WS2 nanosheets by liquid phase exfoliation with assistance of epigallocatechin gallate and study as an additive for high-performance lithium-sulfur batteries

Journal of Colloid and Interface Science 552 (2019) 554–562

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Regular Article

Preparation of MoS2/WS2 nanosheets by liquid phase exfoliation with assistance of epigallocatechin gallate and study as an additive for high-performance lithium-sulfur batteries Hang Zhao a,b, Hao Wu a, Jinhua Wu a,c, Jianlong Li a, Yujie Wang a,d, Yun Zhang a, Heng Liu a,⇑ a

Department of Advanced Energy Materials, College of Materials Science and Engineering, Sichuan University, Chengdu 610064, PR China International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, PR China c Department of Materials Engineering, Sichuan College of Architectural Technology, Deyang 618000, PR China d Research Institute of Natural Gas Technology, Petrochina Southwest Oil & Gas Field Company, Chengdu 610213, PR China b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 23 January 2019 Revised 22 May 2019 Accepted 24 May 2019 Available online 25 May 2019 Keywords: MoS2 WS2 Nanosheets Liquid phase exfoliation Free-standing electrode Lithium-sulfur battery

⇑ Corresponding author. E-mail address: [email protected] (H. Liu). https://doi.org/10.1016/j.jcis.2019.05.080 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

a b s t r a c t The two-dimensional transition metal dichalcogenides have wide application value in many fields. Herein, MoS2 and WS2 nanosheets are produced by liquid phase exfoliation method assisted by epigallocatechin gallate. The effects of epigallocatechin gallate concentration and stripping time are investigated by UV–vis. The morphology and structure characterization of exfoliated nanosheets are studied by XRD, Raman, SEM, HRTEM and AFM, the results showing that the nanosheets have few layers and the exfoliated solution is homogeneous dispersion and stable. The exfoliated nanosheets produced by 2 mg ml
1 epigallocatechin gallate solution for 12 h have five monolayers approximately, presenting a phase transformation from 2H to 1T structure of MoS2 nanosheets. Then, Graphene aerogel composites are prepared with the exfoliated MoS2/WS2 nanosheets, which loaded sulfur and employed as the cathode of Li-S batteries. TEM results reveal that MoS2/WS2 nanosheets are embedded in reduced graphene oxide, and sulfur is evenly distributed in the composites. The composites containing MoS2/WS2 nanosheets achieve outstanding electrochemical performance, during to the polysulfide adsorption capability of MoS2/WS2

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nanosheets and reducing the shuttle effect in Li-S batteries. It demonstrates a good application of exfoliated MoS2/WS2 nanosheets in Li-S batteries. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Two-dimensional (2D) transition-metal dichalchogenides (TMDs, MX2, where M = Mo, W, V, Nb, Ti, Zr; and X = S, Se, Te) have recently attracted much attention in catalysis [1], opto-electronics [2], sensing [3], energy storage [4,5] and biomedicine [6] due to their novel and unique physicochemical properties. The structure of TMDs consist of repeated layers with covalent bond of X-M-X single layer and weak van-der-Waals interaction between layers [7]. When transforming thin bulk to mono-layer structure, TMDs performs indirect-to-direct band gap transition due to thicknessinduced quantum confinement, which provides a high surface area to produce abundant surface-active sites and offer vast advanced applications in numerous fields [8–10]. Generally, single or few layer TMD nanosheets are prepared through bottom-up methods such as chemical vapor deposition (CVD) [11], or top-down methods such as micromechanical cleavage, ion-intercalation exfoliation and liquid phase exfoliation [12– 15]. CVD method can prepare homogeneous TMD nanosheets whereas it requires high temperature and high vacuum. Micromechanical cleavage can synthesize high-quality and highcrystallinity TMD nanosheets while the yield is low. Ion intercalation exfoliation is capable to prepare TMD nanosheets on a large scale, though the approach requiring strong intercalating agents (e.g. n-butyl lithium) is sensitive to environment and easily subjected to structural or electronic properties change [16]. Liquid phase exfoliation method with sonication is environmental friendly due to avoiding hazardous chemicals, providing an effective and potential approach to produce mass TMD nanosheets [17,18]. Lithium-sulfur (Li-S) batteries, as one of promising energy storage system for next generation batteries, demonstrate a high specific capacity of 1675 mAh g
1 and high theoretical energy density of 2600 Wh kg
1 [19,20]. However, one serious problem of Li-S batteries is the shuttle effect between cathode and anode resulting from the long-chain lithium polysulfide (LiPSs, Li2Sn, 4  n  8) produced during charge/discharge process, which reduces the utilization of sulfur and decreases cycle performance of Li-S batteries [21,22]. Recently, diverse strategies have been proposed to address the shuttle effect. Metal oxides and sulfides (Al2O3, MgO, V2O5, MoS2, WS2, CoS2, etc. [23–27]) equipped strong binding energy with polysulfide can accelerate conversion kinetics and alleviate the shuttle effect of LiPSs [28–31]. Fast and effective redox reactions of LiPSs can be carried out while LiPSs are stabilized. This approach improves reaction kinetics and thus promotes capacity retention, high rate and cycle performance of Li-S batteries [32]. Arava investigated the catalysis effect of MoS2 in terms of sulfur reaction chemistry for facilitating conversion of LiPSs [33]. Such previous studies supply clear evidence towards the catalytic effect of MoS2 on sulfur reaction chemistry. The large-exposed active surface of TMDs nanosheets can be effective to facilitate sulfur conversion in Li-S batteries, indicating that TMDs nanosheets are a promising approach to improve the performance of Li-S batteries [34,35]. Herein, plant polyphenols epigallocatechin gallate (EGCG) is employed as stripping agent and sonication is performed to exfoliate bulk MoS2 and WS2. A stable and homogeneous dispersion solution with few-layered MoS2/WS2 (XS2) nanosheets can be obtained. We investigate the effects of stripping agent concentra-

tion and ultrasonic time. Eventually, the prepared nanosheets are used as an additive in Li-S batteries to adsorb LiPSs. A composite consisting of graphene aerogel (GA), XS2 nanosheets and sulfur (GA/XS2-S) is synthesized and regarded as a cathode of Li-S batteries. The XS2 nanosheets act as an electrocatalysts which afford a chemical adsorption of LiPSs and facilitate the sulfur redox reactions, achieving an excellent performance of Li-S batteries. 2. Experimental section 2.1. Materials EGCG was purchased from Aladdin-reagent Ltd. Bulk MoS2, WS2 with a particle size of about 2 lm was purchased from Sigma Aldrich. All chemicals are analytical grade without any further purification. 2.2. Preparation of exfoliated XS2 nanosheets XS2 nanosheets were prepared by adding a certain amount of bulk XS2 powder (XS2, 10 mg ml
1) in the aqueous solution of EGCG (35 ml; EGCG, 0.01–2 mg ml
1) along with exposing to sonication (SCIENTZ-IID, 950 W, pulse of 8 s on and 5 s off) for 10– 720 min with water-cooled bath. Then, the mixture was centrifuged at a speed of 1500 rpm for 30 min to collect the top 3/4 solution and remove the bottom unexfoliated or thick XS2 flakes. The supernatant was further centrifuged at 10,000 rpm for 30 min. After that, the obtained sediment was collected and redistributed into equivalent water (26 ml) by sonication for 10 min, repeating the process three times. The finally prepared XS2 nanosheets dispersion was retained for further application. 2.3. Synthesis of GA/XS2-S composites GA/XS2 was synthesized through one-step hydrothermal method. 50 mg GO and the above 5 ml XS2 dispersion were dispersed in 15 ml deionized (DI) water with sonication for 2 h. Then, 0.1 ml ethylenediamine (EDA) was added into the mixture solution and dispersed evenly. After that, the mixture was heated by water bath at 95 °C for 24 h. The obtained hydrogel was washed by plenty DI water and freeze-dried. Finally, GA/XS2 aerogel was obtained for use. The prepared GA/XS2 aerogel was cut into slices of 2 mm thickness and punched into disks with a diameter of 12 mm. 70 wt% of sulfur in GA/XS2 disks weight ratio was dissolved in 5 ml CS2 reagent and dissolved completely. The GA/XS2 aerogel disks were immersed in CS2 solution and kept at ambient temperature for 12 h. Finally, the dried GA/XS2-S composites were heated at 155 °C for 2 h in airtight. The average sulfur mass loading is about 2–3 mg cm
2. 2.4. Synthesis of lithium polysulfide (Li2S6) Li2S6 solution was synthesized for lithium polysulfide adsorption tests. Typically, the stoichiometric amounts of S and Li2S (molar ratio is 5:1) were dissolved in DOL/DME (1:1 by volume) with magnetically stirring for 48 h at 55 °C in Ar atmosphere. The as-obtained Li2S6 solution was diluted before the adsorption test.

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2.5. Structural and morphological characterization To measure the distinction in concentration of the XS2 suspensions, UV–Vis spectra of the samples were measured on a UV3600 spectrometer, in which the original samples were diluted 20 times for testing. Zeta potential measurement of the XS2 suspension was taken on a Malvern Zetasizer Nano system. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed to examine the morphology of the exfoliated XS2 nanosheets by FEI Tecnai G2 F30 (accelerating voltage, 300 kV). The XS2 nanosheets were obtained for following study by freeze-drying from the exfoliated suspensions. The morphology characterization was investigated by a field-emission scanning electron microscope (FESEM) of Hitachi S4800 (accelerating voltage, 5 kV). Atomic force microscopy (AFM) was carried out on a Bruker dimension icon system to characterize XS2 nanosheets. X-ray diffraction (XRD) patterns were collected using a Bruker D8 Focus X-ray diffractometer (Cu-Ka, k = 1.5406 Å) with a scan rate of 10° min
1. Raman spectra were performed using a Renishaw InVia Reflex Micro-Raman system with a 532 nm excitation laser under ambient conditions at room temperature. Thermogravimetric analysis (TG) was performed by STA 449 F3 thermal analyzer from ambient temperature to 800 °C at a heating rate of 10 °C min
1 under flowing N2 atmosphere. X-ray photoelectron spectroscopy (XPS) analysis was carried out by a Kratos Axis Ultra DLD X-ray photoelectron spectrometer. 2.6. Electrochemical measurements The GA/XS2-S composites were employed as the cathode directly for assembling CR2032 coin cells in an argon-filled glove

box with lithium foil as the anode and a separator of Celgard 2400 membrane. The electrolyte was 1.0 M LiTFSI in DOL/DME (1:1 by volume) with 1 wt% LiNO3 additive. The amount of electrolyte per coin-type cell was about 50 lL. Galvanostatic charge– discharge (GCD) tests and cyclic voltammetry (CV) were carried out on Neware battery tester (BTS-5 V 10 mA) and Princeton Applied Research electrochemistry workstation in the voltage window of 1.5–2.8 V. The CV test was scanned at a rate of 0.1 mV s
1. The specific capacity was calculated based sulfur mass. Electrochemical impedance spectroscopy (EIS) measurements were further tested on Princeton Applied Research with a frequency range of 100 kHz to 0.01 Hz. 3. Results and discussion Schematic illustration of the synthesis process for E-XS2 (X = Mo, W) nanosheets are described in Fig. S1. XS2 nanosheets are obtained by a liquid phase exfoliation procedure of bulk XS2. EGCG is an abundant natural polyphenol which possesses plenty of phenolic hydroxyl groups (inset of Fig. S1). Utilizing strong coordination effect of the phenolic hydroxyl groups with Mo/W atoms in XS2, exfoliated XS2 nanosheets are functionalized with EGCG after sonication. The hydroxyl in EGCG can prevent layered XS2 nanosheets from reaggregating. The exfoliated XS2 solution remains stably at least for 15 days without aggregation (inset of Fig. S1). Ultraviolet–visible (UV–vis) spectra are carried out to evaluate the exfoliation efficiency. Fig. 1a and c demonstrates the UV–Vis spectra of the as-exfoliated MoS2 (Fig. 1a) and WS2 (Fig. 1c), which are sonicated for 2 h in different EGCG concentration solution. The progressively intense absorption characteristic peaks at 676 nm for MoS2 and ~635 nm for WS2 indicate that XS2 exfoliating yield

Fig. 1. (a, c) UV–Vis spectra of exfoliated MoS2 (a) and WS2 (c) produced at different EGCG concentration via 2 h sonication; (e, g) The intensity variation of UV–Vis absorption peak for exfoliated MoS2 (e, 676 nm) and WS2 (g, 635 nm) produced at different EGCG concentration via 2 h sonication; (b, d) UV–Vis spectra of exfoliated MoS2 (b) and WS2 (d) produced at different sonication time in 2 mg ml
1 EGCG solution; (f, h) The intensity variation of UV–Vis absorption peak for exfoliated MoS2 (f, 676 nm) and WS2 (h, 635 nm) produced at different sonication time in 2 mg ml
1 EGCG solution; (i, g) particle size distribution of MoS2 (i) and WS2 (g) exfoliated in 2 mg ml
1 EGCG solution for 12 h; (k, l) XRD patterns (k) and Raman spectra (l) of bulk MoS2 and MoS2 nanosheets exfoliated in 2 mg ml
1 EGCG solution for 12 h.

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grows with increasing EGCG concentration [36]. It achieves an equilibrium at the EGCG concentration of 2 mg ml
1 (Fig. 1e and g). In addition, Fig. 1b and d shows the UV–Vis spectra of exfoliated MoS2 and WS2, respectively, producing at different sonication time in 2 mg ml
1 EGCG solution. The variation trend of intensity for UV–Vis absorption characteristic peak of MoS2 (Fig. 1f) and WS2 (Fig. 1h) demonstrates the gradual increase to an equilibrium of the exfoliated yield with the sonication time increasing. Therefore, the exfoliated solution with higher exfoliation efficiency can be obtained under appropriate longer sonication time and higher EGCG concentration. We also compared the assistant exfoliation efficiency of other polyphenols, including Catechinic, tea polyphenols and bayberry tannin. These results (Fig. S2) show that EGCG delivers the best performance, which could ascribe to the more phenolic hydroxyl groups of EGCG. The particle size distribution of E-MoS2 solution (Fig. 1i) and E-WS2 solution (Fig. 1g) gained in 2 mg ml
1 EGCG solution for 12 h are characterized by laser particle analyzer. Fig. 1i and g shows that both the particle size of the exfoliated MoS2 and WS2 nanosheets are almost about 100–200 nm with a normal distribution. The zeta potentials (Fig. S3) of exfoliated XS2 nanosheets solution are
25.2 mV (for MoS2 suspension) and
18.5 mV (for WS2 suspension) preparing in 2 mg ml
1 EGCG solution for 12 h. The negative zeta potential demonstrates exfoliated XS2 nanosheets are functionalized with EGCG and attached plenty phenolic hydroxyl groups on the surface of XS2 nanosheets [37]. XRD patterns of bulk MoS2 and E-MoS2 nanosheets (2 mg ml
1 EGCG solution, 12 h) are presented in Fig. 1k. The results indicate the MoS2 nanosheets retain a hexagonal in-plane structure, but its main diffraction peaks intensity are significantly weaker than those of bulk MoS2, such as (0 0 2) and (1 0 3) peaks, suggesting the presence of thin-layer MoS2 nanosheets [38]. Raman spectroscopy provide further evidence to estimate the thickness of MoS2 nanosheets. In Fig. 1l, the Raman spectra of bulk MoS2 and MoS2 nanosheets present two vibration modes of A1g and E12g. The A1g peak links to the stretching of S atoms along the c axis,

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and the E12g peak is the in-plane Mo-S bending mode [39]. The E12g and A1g modes for bulk MoS2 are located at 376.79 and 402.82 cm
1, and the peak positions difference between E12g and A1g is 24.73 cm
1. For MoS2 nanosheets, the E12g and A1g modes are located at 376.68 and 401.41 cm
1, whereas the peak positions difference value is 26.03 cm
1. The obvious shift of A1g peak happens because of the weaker van-der-waals force between the sulfur atoms in the adjacent thin-layer MoS2 nanosheets. In addition, compared with bulk MoS2, the intensity of the peak for MoS2 nanosheets reduced significantly. All these properties suggest the existence of thin-layer MoS2 after the exfoliation of bulk MoS2 [40]. Fig. 2a–c and a0 –c0 show the SEM images with different magnification of bulk-MoS2, E-MoS2 and E-WS2 nanosheets produced in 2 mg ml
1 EGCG solution for 12 h sonication, respectively. Typically layered structure of bulk MoS2 with micron sized particle can be found. After 12 h exfoliated progress, the particle size is evidently much smaller than bulk MoS2. From the corresponding TEM images of Fig. 2d, d0 , e and e0 , the lateral dimension of the exfoliated nanosheets is about 100–200 nm, which is consistent with the results of laser particle analysis above. Fig. S5 demonstrates TEM images of MoS2 nanosheets produced in 2 mg ml
1 EGCG solution for 8 h with the lateral dimension of about 200  500 nm, which is slightly larger than the nanosheets sonicated for 12 h. The extra sonication time (4 h) is obviously beneficial to the smaller particle size. Morphologies of the exfoliated nanosheets are further investigated by TEM. Fig. 2f–i provides the HRTEM images of E-MoS2 nanosheets. Among them, Fig. 2g–i presents the magnified images of the selected areas in Fig. 2f. The inset of Fig. 2i shows clear diffraction spots of E-MoS2 nanosheets, displaying a typical 1T phase structure of MoS2 [41]. And Fig. 2j shows the HRTEM image of exfoliated MoS2 for 2 h sonication time, as well as, the inset is the corresponding diffraction spots, presenting a typical 2H hexagonal phase structure of MoS2 [42]. In 2H-MoS2, the Mo atoms locate at a trigonal prismatic environment, while in 1T-MoS2 they exist in an octahedral environment [43]. The arrangement of Mo atoms is depicted by the yellow dots in Fig. 2i and j for clarity.

Fig. 2. (a and a0 ) SEM images with different magnification of bulk-MoS2; SEM images with different magnification of E-MoS2 nanosheets (b and b0 ) and E-WS2 nanosheets (c and c0 ) produced in 2 mg ml
1 EGCG solution for 12 h sonication; TEM images of E-MoS2 nanosheets (d and d0 ) and E-WS2 nanosheets (e and e0 ) produced in 2 mg ml
1 EGCG solution for 12 h sonication; (f) HRTEM image of E-MoS2 nanosheets; (g-i) Magnified images of the selected areas in image f. Inset in Fig. 2i is the corresponding diffraction pattern; (j) HRTEM image of exfoliated MoS2 for 2 h sonication, and inset is the corresponding diffraction pattern; (k and l) AFM image of E-MoS2 nanosheets (k) and E-WS2 nanosheets (l) produced in 2 mg ml
1 EGCG solution for 12 h sonication. Inset presents height profiles along indicated lines.

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Therefore, it can be concluded that long time sonication lead to changes of atomic layout and phase structure in XS2. Additionally, AFM analysis is utilized to confirm the thickness of the exfoliated nanosheets. Fig. 2p and q shows the AFM images of the above E-MoS2 and E-WS2 nanosheets. The inset gives the height profiles along tagged lines, demonstrating that these nanosheets is approximately 3.1 nm, which is about five monolayers of MoS2 (0.62 nm per layer) [44]. Fig. S6 presents the AFM images for E-MoS2 nanosheets exfoliated for 8 h sonication in 2 mg ml
1 EGCG solution. The height of the nanosheets is about 3.9 nm and about 6–7 layers. Therefore, it is confirmed that the exfoliation cuts the bulk MoS2 down to few layers, and lengthening sonication time is an effective way to reduce the layer number of MoS2 nanosheets. Schematic illustration of the synthesis process for GA/XS2-S (X = Mo/W) composites is described in Fig. S7. Firstly, GA/XS2 aerogel is synthesized through hydrothermal method using XS2 nanosheets and GO with EDA as GO reductant. Afterwards, The GA/XS2 aerogel is immersed in sulfur-containing CS2 solution to obtain sulfur-loading composites GA/XS2-S. Fig. S8 just presents the optical images of GA/XS2 aerogel. And the GA/XS2-S can directly be used as electrodes for Li-S batteries. The morphologies of as-synthesized GA, GA/M, GA/W, GA-S, GA/M-S and GA/W-S are characterized by SEM. From the images in Fig. 3a–f and a0 –f0 , it can be seen that the addition of XS2 nanosheets or sulfur exert little influence on the morphology of GA. Therefore, it can be speculated that XS2 nanosheets and sulfur locate in the interspace between reduced graphene oxide (rGO)

layers. The special formed structure is consistent with the before experimental design. On account of that XS2 nanosheets in nanoscale with thin-layer structure, it is easy to trap them in the middle of rGO layers. As a result, the gap between them also contributes to loading more sulfur and wrapping tightly. In order to further investigate GA/M-S composite, we carried out TEM research. As showed in Fig. 3g and h, the MoS2 nanosheets with a size of about 100– 200 nm are evenly dispersed in rGO, and the nanosheets are densely wrapped and anchored by rGO. Fig. 3i and j exhibits the HRTEM images of GA/M-S. We can confirm that there is uniformly distributed MoS2, sulfur and rGO by measuring the lattice distance. The lattice distances of 0.232 nm and 0.279 nm correspond to (1 0 3) and (1 0 0) lattice planes of MoS2, respectively [45]. The 0.242 nm lattice spacing matches well with the (0 4 2) lattice plane of sulfur [46]. Fig. 3j shows a typical rGO structure with clear lattice fringes, which is composed of few layers, and the interlayer distance is estimated to be about 0.347 nm or 0.361 nm. The corresponding EDX elemental mappings of C, Mo and S (Fig. 3k–o) are coincident with the TEM structure. It can be determined that the material in rGO is MoS2 nanosheets, and sulfur is evenly distributed in the composite. The unique rGO-packaged MoS2/WS2 nanosheets composites are like a sandwich structure. It demonstrates the successful synthesis of GA/M-S composite. The crystalline phase composition and structure analysis of the samples are inspected by XRD. Fig. 4a exhibits the XRD patterns of GA, GA/M, GA/W, GA-S, GA/M-S and GA/W-S, respectively. It can been observed that XRD curve of GA shows a hunchback-like peak

Fig. 3. (a–f) and (a0 –f0 ) SEM images with different magnification of GA (a and a0 ), GA/M (b and b0 ), GA/W (c and c0 ), GA-S (d and d0 ), GA/M-S (e and e0 ) and GA/W-S (f and f0 ); (g and h) TEM images of GA/M-S; (i and j) HRTEM images of GA/M-S; (k-o) EDX elemental mapping images of GA/M-S.

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Fig. 4. (a) XRD patterns of GA, GA/M, GA/W, GA-S, GA/M-S and GA/W-S; (b–e) XPS spectra of GA/M-S: survey spectrum (b) and high-resolution spectra of C 1s (c), Mo 3d (d) and S 2p (e); (f) TG curve of GA/M-S.

located at around 24°. After adding MoS2/WS2 nanosheets, the characteristic peaks of MoS2/WS2 around 14° appear in the XRD patterns of GA/M and GA/W, and hold the hunchback-like peak of GA [47]. When sulfur is loaded in the above three samples, the corresponding XRD patterns exhibit the standard diffraction peaks of sulfur (JCPDS 71-0137). In addition, the peak intensity of GA/M-S and GA/W-S is much lower than that of GA-S. It can be derived that sulfur may enter the gap between the XS2 nanosheets and rGO, which reduces detectable crystallization of sulfur in the sample. Therefore, the structure composing of nanosheets and rGO is beneficial to load and wrap sulfur. XPS is used to further identify the bonding state and investigate the accurate surface composition of GA/M-S composite. Fig. 4b–e shows the XPS spectra of GA/M-S. The survey spectrum reveals the composite consists of C, Mo, S and O. In Fig. 4c, the high-resolution spectrum of C 1s could be divided into CAC and CAO bonds. The Mo 3d high-resolution spectrum in Fig. 4d presents the two characteristic peaks at 229.1 eV and 232.2 eV, respectively corresponding to 3d5/2 and 3d3/2 orbitals of Mo4+, respectively. The peaks at 226.9 eV and 227.9 eV belong to 2s orbitals of pure sulfur, proving the existence of sulfur in the composite. The S 2p high-resolution spectrum in Fig. 4e verifies the exhibition of 2p3/2 and 2p1/2 orbitals of S8 in 163.4 eV and 164.6 eV, respectively [47]. In addition, the two peaks in 161.9 eV and 163.3 eV are ascribed to 2p3/2 and 2p1/2 orbitals of S2- for MoS2. TG test is an effective approach to measure the contents of sulfur and XS2 nanosheets in the GA/XS2-S composites. The TG curve of GA/M-S is displayed in Fig. 4f which is tested from room temperature to 700 °C in air atmosphere. The weight loss at 100– 300 °C suggests the evaporation of sulfur [48]. When heating to 600 °C in air, the residual material is MoO3, which is derived from the oxidization of MoS2. The mass of S, MoS2, and rGO are calculated about 77.63%, 4.34% and 18.03%, respectively. The content of MoS2 in the GA/M composite is 19.40%. Analogously, For the TG result of GA/W-S from Fig. S9, the mass of S, WS2, and rGO are calculated about 76.30%, 5.35% and 18.35%, respectively, and the content of WS2 in the GA/W composite is 22.60%.

In order to evaluate the advantages of GA/XS2-S as an electrode for Li-S batteries, coin-cells are assembled. The cyclic voltammograms (CV) and galvanostatic charge/discharge tests are carried out at a potential range of 1.5–2.8 V. The first three cycles CV curves of GA/M-S are presented in Fig. 5a to analyze the electrochemical reaction mechanism. In the first cathodic scan, two peaks at 2.23 and 2.03 V represent the electrochemical reaction process from sulfur element to Li2S. The peaks at 2.23 and 2.03 V are attributed to the formation of long-chains lithium polysulfide (Li2Sn, 4  n  8) and further reduction to short-chain insoluble lithium sulfide (Li2S2/Li2S) [49]. The peak at 2.5 V in the anodic scan is related to the oxidation of Li2S2/Li2S to Li2S8/S8. After activation, in the second cycle, the cathodic peaks potential increases while anodic peak potential decreases. In addition, the curves show high stability and reversibility at the third cycle. To better understand the electrochemical performance of GA/X-S composites, galvanostatic discharge/charge and rate capability are carried out. Fig. 5b presents the rate performances of GA-S, GA/M-S and GA/W-S under current densities from 0.05 to 2 C. The initial specific capacities under a current density of 0.05 C are 909, 943 and 1140 mAh g
1, respectively. The reversible rate capacities of the GA/M-S and GA/W-S composites under current densities of 0.05, 0.1, 0.2, 0.5, 1, 2 C and even back to 0.1 C are higher than those of GA-S, of which GA/W-S presents the best performance. Fig. 5c depicts the corresponding initial charge/ discharge curves of GA/W-S at various rates from 0.05 to 2 C. Generally, the discharge products of Li2S2 and Li2S cannot be completely oxidized at a high rate within the limited time, leading to the gradual accumulation of the insulating discharge products on the electrode surface, blocking the conversion path of electrons and Li-ions. The first charge/discharge profiles of GA-S, GA/M-S and GA/W-S are showed in Fig. 5d. GA/M-S and GA/W-S demonstrate higher discharge plateaus and lower charging platforms than those of GA-S, implying that GA/M-S and GA/W-S deliver a weaker polarization than GA-S, and that GA/W-S probably exhibits the best performance.

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Fig. 5. Electrochemistry performance of GA-S, GA/M-S and GA/W-S: (a) CV curves for the first three cycles of GA/M-S composite recorded at 0.1 mV s
1; Rate capability (b) and the corresponding initial charge/discharge profiles (c) of GA/W-S at various rates from 0.05 to 2 C; (d) The first charge/discharge profiles of GA-S, GA/M-S and GA/W-S; (e) Long-term cycling performance and Coulombic efficiency of GA/W-S hybrid at 0.5 C after three cycles activation at 0.05 C; (f) Electrochemical impedance spectra (EIS) performance before cycling from 100 kHz to 10 MHz at room temperature; (g) Optical images of Li2S6 solutions before and after 12 h contact with the GA, GA/M and GA/W; (h) Optical images of GA/W-S and GA-S cells after 300 cycles at 0.5 C.

Cycling performance at 0.5 C is displayed in Fig. 5e, which further demonstrates the superiority of GA/M-S and GA/W-S. After four cycles activation at 0.05 C, the cells deliver specific initial capacities of 272.7, 461.6 and 678.8 mAh g
1 for GA-S, GA/M-S and GA/W-S, respectively. After 300 cycles, the corresponding reversible capacities retain 324.1, 390.5 and 444.1 mAh g
1, respectively. Actually, the capacity of GA/M-S or GA/W-S could be always higher than that of GA-S. The higher affinity between XS2 and polysulfide, which reduces the shuttle effect is supposed to be the contributive factor. In addition, the unique structure of embedded MoS2/WS2 nanosheets in rGO is also considered to be benefit for sulfur storage. The conductivity of fresh cells is evaluated by electrochemical impedance spectroscopy (EIS) measurement. As shown in Fig. 5f, in high frequency, the intercept of semi-circular loop with Zreal axis represents electrolyte resistance and ohmic resistance (Rs). The semi-circular loop in the mediate frequency ascribes to the charge transfer resistance (R1) and the interface contact resistance (R2). The shortly sharp-increased line at low frequency is explained by Warburg impedance (Zw) associating with the Li+ diffusion in electrode material [50]. The sum of the impedance values of R1 and R2 for GA-S, GA/M-S and GA/W-S

fitted by the equivalent circuit model of Fig. 5f inset are simulated to be 178.1 163.8 and 119.5 X, respectively. It is worth noting that the introduction of MoS2/WS2 nanosheets in GA does not increase but reduce the impedance value. Though MoS2 and WS2 are semiconductors, it may be due to the effect of MoS2/WS2 nanosheets in the microstructure of GA, providing a better conductive structure and a good contact surface for sulfur cathode. Polysulfide adsorption capability of GA, GA/M and GA/W can be clearly observed in Fig. 5g. 4 mg GA, GA/M and GA/W are added to 5 ml Li2S6 solution, respectively. After fully vibrating, the glass bottles are positioned in glove box for 12 h. It can be seen that the colors of GA/M and GA/W solution are apparently faded compared to the pristine solution, whereas the GA solution has the similar color with the pristine solution, indicating that the polysulfide adsorption capability of GA/M and GA/W is intense and effective than that of GA. To better understand the polysulfide adsorption capability in cells directly, as shown in Fig. 5h, we disassemble the cells of GA-S and GA/W-S after 300 cycles at 0.5C. The result shows that the membrane surface of GA/W-S cell is clean for the polysulfide adsorption of WS2 nanosheets, while the membrane surface of GA-S cell is covered with yellowish polysulfide. From the above,

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GA/M-S and GA/W-S deliver a remarkable cycling and rate performance for Li-S batteries compared to GA, due to the strong adsorption capability of MoS2/WS2 nanosheets for polysulfide in the composites to mitigate the shuttle effect, and the unique sandwich structure of rGO packing MoS2/WS2 nanosheets to trap sulfur beneficially.

4. Conclusion A liquid phase exfoliation method with EGCG assistance is utilized to product MoS2/WS2 nanosheets. The effects of EGCG concentration and ultrasonic time on the exfoliation efficiency are investigated. The results show a gradual increase to an equilibrium of the exfoliated yield with the concentration and sonication time increasing. The E-MoS2 and E-WS2 nanosheets produced in 2 mg ml
1 EGCG solution for 12 h have five monolayers approximately and present a uniform particle size distribution of 100–200 nm mostly. GA/XS2-S composites are synthesized via rGO trapping MoS2/WS2 nanosheets and further loading sulfur as a cathode of Li-S batteries. The GA/XS2-S composites perform an excellent electrochemical performance compared to GA-S owing to the polysulfide adsorption capability of MoS2/WS2 nanosheets in the composites which mitigates the shuttle effect in Li-S batteries. The preparation of XS2 nanosheets by liquid phase exfoliation delivers a valuable application in Li-S batteries.

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