Construction of reduced graphene oxide supported molybdenum carbides composite electrode as high-performance anode materials for lithium ion batteries

Construction of reduced graphene oxide supported molybdenum carbides composite electrode as high-performance anode materials for lithium ion batteries

Materials Research Bulletin 73 (2016) 459–464 Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier.c...

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Materials Research Bulletin 73 (2016) 459–464

Contents lists available at ScienceDirect

Materials Research Bulletin journal homepage:

Short communication

Construction of reduced graphene oxide supported molybdenum carbides composite electrode as high-performance anode materials for lithium ion batteries Minghua Chena , Jiawei Zhanga , Qingguo Chena,* , Meili Qia , Xinhui Xiab,* a Key Laboratory of Engineering Dielectric and Applications (Ministry of Education), and School of Applied Science, Harbin University of Science and Technology, Harbin 150080, PR China b State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, and School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China



Article history: Received 25 August 2015 Received in revised form 25 September 2015 Accepted 28 September 2015 Available online 9 October 2015

Metal carbides are emerging as promising anodes for advanced lithium ion batteries (LIBs). Herein we report reduced graphene oxide (RGO) supported molybdenum carbides (Mo2C) integrated electrode by the combination of solution and carbothermal methods. In the designed integrated electrode, Mo2C nanoparticles are uniformly dispersed among graphene nanosheets, forming a unique sheet-on-sheet integrated nanostructure. As anode of LIBs, the as-prepared Mo2C-RGO integrated electrode exhibits noticeable electrochemical performances with a high reversible capacity of 850 mAh g1 at 100 mA g1, and 456 mAh g1 at 1000 mA g1, respectively. Moreover, the Mo2C-RGO integrated electrode shows excellent cycling life with a capacity of 98.6 % at 1000 mA g1 after 400 cycles. Our research may pave the way for construction of high-performance metal carbides anodes of LIBs. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Carbides Composites Nanostructures Energy storage Electrochemical properties

1. Introduction Lithium ion batteries (LIBs), the current “king” of batteries in the market, have been the research focus in the past decades because of their fascinating characteristics such as high working voltage, large energy density, low toxicity and long cycling life [1,2]. However, the LIBs technology still falls short of meeting the demands dictated by the large-volume and high-power of applications [3–5]. In parallel with the research of cathode, to perform best, great efforts are made to search for advanced anodes to replace the current graphite anode with a relative low theoretical specific capacity (372 mAh g1) [6]. In recent years, transition metal carbides (such as molybdenum carbides, Mo2C) are emerging as new kinds of anode materials for LIBs. Among the explored metal carbides candidates, Mo2C is considered as one of the most promising anodes due to its high reversible capacity arising from the reaction: Mo2C + xLi+ + xe ! 3Mo + LixC, and high electrical conductivity (102 S cm1), which is highly favorable for high-rate capability. Currently, porous nanostructured Mo2C with a high surface area and enhanced electrochemical reactivity, is particularly attractive. However, the electrochemical performance

* Corresponding author. Tel.: +86 451 86390778; fax: +86 451 86390779. E-mail addresses: [email protected] (Q. Chen), [email protected] (X. Xia). 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.

of Mo2C is always compromised in the powder formed materials for LIBs because of the extra fabrication process of electrode. The active Mo2C powder materials need to be mixed with polymer binders and additives and processed into electrode pellets. This process introduces supplementary and undesirable interfaces, and increase inner resistance. Therefore, integrated binder-free nanostructured Mo2C electrode is an important issue for their applications. For the integrated electrode design, the active Mo2C materials are growth on current collectors directly. This electrode design ensures good mechanical adhesion and electric connection of the active material to the current collector. Meanwhile, no extra preparation process of electrode and avoid undesirable supplementary interfaces. To further improve the electrochemical performance, integrated Mo2C-based composite electrode design strategy is the new research focus. This hybrid strategy is to combine MoS2 with conductive matrix or coating (such as CNT and graphene). This approach provides better and faster electron transfer path required by high-rate application by introducing highly conductive media [7,8]. Zhu et al. reported integrated molybdenum carbide/ graphitic carbon nanosheets electrode with enhanced capacity and rate capability [9]. Despite some progress achieved in the Mo2C-based electrode, there are no report about fabrication of Mo2C/reduced graphene oxide integrated electrodes and their application in LIBs. In this


M. Chen et al. / Materials Research Bulletin 73 (2016) 459–464

paper, we report reduced graphene oxide (RGO) supported molybdenum carbides (Mo2C) integrated electrode by the combination of solvothermal and carbothermal methods. As anodes of LIBs, the Mo2C-RGO electrode shows impressive electrochemical performances with high capacity and excellent high-rate cycling stability, which are attributed to the synergistic effect between well-dispersed molybdenum carbides and reduced graphene oxide.

2. Experimental 2.1. Preparation of graphene oxide (GO) GO was prepared from natural flake graphite powder according to an improved Hummers method [10]. Typically, 5 g of graphite was added into breaker including 300 ml concentrated H2SO4 and keep stirring at an ice bath. Afterwards, 10 g of KMnO4 was then

Fig. 1. (a) Schematic illustration for preparation of Mo2C nanoparticles dispersed on reduced graphene oxide (Mo2C-RGO). SEM images of GO (b) and Mo2C-RGO (c–e). TEM (f) and HRTEM images (g) of Mo2C-RGO. The insets in Fig. 1g are SAED patterns of nanoparticle.

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added into the above solution gradually under continuous stirring. The mixture was then kept at 90  C and further stirred for 24 h, washed with 5 wt.% HCl and DI water, and freeze-dried for further use. 2.2. Preparation of Mo2C-RGO 0.3 mmol (NH4)6Mo7O244H2O was added into a breaker containing 50 ml DI water and keep ultrasonic for 0.5 h. The mixture was stirred for another 1 h after addition of 5 mmol urea. Afterwards, the aqueous solution (100 mg GO dispersed in 50 ml DI water) was slowly added into the above solution and kept stirring for 12 h at 80  C. Then, the as-prepared solution was freeze-dried, collected and grinded. Finally, the products were pyrolyzed at 800  C in an Ar/H2 atmosphere at flow rate of 100 sccm for 2 h to form Mo2C-RGO hybrid sample. The weight ratio of RGO and Mo2C is around 1:2.5. 2.3. Characterization The morphologies of sample were collected by field-emission scanning electron microscope (FE-SEM JEOL JSM-6700F; JEOL, Tokyo, Japan) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010F) operating at 200 kV. The Raman spectra were recorded on a WITEC-CRM200 Raman system (WITEC, Germany), using 532 nm laser (2.33 eV) as the excitation source. X-ray diffraction (XRD) patterns of the samples were recorded by a Bruker D8 ADVANCE XRD with Cu Ka radiation in the range of 10 to 80 . The X-ray photoelectron spectroscopy (XPS) was employed on a VG ESCALAB 250 spectrometer (Thermo Electron, UK), using Al Ka X-ray source (1486 eV) as the excitation source. 2.4. Battery fabrication and electrochemical measurements The electrochemical measurements were employed on standard CR2032-type coin cells. The samples were prepared by mixing as fabricated Mo2C-RGO with carbon black (super P) as the conductive agent and polyvinylidene fluoride (PVDF) in a weight ratio of 80:10:10 dissolved in N-methyl-2-pyrrolidone (NMP) as the binder. Then, the slurry was then coated onto a copper foil, which the pellets of 12 mm in diameter as the working electrode. The cells were assembled in an argon-filled glove box (Mbraun, Unilab, Germany) by using lithium metal circular foil (0.59 mm thick) was used as the counter-electrode. The separator was polypropylene (PP) micro-porous film (Cellgard 2400), 1 M LiPF6 in


ethylene carbonate (EC) and dimethyl carbonate (DME) (1:1 by volume) as the electrolyte. The mass loading of Mo2C/RGO hybrid in the electrode is 1.3 mg/cm2. The cyclic voltammetry (CV) test was scanned at a scan rate of 0.5 mV s1 between 0 and 3 V using an electrochemical workstation (CHI1760D). Galvanostatic charge/ discharge cycles were conducted on Neware battery tester (0.01 V– 3.0 V vs Li/Li+) at different current densities at room temperature. The cyclic voltammetry was scanned at 0.1 mV s1 using an electrochemistry system (CHI660E). The galvanostatic charge/ discharge tests were conducted on a LAND battery programcontrol test system at room temperature. 3. Results and discussion The two-step fabrication process of Mo2C-RGO integrated electrode is schematically illustrated in Fig. 1a. SEM (Fig. 2b–e) and TEM (Fig. 1f and g) images of Mo2C-RGO reveal the uniform distribution of nanopraticles with size of 50 nm on RGO sheets. As shown in Fig. 2g, the lattice space of 0.21 nm corresponding to Mo2C (1 0 1) can be observed by HRTEM. This is confirmed by Fast Fourier transform (FFT) and selected area electron diffraction (SAED) patterns (see inset in Fig. 1g). The phase and composition of the samples are further monitored by XRD pattern and Raman spectroscopy. XRD patterns of Mo2C-RGO sample are shown in Fig. 2a. It is seen that the broad diffraction peak located at about 26 , corresponding to the peaks of RGO (0 0 2). The strong peaks at 34.4 , 38.0 , 39.4 , 52.1, 61.5 , 69.5 , 74.6 and 75.5 are indexed well as (1 0 0), (0 0 2), (1 0 1), (1 0 2), (11 0), (1 0 3), (11 2), (2 0 1) of Mo2C (JCPDS card no. 350787) and no other purity peaks are observed [11]. Raman spectrum of the Mo2C-RGO is shown in Fig. 2b. Two obvious peaks D band at 1350 and G band at 1590 cm1 are noticed, corresponding to the disordered graphitic carbon and the Eg vibration of the sp2-bonded carbon atoms, respectively [12]. The peak intensity ratios (ID/IG) for the Mo2C-RGO sample is about 1.5, which suggests that most graphene group formed on RGO sheets and oxygenated functions removed through annealing. The electrical conductivity of the RGO sheets could be enhanced, and the charge transfer would be facilitated, correspondingly. Meanwhile, the Raman peaks at 778 and 945 cm1 are matched well with the characteristic Raman spectrum of Mo2C phase. The XRD and Raman analysis is in consistent, indicating the formation of Mo2C-RGO electrode. Fig. 3 presents the XPS spectra of surface chemistry of C, O, and Mo elements. The data of C1s (Fig. 3b) exhibit three peaks at 284.5, 285.3 and 287.0 eV, corresponding to C¼C, C O and O¼C O

Fig. 2. XRD patterns (a) and Raman spectroscopy (b) of Mo2C-RGO electrode.


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Fig. 3. XPS spectra of (a) wide scan, (b) C 1s, (c) O 1s and (d) Mo 3d of Mo2C-RGO electrode.

respectively [13]. For the O1s spectrum (Fig. 3c), three peaks locating at 532.6, 533.3 and 534.6 eV at binding energy, which can be assigned to species C¼O/O¼C O, COH, [14] and MoO respectively. The peak of Mo3d XPS (Fig. 3d) is deconvoluted into two dominant peaks centered at 232.4 eV and 228.3 eV, corresponding to Mo5+/6+ and Mo2+, respectively, which is in agreement with previous data reported for molybdenum carbides [15,16]. The electrochemical performance of Mo2C-RGO integrated electrode is evaluated as anode of LIBs. CV curves of the Mo2C-RGO electrode at a scan rate of 0.5 mV s1 at the first five cycles are shown in Fig. 4a. It can be noticed there is a substantial difference between the first and the subsequent cycles. In the first discharge process, there exhibit a strong irreversible cathodic peak at around 1.65 V, which can be assigned to the decomposition of the electrolyte formation of solid electrolyte interphase (SEI) film on the surface of the electrode, as also elaborated in previous work [17]. In the subsequent cycles, the cathodic lithium insertion mainly occurs at 1.18 and 1.45 V, and the anodic lithium extraction occurs at 1.55 and 1.92 V, which may correspond to Li+ conversion or alloying. The simplified lithium storage mechanism of Mo2C reactions can be expressed as follows. þ

Mo2 C þ xLi þ xe ! 3Mo þ Lix C The CV curves in Fig. 4a for Mo2C-RGO electrode also show another pair of redox peaks located at 0.14 and 0.4 V, which can be assigned to the lithiation and dilithiation of RGO, respectively [18]. It is noted that this plateau position is the same to the previous graphene [19].

Fig. 4b shows the discharge and charge curves of the Mo2C-RGO electrodes for the 1st, 2nd, 10th, 100th and 400th cycles at a current density of 100 mA g1 within a cut-off voltage window of 0.01–3.0 V vs. Li/Li+, respectively. The initial discharge and charge capacities of the 1st cycle are found to be 1060 and 850 mAh g1, respectively, with a first initial coulombic efficiency of more than 80%. The followed sloping drop to low potentials corresponds to initial irreversible loss (e.g., the formation of SEI) [20–22]. There is a broad plateau around 1.25 V observed in the first discharge curve, followed by a continuous voltage sloping curve down to the cut voltage of 0.01 V, corresponding to the typical characteristics of voltage trends for the Mo2C. In the subsequent cycles, the plateaus shifts to higher voltage at 1.4 and 0.9 V, corresponding to the two right-shifted peaks in CV curves owing to the change of crystalline structure after the first cycle. Fig. 4c shows the cyclability of the Mo2C-RGO electrodes. The capacity profiles are nearly flat at discharging rates of 100 and 1000 mA g1. A specific reversible capacity of 856.4 mAh g1 is maintained at 100 mA g1 after 400 cycles, and 456.4 mAh g1 at 1000 mA g1 after 400 cycles, respectively. It exhibits good cycling stability. Fig. 4d shows the capabilities for Mo2C-RGO electrodes in the range of 100–1000 mA g1. Note that the Mo2C-RGO electrode shows good rate performance. Evidently, the capacity Mo2C-RGO electrode is as high as about 450 mAh g1, even upon increasing the current density by 10 times (1000 mA g1), which is comparable to the theoretical capacity of graphite. The good capacity retention and high-rate capability are attributed to its structure of Mo2C nanoparticles embedded in the graphene sheets, which can

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Fig. 4. Electrochemical performance of Mo2C-RGO electrodes: (a) CVs at a scan rate of 0.1 mV s1 between 0.01 and 3.0 V; (b) charge/discharge profiles at the 1st, 2nd, 10th, 100th and 400th cycles; (c) cycling performance at 100 mA g1 and 1000 mA g1; (d) Rate capability at different current densities ranging from 100 mA g1 to 1000 mA g1.

provide large specific surface area to facilitate the transportation of Li ion and electron, and keep stable by buffering the volume expansion. Moreover, the composite structure also can enhance the electrical conductivity for facilitating charge transfer. Therefore, even at high discharge rates, the Mo2C-RGO electrode can show excellent cycling performance.

Heilongjiang Province, China Postdoctoral Science Foundation (2015M570301), Natural Science Foundation (E2015064), Program for Innovative Research Team in University (2013TD008) and Postdoctoral Science Foundation (LBH-Z14120) of Heilongjiang Province of China. References

4. Conclusion In summary, reduced graphene oxide (RGO) supported molybdenum carbides (Mo2C) integrated electrode was reported in this work. The Mo2C nanoparticles with average size of about 50 nm are uniformly dispersed on the surface of RGO. As anode materials for LIBs, the Mo2C-RGO electrode exhibits remarkably enhanced excellent cycling stability, high reversible capacity of 850 mAh g1 after 400 charge/discharge cycles at 100 mA g1, and good rate capability (450 mAh g1 at at a high current rate of 1000 mA g1), which is attributed to the strong coupling effect between graphene and Mo2C nanoparticles. The structural design and fabrication of electrode materials are suggested as a favorable strategy toward the development of high performance electrode for lithium-ion batteries and fuel cells. Acknowledgements The authors acknowledge financial support from Natural Science Foundation of China (51502063,51502263), University Nursing Program for Young Scholars with Creative Talents in

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