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Study on the synthesis and properties of LiV3O8 rechargeable lithium batteries cathode
Electrochimica Acta 47 (2002) 3239 /3243 www.elsevier.com/locate/electacta
Study on the synthesis and properties of LiV3O8 rechargeable lithium batteries cathode G.Q. Liu *, C.L. Zeng, K. Yang State Key Laboratory for Corrosion and Protection, Institute of Metal Research, The Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China Received 2 January 2002; received in revised form 20 March 2002
Abstract A new simple synthetic method was employed to produce LiV3O8 compound in which LiOH, V2O5 and NH4OH were used as the starting reactants. At first, V2O5 reacted with LiOH and NH4OH in liquid solutions to obtain a compound containing Li and V, which was then calcined at 370, 450 and 550 8C for 8 h, respectively. The electrochemical properties of the LiV3O8 compound were studied by galvanostatic charge /discharge, and the highest capacity of 274 mAh g 1 was obtained for the LiV3O8 compound calcined at 370 8C in the range of 1.8 /4.0 V. In the fifteenth cycle, its capacity remained 257 mAh g 1. The inspections by X-ray diffraction and SEM indicated that different calcining temperatures resulted in different structure, which resulted in different discharge capacity. # 2002 Published by Elsevier Science Ltd. Keywords: Lithium vanadate; Rechargeable battery; Cathode; Synthesis; Electrochemical properties
1. Introduction LiV3O8 is a promising cathode material in rechargeable lithium batteries, for it has very attractive characteristics such as high specific energy, good rate capacity, and long cycle life due to its unique crystal structure and its outstanding structure stability [1,2]. A earlier synthetic method of LiV3O8 was high-temperature melting in which Li2CO3 reacted with V2O5 at 680 8C [3]. At such high reaction temperature, the reactants Li2CO3 and V2O5 would evaporate, so their accurate amounts were difficult to control. Another defect of this synthetic method is that the product LiV3O8 had a low capacity of 180 mAh g 1 in the range of 1.8 /4.0 V. In addition, molten V2O5 would cause corrosion to the crucible. Afterwards, many improved methods were proposed, among which the liquid reaction method attracted much
* Corresponding author. Fax: /86-24-2389-3624. E-mail address: [email protected] (G.Q. Liu).
interest [4 /7]. Usually, the compounds containing Li and V were used as starting reactants, and reactions took place in liquid solutions. The liquid reaction method does not need a high reaction temperature, and the product could reach a high capacity. However, these liquid reactions need a long reaction time and additional process, such as stirring and heating. It took much time for these liquid reactions to complete. In this research, a new easy synthetic method was employed to prepare LiV3O8 compound, in which LiOH and V2O5 were blended in water, and then some NH4OH was added to the mixture, while V2O5 were dissolved into solution instantly. After evaporating water, drying and calcining, LiV3O8 was obtained. In the present research, three different LiV3O8 compounds were obtained by calcining at 370, 450 and 550 8C for 8 h, respectively. To compare their electrochemical performances, galvanostatic discharge/charge test was performed. In order to clarify the relationships between the structure and morphology of LiV3O8 and their electrochemical performances, the XRD and SEM were also carried out.
0013-4686/02/$ - see front matter # 2002 Published by Elsevier Science Ltd. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 1 7 4 - 3
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G.Q. Liu et al. / Electrochimica Acta 47 (2002) 3239 /3243
2. Experimental
2.1. Materials LiOH, V2O5 and NH4OH used to prepare LiV3O8 were analytical grade. The electrolyte used to measure discharge /charge capacity, consisting of LiPF6 (1 M)EC/DMC (1:1, volume ratio; EC, ethylene carbonate; DMC, dimethyl carbonate) and the polyvinylidene binder (PVDF), were lithium battery grade.
2.2. Synthesis of LiV3O8 Stoichiometrically weighted LiOH and V2O5 (Li:V / 1:3, molar rate) were blended in some deionized water, and part of V2O5 reacted with LiOH. After NH4OH was added to the above mixture, V2O5 was dissolved completely into solution instantly. The liquid mixture was then put in 80 8C bath to evaporate water. The drying process was performed in vacuum at 120 8C for 4 h, and then calcining process was carried out at 370, 450 and 550 8C for 8 h, respectively. Thermogravimetric analysis (TGA), with a heating rate of 5 8C min 1, was carried out to determine the reaction process.
3. Results and discussion
3.1. Synthetic process analysis of LiV3O8 When LiOH and V2O5 were blended in water, part of V2O5 reacted with LiOH as follows: 2LiOHV2 O5 0 2LiVO3 2LiVO3 2V2 O5 0 2LiV3 O8
(1) (2)
The first reaction took place easily, but the second one proceeded slowly [5]. After ammonia was slowly added to the mixture, solid V2O5 was dissolved instantly by the following reaction: NH4 OHV2 O5 0 NH4 VO3 H2 O
(3)
When the mixture was put in 80 8C bath to remove water, a yellow NH4V3O8 ×/x H2O precipitated. 6NH4 VO3 2NH4 V3 O8 ×xH2 O4NH3 2(1 x)H2 O
(4)
In addition, with the evaporation of NH3, the pH value of solution reduced, and a little brown solid V2O5 were obtained. In the subsequent calcining process, the following reactions took place: NH4 V3 O8 ×xH2 OLiOH
2.3. Electrochemical measurements The electrochemical properties of the products were performed in cells with metallic lithium as the negative electrode. The cathode was separated from the Li anode by a layer of celgard 2300 membrane soaked with the electrolyte. The cathode was a mixture of 85 wt.% active material, 10% conducting carbon black, and 5% PVDF. Before mixing with other materials, the LiV3O8 samples must be ground and sieved by a 400 mesh sieve. The cells were assembled in an argon-filled dry box. Charge / discharge tests were performed at a constant current density of 0.3 mA cm 2, in the range of 1.8 /4.0 V. All the tests were carried out at room temperature (r.t.). The specific capacity values were calculated from the value of the current, the mass of active material in the cathode, and the elapsed time.
0 LiV3 O8 NH3 (1x)H2 O LiVO3 V2 O5 0 LiV3 O8
Fig. 1 represents the result of TGA test. It can be seen that in the range of 130 /320 8C, the weight loss was 17.1%. Above 320 8C, the weight remained stable. From above results, we can conclude that the reactions took place in the range of 130/320 8C. In fact, the weight loss process can be divided into two stages, one was in the range of 130/230 8C with a weight loss of 13.7%, which mainly resulted from the evaporation of NH3. The other stage occurred in the range of 230/
2.4. Structure and morphology analysis of LiV3O8 The precursor and LiV3O8 powders were characterized by X-ray diffraction using a Siemens D5005 X-ray powder diffractometer. X-ray profiles were measured between 10 and 908 2u with a monochromatic CuKa radiation source. The morphologies of the products were examined with a XL-30FEG scanning electron microscope.
(5) (6)
Fig. 1. TGA of the synthetic process, 5 8C min 1.
G.Q. Liu et al. / Electrochimica Acta 47 (2002) 3239 /3243
320 8C with a weight loss of 3.4%, which was caused by the deintercalation of water from NH4V3O8. 3.2. Charge /discharge tests of LiV3O8 compound The discharge and charge curves of LiV3O8 formed at 370 8C are shown in Fig. 2. We can see that the open circuit voltage of LiV3O8 is about 3.22 V. At the beginning of discharge, the voltage drops sharply to 2.92 V. This is a polarization process resulting from non-stable diffusion [8]. In the range of 2.92 /2.10 V, the voltage begins to drop slowly. This is a discharge process in which Li is inserted into the cathode. There are two stages in this process, one is in the range of 2.92 /2.67 V, and the other is in the range of 2.67 /2.10 V, which demonstrates that phase transitions may take place, but further test evidences are needed. When voltage reaches below 2.10 V, the voltage drops rapidly again, which means another polarization process resulting from the retardation of electrode reaction, namely the reduction of the amount of Li inserted into the cathode. Based on the equation, specific capacity /nF / 3.6Mw, where F is Faradic constant and n is the amount of Li inserted into the cathode and Mw represents the molecular weight of LiV3O8, we can obtain that a total of 2.94 Li is inserted into the cathode. Then the cathode becomes Li12.94V3O8 after discharge. In the subsequent charge process, we can see that in the range of 1.8 /2.64 V the voltage goes up dramatically. This is a polarization process that resulted from the retardation of deintercalation of Li from the cathode. When the voltage reaches 2.64 V, the curve ascends gradually, indicating a deintercalating process of Li from LiV3O8. When the voltage reaches 3.5 V, the curve rises steeply, meaning a polarization process resulting from the reduction of deintercalation of Li from the cathode. From Fig. 2, we can also see that the discharge capacity of LiV3O8 reaches 274 mAh g 1 in the first cycle, 269 mAh g 1 in the third cycle and 257 mAh g 1 in the
Fig. 2. Discharge and charge curves of the LiV3O8 synthesized at 370 8C. Current density, 0.3 mA cm 2.
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fifteenth cycle. This result gets close to the result of another liquid reaction method in which the capacity reached about 280 mAh g 1 [7]. From Fig. 3, we can see that the discharge capacity of LiV3O8 synthesized at 450 8C is 264, 259 and 245 mAh g 1, respectively, in the first, third and fifteenth cycle. From Fig. 4, we can see that the discharge capacity of LiV3O8 synthesized at 550 8C is 192, 188 and 175 mAh g 1, respectively, in the first, third and fifteenth cycle. From the above results, it can seen that with increase in synthetic temperature, the discharge capacity of LiV3O8 decreases, which is related to the different structures of LiV3O8 synthesized at different temperature. 3.3. XRD and SEM inspections of LiV3O8 Fig. 5 shows the X-ray patterns of LiV3O8 synthesized at 370, 450 and 550 8C. It can be seen that with increase in temperature the intensity of peaks becomes stronger, which indicates that the crystallization becomes higher.
Fig. 3. Discharge and charge curves of the LiV3O8 synthesized at 450 8C. Current density, 0.3 mA cm 2.
Fig. 4. Discharge and charge curves of the LiV3O8 synthesized at 550 8C. Current density, 0.3 mA cm 2.
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Fig. 5. XRD of the LiV3O8 compounds synthesized at 370, 450 and 550 8C. (a) LiV3O8 compound synthesized at 550 8C; (b) LiV3O8 compound synthesized at 450 8C; (c) LiV3O8 compound synthesized at 370 8C.
Fig. 6. SEM image of the LiV3O8 synthesized at 370 8C.
Fig. 7. SEM image of the LiV3O8 synthesized at 450 8C.
In addition, there is an obvious difference among the three XRD patterns. The relative intensity of (100) peaks at an approximately diffraction angle of 148 is different. The LiV3O8 synthesized at 550 8C has the strongest (100) peak, while that synthesized at 450 8C has a weaker (100) peak, and that synthesized at 370 8C
Fig. 8. SEM image of the LiV3O8 synthesized at 550 8C.
has the weakest (100) peak. It can be concluded that the structure of LiV3O8 depends on the synthetic temperature, and higher synthetic temperature can result in a preferred orientation of crystallites. The intercalation process of Li ion between the layers of the cathode is a diffusion process, in which a long path is not advantageous to intercalation. The preferred orientation is responsible for the reduction of discharge capacity of LiV3O8, as shown in the previous discharge and charge results, and it should be avoided in the synthetic process. Figs. 6/8 show the morphologies of the LiV3O8 particles synthesized at 370, 450 and 550 8C, respectively. From Fig. 6, we can see that the particles of LiV3O8 synthesized at 370 8C are lossely connected, and their dimensions are well distributed. Their sizes are less than 0.4 mm, and most are approximately 0.3 mm. The particles of LiV3O8 synthesized at 450 8C samples are bar-like, with the long sides being less than 0.6 mm and the short sides less than 0.4 mm, as shown in Fig. 7. Fig. 8 shows the morphology of the 550 8C samples. It can be seen that the sizes of particles are very widely dispersive, and the particles are more independent than those synthesized at 450 8C. The particles are bar-like, and their dimensions are also much larger than those synthesized at 450 8C. Most of them have the long sides less than 1 mm, and the short sides less than 0.5 mm. There are several large particles that have long sides of approximately 2/4 mm. The results are in agreement with the XRD results and discharge/charge results. With the increase in temperature, the particles of sample become larger, less uniform, resulting in the reduction of discharge capacity.
4. Conclusion A simple method was proposed to synthesize LiV3O8 in the present study, in which LiOH, V2O5 and NH4OH
G.Q. Liu et al. / Electrochimica Acta 47 (2002) 3239 /3243
were used as starting reactants, and the liquid reactions between them could complete instantly. After evaporating, drying and calcining, the product LiV3O8 was obtained. The different synthetic temperature resulted in the disparity in structure of products, and resulted in their different capacities. With increase in synthetic temperature, the intensity of (100) peak of LiV3O8 became stronger, which was unfavorable for Li to intercalate and deintercalate in the cathode. The product LiV3O8 synthesized at 370 8C exhibited a high capacity of 274 mAh g 1 in the first cycle and a good cycle performance, reaching 257 mAh g 1 in the fifteenth cycle.
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References [1] J. Desilvestro, O. Haas, J. Electrochem. Soc. 137 (1990) 7C. [2] V.L. Piccotto, K. Adendorff, D. Liles, M. Thackeray, Solid State Ionics 62 (1993) 297. [3] G. Pistoia, M. Pasquali, G. Wang, et al., J. Electrochem. Soc. 130 (1983) 1226. [4] G. Pistoia, M. Pasquali, G. Wang, L. Li, J. Electrochem. Soc. 137 (1990) 2365. [5] V. Manev, A. Momchilov, A. Nassalevska, et al., J. Power Sources 54 (1995) 503. [6] K. West, B. Zachau-Christiansen, S. Skaarup, Y. Saidi, J. Barker, I.I. Olsen, J. Electrochem. Soc. 143 (1996) 823. [7] J. Dai, S.F.Y. Li, Z. Gao, J. Electrochem. Soc. 145 (1998) 3059. [8] Z. Quanxing, The Introduction to the Kinetic Process of Electrode(in Chinese), Science publishing house, Beijing, 1976, p. 46.
Study on the synthesis and properties of LiV3O8 rechargeable lithium batteries cathode G.Q. Liu *, C.L. Zeng, K. Yang State Key Laboratory for Corrosion and Protection, Institute of Metal Research, The Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China Received 2 January 2002; received in revised form 20 March 2002
Abstract A new simple synthetic method was employed to produce LiV3O8 compound in which LiOH, V2O5 and NH4OH were used as the starting reactants. At first, V2O5 reacted with LiOH and NH4OH in liquid solutions to obtain a compound containing Li and V, which was then calcined at 370, 450 and 550 8C for 8 h, respectively. The electrochemical properties of the LiV3O8 compound were studied by galvanostatic charge /discharge, and the highest capacity of 274 mAh g 1 was obtained for the LiV3O8 compound calcined at 370 8C in the range of 1.8 /4.0 V. In the fifteenth cycle, its capacity remained 257 mAh g 1. The inspections by X-ray diffraction and SEM indicated that different calcining temperatures resulted in different structure, which resulted in different discharge capacity. # 2002 Published by Elsevier Science Ltd. Keywords: Lithium vanadate; Rechargeable battery; Cathode; Synthesis; Electrochemical properties
1. Introduction LiV3O8 is a promising cathode material in rechargeable lithium batteries, for it has very attractive characteristics such as high specific energy, good rate capacity, and long cycle life due to its unique crystal structure and its outstanding structure stability [1,2]. A earlier synthetic method of LiV3O8 was high-temperature melting in which Li2CO3 reacted with V2O5 at 680 8C [3]. At such high reaction temperature, the reactants Li2CO3 and V2O5 would evaporate, so their accurate amounts were difficult to control. Another defect of this synthetic method is that the product LiV3O8 had a low capacity of 180 mAh g 1 in the range of 1.8 /4.0 V. In addition, molten V2O5 would cause corrosion to the crucible. Afterwards, many improved methods were proposed, among which the liquid reaction method attracted much
* Corresponding author. Fax: /86-24-2389-3624. E-mail address: [email protected] (G.Q. Liu).
interest [4 /7]. Usually, the compounds containing Li and V were used as starting reactants, and reactions took place in liquid solutions. The liquid reaction method does not need a high reaction temperature, and the product could reach a high capacity. However, these liquid reactions need a long reaction time and additional process, such as stirring and heating. It took much time for these liquid reactions to complete. In this research, a new easy synthetic method was employed to prepare LiV3O8 compound, in which LiOH and V2O5 were blended in water, and then some NH4OH was added to the mixture, while V2O5 were dissolved into solution instantly. After evaporating water, drying and calcining, LiV3O8 was obtained. In the present research, three different LiV3O8 compounds were obtained by calcining at 370, 450 and 550 8C for 8 h, respectively. To compare their electrochemical performances, galvanostatic discharge/charge test was performed. In order to clarify the relationships between the structure and morphology of LiV3O8 and their electrochemical performances, the XRD and SEM were also carried out.
0013-4686/02/$ - see front matter # 2002 Published by Elsevier Science Ltd. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 1 7 4 - 3
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2. Experimental
2.1. Materials LiOH, V2O5 and NH4OH used to prepare LiV3O8 were analytical grade. The electrolyte used to measure discharge /charge capacity, consisting of LiPF6 (1 M)EC/DMC (1:1, volume ratio; EC, ethylene carbonate; DMC, dimethyl carbonate) and the polyvinylidene binder (PVDF), were lithium battery grade.
2.2. Synthesis of LiV3O8 Stoichiometrically weighted LiOH and V2O5 (Li:V / 1:3, molar rate) were blended in some deionized water, and part of V2O5 reacted with LiOH. After NH4OH was added to the above mixture, V2O5 was dissolved completely into solution instantly. The liquid mixture was then put in 80 8C bath to evaporate water. The drying process was performed in vacuum at 120 8C for 4 h, and then calcining process was carried out at 370, 450 and 550 8C for 8 h, respectively. Thermogravimetric analysis (TGA), with a heating rate of 5 8C min 1, was carried out to determine the reaction process.
3. Results and discussion
3.1. Synthetic process analysis of LiV3O8 When LiOH and V2O5 were blended in water, part of V2O5 reacted with LiOH as follows: 2LiOHV2 O5 0 2LiVO3 2LiVO3 2V2 O5 0 2LiV3 O8
(1) (2)
The first reaction took place easily, but the second one proceeded slowly [5]. After ammonia was slowly added to the mixture, solid V2O5 was dissolved instantly by the following reaction: NH4 OHV2 O5 0 NH4 VO3 H2 O
(3)
When the mixture was put in 80 8C bath to remove water, a yellow NH4V3O8 ×/x H2O precipitated. 6NH4 VO3 2NH4 V3 O8 ×xH2 O4NH3 2(1 x)H2 O
(4)
In addition, with the evaporation of NH3, the pH value of solution reduced, and a little brown solid V2O5 were obtained. In the subsequent calcining process, the following reactions took place: NH4 V3 O8 ×xH2 OLiOH
2.3. Electrochemical measurements The electrochemical properties of the products were performed in cells with metallic lithium as the negative electrode. The cathode was separated from the Li anode by a layer of celgard 2300 membrane soaked with the electrolyte. The cathode was a mixture of 85 wt.% active material, 10% conducting carbon black, and 5% PVDF. Before mixing with other materials, the LiV3O8 samples must be ground and sieved by a 400 mesh sieve. The cells were assembled in an argon-filled dry box. Charge / discharge tests were performed at a constant current density of 0.3 mA cm 2, in the range of 1.8 /4.0 V. All the tests were carried out at room temperature (r.t.). The specific capacity values were calculated from the value of the current, the mass of active material in the cathode, and the elapsed time.
0 LiV3 O8 NH3 (1x)H2 O LiVO3 V2 O5 0 LiV3 O8
Fig. 1 represents the result of TGA test. It can be seen that in the range of 130 /320 8C, the weight loss was 17.1%. Above 320 8C, the weight remained stable. From above results, we can conclude that the reactions took place in the range of 130/320 8C. In fact, the weight loss process can be divided into two stages, one was in the range of 130/230 8C with a weight loss of 13.7%, which mainly resulted from the evaporation of NH3. The other stage occurred in the range of 230/
2.4. Structure and morphology analysis of LiV3O8 The precursor and LiV3O8 powders were characterized by X-ray diffraction using a Siemens D5005 X-ray powder diffractometer. X-ray profiles were measured between 10 and 908 2u with a monochromatic CuKa radiation source. The morphologies of the products were examined with a XL-30FEG scanning electron microscope.
(5) (6)
Fig. 1. TGA of the synthetic process, 5 8C min 1.
G.Q. Liu et al. / Electrochimica Acta 47 (2002) 3239 /3243
320 8C with a weight loss of 3.4%, which was caused by the deintercalation of water from NH4V3O8. 3.2. Charge /discharge tests of LiV3O8 compound The discharge and charge curves of LiV3O8 formed at 370 8C are shown in Fig. 2. We can see that the open circuit voltage of LiV3O8 is about 3.22 V. At the beginning of discharge, the voltage drops sharply to 2.92 V. This is a polarization process resulting from non-stable diffusion [8]. In the range of 2.92 /2.10 V, the voltage begins to drop slowly. This is a discharge process in which Li is inserted into the cathode. There are two stages in this process, one is in the range of 2.92 /2.67 V, and the other is in the range of 2.67 /2.10 V, which demonstrates that phase transitions may take place, but further test evidences are needed. When voltage reaches below 2.10 V, the voltage drops rapidly again, which means another polarization process resulting from the retardation of electrode reaction, namely the reduction of the amount of Li inserted into the cathode. Based on the equation, specific capacity /nF / 3.6Mw, where F is Faradic constant and n is the amount of Li inserted into the cathode and Mw represents the molecular weight of LiV3O8, we can obtain that a total of 2.94 Li is inserted into the cathode. Then the cathode becomes Li12.94V3O8 after discharge. In the subsequent charge process, we can see that in the range of 1.8 /2.64 V the voltage goes up dramatically. This is a polarization process that resulted from the retardation of deintercalation of Li from the cathode. When the voltage reaches 2.64 V, the curve ascends gradually, indicating a deintercalating process of Li from LiV3O8. When the voltage reaches 3.5 V, the curve rises steeply, meaning a polarization process resulting from the reduction of deintercalation of Li from the cathode. From Fig. 2, we can also see that the discharge capacity of LiV3O8 reaches 274 mAh g 1 in the first cycle, 269 mAh g 1 in the third cycle and 257 mAh g 1 in the
Fig. 2. Discharge and charge curves of the LiV3O8 synthesized at 370 8C. Current density, 0.3 mA cm 2.
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fifteenth cycle. This result gets close to the result of another liquid reaction method in which the capacity reached about 280 mAh g 1 [7]. From Fig. 3, we can see that the discharge capacity of LiV3O8 synthesized at 450 8C is 264, 259 and 245 mAh g 1, respectively, in the first, third and fifteenth cycle. From Fig. 4, we can see that the discharge capacity of LiV3O8 synthesized at 550 8C is 192, 188 and 175 mAh g 1, respectively, in the first, third and fifteenth cycle. From the above results, it can seen that with increase in synthetic temperature, the discharge capacity of LiV3O8 decreases, which is related to the different structures of LiV3O8 synthesized at different temperature. 3.3. XRD and SEM inspections of LiV3O8 Fig. 5 shows the X-ray patterns of LiV3O8 synthesized at 370, 450 and 550 8C. It can be seen that with increase in temperature the intensity of peaks becomes stronger, which indicates that the crystallization becomes higher.
Fig. 3. Discharge and charge curves of the LiV3O8 synthesized at 450 8C. Current density, 0.3 mA cm 2.
Fig. 4. Discharge and charge curves of the LiV3O8 synthesized at 550 8C. Current density, 0.3 mA cm 2.
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Fig. 5. XRD of the LiV3O8 compounds synthesized at 370, 450 and 550 8C. (a) LiV3O8 compound synthesized at 550 8C; (b) LiV3O8 compound synthesized at 450 8C; (c) LiV3O8 compound synthesized at 370 8C.
Fig. 6. SEM image of the LiV3O8 synthesized at 370 8C.
Fig. 7. SEM image of the LiV3O8 synthesized at 450 8C.
In addition, there is an obvious difference among the three XRD patterns. The relative intensity of (100) peaks at an approximately diffraction angle of 148 is different. The LiV3O8 synthesized at 550 8C has the strongest (100) peak, while that synthesized at 450 8C has a weaker (100) peak, and that synthesized at 370 8C
Fig. 8. SEM image of the LiV3O8 synthesized at 550 8C.
has the weakest (100) peak. It can be concluded that the structure of LiV3O8 depends on the synthetic temperature, and higher synthetic temperature can result in a preferred orientation of crystallites. The intercalation process of Li ion between the layers of the cathode is a diffusion process, in which a long path is not advantageous to intercalation. The preferred orientation is responsible for the reduction of discharge capacity of LiV3O8, as shown in the previous discharge and charge results, and it should be avoided in the synthetic process. Figs. 6/8 show the morphologies of the LiV3O8 particles synthesized at 370, 450 and 550 8C, respectively. From Fig. 6, we can see that the particles of LiV3O8 synthesized at 370 8C are lossely connected, and their dimensions are well distributed. Their sizes are less than 0.4 mm, and most are approximately 0.3 mm. The particles of LiV3O8 synthesized at 450 8C samples are bar-like, with the long sides being less than 0.6 mm and the short sides less than 0.4 mm, as shown in Fig. 7. Fig. 8 shows the morphology of the 550 8C samples. It can be seen that the sizes of particles are very widely dispersive, and the particles are more independent than those synthesized at 450 8C. The particles are bar-like, and their dimensions are also much larger than those synthesized at 450 8C. Most of them have the long sides less than 1 mm, and the short sides less than 0.5 mm. There are several large particles that have long sides of approximately 2/4 mm. The results are in agreement with the XRD results and discharge/charge results. With the increase in temperature, the particles of sample become larger, less uniform, resulting in the reduction of discharge capacity.
4. Conclusion A simple method was proposed to synthesize LiV3O8 in the present study, in which LiOH, V2O5 and NH4OH
G.Q. Liu et al. / Electrochimica Acta 47 (2002) 3239 /3243
were used as starting reactants, and the liquid reactions between them could complete instantly. After evaporating, drying and calcining, the product LiV3O8 was obtained. The different synthetic temperature resulted in the disparity in structure of products, and resulted in their different capacities. With increase in synthetic temperature, the intensity of (100) peak of LiV3O8 became stronger, which was unfavorable for Li to intercalate and deintercalate in the cathode. The product LiV3O8 synthesized at 370 8C exhibited a high capacity of 274 mAh g 1 in the first cycle and a good cycle performance, reaching 257 mAh g 1 in the fifteenth cycle.
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References [1] J. Desilvestro, O. Haas, J. Electrochem. Soc. 137 (1990) 7C. [2] V.L. Piccotto, K. Adendorff, D. Liles, M. Thackeray, Solid State Ionics 62 (1993) 297. [3] G. Pistoia, M. Pasquali, G. Wang, et al., J. Electrochem. Soc. 130 (1983) 1226. [4] G. Pistoia, M. Pasquali, G. Wang, L. Li, J. Electrochem. Soc. 137 (1990) 2365. [5] V. Manev, A. Momchilov, A. Nassalevska, et al., J. Power Sources 54 (1995) 503. [6] K. West, B. Zachau-Christiansen, S. Skaarup, Y. Saidi, J. Barker, I.I. Olsen, J. Electrochem. Soc. 143 (1996) 823. [7] J. Dai, S.F.Y. Li, Z. Gao, J. Electrochem. Soc. 145 (1998) 3059. [8] Z. Quanxing, The Introduction to the Kinetic Process of Electrode(in Chinese), Science publishing house, Beijing, 1976, p. 46.