Accepted Manuscript Three-dimensional S-MoS2@α-Fe2O3 nanoparticles composites as lithium-ion battery anodes for enhanced electrochemical performance Cheng-Yu Wu, Wei-En Chang, Yu-Guang Sun, Jyh-Ming Wu, Jenq-Gong Duh PII: S0254-0584(18)30724-7 DOI: 10.1016/j.matchemphys.2018.08.059 Reference: MAC 20902 To appear in: Materials Chemistry and Physics Received Date: 09 May 2018 Accepted Date: 20 August 2018 Please cite this article as: Cheng-Yu Wu, Wei-En Chang, Yu-Guang Sun, Jyh-Ming Wu, Jenq-Gong Duh, Three-dimensional S-MoS2@α-Fe2O3 nanoparticles composites as lithium-ion battery anodes for enhanced electrochemical performance, Materials Chemistry and Physics (2018), doi: 10.1016/j. matchemphys.2018.08.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT Three-dimensional SMoS2@α-Fe2O3 nanoparticles composites as lithium-ion battery anodes for enhanced electrochemical performance Cheng-Yu Wu, Wei-En Chang, Yu-Guang Sun, Jyh-Ming Wu*, JenqGong Duh* Department of Material Science and Engineering, National Tsing-Hua University, Hsinchu, Taiwan Corresponding author: Tel.: +886-3-571-2686. E-mail address: firstname.lastname@example.org (J.G. Duh), email@example.com (J.M. Wu) ACCEPTED MANUSCRIPT Abstract Two-dimensional (2D) structures have received substantial attention in the energy applications. However, a common problem with 2D layered structures is restacking during the charge–discharge process. In this work, a graphene-like structure, molybdenum disulfide (S-MoS2), was synthesized using the hydrothermal method. Moreover, zero-dimensional (0D) α-Fe2O3 nanoparticles were constructed to act as a spacer in the slurry. Subsequently, the three-dimensional (3D) S-MoS2@Fe2O3 nanoparticles were fabricated by the ultrasonic vibration process to improve their electrochemical properties. On the basis of the 3D architecture, the α-Fe2O3 nanoparticles not only prevent the restacking of MoS2 sheets, but also increase accessibility for electrolyte penetration to provide more active sites and lower the diffusion energy barrier for Li+ ions during the charge–discharge process. In the potential window of 3 V to 0.01 V, high-rate charge–discharge and long cycle tests were performed to investigate the electrochemical performance of all electrodes. The electrochemical performance was enhanced through spatial distribution of MoS2. In summary, these S-MoS2@Fe2O3 nanoparticles possess high potential for application as ACCEPTED MANUSCRIPT anode materials in next-generation lithium-ion batteries. Keywords: 3D structure, MoS2, anode, lithium-ion battery 1. Introduction Lithium-ion batteries (LiBs) are widely used in consumer electronics products, such as portable electronics and electric vehicles, and are a promising candidate for next-generation energy storage. The primary anode material in batteries is natural graphite, but it still has a limitation for high energy storage above ~ 372 mAhg-1, although it is quite stable with chemically inert. To meet the requirements of transportation electrification and renewable energy integration, scientists have focused on developing high-working-voltage and high-capacity LiBs. Alloy-type materials, such as Si[2, 3] and Sn, have high capacity owing to their reaction with Li ions to form a lithiated compound. Nevertheless, a common problem of alloy-type materials is volume expansion during the charge/discharge process, which could aggravate the failure of the battery and trigger human hazards. Two-dimensional (2D) structures have received substantial attention in the field of energy[5-9] and catalysis related field[10-13]. The structures are superior to alloy- ACCEPTED MANUSCRIPT type structures and have a high theoretical capacity, resulting in the 2D materials, which may exhibit a great potential for use as substitutes to alloy-type materials. Recently, molybdenum disulfide (MoS2) has become a common anode material for LiBs[14-20]. It comprises transition metal and sulfide layers that are stacked on each other by a weak van der Waals interaction. However, a drawback of the 2D materials is restacking of neighboring materials during the charge–discharge process, which could result in the electrolyte infiltration to prevent ionic transportation in battery. To solve this problem and to improve the cycling performance of the LiBs , several studies have modified 2D materials using various methods, such as coating[21-23], composites[24-37], doping[38, 39] and structural design[22, 40-47]. Jiang et al. used MoS2–carbon composites to create an interoverlapped superstructure that can efficiently improve Li-ion storage and achieve cycling stability. Similarly, Chen et al. demonstrated the development of high-energy-density, high-capacity LiB anodes. In mitigating the restacking effect and enhancing the electrical conductivity by designing the conductive networks and buffer space to provide an accessible approach for prolonging the cycling lifetime of MoS2 anodes. In this work, the MoS2 was used to combine with α-Fe2O3 nanoparticles to ACCEPTED MANUSCRIPT fabricate the LiB anodes. The feature of α-Fe2O3 acted as a spacer in this system. The aim was to realize the chemical performance of MoS2 anodes by designing a fluent pathway for Li ions. These results indicate that 2D structures improved by 3D structure have high potential in high-performance applications in the future. 2. Experimental 2.1. Synthesis of MoS2 The MoS2 was synthesized using the hydrothermal method [11-13]. First, Na2MoO4．2H2O (98% purity) and CS(NH2)2 (99% purity) were added to a deionized (DI) water solution. Subsequently, 1 mL of HCl was slowly added while stirring. After the stirring, a conductive substrate was placed into a Teflon-lined stainless-steel autoclave and maintained at 220 °C for 24 h. Finally, a black bulk MoS2 powder was obtained by washing the substrate. 2.2 Synthesis of graphene-like MoS2 sheets The lithiated MoS2 was prepared using the hydrothermal process and exfoliated by ultra-sonication process. First, an appropriate amount of LiOH was dissolved in ethylene glycol (EG) under constant stirring to obtain a uniform solution. The bulk MoS2 powder was added to the LiOH solution and vigorously stirred for 1 h. ACCEPTED MANUSCRIPT Subsequently, the mixture was transferred into the autoclave and heated at 220 °C for 24 h. Lithiated MoS2 powder was obtained through centrifugation. The product was washed with acetone to eliminate EG and LiOH and added to DI water under ultrasonication for 30 min. Finally, sheet-like MoS2 (S-MoS2) was obtained. 2.3 Synthesis of α-Fe2O3 nanoparticles Calcination process was employed to prepare α-Fe2O3. First, FeCl3·6H2O (reagent grade, ≥98%) was added to DI water under ultrasonic vibration. Next, a red flocculent precipitate appeared immediately when methyl orange (0.05 M) was added to the solution. After that, the product was collected and washed with distilled water several times and then dried under vacuum. Finally, α-Fe2O3 nanoparticles were fabricated through pyrolysis of the product at 800 °C in a furnace for 5 h in atmosphere. 2.4 Fabrication of S-MoS2@Fe2O3 composite A self-assembly process was employed to synthesize the S-MoS2@Fe2O3 composite through ultrasonication. In brief, α-Fe2O3 nanoparticles were added to 100 mL of DI water and then mixed uniformly in an ultrasonic bath for 1 h. Subsequently, the S-MoS2 was added to the solution under stirring for another hour. The ratios of S- ACCEPTED MANUSCRIPT MoS2 and α-Fe2O3 were controlled at 10 and 1 wt%, respectively. The mixture was then ﬁltered and rinsed with DI water. Finally, the samples were collected and dried in a vacuum oven at 60 °C for 24 h to obtain the S-MoS2@Fe2O3 composite. 2.5 Characterization The surface morphology was observed using field-emission scanning electron microscopy (JEOL 7600F, Japan). The phases were identified through high-resolution transmission electron microscopy (HR-TEM, JEOL Cs Corrected Field Emission TEM, Japan) and X-ray diffractometry (XRD, Rigaku TTRAX III, Japan). The electrochemical properties were characterized by using the coin cell method (type 2032) with various electrodes as working electrodes. The Li foil was used as the counter electrode and reference electrode. The electrolyte used was 1 M LiPF6 in an ethylene carbonate and ethyl-methyl carbonate mixed solution (volume ratio 1:1). All cells were assembled in an Ar-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm) to avoid water contamination. The cells were analyzed using the charge–discharge process (ArbinBT2000) in the potential window of 0.01 –3 V, and the cycling test was performed at 0.2 C in the same potential window for 70 cycles. ACCEPTED MANUSCRIPT 3. Results and Discussion The synthesized MoS2 powders, namely bulk-like MoS2 (B-MoS2), sheet-like MoS2 (S-MoS2) and S-MoS2@Fe2O3, are shown in Fig. 1. The B-MoS2 was shown in Figure 1(a), revealing that the B-MoS2 powder did not exhibit a high surface to volume ratio. In contrast, as illustrated in Figures. 1(b) and 1(c), the lithium ions are intercalated into the laminae of MoS2 to form lithiated MoS2. The product was washed under ultrasonication for 30 min with DI water. Finally, S-MoS2 was obtained. The products comprise well-dispersed nanosheets with abundant active surfaces. The reaction formula is shown as follows: LiMoS2 + H2O → S-MoS2 + LiOH + 1/2H2 (1) The α-Fe2O3 nanoparticles have an average size of approximately 20 to 100 nm, as shown in Fig. 1(d). The XRD pattern in Fig. 1(e) shows a single phase of MoS2, including the polycrystalline Fe2O3 structure. As shown in Fig. 2(a), the HR-TEM image illustrates that the α-Fe2O3 was mixed into the MoS2 to form a heterojunction structures. In Fig. 2(b), the spacing between atoms were 0.32 nm and 0.27 nm which is identified to be the (0 0 2) crystal plane of MoS2 sheets and the (1 0 -1 4) crystal plane of Fe2O3, respectively. In addition, the ACCEPTED MANUSCRIPT different fast Fourier transform (FFT) images in the inset of Fig. 2(b) can effectively check the lattice constant as 0.32 nm and 0.27nm, respectively. In Fig. 2(c), The STEM model can be used to identify each element of the sphere structure that is attributed to Fe and O. Furthermore, Mo and S were uniformly distributed around the sphere structure, implying that S-MoS2 and α-Fe2O3 were uniformly mixed without aggregation The electrochemical performance for various types of MoS2 was investigated. As shown in Fig. 3(a), the bulk-like MoS2 anode showed poor cycling stability that dropped markedly within 20 cycles. By contrast, a flower-like S-MoS2 sample exhibited good stability in capacity. After 20 cycles, the remaining capacity was approximately 900 mAhg-1, which can be attributed to the high surface area of the flower-like MoS2. The petal structures mitigate restacking of the material during the charge and discharge processes to prevent capacity fading. Nevertheless, capacity fading became more severe after 20 cycles. Among all the samples, S-MoS2@α-Fe2O3 exhibited the greatest stability in capacity. Adding α-Fe2O3 nanoparticles enhances stability because of the crucial role played by MoS2 anodes in capacity retention. The capacity was 600 mAhg-1 after 70 cycles, which is higher than that of B-MoS2 and S-MoS2, and the fading rate ACCEPTED MANUSCRIPT was lower. Moreover, the coulombic efficiency of S-MoS2@α-Fe2O3 was much higher than that of the other samples. A schematic of the potential mechanism underlying stability improvement is shown in Fig. 3(b). In fact, Bulk-like MoS2 exhibited a lower surface area than the other samples. In fact, its surface area was not sufficiently high for the MoS2 to react with Li+ ions to achieve high capacity. After exfoliation, S-MoS2 exhibited a much higher capacity than bulk-like MoS2 because of their high surface to volume ratio. This resulted in a short migration path of Li+ into the host and provided more electrolyte to infiltrate the electrode. However, this type of 2D structure suffers from the restacking effect. Therefore, α-Fe2O3 was used to create a 3D architecture, including 0D spacer which not only prevented restacking of the MoS2 sheets but also increased accessibility for electrolyte infiltration to provide more active sites and lower the diffusion energy barrier of Li+ ions during the charge–discharge process. As a result, the 3D structure of S-MoS2@Fe2O3 exhibits high stability. Fig. 4(a) shows the cyclic voltammetry curves to clarify the electrochemical behavior of S-MoS2@Fe2O3. In the first cycle, the negative scan began at 1.5 V, which can be attributed to Li-ion insertion into α-Fe2O3 to form LixFe2O3. The peaks at 1.1 V and 0.6 V correspond to MoS2 transfer to LixMoS2 (1.1 V vs Li/Li+) and solid ACCEPTED MANUSCRIPT electrolyte interface formation (0.6 V), respectively. The last peak around 0.3 V indicates LixMoS2 transfer to Mo, which was shifted to a lower voltage after the first cycle. By contrast, the positive scan exhibited two peaks at 1.6 V and 2.3 V, corresponding to the conversion of Mo to MoS2 and Li2S to S, respectively. Subsequently, a new peak at 1.9 V indicates S transfer to Li2S. Fig. 4(b) depicts the voltage profile of S-MoS2@α-Fe2O3, which is in good agreement with the result of cyclic voltammetry test. In order to confirm the capacity of Fe2O3, the Fe2O3 was assembled as the coin cell to test the electrochemical performance. The data is shown in Fig. 4(c). In first cycle, the long plateau that appeared near 0.8 V is attributed to the conversion from Fe(II) to Fe (0) and the decomposition of electrolyte. A small plateau from 1.5 to 2.2 V shown in the charge curve is corresponding to the reverse reaction. To compare the reaction plateau between MoS2 and Fe2O3, the reaction plateau of Fe2O3 is observed below 2.2V. Thus, In Fig. 4b, the contributed capacity of Fe2O3 in the system only provide a little. The main capacity comes from MoS2 itself. This result proves that the capacity of MoS2 slowly decreased by cooperation with α-Fe2O3. To determine the stability of MoS2, the discharge capacities of all samples cycled at various current rates (0.2 C to 5 C) as shown in Fig. 5(a). S-MoS2@Fe2O3 exhibits ACCEPTED MANUSCRIPT more stability in capacity than the other samples because of the smooth aisle of Li+ ions during the charge–discharge process. The capacity of S-MoS2@ Fe2O3 was approximately 500 mAhg-1, which is higher than that of bulk-like MoS2 at a high current rate. The results indicate that an appropriate amount of α-Fe2O3 added to MoS2 can improve the capacity at various current rates because of the reduction of both the Li+ diffusion pathway and electrochemical resistance. The electrochemical impedance spectroscopy was further employed to investigate this result. All the cells were analyzed, and the Nyquist plots are shown in Fig. 5(b). The Nyquist plots can be analyzed by fitting the spectra using the equivalent circuit inset in Fig. 5(b). In the figure, Rs denotes the resistance of the electrolyte, Rct is the charge transfer resistance of the electrode or electrolyte, CPE is the interface capacitance of the electrode or electrolyte, and Wo1 the Warburg impedance. As the semicircular portion of the Nyquist plot shows, the charge transfer resistance of the S-MoS2@Fe2O3 electrode was slightly lower than that of bulklike MoS2. This indicates that the nanoparticle improves MoS2 stability during cycling and thus verifies the improved electrochemical performance of MoS2. This result also confirms that the special design is more beneficial to transportation of charge. As a consequence, this 3D architecture design not only prevents the restacking effect of ACCEPTED MANUSCRIPT MoS2 sheets, but also enhances approachability of electrolyte to penetrate, providing lower diffusion energy barrier for Li+ ions during the charge–discharge process. 4. Conclusions In this study, MoS2 was enhanced through a simple method. The exfoliation processes can effectively enhance the capacity of MoS2. Nevertheless, the restacking phenomenon inevitably affects 2D material, which can aggravate the fading of capacity after a few cycles. Therefore, α-Fe2O3 was used as a spacer to prevent the restacking of 2D material during the cycling process. S-MoS2@Fe2O3 exhibited an initial reversible capacity of approximately 1000 mAhg-1 and a high capacity of approximately 600 mAhg-1 after 70 charge–discharge cycles with a low decay. In addition, it also exhibited excellent capacity in a rate test. The improved anode performance can be attributed to the accessible active sites of 2D structures obtained via the special design and the reduction of the ionic transportation pathway. These results indicate that 2D structures have high potential in high-performance applications in the future. ACCEPTED MANUSCRIPT Fig1. SEM images of various condition (a) B-MoS2, (b) S-MoS2, (c) S-MoS2@Fe2O3 (d) Fe2O3 nano-particles and (e) XRD analysis Fig2. (a,b) HR-TEM images with FFT imges (c)EDS images with SEM model. ACCEPTED MANUSCRIPT Fig3. (a) Reversible capacity and Coulombic efficiency for the 70 cycles are plotted with various electrodes at rate 0.2 C. (b) . The schematic diagram of experimental process and mechanism. ACCEPTED MANUSCRIPT Fig4. (a) The first three cycles of cyclic voltammogram for S-MoS2@Fe2O3 from 3V to 0.01 V versus Li/Li+ at 0.2 mV s-1 scan rate. (b) The voltage profile of SMoS2@Fe2O3 with different cycles and (c)The voltage profile of Fe2O3 with different cycles. . ACCEPTED MANUSCRIPT Fig 5. (a) Capacities of samples MoS2, S-MoS2 and S-MoS2@Fe2O3 subject to charge– discharge under different charging rates. (b) The EIS impedance of MoS2, S-MoS2 and S-MoS2@Fe2O3 after first three cycles. The frequency is ranged from 0.1 MHz to 0.1 Hz. ACCEPTED MANUSCRIPT References  B. Scrosati, J. Garche, Lithium batteries: Status, prospects and future, Journal of Power Sources, 195 (2010) 2419-2430.  C.-Y. Wu, C.-C. Chang, J.-G. 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