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Low temperature synthesis of LiNi0.5Mn1.5O4 spinel
Materials Letters 60 (2006) 1273 – 1275 www.elsevier.com/locate/matlet
Low tempderature synthesis of LiNi0.5Mn1.5O4 spinel Hai-sheng Fang, Zhi-xing Wang ⁎, Xin-hai Li, Hua-jun Guo, Wen-jie Peng School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China Received 25 June 2005; accepted 5 November 2005 Available online 29 November 2005
Abstract Pure LiNi0.5Mn1.5O4 phase was prepared by one-step solid-state reaction at 600 °C in air. TG measurement revealed that the oxygen loss occurred when the mixed precursors were heated above 700 °C. X-ray diffraction (XRD) pattern and scanning electron microscopic (SEM) image indicated that LiNi0.5Mn1.5O4 has cubic spinel structure with small and homogeneous particles. Electrochemical test showed that the prepared LiNi0.5Mn1.5O4 delivered up to 138 mA h g− 1, and the capacity retained 128 mA h g− 1 after 30 cycles. © 2005 Elsevier B.V. All rights reserved. Keywords: Lithium ion batteries; Cathode material; LiNi0.5Mn1.5O4; Solid-state reaction
1. Introduction Recently, several research groups have reported transitionmetal-substituted spinel materials (LiMxMn2−xO4, M: Cr, Co, Fe, Ni, Cu) with high-voltage plateaus above 4.5 V [1–5]. Among these materials, LiNi0.5Mn1.5O4 is the most promising and attractive one because of its good cyclic property and relatively high capacity with a plateau at around 4.7 V [3,6]. So far, a variety of methods were used for preparation of LiNi0.5Mn1.5O4, such as solid-state reaction [4,7,8], sol–gel [4,6,7], co-precipitation [9], emulsion drying [10], composite carbonate process [11], and molten salt [12], combustion [13] and ultrasonic spray pyrolysis method [14]. When LiNi0.5 Mn1.5O4 was synthesized by solid-state method, at least two steps together with high temperature were often employed in the synthesis process. Zhong et al. [4] has synthesized LiNi0.5 Mn1.5O4 by heating it three times at 750 °C with intermediate grinding, followed by a single heating at 800 °C. Idemoto et al. [7] reported that LiNi0.5Mn1.5O4 was prepared by preheating at 600 °C in air, then heating at 800 °C in O2. Both prepared powders delivered low capacity (b 120 mA h g− 1) and clearly exhibited a 4.1 V plateau, though reheating and regrinding, or oxygen flow were applied to the synthesis process. Ohzuku et al. [8] has synthesized LiNi0.5Mn1.5O4 by firing at temperature as high as 1000 °C for 12 h followed by an annealing process at ⁎ Corresponding author. Tel./fax: +86 731 8836633. E-mail address: [email protected] (Z. Wang). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.11.013
700 °C for 48 h at relatively slow heating and cooling rates, and the resulting powders delivered flat voltage profiles at around 4.7 V with high capacity, but the temperature is so high and the synthetic time is too long. To our knowledge, there is no report on LiNi0.5Mn1.5O4 synthesized by one-step solid-state method at low temperature. In the present paper, we describe the preparation of LiNi0.5Mn1.5O4 by one-step solid-state reaction at 600 °C in air, and the property of resulting powders was investigated. 2. Experimental Appropriate amounts of Li2CO3, NiO and electrolytic MnO2 were initially ground in mortar for 1 h, and then the mixture was thoroughly ball-milled for 6 h. Subsequently the mixed precursors were calcined at 600 °C for 24 h in air. The powder X-ray diffraction (XRD, Rint-2000, Rigaku) measurement using CuKα radiation was employed to identify the crystalline phase of the synthesized materials, recorded at room temperature. Chemical composition of the resulting powders was analyzed by atomic absorption spectroscopy (AAS, Vario 6, Analytic Jena AG, Jena, Germany). The particle size and morphology of the LiNi0.5Mn1.5O4 powders was observed by scanning electron microscope (JEOL, JSM-5600LV) with an accelerating voltage of 20 kV. Thermal gravimetric analysis of the precursors was performed with a TA instrument (SDT Q600). The powders were heated at 10 °C/min in a constant flow of extra dry air.
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absorbed by the precursors. Subsequent weight loss around 11% of the mixture is observed over the temperature range of 100 °C to 370 °C, which could be ascribed to the decomposition of Li2CO3 as deduced from its decomposition reaction. No weight loss is observed between
Fig. 1. TG curve of mixed precursors Li2CO3, NiO and electrolytic MnO2.
The electrochemical characterizations were performed using CR2025 coin-type cell. For positive electrode fabrication, the prepared powders were mixed with 20 wt.% of carbon black and 10 wt.% of polyvinylidene fluoride in N-methyl pyrrolidinone until slurry was obtained. And then, the blended slurries were pasted onto an aluminum current collector, and the electrode was dried at 80 °C for 1 day in vacuum. The test cell consisted of the positive electrode and lithium foil negative electrode separated by a porous polypropylene film, and 1 M LiPF6 in EC/EMC/DMC (1:1:1 in volume) as the electrolyte. The assembly of the cells was carried out in a dry Ar-filled glove box. The cells were charged and discharged over a voltage range of 3.5 V to 4.9 V versus Li/Li+ electrode at the rate of 2/7C at room temperature. Cyclic Voltammogram(CV) test was performed on a CHI660A Electrochemical Workstation over a voltage range of 3.5 V to 5.2 V versus Li/Li+ electrode at a scan rate of 0.1 mV S− 1.
Fig. 3. SEM image of LiNi0.5Mn1.5O4.
3. Results and discussion Fig. 1 shows the TG curve of the mixed precursors recorded from 50 °C to 900 °C in air. The weight loss below 100 °C is about 4% of the mixture, which is mainly due to the elimination of water molecules
Fig. 2. Powder X-ray diffraction pattern of LiNi0.5Mn1.5O4.
Fig. 4. The charge–discharge curves of LiNi0.5Mn1.5O4 at the rate of 2/7C.
Fig. 5. Cyclic voltammogram of LiNi0.5Mn1.5O4 electrode scanned at 0.1 mV s− 1.
H. Fang et al. / Materials Letters 60 (2006) 1273–1275
Fig. 6. Electrochemical cycling performance of LiNi0.5Mn1.5O4 cycled at the rate of 2/7C.
370 °C and 700 °C. However, when the temperature arrived at above 700 °C, the weight decreased again. Similar result was obtained in the work reported by Lazarraga et al. [13], and it was due to the oxygen loss. The theoretic product/input weight ratio of the mixed precursors is 89.25%, and when the temperature was below 700 °C, the datum observed from the TG curve is well consistent with this theory value, if the effect of water molecules is considered. Thus, the TG result may indicate that the final product LiNi0.5Mn1.5O4 could be obtained at a temperature below 700 °C, and this presumption was also supported by the results presented below. Fig. 2 shows the XRD pattern of LiNi0.5Mn1.5O4 powders. All peaks can be indexed as a phase-pure cubic spinel structure with a space group of Fd-3m. It has been reported [4] that LiNi0.5Mn1.5O4 would loss oxygen and disproportionate to a spinel and LixNi1−xO when heated above 650 °C. In the present work, low synthesis temperature prevented the occurrence of oxygen loss and facilitated synthesis of pure LiNi0.5Mn1.5O4 phase. From the AAS analysis, the chemical composition of the resulting powders was determined to be Li0.99Ni0.49Mn1.5O4. Fig. 3 shows the SEM image of LiNi0.5Mn1.5O4. Small and homogeneous particles are observed, and the average size is about 150 nm, as is clear in Fig. 3. From the previous literatures, a 4.1 V plateau based on Mn3+/Mn4+ redox couple was often observed in the charge/discharge curves, when the LiNi0.5Mn1.5O4 powders synthesized by conventional solid-state method [4,7]. Even though the same material was obtained by wet methods such as sol–gel method and emulsion drying method, some groups failed to eliminate the 4.1 V plateau from the charge/discharge curves [6,10]. The charge–discharge curves of LiNi0.5Mn1.5O4 cycled between 3.5 V and 4.9 V at the rate of 2/7C are shown in Fig. 4. The LiNi0.5Mn1.5O4 exhibits only 4.7 V region plateau (formed of two very close steps), which is attributed to the Ni2+/Ni4+ redox couple [4], and the 4.1 V plateau is not observed. Disappearing of the 4.1 V plateau also suggests that pure LiNi0.5Mn1.5O4 phase was successfully synthesized in the present work. This result is well in agreement with the XRD pattern. Irreversible capacity is observed which is mainly due to decomposition of electrolyte at high voltage [4]. Similar result is observed in the cyclic voltammogram of LiNi0.5Mn1.5O4 shown in Fig. 5. The broad anodic and cathodic peaks correspond to the Ni2+/Ni4+ redox couple. These peaks confirm the good reversibility of the lithium extraction/insertion reactions in LiNi0.5Mn1.5O4. Fig. 6 shows the electrochemical cycling performance of LiNi0.5Mn1.5O4 at the rate of 2/7C. The maximal discharge capacity reaches 138 mA h g− 1, and the capacity still remains
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128 mA h g− 1 after 30 cycles, which is still higher than the maximal discharge capacity (corresponding to solid-state method) reported by Zhong et al. [4] and Idemoto et al. [7]. Therefore, from the above results, we can conclude that higher capacity LiNi0.5Mn1.5O4 with only one plateau at 4.7 V could be successfully synthesized by onestep solid-state method. The possibility of synthesis of LiNi0.5Mn1.5O4 at low temperature may be associated with two aspects: 1) introduction of ball milling. In the previous works [4,7,8], the mixture of precursors for synthesizing LiNi0.5Mn1.5O4 by solid-state method was not ball milled. When ball milling process was employed into the synthesis route, fine and homogeneous precursors particles can be obtained, and their contact area and reactivity can be greatly enhanced, which highly facilitated the completeness of solid-state reaction and lower the synthesis temperature. 2) Use of electrolytic MnO2. The crystal structure of electrolytic MnO2 is γ-MnO2 which is similar to that of LiNi0.5Mn1.5O4 and has a more open structure, and the structural similarity and open structure may be favorable for the formation of LiNi0.5Mn1.5O4. Thus, it is possible to synthesize LiNi0.5Mn1.5O4 at low temperature by one-step solid-state method.
4. Conclusions LiNi0.5Mn1.5O4 has been prepared by one-step solid-state reaction at 600 °C in air. X-ray diffraction (XRD) pattern showed that LiNi0.5Mn1.5O4 has cubic spinel structure without any impurity such as LixNi1−xO; scanning electron microscopic (SEM) image showed that the particle is about 150 nm in size together with homogeneous distribution. The prepared LiNi0.5 Mn1.5O4 powders can deliver up to 138 mA h g− 1. Acknowledgements This project was sponsored by the Scientific Research Foundation for the Returned Overseas Chinese scholars, State Education Ministry. References [1] C. Sigala, D. Guyomard, A. Verbaere, Y. Piffard, M. Tournoux, Solid State Ionics 81 (1995) 167. [2] H. Kawai, M. Nagata, H. Tukamoto, A.R. West, J. Mater. Chem. 8 (1998) 837. [3] T. Ohzuku, S. Takeda, M. Iwanaga, J. Power Sources 81–82 (1999) 90. [4] Q. Zhong, A. Bonakdarpour, M. Zhang, Y. Gao, J.R. Dahn, J. Electrochem. Soc. 144 (1997) 205. [5] Y. Ein-Eli, W.F. Howard Jr., J. Electrochem. Soc. 144 (1997) L205. [6] X. Wu, S.B. Kim, J. Power Sources 109 (2002) 53. [7] Y. Idemoto, H. Narai, N. Koura, J. Power Sources 119–121 (2003) 125. [8] T. Ohzuku, K. Ariyoshi, S. Yamamoto, J. Ceram. Soc. Jpn., Int. Ed. 110 (2002) 501. [9] R. Alcantara, M. Jaraba, P. Lavela, J.L. Trado, Electrochim. Acta 47 (2002) 1829. [10] S.T. Myung, S. Komaba, N. Kumagai, H. Yashiro, H.T. Chung, T.-H. Cho, Electrochim. Acta 47 (2002) 2543. [11] Y.S. Lee, Y.-K. Sun, S. Ota, T. Miyashita, M. Yoshio, Electrochem. Commun. 4 (2002) 989. [12] J.H. Kim, S.T. Myung, Y.K. Sun, Electrochim. Acta 49 (2004) 219. [13] M.G. Lazarraga, L. Pascual, H. Gadjov, D. Kovacheva, K. Petrov, J.M. Amarilla, R.M. Rojas, M.A. Martin-Luengo, J.M. Rojo, J. Mater. Chem. 14 (2004) 1640. [14] S.H. Park, S.W. Oh, S.T. Myung, Y.C. Kang, Y.K. Sun, Solid State Ionics 176 (2005) 481.
Low tempderature synthesis of LiNi0.5Mn1.5O4 spinel Hai-sheng Fang, Zhi-xing Wang ⁎, Xin-hai Li, Hua-jun Guo, Wen-jie Peng School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China Received 25 June 2005; accepted 5 November 2005 Available online 29 November 2005
Abstract Pure LiNi0.5Mn1.5O4 phase was prepared by one-step solid-state reaction at 600 °C in air. TG measurement revealed that the oxygen loss occurred when the mixed precursors were heated above 700 °C. X-ray diffraction (XRD) pattern and scanning electron microscopic (SEM) image indicated that LiNi0.5Mn1.5O4 has cubic spinel structure with small and homogeneous particles. Electrochemical test showed that the prepared LiNi0.5Mn1.5O4 delivered up to 138 mA h g− 1, and the capacity retained 128 mA h g− 1 after 30 cycles. © 2005 Elsevier B.V. All rights reserved. Keywords: Lithium ion batteries; Cathode material; LiNi0.5Mn1.5O4; Solid-state reaction
1. Introduction Recently, several research groups have reported transitionmetal-substituted spinel materials (LiMxMn2−xO4, M: Cr, Co, Fe, Ni, Cu) with high-voltage plateaus above 4.5 V [1–5]. Among these materials, LiNi0.5Mn1.5O4 is the most promising and attractive one because of its good cyclic property and relatively high capacity with a plateau at around 4.7 V [3,6]. So far, a variety of methods were used for preparation of LiNi0.5Mn1.5O4, such as solid-state reaction [4,7,8], sol–gel [4,6,7], co-precipitation [9], emulsion drying [10], composite carbonate process [11], and molten salt [12], combustion [13] and ultrasonic spray pyrolysis method [14]. When LiNi0.5 Mn1.5O4 was synthesized by solid-state method, at least two steps together with high temperature were often employed in the synthesis process. Zhong et al. [4] has synthesized LiNi0.5 Mn1.5O4 by heating it three times at 750 °C with intermediate grinding, followed by a single heating at 800 °C. Idemoto et al. [7] reported that LiNi0.5Mn1.5O4 was prepared by preheating at 600 °C in air, then heating at 800 °C in O2. Both prepared powders delivered low capacity (b 120 mA h g− 1) and clearly exhibited a 4.1 V plateau, though reheating and regrinding, or oxygen flow were applied to the synthesis process. Ohzuku et al. [8] has synthesized LiNi0.5Mn1.5O4 by firing at temperature as high as 1000 °C for 12 h followed by an annealing process at ⁎ Corresponding author. Tel./fax: +86 731 8836633. E-mail address: [email protected] (Z. Wang). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.11.013
700 °C for 48 h at relatively slow heating and cooling rates, and the resulting powders delivered flat voltage profiles at around 4.7 V with high capacity, but the temperature is so high and the synthetic time is too long. To our knowledge, there is no report on LiNi0.5Mn1.5O4 synthesized by one-step solid-state method at low temperature. In the present paper, we describe the preparation of LiNi0.5Mn1.5O4 by one-step solid-state reaction at 600 °C in air, and the property of resulting powders was investigated. 2. Experimental Appropriate amounts of Li2CO3, NiO and electrolytic MnO2 were initially ground in mortar for 1 h, and then the mixture was thoroughly ball-milled for 6 h. Subsequently the mixed precursors were calcined at 600 °C for 24 h in air. The powder X-ray diffraction (XRD, Rint-2000, Rigaku) measurement using CuKα radiation was employed to identify the crystalline phase of the synthesized materials, recorded at room temperature. Chemical composition of the resulting powders was analyzed by atomic absorption spectroscopy (AAS, Vario 6, Analytic Jena AG, Jena, Germany). The particle size and morphology of the LiNi0.5Mn1.5O4 powders was observed by scanning electron microscope (JEOL, JSM-5600LV) with an accelerating voltage of 20 kV. Thermal gravimetric analysis of the precursors was performed with a TA instrument (SDT Q600). The powders were heated at 10 °C/min in a constant flow of extra dry air.
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absorbed by the precursors. Subsequent weight loss around 11% of the mixture is observed over the temperature range of 100 °C to 370 °C, which could be ascribed to the decomposition of Li2CO3 as deduced from its decomposition reaction. No weight loss is observed between
Fig. 1. TG curve of mixed precursors Li2CO3, NiO and electrolytic MnO2.
The electrochemical characterizations were performed using CR2025 coin-type cell. For positive electrode fabrication, the prepared powders were mixed with 20 wt.% of carbon black and 10 wt.% of polyvinylidene fluoride in N-methyl pyrrolidinone until slurry was obtained. And then, the blended slurries were pasted onto an aluminum current collector, and the electrode was dried at 80 °C for 1 day in vacuum. The test cell consisted of the positive electrode and lithium foil negative electrode separated by a porous polypropylene film, and 1 M LiPF6 in EC/EMC/DMC (1:1:1 in volume) as the electrolyte. The assembly of the cells was carried out in a dry Ar-filled glove box. The cells were charged and discharged over a voltage range of 3.5 V to 4.9 V versus Li/Li+ electrode at the rate of 2/7C at room temperature. Cyclic Voltammogram(CV) test was performed on a CHI660A Electrochemical Workstation over a voltage range of 3.5 V to 5.2 V versus Li/Li+ electrode at a scan rate of 0.1 mV S− 1.
Fig. 3. SEM image of LiNi0.5Mn1.5O4.
3. Results and discussion Fig. 1 shows the TG curve of the mixed precursors recorded from 50 °C to 900 °C in air. The weight loss below 100 °C is about 4% of the mixture, which is mainly due to the elimination of water molecules
Fig. 2. Powder X-ray diffraction pattern of LiNi0.5Mn1.5O4.
Fig. 4. The charge–discharge curves of LiNi0.5Mn1.5O4 at the rate of 2/7C.
Fig. 5. Cyclic voltammogram of LiNi0.5Mn1.5O4 electrode scanned at 0.1 mV s− 1.
H. Fang et al. / Materials Letters 60 (2006) 1273–1275
Fig. 6. Electrochemical cycling performance of LiNi0.5Mn1.5O4 cycled at the rate of 2/7C.
370 °C and 700 °C. However, when the temperature arrived at above 700 °C, the weight decreased again. Similar result was obtained in the work reported by Lazarraga et al. [13], and it was due to the oxygen loss. The theoretic product/input weight ratio of the mixed precursors is 89.25%, and when the temperature was below 700 °C, the datum observed from the TG curve is well consistent with this theory value, if the effect of water molecules is considered. Thus, the TG result may indicate that the final product LiNi0.5Mn1.5O4 could be obtained at a temperature below 700 °C, and this presumption was also supported by the results presented below. Fig. 2 shows the XRD pattern of LiNi0.5Mn1.5O4 powders. All peaks can be indexed as a phase-pure cubic spinel structure with a space group of Fd-3m. It has been reported [4] that LiNi0.5Mn1.5O4 would loss oxygen and disproportionate to a spinel and LixNi1−xO when heated above 650 °C. In the present work, low synthesis temperature prevented the occurrence of oxygen loss and facilitated synthesis of pure LiNi0.5Mn1.5O4 phase. From the AAS analysis, the chemical composition of the resulting powders was determined to be Li0.99Ni0.49Mn1.5O4. Fig. 3 shows the SEM image of LiNi0.5Mn1.5O4. Small and homogeneous particles are observed, and the average size is about 150 nm, as is clear in Fig. 3. From the previous literatures, a 4.1 V plateau based on Mn3+/Mn4+ redox couple was often observed in the charge/discharge curves, when the LiNi0.5Mn1.5O4 powders synthesized by conventional solid-state method [4,7]. Even though the same material was obtained by wet methods such as sol–gel method and emulsion drying method, some groups failed to eliminate the 4.1 V plateau from the charge/discharge curves [6,10]. The charge–discharge curves of LiNi0.5Mn1.5O4 cycled between 3.5 V and 4.9 V at the rate of 2/7C are shown in Fig. 4. The LiNi0.5Mn1.5O4 exhibits only 4.7 V region plateau (formed of two very close steps), which is attributed to the Ni2+/Ni4+ redox couple [4], and the 4.1 V plateau is not observed. Disappearing of the 4.1 V plateau also suggests that pure LiNi0.5Mn1.5O4 phase was successfully synthesized in the present work. This result is well in agreement with the XRD pattern. Irreversible capacity is observed which is mainly due to decomposition of electrolyte at high voltage [4]. Similar result is observed in the cyclic voltammogram of LiNi0.5Mn1.5O4 shown in Fig. 5. The broad anodic and cathodic peaks correspond to the Ni2+/Ni4+ redox couple. These peaks confirm the good reversibility of the lithium extraction/insertion reactions in LiNi0.5Mn1.5O4. Fig. 6 shows the electrochemical cycling performance of LiNi0.5Mn1.5O4 at the rate of 2/7C. The maximal discharge capacity reaches 138 mA h g− 1, and the capacity still remains
1275
128 mA h g− 1 after 30 cycles, which is still higher than the maximal discharge capacity (corresponding to solid-state method) reported by Zhong et al. [4] and Idemoto et al. [7]. Therefore, from the above results, we can conclude that higher capacity LiNi0.5Mn1.5O4 with only one plateau at 4.7 V could be successfully synthesized by onestep solid-state method. The possibility of synthesis of LiNi0.5Mn1.5O4 at low temperature may be associated with two aspects: 1) introduction of ball milling. In the previous works [4,7,8], the mixture of precursors for synthesizing LiNi0.5Mn1.5O4 by solid-state method was not ball milled. When ball milling process was employed into the synthesis route, fine and homogeneous precursors particles can be obtained, and their contact area and reactivity can be greatly enhanced, which highly facilitated the completeness of solid-state reaction and lower the synthesis temperature. 2) Use of electrolytic MnO2. The crystal structure of electrolytic MnO2 is γ-MnO2 which is similar to that of LiNi0.5Mn1.5O4 and has a more open structure, and the structural similarity and open structure may be favorable for the formation of LiNi0.5Mn1.5O4. Thus, it is possible to synthesize LiNi0.5Mn1.5O4 at low temperature by one-step solid-state method.
4. Conclusions LiNi0.5Mn1.5O4 has been prepared by one-step solid-state reaction at 600 °C in air. X-ray diffraction (XRD) pattern showed that LiNi0.5Mn1.5O4 has cubic spinel structure without any impurity such as LixNi1−xO; scanning electron microscopic (SEM) image showed that the particle is about 150 nm in size together with homogeneous distribution. The prepared LiNi0.5 Mn1.5O4 powders can deliver up to 138 mA h g− 1. Acknowledgements This project was sponsored by the Scientific Research Foundation for the Returned Overseas Chinese scholars, State Education Ministry. References [1] C. Sigala, D. Guyomard, A. Verbaere, Y. Piffard, M. Tournoux, Solid State Ionics 81 (1995) 167. [2] H. Kawai, M. Nagata, H. Tukamoto, A.R. West, J. Mater. Chem. 8 (1998) 837. [3] T. Ohzuku, S. Takeda, M. Iwanaga, J. Power Sources 81–82 (1999) 90. [4] Q. Zhong, A. Bonakdarpour, M. Zhang, Y. Gao, J.R. Dahn, J. Electrochem. Soc. 144 (1997) 205. [5] Y. Ein-Eli, W.F. Howard Jr., J. Electrochem. Soc. 144 (1997) L205. [6] X. Wu, S.B. Kim, J. Power Sources 109 (2002) 53. [7] Y. Idemoto, H. Narai, N. Koura, J. Power Sources 119–121 (2003) 125. [8] T. Ohzuku, K. Ariyoshi, S. Yamamoto, J. Ceram. Soc. Jpn., Int. Ed. 110 (2002) 501. [9] R. Alcantara, M. Jaraba, P. Lavela, J.L. Trado, Electrochim. Acta 47 (2002) 1829. [10] S.T. Myung, S. Komaba, N. Kumagai, H. Yashiro, H.T. Chung, T.-H. Cho, Electrochim. Acta 47 (2002) 2543. [11] Y.S. Lee, Y.-K. Sun, S. Ota, T. Miyashita, M. Yoshio, Electrochem. Commun. 4 (2002) 989. [12] J.H. Kim, S.T. Myung, Y.K. Sun, Electrochim. Acta 49 (2004) 219. [13] M.G. Lazarraga, L. Pascual, H. Gadjov, D. Kovacheva, K. Petrov, J.M. Amarilla, R.M. Rojas, M.A. Martin-Luengo, J.M. Rojo, J. Mater. Chem. 14 (2004) 1640. [14] S.H. Park, S.W. Oh, S.T. Myung, Y.C. Kang, Y.K. Sun, Solid State Ionics 176 (2005) 481.