High throughput synthesis of defect-rich MoS2 nanosheets via facile electrochemical exfoliation for fast high-performance lithium storage

Journal of Colloid and Interface Science 542 (2019) 263–268

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Short Communication

High throughput synthesis of defect-rich MoS2 nanosheets via facile electrochemical exfoliation for fast high-performance lithium storage Weiming Wu a, Changsong Zhang a, Limin Zhou a, Shaogang Hou a, Linsen Zhang b,⇑ a b

School of Chemical and Environmental Engineering, Anyang Institute of Technology, Anyang 455000, China School of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 6 December 2018 Revised 29 January 2019 Accepted 2 February 2019 Available online 4 February 2019 Keywords: Electrochemical exfoliation MoS2 Nanosheets Defect-rich Lithium storage

a b s t r a c t A facile and cost-effective method to prepare defect-rich MoS2 nanosheets is developed via an electrochemical exfoliation process. By using bucket-like metallic titanium mesh both as inert anode and container for MoS2 powders, defect-rich thin MoS2 nanosheets can be fast exfoliated from the bulk powders in aqueous sodium sulfate electrolyte under positive potentials. The as-obtained MoS2 nanosheets exhibit excellent cycling capability, high specific capacity, and superior rate performance for lithium storage. Furthermore, it provides a novel and effective method to exfoliate insulative or semiconductive layerstructured bulk powders into defect-rich 2D nanosheets. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction With the rapid demand for energy storage devices, numerous efforts have been dedicated to developing lithium-ion batteries (LIBs) in the recent decades, due to the superior capacity, reasonable cycling capability and environmental benignancy [1–3]. However, high rate performance is additionally required for its recent application in electric and/or hybrid vehicles. Novel structured ⇑ Corresponding author. E-mail address: [email protected] (L. Zhang). https://doi.org/10.1016/j.jcis.2019.02.007 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

electrodes are developed for this aim, and two-dimensional (2D) nanosheets with large-aspect ratio, such as graphene or graphene-analogous materials, are the recent emerging materials for these electrodes [4–10]. Thin MoS2 nanosheets possess more electrochemical active sites, and show more superior capability for lithium storage than the notable graphene and/or reduced graphene oxide (rGO) nanosheets, theoretical capacity of which could be up to 670 mAh g 1, and it is a promising material for the anode of LIBs [7– 15]. Besides, the 2D structure of MoS2 nanosheets favors to alleviate stress and lower energy barrier for ion’s intercalation [7,9].

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Numerous methods have been developed to fabricate 2D MoS2 nanosheets, such as liquid mechanical exfoliation [16–20], chemical vapor deposition (CVD) [21,22], physical vapor deposition [23], hydrothermal or solvothermal synthesis [24] and chemical exfoliation [25]. However, complicated process and/or low efficiency of these methods impede their further utilization in practice. Electrochemical intercalation or exfoliation is an efficient and mild strategy to prepare 2D nanosheets. Although cathodic intercalation and/or exfoliation could exfoliate MoS2 bulk powders into 2D nanosheets with high efficiency [26], Li+ or other cation ions’ intercalation would result in the destroyed pristine crystalline structure, moreover, expensive organic electrolyte is required and the intercalation is usually taken place under inert atmospheres for safety problems, these drawbacks restrict its wide application and further development. By contrast, anodic exfoliation is a simple and cost-effective method to fabricate 2D nanosheets. For instance, few-layered graphene nanosheets are exfoliated effectively from the graphite anode in aqueous sodium sulfate electrolyte without destroying the pristine crystalline, a high yield of 85% is obtained for the nanosheets less than 3 atomic layers’ thick [27]. Few-layered Bi2Se3 and Bi2Te3 nanosheets could also be exfoliated anodically from the corresponding bulk crystals [28]. It is reported that thin MoS2 nanosheets could be exfoliated anodically from the single-crystalline bulk MoS2 electrode [29], however, low pristine conductivity of MoS2 material results in extremely low exfoliating efficiency. It is still challengeable to exfoliate thin MoS2 nanosheets with this facile and moderate method. In this study, in order to improve the exfoliating efficiency, a bucket-like metallic titanium mesh encircled by a filter mesh is designed both as powders’ holder and inert anode for electrochemical exfoliation for the first time, the schematic diagram of which is presented in Fig. 1. By introducing commercial MoS2 powders into the bucket-like titanium mesh, thin MoS2 nanosheets rich in defects are anodically exfoliated from the bulk powders in aqueous sodium sulfate electrolyte, without using the expensive singlecrystalline bulk MoS2 electrode. With the as-prepared defect-rich thin MoS2 nanosheets as anode of LIBs, the nanosheets depict high specific capacity, superior rate performance and excellent cycling performance. Furthermore, it provides a safe, simple and effective method to exfoliate insulative or semiconductive layer-structured materials into thin nanosheets.

2. Experimental 2.1. Preparation MoS2 nanosheets were exfoliated electrochemically from the bulk powders by the following procedures: (1) commercially available MoS2 powders (Tianjin Guangfu Fine Chemical Research Institute) were initially suspended in 1 M sodium sulfate aqueous solution by ultrasonication for about 60 mins, (2) the formed suspension was then poured slowly and carefully into a bucket-like titanium mesh whose wall and bottom were encircled tightly by a bag of polypropylene membrane, the titanium mesh used was 100 mesh and the polypropylene membrane was 125,000 mesh with a bore diameter ca. 0.1 lm, (3) afterwards, the bucked-like titanium mesh containing MoS2 bulk powders was immersed into 1 M sodium sulfate aqueous electrolyte, (4) as schemed in Fig. 1, with the titanium mesh as positive electrode and nickel plate as negative electrode respectively, electrochemical exfoliation of MoS2 bulk powders was proceeded in aqueous sodium sulfate electrolyte under a constant voltage of ca. 20.0 V at room temperature for ca. 4 h, (5) at last, the obtained MoS2 powders was further treated by tip ultrasonication for ca. 2 h. MoS2 nanosheets were collected by vacuum filtration and washed with de-ionized water for several times. Few-layered MoS2 nanosheets were separated from the thick ones by centrifugation with a rotating rate of 3000 rpm in isopropanol solution for 10 min. Few-layered MoS2 nansoheets were obtained by the next step of freeze drying. 2.2. Characterization The micrographs were taken a scanning electronic microscope (SEM) (Zeiss MERLIN Compact) and a transmission electronic microscope (TEM) (JEOL 2100F). The XRD patterns were taken on an X-ray diffractometer (Rigaku D/max-2500PC) by using Cu Ka radiation. 2.3. Electrochemical characterization Electrochemical tests were conducted by using standard CR2032 type coin cells. After mixing the exfoliated few-layered

Fig. 1. Schematic illustration for electrochemical exfoliation of defect-rich MoS2 nanosheets.

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MoS2 nanosheets, acetylene black and poly(vinyl difluoride) (PVDF) in a weight ratio of 80:10:10, the mixed paste was evenly coated onto a Cu foil, and dried at 80 °C under vacuum for ca. 14 h. The working electrodes of the half cells were fabricated by this procedure. The half-cells were assembled in an argon-filled glove box with pure lithium foil as positive electrode, polypropylene as the separator, and a solution of 1 M LiPF6 in ethylene carbonate/ ethyl methyl carbonate/diethyl carbonate (EC/EMC/DEC vol. 1:1:1) containing 1 vol% vinylene carbonate (VC) as the electrolyte. Galvanostatical discharge-charge performances were tested on Land CT2001A system at different current densities in a voltage range of 0.05–3.0 V vs. Li+/Li. Cyclic voltammograms (CV) curves were measured at scan rate of 0.5 mV s 1 in the voltage range of 3–0.05 V (vs. Li+/Li) by using a Solartron 1287 potentiostat.

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formed in the nanosheets. In order to further clarify the microstructure of the nanosheets, HRTEM is performed as displayed in Fig. 2(c) and (d). Notably, it is observed that many dark parts are appeared on the nanosheets, as marked by the red circles, which may be etched by the highly active intermediates or free radicals during the electrolysis of water. These dark parts in the nanosheets could be thought as defects, which are totally different from the defects caused by enriched sulfur atoms [34,35]. The cross section of MoS2 could also be observed in Fig. 2(d), it is clearly found that the nanosheet contains 9 S-Mo-S layers with an interlayer spacing of ca. 0.60 nm, about 4.8 nm in thickness. These results depict that defect-rich thin nanosheets could be exfoliated anodically by this mild and simple method.

3.2. XRD characterizations 3. Results and discussion 3.1. Microstructure of the MoS2 nanosheets Fig. 2(a) shows the SEM micrograph of the as-obtained exfoliated samples. It is found that the samples are mainly composed of rigid thin nanosheets, with a lateral size of tens of nanometers to several micrometers, which are different to the ones fabricated by hydrothermal synthesis [30–32], and similar to the ones prepared by liquid ultrasonic exfoliation [16,33]. Fig. 2(b) displays the TEM micrograph of the exfoliated sample. It is found in Fig. 2 (b) that the exfoliated MoS2 nanosheets are very thin, and a very uneven surface can be observed, it implies that defects may be

Fig. 3 depicts the XRD patterns of the as-exfoliated MoS2 nanosheets and the commercial bulk MoS2 powders. It is found that peaks of the exfoliated nanosheets are matched well with the starting materials of MoS2 bulk powders, which means that the exfoliated nanosheets are MoS2 with 2H phase (JCPDS# 371492) and no phase transformation is occurred during the electrochemical exfoliation, which is different from the nanosheets prepared by cathodic lithium-intercalation [26]. Furthermore, it is found that the peaks of the MoS2 nanosheets are widened notably, it suggests that the bulk powders are exfoliated into smaller sheets and/or smaller powders, which agrees well with the results directly from micrographs as described above.

Fig. 2. SEM (a), TEM (b) and HRTEM (c,d) micrographs of the as-obtained electrochemically exfoliated MoS2 nanosheets.

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8000

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6000

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3.3. Electrochemical performance Fig. 4(a) displays the CV curves of the electrochemically exfoliated MoS2 nanosheets used as anode for lithium storage. It is found that the CV curves are agreed with the previously reported MoS2based anode in general [30,36–41]. For the first CV cycle, two pronounced cathodic peaks are formed at ca. 0.96 V and 0.32 V, which are ascribed to the formation of LixMoS2 and its further reduction to Li2S and Mo respectively [36–38], the anodic peak appeared at about 2.43 V is corresponded to the oxidation of Li2S to S [39]. For the 2nd cycle, a new cathodic peak is emerged at ca. 1.80 V in addition to the weak peak at 0.98 V, it indicates the multi-step mechanism for Li ions’ intercalation [30,40,41]. The 3rd to 5th CV cycles reflect the overall reversible lithiation and delithiation reaction: MoS2 + 4Li M Mo + 2Li2S [36,37]. Fig. 4(b) depicts the charge-discharge profiles of the electrochemically exfoliated MoS2 nanosheets, and the profiles were measured under the constant current density of 0.1 A g 1. For the 1st discharge profile, two plateaus located in the voltage ranges of ca. 1.17–1.06 V and ca. 0.62–0.51 V could be observed, which are attributed to the intercalation of the Li into the interlayers of MoS2 and the conversion of MoS2 to Li2S and Mo respectively [38]. For the 1st charge profile, a plateau between ca. 2.18 and 2.4 V could be found, which agrees well with the cathodic peak in the first CV curve. The discharge and charge profiles of the 5th and the 10th cycles are almost overlapped, suggesting the superior reversibility of the as-obtained MoS2 nanosheets. Fig. 4(c) displays the rate performance of the as-exfoliated MoS2 nanosheets and the commercially available MoS2 bulk powders under different current densities. The specific capacity of the MoS2 nanosheets is 839.2 mAh g 1 under the current densities of 0.1 A g 1 after 10 charge-discharge cycles, and the reversible capacities are 988.1, 847, 755.3, and 610.2, 425.3 and 313 mAh g 1 under the current densities of 0.2, 0.5, 1, 2, 5 and 10 A g 1 respectively. For comparison, MoS2 bulk powders are tested in the same way, and the reversible capacities of 618, 463.8, 298.1, 168.6, 75.3, 20, 11 mAh g 1 are obtained for the similar current densities of 0.1, 0.2, 0.5, 1, 2, 5 and 10 A g 1. It is found that the as-obtained MoS2 nanosheets display much more superior capacity than MoS2 bulk powders for lithium storage, especially at higher current densities above 1 A g 1. The as-exfoliated MoS2 nanosheets also display higher specific capacity than the MoS2 nanosheets synthesized by hydrothermal method [30–32,37], liquid ultrasonic exfoliation [33], or chemical lithium-intercalation [36,42]. It is mainly ascribed to the more active sites for the defect-rich MoS2

nanosheets, which is consistent with the HRTEM results as discussed above. It depicts that the exfoliated MoS2 nanosheets possess the merits of high specific capacity and superior rate performance for lithium storage. It is noteworthy that the capacities at the current density of 0.2 A g 1 are higher than those at the current density of 0.1 A g 1, and the capacity of the MoS2 nanosheets is recovered and further increased from the initial capacity of 839.2 to 899 mAh g 1, when the current density is recovered from 10 A g 1 to 0.1 A g 1. It is mainly caused by the electrochemical activation of MoS2 nanosheets during the lithium intercalation and/or deintercalation cycles [38,40]. Fig. 4(d) exhibits the cycling capability of the electrochemically exfoliated MoS2 nanosheets and the MoS2 bulk powders evaluated at a current density of 1 A g 1. For the as-prepared MoS2 nanosheets, the specific capacity of which is reduced from 762.6 to 715.7 mAh g 1 gradually in the initial 14 charge-discharge cycles, which may be ascribed to the formation of the solidelectrolyte interface (SEI) and/or gel-like polymeric layer through the decomposition of the electrolyte [38,42,43], and then increase gradually from 715.7 to 1348.9 mAh g 1 afterwards until to the 500th cycle, ca. 1.88 times are enhanced for the specific capacity. The trend is different to the MoS2 nanosheets fabricated by hydrothermal, liquid ultrasonic, or chemical lithium-intercalating method, whose specific capacities are usually decreased by increasing the cycling number at the current density of 1 A g 1 [30–33,36,37,42], and is similar to the MoS2-mesoporous carbon composite [40]. The enhanced capacity depicts that the exfoliated MoS2 nanosheets, either the surfaces and/or the cross sections, are activated by lithium intercalation and/or deintercalation during the cycling charge-discharge process [38,40], which argues well with the results as discussed for Fig. 4(c). For comparison, the cycling performance of MoS2 bulk powders is also tested, as indicated in Fig. 4(d), the specific capacities of the bulk powders drop significantly from 349.5 to 102.5 mAh g 1 (70.7% reduction) in the initial 200 charge-discharge cycles, and keep almost unchanged afterwards, it is also found that the capacities of MoS2 bulk powders are much lower than that of the as-obtained MoS2 nanosheets during the whole 500 charge-discharge cycles, e.g., the bulk powders’ specific capacity is only about 8.4% of the as-prepared MoS2 nanosheets’ at the 500th cycle. The cycling capability and the coulombic efficiency (CE) of the exfoliated MoS2 nanosheets is tested under a very high current density of 10 A g 1 as well, as displayed in Fig. 4(e). The specific capacity is initially decreased and then increased gradually to 621.7 mAh g 1 during the first 65 cycles, afterwards, the capacity fluctuates between 610 and 640 mAh g 1, and is kept stable in general until to the 2000th cycle, close to the capacity of the composite of MoS2 nanoplates embedded in the carbon fibers [13]. It is also found that the CE is kept stable at about 100% during the 2000 cycles. These results indicate the excellent cycling performance, high specific capacity and superior rate performance of the electrochemically exfoliated MoS2 nanosheets for lithium storage.

4. Conclusions In conclusion, defect-rich MoS2 nanosheets could be exfoliated electrochemically by using a self-designed bucket-like titanium mesh encircled tightly by a polypropylene membrane as inert anode for holding MoS2 bulk powders. The exfoliated defect-rich MoS2 nanosheets display excellent cycling capability, superior rate performance and high capacity for lithium storage, and may show more superior performance by compositing with other conductive materials. Furthermore, this approach provides a novel and effective avenue to exfoliate insulative or semiconductive layerstructured materials into thin nanosheets.

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