Nanoscale View Article Online
Published on 27 January 2014. Downloaded by National Dong Hwa University Library on 30/03/2014 11:43:26.
PAPER
Cite this: Nanoscale, 2014, 6, 3889
View Journal | View Issue
Preparation of fluorine-doped, carbonencapsulated hollow Fe3O4 spheres as an efficient anode material for Li-ion batteries† Hongbo Geng,ab Qun Zhou,b Yue Pan,a Hongwei Gu*a and Junwei Zheng*b Herein we report the design and synthesis of fluorine-doped, carbon-encapsulated hollow Fe3O4 spheres (h-Fe3O4@C/F) through mild heating of polyvinylidene fluoride (PVDF)-coated hollow Fe3O4 spheres. The spheres exhibit enhanced cyclic and rate performances. The as-prepared h-Fe3O4@C/F shows significantly improved electrochemical performance, with high reversible capacities of over 930 mA h g1 at a rate of 0.1 C after 70 cycles, 800 mA h g1 at a rate of 0.5 C after 120 cycles and 620 mA h g1 at a rate of 1 C
Received 3rd December 2013 Accepted 23rd January 2014
after 200 cycles. This improved lithium storage performance is mainly ascribed to the encapsulation of
DOI: 10.1039/c3nr06409c
the solid electrolyte interface film but also prevents aggregation and drastic volume change of the Fe3O4
www.rsc.org/nanoscale
particles. These spheres thus represent a promising anode material in lithium-ion battery applications.
the spheres with fluorine-doped carbon, which not only improves the reaction kinetics and stability of
Introduction Because of its low cost and minimal environmental impact,1,2 magnetite (Fe3O4) has been widely used in MRI contrast agents,3 water treatment,4 gas sensors5 and catalysts.6 Recently, the promising application of Fe3O4 as an anode material for lithium ion batteries (LIBs) has also attracted considerable attention.7–11 In traditional LIBs, graphite has been used as the anode material. However, graphite has a relatively low capacity of 372 mA h g1, which may reduce the applicability of LIBs in many elds. Compared with commercially available graphite, novel materials have been reported as anode materials with a view to replace commercial graphite, such as titanate, TiO2, Si, graphene and transition metal oxides. Among them, Fe3O4 has a relatively higher operating potential, which could avoid the safety problem of lithium deposition. Moreover, Fe3O4 has a relatively high theoretical capacity of 928 mA h g1 and thus may replace graphite as the LIB anode material.12–15 Unfortunately, Fe3O4 particles suffer from several disadvantages, including low electronic conductivity and a tendency to undergo severe aggregation and large volume variations during the insertion and extraction of lithium ions, all of which may lead to poor cycling performance and rate capability.16–19
a
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, China. E-mail: [email protected]
b
Institute of Chemical Power Sources, Soochow University, Suzhou, 215123, China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Additional TGA, SEM, TEM, HRTEM, EDX spectra and elemental mapping, XRD and electrochemical data. See DOI: 10.1039/c3nr06409c
This journal is © The Royal Society of Chemistry 2014
To date, signicant effort has been made to developing innovations directed at addressing the above shortcomings of Fe3O4. As a result, several novel Fe3O4-based nanostructures have been reported, such as porous or hollow nanospheres,20–23 microspheres,24 particles,25 owers,26 capsules,27 nanocrystals,28 nanospindles,29 nanowires,30 nanorods,31 clusters32 and nanotubes.33 These materials have been produced in an effort to enhance diffusion kinetics by dramatically reducing the diffusion distances associated with lithium-ion transport as well as to prevent the aggregation and drastic volume changes of Fe3O4 nanoparticles. Conductive carbon layer coatings have also been found to represent an efficient surface modication technique34–38 and provide both enhanced electronic conductivity and a buffer layer to restrict volume expansion. There are difficulties associated with these approaches, however, since merely varying the surface area or the structure of magnetite does not solve the serious problem of irreversible capacity loss resulting in poor rate and cycling abilities. Recently, Ma et al.39 have presented a strategy for the synthesis of N-doped, carbonencapsulated Fe3O4, which exhibits excellent capacity retention. Lei et al.40 also have reported that N-doped, carbon-coated Fe3O4 shows improved electronic conductivity and ion permeability in the carbon layer and thus exhibits signicantly superior electrochemical properties. In addition to the use of a carbon coating incorporating nitrogen species, it has been demonstrated in the literature that the application of uorinated graphite results in superior cyclic performance,41,42 which was attributed to the improvement of reaction kinetics and the stability of the solid electrolyte interface (SEI) lms. Despite this, there are very few reports in the literature of carbon coatings that contain uorine.
Nanoscale, 2014, 6, 3889–3894 | 3889
View Article Online
Published on 27 January 2014. Downloaded by National Dong Hwa University Library on 30/03/2014 11:43:26.
Nanoscale
Scheme 1
Paper
The synthetic protocol of h-Fe3O4@C/F spheres.
Herein, we reported a facile method for the preparation of uorine-doped, carbon-encapsulated hollow Fe3O4 (h-Fe3O4@C/F) spheres that show signicantly enhanced cyclic and rate performances. The benecial properties of this material include a high specic reversible capacity (1411 mA h g1), good cycle stability (over 930 mA h g1 at a current rate of 0.1 C aer 70 cycles (1 C ¼ 928 mA h g1)) and good rate performance (around 620 mA h g1 at 1 C), thus surpassing the performance of previously reported Fe3O4 materials based on delicate structures and conductive carbon layer encapsulation. As illustrated in Scheme 1, the synthesis of the h-Fe3O4@C/F spheres was relatively straightforward and was accomplished via simple heat treatment with polyvinylidene diuoride (PVDF). In the initial step, a uniform coating of PVDF was obtained over the surface of hollow Fe3O4 spheres. Subsequently, the hollow Fe3O4@PVDF composites were annealed at 500 C for 4 h to convert the core–shell spheres into h-Fe3O4@C/ F. For comparison purposes, hollow Fe3O4@C (h-Fe3O4@C) spheres were also prepared through a facile hydrothermal method using glucose as the carbon source.
Results and discussion Fig. 1 presents scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of h-Fe3O4 (Fig. 1A and C) and h-Fe3O4@C/F (Fig. 1B and D). The average size of the h-Fe3O4 spheres was about 200 nm and they possessed a rather rough surface (Fig. 1A). The formation of h-Fe3O4 spheres was further conrmed by the TEM images in Fig. 1C. As can be seen from the main TEM image, the uniform h-Fe3O4 spheres had a hollow interior, while the inset indicates that these hollow spheres consisted of nanocrystals approximately 10 nm in diameter. Beneting from the optimization of the reaction conditions, the spherical morphology was well preserved, and the resulting spheres exhibited rough surfaces and hollow interiors (Fig. 1B and D). Furthermore, a uniform and continuous conductive carbon layer can be clearly identied from the high-resolution TEM images (Fig. 1D, inset). The product was found to have a lattice fringe with a spacing (d) of 0.25 nm, which is highly consistent with the expected spacing of the (311) planes of Fe3O4.25 The morphologies of the h-Fe3O4@C samples were similar to those of the as-prepared h-Fe3O4@C/F spheres (Fig. S1†). Energy-dispersive X-ray spectroscopy (EDX) elemental mapping (Fig. S2†) of C, F and Fe in the h-Fe3O4@C/F composite showed concordance between C, F and Fe signals, indicating
3890 | Nanoscale, 2014, 6, 3889–3894
Fig. 1 SEM images of h-Fe3O4 (A) and h-Fe3O4@C/F (B). TEM images of h-Fe3O4 (C) (inset: magnified image of h-Fe3O4) and h-Fe3O4@C/F (D) (inset: HRTEM image of h-Fe3O4@C/F).
the uniform distribution of C and F over the spheres. To further investigate the state of F atoms in h-Fe3O4@C/F, X-ray photoelectron spectroscopy (XPS) was performed. The XPS survey spectrum (Fig. 2A) of the h-Fe3O4@C/F composite contains four dominant peaks at 284.7, 530.5, 687.3 and 712.6 eV, corresponding to C 1s, O 1s, F 1s and Fe 2p respectively. As shown in Fig. 2B, two main peaks centered at 684.7 and 687.3 eV appear in the high-resolution F 1s XPS spectrum of the h-Fe3O4@C/F composite. The 684.7 eV peak is attributed to F atoms semiionically bonded to C atoms while the 687.3 eV peak is assigned to F–C bonds.43 These results conrm that the carbon layer has
Fig. 2 XPS survey spectrum (A) and the high-resolution F 1s spectrum (B) of h-Fe3O4@C/F. Raman spectrum (C) and TGA plot (D) of h-Fe3O4@C/F.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 27 January 2014. Downloaded by National Dong Hwa University Library on 30/03/2014 11:43:26.
Paper
been doped with F atoms. Raman spectral analysis over the range of 800–2000 cm1 (Fig. 2C) was also carried out to demonstrate the presence of a conductive surface carbon layer. Two fundamental peaks are observed at 1590 and 1350 cm1, agreeing with the expected G and D bands of carbon, respectively. The D band at around 1350 cm1 was attributed to the presence of defects and disorders in the lattice, while the G band at 1590 cm1 was related to the E2g phonons of sp2bonded carbon atoms, indicating the presence of a uniform and continuous carbon layer with an amorphous structure.44 To quantify the carbon content in the h-Fe3O4@C/F spheres, thermogravimetric analysis (TGA) (Fig. 2D) was carried out under air at a ramp rate of 5 C min1. The slight weight loss between room temperature and 150 C is about 0.7%, corresponding to the evaporation of absorbed moisture in the samples, whereas the weight gain of about 0.5% for h-Fe3O4@C/ F between 150 and 350 C is likely the result of oxidation of the Fe3O4 in air.45 A major weight loss of 13.8% was clearly observed between 350 and 800 C, induced by oxidizing the carbon into CO2. These results are in accordance with the results of EDX spectroscopy (Fig. S3†). According to the EDX spectrum and XPS spectroscopy, the F atom ratio in the material was 5.25%. TGA analysis under air at a ramp rate of 5 C min1 (Fig. S4†) also determined that the carbon content of the h-Fe3O4@C composite was about 16.5%. The crystal structures of h-Fe3O4 and h-Fe3O4@C/F were subsequently conrmed by X-ray diffraction (XRD) (Fig. 3). The characteristic peaks at 30.36 (220), 35.77 (311), 43.35 (400), 53.76 (422), 57.16 (511) and 62.98 (440) agreed with the standard XRD data for pure Fe3O4 (JCPDS card no. 19-0629, S.G.: ˚ 46 There were no obvious diffraction peaks Fd3m, ao ¼ 8.396 A). corresponding to carbon or other impurity peaks in the XRD pattern (XRD analysis of h-Fe3O4@C in Fig. S5†), which indicates that the carbon coating and F doping did not result in any structural change in h-Fe3O4. The absence of any graphite peak was attributed to the amorphous state of carbon.44 The surface areas of (A) h-Fe3O4, (B) h-Fe3O4@C/F and (C) h-Fe3O4@C
Fig. 3 X-ray diffraction (XRD) patterns of h-Fe3O4 (A) and hFe3O4@C/F (B).
This journal is © The Royal Society of Chemistry 2014
Nanoscale
(Fig. S6†) were measured using the Brunauer–Emmett–Teller (BET) method. The BET specic surface area of h-Fe3O4 was 66 m2 g1, higher than 51 m2 g1 for h-Fe3O4@C and 54 m2 g1 for h-Fe3O4@C/F, which was attributed to the embedment of the carbon and F-doped carbon respectively. The detailed distributions of the sphere diameters of h-Fe3O4 and h-Fe3O4@C/F are shown in Fig. S7A and B† respectively. They have a mean diameter of about 200 nm, corresponding to the SEM images (Fig. 1A and B). The charge–discharge voltage proles for the 1st, 10th, 20th, th 30 , 40th and 50th cycles of h-Fe3O4, h-Fe3O4@C and h-Fe3O4@C/F over the voltage range from 0.05 to 3.00 V at a rate of 0.1 C are shown in Fig. 4A and B and S8,† respectively. In the rst cycle, there were two obvious potential plateaus at about 1.6 and 0.8 V, which are similar to those described in the literature,26,27 suggesting that differing structures and carbon modication do not change the chemical nature of Fe3O4. The potential plateaus at about 0.8 V in the rst discharge and 1.6 V in the rst charge are ascribed to the Fe(III), Fe(II) and Fe redox couples during lithium ion insertion and extraction of the hollow spheres. The sloping curve around 0.05–0.8 V is attributed to the decomposition of the electrolyte to form a gel-like solid electrolyte interface (SEI) lm.25,28,33 The rst discharge curves for the samples show very high discharge specic capacities of 1647, 1714 and 1411 mA h g1 for h-Fe3O4, h-Fe3O4@C and h-Fe3O4@C, with corresponding charge capacities of 1306, 1237 and 1010 mA h g1, respectively. In subsequent cycles, h-Fe3O4@C/F shows enhanced capacity retention, indicating the contribution of the F-doped carbon layer to stabilization of the electrochemical performance of the anode materials.41,42 Fig. 4C shows the cycling performances of the as-prepared spheres at a current rate of 0.1 C aer 50 cycles as anode materials in a LIB. The discharge capacity of h-Fe3O4 decreased dramatically, from 1647 to 194.6 mA h g1. In sharp contrast, the h-Fe3O4@C spheres exhibited a markedly improved cycling
Fig. 4 The charge–discharge voltage profiles over the initial 50 cycles for h-Fe3O4 (A) and h-Fe3O4@C/F (B) at a current rate of 0.1 C. Cycling performances at a current of 0.1 C (C) and rate capability of h-Fe3O4@C/F (D).
Nanoscale, 2014, 6, 3889–3894 | 3891
View Article Online
Published on 27 January 2014. Downloaded by National Dong Hwa University Library on 30/03/2014 11:43:26.
Nanoscale
performance with a reversible capacity of 639.9 mA h g1. More importantly, h-Fe3O4@C/F also retained a high reversible capacity of about 938 mA h g1, much higher than h-Fe3O4, h-Fe3O4@C and the literature reported.21,23 The rate capability of h-Fe3O4@C/F from 0.1 C to 1 C was further investigated, as shown in Fig. 4D, and the results showed stable cyclic performance in discrete steps. With increasing rates from 0.1 C to 1 C, h-Fe3O4@C/F delivered discharge capacities over 930, 880, 800, 740, 700 and 620 mA h g1 at current rates of 0.1, 0.2, 0.4, 0.6, 0.8 and 1 C, respectively. In addition, when the current rate was reduced back to 0.1 C, the capacity eventually recovered to around 930 mA h g1, which conrms the good capacity reversibility of the h-Fe3O4@C/F electrode. To further probe the electrochemical performance of the h-Fe3O4@C/F electrode, we increased the cycle number to 230 cycles (Fig. 5). As expected, h-Fe3O4@C/F manifested satisfactory rate capability. It delivered a reversible capacity of over 930 mA h g1 at a rate of 0.1 C aer 70 cycles, 800 mA h g1 at a rate of 0.5 C aer 120 cycles, 620 mA h g1 at a rate of 1 C aer 200 cycles and nally recovered to around 1000 mA h g1 at a rate of 0.1 C aer 230 cycles, which represents a stable coulombic efficiency of almost 100% aer the rst few cycles. Based on the synthetic process, the stable cyclic performance, high coulombic efficiency and stable rate capability are primarily attributed to the distribution of F and C atoms and to the unique structure of the material, which not only improve the reaction kinetics and stability of the SEI lms, but also prevent any aggregation and drastic volume changes of the Fe3O4 particles.38,39,41,42 In contrast, the h-Fe3O4@C electrode with the glucose-derived carbon layer would lead to lackluster electron conductivity and ion permeability.38,39 Compared with the pure Fe3O4, h-Fe3O4@C showed higher capacity retention, which may be attributed to the protective effect of the carbon layer.35,37 Furthermore, the electrochemical impedance spectra were applied to conrm the contributions of the introduction of the F element. As shown in Fig. S9A,† the Nyquist plots showed a depressed semicircle at the high-frequency region and an inclined straight line at low frequency, corresponding to the charge transfer resistance and Li+ diffusion in the electrode
Paper
material respectively. Due to the presence of carbon and the F-doped carbon, the size of the semicircle was much smaller for h-Fe3O4@C and h-Fe3O4@C/F, indicating that carbon and the F-doped carbon improved the electronic conductivity of the electrode. Compared with h-Fe3O4@C, h-Fe3O4@C/F exhibited a smaller semicircle from the equivalent circuit (inset of Fig. S9B†) and a larger slope (Fig. S9B†). The results conrmed that the F-doped carbon improved the electronic conductivity and Li+ diffusion in the electrode material.
Conclusions In conclusion, we have demonstrated the synthesis of h-Fe3O4@C/F hollow spheres by a facile and convenient route. Beneting from the incorporation of F and C atoms, the sample showed enhanced cyclic performance, high coulombic efficiency and stable rate capability. As an example, the test specimen exhibited a high reversible capacity of over 930 mA h g1 at a rate of 0.1 C aer 70 cycles, 800 mA h g1 at a rate of 0.5 C aer 120 cycles and 620 mA h g1 at a rate of 1 C aer 200 cycles. This dramatic improvement can undoubtedly be attributed to the improved properties of the original magnetite resulting from F-doped carbon encapsulation, which indicates that this unique composite will be of signicant interest with regard to its applications in high-performance Li-ion batteries. This same F-doped carbon encapsulation modication could potentially be applied to other metal oxide electrode materials that suffer from poor conductivity and severe volume expansion.
Experimental Preparation of h-Fe3O4 spheres In a typical procedure,47 1.08 g FeCl3$6H2O, 2.35 g sodium citrate (C6H5O7Na3$2H2O) and 0.72 g urea were dissolved in 80 mL distilled water, aer which polyacrylamide (PAM, 0.6 g) was added with magnetic stirring for 1 h. The resulting mixture was transferred into a 100 mL Teon-lined stainless steel autoclave and held at 200 C for 12 h. The product was washed with ethanol and distilled water and then harvested using a magnetic eld. Preparation of h-Fe3O4@C spheres The as-prepared h-Fe3O4 (0.36 g) was dispersed in 40 mL 0.5 M aqueous glucose solution. The suspension was transferred to a 100 mL Teon-lined stainless steel autoclave and heated at 180 C for 6 h, aer being washed with distilled water and ethanol and dried under vacuum at 80 C overnight. In order to enhance the electrical conductivity of the carbon layer, the resulting material was sintered at 500 C for 4 h under a N2 atmosphere. Preparation of h-Fe3O4@C/F spheres
The long-term rate performance and coulombic efficiency of h-Fe3O4@C/F. Fig. 5
3892 | Nanoscale, 2014, 6, 3889–3894
The obtained h-Fe3O4 spheres (0.5 g) were mixed with 0.1 g PVDF by grinding. The mixture was sintered in a N2 ow for 4 h at 500 C, aer which the h-Fe3O4@C/F spheres were collected.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Paper
Published on 27 January 2014. Downloaded by National Dong Hwa University Library on 30/03/2014 11:43:26.
Material characterization The as-prepared nanostructures were characterized by XRD using an X'Pert-Pro MPD diffractometer (PANalytical, Nether˚ The sphere lands) with a Cu Ka X-ray source (l ¼ 1.540598 A). morphologies were characterized by SEM, TEM and high-resolution TEM (HRTEM). SEM was performed on a Hitachi S-4700 cold eld emission scanning electron microscope operated at 30 kV, TEM (Tecnai G220, FEI, USA) was performed using a Gatan CCD794 camera operated at 200 kV and HRTEM was carried out on a Tecnai G2 F20 S-TWIN microscope with an accelerating voltage of 200 kV. XPS data were obtained using a KRATOS Axis ultra-DLD X-ray photoelectron spectrometer with monochromatic Mg Ka X-ray (1283.3 eV). TGA was performed using a Perkin Elmer TGA 4000 thermogravimetric analyzer and Raman spectra were acquired at room temperature with a confocal Raman microscope (HR-800, Jobin Yvon, France). Electrochemical characterization The electrochemical measurements of obtained products were performed using a coin-type half cell (CR 2016). The working electrode was prepared by mixing 80 wt% of the active material (h-Fe3O4, h-Fe3O4@C or h-Fe3O4@C/F), 10 wt% conductive material (acetylene black, AB) and 10 wt% polymer binder (polyvinylidene diuoride, PVDF) in an N-methyl-2-pyrrolidone (NMP) solvent to form a homogeneous slurry, followed by spreading onto a copper foil. Finally, the copper foil was dried overnight under vacuum at 120 C. The electrolyte was 1 M LiPF6 in a 1 : 1 (w/w) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (Guotaihuarong, Zhangjiagang, China). Cell assembly was performed in an argon-lled glovebox in which moisture and oxygen were both below 0.1 ppm. The cells were charged and discharged between 3.0 and 0.05 V at room temperature.
Acknowledgements H.W.G. acknowledges the nancial support from the National Natural Science Foundation of China (no. 21003092 and 21373006), the Key Project of Chinese Ministry of Education (no. 211064), and the Priority Academic Program Development of Jiangsu Higher Education Institutions. Y.P. acknowledges nancial support from the New Faculty Start Foundation of Soochow University (Q410900413); J.W.Z. acknowledges the nancial support from the Nature Science Foundation of China (no. 20873089 and 20975073), Nature Science Foundation of Jiangsu Province (no. BK2011272), Industry-Academia Cooperation Innovation Fund Projects of Jiangsu Province (no. BY2011130) and Key Laboratory of Lithium Ion Battery Materials of Jiangsu Province, the Project of Scientic and Technologic Infrastructure of Suzhou (SZS201207).
Notes and references 1 P. L. Taberna, S. Mitra, P. Poizot, P. Simon and J.-M. Tarascon, Nat. Mater., 2006, 5, 567. This journal is © The Royal Society of Chemistry 2014
Nanoscale
2 C. T. Yavuz, J. T. Mayo, W. W. Yu and V. L. Colvin, Science, 2006, 314, 964. 3 H. Ho, X. L. Sun and S. H. Sun, Acc. Chem. Res., 2011, 44, 875; Y. Pan, X. W. Du, F. Zhao and B. Xu, Chem. Soc. Rev., 2012, 41, 2912; J. Zhu, Y. Lu, Y. Li, J. Jiang, L. Cheng, Z. Liu, L. Guo, Y. Pan and H. Gu, Nanoscale, 2014, 6, 199; Y. Pan, M. J. C. Long, X. Li, J. Shi, L. Hedstrom and B. Xu, Chem. Sci., 2011, 2, 945–948; J. Wu and C. -C. Chu, J. Mater. Chem. B, 2013, 1, 353; Y. Pan, M. J. C. Long, H. -C. Lin, L. Hedstrom and B. Xu, Chem. Sci., 2012, 3, 3495–3499; Y. Pan, Y. Gao, J. Shi, L. Wang and B. Xu, J, Mater. Chem., 2011, 21, 6804. 4 L. S. Zhong, J. S. Hu, H. P. Liang, A. M. Cao, W. G. Song and L. Y. Wan, Adv. Mater., 2006, 18, 2426. 5 X. L. Hu, J. M. Gong, Q. Li and G. S. Li, Adv. Mater., 2007, 19, 2324. 6 Y. M. Lee, M. A. Garcia, N. A. F. Huls and S. H. Sun, Angew. Chem., Int. Ed., 2010, 122, 1293. 7 A. S. Aric` o, P. Bruce, B. Scrosati, J. M. Tarascon and W. V. Schalkwijk, Nat. Mater., 2005, 4, 366; B. Koo, H. Xiong, M. D. Slater, V. B. Prakapenka, M. Balasubramanian, P. Podsiadlo, C. S. Johnson, T. Rajh and E. V. Shevchenko, Nano Lett., 2012, 12, 2429. 8 C. Xu, Y. Zeng, X. H. Rui, N. Xiao, J. X. Zhu, W. Y. Zhang, J. Chen, W. L. Liu, H. T. Tan, H. H. Hng and Q. Y. Yan, ACS Nano, 2012, 6, 4713; G. Gao, Q. Zhang, K. Wang, H. Son, P. Y. Qiu and D. X. Cui, Nano Energy, 2013, 2, 1010. 9 Z. W. Seh, W. Y. Li, J. J. Cha, G. Y. Zheng, M. T. McDowell, P. C. Hsu and Y. Cui, Nature, 2013, 4, 1331. 10 F. Y. Cheng, Z. L. Tao, J. Liang and J. Chen, Chem. Mater., 2008, 20, 667. 11 L. P. Tan, Z. Y. Lu, H. T. Tan, J. X. Zhu, X. H. Rui, Q. Y. Yan and H. H. Hng, J. Power Sources, 2012, 206, 253. 12 P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J.-M. Tarascon, Nature, 2000, 407, 496; S. Y. Yin, Y. Y. Zhang, J. H. Kong, C. J. Zou, C. M. Li, X. H. Lu, J. Ma, F. Y. C. Boey and X. D. Chen, ACS Nano, 2011, 5, 3831. 13 J. X. Zhu, Z. Y. Yin, D. Yang, T. Sun, H. Yu, H. E. Hoster, H. Zhang and Q. Y. Yan, Energy Environ. Sci., 2013, 6, 987; J. X. Zhu, T. Zhu, X. Z. Zhou, Y. Y. Zhang, X. W. Lou, X. D. Chen, H. Zhang, H. H. Hng and Q. Y. Yan, Nanoscale, 2011, 3, 1084. 14 P. G. Bruce, B. Scrosati and J. M. Tarascon, Angew. Chem., Int. Ed., 2008, 47, 2930; Y. Y. Zhang, Y. X. Tang, S. Y. Yin, Z. Y. Zeng, H. Zhang, C. M. Li, Z. L. Dong, Z. Chen and X. D. Chen, Nanoscale, 2011, 3, 4074. 15 J. S. Chen, C. M. Li, W. W. Zhou, Q. Y. Yan, L. A. Archer and X. W. Lou, Nanoscale, 2009, 1, 280; J. H. Kong, W. A. Yee, Y. F. Wei, L. P. Yang, J. M. Ang, S. L. Phua, S. Y. Wong, R. Zhou, Y. L. Dong, X. Li and X. H. Lu, Nanoscale, 2013, 5, 2967. 16 J. S. Chen, Y. M. Zhang and X. W. Lou, ACS Appl. Mater. Interfaces, 2011, 3, 3276–3279; H. B. Geng, Q. Zhou, J. W. Zheng and H. W. Gu, RSC Adv., 2014, 4, 6430. 17 G. M. Zhou, D. W. Wang, F. Li, L. L. Zhang, N. Li, Z. S. Wu, L. Wen, G. Q. Lu and H. M. Cheng, Chem. Mater., 2010, 22, 5306.
Nanoscale, 2014, 6, 3889–3894 | 3893
View Article Online
Published on 27 January 2014. Downloaded by National Dong Hwa University Library on 30/03/2014 11:43:26.
Nanoscale
18 Y. Z. Su, S. Li, D. Q. Wu, F. Zhang, H. W. Liang, P. F. Gao, C. Cheng and X. L. Feng, ACS Nano, 2012, 6, 8349. 19 C. K. Chan, H. L. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins and Y. Cui, Nat. Nanotechnol., 2008, 3, 31. 20 Q. Q. Xiong, J. P. Tu, Y. Lu, J. Chen, Y. X. Yu, Y. Q. Qiao, X. L. Wang and C. D. Gu, J. Phys. Chem. C, 2012, 116, 6495. 21 Y. Chen, H. Xia, L. Lu and J. M. Xue, J. Mater. Chem., 2012, 22, 5006; Y. Pan, J. H. Gao, B. Zhang, X. X. Zhang and B. Xu, Langmuir, 2010, 26, 4184. 22 H. S. Lim, B. Y. Junga, Y. K. Sunb and K. D. Suha, Electrochim. Acta, 2012, 75, 123. 23 H. J. Kim, K. I. Choi, A. Pan, I. D. Kim, H. R. Kim, K. M. Kim, C. W. Na, G. Z. Cao and J. H. Lee, J. Mater. Chem., 2011, 21, 6549. 24 J. S. Xu and Y. J. Zhu, ACS Appl. Mater. Interfaces, 2012, 4, 4752. 25 C. N. He, S. Wu, N. Q. Zhao, C. S. Shi, E. Z. Liu and J. J. Li, ACS Nano, 2013, 7, 4459. 26 S. L. Jin, H. G. Deng, D. H. Long, X. J. Liu, L. Zhan, X. Y. Liang, W. M. Qiao and L. C. Ling, J. Power Sources, 2011, 196, 3887. 27 W. Cheng, K. B. Tang, Y. X. Qi, J. Sheng and Z. P. Liu, J. Mater. Chem., 2010, 20, 1799. 28 X. L. Jia, Z. Chen, X. Cui, Y. T. Peng, X. L. Wang, G. Wang, F. Wei and Y. F. Lu, ACS Nano, 2012, 6, 9911. 29 W. M. Zhang, X. L. Wu, J. S. Hu, Y. G. Guo and L. J. Wan, Adv. Funct. Mater., 2008, 18, 3941. 30 T. Muraliganth, A. V. Murugan and A. Manthiram, Chem. Commun., 2009, 7360. 31 P. Tartaj and J. M. Amarilla, J. Power Sources, 2011, 196, 2164. 32 S. H. Lee, S. H. Yu, J. E. Lee, Y. E. Sung and T. Hyeon, Nano Lett., 2013, 13, 4249.
3894 | Nanoscale, 2014, 6, 3889–3894
Paper
33 Y. Wu, Y. Wei, J. P. Wang, K. L. Jiang and S. S. Fan, Nano Lett., 2013, 13, 818. 34 R. H. Wang, C. H. Xu, J. Sun, L. Gao and C. C. Lin, J. Mater. Chem. A, 2013, 1, 1794. 35 D. Y. Chen, G. Ji, Y. Ma, J. Y. Lee and J. M. Lu, ACS Appl. Mater. Interfaces, 2011, 3, 3078. 36 W. Wei, S. B. Yang, H. X. Zhou, I. Lieberwirth, X. L. Feng and K. M¨ ullen, Adv. Mater., 2013, 25, 2909. 37 L. L. Wang, J. W. Liang, Y. C. Zhu, T. Mei, X. Zhang, Q. Yang and Y. T. Qian, Nanoscale, 2013, 5, 3627. 38 J. E. Lee, S. H. Yu, D. L. Lee, D. C. Lee, Y. E. Sung and T. Hyeon, Energy Environ. Sci., 2012, 5, 9528. 39 Y. Ma, C. Zhang, G. Jia and J. Y. Lee, J. Mater. Chem., 2012, 22, 7845. 40 C. Lei, F. Han, D. Li, W. C. Li, Q. Sun, X. Q. Zhang and A. H. Lu, Nanoscale, 2013, 5, 1168. 41 T. Nakanima, V. Gupta, Y. Ohzawa, M. Koh, R. N. Singh, A. Tressaud and E. Durand, J. Power Sources, 2002, 104, 108. 42 V. Gupta, T. Nakanima, Y. Ohzawa and H. Iwata, J. Fluorine Chem., 2001, 112, 233. 43 A. Tressaud, F. Moguet, G. Nanse, E. Papirer, V. Gupta and O. P. Bahl, J. Phys. Chem. Solids, 1996, 57, 745. 44 M. Zhang, B. H. Qu, D. Lei, Y. J. Chen, X. Z. Yu, L. B. Chen, Q. H. Li, Y. G. Wang and T. H. Wang, J. Mater. Chem., 2012, 22, 3868. 45 T. Zhu, J. S. Chen and X. W. Lou, J. Phys. Chem. C, 2011, 115, 9814. 46 J. S. Chen, Y. M. Zhang and X. W. Lou, ACS Appl. Mater. Interfaces, 2011, 3, 3276. 47 W. Cheng, K. Tang, Y. Qi, J. Sheng and Z. P. Liu, J. Mater. Chem., 2010, 20, 1799.
This journal is © The Royal Society of Chemistry 2014
Published on 27 January 2014. Downloaded by National Dong Hwa University Library on 30/03/2014 11:43:26.
PAPER
Cite this: Nanoscale, 2014, 6, 3889
View Journal | View Issue
Preparation of fluorine-doped, carbonencapsulated hollow Fe3O4 spheres as an efficient anode material for Li-ion batteries† Hongbo Geng,ab Qun Zhou,b Yue Pan,a Hongwei Gu*a and Junwei Zheng*b Herein we report the design and synthesis of fluorine-doped, carbon-encapsulated hollow Fe3O4 spheres (h-Fe3O4@C/F) through mild heating of polyvinylidene fluoride (PVDF)-coated hollow Fe3O4 spheres. The spheres exhibit enhanced cyclic and rate performances. The as-prepared h-Fe3O4@C/F shows significantly improved electrochemical performance, with high reversible capacities of over 930 mA h g1 at a rate of 0.1 C after 70 cycles, 800 mA h g1 at a rate of 0.5 C after 120 cycles and 620 mA h g1 at a rate of 1 C
Received 3rd December 2013 Accepted 23rd January 2014
after 200 cycles. This improved lithium storage performance is mainly ascribed to the encapsulation of
DOI: 10.1039/c3nr06409c
the solid electrolyte interface film but also prevents aggregation and drastic volume change of the Fe3O4
www.rsc.org/nanoscale
particles. These spheres thus represent a promising anode material in lithium-ion battery applications.
the spheres with fluorine-doped carbon, which not only improves the reaction kinetics and stability of
Introduction Because of its low cost and minimal environmental impact,1,2 magnetite (Fe3O4) has been widely used in MRI contrast agents,3 water treatment,4 gas sensors5 and catalysts.6 Recently, the promising application of Fe3O4 as an anode material for lithium ion batteries (LIBs) has also attracted considerable attention.7–11 In traditional LIBs, graphite has been used as the anode material. However, graphite has a relatively low capacity of 372 mA h g1, which may reduce the applicability of LIBs in many elds. Compared with commercially available graphite, novel materials have been reported as anode materials with a view to replace commercial graphite, such as titanate, TiO2, Si, graphene and transition metal oxides. Among them, Fe3O4 has a relatively higher operating potential, which could avoid the safety problem of lithium deposition. Moreover, Fe3O4 has a relatively high theoretical capacity of 928 mA h g1 and thus may replace graphite as the LIB anode material.12–15 Unfortunately, Fe3O4 particles suffer from several disadvantages, including low electronic conductivity and a tendency to undergo severe aggregation and large volume variations during the insertion and extraction of lithium ions, all of which may lead to poor cycling performance and rate capability.16–19
a
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, China. E-mail: [email protected]
b
Institute of Chemical Power Sources, Soochow University, Suzhou, 215123, China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Additional TGA, SEM, TEM, HRTEM, EDX spectra and elemental mapping, XRD and electrochemical data. See DOI: 10.1039/c3nr06409c
This journal is © The Royal Society of Chemistry 2014
To date, signicant effort has been made to developing innovations directed at addressing the above shortcomings of Fe3O4. As a result, several novel Fe3O4-based nanostructures have been reported, such as porous or hollow nanospheres,20–23 microspheres,24 particles,25 owers,26 capsules,27 nanocrystals,28 nanospindles,29 nanowires,30 nanorods,31 clusters32 and nanotubes.33 These materials have been produced in an effort to enhance diffusion kinetics by dramatically reducing the diffusion distances associated with lithium-ion transport as well as to prevent the aggregation and drastic volume changes of Fe3O4 nanoparticles. Conductive carbon layer coatings have also been found to represent an efficient surface modication technique34–38 and provide both enhanced electronic conductivity and a buffer layer to restrict volume expansion. There are difficulties associated with these approaches, however, since merely varying the surface area or the structure of magnetite does not solve the serious problem of irreversible capacity loss resulting in poor rate and cycling abilities. Recently, Ma et al.39 have presented a strategy for the synthesis of N-doped, carbonencapsulated Fe3O4, which exhibits excellent capacity retention. Lei et al.40 also have reported that N-doped, carbon-coated Fe3O4 shows improved electronic conductivity and ion permeability in the carbon layer and thus exhibits signicantly superior electrochemical properties. In addition to the use of a carbon coating incorporating nitrogen species, it has been demonstrated in the literature that the application of uorinated graphite results in superior cyclic performance,41,42 which was attributed to the improvement of reaction kinetics and the stability of the solid electrolyte interface (SEI) lms. Despite this, there are very few reports in the literature of carbon coatings that contain uorine.
Nanoscale, 2014, 6, 3889–3894 | 3889
View Article Online
Published on 27 January 2014. Downloaded by National Dong Hwa University Library on 30/03/2014 11:43:26.
Nanoscale
Scheme 1
Paper
The synthetic protocol of h-Fe3O4@C/F spheres.
Herein, we reported a facile method for the preparation of uorine-doped, carbon-encapsulated hollow Fe3O4 (h-Fe3O4@C/F) spheres that show signicantly enhanced cyclic and rate performances. The benecial properties of this material include a high specic reversible capacity (1411 mA h g1), good cycle stability (over 930 mA h g1 at a current rate of 0.1 C aer 70 cycles (1 C ¼ 928 mA h g1)) and good rate performance (around 620 mA h g1 at 1 C), thus surpassing the performance of previously reported Fe3O4 materials based on delicate structures and conductive carbon layer encapsulation. As illustrated in Scheme 1, the synthesis of the h-Fe3O4@C/F spheres was relatively straightforward and was accomplished via simple heat treatment with polyvinylidene diuoride (PVDF). In the initial step, a uniform coating of PVDF was obtained over the surface of hollow Fe3O4 spheres. Subsequently, the hollow Fe3O4@PVDF composites were annealed at 500 C for 4 h to convert the core–shell spheres into h-Fe3O4@C/ F. For comparison purposes, hollow Fe3O4@C (h-Fe3O4@C) spheres were also prepared through a facile hydrothermal method using glucose as the carbon source.
Results and discussion Fig. 1 presents scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of h-Fe3O4 (Fig. 1A and C) and h-Fe3O4@C/F (Fig. 1B and D). The average size of the h-Fe3O4 spheres was about 200 nm and they possessed a rather rough surface (Fig. 1A). The formation of h-Fe3O4 spheres was further conrmed by the TEM images in Fig. 1C. As can be seen from the main TEM image, the uniform h-Fe3O4 spheres had a hollow interior, while the inset indicates that these hollow spheres consisted of nanocrystals approximately 10 nm in diameter. Beneting from the optimization of the reaction conditions, the spherical morphology was well preserved, and the resulting spheres exhibited rough surfaces and hollow interiors (Fig. 1B and D). Furthermore, a uniform and continuous conductive carbon layer can be clearly identied from the high-resolution TEM images (Fig. 1D, inset). The product was found to have a lattice fringe with a spacing (d) of 0.25 nm, which is highly consistent with the expected spacing of the (311) planes of Fe3O4.25 The morphologies of the h-Fe3O4@C samples were similar to those of the as-prepared h-Fe3O4@C/F spheres (Fig. S1†). Energy-dispersive X-ray spectroscopy (EDX) elemental mapping (Fig. S2†) of C, F and Fe in the h-Fe3O4@C/F composite showed concordance between C, F and Fe signals, indicating
3890 | Nanoscale, 2014, 6, 3889–3894
Fig. 1 SEM images of h-Fe3O4 (A) and h-Fe3O4@C/F (B). TEM images of h-Fe3O4 (C) (inset: magnified image of h-Fe3O4) and h-Fe3O4@C/F (D) (inset: HRTEM image of h-Fe3O4@C/F).
the uniform distribution of C and F over the spheres. To further investigate the state of F atoms in h-Fe3O4@C/F, X-ray photoelectron spectroscopy (XPS) was performed. The XPS survey spectrum (Fig. 2A) of the h-Fe3O4@C/F composite contains four dominant peaks at 284.7, 530.5, 687.3 and 712.6 eV, corresponding to C 1s, O 1s, F 1s and Fe 2p respectively. As shown in Fig. 2B, two main peaks centered at 684.7 and 687.3 eV appear in the high-resolution F 1s XPS spectrum of the h-Fe3O4@C/F composite. The 684.7 eV peak is attributed to F atoms semiionically bonded to C atoms while the 687.3 eV peak is assigned to F–C bonds.43 These results conrm that the carbon layer has
Fig. 2 XPS survey spectrum (A) and the high-resolution F 1s spectrum (B) of h-Fe3O4@C/F. Raman spectrum (C) and TGA plot (D) of h-Fe3O4@C/F.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 27 January 2014. Downloaded by National Dong Hwa University Library on 30/03/2014 11:43:26.
Paper
been doped with F atoms. Raman spectral analysis over the range of 800–2000 cm1 (Fig. 2C) was also carried out to demonstrate the presence of a conductive surface carbon layer. Two fundamental peaks are observed at 1590 and 1350 cm1, agreeing with the expected G and D bands of carbon, respectively. The D band at around 1350 cm1 was attributed to the presence of defects and disorders in the lattice, while the G band at 1590 cm1 was related to the E2g phonons of sp2bonded carbon atoms, indicating the presence of a uniform and continuous carbon layer with an amorphous structure.44 To quantify the carbon content in the h-Fe3O4@C/F spheres, thermogravimetric analysis (TGA) (Fig. 2D) was carried out under air at a ramp rate of 5 C min1. The slight weight loss between room temperature and 150 C is about 0.7%, corresponding to the evaporation of absorbed moisture in the samples, whereas the weight gain of about 0.5% for h-Fe3O4@C/ F between 150 and 350 C is likely the result of oxidation of the Fe3O4 in air.45 A major weight loss of 13.8% was clearly observed between 350 and 800 C, induced by oxidizing the carbon into CO2. These results are in accordance with the results of EDX spectroscopy (Fig. S3†). According to the EDX spectrum and XPS spectroscopy, the F atom ratio in the material was 5.25%. TGA analysis under air at a ramp rate of 5 C min1 (Fig. S4†) also determined that the carbon content of the h-Fe3O4@C composite was about 16.5%. The crystal structures of h-Fe3O4 and h-Fe3O4@C/F were subsequently conrmed by X-ray diffraction (XRD) (Fig. 3). The characteristic peaks at 30.36 (220), 35.77 (311), 43.35 (400), 53.76 (422), 57.16 (511) and 62.98 (440) agreed with the standard XRD data for pure Fe3O4 (JCPDS card no. 19-0629, S.G.: ˚ 46 There were no obvious diffraction peaks Fd3m, ao ¼ 8.396 A). corresponding to carbon or other impurity peaks in the XRD pattern (XRD analysis of h-Fe3O4@C in Fig. S5†), which indicates that the carbon coating and F doping did not result in any structural change in h-Fe3O4. The absence of any graphite peak was attributed to the amorphous state of carbon.44 The surface areas of (A) h-Fe3O4, (B) h-Fe3O4@C/F and (C) h-Fe3O4@C
Fig. 3 X-ray diffraction (XRD) patterns of h-Fe3O4 (A) and hFe3O4@C/F (B).
This journal is © The Royal Society of Chemistry 2014
Nanoscale
(Fig. S6†) were measured using the Brunauer–Emmett–Teller (BET) method. The BET specic surface area of h-Fe3O4 was 66 m2 g1, higher than 51 m2 g1 for h-Fe3O4@C and 54 m2 g1 for h-Fe3O4@C/F, which was attributed to the embedment of the carbon and F-doped carbon respectively. The detailed distributions of the sphere diameters of h-Fe3O4 and h-Fe3O4@C/F are shown in Fig. S7A and B† respectively. They have a mean diameter of about 200 nm, corresponding to the SEM images (Fig. 1A and B). The charge–discharge voltage proles for the 1st, 10th, 20th, th 30 , 40th and 50th cycles of h-Fe3O4, h-Fe3O4@C and h-Fe3O4@C/F over the voltage range from 0.05 to 3.00 V at a rate of 0.1 C are shown in Fig. 4A and B and S8,† respectively. In the rst cycle, there were two obvious potential plateaus at about 1.6 and 0.8 V, which are similar to those described in the literature,26,27 suggesting that differing structures and carbon modication do not change the chemical nature of Fe3O4. The potential plateaus at about 0.8 V in the rst discharge and 1.6 V in the rst charge are ascribed to the Fe(III), Fe(II) and Fe redox couples during lithium ion insertion and extraction of the hollow spheres. The sloping curve around 0.05–0.8 V is attributed to the decomposition of the electrolyte to form a gel-like solid electrolyte interface (SEI) lm.25,28,33 The rst discharge curves for the samples show very high discharge specic capacities of 1647, 1714 and 1411 mA h g1 for h-Fe3O4, h-Fe3O4@C and h-Fe3O4@C, with corresponding charge capacities of 1306, 1237 and 1010 mA h g1, respectively. In subsequent cycles, h-Fe3O4@C/F shows enhanced capacity retention, indicating the contribution of the F-doped carbon layer to stabilization of the electrochemical performance of the anode materials.41,42 Fig. 4C shows the cycling performances of the as-prepared spheres at a current rate of 0.1 C aer 50 cycles as anode materials in a LIB. The discharge capacity of h-Fe3O4 decreased dramatically, from 1647 to 194.6 mA h g1. In sharp contrast, the h-Fe3O4@C spheres exhibited a markedly improved cycling
Fig. 4 The charge–discharge voltage profiles over the initial 50 cycles for h-Fe3O4 (A) and h-Fe3O4@C/F (B) at a current rate of 0.1 C. Cycling performances at a current of 0.1 C (C) and rate capability of h-Fe3O4@C/F (D).
Nanoscale, 2014, 6, 3889–3894 | 3891
View Article Online
Published on 27 January 2014. Downloaded by National Dong Hwa University Library on 30/03/2014 11:43:26.
Nanoscale
performance with a reversible capacity of 639.9 mA h g1. More importantly, h-Fe3O4@C/F also retained a high reversible capacity of about 938 mA h g1, much higher than h-Fe3O4, h-Fe3O4@C and the literature reported.21,23 The rate capability of h-Fe3O4@C/F from 0.1 C to 1 C was further investigated, as shown in Fig. 4D, and the results showed stable cyclic performance in discrete steps. With increasing rates from 0.1 C to 1 C, h-Fe3O4@C/F delivered discharge capacities over 930, 880, 800, 740, 700 and 620 mA h g1 at current rates of 0.1, 0.2, 0.4, 0.6, 0.8 and 1 C, respectively. In addition, when the current rate was reduced back to 0.1 C, the capacity eventually recovered to around 930 mA h g1, which conrms the good capacity reversibility of the h-Fe3O4@C/F electrode. To further probe the electrochemical performance of the h-Fe3O4@C/F electrode, we increased the cycle number to 230 cycles (Fig. 5). As expected, h-Fe3O4@C/F manifested satisfactory rate capability. It delivered a reversible capacity of over 930 mA h g1 at a rate of 0.1 C aer 70 cycles, 800 mA h g1 at a rate of 0.5 C aer 120 cycles, 620 mA h g1 at a rate of 1 C aer 200 cycles and nally recovered to around 1000 mA h g1 at a rate of 0.1 C aer 230 cycles, which represents a stable coulombic efficiency of almost 100% aer the rst few cycles. Based on the synthetic process, the stable cyclic performance, high coulombic efficiency and stable rate capability are primarily attributed to the distribution of F and C atoms and to the unique structure of the material, which not only improve the reaction kinetics and stability of the SEI lms, but also prevent any aggregation and drastic volume changes of the Fe3O4 particles.38,39,41,42 In contrast, the h-Fe3O4@C electrode with the glucose-derived carbon layer would lead to lackluster electron conductivity and ion permeability.38,39 Compared with the pure Fe3O4, h-Fe3O4@C showed higher capacity retention, which may be attributed to the protective effect of the carbon layer.35,37 Furthermore, the electrochemical impedance spectra were applied to conrm the contributions of the introduction of the F element. As shown in Fig. S9A,† the Nyquist plots showed a depressed semicircle at the high-frequency region and an inclined straight line at low frequency, corresponding to the charge transfer resistance and Li+ diffusion in the electrode
Paper
material respectively. Due to the presence of carbon and the F-doped carbon, the size of the semicircle was much smaller for h-Fe3O4@C and h-Fe3O4@C/F, indicating that carbon and the F-doped carbon improved the electronic conductivity of the electrode. Compared with h-Fe3O4@C, h-Fe3O4@C/F exhibited a smaller semicircle from the equivalent circuit (inset of Fig. S9B†) and a larger slope (Fig. S9B†). The results conrmed that the F-doped carbon improved the electronic conductivity and Li+ diffusion in the electrode material.
Conclusions In conclusion, we have demonstrated the synthesis of h-Fe3O4@C/F hollow spheres by a facile and convenient route. Beneting from the incorporation of F and C atoms, the sample showed enhanced cyclic performance, high coulombic efficiency and stable rate capability. As an example, the test specimen exhibited a high reversible capacity of over 930 mA h g1 at a rate of 0.1 C aer 70 cycles, 800 mA h g1 at a rate of 0.5 C aer 120 cycles and 620 mA h g1 at a rate of 1 C aer 200 cycles. This dramatic improvement can undoubtedly be attributed to the improved properties of the original magnetite resulting from F-doped carbon encapsulation, which indicates that this unique composite will be of signicant interest with regard to its applications in high-performance Li-ion batteries. This same F-doped carbon encapsulation modication could potentially be applied to other metal oxide electrode materials that suffer from poor conductivity and severe volume expansion.
Experimental Preparation of h-Fe3O4 spheres In a typical procedure,47 1.08 g FeCl3$6H2O, 2.35 g sodium citrate (C6H5O7Na3$2H2O) and 0.72 g urea were dissolved in 80 mL distilled water, aer which polyacrylamide (PAM, 0.6 g) was added with magnetic stirring for 1 h. The resulting mixture was transferred into a 100 mL Teon-lined stainless steel autoclave and held at 200 C for 12 h. The product was washed with ethanol and distilled water and then harvested using a magnetic eld. Preparation of h-Fe3O4@C spheres The as-prepared h-Fe3O4 (0.36 g) was dispersed in 40 mL 0.5 M aqueous glucose solution. The suspension was transferred to a 100 mL Teon-lined stainless steel autoclave and heated at 180 C for 6 h, aer being washed with distilled water and ethanol and dried under vacuum at 80 C overnight. In order to enhance the electrical conductivity of the carbon layer, the resulting material was sintered at 500 C for 4 h under a N2 atmosphere. Preparation of h-Fe3O4@C/F spheres
The long-term rate performance and coulombic efficiency of h-Fe3O4@C/F. Fig. 5
3892 | Nanoscale, 2014, 6, 3889–3894
The obtained h-Fe3O4 spheres (0.5 g) were mixed with 0.1 g PVDF by grinding. The mixture was sintered in a N2 ow for 4 h at 500 C, aer which the h-Fe3O4@C/F spheres were collected.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Paper
Published on 27 January 2014. Downloaded by National Dong Hwa University Library on 30/03/2014 11:43:26.
Material characterization The as-prepared nanostructures were characterized by XRD using an X'Pert-Pro MPD diffractometer (PANalytical, Nether˚ The sphere lands) with a Cu Ka X-ray source (l ¼ 1.540598 A). morphologies were characterized by SEM, TEM and high-resolution TEM (HRTEM). SEM was performed on a Hitachi S-4700 cold eld emission scanning electron microscope operated at 30 kV, TEM (Tecnai G220, FEI, USA) was performed using a Gatan CCD794 camera operated at 200 kV and HRTEM was carried out on a Tecnai G2 F20 S-TWIN microscope with an accelerating voltage of 200 kV. XPS data were obtained using a KRATOS Axis ultra-DLD X-ray photoelectron spectrometer with monochromatic Mg Ka X-ray (1283.3 eV). TGA was performed using a Perkin Elmer TGA 4000 thermogravimetric analyzer and Raman spectra were acquired at room temperature with a confocal Raman microscope (HR-800, Jobin Yvon, France). Electrochemical characterization The electrochemical measurements of obtained products were performed using a coin-type half cell (CR 2016). The working electrode was prepared by mixing 80 wt% of the active material (h-Fe3O4, h-Fe3O4@C or h-Fe3O4@C/F), 10 wt% conductive material (acetylene black, AB) and 10 wt% polymer binder (polyvinylidene diuoride, PVDF) in an N-methyl-2-pyrrolidone (NMP) solvent to form a homogeneous slurry, followed by spreading onto a copper foil. Finally, the copper foil was dried overnight under vacuum at 120 C. The electrolyte was 1 M LiPF6 in a 1 : 1 (w/w) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (Guotaihuarong, Zhangjiagang, China). Cell assembly was performed in an argon-lled glovebox in which moisture and oxygen were both below 0.1 ppm. The cells were charged and discharged between 3.0 and 0.05 V at room temperature.
Acknowledgements H.W.G. acknowledges the nancial support from the National Natural Science Foundation of China (no. 21003092 and 21373006), the Key Project of Chinese Ministry of Education (no. 211064), and the Priority Academic Program Development of Jiangsu Higher Education Institutions. Y.P. acknowledges nancial support from the New Faculty Start Foundation of Soochow University (Q410900413); J.W.Z. acknowledges the nancial support from the Nature Science Foundation of China (no. 20873089 and 20975073), Nature Science Foundation of Jiangsu Province (no. BK2011272), Industry-Academia Cooperation Innovation Fund Projects of Jiangsu Province (no. BY2011130) and Key Laboratory of Lithium Ion Battery Materials of Jiangsu Province, the Project of Scientic and Technologic Infrastructure of Suzhou (SZS201207).
Notes and references 1 P. L. Taberna, S. Mitra, P. Poizot, P. Simon and J.-M. Tarascon, Nat. Mater., 2006, 5, 567. This journal is © The Royal Society of Chemistry 2014
Nanoscale
2 C. T. Yavuz, J. T. Mayo, W. W. Yu and V. L. Colvin, Science, 2006, 314, 964. 3 H. Ho, X. L. Sun and S. H. Sun, Acc. Chem. Res., 2011, 44, 875; Y. Pan, X. W. Du, F. Zhao and B. Xu, Chem. Soc. Rev., 2012, 41, 2912; J. Zhu, Y. Lu, Y. Li, J. Jiang, L. Cheng, Z. Liu, L. Guo, Y. Pan and H. Gu, Nanoscale, 2014, 6, 199; Y. Pan, M. J. C. Long, X. Li, J. Shi, L. Hedstrom and B. Xu, Chem. Sci., 2011, 2, 945–948; J. Wu and C. -C. Chu, J. Mater. Chem. B, 2013, 1, 353; Y. Pan, M. J. C. Long, H. -C. Lin, L. Hedstrom and B. Xu, Chem. Sci., 2012, 3, 3495–3499; Y. Pan, Y. Gao, J. Shi, L. Wang and B. Xu, J, Mater. Chem., 2011, 21, 6804. 4 L. S. Zhong, J. S. Hu, H. P. Liang, A. M. Cao, W. G. Song and L. Y. Wan, Adv. Mater., 2006, 18, 2426. 5 X. L. Hu, J. M. Gong, Q. Li and G. S. Li, Adv. Mater., 2007, 19, 2324. 6 Y. M. Lee, M. A. Garcia, N. A. F. Huls and S. H. Sun, Angew. Chem., Int. Ed., 2010, 122, 1293. 7 A. S. Aric` o, P. Bruce, B. Scrosati, J. M. Tarascon and W. V. Schalkwijk, Nat. Mater., 2005, 4, 366; B. Koo, H. Xiong, M. D. Slater, V. B. Prakapenka, M. Balasubramanian, P. Podsiadlo, C. S. Johnson, T. Rajh and E. V. Shevchenko, Nano Lett., 2012, 12, 2429. 8 C. Xu, Y. Zeng, X. H. Rui, N. Xiao, J. X. Zhu, W. Y. Zhang, J. Chen, W. L. Liu, H. T. Tan, H. H. Hng and Q. Y. Yan, ACS Nano, 2012, 6, 4713; G. Gao, Q. Zhang, K. Wang, H. Son, P. Y. Qiu and D. X. Cui, Nano Energy, 2013, 2, 1010. 9 Z. W. Seh, W. Y. Li, J. J. Cha, G. Y. Zheng, M. T. McDowell, P. C. Hsu and Y. Cui, Nature, 2013, 4, 1331. 10 F. Y. Cheng, Z. L. Tao, J. Liang and J. Chen, Chem. Mater., 2008, 20, 667. 11 L. P. Tan, Z. Y. Lu, H. T. Tan, J. X. Zhu, X. H. Rui, Q. Y. Yan and H. H. Hng, J. Power Sources, 2012, 206, 253. 12 P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J.-M. Tarascon, Nature, 2000, 407, 496; S. Y. Yin, Y. Y. Zhang, J. H. Kong, C. J. Zou, C. M. Li, X. H. Lu, J. Ma, F. Y. C. Boey and X. D. Chen, ACS Nano, 2011, 5, 3831. 13 J. X. Zhu, Z. Y. Yin, D. Yang, T. Sun, H. Yu, H. E. Hoster, H. Zhang and Q. Y. Yan, Energy Environ. Sci., 2013, 6, 987; J. X. Zhu, T. Zhu, X. Z. Zhou, Y. Y. Zhang, X. W. Lou, X. D. Chen, H. Zhang, H. H. Hng and Q. Y. Yan, Nanoscale, 2011, 3, 1084. 14 P. G. Bruce, B. Scrosati and J. M. Tarascon, Angew. Chem., Int. Ed., 2008, 47, 2930; Y. Y. Zhang, Y. X. Tang, S. Y. Yin, Z. Y. Zeng, H. Zhang, C. M. Li, Z. L. Dong, Z. Chen and X. D. Chen, Nanoscale, 2011, 3, 4074. 15 J. S. Chen, C. M. Li, W. W. Zhou, Q. Y. Yan, L. A. Archer and X. W. Lou, Nanoscale, 2009, 1, 280; J. H. Kong, W. A. Yee, Y. F. Wei, L. P. Yang, J. M. Ang, S. L. Phua, S. Y. Wong, R. Zhou, Y. L. Dong, X. Li and X. H. Lu, Nanoscale, 2013, 5, 2967. 16 J. S. Chen, Y. M. Zhang and X. W. Lou, ACS Appl. Mater. Interfaces, 2011, 3, 3276–3279; H. B. Geng, Q. Zhou, J. W. Zheng and H. W. Gu, RSC Adv., 2014, 4, 6430. 17 G. M. Zhou, D. W. Wang, F. Li, L. L. Zhang, N. Li, Z. S. Wu, L. Wen, G. Q. Lu and H. M. Cheng, Chem. Mater., 2010, 22, 5306.
Nanoscale, 2014, 6, 3889–3894 | 3893
View Article Online
Published on 27 January 2014. Downloaded by National Dong Hwa University Library on 30/03/2014 11:43:26.
Nanoscale
18 Y. Z. Su, S. Li, D. Q. Wu, F. Zhang, H. W. Liang, P. F. Gao, C. Cheng and X. L. Feng, ACS Nano, 2012, 6, 8349. 19 C. K. Chan, H. L. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins and Y. Cui, Nat. Nanotechnol., 2008, 3, 31. 20 Q. Q. Xiong, J. P. Tu, Y. Lu, J. Chen, Y. X. Yu, Y. Q. Qiao, X. L. Wang and C. D. Gu, J. Phys. Chem. C, 2012, 116, 6495. 21 Y. Chen, H. Xia, L. Lu and J. M. Xue, J. Mater. Chem., 2012, 22, 5006; Y. Pan, J. H. Gao, B. Zhang, X. X. Zhang and B. Xu, Langmuir, 2010, 26, 4184. 22 H. S. Lim, B. Y. Junga, Y. K. Sunb and K. D. Suha, Electrochim. Acta, 2012, 75, 123. 23 H. J. Kim, K. I. Choi, A. Pan, I. D. Kim, H. R. Kim, K. M. Kim, C. W. Na, G. Z. Cao and J. H. Lee, J. Mater. Chem., 2011, 21, 6549. 24 J. S. Xu and Y. J. Zhu, ACS Appl. Mater. Interfaces, 2012, 4, 4752. 25 C. N. He, S. Wu, N. Q. Zhao, C. S. Shi, E. Z. Liu and J. J. Li, ACS Nano, 2013, 7, 4459. 26 S. L. Jin, H. G. Deng, D. H. Long, X. J. Liu, L. Zhan, X. Y. Liang, W. M. Qiao and L. C. Ling, J. Power Sources, 2011, 196, 3887. 27 W. Cheng, K. B. Tang, Y. X. Qi, J. Sheng and Z. P. Liu, J. Mater. Chem., 2010, 20, 1799. 28 X. L. Jia, Z. Chen, X. Cui, Y. T. Peng, X. L. Wang, G. Wang, F. Wei and Y. F. Lu, ACS Nano, 2012, 6, 9911. 29 W. M. Zhang, X. L. Wu, J. S. Hu, Y. G. Guo and L. J. Wan, Adv. Funct. Mater., 2008, 18, 3941. 30 T. Muraliganth, A. V. Murugan and A. Manthiram, Chem. Commun., 2009, 7360. 31 P. Tartaj and J. M. Amarilla, J. Power Sources, 2011, 196, 2164. 32 S. H. Lee, S. H. Yu, J. E. Lee, Y. E. Sung and T. Hyeon, Nano Lett., 2013, 13, 4249.
3894 | Nanoscale, 2014, 6, 3889–3894
Paper
33 Y. Wu, Y. Wei, J. P. Wang, K. L. Jiang and S. S. Fan, Nano Lett., 2013, 13, 818. 34 R. H. Wang, C. H. Xu, J. Sun, L. Gao and C. C. Lin, J. Mater. Chem. A, 2013, 1, 1794. 35 D. Y. Chen, G. Ji, Y. Ma, J. Y. Lee and J. M. Lu, ACS Appl. Mater. Interfaces, 2011, 3, 3078. 36 W. Wei, S. B. Yang, H. X. Zhou, I. Lieberwirth, X. L. Feng and K. M¨ ullen, Adv. Mater., 2013, 25, 2909. 37 L. L. Wang, J. W. Liang, Y. C. Zhu, T. Mei, X. Zhang, Q. Yang and Y. T. Qian, Nanoscale, 2013, 5, 3627. 38 J. E. Lee, S. H. Yu, D. L. Lee, D. C. Lee, Y. E. Sung and T. Hyeon, Energy Environ. Sci., 2012, 5, 9528. 39 Y. Ma, C. Zhang, G. Jia and J. Y. Lee, J. Mater. Chem., 2012, 22, 7845. 40 C. Lei, F. Han, D. Li, W. C. Li, Q. Sun, X. Q. Zhang and A. H. Lu, Nanoscale, 2013, 5, 1168. 41 T. Nakanima, V. Gupta, Y. Ohzawa, M. Koh, R. N. Singh, A. Tressaud and E. Durand, J. Power Sources, 2002, 104, 108. 42 V. Gupta, T. Nakanima, Y. Ohzawa and H. Iwata, J. Fluorine Chem., 2001, 112, 233. 43 A. Tressaud, F. Moguet, G. Nanse, E. Papirer, V. Gupta and O. P. Bahl, J. Phys. Chem. Solids, 1996, 57, 745. 44 M. Zhang, B. H. Qu, D. Lei, Y. J. Chen, X. Z. Yu, L. B. Chen, Q. H. Li, Y. G. Wang and T. H. Wang, J. Mater. Chem., 2012, 22, 3868. 45 T. Zhu, J. S. Chen and X. W. Lou, J. Phys. Chem. C, 2011, 115, 9814. 46 J. S. Chen, Y. M. Zhang and X. W. Lou, ACS Appl. Mater. Interfaces, 2011, 3, 3276. 47 W. Cheng, K. Tang, Y. Qi, J. Sheng and Z. P. Liu, J. Mater. Chem., 2010, 20, 1799.
This journal is © The Royal Society of Chemistry 2014
