Mesoporous Fe2O3 nanoparticles as high performance anode materials for lithium-ion batteries

Electrochemistry Communications 29 (2013) 17–20

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Mesoporous Fe2O3 nanoparticles as high performance anode materials for lithium-ion batteries Jingjing Zhang a, Tao Huang a, Zhaolin Liu b,⁎, Aishui Yu a,⁎ a b

Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200438, China Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 3 Research Link, Singapore 117602, Singapore

a r t i c l e

i n f o

Article history: Received 10 December 2012 Received in revised form 8 January 2013 Accepted 9 January 2013 Available online 17 January 2013 Keywords: Hematite Nanoparticle Mesoporous Lithium-ion battery Anode material

a b s t r a c t This work introduces an effective, inexpensive, and large-scale production approach to the synthesis of Fe2O3 nanoparticles with a favorable configuration that 5 nm iron oxide domains in diameter assembled into a mesoporous network. The phase structure, morphology, and pore nature were characterized systematically. When used as anode materials for lithium-ion batteries, the mesoporous Fe2O3 nanoparticles exhibit excellent cycling performance (1009 mA h g−1 at 100 mA g−1 up to 230 cycles) and rate capability (reversible charging capacity of 420 mA h g−1 at 1000 mA g−1 during 230 cycles). This research suggests that the mesoporous Fe2O3 nanoparticles could be suitable as a high rate performance anode material for lithium-ion batteries. © 2013 Elsevier B.V. All rights reserved.

1. Introduction With the advantages of high energy density, long lifespan and environmental benignity, lithium-ion batteries have become the predominant power source for portable electronics for many years [1]. The continuously surging demand in emerging large-scale energy applications such as low-emission electric vehicles, renewable power plants and electric grids boosts a great deal of interest in seeking for high-performance electrode materials with the capability to store and deliver more energy efficiently. Among the available alternative anode materials, iron oxides (mainly Fe2O3 and Fe3O4) have always been regarded as very appealing candidates because of their much higher capacity than that of conventional graphite (372 mA h g−1), as well as nontoxicity, high corrosion resistance and low processing cost [2–5]. In principle, the lithium storage capacity of iron oxides is mainly delivered through the reversible conversion reaction between lithium ions and metal oxide forming metal nanocrystals dispersed in a Li2O matrix [6–8]. This process usually causes fast capacity fading due to drastic volume change and severe destruction of the electrode during electrochemical cycling. The sluggish kinetics for charge transfer and ionic diffusion, as well as high intrinsic resistance also induces additional performance degradation of metal oxide based electrodes, especially at high current densities. So far, an enormous amount of effort has been made to circumvent the above issues by designing nanostructured Fe2O3 or mixing Fe2O3 particles with various carbon additives [9–12]. Nanostructured anode materials have significant advantages because ⁎ Corresponding authors. Tel./fax: +86 21 51630320. E-mail addresses: [email protected] (Z. Liu), [email protected] (A. Yu). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.01.002

of (i) short Li+ transport distance, (ii) large electrolyte-electrode contact area that can provide faster reactions, and (iii) accommodation of the structural strain generated due to Li+ intercalation [5]. Herein we report a novel material of α-Fe2O3 nanoparticles (~5 nm) with abundant mesopores (~3 nm and 20 nm) via a very simple method which only uses Fe(NO3)3·9H2O and H2O as starting materials. We believe that this synthesis route is suitable for large-scale production. And it is different from the current routes [13–15]. When investigated as anode materials, the as-prepared mesoporous Fe2O3 nanoparticles present high charge–discharge capacity, good cycling performance, and excellent rate capability as a pure active material without any conductive matrix. 2. Experimental 2.1. Preparation of Fe2O3 nanoparticles In a typical experiment, 1.5 g Fe(NO3)3·9H2O was dissolved in 40 mL deionized water and stirred at 125 °C for 1 h. After reaction, the obtained mixture cooled to room temperature naturally. The products were obtained by filtering and sequentially washing with water for several times and then dried in a vacuum oven at 60 °C for 12 h. 2.2. Materials characterization The as-prepared products were characterized by X-ray diffraction (XRD, Bruker D8 Advance, Cu Ka radiation, =1.5406 Å), field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission

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3. Results and discussion

electron microscopy (TEM, JEM-2100F). The specific surface area and pore size distribution of the sample were derived using the multipoint Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) model, respectively (QUDRASORB SI, Quantachrome Instruments U.S.).

3.1. Morphology and structure of the obtained materials Fig. 1a shows the XRD pattern of the Fe2O3 nanoparticles. Eight obvious diffraction peaks can be easily identified for the (012), (104), (110), (113), (024), (116), (214) and (300) planes of the pure hexagonal phase α-Fe2O3 crystalline structure, respectively, which are consistent with the standard spectrum (JCPDS No. 33-0664). FE-SEM and TEM were used to further examine the crystallinity, pore-solid architecture of the as-prepared Fe2O3 (Fig. 1b, c and d). It can be seen that the uniform Fe2O3 nanoparticles are with a diameter of 5 nm, and mesopores formed among the nanoparticles produce a hierarchical porous structure. Nitrogen isothermal adsorption measurements were performed to determine the pore structure and the BET surface area. N2 adsorption–desorption isotherms and pore size distribution (PSD) of Fe2O3 is presented in Fig. 2. It can be seen that the adsorption isotherm of Fe2O3 (Fig. 2a) is of type IV with a distinct hysteresis loop observed in the range of 0.5 to 1.0 P/P0 according to the International Union of Pure and Applied Chemistry (IUPAC) classification, indicative of mesoporous materials. The remarkable nitrogen uptake above the relative pressure of 0.5 is due to the capillary condensation of nitrogen in the mesoporous texture. Fig. 2b shows the PSD curve, mesopores about 3 and 20 nm indicating the mesoporous feature present in the as-obtained Fe2O3 nanoparticles. The surface area of Fe2O3 is 112 m2 g−1.

2.3. Electrochemical measurement Electrochemical tests were performed using a CR2016-type coin cell. An assembled coin cell was composed of lithium as the counter electrode and the working electrode consisting of 50% active material, 30% super P carbon black and 20% polyvinylidene fluoride (PVDF) made on copper foil. Since the poor electronic conductivity, drastic volume change and severe destruction of the electrode during electrochemical cycling of Fe2O3, adding more conductive agent and binder are usually considered. According to the literatures [16–18], the weight ratio (50:30:20) is selected. Coin-type cells were assembled in an argon-filled glove box (Mikarouna, Superstar 1220/750/900) with 1 M LiPF6 solution in ethylene carbonate/diethyl carbonate (EC:DEC=1:1, v/v) as the electrolyte and Celgard 2300 as the separator. The total mass of the active electrode material is about 2–3 mg and the electrode surface area is 1.54 cm2 (Φ14 mm). The galvanostatic charge–discharge tests were performed on a battery test system (Land CT2001A, Wuhan Jinnuo Electronic Co. Ltd.) at a constant current density of 100, 500 and 1000 mA g−1 in the potential range from 0.01 to 3.0 V.

(104)

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Fig. 1. (a) XRD pattern, (b) SEM image, (c) TEM image and (d) HRTEM image of Fe2O3 nanoparticles.

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a 60

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The initial charge–discharge profiles for the Fe2O3 electrodes at the current densities of 100, 500 and 1000 mA g −1 between 3.00 and 0.01 V are shown in Fig. 3a. The initial discharge and charge capacities were measured to be 1706, 1286 mA h g−1 at 100 mA g−1; 1606, 1245 mA h g−1 at 500 mA g−1; and 1577, 1209 mA h g−1 at 1000 mA g−1, corresponding to a Coulombic efficiency of 75.4%, 77.5% and 76.7%, respectively. The discharge capacity decreases slightly with the increasing of current density, and some irreversible capacity loss was observed in the first cycle, which might be due to irreversible electrochemical decomposition of the electrolyte and the subsequent formation of a solid electrolyte interphase (SEI) layer on the surface [4]. In addition, the small plateau at 1.65 V is corresponding to the lithium insertion into the crystal structure of Fe2O3 and the conversion from Fe(III) to Fe(II). The long plateau that appeared near 0.85 V is attributed to the conversion from Fe(II) to Fe(0) and the decomposition of electrolyte [19]. A sloped region from 1.2 to 2.2 V shown in the charge curve is due to the reverse reaction [20]. Therefore, the safety of Fe2O3 is much better than carbon materials due to the higher voltage plateau for Li+ lithiation, avoiding appearing of lithium dendrite. The rate capability of the Fe2O3 nanoparticles is presented in Fig. 3b. No obvious fading of the charge capacity is observed during the first 5 cycles when testing at various rates. In addition, the reversible capacity decreased slowly with the increasing of current density. Even at a high rate of 1000 mA g−1, the specific capacity was still above 450 mA h g −1, and could be recovered to 750 mA h g−1 when

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Fig. 2. The gas (N2) adsorption–desorption isotherm loop (a) and BJH pore size distribution (b) of Fe2O3 nanoparticles.

3.2. Electrochemical performance

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1400 1200 1000 800 600 400 200 0 0

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Cycle number Fig. 3. (a) The initial galvanostatic charge–discharge profiles of Fe2O3 electrodes at 100, 500 and 1000 mA g−1; (b) rate performance of Fe2O3 electrodes (100–1000 mA g−1); (c) cyclic performance of Fe2O3 electrodes at 100, 500 and 1000 mA g−1 (open—discharge, filled— charge).

the current density reduced back to 100 mA g−1. It further confirms that mesoporous Fe2O3 nanoparticles without a carbon matrix as a buffer still have an excellent structural stability. We further evaluated the long life cycle performance of the Fe2O3 anode at different current densities shown in Fig. 3c. The mesoporous Fe2O3 nanoparticles exhibit high retention efficiency with a stable specific capacity of 1009 mA h g−1 after 230 cycles at 100 mA g−1, which is superior to those reported in the literature [21–25]. The charge capacity of Fe2O3 nanoparticles observed after 230 cycles is almost 3 times higher than the reversible capacity of commercial graphite. The Coulombic

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efficiency remains higher than 97%. The excellent capacity retention can be ascribed to the high electro-active surface area of mesoporous Fe2O3 nanoparticles which provides an increased contact between active material and the electrolyte that enhances the electron transport, and accommodates the volume change during charge–discharge process. Meanwhile, the nanocrystalline structure favors a fast kinetic property with facilitating the Li+ insertion/deinsertion. An interesting result is that such nanostructured Fe2O3 also reveals noble electrochemical properties under high current densities. It can achieve sustainable capacities with 540 mA h g−1 at 500 mA g−1, and 420 mA h g−1 at 1000 mA g−1 during 230 cycles, respectively. It is obvious that the capacities increase after 30 cycles. The capacity rise phenomenon has been widely referred in the transition metal oxide electrodes, which is attributed to the reversible growth of a polymeric gel-like film on the surface of the progressively pulverized particles resulting from electrochemical grinding effect [12,26–28]. 4. Conclusions In summary, highly pure α-Fe2O3 nanoparticles with abundant mesopores were prepared via a facile, cost-effective method. The Fe2O3 nanoparticles with a large surface-to-volume ratio exhibit extremely high-rate capability and outstanding cycling performance, which could be attributed to the unique 3D pore-solid architecture. It not only favors relatively short ion and charge diffusion pathways and high transport rates of both lithium ions and electrons, but also can partially accommodate the volume expansion during cycling. They could deliver a high capacity of 1009 mA h g−1 at 100 mA g−1 over 230 cycles and superior high-rate performance. Hence the mesoporous Fe2O3 nanoparticles can be considered as a potential high capacity anode for next generation lithium-ion batteries. Acknowledgments The authors acknowledge funding supports from 973 Program (2013CB934103), the National Natural Science Foundation (No. 21173054) and Science & Technology Commission of Shanghai Municipality (No. 08DZ2270500), China.

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