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Study on the Electrochemical Reaction Mechanism of NiFe2O4 as a High-Performance Anode for Li-Ion Batteries Mobinul Islam,†,§,# Ghulam Ali,†,§,# Min-Gi Jeong,† Wonchang Choi,†,§ Kyung Yoon Chung,*,†,§ and Hun-Gi Jung*,†,§ †

Center for Energy Convergence Research, Green City Technology Institute, Korea Institute of Science and Technology, Hwarangno 14 gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea § Department of Energy and Environmental Engineering, Korea University of Science and Technology, 176 Gajungro, Yuseong-gu, Daejeon 305-350, Republic of Korea S Supporting Information *

ABSTRACT: Nickel ferrite (NiFe2O4) has been previously shown to have a promising electrochemical performance for lithium-ion batteries (LIBs) as an anode material. However, associated electrochemical processes, along with structural changes, during conversion reactions are hardly studied. Nanocrystalline NiFe2O4 was synthesized with the aid of a simple citric acid assisted sol−gel method and tested as a negative electrode for LIBs. After 100 cycles at a constant current density of 0.5 A g−1 (about a 0.5 C-rate), the synthesized NiFe2O4 electrode provided a stable reversible capacity of 786 mAh g−1 with a capacity retention greater than 85%. The NiFe2O4 electrode achieved a specific capacity of 365 mAh g−1 when cycled at a current density of 10 A g−1 (about a 10 Crate). At such a high current density, this is an outstanding capacity for NiFe2O4 nanoparticles as an anode. Ex-situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) were employed at different potential states during the cell operation to elucidate the conversion process of a NiFe2O4 anode and the capacity contribution from either Ni or Fe. Investigation reveals that the lithium extraction reaction does not fully agree with the previously reported one and is found to be a hindered oxidation of metallic nickel to nickel oxide in the applied potential window. Our findings suggest that iron is participating in an electrochemical reaction with full reversibility and forms iron oxide in the fully charged state, while nickel is found to be the cause of partial irreversible capacity where it exists in both metallic nickel and nickel oxide phases. KEYWORDS: NiFe2O4, lithium-ion battery, conversion materials, X-ray absorption spectroscopy, spinel oxide

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LIBs are considered to be promising alternatives.2−8 Moreover, mixed metal oxides comprising more than one transition metal forming an AB2O4 spinel structure, such as MMn2O4 (M = Zn, Co, or Ni) and MCo2O4 (M = Zn, Cu, or Ni), are also appealing because both metal elements can contribute in the electrochemical process and provide higher theoretical capacities.9−13 After the first discharge (lithium insertion process) of the mixed metal oxides in the LIBs, the discharge products can serve as a “self-matrix” buffer for each other in the course of the following reactions, resulting in a superior cycle performance along with enhanced energy density compared with that of the respective bare single oxides.14 Metal ferrites (MFe2O4, M = a bivalent cation) are quite attractive as anode materials for their remarkable electrochemical performance with a high initial discharge capacity. A study found that both mixed and normal spinel structured ferrites incorporate a lower number of Li ions than an inverse

INTRODUCTION Lithium-ion batteries (LIBs) have been recognized as leading power sources for small portable electronics over the past two decades, and they are anticipated to be a dominant candidate for large-scale stationary applications and powering electric vehicles in the future. The development of energy storage devices with enriched energy densities is one of the key intentions for modern day battery research. Unfortunately, despite the widespread commercial success of LIBs, the current existing technology based on lithium-ion intercalation compounds in anode materials (e.g., graphite and Li4Ti5O12) is unable to meet the rapidly growing energy demand because of their limited capacity. Thus, the ambition of achieving greater energy densities has accelerated substantial research activities concentrating on different lithium storage mechanisms within LIBs, such as alloying or conversion reactions.1 Anode materials related to conversion reactions deliver higher capacities compared to intercalation compounds by using all of the oxidation states of the transition metals. So far, several transitional metal oxides (e.g., Mn3O4, Fe2O3, Fe3O4, Co3O4, CuO, and NiO, etc.) exploited as anode materials of © 2017 American Chemical Society

Received: February 8, 2017 Accepted: April 11, 2017 Published: April 11, 2017 14833

DOI: 10.1021/acsami.7b01892 ACS Appl. Mater. Interfaces 2017, 9, 14833−14843

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ACS Applied Materials & Interfaces

sol−gel method. At first, electrochemical tests comprising cyclic voltammetry (CV) and galvanostatic cycling were employed to get a preliminary picture of the conversion reactions in the chosen potential window. In this study, the lithiation mechanism of NiFe2O4 is investigated over a 1.5 cycle to gain knowledge about the factor that reduces the reversibility of this electrode. To describe the lithium insertion and extraction behavior of the anode material in detail, the ex-situ XRD measurements and ex-situ XAS (XANES and EXAFS) study of the Fe and Ni K-edge provided an understanding of the structural transformation and electrode reaction mechanism. The prepared NiFe2O4 anodes exhibited excellent electrochemical properties including a high specific capacity (1190 mAh g−1 at 0.1 A g−1), a stable cycling retention (greater than 85% after 100 cycles at 0.5 A g−1), and a high rate capability (365 mAh g−1 at 10 A g−1).

spinel structured ferrite.15 Among the several ferrites, NiFe2O4 (an inverse spinel ferrite where Ni2+ and half of the Fe3+ cations occupy the octahedral sites) was assumed to be fully reduced by 8 mol of lithium to form Ni, Fe, and Li2O, with a corresponding theoretical capacity of 915 mAh g−1.15,16 Moreover, these two metals are relatively nontoxic and earth-abundant elements. With these advantages, several reports identified NiFe2O4 as an intriguing electrode material implemented in LIBs. It shows a reversible capacity of ∼810 mAh g−1 including cycling over tens of cycles; however, the cycle performance of the oxide was poor.17,18 The reported strategies to mitigate this issue are the design of a one-dimensional distinctive morphology (e.g., nanofibers16 and nanorods19) and forming composites with conductive materials (e.g., carbon20 or graphene21) that can serve as a buffer matrix. However, not much attention has been placed on the structural investigations of the conversion reaction with lithium. As for NiFe2O4, its electrochemical reaction was stated in many previous sources to be NiFe2O4 + 8Li+ + 8e− → 2Fe + Ni + 4Li2O ↔ Fe2O3 + NiO + 8Li+ + 8e− without any experimental confirmation.16−21 By using Raman spectroscopy, Kumar and Mitra suggested the products of the deeply discharged are Li2O, Ni, and Fe, and the recharged materials are NiO and Fe2O3.22 Nevertheless, synchrotronbased X-ray absorption spectroscopy (XAS) is a powerful technique from the perspective of the reaction mechanisms analysis. It is actually sensitive to the local structure and element specific.23−25 In fact, the study of conversion reaction mechanisms of metal oxide materials has long encountered difficulties because the loss of crystallinity within the initial lithiation step has been seen as the major obstacle for analysis. This sometimes makes it difficult to use the conventional techniques as a way to reveal the reaction mechanisms.26,27 XAS is able to reasonably account for samples containing both the amorphous and crystalline phases and can successfully be used to investigate the conversion materials based electrode after lithium insertion/deinsertion where XRD provides no structural information.23 Accordingly, XAS, precisely X-ray absorption near-edge structure (XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy, are ideal techniques for studying oxidation states and the local environment of the iron and nickel atoms of nickel ferrites in the lithium cell. The XANES spectrum results from the excitation of a 1s photoelectron into low-lying unoccupied states of the central atom at the K-edge and demonstrates by a normalized absorbance at specific energy. The K-edge XANES spectra for the transition metal oxide has a sharply rising main absorption edge with high-intensity main absorption peak. Additionally, a small pre-edge peak can be seen if the oxide’s excited atom site has a lack of centro-symmetry.25 Zhou et al. very recently reported the spectroscopic analysis (XANES and EXAFS) for a NiFe2O4 electrode which involves the electronic and local structural changes around Fe and Ni atoms at various states of discharge (lithiation) during the first discharge cycle.28 However, there is an apparent difference between the first and subsequent discharge process, as noticed in previous studies.16−22 Therefore, structural investigations were extended to the second discharge cycle. Additionally, the electrochemical charge/discharge process in the inverse spinel NiFe2O4 system is determined by ex situ X-ray diffraction (XRD), and for the first time, the formation/decomposition of Li2O is clearly spotted. Herein, the nanocrystalline nickel ferrite, NiFe2O4, was synthesized for use as an anode material in LIBs using a simple

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EXPERIMENTAL SECTION

Synthesis and Characterization of NiFe2O4. The ternary metal oxide, NiFe2O4, was synthesized using a sol−gel method. Ni(NO3)2· 6H2O (Kanto Chemical Co.) and Fe(NO3)3·9H2O (Junsei Chemical Co.) were dissolved in distilled water, and then citric acid (Samchun Pure Chemical Co.) was added to this solution (molar ratio of citric acid:total cations of 3:1) with stirring for 1 h at 25 °C. The solutions were kept at 70 °C on a hot plate with continuous stirring until amorphous gels were obtained. This step consists of simple hydrolysis and condensation reactions. Drying is a crucial process to control the final product. The gels were dried at 120 °C for 12 h (optimized condition) in a vacuum oven. After grinding with a mortar and pestle, the resulting powders were calcined at 600 °C for 12 h in air to obtain the pure crystalline phase of NiFe2O4. The structure of the heattreated sample was characterized by X-ray diffraction (XRD, Rigaku Xray diffractometer) equipped with Cu Kα (λ = 1.5418 Å) radiation, and the morphology was characterized by field emission scanning electron microscopy (FE-SEM, Hitachi, S-4200). Further, energyfiltered transmission electron microscopy (EF-TEM, Titan, FEI Corp.) was used to determine the morphology and crystallinity of the asprepared spinel oxide. The Brunauer−Emmett−Teller (BET) test using an N2 sorption isotherm measurement was performed using a Micromeritics apparatus (ASAP 2010) for measuring surface area and average pore diameter. Electrochemical Measurements. Slurries containing the synthesized NiFe2O4 powders, acetylene black (DB100), and poly acrylic acid (35 wt % in H2O) (weight ratio 6:2:2) in ethanol were cast on to a Cu foil (Jin Copper Foil Co. Ltd.) current collector. The electrode thickness was ∼28 μm after roll pressing (25% reduction); a loading amount accounting for only active material of approximately 1.2 mg cm−2 was obtained. These prepared electrodes were vacuum ovendried at 80 °C for 4 h prior to use as the working electrode. Then, the coin-type half cells (CR2032) were assembled with a polypropylene separator, the prepared working electrode, a Li counter electrode, and an electrolyte composed of 1.2 M LiPF6 in diethyl carbonate and ethylene carbonate (1:1 volume ratio). The cells were cycled at different current densities from 0.1 to 10 A g−1 in a voltage window between 0.01 and 3.0 V (vs Li/Li+) at room temperature (25 °C) using a battery testing system (WBCS 3000, WonATech). Ex-situ XRD and X-ray Absorption Spectroscopy: XANES and EXAFS. To investigate the lithium insertion and extraction behavior of the NiFe2O4 electrode, ex-situ XRD patterns of the NiFe2O4 electrodes were collected at selected voltage points during the charge and discharge processes using XRD. Ex-situ XAS (XANES and EXAFS) measurements were performed at selected potentials to determine the oxidation states of Fe and Ni and any structural changes of the active material. For sample preparation, the cells were disassembled at different charged and discharged states in an argonfilled glovebox. After collection, the electrodes were carefully washed with dimethyl carbonate to remove any residual electrolytes.29 The used electrodes were vacuum-sealed in plastic bags. XAS measure14834


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ACS Applied Materials & Interfaces ments of the electrodes were performed at the 1D KIST-PAL beamline of PLS-II, and a Si(111) double-crystal monochromator (detuned to ∼40% of the maximum intensity) was used for energy selection. The energy was scanned from −200 to +1000 eV above the Fe K-edge (7112 eV) and Ni K-edge (8333 eV). The Fe K-edge and Ni K-edge XAS data were measured at room temperature in transmission mode, and pure iron and nickel foils were used as references to calibrate the spectra.30 The X-ray absorption spectra were recorded using an ion chamber detector filled with high-purity nitrogen, and the data were analyzed using the ATHENA package.31 Fourier transformation of the EXAFS spectra was performed in the k-range from 2 to 13 Å−1, and the resulting spectra were plotted in the R-range from 0 to 6.0 Å with the k2 weight.30

electron microscopy (TEM). The typical low-magnification FESEM image of the resultant NiFe2O4 in Figure 2a shows that the product is composed of spherical nanoparticles. The TEM images (Figure 2b and Figure S1) show the nanoparticles with sizes in the range 20−40 nm and the grain boundaries are clearly distinguishable despite the slight agglomeration. From the analysis of the selected area electron diffraction (SAED) patterns, the Miller indices (hkl) assigned from the d-spacings (which were derived from the symmetrically spaced bright ring diameters) indicate the polycrystalline nature of NiFe2O4 as presented in Figure 2b (inset) and match very well to the XRD pattern (Figure 1). The representative high-resolution lattice images show distinct lattice fringes with interplanar spacings of 0.48, 0.25, and 0.29 nm, as shown in Figure 2c−e match well with the d-values corresponding to the (111), (311), and (220) planes, respectively. Thus, the high-resolution TEM results further confirm the spinel NiFe2O4 phase and the compound crystallinity with a very clear set of lattice planes. The N2 adsorption−desorption isotherm of the NiFe2O4 nanoparticles, included in the Supporting Information (Figure S2) with Barrett−Joyner−Halenda (BJH) pore-size distribution curve, can be classified as type-IV with a hysteresis loop reflecting their mesoporous nature.34 The BET specific surface area and pore size distributions of the sample were 27.10 m2 g−1 and 18.5 nm, respectively. The identified porosity and resultant specific surface area are believed to be useful for the electrode− electrolyte contacts, infiltration of electrolyte, and mass transportation during the electrochemical process. Electrochemical Characterization. Before the discharge/ charge electrodes are addressed, the rechargeability of asprepared NiFe2O4 as a conversion anode material was demonstrated. As presented in Figure 3a, a sharp and strong cathodic peak at 0.58 V in a cyclic voltammetry (CV) curve, shifted to 0.86 V in the second cycle, is observed, corresponding to the Fe3+ and Ni2+ reduction into Fe0 and Ni0, as well as the amorphous Li2O and SEI formation. In the reversible anodic scan, the peak at 1.65 V corresponds to the oxidation of metallic iron and nickel, and the partial decomposition of SEI.16,18,22,35 The CV reduction curves from the second cycle onward are identical, specifying the electrode reversibility, but we found an immense difference between the initial and subsequent cycles. This result indicates that the original spinel structure failed to restore after the first cycle. From the second discharge onward, a second cathodic peak is detected at 1.7 V, which is similar to the CV results from previous literature.16−22,28 The source of this peak is unknown at this stage. X-ray absorption spectroscopy: XANES could be helpful to obtain information regarding this point. The galvano static discharge−charge voltage profiles of the NiFe2O4 nanoparticle electrode measured at a current density of 0.1 A g−1 in the range 0.01−3 V are presented in Figure 3b. Different cell potentials are observed during the first and subsequent discharge, which agrees well with the CV curves. In the first discharge process, NiFe2O4 shows a long-range plateau around 0.75 V corresponding to the reduction of Ni2+/Fe3+ during Li insertion. This was assumed because of the constant region of the discharge profile from 0.7 to 0.5 V giving a specific capacity of almost 1000 mAh g−1. Thus, our active material completely converts itself to Ni and Fe to follow the conversion reaction mechanism during discharging. The voltage profile during the following charge and discharge were different, where the average charge and discharge potentials in subsequent cycles were around 1.5 and 1 V, respectively. In the charge curve, the

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RESULTS AND DISCUSSION Structure and Morphology. The structural information for the synthesized NiFe2O4 was determined by XRD as presented in Figure 1. The XRD pattern of the synthesized

Figure 1. Powder XRD pattern of NiFe2O4 calcined at 600 °C synthesized using a sol−gel Method.

sample calcined at 600 °C is identical to the standard values of NiFe2O4 (ICSD No. 00-010-0325), which can be readily indexed as the face-centered cubic structure (with space group of Fd3m). The characterized XRD peaks depicted that the assynthesized NiFe2O4 had a pure crystalline phase due to the homogeneous mixing of cations during the sol hydrolysis and subsequent condensation steps. The lattice parameter calculated from the most intense peak (311) of XRD pattern is a = 8.330 Å, which corresponds to a cell volume of 578.14 Å3 (Table 1) and is in agreement with previously reported Table 1. Structure Parameters of the Prepared Oxide, NiFe2O4 cell parameters

NiFe2O4

a (Å) b (Å) c (Å) cell volume (Å3) lattice structure crystallite size (nm)

8.330 8.330 8.330 578.14 cubic 20

values.16,32,33 The crystallite size of the synthesized NiFe2O4 is ∼20 nm, calculated by using Scherrer’s equation from the broadening of the (220) and (311) reflection of the spinel phase. The morphology of the synthesized NiFe2O4 was observed with scanning electron microscopy (SEM) and transmission 14835


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Figure 2. (a) FE-SEM image and (b) TEM image and corresponding (c, d, and e) high-resolution TEM images of the spinel NiFe2O4. The inset in (b) shows the corresponding selected area diffraction patterns (SAED).

Figure 3. (a) Cyclic voltammetry curves scanned at 0.1 mV s−1 and (b) charge/discharge profiles at 0.1 A g−1 current density for the NiFe2O4 electrode cycled against a Li anode in the voltage window 0.01−3.00 V.

been a general interpretation for transition metal oxides.5,6,16,19,22,37 Ex-situ XRD. For the ex-situ XRD and XAS measurements, the electrochemical experiments were interrupted at the points marked in Figure 4a. Figure 4b depicts the XRD patterns of the NiFe2O4 electrode collected at different depths of the first discharge curve, as marked in Figure 4a, and the results are examined on the basis of NiFe2O4 powder sample XRD pattern. At point 1, all of the main XRD peaks of pristine NiFe2O4 are observed. The conversion of NiFe2O4 from point 1 to 2 provided a highly stable specific capacity, as described in the previous section. The intensity of the Bragg reflections assigned to NiFe2O4 became weaker with lithium insertion (discharge). The reflection corresponding to the formation of Li2O was clearly distinguished by XRD. The intensity of the diffraction peaks of NiFe2O4 disappeared during the lithium insertion from point 2 to 3, signifying a decreased crystallinity. Simultaneously, the existence of a shoulder peak (at ∼44.5−44.8°) adjacent to the peak for the (400) crystal plane of NiFe2O4 (at 2θ = 43.3°, overlapped with the Cu foil peak) confirmed the formation of metallic Fe and Ni and indicated that a conversion reaction

quasi-plateau exhibited near 1.5 V can be attributed to the oxidation of Fe/Ni to their corresponding oxides. During subsequent discharging, the Fe/Ni oxides were possibly reduced to Ni and Fe pure metals. The observed first discharge cycle plateau moved away from the following discharge cycles to 1.0 V. Nevertheless, the subsequent charging cycles almost remain the same as the preceding cycles, which illustrates that the active material becomes stable and indicates that reduced compounds can undoubtedly be converted back to their oxidized state during charging. The electrode showed a low Coulombic efficiency (CE) of 79.3% in the first cycle. The partial disintegration of the SEI film is speculated to be the key reason for the low CE during the first cycle for conversion based oxide anodes.7,11−14,36 A possible reason for exceeding respective theoretical capacity (∼915 mAh g−1, estimating 4Li2O per formula) is the reversible evolution/demolition of a polymer gel-like film (which can reversibly store Li) that developed from the decomposition of the electrolyte when the electrode potential decreases and approaches a very low voltage.37 It has been recognized in previous reports and has 14836


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Figure 4. (a) Galvanostatic cycling curves of the NiFe2O4 electrode against a lithium anode in the 3.0−0.01 V region including the selected discharge/charge depths (depicted by numbers 1−10) at which the experiments were interrupted for the XAS and XRD experiments. Ex-situ XRD pattern evolution of the NiFe2O4 electrode during the first cycle for lithium (b) insertion and (c) extraction.

Figure 5. Normalized (a) Fe K-edge and (b) Ni K-edge XANES spectra and the Fourier transformed (FT) magnitude of (c) Fe and (d) Ni EXAFS spectra for the pristine sample (at OCV) and at a number of different depths of lithium inserted/extracted into the NiFe2O4 electrode during the first discharge/charge process. The corresponding pure metal foils and reference sample spectra were also included and are marked with a broken line. The isosbestic points are shown by circles in (a) and (b). The designated FT peaks are related to the bond distance between the absorbing and backscattering atoms for the first Fe−O/Ni−O shell (at ∼1.5 Å) and second Fe/Ni-metal shells (around 2.5−3.5).

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metallic state. This confirms that as soon as the electrode is completely discharged to 0.01 V, both Fe and Ni transform into their respective metallic states, indicating a complete conversion reaction. During the recharge process (point 4), no obvious shift could be observed in the Fe and Ni K-edge XANES spectra, illustrating that the valences of Fe and Ni remain almost unchanged. Therefore, the results from XAS are consistent with the interpretation from ex-situ XRD: the conversion reaction could not revert in this potential. It is possible that the capacity originates at the initial part of the charge plateau (point 4) due to the nonmetallic reactivity. This non-metal-centered reactivity was previously reported for transition-metal-oxide-based electrodes.37,40 The non-metalcentered charge capacity is supposedly due to a reversible oxidation of the polymer gel-like film that originated from the electrolyte reduction in the time of the lithium insertion process.37 The reversibility of the metallic contributors is evident after the lithium removal up to point 5 because the Fe K-edge XANES spectrum shows an extensive shift of the edge toward higher energy. In contrast, the Ni K-edge XANES spectrum changes comparatively less from point 4 to point 5, as demonstrated in Figure 5b. This result stipulates that it is necessary to extract lithium above 2 V for the comprehensive oxidation of nickel and that the capacity contribution in this region is mainly from the metallic Fe oxidation. The Fe XANES spectrum of the fully recharged electrode (point 6) reverts and almost covers the XANES spectrum recorded at OCV (Figure 5a). A clear isosbestic point at 7149 eV (pointed with a dashed circle in Figure 5a) in the Fe K-edge XANES spectra highlights that two chemical species are present in the reaction system and, during the oxidation−reduction (electrochemical cycling) process, only the component ratio of the species has changed.29 In contrast, the XANES spectrum is at a slightly lower energy than the OCV spectrum for the Ni K-edge (Figure 5b) after fully recharging the electrode to 3.0 V (point 6), which indicates a little limited reversibility of Ni in the applied potential window. The aligning isosbestic points (marked by circles in Figure 5b) observed in the spectra indicate the two phase transition behavior (Ni(2−x)+ ↔ Ni0). The irreversible metallic Ni contents in the fully charged sample were quantified using a linear combination method where Ni metal foil and NiO were used as references, and the fitted spectrum (Figure S4) reveals 4.4 and 95.6% contents, respectively. Therefore, an almost complete transformation of metallic Fe to Fe3+ was detected at the end of the recharge process while the elemental Ni shows the incomplete oxidation (more or less amount remains as metallic Ni) and forms a defective NiOδ (δ < 1) for the NiFe2O4 system. The locations of the k2-weighed Fourier transform (FT) peaks are related to the bond distance between the absorbing and backscattering atoms. Parts c and d of Figure 5 represent the Fe K-edge and Ni K-edge EXAFS spectra (without phase correction) recorded for the pristine sample at an OCV and electrodes at various discharged/charged potential states, respectively, for the first cycle. The EXAFS spectrum of Fe (Figure 5c) shows a first peak at ∼1.5 Å (designated Fe−O), which is due to the contribution from the scattering of nearest oxygens around the Fe atom. The next doublet peak in the Fe EXAFS spectrum is originated by the scattering of the second nearest Fe cation located at both the octahedral and tetrahedral sites.41−43 The split peak characteristics viewing nearby 2.6 Å (designated Fe−B) is due to the single scattering occurrences of Fe ions confined at the octahedral sites, while the second

conformed with the nucleation and growth of metallic Fe and Ni particles occurred. Therefore, after a complete discharge to 0.01 V (point 3), there were no signs of the NiFe2O4 peaks, which indicated that NiFe2O4 was fully reduced at this potential. An extra broad peak around 20−30° was observed in all ex-situ samples including a fresh electrode (Figure S3). This broad peak belongs to the diffraction peak from a carbon additive (acetylene black) and PAA binder.38,39 Figure 4c depicts the ex-situ XRD patterns of the NiFe2O4 electrode at different depths of the first charge curve, as marked in Figure 4a, with that of the standard NiO (ICSD No. 01-0895881), Fe2O3 (ICSD No. 00-039-1346), and NiFe2O4 for comparison. During charging at point 4, the peaks for the metallic Fe and Ni phases still exist with the Li2O peak (marked with a circle) and suggest that the conversion reaction did not revert until that time. When the electrode was charged to point 5, the peaks at 30.24°, 35.63°, and 62.93° indicated Fe2O3 formation and the partial conversion of the discharge product. When the electrode was further charged to point 6, the Bragg reflections became intense and strong crystalline peaks were detected. The emergence of the sharp XRD peaks of the fully charged electrode (at 3.0 V) implies that Fe2O3 and NiO were generated by the complete decomposition of the discharge products and Li2O (which completely disappeared). To the best of our knowledge, these manifestations are observed for the first time in this study by applying XRD in the case of NiFe2O4. X-ray Absorption Spectroscopy: XANES and EXAFS. We performed ex-situ XAS assessments for the electrodes at various lithium insertion/extraction stages during the first one cycle and a half cycles at the points marked in Figure 4a. The changes of the oxidation state of the nickel and iron atoms throughout the electrochemical reactions were inspected by XANES. The absorption edge was produced by the dipoleallowed 1s → 4p transition, which is known as the white line, and the oxidation state of the absorbing atom is defined by its relative position to the reference.30 The data were also recorded for standard materials, such as Fe2O3 and NiO, and this information is plotted as a dotted line to provide a steady comparison of the XANES spectra. Parts a and b of Figure 5 represent the Fe K-edge and Ni Kedge XANES spectra recorded for the pristine sample at an open circuit potential (OCV) and electrodes at various discharged/charged potential states for the first cycle, respectively. As expected, the Fe and Ni K-edge XANES spectra of NiFe2O4 at an OCV show Fe3+ (matched with Fe2O3) and Ni2+ (matched with NiO) oxidation states for the respective elements. The nickel ferrite materials are expected to transform into their respective metallic states for a complete conversion reaction. When NiFe2O4 is discharged to point 1, there is a sudden potential drop from 2.5 to 0.8 V, and Fe Kedge (Figure 5a) and Ni K-edge (Figure 5b) positions remain the same as those measured at an OCV. On discharge (point 1 to 2), the Fe K-edge XANES spectrum shows a significant shift to lower energy (Figure 5a), which reflects the reduction of Fe. The change in the Ni K-edge XANES spectrum at point 2 (Figure 5b) also shows a considerable displacement of the absorption edge to lower energy. Noticeably, Fe shows more reduction than Ni, which indicates that Fe is contributing more in total capacity from point 1 to 2. Both the Fe and Ni K-edge XANES spectra (Figure 5a,b) of the fully discharged electrode (point 3) show a further shift toward lower energy, a clue of farther reduction of iron and nickel, respectively, toward the 14838


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Figure 6. Normalized (a) Fe K-edge and (b) Ni K-edge XANES spectra and Fourier transformed (FT) magnitude of (c) Fe and (d) Ni EXAFS spectra for a number of different depths of lithium inserted/extracted into the NiFe2O4 electrode during the second discharge process after a full charging up to ∼3 V. The pure metal foils and reference sample spectra were also added and are marked with a broken line. The designated FT peaks are related to the bond distance between the absorbing and backscattering atoms for the first Fe−O/Ni−O shell (at ∼1.5 Å) and second Fe/Ni− metal shells (around 2.5−3.5).

subpeak appearing near 3.0 Å (designated Fe-A) originates from single and multiple scattering contributions from Fe ions bounded at the tetrahedral sites.42 The ratio between the amplitudes of the doublet peaks is approximately equal for pristine NiFe2O4, reflecting the qualitatively equal allocation of Fe cations at the tetrahedral and octahedral sites.41,42 In the EXAFS spectra of Ni (Figure 5d), the first peak at approximately 1.5 Å corresponds to the Ni−O interactions, and Ni−Ni interactions corresponds to another peak at ∼2.5 Å.44 There are no changes in the Fe and Ni EXAFS spectra (OCV → point 1), as shown in Figure 5c,d, respectively, which implies that the reduction process was initiated after that point. After the lithium insertion from point 1 to point 2, the EXAFS spectrum of Fe shows significant changes as the doublet peak of Fe−B and Fe-A transforms into a single peak in this region, as revealed in Figure 5c. The intensity of the peaks (Ni−O and Ni−Ni) were significantly reduced at point 2 in the Ni EXAFS spectra (Figure 5d), indicating changes of the local Ni arrangement. The EXAFS spectra of the fully discharged electrode (point 3) show a single peak around 2.0 Å (for both Fe and Ni), which corresponds to the respective metallic species because of the complete electrochemical reduction, as shown in Figure 5c,d, respectively. Both the XANES and EXAFS studies demonstrate that Fe3+ and Ni2+ drive the complete conversion to their respective metallic states at the end of the initial discharge. The Fe and Ni K-edge EXAFS spectra exhibit no change in position in the spectrum corresponding to point 4, as shown in Figure 5c,d, respectively. This result agrees well with XANES result at this stage and provides further evidence regarding nonmetal center activity as discussed in XANES part. The progressing lithium extraction process at point 5 demonstrates the decrease of the peak related to the metal fraction. The peak related to the metallic species of Fe was replaced by iron−oxygen and iron−iron scattering peaks in the Fe EXAFS spectra (Figure 5c). In

contrast, the interesting prominent feature in the Ni EXAFS spectra is the appearance of the Ni metal fraction peak (Figure 5d) after charging the electrode up to point 5. This phenomenon could be explained by the oxidation of Fe, which occurs at a lower potential than the oxidation of Ni. Further lithium extraction up to a final recharge potential of 3 V (point 6) oxidizes nickel, and the Ni−Ni and Ni−O peaks are reinstated at that potential. However, the Ni−Ni peak shows a little peak broadening toward the Ni metal fraction peak (marked by an arrow in Figure 5d), indicating a hindered reoxidation of Ni. The Fourier feature (doublet peak) displayed in the Fe EXAFS spectra at the end of the recharge (point 6) is a characteristic of the tetrahedral and octahedral environments, indicating the formation of cubic γ-Fe2O3.45 Therefore, the initial discharge/charge capacity of NiFe2O4 was due to the nonmetal reactivity and the oxidation/reduction of both Ni and Fe ions; in other words, both metal species act as active elements. Parts a and b of Figure 6 demonstrate the Fe and Ni K-edge XANES spectra of the electrodes, respectively, which correspond to different discharge potential states for the second discharge. Notably, after the initial cycle of lithium insertion/extraction, the subsequent lithium reinsertion exhibits a sloping voltage profile, indicating a significant difference of the reaction mechanism from the initial discharge process. The spectra after lithium insertion to point 7 show two different trends. First, the edge position in the Ni K-edge XANES spectrum shifts considerably to lower energy (Figure 6b). In contrast, no recognizable shift could be observed in the Fe Kedge spectra in comparison with the fully charged ∼3 V state (point 6), clarifying that the valence of Fe was almost unchanged (Figure 6a), as well as that the charge stored in this region is possibly because of Ni ion reduction. The Fe XANES spectra show a slight shift at point 8, but upon further lithium insertion to point 9, it shows an extensive shift toward 14839


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Figure 7. Cyclic performance of (a) a NiFe2O4 electrode at 0.5 A g−1 current density and (b) rate performance of a NiFe2O4 electrode at current densities in the range 0.1−10 A g−1, cycled against a Li anode in the voltage window of 0.01−3.00 V.

predominantly due to Fe ion reduction. Finally, the appearance of peaks related to both metallic Ni and Fe in their respective EXAFS spectra at the end of discharge point 10 (∼0.01 V) verify the complete conversion of the Fe2O3 and NiO phases. The NiFe2O4 electrochemical behaviors are briefly summarized here by coupling the results obtained from the ex-situ XAS and XRD studies. In the course of first discharge process, the pristine NiFe2O4 was reduced to metallic Fe and Ni when discharged to 0.01 V (point 3). During recharging to 3.0 V (point 6), the metallic Fe was oxidized to γ-Fe2O3, but a fraction of metallic Ni remained in the reduced state at the end of the recharge process which leads to the formation of NiOδ (δ < 1). Boesenberg et al. already reported a spectroscopic analysis (XANES and EXAFS) for the NiO electrode and monitored the electronic and structural changes of Ni atoms at various states of charge/discharge.55 The conclusion of present investigation regarding NiO species is very similar to that reported in this literature. However, when charged samples were subjected to a second discharge process, both of the metal oxide phases were completely converted to their respective metallic forms near 0.01 V. The entire electrochemical reaction that occurs can be rewritten like so:

lower energy. In contrast, the Ni XANES spectra show a significant shift at point 8, but upon further lithium insertion to point 9, it shows a negligible change. The fully discharged electrode again shows two different trends after further lithium insertion (point 9 → 10): the Fe XANES spectrum displays a minor displacement to lower energy, but the Ni XANES spectrum shows a significant change of the absorption peak position into lower energy. Parts c and d of Figure 6 represent the Fe K-edge and Ni Kedge EXAFS spectra, respectively, of the electrodes corresponding to different discharge potential states for the second discharge. There is no notable change observed for the Fe EXAFS peaks around 1.45 and 2.6 Å at point 7, as viewed in Figure 6c. Conversely, the Ni EXAFS spectrum exhibits a diminution in intensity for the signals at 1.5 and 2.5 Å, which corresponds to Ni−O and Ni−Ni interactions, respectively, as presented in Figure 6d. This result supports the assumption that the charge storage throughout this region is mostly due to Ni ion reduction. Once again, after the lithium insertion (7 → 8), the Ni EXAFS spectra go through a substantial change: a new peak emerges close to the Ni metal peak (2.0 Å), and the peak from the adjacent Ni−O interaction almost disappears (Figure 6d). Moreover, the Fe EXAFS spectrum demonstrates nominal peak changes (Figure 6c), which roughly indicates that the charge stored in this region could largely be due to Ni ion reduction. Remember that after the first charge cycle, the nickel ferrite spinel is not restored; a mixture of Fe2O3 and NiO along with metallic Ni exists instead. Neither NiO anode46,47 nor Fe2O3 anode48−50 individually showed a second cathodic peak from the second discharge cycle onward. This peak evidenced only in the case of the spinel metal ferrite electrode.16−22,28 However, a similar result was observed in a few previous studies dealing with a NiO/Ni composite electrode in LIBs.51,52 It is found that the locations of cathodic peaks in CV curves (Figure 3a) altered considerably from that of both NiO and Fe2O3, indicating the coordinated effect of Fe2O3 with a NiO/Ni system.53,54 Thus, the origin of the broad peak around 1.7 V can be speculated to be the reduction of NiOδ that commenced at that potential on the basis of XAS results described above. At point 9, a new peak appears close to the Fe metal peak (2.0 Å) in the Fe EXAFS spectrum, and the peak at ∼1.5 Å (designated as the Fe−O interaction) disappeared (Figure 6c). In contrast, the only spectroscopic change for Ni during the lithium insertion at point 9 is a minor increase of the Ni EXAFS peak intensity from the metallic Ni portion (Figure 6d). These findings are compatible with the estimation from the Fe and Ni K-edge XANES spectra and indicate that the capacity that occurs at this portion of the discharge plateau could be

NiFe2O4 + 8Li+ + 8e− → Ni0 + 2Fe 0 + 4Li 2O

(1)

Ni0 + 2Fe 0 + 4Li 2O ↔ (1 − x)NiO + x Ni0 + Fe2O3 + x Li 2O + (8 − 2x)(Li+ + e−)

(0 < x < 1)

(2)

Thus, NiFe2O4 electrochemically reacted with Li according to the mechanism presented in eq 1and eq 2. The outcome of this study revises the reaction mechanism proposed before in different studies.16−22 Additionally, electrochemical cycling potency of the assembled NiFe2O4 electrode was tested in a Li half-cell at a current density of 0.5 A g−1 after precycling at 0.1 A g−1 for the first five cycles (not shown). At these conditions, the NiFe2O4 electrode secured a reversible capacity of ∼786 mAh g−1 after 100 cycles, which was 923 mAh g−1 in the first cycle as presented in Figure 7a. The capacity fading between the first and 100th cycles was less than 15%. The rate performances (Figure 7b) were also evaluated to scrutinize the high-power competency of the synthesized NiFe 2 O4 . The NiFe 2 O 4 electrode was exposed at different current densities and demonstrated excellent rate capabilities at 0.2, 0.5, 1, 3, and 6 A g−1. The average specific capacities at these current densities were 1057, 933, 823, 725, and 523 mAh g−1, respectively, with capacity retentions of 89%, 79%, 69%, 61.0%, and 44% against average specific capacity (∼1186 mAh g−1) at 0.1 A g−1. A 14840



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specific capacity of around 365 mAh g−1 was sustained even at a very high current density of 10 A g−1, which is equivalent to around 10.1 C (1 C = 0.915 A g−1), and where the discharge occurred in only ∼6 minutes. Remarkably, the discharge capacities retrieved to the original values immediately upon returning to a low current density (0.1 A g−1), similar to the initial cycles for the NiFe2O4 electrode. Figure S5 shows the surface morphology of the fresh and cycled (100 cycles) electrodes examined by employing SEM. The smoothness of the NiFe2O4 electrode surface after cycling indicates that the inner stress can be well relieved by using the nanosized NiFe2O4 as the active material. Briefly, the excellent electrochemical performances indicate that the NiFe2O4 nanoparticles synthesized by employing the sol−gel method could be used as high-rate anodes in LIBs. The spherical NiFe2O4 nanoparticles can accommodate Li+ and be converted into Fe0 and Ni0 with the formation of Li2O on account of the small-sized particle and reduced Coulombic interaction of Fe3+ (placed at the tetragonal site) with O 2p. The conversion of nanometric Fe0 and Ni0 (dispersed in amorphous Li2O) into NiO, Fe2O3, and Li+ is not only an energetically favorable process but also kinetically favorable, as observed in the rate capability test in this study. Reports of such exceptional rate capabilities for NiFe2O4 nanoparticles are quite rare. In spite of an encouraging electrochemical performance, NiFe2O4 still suffered from an irreversible capacity loss. Further experiments are underway to mitigate the irreversible capacity loss issue. The obtained structural information for NiFe2O4 could be useful for improving its electrochemical performance.

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01892. Additional images on the powder morphology, nitrogen adsorption−desorption isotherm, fresh electrode XRD data, Ni K-edge linear combination fitting curve, and cycled electrode surface morphology (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +82-2-958-5225. Fax: +82-02-958-5229. E-mail: [email protected]. (K. Y. Chung) *Tel: +82-2-958-5240. Fax: +82-02-958-5229. E-mail: hungi@ kist.re.kr. (H.-G. Jung) ORCID

Hun-Gi Jung: 0000-0002-2162-2680 Author Contributions #

These authors contributed equally to this study.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Council of Science and Technology (CAP-14-2-KITECH) and by the Korea Institute of Science and Technology (KIST) institutional program (2V05540). This research was also supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP). Financial resources were granted by the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20152020106100).

CONCLUSIONS

NiFe2O4 nanoparticles (∼20 nm in diameter) were synthesized using a simple sol−gel method. XRD results confirm successful formation of a single-phase spinel NiFe2O4. The synthesized NiFe2O4, inspected as an anode for LIBs, showed the highest discharge and charge capacities of 1500 and 1190 mAh g−1, respectively, at a current density of 0.1 A g−1 and showed stable cycling performances with a capacity retention of greater than 85% after 100 cycles at a current density of 0.5 A g−1. They also had a superb performance at a high rate, exhibiting charge capacities of 523 and 365 mAh g−1 at high current densities of 6 and 10 A g−1, respectively (about a 6.5 and 10.1 C-rate, respectively). Ex-situ XRD experiments clearly identified that the electrochemical reaction with Li resulted in the complete conversion of NiFe2O4 followed by the formation of Li2O with metallic Fe and Ni. During the recharging, delithiation and releasing electrons from Fe−Ni−Li2O did not cause a nickel ferrite restoration but only the discrete metal oxides, i.e., Fe2O3 and NiO, exist. This well-known phenomenon is distinctly recognized by XRD for the first time in this study. XAS studies at the Ni and Fe K-edges identified that the lithium extraction process of NiFe2O4 in LIBs is fairly disparate from the reaction mechanism reported in prior studies. Analysis unveiled a partially irreversible conversion between Ni0 and Ni2+, which may be a further reason for the low Coulombic efficiency during the first cycle. This work provides insight into the reaction mechanism of NiFe2O4, which showed a high capacity, and provides additional areas on which to focus in further studies.

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