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Preparation and electrochemical behaviour of LiMn2O4 thin film by spray pyrolysis method
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Thin Solid Films 516 (2008) 8295 – 8298
www.elsevier.com/locate/tsf
Preparation and electrochemical behaviour of LiMn2O4 thin film by spray pyrolysis method A. Subramania ⁎, S.N. Karthick, N. Angayarkanni Advanced Materials Research Lab, Department of Industrial Chemistry, Alagappa University, Karaikudi, 630 003, India Received 3 January 2007; received in revised form 18 March 2008; accepted 20 March 2008 Available online 29 March 2008
Abstract Spinel lithium manganese oxide thin films were prepared on a stainless steel substrate by a spray pyrolysis method using a precursor solution of lithium acetate and manganese acetate in 1:1 (v/v) mixture of methanol and distilled water. Thermal analysis of the precursor solution was carried out for determining the required temperature for the phase formation or complete crystallization of the precursor sample. The as-deposited thin films were calcined at different temperatures from 500 to 700 °C to find out the degree of crystallization of LiMn2O4 thin film. The structural property of the prepared thin film was analyzed by X-ray diffraction. The morphology and the grain size of the deposited film were evaluated by scanning electron microscopy. The electrochemical behaviour of the prepared LiMn2O4 thin films was analysed by cyclic voltammetry and charge–discharge studies. The results showed that the prepared LiMn2O4 thin films had good electrochemical properties. © 2008 Elsevier B.V. All rights reserved. Keywords: LiMn2O4 thin film; Spray pyrolysis method; Thin film battery; Microbattery
1. Introduction Fabrication of all solid state microbatteries has received much attention in the recent years, to use as a power source for microelectronic devices. Among these batteries, lithium-ion microbattery made from thin film electrode materials and polymer electrolytes plays an important role. Several metal oxides have been reported to use as cathode material for Limicrobatteries. Among them, cubic spinel lithium manganese oxide has attracted much attention because of its high voltage, low cost and non-toxicity [1,2]. However, the cubic spinel cathode material suffers from Jahn–Teller instability on prolonged cycling [3]. So thin film electrodes were prepared as an useful alternative in order to avoid the cycling stability problem, kinetics of Li+ ion transport, conductivity and high interfacial resistance with porous electrode. Various thin film deposition methods such as chemical vapour deposition [4,5], radio frequency magnetron sputtering [6,7], electron beam evaporation [8], pulse laser deposition [9,10] and electrostatic spray deposition methods [11,12] have ⁎ Corresponding author. Tel.: +91 4565 228836; fax: +91 4565 225202. E-mail address: [email protected] (A. Subramania). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.03.032
been used to prepare LiMn2O4 thin films. However, very recently ultrasonic spray pyrolysis method has also been reported for the preparation of LiMn2O4 cathode material [13,14]. In the present investigation, we have reported a low cost spray pyrolysis method for the preparation of LiMn2O4 thin film cathode material for fabricating lithium-ion microbattery. This method has the advantage of stoichiometry control of the high purity product. The effect of the calcination temperature, spraying time and distance between the spray nozzle and the substrate on the phase formation and microstructure of the thin films were examined and discussed. 2. Experimental details 2.1. Preparation of LiMn2O4 thin film The spray pyrolysis set up has a temperature controller to maintain a constant substrate temperature during the deposition. A spray nozzle was used for spraying the aerosol uniformly on the preheated substrate through gravity feeding mechanism. A pressure gauge was used to measure the pressure of the inflow atmospheric air. Pressurized air and stainless steel were used as the carrier gas and substrate, respectively.
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Stoichiometric amount of lithium acetate and manganese acetate were dissolved in a mixture of 1:1(v/v) methyl alcohol and water to get a homogeneous precursor solution for the preparation of good adhesive lithium manganese oxide thin film. Due to the volatile nature of lithium in the compound, an excess of 10% lithium was added during the preparation of the precursor solution. The stainless steel substrate was first cleaned with acetone, triple distilled water and ultrasonic rinsing for 10 min. A downward spray setup was employed for film formation. Substrate temperature, spraying time, distance between the substrate and spray nozzle, air pressure were optimized to obtain LiMn2O4 thin films. The precursor solution (0.1 M) was sprayed over preheated stainless steel substrate at the spray rate of 5 ml min− 1. In order to avoid a decrease in substrate temperature during the deposition, many cycles were used. Spraying was done with 10 cycles at 100 s interval between the each successive cycles and the spraying time was 15 s/cycle. The optimized preparative conditions are given in Table 1. 2.2. Characterization of LiMn2O4 thin film Thermal analysis of the precursor sample was made using simultaneous thermogravimetry/differential thermogravimetry thermal analyzer (TG/DTA) (Perkin Elmer — Diamond) at a heating rate of 10 °C min− 1 in the range of 800 °C under air atmosphere to find out the phase formation or complete crystallization temperature of the precursor sample. Thickness of the film was measured by a contact probe surface profilometer (Tencor Instrument). X-ray diffraction (XRD) measurements were made from JEOL (Model: JDX 8030) Xray diffractometer using nickel filtered Cu-Kα radiation to identify the phase purity and structure conformity of the prepared thin film. The diffraction patterns were taken at 25 °C in the range of 0° ≤ 2θ ≥ 80° in step scans. The step size and scan rate were set at 0.1° and 2°min− 1, respectively. The deposited thin film was analyzed by scanning electron microscopy (SEM) (JEOL Model: JSM-840A) to examine the morphology and particle size of the prepared thin film by employing an accelerating voltage of 20 kV. Finally, the intercalation and deintercalation behaviour of LiMn2O4 thin films were studied using cyclic voltammetry experiments and further electrochemical studies were done using charge– discharge and cycleability measurements by fabricating a three electrode cell assembly under argon atmosphere in a glove box. The studies were carried out with the LiMn2O4 thin film as the working electrode and Li foil as the reference and counter electrode with PVdF-HFP (Polyvinylidene fluoride-coTable 1 Optimized parameters for the preparation of LiMn2O4 thin film Optimized parameters
Specification
Substrate temperature Substrate to spray nozzle distance Carrier gas Carrier gas pressure Spray rate Annealing temperature
320 °C 20 cm Air 200 kPa 5 ml min− 1 700 °C for 10 min
Fig. 1. TGA-DTA result of LiMn2O4 precursor solution.
hexa fluoropropylene) based micro-porous polymer electrolyte membrane, over the potential range of 3.0–4.5 V at the current density of 0.2 mA/cm2 [2,11]. 3. Results and discussion 3.1. Thermal studies The TGA/DTA curves for the precursor heated at a rate of 10 °C/min are shown in Fig. 1. Curves show two endothermic peaks followed by one exothermic peak. The two endothermic peaks at ~ 60 °C and ~ 120 °C were due to the removal of alcohol and water respectively. The exothermic peak at 305 °C indicates the phase formation or crystallization of LiMn2O4. At temperature N 320 °C, there was no weight loss. This revealed that the decomposition temperature of precursor was 320 °C. Hence, the temperature of the substrate was kept at 320 °C. 3.2. X-ray diffraction studies The X-ray diffraction patterns of LiMn2O4 thin film calcined at different temperatures for 10 min are shown in Fig. 2. For the sample sprayed at 320 °C, the diffraction peaks located at 2θ = 18.68, 36.32, 44.18, 58.47 and 63.79 corresponding to the (111), (311), (400), (511) and (440) planes of spinel LiMn2O4 lattice [15]. The observed peaks became sharper when the annealing temperature was increased by 700 °C, indicates a better crystallinity of LiMn2O4 thin films. The (533) and (622) planes were also obtained at this temperature. With further annealing at 800 °C for 10 min, there was no improvement in the intensity of observed peaks. It shows that the optimum calcination temperature for the formation of well-defined crystalline LiMn2O4 thin film was 700 °C for 10 min and the thickness of the film was 0.5 ± 0.05 μm. 3.3. Scanning electron microscopy studies The direct and the cross sectional views of SEM photograph of LiMn2O4 thin film annealed at 700 °C for 10 min is shown in
A. Subramania et al. / Thin Solid Films 516 (2008) 8295–8298
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Fig. 4. Cyclic voltammogram of Li//LiMn2O4 thin film cell.
Fig. 2. X-ray diffraction patterns of the cubic spinel LiMn2O4 thin films. (a) asdeposited at 320 °C (b) annealing at 500 °C (c) 600 °C (d) 700 °C (e) 800 °C for 10 min in air.
Fig. 3. The surface of the thin film has uniform particle size and well-defined edges with dense surface area and the grain size was estimated to be 0.4–0.7 μm. The approximate thickness of the film was b 0.5 μm. 3.4. Electrochemical studies Fig. 4 shows the cyclic voltammogram of 700 °C annealed LiMn2O4 thin film for 10 min at a scan rate of 0.1 mVs− 1. The measurements were performed in the voltage range of 3.0– 4.5 V. Two pairs of redox peaks are observed indicating a phase change of LiMn2O4 during the intercalation/deintercalation process. The higher potential peak corresponds to Li+ extraction and the lower potential peak corresponds to Li+ insertion. The appearance of two pairs of redox peaks revealed the reversible nature of Li insertion and extraction process. The LiMn2O4 prepared by this spray pyrolysis method exhibits good electrochemical properties. Fig. 5 shows the charge–discharge curves of the prepared LiMn2O4 thin film at the current density of 0.2 mA/cm2 and its theoretical capacity is 148 mAhg− 1. The figure shows that the initial capacity of the cell was near to that of the theoretical one and this initial discharge capacity decreased only by 15% even
Fig. 3. SEM photographs of LiMn2O4 thin film annealed at 700 °C for 10 min. (a) direct view (b) cross sectional view.
Fig. 5. Charge/discharge curve for Li//LiMn2O4 thin film cell.
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results revealed that the thin film calcined at 700 °C for 10 min had a single-phase cubic spinel with a space group Fd3m, which showed good electrochemical reversibility during charge and discharge cycles. References [1] [2] [3] [4] [5] [6] Fig. 6. Relationship between the discharge capacity and cycle number of Li// LiMn2O4 thin film cell in the voltage range of 3.0–4.5 V at a current density of 0.2 mA/cm2.
[7]
after 25th cycle (Fig. 6). It suggests that the prepared LiMn2O4 thin film electrode showed good cycling stability. It may be due to the less porous nature of the prepared thin film electrode material than the bulk electrode material [16]. Hence, the spray pyrolysis method can be used for the preparation of active LiMn2O4 thin film electrode material.
[9]
4. Conclusions LiMn2O4 thin film has been fabricated with good adhesion on the substrate by the spray pyrolysis method. X-ray diffraction
[8]
[10] [11] [12] [13] [14] [15] [16]
I. Taniguchi, Mater. Chem. Phys. 92 (2005) 172. S.-H. Park, S.-T. Myung, Y.-K. Sun, Electrochim. Acta 51 (2006) 4089. A. Yamada, J. Solid State Chem. 122 (1995) 160. H.-S. Moon, S.W. Lee, Y.-K. Lee, J.-W. Park, J. Power Sources 119 (2003) 713. P. Frangaud, R. Nagarajan, D.M. Schleich, D. Vujic, J. Power Sources 54 (1995) 362. H.-S. Moon, W. Lee, P.J. Reucroft, J.-W. Park, J. Power Sources 119-121 (2003) 710. J.B. Bates, N.J. Dudney, D.C. Lubben, G.R. Gruzalski, B.S. Kwak, X. Yu, R.A. Zuhr, J. Power Sources 54 (1995) 58. S.-J. Lee, J.-K. Lee, D.-W. Kim, H.-K. Baik, J. Electrochem. Soc. 143 (1996) L268. C. Julien, E.H. Poniatowski, M.A.C. Lopez, L.E. Alarcon, J.J. Jarquin, Mater. Sci. Eng., B 72 (2000) 36. K.A. Striebel, C.Z. Deng, S.J. Wen, E.J. Carims, J. Electrochem. Soc. 143 (1996) 1821. D. Shu, K. Gopu, K.-B. Kim, K.S. Ryu, S.H. Chang, Solid State Ionics 160 (2003) 227. S. Koike, K. Tatsumi, J. Power Sources 146 (2005) 241. K. Matsuda, I. Taniguchi, J. Power Sources 132 (2004) 156. Z. Bakena, I. Taniguchi, Solid State Ionics 176 (2005) 1027. F.-Y. Shih, K.-Z. Fung, J. Power Sources 159 (2006) 179. A. Subramania, N. Angayarkanni, N. Niruba, T. Vasudevan, Nanotechnology 18 (2007) 05603.
Thin Solid Films 516 (2008) 8295 – 8298
www.elsevier.com/locate/tsf
Preparation and electrochemical behaviour of LiMn2O4 thin film by spray pyrolysis method A. Subramania ⁎, S.N. Karthick, N. Angayarkanni Advanced Materials Research Lab, Department of Industrial Chemistry, Alagappa University, Karaikudi, 630 003, India Received 3 January 2007; received in revised form 18 March 2008; accepted 20 March 2008 Available online 29 March 2008
Abstract Spinel lithium manganese oxide thin films were prepared on a stainless steel substrate by a spray pyrolysis method using a precursor solution of lithium acetate and manganese acetate in 1:1 (v/v) mixture of methanol and distilled water. Thermal analysis of the precursor solution was carried out for determining the required temperature for the phase formation or complete crystallization of the precursor sample. The as-deposited thin films were calcined at different temperatures from 500 to 700 °C to find out the degree of crystallization of LiMn2O4 thin film. The structural property of the prepared thin film was analyzed by X-ray diffraction. The morphology and the grain size of the deposited film were evaluated by scanning electron microscopy. The electrochemical behaviour of the prepared LiMn2O4 thin films was analysed by cyclic voltammetry and charge–discharge studies. The results showed that the prepared LiMn2O4 thin films had good electrochemical properties. © 2008 Elsevier B.V. All rights reserved. Keywords: LiMn2O4 thin film; Spray pyrolysis method; Thin film battery; Microbattery
1. Introduction Fabrication of all solid state microbatteries has received much attention in the recent years, to use as a power source for microelectronic devices. Among these batteries, lithium-ion microbattery made from thin film electrode materials and polymer electrolytes plays an important role. Several metal oxides have been reported to use as cathode material for Limicrobatteries. Among them, cubic spinel lithium manganese oxide has attracted much attention because of its high voltage, low cost and non-toxicity [1,2]. However, the cubic spinel cathode material suffers from Jahn–Teller instability on prolonged cycling [3]. So thin film electrodes were prepared as an useful alternative in order to avoid the cycling stability problem, kinetics of Li+ ion transport, conductivity and high interfacial resistance with porous electrode. Various thin film deposition methods such as chemical vapour deposition [4,5], radio frequency magnetron sputtering [6,7], electron beam evaporation [8], pulse laser deposition [9,10] and electrostatic spray deposition methods [11,12] have ⁎ Corresponding author. Tel.: +91 4565 228836; fax: +91 4565 225202. E-mail address: [email protected] (A. Subramania). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.03.032
been used to prepare LiMn2O4 thin films. However, very recently ultrasonic spray pyrolysis method has also been reported for the preparation of LiMn2O4 cathode material [13,14]. In the present investigation, we have reported a low cost spray pyrolysis method for the preparation of LiMn2O4 thin film cathode material for fabricating lithium-ion microbattery. This method has the advantage of stoichiometry control of the high purity product. The effect of the calcination temperature, spraying time and distance between the spray nozzle and the substrate on the phase formation and microstructure of the thin films were examined and discussed. 2. Experimental details 2.1. Preparation of LiMn2O4 thin film The spray pyrolysis set up has a temperature controller to maintain a constant substrate temperature during the deposition. A spray nozzle was used for spraying the aerosol uniformly on the preheated substrate through gravity feeding mechanism. A pressure gauge was used to measure the pressure of the inflow atmospheric air. Pressurized air and stainless steel were used as the carrier gas and substrate, respectively.
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Stoichiometric amount of lithium acetate and manganese acetate were dissolved in a mixture of 1:1(v/v) methyl alcohol and water to get a homogeneous precursor solution for the preparation of good adhesive lithium manganese oxide thin film. Due to the volatile nature of lithium in the compound, an excess of 10% lithium was added during the preparation of the precursor solution. The stainless steel substrate was first cleaned with acetone, triple distilled water and ultrasonic rinsing for 10 min. A downward spray setup was employed for film formation. Substrate temperature, spraying time, distance between the substrate and spray nozzle, air pressure were optimized to obtain LiMn2O4 thin films. The precursor solution (0.1 M) was sprayed over preheated stainless steel substrate at the spray rate of 5 ml min− 1. In order to avoid a decrease in substrate temperature during the deposition, many cycles were used. Spraying was done with 10 cycles at 100 s interval between the each successive cycles and the spraying time was 15 s/cycle. The optimized preparative conditions are given in Table 1. 2.2. Characterization of LiMn2O4 thin film Thermal analysis of the precursor sample was made using simultaneous thermogravimetry/differential thermogravimetry thermal analyzer (TG/DTA) (Perkin Elmer — Diamond) at a heating rate of 10 °C min− 1 in the range of 800 °C under air atmosphere to find out the phase formation or complete crystallization temperature of the precursor sample. Thickness of the film was measured by a contact probe surface profilometer (Tencor Instrument). X-ray diffraction (XRD) measurements were made from JEOL (Model: JDX 8030) Xray diffractometer using nickel filtered Cu-Kα radiation to identify the phase purity and structure conformity of the prepared thin film. The diffraction patterns were taken at 25 °C in the range of 0° ≤ 2θ ≥ 80° in step scans. The step size and scan rate were set at 0.1° and 2°min− 1, respectively. The deposited thin film was analyzed by scanning electron microscopy (SEM) (JEOL Model: JSM-840A) to examine the morphology and particle size of the prepared thin film by employing an accelerating voltage of 20 kV. Finally, the intercalation and deintercalation behaviour of LiMn2O4 thin films were studied using cyclic voltammetry experiments and further electrochemical studies were done using charge– discharge and cycleability measurements by fabricating a three electrode cell assembly under argon atmosphere in a glove box. The studies were carried out with the LiMn2O4 thin film as the working electrode and Li foil as the reference and counter electrode with PVdF-HFP (Polyvinylidene fluoride-coTable 1 Optimized parameters for the preparation of LiMn2O4 thin film Optimized parameters
Specification
Substrate temperature Substrate to spray nozzle distance Carrier gas Carrier gas pressure Spray rate Annealing temperature
320 °C 20 cm Air 200 kPa 5 ml min− 1 700 °C for 10 min
Fig. 1. TGA-DTA result of LiMn2O4 precursor solution.
hexa fluoropropylene) based micro-porous polymer electrolyte membrane, over the potential range of 3.0–4.5 V at the current density of 0.2 mA/cm2 [2,11]. 3. Results and discussion 3.1. Thermal studies The TGA/DTA curves for the precursor heated at a rate of 10 °C/min are shown in Fig. 1. Curves show two endothermic peaks followed by one exothermic peak. The two endothermic peaks at ~ 60 °C and ~ 120 °C were due to the removal of alcohol and water respectively. The exothermic peak at 305 °C indicates the phase formation or crystallization of LiMn2O4. At temperature N 320 °C, there was no weight loss. This revealed that the decomposition temperature of precursor was 320 °C. Hence, the temperature of the substrate was kept at 320 °C. 3.2. X-ray diffraction studies The X-ray diffraction patterns of LiMn2O4 thin film calcined at different temperatures for 10 min are shown in Fig. 2. For the sample sprayed at 320 °C, the diffraction peaks located at 2θ = 18.68, 36.32, 44.18, 58.47 and 63.79 corresponding to the (111), (311), (400), (511) and (440) planes of spinel LiMn2O4 lattice [15]. The observed peaks became sharper when the annealing temperature was increased by 700 °C, indicates a better crystallinity of LiMn2O4 thin films. The (533) and (622) planes were also obtained at this temperature. With further annealing at 800 °C for 10 min, there was no improvement in the intensity of observed peaks. It shows that the optimum calcination temperature for the formation of well-defined crystalline LiMn2O4 thin film was 700 °C for 10 min and the thickness of the film was 0.5 ± 0.05 μm. 3.3. Scanning electron microscopy studies The direct and the cross sectional views of SEM photograph of LiMn2O4 thin film annealed at 700 °C for 10 min is shown in
A. Subramania et al. / Thin Solid Films 516 (2008) 8295–8298
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Fig. 4. Cyclic voltammogram of Li//LiMn2O4 thin film cell.
Fig. 2. X-ray diffraction patterns of the cubic spinel LiMn2O4 thin films. (a) asdeposited at 320 °C (b) annealing at 500 °C (c) 600 °C (d) 700 °C (e) 800 °C for 10 min in air.
Fig. 3. The surface of the thin film has uniform particle size and well-defined edges with dense surface area and the grain size was estimated to be 0.4–0.7 μm. The approximate thickness of the film was b 0.5 μm. 3.4. Electrochemical studies Fig. 4 shows the cyclic voltammogram of 700 °C annealed LiMn2O4 thin film for 10 min at a scan rate of 0.1 mVs− 1. The measurements were performed in the voltage range of 3.0– 4.5 V. Two pairs of redox peaks are observed indicating a phase change of LiMn2O4 during the intercalation/deintercalation process. The higher potential peak corresponds to Li+ extraction and the lower potential peak corresponds to Li+ insertion. The appearance of two pairs of redox peaks revealed the reversible nature of Li insertion and extraction process. The LiMn2O4 prepared by this spray pyrolysis method exhibits good electrochemical properties. Fig. 5 shows the charge–discharge curves of the prepared LiMn2O4 thin film at the current density of 0.2 mA/cm2 and its theoretical capacity is 148 mAhg− 1. The figure shows that the initial capacity of the cell was near to that of the theoretical one and this initial discharge capacity decreased only by 15% even
Fig. 3. SEM photographs of LiMn2O4 thin film annealed at 700 °C for 10 min. (a) direct view (b) cross sectional view.
Fig. 5. Charge/discharge curve for Li//LiMn2O4 thin film cell.
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results revealed that the thin film calcined at 700 °C for 10 min had a single-phase cubic spinel with a space group Fd3m, which showed good electrochemical reversibility during charge and discharge cycles. References [1] [2] [3] [4] [5] [6] Fig. 6. Relationship between the discharge capacity and cycle number of Li// LiMn2O4 thin film cell in the voltage range of 3.0–4.5 V at a current density of 0.2 mA/cm2.
[7]
after 25th cycle (Fig. 6). It suggests that the prepared LiMn2O4 thin film electrode showed good cycling stability. It may be due to the less porous nature of the prepared thin film electrode material than the bulk electrode material [16]. Hence, the spray pyrolysis method can be used for the preparation of active LiMn2O4 thin film electrode material.
[9]
4. Conclusions LiMn2O4 thin film has been fabricated with good adhesion on the substrate by the spray pyrolysis method. X-ray diffraction
[8]
[10] [11] [12] [13] [14] [15] [16]
I. Taniguchi, Mater. Chem. Phys. 92 (2005) 172. S.-H. Park, S.-T. Myung, Y.-K. Sun, Electrochim. Acta 51 (2006) 4089. A. Yamada, J. Solid State Chem. 122 (1995) 160. H.-S. Moon, S.W. Lee, Y.-K. Lee, J.-W. Park, J. Power Sources 119 (2003) 713. P. Frangaud, R. Nagarajan, D.M. Schleich, D. Vujic, J. Power Sources 54 (1995) 362. H.-S. Moon, W. Lee, P.J. Reucroft, J.-W. Park, J. Power Sources 119-121 (2003) 710. J.B. Bates, N.J. Dudney, D.C. Lubben, G.R. Gruzalski, B.S. Kwak, X. Yu, R.A. Zuhr, J. Power Sources 54 (1995) 58. S.-J. Lee, J.-K. Lee, D.-W. Kim, H.-K. Baik, J. Electrochem. Soc. 143 (1996) L268. C. Julien, E.H. Poniatowski, M.A.C. Lopez, L.E. Alarcon, J.J. Jarquin, Mater. Sci. Eng., B 72 (2000) 36. K.A. Striebel, C.Z. Deng, S.J. Wen, E.J. Carims, J. Electrochem. Soc. 143 (1996) 1821. D. Shu, K. Gopu, K.-B. Kim, K.S. Ryu, S.H. Chang, Solid State Ionics 160 (2003) 227. S. Koike, K. Tatsumi, J. Power Sources 146 (2005) 241. K. Matsuda, I. Taniguchi, J. Power Sources 132 (2004) 156. Z. Bakena, I. Taniguchi, Solid State Ionics 176 (2005) 1027. F.-Y. Shih, K.-Z. Fung, J. Power Sources 159 (2006) 179. A. Subramania, N. Angayarkanni, N. Niruba, T. Vasudevan, Nanotechnology 18 (2007) 05603.