Combustion synthesized rod-like nanostructure hematite with enhanced lithium storage properties

Materials Research Bulletin 61 (2014) 83–88

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Combustion synthesized rod-like nanostructure hematite with enhanced lithium storage properties Q.Q. Xiong, S.J. Shi, H. Tang, X.L. Wang, C.D. Gu, J.P. Tu * State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 18 September 2014 Received in revised form 27 September 2014 Accepted 1 October 2014 Available online 5 October 2014

Fe2O3 nanorods are synthesized by combustion method using alcohol as both solvent and fuel, which is a facile and effective strategy for the large-scale and inexpensive fabrication. The Fe2O3 nanorods are with the well distributed diameters of 20–30 nm and length ranging from 80 to 100 nm. As an anode material for lithium-ion batteries, the Fe2O3 nanorod electrode delivers a high discharge capacity of 761.7 mA h g 1 after 60 cycles at 500 mA g 1, and 727.2 mA h g 1 at a high current density of 2000 mA g 1. The good electrochemical performance is attributed to the sufficient contact of active material and electrolyte, large surface area, and short diffusion length of Li+. ã 2014 Elsevier Ltd. All rights reserved.

Keywords: Hematite Nanorod Combustion synthesis Lithium-ion battery

1. Introduction Nowadays, in order to the success of electric vehicles (EVs), it is key to develop the rechargeable batteries with long cycling life, high capacity, and high rate capability. Among the most popular power sources for portable electronic devices, lithium ion batteries (LIBs), can be successfully used for EVs if their energy density and power density can be enhanced [1]. During the last two decades, extensive efforts have been made to seek for excellent anode materials to satisfy the demand of the better battery performance [2–8]. Poizot et al. firstly proposed transition metal oxides as the new-type anode materials for LIBs [2]. They exhibit much higher theoretical capacities (2 to 3 times) than that of commercial graphite and smaller volume change during the charge–discharge process than that of alloy anodes [9–21]. Among them, hematite (Fe2O3) has been extensively investigated as a very appealing host anode material because of its high capacity (1007 mA h g 1), natural abundance, nontoxicity, and low cost [22–26]. During the discharging–charging process, each unit of Fe2O3 can react with Li+ to form a composite containing Fe nanoclusters embedded in amorphous Li2O matrix and then it reversibly converts back to Fe2O3, which is similar to other transition metal oxides [17,27]. Currently, there are many methods used to synthesize Fe2O3 nanostructures, such as hydrothermal method [28–30], temple method [31,32], spray pyrolysis method [33], microwave heating

* Corresponding author. Tel.: +86 571 87952573; fax: +86 571 87952856. E-mail addresses: [email protected], [email protected] (J.P. Tu). http://dx.doi.org/10.1016/j.materresbull.2014.10.012 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.

method [34,35], and so on. However, by using the hydrothermal process, it is difficult to obtain the Fe2O3 product in one step, always followed by annealing treatment with energy and time consuming [30,36,37]. And the yield is also disappointing to meet the demand of the industrial production. The use of template can successfully prepare various nanostructures, but it is somewhat cumbersome with a complex elaboration process and rather high production cost. As for the spray pyrolysis and microwave heating method, they both need the special and complicated equipment. Therefore, in order to achieve the practical use of Fe2O3, development of a facile and effective strategy for the large-scale and inexpensive fabrication of a high-performance electrode is urgently desirable and of great importance. Combustion synthesis involves an exothermic and selfsustaining chemical reaction between the metal salts and suitable organic fuel [38]. This method has been successfully used to synthesize many kinds of nanopowder materials [39–42]. In fact, combustion, a very outstanding synthesis process, has attracted a good deal of attention, because it saves the time and energy, requires simple equipment, and cheap reagents, and provides rather high production [43]. It meets the demand of the green and sustainable modern industry. In this present work, we aim to report the Fe2O3 nanostructure synthesized by simple and highefficiency combustion method with ethanol as both solvent and fuel. The effect of ethanol on promoting mixture of the reactants to gain a better performance of the materials has been reported in many other works [42,44]. The as-synthesized Fe2O3 nanostructure as the anode material for LIBs, shows greatly enhanced electrochemical performance because of its sufficient contact of

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2.2. Characterization The morphology and microstructure of the products were characterized by X-ray diffraction (XRD, Rigaku D/max 2550 PC, Cu Ka), field emission scanning electron microscopy (SEM, Hitachi S-4700), and transmission electron microscopy (TEM, JEM 200CX at 160 kV, Tecnai G2 F30 at 300 kV). The electrode was prepared by mixing the active materials (85 wt%) with carbon black (10 wt%) and polyvinylidene fluoride (5 wt%). The obtained slurry was coated on a copper foil, followed by drying in a vacuum oven at 90  C for 24 h. The electrochemical tests were performed using a coin-type half cell (CR 2025). Test cells with lithium foil as the counter electrode and a polypropylene (PP) micro-porous film (Cellgard 2300) as the separator were assembled in an argon-filled glove box. Electrolyte consisting of 1 M LiPF6 in ethylene carbonate (EC)–dimethyl carbonate (DMC) (1:1 in volume) was used. The galvanostatic charge–discharge tests were conducted on a LAND battery program-control test system at certain current densities between 0.01 and 3.0 V at room temperature. Cyclic voltammetry (CV) was performed on a CHI660C electrochemical workstation in the potential from 0 to 3.0 V (vs Li+/Li) at a scan rate of 0.1 mV s 1. Electrochemical impedance spectroscopy (EIS) measurements were performed over a frequency range of 100 kHz–10 mHz by applying an AC signal of 5 mV. 3. Results and discussion Fig. 1 shows typical XRD patterns of the two powders. All the diffraction peaks confirm that both the crystal structures are in agreement with the standard values for the rhombohedral phase of Fe2O3 [JCPDS card No. 33-0664]. No impurity is detected from the XRD patterns of the two samples, indicating that they both have a single-phase rhombohedral crystal structure. The comparison on the morphologies of the two products is shown in Fig. 2. The Fe2O3 product synthesized by combustion method exhibits a uniform rod-like morphology (Fig. 2(a)). The relatively high magnified SEM image shows that the Fe2O3 nanorods are with the well distributed diameters of 20–30 nm and length ranging from 80 to 100 nm (Fig. 2(b)). Fig. 2(c) and (d) shows the morphology of Fe2O3 nanoparticles with the nonuniform size of 50–200 nm due to the inhomogeneous precipitation in the drying process. With the combustion method, in such a short reaction time, the raw materials reacted more homogeneously in ethanol solution. The structural features and detailed morphological of the powders are also examined by TEM.

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The Fe2O3 nanorods were synthesized by a combustion method. Firstly, 30 g Fe(NO3)3
9H2O was dissolved in 30 mL of ethanol with stirring. Then the resulting solution was poured into a 100 mL corundum crucible. The crucible was put into a chamber furnace which was pre-heated to 500  C in air, and treated at this temperature for 30 min to obtain the Fe2O3 nanorods. To prepare Fe2O3 nanoparticles by an annealing method for comparison, the same amounts of Fe(NO3)3
9H2O and ethanol were used. The solution was firstly dried in an oven at 90  C. Then it was annealed at 500  C in air for the same reaction time in the synthesis of Fe2O3 nanorods. The average loading of both electrodes was about 1.4 mg cm 2.

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active material and electrolyte, large surface area, and short diffusion length of Li+.

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2 Theta (degree) Fig. 1. XRD pattern of Fe2O3 nanorods and nanoparticles.

Fig. 3(a) further confirms the rod-like structure of Fe2O3 synthesized by combustion method. The HRTEM examination of a Fe2O3 nanorod shown in Fig. 3(b) shows the nanorod has perfect crystallinity with a distinct set of visible lattice fringes. The interplanar spacing is 0.25 nm, corresponding to the (11 0) plane of rhombohedral Fe2O3. The Fe2O3 nanoparticles are also observed in Fig. 3(c) and (d). From the HRTEM (Fig. 3(d)), the space between the adjacent planes is 0.37 nm, corresponding to the (0 1 2) plane of Fe2O3. Fig. 4 shows the nitrogen adsorption and desorption isotherm and the Barrett–Joyner–Halenda (B–J–H) curves of the Fe2O3 nanorod and Fe2O3 nanoparticle. At the relative pressure range of 0.5–1.0, the isotherm exhibits a hysteresis loop, which is an indication of the existence of the porosity in the samples. For Fe2O3 nanorod, the B–J–H curve consists of two peaks. One is around 17 nm which may be the pore in the nanorod. The other peak around 360 nm indicates the interspaces between the nanorods. The Brunauer–Emmett–Teller (B–E–T) surface area of the Fe2O3 nanorod is calculated to be 15.84 m2 g 1. The relatively large specific surface area can offer large contact area between active material and the electrolyte, and promotes Li-ion diffusion. However, the B–E–T surface area of the Fe2O3 nanoparticle is only 7.96 m2 g 1, much smaller than that of the Fe2O3 nanorod. In view of the potential application of Fe2O3 as an anode in LIBs, we investigated their ability to reversibly insert/release lithium. Fig. 5 shows the CV curves of the two Fe2O3 electrodes for the first three cycles at the scan rate of 0.1 mV s 1 in the potential range from 0 to 3.0 V (vs Li/Li+). The observed CV curves of the two electrodes are very similar. As shown in Fig. 5(a), in the first cathodic scan for the Fe2O3 nanorod, there is a sharp reduction peak at about 0.57 V, which can be attributed to the reduction of Fe3+ to Fe0 and the irreversible reaction with the electrolyte. And the anodic peak at about 1.70 V corresponds to the reversible oxidation of Fe0 to Fe3+, which agrees well with previous works [19,33,45]. During the charging process, both the peak current and the integrated area of the anodic peak are decreased. In this step, the conversion of Fe2O3 to Fe and the formation of insulated Li2O are responsible for the irreversible capacity. Compared to Fe2O3 nanoparticle, however, the peak current and areas of all CV peaks of the Fe2O3 nanorod electrode are larger and the CV curves are more stable after the second cycle, indicating that the Fe2O3 nanorod electrode has higher reactivity and capacity. The uniform Fe2O3 nanorod with much larger surface area results in a more adequate reaction and higher electrode utilization. The first two galvanostatic charge–discharge curves for the Fe2O3 nanorod and nanoparticle electrodes at a current density of 500 mA g 1 within a voltage range of 0.01 to 3.0 V are shown in

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Fig. 2. SEM images of (a, b) Fe2O3 nanorods, (c, d) Fe2O3 nanoparticles.

Fig. 6(a). The charge–discharge curves of the two electrodes are nearly the same. At the first discharging curve, there is a voltage plateau at about 0.9 V followed by a sloping curve down to the cut voltage of 0.01 V for both the electrodes, which is similar to the previous results [20,29,30,33], indicating that the different morphologies do not change the electrochemical nature of Fe2O3. However, the second discharge curves of both the electrodes are different from the first ones, which indicates drastic Li+-driven, textural or structural modifications [2]. And for the two electrodes,

the charge voltage plateaus are both higher than the discharge ones, suggesting the polarization (i.e., voltage hysteresis between charge and discharge) due to the limited Li ion diffusion kinetics during the insert/release process [46,47]. Obviously, the Fe2O3 nanorod electrode shows much small voltage differences of the charge–discharge plateaus. It is concluded that the uniform nanorod structure with high surface area induces high reversibility in the insert/release lithium processes. The Fe2O3 nanorod and nanoparticle electrodes in the initial discharge process show

Fig. 3. TEM images of (a) Fe2O3 nanorod, (b) HRTEM image of Fe2O3 nanorod, (c) Fe2O3 nanoparticle, (d) HRTEM image of Fe2O3 nanoparticle.

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specific capacities of 1257.7 and 1206.6 mA h g 1, respectively. The extra capacity compared with the theoretic capacity has been explained as the decomposition of electrolyte to form the SEI layer or further lithium storage via interfacial reaction [48–50]. The high surface area will enhance either/both of the reactions [33]. The cycling performance of Fe2O3 nanorod and nanoparticle electrodes is illustrated in Fig. 6(b). Both the electrodes were tested

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Fig. 6. (a) Charge–discharge profiles of Fe2O3 nanorod and nanoparticle between 0.01 and 3.0 V at current densities of 500 mA g 1, (b) cycling performance of Fe2O3 nanorod and nanoparticle electrodes at a current density of 500 mA g 1, (c) rate performance of Fe2O3 nanorod and nanoparticle electrodes at current density of 100 mA g 1–2000 mA g 1, (d) representative discharge–charge curves at various current rates of Fe2O3 nanorod electrode.

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Fig. 5. CV curves of (a) Fe2O3 nanorod, (b) Fe2O3 nanoparticle for the first three cycles at a scan rate of 0.1 mV s 1 in the potential range of 0–3.0 V (vs Li/Li+).

at a constant current density of 500 mA g 1 charge–discharge cycling up to 60 cycles. It is evident that the Fe2O3 nanorod electrode shows much high cyclability. The discharge capacity decreases slowly and then becomes stable. After the 60th cycle, the Fe2O3 nanorod electrode still delivers a high reversible capacity of 761.7 mA h g 1, which is more than two-fold of the graphite anode capacity (372 mA h g 1). By contrast, the Fe2O3 nanoparticle electrode shows very low reversible capacity (216.6 mA h g 1) and poor cycling stability. Fig. 6(c) shows the rate performance of Fe2O3 nanorod and nanoparticle electrodes at different current densities. The Fe2O3 nanorod illustrates a much better rate performance, in particular, when the current density reaches 2000 mA g 1, the specific capacity of the Fe2O3 nanorod still retains 727.2 mA h g 1. However, the capacity of the Fe2O3 nanoparticle drops dramatically, only 358.6 mA h g 1 at this high current density. It has been reported that, in addition to the intrinsic crystal structure, the high reversible capacity and improved cycle stability and rate performance are greatly related to extrinsic morphology, which has an obvious effect on the reactivity not only in the initial charge–discharge cycle but also in the subsequent cycles. Until now, many nanorod materials show rather good electrochemical performance [51,52]. The presence of high surface area enhances the contact area between active material and electrolyte, rendering an increased number of active sites for electrochemical Li+ storage [53]; the diffusion length of the Li+ is also shortened in uniform tiny nanotod. To the best of our

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delivers high capacity, suggesting the good rate performance of the Fe2O3 nanorod electrode. Nyquist plots for the two electrodes measured at the full charged state after 10 cycles at a current density of 500 mA g 1 illustrate a depressed semicircle from high to medium frequency followed by a straight sloping line at the low-frequency end (Fig. 7(a)). In this work, we select a modified two-parallel diffusion path model. And Fig. 7(b) shows the corresponding equivalent circuit, where Rel indicates the solution resistance; Rsl(i) and Csl(i) stand for the resistance of migration and capacity of the surface-passivating layer, respectively; Rct(i) and Cdl(i) designate the Li+ charge-transfer resistance and double-layer capacitance at the interface between electrolyte and electrode, respectively; and ZW(i) represents the diffusion-controlled Warburg impedance (i = 1, 2) [57]. The correspondence between frequency-dependent impedance and equivalent circuit elements can be logically distinguished after fitting to the equivalent circuit (Fig. 7(b)). It is evident that charge transfer resistance (Rct) of Fe2O3 nanorod electrode is much smaller than that of Fe2O3 nanoparticle. According to the curve fitting, the impedance of Rct is 17.8 V for the Fe2O3 nanorod, whereas it increases to 24.0 V for the Fe2O3 nanoparticle. The SEM images of Fe2O3 nanorod and nanoparticle electrodes after 60 cycles are shown in Fig. 8. Although the Fe2O3 nanorod doesn’t maintain the rod-like morphology, they still separate and are not agglomerated. The preserved morphology would still facilitate Li-ion and electron transportation, which is responsible for better cycle performance. After 30 charge–discharge cycles, the Fe2O3 nanoparticle is pulverized and seriously agglomerated to larger particles. That reduces the contact area between active material and electrolyte, leading to quick capacity decay. 4. Conclusions

Fig. 7. (a) Nyquist plots of Fe2O3 nanorod and nanoparticle electrodes at the full charged state after 10 cycles at current density of 500 mA g 1 in the frequency range from 100 kHz to 10 mHz, (b) the equivalent circuit.

knowledge, Fe2O3 nanorod synthesized in our experiment shows better rate retention and cycling capability than those in the published works before [30,32,36,45,54,55]. Fig. 6(d) shows the representative discharge–charge voltage profiles of the Fe2O3 nanorod electrode at various current densities ranging from 100 mA g 1 to 2000 mA g 1. When the current density increases, the overpotential of the electrode is higher with lower discharge plateau and higher charge potential, which are due to the kinetic effects of the material [56]. However, the discharge and charge curves at different current densities still keeps a similar shape and

Uniform Fe2O3 nanorods were synthesized by combustion synthesis method with alcohol as both solvent and fuel. This method is rather simple and can synthesize hematite nanostructure with good performance in a very short time and in large scale. The Fe2O3 nanorod electrode displays optimal electrochemical performances, which can be owing to its relatively high surface area, good contact of active materials and electrolyte. The environmentally benign, high-efficiency, and facial combustion strategy proposed in our work is suggested as a promising method for preparing energy storage materials. Acknowledgments This work is supported by the National Science and Technology Support Program (2012BAC08B08), the Program for Innovative Research Team in University of Ministry of Education of China

Fig. 8. SEM images of the Fe2O3 nanorod and nanoparticle after 60 cycles at current density of 500 mA g 1.

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