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C nanoplate arrays on Ni foam as binder-free anode for high performance lithium-ion battery
Electrochimica Acta 182 (2015) 398–405
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
3D heterostructure Fe3O4/Ni/C nanoplate arrays on Ni foam as binder-free anode for high performance lithium-ion battery Zhaolin Lia , Hailei Zhaoa,b,* , Jie Wanga , Pengpeng Lva , Zijia Zhanga , Zhipeng Zengc , Qing Xiaa,* a b c
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Lab of New Energy Materials and Technologies, Beijing 100083, China Department of Chemical and Biomolecular Engineering, The University of Akron, OH 44325, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 July 2015 Received in revised form 11 September 2015 Accepted 15 September 2015 Available online 25 September 2015
An open-up network structure assembled by interconnected 3D heterostructure Fe3O4/Ni/C nanoplate arrays on Ni foam is successfully synthesized via a facile hydrothermal method with subsequent CVD heat treatment. When used as a binder free anode material for lithium-ion battery (LIBs), it shows quite a favorable electrochemical performance with high reversible capacity and good rate capability. A high capacity of 832.5 mAh g1 is achieved at 0.3C and a specific capacity of 279 mAh g1 can still be delivered at current density of 4.5C, corresponding to 34% of the capacity at 0.3C. The self-supporting nanoplates are intercrossed and interconnected with robust adhesion on Ni foam, preventing the active material from peeling off during the electrochemical reactions. Ni foam substrate, uniform carbon layer on the nanoplate surface and in-situ formed Ni nanocrystals together play important roles in effectively building a fast 3D electron transport network for electrode reactions. The excellent electrochemical performance makes this composite a promising candidate as anode material for high energy density LIBs. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: 3D heterostructure binder free ferroferric oxide nanoplate arrays anode electrode materials lithium-ion batteries
1. Introduction The electrical energy storage technique will become far more important in the coming decades. The electrochemical battery is one of the most promising energy storage devices considering its portability, high conversion efficiency and no gaseous exhaust. As representative, LIBs have attracted much attention due to the high energy density and long cycle life [1–4]. Besides the successful applications in portable electronic devices including cellphones, laptop computers and digital cameras, it is expected to power electric vehicles (EVs) and hybrid electric vehicles (HEVs) and to store the sustainable energy, e.g. solar, wind, wave and smart grid [5–8]. The latter usage is especially attractive because it addresses the global warming issues. However, the state of the art of LIBs is not yet at such technique level to meet the user unconditioned demands especially in the field of EVs and HEVs, and thus further development of high performance energy storage technique is becoming extremely urgent.
* Corresponding author at: School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100,083, China. Tel.: +86 10 82376837; Fax : +86 10 82 376 837. E-mail address: [email protected] (H. Zhao). http://dx.doi.org/10.1016/j.electacta.2015.09.086 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
Current LIBs commonly use graphite as anode material, which has a limited theoretical capacity of 372 mAh g1 (LiC6) and shows some inherent disadvantages, including poor rate-capability, solvent co-intercalation and unsatisfactory safety feature [9–11]. These problems restrict the design and implementation of high performance lithium-ion batteries. Therefore, currently, intensive efforts are devoted in progress to explore new anode materials with high specific capacity and good safety performance, such as Sn-based [12,13], Si-based [14,15] and transition-metal oxide materials (TMOs, M = Ni, Cu, Co, Fe or Mn) [16–20]. Among these alternatives, TMOs have attracted many interests in virtue of their high theoretical capacity (500–1000 mAh g1) based on a conversion reaction mechanism (2yLi+ + 2ye + MxOy $ xM + yLi2O) [21] instead of an intercalation reaction. In particular, Fe3O4 has been considered as a promising anode material because of its high capacity (926 mAh g1), high electronic conductivity (among the TMOs), nature abundance (low cost), and environmental benignity [19,21]. However, similar to other conversion-based anode materials, the large volume variation that inherently accompanies the lithium uptake and release processes can cause electrode pulverization [18] and further electrical contact loss between active materials and current collector, leading to poor cycling stability of the electrodes. To overcome this issue, the most common strategies employed are to
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downsize particle dimension to the nanoscale level (nanoparticles [22], nanospindles [23], nanobelts [24], nanotubes [25], nanowires [26], nanocubes [27]) and to composite with electrically conductive and electrochemically less active components, such as carbon nanotube, carbon nanofibers or graphene [28]. Lang and Xu [24] prepared porous Fe3O4/C microbelts using simple electrospinning method with subsequent carbonization, which exhibited enhanced rate capability (0.2C: 710 mAh g1; 6C: 189 mAh g1). The carbon coated Fe3O4/CNT grape-cluster nanostructure was reported by Liu et al. [25], which showed a high specific capacity and good rate capability (60 mA g1: 975 mAh g1; 1200 mA g1: 282 mAh g1). Li et al. [26] developed hierarchical nanostructure Fe3O4@C mesoporous nanowires with excellent electrochemical performance (200 mA g1: 990 mAh g1; 1 A g1: 550 mAh g1). Gao et al. [27] reported Fe3O4 nanocubes, which were prepared by an amino acid assisted solvothermal process and presented good rate capability (0.2C: 695.1 mAh g1; 5C: 51 mA g1). The better strain accommodation and the shorter lithium ion diffusion/ electron transport lengths of the nanosized materials ensure the good electrochemical performance of nano-sized Fe3O4 electrode. The high electronic conductivity of conductive substance allows fast and homogeneous electrode reaction kinetics. In addition to aforementioned strategies, developing new anode architectures is also an important approach to achieve excellent electrochemical performance of electrodes. Currently, construction of well-ordered nanostructures (NiO flakes [16], Co3O4 nano-sheets [29], Fe2O3 nanorods [30], Fe3O4 NPs [31]) directly grown on a current collecting substrate has effectively enhanced rate capability and cycling stability of LIBs. Such selfsupported nanostructured TMOs with lots of open spaces in the electrode ensure the large electrolyte/electrode contact area and the well volume expansion release during cycling. Notably, the designed excellent electric contact between active materials and current collecting substrate offers numerous fast electron transport pathways, making the impressive high-rate performance available owing to the reduction of Ohmic electrode polarization. Recently, Varghese et al. [32] prepared vertically aligned NiO nanowalls on nickel foils using a plasma assisted oxidation method, which showed a capacity of 638 mAh g1 after 85 cycles at 1.25C. Fu et al. [18] synthesized self-supporting Co3O4 with lemongrass-like morphology on Ni foam and found that, the capacity maintained 981 mAh g1 after 100 cycles at a rate of 0.5C. Li et al. [31] reported the carbon-wrapped Fe3O4 nanoparticle film on nickel foam which delivered a stable capacity of 600 mAh g1 for 100 cycles at 0.2C. In this work, we report a strategy for the synthesis of a novel binder-free electrode, which contains 3D heterostructure Fe3O4/ Ni/C nanoplate arrays growing on nickel foam. The electrode can be used directly as anode for lithium-ion batteries without any addition of the polymer binder and carbon conductive agent. The carbon coating process on the synthesized precursor yields insitu lots of tiny well-crystallized Ni nanocrystals inside the Fe3O4 nanoplates. The particle surface carbon layer, underneath Ni foam and particle interior Ni nanoparticles can effectively build a fast 3D electron transport network to facilitate the electrode reaction and homogenize the current density on active Fe3O4 particles. The latter can ensure a uniform volume change and thus less stress of active Fe3O4 particles, therefore a stable electrode structure and excellent cycling performance can be expected. The practical electrochemical tests reveal that the as-prepared 3D heterostructure Fe3O4/Ni/C nanoplate arrays exhibit outstanding performance with high capacity and rate-capability. It delivers outstanding cycling performance as high as 832.5 mAh g1 (after cycling at 0.3C for 50 cycles) and good rate capacity up to 279 mAh g1 at 4.5C when evaluated as an anode material for LIBs.
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2. Experimental 2.1. Fabrication procedure 3D heterostructure Fe3O4/Ni/C nanostructured electrode was prepared by a simple hydrothermal process combined with a subsequent chemical vapor deposition (CVD) heat treatment. In a typical synthesis, the nickel foam (5 cm 3 cm) with thickness of 0.35 mm was firstly washed with distilled water and ethanol for several times to remove the surface impurities. Fe(NO3)3 9H2O (5 mmol), NH4F (5 mmol), and CO(NH2)2 (25 mmol) were dissolved in distilled water (70 ml) to form a transparent solution. The obtained solution was transferred into a 100 mL Teflon-lined stainless steel autoclave. After immersing one piece of clean nickel foam into the solution, the autoclave was sealed and heated at 120 C for 5 h. After the autoclave cooled down to room temperature naturally, the precursor was collected, rinsed with distilled water and absolute ethanol, and dried in air at 50 C for 10 h. In order to investigate the growth mechanism of the Fe3O4/Ni/ C nanoplate, the autoclave was kept at 120 C for different time (1 h, 10 h and 20 h). The hydrothermally synthesized nanoplate array precursor was placed in a quartz tube furnace for carbon CVD treatment at 600 C for 3 h. During the CVD process, toluene was used as the carbon source, which was carried by argon gas with the flow rate of 0.1 L min1. After the heat treatment, Fe3O4/Ni/C nanoplate arrays on Ni foam electrode were obtained. For simplicity, the prepared electrode was named as FNC600. The Fe3O4 and carbon components were taken as active materials and the loading of active material on electrode is 2.96 mg cm2. The detailed calculation about active material percentage in Ni foam supported electrode is supplied in supporting information. 2.2. Structure and morphology characterization The phase structure of the products was identified by X-ray diffraction (XRD, Rigaku, D/max-A, Cu Ka, l = 1.5406 Å) and Raman spectroscope (LabRAM HR Evolution, excited by 523 nm laser). The morphologies of the synthesized samples were examined using field-emission scanning electron microscopy (FESEM, Zeiss, SUPRA55). The lattice structure was characterized using highresolution transmission electron microscopy (HRTEM, JEM-2010). 2.3. Electrochemical characterization The prepared Fe3O4/Ni/C nanoplate arrays on Ni foam were used directly as electrode without any additional of binder and conductive agent. Before the coin-type 2032 cells assembly, the Ni foam with Fe3O4/Ni/C plates (disc electrode with diameter of 8 mm) was further dried at 120 C for 24 h in vacuum environment. A Celgard 2400 microporous polypropylene was used as separator, and pure lithium foil as counter electrodes. The non-aqueous electrolyte used was 1 M LiPF6 dissolved in ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) (1:1:1, in vol%). The cells were tested in the voltage range between 0.02 and 3 V with a multichannel battery test system (LAND CT2001A) at room temperature (25 C). The electrochemical impedance spectra (EIS) measurements were carried out in Solartron 1260 Frequency Response Analyzer combined with a Solartron 1287 potentiate in the frequency region from 105 to 0.01 Hz with a signal amplitude 5 mV. 3. Results and discussion The synthesized samples were examined by XRD to identify the crystal structures and phase components. As shown in Fig. 1,
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Fig. 1. (a) XRD patterns of the precursor after hydrothermal reaction and (b) the prepared Fe3O4/Ni/C nanoplate arrays on Ni foam after CVD treatment.
obviously, the strong diffraction peaks of the Ni phase (JCPDS 652865) were detected in both of the precursor and the as-prepared product on account of Ni foam substrate. Fig. 1a shows the XRD pattern of the precursor in which the diffraction peaks completely match with FexNiy(OH)2(CO3)z phase as reported in literature [33]. After CVD treatment, the diffraction peaks (Fig. 1b) are in good accordance with those of Fe3O4 phase (JCPDS 88-0315), indicating the decomposition and carbothermal reduction of precursors taking place during the CVD heat-treatment. Notably, no signals related to carbon were detected because of its amorphous feature and low content. Amorphous carbon in Fe3O4/Ni/C nanoplate arrays on Ni foam was detected by Raman spectroscopy (Fig. 2). In the range of 1050– 1800 cm1, the Raman spectrum exhibits typical characteristics of amorphous carbon: the two characteristic peaks at 1349 cm1 and 1606 cm1 are in good agreement with the typical Raman mode for the D band and G band of Fe3O4/Ni/C nanoplate arrays [17]. The peak intensity ratio between the D and G bands (ID/IG) is used as an index to estimate the degree of crystallization of carbon materials. Here, the ID/IG ratio of 0.83 indicates the low crystallinity of amorphous carbon. The nickel foam with high conductivity was used as the supporting backbone for the growth of the 3D heterostructure Fe3O4/Ni/C nanoplates for electrochemical energy storage. Fig. 3 shows the FESEM images of the Ni foam before and after hydrothermal treatment for different duration time (1, 5, 10 and 20 h). The as-received Ni foam (Fig. 3a) presents a smooth surface with clear Ni grain boundaries. After treated for 1 h, sample shows
Fig. 2. Raman spectra of Fe3O4/Ni/C nanoplate arrays with an excitation laser wavelength of 532 nm.
a rough surface with many fine particles ranging from 150 to 200 nm (Fig. 3b). With prolonged hydrothermal treatment time, the nanoparticles started to grow to nanoplates with lots of space among them (sample-5 h, Fig. 3c). Longer hydrothermal treating time results in larger thickness and higher density of nanoplates. Although the sample-10 h (Fig. 3d) and sample-20 h (Fig. 3e) remain the plate-like structure, the thickness of the nanoplates tends to be much larger, and the structure becomes dense without void space for sample-20 h, which will prevent the electrolyte solution from having sufficient contact with the particle surface and thus retard the electrode reaction kinetics. Based on the morphology observations of the samples with different hydrothermal treatment times, we present a four-step mechanism concerning the growth of 3D heterostructure Fe3O4/ Ni/C nanoplate arrays on Ni foam substrates, as schematically illustrated in Fig. 4a. Step 1: at the beginning, ferric ions are fully coordinated with F to form FeFx(x3) in the reaction solution, as shown in Eq. (1). As the temperature increases in the autoclave, the hydrolysis process of urea takes place around 70 C [34]. As a result, lots of CO32 and NH4+ anions are formed gradually (Eq. (2)). Step 2: with the increase of the concentration of the ions (CO32 and NH4+), ferric ions are released slowly into the reaction system and gradually participate in the formation of FexNiy(OH)2(CO3)zaH2O nuclei on Ni foam substrate (Eq. (3)). Instead of the rapid nucleus formation, the slow-down process is greatly helpful for the substantial contact between nucleus and the smooth substrate. Step 3: on account of the proceeding reaction, the growing nuclei start to assemble along the specific orientation preferentially. As a result, the yellow-brown nanoplate array precursors form and exhibit an open-up network structure. Step 4: the precursor is subjected to carbon CVD processing, in which the complex compound FexNiy(OH)2(CO3)zaH2O is decomposed and reduced to Fe3O4 nanoplates and Ni nanoparticles (Eq. (4)). Fe3+ + nF ! [FeFn](n3) (1)
4H2NCONH2 + 2H2O ! 2NH4+ + CO32 (2)
x[FeFn](n3) + yNi + zCO32 + (xn-2)NH4+ + (a+2)H2O ! FexNiy(OH)2(CO3)zaH2O + (xn-2)NH4F + 2HF (3)
FexNiy(OH)2(CO3)zaH2O + (x/12+y/2)C ! (x/3)Fe3O4 + yNi + (x/12 +y/2+z)CO2 + (a+1)H2O (4) The coated carbon layer on the surface of nanoplates together with the in-situ formed Ni nanoparticles provide a fast 3D electron transport network to facilitate the electrode reaction process. Moreover, the good electronic transport pathway can homogenize the current density of the electrode, and thus make the volume change of active particles upon lithium uptake/removal much uniform, preventing the cracking and pulverization of the active particles. Therefore, excellent electrochemical performance, in terms of cycling stability and rate-capability, can be expected. Fig. 4b shows an optical image including installed coin-type cell, Ni foam, precursor (yellow-brown) and Fe3O4/Ni/C nanoplates (black) on Ni foam. The black color should come from Fe3O4 and carbon. Combined with XRD results (Fig. 1), it is reasonable to state that Fe3O4/Ni/C nanoplates were formed after CVD process. The morphologies of the precursors and the as-prepared 3D heterostructure Fe3O4/Ni/C nanoplate arrays on Ni foam were examined by FESEM. Fig. 5a shows a representative lowmagnification top view of the nanoplate precursors. The Ni foam surface is covered with uniformly well-established nanoplates.
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Fig. 3. FESEM images of Ni foam before and after different hydrothermal reaction durations: (a) 0 h (pure Ni foam), (b) 1 h, (c) 5 h, (d) 10 h and (e) 20 h.
Fig. 4. (a) Schematic mechanism for the direct-growth process of Fe3O4/Ni/C nanoplate arrays on Ni foam; (b) optical image of coin-type battery, Ni foam substrate, precursor grown on Ni foam and Fe3O4/Ni/C nanoplates on Ni foam.
High-magnification images (Fig. 5b and c) reveal that the nanoplate precursors with a smooth surface are interconnected to each other, resulting in the formation of extended open-up network structure. The thickness of precursor nanoplates is ca.
150 nm (Fig. 5c). Fig. 5d and e show FESEM images of the asprepared 3D heterostructure Fe3O4/Ni/C nanoplate arrays. After CVD process, the well distributed and open-up Fe3O4/Ni/C nanoplates are still remained. Obviously, the structure becomes even
Fig. 5. (a)-(c) FESEM images of nanoplate precursors; (d)-(f) FESEM images of Fe3O4/Ni/C nanoplates after CVD treatment.
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open with more void space after heat-treatment, suggesting the occurrence of decomposition and conversion reactions, which is mainly attributed to the decomposition of precursors (CO32 and OH groups) and the reduction of Ni. Compared with nanoplate precursors (Fig. 5c), the heat-treated nanoplates (Fig. 5f) show a rough surface, which is mostly likely resulted from the in-situ generation of Ni nanocrystals. To characterize the internal structure of Fe3O4/Ni/C nanoplate arrays more precisely, TEM and HR-TEM inspections were conducted. The synthesized product exhibits plate-like morphology with some nanoparticles inside, which should be related to metallic Ni as shown in Fig. 6a. In addition, the individual nanoplate presents a core-shell structure with a uniform and continuous amorphous carbon layer on the plate surface. Such core-shell structure can be further confirmed in Fig. 6b, and the thickness of the carbon layer is 3 nm. Furthermore, a lattice spacing of 0.203 nm was observed in both Fig. 6b and c, in a good agreement with the (111) planes of metallic Ni. Besides, the lattice fringes with d-spacing of 0.483, 0.253 and 0.241 nm (Fig. 6b and c) are clearly distinguished, corresponding to the (111), (311) and (222) planes of Fe3O4, respectively. Therefore, from the structural analyses, it is certain that the metallic Ni nanocrystals are distributed in the Fe3O4 matrix. The Ni foam substrate, the carbon layer and the Ni nanocrystals can effectively build a 3D electron transport highway, and the open space between each nanoplate ensures homogenous electrolyte flow. Such unique structure feature makes the better electrochemical performance be expected.
To determine the effectiveness of this unique structure in improving the electrochemical performance, a series of electrochemical evaluations on the Fe3O4/Ni/C electrodes were carried out based on the assembled coin cell. Fig. 7a shows the dischargecharge voltage profiles at the current density of 0.3C (1C = 924 mAh g1) with the voltage windows between 0.02 and 3.0 V. For clarity, the 1st, 2nd, 10th, 30th, and 50th cycles are shown. The voltage profile of the first discharge reaction mainly comprises three regions, the steep voltage drop from the open circuit potential to 0.75 V corresponds to Li+ insertion into the Fe3O4 lattice (Eq. (5)) [35] and the subsequent long discharge plateaus at about 0.75 V is associated with conversion reaction of LixFe3O4 to Fe0 (Eq. (6)) [36], which is then followed by a slope till 0.02 V. The low potential slope is usually considered to relate to the irreversible formation of a gellike film on the surface of Fe3O4 particles [37]. Apparently, in the subsequent cycles, redox reaction (Eq. (7)) of lithium with Fe3O4 presents highly reversible. The cathodic lithium insertion mainly occurs at 1.1 V, and the anodic lithium extraction occurs at 1.75 V which are in consistent with that described in literature for Fe3O4 anodes [36]. Fe3O4 + xLi+ + xe ! LixFe3O4 (5)
LixFe3O4 + (8-x)Li+ + (8-x) e ! 4Li2O + 3Fe (6)
8Li+ + 8e + Fe3O4 $ 4Li2O + 3Fe (7)
Fig. 6. (a) TEM and (b, c) HR-TEM images of Fe3O4/Ni/C nanoplate arrays.
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Ni/C electrode as well as a more adequate space to accommodate the huge volume variation related to lithium insertion and extraction. These advantages are beneficial to accelerating the electrochemical reactions and reducing electrochemical polarization of electrode. Electrochemical impedance spectral (EIS) measurements on electrode materials can give important information about electrode reaction kinetics [38,39]. Fig. 8a shows the Nyquist plots of the Fe3O4/Ni/C electrode after 50 cycles at 50% discharge state. The Nyquist plots present a compressed two semicircles in the high frequency region and an inclined line at low frequency, corresponding to the resistances from surface film, charge transfer and lithium ion diffusion, respectively [40]. The equivalent circuit is presented in inset of Fig. 8a, with which the parameters of each process are simulated and listed in Table 1. The charge transfer resistance (Rct) is higher than film resistance (Rf), suggesting the rate limiting feature of charge transfer process for the electrode
Fig. 7. Electrochemical performance of the Fe3O4/Ni/C anode: (a) Galvanostatic charge/discharge profiles of Fe3O4/Ni/C electrode at 0.3C; (b) cycling performance of Fe3O4/Ni/C anode at a current density of 0.3C; (c) rate performance of Fe3O4/Ni/C anode at various current densities in the potential windows of 0.02-3.0 V versus Li/ Li+.
The cycling performance of the prepared Fe3O4/Ni/C electrode was examined at a constant current rate of 0.3C. Fig. 7b presents the results of discharge-charge galvanostatic cycles of the hybrid composite electrode in the voltage of 0.02 to 3.0 V. The discharge capacity of the Fe3O4/Ni/C electrode in the first cycle is as high as 1184.5 mAh g1 and the corresponding charge capacity is 817 mAh g1, giving an initial coulombic efficiency of ca. 70%. After that, the coulombic efficiency keeps above 99% in the following dischargecharge cycles. The subsequent charge and discharge cycles deliver a very stable capacity, being of 832.5 and 848.7 mAh g1 after 50 cycles. The rate capability of the Fe3O4/Ni/C anode was evaluated by cycling the electrode in a step mode at different current densities ranging from 0.15 to 4.5C. As presented in Fig. 7c, at the current densities of 0.15, 0.3, 0.45, 0.75, 1.5, 3.0C, the electrode delivers capacities of 876, 819, 765, 691, 568, 412 mAh g1, respectively. It is worth noting that a specific capacity of 279 mAh g1 can still be delivered by Fe3O4/Ni/C composite electrode at the current density as high as 4.5C. When the current density finally switched back to its initial value of 0.15C, a charge capacity of 691 mAh g1 is still available, indicating the excellent rate-capability and the good structural stability of the synthesized electrode. These electrochemical results clearly indicate the synthesized Fe3O4/Ni/C composite on Ni foam with such unique open network nanostructure can be a promising electrode for high rate performance LIBs. The outstanding electrochemical performance of the Fe3O4/Ni/ C electrode can be attributed to the following aspects: (1) The nanoplates with thickness of about 150 nm can provide a shorter diffusion distance for both lithium ions and electrons. (2) The insitu grown Fe3O4/Ni/C nanoplates have strong attachment with the current collector, and thus effectively preventing the active material from peeling off. (3) The Ni foam substrate, the uniform carbon layer on the particle surface and the in-situ formed nanosized Ni particles can build a conductive and mechanical 3D framework to improve the reaction kinetics of the synthesized electrode. (4) The open space between nanoplate arrays provide an easy diffusion of the electrolyte into the inner region of the Fe3O4/
Fig. 8. (a) Electrochemical impedance spectra of Fe3O4/Ni/C electrode after 50 cycles at 50% discharge state; (b) relationship between real impedance and reciprocal square root of the lower angular frequencies.
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Table 1 Kinetic parameters of Fe3O4/Ni/C electrodes. Electrode
Re/V
Rf/V
Cf/F
Rct/V
Cdl/F
sw/V s0.5
DLi/cm2 s1
FNC600
2.61
7.12
4.13 107
15.88
1.0 105
12.77
6.85 1012
reaction. To investigate the behavior of Li ion in electrode materials, the relationship between the real impedance (Zre) and the reciprocal square root of the lower angular frequencies (v1/2) is plotted in Fig. 8b. The straight lines are attributed to the diffusion of the lithium ions into the bulk of the electrode material, or the socalled Warburg diffusion. From the Fig. 8b, Warburg impedance coefficient (sw) can be derived from the slope of straight line according to Eq. (8), and the diffusion co-efficient values (DLi) of the lithium ion diffusion into the bulk electrode materials can be calculated using Eq. (9) [40]. Zre = Re + Rf + Rct + sw v1/2 (8) D = 0.5(RT/AF2swC)2 (9) where v is the frequency, R is the gas constant, T is the absolute temperature, A is the surface area of the cathode, F is the Faraday constant, C is the molar concentration of lithium ion [40]. The Li ion diffusion coefficient in the prepared Fe3O4/Ni/C electrode is 6.85 1012 cm2 s1. The EIS results reveal that lithium ion diffusion and charge transfer process are limiting steps for Fe3O4/Ni/C electrode reaction. Therefore, nanostructuring is imperative to the improvement of the Fe3O4-based electrode reaction kinetics because it can shorten the lithium ion diffusion distance and provide more surface area for charge transfer process. Although the prepared Fe3O4/Ni/C electrode shows high specific capacity and excellent rate-capability, its energy density is not satisfied due to the coarse skeleton of Ni foam and the high density of metal Ni. Further optimization of preparation parameters, such as utilization of customized Ni foam with slim skeleton and increase of loading of the active materials on Ni foam, can provide opportunity to increase the energy density of Fe3O4/Ni/C electrode. 5. Conclusion In summary, we have successfully fabricated Fe3O4/Ni/C nanoplate arrays on Ni foam substrate using a novel hydrothermal method with subsequent CVD heat treatment, which are used directly as a binder-free anode for LIBs. The Ni foam-supported hybrid composite electrode has good mechanical properties and higher electrical conductivity as anode, which performs excellent lithium storage characteristics with high capacity, stable cycling performance, and excellent rate capability. It shows a high discharge capacity of 832.5 mAh g1 at 0.3C after 50 cycles. Even at the current density of 4.5C, a capacity of 279 mAh g1 can be reached. The prepared Fe3O4/Ni/C nanoplate arrays with open-up network structure can be an ideal candidate for high-capacity electrode of rechargeable LIBs. Acknowledgements This work was financially supported by National Basic Research Program of China (2013CB934003), National High Technology Research and Development Program of China (2013AA050902), National Nature Science Foundation of China (21273019), Guangdong Industry-Academy-Research Alliance (2013C2FC0015) and
Program of Introducing Talents of Discipline to Universities (B14003). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2015.09.086. References [1] D. Li, L. Ding, H. Chen, S. Wang, Z. Li, M. Zhu, H. Wang, Novel nitrogen-rich porous carbon spheres as a high-performance anode material for lithium-ion batteries, J. Mater. Chem. A 2 (16) (2014) 617. [2] D. Kong, J. Luo, Y. Wang, W. Ren, T. Yu, Y. Luo, Y. Yang, C. Cheng, Threedimensional Co3O4@MnO2 hierarchical nanoneedle arrays: morphology control and electrochemical energy storage, Adv. Funct. Mater. 24 (2014) 3815–3826. [3] Y. Hu, R. Demir-Cakan, M.M. Titirici, J.O. Muller, R. Schlogl, M. Antonietti, J. Maier, Superior storage performance of a Si@SiOx/C nanocomposite as anode material for lithium-ion batteries, Angew. Chem. Int. Ed. 47 (2008) 1645–1649. [4] T. Moritaz, N. Takami, Nano Si Cluster-SiOx-C composite material as highcapacity anode material for rechargeable lithium batteries, J. Electrochem. Soc. 153 (2006) A425–A430. [5] S. Choi, D.S. Jung, J.W. Choi, Scalable fracture-free SiO-C glass coating for robust silicon nanoparticle anodes in lithium secondary batteries, Nano Lett. 14 (2014) 7120–7125. [6] W.W. Lee, J.M. Lee, Novel synthesis of high performance anode materials for lithium-ion batteries (LIBs), J. Mater. Chem. A 2 (2014) 1589–1626. [7] B. Scrosati, J. Garche, Lithium batteries: status, prospects and future, J. Power Sources 195 (2010) 2419–2430. [8] P.J. Hall, E.J. Bain, Energy-storage technologies and electricity generation, Energy Policy 36 (2008) 4352–4355. [9] X. Feng, J. Yang, Q. Lu, J. Wang, Y. Nuli, Facile approach to SiOx/Si/C composite anode material from bulk SiO for lithium ion batteries, Phys. Chem. Chem. Phys. 15 (2013) 14420. [10] J. Tu, Y. Yuan, P. Zhan, H. Jiao, X. Wang, H. Zhu, S. Jiao, Straight forward approach toward SiO2 nanospheres and their superior lithium storage performance, J. Phys. Chem. C 118 (2014) 7357–7362. [11] H. Guo, R. Mao, X. Yang, J. Chen, Hollow nanotubular SiOx templated by cellulose fibers for lithium ion batteries, Electrochim. Acta 74 (2012) 271–274. [12] B. Qu, C. Ma, G. Ji, C. Xu, J. Xu, Y.S. Meng, T. Wang, J.Y. Lee, Layered SnS2-reduced graphene oxide composite?a high-capacity, high-rate, and long-cycle life sodium-ion battery anode material, Adv. Mater. 26 (2014) 3854–3859. [13] X. Zhou, Z. Dai, S. Liu, J. Bao, Y. Guo, Ultra-uniform SnOx/carbon nanohybrids toward advanced lithium-ion battery anodes, Adv. Mater. 26 (2014) 3943– 3949. [14] Y. Ren, M. Li, Si-SiOx-cristobalite/graphite composite as anode for li-ion batteries, Electrochim. Acta 142 (2014) 11–17. [15] X. Huang, J. Yang, S. Mao, J. Chang, P.B. Hallac, C.R. Fell, B. Metz, J. Jiang, P.T. Hurley, J. Chen, Controllable synthesis of hollow Si anode for long-cycle life lithium-ion batteries, Adv. Mater. 26 (2014) 4326–4332. [16] S. Ni, T. Li, X. Lv, X. Yang, L. Zhang, Designed constitution of NiO/Ni nanostructured electrode for high performance lithium ion battery, Electrochim. Acta 91 (2013) 267–274. [17] J. Song, L. Xu, C. Zhou, R. Xing, Q. Dai, D. Liu, H. Song, Synthesis of graphene oxide based CuO nanoparticles composite electrode for highly enhanced nonenzymatic glucose detection, ACS Appl. Mater. Interfaces 5 (12) (2013) 928–12934. [18] L. Zhan, S. Wang, L. Ding, Z. Li, H. Wang, Grass-like Co3O4 nanowire arrays anode with high rate capability and excellent cycling stability for lithium-ion batteries, Electrochim. Acta 135 (2014) 35–41. [19] L. Zhong, J. Hu, H. Liang, A. Cao, W. Song, L. Wan, Self-assembled 3D flowerlike iron oxide nanostructures and their application in water treatment, Adv. Mater. 18 (2006) 2426–2431. [20] A. Debart, A.J. Paterson, J. Bao, P.G. Bruce, a-MnO2 nanowires: a catalyst for the O2 electrode in rechargeable lithium batteries, Angew. Chem. 120 (2008) 4597–4600. [21] B.H. Park, S.B. Park, S.M. Jeong, C.S. Seo, S.W. Park, Electrolytic reduction of spent oxide fuel in a molten LiCl-Li2O system, J. Radioanal. Nucl. Chem. 270 (2006) 575–583. [22] L. Ji, Z. Tan, T.R. Kuykendall, S. Aloni, Fe3O4 nanoparticle-integrated graphene sheets for high-performance half and full lithium ion cells, Phys. Chem. Chem. Phys. 13 (2011) 7170–7177.
Z. Li et al. / Electrochimica Acta 182 (2015) 398–405 [23] W. Zhang, X. Wu, J. Hu, Y. Guo, L. Wan, Carbon coated Fe3O4 nanospindles as a superior anode material for lithium-ion batteries, Adv. Funct. Mater. 18 (2008) 3941–3946. [24] L. Lang, Z. Xu, In situ synthesis of porous Fe3O4/C microbelts and their enhanced electrochemical performance for lithium-ion batteries, ACS Appl. Mater. Interfaces 5 (2013) 1698–1703. [25] J. Liu, J. Ni, Y. Zhao, H. Wang, L. Gao, Grapecluster-like Fe3O4@C/CNT nanostructures with stable Li-storage capability, J. Mater. Chem. A 1 (2013) 12879. [26] H. Li, R. Xu, Y. Wang, B. Qian, H. Wang, L. Chen, H. Jiang, Y. Yang, Y. Xu, In situ synthesis of hierarchical mesoporous Fe3O4@C nanowires derived from coordination polymers for high-performance lithium-ion batteries, RSC Adv. 4 (2014) 51960. [27] H. Cao, R. Liang, D. Qian, J. Shao, M. Qu, L-Serine-assisted synthesis of superparamagnetic Fe3O4 nanocubes for lithuium ion batteries, J. Phys. Chem. C 115 (2011) 24688–24695. [28] W. Zhou, J. Zhu, C. Cheng, J. Liu, H. Yang, C. Cong, C. Guan, X. Jia, H. Fan, Q. Yan, C. Li, T. Yu, A general strategy toward graphene@metal oxide core–shell nanostructures for high-performance lithium storage, Energy Environ. Sci. 4 (2011) 4954. [29] C. Yuan, L. Yang, L. Hou, L. Shen, X. Zhang, X. Lou, Growth of ultrathin mesoporous Co3O4 nanosheet arrays on Ni foam for high-performance electrochemical capacitors, Energy Environ. Sci. 5 (2012) 7883. [30] W. Yang, Z. Gao, J. Wang, B. Wang, L. Liu, Hydrothermal synthesis of reduced graphene sheets/Fe2O3 nanorods composites and their enhanced electrochemical performance for supercapacitors, Solid State Sci. 20 (2013) 46–53. [31] D. Li, X. Li, S. Wang, Y. Zheng, L. Qiao, D. He, Carbon-wrapped Fe3O4 nanoparticle films grown on nickel foam as binder-free anodes for high-rate and long-life lithium storage, ACS Appl. Mater. Interfaces 6 (2014) 648–654.
405
[32] B. Varghese, M.V. Reddy, Y. Zhu, C. Lit, T. Hoong, G.V. Rao, B.V.R. Chowdari, A.T. S. Wee, C.T. Lim, C. Sow, Fabrication of NiO nanowall electrodes for high performance lithium ion battery, Chem. Mater. 20 (2008) 3360–3367. [33] E. Uzunova, D. Klissurski, S. Kassabov, Nickel-iron hydroxide carbonate precursors in the synthesis of high-dispersity oxides, J. Mater. Chem. 4 (1994) 153–159. [34] J. Jiang, J.P. Liu, X.T. Huang, Y.Y. Li, R.M. Ding, X.X. Ji, Y.Y. Hu, Q.B. Chi, Z.H. Zhu, General synthesis of large-scale arrays of one-dimensional nanostructured Co3O4 directly on heterogeneous substrates, Cryst. Growth Des. 10 (2010) 71. [35] B. Yang, J. Zhao, J. Chen, M. He, S. Xu, Controllable synthesis of graphene nanoscroll-wrapped Fe3O4 nanoparticles and their lithium-ion battery performance, RSC Adv. 5 (2015) 57906. [36] Y. Wu, J. Liu, C. Wu, Q. Zhuang, H. Chen, H. Zhang, X. Zhang, Magnetite/carbon nanotubes nanocomposite: facile hydrothermal synthesis and enhanced cycling performance and high-rate capability as anode material for lithiumion batteries, Ionics 21 (2015) 635–643. [37] K. Sun, H. Zhao, J. Yao, S. Zhang, J. Xu, Foam carbon loading Fe3O4 nanoparticles for superior lithium-ion batteries anode material, Ionics 21 (2015) 1901–1908. [38] A.Y. Shenouda, H.K. Liu, Preparation, characterization, and electrochemical performance of Li2CuSnO4 and Li2CuSnSiO6 electrodes for lithium batteries, Journal of The Electrochemical Society 157 (2010) A1183–A1187. [39] K. Xie, Z. Lu, H. Huang, W. Lu, Y. Lai, J. Li, L. Zhou, Y. Liu, Iron supported C@Fe3O4 nanotube array: a new type of 3D anode with low-cost for high performance lithium-ion batteries, J. Mater. Chem. 22 (2012) 5560. [40] L. Li, T. Wang, L. Zhang, Z. Su, C. Wang, R. Wang, Selected-control synthesis of monodisperse Fe3O4@C core–shell spheres, chains, and rings as highperformance anode materials for lithium-ion batteries, Chem. Eur. J 18 (2012) 11417–11422.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
3D heterostructure Fe3O4/Ni/C nanoplate arrays on Ni foam as binder-free anode for high performance lithium-ion battery Zhaolin Lia , Hailei Zhaoa,b,* , Jie Wanga , Pengpeng Lva , Zijia Zhanga , Zhipeng Zengc , Qing Xiaa,* a b c
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Lab of New Energy Materials and Technologies, Beijing 100083, China Department of Chemical and Biomolecular Engineering, The University of Akron, OH 44325, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 July 2015 Received in revised form 11 September 2015 Accepted 15 September 2015 Available online 25 September 2015
An open-up network structure assembled by interconnected 3D heterostructure Fe3O4/Ni/C nanoplate arrays on Ni foam is successfully synthesized via a facile hydrothermal method with subsequent CVD heat treatment. When used as a binder free anode material for lithium-ion battery (LIBs), it shows quite a favorable electrochemical performance with high reversible capacity and good rate capability. A high capacity of 832.5 mAh g1 is achieved at 0.3C and a specific capacity of 279 mAh g1 can still be delivered at current density of 4.5C, corresponding to 34% of the capacity at 0.3C. The self-supporting nanoplates are intercrossed and interconnected with robust adhesion on Ni foam, preventing the active material from peeling off during the electrochemical reactions. Ni foam substrate, uniform carbon layer on the nanoplate surface and in-situ formed Ni nanocrystals together play important roles in effectively building a fast 3D electron transport network for electrode reactions. The excellent electrochemical performance makes this composite a promising candidate as anode material for high energy density LIBs. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: 3D heterostructure binder free ferroferric oxide nanoplate arrays anode electrode materials lithium-ion batteries
1. Introduction The electrical energy storage technique will become far more important in the coming decades. The electrochemical battery is one of the most promising energy storage devices considering its portability, high conversion efficiency and no gaseous exhaust. As representative, LIBs have attracted much attention due to the high energy density and long cycle life [1–4]. Besides the successful applications in portable electronic devices including cellphones, laptop computers and digital cameras, it is expected to power electric vehicles (EVs) and hybrid electric vehicles (HEVs) and to store the sustainable energy, e.g. solar, wind, wave and smart grid [5–8]. The latter usage is especially attractive because it addresses the global warming issues. However, the state of the art of LIBs is not yet at such technique level to meet the user unconditioned demands especially in the field of EVs and HEVs, and thus further development of high performance energy storage technique is becoming extremely urgent.
* Corresponding author at: School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100,083, China. Tel.: +86 10 82376837; Fax : +86 10 82 376 837. E-mail address: [email protected] (H. Zhao). http://dx.doi.org/10.1016/j.electacta.2015.09.086 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
Current LIBs commonly use graphite as anode material, which has a limited theoretical capacity of 372 mAh g1 (LiC6) and shows some inherent disadvantages, including poor rate-capability, solvent co-intercalation and unsatisfactory safety feature [9–11]. These problems restrict the design and implementation of high performance lithium-ion batteries. Therefore, currently, intensive efforts are devoted in progress to explore new anode materials with high specific capacity and good safety performance, such as Sn-based [12,13], Si-based [14,15] and transition-metal oxide materials (TMOs, M = Ni, Cu, Co, Fe or Mn) [16–20]. Among these alternatives, TMOs have attracted many interests in virtue of their high theoretical capacity (500–1000 mAh g1) based on a conversion reaction mechanism (2yLi+ + 2ye + MxOy $ xM + yLi2O) [21] instead of an intercalation reaction. In particular, Fe3O4 has been considered as a promising anode material because of its high capacity (926 mAh g1), high electronic conductivity (among the TMOs), nature abundance (low cost), and environmental benignity [19,21]. However, similar to other conversion-based anode materials, the large volume variation that inherently accompanies the lithium uptake and release processes can cause electrode pulverization [18] and further electrical contact loss between active materials and current collector, leading to poor cycling stability of the electrodes. To overcome this issue, the most common strategies employed are to
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downsize particle dimension to the nanoscale level (nanoparticles [22], nanospindles [23], nanobelts [24], nanotubes [25], nanowires [26], nanocubes [27]) and to composite with electrically conductive and electrochemically less active components, such as carbon nanotube, carbon nanofibers or graphene [28]. Lang and Xu [24] prepared porous Fe3O4/C microbelts using simple electrospinning method with subsequent carbonization, which exhibited enhanced rate capability (0.2C: 710 mAh g1; 6C: 189 mAh g1). The carbon coated Fe3O4/CNT grape-cluster nanostructure was reported by Liu et al. [25], which showed a high specific capacity and good rate capability (60 mA g1: 975 mAh g1; 1200 mA g1: 282 mAh g1). Li et al. [26] developed hierarchical nanostructure Fe3O4@C mesoporous nanowires with excellent electrochemical performance (200 mA g1: 990 mAh g1; 1 A g1: 550 mAh g1). Gao et al. [27] reported Fe3O4 nanocubes, which were prepared by an amino acid assisted solvothermal process and presented good rate capability (0.2C: 695.1 mAh g1; 5C: 51 mA g1). The better strain accommodation and the shorter lithium ion diffusion/ electron transport lengths of the nanosized materials ensure the good electrochemical performance of nano-sized Fe3O4 electrode. The high electronic conductivity of conductive substance allows fast and homogeneous electrode reaction kinetics. In addition to aforementioned strategies, developing new anode architectures is also an important approach to achieve excellent electrochemical performance of electrodes. Currently, construction of well-ordered nanostructures (NiO flakes [16], Co3O4 nano-sheets [29], Fe2O3 nanorods [30], Fe3O4 NPs [31]) directly grown on a current collecting substrate has effectively enhanced rate capability and cycling stability of LIBs. Such selfsupported nanostructured TMOs with lots of open spaces in the electrode ensure the large electrolyte/electrode contact area and the well volume expansion release during cycling. Notably, the designed excellent electric contact between active materials and current collecting substrate offers numerous fast electron transport pathways, making the impressive high-rate performance available owing to the reduction of Ohmic electrode polarization. Recently, Varghese et al. [32] prepared vertically aligned NiO nanowalls on nickel foils using a plasma assisted oxidation method, which showed a capacity of 638 mAh g1 after 85 cycles at 1.25C. Fu et al. [18] synthesized self-supporting Co3O4 with lemongrass-like morphology on Ni foam and found that, the capacity maintained 981 mAh g1 after 100 cycles at a rate of 0.5C. Li et al. [31] reported the carbon-wrapped Fe3O4 nanoparticle film on nickel foam which delivered a stable capacity of 600 mAh g1 for 100 cycles at 0.2C. In this work, we report a strategy for the synthesis of a novel binder-free electrode, which contains 3D heterostructure Fe3O4/ Ni/C nanoplate arrays growing on nickel foam. The electrode can be used directly as anode for lithium-ion batteries without any addition of the polymer binder and carbon conductive agent. The carbon coating process on the synthesized precursor yields insitu lots of tiny well-crystallized Ni nanocrystals inside the Fe3O4 nanoplates. The particle surface carbon layer, underneath Ni foam and particle interior Ni nanoparticles can effectively build a fast 3D electron transport network to facilitate the electrode reaction and homogenize the current density on active Fe3O4 particles. The latter can ensure a uniform volume change and thus less stress of active Fe3O4 particles, therefore a stable electrode structure and excellent cycling performance can be expected. The practical electrochemical tests reveal that the as-prepared 3D heterostructure Fe3O4/Ni/C nanoplate arrays exhibit outstanding performance with high capacity and rate-capability. It delivers outstanding cycling performance as high as 832.5 mAh g1 (after cycling at 0.3C for 50 cycles) and good rate capacity up to 279 mAh g1 at 4.5C when evaluated as an anode material for LIBs.
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2. Experimental 2.1. Fabrication procedure 3D heterostructure Fe3O4/Ni/C nanostructured electrode was prepared by a simple hydrothermal process combined with a subsequent chemical vapor deposition (CVD) heat treatment. In a typical synthesis, the nickel foam (5 cm 3 cm) with thickness of 0.35 mm was firstly washed with distilled water and ethanol for several times to remove the surface impurities. Fe(NO3)3 9H2O (5 mmol), NH4F (5 mmol), and CO(NH2)2 (25 mmol) were dissolved in distilled water (70 ml) to form a transparent solution. The obtained solution was transferred into a 100 mL Teflon-lined stainless steel autoclave. After immersing one piece of clean nickel foam into the solution, the autoclave was sealed and heated at 120 C for 5 h. After the autoclave cooled down to room temperature naturally, the precursor was collected, rinsed with distilled water and absolute ethanol, and dried in air at 50 C for 10 h. In order to investigate the growth mechanism of the Fe3O4/Ni/ C nanoplate, the autoclave was kept at 120 C for different time (1 h, 10 h and 20 h). The hydrothermally synthesized nanoplate array precursor was placed in a quartz tube furnace for carbon CVD treatment at 600 C for 3 h. During the CVD process, toluene was used as the carbon source, which was carried by argon gas with the flow rate of 0.1 L min1. After the heat treatment, Fe3O4/Ni/C nanoplate arrays on Ni foam electrode were obtained. For simplicity, the prepared electrode was named as FNC600. The Fe3O4 and carbon components were taken as active materials and the loading of active material on electrode is 2.96 mg cm2. The detailed calculation about active material percentage in Ni foam supported electrode is supplied in supporting information. 2.2. Structure and morphology characterization The phase structure of the products was identified by X-ray diffraction (XRD, Rigaku, D/max-A, Cu Ka, l = 1.5406 Å) and Raman spectroscope (LabRAM HR Evolution, excited by 523 nm laser). The morphologies of the synthesized samples were examined using field-emission scanning electron microscopy (FESEM, Zeiss, SUPRA55). The lattice structure was characterized using highresolution transmission electron microscopy (HRTEM, JEM-2010). 2.3. Electrochemical characterization The prepared Fe3O4/Ni/C nanoplate arrays on Ni foam were used directly as electrode without any additional of binder and conductive agent. Before the coin-type 2032 cells assembly, the Ni foam with Fe3O4/Ni/C plates (disc electrode with diameter of 8 mm) was further dried at 120 C for 24 h in vacuum environment. A Celgard 2400 microporous polypropylene was used as separator, and pure lithium foil as counter electrodes. The non-aqueous electrolyte used was 1 M LiPF6 dissolved in ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) (1:1:1, in vol%). The cells were tested in the voltage range between 0.02 and 3 V with a multichannel battery test system (LAND CT2001A) at room temperature (25 C). The electrochemical impedance spectra (EIS) measurements were carried out in Solartron 1260 Frequency Response Analyzer combined with a Solartron 1287 potentiate in the frequency region from 105 to 0.01 Hz with a signal amplitude 5 mV. 3. Results and discussion The synthesized samples were examined by XRD to identify the crystal structures and phase components. As shown in Fig. 1,
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Fig. 1. (a) XRD patterns of the precursor after hydrothermal reaction and (b) the prepared Fe3O4/Ni/C nanoplate arrays on Ni foam after CVD treatment.
obviously, the strong diffraction peaks of the Ni phase (JCPDS 652865) were detected in both of the precursor and the as-prepared product on account of Ni foam substrate. Fig. 1a shows the XRD pattern of the precursor in which the diffraction peaks completely match with FexNiy(OH)2(CO3)z phase as reported in literature [33]. After CVD treatment, the diffraction peaks (Fig. 1b) are in good accordance with those of Fe3O4 phase (JCPDS 88-0315), indicating the decomposition and carbothermal reduction of precursors taking place during the CVD heat-treatment. Notably, no signals related to carbon were detected because of its amorphous feature and low content. Amorphous carbon in Fe3O4/Ni/C nanoplate arrays on Ni foam was detected by Raman spectroscopy (Fig. 2). In the range of 1050– 1800 cm1, the Raman spectrum exhibits typical characteristics of amorphous carbon: the two characteristic peaks at 1349 cm1 and 1606 cm1 are in good agreement with the typical Raman mode for the D band and G band of Fe3O4/Ni/C nanoplate arrays [17]. The peak intensity ratio between the D and G bands (ID/IG) is used as an index to estimate the degree of crystallization of carbon materials. Here, the ID/IG ratio of 0.83 indicates the low crystallinity of amorphous carbon. The nickel foam with high conductivity was used as the supporting backbone for the growth of the 3D heterostructure Fe3O4/Ni/C nanoplates for electrochemical energy storage. Fig. 3 shows the FESEM images of the Ni foam before and after hydrothermal treatment for different duration time (1, 5, 10 and 20 h). The as-received Ni foam (Fig. 3a) presents a smooth surface with clear Ni grain boundaries. After treated for 1 h, sample shows
Fig. 2. Raman spectra of Fe3O4/Ni/C nanoplate arrays with an excitation laser wavelength of 532 nm.
a rough surface with many fine particles ranging from 150 to 200 nm (Fig. 3b). With prolonged hydrothermal treatment time, the nanoparticles started to grow to nanoplates with lots of space among them (sample-5 h, Fig. 3c). Longer hydrothermal treating time results in larger thickness and higher density of nanoplates. Although the sample-10 h (Fig. 3d) and sample-20 h (Fig. 3e) remain the plate-like structure, the thickness of the nanoplates tends to be much larger, and the structure becomes dense without void space for sample-20 h, which will prevent the electrolyte solution from having sufficient contact with the particle surface and thus retard the electrode reaction kinetics. Based on the morphology observations of the samples with different hydrothermal treatment times, we present a four-step mechanism concerning the growth of 3D heterostructure Fe3O4/ Ni/C nanoplate arrays on Ni foam substrates, as schematically illustrated in Fig. 4a. Step 1: at the beginning, ferric ions are fully coordinated with F to form FeFx(x3) in the reaction solution, as shown in Eq. (1). As the temperature increases in the autoclave, the hydrolysis process of urea takes place around 70 C [34]. As a result, lots of CO32 and NH4+ anions are formed gradually (Eq. (2)). Step 2: with the increase of the concentration of the ions (CO32 and NH4+), ferric ions are released slowly into the reaction system and gradually participate in the formation of FexNiy(OH)2(CO3)zaH2O nuclei on Ni foam substrate (Eq. (3)). Instead of the rapid nucleus formation, the slow-down process is greatly helpful for the substantial contact between nucleus and the smooth substrate. Step 3: on account of the proceeding reaction, the growing nuclei start to assemble along the specific orientation preferentially. As a result, the yellow-brown nanoplate array precursors form and exhibit an open-up network structure. Step 4: the precursor is subjected to carbon CVD processing, in which the complex compound FexNiy(OH)2(CO3)zaH2O is decomposed and reduced to Fe3O4 nanoplates and Ni nanoparticles (Eq. (4)). Fe3+ + nF ! [FeFn](n3) (1)
4H2NCONH2 + 2H2O ! 2NH4+ + CO32 (2)
x[FeFn](n3) + yNi + zCO32 + (xn-2)NH4+ + (a+2)H2O ! FexNiy(OH)2(CO3)zaH2O + (xn-2)NH4F + 2HF (3)
FexNiy(OH)2(CO3)zaH2O + (x/12+y/2)C ! (x/3)Fe3O4 + yNi + (x/12 +y/2+z)CO2 + (a+1)H2O (4) The coated carbon layer on the surface of nanoplates together with the in-situ formed Ni nanoparticles provide a fast 3D electron transport network to facilitate the electrode reaction process. Moreover, the good electronic transport pathway can homogenize the current density of the electrode, and thus make the volume change of active particles upon lithium uptake/removal much uniform, preventing the cracking and pulverization of the active particles. Therefore, excellent electrochemical performance, in terms of cycling stability and rate-capability, can be expected. Fig. 4b shows an optical image including installed coin-type cell, Ni foam, precursor (yellow-brown) and Fe3O4/Ni/C nanoplates (black) on Ni foam. The black color should come from Fe3O4 and carbon. Combined with XRD results (Fig. 1), it is reasonable to state that Fe3O4/Ni/C nanoplates were formed after CVD process. The morphologies of the precursors and the as-prepared 3D heterostructure Fe3O4/Ni/C nanoplate arrays on Ni foam were examined by FESEM. Fig. 5a shows a representative lowmagnification top view of the nanoplate precursors. The Ni foam surface is covered with uniformly well-established nanoplates.
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Fig. 3. FESEM images of Ni foam before and after different hydrothermal reaction durations: (a) 0 h (pure Ni foam), (b) 1 h, (c) 5 h, (d) 10 h and (e) 20 h.
Fig. 4. (a) Schematic mechanism for the direct-growth process of Fe3O4/Ni/C nanoplate arrays on Ni foam; (b) optical image of coin-type battery, Ni foam substrate, precursor grown on Ni foam and Fe3O4/Ni/C nanoplates on Ni foam.
High-magnification images (Fig. 5b and c) reveal that the nanoplate precursors with a smooth surface are interconnected to each other, resulting in the formation of extended open-up network structure. The thickness of precursor nanoplates is ca.
150 nm (Fig. 5c). Fig. 5d and e show FESEM images of the asprepared 3D heterostructure Fe3O4/Ni/C nanoplate arrays. After CVD process, the well distributed and open-up Fe3O4/Ni/C nanoplates are still remained. Obviously, the structure becomes even
Fig. 5. (a)-(c) FESEM images of nanoplate precursors; (d)-(f) FESEM images of Fe3O4/Ni/C nanoplates after CVD treatment.
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open with more void space after heat-treatment, suggesting the occurrence of decomposition and conversion reactions, which is mainly attributed to the decomposition of precursors (CO32 and OH groups) and the reduction of Ni. Compared with nanoplate precursors (Fig. 5c), the heat-treated nanoplates (Fig. 5f) show a rough surface, which is mostly likely resulted from the in-situ generation of Ni nanocrystals. To characterize the internal structure of Fe3O4/Ni/C nanoplate arrays more precisely, TEM and HR-TEM inspections were conducted. The synthesized product exhibits plate-like morphology with some nanoparticles inside, which should be related to metallic Ni as shown in Fig. 6a. In addition, the individual nanoplate presents a core-shell structure with a uniform and continuous amorphous carbon layer on the plate surface. Such core-shell structure can be further confirmed in Fig. 6b, and the thickness of the carbon layer is 3 nm. Furthermore, a lattice spacing of 0.203 nm was observed in both Fig. 6b and c, in a good agreement with the (111) planes of metallic Ni. Besides, the lattice fringes with d-spacing of 0.483, 0.253 and 0.241 nm (Fig. 6b and c) are clearly distinguished, corresponding to the (111), (311) and (222) planes of Fe3O4, respectively. Therefore, from the structural analyses, it is certain that the metallic Ni nanocrystals are distributed in the Fe3O4 matrix. The Ni foam substrate, the carbon layer and the Ni nanocrystals can effectively build a 3D electron transport highway, and the open space between each nanoplate ensures homogenous electrolyte flow. Such unique structure feature makes the better electrochemical performance be expected.
To determine the effectiveness of this unique structure in improving the electrochemical performance, a series of electrochemical evaluations on the Fe3O4/Ni/C electrodes were carried out based on the assembled coin cell. Fig. 7a shows the dischargecharge voltage profiles at the current density of 0.3C (1C = 924 mAh g1) with the voltage windows between 0.02 and 3.0 V. For clarity, the 1st, 2nd, 10th, 30th, and 50th cycles are shown. The voltage profile of the first discharge reaction mainly comprises three regions, the steep voltage drop from the open circuit potential to 0.75 V corresponds to Li+ insertion into the Fe3O4 lattice (Eq. (5)) [35] and the subsequent long discharge plateaus at about 0.75 V is associated with conversion reaction of LixFe3O4 to Fe0 (Eq. (6)) [36], which is then followed by a slope till 0.02 V. The low potential slope is usually considered to relate to the irreversible formation of a gellike film on the surface of Fe3O4 particles [37]. Apparently, in the subsequent cycles, redox reaction (Eq. (7)) of lithium with Fe3O4 presents highly reversible. The cathodic lithium insertion mainly occurs at 1.1 V, and the anodic lithium extraction occurs at 1.75 V which are in consistent with that described in literature for Fe3O4 anodes [36]. Fe3O4 + xLi+ + xe ! LixFe3O4 (5)
LixFe3O4 + (8-x)Li+ + (8-x) e ! 4Li2O + 3Fe (6)
8Li+ + 8e + Fe3O4 $ 4Li2O + 3Fe (7)
Fig. 6. (a) TEM and (b, c) HR-TEM images of Fe3O4/Ni/C nanoplate arrays.
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Ni/C electrode as well as a more adequate space to accommodate the huge volume variation related to lithium insertion and extraction. These advantages are beneficial to accelerating the electrochemical reactions and reducing electrochemical polarization of electrode. Electrochemical impedance spectral (EIS) measurements on electrode materials can give important information about electrode reaction kinetics [38,39]. Fig. 8a shows the Nyquist plots of the Fe3O4/Ni/C electrode after 50 cycles at 50% discharge state. The Nyquist plots present a compressed two semicircles in the high frequency region and an inclined line at low frequency, corresponding to the resistances from surface film, charge transfer and lithium ion diffusion, respectively [40]. The equivalent circuit is presented in inset of Fig. 8a, with which the parameters of each process are simulated and listed in Table 1. The charge transfer resistance (Rct) is higher than film resistance (Rf), suggesting the rate limiting feature of charge transfer process for the electrode
Fig. 7. Electrochemical performance of the Fe3O4/Ni/C anode: (a) Galvanostatic charge/discharge profiles of Fe3O4/Ni/C electrode at 0.3C; (b) cycling performance of Fe3O4/Ni/C anode at a current density of 0.3C; (c) rate performance of Fe3O4/Ni/C anode at various current densities in the potential windows of 0.02-3.0 V versus Li/ Li+.
The cycling performance of the prepared Fe3O4/Ni/C electrode was examined at a constant current rate of 0.3C. Fig. 7b presents the results of discharge-charge galvanostatic cycles of the hybrid composite electrode in the voltage of 0.02 to 3.0 V. The discharge capacity of the Fe3O4/Ni/C electrode in the first cycle is as high as 1184.5 mAh g1 and the corresponding charge capacity is 817 mAh g1, giving an initial coulombic efficiency of ca. 70%. After that, the coulombic efficiency keeps above 99% in the following dischargecharge cycles. The subsequent charge and discharge cycles deliver a very stable capacity, being of 832.5 and 848.7 mAh g1 after 50 cycles. The rate capability of the Fe3O4/Ni/C anode was evaluated by cycling the electrode in a step mode at different current densities ranging from 0.15 to 4.5C. As presented in Fig. 7c, at the current densities of 0.15, 0.3, 0.45, 0.75, 1.5, 3.0C, the electrode delivers capacities of 876, 819, 765, 691, 568, 412 mAh g1, respectively. It is worth noting that a specific capacity of 279 mAh g1 can still be delivered by Fe3O4/Ni/C composite electrode at the current density as high as 4.5C. When the current density finally switched back to its initial value of 0.15C, a charge capacity of 691 mAh g1 is still available, indicating the excellent rate-capability and the good structural stability of the synthesized electrode. These electrochemical results clearly indicate the synthesized Fe3O4/Ni/C composite on Ni foam with such unique open network nanostructure can be a promising electrode for high rate performance LIBs. The outstanding electrochemical performance of the Fe3O4/Ni/ C electrode can be attributed to the following aspects: (1) The nanoplates with thickness of about 150 nm can provide a shorter diffusion distance for both lithium ions and electrons. (2) The insitu grown Fe3O4/Ni/C nanoplates have strong attachment with the current collector, and thus effectively preventing the active material from peeling off. (3) The Ni foam substrate, the uniform carbon layer on the particle surface and the in-situ formed nanosized Ni particles can build a conductive and mechanical 3D framework to improve the reaction kinetics of the synthesized electrode. (4) The open space between nanoplate arrays provide an easy diffusion of the electrolyte into the inner region of the Fe3O4/
Fig. 8. (a) Electrochemical impedance spectra of Fe3O4/Ni/C electrode after 50 cycles at 50% discharge state; (b) relationship between real impedance and reciprocal square root of the lower angular frequencies.
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Table 1 Kinetic parameters of Fe3O4/Ni/C electrodes. Electrode
Re/V
Rf/V
Cf/F
Rct/V
Cdl/F
sw/V s0.5
DLi/cm2 s1
FNC600
2.61
7.12
4.13 107
15.88
1.0 105
12.77
6.85 1012
reaction. To investigate the behavior of Li ion in electrode materials, the relationship between the real impedance (Zre) and the reciprocal square root of the lower angular frequencies (v1/2) is plotted in Fig. 8b. The straight lines are attributed to the diffusion of the lithium ions into the bulk of the electrode material, or the socalled Warburg diffusion. From the Fig. 8b, Warburg impedance coefficient (sw) can be derived from the slope of straight line according to Eq. (8), and the diffusion co-efficient values (DLi) of the lithium ion diffusion into the bulk electrode materials can be calculated using Eq. (9) [40]. Zre = Re + Rf + Rct + sw v1/2 (8) D = 0.5(RT/AF2swC)2 (9) where v is the frequency, R is the gas constant, T is the absolute temperature, A is the surface area of the cathode, F is the Faraday constant, C is the molar concentration of lithium ion [40]. The Li ion diffusion coefficient in the prepared Fe3O4/Ni/C electrode is 6.85 1012 cm2 s1. The EIS results reveal that lithium ion diffusion and charge transfer process are limiting steps for Fe3O4/Ni/C electrode reaction. Therefore, nanostructuring is imperative to the improvement of the Fe3O4-based electrode reaction kinetics because it can shorten the lithium ion diffusion distance and provide more surface area for charge transfer process. Although the prepared Fe3O4/Ni/C electrode shows high specific capacity and excellent rate-capability, its energy density is not satisfied due to the coarse skeleton of Ni foam and the high density of metal Ni. Further optimization of preparation parameters, such as utilization of customized Ni foam with slim skeleton and increase of loading of the active materials on Ni foam, can provide opportunity to increase the energy density of Fe3O4/Ni/C electrode. 5. Conclusion In summary, we have successfully fabricated Fe3O4/Ni/C nanoplate arrays on Ni foam substrate using a novel hydrothermal method with subsequent CVD heat treatment, which are used directly as a binder-free anode for LIBs. The Ni foam-supported hybrid composite electrode has good mechanical properties and higher electrical conductivity as anode, which performs excellent lithium storage characteristics with high capacity, stable cycling performance, and excellent rate capability. It shows a high discharge capacity of 832.5 mAh g1 at 0.3C after 50 cycles. Even at the current density of 4.5C, a capacity of 279 mAh g1 can be reached. The prepared Fe3O4/Ni/C nanoplate arrays with open-up network structure can be an ideal candidate for high-capacity electrode of rechargeable LIBs. Acknowledgements This work was financially supported by National Basic Research Program of China (2013CB934003), National High Technology Research and Development Program of China (2013AA050902), National Nature Science Foundation of China (21273019), Guangdong Industry-Academy-Research Alliance (2013C2FC0015) and
Program of Introducing Talents of Discipline to Universities (B14003). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2015.09.086. References [1] D. Li, L. Ding, H. Chen, S. Wang, Z. Li, M. Zhu, H. Wang, Novel nitrogen-rich porous carbon spheres as a high-performance anode material for lithium-ion batteries, J. Mater. Chem. A 2 (16) (2014) 617. [2] D. Kong, J. Luo, Y. Wang, W. Ren, T. Yu, Y. Luo, Y. Yang, C. Cheng, Threedimensional Co3O4@MnO2 hierarchical nanoneedle arrays: morphology control and electrochemical energy storage, Adv. Funct. Mater. 24 (2014) 3815–3826. [3] Y. Hu, R. Demir-Cakan, M.M. Titirici, J.O. Muller, R. Schlogl, M. Antonietti, J. Maier, Superior storage performance of a Si@SiOx/C nanocomposite as anode material for lithium-ion batteries, Angew. Chem. Int. Ed. 47 (2008) 1645–1649. [4] T. Moritaz, N. Takami, Nano Si Cluster-SiOx-C composite material as highcapacity anode material for rechargeable lithium batteries, J. Electrochem. Soc. 153 (2006) A425–A430. [5] S. Choi, D.S. Jung, J.W. Choi, Scalable fracture-free SiO-C glass coating for robust silicon nanoparticle anodes in lithium secondary batteries, Nano Lett. 14 (2014) 7120–7125. [6] W.W. Lee, J.M. Lee, Novel synthesis of high performance anode materials for lithium-ion batteries (LIBs), J. Mater. Chem. A 2 (2014) 1589–1626. [7] B. Scrosati, J. Garche, Lithium batteries: status, prospects and future, J. Power Sources 195 (2010) 2419–2430. [8] P.J. Hall, E.J. Bain, Energy-storage technologies and electricity generation, Energy Policy 36 (2008) 4352–4355. [9] X. Feng, J. Yang, Q. Lu, J. Wang, Y. Nuli, Facile approach to SiOx/Si/C composite anode material from bulk SiO for lithium ion batteries, Phys. Chem. Chem. Phys. 15 (2013) 14420. [10] J. Tu, Y. Yuan, P. Zhan, H. Jiao, X. Wang, H. Zhu, S. Jiao, Straight forward approach toward SiO2 nanospheres and their superior lithium storage performance, J. Phys. Chem. C 118 (2014) 7357–7362. [11] H. Guo, R. Mao, X. Yang, J. Chen, Hollow nanotubular SiOx templated by cellulose fibers for lithium ion batteries, Electrochim. Acta 74 (2012) 271–274. [12] B. Qu, C. Ma, G. Ji, C. Xu, J. Xu, Y.S. Meng, T. Wang, J.Y. Lee, Layered SnS2-reduced graphene oxide composite?a high-capacity, high-rate, and long-cycle life sodium-ion battery anode material, Adv. Mater. 26 (2014) 3854–3859. [13] X. Zhou, Z. Dai, S. Liu, J. Bao, Y. Guo, Ultra-uniform SnOx/carbon nanohybrids toward advanced lithium-ion battery anodes, Adv. Mater. 26 (2014) 3943– 3949. [14] Y. Ren, M. Li, Si-SiOx-cristobalite/graphite composite as anode for li-ion batteries, Electrochim. Acta 142 (2014) 11–17. [15] X. Huang, J. Yang, S. Mao, J. Chang, P.B. Hallac, C.R. Fell, B. Metz, J. Jiang, P.T. Hurley, J. Chen, Controllable synthesis of hollow Si anode for long-cycle life lithium-ion batteries, Adv. Mater. 26 (2014) 4326–4332. [16] S. Ni, T. Li, X. Lv, X. Yang, L. Zhang, Designed constitution of NiO/Ni nanostructured electrode for high performance lithium ion battery, Electrochim. Acta 91 (2013) 267–274. [17] J. Song, L. Xu, C. Zhou, R. Xing, Q. Dai, D. Liu, H. Song, Synthesis of graphene oxide based CuO nanoparticles composite electrode for highly enhanced nonenzymatic glucose detection, ACS Appl. Mater. Interfaces 5 (12) (2013) 928–12934. [18] L. Zhan, S. Wang, L. Ding, Z. Li, H. Wang, Grass-like Co3O4 nanowire arrays anode with high rate capability and excellent cycling stability for lithium-ion batteries, Electrochim. Acta 135 (2014) 35–41. [19] L. Zhong, J. Hu, H. Liang, A. Cao, W. Song, L. Wan, Self-assembled 3D flowerlike iron oxide nanostructures and their application in water treatment, Adv. Mater. 18 (2006) 2426–2431. [20] A. Debart, A.J. Paterson, J. Bao, P.G. Bruce, a-MnO2 nanowires: a catalyst for the O2 electrode in rechargeable lithium batteries, Angew. Chem. 120 (2008) 4597–4600. [21] B.H. Park, S.B. Park, S.M. Jeong, C.S. Seo, S.W. Park, Electrolytic reduction of spent oxide fuel in a molten LiCl-Li2O system, J. Radioanal. Nucl. Chem. 270 (2006) 575–583. [22] L. Ji, Z. Tan, T.R. Kuykendall, S. Aloni, Fe3O4 nanoparticle-integrated graphene sheets for high-performance half and full lithium ion cells, Phys. Chem. Chem. Phys. 13 (2011) 7170–7177.
Z. Li et al. / Electrochimica Acta 182 (2015) 398–405 [23] W. Zhang, X. Wu, J. Hu, Y. Guo, L. Wan, Carbon coated Fe3O4 nanospindles as a superior anode material for lithium-ion batteries, Adv. Funct. Mater. 18 (2008) 3941–3946. [24] L. Lang, Z. Xu, In situ synthesis of porous Fe3O4/C microbelts and their enhanced electrochemical performance for lithium-ion batteries, ACS Appl. Mater. Interfaces 5 (2013) 1698–1703. [25] J. Liu, J. Ni, Y. Zhao, H. Wang, L. Gao, Grapecluster-like Fe3O4@C/CNT nanostructures with stable Li-storage capability, J. Mater. Chem. A 1 (2013) 12879. [26] H. Li, R. Xu, Y. Wang, B. Qian, H. Wang, L. Chen, H. Jiang, Y. Yang, Y. Xu, In situ synthesis of hierarchical mesoporous Fe3O4@C nanowires derived from coordination polymers for high-performance lithium-ion batteries, RSC Adv. 4 (2014) 51960. [27] H. Cao, R. Liang, D. Qian, J. Shao, M. Qu, L-Serine-assisted synthesis of superparamagnetic Fe3O4 nanocubes for lithuium ion batteries, J. Phys. Chem. C 115 (2011) 24688–24695. [28] W. Zhou, J. Zhu, C. Cheng, J. Liu, H. Yang, C. Cong, C. Guan, X. Jia, H. Fan, Q. Yan, C. Li, T. Yu, A general strategy toward graphene@metal oxide core–shell nanostructures for high-performance lithium storage, Energy Environ. Sci. 4 (2011) 4954. [29] C. Yuan, L. Yang, L. Hou, L. Shen, X. Zhang, X. Lou, Growth of ultrathin mesoporous Co3O4 nanosheet arrays on Ni foam for high-performance electrochemical capacitors, Energy Environ. Sci. 5 (2012) 7883. [30] W. Yang, Z. Gao, J. Wang, B. Wang, L. Liu, Hydrothermal synthesis of reduced graphene sheets/Fe2O3 nanorods composites and their enhanced electrochemical performance for supercapacitors, Solid State Sci. 20 (2013) 46–53. [31] D. Li, X. Li, S. Wang, Y. Zheng, L. Qiao, D. He, Carbon-wrapped Fe3O4 nanoparticle films grown on nickel foam as binder-free anodes for high-rate and long-life lithium storage, ACS Appl. Mater. Interfaces 6 (2014) 648–654.
405
[32] B. Varghese, M.V. Reddy, Y. Zhu, C. Lit, T. Hoong, G.V. Rao, B.V.R. Chowdari, A.T. S. Wee, C.T. Lim, C. Sow, Fabrication of NiO nanowall electrodes for high performance lithium ion battery, Chem. Mater. 20 (2008) 3360–3367. [33] E. Uzunova, D. Klissurski, S. Kassabov, Nickel-iron hydroxide carbonate precursors in the synthesis of high-dispersity oxides, J. Mater. Chem. 4 (1994) 153–159. [34] J. Jiang, J.P. Liu, X.T. Huang, Y.Y. Li, R.M. Ding, X.X. Ji, Y.Y. Hu, Q.B. Chi, Z.H. Zhu, General synthesis of large-scale arrays of one-dimensional nanostructured Co3O4 directly on heterogeneous substrates, Cryst. Growth Des. 10 (2010) 71. [35] B. Yang, J. Zhao, J. Chen, M. He, S. Xu, Controllable synthesis of graphene nanoscroll-wrapped Fe3O4 nanoparticles and their lithium-ion battery performance, RSC Adv. 5 (2015) 57906. [36] Y. Wu, J. Liu, C. Wu, Q. Zhuang, H. Chen, H. Zhang, X. Zhang, Magnetite/carbon nanotubes nanocomposite: facile hydrothermal synthesis and enhanced cycling performance and high-rate capability as anode material for lithiumion batteries, Ionics 21 (2015) 635–643. [37] K. Sun, H. Zhao, J. Yao, S. Zhang, J. Xu, Foam carbon loading Fe3O4 nanoparticles for superior lithium-ion batteries anode material, Ionics 21 (2015) 1901–1908. [38] A.Y. Shenouda, H.K. Liu, Preparation, characterization, and electrochemical performance of Li2CuSnO4 and Li2CuSnSiO6 electrodes for lithium batteries, Journal of The Electrochemical Society 157 (2010) A1183–A1187. [39] K. Xie, Z. Lu, H. Huang, W. Lu, Y. Lai, J. Li, L. Zhou, Y. Liu, Iron supported C@Fe3O4 nanotube array: a new type of 3D anode with low-cost for high performance lithium-ion batteries, J. Mater. Chem. 22 (2012) 5560. [40] L. Li, T. Wang, L. Zhang, Z. Su, C. Wang, R. Wang, Selected-control synthesis of monodisperse Fe3O4@C core–shell spheres, chains, and rings as highperformance anode materials for lithium-ion batteries, Chem. Eur. J 18 (2012) 11417–11422.