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graphene and its lithium-storage performance
Electrochimica Acta 113 (2013) 212–217
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
One-pot synthesis of Fe2 O3 /graphene and its lithium-storage performance Jing Ye a,b , Jun Zhang b,∗ , Fengxian Wang b , Qingmei Su b , Gaohui Du b,∗ a b
Chuyang Honors College, Zhejiang Normal University, Jinhua 321004, China Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China
a r t i c l e
i n f o
Article history: Received 19 May 2013 Received in revised form 18 August 2013 Accepted 17 September 2013 Available online xxx Keywords: Ferric oxide Graphene Nanocomposite Lithium ion battery Electrochemical impedance spectrum
a b s t r a c t Fe2 O3 /graphene composite was synthesized by a facile hydrothermal method. SEM and TEM characterizations show that Fe2 O3 particles with size of 40–60 nm distributed uniformly on the surface of the graphene. As an anode material for Li-ion batteries (LIBs), Fe2 O3 /graphene delivered discharge and charge capacity of 1369 mAh g−1 and 899 mAh g−1 respectively in the first cycle with an initial Coulombic efficiency of 65.7%, which was much better than the bare Fe2 O3 nanoparticles electrode. Enhanced rate capacity and cycling stability were also observed for Fe2 O3 /graphene composite. A capacity of 559 mAh g−1 was maintained after 50 discharge–charge cycles. Uniform dispersion of Fe2 O3 , high conductivity and specific surface area were responsible for the enhancement of electrochemical property. Electrochemical impedance spectrum results revealed that the improved electrochemical performance of Fe2 O3 /graphene can be attributed to fast migration of Li+ through surface film and charge transfer on active material/electrolyte interfaces. The synthesis approach presents a promising route for large-scale production of Fe2 O3 /graphene composite as electrode materials for Li-ion batteries. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Rechargeable Li-ion batteries have attracted worldwide interests for its prominent application in portable electronic devices and hybrid electric vehicles. Numerous researches on Li-ion batteries (LIBs) are carried out in order to meet the increasing demand for higher energy density batteries. Transition metal oxides (such as NiO [1,2,3], Co3 O4 [4,5], CuO [6,7,8], etc.) have drawn plentiful attention as anodes for LIBs due to their high theoretical energy densities. Among various transition metal oxides, hematite (Fe2 O3 ) is one of the most promising anode materials, because of its nontoxicity, good stability, and low cost [9–11]. However, transition metal oxides including Fe2 O3 suffer severe loss of capacity with cycling owing to their poor electrical conductivity and large volume expansion/shrinkage during Li+ insertion/extraction processes. This phenomenon is more apparent at high rates [12,13]. One of the approaches to inhibit pulverization of transition metal oxide during cycling is to fabricate nanostructures, which is also beneficial to the infiltration of electrolyte. Nanoparticles can also shorten the Li+ insertion/extraction pathways significantly [14,15]. However, nanoparticles are prone to aggregate, resulting poor electrical contact between particles and
∗ Corresponding authors. Tel.: +86 579 82283897; fax: +86 579 82282595. E-mail addresses: [email protected], [email protected] (J. Zhang), [email protected] (G. Du). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.09.103
hence fast degradation of capacity, which counteracts their advantage as anode for lithium ion batteries. To solve these problems, researchers attend to synthesize nanostructured oxide/conductor composites, such as oxide/metal [16], oxide/CNT [17], oxide/PANI [18], etc. Recently, a new form of carbonaceous materials – graphene has been attracting extensive attention for its unique physical and chemical characteristics. Graphene is demonstrated as an outstanding “matrix” material in composite for its high theoretical surface of 2630 m2 /g, preeminent electronic conductivity and electron mobility, terrific chemical stability and suppleness. On one hand, the incorporation of nanoparticles and graphene nanosheets (GNS) will generate a porous network, providing excellent electron-conducting and ion-transporting pathways. On the other hand, the nanostructured metal oxides loaded onto GNS can act as spacers to prevent the re-stacking of GNS during cycling process [19–21]. Therefore, graphene–metal oxide composites become an attractive topic for researchers as to increase the cycling performance and rate capabilities of LIBs [22–25]. However, among the reported researches, most synthesizing approaches consist of mainly two steps, which require the reduction of graphene oxide by thermal annealing or adding toxic hydrazine [26,27]. The experiment was complicated and not environmentally friendly. Thus we believe it is worthwhile to explore a simple and nontoxic way to synthesize Fe2 O3 /graphene composite as anode for LIBs. In this work, we proposed a facile one-pot hydrothermal process to prepare Fe2 O3 /graphene composite without any template or
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organic solvent, reducing the experimental pollution and the cost as well compared to previous work [11,28]. As LIBs electrode material, Fe2 O3 /graphene composite exhibited an excellent performance in lithium storage and rate capability. The whole process was simple, practical and economic. The synthesis approach presents a promising route for large-scale production of Fe2 O3 /graphene composite as electrode materials for Li-ion batteries.
was used as the counter electrode. The cells were charged and discharged between 0.01 and 3 V (vs. Li/Li+ ) at various current densities. Cyclic voltammetry (CV) was performed on a CHI604D electrochemical workstation at a scan rate of 0.1 mV s−1 . All tests were performed at room temperature.
2. Experimental
XRD analysis of the sample was shown in Fig. 1(a). For both samples (the pure Fe2 O3 and the Fe2 O3 /graphene composite), characteristic diffraction peaks are in good accordance with hexagonal ˚ c = 13.75 A˚ (standard crystal structure ␣-Fe2 O3 with a = 5.035 A, hematite PCPDS 33-0664). It is worth mentioning that the characteristic (0 0 2) peak of carbon is shadowed by the (0 1 2) peak of crystals at the similar degree so that (0 1 2) peak of Fe2 O3 /graphene is sharper than that of pure Fe2 O3 . No evident peaks of impurities are detected in the samples. The crystal grain size of Fe2 O3 is calculated to be ∼45 nm using the Scherrer equation. The weight ratio of Fe2 O3 and graphene was determined by TGA in air as shown in Fig. 2(b). The graphene content was estimated to 12% according to TGA. The increase of sample quality in less than 200 ◦ C can be attributed to the oxidation of graphene in the air. Fig. 2 shows the scanning electron microcopy (SEM) and transmission electron microscopy (TEM) images of Fe2 O3 and Fe2 O3 /graphene composite. The sizes of the Fe2 O3 nanoparticles of both samples are about 40–60 nm, which is consistent with the XRD results. Fig. 2b shows that the Fe2 O3 nanoparticles are distributed uniformly on the surface of graphene, which can be further confirmed by the TEM image (Fig. 2c). Fig. 2d shows the HRTEM image of Fe2 O3 nanoparticle on graphene. An interlayer distance of 0.27 nm are observed, which agrees well with the spacing between (1 1 0) planes of Fe2 O3 crystals. The nanoparticles are found to be monocrystals from the HRTEM and diffraction results. Fig. 3 shows the electrochemical properties of Fe2 O3 and Fe2 O3 /graphene composite. Fig. 3a presents the first cycle of CV curve of Fe2 O3 and Fe2 O3 /graphene. The cathodic peaks at 1.52 V for Fe2 O3 /graphene and 1.48 V for the bare Fe2 O3 represent the Li+ intercalation to hexagonal Fe2 O3 [10]:
2.1. Preparation of graphene Graphene oxide was synthesized from purified natural graphite by a modified Hummers method [29]. Briefly, on the preoxidation process, 1 g graphite powder, 1 g K2 S2 O8 and 1 g P2 O5 was added to 25 ml 98% H2 SO4 , stirring at the temperature of 35 ◦ C for 6 h. The resultant was washed by deionized (DI) water until the pH of the eluate became neutral, then dried at 60 ◦ C for 24 h. In the reoxidation process, the above product was added to 50 ml 98% H2 SO4 in an ice bath, 5 g KMnO4 was gradually added with stirring. After stirred for 30 min, the mixture was heated to 35 ◦ C and kept for 6 h. Then 100 ml DI water was added with the temperature rising to 98 ◦ C for 15 min. The reactant was washed by 1:10 diluted hydrochloric acid and DI water until the supernatant liquor is neutral. The obtained yellow and viscous liquid was dried at 60 ◦ C for 24 h and graphene oxide was received. Graphene oxide was then reduced to graphene by calcining at 400 ◦ C in nitrogen flow for 2 h and most oxygen groups were successfully removed [30]. The as-prepared graphene is single or few-layer with lots of wrinkles. 2.2. Preparation of Fe2 O3 and Fe2 O3 /graphene composite Fe2 O3 /graphene composite was fabricated by a one-pot hydrothermal method. 0.067 g graphene was dispersed in 16 ml DI water by ultrasonication, then 3.5 ml HAc and 0.317 g iron chloride (FeCl3 ) were added into the suspension while stirring, after which 3.5 ml ammonia water (NH3 ·H2 O) was added slowly and sequentially. The solution was then transferred to a 25 ml Teflon-lined stainless steel autoclave and heated at 160 ◦ C for 48 h. After the mixture cooled to room temperature naturally, the brown loose precipitate was collected by centrifugation, washed with deionized water, and dried under vacuum at 60 ◦ C for 12 h. 0.132 g Fe2 O3 /graphene composite was obtained. For comparison, bare Fe2 O3 nanoparticles were prepared by a similar method except for the absence of graphene. 2.3. Materials characterizations The composition, morphology and microstructures of the products were analyzed using powder X-ray diffraction (XRD, Cu K␣ radiation, Philips PW3040/60), scanning electron microscopy (SEM, Hitachi S-4800) and high-resolution transmission electron microscope (TEM, JEOL-2100F). The thermogravimetric analysis (TGA) measurement were conducted on a Netzsch STA 449C thermal analyzer. As for the electrochemical characterization, the experiments were performed at a CT2001A Land battery tester at room temperature. To prepare the LIB anode, the electrode slurry was made by mixing the active material, acetylene black and polyvinylidene fluoride (PVDF) in a weight ratio of 75:15:10 in N-methyl pyrrolidone with stirring for 2 h. The working electrodes were made by spreading the slurry onto a Ni foam current collector (˚ 14 mm) and dried at 120 ◦ C under vacuum overnight. The average loading mass of the active material is 1.03 mg cm−2 in terms of Fe2 O3 . The electrolyte solution was 1 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1, v/v). Pure Li foil (Aldrich)
3. Results and discussion
Fe2 O3 + 2Li+ + 2e− → Li2 (Fe2 O3 )
(1)
Li2 (Fe2 O3 ) + 4Li+ + 4e− → 2Fe0 + 3Li2 O
(2)
Fe2 O3 + 6Li+ + 6e− ↔ 2Fe + 3Li2 O
(3)
The broad cathodic peaks located at 0.1–0.65 V can be attributed to the stepwise reduction of Fe3+ by Li to nanometer-sized Fe metal (Fe0 ) with the formation of amorphous Li2 O, and the irreversible reaction with the electrolyte to form solid electrolyte interface (SEI) film [31], which could be expressed by Eq. (2). The anodic peaks of the two materials are both at 1.75 V. The storage mechanism is based on a well-known reversible Li-ion reaction with transitional metal oxides as described by Eq. (3) [32,33]. Fig. 3b shows that the main reduction peak shifts to 0.65 V for the composite since the 2nd cycle. The CV curve of the 3rd cycle is similar to that of the 2nd cycle, indicating that the electrochemical reversibility of the obtained composites is gradually stabilized after the 2nd cycle. The charge/discharge profiles of the bare Fe2 O3 nanoparticles and the Fe2 O3 /graphene composite are shown in Fig. 3c and d. Insets are the dQ/dV curves of the first discharge profile. The electrode based on Fe2 O3 /graphene composite delivers a discharge capacity of 1369 mAh g−1 and a charge capacity of 899 mAh g−1 at the 1st cycle with a Coulombic efficiency of 65.7%, which is comparable with the Fe2 O3 nanoparticle/graphene composite prepared using hydrothermal method [34] but lower than the rice-on-sheet Fe2 O3 /graphene composite prepared by a microwave-assisted
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Fig. 1. (a) XRD patterns of Fe2 O3 nanoparticles on GNS and pure Fe2 O3 nanoparticles, (b) thermogravimetric analysis of Fe2 O3 /graphene composite in air.
hydrothermal technique [14]. The Coulombic efficiency rapidly increases to 95% in the 2nd cycle, and remains stable in the following cycles. In contrast, the bare Fe2 O3 electrode exhibits discharge and charge capacity of 1236 mAh g−1 and 493 mAh g−1 , respectively in the first cycle with a Coulombic efficiency of only 39.9%. For the Fe2 O3 /graphene composite, an additional peak is found in the dQ/dV curve when the discharge voltage down to 0.15 V, which delivers ∼130 mAh g−1 (versus the composite). This part of capacity could be partially attributed to the contribution from graphene. It suggests that, for the first discharge, bare Fe2 O3 delivers a comparable capacity with Fe2 O3 nanoparticles on graphene sheet. The enhancement effect of graphene to the capacity is mainly reflected in the Coulombic efficiency and capacity retention. Fig. 4a presents the cycling performance of bare Fe2 O3 and Fe2 O3 /graphene composite at a current density of 50 mA g−1 . After 50 discharge–charge cycles, the Fe2 O3 /graphene composite still maintains a capacity of 559 mAh g−1 , while bare Fe2 O3 exhibits a poor capacity of only 76 mAh g−1 . It achieves a comparable stability
with the previous report with the same morphology of particle-onsheet or nanorod-on-sheet [35], but inferior than the rice-shaped Fe2 O3 on graphene composite, which could be attributed to the agglomeration effect [14]. Notably, the reversible capacity of Fe2 O3 /graphene composite slightly increases from the 2nd cycle, and the Coulombic efficiency reaches to 99.1% after 50 cycles, which could be ascribed to the gradual activation of GNS in the composite during the first several cycles [1]. The Fe2 O3 /graphene composite displays good rate performance as well (Fig. 4b). It delivers discharge capacities of ∼450 mAh g−1 and 300 mAh g−1 at current densities of 200 mA g−1 and 300 mA g−1 , respectively. When the discharge rate decreases back to 50 mA g−1 after 50 cycles, the capacity recovers to 750 mAh g−1 , indicating good rate endurance. The Fe2 O3 /graphene composite prepared in this work exhibits better electrochemical performance than the previous report with the same content of graphene (12 wt%) [36]. From the above analysis, it is believed that GNS can provide a highly conductive matrix for the diffusion of electrons and lithium
Fig. 2. (a) SEM image of Fe2 O3 ; (b) SEM image of Fe2 O3 /graphene; (c) TEM image of Fe2 O3 /graphene; (d) TEM image of Fe2 O3 /grapheme in high resolution.
J. Ye et al. / Electrochimica Acta 113 (2013) 212–217
(c) 3.0
0.5
Current (mA g )
+
Potential (V vs Li/Li )
(a)
0.0
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-0.5 -1.0 Fe2O3 /graphene Fe2O3
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2.5 2.0 1.5 1.0 0.5 0.0
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+
Potential /V (vs. Li /Li)
(b)
(d)
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Potential (V vs Li/Li )
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Current (mA g )
0.2 0.0 -0.2 -0.4
1st 2nd 3rd
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0.0
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1.0
1.5
2.0
2.5
3rd
3rd
3.0
+
Potential (V vs. Li /Li) Fig. 3. Electrochemical performance of bare Fe2 O3 and Fe2 O3 /graphene composite. (a) The comparison of CV curves of bare Fe2 O3 and Fe2 O3 /graphene, (b) first three CV curves of Fe2 O3 /graphene composite, (c) typical charge–discharge curves of Fe2 O3 /graphene, (d) typical charge–discharge curves of Fe2 O3 . Insets are the corresponding differential capacity versus voltage plots of the first discharge profiles in (c) and (d).
100 1500
80 60
1000
40
Fe2O3/graphene
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20 Fe2O3
Coulombic efficiency (%)
Specific capacity (mAh/g)
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Specific capacity (mAh g )
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Fe2O3 discharge
1000 100 mA g
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-1
500 300 mA g
0
0
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-1
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Cycle number Fig. 4. (a) the comparison of the cycling performance of Fe2 O3 /graphene and bare Fe2 O3 , (b) rate capacity of Fe2 O3 /graphene and Fe2 O3 .
ions during the lithium insertion and extraction reactions. To further understand the mechanism for the improved cycling stability of Fe2 O3 /graphene, impedance measurements were carried out in the frequency range from 0.01 Hz to 100 kHz at room temperature at the open-circuit voltage of 2.327 V. Fig. 5a shows the experimental EIS data and fitting plots with a simulating model illustrated in Fig. 5b. REL indicates the electrolyte resistance; RCT and RSF stands for the charge transfer and surface film resistance, respectively; QCPE designates a constant phase element; Ws refers to suitable diffusional components like Warburg impedance. The impendence of CPE is defined as ZCPE = 1/[B(jw)a ] where w is the angular frequency, and B and a are constants. The value of a gives the degree of distortion of the impedance spectra and when a is more close to 1, CPE is more close to an ideal capacitor. The excellent fitness between the simulated curve and the experimental EIS data indicates the accuracy of the simulated model. From the EIS plots, both impedance spectra consist of semicircles in the high and medium frequency region and a following line in low frequency. As is known, the high frequency region in the EIS plots is indexed to the migration impedance of lithium ions through the surface layer, the medium frequency region corresponds to the charge-transfer impedance in the film-solution interface, and the low frequency region reflects the solid-state diffusion impedance of lithium ions in the active material [37,38]. The Fe2 O3 /graphene electrode shows smaller semicircles in both high and medium frequency than Fe2 O3 electrode, which indicates the lithium ions and electrons can transfer more easily on Fe2 O3 /graphene/electrolyte interface. The decreasing charge-transfer resistance results in the enhanced electrode process kinetics and improved electrochemical performance of Fe2 O3 /graphene electrode. The fitting parameters are given in Table 1. All parameters of Fe2 O3 /graphene electrode are better than that of Fe2 O3 electrode
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Acknowledgments This work was supported by the National Science Foundation of China (No. 21203168), the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET11-1081), and the Science Technology Department of Zhejiang Province (Grant No. 2013C31070).
References
Fig. 5. EIS plots of electrodes made from Fe2 O3 /graphene composite and bare Fe2 O3 nanoparticles at open-circuit voltage of 2.327 V (a), and the equivalent circuit to fit the experimental data (b).
Table 1 Impedance parameters of the cells fabricated with Fe2 O3 /graphene composite and bare Fe2 O3 nanoparticles as the cathode. Parameters
Fe2 O3 /graphene composite
Bare Fe2 O3
REL () RCT + RSF () QCPE (F) ˛
4.40 290.7 294.7 0.77
5.37 1107 436.8 0.61
quantificationally. For the Fe2 O3 /graphene electrode, the sum of charge transfer (RCT ) and surface film resistance (RSF ) is 290.7 , much smaller than that of the bare Fe2 O3 nanoparticles electrode’s 1107 , the corresponding CPE is 294.7 F and 436.8 F. The electrolyte resistance (REL ) in Fe2 O3 /graphene electrode is 0.97 less than that in bare Fe2 O3 nanoparticles electrode. Therefore, in this work, it is considered that the kinetics of Li+ migration and charge transfer on active material/electrolyte interfaces is the key point to influence the electrochemical performances of the as-prepared Fe2 O3 /graphene electrode. 4. Conclusions In summary, we report a simple method to fabricate Fe2 O3 /graphene composite as a high-performance anode material for LIBs. The Fe2 O3 /graphene composite delivered discharge and charge capacity of 1369 mAh g−1 and 899 mAh g−1 respectively in the first cycle with an initial Coulombic efficiency of 65.7%, which was much better than the bare Fe2 O3 nanoparticles electrode. A good rate capacity and cycling performance were also observed for the Fe2 O3 /graphene composite. The enhanced electrochemical performance of Fe2 O3 /graphene can be ascribed to fast migration of Li+ through surface film and charge transfer on active material/electrolyte interfaces. The synthesis approach presents a promising route for a large-scale production of Fe2 O3 /graphene composite as electrode materials for Li-ion batteries, which can be also extended to other transition metal oxides.
[1] L.Q. Tao, J.T. Zai, K.X. Wang, Y.H. Wan, H.J. Zhang, C. Yu, Y.L. Xiao, X.F. Qian, 3D-hierarchical NiO-graphene nanosheet composites as anodes for lithium ion batteries with improved reversible capacity and cycle stability, RSC Advances 2 (2012) 3410. [2] Y. Xia, W.K. Zhang, Z. Xiao, H. Huang, H.J. Zeng, X.R. Chen, F. Chen, Y.P. Gan, X.Y. Tao, Biotemplated fabrication of hierarchically porous NiO/C composite from lotus pollen grains for lithium-ion batteries, Journal of Materials Chemistry 22 (2012) 9209. [3] X.H. Huang, J.P. Tu, C.Q. Zhang, F. Zhou, Hollow microspheres of NiO as anode materials for lithium-ion batteries, Electrochimica Acta 55 (2010) 8981. [4] H. Huang, W.J. Zhu, X.Y. Tao, Y. Xia, Z.Y. Yu, J.W. Fang, Y.P. Gan, W.K. Zhang, Nanocrystal-constructed mesoporous single-crystalline Co3 O4 nanobelts with superior rate capability for advanced lithium-ion batteries, ACS Applied Materials and Interfaces 4 (2012) 5974. [5] Y. Lu, Y. Wang, Y.Q. Zou, Z. Jiao, B. Zhao, Y.Q. He, M.H. Wu, Macroporous Co3 O4 platelets with excellent rate capability as anodes for lithium ion batteries, Electrochemistry Communications 12 (2010) 101. [6] X.P. Gao, J.L. Bao, G.L. Pan, H.Y. Zhu, P.X. Huang, F. Wu, D.Y. Song, Preparation and electrochemical performance of polycrystalline and single crystalline CuO nanorods as anode materials for Li ion battery, The Journal of Physical Chemistry B 108 (2004) 5547. [7] J.Y. Xiang, J.P. Tu, L. Zhang, Y. Zhou, X.L. Wang, S.J. Shi, Simple synthesis of surface-modified hierarchical copper oxide spheres with needle-like morphology as anode for lithium ion batteries, Electrochimica Acta 55 (2010) 1820. [8] Y.J. Mai, X.L. Wang, J.Y. Xiang, Y.Q. Qiao, D. Zhang, C.D. Gu, J.P. Tu, CuO/graphene composite as anode materials for lithium-ion batteries, Electrochimica Acta 56 (2011) 2306. [9] J. Chen, L. Xu, W. Li, X. Gou, ␣-Fe2 O3 nanotubes in gas sensor and lithium-ion battery applications, Advanced Materials 17 (2005) 582. [10] M.V. Reddy, T. Yu, C.H. Sow, Z.X. Shen, C.T. Lim, G.V.S. Rao, B.V.R. Chowdari, ␣Fe2 O3 nanoflakes as an anode material for Li-ion batteries, Advanced Functional Materials 17 (2007) 2792. [11] X. Xu, R. Cao, S. Jeong, J. Cho, Spindle-like mesoporous alpha-Fe2 O3 anode material prepared from MOF template for high-rate lithium batteries, Nano Letters 12 (2012) 4988. [12] X.J. Zhu, Z.P. Guo, P. Zhang, G.D. Du, R. Zeng, Z.X. Chen, S. Li, H.K. Liu, Highly porous reticular tin–cobalt oxide composite thin film anodes for lithium ion batteries, Journal of Materials Chemistry 19 (2009) 8360. [13] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W. van Schalkwijk, Nanostructured materials for advanced energy conversion and storage devices, Nature Materials 4 (2005) 366. [14] Y.Q. Zou, J. Kan, Y. Wang, Fe2 O3 -graphene rice-on-sheet nanocomposite for high and fast lithium ion storage, Journal of Physical Chemistry C 115 (2011) 20747. [15] Q.Q. Xiong, J.P. Tu, Y. Lu, J. Chen, Y.X. Yu, Y.Q. Qiao, X.L. Wang, C.D. Gu, Synthesis of hierarchical hollow-structured single-crystalline magnetite (Fe3 O4 ) microspheres: the highly powerful storage versus lithium as an anode for lithium ion batteries, Journal of Physical Chemistry C 116 (2012) 6495. [16] X.H. Huang, J.P. Tu, Z.Y. Zeng, J.Y. Xiang, X.B. Zhao, Nickel foam-supported porous NiO/Ag film electrode for lithium-ion batteries, Journal of the Electrochemical Society 155 (2008) A438. [17] J.Y. Xiang, J.P. Tu, J. Zhang, J. Zhong, D. Zhang, J.P. Cheng, Incorporation of MWCNTs into leaf-like CuO nanoplates for superior reversible Li-ion storage, Electrochemistry Communications 12 (2010) 1103. [18] X.H. Huang, J.P. Tu, X.H. Xia, X.L. Wang, J.Y. Xiang, Nickel foam-supported porous NiO/polyaniline film as anode for lithium ion batteries, Electrochemistry Communications 10 (2008) 1288. [19] H. Kim, D.H. Seo, S.W. Kim, J. Kim, K. Kang, Highly reversible Co3 O4 /graphene hybrid anode for lithium rechargeable batteries, Carbon 49 (2011) 326. [20] Z.S. Wu, W.C. Ren, L. Wen, L.B. Gao, J.P. Zhao, Z.P. Chen, G.M. Zhou, F. Li, H.M. Cheng, Graphene anchored with Co3 O4 nanoparticles as anode of lithium ion batteries with enhanced reversible capacity and cyclic performance, ACS Nano 4 (2010) 3187. [21] Y.Q. Zou, Y. Wang, NiO nanosheets grown on graphene nanosheets as superior anode materials for Li-ion batteries, Nanoscale 3 (2011) 2615. [22] Y. He, L. Huang, J.S. Cai, X.M. Zheng, S.G. Sun, Structure and electrochemical performance of nanostructured Fe3 O4 /carbon nanotube composites as anodes for lithium ion batteries, Electrochimica Acta 55 (2010) 1140. [23] L. Wang, Y. Yu, P.C. Chen, D.W. Zhang, C.H. Chen, Electrospinning synthesis of C/Fe3 O4 composite nanofibers and their application for high performance lithium-ion batteries, Journal of Power Sources 183 (2008) 717.
J. Ye et al. / Electrochimica Acta 113 (2013) 212–217 [24] W.M. Zhang, X.L. Wu, J.S. Hu, Y.G. Guo, L.J. Wan, Carbon coated Fe3 O4 nanospindles as a superior anode material for lithium-ion batteries, Advanced Functional Materials 18 (2008) 3941. [25] L.Q. Lu, Y. Wang, Sheet-like and fusiform CuO nanostructures grown on graphene by rapid microwave heating for high Li-ion storage capacities, Journal of Materials Chemistry 21 (2011) 17916. [26] J. Zhu, T. Zhu, X. Zhou, Y. Zhang, X.W. Lou, X. Chen, H. Zhang, H.H. Hng, Q. Yan, Facile synthesis of metal oxide/reduced graphene oxide hybrids with high lithium storage capacity and stable cyclability, Nanoscale 3 (2011) 1084. [27] X. Zhu, Y. Zhu, S. Murali, M.D. Stoller, R.S. Ruoff, Nanostructured reduced graphene oxide/Fe2 O3 composite as a high-performance anode material for lithium ion batteries, ACS Nano 5 (2011) 3333. [28] S.Y. Liu, J. Xie, Q. Pan, C.Y. Wu, G.S. Cao, T.J. Zhu, X.B. Zhao, Graphene anchored with nanocrystal Fe2 O3 with improved electrochemical Li-storage properties, International Journal of Electrochemical Science 7 (2012) 354. [29] N.I. Kovtyukhova, P.J. Ollivier, B.R. Martin, T.E. Mallouk, S.A. Chizhik, E.V. Buzaneva, A.D. Gorchinskiy, Layer-by-layer assembly of ultrathin composite films from micro-sized graphite oxide sheets and polycations, Chemistry of Materials 11 (1999) 771. [30] Z.L. Wang, D. Xu, Y. Huang, Z. Wu, L.M. Wang, X.B. Zhang, Facile, mild and fast thermal-decomposition reduction of graphene oxide in air and its application in high-performance lithium batteries, Chemical Communications 48 (2012) 976.
217
[31] G. Zhou, D.W. Wang, L.C. Yin, N. Li, F. Li, H.M. Cheng, Oxygen bridges between NiO nanosheets and graphene for improvement of lithium storage, ACS Nano 6 (2012) 3214. [32] J. Ma, D. Lei, X. Duan, Q. Li, T. Wang, A. Cao, Y. Mao, W. Zheng, Designable fabrication of flower-like SnS2 aggregates with excellent performance in lithium-ion batteries, RSC Advances 2 (2012) 3615. [33] C. Shi, T. Cao, C.B. Cao, D.J. Wu, Y.A. Li, M.N. Wang, W. Mo, eta-Fe2 O3 spindles as anode materials for lithium-ion batteries with excellent stable and high electrochemical performance, Materials Letters 83 (2012) 35. [34] L.L. Tian, Q.C. Zhuang, J. Li, C. Wu, Y.L. Shi, S.G. Sun, The production of selfassembled Fe2 O3 -graphene hybrid materials by a hydrothermal process for improved Li-cycling, Electrochimica Acta 65 (2012) 153. [35] X.Y. Xue, C.H. Ma, C.X. Cui, L.L. Xing, High lithium storage performance of alphaFe2 O3 /graphene nanocomposites as lithium-ion battery anodes, Solid State Sciences 13 (2011) 1526. [36] W. Xiao, Z.X. Wang, H.J. Guo, X.H. Li, J.X. Wang, S.L. Huang, L. Gan, Fe2 O3 particles enwrapped by graphene with excellent cyclability and rate capability as anode materials for lithium ion batteries, Applied Surface Science 266 (2013) 148. [37] J.Y. Xiang, J.P. Tu, Y.Q. Qiao, X.L. Wang, J. Zhong, D. Zhang, C.D. Gu, Electrochemical impedance analysis of a hierarchical CuO electrode composed of self-assembled nanoplates, Journal of Physical Chemistry C 115 (2011) 2505. [38] J.Y. Xiang, X.L. Wang, X.H. Xia, J. Zhong, J.P. Tu, Fabrication of highly ordered porous nickel phosphide film and its electrochemical performances toward lithium storage, Journal of Alloys and Compounds 509 (2011) 157.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
One-pot synthesis of Fe2 O3 /graphene and its lithium-storage performance Jing Ye a,b , Jun Zhang b,∗ , Fengxian Wang b , Qingmei Su b , Gaohui Du b,∗ a b
Chuyang Honors College, Zhejiang Normal University, Jinhua 321004, China Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China
a r t i c l e
i n f o
Article history: Received 19 May 2013 Received in revised form 18 August 2013 Accepted 17 September 2013 Available online xxx Keywords: Ferric oxide Graphene Nanocomposite Lithium ion battery Electrochemical impedance spectrum
a b s t r a c t Fe2 O3 /graphene composite was synthesized by a facile hydrothermal method. SEM and TEM characterizations show that Fe2 O3 particles with size of 40–60 nm distributed uniformly on the surface of the graphene. As an anode material for Li-ion batteries (LIBs), Fe2 O3 /graphene delivered discharge and charge capacity of 1369 mAh g−1 and 899 mAh g−1 respectively in the first cycle with an initial Coulombic efficiency of 65.7%, which was much better than the bare Fe2 O3 nanoparticles electrode. Enhanced rate capacity and cycling stability were also observed for Fe2 O3 /graphene composite. A capacity of 559 mAh g−1 was maintained after 50 discharge–charge cycles. Uniform dispersion of Fe2 O3 , high conductivity and specific surface area were responsible for the enhancement of electrochemical property. Electrochemical impedance spectrum results revealed that the improved electrochemical performance of Fe2 O3 /graphene can be attributed to fast migration of Li+ through surface film and charge transfer on active material/electrolyte interfaces. The synthesis approach presents a promising route for large-scale production of Fe2 O3 /graphene composite as electrode materials for Li-ion batteries. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Rechargeable Li-ion batteries have attracted worldwide interests for its prominent application in portable electronic devices and hybrid electric vehicles. Numerous researches on Li-ion batteries (LIBs) are carried out in order to meet the increasing demand for higher energy density batteries. Transition metal oxides (such as NiO [1,2,3], Co3 O4 [4,5], CuO [6,7,8], etc.) have drawn plentiful attention as anodes for LIBs due to their high theoretical energy densities. Among various transition metal oxides, hematite (Fe2 O3 ) is one of the most promising anode materials, because of its nontoxicity, good stability, and low cost [9–11]. However, transition metal oxides including Fe2 O3 suffer severe loss of capacity with cycling owing to their poor electrical conductivity and large volume expansion/shrinkage during Li+ insertion/extraction processes. This phenomenon is more apparent at high rates [12,13]. One of the approaches to inhibit pulverization of transition metal oxide during cycling is to fabricate nanostructures, which is also beneficial to the infiltration of electrolyte. Nanoparticles can also shorten the Li+ insertion/extraction pathways significantly [14,15]. However, nanoparticles are prone to aggregate, resulting poor electrical contact between particles and
∗ Corresponding authors. Tel.: +86 579 82283897; fax: +86 579 82282595. E-mail addresses: [email protected], [email protected] (J. Zhang), [email protected] (G. Du). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.09.103
hence fast degradation of capacity, which counteracts their advantage as anode for lithium ion batteries. To solve these problems, researchers attend to synthesize nanostructured oxide/conductor composites, such as oxide/metal [16], oxide/CNT [17], oxide/PANI [18], etc. Recently, a new form of carbonaceous materials – graphene has been attracting extensive attention for its unique physical and chemical characteristics. Graphene is demonstrated as an outstanding “matrix” material in composite for its high theoretical surface of 2630 m2 /g, preeminent electronic conductivity and electron mobility, terrific chemical stability and suppleness. On one hand, the incorporation of nanoparticles and graphene nanosheets (GNS) will generate a porous network, providing excellent electron-conducting and ion-transporting pathways. On the other hand, the nanostructured metal oxides loaded onto GNS can act as spacers to prevent the re-stacking of GNS during cycling process [19–21]. Therefore, graphene–metal oxide composites become an attractive topic for researchers as to increase the cycling performance and rate capabilities of LIBs [22–25]. However, among the reported researches, most synthesizing approaches consist of mainly two steps, which require the reduction of graphene oxide by thermal annealing or adding toxic hydrazine [26,27]. The experiment was complicated and not environmentally friendly. Thus we believe it is worthwhile to explore a simple and nontoxic way to synthesize Fe2 O3 /graphene composite as anode for LIBs. In this work, we proposed a facile one-pot hydrothermal process to prepare Fe2 O3 /graphene composite without any template or
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organic solvent, reducing the experimental pollution and the cost as well compared to previous work [11,28]. As LIBs electrode material, Fe2 O3 /graphene composite exhibited an excellent performance in lithium storage and rate capability. The whole process was simple, practical and economic. The synthesis approach presents a promising route for large-scale production of Fe2 O3 /graphene composite as electrode materials for Li-ion batteries.
was used as the counter electrode. The cells were charged and discharged between 0.01 and 3 V (vs. Li/Li+ ) at various current densities. Cyclic voltammetry (CV) was performed on a CHI604D electrochemical workstation at a scan rate of 0.1 mV s−1 . All tests were performed at room temperature.
2. Experimental
XRD analysis of the sample was shown in Fig. 1(a). For both samples (the pure Fe2 O3 and the Fe2 O3 /graphene composite), characteristic diffraction peaks are in good accordance with hexagonal ˚ c = 13.75 A˚ (standard crystal structure ␣-Fe2 O3 with a = 5.035 A, hematite PCPDS 33-0664). It is worth mentioning that the characteristic (0 0 2) peak of carbon is shadowed by the (0 1 2) peak of crystals at the similar degree so that (0 1 2) peak of Fe2 O3 /graphene is sharper than that of pure Fe2 O3 . No evident peaks of impurities are detected in the samples. The crystal grain size of Fe2 O3 is calculated to be ∼45 nm using the Scherrer equation. The weight ratio of Fe2 O3 and graphene was determined by TGA in air as shown in Fig. 2(b). The graphene content was estimated to 12% according to TGA. The increase of sample quality in less than 200 ◦ C can be attributed to the oxidation of graphene in the air. Fig. 2 shows the scanning electron microcopy (SEM) and transmission electron microscopy (TEM) images of Fe2 O3 and Fe2 O3 /graphene composite. The sizes of the Fe2 O3 nanoparticles of both samples are about 40–60 nm, which is consistent with the XRD results. Fig. 2b shows that the Fe2 O3 nanoparticles are distributed uniformly on the surface of graphene, which can be further confirmed by the TEM image (Fig. 2c). Fig. 2d shows the HRTEM image of Fe2 O3 nanoparticle on graphene. An interlayer distance of 0.27 nm are observed, which agrees well with the spacing between (1 1 0) planes of Fe2 O3 crystals. The nanoparticles are found to be monocrystals from the HRTEM and diffraction results. Fig. 3 shows the electrochemical properties of Fe2 O3 and Fe2 O3 /graphene composite. Fig. 3a presents the first cycle of CV curve of Fe2 O3 and Fe2 O3 /graphene. The cathodic peaks at 1.52 V for Fe2 O3 /graphene and 1.48 V for the bare Fe2 O3 represent the Li+ intercalation to hexagonal Fe2 O3 [10]:
2.1. Preparation of graphene Graphene oxide was synthesized from purified natural graphite by a modified Hummers method [29]. Briefly, on the preoxidation process, 1 g graphite powder, 1 g K2 S2 O8 and 1 g P2 O5 was added to 25 ml 98% H2 SO4 , stirring at the temperature of 35 ◦ C for 6 h. The resultant was washed by deionized (DI) water until the pH of the eluate became neutral, then dried at 60 ◦ C for 24 h. In the reoxidation process, the above product was added to 50 ml 98% H2 SO4 in an ice bath, 5 g KMnO4 was gradually added with stirring. After stirred for 30 min, the mixture was heated to 35 ◦ C and kept for 6 h. Then 100 ml DI water was added with the temperature rising to 98 ◦ C for 15 min. The reactant was washed by 1:10 diluted hydrochloric acid and DI water until the supernatant liquor is neutral. The obtained yellow and viscous liquid was dried at 60 ◦ C for 24 h and graphene oxide was received. Graphene oxide was then reduced to graphene by calcining at 400 ◦ C in nitrogen flow for 2 h and most oxygen groups were successfully removed [30]. The as-prepared graphene is single or few-layer with lots of wrinkles. 2.2. Preparation of Fe2 O3 and Fe2 O3 /graphene composite Fe2 O3 /graphene composite was fabricated by a one-pot hydrothermal method. 0.067 g graphene was dispersed in 16 ml DI water by ultrasonication, then 3.5 ml HAc and 0.317 g iron chloride (FeCl3 ) were added into the suspension while stirring, after which 3.5 ml ammonia water (NH3 ·H2 O) was added slowly and sequentially. The solution was then transferred to a 25 ml Teflon-lined stainless steel autoclave and heated at 160 ◦ C for 48 h. After the mixture cooled to room temperature naturally, the brown loose precipitate was collected by centrifugation, washed with deionized water, and dried under vacuum at 60 ◦ C for 12 h. 0.132 g Fe2 O3 /graphene composite was obtained. For comparison, bare Fe2 O3 nanoparticles were prepared by a similar method except for the absence of graphene. 2.3. Materials characterizations The composition, morphology and microstructures of the products were analyzed using powder X-ray diffraction (XRD, Cu K␣ radiation, Philips PW3040/60), scanning electron microscopy (SEM, Hitachi S-4800) and high-resolution transmission electron microscope (TEM, JEOL-2100F). The thermogravimetric analysis (TGA) measurement were conducted on a Netzsch STA 449C thermal analyzer. As for the electrochemical characterization, the experiments were performed at a CT2001A Land battery tester at room temperature. To prepare the LIB anode, the electrode slurry was made by mixing the active material, acetylene black and polyvinylidene fluoride (PVDF) in a weight ratio of 75:15:10 in N-methyl pyrrolidone with stirring for 2 h. The working electrodes were made by spreading the slurry onto a Ni foam current collector (˚ 14 mm) and dried at 120 ◦ C under vacuum overnight. The average loading mass of the active material is 1.03 mg cm−2 in terms of Fe2 O3 . The electrolyte solution was 1 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1, v/v). Pure Li foil (Aldrich)
3. Results and discussion
Fe2 O3 + 2Li+ + 2e− → Li2 (Fe2 O3 )
(1)
Li2 (Fe2 O3 ) + 4Li+ + 4e− → 2Fe0 + 3Li2 O
(2)
Fe2 O3 + 6Li+ + 6e− ↔ 2Fe + 3Li2 O
(3)
The broad cathodic peaks located at 0.1–0.65 V can be attributed to the stepwise reduction of Fe3+ by Li to nanometer-sized Fe metal (Fe0 ) with the formation of amorphous Li2 O, and the irreversible reaction with the electrolyte to form solid electrolyte interface (SEI) film [31], which could be expressed by Eq. (2). The anodic peaks of the two materials are both at 1.75 V. The storage mechanism is based on a well-known reversible Li-ion reaction with transitional metal oxides as described by Eq. (3) [32,33]. Fig. 3b shows that the main reduction peak shifts to 0.65 V for the composite since the 2nd cycle. The CV curve of the 3rd cycle is similar to that of the 2nd cycle, indicating that the electrochemical reversibility of the obtained composites is gradually stabilized after the 2nd cycle. The charge/discharge profiles of the bare Fe2 O3 nanoparticles and the Fe2 O3 /graphene composite are shown in Fig. 3c and d. Insets are the dQ/dV curves of the first discharge profile. The electrode based on Fe2 O3 /graphene composite delivers a discharge capacity of 1369 mAh g−1 and a charge capacity of 899 mAh g−1 at the 1st cycle with a Coulombic efficiency of 65.7%, which is comparable with the Fe2 O3 nanoparticle/graphene composite prepared using hydrothermal method [34] but lower than the rice-on-sheet Fe2 O3 /graphene composite prepared by a microwave-assisted
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Fig. 1. (a) XRD patterns of Fe2 O3 nanoparticles on GNS and pure Fe2 O3 nanoparticles, (b) thermogravimetric analysis of Fe2 O3 /graphene composite in air.
hydrothermal technique [14]. The Coulombic efficiency rapidly increases to 95% in the 2nd cycle, and remains stable in the following cycles. In contrast, the bare Fe2 O3 electrode exhibits discharge and charge capacity of 1236 mAh g−1 and 493 mAh g−1 , respectively in the first cycle with a Coulombic efficiency of only 39.9%. For the Fe2 O3 /graphene composite, an additional peak is found in the dQ/dV curve when the discharge voltage down to 0.15 V, which delivers ∼130 mAh g−1 (versus the composite). This part of capacity could be partially attributed to the contribution from graphene. It suggests that, for the first discharge, bare Fe2 O3 delivers a comparable capacity with Fe2 O3 nanoparticles on graphene sheet. The enhancement effect of graphene to the capacity is mainly reflected in the Coulombic efficiency and capacity retention. Fig. 4a presents the cycling performance of bare Fe2 O3 and Fe2 O3 /graphene composite at a current density of 50 mA g−1 . After 50 discharge–charge cycles, the Fe2 O3 /graphene composite still maintains a capacity of 559 mAh g−1 , while bare Fe2 O3 exhibits a poor capacity of only 76 mAh g−1 . It achieves a comparable stability
with the previous report with the same morphology of particle-onsheet or nanorod-on-sheet [35], but inferior than the rice-shaped Fe2 O3 on graphene composite, which could be attributed to the agglomeration effect [14]. Notably, the reversible capacity of Fe2 O3 /graphene composite slightly increases from the 2nd cycle, and the Coulombic efficiency reaches to 99.1% after 50 cycles, which could be ascribed to the gradual activation of GNS in the composite during the first several cycles [1]. The Fe2 O3 /graphene composite displays good rate performance as well (Fig. 4b). It delivers discharge capacities of ∼450 mAh g−1 and 300 mAh g−1 at current densities of 200 mA g−1 and 300 mA g−1 , respectively. When the discharge rate decreases back to 50 mA g−1 after 50 cycles, the capacity recovers to 750 mAh g−1 , indicating good rate endurance. The Fe2 O3 /graphene composite prepared in this work exhibits better electrochemical performance than the previous report with the same content of graphene (12 wt%) [36]. From the above analysis, it is believed that GNS can provide a highly conductive matrix for the diffusion of electrons and lithium
Fig. 2. (a) SEM image of Fe2 O3 ; (b) SEM image of Fe2 O3 /graphene; (c) TEM image of Fe2 O3 /graphene; (d) TEM image of Fe2 O3 /grapheme in high resolution.
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(c) 3.0
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Potential (V vs. Li /Li) Fig. 3. Electrochemical performance of bare Fe2 O3 and Fe2 O3 /graphene composite. (a) The comparison of CV curves of bare Fe2 O3 and Fe2 O3 /graphene, (b) first three CV curves of Fe2 O3 /graphene composite, (c) typical charge–discharge curves of Fe2 O3 /graphene, (d) typical charge–discharge curves of Fe2 O3 . Insets are the corresponding differential capacity versus voltage plots of the first discharge profiles in (c) and (d).
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80 60
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ions during the lithium insertion and extraction reactions. To further understand the mechanism for the improved cycling stability of Fe2 O3 /graphene, impedance measurements were carried out in the frequency range from 0.01 Hz to 100 kHz at room temperature at the open-circuit voltage of 2.327 V. Fig. 5a shows the experimental EIS data and fitting plots with a simulating model illustrated in Fig. 5b. REL indicates the electrolyte resistance; RCT and RSF stands for the charge transfer and surface film resistance, respectively; QCPE designates a constant phase element; Ws refers to suitable diffusional components like Warburg impedance. The impendence of CPE is defined as ZCPE = 1/[B(jw)a ] where w is the angular frequency, and B and a are constants. The value of a gives the degree of distortion of the impedance spectra and when a is more close to 1, CPE is more close to an ideal capacitor. The excellent fitness between the simulated curve and the experimental EIS data indicates the accuracy of the simulated model. From the EIS plots, both impedance spectra consist of semicircles in the high and medium frequency region and a following line in low frequency. As is known, the high frequency region in the EIS plots is indexed to the migration impedance of lithium ions through the surface layer, the medium frequency region corresponds to the charge-transfer impedance in the film-solution interface, and the low frequency region reflects the solid-state diffusion impedance of lithium ions in the active material [37,38]. The Fe2 O3 /graphene electrode shows smaller semicircles in both high and medium frequency than Fe2 O3 electrode, which indicates the lithium ions and electrons can transfer more easily on Fe2 O3 /graphene/electrolyte interface. The decreasing charge-transfer resistance results in the enhanced electrode process kinetics and improved electrochemical performance of Fe2 O3 /graphene electrode. The fitting parameters are given in Table 1. All parameters of Fe2 O3 /graphene electrode are better than that of Fe2 O3 electrode
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Acknowledgments This work was supported by the National Science Foundation of China (No. 21203168), the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET11-1081), and the Science Technology Department of Zhejiang Province (Grant No. 2013C31070).
References
Fig. 5. EIS plots of electrodes made from Fe2 O3 /graphene composite and bare Fe2 O3 nanoparticles at open-circuit voltage of 2.327 V (a), and the equivalent circuit to fit the experimental data (b).
Table 1 Impedance parameters of the cells fabricated with Fe2 O3 /graphene composite and bare Fe2 O3 nanoparticles as the cathode. Parameters
Fe2 O3 /graphene composite
Bare Fe2 O3
REL () RCT + RSF () QCPE (F) ˛
4.40 290.7 294.7 0.77
5.37 1107 436.8 0.61
quantificationally. For the Fe2 O3 /graphene electrode, the sum of charge transfer (RCT ) and surface film resistance (RSF ) is 290.7 , much smaller than that of the bare Fe2 O3 nanoparticles electrode’s 1107 , the corresponding CPE is 294.7 F and 436.8 F. The electrolyte resistance (REL ) in Fe2 O3 /graphene electrode is 0.97 less than that in bare Fe2 O3 nanoparticles electrode. Therefore, in this work, it is considered that the kinetics of Li+ migration and charge transfer on active material/electrolyte interfaces is the key point to influence the electrochemical performances of the as-prepared Fe2 O3 /graphene electrode. 4. Conclusions In summary, we report a simple method to fabricate Fe2 O3 /graphene composite as a high-performance anode material for LIBs. The Fe2 O3 /graphene composite delivered discharge and charge capacity of 1369 mAh g−1 and 899 mAh g−1 respectively in the first cycle with an initial Coulombic efficiency of 65.7%, which was much better than the bare Fe2 O3 nanoparticles electrode. A good rate capacity and cycling performance were also observed for the Fe2 O3 /graphene composite. The enhanced electrochemical performance of Fe2 O3 /graphene can be ascribed to fast migration of Li+ through surface film and charge transfer on active material/electrolyte interfaces. The synthesis approach presents a promising route for a large-scale production of Fe2 O3 /graphene composite as electrode materials for Li-ion batteries, which can be also extended to other transition metal oxides.
[1] L.Q. Tao, J.T. Zai, K.X. Wang, Y.H. Wan, H.J. Zhang, C. Yu, Y.L. Xiao, X.F. Qian, 3D-hierarchical NiO-graphene nanosheet composites as anodes for lithium ion batteries with improved reversible capacity and cycle stability, RSC Advances 2 (2012) 3410. [2] Y. Xia, W.K. Zhang, Z. Xiao, H. Huang, H.J. Zeng, X.R. Chen, F. Chen, Y.P. Gan, X.Y. Tao, Biotemplated fabrication of hierarchically porous NiO/C composite from lotus pollen grains for lithium-ion batteries, Journal of Materials Chemistry 22 (2012) 9209. [3] X.H. Huang, J.P. Tu, C.Q. Zhang, F. Zhou, Hollow microspheres of NiO as anode materials for lithium-ion batteries, Electrochimica Acta 55 (2010) 8981. [4] H. Huang, W.J. Zhu, X.Y. Tao, Y. Xia, Z.Y. Yu, J.W. Fang, Y.P. Gan, W.K. Zhang, Nanocrystal-constructed mesoporous single-crystalline Co3 O4 nanobelts with superior rate capability for advanced lithium-ion batteries, ACS Applied Materials and Interfaces 4 (2012) 5974. [5] Y. Lu, Y. Wang, Y.Q. Zou, Z. Jiao, B. Zhao, Y.Q. He, M.H. Wu, Macroporous Co3 O4 platelets with excellent rate capability as anodes for lithium ion batteries, Electrochemistry Communications 12 (2010) 101. [6] X.P. Gao, J.L. Bao, G.L. Pan, H.Y. Zhu, P.X. Huang, F. Wu, D.Y. Song, Preparation and electrochemical performance of polycrystalline and single crystalline CuO nanorods as anode materials for Li ion battery, The Journal of Physical Chemistry B 108 (2004) 5547. [7] J.Y. Xiang, J.P. Tu, L. Zhang, Y. Zhou, X.L. Wang, S.J. Shi, Simple synthesis of surface-modified hierarchical copper oxide spheres with needle-like morphology as anode for lithium ion batteries, Electrochimica Acta 55 (2010) 1820. [8] Y.J. Mai, X.L. Wang, J.Y. Xiang, Y.Q. Qiao, D. Zhang, C.D. Gu, J.P. Tu, CuO/graphene composite as anode materials for lithium-ion batteries, Electrochimica Acta 56 (2011) 2306. [9] J. Chen, L. Xu, W. Li, X. Gou, ␣-Fe2 O3 nanotubes in gas sensor and lithium-ion battery applications, Advanced Materials 17 (2005) 582. [10] M.V. Reddy, T. Yu, C.H. Sow, Z.X. Shen, C.T. Lim, G.V.S. Rao, B.V.R. Chowdari, ␣Fe2 O3 nanoflakes as an anode material for Li-ion batteries, Advanced Functional Materials 17 (2007) 2792. [11] X. Xu, R. Cao, S. Jeong, J. Cho, Spindle-like mesoporous alpha-Fe2 O3 anode material prepared from MOF template for high-rate lithium batteries, Nano Letters 12 (2012) 4988. [12] X.J. Zhu, Z.P. Guo, P. Zhang, G.D. Du, R. Zeng, Z.X. Chen, S. Li, H.K. Liu, Highly porous reticular tin–cobalt oxide composite thin film anodes for lithium ion batteries, Journal of Materials Chemistry 19 (2009) 8360. [13] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W. van Schalkwijk, Nanostructured materials for advanced energy conversion and storage devices, Nature Materials 4 (2005) 366. [14] Y.Q. Zou, J. Kan, Y. Wang, Fe2 O3 -graphene rice-on-sheet nanocomposite for high and fast lithium ion storage, Journal of Physical Chemistry C 115 (2011) 20747. [15] Q.Q. Xiong, J.P. Tu, Y. Lu, J. Chen, Y.X. Yu, Y.Q. Qiao, X.L. Wang, C.D. Gu, Synthesis of hierarchical hollow-structured single-crystalline magnetite (Fe3 O4 ) microspheres: the highly powerful storage versus lithium as an anode for lithium ion batteries, Journal of Physical Chemistry C 116 (2012) 6495. [16] X.H. Huang, J.P. Tu, Z.Y. Zeng, J.Y. Xiang, X.B. Zhao, Nickel foam-supported porous NiO/Ag film electrode for lithium-ion batteries, Journal of the Electrochemical Society 155 (2008) A438. [17] J.Y. Xiang, J.P. Tu, J. Zhang, J. Zhong, D. Zhang, J.P. Cheng, Incorporation of MWCNTs into leaf-like CuO nanoplates for superior reversible Li-ion storage, Electrochemistry Communications 12 (2010) 1103. [18] X.H. Huang, J.P. Tu, X.H. Xia, X.L. Wang, J.Y. Xiang, Nickel foam-supported porous NiO/polyaniline film as anode for lithium ion batteries, Electrochemistry Communications 10 (2008) 1288. [19] H. Kim, D.H. Seo, S.W. Kim, J. Kim, K. Kang, Highly reversible Co3 O4 /graphene hybrid anode for lithium rechargeable batteries, Carbon 49 (2011) 326. [20] Z.S. Wu, W.C. Ren, L. Wen, L.B. Gao, J.P. Zhao, Z.P. Chen, G.M. Zhou, F. Li, H.M. Cheng, Graphene anchored with Co3 O4 nanoparticles as anode of lithium ion batteries with enhanced reversible capacity and cyclic performance, ACS Nano 4 (2010) 3187. [21] Y.Q. Zou, Y. Wang, NiO nanosheets grown on graphene nanosheets as superior anode materials for Li-ion batteries, Nanoscale 3 (2011) 2615. [22] Y. He, L. Huang, J.S. Cai, X.M. Zheng, S.G. Sun, Structure and electrochemical performance of nanostructured Fe3 O4 /carbon nanotube composites as anodes for lithium ion batteries, Electrochimica Acta 55 (2010) 1140. [23] L. Wang, Y. Yu, P.C. Chen, D.W. Zhang, C.H. Chen, Electrospinning synthesis of C/Fe3 O4 composite nanofibers and their application for high performance lithium-ion batteries, Journal of Power Sources 183 (2008) 717.
J. Ye et al. / Electrochimica Acta 113 (2013) 212–217 [24] W.M. Zhang, X.L. Wu, J.S. Hu, Y.G. Guo, L.J. Wan, Carbon coated Fe3 O4 nanospindles as a superior anode material for lithium-ion batteries, Advanced Functional Materials 18 (2008) 3941. [25] L.Q. Lu, Y. Wang, Sheet-like and fusiform CuO nanostructures grown on graphene by rapid microwave heating for high Li-ion storage capacities, Journal of Materials Chemistry 21 (2011) 17916. [26] J. Zhu, T. Zhu, X. Zhou, Y. Zhang, X.W. Lou, X. Chen, H. Zhang, H.H. Hng, Q. Yan, Facile synthesis of metal oxide/reduced graphene oxide hybrids with high lithium storage capacity and stable cyclability, Nanoscale 3 (2011) 1084. [27] X. Zhu, Y. Zhu, S. Murali, M.D. Stoller, R.S. Ruoff, Nanostructured reduced graphene oxide/Fe2 O3 composite as a high-performance anode material for lithium ion batteries, ACS Nano 5 (2011) 3333. [28] S.Y. Liu, J. Xie, Q. Pan, C.Y. Wu, G.S. Cao, T.J. Zhu, X.B. Zhao, Graphene anchored with nanocrystal Fe2 O3 with improved electrochemical Li-storage properties, International Journal of Electrochemical Science 7 (2012) 354. [29] N.I. Kovtyukhova, P.J. Ollivier, B.R. Martin, T.E. Mallouk, S.A. Chizhik, E.V. Buzaneva, A.D. Gorchinskiy, Layer-by-layer assembly of ultrathin composite films from micro-sized graphite oxide sheets and polycations, Chemistry of Materials 11 (1999) 771. [30] Z.L. Wang, D. Xu, Y. Huang, Z. Wu, L.M. Wang, X.B. Zhang, Facile, mild and fast thermal-decomposition reduction of graphene oxide in air and its application in high-performance lithium batteries, Chemical Communications 48 (2012) 976.
217
[31] G. Zhou, D.W. Wang, L.C. Yin, N. Li, F. Li, H.M. Cheng, Oxygen bridges between NiO nanosheets and graphene for improvement of lithium storage, ACS Nano 6 (2012) 3214. [32] J. Ma, D. Lei, X. Duan, Q. Li, T. Wang, A. Cao, Y. Mao, W. Zheng, Designable fabrication of flower-like SnS2 aggregates with excellent performance in lithium-ion batteries, RSC Advances 2 (2012) 3615. [33] C. Shi, T. Cao, C.B. Cao, D.J. Wu, Y.A. Li, M.N. Wang, W. Mo, eta-Fe2 O3 spindles as anode materials for lithium-ion batteries with excellent stable and high electrochemical performance, Materials Letters 83 (2012) 35. [34] L.L. Tian, Q.C. Zhuang, J. Li, C. Wu, Y.L. Shi, S.G. Sun, The production of selfassembled Fe2 O3 -graphene hybrid materials by a hydrothermal process for improved Li-cycling, Electrochimica Acta 65 (2012) 153. [35] X.Y. Xue, C.H. Ma, C.X. Cui, L.L. Xing, High lithium storage performance of alphaFe2 O3 /graphene nanocomposites as lithium-ion battery anodes, Solid State Sciences 13 (2011) 1526. [36] W. Xiao, Z.X. Wang, H.J. Guo, X.H. Li, J.X. Wang, S.L. Huang, L. Gan, Fe2 O3 particles enwrapped by graphene with excellent cyclability and rate capability as anode materials for lithium ion batteries, Applied Surface Science 266 (2013) 148. [37] J.Y. Xiang, J.P. Tu, Y.Q. Qiao, X.L. Wang, J. Zhong, D. Zhang, C.D. Gu, Electrochemical impedance analysis of a hierarchical CuO electrode composed of self-assembled nanoplates, Journal of Physical Chemistry C 115 (2011) 2505. [38] J.Y. Xiang, X.L. Wang, X.H. Xia, J. Zhong, J.P. Tu, Fabrication of highly ordered porous nickel phosphide film and its electrochemical performances toward lithium storage, Journal of Alloys and Compounds 509 (2011) 157.