Porous Iron Cobaltate Nanoneedles Array on Nickel Foam as Anode

Dec 29, 2015 - When the as-prepared porous FeCo2O4 nanoneedles array with a high surface area of 58.49 m2 g–1 was applied as binder-free electrode i...
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Porous Iron Cobaltate Nanoneedles Array on Nickel Foam as Anode Materials for Lithium-Ion Batteries with Enhanced Electrochemical Performance Li Liu, Huijuan Zhang, Yanping Mu, Jiao Yang, and Yu Wang* The State Key Laboratory of Mechanical Transmissions and the School of Chemistry and Chemical Engineering, Chongqing University, 174 Shazheng Street, Shapingba District, Chongqing 400044, P.R. China S Supporting Information *

ABSTRACT: A monocrystalline and porous FeCo2O4 nanoneedles array growing directly on a nickel foam substrate was obtained by a hydrothermal technique accompanying with combustion of the one-dimensional precursor. The average length of the FeCo2O4 nanoneedles is approximately 2 μm, while the diameter of the root segment of the nanoneedle can be estimated to be around 100 nm, which gradually reduces to only several nanometers at the top. When the as-prepared porous FeCo2O4 nanoneedles array with a high surface area of 58.49 m2 g−1 was applied as binder-free electrode in lithium-ion batteries, it exhibited satisfactory electrochemical performance, such as outstanding reversibility (Coulombic efficiency of approximately 92−95%), high specific capacity (1962 mAh g−1 at the current density of 100 mA g−1), and excellent rate performance (discharge capacity of 875 mAh g−1 at the current density of 2000 mA g−1), due to the various favorable conditions. Undoubtedly, the simple but effective strategy can be expanded to other high-performance binary metal-oxide materials. KEYWORDS: FeCo2O4, nanoneedles array, lithium-ion batteries, electrochemical performance, nickel foam

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INTRODUCTION Consumption of fossil fuels and rising environmental issues have triggered an expansion in the exploration of sustainable energy sources and effective conversion/storage techniques.1 In various energy storage equipment, lithium-ion batteries (LIBs) have appealed to worldwide attention,2 because high-capacity LIBs play a significant role in meeting the mounting power demands, which originate from the prompt development in electronics.3 As is well-known, the high-capacity anode materials with low resistance and high safety have a crucial effect on improving power and energy density for LIBs.4 Many materials, such as metal, metal oxide, nonmetal, and multielement oxide have been considered to substitute graphite.5 Among them, transition metal oxides (for instance, SnO2,6 TiO2,7,8 Fe3O4,9 MnO2,10 Co3O4,11,12 and so on) have been widely explored as the anode materials for LIBs.13,14 In this respect, spinel Co3O4 is in the good graces of various researchers as anode materials of LIBs because it can be easily prepared by various synthetic strategies (for instance, the solvothermal method,15 the low-temperature molten salt method16), and it owns a high theoretical capacity of 890 mAh g−1 (relative to that of 372 mAh g−1 for graphite). Therefore, spinel Co3O4 is highly anticipated to meet the requirements for storage and utilization of clean energy.17 However, there still exists an immense disadvantage for Co3O4 that is pertinent to the use of toxic element cobalt. © 2015 American Chemical Society

Furthermore, the high cost of the cobalt element also makes it not an ideal anode material in practical applications.18 Recently, there is a new trend that other 3d-transition metals (A) partly substitute the B in spinel B3O4 to form binary transition metal oxides, described by the general formula AB2O4, where A and B stand for tetrahedral and octahedral cation sites in a cubic close packing of oxygens, respectively.19 Among the currently available binary transition-metal oxides, cobalt-containing spinel oxides (MCo2O4, where M = Cu,20 Mn,21 Ni,22 Zn,23 or Mg24) have attracted comprehensive attention because of their prominent physicochemical properties, which can be applied to many technological fields, ranging from catalysts, sensors, electrode materials to electrochemical devices.25 Thus, it is an advisible strategy to replace Co in Co3O4 by eco-friendly and cheaper alternative elements (for example, Zn, Cu, Ni, Mg, Fe, etc.) without sacrificing the electrochemical properties.24,26−28 Iron (Fe) is 50 times less expensive than cobalt because of the more abundant content in nature, and iron-containing oxides possess a low operating voltage as well as the excellent electronic conductivity.29 Thus, the introduction of Fe species into cobalt-based oxide electrodes may be an effective strategy to improve the Received: October 26, 2015 Accepted: December 29, 2015 Published: December 29, 2015 1351

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act as precipitates to release the CO32− and OH− anions, which could react with metal cations (Fe2+ and Co2+) with ethanol as a surfactant. The relevant reactions can be described in eqs 1−5. A piece of preprocessed nickel foam and the mixture solution undergo a hydrothermal process, wherein the surface of Ni foam provides an ideal interface for nucleation and guarantees a homogeneous precipitation of the hybrid iron/ cobalt precursors on Ni foam. The second step is a postannealing process in air. The (Fe, Co) bimetallic hydroxide carbonate precursors are thermally transformed to spinel FeCo2O4 nanoneedles array on Ni foam via a facile calcination in air at a relatively low temperature, as depicted in eq 6. In this process, which accompanies the release of H2O and CO2 gas, a large number of pores within the FeCo2O4 nanoneedles are formed.

performance of lithium ion batteries.30 It is a problem, however, that the large volume change, the severe aggregation, and the high contact resistance of the materials are observed during the Li+ insertion/extraction process, resulting in capacity fade and poor cycle performance, which limits its application for an energy storage system.31 The morphology and microstructure of an anode material make a great difference on the electrochemical performances.32 In order to remedy the above-mentioned drawbacks of binary transition-metal oxides, efforts have been roughly devoted to three aspects. First, it is necessary to consider the fabrication of one-dimensional nanostructural binary transition-metal oxides. The confining dimension effect and high surface area of nanostructures are advantageous to reduce electron and lithium ions diffusion paths and increase active sites for Li insertion/ extraction reactions. Therefore, one-dimensional (1D) binary transition-metal oxides, like nanofibers,33 nanowires,34 nanotubes,35 and so forth, display excellent electrochemical properties. Besides, the coexistence of two different cations in a single-crystal structure could improve the stability.36 Second, it is important to optimize the architecture of binary transitionmetal oxides with porous voids.37 The cyclability of nanostructured electrodes may be enhanced owing to the sufficient spaces to relax the large volume changes during continuous charge/discharge processes.10 Third, it is imperative to directly grow the active materials on some three-dimensional conductive substrates, such as Ni foam.28 By using the 1D nanoneedles array/Ni foam as a new class of binder-free anode and the current collectors to replace traditional metal current collectors like copper and aluminum, the highly flexible lithium batteries with enhanced electrochemical performance can be fabricated and can be used for the flexible/bendable electronics for the next generation of electronic devices, such as rollup displays, smart electronics, and wearable devices.38,39 Given all this, in this work, we design a facile and energysaving one-step hydrothermal strategy, accompanied by heat treatment at 550 °C under air atmosphere for 200 min, to synthesize 1D porous binary transition-metal oxide nanoneedles array on conductive substrates (nickel foam), applied as the binder- and conductive-agent-free anode materials for LIBs. As far as we know, it is the first time to report that 1D porous FeCo2O4 and CoFe2O4 nanoneedles array directly grown on nickel foam substrate were used as anode materials for their lithium-storage properties. The effects of the component and morphology on the electrochemical performances are also investigated by comparison with the behavior of FeCo2O4 nanoneedles array, CoFe2O4 nanoneedles array, and FeCo2O4 bulks prepared by a similar method. Encouragingly, owing to the unique properties of this porous FeCo2O4 nanoneedles array, such as high surface areas, good crystallinity, excellent conductivity, and direct growth on conductive substrates, they exhibit high reversible capacity, superior cyclability, and perfect rate capability. Notably, this general preparation approach could also be applied to synthesize other multicomponent transition-metal oxide nanoneedles arrays on conductive substrate by simple substitution for the raw materials.

NH4F → F− + NH4 +

(1)

CO(NH 2)2 + H 2O → CO2 + 2NH3

(2)

CO2 + H 2O → H 2CO3 → CO32 − + 2H+

(3)

NH3 + H 2O → NH3·H 2O → NH4 + + OH−

(4)

(Co2 + + Fe2 +) + xOH− + 0.5(2 − x)CO32 − + nH 2O → (Co, Fe)(OH)x (CO3)0.5(2 − x) ·nH 2O

(5)

(2Co, Fe)(OH)x (CO3)0.5(2 − x) ·nH 2O + 1.5O2 → FeCo2O4 + (n + 0.5x)H 2O + (1 − 0.5x)CO2

(6)

The compositions of the as-synthesized samples are investigated by the powerful techniques X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). As is wellknown, XRD analysis is an important technique to characterize the phase purity, crystallographic structure, and crystallinity of the as-prepared samples. Figure S1a reveals the XRD pattern of the (Fe, Co) bimetallic hydroxide carbonate precursors ((Co, Fe) (OH)x(CO3)0.5(2−x)·nH2O). From Figure S1a, it can be seen that all the diffraction peaks of the (Fe, Co) bimetallic hydroxide carbonate precursors match well with the standard Joint Committee on Powder Diffraction Standards (JCPDF) cards of Co(CO3)0.5(OH)2·0.11H2O (No. 48-0083) and Fe2(CO3) (OH) (No. 52-0163), except for three distinct peaks originated from nickel foam (JCPDF card No. 04-0850). Figure 1a displays the wide-angle XRD pattern of the FeCo2O4 nanoneedles array supported on nickel foam. Excluding the peaks marked as “heart” and “shamrock”, stemmed from Ni and NiO, respectively, the whole diffraction peaks in this pattern can be easily indexed to spinel FeCo2O4 phase with the space group of Fd3m, in good accordance with the previous literatures.24,40 Besides, very sharp peaks could be observed, indicating the good crystallinity of the FeCo2O4 nanoneedles array. No other diffraction peaks from parental hydroxide carbonate precursors and phase separation from cobalt oxide or iron oxide are detected, demonstrating the absence of any impurities. Meanwhile, the results also indicate that the precursors are completely decomposed to pure spinel FeCo2O4 phase after calcinations. In order to further certify the elemental ratio and chemical compositions of the FeCo2O4 nanoneedles array on nickel foam, energy-dispersive X-ray spectrometry (EDS) microanalysis is conducted, and the relevant results are displayed in Figure S1b. As depicted in

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RESULTS AND DISCUSSION Herein, the synthetic procedure of porous FeCo2O4 nanoneedles array on nickel foam substrate includes two steps. The first step is to form the mixture of iron/cobalt hydroxide carbonate salts, that is to say, Co(CO3)0.5(OH)2·0.11H2O and Fe2(CO3) (OH). During the synthesis, CO(NH2)2 and NH4F 1352

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FeCo2O4 nanoneedles array own a mixed composition including Fe3+, Co2+, Co3+, and O2−, confirming the formation of FeCo2O4 and supporting the results of XRD and EDS. In addition, for the purpose of investigating the effect of Fe content on the electrochemical performances, CoFe2 O4 nanoneedles array on Ni foam are also synthesized by analogous experimental methods. Figure S1c displays the XRD pattern of the CoFe2O4 nanoneedles array, in which all the diffraction peaks apart from several sharp peaks resulting from Ni foam (denoted as specific symbol) could coincide well with the cubic CoFe2O4 spinel phase. Similarly, the EDS data of CoFe2O4 nanoneedles array (Figure S1d) also exhibits the signals of Fe, Co, Ni (comes from Ni foam), and O with the atomic ratio of approximately 2:1:4 (Fe: Co: O), which are consistent with the element proportions in the raw materials. As we all know, the ICP-AES is a trustworthy characterization technique to confirm the exact composition of the materials. Thus, the accurate elemental ratios of Fe/Co/O in the FeCo2O4 and CoFe2O4 nanoneedles array are obtained by the ICP-AES, as exhibited in Table S1. As is expected, the elemental ratios of Fe/Co in the FeCo2O4 and CoFe2O4 nanoneedles array are nearly 1:2 and 2:1, respectively, conforming the results of EDS and XPS. However, the contents of the oxygen element in both FeCo2O4 and CoFe2O4 are a little higher than the theoretical value, which is associated with the formation of NiO in the nickel foam. Field emission scanning electron microscopy (FESEM) is employed to characterize the morphological and detailed structural characteristics of the as-obtained samples, as expressed in Figure 2. Figure 2a displays the panoramic SEM

Figure 1. (a) XRD pattern of FeCo2O4 nanoneedles array on nickel foam substrate; XPS spectra of (b) O 1s, (c) Fe 2p, (d) Co 2p after calibrating by C 1s.

Figure S1b, the composites contain four kinds of elements including Fe, Co, Ni and O, wherein Ni element comes from the nickel foam substrate, and the atomic ratio of Fe, Co, O is approximately 1:2:4 in good agreement with the stoichiometric ratio of FeCo2O4, suggesting the formation of pure FeCo2O4. The elemental chemical state and the chemical composition on the surface of the FeCo2O4 nanoneedles array are studied by Xray photoelectron spectroscopy (XPS), as exhibited in Figure 1b−d. All binding energies are calibrated for specimen charging by referencing the C 1s peak at 284.6 eV. Figure 1b manifests the high-resolution O 1s spectrum, in which a main peak and a low-intensity broad peak are observed. By using the Gaussian fitting method, the O 1s emission spectrum is well-fitted and displays three fitting peaks, representing for three types of oxygen species labeled as O1, O2, and O3, respectively. As depicted in Figure 1b, the fitting peak of O1 at a binding energy of 529.8 eV corresponds to a typical metal−oxygen bond, on the basis of existing reports.41 Besides, the fitting peak of O2 is located at 530.7 eV and is caused by a large number of deflection sites with low oxygen coordination in the materials.42 In addition, the fitting peak of O3 at 531.8 eV is usually associated with the multiplicity of physi- and chemisorbed water at/within the surface.41 Figure 1c elucidates the XPS spectrum of Fe 2p, in which two main fitting peaks at the binding energy of 711.5 and 724.6 eV with two “shoulder” weak shakeup satellite peaks (denoted as Sat.) at 717.4 and 733.4 eV are observed, in good accordance with spin−orbit peaks of the Fe 2p3/2 and Fe 2p1/2, implying the appearance of Fe3+.43 Likewise, for the Co 2p core level, as displayed in Figure 1d, there are two main peaks corresponding to Co 2p3/2 and Co 2p1/2, which are divided into four subpeaks after fitting. In the Co 2p spectrum, the fitting peaks at 779.4 and 794.6 eV with the satellite peak of 786.7 eV are indexed to Co3+,44 while the fitting peaks at 781.1 and 795.9 eV with the satellite peak of 804.2 eV are related with Co2+.45 It can be seen that the spin− orbit splitting is approximately 15 eV, indicating these signals are perhaps attributed to Co 2p3/2 and Co 2p1/2 levels, respectively. As a result, according to the peak widths of the signals Co 2p3/2 and Co 2p1/2 as well as the distances of satellite peaks, it is most probable that Co3+ and Co2+ coexist in the present samples. The above XPS results testify that the

Figure 2. (a) SEM image of the (Fe, Co) bimetallic hydroxide carbonate precursors; (b−d) gradually magnified SEM images of FeCo2O4 nanoneedles array on nickel foam substrate. The inset of panel d displays the cross section of FeCo2O4 nanoneedles array exfoliation from nickel foam after a long time of powerful ultrasonic.

image of the (Fe, Co) bimetallic hydroxide carbonate precursors, in which the nanoneedle precursors uniformly, completely, and densely cover the surface of the Ni foam over a large area, indicating the large-scale samples have been obtained on Ni foam. Figure 2b−d are the gradually magnified SEM images of FeCo2O4 nanoneedles array on nickel foam substrate, providing clearer information about the final products. As displayed in Figure 2b, the array structure and morphology of the FeCo2O4 nanoneedles are perfectly retained after annealing. 1353

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array from the nickel foam substrate for the further Brunauer− Emmett−Teller (BET) gas-sorption measurements. The nitrogen adsorption isotherm and its corresponding pore-size disctribution are displayed in Figure S2 and the inset, respectively. As depicted in Figure S2, the nitrogen adsorption−desorption isotherm of the FeCo2O4 nanoneedles array can be classified as type-IV with a type-H1 hysteresis loop in the range of ca. 0.6−1.0 p/p0 according to the IUPAC classifications of hysteresis loops, which infers a reflection of a typical mesoporous microstructure.47 Based on the Barrett− Joyner−Halenda (BJH) model recorded from the inset of Figure S2, a narrow pore size distribution (5−10 nm) is mainly centered at 5.8 nm, which is consistent with the pore size highlighted in Figure 3c. The mesoporous structure could facilitate the fast penetration of electrolyte, which is beneficial to the sustenance of a high Li+ flux across the solid/liquid interface. In addition, the surface area of FeCo2O4 nanoneedles array is 58.49 m2 g−1, as revealed in Figure S2. The relatively large surface area is in favor of the interfacial contact between electrolyte and electrode materials, which could contribute to a considerable improvement in the electrochemical properties. Figure 3d is the HRTEM image of an individual FeCo2O4 nanoneedle, indicating the high crystallinity. Nanocrystals are randomly selected, and there exists an appearance of two groups of crystal planes with the neighboring interlayer distances of 0.28 and 0.47 nm, which are indexed to the (220) and (311) planes of FeCo2O4 spinel phase, respectively. The result is consistent with that of XRD analysis. Besides, the corresponding selected area electron diffraction (SAED) pattern (the inset of Figure 3d) displays some legible spots, inferring the monocrystalline nature of the FeCo2O4 nanoneedle. Similarly, the detailed morphological and structure features of CoFe2O4 nanoneedles array are also researched by SEM and TEM, as exhibited in Figure S3. According to Figure S3a and S3b, a large area covered by CoFe2O4 nanoneedles with a uniform length of about 1.5−2 μm and diameter of about 60 nm is readily observed. The similar results are confirmed by the TEM image in Figure S3c. The HRTEM image, as presented in Figure S3d, provides some clear crystal lattice with the interplanar spacing of 0.24 nm corresponding to the (311) crystal plane of CoFe2O4, which confirms the successful synthesis of CoFe2O4 nanoneedles array on Ni foam. To research the possible applications of the FeCo2O4 nanoneedles array in LIBs, the corresponding electrochemical tests are conducted under ambient temperatures using twoelectrode coin cells with FeCo2O4 nanoneedles array on Ni foam acting as the binder-free working electrode and lithium foils serving as both the counter electrode and the reference electrode. Notably, the direct growth of FeCo2O4 nanoneedles array on Ni foam guarantees robust mechanical adhesion and excellent electrical contact with the current collector (nickel foam) in such an additive-free electrode. The electrochemical performances of FeCo2O4 nanoneedles array are demonstrated in Figure 4 to evaluate the lithium storage property. Figure 4a presents the initial three cyclic voltammetry (CV) curves of the FeCo2O4 nanoneedles array electrode obtained at a scan rate of 0.1 mV s−1 and a potential window of 0.01−3.0 V. From Figure 4a, the redox peaks of the first cycle are obviously different than those of subsequent cycles, and there is no considerable distinction between the second and third cycles. In the initial reaction stage, the FeCo2O4 nanoneedles are electrochemically discharged with lithium metal accompanied by the Li ion insertion into the crystal lattice of FeCo2O4, resulting in the

Figure 2c reveals the FeCo2O4 nanoneedles array from a different angle by tilting the sample, offering a clear horizon of the needle-like morphology. The average length of the FeCo2O4 nanoneedles is approximately 2 μm, and the diameters change from 70 to 100 nm, as exhibited in Figure 2d and its inset. Besides, the structure, from the root to the roof of nanoneedle with different sizes, is beneficial to fast transmission of electron and diffusion of lithium ion. Transmission electron microscopy (TEM) is an effective characterization tool to provide more insight into the morphology and microstructure of the materials. Therefore, we relied on the TEM to authenticate the structural and crystallographic properties of the as-prepared FeCo2O4 nanoneedles. The low-magnification TEM image of (Fe, Co) bimetallic hydroxide carbonate precursors is revealed in Figure 3a, in which the

Figure 3. (a) TEM image of the (Fe, Co) bimetallic hydroxide carbonate precursors; (b) TEM image of FeCo2O4 nanoneedles array exfoliation from nickel foam substrate; (c) the amplifying TEM image of individual FeCo2O4 nanoneedle; (d) HRTEM image of FeCo2O4 nanoneedle (the inset is the corresponding SAED pattern of FeCo2O4 nanoneedle).

morphology is good agreement with the SEM image in Figure 2a. Figure 3b is the representative high-resolution TEM image of FeCo2O4 nanoneedles after burning at 550 °C for 200 min, obtained by scraping from the Ni foam substrate. From Figure 3b, it can be seen that the average length of the FeCo2O4 nanoneedles is approximately 2 μm, whereas the diameter of the root segment of the nanoneedle can be estimated to be around 100 nm and only several nanometers at the top. Those results have no differences with those of SEM images in Figure 2. The formation of the nanoneedle shape could be related to the depletion of precursor during the growth process. Besides, a large number of pores can be easily observed in the nanoneedles, also verified by Figure 3c. Figure 3c clearly shows that the individual FeCo2O4 nanoneedle consists of abundant mesoporous structure, caused by the fact that the gases like H2O (g) and CO2 are released and lost during the decomposition or oxidation processes of the intermediates through thermal annealing.46 Figure 3c indicates that the pores are irregular and vary in the range of 5−10 nm. Actually, the pores are formed by the interconnection of some irregular shaped nanoparticles with an average particle size of 15 nm. To get better insight into the porosity and specific surface areas of the as-synthesized FeCo2O4 nanoneedles array, a powerful ultrasonic is employed to peel off the FeCo2O4 nanoneedles 1354

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initial charge and discharge capacities of the FeCo2O 4 nanoneedles array electrode are 2019 and 2573 mAh g−1, respectively, with the initial Coulombic efficiency of 78.5%. It is well-documented that the extra capacity at the first discharge results from the inactive process of some Li+ embedded into amorphous Li2O and the formation of the SEI floor on the interface of electrode. Additionally, there is a phenomenon that potential plateaus in the following discharge profiles is higher than that of the first discharge profile, simultaneously, accompanied by a sloping profile and capacity loss, which could be explained by eq 1. Moreover, the discharge capacities at the 2nd, 50th, 100th, and 200th cycles are 1962, 1918, 1715, and 1335 mAh g−1, respectively, indicating the admirable capacity retention of the FeCo2O4 nanoneedles array electrode. Figure 4c illustrates the Coulombic efficiency, charge− discharge capacity, and reversibility of the FeCo2O4 nanoneedles array electrode at a current rate of 100 mAh g−1. In Figure 4c, the discharge capacities of the FeCo2O4 nanoneedles array electrode reveal a stable trend within 100 cycles. Thereafter, an obviously decreasing trend of the discharge capacities is detected. Interestingly, the discharge capacities mildly rise after the ∼170th cycle, which could be ascribed to the reversible formation of a polymeric gel-like film originating from the slow kinetic activation process of the internal FeCo2O4 nanoneedles array. It is worth noting that the Coulombic efficiencies of the FeCo2O4 nanoneedles array electrode maintain at a high level of 92−95% even though the capacities are diminished, suggesting the admirable reversibility. As is well-known, the rate performance plays a significant role in evaluating the total properties. Thus, the rate performance of the FeCo2O4 nanoneedles array is also investigated and exhibited in Figure 4d. When the current densities increase step by step from 100 mA g−1 to 200, 500, 1000, and 2000 mA g−1 for 20 cycles, the corresponding average discharge capacities of the FeCo2O4 nanoneedles array electrode are 1951, 1618, 1488, 1174, and 875 mAh g−1, respectively. After the high-rate cycles, the current density comes back to 100 mA g−1, and the discharge capacity bounces back to 1594 mAh g−1 with a capacity retention of 81.7%, indicating the admirable rate performance. At the same time, in order to demonstrate the effects of morphology and Fe-content on the electrochemical performance, CoFe2O4 nanoneedles array and FeCo2O4 bulks are also prepared and investigated by multiple-step charge− discharge process at several different current densities from 100 mA g−1 to 2000 mA g−1. However, the discharge capacities of 1712, 1504, 1202, 870, and 526 mAh g−1 for CoFe2O4 nanoneedles array, versus 1698, 1438, 1061, 655, and 323 mAh g−1 for FeCo2O4 bulks, respectively, are acquired at the same current densities. Furthermore, the discharge capacities of CoFe2O4 nanoneedles array and FeCo2O4 bulks restore to 1284 and 947 mAh g−1, respectively, when the current density goes back to 100 mA g−1. Distinctly, the rate performance of FeCo2O4 nanoneedles array electrode is notably superior to CoFe2O4 nanoneedles array and FeCo2O4 bulks. To better research the stability of the FeCo2O4 nanoneedles array, CoFe2O4 nanoneedles array, and FeCo2O4 bulks, the charge− discharge experiments at the current density of 100 mAh g−1 with longer cycles are carried out, and the corresponding results are displayed in Figure S4. Obviously, the stability of FeCo2O4 nanoneedles array electrode is also satisfactory, compared to the CoFe2O4 nanoneedles array and FeCo2O4 bulks. Notably, the specific capacities of FeCo2O4 nanoneedles array hardly change within 100 cycles. Then a visible decline of specific

Figure 4. (a) Cyclic voltammograms (CVs) of the FeCo2O4 nanoneedles array at a scan rate of 0.1 mV s−1 and a voltage window of 0.01−3.0 V; (b) Galvanostatic charge−discharge profiles of the FeCo2O4 nanoneedles array at a current density of 100 mA g−1; (c) Cycle performance and Coulombic efficiency of FeCo2O4 nanoneedles array at a current density of 100 mA g−1; (d) Rate performances of FeCo2O4 nanoneedles array, CoFe2O4 nanoneedles array, and FeCo2O4 bulks at various current densities.

distortion of the crystal structure, and the distortion further evolves into the destruction of crystal lattice of FeCo2O4 with the increase of intercalated Li+, leading to the formation of amorphous phase Li2O and the acquisition of (Fe, Co) metal nanoparticles. The fact could explain the changes of CV curves between the first cycle and following cycles. In the first cycle, an irreversible and broad cathodic peak is detected at 0.48 V, which is ascribed to the reduction process of FeCo2O4 to Fe and Co by the insertion of lithium and the formation of amorphous Li2O (the electrochemical process in eq 7). In addition, an accompanying reduction peak of about 0.3 V is related to the formation of a solid electrolyte interface (SEI) layer caused by the decomposition of organic electrolyte. In the anodic scan of the first cycle, the oxidation peak of 2.10 V is observed, corresponding to the oxidation reaction and formation of FeO and CoO. Compared with the first cycle, the reduction peaks in the second and third cycles are shifted to a higher potential at ∼0.88 V, and the anodic peak positions move to 2.00 V, attributing to the reduction and oxidation reactions of the iron oxides and cobalt oxides, described by eqs 8−10. The changes of redox peaks in the following cycles might be relevant to some activation process originated from the Li+ insertion in the first cycle, as discovered in previous reports.48,49 What’s more, the CV profiles of the subsequent cycles tend to overlap, inferring the excellent reversibility of lithium storage. FeCo2O4 + 8Li+ + 8e− → Fe + 2Co + 4Li 2O

(7)

Fe + Li 2O ↔ 2Li+ + FeO + 2e−

(8)

Co + Li 2O ↔ 2Li+ + CoO + 2e−

(9) +

CoO + 1/3Li 2O ↔ 1/3Co3O4 + 2/3Li + 2/3e

−

(10)

Representative charge−discharge profiles of the 1st, 2nd, 50th, 100th, and 200th cycles at a current density of 100 mA g−1 with the potential window of 0.01−3.0 V vs Li+/Li are presented in Figure 4b. As displayed in Figure 4b, the smooth charge−discharge curves are observed, illustrating the potential stability of the electrode structure in the voltage window. The 1355

DOI: 10.1021/acsami.5b10237 ACS Appl. Mater. Interfaces 2016, 8, 1351−1359

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substitution the Fe ions for Co ions in the spinel Co3O4, owns admirable electrochemical performance for LIBs. As discussed above, several contributing factors are indispensable to improve electrochemical performance, such as the high specific capacity, remarkable rate performance, excellent reversibility, and long cycle life. First, the existence of abundant pores in the FeCo2O4 nanoneedles array enlarges the electrode−electrolyte contact areas for enhanced migration of lithium ions through the interface, enhancing the rate performance. Meanwhile, the porous structure could remit the stress caused by volumetric expansion and contraction in the repeated Li+ insertion and extraction processes, contributing to the favorable cyclability and stability. Second, the novel morphology of the FeCo2O4 nanoneedles array, of which the diameters from the root to the roof in the nanoneedles gradually decrease to only several nanometers, is advantageous for the fast transmission of electrons and the diffusion of the lithium ion due to the shorter transport pathway, enhancing the kinetics. Third, the fact that FeCo2O4 nanoneedles array directly adhere on nickel foam not only provides a high active mass loading but also enhances the electronic conductivity of the binder-free electrode. Moreover, when the FeCo2O4 nanoneedles array on nickel foam are directly used as the binder-free electrode, they can decrease the contact resistance brought by the binder agent. In addition, the FeCo2O4 nanoneedles separately grow on the nickel foam to form the arrays, which is benefit to avoiding the aggregation and pulverization of the electrode material, increasing the stability. Finally, cobalt-based oxides possess preferable electrochemical performance relative to the iron-base oxides. However, iron owns the superior electrical conductivity. Therefore, Fe partly substitutes for Co in Co3O4 to form FeCo2O4, which could integrate the advantages of the two elements to improve the electrochemical properties and decrease the cost of production. Bearing those in mind, the electrochemical performance of FeCo2O4 nanoneedles array on nickel foam can be tremendously improved by those features.

capacities is detected between 120 and150 cycles, which is ascribed to the fact that the stand-up FeCo2O4 nanoneedles on the top of the Ni foam collapse. After 170 cycles, the discharge capacities gradually increase, because the slow kinetic activation process of the FeCo2O4 nanoneedles array on the internal and bottom of the Ni foam leads to the formation of the polymeric gel-like film. Finally, the capacities slowly decrease with the increase of the cycling number. However, the specific capacities of CoFe2O4 nanoneedles array sharply decrease after 50 cycles, and then gradually reduce after 150 cycles. For FeCo2O4 bulks, the specific capacities severely decrease consistently. After 350 charge−discharge cycles, the discharge capacity of FeCo2O4 nanoneedles array remains at 1129 mAh g−1 with a capacity retention of 57.5%, which is higher than 36.8% for CoFe2O4 nanoneedles array, 19.3% for FeCo2O4 bulks and the values reported in recent literatures, as presented in Table S2. Figure 5

Figure 5. Nyquist plots of the FeCo2O4 nanoneedles array and the CoFe2O4 nanoneedles array for (a) 1st cycle and (b) 200th cycle.

displays the SEM images of the FeCo2O4 nanoneedles array on nickel foam after 200 and 350 cycles. It can be seen that the morphology of the FeCo2O4 nanoneedle has little change after 200 charge−discharge cycles, as shown in Figure S5a. Additionally, after 350 cycles, the orderly arrays structure is roughly maintained, except for the slight agglomeration (Figure S5b), indicating the excellent stability of structure. The electrochemical impedance spectra (EIS) of FeCo2O4 and CoFe2O4 nanoneedles array were also comparatively recorded in Figure 5 before and after charge−discharge tests. The corresponding Nyquist plots of both FeCo2O4 and CoFe2O4 electrodes are similar, which are consist of a semicircle at high frequency and a straight line with a slope of about 45 deg at low frequency. The high frequency semicircle is assigned to the electrochemical reaction and the semicircle diameter is on behalf of the value of charge transfer resistance (Rct). Obviously, according to Figure 5a, the semicircle diameter of FeCo2O4 nanoneedles array electrode is a little bigger than that of the CoFe2O4 nanoneedles array electrode, on account of the lower Fe content in FeCo2O4 nanoneedles array electrode. Fe-contained composites possess excellent electronic conductivity, and therefore, it is reasonable that the higher the Fe content, the smaller the charge transfer resistance. However, the value of Rct for CoFe2O4 nanoneedles array electrode apparently becomes bigger after 200 charge− discharge cycles, compared with that of FeCo2O4 nanoneedles array, as revealed in Figure 5b. The results illustrate that the stability and reversibility of FeCo2O4 nanoneedles array electrode are superior to those of CoFe2O4 nanoneedles array electrode, which coincides well with the conclusion from Figure 4 and S4. The above consequences infer that the 1D FeCo2O4 nanoneedles array, which could be considered as the partial

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CONCLUSION In summary, we have synthesized FeCo2O4 nanoneedles array on nickel foam through a facile, green and energy-saving preparation technique. The as-obtained FeCo2O4 nanoneedles array with the high surface area of 58.49 m2 g−1 and an average pore distribution of 5.85 nm display the promising electrochemical performance in terms of the high specific capacity (1962 mAh g−1 at the current density of 100 mA g−1), remarkable reversibility (Coulombic efficiency of approximately 92−95%), and outstanding rate performance (discharge capacity of 875 mAh g−1 at the current density of 2000 mA g−1), because of the effective conductive transport path, reduced electron- and ion-transport pathways, as well as directly growth on nickel foam for binder-free electrode. There is no doubt that FeCo2O4 nanoneedles array directly grown on nickel foam have great potential applications in high-performance storage systems.

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EXPERIMENTAL SECTION

Materials. All materials or chemicals were analytical grade reagents without any further purification. Deionized water was employed during the whole experimental process. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 99.9%, Aldrich), ferrous chloride tetrahydrate (FeCl2· 4H2O, 99.9%, Aldrich), urea (CO(NH2)2, 99.9%, Aldrich), ammonium 1356

DOI: 10.1021/acsami.5b10237 ACS Appl. Mater. Interfaces 2016, 8, 1351−1359

Research Article

ACS Applied Materials & Interfaces fluoride (NH4F, 99.9%, Aldrich), anhydrous ethanol (Fisher Chemical, 99.99%), and nickel foam. Materials Synthesis. FeCo2O4 and CoFe2O4 nanoneedles array were synthesized directly on nickel foam by a one-step hydrothermal synthesis accompanied by the low-temperature calcination. At first, some preparation work should be carried out before the reaction. A piece of nickel foam (1 × 4 cm2 in sizes) was prepared and repeatedly cleaned by an ultrasound bath for 15 min using 3 M HCl aqueous solution, with the purpose of eliminating the possible nickel oxide layer on the surface of nickel foam. Then it was ultrasonically washed with deionized water and anhydrous ethanol for several times (15 min of successive sonication in each time). In the synthesis, deionized water (20 mL), anhydrous ethanol (5 mL), 1 M Co(NO3)2 aqueous solution (2 mL), and 1 M CO(NH2)2 aqueous solution (6 mL) were mixed step-by-step under continuous stirring with time intervals of about 1− 2 min. After that, 198.81 mg of FeCl2·4H2O and 74.08 mg of NH4F were added into the above mixture solution, respectively. At the moment, a distinct color change of resultant mixture could be observed from light pink into orange. Then, the resultant mixture was constantly stirred for 20 min, and the as-obtained mixture solution was poured into a 45 mL polytetrafluoroethylene (Teflon)-lined stainlesssteel autoclave. Additionally, a piece of pretreated nickel foam was obliquely inserted into the mixture solution in the Teflon holder. Following that, the autoclave was heated at 120 °C for 9 h in an electric oven. When it was naturally chilled to room temperature in air, the brown Co−Ni bimetallic hydroxide carbonate salts precursors grown directly on the nickel foam were obtained. Afterward, deionized water and anhydrous ethanol were used to immerse the produced materials with the assistance of ultrasonication. Next, moisture was removed from the as-synthesized precursors for subsequent applications in a vacuum dryer at 60 °C overnight. Following that, the as-prepared precursor was put into a quartz tube and calcined at 550 °C for 200 min under air atmosphere with a heating rate of 5 °C min−1 to obtain the porous FeCo2O4 nanoneedles array. After calcination, the brown precursors turned into black FeCo2O4. The loading density of FeCo2O4 nanoneedles array attached on nickel foam was calculated as 1.68 mg cm−2, which was obtained by weighing the mass of the bare nickel foam (treated at 550 °C for 200 min in air) in a high-precision analytical balance (Sartorius, max weight 5100 mg, d = 0.001 mg). For the synthesis of CoFe2O4 nanoneedles array on nickel foam, 1 M Co(NO3)2 aqueous solution (1 mL) and 397.62 mg of FeCl2·4H2O was used in the process. The other reagents, dosage, and reaction conditions are the same with the FeCo2O4 nanoneedles array. As for the preparation of FeCo2O4 bulks, excluding that the autoclave without a piece of nickel foam was heated for 14 h at 180 °C in an electric oven, the other conditions are the same as that of FeCo2O4 nanoneedles array. Characterization of Materials. the crystal texture and purity of the as-synthesized samples were characterized by powder X-ray diffraction (Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.54184 Å)). The compositions of the samples and accurate contents of elements were examined by energy-dispersive Xray spectroscopy (EDS) attached to field-emission SEM (JEOL JSM7800F) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES, iCAP 6300 Duo). The valence states of elements in the samples were identified by using the X-ray photoelectron spectrometery with an ESCALAB250 analyzer (XPS). The morphology and architectural features of the samples were determined by field-emission scanning electron microscopy (SEM, JEOL JSM-7800F) and transmission electron microscopy (TEM, Philips, Tecnai, F30, 300 kV). The surface area and pore size of the samples were measured by Brunauer−Emmett−Teller surface area measurement (BET, Quantachrome Autosorb-6B surface area and Pore size analyzer). Electrochemical Measurements. The tests of the properties of LIBs were performed under indoor temperatures using two-electrode half-cells, wherein the lithium foil synchronously served as both the counter electrode and the reference electrode. A piece of nickel foam containing 1D porous FeCo2O4 nanoneedles array was utilized as the working electrode without any conducting materials or organic binder. Additionally, a polypropylene microporous film was implied as the

separator. The electrolyte consisted of 1 M LiPF6, DMC, EC, and DEC. The assembly of the cells was carried out in an argon-filled glovebox with both moisture and oxygen content below 0.1 ppm. A Neware battery testing system (model 5 V 5 mA) was used to test the galvanostatic charge−discharge cycling of the 1D porous FeCo2O4 nanoneedles array/Ni foam in a voltage window of 0.01−3.0 V at various current density. The CV data were recorded at a scanning rate of 0.1 mV s−1 and a same potential window with the galvanostatic tests, using an Autolab with the model AUT71740. For the electrochemical impedance spectroscopy (EIS) measurements, the open circuit potential (OCP) was determined first, and then the AC potential was set at ±10 mV (rms) around the OCP. The electrochemical analyzer (model CHI604E) was conducted to collect the impedance data at the frequency range of 0.01 to 100 Hz for following model fitting.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10237. XRD patterns, EDS spectra and SEM images of precursors and CoFe2O4 nanoneedles array, BET of the FeCo2O4 nanoneedles array, SEM images of FeCo2O4 nanoneedles array after electrochemical tests (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mails for Y.W.: [email protected]; prospectwy@gmail. com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Thousand Young Talents Program of the Chinese Central Government (Grant No. 0220002102003), National Natural Science Foundation of China (NSFC, Grant No. 21373280, 21403019), the Fundamental Research Funds for the Central Universities (0301005202017), Beijing National Laboratory for Molecular Sciences (BNLMS), and Hundred Talents Program at Chongqing University (Grant No. 0903005203205).

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REFERENCES

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