SnS2

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C: Energy Conversion and Storage; Energy and Charge Transport 2

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Interface Synergistic Effect from Layered Metal Sulfides of MoS/SnS Van der Waals Heterojunction with Enhanced Li-Ion Storage Performance Xiao-Lei Man, Pei Liang, Haibo Shu, Lin Zhang, Dan Wang, Dongliang Chao, Zugang Liu, Xiaoqing Du, Houzhao Wan, and Hao Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09225 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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The Journal of Physical Chemistry

Interface Synergistic Effect from Layered Metal Sulfides of MoS2/SnS2 Van der Waals Heterojunction with Enhanced Li-ion Storage Performance Xiaolei Mana, Pei Lianga*, Haibo Shua, Lin Zhanga, Dan Wanga, Dongliang Chaob, Zugang Liua, Xiaoqing Duc, Houzhao Wand, Hao Wangd

a.

College of Optical and Electronic Technology, China Jiliang University, 310018， Hangzhou, China

b. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore c.

School of Materials Science and Energy Engineering, Foshan University, Foshan, Guangdong, 528000, China

d. Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics and Electronic Science, Hubei University, Wuhan 430062, China

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ABSTRACT: Layered metal sulfides (LMSs) with larger interlayer spacing is suitable for Li+ intercalation/extraction, which possess relatively higher theoretical specific capacity than commercial graphite. However, pure LMS shows inherent low conductivity and irreversible huge volume expansion in lithium uptakes. In this work, we introduce an interesting van der Waals heterojunction between two popular LMSs, i.e., MoS2/SnS2 van der Waals heterojunction grown on reduced graphene oxide (MoS2/SnS2-rGO), which is synthesized by a facile hydrothermal process. The MoS2/SnS2-rGO nanocomposite exhibits remarkable interface synergistic effect due to the weak van der Waals interaction on their nanocrystalline, which leads to an enhanced energy storage performance compared with MoS2-rGO and SnS2-rGO. The MoS2/SnS2rGO exhibits better cycling stability of 894 mAh g-1 at 200 mA g-1 after 55 cycles and excellent rate performance of 590 mAh g-1 at 1 A g-1 for LIBs. This work proposes that weaker van der Waals interaction between the LMS can induce much more layer space for Li+ in LIBs, which might be another way to improve the storage performance of LIBs.

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1. INTRODUCTION Rechargeable lithium ion batteries (LIBs) have become one of the most popular energy storage equipment in modern society. The energy storage performance of commercial LIBs is increasingly incapable of meeting the demand (e.g., power density, energy density, rate capability and cycle life, etc.).1 Layered metal sulfides (LMS) are composed of “S atom - metal atom - S atom” sandwich-like layers, which are combined by weak van der Waals forces.2 Compared to graphite anode materials, LMS possesses larger interlayer spacing and higher theoretical specific capacity such as SnS 2 (~645 mAh g-1), MoS2 (~670 mAh g-1), VS2, WS2.3-5 Among these, SnS2 is one of the conversion and alloying type sulfides. It is alloying to form Li4.4Sn after the intercalation and conversion during the discharge.6 In term of MoS2, it belongs to conversion type sulfides and does not have the alloying process. Besides, the interlayer spacing of SnS2 and MoS2 is separately 0.59 nm and 0.62 nm, which is larger than the size of Li+ (0.212 nm). It is compatible for Li+ to intercalate the interlayers. However, tremendous reports have clearly stated that pure SnS2 (or MoS2) possess inherent low electrical conductivity and also are accompanied by irreversible volume expansion during the cycling, which eventually results in poor performance.7-8 Highly conductive and flexible graphene is a superior choice to ameliorate LIBs’ capability. Several groups’ research discovered that diverse nanostructures complexes of LMS and graphene prepared through different methods (e.g., microwave, solvothermal and chemical vapor deposition) exhibited better rate performance, longer cycle life and better cycle stability than pristine LMS.9-13 Ye et al. created MoS2/reduced graphene 3



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oxide (MoS2/rGO) composites, in which fewer layers MoS2 were uniformly distributed and anchored on the surface of rGO, displayed a higher specific capacity of 1102 mAh g-1 at 0.05 A g-1 after 140 cycles and remarkably enhanced rate capability of 469 mAh g-1 at 2.5 A g-1 than pristine MoS2 as anode for LIBs.14 All of these confirmed that graphene could improve electrical conductivity and buffer volumetric change of the compound materials. Moreover, atomic doped graphene (e.g., N and S) and 3D graphene possess higher conductivity and are applied in composite materials to enhance LIBs’ electrochemical activity.15-17 Zheng et al. demonstrated that SnS2/sulfur-doped reduced graphene oxide (SnS2/S-doped rGO) complexes displayed 532 mAh g-1 during 600 cycles at 5.0 A g-1 for LIBs.18 Compared with single LMS, much efforts focus on van der Waals heterojunction LMS and metal oxides (LMS/MO). Research results show LMS/MO exhibits better energy storage performance than single LMS, SnO2/SnS2, SnS2/Co3O4, TiO2-MoS2, MoO3@SnS2, for instance.19-22 Besides, double LMS nanocomposite such as SnS2/Sb2S3@rGO, Ce2S3/MoS2, have also attracted much attention for their unique advantages.23-24 The special structure of LMS van der Waals heterojunction could provide faster ion diffusion channel and lower diffusion resistance. Moreover, the interface synergistic effect could promote the electrochemical performance. Chen et al. use cetyltrimethylammonium bromide (CTAB) as surface active agent to prepare the MoS2/WS2-nitrogen doped graphene (MWG), in which CTAB is treated as an intermediate contact to electrostatically combine anion containing transition-metal elements (MoS42- and WS42-) and GO with negative functional groups, and electrochemical measurements demonstrated that MWG held a better cycling stability of ~1066 mAh g-1 at 100 mA g-1 during 100 cycles and higher rate capacity of ~770 4


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mAh g-1 at 5 A g-1 as anode for LIBs.25 In Jiang’s research Sn4+ and MoO42- are sequentially banded together with negative GO to make the SnS2 and MoS2 nanoparticles reasonably distribute on the interface of graphene sheet (MoS2/SnS2-GS) which is served as anode material for LIBs.26 As Jiang’s research show that the MoS2/SnS2 show good performance of energy storage, but the mechanism of the reason why these MoS2/SnS2 structure better than the sole one is still not clear. Herein, we combined the SnS2, MoS2 and graphene together to prepare flaky nanocrystalline MoS2/SnS2 van der Waals heterojunction supported by reduced graphene oxide (MoS2/SnS2-rGO) via a facile hydrothermal reaction. The interface synergistic effect of MoS2/SnS2 van der Waals heterojunction can effectively enhance electrochemical performances of the nanocomposite. The MoS2/SnS2-rGO could provide faster Li+ diffusion from the electrode to electrolyte for the improvement of rate capacity and the buffer matrix of the large volume change for enhancing the cycling stability. Electrochemical tests showed the MoS2/SnS2-rGO displays better cycling stability of 894 mAh g-1 at 200 mA g-1 after 55 cycles than MoS2-rGO and SnS2-rGO. Moreover, the samples possessed excellent rate performance of 590 mAh g-1 at 1 A g-1 for LIBs. 2. EXPERIMETAL SECTION 2.1 Chemical materials Graphite powder (>99.95 %), potassium permanganate (AR, KMnO4), sodium nitrate (NaNO3), Tin (IV) chloride pentahydrate (SnCl4·5H2O), sodium molybdate dihydrate (Na2MoO4·2H2O), thioacetamide (TAA) were purchased from Aladdin (http://www.aladdin-e.com/). Other reagents, such as sulfuric acid (H2SO4), hydrogen 5



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peroxide (H2O2) and dilute hydrochloric acid (HCl, 25wt%) used in this research were without further purification. 2.2 Synthesis of GO Graphene oxide (GO) was prepared via a modified Hummers’ method.27 Typically, 1 g natural graphite powder (Graphite, 99.95 %) and 1 g NaNO3 were successively mixed into 50 mL icy H2SO4 with strongly stirring and then kept for 30 min. 5 g KMnO4 was slowly added with ice bath for 30 min. the obtained suspension was uninterruptedly stirred at 30 oC for 12 h. Then 100 ml deionized water was slowly dropped in the suspension. Next, an excessive dose of H2O2 was added until the color of the suspension became bright yellow. Finally, the suspension was centrifuged and washed with HCl. Finally, GO was obtained by lyophilizing for 24 hours. 2.3 Synthesis of MoS2/SnS2-rGO The MoS2/SnS2-rGO composites were synthesized by a facile hydrothermal method. Concretely, 40 mg as-prepared GO was ultrasonically dispersed in 60 mL deionized water for an hour to ensure the GO suspension is uniform. Then, 1 mmol SnCl4·5H2O was added into the GO suspension. After constantly vigorous stirring for 30 min, 1 mmol Na2MoO4·2H2O was put into the suspension by continuously stirring for another 30 min, then 10 mmol TAA was mixed under stirring for 15 min to make it dissolve completely. Finally, the mixture suspension was transferred into a Teflon-lined stainless-steel autoclave (100 mL) and heated at 200 oC for 16 h. After naturally cooling to room temperature, the obtained composite was collected by centrifugation, washed with absolute ethanol and deionized water for several times and freeze-dried for 24 h.

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2.4 Synthesis of SnS2-rGO and MoS2-rGO For the comparison, the synthesis process of SnS2-rGO and MoS2-rGO is similar to that of the MoS2-SnS2-rGO. In the case of SnS2-rGO, the only difference is that 2 mmol SnCl4·5H2O were used without Na2MoO4·2H2O. For MoS2-rGO, it was synthesized by only adding 2 mmol Na2MoO4·2H2O without SnCl4·5H2O. 2.5 Instruments and characterization Field emission scanning electron microscopy (SEM, Hitachi, SU8010 FE-SEM) and transmission scanning electron microscopy (TEM, Philips, TecnaiG2 20) were used to characterize the morphology of the as-synthesized MoS2/SnS2 Van der Waals heterojunction. The Crystal structures were analyzed using power X-ray diffraction (XRD) and were collected on a Bruker D2 PHASER (Cu Kα radiation, λ=0.1541 nm). MoS2/SnS2 Van der Waals heterojunction surface elemental states were determined by Raman spectra (Renishaw, Invia Reflex Raman with a 532 & 785 nm excitation wavelength) and X-ray photoelectron spectroscopy (XPS, KRATOS Axis ultra-DLD). 2.6 Electrochemical measurements In term of the work electrode, the active materials (80 wt.%), carbon black (10 wt.%) and polyvinylidene fluoride binder (PVDF, 10 wt.%) were mixed strongly in N-methyl2-pyrrolidone (NMP) to form homogeneous slurry. Then the obtained slurry coated onto the rough surface of copper foil with the thickness of about 100 nm used as the current collector. Coated copper foil was dried at 80 °C for half an hour in the air and subsequently kept drying at 110 °C for 12 h in the vacuum. The diameter of the work electrode slices is 12 mm and the mass of active materials was about 2 mg. After that, 7



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the CR2032 coin-type cells were encapsulated in a glove box (pure argon-filled, O2 < 0.1 ppm, H2O < 0.1 ppm), in which lithium metal foil was used as the counter electrode, Celgard 2500 as the separator, a solution of 1.0 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC)/dimethyl carbonate (DMC) (1:1:1 by volume ratio) as the electrolyte. After assembled, the coin-type cells were statically placed overnight. All of the electrochemical tests carried out at the room temperature. Galvanostatic charge/discharge performance of the cells was conducted through using LAND BT2013A battery test system with a 0.01-3.0 V voltage window. Cyclic voltammetry (CV) test was performed on a voltage range of 0.01-3.0 V at various rates and electrochemical impedance spectroscopy (EIS) was tested in frequency window from 100 kHz to 0.01 Hz by employing a CHI660E (Chenhua, Shanghai, China) electrochemical workstation. 3. RESULTS AND DISCUSSION As illustrated in Figure 1, the synthesis process of MoS2/SnS2-rGO is schematically explained. The growth mechanism of MoS2/SnS2-rGO seems reasonable to be explained in this way. First of all, electronegative GO is united with Sn4+ cation by electrostatic interaction to form GO-Sn4+ complex compound. Then MoO42- anion is introduced to obtain GO-Sn4+-MoO42- complex compound, this procedure is relatively facile owing to the existence of Sn4+ cation. Next mixing the sulfur source (TAA) makes the Sn4+ and MoO42- nucleate into SnS2 and MoS2, respectively. The nucleation mechanism could be interpreted as two steps: firstly, TAA hydrolyzed into CH3C(S)NH2 and H2S (eq 1); secondly, the reaction of nucleation into SnS2 and MoS2, 8


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Figure 1. Schematic diagram of the synthetic process for MoS 2/SnS2-rGO nanocomposites. in which is attributed to the existence of H2S (eq 2).28-29 In this process, adding an excessive of TAA could make GO reduced to rGO. CH3C(S)NH2+H2O = CH3C(O)NH2+H2S Sn4++2H2S = SnS2 +4H+

（1）

（2）

Figure 2a presents the XRD pattern of MoS2-rGO, SnS2-rGO and MoS2/SnS2-rGO. There are three weak and broad peaks of MoS2-rGO at the 2θ =14.6°, 32.4°, 39.6° corresponding to the 2H-phase hexagonal MoS2 (JCPDS: 37-1492) planes of (002), (100) and (103), respectively. It indicates that MoS2 has low crystallinity. The main sharp and narrow diffraction peaks of SnS2-rGO are at the 2θ = 15.1°, 28.5°, 32.2°, 50.0° and 52.5°, which matches well with the (001), (100), (101), (110) and (111) planes of 9



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Figure 2. (a) The XRD patterns of MoS2-rGO, SnS2-rGO and MoS2/SnS2-rGO. (b) The Raman spectra of GO, MoS2-rGO, SnS2-rGO and MoS2/SnS2-rGO. (c) The survey spectra of MoS2/SnS2-rGO. (d) The Sn 3d spectra of MoS2/SnS2-rGO. (e) The Mo 3d spectra of MoS2/SnS2-rGO. (f) The C 1s spectra of MoS2/SnS2-rGO. 2T-phase hexagonal SnS2 (JCPDS: 23-0677). It suggests that SnS2-rGO possesses better crystallinity. For the XRD pattern of MoS2/SnS2-rGO nanocomposite, the main 10


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emerging diffraction peaks are located at the 2θ angle of 14.9°, 28.9°, 31.5°, 33.1°, 39.5°, 50.6° and 60.1°. The peaks of 14.9°, 28.9°, 31.5°, 50.6° and 60.1° are closer the angles of 2T-phase hexagonal SnS2, the intensity of peaks is weaker than SnS2-rGO yet. It shows that the crystallinity of MoS2/SnS2-rGO is lower and the grain size is smaller than that of SnS2. The peaks of 33.1° and 39.5° are closer the angles of 2H-phase hexagonal MoS2. It is interesting that the angle of (002) plane has disappeared, which results from the inhibition of rGO. In order to further find out the phase composition of MoS2/SnS2-rGO, the Raman spectroscopy characterizations of MoS2/SnS2-rGO are given in Figure S1. The MoS2SnS2-rGO displays three characteristic peaks. A strong peak at 308.9 cm-1 belonging to the A1g mode of SnS2, which is in accordance with previous studies.30-31 The others originate from the defect-induced D band at 1326.6 cm-1 and the symmetry-allowed G band at 1597.6 cm-1, respectively.16, 32-34 Besides, the intensity ratios of D band and G band (ID/IG) for GO, MoS2-rGO, SnS2-rGO and MoS2/SnS2-rGO are presented in Figure 2b. The ID/IG ratio of MoS2-rGO (1.35), SnS2-rGO (1.26) and MoS2/SnS2-rGO (1.54) are all larger than that of GO (1.13). It indicates that GO is exactly reduced to rGO during the synthesis process. To further identify the existence of SnS2 and MoS2, the XPS was employed to detect the chemical valence states and bonding states of the elements in MoS2/SnS2rGO complex structure. The full scan survey XPS spectra of MoS2/SnS2-rGO is showed in Figure 2c. It presents Sn, Mo, S, O and C elements exist in MoS2/SnS2-rGO. There are two peaks at 494.7 eV and 486.2 eV in Sn 3d spectra in Figure 2d, which belong to the binding energy of Sn 3d3/2 and Sn 3d5/2, respectively.23, 35 In Figure 2e, the Mo 3d spectra has four distinct peaks. The peaks at 232.2 eV and 228.5 eV are 11



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identified to be the binding energy of Mo 3d3/2 and Mo 3d5/2, respectively.36-37 Besides, the chemical bonding of Mo-O and binding energy of S 2s contributed to the peak at 235.4 eV and 225.8 eV, respectively.38 The XPS indicate that the peaks of Sn 3d located at 485 and 495 eV, while the peaks of Mo 3d located at 229 and 233 eV, which explains the co-existence of Sn4+ and Mo4+ in MoS2/SnS2-rGO. The C 1s spectra is presented in Figure 2f. The peak at 284.7 eV is associated to the binding energy of CC or C=C. Moreover, three weak peaks at 286.0 eV, 286.9 eV and 288.4 eV are separately traceable to the effect of C-O, C=O and O-C=O, which are contributed by rGO.26 All these indicate that the nanocomposite composed of MoS2, SnS2 and rGO. The micromorphology of MoS2-rGO, SnS2-rGO and MoS2/SnS2-rGO is tested by SEM. The SEM of MoS2-rGO (Figure S2a-b), SnS2-rGO (Figure S2c-d) show that the apparent agglomeration does not occur. As showed in Figure 3a-b, the flexible rGO and flaky SnS2/MoS2 can be clearly observed. The TEM image (Figure 3c and Figure S3a) of the MoS2/SnS2-rGO further conveys that the MoS2/SnS2 van der Waals heterojunction are well-uniformly grown on the surface of rGO and has the smaller sizes of 50-150 nm, which could provide the shorter diffusion path and increases the contact area between the electrolyte and the electrode. It is easy to find that the MoS2/SnS2 nanosheets are tightly grown on the surface of fewer layer rGO in Figure S3b. The MoS2/SnS2 van der Waals heterojunction grown in vertical and horizontal direction can be found, corresponding to the atomic structure of Figure 3e and Figure 3f, respectively. The crystal planes of the as-obtained MoS2/SnS2-rGO are presented in Figure 3d of high resolution TEM (HRTEM). It shows that the lattice fringes with a distance of 0.185 nm and 0.227 nm, which could be separately indexed to the (105)

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Figure 3. (a) and (b) the SEM image of MoS2/SnS2-rGO. (c) TEM image and (d) HRTEM image of MoS2/SnS2-rGO. (e) and (f) the schematic illustrations of the vertical and horizontal heterostructure of MoS2/SnS2, respectively. and (103) planes of hexagonal MoS2. The (102) and (100) plane of hexagonal SnS2 correspond to the distance of 0.213 nm and 0.316 nm, respectively. These images further explain the co-existence of SnS2 and MoS2 in the objective MoS2/SnS2-rGO. The electrochemical performance of the obtained MoS2-rGO, SnS2-rGO and MoS2/SnS2-rGO as anode for LIBs was tested by the cyclic voltammetry (CV) at a scan rate of 0.1 mV/s with a potential window of 0.01 V to 3.0 V vs Li/Li+ and discharge13



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charge experiments at the current density of 200 mA g-1. The CV curves of MoS2-rGO electrode are exhibited in Figure 4a. In the first discharge cycle, distinct cathodic peaks can be noticed at about 1.63 V, 0.52 V and 0.21 V. The peak at 1.63 V is attributed to Li+ intercalating into MoS2 to form LixMoS2 (i.e. MoS2 + xLi+ + xe− → LixMoS2), which results in the phase change of 2H-MoS2 into 1T-MoS2.39-41 The conversion of LixMoS2 to Mo and Li2S leads to the peak at 0.52 V (i.e. LixMoS2 + (4-x)Li+ → Mo + 2 Li2S) and the peak at 0.21 V is assigned to the formation of a solid electrolyte interphase (SEI) layer.8, 39-41 In the first charge scan, two anodic peaks at 1.84 V and 2.37 V are considered to be the result from the oxidation of Li2S to S (i.e. Li2S - 2e− → S + 2Li+).40-41 In the next scan, the peak at 1.63 V disappeared and two peaks at 1.84 V and 1.34 V are observed, which are ascribed to the transformation of electrode material from MoS2 to Mo and S after the first cycle.26 Concretely, the lithiation of S to form Li2S and the combination of Li+ with Mo cause the formation of peaks at 1.84 V and 1.34 V, respectively.41-42 The discharge/charge potential profiles of MoS2-rGO (Figure 4b) at 200 mA g-1 is in good agreement with the peaks of CV curves. In the first cathodic cycle of SnS2-rGO (Figure 4c), the intercalation of Li+ into SnS2 without phase decomposition (i.e. SnS2 + xLi+ + xe− → LixSnS2) and the decomposition of SnS2 into Sn together with Li2S (i.e. SnS2 + 4Li+ + 4e- ↔ Sn + 2Li2S) correspond to the peaks of 1.82 V and 1.60 V, respectively.43-44 The peak at 1.14V results from the formation of SEI layer based on the previously reported studies, which brings about the smaller irreversible capacity.26, 45 The weak peak at 0.31V in the first cathodic scan and the strong peak at 0.55 V in the first anodic scan signify the alloying and dealloying of LiSn ( i.e. Sn + 4.4 Li+ + 4.4e- ↔ Li4.4Sn), which caused by the larger volume change.26, 43

Other three peaks (i.e. 1.87V, 2.18V and 2.36V) indicate that the oxidation of Sn to 14


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Figure 4. (a), (c) and (e) the CV curves of MoS2-rGO, SnS2-rGO and MoS2/SnS2-rGO at the scan rate of 0.1mV/s, respectively. (b), (d) and (f) discharge-charge voltage profiles of MoS2-rGO, SnS2-rGO and MoS2/SnS2-rGO at a current density of 200mA g-1, respectively. SnS2 is progressive in the anodic scan.44 The discharge/charge potential profiles of SnS2-rGO (Figure 4d) is in good consistence with the results of CV curves. For the CV curves of MoS2/SnS2-rGO (Figure 4e), the significant peaks of MoS2/SnS2-rGO is 15



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jointly contributed by MoS2 and SnS2. In the first cathodic cycle, five reduction peaks are clearly visible at 1.73 V, 1.61 V, 1.36 V, 0.94 V and 0.52 V. Among these, the peaks of 1.73 V and 1.61 V are considered to be the formation of LixSnS2 and LixMoS2, respectively. The peak at 1.36 V is ascribed to the decomposition of SnS2 and MoS2. The peak at 0.94 V is associated with the formation of SEI layer. In the first cycle, the reduction peak at 0.52 V and the oxidation peak at 0.55 V are assigned to the alloying and dealloying of Li-Sn. Two oxidation peaks at 1.89 V and 2.35 V originate from the oxidation of Li2S. The process of alloying and dealloying of SnS2 is not accompanied by obvious electrochemical reaction of MoS2, it means that the large volume change could be relieved at a certain extent.26 In following scans, the disappearance of partial peaks at 1.73 V, 1.61 V, 1.36 V, 0.94 V and 0.52 V indicates that there are some irreversible reactions. The discharge/charge potential profiles of MoS2/SnS2-rGO further prove the results in CV curves (Figure 4f). The discharge/charge potential profiles of MoS2/SnS2-rGO at different cycles are presented in Figure 5a, the discharge and charge capacities have attenuation with a large margin in the first three cycles, which is attributed to the formation of SEI layer and the decomposition of electrolyte. In the next cycles, MoS2/SnS2-rGO could maintain consistent voltage platforms and possess the smaller capacity attenuation. In Figure 5b, the cycling performances of the MoS2-rGO, SnS2-rGO and MoS2/SnS2-rGO were evaluated at the current density of 200 mA g-1. The initial discharge and charge capacities of the MoS2/SnS2-rGO electrode are 1352.4 and 1128.5 mAh g-1, respectively. An initial Coulombic efficiency of the MoS2/SnS2-rGO electrode is 84.1 %, which is higher than that of MoS2-rGO electrode (78.7 %) and SnS2-rGO electrode (80.2 %), the Coulombic efficiency of MoS2/SnS2-rGO electrode swiftly 16


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Figure 5. (a) The discharge-charge voltage profiles of MoS2-rGO, SnS2-rGO and MoS2/SnS2-rGO at different cycles. (b) The cycling performance of MoS2-rGO, SnS2rGO and MoS2/SnS2-rGO at 200mA g-1. (c) The rate capability of MoS2-rGO, SnS2rGO and MoS2/SnS2-rGO at different current densities. (d) the Nyquist plots of MoS2rGO, SnS2-rGO and MoS2/SnS2-rGO. increases to 99.7 % after the following five cycles. These results are ascribed to the interface synergistic effect of MoS2/SnS2 for enhanced electrochemical activity. In the subsequent cycles, the MoS2/SnS2-rGO electrode exhibits better cycling performance with less decay of capacity per cycle with a high residual capacity of ~894 mAh g-1 after 55 cycles, in which the rGO works as the buffer material to prevent the structural collapse of MoS2/SnS2. Compared to the MoS2/SnS2-rGO electrode, MoS2-rGO electrode and SnS2-rGO electrode have worse cycling stability with the low retention 17



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capacity of 591.5 and 710.3 mAh g-1 after 55 cycles, respectively. In the final several cycles, they have a quick drop of capacity, resulting from the structural cracking of large volume change. Therefore, MoS2/SnS2-rGO electrode possess high specific capacity and cycling stability. The rate capability of MoS2-rGO, SnS2-rGO and MoS2/SnS2-rGO electrode was tested at current density of 0.1, 0.3, 0.5 and 1.0 A g-1 in the voltage range of 0.01-3.0 V in Figure 5c. The specific discharge capacities at the current density of 0.1, 0.3, 0.5 and 1.0 A g-1are about 1422, 990.1, 848.5 and 635.6 mAh g-1, which are higher than that of MoS2-rGO (1189.7, 670.7, 610.6, 499.9 and 781.7 mAh g-1) and SnS2-rGO (1257.6, 775.3, 685.4, 549.8 and 760.7 mAh g-1). It results from the faster Li+ diffusion of the interface synergistic effect of MoS2/SnS2 van der Waals heterojunction. The retention rate of capacity is 44.7 % at 1.0 A g-1, which is larger than that of MoS2-rGO (42%) and SnS2-rGO (43.7%), indicating that MoS2/SnS2-rGO electrode displays a better rate performance. When the current density falls to 0.1 A g-1 again, the remaining capacity (only 959.9 mAh g-1) cannot be achieved again with quickly fading and finally the retention rate of capacity is only 62.7 %, although MoS2/SnS2-rGO electrode still possesses higher capacity than MoS2-rGO and SnS2-rGO. The result is assigned to the structural cracking of electrode at large current density of 1.0 A g-1. In summary, MoS2/SnS2-rGO electrode has better electrochemical performance compared with other related samples shown in Table S1. To better investigate the superior electrochemical performance of the nanocomposites, EIS was measured at the frequency range of 100 kHz to 0.01Hz under open circuit voltage conditions. The Nyquist plots composed of a semicircle in the high frequency region and an inclined line in the low frequency rage are presented in the Figure 5d. An equivalent circuit model showed in the insert map (Figure 5d). The 18


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charge-transfer impedance (Rct) represents the transfer impedance of Li+ from the interface of the electrode to the electrolyte and the Warburg impedance means the resistance from the diffusion process of Li+ in electrode materials.15, 41, 46 The numerical value of semicircular diameter for MoS2/SnS2-rGO electrode (60 ohm) is smaller than that of MoS2-rGO (132 ohm) and SnS2-rGO (105 ohm) electrode, indicating the MoS2/SnS2-rGO electrode has lower Rct and higher conductivity. The slope of straight line for MoS2/SnS2-rGO electrode is smaller than that of MoS2-rGO and SnS2-rGO electrode, suggesting to the MoS2/SnS2-rGO electrode has higher Warburg impedance. Thus, the interface synergistic effect of MoS2/SnS2 van der Waals heterojunction can effectively enhance electrochemical performance of the electrode. 4. CONCLUSION In summary, we synthesized lamellar MoS2/SnS2 van der Waals heterojunction with support on the surface of reduced graphene oxide nanocomposites (MoS2/SnS2-rGO) by a simple hydrothermal process. The structure of MoS2/SnS2 van der Waals heterojunction displays the interface synergistic effect on the electrochemical performances. The MoS2/SnS2-rGO could provide the faster Li+ diffusion from electrode to electrolyte for the improvement of rate capacity and the buffer matrix of the large volume change for enhancing the cycling stability. The MoS2/SnS2-rGO possessed the excellent rate performance of 590 mAh g-1 at 1 A g-1 for LIBs. Besides, it displayed better cycling stability of 894 mAh g-1 at 200m A g-1 after 55 cycles. ASSOCIATED CONTENT Supporting Information

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The Raman spectrum are presented in the Figure S1. The SEM images are presented in the Figure S2, The TEM images for the samples are presented in the Figure S3. The comparison to related samples as LIB anode in the Table S1. AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (P. Liang)

Present Addresses College of Optical and Electronic Technology, China Jiliang University, 310018 Hangzhou, China Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work was carried out at National Supercomputer Centre in Shenzhen. The Project was supported by the National Science Foundation for Young Scholars of China (Grant No.31000316). The Project was supported by the National Science Foundation of China (Grant No.61775201). The Project was supported by the Zhejiang province university students in scientific and technological innovation activities (No. 2016R409011), and the Science and technology project of Zhejiang Province (No. 2016C33026).

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SnS2

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