A novel MoS2@C framework architecture composites with three-dimensional cross-linked porous carbon supporting MoS2 nanosheets for sodium storage

Journal Pre-proof A novel MoS2@C framework architecture composites with three-dimensional crosslinked porous carbon supporting MoS2 nanosheets for sodium storage Xia Zhang, Ting Ma, Tao Fang, Yunzhi Gao, Shuai Gao, Wenwen Wang, Lixia Liao PII:

S0925-8388(19)34067-8

DOI:

https://doi.org/10.1016/j.jallcom.2019.152821

Reference:

JALCOM 152821

To appear in:

Journal of Alloys and Compounds

Received Date: 1 August 2019 Revised Date:

3 October 2019

Accepted Date: 27 October 2019

Please cite this article as: X. Zhang, T. Ma, T. Fang, Y. Gao, S. Gao, W. Wang, L. Liao, A novel MoS2@C framework architecture composites with three-dimensional cross-linked porous carbon supporting MoS2 nanosheets for sodium storage, Journal of Alloys and Compounds (2019), doi: https:// doi.org/10.1016/j.jallcom.2019.152821. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.

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A novel MoS2@C framework architecture composites with threedimensional cross-linked porous carbon supporting MoS2 nanosheets for sodium storage Xia Zhang1, Ting Ma1, Tao Fang1*, Yunzhi Gao2, Shuai Gao1, Wenwen Wang1, Lixia Liao1* 1. College of Chemistry, Chemical Engineering and Resource Utilization, Northeast Forestry University, Harbin 150040, China 2. School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China *Corresponding author, E-mail: [email protected], [email protected] Abstract The exploration of appropriate electrode materials for high-performance sodium-ion batteries (SIBs) is deemed to a critical technological challenge to satisfy the everincreasing demand for energy storage and conversion market. Herein, molybdenum sulfide (MoS2) nanosheets anchored on porous carbon (MoS2@C) were synthesized via a facile and controllable method. The novel three-dimensional (3D) honeycomb-like hierarchical structure is capable of offering reduced ion diffusion pathways, buffering volume expansion, as well as stabilizing interface. Benefiting from the 3D ion diffusion channels formed by this unique architecture, a high intercalation pseudocapacitance (77% at the scan rate of 1.5 mV s-1) is achieved, which is responsible for the good rate capability (about 372 and 225 mAh g-1 at 0.05 and 2 A g-1, respectively). Moreover, the composites deliver the long cycle performance (233 mAh g-1 at 0.5 A g-1 over 200 cycles) compared with the pristine MoS2 electrode. Furthermore, 3D roughness reconstruction of the MoS2@C electrode well reveals the integrity of the hierarchical structure after Na+ insertion. Our work paves a new way for the application of MoS2@C and opens up new opportunities for the broader design of SIBs anodes.

Keywords: Sodium ion battery; Hierarchical structure; Pseudocapacitance

1

1. Introduction Recently, intensive researches focused on rechargeable secondary batteries are moving to develop large scale energy storage systems (ESSs) such as electric vehicles and portable electronic devices. However, concerns over cost, safety and restricted lithium source for the potential application of high energy density output equipment remain to be answered [1]. Considering of the natural richness of Na sources, sodium ion battery is stepping up into the electrochemical storage stage. Moreover, the redox potential of Na/Na+ is -2.71 V versus the standard hydrogen electrode while the ion intercalation chemistry is similar to that of lithium which makes sodium the most advantageous after lithium in the practical application of battery [2-5]. Nevertheless, the radius of Na+ is about 34% larger than that of Li+, therefore, the electrode materials such as graphite that functions well in LIBs have been so far considered unsuitable for Na storage due to insufficient interlamellar spacing [6-9]. It is highly imperative that measures should be taken to find a suitable electrode material to accommodate larger Na+ and promote reversible insertion-extraction of Na ion [10]. Currently, 2D-layered transition metal dichalcogenides (TMDs) with analogous structures to graphite, such as MoS2 [11], SnS2 [12], and WS2 [13], have been extensively explored as SIB anode materials. Among them, MoS2 presents the enormous prospect in the energy storage field owing to its rather large layer distance (~0.615 nm) and high theoretical capacity (670 mAh g-1) [14]. Specifically, MoS2 is a well-defined layered structure with S-Mo-S motifs accumulated together by van der Waals forces, which helps to reversible intercalation/deintercalation for alkali ions [3]. However, the commercial application of MoS2 electrode materials is still hindered by the following factors. First, the poor intrinsic conductivity results in sluggish intercalation kinetics [2]. Then, the drastic volume variations are unavoidable upon the charge/discharge process, which may result in the severe pulverization of active materials, low electrical contact and finally lead to electrode collapse. Finally, because of the high surface energy and inter-layer van der Waals force, two-dimensional (2D) materials are liable to be stacked and restacked, which may lead to fast capacity decay [15]. Various strategies have been employed to overcome those limitations, such as incorporating them with advanced carbon matrixes, nanostructuring MoS2, and expanding their interlayer space [16]. Among these 2

approaches, dispersing MoS2 nanosheets anchored on carbon matrixes with high conductivity has been proven to be an available method to enhance electrochemical performance in SIBs. Recently, numerous conductive carbonaceous materials have been developed to ameliorate the structural stability of MoS2 based anode, such as carbon fiber [17], Ndoped graphene [18], carbon paper [3], and carbon cloth [19]. Unfortunately, it is difficult to achieve large-scale production due to the complicated manufacturing process and the high cost. More importantly, a two-dimensional carbon structure, such as graphene [20] has been widely studied due to its good electronic conductivity and flexibility [21]. However, the lack of good inter-sheet connections between isolated graphene sheets disrupts the continuous passage of ion transport, severely inhibiting the inherent high electrical conductivity and mechanical strength of individual graphene sheets [20]. Generally, the electrochemical performance of the electrode is directly related to the morphology of active substance and contact ability between carbon framework and MoS2 nanosheet. Therefore, achieving a robust 3D hierarchical structure for the construction of MoS2@C with excellent interfacial contact remains a huge challenge. Herein, we propose a facile and low-cost method to synthesize a 3D hierarchical MoS2@C structure with MoS2 nanosheets evenly anchoring on 3D cross-linked porous carbon network (3DPC). In MoS2@C composite electrode, MoS2 nanosheets tightly adhere to the 3D carbon skeleton, ensuring the integrity of the composite electrode upon repeated sodiation/desodiation. Furthermore, the desirable integration of MoS2 and 3DPC favorably offers extra free void space for alleviating the volume changes and boosts pathway for the transmission of ions and electron. Based on the above merits, MoS2@C displays an attractive electrochemical performance in terms of cycle stability, reversible specific capacity and rate capability when served as an anode material of SIBs. Our work digs out the sodium ion storage mechanism and paves a new way for designing of the porous carbon in alkali secondary battery. 2. Experimental Section 2.1. Material synthesis 2.1.1 Synthesis of 3D porous carbon 3

3D porous carbon was prepared by chemical blowing the polyvinylpyrrolidone (PVP) with iron nitrate [22]. Specifically, 1.0 g of PVP (K-30) was dissolved in 30 mL deionized water by vigorous magnetic stirring at room temperature. Then, 1.5 g of iron (III) nitrate nonahydrate (Fe(NO3)3·9H2O) was then added into the above PVP solution. The as-obtained mixture was maintained at 90 oC under stirring to evaporate the water solvent. The resultant product was ground using mortar, which was then transferred to a quartz boat at 700 oC for 1 h with a steady flow of nitrogen (N2). Finally, the sample was immersed in a 30 mL 2 M hydrochloric acid (HCl) solution for 10 h at 120 oC to etch Fe(NO3)3·9H2O templates, and then obtain the 3D porous carbon material. 2.1.2 Synthesis of MoS2@C and flower-like MoS2 nanosheets MoS2 nanosheets loaded on 3D porous carbon (MoS2@C) were prepared by hydrothermal method. Briefly, sodium molybdate dihydrate (Na2MoO4·2H2O, 1.2 g) and sulfocarbamide (CH4N2S, 1.6 g) were dispersed in ultrapure water to form a homogeneous solution. Subsequently, 0.6 g of oxalic acid (C6H8O7) and an appropriate amount of 3D porous carbon were added into the above liquor, magnetically stirring for around 30 min. The mixture solution was then transferred into a 100 mL Teflon-lined stainless-steel autoclave and treated at 180 °C for 24 h. After being cooled naturally, the black precipitate was generated and adhered to the inner wall of the lining. MoS2@C material was collected by ultrasonic cleaning with distilled water and ethanol respectively for several times, and then MoS2@C was dried in a vacuum oven at 50 oC. MoS2 nanoflower was prepared by a similar approach, which was hydrothermally treated the mixture of Na2MoO4·2H2O, CH4N2S and C6H8O7 without adding 3D porous carbon, MoS2 nanoflower was dealt with distilled water and ethanol for several times. To improve MoS2 crystallinity, the as-obtained samples were annealed at 800 oC for an hour under argon (Ar) flow. 2.2 Electrochemical Measurements For electrochemical testing, MoS2/MoS2@C was mixed with conductive carbon black (Super P) and polyvinylidene fluoride binder (PVDF) in N-methyl-2-pyrrolidone(NMP), with mass ratio of 7:2:1, to produce a homogeneous slurry, which was then coated onto pure Cu foil and dried at 60 oC in vacuum oven for 12 h. CR2025-type half-coin cells employing sodium metal foil as counter electrode were assembled in an argon-filled 4

glovebox. The electrolyte was 1 M sodium perchlorate (NaClO4) dissolved in ethylene carbonate and diethyl carbonate (EC: DEC, 1:1 in volume) with 8 vol% fluoroethylene carbonate (FEC). Galvanostatic cycling was conducted with Neware Battery Testing System in the potential range of 0.01-2.5 V versus Na/Na+ at different current densities. The cyclic voltammetry (CV) with a sweep rate of 0.1 mV s-1 at 0.01-2.5 V and electrochemical impedance spectroscopy (EIS) measurements in the frequency range of 100 kHz to 0.01 Hz were carried out using CHI 660D electrochemical workstation. The equilibrium potentials of cells were acquired by the galvanostatic intermittent titration technique (GITT) measurements. All cells were charged for 10 min at a current density of 50 mA g-1 and then followed by a 1 h relaxation. 2.3 Characterization: The morphologies of MoS2 and MoS2@C materials were characterized by scanning electron microscopy (SEM, JEOL JSM-7500F) and transmission electron microscopy (TEM, JEOL JEM-2100). X-ray diffraction (XRD) patterns were acquired by the Panalytical diffractometer (X’PERT) using Cu Kα radiation at a scan rate of 0.1° s-1 in the range of 5-90o. The thermal stability of the MoS2 and MoS2@C was assessed by the thermogravimetric analysis (TGA, STA449F3) with the heating rate of 10 oC min-1 in the air atmosphere from the room temperature to 800 oC. Raman spectroscopy was obtained by Renishaw inVia with 532 nm laser excitation. The surface electronic states of the materials were tested by X-ray photoelectron spectroscopy (XPS, PHI-5000CESCA) with Al Kα radiation. The specific surface area and pore size distribution were performed by nitrogen adsorption-desorption using Micromeritics ASAP2020. 3．Results and discussion Structural and Morphological study The overall fabrication strategy of MoS2@C is schematically illustrated in Scheme 1. Firstly, the 3DPC with a hierarchical structure is obtained using chemical blowing and carbonization of PVP under the auxiliary action of iron nitrate at high temperatures. Meanwhile, the MoS2 nanoflower is prepared by a facile one-step hydrothermal method [23]. Then, adding a proper amount of 3DPC as the precursor, MoS2 nanosheets are insitu uniformly grown on the 3DPC via the hydrothermal reaction, which is labeled as

5

MoS2@C. MoS2 nanosheets would in-situ grow on the carbon skeleton, forming a novel three-dimensional structure. The morphology and microstructure of the as-obtained bare MoS2, 3DPC, and MoS2@C are systematically characterized by SEM at different magnifications. As presented in Fig. 1a-c, the typical MoS2 nanoflower structure is observed, and the average diameter is about 0.5-0.7 µm. MoS2 nanosheets randomly intersect together and closely aggregate into nanoflower structure [24]. The SEM images of three-dimensional porous carbon are shown in Fig. 1d-f. They are made of cross-linked carbon nanosheets with continuous macropores up to several micrometers [25]. After one-step hydrothermal treatment, the building blocks MoS2 nanosheets freely in-situ grow on 3D carbon framework and self-assembly form successive and interconnected 3D hierarchical structure (MoS2@C) as shown in Fig. 1g-i. This cross-linked structure effectively prevents the aggregation of MoS2 nanosheets and further ensures the MoS2 nanoparticles in intimate contact with electrolyte [26]. Specifically, a homogeneous carbon skeleton network with a thickness of about 500 nm is formed and this novel structure would offer enough void space to alleviate the volume expansion of MoS2 nanosheets upon cycling. The unique architecture is further investigated by TEM and HRTEM, as shown in Fig. 2a-i. The multi-layer MoS2 presents innumerable thin nanosheets, which are closely interconnected with each other to form a spherical structure illustrated in Fig. 2a and b. The TEM images (Fig. 2d and e) show the unique bubble-like porous structure, which would present many active sites for the growth of MoS2 nanosheets. Moreover, as shown in Fig. 2g and h, it is distinctly observed that MoS2 curled nanosheets are successfully anchored on the surface of the 3D framework, which is in accordance with the results of SEM images. HRTEM is performed to further characterize the structural characteristics of products [27]. The carbon shells with a layer distance of 0.32 nm are found in the HRTEM image (Fig. 2f), which is consistent with the previous literature [22]. As is displayed in Fig. 2c and i, the lattice fringe of 0.608 and 0.638 nm can be explicitly observed for MoS2 and MoS2@C, respectively. The expanded layer distance would be conducive to the insertion and extraction of sodium ion in the long-term cycle. XRD is used to investigate the phase of the pure MoS2 and MoS2@C in Fig. 3a. The observed diffraction peaks of MoS2@C can be indexed to the phase of the hexagonal 2H6

MoS2 (JCPDS no. 37-1492) [17]. Compared with the MoS2@C, there are two new peaks in the low-angle region (8.9° and 17.5°) in pure MoS2, indexed to (001) and (002) reflections which is mainly ascribed to the emergence of a different layered structure when they are in a hydrothermal reaction [28,29]. It is noted that the (002) peak of the MoS2@C, in contrast to pure MoS2, exhibits a shift to the lower scattering angle, which is ascribed to the fact that the porous carbon expands the interlayer spacing. As presented in Fig. 3b, the content of the MoS2 in the sample of MoS2@C is conducted by the thermogravimetric analysis (TGA) from room temperature to 800 ℃. The rapid weight change (about 10.7 wt%) between 330 ℃ and 500 ℃ of bare MoS2 is attributed to the fact that MoS2 is oxidized to MoO3 [30]. For MoS2@C, the mass loss before 200 ℃ is mainly owing to the evaporation of moisture and the other organic impurities adsorbed on the material surface. From 200 ℃ to 500 ℃, MoS2@C undergoes an obvious weight loss, resulting from the combustion of carbon and the oxidation of MoS2 [31]. According to TGA analysis, the overall loading ratio of MoS2 is calculated to be approximately 75.1 wt% in the composite of MoS2@C. Fig. 3c and d display representative Raman spectra excited by 532 nm line for MoS2@C, MoS2, and 3DPC samples, respectively. As shown in Fig. 3d, the distinct peaks at 1343.9 and 1595.3 cm-1 may be indexed to the D (disordered) band and G (graphite) band of porous carbon material, respectively [14]. The ratio of ID and IG is around 1.33, indicating that the disordered structure of carbon nanosheets is dominant. Meanwhile, Fig. 3c exhibits peaks at around 380.5, 406.9 cm-1 for MoS2 and 376.6, 401.9 cm-1 for MoS2@C, which correspond to in-plane E11g, out-of-plane A1g vibration modes, separately [3]. According to previous literatures [32-35], the A1g peak of MoS2 is downshifted with the decreasing number of layers. It is noted that the A1g peak moves from 406.9 cm-1 to 401.9 cm-1 after loading carbon in Fig. 3c which further proves that the MoS2 in MoS2@C composite are few-layer nanosheets. Furthermore, there are two characteristic peaks centered at 1348.6 and 1577.5 cm-1 in MoS2@C composite, which are ascribed to the D-band and G-band of porous carbon material, respectively. This verifies the presence of the carbon framework. XPS is utilized to investigate the chemical configuration and the surface electronic states of MoS2@C (Fig. 3e-h) and pure MoS2 (Fig. S1a and b). Fig. 3e exhibits the XPS 7

survey spectrum, which displays the existence of S, Mo, C and O elements in the MoS2@C sample without other impurities. As exhibited in Fig. 3f, three peaks at about 225.40, 228.39 and 232.08 eV observed in the high-resolution Mo 3d spectrum of MoS2@C are attributed to S 2s, Mo4+ 3d5/2, and Mo4+ 3d3/2 respectively. Meanwhile, the characteristic peak centered at 235.70 eV is assigned to Mo6+ 3d3/2, which is formed by surface oxidation from Mo4+ to Mo6+ in the air [36]. The S 2p region is shown in Fig. 3g, in which the two peaks located at 161.42 and 168.39 eV are ascribed to S 2p3/2 and S 2p1/2, respectively. Also, Fig. 3h reveals the C 1s spectrum, suggesting three main peaks centered at about 284.68, 286.40 and 288.55 eV, corresponding to C-C, C-O, and C=O, respectively [37]. The nitrogen adsorption-desorption measurement shown in Fig. 3i and j was performed to acquire the surface nature of the 3D porous carbon, MoS2, and MoS2@C. BrunauerEmmett-Teller (BET) characterization in Fig. 3i demonstrates a feature of type IV isotherms, indicating that the 3D carbon framework is a typical mesoporous structure with a surface area up to 604 m2 g-1 which would provide more active site for MoS2 nanosheets. As shown in Fig. 3j, MoS2@C has a specific surface area of 9.1 m2 g-1, larger than that of MoS2 of 4.2 m2 g-1, which can promote the infiltration of electrolyte. Besides, from Barrett-Joyner-Halenda (BJH) analysis, we can acquire that the MoS2 and MoS2@C show mean pore size of 38.3 and 52.6 nm respectively. The larger pore size can enhance the diffusion of Na+ ion, which is beneficial for electrochemical performance. Kinetics and quantitative analysis The electrochemical performance of the MoS2@C electrode for sodium-ion batteries was investigated. The typical cyclic voltammetry (CV) curves of the initial three cycles for the MoS2@C electrode in Fig. 4a were determined at a sweeping speed of 0.1 mV s-1 with the voltage window from 0.01 to 2.5 V. For the first cathodic scan, the irreversible peak positioned at about 0.802 V can be ascribed to the decomposition of electrolyte, causing the formation of solid electrolyte interphase (SEI) [38-40]. The cathodic peak located at about 0.705 V corresponds to the phase transition of MoS2 (MoS2 + xNa+ →NaxMoS2) and the peak below 0.4 V is associated with the formation of Mo and Na2S (NaxMoS2 + (4-x) Na+ → 2Na2S + Mo) [41]. For the following anodic scan, the peak centered at around 0.13 and 1.82 V can be attributed to the reconstruction of NaxMoS2 8

and the formation of MoS2. For the following cycles, the reduction and oxidation curves almost overlap, revealing the outstanding reversibility of the MoS2@C electrode [15]. MoS2@C and MoS2 electrodes were galvanostatic charged/discharged at the current density of 50 mA g-1 and the corresponding charge/discharge curve and cycling performance were exhibited in Fig. 4b-c and Fig. S2b. In the first cycle, the discharge and charge capacities of MoS2@C electrode are 729 mAh g-1 and 438 mAh g-1, respectively, and an irreversible capacity loss is about 40%, which can mainly be ascribed to the inevitable formation of inorganic solid electrolyte interface (SEI) film and the decomposition of the electrolyte. This is a universal phenomenon for most anode materials [36,41]. It is noteworthy that, in the subsequent cycles, the coulombic efficiency of the MoS2@C electrode increases to more than 95% and no significant capacity decay is observed. Also, the reversible capacity of the MoS2@C electrode is maintained at around 305 mAh g-1 after 150 cycles, while the MoS2 electrode suffers rapidly capacity loss, only retains 153 mAh g-1, demonstrating that the unique 3DPC provides robust support for MoS2 nanosheets. The rate performance of the MoS2@C and MoS2 electrode is evaluated, as shown in Fig. 4d and Fig. S2c. When cycled at the current densities of 50, 100, 200, 500, 1000 and 2000 mA g-1, the MoS2@C electrode delivers the discharge capacities of 372, 320, 300, 274, 255, 225 mAh g-1, respectively, which are superior to pure MoS2. More importantly, once the current density returns to 50 mA g-1, the specific capacity can almost totally recover, indicating excellent sodium ion diffusion kinetics and favorable structure reversibility. The rate capability of MoS2@C is significantly better than other anodes materials previously reported, such as MoS2/graphene[1], MoS2@nitrogen-doped graphene[18], Nb2O5@RGO[42], MoS2/C hybrids[43], TiS2 nanoplates[44], graphene paper[45], hard carbon/graphene[46] for SIBs. (Fig. S3) Furthermore, in order to verify the long-term cycle stability of MoS2@C electrode (Fig. 4e), all the samples are further analyzed at a high current density of 500 mA g-1, after 200 cycles, the capacity can still maintain at 233 mAh g-1 which is far higher than 84 mAh g-1 of pure MoS2 electrode (Fig.S2d). This enhanced electrochemical performance might be attributed to a synergistic effect between MoS2 nanosheets and 3D carbon framework: 3DPC with hierarchical porosity serves as a high-speed 3D pathway 9

to promote electron transportation and electrolyte infiltration, and the novel structure formed by layered MoS2 and 3DPC inhibits the aggregation of nanosheets and improves the conductivity. In order to explore the volume change for our samples, we have carried out TEM characterization of the electrode after the initial cycle to investigate the interlayer space of (002). As shown in Fig. S4a-d, the interlamellar spacing of MoS2@C and pure MoS2 is 6.27, 9.18 Å after the initial cycle, separately. Consequently, the volume change in the c direction is about 3.1%, 43.9% for MoS2@C and pure MoS2, which demonstrate that the volume expansion of the MoS2@C electrode material is effectively alleviated [47]. To further explain the superior electrochemical characteristics of MoS2@C, CV measurements were conducted at different scan rates ranging from 0.4 to 1.5 mV s-1 [48]. Fig. 5a exhibits the CV curves of MoS2@C whose shapes can maintain well with increasing scan rates, which can be ascribed to the improved surface diffusion process [49]. Generally, pseudo-capacitance is a kind of electrochemical reaction which occurs near the surface. There are three main reaction mechanisms including: (i) the faradaic contribution from the Na+ insertion process. (ii) the surface pseudocapacitance effect. (iii) the non-faradaic contribution from the double-layer capacitors [11,50]. According to previous literature [51,52], the current (i) and sweep speed (v) are in accordance with the following power-law relationship (1)

i = avb

(1)

Where i is the measured current density, v represents scan rate, and a and b are adjustable values. Based on former research [53], when the b-value is about 1, the reaction process is mainly controlled by capacitance, while the b-value is close to 0.5, diffusion-controlled dominates the whole process. As displayed in Fig 5b, the calculated b values of anodic and cathodic current peaks are separately 0.903 and 0.989, which can be acquired from the slope of the logarithm of the peak current density versus the logarithm of the scan rate, illustrating that Na+ storage mechanism of MoS2@C is dominantly controlled by the capacitive process. More specifically, the total current can be separated into two parts [54,55]

i = k1v + k2v1/2

10

(2)

Here, k1v and k2v1/2 stand for the capacitive-controlled contribution and the diffusioncontrolled contribution respectively. With the gradual increase of sweep speed (from 0.4 to 1.5 mV s-1), the pseudo-capacitance contribution increase from 67% to 77%, as shown in Fig. 5d-i. The high capacitive contribution can be ascribed to the hierarchical structure of MoS2@C which can shorten the transport path of Na+ and enhance the mass transfer rate. For comparison, the capacitance contribution to the MoS2 electrode (from 52% to 68%) is also investigated as displayed in Fig. S5a-f, which is relatively smaller than that of MoS2@C at the same sweep speed. EIS measurement is carried out before charging/discharging and after 50 and 100 cycles at 50 mA g-1. Fig. 5c and Fig.S6c display the EIS profiles of MoS2@C and MoS2. The semicircular in the high-medium frequencies can be regarded as two parts corresponding to SEI film resistance (Rs) and charge-transfer impedance (Rct), respectively [56,15]. The low-frequency sloping line represents the Warburg impedance (Zw), which can be correlated with solid-state diffusion of Na+ in the bulk materials. The kinetic parameters of two electrodes are obtained from the equivalent circuit in Fig. S6d. Before discharging and after 50 and 100 cycles, the fitted Rct values are 33, 113 and 162 Ω for MoS2@C electrode and 103, 217 and 406 Ω for MoS2 electrode, respectively. It is demonstrated that the MoS2@C electrode shows a smaller charge-transfer resistance than MoS2, suggesting a faster electron transfer rate occurred on the MoS2@C electrode/electrolyte interface. To further understand the Na-diffusion kinetics behavior, GITT was employed to investigate the chemical diffusion coefficient of Na+ (DNa+) for the MoS2 and MoS2@C electrodes. Fig. 6a and b show the GITT profiles of MoS2@C and MoS2 electrodes in the discharge process at a current density of 50 mA g-1 between 0.01-2.5 V. According to the following equation [57,58], we can calculate the DNa+ mV ∆E m )2 ( s )2 πτ MA ∆E

DNa + = 4 (

(3)

τ

Where τ is pulse duration; m and M correspond to the mass and molar weight of active substance; Vm represents molar volume; A is the electrode/ electrolyte contact area; ∆Eτ and ∆Es are potential variations caused by a constant current pulse and quasi-equilibrium

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potential, respectively. Based on eq3 and GITT tests, DNa+ at different potentials is obtained, as shown in Fig. 6c. Obviously, the DNa+ of the MoS2@C (10-13 to 10-12 cm2 s-1) is in a one order magnitude higher than pure MoS2 (10-14 to 10-13 cm2 s-1). The higher DNa+ is ascribed to the following two reasons that the enlarged interlamellar spacing can make Na+ easily shuttle in the bulk materials and 3D ion diffusion channels can also shorten the transport path of Na+. 3D roughness reconstruction is applied to evaluate the morphology feature of the surface of MoS2@C and MoS2 electrodes before and after the first discharge, as shown in Fig. 7a-d and Fig. S7a-d, respectively. Fig. 7a,b and Fig. S7a,b exhibit the 3D images of MoS2@C and MoS2 electrode before discharging, and the surface of electrodes are both relatively flat. After discharging to 0.01V, the surface roughness of the electrodes increases with the insertion of sodium ions. Interestingly, the MoS2@C electrode (Fig. 7c and d) has a comparatively flat surface, and the height changes uniformly. In contrast, the surface of the MoS2 electrode (Fig. S7c and d) is disorganized, accompanied by the formation of large agglomeration. These differences may be due to the fact that the 3D carbon skeleton provides a powerful matrix for MoS2 nanosheets to make MoS2 more uniform. And the agglomeration of the pristine MoS2 material is unfavorable to the transport of sodium ion and electron.

4. Conclusion To summarize, a facile and effective method has been developed for fabricating a novel 3D porous interconnected architecture (MoS2@C) with few-layer MoS2 nanosheets loading on 3D cross-linked hollow skeleton. In such a 3D structure, the carbon matrix offers powerful support for MoS2 nanosheets, which enables to improve mechanical stability and

electrical

conductivity of

electrodes

upon

sodiation/desodiation.

Furthermore, the interconnected porous structure designed here can facilitate the diffusion paths of ions and electrons, and be favorable for the electrolyte wettability. When adopting MoS2@C as the electrode for SIBs, compared with pure MoS2, MoS2@C displays much enhanced cyclability with an outstanding reversible capacity of 305 mAh g-1 after 150 cycles at 50 mA g-1 and remarkable rate performance (372 and 225 mAh g-1 at 0.05 and 2 A g-1, respectively). Also, the superior ion diffusion behavior of the MoS2@C electrode was comprehensively studied by the GITT technique to evaluate the 12

Na-ion diffusion coefficient (DNa+) upon sodiation, exhibiting a one order magnitude higher than MoS2. This work may help to dig out Na-storage mechanism in the lamella materials and open up new opportunities for using porous carbon materials in the SIBs regime.

Conflicts of interest There are no conflicts to declare.

Acknowledgment The study was supported by the financial support from the Natural Science Foundation of China (51602046, 21875057), Fundamental Research Funds for the Central Universities (2572018BC30, 2572015CB23), China Postdoctoral Science Foundation (2014M561312) and Heilongjiang Postdoctoral Foundation (LBH-Z14014).

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Figure

19

Scheme 1. Schematic illustration of the preparation of MoS2@C.

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Fig. 1. SEM images of (a-c) the MoS2 nanoflower, (d-f) the porous 3DPC and (g-i) the novel MoS2@C architecture.

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Fig. 2. TEM and HRTEM images of (a-c) MoS2, (d-f) 3DPC and (g-i) MoS2@C.

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Fig. 3. (a) XRD patterns of the pure MoS2 and MoS2@C sample. (b) TGA curves of the pure MoS2 and MoS2@C product in the air at the heating rate of 10 oC min-1. (c-d) Raman spectra of the MoS2@C, MoS2, and 3DPC. (e) XPS survey spectrum of MoS2@C. High-resolution XPS spectra of Mo 3d (f), S 2p (g), C 1s (h). (i-j) N2 adsorptiondesorption isotherm of 3DPC, MoS2@C and MoS2. The inset indicates the pore size distribution of 3DPC, MoS2@C and MoS2.

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Fig. 4. (a) CV curves of the MoS2@C electrode at potential range of 0.01 to 2.5 V and sweep rate of 0.1 mV s-1. (b) Charge-discharge and (c) cycling performance profiles of the MoS2@C electrode at a current density of 50 mA g-1. (d) Rate capability of the MoS2@C electrode. (e) Cycling performance of MoS2@C for 200 cycles at a current density of 0.5 A g-1.

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Fig. 5. (a) CV curves of the MoS2@C at different sweep rates from 0.4 to 1.5 mV s-1. (b) b values based on log(i) versus log(v) plots at different oxidation and reduction states of MoS2@C. (c) EIS curves of MoS2@C electrode before discharging and after 50 and 100 cycles at 50 mA g-1. (d-h) Capacitive contribution of MoS2@C at different scan rate from 0.4 to 1.5 mV s-1. (i) Normalized contribution ratio of capacitive capacities at different scan rate.

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Fig.6. (a-b) GITT curves of the MoS2@C and pure MoS2 electrode in the discharge process. (c) The calculated Na+ diffusion coefficients in the discharging process for MoS2@C and MoS2 electrode.

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Fig.7. The 3D roughness reconstruction of MoS2@C. (a) 2D image of MoS2@C electrode before discharging. (b) 3D reconstruction of MoS2@C electrode before discharging. (c) 2D image of MoS2@C electrode after discharging to 0.01 V. (d) 3D reconstruction of MoS2@C electrode after discharging to 0.01 V. Color-bar represents the surface heights relative to the datum at the mean height value.

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MoS2 nanosheets anchored on porous carbon (MoS2@C) were synthesized via a facile and controllable method. Benefiting from the three-dimensional (3D) ion diffusion channels, a high intercalation pseudocapacitance is achieved, which is responsible for the good rate capability. 3D roughness reconstruction of the MoS2@C electrode well reveals the integrity of the hierarchical structure after Na+ insertion.