Electrochemical Property of Cobalt Vanadium Oxide CoV 3 O 8 for Lithium-Ion Battery

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1 Asian J. Energy Environ., Vol. 8, Issue 1 and 2, (2007), pp Electrochemical Property of Cobalt Vanadium Oxide CoV 3 O 8 for Lithium-Ion Battery S. Ichikawa *, M. Hibino and T. Yao Department of Fundamental Energy Science, Graduate School of Energy Science, Kyoto University Yoshida, Sakyo-ku, Kyoto , Japan *Corresponding author: Shin-nosuke@t27a0221.mbox.media.kyoto-u.ac.jp Abstract: Cobalt vanadium oxide CoV 3 O 8 possessing tunnel space along the c axis in the crystal structure was synthesized by a solid state reaction and structurally analyzed by the Rietveld method. The electrochemical lithium insertion-extraction and cycle properties were investigated. CoV 3 O 8 exhibited different electrochemical behaviors between the first cycle and the later. This oxide showed discharge profile with small potential change in the first discharge process, in which amorphous substance was found to be generated by lithium insertion into CoV 3 O 8 from XRD measurement. In contrast, relatively large potential change was observed in discharge-charge profile of the second cycle or later. Lithium was possibly inserted into and extracted from the amorphous phase generated during the first discharge process. As a result, the sample showed reversible lithium insertion and good cycle performance after the second cycle. Copyright 2007 By the Joint Graduate School of Energy and Environment 33

2 S. Ichikawa, M. Hibino and T. Yao Keywords: cobalt vanadium oxide, CoV 3 O 8, solid-state reaction, lithium-ion battery, cathode material. 1. INTRODUCTION Transition metal oxides have been extensively studied in terms of electrode materials for lithium-ion batteries. Among them, vanadium oxides are one of the most promising materials due to availability, low toxicity, and low cost [1-3]. These vanadium oxides, however, are liable to collapse structurally by repetitions of discharge and charge, leading to capacity fade. It is well known that the structure and morphology of vanadium oxide can strongly influence its electrochemical performance [4]. The preparation in the form of nanoparticles [5], xerogels [6-8], or aerogels [9], is one approach for an improvement of such a poor cyclability. The preparation and application of V 2 O 5 -based composite, such as MO-V 2 O 5 (M: Ni, Ti) is another approach to improve the cyclability [10,11]. A vanadium-based complex oxide that has intercalation sites and conduction paths of lithium ion is also one of the candidates. Previously, hydrothermal synthesis and the structure of a cobalt vanadium oxide CoV 3 O 8 have been reported by Y. Oka and T. Yao et al. [12]. The CoV 3 O 8 has tunnel space along the c-axis in the crystal structure (Fig. 1) and is expected to have good electrochemical performance as an electrode material for lithium-ion battery. However, hydrothermal method allowed us to obtain only a small amount of CoV 3 O 8. For practical use of CoV 3 O 8, a method to synthesize CoV 3 O 8 by a solid state reaction was developed, since the reaction enables us 34 Asian J. Energy Environ., Vol 8, Issue 1 and 2, (2007), pp

3 Electrochemical Property of Cobalt Vanadium Oxide CoV 3 O 8 for Lithium-Ion Battery obtain enough amount of the product easily, and the obtained CoV 3 O 8 was examined as electrode materials for lithium-ion batteries [13-15]. CoV 3 O 8 exhibited reversible discharge-charge performance after the second cycle, although irreversible capacity appeared in the first cycle. After the report of Ref , we carried out additional study about CoV 3 O 8, and could get information on the relation between the electrochemical behavior and the structural change in the first cycle of CoV 3 O 8 [16]. In the discharge process of CoV 3 O 8, lithium intercalation reaction probably occurred at low lithium content. Further lithium insertion induced an amorphous substance and another crystalline substance. In the charge process of CoV 3 O 8, lithium was extracted mainly from the amorphous substance. In the present study, we synthesize CoV 3 O 8 by a solid state reaction, and the obtained CoV 3 O 8 is examined as an electrode material for lithium-ion battery. After the multicycle discharge-charge test for CoV 3 O 8, powder X-ray diffraction (XRD) pattern is taken on the sample on a discharged state. The electrochemical behavior is discussed in view of structural change in CoV 3 O METHODS 2.1 Sample preparation The CoV 3 O 8 sample was synthesized by a solid state reaction. Cobalt oxide (CoO), vanadium dioxide (V 2 O 4 ), and vanadium pentoxide (V 2 O 5 ) were mixed in the molar ratio of Co:V:O = 1:3:8 for 5 h in an alumina mortar. Then, the mixture was pressed cold-isostatically into a pellet. The pellet was sealed in a vacuum silica tube and thermally treated at 600 C for 72h to produce the CoV 3 O 8 sample. Asian J. Energy Environ., Vol 8, Issue 1 and 2, (2007), pp

4 S. Ichikawa, M. Hibino and T. Yao Figure 1. Polyhedral representation of the crystal structure of CoV 3 O 8 projected onto ab plane. 2.2 Electrochemical measurement Electrochemical measurements were carried out using a cell with three electrodes-the working, counter, and reference electrodes. The working electrode was fabricated from a mixture containing an active material, acetylene black used as the conducting additive, and poly(tetrafluoroethylene) used as a binder, in the weight ratio of 80:15:5. This mixture was pressed on a Ni mesh, which served as a current collector. The counter and reference electrodes were fabricated by pressing lithium metal on Ni mesh current collectors. The electrolyte was 1 mol dm 3 LiClO 4 dissolved in ethylene carbonate 36 Asian J. Energy Environ., Vol 8, Issue 1 and 2, (2007), pp

5 Electrochemical Property of Cobalt Vanadium Oxide CoV 3 O 8 for Lithium-Ion Battery and 1,2-dimethoxyethane (1:1 v/v). Cyclic voltammetry was performed between 1.5 and 4.0 V at a scan rate 0.1 mv s -1. Discharge-charge cycle test was carried out between 1.5 and 4.0 V vs. Li + /Li with constant current density of 80 ma g X-ray diffraction and the Rietveld method Powder X-ray diffraction (XRD) pattern was taken on the sample. The crystal structure was analyzed by the Rietveld method using the RIEVEC program [17-19]. For the sample after the 21st discharge, the working electrode of electrochemical cell was detached from the cell, placed on a zero background plate of quartz and set in a sealed holder in an argon gas system. This holder was fixed to a diffractometer for XRD measurement. 3. RESULTS AND DISCUSSION 3.1 X-ray diffraction and Rietveld method The result of the Rietveld refinement for the CoV 3 O 8 sample obtained by solid-state reaction is shown in Fig. 2, and the refined parameters are listed in Table 1. The observed XRD pattern contained 411 reflections. The calculated XRD pattern agreed with the observed one. The reliability indexes were R wp = 0.115, R B = 0.032, RF = 0.026, and GOF = It was indicated that the CoV 3 O 8 obtained from CoO, V 2 O 4 and V 2 O 5 in this study had the same crystal structure as those of the samples obtained in Ref Asian J. Energy Environ., Vol 8, Issue 1 and 2, (2007), pp

6 S. Ichikawa, M. Hibino and T. Yao Figure 2. The result of the Rietveld analysis of CoV 3 O 8. The 411 vertical marks in the middle show positions calculated for Bragg reflection. The trace in the bottom is a plot of the difference: observed minus calculated. 3.2 Electrochemical measurements Figure 3 shows a cyclic voltammogram of the CoV 3 O 8. In the first cathodic process, three reduction peaks at 2.4, 2.1, and 1.95 V appeared. This indicated that three kinds of electrochemical reaction proceeded. In contrast, in the first anodic process, one broad oxidation peak around 2.7 V was observed. The first anodic curve was different from the first cathodic one. The integration value of 38 Asian J. Energy Environ., Vol 8, Issue 1 and 2, (2007), pp

7 Electrochemical Property of Cobalt Vanadium Oxide CoV 3 O 8 for Lithium-Ion Battery cathodic current was 122 ma h g -1, and that of anodic current was 89 ma h g -1. This showed that the irreversible reaction occurred in the first cathodic process, resulting in irreversible capacity in the first cycle. In our previous study, following lithium intercalation in CoV 3 O 8, the amorphous substance and the other crystalline substance were generated during first discharging [16]. Therefore, the first reduction peak at 2.4 V possibly corresponded to lithium intercalation reaction, and the next two reduction peaks at 2.1 V and at 1.95 V possibly corresponded to the generation reaction. In the second cycle, one broad reduction peak and the same broad oxidation peak as that in the first anodic process appeared. This implied that reversible one-step reaction proceeded in the second cycle. Table 1. Crystallographic parameters of CoV 3 O 8 refined by the Rietveld analysis. Data : Cu Kα??2θ = / (411 reflections) R wp =?0.115, R B = 0.032, R F = 0.026, GOF = 1.77 Crystal system - space group : Orthorhombic - Ibam a = (1) nm, b = (1) nm, c = (1) nm Atoms Wyckoff site x y z occupancy Co-V(1) 16k (2) (2) (3) 0.5 for each V(2) 8j (2) (3) 0 1 V(3) 8j (3) (3) 0 1 O(1) 16k (4) (6) (7) 1 O(2) 8j (7) (8) 0 1 O(3) 8j (6) (8) 0 1 O(4) 8j (6) (10) 0 1 O(5) 16k (6) (24) (21) 0.5 O(6) 16k (4) (5) (6) 1 Asian J. Energy Environ., Vol 8, Issue 1 and 2, (2007), pp

8 S. Ichikawa, M. Hibino and T. Yao Figure 3. Cyclic voltammogram of CoV 3 O 8 at a scan rate 0.1 mv s -1. Figure 4 shows discharge-charge curves of CoV 3 O 8 at the first, second and 20th cycles, and Fig. 5 shows specific capacity of discharge and charge for CoV 3 O 8 as a function of cycle number. In the first discharge, three potential plateaus were observed around 2.4, 2.1 and 1.95 V. In the first discharge process, three kinds of electrochemical reaction seemed to proceed around 2.4, 2.1 and 1.95 V. In the first charge process, the potential was distributed around 2.7 V. The first charge curve was different from the first discharge one. In the second and later discharge-charge cycles, electrochemical reaction proceeded in one step. These results corresponded to the above cyclic voltammetric observation. After the second cycle, the coulombic efficiency was almost unity. It indicated that lithium reacted electrochemically with the sample reversibly. As a result, this material exhibited good discharge-charge cycle performance. 40 Asian J. Energy Environ., Vol 8, Issue 1 and 2, (2007), pp

9 Electrochemical Property of Cobalt Vanadium Oxide CoV 3 O 8 for Lithium-Ion Battery Figure 4. Discharge-charge curves of CoV 3 O 8 at a current of 80 ma g -1. Figure 5. Specific capacity of discharge and charge of CoV 3 O 8 as a function of cycle number. Asian J. Energy Environ., Vol 8, Issue 1 and 2, (2007), pp

10 S. Ichikawa, M. Hibino and T. Yao 3.4 X-ray diffraction Figure 6 shows the XRD profile of the electrode sample after the 21st discharge. The electrode was on a discharged state and lithium ion was left inserted. Lithium insertion induced slight variation in the peak positions of CoV 3 O 8, thereby changing the lattice parameters. The a-axis elongated from (1) to (9) nm, the b-axis shortened from (1) to (5) nm, and the c-axis shrunk slightly from (1) to (5) nm. These behaviors were similar to those observed in our previous study, in which the a-axis elongated from (2) to (10) nm, the b-axis shortened from (2) to (4) nm, and the c-axis shrunk slightly from (2) to (5) nm during discharging up to 80 ma h g -1 at a current of 2 ma g -1 [16]. In the discharge process at a current of 80 ma g -1, lithium intercalation in CoV 3 O 8 possibly occurred. The peak intensities of CoV 3 O 8 declined, while additional unidentified peaks appeared at 43.8 o, 63.5 o, and 80.2 o in the profile of the sample after 21st discharge. These peaks can be indexed as 200, 220, and 222, respectively by using a cubic cell with a = (3) nm. The extinction rule implied that the unit cell is a face-centered cubic, in which the 111 and 311 peaks were probably buried in the peak of CoV 3 O 8 and the 220 peak of the current collector Ni respectively. Halo peaks were also observed at around 30 and 60 (Fig. 6(b)). Such crystalline peaks and halo peaks also appeared in the XRD profiles of lithiated CoV 3 O 8 in our previous study [16]. Therefore, the same crystalline substance and amorphous substance were probably generated in the discharge process of CoV 3 O 8 in this study and in our previous study. 42 Asian J. Energy Environ., Vol 8, Issue 1 and 2, (2007), pp

11 Electrochemical Property of Cobalt Vanadium Oxide CoV 3 O 8 for Lithium-Ion Battery In the electrochemical measurement of CoV 3 O 8 (Fig. 4), the sample showed large slope after the second cycle, compared to normal crystalline electrode materials that lithium can be inserted into and extracted from. Our previous study indicated that the large slope of the open circuit potential in the first charge process of CoV 3 O 8 was mainly due to the extraction of the amorphous substance. Therefore, the profile of second discharge-charge cycle or later was possibly due to lithium insertion into and extraction from the amorphous material which was generated during the discharge process. Such a large change in the potential is probably caused by the wide distribution of the lithium site energy in the amorphous substance [20]. Good reversibility may be responsible for the newly generated amorphous substance due to its structural flexibility with respect to lithium insertion and extraction. Though lithium was reversibly inserted into the amorphous phase, the contact condition between the amorphous material and acetylene black must be unfavorable, leading to capacity decrease. In this study, since CoV 3 O 8 had partly changed into the amorphous phase, not fully yet, the production of the amorphous substance proceeded during repeated discharges. That could be one of main causes of gradual reduction of capacity by cycle repetition. Asian J. Energy Environ., Vol 8, Issue 1 and 2, (2007), pp

12 S. Ichikawa, M. Hibino and T. Yao Figure 6. (a) XRD profiles of CoV 3 O 8, and the sample after 21st discharge, and (b) the magnified profiles of these samples for clarification of the base lines. Triangles indicate unidentified peaks. 44 Asian J. Energy Environ., Vol 8, Issue 1 and 2, (2007), pp

13 Electrochemical Property of Cobalt Vanadium Oxide CoV 3 O 8 for Lithium-Ion Battery 4. CONCLUSIONS CoV 3 O 8, which has tunnel space along the c axis in the crystal structure, was synthesized by a solid state reaction. Both cyclic voltammetry and multicycle discharge-charge measurement indicated that lithium was inserted into and extracted from the sample reversibly after the second cycle and that the sample exhibited good dischargecharge cycle performance. For the first lithium insertion into CoV 3 O 8, the crystalline CoV 3 O 8 changed into an amorphous substance and another crystalline substance. The amorphous substance is allowed to discharge and charge reversibly after the second cycle. The good cycle performance of CoV 3 O 8 may be caused by lithium insertion into and extraction from the generated amorphous, because the structure of amorphous can be flexible with respect to lithium insertion and extraction. We concluded that CoV 3 O 8 is promising for an electrode material of lithium-ion battery. 5. REFERENCES [1] Cocciantelli, J.M., Doumerc, J.P., Pouchard, M., Broussely, M. and Labat, J. (1991) Crystal chemistry of electrochemically inserted Li x V 2 O 5, J. Power Sources, 34, pp [2] Cocciantelli, J.M., Menetrier, M., Delmas, C., Doumerc, J.P., Pouchard, M. and Hagenmuller, P. (1992) Electrochemical and structural characterization of lithium intercalation and deintercalation in the γ-liv 2 O 5, Solid State Ionics, 50, pp [3] Delmas, C., Cognac-Auradou, H., Cocciantelli, J.M., Menetrier, M. and Doumerc, J.P. (1994) The Li x V 2 O 5 system: An overview of the structure modifications induced by the lithium intercalation, Solid State Ionics, 69, pp Asian J. Energy Environ., Vol 8, Issue 1 and 2, (2007), pp

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