Rapid Synthetic Route to Nanocrystalline Carbon-Mixed Metal Oxide

Publication Date (Web): April 8, 2016 ... A rapid synthetic route to nanocrystalline carbon-incorporated mixed metal oxide nanocomposites with enhance...
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Rapid Synthetic Route to Nanocrystalline Carbon-Mixed Metal Oxide Nanocomposites with Enhanced Electrode Functionality Jang Mee Lee,† Tae Ha Gu,† Nam Hee Kwon, Seung Mi Oh, and Seong-Ju Hwang* Department of Chemistry and Nanoscience, College of Natural Sciences, Ewha Womans University, Seoul 03760, Korea S Supporting Information *

ABSTRACT: A rapid synthetic route to nanocrystalline carbon-incorporated mixed metal oxide nanocomposites with enhanced electrode performance for lithium ion batteries is developed by applying a very short heat-treatment of layered double hydroxide (LDH) precursor under C2H2 flow. Employing C2H2 atmosphere makes possible the rapid synthesis of nanocrystalline C− NiO−NiFe2O4 nanocomposite via the calcination of the Ni−Fe−LDH precursor at 300 °C in a very short period of 5 min. In the case of ambient atmosphere, a prolonged calcination time of several hours is demanded to induce a complete phase transformation from Ni−Fe−LDH to electrochemically active NiO−NiFe2O4 nanocomposite, highlighting the usefulness of C2H2 atmosphere in promoting the formation of mixed metal oxide nanocomposite. The present C−NiO−NiFe2O4 nanocomposite shows much better anode performance for lithium ion batteries with greater discharge capacity and better cyclability than do the NiO−NiFe2O4 nanocomposites prepared by the prolonged calcination of LDH under ambient atmosphere. The superior electrode activity of the present C−NiO−NiFe2O4 nanocomposite is attributable to the optimization of charge transfer induced by the enhanced electrical conductivity and a short diffusion length of Li ion. The present C2H2-assisted phase transition of LDH precursor provides a convenient, economic, and scalable synthetic way to carbon−mixed metal oxide nanocomposites with promising electrode performance for lithium ion batteries.

1. INTRODUCTION Over the past decade, nanocrystalline transition metal oxides receive prime attention as alternative anode materials for replacing currently commercialized graphite because of their high lithium storage capacity and facile synthesis.1−6 However, severe volume change during electrochemical cycling and poor electrical conductivity of transition metal oxides limit their electrode performance, especially under high current density conditions.7 The composite formation between two different metal oxides can provide an efficient way to improve the electrode performance of metal oxide via the relief of volume change during electrochemical cycling.8−11 One of the most effective methods to synthesize mixed metal oxide nanocomposites is the heat-treatment of layered double hydroxide (LDH), since the LDH material has a general chemical formula of [A2+1−xB3+x(OH)2]x+[Cn−x−n]x−·zH2O, where A2+, B3+, and Cn− present divalent metal cations, trivalent metal cations, and interlayer anions, respectively.12,13 The heat-treatment of this material at elevated temperature yields mixed AO−AB2O4 nanocomposites consisting of intimately coupled metal oxide components, as well-documented by many groups.14−16 A high flexibility in cation composition renders the LDH material a useful precursor for the synthesis of mixed metal oxide nanocomposite with tunable chemical formula.17−19 However, the resulting nanocomposites prepared by the calcination of LDH precursor at elevated temperature still suffer from marked capacity fading, since high-temperature calcination yields wellcrystalline metal oxide crystal with enlarged particle size © XXXX American Chemical Society

showing poor structural stability to accommodate severe volume change during the electrochemical cycling.7 Taking into account the fact that nanocrystalline metal oxide is more tolerable to volume change than microcrystalline homologue,20−22 the control of crystal size would be critical in improving the electrode performance of mixed metal oxide nanocomposite. It is therefore recommendable to induce the phase transition of LDH to metal oxides under mild condition such as a short heat-treatment at low temperature. For this purpose, the loading of C2H2 atmosphere is supposed to be effective since the acetylene molecule has a high reactivity to promote the phase transition of LDH to mixed metal oxide under the mild condition. The incorporation of carbon inside the LDH lattice is supposed to create oxygen vacancy and thus to facilitate the formation of oxygen-deficient LDH lattice that easily transforms to mixed metal oxide. Similarly, the phase transition of titanium oxide can be promoted by carbon incorporation.23,24 Additionally the reaction of LDH with C2H2 gives rise to the deposition of carbon species on the product nanocomposite, which is highly effective in improving the electrical conductivity and rate characteristic of the resulting material. Thus, the application of C2H2 atmosphere for the heat-treatment of LDH makes possible a rapid synthesis of nanocrystalline carbon−metal oxide nanocomposites with Received: January 26, 2016 Revised: March 24, 2016

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energy dispersive spectrometry (EDS)-elemental mapping with an energy dispersive X-ray spectrometer equipped with an FESEM machine. The content of carbon incorporated in the C− NiO−NiFe2O4 nanocomposite was estimated with thermogravimetric analysis (TGA). The chemical bonding natures of the present nanocomposites were investigated with micro-Raman spectroscopy. All the present micro-Raman spectra were measured with a JY LabRam HR spectrometer, in which an Ar laser with a wavelength of 514.5 nm was used as the excitation source. X-ray absorption near-edge structure (XANES) spectroscopic measurements were carried out at Fe K-edge and Ni K-edge with the extended X-ray absorption fine structure (EXAFS) facility installed at beamline 10C at the Pohang Accelerator Laboratory (PAL, Pohang, Korea, 3.0 GeV, and 300 mA). All the spectra were measured in transmission mode from the thin layer of powder samples deposited on transparent adhesive tapes using gas-ionization detectors. The measurements were carried out at room temperature with a Si(111) single crystal monochromator. The energies of all the measured spectra were carefully calibrated by measuring Fe or Ni metal foil simultaneously. The experimental spectra were analyzed by the standard procedure reported previously.27 The surface areas and porosity of the present nanocomposites were probed with N2 adsorption−desorption isotherm measurements at liquid nitrogen temperature (Micromeritics ASAP 2020). To activate the pores in these materials, all the samples were degassed at 150 °C for 5 h under vacuum. 2.3. Electrochemical Measurements. The anode performances of the obtained nanocomposites were examined by performing galvanostatic charge−discharge cycles. The electrochemical cycling test was done with the button-type half-cell of Li-1 M LiPF6 in ethylene carbonate (EC):diethylcarbonate (DEC) (50:50 v/v)-active material. For the preparation of the composite electrode, the active material (70%) was thoroughly mixed with Super P (20%) and PVDF (10%) in N-methyl-2pyrrolidinone (NMP). The resulting slurry was deposited on Cu-foil with the doctor-blade method and dried at 100 °C for 12 h. The resulting composite electrode was roll-pressed and cut into a disk. The counter lithium electrode and composite electrode were assembled into 2016 coin-type cell in Ar-filled glovebox. All the electrochemical experiments were performed in galvanostatic mode with a WonA Tech multichannel battery cycler in the voltage range of 0.01−3.0 V with 100 mA g−1. The variations of the charge-transfer properties of the present nanocomposites upon the electrochemical cycling were investigated by performing electrochemical impedance spectroscopy (EIS) analysis using an IVIUM impedance analyzer in the frequency region of 0.01−105 Hz.

enhanced electrode performance. Although there are many reports about the preparation of anode material for lithium ion batteries through the calcination of LDH precursor,14−16,25−27 we are unaware of other reports about the C2H2-assisted synthesis of carbon-mixed metal oxide nanocomposites from an LDH precursor and their application as lithium ion electrodes. In the present work, nanocrystalline C−NiO−NiFe2O4 nanocomposite can be synthesized within 5 min by the heattreatment of Ni−Fe−LDH precursor in C2H2 atmosphere. To probe the role of C2H2 atmosphere, the carbon-free NiO− NiFe2O4 nanocomposite is also prepared by the same synthetic process except for the absence of C2H2 atmosphere. Additionally, the effect of crystallinity on the electrode performances of mixed metal oxide nanocomposites is investigated by preparing the nanocomposites at higher temperature for longer calcination time. All the obtained nanocomposites are tested as anode materials for lithium ion batteries to examine their electrochemical activity. The crystal structures, chemical bonding natures, and charge-transport properties of the present nanocomposites are studied to understand the origin of the improved electrode performance of these materials.

2. EXPERIMENTAL SECTION 2.1. Synthesis. The Ni−Fe−LDH precursor was synthesized by conventional coprecipitation methods, as reported previously.16 Nickel nitrate (Ni(NO3)2·6H2O, 0.165 mol) and iron nitrate (Fe(NO3)3·6H2O, 0.0825 mol) were dissolved in 250 mL of decarbonated water with vigorous stirring. Then 20 mL of 1 M NaOH solution was added dropwise to adjust the pH to 8. After the mixing process was finished, the resulting slurry was kept for 1 h at room temperature and then heated at 80 °C for 24 h. The obtained materials were filtered and washed with decarbonated water. The C−NiO−NiFe2O4 nanocomposite was synthesized by heat-treatment of Ni−Fe− LDH at 300 °C with a heating rate of 10 °C min−1 under the flow of 0.2% C2H2 gas diluted with Ar gas for 5 min, and the resulting product was then quickly quenched to room temperature. As a reference, carbon-free mixed metal oxide NiO−NiFe2O4 nanocomposite was also prepared by calcination of Ni−Fe−LDH in ambient atmosphere not applying C2H2 gas at the same heating temperature and heating time. (Hereafter this material is denoted as NiO−NiFe2O4-1.) Additionally, the other reference NiO−NiFe2O4 nanocomposites were obtained by applying heat-treatment in ambient atmosphere at different time and temperature, i.e., a longer heating time of 3 h at 300 and 500 °C (Hereafter these materials are denoted as NiO− NiFe2O4-2 and NiO−NiFe2O4-3, respectively). All these reference materials were restored by quick quenching to room temperature. In the case of calcination under C2H2 flow, it was not possible to prepare the C−NiO−NiFe2O4 nanocomposite at temperatures higher than 300 °C, because the high reduction power of C2H2 led to the formation of metallic Ni species. 2.2. Characterization. The crystal structures of the present materials were studied with powder X-ray diffraction (XRD) analysis (Rigaku, λ = 1.5418 Å, 25 °C). The crystal morphologies of the present materials were characterized with field emission-scanning electron microscopy (FE-SEM) analysis using a Jeol JSM-6700F electron microscope and transmission electron microscopy (TEM) using a Jeol JEM2100F electron microscope working at an electrical potential of 200 kV. The elemental distributions and chemical compositions of the present nanocomposite materials were examined by

3. RESULTS AND DISCUSSION 3.1. Powder XRD Analysis. The powder XRD pattern of the carbon-incorporated C−NiO−NiFe2O4 nanocomposite is presented in Figure 1, together with those of the carbon-free NiO−NiFe2O4 nanocomposites (NiO−NiFe2O4-1, NiO− NiFe2O4-2, and NiO−NiFe2O4-3), the precursor Ni−Fe− LDH, and the references of NiO and NiFe2O4. The pristine Ni−Fe−LDH displays a series of well-developed Bragg reflections, which can be well-indexed with the hexagonal layered LDH structure having rhombohedral structure.13 The other NiO and NiFe2O4 references show typical Bragg reflections of rocksalt and cubic spinel structures, respectively. Even with a short calcination time of 5 min, the present C− NiO−NiFe2O4 nanocomposite exhibits broad but distinct XRD B

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°C min−1. The resulting nanocomposite displays distinct XRD patterns of the NiO and NiFe2O4 phases, which is quite similar to that of the C−NiO−NiFe2O4 nanocomposite prepared with a slower heating rate of 10 °C min−1 (see Figure S1 of the Supporting Information). This result clearly demonstrates that the mixed metal oxide nanocomposites can be obtained with a much shorter heat-treatment under C2H2 flow. In the case of heat-treatment in ambient atmosphere, the prolongation of calcination time to 3 h leads to the advent of the XRD peaks of NiO and NiFe2O4 phases for the NiO− NiFe2O4-2 nanocomposite, indicating that a prolonged calcination for >3 h is demanded to complete the phase transformation of Ni−Fe−LDH to NiO and NiFe2O4 at 300 °C in ambient atmosphere. The elevation of heating temperature to 500 °C induces the enhancement and sharpening of the XRD peaks of NiO and NiFe2O4 phases for the NiO−NiFe2O43 nanocomposite, reflecting an enlargement of particle size and an enhancement of the crystallinity of these metal oxide phases. 3.2. FE-SEM, TEM, EDS, and TGA Analyses. The morphological evolution of the precursor Ni−Fe−LDH upon the calcination process is examined with FE-SEM (see the top panel of Figure 2). The as-prepared Ni−Fe−LDH shows layered plate-like crystallite with the particle size of several micrometers. The C−NiO−NiFe2O4 nanocomposite, as well as the NiO−NiFe2O4-2 and NiO−NiFe2O4-3 ones, displays a morphological change to aggregates of small crystallites, confirming the phase transition from LDH to mixed metal oxide nanocomposite. Such a morphological variation upon the heat treatment is less obvious for the NiO−NiFe2O4-1 nanocomposite, reflecting the incomplete phase transition of the LDH precursor.

Figure 1. Powder XRD patterns of (a) NiO, (b) NiFe2O4, (c) the precursor Ni−Fe−LDH, and the nanocomposites of (d) C−NiO− NiFe2O4, (e) NiO−NiFe2O4-1, (f) NiO−NiFe2O4-2, and (g) NiO− NiFe2O4-3. In the data (d−g), the triangle, circle, and square symbols represent the Bragg reflections of the Ni−Fe−LDH, NiO, and NiFe2O4 phases, respectively.

peaks of NiO and NiFe2O4 phases without those of the precursor LDH, highlighting the complete phase transition of LDH to mixed metal oxides. Conversely, the broad Bragg reflections of the LDH phase are still discernible for the NiO− NiFe2O4-1 nanocomposite calcined in air at 300 °C for 5 min, indicating the incomplete transformation of the LDH lattice into the mixed metal oxides of NiO and NiFe2O4 in ambient atmosphere. This finding clearly demonstrates that the C2H2 atmosphere plays a crucial role in the rapid phase transition from LDH to mixed metal oxides. To probe the effect of heating rate on the phase transition behavior of LDH under C2H2 flow, additional C−NiO−NiFe2O4 nanocomposite was prepared at the same condition with a faster heating rate of 30

Figure 2. (Top) FE-SEM and (bottom) TEM images of (a) the precursor of Ni−Fe−LDH and the nanocomposites of (b) C−NiO−NiFe2O4, (c) NiO−NiFe2O4-1, (d) NiO−NiFe2O4-2, and (e) NiO−NiFe2O4-3. C

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Figure 3. Elemental maps of the nanocomposites of (a) C−NiO−NiFe2O4, (b) NiO−NiFe2O4-1, (c) NiO−NiFe2O4-2, and (d) NiO−NiFe2O4-3.

mixed metal oxides. This finding confirms the critical role of C2H2 in facilitating the formation of mixed metal oxide from the LDH precursor. The reaction of LDH with C2H2 leads to the creation of oxygen vacancy with the formation of metastable oxygen-deficient LDH phase, which facilitates the transformation from LDH to mixed metal oxide. In contrast to the significant improvement of crystallinity, there are no marked changes in the morphology of the resulting nanocomposites, depending on the partial pressure of C2H2 (Figure S4 of the Supporting Information). 3.3. Micro-Raman Spectroscopic Analysis. The chemical bonding natures of metal oxide components and incorporated carbon species in the present nanocomposites are probed with micro-Raman spectroscopy. Figure 4 presents

The TEM images of the present nanocomposites are illustrated in the bottom panel of Figure 2. All the present nanocomposites except NiO−NiFe2O4-1 commonly exhibit the formation of aggregates of spherical nanoparticles. The NiO and NiFe2O4 nanoparticles in the nanocomposites calcined at 300 °C have an average particle size of 5−10 nm, which is compatible with the results of XRD and FE-SEM. As the heating temperature increases to 500 °C, the particle size of mixed metal oxides becomes larger from 5−10 to 10−20 nm, confirming the enlargement of nanoparticles at elevated temperature. This is in good agreement with the powder XRD results (Figure 1). The homogeneous formation of C−NiO−NiFe2O4 and NiO−NiFe2O4 nanocomposites is evidenced by EDS-elemental mapping analysis. As can be seen clearly from Figure 3, both the metal component elements of Ni and Fe exist uniformly in the entire region of the present materials, confirming the nanoscale mixing of two kinds of metal oxides. According to the EDS analysis, the ratio of Ni/Fe in the present nanocomposite is determined to be 1.67, which corresponds to the relative concentration of NiO:NiFe2O4 = 2.34:1. The carbon content of C−NiO−NiFe2O4 nanocomposite is estimated by TGA (see Figure S2 of the Supporting Information). While NiO− NiFe2O4-1 nanocomposite shows notable mass loss in the temperatures of ∼100 and ∼350 °C corresponding to the evolution of H2O from OH− group and CO2 from intercalated CO32− species, the carbon-containing nanocomposite exhibits an additional distinct weight loss at ∼300 °C, which is assigned as the oxidation of incorporated carbon species. From this mass loss, the carbon content of the C−NiO−NiFe2O4 nanocomposite is calculated to be ∼10%. To better understand the role of C2H2 in the phase transition of LDH precursor caused by heat-treatment, several carbonincorporated C−NiO−NiFe2O4 nanocomposites were also prepared by employing several partial pressures of C2H2 such as 0.1, 0.2, 0.4, and 0.8%. As shown in Figure S3 of the Supporting Information, the increase of the partial pressure of C2H2 enhances the XRD peaks of NiO and NiFe2O4 phases, indicating the promotion of the phase transition from LDH to

Figure 4. Micro-Raman spectra of (a) the precursor of Ni−Fe−LDH, (b) NiO, (c) NiFe2O4, and the nanocomposites of (d) C−NiO− NiFe2O4, (e) NiO−NiFe2O4-1, (f) NiO−NiFe2O4-2, and (g) NiO− NiFe2O4-3.

the micro-Raman spectra of the C−NiO−NiFe2O4 and NiO− NiFe2O4 nanocomposites and the reference spectra of the precursor Ni−Fe−LDH, NiO, and NiFe2O4. The carbonincorporated C−NiO−NiFe2O4 nanocomposite displays two intense Raman peaks at ∼1330 and ∼1600 cm−1, which are assigned as the D and G vibration bands of carbon species.28 D

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The Journal of Physical Chemistry C The observation of these peaks provides strong evidence for the incorporation of carbon species in the NiO−NiFe2O4 nanocomposite. These carbon-related features are absent in the Raman spectra of the other carbon-free materials. In the low wavenumber region, in which the vibrations of oxygen and metal components appear, the precursor Ni−Fe−LDH shows several phonon lines of LDH lattice at ∼460, ∼520, ∼700, and ∼1040 cm−1 corresponding to the symmetric Ni−OH stretching mode, the vibrations of the Ni−O stretching mode, symmetric Fe−O stretching, and the symmetric stretching of interlayer carbonate ions, respectively.29 The reference NiFe2O4 exhibits several peaks at ∼300, ∼ 490, ∼ 580, and ∼700 cm−1 corresponding to four Raman-active A1g + Eg + 2T2g modes of cubic spinel phase,30 whereas a quite broad spectral feature is observed at ∼530 cm−1 for the reference NiO. Despite a short calcination time of 5 min, the C−NiO− NiFe2O4 nanocomposite prepared in C2H2 flow exhibits the typical phonon lines of NiO and NiFe2O4 phases, confirming the complete phase transition of Ni−Fe−LDH to these two kinds of metal oxides. Among the carbon-free NiO−NiFe2O4 nanocomposites calcined in ambient atmosphere, only the NiO−NiFe2O4-3 nanocomposite heated at 500 °C for 3 h shows similar Raman spectrum to the C−NiO−NiFe2O4 nanocomposite in low wavenumber region. Conversely, the Raman spectra of the other NiO−NiFe2O4-1 and NiO− NiFe2O4-2 nanocomposites are notably different from that of C−NiO−NiFe2O4 but somewhat similar to that of the pristine Ni−Fe−LDH, reflecting the incomplete phase transformation of the precursor LDH. This result confirms that the application of C2H2 atmosphere is highly effective in promoting the formation of NiO−NiFe2O4 nanocomposite from the precursor Ni−Fe−LDH, even with low heating temperature and short calcination time. 3.4. XANES Analysis. The oxidation states and local atomic arrangements of nickel and iron ions in the present nanocomposites are examined using Fe K-edge and Ni K-edge XANES analysis. The top panel of Figure 5 plots the Fe K-edge spline XANES spectra and their second derivatives of the asprepared Ni−Fe−LDH and their calcined nanocomposites, as compared with those of references NiFe2O4, FeO, and Fe2O3. The precursor Ni−Fe−LDH shows similar edge-position to those of the references NiFe2O4 and Fe2O3, indicating the trivalent oxidation state of Fe3+ in this material. All the calcined nanocomposites retain the original edge energy of the precursor Ni−Fe−LDH, highlighting the maintenance of trivalent Fe3+ oxidation state upon heat-treatment with and without C2H2 flow. In the pre-edge region, only a weak peak P corresponding to the dipole-forbidden 1s → 3d transition appears for the references FeO and Fe2O3.30 Since the spectral weight of this pre-edge peak sensitively reflects the structural distortion from regular octahedral symmetry,32 this observation strongly suggests the iron ions in the references FeO and Fe2O3 are stabilized in octahedral symmetry. Similarly, the precursor Ni− Fe−LDH material shows very weak pre-edge peak P, confirming the octahedral symmetry of Fe ion. As can be seen from the second derivatives, the reference NiFe2O4 displays a somewhat stronger pre-edge peak P than do the references FeO and Fe2O3, indicating the stabilization of Fe ions in the tetrahedral and octahedral sites of the inverse-spinel structure of NiFe2O4. Similarly, the C−NiO−NiFe2O4 nanocomposite shows an intense pre-edge peak P, whose intensity is comparable to that of NiFe2O4, strongly suggesting the creation

Figure 5. (Top) Fe K-edge and (bottom) Ni K-edge (left) spline XANES spectra and (right) the corresponding second derivatives of (a) the as-prepared Ni−Fe−LDH and the nanocomposites of (b) C− NiO−NiFe2O4, (c) NiO−NiFe2O4-1, (d) NiO−NiFe2O4-2, and (e) NiO−NiFe2O4-3, and the references of (f) NiFe2O4, (g) FeO/NiO, and (h) Fe2O3.

of tetrahedral Fe species and the formation of NiFe2O4 phase. In the cases of the NiO−NiFe2O4 nanocomposites calcined in ambient atmosphere, this pre-edge peak P becomes stronger with an increase in heating temperature and time, reflecting the promoted formation of NiFe2O4 phase. In the main-edge region, there are several features, A, B, and C, related to the dipole-allowed 1s → 4p transitions.31,33 As presented in the second derivatives, the spectral feature of the C−NiO−NiFe2O4 nanocomposite is nearly identical to that of the reference NiFe2O4 but quite different from that of the pristine Ni−Fe−LDH, confirming the complete phase transformation of Ni−Fe−LDH in C2H2 atmosphere. In the cases of the NiO−NiFe2O4 nanocomposites calcined in ambient atmosphere, their spectral shapes become more similar to that of the NiFe2O4 with an increase in heating temperature and time. That is, the NiO−NiFe2O4-1 material shows somewhat close spectral feature to that of the pristine Ni− Fe−LDH, whereas a significant spectral similarity is discernible between the NiO−NiFe2O4-3 nanocomposite and NiFe2O4. This result confirms that the application of prolonged heatE

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Figure 6. (Left) N2 adsorption−desorption isotherms and the size distribution curves of (middle) micropores and (right) mesopores for (a) C− NiO−NiFe2O4 (circles), (b) NiO−NiFe2O4-1 (triangles), (c) NiO−NiFe2O4-2 (squares), and (d) NiO−NiFe2O4-3 (diamonds). In the left panel, the open and close symbols represent the data of adsorption and desorption processes, respectively.

classification, reflecting the generation of open slit-shaped capillaries with very wide bodies and narrow short necks. An increase of the heating temperature to 500 °C displaces the onset of hysteresis into a high pressure region with an H1-type hysteresis loop, indicating the agglomeration of spheroidal nanoparticles.35 This result clearly demonstrates the importance of heating temperature in controlling the pore structure of the calcined nanocomposites. The surface areas of the present nanocomposites are calculated on the basis of the Brunauer−Emmett−Teller (BET) equation. The C−NiO− NiFe2O4, NiO−NiFe2O4-1, and NiO−NiFe2O4-2 nanocomposites have expanded surface areas of 162, 172, and 136 m2 g−1, respectively, which are much greater than that of the NiO− NiFe2O4-3 nanocomposite prepared at higher temperature (53 m2 g−1). This result underscores that the lowering of calcination temperature is beneficial in maximizing the porosity of the nanocomposites through the stabilization of nanocrystalline metal oxides. The size distributions of the micropores in the present nanocomposites are analyzed using the MP method. Micropore hydraulic radii of ∼1−2 nm with significant microporosity are observed for the C−NiO−NiFe2O4, NiO−NiFe2O4-1, and NiO−NiFe2O4-2 nanocomposites calcined at 300 °C (see the middle panel of Figure 6). Conversely, no distinct micropore is observable for the NiO−NiFe2O4-3 nanocomposite calcined at 500 °C, indicating the significant destruction of micropores during the calcination at elevated temperature. Additionally, the size distribution curves of mesopores are calculated on the basis of the Barrett−Joyner−Halenda (BJH) method. As can be seen clearly from the right panel of Figure 6, the C−NiO−NiFe2O4, NiO−NiFe2O4-1, and NiO−NiFe2O4-2 nanocomposites commonly possess mesopores with a size of ∼3−5 nm. An increase of heating temperature to 500 °C causes a significant enlargement of mesopores for the NiO−NiFe2O4-3 nanocomposite. This result clearly demonstrates that the increase of heating time and temperature makes mesopores greater, highlighting the prominent effect of heating condition on the size and distribution of mesopores. Among the calcined nanocomposites under investigation, the C−NiO−NiFe2O4 nanocomposite possesses the smallest size of micropores and mesopores with narrow distribution, underscoring the usefulness of C2H2 atmosphere in stabilizing small-sized pores. 3.6. Electrochemical Measurement. The carbon-incorporated C−NiO−NiFe2O4 nanocomposite and the carbon-free NiO−NiFe2O4 nanocomposites are applied as anode materials for lithium-ion batteries. The potential profiles of all the

treatment at elevated temperature is demanded to induce the complete phase transition from the Ni−Fe−LDH to the spinel NiFe2O4 phase in ambient atmosphere. The Ni K-edge XANES spectra of the as-prepared Ni−Fe− LDH and their calcined nanocomposites and their second derivatives are presented in the bottom panel of Figure 5, as compared with the reference spectra of NiFe2O4 and NiO. The edge energies of all the present nanocomposites as well as the precursor Ni−Fe−LDH are nearly the same as that of the reference NiO, confirming the divalent Ni2+ oxidation state of these compounds. In the pre-edge region, all the present nanocomposites and the precursor Ni−Fe−LDH commonly display a weak peak P related to a dipole-forbidden transition from core 1s to unoccupied 3d levels. The weak intensity of these materials clearly demonstrates the octahedral local symmetry of Ni ions. All the materials under investigation demonstrate several main-edge peaks, A, B, and C, corresponding to the dipole-allowed 1s → 4p transitions.34 As can be seen clearly from the second derivatives, the spectral feature of the C−NiO−NiFe2O4 nanocomposite is almost the same as the summation of the spectral features of NiO and NiFe2O4 phases, underscoring the phase transition of the Ni−Fe−LDH to mixed metal oxides of NiO−NiFe2O4. While the NiO−NiFe2O4-1 nanocomposite exhibits nearly identical spectral shape to the precursor Ni−Fe−LDH, the increase of heating temperature and time makes the spectral features similar to the merged spectral features of NiO and NiFe2O4, confirming the phase transformation into mixed NiO−NiFe2O4 nanocomposite with elongated heat-treatment at elevated temperature. 3.5. N2 Adsorption−Desorption Isotherm Measurements. The N2 adsorption−desorption isotherms of all the present nanocomposites are illustrated in the left panel of Figure 6. The C−NiO−NiFe2O4, NiO−NiFe2O4-1, and NiO− NiFe2O4-2 nanocomposites prepared at low temperature of 300 °C can adsorb a considerable amount of N2 molecules in the low pp0−1 region of 0.4, a distinct hysteresis occurs for the nanocomposites of C−NiO−NiFe2O4, NiO−NiFe2O4-1, and NiO−NiFe2O4-2, indicating the presence of mesopores in these materials.35,36 The observed isotherm behaviors of the present nanocomposites are classified as the BDDT type I and IV shape of isotherms and H2-type hysteresis loop in the IUPAC F

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close to the theoretical value of 777 mAh g−1, clearly demonstrating the usefulness of C2H2 treatment in optimizing the electrode activity of the calcined NiO−NiFe2O4 nanocomposite. The left panel of Figure 8 plots the cycling performance of all the present nanocomposites at a current density of 100 mA g−1.

nanocomposites recorded in the range of 0.01−3.0 V at 100 mA g−1 are plotted in Figure 7. A similar shape of discharge−

Figure 8. (Left) Discharge−charge capacity plots and (right) rate capability plots of C−NiO−NiFe2O4 (circles), NiO−NiFe2O4-1 (triangles), NiO−NiFe2O4-2 (squares), and NiO−NiFe2O4-3 (diamonds) at a current density of 100 mA g−1. The open and close symbols represent the data of discharging and charging processes, respectively.

Among the carbon-free NiO−NiFe2O4 nanocomposites, the materials prepared at 300 °C show better electrode performance than that materials prepared at 500 °C. This result underscores that the nanocrystalline nature with smaller particle size by the calcination at mild temperature can offer short diffusion path for Li ion transport on the electrode performance of the resulting nanocomposites. The effect of the elevation of heating temperature on the electrode performance of nanocomposites is further investigated by preparing additional NiO−NiFe2O4 nanocomposites at higher temperatures of 700 and 900 °C for 3 h. As can be seen clearly from Figure S5 of the Supporting Information, both the calcined nanocomposites display even poorer electrode performances than does the NiO−NiFe2O4-3 nanocomposite, confirming the detrimental effect of high heating temperature on the electrode activity of the calcined nanocomposites. As shown in the left panel of Figure 8, all the NiO−NiFe2O4 nanocomposites commonly suffer from severe capacity fading. This phenomenon is ascribable to the low electrical conductivity of metal oxide, leading to poor Li+ diffusion on the surface of NiO−NiFe2O4 nanocomposites. In comparison with these carbon-free nanocomposites, the carbon-incorporated C−NiO−NiFe2O4 nanocomposite exhibits much greater discharge capacity and much better cyclability. The incorporation of highly conductive carbon into the mixed metal oxide nanocomposites can effectively improve the electronic conductivity of active materials, resulting in the enhancement of Li+ ion transports on the surface of metal oxide and increase of stability during the electrochemical charge−discharge process.43,44 The formation of nanocrystalline metal oxides with optimized pore structure upon the short calcination under C2H2 flow is also responsible for the observed superior electrode performance of the C− NiO−NiFe2O4 nanocomposite over the other NiO−NiFe2O4 nanocomposites. The beneficial effect of the calcination under C2H2 flow on the electrode performance is more distinct for the high current density condition. As illustrated in the right panel of Figure 8, the carbon-incorporated C−NiO−NiFe2O4 nanocomposite

Figure 7. Discharge−charge potential profiles of (a) C−NiO− NiFe2O4, (b) NiO−NiFe2O4-1, (c) NiO−NiFe2O4-2, and (d) NiO− NiFe2O4-3 at a current density of 100 mA g−1.

charge potential profile occurs for all the present nanocomposites, strongly suggesting that the electrode activities of these materials commonly originate from the electrochemically active components of NiO and NiFe2O4.37−40 In the first discharge cycle, all the present materials display the plateau of ∼0.8 V corresponding to the reduction of Ni2+/Fe3+ ions to Ni/ Fe elements and the accompanying formation of Li2O, which associate with the decomposition of electrolyte.41,42 After the first cycle, the severe capacity loss occurs commonly for all the present nanocomposites, which is attributable to the irreversible decomposition of the electrolyte and the formation of a solid electrolyte interphase (SEI) layer.41 On the basis of the theoretical capacities of NiO (718 mAh g−1) and NiFe2O4 (915 mAh g−1), and the relative population of NiO and NiFe2O4 phases in the present nanocomposite (2.34:1), the theoretical capacity of this material is calculated to be 777 mAh g − 1 for t he chemical formula o f (NiO)0.7(NiFe2O4)0.3, which originates from the reversible uptake-removal of 1.4Li+ ions for the NiO component and 2.4Li+ ions for the NiFe2O4 component according to the following electrochemical reactions. NiO + 2Li+ + 2e− ↔ Ni + Li 2O

(1)

NiFe2O4 + 8Li+ + 8e− ↔ Ni + 2Fe + 4Li 2O

(2)

After the 30th cycle, the carbon-incorporated C−NiO− NiFe2O4 nanocomposite retains a large discharge capacity of 733 mAh g−1, which is much greater than those of the other nanocomposites (308, 203, and 150 mAh g−1 for NiO− NiFe2O4-1, NiO−NiFe2O4-2, and NiO−NiFe2O4-3, respectively), underscoring the superior electrode performance of carbon-incorporated nanocomposite. The obtained capacity of the C−NiO−NiFe2O4 nanocomposite (733 mAh g−1) is quite G

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respect to the real axis is determined to be 5.8, 37.5, 6.17, and 78.5 Ω for the C−NiO−NiFe2O4, NiO−NiFe2O4-1, NiO− NiFe2O4-2, and NiO−NiFe2O4-3, respectively. The carbonincorporated C−NiO−NiFe2O4 nanocomposite shows a much closer intercept to zero than do the other carbon-free NiO− NiFe2O4 nanocomposites, indicating the higher electronic conductivity between C−NiO−NiFe2O4 and substrate. Also the C−NiO−NiFe2O4 material exhibits the smallest diameter of the semicircle, highlighting the lowest charge-transfer resistance among the present nanocomposites. This finding clearly demonstrates that employing C2H2 atmosphere is quite effective in enhancing the electrical conductivity of the calcined nanocomposite and also in improving the charge-transfer characteristics of this material. Among the present carbon-free NiO−NiFe2O4 nanocomposites, the material calcined at 500 °C shows much higher resistance compared with the homologues prepared at a low temperature of 300 °C, confirming the detrimental effect of high calcination temperature on the electrical conductivity of the calcined nanocomposite. The observed order of electrical conductivity and charge-transfer is in good agreement with the relative cyclabilities of the present nanocomposites. This result clearly demonstrates that the improvement of electrical conductivity and charge transport is mainly responsible for the optimization of the electrode performance of a nanocomposite upon heattreatment under C2H2 flow.

display much better rate characteristics compared with the carbon-free nanocomposites. At the high current density of 1600 mA g−1, the C−NiO−NiFe2O4 nanocomposite delivers the large discharge capacity of ∼500 mAh g−1, which is much greater than that of the NiO−NiFe2O4-2 nanocomposite (∼150 mAh g−1). Furthermore, a capacity of ∼750 mAh g−1 is fully recovered after the rate is restored to the initial 100 mAh g−1, demonstrating a very good reversibility. The present experimental findings provide strong evidence for the merit of C2H2 atmosphere in optimizing the electrode functionality of the calcined nanocomposite. Additionally, we investigated the effect of the crystal morphology of the precursor LDH on the electrochemical property of the resulting nanocomposite. The reference C− NiO−NiFe2O4 nanocomposite was prepared with the welldispersed LDH precursor subjected to mechanical grinding for >30 min. According to the FE-SEM and dynamic light scattering (DLS) analyses, the mechanically ground LDH precursor shows well-dispersed morphology with homogeneous smaller particle size (see Figures S6 and S7 of the Supporting Information). This result indicates the usefulness of mechanical grinding in preparing homogeneous LDH precursor. Despite the significant variation of the particle dispersion and morphology of the LDH precursors, the mechanical grinding of the precursors has negligible influence on the electrode performance of the resulting C−NiO−NiFe2O4 nanocomposites. As illustrated in Figure S8 of the Supporting Information, newly prepared C−NiO−NiFe 2O 4 nanocomposite with mechanically ground LDH precursor exhibits notably greater initial capacities than the homologue prepared with the asprepared LDH precursor. However, after initial several cycles, there is no significant difference in the charge and discharge capacities of both the materials, underscoring the negligible influence of the dispersion of LDH precursor on the electrode performance of the calcined nanocomposites. 3.7. EIS Analysis. The charge transport behaviors of the present C−NiO−NiFe2O4 and NiO−NiFe2O4 nanocomposites during electrochemical cycling were investigated with EIS analysis. As plotted in Figure 9, similar Nyquist plots showing a

4. CONCLUSIONS A rapid and facile route to efficient electrode material of carbon-incorporated mixed metal oxide nanocomposite can be developed by the heat-treatment of LDH precursor under C2H2 atmosphere. The carbon-incorporated C−NiO−NiFe2O4 nanocomposite obtained with the Ni−Fe−LDH precursor can deliver a large discharge capacity of ∼730 mA h g−1, which is far superior to that of carbon-free NiO−NiFe2O4 nanocomposite prepared at higher temperature with longer reaction time. The superior electrode activity of the present nanocrystalline C− NiO−NiFe2O4 nanocomposite can be interpreted as results of the increase of electrical conductivity and the enhanced stability of crystal structure and morphology caused by the incorporation of carbon and the good maintenance of nanocrystalline nature of metal oxides. The present study clearly demonstrates that the loading of C2H2 atmosphere is fairly effective in exploring efficient nanocomposite electrode materials using the LDH precursors in a short period of reaction time. Taking into account diverse and useful functionalities of the LDH materials and their calcined derivatives as electrocatalysts, photocatalysts, CO2 adsorbents, etc.,46 the present synthetic strategy employing C2H2 atmosphere can provide useful and economic methodology to explore many promising functional materials via the optimization of surface property and electrical conductivity. Our current project is to explore efficient adsorbent and catalyst materials via the heat-treatment of the LDH materials under C2H2 flow.

Figure 9. EIS data of electrochemically cycled electrode materials of C−NiO−NiFe2O4 (circles), NiO−NiFe2O4-1 (triangles), NiO− NiFe2O4-2 (squares), and NiO−NiFe2O4-3 (diamonds) in (left) the high-frequency region and (right) the high−medium frequency region.

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semicircle in the high−medium frequency region appear for all the electrochemically cycled nanocomposites. The starting and terminating points of semicircles in the high−medium frequency region reflect electrical conduction between substrate and active electrode material, and the combined effect of SEI film and charge transfer resistance in the electrode−electrolyte interface, respectively.40,45 The high-frequency intercept with

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00841. Powder XRD of the C−NiO−NiFe2O4 nanocomposite prepared with a rapid heating rate of 30 °C min−1. TGA curves of the C−NiO−NiFe2O4 and NiO−NiFe2O4 H

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nanocomposites calcined at 300 °C for 5 min and discharge−charge capacity plots of NiO−NiFe2O4 nanocomposites calcined at 700 and 900 °C under Ar atmosphere. Powder XRD and FE-SEM data of the nanocomposites prepared with several partial pressures of C2H2. FE-SEM data and DLS curves of the LDH precursor with and without mechanical grinding. Discharge−charge capacity plots of the nanocomposites prepared with mechanically ground LDH precursor. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: +82-2-3277-4370; fax: +82-2-3277-3419. Author Contributions †

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. NRF-2014R1A2A1A10052809) and by the Global Frontier R&D Program (2013-073298) on Center for Hybrid Interface Materials (HIM). The experiments at PAL were supported in part by MOST and POSTECH.

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