Synthesis and electrochemical performance of Fe2(MoO4)3 cathode for sodium-Ion batteries - Nguyen Van Tu

Vietnam Journal of Chemistry, International Edition, 54(4): 424-428, 2016 DOI: 10.15625/0866-7144.2016-00340 424 Synthesis and electrochemical performance of Fe2(MoO4)3 cathode for sodium-ion batteries Nguyen Van Tu 1,2* , Phan Quang Quy 3 1 State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Material Science and Engineering, Wuhan University of Technology, Wuhan 430070, P. R. China 2 Institute for Chemistry and Materials, Military Institute of Science and Technology 3 Hanoi University of Industry Received 29 January 2016; Accepted 12 August 2016 Abstract In this articles, Fe2(MoO4)3 was prepared by precipitation method and used as cathode materials for sodium-ion batteries. The crystalline structure of the sample was characterized by a powder X-ray diffraction spectroscopy (XRD), the morphologies of the sample were observed by the field emission scanning electron microscopy (SEM), the binding state of elements in sample was analyzed by X-ray photoelectron spectroscopy (XPS). The electrochemical properties were investigated in CR2025 coin type cell with a metal sodium foil as the anode electrode. As the charge/discharge current density at 0.1 C, the initial specific capacity of Fe2(MoO4)3 is 82.50 mAhg -1 , and remains 67.92 mAhg -1 after 15 cycles. Keywords. NASICON, iron molybdate, sodium ion battery, cathode material. 1. INTRODUCTION Sodium-ion batteries (SIBs) are the most promising alternatives to lithium-ion batteries due to the low cost and abundance of sodium element in the earth. The chemical similarity of sodium ion toward lithium ion enables some electrode materials used in Li-ion batteries (LIBs) to be applied for SIBs. Specialty for the application in the large-scale energy storage for smart grid and solar/wind energy, and the problem of low-cost would be a big challenge [1]. Since the 1980s, many cathode materials for sodium-ion battery have been identified, such as NaxMO2 [2], NaFePO4 [3], MF3 [4], NaMnPO4 [5], Na2MPO4F [6], Na3M2(PO4)3F [7] (M = Fe, V, Mn), V2O5 [8], Na3V2(PO4)3 [9], NASICON (Na + superionic conductor) compounds, and organic compounds [10]. But the limitations of the material are the insertion/extraction of Na + ions which is difficult, due to large size of Na + (1.02 Å). It is a big challenge for the research community to find suitable cathode materials, which can be used in sodium-ion batteries. Though Na + ion has larger ionic size, the insertion/extraction of it is easy and fast in the structure of NASICON compounds due to the 3D structure of Fe2(MoO4)3. NASICON-Fe2(MoO4)3 has been identified as a potential candidate for sodium storage due to the low price, non-toxicity of iron and its open three dimensions framework. However, its poor cycle-ability and low electric conductivity limit its further applications [11]. In this paper there have been studied the preparation, characterization and electrochemical properties of four Fe2(MoO4)3 samples. The relativity between structure and properties are also analyzed. A reasonable explanation for the mechanism to improve the electrochemical performance is given in our work. 2. EXPERIMENTAL 2.1. Synthesis Fe2(MoO4)3 powder Monoclinic Fe2(MoO4)3 powder was synthesized by a precipitation method described elsewhere [2]. 0.983 g (NH4)6Mo7O24·4H2O was dissolved in 20 mL distilled water, referring as solution A. 1g Fe(NO3)3·9H2O was dissolved in 20 mL distilled water, referring as solution B. Then the solution B was slowly added to solution A under continuous stirring and HNO3 was added to adjust the pH value to 2, 3, 4 and 5. Later, the obtained solution was VJC, 54(4) 2016 Nguyen Van Tu, et al. 425 heated to 95-100 o C for 2 hours. Finally, the precipitate was then aged, filtered, washed and calcined at 500-650 °C in air for 25 hours. The sample was allowed to cool in furnace till room temperature. 2.2. Characterization The crystalline structure of the sample was characterized by a powder X-ray diffraction spectroscopy (XRD, PertrPro PANalytical, Netherlands) equipped with CuK radiation (1.5418 Å). The morphologies of the sample were observed by the field emission scanning electron microscopy (SEM, JSM-6700F, JEOL, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) measurements were acquired using a VG Multilab 2000, with AlK the as the radiation source. All XPS spectra were corrected by the C1s line at 284.8 eV. The electrochemical properties were investigated in CR2025 coin type cell with a metal sodium foil as the anode electrode. The NaClO4 (Aldrich, 99.99 wt%) and the solvent propylene carbonate were used as an electrolyte (1 mol/L). The working electrode was prepared by spreading the slurry of the Fe2(MoO4)3 (80 wt%), acetylene black (15 wt%), and binder polytetrafluoroethylene (PTFE) (5wt%) on Ni mesh. Polypropylene micro- porous film (Cellgard 2300) is used as a separator. The cells were assembled in an argon-filled glove box at room temperature. For galvanostatic charge- discharge test were carried out on a battery test system (Land BT2000, Wuhan, China). The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were measured by Autolab Potentiostat 30. 3. RESULTS AND DISCUSSION 3.1. Morphology and structure XRD patterns of the samples are shown in figure 1. It can be observed that the peaks are well indexed of monoclinic structure of Fe2(MoO4)3 (JCPDS 01- 072-0935), as impurities of MoO3, Fe3O4. XRD patterns of Fe2(MoO4)3 were successfully indexed with a monoclinic lattice using the program Jade 6.5. The unit cell lattice parameters of all the experimental Fe2(MoO4)3 phases are summarized in table 1. On the table 1, the lattice parameters changed little with different pH values of conditional preparation. The results of XRD analysis indicate Fe2(MoO4)3 sample prepared at pH = 4 is pure than every samples. Figure 1: XRD patterns of the samples at different pH values and pure Fe2(MoO4)3 (JCPDS 01-072-0935) Table 1: Refined unit cell lattice parameters for Fe2(MoO4)3 cell at different pH values pH a(Å) b (Å) c (Å) ( o ) V(Å 3 ) 2 15.7251 9.1971 18.2505 125.529 2148.98 3 15.7272 9.1970 18.2520 125.532 2150.89 4 15.7266 9.1967 18.2514 125.539 2148.04 5 15.7271 9.1969 18.2498 125.452 2147.97 The SEM images of Fe2(MoO4)3 samples at different pH values are shown in figure 2. It can be observed that the particles are uniform and fine sizes. Particle sizes are about 0.2-1 m and the particle sizes are observed to be smaller at lower pH. This issue may be explained by precipitation of MoO3 at lower pH value. From results of XRD and SEM analysis, we select synthetic condition of Fe2(MoO4)3 at pH = 4 for all experiments. Figure 2: SEM images of Fe2(MoO4)3 samples. (a) pH = 5, (b) pH = 4, (c) pH = 3, (d) pH = 2 VJC, 54(4) 2016 Synthesis and electrochemical performance of... 426 1.5 2.0 2.5 3.0 3.5 -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 Fe 2 (MoO 4 ) 3 - powder c u rr e n t (m A ) Potential (V vs Na + /Na) 1st cycle 2nd cycle 3rd cycle (a) A B C D 10 20 30 40 50 60 70 80 90 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 0.01C, 0.05C, 0.1C-Discharge 0.01C, 0.05C, 0.1C- Charge V o lt a g e (V ) Specific capacity (mAhg -1 ) (b) The formation of Fe2(MoO4)3 is further investigated using X-ray photoelectron spectroscopy (XPS). It is well known that the electrochemical properties of the sample are related with their sizes and phases as well as their chemical binding states. The XPS survey spectra in figure 3a show that Fe, Mo, and O elements coexist. Further, the spectra of Fe2p and Mo3d in figure 3c and figure 3d show the characteristic peaks of Fe 3+ state and Mo 6+ state located at 711,79 eV and 232.8 eV, respectively (the states of Fe 3+ and Mo 6+ in Fe2(MoO4)3). Figure 3: X-ray photoelectron spectra of Fe2(MoO4)3 powder (a) Survey spectra; (b) Fe2p spectra, (c) Mo3d spectra 3.2. Electrochemical performances and reaction mechanism The cyclic voltammetry curves of the Fe2(MoO4)3 between 1.5 and 3.5 V at a scan rate of 0.1 mV s -1 are shown in figure 4a. Two cathodic current peaks at 2.61 and 2.52 V are observed in the first reduction process and shifted to 2.62 V and 2.53 V in the subsequent ones. During all oxidation processes, there are two corresponding anodic peaks at 2.56 and 2.71 V. The intensities of peaks are well maintained in all subsequent cycles. The results are in a good agreement with the galvanostatic cycling profiles and indicate a reversible two-step electrochemical reaction mechanism of Fe2(MoO4)3 with sodium. This result is consistent with our previous report, and it can be expressed as follows [11]. Fe2(MoO4)3 + xNa + + xe -→NaxFe2(MoO4)3 (the first discharge process, x = 1, 2) (1) NaxFe2(MoO4)3 Fe2(MoO4)3 + xNa + + xe - (discharge/charge process) (2) On the figure 4a, the oxidation/reduction reactions between Fe2(MoO4)3 and NaxFe2(MoO4)3 are shown as follow: At peak A (2.56 V), anodic process: Na2Fe2(MoO4)3 - 1e - = NaFe2(MoO4)3 + Na + (3) At peak B (2.71 V), anodic process: NaFe2(MoO4)3 - 1e - = Fe2(MoO4)3 + Na + (4) At peak C (2.61 V), cathodic process: Fe2(MoO4)3 + 1e - + Na + = NaFe2(MoO4)3 (5) At peak D (2.52 V), cathodic process: NaFe2(MoO4)3 + 1e - + Na + = Na2Fe2(MoO4)3 (6) These results clearly reveal that the insertion/extraction of Na + ions occur inside Fe2(MoO4)3. Figure 4: (a) The first cyclic voltammetry curves of Fe2(MoO4)3 powder electrode at a voltage sweep rate of 0.1 mVs -1 , (b) Galvanostatic curves of Fe2(MoO4)3/Na cell at a current rate of 0.1 C Figure 4b shows the first charge/discharge profiles of Fe2(MoO4)3/Na cell at a current rates of 0.01, 0.05, 0.1 C. The open circuit voltage (OCV) of Fe2(MoO4)3/Na cell is 2.72 V. The discharge capacities of Fe2(MoO4)3 at 0.1 C is about 82.6 mAh/g and 81.8 mAh/g, respectively, corresponding to about 2.0 Na + per formula unit (p.f.u), which means completely transformed the Fe 3+ to Fe 2+ . Figure 4c clearly shows that the Fe2(MoO4)3 electrode may be the charge/discharge at 0.1 C, the initial specific capacity of Fe2(MoO4)3 is 82.5 mAh/g, and remains 67.92 mAh/g after 15 cycles. Figure 4d shows the Nyquist plots of Fe2(MoO4)3 cathode after 3 cycles at 9 mA/g in the frequency range between 100 kHz and 0.1 Hz at VJC, 54(4) 2016 Nguyen Van Tu, et al. 427 0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 800 -Z '' ( Z'( Fe 2 (MoO 4 ) 3 powder(d) 10 30 40 50 60 70 80 S p ec if ic c ap ac it iy ( m A h g -1 ) Cycle number (c) Discharge/Charge Fe 2 (MoO 4 ) 3 powder at 0.1C Figure 4 (c): The specific capacities of Fe2(MoO4)3 powder at 0.1 C (Electrolyte is 1 M NaClO4 in propylene carbonate (PC), (d) EIS plots of Fe2(MoO4)3 powder after 3 cycles at 9 mA/g in the frequency range between 100 kHz and 0.1 Hz at open circuit voltage (OCV) with 5 mV amplitude voltage (Inset shows the equivalent circuits corresponding to the Nyquist plots) open circuit voltage (OCV) with 5 mV amplitude voltage. The semicircles at high to medium frequency are mainly related to a complex reaction process at the electrolyte/cathode interface. The inclined line in the lower frequency region is attributed to the Warburg impedance, which is associated with sodium-ion diffusion in the Fe2(MoO4)3 electrode. The impedance spectra fitted using an equivalent circuit in which Re represents the total resistance of electrolyte, electrode and separator; Rf and CPE1 are related to the diffusion resistance of Na-ions through the solid electrolyte interface (SEI) layer and the corresponding constant phase element (CPE); Rct and CPE2 correspond to the charge transfer resistance and the corresponding CPE; Zw is Warburg impedance [12]. The exchange current density is calculated using the following equation. i o = RT/nFRct (7) The fitting results of Re, Rf, Rct and i o as shown in table 2 indicate that the Rf and Rct values of Fe2(MoO4)3 cathode are big. It can be confirmed that the decrease of charge transfer resistance is beneficial to the kinetic behaviors during charge/discharge process. Since the Fe2(MoO4)3 shows big resistance and the small exchange current density, it is suggested that Fe2(MoO4)3 powder significantly unimproved the performance of the sodium-ion battery. 4. CONCLUSION Fe2(MoO4)3 was prepared by precipitation method and used materials cathode for sodium-ion batteries. For the technological condition, the Fe2(MoO4)3 electrode may be the charge/discharge at 0.1 C, the initial specific capacity of Fe2(MoO4)3 is 82.5 mAh/g, and remains 67.92 mAh/g after 15 cycles. Synthesis of Fe2(MoO4)3 samples may be used for material cathode for sodium-ion batteries. Table 2: Impedance parameters calculated from equivalent circuits Sample Re ( ) Rf ( ) Rct ( ) i o (mAcm -2 ) Fe2(MoO4)3 powder (pH = 4) 10.55 120.86 265.60 4.831×10 -5 REFERENCES 1. Komaba S, Yabuuchi N, Kubota K, et al. Research development on sodium-ion batteries, Chemical Reviews, 114, 11636-11682 (2014). 2. Slater M D, Kim D H, Lee E, et al. Sodium-ion batteries. Advanced Functional Materials, 23, 947-958 (2013). 3. Saiful Islam M, Fisher-Craig A J. Lithium and sodium battery cathode materials: Computational insights into voltage, diffusion and nanostructural properties, Chemical Society Reviews, 43, 185-204 (2014). 4. Chevrier V. L., Ceder G. Challenges for Na-ion negative electrodes batteries and energy storage. Journal of the Electrochemistry Society, 158, A1011-A1014 (2011). 5. Zaghib K., Trottier J., Hovington P., et al. Characterization of Na-based phosphate as electrode materials for electrochemical cells. Journal of Power Sources, 196, 9612-9617 (2011). 6. Zheng Y., Zhang P., Wu S. Q., et al. First- principles investigations on the Na2MnPO4F as a cathode material for Na-ion batteries, Journal of the Electrochemistry Society, 160, A927-A932 (2013). 7. Recham N., Chotard J. N., Dupont L., et al. Ionothermal synthesis of sodium-based fluorophosphate cathode materials, Journal of the Electrochemistry Society, 156, A993-A999 (2009). VJC, 54(4) 2016 Synthesis and electrochemical performance of... 428 8. Tepavcevic S., Xiong H., R. Stamenkovic V. R., et al. Nanostructured bilayered vanadium oxide electrodes for rechargeable sodium-ion batteries, ACS Nano, 6(1), 530-538 (2012). 9. Jian Z. L., Zhao L., Chen W., et al. Carbon coated Na3V2(PO4)3 as novel electrode material for sodium ion batteries, Electrochemistry Communications, 14, 86-89 (2012). 10. Brian L. Ellis, Linda F. Nazar. Sodium and sodium-ion energy storage batteries, Solid State and Materials Science, 16, 168-177 (2012). 11. Shirakawa J., Nakaya M., Wakihar M., et al. Changes in electronic structure upon lithium insertion into Fe2(SO4)3 and Fe2(MoO4)3 investigated by X-ray absorption spectroscopy, Journal of Physical Chemistry B, 11, 1424-1430 (2007). 12. Nguyen V T, Liu Y L, Chen W, et al. Synthesis and electrochemical performance of Fe2(MoO4)3/carbon nanotubes nanocomposite cathode material for sodium-ion battery, ECS Journal of Solid State Science and Technology, 4(5), M25-M29 (2015). Corresponding author: Nguyen Van Tu Institute for Chemistry and Materials Military Institute of Science and Technology 17, Hoang Sam, Nghia Do, Cau Giay, Hanoi, Vietnam E-mail: nguyenvantu882008@yahoo.com.