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Synthesis and characteristics of Li3V2(PO4)3 as cathode materials for lithium-ion batteries
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Solid State Ionics 179 (2008) 1679 – 1682 www.elsevier.com/locate/ssi
Synthesis and characteristics of Li3V2(PO4)3 as cathode materials for lithium-ion batteries Xianjun Zhu a,⁎, Yunxia Liu a , Liangmei Geng a , Longbin Chen a , Hanxing Liu b,⁎, Minghe Cao b a
b
College of Chemistry, Central China Normal University, Wuhan, Hubei 430079, China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, Hubei 430070, China Received 12 April 2007; received in revised form 22 November 2007; accepted 29 November 2007
Abstract Monoclinic lithium vanadium phosphate, Li3V2(PO4)3, was synthesized by carbon-thermal reduction (RTC) under Ar atmosphere. The influence of sintering temperatures on the synthesis of Li3V2(PO4)3 has been investigated using X-ray diffraction (XRD), SEM and electrochemical methods. XRD patterns show that the Li3V2(PO4)3 compound with monoclinic crystal structure begins to appear at the temperature of less than 900 °C. As the temperature was ≥900 °C, pure monoclinic Li3V2(PO4)3 phase can be obtained. SEM results indicate that the particle size of as-prepared samples increased with the sintering temperature increase, as well as the presence of minor carbon particles on the surface of the sample particles, which are very useful to enhance the electronic conductivity of Li3V2(PO4)3. Charge/discharge tests show the 900 °C-sample exhibits the highest initial discharge capacity of 119.3 mAh/g at 10 mA/g in the voltage range of 3.0–4.2 V with good capacity retention. CV experiment exhibits that there are three anodic peaks at 3.61, 3.69 and 4.09 V on lithium extraction as well as three cathodic peaks at 3.58, 3.66 and 4.03 V on lithium reinsertion at 0.05 mV/s between 3.0 and 4.3 V. It is suggested that the optimal sintering temperature is 900 °C in order to obtain pure monoclinic Li3V2(PO4)3 with good electrochemical performance by CRT method, and the monoclinic Li3V2(PO4)3 can be used as candidate cathode materials for lithium-ion batteries. © 2007 Elsevier B.V. All rights reserved. Keywords: Li3V2(PO4)3; Carbon-thermal reduction; Lithium-ion batteries; Cathode materials; Cyclic voltammetry
1. Introduction Lithium-ion batteries are considered to be the most advanced energy storage systems. Transition metal oxides have been the focus of a wide development effort as cathode materials [1–4]. In an intensive search for alternative materials, lithium conducting phosphates, Li3M2(PO4)3, and materials based on these compounds have emerged as the most promising candidates [5–8]. Of these materials, monoclinic Li3V2(PO4)3 [9], unlike the rhombohedral one [10], exhibits a complex series of two-phase transitions on Li extraction, followed by a solid solution regime on lithium reinsertion. The reversible cycling of all three lithiums from Li3V2 (PO4)3 would correspond to a theoretical capacity of 197 mAh/g
[11–13], which is the highest for all phosphates reported. Usually, monoclinic Li3V2(PO4)3 was synthesized by solid state reaction using hydrogen reduction method [14], which needs high reaction conditions. It is difficult to obtain monoclinic Li3V2 (PO4)3 sample with small particle and homogeneous distribution, which is critical to its electrochemical performance. In this context, monoclinic Li3V2(PO4)3 was synthesized by carbon-thermal reduction(CTR) [15]. Residual carbon left over from the CTR reaction is useful for stabilization of its electrochemical properties. It is investigated for the influence of sintering temperatures on the structure, the morphology and the electrochemical properties of monoclinic Li3V2(PO4)3. 2. Experimental
⁎ Corresponding authors. Tel.: +86 27 62178945, 87653330. E-mail addresses: [email protected] (X. Zhu), [email protected] (H. Liu). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.11.025
The monoclinic Li3V2(PO4)3 samples were prepared by carbon-thermal reduction method, using a solid state reaction
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X. Zhu et al. / Solid State Ionics 179 (2008) 1679–1682
Fig. 1 shows the XRD patterns of Li3V2(PO4)3 samples synthesized by CRT method at different temperatures. As shown in Fig. 1a and b, Li3V2(PO4)3 phase cannot be observed for the raw materials at room temperature and for the sample preheated at the temperature of 300 °C. The characteristic peaks of monoclinic Li3V2(PO4)3 come to appear at the precalcined temperature of 600 °C, indicating that monoclinic Li3V2(PO4)3 begin to form at 600 °C. (Fig. 1c). When the sintering temperature increased to 850 °C (Fig. 1d), monoclinic Li3V2(PO4)3 becomes the dominant phase, but the impurity phase of Li3PO4 exists obviously. Further increasing the sintering temperature to higher than 900 °C, purity Li3V2(PO4)3 with monoclinic crystal structure was obtained (Fig. 1e and f). The morphology of Li3V2(PO4)3 samples sintered at three different temperatures is shown in Fig. 2. As seen from Fig. 2a, Fig. 1. XRD patterns of Li3V2(PO4)3 samples at different temperatures.
by mixing stoichiometric amounts of LiOH·H2O, V2O5 and NH4H2PO4 as well as a 25% mass excess of super high area carbon over the stoichiometric reaction. The mixture was initially heated to 300 °C in air for 3 h to expel H2O and NH3, reground, pelletized, and then fired to 600 °C and held at this temperature for 16 h under a flowing Ar atmosphere. The resulting product was then ground, pelletized and sintered for 24 h at the temperatures ranging from 850 °C to 950 °C under a flowing Ar atmosphere. The completion of the reaction and the phase purity of the samples were confirmed by XRD using an X'pert powder X-ray diffractometer (Philip, Holland). Thermal analysis of the raw material mixture was performed between room temperature and 1000 °C at a heating rate of 10 °C·min− 1 under argon flow (20 ml·min− 1). The morphology and particle size of the samples was observed with field emission scanning electron microscope (JEOL, JSM6700F). The charge/discharge tests were performed using CR2016 coin-type cell by an automatic battery tester system (Land®, China). The test cell consisted of the positive electrode and lithium foil negative electrode separated by Celgard 2300 separator, and 1 M LiPF6 in EC, EMC and DMC (1:1:1 by volume) as the electrolyte. For the positive electrode, it was composed of the Li3V2(PO4)3 sample, acetylene black and PTFE in weight ratio of 80:15:5. Charge/discharge measurements were performed in the voltage range of 3.0–4.2 V at the current density of 10 mA/g. Cyclic voltammograms were tested using CHI650A electrochemical analyzer (Shanghai, China) at a scan rate of 0.05 mV/s in the voltage range of 3.0–4.3 V. 3. Results and discussion In order to determine preheating and calcining temperature of the raw material mixture, TG and DTA were performed. The result shows that, in order to obtain complete crystallization of monoclinic Li3V2(PO4)3, the mixture must be calcined at 900 °C for 24 h as evidenced by the following X-ray diffraction studies.
Fig. 2. SEM images of Li3V2(PO4)3 samples at different sintering temperatures.
X. Zhu et al. / Solid State Ionics 179 (2008) 1679–1682
Fig. 3. The initial charge/discharge curves (a) and cyclic performances (b) of Li3V2(PO4)3 samples synthesized at different sintering temperature.
the particle of the sample synthesized at 850 °C has a uniform dumbbell-like shape with an average particle size of 1 μm. This sintering temperature of 850 °C cannot be high enough to synthesize the pure phase of monoclinic Li3V2(PO4)3 sample as indicated by XRD. When the sample sintered at 900 °C (Fig. 2b), the particles with an average particle size of about 2 μm, which are larger than that of 850 °C-particles, are observed. As shown in Fig. 2, all the SEM images show that there are some small particles with the size of less than 10 nm on the surface of the particles, which are attributed to the residual carbon particles left over from CRT reaction. This is to say, it is the result as a direct consequence of the use of a 25% mass excess of carbon during the CRT reaction and produces a composite powder with superior electronic conductivity [15]. Further increasing the sintering temperature to 950 °C (Fig. 2c), the morphology of the 950 °C-particle shows particle growth with the particle size of ∼ 3 μm. Fig. 3 shows the initial charge/discharge curves and cycling performance of Li3V2(PO4)3 samples synthesized at 850, 900 and 950 °C at the current density of 10 mA/g in the voltage range of 3.0 and 4.2 V. As shown in Fig. 3a, the initial charge and discharge capacities of 850 °C-sample are 111.4 and 103.3 mAh/g, respectively, so the first columbic efficiency is 88.4%. Fig. 3b shows its discharge capacity decreases rapidly,
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from 103.3 mAh/g of the first cycle to 88.4 mAh/g of the 20th cycle, indicating that its capacity retention is 85.6% of the initial discharge capacity. These results are attributed to the incomplete Li3V2(PO4)3 crystalline with little impurities as indicated by XRD. For 900 °C-sample, the first charge and discharge capacity are 122.4 and 119.3 mAh/g, respectively, and the columbic efficiency is as high as 97.5%. After 20 cycles, 93.4% capacity retention can be held. For 950 °C-sample, the first charge and discharge capacities are 120.7 and 112.8 mAh/g, respectively, so its initial columbic efficiency is 93.5% with 86.1% capacity retention up to 20 cycles. Of all the three samples, the first discharge capacity and the 20th discharge capacity retention of 900 °C-sample are the highest. This may be the formation of pure monoclinic Li3V2(PO4)3 coated carbon particles with relatively high specific surface area. And 950 °Csample has lower initial discharge capacity and lower capacity retention due to larger particle size, resulting in lithium diffusion that is difficult and slow during lithium insertion/desertion from Li3V2(PO4)3 host structure. It has been reported that monoclinic Li3V2(PO4)3 can be as a high rate cathode, and the fast-ion conducting Nascion structure can reversibly extracted of all three lithium ion from the host materials [13]. This is related to the morphology and specific surface area of obtained samples. Considering the electronic conductivity of active materials Li3V2(PO4)3, the sample with a larger particle size has a relative lower specific area, and the electronics have to diffuse over longer distance between the surface and center during lithium insertion or extraction, resulting in the active material near the center of particle contributing very little to the lithium insertion/reinsertion reaction. The sample with smaller particles, which has relative higher specific surface area, will have better electrochemical performance. To our experimental results, it is suggested that the better performance of monoclinic Li3V2(PO4)3 can be obtained with optimizing powder characteristics at an optimal sintering temperature of 900 °C.
Fig. 4. CV curve of 900 °C-Li3V2(PO4)3 sample at a scan rate of 0.05 mV/s in the voltage range of 3.0–4.3 V for the first cycle.
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Fig. 4 shows the cyclic voltammetry (CV) for 900 °C-sample between 3.0 and 4.3 V at a scanning rate of 0.05 mV/s. As known, CV indicates the oxidation/reduction potential at which lithium ion was extracted from the lattice or inserted into the lattice as well as phase transitions. As shown in Fig. 4, during the positive scan, there are three anodic peaks at 3.61, 3.69 and 4.09 V, which was corresponded to the lithium extraction as the following stoichiometric ranges: x = 0.0–0.50, 0.50–1.00, and 1.00–2.00 in Li3 − xV2(PO4)3, respectively. During the negative scan, there are correspondingly three cathodic peaks at 3.58, 3.66 and 4.03 V due to lithium insertion. It is well known that each anodic or cathodic peak in CV curve stands for two-phase transitions. The results are in good agreement with the first charge–discharge study (Fig. 3a). 4. Conclusions Monoclinic Li3V2(PO4)3 has been successfully synthesized by a carbon-thermal reduction (CRT) method under Ar atmosphere. The influence of sintering temperatures between 850 °C and 950 °C on the synthesis of Li3V2(PO4)3 has been investigated. Charge/discharge tests show the 900 °C-sample exhibits the highest initial discharge capacity of 119.3 mAh/g and good capacity retention at 10 mA/g in the voltage range of 3.0–4.2 V. It is suggested that the optimal sintering temperature is 900 °C in order to obtain pure monoclinic Li3V2(PO4)3 with good electrochemical performance by CRT method, and the monoclinic Li3V2(PO4)3 can be used as candidate cathode materials for lithium-ion batteries.
Acknowledgements This research was sponsored by the Natural Science Foundation of Hubei Province (No. 2006ABA317) and the Foundation for Innovative Research Team of Hubei Province (No. 2005ABC004). References [1] S. Megahed, B. Scrosati, J. Power Sources 51 (1994) 79. [2] M.S. Whittingham, Chem. Rev. 104 (2004) 4271. [3] J. Breger, N. Dupre, P.J. Chupas, P.L. Lee, T. Proffen, J.B. Parise, C.P. Grey, J. Am. Chem. Soc. 127 (2005) 7529. [4] C. Yang, T.A. Arunkumar, A. Manthiram, Solid State Ionics 177 (2006) 863. [5] A.K. Padhi, K. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188. [6] K.S. Nanjundaswamy, A.K. Padhi, J.B. Goodenough, S. Okada, H. Ohtsuka, H. Arai, J. Yamaki, Solid State Ionics 92 (1996) 1. [7] A.K. Padhi, V. Manivannan, J.B. Goodenough, J. Electrochem. Soc. 145 (1998) 1518. [8] A. Yamada, S.C. Chung, K. Hinokuma, J. Electrochem. Soc. 148 (2001) A224. [9] H. Huang, S.C. Yin, T. Kerr, N. Taylor, L.F. Nazar, Adv. Mater. 14 (2002) 1525. [10] J. Gaubicher, C. Wurm, G. Goward, C. Masquelier, L. Nazar, Chem. Mater. 12 (2000) 3240. [11] M.Y. Saidi, J. Barker, H. Huang, J.L. Swoyer, G. Adamson, Electrochem. Solid-State Lett. 5 (2002) A149. [12] S.C. Yin, H. Grondey, P. Strobel, H. Huang, L.F. Nazar, J. Am. Chem. Soc. 125 (2003) 326. [13] S.C. Yin, P.S. Strobel, H. Grondey, L.F. Nazar, Chem. Mater. 16 (2004) 1456. [14] D. Morgan, G. Ceder, M.Y. Saidi, J. Barker, J. Swoyer, H. Huang, G. Adamson, J. Power Sources 119 (2003) 755. [15] J. Barker, M.Y. Saidi, J.L. Swoyer, J. Electrochem. Soc. 150 (2003) A684.
Solid State Ionics 179 (2008) 1679 – 1682 www.elsevier.com/locate/ssi
Synthesis and characteristics of Li3V2(PO4)3 as cathode materials for lithium-ion batteries Xianjun Zhu a,⁎, Yunxia Liu a , Liangmei Geng a , Longbin Chen a , Hanxing Liu b,⁎, Minghe Cao b a
b
College of Chemistry, Central China Normal University, Wuhan, Hubei 430079, China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, Hubei 430070, China Received 12 April 2007; received in revised form 22 November 2007; accepted 29 November 2007
Abstract Monoclinic lithium vanadium phosphate, Li3V2(PO4)3, was synthesized by carbon-thermal reduction (RTC) under Ar atmosphere. The influence of sintering temperatures on the synthesis of Li3V2(PO4)3 has been investigated using X-ray diffraction (XRD), SEM and electrochemical methods. XRD patterns show that the Li3V2(PO4)3 compound with monoclinic crystal structure begins to appear at the temperature of less than 900 °C. As the temperature was ≥900 °C, pure monoclinic Li3V2(PO4)3 phase can be obtained. SEM results indicate that the particle size of as-prepared samples increased with the sintering temperature increase, as well as the presence of minor carbon particles on the surface of the sample particles, which are very useful to enhance the electronic conductivity of Li3V2(PO4)3. Charge/discharge tests show the 900 °C-sample exhibits the highest initial discharge capacity of 119.3 mAh/g at 10 mA/g in the voltage range of 3.0–4.2 V with good capacity retention. CV experiment exhibits that there are three anodic peaks at 3.61, 3.69 and 4.09 V on lithium extraction as well as three cathodic peaks at 3.58, 3.66 and 4.03 V on lithium reinsertion at 0.05 mV/s between 3.0 and 4.3 V. It is suggested that the optimal sintering temperature is 900 °C in order to obtain pure monoclinic Li3V2(PO4)3 with good electrochemical performance by CRT method, and the monoclinic Li3V2(PO4)3 can be used as candidate cathode materials for lithium-ion batteries. © 2007 Elsevier B.V. All rights reserved. Keywords: Li3V2(PO4)3; Carbon-thermal reduction; Lithium-ion batteries; Cathode materials; Cyclic voltammetry
1. Introduction Lithium-ion batteries are considered to be the most advanced energy storage systems. Transition metal oxides have been the focus of a wide development effort as cathode materials [1–4]. In an intensive search for alternative materials, lithium conducting phosphates, Li3M2(PO4)3, and materials based on these compounds have emerged as the most promising candidates [5–8]. Of these materials, monoclinic Li3V2(PO4)3 [9], unlike the rhombohedral one [10], exhibits a complex series of two-phase transitions on Li extraction, followed by a solid solution regime on lithium reinsertion. The reversible cycling of all three lithiums from Li3V2 (PO4)3 would correspond to a theoretical capacity of 197 mAh/g
[11–13], which is the highest for all phosphates reported. Usually, monoclinic Li3V2(PO4)3 was synthesized by solid state reaction using hydrogen reduction method [14], which needs high reaction conditions. It is difficult to obtain monoclinic Li3V2 (PO4)3 sample with small particle and homogeneous distribution, which is critical to its electrochemical performance. In this context, monoclinic Li3V2(PO4)3 was synthesized by carbon-thermal reduction(CTR) [15]. Residual carbon left over from the CTR reaction is useful for stabilization of its electrochemical properties. It is investigated for the influence of sintering temperatures on the structure, the morphology and the electrochemical properties of monoclinic Li3V2(PO4)3. 2. Experimental
⁎ Corresponding authors. Tel.: +86 27 62178945, 87653330. E-mail addresses: [email protected] (X. Zhu), [email protected] (H. Liu). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.11.025
The monoclinic Li3V2(PO4)3 samples were prepared by carbon-thermal reduction method, using a solid state reaction
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X. Zhu et al. / Solid State Ionics 179 (2008) 1679–1682
Fig. 1 shows the XRD patterns of Li3V2(PO4)3 samples synthesized by CRT method at different temperatures. As shown in Fig. 1a and b, Li3V2(PO4)3 phase cannot be observed for the raw materials at room temperature and for the sample preheated at the temperature of 300 °C. The characteristic peaks of monoclinic Li3V2(PO4)3 come to appear at the precalcined temperature of 600 °C, indicating that monoclinic Li3V2(PO4)3 begin to form at 600 °C. (Fig. 1c). When the sintering temperature increased to 850 °C (Fig. 1d), monoclinic Li3V2(PO4)3 becomes the dominant phase, but the impurity phase of Li3PO4 exists obviously. Further increasing the sintering temperature to higher than 900 °C, purity Li3V2(PO4)3 with monoclinic crystal structure was obtained (Fig. 1e and f). The morphology of Li3V2(PO4)3 samples sintered at three different temperatures is shown in Fig. 2. As seen from Fig. 2a, Fig. 1. XRD patterns of Li3V2(PO4)3 samples at different temperatures.
by mixing stoichiometric amounts of LiOH·H2O, V2O5 and NH4H2PO4 as well as a 25% mass excess of super high area carbon over the stoichiometric reaction. The mixture was initially heated to 300 °C in air for 3 h to expel H2O and NH3, reground, pelletized, and then fired to 600 °C and held at this temperature for 16 h under a flowing Ar atmosphere. The resulting product was then ground, pelletized and sintered for 24 h at the temperatures ranging from 850 °C to 950 °C under a flowing Ar atmosphere. The completion of the reaction and the phase purity of the samples were confirmed by XRD using an X'pert powder X-ray diffractometer (Philip, Holland). Thermal analysis of the raw material mixture was performed between room temperature and 1000 °C at a heating rate of 10 °C·min− 1 under argon flow (20 ml·min− 1). The morphology and particle size of the samples was observed with field emission scanning electron microscope (JEOL, JSM6700F). The charge/discharge tests were performed using CR2016 coin-type cell by an automatic battery tester system (Land®, China). The test cell consisted of the positive electrode and lithium foil negative electrode separated by Celgard 2300 separator, and 1 M LiPF6 in EC, EMC and DMC (1:1:1 by volume) as the electrolyte. For the positive electrode, it was composed of the Li3V2(PO4)3 sample, acetylene black and PTFE in weight ratio of 80:15:5. Charge/discharge measurements were performed in the voltage range of 3.0–4.2 V at the current density of 10 mA/g. Cyclic voltammograms were tested using CHI650A electrochemical analyzer (Shanghai, China) at a scan rate of 0.05 mV/s in the voltage range of 3.0–4.3 V. 3. Results and discussion In order to determine preheating and calcining temperature of the raw material mixture, TG and DTA were performed. The result shows that, in order to obtain complete crystallization of monoclinic Li3V2(PO4)3, the mixture must be calcined at 900 °C for 24 h as evidenced by the following X-ray diffraction studies.
Fig. 2. SEM images of Li3V2(PO4)3 samples at different sintering temperatures.
X. Zhu et al. / Solid State Ionics 179 (2008) 1679–1682
Fig. 3. The initial charge/discharge curves (a) and cyclic performances (b) of Li3V2(PO4)3 samples synthesized at different sintering temperature.
the particle of the sample synthesized at 850 °C has a uniform dumbbell-like shape with an average particle size of 1 μm. This sintering temperature of 850 °C cannot be high enough to synthesize the pure phase of monoclinic Li3V2(PO4)3 sample as indicated by XRD. When the sample sintered at 900 °C (Fig. 2b), the particles with an average particle size of about 2 μm, which are larger than that of 850 °C-particles, are observed. As shown in Fig. 2, all the SEM images show that there are some small particles with the size of less than 10 nm on the surface of the particles, which are attributed to the residual carbon particles left over from CRT reaction. This is to say, it is the result as a direct consequence of the use of a 25% mass excess of carbon during the CRT reaction and produces a composite powder with superior electronic conductivity [15]. Further increasing the sintering temperature to 950 °C (Fig. 2c), the morphology of the 950 °C-particle shows particle growth with the particle size of ∼ 3 μm. Fig. 3 shows the initial charge/discharge curves and cycling performance of Li3V2(PO4)3 samples synthesized at 850, 900 and 950 °C at the current density of 10 mA/g in the voltage range of 3.0 and 4.2 V. As shown in Fig. 3a, the initial charge and discharge capacities of 850 °C-sample are 111.4 and 103.3 mAh/g, respectively, so the first columbic efficiency is 88.4%. Fig. 3b shows its discharge capacity decreases rapidly,
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from 103.3 mAh/g of the first cycle to 88.4 mAh/g of the 20th cycle, indicating that its capacity retention is 85.6% of the initial discharge capacity. These results are attributed to the incomplete Li3V2(PO4)3 crystalline with little impurities as indicated by XRD. For 900 °C-sample, the first charge and discharge capacity are 122.4 and 119.3 mAh/g, respectively, and the columbic efficiency is as high as 97.5%. After 20 cycles, 93.4% capacity retention can be held. For 950 °C-sample, the first charge and discharge capacities are 120.7 and 112.8 mAh/g, respectively, so its initial columbic efficiency is 93.5% with 86.1% capacity retention up to 20 cycles. Of all the three samples, the first discharge capacity and the 20th discharge capacity retention of 900 °C-sample are the highest. This may be the formation of pure monoclinic Li3V2(PO4)3 coated carbon particles with relatively high specific surface area. And 950 °Csample has lower initial discharge capacity and lower capacity retention due to larger particle size, resulting in lithium diffusion that is difficult and slow during lithium insertion/desertion from Li3V2(PO4)3 host structure. It has been reported that monoclinic Li3V2(PO4)3 can be as a high rate cathode, and the fast-ion conducting Nascion structure can reversibly extracted of all three lithium ion from the host materials [13]. This is related to the morphology and specific surface area of obtained samples. Considering the electronic conductivity of active materials Li3V2(PO4)3, the sample with a larger particle size has a relative lower specific area, and the electronics have to diffuse over longer distance between the surface and center during lithium insertion or extraction, resulting in the active material near the center of particle contributing very little to the lithium insertion/reinsertion reaction. The sample with smaller particles, which has relative higher specific surface area, will have better electrochemical performance. To our experimental results, it is suggested that the better performance of monoclinic Li3V2(PO4)3 can be obtained with optimizing powder characteristics at an optimal sintering temperature of 900 °C.
Fig. 4. CV curve of 900 °C-Li3V2(PO4)3 sample at a scan rate of 0.05 mV/s in the voltage range of 3.0–4.3 V for the first cycle.
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Fig. 4 shows the cyclic voltammetry (CV) for 900 °C-sample between 3.0 and 4.3 V at a scanning rate of 0.05 mV/s. As known, CV indicates the oxidation/reduction potential at which lithium ion was extracted from the lattice or inserted into the lattice as well as phase transitions. As shown in Fig. 4, during the positive scan, there are three anodic peaks at 3.61, 3.69 and 4.09 V, which was corresponded to the lithium extraction as the following stoichiometric ranges: x = 0.0–0.50, 0.50–1.00, and 1.00–2.00 in Li3 − xV2(PO4)3, respectively. During the negative scan, there are correspondingly three cathodic peaks at 3.58, 3.66 and 4.03 V due to lithium insertion. It is well known that each anodic or cathodic peak in CV curve stands for two-phase transitions. The results are in good agreement with the first charge–discharge study (Fig. 3a). 4. Conclusions Monoclinic Li3V2(PO4)3 has been successfully synthesized by a carbon-thermal reduction (CRT) method under Ar atmosphere. The influence of sintering temperatures between 850 °C and 950 °C on the synthesis of Li3V2(PO4)3 has been investigated. Charge/discharge tests show the 900 °C-sample exhibits the highest initial discharge capacity of 119.3 mAh/g and good capacity retention at 10 mA/g in the voltage range of 3.0–4.2 V. It is suggested that the optimal sintering temperature is 900 °C in order to obtain pure monoclinic Li3V2(PO4)3 with good electrochemical performance by CRT method, and the monoclinic Li3V2(PO4)3 can be used as candidate cathode materials for lithium-ion batteries.
Acknowledgements This research was sponsored by the Natural Science Foundation of Hubei Province (No. 2006ABA317) and the Foundation for Innovative Research Team of Hubei Province (No. 2005ABC004). References [1] S. Megahed, B. Scrosati, J. Power Sources 51 (1994) 79. [2] M.S. Whittingham, Chem. Rev. 104 (2004) 4271. [3] J. Breger, N. Dupre, P.J. Chupas, P.L. Lee, T. Proffen, J.B. Parise, C.P. Grey, J. Am. Chem. Soc. 127 (2005) 7529. [4] C. Yang, T.A. Arunkumar, A. Manthiram, Solid State Ionics 177 (2006) 863. [5] A.K. Padhi, K. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188. [6] K.S. Nanjundaswamy, A.K. Padhi, J.B. Goodenough, S. Okada, H. Ohtsuka, H. Arai, J. Yamaki, Solid State Ionics 92 (1996) 1. [7] A.K. Padhi, V. Manivannan, J.B. Goodenough, J. Electrochem. Soc. 145 (1998) 1518. [8] A. Yamada, S.C. Chung, K. Hinokuma, J. Electrochem. Soc. 148 (2001) A224. [9] H. Huang, S.C. Yin, T. Kerr, N. Taylor, L.F. Nazar, Adv. Mater. 14 (2002) 1525. [10] J. Gaubicher, C. Wurm, G. Goward, C. Masquelier, L. Nazar, Chem. Mater. 12 (2000) 3240. [11] M.Y. Saidi, J. Barker, H. Huang, J.L. Swoyer, G. Adamson, Electrochem. Solid-State Lett. 5 (2002) A149. [12] S.C. Yin, H. Grondey, P. Strobel, H. Huang, L.F. Nazar, J. Am. Chem. Soc. 125 (2003) 326. [13] S.C. Yin, P.S. Strobel, H. Grondey, L.F. Nazar, Chem. Mater. 16 (2004) 1456. [14] D. Morgan, G. Ceder, M.Y. Saidi, J. Barker, J. Swoyer, H. Huang, G. Adamson, J. Power Sources 119 (2003) 755. [15] J. Barker, M.Y. Saidi, J.L. Swoyer, J. Electrochem. Soc. 150 (2003) A684.