Improving the Anode Performance of WS2 through a Self-Assembled

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Improving the Anode Performance of WS through Self-Assembled Double Carbon Coating Yichen Du, Xiaoshu Zhu, Ling Si, Yafei Li, Xiaosi Zhou, and Jianchun Bao J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 23 Jun 2015 Downloaded from http://pubs.acs.org on June 24, 2015

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The Journal of Physical Chemistry

Improving the Anode Performance of WS2 through Self-Assembled Double Carbon Coating Yichen Du,† Xiaoshu Zhu,‡ Ling Si,† Yafei Li,† Xiaosi Zhou,*,† and Jianchun Bao*,† †

Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China ‡

Center for Analysis and Testing, Nanjing Normal University, Nanjing 210023, P. R. China

Abstract: Tungsten disulfide, which possesses a well-defined layered structure, has been intensively studied as an anode material for lithium-ion batteries, but it usually suffers from poor cycling stability due to its large volume changes during lithium insertion and extraction processes. Herein, we develop a self-assembled double carbon coating to enhance the anode performance of WS2 via a self-assembly process between oleylamine-coated WS2 nanosheets and graphene oxide and subsequent pyrolysis treatment. When employed as an anode material for lithium-ion batteries, the as-prepared WS2@C/reduced graphene oxide (WS2@C/RGO) composite exhibits excellent cycling stability and rate capability when compared with WS2@C nanosheets. A reversible capacity of 486 mA h g−1 and around 90% capacity retention were obtained after 200 cycles at a current density of 0.5 A g−1. Even under 10 A g−1 a high reversible capacity of 126 mA h g−1 can be retained. The good electrochemical performance could be attributed to the external electronically conductive and flexible RGO coating in addition to the surface carbon layer and the uniform distribution of WS2 nanosheets. The self-assembled dual carbon coating strategy is facile yet effective, and it may be applied to other high-capacity anode materials with huge volume changes and poor electrical conductivities.

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Keywords: Duplicate carbon coating, self-assembly, anode, lithium-ion batteries, tungsten disulfide

1. Introduction Rechargeable lithium-ion batteries (LIBs) are crucial for portable electronics, electric vehicles, and grid-scale energy storage systems.1−6 Owing to the rapid development of such technological applications, LIBs with high energy density and long cycle life as well as low cost are in high demand.7−14 High-energy-density LIBs can be gained by employing electrode materials with higher specific capacities than current commercial counterparts.15−21 Compared to the graphite anodes, which possess a theoretical specific capacity of 372 mA h g−1,22 transition metal dichalcogenides (TMDs) have received considerable attention as potential high-capacity anode materials.23−29 In particular, both MoS2 and WS2 have become the hotspot materials for rechargeable batteries due to their high theoretical specific capacities and two-dimensional layered structure.30−39 Additionally, in comparison with MoS2, WS2 has advantage for highpower Li ion batteries because of its high electrical conductivity.40 However, the practical application of WS2 in LIBs is severely hampered by the poor cycling performance, similar to MoS2.41−43 The main reason is that WS2 undergoes large volume changes during the lithiation and delithiation processes that leads to pulverization of WS2 particles, loss of electrical contact, and continuous formation of a thick solid electrolyte interphase (SEI) on WS2 surfaces, resulting in a rapid capacity fading.32,44,45 To overcome this problem, tremendous efforts have been devoted to designing materials that can accommodate the large volume change, including the utilization of nanosized WS2 particles, tubes, or sheets, the fabrication of porous WS2, and decorating with carbon.41,46−53 Among the various strategies, WS2 nanoparticles encased by carbon layers are appealing in enhancing the cycling stability. On one hand, reducing the size of


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WS2 particles to nanoscale can alleviate the mechanical stress during Li insertion/extraction processes, thus leading to less fractures and particle pulverization. On the other hand, the carbon coating with moderate Li+ diffusion and electron transport kinetics also play a structural buffering role in mitigating the interior strain caused by the significant volume changes of WS2. Such a strategy often gives rise to increased capacity in initial cycles but does not imply to obtain long cycle life and high power density due to the inevitable aggregation of WS2 nanoparticles. It remains a great challenge to improve both the cycling stability and rate capability of WS2 anodes. Graphene is an ideal candidate to encapsulate active materials for Li storage owing to its excellent conductivity, large surface area, high flexibility, and prominent chemical stability.54−55 In recent years, various graphene-based composites have been synthesized as electrode materials.56−59 Compared to well-known carbon nanotubes, graphene sheets are helpful to achieve good dispersion of nanosized active materials (denoted as NAMs) and ensure a high electrical conductivity of the whole electrode, which is favorable for the aim of high rate capability. However, NAMs anchored on graphene surface may experience continual growth of SEI films if they are directly exposed to the electrolyte during cycling.60,61 Therefore, if a strategy could confine NAMs in individual carbon shells and then sandwich the carbon-coated NAMs between RGO sheets, the above two issues including unstable SEI film and particle aggregation would be addressed and thus remarkable anode performance would be expected. Herein, we design a double carbon coating for WS2 anode through a self-assembly process between oleylamine (OLA)-coated WS2 nanosheets and graphene oxide and subsequent pyrolysis treatment. The self-assembled duplicate carbon coating consists of OLA-derived surface carbon layer and external electronically conductive and flexible RGO shell, both of



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which can not only effectively avoid the direct contact between WS2 nanosheets and the electrolyte but also impede the aggregation and accommodate the large volume changes of WS2 nanosheets during cycling. When evaluated as an anode material for LIBs, the double carboncoated WS2 nanosheets exhibit superior lithium storage properties in terms of high capacity, stable cyclability, and excellent rate capability. Moreover, due to its versatility the duplicate carbon coating strategy may be extended to other high-capacity anode materials with large volume expansions and poor electrical conductivities.

2. Experimental Section 2.1. Synthesis of the WS2@C/RGO composite Oleylamine (OLA)-coated WS2 nanosheets were prepared via a slightly modified colloidal synthesis method.62 In a typical large-scale synthesis, 90 mL of OLA in a 250 mL three-neck flask was degassed for 1 h under vacuum at 60 oC, and then heated up to 320 oC under argon atmosphere. Meanwhile, 300 mg of WCl6 (0.75 mmol) and 1.8 mL of oleic acid were mixed in a tube and sonicated until complete dissolution. The solution color changed from dark blue to dark brown during the WCl6 dissolution process. After the solution was bubbled with nitrogen, 30 mL of OLA was added into the tube, which caused a second color change from dark brown to light yellow. Before injection, 1.44 mL of CS2 was quickly added, resulting in a temperature increase and a further color change to orange. The solution immediately became sticky and gradually solidified over time. The obtained precursor mixture was injected dropwise into the hot OLA solution within 30 min by using a syringe pump (KD Scientific, KDS 100). After injection, the heating mantle was removed to allow the reaction mixture to cool rapidly to room temperature. Afterwards, the black sample was collected and washed with hexane and isopropanol several times, and dried under vacuum at 70 oC overnight. The as-formed OLA-coated WS2 nanosheets


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were then dispersed in 50 mL of ethanol, followed by addition into 15 mL of 3.0 mg mL−1 GO/ethanol dispersion under sonication. Immediately, an obvious aggregation occurred. The resulting precipitate OLA-coated WS2/GO was collected by centrifugation, and dried under vacuum at 70 oC overnight. To obtain the WS2@C/RGO composite, the OLA-coated WS2/GO was annealed at 500 oC for 2 h under argon flow. For comparison, the WS2@C composite was prepared by the same procedures as for WS2@C/RGO except that GO was not added. 2.2. Materials Characterization X-ray diffraction (XRD) pattern was recorded on a Rigaku D/max 2500/PC diffractometer using Cu Kα radiation. Raman measurement was performed on a Labram HR800 with a laser wavelength of 514.5 nm. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observations were carried out on a JEOL JEM-2100F transmission electron microscope operated at 200 kV. Scanning transmission electron microscopy (STEM) meaurement as well as elemental mapping analyses were taken on the JEOL JEM-2100F transmission electron microscope equipped with a Thermo Fisher Scientific energy-dispersive Xray spectrometer. X-ray photoelectron spectroscopy (XPS) measurement was determined on an ESCALab250Xi electron spectrometer from VG Scientific using 300 W Al Kα radiation. Thermogravimetric analysis (TGA) was conducted on a NETZSCH STA 449 F3 under air flow with a heating rate of 10 oC min−1 from room temperature to 800 oC. Assuming complete combustion of carbon and conversion from WS2 to WO3, the contents of WS2 in the WS2@C nanosheets and WS2@C/RGO composite can be calculated according to the following equation:

2.3. Electrochemical Measurements



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Electrochemical experiments were performed by using CR2032 coin cells. To fabricate working electrodes, WS2@C/RGO or WS2@C was mixed with Super-P carbon black and poly(vinylidene fluoride) with a weight ratio of 80:10:10 in N-methyl-2-pyrrolidone using a mortar and pestle. The resulting homogeneous slurry was pasted onto pure Cu foil (99.9 %, Goodfellow) and then dried in a vacuum oven at 80 oC overnight. The mass loading of active material was typically 1.0−1.2 mg cm−2. The electrolyte for all tests was 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 v/v). Glass fibers (GF/D) from Whatman and pure lithium metal foil were employed as separators and counter electrode, respectively. The coin cells were assembled in an argon-filled glovebox (H2O, O2 < 0.1 ppm, MBraun, Germany). The charge and discharge measurements of the batteries were galvanostatically carried out on a Land CT2001A multichannel battery testing system in the fixed voltage range of 0.005−3 V vs Li+/Li at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted on a PARSTAT 4000 electrochemical workstation. CV was measured at a scan rate of 0.1 mV s−1 while EIS was investigated by applying a sine wave with amplitude of 10.0 mV over the frequency range from 100 kHz to 100 mHz.

3. Results and Discussion Figure 1 shows a schematic illustration of the synthesis process for the WS2@C/RGO composite. In a typical synthesis, OLA-coated WS2 nanosheets were fabricated using a colloidal synthesis method based on the reaction between WCl6 and CS2, as reported recently.62 The asprepared OLA-coated WS2 nanosheets (Supporting Information, Figure S1) were subjected to dispersion in ethanol and subsequently mixed with GO/ethanol dispersion under sonication, resulting in an obvious aggregation (Supporting Information, Figure S2). The resulting precipitate OLA-coated WS2/GO (Supporting Information, Figure S3) was collected and dried


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under vacuum, followed by annealing under argon atmosphere at 500 oC for 2 h, which yield a double carbon-coated hybrid material WS2@C/RGO. Additionally, OLA-coated WS2 nanosheets were directly annealed under the same conditions to achieve the WS2@C nanosheets. Detailed synthetic procedures can be found in the Experimental Section.

Figure 1. Schematic illustration of the formation procedures for the WS2@C/RGO composite. The X-ray diffraction (XRD) patterns for the as-prepared WS2@C nanosheets and WS2@C/RGO composite are shown in Figure 2a. The XRD peaks of the WS2@C nanosheets are in good agreement with hexagonal WS2 (JCPDS card No. 08-0237). No apparent peaks corresponding to carbon are observed in the XRD pattern, suggesting that the carbon in the sample is amorphous. In contrast to the WS2@C nanosheets, the XRD pattern of the WS2@C/RGO composite shows two new broad peaks centered at 2θ = 8.7o and 17.8o, which are attributed to the (002) diffraction of the layer-by-layer structure and the (101) diffraction for RGO sheets, respectively.63 From the 2θ degree of the (002) peak, the d spacing of RGO sheets is calculated to be about 1.02 nm, which is consistent with the HRTEM observation (see below), indicating that the WS2@C nanosheets are sandwiched by RGO sheets. In addition, Raman spectra show that two characteristic peaks indexed to RGO can be detected in the sample of WS2@C/RGO but not in the WS2@C nanosheets, further confirming the presence of RGO coating in WS2@C/RGO. The weight fractions of WS2 in the WS2@C nanosheets and



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WS2@C/RGO composite were determined by TGA (Supporting Information, Figure S4). Assuming complete oxidation of the carbon coating and further conversion from WS2 to WO3, based on the final weight remaining of the samples, the weight percentages of WS2 in the WS2@C nanosheets and WS2@C/RGO composite are ~83 wt % and ~72 wt %, respectively. XPS results show that the starting material graphene oxide is transformed into RGO after pyrolysis at 500 oC (Supporting Information, Figure S5). The XPS also reveals the presence of WS2 with a low surface oxidation in the composite. Furthermore, the porous structure of WS2@C/RGO is examined by nitrogen adsorption−desorption measurement. As shown in Figure S6a (Supporting Information), the nitrogen adsorption−desorption isotherms present as a type IV with an associated H3 type hysteresis loop, indicating a mesoporous characteristic of the composite. The pore size distribution (Supporting Information, Figure S6b) calculated by the Barrett−Joyner−Halenda (BJH) method confirms the existence of mesopores of 2−5 nm in size. Besides, such a composite shows a low Brunauer−Emmett−Teller (BET) specific surface area of 43.7 m2 g−1, suggesting that the WS2@C nanosheets are well encapsulated by RGO sheets.


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Figure 2. (a) XRD patterns of the WS2@C nanosheets and WS2@C/RGO composite and standard XRD pattern of WS2 (JCPDS card No. 08-0237). (b) Raman spectra of the WS2@C nanosheets and WS2@C/RGO composite. The morphology and structure of the WS2@C nanosheets and WS2@C/RGO composite were investigated by transmission electron microscopy (TEM). Figure 3a shows a typical TEM image of the WS2@C nanosheets, which present an interconnected round-shaped morphology with a mean diameter of around 150 nm. The selected area electron diffraction (SAED) pattern (Figure 3b) also reveals that the nanosheets have a hexagonal structure. The computed interlayer distances from the SAED pattern are 0.27 and 0.16 nm, which match well with the d spacing values for the (100) and (110) planes of hexagonal WS2, respectively. As shown in Figure S7, the elemental mapping images clearly illustrate a good combination between WS2 nanosheets



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and the OLA-derived carbon coating, indicating that the WS2 nanosheets are well-enwrapped within the carbon shells. In the high-resolution TEM (HRTEM) image (Figure 3c), crystalline planes of WS2 (002) and (100) with respective interlayer spacing of 0.62 and 0.27 nm can be clearly observed. Figure 3d shows a TEM image of the as-synthesized WS2@C/RGO composite. It can clearly be seen that the WS2@C nanosheets are well-dispersed between RGO sheets. The low agglomeration of WS2@C nanosheets in the composite (Figure 3d) as compared with the RGO-free WS2@C nanosheets (Figure 3a) suggests that the weak interaction is effective during the self-assembly process between OLA-coated WS2 nanosheets and GO, which leads to a uniform dispersion of the WS2@C nanosheets in RGO networks after pyrolysis, and must play an essential role in improving the electrochemical performance of the WS2@C/RGO composite. As displayed in Figure 3d inset, the SAED pattern of the WS2@C/RGO composite also shows a hexagonal structure. In the HRTEM images of the composite (Figure 3e,f), lattice planes of WS2 (100) and RGO (002) with respective interlayer distance of 0.27 and 1.02 nm can be clearly identified, which further confirm that the WS2@C nanosheets are well-sandwiched by graphene sheets, resulting in an interlayer-expanded RGO sheets. Thus both the surface carbon layer and the RGO shell build a double carbon coating for the encapsulated WS2 nanosheets. Moreover, the edge of carbon, tungsten, and sulfur EDS mappings in Figure S8b−d well-match with the result shown in the STEM image (Supporting Information, Figure S8a), suggesting that the WS2 nanosheets are homogeneously distributed throughout the composite. Such a duplicate carbon coating in combination with the uniform dispersion of WS2 nanosheets in the composite, probably give rise to high-performance lithium storage properties.


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Figure 3. (a−c) TEM image, SAED pattern, and HRTEM image of the WS2@C nanosheets. (d−f) TEM image, SAED pattern (inset), and HRTEM images of the WS2@C/RGO composite. Figure 4a shows the first three cyclic voltammetry (CV) curves of the WS2@C/RGO electrode from 0.005 to 3.0 V vs Li+/Li at a scan rate of 0.1 mV s−1. In the first cycle, two pronounced cathodic peaks at 0.69 and 0.01 V can be clearly observed, corresponding to the generation of solid electrolyte interphase (SEI) layer as well as the insertion of lithium into the self-assembled double carbon coating and the reduction of WS2 to form a lithium sulfide phase along with W nanoparticles, respectively.41 The first anodic scan displays a broad peak at 1.6 V and a strong peak at 2.3 V, which can be assigned to the delithiation process from the dual carbon coating and LixWS2 host, respectively, in accordance with previously reported characteristics of WS2 electrode.32 Negligible changes can be observed for the subsequent two cycles, suggesting good structural maintenance and lithium storage reversibility. The difference during the initial three cycles may be caused by the generation of SEI film and local structure



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rearrangement in the electrode to accommodate the stress formed during the charge/discharge processes.

Figure 4. (a) CV curves of the first three cycles of the WS2@C/RGO electrode. (b) Galvanostatic charge−discharge profiles of different cycles for the WS2@C/RGO electrode. (c) Cycling performances of the WS2@C and WS2@C/RGO electrodes. (d) Rate capability of the the WS2@C and WS2@C/RGO electrodes. Figure 4b displays the charge−discharge voltage profiles cycled at a current density of 0.5 A g−1 in the voltage range of 0.005−3 V vs Li+/Li. The charge and discharge profiles present relatively sloped plateaus, which is consistent with the broad peaks observed during CV scans. The initial charge and discharge capacities are 542 and 801 mA h g−1, respectively, based on the total mass of the WS2@C/RGO composite. The large initial discharge capacity of the composite is generally ascribed to the production of SEI layer on the surface of the electrode owing to electrolyte decomposition, similar to that reported in nanosized TMD-based anode materials.31−33


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Furthermore, the RGO coating with high surface area probably also contributes to this high discharge capacity.12,18,56 Despite a large irreversible capacity loss occurred in the first cycle, the reversibility of the capacity was greatly enhanced, with an average Coulombic efficiency of >99% for up to 200 cycles after the second cycle (Supporting Information, Figure S9). The initial irreversible capacity loss could be attributed to irreversible Li insertion into composite, in addition to the formation of SEI layer on the surface of the WS2@C/RGO electrode.44,52 Also, the WS2@C nanosheets were tested for comparison. As shown in Figure 4c, the WS2@C/RGO composite exhibits stable cycling performance and higher specific capacities than the WS2@C nanosheets. After 200 cycles under 0.5 A g−1, the WS2@C/RGO composite still retains a reversible capacity of 486 mA h g−1, namely, ~90% retention of the specific capacity in the second cycle, which is much higher than the theoretical capacity of graphite. However, the WS2@C nanosheets only deliver a specific capacity of 319 mA h g−1 after 200 cycles, suggesting that the self-assembled double carbon coating is indeed favorable for improving the anode performance of the WS2 nanosheets. Besides the achieved high specific capacity and superior cycling stability, the WS2@C/RGO electrode shows satisfactory rate capability as well. As displayed in Figure 4d, on increasing current densities from 0.5 to 1, 2, 5, and 10 A g−1, reversible capacities of 513, 402, 309, 200, and 126 mA h g−1 are achieved, respectively. Importantly, after the continuous cycling with increasing current densities, a specific capacity as high as 428 mA h g−1 could be recovered at a current density of 0.5 A g−1, suggesting an excellent lithium storage reversibility. It is worth mentioning that the WS2@C/RGO electrode exhibits much higher capacities than the WS2@C electrode under all used current densities, in accordance with the cycling performances (Figure 4c).



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Figure 5. (a) Geometric structures of OLA and GO. (c) Ab initio simulation demonstrating the most stable configuration and calculated adsorption energy of OLA with GO. The corresponding adsorption energy is shown in parentheses. The black, white, red, and blue spheres represent C, H, O, and N, respectively. To elucidate the self-assembly process between OLA-coated WS2 nanosheets and GO, we performed ab initio simulation using density functional theory to calculate the weak interaction between OLA and GO. Figure 5a,b shows the geometric structures of OLA and GO. Usually, heteroatoms with lone electron pairs, such as nitrogen, oxygen, or sulfur atoms are able to generate hydrogen bonding with hydroxyl and carboxylic groups of GO. For simplicity, we used the repeat unit of GO as the modeling molecule to calculate the adsorption energy to reveal the interaction between OLA and GO. This simulation can offer a qualitative explanation on the formation of robust double carbon coating. The result is demonstrated in Figure 5c. As for OLA, we can see that the nitrogen atom strongly binds with the hydrogen atom in the functional groups of GO. This very stable configuration represents a high adsorption energy of 3.17 eV, which is much higher than that between PVP and GO (1.58 eV).10 The strong adsorption affinity of OLA with GO can effectively bind the WS2 nanosheets within GO and thus consequently produce self-


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assembled duplicate carbon coating for WS2 nanosheets, bringing about a more stable cycling performance in comparison to the WS2@C nanosheets, as shown in Figure 4c. In order to study the structural stability during cycling, we carried out TEM characterization on the as-made WS2@C/RGO electrode after 200 cycles. As displayed in Figure S10a (Supporting Information), the hybrid nanostructure is almost maintained without apparent cracking. The structure is evidently robust enough to tolerate repeated Li uptake/release processes, resulting in good cycling stability as well as superior rate capability. Furthermore, the TEM image displays that some W nanoparticles are uniformly distributed in the RGO matrix, as verified by the SAED pattern and HRTEM image (Supporting Information, Figure S10b−d). This indicates that the first reaction between Li and WS2 to produce the Li2S phase together with W nanoparticles is not fully reversible, leading not only to the formation of W nanoparticles, but also to the initial capacity loss. The significantly enhanced electrochemical performance of the WS2@C/RGO composite in comparison with the WS2@C nanosheets might be related to the unique structural design. On one hand, the duplicate carbon coating serves as a buffer that plays a key role in accommodating the large volume changes and minimizing the contact area between WS2 and the electrolyte, facilitating the generation of stable SEI layer, which contributes to the excellent cycling performance. On the other hand, the dual carbon coating, which acts as the flexible and electronically conductive networks, affords uniform dispersion of the WS2 nanosheets and ensures a high electrical conductivity of the whole electrode, thus exhibiting significantly improved specific capacity, cycling performance, and rate capability. The increased SEI film stability of the WS2@C/RGO composite was verified by electrical impedance spectroscopy (EIS) measurements. Figure S11 (Supporting Information) compares



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the Nyquist plots for the WS2@C/RGO composite and WS2@C nanosheets. Obviously, the WS2@C/RGO electrode delivers a much lower SEI resistance Rf than that of the WS2@C electrode (62.04 vs 89.26 Ω) (Supporting Information, Table S1) based on the modified Randles equivalent circuit shown in the inset of Figure S11. This result suggests that the WS2@C/RGO electrode has a more stable SEI film and relatively faster electrochemical kinetics, giving rise to higher specific capacity and better rate capability as compared with the RGO-free WS2@C nanosheets.

4. Conclusions In summary, we have successfully prepared a WS2@C/RGO composite via a facile selfassembly approach, resulting in double carbon coating for WS2 nanosheets through OLA-derived surface carbon layer and external electronically conductive and flexible RGO shell. The key to its realization is the strong adsorption affinity between OLA-coated WS2 nanosheets and GO, which enables a uniform embedding of carbon-coated WS2 nanosheets into RGO sheets after pyrolysis. When employed as an anode material for LIBs, the WS2@C/RGO composite exhibits excellent lithium storage properties as compared to RGO-free WS2@C nanosheets, which is attributed to the external electronically conductive and elastic RGO networks as well as the surface carbon layer and the uniform distribution of WS2 nanosheets. Such findings show the usefulness of dual carbon coating on enhancing the cycling stability and rate capability of highcapacity anode materials with large volume expansions and poor electrical conductivities.

Supporting Information TEM image of OLA-coated WS2 nanosheets, photograph/TEM image of OLA-coated WS2/GO, TGA curves of WS2@C and WS2@C/RGO, XPS spectra of GO and WS2@C/RGO, nitrogen adsorption−desorption isotherms and pore-size distribution of WS2@C/RGO, elemental mapping


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images of WS2@C and WS2@C/RGO, Coulombic efficiency plot for the WS2@C/RGO electrode, TEM/HRTEM images and SAED pattern of the cycled WS2@C/RGO, and Nyquist plots of the WS2@C and WS2@C/RGO electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Authors *(X.Z.) E-mail: [email protected]. Phone/Fax: +86-25-85891027 *(J.B.) E-mail: [email protected]. Phone/Fax: +86-25-85891936 Notes The authors declare no competing financial interest.

Acknowledgements. This work was supported by the National Natural Science Foundation of China (Grant Nos. 21471081 and 21171096), the Natural Science Foundation of Jiangsu Province of China (BK20140915), the Scientific Research Foundation for Advanced Talents of Nanjing Normal University (2014103XGQ0073), the financial support from the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Program of Jiangsu Collaborative Innovation Center of Biomedical Functional Materials.

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Improving the Anode Performance of WS2 through a Self-Assembled

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