FeWO4: An Anode Material for Sodium-Ion Batteries

Wei Wang, Weiyi Xiong, He Sun, Shuqiang Jiao*

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, PR China *Corresponding author: Email address: sjiao@ustb.edu.cn

Keywords: FeWO4, sodium-ion battery, anode

Abstract

Recently, the eco-friendly sodium-ion batteries with low price show huge potential for electrical energy storage. A facile strategy to synthesize FeWO4 has been carried out through a solid state method. The as-prepared powders are characterized by the X-ray diffraction and scanning electron microscopy. The electrochemical properties of this material as an anode electrode for sodium-ion batteries have been characterized by galvanostatic charge–discharge measurements. The galvanostatic charge–discharge measurements, using the as-prepared FeWO4 as a working electrode with a voltage range of 0.01–2.5 V vs. Na+/Na has disclosed an excellent electrochemical performance. Even under a high current density, the discharge capacity can still maintain at a relatively high level. All results indicate that this material is a good candidate for an anode material of sodium-ion batteries.

Introduction

Renewable energy sources such as solar, wind and hydropower have been widely used to generate electricity without producing carbon dioxide [1-5]. However, the electricity generated from these renewable sources is intermittent, and effective delivery of uninterrupted electricity requires new power sources for portable electronic devices [6, 7]. Due to their high energy density and power density, lithium-ion batteries have been the main power source for portable devices nowadays [8-10].Nevertheless, lithium-ion batteries have disadvantages including cost, lithium supply, reliability, stability and safety [11-13]. Sodium-ion batteries have attracted much attention as an alternative device to lithium-ion batteries, especially in large scale systems [14, 15]. Recently, the development of sodium-ion batteries is rapid and many materials for sodium-ion batteries have been extensively investigated, such as NaNi1/3Mn1/3Co1/3O2, Na2Ti3O7, NaMnFe2(PO4)3, V2O5, Na3V2(PO4)3, Na2V6O16,NaFeF3 and NiCo2O4 [16-24].

Metal tungstates belong to an important family of inorganic functional materials with potential application in various fields [25]. Among them, FeWO4 is a very important electrode material for its good electron transport performance, which has been applied in catalysts, optical fibers, humidity sensors, laser hosts, scintillation detectors, phase-change optical recording devices, pigments, etc [26-31]. Quite recently, as an anode material, FeWO4 has been used for Li-ion batteries [32, 33].However, the application in Na-ion batteries has not been reported. Herein, a solid

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TMS2014 Annual Meeting Supplemental ProceedingsTMS (The Minerals, Metals & Materials Society), 2014

state method was employed for the synthesis of FeWO4. The as-prepared FeWO4 exhibited good electrochemical performance.

Experimental

All materials and chemicals were purchased commercially and used as received.

The FeWO4 powder was prepared through a solid state reaction. The iron oxide (Fe2O3) and tungsten trioxide (WO3) were used as the starting materials. Initially, the two starting materials were weighted in a Fe:W mole ratio of 1:1. Then the compounds were mixed together in an agate mortar and grinded for above 0.5 h. The mixture was dispersed in 30 mL of anhydrous alcohol and put into an ultrasonic vibration generator to mix uniformly. Then the mixture above was dried at 60 °C in an oven until all anhydrous alcohol evaporated. The obtained powder was heated in a tube furnace at 650 °C for 5 h in hydrogen atmosphere.

The structure and morphology of the as prepared powders were characterized by X-ray diffraction measurement (XRD, Rigaku, D/max-RB) and scanning electron microscopy (SEM, CAMBRIDGE, S-360).

The electrochemical performances of the obtained powder were carried out with coin-type cell CR2032. The working electrode was fabricated by mixing the active material FeWO4, acetylene black and teflon (poly(tetrafluoroethylene), PTFE) binder in a weight ratio of 75:15:10. The above mixture was put into an ultrasonic vibration generator to mix uniformly. Then the mixture was dried at 60 °C in an oven until they became slurries and then coated uniformly on a copper sheet. The electrode was dried under air at 120 °C for above 12 h. Tablet machine was used to make the electrode smooth and thin and after that, the electrode was cut into rounded pieces with a diameter of about 8 mm. The active material FeWO4 electrode was used as the working electrode, and Na foil was used for both counter and reference electrodes. The electrolyte used was a 1.0 M NaClO4 in a propylene carbonate (PC) electrolyte solution. A glass fiber (GF/D) from Whatman was used as the separator. The cells were assembled and sealed in a glove box filled with high-purity argon gas. A galvanostatic experiment was performed in the voltage range of 0.01 and 2.5 V versus Na/Na+.

Results and discussion

XRD patterns of the obtained FeWO4 are shown in Figure 1(a). The main

diffraction peaks are indexed to JCPDS74-1130. It is worth noting that, some small impurity peaks emerged, which is indexed for Fe2O3. Figure 1(b) shows the SEM image of FeWO4. It can be seen from figure1(b)that the size of the particles is in the range of 300 nm to 800 nm.

Figure 2 shows the crystal structure of FeWO4. In the structure of FeWO4, with two formulas per unit cell, every metal atom is surrounded by six oxygen atoms: zigzag chains of oxygen octahedra coordinating the metal ions are aligned along the c axis [34]. Iron, tungsten and oxygen atoms occupy the 2f, 2e and 4g sites, respectively.

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Figure 1. (a) X-ray diffraction patterns of the as-prepared FeWO4 synthesized by solid state reaction. (b) SEM image of FeWO4 powder.

Figure 2. The crystal structure of FeWO4. Figure 3 displays the galvanostatic charge/discharge profiles at a current density

of 20 mAg-1 in the initial three cycles. The cut-off voltage is set in the range of 0.01 to 2.5 V vs Na+/Na. The charge and discharge capacities for the initial cycle are 115 mAhg-1 and 116 mAhg-1, respectively. It is found that the charge and discharge capacity loss is very little, which demonstrates a high reversibility of this material. The voltage plateau of the discharge curves is below 0.5 V, which is good for lithium-ion batteries as an anode material.

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Figure 3. The initial three charge and discharge curves of FeWO4 at a current density of 20 mAg-1 between 0.01 and 2.5 V (vs. Na+/Na).

The rate performance of the obtained material was also investigated through varying the charge/discharge rate. Figure 4 shows the initial cycles of typical charge (extraction of Na+ from the active material) and discharge (insertion of Na+) profiles of FeWO4 at different current densities from 20 to 100 mAg-1. All galvanostatic charge/discharge profiles are similar to each other. As the current density increases, the charge and discharge capacities decrease. Due to the polarization of the electrode, the charge curve moves to higher potential and the discharge curve moves to lower potential. At 50 mAg-1, the charge and discharge capacities are 103 mAhg-1 and 106 mAhg-1, respectively. Even at a high current density of 100 mAg-1, this material can still retain a discharge capacity of 90 mAhg-1 and recover a charge capacity of 89 mAhg-1, yielding high reversible capacities.

Figure 4. Cell potential as a function of specific capacity for the initial cycle under a range of charge/discharge rates from 20 to 100 mAg-1.

The cycle performance of FeWO4 over 20 cycles with different current densities is shown in Figure 5. It is worth noting that the discharge capacity of the first few cycles fades more rapidly than other cycles, which can be attributed to the formation of SEI. The capacity loss for the following cycles is steady and slow. After 20 cycles, the discharge capacity is 104 mAhg-1 at 20 mAg-1. Even at a high current density of

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100 mAg-1, the obtained material can still deliver 86 mAhg-1 over 20 cycles. The good performance is due to the small size of the as-prepared particles, which can significantly increase the contact area between the active material and the liquid electrolyte and provide shorter diffusion distance, maintaining the original volume and realizing good electrochemical performance.

Figure 5. Discharge behavior of the as-prepared battery at different current densities from 20 mAg-1 to 100mAg-1 over 20 cycles.

Conclusions

In summary, FeWO4 particles have been successfully synthesized by solid state

reaction, which shows good charge and discharge property. Over 20 cycles, the discharge capacity was maintained at 104 mAhg-1 at the current density of 20 mAg-1. Even at a high current density of 100 mAg-1, the discharge capacity still retained 86 mAhg-1. If further optimized, it is believed that the FeWO4 particles can be used as high-rate anode material in large-scale energy storage.

Acknowledgments

The work was supported by the National Natural Science Foundation of China

(No.51322402), the Program for New Century Excellent Talents in University, Ministry of Education of China (NCET-2011-0577), and the Fundamental Research Funds for the Central Universities (FRF-TP-12-002B).

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