Yining Maab,
Shidong Jia,
Huaijuan Zhouab,
Shuming Zhangab,
Rong Lia,
Jingting Zhuab,
Wenjing Liab,
Hehe Guoab and
Ping Jin*ac
aState Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. E-mail: p-jin@mail.sic.ac.cn; Fax: +86 21 69906208; Tel: +86 21 69906208
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
cMaterials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8560, Japan
First published on 19th October 2015
A novel ammonium vanadium bronze (NH4)0.6V2O5 has been successfully synthesized via a simple hydrothermal treatment and its electrochemical performance is investigated. The as-synthesized material was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectrum, Raman spectrum, X-ray photoelectron spectroscopy (XPS), element analysis (EA), cyclic voltammetry (CV) and galvanostatic charge/discharge cycling test. The results revealed that a pure novel phase (NH4)0.6V2O5 was obtained with square brick-like morphology. Preparation conditions such as amount of reducing agent, temperature and reaction time have been investigated to obtain the pure phase. (NH4)0.6V2O5 square bricks are tested as a cathode material for lithium-ion batteries. It has an excellent lithium ion insertion/extraction ability with a high specific discharge capacity of 280.2 mA h g−1 and 244.3 mA h g−1 during 1.0–3.8 V at the current densities of 10 mA g−1 and 20 mA g−1, respectively.
Since NH4V4O10 was firstly used as cathode material in a rechargeable lithium battery8 in 2006, ammonium vanadium bronzes have attracted considerable attention. Flake-like NH4V3O8·0.2H2O had the greatest discharge capacity of 225.9 mA h g−1 during 1.8–4.0 V and remained 209.4 mA h g−1 after 30 cycles.16 Sarkar reported that NH4V4O10 along with CMC/alginate binder exhibited discharge capacity of 200 mA h g−1 at current rate of 1000 mA g−1 superior to the performance of PVDF-based cathode.17 Except for single metal vanadium bronzes, bi-cation intercalated (NH4)0.26Na0.14V2O5 (ref. 18) and (NH4)0.83Na0.43V4O10·0.26H2O flowers19 have also been obtained through hydrothermal treatment and functioned as stable lithium battery electrodes. From the above, we conclude that vanadium oxide bronzes have a variety of types and structures and are potential for rechargeable lithium-ion batteries. It would be of great significance to prepare novel structured vanadium bronze with excellent electrochemical properties to enhance competitiveness among the wide range of cathode materials world.
Herein, we report a new brick-like ammonium vanadium bronze (NH4)0.6V2O5 by hydrothermal reduction of NH4VO3 in an acidic solution. In order to obtain the pure phase, preparation conditions, such as amount of reducing agent, temperature and reaction time, have been optimized. The electrochemical properties of (NH4)0.6V2O5 were investigated by means of galvanostatic charge–discharge cycling and cyclic voltammetry.
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Fig. 1 (a) XRD pattern, (b) SEM image, (c) SAED pattern and (d) EDS spectrum of the obtained powder by hydrothermal method. |
In Fig. 1c, the selected area electron diffraction (SAED) pattern taken from one square brick indicates that it is single crystalline. The element composition of the square brick was investigated by energy-dispersive spectrometer (EDS) (Fig. 1d). The result shows the product contains the elements of N, V and O. To further confirm the undiscovered compound, we carried out FT-IR, Raman spectrum and XPS measurements.
The FT-IR spectrum of the obtained powder is shown in Fig. 2a. The absorption bands at ∼3187 and ∼1394 cm−1 are assigned to asymmetric stretching vibration and symmetric bending vibration corresponding to the N–H mode of NH4+ group.20 The bands at ∼994 and ∼934 cm−1 are attributed to VO stretching of distorted octahedral and distorted square-pyramids,21 while those at ∼786, ∼574 cm−1 and ∼482 cm−1can be contributed to asymmetric and symmetric stretching vibration of V–O–V bond, respectively.17,22 The spectrum does not exhibit the adsorption bands at ∼3434 cm−1 and ∼1625 cm−1 which correspond to the O–H stretching vibration and bending vibration of the crystal water,19 inferring that it is a pure phase without water of crystallization.
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Fig. 2 (a) FT-IR spectrum, (b) Raman spectrum, (c) wide survey XPS spectrum and (d) V 2p spectrum of the as-synthesized sample. |
To further identify the obtained phase, we carried out Raman scattering. As is shown in Fig. 2b, Raman spectrum exhibits a series of bands at 136, 197, 281, 302, 405, 473, 520, 689, and 990 cm−1, which are well consistent with those of V2O5.23,24 The peak at 990 cm−1 is assigned to the stretching mode of vanadyl oxygen indicating the attainment of V2O5 characteristic mode.25 The bands at 689 cm−1 and 520 cm−1 are attributed to the stretching vibrations of doubly coordinated oxygen (V–O2) and triply coordinated oxygen (V–O3), respectively.26 The peaks at 405 and 281 cm−1 are assigned to VO bending modes and the bands at 473 and 302 cm−1 belongs to V–O–V bending modes.26 While the other two peaks recorded at 136 and 197 cm−1 correspond to the lattice vibration of [V2O5].26 From the results of unknown XRD, single crystalline SAED pattern, EDS, Raman scattering and FT-IR spectrum of the obtained sample, we conclude that the compound is an undiscovered type of ammonium vanadium bronzes and its chemical formula is (NH4)xV2O5.
To further confirm the oxidation state of vanadium in the as-synthesized product, X-ray photoelectron spectroscopy measurements were carried out and the results were exhibited in Fig. 2c and d. The wide survey XPS spectrum implies that the surface of prepared sample consists of N, V and O, which is consistent with the conclusion from EDS in Fig. 1d. The XPS spectrum of the V 2p3/2 peak in Fig. 2d is composed of two peaks at 516.5 and 517.7 eV, corresponding to V4+ and V5+, respectively, which confirms the presence of mix-valence vanadium bronze.25 The average valence of vanadium is +4.6 calculated from XPS data by their peak area ratios. In order to confirm the chemical formula, element analysis (EA) was performed to analyze the content of N. The result shows the weight content of N is 4.33%. Based on the above analysis, the formula of new ammonium vanadium bronze can be expressed as (NH4)0.6V2O5. The average valance of vanadium is +4.7 calculated from the element analysis which is close to the result of XPS. Since XPS is a semi-quantitative method, the formula is subject to the result of element analysis.
Fig. 3 shows the X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) images of as-synthesized powders prepared by different amounts of formic acid assisted hydrothermal treatment. As 0.15 g formic acid was added, the final product was a mixture of NH4V4O10 (JCPDS card no. 31-0075) and (NH4)0.6V2O5 (Fig. 3a). Accordingly, the SEM image in Fig. 3b shows two morphologies: a nanobelt one and a square brick one, corresponding to the NH4V4O10 (ref. 17, 21and 28) and (NH4)0.6V2O5, respectively. When the formic acid was increased to 0.4 g, the pure (NH4)0.6V2O5 phase was obtained and no other impure peaks in Fig. 3a and morphologies in Fig. 3c could be observed. With further increasing to 1.0 g, the XRD pattern confirms that the powders consist of two phase (NH4)0.6V2O5 and VO2 (D).29 The SEM image also identifies the (NH4)0.6V2O5 square bricks circled by red squares and VO2 (D) micro-flowers as shown in Fig. 3d. (NH4)0.6V2O5 is more likely to be an intermediate phase between NH4V4O10 and VO2 (D). From the analysis above, the optimal amount of formic acid to synthesize (NH4)0.6V2O5 is 0.4 g.
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Fig. 3 (a) XRD patterns and (b–d) SEM images of hydrothermal reaction samples using different amounts of formic acid: (b) 0.15 g, (c) 0.4 g, (d) 1.0 g. |
The reaction was also carried out at different temperatures to study the temperature effect. NH4VO3 could not react with formic acid completely below 250 °C even though the reaction time was 8 h, which is ascribed to the weak reducing ability of formic acid (Fig. 4). When the temperature reached 250 °C or higher, pure (NH4)0.6V2O5 phase was acquired. Reaction time was varied to unravel the phase evolution while the reaction temperature was kept at 250 °C (Fig. 5). After continuous reaction for 4 h, nanobelts NH4V4O10 and square bricks (NH4)0.6V2O5 simultaneously appeared (Fig. 5a and b) which is similar to the consequence of less amount of the reducing agent. Pure phase (NH4)0.6V2O5 was obtained after 8 h or longer time as shown in Fig. 5c and d. Based on the analysis above, it comes to the conclusion that the optimum condition to obtain pure phase (NH4)0.6V2O5 is that 0.4 g formic acid reacts with NH4VO3 at 250 °C for 8 h under hydrothermal treatment.
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Fig. 4 XRD patterns of hydrothermal reaction samples at different temperatures ranging from 240 °C to 270 °C. |
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Fig. 5 (a) XRD patterns and (b–d) SEM images of hydrothermal reaction samples treated under different reaction times: (b) 4 h, (c) 8 h, (d) 24 h. |
Fig. 6b and c display typical discharge/charge curves and cycling performance of (NH4)0.6V2O5 electrodes at 10 mA g−1 during 1.0–3.8 V. From Fig. 6b, the discharge curves comprise three voltage plateaus at ∼2.5, 2.1, and 1.6 V, which is consistent with the result of CVs, whereas the subsequent charge process exhibits S-shaped curves. Apart from the first discharge/charge curves, the other two pairs of curves show exactly similar shape, which is in good accordance with CV. It should be noted that the coulombic efficiency of the first three cycles are less than 100%. The mechanism may be more complicated, especially arising from the irreversible loss of charge capacity, which may be caused by poor electrical conductivity, the formation of solid electrolyte interface (SEI) in the first cycle, loss of some material contacting with the current collector (the structure morphology changes during Li insertion) or sluggishness of the transformation in the large particles. Moreover, the discharge curves of the second and third cycles are different from the initial discharge profile which attributed to the formation of solid electrolyte interface (SEI) with activation process and the wetting of active electrode at the beginning. Similar phenomenon has also found in other vanadium-based materials, such as K0.25V2O5,11 Ag0.33V2O5,30 NH4V4O10,17 etc.
The galvanostatic charge/discharge cycling performance of (NH4)0.6V2O5 was shown in Fig. 6c. It is demonstrated that the electrode can charge and discharge in the organic electrolyte, inferring that Li+ can reversibly insert into and extract from (NH4)0.6V2O5 crystalline. The initial discharge capacity is 280.2 mA h g−1 and after 30 cycles, the brick-like (NH4)0.6V2O5 maintains a capacity of 152.8 mA h g−1. It is worth mentioning that there is a large capacity loss during the first two cycles which is probably on account of the slight structure rearrangement stemming from the new lithium ions insertion and extraction.
Fig. 6d shows the rate performance of (NH4)0.6V2O5 cathode material at varying current rates. The material delivers discharge capacity of 244.3 mA h g−1 at 20 mA g−1 which possesses a better performance than that of NH4V4O10.8 On increasing current densities from 50, 100 and 200 mA g−1, capacities of 138.8, 89.8, 48.9 mA g−1 are obtained, respectively. It should be noted that after the continuous cycling with increasing and decreasing current densities, a specific capacity of 186.3 mA h g−1 could be recovered at a current of 20 mA g−1 which confirms the Li+ storage reversibility.
From the above, we can conclude that the brick-like (NH4)0.6V2O5 exhibited a high specific discharge capacity of 280.2 mA h g−1 but degraded quickly after several cycles. The decay mechanism might be attributed to the structural instability of (NH4)0.6V2O5 and sluggishness of the transformation in the large particles. The capacity fading may be due to the accommodation of large amount of lithium during Li+ intercalation and deintercalation which results in disintegration of active materials from the current collector, possibly active particle–particle contact loss or its structural collapse during long term cycling.17 The morphology study of (NH4)0.6V2O5 electrodes after the 5th and 30th cycle are shown in Fig. 7. It can be seen in Fig. 7a that visible cracks circled by red ellipses can be clearly seen on the surface of the square bricks implying that the electrode began to disintegrate. At the same time, the surface of the bricks became rough. After the 30th cycle, the square brick disintegrated to irregular shaped pieces and the surface was wrinkled which confirms that the structural stability of (NH4)0.6V2O5 electrode is poor.
Large particle size might be another factor to cause capacity fading. Particle morphology and particle size are very important factors to determine the electrochemical performance of electrode materials.16 Large particle would increase the Li ions' diffusion distance and make transformation sluggishness resulting in inferior electrochemical performance.
Doping heteroatoms such as Cu can not only act as pillars between layers but also stabilize the structure and improve the electronic conductivity to enhance the electrochemical performance.2 It is expected that the fast capacity fading and bad rate capability of (NH4)0.6V2O5 electrode will be improved by doping heteroatoms.
According to the results above, it is thereby expected that as-synthesized (NH4)0.6V2O5 square brick with high specific capacity could be a promising cathode for lithium-ion battery.
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