Self-assembly of hierarchical MoSx/CNT nanocomposites (2

Two dimension (2D) layered molybdenum disulfide (MoS2) has emerged as a promising candidate for the anode material in lithium ion batteries (LIBs). Herein, 2D MoSx (2 ≤ x ≤ 3) nanosheet-coated 1D multiwall carbon nanotubes (MWNTs) nanocomposites with hierarchical architecture were synthesized via a high-throughput solvent thermal method under low temperature at 200°C. The unique hierarchical nanostructures with MWNTs backbone and nanosheets of MoSx have significantly promoted the electrode performance in LIBs. Every single MoSx nanosheet interconnect to MWNTs centers with maximized exposed electrochemical active sites, which significantly enhance ion diffusion efficiency and accommodate volume expansion during the electrochemical reaction. A remarkably high specific capacity (i.e., > 1000 mAh/g) was achieved at the current density of 50 mA g−1, which is much higher than theoretical numbers for either MWNTs or MoS2 along (~372 and ~670 mAh/g, respectively). We anticipate 2D nanosheets/1D MWNTs nanocomposites will be promising materials in new generation practical LIBs.

Two dimension (2D) layered molybdenum disulfide (MoS 2 ) has emerged as a promising candidate for the anode material in lithium ion batteries (LIBs). Herein, 2D MoS x (2 # x # 3) nanosheet-coated 1D multiwall carbon nanotubes (MWNTs) nanocomposites with hierarchical architecture were synthesized via a high-throughput solvent thermal method under low temperature at 2006C. The unique hierarchical nanostructures with MWNTs backbone and nanosheets of MoS x have significantly promoted the electrode performance in LIBs. Every single MoS x nanosheet interconnect to MWNTs centers with maximized exposed electrochemical active sites, which significantly enhance ion diffusion efficiency and accommodate volume expansion during the electrochemical reaction. A remarkably high specific capacity (i.e., . 1000 mAh/g) was achieved at the current density of 50 mA g 21 , which is much higher than theoretical numbers for either MWNTs or MoS 2 along (,372 and ,670 mAh/g, respectively). We anticipate 2D nanosheets/1D MWNTs nanocomposites will be promising materials in new generation practical LIBs. A dvanced energy storage technology is the key to manage the energy supply and demand. Lithium ion batteries (LIBs) have attracted increasing research interests and become one of the main power sources for portable electronic devices and electric vehicles due to its high energy densities, no memory effect, and good cycling stability compared to other alternatives 1 . In commercial LIBs, graphite and lithium metal oxides are commonly employed as the negative (anode) and positive (cathode) electrode materials, respectively. Lithium is the lightest metal that delivers high energy density per electron with a theoretical electrochemical capacity of Li to Li 1 is 3860 mAh/g 2 . However, further advancements in the state-of-the art LIBs are still bottlenecked by the limitation in the anode materials associated with limited capacity (i.e., graphite, ,372 mAh/g), lack of shape flexibility and low ion/electron conductivity 3,4 . In the past few years, substantial research efforts have been devoted in developing high performance LIBs electrodes. Various carbon nanomaterials, such as one dimension (1D) carbon nanotubes (CNTs) 5,6 , two dimension (2D) graphene nanosheets 7,8 , three dimension (3D) graphene foam 9,10 , have all been investigated as the anode materials in reversible storage of Li 1 , due to their outstanding electronic conductivities, high charge mobilities and large specific surface areas. As one of the crystalline form of carbon, 1D CNTs has high electric conductivity, good mechanical property, chemical stability and reversible redox reaction capability, which makes it a promising candidate as lithium insertion hosts for LIBs.
The nanostructured multifunctional heterostrucutres have been proved to work synergistically with both high capacity and good cyclability [11][12][13][14] . Molybdenum disulfide (MoS 2 ), an inorganic graphite analogue, belongs to the layered transition-metal dichalcogenide (LTMDs) family. The weak van der Waals interaction between MoS 2 layers allows the Li 1 ions to diffuse without a significant increase in volume expansion and prevent the pulverization problem of active materials caused by the repeatly lithiation and delithiation process. The promising potential of MoS 2 serving as an anode materials for LIBs is widely reported in the literature due to its attractive specific capacity [15][16][17][18][19][20][21] . Theoretically the conversion reaction between Li ions and MoS 2 leads to four moles of lithium incorporation per mole of MoS 2 accounting for 670 mA h g -1 lithium storage capacity that is ,1.8 times higher than the graphite electrode 20 . With all these significant advantages, MoS 2 has attracted lots of research interests and became a promising material as an anode material in LIBs [17][18][19] . Various methods have been reported for the synthesis of MoS 2 including the gas-phase reaction of MoO 3 with H 2 S or S vapor 22,23 , thermal decomposition of ammonium thiomolybdate 24,25 , and solvent thermal method 26,27 .
The solvent thermal process is an important wet chemistry synthesis method and has been widely used to prepare various nanomaterials or nanocomposites. It has been reported CNTs favored the growth of the tubular MoS 2 on the surface of carbon nanotube side walls and promoted the formation of tubular MoS 2 layers with high crystallinity [27][28][29] , CNTs/MoS 2 composites have also been prepared by the simple solvothermal method 30,31 . For example, tubular MoS 2 layers coating on CNTs were synthesized by the hydrothermal reaction between Na 2 MoO 4 and CS(NH 2 ) 2 with the presence of CNTs 12 . The surface area of MoS 2 is limited by the surface area of CNTs. Nevertheless, when aqueous solvent is used, CNTs need to be treated by refluxing in high concentrated strong acid in order to improve the wetting between CNTs and MoS 2 precursor 28 . This acidic treatment will introduce defects in CNTs and negatively affect the electrical properties of CNTs. MoS 2 /CNTs with a design of 2D MoS 2 nanoflakes surrounded by a coating of CNTs was synthesized by using Na 2 MoO 4 and KSCN as reactant and ethylene glycol as solvent in the presence of CNTs 27 . These composites show higher capacity and improved cycling stability compared to pure MoS 2 . The MoS 2 nanoflakes synthesized are relatively thick and randomly attached to CNTs, which causes a continues capacity fading during cycles 27 . Wang et al. prepared MoS 2 overlayers supported on coaxial CNTs by wet-chemistry process and studied the reversible lithium-storage behaviors of this composite 32 . A reversible capacity of 400 mAh/g was achieved; however this value is much smaller than the noncoaxial MoS 2 /CNTs composite.

Results
Herein, we report a unique MoS x /CNTs (2 # x # 3) nanostructure synthesized by simple solvent thermal method at low temperature (200uC) using (NH4) 2 MoS 4 as single reactant and N,N-dimethylformamide (DMF) as solvent in the presence of MWNTs. The synthesized MoS x /MWNTs composites are different from the previous report for MoS 2 sheath/CNT-core nanoarchitecture 32 , the MoS x layers are not confined to the MWNTs surface, but extend the layered structure out of the cylindrical tubules (as shown in Figure S1). To understand the forming of hierarchical architecture, the morphology and lattice structure of as prepared MoS x /MWTNs composite was compared with the samples treated under elevated temperature.  21 . It was also found that the Raman signature of MoS 2 dramatically increased after thermal annealing, which suggests the formation of highly crystallized MoS 2 layers. This is agreed with the result of HRTEM.
X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical states of Mo and S in the MoS x /MWNTs nanocomposites. Figure 2 displays the XPS characterization of the samples before and after thermal annealing at 800uC under Ar protecting environment. The high-resolved XPS spectra shows the binding energies of Mo 3d 3/2, Mo 3d 5/2, S 2p K and S 2p 3/2 peaks in the thermal annealed MoS x /MWNTs are located at 232.4, 229.2, 163.3 and 162.1 eV, respectively, indicating that Mo 41 existed in the annealed MoS x /MWNTs 32 . The stoichiometric ratio of S:Mo estimated from the respective integrated peak area of XPS spectra is ,2.125 suggesting the structure is close to MoS 2 . For the as prepared MoS x /MWNTs two broaden peaks centered at ,232.5 and ,228.9 eV, in addition to the XPS peaks for MoS 2 structure, other sets of peaks are also observed. The higher energy shift of Mo 3d 3/2 and 3d 5/2 doublet are associated with higher valence states. The observation of Mo 3d 3/2 and Mo 3d 5/2 peaks at 233.6 and 230.5 eV with separation energies close to 3.1 eV can be attributed to the presence of Mo 51 ions 33,34 . For the non-annealed MoS x /MWNTs the S 2p spectra can be interpreted in terms of two doublets, with S 2p3/2 binding energies of 161.7 and 163.2 eV. Compared to the thermal annealed samples, the additional S 2p1/2 and 2p3/2 energies located at 164.3 and 163.2 eV can be assigned to the binding energies of apical S 22 or bridging disulfide S 2 22 ligands. The S 2p spectrum that can be fit with two S 2p doublets, which is similar to those of amorphous MoS 3 35,36 . The presence of bridging apical S 22 or bridging S 2 22 is in good consistence with the TEM analyses in Figure 1 (B), which reveals that the MoS x obtained are basically semicrystalline. Furthermore, the S/Mo elemental ratio estimated from the integrated peak area of XPS spectra is ,3.0 which also suggests the as grown MoS x is stoichiometrically close to MoS 3 . The thermal decomposition of (NH4) 2 MoS 4 is accompanied by molybdenum-sulfur redox processes, which include the oxidation of S 22 ligands of the MoS 4 22 anion and the reduction of Molybdenum metal from Mo VI to Mo IV , and various thermal decomposition intermediate may exist 37 . The XPS results confirm the presents of MoS 3 while the Raman spectra from the as prepared samples show smaller but visible Raman Peaks of MoS 2 at 376 and 402 cm 21 (as shown in Figure 1(E)). Therefore, the exact phase of the MoS x /MWNTs compound is suggested to be a mixture of MoS 2 and MoS 3.
The growth mechanism of MoS x /MWNTs layered structures were also investigated by varying the Mo/Carbon ratio in the precursor.

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Considering the electrodes with special hierarchical nanocomposites are advantageous to LIBs, we investigate the lithium storage properties of as-prepared MoS x /MWNTs using half-cell configuration. Figure 4 shows the electrochemical performance of MoS x / MWNTs as anode materials. Figure 4 (A) illustrates the first, second, fifth and tenth discharge/charge voltage profiles of the MoS x / MWNTs composite electrode in the voltage range of 0.01 to 3 V (vs. Li/Li 1 ). During the first discharge, the initial discharge capacity between 2.0 to 1.5 V can be attributed in part to the reaction of residual carbon (MWNTs) surface functional group 38 and in part to lithium insertion into the MoS x /MWNTs composites forming Li n MoS x (0 , n , 4) 39 , according to the reaction MoS x 1 nLi 1 1 ne 2 R Li n MoS x 27,40 . We note that it is previously proposed that a better formulation for MoS 3 would be Mo V 2 (S 2 22 )(S 22 ) 4 , therefore, the reduction of sulfur during initial discharge can also be considered here 39 . Following this, the capacity between 1.0 to 0.5 V can be attributed to the conversion reaction process MoS x 1 2xLi 1 1 2xe 2 R Mo 1 xLi 2 S 41-43 . The metal sulfide reacts with lithium ions forming metal nanoparticles and insoluble Li 2 S matrix 20 . It was argued that the nanosized metal particles promote the reversible reaction which is responsible for the reversible lithium-storage capacity, therefore the phase segregation of transition metals should be limited in order to improve the cycling stability 32 . The sloping plateau at the lower voltage region (below 0.5 V) includes the contribution from  the formation of a solid electrolyte interface (SEI) and the gel-like polymeric layer on the surface of the active materials 44 . In the subsequent charge process, a plateau at ,1.3 V and the sloping region above 2.2 V are attributed to the oxidation of Mo particles to MoS x and the oxidation of Li 2 S to form S, respectively 42,45,46 . We note that lithium extraction from the Li n MoS x phase should also be considered here 27,39,40 . The initial discharge and charge capacities are found to be 1549 and 1159 mAhg 21 , respectively. (with a Coulombic efficiency of 74.8%).The irreversible capacity loss of approximately 25.1% in the 1 st cycle can be mainly attributed to the irreversible processes including the electrolyte decomposition and inevitable formation of the SEI, which have been observed for nanosized anode materials 47 . During the 2 nd cycle, the discharge capacity decreases to 1154 mAh/g with a corresponding charge capacity of 1126 mAh/g, leading to a much higher Coulombic efficiency of 97.5%. This value further increased to 99.6% in the 5 th cycle and still maintained above 98.6% at the 10 th cycle. To further clarify the electrochemical process of the MoS x /MWNTs composite, cyclic voltammograms (CV) measurement of the first three cycles in the voltage range of 3.0 -0.01 V with a scan rate of 0.1 mVs 21 was shown in Figure 4 (B). In the first cycle a very small reduction peak at ,1.80 V was found, which can be related to the reaction of residual carbon surface functional group 38 , in part to lithium insertion into the MoS x structure forming Li n MoS x 39 , and the reduction of traced sulfur 39 . A pronounced reduction peak at ,0.50 V was observed in the first cycle, however for the subsequent cycles, the peak at ,0.50 V disappeared. This process has been attributed to the decomposition of MoS x into Mo nanoparticles embedded in a Li 2 S matrix through the conversion process 42,43 . Upon the anodic scan, the oxidation peak at ,1.5 V can be in part attributed to the oxidation of Mo to MoS 2 followed by a anodic peak at 2.3 V associated with the oxidation of Li 2 S into S 42,43,45 . In addition, lithium extraction from Li n MoS x could contribute to these anodic processes depending on the stoichiometry of the LinMoS x 39 . During the 2 nd CV scan, a pair of reduction peak at ,1.3 V and ,1.80 V together with two corresponding oxidation peaks at , 1.5 and 2.3 V for the MoS x /MWNTs composite became distinct. The reduction peak at ,1.3 V can be related to the intercalation of Li 1 into the MoS x lattice While, the oxidation peaks at ,1.48 V and 2.28 V correspond to the extraction of Li 1 from Li n MoS x lattice and the oxidation of Li 2 S, respectively 40 . Figure 4 (C) shows the cycling stability of the MoS x /MWNTs electrode compared to the pristine MWNTs. The specific capacity of the MoS x /MWNTs composite with a Mo/C molar ratio of 151 is above 1000 mAh/g which is more than 4 times larger than the pristine MWNTs electrodes under current density of 50 mA/g. The specific capacities of MoS x /MWNTs composites with various Mo/ C molar ratios are shown in supporting information Figure S2. Even at a very high current density of 1000 mAg 21 , the composite electrode can still deliver a capacity of 358 mAhg 21 , which is comparable with the theoretical capacity of graphite (372 mAh g 21 ). Furthermore, after the current density returns from 2000 mAg 21 to 50 mAg 21 , the specific capacity of MoS x /MWNTs electrode can recover to 1087 mAhg 21 and remain 1098 mAhg 21 after 10 cycles. Our MoS x /CNTs have shown a remarkably high reversible specific capacity (i.e., . 1000 mAh/g) at the current density of 50 mA g 21 , which is much larger than the ''theoretical'' capacity value of MoS 2 (670 mAh/g assuming 4 lithium ions per MoS 2 ) and CNTs along. We note that specific capacity of MoS 2 higher than 670 mAh/g is well-documented in the literature 45,48,15 . It was shown that MoS 2 can take up to 8 lithium ions with major capacity between 0.01 to 1.0 V vs. Li/Li 1 15 , which corresponds to a theoretical capacity up to 1334 mAh/g. It is believed that the lithium ions can be stored in different defect sites of the MoS 2 depending on the morphology of the material 15 . In addition, Kartick et al. reported that MoS 2 /CNT composites prepared by dry grinding method can achieve a reversible storage capacity around 1000 mAh/g 49 and X. Cao et al. reported that the MoS 2 layers grown on CVD-G has a reversible capacity above 1000 mAh/g 50 . We believe that the high capacity observed in our study is associated with the unique material structure and defect distribution of MoS x /CNT. It worth mentioning that the MoS x / MWNTs composites had better rate performance compared to the reported single-layer MoS 2 -graphene composites 40 and much improved cycling stability than the MoS 2 electrodes 27,40 . As demonstrated by the schematical illustration image in Figure 5, the high rate capability can be attributed to the unique hierarchical nanoarchitecture of MoS x /MWNTs which provide structural stability and transport advantages for both electrons and lithium ions. The Li 1 ion from the surrounding of MoS x /MWNTs have sufficient contact with the Li accommodate layers, and the exposed MoS x edges provides abundant intercalations tunnels. The MWNTs provide fast electronic conduction channels and ensure the individual high specific MoS x layerelectrically connected during charge/discharge cycles, meanwhile the Li 1 are accommodated in the metal sulfide layers.
In conclusion, the outstanding performance of hierarchical composites based anode material is attributable to the unique synergy at the nanoscale between 1D CNT and Li 1 hosting 2D nanoseets . The CNTs provide high conductance channels and ensure the individual high specific MoS x layerelectrically connected during charge/discharge cycles, meanwhile the Li 1 are accommodated in the metal sulfide layers. Moreover, the designed hierarchical structure with maximized surface and increased layer distance of S-Mo-S have resulted in less strain and smaller intercalation barrier of Li ions, which maintain the high lithium storage in reversible capacities, stable cycling lifetime, and excellent rate performances. Other promising applications are also anticipated to arise that take advantage of the abundant active MoS x edges as catalysts [51][52][53][54] .

Methods
Preparation of MoS x /MWNTs nanocomposite. The multi-walled carbon nanotubes (MWNTs), L-MWNTs-60100, were purchased from Shen-zhen Nanotech Port Co., Ltd, Shenzhen, China. The (NH4) 2 MoS 4 powder and N,N-dimethylformamide (DMF) were purchased from Sigma-Aldrich. All chemicals and raw materials were directly used without further purification. The MWNTs/MoS 2 hybrid was prepared by a solvent thermal process. In a typical experiment, 220 mg (NH4) 2 MoS 4 powder (Sigma-Aldrich) and 100 mg MWNTs were mixed and dispersed into 30 ml of N,Ndimethylformamide (DMF) in a 40 ml Teflon autoclave. After that, the solution was sonicated at room temperature for approximately 10 mins until homogeneous solution was achieved. Then the autoclave was sealed tightly and heated at 200uC for 10 hours under autogenous pressure without intentional control of ramping and cooling rate. After cooled down to room temperature, the product was extracted by centrifugation at 10,000 rpm for 5 min. To remove the unreacted molecules and most of the DMF residuals the product was dispersed in DI water and recollected by centrifugation, this washing step was repeated for at least 5 times, the final products was MWNTs/MoS x nano composite.
Materials characterization. X-ray photoelectron spectroscopy (XPS) analysis was performed on a KRATOS AXIS ULTRA-DLD spectrometer with a monochromatic Al K a1 radiation (hv 5 1486.6 eV). The morphologies and microstructures of the products were characterized by transmission electron microscopy (TEM) and high resolution TEM (HRTEM) on a JEM 2100F microscope. The Raman spectra were obtained by using WITec CRM 200 confocal Raman microscopy system with a laser wavelength of 488 nm and spot size of 0.5 mm. To calibrate the wavenumber, the Si peak at 520 cm 21 was used as a reference.
Electrochemical measurements. The electrochemical performance of MWNTs/ MoS x nanocomposites electrode was measured with a half-cell lithium ion battery (LIBs) configuration. The 2032 coin-type cells were assembled in an argon-filled glove-box with both of the moisture and oxygen level less than 1 ppm. The working www.nature.com/scientificreports SCIENTIFIC REPORTS | 3 : 2169 | DOI: 10.1038/srep02169 electrode material slurry were prepared by mixing MWNTs/MoS x , carbon black and poly(vinyldifluoride) (PVDF) at a weight ratio of 80510510, several drops of Nmethylpyrrolidone (NMP) solvent was added into the mixture to prepare the active materials slurry. The resulting slurry was then uniformly pasted onto Ni foam , with mass loading of 4 , 6 mg. Lithium sheet was used as anodes and 1 M LiPF 6 in a 1/1 (volume ratio) mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC) as electrolyte. CegardH 2400 was used as the separator of the battery. The cells were tested on a NEWARE multi-channel battery test system with galvanostatic charge and discharge in the voltage range between 0.01 and 3.0 V vs. Li/Li 1 at various current density at room temperature. The cyclic Voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were tested on an electrochemical workstation (VMP3, Bio-Logic).