Hierarchical Nanocomposite of Hollow N-Doped Carbon Spheres

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Hierarchical Nanocomposite of Hollow N-doped Carbon Spheres Decorated with Ultrathin WS Nanosheets for High-Performance Lithium-Ion Battery Anode 2

Xiaohui Zeng, Zhengping Ding, Cheng Ma, Laidi Wu, Jiatu Liu, Libao Chen, Douglas G. Ivey, and Weifeng Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04770 • Publication Date (Web): 06 Jul 2016 Downloaded from http://pubs.acs.org on July 6, 2016

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ACS Applied Materials & Interfaces

Hierarchical Nanocomposite of Hollow N-doped Carbon Spheres Decorated with Ultrathin WS2 Nanosheets for High-Performance Lithium-Ion Battery Anode

Xiaohui Zeng1, Zhengping Ding1, Cheng Ma 1, Laidi Wu1, Jiatu Liu1, Libao Chen1, Douglas G. Ivey2, Weifeng Wei 1,*

1

State Key Laboratory of Powder Metallurgy, Central South University, Changsha

410083, China 2

Department of Chemical and Materials Engineering, University of Alberta,

Edmonton, Alberta T6G 1H9, Canada

*

To whom correspondence should be addressed: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT Hierarchical nanocomposite of ultrathin WS2 nanosheets uniformly attached on the surface of hollow nitrogen-doped carbon spheres (WS2@HNCSs) was successfully fabricated via a facile synthesis strategy. When evaluated as an anode material for LIBs, the hierarchical WS2@HNCSs exhibits a high specific capacity of 801.4 mA h g-1 at 0.1 A g-1, excellent rate capability (545.6 mA h g-1 at a high current density of 2 A g-1) and great cycling stability with a capacity retention of 95.8% after 150 cycles at 0.5 A g-1. The Li-ion storage properties of our WS2@HNCSs nanocomposite are much better than those of previously most reported WS 2-based anode materials. The impressive electrochemical performance is attributed to the robust nanostructure and the favorable synergistic effect between the ultrathin (3-5 layers) WS2 nanosheets and the highly conductive hollow N-doped carbon spheres. The hierarchical hybrid can simultaneously facilitate fast electron/ion transfer, effectively accommodate mechanical stress from cycling, restrain agglomeration and enable full utilization of the active materials. These characteristics make WS2@HNCSs a promising anode material for high-performance LIBs.

KEYWORDS: WS2, hierarchical structure, hollow conductive material, synergistic effect, lithium-ion storage

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1. INTRODUCTION Transition metal dichalcogenides (TMDs) MX2 (M = Mo, W, V, Ti; X = S, Se) with layered structures have attracted great interest for fundamental research and technological applications in recent years.1-4 In TMDs, the metals and chalcogens are bonded by strong covalent bonding within layers, while the individual layers are further stacked by weak van der Waals interactions.5-6 The unique physicochemical properties of TMDs open up the possibility for numerous applications in integrated circuits, field effect transistors, photodetectors, electrocatalysis and energy storage. 7-14 In particular, TMDs with graphene-like layered structures possess higher electrical conductivities (than those of corresponding metal oxides) and higher theoretical capacities (than those of carbon/graphite-based materials), giving them significant advantages as electrode materials in the field of reversible energy storage, especially for lithium-ion batteries (LIBs).15-18 Tungsten disulfide (WS2), a typical layered transition metal dichalcogenide, has received increasing attention in energy conversion and storage.19-21 Compared with the widely studied MoS 2 for Li+ storage, WS2 has a higher intrinsic electrical conductivity and has shown to be a promising anode candidate for Li-ion batteries.22 Additionally, the interlayer spacing of WS 2 (0.62 nm) is much larger than that of commercial graphite (0.34 nm), which significantly facilitates the reversible intercalation/deintercalation processes of Li ions. 15,

20-21

However, WS 2 still suffers

from rapid capacity fading and poor rate performance because of its large volume fluctuation and slow rate of charge transfer during charge/discharge.23-25 Moreover, due to strong interlayer π-π interaction, the inherent layer stacking and agglomeration in WS2 decrease the number of active sites, and consequently, restrict the sufficient utilization of active materials.19,

26

Therefore, seeking desired structure and

morphology of WS2 with enhanced conductivity is greatly demanded to improve the electrochemical performance. Fabricating nanosized WS2 of diverse geometries and incorporating WS 2 into conductive carbonaceous matrixes have been widely employed to address these 3



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challenges. Various nanostructures of WS 2, such as nanoflakes, ordered mesoporous, graphene-like nanosheets have been studied to attain large electrolyte-electrode interface and reduced ion diffuse pathway. 27-29 Especially, compositing pristine WS 2 with conductive carbonaceous matrixes like carbon nanotubes, graphene, carbon nanofibers and porous carbon is an efficient approach to buffer the volume expansion and to promote electron/ion transport and, thus, enhance the corresponding electrochemical performance. 5,

15, 21, 30-31

Among numerous alternative carbon

matrixes, hollow carbon materials have attracted tremendous interest on the basis of their high surface areas, high stabilities and enhanced kinetics. 32-36 The hollow structured carbon matrixes can not only boost Li+ transport by offering a large surface area and a short diffusion distance, but also restrain agglomeration of active particles and provide space for buffering vol ume change. 37-38 It has been suggested that the rational design of assembling layered TMDs with hollow carbon matrixes would significantly improve electrochemical properties. 39-40 For example, Yu et al. have prepared a nanosheets-on-box nanostructure composed of MoS2 nanosheets supported on hollow carbon nanoboxes, which shows enhanced lithium storage and electrocatalytic properties.41 However, the preparation of hierarchical nanocomposite by combining WS2 nanosheets with hollow carbon matrixes has not been investigated as anode materials for LIBs. Inspired by these considerations, herein we fabricate for the first time a hierarchical structure by directly decorating WS 2 nanosheets on hollow nitrogen-doped carbon spheres (WS2@HNCSs) through a surfactant-free hydrothermal way. In this structure design, the ultrathin WS2 nanosheets in the nanocomposite possess a high surface area and shorten the diffusion distances of ions and electrons. The N-doped carbon shells serve as three-dimensional carbon networks in the hybrid structure which not only offer a beneficial conductive environment, but also suppress the agglomeration of WS 2 nanosheets effectively. Furthermore, both the hollow structure of the carbon spheres and the ultrathin nanosheet structure of WS2 can significantly accommodate mechanical stress during long-term cycling. Benefiting from the highly stable structure and the favorable synergistic effect between WS 2 nanosheets and hollow 4


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carbon spheres, the unique hierarchical WS2@HNCSs nanocomposite yields high specific capacity, excellent rate capability and enhanced cycling stability.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Hollow N-dope d Carbon Spheres (HNCSs). Hollow N-doped carbon spheres were prepared by employing SiO2 nanospheres as the templates and dopamine as the carbon source. First, SiO2 nanospheres with uniform diameters (~200 nm) were produced by the Stöber method.42 In a typical synthesis process, 3 mL of ammonia aqueous solution (28 wt %) was added to 40 mL of an ethanol-water mixture (ethanol/water ratio = 7:1) with stirring for 30 minutes. Subsequently, 1.5 mL of tetraethyl orthosilicate (TEOS) was quickly added to the above solution. The mixture was vigorously stirred at room temperature for 24 h to obtain SiO 2 nanospheres. Then, 200 mg of SiO 2 nanospheres and 400 mg of dopamine (DA) were dispersed in a tris-buffer solution (100 mL, 10 mM, pH = 8.5) with stirring for 24 h. The as-obtained SiO2@PDA core@shell nanospheres were carbonized at 500 ℃ for 3 h under Ar flow. The silica cores were then removed by dispersing the powder in 2 M NaOH at 80 ℃

for 5 h. The HNCSs were harvested via several

rinse-centrifugation cycles with deionized water and ethanol and dried at 60 ℃ overnight. 2.2. Synthesis of WS2@HNCSs Nanocomposite. 100 mg of as-synthesized HNCSs were first ultrasonically dispersed in 30 mL deionized water to form a suspension, followed by the dissolution of 0.8923 g of WCl 6 and 1.6904 g of thioacetamide. Then the mixture was dispersed by sonication for 30 min, transferred into a 50 mL Teflon-lined autoclave and reacted at 240 ℃ for 24 h. During the hydrothermal process, thioacetamide acts as the source of sulfur as well as the reductant. The resulting product was collected by centrifugation and subsequently washed with deionized water and ethanol several times. For comparison purposes, we also prepared pristine WS2 nanosheets through the same hydrothermal reaction condition, but without the addition of HNCSs. 5



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2.3. Materials Characterization. Field emission scanning electron microscopy (FE-SEM, Nova NanoSEM 230) and field emission transmission electron microscopy (FE-TEM, FEI Titan G2 60-300 equipped with an aberration-corrector) were used to evaluate the microscopic features and elemental compositions of the products. Powder X-ray diffraction (XRD, Rigaku D/Max-2500 Diffractometer with Cu Kα radiation) was employed to identify the crystal structure of the samples. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB 250Xi X-ray photoelectron spectrometer. Curve fitting and background subtraction were achieved with Thermo Avantage Version 5.52 software. Raman spectra were collected on a LabRAM HR Raman microscope with a laser excitation wavelength of 532 nm. Thermogravimetric analysis (TGA, SDTQ600) was conducted under air flow with a temperature ramp rate of 10 ℃ min-1. Nitrogen adsorption measurements were performed using a Quantachrome instrument (Quabrasorb SI-3MP) at 77 K. 2.4. Electroche mical Measure ments. Electrochemical measurements were conducted using CR2016 coin-type half-cells assembled in an Ar-filled glove box. The working electrode was prepared by mixing active materials (WS2@HNCSs composite or bare WS 2 nanosheets), acetylene black and carboxymethyl cellulose (CMC) binder at a weight ratio of 8:1:1 in an ethanol-water mixture (ethanol/water ratio = 2:3) to form a slurry. Subsequently, the slurry was coated onto a Cu current collector and dried in a vacuum oven at 60 ℃ overnight to obtain as-prepared anodes. The whole weight of the nanocomposite material was used to calculate specific capacities. Lithium metal served as the counter electrode and a Celgard 2500 was used as the separator. The electrolyte was 1 M LiPF 6 in a mixture of ethylene carbonate and dimethyl carbonate (EC/DMC, 1:1 in volume). Cyclic voltammetry (CV) curves were collected at a scan rate of 0.1 mV s-1 in the voltage range of 0.005 to 3.0 V. The cells were galvanostatically charged and discharged using a battery testing system (LANHE CT2001A, Wuhan LAND Electronics Co., P. R. China) between 0.005 and 3.0 V at different current densities. Electrochemical impedance spectroscopy (EIS) measurements were carried out on a PARSTAT 4000 potentiostat with a 10 mV AC amplitude over the frequency range of 100 kHz to 1 Hz. 6


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3. RESULTS AND DISCUSSION 3.1. Material Synthesis and Characterization. Scheme 1 shows a schematic illustration of the synthesis process of WS 2@HNCSs nanocomposite. Briefly, these hierarchical WS2@HNCSs anode

materials were synthesized by a

facial

solution-phase self-polymerize, solid-phase carbonization and hydrothermal reaction route. SiO 2 nanospheres with uniform diameters of about 200 nm are the templates to prepare hollow nitrogen-doped carbon spheres (HNCSs). Dopamine, a nitrogenous biomolecule that can homogeneously self-polymerize on almost any surface in an alkaline solution, was chosen as the carbon source. 43 The morphology of intermediate products are provided in Fig S1. The obtained HNCSs can be evenly dispersed in DI water for subsequent attachment of WS 2 nanosheets. There exist abundant oxygen-containing functional groups on the surface of the HNCSs. Zeta potential result shows that the hollow N-doped carbon spheres are negatively charged (-45 mv) on their surfaces (Fig. S2). Due to intense hydrolysis of W 6+ in aqueous solution, huge amount of protons and (WO x)y- are produced, forming hydrogen-bond interactions between electronegative functional groups on HNCS and (WO x)y-. During the hydrothermal reaction, (WO x)y- on HNCS is supposed to react with sulfur source in solution, producing WS2 in site on the surface HNCS. Consequently, ultrathin WS 2 nanosheets are successfully decorated on the surface of hollow carbon spheres, resulting in the formation of hierarchical WS2@HNCSs nanocomposite.

Sche me 1 Schematic illustration of the synthesis procedure for WS2@HNCSs nanocomposite. 7



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Field emission scanning electron microscopy (FESEM) images (Fig. 1) clearly show the morphology of pristine WS2 sample and WS2@HNCSs nanocomposite with different magnifications. As shown in Fig. 1a and 1b, the as-prepared WS2 has a flower-like structure composed of aggregated nanosheets as a result of serious agglomeration and stacking during hydrothermal process. With the participation of HNCSs, the ultrathin WS 2 nanosheets with curled shape are closely attached to the surface of the carbon spheres (Fig. 1c and 1d). No WS 2 aggregation are observed, suggesting that the negatively charged HNCSs can be used as an efficient substrate for the nucleation and growth of WS 2 nanosheets. The uniform spherical morphology of WS 2@HNCSs nanocomposite proves the incorporation of HNCSs can effectively prevent the agglomeration of WS2 nanosheets, which is beneficial in terms of cycling stability. In addition, the BET analysis shows that the as-obtained WS2@HNCSs possesses a higher specific surface area of 36.5 m2 g-1 compared with pure WS 2 (21.4 m2 g-1; Fig. S3).

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Fig. 1 FESEM secondary electron (SE) images of the as-prepared (a, b) WS 2 nanosheets and (c, d) WS2@HNCSs nanocomposite. The morphology and microstructure of samples were inspected in detail by TEM observation. The TEM images (Fig. 2a and 2b) of pristine WS2 reveal the thick stacking structure of WS 2 nanosheets, imposing restrictions on the sufficient utilization of active materials. Fig. 2c and 2d confirm the hierarchical architecture and that the surface of the carbon spheres is uniformly decorated with ultrathin WS 2 nanosheets. A high-resolution TEM image (Fig. 2e) shows the intertwined WS 2 nanosheets on HNCSs typically consist of only three to five layers. The interlayer spacing in Fig. 2f is measured as 0.62 nm, corresponding to the (002) lattice spacing of hexagonal WS 2. Fig. 2f shows the WS 2 nanosheets in the nanocomposite with an interplanar spacing of 0.27 nm matching well with the (100) d spacing of 2H-WS 2. A series of diffraction rings in the corresponding selected area electron diffraction (SAED) pattern (Fig. 2g) confirm the polycrystalline structure of the hexagonal WS 2 9



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phase. Energy-dispersive X-ray spectroscopy (EDS) mapping analysis was also carried out, as shown in Fig. 3. A homogenous distribution of W, S, C and N on a typical nanosheets-on-sphere structure further confirms that the WS2 nanosheets are uniformly distributed on the surface of HNCSs.

Fig. 2 TEM BF images of (a, b) pure WS 2 and (c-f) WS2@HNCSs nanocomposite. SAED pattern (g) of WS2@HNCSs nanocomposite.

Fig. 3 High angle annular dark-field (HAADF) STEM image and corresponding EDS elemental maps for WS2@HNCSs composite. 10


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The phase structure of WS2 and WS2@HNCSs nanocomposite was investigated by powder X-ray diffraction (PXRD). As shown in Fig. 4a, the broad diffraction peaks at 2θ values of 14.2°, 28.7°, 32.1°, 43.0°, 48.2°, 56.9°and 68.0°correspond respectively to the (002), (004), (100), (006), (105), (110) and (114) planes of hexagonal 2H-WS 2 (JCPDS card No. 84-1398), demonstrating successful conversion of the tungsten precursor to layered WS 2 with no discernible impurities. The peak broadening and distinct asymmetry of (100) and (110) peaks suggest the formation of a nanosized WS 2 structure with ultrathin thickness.19, 22 Comparing with bare WS 2, the intensity of the (002) peak in WS2@HNCSs composite is weaker, indicating that the introduction of carbon spheres reduces the stacking of WS 2 layers.22 No diffraction peaks for carbon spheres were detectable in the XRD patterns of the WS 2@HNCSs, due to the amorphous nature of carbon spheres (Fig. S4). Thermogravimetric analysis (TGA) was carried out to determine the loading of WS 2 in WS 2@HNCSs (Fig. 4b). In the case of pure WS 2 nanosheets, a total weight loss of 6.5 wt% results from the oxidation of WS2 into WO3 in air, which is in accordance with the calculated value. 44 For the composite materials, the main weight loss between 300 ℃ and 550 ℃ represents the decomposition of HNCSs. The mass loading of WS 2 in the WS 2@HNCSs is estimated to be approximately 77.3 wt%.

Fig. 4 Powder XRD patterns (a) and TG curves (b) for WS2@HNCSs nanocomposite and pure WS2.

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The Raman spectra of the WS 2@HNCSs and WS 2 are shown in Fig. 5a. Two characteristic peaks for the WS 2 phase are located at 350 and 414 cm-1, corresponding to the in-plane vibrational mode (E 12g) and the out-of-plane vibrational mode (A 1g), respectively. Commonly, the relative intensity ratio of the two peaks is an indication of the number of WS 2 layers. The high IE/IA value for WS2@HNCSs composite (IE/IA = 1.51) compared with that for pure WS 2 sample (IE/IA = 1.05 ) demonstrates that the WS 2 nanosheets in the composite are ultrathin few layers.45 The D-band related to defects and the G-band associated with vibration of sp2 graphitic crystallites in the carbon matrix for the WS2@HNCSs composite were observed at 1350 and 1583 cm-1, respectively. To further characterize surface element composition and bonding state of the as-prepared WS2@HNCSs, X-ray photoelectron spectroscopy (XPS) was conducted. As shown in the XPS survey spectrum (Fig. 5b), W, S, C, N and O can be clearly detected. The binding energies for the W 4f7/2, 4f5/ 2 and 5p 3/2 peaks located at 33.1, 35.2 and 38.6 eV, respectively, confirm the presence of tetravalent W in WS 2 (Fig. 5c). Another weak peak centered at 36.4 eV is assigned to the W-O bond, indicating slight surface oxidation.21 The binding energies for the S 2p 3/2 and 2p 1/2 peaks located at 162.7 and 163.9 eV are attributed to S2- in WS 2 (Fig. 5d). The deconvolution of the C 1s peaks (Fig. 5e) confirms the presence of oxygen-containing functional groups. Peaks at 284.6, 285.1, 286.0 and 287.5 eV are ascribed to C-C, C-N, C-O and C=O, respectively.21 The N 1s peaks are fitted to three different components, corresponding to pyridinic (N-6), pyrrolic (N-5) and quaternary (N-Q) nitrogen (Fig. 5f). 46-48 These peaks are located at 398.7, 400.2 and 401.8 eV, respectively, which prove the N-doping in the carbon spheres. It has been shown that both N-Q nitrogen and N-6 nitrogen are sp2 hybridized, giving rise to an improvement in the electronic conductivity of the carbon networks. 5, 46 The high-resolution spectra of O 1s is also provided in Figure S5.

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Fig. 5 Raman spectra for WS 2@HNCSs nanocomposite and pure WS 2 (a). XPS spectra for the WS 2@HNCSs: Survey spectrum (b) and high-resolution spectra of W 4f 5p (c), S 2p (d), C 1s (e) and N 1s (f).

3.2.

Electroche mical

Performance.

The

electrochemistry

properties

of

WS 2@HNCSs nanocomposite as an anode material for LIBs were investigated using a two-electrode cell with Li metal as the counter electrode. Figure 6a shows the initial 13



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three CVs curves of the WS 2@HNCSs anode. In the first cathodic process, two reduction peaks at 1.7 and 1.6 V are assigned to the formation of Li xWS 2 during lithium insertion into WS2. 21, 49 The following strong reduction peak at 0.4 V can be ascribed to conversion reaction of Li with WS 2, accompanied by the decomposition of electrolyte.15 From the subsequent cycles, the reduction peak at 0.4 V disappears, while the reduction peaks at 1.7 V and 1.6 V shift to 2.1 V and 1.4 V, respectively, implying improved reversibility with cycling. During anodic scans, two oxidation peaks at around 1.8 V and 2.3 V are observed, corresponding with the lithium extraction processes from Li xWS 2 host. 15, 22, 27, 49 CV curves of pure WS2 are shown in Fig. S6a. Both WS 2@HNCSs nanocomposite and pure WS 2 display typical CV characteristics of WS 2 anodes. However, the redox peaks from the WS2@HNCSs electrode show obviously higher specific current than that of pure WS 2, indicating enhanced electrochemical reactivity and electrical conductivity of the hierarchical nanocomposite. The typical galvanostatic charge/discharge voltage profiles of WS2@HNCSs nanocomposite are shown in Fig. 6b. The plateaus in the voltage profiles represent the very same electrochemical processes found in CV curves. The hybrid anode delivers an initial discharge capacity of 734.8 mA h g-1 and a subsequent charge capacity of 559.3 mA h g-1 with a first-cycle Coulombic efficiency (CE) of 76.1%. The irreversible capacity loss may be due to the decomposition of the electrolyte and the formation of a solid-electrolyte interface layer during the first cycle. The Coulombic efficiency of the 2nd cycle attains 96.5 %, with discharge and charge capacities of 659.3 mA h g-1 and 636.1 mA h g-1. In the subsequent cycles, WS 2@HNCSs electrode exhibits stable cycling performance, and a high reversible capacity (677.1 mA h g-1) is maintained after 100 cycles. Comparatively, the charge/discharge voltage profiles of pure WS2 sample are also tested (Fig. S6b), which show sharply capacity loss upon prolonged cycling. It can be seen that the hierarchical nanocomposite possesses superior Li + insertion and extraction capability and electrochemical reversibility to pure WS2 nanosheets. 14


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Fig. 6 (a) Cyclic voltammograms for WS2@HNCSs over a voltage range of 0.005-3.0 V at a scan rate of 0.1 mV/s. (b) Charge/discharge voltage profiles of WS 2@HNCSs at a current density of 0.5 A g -1. (c) Rate capabilities of WS 2@HNCSs and pristine WS 2 at various current densities ranging from 0.1 to 2 A g-1. (d) Galvanostatic cycling performance of WS 2@HNCSs and pristine WS2 and Coulombic efficiency of WS2@HNCSs at a current density of 0.5 A g-1. To evaluate the rate capability, the WS2@HNCSs and pristine WS2 were cycled at various current densities ranging from 0.1 to 2 A g -1 (Fig. 6c). The WS2@HNCSs attains a high discharge capacities of 801.4 mA h g-1 at 0.1 A g-1. The capacity decreases gradually to 545.6 mA h g-1 at 2 A g-1. Remarkably, after 50 cycles at different current densities, a high capacity of about 834.6 mA h g -1 is achieved and remains stable without fading for up to 120 cycles at 0.1 A g-1. In contrast, pure WS 2 anode exhibits low capacities and poor rate performance. As a result of large volume change and strong aggregation, the capacity of pure WS 2 cannot recover to its initial level and fades rapidly with cycling after high rate testing. The electrochemical performance in terms of rate stability and cycling stability for HNCSs is shown in Fig. S7. The whole weight of the nanocomposite material was used to calculate specific 15



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capacities. To analyze the contribution of WS2 (77.3 wt %) and HNCSs (22.7 wt %) to the overall capacity, the normalized capacity of WS 2 are presented in Table S1. The normalized capacity of WS2 in the composite anode is markedly higher than theoretical specific capacity (433 mA h g-1). This result clearly demonstrates that the WS 2 in WS2@HNCSs composite has been effectively utilized and its electrochemical activity has been significantly improved, owing to its fast reaction kinetics, better stability and large contact area between the electrode material and electrolyte. The high normalized capacity of WS 2 is resulted from additional lithium storage sites, as reported previously.28, 30, 50-51 The discharge-charge cycling performances of WS 2@HNCSs nanocomposite and WS 2 nanosheets are presented in Fig. 6d. Notably, as-prepared WS2@HNCSs shows high specific capacity and most importantly enhanced cycling stability compared with bare WS2. After 150 cycles, a high discharge capacity of 631.6 mA h g -1 is maintained and the corresponding capacity retention measured after the first cycle is as high as 95.8%. For pure WS 2 sample, almost 70% decline of specific capacities is observed, suggesting serious structural collapse. The SEM images of WS 2 after cycling for 150 cycles have been provided as Figure S8. In addition, the Columbic efficiency of WS 2@HNCSs electrode is increased to above 98% after initial several cycles, giving evidence of its excellent electrochemical reversibility. During initial cycles, the enhancement of capacity can be ascribed to the growth of a gel-like polymeric layer and electrochemical activation of the composite electrode, which is frequently found in other metal sulfide based electrode materials. 5,

49

Moreover, WS2@HNCSs

nanocomposite can be cycled stably with a large capacity at 0.1 A g-1 and 1 A g-1 (Fig. S9). The reversible capacity of 753.2 and 537.1 mA h g-1 remains after 100 cycles at 0.1 A g-1 and 1 A g-1, respectively. The SEM images and TEM images of WS 2@HNCSs electrode after cycling at 0.5 A g-1 for 150 cycles are shown in Fig. S10 and Fig. S11, respectively. The morphologies of WS 2@HNCSs electrode after cycling were well maintained without obvious cracking, demonstrating that the robust hierarchical construction has excellent structural stability and can accommodate stress from cycling. 16


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Fig. 7 Nyquist plots for WS 2@HNCSs nanocomposite and WS2 nanosheets, and applied equivalent circuit model (inset). Electrochemical impedance spectroscopy (EIS) measurements were conducted to further understand the excellent electrochemical performance of WS 2@HNCSs composite. The Nyquist plots of the WS 2@HNCSs and WS2 electrodes in Fig. 7 both display a compressed semicircle in the medium frequency region and an inclined straight line in the low frequency region. Obviously, the charge-transfer resistance value (Rct) of WS 2@HNCSs (59.64 Ω) is much lower than that of pure WS 2 (183.2 Ω), which was obtained by fitting data according to the equivalent circuit model shown in the inset of Fig. 7. The reduced Rct value indicates that the WS2@HNCSs can remarkably enhance electron transport resulting from the close contact between ultrathin WS 2 nanosheets and conductive hollow carbon spheres, resulting in an improved electrochemical performance. It is deserved to be mentioned that the Li-ion storage properties of our hierarchical WS 2@HNCSs nanocomposite are much better than those of previously most reported WS 2-based anode materials (Table S2). The impressive electrochemical performance of WS2@HNCSs can be assigned to the rational design of the hierarchical nanostructure. First, N-doping can significantly increase the electrical conductivity of 17



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the three-dimensional carbon networks. Second, for the uniform composite structure, the HNCSs effectively restrain the agglomeration of WS 2 nanosheets, thus ensuring full utilization of active materials and maintaining the large capacity during charge-discharge. Last but not least, both the hollow structure of carbon spheres and the ultrathin nanosheet structure of WS 2 can effectively accommodate the mechanical stress caused by the large volume change and prevent the crumbling of electrode material during continuous cycling.

4. CONCLUSIONS In summary, a novel hybrid nanostructure of WS 2@HNCSs has been successfully synthesized through a facile synthesis strategy. The as-obtained WS2@HNCSs displays a hierarchical architecture with ultrathin WS 2 nanosheets firmly attached on the surface of hollow nitrogen-doped carbon spheres. When evaluated as an anode material for LIBs, the WS 2@HNCSs exhibits high specific capacity, excellent rate capability and enhanced cycling stability. The enhanced electrochemical performance can be attributed to the robust composite structure and the synergistic effect between the ultrathin WS 2 nanosheets with high surface area and the highly conductive hollow N-doped carbon spheres. The distinctive structural characteristics of WS 2@HNCSs nanocomposite make it a promising anode material for next-generation high performance LIBs and this efficient synthesis approach may also be applied for other TMD materials.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Recruitment Program of Global Youth Experts, the National Natural Science Foundation of China (51304248), the Program for New Century Excellent Talents in University (NCET-11-0525), the Doctoral Fund of the Ministry of Education of China (20130162110002), the Program for Shenghua Overseas Talents from Central South University and the State Key Laboratory of Powder Metallurgy at Central South University.

ASSOCIATED CONTENT Supporting Information The morphology of intermediate products, Zeta potential result of hollow N-doped carbon spheres (HNCSs), N2 adsorption-desportion isotherms and pore-size distribution of pure WS 2 and WS2@HNCSs nanocomposite, XRD pattern of HNCSs, XPS high-resolution spectra of O 1s, CV curves and charge/discharge voltage profiles of pure WS 2, rate capability and cycle performance of HNCSs, SEM images of WS 2 after cycling, cycle performance of WS 2@HNCSs composite at 0.1 A g-1 and 1 A g-1, SEM images of WS 2@HNCSs composite before cycling and after cycling for 150 cycles, TEM images, SAED pattern and EDS elemental maps of WS2@HNCSs electrode material after cycling for 150 cycles, table of normalized capacity of WS 2 nanosheets at different current densities, and list of electrochemical properties of WS 2 based anodes.

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Table of Contents Graphic

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Hierarchical Nanocomposite of Hollow N-Doped Carbon Spheres

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