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Synthesis of [email protected]3O4-C hybrid nanocables as anode materials with enhanced electrochemical performance for lithium ion batteries
Electrochimica Acta 176 (2015) 1332–1337
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Synthesis of CNT@Fe3O4-C hybrid nanocables as anode materials with enhanced electrochemical performance for lithium ion batteries Yucheng Donga,b , Kamruzzaman Mda , Ying-San Chuia , Yang Xiaa , Chenwei Caoa , Jong-Min Leeb,* , Juan Antonio Zapiena,* a b
Center of super-Diamond and Advanced Films (COSDAF), Department of Physics and Materials Science, City University of Hong Kong, Hong Kong School of Chemical & Biomedical Engineering, Nanyang Technological University, Nayang Drive, Singapore 639798
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 May 2015 Received in revised form 24 July 2015 Accepted 26 July 2015 Available online 29 July 2015
We report a simple method to prepare hierarchical structures of carbon-coated Fe3O4 nanoparticles decorated on conductive multi-walled carbon nanotubes (CNTs) backbone as advanced anode materials for lithium ion batteries. The CNTs backbone serves as a shape template and conducting network to facilitate electron and lithium ion transport, while the carbon coating acts as a buffering layer to maintain the structural integrity of the electrode upon cycling. The prepared CNT@Fe3O4-C hybrid nanocables electrode exhibits excellent electrochemical performance with long-term cycling stability and high specific capacity of 1080 mAh g1 after 700 cycles at a current density of 500 mA g1, and good rate capability, which could be attributed to their unique structural features as well as the improved mechanical stability enabled by the outer carbon coating layer. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Hybrid nanocables carbon nanotubes Fe3O4 anode materials lithium ion batteries
1. INTRODUCTION Lithium ion batteries (LIBs), widely used as power sources for portable electronics, are now expanding their applications to electrical/hybrid vehicles and large scale grid storages [1–3]. However, graphite is the currently commercially anode material for LIBs and possess inherent limitations because of its limited specific capacity of 372 mAh g1 [4], thus stimulating intense global research interests for high performance electrode materials with high specific capacity to satisfy the ever-increasing market demand [5,6]. Transition metal oxides are considered as potential candidates to replace graphite-based anodes in LIBs due to their high theoretical capacity (500-1000 mAh g1) [7–10]. Among them, Fe3O4 is considered as the most promising anode candidate because of its high theoretical capacity (924 mAh g1), environmental benignity, low cost, and natural abundance [11–14]. However, a major limitation for the practical application of Fe3O4-based anode materials is that it suffers from poor capacity retention caused by drastic volume variation which results in the pulverization of the anode and the fracture of the solid-electrolyte interphase (SEI) layer upon cycling. In addition, the intrinsic kinetic limitations of charge transfer and ionic diffusion of the Fe3O4-
* Corresponding authors. E-mail addresses: [email protected] (J.-M. Lee), [email protected] (J.A. Zapien). http://dx.doi.org/10.1016/j.electacta.2015.07.144 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
based electrodes lead to additional performance degradation [7,15–17]. Many efforts have been made to circumvent the above issues by designing many kinds of nanostructured materials [18– 22]. Small-sized nanoparticles can provide short diffusion path for lithium ion and accommodate mechanical stress associate with volume variation during cycling [23–25]. However, the high external surface area of nanoparticle improves the contact area between the electrolyte and electrode, the additional exposure of active material results in the possibility of significant sub-reactions with the electrolyte [26]. The repeated SEI growth caused by volume change upon each cycle leads to continuous SEI formation, and the resulting thick SEI layer can retard the diffusion of lithium ion thus leading to larger irreversible capacity [27]. The use of a carbon coating on the nanoparticles’ surface is considered an effective way to improve the mechanical strength and conductivity of the electrode, as well as the stability of the SEI layer by avoiding direct electrode/electrolyte contact [28–31]. Another widely utilized approach to improve the electronic conductivity of the LIBs’ electrode is to combine nanostructured materials with a highly conductive matrix to form a hybrid nanostructure [32–35]. Among various conductive matrixes, onedimensional CNTs networks have been shown to provide enhanced electrical conductivity, large surface-to-volume ratio, high mechanical strength, and good chemical stability and are considered as promising conducting substrates for energy storage system [36–44]. As a result, it has been proposed to design hierarchical structures of small-sized Fe3O4 nanoparticles attached onto
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conductive CNTs substrate, further coated with a carbon layer to improve the electrochemical performance of the hybrid composites as anode materials for LIBs [45–47]. The flexible CNTs backbone provides both a high electronic conductive network to facilitate charge transfer and inner buffering matrix to alleviate volume variation during lithiation/delithiation process [45,46]. Meanwhile, the outmost carbon coating sheath serves as a structural buffer layer to accommodate the internal strain changes during lithium uptake, and protective shell to prevent the breaking down of the Fe3O4 nanoparticles from the CNTs substrate during discharge/charge process [48]. Gao’s and Nie’s groups reported the similar architectural structures as anode materials for LIBs, these hybrid composites delivered enhanced electrochemical performance [45,46]. However, the uniformity of the small-sized Fe3O4 nanoparticles destributed onto CNT backbone and the outmost thin carbon sheath should be revised to improve the long-term stability of the electrode. We report an effective method to realize the full potential of the active materials by the integration of these advantageous structural features by the fabrication of small-sized Fe3O4 nanoparticles (10 nm) uniformly decorated onto conductive CNTs substrate, which are then coated with a thin carbon layer to form hierarchical CNT@Fe3O4-C hybrid nanocables. Benefiting from the improved structural stability and charge transfer, the hybrid composites of the CNT@Fe3O4-C electrode exhibits enhanced electrochemical performance with excellent long-term cycling stability and high specific capacity of 1080 mAh g1 after 700 cycles at a current density of 500 mA g1. 2. EXPERIMENTAL The multi-walled CNTs, with diameter 20-60 nm and several micrometer in length, were pre-treated by nitric acid as illustrated in the supporting information. The CNT@Fe3O4-C hybrid nanocables were prepared by a hydrothermal method. In a typical synthesis, 25 mg acid-treated CNTs were dissolved in 40 mL deionized water by mild ultra-sonication for 1 h, then 1.059 g Iron (III) acetylacetonate (Fe(acac)3; 97%, Aldrich) and 0.384 g critic acid (C6H8O7) were added into the resultant solution while stirring at 80 C for 2 h to form a crimson solution. The resultant solution was transferred to a Teflon-lined stainless steel autoclave (50 mL in volume) and heated at 200 C for 12 h. The products were centrifuged and washed several times by alcohol and deionized water before drying at 60 C for 12 h in an electric oven. For the carbon coating of CNT@Fe3O4 hierarchical structures, a certain
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amount of CNT@Fe3O4 powder (50 mg) was dispersed in 40 mL of aqueous solution of glucose (0.06 M). The resultant suspension was transferred to a 50 mL Teflon-line stainless steel autoclave and heated at 180 C for 6 h. The obtained products were collected and dried. The resulting product was calcined at 500 C for 3 h in a tube furnace under a continuous argon gas atmosphere to obtain crystalline CNT@Fe3O4-C hybrid nanocables. For comparison, the Fe3O4 composites were fabricated under the similar procedure but without the addition of acid-treated CNTs and glucose. The crystallographic characteristics of the products were studied by X-ray diffraction (XRD, Philips PW 1830); which also enabled particle size estimation using the Scherrer equation, L = Kl/bcosu, where K is the Scherrer constant (0.89), l is the X-ray wavelength, b is the line broadening at half the maximum intensity (FWHM), u is the Bragg diffraction angle. Raman measurements were conducted on a Renishaw in Via 2000 spectrometer using an excited HeNe laser (17 mW and 628 nm) with an accumulation/collection time of 10 S. Thermogravimetric analysis (TG, Q50) was performed from room temperature to 700 C with a heating rate of 10 C min1 to evaluate the content of carbon under air atmosphere. The morphological characteristics were investigated by scanning electron microscopy (SEM; Philips, XL 30FEG), transmission electron microscopy (TEM; Philips, CM20), and highresolution TEM (HRTEM; CM200 FEG). The working electrodes were prepared by mixing 80 wt% active material, 10 wt% acetylene carbon black, and 10 wt% polyvinylidene fluoride (PVDF) binder dissolved in 1-methyl-2-pyrrolidinone (NMP) solvent to form a slurry. The resultant slurry was uniformly coated on a copper foil current collector, and then dried at 100 C for 10 h in vacuum to remove the solvent. The electrochemical measurements were carried out using coin-type cells (2032) assembled in an argon-filled glovebox with lithium metal foil (Aldrich, USA) as counter/reference electrode, Celgard 2032 (Celgard, Inc., USA) was used as the separator while LiPF6 (1 mol L1), dissolved in a mixture of ethylene carbonate/dimethyl carbonate (1:1 in volume), was used as the electrolyte. Cyclic voltammograms (CVs) measurements were performed using a CHI-660C electrochemical workstation at a scanning rate of 0.2 mV s1 in a potential range from 5 mV to 3.0 V. The electrochemical impedance spectroscopy (EIS) was carried out on a ZAHNER-elektrik IM6 over a frequency range from 100 kHz to 10 mHz. Galvanostatic discharge/charge cycles were tested in the voltage range from 5 mV to 3.0 V using different constant current densities on an Arbin Instruments (BT 2000, College Station, Texas,
Fig. 1. (a) XRD patterns of pristine CNTs and CNT@Fe3O4-C hybrid nanocables, and (b) Raman spectra of acid-treated CNTs, CNT@Fe3O4 composites, and CNT@Fe3O4-C hybrid nanocables.
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USA) battery cycler at room temperature. Note that the specific capacity was calculated by the Fe3O4 mass of the composites. 3. RESULTS AND DISCUSSION The XRD patterns of the pristine CNTs and heat treated CNT@Fe3O4-C hybrid nanocables are shown in Fig. 1(a). Two diffraction peaks at 26.1 and 42.9 are clearly seen which are associated with the graphitic structure, planes (200) and (100) (PDF, 00-001-0640), respectively, of the MWCNT. For the CNT@Fe3O4-C hybrid nanocables, the diffraction peaks and relative intensities are in good agreement with those of Fe3O4 (Magnetite, PDF, 00-011-0614). The strong diffraction peaks indicate good crystallinity of Fe3O4 phases. The average Fe3O4 nanoparticle size of the as-synthesized samples can be estimated to be 9.7 nm by the Scherrer’s formula from the (311) crystal plane. The graphite diffraction peak (#) can be detected in the CNT@Fe3O4-C hybrid nanocables XRD pattern due to the presence of the CNT in the composites. The Raman spectra of the acid-treated CNTs, CNT@Fe3O4 and CNT@Fe3O4-C hybrid nanocables are presented in Fig. 1(b) to obtain structural change information during the preparation process. These three samples possess two strong and broad peaks centered at 1350 and 1585 cm1, which are corresponding to D band and G band, respectively, of acid-treated CNT [49]. It is noteworthy that the intensity ratio of D and G band (ID/IG) for acidtreated CNTs, CNT@Fe3O4 and CNT@Fe3O4-C hybrid nanocables are 1.36, 1.55 and 1.16, respectively. The ID/IG of CNT@Fe3O4 composites increased by comparison to the acid-treated CNTs due to the increased structure defects and the strong binding interaction between Fe and O [50]. The ID/IG of CNT@Fe3O4-C hybrid nanocables decreased to 1.16, which demonstrates the existence of amorphous carbon shells. In addition, the Raman spectrum of CNT@Fe3O4 composites exhibits the typical peaks of Fe3O4 at 346, 488, and 686 cm1 from Eg, T2g, and A1g vibration modes, respectively [51]. For the CNT@Fe3O4-C hybrid nanocables, the Raman peaks of Fe3O4 cannot be detected after the carbon layer coated onto the overall architecture of the CNT@Fe3O4 composites, which is consistent with a uniform covering of carbon layer in this sample. The approximate proportions of carbon content in the CNT@Fe3O4 composites and CNT@Fe3O4-C hybrid nanocables were investigated by TG analysis under air atmosphere as shown in Fig. 2(a) and 2(b), respectively. The TG curve of CNT@Fe3O4 composites in Fig. 2(a) show a major mass loss of 15.4 wt% in the temperature range of 400-550 C and the presence of an exothermic peak at 502 C which could be ascribed to the oxidation of CNTs; however, considering that the oxidation of
Fe3O4 to Fe2O3 leads to a mass increase of 3.4 wt% and happens in the temperature range of 160-450 C based on the TG spectrum in Fig. S3 (see ESI for details), the carbon content in the CNT@Fe3O4 composites is estimated to be 18.8 wt%. In contrast, the CNT@Fe3O4-C hybrid nanocables in Fig. 2(b) presents two mass losses. The first one, between 250 and 390 C, results from burning of the outside carbon sheath and the oxidation of Fe3O4, so that the carbon content can be estimated to be 17.6 wt%; while the second mass loss of 16.7 wt%, from 390 to 550 C, should be attributed to the oxidation of Fe3O4 and burning of CNTs. The structural morphologies of as-prepared product were characterized by SEM, TEM, and HRTEM. The typical SEM and TEM images of the acid-treated CNTs are shown in Fig. S1 (see ESI for details). It is can be seen that the functionalized CNTs present smooth surface with outer diameter in the 20-70 nm range. Fig. S2 (see ESI for details) shows the SEM images of the CNT@Fe3O4-C hybrid nanocables with coarse surfaces. The TEM image of CNT@Fe3O4 composites in Fig. 3(a) clearly exhibits the Fe3O4 nanoparticles attached on the surface of CNTs to form intercrossing networks. No bare CNTs or freestanding Fe3O4 nanoparticles are found in this sample. Fig. 3(b) shows the TEM image of a single CNT@Fe3O4 composites, which reveals the smallsized Fe3O4 nanoparticles closely anchored on CNTs to form a rough and dense Fe3O4 shell. The underneath conductive CNT core would benefit the fast electron transfer and maintain the structural integrity. As shown in Fig. 3(c), no further change in morphology is observed after carbon coating of the overall CNT@Fe3O4 composites to form CNT@Fe3O4-C hybrid nanocables, which is expected form a uniform carbon layer coating. The result was further confirmed by the HRTEM image in Fig. 3(f), a uniform carbon layer with several nanometers thickness can be found surround the Fe3O4 nanoparticle. The uniform carbon sheath plays an important role in forming stable SEI layer, avoiding the detachment of Fe3O4 nanoparticles from CNTs, and improving the conductivity of the electrode. Finally, the HRTEM images in Fig. 3(d) and 3(e) show high magnification details of the CNT being closely decorated with Fe3O4 nanoparticles with an average diameter of 10 nm which is in good agreement with the estimated crystalline size from XRD data (Fig. 1(a)) using Scherrer’s equation. It is should be noted that the Fe3O4 nanoparticles are strongly anchored on CNTs substrate ever after sonication treatment during the preparation of TEM specimens. The potential of the CNT@Fe3O4-C hybrid nanocables as anode materials for LIB was investigated by carrying out a series of electrochemical measurements. Fig. 4(a) shows cyclic voltammograms of a CNT@Fe3O4-C electrode at a scan rate of 0.2 mV s1 in a potential range between 0.005 and 3.0 V versus Li/Li+ for the first three cycles. In the cathodic process of the first scan, the minor
Fig. 2. TG and DTG spectra of (a) CNT@Fe3O4 composites and (b) CNT@Fe3O4-C hybrid nanocables under air atmosphere.
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Fig. 3. (a) TEM image of CNT@Fe3O4 composites; TEM images of (b) single CNT@Fe3O4 composites and (c) CNT@Fe3O4-C hybrid nanocables; HRTEM images of (d,e) CNT@Fe3O4 composites and (f) CNT@Fe3O4-C hybrid nanocables.
peak located at 1.53 V, which can be ascribed to the irreversible with the electrolyte [52]. The two peaks are observed at 0.80 and 0.50 V, respectively, corresponding to the two steps of the lithiation reactions of Fe3O4 [step 1, Fe3O4 + 2Li+ + 2e ! Li2(Fe3O4); and step 2, Li2(Fe3O4) + 6Li+ + 6e ! 3Fe0 + 4Li2O], and the formation of SEI layer on the electrode surface due to the irreversible reaction with the electrolyte [53,54]. During the first anodic process, a broad peak at 1.67 V and a shoulder peak at 1.86 V that can be attributed to the oxidation of Fe0 to Fe3O4 (3Fe0 + 4 Li2O ! Fe3O4 + 8Li+ + 8e) [55]. The stepwise process of the broad peak cannot be distinguished most possibly due to kinetic effects [46]. After the first cycle, both anodic and cathodic peaks shift to higher voltage and the corresponding current peaks decrease revealing the existence of an irreversible capacity loss in the initial lithiation-delithiation process. The change of peak intensity and integrated areas for both cathodic and anodic peaks are very limited after the first cycle, suggesting that lithium ions insertion/ extraction takes place to the same extent. These similar characteristics are also observed in the in the voltage range between 0.005 to 3.0 V at a current density galvanostatic discharge/charge profiles for the first three cycles of 100 mA g1 as shown in Fig. S4 (see ESI for details). For the confirmation of the potential applicability of the electrodes, the comparative cycling performance of the CNT@Fe3O4-C, CNT@Fe3O4, and Fe3O4 electrodes were tested as anode materials for LIBs at a current density of 100 mA g1 for the first 5 cycles and then at 500 mA g1 for the following cycles, as shown in Fig. 4(b). The reversible capacity of the CNT@Fe3O4-C electrode is 1055 mAh g1 in the first cycle, and begins to increase from the sixth cycle until reaching 1085 mAh g1 at 300 cycles. The CNT@Fe3O4-C electrode exhibited excellent long-term cycling performance, and while its capacity undergoes a slightly decrease
it steadily increases to and stabilize at 1080 mAh g1 after 220 cycles remaining at this level up to 700 cycles. The Coulombic efficiency is consistently over 98% from the 10th to the 250th cycle, then over 99% in the following cycles. The relative low increased Coulombic efficiency (>99% after 250 cycles) is attributed to the parasitic surface reactions in the electrode. The CNT@Fe3O4 electrode shows a similar phenomenon while, however, capacity recovery is more gradual and stabilizes at 770 mAh g1 after 400 cycles. In contrast, the reversible capacity of Fe3O4 electrode rapidly decrease from 880 mAh g1 to 130 mAh g1, and then increased to 370 mAh g1. The results in Fig. 4(b) clearly demonstrate that the cycling performance of Fe3O4 electrode is greatly enhanced by the uniform decoration of Fe3O4 nanoparticles on CNTs substrate to form CNT@Fe3O4 composites, and that the performance of such composites is enhanced even further be the uniform carbon coating to form CNT@Fe3O4-C with excellent long-term stability. The phenomenon of capacity increasing trend has been observed in the nanostructured Fe3O4 and carbon composites electrodes [27,39,45,56,57], but there is so for no consensus on the reason. We propose the following possibilities: (i) During the cycling process, the active materials are rearranged gradually to provide more accessible active sites for the insertion of lithium ions, this long-term activation process allows the full utilization of active materials [27,45]. (ii) The reversible growth of the polymeric or gel-like film on the electrode surface by the kinetically activated electrolyte degradation is likely considered as possible reasons for the capacity increase, because the SEI layer could supply excess lithium ion storage sites by a so-called “pseudocapacitance-type behavior”, especially in the low potential region [58]. (iii) The generation of Fe nanoparticles accompanied with irreversible reactions resulting in capacity decrease in the initial
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Fig. 4. (a) Cyclic voltammograms of CNT@Fe3O4-C hybrid nanocables. (b) Cycling performance of CNT@Fe3O4-C, CNT@Fe3O4, and Fe3O4 electrodes at a current density of 500 mA g1. (c) Rate performance of CNT@Fe3O4-C and CNT@Fe3O4 electrodes at various current densities. (d) Nyquist plots (Z0 vs. –Z00 ) of CNT@Fe3O4-C, CNT@Fe3O4, and Fe3O4 electrodes.
electrochemical cycles [59]. The metallic nanoparticles will increase the conductivity of the whole electrodes, which leads to the capacity increase in the following cycles [60]. To further investigate the electrochemical performance of the CNT@Fe3O4-C hybrid nanocables electrode, the rate performances of different samples were evaluated for 10 cycles at various current densities, as shown in Fig. 4(c). As expected, the CNT@Fe3O4-C electrode exhibited a better rate performance compared with CNT@Fe3O4 electrode. Even at a high current density of 2000 mA g1, the CNT@Fe3O4-C electrode delivers a reversible capacity of 405 mAh g1, which is higher than the theoretical capacity of commercial graphite (372 mAh g1). In contrast, only 303 mAh g1 of reversible capacity was delivered by the CNT@Fe3O4 electrode at 2000 mA g1. Moreover, when the current density was reduced to 200 mA g1, the specific capacity of the CNT@Fe3O4-C electrode recovered to 900 mAh g1, indicating the structural stability of the electrode even after the high rate cycling. To verify the positive role of CNTs networks and outside carbon layers in the hybrid composites, EIS measurements were performed in the frequency range between 100 kHz and 10 mHz; the resulting Nyquist plots (Z0 vs. –Z00 ) for the CNT@Fe3O4-C, CNT@Fe3O4, and Fe3O4 electrodes are compared in Fig. 4(d). The Nyquist plot consists of a depressed semicircle in the high to medium frequency region and a sloping line in the low-frequency region. The semicircle at the high frequency region is attributed to lithium ions transport resistance through the SEI film; while that at the mid-frequency is related to the charge transfer impedance at the electrodeelectrolyte interface (Rct) which is considered as a large proportion of overall cell’s kinetic impedance [61,62]. The linear region known as Warburg impedance corresponds to diffusion of lithium ions in the lattice of Fe3O4 crystals [63–67]. Analysis of the Nyquist’s plots
in Fig. 4(d) reveals that the Rct of the CNT@Fe3O4-C, CNT@Fe3O4, and Fe3O4 electrodes are 180, 245, and 370 V, respectively. The reduced charge transfer resistance of the CNT@Fe3O4-C electrode can be attributed to the conductive CNTs networks and carbon coating layers. Moreover, the largest slop in the low frequency region for the CNT@Fe3O4-C electrode suggests an improvement in the conversion reaction and diffusivity of lithium ions in the electrode materials, which further demonstrates the significance of the designed hybrid coaxial nanostructures. It should be noted that the cycling stability of the CNT@Fe3O4-C hybrid nanocables electrode presented in this work is better than that of previously reported the hierarchical structure of the carbon coated Fe3O4/CNTs composites [45–48]. The enhanced electrochemical performance originates from their multiple structural features. First, the electronic conductivity of the hybrid composites is greatly enhanced by the hierarchically inner conductive CNTs backbone networks and outside carbon coating shells. Second, the small-sized Fe3O4 nanoparticles (10 nm) allow fast diffusion of lithium ions and transport electrons while their large surface area provide efficient interface for the electrochemical reaction. Also, the carbon coating of the complete hierarchical structure avoids direct contact between the active materials and electrolyte thus minimizing the possibility of sub-reactions of the electrode with the electrolyte that otherwise might result given the high external surface area of the small-sized Fe3O4 nanoparticles. Finally, the inner CNTs backbones and outer carbon coatings improve the structural integrity and robustness of the electrode by suppressing volume variation and aggregation during cycling. These results clearly demonstrate the potential of the CNT@Fe3O4-C hybrid nanocables as high performance anode materials for LIBs.
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4. CONCLUSION In this work, the hierarchical structure of Fe3O4 nanoparticles decorated onto conductive CNTs substrate with carbon coating layer was prepared successfully and tested as anode materials for LIBs. In this architecture, the CNTs backbone and carbon coating layer not only provide electrical network to facilitate electron and lithium ion transport, but also improve the mechanical strength against deformation to maintain the structural integrity of the electrode upon electrochemical cycling. Furthermore, the carbon coating on Fe3O4 nanoparticles prevents direct contact between the electrode and electrolyte for the benefit of forming a stable SEI layer. As a result, the hierarchical CNT@Fe3O4-C electrode exhibits improved electrochemical performance compared with the CNT@Fe3O4 and Fe3O4 anode materials due to the synergistic effects between the conductive inner CNT/outer carbon coating and uniform distributed Fe3O4 nanoparticles. Specifically, the CNT@Fe3O4-C electrode delivered excellent long-term cycling performance and high specific capacity of 1080 mAh g1 after 700 cycles at a current density of 500 mA g1. The simple fabrication route described can be extended to fabricate other hybrid composites with nanocable architectures for high-performance battery applications. ACKNOWLEDGEMENT The authors acknowledge support by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. CityU 122812). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2015.07.144. References [1] J.M. Tarascon, M. Armand, Nature 414 (2011) 359. [2] P.G. Bruce, B. Scrosati, J.-M. Tarascon, Angewandte Chemie International Edition 47 (2008) 2930. [3] B. Dunn, H. Kamath, J.-M. Tarascon, Science 334 (2011) 928. [4] N.A. Kaskhedikar, J. Maier, Advanced Materials 21 (2009) 2664. [5] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Advanced Materials 22 (2010) E28. [6] H.L. Wang, L.F. Cui, Y.A. Yang, H.S. Casalongue, J.T. Robinson, Y.Y. Liang, Y. Cui, H. J. Dai, Journal of the Americal Chemical Society 132 (2010) 13978. [7] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496. [8] X.W. Lou, Y. Wang, C.L. Yuan, J.Y. Lee, L.A. Archer, Advanced Materials 18 (2006) 2325. [9] Y.G. Li, B. Tan, Y.Y. Wu, Nano Letters 8 (2008) 265. [10] O. Waser, M. Hess, A. Güntner, P. Novák, S.E. Pratsinis, Journal of Power Sources 241 (2013) 415. [11] P.L. Taberna, S. Mitra, P. Poizot, P. Simon, J.-M. Tarascon, Nature Materials 5 (2006) 567. [12] S. Mitra, P. Poizot, A. Finke, J.-M. Tarascon, Advanced Functional Materials 16 (2006) 2281. [13] S.Q. Wang, J.Y. Zhang, C.H. Chen, Journal of Power Sources 195 (2010) 5379. [14] Y.C. Dong, R.G. Ma, M.J. Hu, H. Cheng, Q.D. Yang, Y.Y. Li, J.A. Zapien, Physical Chemistry Chemical Physics 15 (2013) 7174. [15] W.J. Zhang, Journal of Power Sources 196 (2011) 13. [16] L.W. Ji, Z.K. Tan, T.R. Kuykendall, S. Aloni, S.D. Xun, E. Lin, V. Battaglia, Y.G. Zhang, Physical Chemistry Chemical Physics 13 (2011) 7170. [17] Y. Chen, H. Xia, L. Lu, J.M. Xue, Journal of Material Chemistry 22 (2012) 5006. [18] X.W. Lou, C.M. Li, L.A. Archer, Advanced Materials 21 (2009) 2536. [19] T. Kavitha, H. Yuvaraj, Journal of Material Chemistry 21 (2011) 15686. [20] Y. Han, Y.J. Wang, L. Li, Y.P. Wang, L.F. Jiao, H.T. Yuan, S.X. Liu, Electrochimica Acta 56 (2011) 3175.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Synthesis of CNT@Fe3O4-C hybrid nanocables as anode materials with enhanced electrochemical performance for lithium ion batteries Yucheng Donga,b , Kamruzzaman Mda , Ying-San Chuia , Yang Xiaa , Chenwei Caoa , Jong-Min Leeb,* , Juan Antonio Zapiena,* a b
Center of super-Diamond and Advanced Films (COSDAF), Department of Physics and Materials Science, City University of Hong Kong, Hong Kong School of Chemical & Biomedical Engineering, Nanyang Technological University, Nayang Drive, Singapore 639798
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 May 2015 Received in revised form 24 July 2015 Accepted 26 July 2015 Available online 29 July 2015
We report a simple method to prepare hierarchical structures of carbon-coated Fe3O4 nanoparticles decorated on conductive multi-walled carbon nanotubes (CNTs) backbone as advanced anode materials for lithium ion batteries. The CNTs backbone serves as a shape template and conducting network to facilitate electron and lithium ion transport, while the carbon coating acts as a buffering layer to maintain the structural integrity of the electrode upon cycling. The prepared CNT@Fe3O4-C hybrid nanocables electrode exhibits excellent electrochemical performance with long-term cycling stability and high specific capacity of 1080 mAh g1 after 700 cycles at a current density of 500 mA g1, and good rate capability, which could be attributed to their unique structural features as well as the improved mechanical stability enabled by the outer carbon coating layer. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Hybrid nanocables carbon nanotubes Fe3O4 anode materials lithium ion batteries
1. INTRODUCTION Lithium ion batteries (LIBs), widely used as power sources for portable electronics, are now expanding their applications to electrical/hybrid vehicles and large scale grid storages [1–3]. However, graphite is the currently commercially anode material for LIBs and possess inherent limitations because of its limited specific capacity of 372 mAh g1 [4], thus stimulating intense global research interests for high performance electrode materials with high specific capacity to satisfy the ever-increasing market demand [5,6]. Transition metal oxides are considered as potential candidates to replace graphite-based anodes in LIBs due to their high theoretical capacity (500-1000 mAh g1) [7–10]. Among them, Fe3O4 is considered as the most promising anode candidate because of its high theoretical capacity (924 mAh g1), environmental benignity, low cost, and natural abundance [11–14]. However, a major limitation for the practical application of Fe3O4-based anode materials is that it suffers from poor capacity retention caused by drastic volume variation which results in the pulverization of the anode and the fracture of the solid-electrolyte interphase (SEI) layer upon cycling. In addition, the intrinsic kinetic limitations of charge transfer and ionic diffusion of the Fe3O4-
* Corresponding authors. E-mail addresses: [email protected] (J.-M. Lee), [email protected] (J.A. Zapien). http://dx.doi.org/10.1016/j.electacta.2015.07.144 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
based electrodes lead to additional performance degradation [7,15–17]. Many efforts have been made to circumvent the above issues by designing many kinds of nanostructured materials [18– 22]. Small-sized nanoparticles can provide short diffusion path for lithium ion and accommodate mechanical stress associate with volume variation during cycling [23–25]. However, the high external surface area of nanoparticle improves the contact area between the electrolyte and electrode, the additional exposure of active material results in the possibility of significant sub-reactions with the electrolyte [26]. The repeated SEI growth caused by volume change upon each cycle leads to continuous SEI formation, and the resulting thick SEI layer can retard the diffusion of lithium ion thus leading to larger irreversible capacity [27]. The use of a carbon coating on the nanoparticles’ surface is considered an effective way to improve the mechanical strength and conductivity of the electrode, as well as the stability of the SEI layer by avoiding direct electrode/electrolyte contact [28–31]. Another widely utilized approach to improve the electronic conductivity of the LIBs’ electrode is to combine nanostructured materials with a highly conductive matrix to form a hybrid nanostructure [32–35]. Among various conductive matrixes, onedimensional CNTs networks have been shown to provide enhanced electrical conductivity, large surface-to-volume ratio, high mechanical strength, and good chemical stability and are considered as promising conducting substrates for energy storage system [36–44]. As a result, it has been proposed to design hierarchical structures of small-sized Fe3O4 nanoparticles attached onto
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conductive CNTs substrate, further coated with a carbon layer to improve the electrochemical performance of the hybrid composites as anode materials for LIBs [45–47]. The flexible CNTs backbone provides both a high electronic conductive network to facilitate charge transfer and inner buffering matrix to alleviate volume variation during lithiation/delithiation process [45,46]. Meanwhile, the outmost carbon coating sheath serves as a structural buffer layer to accommodate the internal strain changes during lithium uptake, and protective shell to prevent the breaking down of the Fe3O4 nanoparticles from the CNTs substrate during discharge/charge process [48]. Gao’s and Nie’s groups reported the similar architectural structures as anode materials for LIBs, these hybrid composites delivered enhanced electrochemical performance [45,46]. However, the uniformity of the small-sized Fe3O4 nanoparticles destributed onto CNT backbone and the outmost thin carbon sheath should be revised to improve the long-term stability of the electrode. We report an effective method to realize the full potential of the active materials by the integration of these advantageous structural features by the fabrication of small-sized Fe3O4 nanoparticles (10 nm) uniformly decorated onto conductive CNTs substrate, which are then coated with a thin carbon layer to form hierarchical CNT@Fe3O4-C hybrid nanocables. Benefiting from the improved structural stability and charge transfer, the hybrid composites of the CNT@Fe3O4-C electrode exhibits enhanced electrochemical performance with excellent long-term cycling stability and high specific capacity of 1080 mAh g1 after 700 cycles at a current density of 500 mA g1. 2. EXPERIMENTAL The multi-walled CNTs, with diameter 20-60 nm and several micrometer in length, were pre-treated by nitric acid as illustrated in the supporting information. The CNT@Fe3O4-C hybrid nanocables were prepared by a hydrothermal method. In a typical synthesis, 25 mg acid-treated CNTs were dissolved in 40 mL deionized water by mild ultra-sonication for 1 h, then 1.059 g Iron (III) acetylacetonate (Fe(acac)3; 97%, Aldrich) and 0.384 g critic acid (C6H8O7) were added into the resultant solution while stirring at 80 C for 2 h to form a crimson solution. The resultant solution was transferred to a Teflon-lined stainless steel autoclave (50 mL in volume) and heated at 200 C for 12 h. The products were centrifuged and washed several times by alcohol and deionized water before drying at 60 C for 12 h in an electric oven. For the carbon coating of CNT@Fe3O4 hierarchical structures, a certain
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amount of CNT@Fe3O4 powder (50 mg) was dispersed in 40 mL of aqueous solution of glucose (0.06 M). The resultant suspension was transferred to a 50 mL Teflon-line stainless steel autoclave and heated at 180 C for 6 h. The obtained products were collected and dried. The resulting product was calcined at 500 C for 3 h in a tube furnace under a continuous argon gas atmosphere to obtain crystalline CNT@Fe3O4-C hybrid nanocables. For comparison, the Fe3O4 composites were fabricated under the similar procedure but without the addition of acid-treated CNTs and glucose. The crystallographic characteristics of the products were studied by X-ray diffraction (XRD, Philips PW 1830); which also enabled particle size estimation using the Scherrer equation, L = Kl/bcosu, where K is the Scherrer constant (0.89), l is the X-ray wavelength, b is the line broadening at half the maximum intensity (FWHM), u is the Bragg diffraction angle. Raman measurements were conducted on a Renishaw in Via 2000 spectrometer using an excited HeNe laser (17 mW and 628 nm) with an accumulation/collection time of 10 S. Thermogravimetric analysis (TG, Q50) was performed from room temperature to 700 C with a heating rate of 10 C min1 to evaluate the content of carbon under air atmosphere. The morphological characteristics were investigated by scanning electron microscopy (SEM; Philips, XL 30FEG), transmission electron microscopy (TEM; Philips, CM20), and highresolution TEM (HRTEM; CM200 FEG). The working electrodes were prepared by mixing 80 wt% active material, 10 wt% acetylene carbon black, and 10 wt% polyvinylidene fluoride (PVDF) binder dissolved in 1-methyl-2-pyrrolidinone (NMP) solvent to form a slurry. The resultant slurry was uniformly coated on a copper foil current collector, and then dried at 100 C for 10 h in vacuum to remove the solvent. The electrochemical measurements were carried out using coin-type cells (2032) assembled in an argon-filled glovebox with lithium metal foil (Aldrich, USA) as counter/reference electrode, Celgard 2032 (Celgard, Inc., USA) was used as the separator while LiPF6 (1 mol L1), dissolved in a mixture of ethylene carbonate/dimethyl carbonate (1:1 in volume), was used as the electrolyte. Cyclic voltammograms (CVs) measurements were performed using a CHI-660C electrochemical workstation at a scanning rate of 0.2 mV s1 in a potential range from 5 mV to 3.0 V. The electrochemical impedance spectroscopy (EIS) was carried out on a ZAHNER-elektrik IM6 over a frequency range from 100 kHz to 10 mHz. Galvanostatic discharge/charge cycles were tested in the voltage range from 5 mV to 3.0 V using different constant current densities on an Arbin Instruments (BT 2000, College Station, Texas,
Fig. 1. (a) XRD patterns of pristine CNTs and CNT@Fe3O4-C hybrid nanocables, and (b) Raman spectra of acid-treated CNTs, CNT@Fe3O4 composites, and CNT@Fe3O4-C hybrid nanocables.
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USA) battery cycler at room temperature. Note that the specific capacity was calculated by the Fe3O4 mass of the composites. 3. RESULTS AND DISCUSSION The XRD patterns of the pristine CNTs and heat treated CNT@Fe3O4-C hybrid nanocables are shown in Fig. 1(a). Two diffraction peaks at 26.1 and 42.9 are clearly seen which are associated with the graphitic structure, planes (200) and (100) (PDF, 00-001-0640), respectively, of the MWCNT. For the CNT@Fe3O4-C hybrid nanocables, the diffraction peaks and relative intensities are in good agreement with those of Fe3O4 (Magnetite, PDF, 00-011-0614). The strong diffraction peaks indicate good crystallinity of Fe3O4 phases. The average Fe3O4 nanoparticle size of the as-synthesized samples can be estimated to be 9.7 nm by the Scherrer’s formula from the (311) crystal plane. The graphite diffraction peak (#) can be detected in the CNT@Fe3O4-C hybrid nanocables XRD pattern due to the presence of the CNT in the composites. The Raman spectra of the acid-treated CNTs, CNT@Fe3O4 and CNT@Fe3O4-C hybrid nanocables are presented in Fig. 1(b) to obtain structural change information during the preparation process. These three samples possess two strong and broad peaks centered at 1350 and 1585 cm1, which are corresponding to D band and G band, respectively, of acid-treated CNT [49]. It is noteworthy that the intensity ratio of D and G band (ID/IG) for acidtreated CNTs, CNT@Fe3O4 and CNT@Fe3O4-C hybrid nanocables are 1.36, 1.55 and 1.16, respectively. The ID/IG of CNT@Fe3O4 composites increased by comparison to the acid-treated CNTs due to the increased structure defects and the strong binding interaction between Fe and O [50]. The ID/IG of CNT@Fe3O4-C hybrid nanocables decreased to 1.16, which demonstrates the existence of amorphous carbon shells. In addition, the Raman spectrum of CNT@Fe3O4 composites exhibits the typical peaks of Fe3O4 at 346, 488, and 686 cm1 from Eg, T2g, and A1g vibration modes, respectively [51]. For the CNT@Fe3O4-C hybrid nanocables, the Raman peaks of Fe3O4 cannot be detected after the carbon layer coated onto the overall architecture of the CNT@Fe3O4 composites, which is consistent with a uniform covering of carbon layer in this sample. The approximate proportions of carbon content in the CNT@Fe3O4 composites and CNT@Fe3O4-C hybrid nanocables were investigated by TG analysis under air atmosphere as shown in Fig. 2(a) and 2(b), respectively. The TG curve of CNT@Fe3O4 composites in Fig. 2(a) show a major mass loss of 15.4 wt% in the temperature range of 400-550 C and the presence of an exothermic peak at 502 C which could be ascribed to the oxidation of CNTs; however, considering that the oxidation of
Fe3O4 to Fe2O3 leads to a mass increase of 3.4 wt% and happens in the temperature range of 160-450 C based on the TG spectrum in Fig. S3 (see ESI for details), the carbon content in the CNT@Fe3O4 composites is estimated to be 18.8 wt%. In contrast, the CNT@Fe3O4-C hybrid nanocables in Fig. 2(b) presents two mass losses. The first one, between 250 and 390 C, results from burning of the outside carbon sheath and the oxidation of Fe3O4, so that the carbon content can be estimated to be 17.6 wt%; while the second mass loss of 16.7 wt%, from 390 to 550 C, should be attributed to the oxidation of Fe3O4 and burning of CNTs. The structural morphologies of as-prepared product were characterized by SEM, TEM, and HRTEM. The typical SEM and TEM images of the acid-treated CNTs are shown in Fig. S1 (see ESI for details). It is can be seen that the functionalized CNTs present smooth surface with outer diameter in the 20-70 nm range. Fig. S2 (see ESI for details) shows the SEM images of the CNT@Fe3O4-C hybrid nanocables with coarse surfaces. The TEM image of CNT@Fe3O4 composites in Fig. 3(a) clearly exhibits the Fe3O4 nanoparticles attached on the surface of CNTs to form intercrossing networks. No bare CNTs or freestanding Fe3O4 nanoparticles are found in this sample. Fig. 3(b) shows the TEM image of a single CNT@Fe3O4 composites, which reveals the smallsized Fe3O4 nanoparticles closely anchored on CNTs to form a rough and dense Fe3O4 shell. The underneath conductive CNT core would benefit the fast electron transfer and maintain the structural integrity. As shown in Fig. 3(c), no further change in morphology is observed after carbon coating of the overall CNT@Fe3O4 composites to form CNT@Fe3O4-C hybrid nanocables, which is expected form a uniform carbon layer coating. The result was further confirmed by the HRTEM image in Fig. 3(f), a uniform carbon layer with several nanometers thickness can be found surround the Fe3O4 nanoparticle. The uniform carbon sheath plays an important role in forming stable SEI layer, avoiding the detachment of Fe3O4 nanoparticles from CNTs, and improving the conductivity of the electrode. Finally, the HRTEM images in Fig. 3(d) and 3(e) show high magnification details of the CNT being closely decorated with Fe3O4 nanoparticles with an average diameter of 10 nm which is in good agreement with the estimated crystalline size from XRD data (Fig. 1(a)) using Scherrer’s equation. It is should be noted that the Fe3O4 nanoparticles are strongly anchored on CNTs substrate ever after sonication treatment during the preparation of TEM specimens. The potential of the CNT@Fe3O4-C hybrid nanocables as anode materials for LIB was investigated by carrying out a series of electrochemical measurements. Fig. 4(a) shows cyclic voltammograms of a CNT@Fe3O4-C electrode at a scan rate of 0.2 mV s1 in a potential range between 0.005 and 3.0 V versus Li/Li+ for the first three cycles. In the cathodic process of the first scan, the minor
Fig. 2. TG and DTG spectra of (a) CNT@Fe3O4 composites and (b) CNT@Fe3O4-C hybrid nanocables under air atmosphere.
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Fig. 3. (a) TEM image of CNT@Fe3O4 composites; TEM images of (b) single CNT@Fe3O4 composites and (c) CNT@Fe3O4-C hybrid nanocables; HRTEM images of (d,e) CNT@Fe3O4 composites and (f) CNT@Fe3O4-C hybrid nanocables.
peak located at 1.53 V, which can be ascribed to the irreversible with the electrolyte [52]. The two peaks are observed at 0.80 and 0.50 V, respectively, corresponding to the two steps of the lithiation reactions of Fe3O4 [step 1, Fe3O4 + 2Li+ + 2e ! Li2(Fe3O4); and step 2, Li2(Fe3O4) + 6Li+ + 6e ! 3Fe0 + 4Li2O], and the formation of SEI layer on the electrode surface due to the irreversible reaction with the electrolyte [53,54]. During the first anodic process, a broad peak at 1.67 V and a shoulder peak at 1.86 V that can be attributed to the oxidation of Fe0 to Fe3O4 (3Fe0 + 4 Li2O ! Fe3O4 + 8Li+ + 8e) [55]. The stepwise process of the broad peak cannot be distinguished most possibly due to kinetic effects [46]. After the first cycle, both anodic and cathodic peaks shift to higher voltage and the corresponding current peaks decrease revealing the existence of an irreversible capacity loss in the initial lithiation-delithiation process. The change of peak intensity and integrated areas for both cathodic and anodic peaks are very limited after the first cycle, suggesting that lithium ions insertion/ extraction takes place to the same extent. These similar characteristics are also observed in the in the voltage range between 0.005 to 3.0 V at a current density galvanostatic discharge/charge profiles for the first three cycles of 100 mA g1 as shown in Fig. S4 (see ESI for details). For the confirmation of the potential applicability of the electrodes, the comparative cycling performance of the CNT@Fe3O4-C, CNT@Fe3O4, and Fe3O4 electrodes were tested as anode materials for LIBs at a current density of 100 mA g1 for the first 5 cycles and then at 500 mA g1 for the following cycles, as shown in Fig. 4(b). The reversible capacity of the CNT@Fe3O4-C electrode is 1055 mAh g1 in the first cycle, and begins to increase from the sixth cycle until reaching 1085 mAh g1 at 300 cycles. The CNT@Fe3O4-C electrode exhibited excellent long-term cycling performance, and while its capacity undergoes a slightly decrease
it steadily increases to and stabilize at 1080 mAh g1 after 220 cycles remaining at this level up to 700 cycles. The Coulombic efficiency is consistently over 98% from the 10th to the 250th cycle, then over 99% in the following cycles. The relative low increased Coulombic efficiency (>99% after 250 cycles) is attributed to the parasitic surface reactions in the electrode. The CNT@Fe3O4 electrode shows a similar phenomenon while, however, capacity recovery is more gradual and stabilizes at 770 mAh g1 after 400 cycles. In contrast, the reversible capacity of Fe3O4 electrode rapidly decrease from 880 mAh g1 to 130 mAh g1, and then increased to 370 mAh g1. The results in Fig. 4(b) clearly demonstrate that the cycling performance of Fe3O4 electrode is greatly enhanced by the uniform decoration of Fe3O4 nanoparticles on CNTs substrate to form CNT@Fe3O4 composites, and that the performance of such composites is enhanced even further be the uniform carbon coating to form CNT@Fe3O4-C with excellent long-term stability. The phenomenon of capacity increasing trend has been observed in the nanostructured Fe3O4 and carbon composites electrodes [27,39,45,56,57], but there is so for no consensus on the reason. We propose the following possibilities: (i) During the cycling process, the active materials are rearranged gradually to provide more accessible active sites for the insertion of lithium ions, this long-term activation process allows the full utilization of active materials [27,45]. (ii) The reversible growth of the polymeric or gel-like film on the electrode surface by the kinetically activated electrolyte degradation is likely considered as possible reasons for the capacity increase, because the SEI layer could supply excess lithium ion storage sites by a so-called “pseudocapacitance-type behavior”, especially in the low potential region [58]. (iii) The generation of Fe nanoparticles accompanied with irreversible reactions resulting in capacity decrease in the initial
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Fig. 4. (a) Cyclic voltammograms of CNT@Fe3O4-C hybrid nanocables. (b) Cycling performance of CNT@Fe3O4-C, CNT@Fe3O4, and Fe3O4 electrodes at a current density of 500 mA g1. (c) Rate performance of CNT@Fe3O4-C and CNT@Fe3O4 electrodes at various current densities. (d) Nyquist plots (Z0 vs. –Z00 ) of CNT@Fe3O4-C, CNT@Fe3O4, and Fe3O4 electrodes.
electrochemical cycles [59]. The metallic nanoparticles will increase the conductivity of the whole electrodes, which leads to the capacity increase in the following cycles [60]. To further investigate the electrochemical performance of the CNT@Fe3O4-C hybrid nanocables electrode, the rate performances of different samples were evaluated for 10 cycles at various current densities, as shown in Fig. 4(c). As expected, the CNT@Fe3O4-C electrode exhibited a better rate performance compared with CNT@Fe3O4 electrode. Even at a high current density of 2000 mA g1, the CNT@Fe3O4-C electrode delivers a reversible capacity of 405 mAh g1, which is higher than the theoretical capacity of commercial graphite (372 mAh g1). In contrast, only 303 mAh g1 of reversible capacity was delivered by the CNT@Fe3O4 electrode at 2000 mA g1. Moreover, when the current density was reduced to 200 mA g1, the specific capacity of the CNT@Fe3O4-C electrode recovered to 900 mAh g1, indicating the structural stability of the electrode even after the high rate cycling. To verify the positive role of CNTs networks and outside carbon layers in the hybrid composites, EIS measurements were performed in the frequency range between 100 kHz and 10 mHz; the resulting Nyquist plots (Z0 vs. –Z00 ) for the CNT@Fe3O4-C, CNT@Fe3O4, and Fe3O4 electrodes are compared in Fig. 4(d). The Nyquist plot consists of a depressed semicircle in the high to medium frequency region and a sloping line in the low-frequency region. The semicircle at the high frequency region is attributed to lithium ions transport resistance through the SEI film; while that at the mid-frequency is related to the charge transfer impedance at the electrodeelectrolyte interface (Rct) which is considered as a large proportion of overall cell’s kinetic impedance [61,62]. The linear region known as Warburg impedance corresponds to diffusion of lithium ions in the lattice of Fe3O4 crystals [63–67]. Analysis of the Nyquist’s plots
in Fig. 4(d) reveals that the Rct of the CNT@Fe3O4-C, CNT@Fe3O4, and Fe3O4 electrodes are 180, 245, and 370 V, respectively. The reduced charge transfer resistance of the CNT@Fe3O4-C electrode can be attributed to the conductive CNTs networks and carbon coating layers. Moreover, the largest slop in the low frequency region for the CNT@Fe3O4-C electrode suggests an improvement in the conversion reaction and diffusivity of lithium ions in the electrode materials, which further demonstrates the significance of the designed hybrid coaxial nanostructures. It should be noted that the cycling stability of the CNT@Fe3O4-C hybrid nanocables electrode presented in this work is better than that of previously reported the hierarchical structure of the carbon coated Fe3O4/CNTs composites [45–48]. The enhanced electrochemical performance originates from their multiple structural features. First, the electronic conductivity of the hybrid composites is greatly enhanced by the hierarchically inner conductive CNTs backbone networks and outside carbon coating shells. Second, the small-sized Fe3O4 nanoparticles (10 nm) allow fast diffusion of lithium ions and transport electrons while their large surface area provide efficient interface for the electrochemical reaction. Also, the carbon coating of the complete hierarchical structure avoids direct contact between the active materials and electrolyte thus minimizing the possibility of sub-reactions of the electrode with the electrolyte that otherwise might result given the high external surface area of the small-sized Fe3O4 nanoparticles. Finally, the inner CNTs backbones and outer carbon coatings improve the structural integrity and robustness of the electrode by suppressing volume variation and aggregation during cycling. These results clearly demonstrate the potential of the CNT@Fe3O4-C hybrid nanocables as high performance anode materials for LIBs.
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4. CONCLUSION In this work, the hierarchical structure of Fe3O4 nanoparticles decorated onto conductive CNTs substrate with carbon coating layer was prepared successfully and tested as anode materials for LIBs. In this architecture, the CNTs backbone and carbon coating layer not only provide electrical network to facilitate electron and lithium ion transport, but also improve the mechanical strength against deformation to maintain the structural integrity of the electrode upon electrochemical cycling. Furthermore, the carbon coating on Fe3O4 nanoparticles prevents direct contact between the electrode and electrolyte for the benefit of forming a stable SEI layer. As a result, the hierarchical CNT@Fe3O4-C electrode exhibits improved electrochemical performance compared with the CNT@Fe3O4 and Fe3O4 anode materials due to the synergistic effects between the conductive inner CNT/outer carbon coating and uniform distributed Fe3O4 nanoparticles. Specifically, the CNT@Fe3O4-C electrode delivered excellent long-term cycling performance and high specific capacity of 1080 mAh g1 after 700 cycles at a current density of 500 mA g1. The simple fabrication route described can be extended to fabricate other hybrid composites with nanocable architectures for high-performance battery applications. ACKNOWLEDGEMENT The authors acknowledge support by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. CityU 122812). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2015.07.144. References [1] J.M. Tarascon, M. Armand, Nature 414 (2011) 359. [2] P.G. Bruce, B. Scrosati, J.-M. Tarascon, Angewandte Chemie International Edition 47 (2008) 2930. [3] B. Dunn, H. Kamath, J.-M. Tarascon, Science 334 (2011) 928. [4] N.A. Kaskhedikar, J. Maier, Advanced Materials 21 (2009) 2664. [5] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Advanced Materials 22 (2010) E28. [6] H.L. Wang, L.F. Cui, Y.A. Yang, H.S. Casalongue, J.T. Robinson, Y.Y. Liang, Y. Cui, H. J. Dai, Journal of the Americal Chemical Society 132 (2010) 13978. [7] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496. [8] X.W. Lou, Y. Wang, C.L. Yuan, J.Y. Lee, L.A. Archer, Advanced Materials 18 (2006) 2325. [9] Y.G. Li, B. Tan, Y.Y. Wu, Nano Letters 8 (2008) 265. [10] O. Waser, M. Hess, A. Güntner, P. Novák, S.E. Pratsinis, Journal of Power Sources 241 (2013) 415. [11] P.L. Taberna, S. Mitra, P. Poizot, P. Simon, J.-M. Tarascon, Nature Materials 5 (2006) 567. [12] S. Mitra, P. Poizot, A. Finke, J.-M. Tarascon, Advanced Functional Materials 16 (2006) 2281. [13] S.Q. Wang, J.Y. Zhang, C.H. Chen, Journal of Power Sources 195 (2010) 5379. [14] Y.C. Dong, R.G. Ma, M.J. Hu, H. Cheng, Q.D. Yang, Y.Y. Li, J.A. Zapien, Physical Chemistry Chemical Physics 15 (2013) 7174. [15] W.J. Zhang, Journal of Power Sources 196 (2011) 13. [16] L.W. Ji, Z.K. Tan, T.R. Kuykendall, S. Aloni, S.D. Xun, E. Lin, V. Battaglia, Y.G. Zhang, Physical Chemistry Chemical Physics 13 (2011) 7170. [17] Y. Chen, H. Xia, L. Lu, J.M. Xue, Journal of Material Chemistry 22 (2012) 5006. [18] X.W. Lou, C.M. Li, L.A. Archer, Advanced Materials 21 (2009) 2536. [19] T. Kavitha, H. Yuvaraj, Journal of Material Chemistry 21 (2011) 15686. [20] Y. Han, Y.J. Wang, L. Li, Y.P. Wang, L.F. Jiao, H.T. Yuan, S.X. Liu, Electrochimica Acta 56 (2011) 3175.
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