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Design of a Nitrogen-Doped, Carbon-Coated Li [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) A facile solution-based synthesis and characterization of a nitrogen-doped, carbon-coated Li4 Ti5 O12 (NC-LTO) nanocomposite is reported. The mesoporous TiO2 nanoparticles (NP-TiO2) are first prepared by using nanocrystalline cellulose (NCC) as a template and subsequently transform in situ into an NC-LTO nanocomposite with a core-shell structure by using the ionic liquid 1-ethyl-3-methylimdazolium tricyanomethanide as the carbon source. Various state-of-the-art techniques, including field-emission SEM, TEM, scanning transmission electron microscopy, XRD, X-ray photoelectron spectroscopy, and thermogravimetric analysis, were performed to characterize the morphologies, structures, and compositions. Such NC-LTO nanocomposites have a well-defined LTO core and thin uniform carbon shell with a thickness of 1-2 nm. Electrochemical tests reveal that the NC-LTO nanocomposite delivers a reversible capacity of 171.5 mAhg-1 at 0.2 C, and shows remarkable rate capability by maintaining 63% of the capacity at 60 C (vs. 0.2 C), as well as excellent cycling stability with a capacity retention of 95% after 300 cycles at a rate of 10 C. The excellent electrochemical performance is attributed exclusively to the well-defined core-shell nanostructure and high electric conductivity. The nanosized LTO core can significantly shorten the transport lengths of lithium ions and the admirable electric conductivity of the nitrogen-doped carbon shell can act as an "expressway" for electrons and lithium ions to transport them between the anode material core and the electrolytes. Keywords: carbon . electrochemistry . lithium . nanoparticles . nitrogen Introduction With increasing concerns about global warming and climate change caused by the burning of fossil fuels, there is a strong and growing demand for the development of renewable energy sources. Lithium-ion batteries (LIBs) are the most advanced electrochemical energy storage and conversion systems for a wide range of applications, including electric vehicles (EVs), hybrid electric vehicles (HEVs), and smart grids, but their performance still lies behind that of the demands of consumers.[ 1-6] Current LIBs, which use graphite as the anode in a full cell configuration, exhibit poor rate performance owing to their low lithium diffusion coefficient and present serious safety issues because of potential solid electrolyte interphase (SEI) film formation.[3, 7-9] Therefore, developing advanced materials with high rate capabilities and better safety becomes an essential component of the current endeavor for the next generation of LIBs. Compared with graphite, spinel Li4Ti5O12 exhibits a flat and high potential at approximately 1.5 V (vs. Li/Li+), which circumvents the formation of the SEI and suppresses lithium dendrite deposition on the surface of the anode electrode during charge and discharge.[10,11] Typically, Li4Ti5O12 can accommodate three Li+ ions during the intercalation process to form Li7Ti5O12 and results in a theoretical specific capacity of 175 mAhg1. One of its most interesting properties is that lithium insertion occurs with very little expansion of its unit cell, which makes Li4Ti5O12 a uniquely robust anode.[12-16] For most electrodes, notably main group metal based anodes, such as tin, changes in crystallographic volume upon lithium insertion lead to particle isolation and capacity decaying upon cycling; a key failure mechanism in LIBs.[12, 17, 18] Furthermore, Li4Ti5O12 also has inherent high Li+ diffusivity owing to the three-dimensional channels of the spinel structure. However, spinel Li4Ti5O12 suffers from its insulting properties that arise from the empty Ti 3d state, with a band energy of approximately 2 eV, and it greatly constrains the electrochemical performance, especially at high charge-discharge rates, which may induce the high level of polarization.[3, 19-21] To date, great endeavors have been devoted to optimize Li4Ti5O12 from new synthesis routes to surface modification to address this problem, for instance, carbon coating by using the ball-milling method[9, 20] or chemical vapor deposition;[22] the fabrication of hybrids with multiwalled carbon nanotubes [23] or graphene,[24] nitridation,[25] hydrogenation,[10] electrospinning Li4Ti5O12/C nanofibers,[19, 26] constructing nanostructured Li4Ti5O12 with various morphologies,[27-30] and doping of the Li4Ti5O12 with supervalent cations.[19, 31-35] These optimization strategies can significantly enhance electrochemical performance. Nevertheless, it is believed that major advances in preparation and nanostructure design are required to further improve the performance of Li4Ti5O12-based LIB systems. Recently, nanostructured electrode materials have attracted much attention because of their improved kinetic performance by reducing the transport length of lithium ions and electrons.[ 7, 36] Jaiswal et al. synthesized Li4Ti5O12 nanoparticles (NPs) by using a facile solvothermal technique; these Li4Ti5O12 NPs possessed short diffusion paths and a large contact area at the electrode/electrolyte interface.[37] Wang et al. reported an in situ polymerization restriction method for the synthesis of a LiFePO4/carbon nanocomposite, which effectively enhanced its electrochemical performance.[4] Shen and co-workers developed a novel, yet generally applicable, strategy for the fabrication of optimum nanostructured materials by a simple solidstate reaction without the addition of external carbon sources; these materials exhibited superior rate capability and excellent cycling stability.[14] Furthermore, it has been widely reported that the introduction of nitrogen into a carbon structure can be considered as a tool for the tuning of carbon properties and improved electric conductivity.[38] However, the synthesis of highly crystalline nanostructured materials completely coated with nitrogen-doped carbon (or a nitrogen-doped carbon shell) remains a significant challenge, according to previous studies.[9] Ionic liquids (ILs) and nitrogenous substances have the advantage that they are both liquid and, owing to the salt-like structure of the substances, exhibit a negligible vapor pressure.[38] Thus, it is easier to cover the nanomaterials completely and endure a wide temperature range of 400- 1000 8C without drastic solvent evaporation in the pyrolysis process. These are almost perfect precursor properties that enable simple processing and are favorable for the formation of a uniform coating layer on the surface of the materials. Moreover, the thickness and composition of the coating layer can be easily controlled by the quantity of ILs added and by selecting different ILs, respectively. Hence, it is a great advantage to use ILs as a precursor to synthesize nitrogen-doped carbon-coated nanocomposites with a core-shell structure. Herein, we report the synthesis of a nitrogen-doped, carboncoated Li4Ti5O12 nanocomposite with a core-shell structure by using the IL 1-ethyl-3-methylimdazolium tricyanomethanide ([EMIM][TCCN]) as the precursor. For Li4Ti5O12 completely coated with nitrogen-doped carbon composite electrode, the outer nitrogen-doped carbon acts as a shell and the nanosized Li4Ti5O12 particle acts as a core and active material. The nitrogen-doped carbon shell serves as a conductive network through the whole material; this is favorable for lithium ion transport across the interface between the active materials and the electrolyte. Benefiting from a core- shell nanostructure with significantly shortened transport lengths for the lithium ions and admirable electric conductivity, the as-prepared nitrogen-doped, carbon-coated Li4Ti5O12 (NCLTO) nanocomposite shows superior electrochemical performances in terms of specific capacity, cycling performance, and rate performance, if used as an anode material for LIBs. Results and Discussion The synthesis of the NC-LTO nanocomposite is illustrated in Figure 1. The NPTiO2 was formed first by using nanocrystalline cellulose (NCC) as a hard template. Afterwards, through a low-temperature hydrothermal treatment in a solution of LiOH, NPTiO2 was transferred in situ to amorphous LTO, which was then completely mixed with [EMIM][TCCN]. A calcination treatment in argon was used to convert the precursor into the LTO nanocomposite with a nitrogen-doped carbon coating. The morphology and microstructure of NPTiO2 were investigated by using SEM and TEM, as shown in Figure 2. Representative NPs appear to be loose and softagglomerates with a uniform size (Figure 2 a). In the low-magnification TEM image of a representative self-assembled mesoporous TiO2 NP (Figure 2 b), the light regions suggest that pores existed in NP TiO2 and dispersed well. Figure 2 c reveals that the TiO2 NPs are connected with each other and the pores are the interparticle voids. In addition, the size of individual TiO2 NPs is approximately 5 nm in diameter. Figure 3 a shows the XRD pattern of the NC-LTO nanocomposite. The major identified peaks at 2q=18.4, 35.6, 43.3, 47.4, 57.2, 62.8, and 66.18 correspond to the (111), (311), (400), (331), (333), (440), and (531) planes of face-centered cubic spinel Li4Ti5O12 with the Fd3m space group, which can be perfectly indexed to JCPDS card no. 49-0207. No other secondary or parasitic phase peaks, such as TiO2 and/or Li2TiO3, were detected; this indicates the purity of the product. The broad diffraction peaks indicate that the NC-LTO nanocomposite has a small grain size, which can contribute to the enhanced electrochemical performance.[40, 41] Electron microscopy confirms that the materials obtained after heat treatment are homogenous and that, despite the small size of the crystals, the samples appear to be fully crystalline and are composed of spinel nanocrystals interconnected with each other (Figure 3b-f). Compared with the precursor NPTiO2 (Figure 2a), the grain size of the NC-LTO NPs increases with the hydrothermal process, but the character of the precursor framework is preserved, as shown in Figure 3b and c. Between the small crystals, pores were formed and provided a large volume for electrolyte storage to ensure Li+ diffusion in channels across the NC-LTO nanocomposite anode. This provides remarkable rate capability and cycling performance.[42] High-magnification TEM images (Figure 3 c-e) reveal that a thin uniform amorphous carbon shell (approximately 1-2 nm thick) forms from the carbonization of [EMIM]- [TCCN], which coats every crystallite to form a carbon-coated LTO core-shell structure. The thin outer carbon layer with high conductivity could act as an "expressway" for electrons and lithium ions to transport them between the anode material core and the electrolytes. According to the thermogravimetric (TG) result (Figure S1 in the Supporting Information), the percentage of pyrolytic carbon in the NC-LTO nanocomposite was 6.4 wt%. As further characterized in Figure 3 f, the inner nanostructure clearly presents fine lattice fringes. The interplanar spacings of the lattice fringes are measured to be 0.48 nm, which is in good agreement with a d spacing of 0.484 nm associated with the (111) direction of spinel LTO. A typical high-resolution X-ray photoelectron spectrum of N 1 s for the NC-LTO nanocomposite and LTO NPs is shown in Figure 4 a. Peaks at binding energies of 398.3 and 399.5 eV can be attributed to pyridinic nitrogen (N1) and pyrrolic nitrogen (N2), respectively, and the peak at 400.8 eV indicates the presence of quaternary nitrogen (N3).[43-45] Furthermore, an additional peak at 396.7 eV is observed in the N 1s spectrum, which belongs to TiN. According to reference [46], the binding energy of TiN is 397.0 eV, so the slight shiftof the peak may be caused by the formation of a certain amount of Ti-N-C-like compound.[47] The existence of TiN with a metallic conductivity formed on the surface of the active materials is beneficial for electron conduction and contributes significantly to the rate performance, as demonstrated below.[25] The Raman spectrum of the NC-LTO nanocomposite shows two bands at ñ=1350 and 1590 cm1, which correspond to the D and G bands, respectively, of carbon (Figure S2). High-angle annular dark-field STEM elemental mapping in Figure 4b confirms the homogeneous distribution of carbon and nitrogen on/inside the NC-LTO nanocomposite. According to the above analysis of the X-ray photoelectron spectroscopy (XPS) and Raman data, the structure of [EMIM][TCCN] would be transformed into nitrogen- doped carbon after carbonization, as shown in Figure 4 c. Galvanostatic discharge (Li+ insertion)-charge (Li+ extraction) measurements were performed over a voltage range from 1.0 to 2.5 V to evaluate the electrochemical performance of the as-prepared NC-LTO nanocomposite. It is striking that a flat voltage plateau at the potential of approximately 1.55 V is observed (Figure 5a); this corresponds to the two-phase insertion reaction between Li4Ti5O12 and Li7Ti5O12.[48] The sloping voltage curves at the beginning (or end) of the discharge curve indicate a single-phase insertion of Li4+dTi5O12 (or Li7gTi5O12). The increase in the single-phase region was attributed to tailored particle size, as reported previously.[49] Figure 5b shows the discharge capacities against different current rates, each sustained for 10 cycles. A stable cyclic performance was obtained for all rates. As the current rate increased from 1 to 5, 10, and 30 C, the discharge capacity decreased slightly from 163 to 150, 141, and 125 mAhg1, respectively. At a high rate of 60 C, the delivered capacity was still approximately 63% of the value achieved at 0.2 C; this indicates the excellent rate capability of the material. A specific capacity of approximately 157 mAhg1 was recovered when the current rate reduced back to 1 C after 60 cycles at higher rates. Although there is only 6.4 wt% nitrogen-doped carbon in the electrode and no ancillary materials, the rate performance of the electrode is much better than that of hierarchically porous Li4Ti5O12,[50] carbon-coated Li4Ti5O12,[20] or TiN-coated Li4Ti5O12,[25] as well as Li4Ti5O12-carbon nanotube[23] and Li4Ti5O12-graphene composites.[24] For comparison, we also performed electrochemical experiments for LTO NP electrodes without a nitrogen-doped carbon coating. As seen from Figure 5 c, the NC-LTO nanocomposite clearly exhibited a much higher storage capacity and much better rate capability than that of the LTO NPs. At a low rate of 0.2 C, the LTO NPs delivered a discharge capacity of nearly 170 mAhg1. However, as the rate increased, the discharge capacities of the LTO NPs dropped dramatically. Notably, at a rate of 60 C, the capacity of NC-LTO nanocomposite was nearly five times greater than that of LTO NPs. Another excellent property of the NC-LTO nanocomposite electrode is its superior cycling performance with very slight capacity decay (Figure 5 d). After 300 cycles, the discharge capacity of the NC-LTO nanocomposite was 134.6 mAhg1 with a capacity retention of 95.1 %; this demonstrates the prefect structural stability of Li4Ti5O12 and rapid ionic and electronic conduction in the electrode owing to the nitrogen- doped carbon coatings derived from the carbonation of [EMIM][TCCN]. The Coulombic efficiency (calculated from the discharge-charge capacity) remained constant at approximately 100 %; this indicated that the electrochemical Li+ insertion/ extraction process was completely reversible, even at high rates. To understand why NC-LTO nanocomposite electrodes exhibit such superior electrochemical performance to that of LTO NP electrodes, electrochemical impedance spectroscopy (EIS) measurements were performed after 30 cycles. Nyquist plots (Figure S3 in the Supporting Information) showed that both spectra consist of a depressed semicircle in the high-frequency region and an oblique straight line in the low-frequency region. Moreover, the diameter of the semicircle for the NCLTO nanocomposite electrode was much smaller than that of the LTO NP electrode; thus suggesting that NC-LTO nanocomposite electrodes possess lower contact and charge-transfer impedances. The exact kinetic differences between the NC-LTO nanocomposite and LTO NPs were inspected by modeling alternating current (AC) impedance spectra based on the Randles equivalent circuit (summarized in Table S1). From the equivalent circuit, Rs represents the ohmic resistance of the electrolyte and cell components and Rct corresponds to charge-transfer resistance at the interface between the electrode and electrolyte.[51] The values of Rs and Rct were 2.6 and 40.3 W, respectively, in the case of the NC-LTO nanocomposite, which were significantly lower than those of LTO NPs (4.4 and 95.1 W; Table S1). This result confirmed that the nitrogendoped carbon shells not only preserved the high conductivity of the overall electrode, but also largely improved the electrochemical activity of electrode materials during the cycle processes. Conclusion We have developed an efficient fabrication strategy toward a high-performance LIB negative electrode by designing a nitrogen- doped carbon-coated Li4Ti5O12 nanocomposite with a core-shell structure. By taking advantage of the inner nanosized LTO core, which could significantly shorten the transport lengths of lithium ions, and the outer thin conductive nitrogen- doped carbon, which could provide fast electronic and ionic transfer channels, the NC-LTO nanocomposite electrodes demonstrated superior lithium-storage capability with a high capacity and exceptional rate capability. These results indicated their great potential application as promising candidates for the development of high-performance, advanced lithium batteries directed to the HEV, EV, and smart grids markets. In addition to the application of spinel Li4Ti5O12, we anticipate that ILs will provide broad applications in other electrode materials for electrochemical instruments. Experimental Section Synthesis of NPTiO2 using NCC as template All of the reactants and solvents were of analytical grade and used without further purification. The NCC template solution was prepared as reported previously[39] with modifications: NCC (2.0 g) was immersed in distilled water (250 mL) for 24 h at room temperature, then the mixture was exposed to ultrasound treatment for 10 min, and centrifuged for 10 min; the top layer of the suspension was the resulting NCC template solution. To synthesize NPTiO2, the concentration of the NCC template solution was diluted to 0.1%. In a typical procedure, high-purity TiCl4 was slowly added to the 0.1% NCC template solution cooled on an ice/water bath under vigorous stirring. Subsequently, the mixture was stirred for 1 h at a speed of 300 rmin1 and then ammonium sulfate and hydrochloric acid catalyst were slowly dropped into the mixture with a molar ratio of n(TiCl4)/n((NH4)2SO4)/n(H+) of 1:2:10. The reaction was kept for 0.5 h at a temperature of 58C. The solution was then heated to 988C and reacted for 1 h before ammonium hydroxide was added to the solution to give pH 8 and the reaction was kept for another 1 h. Finally, the solution was removed and aged for 12 h under ambient temperature. The white precipitate was separated by filtration, washed several times with deionized water and ethanol to remove the chloride ions, before being dried at 1108C for 10 h. Finally, NPTiO2 was obtained after the white powder was calcinated at 4008C for 2 h in air. Synthesis of NC-LTO nanocomposite with a core-shell structure The NC-LTO nanocomposite was synthesized by a one-pot hydrothermal procedure with NPTiO2 as the precursor combined with heat treatment. Typically, NPTiO2 (0.2 g) was added to a 3m solution of LiOH (50 mL) at ambient temperature. The solution was mixed completely by using magnetic stirrer for about 0.5 h, then the solution was transferred to an 80 mL Teflon-lined stainless-steel autoclave, and placed in an oven at 90 8C for 10 h. The white precipitate was separated by centrifugation and washed several times with deionized water and ethanol to remove excess hydroxide before being dried at 80 8C for 6 h. Subsequently, [EMIM][TCCN] was mixed with the precursor. To form the NC-LTO nanocomposite, the mixture was heated at 600 8C for 2 h in a horizontal tube furnace under an argon atmosphere. For comparison, pure LTO NPs were obtained by the same process without using [EMIM][TCCN] as the N-doped carbon precursor. The microstructural properties of the resultant samples were obtained by SEM (HITACHI S-4800), energy-dispersive X-ray spectroscopy (XPS; Phoenix), and TEM (JEOL JEM-2010). The crystal structure was characterized by XRD (Bruker D8 advance) with CuKa radiation. TG analyses were performed on a TG instrument (NETZSCH STA 409 PC) at a heating rate of 5 8Cmin1 in air from 30 to 7008C. XPS analysis was performed on a PerkinElmer PHI 550 spectrometer with AlKa (1486.6 eV) as the X-ray source. Raman spectra were collected by using a Renishaw 2000 system with an argon ion laser (514.5 nm) and a charge-coupled device detector. Electrochemical evaluations were performed by galvanostatic cycling in a CR2016-type coin cell. The working electrodes were formed by mixing 80 wt% active materials, 10 wt% carbon black, and 10 wt% polyvinylidene fluoride dissolved in N-methylpyrrolidinone, and pasting the mixture on a copper-foil current collector. Afterwards, the electrode was dried under vacuum at 1108C for 12 h. The cells were assembled with the as-prepared cathode, lithium metal as the anode, and polypropylene film as the separator. The electrolytes were a 1 molL1 solution of LiPF6 in a 1:1 (v/v) mixture of ethylene carbonate and dimethyl carbonate. All of the assembly processes for the test cells were performed in an argonfilled glove box. Galvanostatic charge-discharge experiments were performed at different current densities between 1.0 and 2.5 V (vs. Li/Li+) by using a CT2001A cell test instrument (LAND Electronic Co.) Acknowledgements This study was supported by funding from the "973" project (no. 2014CB239701), the National Natural Science Foundations of China (no. 21173120), the Natural Science Foundations of Jiangsu Province (no. BK2011030), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (no. 20060287026). L.S. thanks the Jiangsu Innovation Program for Graduate Education (no. CXZZ11_0204) and the Outstanding Doctoral Dissertation in NUAA (no. BCXJ11-10). [1] J. M. Tarascon, M. Armand, Nature 2001, 414, 359- 367. [2] X. L. Wu, L. Y. Jiang, F. F. Cao, Y. G. Guo, L. J. Wan, Adv. Mater. 2009, 21, 2710 -2714. [3] H. G. Jung, S. T. Myung, C. S. Yoon, S. B. Son, K. H. Oh, K. Amine, B. Scrosati, Y. K. Sun, Energy Environ. Sci. 2011, 4, 1345 -1351. [4] Y. G. Wang, Y. R. Wang, E. Hosono, K. X. Wang, H. S. Zhou, Angew. Chem. 2008, 120, 7571 - 7575; Angew. Chem. Int. Ed. 2008, 47, 7461 -7465. [5] D. Lepage, C. Michot, G. X. Liang, M. Gauthier, S. B. Schougaard, Angew. Chem. 2011, 123, 7016 -7019. [6] G. X. Wang, H. Liu, J. Liu, S. Z. Qiao, G. Q. M. Lu, P. Munroe, H. Ahn, Adv. Mater. 2010, 22, 4944 - 4948. [7] L. F. Shen, X. G. Zhang, E. Uchaker, C. Z. Yuan, G. Z. Cao, Adv. Energy Mater. 2012, 2, 691- 698. [8] S. B. Yang, X. L. Feng, K. Mllen, Adv. Mater. 2011, 23, 3575 -3579. [9] H. S. Li, L. F. Shen, X. G. Zhang, J. Wang, P. Nie, Q. Che, B. Ding, J. Power Sources 2013, 221, 122- 127. [10] L. F. Shen, E. Uchaker, X. G. Zhang, G. Z. Cao, Adv. Mater. 2012, 24, 6502 -6506. [11] Y. Q. Wang, L. Gu, Y. G. Guo, H. Li, X. Q. He, S. Tsukimoto, Y. Ikuhara, L. J. Wan, J. Am. Chem. Soc. 2012, 134, 7874 -7879. [12] E. M. Sorensen, S. J. Barry, H. K. Jung, J. R. Rondinelli, J. T. Vaughey, K. R. Poeppelmeier, Chem. Mater. 2006, 18, 482- 489. [13] S. G. Ri, L. Zhan, Y. Wang, L. H. Zhou, J. Hu, H. L. Liu, Electrochim. Acta, 2013, 109, 389 -394. [14] L. F. Shen, H. S. Li, E. Uchaker, X. G. Zhang, G. Z. Cao, Nano Lett. 2012, 12, 5673 -5678. [15] L. Yu, H. B. Wu, X. W. (David) Lou, Adv. Mater. 2013, 25, 2296 -2300. [16] M. M. Thackeray, J. Electrochem. Soc. 1995, 142, 2558 -2563. [17] S. M. Paek, E. Yoo, I. Honma, Nano Lett. 2009, 9, 72-75. [18] J. S. Chen, Y. L. Cheah, Y. T. Chen, N. Jayaprakash, S. Madhavi, Y. H. Yang, X. W. Lou, J. Phys. Chem. C 2009, 113, 20504 -20508. [19] H. S. Li, L. F. Shen, X. G. Zhang, P. Nie, L. Chen, K. Xu, J. Electrochem. Soc. 2012, 159, A426 -A430. [20] G. N. Zhu, H. J. Liu, J. H. Zhuang, C. X. Wang, Y. G. Wang, Y. Y. Xia, Energy Environ. Sci. 2011, 4, 4016 -4022. [21] T. Brousse, P. Fragnaud, R. Marchand, D. M. Schleich, O. Bohnke, K. West, J. Power Sources 1997, 68, 412. [22] L. Cheng, J. Yan, G. N. Zhu, J. Y. Luo, C. X. Wang, Y. Y. Xia, J. Mater. Chem. 2010, 20, 595- 602. [23] L. F. Shen, C. Z. Yuan, H. J. Luo, X. G. Zhang, K. Xu, F. Zhang, J. Mater. Chem. 2011, 21, 761 -767. [24] L. F. Shen, C. Z. Yuan, H. J. Luo, X. G. Zhang, S. D. Yang, X. J. Lu, Nanoscale 2011, 3, 572- 574. [25] K. S. Park, A. Benayad, D. J. Kang, S. G. Doo, J. Am. Chem. Soc. 2008, 130, 14930 -14931. [26] N. Zhua, W. Liu, M. Q. Xue, Z. Xie, D. Zhao, M. N. Zhang, J. T. Chen, T. B. Cao, Electrochim. Acta 2010, 55, 5813 -5818. [27] Y. Qiao, X. L. Hu, Y. Liu, Y. H. Huang, Electrochim. Acta 2012, 63, 118- 123. [28] Y. F. Tang, L. Yang, Z. Qiu, J. S. Huang, J. Mater. Chem. 2009, 19, 5980 - 5984. [29] B. Zhang, Y. S. Liu, Z. D. Huang, S. Oh, Y. Yu, Y. W. Mai, J. K. Kim, J. Mater. Chem. 2012, 22, 12133 - 12140. [30] J. Z. Chen, L. Yang, S. H. Fang, S. I. Hirano, K. Tachibana, J. Power Sources 2012, 200, 59 -66. [31] Y. J. Bai, C. Gong, N. Lun, Y. X. Qi, J. Mater. Chem. A 2013, 1, 89-96. [32] Z. H. Wang, G. Chen, J. Xu, Z. S. Lv, W. Q. Yang, J. Phys. Chem. Solids 2011, 72, 773- 778. [33] C. Y. Lin, Y. R. Jhan, J. G. Duh, J. Alloys Compd. 2011, 509, 6965- 6968. [34] T. F. Yi, J. Shu, Y. R. Zhu, X. D. Zhu, R. S. Zhu, A. N. Zhou, J. Power Sources 2010, 195, 285 -288. [35] B. Zhang, H. D. Du, B. H. Li, F. Y. Kang, Electrochem. Solid-State Lett. 2010, 13, A36-A38. [36] H. Liu, G. X. Wang, J. Liu, S. Z. Qiao, H. Ahn, J. Mater. Chem. 2011, 21, 3046 -3052. [37] A. Jaiswal, C. R. Horne, O. Chang, W. Zhang, W. Kong, E. Wang, T. Chern, M. M. Doeff, J. Electrochem. Soc. 2009, 156, A1041-A1046 [38] J. P. Paraknowitsch, A. Thomas, M. Antonietti, J. Mater. Chem. 2010, 20, 6746 -6758. [39] K. E. Shopsowitz, W. Y. Hamad, M. J. MacLachlan, Angew. Chem. 2011, 123, 11183 -11187; Angew. Chem. Int. Ed. 2011, 50, 10991 -10995. [40] J. M. Feckl, K. Fominykh, M. Doblinger, D. Fattakhova-Rohlfing, T. Bein, Angew. Chem. 2012, 124, 7577 - 7581; Angew. Chem. Int. Ed. 2012, 51, 7459 -7463. [41] J. Lim, E. Choi, V. Mathew, D. Kim, D. Ahn, J. Gim, S. H. Kang, J. Kim, J. Electrochem. Soc. 2011, 158, A275 -A280. [42] N Li, G. M. Zhou, F. Li, L. Wen, H-M. Cheng, Adv. Funct. Mater. 2013, DOI: 10.1002/adfm.201300495 [43] H. S. Li, L. F. Shen, K. B. Yin, J. Ji, J. Wang, X. Y. Wang, X. G. Zhang, J. Mater. Chem. A 2013, 1, 7270 -7276. [44] T. C. Nagaiah, A. Bordoloi, M. D. Snchez, M. Muhler, W. Schuhmann, ChemSusChem 2012, 5, 637 -641. [45] L. Qie, W. M. Chen, Z. H. Wang, Q. G. Shao, X. Li, L. X. Yuan, X. L. Hu, W. X. Zhang, Y. H. Huang, Adv. Mater. 2012, 24, 2047 - 2050. [46] N. C. Saha, H. G. Tompkins, J. Appl. Phys. 1992, 72, 3072 -3079. [47] L. Zhao, Y. S. Hu, H. Li, Z. X. Wang, L. Q. Chen, Adv. Mater. 2011, 23, 1385 -1388. [48] H. S. Zhou, D. L. Li, M. Hibino, I. Honma, Angew. Chem. 2005, 117, 807 - 812; Angew. Chem. Int. Ed. 2005, 44, 797 -802. [49] Y. Wang, H. Liu, K. Wang, H. Eiji, Y. Wang, H. Z. Wang, J. Mater. Chem. 2009, 19, 6789 -6795. [50] L. F. Shen, C. Z. Yuan, H. J. Luo, X. G. Zhang, K. Xu, Y. Y. Xia, J. Mater. Chem. 2010, 20, 6998 -7004. [51] S. B. Yang, X. L. Feng, S. Ivanovici, K. Mllen, Angew. Chem. 2010, 122, 8586 -8589. Received: September 12, 2013 Published online on November 29, 2013 Hongsen Li, Laifa Shen, Jie Wang, Bing Ding, Ping Nie, Guiyin Xu, Xiaoyan Wang, and Xiaogang Zhang*[a] [a] Dr. H. Li, Dr. L. Shen, J. Wang, Dr. B. Ding, Dr. P. Nie, G. Xu, X. Wang, Prof. X. Zhang College of Materials Science and Engineering Nanjing University of Aeronautics and Astronautics Nanjing, 210016 (P. R. China) Fax: (+86) 025-52112626 E-mail: [email protected] * Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201300316. (c) 2014 Blackwell Publishing Ltd. |