Layer-by-Layer Hybrids of MoS2 and Reduced Graphene Oxide for Lithium Ion Batteries

Electrochimica Acta 147 (2014) 392–400

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Layer-by-Layer Hybrids of MoS2 and Reduced Graphene Oxide for Lithium Ion Batteries Yu Jing a,b , Edwin O. Ortiz-Quiles a , Carlos R. Cabrera a, *, Zhongfang Chen a, *, Zhen Zhou b, * a

Department of Chemistry, Institute for Functional Nanomaterials, University of Puerto Rico, Rio Piedras Campus, San Juan, PR 00931, USA Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, PR China b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 13 July 2014 Received in revised form 12 September 2014 Accepted 26 September 2014 Available online 28 September 2014

Two-dimensional MoS2 shows great potential for effective Li storage due to its good thermal and chemical stability, high theoretical capacity, and experimental accessibility. However, the poor electrical conductivity and the restacking tendency significantly restrict its applications to lithium ion batteries (LIBs). To overcome these problems, we introduced reduced graphene oxides (rGO) to the intercalationexfoliation preparation process of few-layered MoS2 and obtained layer-by-layer MoS2/rGO hybrids. With the addition of rGO, the restacking of MoS2 layers was apparently inhibited, and MoS2 with 1  3 layers was obtained in the composite. Due to the positive role of rGO, MoS2/rGO hybrids exhibited highly enhanced cyclic stability and high-rate performances as LIB anodes in comparison with bare MoS2 layers or bulk MoS2. Moreover, the experimental results were well interpreted through density functional theory computations. ã 2014 Elsevier Ltd. All rights reserved.

Keywords: molybdenum disulfide reduced graphene oxide composite synergetic effect Li ion batteries

1. Introduction Two-dimensional (2D) materials, including graphene and its inorganic analogs, have been attracting great research enthusiasm in the field of lithium ion batteries (LIBs) due to their excellent electronic and structural properties [1–8]. As revealed by extensive computational studies [9–18], 2D materials, especially monolayer structures, can provide larger surface areas for Li adsorption and lower energy barriers for Li diffusion than three-dimensional (3D) bulks. Therefore, 2D materials can provide fast charge/discharge rates and high specific capacities if served as LIB electrode materials. Inspired by these theoretical predictions, superior electrochemical performances of 2D materials have been demonstrated in abundant experiments [19–25]. Among those 2D inorganic materials [7,26], MoS2 stands out for its high thermal stability and chemical inertness. MoS2 monolayer has been feasibly prepared through many routes, such as hydrothermal method, liquid exfoliation, and chemical vapor deposition [27–37]. Especially, MoS2 monolayer has a theoretical

* Corresponding authors. Tel.: +86 2223503623. E-mail addresses: [email protected] (C.R. Cabrera), [email protected] (Z. Chen), [email protected] (Z. Zhou). http://dx.doi.org/10.1016/j.electacta.2014.09.132 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.

capacity of 670 mAh/g, which is much superior to the traditional graphite anode (372 mAh/g). In recent years, 2D MoS2 has been widely investigated as LIB electrodes [38–42]. However, MoS2 monolayer is a semiconductor with a direct band gap of 1.8 eV [43,44]. The lack of good electronic conductivity would limit the electrochemical performances of 2D MoS2. Moreover, MoS2 monolayer obtained from hydrothermal process or liquid exfoliation suffered seriously from the restacking problem [45–47], which would directly suppress the charming advantages of 2D MoS2. The above mentioned problems of 2D MoS2 can be satisfactorily solved by introducing graphene, which has an extraordinarily high electronic conductivity. Recently there have been several attempts on preparing MoS2/graphene composites for applications to LIB anodes and sodium-ion battery electrodes [48–58]. For example, Chang et al. successfully prepared MoS2/graphene composites with MoS2 nanosheets (3–6 layers) dispersed randomly on the surface of graphene via an L-cysteine assisted hydrothermal method [48]. The obtained MoS2/graphene composites exhibited high capacity of 1100 mAh/g (at current density of 100 mA/g), good cyclic stability (no fading after 100 cycles), and excellent high rate performances. Moreover, they also developed an in-situ reduction method to prepare MoS2/graphene composites for LIBs [49]. Since MoS2 monolayer was anchored onto graphene, the restacking problem of MoS2 layers could be effectively alleviated, and the

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obtained composites contained a homogeneously dispersed 4– 5 layers of MoS2. As a result, the MoS2/graphene composites showed a high reversible capacity of 1300 mAh/g over 50 cycles. In order to provide a clear comparison of previous results, we summarized the electrochemical performances of several MoS2/ graphene systems as anodes for LIBs in Table S1 (Supplementary Data) in detail. Generally, the effective electron transport and structure stability of the composites were attributed to the interactions between graphene and MoS2 and the so-called synergistic effect [48,50]. However, more efforts should be made to prepare the composites of less stacked MoS2 layers and graphene in facile processes. Meanwhile, a better understanding of the interaction between graphene and MoS2 is necessary to figure out how graphene works to improve the electrochemical performances of 2D MoS2. In this work, via a chemical lithiation process combined with exfoliation in deionized water, we prepared MoS2/rGO (reduced graphene oxide) hybrids of less stacked MoS2 layers (1–3 layers) and sizable rGO sheets, and utilized them as LIB anodes. Due to the improved surface interaction between MoS2 layers and rGO and the enhanced electrical conductivity, MoS2/rGO hybrids delivered rather high initial capacity. Nevertheless, after the first charging/ discharging cycle, the lithiation/delithiation mechanism became different and graphene mainly worked as a conductive substrate stabilizing sulfur on its surface. As a result, MoS2/rGO hybrids exhibited high capacity and good cyclic stability superior to those of bare MoS2 obtained with the same method (exf-MoS2) or bulk MoS2. To assist the interpretation of our experimental findings, density functional theory (DFT) computations were performed, and provided a deep understanding of the functions of graphene in different charging/discharging processes. 2. Experimental and Computational Details

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Scanning electron microscope (SEM) was conducted on FEI Nanosem 430 field-emission gun scanning electron microscope. Selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy (Horiba-Jobin T64000, 532 nm wavelength excitation), elementary analyzer (vario EL CUBE, elementar) and X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos Analytical) were also adopted to characterize the prepared samples. 2.3. Electrochemical measurements All electrochemical measurements were performed within 2025 coin cells. In the cells, the electrolyte was a 1.0 M LiPF6 dissolved in a mixture of ethylenecarbonate (EC), dimethylcarbonate (DMC), and diethylcarbonate (DEC) (1:1:1 in volume). Li metal worked as the counter electrode and MoS2/rGO hybrids as the working electrode. The working electrode was fabricated by coating the mixture of MoS2/rGO, acetylene black and 0.02 M polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidinone (80:10:10 wt%) on a cupper foil. After that, the cupper foil was dried in vacuum at 80  C for 2 h and then at 110  C over night. The cells were finally assembled in an Ar-filled (O2 and H2O < 1 ppm) glove box. Galvanostatic charge/discharge cycles were performed on a LAND-CT2001A battery tester between 0.01 and 3.00 V (vs. Li/Li+) at various current densities. Cyclic voltammetry (CV) measurements were performed on an electrochemical workstation (CHI600C) over the same potential range of 0.01 and 3.00 V (vs. Li/Li+) at a scan rate of 0.10 mV/s. Electrochemical impedance spectroscopy (EIS) was conducted on a Zahner-Elektrik IM6e electrochemical workstation. The Nyquist plots were recorded potentiostatically by applying an AC signal of 5 mV amplitude in the frequency range of 100 kHz to 10 mHz. All electrochemical measurements were performed at room temperature.

2.1. Material Preparation 2.4. Computational details As described in more detail in the Supplementary Data, rGO was prepared through a modified Hummer method followed by chemical reduction in hydrazine hydrate. Bulk MoS2 (0.25 g) and rGO (0.05 g) in a weight ratio of 5:1 were added to a round-bottle flask. The flask was then transferred into a glove box, and injected with 10 mL n-butyllithium (n-BuLi) with the protection of Ar [59,60]. The lithiation reaction lasted for 18 h with magnetic agitation at ambient temperature. After full lithiation, the mixture was filtrated with a vacuum solution filter and washed with hexane, and then the obtained lithiated MoS2/rGO (LixMoS2/rGO) powder was transferred into a glass sample vial. Note that all these operations must be conducted in the glove box with the protection of Ar. After lithiation, LixMoS2/rGO powder (0.1 g) was dispersed into deionized water and sonicated for 2 h at 20  C. When the dispersion was placed overnight, the stacked MoS2 layers resided at the bottom, and the exfoliated MoS2 and rGO few layers suspended in the upper liquid. The exfoliated MoS2 and rGO in the upper liquid were collected by filtering in a vacuum solution filter. After thoroughly washed with deionized water (18.2 MV cm, Nanopure Diamond, Barnstead) and freeze-dried at 54  C in vacuum (16–17 ppm), exfoliated MoS2/rGO hybrids were obtained. Exf-MoS2 was synthesized by the same method without the addition of rGO.

DFT computations were performed by using an all-electron method within a generalized gradient approximation (GGA) for the exchange-correlation term, as implemented in the DMol3 code [61,62]. The double numerical plus d functions (DND) basis set and PBE functional were adopted [63]. Especially, to accurately account for the long-rang electrostatic interactions between Li atoms with high concentrations, we adopted the PBE + D2 method with the Grimme vdW correction [64]. Self-consistent field (SCF) computations were performed with a convergence criterion of 10 6 a.u.

2.2. Material Characterization The crystalline structures of MoS2/rGO hybrids, exf-MoS2 and bulk MoS2 were characterized by X-ray diffraction (XRD) (Rigaku D/Max III diffractometer with Cu Ka radiation, l = 1.5418 Å).

Fig. 1. (a) XRD patterns of bulk MoS2, exfoliated MoS2 nanosheets and exfoliated MoS2/rGO hybrids; (b) Magnified (0 0 2) peaks of bulk MoS2, exfoliated MoS2 nanosheets and exfoliated MoS2/rGO hybrids.

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on the total energy and electron density. To ensure high-quality numerical results, we chose the real-space global orbital cutoff radius as high as 5.1 Å in all computations. The Brillouin zones were sampled with 4  4  1 k points. 3. Results and Discussion 3.1. Characterization of MoS2/rGO hybrids Fig. 1a shows the XRD patterns of MoS2/rGO hybrids and exfMoS2 compared with bulk MoS2. All the peaks can be ascribed to

hexagonal phase of MoS2 (JCPDS 37-1492). After lithiationexfoliation, both MoS2/rGO hybrids and exf-MoS2 show lower peak intensity, indicating the decrease of crystallinity. For MoS2/ rGO hybrids, there is a weak wide peak around 24 , which can be ascribed to few-layer rGO. From the magnified (002) peaks (Fig. 1b), it can be seen that the peaks of MoS2/rGO hybrids and exf-MoS2 shift a little to the left, implying the increase of interlayer distance according to the Bragg law. The broadening and downshift of (002) peak in both exf-MoS2 and MoS2/rGO hybrids can also be attributed to the dimension reduction in c direction. Therefore, the XRD results indicate that the interlayer distance of MoS2 was

Fig. 2. SEM images (a and b) of MoS2/rGO hybrids; HRTEM images (c and d) of MoS2/rGO hybrids (1L, 2L and 3L represent monolayer, bilayer and trilayer MoS2, respectively); Magnified HRTEM image (e) and (f) SAED of the selected area in (d).

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enlarged after lithiation-exfoliation process, and the thickness of MoS2 layers could be further reduced in MoS2/rGO hybrids due to the presence of rGO. The SEM images of exfoliated MoS2/rGO hybrids (Fig. 2a and b) show that the size of exfoliated MoS2/rGO flakes distributes in a rather wide range. To better understand the morphology and microstructure of the prepared MoS2/rGO hybrids, we performed HRTEM observations. As labeled in Fig. 2c, the interlayer distance of MoS2 in MoS2/rGO hybrids is 0.63 nm (1.26 nm for two interlayer spaces), slightly larger than that of MoS2 bulk (0.62 nm). Besides trilayer structure, monolayer and bilayer structures of MoS2 can also be identified as labeled in Fig. 2c. As illustrated in Fig. 2d, large-area MoS2 layers were loaded on the surface of graphene, resulting in a layer-by-layer structure of MoS2/rGO hybrids. With the flat overlay structure, MoS2 and rGO layers can interact efficiently with each other. On the basis of magnified HRTEM image (Fig. 2e), the lattice parameters were estimated as a = 3.04 Å and b = 3.07 Å, which agree well with the values of a and b lattices in (002) cardinal plane of MoS2 [54]. The SAED spectroscopy of MoS2/ rGO hybrids (Fig. 2f) further confirms the presence of MoS2 layers arranged in the (002) crystal plane in the hybrids. In sharp contrast, without the presence of rGO, the obtained exf-MoS2 layers suffered from a serious restacking problem (Fig. S1, Supplementary Data). Therefore, the participation of rGO can suppress the stacking of MoS2 layers, and MoS2 layers can be well loaded on the surface of rGO forming a layer-by-layer hybrid structure. Raman spectroscopy was conducted to further identify the composition of the prepared hybrids. As shown in Fig. S2, two typical peaks corresponding to D and G bands appear at 1342.5 cm 1 and 1595.2 cm 1 with strong intensity, corresponding to sp3 and sp2 carbon atoms, respectively, indicating the existence of rGO with partially functionalized carbon [65]. The peaks at 377.3 cm 1 and 404.0 cm 1 corresponding to E2 g and A1 g modes of MoS2 can also be observed in the magnified image [66]. In addition, XPS was also adopted to verify the existence of MoS2 and rGO. As shown in Fig. 3a, there are two doublets of Mo-

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3d signal. The doublets, Mo3d5/2 and Mo3d3/2, with the binding energy of 228.9 eV and 232.1 eV, can be assigned to Mo4+. Meanwhile the signals of S-2p, C-1s and O-1s (Fig. 3b, c, d) can also be detected in XPS. Especially, the C-1s peak (Fig. 3c) can be deconvoluted into four peaks at 284.6, 285.2, 285.7 and 288.8 eV, corresponding to sp2 carbon, sp3 carbon, carbon in C-O single bond, and carbon in C =O double bond, respectively. The XPS results also indicate the partial functionalizations of rGO. We also examined the content of graphene in MoS2/rGO hybrids. Fig. S3 shows the mapping of element distribution in MoS2/rGO hybrids. As revealed, C is homogeneously distributed in the hybrids. In addition, according to the results of elementary analysis, the C content is 20.78 wt.%, which should be entirely contributed by rGO. Therefore, the coexistence of MoS2 and rGO as well as the layerby-layer structure of the prepared hybrids can be sufficiently verified by means of XRD, TEM, Raman and XPS. To better describe the synthetic process and the formation of MoS2/rGO hybrids, the schematic illustration of the rGO involved intercalation-exfoliation process is shown in Fig. 4. In details, during the intercalation process, highly active Li+ ions from n-BuLi slip into the interlayer space of layered MoS2 and enlarge the interlayer distance, contributing to the formation of LixMoS2/rGO. During exfoliation, since LixMoS2/rGO powder is dispersed in a large amount of water, the Li in the interlayer space will react drastically with water and produce vast hydrogen, and therefore force the stacked MoS2 layers to be separated ones. After the severe reactions, the suspended MoS2 layers tend to restack together. However, with the existence of rGO layers, the restacking tendency will be effectively obstructed and suppressed, thus contributing to a hybrid structure of few-layer MoS2 and rGO. 3.2. Electrochemical measurements of MoS2/rGO hybrids. Due to the layer-by-layer structure and efficient contact between MoS2 and rGO, the as-obtained MoS2/rGO hybrids can

Fig. 3. XPS of (a) Mo 3d, (b) S 2p, (c) C 1s and (d) O 1s binding energy regions for MoS2/rGO hybrids.

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Fig. 4. Schematic illustration of the lithiation–exfoliation synthesis process.

facilitate the Li+ adsorption and diffusion, leading to a fast lithiation/delithiation process. To reveal the advantages of MoS2/ rGO hybrids as Li+ storage media in comparison to exf-MoS2 and bulk MoS2, we conducted comprehensive electrochemical measurements. Galvanostatic charge/discharge cycles of MoS2/rGO hybrids were first conducted in comparison with those of exf-MoS2 and bulk MoS2. As shown in Fig. 5a, MoS2/rGO hybrids show a high initial discharging capacity of almost 1800 mAh/g at the current density of 100 mA/g. However, due to the partially irreversible lithium storage on the surface/interlayer space as discussed above, MoS2/rGO hybrids exhibit a low charging capacity of about 940 mAh/g and a low coulombic efficiency (52%) at the first cycle. The high irreversible capacity in the initial discharging process can also be attributed to the formation of solid electrolyte interface (SEI) film [48,67], due to the large surface area of MoS2/rGO hybrids. After that, the discharging capacities of MoS2/rGO are stabilized at approximate 1000 mAh/g with a small fluctuation, while the charging capacities increase slightly and then are also stabilized at around 1000 mAh/g. Besides the high specific capacity, MoS2/rGO also exhibits excellent cyclic stabilities and sustains a high capacity of 940 mAh/g after 75 cycles, with practically no degradation after the 10th cycle. Meanwhile, the coulombic efficiencies of MoS2/rGO are stabilized at 9698% from the 10th cycle to the 75th cycle (Fig. S4), indicating a good reversibility of the electro-active material. In comparison, the charging/discharging capacities of exf-MoS2 and bulk MoS2 are much lower, and the capacity degradation as well as the highly fluctuating coulombic efficiencies of exf-MoS2 (Fig. S4) indicate their poor stability and reversibility. The enhanced electrochemical performances of MoS2/rGO hybrids are distinctive and comparable to those of previous reports [48,49].

The significant increase of charging capacity of MoS2/rGO hybrids over exf-MoS2 and bulkMoS2 highlights the key role of rGO. In MoS2/rGO hybrids, there are some irreversible Li+ intercalation in the interlayer and surface of MoS2 after the first cycle; however, in the subsequent cycles, the initial crystallized MoS2 becomes amorphous Mo and S [41], and the excessive lithium stored in the interlayer and adsorbed on the surface will be released gradually and contribute to increasing charging capacities. Meanwhile, due to the existence of surface functionalities as indicated by the Raman and XPS analyses, rGO can also contribute to some irreversible capacity in the initial cycles [68]. It is the presence of rGO that contributes significantly to the initial capacity enhancement of MoS2 in MoS2/rGO hybrids. With the existence of rGO, the thickness of MoS2 was limited to 1–3 layers, leading to an effective material utilization during the charging/discharging processes. Meanwhile, the expansion at the graphene-MoS2 interlayer distance caused by initial lithiation can be highly facilitated by the weak van der Waals (vdW) and electrostatic interactions of the two materials [69]. Therefore, MoS2/rGO can exhibit a rather higher initial discharging capacity than exf-MoS2 or bulk MoS2. Actually, the effect between rGO and MoS2 is extensively interpreted as a synergistic effect [48,50]. At different high current densities, MoS2/rGO still exhibits higher capacity and stability than exf-MoS2 or bulk MoS2, as shown in Fig. 5b. When modulating the current density from 2000 mA/g to 100 mA/g, MoS2/rGO hybrids can achieve a full recover and sustain high reversible capacity of almost 1000 mAh/g. These good high rate performances of MoS2/rGO are essentially ascribed to the effective electrical conductivity and the stable carbon network upholder provided by rGO.

Fig. 5. (a) Charge–Discharge cyclic performances of MoS2/rGO hybrids, exf-MoS2 and bulk MoS2 at 100 mA/g; (b) Rate performances of MoS2/rGO hybrids, exf-MoS2 and bulk MoS2 at different current densities.

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Fig. 6. (a) Cyclic voltammograms for MoS2/rGO hybrids at a scan rate of 0.1 mV/s; (b) Charge–discharge curves for MoS2/rGO hybrids (current density 100 mA/g).

Note that the C content in MoS2/rGO is 20.78 wt%, we can deduce that the weight ratio of MoS2 and rGO can be approximately 4:1 or less. Since the theoretical capacity of graphene is 670 mAh/g and the capacity of rGO is lower, the higher capacity of MoS2/rGO is devoted by MoS2 with the synergistic interaction between MoS2 and rGO, instead of the unilateral contribution of rGO. Nevertheless, the capacity of MoS2/rGO was calculated by counting the total mass of MoS2 and rGO, which is different from previous reports [48,49]. However, the results are manifested to elucidate the much enhanced electrochemical performances of MoS2/rGO over exf-MoS2 or bulkMoS2. In order to better understand the lithiation/delithiation processes of MoS2, we also conducted CVs of MoS2/rGO (Fig. 6a). Upon the first discharge, three cathodic peaks appear at 1.1 V, 0.7 V and 0.5 V, which correspond to different lithiation processes in the first cycle. In detail, the 1.1 V peak corresponds to the Li intercalation into MoS2 interlayer space or/and Li adsorption on the surface of MoS2 layers, thus the formation of LixMoS2. The 0.7 V peak corresponds to additional Li insertion into the expanded space and edge effects of MoS2 layers, and the 0.5 V peak indicates the irreversible conversion reaction of LixMoS2 into Li2S and Mo metal [67,70,71]. Similarly, the anodic peaks (at 1.05 V and 2.3 V) in the first cycle stand for two delithiation processes: the 1.05 V peak corresponds to the Li deintercalation from the interlayer space or surface of MoS2, while the 2.3 V peak represents the conversion of Li2S into sulfur (S82 ). In the following cycles, the cathodic and anodic peaks overlap well, and center at 1.9 V and 2.3 V, respectively. The cathodic peaks at 0.5 V/0.7 V and anodic peak at 1.05 V disappear after the first cycle, indicating that the lithiation/delithiation processes after the first cycle are dominated by the conversion between Li2S and sulfur (S82 ) akin to Li-S batteries [41,67,71]. In the second and third cycles, the weak cathodic peaks at 1.0 V and 0.75 V appear, which can be ascribed to the association of Li with Mo metal [67]. Similar peak shift indicating altered lithiation/delithiation processes were also observed from the CV curves of exf-MoS2 and bulk MoS2 (Fig. S5). Fig. 6b shows the capacity-voltage profiles of MoS2/rGO hybrids in the initial three cycles. Consistent with the results of CV curves, there are two plateaus around 1.1 V and 0.5 V during the first discharging process, indicating the Li intercalation in surface/ interlayer space and the following conversion of MoS2 into Li2S and Mo. These two discharging plateaus disappear in subsequent cycles, indicating that the lithiation process in the first cycle is irreversible. After the first cycle, the charging and discharging plateaus at 1.9 V and 2.3 V overlap well, which suggest a stable and reversible conversion between Li2S and sulfur. These results agree well with previous investigations [67,70]. Moreover, the

differential capacity curves of MoS2/rGO in the initial three cycles (Fig. S6) also indicate the same lithiation/delithiation mechanisms with CV curves. To further investigate the variation of electrode materials with lithiation/delithiation, we conducted XPS of MoS2/rGO electrodes after different cycles. As shown in Fig. S7, the full wide survey peaks of MoS2/rGO electrodes after 1 cycle, 5 cycles and 10 cycles are presented in comparison. Since the peak differences after different cycles mainly locate in the range of 450–0 eV, we present the enlarged region in Fig. 7 to obtain a better illustration. As shown in Fig. 7, the peaks of Mo3d and S2p which indicate the existence of MoS2 decrease with increasing cycles. Meanwhile, peaks of Li1s, Mo4d and S3s appear and increase with cycles. These results demonstrate the conversion from MoS2 to Mo and S after lithiation/delithiation and the conversion is accomplished gradually with cycles, as implied by the CV results. The peaks of P that comes from the electrolyte were also detected in the XPS results. Therefore, with charging and discharging cycles, the active electrode materials convert gradually from MoS2/rGO into Mo and S with the coexistence of rGO. Especially, the existence of Mo3p peaks indicates that some Mo was oxidized into MoO3 after cycling. As reported by Sen et al. [71], the formation of molybdenum oxides after different cycles was also detected by Raman spectroscopy conducted under ex situ mode. Note that, the XPS measurement in our study was not conducted under the ex situ

Fig. 7. XPS peaks of in the range of 450–0 eV for MoS2/rGO after different charging– discharging cycles.

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resistance of MoS2/rGO hybrids decreased from 340 V to 157 V. Due to the conductivity enhancement with cycling, it is understandable that MoS2/rGO hybrids exhibit improved capacity and cycling stability upon charging. Besides facilitating effective Li intercalation and adsorption and contributing to high initial discharging capacity in the first cycle, rGO also plays a rather important role in the subsequent cycles. In the following cycles, sulfur works as the active material in Li storage [41]. Due to the high contact with rGO, Mo/rGO can work as a stable, effective and highly conductive catalyst for the conversion between sulfur and Li2S. Therefore, MoS2/rGO hybrids can exhibit high capacity, excellent cyclic stability and high rate performance. 3.3. DFT computations on a graphene/MoS2 hetero-structure.

Fig. 8. Nyquist plots for (a) bulk MoS2, exf-MoS2, MoS2/rGO before charging/ discharging and (b) MoS2/rGO hybrids before cycles, after one cycle and 10 cycles, in the frequency range of 100 kHz to 10 mHz.

mode, and the possibility for the oxidation of Mo in air cannot be eliminated. With the presence of rGO, the electrochemical performances of MoS2/rGO hybrids can be significantly improved. By investigating the lithiation/delithiation processes of MoS2 in different cycles, we can have a general idea on how graphene helps to improve the electrochemical performances of MoS2 in the hybrids. First, the addition of rGO can inhibit the restacking of MoS2 layers and contribute to more accessible Li adsorption and diffusion in the initial discharging process. Second, the addition of rGO can enhance the electrical conductivity of MoS2/rGO effectively and providing a stable and conductive network for the following Li-S battery reactions. The improved electrical conductivity of MoS2/rGO can be well illustrated by the electrochemical impedance measurements. As shown in Fig. 8a, all these three materials exhibited semicircles at high frequency. The radius of the semicircle is a direct reflection of the equivalent series resistance in the electrode/electrolyte system, which is mainly contributed by the electrolyte resistance and the electronic resistance of the electrode. Within the same electrolyte, a smaller radius of the semicircle indicates a lower electronic resistance of the electrode. Therefore, MoS2/rGO hybrids possess a lower electronic resistance (340 V) than the other two materials (677 V for exf-MoS2 and 1160 V for bulk MoS2). After different lithiation/delithiation cycles, the electronic resistance of MoS2/rGO hybrids (Fig. 8b) decreases, since the effective materials in the electrode have transferred from MoS2 into Mo and S with the support of rGO. After 10 cycles, the electrical

To gain a deeper understanding on the interlayer interaction between graphene and MoS2, DFT computations were performed. We constructed a graphene/MoS2 hetero-structure by pairing a graphene monolayer and a MoS2 monolayer together. Considering the difference in lattice parameters between graphene (2.46 Å) and MoS2 (3.16 Å), a 5  5 supercell of graphene and a 4  4 supercell of MoS2 were employed to minimize the lattice mismatch (Fig. 9a and 9b). The bilayer structure adopts the same lattice parameters as graphene, which imposes only a 2.7% strain to MoS2. According to our computations, the energetically most stable stacking pattern of graphene/MoS2 has as many carbon atoms as possible pointing to the top of Mo atom, resulting in a 2.31 eV binding energy and a 3.34 Å distance between graphene and S atomic plane. For comparison, the binding energies of graphene bilayer in a 5  5 supercell and MoS2 bilayer in a 4  4 supercell are 3.3 and 2.56 eV, respectively. Therefore, the interaction between graphene/MoS2 heterostructures is weaker than that of graphene layers, but much comparable to that of MoS2 bilayers. Thus, graphene/MoS2 hybrids can compete with the restacking of MoS2 layers, and the addition of graphene can inhibit the restacking of MoS2. We also computed the band structure of graphene/MoS2 bilayer. As shown in Fig. 9c, graphene/MoS2 bilayer is semi-metallic with valence band maximum (VBM) and conduction band minimum (CBM) touching each other at the Fermi level. The states at the Fermi level are contributed solely by graphene, which demonstrates that the addition of graphene can significantly increase the conductivity of MoS2. Since after the first cycle, the active anode material becomes sulfur instead of MoS2, we computationally investigated the stability of one S8 molecule on the surface of graphene in a 5  5 supercell. As illustrated in Fig. 9d and e, the S8 molecule can be adsorbed on the surface of graphene right above a six-member ring, with a vertical distance of 3.25 Å. As calculated, the binding energy is 0.64 eV, denoting that the S8 molecules can be effectively

Fig. 9. Schematic illustrations of graphene/MoS2 bilayer hybrid in side view (a) and top view (b); (c) band structure of graphene/MoS2 bilayer hybrid; Schematics of a S8 molecule on the surface of graphene in side view (d) and top view (e).

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stabilized on the surface of graphene, thus in the following cycles, graphene will work as a favorably conductive substrate for the conversion between Li2S and S, and facilitate the charging and discharging processes. These computational simulations are highly in accord with the experimental results, and can provide further understanding of the synergistic effect between MoS2 and graphene in the initial electrochemical processes, and the role of graphene in the subsequent cycles. 4. Conclusion

[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

In summary, we prepared MoS2/rGO hybrids by an rGOinvolved lithiation-exfoliation method, for the first time. Due to the preferable interaction between MoS2 and rGO layers, the restacking of MoS2 can be effectively inhibited. Favored by the few-layer hybrid structure and the existence of rGO, MoS2/rGO hybrids exhibited much higher initial capacity (1300 mAh/g at the current density of 100 mA/g) than exf-MoS2 or bulk MoS2 as LIB anodes. In the subsequent cycles, MoS2/rGO hybrids were changed into Mo and sulfur/rGO and worked like a Li-S battery. The existence of rGO can provide a highly conductive and beneficial network for the conversion between Li2S and sulfur. As a result, MoS2/rGO hybrids exhibited higher capacity, better cyclic stability and high rate performance than exf-MoS2 or bulk MoS2. To better understand the interactions between graphene and MoS2, DFT computations were conducted to simulate the MoS2/graphene bilayer system and the adsorption of S8 molecule on graphene sheet, respectively. As expected, the computational simulations were highly consistent with our experimental results, and the positive functions of graphene in MoS2/graphene hybrids, in the initial charging/ discharging process and the subsequent cycles, were significantly revealed. By investigating MoS2/graphene hybrids for Li storage, we hope that our results can provide further understanding of the interactions between graphene and 2D graphene analogs and promote more efforts to the explorations of novel 2D materials for Li ion batteries.

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Appendix A. Supplementary data

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Supplementary data related to this article can be found at http://dx.doi.org/10.1016/xx Acknowledgement

[24] [25] [26] [27] [28] [29] [30]

[31] [32] [33] [34] [35] [36]

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Support in USA by Department of Defense (Grant W911NF-121-0083), the NSF-NSEC Center for Hierarchical Manufacturing Grant (No. CHM-CMMI-0531171) and NSF-EPSCoR Institute for Functional Nanomaterials (IFN) Grant (No. EPS-1002410, EPS1010094) as well as in China by NSFC (21073096 and 21421001), 111 Project (B12015) and MOE Innovation Team (IRT13022) is gratefully acknowledged.

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