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rGO composites with enhanced electrochemical properties for lithium-ion batteries
Accepted Manuscript Title: Hierarchical architecture of ReS2 /rGO composites with enhanced electrochemical properties for lithium-ion batteries Author: Fei Qi Yuanfu Chen Binjie Zheng Jiarui He Qian Li Xinqiang Wang Jie Lin Jinhao Zhou Bo Yu Pingjian Li Wanli Zhang PII: DOI: Reference:
S0169-4332(17)30994-7 http://dx.doi.org/doi:10.1016/j.apsusc.2017.03.296 APSUSC 35672
To appear in:
APSUSC
Received date: Revised date: Accepted date:
2-1-2017 30-3-2017 31-3-2017
Please cite this article as: F. Qi, Y. Chen, B. Zheng, J. He, Q. Li, X. Wang, J. Lin, J. Zhou, B. Yu, P. Li, W. Zhang, Hierarchical architecture of ReS2 /rGO composites with enhanced electrochemical properties for lithium-ion batteries, Applied Surface Science (2017), http://dx.doi.org/10.1016/j.apsusc.2017.03.296 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.
Hierarchical architecture of ReS2/rGO composites with enhanced electrochemical properties for lithium-ion batteries
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Fei Qi, Yuanfu Chen*, Binjie Zheng, Jiarui He, Qian Li, Xinqiang Wang, Jie Lin,
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Jinhao Zhou, Bo Yu, Pingjian Li, and Wanli Zhang
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of
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Electronic Science and Technology of China, Chengdu 610054, P. R. China.
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*Corresponding authors.
E-mail address: [email protected].
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Tel.: +86 028 83202710.
Postal address: State Key Laboratory of Electronic Thin Films and Integrated
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Devices, University of Electronic Science and Technology of China,
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Chengdu 610054, P. R. China.
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Graphical abstract
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Highlights 1. The ReS2/rGO composites have been synthesized by a facile one-pot method. 2. The ReS2/rGO composites exhibit hierarchical architecture. 3. The ReS2/rGO composites deliver better electrochemical performances than ReS2.
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4. The enhanced performance is due to porous and conductive structure of ReS2/rGO.
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ABSTRACT Rhenium disulfide (ReS2) two-dimensional (2D) semiconductor, has attracted more and more attention due to its unique anisotropic electronic, optical, mechanical properties. However, the facile synthesis and electrochemical property of ReS2 and its composite are still necessary to be researched. In this study, for the first time, the
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ReS2/reduced graphene oxide (rGO) composites have been synthesized through a
facile and one-pot hydrothermal method. The ReS2/rGO composites exhibit a
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hierarchical, interconnected, and porous architecture constructed by nanosheets. As anode for lithium-ion batteries, the as-synthesized ReS2/rGO composites deliver a
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large initial capacity of 918 mAh g-1 at 0.2 C. In addition, the ReS2/rGO composites exhibit much better electrochemical cycling stability and rate capability than that of
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bare ReS2. The significant enhancement in electrochemical property can be attributed to its unique architecture constructed by nanosheets and porous structure, which can
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allow for easy electrolyte infiltration, efficient electron transfer and ionic diffusion. Furthermore, the graphene with high electronic conductivity can provide good
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conductive passageways. The facile synthesis approach can be extended to prepare other 2D transition metal dichalcogenides semiconductors for energy storage and
Keywords:
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catalytic application.
rhenium
disulfide;
transition
metal
dichalcogenides;
graphene;
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lithium-ion batteries.
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1. Introduction With the development of hybrid vehicles and portable electronic devices, the capacities of the energy storage devices need be enhanced to meet the requirements of the future electronics industry [1-4]. Lithium-ion batteries exhibit many advantages, such as, great safety performance, high energy density, long cycle life, high capability
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of rapid charge/discharge and less pollution [5, 6]. The electrode materials play an
important role in the development of the lithium-ion batteries. The transition metal
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dichalcogenides (TMDs) as anode materials have attracted increasing attention in
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recent years due to their excellent electrochemical properties [7-12].
As one kind of the TMDs, rhenium disulfide (ReS2) has many distinctive characteristics due to its unique crystal structure [13, 14]. These favorable properties
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have been demonstrated in field effect transistors [15], photodetectors [16], digital inverters [17], and electrocatalysts [18]. According to theoretical calculation, the ReS2
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has theoretical special capacity of 430 mAh g-1 which is larger than the commercial graphite material (372 mAh g-1) for lithium-ion batteries [19, 20]. In addition, the
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ReS2 bulk material with layered structure has an interlayer distance of 0.614 nm which is larger than that of graphite (0.335 nm) [19]. Therefore, the ReS2 possesses
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much weaker interlayer coupling and a unique distorted octahedral (1T) structure with triclinic system [13]. The extremely weak interlayer coupling and larger interlayer distance supply a possibility for massive lithium ions to efficiently diffuse without a significant increase in volume, which is beneficial for the electrochemical
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performance of lithium-ion batteries [21, 22]. However, the low electrical conductivity and poor cycling stability still challenge the application of TMDs anode electrodes.
In order to address such issues, many studies about MoS2 and WS2 for lithium-ion batteries have been reported. However, there are very few reports on ReS2 to be used as anode for lithium-ion batteries [19]. According to previous reports, some effective strategies can be dedicated to enhance the performance of ReS2 for lithium-ion batteries. On the one hand, nanostructurization will be an effective method to improve the electrochemical performance of TMDs-based electrode materials, e.g. MoS2.[23]
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On the other hand, a porous and conductive architecture, which is usually supported by carbon materials, such as carbon nanotubes and graphene, could be favorable to electrolyte infiltration, faster ion migration and electron transportation throughout the whole electrodes [24]. In particular, two-dimensional graphene has extremely high electrical conductivity, outstanding mechanical strength, large surface area,
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preeminent chemical stability [25-28]. These advantages make graphene a promising matrix for improving the electrochemical performance of TMDs-based electrode
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materials. Nevertheless, up to now, to fabricate nanostructured ReS2/reduced graphene
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oxide (rGO) composites with porous structure has not been realized yet.
In this study, for the first time, the ReS2/rGO composites with hierarchical architecture have been synthesized through a facile and one-pot hydrothermal method.
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The ReS2/rGO composites exhibit an interconnected porous structure constructed by nanosheets. When used as anode material for lithium-ion batteries, the ReS2/rGO
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composites deliver better electrochemical performances than that bare ReS2. This
batteries. 2. Experimental
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study provides great promise for practical applications of ReS2 in lithium-ion
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2.1 Synthesis of ReS2/rGO
The ReS2/rGO composites were prepared as the following procedures. Firstly, the graphene oxide was prepared through our previous procedures.[29] Then, 100 mg graphene oxide was dispersed into 60 ml deionized (DI) water
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and sonicated for 2 hours. Secondly, 536 mg ammonium perrhenate, 417 mg hydroxylamine hydrochloride, and 685 mg thiourea were dissolved in 60 ml graphene oxide solution. Then the mixture was stirred for 2 hours and transferred to a 100 ml stainless steel autoclave for hydrothermal reaction, which was then heated up to 240 °C and kept for 24 h. After cooling, the black powder of ReS2/rGO was repeatedly washed with DI water and ethanol. The final product was collected by drying. As comparison, the bare ReS2 was synthesized by the same process without graphene oxide. 2.2 Materials characterization
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The X-ray diffraction (XRD) patterns of the products were recorded on Rigaku D/MAX-rA diffractometer with non-monochromated Cu Kα X-Ray source. The Raman spectrum was acquired at room temperature with excitation laser lines of 514 nm (Renishaw). The morphologies, structures, and elemental compositions of obtained samples were investigated by using field emission
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scanning electron microscope (FESEM, JSM-7000F, JEOL), transmission
electron microscope (TEM, TECNAI G2 F20 S-TWIN), and X-ray
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photoelectron spectroscopy (Kratos XSAM800, Al Kα radiation (144 W, 12 mA,
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12 kV)). 2.3 Electrochemical measurements
The anodes were made of 80 wt% active materials, 10 wt% carbon black
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(super P Timcal) and 10 wt% poly (vinylidene fluoride) (PVDF) binder. The mixtures were dispersed in N-methyl-2- pyrrolidinone (NMP) to form slurry.
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Then the slurry was spread on the Cu foil and dried at 120 °C in vacuum for 12 h. The coin-type half cell (CR2025) was assembled in an argon-filled glove box
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with water and oxygen content below 0.5 ppm, using Li foil as counter electrode. 1 M LiPF6 in a 1:1 v/v mixture of ethylene carbonate (EC) and
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dimethyl carbonate (DMC) was used as the electrolyte and Celgard 2400 was used as the separator. The charge/discharge tests between 0.01 to 3.0 V vs. Li/Li+ were performed on LAND CT2001A. Cyclic voltammetry (CV) measurements at a scan rate of 0.2 mV s-1 in the voltage range of 0 to 3.0 V vs.
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Li/Li+ and electrochemical impedance spectroscopy (EIS) using an AC voltage of in the frequency range of 100k Hz to 0.01Hz were performed by electrochemical workstation (CHI660D). 3. Results and discussion To study the crystal structure and composition of hydrothermal synthesis of ReS2/rGO, XRD patterns were performed. As shown in Fig. 1a, the reflection peaks at 14.5°, 32.7°, 44.6°, 57.8°, respectively corresponding to the lattice planes of (100), (002), (300), (-122) for the triclinic phase ReS2 (ReS2, PDF#82-1379), can be detected [30]. The diffraction peaks of rGO cannot be clearly observed due to the low rGO
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content [5, 20]. The ReS2/rGO composites were then characterized by the Raman spectroscopy. As displayed in Fig. 1b, two characteristic Raman modes at 161.7 cm-1 and 211.4 cm-1 result from the in-plane (Eg) and out-of-plane (Ag) vibrational modes of ReS2, respectively [19]. One can also observe that there are additionally several Raman peaks in the range of 100~400 cm-1, because of the unique asymmetry in the
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distorted 1T structure for ReS2 [31]. In addition, the D and G bands of graphene located at 1358 cm-1 and 1596 cm-1 are observed, which confirmed the presence of
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rGO. The XRD patterns and Raman spectrum characterization of synthesized ReS2
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are also shown in Fig. S1.
XPS was utilized to examine the elemental composition and bonding configurations of the ReS2/rGO composites. The Fig. 2a shows the XPS scan survey of ReS2/rGO
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composites. The C-C and C-O chemical bonds of rGO at 284.9 eV and 286.2 eV are clearly observed in Fig. 2b, which also confirmed the presence of rGO. The bonding
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configurations of rhenium (Re) have been researched by high-resolution Re 4f XPS spectrum exhibited in Fig. 2c. One can see that two characteristic peaks are located at
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42.3 eV and 44.7 eV, corresponding to the core 4f7/2 and 4f5/2 level peaks of Re4+, respectively. As shown in Fig. 2d, two characteristic peaks at 162.6 eV and 163.7 eV
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for the bonding configurations of sulfur (S) are detected, corresponding to the core 2p3/2 and 2p1/2 level peaks of S2-, respectively. All the results are consistent with the reported data for ReS2. In addition, the Re and S ratio acquired from XPS is equal to 1:2, which is the stoichiometric values of ReS2.
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The morphologies of ReS2 and ReS2/rGO samples synthesized by our preparation procedure were checked using a SEM technique. The synthesized bare ReS2 shows microsphere structure as displayed in the Fig. 3a. The ReS2 microspheres exhibit a poor dispersions and severely aggregate. The SEM morphology of ReS2/rGO composites is shown in Fig. 3b. The ReS2/rGO composites deliver flexible nanosheets and curly structure. The bare ReS2 with microsphere architecture might be assigned to the absence of nucleate sites during growth process. When the rGO is introduced, the graphene nanosheets play a role of good template and greatly inhibit the restack and agglomeration of ReS2 layers [32]. The hierarchical architecture of ReS2/rGO
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composites was carefully examined by TEM and HRTEM. The Fig. 3c is the TEM image of ReS2/rGO composites, which exhibit two-dimensional nanoflake architecture. From Fig. 3d, it can be seen that the typical ReS2 layers with an interlayer distance of 0.62 nm corresponding to (100) lattice plane, are grown on the surface of graphene layers with the interlayer of 0.38 nm in the hierarchical structure.
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The ultrathin characteristics of ReS2/rGO composites are favorable for acquiring fast
electron transportation and short passways for lithium ion diffusion. Furthermore,
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graphene with flexible nature in the hierarchical structure of ReS2/rGO composites
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can also effectively restrict the excessive volume expansion during discharge/charge process. To confirm the uniform combination of ReS2 and rGO, energy dispersive X-ray spectroscope (EDX) integrated with SEM apparatus elemental mapping was
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performed as exhibited in Fig. S2. It can be clearly seen that the Re and S elemental mapping patterns well match with that of C element, indicating the uniform
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distribution of ReS2 in the ReS2/rGO composites.
To evaluate the electrochemical properties of ReS2/rGO, a series of electrochemical
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measurements were carried out. Fig. 4a shows the CV curves of ReS2/rGO composites at a scan rate of 0.2 mV s-1 between 0 V and 3.0 V vs. Li/Li+ of the first three cycles.
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During the first cathodic sweep, the electrode exhibits a reduction peak at ~0.78 V which can be related with the formation of LixReS2 [19]. During the anodic sweep, the electrode exhibits an obvious peak at ~2.35 V, showing a reversible redox reaction [19]. The Li+ insertion/extraction properties of ReS2/rGO as anode material were
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carried out by galvanostatic charge/discharge measurements. Fig. 4b shows the first several discharge/charge curves of the ReS2/rGO in the range of 0.01~3.0 V vs. Li/Li+ at a current rate of 0.2 C (1 C = 430 mA g-1). The first discharge and charge capacities of ReS2/rGO are 1301 and 922 mAh g-1, indicating a Coulombic efficiency of 71%. The capacity loss might result from the formation of solid electrolyte interphase (SEI) layer and the irreversible conversion process between lithium and ReS2 during the first lithiation/delithiation process.[1, 22] The rate performances of the ReS2 and ReS2/rGO at various C-rates from 0.2 C to 2 C are illustrated in Fig. 4c. The synthesized ReS2 exhibits poor rate performance for
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lithium-ion batteries. On the contrary, the ReS2/rGO anode shows relatively outstanding rate capability with specific capacities of 918, 728, 593, 480, 383 mAh g-1 at 0.2, 0.5, 1, 1.5, 2 C, respectively. The cycling performances of ReS2, rGO and ReS2/rGO at a current of 0.2 C are displayed in Fig. 4d. It clearly shows a relatively stable cyclic performance of ReS2/rGO with 2nd discharge capacity of 885 mAh g-1
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and maintaining discharge capacity of 745 mAh g-1 after 50 cycles. In contrast, the 2nd discharge capacities of bare ReS2, and rGO are 604, and 310 mAh g-1, respectively,
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which maintain 147, and 241 mAh∙g-1 after 50 cycles. It is noted that the capacity of
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rGO in this study is lower than the previous report [33], which might be caused by different synthesis methods. From Fig. 4d, it indicates that ReS2/rGO exhibits better cycling stability than ReS2. The direct capacity contribution of rGO on the total
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capacity of ReS2/rGO (with 6.2 wt% of rGO) can be simply estimated as 2.2%, which is very low and similar to some previous reports [5, 8, 20]. So, in this study, the key
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role of rGO on the capacity of ReS2/rGO is not from the direction capacity contribution of rGO, but from the following synergy effects induced by rGO. The
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excellent conductivity of rGO ensures the high conductivity of ReS2/rGO electrode, which will facilitate the fast charge migration during the charge/discharge
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process[34-37]; the incorporation of flexible rGO nanosheets can accommodate the volume change during the discharge/charge process[38-40]; the rGO nanosheets will also greatly inhibit the re-stack and agglomeration of ReS2 layers, resulting in the formation of hierarchical structure with large surface area and porous architecture,
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which can provide many more electrochemically active sites resulting larger capacity [41-44]. In addition, the ReS2/rGO with porous structure could not only be favorable for electrolyte permeation and fast lithium ions transportation throughout the whole electrode, but also effectively accommodate the volume variation and structural strains accompanying the lithium intercalation/deintercalation process. Furthermore, the flexible nature of such ultrathin ReS2/rGO nanosheets enhances the robustness of the electrode structure. And, the conductive rGO can be well charge-transfer channels in the ReS2/rGO composites. It is noted that the reversible capacity of ReS2/rGO composites is much higher than
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the theoretical value of ReS2 (430 mAh g-1), which had been previously reported in many papers on TMDs-based anodes [2, 45, 46]. The extra capacity may be attributed to these factors. Firstly, the nanomaterial anodes usually present higher capacities than those of bulk materials or large-size materials. The nanostructured ReS2/rGO can provide more active sites for electrochemical reaction, leading to enhanced practical
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capacity [47-49]. Secondly, abundant defect sites including dislocations, vacancies and distortions in the ReS2/rGO nanostructure provide active sites for efficient
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intercalation of lithium ions [1, 8]. Thirdly, the conductive rGO network can provide
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the passageways for electrons and lithium ions for increasing the conductivity of electrode, resulting in higher capacity of ReS2/rGO [5, 20, 50].
To better understand the reason why the synthesized ReS2/rGO shows good
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electrochemical properties, electrochemical impedance spectroscopy (EIS) was measured with fresh cells. Fig. 5 illustrates the Nyquist plots of ReS2 and ReS2/rGO
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electrodes. The equivalent circuit diagram of EIS for the two electrodes is shown in the inset of Fig. 5. According to the previous reports,[7, 51] the high-to-medium
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frequency semicircle owes to the charge-transfer resistance (Rct) and constant phase capacitance of electrode/electrolyte interface. The slope line in the low frequency
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region representing the Warburg impedance is related with the lithium ion diffusion process. The fitting EIS results indicate that the Rct of the ReS2/rGO (22.6 Ω) is smaller than that of the ReS2 (127.5 Ω), which is attributed to the graphene within the composite enhancing the conductivity of electrode. The larger slope of the low
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frequency incline line for ReS2/rGO also reveals that the faster lithium ions diffusion in the anode electrode. 4. Conclusions
In summary, we first synthesized ReS2/rGO composites through a facile one-pot hydrothermal method.
The ReS2/rGO composites exhibit hierarchical and
interconnected porous architecture constructed by nanosheets. The ReS2/rGO composites anodes for lithium-ion batteries exhibit a good cycling stability and rate performance. At a current of 0.2C, the ReS2/rGO anode delivers a large initial capacity of 918 mAh g-1. With an increase of current from 0.2 to 2C, the ReS2/rGO
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anode exhibits better rate performance than that of ReS2. These excellent electrochemical properties can be attributed to the weak interlayer coupling of ReS2, the porous and hierarchical nanostructure, and the high electronic conductivity of graphene. The work provides a choice project for synthesis ReS2 and broadens the application of ReS2 for lithium ion batteries.
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Acknowledgments
The research was supported by the National High Technology Research and
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Development Program of China (Grant No. 2015AA034202), the National Natural
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Science Foundation of China (Grant No. 51372033), the 111 Project (Grant No. B13042), and the Fundamental Research Funds for the Central Universities (Grant No.
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ZYGX2013Z001). References
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Figures
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Figure captions Fig. 1. (a) XRD pattern and (b) Raman spectrum of synthesized ReS2/rGO. Fig. 2. (a) XPS spectrum of ReS2/rGO. High-resolution XPS spectra of the (b) C 1s sate, (c) Re 4f state and (d) S 2p state for synthesized ReS2/rGO. Fig. 3. SEM and TEM characterizations of synthesized ReS2 and ReS2/rGO. SEM
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images of (a) ReS2 and (b) ReS2/rGO. (c) TEM image and (d) High-resolution TEM image of ReS2/rGO.
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Fig. 4. The electrochemical performances. (a) CV curves of the first three cycles, (b)
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selected discharge-charge profiles at 0.2 C of ReS2/rGO. (c) Rate and (d) cycling performances at 0.2 C of ReS2 and ReS2/rGO. (c) Rate performances of ReS2/rGO and ReS2. (d) Cycling performances of ReS2/rGO, ReS2, and rGO at 0.2 C.
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shows the equivalent circuit diagram of EIS.
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Fig. 5. Nyquist plots of fresh ReS2 and ReS2/rGO electrodes. The inset of Fig. 5
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S0169-4332(17)30994-7 http://dx.doi.org/doi:10.1016/j.apsusc.2017.03.296 APSUSC 35672
To appear in:
APSUSC
Received date: Revised date: Accepted date:
2-1-2017 30-3-2017 31-3-2017
Please cite this article as: F. Qi, Y. Chen, B. Zheng, J. He, Q. Li, X. Wang, J. Lin, J. Zhou, B. Yu, P. Li, W. Zhang, Hierarchical architecture of ReS2 /rGO composites with enhanced electrochemical properties for lithium-ion batteries, Applied Surface Science (2017), http://dx.doi.org/10.1016/j.apsusc.2017.03.296 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.
Hierarchical architecture of ReS2/rGO composites with enhanced electrochemical properties for lithium-ion batteries
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Fei Qi, Yuanfu Chen*, Binjie Zheng, Jiarui He, Qian Li, Xinqiang Wang, Jie Lin,
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Jinhao Zhou, Bo Yu, Pingjian Li, and Wanli Zhang
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of
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Electronic Science and Technology of China, Chengdu 610054, P. R. China.
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*Corresponding authors.
E-mail address: [email protected].
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Tel.: +86 028 83202710.
Postal address: State Key Laboratory of Electronic Thin Films and Integrated
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Devices, University of Electronic Science and Technology of China,
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Chengdu 610054, P. R. China.
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Graphical abstract
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Highlights 1. The ReS2/rGO composites have been synthesized by a facile one-pot method. 2. The ReS2/rGO composites exhibit hierarchical architecture. 3. The ReS2/rGO composites deliver better electrochemical performances than ReS2.
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4. The enhanced performance is due to porous and conductive structure of ReS2/rGO.
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ABSTRACT Rhenium disulfide (ReS2) two-dimensional (2D) semiconductor, has attracted more and more attention due to its unique anisotropic electronic, optical, mechanical properties. However, the facile synthesis and electrochemical property of ReS2 and its composite are still necessary to be researched. In this study, for the first time, the
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ReS2/reduced graphene oxide (rGO) composites have been synthesized through a
facile and one-pot hydrothermal method. The ReS2/rGO composites exhibit a
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hierarchical, interconnected, and porous architecture constructed by nanosheets. As anode for lithium-ion batteries, the as-synthesized ReS2/rGO composites deliver a
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large initial capacity of 918 mAh g-1 at 0.2 C. In addition, the ReS2/rGO composites exhibit much better electrochemical cycling stability and rate capability than that of
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bare ReS2. The significant enhancement in electrochemical property can be attributed to its unique architecture constructed by nanosheets and porous structure, which can
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allow for easy electrolyte infiltration, efficient electron transfer and ionic diffusion. Furthermore, the graphene with high electronic conductivity can provide good
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conductive passageways. The facile synthesis approach can be extended to prepare other 2D transition metal dichalcogenides semiconductors for energy storage and
Keywords:
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catalytic application.
rhenium
disulfide;
transition
metal
dichalcogenides;
graphene;
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lithium-ion batteries.
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1. Introduction With the development of hybrid vehicles and portable electronic devices, the capacities of the energy storage devices need be enhanced to meet the requirements of the future electronics industry [1-4]. Lithium-ion batteries exhibit many advantages, such as, great safety performance, high energy density, long cycle life, high capability
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of rapid charge/discharge and less pollution [5, 6]. The electrode materials play an
important role in the development of the lithium-ion batteries. The transition metal
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dichalcogenides (TMDs) as anode materials have attracted increasing attention in
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recent years due to their excellent electrochemical properties [7-12].
As one kind of the TMDs, rhenium disulfide (ReS2) has many distinctive characteristics due to its unique crystal structure [13, 14]. These favorable properties
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have been demonstrated in field effect transistors [15], photodetectors [16], digital inverters [17], and electrocatalysts [18]. According to theoretical calculation, the ReS2
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has theoretical special capacity of 430 mAh g-1 which is larger than the commercial graphite material (372 mAh g-1) for lithium-ion batteries [19, 20]. In addition, the
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ReS2 bulk material with layered structure has an interlayer distance of 0.614 nm which is larger than that of graphite (0.335 nm) [19]. Therefore, the ReS2 possesses
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much weaker interlayer coupling and a unique distorted octahedral (1T) structure with triclinic system [13]. The extremely weak interlayer coupling and larger interlayer distance supply a possibility for massive lithium ions to efficiently diffuse without a significant increase in volume, which is beneficial for the electrochemical
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performance of lithium-ion batteries [21, 22]. However, the low electrical conductivity and poor cycling stability still challenge the application of TMDs anode electrodes.
In order to address such issues, many studies about MoS2 and WS2 for lithium-ion batteries have been reported. However, there are very few reports on ReS2 to be used as anode for lithium-ion batteries [19]. According to previous reports, some effective strategies can be dedicated to enhance the performance of ReS2 for lithium-ion batteries. On the one hand, nanostructurization will be an effective method to improve the electrochemical performance of TMDs-based electrode materials, e.g. MoS2.[23]
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On the other hand, a porous and conductive architecture, which is usually supported by carbon materials, such as carbon nanotubes and graphene, could be favorable to electrolyte infiltration, faster ion migration and electron transportation throughout the whole electrodes [24]. In particular, two-dimensional graphene has extremely high electrical conductivity, outstanding mechanical strength, large surface area,
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preeminent chemical stability [25-28]. These advantages make graphene a promising matrix for improving the electrochemical performance of TMDs-based electrode
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materials. Nevertheless, up to now, to fabricate nanostructured ReS2/reduced graphene
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oxide (rGO) composites with porous structure has not been realized yet.
In this study, for the first time, the ReS2/rGO composites with hierarchical architecture have been synthesized through a facile and one-pot hydrothermal method.
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The ReS2/rGO composites exhibit an interconnected porous structure constructed by nanosheets. When used as anode material for lithium-ion batteries, the ReS2/rGO
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composites deliver better electrochemical performances than that bare ReS2. This
batteries. 2. Experimental
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study provides great promise for practical applications of ReS2 in lithium-ion
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2.1 Synthesis of ReS2/rGO
The ReS2/rGO composites were prepared as the following procedures. Firstly, the graphene oxide was prepared through our previous procedures.[29] Then, 100 mg graphene oxide was dispersed into 60 ml deionized (DI) water
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and sonicated for 2 hours. Secondly, 536 mg ammonium perrhenate, 417 mg hydroxylamine hydrochloride, and 685 mg thiourea were dissolved in 60 ml graphene oxide solution. Then the mixture was stirred for 2 hours and transferred to a 100 ml stainless steel autoclave for hydrothermal reaction, which was then heated up to 240 °C and kept for 24 h. After cooling, the black powder of ReS2/rGO was repeatedly washed with DI water and ethanol. The final product was collected by drying. As comparison, the bare ReS2 was synthesized by the same process without graphene oxide. 2.2 Materials characterization
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The X-ray diffraction (XRD) patterns of the products were recorded on Rigaku D/MAX-rA diffractometer with non-monochromated Cu Kα X-Ray source. The Raman spectrum was acquired at room temperature with excitation laser lines of 514 nm (Renishaw). The morphologies, structures, and elemental compositions of obtained samples were investigated by using field emission
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scanning electron microscope (FESEM, JSM-7000F, JEOL), transmission
electron microscope (TEM, TECNAI G2 F20 S-TWIN), and X-ray
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photoelectron spectroscopy (Kratos XSAM800, Al Kα radiation (144 W, 12 mA,
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12 kV)). 2.3 Electrochemical measurements
The anodes were made of 80 wt% active materials, 10 wt% carbon black
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(super P Timcal) and 10 wt% poly (vinylidene fluoride) (PVDF) binder. The mixtures were dispersed in N-methyl-2- pyrrolidinone (NMP) to form slurry.
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Then the slurry was spread on the Cu foil and dried at 120 °C in vacuum for 12 h. The coin-type half cell (CR2025) was assembled in an argon-filled glove box
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with water and oxygen content below 0.5 ppm, using Li foil as counter electrode. 1 M LiPF6 in a 1:1 v/v mixture of ethylene carbonate (EC) and
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dimethyl carbonate (DMC) was used as the electrolyte and Celgard 2400 was used as the separator. The charge/discharge tests between 0.01 to 3.0 V vs. Li/Li+ were performed on LAND CT2001A. Cyclic voltammetry (CV) measurements at a scan rate of 0.2 mV s-1 in the voltage range of 0 to 3.0 V vs.
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Li/Li+ and electrochemical impedance spectroscopy (EIS) using an AC voltage of in the frequency range of 100k Hz to 0.01Hz were performed by electrochemical workstation (CHI660D). 3. Results and discussion To study the crystal structure and composition of hydrothermal synthesis of ReS2/rGO, XRD patterns were performed. As shown in Fig. 1a, the reflection peaks at 14.5°, 32.7°, 44.6°, 57.8°, respectively corresponding to the lattice planes of (100), (002), (300), (-122) for the triclinic phase ReS2 (ReS2, PDF#82-1379), can be detected [30]. The diffraction peaks of rGO cannot be clearly observed due to the low rGO
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content [5, 20]. The ReS2/rGO composites were then characterized by the Raman spectroscopy. As displayed in Fig. 1b, two characteristic Raman modes at 161.7 cm-1 and 211.4 cm-1 result from the in-plane (Eg) and out-of-plane (Ag) vibrational modes of ReS2, respectively [19]. One can also observe that there are additionally several Raman peaks in the range of 100~400 cm-1, because of the unique asymmetry in the
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distorted 1T structure for ReS2 [31]. In addition, the D and G bands of graphene located at 1358 cm-1 and 1596 cm-1 are observed, which confirmed the presence of
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rGO. The XRD patterns and Raman spectrum characterization of synthesized ReS2
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are also shown in Fig. S1.
XPS was utilized to examine the elemental composition and bonding configurations of the ReS2/rGO composites. The Fig. 2a shows the XPS scan survey of ReS2/rGO
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composites. The C-C and C-O chemical bonds of rGO at 284.9 eV and 286.2 eV are clearly observed in Fig. 2b, which also confirmed the presence of rGO. The bonding
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configurations of rhenium (Re) have been researched by high-resolution Re 4f XPS spectrum exhibited in Fig. 2c. One can see that two characteristic peaks are located at
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42.3 eV and 44.7 eV, corresponding to the core 4f7/2 and 4f5/2 level peaks of Re4+, respectively. As shown in Fig. 2d, two characteristic peaks at 162.6 eV and 163.7 eV
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for the bonding configurations of sulfur (S) are detected, corresponding to the core 2p3/2 and 2p1/2 level peaks of S2-, respectively. All the results are consistent with the reported data for ReS2. In addition, the Re and S ratio acquired from XPS is equal to 1:2, which is the stoichiometric values of ReS2.
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The morphologies of ReS2 and ReS2/rGO samples synthesized by our preparation procedure were checked using a SEM technique. The synthesized bare ReS2 shows microsphere structure as displayed in the Fig. 3a. The ReS2 microspheres exhibit a poor dispersions and severely aggregate. The SEM morphology of ReS2/rGO composites is shown in Fig. 3b. The ReS2/rGO composites deliver flexible nanosheets and curly structure. The bare ReS2 with microsphere architecture might be assigned to the absence of nucleate sites during growth process. When the rGO is introduced, the graphene nanosheets play a role of good template and greatly inhibit the restack and agglomeration of ReS2 layers [32]. The hierarchical architecture of ReS2/rGO
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composites was carefully examined by TEM and HRTEM. The Fig. 3c is the TEM image of ReS2/rGO composites, which exhibit two-dimensional nanoflake architecture. From Fig. 3d, it can be seen that the typical ReS2 layers with an interlayer distance of 0.62 nm corresponding to (100) lattice plane, are grown on the surface of graphene layers with the interlayer of 0.38 nm in the hierarchical structure.
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The ultrathin characteristics of ReS2/rGO composites are favorable for acquiring fast
electron transportation and short passways for lithium ion diffusion. Furthermore,
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graphene with flexible nature in the hierarchical structure of ReS2/rGO composites
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can also effectively restrict the excessive volume expansion during discharge/charge process. To confirm the uniform combination of ReS2 and rGO, energy dispersive X-ray spectroscope (EDX) integrated with SEM apparatus elemental mapping was
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performed as exhibited in Fig. S2. It can be clearly seen that the Re and S elemental mapping patterns well match with that of C element, indicating the uniform
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distribution of ReS2 in the ReS2/rGO composites.
To evaluate the electrochemical properties of ReS2/rGO, a series of electrochemical
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measurements were carried out. Fig. 4a shows the CV curves of ReS2/rGO composites at a scan rate of 0.2 mV s-1 between 0 V and 3.0 V vs. Li/Li+ of the first three cycles.
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During the first cathodic sweep, the electrode exhibits a reduction peak at ~0.78 V which can be related with the formation of LixReS2 [19]. During the anodic sweep, the electrode exhibits an obvious peak at ~2.35 V, showing a reversible redox reaction [19]. The Li+ insertion/extraction properties of ReS2/rGO as anode material were
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carried out by galvanostatic charge/discharge measurements. Fig. 4b shows the first several discharge/charge curves of the ReS2/rGO in the range of 0.01~3.0 V vs. Li/Li+ at a current rate of 0.2 C (1 C = 430 mA g-1). The first discharge and charge capacities of ReS2/rGO are 1301 and 922 mAh g-1, indicating a Coulombic efficiency of 71%. The capacity loss might result from the formation of solid electrolyte interphase (SEI) layer and the irreversible conversion process between lithium and ReS2 during the first lithiation/delithiation process.[1, 22] The rate performances of the ReS2 and ReS2/rGO at various C-rates from 0.2 C to 2 C are illustrated in Fig. 4c. The synthesized ReS2 exhibits poor rate performance for
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lithium-ion batteries. On the contrary, the ReS2/rGO anode shows relatively outstanding rate capability with specific capacities of 918, 728, 593, 480, 383 mAh g-1 at 0.2, 0.5, 1, 1.5, 2 C, respectively. The cycling performances of ReS2, rGO and ReS2/rGO at a current of 0.2 C are displayed in Fig. 4d. It clearly shows a relatively stable cyclic performance of ReS2/rGO with 2nd discharge capacity of 885 mAh g-1
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and maintaining discharge capacity of 745 mAh g-1 after 50 cycles. In contrast, the 2nd discharge capacities of bare ReS2, and rGO are 604, and 310 mAh g-1, respectively,
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which maintain 147, and 241 mAh∙g-1 after 50 cycles. It is noted that the capacity of
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rGO in this study is lower than the previous report [33], which might be caused by different synthesis methods. From Fig. 4d, it indicates that ReS2/rGO exhibits better cycling stability than ReS2. The direct capacity contribution of rGO on the total
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capacity of ReS2/rGO (with 6.2 wt% of rGO) can be simply estimated as 2.2%, which is very low and similar to some previous reports [5, 8, 20]. So, in this study, the key
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role of rGO on the capacity of ReS2/rGO is not from the direction capacity contribution of rGO, but from the following synergy effects induced by rGO. The
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excellent conductivity of rGO ensures the high conductivity of ReS2/rGO electrode, which will facilitate the fast charge migration during the charge/discharge
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process[34-37]; the incorporation of flexible rGO nanosheets can accommodate the volume change during the discharge/charge process[38-40]; the rGO nanosheets will also greatly inhibit the re-stack and agglomeration of ReS2 layers, resulting in the formation of hierarchical structure with large surface area and porous architecture,
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which can provide many more electrochemically active sites resulting larger capacity [41-44]. In addition, the ReS2/rGO with porous structure could not only be favorable for electrolyte permeation and fast lithium ions transportation throughout the whole electrode, but also effectively accommodate the volume variation and structural strains accompanying the lithium intercalation/deintercalation process. Furthermore, the flexible nature of such ultrathin ReS2/rGO nanosheets enhances the robustness of the electrode structure. And, the conductive rGO can be well charge-transfer channels in the ReS2/rGO composites. It is noted that the reversible capacity of ReS2/rGO composites is much higher than
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the theoretical value of ReS2 (430 mAh g-1), which had been previously reported in many papers on TMDs-based anodes [2, 45, 46]. The extra capacity may be attributed to these factors. Firstly, the nanomaterial anodes usually present higher capacities than those of bulk materials or large-size materials. The nanostructured ReS2/rGO can provide more active sites for electrochemical reaction, leading to enhanced practical
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capacity [47-49]. Secondly, abundant defect sites including dislocations, vacancies and distortions in the ReS2/rGO nanostructure provide active sites for efficient
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intercalation of lithium ions [1, 8]. Thirdly, the conductive rGO network can provide
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the passageways for electrons and lithium ions for increasing the conductivity of electrode, resulting in higher capacity of ReS2/rGO [5, 20, 50].
To better understand the reason why the synthesized ReS2/rGO shows good
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electrochemical properties, electrochemical impedance spectroscopy (EIS) was measured with fresh cells. Fig. 5 illustrates the Nyquist plots of ReS2 and ReS2/rGO
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electrodes. The equivalent circuit diagram of EIS for the two electrodes is shown in the inset of Fig. 5. According to the previous reports,[7, 51] the high-to-medium
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frequency semicircle owes to the charge-transfer resistance (Rct) and constant phase capacitance of electrode/electrolyte interface. The slope line in the low frequency
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region representing the Warburg impedance is related with the lithium ion diffusion process. The fitting EIS results indicate that the Rct of the ReS2/rGO (22.6 Ω) is smaller than that of the ReS2 (127.5 Ω), which is attributed to the graphene within the composite enhancing the conductivity of electrode. The larger slope of the low
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frequency incline line for ReS2/rGO also reveals that the faster lithium ions diffusion in the anode electrode. 4. Conclusions
In summary, we first synthesized ReS2/rGO composites through a facile one-pot hydrothermal method.
The ReS2/rGO composites exhibit hierarchical and
interconnected porous architecture constructed by nanosheets. The ReS2/rGO composites anodes for lithium-ion batteries exhibit a good cycling stability and rate performance. At a current of 0.2C, the ReS2/rGO anode delivers a large initial capacity of 918 mAh g-1. With an increase of current from 0.2 to 2C, the ReS2/rGO
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anode exhibits better rate performance than that of ReS2. These excellent electrochemical properties can be attributed to the weak interlayer coupling of ReS2, the porous and hierarchical nanostructure, and the high electronic conductivity of graphene. The work provides a choice project for synthesis ReS2 and broadens the application of ReS2 for lithium ion batteries.
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Acknowledgments
The research was supported by the National High Technology Research and
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Development Program of China (Grant No. 2015AA034202), the National Natural
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Science Foundation of China (Grant No. 51372033), the 111 Project (Grant No. B13042), and the Fundamental Research Funds for the Central Universities (Grant No.
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ZYGX2013Z001). References
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Figures
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Figure captions Fig. 1. (a) XRD pattern and (b) Raman spectrum of synthesized ReS2/rGO. Fig. 2. (a) XPS spectrum of ReS2/rGO. High-resolution XPS spectra of the (b) C 1s sate, (c) Re 4f state and (d) S 2p state for synthesized ReS2/rGO. Fig. 3. SEM and TEM characterizations of synthesized ReS2 and ReS2/rGO. SEM
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images of (a) ReS2 and (b) ReS2/rGO. (c) TEM image and (d) High-resolution TEM image of ReS2/rGO.
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Fig. 4. The electrochemical performances. (a) CV curves of the first three cycles, (b)
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selected discharge-charge profiles at 0.2 C of ReS2/rGO. (c) Rate and (d) cycling performances at 0.2 C of ReS2 and ReS2/rGO. (c) Rate performances of ReS2/rGO and ReS2. (d) Cycling performances of ReS2/rGO, ReS2, and rGO at 0.2 C.
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shows the equivalent circuit diagram of EIS.
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Fig. 5. Nyquist plots of fresh ReS2 and ReS2/rGO electrodes. The inset of Fig. 5
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