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C composite with hollow spheres in porous 3D-nanostructure as anode material for the lithium-ion batteries
Journal of Power Sources 363 (2017) 161e167
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Fe3O4/C composite with hollow spheres in porous 3D-nanostructure as anode material for the lithium-ion batteries Zhao Yang a, Danyang Su a, Jinping Yang a, b, Jing Wang a, b, * a b
School of Materials Science and Technology, North China University of Science and Technology, Hebei, Tangshan 063210, China Key Laboratory of Inorganic Material of Hebei Province, Tangshan 063210, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The composite with hollow spheres in porous 3D nanostructure was prepared. This structure endowed composite better volume buffer and electric conductivity. The Fe3O4/C composite exhibited excellent electrochemical properties.
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Article history: Received 3 March 2017 Received in revised form 2 June 2017 Accepted 23 July 2017
3d transition-metal oxides, especially Fe3O4, as anode materials for the lithium-ion batteries have been attracting intensive attentions in recent years due to their high energy capacity and low toxicity. A new Fe3O4/C composite with hollow spheres in porous three-dimensional (3D) nanostructure, which was synthesized by a facile solvothermal method using FeCl3$6H2O and porous spongy carbon as raw materials. The specific surface area and microstructures of composite were characterized by nitrogen adsorption-desorption isotherm method, FE-SEM and HR-TEM. A homogeneous distribution of hollow Fe3O4 spheres (diameter ranges from 120 nm to 150 nm) in the spongy carbon (pore size > 200 nm) conductive 3D-network significantly reduced the lithium-ion diffusion length and increased the electrochemical reaction area, and further more enhanced the lithium ion battery performance, such as discharge capacity and cycle life. As an anode material for the lithium-ion battery, the title composite exhibit excellent electrochemical properties. The Fe3O4/C composite electrode achieved a relatively high reversible specific capacity of 1450.1 mA h g1 in the first cycle at 100 mA g1, and excellent rate capability (69% retention at 1000 mA g1) with good cycle stability (only 10% loss after 100 cycles). © 2017 Published by Elsevier B.V.
Keywords: Iron oxide Spongy carbon Three-dimensional nanostructure Solvothermal method Lithium ion battery
1. Introduction Recently, compared with the nickel-cadmium (Ni-Cd) batteries and nickel-metal hydride (Ni-MH) batteries, LIBs as potential power sources for pure electric vehicles (EVs), hybrid electric vehicles
* Corresponding author. School of Materials Science and Technology, North China University of Science and Technology, Hebei, Tangshan 063210, China. E-mail address: [email protected] (J. Wang). http://dx.doi.org/10.1016/j.jpowsour.2017.07.080 0378-7753/© 2017 Published by Elsevier B.V.
(HEVs) and plug in hybrid electric vehicles (PHEVs) have also been recognized due to their lighter weight, lower self-discharge property, higher energy density [1,2]. In order to meet the increasing demand intensified investigations are required to develop a newgeneration Li-ion batteries with dramatically improved performances, including specific energy and volumetric energy density, cyclability, charging rate, stability, and safety [3,4]. Graphite as the commercial anode material for lithium-ion batteries, however, allows insertion-way of only one Li-ion within six carbon atoms, with
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a resulting low theoretical gravimetric capacity (372 mA h g1). This low theoretical capacity restricts the applications of LIBs [5e7]. High theoretical specific capacities (e.g. > 600 mA h g1) has been proposed for the transition metal oxides based on a novel conversion mechanism, including MnO2 [8,9], Fe3O4 [10,11], Fe2O3 [12,13], CoO [14], Co3O4 [15,16], and NiO [17], and making these oxides promising anode materials for high performance LIBs. Among transition metal oxides, Fe3O4 exhibits high electrical conductivity (2*104 S m1), high theoretical capacity (924 mA h g1), and low conversion potential, making it one of the most promising anode candidate for LIBs [18,19]. However, a major challenge associated with Fe3O4 as anode material for LIBs is the large volume change (about 100%) during the processes of insertion and de-insertion of Liþ [20,21]. Moreover, Fe3O4 suffers from the same problem as other transition metal oxides, such as surface instability and low initial reversibility for the pulverization of active materials, which are attributed to apparent phase and morphology changes accompanied by typical conversion reactions [22e24]. However, versatile controlled synthetic methods for low dimensional Fe3O4 nanostructures have been developed that may improve its unsatisfactory electrochemical performance. Investigations on nanocomposite reveal that carbon materials are beneficial to electrode material of transition metal oxides in two main ways [25e31]: Firstly by increasing the electrical conductivity, and secondly by reducing the volume change via accommodating the stresses, which are arose from volume increasing [32,33]. Such as Dong et al. [34] reported a preparation of hierarchical structures of carbon-coated Fe3O4 nanoparticles decorated on conductive multi-walled carbon nanotubes (CNTs) backbone (CNT@Fe3O4-C) with a reversible capacity of 1080 mA h g1 at 500 mA g1 after 700 cycles. Graphene-wrapped Fe3O4 hollow nanospheres [35] had been synthesized with a higher reversible capacity than CNT@Fe3O4-C. Fe3O4 nanoparticles & carbon composite of 1D-nanostructure demonstrated high reversible capacity at high current density and long cycling-life. Meanwhile, Fe3O4 composite of 3D-nanostructure with graphene (2D carbon
material) exhibited excellent reversible capacity. Above all, the specific surface area and porosity are essential factors that affect electrochemical performances of carbon materials, 3D-nanostructure porous carbon materials exist potential application prospect in composite as anode material for LIBs [4]. In this study, we designed and synthesized a special 3D-nanostructure with a hollow Fe3O4 spheres into spongy carbon matrix, by a facile and disposable solvothermal method. The hollow spheres in porous unique architecture with 3-dimensional void can relieve the huge volume variation of Fe3O4 during cycling processes, and prevent the aggregation of hollow Fe3O4 spheres from aggregation. Furthermore, nano-porous spongy carbon could offer electronic conductivity. Herein, the title composite was found to exhibit a highly reversible capacity and excellent cycle performance as anode material for LIBs. 2. Experimental 2.1. Synthesis The schematic synthesis process pattern of title composite was illustrated in Fig. 1. The composite was prepared in two steps. The spongy carbon was obtained, and then the hollow Fe3O4 spheres were decorated on the spongy carbon by solvothermal reaction. The mixture of CaCO3 and phenolic resin was carbonization at 800 C in Ar atmosphere for 2 h. After the treatment of acid (0.1 mol L1 HCl) corroding, the solution was dried at 80 C for 10 h and the spongy carbon was obtained. The spongy carbon, FeCl3$6H2O, PVP, and carbamide were dissolved in 45 mL of C2O2H6 solution. The Fe3O4/C composite was synthesized by the solvothermal method at 200 C for 24 h. 2.2. Characterization Phase analysis was conducted using powder X-ray diffraction (XRD, D/MAX2500PC) with Cu Ka radiation. The particle
Fig. 1. Schematic illustration of Fe3O4/C composite preparation.
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morphology was marked by a high-resolution transmission electron microscope (HR-TEM, FEI Talos F200X) with an accelerating voltage of 200 kV. Raman measurements were conducted on a Laser Raman spectrometer (Thermo DXR) with an accumulation/ collection time of 10s. Specimen morphologies were characterized using scanning electron microscope (SEM, S-4800, Hitachi, Japan). The chemical composition of the composite was established using an energy dispersive X-ray spectrometer (EDX) acquiring at 200 kV, gun lens 3.6 and spot size 6. N2 adsorption/desorption measurements were performed by using a Quantachrome instrument (QUADRASORB SI-MP) at 77.3 K. Thermogravimetric analysis (TGA) with thermogravimetric analysis instrument (NETZSCH STA 449C) was conducted to determine the weight percentage of Fe3O4 and C in the composite. Fourier transform infrared spectroscopy (FTIR) analyses were recorded on a VERTEX70 spectrometer (Bruker) using KBr pellets. 2.3. Electrochemical characterization The working electrodes were made of the Fe3O4/C composite, acetylene black, polyvinylidene difluoride (PVDF) binder mixed at a weight ratio of 70:15:15 in binder (N-methylpyrrolidone, NMP) and pasted on Cu foil. Pure lithium sheets and polypropylene film (Celgard2400) were used as counter electrode and separator. The electrolyte was a solution of LiPF6 (1.0 M) in a mixture of ethylene carbonate, dimethyl carbonate and diethyl carbonate (EC-DMCDEC, 1:1:1 in volume) [36]. Galvanostatic cycling tests were conducted in the range of 0.1e2.5 V vs. Li/Liþ at different current density by a LAND CT2001A battery test system. Cyclic voltammetry (CV) was implemented on a Arbin BT2000 electrochemical workstation at a scan rate of 0.1 mV s1 between 0 and 3 V. 3. Results and discussion XRD patterns of the spongy carbon, pure hollow Fe3O4 spheres, and Fe3O4/C composite and Raman spectra of spongy carbon and Fe3O4/C composite were shown as Fig. 2. For giving the direct evidence to confirm that the HCl solution could completely etch CaCO3 and CaO (CaCO3 under high temperature produces). The broad peak at 23 was a typical XRD pattern of amorphous carbon (PDF NO.50e0926), as shown in Fig. 2(a), which could be indexed to carbon with a hexagonal crystal lattice. For pure hollow Fe3O4 spheres specimen, all diffraction peaks in the patterns were indexed to a cubic phase of Fe3O4 in the Fd-3m space group
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(a ¼ 0.839 nm, PDF No. 65e3107) [18]. There were only the crystalline diffraction peaks of Fe3O4, and no other impurity phases were observed in the in the Fe3O4/C composite. In Fig. 2(b), a pair of peaks in the range of 800e2000 cm1 is observed for both samples of spongy carbon and Fe3O4/C composite by Raman spectroscopy. Two strong and broad peaks centered at 1335 cm1 and 1600 cm1 which were corresponding to D band and G band, respectively. The D band corresponded to defects and disordered portions of carbon (sp3-corrdinated), while the G band is associated with the ordered sp2 carbon [37]. The intensity ratio of D and G band (ID/IG) for spongy carbon and Fe3O4/C composite are 0.99 and 0.93, indicating that the spongy carbon possessed lower graphitized degree. In addition, the peaks at around 664 cm1 and 588 cm1 are designated as the typical of Fe3O4, which also confirmed the reliability of the XRD analysis result [32,34,37]. The morphology and micro-structure of the specimens were examined by FESEM (Fig. 3). Abundant macroporous (diameter ranges from 50 nm to 100 nm) in spongy carbon were observed in Fig. 3(a), the hierarchical porous structure is composed of numerous mesopores in the interconnected macroporous walls. As shown in Fig. 3(b), the specimen of Fe3O4 consisted of a large quantity of spheres with an average diameter of ca. 100e120 nm. The hollow spheres in porous 3D-nanostructure had been successfully prepared, and hollow Fe3O4 spheres were embedded into the coarse surfaces of spongy carbon. The probable formation process of this hollow spheres in porous 3D-nanostructure can be interpreted as follow: 1) The sponginess structure of carbon with macroporous was formed in the process of corrosion of CaO and CaCO3, as illustrated in Fig. 1; 2) The nucleation and growth of Fe3O4 could fill the empty pores space at solvothermal system; 3) The hollow spheres in porous between the Fe3O4 and spongy carbon formed after the Fe3O4 crystal agglomeration and growth into hollow spheres on the surface of spongy carbon. Preferential growth of Fe3O4 spheres on the spongy carbon was exhibited little variation in particle in Fig. 3(c), probably due to the great and special surface of the spongy carbon, which provided the nucleation boundary for the Fe3O4 spheres, compared with pure Fe3O4 spheres shown in Fig. 3(b). Moreover, under the high multiplying power observation (shown in Fig. 3(d)), the Fe3O4 spheres with hollow structure were clearly visible. The underneath conductive spongy carbon would benefit the fast electron transfer and maintain the structural stability. In order to figure out the distribution of hollow Fe3O4 spheres in C matrix, energy dispersive X-ray spectrometer (EDS) mappings of
Fig. 2. (a)XRD spectrum of the specimens: spongy carbon, hollow Fe3O4 spheres, Fe3O4/C composite. (b) Raman spectra of spongy carbon and Fe3O4/C composite.
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Fig. 3. SEM image of the specimens: spongy carbon(a), hollow Fe3O4 spheres(b), Fe3O4/C composite(c), (d).
Fig. 4. TEM image of Fe3O4/C composite (a); Elemental mappings of Fe (b), O (c), C (d); HRTEM image of Fe3O4/C composite (e).
Fe, O and C elements were measured (Fig. 4). It indicated that the Fe (b) and O (c) content were higher in the region of the Fe3O4 spheres than in others. Carbon (d) formed the spongy structure and the carbon network (electron microscope) of the EDS mappings. This structure model was not only an available method to prevent the Fe3O4 nanoparticles aggregation but also benefited to improve the contact state between nanoparticles, electrolyte and transportation of Liþ. The spongy carbon would act as a buffer to accommodate the volume change of the Fe3O4 spheres, and this effect was limited as the outward expansion of the Fe3O4 nanoparticles. The lattice spacing of the composite specimen implied in the HRTEM image (Fig. 4(e)) was 0.4816 nm, which could be well-matched to (111) [37]lattice plane of cubic Fe3O4. The specific surface area and pore size distribution of the asprepared spongy carbon and Fe3O4/C composite were analyzed by
nitrogen adsorption-desorption isotherm method at 77 K. As illustrated in Fig. 5(a), the isotherm profile of spongy carbon should be categorized as a combined I/IV type with a H4 hysteresis loop at relative pressure P/Po of 0.2e0.95, indicating the presence of slitlike mesoporous, according to the IUPAC classification. However, low adsorption quantity at relatively low pressure (P/Po of 0.1e0.8) and a sharp increase of adsorption quantity at the relative pressure P/Po of 0.8e1.0 can be observed, demonstrating the existence of macropores which can be observed in the SEM picture. The Fe3O4/C composite exhibited clear hysteresis loop that is the property of the type IV adsorption isotherms with a H3 hysteresis loop at P/Po of 0.1e1.0. Compared with spongy carbon, the increase of adsorption quantity of the title composite at the relative pressure was seen to be uniformly slow, indicating the disappearance of macropores and the decrease of mesoporous [38]. The BET surface area and pore size
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Fig. 5. (a) N2 adsorption-desorption isotherms of spongy carbon and Fe3O4/C composite; (b) pore-size distribution curve of spongy carbon and Fe3O4/C composite; (c) FTIR spectra of Fe3O4/C composite; (d) TGA weight loss curve and DTA curve of Fe3O4/C composite.
of spongy carbon and Fe3O4/C composite were found to be 195.148 m3 g1 nm1 and 884.739 m3 g1 nm1,3e10 nm and 3e10 nm in Fig. 5(b), respectively. The high specific surface area and the broad pore size distribution can provide efficient transport of Liþ and rapid transport of electrolyte [39]. To further demonstrate the structure of Fe3O4/C composite, Fig. 5(c) illustrated the FTIR spectrum of as prepared title composite in the wavelength range of 4000-400 cm1. The absorption peaks at 586 cm1, and 1400 cm1 were assigned to be stretching vibration modes of Fe-O bonds. The peak at around 1098 cm1 was found in Fe3O4/C composite, which was attributed to the vibration of CeO in FeeOeC bond according to the previous research [40]. The peak around 1625 cm1 was corresponding to the C¼C bonds bending vibrations in spongy carbon. C-O stretching vibration of CO2 can be assigned for the appearance of the twin peaks whose position were located at 2339 cm1 and 2363 cm1. During the synthesized process, CO2 is produced and adsorbed on the composite surface due to the carbonization of organic groups in precursor and the existence of the circumstance in air. The band at 3240 cm1 was contributed by the stretching vibration of H-O bonds in the H2O molecule in the air. The carbon content of the Fe3O4/C composite was disclosed by TGA/DTA analysis. Shown in Fig. 5(d), as there is no peak in the DTA curve, the initial weight loss of ca. 2 wt % from 20 to 300 C should dependent on the evaporation of the weakly adsorbed water and solvent as well as the oxidation of some impurity [34]. With increasing temperature, an obvious weight loss occurs near 300 Cextending to 550 C, accompanying with one exothermic peaks at 390.0 C in the DTA curve. From 300 to 500 Cthe weight loss of ca. 33 wt %, the carbon content in the title composite can be derived to be about 33 wt % and the mass ratio of the Fe3O4 is about 67%. Fig. 6(a) showed the cycling performance of the Fe3O4/C composite, spongy carbon and hollow Fe3O4 spheres electrodes in the range of 0.01e2.5 V. The reversible capacity of the first cycle was
1445.1 mAh g1 and 1300 mAh g1 until 100 cycles at 100 mA g1(based on the total mass of the composite), and the low coulombic efficiency in the first cycle, which was commonly explained by the SEI layer formation resulted from electrolyte decomposition and irreversible trapping of Li in the Fe3O4 crystal lattice. The electrode exhibited an outstanding cycle life, while its specific capacity stabilized at 1320 mAh g1 after 12 cycles remaining at this leveled up to 100 cycles. The capacity of hollow Fe3O4 spheres was 680 mAh g1 after 60 cycles at 100 mA g1. Obviously, the capacity of spongy carbon was 1080 mAh g1 after 60 cycles higher than graphite with 3D-nanostructure pores. The reversible capacity value of composite markedly exceeded the theoretical capacity of Fe3O4 or carbon, which can be ascribed as: Reactions “6C þ Liþþ e /C6Lix” offers a theoretical capacity of 372 mAh g1 [41,42]. The spongy carbon was prepared by pyrolyzing phenolic resin, which should exist poly(p-phenylene) (PPP)based carbon [43]. In fact, the most saturated state may be C2Lix, because of sites of Li occupation supplied by benzene ring of PPPbased carbon, so that the calculated specific capacity is 3862 mAh g1 for Li metal [42,44e46]. Fig. 6(b) showed the corresponding galvanostatic discharge/ charge current rate of 0.1 A g1, where two plateaus at 0.8 and 1.63 V (vs. Li/Liþ) were attributed to the conversion reaction of discharge/charge process, in common with the cyclic voltammograms result. The cyclic voltammograms for the initial 3 cycles at a scan rate of 0.1 mV s1 between 0.01 and 3.0 V at room temperature were shown in Fig. 6(c). Respectively, where the peaks at 1.5 and 0.72 V (vs Li/Liþ) in the first cycle charge process were attributed to the conversion reaction of solid electrolyte interface (SEI) film formation, the reduction of Fe3O4 and the insertion of Liþ in the Fe3O4 lattice [34]. During following cycle with Liþ insertion process, the peaks were observed at 0.8 V and 1.45 V, which could be attributed only to the reduction of Fe3O4 to Fe (Fe3þ / Fe2þ / Fe). In the Li extraction process, two peaks at 1.63 V and 1.9 V were observed,
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Fig. 6. (a) Cycling performances of Fe3O4/C composite, spongy carbon and hollow Fe3O4 spheres. (b) Discharge/charge profile of Fe3O4/C composite. (c) CV curves of Fe3O4/C composite electrode at a scan rate of 0.1 mV s1. (d) Rate performance of Fe3O4/C composite from 0.1 A g1 to 1 A g1.
attributed to the electrochemical oxidation reaction of Fe to Fe3O4 [47, 48]. When increasing the specific current, the Fe3O4/C composite anode materials displayed highly reversible capacities of 1160, 1060, and 900 mAh g1 at 0.2, 0.5, and 1 A g1, respectively, as shown in Fig. 6(d). When the test current was set back to 0.1 A g1 after 20 cycles, capacity recovered to 1150 mAh g1 at the 25th cycle, indicating an excellent tolerance under highly specific current of title composite. 4. Conclusions The Fe3O4/C composite with hollow spheres in porous 3Dnanostructure was synthesized by facile solvothermal reaction. The hollow sphere structure of Fe3O4 was successfully prepared and dispersed on the surface of porous spongy carbon, which were helpful to form stabilize electrochemical performance, and increased cycle life. The highly reversible capacity was 1450.1 mAh g1 in the first cycle and 1300 mAh g1 until 100 cycles at 100 mA g1 in the range of 0.01e2.5 V. This synthesis approach and structure design strategy are feasible and scalable, which can be applied to other transition metal oxides and carbon containing composite for LIBs applications. Acknowledgments This work was supported by the National Natural Science Foundation of China (NO. 53171036) and the Scientific Research
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Fe3O4/C composite with hollow spheres in porous 3D-nanostructure as anode material for the lithium-ion batteries Zhao Yang a, Danyang Su a, Jinping Yang a, b, Jing Wang a, b, * a b
School of Materials Science and Technology, North China University of Science and Technology, Hebei, Tangshan 063210, China Key Laboratory of Inorganic Material of Hebei Province, Tangshan 063210, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The composite with hollow spheres in porous 3D nanostructure was prepared. This structure endowed composite better volume buffer and electric conductivity. The Fe3O4/C composite exhibited excellent electrochemical properties.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2017 Received in revised form 2 June 2017 Accepted 23 July 2017
3d transition-metal oxides, especially Fe3O4, as anode materials for the lithium-ion batteries have been attracting intensive attentions in recent years due to their high energy capacity and low toxicity. A new Fe3O4/C composite with hollow spheres in porous three-dimensional (3D) nanostructure, which was synthesized by a facile solvothermal method using FeCl3$6H2O and porous spongy carbon as raw materials. The specific surface area and microstructures of composite were characterized by nitrogen adsorption-desorption isotherm method, FE-SEM and HR-TEM. A homogeneous distribution of hollow Fe3O4 spheres (diameter ranges from 120 nm to 150 nm) in the spongy carbon (pore size > 200 nm) conductive 3D-network significantly reduced the lithium-ion diffusion length and increased the electrochemical reaction area, and further more enhanced the lithium ion battery performance, such as discharge capacity and cycle life. As an anode material for the lithium-ion battery, the title composite exhibit excellent electrochemical properties. The Fe3O4/C composite electrode achieved a relatively high reversible specific capacity of 1450.1 mA h g1 in the first cycle at 100 mA g1, and excellent rate capability (69% retention at 1000 mA g1) with good cycle stability (only 10% loss after 100 cycles). © 2017 Published by Elsevier B.V.
Keywords: Iron oxide Spongy carbon Three-dimensional nanostructure Solvothermal method Lithium ion battery
1. Introduction Recently, compared with the nickel-cadmium (Ni-Cd) batteries and nickel-metal hydride (Ni-MH) batteries, LIBs as potential power sources for pure electric vehicles (EVs), hybrid electric vehicles
* Corresponding author. School of Materials Science and Technology, North China University of Science and Technology, Hebei, Tangshan 063210, China. E-mail address: [email protected] (J. Wang). http://dx.doi.org/10.1016/j.jpowsour.2017.07.080 0378-7753/© 2017 Published by Elsevier B.V.
(HEVs) and plug in hybrid electric vehicles (PHEVs) have also been recognized due to their lighter weight, lower self-discharge property, higher energy density [1,2]. In order to meet the increasing demand intensified investigations are required to develop a newgeneration Li-ion batteries with dramatically improved performances, including specific energy and volumetric energy density, cyclability, charging rate, stability, and safety [3,4]. Graphite as the commercial anode material for lithium-ion batteries, however, allows insertion-way of only one Li-ion within six carbon atoms, with
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a resulting low theoretical gravimetric capacity (372 mA h g1). This low theoretical capacity restricts the applications of LIBs [5e7]. High theoretical specific capacities (e.g. > 600 mA h g1) has been proposed for the transition metal oxides based on a novel conversion mechanism, including MnO2 [8,9], Fe3O4 [10,11], Fe2O3 [12,13], CoO [14], Co3O4 [15,16], and NiO [17], and making these oxides promising anode materials for high performance LIBs. Among transition metal oxides, Fe3O4 exhibits high electrical conductivity (2*104 S m1), high theoretical capacity (924 mA h g1), and low conversion potential, making it one of the most promising anode candidate for LIBs [18,19]. However, a major challenge associated with Fe3O4 as anode material for LIBs is the large volume change (about 100%) during the processes of insertion and de-insertion of Liþ [20,21]. Moreover, Fe3O4 suffers from the same problem as other transition metal oxides, such as surface instability and low initial reversibility for the pulverization of active materials, which are attributed to apparent phase and morphology changes accompanied by typical conversion reactions [22e24]. However, versatile controlled synthetic methods for low dimensional Fe3O4 nanostructures have been developed that may improve its unsatisfactory electrochemical performance. Investigations on nanocomposite reveal that carbon materials are beneficial to electrode material of transition metal oxides in two main ways [25e31]: Firstly by increasing the electrical conductivity, and secondly by reducing the volume change via accommodating the stresses, which are arose from volume increasing [32,33]. Such as Dong et al. [34] reported a preparation of hierarchical structures of carbon-coated Fe3O4 nanoparticles decorated on conductive multi-walled carbon nanotubes (CNTs) backbone (CNT@Fe3O4-C) with a reversible capacity of 1080 mA h g1 at 500 mA g1 after 700 cycles. Graphene-wrapped Fe3O4 hollow nanospheres [35] had been synthesized with a higher reversible capacity than CNT@Fe3O4-C. Fe3O4 nanoparticles & carbon composite of 1D-nanostructure demonstrated high reversible capacity at high current density and long cycling-life. Meanwhile, Fe3O4 composite of 3D-nanostructure with graphene (2D carbon
material) exhibited excellent reversible capacity. Above all, the specific surface area and porosity are essential factors that affect electrochemical performances of carbon materials, 3D-nanostructure porous carbon materials exist potential application prospect in composite as anode material for LIBs [4]. In this study, we designed and synthesized a special 3D-nanostructure with a hollow Fe3O4 spheres into spongy carbon matrix, by a facile and disposable solvothermal method. The hollow spheres in porous unique architecture with 3-dimensional void can relieve the huge volume variation of Fe3O4 during cycling processes, and prevent the aggregation of hollow Fe3O4 spheres from aggregation. Furthermore, nano-porous spongy carbon could offer electronic conductivity. Herein, the title composite was found to exhibit a highly reversible capacity and excellent cycle performance as anode material for LIBs. 2. Experimental 2.1. Synthesis The schematic synthesis process pattern of title composite was illustrated in Fig. 1. The composite was prepared in two steps. The spongy carbon was obtained, and then the hollow Fe3O4 spheres were decorated on the spongy carbon by solvothermal reaction. The mixture of CaCO3 and phenolic resin was carbonization at 800 C in Ar atmosphere for 2 h. After the treatment of acid (0.1 mol L1 HCl) corroding, the solution was dried at 80 C for 10 h and the spongy carbon was obtained. The spongy carbon, FeCl3$6H2O, PVP, and carbamide were dissolved in 45 mL of C2O2H6 solution. The Fe3O4/C composite was synthesized by the solvothermal method at 200 C for 24 h. 2.2. Characterization Phase analysis was conducted using powder X-ray diffraction (XRD, D/MAX2500PC) with Cu Ka radiation. The particle
Fig. 1. Schematic illustration of Fe3O4/C composite preparation.
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morphology was marked by a high-resolution transmission electron microscope (HR-TEM, FEI Talos F200X) with an accelerating voltage of 200 kV. Raman measurements were conducted on a Laser Raman spectrometer (Thermo DXR) with an accumulation/ collection time of 10s. Specimen morphologies were characterized using scanning electron microscope (SEM, S-4800, Hitachi, Japan). The chemical composition of the composite was established using an energy dispersive X-ray spectrometer (EDX) acquiring at 200 kV, gun lens 3.6 and spot size 6. N2 adsorption/desorption measurements were performed by using a Quantachrome instrument (QUADRASORB SI-MP) at 77.3 K. Thermogravimetric analysis (TGA) with thermogravimetric analysis instrument (NETZSCH STA 449C) was conducted to determine the weight percentage of Fe3O4 and C in the composite. Fourier transform infrared spectroscopy (FTIR) analyses were recorded on a VERTEX70 spectrometer (Bruker) using KBr pellets. 2.3. Electrochemical characterization The working electrodes were made of the Fe3O4/C composite, acetylene black, polyvinylidene difluoride (PVDF) binder mixed at a weight ratio of 70:15:15 in binder (N-methylpyrrolidone, NMP) and pasted on Cu foil. Pure lithium sheets and polypropylene film (Celgard2400) were used as counter electrode and separator. The electrolyte was a solution of LiPF6 (1.0 M) in a mixture of ethylene carbonate, dimethyl carbonate and diethyl carbonate (EC-DMCDEC, 1:1:1 in volume) [36]. Galvanostatic cycling tests were conducted in the range of 0.1e2.5 V vs. Li/Liþ at different current density by a LAND CT2001A battery test system. Cyclic voltammetry (CV) was implemented on a Arbin BT2000 electrochemical workstation at a scan rate of 0.1 mV s1 between 0 and 3 V. 3. Results and discussion XRD patterns of the spongy carbon, pure hollow Fe3O4 spheres, and Fe3O4/C composite and Raman spectra of spongy carbon and Fe3O4/C composite were shown as Fig. 2. For giving the direct evidence to confirm that the HCl solution could completely etch CaCO3 and CaO (CaCO3 under high temperature produces). The broad peak at 23 was a typical XRD pattern of amorphous carbon (PDF NO.50e0926), as shown in Fig. 2(a), which could be indexed to carbon with a hexagonal crystal lattice. For pure hollow Fe3O4 spheres specimen, all diffraction peaks in the patterns were indexed to a cubic phase of Fe3O4 in the Fd-3m space group
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(a ¼ 0.839 nm, PDF No. 65e3107) [18]. There were only the crystalline diffraction peaks of Fe3O4, and no other impurity phases were observed in the in the Fe3O4/C composite. In Fig. 2(b), a pair of peaks in the range of 800e2000 cm1 is observed for both samples of spongy carbon and Fe3O4/C composite by Raman spectroscopy. Two strong and broad peaks centered at 1335 cm1 and 1600 cm1 which were corresponding to D band and G band, respectively. The D band corresponded to defects and disordered portions of carbon (sp3-corrdinated), while the G band is associated with the ordered sp2 carbon [37]. The intensity ratio of D and G band (ID/IG) for spongy carbon and Fe3O4/C composite are 0.99 and 0.93, indicating that the spongy carbon possessed lower graphitized degree. In addition, the peaks at around 664 cm1 and 588 cm1 are designated as the typical of Fe3O4, which also confirmed the reliability of the XRD analysis result [32,34,37]. The morphology and micro-structure of the specimens were examined by FESEM (Fig. 3). Abundant macroporous (diameter ranges from 50 nm to 100 nm) in spongy carbon were observed in Fig. 3(a), the hierarchical porous structure is composed of numerous mesopores in the interconnected macroporous walls. As shown in Fig. 3(b), the specimen of Fe3O4 consisted of a large quantity of spheres with an average diameter of ca. 100e120 nm. The hollow spheres in porous 3D-nanostructure had been successfully prepared, and hollow Fe3O4 spheres were embedded into the coarse surfaces of spongy carbon. The probable formation process of this hollow spheres in porous 3D-nanostructure can be interpreted as follow: 1) The sponginess structure of carbon with macroporous was formed in the process of corrosion of CaO and CaCO3, as illustrated in Fig. 1; 2) The nucleation and growth of Fe3O4 could fill the empty pores space at solvothermal system; 3) The hollow spheres in porous between the Fe3O4 and spongy carbon formed after the Fe3O4 crystal agglomeration and growth into hollow spheres on the surface of spongy carbon. Preferential growth of Fe3O4 spheres on the spongy carbon was exhibited little variation in particle in Fig. 3(c), probably due to the great and special surface of the spongy carbon, which provided the nucleation boundary for the Fe3O4 spheres, compared with pure Fe3O4 spheres shown in Fig. 3(b). Moreover, under the high multiplying power observation (shown in Fig. 3(d)), the Fe3O4 spheres with hollow structure were clearly visible. The underneath conductive spongy carbon would benefit the fast electron transfer and maintain the structural stability. In order to figure out the distribution of hollow Fe3O4 spheres in C matrix, energy dispersive X-ray spectrometer (EDS) mappings of
Fig. 2. (a)XRD spectrum of the specimens: spongy carbon, hollow Fe3O4 spheres, Fe3O4/C composite. (b) Raman spectra of spongy carbon and Fe3O4/C composite.
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Fig. 3. SEM image of the specimens: spongy carbon(a), hollow Fe3O4 spheres(b), Fe3O4/C composite(c), (d).
Fig. 4. TEM image of Fe3O4/C composite (a); Elemental mappings of Fe (b), O (c), C (d); HRTEM image of Fe3O4/C composite (e).
Fe, O and C elements were measured (Fig. 4). It indicated that the Fe (b) and O (c) content were higher in the region of the Fe3O4 spheres than in others. Carbon (d) formed the spongy structure and the carbon network (electron microscope) of the EDS mappings. This structure model was not only an available method to prevent the Fe3O4 nanoparticles aggregation but also benefited to improve the contact state between nanoparticles, electrolyte and transportation of Liþ. The spongy carbon would act as a buffer to accommodate the volume change of the Fe3O4 spheres, and this effect was limited as the outward expansion of the Fe3O4 nanoparticles. The lattice spacing of the composite specimen implied in the HRTEM image (Fig. 4(e)) was 0.4816 nm, which could be well-matched to (111) [37]lattice plane of cubic Fe3O4. The specific surface area and pore size distribution of the asprepared spongy carbon and Fe3O4/C composite were analyzed by
nitrogen adsorption-desorption isotherm method at 77 K. As illustrated in Fig. 5(a), the isotherm profile of spongy carbon should be categorized as a combined I/IV type with a H4 hysteresis loop at relative pressure P/Po of 0.2e0.95, indicating the presence of slitlike mesoporous, according to the IUPAC classification. However, low adsorption quantity at relatively low pressure (P/Po of 0.1e0.8) and a sharp increase of adsorption quantity at the relative pressure P/Po of 0.8e1.0 can be observed, demonstrating the existence of macropores which can be observed in the SEM picture. The Fe3O4/C composite exhibited clear hysteresis loop that is the property of the type IV adsorption isotherms with a H3 hysteresis loop at P/Po of 0.1e1.0. Compared with spongy carbon, the increase of adsorption quantity of the title composite at the relative pressure was seen to be uniformly slow, indicating the disappearance of macropores and the decrease of mesoporous [38]. The BET surface area and pore size
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Fig. 5. (a) N2 adsorption-desorption isotherms of spongy carbon and Fe3O4/C composite; (b) pore-size distribution curve of spongy carbon and Fe3O4/C composite; (c) FTIR spectra of Fe3O4/C composite; (d) TGA weight loss curve and DTA curve of Fe3O4/C composite.
of spongy carbon and Fe3O4/C composite were found to be 195.148 m3 g1 nm1 and 884.739 m3 g1 nm1,3e10 nm and 3e10 nm in Fig. 5(b), respectively. The high specific surface area and the broad pore size distribution can provide efficient transport of Liþ and rapid transport of electrolyte [39]. To further demonstrate the structure of Fe3O4/C composite, Fig. 5(c) illustrated the FTIR spectrum of as prepared title composite in the wavelength range of 4000-400 cm1. The absorption peaks at 586 cm1, and 1400 cm1 were assigned to be stretching vibration modes of Fe-O bonds. The peak at around 1098 cm1 was found in Fe3O4/C composite, which was attributed to the vibration of CeO in FeeOeC bond according to the previous research [40]. The peak around 1625 cm1 was corresponding to the C¼C bonds bending vibrations in spongy carbon. C-O stretching vibration of CO2 can be assigned for the appearance of the twin peaks whose position were located at 2339 cm1 and 2363 cm1. During the synthesized process, CO2 is produced and adsorbed on the composite surface due to the carbonization of organic groups in precursor and the existence of the circumstance in air. The band at 3240 cm1 was contributed by the stretching vibration of H-O bonds in the H2O molecule in the air. The carbon content of the Fe3O4/C composite was disclosed by TGA/DTA analysis. Shown in Fig. 5(d), as there is no peak in the DTA curve, the initial weight loss of ca. 2 wt % from 20 to 300 C should dependent on the evaporation of the weakly adsorbed water and solvent as well as the oxidation of some impurity [34]. With increasing temperature, an obvious weight loss occurs near 300 Cextending to 550 C, accompanying with one exothermic peaks at 390.0 C in the DTA curve. From 300 to 500 Cthe weight loss of ca. 33 wt %, the carbon content in the title composite can be derived to be about 33 wt % and the mass ratio of the Fe3O4 is about 67%. Fig. 6(a) showed the cycling performance of the Fe3O4/C composite, spongy carbon and hollow Fe3O4 spheres electrodes in the range of 0.01e2.5 V. The reversible capacity of the first cycle was
1445.1 mAh g1 and 1300 mAh g1 until 100 cycles at 100 mA g1(based on the total mass of the composite), and the low coulombic efficiency in the first cycle, which was commonly explained by the SEI layer formation resulted from electrolyte decomposition and irreversible trapping of Li in the Fe3O4 crystal lattice. The electrode exhibited an outstanding cycle life, while its specific capacity stabilized at 1320 mAh g1 after 12 cycles remaining at this leveled up to 100 cycles. The capacity of hollow Fe3O4 spheres was 680 mAh g1 after 60 cycles at 100 mA g1. Obviously, the capacity of spongy carbon was 1080 mAh g1 after 60 cycles higher than graphite with 3D-nanostructure pores. The reversible capacity value of composite markedly exceeded the theoretical capacity of Fe3O4 or carbon, which can be ascribed as: Reactions “6C þ Liþþ e /C6Lix” offers a theoretical capacity of 372 mAh g1 [41,42]. The spongy carbon was prepared by pyrolyzing phenolic resin, which should exist poly(p-phenylene) (PPP)based carbon [43]. In fact, the most saturated state may be C2Lix, because of sites of Li occupation supplied by benzene ring of PPPbased carbon, so that the calculated specific capacity is 3862 mAh g1 for Li metal [42,44e46]. Fig. 6(b) showed the corresponding galvanostatic discharge/ charge current rate of 0.1 A g1, where two plateaus at 0.8 and 1.63 V (vs. Li/Liþ) were attributed to the conversion reaction of discharge/charge process, in common with the cyclic voltammograms result. The cyclic voltammograms for the initial 3 cycles at a scan rate of 0.1 mV s1 between 0.01 and 3.0 V at room temperature were shown in Fig. 6(c). Respectively, where the peaks at 1.5 and 0.72 V (vs Li/Liþ) in the first cycle charge process were attributed to the conversion reaction of solid electrolyte interface (SEI) film formation, the reduction of Fe3O4 and the insertion of Liþ in the Fe3O4 lattice [34]. During following cycle with Liþ insertion process, the peaks were observed at 0.8 V and 1.45 V, which could be attributed only to the reduction of Fe3O4 to Fe (Fe3þ / Fe2þ / Fe). In the Li extraction process, two peaks at 1.63 V and 1.9 V were observed,
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Fig. 6. (a) Cycling performances of Fe3O4/C composite, spongy carbon and hollow Fe3O4 spheres. (b) Discharge/charge profile of Fe3O4/C composite. (c) CV curves of Fe3O4/C composite electrode at a scan rate of 0.1 mV s1. (d) Rate performance of Fe3O4/C composite from 0.1 A g1 to 1 A g1.
attributed to the electrochemical oxidation reaction of Fe to Fe3O4 [47, 48]. When increasing the specific current, the Fe3O4/C composite anode materials displayed highly reversible capacities of 1160, 1060, and 900 mAh g1 at 0.2, 0.5, and 1 A g1, respectively, as shown in Fig. 6(d). When the test current was set back to 0.1 A g1 after 20 cycles, capacity recovered to 1150 mAh g1 at the 25th cycle, indicating an excellent tolerance under highly specific current of title composite. 4. Conclusions The Fe3O4/C composite with hollow spheres in porous 3Dnanostructure was synthesized by facile solvothermal reaction. The hollow sphere structure of Fe3O4 was successfully prepared and dispersed on the surface of porous spongy carbon, which were helpful to form stabilize electrochemical performance, and increased cycle life. The highly reversible capacity was 1450.1 mAh g1 in the first cycle and 1300 mAh g1 until 100 cycles at 100 mA g1 in the range of 0.01e2.5 V. This synthesis approach and structure design strategy are feasible and scalable, which can be applied to other transition metal oxides and carbon containing composite for LIBs applications. Acknowledgments This work was supported by the National Natural Science Foundation of China (NO. 53171036) and the Scientific Research
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