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Self-assembly of Fe3O4 nanorods on graphene for lithium ion batteries with high rate capacity and cycle stability
Electrochemistry Communications 28 (2013) 139–142
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Self-assembly of Fe3O4 nanorods on graphene for lithium ion batteries with high rate capacity and cycle stability Aiping Hu a, Xiaohua Chen a,⁎, Yuanhong Tang a, Qunli Tang a, Lei Yang a, Shaopeng Zhang b a b
College of Materials Science and Engineering, Hunan University, Changsha 410082, China Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 14 December 2012 Received in revised form 21 December 2012 Accepted 21 December 2012 Available online 4 January 2013 Keywords: Fe3O4 nanorods Graphene sheets Composites Anode materials
a b s t r a c t Fe3O4 nanorod graphene composites (FNGC) have been successfully prepared via in situ self-assembly by mild chemical reduction of graphite oxide and (NH4)2Fe(SO4)2 in water with hydrazine as reducing agent under normal pressure. Scanning electron microscopy and transmission electron microscopy observations confirmed that the as-formed Fe3O4 nanorods, about 11 nm in diameter and more than 100 nm in length, were uniformly anchored on graphene nanosheets. Electrochemical investigation showed that the FNGC exhibited improved cycling stability and superior rate capacity in comparison with Fe3O4 nanoparticles. A charge specific capacity of 867 mA h g−1 was maintained with only 5% capacity loss after the 100th cycle at 1 C. At a current density of 5 C, its charge capacity was 569 mA h g −1. The results suggested that FNGC is a promising candidate for practical application as lithium ion battery anode material. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, lithium ion batteries (LIBs) have attracted extensive attention because of their high voltage, high specific energy, and long working life. Fe3O4 has been intensively investigated as a promising anode material for LIBs owing to its high capacity, abundance, environmental benignity, and low cost [1]. However, it suffers from poor cycling performance and low rate capacity because of a large volume change and serious aggregation during cycling processes. To circumvent these obstacles, designing nanostructures and particle mixtures with particular conductive additives such as hollow Fe3O4/C spheres [2], Fe3O4 helical carbon nanofibers [3] and Fe3O4 nanoparticles/ carbon nanotubes [4] has been employed to improve the reversible capacity and rate capability of the resulting electrodes. Graphene could be an ideal matrix for embedding functional substances owing to its superior electronic conductivity, remarkable structural flexibility, and high specific surface area [5]. Numerous graphenebased inorganic nanocomposites with metal oxide (MO) nanoparticles such as Fe3O4 [6,7], SnO2 [8], and Co3O4 [9] have been successfully synthesized. These nanocomposites have shown enhanced electrochemical properties. Of particular interest, recent works have illustrated that 1D metal oxides such as Co3O4 nanorod graphene nanosheet (GNS) nanocomposites show remarkably superior rate capacities than those of nanoparticle GNS nanocomposites [10]. This is attributed to the fact that the 1D metal oxide structure is more beneficial in not only preventing the aggregation of metal oxide nanocrystals because of confinement effects, but also in reducing the stacking degree of GNS because of its ⁎ Corresponding author. Tel.: +86 73188821610; fax: +86 73188823554. E-mail address: [email protected] (X. Chen). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2012.12.024
larger aspect ratio, thereby providing an excellent ion diffusion and electronic conduction pathway [10]. Until now, there have been no reports of 1D Fe3O4 structure GNS nanocomposites for LIB anodes. Herein, Fe3O4 nanorod GNS composites (FNGCs) were prepared via in situ self-assembly by a facile strategy, where graphite oxide (GO) and (NH4)2Fe(SO4)2 were reacted with hydrazine under atmospheric pressure. The obtained FNGCs exhibited superior rate capability, improved cycling stability, and high reversible lithium storage capacity. This procedure is a versatile route to prepare well controlled metal oxide GNS composites. 2. Experimental All chemicals were analytical grade and used without further purification. GO used in this work was prepared from natural flake graphite powder according to a modified Hummers method [11]. The resultant homogeneous brown GO was dispersed in water. In a typical procedure, 3 mL of 5 M NaOH was added to a 200 mL aqueous solution containing 80 mg GOs in a beaker, yielding a homogeneous yellow-brown dispersion. The dispersion was sonicated for 30 min, after which a 20 mL aqueous solution containing 1.16 g (NH4)2Fe(SO4)2 was added to it dropwise with further sonication and vigorous agitation for 30 min. Hydrazine (10 mL) was then added and the beaker was sealed with a plastic film. The solution was heated in an oven at 100 °C for 10 h, after which the product was isolated by magnetic separation and washed with water to pH 7. The resulting black product was dried in vacuum at 60 °C for 24 h and then calcined in a quartz tube at 450 °C for 30 min under argon atmosphere. For comparison, pure Fe3O4 nanoparticles were also prepared using a similar procedure without any GO.
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The phases of the products were characterized by X-ray powder diffraction (XRD, Siemens D5000 X-ray diffractometer with Cu·Kα, λ = 0.15406 nm, 40 kV, and 50 mA). The morphology and structure of the products were observed by field emission scanning electron microscopy (FESEM, JSM 6700F) and field emission transmission electron microscopy (FETEM, JEM 2010F). FT-IR spectra were recorded on a Nicolet 6700 FT-IR spectrometer (Thermo Electron Scientific Instruments, U.S.). Thermogravimetric (TG) analysis was performed on a thermal analyzer (Netzsch STA449C, Germany). Electrochemical experiments were carried out using CR2032-type coin cells. The working electrodes were prepared using a mixture of active material (Fe3O4 nanoparticles or FNGC (containing 75% Fe3O4)), acetylene black, and polyvinylidene difluoride (PVDF) at a weight ratio of 80:10:10, dispersed in N-methyl-2-pyrrolidone (NMP) to form a slurry. The slurry was uniformly spread on Cu foil with a blade and dried at 120 °C in a vacuum oven. The amount of active material in each electrode was 0.48 mg cm−2. Model cells were fabricated using lithium foil as counter electrode and a solution of 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1:1:1, wt.%) as the electrolyte. The CR2032-type coin cells were assembled in an argon-filled glove box. Galvanostatic cycling experiments were performed on a Land CT2001A battery test system in the voltage range 0.01–3.00 V versus Li+/Li at 25 °C. The mass of the entire composite was included when the current density was calculated. 3. Results and discussion Fig. 1a shows the XRD patterns of GO and FNGC. The main diffraction peaks of the FNGC pattern can be indexed to Fe3O4 with an inverse spinel structure (JCPDS card no.19-0629). Furthermore, no obvious characteristic diffraction peaks of GO (2θ = 10.4°) and graphite (2θ = 26.5°) were observed, which suggested that the GO was reduced and that the stacking of GNS in the FNGC was disordered [12]. TG analysis indicated that the amount of Fe3O4 in the FNGC was about 75 wt.% (Fig. 1b). The FT-IR spectra of graphite, GO, and FNGC are presented in Fig. 1c. Upon oxidation of graphite to GO, the observed representative peaks of GO confirmed the presence of the oxygen-containing functional groups
(\OH, \COOH, C\O\C) in carbon frameworks, including bands at 1067 cm−1 (C\O stretching vibration of epoxide), 1223 cm−1 (C\O stretching vibration of C\OH), and 1720 cm−1 (C_O stretching of carbonyl groups). In contrast, in the FT-IR spectrum of the FNGC (Fig. 1c), the peaks at 1223 cm−1 and 1720 cm−1 vanished, and a new absorption band was observed at 1564 cm−1, attributed to the aromatic skeletal C_C stretching vibration of GNS [10]. This result revealed that the \OH and \C_O groups of GO had been reduced by hydrazine to form some conjugated regions. Notably, the presence of the absorption peaks around 1067 cm−1 and 885 cm−1 in the FT-IR curve of the FNGC indicated that the epoxide groups of the GO were not reduced and remained at the edges of the GNS in the FNGC [13]. The residual oxygenated groups made it possible for the Fe3O4 nanorods to firmly bond to the surface of GNS through covalent bond between Fe and O [14,15]. The morphology and nanostructure of the FNGC were characterized by SEM and TEM. In low magnification SEM image of FNGC (Fig. 1d), the outline of both graphene and Fe3O4 nanorods can be clearly observed. The Fe3O4 nanorods were uniformly covered with GNS, forming the composites. No significant aggregation of the Fe3O4 nanorods was observed in the high magnification SEM image of the FNGC (inset in Fig. 1d). From TEM (Fig. 1e), the sheet-like products were attributed to GNS because of its thin structure, while the rod-like products were Fe3O4 of about 11 nm in diameter and more than 100 nm in length. It was clear that the two-dimensional GNS were well decorated by a large number of Fe3O4 nanorods. The selected area electron diffraction (SAED) pattern (inset in Fig. 1e) clearly demonstrated the polycrystalline nature of the obtained Fe3O4 nanorods. The first charge/discharge voltage profiles at a current density of 92.8 mA g −1 are shown in Fig. 2a. The FNGC shows a long discharge plateau voltage at 0.8 V, similar to that of Fe3O4, which was attributed to the conversion reaction: Fe3O4 + 8Li + + 8e − ↔ 3Fe 0 + 4Li2O. The sloping curve from 0.80 V to 0.01 V was mainly attributed to the formation of SEI film and accommodation of Li + in GNS, which is consistent with the literature [16]. The initial discharge and charge capacities of FNGC were 1538 mA h g −1 and 925 mA h g −1, respectively, higher than those of Fe3O4 nanoparticles (1066 and 767 mA h g −1). Fig. 2b and d show specific capacities at different current densities of the cell for FNGC and Fe3O4 nanoparticles, respectively. From the
Fig. 1. a) XRD patterns of GO and FNGC;. b) TG and DSC curves of FNGC in air atmosphere;. c) FT-IR spectra of graphite, GO, and FNGC;. d) SEM images of FNGC;. e) TEM image and SAED of FNGC.
A. Hu et al. / Electrochemistry Communications 28 (2013) 139–142
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Fig. 2. a) Charge and discharge profiles of Fe3O4 and FNGC at a current density of 92.8 mA g−1; b) rate performance of FNGC at current densities between 92.8 and 4640 mAg−1; c) special capacities versus cycle number between 0.01 and 3 V at 928 mA g−1; and d) capacity delivered upon cycling of the Fe3O4 electrode at current densities between 92.8 and 464 mA g−1.
second and following cycles, the FNGC electrode (Fig. 2b) clearly exhibited much higher capacities than the Fe3O4 nanoparticle electrode. After five cycles, it exhibited a high reversible discharge and charge capacity of 1046 mA h g −1 and 1006 mA h g −1, respectively, and the Coulombic efficiency rapidly increased from 60.2% in the first cycle to 96.2% in the fifth cycle. Importantly, the reversible charge capacity slightly increased with cycle number, similar to a previous report on Co3O4 nanorod GNS nanocomposites, which may be attributed to the gradual activation of nanocomposites during the first several cycles [10]. In contrast, the reversible charge and discharge capacity of the Fe3O4 nanoparticle (Fig. 2d) rapidly dropped to 630 mA h g −1 and 610 mA h g −1, respectively, after five cycles. A fascinating enhancement for the FNGC is its remarkable rate capability compared with the Fe3O4 electrode. As shown in Fig. 2b, when the same FNGC cell was tested for five cycles at different current densities between 92.8 mA g −1 (0.1 C) and 4840 mA g −1 (5 C), it exhibited an excellent high rate capability. At current densities of 185.6 (0.2 C), 464 (0.5 C), 928 (1 C), 1856 (2 C), and 4864 mA g −1 (5 C), the FNGC delivered high reversible charge specific capacities of 979, 923, 878, 800, and 569 mA h g −1, corresponding to charge capacity losses of 0.3, 6.6, 10.6, 19.5, and 42.1%, respectively. Remarkably, when the current density was returned to its initial value of 0.1 C after 30 cycles, the capacity recovered to the initial capacity or even a little higher (the reversible charge capacity was 1136 mA h g −1 for the 36th cycle), which illustrated that the structure of the anode material was not damaged. In contrast, from Fig. 2d, the Fe3O4 nanoparticle electrode exhibited a poor rate performance (the charge capacity was 74.8 mA h g −1 at a current of 464 mA g −1). More importantly, a comparison indicated that the 800 mA h g −1 rate capacity of the FNGC at 2 C was much larger than those previously reported for materials such as Fe3O4 nanoparticle GNS composites (410 mA h g −1 at a current density of 1000 mA g −1) [17]. As shown in Fig. 2c, the FNGC electrode exhibited a stable specific capacity as high as 867 mA h g −1 after the 100th cycle at 1 C with a capacity loss of only 5%. Remarkably, compared with Fe3O4 nanoparticle GNS composites (600 mA h g−1 at a current density of 500 mA g −1 after 10 cycles) [17], the FNGC had better capacity retention and cycling
stability at high current rate. This result can be ascribed to the FNGC structure. First, GNS, which acts as an elastic flexible support in FNGC, can maintain the structural integrity of the FNGC electrode and improve its cycle stability by suppressing volume change and particle agglomeration during intercalation/delithiation cycles because of the intimate interaction (Fe–O bond) between the GNS and Fe3O4 nanorods. In addition, the Fe3O4 nanorods were anchored on the surface of the GNS, which may have improved the rate performance because of the high electronic conductivity of GNS and the short path length of the Fe3O4 nanorods for Li+ transportation. 4. Conclusions In summary, a FNGC has been directly prepared from GO and (NH4)2Fe(SO4)2 at normal pressure. Fe3O4 nanorods of about 11 nm in diameter and more than 100 nm in length were found to be homogeneously embedded on graphene sheets in the composite. The FNGC exhibited remarkable lithium storage properties including high rate capability, reversible capacity, and good cycle performance, which makes it a suitable and promising anode material for LIBs. Acknowledgments This work was supported by the Natural Science Foundation of China (NSFC, nos. 50972043, 51154001, 51272073 and 11104066). References [1] Z.M. Cui, L.Y. Jiang, W.G. Song, Y.G. Guo, Chemistry of Materials 21 (2009) 1162. [2] Q.M. Zhang, Z.C. Shi, Y.F. Deng, J. Zheng, G.C. Liu, G.H. Chen, Journal of Power Sources 197 (2012) 305. [3] S. Ren, R. Prakash, D. Wang, V.S.K. Chakravadhanula, M. Fichtner, ChemSusChem 5 (2012) 1397. [4] Y. He, L. Huang, J.S. Cai, X.M. Zheng, S.G. Sun, Electrochimica Acta 55 (2010) 1140. [5] Y.H. Liu, J.S. Xue, T. Zheng, J.R. Dahn, Carbon 34 (1996) 193. [6] M. Zhang, M.Q. Jia, Y.H. Jin, Applied Surface Science 261 (2012) 298. [7] M. Sathish, T. Tomai, I. Honma, Journal of Power Sources 217 (2012) 85. [8] S.M. Paek, E.J. Yoo, I. Honma, Nano Letters 9 (2009) 72. [9] B.J. Li, H.Q. Cao, J. Shao, G.Q. Li, M.Z. Qu, G. Yin, Inorganic Chemistry 50 (2011) 1628.
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[10] L.Q. Tao, J.T. Zai, K.X. Wang, H.J. Zhang, M. Xu, J. Shen, Y.Z. Su, X.F. Qian, Journal of Power Sources 202 (2012) 230. [11] W.S. Hummers Jr., R.E. Offeman, Journal of the American Chemical Society 80 (1958) 1339. [12] Z.S. Wu, W. Ren, L. Wen, L. Gao, J. Zhao, Z. Chen, G. Zhou, F. Li, H.M. Cheng, ACS Nano 4 (2010) 3187. [13] M. Acik, G. Lee, C. Mattevi, M. Chhowalla, K. Cho, Y.J. Chabal, Nature Materials 9 (2010) 840.
[14] J. Su, M.H. Cao, L. Ren, C.W. Hu, Journal of Physical Chemistry C 115 (2011) 14469. [15] H. Wang, Y. Yang, Y. Liang, L. Cui, H.S. Casalongue, Y. Li, G. Hong, Y. Cui, H. Dai, Angewandte Chemie International Edition 50 (2011) 1. [16] Z.S. Wu, G.M. Zhou, L.C. Yin, W.C. Ren, F. Li, H.M. Cheng, Nano Energy 1 (2012) 107. [17] P.C. Lian, X.F. Zhu, H.F. Xiang, W.S. Yang, H.H. Wang, Electrochimica Acta 56 (2010) 834.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Self-assembly of Fe3O4 nanorods on graphene for lithium ion batteries with high rate capacity and cycle stability Aiping Hu a, Xiaohua Chen a,⁎, Yuanhong Tang a, Qunli Tang a, Lei Yang a, Shaopeng Zhang b a b
College of Materials Science and Engineering, Hunan University, Changsha 410082, China Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 14 December 2012 Received in revised form 21 December 2012 Accepted 21 December 2012 Available online 4 January 2013 Keywords: Fe3O4 nanorods Graphene sheets Composites Anode materials
a b s t r a c t Fe3O4 nanorod graphene composites (FNGC) have been successfully prepared via in situ self-assembly by mild chemical reduction of graphite oxide and (NH4)2Fe(SO4)2 in water with hydrazine as reducing agent under normal pressure. Scanning electron microscopy and transmission electron microscopy observations confirmed that the as-formed Fe3O4 nanorods, about 11 nm in diameter and more than 100 nm in length, were uniformly anchored on graphene nanosheets. Electrochemical investigation showed that the FNGC exhibited improved cycling stability and superior rate capacity in comparison with Fe3O4 nanoparticles. A charge specific capacity of 867 mA h g−1 was maintained with only 5% capacity loss after the 100th cycle at 1 C. At a current density of 5 C, its charge capacity was 569 mA h g −1. The results suggested that FNGC is a promising candidate for practical application as lithium ion battery anode material. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, lithium ion batteries (LIBs) have attracted extensive attention because of their high voltage, high specific energy, and long working life. Fe3O4 has been intensively investigated as a promising anode material for LIBs owing to its high capacity, abundance, environmental benignity, and low cost [1]. However, it suffers from poor cycling performance and low rate capacity because of a large volume change and serious aggregation during cycling processes. To circumvent these obstacles, designing nanostructures and particle mixtures with particular conductive additives such as hollow Fe3O4/C spheres [2], Fe3O4 helical carbon nanofibers [3] and Fe3O4 nanoparticles/ carbon nanotubes [4] has been employed to improve the reversible capacity and rate capability of the resulting electrodes. Graphene could be an ideal matrix for embedding functional substances owing to its superior electronic conductivity, remarkable structural flexibility, and high specific surface area [5]. Numerous graphenebased inorganic nanocomposites with metal oxide (MO) nanoparticles such as Fe3O4 [6,7], SnO2 [8], and Co3O4 [9] have been successfully synthesized. These nanocomposites have shown enhanced electrochemical properties. Of particular interest, recent works have illustrated that 1D metal oxides such as Co3O4 nanorod graphene nanosheet (GNS) nanocomposites show remarkably superior rate capacities than those of nanoparticle GNS nanocomposites [10]. This is attributed to the fact that the 1D metal oxide structure is more beneficial in not only preventing the aggregation of metal oxide nanocrystals because of confinement effects, but also in reducing the stacking degree of GNS because of its ⁎ Corresponding author. Tel.: +86 73188821610; fax: +86 73188823554. E-mail address: [email protected] (X. Chen). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2012.12.024
larger aspect ratio, thereby providing an excellent ion diffusion and electronic conduction pathway [10]. Until now, there have been no reports of 1D Fe3O4 structure GNS nanocomposites for LIB anodes. Herein, Fe3O4 nanorod GNS composites (FNGCs) were prepared via in situ self-assembly by a facile strategy, where graphite oxide (GO) and (NH4)2Fe(SO4)2 were reacted with hydrazine under atmospheric pressure. The obtained FNGCs exhibited superior rate capability, improved cycling stability, and high reversible lithium storage capacity. This procedure is a versatile route to prepare well controlled metal oxide GNS composites. 2. Experimental All chemicals were analytical grade and used without further purification. GO used in this work was prepared from natural flake graphite powder according to a modified Hummers method [11]. The resultant homogeneous brown GO was dispersed in water. In a typical procedure, 3 mL of 5 M NaOH was added to a 200 mL aqueous solution containing 80 mg GOs in a beaker, yielding a homogeneous yellow-brown dispersion. The dispersion was sonicated for 30 min, after which a 20 mL aqueous solution containing 1.16 g (NH4)2Fe(SO4)2 was added to it dropwise with further sonication and vigorous agitation for 30 min. Hydrazine (10 mL) was then added and the beaker was sealed with a plastic film. The solution was heated in an oven at 100 °C for 10 h, after which the product was isolated by magnetic separation and washed with water to pH 7. The resulting black product was dried in vacuum at 60 °C for 24 h and then calcined in a quartz tube at 450 °C for 30 min under argon atmosphere. For comparison, pure Fe3O4 nanoparticles were also prepared using a similar procedure without any GO.
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The phases of the products were characterized by X-ray powder diffraction (XRD, Siemens D5000 X-ray diffractometer with Cu·Kα, λ = 0.15406 nm, 40 kV, and 50 mA). The morphology and structure of the products were observed by field emission scanning electron microscopy (FESEM, JSM 6700F) and field emission transmission electron microscopy (FETEM, JEM 2010F). FT-IR spectra were recorded on a Nicolet 6700 FT-IR spectrometer (Thermo Electron Scientific Instruments, U.S.). Thermogravimetric (TG) analysis was performed on a thermal analyzer (Netzsch STA449C, Germany). Electrochemical experiments were carried out using CR2032-type coin cells. The working electrodes were prepared using a mixture of active material (Fe3O4 nanoparticles or FNGC (containing 75% Fe3O4)), acetylene black, and polyvinylidene difluoride (PVDF) at a weight ratio of 80:10:10, dispersed in N-methyl-2-pyrrolidone (NMP) to form a slurry. The slurry was uniformly spread on Cu foil with a blade and dried at 120 °C in a vacuum oven. The amount of active material in each electrode was 0.48 mg cm−2. Model cells were fabricated using lithium foil as counter electrode and a solution of 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1:1:1, wt.%) as the electrolyte. The CR2032-type coin cells were assembled in an argon-filled glove box. Galvanostatic cycling experiments were performed on a Land CT2001A battery test system in the voltage range 0.01–3.00 V versus Li+/Li at 25 °C. The mass of the entire composite was included when the current density was calculated. 3. Results and discussion Fig. 1a shows the XRD patterns of GO and FNGC. The main diffraction peaks of the FNGC pattern can be indexed to Fe3O4 with an inverse spinel structure (JCPDS card no.19-0629). Furthermore, no obvious characteristic diffraction peaks of GO (2θ = 10.4°) and graphite (2θ = 26.5°) were observed, which suggested that the GO was reduced and that the stacking of GNS in the FNGC was disordered [12]. TG analysis indicated that the amount of Fe3O4 in the FNGC was about 75 wt.% (Fig. 1b). The FT-IR spectra of graphite, GO, and FNGC are presented in Fig. 1c. Upon oxidation of graphite to GO, the observed representative peaks of GO confirmed the presence of the oxygen-containing functional groups
(\OH, \COOH, C\O\C) in carbon frameworks, including bands at 1067 cm−1 (C\O stretching vibration of epoxide), 1223 cm−1 (C\O stretching vibration of C\OH), and 1720 cm−1 (C_O stretching of carbonyl groups). In contrast, in the FT-IR spectrum of the FNGC (Fig. 1c), the peaks at 1223 cm−1 and 1720 cm−1 vanished, and a new absorption band was observed at 1564 cm−1, attributed to the aromatic skeletal C_C stretching vibration of GNS [10]. This result revealed that the \OH and \C_O groups of GO had been reduced by hydrazine to form some conjugated regions. Notably, the presence of the absorption peaks around 1067 cm−1 and 885 cm−1 in the FT-IR curve of the FNGC indicated that the epoxide groups of the GO were not reduced and remained at the edges of the GNS in the FNGC [13]. The residual oxygenated groups made it possible for the Fe3O4 nanorods to firmly bond to the surface of GNS through covalent bond between Fe and O [14,15]. The morphology and nanostructure of the FNGC were characterized by SEM and TEM. In low magnification SEM image of FNGC (Fig. 1d), the outline of both graphene and Fe3O4 nanorods can be clearly observed. The Fe3O4 nanorods were uniformly covered with GNS, forming the composites. No significant aggregation of the Fe3O4 nanorods was observed in the high magnification SEM image of the FNGC (inset in Fig. 1d). From TEM (Fig. 1e), the sheet-like products were attributed to GNS because of its thin structure, while the rod-like products were Fe3O4 of about 11 nm in diameter and more than 100 nm in length. It was clear that the two-dimensional GNS were well decorated by a large number of Fe3O4 nanorods. The selected area electron diffraction (SAED) pattern (inset in Fig. 1e) clearly demonstrated the polycrystalline nature of the obtained Fe3O4 nanorods. The first charge/discharge voltage profiles at a current density of 92.8 mA g −1 are shown in Fig. 2a. The FNGC shows a long discharge plateau voltage at 0.8 V, similar to that of Fe3O4, which was attributed to the conversion reaction: Fe3O4 + 8Li + + 8e − ↔ 3Fe 0 + 4Li2O. The sloping curve from 0.80 V to 0.01 V was mainly attributed to the formation of SEI film and accommodation of Li + in GNS, which is consistent with the literature [16]. The initial discharge and charge capacities of FNGC were 1538 mA h g −1 and 925 mA h g −1, respectively, higher than those of Fe3O4 nanoparticles (1066 and 767 mA h g −1). Fig. 2b and d show specific capacities at different current densities of the cell for FNGC and Fe3O4 nanoparticles, respectively. From the
Fig. 1. a) XRD patterns of GO and FNGC;. b) TG and DSC curves of FNGC in air atmosphere;. c) FT-IR spectra of graphite, GO, and FNGC;. d) SEM images of FNGC;. e) TEM image and SAED of FNGC.
A. Hu et al. / Electrochemistry Communications 28 (2013) 139–142
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Fig. 2. a) Charge and discharge profiles of Fe3O4 and FNGC at a current density of 92.8 mA g−1; b) rate performance of FNGC at current densities between 92.8 and 4640 mAg−1; c) special capacities versus cycle number between 0.01 and 3 V at 928 mA g−1; and d) capacity delivered upon cycling of the Fe3O4 electrode at current densities between 92.8 and 464 mA g−1.
second and following cycles, the FNGC electrode (Fig. 2b) clearly exhibited much higher capacities than the Fe3O4 nanoparticle electrode. After five cycles, it exhibited a high reversible discharge and charge capacity of 1046 mA h g −1 and 1006 mA h g −1, respectively, and the Coulombic efficiency rapidly increased from 60.2% in the first cycle to 96.2% in the fifth cycle. Importantly, the reversible charge capacity slightly increased with cycle number, similar to a previous report on Co3O4 nanorod GNS nanocomposites, which may be attributed to the gradual activation of nanocomposites during the first several cycles [10]. In contrast, the reversible charge and discharge capacity of the Fe3O4 nanoparticle (Fig. 2d) rapidly dropped to 630 mA h g −1 and 610 mA h g −1, respectively, after five cycles. A fascinating enhancement for the FNGC is its remarkable rate capability compared with the Fe3O4 electrode. As shown in Fig. 2b, when the same FNGC cell was tested for five cycles at different current densities between 92.8 mA g −1 (0.1 C) and 4840 mA g −1 (5 C), it exhibited an excellent high rate capability. At current densities of 185.6 (0.2 C), 464 (0.5 C), 928 (1 C), 1856 (2 C), and 4864 mA g −1 (5 C), the FNGC delivered high reversible charge specific capacities of 979, 923, 878, 800, and 569 mA h g −1, corresponding to charge capacity losses of 0.3, 6.6, 10.6, 19.5, and 42.1%, respectively. Remarkably, when the current density was returned to its initial value of 0.1 C after 30 cycles, the capacity recovered to the initial capacity or even a little higher (the reversible charge capacity was 1136 mA h g −1 for the 36th cycle), which illustrated that the structure of the anode material was not damaged. In contrast, from Fig. 2d, the Fe3O4 nanoparticle electrode exhibited a poor rate performance (the charge capacity was 74.8 mA h g −1 at a current of 464 mA g −1). More importantly, a comparison indicated that the 800 mA h g −1 rate capacity of the FNGC at 2 C was much larger than those previously reported for materials such as Fe3O4 nanoparticle GNS composites (410 mA h g −1 at a current density of 1000 mA g −1) [17]. As shown in Fig. 2c, the FNGC electrode exhibited a stable specific capacity as high as 867 mA h g −1 after the 100th cycle at 1 C with a capacity loss of only 5%. Remarkably, compared with Fe3O4 nanoparticle GNS composites (600 mA h g−1 at a current density of 500 mA g −1 after 10 cycles) [17], the FNGC had better capacity retention and cycling
stability at high current rate. This result can be ascribed to the FNGC structure. First, GNS, which acts as an elastic flexible support in FNGC, can maintain the structural integrity of the FNGC electrode and improve its cycle stability by suppressing volume change and particle agglomeration during intercalation/delithiation cycles because of the intimate interaction (Fe–O bond) between the GNS and Fe3O4 nanorods. In addition, the Fe3O4 nanorods were anchored on the surface of the GNS, which may have improved the rate performance because of the high electronic conductivity of GNS and the short path length of the Fe3O4 nanorods for Li+ transportation. 4. Conclusions In summary, a FNGC has been directly prepared from GO and (NH4)2Fe(SO4)2 at normal pressure. Fe3O4 nanorods of about 11 nm in diameter and more than 100 nm in length were found to be homogeneously embedded on graphene sheets in the composite. The FNGC exhibited remarkable lithium storage properties including high rate capability, reversible capacity, and good cycle performance, which makes it a suitable and promising anode material for LIBs. Acknowledgments This work was supported by the Natural Science Foundation of China (NSFC, nos. 50972043, 51154001, 51272073 and 11104066). References [1] Z.M. Cui, L.Y. Jiang, W.G. Song, Y.G. Guo, Chemistry of Materials 21 (2009) 1162. [2] Q.M. Zhang, Z.C. Shi, Y.F. Deng, J. Zheng, G.C. Liu, G.H. Chen, Journal of Power Sources 197 (2012) 305. [3] S. Ren, R. Prakash, D. Wang, V.S.K. Chakravadhanula, M. Fichtner, ChemSusChem 5 (2012) 1397. [4] Y. He, L. Huang, J.S. Cai, X.M. Zheng, S.G. Sun, Electrochimica Acta 55 (2010) 1140. [5] Y.H. Liu, J.S. Xue, T. Zheng, J.R. Dahn, Carbon 34 (1996) 193. [6] M. Zhang, M.Q. Jia, Y.H. Jin, Applied Surface Science 261 (2012) 298. [7] M. Sathish, T. Tomai, I. Honma, Journal of Power Sources 217 (2012) 85. [8] S.M. Paek, E.J. Yoo, I. Honma, Nano Letters 9 (2009) 72. [9] B.J. Li, H.Q. Cao, J. Shao, G.Q. Li, M.Z. Qu, G. Yin, Inorganic Chemistry 50 (2011) 1628.
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