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Apr 22, 2017 - benignity.1−4 Sodium ion batteries (SIBs) have attracted attention due to their low cost and abundant resource compared to LIBs.5,6 H...
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Research Article pubs.acs.org/journal/ascecg

MoS2 Decorated Fe3O4/Fe1−xS@C Nanosheets as High-Performance Anode Materials for Lithium Ion and Sodium Ion Batteries Qichang Pan,† Fenghua Zheng,† Xing Ou,† Chenghao Yang,*,† Xunhui Xiong,† Zhenghua Tang,† Lingzhi Zhao,§ and Meilin Liu†,‡ †

Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research Institute, School of Environment and Energy, South China University of Technology, Higher Education Mega Center, Guangzhou 510006, China ‡ School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States § Guangdong Provincial Engineering Technology Research Center for Low Carbon and Advanced Energy Materials, South China Normal University, Guangzhou 510631, China S Supporting Information *

ABSTRACT: Fe3O4/Fe1−xS@C@MoS2 nanosheets consisting of Fe3O4/ Fe1−xS nanoparticles embedded in carbon nanosheets and coated by MoS2 were synthesized via a facile and scalable strategy with assistance of NaCl template. With Fe3O4/Fe1−xS@C@MoS2 nanosheets composite as an anode for LIBs and SIBs, the Fe3O4/Fe1−xS@C@MoS2 nanosheets composite shows outstanding electrochemical performance because of the synergistic effects of the Fe3O4/Fe1−xS nanoparticles, carbon nanosheets and MoS2. In this unique constructed architecture, on one hand, the carbon nanosheets can avoid the direct exposure of Fe3O4/Fe1−xSNPs to the electrolyte; on the other hand, the carbon nanosheets can buffer the volume change of Fe3O4/ Fe1−xS NPs as well as suppress the aggregation of Fe3O4/Fe1−xS NPs during the cycling processes. Moreover, MoS2 can offer high interfacial contact areas between active materials/electrolyte, resulting in rapid charge transfer and higher capacity. As a consequence, Fe3O4/Fe1−xS@C@MoS2 nanosheets exhibit high reversible capacity of 1142 mAhg−1 after 700 cycles at 1.0 A g−1 and 640 mA h g−1 at 5.0 A g−1for LIBs, 402 mA h g−1 after 1000 cycles at 1.0 A g−1and 355 mA h g−1 at 2.0 A g−1 for SIBs, respectively. This outstanding electrochemical performance indicated that the Fe3O4/Fe1−xS@C@MoS2 nanosheets have potential as anode for high-performance LIBs and SIBs. KEYWORDS: Fe3O4/Fe1−xS, MoS2, Nanosheets, Lithium ion batteries, Sodium ion batteries

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INTRODUCTION Recently, lithium ion batteries (LIBs) have become the most attractive technologies for electric vehicles and portable electronics due to their high energy density and environmental benignity.1−4 Sodium ion batteries (SIBs) have attracted attention due to their low cost and abundant resource compared to LIBs.5,6 However, the energy densities and cycle life of both LIBs and SIBs are insufficient to satisfy completely social needs and has become a critical issue. Therefore, to solve these problems, it is necessary to develop an advanced electrode materials with high performance for both LIBs and SIBs.7,8 In the case of anode materials, Fe-based anodes are promising anode materials in both LIBs and SIBs because of its high capacity and environmental friendliness.9,10 However, the practical application of Fe-based anodes are still limited by rapid capacity fading because of the serious volume expansion during the Li+ insertion/extraction processes, which cause pulverization of Fe-based anodes and lead to poor cycle life.11,12 To circumvent these problems, many efforts have been made to © 2017 American Chemical Society

enhance the electrochemical performance by designed nanostructured materials or hybridized with carbon-based material. Therefore, nanostructured Fe-based anode materials with various morphologies have been developed, such as nanosheets,13 nanorods14 and hollow nanostructures.15 And the electrochemical performance of Fe-based anodes have been much improved thank to that can inhibit the volume expansion among the charge/discharge process. On the other hand, the nanoscale sized electrode materials also cause shorter path lengths for the transport of electrons and Li+/Na+. Another widely used method is to fabricate hybrid materials composed of nanoscale Fe-based materials and carbon. So, a lot of Febased hybrids such as Fe3O4 nanoparticles embedded in carbon,16 graphene/Fe3O4,17 Fe1−xS/C nanocomposites18 and FeS nanoparticles/carbon nanosheets composite19 have been developed. These carbon can accommodate the volume Received: January 12, 2017 Revised: April 13, 2017 Published: April 22, 2017 4739

DOI: 10.1021/acssuschemeng.7b00119 ACS Sustainable Chem. Eng. 2017, 5, 4739−4745

Research Article

ACS Sustainable Chemistry & Engineering variation of nanoparticles during Li+/Na+ insertion/extraction process, and also lead to good conductivity, so the electrochemical performance of the Fe-based anodes have been remarkably improved. Recently, MoS2, a transition metal sulfide, which features a layered structure resembling that of graphite, is a promising anode material for both LIBs and SIBs.20,21 However, MoS2 anode is plagued with low intrinsic electric conductivity and the large strain upon cycling that induces low rate capability and fast capacity decay.22 Therefore, several strategies have been taken to solve these problems. Synthesize nanostructured MoS2 and decorated with nanoparticles or carbonaceous materials were effective strategies to overcome this issues.23−26 Moreover, MoS2 has high mechanical strength, which even can comparable to steel. Therefore, MoS2 nanosheets can prevent the severe structural deformation of some metal oxides with huge volume change during the cycling process due to such strong mechanical behavior.27 Herein, we developed a facile strategy to fabricate Fe3O4/ Fe1−xS nanoparticles embedded in carbon nanosheets and uniformly coated with MoS2nanosheets (designated as Fe3O4/ Fe1−xS@C@MoS2 nanosheets) as anode material with superior electrochemical performance for LIBs and SIBs. The novel process for fabricating Fe3O4/Fe1−xS@C@MoS2nanosheets involves in situ preparation Fe3O4 nanoparticles embedded in carbon nanosheets and uniformly coated with MoS2 nanosheets and the Fe3O4 were sulfurated simultaneously under hydrothermal conditions. In this constructed 2D architecture, on one hand, the carbon nanosheets can inhibit the aggregation of Fe3O4/Fe1−xS NPs and accommodate the volume change. On the other hand, the MoS2nanosheets and carbon nanosheets can avoid the direct exposure of Fe3O4/Fe1−xS nanoparticles to the electrolyte and buffer the volume change but also benefits fast ion intercalation/extraction and provides structural stability during Li+/Na+ insertion/extraction processes. Finally, the novel Fe3O4/Fe1−xS@C@MoS2 nanosheets exhibit outstanding cycling performances and rate capability for both LIBs and SIBs.

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XPS analysis was performed on an ESCALab220i-XL electron spectrometer at room temperature. Electrochemical Measurements. The electrochemical properties of Fe3O4/Fe1−xS@C@MoS2 composite and Fe3O4@C composite was carried out using coin type half-cell (CR2025 for LIBs and CR2032 for SIBs). The working electrode composed with Fe3O4/Fe1−xS@C@ MoS2/Fe3O4@C, acetylene black and polyvinylidene fluoride with mass ratio 70:15:15 were mixed into with NMP form homogeneous slurry. Lithium metal foil and sodium metal foil as both counter electrodes for LIBs and SIBs, respectively. The electrolyte was LiPF6 (1 M) dissolved in EC/DEC (1:1 vol %) for LIBs and EC/DMC (1:1 vol %) containing 1 M NaClO4 with 5% FEC additive as the electrolyte for SIBs. Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) were tested with a CHI660D electrochemical workstation. The discharge and charge measurements were performed on aLAND CT2001A test systems between 3.00 and 0.01 V at 25 °C.

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RESULTS AND DISCUSSION The crystallographic structures of the Fe3O4@C and Fe3O4/ Fe1−xS@C@MoS2 were identified by X-ray powder diffraction (XRD). Figure S1A (Supporting Information) shown the XRD pattern of as-synthesized Fe3O4@C; every peaks in the XRD pattern can be readily attributed to the cubic phase of Fe3O4 (JCPDS No. 79-0419) without any impurity.28 For Fe3O4/ Fe1−xS@C@MoS2 (Figure S1B, Supporting Information), it is of note that after the hydrothermal coating reaction, a series of strong peaks at 29.9°, 33.8°, 43.7° and 53.1° can be obtained, which can be attributed to the (200), (205), (2010) and (220) planes of the Fe1−xS (JCPDS Card no. 29-0724), respectively.18 In addition, the peaks at 35.5°, 57.0° and 62.5° can be indexed to the (311), (511) and (440) planes of the cubic phase of Fe3O4 (JCPDS No. 79-0419). On the other hand, the diffraction peak at (002) can be indexed to the MoS2 (JCPDS No. 37−1492).29 Therefore, all these results indicating that Fe3O4 are sulfided into Fe1−xS but not completely in the hydrothermal reaction, and also prove that successful coating of the MoS2 in the Fe3O4/Fe1−xS@C@MoS2 nanocomposite. Moreover, Fe3O4/Fe1−xS@C@MoS2 composite was further studied by Raman spectroscopy and the results as shown in Figure S2 (Supporting Information). Figure S2A (Supporting Information) indicates two characteristic peaks of carbon, located around at 1350 and 1580 cm−1, corresponding to the D and G bands for carbon materials, respectively.30 In addition, two peaks at around 383 and 405 cm−1 can be found in the Fe3O4/Fe1−xS@C@MoS2 for MoS2 (Figure S2B, Supporting Information).31 The weight fraction of Fe3O4 in the Fe3O4@C nanosheets, Fe3O4/Fe1−xS and MoS2 in the Fe3O4/Fe1−xS@C@MoS2 nanocomposite could be confirmed by TGA measurements. From the results as shown in Figure S3 (Supporting Information), from 300 to 700 °C, Fe3O4 and Fe1−xS were oxidized to Fe2O3 slowly, MoS2 was oxidized to MoO3, and carbon was burned to form CO2. Thus, this confirms that Fe3O4 accounts for 80.42 wt % of the Fe3O4@C nanocomposites, Fe3O4/Fe1−xS and MoS2 account for 57.08 and 27.8 wt % of the Fe3O4/Fe1−xS@C@MoS2 nanocomposite, respectively. XPS spectra were conducted to investigate the chemical composition and chemical bonding state of the Fe3O4@C and Fe3O4/Fe1−xS@C@MoS2, respectively. Figure S4A (Supporting Information) is the survey spectra of the Fe3O4@C and confirm that the composite composed of Fe, C and O. Furthermore, Figure 1A exhibits the overall XPS spectrum of the Fe3O4/Fe1−xS@C@MoS2 composite, suggesting that the sample is mainly composed of Fe, C, O, Mo and S

EXPERIMENTAL SECTION

Synthesis of Fe3O4@C Nanosheets. First, 1.8 g of Fe(NO3)3· 9H2O, 2 g of citric acid and 15 g of NaCl were mixed with 50 mL of deionized water, and then subjected to freeze-drying to obtain dry gel composite. Second, the obtained dry gel composite was calcined at 400 °C under Ar atmosphere for 2 h. Finally, the NaCl was removed by using distilled water and dry at 80 °C to obtain Fe3O4@C nanosheets composite. Synthesis of Fe3O4/Fe1−xS@C@MoS2 Nanosheets. The obtained Fe3O4@C nanosheets (0.3 g) were dispersed into 80 mL of deionized water by ultrasonication, followed by the addition of Na2MoO4·2H2O (0.15 g) and L-cysteine (0.6 g). After stirring for 30 min, the solution were transferred to a 100 mL Teflon-lined stainlesssteel autoclave, and kept in an electric oven at 220 °C for 24 h. The precipitates were collected by filtration, washed with deionized water and dried at 80 °C for 24 h. Then Fe3O4/Fe1−xS@C@MoS2 nanosheets were further treated at 600 °C in Ar atmosphere for 2 h. For comparison, the Fe3O4@C nanosheets were also annealed at 600 °C for 2 h under Ar atmosphere. Characterization. Morphology of the samples was observed using FESEM (Hitachi S-4800), TEM and HRTEM (TEM, JEM-2010, JEOL, 200 kV). The crystal phase of the all samples was evaluated by XRD (Rigaku D/max 2500). TG analysis was performed with a PerkinElmer (TA Instruments) with a temperature ramp of 5 °C min−1 under air atmosphere. A Raman spectrum was recorded on the LabRAM HR Raman spectrometer with a laser wavelength of 633 nm. 4740

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MoS2 nanosheets are inherited from the sheet-like architectures of Fe3O4@C nanosheets (Figure 2C,D). But the surfaces of Fe3O4/Fe1−xS@C@MoS2 nanosheets become much smoother after hydrothermal reaction, indicating that the surface of nanosheets is successfully coated with MoS2. The SEM and EDX mapping of Fe3O4/Fe1−xS@C@MoS2 nanosheets are shown in FigureS5 (Supporting Information). Carbon (C), iron (Fe), oxgen (O), molybdenum (Mo) and sulfur (S) were identified and uniformly distributed in the Fe3O4/Fe1−xS@C@ MoS2 nanosheets. The Fe3O4/Fe1−xS@C@MoS2 and Fe3O4@C composite were further studied by TEM and high-magnification HRTEM. TEM images of Fe3O4@C exhibits that the Fe3O4 nanoparticles embedded in carbon nanosheets (Figure S6A,B, Supporting Information), and the HRTEM images show that the lattice fringes with d-spacings of 0.25 nm, which corresponding to (222) plane of Fe3O4 (Figure S6C,D, Supporting Information). Figure 3A,B shows that the Fe3O4/ Figure 1. XPS spectra of the as-synthesized Fe3O4/Fe1−xS@C@MoS2: (A) survey spectra, and (B) Fe 2p, (C) Mo 3d and (D) S 2p core level spectra.

elements. Figure S4B (Supporting Information) and Figure 1B exhibit the high-resolution XPS Fe 2p spectrum of Fe3O4/C and Fe3O4/Fe1−xS@C@MoS2; there are two peaks at around 711.4 and 724.8 eV in the two samples, in accord with Fe 2p3/2 and Fe 2p1/2, suggesting there exist the Fe3O4 in Fe3O4/C and Fe3O4/Fe1−xS@C@MoS2. On the other hand, according to the spectrum of Fe 2p is difficult to confirm the Fe1−xS phase in Fe3O4/Fe1−xS@[email protected],32 Moreover, as shown in Figure 1C, two peaks located at around 232.7 and 229.5 eV are observed, which are characteristic of the Mo 3d3/2, Mo 3d5/2, respectively, and are attributed to Mo4+ in MoS2. As described in Figure 1D, two peaks located at 161.9 and 163 eV, which are ascribed to S 2p3/2 and S 2p1/2 for MoS2 or Fe1−xS in the composite, respectively.33 The morphology and structure of the Fe3O4@C and Fe3O4/ Fe1−xS@C@MoS2 are examined by SEM. Fe3O4@C exhibits a sheets-like morphology with abundant and uniform nanocrystals embedded in nanosheets (Figure 2A,B). Similar to SEM images of the Fe3O4@C nanosheets, the Fe3O4/Fe1−xS@C@

Figure 3. TEM (A, B, C) and HRTEM (D, E, F) image of the Fe3O4/ Fe1−xS@C@MoS2 composite.

Fe1−xS@C@MoS2 exhibits well-maintained nanosheets-structure, and the Fe3O4/Fe1−xS nanoparticles are wrapped fully by carbon nanosheets, in good agreement with SEM observed. Moreover, the Fe3O4/Fe1−xS and carbon nanosheets were covered by thin MoS2 nanosheets, indicating that the MoS2 nanosheets are grown tightly on the surface of carbon nanosheets after a facile hydrothermal process (Figure 3C). HRTEM images (Figure 3D,E,F) of Fe3O4/Fe1−xS@C@MoS2 nanosheets show that the lattice fringes with d-spacings of 0.48, 0.63 and 0.29 nm can be clearly observed, which can be attributed to the (111) plane of Fe3O4, (002) plane of MoS2 and the (220) plane of Fe1−xS, respectively.18,34,35

Figure 2. SEM images of (A, B) Fe3O4@C and (C, D) Fe3O4/ Fe1−xS@C@MoS2 composite. 4741

DOI: 10.1021/acssuschemeng.7b00119 ACS Sustainable Chem. Eng. 2017, 5, 4739−4745

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discharge capacities were 608, 525, 476, 430 and 366 mA h g−1 at 0.2, 0.5, 1.0, 2.0 and 5.0 A g−1, respectively. Especially, after 50 cycles even up to 5.0 A g−1, a high discharge capacity of 610 mAh g−1 was recovered when return to 0.2 A g−1. Moreover, the long-cycle measurement was carried out at 1.0 A g−1, as shown in Figure S9C (Supporting Information), the Fe3O4@C composite electrode exhibited a high discharge capacity of 580 mAh g−1 after 800 cycles, indicating a high stability cycles at high rates. Figure 4A shows the cycle performance of Fe3O4/Fe1−xS@ C@MoS2 composite at 0.2 A g−1. The as-prepared Fe3O4/

The electrochemical performances of Fe3O4@C nanosheets and Fe3O4/Fe1−xS@C@MoS2 nanosheets for LIBs were evaluated based on CR2025 using Li metal as the counter and reference electrode. Figure S7A (Supporting Information) presents cyclic voltammogram of Fe3O4@C composite, in agreement with previous literature.16,17 In the initial cycle, a large cathodic peak at around 0.60 V can be obtained due to the reduction of Fe3+ and Fe2+ to Fe0, as well as formation of SEI layer. For the anodic scan, two peaks at around 1.64 and 1.85 V belong to the reversible oxidation of Fe0 to Fe2+ and Fe3+, respectively. Moreover, three peaks at around 0.8 V in discharge process, and at 1.6 and 1.8 V in charge process for the subsequent two cycles of theFe3O4@C electrode almost overlap, indicating the good electrochemical reversibility of the Fe3O4@C electrode.36 Moreover, Figure S7B (Supporting Information) shows the cyclic voltammogram curves of the Fe3O4/Fe1−xS@C@MoS2 nanosheets electrode at the same conditions. A sharp reduction peak at around 1.2 V can be assigned to the following reaction:18 Fe1 − xS + 2Li + 2e− → Li 2S + Fe

In the first cycle, a series of reduction peaks at 0.4−1.2 V were assigned to the formation of the SEI layer and insertion of Li+ into the interlayer space of MoS2 to form LixMoS2, and the reduction of LixMoS2 to Li2S and metallic Mo nanoparticles via a conversion reaction.37 A strong anodic peak at 1.95 V was found because of the oxidation of Fe to Li2−xFeS2, as for a oxidation peak at 2.33 V, which can be owing to the oxidation of Li2S to S and Li+. In the next two cycles, two reduction peaks appeared at around 1.36 and 1.91 V, suggesting that the formation of Li2−xFeS2 and the reversible delithiation process (Li2−xFeS2 to Li2FeS2). In addition, it can be noted that the subsequent CV curves of Fe3O4/Fe1−xS@C@MoS2 electrode almost overlap after first cycle, indicating the excellent retention of structural stability for the Fe3O4/Fe1−xS@C@MoS2 during the cycling process. Typical charge/discharge profiles of Fe3O4@C and Fe3O4/ Fe1−xS@C@MoS2at 0.2 A g−1 as shown in Figure S8 (Supporting Information). As shown in Figure S8A (Supporting Information), a voltage plateau at 0.75 V can be found during the first discharge process was ascribed to formation of SEI layers, consistent with CV results, whereas the two plateaus at 1.7 and 0.9 V in the second charge and discharge processes were ascribed to the conversion reactions between Fe3O4 and Li. Figure S8B (Supporting Information) shows the charge/ discharge profiles of the Fe3O4/Fe1−xS@C@MoS2 nanocomposites for the first and second cycles under 0.2 A g−1. A large plateau at around 1.3 V was observed, which can be ascribed to the reaction between Fe1−xS and lithium and formation of Fe, Li2S and Li-rich phases. Turn to charging process, a plateau between 1.8 and 1.9 V can be observed, which is assigned to the oxidation process of Fe to Li2FeS2. And a plateau at around 2.3 V is due to the oxidation of Li2S into S and Li+. Figure S9A (Supporting Information) reveals the cycling performance of Fe3O4@C at 0.2 A g−1, the Fe3O4@C composite delivers a relatively cycling stability. A reversible capacity of 621 mAh g−1 can be obtained after 100 cycles at 0.2 A g−1, and also remain high average Coulombic efficiency. In addition, to evaluate the capability of the Fe3O4@C composite, the composite was tested at different current densities from 0.2 to 5.0 A g−1 (Figure S9B, Supporting Information), the

Figure 4. Electrochemical evaluation of the Fe3O4/Fe1−xS@C@MoS2 electrode for LIBs: (A) Cycling performance of the Fe3O4/Fe1−xS@ C@MoS2 composite at a current density of 0.2 Ag1−. (B) Rate capability performance of the electrodes of Fe3O4/Fe1−xS@C@MoS2 composite. (C) Long-term cycling performance of the Fe3O4/Fe1−xS@ C@MoS2 composite at 1.0 A g−1.

Fe1−xS@C@MoS2 electrode shows a high discharge and charge capacities of 1495 and 1138 mAh g−1, corresponding to a high initial Coulombic efficiency of 76%. Moreover, the capacity of the Fe3O4/Fe1−xS@C@MoS2 composite is higher than the theoretical capacity of MoS2 or Fe3O4/Fe1−xS, which probably caused by the favorable synergistic effect between MoS2 andFe3O4/Fe1−xS@C nanosheets, and the similar phenomenon has also been observed in previous reports.23,31 After the first cycle, the Coulombic efficiency of the composite increases dramatically to 95% and stably maintained at about 98% in subsequent cycles. After 100 cycles, the Fe3O4/Fe1−xS@C@ MoS2 delivers a high discharge capacity of 1003 mAh g−1, which is 82.96% retention of the second cycle and demonstrates a slow capacity decay of only 0.17% per cycle. The rate performance of the Fe 3 O 4 /Fe 1−x S@C@MoS 2 composite is then investigated at various current densities (Figure 4B). Fe3O4/Fe1−xS@C@MoS2 composite delivers a high discharge capacity of 1050 mAh g−1 at 0.2 A g−1. As the current density increases, the capacity decreases slightly. When the current densities increase from 0.5 to 5.0 A g−1, the discharge capacities were 974, 904, 805 and 640 mA h g−1 at 0.5, 1.0, 2.0, and 5.0 A g−1, respectively. Remarkably, a high capacity of 1020 mAhg−1 can be obtained when reduced to 0.2 A g−1, which exhibits outstanding rate capability. Moreover, to further confirm the superior performance of the Fe3O4/ Fe1−xS@C@MoS2 composite, the long-term cycling of the Fe3O4/Fe1−xS@C@MoS2 composite at 1.0 A g−1 has been tested, and the result is shown in Figure 4C. The discharge capacity of the Fe3O4/Fe1−xS@C@MoS2 composite was 1142 4742

DOI: 10.1021/acssuschemeng.7b00119 ACS Sustainable Chem. Eng. 2017, 5, 4739−4745

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ACS Sustainable Chemistry & Engineering mAh g−1 after 700 cycles. All of these strongly confirmed that the Fe3O4/Fe1−xS@C@MoS2 composite exhibited excellent cycle and rate performance, which can be owing to the synergistic effects of the Fe3O4/Fe1−xS nanoparticles, carbon nanosheets and MoS2. After the first discharge process, MoS2 turn into Li2S and Mo, formed Li2S/C. So, the smaller Mo and S embedded in the carbon nanosheets, which can provide additional Li storage capacity and stable the structure of the composite, resulting in excellent cyclic performance and rate capability.38−40 Moreover, the capacity of the composite has a continuous increasing from 150 to 400 cycles, which can attributed to more and more available site from both MoS2 and Fe3O4/Fe1−xS inside electrode participated into the reversible reaction to contribute capacity or formation of a gel-like polymeric layer and possibly interfacial lithium storage as well as electrochemical activation of the composite during the discharge and charge process.41,42 Furthermore, the Fe3O4/Fe1−xS@C@MoS2 composite was further tested as anode materials for SIBs. It can be seen that the CV curve of Fe3O4/Fe1−xS@C@MoS2 composite is similar to the LIBs (Figure S10A, Supporting Information), whereas with lower voltage peaks due to the difference in the thermodynamics and kinetics for the insertion of lithium and sodium ions.20,43 The first discharge and charge capacity of Fe3O4/Fe1−xS@C@MoS2 composite are 1200.7 and 721.1 mAh g−1, respectively, and corresponding to the Coulombic efficiency is 62% for the first cycle (Figure S10B, Supporting Information). Moreover, the Fe3O4/Fe1−xS@C@MoS2 composite also displays outstanding cycling and rate performance when tested for SIBs. As shown in Figure 5A, Fe3O4/Fe1−xS@

high reversible capacity of 402 mAh g−1 still be retained even after 1000 cycles (Figure 5C). The SEM images of Fe3O4/Fe1−xS@C@MoS2 composite as anode materials after cycling for LIBs and SIBs have been investigated to understand further the excellent electrochemical performance. As shown in Figure S11 (Supporting Information), the Fe3O4/Fe1−xS@C@MoS2 composite maintains the sheets-like morphology after 100 cycles at 0.2 A g−1 for LIBs and SIBs. Therefore, the SEM images Fe3O4/Fe1−xS@C@ MoS2 composite after cycling strongly demonstrated that the excellent cycling and rate performance for LIBs and SIBs, and the performance comparisons between our work and others similar materials as anode materials for LIBs or SIBs as shown in Table S1 (Supporting Information). To investigate further the superior electrochemical performance of the Fe3O4/Fe1−xS@C@MoS2 electrode for LIBs and SIBs, electrochemical impedance spectroscopy (EIS) was measured to study the mechanism for the improved electrochemical performance in the Fe3O4/Fe1−xS@C@MoS2 electrode. As shown in Figure 6, all these curves demonstrate

Figure 6. Nyquist plots of the Fe3O4/Fe1−xS@C@MoS2 electrode tested before and after 100th cycles at 200 mA g−1 for (A) LIBs and (B) SIBs.

identical shapes, with a semicircle and straight line appearing in the high- and low-frequency region, respectively. The semicircle appeared in the high-frequency range, which was ascribed to the charge-transfer resistance (Rct) occurring between electrode materials and electrolyte. However, the charge transfer resistance (Rct) presents adecreasing trend along with the cycles. The reduction of Rct indicates the activation and improved kinetics of the electrochemical reaction, and also suggested that the charge-transfer kinetics of the hybrid electrode did not degrade after 100 cycles, demonstrating the structural stability of the Fe3O4/Fe1−xS@C@MoS2 electrode.44,45 All of these results strongly confirmed that the Fe3O4/ Fe1−xS@C@MoS2 composite exhibits outstanding electrochemical performance for LIBs and SIBs. These outstanding properties can be assigned to three factors: (1) The small size of Fe3O4/Fe1−xS nanoparticles can not only offer lots of active sites for Li+ and Na+ but also shorten pathway for Li+/Na+ transport, leading to excellent rate capability and high reversible capacity. (2) The carbon nanosheets can buffer the volume change of the Fe3O4/Fe1−xS nanocrystals during the Li+/Na+ insertion/extraction process, and prevents the aggregation ofFe3O4/Fe1−xS nanocrystals as well as maintaining the structural stability of the electrode.46 (3) On one hand, MoS2 can provide a lot of active sites for hosting Li+/Na+. On the other hand, it can also greatly shorten the diffusion distance of Li+/Na+, which benefit to enhance the rate capability and improved the Coulombic efficiency.47−49

Figure 5. Electrochemical performance of Fe3O4/Fe1−xS@C@MoS2 composite for SIBs: (A) Cycle performance of Fe3O4/Fe1−xS@C@ MoS2 at current density of 0.2 A g−1. (B) Rate capability of Fe3O4/ Fe1−xS@C@MoS2 at various current densities. (C) Long-term cycling performance Fe3O4/Fe1−xS@C@MoS2 at current density of 1.0 A g−1.

C@MoS2 composite delivered a high reversible discharge capacity of 504 mAh g−1 after 100 cycles at 0.2 A g−1 and a Coulombic efficiency exceeding 97% was estimated. Furthermore, Fe3O4/Fe1−xS@C@MoS2 composite displayed excellent rate performance for sodium storage as average reversible capacity of 650, 550, 475, 420 and 355 mAh g−1 at 0.1, 0.2, 0.5, 1.0 and 2.0 A g−1, respectively (Figure 5B). In addition, long cycling performance at 1.0 A g−1 is also evaluated for SIBs, a 4743

DOI: 10.1021/acssuschemeng.7b00119 ACS Sustainable Chem. Eng. 2017, 5, 4739−4745

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CONCLUSIONS In conclusion, MoS2 coat carbon nanosheets encapsulated Fe3O4/Fe1−xS NPs were successful fabricated by a facile method using NaCl as template. Fe 3 O 4 /Fe 1−x S@C@ MoS2composite shows significantly improved electrochemical performance due to the carbon nanosheets maintaining the structural stability and the incorporation of MoS2 with improved lithium/sodium storage as well as shorten the diffusion path of Li+/Na+. Finally, when evaluated as anode materials for LIBs, this Fe3O4/Fe1−xS@C@MoS2 composite exhibits a high reversible capacity of 1142 mAh g−1 after 700 cycles at 1.0 A g−1 and an excellent high-rate capability (640 mAh g−1 at 5.0 A g−1). For SIBs test, a capacity of 402 mAh g−1 after 1000 cycles at 1.0 A g−1 was retained and a capacity of 355 mAh g−1 could be achieved at 2.0 A g−1. The excellent electrochemical performances indicated its potential for LIBs and SIBs applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00119. XRD patterns of Fe3O4@C and Fe3O4/Fe1−xS@C@ MoS2, Raman spectrum of Fe3O4/Fe1−xS@C@MoS2, TG curves of Fe3O4@C and Fe3O4/Fe1−xS@C@MoS2, XPS spectra of Fe3O4@C, EDS mapping of Fe3O4/ Fe 1−xS@C@MoS 2, TEM and HRTEM images of Fe 3 O 4 @C, CV curves of Fe 3 O 4@C and Fe 3 O 4 / Fe1−xS@C@MoS2, discharge/charge profiles of Fe3O4@ C and Fe3O4/Fe1−xS@C@MoS2, cycling and rate performance of Fe3O4@C, CV and charge/discharge curves of Fe3O4/Fe1−xS@C@MoS2 for SIBs, electrochemical performances of various FeS-based materials for lithium or sodium ion batteries, SEM images of Fe3O4/ Fe1−xS@C@MoS2 for LIBs and SIBs after cycling (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected] (C. H. Yang). ORCID

Chenghao Yang: 0000-0002-3214-328X Meilin Liu: 0000-0002-6188-2372 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from Natural Science Foundation of China (51402109), Project of Public Interest Research and Capacity Building of Guangdong Province (2014A010106007), Pearl River S&T NovaProgram of Guangzhou (201506010030), Guangdong Innovative and Entrepreneurial Research Team Program (No. 2014ZT05N200) and Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2016A030306010).

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DOI: 10.1021/acssuschemeng.7b00119 ACS Sustainable Chem. Eng. 2017, 5, 4739−4745

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ACS Sustainable Chemistry & Engineering

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