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Facile preparation and electrochemical properties of carbon coated Fe3O4 as anode material for lithium-ion batteries
Journal of Power Sources 259 (2014) 92e97
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Facile preparation and electrochemical properties of carbon coated Fe3O4 as anode material for lithium-ion batteries Pengpeng Lv a, Hailei Zhao a, b, *, Zhipeng Zeng a, Jie Wang a, Tianhou Zhang a, Xingwang Li a a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Lab of New Energy Materials and Technologies, Beijing 100083, China
h i g h l i g h t s Nano-sized Fe3O4/C composite was prepared via a facile and productive route. Fe3O4/C composite is composed of Fe3O4 nanoparticles and carbon coating layer. Fe3O4/C electrode exhibits high specific capacity and stable cycling performance. Fe3O4/C electrode displays good rate-capability.
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Article history: Received 17 November 2013 Received in revised form 21 January 2014 Accepted 21 February 2014 Available online 3 March 2014
Carbon coated Fe3O4 nanocomposite (Fe3O4/C) is synthesized via a simple solegel route and a subsequent carbon CVD process, with Fe2O3 xerogel as intermediate product. The nanoporous Fe2O3 xerogel is reduced to Fe3O4 during the CVD process. The prepared Fe3O4/C composite presents a well-distributed nanostructure composing of Fe3O4 nanoparticles coated with carbon layer. The electrode exhibits a stable reversible capacity of over 850 mAh g1 at 0.1 A g1, excellent cycling performance and good rate capability. Both of the nano-scale particle size of Fe3O4 and the carbon layer contribute to the excellent electrochemical performance of Fe3O4/C. An increase in electrode capacity with cycling is observed for the prepared Fe3O4/C composite when cycled at 50 C, which is similar to other reported transition metal oxides. The preparation process of Fe3O4/C composite is facile, mild and productive. Ó 2014 Elsevier B.V. All rights reserved.
Keywords: Solegel Carbon coating Magnetite Xerogel Anode Lithium-ion batteries
1. Introduction As a kind of clean energy storage devices, lithium-ion batteries (LIBs) have been widely utilized in portable electronic devices, electric tools, and are beginning to be used in electric vehicles and hybrid electric vehicles [1]. For mobile application, a significant advance in energy storage density, power density and cycle life is required. Carbonaceous materials, especially graphite, are used as anodes in commercial LIBs because of their long cycle life, relative safety, low cost as well as a theoretical capacity of 372 mAh g1 for LiC6. To satisfy the increasing demands for high energy density batteries, various materials with higher specific capacity have been developed as new anode candidates, such as * Corresponding author. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. Tel./fax: þ86 10 82376837. E-mail addresses: [email protected], [email protected] (H. Zhao). http://dx.doi.org/10.1016/j.jpowsour.2014.02.081 0378-7753/Ó 2014 Elsevier B.V. All rights reserved.
silicon-based materials [2e5], intermetallic alloys [6], and transition metal oxides [7e12]. Among the transition metal oxides (MO, M ¼ Co, Mo, Ni, Cu, Fe, etc.), Fe3O4 as anode material is attracting much attention recently due to its high theoretical capacity of 926 mAh g1, environmental friendliness, low toxicity and natural abundance [13e15]. Additionally, Fe3O4 possesses higher electronic conductivity (2 104 S m1) than other transition metal oxides [15], which is desirable for electrode reaction. However, like other transition metal oxides, Fe3O4 also suffers from large volume change during lithiation/delithiation process, which leads to poor cycling stability with the active particle cracking or pulverization of electrodes. In order to improve the electrochemical performance of Fe3O4 anode, many approaches have been employed, which can be mainly divided into two categories. One is the preparation of nanostructured materials such as nanospheres [16e18], hollow spheres [19,20], nanowires [21], nanorods [22] and nanospindle
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Scheme 1. Schematic illustration of the synthesis route of Fe3O4/C nano-composite.
[14]. Due to the large surface-to-volume ratio and small dimensions, the nanostructured electrode can partially alleviate the mechanical stress caused by large volume change, thus improve the cycling stability of Fe3O4 anode [23]. Chen et al. [20] produced porous hollow Fe3O4 beads by a solvothermal route, which exhibited a reversible capacity of 500 mAh g1 at 100 mA g1 for 50 cycles. Hierarchical hollow Fe3O4 micropheres were prepared by Xiong et al. [24], which presented a high specific capacity of 851 mAh g1 at 1 C and a stable cycling performance for 50 cycles. The other effective strategy is fabricating Fe3O4-based composites by introducing a second component, which acts as a buffer to reduce the large volume change and prevent active nanoparticles from aggregation during electrochemical cycling, leading to an improved cycling stability. Carbon is frequently used as the buffer component to enhance the electrochemical performance of Fe3O4 because of its high tolerance to mechanical stress and good electronic conductivity [13]. Yuan et al. [25] prepared mesopores Fe3O4@C microcapsules by hydrothermal route, which demonstrated a high specific capacity of 1010 mAh g1 for 50 cycles. Graphene anchored with Fe3O4 nanoparticles were prepared at very dilute and hydrothermal conditions, which exhibited a reversible capacity of 675 mAh g1 after 50 cycles at 50 mA g1 [26]. Zhou et al. produced graphene-wrapped Fe3O4 composite through in situ reduction of iron hydroxide existing between graphene nanosheets, which showed an excellent cycling stability and rate capability [27]. In this work, we use Fe2O3 xerogel as precursor for the first time to prepare Fe3O4/C nanocomposite used as anode material for lithium-ion batteries. The nano-porous feature of Fe2O3 xerogel can make the preparation of nanosized Fe2O3 particles much easier with a simple grinding process. The solegel process was employed for the preparation of Fe2O3 xerogel, which allows a high productivity. With chemical vapor deposition (CVD) of carbon, the Fe2O3 can be reduced to Fe3O4 and at the same time carbon coated nanosized Fe3O4 (Fe3O4/C) was in situ produced. The preparation of Fe3O4/C nano-composite is facile, mild and mass-productive. The structure characteristics and the electrochemical properties of Fe3O4/C were investigated.
basic catalyst was introduced, which made the solution rapidly change to an intense dark redebrown color, accompanying by a significant heat generation. Gel formation occurred within several minutes. The resulted gel was aged in ethanol for 48 h under ambient conditions. Then fresh ethanol was used to replace the aging solution and this step was repeated every 12 h for 3 times to remove the unreacted chemicals and ensure a complete solution exchange. After solvent exchange, the wet gel was dried at room temperature for 24 h and then oven-dried at 80 C for 48 h to obtain Fe2O3 xerogel. After grinding, the Fe2O3 xerogel was heated at 600 C in air for 3 h to prepare Fe2O3 powders, which were transferred into a reaction tube for carbon coating by a chemical vapor deposition process. A toluene vapor carried by argon gas was flowed through the reaction tube at a rate of 0.1 L min1. The reaction temperature was maintained at 800 C for 30 min to obtain Fe3O4/C composite.
2. Experimental 2.1. Preparation of carbon coated Fe3O4 nano-composite The preparation of the nano-sized Fe3O4/C can be briefly described by the following process. The schematic illustration is shown in Scheme 1. The Fe2O3 xerogel precursor was prepared via a solegel route and a subsequent ambient drying process. In a typical experiment, 2.43 g of FeCl3 (0.015 mol) was dissolved in 100 ml of ethanol under magnetic agitation to form a clear redeorange solution, in which 2.7 ml of distilled water (0.15 mol) was then added. After stirring for 15 min, 10.48 ml of propylene oxide (0.15 mol) as
Fig. 1. (a) XRD patterns of Fe2O3 and Fe3O4/C samples. Also shown are JCPDS patterns for a-Fe2O3 (no. 89-0597) and Fe3O4 (no. 89-0688). (b) FTIR spectrum of Fe3O4/C sample.
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2.2. Structure characterization The phase identification of samples was carried out by using a powder X-ray diffraction (XRD, Rigaku, D/max-A, Cu Ka, l ¼ 1.5406 A) over 2q degree from 20 e80 . Infrared spectrum of the sample was measured on a Fourier transform infrared spectroscope (FTIR, NEXUS FT-IR670). To get the carbon content in Fe3O4/C sample, thermogravimetric (TG) analysis was carried out on an apparatus (Netzsch STA449C) with a heating rate of 10 C per minute from room temperature to 1000 C in air atmosphere. The particle morphology was observed by field emission scanning electron microscope (FESEM, SUPRA55) and high resolution transmission electron microscope (HRTEM, JEM2010). Scanning electron microscope (SEM, LEO-1450) was employed to observe the morphology of the electrodes before and after cycling. 2.3. Electrochemical measurements The electrochemical performances of the as-prepared samples as anode material for lithium-ion batteries were evaluated by using two-electrode half-cells. The working electrode was fabricated by mixing the active material (Fe3O4/C) with acetylene black (AB) as conductive agent and carboxymethylcellulose (CMC) as
binder together at a mass ratio of 80:10:10, using distilled water as the solvent. The resultant slurry was uniformly pasted on a copper foil. After drying in air at ambient temperature and then in a vacuum environment at 80 C for 6 h, the electrode film was pressed and punched into circular discs with the diameter of 8 mm. The discs were weighted and dried again at 120 C for 24 h in vacuum. The test cells were finally assembled in an argon-filled glove box with metal lithium-foil as the counter electrode, porous polypropylene (Celgard 2400) film as the separator and 1 M LiPF6 in a non-aqueous solution of ethylene carbonate (EC), diethylcarbonate (DEC), and dimethyl carbonate (DMC) (1:1:1 by volume) as the electrolyte. The galvanostatic cycling tests were carried out with LAND CT2001A tester (Wuhan, China) at different current densities in the voltage range of 0.02e3 V versus Liþ/Li. The cyclic voltammetry (CV) test was performed on an Arbin BT2000 battery testing system (Arbin Instrument Inc.) in the voltage range of 0.02e3 V versus Liþ/Li at a scan rate of 0.1 mV s1 at room temperature by using three-electrode cell. To investigate the structural integrity of electrode film, the Swagelok-type cell with Fe3O4/C electrode was disassembled inside an argon-filled glove box after charge/discharged at 0.1 A g1 for 60 cycles, and then the electrode was washed carefully with DMC in order to remove electrolyte salt residues. The dried electrode film was subjected to SEM observation.
Fig. 2. Morphological characterizations of Fe2O3 and Fe3O4/C samples: optical microscope images of Fe2O3 (a) and Fe3O4/C (b), FESEM images of Fe2O3 (c) and Fe3O4/C (d, e), HRTEM image of Fe3O4/C (f). The inset in (f) is the corresponding SAED pattern of Fe3O4.
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Fig. 3. (a) Cyclic voltammetry curves, (b) discharge/charge voltage profiles and (c) cycling performance of Fe3O4/C electrode.
3. Results and discussion To clarify the crystal structure of the prepared Fe2O3 and Fe3O4/ C composite, XRD measurements were carried out. Fig. 1a illustrates the XRD patterns of the two samples. As observed, the diffraction peaks of Fe2O3 can be indexed well with that of hematite (JCPDS File Card No. 89-0597). After CVD process, Fe2O3
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was reduced by carbon and transformed into Fe3O4, whose XRD pattern matches well with that of magnetite (JCPDS File Card No. 89-0688). However, no obvious diffraction peaks attributed to crystalline carbon can be observed. In order to confirm the existence of carbon in the Fe3O4-based sample after CVD process, FTIR spectroscopy was used to identify the functional groups of Fe3O4/C composite. As shown in Fig. 1b, the characteristic peak at 567 cm1 is in well agreement with those reported for the stretching mode of FeeO in Fe3O4 [28,29]. The peak at 1598 cm1 corresponds to C]C vibrations [30e32], which is a result of carbonization of toluene during the CVD process. The carbon content in the Fe3O4/C composite can be derived to be ca. 2.37% with TG analysis. The results of the XRD and FTIR measurements indicate the amorphous feature of carbon in the composite, which can also be confirmed by HRTEM observation. The morphological characterizations of Fe2O3 and Fe3O4/C samples were carried out and the results are illustrated in Fig. 2. The Fe2O3 powder shows darkered color (Fig. 2a), while after CVD process the powder color changes into black (Fig. 2b), consistent with the XRD and FTIR results (Fig. 1). FESEM observation reveals that both samples present similar morphology composed of welldistributed granular particles with diameter ranging in 80e 160 nm (Fig. 2cee). The HRTEM image of Fe3O4/C, displayed in Fig. 2f, shows a clear morphology of the particle edge. The component of Fe3O4 is highly crystallized, as confirmed by the distinct lattice fringes and the corresponding SAED pattern. The lattice interplanar spacing of 0.298 nm can be identified, which is in well agreement with that of Fe3O4 (220) [33]. No crystalline lattice but a turbostratic structure is observed in the outer layer, indicating that the amorphous feature of carbon coating layer, which is consistent with the XRD result. The thickness of carbon coating layer on the surface of the Fe3O4 particles is around 5 nm. In order to test the applicability of Fe3O4/C as anode in lithiumion batteries, the electrochemical properties of the prepared Fe3O4/ C were investigated. Cyclic voltammetry measurement was carried out to understand the electrochemical reaction process of Fe3O4/C. Fig. 3a shows the CV curves of Fe3O4/C electrode for the first three cycles in the voltage range between 3 and 0.02 V. In the first cycle, an obvious cathodic peak at about 0.61 V and two anodic peaks at 1.67 and 1.88 V are observed. For the subsequent cycles, both of the cathodic and anodic peaks shift positively, with a big shifting (to ca. 0.96 V) for the cathodic peak but a slight shifting for the anodic peaks. A large irreversible capacity lose occurs during first cycle according to the integrated peak area. One most recent work on Fe2O3 electrode [34] reveals the existence of LiFeO2 in the delithiated electrode, which should be
Fig. 4. SEM images of the Fe3O4/C electrodes before (a) and after (b) 60 cycles.
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responsible for the large initial irreversible capacity loss. Based on this work, the lithiation/delithiation process of Fe3O4 can be deduced as Eqs. (1) and (2). Fe3O4 þ 8Liþ þ 8e / 4Li2O þ 3Fe
(1)
4Li2O þ 3Fe 4 2LiFeO2 þ Fe þ 6Liþ þ 6e
(2)
In the first lithiation process, reaction (1) occurs, which is irreversible and corresponds to a theoretical specific capacity of 924 mAh g1. During the delithiation process, reaction (2) takes place, which is reversible and can deliver a specific capacity of 693 mAh g1. The different lithiation reactions for the first and subsequent cycles result in the difference in the cathodic peak potential, while the formation of LiFeO2 is accounted for the initial irreversible capacity loss. Based on the proposed lithiation/delithiation mechanism, reactions (1) and (2), the first coulombic efficiency of Fe3O4 anode material is 75%, which is much close to the practical value of 70%. On the other hand, however, either the initial or the following specific capacity is much higher than the theoretical values, 924 and 693 mAh g1, respectively, which can be attributed to the formation of gel-like SEI film on the surface of Fe3O4 electrodes [35e38]. The formation of this gel-like film is proved to be partially reversible, thus contributing much to the reversible capacity of electrode. This phenomenon is frequently observed in transition metal oxides [39e41]. They usually deliver a higher specific capacity than its theoretical value. Of course, the proposed lithiation/delithiation mechanism of Fe3O4 shown in reactions (1) and (2) needs definitely to be further investigated and confirmed. The typical discharge/charge curves of Fe3O4/C for the 1st, 2nd, 10th, 50th and 60th cycle tested at 0.1 A g1 over a potential window of 0.02e3 V are shown in Fig. 3b. The electrode delivers an
Fig. 5. (a) Rate-capability and (b) corresponding discharge/charge voltage profiles of Fe3O4/C electrode.
initial discharge capacity of 1221 mAh g1 and a reversible capacity of 856 mAh g1, leading to an initial coulombic efficiency of around 70%. The first discharge voltage profile shows a steep voltage drop from about 2 to 0.8 V, and then a long voltage plateau at 0.8 V was observed, which corresponds to the reduction reaction of Fe3O4 to Fe0, as presented by Eq. (1). The delithiation reaction occurs at ca. 1.7 V, as depicted by Eq. (2). Not all the lithium ions can be extracted from the lithiated electrode, some lithium is remained in the form of LiFeO2 [34]. This reaction is reversible and differs from the reaction (1), which results in the lithiation potential increasing to a higher value in the following cycles. It should be emphasized that no obvious change in both charge and discharge profiles is observed even after 60 cycles, which indicates the excellent reversibility of electrode reaction of the prepared Fe3O4/C composite. Fig. 3c shows the cycling performance of Fe3O4/C at 0.1 A g1 for 60 cycles. As observed, the Fe3O4/C composite electrode demonstrates a high reversible capacity of ca. 850 mAh g1 and an excellent cyclic retention without any tendency of degradation. Furthermore, the coulombic efficiency rapidly increases from 70% for the first cycle to 95% for the second cycle and keeps almost 100% in the subsequent cycles. The high coulombic efficiency during subsequent discharge/charge cycles indicates a stable SEI film on the electrode surface. With respect to the excellent cycling performance, several reasons should be accounted for. Firstly, the nano feature of Fe3O4 particles is beneficial to accommodating the mechanical strain caused by lithium uptake and removal processes. Secondly, the carbon coating layer can act not only as a barrier to prevent Fe3O4 nanoparticles from aggregation but also as a buffer to accommodate the volume change during discharge/charge process, ensuring an enhanced structural stability and an excellent cycling performance [42]. The good structural stability of Fe3O4/C electrode can be reflected in the image of cycled electrode. Fig. 4 shows the SEM images of electrodes before and after cycling. Both of the electrodes display homogeneous film morphology, and no cracks and breaks are observed after cycling at 0.1 A g1 for 60 cycles (Fig. 4b). Besides the cycling performance, the rate capability is also one of the most important properties for high performance anodes of lithium-ion batteries. To evaluate the rate capability, the Fe3O4/C electrode was cycled at different current densities from 0.1 to 1 A g1, and then back to the initial moderate current density of 0.1 A g1. As shown in Fig. 5a, the reversible specific capacities of electrode cycling at progressively increasing current densities were ca. 835, 790, 730, 660, 610 and 570 mAh g1, respectively. Even at 1 A g1, the specific capacity is much higher than the theoretical
Fig. 6. Cycling performances of Fe3O4/C electrode tested at 50 and 25 C.
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specific capacity of graphite. When the current density switches back from 1 to 0.1 A g1, the reversible capacity recovers to ca. 750 mAh g1, indicating a good recoverability of electrode. The high coulombic efficiency of all cycles (almost 100%) at different current densities indicates a stable SEI film present on the surface of the electrode. Fig. 5b shows the typical discharge/charge voltage profiles of Fe3O4/C electrode at different current densities. Different from the obvious variation in specific capacity, only small changes can be observed in charge/discharge potential plateaus with increasing C-rates. This phenomenon probably implies that the synthesized Fe3O4/C electrode has a good electronic conductivity but low lithium ionic conductivity or slow electrode reaction process. The effect of temperature on the specific capacity and cycling performance of Fe3O4/C electrode was examined by cycling the electrode at 50 and 25 C, respectively. The results are shown in Fig. 6. At two testing temperatures, the Fe3O4/C electrode delivers similar initial charge/discharge capacities and therefore similar initial coulombic efficiency. For the subsequent cycles, however, the electrode cycled at 50 C shows a progressive capacity increase while that at 25 C displays a relatively stable cycling performance. A capacity difference of about 100 mAh g1 can be observed after 60 cycles. The capacity increase phenomenon is often encountered in other transition metal oxides [43e45], which is usually attributed to the reversible formation of a gellike polymer film on the Fe3O4 particles. The gel-like polymer film could simply be the result of the catalytically enhanced electrolyte decomposition, which can be formed and dissolved reversibly with cycling [45]. The formation of gel-like film is also thermally activated [46,47] and contributes much to the specific capacity of electrode. Nevertheless, this will bring some difficulty for battery design. 4. Conclusions Carbon coated Fe3O4 nanocomposite was synthesized by a solegel route and a subsequent carbon CVD process. The Fe2O3 xerogel was firstly prepared as precursor, which was then reduced to Fe3O4 in the carbon CVD process. The nano and porous features of Fe2O3 xerogel is essential for the preparation of the nanosized Fe3O4 particles coated by a thin carbon layer. As anode material for lithium-ion batteries, the Fe3O4/C electrode shows high specific capacity (w850 mAh g1), excellent cycling stability and good rate capability, which are associated with the combined effect of the nanostructure and the coating carbon layer. The carbon layer plays important roles in the composite, which can accommodate volume change during discharge/charge cycles, and provide good electronic conductivity for electrode reaction. The capacity increase phenomenon is observed for the Fe3O4/C electrode cycled at 50 C, which is similar to other reported transition metal oxides. These results suggest that the Fe3O4/C composite is a promising anode material for lithium-ion batteries. The preparation process of Fe3O4/C composite is facile and productive, and therefore suitable for other oxide-based electrode materials. Acknowledgments This work was financially supported by National Basic Research Program of China (2013CB934003), National Nature Science Foundation of China (21273019) and the Fundamental Research Funds for the Central Universities (FRF-MP-12-006B).
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Facile preparation and electrochemical properties of carbon coated Fe3O4 as anode material for lithium-ion batteries Pengpeng Lv a, Hailei Zhao a, b, *, Zhipeng Zeng a, Jie Wang a, Tianhou Zhang a, Xingwang Li a a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Lab of New Energy Materials and Technologies, Beijing 100083, China
h i g h l i g h t s Nano-sized Fe3O4/C composite was prepared via a facile and productive route. Fe3O4/C composite is composed of Fe3O4 nanoparticles and carbon coating layer. Fe3O4/C electrode exhibits high specific capacity and stable cycling performance. Fe3O4/C electrode displays good rate-capability.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 November 2013 Received in revised form 21 January 2014 Accepted 21 February 2014 Available online 3 March 2014
Carbon coated Fe3O4 nanocomposite (Fe3O4/C) is synthesized via a simple solegel route and a subsequent carbon CVD process, with Fe2O3 xerogel as intermediate product. The nanoporous Fe2O3 xerogel is reduced to Fe3O4 during the CVD process. The prepared Fe3O4/C composite presents a well-distributed nanostructure composing of Fe3O4 nanoparticles coated with carbon layer. The electrode exhibits a stable reversible capacity of over 850 mAh g1 at 0.1 A g1, excellent cycling performance and good rate capability. Both of the nano-scale particle size of Fe3O4 and the carbon layer contribute to the excellent electrochemical performance of Fe3O4/C. An increase in electrode capacity with cycling is observed for the prepared Fe3O4/C composite when cycled at 50 C, which is similar to other reported transition metal oxides. The preparation process of Fe3O4/C composite is facile, mild and productive. Ó 2014 Elsevier B.V. All rights reserved.
Keywords: Solegel Carbon coating Magnetite Xerogel Anode Lithium-ion batteries
1. Introduction As a kind of clean energy storage devices, lithium-ion batteries (LIBs) have been widely utilized in portable electronic devices, electric tools, and are beginning to be used in electric vehicles and hybrid electric vehicles [1]. For mobile application, a significant advance in energy storage density, power density and cycle life is required. Carbonaceous materials, especially graphite, are used as anodes in commercial LIBs because of their long cycle life, relative safety, low cost as well as a theoretical capacity of 372 mAh g1 for LiC6. To satisfy the increasing demands for high energy density batteries, various materials with higher specific capacity have been developed as new anode candidates, such as * Corresponding author. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. Tel./fax: þ86 10 82376837. E-mail addresses: [email protected], [email protected] (H. Zhao). http://dx.doi.org/10.1016/j.jpowsour.2014.02.081 0378-7753/Ó 2014 Elsevier B.V. All rights reserved.
silicon-based materials [2e5], intermetallic alloys [6], and transition metal oxides [7e12]. Among the transition metal oxides (MO, M ¼ Co, Mo, Ni, Cu, Fe, etc.), Fe3O4 as anode material is attracting much attention recently due to its high theoretical capacity of 926 mAh g1, environmental friendliness, low toxicity and natural abundance [13e15]. Additionally, Fe3O4 possesses higher electronic conductivity (2 104 S m1) than other transition metal oxides [15], which is desirable for electrode reaction. However, like other transition metal oxides, Fe3O4 also suffers from large volume change during lithiation/delithiation process, which leads to poor cycling stability with the active particle cracking or pulverization of electrodes. In order to improve the electrochemical performance of Fe3O4 anode, many approaches have been employed, which can be mainly divided into two categories. One is the preparation of nanostructured materials such as nanospheres [16e18], hollow spheres [19,20], nanowires [21], nanorods [22] and nanospindle
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Scheme 1. Schematic illustration of the synthesis route of Fe3O4/C nano-composite.
[14]. Due to the large surface-to-volume ratio and small dimensions, the nanostructured electrode can partially alleviate the mechanical stress caused by large volume change, thus improve the cycling stability of Fe3O4 anode [23]. Chen et al. [20] produced porous hollow Fe3O4 beads by a solvothermal route, which exhibited a reversible capacity of 500 mAh g1 at 100 mA g1 for 50 cycles. Hierarchical hollow Fe3O4 micropheres were prepared by Xiong et al. [24], which presented a high specific capacity of 851 mAh g1 at 1 C and a stable cycling performance for 50 cycles. The other effective strategy is fabricating Fe3O4-based composites by introducing a second component, which acts as a buffer to reduce the large volume change and prevent active nanoparticles from aggregation during electrochemical cycling, leading to an improved cycling stability. Carbon is frequently used as the buffer component to enhance the electrochemical performance of Fe3O4 because of its high tolerance to mechanical stress and good electronic conductivity [13]. Yuan et al. [25] prepared mesopores Fe3O4@C microcapsules by hydrothermal route, which demonstrated a high specific capacity of 1010 mAh g1 for 50 cycles. Graphene anchored with Fe3O4 nanoparticles were prepared at very dilute and hydrothermal conditions, which exhibited a reversible capacity of 675 mAh g1 after 50 cycles at 50 mA g1 [26]. Zhou et al. produced graphene-wrapped Fe3O4 composite through in situ reduction of iron hydroxide existing between graphene nanosheets, which showed an excellent cycling stability and rate capability [27]. In this work, we use Fe2O3 xerogel as precursor for the first time to prepare Fe3O4/C nanocomposite used as anode material for lithium-ion batteries. The nano-porous feature of Fe2O3 xerogel can make the preparation of nanosized Fe2O3 particles much easier with a simple grinding process. The solegel process was employed for the preparation of Fe2O3 xerogel, which allows a high productivity. With chemical vapor deposition (CVD) of carbon, the Fe2O3 can be reduced to Fe3O4 and at the same time carbon coated nanosized Fe3O4 (Fe3O4/C) was in situ produced. The preparation of Fe3O4/C nano-composite is facile, mild and mass-productive. The structure characteristics and the electrochemical properties of Fe3O4/C were investigated.
basic catalyst was introduced, which made the solution rapidly change to an intense dark redebrown color, accompanying by a significant heat generation. Gel formation occurred within several minutes. The resulted gel was aged in ethanol for 48 h under ambient conditions. Then fresh ethanol was used to replace the aging solution and this step was repeated every 12 h for 3 times to remove the unreacted chemicals and ensure a complete solution exchange. After solvent exchange, the wet gel was dried at room temperature for 24 h and then oven-dried at 80 C for 48 h to obtain Fe2O3 xerogel. After grinding, the Fe2O3 xerogel was heated at 600 C in air for 3 h to prepare Fe2O3 powders, which were transferred into a reaction tube for carbon coating by a chemical vapor deposition process. A toluene vapor carried by argon gas was flowed through the reaction tube at a rate of 0.1 L min1. The reaction temperature was maintained at 800 C for 30 min to obtain Fe3O4/C composite.
2. Experimental 2.1. Preparation of carbon coated Fe3O4 nano-composite The preparation of the nano-sized Fe3O4/C can be briefly described by the following process. The schematic illustration is shown in Scheme 1. The Fe2O3 xerogel precursor was prepared via a solegel route and a subsequent ambient drying process. In a typical experiment, 2.43 g of FeCl3 (0.015 mol) was dissolved in 100 ml of ethanol under magnetic agitation to form a clear redeorange solution, in which 2.7 ml of distilled water (0.15 mol) was then added. After stirring for 15 min, 10.48 ml of propylene oxide (0.15 mol) as
Fig. 1. (a) XRD patterns of Fe2O3 and Fe3O4/C samples. Also shown are JCPDS patterns for a-Fe2O3 (no. 89-0597) and Fe3O4 (no. 89-0688). (b) FTIR spectrum of Fe3O4/C sample.
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2.2. Structure characterization The phase identification of samples was carried out by using a powder X-ray diffraction (XRD, Rigaku, D/max-A, Cu Ka, l ¼ 1.5406 A) over 2q degree from 20 e80 . Infrared spectrum of the sample was measured on a Fourier transform infrared spectroscope (FTIR, NEXUS FT-IR670). To get the carbon content in Fe3O4/C sample, thermogravimetric (TG) analysis was carried out on an apparatus (Netzsch STA449C) with a heating rate of 10 C per minute from room temperature to 1000 C in air atmosphere. The particle morphology was observed by field emission scanning electron microscope (FESEM, SUPRA55) and high resolution transmission electron microscope (HRTEM, JEM2010). Scanning electron microscope (SEM, LEO-1450) was employed to observe the morphology of the electrodes before and after cycling. 2.3. Electrochemical measurements The electrochemical performances of the as-prepared samples as anode material for lithium-ion batteries were evaluated by using two-electrode half-cells. The working electrode was fabricated by mixing the active material (Fe3O4/C) with acetylene black (AB) as conductive agent and carboxymethylcellulose (CMC) as
binder together at a mass ratio of 80:10:10, using distilled water as the solvent. The resultant slurry was uniformly pasted on a copper foil. After drying in air at ambient temperature and then in a vacuum environment at 80 C for 6 h, the electrode film was pressed and punched into circular discs with the diameter of 8 mm. The discs were weighted and dried again at 120 C for 24 h in vacuum. The test cells were finally assembled in an argon-filled glove box with metal lithium-foil as the counter electrode, porous polypropylene (Celgard 2400) film as the separator and 1 M LiPF6 in a non-aqueous solution of ethylene carbonate (EC), diethylcarbonate (DEC), and dimethyl carbonate (DMC) (1:1:1 by volume) as the electrolyte. The galvanostatic cycling tests were carried out with LAND CT2001A tester (Wuhan, China) at different current densities in the voltage range of 0.02e3 V versus Liþ/Li. The cyclic voltammetry (CV) test was performed on an Arbin BT2000 battery testing system (Arbin Instrument Inc.) in the voltage range of 0.02e3 V versus Liþ/Li at a scan rate of 0.1 mV s1 at room temperature by using three-electrode cell. To investigate the structural integrity of electrode film, the Swagelok-type cell with Fe3O4/C electrode was disassembled inside an argon-filled glove box after charge/discharged at 0.1 A g1 for 60 cycles, and then the electrode was washed carefully with DMC in order to remove electrolyte salt residues. The dried electrode film was subjected to SEM observation.
Fig. 2. Morphological characterizations of Fe2O3 and Fe3O4/C samples: optical microscope images of Fe2O3 (a) and Fe3O4/C (b), FESEM images of Fe2O3 (c) and Fe3O4/C (d, e), HRTEM image of Fe3O4/C (f). The inset in (f) is the corresponding SAED pattern of Fe3O4.
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Fig. 3. (a) Cyclic voltammetry curves, (b) discharge/charge voltage profiles and (c) cycling performance of Fe3O4/C electrode.
3. Results and discussion To clarify the crystal structure of the prepared Fe2O3 and Fe3O4/ C composite, XRD measurements were carried out. Fig. 1a illustrates the XRD patterns of the two samples. As observed, the diffraction peaks of Fe2O3 can be indexed well with that of hematite (JCPDS File Card No. 89-0597). After CVD process, Fe2O3
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was reduced by carbon and transformed into Fe3O4, whose XRD pattern matches well with that of magnetite (JCPDS File Card No. 89-0688). However, no obvious diffraction peaks attributed to crystalline carbon can be observed. In order to confirm the existence of carbon in the Fe3O4-based sample after CVD process, FTIR spectroscopy was used to identify the functional groups of Fe3O4/C composite. As shown in Fig. 1b, the characteristic peak at 567 cm1 is in well agreement with those reported for the stretching mode of FeeO in Fe3O4 [28,29]. The peak at 1598 cm1 corresponds to C]C vibrations [30e32], which is a result of carbonization of toluene during the CVD process. The carbon content in the Fe3O4/C composite can be derived to be ca. 2.37% with TG analysis. The results of the XRD and FTIR measurements indicate the amorphous feature of carbon in the composite, which can also be confirmed by HRTEM observation. The morphological characterizations of Fe2O3 and Fe3O4/C samples were carried out and the results are illustrated in Fig. 2. The Fe2O3 powder shows darkered color (Fig. 2a), while after CVD process the powder color changes into black (Fig. 2b), consistent with the XRD and FTIR results (Fig. 1). FESEM observation reveals that both samples present similar morphology composed of welldistributed granular particles with diameter ranging in 80e 160 nm (Fig. 2cee). The HRTEM image of Fe3O4/C, displayed in Fig. 2f, shows a clear morphology of the particle edge. The component of Fe3O4 is highly crystallized, as confirmed by the distinct lattice fringes and the corresponding SAED pattern. The lattice interplanar spacing of 0.298 nm can be identified, which is in well agreement with that of Fe3O4 (220) [33]. No crystalline lattice but a turbostratic structure is observed in the outer layer, indicating that the amorphous feature of carbon coating layer, which is consistent with the XRD result. The thickness of carbon coating layer on the surface of the Fe3O4 particles is around 5 nm. In order to test the applicability of Fe3O4/C as anode in lithiumion batteries, the electrochemical properties of the prepared Fe3O4/ C were investigated. Cyclic voltammetry measurement was carried out to understand the electrochemical reaction process of Fe3O4/C. Fig. 3a shows the CV curves of Fe3O4/C electrode for the first three cycles in the voltage range between 3 and 0.02 V. In the first cycle, an obvious cathodic peak at about 0.61 V and two anodic peaks at 1.67 and 1.88 V are observed. For the subsequent cycles, both of the cathodic and anodic peaks shift positively, with a big shifting (to ca. 0.96 V) for the cathodic peak but a slight shifting for the anodic peaks. A large irreversible capacity lose occurs during first cycle according to the integrated peak area. One most recent work on Fe2O3 electrode [34] reveals the existence of LiFeO2 in the delithiated electrode, which should be
Fig. 4. SEM images of the Fe3O4/C electrodes before (a) and after (b) 60 cycles.
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responsible for the large initial irreversible capacity loss. Based on this work, the lithiation/delithiation process of Fe3O4 can be deduced as Eqs. (1) and (2). Fe3O4 þ 8Liþ þ 8e / 4Li2O þ 3Fe
(1)
4Li2O þ 3Fe 4 2LiFeO2 þ Fe þ 6Liþ þ 6e
(2)
In the first lithiation process, reaction (1) occurs, which is irreversible and corresponds to a theoretical specific capacity of 924 mAh g1. During the delithiation process, reaction (2) takes place, which is reversible and can deliver a specific capacity of 693 mAh g1. The different lithiation reactions for the first and subsequent cycles result in the difference in the cathodic peak potential, while the formation of LiFeO2 is accounted for the initial irreversible capacity loss. Based on the proposed lithiation/delithiation mechanism, reactions (1) and (2), the first coulombic efficiency of Fe3O4 anode material is 75%, which is much close to the practical value of 70%. On the other hand, however, either the initial or the following specific capacity is much higher than the theoretical values, 924 and 693 mAh g1, respectively, which can be attributed to the formation of gel-like SEI film on the surface of Fe3O4 electrodes [35e38]. The formation of this gel-like film is proved to be partially reversible, thus contributing much to the reversible capacity of electrode. This phenomenon is frequently observed in transition metal oxides [39e41]. They usually deliver a higher specific capacity than its theoretical value. Of course, the proposed lithiation/delithiation mechanism of Fe3O4 shown in reactions (1) and (2) needs definitely to be further investigated and confirmed. The typical discharge/charge curves of Fe3O4/C for the 1st, 2nd, 10th, 50th and 60th cycle tested at 0.1 A g1 over a potential window of 0.02e3 V are shown in Fig. 3b. The electrode delivers an
Fig. 5. (a) Rate-capability and (b) corresponding discharge/charge voltage profiles of Fe3O4/C electrode.
initial discharge capacity of 1221 mAh g1 and a reversible capacity of 856 mAh g1, leading to an initial coulombic efficiency of around 70%. The first discharge voltage profile shows a steep voltage drop from about 2 to 0.8 V, and then a long voltage plateau at 0.8 V was observed, which corresponds to the reduction reaction of Fe3O4 to Fe0, as presented by Eq. (1). The delithiation reaction occurs at ca. 1.7 V, as depicted by Eq. (2). Not all the lithium ions can be extracted from the lithiated electrode, some lithium is remained in the form of LiFeO2 [34]. This reaction is reversible and differs from the reaction (1), which results in the lithiation potential increasing to a higher value in the following cycles. It should be emphasized that no obvious change in both charge and discharge profiles is observed even after 60 cycles, which indicates the excellent reversibility of electrode reaction of the prepared Fe3O4/C composite. Fig. 3c shows the cycling performance of Fe3O4/C at 0.1 A g1 for 60 cycles. As observed, the Fe3O4/C composite electrode demonstrates a high reversible capacity of ca. 850 mAh g1 and an excellent cyclic retention without any tendency of degradation. Furthermore, the coulombic efficiency rapidly increases from 70% for the first cycle to 95% for the second cycle and keeps almost 100% in the subsequent cycles. The high coulombic efficiency during subsequent discharge/charge cycles indicates a stable SEI film on the electrode surface. With respect to the excellent cycling performance, several reasons should be accounted for. Firstly, the nano feature of Fe3O4 particles is beneficial to accommodating the mechanical strain caused by lithium uptake and removal processes. Secondly, the carbon coating layer can act not only as a barrier to prevent Fe3O4 nanoparticles from aggregation but also as a buffer to accommodate the volume change during discharge/charge process, ensuring an enhanced structural stability and an excellent cycling performance [42]. The good structural stability of Fe3O4/C electrode can be reflected in the image of cycled electrode. Fig. 4 shows the SEM images of electrodes before and after cycling. Both of the electrodes display homogeneous film morphology, and no cracks and breaks are observed after cycling at 0.1 A g1 for 60 cycles (Fig. 4b). Besides the cycling performance, the rate capability is also one of the most important properties for high performance anodes of lithium-ion batteries. To evaluate the rate capability, the Fe3O4/C electrode was cycled at different current densities from 0.1 to 1 A g1, and then back to the initial moderate current density of 0.1 A g1. As shown in Fig. 5a, the reversible specific capacities of electrode cycling at progressively increasing current densities were ca. 835, 790, 730, 660, 610 and 570 mAh g1, respectively. Even at 1 A g1, the specific capacity is much higher than the theoretical
Fig. 6. Cycling performances of Fe3O4/C electrode tested at 50 and 25 C.
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specific capacity of graphite. When the current density switches back from 1 to 0.1 A g1, the reversible capacity recovers to ca. 750 mAh g1, indicating a good recoverability of electrode. The high coulombic efficiency of all cycles (almost 100%) at different current densities indicates a stable SEI film present on the surface of the electrode. Fig. 5b shows the typical discharge/charge voltage profiles of Fe3O4/C electrode at different current densities. Different from the obvious variation in specific capacity, only small changes can be observed in charge/discharge potential plateaus with increasing C-rates. This phenomenon probably implies that the synthesized Fe3O4/C electrode has a good electronic conductivity but low lithium ionic conductivity or slow electrode reaction process. The effect of temperature on the specific capacity and cycling performance of Fe3O4/C electrode was examined by cycling the electrode at 50 and 25 C, respectively. The results are shown in Fig. 6. At two testing temperatures, the Fe3O4/C electrode delivers similar initial charge/discharge capacities and therefore similar initial coulombic efficiency. For the subsequent cycles, however, the electrode cycled at 50 C shows a progressive capacity increase while that at 25 C displays a relatively stable cycling performance. A capacity difference of about 100 mAh g1 can be observed after 60 cycles. The capacity increase phenomenon is often encountered in other transition metal oxides [43e45], which is usually attributed to the reversible formation of a gellike polymer film on the Fe3O4 particles. The gel-like polymer film could simply be the result of the catalytically enhanced electrolyte decomposition, which can be formed and dissolved reversibly with cycling [45]. The formation of gel-like film is also thermally activated [46,47] and contributes much to the specific capacity of electrode. Nevertheless, this will bring some difficulty for battery design. 4. Conclusions Carbon coated Fe3O4 nanocomposite was synthesized by a solegel route and a subsequent carbon CVD process. The Fe2O3 xerogel was firstly prepared as precursor, which was then reduced to Fe3O4 in the carbon CVD process. The nano and porous features of Fe2O3 xerogel is essential for the preparation of the nanosized Fe3O4 particles coated by a thin carbon layer. As anode material for lithium-ion batteries, the Fe3O4/C electrode shows high specific capacity (w850 mAh g1), excellent cycling stability and good rate capability, which are associated with the combined effect of the nanostructure and the coating carbon layer. The carbon layer plays important roles in the composite, which can accommodate volume change during discharge/charge cycles, and provide good electronic conductivity for electrode reaction. The capacity increase phenomenon is observed for the Fe3O4/C electrode cycled at 50 C, which is similar to other reported transition metal oxides. These results suggest that the Fe3O4/C composite is a promising anode material for lithium-ion batteries. The preparation process of Fe3O4/C composite is facile and productive, and therefore suitable for other oxide-based electrode materials. Acknowledgments This work was financially supported by National Basic Research Program of China (2013CB934003), National Nature Science Foundation of China (21273019) and the Fundamental Research Funds for the Central Universities (FRF-MP-12-006B).
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