Synthesis and electrochemical performance of V2O5 doped MoO3 cathode materials

Journal of Alloys and Compounds 486 (2009) 672–676

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Synthesis and electrochemical performance of V2 O5 doped MoO3 cathode materials Xin Wei, Lifang Jiao ∗ , Sichen Liu, Junli Sun, Wenxiu Peng, Haiyan Gao, Yuchang Si, Huatang Yuan Institute of New Energy Material Chemistry, Engineering Research Center of Energy Storage & Conversion (Ministry of Education) and Key Laboratory of Energy-Material Chemistry (Tianjin), Nankai University, 94 Weijin Road, Tianjin 300071, PR China

a r t i c l e

i n f o

Article history: Received 8 June 2009 Received in revised form 5 July 2009 Accepted 6 July 2009 Available online 14 July 2009 Keywords: MoO3 V2 O5 Doping Cathode materials Lithium-ion batteries

a b s t r a c t A series of cathode materials for lithium-ion batteries with the formula (MoO3 )1−x (V2 O5 )x (x = 0.00, 0.01, 0.02, 0.03, 0.04) were synthesized by high-temperature solid-state method. The effects of V2 O5 on the structural, morphology and electrochemical properties of the cathode materials were investigated through X-ray diffraction (XRD), scanning electron microscopy (SEM), charge–discharge test, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments. The results show that cathode materials exhibit better electrochemical performance after V2 O5 doping. (MoO3 )0.98 (V2 O5 )0.02 shows the highest first discharge capacity of 280.6 mAh g−1 and the best cycling reversibility. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Many researchers have deeply investigated lithium-ion batteries due to their high energy density, flexible, lightweight design and long life span [1,2]. The cathode and anode materials are important components of lithium-ion batteries. It is all known that cathode materials have great influence on the capacity of lithium-ion batteries and thus improving its performance is especially essential. Among numerous cathode materials, transition metal oxides have been widely investigated because of their unique layered structures allowing reversible insertion/deinsertion of lithium ions [1–3]. As an important transition metal oxide, molybdenum trioxide (MoO3 ) is of special interest due to its layered crystal structure and potential applied value in material chemistry field. The layered structure of orthorhombic MoO3 exists tetrahedron and octahedron cavity and the size of the channel fit intercalation/deintercalation of lithium ion [4]. It has been reported that MoO3 can accommodate around 1.5 lithium/molybdenum [5,6]. However, poor cycling reversibility limits the development of MoO3 as a cathode material. So many researchers managed to find methods to improve this disadvantage. Lee et al. [7] recently reported that MoO3 nanoparticles which are obtained by hot-

wire chemical vapor deposition technique displayed high capacity reversible cycling. Several groups [8–10] showed that doping and modifying processes were effective way to improve the electrochemical performance of MoO3 powders and doping other metal oxides was received much attention. V2 O5 , another important cathode material for lithium-ion batteries, is of considerable interest, because of its unique features such as high electrochemical activity, stability and energy density [11,12]. Chou et al. [13] recently reported V2 O5 nanomaterial cathode with high capacity, safety and enhanced cyclability. V2 O5 with several discharge platforms limited its further application of lithium-ion batteries. Chernova et al. [14] lately investigated layered vanadium and molybdenum oxides in their application in lithium-ion batteries. Some researchers [15–20] have investigated the properties of MoO3 –V2 O5 , such as the structural, gas-sensing properties, the electrochemical properties, the interaction and mechanism of MoO3 and V2 O5 . But no attempts have been made so far to study on V2 O5 doped MoO3 as cathode material for rechargeable lithium batteries. In this work, different amounts of V2 O5 were doped into MoO3 by high-temperature solid-state method. The effects of V2 O5 on the structural, morphology and electrochemical properties of cathode materials were thoroughly investigated. 2. Experimental

∗ Corresponding author. Tel.: +86 22 23498089; fax: +86 22 23502604. E-mail address: [email protected] (L. Jiao). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.030

MoO3 sol. and V2 O5 sol. were respectively prepared by an ion exchange of (NH4 )6 Mo7 O24 ·4H2 O (≥99.0%) and NH4 VO3 (≥99.0%) through a cation exchange resin (from Tianjin NanKai Hecheng S&T Co., Ltd.). MoO3 sol. was mixed with

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Table 2 The columbic efficiencies of the second charge and discharge cycle for the electrodes made from the as-prepared (MoO3 )1−x (V2 O5 )x [x = 0.00 (a), 0.01 (b), 0.02 (c), 0.03 (d), and 0.04 (e)].

Fig. 1. X-ray diffraction pattern of (MoO3 )1−x (V2 O5 )x [x = 0.00 (a), 0.01 (b), 0.02 (c), 0.03 (d), and 0.04 (e)].

V2 O5 sol. in different molar ratio [(MoO3 )1−x (V2 O5 )x (x = 0.00, 0.01, 0.02, 0.03, 0.04)] and stirred mechanically to dry. Then the powders were calcined at 600 ◦ C for 10 h. The XRD patterns of the synthesized samples were investigated using a Rigaku D/MAX-2500 powder diffractometer with a graphite monochromatic Cu K␣ radiation ( = 0.15406 nm) in the 2 range of 3–80◦ . Scanning electron microscope (SEM) images of the samples were obtained on the FEG SEM Sirion scanning electron microscope used to observe morphology, size and distribution. Electrochemical measurements were carried out with lithium metal as anode electrodes and cathode materials were fabricated by mixing 80% of the (MoO3 )1−x (V2 O5 )x powder with 10% carbon black and 10% PTFE. The electrolytes were 1 M LiPF6 dissolved in EC + DMC (1:1 volume ratio) and separator was Celgard 2300 film. The testing cells were assembled in an argon-filled glove box (home-made) in which water and oxygen concentration was kept less than 5 ppm. The discharge–charge tests were run at a current density of 50 mA g−1 at the potential range of 1.5–4.0 V. All the tests were performed at room temperature. Cyclic voltammetry (CV) tests and EIS experiments were performed on a CHI660B electrochemical workstation. CV was tested at a scan rate of 0.1 mV s−1 on the voltage range 1.5–4.0 V (vs. Li|Li+ ). In the EIS experiments, ac perturbation signal was ±5 mV and the frequency range was from 10 mHz to 105 Hz. The impedance spectrums were analyzed by using Z-View software from Scribner Associates.

3. Results and discussion The typical XRD patterns of as-prepared compounds (MoO3 )1−x (V2 O5 )x (x = 0.00, 0.01, 0.02, 0.03, 0.04) are shown in Fig. 1. A good agreement between our data and those of MoO3 file (JCPDS data number 65-2421 card) corresponding to the MoO3 with orthorhombic symmetry is observed in Fig. 1a. No other peaks for impurities are detected, which indicated the pure MoO3 powder obtained. The position of the characteristic peaks of the pristine and doped samples is consistent, whereas the peak intensity is different. The intensity of (1 1 0) (2 2 0) (3 3 0) for doped samples is much smaller than that for pristine sample. It may be due to the change of grain size and solid-solid interTable 1 The cell parameters of as-prepared (MoO3 )1−x (V2 O5 )x [x = 0.00 (a), 0.01 (b), 0.02 (c), 0.03 (d), and 0.04 (e)]. Sample

a

b

c

(a) (b) (c) (d) (e)

13.83611 13.86040 13.86868 13.86040 13.86098

3.69698 3.70002 3.69729 3.69872 3.69495

3.95720 3.96103 3.96371 3.95822 3.96196

Samples

a

b

c

d

e

Columbic efficency (%)

99.76

96.84

99.77

97.44

98.56

action during mixing and high-treating process. Table 1 shows the cell parameters of as-prepared samples. It can be observed that the cell parameters of a-direction obviously increase after V2 O5 doping. The characteristic peaks of V2 O5 cannot be found in Fig. 1, probably indicating that V2 O5 has intercalated the lattice of MoO3 . Fig. 2 shows the SEM images of (MoO3 )1−x (V2 O5 )x (x = 0.00, 0.01, 0.02, 0.03, 0.04) powders at the same magnification. Fig. 2a is overall view of the pristine MoO3 sample. It can be observed that a large number of long and uniform microrods are distributed homogeneously. The average length of the microrods is about 8–15 ␮m. Fig. 2a is overall view of V2 O5 and small particles can be observed. Compared Fig. 2b–e with Fig. 2a, grain size of doped samples is smaller than pristine sample. (MoO3 )0.98 (V2 O5 )0.02 has the smallest grain size. The change of morphology is probably due to the solid–solid thermal interaction after mixing of two kinds of sol. Fig. 3 shows the first discharge curves (A), the second cycle charge–discharge curves (B) and columbic efficiencies (Table 2) of the electrodes made from as-prepared compounds (MoO3 )1−x (V2 O5 )x (x = 0.00, 0.01, 0.02, 0.03, 0.04) in the potential range of 1.5–4.0 V at the current density of 50 mA g−1 . The first discharge capacity of the pristine MoO3 electrode (Fig. 3Aa) is 227.3 mAh g−1 . While doped samples have higher first discharge capacity than pristine sample. (MoO3 )0.98 (V2 O5 )0.02 is the highest one, with the capacity of 280.6 mAh g−1 . We can come to the same conclusion from the second cycle charge–discharge curves. Fig. 4 shows the curves of discharge capacity vs. the cycle number for the electrode materials made from (MoO3 )1−x (V2 O5 )x (x = 0.00, 0.01, 0.02, 0.03, 0.04). It can be observed that discharge capacity of doped samples is much higher than that of pristine sample. It shows that capacity and cycling stability of the cathode material have improved after V2 O5 doping. (MoO3 )0.98 (V2 O5 )0.02 , first specific discharge capacity of 280.6 mAh g−1 and 170.1 mAh g−1 after 50 cycles, exhibits the highest capacity and best cycling stability of all the as-prepared samples. The second cyclic voltammogram curves of the electrodes made from (MoO3 )1−x (V2 O5 )x (x = 0.00, 0.01, 0.02, 0.03, 0.04) samples are shown in Fig. 5. In the cathodic polarization process of the second cycle for the electrodes made from pristine MoO3 (Fig. 5a), one peak is located at 2.72 V versus Li+ |Li, corresponding to the lithium intercalation processes. In the following anodic polarization, one peak is observed at 2.19 V, corresponding to the lithium extraction processes. Compared with Fig. 5a, the other curves show that both the area of CV curves and peak current for doped samples are much larger than that for pristine sample. What is more, the potential interval between oxidation peak and reduction peak for doped samples is smaller, which is an important parameter to value the electrochemical reaction reversibility [21]. It indicates the improvement of discharge capacity and electrode reaction reversibility after V2 O5 doping. Of all the doped samples, (MoO3 )0.98 (V2 O5 )0.02 sample, the sharpest oxidation peak at 2.62 V vs. Li+ |Li and reduction peaks at 2.22 V vs. Li+ |Li, has largest area of CV curves and peak current and smallest potential interval between oxidation peak and reduction peak. Doped samples exhibit much higher discharge capacity probably because V2 O5 can accommodate up to 2.6 lithium/vanadium [22] while MoO3 can accommodate around 1.5 lithium/molybdenum

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Fig. 2. Typical SEM images of V2 O5 sol. (a ) and (MoO3 )1−x (V2 O5 )x [x = 0.00 (a), 0.01 (b), 0.02 (c), 0.03 (d), and 0.04 (e)] at the same magnification.

[5–7] during the electrochemical cycling. Several discharge platforms of V2 O5 are not shown in Figs. 4 and 5 and the characteristic platform of MoO3 only can be observed. That may be because mutual inhibition and influence of two kinds of metal oxides. The results show that (MoO3 )0.98 (V2 O5 )0.02 has the smallest particle size and exhibits the best electrochemical performance. It is probably because that morphology effects on improvement of the electrochemical activities [23] and small particles would accommodate more easily the structural changes occurring during the cycling process where lithium are inserted and extracted [24]. Fig. 6A shows the ac impedance of cells with (MoO3 )1−x (V2 O5 )x (x = 0.00, 0.01, 0.02, 0.03, 0.04) samples at the cut-off potential of 2.3 V at the second discharge process. The impedance spectrums show a high frequency semicircle and a low frequency tail, indicating the double layer response at the electrode/sample

interface and the diffusion of lithium ions in the solid matrix. The impedance plots are fitted using the equivalent circuit model (Fig. 6B). The equivalent circuit model includes electrolyte resistance Rs , a constant phase element (CPE) associated with the interfacial resistance, charge-transfer resistance Rct , and the Warburg impedance (Zw ) related to the diffusion of lithium ions in the solid oxide matrix. According to Chen et al. [25] studies on EIS of lithium-ion cells, cathode impedance is mainly attributed to the cell impedance, especially charge-transfer resistance. Table 3 Table 3 Comparison of EIS data for (MoO3 )1−x (V2 O5 )x [x = 0.00 (a), 0.01 (b), 0.02 (c), 0.03 (d), and 0.04 (e)]. Sample

a

b

c

d

e

Rct ()

215.0

212.1

135.1

142.5

186.6

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Fig. 5. Comparison of CV for the electrodes made from the as-prepared (MoO3 )1−x (V2 O5 )x [x = 0.00 (a), 0.01 (b), 0.02 (c), 0.03 (d), and 0.04 (e)].

Fig. 3. Initial discharge curves (A) and the second cycle charge–discharge curves (B) for the electrodes made from the as-prepared (MoO3 )1−x (V2 O5 )x [x = 0.00 (a), 0.01 (b), 0.02 (c), 0.03 (d), and 0.04 (e)].

shows the Rct of all the samples. It can be observed that the Rct of all the doped samples are much smaller than pristine sample. The Rct of (MoO3 )0.98 (V2 O5 )0.02 cathode (c) (135.1 ) is the smallest. It indicates that the conductivity of the cathode material has been

Fig. 6. (A) Nyquist plots of the electrodes made from the as-prepared (MoO3 )1−x (V2 O5 )x [x = 0.00 (a), 0.01 (b), 0.02 (c), 0.03 (d), and 0.04 (e)] and (B) equivalent circuit model.

improved and the charge transfer process becomes more easy after V2 O5 doping. 4. Conclusions

Fig. 4. Comparison of cycling stability curves of the electrodes made from the asprepared (MoO3 )1−x (V2 O5 )x [x = 0.00 (a), 0.01 (b), 0.02 (c), 0.03 (d), and 0.04 (e)].

In this paper, a series of (MoO3 )1−x (V2 O5 )x materials were successfully synthesized and investigated. Results showed that the electrochemical performance of the cathode material was improved after V2 O5 doping. In addition, cells fabricated with (MoO3 )0.98 (V2 O5 )0.02 cathode materials showed a highest discharge capacity of 280.6 mAh g−1 and lowest charge-transfer resistance (135.1 ) than any other samples. This kind of doped materials was successful to improve the discharge capacity and the cycling reversibility of MoO3 cathode material. It is meaningful to

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enlarge the appliance of cathode material in lithium-ion batteries and valuable to further research. Acknowledgments This work was supported by the Natural Science Fund of Tianjin (06YFJMJC04900), National Science Foundation of China (20673062, 20801059). References [1] [2] [3] [4]

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