Rapid microwave assisted hydrothermal synthesis of porous α-Fe₂O₃ nanostructures as stable and high capacity negative electrode for lithium and sodium ion batteries

Abstract

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.

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1. Introduction

Lithium ion batteries (LIBs) with high energy densities and high voltages have become the main rechargeable power source for portable electronic devices and, more recently, for electric vehicles (EVs) and hybrid electric vehicles (HEVs).¹ However, the scarcity of lithium resources in the Earth's crust and high processing cost have limited their large scale applications.^2,3 In this regard, sodium ion batteries (SIBs) are found to be an attractive alternative to LIBs because of the high abundance and low cost of sodium.^4,5 Furthermore, the similarities between Li⁺ and Na⁺ intercalation reactions favour the use of analogous materials, which could match both the applications.¹ The ionic radius of Na⁺ (0.95 Å) is almost 1.6 times larger than that of Li⁺ (0.60 Å), which undoubtly restricts the development of electrode materials for SIBs. In recent years, many positive electrode materials have been investigated for both LIBs and SIBs.^6–11 So far, a few varieties of materials, as a viable negative electrode, have been developed and studied for LIBs and SIBs. Graphite usage as negative electrode in LIBs has limitations, such as low Li storage capability (372 mA h g⁻¹) and safety problems due to lithium plating on its surface.¹² Also, it is not suitable as negative electrode in SIBs, because of its inability to intercalate Na atoms, dendrites formation and safety issues.^13,14 Thus, the development of new negative electrode materials with high storage capacity and power density has become an urgent task to meet the ever growing performance demand. Recently, metal oxides (NiO, CuO, Mn₂O₃, Co₃O₄, TiO₂, α-Fe₂O₃, etc.) have received great interest as negative electrode materials in LIBs and SIBs,^1,15–19 because of their improved electrochemical performance. Hematite α-Fe₂O₃ has attracted much interest as negative electrode material in both LIBs and SIBs, because of its low cost, high stability under ambient conditions, non-toxicity, high resistivity towards corrosion and environmentally benign.²⁰ The performance of α-Fe₂O₃ as negative electrode in secondary batteries strongly depends on the structure, particle size and morphology.^21,22 Recently, nanostructured porous materials have paid much attention as negative electrodes in LIBs because, the nanosize particles provide shorter diffusion lengths for ions and high surface area contact with electrolyte for electron transfer, leading to an increased rate capability, while the porous nature provides facile strain relaxation during charge–discharge reactions, leading to an improved cycleability.^23,24 So, research efforts have been focused for the development of nanostructured porous materials with different morphologies, such as nanospheres, nanorods, nanowires, nanofibers, etc.^25–29

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, W_0.4Mo_0.6O₃ 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.

2. Experimental

2.1 Synthesis

Hematite porous α-Fe₂O₃ nanostructures were prepared by using ferric chloride (FeCl₃) and sodium hydroxide as the starting materials. In the typical process, 0.008 moles of FeCl₃ (Qualigens) was dissolved in 50 ml of deionized water at room temperature under stirring condition. Then, 0.50 g (0.0125 moles) of solid NaOH (CDH chemicals, LR) pellets were added to the above solution and continued stirring for 10 minutes. The obtained solution was transferred to a 100 ml Teflon liner vessel and the vessel was covered with an outer high strength sleeve made up of advanced composite material. The vessel was sealed and placed on a rotating table inside the microwave accelerated reaction system (MARS, CEM Corporation, USA). A temperature sensor was inserted into the reaction vessel to monitor the vessel temperature and the complete system was maintained at 140 °C for 60 minutes for microwave hydrothermal reaction. After the reaction, the vessel was allowed to cool down to room temperature and the precipitated product was washed with water followed by acetone washing through high speed centrifugation. The obtained powder was dried at 60 °C overnight and subsequently heated at 350 °C for 1 h in air. The schematic representation of the reaction mechanism for the formation of porous nanostructured α-Fe₂O₃ particles in the presence of base catalyst (NaOH) under microwave irradiation is shown in Fig. 1. The usage of NaOH, as a base catalyst, and microwave irradiation, leading to the fast formation of ultrafine (nanosize) α-Fe₂O₃ spherical particles. The addition of base catalyst (NaOH) can accelerate the hydrolysis reaction and may form ultrafine α-Fe₂O₃ particles.⁴⁸ In the subsequent process, to minimize the surface free energy of the system, the ultrafine α-Fe₂O₃ particles may continuously assemble into the porous spherical structure. Further, the drying process leads to the formation of densified porous α-Fe₂O₃ nanospheres structure. The formation of ultrafine (nanosize) α-Fe₂O₃ particles (20–30 nm) and agglomerated porous nanostructured α-Fe₂O₃ particles (∼350 nm) are further confirmed from the observed FESEM/TEM images, as shown in Fig. 3.

Fig. 1 Schematic mechanism for the growth of porous α-Fe₂O₃ nanostructures by microwave assisted hydrothermal process.

2.2 Characterization

The crystalline phase of the as-obtained product was identified using X-ray powder diffractometer (X'pert PRO MPD, PANalytical, Philips) with Cu Kα radiation of wavelength λ = 0.154 nm. The structural co-ordination of the sample was studied using Fourier transform infrared (FTIR) spectroscopy (Shimadzu FTIR-8000 spectrometer). Raman spectrum of the obtained product was recorded at room temperature on Renishaw in Via Raman Microscope (Ar–Ne Laser source, 514 nm line radiation). The morphology of the synthesized sample was examined on field-emission scanning electron microscopy (FE-SEM, HITACHI, S4800, Japan) and transmission electron microscopy (Tecnai, G2 F30, FEI with 300 kV).

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.

3. Results and discussion

Powder X-ray diffraction pattern of α-Fe₂O₃ sample along with standard JCPDS data are shown in Fig. 2a. From Fig. 2a, the observed diffraction peaks indicate the formation of hematite type rhombohedral structured α-Fe₂O₃ with R3̄c space group and it was confirmed by comparing the observed diffraction pattern with the standard JCPDS data (card no. 87-1166). The observed diffraction peaks at 24.1°, 33.2°, 35.6°, 49.5°, 54.1° and 64.1° are respectively indexed to (012), (104), (110), (024), (116) and (330) planes, which also confirm the formation of α-Fe₂O₃ with rhombohedral structure. The average crystallite size of the sample was calculated using Scherrer's formula

D = 0.9λ/β cos θ (1)

where, λ is the X-ray wavelength and β is the full-width at half maximum (FWHM) of the diffraction line. The average crystallite size of the α-Fe₂O₃ sample is found to be 28 nm.

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 A_1g and three E_g 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. 3 FE-SEM images (a and b) and TEM image (c) of α-Fe₂O₃ sample.

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 1^st cycle, in 5^th, 10^th, 15^th and 20^th 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 5^th–20^th 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 Na_xFe₂O₃ 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 5^th–20^th 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 Li_xFe₂O₃ 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⁻ → Li_xFe₂O₃ (2)

Li_xFe₂O₃ + (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 10^th, 20^th and 30^th cycles, only one potential plateau was observed with a decrease in discharge capacity. The discharge capacities of α-Fe₂O₃ in the 1^st, 10^th, 20^th and 30^th 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 2^nd 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 2^nd 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 2^nd 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. 7 Charge–discharge curves (a) and cycleability (b) of α-Fe₂O₃ sample at 0.1 C rate vs. Na/Na⁺.

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.

4. Conclusions

Hematite porous α-Fe₂O₃ nanostructures were prepared in short time by rapid microwave assisted hydrothermal process. The XRD studies confirm the rhombohedral structure of the prepared α-Fe₂O₃ sample. The formation of porous α-Fe₂O₃ nanostructures with uniform particle size were confirmed from FE-SEM and TEM results. For use as negative electrode in Li-ion batteries, the prepared porous α-Fe₂O₃ nanostructures show a discharge capacity of 799 mA h g⁻¹ for the 30^th cycle at 0.1 C rate. Furthermore, porous α-Fe₂O₃ nanostructures vs. Na/Na⁺ show a stable discharge capacity of 300 mA h g⁻¹ with excellent cycleability over 100 cycles. The morphology of α-Fe₂O₃ nanospheres with porous nature and interconnected monocrystallites structure is responsible for good electrochemical performance in both Li and Na ion batteries.

Acknowledgements

NS is grateful to UGC, Govt. of India for providing financial support in the form of research project [no. 39-460/2010(SR)]. BNR is thankful to DST, Govt. of India for awarding the INSPIRE fellowship (no. DST/INSPIRE Fellowship/2011/241) for pursuing the doctoral degree. Authors thankful to central instrumentation facility (CIF), Pondicherry University for providing Raman Spectroscopy facility.

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