B. Nageswara Raoa,
P. Ramesh Kumara,
O. Padmaraja,
M. Venkateswarlub and
N. Satyanarayana*a
aDepartment of Physics, Pondicherry University, Pondicherry-605014, India. E-mail: nallanis2011@gmail.com; Tel: +91-413-2654404
bR&D, Amara Raja Batteries, Tirupati-517520, India
First published on 7th April 2015
Hematite porous α-Fe2O3 nanostructures were prepared within 60 minutes by a rapid microwave assisted hydrothermal process. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy studies confirm the phase and structural co-ordination of α-Fe2O3, respectively. Scanning electron microscopy (SEM) images reveal the formation of well defined uniform shaped α-Fe2O3 nanospheres. The transmission electron microscopy (TEM) image shows the porous nature of the α-Fe2O3 nanospheres with an interconnected monocrystallites structure. Swagelok type lithium and sodium batteries were fabricated using the developed porous α-Fe2O3 nanostructures as a negative electrode material. As a negative electrode material in Li-ion cells, the porous α-Fe2O3 nanostructures deliver a second cycle discharge capacity of 1000 mA h g−1 at a 0.1 C rate. At a higher current density of 5 C, the porous α-Fe2O3 nanostructures exhibit a specific capacity of 264 mA h g−1. In the case of Na-ion batteries, the porous α-Fe2O3 nanostructures as negative electrode exhibit a reversible capacity of 300 mA h g−1 with excellent cycleability and coulombic efficiency at 0.1 C up to 100 cycles. The observed excellent electrochemical performance is attributed to the porous nature and uniform shape of the α-Fe2O3 nanostructures that are composed of interconnected monocrystallites. Importantly, the architecture of α-Fe2O3 nanostructures, comprising interconnected monocrystallites could be developed within a short time by a rapid microwave synthesis process and it can be applied to develop different morphological nanostructures for better energy storage device applications.
Several innovative methods have been developed for the synthesis of nanostructured materials, such as sol–gel, quasi-emulsion-templated, electrostatic spray deposition, hydrothermal, etc.30–35 Sun et al. have reported the electrochemical performance of mesoporous nanostructured α-Fe2O3 obtained by soft template method. The prepared mesoporous α-Fe2O3 delivered a higher specific capacity.36 As reported by D. Chen et al., α-Fe2O3 nanoparticles prepared by a simple wet chemistry route showed good electrochemical performance.26 However, longer reaction times, nonuniform distribution of particles and low yield are recognized as the limitations of above preparation methods. Recently, microwave assisted hydrothermal/solvothermal method has been recognized as a potential technique to overcome the above mentioned limitations. Microwave synthesis has many advantages: (i) shorter reaction times (it can reduce the reaction times from several hours to minutes), (ii) low temperature synthesis, (iii) environmentally friendly and energy saving, (iv) high productivity and efficiency, (v) uniform distribution of particles, (vi) better reproducibility and large scalability, (vii) excellent control over experimental parameters (provides better understanding of the influence of synthesis conditions on nucleation and growth mechanism), (viii) its a versatile approach and can be used effectively and efficiently to develop nanocrystalline materials with different morphologies.37–39 A. V. Murugan et al. have reported the microwave assisted hydrothermal and solvothermal synthesis of LiFePO4 nanorods as positive electrode material for Li ion batteries.40 T. Muraliganth et al. group could prepare nanostructured Li2MSiO4 (M = Fe, Mn) positive electrodes for Li ion batteries by facile microwave assisted solvothermal process.41 More recently, nanostructured metal oxides (Co3O4/graphene, Mo3O4/graphene, W0.4Mo0.6O3 and NiO) prepared by microwave synthesis method exhibited good electrochemical performance as negative electrodes for Li ion batteries.42–45 As reported by K. Chen et al., hematite Fe2O3 cube-like structures prepared by microwave hydrothermal route exhibited a high discharge capacity as negative electrode for Li ion battery.46 R. Indrajothi et al. and G. Y. Zhang et al. studies revealed that the mesoporous nanostructures are formed in the presence of base catalysts.23,47 Also, it is revealed that the porous nanostructures are effective in enhancing the electrochemical performance of Li ion batteries.23,36 Hence, we are motivated to develop porous α-Fe2O3 nanostructures in the presence of base catalyst (NaOH) by microwave assisted hydrothermal method and study their efficiency as negative electrode in LIBs and SIBs. For the first time, we report the synthesis of porous α-Fe2O3 nanostructures by using a simple inorganic sodium hydroxide (NaOH). The synthesized sample was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) techniques. Swagelok cell type lithium and sodium batteries were fabricated using the newly developed porous α-Fe2O3 nanostructures as negative electrode material. The electrochemical performance of the fabricated lithium and sodium batteries using the newly developed nanocrystalline α-Fe2O3 sample was studied by cycling the cells in the voltage range 0.002–3 V, at different C-rates, at ambient conditions.
Fig. 1 Schematic mechanism for the growth of porous α-Fe2O3 nanostructures by microwave assisted hydrothermal process. |
For the fabrication of the composite electrode, 70% active material, 20% super P carbon and 10% Na-alginate in deionized water as solvent were used. The above mixture was grounded several hours to obtain a well mixed slurry. The slurry was coated onto a Cu-foil and dried at 100 °C overnight. The dried foil was cut into circular electrode disks. The Swagelok-type cells were assembled in an argon filled glove box (Vacuum Atmospheres Co., USA) by using Whatman GF/D borosilicate glass-fiber as separator. Lithium metal (for lithium ion battery) and sodium metal (for sodium ion battery) were used as reference electrodes. 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 in volume) was used as electrolyte for lithium ion battery (LIB). 1 M NaClO4 in propylene carbonate (PC) was used as electrolyte for sodium ion battery (SIB). The cyclic voltammetry (CV) curves were recorded at 0.2 mV s−1 scan rate, at ambient conditions, on a Novocontrol electrochemical test station (POT/GAL). The cycling performance of the cells was recorded at different C-rates on a battery testing system (model MCV4, Bitrode, USA) in the voltage range 0.002–3 V, at ambient conditions.
D = 0.9λ/βcosθ | (1) |
Fig. 2 (a) XRD pattern, (b) FTIR spectrum and (c) Raman spectrum of the as-synthesized α-Fe2O3 sample. |
Fig. 2b shows the FTIR spectrum of as-prepared α-Fe2O3 sample. As shown in Fig. 2b, the IR peaks observed at 480 cm−1 and 573 cm−1 are attributed to the stretching vibrations of Fe–O in FeO4 tetrahedron and FeO6 octahedron, respectively.49 Raman spectrum of the as-prepared α-Fe2O3 sample is shown in Fig. 2c. From Fig. 2c, the Raman scattering peaks observed at 211, 279, 393, 480 and 593 cm−1 are assigned to two A1g and three Eg Raman modes in a typical hematite α-Fe2O3 structure.50
The field-emission scanning electron microscopy (FE-SEM) images of α-Fe2O3 sample are shown in Fig. 3a and b. The FE-SEM images show the formation of uniformly distributed nanospheres. The nanospheres have a diameter of about 350 nm. The FE-SEM images reveal that the uniform heating of the precursor solution by microwave irradiation leads to the formation of uniformly distributed α-Fe2O3 nanospheres. The transmission electron microscopy (TEM) image (Fig. 3c) clearly shows the porous structure of α-Fe2O3 nanospheres and the nanospheres are consisted of densely packed small nanoparticle subunits. The small nanoparticle subunits have particle size distribution in the range of 20–30 nm. The size of small particle subunits is comparable to the average crystallite size deduced from XRD analysis, which may indicate that each subunit is a monocrystallite and the nanospheres are composed of interconnected monocrystallites.
Fig. 4 shows the cyclic voltammetry (CV) curves at different cycles with 0.2 mV s−1 scan rate obtained at room temperature for the as prepared α-Fe2O3 sample. From Fig. 4a, in the first cycle of cathodic polarization process, a spiky peak was observed at 0.54 V vs. Li/Li+, ascribed to the complete reduction of iron from Fe3+ to Fe0, formation of Li2O and decomposition of the electrolyte to form solid electrolyte interface (SEI) film.12,51 During the first anodic scan, two broad overlapping peaks were observed at 1.65 V and 1.83 V, which correspond to the oxidation of Fe0 to Fe2+ and further oxidation to Fe3+.12,36 The observed redox peaks are comparable to the literature reports.12,36,51 Compared to 1st cycle, in 5th, 10th, 15th and 20th cycles, the anodic polarization shows two broad peaks with decrease in peak intensity, which indicates good reversible reaction of Fe0 to Fe2+ and Fe2+ to Fe3+. Whereas, the cathodic polarization shows one peak at 0.8 V with decrease in peak intensity and area, indicating the irreversible phase transformation with the formation of SEI film in the first cycle.12,52 It reveals that the oxidation and reduction processes proceed with some irreversibility. The observed decrease in peak intensity implies the decrease of capacity with cycling. During 5th–20th cycles, the current of the anodic and cathodic peaks remains nearly the same, suggesting good reversibility and structure stability during charge–discharge reactions.
Fig. 4 Cyclic voltammograms of (a) α-Fe2O3 nanospheres vs. Li/Li+ and (b) α-Fe2O3 nanospheres vs. Na/Na+ at a scan rate of 0.2 mV s−1. |
From Fig. 4b, during the first cathodic scan, α-Fe2O3 exhibits three different reductive peaks at 1.1, 0.8 and 0.4 V vs. Na/Na+. The observed reduction peaks correspond to initial insertion of Na+ into α-Fe2O3, which leads to the formation of NaxFe2O3 compound and a conversion of the iron oxide accompanied by SEI film formation.1,53 In the first anodic scan, two weak peaks were detected around 0.7 and 1.5 V vs. Na/Na+, corresponding to the double oxidation process (i.e. Fe0/Fe2+ and Fe2+/Fe3+) that enabled iron oxide regeneration.1 From 5th–20th cycles, the cathodic peaks at 1.1 and 0.8 V were disappeared and the peak at 0.4 V shifted to 0.6 V with reduced peak intensity, which indicate some irreversible phase transformation due to SEI film formation in the first cycle. Anodic scan shows two peaks with decreased peak intensities, which represent the good reversible double oxidation of iron. The α-Fe2O3 structure facilitates the intercalation process before the conversion reaction due to the stable interface while using Na-alginate binder.54
The charge–discharge curves of α-Fe2O3 sample vs. Li/Li+, collected at 0.1 C-rate are shown in Fig. 5a. In the first cycle of the discharge curve, three distinct regions were observed and labelled as I, II and III. Initially, the cell was discharged from open circuit voltage (OCV) to insert Li+ ions into α-Fe2O3 structure as given in eqn (2).22 In region I, a plateau with slope is observed at 0.96 V, which corresponds to the phase transformation from hexagonal LixFe2O3 to cubic Li2Fe2O3 (eqn (3)).12,21,22,36 In II region, a potential plateau is observed at 0.85 V, which is due to the complete reduction of iron from Fe3+ to Fe0 (eqn (4)).12,21,22 It indicates the dispersion of Fe nanocrystals into Li2O matrix. The III region attributes to the reduction of electrolyte below 0.8 V and formation of SEI film.12 An interfacial reduction at the Fe–Li2O boundary leads to further lithium storage, which results in higher discharge capacity of 1364 mA h g−1 (8.1 moles of lithium per one mole of α-Fe2O3) than the theoretical capacity of 1007 mA h g−1 (6 moles of lithium per one mole of α-Fe2O3).
Fe2O3 + xLi+ + xe− → LixFe2O3 | (2) |
LixFe2O3 + (2 − x)Li+ + (2 − x)e− → Li2Fe2O3 (cubic) | (3) |
Li2Fe2O3 (cubic) + 4Li+ + 4e− → 2Fe + 3Li2O | (4) |
Fig. 5 Galvanostatic charge–discharge curves (a) and cycleability (b) of α-Fe2O3 sample at 0.1 C rate vs. Li/Li+. |
During the 10th, 20th and 30th cycles, only one potential plateau was observed with a decrease in discharge capacity. The discharge capacities of α-Fe2O3 in the 1st, 10th, 20th and 30th cycles are 1364, 941, 876 and 799 mA h g−1, respectively. Fig. 5b shows the cycling performance of α-Fe2O3 sample at 0.1 C-rate. As shown in Fig. 5b, the material delivers a first cycle discharge and charge capacities of 1364 and 1031 mA h g−1, respectively, with 24.5% irreversible capacity loss and a second cycle discharge capacity of 1000 mA h g−1. M. V. Reddy et al. have synthesized the α-Fe2O3 nanoflakes by thermal treatment method and the obtained α-Fe2O3 nanoflakes delivered a stable discharge capacity of (680 ± 20) mA h g−1, at 0.1 C-rate.25 L. Ji et al. have studied the electrochemical performance of α-Fe2O3 nanoparticle-loaded carbon nanofibers composite synthesized by electrospinning method. The results showed that the 2nd cycle discharge capacity is <600 mA h g−1, at 50 mA g−1.29 In our study, the porous α-Fe2O3 nanostructures synthesized by rapid microwave assisted hydrothermal method delivered a 2nd cycle discharge capacity of 1000 mA h g−1 at 0.1 C, which is 30% higher than the stable discharge capacity of α-Fe2O3 nanoflakes.25 In addition, the 2nd cycle discharge capacity of porous α-Fe2O3 nanostructures is 63% higher than the theoretical capacity of graphite (372 mA h g−1), which is currently used as negative electrode material in commercial Li-ion batteries. It can also be observed that the material shows identical charge and discharge capacity values in 30 cycles, which indicates an excellent reversibility of the material.
Further, the α-Fe2O3 cells were cycled at different (1, 5, 10 and 20 C) C-rates and the corresponding capacity vs. voltage curves are shown in Fig. 6a. The observed discharge capacities at 1, 5, 10 and 20 C-rates are 590, 267, 157 and 63 mA h g−1, respectively. The cycleability of α-Fe2O3 sample at 1, 5, 10 and 20 C rates is compared in Fig. 6b. From Fig. 6b, a fade of capacity is observed with increase of C-rate from 1 to 5, 10, and 20 C. At 1 C, the discharge capacity lost per cycle is 1.7%, and 0% capacity lost per cycle is observed at 5, 10 and 20 C-rates. The decrease in the rate of capacity fade per cycle with increase of C-rate indicates an enhanced reversibility at high C-rates. The comparison of specific capacity at 1 C-rate (Fig. 6b) shows that the porous α-Fe2O3 nanostructures could be able to retain their initial capacity values even after cycling at higher rates, which indicates an excellent reversibility of the material.
Fig. 6 Voltage vs. specific capacity curves (a) and cycling performance (b) of α-Fe2O3 at different (1, 5, 10 and 20 C) rates vs. Li/Li+. |
Fig. 7 shows the charge–discharge curves and cycle life of α-Fe2O3 vs. Na/Na+, at 0.1 C-rate. From Fig. 7a, the first cycle discharge curve displayed all the characteristic features of sodiation of iron oxide and confirmed the previous cyclic voltammetry (CV) results in Fig. 4b. As shown in Fig. 7a, the first cycle discharge curve exhibits the potential plateaus, indicating the Na+ insertion, reduction of iron and SEI film formation.1 From Fig. 7b, the material shows a high stable reversible capacity of 300 mA h g−1 with excellent cycleability and coulombic efficiency over 100 cycles. An earlier report on an electrodeposited thin films of Fe2O3 cycled at 5 μA cm−2, between 0.05 and 3.0 V vs. Na/Na+ completely lost its capacity in 8 cycles.54 Recently, M. Valvo et al. report on Fe2O3 particles cycled between 0.05 and 2.8 V vs. Na/Na+, at 130 mA g−1 exhibited a capacity of 250 mA h g−1 after 60 cycles.55 In the present study, the porous α-Fe2O3 nanostructures cycled at 0.1 C-rate exhibited high reversible capacity of 300 mA h g−1 and an excellent cycleability up to 100 cycles. Fig. 8 shows the rate capability of α-Fe2O3 sample at different C-rates. From Fig. 8b, the observed discharge capacities at 1, 2, 5 and 10 C-rates are 150, 105, 32 and 14 mA h g−1, respectively. From Fig. 8b, it can be seen that the porous α-Fe2O3 nanostructures exhibit stable cycleability at high current densities of 1, 2, 5 and 10 C. Even after cycling at higher C-rates, the material is capable to retain the initial capacity values at 1 C rate, which indicates an excellent reversibility of the material.
Fig. 8 Voltage vs. specific capacity curves (a) and cycling performance (b) of α-Fe2O3 at different (1, 2, 5 and 10) C rates vs. Na/Na+. |
The good cycleability and rate capability of porous α-Fe2O3 nanostructures may be attributed to their uniform shape, porous nature and the interconnected monocrystallites structure, which facilitate the better electron and ion transfer in the Li or Na ion batteries. Generally, lithium or sodium ion batteries include both electron and Li+ or Na+ ion transfer during charge–discharge process. A schematic diagram for the transport of electrons and Li+ or Na+ ions through the interconnected monocrystals of the porous α-Fe2O3 nanostructures is shown in Fig. 9. The electron transfer takes place through the grain boundaries and the interconnected structure of α-Fe2O3 nanocrystals is expected to enhance the electron kinetics. The interconnected structure of TiO2 nanocrystals also exhibited good rate capability and cycleability as reported by J. Shen et al.56 The small particle subunits or monocrystallites reduce the diffusion lengths and provide a fast and efficient transport for Li+ or Na+ ions. The electrolyte filled in the pore network will enable the rapid transport of ions. In addition, the pores also provide room for facile strain relaxation during charge–discharge processes, which leads to an improved cycleability. The improved kinetic properties of electrons and Li+ or Na+ ions are responsible for an excellent cycleability and rate capability of the α-Fe2O3 electrode. The results show that microwave assisted hydrothermal process is a potential technique to develop morphology controlled α-Fe2O3 in short time with good electrochemical performance in both Li and Na ion battery applications.
Fig. 9 Schematic representation of electron and Li+ or Na+ ion transport through the interconnected monocrystallites of α-Fe2O3 nanospheres electrode. |
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