Accepted Manuscript Hydrothermal synthesis of Co-doped-MoS2/reduced graphene oxide hybrids with enhanced electrochemical lithium storage performances Shurui Xu, Qing Zhu, Tao Chen, Weixiang Chen, Jianbo Ye, Jianguo Huang PII: S0254-0584(18)30712-0 DOI: 10.1016/j.matchemphys.2018.08.048 Reference: MAC 20891 To appear in: Materials Chemistry and Physics Received Date: 22 January 2018 Accepted Date: 19 August 2018 Please cite this article as: Shurui Xu, Qing Zhu, Tao Chen, Weixiang Chen, Jianbo Ye, Jianguo Huang, Hydrothermal synthesis of Co-doped-MoS2/reduced graphene oxide hybrids with enhanced electrochemical lithium storage performances, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.08.048 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT Graphical Abstract (Figure) ACCEPTED MANUSCRIPT Hydrothermal synthesis of Co-doped-MoS2/reduced graphene oxide hybrids with enhanced electrochemical lithium storage performances Shurui Xu, Qing Zhu, Tao Chen, Weixiang Chen*, Jianbo Ye, Jianguo Huang Department of Chemistry, Zhejiang University, Hangzhou 310027, P.R. China Abstract This work reports a facile one-pot hydrothermal route to fabricate Co-dopedMoS2/reduced graphene oxides (RGO) hybrids. The effects of Co-doping on the morphology, microstructure and electrochemical lithium storage performance of these hybrids are investigated. It is demonstrated that the rational Co-doping can change the morphologies and microstructures of the hybrids. Especially, the Co-dopedMoS2/RGO-2 prepared with 1:4 mole ratio of CoCl2 to Na2MoO4 in the hydrothermal solution shows that the numerous Co-doped MoS2 layers with shorter crystal fringes and more defect sites are well anchored on the surface of RGO, resulting in the significantly enhanced electrochemical performance for reversible lithium storage. In comparison with the MoS2/RGO, the Co-doped-MoS2/RGO-2 can not only deliver much larger reversible capacity of 1236 mAh g-1 with good cyclic stability at the current density of 100 mA g-1, but also exhibit significantly enhanced high-rate capability of 895 mAh g-1 at a high current density of 1000 mA g-1. The Co-dopedMoS2/RGO-2 also shows much higher Coulombic efficiency of 89.2% at the first cycle than MoS2/RGO (67.2%). The greatly enhanced electrochemical performance is ascribed to its robust heterostructure by hybridizing of Co-doped MoS2 layers and RGO sheets. Keywords? Molybdenum disulfide; cobalt doping; reduced graphene oxide sheets; hydrothermal route; lithium ion battery. ------------------- *Corresponding author: Fax: 86-571-87951895; Tel: 86-571-87951352. E-mail address: [email protected] (W. X. Chen) 1 ACCEPTED MANUSCRIPT 1. Introduction As one typical example of advanced electrochemical energy storage systems, lithium ion batteries (LIBs) with high energy density, low self-discharge and long service life have become the dominant power sources for portable electronic products and electric vehicles [1-3]. However, the performance of current LIBs cannot meet ever-growing demands for superior energy density and power density. In principle, the performance of LIBs largely depends on electrode materials [4, 5]. Even if graphite is currently used as anode material for most commercial LIBs because of its excellent cycling stability, its low specific capacity (theoretical capacity of 372 mAh g-1) and poor rate-capability limit the development of next-generation LIBs. Therefore, it is essential to devote more efforts to design and create novel anode materials with improved lithium storage performances to satisfy the ever-increasing performance demands. [6, 7] Recently, nanostructured MoS2 materials have attracted a great deal of attention on account of their unique two-dimension (2D) morphology, fascinating properties and various applications . MoS2 comprises of a molybdenum layer located between two sulfur layers features a layered structure, in which S-Mo-S atoms are covalently bonded to form closed packed hexagonal structures that are stacked together through van der waals interactions. The interlayer spacing of MoS2 (0.63 nm) is remarkably greater than that of graphite (0.33 nm), indicating that MoS2 is a kind of suitable host among the numerous candidates for LIBs [10, 11]. For instance, the single- or fewlayer MoS2 sheets have been revealed to exhibit high reversible specific capacity of 900-1000 mAh g-1 for electrochemical lithium storage [12, 13]. Nevertheless, MoS2 sheets have their own shortcomings as electrode materials of LIBs. The van der waals 2 ACCEPTED MANUSCRIPT interaction between basic plane will result in aggregation of MoS2 layers, which will reduce the stability of 2D nanostructure, leading to the poor cyclic performance . Additionally, as a typical semiconductor , the low conductivity of MoS2 will greatly limits its practical application as electrode materials. What?s more, during intercalation/extraction, the volume changes give birth to concomitant mechanical stress, which leads to a conspicuous degradation and loss of contact between active material and current collector. These are the major problems faced by practical applications of MoS2-based materials in LIBs [12, 13]. As one of the effective methods to settle these problems, hybridization with conductive substrates (such as graphene, carbon nanotubes and conducting polymers) has drawn considerable attention . Owing to its superior electrical conductivity, high charge mobility, large specific surface area and intrinsic flexibility, graphene is believed as one of the most promising carbon matrices . It is well verified that the horizontal growth of 2D materials on graphene can intensify interaction between 2D nanosheets and graphene, which can increase the electrical conductivity and avoid the phenomenon of aggregation and pulverization, resulting in the enhanced performances for electrochemical energy storage . Besides, 2D MoS2 nanosheets are similar to graphene in the microstructure and morphology. Naturally, graphene can be employed as a suitable matrix to support MoS2 layers to form a sheet-on-sheet heterostructural hybrid, which can greatly improve its conductivity and enhance electron transfer rate, resulting in significantly improved electrochemical performance for reversible lithium storage . The robust heterostructure of MoS2/RGO hybrids not only ameliorates lithium ion accommodation, but also shortens the transport pathway of ions, leading to high specific capacity and enhanced rate capability . In addition, the intrinsic flexibility of graphene can dramatically mitigate the 3 ACCEPTED MANUSCRIPT volume change during long-term charge/discharge cycling . Therefore, this kind of heterostructural MoS2/graphene hybrids could exhibits significantly enhanced electrochemical performance for LIBs. In our previous work , the layered MoS2/graphene composite prepared by hydrothermal route and annealing in H2/N2 manifested a high specific capacity of about 1100 mAh g-1 at a current of 100 mA g-1. Few-layered MoS2/graphene composite synthesized with the assistance of a watersolvable supramolecule (N-methylimidazole pillararene) delivered a high reversible specific capacity of 1100-1200 mAh g-1 at a current density of 100 mA g-1 with stable cyclic performance and enhanced rate capability for electrochemical lithium storage . On the other hand, heteroatom doping has been demonstrated one general strategy to change the microstructure and increase the electrical conductivity of metal sulfides and oxides . It has been reported that rational cation-doping (such as Ni2+, Co2+) could adjust the structure and morphology of MoS2 nanomaterials, resulting in improved electrochemical properties [26, 27]. Change in crystal engineering leads to cobalt or nickel atoms being located exclusively at the sulfur edges; hence achieving a high density of active sites [28-30]. Density functional theory (DFT) calculation and electrochemical measurement have revealed that Co-doping could greatly increase the activity of MoS2 for hydrogen evolution reaction (HER) . Dai et al have demonstrated that the optimized Co-doped MoS2 catalyst fabricated by a depositionprecipitation method showed superior electrochemical activity and excellent stability for HER . Zhang et al have also reported that the 3D defect-rich MoS2 nanomesh/RGO foam (5 wt% RGO to MoS2 and 2 mol% Co to Mo) achieved by a one-pot hydrothermal reaction exhibited prominent activity for HER . In our previous work , Co-doped MoS2/graphene hybrids synthesized through a one-pot 4 ACCEPTED MANUSCRIPT hydrothermal method showed a remarkable electrocatalytic activity toward HER. Codoped MoS2 is mostly applied for HER. We believe that hybridizing of Co-doped MoS2 layers with RGO sheets can significantly improve the electrochemical performance for reversible lithium storage. Herein, we present a facile one-pot hydrothermal route to synthesize Co-doped MoS2/RGO hybrids with different mole ratios (1:9, 1:4, 3:7, 1:1) of Co:Mo as anode materials for LIBs. The morphologies and microstructures of these hybrids were examined using X-ray diffraction (XRD) patterns, scanning electron microscope (SEM) images, high-resolution transmission electron microscopy (HRTEM) and Xray photoelectron spectroscopy (XPS). The effects of Co-doping on the microstructure and the electrochemical performance of MoS2/RGO for lithium ion storage were investigated. The results indicate that rational Co-doping can change microstructures and morphologies of Co-doped-MoS2/RGO hybrids, leading to significant improvement in their electrochemical performance. Especially, the Codoped-MoS2/RGO-2 hybrid with 1:4 mole ratio of Co:Mo shows a high reversible specific capacity of 1236 mAh g-1 with stable cyclic performance and enhanced highrate capability. The Co-doped-MoS2/RGO-2 hybrid with excellent electrochemical lithium storage property will hold high potential as host electrode material in LIB. 2. Experimental section 2.1 Synthesis of Co-doped-MoS2/RGO hybrids The natural graphite power (Shanghai Colloid Chemical Plant, China) was used for preparing graphene oxide sheets (GOS) by the modified Hummers method, the details of which were described elsewhere [18, 33], Co-doped-MoS2/RGO hybrids were synthesized by one-pot hydrothermal reaction. 20 mL of the solution containing 5 ACCEPTED MANUSCRIPT CoCl2�2O (0.15, 0.30, 0.45 or 0.75 mmol) was dropped in 20 mL suspension of GOS (3.0 mmoL) along with vigorous stirring. Due to a large amount of oxygencontaining functional groups (such as ?COO, C=O, ?OH), Co2+ ions could be well absorbed on the surface of GOS by coordination with these functional groups after stirring of 12 h at room temperature. Then, the mixed solution of Na2MoO4�2O (1.35, 1.20, 1.05 or 0.75 mmol) and L-cysteine (7.50 mmol) in 35 mL deionized water was added into the above suspension. The final mixed suspension was stirred for another 2.0 h. After that, the mixture was transferred into 100 ml Teflon-lined stainless steel autoclave, which was heated to 240 癈 and kept this temperature for 24 h. The black edimentation was collected by the centrifugation and washed for several times with water, and freeze-dried for 48 h. Finally, four hybrid samples were obtained, which are denoted as Co-doped-MoS2/RGO-1, Co-doped-MoS2/RGO-2, Co-doped-MoS2/RGO-3 and Co-doped-MoS2/RGO-4, respectively, prepared with different mole ratios (1:9, 1:4, 3:7, 1:1) of CoCl2:Na2MoO4 in hydrothermal reaction solution. For the control experiment, MoS2/RGO hybrid was prepared by the same hydrothermal route except for adding CoCl2�2O. 2.2 Characterizations The crystal structures of samples were characterized by XRD on a Thermo X?TRA X-ray diffractometer with Cu ?? radiation (?=0.154056 nm). The morphologies were observed by using a SIRION-100 field emission scanning electron microscop (FE-SEM). Thermogravimetric analysis (TGA) was carried out with a NETZSCH STA 409 PC apparatus at a heating rate of 5 oC min-1 in flowing air. High resolution transmission electron microscope (HRTEM) characterization was performed by using a JEOL JFL-2010 TEM operating at 200 kV. In order to HRTEM observation, sample was well dispersed in ethanol and drop-casted onto a 200 mesh 6 ACCEPTED MANUSCRIPT copper grid coated with holey carbon. X-ray photoelectron spectrum (XPS) analysis was performed on a PHI 5000 Versaprobe system using monochromatic Al K? radiation (1486.6 eV). All binding energies were referenced to the C 1s peak at 284.6 eV. 2.3 Electrochemical measurements The electrochemical measurements were carried out by using CR2032 buttontype cells with lithium foil served as both counter electrode and reference electrode. The working electrode was prepared by a slurry coating procedure. The slurry consisted of 80 wt% active material (MoS2/RGO or Co-doped-MoS2/RGO hybrid), 10 wt% acetylene black and 10 wt% polyvinylidene fluorides dispersed in N-methyl-2pyrrolidinone. The obtained slurry was spread on a copper foil, dried at 120 癈 for 12 h under vacuum, and then pressed to form the working electrode. The test cells were assembled in an argon-glove box, in which both the moisture and oxygen contents were below 1.0 ppm. A polypropylene film (Celgard-2300) was used as the separator. 1.0 mol L-1 LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate (EC/DMC, 1:1 in volume) was employed as electrolyte. Cyclic voltammetry was implemented on an electrochemical workstation (CHI 660E) at a scanning rate of 0.5 mV s-1 in a potential windrow of 0.005-3.00 V. Galvanostatic charging/discharging cycles were carried out on a LAND 2001A Battery Tester in a voltage range of 0.0053.00 V at various current densities. Electrochemical impendence spectroscopy (EIS) was performed on the electrochemical workstation (CHI 660E) by applying a sine wave with an amplitude of 5.0 mV over the frequency range from 200 kHz to 0.01 Hz 3. Results and discussions Fig. 1 7 ACCEPTED MANUSCRIPT Fig. 1 shows the XRD patterns of the MoS2/RGO and Co-doped-MoS2/RGO samples prepared by one-pot hydrothermal route. As illustrated in Fig. 1(a), even if three broad and poor peaks at 32.3o, 43.1o and 57.0o can be observed, respectively corresponding to (100), (006) and (110) of 2H-MoS2 (JCPDS No. 37-1492), the peak at 2?=14.3� attributed to (002) plane of 2H-MoS2 cannot be found. The fact indicates the extremely low crystallinity of MoS2 in as-prepared sample and poor-stacking of MoS2 along c-axis. Fig. 1(b, c) shows that both Co-doped-MoS2/RGO-1 and Codoped-MoS2/RGO-2 almost display the same XRD patterns in comparison with MoS2/RGO. The reflections of CoS2 hardly appear form the Co-doped-MoS2/RGO-1 and Co-doped-MoS2/RGO-2 hybrids. This means that that when the hydrothermal solution contains 0.15 mol or 0.30 mmol CoCl2 with CoCl2:Na2MoO4 of 1:9 or 1:4 in mole ratio, Co2+ ions can substitute for Mo form the Co-doped MoS2 layers by the simultaneous hydrothermal reaction of Co2+ and MoO42- with L-cysteine. From the XRD pattern of Co-doped-MoS2/RGO-3 and Co-doped-MoS2/RGO-4, the sharp peaks at 27.8 o, 32.1o, 36.2, 39.9o, 46.6o, 55.0o, 60.4o and 63.0 o can be found, which are respectively induced to (111), (200), (210), (211), (220), (311) (023) and (321) planes of CoS2 (JCPDS No. 65-3322). The fact indicates that there are CoS2 phase in both Co-doped-MoS2/RGO-3 and Co-doped-MoS2/RGO-4. It is due to that the excessive Co2+ ions cannot be doped into MoS2 lattices and are transferred to CoS2 particles during hydrothermal reaction. It is worth notice that all samples display a pronounced peak at around 9.0o (marked by #1) and a poor peak at 17.2o (marked by #2). By using Bragg's equation, the d-spacing of peak #1 is about 0.98 nm, it may be regarded as the distance of adjacent MoS2 layers. The interlayer expansion can be attributed to the incorporation of RGO and trapping of foreign species, such as NH4+ ions that are generated from the L-cysteine 8 under hydrothermal condition ACCEPTED MANUSCRIPT (HSCH2CHNH2COOH+H2O?CH3COCOOH+NH4++S2-) [25, 34]. The interlayer spacing of the reflection #2 at 2?=17.2o can be calculated to be about 0.52 nm, which can be ascribed to the spacing between MoS2 layer and RGO sheet as described in Fig 1(f). In addition, it can be found from Fig. 1 that the intensity of peak #2 at 2?=17.2o becomes more and more poor with increasing of Co2+ content in the hydrothermal solution. In particular, the peak #2 is too weak to be observed from the Co-dopedMoS2/RGO-4. It is due to that the excessive Co2+ ions preferentially react with Lcysteine to form CoS2 particles under hydrothermal condition, disturbing the wellgrowth of MoS2 layers on the surface of RGO. Finally, the peak at 2?=26.2�, induced to the restacked graphene (JCPDS No. 75-1621), does not appear for all the samples, which means that the RGO is synchronously prevented from restacking due to the hybridization with MoS2 or Co-doped MoS2 layers. Fig. 2 The general morphological features of as-prepared samples are characterized by SEM as shown in Fig 2. It is observed from Fig. 2(a) that the MoS2/RGO hybrid displays a graphene-like architecture consisted of the curved ultrathin flakes, from which the individual MoS2 flakes or particles can not be found, indicating that MoS2 layers uniformly growth on the surface of RGO to form sheet-on-sheet heterostructure. As shown in Fig 2(b, c), Co-doped-MoS2/RGO-1 and Co-dopedMoS2/RGO-2 hybrids display different morphological features to some extent compared to MoS2/RGO. It can be found from Fig. 2(b, c) that the curled MoS2 layers are well anchored on the surface of RGO. Especially, as shown in Fig. 2(c), numerous curled MoS2 layers can be clearly observed on the surface of RGO from the Codoped-MoS2/RGO-2 sample. Of course, these MoS2 layers should be Co-doped MoS2 for these two samples. The fact demonstrates that the rational Co-doping alters the 9 ACCEPTED MANUSCRIPT morphology of MoS2/RGO hybrid. When the hydrothermal solution contains 0.45 mol of CoCl2 and 1.05 mmol of Na2MoO4 (Co:Mo=3:7 in mole), a small amount of CoS2 particles are dispersed in the Co-doped-MoS2/RGO-3 sample as shown in Fig. 2(d). Especially, when the hydrothermal solution contains 0.75 mol of CoCl2 and 0.75 mmol of Na2MoO4 (Co:Mo=1:1 in mole), the as-prepared Co-doped-MoS2/RGO-4 clearly shows that quite a few of CoS2 particles with the sizes of 40-90 nm disperse on the curled graphene-like flakes as shown in Fig. 2(e), which agrees with its XRD analysis as shown in Fig.1 (e). Fig. 3 HRTEM characterization is carried out to obtain more structural details of the samples. Fig. 3(a, b) exhibits that interlaced MoS2 layers are scattered on RGO, which displays a few-layered structure with d-spacing of 0.97 nm. HRTEM images of Fig. 3(c-h) shows that the microstructures of Co-doped-MoS2/RGO-1, Co-dopedMoS2/RGO-2 and Co-doped-MoS2/RGO-3 are on the whole similar to that of MoS2/RGO. However, the three Co-doped-MoS2/RGO hybrids show that the MoS2 layers display shorter crystal fringes with less layer number and more exposed edges, particularly for Co-doped-MoS2/RGO-2 as shown in Fig. 3(e, f). In addition, the interlayer spacing of 0.96, 0.97 nm and 0.94 nm in Fig. 3(d, f, h) are consistent with their XRD analysis. No crystal fringes of CoS2 can be found from both Co-dopedMoS2/RGO-1 and Co-doped-MoS2/RGO-2, indicating the well-doping of Co2+ ions into MoS2 lattices. Even if HRTEM image of Co-doped-MoS2/RGO-3 hardly presents the crystal fringes of CoS2 as shown Fig.3 (g, h), its XRD pattern shows the peaks of CoS2 phase as shown in Fig. 1 (d). For the Co-doped-MoS2/RGO-4, quite a few CoS2 particles can be clearly viewed from Fig. 3(i, j). The lattice spacing of 0.27 nm is assigned to the (200) planes of CoS2 (JCPDS No. 65-3322). 10 ACCEPTED MANUSCRIPT Fig. 4. In order to investigate the specific capacity contribution of MoS2 (or Co-doped MoS2) and RGO, TGA was employed to determine the content of RGO in the different hybrids as shown in Fig. 4. Fig. 4(a) shows the MoS2/RGO hybrid displays three mass losses in the TGA curve. The first mass loss from room temperature to 90-100 oC should be due to the volatilization of the trace moisture. The second appeared at about 230 oC is attributed to the removal of oxygen-containing groups of RGO. The third is a large continuous weight loss in the range of 350-470 oC, which is caused by the decomposition of RGO and the oxidation of MoS2 to MoO3 in the flowing air. As shown in Fig. 4(b-e), the Co-doped-MoS2/RGO hybrids exhibit the similar TGA curves to that of MoS2/RGO, except for a small mass loss in the range of 530-600 oC. The RGO would be completely oxidized to CO2 under 700 oC in air . The finally remained substance should be MoO3 for MoS2/RGO sample or Co3O4 and MoO3 for Co-doped-MoS2/RGO hybrids. Therefore, the contents of RGO can be calculated to be respectively about 18.7wt% for MoS2/RGO, 22.1wt% for Co-doped-MoS2/rGO-1, 22.7wt% for Co-doped-MoS2/rGO-2, 23.8wt% for Co-doped-MoS2/rGO-3 and 24.6wt% for Co-doped-MoS2/rGO-4. Fig. 5 XPS was used to further analyze the elemental composition and chemical state of the typical sample, Co-doped-MoS2/RGO-2 hybrid. As shown in Fig. 5(a), the survey spectroscopy reveals that the Co-doped-MoS2/RGO-2 sample consists of C, O, S, Mo and Co elements. The high-resolution scan of C 1s in Fig. 5(b) exhibits four peaks located at 284.6 eV, 285.4 eV, 286.4 eV and 288.6 eV, which are ascribed to the carbon-functional groups C=C, C?O, C?O?C and C=O, respectively . Fig. 5(c) states that the S 2p peak can be deconvoluted to four peaks. The two peaks at 161.2 11 ACCEPTED MANUSCRIPT eV and 162.4 eV manifest the S 2p3/2 and S 2p1/2 for S2-, while the other two peaks at 163.4 eV and 164.1 eV can be attributed to the S 2p3/2 and S 2p1/2 for apical S2ligands or/and bridging disulfides S22- [36-38]. The high-resolution XPS of the hybrid in the Mo 3d region can be divided into six peaks. One peak at 225.7 eV actually is assigned to the S 2s of MoS2. Two main peaks arising from 228.4 eV and 231.7 eV correspond to Mo (IV) 3d5/2 and Mo (IV) 3d3/2 binding energy, respectively. The two relatively small peaks at 232.7.0 eV (Mo 3d5/2) and 235.6 eV (Mo 3d3/2) suggest that Mo (IV) is partially oxidized to Mo (VI) in the air [39-41]. Moreover, a shoulder peak at 229.4 eV manifests that Mo (VI) was partially reduced to Mo (V) [40-42]. The Co 2p XPS spectra in Fig. 4(e) can be well fitted with two spin?orbit doublets, which are characteristic of Co2+ and Co3+, and two shake-up satellites (identified as ??Sat.??). The two prominent peaks at 779.5 eV and 798.3 eV, attributed to Co 2p3/2 and Co 2p1/2 of Co2+ species, are very near to the binding energy of the Co species in CoS2 [43-45]. The slight difference is due to the Co doping into the MoS2 lattice. Even if CoS2 phase in the Co-doped-MoS2/RGO-2 hybrid can not be detected by XRD, SEM and HRTEM, a XPS peak for Co2+ can be observed, indicating that Co2+ ions can be well doped into the MoS2 lattice. The quantitative analysis of the XPS peak states that the atomic ratio of Mo:Co:S is about 0.80:0.23:2.5, which generally agrees with the mole ratios of CoCl2 and Na2MoO4 in the hydrothermal solution and also is approaching to the stoichiometry of Co0.2Mo0.8S2. Fig. 6. To clearly reveal the redox reaction of the MoS2/RGO and Co-doped-MoS2/RGO hybrids for reversible lithium storage, the cycle voltammetry (CV) was performed in the potential windrow of 0.005-3.0 V at a scan rate of 0.5 mV s-1. Fig. 6(a) shows that two conspicuous peaks appear in the 1st cathodic scanning. The peak at 1.40 V is 12 ACCEPTED MANUSCRIPT ascribed to the intercalation of lithium ion into the MoS2 lattices (MoS2+xLi+ +xe?LixMoS2) [10, 46]. Lithium intercalates into the S slab, and the van der Waals S?S bonds are broken to be replaced by Li?S bonds, resulting in a phase change from 2H to 1T. The peak at 0.40 V is attributed to the conversion reaction of LixMoS2 into metallic Mo and Li2S matrix (LixMoS2+(4-x)Li++(4-x)e ? Mo+2Li2S) [47, 48]. These metallic Mo clusters are highly uniformly embedded in the Li2S matrix in atomic level. There are abundant interface phases between Mo clusters and Li2S matrix. In the subsequent anodic sweep, two peaks can be distinguished at about 1.92 V and 2.28 V. The peak at 1.92 V can be attributed to the removal of lithium associated with Mo or partial oxidation of Mo [46, 47]. The peak at 2.28 V is assigned to the delithation of Li2S (Li2S ? S+2Li++2e-). At the end of the 1st cycle, most of the MoS2 should be converted to Mo and S (MoS2+4Li++4e 2Li2S+Mo and Li2S?S+2Li++2e) . In the 2nd and 3rd cathodic sweep, the two peaks at 1.4 V and 0.40 V do not appear, while two new peaks at about 2.0 V and 1.37 V arise. The peak at 2.0 V pairs with anodic peak at 2.28 V is the reduction of S forming Li2S (S+2Li++2e-? Li2S). The peak at 1.37 V is attributed to the association of Li+ ions with molybdenum . The increasing cathodic current below 0.3 V is due to the lithium storage in the defect sites and the interfaces between Mo clusters and Li2S. As shown in Fig. 6(b, c, d), Co-doped-MoS2/RGO-1, Co-doped-MoS2/RGO-2 and Codoped-MoS2/RGO-3 electrodes exhibit the similar CVs feature compared to MoS2/RGO, except for that at the 1st cathodic sweep, the peaks of Li+ insertion slightly move from 1.40 V to lower potential (1.12 V, 1.08 V or 0.90 V). For Codoped-MoS2/RGO-4 hybrid electrode, Fig. 6(e) shows the three reduced peaks at 1.30 V, 0.95 V and 0.55 V at the 1st cathodic sweep. Since XRD, SEM and HRTEM characterizations have demonstrated that there are quite a few CoS2 particles in Co13 ACCEPTED MANUSCRIPT doped-MoS2/RGO-4 hybrid, the three reduced peaks should be attributed to the first lithium storage process of CoS2 and Co-doped-MoS2/RGO, including intercalation of lithium ions into CoS2 and Co-doped MoS2 lattices (MS2+xLi++xe?LixMS2, M=Co, Mo)  and the successive conversion reaction into metallic Co and Mo embedded in a Li2S matrixes (LixMS2+(4-x)Li++(4-x)e ?M+2Li2S, M=Co, Mo) [49, 50]. Fig. 7 Fig. 7 shows the first three charge/discharge voltage curves of MoS2/RGO and Co-doped-MoS2/RGO hybrid electrodes at the current density of 100 mA g-1 between 0.005 V and 3.00 V. As shown in Fig. 7(a), in the 1st discharge (lithiation process), the plateau at 1.5 V represents the insertion of lithium ions into MoS2 lattice to form LixMoS2 with the change of phase from 2H to 1T and the variation of plateau is due to the poor crystallinity of MoS2. Another plateau at around 0.65 V is indicative of the conversion reaction of LixMoS2 to generate metallic Mo highly embedded in Li2S matrixes. The sloping potential curve below 0.4 V is attributed to lithium storage in the defect sites of active material and the interfaces between metallic Mo and Li2S matrix. During the 2nd and 3rd discharge, there are two plateaus located at 2.1 V and 1.5 V, which are ascribed to the lithation of S forming Li2S and association of lithium ions with molybdenum. In the charge process (delithiation), MoS2/RGO electrode displays two plateaus at 1.85 V and 2.20 V, which well agree with its CVs. As pictured in Fig. 7(b, c, d, e), the four Co-coped-MoS2/RGO hybrids almost exhibit the sample voltage plateau features in their charge/discharge curves compared to the MoS2/RGO electrode, expect for that the 1st discharge curve of the Co-copedMoS2/RGO-4 is different form those of other samples due to the existence of quite a few of CoS2 particles. Fig. 7 also shows that the Co-coped-MoS2/RGO-2 hybrid exhibits higher specific capacity for electrochemical lithium storage than other 14 ACCEPTED MANUSCRIPT samples. As shown Fig. 7(a), at the first cycle, MoS2/RGO delivers an initial discharge capacity of 1382.6 mAh g-1 and a reversible charge capacity of 929.3 mAh g-1 with a Coulombic efficiency of 67.2%. The reversible specific capacity delivered by MoS2/RGO is much higher the theoretical value of MoS2 (670 mAh/g), which is due to the defects or disorder structures of MoS2 layers dispersed on RGO and the synergism between MoS2 layers and RGO. The irreversible capacity loss in the first cycle is mainly caused by the formation of a solid electrolyte interface (SEI) film and other the incomplete conversion reaction such as reduction of oxygen-containing groups [47, 50]. Besides, during the charge/discharge process, a small amount of lithium ions are trapped in the defect sites and hardly delithated, leading to irreversible capacity. Fig. 7(c) shows that the Co-coped-MoS2/RGO-2 can deliver an initial discharge capacity of 1385.3 mAh g-1 and a reversible capacity of 1235.2 mAh g-1 with a Coulombic efficiency of 89.2%. The corresponding values are respectively 1292.5 mAh g-1 and 1120.6 mAh g-1 with a Coulombic efficiency of 86.7% for Cocoped-MoS2/RGO-1, 1171.6 mAh g-1 and 1020.5 mAh g-1 with a Coulombic efficiency of 87.1% for Co-coped-MoS2/RGO-3, 1173.5 mAh g-1 and 905.6 mAh g-1 with a Coulombic efficiency of 77.2% for Co-coped-MoS2/RGO-4. It is concluded that rational Co-doping not only increases the reversible specific capacity, but also greatly improves the Coulombic efficiency at the first cycle, which is one of important factors for the practical application in LIB. Especially, the first Coulombic efficiency of Co-doped-MoS2/RGO-2 is up to 89.2%, which is much higher than that (67.2%) of MoS2/RGO in this work and also higher than other MoS2/RGO composites reported elsewhere [27, 53, 54]. In comparison with MoS2/rGO, the rational Co-doping can alter the microstructure and morphology, which facilitates the diffusion of ions during electrode reaction and the accessibility of electrolyte into active materials. In addition, 15 ACCEPTED MANUSCRIPT it was reported that the Co-doping greatly enhanced the conductivity of MoS2 , which would further reduce charge-transfer resistance of Co-doped-MoS2/RGO-2 electrode for reversible lithium storage. Therefore, Co-doped-MoS2/RGO-2 not only delivers higher reversible specific capacity, but also exhibits higher Coulombic efficiency at the 1st cycle than MoS2/RGO. At the 2nd and 3rd cycle, the Coulombic efficiencies increase up to above 96% for all electrodes as shown in Fig. 7. Fig. 8. Fig. 8(a) illustrates the cyclic performance of MoS2/RGO and Co-dopedMoS2/RGO electrodes for reversible lithium storage at a current density of 100 mA g1. It has been reported that the MoS2 nanostructures prepared by different methods including hydrothermal routes could deliver a specific capacity of 800-1000 mAh g-1 [12, 13]. But the pristine MoS2 exhibits a poor cyclic stability due to its agglomeration and pulverization during repeat lithiation/delithation process, leading to the destruction of electrode and the loss of capacity [12, 13]. The hybridization with graphene (or RGO) nanosheets has been demonstrated an effective strategy to improved the cyclic durability. Thus, it is reasonable that all electrodes exhibit good cyclic stability as shown Fig. 8(a). It can also be observed in Fig. 8(a) that after 5-10 cycles, Co-doped-MoS2/RGO-2 electrode exhibits a reversible capacity of 1236 mAh g-1, which is much larger that of MoS2/RGO (898 mAh g-1), and also larger than those of Co-doped-MoS2/RGO-1 (1085 mAh g-1), Co-doped-MoS2/RGO-3 (1019 mAh g-1) and Co-doped-MoS2/RGO-4 (902 mAh g-1). The specific capacity of RGO prepared by L-cysteine assisted hydrothermal method is about 860 mAh g-1 . According to the contents of RGO in the hybrids, the specific capacity contribution of MoS2 or Codoped-MoS2 can be calculated to be 907, 1149, 1341, 1068 and 917 mAh g-1, respectively, for MoS2/RGO, Co-doped-MoS2/RGO-1, Co-doped-MoS2/RGO-2, Co16 ACCEPTED MANUSCRIPT doped-MoS2/RGO-3 and Co-doped-MoS2/RGO-4. It was reported that the specific capacity contribution of the exfoliated MoS2 sheets in MoS2/PEO composite was 1131 mAh g-1, due to that the more defects or disorder structures of the exfoliated MoS2 greatly enhanced the lithium accommodate capability. One can be found that the specific capacity contributions of Co-doped MoS2 in Co-doped-MoS2/RGO-1 and Co-doped-MoS2/RGO-2 are very closed to or higher than that (1131 mAh/g) of the exfoliated MoS2 in MoS2/PEO. The higher specific capacity contribution of Codoped-MoS2 in Co-doped-MoS2/RGO-2 should be attributed to that Co-doped-MoS2 layers exhibit more defects or disorder structures than the exfoliated MoS2 sheets. Another reason for such high specific capacity of the Co-doped-MoS2/RGO-2 is due to the synergistic effects between Co-doped MoS2 layers and RGO, as well as the enhanced conductivity of Co-doped MoS2. At the 100th cycle, Co-doped- MoS2/RGO-2 still retains a reversible specific capacity of 1223 mAh g-1, indicating its excellent cyclic stability. High-rate capability is an important aspect to obtain high power density in LIB. Fig. 8(b) shows the rate cycling behavior of the MoS2/RGO and Co-dopedMoS2/RGO electrodes at different current densities. It can be seen from Fig. 8(b) that at a high current density of 1000 mA g-1, Co-doped-MoS2/RGO-2 can delivers a reversible capacity of 894 mAh g-1, which is larger that of MoS2/RGO (692 mAh g-1), Co-doped-MoS2/RGO-1 (795 mAh g-1) and Co-doped-MoS2/RGO-3 (750 mAh g-1). The fact indicates that rational Co-doping can significantly improve the rate capability of MoS2/RGO based electrode materials. In addition, Fig. 8(b) also shows that when the current density return to 100 mA g-1 from 1000 mA g-1, all electrodes can recover the specific capacity to their original values, indicating that they still can keep good cyclic durability after the cycling at the different current densities. 17 ACCEPTED MANUSCRIPT The results of electrochemical measurements reveal that the rational Co-doping can further improve the electrochemical performance of MoS2/RGO hybrid, because the rational Co-doping changes the microstructure and morphology of the hybrid. These changes not only provide more sites for the lithium accommodate, but more facilitates the diffusion of ions and the access of electrolyte. Among of all samples in this work, the Co-doped-MoS2/RGO-2 hybrid displays best electrochemical property. It not only delivers a high specific capacity of 1236 mAh g-1 with stable cyclic performance, but also exhibits significantly enhanced high-rate capability. In order to well understand the remarkable electrochemical performance of Codoped-MoS2/RGO-2 hybrid in comparison with MoS2/RGO and gain insight into their kinetics characteristics of the electrochemical lithium storage process, EIS of MoS2/RGO and Co-doped-MoS2/RGO-2 electrodes are analyzed after 10 cycles as shown in Fig. 8(c). Fig. 8(d) is the corresponding equivalent circuits for EIS fitting by using the Z-view software. Fig. 8(c) shows that the Nyquist plots of MoS2/RGO and Co-doped-MoS2/RGO-2 electrodes consist of an incline line in the low-frequency region and two depressed semicircles in the medium- and high-frequency region. The incline line in the low-frequency region corresponds to Warburg impedance (Zw), reflecting the diffusion process of lithium ions from electrolyte into the bulk of the electrode material. The semicircle in the high-frequency region is related to the interface resistance (Rf) and capacitance (CPE1) of electrode active particles, while the semicircle in the medium-frequency region reflects the charge-transfer resistance (Rct) of the electrode reaction and the electrochemical double layer capacitance (CPE2) between electrode materials and electrolyte [55, 56]. The parameter Re corresponds to internal Ohmic resistance of test cell, which contains the resistance of current collector, electrolyte and contact resistance. As shown in Fig. 8(b), the EIS 18 ACCEPTED MANUSCRIPT fitting results well agree with the experimental date. The values of Re, Rf and Rct are respectively 28.8 ?, 16.3 ? and 8.1 ? for Co-doped-MoS2/RGO-2 electrode, and 29.9 ?, 21.2 ?, 12.9 ? for MoS2/RGO electrode. It can be clearly found that the Codoped-MoS2/RGO-2 electrode exhibits lower Rct than MoS2/RGO, manifesting that Co-doping effectively improve charge-transfer process for reversible lithium storage. As well known, the incorporation of graphene (or RGO) can greatly promote the charge transfer process of electrode reaction due to its superior conductivity and high charge mobility, resulting in the great reduce of charge-transfer resistance of MoS2/RGO electrode. Thus, in this work, MoS2/RGO electrode also shows a low Rct of 12.9 ?, which is much lower than that (71.5 ?) of the pristine MoS2 electrode prepared through the similar hydrothermal route in our previous work . In comparison with MoS2/RGO, the further improvement in the electrode kinetics of Codoped-MoS2/RGO-2 should be attributed to the rational Co-doping. It has been reported that the Co-doping could greatly improve the conductivity of MoS2 . At room temperature, only 3% Co-doping can increased the conductivity of MoS2 layers by one order of magnitude . Therefore, the rational Co-doping can further reduces charge-transfer resistance of MoS2/RGO-based electrode for reversible lithium storage. Additionally, the current work also demonstrates that the rational Co-doping can change the microstructure and morphology of MoS2/RGO hybrids, facilitating the diffusion of ions during electrode reaction and the accessibility of electrolyte into active materials. Therefore, the electrochemical lithium storage performance of Codoped-MoS2/RGO-2 is significantly improved in comparison with MoS2/RGO. 4. Conclusions In summary, this work has presented a facial one-pot hydrothermal route to 19 ACCEPTED MANUSCRIPT fabricate Co-doped-MoS2/RGO hybrids by simultaneous hydrothermal reaction of Co2+ ions and MoO42- with L-cysteine in the presence of GOS. It is demonstrated that the rational Co-doping can change the morphology and microstructure of the Codoped-MoS2/RGO hybrids, resulting in significantly enhanced electrochemical lithium storage property. When the mole ratio of CoCl2:Na2MoO4 in the hydrothermal solution is 1:9 or 1:4, Co2+ ions can effectively dope into MoS2 lattices. When the mole ratio of CoCl2:Na2MoO4 in the hydrothermal solution is 3:7 or 1:1, the excessive Co2+ ions would be transferred into CoS2 particles dispersed in the hybrids. Electrochemical measurements show that the Co-doped-MoS2/RGO-2 hybrid with the Co:Mo of 1:4 in mole exhibits a high reversible capacity of 1236 mAh g-1 at the current density of 100 mA g-1 with excellent cyclic stability and enhanced high-rate capability of 895 mAh g-1 at the current density of 1000 mA g-1. In addition, the Codoped-MoS2/RGO-2 exhibits much higher Coulombic efficiency of 89.2% at the first cycle than that of MoS2/RGO (67.2%). The superior electrochemical performance of Co-doped-MoS2/RGO-2 is attributed to the more defects or disorder structures of Codoped MoS2 layers dispersed on the surface RGO, which greatly enhance lithium accommodation capability. In addition, due to the synergistic effects between Codoped-MoS2 layers and RGO, Co-doped-MoS2/RGO-2 exhibits significantly enhanced high-rate capability and electrode reaction kinetics, including low chargetransfer resistance and improved CE in the 1st cycle. The present results turn out that the Co-doped-MoS2/RGO-2 hybrid can be used as a promising electrode material for high-performance LIBs. Acknowledgements This work is financially supported by the Natural Science Foundation of China 20 ACCEPTED MANUSCRIPT (21473156), the Science and Technology Project of Zhejiang Province of China (2015C01001), the International Science and Technology Cooperation Program of China (2012DFG42100) and the Fundamental Research Funds for the Central Universities (2017XZZX008-06). References  J.B. Goodenough, K.S. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167-1176.  J. Liang, F. Li, H. M. Cheng, High-capacity lithium ion batteries: bridging future and current, Energy Storage Mater. 4 (2016) A1-A2.  L. Cong, H. Xie, J. Li, Hierarchical structures based on two-dimensional nanomaterials for rechargeable lithium batteries, Adv. Energy Mater. 7 (2017) 1601906.  L. Peng, Y. Zhu, D. Chen, R.S. Ruoff, G. Yu, Two-dimensional materials for beyondlithium-ion batteries, Adv. Energy Mater. 6 (2016) 160025.  Y. Sun, N. Liu, Y. Cui, Promises and challenges of nanomaterials for lithium-based rechargeable batteries, Nature Energy 1 (2016) 16071.  H. S. Fan, H Yu, Y. F. Zhang, J. Guo, Z. Wang, H. Wang, X. Hao, N. Zhao, H. B. Geng, Z. F. Dai, Q. Y. Yan, J. Xu, From zinc-cyanide hybrid coordination polymers to hierarchical yolk-shell structures for high-performance and ultra-stable lithium-ion batteries, Nano Energy, 33 (2017) 168-176  H. Yu, H. S. Fan, X. L. Wu, H. W. Wang, Z. Z. Luo, H. T. Tan, B. Yadian, Y. Z. Huang, Q. Y. Yan, Diffusion induced concave [email protected] heterostructures for high performance lithium ion battery anode, Energy Storage Materials, 4 (2016) 145-153.  W. Zhang, K. Huang, A review of recent progress in molybdenum disulfide-based supercapacitors and batteries, Inorg. Chem. Front. 4 (2017) 1602-1620.  R. Tenne, Advances in the synthesis of inorganic nanotubes and fullerene-like nanoparticles, Angew. Chem. Int. Ed. 42 (2003) 5124-5132.  T. Stephenson, Z. Li, B. Olsenab, D. Mitlin, Lithium ion battery applications of molybdenum disulfide (MoS2) nanocomposites, Energy Environ. Sci. 7 (2014) 209-231.  H. Yu, C. Ma, B. Ge, Y. Chen, Z. Xu, C. Zhu, C. Li, Q. Ouyang, P. Gao, b. Jianqi Li, C. Sun, L. Qi, Y. Wang, F. Li, Three-dimensional hierarchical architectures constructed by 21 ACCEPTED MANUSCRIPT graphene/MoS2 nanoflake arrays and theirrapid charging/discharging properties as lithium-ion battery anodes, Chem. Eur. J. 19 (2013) 5818-5823.  J. Xiao, D. Choi, L. Cosimbescu, P. Koech, J. Liu, J.P. Lemmon, Exfoliated MoS2 nanocomposite as an anode material forlithium ion batteries, Chem. Mater. 22 (2010) 4522-4524.  G. Du, Z. Guo, S. Wang, R. Zeng, Z. Chen, H. Liu, Superior stability and high capacity of restacked molybdenum disulfide as anode material for lithium ion batteries, Chem. Commun. 46 (2010) 1106-1108.  P. Wang, H. Sun, Y. Ji, W. Li, X. Wang, Three-dimensional assembly of single-layered MoS2, Adv. Mater. 26 (2014) 964-969.  Q.H. Wang, K. Kalantar-Zadeh, A. Kis, Jonathan N. Coleman, M.S. Strano1, Electronics and optoelectronics of two-dimensional transition metal dichalcogenides, Nature Nanotech. 7 (2012) 699-712.  G. Li, X. Zeng, T. Zhang, W. Ma, W. Li, M. Wang, Facile synthesis of hierarchical hollow MoS2 nanotubes as anode materials for high-performance lithium-ion batteries, CrystEngComm. 16 (2014) 10754-10759.  P. Xiong, J. Zhu, L. Zhang, X. Wang, Recent advances in graphene-based hybrid nanostructures for electrochemical energy storage, Nanoscale Horiz. 1 (2016) 340-374.  K. Chang, W. Chen, L-cysteine-assisted synthesis of layered MoS2/graphene composites with excellent electrochemical performances for lithium ion batteries, ACS Nano 5 (2011) 4720-4728.  S.B. Patil, K. Adpakpang, S.M. Oh, J.M. Lee, S.-J. Hwang, Reductive hybridization route with exfoliated graphene oxide and MoS2 nanosheets to efficient electrode materials, Electrochim. Acta 176 (2015) 188-196.  D. Xie, W.J. Tang, X.H. Xia, D.H. Wang, D. Zhou, F. Shi, X.L. Wang, C.D. Gu, J.P. Tu, Integrated 3D porous C-MoS2/nitrogen-doped graphene electrode for high capacity and prolonged stability lithium storage, J. Power Sources 296 (2015) 392-399.  J. He, C. Zhang, H. Du, S. Zhang, P. Hu, Z. Zhang, Y. Ma, C. Huang, G. Cui, Engineering vertical aligned MoS2 on graphene sheet towards thin film lithium ion battery, Electrochim Acta 178 (2015) 476-483.  Y. Sun, Q. Wu, G. Shi, Graphene based new energy materials, Energy Environ. Sci. 4 (2011) 1113-1132.  D.H. Youn, C. Jo, J.Y. Kim, J. Lee, J.S. Lee, Ultrafast synthesis of MoS2 or WS222 ACCEPTED MANUSCRIPT reduced graphene oxide composites via hybrid microwave annealing for anode materials of lithium ion batteries, J. Power Sources 295 (2015) 228-234.  Z. Yu, J. Ye, W. Chen, S. Xu, F. Huang, Fabrication of few-layer molybdenum disulfide/reduced graphene oxide hybrids with enhanced lithium storage performance through a supramolecule-mediated hydrothermal route, Carbon 114 (2017) 125-133.  Y. Jung, Y. Zhou, J.J. Cha, Intercalation in two-dimensional transition metal chalcogenides, Inorg. Chem. Front. 3 (2016) 452-463.  K. Zhang, H.-J. Kim, J.-T. Lee, G.-W. Chang, X. Shi, W. Kim, M. Ma, K.-j. Kong, J.-M. Choi, M.-S. Song, J.H. Park, Unconventional pore and defect generation in molybdenum disulfide: application in high-rate lithium-ion batteries and the hydrogen evolution reaction, ChemSusChem 7 (2014) 2489-2495.  X. Zhang, R. Zhao, Q. Wu, W. Li, C. Shen, L. Ni, H. Yan, G. Diao, M. Chen, Petal-like MoS2 nanosheets space-confined in hollow mesoporous carbon spheres for enhanced lithium storage performance, ACS Nano 11 (2017) 8429-8436.  J. Bonde, P.G. Moses, T.F. Jaramillo, J.K. N鴕skov, I. Chorkendorff, Hydrogen evolution on nano-particulate transition metal sulfides, Faraday Discuss. 140 (2008) 219231  M. Brorson, A. Carlsson, H. Tops鴈, The morphology of MoS2, WS2, Co-Mo-S, Ni-MoS and Ni-W-S nanoclusters in hydrodesulfurization catalysts revealed by HAADFSTEM, Catal. Today 123 (2007) 31-36.  J.V. Lauritsen, S. Helveg, E. L鎔sgaard, I. Stensgaard, B.S. Clausen, H. Tops鴈, F. Besenbacher, Atomic-scale structure of Co-Mo-S nanoclusters in hydrotreating catalysts, J. Catal. 197 (2001) 1-5.  X. Dai, K. Du, Z. Li, M. Liu, Y. Ma, H. Sun, X. Zhang, Y. Yang, Co-doped MoS2 nanosheets with the dominant CoMoS phase coated on carbon as an excellent electrocatalyst for hydrogen evolution, ACS Appl. Mater. Interfaces 7 (2015) 2724227253.  J. Ye, W. Chen, S. Xu, Z. Yu, S. Hou, Synthesis of Co-doped MoS2/graphene hybrids as enhanced electrocatalysts for the hydrogen evolution reaction, RSC Adv. 6 (2016) 104925-104932.  B.W.S. HUMMERS, JR., R.E. OFFEMA, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339.  J. Guo, H. Zhu, Y. Sun, L. Tang, X. Zhang, Doping MoS2 with graphene quantum dots: 23 ACCEPTED MANUSCRIPT structural and electrical engineering towards enhanced electrochemical hydrogen evolution, Electrochim. Acta 211 (2016) 603-610.  X. Zheng, J. Xu, K. Yan, H. Wang, Z. Wang, S. Yang, Space-confined growth of MoS2 nanosheets within graphite: the layered hybrid of MoS2 and graphene as an active catalyst for hydrogen evolution reaction, Chem. Mater. 26 (2014) 2344-2353.  P. K. Chow, E. Sing, B. C. Viana, J. Gao, J. Luo, J. Li, Z. Lin, A.L. Elias, Y.F. Shi, Z.K. Wang, M. Terrones, N. Koratkar, Wetting of mono and few-layered WS2 and MoS2 films supported on Si/SiO2 substrates, ACS Nano. 9 (2015) 3023-3031.  J. Gao, B. Li, J. Tan, P. Chow, T.-M. Lu, N. Koratkar, Aging of transition metal dichalcogenide monolayers, ACS Nano 10 (2016) 2628-2635.  P. Li, Z. Yang, J. Shen, H. Nie, Q. Cai, L. Li, M. Ge, C. Gu, X.a. Chen, K. Yang, L. Zhang, Y. Chen, S. Huang, Subnanometer molybdenum sulfide on carbon nanotubes as a highly active and stable electrocatalyst for hydrogen evolution reaction, ACS Appl. Mater. Interfaces 8 (2016) 3543-3550.  J.D. Benck, Z. Chen, L.Y. Kuritzky, A.J. Forman, T.F. Jaramillo, Amorphous molybdenum sulfide catalysts for electrochemical hydrogen production: insights into the origin of their catalytic activity, ACS Catal. 2 (2012) 1916-1923.  S. Zhao, C. Li, L. Wang, N. Liu, S. Qiao, B. Liu, H. Huang, Y. Liu, Z. Kang, Carbon quantum dots modified MoS2 with visible-light-induced high hydrogen evolution catalytic ability, Carbon 99 (2016) 599-606.  Y.J. Tang, Y. Wang, X.L. Wang, S.L. Li, W. Huang, L.Z. Dong, C.H. Liu, Y.F. Li, Y.Q. Lan, Molybdenum disulfide/nitrogen-doped reduced graphene oxide nanocomposite with enlarged interlayer spacing for electrocatalytic hydrogen evolution, Adv. Energy Mater. 6 (2016) 1600116.  X.J. Lv, G.W. She, S.X. Zhou, Y.M. Li, Highly efficient electrocatalytic hydrogen production by nickel promoted molybdenum sulfide microspheres catalysts, RSC Adv. 3 (2013) 21231.  M.S. Faber, R. Dziedzic, M.A. Lukowski, N.S. Kaiser, Q. Ding, S. Jin, Highperformance electrocatalysis using metallic cobalt pyrite (CoS2) micro- and nanostructures, J. Am. Chem. Soc. 136 (2014) 10053-10061.  M. Cab醤-Acevedo, M.L. Stone, J.R. Schmidt, J.G. Thomas, Q. Ding, H.-C. Chang, M.L. Tsai, J.-H. He, S. Jin, Efficient hydrogen evolution catalysis using ternary pyrite-type cobalt phosphosulphide, Nature Mater. 14 (2015) 1245-1251. 24 ACCEPTED MANUSCRIPT  D.C. Higgins, F.M. Hassan, M.H. Seo, J.Y. Choi, M.A. Hoque, D.U. Lee, Z. Chen, Shape-controlled octahedral cobalt disulfide nanoparticles supported on nitrogen and sulfur-doped graphene/carbon nanotube composites for oxygen reduction in acidic electrolyte, J. Mater. Chem. A 3 (2015) 6340-6350.  L. Ma, J. Ye, W. Chen, J. Wang, R. Liu, J.Y. Lee, Synthesis of few-layer MoS2-graphene composites with superior electrochemical lithium-storage performance by an ionicliquid-mediated hydrothermal route, ChemElectroChem. 2 (2015) 538-546.  K. Chang, W. Chen, Single-layer MoS2/graphene dispersed in amorphous carbon: towards high electrochemical performances in rechargeable lithium ion batteries, J. Mater. Chem. 21 (2011) 17175-117184.  Q. Wang, L. Jiao, Y. Han, H. Du, W. Peng, Q. Huan, D. Song, Y. Si, Y. Wang, H. Yuan, CoS2 hollow spheres: fabrication and their application in Lithium-Ion batteries, J. Phys. Chem. C. 115 (2011) 8300-8304.  X. Fang, X. Yu, S. Liao, Y. Shi, Y.-S. Hu, Z. Wang, G.D. Stucky, L. Chen, Lithium storage performance in ordered mesoporous MoS2 electrode material, Microporous and Mesoporous Mater. 151 (2012) 418-423.  X. Sun, Y. Li, Hollow carbonaceous capsules from glucose solution, J. Colloid Interface Sci. 291 (2005) 7-12.  J. Pan, C. Song, X. Wang, X. Yuan, Y. Fang, C. Guo, W. Zhao, F. Huang, Intermediate bands of MoS2 enabled by Co doping for enhanced hydrogen evolution, Inorg. Chem. Front. 4 (2017) 1895-1899.  K. Chang, Z. Wang, G. C. Huang, H. Li, W. X. Chen, J. Y. Leeb, Few-layer SnS2/graphene hybrid with exceptional electrochemical performance as lithium-ion battery anode, J Power Sources 201 (2012) 259- 266  J. Wang, J. Liu, D. Chao, J. Yan, J. Lin, Z.X. Shen, Self-assembly of honeycomb-like MoS2 nanoarchitectures anchored into graphene foam for enhanced lithium-ion storage, Adv. Mater. 26 (2014) 7162-7169.  Y. Chao, R. Jalili, Y. Ge, C. Wang, T. Zheng, K. Shu, G.G. Wallace, Self-assembly of flexible free-standing 3D porous MoS2 reduced graphene oxide structure for highperformance lithium-ion batteries, Adv. Funct. Mater. 27 (2017) 1700234.  G. Huang, T. Chen, W. Chen, Z. Wang, K. Chang, L. Ma, F. Huang, D. Chen, J.Y. Lee, Graphene-Like MoS2/graphene composites: cationic surfactant-assisted hydrothermal synthesis and electrochemical reversible storage of lithium, Small 9 (2013) 3693-3703. 25 ACCEPTED MANUSCRIPT  L. Hu, Y. Ren, H. Yang, Q. Xu, Fabrication of 3D hierarchical MoS2/polyaniline and MoS2/C architectures for lithium-ion battery applications, ACS Appl. Mater. Interfaces 6 (2014) 14644-14652. 26 ACCEPTED MANUSCRIPT Graphical Abstract (text) Co-doped-MoS2/reduced graphene oxides (RGO) hybrids are synthesized by facile one-pot hydrothermal method. The Co-doped-MoS2/RGO hybrid prepared with 1:4 mole ratio of CoCl2 to Na2MoO4 in the hydrothermal solution delivers a reversible capacity as high as 1236 mAh g-1 with the excellent cyclic stability and enhanced high-rate capability for electrochemical lithium storage. ACCEPTED MANUSCRIPT 023 321 311 111 Intensity/a.u. ?? 220 210 211 200 ?? e 10 20 30 40 2?/deg Fig. 1. 110 006 100 d c 50 60 b a 70 ACCEPTED MANUSCRIPT Fig. 2. ACCEPTED MANUSCRIPT Fig. 3. ACCEPTED MANUSCRIPT 100 Weight loss /% 90 80 (a) (b) (c) 70 (a) MoS2/RGO 60 (c) Co-doped-MoS2/RGO-2 (b) Co-doped-MoS2/RGO-1 (d) Co-doped-MoS2/RGO-3 50 40 (e) Co-doped-MoS2/RGO-4 0 (d) (e) 100 200 300 400 500 600 700 o Temperature / C Fig. 4 ACCEPTED MANUSCRIPT (a) O1s Intensity (a.u.) C1s Mo3d 5/2 Mo3d 5/2 S2s Co2p Mo3p 3/2 N1s S2p O2s Mo4p Mo4s Mo3s 800 (b) 600 400 200 Binding energy (eV) (c) C=C 0 2- S 2P 2- 3/2 S 2P Intensity (a.u.) Intensity (a.u.) 1/2 C-O C=O C-O-C Mo(VI)3d3/2 Mo(V)3d5/2 S2s 240 236 232 228 Binding energy (eV) 1/2 164 162 160 Binding energy (eV) Fig. 5 Co 2+ Co sat. 810 224 158 3+ (e) Mo(IV)3d5/2 Mo(VI)3d5/2 S2 2P Intensity (a.u.) Intensity (a.u.) Mo(IV)3d3/2 3/2 2- 166 294 292 290 288 286 284 282 280 Binding energy (eV) (d) 2- S2 2P 2+ Co 2p1/2 2p3/2 3+ Co sat. 800 790 780 Binding energy (eV) 770 ACCEPTED MANUSCRIPT -1 0.0 -0.5 -1.0 1st 2nd 3rd -1.5 2.0 Current/(mA mg ) 1.5 0.0 0.5 1.0 1.5 2.0 2.5 Potential/(V vs Li+/Li) 0.0 -0.5 -1.0 0.0 -0.5 -1.0 1st 2nd 3rd -1.5 0.5 1.0 1.5 2.0 2.5 Potential/(V vs. Li+/Li) 0.0 0.5 1.0 1.5 2.0 2.5 + Potential/(V vs. Li /Li) (d) 0.5 0.0 -0.5 -1.0 1st 2nd 3rd -1.5 0.0 0.5 1.0 1.5 2.0 2.5 Potential/(V vs. Li+/Li) 2.0 -1 Current/(mA mg ) 1.5 (e) 1.0 0.5 0.0 -0.5 -1.0 1st 2nd 3rd -1.5 -2.0 0.0 0.5 1.0 1.5 2.0+ 2.5 Potential/(V vs. Li /Li) Fig. 6. 3.0 1.0 -2.0 3.0 1st 2nd 3rd -1.5 1.5 0.5 0.0 0.5 2.0 (c) (b) 1.0 -2.0 3.0 1.0 -2.0 Current/(mA mg ) 1.5 0.5 -2.0 -1 2.0 (a) 1.0 -1 Current/(mA mg ) 1.5 Current/(mA mg-1) 2.0 3.0 3.0 ACCEPTED MANUSCRIPT 3.5 2.5 Potential /V 2.0 1.5 3rd 1.0 0.0 1st disc harg e 2nd 0 3.5 300 Potential /V 2.0 1.5 1.0 0.0 2nd 1st 0 1.5 2nd 3rd disch arge 1st 1.0 0 3rd dis cha rge 300 3.5 (e) 3.0 1.0 3rd 2nd 1st 0 300 2nd 1st ge ar ch 1.5 3rd 0.5 0.0 1st dis cha rge 2nd 0 300 600 900 1200 Capacity /mAh g-1 Fig. 7? disc harg e 600 900 -1 1200 Capacity /(mAh g ) 2.5 1.0 2nd 1st 1.5 3rd 2.0 1500 ge ar ch 2.0 0.0 1500 3rd 2.5 0.5 300 600 900 1200 -1 Capacity /(mAh g ) 600 900 -1 1200 Capacity /(mAh g ) (d) 3.0 1st 0.5 e arg ch 2.0 3.5 e arg ch 2nd 1st 2.5 0.0 1500 3rd 2nd 2.5 3rd 0.5 600 900 1200 -1 Capacity/(mAh g ) (c) 3.0 (b) 3.0 ge ar ch Potential /V Potential/V 2nd 3rd 1st 0.5 Potential /V 3.5 (a) 3.0 1500 1500 ACCEPTED MANUSCRIPT 1200 1000 800 (1) MoS2/RGO 600 (2) Co-doped-MoS2/RGO-1 400 (4) Co-doped-MoS2/RGO-3 200 160 (3) Co-doped-MoS2/RGO-2 (5) Co-doped-MoS2/RGO-4 0 20 40 60 80 Cycle number MoS2/RGO(experiment date) 140 MoS2/RGO(fitted date) 120 Co-doped-MoS2/RGO-2 (fitted date) (3) (2) (4) (1) (5) 1400 (c) Co-doped-MoS2/RGO-2(experiment date) -Zimg /? 80 60 40 20 0 20 40 60 80 100 120 140 160 Zreal /? Fig. 8 100 mA g -1 1000 800 600 400 200 100 100 0 1200 (b) 0m A g -1 20 0 5 0 mA - 1 g 0 1 0 mA 00 g 1 mA g -1 -1 charge discharge 10 1400 Capacity /(mAh g ) 1600 (a) Capacity /(mAh g-1) 1600 MoS2/RGO Co-doped-MoS2/RGO-1 Co-doped-MoS2/RGO-2 Co-doped-MoS2/RGO-3 0 10 20 30 40 50 60 70 80 90 100 Cycle number ACCEPTED MANUSCRIPT Figure Captions Fig. 1. XRD patterns of the (a) MoS2/RGO, (b) Co-doped-MoS2/RGO-1, (c) Codoped-MoS2/RGO-2, (d) Co-doped-MoS2/RGO-3 and (e) Co-doped-MoS2/RGO-4 hybrids prepared by hydrothermal method; (f) Schematic illustration of microstructures of MoS2/RGO. Fig. 2. SEM images of (a) the MoS2/RGO, (b) Co-doped-MoS2/RGO-1 (c) Co-dopedMoS2/RGO-2, (d) Co-doped-MoS2/RGO-3 and (e) Co-doped-MoS2/RGO-4 hybrids prepared by hydrothermal method. Fig. 3. TEM/HRTEM images of the (a, b) MoS2/RGO, (c, d) Co-doped-MoS2/RGO-1, (e, f) Co-doped-MoS2/RGO-3, (g, h) Co-doped-MoS2/RGO-3 and (i, j) Co-dopedMoS2/RGO-4 hybrids prepared by hydrothermal method. Fig. 4. TGA curves of the MoS2/RGO and Co-doped-MoS2/RGO hybrids. Fig. 5. XPS high-resolution scans of (a) survey, (b) C 1s, (c) S 2p, (d) Mo 3d and (e) Co 2p of the Co-doped-MoS2/RGO-2 hybrid. Fig. 6. Cyclic voltammograms of (a) the MoS2/RGO, (b) Co-doped-MoS2/RGO-1, (c) Co-doped-MoS2/RGO-2, (d) Co-doped-MoS2/RGO-3 and (e) Co-doped-MoS2/RGO-4 hybrid electrodes at a scan rate of 0.5 mV s-1. Fig. 7?Galvanostatic discharge/charge voltage profiles of the (a) MoS2/RGO, (b) Co-doped-MoS2/RGO-1, (c) Co-doped-MoS2/RGO-2, (d) Co-doped-MoS2/RGO-3 and (e) Co-doped-MoS2/RGO-4 electrodes for the first three cycles at a current density of 100 mA g-1. Fig. 8. (a) Cycling performances of MoS2/RGO and Co-doped-MoS2/RGO electrodes at current density of 100 mA g-1; (b) Rate capabilities of MoS2/RGO and Co-dopedMoS2/RGO electrodes at different current densities; (c) Nyquist plots of the MoS2/RGO and Co-doped-MoS2/RGO-2 electrodes obtained by applying a sine wave with amplitude of 5.0 mV over the frequency range from 200 kHz to 0.01 Hz and (c) equivalent circuit model of the studied system (CPE represents the constant phase element). ACCEPTED MANUSCRIPT Highlights 1. Co-doped-MoS2/RGO hybrids are synthesized by one-pot hydrothermal method. 2. Co-doping changes the morphology and microstructure of the hybrids. 3. Co-doped-MoS2/RGO can deliver a reversible capacity as high as 1236 mAh g-1. 4. The hybrid exhibits significantly enhanced high-rate capability.