Synthesis of MoO2 nanosheets by an ionic liquid route and its electrochemical properties

Journal of Alloys and Compounds 580 (2013) 358–362

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Synthesis of MoO2 nanosheets by an ionic liquid route and its electrochemical properties Huixing Zhang, Lingxing Zeng, Xiaomin Wu, Lifang Lian, Mingdeng Wei ⇑ Institute of Advanced Energy Materials, Fuzhou University, Fuzhou, Fujian 350002, China

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Article history: Received 4 May 2013 Received in revised form 8 June 2013 Accepted 15 June 2013 Available online 27 June 2013 Keywords: MoO2 nanosheets Ionic liquid Anode materials Lithium-ion batteries Electrochemical properties

a b s t r a c t MoO2 nanosheets were synthesized for the first time by a solvothermal route using an ionic liquid in the reaction system. The synthesized nanocomposites were characterized by XRD, FTIR, SEM, and TEM measurements. It was found that the ionic liquid composed of ethylene glycol and choline chloride (ChCl) acted as a solvent and a reactive reagent during the reaction process. Based on a series of experimental results, the formation mechanism of nanosheets was also discussed. Furthermore, MoO2 nanosheets were used as anode materials for Li-ion intercalation and exhibited large reversible capacity, high rate performance and good cycling stability. For instance, a high reversible capacity of 620 mA h g 1 can be remained after 20 cycles at 100 mA g 1. This might be contributed to the intrinsic characteristics of MoO2 nanosheets, which offered a shorter path for Li+ and electron transport, leading to the improved capacity and enhanced rate capability. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Recently, molybdenum dioxide (MoO2) has been extensively studied as an anode material in lithium-ion batteries (LIBs). MoO2 crystallizes mainly in three polymorphic forms: hexagonal phase (P63/mmc) [1], tetragonal phase (P42/mnm) [2] and monoclinic phase (P21) [3], of which monoclinic MoO2 has a large theoretical specific capacity of 838 mA h g 1 as an anode material for LIBs and low metallic electrical resistivity of 8.8  10 5 Xm in bulk samples. It was reported that the electrochemical performances of MoO2 nanostructures in LIB were greatly influenced by their morphologies and size, due to a huge and uneven volume variation during lithium-ion intercalation/deintercalation into/out of anode material, namely, electrode pulverization [4]. Thus, the development of new type MoO2 nanostructures for enhancing electrochemical performance is still a great challenge. To date, the well-defined MoO2 nanostructures, including nanowires, nanoparticles, nanorods, nanospheres and nanocrystals, have been successfully synthesized by several different synthetic approaches, such as electrospinning [5], solid reduction reaction [6], hydrothermal reaction and solvothermal route [7]. Generally, the size, shape and the crystallinity of the products can be controlled easily by the solvothermal route compared with the hydrothermal reaction. Recently, a lot of nanostructured materials, such

⇑ Corresponding author. Fax: +86 591 83753180. E-mail address: [email protected] (M. Wei). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.06.100

as ZnO [8], TiO2 [9], Co2O3 [10], metal [11], hematite [12], metal chalcogenide [13], CoPt [14], silica [15] and carbon materials [16] have been synthesized by using a solvothermal route, in which the ionic liquids (ILs) were used in the reaction system. ILs are salts in the liquid state which have a low melting point and vapor pressure. However, the commonly used ILs have complex structures which are difficult to be synthesized, resulting in the limitation of potential applications. In the present work, choline chloride– ethylene glycol ionic liquid with simple components was first applied in the synthesis of MoO2 nanosheets. Furthermore, the synthesized MoO2 nanosheets were used as an electrode material for lithium-ion intercalation. 2. Experimental 2.1. Synthesis and characterizations of MoO2 nanosheets The preparation of ionic liquid was described by Gutierrez et al. [17] Ethylene glycol (EG) and choline chloride (ChCl) were mixed in a molar ratio of 2, and then the mixture was heated in the oil bath at 120 °C for 12 h under stirring. After cooled to room temperature, a clear solution of ionic liquid (EG–ChCl IL) was finally obtained. The obtained IL was used as a precursor in the solvothermal reaction. Typically, 0.5 g of ammonium heptamolybdate (AHM) was dispersed in IL with different volumes (4, 6, 8 mL), to obtain three mixtures, and then these mixtures were transferred into a 50 mL Teflon-lined autoclave respectively and kept at 180 °C for 24 h. The precipitations with a brown color were collected and then washed with ethanol and water alternatively for several times, and dried at 70 °C for 6 h. After the precipitations were calcined at 700 °C in Ar atmosphere for 6 h, the finial products were obtained. For simplification, the MoO2 samples synthesized with 4, 6 and 8 mL of ionic liquid were indicated as MoO2-4, MoO2-6 and MoO2-8, respectively.

H. Zhang et al. / Journal of Alloys and Compounds 580 (2013) 358–362 X-ray powder diffraction (XRD) data were collected by using a PANalytical X’Pert Pro diffractometer (Co Ka radiation, Panalytical X’Celerator detector) and then transformed to Cu Ka data. The Raman spectra were recorded on a Renishaw in Via Laser Raman Co-focal Microspectrometry. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were taken on a Hitachi S4800 and a Tecnai G2 FEI F20S-TWIN, respectively. X-ray photoelectron spectroscopy (XPS) measurement was performed on a Lab250 spectrometer with monochromatic Al Ka as the X-ray source. All of the binding energies were calibrated by the C 1 s peak at 284.6 eV. Thermogravimetric analysis was carried out on a PerkinElmer thermo gravimetric analyzer with the heating rate of 10 °C min 1 in an air flow rate of 100 ml min 1. Fourier Transform infrared (FTIR) spectra were recorded on a PE SPECTRUM2000 using the KBr pellet technique.

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2.2. Electrochemical measurements For the electrochemical measurements, active material, polyvinylidene fluoride (PVDF powder as a binder) and acetylene back carbon (AB as the conductive assistant materials) were mixed with a ratio of 8:1:1. The mixture was spread and pressed on Cu circular sheets as the working electrode (WE), and dried at 120 °C for 24 h under vacuum condition. Metallic lithium foils were used as the negative electrodes. The electrolyte was 1 M LiPF6 in a 1:1:1 (volume ratio) mixture of ethylene carbonate (EC), ethylene methyl carbonate (EMC) and dimethyl carbonate (DMC). The separator was a UP 3093 (Japan) microporous polypropylene membrane. The cells were assembled in a glove box filled with highly pure argon gas (O2 and H2O levels <1 ppm), and charge/discharge tests were performed in the voltage range of 0.02–3 V vs Li/Li+ at different current densities on a Land CT 2001A automatic battery tester (Wuhan, China). Cyclic voltammetry (CV) measurements were performed on a CHI660C electrochemical workstation at a scan rate of 0.5 mV s–1 in the range of 0.02–3.0 V vs Li/Li+.

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Fig. 2. SEM images of MoO2 nanomaterials obtained using different amounts of ionic liquid: (a) MoO2-4, (b) MoO2-6 and (c) MoO2-8.

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3. Results and discussion Fig. 1 shows XRD patterns and Raman spectra of the samples without and with calcination. It can be seen from Fig. 1a that no pronounced peak was found except a broad peak at ca. 2h = 10° for all samples, indicating these precursors have a layered structure. As depicted in Fig. 1b, several sharp peaks were detected for the samples calcined at 700 °C for 6 h in Ar and all reflection peaks can be indexed to a monoclinic MoO2 phase (JCPDS 731249). As depicted in Fig. 1c, the peaks at 819 and 992 cm 1 were ascribed to MoO2, while two peaks at 1341 and 1586 cm 1 were assigned to C [18], indicating that the synthesized samples were the composites of MoO2 with C. Fig. 2 shows SEM images of three samples obtained using different amounts of IL precursor. It clearly shows that only irregular particles were observed for the sample MoO2-4, while MoO2-6 exhibited a sheet-like morphology, as depicted in Fig. 2b. After the IL amount was increased in the reaction system, a large number of nanosheets were formed for the sample MoO2-8, as shown in Fig. 2c. Fig. 3 shows TEM images of MoO2-8 nanosheets. It can be seen from Fig. 3a that these nanosheets tended to aggregate together. These nanosheets were very thin and the thickness was found to be ca. 10 nm, as depicted in Fig. 3b. A HRTEM image in Fig. 3c indicates that the surface of nanosheets was coated with a thin layer of carbon. It is also found that these nansheets were highly crystalline and the lattice fringe was found to be ca. 0.343 nm, corresponding to a d011-spacing in the XRD pattern of monoclinic MoO2. The composition and oxidation states of MoO2-8 nanosheets were investigated by XPS measurement. As shown in Fig. 4a, the elements of Mo, O and C in the sample were detected. Two weak peaks of Cl 2p1/2 and 2p3/2 in Fig. 4b indicate the existence of small amount of Cl , while Cl peaks can hardly be detected in the widerange XPS in Fig. 4a. The residue Cl might be ascribed to the incomplete washing. As presented in Fig. 4c, the peak at 232.2 eV is attributed to Mo6+, while another one at 235.2 eV is due to Mo4+ [19]. The result indicates that the precursor might be partially reduced during the solvothermal reaction process. Meanwhile, C1s peaks in Fig. 4d indicate that the co-existence of carbon in the precursor. FTIR spectrum of MoO2-8 precursor is shown in Fig. 5. The broad peak at ca. 3300–3500 cm 1, and the peaks at 1085 and 1040 cm 1 could be assigned to OH or C–C–O stretching vibrations associated with the N–C2H4–OH group in choline cation [20,21]. The peaks at 2930, 2875 and 1490 cm 1 were associated with the CH2 stretching and bending vibrations [17], whereas the peak at 1625 cm 1 was the stretch vibration of C@O group [22]. As a result of hydrogen bond formed between O–H and Cl , the mC–O of ChCl was shifted to lower wavenumber of 915 cm 1, which often occurred in non-aqueous solutions [21]. The peak at 1000 cm 1 was due to Mo–O vibration mode and other peaks at 750, 800 and 1000 cm–1 might be assigned to Mo–O vibration mode [23,24]. Based on the results mentioned above, it can be concluded that the ionic liquid acted as a solvent as well as a reactive reagent in the formation of MoO2 nanosheets. Firstly, AHM which is non-soluble in EG–ChCl, reacted with EG–ChCl in the solvothermal conditions and a layered intermediate product ((NH4)2O)x
MoO3
yH2O [25] was formed. At high temperature, Mo@O can be reduced by OH groups of EG or ChCl, which might be responsible for the oxidation states variation from 6+ to 4+ [19]. Secondly, the layered structure intermediate was exfoliated by intercalation of amine salts R4N+, resulted in the formation of sheet-like precursor. [26– 27]. When heated at 700 °C in Ar atmosphere, the precursor decomposed and MoO2 nanoparticles together with amorphous carbon were formed simultaneously. Thus, it was deduced that

the ionic liquid with the OH and R4N+ groups might play an important role in the formation of MoO2 nanosheets. To evaluate the electrochemical performances, the cycling stability, charge–discharge profile (voltage/capacity curves) and CV curves were measured. As shown in Fig. 6a the battery made

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Fig. 3. TEM images of MoO2-8 nanosheets. Inset: the corresponding selected area electron diffraction (SAED) pattern.

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Fig. 4. XPS spectra for the MoO2-8: (a) survey spectrum and high-resolution, (b) Cl 2p, (c) Mo 3d, and (d) C 1 s spectra.

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of MoO2-8 nanosheets exhibited a steady specific capacity of 620 mA h g 1 at a current density of 100 mA g 1 in the voltage range of 0.02–3.0 V for 20 cycles, while the specific capacity of MoO2-4 and MoO2-6 quickly faded to less than 200 mA h g 1. Obviously, the specific capacity retention and cycling performance of the MoO2-8 nanosheets were significantly prior to MoO2-4 and MoO2-6 nanomaterials. This might be attributed to the facts that

the distance for the transfer of Li+ ion and electron is shorter in the electrode composed of nanosheets, and that the carbon layer on the surface of nanosheets could improve the electrical conductivity and buffer the volume change during the insertion and deinsertion of Li+. Fig. 6b depicts the charge–discharge profiles of MoO2-8 nanosheets for the 1st, 2nd, 4th, and 10th cycle at a current density of 100 mA g 1. The discharge profiles between 5th and 10th cycle were overlapped, indicating the excellent reversible performance. The electrochemical behaviors of MoO2-8 nanosheets were also characterized by CV measurement at a scanning rate of 0.5 mV s 1 in the voltage range of 0.02–3.0 V, as shown in Fig. 6c. Two pairs of peaks (cathodic/anodic) appeared at the potentials (V) of (1.25/1.50) and (1.51/1.75), representing two stages of reversible electrochemical lithium intercalation/deintercalation into/out of the electrode and a transition between monoclinic phase and orthogonal phase in the partially lithiated oxide LixMoO2 [28–30]. The reversibility reflected by CV curves was in agreement with the charge–discharge profile properties. Fig. 6d shows the rate capacity of MoO2-8 nanosheets at different current rates of 100, 200, 400 and 800 mA g 1, respectively. It kept a high reversible capacity after 5th cycle at 100 mA g 1. The following reversible capacities were 560, 460, 350 mA h g 1 at the current densities of 200, 400 and 800 mA g 1, respectively. The good performance at different rates of charging–discharging may be due to due to the nanosheets with a thin thickness and carbon buffer layer on the surface of nanosheets. These two factors probably improved greatly the transfer of electron and Li+ in the electrode and resulted in the enhanced electrochemical properties.

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Fig. 6. (a) Cycling performance of MoO2 nanomaterials synthesized with different amounts of ionic liquid. (b) Charge–discharge profiles, (c) CV curves at a scan rate of 0.5 mV s 1 in 0.02–3.0 V, and (d) rate capability of MoO2-8 nanosheets.

4. Conclusions In summary, the monoclinic MoO2 nanosheets were successfully synthesized for the first time by using an ionic liquid as a solvent and reactive reagent. These formed materials might be attributed to layered structural precursor being exfoliated by intercalation of amine salts R4N+. Furthermore, MoO2 nanosheets were used as anode materials in the rechargeable LIBs and exhibited a large reversible capacity, high rate performance and good cycling stability. These performances might be ascribed to the facts that the nanosheets with a thin thickness could improved greatly the transfer of electron and Li+ in the electrode material and buffering effect of carbon layer on the surface of nanosheets. References [1] L. Yang, Q. Gao, Y. Zhang, Y. Tang, Y. Wu, Electrochem. Commun. 10 (2008) 118. [2] Y. Shi, B. Guo, S. Corr, Q. Shi, Y. Hu, K. Heier, L. Chen, R. Seshadri, G. Stucky, Nano Lett. 9 (2009) 4215. [3] L. Yang, Q. Gao, Y. Tang, Y. Wu, R. Holze, J. Power Sources 179 (2008) 357. [4] S. Lee, Y. Kim, R. Deshpande, P. Parilla, E. Whitney, D. Gillaspie, K. Jones, A. Mahan, S. Zhang, A. Dillon, Adv. Mater. 20 (2008) 3627. [5] W. Luo, X. Hu, Y. Sun, Y. Huang, Phys. Chem. Chem. Phys. 13 (2011) 16735. [6] Y. Sun, X. Hu, J. Yu, Q. Li, W. Luo, L. Yuan, W. Zhang, Y. Huang, Energy Environ. Sci. 4 (2011) 2870.

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