- Email: [email protected]
rGO nanocomposites by microwave autoclave as superior anodes for sodium-ion batteries
Journal of Power Sources 280 (2015) 107e113
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Rapid synthesis of a-Fe2O3/rGO nanocomposites by microwave autoclave as superior anodes for sodium-ion batteries Zhi-Jia Zhang a, Yun-Xiao Wang a, Shu-Lei Chou a, Hui-Jun Li b, *, Hua-Kun Liu a, Jia-Zhao Wang a, * a b
Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia Faculty of Engineering, University of Wollongong, Wollongong, NSW 2522, Australia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
a-Fe2O3/rGO nanocomposite is synthesized through ultrafast microwave hydrothermal method. a-Fe2O3 nanoparticle with numerous nanovoids is wrapped up in crumpled graphene sheets. The nanovoids effect tolerating the volume change during sodium alloying/dealloying reactions. Significantly improved Na-storage capability in terms of cycling performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 November 2014 Received in revised form 19 December 2014 Accepted 14 January 2015 Available online 15 January 2015
a-Fe2O3/reduced graphene oxide (rGO) nanocomposites were successfully synthesized within 15 min
through a facile, environmentally friendly microwave hydrothermal method. From field emission scanning electron microscopy and transmission electron microscopy, it can be determined that the a-Fe2O3 nanoparticles, around 50 nm in diameter, are uniformly anchored on the graphene nanosheets. The asobtained a-Fe2O3/rGO nanocomposites were applied as anode materials in sodium-ion batteries, which could deliver capacity of ~310 mAh g1 after 150 cycles at 100 mA g1. © 2015 Published by Elsevier B.V.
Keywords: a-Fe2O3 Reduced graphene oxide Microwave autoclave Sodium-ion battery
1. Introduction Over the past several decades, energy storage and conversion have drawn a great deal of attention from the scientific community and industry. Lithium-ion batteries (LIBs) have been demonstrated
* Corresponding authors. E-mail addresses: [email protected] (J.-Z. Wang). http://dx.doi.org/10.1016/j.jpowsour.2015.01.092 0378-7753/© 2015 Published by Elsevier B.V.
(H.-J.
Li),
[email protected]
to be one of the most successful examples in commercial applications [1e3]. Their large-scale applications, however, would be limited due to the scarcity of lithium resources in the Earth's crust. In this regard, sodium-ion batteries (SIBs) have great advantages because of the abundance and low cost of sodium, and thus, they have become an alternative to LIBs [4e7]. The radius of sodium ion (0.95 Å) is about 1.6 times larger than that of lithium ion (0.6 Å), however, which is undoubtedly restricted the development of electrode materials of SIBs, especially at anode materials. In the past decade, cathode materials for SIBs
108
Z.-J. Zhang et al. / Journal of Power Sources 280 (2015) 107e113
have already attained fruitful results, mainly including three categories: sodium transition metal oxides [8,9], transition metal fluorides [10,11], and phosphates [12,13]. In contrast, the promising anodes for SIBs are confined a limited kind of materials according to recent reports, such as hard carbons [14,15], Ti-based compounds (i.e. TiO2 [16], Li4Ti5O12 [17], P2eNa0.66[Li0.22Ti0.78]O2 [18], Na2Ti3O7 [19], etc.), sodium Alloys (i.e. Sn [20], P [21], Sb [22], Sn4þxP3 [23], etc.). The disadvantages of anodes, including the large volume change during cycling, the poor electronic conductivity, and unclear sodiation/desodiation mechanisms, urgently need to be indepth investigated for acquiring high electrochemical performance. Therefore, the research on anode materials is one of the most important issues for developing SIBs. Iron oxides have many excellent advantages as potential anode materials, such as low cost, abundance, and high capacity [24]. In the past ten years, they have been widely studied as anode materials for LIBs [25e27]. Recently, they were also introduced as anodes for SIBs [28,29]. Komaba and his co-workers reported that a-Fe2O3 nanocrystals were highly electrochemically active in sodium salt electrolyte, and have excellent capacity retention of 170 mAh g1 at a rate of 100 mA g1 [30]. More recently, Zhou's group reported that Fe2O3@graphene nanosheets could deliver about 400 mAh g1 and remain stable over 200 cycles at a current density of 100 mA g1 [31]. The synthesis of iron oxides or iron oxide-based composites, however, usually required a long reaction time. Thus, exploring the synthesis of iron oxide-based composites is an effective method for obtaining low-cost, high-performance anode materials for SIBs. Herein, we have employed a novel microwave autoclave strategy, which combines the advantages of high productivity and efficiency with low temperature and risk. Most importantly, the microwave autoclave strategy is environmentally friendly and energy saving, successfully producing a composite material of wellorganized a-Fe2O3 encapsulated in graphene sheets. In comparison with the traditional hydrothermal method [31e36], the microwave hydrothermal method can expedite the kinetics of crystallization by causing rapid nucleation and growth, which can dramatically decrease the reaction time from 10 h or even several days via the conventional hydrothermal method to 15 min, as reported here. Therefore it can save a large amount of energy and offers great possibilities for large-scale reactions. Furthermore, the microwave hydrothermal method can be used effectively and efficiently for preparing various multifunctional nanomaterials with intriguing morphologies, such as nanowires [37,38], nanoplates [39], nanospheres [40], and nanoporous networks [41]. In this work, a-Fe2O3/reduced graphene oxide (a-Fe2O3/rGO) composites were synthesized by a microwave hydrothermal method and investigated as anode material for SIBs. This synthetic method has several important merits, such as being rapid, reductant-free, low cost and environmentally friendly. Moreover, the as-synthesized a-Fe2O3/rGO composite could deliver a discharge capacity of 310 mAh g1 over 150 cycles at a current of 100 mA g1, as well as having the improved rate capability compared with the pure a-Fe2O3 in SIBs.
GO was added to the solution followed by ultrasonic dispersion for 2 h. The solution was transferred into sealed Teflon vessels and reacted for 15 min at 180 C using a Milestone Microsynth Microwave Labstation (Germany). After cooling down naturally and washing 4 times with distilled water and ethanol, the black product, designated as a-Fe2O3/rGO1, was dried at 60 C in a vacuum oven for 12 h. The a-Fe2O3/rGO2 nanocomposite was prepared by the same method, except that the GO amount was 50 mg. To prepare pure a-Fe2O3 and rGO as a comparison, the same amounts of FeCl3 and rGO were used alone, followed by the same conditions and procedures applied in the synthesis of the composite. 2.2. Materials characterization The products, Fe2O3, a-Fe2O3/rGO1, and a-Fe2O3/rGO2, were analyzed by X-ray diffraction (XRD; GBC MMA) with Cu Ka radiation; Raman spectroscopy (JobinYvon HR800) employing a 10 mW helium/neon laser at 632.8 nm; field emission scanning electron microscopy (FESEM; JEOL 7500); and transmission electron microscopy (TEM; JEOL JEM-2011, 200 kV) with high-resolution TEM (HRTEM). Thermogravimetric analysis (TGA) was performed by using a SETARAM Thermogravimetric Analyzer (France) in air to determine the changes in sample weight with increasing temperature and to estimate the amount of rGO in the sample. 2.3. Electrochemical measurements The electrodes were prepared by mixing 80 wt.% a-Fe2O3, or aFe2O3/rGO composite with 15 wt.% carbon black and 5 wt.% carboxymethyl cellulose (CMC) binder. The slurry was spread onto Cu foil substrates. The coated electrodes were dried at 80 C in a vacuum oven for 24 h to remove water molecules. The electrodes were then pressed using a disc with a diameter of 14 mm to enhance the contact between the Cu foil, active materials, and conductive carbon. Subsequently, the electrodes were cut to a 1 1 cm2 size. The average active material loading rate was ~2 mg cm2. CR 2032 cointype cells were assembled in an Ar-filled glove box (Mbraun, Unilab, Germany), using sodium metal foil or lithium metal as the counter electrode and glass fiber as separator. The electrolyte was 1 M NaClO4 or LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). The cells were galvanostatically charged and discharged in the range of 0.05e2.7 V at different current
2. Experimental 2.1. Material synthesis Graphene oxide (GO) was prepared from graphite powder (Aldrich, powder, <20 mm, synthetic) by a modified Hummers' method [42]. The a-Fe2O3/rGO composites were prepared by using our previously reported microwave hydrothermal (MH) method [43]. For synthesizing a-Fe2O3/rGO composite, 0.01 mol FeCl3 (SigmaeAldrich) was dissolved in distilled water to obtain a final concentration of 0.1 M in a beaker. After stirring for 15 min, 100 mg
Fig. 1. XRD patterns of rGO, pure a-Fe2O3, a-Fe2O3/rGO1 nanocomposite, and a-Fe2O3/ rGO2 nanocomposite.
Z.-J. Zhang et al. / Journal of Power Sources 280 (2015) 107e113
109
densities using a computer-controlled charger system manufactured by Land Battery Testers. The calculation of specific capacity is based on the mass of active material (pure a-Fe2O3, or a-Fe2O3/rGO composite). A Biologic VMP-3 electrochemical workstation was used to perform electrochemical impedance spectroscopy (EIS; ac amplitude 5 mV, frequency range 100 kHze0.01 Hz). 3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of the samples. The XRD peaks of the composites can be indexed to standard aFe2O3 (hematite structure; JCPDS card No. 33e0664) without any impurity (Fig. 1a). For the rGO, the diffraction peaks at 2q z 24.5 and 42.8 can be attributed to the graphite-like (002) and (100) planes, respectively [44]. The corresponding peaks of rGO in aFe2O3/rGO1 and a-Fe2O3/rGO2 overlap with the a-Fe2O3 peaks [(012) and (202)]. In comparison to the pure a-Fe2O3, the peaks of the composites are broadened and have lower intensities, indicating that the addition of graphene restrains the growth of a-Fe2O3 particles. As can be seen from Fig. 1, the large full width at half maximum (FWHM) of a-Fe2O3/rGO2 shows that the a-Fe2O3 particle has a small size or is composed by smaller primary particles. In Fig. 2, the Raman spectra of rGO, pure a-Fe2O3, a-Fe2O3/rGO1 nanocomposite, and a-Fe2O3/rGO2 nanocomposite are plotted for comparison. The Raman spectra of the a-Fe2O3/rGO1 and a-Fe2O3/ rGO2 composites display their main peaks at 227, 293, 412, 498, 610 and 626 cm1, which correspond to the peaks for the a-Fe2O3 particles [45,46], and another two peaks at around 1355 cm1 and 1597 cm1, which are identified as the D-band and G-band of graphene, respectively [47]. In addition, the peak at 1322 cm1 (aFe2O3) and the one at 1355 cm1 (rGO) overlap in the Raman spectra of the composite. The peak at 1322 cm1 (a-Fe2O3) is the characteristic band assigned to a two-magnon scattering, resulting from the interaction of two magnons created on antiparallel close spin sites [48]. In agreement with the XRD results, this indicates that the a-Fe2O3/rGO composites were successfully synthesized via the microwave hydrothermal method from graphite oxide and FeCl3 powder. Furthermore, different intensities of the XRD and Raman peaks between a-Fe2O3/rGO1 and a-Fe2O3/rGO2 are observed, which can probably be ascribed to the different ratios of a-Fe2O3 in the composites. For quantifying the amount of rGO in the a-Fe2O3/rGO1 and aFe2O3/rGO2 nanocomposites, TGA was carried out in air. Fig. 3
Fig. 2. Raman spectra of rGO, pure a-Fe2O3, a-Fe2O3/rGO1 nanocomposite, and aFe2O3/rGO2 nanocomposite.
Fig. 3. TGA curves of rGO, pure a-Fe2O3, a-Fe2O3/rGO1 nanocomposite, and a-Fe2O3/ rGO2 nanocomposite.
shows the TGA curves of the a-Fe2O3/rGO1 and a-Fe2O3/rGO2 nanocomposites along with those of pure a-Fe2O3 and rGO powder when heated from 40 to 800 C at a rate of 10 C min1 in air. rGO powder burns out at ~750 C, while the pure a-Fe2O3 powder maintains a constant weight throughout the temperature range used for this experiment. The a-Fe2O3/rGO1 and a-Fe2O3/rGO2 nanocomposites show a single-step weight loss at a temperature of ~500 C, which corresponds to the burning of rGO. There is no further weight loss after the initial decomposition of rGO. Therefore, the change in weight before and after the burning of rGO directly translates into the amount of rGO in the a-Fe2O3/rGO. Using this method, it can be estimated that the amounts of rGO in the a-Fe2O3/rGO1 and a-Fe2O3/rGO2 composites were 13.7 wt.% and 20.8 wt.%, respectively. The morphology and microstructure of the samples were observed via FESEM (Figure S1, Appendix A. Supplementary Information (ASI)) and TEM (Fig. 4). As shown in Figure S1, the nanosized a-Fe2O3 particles are well crystallized, and the particle size is ~300 nm. The particle size of a-Fe2O3 in the a-Fe2O3/rGO1 composite is similar to that in the pure a-Fe2O3 sample, while the aFe2O3 particle is composed by numerous ultrasmall primary particles which are only several nanometers (Figure S1b and Figure S1e). This phenomenon is mostly caused by the reduction process of GO. a-Fe2O3 particles are only partially anchored on the rGO nanosheets, possibly due to the low amount of rGO (13.7 wt.%). When the rGO content is increased to 20.8 wt%, the a-Fe2O3 nanoparticles are fully wrapped up in the crumpled micrometersize graphene nanosheets, and the a-Fe2O3 particle size dramatically decreases to about 50 nm (Figure S1c and Figure S1f). TEM investigations further revealed that the nanosized a-Fe2O3 particles are uniformly dispersed on the graphene nanosheets (Fig. 4a and b). Fig. 4b shows that numerous nanovoids exist in the a-Fe2O3 nanoparticles, mostly due to the high energy microwave treatment [49,50]. The nanovoids are extremely favorable for tolerating the large volume change during sodium alloying/dealloying reactions. In Fig. 4c, HRTEM image shows the (110) lattice fringes, with the dspacing measured to be 0.251 nm. Meanwhile, the selected area electron diffraction (SAED) pattern (Fig. 4d) shows two sets of diffraction patterns: 1) a-Fe2O3 phase, which can be indexed as (01 0), (1 00), and (110); 2) polycrystalline diffraction rings corresponding to the graphene and support films. In addition, the SAED pattern of a-Fe2O3 shows a series of weak diffraction spots for other directions, due to the existence of the nanovoids. In this optimized
110
Z.-J. Zhang et al. / Journal of Power Sources 280 (2015) 107e113
Fig. 4. TEM images of a-Fe2O3/rGO2 nanocomposite at low magnification (a), at higher magnification (b), at high resolution (HRTEM), (c) and the corresponding SAED pattern (d) from (b).
structure, the rGO matrix can form an effective conducting network and facilitate the formation of a stable solid electrolyte inter-phase (SEI) layer as well. The as-prepared a-Fe2O3/RGO composites were first tested as anode for LIBs, where they showed high capacity and cycling performance (with a-Fe2O3/RGO2 delivering about 1168 mAh g1 over 100 cycles at 100 mA g1). Furthermore, the a-Fe2O3/rGO2 composite electrode also exhibits excellent rate capability details shown in Figure S2 (ASI). Therefore, a-Fe2O3/RGO composites were expected to deliver a good electrochemical performance in SIBs. And a series tests were then carried out on a-Fe2O3/rGO anode in SIBs. Fig. 5 and Figure S3 (ASI) show typical chargeedischarge curves for different cycles of a-Fe2O3 and a-Fe2O3/rGO composites in cointype half-cells using sodium as the counter and reference electrode between 0.05 and 3.0 V (vs. Naþ/Na). a-Fe2O3/rGO2 composite shows an irreversible plateau (~0.5 V) at the first discharge process, due to the irreversible reaction between a-Fe2O3/rGO2 and sodium ions and the formation of the SEI layer. This is responsible for the low initial coulombic efficiency of the cycling performance (Fig. 6a). As shown in Fig. 5, the first discharge capacity of a-Fe2O3/rGO2 is around 1170 mAh g1. In contrast, the a-Fe2O3 and a-Fe2O3/rGO1 composite electrodes show lower initial discharge and discharge capacities (1010 and 543 mAh g1, respectively), and the reversible capacity in the following cycles is much lower than for the a-Fe2O3/ rGO2 composite electrode, being maintained at 37, 165 and 310 mAh g1, respectively (Fig. 5 and Figure S3). The reasons of the high initial discharge and charge capacities of the a-Fe2O3/rGO2 composite electrodes can be explained by: a) rGO was reported to be able accommodate sodium ions according our previous report [51]. It exhibited a high discharge capacity (1695 mAh g1, at a current density of 50 mA g1) at the first cycle, due to decomposition of electrolyte and irreversible reactions. b) The a-Fe2O3/rGO2 composite (rGO, 20.8 wt.%) contains higher amount of rGO than that of the a-Fe2O3/rGO1 composite (rGO, 13.7 wt.%). It is
Fig. 5. 1st, 3rd, and 10th cycle charge/discharge curves for a-Fe2O3/rGO2 nanocomposite. The inset shows charge/discharge curves for other selected higher cycles.
reasonable that a-Fe2O3/rGO2 delivers higher initial discharge and charge capacities with more sodium ions absorbed in rGO matrix. The unique architecture and extremely small a-Fe2O3 nanoparticles are responsible for the superior electrochemical properties of a-Fe2O3/rGO2. As can be seen from Fig. 6a, the cycling performance was tested at 0.1C (100 mA g1) over the range of 0.05e3.0 V. In contrast, the a-Fe2O3 electrode delivered a poor cycling performance with much lower capacity (after 50 cycles, ~10 mAh g1), which demonstrates the crucial effect of the architecture for the composite. The poor performance of the a-Fe2O3 electrode is mainly ascribed to the large crystallized a-Fe2O3 particles and the longer diffusion paths due to the absence of graphene. The capacity remains at 310 mAh g1 over 150 cycles, and
Z.-J. Zhang et al. / Journal of Power Sources 280 (2015) 107e113
Fig. 6. (a) Cycling performance of pure a-Fe2O3, and a-Fe2O3/rGO1 and a-Fe2O3/rGO2 nanocomposites at the current density of 100 mA g1; (b) high-rate capability of aFe2O3/rGO1 and a-Fe2O3rGO2 nanocomposites.
the coulombic efficiency is nearly 100% for a-Fe2O3/rGO2 electrode. The a-Fe2O3/rGO1 electrode, which has larger a-Fe2O3 particles and a lower amount rGO, was also evaluated for comparison. Compared with a-Fe2O3/rGO2, this electrode shows higher initial capacity, but inferior capacity retention and worse cycling stability. The
111
discharge capacity declines to 165 mAh g1 after 150 cycles. Furthermore, the rate capability of the a-Fe2O3/rGO1 and a-Fe2O3/ rGO2 composites at different current rates is evaluated in Fig 6b. The a-Fe2O3/rGO2 composite electrode exhibits excellent rate capability. When the electrode was tested at 50 mA g1 (0.05C), the discharge capacity was about 1136 mAh g1, and then when the current rate was increased rapidly to 2000 mA g1 (2C), the corresponding capacity was maintained at 77 mAh g1. More importantly, the capacity can recover to a great extent, to around 370 mA h g1 at 0.1C, which is consistent with the obtained results for electrode cycling at 0.1C shown in Fig. 6a. Electrochemical impedance spectroscopy (EIS) measurements were also carried out to compare the impedance differences in aFe2O3, a-Fe2O3/rGO1 and a-Fe2O3/rGO2. The Nyquist plots were collected from 100 kHz to 10 mHz on the coin-cell batteries after charge/discharge for 50 cycles. As shown in Fig. 7, the EIS spectra are fitted by an equivalent circuit and exhibit two compressed semicircles followed by a linear part [52,53]. The fitted parameters show that the cells with a-Fe2O3/rGO1 and a-Fe2O3/rGO2 electrode possess similar uncompensated resistance (Rl) of 6.5 and 2.3 U respectively, which is much lower than that of the pure a-Fe2O3 electrode (76.4 U) due to its poor conductivity. In contrast, the film resistance (Rf) of the SEI and the charge transfer resistance (Rct) show significant differences in the three different coin cells. The cells with a-Fe2O3/rGO2 show a lower Rf (15.3 U) than those for aFe2O3 and a-Fe2O3/rGO1 (142.6 and 44.5 U, respectively), indicating the formation of a thinner SEI layer because the a-Fe2O3 particles are uniformly wrapped with graphene, which is in agreement with the much higher capacity in Fig. 6a and Figure S2. Meanwhile, the Rct of the a-Fe2O3/rGO2 electrode is much smaller than those of the a-Fe2O3 and a-Fe2O3/rGO1 electrodes (249.5, 1218.6 and 498.8 U, respectively). The results indicate that Naþ suffers lower kinetics in a-Fe2O3/rGO1 electrode than in the a-Fe2O3/rGO2 electrode because of its unique architecture. A morphological study of the electrodes before cycling and after 150 cycles was also conducted. As shown in Fig. 8a-c, the electrodes before cycling show a similar smooth surface, while after cycling, the electrode morphology shows big differences. Fig. 8d is a FESEM image showing the surface of the a-Fe2O3 after 150 cycles. Large agglomerations of particles 20 mm in size can be clearly observed on the surface of the electrode. The a-Fe2O3/rGO1 electrode surface also exhibits agglomerations of particles in Fig. 8e, but the electrode keeps a part of the before cycling morphology. The a-Fe2O3/rGO2 electrode surface is much smoother in Fig. 8f. There are no clear cracks or particle agglomeration, suggesting the good structural stability of the composite electrode. In addition, some fibers can be seen in Fig. 8d and f, which came from the separator (glass fiber). This excellent stability of the electrode should be attributed to the presence of the special nanostructure, in which the a-Fe2O3 nanoparticles were wrapped up in graphene nanosheets. The graphene nanosheets could work as an effective conducting network, and the nanovoids contained in the a-Fe2O3 particles prevent large volume changes during sodium alloying/dealloying reactions [54,55]. Therefore, the special nanostructure could prevent agglomeration and pulverization of the a-Fe2O3 electrode [56]. Moreover, the a-Fe2O3/rGO2 electrode shows excellent cycling stability. 4. Conclusions
Fig. 7. Impedance plots of pure a-Fe2O3, and a-Fe2O3/rGO1 and a-Fe2O3/rGO2 nano composites, after cycling over 50 cycles at 25 C at frequencies from 100 kHz to 10 mHz. The equivalent circuit is shown in the inset.
In summary, a-Fe2O3/rGO composites have been successfully synthesized via an ultrafast and environmentally friendly microwave hydrothermal method. The a-Fe2O3 nanoparticles are wrapped up in crumpled micrometer-size graphene sheets. This nanoarchitecture not only can ensure intimate contact between the
112
Z.-J. Zhang et al. / Journal of Power Sources 280 (2015) 107e113
Fig. 8. FESEM images of the electrode surface of pure a-Fe2O3 (a and d), a-Fe2O3/rGO1 nanocomposite (b and e), and a-Fe2O3/rGO2 nanocomposite (c and f) before (a, b, and c) and after (d, e, and f) 150 cycles.
electrolyte and the active materials, but also high electronic conductivity of the electrode. The a-Fe2O3/rGO composites show significantly improved cycling performance and rate capability compared with the bare a-Fe2O3 electrode due to the active function of the graphene nanosheets in the composites. The experimental results show that the well-designed a-Fe2O3/rGO composite electrode has great potential as an anode for sodium ion batteries. Acknowledgments Financial support was provided by an Australian Research Council (ARC) Discovery Project (DP100103909). Zhijia Zhang is grateful to the China Scholarship Council (CSC) for scholarship support. This research used equipment that was funded by Australian Research Council Grant LE0237478 and is located at the UOW Electron Microscopy Centre. Many thanks are owed to Dr. Tania Silver for critical reading of the manuscript. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.01.092.
References [1] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci. 4 (2011) 3243e3262. [2] A.D. Roberts, X. Li, H. Zhang, Chem. Soc. Rev. 43 (2014) 4341e4356. [3] A. Ikram, S. Sahai, S. Rai, S. Dass, R. Shrivastav, V.R. Satsangi, J. Power Sources 267 (2014) 664e672. [4] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Funct. Mater. 23 (2013) 947e958. [5] S.W. Kim, D.H. Seo, X. Ma, G. Ceder, K. Kang, Adv. Energy Mater. 2 (2012) 710e721. [6] Y.X. Wang, Y.G. Lim, M.S. Park, S.L. Chou, J.H. Kim, H.K. Liu, S.X. Dou, Y.J. Kim, J. Mater. Chem. A 2 (2014) 529e534. [7] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Chem. Rev. 114 (2014) 11636e11682. [8] N. Yabuuchi, M. Kajiyama, J. Iwatate, H. Nishikawa, S. Hitomi, R. Okuyama, R. Usui, Y. Yamada, S. Komaba, Nat. Mater. 11 (2012) 512e517. [9] Z. Jian, W. Han, X. Lu, H. Yang, Y.S. Hu, J. Zhou, Z. Zhou, J. Li, W. Chen, D. Chen, Adv. Energy Mater. 3 (2013) 156e160. lez, [10] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonza T. Rojo, Energy Environ. Sci. 5 (2012) 5884e5901. [11] I.D. Gocheva, M. Nishijima, T. Doi, S. Okada, J.I. Yamaki, T. Nishida, J. Power Sources 187 (2009) 247e252. [12] Z. Jian, L. Zhao, H. Pan, Y.S. Hu, H. Li, W. Chen, L. Chen, Electrochem. Commun. 14 (2012) 86e89. [13] K.T. Lee, T. Ramesh, F. Nan, G. Botton, L.F. Nazar, Chem. Mater. 23 (2011) 3593e3600. [14] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Funct. Mater. 23 (2013) 947e958. [15] S. Komaba, W. Murata, T. Ishikawa, N. Yabuuchi, T. Ozeki, T. Nakayama,
Z.-J. Zhang et al. / Journal of Power Sources 280 (2015) 107e113 A. Ogata, K. Gotoh, K. Fujiwara, Adv. Funct. Mater. 21 (2011) 3859e3867. [16] H. Xiong, M.D. Slater, M. Balasubramanian, C.S. Johnson, T. Rajh, J. Phys. Chem. Lett. 2 (2011) 2560e2565. [17] X. Yu, H. Pan, W. Wan, C. Ma, J. Bai, Q. Meng, S.N. Ehrlich, Y.S. Hu, X.Q. Yang, Nano Lett. 13 (2013) 4721e4727. [18] Y. Wang, X. Yu, S. Xu, J. Bai, R. Xiao, Y.S. Hu, H. Li, X.Q. Yang, L. Chen, X. Huang, Nat. Commun. 4 (2013) 2365. [19] P. Senguttuvan, G. Rousse, V. Seznec, J.M. Tarascon, M.R. Palacin, Chem. Mater. 23 (2011) 4109e4111. [20] H. Zhu, Z. Jia, Y. Chen, N. Weadock, J. Wan, O. Vaaland, X. Han, T. Li, L. Hu, Nano Lett. 13 (2013) 3093e3100. [21] M. Dahbi, N. Yabuuchi, K. Kubota, K. Tokiwa, S. Komaba, Phys. Chem. Chem. Phys. 16 (2014) 15007e15028. [22] J. Qian, Y. Chen, L. Wu, Y. Cao, X. Ai, H. Yang, Chem. Commun. 48 (2012) 7070e7072. [23] W. Li, S.L. Chou, J.Z. Wang, J.H. Kim, H.K. Liu, S.X. Dou, Adv. Mater. 26 (2014) 4037e4042. [24] J. Chen, L. Xu, W. Li, X. Gou, Adv. Mater. 17 (2005) 582e586. [25] W.M. Zhang, X.L. Wu, J.S. Hu, Y.G. Guo, L.J. Wan, Adv. Funct. Mater. 18 (2008) 3941e3946. [26] M.H. Oh, T. Yu, S.H. Yu, B. Lim, K.T. Ko, M.G. Willinger, D.H. Seo, B.H. Kim, M.G. Cho, J.H. Park, K. Kang, Y.E. Sung, N. Pinna, T. Hyeon, Science 340 (2013) 964e968. [27] Li L., Wu H.B., Yu L., Madhavi S., Lou X.W., Adv. Mater. Interfaces doi:10.1002/ admi.201400050. €refors, K. Edstro €m, J. Power Sources 245 [28] M. Valvo, F. Lindgren, U. Lafont, F. Bjo (2014) 967e978. pez, P. Lavela, G.F. Ortiz, J.L. Tirado, Electrochem. Commun. 27 (2013) [29] M.C. Lo 152e155. [30] S. Komaba, T. Mikumo, N. Yabuuchi, A. Ogata, H. Yoshida, Y. Yamada, J. Electrochem. Soc. 157 (1) (2010) A60eA65. [31] Z. Jian, B. Zhao, P. Liu, F. Li, M. Zheng, M. Chen, Y. Shi, H. Zhou, Chem. Commun. 50 (2014) 1215e1217. [32] J.Z. Wang, C. Zhong, D. Wexler, N.H. Idris, Z.X. Wang, L.Q. Chen, H.K. Liu, Chem. Eur. J. 17 (2011) 661e667. [33] X. Zhu, Y. Zhu, S. Murali, M.D. Stoller, R.S. Ruoff, ACS Nano 5 (4) (2011)
113
3333e3338. [34] H. Wang, J.T. Robinson, G. Diankov, H. Dai, J. Am. Chem. Soc. 132 (10) (2010) 3270e3271. [35] J. Li, S. Xiong, Y. Liu, Z. Ju, Y. Qian, Nano Energy 2 (2013) 1249e1260. [36] J. Li, J. Wang, X. Liang, Z. Zhang, H. Liu, Y. Qian, S. Xiong, ACS Appl. Mater. Interfaces 6 (1) (2013) 24e30. [37] C. Zhong, J.Z. Wang, D. Wexler, H.K. Liu, Carbon 66 (2014) 637e645. [38] J. Xu, S.L. Chou, C. Zhou, Q.F. Gu, H.K. Liu, S.X. Dou, J. Power Sources 246 (2014) 124e131. [39] S.L. Chou, J.Z. Wang, J.Z. Sun, D. Wexler, M. Forsyth, H.K. Liu, D.R. MacFarlane, S.X. Dou, Chem. Mater. 20 (22) (2008) 7044e7051. ~ a, H.K. Liu, H. Li, J. Wang, Y. Wu, Nano Energy 8 (2014) [40] Y. Shi, J. Gao, H.D. Abrun 297e304. [41] C. Zhong, J. Wang, Z. Chen, H. Liu, J. Phys. Chem. C 115 (2011) 25115e25120. [42] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [43] Z.J. Zhang, J.Z. Wang, S.L. Chou, H.K. Liu, K. Ozawa, H.J. Li, Electrochim. Acta 108 (2013) 820e826. [44] S.Y. Liu, J. Xie, Q. Pan, C.Y. Wu, G.S. Cao, T.J. Zhu, X.B. Zhao, Int. J. Electrochem. Sci. 7 (2012) 354e362. [45] H. Yang, X. Mao, Y. Guo, D. Wang, G. Ge, R. Yang, X. Qiu, Y. Yang, Ch Wang, Y. Wang, G. Liu, CrystEngComm 12 (2010) 1842e1849. [46] X.Y. Xue, C.H. Ma, C.X. Cui, L.L. Xing, Solid State Sci. 13 (2011) 1526e1530. [47] Z. Tai, X. Yan, Q. Xue, J. Electrochem. Soc. 159 (10) (2012) A1702eA1709. [48] D. Bersani, P.P. Lottici, A. Montenero, J. Raman Spectrosc. 30 (1999) 355e360. [49] J. Ma, J. Teo, L. Mei, Z. Zhong, Q. Li, T. Wang, X. Duan, J. Lian, W. Zheng, J. Mater. Chem. 22 (2012) 11694e11700. [50] J. Ma, T. Wang, X. Duan, J. Lian, Z. Liu, W. Zheng, Nanoscale 3 (2011) 4372e4375. [51] Y.X. Wang, S.L. Chou, H.K. Liu, S.X. Dou, Carbon 57 (2013) 202e208. [52] Z.J. Zhang, Q.Y. Zeng, S.L. Chou, X.J. Li, H.J. Li, K. Ozawa, H.K. Liu, J.Z. Wang, Electrochim. Acta 133 (2014) 570e577. [53] H. Ma, S. Zhang, W. Ji, Z. Tao, J. Chen, J. Am. Chem. Soc. 130 (2008) 5361e5367. [54] C.Z. Wu, K. Yu, S. Zhang, Y. Xie, J. Phys. Chem. C 112 (2008) 11307e11313. [55] S. Zeng, K. Tang, T.W. Li, Z.H. Liang, D. Wang, Y. Wang, W. Zhou, J. Phys. Chem. C 111 (2007) 10217e10225. [56] L. Li, A.R.O. Raji, J.M. Tour, Adv. Mater. 25 (2013) 6298e6302.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Rapid synthesis of a-Fe2O3/rGO nanocomposites by microwave autoclave as superior anodes for sodium-ion batteries Zhi-Jia Zhang a, Yun-Xiao Wang a, Shu-Lei Chou a, Hui-Jun Li b, *, Hua-Kun Liu a, Jia-Zhao Wang a, * a b
Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia Faculty of Engineering, University of Wollongong, Wollongong, NSW 2522, Australia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
a-Fe2O3/rGO nanocomposite is synthesized through ultrafast microwave hydrothermal method. a-Fe2O3 nanoparticle with numerous nanovoids is wrapped up in crumpled graphene sheets. The nanovoids effect tolerating the volume change during sodium alloying/dealloying reactions. Significantly improved Na-storage capability in terms of cycling performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 November 2014 Received in revised form 19 December 2014 Accepted 14 January 2015 Available online 15 January 2015
a-Fe2O3/reduced graphene oxide (rGO) nanocomposites were successfully synthesized within 15 min
through a facile, environmentally friendly microwave hydrothermal method. From field emission scanning electron microscopy and transmission electron microscopy, it can be determined that the a-Fe2O3 nanoparticles, around 50 nm in diameter, are uniformly anchored on the graphene nanosheets. The asobtained a-Fe2O3/rGO nanocomposites were applied as anode materials in sodium-ion batteries, which could deliver capacity of ~310 mAh g1 after 150 cycles at 100 mA g1. © 2015 Published by Elsevier B.V.
Keywords: a-Fe2O3 Reduced graphene oxide Microwave autoclave Sodium-ion battery
1. Introduction Over the past several decades, energy storage and conversion have drawn a great deal of attention from the scientific community and industry. Lithium-ion batteries (LIBs) have been demonstrated
* Corresponding authors. E-mail addresses: [email protected] (J.-Z. Wang). http://dx.doi.org/10.1016/j.jpowsour.2015.01.092 0378-7753/© 2015 Published by Elsevier B.V.
(H.-J.
Li),
[email protected]
to be one of the most successful examples in commercial applications [1e3]. Their large-scale applications, however, would be limited due to the scarcity of lithium resources in the Earth's crust. In this regard, sodium-ion batteries (SIBs) have great advantages because of the abundance and low cost of sodium, and thus, they have become an alternative to LIBs [4e7]. The radius of sodium ion (0.95 Å) is about 1.6 times larger than that of lithium ion (0.6 Å), however, which is undoubtedly restricted the development of electrode materials of SIBs, especially at anode materials. In the past decade, cathode materials for SIBs
108
Z.-J. Zhang et al. / Journal of Power Sources 280 (2015) 107e113
have already attained fruitful results, mainly including three categories: sodium transition metal oxides [8,9], transition metal fluorides [10,11], and phosphates [12,13]. In contrast, the promising anodes for SIBs are confined a limited kind of materials according to recent reports, such as hard carbons [14,15], Ti-based compounds (i.e. TiO2 [16], Li4Ti5O12 [17], P2eNa0.66[Li0.22Ti0.78]O2 [18], Na2Ti3O7 [19], etc.), sodium Alloys (i.e. Sn [20], P [21], Sb [22], Sn4þxP3 [23], etc.). The disadvantages of anodes, including the large volume change during cycling, the poor electronic conductivity, and unclear sodiation/desodiation mechanisms, urgently need to be indepth investigated for acquiring high electrochemical performance. Therefore, the research on anode materials is one of the most important issues for developing SIBs. Iron oxides have many excellent advantages as potential anode materials, such as low cost, abundance, and high capacity [24]. In the past ten years, they have been widely studied as anode materials for LIBs [25e27]. Recently, they were also introduced as anodes for SIBs [28,29]. Komaba and his co-workers reported that a-Fe2O3 nanocrystals were highly electrochemically active in sodium salt electrolyte, and have excellent capacity retention of 170 mAh g1 at a rate of 100 mA g1 [30]. More recently, Zhou's group reported that Fe2O3@graphene nanosheets could deliver about 400 mAh g1 and remain stable over 200 cycles at a current density of 100 mA g1 [31]. The synthesis of iron oxides or iron oxide-based composites, however, usually required a long reaction time. Thus, exploring the synthesis of iron oxide-based composites is an effective method for obtaining low-cost, high-performance anode materials for SIBs. Herein, we have employed a novel microwave autoclave strategy, which combines the advantages of high productivity and efficiency with low temperature and risk. Most importantly, the microwave autoclave strategy is environmentally friendly and energy saving, successfully producing a composite material of wellorganized a-Fe2O3 encapsulated in graphene sheets. In comparison with the traditional hydrothermal method [31e36], the microwave hydrothermal method can expedite the kinetics of crystallization by causing rapid nucleation and growth, which can dramatically decrease the reaction time from 10 h or even several days via the conventional hydrothermal method to 15 min, as reported here. Therefore it can save a large amount of energy and offers great possibilities for large-scale reactions. Furthermore, the microwave hydrothermal method can be used effectively and efficiently for preparing various multifunctional nanomaterials with intriguing morphologies, such as nanowires [37,38], nanoplates [39], nanospheres [40], and nanoporous networks [41]. In this work, a-Fe2O3/reduced graphene oxide (a-Fe2O3/rGO) composites were synthesized by a microwave hydrothermal method and investigated as anode material for SIBs. This synthetic method has several important merits, such as being rapid, reductant-free, low cost and environmentally friendly. Moreover, the as-synthesized a-Fe2O3/rGO composite could deliver a discharge capacity of 310 mAh g1 over 150 cycles at a current of 100 mA g1, as well as having the improved rate capability compared with the pure a-Fe2O3 in SIBs.
GO was added to the solution followed by ultrasonic dispersion for 2 h. The solution was transferred into sealed Teflon vessels and reacted for 15 min at 180 C using a Milestone Microsynth Microwave Labstation (Germany). After cooling down naturally and washing 4 times with distilled water and ethanol, the black product, designated as a-Fe2O3/rGO1, was dried at 60 C in a vacuum oven for 12 h. The a-Fe2O3/rGO2 nanocomposite was prepared by the same method, except that the GO amount was 50 mg. To prepare pure a-Fe2O3 and rGO as a comparison, the same amounts of FeCl3 and rGO were used alone, followed by the same conditions and procedures applied in the synthesis of the composite. 2.2. Materials characterization The products, Fe2O3, a-Fe2O3/rGO1, and a-Fe2O3/rGO2, were analyzed by X-ray diffraction (XRD; GBC MMA) with Cu Ka radiation; Raman spectroscopy (JobinYvon HR800) employing a 10 mW helium/neon laser at 632.8 nm; field emission scanning electron microscopy (FESEM; JEOL 7500); and transmission electron microscopy (TEM; JEOL JEM-2011, 200 kV) with high-resolution TEM (HRTEM). Thermogravimetric analysis (TGA) was performed by using a SETARAM Thermogravimetric Analyzer (France) in air to determine the changes in sample weight with increasing temperature and to estimate the amount of rGO in the sample. 2.3. Electrochemical measurements The electrodes were prepared by mixing 80 wt.% a-Fe2O3, or aFe2O3/rGO composite with 15 wt.% carbon black and 5 wt.% carboxymethyl cellulose (CMC) binder. The slurry was spread onto Cu foil substrates. The coated electrodes were dried at 80 C in a vacuum oven for 24 h to remove water molecules. The electrodes were then pressed using a disc with a diameter of 14 mm to enhance the contact between the Cu foil, active materials, and conductive carbon. Subsequently, the electrodes were cut to a 1 1 cm2 size. The average active material loading rate was ~2 mg cm2. CR 2032 cointype cells were assembled in an Ar-filled glove box (Mbraun, Unilab, Germany), using sodium metal foil or lithium metal as the counter electrode and glass fiber as separator. The electrolyte was 1 M NaClO4 or LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). The cells were galvanostatically charged and discharged in the range of 0.05e2.7 V at different current
2. Experimental 2.1. Material synthesis Graphene oxide (GO) was prepared from graphite powder (Aldrich, powder, <20 mm, synthetic) by a modified Hummers' method [42]. The a-Fe2O3/rGO composites were prepared by using our previously reported microwave hydrothermal (MH) method [43]. For synthesizing a-Fe2O3/rGO composite, 0.01 mol FeCl3 (SigmaeAldrich) was dissolved in distilled water to obtain a final concentration of 0.1 M in a beaker. After stirring for 15 min, 100 mg
Fig. 1. XRD patterns of rGO, pure a-Fe2O3, a-Fe2O3/rGO1 nanocomposite, and a-Fe2O3/ rGO2 nanocomposite.
Z.-J. Zhang et al. / Journal of Power Sources 280 (2015) 107e113
109
densities using a computer-controlled charger system manufactured by Land Battery Testers. The calculation of specific capacity is based on the mass of active material (pure a-Fe2O3, or a-Fe2O3/rGO composite). A Biologic VMP-3 electrochemical workstation was used to perform electrochemical impedance spectroscopy (EIS; ac amplitude 5 mV, frequency range 100 kHze0.01 Hz). 3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of the samples. The XRD peaks of the composites can be indexed to standard aFe2O3 (hematite structure; JCPDS card No. 33e0664) without any impurity (Fig. 1a). For the rGO, the diffraction peaks at 2q z 24.5 and 42.8 can be attributed to the graphite-like (002) and (100) planes, respectively [44]. The corresponding peaks of rGO in aFe2O3/rGO1 and a-Fe2O3/rGO2 overlap with the a-Fe2O3 peaks [(012) and (202)]. In comparison to the pure a-Fe2O3, the peaks of the composites are broadened and have lower intensities, indicating that the addition of graphene restrains the growth of a-Fe2O3 particles. As can be seen from Fig. 1, the large full width at half maximum (FWHM) of a-Fe2O3/rGO2 shows that the a-Fe2O3 particle has a small size or is composed by smaller primary particles. In Fig. 2, the Raman spectra of rGO, pure a-Fe2O3, a-Fe2O3/rGO1 nanocomposite, and a-Fe2O3/rGO2 nanocomposite are plotted for comparison. The Raman spectra of the a-Fe2O3/rGO1 and a-Fe2O3/ rGO2 composites display their main peaks at 227, 293, 412, 498, 610 and 626 cm1, which correspond to the peaks for the a-Fe2O3 particles [45,46], and another two peaks at around 1355 cm1 and 1597 cm1, which are identified as the D-band and G-band of graphene, respectively [47]. In addition, the peak at 1322 cm1 (aFe2O3) and the one at 1355 cm1 (rGO) overlap in the Raman spectra of the composite. The peak at 1322 cm1 (a-Fe2O3) is the characteristic band assigned to a two-magnon scattering, resulting from the interaction of two magnons created on antiparallel close spin sites [48]. In agreement with the XRD results, this indicates that the a-Fe2O3/rGO composites were successfully synthesized via the microwave hydrothermal method from graphite oxide and FeCl3 powder. Furthermore, different intensities of the XRD and Raman peaks between a-Fe2O3/rGO1 and a-Fe2O3/rGO2 are observed, which can probably be ascribed to the different ratios of a-Fe2O3 in the composites. For quantifying the amount of rGO in the a-Fe2O3/rGO1 and aFe2O3/rGO2 nanocomposites, TGA was carried out in air. Fig. 3
Fig. 2. Raman spectra of rGO, pure a-Fe2O3, a-Fe2O3/rGO1 nanocomposite, and aFe2O3/rGO2 nanocomposite.
Fig. 3. TGA curves of rGO, pure a-Fe2O3, a-Fe2O3/rGO1 nanocomposite, and a-Fe2O3/ rGO2 nanocomposite.
shows the TGA curves of the a-Fe2O3/rGO1 and a-Fe2O3/rGO2 nanocomposites along with those of pure a-Fe2O3 and rGO powder when heated from 40 to 800 C at a rate of 10 C min1 in air. rGO powder burns out at ~750 C, while the pure a-Fe2O3 powder maintains a constant weight throughout the temperature range used for this experiment. The a-Fe2O3/rGO1 and a-Fe2O3/rGO2 nanocomposites show a single-step weight loss at a temperature of ~500 C, which corresponds to the burning of rGO. There is no further weight loss after the initial decomposition of rGO. Therefore, the change in weight before and after the burning of rGO directly translates into the amount of rGO in the a-Fe2O3/rGO. Using this method, it can be estimated that the amounts of rGO in the a-Fe2O3/rGO1 and a-Fe2O3/rGO2 composites were 13.7 wt.% and 20.8 wt.%, respectively. The morphology and microstructure of the samples were observed via FESEM (Figure S1, Appendix A. Supplementary Information (ASI)) and TEM (Fig. 4). As shown in Figure S1, the nanosized a-Fe2O3 particles are well crystallized, and the particle size is ~300 nm. The particle size of a-Fe2O3 in the a-Fe2O3/rGO1 composite is similar to that in the pure a-Fe2O3 sample, while the aFe2O3 particle is composed by numerous ultrasmall primary particles which are only several nanometers (Figure S1b and Figure S1e). This phenomenon is mostly caused by the reduction process of GO. a-Fe2O3 particles are only partially anchored on the rGO nanosheets, possibly due to the low amount of rGO (13.7 wt.%). When the rGO content is increased to 20.8 wt%, the a-Fe2O3 nanoparticles are fully wrapped up in the crumpled micrometersize graphene nanosheets, and the a-Fe2O3 particle size dramatically decreases to about 50 nm (Figure S1c and Figure S1f). TEM investigations further revealed that the nanosized a-Fe2O3 particles are uniformly dispersed on the graphene nanosheets (Fig. 4a and b). Fig. 4b shows that numerous nanovoids exist in the a-Fe2O3 nanoparticles, mostly due to the high energy microwave treatment [49,50]. The nanovoids are extremely favorable for tolerating the large volume change during sodium alloying/dealloying reactions. In Fig. 4c, HRTEM image shows the (110) lattice fringes, with the dspacing measured to be 0.251 nm. Meanwhile, the selected area electron diffraction (SAED) pattern (Fig. 4d) shows two sets of diffraction patterns: 1) a-Fe2O3 phase, which can be indexed as (01 0), (1 00), and (110); 2) polycrystalline diffraction rings corresponding to the graphene and support films. In addition, the SAED pattern of a-Fe2O3 shows a series of weak diffraction spots for other directions, due to the existence of the nanovoids. In this optimized
110
Z.-J. Zhang et al. / Journal of Power Sources 280 (2015) 107e113
Fig. 4. TEM images of a-Fe2O3/rGO2 nanocomposite at low magnification (a), at higher magnification (b), at high resolution (HRTEM), (c) and the corresponding SAED pattern (d) from (b).
structure, the rGO matrix can form an effective conducting network and facilitate the formation of a stable solid electrolyte inter-phase (SEI) layer as well. The as-prepared a-Fe2O3/RGO composites were first tested as anode for LIBs, where they showed high capacity and cycling performance (with a-Fe2O3/RGO2 delivering about 1168 mAh g1 over 100 cycles at 100 mA g1). Furthermore, the a-Fe2O3/rGO2 composite electrode also exhibits excellent rate capability details shown in Figure S2 (ASI). Therefore, a-Fe2O3/RGO composites were expected to deliver a good electrochemical performance in SIBs. And a series tests were then carried out on a-Fe2O3/rGO anode in SIBs. Fig. 5 and Figure S3 (ASI) show typical chargeedischarge curves for different cycles of a-Fe2O3 and a-Fe2O3/rGO composites in cointype half-cells using sodium as the counter and reference electrode between 0.05 and 3.0 V (vs. Naþ/Na). a-Fe2O3/rGO2 composite shows an irreversible plateau (~0.5 V) at the first discharge process, due to the irreversible reaction between a-Fe2O3/rGO2 and sodium ions and the formation of the SEI layer. This is responsible for the low initial coulombic efficiency of the cycling performance (Fig. 6a). As shown in Fig. 5, the first discharge capacity of a-Fe2O3/rGO2 is around 1170 mAh g1. In contrast, the a-Fe2O3 and a-Fe2O3/rGO1 composite electrodes show lower initial discharge and discharge capacities (1010 and 543 mAh g1, respectively), and the reversible capacity in the following cycles is much lower than for the a-Fe2O3/ rGO2 composite electrode, being maintained at 37, 165 and 310 mAh g1, respectively (Fig. 5 and Figure S3). The reasons of the high initial discharge and charge capacities of the a-Fe2O3/rGO2 composite electrodes can be explained by: a) rGO was reported to be able accommodate sodium ions according our previous report [51]. It exhibited a high discharge capacity (1695 mAh g1, at a current density of 50 mA g1) at the first cycle, due to decomposition of electrolyte and irreversible reactions. b) The a-Fe2O3/rGO2 composite (rGO, 20.8 wt.%) contains higher amount of rGO than that of the a-Fe2O3/rGO1 composite (rGO, 13.7 wt.%). It is
Fig. 5. 1st, 3rd, and 10th cycle charge/discharge curves for a-Fe2O3/rGO2 nanocomposite. The inset shows charge/discharge curves for other selected higher cycles.
reasonable that a-Fe2O3/rGO2 delivers higher initial discharge and charge capacities with more sodium ions absorbed in rGO matrix. The unique architecture and extremely small a-Fe2O3 nanoparticles are responsible for the superior electrochemical properties of a-Fe2O3/rGO2. As can be seen from Fig. 6a, the cycling performance was tested at 0.1C (100 mA g1) over the range of 0.05e3.0 V. In contrast, the a-Fe2O3 electrode delivered a poor cycling performance with much lower capacity (after 50 cycles, ~10 mAh g1), which demonstrates the crucial effect of the architecture for the composite. The poor performance of the a-Fe2O3 electrode is mainly ascribed to the large crystallized a-Fe2O3 particles and the longer diffusion paths due to the absence of graphene. The capacity remains at 310 mAh g1 over 150 cycles, and
Z.-J. Zhang et al. / Journal of Power Sources 280 (2015) 107e113
Fig. 6. (a) Cycling performance of pure a-Fe2O3, and a-Fe2O3/rGO1 and a-Fe2O3/rGO2 nanocomposites at the current density of 100 mA g1; (b) high-rate capability of aFe2O3/rGO1 and a-Fe2O3rGO2 nanocomposites.
the coulombic efficiency is nearly 100% for a-Fe2O3/rGO2 electrode. The a-Fe2O3/rGO1 electrode, which has larger a-Fe2O3 particles and a lower amount rGO, was also evaluated for comparison. Compared with a-Fe2O3/rGO2, this electrode shows higher initial capacity, but inferior capacity retention and worse cycling stability. The
111
discharge capacity declines to 165 mAh g1 after 150 cycles. Furthermore, the rate capability of the a-Fe2O3/rGO1 and a-Fe2O3/ rGO2 composites at different current rates is evaluated in Fig 6b. The a-Fe2O3/rGO2 composite electrode exhibits excellent rate capability. When the electrode was tested at 50 mA g1 (0.05C), the discharge capacity was about 1136 mAh g1, and then when the current rate was increased rapidly to 2000 mA g1 (2C), the corresponding capacity was maintained at 77 mAh g1. More importantly, the capacity can recover to a great extent, to around 370 mA h g1 at 0.1C, which is consistent with the obtained results for electrode cycling at 0.1C shown in Fig. 6a. Electrochemical impedance spectroscopy (EIS) measurements were also carried out to compare the impedance differences in aFe2O3, a-Fe2O3/rGO1 and a-Fe2O3/rGO2. The Nyquist plots were collected from 100 kHz to 10 mHz on the coin-cell batteries after charge/discharge for 50 cycles. As shown in Fig. 7, the EIS spectra are fitted by an equivalent circuit and exhibit two compressed semicircles followed by a linear part [52,53]. The fitted parameters show that the cells with a-Fe2O3/rGO1 and a-Fe2O3/rGO2 electrode possess similar uncompensated resistance (Rl) of 6.5 and 2.3 U respectively, which is much lower than that of the pure a-Fe2O3 electrode (76.4 U) due to its poor conductivity. In contrast, the film resistance (Rf) of the SEI and the charge transfer resistance (Rct) show significant differences in the three different coin cells. The cells with a-Fe2O3/rGO2 show a lower Rf (15.3 U) than those for aFe2O3 and a-Fe2O3/rGO1 (142.6 and 44.5 U, respectively), indicating the formation of a thinner SEI layer because the a-Fe2O3 particles are uniformly wrapped with graphene, which is in agreement with the much higher capacity in Fig. 6a and Figure S2. Meanwhile, the Rct of the a-Fe2O3/rGO2 electrode is much smaller than those of the a-Fe2O3 and a-Fe2O3/rGO1 electrodes (249.5, 1218.6 and 498.8 U, respectively). The results indicate that Naþ suffers lower kinetics in a-Fe2O3/rGO1 electrode than in the a-Fe2O3/rGO2 electrode because of its unique architecture. A morphological study of the electrodes before cycling and after 150 cycles was also conducted. As shown in Fig. 8a-c, the electrodes before cycling show a similar smooth surface, while after cycling, the electrode morphology shows big differences. Fig. 8d is a FESEM image showing the surface of the a-Fe2O3 after 150 cycles. Large agglomerations of particles 20 mm in size can be clearly observed on the surface of the electrode. The a-Fe2O3/rGO1 electrode surface also exhibits agglomerations of particles in Fig. 8e, but the electrode keeps a part of the before cycling morphology. The a-Fe2O3/rGO2 electrode surface is much smoother in Fig. 8f. There are no clear cracks or particle agglomeration, suggesting the good structural stability of the composite electrode. In addition, some fibers can be seen in Fig. 8d and f, which came from the separator (glass fiber). This excellent stability of the electrode should be attributed to the presence of the special nanostructure, in which the a-Fe2O3 nanoparticles were wrapped up in graphene nanosheets. The graphene nanosheets could work as an effective conducting network, and the nanovoids contained in the a-Fe2O3 particles prevent large volume changes during sodium alloying/dealloying reactions [54,55]. Therefore, the special nanostructure could prevent agglomeration and pulverization of the a-Fe2O3 electrode [56]. Moreover, the a-Fe2O3/rGO2 electrode shows excellent cycling stability. 4. Conclusions
Fig. 7. Impedance plots of pure a-Fe2O3, and a-Fe2O3/rGO1 and a-Fe2O3/rGO2 nano composites, after cycling over 50 cycles at 25 C at frequencies from 100 kHz to 10 mHz. The equivalent circuit is shown in the inset.
In summary, a-Fe2O3/rGO composites have been successfully synthesized via an ultrafast and environmentally friendly microwave hydrothermal method. The a-Fe2O3 nanoparticles are wrapped up in crumpled micrometer-size graphene sheets. This nanoarchitecture not only can ensure intimate contact between the
112
Z.-J. Zhang et al. / Journal of Power Sources 280 (2015) 107e113
Fig. 8. FESEM images of the electrode surface of pure a-Fe2O3 (a and d), a-Fe2O3/rGO1 nanocomposite (b and e), and a-Fe2O3/rGO2 nanocomposite (c and f) before (a, b, and c) and after (d, e, and f) 150 cycles.
electrolyte and the active materials, but also high electronic conductivity of the electrode. The a-Fe2O3/rGO composites show significantly improved cycling performance and rate capability compared with the bare a-Fe2O3 electrode due to the active function of the graphene nanosheets in the composites. The experimental results show that the well-designed a-Fe2O3/rGO composite electrode has great potential as an anode for sodium ion batteries. Acknowledgments Financial support was provided by an Australian Research Council (ARC) Discovery Project (DP100103909). Zhijia Zhang is grateful to the China Scholarship Council (CSC) for scholarship support. This research used equipment that was funded by Australian Research Council Grant LE0237478 and is located at the UOW Electron Microscopy Centre. Many thanks are owed to Dr. Tania Silver for critical reading of the manuscript. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.01.092.
References [1] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci. 4 (2011) 3243e3262. [2] A.D. Roberts, X. Li, H. Zhang, Chem. Soc. Rev. 43 (2014) 4341e4356. [3] A. Ikram, S. Sahai, S. Rai, S. Dass, R. Shrivastav, V.R. Satsangi, J. Power Sources 267 (2014) 664e672. [4] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Funct. Mater. 23 (2013) 947e958. [5] S.W. Kim, D.H. Seo, X. Ma, G. Ceder, K. Kang, Adv. Energy Mater. 2 (2012) 710e721. [6] Y.X. Wang, Y.G. Lim, M.S. Park, S.L. Chou, J.H. Kim, H.K. Liu, S.X. Dou, Y.J. Kim, J. Mater. Chem. A 2 (2014) 529e534. [7] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Chem. Rev. 114 (2014) 11636e11682. [8] N. Yabuuchi, M. Kajiyama, J. Iwatate, H. Nishikawa, S. Hitomi, R. Okuyama, R. Usui, Y. Yamada, S. Komaba, Nat. Mater. 11 (2012) 512e517. [9] Z. Jian, W. Han, X. Lu, H. Yang, Y.S. Hu, J. Zhou, Z. Zhou, J. Li, W. Chen, D. Chen, Adv. Energy Mater. 3 (2013) 156e160. lez, [10] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonza T. Rojo, Energy Environ. Sci. 5 (2012) 5884e5901. [11] I.D. Gocheva, M. Nishijima, T. Doi, S. Okada, J.I. Yamaki, T. Nishida, J. Power Sources 187 (2009) 247e252. [12] Z. Jian, L. Zhao, H. Pan, Y.S. Hu, H. Li, W. Chen, L. Chen, Electrochem. Commun. 14 (2012) 86e89. [13] K.T. Lee, T. Ramesh, F. Nan, G. Botton, L.F. Nazar, Chem. Mater. 23 (2011) 3593e3600. [14] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Funct. Mater. 23 (2013) 947e958. [15] S. Komaba, W. Murata, T. Ishikawa, N. Yabuuchi, T. Ozeki, T. Nakayama,
Z.-J. Zhang et al. / Journal of Power Sources 280 (2015) 107e113 A. Ogata, K. Gotoh, K. Fujiwara, Adv. Funct. Mater. 21 (2011) 3859e3867. [16] H. Xiong, M.D. Slater, M. Balasubramanian, C.S. Johnson, T. Rajh, J. Phys. Chem. Lett. 2 (2011) 2560e2565. [17] X. Yu, H. Pan, W. Wan, C. Ma, J. Bai, Q. Meng, S.N. Ehrlich, Y.S. Hu, X.Q. Yang, Nano Lett. 13 (2013) 4721e4727. [18] Y. Wang, X. Yu, S. Xu, J. Bai, R. Xiao, Y.S. Hu, H. Li, X.Q. Yang, L. Chen, X. Huang, Nat. Commun. 4 (2013) 2365. [19] P. Senguttuvan, G. Rousse, V. Seznec, J.M. Tarascon, M.R. Palacin, Chem. Mater. 23 (2011) 4109e4111. [20] H. Zhu, Z. Jia, Y. Chen, N. Weadock, J. Wan, O. Vaaland, X. Han, T. Li, L. Hu, Nano Lett. 13 (2013) 3093e3100. [21] M. Dahbi, N. Yabuuchi, K. Kubota, K. Tokiwa, S. Komaba, Phys. Chem. Chem. Phys. 16 (2014) 15007e15028. [22] J. Qian, Y. Chen, L. Wu, Y. Cao, X. Ai, H. Yang, Chem. Commun. 48 (2012) 7070e7072. [23] W. Li, S.L. Chou, J.Z. Wang, J.H. Kim, H.K. Liu, S.X. Dou, Adv. Mater. 26 (2014) 4037e4042. [24] J. Chen, L. Xu, W. Li, X. Gou, Adv. Mater. 17 (2005) 582e586. [25] W.M. Zhang, X.L. Wu, J.S. Hu, Y.G. Guo, L.J. Wan, Adv. Funct. Mater. 18 (2008) 3941e3946. [26] M.H. Oh, T. Yu, S.H. Yu, B. Lim, K.T. Ko, M.G. Willinger, D.H. Seo, B.H. Kim, M.G. Cho, J.H. Park, K. Kang, Y.E. Sung, N. Pinna, T. Hyeon, Science 340 (2013) 964e968. [27] Li L., Wu H.B., Yu L., Madhavi S., Lou X.W., Adv. Mater. Interfaces doi:10.1002/ admi.201400050. €refors, K. Edstro €m, J. Power Sources 245 [28] M. Valvo, F. Lindgren, U. Lafont, F. Bjo (2014) 967e978. pez, P. Lavela, G.F. Ortiz, J.L. Tirado, Electrochem. Commun. 27 (2013) [29] M.C. Lo 152e155. [30] S. Komaba, T. Mikumo, N. Yabuuchi, A. Ogata, H. Yoshida, Y. Yamada, J. Electrochem. Soc. 157 (1) (2010) A60eA65. [31] Z. Jian, B. Zhao, P. Liu, F. Li, M. Zheng, M. Chen, Y. Shi, H. Zhou, Chem. Commun. 50 (2014) 1215e1217. [32] J.Z. Wang, C. Zhong, D. Wexler, N.H. Idris, Z.X. Wang, L.Q. Chen, H.K. Liu, Chem. Eur. J. 17 (2011) 661e667. [33] X. Zhu, Y. Zhu, S. Murali, M.D. Stoller, R.S. Ruoff, ACS Nano 5 (4) (2011)
113
3333e3338. [34] H. Wang, J.T. Robinson, G. Diankov, H. Dai, J. Am. Chem. Soc. 132 (10) (2010) 3270e3271. [35] J. Li, S. Xiong, Y. Liu, Z. Ju, Y. Qian, Nano Energy 2 (2013) 1249e1260. [36] J. Li, J. Wang, X. Liang, Z. Zhang, H. Liu, Y. Qian, S. Xiong, ACS Appl. Mater. Interfaces 6 (1) (2013) 24e30. [37] C. Zhong, J.Z. Wang, D. Wexler, H.K. Liu, Carbon 66 (2014) 637e645. [38] J. Xu, S.L. Chou, C. Zhou, Q.F. Gu, H.K. Liu, S.X. Dou, J. Power Sources 246 (2014) 124e131. [39] S.L. Chou, J.Z. Wang, J.Z. Sun, D. Wexler, M. Forsyth, H.K. Liu, D.R. MacFarlane, S.X. Dou, Chem. Mater. 20 (22) (2008) 7044e7051. ~ a, H.K. Liu, H. Li, J. Wang, Y. Wu, Nano Energy 8 (2014) [40] Y. Shi, J. Gao, H.D. Abrun 297e304. [41] C. Zhong, J. Wang, Z. Chen, H. Liu, J. Phys. Chem. C 115 (2011) 25115e25120. [42] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [43] Z.J. Zhang, J.Z. Wang, S.L. Chou, H.K. Liu, K. Ozawa, H.J. Li, Electrochim. Acta 108 (2013) 820e826. [44] S.Y. Liu, J. Xie, Q. Pan, C.Y. Wu, G.S. Cao, T.J. Zhu, X.B. Zhao, Int. J. Electrochem. Sci. 7 (2012) 354e362. [45] H. Yang, X. Mao, Y. Guo, D. Wang, G. Ge, R. Yang, X. Qiu, Y. Yang, Ch Wang, Y. Wang, G. Liu, CrystEngComm 12 (2010) 1842e1849. [46] X.Y. Xue, C.H. Ma, C.X. Cui, L.L. Xing, Solid State Sci. 13 (2011) 1526e1530. [47] Z. Tai, X. Yan, Q. Xue, J. Electrochem. Soc. 159 (10) (2012) A1702eA1709. [48] D. Bersani, P.P. Lottici, A. Montenero, J. Raman Spectrosc. 30 (1999) 355e360. [49] J. Ma, J. Teo, L. Mei, Z. Zhong, Q. Li, T. Wang, X. Duan, J. Lian, W. Zheng, J. Mater. Chem. 22 (2012) 11694e11700. [50] J. Ma, T. Wang, X. Duan, J. Lian, Z. Liu, W. Zheng, Nanoscale 3 (2011) 4372e4375. [51] Y.X. Wang, S.L. Chou, H.K. Liu, S.X. Dou, Carbon 57 (2013) 202e208. [52] Z.J. Zhang, Q.Y. Zeng, S.L. Chou, X.J. Li, H.J. Li, K. Ozawa, H.K. Liu, J.Z. Wang, Electrochim. Acta 133 (2014) 570e577. [53] H. Ma, S. Zhang, W. Ji, Z. Tao, J. Chen, J. Am. Chem. Soc. 130 (2008) 5361e5367. [54] C.Z. Wu, K. Yu, S. Zhang, Y. Xie, J. Phys. Chem. C 112 (2008) 11307e11313. [55] S. Zeng, K. Tang, T.W. Li, Z.H. Liang, D. Wang, Y. Wang, W. Zhou, J. Phys. Chem. C 111 (2007) 10217e10225. [56] L. Li, A.R.O. Raji, J.M. Tour, Adv. Mater. 25 (2013) 6298e6302.