DOI: 10.1002/chem.201304713
Full Paper
& Lithium-Ion Batteries
Iron Fluoride Hollow Porous Microspheres: Facile Solution-Phase Synthesis and Their Application for Li-Ion Battery Cathodes Jun Liu,*[a, b] Wei Liu,[a, b] Shaomin Ji,*[a, b] Yanling Wan,[a, b] Mingzhe Gu,[a, b] Huaqi Yin,[a, b] and Yichun Zhou[a, b]
Abstract: Iron fluoride cathodes have been attracting considerable interest due to their high electromotive force value of 2.7 V and their high theoretical capacity of 237 mA h g1 (1 e transfer). In this study, uniform iron fluoride hollow porous microspheres have been synthesized for the first time by using a facile and scalable solution-phase route.
Introduction In recent decades, lithium-ion batteries (LIBs) have successfully captured the portable electronic market. However, to satisfy the fast-growing demand for lightweight and high-capacity electrical storage, such as in electric vehicles and power storage from renewable energy resources, great improvements in safe, low-cost and high-energy-density LIBs are urgently needed.[1–5] Iron trifluoride, first reported by Arai and co-workers as an electrode material,[6] has been regarded as one of the possible candidates due to its high electromotive force value of 2.7 V, high theoretical capacity of 237 mA h g1 (1 e transfer) and 712 mA h g1 (3 e transfer), abundant sources, and better safety.[7, 12] Unfortunately, its implementation in LIBs is seriously limited by at least two issues: 1) the iron fluoride bonds in FeF3 are strongly polarized because of the high electronegativity of fluorine, which results the slow diffusion of Li ions and poor electron conductivity; and 2) the drastic volume variation during the Li + intercalation and deintercalation processes, which decreases the stability of the cycle.[6–11] Thus, the electrode performance of bulk FeF3 is extremely unsatisfactory, although FeF3 has open structures with three-dimensional pathways for lithium insertion. The current strategy to solve these difficulties is to mix FeF3 with conductive materials, such as carbon nanotubes, graphite, and conductive agents.[8, 11–12] For
[a] Dr. J. Liu, W. Liu, Dr. S. Ji, Y. Wan, M. Gu, H. Yin, Prof. Dr. Y. Zhou Key Laboratory of Low Dimensional Materials & Application Technology Ministry of Education, Xiangtan University Xiangtan 411105 (P.R. China) E-mail: [email protected] [email protected] [b] Dr. J. Liu, W. Liu, Dr. S. Ji, Y. Wan, M. Gu, H. Yin, Prof. Dr. Y. Zhou Faculty of Materials, Optoelectronics and Physics Xiangtan University, Xiangtan 411105 (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304713. Chem. Eur. J. 2014, 20, 5815 – 5820
These uniform porous and hollow microspheres show a high specific capacity of 210 mA h g1 at 0.1 C, and excellent rate capability (100 mA h g1 at 1 C) between 1.7 and 4.5 V versus Li/Li + . When in the range of 1.3 to 4.5 V, stable capacity was achieved at 350 mA h g1 at a current of 50 mA g1.
example, Kang et al.[8] fabricated carbon nanotube–FeF3 nanocomposites, which present a high rate capability (150 mA h g1 at a current rate of 500 mA g1) due to the enhanced electronic and ionic transport in the electrode. Recently, we have developed the first synthesis of trifluoride cathode nanoparticles on highly conducting graphene sheets with desired size and morphology, which showed superior performance with a stable specific capacity of about 210 mA h g1 at a rate of 0.2 C and high power capability that delivered about 114 mA h g1 even at a rate of 5 C.[12] The above successes can be attributed to the carbon material, which not only increases the electrical conductivity of particle–particle contacts to improved the rate performance but also provides a physical buffering layer for the volume change to enhance the cycling performance. As we know, the performance of cathode/anode materials, such as specific capacity, cycling stability, and rate capability, is also affected by the crystallinity, particle size, and morphology. Thus, researchers have tried to enhance the function of iron fluoride by controlling the structure of this material. Maier and co-workers[13] reported a nanometer-sized FeF3·0.33H2O cathode prepared by low-temperature ionic-liquid-based synthesis route. The nanometer-sized FeF3·0.33H2O delivered about 160 and 126 mA h g1 at current densities of 14 and 71 mA g1, respectively (1.6–4.5 V). Sun’s group synthesized porous FeF3 nanospheres by solvent exchange that delivered a high rate capacity of 180 mA h g1 at 50 mA g1 and 127 mA h g1 at 500 mA g1.[14] Hollow micro-/nano-hybrids with porous structures have attracted fast-growing attention with the aim of building LIBs with high energy density and long cycle life.[15–18] According Lou et al.,[15] this architecture can increase the surface area, lessen the compounded internal stresses, and prevent nanoparticles from agglomerating. Numerous efforts have been devoted to the synthesis of various hollow structures with great progress achieved.[16–20] For example, Wan et al.[17] designed Sn nanoparticles encapsulated in elastic hollow carbon nano-
5815
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper spheres (TNHCs) based on SiO2 colloid templates. Our group also used a soft-template route to synthesize uniform hollow carbon microspheres embedded with transition-metal oxide nanocrystals.[20] However, template methods for constructing these hollow structures are usually time consuming and costly because of the need for the fabrication of templates and the multi-step synthesis process. Moreover, templates are sometimes not easy to remove. Therefore, researchers have tried to develop facile, scalable template-free approaches for the rational synthesis of hollow structures.[21–24] Lou’s group reported various VO2 hollow microspheres, including yolk-shelled, multi-shelled, and single-shelled structures, through Ostwald ripening method.[23] Yan’s group fabricated a series of novel hollow structures, such as strain- Figure 1. Schematic of the fabrication process of hollow-structured FeF3·0.33 H2O microspheres based on Ostwald ripening and solid transformation processes: a) full view; b) cross-sectional view. released hybrid multilayer Ge/Ti nanomembranes[25] and naturally rolled-up C/Si/C trilayer nanomembranes,[26] by using a electron-beam deposition method. a porous structure under the inside-out Ostwald ripening. A seHowever, the uniform hollow and porous structured FeF3 requentially extended solvothermal reaction, the outward Ostported is rather limited due to the decomposition of FeF3 into wald ripening makes the porous solid particles transform into Fe2O3 and FeF2 during the high-temperature sintering progress, hollow Fe1.9F4.75·0.95 H2O microspheres. The formation of the even under a protective atmosphere, and the high solubility of hollow structure also accompanies further growth of the nanoFeF3·3 H2O (5.92 g/100 g at 25 8C).[27–29] Herein, we present crystallines and the increasing diameter of the particles. We a facile solution-phase approach for synthesis of uniform first examined the morphology of the as-prepared hollow porous iron fluoride microspheres. The formation FeF3·0.33 H2O products and their precursors by using scanning mechanism of these porous hollow microspheres is a combinaelectron microscopy (SEM). The SEM images in Figure 2a and tion of the outward Ostwald ripening and phase-transformab show that the solid Fe1.9F4.75·0.95 H2O precursors have quite good homogeneity, rough surfaces, and an average diameter tion processes. The resultant FeF3·3 H2O porous hollow structures exhibit a discharge capacity of about 210 mA h g1, with of about 0.8 mm. The porous particles in Figure 2c and d have excellent cycling stability and superior rate capability. a loose and porous outside layer, but the inside layer is tight. This result reveals that the progress of Ostwald ripening is from inside to outside. In the prolonged solvothermal reaction, Results and Discussion hollow microspheres were also obtained by the outward Ostwald ripening process.[15, 19] In Figure 2e and f, we can see that A possible synthesis mechanism for hydrated iron-based fluorides is shown in Figure 1 and the detailed description is prethese hollow microspheres are mainly comprised of approxisented in the Experimental Section. First, a mixed solution of mately 10 to 20 nm particles, which grew slightly compared Fe(NO3)3·9 H2O and HF generated an FeF63 complex,[29] and with the precursors (Figure 2a and b). We presume that the change in size might be attributed to Ostwald ripening and the color changed from bright orange (Fe3 + dissolved in ethathe growth of larger crystals from those of smaller size, which nol) to colorless (FeF63) upon mixing the Fe3 + /ethanol and have a higher solubility than the larger ones.[30] the HF/ethanol solutions. After being sealed and heated, the 3 FeF6 complex transformed to metastable Fe1.9F4.75·0.95 H2O Hydration water-induced crystallization is also presented in nanocrystals, and then aggregated to form the X-ray diffraction (XRD) profiles of hydrated iron-based fluoride Fe1.9F4.75·0.95 H2O solid microspheres. When the solution was hollow porous microspheres (Figure 3) and solid microspheres (Figure S1 in the Supporting Information). Well-defined XRD heated at 200 8C for 30 min, these solid spheres transformed to Chem. Eur. J. 2014, 20, 5815 – 5820
www.chemeurj.org
5816
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper no. 28-0483, a = 10.35 ). Notably, after vacuum treatment the grain corresponds to orthorhombic FeF3·0.33 H2O (JCPDS card no. 76-1262, a = 7.423, b = 12.73, c = 7.526 ) with less hydration water (Figure 3b). The grain size is little smaller than that of Fe1.9F4.75·0.95 H2O. The widening of the XRD peaks is mainly associated with the removal of hydration water under vacuum treatment. According research by Maier et al.,[13] Fe1.9F4.75·0.95 H2O is composed of characteristic structures of both FeF2 and FeF3, and the FeF2 crystals are mainly close to the surface of the particles. When heated at 170 8C, the FeF2 particles were transformed into FeF3, probably due to a little remaining oxygen. The morphology and microstructure details of these FeF3·0.33 H2O porous and hollow microspheres were further characterized by transmission electron microscopy (TEM), as shown in Figure 4. Figure 4a shows a typical low-magnification
Figure 2. SEM images of iron fluoride microstructure: a, b) Fe1.9F4.75·0.95 H2O solid microspheres; c, d) porous-structured Fe1.9F4.75·0.95 H2O microspheres; e, f) ripened Fe1.9F4.75·0.95 H2O hollow microspheres; f, g) FeF3·0.33 H2O hollow microspheres obtained by heating at 170 8C.
Figure 4. TEM characterizations of the FeF3·0.33 H2O hollow porous microspheres cathode: a, b) TEM images of FeF3·0.33 H2O hollow porous microspheres at different magnifications; c) HRTEM image of the lattice planes of FeF3·0.33 H2O; d) SAED pattern of a single nanoparticle, which indicates its single-crystalline character.
Figure 3. XRD patterns of a) Fe1.9F4.75·0.95 H2O hollow porous microsphere precursors and b) FeF3·0.33 H2O hollow porous microsphere products obtained by heating at 170 8C.
peaks (Figure 3a) were assigned to the product without vacuum treatment, that is, cubic Fe1.9F4.75·0.95 H2O (JCPDS card Chem. Eur. J. 2014, 20, 5815 – 5820
www.chemeurj.org
image of FeF3·0.33 H2O hollow microspheres, which indicates that the product is composed of hollow microspheres with uniform size. It can be observed that the contrast between the central portion and the edge of microspheres strongly supports the formation of hollow structures. A higher magnification TEM image (Figure 4b) reveals that the porous shell is a hierarchical nanostructure composed of dozens of plate-like FeF3·0.33 H2O nanoparticles with a size of several tens of nanometers. As shown in Figure 4c, a lattice plane with an interplanar distance of 0.63 nm was observed in the lattice fringe, which corresponds to the (020) plane of the FeF3·0.33 H2O monoclinic crystalline structure. The single-crystalline porous
5817
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper nanostructures can be confirmed by the selected-area electron diffraction (SAED) pattern of a single nanoparticles subunit (Figure 4d). Nitrogen sorption isotherms were provided to investigate the porous structure and the Brunauer–Emmett– Teller (BET) surface areas of these iron fluoride hollow microspheres, and the data are shown in Figure 5. According to the
Figure 5. The N2 adsorption–desorption isotherms and corresponding pore size distribution (inset) of mesoporous FeF3·0.33 H2O hollow microspheres, calculated by using the BJH method.
corresponding Barrett–Joyner– Halenda (BJH) plots (Figure 5, inset) recorded from the nitrogen isotherms of the hollow porous FeF3·0.33 H2O samples, the average pore size is about 5 nm, which further confirms that the sample has porous material characteristics. The BET specific surface areas and pore volumes of the samples were 22.7 m2 g1 and 0.118 cm3 g1, respectively. The inner void and porous shell have two main advantages: one is that the dense pores in shell allow electrolyte to penetrate and make close contact with the inner–outer surface, which results in a higher surface area and a shorter transport length for Li ions; the other is the hollow core, which can serve as a good cushion for the material volume changes during Li ion insertion/extraction and enhance cycling performance.[15] To explore the efficiency of our approach, both porous hollow microspheres and solid Chem. Eur. J. 2014, 20, 5815 – 5820
microspheres were tested as cathodes in a CR2025 coin-type cell. Initially, as shown in Figure S2 in the Supporting Information, the galvanostatic charge–discharge process for FeF3·0.33 H2O was performed in a voltage range of 1.5 to 4.5 V with a current density of 50 mA g1. The first discharge capacity achieved 402 mA h g1, which approximately corresponds to the insertion of 1.7 Li per formula, and the discharge profile can be divided into two parts. The initial sloped area between 4.5 and 1.7 V stems from Li + insertion into FeF3·0.33 H2O through a two-phase reaction to form Li0.5FeF3 and then through a single-phase reaction to form LiFeF3 (FeF3 + Li + + e$LiFeF3). This process appears to be fully reversible, that is, the structure of pristine FeF3 can be maintained during the Li ion insertion process. A reaction plateau appeared at 1.5 V, which is due to the decomposition of formed LiFeF3 to LiF and Fe metal through a conversion reaction (LiFeF3 + 2 Li + + 2e$Fe + 3 LiF).[11–13] This process was also identified by a cyclic voltammogram test at 1.5 to 4.5 V in Figure S3 in the Supporting Information. Therefore, we first chose the potential window to be from 1.7 to 4.5 V to ensure that only the insertion reaction took place. Figure 6a shows the charge–discharge voltage profiles of FeF3·0.33 H2O solid microspheres and porous hollow microspheres at 1.7 to 4.5 V. The hollow porous materials have a high initial discharge capacity 210 mA h g1at 0.1 C, whereas solid microspheres give a value of just 190 mA h g1. Hollow porous materials also exhibit good rate capability of 210, 165, 124, 100, 78, and 57 mA h g1 at 0.1, 0.2, 0.5, 1, 2, and 5 C, respectively (see Figure 6c). However, solid FeF3·0.33 H2O
Figure 6. Electrochemical performance of the FeF3·0.33 H2O cathode measured in the voltage range of 1.7–4.5 V: a) voltage–capacity curves of FeF3·0.33 H2O solid and hollow microspheres at 0.1 C in the first discharge/charge cycle; b) cycling performance of FeF3·0.33 H2O hollow microspheres at different rates (increasing from 0.1 to 5 C); c) discharge–charge curves of FeF3·0.33 H2O hollow microspheres at different rates (increasing from 0.1 to 5 C); d) cycling performance of FeF3·0.33 H2O hollow microspheres at a current density of 1 C.
www.chemeurj.org
5818
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper
Figure 7. a) Voltage–capacity curves for FeF3·0.33 H2O hollow and porous microspheres cycled at 1.3–4.5 V at a current density of 50 mA g1; b) variation in specific capacities vs. cycle number for FeF3·0.33 H2O hollow and solid microspheres at 1.7–4.5 V.
space for the storage of lithium ions, which is beneficial for enhancing the specific capacity of the battery. Thus, the hollow porous FeF3·0.33 H2O microspheres present higher initial capacity (210 mA h g1 at 0.1 C) compared with solid microspheres (180 mA h g1) in Figure 7b. Second, the better rate capabilities of the hollow porous FeF3·0.33 H2O microspheres are ascribed to their hollow nature, which provides a large surface area and porous structure and affording a shortened diffusion path length. Third, the void space in hollow structures can accommodate the volume change during lithium insertion–deinsertion, which improves the cycling performance. To make a further investigation, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to clarify the energy storage redox mechanism of FeF3·0.33 H2O (Figure 8). As shown in Figure 8a, the potential difference between the cathodic and anodic peaks of FeF3·0.33 H2O hollow spheres is only about 0.3 V (cathodic peak: 2.9 V; anodic peak: 3.2 V). However, solid FeF3·0.33 H2O has a distinct anodic peak at 4.5 V (Figure S4 in the Supporting Information), which results a short charge plateau in the high voltage section. As we know, this huge potential hysteresis, caused by polarization, will also induce a large energy loss during battery operation. The hollow inner cores and porous shells designed herein can effectively decrease the polarization by providing a larger contact surface between the electrolyte and the cathode material and shortening the transport length for Li ions and electrons. Figure 8b shows the EIS curve, which was used to study the dynamics for lithium insertion and extraction. Generally, the high-frequency semicircle results from the charge-transfer process and the straight line is attributed to the diffusion of lithium ions in the particles.[31–33] According to the Nyquist plots, the EIS is fitted by using ZSimpWin analysis software.[33] Before
samples show corresponding capacities of 180 (0.1 C), 114 (0.2 C), 82 (0.5 C), 65 (1 C), 52 (2 C), and 37 mA h g1 (5 C) in Figure 7b, and there is an obvious capacity decrease at low current density regions. This might be due to the potential and the drastic volume variation during the Li uptake-and-release process.[13] Compared with the solid particles, the performance of these porous hollow microspheres in both cycling capability and rate capacity has been greatly enhanced as shown in Figure 7b. At low rates, the hollow porous cathode can still deliver a reversible capacity of 180 mA h g1 after eight cycles and there is excellent capacity retention after 45 cycles at a current rate of 1 C (Figure 6d). Moreover, when worked at 1.3 to 4.5 V, the hollow architectured FeF3·0.33 H2O cathode can deliver a high capacity of 412 mA h g1 and stabilized discharge capacities stay at about 350 mA h g1 (Figure 7a). The improvement in performance for microspheres cathode: a) the CV curve of the porous hollow architecture Figure 8. Electrochemical performance of the FeF3·0.33 H2O hollow FeF3·0.33 H2O hollow microspheres at a scan rate of 0.1 mV s1; b) the EIS data at open circuit potential with an AC can also be explained in terms voltage of 5 mV amplitude with a frequency range of 0.1 Hz to 82.5 KHz; c) the equivalent circuit for fitting the imof three factors. First, the porous pedance data with the resistance of the electrolytes (Rs), in which Rct is the resistance for Li + migration through shell structure may provide extra the surface film; Ci is the surface film capacitance; and Zw is the Warburg impedance. Chem. Eur. J. 2014, 20, 5815 – 5820
www.chemeurj.org
5819
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper the cycles, the hollow sample shows lower charge-transfer resistance (56.2 W, Figure 8b) than the solid sample (119.6 W, Figure S5 in the Supporting Information) because the hollow structure can provide more reaction sites. After 40 cycles at different current densities, the charge-transfer resistance of the solid sample electrode increased from 119.6 to 266 W (Figure S5 in the Supporting Information). For the hollow cathode, the charge-transfer resistance increases to only 66.1 W (Figure 8b), which is still far lower than the solid sample. This indicates that hollow porous microspheres could effectively enhance the ion diffusion ability, which makes the charge-transfer reaction easier.
Conclusion We have developed the first synthesis of hollow porous FeF3·0.33 H2O via Ostwald ripening. These porous hollow microspheres as cathode materials exhibit good rate capability and cycling performance, which could be attributed to their porous and hollow structure, which facilitates electrolyte transport, shortens electronic and ionic pathways within the electrode, and provides a larger surface area. These encouraging results may open up new opportunities to promote long-term endeavors in developing high-capacity cathodes for rechargeable Li-ion batteries.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (11202177, 51202207). Keywords: cathode · electrochemistry · hollow spheres · nanostructures · solid-state structures [1] A. S. Aric, P. Bruce, B. Scrosati, J. M. Tarascon, W. V. Schalkwijk, Nat. Mater. 2005, 4, 366 – 377. [2] O. K. Park, Y. Cho, S. Lee, H. C. Yoo, H. K. Song, J. Cho, Energy Environ. Sci. 2011, 4, 1621 – 1633. [3] Y. Yu, C. Yan, L. Gu, X. Lang, K. Tang, L. Zhang, Y. Hou, Z. Wang, M. Chen, O. G. Schmidt, Adv. Energy Mater. 2013, 3, 281 – 285. [4] Y. Yu, L. Gu, X. Lang, C. Zhu, T. Fujita, M. Chen, J. Maier, Adv. Mater. 2011, 23, 2443 – 2447. [5] Y. K. Sun, Z. Chen, H. J. Noh, D. J. Lee, H. G. Jung, Y. Ren, S. Wang, C. S. Yoon, S. T. Myung, K. Amine, Nat. Mater. 2012, 11, 942 – 947. [6] H. Arai, S. Okada, Y. Sakurai, J. Yamaki, J. Power Sources 1997, 68, 716 – 719.
Chem. Eur. J. 2014, 20, 5815 – 5820
www.chemeurj.org
[7] F. Badway, N. Pereira, F. Cosandey, G. G. Amatucci, J. Electrochem. Soc. 2003, 150, A1209. [8] S. W. Kim, D. H. Seo, H. Gwon, J. Kim, K. Kang, Adv. Mater. 2010, 22, 5260 – 5264. [9] T. Li, L. Li, Y. L. Cao, X. P. Ai, H. X. Yang, Phys. Chem. C. 2010, 114, 3190 – 3195. [10] J. Cabana, L. Monconduit, D. Larcher, M. R. Palacin, Adv. Mater. 2010, 22, E170 – E192. [11] D. L. Ma, Z. Y. Cao, H. G. Wang, X. L. Huang, L. M. Wang, X. B. Zhang, Energy Environ. Sci. 2012, 5, 8538 – 8542. [12] J. Liu, Y. L. Wan, W. Liu, Z. S. Ma, S. M. Ji, J. B. Wang, Y. C. Zhou, P. Hodgson, Y. C. Li, J. Mater. Chem. A 2013, 1, 1969 – 1975. [13] C. Li, L. Gu, S. Tsukimoto, P. A. Van Aken, J. Maier, Adv. Mater. 2010, 22, 3650 – 3654. [14] Q. X. Chu, Z. C. Xing, J. Q. Tian, X. B. Ren, A. M. Asiri, A. O. Al-Youbi, K. A. Alamry, X. P. Sun, J. Power Sources 2013, 236, 188 – 191. [15] X. W. Lou, L A. Archer, Z. Yang, Adv. Mater. 2008, 20, 3987 – 4019. [16] J. Liu, Y. Wan, C. Liu, W. Liu, S. Ji, Y. Zhou, J. Wang, Euro. J. Inorg. Chem. 2012, 24, 3825 – 3829. [17] W. M. Zhang, J. S. Hu, Y. G. Gou, S. F. Zheng, L. S. Zhong, W. G. Song, L. J. Wan, Adv. Mater. 2008, 20, 1160 – 1165. [18] L. Yu, H. B. Wu, X. W. Lou, Adv. Mater. 2013, 25, 2296 – 2300. [19] G. Zhang, L. Yu, H. B. Wu, H. E. Hoster, X. W. Lou, Adv. Mater. 2012, 24, 4609 – 4613. [20] J. Liu, W. Liu, S. Ji, Y. Zhou, P. Hodgson, Y. Li, Chem. Eur. J. 2013, 19, 9811 – 9816. [21] J. Liu, H. Xia, D. Xue, L. Lu, J. Am. Chem. Soc. 2009, 131, 12086 – 12087. [22] L. Zhou, D. Zhao, X. W. Lou, Angew. Chem. 2011, 123, 1 – 1; Angew. Chem. Int. Ed. 2011, 50, 1 – 4. [23] A. Pan, H. B. Wu, L. Yu, X. W. Lou, Angew. Chem. Int. Ed. 2013, 125, 2282 – 2286. [24] J. Liu, W. Liu, Y. Wan, S. Ji, J. Wang, Y. Zhou, RSC Adv. 2012, 2, 10470 – 10474. [25] C. Yan, W. Xi, W. Si, J. Deng, O. G. Schmidt, Adv. Mater. 2013, 25, 539 – 544. [26] J. Deng, H. Ji, C. Yan, J. Zhang, W. Si, S. Baunack, S. Oswald, Y. Mei, O. G. Schmidt, Angew. Chem. Int. Ed. 2013, 52, 2326 – 2330. [27] N. Yabuuchi, M. Sugano, Y. Yamakawa, I. Nakai, K. Sakamoto, H. Muramatsu, S. Komaba, J. Mater. Chem. 2011, 21, 10035 – 10041. [28] C. Li, C. Yin, L. Gu, R. E. Dinnebier, X. Mu, P. A. van Aken, J. Maier, J. Am. Chem. Soc. 2013, 135, 11425 – 11428. [29] L. Li, Y. Yu, F. Meng, Y. Tan, R. J. Hamers, S. Jin, Nano lett. 2012, 12, 724 – 731. [30] S. R. Challa, A T. Delariva, T. W. Hansen, S. Helveg, J. Sehested, P L. Hansen, F. Garzon, A K. Datye, J. Am. Chem. Soc. 2011, 133, 20672 – 20675. [31] J. Xie, T. Tanaka, N. Imanishi, T. Matsumura, A. Hirano, Y. Takeda, O. Yamamoto, J. Power Sources 2008, 180, 576 – 581. [32] C. Ho, I. Raistrick, R. Huggins, J. Electrochem. Soc. 1980, 127, 343 – 335. [33] W. Liu, J. Liu, K. Chen, S. Ji, Y. Wan, Y. Zhou, D. Xue, P. Hodgson, Y. Li, Chem. Eur. J. 2014, 20, 824 – 830. Received: December 2, 2013 Revised: February 9, 2014 Published online on March 24, 2014
5820
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper
& Lithium-Ion Batteries
Iron Fluoride Hollow Porous Microspheres: Facile Solution-Phase Synthesis and Their Application for Li-Ion Battery Cathodes Jun Liu,*[a, b] Wei Liu,[a, b] Shaomin Ji,*[a, b] Yanling Wan,[a, b] Mingzhe Gu,[a, b] Huaqi Yin,[a, b] and Yichun Zhou[a, b]
Abstract: Iron fluoride cathodes have been attracting considerable interest due to their high electromotive force value of 2.7 V and their high theoretical capacity of 237 mA h g1 (1 e transfer). In this study, uniform iron fluoride hollow porous microspheres have been synthesized for the first time by using a facile and scalable solution-phase route.
Introduction In recent decades, lithium-ion batteries (LIBs) have successfully captured the portable electronic market. However, to satisfy the fast-growing demand for lightweight and high-capacity electrical storage, such as in electric vehicles and power storage from renewable energy resources, great improvements in safe, low-cost and high-energy-density LIBs are urgently needed.[1–5] Iron trifluoride, first reported by Arai and co-workers as an electrode material,[6] has been regarded as one of the possible candidates due to its high electromotive force value of 2.7 V, high theoretical capacity of 237 mA h g1 (1 e transfer) and 712 mA h g1 (3 e transfer), abundant sources, and better safety.[7, 12] Unfortunately, its implementation in LIBs is seriously limited by at least two issues: 1) the iron fluoride bonds in FeF3 are strongly polarized because of the high electronegativity of fluorine, which results the slow diffusion of Li ions and poor electron conductivity; and 2) the drastic volume variation during the Li + intercalation and deintercalation processes, which decreases the stability of the cycle.[6–11] Thus, the electrode performance of bulk FeF3 is extremely unsatisfactory, although FeF3 has open structures with three-dimensional pathways for lithium insertion. The current strategy to solve these difficulties is to mix FeF3 with conductive materials, such as carbon nanotubes, graphite, and conductive agents.[8, 11–12] For
[a] Dr. J. Liu, W. Liu, Dr. S. Ji, Y. Wan, M. Gu, H. Yin, Prof. Dr. Y. Zhou Key Laboratory of Low Dimensional Materials & Application Technology Ministry of Education, Xiangtan University Xiangtan 411105 (P.R. China) E-mail: [email protected] [email protected] [b] Dr. J. Liu, W. Liu, Dr. S. Ji, Y. Wan, M. Gu, H. Yin, Prof. Dr. Y. Zhou Faculty of Materials, Optoelectronics and Physics Xiangtan University, Xiangtan 411105 (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304713. Chem. Eur. J. 2014, 20, 5815 – 5820
These uniform porous and hollow microspheres show a high specific capacity of 210 mA h g1 at 0.1 C, and excellent rate capability (100 mA h g1 at 1 C) between 1.7 and 4.5 V versus Li/Li + . When in the range of 1.3 to 4.5 V, stable capacity was achieved at 350 mA h g1 at a current of 50 mA g1.
example, Kang et al.[8] fabricated carbon nanotube–FeF3 nanocomposites, which present a high rate capability (150 mA h g1 at a current rate of 500 mA g1) due to the enhanced electronic and ionic transport in the electrode. Recently, we have developed the first synthesis of trifluoride cathode nanoparticles on highly conducting graphene sheets with desired size and morphology, which showed superior performance with a stable specific capacity of about 210 mA h g1 at a rate of 0.2 C and high power capability that delivered about 114 mA h g1 even at a rate of 5 C.[12] The above successes can be attributed to the carbon material, which not only increases the electrical conductivity of particle–particle contacts to improved the rate performance but also provides a physical buffering layer for the volume change to enhance the cycling performance. As we know, the performance of cathode/anode materials, such as specific capacity, cycling stability, and rate capability, is also affected by the crystallinity, particle size, and morphology. Thus, researchers have tried to enhance the function of iron fluoride by controlling the structure of this material. Maier and co-workers[13] reported a nanometer-sized FeF3·0.33H2O cathode prepared by low-temperature ionic-liquid-based synthesis route. The nanometer-sized FeF3·0.33H2O delivered about 160 and 126 mA h g1 at current densities of 14 and 71 mA g1, respectively (1.6–4.5 V). Sun’s group synthesized porous FeF3 nanospheres by solvent exchange that delivered a high rate capacity of 180 mA h g1 at 50 mA g1 and 127 mA h g1 at 500 mA g1.[14] Hollow micro-/nano-hybrids with porous structures have attracted fast-growing attention with the aim of building LIBs with high energy density and long cycle life.[15–18] According Lou et al.,[15] this architecture can increase the surface area, lessen the compounded internal stresses, and prevent nanoparticles from agglomerating. Numerous efforts have been devoted to the synthesis of various hollow structures with great progress achieved.[16–20] For example, Wan et al.[17] designed Sn nanoparticles encapsulated in elastic hollow carbon nano-
5815
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper spheres (TNHCs) based on SiO2 colloid templates. Our group also used a soft-template route to synthesize uniform hollow carbon microspheres embedded with transition-metal oxide nanocrystals.[20] However, template methods for constructing these hollow structures are usually time consuming and costly because of the need for the fabrication of templates and the multi-step synthesis process. Moreover, templates are sometimes not easy to remove. Therefore, researchers have tried to develop facile, scalable template-free approaches for the rational synthesis of hollow structures.[21–24] Lou’s group reported various VO2 hollow microspheres, including yolk-shelled, multi-shelled, and single-shelled structures, through Ostwald ripening method.[23] Yan’s group fabricated a series of novel hollow structures, such as strain- Figure 1. Schematic of the fabrication process of hollow-structured FeF3·0.33 H2O microspheres based on Ostwald ripening and solid transformation processes: a) full view; b) cross-sectional view. released hybrid multilayer Ge/Ti nanomembranes[25] and naturally rolled-up C/Si/C trilayer nanomembranes,[26] by using a electron-beam deposition method. a porous structure under the inside-out Ostwald ripening. A seHowever, the uniform hollow and porous structured FeF3 requentially extended solvothermal reaction, the outward Ostported is rather limited due to the decomposition of FeF3 into wald ripening makes the porous solid particles transform into Fe2O3 and FeF2 during the high-temperature sintering progress, hollow Fe1.9F4.75·0.95 H2O microspheres. The formation of the even under a protective atmosphere, and the high solubility of hollow structure also accompanies further growth of the nanoFeF3·3 H2O (5.92 g/100 g at 25 8C).[27–29] Herein, we present crystallines and the increasing diameter of the particles. We a facile solution-phase approach for synthesis of uniform first examined the morphology of the as-prepared hollow porous iron fluoride microspheres. The formation FeF3·0.33 H2O products and their precursors by using scanning mechanism of these porous hollow microspheres is a combinaelectron microscopy (SEM). The SEM images in Figure 2a and tion of the outward Ostwald ripening and phase-transformab show that the solid Fe1.9F4.75·0.95 H2O precursors have quite good homogeneity, rough surfaces, and an average diameter tion processes. The resultant FeF3·3 H2O porous hollow structures exhibit a discharge capacity of about 210 mA h g1, with of about 0.8 mm. The porous particles in Figure 2c and d have excellent cycling stability and superior rate capability. a loose and porous outside layer, but the inside layer is tight. This result reveals that the progress of Ostwald ripening is from inside to outside. In the prolonged solvothermal reaction, Results and Discussion hollow microspheres were also obtained by the outward Ostwald ripening process.[15, 19] In Figure 2e and f, we can see that A possible synthesis mechanism for hydrated iron-based fluorides is shown in Figure 1 and the detailed description is prethese hollow microspheres are mainly comprised of approxisented in the Experimental Section. First, a mixed solution of mately 10 to 20 nm particles, which grew slightly compared Fe(NO3)3·9 H2O and HF generated an FeF63 complex,[29] and with the precursors (Figure 2a and b). We presume that the change in size might be attributed to Ostwald ripening and the color changed from bright orange (Fe3 + dissolved in ethathe growth of larger crystals from those of smaller size, which nol) to colorless (FeF63) upon mixing the Fe3 + /ethanol and have a higher solubility than the larger ones.[30] the HF/ethanol solutions. After being sealed and heated, the 3 FeF6 complex transformed to metastable Fe1.9F4.75·0.95 H2O Hydration water-induced crystallization is also presented in nanocrystals, and then aggregated to form the X-ray diffraction (XRD) profiles of hydrated iron-based fluoride Fe1.9F4.75·0.95 H2O solid microspheres. When the solution was hollow porous microspheres (Figure 3) and solid microspheres (Figure S1 in the Supporting Information). Well-defined XRD heated at 200 8C for 30 min, these solid spheres transformed to Chem. Eur. J. 2014, 20, 5815 – 5820
www.chemeurj.org
5816
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper no. 28-0483, a = 10.35 ). Notably, after vacuum treatment the grain corresponds to orthorhombic FeF3·0.33 H2O (JCPDS card no. 76-1262, a = 7.423, b = 12.73, c = 7.526 ) with less hydration water (Figure 3b). The grain size is little smaller than that of Fe1.9F4.75·0.95 H2O. The widening of the XRD peaks is mainly associated with the removal of hydration water under vacuum treatment. According research by Maier et al.,[13] Fe1.9F4.75·0.95 H2O is composed of characteristic structures of both FeF2 and FeF3, and the FeF2 crystals are mainly close to the surface of the particles. When heated at 170 8C, the FeF2 particles were transformed into FeF3, probably due to a little remaining oxygen. The morphology and microstructure details of these FeF3·0.33 H2O porous and hollow microspheres were further characterized by transmission electron microscopy (TEM), as shown in Figure 4. Figure 4a shows a typical low-magnification
Figure 2. SEM images of iron fluoride microstructure: a, b) Fe1.9F4.75·0.95 H2O solid microspheres; c, d) porous-structured Fe1.9F4.75·0.95 H2O microspheres; e, f) ripened Fe1.9F4.75·0.95 H2O hollow microspheres; f, g) FeF3·0.33 H2O hollow microspheres obtained by heating at 170 8C.
Figure 4. TEM characterizations of the FeF3·0.33 H2O hollow porous microspheres cathode: a, b) TEM images of FeF3·0.33 H2O hollow porous microspheres at different magnifications; c) HRTEM image of the lattice planes of FeF3·0.33 H2O; d) SAED pattern of a single nanoparticle, which indicates its single-crystalline character.
Figure 3. XRD patterns of a) Fe1.9F4.75·0.95 H2O hollow porous microsphere precursors and b) FeF3·0.33 H2O hollow porous microsphere products obtained by heating at 170 8C.
peaks (Figure 3a) were assigned to the product without vacuum treatment, that is, cubic Fe1.9F4.75·0.95 H2O (JCPDS card Chem. Eur. J. 2014, 20, 5815 – 5820
www.chemeurj.org
image of FeF3·0.33 H2O hollow microspheres, which indicates that the product is composed of hollow microspheres with uniform size. It can be observed that the contrast between the central portion and the edge of microspheres strongly supports the formation of hollow structures. A higher magnification TEM image (Figure 4b) reveals that the porous shell is a hierarchical nanostructure composed of dozens of plate-like FeF3·0.33 H2O nanoparticles with a size of several tens of nanometers. As shown in Figure 4c, a lattice plane with an interplanar distance of 0.63 nm was observed in the lattice fringe, which corresponds to the (020) plane of the FeF3·0.33 H2O monoclinic crystalline structure. The single-crystalline porous
5817
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper nanostructures can be confirmed by the selected-area electron diffraction (SAED) pattern of a single nanoparticles subunit (Figure 4d). Nitrogen sorption isotherms were provided to investigate the porous structure and the Brunauer–Emmett– Teller (BET) surface areas of these iron fluoride hollow microspheres, and the data are shown in Figure 5. According to the
Figure 5. The N2 adsorption–desorption isotherms and corresponding pore size distribution (inset) of mesoporous FeF3·0.33 H2O hollow microspheres, calculated by using the BJH method.
corresponding Barrett–Joyner– Halenda (BJH) plots (Figure 5, inset) recorded from the nitrogen isotherms of the hollow porous FeF3·0.33 H2O samples, the average pore size is about 5 nm, which further confirms that the sample has porous material characteristics. The BET specific surface areas and pore volumes of the samples were 22.7 m2 g1 and 0.118 cm3 g1, respectively. The inner void and porous shell have two main advantages: one is that the dense pores in shell allow electrolyte to penetrate and make close contact with the inner–outer surface, which results in a higher surface area and a shorter transport length for Li ions; the other is the hollow core, which can serve as a good cushion for the material volume changes during Li ion insertion/extraction and enhance cycling performance.[15] To explore the efficiency of our approach, both porous hollow microspheres and solid Chem. Eur. J. 2014, 20, 5815 – 5820
microspheres were tested as cathodes in a CR2025 coin-type cell. Initially, as shown in Figure S2 in the Supporting Information, the galvanostatic charge–discharge process for FeF3·0.33 H2O was performed in a voltage range of 1.5 to 4.5 V with a current density of 50 mA g1. The first discharge capacity achieved 402 mA h g1, which approximately corresponds to the insertion of 1.7 Li per formula, and the discharge profile can be divided into two parts. The initial sloped area between 4.5 and 1.7 V stems from Li + insertion into FeF3·0.33 H2O through a two-phase reaction to form Li0.5FeF3 and then through a single-phase reaction to form LiFeF3 (FeF3 + Li + + e$LiFeF3). This process appears to be fully reversible, that is, the structure of pristine FeF3 can be maintained during the Li ion insertion process. A reaction plateau appeared at 1.5 V, which is due to the decomposition of formed LiFeF3 to LiF and Fe metal through a conversion reaction (LiFeF3 + 2 Li + + 2e$Fe + 3 LiF).[11–13] This process was also identified by a cyclic voltammogram test at 1.5 to 4.5 V in Figure S3 in the Supporting Information. Therefore, we first chose the potential window to be from 1.7 to 4.5 V to ensure that only the insertion reaction took place. Figure 6a shows the charge–discharge voltage profiles of FeF3·0.33 H2O solid microspheres and porous hollow microspheres at 1.7 to 4.5 V. The hollow porous materials have a high initial discharge capacity 210 mA h g1at 0.1 C, whereas solid microspheres give a value of just 190 mA h g1. Hollow porous materials also exhibit good rate capability of 210, 165, 124, 100, 78, and 57 mA h g1 at 0.1, 0.2, 0.5, 1, 2, and 5 C, respectively (see Figure 6c). However, solid FeF3·0.33 H2O
Figure 6. Electrochemical performance of the FeF3·0.33 H2O cathode measured in the voltage range of 1.7–4.5 V: a) voltage–capacity curves of FeF3·0.33 H2O solid and hollow microspheres at 0.1 C in the first discharge/charge cycle; b) cycling performance of FeF3·0.33 H2O hollow microspheres at different rates (increasing from 0.1 to 5 C); c) discharge–charge curves of FeF3·0.33 H2O hollow microspheres at different rates (increasing from 0.1 to 5 C); d) cycling performance of FeF3·0.33 H2O hollow microspheres at a current density of 1 C.
www.chemeurj.org
5818
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper
Figure 7. a) Voltage–capacity curves for FeF3·0.33 H2O hollow and porous microspheres cycled at 1.3–4.5 V at a current density of 50 mA g1; b) variation in specific capacities vs. cycle number for FeF3·0.33 H2O hollow and solid microspheres at 1.7–4.5 V.
space for the storage of lithium ions, which is beneficial for enhancing the specific capacity of the battery. Thus, the hollow porous FeF3·0.33 H2O microspheres present higher initial capacity (210 mA h g1 at 0.1 C) compared with solid microspheres (180 mA h g1) in Figure 7b. Second, the better rate capabilities of the hollow porous FeF3·0.33 H2O microspheres are ascribed to their hollow nature, which provides a large surface area and porous structure and affording a shortened diffusion path length. Third, the void space in hollow structures can accommodate the volume change during lithium insertion–deinsertion, which improves the cycling performance. To make a further investigation, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to clarify the energy storage redox mechanism of FeF3·0.33 H2O (Figure 8). As shown in Figure 8a, the potential difference between the cathodic and anodic peaks of FeF3·0.33 H2O hollow spheres is only about 0.3 V (cathodic peak: 2.9 V; anodic peak: 3.2 V). However, solid FeF3·0.33 H2O has a distinct anodic peak at 4.5 V (Figure S4 in the Supporting Information), which results a short charge plateau in the high voltage section. As we know, this huge potential hysteresis, caused by polarization, will also induce a large energy loss during battery operation. The hollow inner cores and porous shells designed herein can effectively decrease the polarization by providing a larger contact surface between the electrolyte and the cathode material and shortening the transport length for Li ions and electrons. Figure 8b shows the EIS curve, which was used to study the dynamics for lithium insertion and extraction. Generally, the high-frequency semicircle results from the charge-transfer process and the straight line is attributed to the diffusion of lithium ions in the particles.[31–33] According to the Nyquist plots, the EIS is fitted by using ZSimpWin analysis software.[33] Before
samples show corresponding capacities of 180 (0.1 C), 114 (0.2 C), 82 (0.5 C), 65 (1 C), 52 (2 C), and 37 mA h g1 (5 C) in Figure 7b, and there is an obvious capacity decrease at low current density regions. This might be due to the potential and the drastic volume variation during the Li uptake-and-release process.[13] Compared with the solid particles, the performance of these porous hollow microspheres in both cycling capability and rate capacity has been greatly enhanced as shown in Figure 7b. At low rates, the hollow porous cathode can still deliver a reversible capacity of 180 mA h g1 after eight cycles and there is excellent capacity retention after 45 cycles at a current rate of 1 C (Figure 6d). Moreover, when worked at 1.3 to 4.5 V, the hollow architectured FeF3·0.33 H2O cathode can deliver a high capacity of 412 mA h g1 and stabilized discharge capacities stay at about 350 mA h g1 (Figure 7a). The improvement in performance for microspheres cathode: a) the CV curve of the porous hollow architecture Figure 8. Electrochemical performance of the FeF3·0.33 H2O hollow FeF3·0.33 H2O hollow microspheres at a scan rate of 0.1 mV s1; b) the EIS data at open circuit potential with an AC can also be explained in terms voltage of 5 mV amplitude with a frequency range of 0.1 Hz to 82.5 KHz; c) the equivalent circuit for fitting the imof three factors. First, the porous pedance data with the resistance of the electrolytes (Rs), in which Rct is the resistance for Li + migration through shell structure may provide extra the surface film; Ci is the surface film capacitance; and Zw is the Warburg impedance. Chem. Eur. J. 2014, 20, 5815 – 5820
www.chemeurj.org
5819
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper the cycles, the hollow sample shows lower charge-transfer resistance (56.2 W, Figure 8b) than the solid sample (119.6 W, Figure S5 in the Supporting Information) because the hollow structure can provide more reaction sites. After 40 cycles at different current densities, the charge-transfer resistance of the solid sample electrode increased from 119.6 to 266 W (Figure S5 in the Supporting Information). For the hollow cathode, the charge-transfer resistance increases to only 66.1 W (Figure 8b), which is still far lower than the solid sample. This indicates that hollow porous microspheres could effectively enhance the ion diffusion ability, which makes the charge-transfer reaction easier.
Conclusion We have developed the first synthesis of hollow porous FeF3·0.33 H2O via Ostwald ripening. These porous hollow microspheres as cathode materials exhibit good rate capability and cycling performance, which could be attributed to their porous and hollow structure, which facilitates electrolyte transport, shortens electronic and ionic pathways within the electrode, and provides a larger surface area. These encouraging results may open up new opportunities to promote long-term endeavors in developing high-capacity cathodes for rechargeable Li-ion batteries.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (11202177, 51202207). Keywords: cathode · electrochemistry · hollow spheres · nanostructures · solid-state structures [1] A. S. Aric, P. Bruce, B. Scrosati, J. M. Tarascon, W. V. Schalkwijk, Nat. Mater. 2005, 4, 366 – 377. [2] O. K. Park, Y. Cho, S. Lee, H. C. Yoo, H. K. Song, J. Cho, Energy Environ. Sci. 2011, 4, 1621 – 1633. [3] Y. Yu, C. Yan, L. Gu, X. Lang, K. Tang, L. Zhang, Y. Hou, Z. Wang, M. Chen, O. G. Schmidt, Adv. Energy Mater. 2013, 3, 281 – 285. [4] Y. Yu, L. Gu, X. Lang, C. Zhu, T. Fujita, M. Chen, J. Maier, Adv. Mater. 2011, 23, 2443 – 2447. [5] Y. K. Sun, Z. Chen, H. J. Noh, D. J. Lee, H. G. Jung, Y. Ren, S. Wang, C. S. Yoon, S. T. Myung, K. Amine, Nat. Mater. 2012, 11, 942 – 947. [6] H. Arai, S. Okada, Y. Sakurai, J. Yamaki, J. Power Sources 1997, 68, 716 – 719.
Chem. Eur. J. 2014, 20, 5815 – 5820
www.chemeurj.org
[7] F. Badway, N. Pereira, F. Cosandey, G. G. Amatucci, J. Electrochem. Soc. 2003, 150, A1209. [8] S. W. Kim, D. H. Seo, H. Gwon, J. Kim, K. Kang, Adv. Mater. 2010, 22, 5260 – 5264. [9] T. Li, L. Li, Y. L. Cao, X. P. Ai, H. X. Yang, Phys. Chem. C. 2010, 114, 3190 – 3195. [10] J. Cabana, L. Monconduit, D. Larcher, M. R. Palacin, Adv. Mater. 2010, 22, E170 – E192. [11] D. L. Ma, Z. Y. Cao, H. G. Wang, X. L. Huang, L. M. Wang, X. B. Zhang, Energy Environ. Sci. 2012, 5, 8538 – 8542. [12] J. Liu, Y. L. Wan, W. Liu, Z. S. Ma, S. M. Ji, J. B. Wang, Y. C. Zhou, P. Hodgson, Y. C. Li, J. Mater. Chem. A 2013, 1, 1969 – 1975. [13] C. Li, L. Gu, S. Tsukimoto, P. A. Van Aken, J. Maier, Adv. Mater. 2010, 22, 3650 – 3654. [14] Q. X. Chu, Z. C. Xing, J. Q. Tian, X. B. Ren, A. M. Asiri, A. O. Al-Youbi, K. A. Alamry, X. P. Sun, J. Power Sources 2013, 236, 188 – 191. [15] X. W. Lou, L A. Archer, Z. Yang, Adv. Mater. 2008, 20, 3987 – 4019. [16] J. Liu, Y. Wan, C. Liu, W. Liu, S. Ji, Y. Zhou, J. Wang, Euro. J. Inorg. Chem. 2012, 24, 3825 – 3829. [17] W. M. Zhang, J. S. Hu, Y. G. Gou, S. F. Zheng, L. S. Zhong, W. G. Song, L. J. Wan, Adv. Mater. 2008, 20, 1160 – 1165. [18] L. Yu, H. B. Wu, X. W. Lou, Adv. Mater. 2013, 25, 2296 – 2300. [19] G. Zhang, L. Yu, H. B. Wu, H. E. Hoster, X. W. Lou, Adv. Mater. 2012, 24, 4609 – 4613. [20] J. Liu, W. Liu, S. Ji, Y. Zhou, P. Hodgson, Y. Li, Chem. Eur. J. 2013, 19, 9811 – 9816. [21] J. Liu, H. Xia, D. Xue, L. Lu, J. Am. Chem. Soc. 2009, 131, 12086 – 12087. [22] L. Zhou, D. Zhao, X. W. Lou, Angew. Chem. 2011, 123, 1 – 1; Angew. Chem. Int. Ed. 2011, 50, 1 – 4. [23] A. Pan, H. B. Wu, L. Yu, X. W. Lou, Angew. Chem. Int. Ed. 2013, 125, 2282 – 2286. [24] J. Liu, W. Liu, Y. Wan, S. Ji, J. Wang, Y. Zhou, RSC Adv. 2012, 2, 10470 – 10474. [25] C. Yan, W. Xi, W. Si, J. Deng, O. G. Schmidt, Adv. Mater. 2013, 25, 539 – 544. [26] J. Deng, H. Ji, C. Yan, J. Zhang, W. Si, S. Baunack, S. Oswald, Y. Mei, O. G. Schmidt, Angew. Chem. Int. Ed. 2013, 52, 2326 – 2330. [27] N. Yabuuchi, M. Sugano, Y. Yamakawa, I. Nakai, K. Sakamoto, H. Muramatsu, S. Komaba, J. Mater. Chem. 2011, 21, 10035 – 10041. [28] C. Li, C. Yin, L. Gu, R. E. Dinnebier, X. Mu, P. A. van Aken, J. Maier, J. Am. Chem. Soc. 2013, 135, 11425 – 11428. [29] L. Li, Y. Yu, F. Meng, Y. Tan, R. J. Hamers, S. Jin, Nano lett. 2012, 12, 724 – 731. [30] S. R. Challa, A T. Delariva, T. W. Hansen, S. Helveg, J. Sehested, P L. Hansen, F. Garzon, A K. Datye, J. Am. Chem. Soc. 2011, 133, 20672 – 20675. [31] J. Xie, T. Tanaka, N. Imanishi, T. Matsumura, A. Hirano, Y. Takeda, O. Yamamoto, J. Power Sources 2008, 180, 576 – 581. [32] C. Ho, I. Raistrick, R. Huggins, J. Electrochem. Soc. 1980, 127, 343 – 335. [33] W. Liu, J. Liu, K. Chen, S. Ji, Y. Wan, Y. Zhou, D. Xue, P. Hodgson, Y. Li, Chem. Eur. J. 2014, 20, 824 – 830. Received: December 2, 2013 Revised: February 9, 2014 Published online on March 24, 2014
5820
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
