Fe3O4 Nanorod Array

Jul 5, 2017 - Synopsis. A facile hydrolysis-coupled redox reaction followed by postheating reduction has been designed to prepare a unique 3D Cu/Fe3O4...
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Hydrolysis-Coupled Redox Reaction to 3D Cu/Fe3O4 Nanorod Array Electrodes for High-Performance Lithium-Ion Batteries Heyun Gu,†,∥ Yingmeng Zhang,†,∥ Mengqiu Huang,†,∥ Fei Chen,† Zeheng Yang,† Xiaoming Fan,† Sheng Li,† Weixin Zhang,*,† Shihe Yang,*,‡ and Mei Li§ †

School of Chemistry and Chemical Engineering, Anhui Key Laboratory of Controllable Chemical Reaction & Material Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, People’s Republic of China ‡ Department of Chemistry, William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China § School of Chemistry, Centre for Organized Matter Chemistry, University of Bristol, Bristol BS8 1TS, U.K. S Supporting Information *

ABSTRACT: A facile hydrolysis-coupled redox (HCR) reaction followed by postheating reduction has been designed to prepare unique 3D Cu/Fe3O4 core− shell nanorod array anodes. Fe2+ ions from fresh FeSO4 solution have been hydrolyzed and oxidized to form an Fe(OH)3 shell on the surface of Cu(OH)2 nanorods; meanwhile the resulting acidic environment induces the reduction of Cu(OH)2 to Cu2O, which realizes an unusual redox reaction between Fe2+ ions and Cu(OH)2. The reaction procedure and thermodynamics possibility between Fe2+ ions and Cu(OH)2 nanorod arrays are discussed from the aspect of electrode potentials. After postheating reduction in Ar/H2, the obtained 3D architecture of Cu current collector serves as a stout support for the Fe3O4 shell to form nanorod array anodes without using any binders or conducting agents. The resulting highly stable core−shell structure facilitates rapid and high-throughput transport pathways for ions/electrons and allows better accommodation of volume change during the repeated lithiation/delithiation. Its application as anodes in combination with LiNi0.5Mn1.5O4 cathodes for full cells demonstrates superior rate capability, enhanced energy density, and long cycling life.

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INTRODUCTION With their rapid penetration into portable electronics, electric vehicles, and hybrid electric vehicles, rechargeable lithium-ion batteries (LIBs) are facing ever more challenges in raising energy density, cycle life, safety, and other performance metrics. The search for a new electrode configuration that could improve the energy density and cycle life of LIBs is one of today’s most challenging issues for next-generation LIBs.1,2 Transition-metal oxides (TMOs) with high capacities and high safety have been regarded as promising candidates to replace commercial graphitean intercalation/deintercalation type anode material, which only delivers a theoretical specific capacity of 372 mAh g−1 and lower rate performance due to safety concerns.3,4 One major problem of the TMO-based conversion-type anodes is their severe capacity fading and poor cycling life in comparison with graphite, which partially results from the limited conductivity and the lithiation/delithiation deterioration of the electrode associated with the volume change and pulverization (MOx + 2xLi ↔ M + xLi2O). Some reported strategies could improve the electrochemical performance of TMO anodes by facilitating the electron and lithium ion transport to some extent: for instance, synthesizing TMO nanomaterials with controlled morphologies and integrating a carbonaceous matrix into active materials.5,6 © 2017 American Chemical Society

Furthermore, the design of TMO nanoarrays directly grown on plane (2D) current collectors can more or less solve the problems of the volume strain and the electrical conductivity, but the kinetic limitations are still acute for conversion reactions, owing to the poor electronic/ionic conductivity of the MxOy/M0/Li2O matrix.7−13 To promote current collector/active material surface contacts, three-dimensional (3D) nanoarray current collectors instead of plane current collectors have attracted much attention in recent years. Except for the good accommodation of the large volume change, each particle of active material almost has its “own” current collector, which will be beneficial to shorten the diffusion length for ions and provide rapid transport pathways for electrons.14−16 Among a group of TMOs, Fe3O4 anode materials have attracted interest due to its high theoretical capacity (926 mAh g−1), relatively high electronic conductivity (2 × 10−4 S m−1), environmental friendliness, and so forth.17−21 A 3D nanostructured Cu/Fe3O4 electrode was reported to be prepared by direct electrodeposition of Cu nanorod arrays onto a copper foil with the help of an anodic aluminum oxide (AAO) membrane, followed Received: January 21, 2017 Published: July 5, 2017 7657

DOI: 10.1021/acs.inorgchem.7b00112 Inorg. Chem. 2017, 56, 7657−7667

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Inorganic Chemistry

substrate can be directly used as nanoarray electrodes in LIBs. The electrochemical properties of the as-prepared Cu/Fe3O4 electrode were investigated by using coin-type cells (CR2032) with Cu/Fe3O4 nanorod arrays as working electrodes and metallic lithium disks as counter electrodes. The liquid electrolyte used was 1.0 M LiPF6 in a 1/ 1 (by volume) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), and the separator was Celgard 2400 microporous polypropylene membrane. The coin-type cells were assembled in an Ar-filled dry glovebox. Finally, galvanostatic charge and discharge tests were conducted between 0.01 and 3.0 V on a multichannel battery tester (Newware Battery Test System, Shenzhen Neware Electronic Co, People’s Republic of China) at different currency rates (1 C = 1000 mA g−1) at 25 °C. The cyclic voltammetry (CV) tests were carried out on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co. Ltd.) in the voltage range of 3.0−0.01 V at a 0.1 mV s−1 scan rate. In order to highlight the electrochemical performance of the 3D Cu/Fe3O4 nanorod array electrodes, a bare Fe3O4 electrode was obtained by mixing the Fe3O4 nanorod powder with carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 7/2/1 and then dispersing the mixture in N-methylpyrrolidinone (NMP) solvent. The obtained slurry was then cast onto the Cu current collector and dried for 12 h in an oven at 100 °C under vacuum. The resulting electrode film was subsequently pressed and punched into a circular disk with a diameter of 12 mm. The battery assembly process and electrochemical properties test conditions are the same for the Fe3O4 electrode and 3D Cu/Fe3O4 nanorod array electrode. Assembly of the 3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 Full Cell and Electrochemical Measurements. A detailed procedure of the assembly and testing process for Cu/Fe3O4 electrodes is as follows. The LiNi0.5Mn1.5O4 material was obtained from BASF SE. The LiNi0.5Mn1.5O4 electrode was obtained by mixing the LiNi0.5Mn1.5O4 powder with carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 8/1/1 and then dispersing the mixture in Nmethylpyrrolidinone (NMP) solvent. The obtained slurry was then cast onto the Al current collector and dried for 12 h in an oven at 100 °C under vacuum. The resulting electrode film was subsequently pressed and punched into a circular disk with a diameter of 12 mm. The 3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 full cells were assembled after the preconditioning procedures.26−28 Prior to the full cell testing, the Fe3O4 (3D Cu/Fe3O4 nanorod arrays) anode was cycled at 0.1 C (1 C = 1000 mA g−1) in the potential range of 0.01−3.00 V (versus Li/Li+) for five cycles and then charged to 3.0 V in a 3D Cu/Fe3O4/Li halfcell. The LiNi0.5Mn1.5O4 cathode was also cycled at 0.1 C (1C = 140 mA g−1) in the potential range of 3.40−4.90 V (versus Li/Li+) for five cycles and then discharged to 3.4 V in a LiNi0.5Mn1.5O4/Li half cell. In order to take full advantage of the capacity of the LiNi0.5Mn1.5O4 cathode in 3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 full cell, the anode to cathode capacity ratio is ∼1.2, calculated according to the practical capacities of the two electrodes in half cells: i.e., a capacity of about 0.66 mAh for the LiNi0.5Mn1.5O4 cathode obtained at the fifth cycle at 0.1 C and a capacity of 0.81 mAh for the Cu/Fe3O4 anode obtained at the fifth cycle at 0.1 C. Hence, the LiNi0.5Mn1.5O4 cathode in principle limits the capacity of the full cell. The operating voltage window of the full-cell testing was determined by the limited capacity of the LiNi0.5Mn1.5O4 cathode corresponding to the LiNi0.5Mn1.5O4/Li halfcell testing. First, when the full cell was charged to a capacity of 0.66 mAh, which is equal to the capacity of the LiNi0.5Mn1.5O4/Li half cell, the upper cutoff voltage of full cell at this moment was 4.306 V. Afterward, when the full cell was discharged to the capacity 0.66 mAh, the lower cutoff voltage of the full cell at this moment was 0.249 V. As a result, the voltage window of the LiNi0.5Mn1.5O4-limited full cell was 0.25−4.31 V. Mass Determination of Fe3O4 Active Materials in the 3D Cu/ Fe3O4 Nanorod Arrays on Cu Substrate. The weight of Fe3O4 in the 3D Cu/Fe3O4 nanorod arrays was determined on the basis of the following method. First, the bare Fe(OH)3 material was obtained when the Cu2O content in the precursor of Cu2O/Fe(OH)3 (m1) was dissolved entirely in dilute ammonia solution, and then the Fe(OH)3 (m2) was obtained by scraping it from the copper substrate and

by the electrodeposition of nanosized Fe3O4, which displayed a 6-fold improvement in power density over a 2D planar electrode.18 In addition, template-assisted and subsequent electrochemical deposition methods were also reported to prepare 3D metal/Fe3O4 nanostructured electrodes.19−21 However, it still remains a challenge to rationally design and easily fabricate 3D metal/Fe3O4 nanoarrays with well-defined scaffolds for practical electrode manufacture. Herein, we report a unique anode configuration with a 3D Cu/Fe3O4 nanorod array structure, which is prepared via a facile hydrolysis-coupled redox (HCR) reaction followed by a simple postheating reduction. The obtained 3D Cu/Fe3O4 nanorod array anode contributes greatly to the enhanced rate performance and improved cycle stability for LIBs, due to the conductive Cu core of nanorod arrays directly grown from the planar substrate forming a 3D current collector, which ensures that Fe3O4 nanoparticles have a direct electron transport pathway to their “own” current collector (the nanorod core), allowing for effective electron conduction and significant interfacial resistance reduction.

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EXPERIMENTAL SECTION

Preparation of the 3D Cu/Fe3O4 Nanorod Arrays. The typical procedure used to prepare 3D Cu/Fe3O4 nanorod arrays on the copper substrate is as follows. The synthesis of Cu(OH)2 nanorod arrays grown on copper substrate (diameter 12 mm) was carried out as in our previous reports.22−24 The as-prepared Cu(OH)2 nanorod arrays on Cu substrate (diameter 12 mm) were immersed in 25 mL of FeSO4 solution (5 mmol L−1) for 12 h. Then the sample was rinsed with distilled water and absolute ethanol, respectively, and dried in air. The dried sample was loaded into a quartz boat and positioned in the middle of a quartz tube. After purging with Ar/H2 for 30 min to remove O2 in the quartz tube, the furnace was heated and maintained at 180 °C for 3 h with a heating rate of 1 °C min−1 and then left at 350 °C for 3 h. Afterward, the furnace was cooled to room temperature naturally, and black films on the copper substrates were obtained. Preparation of Fe3O4 Nanorod Powder. First, the precursor of the Fe3O4 powder was prepared by a hydrothermal method.25 In a typical synthesis process, 0.972 g of iron chloride (FeCl3; 6 mmol) and 0.9 g of urea (CO(NH2)2; 15 mmol) were dissolved in 45 mL of distilled water by magnetic stirring. The solution was sealed in a 60 mL Teflon-lined stainless steel autoclave and kept at 120 °C for 6 h. After it was cooled to room temperature, the precipitate was washed three times with distilled water and another three times with ethanol and then dried in a vacuum oven at 80 °C for 12 h. Then the crystalline Fe3O4 nanorods were obtained by heating the precursor at 180 °C for 3 h and further at 350 °C for 3 h under an Ar/H2 atmosphere, which were the same conditions as those for the preparation of 3D Cu/Fe3O4 nanorod arrays. Sample Characterization. The as-synthesized samples were characterized by X-ray powder diffraction (XRD) on a Rigaku D/ max-γB X-ray diffractometer (Japan) with a Cu Kα radiation source (λ = 0.15418 nm) operated at 40 kV and 80 mA. Field-emission scanning electron microscopy (FESEM) measurements were conducted on a Hitachi SU8020 scanning electron microscope operated at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) images and the selected area electron diffraction (SAED) patterns were taken on Hitachi H-800 and JEOL 2010 transmission electron microscopes operated at an accelerating voltage of 200 kV. The Brunauer−Emmett−Teller (BET) special surface areas of the samples were determined by nitrogen adsorption and desorption isotherms and measured at the temperature of liquid nitrogen on a NOVA 2200e surface area analyzer. X-ray photoelectron spectra (XPS) were obtained on a ESCALAB 250Xi X-ray photoelectron spectrometer with Al Kα radiation as the excitation source. Electrochemical Measurements of the 3D Cu/Fe3O4 Nanorod Array Half Cell. The Cu/Fe3O4 nanorod arrays on the copper 7658

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Figure 1. Fabrication of 3D Cu/Fe3O4 nanorod arrays on copper substrates.

Figure 2. (a) XRD patterns of as-prepared samples on copper substrates. (b) FESEM image of Cu2O/Fe(OH)3 nanorod arrays. Inset: TEM image of a single Cu2O/Fe(OH)3 nanorod. (c, d) FESEM images of Cu/Fe3O4 nanorod arrays, with (c) giving a top view. Inset of (d): TEM image of a single Cu/Fe3O4 nanorod. (e−i) SEM images, corresponding to elemental mappings, and SEM-EDX spectrum of elements for a single Cu/Fe3O4 nanorod on a Si carrier. weighing it. Second, we knew the total moles of Fe3+ and converted it to the molar mass of Fe3O4, and then the weight of Fe3O4 (m3) was

obtained by calculation. The weight of Fe3O4 on the copper substrate was calculated using the equation 7659

DOI: 10.1021/acs.inorgchem.7b00112 Inorg. Chem. 2017, 56, 7657−7667

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Inorganic Chemistry m3 =

m2 MFe(OH)3

×

m2 1 1 × MFe3O4 = × × 231.5 = 0.72m2 3 106.9 3

SHE) is higher than that of Cu2+/Cu+ (0.159 V vs SHE), which often causes the redox reaction between Fe3+ ions and Cu+ ions.29,30 However, it is interesting to find that the galvanic reaction in this study could be driven by the difference in the reduction potentials of the Fe(OH)3/Fe2+ (−0.546 V) and Cu(OH)2/Cu2O (−0.346 V) pairs, which were calculated on the basis of the acidic experimental conditions (Appendix 1 in the Supporting Information). Thus, Cu(OH)2 can be reduced by Fe2+ ions and changed into Cu2O at room temperature, as observed in our experiment. The starting hydrolysis of the Fe2+ ions from fresh FeSO4 solution forms a Fe(OH)3 shell32 at the interfaces of the Cu(OH)2 nanorod/solution, and at the same time the released H+ ions (hydrolysis byproduct) create an acidic environment to induce the redox reaction. The reducing capability of Fe2+ ions in an acidic environment is dramatically increased, which can reduce and transform the Cu(OH)2 nanorod arrays into a Cu2O nanorod core inside the Fe(OH)3 shell. The detailed mechanism for the hydrolysis-coupled redox (HCR) strategy is shown in Figure 3.

The loading mass of active Fe3O4 in the 3D Cu/Fe3O4 nanorod arrays on the copper substrate (3.14 × 0.62 = 1.13 cm2) used in the coin-type cells was typically 0.8 mg.

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RESULTS AND DISCUSSION Preparation and Characterization of the 3D Cu/Fe3O4 Nanorod Arrays. The hydrolysis-coupled redox (HCR) strategy for preparing 3D Cu/Fe3O4 nanorod arrays on a copper substrate has been accomplished via several steps (Figure 1 and Figure S1 in the Supporting Information). First, the Cu(OH)2 nanorod arrays (Figure 2a(i) and Figure S2 in the Supporting Information), prepared on copper substrate according to our previous reports,22−24 reacted with the Fe2+ ions (from fresh FeSO4 solution) by an HCR reaction to give the intermediate Cu2O/Fe(OH)3 nanorod arrays, and then a subsequent postheating reduction was used to fabricate the 3D Cu/Fe3O4 nanorod arrays. The starting hydrolysis of the Fe2+ ions can cause Fe(OH)3 deposition at the interfaces of the Cu(OH)2 nanorod/solution and create an acidic environment with the released H+ ions; thus, a redox reaction between Fe2+ ions and Cu(OH)2 was induced in the acidic environment. The reducing capability of Fe2+ ions in an acidic environment was dramatically increased, which reduced and transformed the Cu(OH)2 nanorod arrays in situ into the cubic phase of a Cu2O nanorod core inside the Fe(OH)3 shell with low crystallinity, as shown by its XRD pattern (Figure 2a(ii)) and its TEM image (Figure 2b).29−31 Finally due to the reductive post-heattreatment atmosphere (Ar/H2), 3D Cu/Fe3O4 nanorod arrays on copper substrate can be fabricated, as shown by the XRD pattern (Figure 2a(iii)) and the FESEM images (Figure 2c,d). The reactions involved are 2Fe2 + + 3H 2O + 2Cu(OH)2 (s) → 2Fe(OH)3 (s) + Cu 2O + 4H+(l)

(1)

Cu 2O + H 2 → 2Cu + H 2O

(2)

6Fe(OH)3 + H 2 → 2Fe3O4 + 10H 2O

(3)

Figure 3. Detailed mechanism for the hydrolysis-coupled redox (HCR) strategy to prepare the intermediate Cu2O/Fe(OH)3 nanorod arrays on a copper substrate.

The final product Cu/Fe3O4 nanorod arrays with diameters of 500−800 nm display morphology similar to the Cu2O/ Fe(OH)3 nanorod arrays except for a slightly smaller diameter due to the volume shrinkage by a thermal treatment. The TEM image (inserted in Figure 2d) shows the preserved morphology of the hierarchical core−shell nanostructure with a layer of numerous thin Fe3O4 nanosheets instead of Fe(OH)3 nanoflakes (inserted in Figure 2b) assembled on the Cu nanorod surfaces. The FESEM elemental mappings of the Cu/Fe3O4 core/shell nanorod arrays (Figure 2e−h) show that the Fe and O element signals distribute uniformly and clearly over the entire nanorod region (∼800 nm), whereas the Cu signals dominate the middle of the nanorod (∼500 nm), which to some extent reveals the Fe3O4 shell growth on the Cu core surface. The elements in the SEM-EDX spectrum (Figure 2i) include Cu, Fe, and O except for the Si carrier, and the Fe/Cu mole ratio (close to 1.09) is almost consistent with the stoichiometric ratio of the redox reaction between Cu(OH)2 and Fe2+ ions (eq 1). Generally, the redox reaction between Fe2+ ions and Cu(OH)2 could not take place directly, because the standard reduction potential of the Fe3+/Fe2+ redox pair (0.771 V vs

To better understand the formation process of the core− shell Cu2O−Fe(OH)3 hybrid nanorod arrays on copper substrate, time-dependent analysis of the reaction process was performed. The gradual consumption of Cu(OH)2 nanorods and formation of Cu2O and Fe(OH)3 were monitored by XRD analysis (Figure 4a). The XRD pattern of the sample that was prepared by immersing the Cu(OH)2 nanorod arrays in FeSO4 solution for 3 h can be indexed to a mixture of cubic Cu2O (JCPDS No. 05-0667) and orthorhombic Cu(OH)2 (JCPDS No. 13-0420). After 6 h, the diffraction peaks of the sample could be indexed to cubic Cu2O, almost without any obvious Cu(OH)2. When the time was prolonged to 12 h, the diffraction peaks of the sample could be completely indexed to cubic Cu2O, which indicates that the Cu(OH)2 nanorod arrays have been totally reduced to Cu2O. Figure 4b shows the pH variation of the FeSO4 solution during the preparation process of the Cu2 O/Fe(OH) 3 intermediate on copper substrate (Figure 4b(iii)). In contrast, the pH variation was also measured in a reaction system using blank (Figure 4b(i)), bare copper substrate (Figure 4b(ii)), or 7660

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Figure 4. (a) XRD patterns of the samples prepared by the reaction between Cu(OH)2 nanorod arrays on copper substrate and FeSO4 solution with increasing reaction time. (b) pH-dependence studies of FeSO4 solution in different reaction systems: (i) blank; (ii) bare Cu foil; (iii) Cu(OH)2 array; (iv) Cu(OH)2 powder.

Figure 5. X-ray photoelectron spectra (XPS) of the 3D Cu/Fe3O4 nanorod arrays which were lightly scraped from the Cu substrate: (a) survey spectra; (b) Fe 2p (insert: Fe 2p3/2); (c) O 1s; (d) Cu 2p (insert: Cu LMM).

Cu(OH)2 powder (Figure 4b(iv)) instead of the Cu(OH)2 array on copper substrate. The initial pH value of the solution is found to be about 4.47. For the reaction system containing the Cu(OH)2 array on copper substrate (Figure 4b(iii)), the pH value went down quickly to about 4.01 for the first 4 h, corresponding to the hydrolysis and oxidation of Fe2+ ions and generation of H+ ions. Then, the generation of H+ ions began to slow down when Cu(OH)2 continued to consume the generated H+ ions, corresponding to the slow pH decline on the pH variation curve for the following 8 h (4−12 h). After 12 h of reaction, the pH tended to reach a stable value around 3.93, corresponding to the end of the HCR reaction.

There exists a similar variation tendency of pH value for the Cu(OH)2 powder containing system (Figure 4b(iv)). In contrast, for the blank (Figure 4b(i)) reaction system, the pH value dropped gradually during the entire hydrolyzing process and finally reached a stable value at about 4.11, indicating the dynamic balance of the hydrolysis reaction. This pH variation process characterizes the typical hydrolysis process of the reaction system. It can be observed that, with the generation of the H+ ions by a redox reaction (eq 1), the pH values for the reaction system with the Cu(OH)2 (array (Figure 4b(iii)) or powder (Figure 4b(iv))) were always lower than those without Cu(OH)2 (blank Figure 4b(i) or bare Cu foil (Figure 4b(ii))) during the hydrolysis process. The above discussion is quite 7661

DOI: 10.1021/acs.inorgchem.7b00112 Inorg. Chem. 2017, 56, 7657−7667

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Figure 6. (a) Cyclic voltammetric curves of 3D Cu/Fe3O4 nanorod arrays for the first through fifth cycles at a scan rate of 0.1 mV s−1 in the potential range of 0.01−3.00 V (versus Li/Li+). (b) Charge/discharge profiles of a 3D Cu/Fe3O4 nanorod array electrode in the voltage range 0.01−3.00 V at various current densities. (c) Rate performance comparison of as-prepared samples at various current densities. (d) Cycling performance comparison of as-prepared electrodes at a current density of 0.5 C for 200 cycles. (e) Cycling performance of as-obtained Cu/Fe3O4 nanorod arrays at current densities of 1, 2, and 5C on charging at 1C (1 C = 1000 mA g−1). (f) Cycling performance of Fe3O4 nanorod powders at current densities of 1, 2, and 5 C on charging at 1 C (1 C = 1000 mA g−1).

survey-scan XPS spectrum (Figure 5a) indicates the obvious existence of Cu, Fe, O, and C elements. The C 1s line of carbon contamination appears at 284.8 eV. For the Fe 2p spectrum (Figure 5b), two peaks at 724.1 and 711.2 eV correspond to the Fe 2p1/2 and Fe 2p3/2 peaks of Fe3O4, respectively.33−37 Moreover, the Fe 2p3/2 peak inserted can be well fitted with two components with a major part at 710.6 eV and a minor part at 712.4 eV, which can be ascribed to Fe2+ and Fe3+, respectively.34 Furthermore, there is no shakeup satellite peak situated at ∼719 eV, the fingerprint of the electronic structure of γ-Fe2O3, which clearly indicates that the product is pure magnetic Fe3O4.36,37 The O 1s spectrum (Figure 5c) indicates the existence of the lattice oxygen in the metal oxide (Fe3O4) and adsorbed oxygen (O2) with binding energies of 530.4 and 531.7 eV, respectively.37,38 Two peaks at 932.7 and 952.5 eV in

consistent with the formation mechanism of Cu2O/Fe(OH)3 nanorod arrays. The results indicate that the pH values vary from 4.5 to 3.9 during the entire reaction. The reaction systems with Cu(OH)2 nanorod arrays or Cu(OH)2 powders always exhibit lower pH values in comparison to the systems with a bare copper substrate or only FeSO4 aqueous solution. For the former systems, Cu(OH)2 can react with the Fe2+ ions to generate more H+ ions (eq 1), which makes the final pH values of the reaction solution much lower. However, for the latter systems without Cu(OH)2, only an ordinary hydrolysis of Fe2+ ions proceeds with slower pH value decrease to a higher final pH value in comparison to the former case. The XPS analysis (Figure 5) results of the as-prepared Cu/ Fe3O4 nanorod arrays lightly scraped from the Cu substrate are in good agreement with the XRD analysis (Figure 2a). The 7662

DOI: 10.1021/acs.inorgchem.7b00112 Inorg. Chem. 2017, 56, 7657−7667

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Figure 7. (a) Schematic illustration of utilization of a 3D Cu/Fe3O4 nanorod array for construction of a 3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 full cell. (b−d) Charge−discharge curves, rates, and cycling performance of the 3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 full cell (1 C = 140 mA g−1).

cycles between 0.01 and 3.00 V at a constant current density of 0.1 C (Figure S5a in the Supporting Information). Three peaks at 1.65, 1.02, and 0.68 V observed during the first cathodic scan could be ascribed to the two lithiation reactions of Fe3O4 involving the reduction of Fe3+ to Fe2+ and Fe2+ to Fe0, as well as the formation of a solid−electrolyte interface (SEI) film.5,10 Meanwhile, two anodic peaks at about 1.64 and 1.89 V correspond to the reversible oxidation of Fe0 to Fe2+/Fe3+.41−44 The general overlapping of CV curves from the second cycle implies the good reversibility and stability of the electrochemical reaction. The rate capability of the 3D Cu/Fe3O4 nanostructure electrode is illustrated in Figure 6b,c. The cell was first cycled at a rate of 0.1 C, and then the rate was increased to 0.2, 0.5, 1, 2, 5, and 10, and 15 C, finally decreased to 10, 5, 2, 1, 0.5, 0.2, and 0.1 C, respectively. Along with the increasing rate, the corresponding discharge capacities were measured respectively at 1024.3, 956.7, 886.5, 816.2, 773.0, 581.1, 448.6, and 289.2 mAh g−1, which were quite stable at each rate. The discharge capacities were correspondingly increasing to 1070.7 mAh g−1 when the rate came back to 0.1 C. The slightly increasing capacity might be attributed to a reversible formation of the gellike polymer layer.45 For comparison, the rate performance of an as-prepared Fe3O4 nanorod powder electrode fabricated

the Cu 2p spectrum (Figure 5d) correspond to Cu 2p3/2 and Cu 2p1/2, respectively, and the Cu LMM Auger peak inserted at a kinetic energy of 916.4 eV corresponds to an Auger parameter of 1849.1 eV, which is consistent with metallic Cu0.39,40 The HCR strategy for preparing Cu/Fe3O4 nanorod arrays on a Cu substrate has also been proved for the powder form preparation of Cu/Fe3O4 nanorod bundles (Figure S3 in the Supporting Information). The results show that the copper substrate does not take part in the hydrolysis and redox reactions but is a necessary support for the formation of Cu/ Fe3O4 nanorod arrays. Electrochemical Performances of the 3D Cu/Fe3O4 Nanorod Arrays. To demonstrate the advantages of the 3D nanorod array current collector in comparison with Fe3O4 nanorod powder (Figure S4 in the Supporting Information), the as-prepared 3D Cu/Fe3O4 nanorod arrays on copper substrate can be directly used as electrodes for LIBs. The 3D Cu/Fe3O4 nanorod arrays obtained in a reaction time of 12 h duringthe HCR reaction stage have been used as typical samples for electrochemical measurements. The observations in the cyclic voltammogram (CV) curves of 3D Cu/Fe3O4 nanorod arrays (Figure 6a) are quite consistent with the potential plateaus and reproducibility of the galvanostatic discharging/charging curves for the first through the tenth 7663

DOI: 10.1021/acs.inorgchem.7b00112 Inorg. Chem. 2017, 56, 7657−7667

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Inorganic Chemistry

325.26 mAh g−1 at 15 C, slightly higher than that for the sample with a reaction time of 12 h. However, among the three samples, the sample with a reaction time of 12 h shows the best cycling performance, with the largest discharge capacity of 791.8 mAh g−1 at 1 C after 500 cycles. As a result, we chose the sample with a reaction time of 12 h to assemble a complete Liion cell. The advantages of the 3D Cu/Fe3O4 nanorod array electrode for superior rate capability and high reversibility capacity in comparison with the conventional 2D plane electrode can be ascribed to the following points (Figure 7a). (1) The 3D Cu nanoarray current collector in a Cu/Fe3O4 nanorod array electrode (Figure S9 in the Supporting Information) provides a large contact surface for better dispersion of Fe3O4 nanoparticles to efficiently maintain the structural integrity of the active material, which is very important to keep high capacity retention upon hundreds of lithiations/delithiations. (2) More importantly, the nanorod array current collector constructed from metallic Cu with good conductivity can provide efficient transport pathways for electrons through the entire electrode architecture, which is the key to the excellent rate performance. (3) There is enough free space within and between neighboring nanorods to buffer the stresses caused by volume changes during the discharge/charge process and prevent the agglomeration of pulverized Fe3O4 nanoparticles, thus extending the cycling life of the electrode. (4) The Fe 3 O 4 nanostructures assembled on the 3D Cu nanorod arrays with proper specific surface area and porous structure (Figure S12 in the Supporting Information) can ensure efficient electrolyte penetration and enhance the contact area between the active material and electrolyte, which could improve the performance of LIBs. Electrochemical Performances of the 3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 Full Cell. To further demonstrate the potential application of the 3D Cu/Fe3O4 nanorod array, it was directly used as the anode for a full cell. The 3D Cu/Fe3O4 nanorod array as an anode and a LiNi0.5Mn1.5O4 (BASF SE, its XRD pattern, FESEM images, and half-cell performance are shown in Figure S13 in the Supporting Information) cathode were assembled into a complete Li-ion cell (cathode limited). Figure 7b shows the charge−discharge curves of the 3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 full cell at various rates corresponding to the LiNi0.5Mn1.5O4 electrode (1 C = 140 mA g−1) between 0.25 and 4.30 V versus Li/Li+. Interestingly, the voltage profiles of the 3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 full cell behave similarly to the LiNi0.5Mn1.5O4/Li half-cell due to the inherent voltage characteristics of LiNi0.5Mn1.5O4. The working voltage of the battery is about 2.75 V; the overall reaction of the complete Liion cell (3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 full cell) is expected to evolve as shown in eq 4):

from Fe3O4 nanorod powder (Figure S4 in the Supporting Information) blended with a polymer binder and carbon black paste showed that the Fe3O4 nanorod powder could only deliver a discharge capacity of 250 mAh g−1 at a rate of 10 C and hardly presented any discharge capacity at a current rate of 15 C (Figure 6c and Figure S5b,c in the Supporting Information). Obviously, the 3D Cu/Fe3O4 nanorod array electrode displayed much slower capacity decay than the Fe3O4 electrode with increasing rates. The 3D Cu/Fe3O4 nanostructure array electrode also shows overpotential advantages (with lower overpotential) in comparison with the Fe3O4 nanorod powder electrode (Figure S6 in the Supporting Information), indicating lower polarization of the array electrode at high rates. Figure 6d respectively shows a cycling performance comparison between the 3D Cu/Fe3O4 nanorod array and Fe3O4 nanorod powder electrodes at a current density of 0.5 C. For the 3D Cu/Fe3O4 nanorod array electrode, the initial Coulombic efficiency was around 71%, and the Coulombic efficiency remained nearly at 100% during the entire cycling process except for the first few cycles and almost no capacity faded after 200 cycles (Figure S5d), suggesting reversible lithium insertion/extraction associated with efficient transportation of ions and electrons in the electrodes. In contrast, the Fe3O4 electrode fabricated from the Fe3O4 nanorod powder showed poor cycling performance (Figure 6d) and Coulombic efficiency (Figure S5d in the Supporting Information). To further highlight the cycling stability (Figure 6e), the 3D Cu/Fe3O4 nanorod electrode was subjected to charging at 1 C and discharging at 1, 2, and 5 C, respectively. Then discharge capacities of 791.8, 691.4, and 543.5 mAh g−1 can be obtained after 500 cycles at rates of 1, 2, and 5 C, respectively. However, for the Fe3O4 nanorod powder electrode, only 49.6%, 69.5%, and 9.9% of capacity retention was obtained at 1, 2, and 5 C for 200 cycles, respectively (Figure 6f). These results reveal the higher rate capability and longer cycling life of 3D Cu/Fe3O4 nanorod arrays even at high current rate.18−21 The charge transfer resistances (Rct) for the 3D Cu/Fe3O4 nanorod array anode after the 3rd and 100th cycles are 80.6 and 132.4 Ω, respectively, which are smaller than those of 109.8 and 383.4 Ω for the Fe3O4 nanorod powder (Figure S7a,b in the Supporting Information). The Cu/Fe3O4 nanorod array anode possesses lower contact and charge-transfer resistances during repeated lithium cycling processes. After 100 cycles at 0.5 C, the 3D Cu/Fe3O4 nanorod array anode shows not much morphological change of the nanoarrays (Figure S8 in the Supporting Information), further confirming the chemical and mechanical stability of the 3D Cu/Fe3O4 nanorod array electrode. This could be ascribed to the presence of the inside 3D Cu nanorod core array (Figure S9 in the Supporting Information) which can effectively strengthen the nanorod array structure, thereby preventing the pulverization of the electrode and aggregation of Fe3O4 nanoparticles upon lithium uptake/release. Moreover, the 3D Cu/Fe3O4 nanorod array anode is also electrochemically stable during charging/ discharging, and no obvious mixed-metal oxide formation can be observed (Figure S10 in the Supporting Information). The electrochemical performances of 3D Cu/Fe3O4 nanorod arrays obtained at different reaction times of 6 and 24 h during the HCR reaction stage are illustrated in Figure S11 in the Supporting Information. On comparison of the rate performance of the three samples with reaction times of 6, 12, and 24 h, it can be seen that the sample with a reaction time of 6 h has better rate capability, with the maximum discharge capacity of

(x /8)Fe3O4 + LiNi 0.5Mn1.5O4 charge

⎯⎯⎯⎯⎯⇀ (x 3/8)Fe + (x /2)Li 2O + Li1 − xNi 0.5Mn1.5O4

↽⎯⎯⎯⎯⎯⎯⎯⎯ discharge

(4)

As shown in Figure 7b and Figure S14a in the Supporting Information, the 3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 full cell could deliver a stable discharge specific energy density of about 327.6 Wh kg−1 at a rate of 0.2 C after 10 cycles and even exhibits 128.2 Wh kg−1 at a much higher rate of 20 C, referenced to the total mass of both cathode and anode. When the current density is decreased from 20 to 0.2 C at the end of the rate 7664

DOI: 10.1021/acs.inorgchem.7b00112 Inorg. Chem. 2017, 56, 7657−7667

Inorganic Chemistry

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cycling, the discharge energy density of the full cell can come back to 322.7 Wh kg−1 (Figure 7c). The cycling performance of the 3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 full cell at a rate of 0.5 C after rate performance is also shown in Figure 7d and Figure S14b in the Supporting Information. The 3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 full cell delivers a discharge specific energy density of 268.2 Wh kg−1 with a energy density retention of 94% after 400 cycles at 0.5 C rate. The rate capability of 3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 full cell is not as consistent with those of LiNi0.5Mn1.5O4/Li half cells. This may be ascribed to the conversion reaction ratio of the Fe3O4 anode and the lithiation/delithiation degree of the LiNi0.5Mn1.5O4 cathode.46 The discharge energy density was estimated by calculating the product of the discharge capacity and the average plateau.47 These results compete well with the reported full cells assembled by TMO anodes prepared on 2D planar current collectors. For example, our group reported a discharge energy density of about 217 Wh kg−1 at a rate of 0.1 C for the CuO/ LiNi0.5Mn1.5O4 full cell, in which the CuO nanorod array anode was obtained from the Cu(OH)2 nanorod array precursor grown from Cu substrate.46 On the basis of the outstanding performance of the 3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 full cell, the as-prepared 3D Cu/Fe3O4 nanorod array is a promising anode for next-generation Li-ion cells.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00112. Optical image illustrating the color evolution from Cu foil to the 3D Cu/Fe3O4 nanorod arrays, FESEM image of Cu(OH)2 nanorod arrays on copper substrate, TEM image of a single Cu(OH)2 nanorod, XRD pattern and SEM images of the as-prepared Cu/Fe3O4 powder, XRD patterns and FESEM images of as-prepared Fe3O4 nanorod powder, galvanostatic discharging/charging curves of as-prepared a 3D Cu/Fe3O4 nanorod array electrode at a current density of 0.1 C, galvanostatic discharging/charging curves of an as-prepared Fe3O4 nanorod powder electrode at a current density of 0.1 C and at series of current densities, Coulombic efficiency comparison between as-prepared electrodes at current densities of 0.5 C for 200 cycles, discharge profiles at various C rates and the derivative of the dQ/dV plots of the 3D Cu/Fe3O4 nanorod array electrode and the Fe3O4 powder electrode, electrochemical impedance spectroscopy (EIS) results of the 3D Cu/Fe3O4 nanorod array and Fe3O4 nanorod powder after the 3rd and 100th cycles at 0.5 C rate, FESEM images of 3D Cu/Fe3O4 electrode after 100 cycles at 0.5 C rate, XRD pattern and FESEM image of 3D Cu nanorod arrays on copper substrate, X-ray photoelectron spectra (XPS) for the 3D Cu/Fe3O4 nanorod arrays electrode after cycling for 3 times at 0.1 C and after cycling for 20 times at 1 C, N2 absorption−desorption isothermal curves of as-prepared 3D Cu/Fe3O4 nanorod arrays and Fe3O4 nanorod powder, XRD pattern and FESEM image of LiNi0.5Mn1.5O4 cathode material, charge/discharge profiles of LiNi0.5Mn1.5O4 cathode material at various current densities, rate and cycling performances of the LiNi0.5Mn1.5O4/Li half cell, charge/discharge profiles at various current densities and rate and cycling performance of the 3D Cu/Fe3O4 vs LiNi0.5Mn1.5O4 full cell, kinetic analysis of the redox reaction between Cu(OH)2 and Fe2+ solution (PDF)

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CONCLUSIONS In summary, we have successfully fabricated 3D Cu/Fe3O4 nanorod arrays based on a facile hydrolysis-coupled redox (HCR) reaction followed by postheating reduction. Fe2+ ions are hydrolyzed and oxidized to form an Fe(OH)3 shell on the surface of Cu(OH)2 nanorods; meanwhile the resulting acidic environment induces the reduction of Cu(OH)2 to Cu2O, which realizes the unusual redox reaction between Fe2+ ions and Cu(OH)2. After postheating reduction, the final 3D architecture of the Cu current collector serves as a stout support for the Fe3O4 shell to form nanorod array anodes for LIBs, which can exhibit high capacity, outstanding cycle stability (543.5 mAh g−1 at 5 C up to 500 cycles), and superior rate capability (289.2 mAh g−1 at 15 C). Moreover, a Li-ion full cell assembled with the 3D Cu/Fe3 O4 as the anode and LiNi0.5Mn1.5O4 as the cathode has demonstrated a high discharge specific energy density of about 327.6 Wh kg−1 at 0.2 C with an average working voltage at 2.75 V, and it could be cycled at 0.5 C rate for more than 400 cycles with a discharge energy density of about 268.2 Wh kg−1. The superior electrochemical performance can be mainly attributed to the advantages of the 3D current collector with Cu nanorod array cores: providing a large contact surface for better dispersion and structural maintenance of Fe3O4 nanoparticles, offering efficient transport pathways for electrons through the entire electrode, accommodating the large volume change, and preventing the agglomeration of pulverized Fe3O4 nanoparticles. The full-cell results suggest that the 3D Cu/Fe3O4 nanorod arrays could be promising anodes for high energy density lithium-ion batteries. This new electrode configuration based on a 3D metal substrate not only provides a cost-effective way to improve the energy density and cycle life of LIBs, which is one of today’s most challenging issues for next-generation LIBs, but also shows great potential as promising electrodes for wearable power devices and other electrochemical devices due to its flexible and conducting 3D metal substrate.

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AUTHOR INFORMATION

Corresponding Authors

*W.Z.: tel, +86 551 62901454; fax, + 86 551 62901450; e-mail, [email protected]. *S.Y.: e-mail, [email protected]. ORCID

Weixin Zhang: 0000-0001-6979-8901 Author Contributions ∥

H.G., Y.Z., and M.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (NSFC Grants 91534102 and 21271058), Anhui Provincial Science and Technology Department (1501021013), and Intelligent Manufacturing Institute of Hefei University of Technology (IMICZ2015104). 7665

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