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Synthesis of CuV2O6 as a cathode material for rechargeable lithium batteries from V2O5 gel
Materials Chemistry and Physics 98 (2006) 71–75
Synthesis of CuV2O6 as a cathode material for rechargeable lithium batteries from V2O5 gel Xiaoyu Cao, Jinggang Xie, Hui Zhan, Yunhong Zhou ∗ College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China Received 22 April 2005; received in revised form 27 August 2005; accepted 28 August 2005
Abstract CuV2 O6 is a very promising cathode material for rechargeable lithium batteries. By a soft chemistry method, CuV2 O6 is successfully synthesized from V2 O5 hydrogel and Cu2 O powder. CuV2 O6 with different degrees of crystallinity are obtained by heating CuV2 O6 precursor at various temperatures. XRD, TG–DTA, TEM and SEM experiments are conducted to characterize its physical properties, and the electrochemical properties have been investigated by galvanostatic charge–discharge experiments. As a result, CuV2 O6 annealed at 550 ◦ C has smaller crystal lattice constants and better electrochemical properties compared to the sample synthesized by the conventional solid-state method. © 2006 Published by Elsevier B.V. Keywords: Copper vanadiun oxides; V2 O5 gel; Soft chemistry; Rechargeable lithium batteries; Cathode
1. Introduction While Li-ion technology is enjoying great commercial success, many researchers are devoting large efforts to search more cost-effective oxide cathodes for advanced lithium batteries, to replace technically excellent but expensive lithium cobalt oxide. Transition metal vanadates, such as copper vanadium oxides (CVO), are of interest as lithium insertion hosts [1–9], where CuV2 O6 has a triclinic structure with space group P 1¯ [10,11] and can afford a layered space where Li ion can be accommodated. Among the present CVO systems, CuV2 O6 can deliver the highest discharge specific capacity and energy density. Like other vanadium-based cathode materials such as Li1.2 V3 O8 and Ag2 V4 O11 [12–14], the electrochemical property of CuV2 O6 highly depends on the synthetic and processing methods. In the case of the synthesis of CuV2 O6 , two typical methods have been reported. One method is the conventional high temperature solid-state reaction [1]. In this reaction, copper oxide (CuO) and vanadium pentoxide (V2 O5 ) are mixed in a molar ratio of 1:1, and then the mixture is melted in an alumina crucible at 620 ◦ C in air for 48.5 h. This method requires a large thermal energy and long reaction time. The other method is coprecipitation reaction in which NH4 VO3 reacted with desired ∗
Corresponding author. Tel.: +86 27 68756931; fax: +86 27 68754067. E-mail address: [email protected] (Y. Zhou).
0254-0584/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2005.08.068
stoichiometric Cu(NO3 )2 in aqueous solution at room temperature and CuV2 O6 is obtained by heating the air-dried precipitation Cu(VO3 )2 ·nH2 O [2,3]. Over the past years, soft chemistry method (chimie douce) has opened a new avenue of research towards material sciences by showing its ability to prepare some materials with peculiar properties [2–4,15,16]. The essence of soft chemistry is that raw materials can be mixed homogeneously at nano-level and react at mild conditions so that products with excellent performance can be obtained at much lower temperature. In this work, CuV2 O6 is successfully synthesized for the first time with V2 O5 hydrogel and Cu2 O powder. V2 O5 gel is an ionic layered compound made up of V2 O5 ·nH2 O layered sheets that have a microstructure resembling that of the crystalline o-V2 O5 [17,18]. It is expected that many vanadium-based compounds conventionally prepared from crystalline V2 O5 may be obtained from V2 O5 gel. 2. Experimental V2 O5 gel was prepared by polycondensation of vanadic acid that was obtained by passing 0.5 mol L−1 sodium metavanadate (Fluka, >98%) aqueous solution through a proton exchange resin (Dowex 50 WX2 100 mesh). The collected effluent, decavanadic acid solution, resulted in the formation of homogeneous vanadium pentoxide hydrogel for 1 or 2 days aging with no agitation at room temperature. By weighing the crystalline o-V2 O5 before and after heating some of the V2 O5 hydrogel at 360 ◦ C in air, one could determine the concentration of the V2 O5 hydrogel precisely and easily [19]. When stoichiometric
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Cu2 O (ACROS, >97%) powder was added directly into V2 O5 gel, some green flocculation was formed. Then the mixture was treated for 1 h with ultrasonic irradiation to disperse the reactants followed by another normal stirring for 3 h and a green-brown flocculent precipitation was obtained. After being air-dried at 100 ◦ C, the resulting CuV2 O6 precursor was heated at 250, 300, 400, 500 and 550 ◦ C in air for 10 h to get the final products of CuV2 O6 , respectively. For preparing the reference material of CuV2 O6 , the mixed powders of V2 O5 and CuO (AR) were heated at 620 ◦ C in air for 48.5 h [1]. The V2 O5 was obtained by heating NH4 VO3 (AR) at 450 ◦ C for 24 h, which gives rise to V2 O5 with a higher reactive activity. The ultrasonicator (Kesheng Ultrasonics Instrument, China) used was a KS600 type ultrasonic cell grinder (600 W, 20 kHz). TEM photography was conducted on a JEM-100(XII) microscope when the CuV2 O6 flocculent precipitation was first formed. Thermogravimetry (TG) and differential thermal analysis (DTA) were performed using WCT-1A (Beijing Optical Instruments) in air at a heating rate of 10 ◦ C min−1 . Powder X-ray diffraction (XRD) experiments were carried out using a SHIMADZU XRD-6000 X-ray diffractometer with Cu K␣ radiation. The cathode sheet composed of 80 wt.% active material, 10 wt.% acetylene black (AB) and 10 wt.% polytetrafluoethylene (PTFE) binder. A stainless-steel mesh acted as the current collector. In the CR2016 coin cell, the lithium anode and the cathode were separated by Celgard-2400. The electrolyte was 1 mol L−1 LiClO4 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) solution with a volumetric ratio of 1:1. Coin cells were assembled in the glove box (MECAFLEX, MECABOX80-1 s) filled with high-purified argon gas. All the coin cells were tested by means of a “Land” battery instrument operating in a galvanostatic mode with a current rate of 30 mA g−1 and cut-off potential of 3.6–2 V versus Li+ /Li at ambient temperature.
3. Results and discussion Fig. 1 shows the TEM photograph of CuV2 O6 flocculent precipitation obtained by mixing V2 O5 gel with Cu2 O powder. It shows a network structure resembling that of V2 O5 gel [12] and the observation implies that the reaction is homogeneous with the help of ultrasonic irradiation. This is because that V2 O5 gel can behave as a versatile host structure for the intercalation of a wide range of ionic or molecular species, and it is easy for copper ions to intercalate into layers of V2 O5 gel at room temperature. The XRD patterns of samples heated at different temperatures are shown in Fig. 2. The main diffraction peaks are consistent
Fig. 1. TEM photograph of CuV2 O6 flocculent precipitation obtained by reacting V2 O5 gel with Cu2 O.
Fig. 2. Evolution of XRD patterns of CuV2 O6 heated at different temperatures.
with the pattern of JCPDS card (30-0513). From Fig. 2, it can be seen that some visible diffraction peaks appear in the pattern recorded at 250 ◦ C. By increasing the heating temperature, the crystallinity of the sample increases and main diffraction peaks appears sharper. The XRD result agrees well with our expectation that CuV2 O6 can be successfully prepared from the new route and the temperature required for the formation of CuV2 O6 is lowered compared to the conventional solid-state method. It is interesting that Cu+ is oxidized to Cu2+ in this process, which is somewhat similar to the synthesis of copper pyrovanadate [20] and implies that some oxido-reduction reactions take place. The crystal lattice parameters of the compounds, calculated by means of least-squares method in terms of triclinic structures are presented in Table 1, which shows that lattice constants of “soft-synthesis” sample at 550 ◦ C are smaller than those of “solid-synthesis” sample. Fig. 3 shows the TG–DTA curves of the precursor air-dried at 100 ◦ C. Upon heating, the CuV2 O6 precursor continuously loses weight until 340 ◦ C. Three distinct endothermal peaks are observed near 64, 145 and 252 ◦ C on DTA curve corresponding to a sample weight loss of 4.9, 12.71 and 18.55%, respectively. The weight losses are possibly due to the liberation of the water molecules. In addition, there are two exothermic peaks at 202
Fig. 3. Thermal analysis traces of precursor air-dried at 100 ◦ C.
X. Cao et al. / Materials Chemistry and Physics 98 (2006) 71–75
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Table 1 The crystal lattice constants of CuV2 O6 obtained by different synthetic methods
550 ◦ C 620 ◦ C
˚ a (A)
˚ b (A)
˚ c (A)
α (◦ )
β (◦ )
γ (◦ )
˚ 3) V (A
9.1655(8) 9.1911(8)
3.5549(4) 3.5641(4)
6.4845(6) 6.4975(6)
92.2762(92) 92.0787(93)
110.4580(72) 110.6166(72)
91.8106(97) 91.9309(91)
197.5664(479) 198.8344(466)
and 340 ◦ C. The first broad exothermic peak may be explained by the energy liberation due to losing water molecule from the structure of CuV2 O6 . The second exothermic peak should be ascribed to the further crystallization of CuV2 O6 . The morphologies of CuV2 O6 annealed at different temperatures are illustrated by scanning electron micrographs (SEM) in Fig. 4. The particle shape changes with the treating temperature. The grains of sample treated at 250 ◦ C are agglomerated and the sample obtained at 300 ◦ C is dispersed into about 200 nm small grains due to dehydration. With the temperature further increas-
ing, grains of samples become larger. Fig. 4e and f show the micrographs of CuV2 O6 obtained by “soft-synthesis” route at 550 ◦ C and “solid-state synthesis” at 620 ◦ C, respectively. It can be found that the particle size of the former is about 2 m and distributed in a narrow range, but the latter consists of many larger platelets shaped particle with a size of more than 5 m. The CuV2 O6 cathode material has high specific capacity compared to other vanadium-based cathode materials. Fig. 5 shows the first discharge curves of the CuV2 O6 samples obtained at different temperatures. As shown in Fig. 5, the samples
Fig. 4. SEM of samples annealed at different temperatures: (a) 250 ◦ C; (b) 300 ◦ C; (c) 400 ◦ C; (d) 500 ◦ C; (e) 550 ◦ C; (f) 620 ◦ C (solid-state synthesis).
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Fig. 5. The first discharge curves of samples treated at different temperatures (30 mA g−1 ).
heated at 300–550 ◦ C have a first discharge capacity of 349, 343 and 336 mAh g−1 , respectively. The discharge capacity of a sample obtained by the high temperature solid-state method is 329 mAh g−1 , which is slightly lower than those of samples made by the soft chemistry method. Furthermore, in the case of “soft-synthesis” sample obtained at 300 ◦ C, two plateaus appear clearly near 2.95 and 2.23 V in the discharge curve, as shown in Fig. 5. With the temperature increasing, the two plateaus shift in the lower potential direction, which may be explained by the ions transports rate [21]. As the electrochemical performance of product is closely connected with its particle size and particle size distribution. The smaller grain size results in a shorter diffusion path for Li+ ions and effectively restrains concentration polarization. As being discussed above, the higher heating temperature will lead to the formation of larger-sized grains and it makes Li+ ions diffusion become the control step for the overall electrochemical reaction. However, for the high temperature solid-state product, the second discharge plateau is higher than that of “soft-synthesis” sample, and we cannot give an exact explanation for the phenomena. As far as cycleability of “soft-synthesis” sample is concerned, it should be noted that the cycleability of samples improves with increasing temperature. The sample obtained at 550 ◦ C can not only exhibit high specific capacity but also a good cycle performance. This result may be ascribed to a stabilizing structure due to the shrinkage of the cell volume compared to the solid-state product. Fig. 6 shows the typical discharge–charge curves of sample prepared at 550 ◦ C by the “soft-synthesis” route and by the high temperature solid-state synthesis method. From Fig. 6, it is observed that the “soft-synthesis” sample shows a much better cycleabilty with a capacity of 210 mAh g−1 after 20 cycles, but capacity fading of the “solid-state synthesis” sample is severe, only a capacity of 145 mAh g−1 is achieved after 20 cycles. In addition, it can be seen that the discharge curve of the first cycle is different from that of the subsequent cycles, which indicates that a structure modification may take place after the first discharge. However, whatever “soft-synthesis” sample or “solid-state synthesis” sample, capacity loss is severe between the first and the
Fig. 6. Discharge–charge curves of CuV2 O6 treated at different temperatures (30 mA g−1 , 3.6–2 V): (a) 550 ◦ C; (b) 620 ◦ C(solid-state synthesis).
Fig. 7. The variation of specific energy and average discharge voltage of samples vs. cycle number.
second discharge. So decreasing the capacity loss is a very crucial problem that needed to be solved. Fig. 7 further shows the variation of specific energy and average discharge voltage (ADV) of samples with cycle num-
X. Cao et al. / Materials Chemistry and Physics 98 (2006) 71–75
ber. From Fig. 7, it is observed that ADV of “soft-synthesis” sample obtained at 550 ◦ C is about 2.6 V and higher than that by the solid-state method. The first discharge specific energy of “soft-synthesis” sample is 800 and 600 Wh kg−1 can be maintained after 20 cycles. Comparatively, specific energy of “solid-state synthesis” sample decreases rapidly and only retains 400 Wh kg−1 after 20 cycles. These results indicate that two different synthesis method results in the difference of the electrochemical performance, which shows that small cell volume is favorable to stabilize crystal structure during the charge–discharge process. 4. Conclusion CuV2 O6 is one of the promising cathode candidates in rechargeable lithium batteries and is successfully synthesized for the first time via a soft chemistry method in this work. Owing to the advantages of the soft chemistry method, the stability of thusprepared material is better than that of the “solid-state”sample. The sample annealed at 550 ◦ C for 10 h exhibits both high discharge capacity and excellent cycle performance. To further improve the cycle performance of CuV2 O6 , further work in understanding the mechanism of CuVO reacting with lithium during the discharge–charge process needs to be done. References [1] Y. Sakurai, H. Ohtsuka, J.I. Yamaki, J. Electrochem. Soc. 135 (1988) 32.
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[2] E. Andrukaitis, J. Power Sources 68 (1997) 656. [3] E. Andrukaitis, J.P. Cooper, J.H. Smit, J. Power Sources 54 (1995) 465. [4] F. Coustier, G. Jarero, S. Passerini, W.H. Smyrl, J. Power Sources 83 (1999) 9. [5] M. Kamata, G. Oriji, Y. Katamaya, T. Miura, T. Kishi, Solid State Ionics 146 (2002) 95. [6] M. Morcrette, P. Rozier, L. Dupont, E. Mugnier, L. Sannier, J. Galy, J.M. Tarascon, Nat. Mater. 2 (2003) 755. [7] M. Eguchi, T. Iwamoto, T. Miura, T. Kishi, Solid State Ionics 89 (1996) 109. [8] F. Garcia-Alvarado, J.M. Tarascon, B. Wilkens, J. Electrochem. Soc. 139 (1992) 3206. [9] M. Eguchi, A. Komamura, T. Miura, T. Kishi, Electrochim. Acta 41 (1996) 857. [10] C. Calvo, D. Manolescu, Acta. Crystallogr. B 29 (1973) 1743. [11] A.V. Prokofiev, R.K. Kremer, W. Assmus, J. Cryst. Growth 231 (2001) 498. [12] J.G. Xie, J.X. Li, H. Zhan, Y.H. Zhou, Mater. Lett. 57 (2003) 2682. [13] J.G. Xie, J.X. Li, Z.X. Dai, H. Zhan, Y.H. Zhou, J. Mater. Sci. 39 (2004) 2565. [14] J.G. Xie, X.Y. Cao, J.X. Li, H. Zhan, Y.Y. Xia, Y.H. Zhou, Ultrasonics Sonochem. 12 (2005) 289. [15] J. Livage, Solid State Ionics 86–88 (1996) 935. [16] J. Pereira-Ramos, J. Power Sources 54 (1995) 120. [17] J. Legendre, J. Livage, J. Colloids Interf. Sci. 94 (1983) 75. [18] J. Legendre, P. Aldebert, N. Baffer, J. Livage, J. Colloids Interf. Sci. 94 (1983) 84. [19] H.W. Zanbdergen, A.M. Crespi, P.M. Skarstad, J.F. Vente, J. Solid State Chem. 110 (1994) 167. [20] K. Melghit, A.H. Yahaya, I.I. Yaaccob, Mater. Lett. 57 (2003) 1423. [21] J. Gao, C.Y. Jiang, C.R. Wan, J. Power Sources 125 (2004) 90.
Synthesis of CuV2O6 as a cathode material for rechargeable lithium batteries from V2O5 gel Xiaoyu Cao, Jinggang Xie, Hui Zhan, Yunhong Zhou ∗ College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China Received 22 April 2005; received in revised form 27 August 2005; accepted 28 August 2005
Abstract CuV2 O6 is a very promising cathode material for rechargeable lithium batteries. By a soft chemistry method, CuV2 O6 is successfully synthesized from V2 O5 hydrogel and Cu2 O powder. CuV2 O6 with different degrees of crystallinity are obtained by heating CuV2 O6 precursor at various temperatures. XRD, TG–DTA, TEM and SEM experiments are conducted to characterize its physical properties, and the electrochemical properties have been investigated by galvanostatic charge–discharge experiments. As a result, CuV2 O6 annealed at 550 ◦ C has smaller crystal lattice constants and better electrochemical properties compared to the sample synthesized by the conventional solid-state method. © 2006 Published by Elsevier B.V. Keywords: Copper vanadiun oxides; V2 O5 gel; Soft chemistry; Rechargeable lithium batteries; Cathode
1. Introduction While Li-ion technology is enjoying great commercial success, many researchers are devoting large efforts to search more cost-effective oxide cathodes for advanced lithium batteries, to replace technically excellent but expensive lithium cobalt oxide. Transition metal vanadates, such as copper vanadium oxides (CVO), are of interest as lithium insertion hosts [1–9], where CuV2 O6 has a triclinic structure with space group P 1¯ [10,11] and can afford a layered space where Li ion can be accommodated. Among the present CVO systems, CuV2 O6 can deliver the highest discharge specific capacity and energy density. Like other vanadium-based cathode materials such as Li1.2 V3 O8 and Ag2 V4 O11 [12–14], the electrochemical property of CuV2 O6 highly depends on the synthetic and processing methods. In the case of the synthesis of CuV2 O6 , two typical methods have been reported. One method is the conventional high temperature solid-state reaction [1]. In this reaction, copper oxide (CuO) and vanadium pentoxide (V2 O5 ) are mixed in a molar ratio of 1:1, and then the mixture is melted in an alumina crucible at 620 ◦ C in air for 48.5 h. This method requires a large thermal energy and long reaction time. The other method is coprecipitation reaction in which NH4 VO3 reacted with desired ∗
Corresponding author. Tel.: +86 27 68756931; fax: +86 27 68754067. E-mail address: [email protected] (Y. Zhou).
0254-0584/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2005.08.068
stoichiometric Cu(NO3 )2 in aqueous solution at room temperature and CuV2 O6 is obtained by heating the air-dried precipitation Cu(VO3 )2 ·nH2 O [2,3]. Over the past years, soft chemistry method (chimie douce) has opened a new avenue of research towards material sciences by showing its ability to prepare some materials with peculiar properties [2–4,15,16]. The essence of soft chemistry is that raw materials can be mixed homogeneously at nano-level and react at mild conditions so that products with excellent performance can be obtained at much lower temperature. In this work, CuV2 O6 is successfully synthesized for the first time with V2 O5 hydrogel and Cu2 O powder. V2 O5 gel is an ionic layered compound made up of V2 O5 ·nH2 O layered sheets that have a microstructure resembling that of the crystalline o-V2 O5 [17,18]. It is expected that many vanadium-based compounds conventionally prepared from crystalline V2 O5 may be obtained from V2 O5 gel. 2. Experimental V2 O5 gel was prepared by polycondensation of vanadic acid that was obtained by passing 0.5 mol L−1 sodium metavanadate (Fluka, >98%) aqueous solution through a proton exchange resin (Dowex 50 WX2 100 mesh). The collected effluent, decavanadic acid solution, resulted in the formation of homogeneous vanadium pentoxide hydrogel for 1 or 2 days aging with no agitation at room temperature. By weighing the crystalline o-V2 O5 before and after heating some of the V2 O5 hydrogel at 360 ◦ C in air, one could determine the concentration of the V2 O5 hydrogel precisely and easily [19]. When stoichiometric
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X. Cao et al. / Materials Chemistry and Physics 98 (2006) 71–75
Cu2 O (ACROS, >97%) powder was added directly into V2 O5 gel, some green flocculation was formed. Then the mixture was treated for 1 h with ultrasonic irradiation to disperse the reactants followed by another normal stirring for 3 h and a green-brown flocculent precipitation was obtained. After being air-dried at 100 ◦ C, the resulting CuV2 O6 precursor was heated at 250, 300, 400, 500 and 550 ◦ C in air for 10 h to get the final products of CuV2 O6 , respectively. For preparing the reference material of CuV2 O6 , the mixed powders of V2 O5 and CuO (AR) were heated at 620 ◦ C in air for 48.5 h [1]. The V2 O5 was obtained by heating NH4 VO3 (AR) at 450 ◦ C for 24 h, which gives rise to V2 O5 with a higher reactive activity. The ultrasonicator (Kesheng Ultrasonics Instrument, China) used was a KS600 type ultrasonic cell grinder (600 W, 20 kHz). TEM photography was conducted on a JEM-100(XII) microscope when the CuV2 O6 flocculent precipitation was first formed. Thermogravimetry (TG) and differential thermal analysis (DTA) were performed using WCT-1A (Beijing Optical Instruments) in air at a heating rate of 10 ◦ C min−1 . Powder X-ray diffraction (XRD) experiments were carried out using a SHIMADZU XRD-6000 X-ray diffractometer with Cu K␣ radiation. The cathode sheet composed of 80 wt.% active material, 10 wt.% acetylene black (AB) and 10 wt.% polytetrafluoethylene (PTFE) binder. A stainless-steel mesh acted as the current collector. In the CR2016 coin cell, the lithium anode and the cathode were separated by Celgard-2400. The electrolyte was 1 mol L−1 LiClO4 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) solution with a volumetric ratio of 1:1. Coin cells were assembled in the glove box (MECAFLEX, MECABOX80-1 s) filled with high-purified argon gas. All the coin cells were tested by means of a “Land” battery instrument operating in a galvanostatic mode with a current rate of 30 mA g−1 and cut-off potential of 3.6–2 V versus Li+ /Li at ambient temperature.
3. Results and discussion Fig. 1 shows the TEM photograph of CuV2 O6 flocculent precipitation obtained by mixing V2 O5 gel with Cu2 O powder. It shows a network structure resembling that of V2 O5 gel [12] and the observation implies that the reaction is homogeneous with the help of ultrasonic irradiation. This is because that V2 O5 gel can behave as a versatile host structure for the intercalation of a wide range of ionic or molecular species, and it is easy for copper ions to intercalate into layers of V2 O5 gel at room temperature. The XRD patterns of samples heated at different temperatures are shown in Fig. 2. The main diffraction peaks are consistent
Fig. 1. TEM photograph of CuV2 O6 flocculent precipitation obtained by reacting V2 O5 gel with Cu2 O.
Fig. 2. Evolution of XRD patterns of CuV2 O6 heated at different temperatures.
with the pattern of JCPDS card (30-0513). From Fig. 2, it can be seen that some visible diffraction peaks appear in the pattern recorded at 250 ◦ C. By increasing the heating temperature, the crystallinity of the sample increases and main diffraction peaks appears sharper. The XRD result agrees well with our expectation that CuV2 O6 can be successfully prepared from the new route and the temperature required for the formation of CuV2 O6 is lowered compared to the conventional solid-state method. It is interesting that Cu+ is oxidized to Cu2+ in this process, which is somewhat similar to the synthesis of copper pyrovanadate [20] and implies that some oxido-reduction reactions take place. The crystal lattice parameters of the compounds, calculated by means of least-squares method in terms of triclinic structures are presented in Table 1, which shows that lattice constants of “soft-synthesis” sample at 550 ◦ C are smaller than those of “solid-synthesis” sample. Fig. 3 shows the TG–DTA curves of the precursor air-dried at 100 ◦ C. Upon heating, the CuV2 O6 precursor continuously loses weight until 340 ◦ C. Three distinct endothermal peaks are observed near 64, 145 and 252 ◦ C on DTA curve corresponding to a sample weight loss of 4.9, 12.71 and 18.55%, respectively. The weight losses are possibly due to the liberation of the water molecules. In addition, there are two exothermic peaks at 202
Fig. 3. Thermal analysis traces of precursor air-dried at 100 ◦ C.
X. Cao et al. / Materials Chemistry and Physics 98 (2006) 71–75
73
Table 1 The crystal lattice constants of CuV2 O6 obtained by different synthetic methods
550 ◦ C 620 ◦ C
˚ a (A)
˚ b (A)
˚ c (A)
α (◦ )
β (◦ )
γ (◦ )
˚ 3) V (A
9.1655(8) 9.1911(8)
3.5549(4) 3.5641(4)
6.4845(6) 6.4975(6)
92.2762(92) 92.0787(93)
110.4580(72) 110.6166(72)
91.8106(97) 91.9309(91)
197.5664(479) 198.8344(466)
and 340 ◦ C. The first broad exothermic peak may be explained by the energy liberation due to losing water molecule from the structure of CuV2 O6 . The second exothermic peak should be ascribed to the further crystallization of CuV2 O6 . The morphologies of CuV2 O6 annealed at different temperatures are illustrated by scanning electron micrographs (SEM) in Fig. 4. The particle shape changes with the treating temperature. The grains of sample treated at 250 ◦ C are agglomerated and the sample obtained at 300 ◦ C is dispersed into about 200 nm small grains due to dehydration. With the temperature further increas-
ing, grains of samples become larger. Fig. 4e and f show the micrographs of CuV2 O6 obtained by “soft-synthesis” route at 550 ◦ C and “solid-state synthesis” at 620 ◦ C, respectively. It can be found that the particle size of the former is about 2 m and distributed in a narrow range, but the latter consists of many larger platelets shaped particle with a size of more than 5 m. The CuV2 O6 cathode material has high specific capacity compared to other vanadium-based cathode materials. Fig. 5 shows the first discharge curves of the CuV2 O6 samples obtained at different temperatures. As shown in Fig. 5, the samples
Fig. 4. SEM of samples annealed at different temperatures: (a) 250 ◦ C; (b) 300 ◦ C; (c) 400 ◦ C; (d) 500 ◦ C; (e) 550 ◦ C; (f) 620 ◦ C (solid-state synthesis).
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Fig. 5. The first discharge curves of samples treated at different temperatures (30 mA g−1 ).
heated at 300–550 ◦ C have a first discharge capacity of 349, 343 and 336 mAh g−1 , respectively. The discharge capacity of a sample obtained by the high temperature solid-state method is 329 mAh g−1 , which is slightly lower than those of samples made by the soft chemistry method. Furthermore, in the case of “soft-synthesis” sample obtained at 300 ◦ C, two plateaus appear clearly near 2.95 and 2.23 V in the discharge curve, as shown in Fig. 5. With the temperature increasing, the two plateaus shift in the lower potential direction, which may be explained by the ions transports rate [21]. As the electrochemical performance of product is closely connected with its particle size and particle size distribution. The smaller grain size results in a shorter diffusion path for Li+ ions and effectively restrains concentration polarization. As being discussed above, the higher heating temperature will lead to the formation of larger-sized grains and it makes Li+ ions diffusion become the control step for the overall electrochemical reaction. However, for the high temperature solid-state product, the second discharge plateau is higher than that of “soft-synthesis” sample, and we cannot give an exact explanation for the phenomena. As far as cycleability of “soft-synthesis” sample is concerned, it should be noted that the cycleability of samples improves with increasing temperature. The sample obtained at 550 ◦ C can not only exhibit high specific capacity but also a good cycle performance. This result may be ascribed to a stabilizing structure due to the shrinkage of the cell volume compared to the solid-state product. Fig. 6 shows the typical discharge–charge curves of sample prepared at 550 ◦ C by the “soft-synthesis” route and by the high temperature solid-state synthesis method. From Fig. 6, it is observed that the “soft-synthesis” sample shows a much better cycleabilty with a capacity of 210 mAh g−1 after 20 cycles, but capacity fading of the “solid-state synthesis” sample is severe, only a capacity of 145 mAh g−1 is achieved after 20 cycles. In addition, it can be seen that the discharge curve of the first cycle is different from that of the subsequent cycles, which indicates that a structure modification may take place after the first discharge. However, whatever “soft-synthesis” sample or “solid-state synthesis” sample, capacity loss is severe between the first and the
Fig. 6. Discharge–charge curves of CuV2 O6 treated at different temperatures (30 mA g−1 , 3.6–2 V): (a) 550 ◦ C; (b) 620 ◦ C(solid-state synthesis).
Fig. 7. The variation of specific energy and average discharge voltage of samples vs. cycle number.
second discharge. So decreasing the capacity loss is a very crucial problem that needed to be solved. Fig. 7 further shows the variation of specific energy and average discharge voltage (ADV) of samples with cycle num-
X. Cao et al. / Materials Chemistry and Physics 98 (2006) 71–75
ber. From Fig. 7, it is observed that ADV of “soft-synthesis” sample obtained at 550 ◦ C is about 2.6 V and higher than that by the solid-state method. The first discharge specific energy of “soft-synthesis” sample is 800 and 600 Wh kg−1 can be maintained after 20 cycles. Comparatively, specific energy of “solid-state synthesis” sample decreases rapidly and only retains 400 Wh kg−1 after 20 cycles. These results indicate that two different synthesis method results in the difference of the electrochemical performance, which shows that small cell volume is favorable to stabilize crystal structure during the charge–discharge process. 4. Conclusion CuV2 O6 is one of the promising cathode candidates in rechargeable lithium batteries and is successfully synthesized for the first time via a soft chemistry method in this work. Owing to the advantages of the soft chemistry method, the stability of thusprepared material is better than that of the “solid-state”sample. The sample annealed at 550 ◦ C for 10 h exhibits both high discharge capacity and excellent cycle performance. To further improve the cycle performance of CuV2 O6 , further work in understanding the mechanism of CuVO reacting with lithium during the discharge–charge process needs to be done. References [1] Y. Sakurai, H. Ohtsuka, J.I. Yamaki, J. Electrochem. Soc. 135 (1988) 32.
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