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Effect of K-doping on the Electrochemical Performance of Ca3Co4O9 Anode for Li-ion Batteries
J. Mater. Sci. Technol., 2010, 26(7), 669-672.
Effect of K-doping on the Electrochemical Performance of Ca3 Co4 O9 Anode for Li-ion Batteries Jina Cao, Hongquan Liu, Jian Xie, Gaoshao Cao and Xinbing Zhao† Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China [Manuscript received October 9, 2009, in revised form December 7, 2009]
Ca3 Co4 O9 (CCO) and Ca2.95 K0.05 Co4 O9 (CKCO) powders have been prepared by the polyacrylamide gel method. CKCO shows increased capacity and better cycling stability compared with CCO. After cycled for 50 cycles at 0.5 C, CKCO retains a capacity of 223 mAh·g−1 , almost twice than CCO. The electrochemical impedance spectroscopy (EIS) tests shows the CKCO sample has a lower initial charge transfer resistance (Rct ) and undergoes smaller Rct change during cycling than the CCO sample, indicating improved electrochemical performance by K-doping. KEY WORDS: Lithium-ion battery; Anode; Ca3 Co4 O9 ; K-doping
1. Introduction Lithium-ion battery has been considered as a promising power source for modern electronic devices due to its highest energy density among the commercial rechargeable batteries[1] . Graphite has been the main and most popular anode material. However, its theoretical capacity is limited to only 372 mAh·g−1 , because six carbon atoms only absorb one lithium atom to form LiC6 [2–4] . A series of nanosized transition-metal oxides such as CoO[5,6] , NiO[7] , Co3 O4 [8,9] , SnO2 [10] , CuO[11] , which exhibit reversible capacities about three times larger than graphite, are considered as alternatives to carbon to improve capacity and energy density[12] . Among these materials, cobalt oxides (CoO and Co3 O4 ) demonstrated the good electrochemical properties as lithium-storage materials. Recently, some spinel cobalt-based oxides MCo2 O4 (M =Ni[13] , Cu[14] , Mg[15] , Zn[16] ) were investigated as Li-ion battery anode materials, where one Co atom in Co3 O4 was replaced by cheaper and more eco-friendly metal M . Because of their high thermal stability, oxidation resistance, and reduced toxicity[17,18] , misfit-layered † Corresponding author. Prof., Ph.D.; Tel.: +86 571 87951451; Fax: +86 571 87951451; E-mail address: [email protected] (X.B. Zhao).
cobaltite (Ca3 Co4 O9 ) has received special attention as a promising thermoelectric material. In 2007, Kim et al.[19] first reported that layered oxide Ca3 Co4 O9 could act as a Li-ion battery anode with excellent cycling performance and rate capability. Unlike some transition metal oxides (such as NiO), the inactive CaO layer in Ca3 Co4 O9 cannot contribute to Listorage capacity, but it has some positive effect on the cycling stability of Ca3 Co4 O9 . In this work, Ca3 Co4 O9 and Ca2.95 K0.05 Co4 O9 were synthesized by the polyacrylamide gel method. Part of the Ca was replaced by K with an attempt to investigate the Kdoping on electrochemical performance for the new anode material Ca3 Co4 O9 . 2. Experimental Ca3 Co4 O9 and Ca2.95 K0.05 Co4 O9 powders were prepared by the polyacrylamide gel method. Stoichiometric Ca(CH3 COO)2 ·H2 O and Co(CH3 COO)2 ·4H2 O were dissolved in distilled water. KNO3 dissolved in deionized water was put into the above solution. The pH value of the solution was kept at 6. After a complete dissolution, acrylamide (monomer) and N, N -methylenebisacrylamide (crosslinking agent) were added into the above mixed solution. A uniform gel was ob-
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J.N. Cao et al.: J. Mater. Sci. Technol., 2010, 26(7), 669–672
Fig. 2 Structural model of CCO
Fig. 1 XRD patterns of the CCO and CKCO powders
tained once the initiator was added at 348 K. The water in the gel was removed by microwave heating. Then it was ground and calcined at about 973 K for 3 h in order to gain the precursor powders. For simplicity, Ca3 Co4 O9 and Ca2.95 K0.05 Co4 O9 are named CCO and CKCO, respectively. The crystal structure of the obtained samples was analyzed by powder X-ray diffraction (XRD) performed on a Rigaku D/MAX-2550PC diffractometer (Japan) using CuKα radiation (λ=0.15406 nm) in the range of 2θ=5–60 deg. with a step size of 0.02 deg. The morphology of the powder was observed by a Hitachi S-4800 field emission scanning electron microscope (FESEM, Japan). The working electrode was prepared with the active material, acetylene black and polyvinylidene fluoride (PVDF) binder in a weight ratio of 75:15:10. The electrochemical performance was evaluated with coin-type cell CR2025 using the working electrode as the cathode, a metal lithium foil as the anode, a polypropylene film (Celgard 2325) as the separator and a solution of 1 mol/L LiPF6 -EC+DMC (1:1 by volume) as the electrolyte. Cell assembling and sealing were carried out in an Ar-filled glove box. Galvanostatic cycling was performed between 0.01 and 3 V at 0.5 C on a Lisun PCBT-138-32D battery tester. A cyclic voltammogram (CV) test was carried out at a scan rate of 1 mV·s−1 . 3. Results and Discussion XRD patterns of samples are shown in Fig. 1. All the peaks are in good agreement with the patterns of Ca3 Co4 O9 without any impurities. It suggests that single phase compounds are obtained and K-doping does not change the Ca3 Co4 O9 structure. As seen from the inset of Fig. 1, the diffraction peaks of the CKCO sample shift to the low angle due to the doping of K with a larger ion radius. This indicates that K is introduced into the lattice. A structural model for the cobaltite “Ca3 Co4 O9 ” is shown in Fig. 2. This compound is a misfit-
Fig. 3 SEM images of samples CCO (a) and CKCO (b)
layered oxide consisting of two monoclinic subsystems with identical a, c parameters, but different b parameters (a=0.48376(7) nm, c=1.0833(1) nm, b1 =0.45565(6) nm, and b2 =0.28189(4) nm). The structure is built up from the stacking along the c axis of alternating triple rock-salt-type layers of [CoCa2 O3 ]- (that is [CaO-CoO-CaO]-) and single CdI2 -type [CoO2 ]- layers[20,21] . Two different sets of Co-O distances are involved, which are interpreted as the existence of cobalt with three different oxidation states 2+ , 3+ , and 4+ . The layered crystal structure of Ca3 Co4 O9 is similar to that of layered rock-salt structure LiCoO2 , consisting of alternate cobalt and lithium layers, and this structure is effective to the cycling performance of the materials. Because of the different valence, K-doping induces a transformation of a little fraction of Co ion from Co3+ to Co4+ . However, K-doping does not change the crystal structure of Ca3 Co4 O9 . Figure 3 shows the SEM images of
J.N. Cao et al.: J. Mater. Sci. Technol., 2010, 26(7), 669–672
Fig. 4 Initial charge-discharge profiles at 0.1 C rate (a) and cycling stability from the third cycle at 0.5 C (b) of the CCO and CKCO samples
CCO and CKCO. It can be seen that there are no obvious change on the surface morphology after Kdoping, and every particle is composed of many thin layers of different thickness. The electrochemical deintercalation-intercalations of the two samples were carried out between 0.01 and 3.0 V. Figure 4(a) shows the typical first chargedischarge and the second discharge curves at 0.1 C. Note that a long plateau at 0.75 V can be observed during the first discharge process for both samples, delivering a capacity around 1000 mAh·g−1 , which is greatly higher than the theoretical capacity of 643 mAh·g−1 according to the following reaction[19] : Ca3 Co4 O9 +12Li → 3CaO+4Co+6Li2 O (1st discharge) (1) The additional capacity comes from the decomposition of electrolyte and the formation of the solid electrolyte interface (SEI). During the first charge, the Kdoped sample gives a capacity of 485 mAh·g−1 , while the capacity of the undoped one is only 385 mAh·g−1 . This is due to the fact that the substitution K+ for Ca2+ can induce a transformation of a little fraction of Co ion from Co3+ to Co4+ , resulting in an increase of the capacity. Figure 4(b) compares the cycling stability between CCO and CKCO at 0.5 C in 0.01–3.0 V. As seen in the figure, K-doping can improve the cycling stability.
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Fig. 5 EIS plots of CCO (a) and CKCO (b) after different cycles
Fig. 6 Equivalent circuit used for the fitting of the impedance spectra
After cycled for 50 cycles at 0.5 C, CKCO retains a capacity of 223 mAh·g−1 , almost twice than CCO. It is suggested that the formation of K2 O can not only reduce the charge transfer resistance but also buffer the volume changes upon cycling. In order to understand the different electrochemical behaviors between the two samples, electrochemical impedance spectroscopy (EIS) measurement was carried out. Figure 5 shows the Nyquist plots of the two samples. An equivalent circuit is used to fit the plots as shown in Fig. 6, where Rs is the electrolyte resistance, Rsf CP Esf couple is surface film (SEI) resistance and capacitance, Rct CP Edl couple is charge transfer resistance and the double layer capacitance, and the W1 is the Warburg impedance indicating the Li-ion diffusion in the bulk material. The fitting values are summarized in Table 1. Note that, the CKCO sample shows a higher SEI resistance than the CCO sample, in agreement with the higher initial irreversible
J.N. Cao et al.: J. Mater. Sci. Technol., 2010, 26(7), 669–672
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Table 1 Fitting results of the Nyquist plots using the equivalent circuit Samples/cycle CCO/2nd CCO/12th CCO/32nd CCO/52nd CKCO/2nd CKCO/12th CKCO/32nd CKCO/52nd
Rs /Ω 4.23 5.56 6.65 7.40 7.65 7.08 7.78 7.67
Rsf /Ω 3.40 2.34 3.89 5.72 7.55 7.54 6.40 7.05
Rct /Ω 40.78 32.05 42.67 53.26 28.31 26.15 31.08 30.20
capacity. It can be seen that CKCO shows lower charge transfer resistance Rct and slower Rct increase during cycling than CCO. Again, this is in well consistent with the results of the electrochemical performance. 4. Conclusion CCO and CKCO powders have been prepared by the polyacrylamide gel method. K-doping does not change the crystal structure of CKCO. SEM images show that CKCO is stacked from thin layers with a thickness about 20–50 nm. Compared with CCO, CKCO delivers a higher initial capacities and better capacity retention ability. It is found that CKCO shows lower Rct and slower Rct increase during cycling than CCO, which can account for its improved electrochemical performance. In addition, the presence of K2 O can alleviate the volume change during cycling. REFERENCES [1 ] J.M. Tarascon and M. Armand: Nature, 2001, 414, 359. [2 ] B. Scrosati: Electrochim. Acta, 2000, 45, 2461.
[3 ] K. Sawai, Y. Iwakoshi and T. Ohzuku: Solid State Ion., 1994, 69, 273. [4 ] R. Yazami and D. Guerard: J. Power Sources, 1993, 43-44, 39. [5 ] H. Qiao, L.F. Xiao, Z. Zheng, H.W. Liu, F.L. Jia and L.Z. Zhang: J. Power Sources, 2008, 185, 486. [6 ] F. Li, Q.Q. Zou and Y.Y. Xia: J. Power Sources, 2008, 177, 546. [7 ] X.H. Huang, J.P. Tu , C.Q. Zhang, X.T. Chen, Y.F. Yuan and H.M. Wu: Electrochim. Acta, 2007, 52, 4177. [8 ] Y. Liu, C.H. Mi, L.H. Su and X.G. Zhang: Electrochim. Acta, 2008, 53, 2507. [9 ] X.W. Lou, D. Deng, J.Y. Lee, J. Feng and L.A. Archer: Adv. Mater., 2008, 20, 258. [10] Z. Ying, Q. Wan, H. Cao, Z.T. Song and S.L. Feng: Appl. Phys. Lett., 2005, 87, 113108. [11] L.B. Chen, N. Lu, C.M. Xu, H.C. Yu and T.H. Wang: Electrochim. Acta, 2009, 54, 4198. [12] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J.M. Tarascon: Nature, 2000, 407, 496. [13] B. Chi, H. Lin, J.B. Li, N. Wang and J. Yang: Int. J. Hydrogen Energy, 2006, 31, 1210. [14] Y. Sharma, N. Sharma, G.V. Subba Rao and B.V.R. Chowdari: J. Power Sources, 2007, 173, 495. [15] Y. Sharma, N. Sharma, G.V. Subba Rao and B.V.R. Chowdari: Solid State Ion., 2008, 179, 587. [16] C. Al, M. Yin, C. Wang and J. Sun: J. Mater. Sci., 2004, 39, 1077. [17] S. Li, R. Funahashi, I. Matsubara, K. Ueno and H. Yamada: J. Mater. Chem., 1999, 9, 1659. [18] I. Matsubara, R. Funahashi, T. Takeuchi, S. Sodeoka, T. Shimizu and K. Ueno: Appl. Phys. Lett., 2001, 78, 3627. [19] D.W. Kim, Y.D. Ko, J.G. Park and B.K. Kim: Angew. Chem. Int. Ed., 2007, 46, 6654. [20] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer and B. Raveau: Phys. Rev. B, 2001, 62, 166. [21] Y. Masuda, D. Nagahama, H. Itahara, T. Tani, W.S. Seo and K. Koumoto: J. Mater. Chem., 2003, 13, 1094.
Effect of K-doping on the Electrochemical Performance of Ca3 Co4 O9 Anode for Li-ion Batteries Jina Cao, Hongquan Liu, Jian Xie, Gaoshao Cao and Xinbing Zhao† Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China [Manuscript received October 9, 2009, in revised form December 7, 2009]
Ca3 Co4 O9 (CCO) and Ca2.95 K0.05 Co4 O9 (CKCO) powders have been prepared by the polyacrylamide gel method. CKCO shows increased capacity and better cycling stability compared with CCO. After cycled for 50 cycles at 0.5 C, CKCO retains a capacity of 223 mAh·g−1 , almost twice than CCO. The electrochemical impedance spectroscopy (EIS) tests shows the CKCO sample has a lower initial charge transfer resistance (Rct ) and undergoes smaller Rct change during cycling than the CCO sample, indicating improved electrochemical performance by K-doping. KEY WORDS: Lithium-ion battery; Anode; Ca3 Co4 O9 ; K-doping
1. Introduction Lithium-ion battery has been considered as a promising power source for modern electronic devices due to its highest energy density among the commercial rechargeable batteries[1] . Graphite has been the main and most popular anode material. However, its theoretical capacity is limited to only 372 mAh·g−1 , because six carbon atoms only absorb one lithium atom to form LiC6 [2–4] . A series of nanosized transition-metal oxides such as CoO[5,6] , NiO[7] , Co3 O4 [8,9] , SnO2 [10] , CuO[11] , which exhibit reversible capacities about three times larger than graphite, are considered as alternatives to carbon to improve capacity and energy density[12] . Among these materials, cobalt oxides (CoO and Co3 O4 ) demonstrated the good electrochemical properties as lithium-storage materials. Recently, some spinel cobalt-based oxides MCo2 O4 (M =Ni[13] , Cu[14] , Mg[15] , Zn[16] ) were investigated as Li-ion battery anode materials, where one Co atom in Co3 O4 was replaced by cheaper and more eco-friendly metal M . Because of their high thermal stability, oxidation resistance, and reduced toxicity[17,18] , misfit-layered † Corresponding author. Prof., Ph.D.; Tel.: +86 571 87951451; Fax: +86 571 87951451; E-mail address: [email protected] (X.B. Zhao).
cobaltite (Ca3 Co4 O9 ) has received special attention as a promising thermoelectric material. In 2007, Kim et al.[19] first reported that layered oxide Ca3 Co4 O9 could act as a Li-ion battery anode with excellent cycling performance and rate capability. Unlike some transition metal oxides (such as NiO), the inactive CaO layer in Ca3 Co4 O9 cannot contribute to Listorage capacity, but it has some positive effect on the cycling stability of Ca3 Co4 O9 . In this work, Ca3 Co4 O9 and Ca2.95 K0.05 Co4 O9 were synthesized by the polyacrylamide gel method. Part of the Ca was replaced by K with an attempt to investigate the Kdoping on electrochemical performance for the new anode material Ca3 Co4 O9 . 2. Experimental Ca3 Co4 O9 and Ca2.95 K0.05 Co4 O9 powders were prepared by the polyacrylamide gel method. Stoichiometric Ca(CH3 COO)2 ·H2 O and Co(CH3 COO)2 ·4H2 O were dissolved in distilled water. KNO3 dissolved in deionized water was put into the above solution. The pH value of the solution was kept at 6. After a complete dissolution, acrylamide (monomer) and N, N -methylenebisacrylamide (crosslinking agent) were added into the above mixed solution. A uniform gel was ob-
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J.N. Cao et al.: J. Mater. Sci. Technol., 2010, 26(7), 669–672
Fig. 2 Structural model of CCO
Fig. 1 XRD patterns of the CCO and CKCO powders
tained once the initiator was added at 348 K. The water in the gel was removed by microwave heating. Then it was ground and calcined at about 973 K for 3 h in order to gain the precursor powders. For simplicity, Ca3 Co4 O9 and Ca2.95 K0.05 Co4 O9 are named CCO and CKCO, respectively. The crystal structure of the obtained samples was analyzed by powder X-ray diffraction (XRD) performed on a Rigaku D/MAX-2550PC diffractometer (Japan) using CuKα radiation (λ=0.15406 nm) in the range of 2θ=5–60 deg. with a step size of 0.02 deg. The morphology of the powder was observed by a Hitachi S-4800 field emission scanning electron microscope (FESEM, Japan). The working electrode was prepared with the active material, acetylene black and polyvinylidene fluoride (PVDF) binder in a weight ratio of 75:15:10. The electrochemical performance was evaluated with coin-type cell CR2025 using the working electrode as the cathode, a metal lithium foil as the anode, a polypropylene film (Celgard 2325) as the separator and a solution of 1 mol/L LiPF6 -EC+DMC (1:1 by volume) as the electrolyte. Cell assembling and sealing were carried out in an Ar-filled glove box. Galvanostatic cycling was performed between 0.01 and 3 V at 0.5 C on a Lisun PCBT-138-32D battery tester. A cyclic voltammogram (CV) test was carried out at a scan rate of 1 mV·s−1 . 3. Results and Discussion XRD patterns of samples are shown in Fig. 1. All the peaks are in good agreement with the patterns of Ca3 Co4 O9 without any impurities. It suggests that single phase compounds are obtained and K-doping does not change the Ca3 Co4 O9 structure. As seen from the inset of Fig. 1, the diffraction peaks of the CKCO sample shift to the low angle due to the doping of K with a larger ion radius. This indicates that K is introduced into the lattice. A structural model for the cobaltite “Ca3 Co4 O9 ” is shown in Fig. 2. This compound is a misfit-
Fig. 3 SEM images of samples CCO (a) and CKCO (b)
layered oxide consisting of two monoclinic subsystems with identical a, c parameters, but different b parameters (a=0.48376(7) nm, c=1.0833(1) nm, b1 =0.45565(6) nm, and b2 =0.28189(4) nm). The structure is built up from the stacking along the c axis of alternating triple rock-salt-type layers of [CoCa2 O3 ]- (that is [CaO-CoO-CaO]-) and single CdI2 -type [CoO2 ]- layers[20,21] . Two different sets of Co-O distances are involved, which are interpreted as the existence of cobalt with three different oxidation states 2+ , 3+ , and 4+ . The layered crystal structure of Ca3 Co4 O9 is similar to that of layered rock-salt structure LiCoO2 , consisting of alternate cobalt and lithium layers, and this structure is effective to the cycling performance of the materials. Because of the different valence, K-doping induces a transformation of a little fraction of Co ion from Co3+ to Co4+ . However, K-doping does not change the crystal structure of Ca3 Co4 O9 . Figure 3 shows the SEM images of
J.N. Cao et al.: J. Mater. Sci. Technol., 2010, 26(7), 669–672
Fig. 4 Initial charge-discharge profiles at 0.1 C rate (a) and cycling stability from the third cycle at 0.5 C (b) of the CCO and CKCO samples
CCO and CKCO. It can be seen that there are no obvious change on the surface morphology after Kdoping, and every particle is composed of many thin layers of different thickness. The electrochemical deintercalation-intercalations of the two samples were carried out between 0.01 and 3.0 V. Figure 4(a) shows the typical first chargedischarge and the second discharge curves at 0.1 C. Note that a long plateau at 0.75 V can be observed during the first discharge process for both samples, delivering a capacity around 1000 mAh·g−1 , which is greatly higher than the theoretical capacity of 643 mAh·g−1 according to the following reaction[19] : Ca3 Co4 O9 +12Li → 3CaO+4Co+6Li2 O (1st discharge) (1) The additional capacity comes from the decomposition of electrolyte and the formation of the solid electrolyte interface (SEI). During the first charge, the Kdoped sample gives a capacity of 485 mAh·g−1 , while the capacity of the undoped one is only 385 mAh·g−1 . This is due to the fact that the substitution K+ for Ca2+ can induce a transformation of a little fraction of Co ion from Co3+ to Co4+ , resulting in an increase of the capacity. Figure 4(b) compares the cycling stability between CCO and CKCO at 0.5 C in 0.01–3.0 V. As seen in the figure, K-doping can improve the cycling stability.
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Fig. 5 EIS plots of CCO (a) and CKCO (b) after different cycles
Fig. 6 Equivalent circuit used for the fitting of the impedance spectra
After cycled for 50 cycles at 0.5 C, CKCO retains a capacity of 223 mAh·g−1 , almost twice than CCO. It is suggested that the formation of K2 O can not only reduce the charge transfer resistance but also buffer the volume changes upon cycling. In order to understand the different electrochemical behaviors between the two samples, electrochemical impedance spectroscopy (EIS) measurement was carried out. Figure 5 shows the Nyquist plots of the two samples. An equivalent circuit is used to fit the plots as shown in Fig. 6, where Rs is the electrolyte resistance, Rsf CP Esf couple is surface film (SEI) resistance and capacitance, Rct CP Edl couple is charge transfer resistance and the double layer capacitance, and the W1 is the Warburg impedance indicating the Li-ion diffusion in the bulk material. The fitting values are summarized in Table 1. Note that, the CKCO sample shows a higher SEI resistance than the CCO sample, in agreement with the higher initial irreversible
J.N. Cao et al.: J. Mater. Sci. Technol., 2010, 26(7), 669–672
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Table 1 Fitting results of the Nyquist plots using the equivalent circuit Samples/cycle CCO/2nd CCO/12th CCO/32nd CCO/52nd CKCO/2nd CKCO/12th CKCO/32nd CKCO/52nd
Rs /Ω 4.23 5.56 6.65 7.40 7.65 7.08 7.78 7.67
Rsf /Ω 3.40 2.34 3.89 5.72 7.55 7.54 6.40 7.05
Rct /Ω 40.78 32.05 42.67 53.26 28.31 26.15 31.08 30.20
capacity. It can be seen that CKCO shows lower charge transfer resistance Rct and slower Rct increase during cycling than CCO. Again, this is in well consistent with the results of the electrochemical performance. 4. Conclusion CCO and CKCO powders have been prepared by the polyacrylamide gel method. K-doping does not change the crystal structure of CKCO. SEM images show that CKCO is stacked from thin layers with a thickness about 20–50 nm. Compared with CCO, CKCO delivers a higher initial capacities and better capacity retention ability. It is found that CKCO shows lower Rct and slower Rct increase during cycling than CCO, which can account for its improved electrochemical performance. In addition, the presence of K2 O can alleviate the volume change during cycling. REFERENCES [1 ] J.M. Tarascon and M. Armand: Nature, 2001, 414, 359. [2 ] B. Scrosati: Electrochim. Acta, 2000, 45, 2461.
[3 ] K. Sawai, Y. Iwakoshi and T. Ohzuku: Solid State Ion., 1994, 69, 273. [4 ] R. Yazami and D. Guerard: J. Power Sources, 1993, 43-44, 39. [5 ] H. Qiao, L.F. Xiao, Z. Zheng, H.W. Liu, F.L. Jia and L.Z. Zhang: J. Power Sources, 2008, 185, 486. [6 ] F. Li, Q.Q. Zou and Y.Y. Xia: J. Power Sources, 2008, 177, 546. [7 ] X.H. Huang, J.P. Tu , C.Q. Zhang, X.T. Chen, Y.F. Yuan and H.M. Wu: Electrochim. Acta, 2007, 52, 4177. [8 ] Y. Liu, C.H. Mi, L.H. Su and X.G. Zhang: Electrochim. Acta, 2008, 53, 2507. [9 ] X.W. Lou, D. Deng, J.Y. Lee, J. Feng and L.A. Archer: Adv. Mater., 2008, 20, 258. [10] Z. Ying, Q. Wan, H. Cao, Z.T. Song and S.L. Feng: Appl. Phys. Lett., 2005, 87, 113108. [11] L.B. Chen, N. Lu, C.M. Xu, H.C. Yu and T.H. Wang: Electrochim. Acta, 2009, 54, 4198. [12] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J.M. Tarascon: Nature, 2000, 407, 496. [13] B. Chi, H. Lin, J.B. Li, N. Wang and J. Yang: Int. J. Hydrogen Energy, 2006, 31, 1210. [14] Y. Sharma, N. Sharma, G.V. Subba Rao and B.V.R. Chowdari: J. Power Sources, 2007, 173, 495. [15] Y. Sharma, N. Sharma, G.V. Subba Rao and B.V.R. Chowdari: Solid State Ion., 2008, 179, 587. [16] C. Al, M. Yin, C. Wang and J. Sun: J. Mater. Sci., 2004, 39, 1077. [17] S. Li, R. Funahashi, I. Matsubara, K. Ueno and H. Yamada: J. Mater. Chem., 1999, 9, 1659. [18] I. Matsubara, R. Funahashi, T. Takeuchi, S. Sodeoka, T. Shimizu and K. Ueno: Appl. Phys. Lett., 2001, 78, 3627. [19] D.W. Kim, Y.D. Ko, J.G. Park and B.K. Kim: Angew. Chem. Int. Ed., 2007, 46, 6654. [20] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer and B. Raveau: Phys. Rev. B, 2001, 62, 166. [21] Y. Masuda, D. Nagahama, H. Itahara, T. Tani, W.S. Seo and K. Koumoto: J. Mater. Chem., 2003, 13, 1094.