1Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials & Ministry-of-Education Key Laboratory for Synthesis and Applications of Organic Functional Molecules, Hubei University, Wuhan, P. R. China
Received date: 24/02/2017; Accepted date: 06/03/2017; Published date: 12/03/2017
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Three dimensional (3D) Bi2WO6 microspheres are successfully synthesized by the hydrothermal method using Bi(NO3)3·5H2O and NaWO4·2H2O as raw materials. The structure and morphology of the samples are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The results show that the Bi2WO6 takes on morphology of microspheres. As a novel anode material for lithium-ion batteries (LIBs), the as-prepared Bi2WO6 electrode exhibits both high reversible capacity and good cycling performance at room temperature under a potential window from 3.0 to 0.01 V (vs. Li+/Li) at current density of 100 mAg−1. The Bi2WO6 anode has initial discharge capacity as 1218 mAhg−1 and retained its reversible capacity at 922 mAhg−1 after 129 cycles. The Bi2WO6 with morphology of microspheres is a promising anode material for the lithium-ion battery application.
Chemical synthesis, Inorganic compound, Electrochemical properties, Anode material
In recent years, lithium ion batteries (LIBs) have conquered the market of portable electronics because of their advantages of long lifetime, high energy density, etc. [1-3]. However, commercial graphite-based anodes possess a low theoretical specific capacity (372 mAhg-1) which can’t meet the increasing requirements for higher energy density , thus limiting its range of applications. In addition, safety problems might occur when lithium ion batteries are charged-discharged at high rates .Therefore, new anode materials should be developed.
Intensive researches have been conducted to search for new electrode materials with higher capacities than that of traditional materials in recent years, for instance, CaWO4 , FeWO4 , ZnWO4 , LiFe(WO4)2  and Bi2WO6  have been investigated as anode materials for LIBs. In near past years, Bi-W-O systems have also attracted a lot of interests because of the high theoretical capacity (670 mAhg-1). Zhang et al. investigated Bi2WO6 composite as anode material for lithium-ion batteries, and its reversible capacity was maintained as 427.8 mAhg-1 at current density of 50 mAg-1 . Although Wu et al. synthesized Bi2WO6 with nanoplate-built hierarchical nest-like structure, the electrochemical properties were not satisfied . In our work, 3D-Bi2WO6 microspheres were synthesized by a facile hydrothermal method. Its electrochemical performances as an anode material were studied.
The precursor of Bi2WO6 microspheres was synthesized by a hydrothermal method using salts (Bi(NO3)3·5H2O, Na2WO4·2H2O) as metal source and Polyvinylpyrrolidone (PVP) as the surfactant. All the reagents were analytical grade (A.R.) reagents and used without further purification. Deionized water was used in all experiments. In a typical synthesis process, Bi(NO3)3·5H2O (2.5 mmol) and PVP (0.5 g) were dissolved in 40 mL distilled water under magnetic stirring to form a mixed solution. Na2WO4·2H2O (1.25 mmol) was slowly added into the mixed solution. After magnetic stirring for an hour, the mixed solution was transferred into a 50 mL teflon-lined stainless steel autoclave and annealed at 180°C for 24 h.The precursor was washed with distilled water and ethanol for several times, and dried under vacuum at 60°C for 12 h. The precursor was heated at 400°C in air for 3 hours to obtain expected sample.
Characterization of the Samples
The crystal structure of the Bi2WO6 was characterized by X-ray diffraction (XRD) using Cu Kα radiation source (Bruker AXS, D8 diffractometer). The particle sizes of the samples were observed by scanning electron microscopy (SEM; JEOL JSM, 6510V). Energy dispersive spectroscopy (EDS) was performed to detect the contents of metallic elements for expected compounds. The valences of elements in samples were analyzed using an X-ray photoelectron spectrometer (XPS, Escalab 250Xi) with a multichannel detector, which can withstand high photonic energies from 0.1 to 3 keV.
Charge/discharge tests were performed using CR2025 coin-type cells in an automatic battery tester system (Neware China). The working electrodes were made from a mixture of 60 wt.% as-prepared powders, 30 wt.% acetylene black and 10wt.% poly(vinylidene fluoride) (PVDF) binder. Prior to being used, the electrodes were dried at 120°C in a vacuum furnace for 24 hours. Lithium foil was used as the counter electrode, and the electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC) (1:1, v/v). All test cells were assembled in an argon-filled glove box containing less than 1 ppm each of oxygen and moisture. The cells were discharged and charged at a constant current of 100 mAg-1 over a voltage range of 0.01~3.00 V (vs. Li+/Li) at room temperature. The typical masses of electrode materials used in the experiment were ranged from 5 to 8 mg. Electrochemical impedance spectroscopy (EIS) experiments and cyclic voltammetry (CV) experiments were conducted using a CHI 600E electrochemical workstation.
Figure 1a presents the XRD patterns of the Bi2WO6 microstructures, in which the major diffraction peaks appeared at 2θ (28.586°, 33.026°, 47.305°, 56.027°, 58.355° and 68.423°) can be respectively correspond to the lattice planes (1 0 3 ), (2 0 0), (2 2 0), (3 0 3), (1 0 7) and (4 0 0) of a hexagonal Bi2WO6 structure which belongs to a space group of P6/mmm (191) (JCPDS card No. is 26-1044, a = b = 5.48Å, c = 11.500Å, and α=β=90°, γ=120°). The Bi2WO6 has high crystallinity. In addition, no characteristic peaks of the other impurities were observed, which indicated that pure phase of Bi2WO6 was formed.
Figure 1b and 1c present the scanning electron microscope (SEM) images of the as-prepared Bi2WO6 sample. Figure 1b is a typical low-magnification SEM image of the as-synthesized Bi2WO6 sample, from which it can be seen that the aggregated particles have a sphere morphology with an average diameter of 700~800 nm. Figure 1c shows a typical surface image of the microspheres at a higher magnification, in which a smooth surface can be clearly observed.
Chemical analysis using Energy dispersive X-ray spectroscopy (EDS) indicates the presence of Bi, W, O (elements of Bi2WO6 sample), and no other elements existed as observed in Figure 1d. It can be seen in Figure 1e, the composites contained the elements Bi, W at the molar ratio of Bi : W = 57.62:29.86 (in At.%), thus, the molar ratio of Bi, W in BWO sample could be calculated to be 1.9:1, which basically agrees with the theoretical value of 2:1.
The expected oxidation states of the elements in the Bi2WO6 sample were further determined using X-ray photoelectron spectroscopy (XPS) spectra (Figure 2).The Bi 4f peaks of the sample appeared at ca. 163. 5eV and ca. 158.7 eV, which corresponded with Bi3+ according to the previous results. The W4f peaks of the sample appeared at 36. 5eV, 34.5 eV, indicating W6+ in the sample, which was in agreement with the reported value in the literature . The O 1s binding energy (Figure 2c) of 529.6 eV was in agreement with literature values of the bulk for Bi2WO6. Consequently, the as-synthesized product could be determined as pure orthorhombic Bi2WO6 based on the results of XPS measurements.
Figure 3a shows the charge/discharge profiles of Bi2WO6 electrode at a current rate of 100 mAg-1 for the 1st, 2nd, and 5th cycles (0.01–3 V). The first Li-insertion and extraction capacity are as high as 1218 and 805 mAg-1, respectively, with a coulombic efficiency of 66.1%. This can be attributed to the difficult dissolution of the SEI, as well as other factors, such as the intrinsic nature of the materials, kinetic limitations and cation deficient . In addition, the discharge capacities of the Bi2WO6 anode in the 2nd, 5th cycles are 861, 806 mAhg-1, respectively; and the charge capacities of the anode in the 2nd, 5th cycles are 805, 757 mAhg-1, respectively.
Figure 3: (a) Selected discharge and charge curves of the as-synthesized Bi2WO6 electrode at a current density of 100 mAg-1; (b) cycling performance of the as-synthesized Bi2WO6 electrode at a current density of 100 mAg-1; (c) rate capability of the as-synthesized Bi2WO6 electrode at different current densities (100, 300, 500 and 100 mAg-1). The test potential window is between 0.01 and 3.0 V (vs. Li+/Li). (d)CV curves of Bi2WO6 film electrodes at various scanning rates between 0.01V and 3V.
Figure 3b presents the cyclic stability and coulombic efficiency of the Bi2WO6 electrode at a current density of 100 mAg-1, and maintains a capacity as high as 922 mAhg-1 after 129 cycles. The capacity increasing phenomenon during cycle process has been found in many anode materials containing metal elements, while there is no accurate explanation for this phenomenon so far. Some authors ascribed this phenomenon to the reversible growth of a polymeric gel-like film resulting from kinetically activated electrolyte degradation and the larger specific surface area obtained from fragmentation of the structure during the cycling process .
The rate capability of the Bi2WO6 electrode at various charge-discharge rates is shown in Figure 3c. Typically, the discharge capacities of the Bi2WO6 electrode at the currents densities of 100, 300 and 500 mAg-1 are 887, 763 and 565 mAhg-1, respectively. Furthermore, it is worth to note that the capacity can also be restored to ~856 mAhg-1 when the current density is returned to 100 mAg-1.
The cyclic voltammograms of the three cycles for annealed Bi2WO6 thin film over a potential range of 1.0–4.0 V are shown in Figure 3d. For the first discharge, the peak at about 2.2 V can be ascribed to the initially inserting of Li into BWO and the reduction of the Bi and W metals with the formation of Li2O. The following two peaks between 0.5 and 1 V van be correspond to the two steps of alloying processes between Li and Bi . For the first charge process, one peak at about 0.9 V and other peak between 2 and 3 V correspond to the two steps oxidation processes of Bi and W.
During charge and discharge process, the revisable alloying processes lead Bi2WO6 to behave excellent cycle ability, which can be seen from CV curves.
In summary, a facile one-pot hydrothermal method combined the heat-treatment was used to synthesize the 3D-Bi2WO6 microspheres. As a novel anode material for the lithium ion battery, the as-prepared Bi2WO6 electrode exhibits both high reversible capacity and good cycling performance at room temperature under a potential window from 3.0 to 0.01 V (vs. Li+/Li) at current density of 100 mAg−1.The material (Bi2WO6) is a promising and novel anode material for lithium ion battery applications. Besides, this work provides a new and simple approach to synthesize inorganic functional materials.
The study was funded by guizhou education department for quality program project (2011).