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Monolayer MoS2-Graphene Hybrid Aerogels with Controllable Porosity for Lithium-Ion Batteries with High Reversible Capacity Lianfu Jiang, Binghui Lin, Xiaoming Li, Xiufeng Song, Hui Xia, Liang Li, and Haibo Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10692 • Publication Date (Web): 13 Jan 2016 Downloaded from http://pubs.acs.org on January 19, 2016

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ACS Applied Materials & Interfaces

Monolayer MoS2-Graphene Hybrid Aerogels with Controllable Porosity for Lithium-Ion Batteries with High Reversible Capacity Lianfu Jiang,†§ Binghui Lin,§ Xiaoming Li,§ Xiufeng Song,§ Hui Xia,§* Liang Li,┴* Haibo Zeng§* †1

State Key Laboratory of Mechanics and Control of Mechanical Structures & College of Materials Science and Technology, Nanjing University of Aeronautics & Astronautics, Nanjing 210016, China §

Institute of Optoelectronics & Nanomaterials, Jiangsu Key Laboratory of Advanced Micro & Nano Materials and Technology, College of Material Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China ┴

College of Physics, Optoelectronics and Energy, Center of Energy Conversion Materials & Physics, Jiangsu Key Laboratory of Thin Films, Soochow University, Suzhou 215006, China *Correspondence and requests for materials should be addressed to ([email protected]), H. X ([email protected]) and L. L ([email protected])

TOC:

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ACS Paragon Plus Environment

H.B.Z.


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ABSTRACT: Monolayer MoS2 nanosheets (NSs) are promising anode materials for lithium-ion batteries because all redox reactions take place at the surface without lithium ion diffusion limit. However, the expanded bandgap of monolayer MoS2 NSs (~1.8 eV) compared to their bulk counterparts (~1.2 eV) and restacking tendency due to the van der Waals forces result in poor electron transfer and loss of the structure advantage. Here, a facile approach is developed to fabricate the MoS2-graphene aerogels comprising controlled three-dimensional (3D) porous architectures constructed by interconnected monolayer MoS2-graphene hybrid NSs. The robust 3D architectures combining with the monolayer feature of the hybrid NSs not only prevent the MoS2 and graphene NSs from restacking, but also enable fast electrode kinetics due to the surface reaction mechanism and highly conductive graphene matrix. As a consequence, the 3D porous monolayer MoS2-graphene composite aerogels exhibit a large reversible capacity up to 1200 mAh g−1 as well as outstanding cycling stability and rate performance, making them promising as advanced anode materials for lithium-ion batteries. KEYWORDS: Monolayer MoS2; graphene; aerogels; Porosity; lithium-ion batteries

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INTRODUCTION In the past decade, lithium-ion batteries have monopolized portable electric device markets such as mobile phones and laptop computers due to their superior energy storage performance compared with other rechargeable battery technologies. Most recently, lithium-ion batteries have attracted growing attention as power supplies

for

electric

vehicles

and

hybrid

electric

vehicles.

However,

commercialization of these batteries in the automotive industries requires further improvement in energy density and power density. Therefore, the development of new electrode materials which can store and deliver more energy holds the key for the next-generation high performance lithium-ion batteries. Graphite is the most widely used anode material for commercial lithium-ion batteries because of its flat potential profile, high columbic efficiency, and good cycling stability. However, the relatively low theoretical capacity (372 mAh g-1) greatly limits the energy density of lithium-ion batteries.1-2 Tremendous efforts have been devoted to searching for alternative anode materials with larger capacity. After the pioneering work by Tarascon and coworkers who revealed that the transition metal oxides could be high capacity anode materials due to the conversion reaction, many researchers have investigated various conversion reaction compounds, such as metal nitrides, sulfides, fluorides, and oxides.3-4 Recently, two-dimensional layered transition-metal dichalcogenides (LTMDs) with analogous structures to graphite, such as MoS2, MoSe2, WS2 and WSe2, have been reported as potential anode materials for lithium-ion batteries.5-10 Among LTMDs, MoS2, possessing a typical layered structure 3



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consisting of S–Mo–S layers separated by Van der Waals interactions, received great attention due to its large capacity. Monolayer or few-layer MoS2 NSs have been reported to facilitate fast lithium ion intercalation/deintercalation process, resulting in an increase in the reversible capacity of MoS2.11-15 Especially for the monolayer MoS2 NSs, the monolayer two-dimensional structure could maximize the surface area for fast charge storage reaction without lithium ion diffusion limit. However, the electrochemical performance of few-layer or monolayer MoS2 NSs is still impeded by their inherent limitations.16-17 Despite the large capacity, the exfoliated MoS2 NSs often suffer from poor cycling stability and rate capability, which are attributed to the poor electronic conductivity of MoS2, large volume change, and restacking of MoS2 NSs during the cycling. To address these issues, many strategies have been developed to improve the electrochemical performance of MoS2 NSs by using carbonaceous materials as the conductive matrix. Graphene has established itself as the most promising matrix to construct MoS2-graphene composites for improving the electrochemical performance of MoS2 due to its high electrical conductivity, good flexibility, and high chemical stability. The large and flexible graphene sheet could act as a scaffold to facilitate the selective growth of MoS2 sheets, while promoting the dispersion of MoS2 over the graphene surface.18-19 The improved electrochemical performance of the few-layer MoS2-graphene composites as anodes for lithium-ion batteries has been demonstrated in previous works.13,14 The structural and morphological compatibility between monolayer MoS2 NSs and graphene NSs could enable great integration and 4


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synergistic interaction, resulting in better electrochemical performance than the sum of individual components. However, the monolayer MoS2-graphene heterostructures were rarely reported due to the restacking tendency of monolayer MoS2 NSs during the synthesis process. The 2D hybrid NSs structure is still not ideal for the electrode design because these NSs tend to self-aggregate due to the high surface energy, reducing the contact area between electrode and electrolyte during the cycling.20-21 Constructing 3D architectures using the 2D materials as building blocks has been proved to be an effective approach to solve this problem. Gong et al. reported 3D architectures by employing the 2D NSs of MoS2 and graphene as co-building blocks via their controllable assembly.5 However, good interaction between graphene and MoS2 NSs was not achieved in this composite so that the MoS2 NSs were only embedded in the 3D graphene structure and started to restack heavily as the MoS2 content increased in the composite. Nevertheless, the 3D MoS2-graphene composite exhibited improved electrochemical performance compared to the bulk MoS2 powder and exfoliated MoS2 NSs. To suppress the restacking or aggregation for both MoS2 and graphene NSs, in this work, we developed a facile strategy to construct 3D porous architectures by using monolayer MoS2-graphene heterostructures as building blocks. Due to the analogous microstructure and morphology between monolayer MoS2 and graphene NSs, the MoS2-graphene heterostructures would exhibit maximal structural compatibility and strong interaction between MoS2 and graphene NSs.5,

13-14, 19

These monolayer

MoS2-graphene hybrid NSs are interconnected with each other, forming the 5



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MoS2-grpahene composite aerogels with a 3D porous architecture. This unique 3D architecture is effective to suppress the restacking and aggregation of MoS2 and graphene NSs. Even when the mass ratio between MoS2 and graphene was increased to 9:1, the NSs morphology and the 3D porous architecture were retained without obvious aggregation. Apart from the excellent structural stability, this 3D porous architecture possesses several advantages as anodes for lithium-ion batteries, which include (a) fast redox reactions without lithium ion intercalation/deintercation limitation, (b) fast electron transfer through the electrode due to the highly conductive graphene matrix, and (c) large electrode/electrolyte contact area due to the porous structure. Therefore, the monolayer MoS2-graphene hybrid aerogels exhibit a high reversible capacity up to1200 mAh g-1 at a current density of 100 mA g-1 as well as excellent cycling stability and rate capability.

EXPERIMENTAL SECTION Synthesis of 3D Monolayer MoS2-Graphene Hybrid Aerogels. Monolayer MoS2 NSs were synthesized by a modified Li-intercalation method as reported in our previous work.9 Graphene oxide (GO) NSs were synthesized from natural graphite by a modified Hummers method. In a typical synthesis of the 3D porous monolayer MoS2-graphene hybrid NSs aerogels, 40 mg of monolayer MoS2 NS powder was dispersed in 10 mL of IPA/water (45 vol%) mixed solvent in a 50 mL flask and sonicated for 30 min. After that, a proper amount of GO NSs suspension (2 mg mL-1) was added into the above solution and sonicated for another 30 min. The well mixed 6


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MoS2 and graphene NS dispersion was transferred into a Telfon-lined autoclave and hydrothermally treated at 180 °C for 24 h. The as-prepared sample was chemically reduced by N2H4·H2O at 90 oC for 12 h to get MoS2-graphene composite. After cooling to room temperature, the sample was freeze-dried overnight to obtain the final products. In order to obtain MoS2-graphene composites with different MoS2 contents, different amounts of GO suspension were used for the precursors during the synthesis. Characterization. The structure and morphology of different samples were characterized by X-ray diffraction (XRD, Bruker D8 Advance diffractometer), Raman spectroscopy (Renishaw), field-emission scanning electron microscopy (FESEM, Hitach, SU8010), atomic force microscopy (AFM, Bruker Multimode 8 Atomic Force Microscope), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM, Tecnai G2 F30), and X-ray photoelectron spectroscopy (XPS, PHI QuanteraⅡspectrometer). Nitrogen sorption isotherms and BET surface area were measured at 77 K with a quantachrome NOVA analyzer (USA). Thermogravimetric analysis (TGA) was carried out with a TA Q6000 with a heating rate of 20 °C min−1 in flowing air. Electrochemical Performance Test. Lab-made Swagelok cells were used for the electrochemical measurements. The working electrodes were prepared by casting a slurry of 80 wt% active material, 10 wt% super P and 10 wt% polyvinylidenefluoride in N-methyl-2-pyrrolidone on copper foils. The prepared electrodes were then dried at 120oC in vacuum for 12 h and pressed at 200 kg cm-2. The cells were assembled in a glove box, using Li foil as both counter and reference electrodes, Celgard-2400 7



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separator, and 1.0 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 by volume) as the electrolyte. Galvanostatic charge and discharge measurements were carried out between 0.01 and 3 V vs. Li/Li+ at variuos current densities using a LAND 2001A battery tester. Cyclic voltammetry (CV) measurements were carried out on an electrochemical workstation (CHI660D) in the potential range between 0.01 and 3 V vs. Li/Li+ at a scan rate of 0.5 mV s-1. Electrochemical impedance spectroscopy (EIS) was carried out on an electrochemical workstation (CHI660D) by applying a AC amplitude of 0.5 mV in the frequency range between 100 kHz to 0.01 Hz.

RESULTS AND DISCUSSION Fabrication of 3D Monolayer MoS2-Graphene Hybrid Aerogels. We developed a modified hydrothermal method for fabricating 3D porous monolayer MoS2-graphene hybrid NSs aerogels as illustrated in Figure 1. Firstly, monolayer MoS2 and graphene oxide (GO) NSs were prepared by the modified Li-intercalation (Figure S1) and Hummers (Figure S2) methods, respectively.22-23 Once mixing together, the rich functional groups on the GO surface induce the strong interaction between the two type of NSs and thus form the MoS2-GO hybrid NSs. Finally, the hybrid NSs were further hydrothermally treated to form the 3D architecture and converted to the 3D porous MoS2-graphene hybrid NSs aerogels by chemical reduction. The monolayer MoS2 NSs have better structural and morphological compatibility to 8


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graphene NSs and few-layer MoS2 NSs. Therefore, the expected strong interaction tends to form the 3D porous MoS2-graphene hybrid architecture. This interaction is also critical to suppress the restacking for both 2D materials during the 3D architecture fabrication. The early work reported the fabrication of 3D architectures by using few-layer MoS2 and graphene NSs as building blocks.5 However, the MoS2 NSs were only embedded in the 3D graphene matrix without forming an ideal MoS2-graphene hybrid NSs structure. The lack of strong interaction between MoS2 and graphene leaded to obvious restacking of MoS2 NSs as the MoS2 content increased in the composite.5 In the present work, the use of monolayer MoS2 NSs greatly improves the interaction between the two materials, which results in stable 3D porous architectures without obvious restacking of MoS2 NSs even for the composites with high MoS2 contents. The morphology and microstructure of the as-prepared MoS2-graphene hybrid NSs aerogels with various MoS2 contents were characterized by scanning electron microscopy (SEM). As shown in Figure 2, the MoS2 contents in the aerogels play an important role in tuning the morphology and microstructure. Figure 2a shows the SEM image of the pure graphene aerogels, revealing a 3D porous structure constructed by interconnected large lamellar NSs with micropores ranging from tens to hundreds of micrometers (Figure S3). As the MoS2 content increases to above 60 wt%, the composite aerogels exhibit a well-constructed 3D foam structure with more uniform pores ranging from several nanometers to micrometers and many smaller hybrid NSs as building blocks (Figure 2b-2f, Figure S4-S8). It is worth noting that the 9



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3D porous architecture of the composite aerogels was well retained even when the MoS2 content reached 90 wt%, exhibiting greatly improved structural stability than the previously reported MoS2-graphene 3D structure. The EDS spectra and element mapping images of the MoS2-graphene aerogels show that the Mo and C elements are uniformly distributed in the 3D architecture (Figure S9 and S10), indicating the basic building blocks are the hybrid MoS2-grpahene NSs but not individual MoS2 or graphene NSs. To determine the exact MoS2 contents in different composites, TGA measurements were carried out from room temperature to 600 °C in flowing air with a ramp rate of 10 °C min−1. As shown in Figure S11, the MoS2 contents in different composites are close to the theoretical values. The XRD patterns of the 3D MoS2-graphene hybrid NSs aerogels (with 80 wt% MoS2) and the pristine MoS2 powder for comparison are shown in Figure S12. The disappearance of graphene (002) peak and the low intensity of MoS2 (002) peak indicate that the MoS2-graphene hetero-interface is effective to suppress the restacking of the two individual NSs. 24-26

The nitrogen adsorption-desorption isotherms and the pore size distribution of the

3D MoS2-graphene hybrid NSs aerogels (with 80 wt% MoS2) are shown in Figure S12. It is noted that the composite shows a typical type-IV isotherms, indicating the existence of mesopores in the composite. The Brunauer−Emmett−Teller (BET) specific surface area of MoS2-graphene aerogels is about 105 m2g−1, which is much larger than reported values for directly dried MoS2 NSs. In combination with the SEM results, we can see that the large surface area of the 3D architecture is originated from 10


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the interconnected hybrid NSs, giving rise to the multilevel pores in the aerogels. This unique porous structure and the ultrathin NSs building blocks are beneficial for electrolyte penetration and lithium ion transport in the electrode. To further characterize the MoS2 NSs-graphene aerogels, Raman and XPS results confirm that the MoS2 NSs is of 2H phase structure in the 3D architectures as shown in Figure S13.27-29 Figure 3a shows the atomic force microscopy (AFM) image of the typical monolayer MoS2 NSs. The inserted height profile reveals an average sheet thickness of about 0.8 nm, which is slightly larger than the theoretical values (0.65-0.7 nm) reported for mechanically exfoliated monolayer MoS2.30 The discrepancy could be explained by the presence of adsorbed water or gas molecules trapped between the sample and substrate.31 The height profile in Figure 3b shows an average sheet thickness of about 1 nm for GO (Fig. 3b), indicating the single-layer state. Based on the statistical analysis for more than 100 samples, about 95% of the MoS2 NSs and 98% of the graphene oxide NSs are monolayer in thickness. The lateral sizes of these NSs are typically in the ranges from 0.5 to 2µm for MoS2, and from one to several micrometers for graphene oxide. The general morphology and microstructure of the 3D MoS2-graphene hybrid NSs aerogels were further investigated by transmission electron microscopy (TEM). The low-magnification TEM image in Figure 3c further validates the 3D porous architecture for the MoS2-graphene hybrid NSs aerogels with multilevel pores. The high-resolution TEM (HRTEM, Figure 3d) image shows clear MoS2 NSs with a 11



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monolayer thickness and many exposed active edges. As shown in Figure 3d, many monolayer MoS2 NSs are closely anchored on the graphene NSs, forming the hybrid NSs heterostructure.31 Scanning transmission electron microscope (STEM) and elemental mapping images of Mo, S, C, and O elements for the 3D MoS2-graphene hybrid NSs aerogels are shown in Figure 3e and f, which further confirm the uniform distribution of MoS2 and graphene in the whole 3D architecture. Electrochemical Performances of Hybrid Aerogels. For evaluating electrochemical properties, the pristine MoS2 NSs and the 3D MoS2-graphene hybrid NSs aerogels with different MoS2 contents were investigated as anode materials of lithium-ion batteries. Figure 4a and 4b show the CV curves and charge/discharge curves for the pristine MoS2 NSs electrode for the initial three cycles. As shown in Figure 4a, two cathodic peaks at about 0.9 V and 0.3 V, and two anodic peaks at about 1.9 and 2.5 V can be clearly observed in the CV curve for the initial cycle. The 0.9 V peak is attributed to the insertion of Li+ ions into the MoS2 to form LixMoS2, accompanied by a phase transition from 2H (trigonal prismatic) to 1T (octahedral). 32-34 The 0.3 V peak is explained by the further conversion reaction from LixMoS2 into Mo and Li2S. The 1.9 V small peak for the oxidation process corresponds to the partial oxidation of Mo to form MoS2, while the 2.5 V broad peak is attributed to the delithiation process of Li2S and the formation of S according to literature.33-34 After the first cycle, three cathodic peaks are found at 1.8, 0.9, and 0.3 V, respectively, which are attributed to the reduction of MoS2 and the conversion from S8 to polysulfides and then to Li2S. The charge/discharge curves of the pristine MoS2 12


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NSs electrode (Figure 4b) shows an initial discharge and charge capacities of about 1280 and 1040 mAh g-1, respectively, with a coulombic efficiency of about 81%. The irreversible capacity loss for the initial cycle resulted from some irreversible reactions such as the formation of solid electrolyte interface (SEI) film. The CV curves of the 3D MoS2-graphene composite electrode show similar redox peaks as the exfoliated MoS2 NSs electrode (Figure 4c and Figure S14). Since no redox peaks are observed from the CV curves of the pristine graphene electrode, those redox peaks for the composites can only be attributed to MoS2. As the MoS2 content increases in the composites, the reversible capacity of the composites increases first, reaches a maximum at 80 wt% MoS2, and starts to decrease when the MoS2 content is above 90 wt%. As shown in Figure 4d, the initial discharge and charge capacities of the 3D MoS2-graphene composite electrode (80 wt% MoS2) are 1403 and 1185 mAh g-1, respectively, with a coulombic efficiency of about 85%. When the MoS2 content is low (such as 40 wt%), the reversible capacity of the composite will be low because the reversible capacity of the pristine graphene electrode is much lower compared to that of the exfoliated MoS2 NSs electrode. At a proper composition with 80 wt% MoS2, the synergetic effects between graphene and MoS2 NSs are maximized to achieve a large reversible capacity up to 1200 mAh g-1. However, further increasing the MoS2 content may induce the restacking of the MoS2 NSs in the composite, which will reduce the utilization of MoS2 NSs and result in lower reversible capacity. As illustrated in Figure 5a, the monolayer MoS2-graphene NSs heterostructure can provide both good structural stability and fast charge transport, which are beneficial to 13



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improve the cycle performance and rate performance for the electrode. Figure 5b compares the cycle performance of the 3D MoS2-graphene composite electrodes, the exfoliated MoS2 NSs electrode, and the pure graphene electrode. The pristine MoS2 NSs electrode show poor cycle performance with only 58% reversible capacity of the initial cycle retained after 200 cycles. The cycle performance is greatly improved as the graphene is incorporated into MoS2 because the graphene NSs function as perfect buffer layers to accommodate the volume change induced large strain in MoS2 NSs during the charge and discharge. Specifically, the 3D MoS2-graphene composite with 80 wt% MoS2 exhibits excellent capacity retention of 95% after 200 cycles. In addition to excellent cycling stability, the 3D MoS2-graphene composite electrodes also exhibit superior rate capability over the pristine MoS2 NSs electrode as shown in Figure 5c. Although the pristine MoS2 NSs electrodes exhibit a large reversible capacity of about 1000 mAh g-1 at a small current density of 100 mA g-1, the reversible capacity reduces quickly as the current density increases. At a current density of 2000 mA g-1, the exfoliated MoS2 NSs electrode can only deliver a reversible capacity of about 436 mAh g-1. As a stark contrast, the 3D MoS2-graphene composite electrode with 80 wt% MoS2 can still deliver a large reversible capacity of about 780 mAh g-1 at the current density of 2000 mA g-1. Moreover, upon the reversal of the current density from 2000 to 100 mA g-1, the reversible capacity of the 3D MoS2-graphene composite electrode can return to its original value, indicating excellent reversibility. Results reported here are among the best performances from theaspect of capability, rate and cycle performances (FigureS15). 14


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In order to further understand the improved rate capability of the MoS2-graphene composite electrode, EIS measurements were conducted for different electrodes. Corresponding Nyquist plots are shown in Figure 5d. All EIS spectra consist of a single depressed semicircle in the high-medium frequency region and a straight line at the low frequency region. Obviously, the charge transfer resistance associated with the semicircle decreases as more graphene is incorporated into MoS2, indicating rapid charge transfer reaction for Li+ insertion and extraction for the MoS2-graphene composites. The fast charge transfer capability leads to the improved rate performance of the 3D MoS2-graphene composite electrodes. Based on the above results, the superior electrochemical performance of the MoS2-graphene composite electrode are owing to its unique 3D porous architecture constructed by interconnected 2D monolayer MoS2-graphene heterostructure. The present 3D porous architecture possesses good structural stability and greatly suppresses the restacking of MoS2 or graphene NSs, retaining the monolayer feature for MoS2 NSs after hybridization with graphene. The graphene NSs, functioning as macromolecular surfactants, can effectively stabilize MoS2 NSs with high surface energy, forming the uniform hybrid NSs heterostructure. The continuous capacity fading for the metal oxides or metal sulfides is usually due to electrode pulverization caused by the

large

strain associated with big volume change

during

lithiation/delithiation processes. The pristine MoS2 NSs tend to restack and easily go through the pulverization process, which will cause the loss of electrical contact between active material and current collector, thus leading to continuous capacity loss. 15



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The smart 3D porous architecture design effectively alleviate this problem because the porous structure provides extra volume to accommodate the volume change and the graphene matrix functions well as the strain buffer layer to improve the electrode stability. Apart from the improved cycling stability, the 3D porous architecture constructs fast electron pathways between MoS2 and current collector, enabling excellent rate capability for the composite electrode.

CONCLUSION In summary, 3D porous monolayer MoS2-graphene hybrid NSs aerogels have been successfully

prepared

by

a

facile

hydrothermal

method.

The

structural

characterization confirms the 3D porous architecture is constructed by interconnected monolayer MoS2-graphene hybrid NSs. The 3D porous architecture and the strong interaction between monolayer MoS2 and graphene NSs effectively suppress the restacking of both NSs and greatly improve the structural stability of the composite. The electrochemical performance of the pristine MoS2 NSs are significantly enhanced by hybridization with graphene. The mass ratio between MoS2 and graphene in the composite plays a key role in determining the electrochemical performance. The MoS2-graphene composite with 80 wt% MoS2 exhibits the best electrochemical performance with a large reversible capacity up to 1200 mAh g-1 as well as excellent cycle performance and rate capability. The outstanding electrochemical performance of the 3D porous monolayer MoS2-graphene hybrid NSs aerogels are attributed to the 16


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unique 3D porous architecture, providing good structural stability, large Li+ reaction sites, and fast electrode kinetics. Our results suggest that present 3D hybrids are promising as anode materials for the next-generation of high-performance lithium-ion batteries

ASSOCIATED CONTENT Supporting Information Figures S1−S14(AFM, SEM, TGA, XRD, XPS and detailed electrochemical results). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author H. Z. email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by National Key Basic Research Program of China (2014CB931702), NSFC (51572128), NSFC-RGC (5151101197), and PAPD of Jiangsu Higher Education Institutions.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure captions Figure 1. Schematic illustration of the fabrication of 3D porous monolayer MoS2-graphene hybrid aerogels. Figure 2.Typical SEM images of the monolayer MoS2-graphene hybrid aerogels with different MoS2 contents of (a) 0 wt%, (b) 40 wt%, (c) 60 wt%, (d) 80 wt%, (e) 84 wt%, and (f) 90 wt%. The inserts show the corresponding digital photographs. Figure 3. (a) AFM image of the monolayer MoS2 NSs. (b) AFM image of the exfoliated GO sheets on mica surfaces. (c) Low-magnification and (d) high-magnification TEM images of the MoS2-graphene hybrid NSs. (e) Typical STEM image of the MoS2-graphene hybrid NSs. (f) Corresponding elemental mapping images of S, O, Mo, and C. Figure 4. Electrochemical Performances of Monolayer MoS2-Graphene Hybrid Aerogels. (a) CV curves of the exfoliated MoS2 NSs electrode at a scanning rate of 0.5 mV s-1 for the first three cycles. (b) The first three charge/discharge curves of the exfoliated MoS2 NSs electrode at a current density of 100 mA g-1. (c) CV curves of the MoS2-graphene composite (80 wt% MoS2) electrode at a scanning rate of 0.5 mV s-1 during the first three cycles. (d) The first three charge/discharge curves of the MoS2-graphene composite (80 wt% MoS2) electrode at a current density of 100 mA g-1. Figure 5. (a) Schematic illustration of monolayer MoS2-graphene hybrid NSs heterostructure with fast electron and Li+ ion transport. (b) Rate performances (c) Cycle performances, and (d) Nyquist plots of the 3D MoS2-graphene composite electrodes, the pristine graphene electrodes, and the pristine MoS2 NSs electrodes.

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