Hydrothermal synthesis, self-assembly and electrochemical performance of α-Fe2O3 microspheres for lithium ion batteries

Hydrothermal synthesis, self-assembly and electrochemical performance of α-Fe2O3 microspheres for lithium ion batteries

Available online at www.sciencedirect.com CERAMICS INTERNATIONAL Ceramics International 40 (2014) 10283–10290 www.elsevier.com/locate/ceramint Hydr...

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Available online at www.sciencedirect.com

CERAMICS INTERNATIONAL

Ceramics International 40 (2014) 10283–10290 www.elsevier.com/locate/ceramint

Hydrothermal synthesis, self-assembly and electrochemical performance of α-Fe2O3 microspheres for lithium ion batteries Hanfeng Liang, Wei Chen, Yiwen Yao, Zhoucheng Wangn, Yong Yang College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China Received 21 December 2013; received in revised form 24 February 2014; accepted 27 February 2014 Available online 7 March 2014

Abstract In this paper, we report the hydrothermal synthesis, structural characterization and electrochemical properties of uniform α-Fe2O3 microspheres. In the synthetic procedure, the surfactant, sodium citrate, was found to be crucial to the formation of α-Fe2O3 microspheres. In addition, by simply varying the concentration of sodium citrate, cylindrical α-Fe2O3 microstructures could be readily obtained. On the basis of time-dependent experimental result, a possible formation mechanism of the α-Fe2O3 microspheres was proposed. The as-obtained α-Fe2O3 microspheres were then evaluated as an anode material for lithium ion batteries. And the result shows that the material exhibited an initial discharge capacity as high as 1453.4 mAh g  1. After 50 cycles, the reversible discharge capacity was 489.5 mAh g  1, which was much higher than that of commercial graphite. This work provides an additional strategy to prepare self-assembled structures with tailored morphology. And the results are helpful to further understand the mechanism of self-organization and expand the applications of iron oxide micromaterials. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Hematite; Self-assembly; Electrochemical properties; Lithium-ion battery

1. Introduction The fabrication of micro- and nano-structural materials with controlled size and tailored shape has been attracting growing interest since the physiochemical properties of materials are largely determined by their size and shape [1]. Therefore, the ability to manipulate the size and morphology of micro- and nano-materials is important for discovering and making use of their novel properties. In particular, it has become a hot topic to assemble low dimensional nanosized materials into organized and designed structures through self-assembly, for the assemblies usually exhibit novel properties different from their bulk or discrete counterparts and therefore show potential applications in many areas [2]. The self-assembly is a spontaneous process, and has become an efficient “bottomup” route for the fabrication of complicated architectures [3]. For instance, hierarchical α-Fe2O3 [4], ZnO [5], Bi2WO6 [6], n

Corresponding author. Tel.|fax: þ 86 592 218 0738. E-mail address: [email protected] (Z. Wang).

http://dx.doi.org/10.1016/j.ceramint.2014.02.120 0272-8842 & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

TiO2 [7] and CdMoO4 [8] have been successfully prepared. Nonetheless, it still remains a big challenge to develop simple and reliable synthetic methods for self-assembled architectures with controlled morphology. As the most stable iron oxide under ambient conditions, hematite (α-Fe2O3) has been extensively studied and widely used as catalysts [9,10], pigments [11], and gas sensors [12,13], owing to its low cost, high resistance to corrosion, and nontoxicity. Furthermore, it has also been shown to act as an anode material for lithium ion batteries (LIB) [14–19]. It can react with six Li per formula unit (Fe2O3 þ 6Li þ þ 6e  2 2Fe þ 3Li2O), exhibiting good battery characteristics with a very high theoretical capacity of  1007 mAh g  1, which is much higher than that of commercial carbonaceous anode materials (e.g. maximum of 372 mAh g  1 for graphite). To date, various iron oxide structures, such as 1D (spindles, rings, fibers) [20–22], 2D (snowflake-like, plates) [23,24], and 3D (flower-like, spheres) [4,13] structures have been prepared, expecting to enhance their performance in currently existing applications. Nonetheless, the self-assembly of low-dimensional building units

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into complex three-dimensional ordered structures is still considerably difficult. To understand the mechanism of selforganisation and expand the applications of iron oxide materials, self-assembled iron oxide 3D structures need to be explored in more detail. Herein, we report the preparation of self-assembled α-Fe2O3 microspheres via a sodium citrate-assisted hydrothermal route. To obtain the metal oxides by hydrothermal reactions, a basic environment is normally required. In general, the basic environment is achieved by employing NaOH, urea or ammonia. For example, Wang et al. demonstrated the synthesis of porous hematite nanospheres using urea as precipitator [13]. While in this work, NaOH and urea were used simultaneously. We previously demonstrated that in a NaOH and urea co-exist system, the NaOH could provide the basic environment while the urea might benefit the self-assembly of flower-like α-Fe2O3 microstructures [4]. Herein, we used a similar procedure, aiming to prepare self-assembled α-Fe2O3 microstructures but with different morphologies with the assistance of surfactants (sodium citrate in this case). As expected, self-assembled α-Fe2O3 microspheres were obtained by the hydrothermal reaction. And it was found that the surfactant had a great influence on the morphology of the product. By simply varying the concentration of sodium citrate, α-Fe2O3 microcylinders could be readily obtained. On the basis of experimental result, a possible formation mechanism of α-Fe2O3 microstructures was proposed. Furthermore, the electrochemical properties of the α-Fe2O3 microspheres were also investigated.

microscopy (SEM) images were obtained using a LEO 1530 microscope (Germany). Transmission electron microscopy (TEM), High-resolution TEM (HRTEM) images and corresponding selected area electron diffraction (SAED) patterns were taken on a JEOL JEM-2100 transmission electron microscope at an accelerating voltage of 200 kV. 2.3. Electrochemical measurements

2. Experimental details

For electrochemical measurements, a typical slurry was obtained by grinding a mixture of α-Fe2O3, carbon black and polyvinylidene fluoride (PVDF) dissolved in N-methyl-2pyrrolidinone (NMP) with a weight ratio of 70:15:15. It was then spread onto a piece of copper foil (1 cm2) to form an electrode. After the electrode was dried at 100 1C for 4 h under vacuum, it was compressed at a rate of about 150 kg cm  2 and then weighed. The electrochemical behavior of the material was examined via CR 2025 coin cells with a lithium metal counter electrode, Celgard 2700 membrane separator, and an electrolyte consisting of 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and methyl ethyl carbonate (EMC) (1:1 by volume). The cells were assembled in an argon-filled glove box (Mbraun, Unilab, Germany). Galvanostatic charge– discharge experiments were carried out between a cell voltage of 0.01–3.0 V and at a current density of 100 mA g  1. Cyclic voltammetric (CV) curves were collected at 0.1 mV s  1 within the range of 0.01–3.0 V. In the electrochemical impedance spectroscopy (EIS) measurement, the amplitude of the alternating voltage signal was 5 mV and the frequency range was between 100 kHz and 10 mHz. Both the CV and EIS measurements were carried out on an electrochemistry workstation (CHI660C).

2.1. Synthesis

3. Results and discussion

All reagents were of analytical grade and used without further purification. In a typical experiment, 1 mmol of K3[Fe(CN)6] and 1.7 mmol of sodium citrate were dissolved in 10 mL of deionized water to form a homogeneous solution. Subsequently, 10 mL NaOH aqueous were introduced into the above homogeneous solution. The final concentrations of the raw materials in solution are K3[Fe(CN)6], 50 mM; sodium citrate, 85 mM; and NaOH, 150 mM. The whole mixture was irradiated by ultrasonic waves for 5 min and then transferred into a Teflon-lined stainless steel autoclave of 50 mL capacity, sealed and maintained at 180 1C for 12 h. When the reaction was completed, the autoclave was cooled to room temperature naturally. The product was filtered and washed several times with distilled water and absolute ethanol, and finally dried in a vacuum oven at 60 1C for 12 h.

The crystal structure and phase purity of the α-Fe2O3 microspheres were examined by X-ray diffraction. Fig. 1 displays a representative XRD pattern of the as-prepared α-Fe2O3 sample. All the diffraction peaks can be perfectly indexed as the hematite α-Fe2O3 with cell constants a ¼ b ¼ 5.038 Å and c ¼ 13.78 Å (space group: R 3c), which are consistent with the values in the literature (JCPDS Card no. 89-0598). The strong diffraction peaks reveal that the as-prepared sample is well crystallized. The size and morphology of the products were examined by SEM measurements. Fig. 2a shows a typical SEM image of the as-prepared sample, which indicates the high-yield growth and good uniformity of the product. Fig. 2b presents a high magnification SEM image of microspheres, which clearly demonstrates that the microspheres are with a diameter of ca. 6.8 μm. Careful observation also finds that these microspheres are with highly rough surfaces, which consists of many randomly arranged rice-like particles with thicknesses of ca. 600 nm and lengths of ca. 1.5 μm. Deep crevices (ca. 400 nm) were naturally formed between these particles. The morphology and structure of the α-Fe2O3 sample were further detected by TEM, SAED, and HRTEM. From the TEM image of one

2.2. Characterization The phase purity of the products was examined by X-ray powder diffraction (XRD) using an X’pert PRO X-ray diffractometer at a voltage of 40 kV and a current of 30 mA with Cu Kα radiation (λ ¼ 1.54056 ˚). Scanning electron

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individual α-Fe2O3 microsphere shown in Fig. 2c, the spherical morphology and rough surface can be seen clearly, which agrees with the SEM result. Fig. 2d shows a closer TEM observation of the area marked by square A in Fig. 2c. Bright and dark stripes alternating with each other can be observed as indicated by the arrow, which further confirms the surface of the microspheres is consist of many rice-like particles. The corresponding SEAD pattern (inset of Fig. 2d) clearly suggests a single-crystal nature of α-Fe2O3 particles. Fig. 2e shows the corresponding HRTEM image of the area marked by square B in Fig. 2d. The lattice fringes are clear visible with a spacing of 0.27 nm, corresponding to the spacing of the (104) planes of α-Fe2O3. To understand the growth mechanism of the microspheres, we carried out time-dependent experiments, and the SEM

Fig. 1. XRD pattern of the as-prepared α-Fe2O3 microspheres.

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images of the products obtained at different reaction time are shown in Fig. 3. As shown in Fig. 3a, at the early stage of the hydrothermal reaction, the product is composed of many microplates. When the reaction time is prolonged to 8 h, two other architectures, flower-like and spherical microstructures, can be observed as indicated by arrow 1 and arrow 2 in Fig. 3b, respectively. Further increasing the reaction time to 10 h, the amount of microspheres increases at the expense of the microplates and flower-like microstructures as shown in Fig. 3c, indicating a process of morphological evolution. Eventually, the sample is composed almost entirely of microspheres (Fig. 3d). From this point, the size and morphology of the product remains the same even at a longer reaction time. It is well accepted that the usage of templates can induce the formation of hierarchical and complex micro- and nanostructures. In this work, sodium citrate was found to be essential to the formation of α-Fe2O3 spherical microstructures. The reaction performed in the absence of sodium citrate only yields irregular crystals (Fig. 4a). To understand the role that sodium citrate played, several experiments involving different dosages of sodium citrate were performed. The result shows that microspheres are obtained under a low concentration of sodium citrate (e.g. 50 mM, Fig. 4b). In contrast, spindle- and cylinder-like microstructures can be formed if the concentration of sodium citrate is relatively high (e.g. 127 mM, Fig. 4c). While further increase in the concentration of sodium citrate (e.g. 212 mM) leads to the formation of microcylinders (Fig. 4d). It seems like the morphology of the product evolves from spherical to spindle-like and then to cylindrical as the concentration of sodium citrate increases. The structure of the α-Fe2O3 microcylinders was further investigated by SEM and TEM. Fig. 5a shows the SEM image of the α-Fe2O3 crystals prepared with 212 mM sodium citrate.

Fig. 2. (a and b) SEM images of α-Fe2O3 microspheres. (c) TEM image of one individual microsphere. (d) Enlarged TEM image of A area (indicated by a square in panel c), and the inset is the corresponding SAED. (e) High-resolution TEM image of B area (indicated by a square in panel d).

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Fig. 3. SEM observations of the products at different reaction stages: (a) 6 h, (b) 8 h, (c) 10 h, and (d) 12 h. All the scale bars are 10 μm.

Fig. 4. SEM images of the samples obtained after hydrothermal reaction at 180 1C for 12 h with different concentrations of sodium citrate at (a) 0 mM, (b) 85 mM, (c) 127 mM, and (d) 212 mM while the concentration of NaOH was fixed at 150 mM.

The cylindrical α-Fe2O3 particles are approximately 4 μm in length and 3.5 μm in diameter. Further observation shows that the surfaces of the cylinders are rough (Fig. 5b). From the TEM image of one individual α-Fe2O3 particle shown in Fig. 5c, the cylindrical morphology and rough profile can be seen clearly, which agrees with the SEM result. A closer TEM observation of the head part of an individual particle indicates that there are many octahedral nanoparticles attached to the

profile of the cylinder (inset of Fig. 5c). Fig. 5d shows the TEM image of a single octahedron. Noticeably, the corresponding SEAD pattern clearly suggests a single-crystal nature of the α-Fe2O3 octahedral nanoparticles (inset of Fig. 5d). Fig. 5e shows the HRTEM image taken from the area marked by square A. The typical lattice fringe spacing is determined to be 0.36 nm, which is close to the (012) lattice spacing of the rhombohedral hematite.

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Fig. 5. (a and b) SEM images of α-Fe2O3 microcylinders. (c) TEM image of one individual microcylinder. (d) Enlarged TEM image of the head part of microcylinder, and the inset is the corresponding SAED. (e) High-resolution TEM image of A area (indicated by a square in panel d).

As we know, citrate is an important biological ligand for metal ions. It can form strong complexes with metal ions (e.g. Bi3 þ and Co2 þ ions) [25,26]. In this system, the introduction of citrate is to form the complex with Fe3 þ ions, which helps to control the precipitation process. The formation process of α-Fe2O3 crystals includes the three steps [Eq. (1)], and the growth process may be determined by the first step because of the weak dissociation tendency of [Fe(CN)6]3  ions [27]. ½FeðCNÞ6 3  -Fe3 þ -FeOOH=FeðOHÞ3 -α  Fe2 O3

ð1Þ

During the synthetic process, on the one hand, the chelation of citrate with iron ions decreases the free Fe3 þ concentration in solution and results in a slow generation of Fe2O3 nanoparticles. The relatively slow reaction is favorable for separating the growth step from the nucleation step and responsible for the oriented growth [28]. On the other hand, as in many surfactant-assisted or ligand-mediated synthesis of shape-controlled materials [29,30], citrate can also serve as a shape modifier and controller, which may bind to certain crystal facets of the Fe2O3 particles. From the crystal structure, Fe2O3 can be described as a number of Fe3 þ and O2  arranged alternatively parallel to the c plane. The oppositely charged ions produce positively charged Fe3 þ and negatively charged O2  polar surfaces. In citrate solution, citrate ions presumably bind to the Fe3 þ polar planes through the COO– and –OH functions. This surface interaction can inhibit Fe2O3 crystals elongate perpendicular to these planes, resulting in the formation of the microplates, which is confirmed by the timedependent experiments of α-Fe2O3 microspheres. The similar phenomenon can be observed during the growth of ZnO hexagonal microplates [28]. This effect is much more prominent at high citrate concentrations. With the gradual growth of the microplates, the cylindrical α-Fe2O3 crystals are finally formed.

Fig. 6. Shape evolution of α-Fe2O3 crystals under hydrothermal condition with different concentrations of sodium citrate.

However, at low concentrations of citrate, the morphology of α-Fe2O3 crystals changes from plate-like to spherical by a selfassembly way. The interaction between surface-adsorbed citrate ions would be instead of that between the particles themselves, which may force the nanoparticles to be assembled. Thus, the formation of the α-Fe2O3 microspheres under hydrothermal conditions may result from the nucleation and continuous assembly of the particles assisted by sodium citrate [31]. On the basis of the investigations described above, it is possible to interpret the growth process of the α-Fe2O3 microstructures. As shown in Fig. 6, at an early reaction stage, the primary α-Fe2O3 nanocrystals are formed through conventional nucleation and a subsequent crystal growth process. The chelation of citrate with iron ions decreases the free Fe3 þ concentration in solution, which helps to separate the nucleation from the crystal growth process. And the growth rate of the nanocrystals is relative slow. The gradual attachment of constituent species onto the formed particles leads to the formation of anisotropic plates. In a low concentration of

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Fig. 7. The electrochemical properties of the as-prepared α-Fe2O3 microspheres. (a) Cyclic voltammograms in the range of 0.01–3.0 V (scanning rate 0.1 mV s  1). (b) Nyquist plots in the open circuit voltage before and after CV measurements. (c) First cycle charge–discharge profile. (d) The electrochemical cycling performance at current density of 100 mA g  1.

sodium citrate, instead of piling up to form columns [32,33], the microplates tend to wrinkle up and form the notched template for further development. The following crystal growth is then confined within the concave portion on both sides of the template, resulting in the formation of a flower-like microstructure. Similar phenomenon has also been observed in the formation process of the Co3O4 semiconvex superstructures [3]. As the reaction goes on, the flower-like microstructures become compact due to the self-assembly process. Finally, the flower-like microstructures are further assembled in the presence of sodium citrate, leading to the formation of microspheres. While in a high concentration of sodium citrate, with the gradual growth of the microplates along c plane, the cylindrical α-Fe2O3 crystals are finally formed. The as-prepared α-Fe2O3 microspheres were then evaluated as anode materials for lithium ion batteries. Fig. 7a shows the CV profile of electrode made from the as-prepared α-Fe2O3 microspheres. It can be seen that there is a substantial difference between the first and the subsequent cycles. For the first cycle, a spiky cathodic peak around 0.55 V can be observed, which is associated to the lithium insertion in the crystal structure of α-Fe2O3, the reduction of Fe3 þ to Fe0, and the irreversible reduction reaction of electrolyte [34]. Meanwhile, one main peak is recorded at about 1.60 V in the anodic process, corresponding to the reversible oxidation process of

Fe0 to Fe3 þ . For the second cycle, the peak intensities decrease; this can be attributed to the extraction of Li from Li2O and the formation of an irreversible SEI layer [18]. The cathodic peak also shifts to a more positive potential 0.86 V, resulting from the decrease of the polarization. The third CV curve is almost the same as the second one, showing the good reversibility of the reduction and oxidation in the microspheres. This may be due to the hierarchical structure, which shortens the diffusion length of the lithium ions and thereby ensures the complete reversibility of the lithium intercalation process [35]. The Nyquist plots of the α-Fe2O3 electrode obtained before and after the CV experiments on the α-Fe2O3 electrode are shown in Fig. 7b. A semicircle in the high frequency region followed by a straight line in the low frequency region is observed for both plots, which is a typical blocking-type behavior of thin-film electrodes [36]. The semicircle at moderate frequency region is assigned to the charge-transfer process of lithium ions at the Fe2O3/electrolyte interface, while the line in the lower frequency region is ascribed to the diffusion of lithium ions in the active anode material. Before the CV experiments, the semicircle is about 340 Ω cm2 in terms of resistance, while the semicircle becomes much smaller and the resistance value decreases to about 25 Ω cm2 after the CV experiments. This indicates that the charge

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Table 1 Comparison of electrochemical performance of different electrode materials. Electrode material

Specific capacity (mAh g  1)

α-Fe2O3 microspheres α-Fe2O3 microcuboids Porous α-Fe2O3 microspheres Hollow [email protected] spheres Fe2O3/graphene Macroporous α-Fe2O3 submicron spheres α-Fe2O3 microboxes

0.01–3 V: 0.01–3 V: 0.01–3 V: 0.05–3 V: 0.02–3 V: 0.01–3 V: 0.01–3 V:

1454 at 100 mA g  1 –1100 at 20 mA g  1 1477 at 100 mA g  1 1208 at 100 mA g  1 –1500 at 200 mA g  1 –1450 at 1 C –1200 at 200 mA g  1

transfer at the electrode–electrolyte interface is an easier process after the CV cycles. Galvanostatic charge–discharge experiments were carried out at a current density of 100 mA g  1 to evaluate the electrochemical performance of the α-Fe2O3 electrode. As revealed in Fig. 7c, only a long potential plateau at around 0.85 V is observed for the α-Fe2O3 electrode, corresponding to the formation process of Fe and Li2O phases [34,37]. The initial discharge capacity of the α-Fe2O3 microspheres is 1453.4 mAh g  1, corresponding to 8.7 Li per α-Fe2O3, which is much larger than the theoretical capacity of 1007 mAh g  1 (6Li per α-Fe2O3). The phenomenon that the first discharge capacity considerably exceeds the theoretical capacity has been widely reported for transition metal oxides [38,39]. The extra capacity over the theoretical value could be contributed to the electrolyte being reduced at low voltage (generally below 0.8 V vs Li þ /Li) to form a solid electrolyte interphase (SEI) layer and possibly interfacial lithium storage [40]. Fig. 7d presents the cycling behavior of the α-Fe2O3 electrode. The obvious capacity decay in the first 40 cycles may be caused by the complicated side-reactions and/or irreversible reactions [40]. Subsequently, the profile shows continuous tiny capacity drops during the repeated charge/ discharge process. And the reversible discharge capacity is 489.5 mAh g  1 after 50 cycles. The capacity retention of the as-prepared α-Fe2O3 microspheres is better than that of commercial graphite and also some micrometric α-Fe2O3 samples reported although the cyclic stability needs to further improve (Table 1). One efficient way to improve the cyclic stability is to coat the α-Fe2O3 microspheres with carbon or other conductive materials (Table 1). 4. Conclusions Herein, we reported the feasible synthesis of α-Fe2O3 microspheres by a convenient hydrothermal method assisted by sodium citrate. The role sodium citrate played can be attributed to two processes in our system. One is the chelation of citrate with iron ions, which decreases the free Fe3 þ concentration in solution and results in a slow generation of α-Fe2O3 nanoparticles. The other one is that citrate can serve as a shape modifier and controller. The formation process of hematite microspheres was carefully investigated by SEM and TEM, and the results show that the microspheres were formed in a low concentration of sodium citrate through a combined

Capacity after cycling (mAh g  1)

Reference

490 after 50 cycles – 400 after 25 cycles – 400 after 50 cycles 893 after 100 cycles 800 after 100 cycles 1300 after 100 cycles 945 after 30 cycles

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