁻¹ after 20 cycles), and extremely-excellent cycling stability, for the MoS₂/VACNTs electrode can still deliver a discharge capacity of 512 mA h g⁻¹ after 1000 cycles at 5000 mA g⁻¹, compared with pristine MoS₂ (negligible discharge capacity at the 70th cycle). Such high electrical properties can mainly be attributed to the unique well-directed pore-morphology which provides low-resistant shortest diffusion pathways upon the high-conductive VACNTs to accelerate ion/electron movement. Moreover, the elastic spare-space inside/outside VACNTs as a buffer factor effectively restrains large volumetric change from MoS₂ during the charge/discharge process. It can be determined that such a structure is attractive to achieve extremely-excellent cycling stability lithium-ion anodes.

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1 Introduction

Batteries, the major power sources for various portable electronic devices and hybrid electric vehicles, have already become the benchmark for the development level in advanced energy storage technology.¹ Regardless of other alternatives, lithium-ion batteries (LIBs) are considered as the most promising renewable energy source due to their high energy density, long service life and unblemished safety record.^2,3 Nevertheless, it must be pointed out that LIBs existing today have still not fulfilled their ultimate potential in either power densities or cycling perdurability. In these two aspects, the sorts and structures of electrode materials play a strategic role.⁴ Thereby, for the sake of meeting higher requirements of applications with ever-increasing energy demands, it is deeply critical to develop high-performance electrode materials to achieve next-generation strong-powering and long-lasting rechargeable LIBs.

Over the past several decades, transition-mental sulfides (MSs) have been extensively studied in different areas, such as dry lubricants,⁵ solar cells,^6,7 catalysis,^8,9 super-capacitors,¹⁰ and field-effect transistors,¹¹ etc. While used for LIBs anodes, MSs usually show higher theoretical lithium storage capacity than traditional commercial graphite anodes (372 mA h g⁻¹).^12,13 Furthermore, it is worth noting that active sites producing by MSs electrode would be more than those from transition-metal oxides during lithiation/delithiation process.^12,14,15 As one of 2D layer-structured MSs, molybdenum disulfide (MoS₂) prevailing in energy conversion and storage exhibits high theoretical capacity of 670 mA h g⁻¹, excellent rate capability and low cost, for typically layered morphology enlarging its specific surface area and the weak van der Waals connecting between its layers facilitate Li⁺ migration during charging/discharging process.^15–17 Furthermore, MoS₂ itself contains a large amount and concentration of metallic 1T phases and short-range defects which can be electrochemically intercalated ions such as Li⁺, Na⁺ and K⁺ with extraordinary efficiency to further expand its practical capacity.^18,19 Unfortunately, in the practice operation of lithium intercalation, it is typically difficult to achieve rapid transference for electrons between two adjoining MoS₂ nanosheets, which could be ascribed to extremely limited conductivity in so-called c-direction and its intrinsic tendency to reaggregate or restack, thus leading to significant capacity loss.^15,20 As well as, the MoS₂ anodes experience a lower first-cycle coulombic efficiency and a shorter life-time induced by large volumetric change in the c-direction during Li⁺ insertion/extraction process.^5,21 Worse still, huge volume change is likely to bring MoS₂ pulverization problem which could accelerate capacity attenuation as well.

To overcome these problems, recent significant investigation drives hot-concern on the MoS₂/C based nanocomposites. These composites mainly include few-layer MoS₂ anchored on carbon nanosheet,¹⁵ MoS₂ coated 3D graphene networks,^12,22,23 and MoS₂ grown on the surface of carbon nanotubes (CNTs) nanocomposites,^24–26 etc. Hereinto, CNTs have been expected most profitable for electrode materials with excellent performance due to their original morphology: helically arranged graphitic layers, one-dimensional (1D) structured central canals and entanglement formation. These structure characteristics entail CNTs incorporated in composites serving as superb conductors with mechanical and chemical stability, high accessible surface area and excellent electrical conductivity.^27,28 For instance, Shyamal K. Das fabricated carbon–CNT–MoS₂ composites by one-step hydrothermal method and these composites retain a discharge capacity of 522 mA h g⁻¹ with 98% columbic efficiency after 50 cycles at 100 mA g⁻¹.²⁹ Moreover, Junsong Chen reported MoS₂ Nanosheets on CNTs backbone by glucose-assisted hydrothermal method and these MoS₂/CNTs exhibit capacity up to 957 mA h g⁻¹ after 1st cycle at 100 mA g⁻¹, while after 60 cycles, a reversible capacity as high as 698 mA h g⁻¹ can still be retained.²⁴ However, these 1D CNTs randomly entangled provide only curved channels for ion migrating, which extend ion mobile distance and hinder electrolyte fast permeating. Therefore, it is extraordinarily significant to develop vertically aligned CNTs which possess one-direction oriented pore structures. These vertical pore structures could not only permit the electrolyte to efficiently permeate active materials but also facilitate Li⁺/electron transport by decreasing the diffusion distance. Meanwhile, the elastic spare-space inside/outside nanotubes can alleviate internal stress induced by the volume expansion/contraction of these electrochemically active phases.³⁰

In this paper, we fabricate nitrogen doped VACNTs with high conductivity through chemical vapor deposition (CVD) technology using xylene as carbon source, cyclohexylamine as nitrogen source and ferrocene as iron catalyst. And then MoS₂ nanosheets could be introduced into VACNTs forest through a simple hydrothermal method named nebulization-assisted infiltration using ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O) and thiourea (CH₄N₂S) as sources, showed in Scheme 1. This process consists of two steps: one is using spray to produce small droplets which could be infiltrated into VACNTs due to gravity; the other is pyrolyzing above sources under 750 °C to synthesis MoS₂: the (NH₄)₆Mo₇O₂₄ is decomposed to ultrathin MoO_x, CH₄N₂S is pyrolyzed to H₂S, and then, ultrathin MoO_x nanosheets react with H₂S to form few-layer MoS₂ nanosheets.¹⁵ Moreover, we design three groups by spraying for 20, 30, and 40 minutes, in which MoS₂ contents are 37.15%, 52%, and 59.9%, marked MoS₂/VACNTs-1, MoS₂/VACNTs-2, and MoS₂/VACNTs-3, respectively. Plus, we synthesize pristine MoS₂ as control group by hydrothermal method. Most importantly, their advanced electrical performance of LIBs effectively outperforms all the existing MoS₂-based materials.

Scheme 1 Schematic illustration of the fabrication process for high-quality 3D structure-based MoS₂/VACNTs.

2 Materials and experimental methods

2.1. Preparation of MoS₂ embedded into 3D vertically aligned CNTs

3D vertically aligned CNTs are synthesized via CVD method. Firstly, ferrocene (96.75 mM) was dissolved in mixed solution of xylene and cyclohexylamine (65 : 35 in the volumetric ratio) in a round bottom flask, while atomized using a KQ-300DE ultrasonic nebulizer (Kunshan Ultrasonic Instrument Co., Ltd., China) in an ultrasonic bath for 15 minutes. Secondly, completely-dissolved solution was atomized by the ultrasonic nebulizer and passed through a quartz tube (45 mm in internal diameter, 1000 mm in length), in which quartz slice (30 mm in length, 30 mm in width, 3 mm in height) was placed directly above thermocouple as the substrate to grow samples at 900 °C provided by an open vacuum/atmosphere tube furnace (SK-G05123K Tianjin Zhonghuan Experiment Electric Stove Co. Ltd., China) with argon flowing (750 sccm) as carrier gas for 60 minutes. When reaction completed, ultrasonic nebulizer could be stopped and argon flow regulated down to 150 sccm. Lastly, the above-synthesized sample would be permeated in nitric acid (70%, 30 mL) for 9 h at 120 °C to introduce oxygen active functional groups –COO– and –OH which can be used for the absorption of ionogenic groups,²³ followed by deionized water washing until pH became neutral, and then was dried in vacuum at 80 °C overnight.³¹

The synthesis scheme for MoS₂ nanosheets embedded in the 3D VACNTs is as follows: 0.02 mmol mL⁻¹ (NH₄)₆Mo₇O₂₄·4H₂O and 0.25 mmol mL⁻¹ CH₄N₂S were added to 40 mL deionized water in a round bottom flask followed by thoroughly mixed stirring. And then this dispersed even solution was displayed on the ultrasonic nebulizer which generated a mist of droplets infiltrating into above-obtained 3D structure VACNTs at room temperature with an argon carrying gas at 700 sccm. After nebulization, the resulting samples were heated 2 h at 750 °C under 5% H₂/95% Ar at 150 sccm, converted to MoS₂, followed by distilled water washing several times, finally drought at 60 °C under vacuum for 12 h. The detailed reaction equations (eqn (1)–(3)) are described as follows:¹⁵

3(NH₄)₆Mo₇O₂₄·4H₂O → (14x − 24)NH₃ + (21 − 7x)N₂ + 21MoO_x + (84 − 21x)H₂O (1)

CH₄N₂S + 2H₂O → 2NH₃ + H₂S + CO₂ (2)

4MoO_x + (x + 6)H₂S → 4MoS₂ + (x − 2)SO₃ + (x + 6)H₂O (3)

Pristine MoS₂ were synthesized through usually hydrothermal process: 0.35 g (NH₄)₆Mo₇O₂₄·4H₂O and 0.35 g CH₄N₂S were dissolved in 40 mL distilled water and put in the autoclave at 180 °C by 12 h. After reaction, on cooling to room temperature, the sample was put in 50 °C ovens after filtering.

2.2. Materials characterizations

The crystal structures of all the samples were identified using X-ray diffraction (XRD) conducted on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation and the data collected from 10° to 80°. The morphologies were characterized by field-emission scanning electron microscope (SEM) at 5 kV on an S-4800 microscope equipment (Hitachi, Japan) and transmission electron microscope (TEM) images were obtained with a JEM-2100F TEM (JEOL, Japan) operating at 200 kV. Thermogravimetric (TG) analysis was carried out on a TG/DTA7300 thermal analyzer with a heating rate of 5 °C min⁻¹ in flowing atmosphere from room temperature to 800 °C to calculate MoS₂ content in each experiment.

2.3. Electrochemical measurements

The electrochemical properties of the samples were measured using CR2025-type coin cells assembled in a high-purity-Ar filled glove box with lithium foils as the counter electrodes. Celgard 2300 microporous polypropylene membrane and 1 M LiPF₆ in ethylene carbonate/dimethyl carbonate (1 : 1 in volume) were used as the separator and electrolyte, respectively. The working electrode was prepared by mixing active material, polyvinylidene fluoride and carbon black with a weight ratio of 70 : 20 : 10 in N-methylpyrrolidone (NMP) with constant stirring for 6 h to form homogeneous slurry. The well mixed slurry was then spread on Cu foil and dried at 80 °C in a vacuum oven for 12 h. The charge–discharge tests were performed between 0.01 V and 3 V on a Land CT2001A batterytest system (Wuhan, China). Electrochemical impedance spectroscopy (EIS) was performed on an electrochemical workstation (CHI660E) in the frequency range from 100 kHz to 0.01 Hz with amplitude of 5.0 mV. Cyclic voltammetry (CV) profiles were carried out on CHI660E at a scanning rate of 0.1 mV s⁻¹ between 0 V and 3.0 V (vs. Li/Li⁺).

3 Results and discussion

3.1. Morphology and structure characterization

Fig. 1 shows the XRD patterns of VACNTs, pristine MoS₂ and MoS₂/VACNTs composites. The peaks at 2θ = 13.9°, 33.5°, 37.1° and 59.3° can be indexed to (002), (100), (103) and (110) planes of MoS₂, and all the diffraction peaks agree well with characteristic peaks of MoS₂ in previous valuable literature.²⁹ It is worth noting that the strong peak at 26° could be ascribed to (002) reflection of the VACNTs.³¹ The XRD results confirm the presence of MoS₂ embedded in VACNTs and exhibit high purity of MoS₂ in these composites compared to the pristine one because of no significant differences between them in the XRD patterns besides (002) reflection. In addition, there are different diffraction intensities of MoS₂ characteristic peaks, meaning various levels of content in different experimental groups, which could correspond with TG analysis undersides.

Fig. 1 XRD patterns of the ACNTs, MoS₂, MoS₂/VACNTs-1, MoS₂/VACNTs-2, and MoS₂/VACNTs-3.

The SEM images of 3D VACNTs, pristine MoS₂ and MoS₂/VACNTs composites are shown in Fig. 2. Fig. 2a and b present the morphological characterization of 3D-matrix VACNTs forest. This forest structure is consisted of one-direction oriented carbon nanotubes with approximate 80 nm in diameters. As showed in Fig. 2c–e, the MoS₂ nanosheets are adhered to the surface of the VACNTs strictly and homogeneously owing to the interaction between oxygen containing functional groups and MoS₂ nanosheets,³¹ and the content of MoS₂ increases as the nebulization time increases. Particularly, these samples still keep vertical and well dispersed compared with VACNTs, which proves this hydrothermal method with the help of nebulization-assisted infiltration is feasible. Fig. 2f indicate that the pristine MoS₂ synthesized by hydrothermal method shows a flower-like shape and these flowers are stacked of MoS₂ nanosheets. From TEM we can further ascertain the whole MoS₂ nanosheets stack and wrap on the surface of VACNTs and so-formed layers of active materials has a uniform distribution as shown in Fig. 3a and b (Fig. S1†). The C, Mo and S element mappings (Fig. 3c–e) of the as-formed 3D MoS₂/VACNTs further give the proof of the even distribution of MoS₂. It can be believed that MoS₂ embedded uniformly into VACNTs can effectively avoid aggregation and restacking of MoS₂ nanosheets, subsequently ensure the whole structural integrity, eventually lead to good cycling stability (Fig. S2†).

Fig. 2 Detailed microstructure characterization from SEM of (a and b) VACNTs, (c) MoS₂/VACNTs-1, (d) MoS₂/VACNT S-2, (e) MoS₂/VACNTs-3 and (f) pristine MoS₂.

Fig. 3 (a and b) TEM micrograph of MoS₂/VACNTs-2; (c–e) the C, Mo and S element mappings.

3.2. Electrochemical performance

TG analysis was carried out from room temperature to 800 °C under air to determine the MoS₂ content contained in MoS₂/VACNTs composites. As shown in Fig. 4a, these TG curves display one distinct region with weight loss from 400 °C to 550 °C, due to the oxidation of MoS₂ to MoO₃ as well as combustion of VACNTs.^14,30 The content of MoS₂ in composites materials can be precisely calculated from this equation: affirming loss content from MoS₂ to MoO₃ is 40% according to MoS₂ TG curve and from VACNTs to CO₂ is 100% seen from VACNTs TG curve, one has 4.0314 × 40% + 4.0314(1 − x) = 67.8% × 4.0314. Therefore, x = 0.52. On the basis of accurate calculations, the MoS₂ contents of the MoS₂/VACNTs-1, MoS₂/VACNTs-2 and MoS₂/VACNTs-3 are determined to be around 37.15%, 52%, and 59.9%, by weight, respectively.

Fig. 4 (a) TG curves of VACNTs, pristine MoS₂ and MoS₂/VACNTs composites; (b and c) representative CV curves of MoS₂/VACNTs-2 and pristine MoS₂ in the initial six cycles; (d) cycling performance of MoS₂/VACNTs-1, MoS₂/VACNTs-2, MoS₂/VACNTs-3 and pristine MoS₂ at different current densities; (e) charge–discharge voltage profiles of MoS₂/VACNTs-2 at first cycle at different current density; (f) electrochemical impedance spectra of MoS₂/VACNTs-1, MoS₂/VACNTs-2, MoS₂/VACNTs-3 and pristine MoS₂.

Fig. 4b and c show the representative cyclic voltammograms (CV) of MoS₂/VACNTs-2 and pristine MoS₂ from the 1st to the 6th cycle and the CV patterns are consistent with previous reports. In the first cathodic sweep, the peak at approximate 1.1 V is attributed to the transformation of MoS₂ from trigonal prisms to octahedral phases for lithium storage in the MoS₂/VACNTs composites.²⁴ Besides, the peak at 0.5 V can be attributed to the conversion reaction of MoS₂ to Li₂S.²⁴ Remarkably, these two reduction peaks disappear in subsequent reduction cycles. Instead, two new produced peaks at 2.0 V and 1.2 V might correspond to the insertion of Li⁺ ions into the MoS₂ nanosheets and the conversion reaction shown above, respectively. Such a modification in CV curve shape is common to electrode materials based on the conversion reaction mechanism and reflects permanent structural change during the first cycle. In the first six anodic sweeps, there is a pronounced oxidation peak centered at 2.3 V in Fig. 4b, which is typically attributed to the oxidation of Li₂S to S, while a series of broad and weak oxidation peaks ranging from 1.4 V to 1.8 V should be attributed to the delithiation of Li_XMoS₂, moreover, peaks at about 0.1 V reveal the reversible lithium storage in VACNTs. Fortunately, CV curves for subsequent cycles almost overlap, which indicates excellent structural stability of these MoS₂/VACNTs during the charge/discharge process compared with pristine MoS₂ seen from Fig. 4c exhibiting some irreversible decay. In the first discharge process as shown in Fig. 4c, the peaks at 1.5 V and 0.5 V are attributed to MoS₂ phase change for lithium storage and conversion of MoS₂ to Li₂S and molybdenum metal, respectively.⁵ In the first charge process, the doublet centered at 1.6 V as well as a large broad peak at 2.3 V are attributed to delithiation of Li_XMoS₂ and the formation of sulphur.⁵ Besides, in the subsequent cycling the dominant cathodic peak at around 0.5 V gets weaker indicating the consumption of the active materials, i.e. Li_XMoS₂ and MoS₂.⁵

The rate performances of all the materials were evaluated at discharge rates from 100 to 2000 mA g⁻¹, as shown in Fig. 4d. From the rates profiles, an improved rate property of MoS₂/VACNTs-2 can be clearly seen. The reversible capacity of MoS₂/VACNTs-2 is stable at about 1078 mA h g⁻¹ after 1st cycle at the current density of 100 mA g⁻¹. As the discharge rates increase, the MoS₂/VACNTs-2 shows the highest discharge capacity at any current density: 997 mA h g⁻¹ at 200 mA g⁻¹, 927 mA h g⁻¹ at 500 mA g⁻¹, 864 mA h g⁻¹ at 1000 mA g⁻¹ and 789 mA h g⁻¹ at 2000 mA g⁻¹ sustaining over 95% coulombic efficiency during each cycling. In contrast, MoS₂/VACNTs-1, MoS₂/VACNTs-3 and pure MoS₂ nanosheets retained capacities of only 532, 677 and 288 mA h g⁻¹ at 2000 mA g⁻¹ with capacities of 1762, 1429 and 1367 mA h g⁻¹ for the first cycle, respectively. Moreover, this improved rate performance of MoS₂/VACNTs-2 can be observed from charge–discharge voltage curves in Fig. 4e.

Fig. 4f shows the electrochemical impedance spectra (EIS) responses for all of the composites. It can be seen that the impedance spectra of all the electrodes consists of a semicircle in the high-medium frequency region and a slanted line in the low frequency region. The semicircle is related to the charge transfer resistance (R_ct) at the electrode/electrolyte interface and the linear represents the diffusion impedance of lithium ions in the MoS₂.^18,30 As shown in Fig. 4f, the MoS₂/VACNTs composites exhibit significant reduction of the charge transfer resistance in comparison to the pristine MoS₂, indicating that VACNTs could indeed improve the kinetics of the MoS₂. In addition, compared to the MoS₂/VACNTs-2 and MoS₂/VACNTs-3, the semicircle of MoS₂/VACNTs-1 is smaller. The possible reason is that the increased content of VACNTs in composite would make the transport pathways for electron and Li⁺ to the MoS₂ particles become more complicated, hence, exhibits a relatively improved conductivity.

Fig. 5a and c show cycling performance of MoS₂/VACNTs-1, MoS₂/VACNTs-2, MoS₂/VACNTs-3 and pristine MoS₂ at 1000 mA g⁻¹ and 5000 mA g⁻¹, respectively. It can be seen that MoS₂/VACNTs composites possess excellent cycle stability compared to pristine MoS₂ that synthesized by hydrothermal process, for the capacities of MoS₂/VACNTs remains unchanged after first cycle while pristine MoS₂ shows negligible capacity after 60 cycles. Moreover, MoS₂/VACNTs-2 exhibits better electrochemical properties retaining capacities of 817 mA h g⁻¹ at 1000 mA g⁻¹ and 512 mA h g⁻¹ at 5000 mA g⁻¹ with capacities of 1624 and 478 mA h g⁻¹ for the first cycle and above 95% coulombic efficiency after the first five cycles, while MoS₂/VACNTs-1 and MoS₂/VACNTs-3 retain capacities of 579, 793 mA h g⁻¹ at 1000 mA g⁻¹ and 397, 407 mA h g⁻¹ at 5000 mA g⁻¹ with capacities of 1494, 1449 and 507, 574 mA h g⁻¹ for the first cycle, respectively. The MoS₂/VACNTs-2 electrode also displays excellent cycling stability even at other current density (100, 300 and 500 mA g⁻¹), as illustrated in Fig. 5b. The discharge capacity of the MoS₂/VACNTs-2 achieved capacity of about 969, 868 and 815 mA h g⁻¹ after 100 cycles at 100, 300 and 500 mA g⁻¹, respectively. In addition, the corresponding coulombic efficiency of MoS₂/VACNTs-2 at 500 mA g⁻¹ rapidly rises from 71% to 99% after five cycles.

Fig. 5 Cycling performance of MoS₂/VACNTs-1, MoS₂/VACNTs-2, MoS₂/VACNTs-3 and pristine MoS₂ at 1000 mA g⁻¹ (a) and 5000 mA g⁻¹ (c); (b) the cycling stability of MoS₂/VACNTs-2 electrode at 100, 300 and 500 mA g⁻¹, respectively.

Above results have demonstrated that MoS₂/VACNTs-2 showed improved properties towards the storage of Li⁺. The improvement of the electrochemical performance, especially the outstanding cycling stability, could be attributed to following reasons based on their microstructure. Firstly, highly conductive 3D VACNTs with well-defined regular pore structure can provide 1D electron-transport pathways that will facilitate the electronic and Li⁺ diffusion rate, and decrease the polarization at a large current density. Secondly, the VACNTs with appropriate inter-tube space serve as buffer matrix to accommodate the volume expansion/contraction of MoS₂ and stabilize the electrode structure during the lithiation/delithiation process.

4 Conclusions

This work sheds light-sight on the excellent cyclic stability anodes for LIBs associated with MoS₂ active substance. It is demonstrated effective way to use VACNTs as conductive additives to improve electrical performance in spite of other MoS₂/C composites. To synthesize this MoS₂/VACNTs composite, we have adopted a simple hydrothermal method with the nebulization-assisted infiltration. So-obtained composites possess original single-direction-oriented hollow based 3D arrays which plays a dominant role in their excellent electrochemical properties. Moreover, these excellent properties in LIBs anodes also take the advantages of synergistic effects with MoS₂ and VACNTs in terms of high theoretical capacity from MoS₂ and high electronic conductivity from VACNTs, respectively. Here, we present detailed and critical analyses of their electrical properties. Reversible capacities of 1078 mA h g⁻¹ at 100 mA g⁻¹ have achieved for these MoS₂/VACNTs composites. Furthermore, an impressive capacities of 789 mA h g⁻¹ at 2000 mA g⁻¹ was exhibited for their good rate capability. Mostly important, such-obtained MoS₂/VNCATs composites exhibits extremely-excellent cycling stability, for the MoS₂/VACNTs electrode can still deliver a discharge capacity of 512 mA h g⁻¹ after 1000 cycles at 5000 mA g⁻¹, compared with pristine MoS₂ (negligible discharge capacity at the 70th cycle). Thus it can be see, this work greatly improves the prevailing problem of capacity attenuation and the intimate contact of MoS₂ and VACNTs can effectively avoid the aggregation and restacking of MoS₂ during the lithium insertion and extraction process, thus remarkably enhancing the structural integrity of electrode, apart from promising lithium-ion battery anode, the MoS₂/VACNTs composite also has immense potential for applications in other areas such as supercapacitor, catalysis, and sensors.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51272073, 51572078 and 51541203) and the Scientific Research Fund of Hunan Province (2015JJ2033).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17042k

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