CMK-5 nanocomposite for rechargeable

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In situ synthesis of WSe2@CMK-5 nanocomposite for rechargeable lithium-ion batteries with a long-term cycling stability Jianbiao Wang, Lin Chen, Lingxing Zeng, Qiao-Hua Wei, and Mingdeng Wei ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03694 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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ACS Sustainable Chemistry & Engineering

In situ synthesis of WSe2/CMK-5 nanocomposite for rechargeable lithium-ion batteries with a long-term cycling stability Jianbiao Wang,a,b Lin Chen,a,b Lingxing Zeng,c Qiaohua Wei,b Mingdeng Wei a, b * a

State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Gongye Road, Gulou district, Fuzhou 350002, P. R. China

b

Institute of Advanced Energy Materials, Fuzhou University, Gongye Road, Gulou district, Fuzhou 350002, P. R. China c

College of Environment Science and Engineering, Fujian Normal University, 8 Shangsan Road, Changsan district, Fuzhou, Fujian 350007, China. *Corresponding Author. E-mail: [email protected]; Tel/Fax: 86 591 83753180

ABSTRACT Transition metal dichalcogenides (TMDs) have received intensive interests in lithium-ion batteries owing to their unique lithium ion storage ability when evaluated as anode materials. In the present work, a nanocomposite of WSe2/CMK-5 was successfully fabricated via a nanocasting route, introducing the unique structure of mesoporous carbon (CMK-5) as a nanorecator. Benefiting from a synergetic effect of WSe2 nanosheets and mesoporous carbon, WSe2/CMK-5 hybrid electrode exhibited large reversible capacity, high rate performance and excellent long-term cycling stability. For instance, a specific capacity of 490 mA h g-1 can be remained even after

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600 cycles at a current density of 0.5 A g-1. KEYWORDS: WSe2/CMK-5 nanocomposite, anode, lithium-ion batteries, long-term cycling stability, electrochemical property INTRODUCTION To alleviate the escalating severe energy crisis, rechargeable lithium-ion batteries (LIBs) has widely been considered as one of the dominant power sources for portable electronic devices and electronic vehicles (EVs), depending on their high energy density, long cycling life and environmental benignity.1-4 Nevertheless, graphite, as a commercial anode material, can’t meet the demands in electric devices. Therefore, it is a pressing need to explore new electrode materials. In recent years, two-dimensional (2D) layered transition metal dichalcogenide materials (TMDs) MX2 (M＝Mo, W, X＝S, Se) have extensively been investigated and employed in the field of energy storage due to their excellent electrochemical performance and unique structure.5-10 WSe2, as a vital member of TMDs, has attracted considerable interests owing to its unique properties (an ultralow thermal conductivities 0.05 W m-1 K-1, smaller band gap of 1.6 eV, and highly hydrophobic sticky surfaces) which was used in many fields as a functional material.11-18 Similar to MoSe2, the adjacent layers spacing of WSe2 is 0.648 nm which is about two times larger than the interlayer space of graphite (0.335 nm), valuably facilitating reversible ions (Li+, Na+ and Mg+) intercalation/extraction. However, since the poor electrical conductivity and volume change during the electrochemical reaction process, WSe2 suffers from rapid degradation performance in energy storage devices. Significant efforts have attempted


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to tackle the above-mentioned issues, including the fabrication of WSe2 nanowires and nanosheets etc.19,20 Although these methods have been proved to be effective, it is still a big challenge to enhance the long-term cycling performance of WSe2-based electrode materials through the compositional and structural engineering. Therefore, a facile and scalable fabrication strategy is imperative for preparing the long-term cycling performance of WSe2-based electrode materials in LIBs. Recently, ordered mesoporous carbon has been widely introduced to construct hybrid electrode materials in the field of energy storage and conversion,21-39 due to its distinguish properties such as large surface area, high electric conductivity and so on. Herein, we take advantage of the CMK-5, a highly ordered mesoporous carbon, as a nanoreactor to confine the growth of WSe2 nanosheets within their mesochannels through a nanocasting route. As a result, WSe2/CMK-5 nanocomposite manifests exceptional electrochemical performance when evaluated as an anode for LIBs, including large reversible capacity, superior high-rate capability, and outstanding long-term cycling stability, suggesting a strong synergetic effect between the CMK-5 and WSe2 nanoparticles. To the best of our knowledge, the reports on the hybrid of WSe2 and mesoporous carbon for LIBs have not been concerned, and make WSe2 a promising potential anode material for wide application in energy storage. EXPERIMENTAL Synthesis of samples The scheme of the synthesized procedure for WSe2/CMK-5 is displayed in Fig. S1 (Supporting information).43,44 CMK-5, a highly ordered crystalline mesoporous



carbon, was used as a nanoreactor, which was synthesized by using SBA-15 silica as a hard template and furfuryl alcohol as a carbon source.40,41 As for the synthesis procedure of SBA-15, it is analogous to the previous reported literature.42,45,46 Selenium powder and phosphotungstic acid (PTA) are regarded as Se and W sources, respectively. The detail synthetic procedure of WSe2/CMK-5 nanocomposite was described as followings: firstly, 40 mg of CMK-5 powder was treated using a concentrated nitric acid at 60 oC for 40 min in order to increase its hydrophilicity, and then washed with deionized water until the pH comes to almost neutral. Secondly, 60 mg of PTA was dissolved in 10 ml of deionized water, and subsequently the treated CMK-5 was dispersed to the solvent under ultrasonication for 1 h to make PTA easily encapsulated into the channels of CMK-5. The following step was to let the mixture vigorously stirred at room temperature until water was evaporated to form an intermediate of PTA/CMK-5. Finally, the intermediate and Se powder were transferred to a tube furnace and calcined at 600 oC for 6 h under the atmosphere of 5% H2/95% Ar flow, the hydrogen gas was first reacted with selenium powder to form H2Se and then in situ converted to the final product (WSe2/CMK-5). In comparison/n, the bulk WSe2 was also synthesized under the same condition except the matrix of CMK-5. Characterization of samples To characterize the characteristic of the composite of WSe2/CMK-5, X-ray diffraction (XRD) patterns were recorded on a Rigaku Ultima IV with the Cu Ka radiation (1.5418 A) to analyze the crystal phase properties of the samples. Scanning electron


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microscopy (SEM) was conducted on a Hitachi 4800 instrument and transmission electron microscopy (TEM) was taken on a FEI F20 S-TWIN instrument to obtain the morphology of the obtained products. X-ray photoelectron spectroscopy (XPS) was implemented on a Thermo Scientific ESCALAB 250 instrument to analyze the chemical and oxidation states of W and Se in the hybrid material within the region of 0-800 eV. N2 adsorption–desorption analysis was carried out on a Micromeritics ASAP 2020 instrument to collect pore volumes and pore size distributions. Thermogravimetric analysis (TGA) was performed using a CHNS/O analyzer (PE 2400II, Perkin-Elmer, America) in air atmosphere to determine the actual amount of carbon in the nanocomposite. Electrochemical measurements WSe2, CMK-5, and WSe2/CMK-5 nanocomposite were prepared with super carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 80:10:10 and subsequently diluted with N-methylpyrrolidone (NMP), respectively. Then, the obtained final homogeneous slurry was separately casted on the copper foil as the working electrodes, and dried at 110 oC overnight in the vacuum oven. The coin cells were assembled in the glove box at the atmosphere of pure argon, using a metallic lithium foil as the counter and reference electrode, LiPF6 (1 M) as the electrolyte consisted with EC, EMC and DMC with a weight ratio of 1:1:1, the microporous polypropylene membrane ( Celgard 2400 ) as the separator. The electrochemical workstation (Land CT 2001A, Wuhan, China) was applied to collect galvanostatic charge–discharge curves and cycling performance within the voltage region between



0.01 and 3.0 V (vs. Li+/Li). The specific capacity for WSe2/CMK-5 composite was calculated on the basis of the total mass of WSe2 and CMK-5 as a whole. Cyclic voltammomrams (CV) were performed on an electrochemical workstation (Chenhua CHI660c, Shanghai, China) at a sweep rate of 0.5 mV s-1 in the voltage range from 0.01 to 3 V. The electrochemical workstation of IM6 (Zahner Elektrik, Germany) was used to test electrochemical impedance spectra (EIS), to further confirm the excellent performance of the WSe2/CMK-5 nanocomposite, compared with WSe2 and CMK-5, the electrochemical performances of CMK-5 and WSe2 were also tested under the same condition. RESULTS AND DISCUSSION The wide angle powder X-ray diffraction patterns of all samples are depicted in Fig.1.

Fig. 1 XRD patterns of bulk WSe2, WSe2/CMK-5 and CMK-5 nanocomposite, respectively. Fig. 1 shows that all diffraction peaks of WSe2 and WSe2/CMK-5 nanocomposite (within the range from 10 to 80o) could respectively be assigned to (002), (100), (103),


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(105), (110), (200) planes of the hexagonal phase of WSe2 (JCPDS: 89-5257).. Compared with the pristine WSe2, the diffractions of the WSe2/CMK-5 nanocomposite show weaker intensity, which may be attributed to the confine effect of the tunnels of CMK-5,21,47 besides, there is no obvious peak appeared in the range from 20o to 30o, indicating the few carbon in the nanocomposite.

Figure. 2 (a, b) SEM images and (c, d) TEM images of WSe2/CMK-5 nanocomposite, and (e) the corresponding elemental mapping results for Se (green), W ( yellow) and C (red). As depicted in Fig.2 (a-b), SEM images of WSe2/CMK-5 nanocomposite still preserves its rod like morphology, compared with pristine CMK-5 shape (FigS3). And few nanosheets can be observed on the surface of mesoporous carbon. In Fig.2(c-d), highly ordered hexagonal arrangement of mesochannels and the WSe2 nanoplates confined in the mesochannels of CMK-5 can be observed. In detail, it can be found that the lattice fringe is 0.678 nm in Fig.2c, which corresponds to the d002



spacing of WSe2 and may be attributed to the twisted and nanosized WSe2 crystal structure or the strain from the layer curvature.8,48 Also, Fig.2d shows the layers of WSe2 was reduced within a few layers (

CMK-5 nanocomposite for rechargeable

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