Hollow LiNi0.8Co0.1Mn0.1O2−MgO Coaxial Fibers: Sol−Gel Method

20176

J. Phys. Chem. C 2008, 112, 20176–20180

Hollow LiNi0.8Co0.1Mn0.1O2-MgO Coaxial Fibers: Sol-Gel Method Combined with Co-electrospun Preparation and Electrochemical Properties Yuanxiang Gu and Fangfang Jian* The Laboratory of New Materials and Functional Compounds, Qingdao UniVersity of Science and Technology, Qingdao 266042, P. R. China ReceiVed: September 24, 2008; ReVised Manuscript ReceiVed: October 21, 2008

Hollow LiNi0.8Co0.1Mn0.1O2-MgO coaxial fibers were prepared by employing the sol-gel proces combined with the coaxial electrospinning technique. The hollow coaxial fibers had a shell thickness of 30-60 nm and wall thicknesses of 300-500 nm. The polyvinyl pyrrolidone (PVP) had an important role in the formation of hollow structure. The electrochemical properties of hollow LiNi0.8Co0.1Mn0.1O2-MgO coaxial fibers were investigated by charge-discharge experiments. The hollow coaxial fibers as electrode materials showed a high discharge capacity and excellent cycle stability. 1. Introduction Hollow inorganic tubules of nano or micro diameter have recently become of particular interest with respect to their unique properties.1 Hollow tubules as one of the most important nanostructures are useful in a wealth of potential applications related to microfluidics, catalysis, drug release, sensing, energy storage, and so forth.2 However, the simplex structure of the hollow tubes cannot meet the demand for the multifunctionality of one-dimensional (1D) materials in future application of nanodevices. Therefore, it is necessary to engineer hollow tubes into hierarchical structures with multiple functionalities. The research into one-dimensional core-shell materials is also a hot subject and provides additional opportunities for enhancing the functionality of 1D nano- and microstructures.3 The formation of shells on cores provides a natural vehicle to incorporate different materials into the same structure and thus to enhance the properties of the core materials.4 Therefore, the synthesis of hollow tubes with coaxial structure is particularly challenging because they will integrate the advantages of both hollow tubes and core-shell materials and are expected to display novel functions. Lately, enormous effort has been made to construct coaxial tubular nanoarchitectures focused mainly on carbon nanotubes and fullerence derivatives, which offers a promising direction for enhanced properties of materials through cooperative contribution of each component.5 However, the fabrication of coaxial tubular nanoarchitectures still remains a great challenge, and there have been very few reports on the synthesis of inorganic hollow tubes with core-shell structure by sol-electrospinning. Electrospinning is a simple, versatile, and useful technique for generating long, ultrathin fibers made of various materials.6 Particularly, coaxial electrospinning has been extensively explored for generating complex nanostructures having controllable hierarchical features, such as core-shell and hollow tubes.7 In this work, we have put forth a successful attempt to prepare hollow fibers with core-shell structure by coaxial electrospinning combined with the sol-gel method. The sol-coaxial electrospinning provides a simple route to develop hollow coaxial fibers and enriches the electrospinning technique to create nanostructured materials with new morphologies. In addition, the meaning of this work is to develop a hollow * Corresponding author. E-mail: [email protected].

Figure 1. The XRD pattern of hollow LiNi0.8Co0.1Mn0.1O2-MgO coaxial fibers.

LiNi0.8Co0.1Mn0.1O2-MgO coaxial fiber cathode with a high capacity and good cycle performance, providing a promising cathode material for high-performance lithium ion batteries. 2. Experiment All the reagents were analytical grade and were used without further purification. In a typical synthesis, 0.0315 mol of lithium acetate (Li(CH3COO) · H2O), 0.024 mol of nickel acetate (Ni(CH3COO)2 · 4H2O), 0.003 mol of cobalt acetate (Co(CH3COO)2 · 4H2O), and 0.003 mol of manganese acetate (Mn(CH3COO)2 · 4H2O) were dissolved into 100 mL of distilled water, and 0.036 mol of citric acid (C6H8O7 · H2O) was added into 30 mL of distilled water. After the two solutions were mixed by stirring, polyvinyl pyrrolidone (PVP, K30) was added into the solution under stirring. Twenty milliloles (4.289 g) of magnesium acetate (Mg(CH3COO)2 · H2O) and 24.0 mmol (5.043 g) of citric acid (C6H8O7 · H2O) were added into 60.0 mL of distilled water to give a clear solution, then the two resulting solutions were aged at 70 °C until they became viscous, transparent sols. The coaxial electrospinning setup was used in this study. By varying the viscosity of the sols with the aging time, the LiNi0.8Co0.1Mn0.1O2 and MgO optimal sol viscosity for the fabrication of core-shell fibers was determined to be 3.0 and 4.0 Pa · s, respectively. The MgO sol was added into the outer syringe, and the LiNi0.8Co0.1Mn0.1O2 sol was poured into the inner tube. The coaxial fibers were electrospun and deposited on a stainless steel collector. To obtain hollow

10.1021/jp808468x CCC: $40.75  2008 American Chemical Society Published on Web 11/24/2008

Hollow LiNi0.8Co0.1Mn0.1O2-MgO Coaxial Fibers

J. Phys. Chem. C, Vol. 112, No. 51, 2008 20177

Figure 2. SEM images of the LiNi0.8Co0.1Mn0.1O2-MgO xerogel fibers (a) and hollow LiNi0.8Co0.1Mn0.1O2-MgO coaxial fibers after being calcined at 750 °C: low magnification (b), high magnification (c).

steel net was used as the reference and counter electrodes. The electrolyte was 1.0 mol · dm-3 LiPF6 solution in ethylene carbonate and diethylene carbonate with a volume ratio of 1/1. The charge-discharge cycles were tested on a Land Series battery test system with a current density of 20.0 mA/g in the potential range of 3.0-4.3 V. 3. Results and Discussion

Figure 3. The TEM (a) and HR-TEM (b) images of hollow LiNi0.8Co0.1Mn0.1O2-MgO coaxial fibers after being calcined at 750 °C.

TABLE 1: The Influence of PVP Addition on the Morphology of LiNi0.8Co0.1Mn0.1O2-MgO and LiNi0.8Co0.1Mn0.1O2 Fibers [PVP]/[M]

morphology of the composite fibers

morphology of the bare fibers

0 0.2 0.4 0.6

solid, round solid, round hollow hollow

belt-like solid, round solid, round hollow

LiNi0.8Co0.1Mn0.1O2-MgO coaxial fibers, the xerogel fibers were heated at 550 °C for 2.0 h at a rate of 1.0 °C · min-1 from room temperature to 550 °C, then heated at 750 °C for 8.0 h at a heating rate of 3.0 °C · min-1 from 550 to 750 °C in an oxygen atmosphere. As a comparison, the bare fibers were also electrospun by a single spinneret under the same conditions. The morphology and microstructure of the fibers were determined by a JEOL JSM-6700F field-emission scanning electron microscopy (Hitachi S-520), transmission electron microscopy (TEM, model H-800) and high-resolution TEM (HR-TEM, GEOL-2010) with an accelerating voltage of 200 kV. The crystal structure of the resulting products was examined using an X-ray diffraction (XRD, Rigaku, D-max-γA XRD with Cu KR radiation, λ ) 0.151 48 nm). The BET surface area was measured on an SSA-3500 micromeritics using the nitrogen adsorption-desorption method. The chemical compositions of the resulting fibers were analyzed by an inductively coupled plasma atomic emission spectroscopy (PE instrument ICP-OES Optima 2000 DV). Electrochemical properties were investigated by using a laboratory cell assembled in a glovebox filled with argon. The working electrode was prepared by loading the fibers into a bag made up of a Ni net connected with a 0.2 mm Ni wire as down lead and pressed under a pressure of 10.0 MP after being dried at 120 °C for 12 h. The lithium foil pressed onto the stainless

The XRD pattern of hollow LiNi0.8Co0.1Mn0.1O2-MgO coaxial fibers (Figure 1) indicates their hexagonal R-NaFeO2 structure nature (space group: R3jm) and good degree of crystallinity of the composite fibers;8 no obvious differences can be observed between the XRD patterns of hollow LiNi0.8Co0.1Mn0.1O2-MgO coaxial fibers and the bare LiNi0.8Co0.1Mn0.1O2 fibers (not shown in Figure 1). These results shows that, as a stable compound, MgO is just coated on the surface of the LiNi0.8Co0.1Mn0.1O2 material without changing the LiNi0.8Co0.1Mn0.1O2 crystal structure.9 Diffraction peaks derived from MgO were not observed because of the poor crystallization and relatively small amount of MgO. As shown in Figure 2a, the xerogel LiNi0.8Co0.1Mn0.1O2-MgO fibers had a uniform and smooth surface and diameters of 3-4 µm. From the inserted SEM image of the LiNi0.8Co0.1Mn0.1O2-MgO xerogel fibers (the inset in Figure 2a), it could be seen that the as-prepared composite fibers were solid and had circinal cross section without hollow structure. After calcinations, the LiNi0.8Co0.1Mn0.1O2-MgO xerogel fibers transformed into hollow tubes with open ends (Figure 2b, c), which clearly shrank in diameter, as compared to the xerogel fibers, as a result of the removal of organic compounds. Hollow LiNi0.8Co0.1Mn0.1O2-MgO coaxial fibers had an outer diameter of 1-2 µm. The high-magnification SEM images (Figure 2c) indicate that the inner and outer surfaces of the hollow coaxial fibers become coarse, and some pores exist in the wall as a result of the decomposition of the organics during the heattreating process. The LiNi0.8Co0.1Mn0.1O2 and MgO precursor sols are miscible, so it is difficult to distinguish the interface of the composite fibers from the SEM images before and after being calcined. However, when the sintered fibers were treated ultrasonically, the uniform coating was distinctly presented in the SEM images of the typically broken, hollow LiNi0.8Co0.1Mn0.1O2-MgO coaxial fibers (inset in Figure 2c). To further confirm the structure of hollow LiNi0.8Co0.1Mn0.1O2-MgO coaxial fibers, the TEM image is given in Figure 3a. The contrast difference between the center and edge of the fiber image also reveals the hollow structure of the coaxial fibers. The HR-TEM image (Figure 3b) indicates that the image contrast between the core and shell can be clearly seen, and the sharp boundary of the core-shell structure along the fiber’s axis indicates that the LiNi0.8Co0.1Mn0.1O2 fiber is coated by a

20178 J. Phys. Chem. C, Vol. 112, No. 51, 2008

Gu and Jian

Figure 4. SEM pictures of the hollow coaxial fibers obtained at different molar ratios of [PVP]/[M]: (a) 0.3, (b) 0.4, (c) 0.5, and (d) 0.6.

Figure 5. The initial charge-discharge curves (a) and capacity retention versus cycle number plots (b) of three fiber electrodes at a current density of 20.0 mA/g.

Figure 6. SEM picture of the hollow coaxial fibers after 50 cyclings.

uniform MgO layer. Typically, the hollow composite fibers had a shell thickness of 30-60 nm, and wall thickness of 300-500 nm. In addition, the HR-TEM image of the hollow coaxial fiber revealed a high crystallinity of the LiNi0.8Co0.1Mn0.1O2 core and a poor crystallinity of the MgO shell due to higher crystallization temperature of MgO. To investigate the formation of the hollow structure, the LiNi0.8Co0.1Mn0.1O2 sols with different molar ratios of [PVP] to [M] (repeating units, M ) Ni2+ + Co2+ + Mn2+) were electrospun. The morphology and structure of the as-prepared coaxial and bare fibers are summarized in Table 1, and their TEM and SEM pictures were shown in Figure S1 of the Supporting Information. It was found that the MgO shell and PVP-addition played an important role in the formation of the hollow structure in the coaxial fiber. PVP is a water-soluble

polymer and has strong polarity.10 PVP could interact with the citrate by strong hydrogen binding, which influenced the textural properties (i.e., pore size, porosity, and surface tension) of the LiNi0.8Co0.1Mn0.1O2 sol.11 Furthermore, the textural properties of the sol had significant effects on the desiccation and sinter of the xerogel fibers. During the heat-treating process, the MgO shell as a rigid “skin” at the air-fiber interface could have enough strength to resist the collapse due to the surface shrinkage of the composite fiber.11a Therefore, it is easier for the hollow structure to form in the coaxial fibers than in the bare fibers because of the fomation of the rigid MgO shell. After the formation of the rigid MgO “skin”, the liquid within the fibers could escape only to the surface of the xerogel fibers by evaporation. When enough PVP was added, the elimination rate of the liquid within the fiber would be increased greatly as a result of the decrease in the surface tension of the LiNi0.8Co0.1Mn0.1O2 sol by the PVP-addition,11 which would be higher than the maximum rate of the shrinkage of the fiber. Therefore, there was the concentration gradient of the sol particles due to the evaporation of the liquid in the fiber, which drove the sol particles to the surface of the fiber. Thus, a hollow structure would form in the fiber. Along with the decomposition of organic materials, the inner diameter of the hollow tube continually grew until the organic materials completely decomposed. As for the free PVP and containing a small quantity of PVP xerogel fibers, the elimination rate of the liquid through the fiber was lower than the maximum rate of the shrinkage, so no hollow structure could come into being, and thus, solid fibers formed. To further investigate the effect of PVP and MgO shell on the formation of hollow coaxial fibers, a series of experiments were carried out. It could be found that the thickness of the MgO shell had no obvious effect on the inner diameter of the

Hollow LiNi0.8Co0.1Mn0.1O2-MgO Coaxial Fibers hollow coaxial fibers because the MgO shell as a rigid skin simply increased the strength to resist the collapse and shrinkage and could not change the textural properties of the sol. However, the PVP addition led to the hollow structure of the composite fibers, and the channel diameter of the hollow coaxial fibers could be controlled by changing the amount of PVP. The hollow coaxial fibers obtained at different molar ratios of [PVP]/[M] were prepared. Their corresponding SEM images are shown in Figure 4. When the molar ratio of [PVP]/[M] ranged from 0.3 to 0.6, it could be found that the inner diameter of the hollow coaxial fibers gradually increased with the increase in the amount of PVP. When the molar ratio of [PVP]/[M] was above 0.6, the inner diameter of the hollow coaxial fibers had no obvious change with the increasing of the PVP, and furthermore, too much PVP did not make for the formation of uniform hollow coaxial fibers because the LiNi0.8Co0.1Mn0.1O2 sols with too much PVP did not favor the electrospinning of uniform coaxial fibers because of its poor spinnability. Electrochemical properties of hollow core-shell fibers compared to those of solid coaxial fibers and hollow and solid bare fibers (Figure 5a and b) were studied in an electrochemical halfcell with lithium metal as the counter electrode. Figure 5a shows the initial charge-discharge curves of four fiber electrodes at a constant 20 mA/g at voltages of 3.0 to 4.3 V. The hollow LiNi0.8Co0.1Mn0.1O2-MgO core-shell fiber electrode showed an initial discharge capacity of 195 mAh/g, whereas solid LiNi0.8Co0.1Mn0.1O2-MgO coaxial fibers and hollow and solid LiNi0.8Co0.1Mn0.1O2 fibers delivered discharge capacities of 184, 192, and 186 mAh/g, respectively. Therefore, it is seen that the hollow LiNi0.8Co0.1Mn0.1O2-MgO core-shell fiber electrode had a higher discharge capacity than those of solid fibers. The capacity retention vs cycle number plots for the three electrodes are shown in Figure 5b. After 50 cyclings, the discharge capacities of the four fiber electrodes were 174, 159, 141, and 126 mAh/g, which corresponded to 89.2%, 86.4%, 70.0%, and 67.7% of their initial capacities, respectively. It was apparent that the hollow LiNi0.8Co0.1Mn0.1O2-MgO coaxial fiber electrode had superior electrochemical performance, including higher discharge capacity and better cycle stability. The results suggested that the special morphology of the hollow LiNi0.8Co0.1Mn0.1O2-MgO fibers had a remarkable influence on their electrochemical performance. It could be believed that significantly improved electrochemical properties of the hollow LiNi0.8Co0.1Mn0.1O2-MgO core-shell fiber electrode might result from the following two effects, except for the structure-stabilizing effect of the MgO coating.12 One is the relatively high specific surface area of the hollow tubular structure. The surface area measurements gave hollow LiNi0.8Co0.1Mn0.1O2-MgO fibers a BET surface area of 30.45 m2/g, which was much larger than those of the solid LiNi0.8Co0.1Mn0.1O2-MgO core-shell (19.31 m2/g) and bare LiNi0.8Co0.1Mn0.1O2 fibers (17.95 m2/g). It was well-known that the surface area of the electrode dramatically affected the Li+ intercalation rate and capacity.13 The high surface area available from both the interior and exterior of hollow fibers with a core-shell structure could offer more active positions for lithium ion intercalating/deintercalating and reduce the Li+ intercalation rate density per unit area, which lowered the real current density and delayed the capacity loss associated with the concentration polarization to higher current density.14 Another factor concerned faster kinetics related to tubular structure of electrode. The hollow structure of the fibers, which allowed the solid state diffusion and intercalation/deintercalation to occur easily, contributed to higher capacities and better reversibility.15 The

J. Phys. Chem. C, Vol. 112, No. 51, 2008 20179 additional intrawall space not only could provide extra routes for Li+ transport/insertion but also could enhance dimensional susceptibility for the hollow tubule structure to undergo a large volume change incurred during charge-discharge cycles.16 The SEM image (Figure 6) of the hollow LiNi0.8Co0.1Mn0.1O2-MgO core-shell fiber electrode shows that, after 50 discharge-charge cycles, hollow LiNi0.8Co0.1Mn0.1O2-MgO core-shell fibers still retained the hollow coaxial fiber structure. Thus, the primary crystal structure and 1D hollow morphology of hollow LiNi0.8Co0.1Mn0.1O2-MgO core-shell fibers were retained during the cycling process, which was of great significance for holding the capacity of the cathode materials. 4. Conclusions In this paper, hollow LiNi0.8Co0.1Mn0.1O2-MgO core-shell fibers were successfully fabricated for the first time via the sol-gel two-capillary spinneret electrospinning technique. Hollow LiNi0.8Co0.1Mn0.1O2-MgO coaxial fibers had an outer diameter of 1-2 µm with a wall thickness of 300-500 nm and shell thickness of 30-60 nm. Coaxial electrospinning of LiNi0.8Co0.1Mn0.1O2 and MgO sols led to the formation of the core-shell structure, and the PVP addition resulted in the formation of a tubular structure of the core-shell fibers during the calcined process. As a cathode material, the hollow LiNi0.8Co0.1Mn0.1O2-MgO core-shell fibers showed superior electrochemical performance, including a high discharge capacity and excellent cycle stability. Acknowledgment. The authors gratefully acknowledge the Doctoral Fund of Qingdao University of Science and Technology (Grant 0022310). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) McCann, J. T.; Li, D.; Xia, Y. J. Mater. Chem. 2005, 15, 735. (b) Ogihara, H.; Sadakane, M.; Nodasaka, Y.; Ueda, W. Chem. Mater. 2006, 18, 4981. (c) Li, X. H.; Shao, C. L.; Liu, Y. C. Langmuir 2007, 23, 10920. (d) Zhai, T.; Gu, Z.; Dong, Y.; Zhong, H.; Ma, Y.; Fu, H.; Li, Y.; Yao, J. J. Phys. Chem. C 2007, 111, 11604. (e) Li, J.; Dai, H.; Zhong, X.; Zhang, Y.; Cao, X. AdV. Funct. Mater. 2007, 9, 205. (2) (a) Mueller, R.; Dahne, L.; Fery, A. J. Phys. Chem. B 2007, 111, 8547. (b) McCann, J. T.; Li, D.; Xia, Y. J. Mater. Chem. 2005, 15, 735. (c) Ras, R. H. A.; Ruotsalainen, T.; Laurikainen, K.; Linderb, M. B.; Ikkala, O. Chem. Commun. 2007, 1, 3–1366. (d) Tenne, R. Angew. Chem., Int. Ed. 2003, 42, 5124. (3) (a) Shen, G.; Chen, D.; Lee, C. J. J. Phys. Chem. C 2007, 111, 5673. (b) Kim, D.-W.; Hwang, I.-S.; Kwon, S. J.; Kang, H.-Y.; Park, K.S.; Choi, Y.-J.; Park, J.-G. Nano. Lett. 2007, 7, 3041. (c) Jin, M.; Zhang, X.; Emeline, A. V.; Liu, Z. L.; Tryk, D. A.; Murakami, T.; Fujishima, A. Chem. Comm. 2006, 43, 4483. (d) Ota, J.; Srivastava, S. K. J. Phys. Chem. C 2007, 111, 12260. (4) (a) Lu, Y.; McLellan, J.; Xia, Y. Langmuir 2004, 20, 3464. (b) Fleming, M. S.; Mandal, T. K.; Walt, D. R. Chem. Mater. 2001, 13, 2210. (c) Park, W. I.; Yoo, J.; Kim, D.-W.; Yi, G.-C. J. Phys. Chem. B 2006, 110, 1516. (5) Li, X.; Liu, Y.; Fu, L.; Cao, L.; Wei, D.; Wang, Y.; Yu, G. J. Phys. Chem. C 2007, 111, 7661. (b) Wang, Y.; Zeng, H. C.; Lee, J. Y. AdV. Mater. 2006, 18, 645. (c) Lee, K. J.; Oh, J. H.; Kim, Y.; Jang, J. Chem. Mater. 2006, 18, 5002. (d) Guo, Y.-G.; Hu, J.-S.; Liang, H.-P.; Wan, L.-J.; Bai, C.-L. AdV. Funct. Mater. 2005, 15, 196. (6) (a) Katta, P.; Alessandro, M.; Ramsier, R. D.; Chase, G. G. Nano Lett. 2004, 4, 2215. (b) Bognitzki, M.; Czado, W.; Frese, T.; Schaper, A.; Hellwig, M.; Steinhart, M.; Greiner, A.; Wendorff, J. H. AdV. Mater. 2001, 13, 70. (c) Li, D.; Xia, Y. Nano Lett. 2003, 3, 555. (d) Li, D.; Ouyang, G.; McCann, J.; Xia, Y. Nano Lett. 2005, 5, 913. (7) (a) Lee, S. W.; Kim, Y. U.; Choi, S.-S.; Park, T. Y.; Joo, Y. L.; Lee, S. G. Mater. Lett. 2007, 61, 889. (b) Sun, Z.; Zussman, E.; Yarin, A. L.; Wendorff, J. H.; Greiner, A. AdV. Mater. 2003, 15, 1929. (c) Liu, Z.; Sun, D. D.; Guo, P.; Leckie, J. O. Nano Lett. 2007, 7, 1081. (d) Li, D.; Xia, Y. Nano Lett. 2004, 4, 933. (e) Loscertales, I. G.; Barero, A.; Guerrero,

20180 J. Phys. Chem. C, Vol. 112, No. 51, 2008 I.; Cortijo, R.; Marquez, M.; Ganan-Calvo, A. M. Science 2002, 295, 1695. (f) Loscertales, I. G.; Barrero, A.; Marquez, M.; Spretz, R.; Velarde-Ortiz, R.; Larson, G. J. Am. Chem. Soc. 2004, 126, 5376. (8) (a) Sun, Y.-K.; Myung, S.-T.; Kim, M.-H.; Prakash, J.; Amine, K. J. Am. Chem. Soc. 2005, 127, 13411. (b) Sun, Y.-K.; Myung, S.-T.; Park, B.-C.; Amine, K. Chem. Mater. 2006, 18, 5159. (9) Kweon, H.-J.; Park, J. J.; Seo, J. W.; Kim, G.; Jung, B.; Lim, H. S. J. Power Sources 2004, 126, 156. (10) (a) Kozuka, H.; Takenaka, S.; Tokita, H.; Okubayashi, M. J. Eur. Ceram. Soc. 2004, 24, 1585. (b) Lu, C.; Dimov, S. S.; Lipson, R. H. Chem. Mater. 2007, 19, 5018. (11) (a) Gu, Y.; Chen, D.; Jiao, X. J. Sol-Gel. Sci. Technol. 2007, 43, 245. (b) Hench, L. L.; West, J. K. Chem. ReV. 1990, 90, 33. (12) (a) Zhao, H.; Gao, L.; Qiu, W.; Zhang, X. J. Power Sources 2004, 132, 195. (b) Kweon, H.-J.; Kim, S. J.; Park, D. G. J. Power Sources 2000, 88, 255.

Gu and Jian (13) (a) Zhang, S.; Li, W.; Li, C.; Chen, J. J. Phys. Chem. B 2006, 110, 24855. (b) Li, B.; Rong, G.; Xie, Y.; Huang, L.; Feng, C. Inorg. Chem. 2006, 45, 64040. (c) Takayuki, D.; Yasutoshi, I.; Takeshi, A.; Zempachi, O. Chem. Mater. 2005, 17, 1580. (14) (a) Patrissi, C. J.; Martin, C. R. J. Electrochem. Soc. 1999, 146, 3176. (b) Zhou, Y.; Cao, L.; Zhang, F.; He, B.; Li, H. J. Electrochem. Soc. 2003, 150, A1246. (c) Yan, H.; Sokolov, S.; Lytle, J. C.; Stein, A.; Zhang, F.; Smyrl, W. H. J. Electrochem. Soc. 2003, 150, A1102. (15) (a) Wang, Q.; Wen, Z.; Li, J. Inorg. Chem. 2006, 45, 6944. (b) Wang, Y.; Su, F.; Lee, J. Y.; Zhao, X. S. Chem. Mater. 2006, 18, 1347. (16) (a) Wang, Y.; Zeng, H. C.; Lee, J. Y. AdV. Mater. 2006, 18, 645. (b) Wang, Y.; Lee, J. Y.; Zeng, H. C. Chem. Mater. 2005, 17, 3899.

JP808468X