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(002)-oriented WS2 with high crystalline with enhanced capacity as anode material for sodium ion batteries
Accepted Manuscript (002)-oriented WS2 with high crystalline with enhanced capacity as anode material for sodium ion batteries Xin Wang, Jianfeng Huang, Jiayin Li, Liyun Cao, Wei Hao, Zhanwei Xu, Qian Kang PII:
S0925-8388(16)33563-0
DOI:
10.1016/j.jallcom.2016.11.095
Reference:
JALCOM 39591
To appear in:
Journal of Alloys and Compounds
Received Date: 8 August 2016 Revised Date:
6 November 2016
Accepted Date: 8 November 2016
Please cite this article as: X. Wang, J. Huang, J. Li, L. Cao, W. Hao, Z. Xu, Q. Kang, (002)-oriented WS2 with high crystalline with enhanced capacity as anode material for sodium ion batteries, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.11.095. 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.
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Graphical abstract:
(002)-oriented WS2 nanosheets with high crystalline have large interlayer
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spacing and high crystallinity. This crystal nanostructure is beneficial for improving capacity and stable capacity retention. The good electrochemical performance is attributed to (002)-oriented growth WS2 with large interlayer spacing and stable
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structure, accelerating Na+ insertion/extraction.
ACCEPTED MANUSCRIPT (002)-oriented WS2 with high crystalline with enhanced capacity as anode material for sodium ion batteries Xin Wang a, Jianfeng Huang a,*, Jiayin Li a,*, Liyun Cao a, Wei Hao b, Zhanwei Xu a,
School of Material Science and Engineering, Shaanxi University of Science and
Technology, Xi’an, Shaanxi 710021, P. R. China b
School of Materials Science and Engineering, Shanghai Jiao Tong University, 800
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Dongchuan Road, Shanghai 200240, P. R. China
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a
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Qian Kang a
Abstract: Different crystal nanostructural WS2 sheets play a significant role in the improvement of their electrochemical performances as anode for sodium ion batteries (SIBs). Here we investigate that WS2 nanosheets with diverse crystal planes are
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fabricated by a simple two-step method. Especially, (002)-oriented WS2 nanosheets with broad interlayer spacing are achieved by a facile controlling the sulfurized time (1 h, 2 h and 3 h). This (002)-oriented crystal structure for WS2 electrode can deliver
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the better cyclic stability (85% capacity retention after 100 cycles at 100 mA g-1) and
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the higher rate performance (capacity of 148 mA h g-1 at the current density of 2000 mA g-1) than other (100)-oriented WS2 nanosheets. Further investigations indicate that (002)-oriented WS2 nanosheets with high crystallinity possess stable structure, which can provide effective channels for accommodating more Na+ and accelerate the
*
Corresponding author: Tel./fax: +86 29 86168802
E-mail addresses: [email protected] (Jianfeng Huang). *
Corresponding author: Tel./fax: +86 29 86168802
E-mail addresses: [email protected] (Jiayin Li). 1
ACCEPTED MANUSCRIPT ionic/electron transportation. This method reveals that the structural design and fabrication of different crystal orientation can be considered as potential ways to improving the electrochemical performances of WS2 anode, as well as that of other
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anodes in SIBs. Keywords: Tungsten disulfide; Crystal plane; Insertion/extraction; Electrochemical performance; Sodium ion batteries.
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1. Introduction
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In the past decade, research on graphene has triggered explosive interests in two-dimensional (2D) materials and van der Waals solids, such as hexagonal boron nitride and transition metal dichalcogenides (TMDs) [1, 2]. Particularly, TMDs were widely applied in the electronic industry and energy storage because of their excellent
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electronic properties, ranging from semiconducting to superconducting. Among the TMDs, MoS2 and WS2 have attracted more attention due to their special crystal structure [3-5].
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WS2 with S-W-S layers are stacked together to form 2D structure through weak
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van der Waals interaction. Recently, layered WS2 materials have shown wide application prospects for sodium ion batteries (SIBs) [6-8]. However, WS2 materials suffer from poor cycling performance for Na+ storage, which results from the occurrence of large volume expansions during Na+ insertion/desertion [9]. To overcome this issue, most researchers have reported that graphene-based materials or carbon nanotubes (CNTs) were incorporated into WS2 to improve Na+ storage performances [10]. In addition, these methods are considered as effective ways to 2
ACCEPTED MANUSCRIPT address the above issue. Nevertheless, these processes are complicated and the carbon materials used in the composites are expensive and difficult to obtain [6, 7, 11]. So far, optimizing the electrochemical performances of this material can be achieved by
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tailoring the different crystal nanostructures, which is still an urgent task [12-14]. Herein, different crystal structural WS2 nanosheets were fabricated and applied as high capacity retention anode in SIBs. The (002) d-spacing WS2 nanosheets with
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high crystallinity possess good electrochemical performances due to their fast
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ionic/electron transportation. The crystal structures of WS2 nanosheets were controlled by tailoring sulfurized time, and the emphasis is on the relationship between exposed crystal plane of WS2 nanosheets and electrochemical performances as anode for SIBs. It can offer a promising guidance for (002)-oriented crystal plane
SIBs platforms. 2. Experimental
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WS2 nanosheets with high crystallinity as a good candidate anode for next-generation
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2.1 Synthesis of WS2 nanosheets
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WO3 were prepared by a precipitation reaction. Firstly, 1.5 g W powders (A.R.) were slowly dissolved into 30 mL H2O2 (30 wt% in purity). The temperature of the solution was kept at 25~40 °C, because the reaction is exothermic. After 1 h reaction, the metatungstic acid suspension was formed [15]. Secondly, 5 mL isopropyl alcohol (IPA) was added into the above suspension with magnetic stirring to produce the precursor solution. The precursor solution was dried at 60 °C to obtain the WO3 precursor. Then, the above WO3 precursor were grinded with thioacetamide (the molar 3
ACCEPTED MANUSCRIPT ratio of 1 : 20) and loaded into a large alumina boat with a cover. This boat was heated in a tube furnace at 800 °C for 1 h, 2 h and 3 h, respectively. The WS2 nanosheets were obtained finally. The above three samples are marked as 1 h, 2 h and 3 h sample
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or electrode in the paper, respectively. 2.2 Characterizations
The crystal structures of the as-prepared powders were characterized by a powder
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X-ray diffraction (XRD, Rigaku D/max-2000) with Co Kα radiation (k= 0.1789 nm).
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The morphologies of the as-prepared samples were observed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscope (TEM, FEI Tecnai G2 F20). The X-ray photoelectron spectroscopy (XPS, PHI-5400) spectrum was performed on a Surface Science Instruments Spectrometer
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focused monochromatic Al Kα radiation 1486.6 eV.
2.3 Electrode preparation and electrochemical measurements The capacities and cycling performances of the as-prepared WS2 samples were
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determined with 2032-type coin cells. The electrodes were prepared from a mixture
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containing 70 wt% active material, 20 wt% Super P and 10 wt% sodium carboxymethyl cellulose (Na-CMC) binder. Sodium metal and glass fiber were used as a counter electrode and separator, respectively. The electrolyte was a solution of 1 M NaPF6 (Aldrich) in a 1:1 volume mixture of ethylene carbonate/dimethyl carbonate (EC/DMC). Moreover, the charge-discharge characteristics of the as-prepared samples were determined by cycling in the potential range 0.0-3.0 V at fixed current densities. Cyclic voltammetry (CV) profiles were performed on an electrochemical workstation 4
ACCEPTED MANUSCRIPT with a three-electrode system (CHI660E, Shanghai Chenhua). CV curves were recorded between 0.005 and 3.0 V at a scanning rate of 0.10 mV s-1. Electrochemical impedance was measured with electrochemical impedance spectroscopy (EIS) over a
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frequency range of 100 kHz-0.01 Hz. All electrochemical tests were conducted at room temperature.
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3. Results and discussion
Fig. 1 XRD patterns of WS2 samples synthesized with different sulfurized time. Fig. 1 shows the XRD patterns of the as-synthesized products with different
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sulfurized time by a typical diffraction pattern of hexagonal WS2 (PDF NO. 08-0237).
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There are no peaks of impurities, such as the existence of WO3. In the as-synthesized products, the crystal phase is pure WS2 crystallite. Noticeably, the relative intensity of (002) diffraction peak is higher than (100) peak in the 2 h reaction samples. Based on the Scherrer’s equation ( D =
kλ ), the average grain size of the three samples can β cos θ
be reasonably estimated to be 48 nm, 28 nm and 50 nm, respectively. Interestingly, the crystallinities of the samples synthesized with different sulfurized time are also different. The intensity of the diffraction peaks of 2 h samples is the highest among 5
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three WS2 samples, indicating that more crystalline materials exist in this sample [16].
Fig. 2 SEM and TEM images of WS2 samples synthesized at 800 °C with different
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sulfurized time: (a-c) 1 h, (d-f) 2 h, (g-i) 3 h. To investigate the growth mechanism of WS2 nanosheets, time-dependent
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experiments have been performed and the as-received products at different growth stages were examined by SEM and TEM observations. As shown in Fig. 2a-2c, the
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sample obtained at an early stage (1 hour) consists of many nanoparticles, which are assembled with disordered and large nanosheets (~200 nm). When the reaction time was prolonged to 2 hours, these WS2 nanosheets started to display good dispersibility and exhibit (002)-oriented growth (Fig. 2d-2f). With the reaction time passing (3 hours), small nanoparticles (~100 nm) are agglomerated together to form thick and large nanosheet (~200 nm). It is clear that the amount of highly crystalline WS2 increases quickly when sulfurized time is 2 h. The high resolution TEM (HRTEM) 6
ACCEPTED MANUSCRIPT images show that the as-prepared WS2 samples are well crystallized and the lattice fringes are distinct. Both of the samples sulfurized with 1 h and 3 h exhibit (100)-oriented growth. At the same time, (002)-oriented crystal plane is examined in
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the 2 h sample.
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Fig. 3 XPS spectra of the WS2 sample synthesized with 2 h: broad scan spectra (a), W 4f (b), S 2p (c).
To confirm the presence of W, S and C elements in the 2 h sample, the broad XPS spectra are displayed in Fig. 3a. In addition, the high resolution XPS spectra of
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W 4f and W 5p are shown in Fig. 3b. The W/S atomic ratio is about 1 : 2 for all the three samples. The peaks located at 32.9, 35.1 and 38.0 eV are assigned to W 4f7/2, W
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4f5/2 and W 5p3/2, respectively. These peaks demonstrate the oxidation state of W4+. In terms of Fig. 3c, two S 2p peaks of S 2p5/2 and S 2p3/2 at 162.4 and 163.8 eV are
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characteristic of S2- in WS2 [17-19]. All the peak locations in the XPS spectra are well consistent with those of WS2 in the literature reports [20-22].
Fig. 4 CVs of the as-synthesized electrodes performed over a potential window from 0 7
ACCEPTED MANUSCRIPT to 3 V at a scan rate of 0.1 mV s-1: (a) 1 h, (b) 2 h, (c) 3 h. The cyclic voltammograms (CVs) show the positions of redox peaks for the reversible redox reactions. According to Fig. 4, the redox peak positions of three
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samples are almost same, which indicates that the same redox reactions occur in all three samples. Notably, the peak locations of initial three cycles are well overlapped in the 2 h sample (Fig. 4b). In the first reduction cycle, the small peak around 0.5 V is
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related to the formation of a solid electrolyte interphase (SEI). While the broad
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oxidation peaks located at about 1.9 V can be ascribed to the extraction of Na ions [23]. In the following cycles, the position of peaks almost overlap. From Fig. 4a and Fig. 4c (the 1 h and 3 h electrodes), the peaks located at ~0.4 V correspond to the formation of SEI in the first reduction reaction, while the oxidation peaks around 2 V
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can be ascribed to extraction of Na ions and back conversion reaction ( W+2Na 2S → WS2 +4Na ) in the WS2 nanosheets [24]. However, the redox peaks become broad and shift a little in the following cycles. These phenomena reveal that
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the polarization has been avoided and the kinetics of ionic/electronic transportation is
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remarkably enhanced in the 2 h electrode [25].
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5
Electrochemical
performance
of
the
as-prepared
electrodes:
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charge/discharge cycling tests of the three electrodes at 100 mA g-1 (a); rate performances at different applied current densities from 100 to 2000 mA g-1 (b); 8
ACCEPTED MANUSCRIPT charge/discharge curves of the three WS2 electrodes at 1st cycle (c). The cycling stability of the 1 h, 2 h and 3 h WS2 electrodes at 100 mA g-1 are investigated, as presented in Fig. 5a. By comparison, both 1 h and 2 h electrodes
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possess good cycling performances. The capacity of 2 h electrode is considerably higher than 1 h electrode and its capacity retention is 85% after 100 cycles. However, capacity fading happened in the cycling tests of the 3 h electrode, which may be due
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to the long sulfurized time resulting in instability volume excursions. For the sake of
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investigating their rate performances (Fig. 5b), different current densities from 100 to 2000 mA g-1 are employed on the samples. According to 3 h WS2 electrode, the capacity of 71 mA h g-1 is achieved at 2000 mA g-1. Whereas the improved rate capacity of 148 mA h g-1 is obtained at 2000 mA g-1 in the 2 h electrode. However,
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further decreasing the sulfurized time to 1 h leads to a rapidly decayed capacity to <10 mA h g-1 at 2000 mA g-1 because of the small interlayer spacing of the exposed (100) crystal plane detected by HRTEM observation, which may not provide favorable
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channel for Na+ to insertion/extraction process. The charge/discharge curves of the
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three electrodes at 1st cycle are shown in Fig. 5c. The specific capacity of the 2 h electrode was higher than the capacities of 1 h and 3 h electrodes. The Na+ storage capacities in WS2 can be ascribed to the following mechanisms: intercalation between WS2 layers and reversible conversion reaction in the WS2. It can be concluded that the sloping above ~0.5 V voltage discharge storage behavior is attributed to the reversible conversion reaction in the WS2. Additionally, the nearly flat plateau at potentials below ~0.5 V discharge storage mechanism is the binding of intercalation between 9
ACCEPTED MANUSCRIPT WS2 layers. The insertion capacity plays a dominant role in the whole discharge capacity and the insertion reaction is the main electrochemical behavior in the cycling
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process.
Fig. 6 EIS spectra of 1 h (a), 2 h (b) and 3 h (c) electrodes after 3rd and 100th cycle,
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Equivalent circuit for electrodes (d).
The charge transfer resistance of the three electrodes after 3rd and 100th cycle derived from the EIS measurements is shown in Fig. 6. It can be revealed that the
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WS2 electrode prepared with 2 h sulfurized time can effectively reduce charge transfer
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resistance from 3rd to 100th cycle (3743 to 493 Ω). On the contrary, the charge transfer resistance of the 1 h electrode increased when compared with those of 3rd and 100th cycle. Obviously, the conductivity of 2 h electrode is increased favorably, which is consistent with the results discussed in Fig. 4. Consequently, the optimal sulfurized time (2 hours) for the preparation of WS2 electrode is obtained, indicating that the favorable crystal nanostructures can be fabricated by controlling the sulfurized time to promote good conductivity and crystallinity, even achieving 10
ACCEPTED MANUSCRIPT (002)-oriented growth crystal structure for Na+ migration accessibly. In a word, the
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as-fabricated 2 h electrode possesses improved rate performance and cycling stability.
Fig. 7 Ex-situ XRD patterns of 1 h, 2 h and 3 h electrodes after 100 cycles.
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To elucidate the effects of (002) and (100)-oriented crystal structures on Na+ insertion/extraction, ex-situ XRD patterns of 1 h, 2 h and 3 h electrodes after 100 cycles are shown in Fig. 7. (002) diffraction peaks shift to large angles and (100) diffraction peaks vanish completely. The shift to high angles of the reflection indicates
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lattice shrinkage along the (002) oritentation, which may be caused by the insertion of Na ions. The disappearance of (100) diffraction peaks demonstrates that the WS2
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phase becomes amorphous after 100 cycles [16, 26]. There is no obvious difference in these ex-situ XRD patterns. The capacities of 1 h and 3 h WS2 electrodes are low and
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fading fast (Fig. 5a) because of the amorphization during cycling. This phenomenon reveals that the high crystallinity and large (002) interlayer spacing of the 2 h electrode can keep its stable structure and provide effective channels for accommodating more Na+.
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Fig. 8 Schematic illustration for the effect of different exposed crystal plane on Na+
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insertion/extraction during charge/discharge process.
In order to further clarify Na+ insertion/extraction in different exposed crystal plane of WS2 electrodes, Fig. 8 illustrates the effect of different crystal structures on Na+ insertion/extraction during charge/discharge process. The WS2 electrode with
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(002)-oriented growth exhibits large interlayer spacing, offering sufficient space for more Na+ insertion. Simultaneously, this large interlayer spacing can provide effective
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channels for Na+ insertion/extraction without destroying the original structure. This is the main reason for (002)-oriented 2 h WS2 electrode with high and stable capacity.
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However, the 0.27 nm interlayer spacing of (100)-oriented crystal structure of WS2 electrode is too small to insert Na+ during the discharge process. With the Na+ insertion/extraction process, the narrow channel may lead to the destruction of the original structure. Thus, the capacities of 1 h and 3 h electrodes are low and fading fast. It agrees well with XRD, TEM analyses and discussions for the electrochemical performances. Finally, the (002)-oriented WS2 electrode can maintain its stable structure and high capacity. 12
ACCEPTED MANUSCRIPT 4. Conclusions In summary, different crystal nanostructures and crystallinities of WS2 are successfully fabricated by a two-step method consisting of precipitation reaction and
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sulfurization process, which can be achieved by tailoring sulfurized time. The investigation of electrochemical performances reveals that 2 h WS2 electrode possesses much better electrochemical performance than the 1 h and 3 h WS2
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electrodes. Furthermore, the 2 h WS2 electrode has high capacity retention 85% after
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100 cycles at the current density of 100 mA g-1 as well as a superior capacity of 148 mA h g-1 at 2000 mA g-1. This better electrochemical performance is attributed to the broad (002)-oriented crystal interlayer spacing and high crystallinity of 2 h WS2 electrode, which can facilitate faster Na ions insertion/extraction behavior.
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Consequently, high crystallinity and oriented growth TMDs materials with large interlayer spacing can provide fast kinetics for Na+ insertion/ extraction as anode in SIBs.
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Acknowledgements
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This work has been supported by the 973 Special Preliminary Study Plan (2014CB260411), National Natural Science foundation of China (No. 51672165, No. 51472152), Innovation Team Assistance Foundation of Shaanxi University of Science & Technology (TD12-05), Fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP201636), China Postdoctoral Science Foundation (2016M592897XB) and Graduate Innovation Foundation of Shaanxi University of Science and Technology. The authors are also grateful to Dr. Hao from Shanghai Jiao 13
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ACCEPTED MANUSCRIPT Highlights: 1. Different crystal nanostructures WS2 are fabricated by controlling sulfurized time. 2. (002)-oriented WS2 with high crystalline has enhanced capacity and stability as anode in SIBs.
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3. (002)-oriented WS2 with large interlayer spacing are beneficial for Na+ insertion/extraction.
4. High crystallinity WS2 with (002) oritentation growth can maintain original stable
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structure.
S0925-8388(16)33563-0
DOI:
10.1016/j.jallcom.2016.11.095
Reference:
JALCOM 39591
To appear in:
Journal of Alloys and Compounds
Received Date: 8 August 2016 Revised Date:
6 November 2016
Accepted Date: 8 November 2016
Please cite this article as: X. Wang, J. Huang, J. Li, L. Cao, W. Hao, Z. Xu, Q. Kang, (002)-oriented WS2 with high crystalline with enhanced capacity as anode material for sodium ion batteries, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.11.095. 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.
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Graphical abstract:
(002)-oriented WS2 nanosheets with high crystalline have large interlayer
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spacing and high crystallinity. This crystal nanostructure is beneficial for improving capacity and stable capacity retention. The good electrochemical performance is attributed to (002)-oriented growth WS2 with large interlayer spacing and stable
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structure, accelerating Na+ insertion/extraction.
ACCEPTED MANUSCRIPT (002)-oriented WS2 with high crystalline with enhanced capacity as anode material for sodium ion batteries Xin Wang a, Jianfeng Huang a,*, Jiayin Li a,*, Liyun Cao a, Wei Hao b, Zhanwei Xu a,
School of Material Science and Engineering, Shaanxi University of Science and
Technology, Xi’an, Shaanxi 710021, P. R. China b
School of Materials Science and Engineering, Shanghai Jiao Tong University, 800
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Dongchuan Road, Shanghai 200240, P. R. China
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a
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Qian Kang a
Abstract: Different crystal nanostructural WS2 sheets play a significant role in the improvement of their electrochemical performances as anode for sodium ion batteries (SIBs). Here we investigate that WS2 nanosheets with diverse crystal planes are
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fabricated by a simple two-step method. Especially, (002)-oriented WS2 nanosheets with broad interlayer spacing are achieved by a facile controlling the sulfurized time (1 h, 2 h and 3 h). This (002)-oriented crystal structure for WS2 electrode can deliver
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the better cyclic stability (85% capacity retention after 100 cycles at 100 mA g-1) and
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the higher rate performance (capacity of 148 mA h g-1 at the current density of 2000 mA g-1) than other (100)-oriented WS2 nanosheets. Further investigations indicate that (002)-oriented WS2 nanosheets with high crystallinity possess stable structure, which can provide effective channels for accommodating more Na+ and accelerate the
*
Corresponding author: Tel./fax: +86 29 86168802
E-mail addresses: [email protected] (Jianfeng Huang). *
Corresponding author: Tel./fax: +86 29 86168802
E-mail addresses: [email protected] (Jiayin Li). 1
ACCEPTED MANUSCRIPT ionic/electron transportation. This method reveals that the structural design and fabrication of different crystal orientation can be considered as potential ways to improving the electrochemical performances of WS2 anode, as well as that of other
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anodes in SIBs. Keywords: Tungsten disulfide; Crystal plane; Insertion/extraction; Electrochemical performance; Sodium ion batteries.
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1. Introduction
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In the past decade, research on graphene has triggered explosive interests in two-dimensional (2D) materials and van der Waals solids, such as hexagonal boron nitride and transition metal dichalcogenides (TMDs) [1, 2]. Particularly, TMDs were widely applied in the electronic industry and energy storage because of their excellent
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electronic properties, ranging from semiconducting to superconducting. Among the TMDs, MoS2 and WS2 have attracted more attention due to their special crystal structure [3-5].
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WS2 with S-W-S layers are stacked together to form 2D structure through weak
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van der Waals interaction. Recently, layered WS2 materials have shown wide application prospects for sodium ion batteries (SIBs) [6-8]. However, WS2 materials suffer from poor cycling performance for Na+ storage, which results from the occurrence of large volume expansions during Na+ insertion/desertion [9]. To overcome this issue, most researchers have reported that graphene-based materials or carbon nanotubes (CNTs) were incorporated into WS2 to improve Na+ storage performances [10]. In addition, these methods are considered as effective ways to 2
ACCEPTED MANUSCRIPT address the above issue. Nevertheless, these processes are complicated and the carbon materials used in the composites are expensive and difficult to obtain [6, 7, 11]. So far, optimizing the electrochemical performances of this material can be achieved by
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tailoring the different crystal nanostructures, which is still an urgent task [12-14]. Herein, different crystal structural WS2 nanosheets were fabricated and applied as high capacity retention anode in SIBs. The (002) d-spacing WS2 nanosheets with
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high crystallinity possess good electrochemical performances due to their fast
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ionic/electron transportation. The crystal structures of WS2 nanosheets were controlled by tailoring sulfurized time, and the emphasis is on the relationship between exposed crystal plane of WS2 nanosheets and electrochemical performances as anode for SIBs. It can offer a promising guidance for (002)-oriented crystal plane
SIBs platforms. 2. Experimental
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WS2 nanosheets with high crystallinity as a good candidate anode for next-generation
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2.1 Synthesis of WS2 nanosheets
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WO3 were prepared by a precipitation reaction. Firstly, 1.5 g W powders (A.R.) were slowly dissolved into 30 mL H2O2 (30 wt% in purity). The temperature of the solution was kept at 25~40 °C, because the reaction is exothermic. After 1 h reaction, the metatungstic acid suspension was formed [15]. Secondly, 5 mL isopropyl alcohol (IPA) was added into the above suspension with magnetic stirring to produce the precursor solution. The precursor solution was dried at 60 °C to obtain the WO3 precursor. Then, the above WO3 precursor were grinded with thioacetamide (the molar 3
ACCEPTED MANUSCRIPT ratio of 1 : 20) and loaded into a large alumina boat with a cover. This boat was heated in a tube furnace at 800 °C for 1 h, 2 h and 3 h, respectively. The WS2 nanosheets were obtained finally. The above three samples are marked as 1 h, 2 h and 3 h sample
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or electrode in the paper, respectively. 2.2 Characterizations
The crystal structures of the as-prepared powders were characterized by a powder
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X-ray diffraction (XRD, Rigaku D/max-2000) with Co Kα radiation (k= 0.1789 nm).
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The morphologies of the as-prepared samples were observed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscope (TEM, FEI Tecnai G2 F20). The X-ray photoelectron spectroscopy (XPS, PHI-5400) spectrum was performed on a Surface Science Instruments Spectrometer
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focused monochromatic Al Kα radiation 1486.6 eV.
2.3 Electrode preparation and electrochemical measurements The capacities and cycling performances of the as-prepared WS2 samples were
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determined with 2032-type coin cells. The electrodes were prepared from a mixture
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containing 70 wt% active material, 20 wt% Super P and 10 wt% sodium carboxymethyl cellulose (Na-CMC) binder. Sodium metal and glass fiber were used as a counter electrode and separator, respectively. The electrolyte was a solution of 1 M NaPF6 (Aldrich) in a 1:1 volume mixture of ethylene carbonate/dimethyl carbonate (EC/DMC). Moreover, the charge-discharge characteristics of the as-prepared samples were determined by cycling in the potential range 0.0-3.0 V at fixed current densities. Cyclic voltammetry (CV) profiles were performed on an electrochemical workstation 4
ACCEPTED MANUSCRIPT with a three-electrode system (CHI660E, Shanghai Chenhua). CV curves were recorded between 0.005 and 3.0 V at a scanning rate of 0.10 mV s-1. Electrochemical impedance was measured with electrochemical impedance spectroscopy (EIS) over a
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frequency range of 100 kHz-0.01 Hz. All electrochemical tests were conducted at room temperature.
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3. Results and discussion
Fig. 1 XRD patterns of WS2 samples synthesized with different sulfurized time. Fig. 1 shows the XRD patterns of the as-synthesized products with different
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sulfurized time by a typical diffraction pattern of hexagonal WS2 (PDF NO. 08-0237).
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There are no peaks of impurities, such as the existence of WO3. In the as-synthesized products, the crystal phase is pure WS2 crystallite. Noticeably, the relative intensity of (002) diffraction peak is higher than (100) peak in the 2 h reaction samples. Based on the Scherrer’s equation ( D =
kλ ), the average grain size of the three samples can β cos θ
be reasonably estimated to be 48 nm, 28 nm and 50 nm, respectively. Interestingly, the crystallinities of the samples synthesized with different sulfurized time are also different. The intensity of the diffraction peaks of 2 h samples is the highest among 5
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three WS2 samples, indicating that more crystalline materials exist in this sample [16].
Fig. 2 SEM and TEM images of WS2 samples synthesized at 800 °C with different
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sulfurized time: (a-c) 1 h, (d-f) 2 h, (g-i) 3 h. To investigate the growth mechanism of WS2 nanosheets, time-dependent
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experiments have been performed and the as-received products at different growth stages were examined by SEM and TEM observations. As shown in Fig. 2a-2c, the
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sample obtained at an early stage (1 hour) consists of many nanoparticles, which are assembled with disordered and large nanosheets (~200 nm). When the reaction time was prolonged to 2 hours, these WS2 nanosheets started to display good dispersibility and exhibit (002)-oriented growth (Fig. 2d-2f). With the reaction time passing (3 hours), small nanoparticles (~100 nm) are agglomerated together to form thick and large nanosheet (~200 nm). It is clear that the amount of highly crystalline WS2 increases quickly when sulfurized time is 2 h. The high resolution TEM (HRTEM) 6
ACCEPTED MANUSCRIPT images show that the as-prepared WS2 samples are well crystallized and the lattice fringes are distinct. Both of the samples sulfurized with 1 h and 3 h exhibit (100)-oriented growth. At the same time, (002)-oriented crystal plane is examined in
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the 2 h sample.
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Fig. 3 XPS spectra of the WS2 sample synthesized with 2 h: broad scan spectra (a), W 4f (b), S 2p (c).
To confirm the presence of W, S and C elements in the 2 h sample, the broad XPS spectra are displayed in Fig. 3a. In addition, the high resolution XPS spectra of
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W 4f and W 5p are shown in Fig. 3b. The W/S atomic ratio is about 1 : 2 for all the three samples. The peaks located at 32.9, 35.1 and 38.0 eV are assigned to W 4f7/2, W
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4f5/2 and W 5p3/2, respectively. These peaks demonstrate the oxidation state of W4+. In terms of Fig. 3c, two S 2p peaks of S 2p5/2 and S 2p3/2 at 162.4 and 163.8 eV are
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characteristic of S2- in WS2 [17-19]. All the peak locations in the XPS spectra are well consistent with those of WS2 in the literature reports [20-22].
Fig. 4 CVs of the as-synthesized electrodes performed over a potential window from 0 7
ACCEPTED MANUSCRIPT to 3 V at a scan rate of 0.1 mV s-1: (a) 1 h, (b) 2 h, (c) 3 h. The cyclic voltammograms (CVs) show the positions of redox peaks for the reversible redox reactions. According to Fig. 4, the redox peak positions of three
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samples are almost same, which indicates that the same redox reactions occur in all three samples. Notably, the peak locations of initial three cycles are well overlapped in the 2 h sample (Fig. 4b). In the first reduction cycle, the small peak around 0.5 V is
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related to the formation of a solid electrolyte interphase (SEI). While the broad
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oxidation peaks located at about 1.9 V can be ascribed to the extraction of Na ions [23]. In the following cycles, the position of peaks almost overlap. From Fig. 4a and Fig. 4c (the 1 h and 3 h electrodes), the peaks located at ~0.4 V correspond to the formation of SEI in the first reduction reaction, while the oxidation peaks around 2 V
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can be ascribed to extraction of Na ions and back conversion reaction ( W+2Na 2S → WS2 +4Na ) in the WS2 nanosheets [24]. However, the redox peaks become broad and shift a little in the following cycles. These phenomena reveal that
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the polarization has been avoided and the kinetics of ionic/electronic transportation is
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remarkably enhanced in the 2 h electrode [25].
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5
Electrochemical
performance
of
the
as-prepared
electrodes:
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charge/discharge cycling tests of the three electrodes at 100 mA g-1 (a); rate performances at different applied current densities from 100 to 2000 mA g-1 (b); 8
ACCEPTED MANUSCRIPT charge/discharge curves of the three WS2 electrodes at 1st cycle (c). The cycling stability of the 1 h, 2 h and 3 h WS2 electrodes at 100 mA g-1 are investigated, as presented in Fig. 5a. By comparison, both 1 h and 2 h electrodes
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possess good cycling performances. The capacity of 2 h electrode is considerably higher than 1 h electrode and its capacity retention is 85% after 100 cycles. However, capacity fading happened in the cycling tests of the 3 h electrode, which may be due
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to the long sulfurized time resulting in instability volume excursions. For the sake of
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investigating their rate performances (Fig. 5b), different current densities from 100 to 2000 mA g-1 are employed on the samples. According to 3 h WS2 electrode, the capacity of 71 mA h g-1 is achieved at 2000 mA g-1. Whereas the improved rate capacity of 148 mA h g-1 is obtained at 2000 mA g-1 in the 2 h electrode. However,
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further decreasing the sulfurized time to 1 h leads to a rapidly decayed capacity to <10 mA h g-1 at 2000 mA g-1 because of the small interlayer spacing of the exposed (100) crystal plane detected by HRTEM observation, which may not provide favorable
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channel for Na+ to insertion/extraction process. The charge/discharge curves of the
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three electrodes at 1st cycle are shown in Fig. 5c. The specific capacity of the 2 h electrode was higher than the capacities of 1 h and 3 h electrodes. The Na+ storage capacities in WS2 can be ascribed to the following mechanisms: intercalation between WS2 layers and reversible conversion reaction in the WS2. It can be concluded that the sloping above ~0.5 V voltage discharge storage behavior is attributed to the reversible conversion reaction in the WS2. Additionally, the nearly flat plateau at potentials below ~0.5 V discharge storage mechanism is the binding of intercalation between 9
ACCEPTED MANUSCRIPT WS2 layers. The insertion capacity plays a dominant role in the whole discharge capacity and the insertion reaction is the main electrochemical behavior in the cycling
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process.
Fig. 6 EIS spectra of 1 h (a), 2 h (b) and 3 h (c) electrodes after 3rd and 100th cycle,
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Equivalent circuit for electrodes (d).
The charge transfer resistance of the three electrodes after 3rd and 100th cycle derived from the EIS measurements is shown in Fig. 6. It can be revealed that the
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WS2 electrode prepared with 2 h sulfurized time can effectively reduce charge transfer
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resistance from 3rd to 100th cycle (3743 to 493 Ω). On the contrary, the charge transfer resistance of the 1 h electrode increased when compared with those of 3rd and 100th cycle. Obviously, the conductivity of 2 h electrode is increased favorably, which is consistent with the results discussed in Fig. 4. Consequently, the optimal sulfurized time (2 hours) for the preparation of WS2 electrode is obtained, indicating that the favorable crystal nanostructures can be fabricated by controlling the sulfurized time to promote good conductivity and crystallinity, even achieving 10
ACCEPTED MANUSCRIPT (002)-oriented growth crystal structure for Na+ migration accessibly. In a word, the
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as-fabricated 2 h electrode possesses improved rate performance and cycling stability.
Fig. 7 Ex-situ XRD patterns of 1 h, 2 h and 3 h electrodes after 100 cycles.
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To elucidate the effects of (002) and (100)-oriented crystal structures on Na+ insertion/extraction, ex-situ XRD patterns of 1 h, 2 h and 3 h electrodes after 100 cycles are shown in Fig. 7. (002) diffraction peaks shift to large angles and (100) diffraction peaks vanish completely. The shift to high angles of the reflection indicates
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lattice shrinkage along the (002) oritentation, which may be caused by the insertion of Na ions. The disappearance of (100) diffraction peaks demonstrates that the WS2
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phase becomes amorphous after 100 cycles [16, 26]. There is no obvious difference in these ex-situ XRD patterns. The capacities of 1 h and 3 h WS2 electrodes are low and
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fading fast (Fig. 5a) because of the amorphization during cycling. This phenomenon reveals that the high crystallinity and large (002) interlayer spacing of the 2 h electrode can keep its stable structure and provide effective channels for accommodating more Na+.
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Fig. 8 Schematic illustration for the effect of different exposed crystal plane on Na+
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insertion/extraction during charge/discharge process.
In order to further clarify Na+ insertion/extraction in different exposed crystal plane of WS2 electrodes, Fig. 8 illustrates the effect of different crystal structures on Na+ insertion/extraction during charge/discharge process. The WS2 electrode with
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(002)-oriented growth exhibits large interlayer spacing, offering sufficient space for more Na+ insertion. Simultaneously, this large interlayer spacing can provide effective
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channels for Na+ insertion/extraction without destroying the original structure. This is the main reason for (002)-oriented 2 h WS2 electrode with high and stable capacity.
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However, the 0.27 nm interlayer spacing of (100)-oriented crystal structure of WS2 electrode is too small to insert Na+ during the discharge process. With the Na+ insertion/extraction process, the narrow channel may lead to the destruction of the original structure. Thus, the capacities of 1 h and 3 h electrodes are low and fading fast. It agrees well with XRD, TEM analyses and discussions for the electrochemical performances. Finally, the (002)-oriented WS2 electrode can maintain its stable structure and high capacity. 12
ACCEPTED MANUSCRIPT 4. Conclusions In summary, different crystal nanostructures and crystallinities of WS2 are successfully fabricated by a two-step method consisting of precipitation reaction and
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sulfurization process, which can be achieved by tailoring sulfurized time. The investigation of electrochemical performances reveals that 2 h WS2 electrode possesses much better electrochemical performance than the 1 h and 3 h WS2
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electrodes. Furthermore, the 2 h WS2 electrode has high capacity retention 85% after
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100 cycles at the current density of 100 mA g-1 as well as a superior capacity of 148 mA h g-1 at 2000 mA g-1. This better electrochemical performance is attributed to the broad (002)-oriented crystal interlayer spacing and high crystallinity of 2 h WS2 electrode, which can facilitate faster Na ions insertion/extraction behavior.
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Consequently, high crystallinity and oriented growth TMDs materials with large interlayer spacing can provide fast kinetics for Na+ insertion/ extraction as anode in SIBs.
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Acknowledgements
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This work has been supported by the 973 Special Preliminary Study Plan (2014CB260411), National Natural Science foundation of China (No. 51672165, No. 51472152), Innovation Team Assistance Foundation of Shaanxi University of Science & Technology (TD12-05), Fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP201636), China Postdoctoral Science Foundation (2016M592897XB) and Graduate Innovation Foundation of Shaanxi University of Science and Technology. The authors are also grateful to Dr. Hao from Shanghai Jiao 13
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ACCEPTED MANUSCRIPT Highlights: 1. Different crystal nanostructures WS2 are fabricated by controlling sulfurized time. 2. (002)-oriented WS2 with high crystalline has enhanced capacity and stability as anode in SIBs.
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3. (002)-oriented WS2 with large interlayer spacing are beneficial for Na+ insertion/extraction.
4. High crystallinity WS2 with (002) oritentation growth can maintain original stable
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structure.