Materials Letters 198 (2017) 4–7
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Synthesis and electrochemical performance of a-ZnMoO4 nanoparticles as anode material for lithium ion batteries Jie Fei ⇑, Qianqian Sun, Jiayin Li, Yali Cui, Jianfeng Huang, Wenle Hui, Hailing Hu School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China
a r t i c l e
i n f o
Article history: Received 5 September 2016 Received in revised form 22 March 2017 Accepted 28 March 2017 Available online 31 March 2017 Keywords: Zinc molybdate Electrical properties Electronic materials Lithium ion battery Nanoparticles
a b s t r a c t The a-ZnMoO4 nanoparticles were successfully synthesized and first time used as anode material for lithium ion batteries (LIBs). The as-prepared a-ZnMoO4 exhibits excellent electrochemical performance, including a very stable capacity of 389.0 mA h g 1 at 50 mA g 1 with capacity loss of 0.2% per cycle for the 2nd cycle, excellent rate capability with discharge capacity of 207.4 mA h g 1 at 500 mA g 1, and well reversibility of 382.6 mA h g 1 when the current density turns back to 50 mA g 1. All these results suggest high potential of a-ZnMoO4 used as anode material in future application for LIBs. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Transition metal oxides (TMOs) have long been attracted extensive research interest as alternative anode materials for lithium ion batteries (LIBs) due to their high reversible capacities and safe potential relative to Li+/Li . Typically, Mo element exists in various oxidation states ranging from +6 to 0 during the electron transfer process, thus they display large capacities at the beginning of electrochemical process. Therefore, metal molybdenums, such as XMoO4 (X = Mn, Ni, Cu, Fe and Co) [2–6], become appealing anode materials due to their varied structures, high theoretical capacity and abundant in nature. Metal molybdenums display large capacity at the beginning of electrochemical process, however, high capacity cannot be sustained on long term cycling due to their conventional bulk and poor electronic conductivity. Therefore, their practical application in LIBs are still limited. It has been widely acknowledged that nanosized materials provide higher electrochemical performance both in cathode and anode by decreasing lithium ion diffusion distances, increasing the contact area between the electrode and the electrolyte . Thus, nanosizing metal molybdenums is an effective way to release their higher capacities and maintain an excellent electrochemical performance. Among various metal molybdates, zinc molybdate (ZnMoO4) has two types of structures: alpha and beta [8,9]. In the last years, ⇑ Corresponding author. E-mail addresses: [email protected]
(J. Fei), [email protected]
(J. Huang). http://dx.doi.org/10.1016/j.matlet.2017.03.160 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
ZnMoO4 has been widely applied in luminescence , red/green phosphors for light-emitting diode , cryogenic/bolometric scintillating detactors , anticorrosive paints  and humidity sensors . As an anode material for LIBs, ZnMoO4 has a theoretical capacity of 951.6 mA h g 1 due to its high valences of Zn (+2) and Mo (+6). ZnMoO4 has been proved its possibility of electrochemical intercalation of lithium , but its electrochemical performance as an anode material for LIBs has not been revealed up to now. From previous reports, b-ZnMoO4 always used as photocatalytic material . In this work, we synthesized a-ZnMoO4 nanoparticles by a facile co-precipitation method with subsequent calcined and investigated its electrochemical performance as an anode material for the first time. The as-prepared a-ZnMoO4 nanoparticles demonstrated excellent cycling stability, well rate capability and excellent reversibility as an anode material for LIBs. 2. Experimental section 2.1. Preparation of a-ZnMoO4 The a-ZnMoO4 nanoparticles were successfully synthesized via a co-precipitation method using (NH4)6Mo7O244H2O and Zn (NO3)26H2O as the starting compounds. In a typical synthesis, (NH4)6Mo7O244H2O (2 mmol) was dissolved in deionized water (200 mL) completely. NH3H2O (8–12 mL, 2 mol L 1) was added to form a mixed solution. Under vigorous agitation, an aqueous solution (100 mL) containing Zn(NO3)26H2O (14 mmol, the mole ratio of Mo6+ and Zn2+ is 1.0) was slowly dropped into the above
J. Fei et al. / Materials Letters 198 (2017) 4–7
solution at room temperature. After stirring the mixed solution for 2 h, the precipitate was filtered, washed with deionized water and absolute ethanol, then dried in oven at 60 °C for 4 h. Finally, the aZnMoO4 nanoparticles were obtained by a subsequent calcined at 600 °C for 3 h. 2.2. Material characterizations The crystallographic phase of as-prepared sample was characterized by X-ray diffraction (D/max-2200, Rigaku, Japan) on an Ultima IV X-ray diffractometer with Cu Ka radiation (k = 0.15406 nm) in the 2h range from 10° to 50°. Particle size and morphology of the as-prepared sample were observed by scanning electron microscope (SEM, S-4800, Hitachi, Japan) and transmission electron microscopy (TEM, JEM-3010). 2.3. Electrochemical measurements The electrochemical performance was measured by CR2032type coin cell. The electrode material was prepared by mixing 80 wt% active material, 10 wt% acetylene black, 5 wt% sodium carboxyl methyl cellulose and 5 wt% polyacrylic acid in deionized water. The mixed slurry was coated on a copper foil. The cell was assembled in a glove box filled with highly pure argon gas (O2 and H2O levels <0.5 ppm), 1 mol L 1 LiPF6 in ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) (1:1:1 in volume) as an electrolyte. Cyclic voltammetry (CV) was tested on an electrochemical station (CHI660E, Chenhua, China). The charge/discharge tests were carried out using Neware battery testing system (Neware Technology Ltd). 3. Results and discussion Fig. 1 shows the XRD pattern of the synthesized sample, the standard XRD pattern of a-ZnMoO4 (triclinic phase with the cell parameters of a = 8.368 Å, b = 9.692 Å, c = 6.964 Å, a = 106.872°, b = 101.726° and c = 96.734°, space group P-1(2), JCPDS No. 35– 0765) as comparison. All diffraction peaks of the sample correspond well with triclinic a-ZnMoO4 phase, and no peaks of other phases are detected in the patterns, confirming the high purity of the sample. It indicates that a-ZnMoO4 are obtained through facile co-precipitation method by subsequently calcined. The morphologies of a-ZnMoO4 are observed by SEM and TEM measurements. Fig. 2a shows the SEM image of the prepared aZnMoO4. The sample shows irregular shape and agglomerates to a big bulk. The morphological property of a-ZnMoO4 is further examined by TEM in Fig. 2b. The sample presents dispersed and homogeneous morphology with 200–300 nm in diameter. The high resolution TEM image in Fig. 2c presents the lattice spacing of
Fig. 1. XRD pattern of the as-synthesized sample.
Fig. 2. SEM (a) and TEM (b and c) images of a-ZnMoO4.
about 0.331 nm. This is agreement with d ( 2 2 0) spacing in the XRD patterns, which the diffraction peak is 26.9°. Fig. 3a illustrates CV curves of a-ZnMoO4. Compare the curves, the peak around 0.22 and 0.28 V gradually disappeared, which can be attributed to the irreversible formation of solid electrolyte interfaces . At the first cathodic sweep, the broad peak at 1.4 V can be attributed to the formation of LixZnMoO4 and the reduction of Mo6+ to Mo, the peak around 0.51 V can be the reduction of Zn2+ to Zn. In the anodic branch, all the curves have two little intensity peaks around 0.72 and 1.45 V, which correspond to the oxidation of Zn to Zn2+ and Mo to Mo6+ . In the subsequent cycles, redox peak located at 1.5 V can be attributed to the Li intercalates with MoO3 and the formation of LixMoO3, while 0.50 V represents Li reacts with LixMoO3 and the reduction of Zn2+ to Zn. Fig. 3b shows galvanostatic discharge/charge profiles of aZnMoO4 at 50 mA g 1. In the first discharge, the voltage plateaus locate at 0.80 and 0.41 V, while in the following discharge, the voltage plateau maintains at 0.73 V. The initial discharge and charge capacities are 1112.7 and 488.9 mA h g 1 respectively, corresponding to an initial coulombic efficiency of 43.9%, which can be mainly attributed to the side reactions during the first lithiation process. The cycle performance of a-ZnMoO4 is depicted in Fig. 3c. The discharge specific capacity of the electrode gradually change from 491.3 mA h g 1 to 389.1 mA h g 1 between the 2nd and 90th cycles, with a capacity loss of 0.2% per cycle, and remain at 389.0 mA h g 1, indicating the cycling stability of the electrode. Simultaneously, the coulombic efficiency can be sustained at above 99%, suggesting excellent electrochemical reversibility during the Li+ lithiation/delithiation process. To further demonstrate the electrochemical performance of a-ZnMoO4, rate capability at current densities from 50 to 500 mA g 1 are shown in Fig. 3d. a-ZnMoO4 electrode delivers a discharge capacity of 398.8, 371.6, 352.6, 325.9, 284.6, 255.4 mA h g 1 at 50, 100, 150, 200, 300, 400 mA g 1. The discharge capacity still remain at 207.4 mA h g 1 even when the current density increases to 500 mA g 1. The discharge capacity can be recovered to 382.6 mA h g 1 when the current density turns back to 50 mA g 1, indicating excellent rate capability and good reversibility of a-ZnMoO4 as an anode material for LIBs. The discharge/charge curves of a-ZnMoO4 at different current densities are illustrated in Fig. 3e. All the curves show similar shape at various current densities with one obvious discharge plateaus around 0.41–0.73 V and the corresponding charge plateaus around 1.28– 1.62 V. The predominant plateaus observed during discharge/ charge are in good agreement with the CV curves.
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Fig. 3. Electrochemical properties of a-ZnMoO4 nanoparticles. (a) CV curves; (b) galvanostatic discharge/charge profiles; (c) cycling performance; (d) rate capability; (e) Typical DISCHARGE/charge curves of a-ZnMoO4 at different current densities.
4. Conclusion In summary, a-ZnMoO4 nanoparticles had been successfully synthesized and used as an anode material for the first time in LIBs. The as-prepared a-ZnMoO4 nanoparticles present a very stable capacity of 389.0 mA h g 1 at 50 mA g 1 with capacity loss of 0.2% per cycle for the 2nd cycle. Furthermore, the discharge capacity of a-ZnMoO4 nanoparticles remain 207.4 mA h g 1 even at 500 mA g 1 and can be recovered to 382.6 mA h g 1 when the current density turns back to 50 mA g 1. Therefore, the a-ZnMoO4
nanoparticles are favorable for the development of the alternative anode materials for LIBs.
Acknowledgements This work was supported by the science and technology project of the Young Star of Shaanxi Province (2014KJXX-68), the Scientific Research Project of Shaanxi Education Department (14Jk1104). The technology plan of Wei yang District (201504) and the Innovation
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