Article Cite This: ACS Appl. Energy Mater. 2019, 2, 4801−4812
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Examining the Role of Anisotropic Morphology: Comparison of FreeStanding Magnetite Nanorods versus Spherical Magnetite Nanoparticles for Electrochemical Lithium-Ion Storage Coray L. McBean,† Lei Wang,§ Dominic Moronta,† Alexis Scida,† Luyao Li,† Esther S. Takeuchi,†,‡,§ Kenneth J. Takeuchi,† Amy C. Marschilok,*,†,‡,§ and Stanislaus S. Wong*,†
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†
Department of Chemistry and ‡Department of Materials Science and Chemical Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794, United States § Energy and Photon Sciences Directorate, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *
ABSTRACT: As a matter of synthetic novelty, Fe3O4 (magnetite) nanorods (NRs) have been successfully generated by using a reproducible four-step protocol, wherein goethite is initially produced, morphologically tuned, chemically treated with a passivating agent to reduce aggregation, and ultimately converted to magnetite by thermal annealing within a reductive atmosphere. Our equally important objective was in correlating electrochemical behavior with the unique morphology of these Fe3O4 anode materials. As such, both conventionally coated and binderfree electrodes were tested using as-prepared magnetite NRs and nanoparticles (NPs) with controlled crystallite size as the active materials. Our study revealed that both the NR and NP Fe3O4 materials were amenable to effective binder-free electrode design. For the conventionally coated electrodes, the NR electrodes demonstrated an improved rate capability using a sequential discharge/charge current density profile as compared with that for corresponding NP electrodes. Most significantly, within the cycling stability test, the NR electrode delivered a high and stable capacity with a superior capacity retention relative to that of the NP for more than 50 cycles in half cells and 100 cycles in full cells. These data in particular showcase the undeniable benefits of the anisotropic structure of the material. KEYWORDS: magnetite, nanorods, morphology, anode, Li-ion battery hydrothermal reaction process.11 However, the presence of organic, long-chained capping agents typically adds structural and electronic complexity to the system and may therefore be deleterious to applications such as energy storage. Hence, a protocol to produce morphologically controlled magnetite that does not rely either on the use of capping agents or more complex protocols (such as involving the use of pulsed laser deposition)12 would be highly desirable. Nonetheless, whereas the synthesis of one-dimensional (1D) magnetite can be challenging, goethite (α-FeOOH) crystals can be arranged into a high aspect ratio structure under disparate reaction conditions. Our novel contributions therefore can be summarized as follows. First, we generated magnetite nanowires through the mediation of a goethite intermediate. Second, we tested the dependence of the electrochemical behavior upon morphology by comparing the electrochemical performance of a 1-D, anisotropic nanorod
1. INTRODUCTION Magnetite (Fe3O4) has long been regarded as a promising anode material for lithium ion batteries, because of a number of factors including its high theoretical capacity of 924 mAh/g, its low cost and comparatively high abundance, and its overall relative nontoxicity.1−4 As a material that can undergo both insertion and conversion processes, with multiple electron transfers per formula unit,5 Fe3O4 offers the opportunity to significantly exceed the theoretical capacity of conventional insertion-based anode materials, such as graphite. However, because of the dense crystallographic structure of Fe3O4, nanostructuring of the material is essential for promoting fast ion transport.6,7 Furthermore, issues associated with the nanoscale morphology of Fe3O4 coupled with its interactions with various electrode components (i.e., carbon, nature of the binder) may significantly impact upon the observed electrochemistry.8 The synthesis of morphologically complex magnetite is nontrivial, since it generally requires the use of capping agents to influence crystallite growth.9,10 For example, magnetite nanorods can be generated by a poly(ethylene glycol)-assisted © 2019 American Chemical Society
Received: March 3, 2019 Accepted: June 7, 2019 Published: June 22, 2019 4801
DOI: 10.1021/acsaem.9b00456 ACS Appl. Energy Mater. 2019, 2, 4801−4812
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ACS Applied Energy Materials (NR) morphology versus that of the corresponding “standard” spherical motifs of Fe3O4. In our novel synthesis protocol, goethite was used as a stepping stone toward creating crystalline, pure magnetite. We should also note that a similar type of principle was at play in recent work, describing not only the topotactic transformation of goethite nanowires into their hematite (α-Fe2O3) analogues13 but also the irreversible, heatdriven (>530 K) conversion of hydrothermally produced goethite nanowires into their hematite counterparts.14 Recent reports have indicated that the structural design of very specific nanostructure motifs can favorably influence the observed electrochemistry.15−17 Magnetite of various morphologies, including but not limited to nanoparticles,4 hollow structures,18 polyhedra,19 and nanowires,20 have been synthesized, and their corresponding electrochemical behavior has been assessed. To highlight the significance of size, structure, and morphological motif, it is worth noting that the hollow structures evinced very favorable electrochemical performance data, likely because of (i) the empty space within the motif, which alleviates the stress caused by volume changes during the conversion reaction, as well as (ii) the smaller nanoparticulate constituents of the motif itself, which result in a correspondingly small diffusion pathway for subsequent Li-ion transport, thereby providing for improved kinetics.3 It is known that lithiation of Fe3O4 to form Li2Fe3O4 is nominally accompanied by a volume expansion of 15%, as determined by an X-ray absorption spectroscopy analysis of Fe3O4 nanoparticle materials. This finding is consistent with density functional theory models.21 To highlight the structural effect, as-synthesized Fe3O4 hollow spheres exhibited a high initial capacity of 1614 mAh/g and retained 65% of the initial capacity after 100 cycles at 500 mA/g, whereas solid spherical control samples only delivered about 800 mAh/g in terms of capacity.22 However, the crystallite size of these two samples was not systematically controlled in this prior work. Hence, the improvement in functional capacity and capacity retention could likely be attributed to a synergistic effect involving both morphology and crystallite size. Herein, to (i) determine whether a particular morphology of the Fe3O4 anode material is intrinsically better than that any of the others and to (ii) deconvolute the effect of morphology versus that of crystallite size, both 0-D nanoparticles and 1-D nanorods of magnetite with controlled crystallite sizes have been systematically synthesized, and their individual electrochemical behaviors have been subsequently compared. Specifically, our work herein has entailed a detailed evaluation of voltammetry, capacity retention, and rate capability data within lithium-based half cells, coupled with a preliminary investigation of full cells incorporating Fe3O4 anodes and lithium cobalt oxide cathodes. Moreover, from a complementary perspective, our Fe3O4 nanorod synthesis approach developed herein possesses a number of advantages. First, one can potentially produce gramscale quantities of goethite nanorods using a relatively facile protocol, and by extension, correspondingly large amounts of magnetite may be obtained in such a manner. Second, the protocol is reasonably simple and robust. For example, as we shall demonstrate in this study, the morphology of goethite, and ultimately that of magnetite itself, is determined by a short, simple, and straightforward hydrothermal step. Third, we have shown that we can control the specific dimensions of our asprepared 1-D nanostructures, merely by toggling and manipulating reaction conditions, as highlighted by additional
data within the Supporting Information. Hence, this Fe3O4 material is appealing as an “insertion−conversion” material, based on the focus on an environmentally sustainable metal center (Fe), useful for future applications necessitating high energy density.
2. EXPERIMENTAL DETAILS Synthesis of Fe3O4 Nanorods (NRs). Our synthesis of magnetite structures occurs as a series of three steps. In the initial first step, goethite is produced by the dropwise addition of a solution, composed of 405 mg of FeCl3·4H2O (99%, Sigma-Aldrich), dissolved in 20 mL of distilled, deionized (DI) H2O, within a basic solution of 800 mg of NaOH (99%, EMD Industries), which itself is solubilized in 30 mL of DI H2O. An orange-brown precipitate subsequently forms. After stirring for 30 min, the mixture is evenly divided between two 45 mL Teflon autoclave vessels, sealed, and then placed into a preheated oven for processing at 160 °C for 3.5 h. The autoclaves are subsequently cooled to room temperature by leaving them under ambient environmental conditions. In the second phase of the synthesis, goethite is then coated with (3-aminopropyl)triethoxysilane (APTES, 99%, AcroSeal). The asformed precipitate is washed in DI H2O for three times, prior to dispersal in 20 mL of a solution, consisting of a 4:1 mixture of ethanol in DI H2O. The dispersion is subsequently sonicated, while an aliquot of 1.6 mL of APTES is added in dropwise. The mixture is then left to stir at 200 rpm for 24 h at 70 °C. Subsequently, the mixture is allowed to cool to room temperature and then washed with ethanol several times via centrifugation. The precipitate is later dried under ambient conditions. The silanized goethite is ultimately converted to magnetite via thermal degradation in a reducing atmosphere. Specifically, a porcelain combustion boat is loaded with the orange goethite powder and then placed inside a tube furnace. The quartz tube is purged with a reducing gaseous mixture of Ar:H2 (5% H2, Airgas) for 45 min, prior to heat treatment to 300 °C at a rate of 15 °C/min. The mixture is then isothermally processed at 300 °C for 30 min under a constant flow of the reducing gas mixture. The furnace and its contents are allowed to cool naturally to room temperature within the furnace. The gas flow is shut off once the temperature has dropped to 120 °C. Synthesis of Fe3O4 Nanoparticle Control Sample. Fe3O4 nanoparticles (NPs) were synthesized by using a precipitation method, analogous in nature to that noted in a previous report.23 Briefly, a solution of iron(II) chloride tetrahydrate (FeCl2·4H2O) was added dropwise to an ammonium hydroxide solution under N2 flow. The reaction mixture was subsequently stirred overnight under air. The isolated black precipitate was further washed with deionized water and subsequently dried, prior to structural characterization. X-ray Diffraction. Fe3O4 samples were characterized by X-ray powder diffraction (XRD) using a Rigaku SmartLab X-ray powder diffractometer. Cu Kα radiation (1.54 Å) was utilized with a Bragg− Brentano focusing geometry. The full width at half-maximum (fwhm) of the (311) peak was determined by using the Peak Fit software. Average crystallite sizes were calculated by using the Scherrer equation, after correcting for instrumental broadening with a lanthanum hexaboride (LaB6) standard. Electron Microscopy. As-synthesized goethite and magnetite nanorod aggregates were initially analyzed by using low-magnification transmission electron microscopy (TEM). Images were collected at an accelerating voltage of 120 kV with the JEOL JEM-1400 instrument, equipped not only with a 2048 × 2048 Gatan CCD digital camera but also with an Apollo XLTSUTW detector, which enabled EDS mapping. Better quality data, consisting of high-resolution transmission electron microscopy (HRTEM) images and accompanying selected area electron diffraction (SAED) patterns, were acquired with a JEOL 3000F microscope, equipped with a field emission gun, operating at an accelerating voltage of 300 kV. Samples were prepared in an identical manner for both low- and high-resolution TEM analysis. Specifically, a dilute dispersion of the powdered analyte in ethanol was sonicated, and then 2−3 drops of this solution were 4802
DOI: 10.1021/acsaem.9b00456 ACS Appl. Energy Mater. 2019, 2, 4801−4812
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range of 100 kHz−10 mHz with a 10 mV amplitude at 30 °C. Analysis of the impedance measurements was conducted by using ZView software.
applied onto a 300 copper mesh TEM grid, coated with a thin film of lacey carbon. The structural and morphological details of electrodes coated with either as-prepared, representative Fe3O4 NPs or NRs were probed both before and after electrochemical cycling tests using an analytical high-resolution SEM (JEOL 7600F) instrument, operating at an accelerating voltage of 10 kV. To prepare these samples for SEM characterization, these variously coated electrodes were directly attached onto a conductive carbon tape for subsequent image acquisition. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) experiments were performed using a TGA Q500 (TA Instruments), within an “extra dry” air environment. Specifically, asprepared magnetite samples were prepared for TGA analysis by first loading 1.9 mg of the powder product within a Pt boat, followed by the application of a controlled heating profile. A typical heating profile involves ramping of the temperature at a rate of 10 °C min−1 to as much as 800 °C. As a control experiment, we analyzed an empty Pt boat. Fourier Transform Infrared Spectroscopy. The lack of any organic surface functionality for the magnetite clusters was confirmed by using the results of infrared spectroscopy. Specifically, an infrared percent reflectance profile was obtained by using a Nexus 670 instrument, fitted with a Smart Orbit diamond ATR accessory, a KBr beam splitter, and a DTGS KBr detector. A background spectrum composed of air was taken in the range spanning 500−4000 cm−1, prior to sample analysis. Powdered magnetite nanorod aggregates were placed directly onto the diamond window and then pressed into place under a reproducible pressure with the ATR tower accessory. The final IR reflectance profile represented the net culmination of 64 individual scans. Electrochemical Characterization. Binder-Free Electrode. Pristine MWNTs (Cheap Tubes Inc., >95 wt %) and either asprepared Fe3O4 nanoparticles (NPs) or nanorods (NRs) were combined via physical sonication in ethanol. The final product was collected by vacuum filtration, washed with deionized water and ethanol, and dried to obtain the resulting Fe3O4−CNT electrode. Electrodes with 16 mm diameter were formed directly after the filtration process. The final amount of CNTs within the electrodes was controlled to be 50 wt %. Conventionally Coated Electrode. Coatings of either Fe3O4 NPs or NRs, incorporating 70% Fe3O4, 20% carbon black, and 10% poly(vinylidene fluoride) (PVdF) binder, were cast onto a copper foil. As-prepared electrodes were used to assemble “two-electrode”, stainless-steel experimental-type coin cells. The half cells were assembled in an electrolyte, containing 1.0 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate and dimethyl carbonate (30/70, v/v), using lithium metal as the corresponding anode. Cyclic voltammetry (CV) data were collected by using a two-electrode configuration, wherein the reference and counter electrodes both consisted of lithium metal. Voltage limits for the CV test were 0.05 and 3.0 V at a scan rate of 0.2 mV/s. Rate capability and cycling tests were conducted by using a Maccor battery tester at 30 °C. Li/Fe3O4 cells were discharged and charged within a voltage window between 3.0 and 0.05 V. A rate capability test was performed with discharge and charge current densities applied in the sequence of 200, 400, 800, 1600, and 200 mA/gFe3O4 for 10 cycles each. Two cells of each cell type were tested to obtain average rate capability results. To further assess the cycling stability, the various cells were cycled at a current density of 200 mA/gFe3O4 for an additional 100 cycles. Full cells were assembled by using Fe3O4 NP and NR electrodes as anodes and LiCoO2−CNT binder-free electrodes as cathodes, with the extent of electrode loading chosen to match Li equivalents. The voltage window was selected, based upon the potential difference between the discharge and charge voltage platforms, noted with the LiCoO2/Li and Fe3O4/Li half cells, respectively. Specifically, the fabricated coin cell type full cell was slightly anode limited and cycled at a current density of 200 mA/gFe3O4 within a voltage window between 1.2 and 3.0 V. Electrochemical impedance spectroscopy (EIS) data were collected over a frequency
3. RESULTS AND DISCUSSION Elucidation of the Synthesis and Morphology of the Products. In this study, we have been inspired by a recent report in which a quasi-linear growth of goethite was reported under basic conditions.24 Specifically, that work implied that in the early growth stages, a morphological mixture consisting of a majority of nanoscale spheres, measuring 2−4 nm in diameter, coupled with a relatively small amount of nanorods, eventually transformed, due to a combination of “aging” over the course of several days with controlled heating to 90 °C, to a medium composed of a higher proportion of ever-growing, longer anisotropic nanorod clusters. Upon attempting a variation of this initial protocol ourselves, our TEM micrographs, shown in Figure 1, suggest that ∼96% of as-prepared goethite particles precipitated as nanoscale grains, i.e., “nanograins”, at room temperature under alkaline conditions.
Figure 1. (A) Typical TEM micrograph and corresponding (B) XRD profile for goethite that had precipitated and aged under alkaline conditions for 10 min.
Furthermore, as the goethite nanograins underwent a hydrothermal reaction at 160 °C under alkaline conditions, the relative number of nanorods increased, an observation which could be attributed to an Ostwald ripening process. From both low- and high-magnification TEM images (Figures 2A and 2B), we found that the morphology of the isolated precipitate altered to some degree after treatment in an autoclave at 160 °C for 30 min. Specifically, the goethite precipitate consisted of ∼23% linear nanostructures and ∼77% grain-like nanoscale motifs. After 40 min of hydrothermal reaction at 160°C, nanorod clusters formed in approximately equal proportion to grain-like motifs (Figures 2C and 2D). In fact, ∼45% of the observed precipitate was composed of clusters of nanorods. Each aggregate cluster possessed a width of 58.8 ± 25.1 nm and a corresponding length of 402.4 ± 108.6 nm. After a reaction interval of 50 min, the vast majority (∼94%) of the sample could be ascribed to linear nanorod aggregates (Figures 2E and 2F). The width of these clusters was effectively unchanged, measuring ∼56.0 ± 21.1 nm, though the corresponding lengths had increased to 508.8 ± 91.9 nm. It is reasonable to hypothesize that via the mediation of an Ostwald ripening process, smaller nanograin “building blocks” preferentially adjoined onto the ends of existing, linear nanorods, thereby resulting in the ongoing growth of larger clusters. Ultimately, we noted that a near-complete consumption of nanoscale spheres was achieved after a heat treatment of 3.5 h, 4803
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reaction. These nanostructures can best be described as nanoclusters composed of aligned nanorods, characterized by an overall length of 479.1 ± 6.1 nm and an aggregate width of 78.4 ± 15.3 nm. Each cluster appears to consist of individual, constituent nanorods measuring 14.4 ± 3.4 nm in diameter. We found that after ∼50 min of reaction time within the autoclave, growth in the length of the clusters appeared to have halted, and that, coincidentally, almost all of the observable zero-dimensional nanograins appeared to have been consumed. These observations confirmed that likely the attachment of freely available nanograin “component units” contributed to the ever-growing length of the isolated nanorods; therefore, according to this scenario, the lack of availability of these zerodimensional particles was responsible for the limit to nanorod extension. Moreover, we also noted a small but perceptible increase in apparent average diameter of the nanorods, which might have reasonably resulted from the aggregation of initially isolated nanoparticulate clusters. Such an aggregation might have been favored by either the concentration or drying of a nanoparticle suspension.26 Nevertheless, unwanted and uncontrollable aggregation of these aligned nanorod clusters could be a major problem if goethite were either kept in storage for long periods of time or immediately converted to magnetite through reductive thermal annealing. To avoid this harmful and undesirable outcome, we made use of the fact that surface modification of nanoparticles with organic species, such as ionic ligands, hydrocarbons, polymers, and micelles as illustrative examples, can decrease and diminish interparticle aggregation.27−30 Moreover, it was known that coating nanoparticles with inorganic species such as silica was also an effective strategy for isolating individual particles.31 In this vein, our as-prepared goethite nanorod clusters were incubated within an aqueous ethanolic solution, containing APTES, for an overnight period at 70 °C to functionalize the goethite surface with amine-terminated organosilanes. We hoped therefore to initiate full derivatization of the goethite, prior to a subsequent heat treatment at 300 °C. Indeed, a similar process, motivated by an analogous inclination to inhibit aggregation, had been previously used to coat not only Fe3O4 particles but also Fe particles.32−34 In our experiments, we found that coating goethite nanorod clusters with APTES not only prevented their further aggregation under ambient conditions but also, more importantly, was equally effective at preventing massing of sample during the necessary thermal reductive annealing process needed to convert the goethite intermediate into magnetite. Figure S1 provides TEM images of APTES-functionalized goethite and the subsequent magnetite product obtained after thermal annealing. Associated EDS mapping data suggests that the relative quantity of Si did not change during the reaction, implying that a small amount of Si remained on the magnetite surface. It is worth emphasizing that the chemically relevant reaction steps leading to successful magnetite formation are summarized in steps 1 and 4 of Figure 4. Whereas step 2 was key to achieving the correct nanorod morphology, the sole function of modifying the goethite surface with APTES (step 3) was to ensure that the resulting magnetite by comparison with the goethite precursor remained relatively dispersed, unaggregated, and hence processable from a battery perspective. In the TEM images of Figures 5A and 5B, the 1-D morphology of typical “clusters of aligned nanorods” was
Figure 2. (A, C, E) Low-magnification and (B, D, F) highmagnification TEM images of the resulting intermediate goethite nanoparticles after successive hydrothermal reactions of (A, B) 30, (C, D) 40, and (E, F) 50 min.
at which point only 1-D nanorod clusters were discernible from our TEM images. Overall, our results are consistent with previous work in that it is known that 1-D growth is promoted by both high-temperature and high-pH conditions.25 Moreover, our data confirm the notion that lower-dimensional nanoparticulate motifs can be considered as intermediates along the pathway to conversion into the desired 1-D goethite. TEM images in Figure 3 highlight representative goethite nanostructures that formed within the autoclave after 3.5 h of
Figure 3. Typical (A) low-magnification and (B) high-magnification TEM micrographs of FeOOH goethite intermediate structures. 4804
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Figure 4. Proposed reaction schematic, summarizing our novel synthesis process.
generally retained. Nonetheless, the clusters did become significantly more porous, though the dimensions of the asprepared magnetite product remained very close to that of the intermediary goethite. Specifically, our aggregate clusters were characterized by widths and lengths of 90.3 ± 26.5 nm and 484.3 ± 86.7 nm, respectively. Individual, constituent nanorod widths measured 16.7 ± 4.9 nm in diameter. The increase in cluster width coupled with a rise in porosity for magnetite as compared with the goethite intermediate (Figure 3) may have originated from a sintering process, emanating from the annealing step. To account for these observations, it is proposed that the brief 30 min heating step may have given rise not only to (i) sample dehydration and mass loss (with little volume change) but also to (ii) the concomitant
Figure 5. Typical low-magnification (A) and high-magnification (B) TEM micrographs of our as-prepared magnetite nanorods.
Figure 6. XRD patterns associated with as-prepared Fe3O4 (A) NPs and (B) NRs. 4805
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ACS Applied Energy Materials crystallographic rearrangement associated with the conversion of goethite into magnetite. The purity and crystallinity of our as-prepared Fe3O4 NPs and NRs were initially characterized by using XRD (Figure 6). The XRD patterns associated with both the NPs and NRs match well with the standard cubic Fe3O4 phase (PDF # 97007-7589). No observable impurities were detected in either of the as-prepared materials, and the crystallite size was estimated to be 25 nm for both samples by using the Debye−Scherrer equation. Significantly, thermogravimetric analysis data displayed in Figure 7 suggests that little if any volatile species or organic
Figure 8. Representative SEM images of pristine conventionally coated electrodes, imaged under different magnifications and prepared by using (A−C) Fe3O4 NPs and (D−F) Fe3O4 NRs as active materials.
blade-like morphology, measuring 305 ± 64 nm in length and 43 ± 6 nm in diameter. The likelihood of porosity was possibly induced by the removal of H2O during the annealing process; its presence can potentially promote the diffusion of Li ions within the electrode and enable a durable high-rate capability.35 Electrochemical Characterization. The as-prepared Fe3O4 NPs and NRs were initially tested within a binder-free electrode regime. No appreciable difference was noted within the cyclic voltammetry data and charge−discharge profiles, as illustrated in Figure S5. The average rate capability results (Figure S6) suggested that, by comparison with the Fe3O4 NP control sample, the Fe3O4 NR-based electrode delivered higher capacities under low discharge current densities, such as 200 mA/gFe3O4 and 400 mA/gFe3O4, but lower capacities under high discharge current densities of 800 mA/gFe3O4 and 1600 mA/ gFe3O4. It is worth noting that this observed difference in the measured capacities delivered by binder-free electrodes based on Fe3O4 NPs and NRs, respectively, is rather minimal. Subsequently, as-prepared Fe3O4 NPs and NRs were further investigated as components of electrodes, generated by a conventional coating method. Detailed results associated with these “traditionally prepared” electrodes are presented in the following discussion. Cyclic Voltammetry. Representative cyclic voltammograms of the two coated electrodes, containing either Fe3O4 NPs or NRs from cycles 1−5, are displayed in Figures 9A and 9B, respectively. In the first cycle, the cathodic process was characterized by three major reduction peaks. The first peak located at ∼1.55 V is associated with the insertion of the Li ion into the interstitial octahedral sites of the cubic Fe3O4 structure and reduction of one electron equivalent of Fe3+ to Fe2+.36,37 It was noted that this peak was more noticeable in the Fe3O4 NPbased electrode (Figure 9A) as compared with its NR counterpart (Figure 9B). The second peak, situated at around 1.0 V, can be ascribed to additional Li+ insertion and the associated phase change from inverse-spinel Fe3O4 to a FeOlike rock-salt structure.38 The third peak, appearing at ∼0.7 V, can be attributed not only to the conversion of Fe2+ to Fe0 but also to the formation of a solid electrolyte interphase (SEI) and amorphous Li2O. The two broad overlapping anodic peaks centered at ∼1.6 and ∼1.8 V have been previously identified as oxidation peaks associated with the conversion of Fe0+ to Fe2+.36,38 In cycle 2 and subsequent cycles, an obvious shift to the higher voltage ranges was noted for both the cathodic and anodic peaks, an
Figure 7. A mass-resolved thermal degradation profile suggests that very little volatile species was present on the surfaces of our asprepared nanorods.
functionality remained on the surface of the thermally annealed magnetite, relevant for their subsequent electrochemical performance. Specifically, it is hypothesized that surface moisture and volatile species represented only 4.1% (with 1.2% moisture) of the overall sample. The notion of little if any organic residuals were present and left over after the annealing process was corroborated by reflective FTIR analysis, as shown in Figure S2, wherein the acquired IR spectrum in the region of 1000−3500 cm−1 remained devoid of any evident peaks that might have been convincingly ascribed to organic functionalities. Therefore, assuming that 95.9% of the final mass is indeed magnetite, then the protocol, as described, yields relatively surface-bare magnetite nanorod clusters with an overall yield of 80%. It is worth mentioning that under different reaction conditions, relatively more isolated, homogeneous magnetite morphologies as opposed to clusters of smaller nanorods could be achieved. For these motifs (Figure S3), we found that with these nanorod species, the relationship of height (H), width (W), and length (L) was such that H < W < L, implying that what we had actually formed in effect consisted of rectangular prisms. Morphology of Conventionally Coated Electrodes. Typical SEM images (under different magnifications) of electrodes, coated with either Fe3O4 NPs or NRs, prior to cycling, are displayed in Figure 8. Aggregates of Fe3O4 NPs with an overall size distribution of 0.5−8 μm were noted within the NP electrodes themselves (A−C). On the basis of the data, it can be plausibly deduced that constituent Fe3O4 NP within the individual aggregates measured 25 ± 6 nm on average in diameter, whereas the carbon black materials appeared to consist of individual granules with an average size of 40 nm. By contrast with the NP-coated electrode, the NRcoated electrode (D−F) demonstrated a more uniform distribution of Fe3O4 NRs within the underlying black carbon matrix. Indeed, individual Fe3O4 NR possessed a porous, 4806
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Figure 9. (A, B) Cyclic voltammetry, (C) rate capability, (D) cycling stability, (E, F) voltage profiles, and (G, H) differential capacity as a function of potential (dQ/dV) of the electrodes, constructed from Fe3O4 NP and NR, respectively.
∼0.8 V is visible in both electrodes, which can be attributed to a FeO to Fe0 metal conversion process. After cycle 1, the successive anodic peaks observed are analogous to that noted with cycle 2 with only minimal variations in the associated peak positions and peak intensities, suggesting that the
observation possibly attributed to the polarization of the electrodes. In addition, the decrease in the cathodic peak intensity suggests the occurrence of a certain degree of an irreversible redox reaction process taking place. In the subsequent cycles, only one reduction peak positioned at 4807
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Figure 10. Nyquist plot as well as the linear fit of the Warburg region from ac impedance shown for electrodes based on Fe3O4 NP and NR electrodes, measured both (A, B) before cycling and (C, D) after 50 cycles.
discharge/charge process is reversible. What is worth noting is the similarity in the profiles associated with the NR and (the completely Si-free) NP cyclic voltammograms, implying that there is “little-to-no” effect of Si impurities upon the observed electrochemical performance of the NR. Rate Capability and Cycling Stability. Rate capability measurements were performed on both Fe3O4 NP and NR electrodes for 50 cycles at charge/discharge current densities in the sequence of 200, 400, 800, 1600, and 200 mA/gFe3O4 within a cutoff voltage window of 3−0.05 V (Figure 9C). The average capacities were presented for the first 24 cycles for both the NP and NR electrodes. At 200 mA/g, the NP- and NR-based electrodes delivered very similar capacities of 999 and 1003 mAh/g in cycle 10, respectively. When the current density was increased to 400 mA/g, the capacity decreased to 841 mAh/g for the NP electrode, whereas the corresponding capacity of the NR electrode was found to be higher at 966 mAh/g. In cycle 30, where the current density was increased to 800 mA/g, a drastic capacity loss was noticed with the NP electrode, since it delivered a capacity of only 562 mAh/g. The rapid capacity fade of the NP electrode can be ascribed to the volume expansion due to the conversion reaction, which would have compromised the structural integrity of NP electrodes, thereby leading to a loss of electrical contact between the active materials with the conductive carbon. By contrast, the analogous capacity delivered by the NR electrode was measured at 807 mAh/g, which was 245 mAh/g higher than that of the NP electrode. We can likely ascribe this performance improvement to the porous structure of the NRs, which would have led to favorable accommodation of the volume expansion. Subsequently, at the highest current density of 1600 mA/g, the NP and NR electrodes delivered capacities of 384 and 584
mAh/g, respectively. When the current density was returned to 200 mA/g, the capacity of the Fe3O4 NP electrode recovered to 523 mAh/g (i.e., 56% retention from cycles 2 to 50). By comparison, the Fe3O4 NR electrode yielded a capacity of 755 mAh/g, corresponding to a capacity retention of 88%. Our results unambiguously suggested that the NR electrode demonstrated a superior rate capability to its NP counterpart. To further assess the cycling stability of both electrodes, cells were subsequently cycled for an additional 100 cycles at a constant current density of 200 mA/g (Figure 9D). It was noted that the NR electrode delivered a stable capacity with minimal fading during the first 40 cycles. Specifically, from cycles 2 to 40, capacity retention was 88% for the NR electrode and 61% for the corresponding NP control sample. However, capacity fading was subsequently observed for the NR electrode, whereas the capacity of the NP electrode maintained relatively the same value, even after cycle 40. In fact, similar capacities of 388 and 365 mAh/g were provided by the NR and NP electrodes, respectively, at cycle 100. To further investigate the reaction kinetics and provide a better understanding of the reduced capacity after cycle 40 for the NR electrode, EIS data were collected before and after 50 galvanostatic cycles, denoting a set of experiments which will be discussed in detail in a later section. Voltage Profiles. Representative discharge−charge voltage profiles of the two Fe3O4 coating electrodes acquired, at cycle 1, 11, 21, and 31, are depicted in Figures 9E and 9F. With the NP electrode, the discharge capacities were 1371, 977, 770, and 513 mAh/g, measured at discharge current densities of 200, 400, 800, and 1600 mA/g, respectively. With the NR electrode, the corresponding discharge capacities were found to be 1137, 903, 871, and 712 mAh/g, indicating that the NR electrode gave rise to a much improved high-rate capability with current densities assessed above 800 mA/g. 4808
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only half of that associated with the NP electrode (3.36 × 10−12 cm−2 s−1). The reduced charge transfer and Li-ion diffusion efficiency of the Fe3O4 NR electrode after 50 cycles might suggest some degree of irreversible structural degradation. To validate our assumption, SEM images of both electrodes after 50 cycles were acquired to probe structural evolution during the cycling process. Structural Evolution after Discharge/Charge Processes. After cycling for 50 cycles, the morphology of the NP remained approximately the same (Figure 11A−C), whereas
To provide more information about the structural transformations occurring during the charge/discharge process, the differential capacity as a function of potential (dQ/dV) plots of both the NP and NR electrodes were shown in Figures 9G and 9H, in which the plateaus in the voltage profiles appear as clearly identifiable peaks. These peaks are often associated with phase transitions of the electrode material, and the appearance of the cathodic and the anodic curves yields information about the reversibility of the electrode reaction. It is clear that both the NP and NR cells evinced similar dQ/dV peak features in the initial cycle. It has been previously reported that a single cathodic peak positioned at 0.1 V can be assigned to lithiation of crystalline Si, whereas a single anodic peak centered at 0.43 V can be ascribed to delithiation of the crystalline Li15Si4; these features are often noted in Si-based electrode materials.39 Nevertheless, both peaks are absent in the NR dQ/dV plot, shown in Figure 9H, thereby indicating that the surface treatment by silane does not necessarily contribute to the observed capacities within the resulting NR cell. Indeed, the NR cell demonstrated a better reversibility by comparison with its NP counterpart, as demonstrated by the smaller intensity decrease, noted with both cathodic and anodic peaks from cycles 11 to 31. Electrochemical Impedance Spectra (EIS). To gain further insights into the reaction kinetics, EIS data were collected before and after 50 galvanostatic cycles under a discharge/ charge current density of 200 mA/g, as shown in Figures 10A and 10C. The impedance spectra analyzed before and after cycling were fit to the equivalent circuit models, displayed in Figures S7A and S7B, respectively. In both models, R1 is the ohmic resistance, CPE is a constant phase element, R2 and R3 are the charge transfer resistance, and Wo is the Warburg element. In the low frequency region, the Warburg coefficient (σw), which is inversely proportional to the ion-diffusion coefficient, was determined from the slope of Z′ versus ω−1/2 (eq 1), as displayed in Figure 10B,D. The effective lithium-ion diffusion coefficient (DLi+) was determined from eq 2, in which R is the gas constant, T is the absolute temperature, A is the contact area of the electrode, n is the number of electrons per molecule, F is the Faraday constant, C is the concentration of Li+ ions, and σ is the Warburg coefficient. All of the fitted results are summarized in Table S1. Zre ∝ σω−1/2
(1)
D Li+ = (R2T 2)/(2A2 n 4F 4C 2σ 2)
(2)
Figure 11. Representative SEM images under different magnifications of electrodes, which had been constructed using (A−C) Fe3O4 NPs and (D−F) Fe3O4 NRs as active materials and which had been cycled at a current density of 200 mA/g for 50 cycles.
the surface of the Fe3O4 NPs and the carbon particles appeared to be coated with some insulating species, possibly corresponding to the solid electrolyte interphase (SEI) layer and the presence of amorphous Li2O. SEM images associated with the NR electrode after 50 cycles were rather dim in intensity (Figure 11D−F), implying that the electrical conductivity of the NR electrode was much lower than that of the corresponding NP electrode, a finding consistent with our observation of the larger charge transfer resistance of the NR electrode, after cycling in the EIS data (Figure 10). In addition, it was noted that although the morphology of the 1-D NR motifs were significantly altered after cycling, the NR aspect ratio had significantly reduced, relative to the original parent sample prior to cycling. This observation could be potentially attributed to structural alteration and fragmentation, induced by the large volume expansion during the 50 cycles. The improved capacity retention for the NRs over the first 40 cycles indicates that our as-prepared porous Fe3O4 NRs can better accommodate for the volume change associated with the (de)lithiation process as compared with the Fe3O4 NPs. However, after 50 cycles and beyond, the extended cycling conditions may have resulted in an apparent breakdown or distortion of the porous structure, thereby leading to a decrease in both electron and ion conductivity. To provide a preliminary assessment of the function of these Fe3O4 anode materials within a full cell configuration, coin cells were assembled using commercial LiCoO2 as the cathode and Fe3O4 NP and NR as the anode. The preliminary cycling tests were conducted at a relatively high current density of 200 mA/g within a voltage window of 1.2−3.0 V, which was determined based upon the half cell performance of the Fe3O4 and LiCoO2 electrodes. The observed capacities shown in Figure 12 were calculated, based upon the active mass of Fe3O4 within the anodes. During the charging process of the full cells, which corresponded to the discharging process within the
Before cycling (Figure 4A), the NR electrode yielded a smaller charge transfer resistance, measuring 9.7 ohm, which was less than that of the corresponding NP electrode (12.9 ohm). The favorable charge transfer behavior could possibly be attributed to the more homogeneous and uniform dispersion of Fe3O4 NRs within the conductive carbon network, thereby leading to an improved electrical contact. After cycling for 50 cycles with a current density of 200 mA/g, the charge transfer resistance (Rct) of the NRs increased to 50.6 ohm, whereas the Rct of the NP electrode (13.1 ohm) remained similar to its pristine state, prior to the cycling process. With respect to lithium-ion diffusion efficiency, the DLi+ was similar to each other within the Fe3O4 NP and NR electrodes before cycling, measured at 7.05 × 10−10 and 5.79 × 10−10 cm−2 s−1, respectively. After 50 cycles, the Li-ion diffusion coefficients decreased for both electrodes, with the DLi+ of NR electrode calculated to be 1.74 × 10−12 cm−2 s−1, which was 4809
DOI: 10.1021/acsaem.9b00456 ACS Appl. Energy Mater. 2019, 2, 4801−4812
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ACS Applied Energy Materials
Figure 12. Cycling data for Fe3O4/LiCoO2 full cells obtained at a current density of 200 mA/g. (A) Cycle number versus capacity for cycles 1− 100. (B, C) Voltage curves for cycles 1, 10, 50, and 100 associated with the (B) NP and (C) NR cells.
before and after the cycling process for the NR and NP electrodes, respectively. With respect to the conventionallycoated electrodes, the NR electrodes exhibited a better cycling stability and an improved rate capability within the initial 40 cycles, a finding attributed not only to (1) the more uniform and homogeneous dispersion of the Fe3O4 within the Ketjen black carbon matrix, thereby leading to a smaller charge transfer resistance before cycling, but also to (2) the porous Fe3O4 NR morphology, which can accommodate for the volume change of the active materials during the initial cycling conditions. However, after extended cycling, the charge transfer resistance of the NRs appeared to increase to almost 4 times that of the NP sample, whereas the corresponding DLi+ was found to be half that noted with the NP electrode. These observations may be possibly ascribed not only to the relatively insulating SiO2 coating accumulating on the Fe3O4 NR surface but also to the structural damage incurred by the NRs, thereby resulting in the observed capacity fade within the NR electrodes after 50 cycles.
Fe3O4/Li half cells, capacity fading was noted for both NP and NR cells from cycles 1 to 40, a finding consistent with the observation in half-cell testing, as shown in Figure 9D. The NR full cell delivered an ∼114 mAh/g charge capacity at cycle 100 and consistently demonstrated an extra 70 mAh/g of capacity, especially as compared with its NP analogue. Additional optimization of the electrode configuration will still be needed in order for the Fe3O4/LiCoO2 full cell design to realize its full potential.
4. CONCLUSIONS We have successfully generated high aspect ratio, relatively surface-bare, one-dimensional magnetite nanostructures using a reproducible, facile, and straightforward protocol. Specifically, clusters of aligned nanorods have been synthesized using a four-step protocol, wherein goethite is initially produced, morphologically tuned, chemically treated with a passivating agent to reduce aggregation, and ultimately converted to magnetite by thermal annealing within a reductive atmosphere with a relatively high overall yield of ∼80%. An equally significant objective of our study was in correlating electrochemical behavior with the unique morphology of Fe3O4-based anode materials. Specifically, both conventionally coated and binder-free electrodes were generated by using as-prepared Fe3O4 NRs and NPs with controlled crystallite size as active materials. To understand the mechanism underlying the initially high capacity within the first 40 cycles but limited electrochemical reversibility of the NR electrodes with a regime of extended cycles, both EIS data and SEM images with spatially resolved features were acquired. Specifically, these complementary spectroscopy and microscopy results were obtained both
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00456. Background and synthesis discussion; TEM, EDS, FTIR, HRTEM, and cyclic voltammetry data; electrical circuit schematic; and a “resistance and lithium diffusion coefficient” data table (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. 4810
DOI: 10.1021/acsaem.9b00456 ACS Appl. Energy Mater. 2019, 2, 4801−4812
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ACS Applied Energy Materials *E-mail:
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(9) Gao, G.; Liu, X.; Shi, R.; Zhou, K.; Shi, Y.; Ma, R.; TakayamaMuromachi, E.; Qiu, G. Shape-Controlled Synthesis and Magnetic Properties of Monodisperse Fe3O4 Nanocubes. Cryst. Growth Des. 2010, 10, 2888−2894. (10) Li, W. L.; Lee, S. S.; Wu, J. W.; Hinton, C. H.; Fortner, J. D. Shape and Size Controlled Synthesis of Uniform Iron Oxide Nanocrystals Through New Non-Hydrolytic Routes. Nanotechnology 2016, 27, 324002. (11) Harraz, F. A. Polyethylene Glycol-Assisted Hydrothermal Growth of Magnetite Nanowires: Synthesis and Magnetic Properties. Phys. E 2008, 40, 3131−3136. (12) Zhang, D.; Liu, Z.; Han, S.; Li, C.; Lei, B.; Stewart, M. P.; Tour, J. M.; Zhou, C. Magnetite Core-Shell Nanowires: Synthesis and Magnetoresistance. Nano Lett. 2004, 4, 2151−2155. (13) Meng, F.; Morin, S. A.; Jin, S. Rational Solution Growth of alpha-FeOOH Nanowires, Driven by Screw Dislocations and Their Conversion to alpha-Hematite Nanowires. J. Am. Chem. Soc. 2011, 133, 8408−8411. (14) Pregelj, M.; Umek, P.; Drolc, B.; Jancar, B.; Jaglicic, Z.; Dominko, R.; Arcon, D. Synthesis, Structure, and Magnetic Properties of Iron-Oxide Nanowires. J. Mater. Res. 2006, 21, 2955−2962. (15) Wang, L.; Yue, S.; Zhang, Q.; Zhang, Y.; Li, Y. R.; Lewis, C. S.; Takeuchi, K. J.; Marschilok, A. C.; Takeuchi, E. S.; Wong, S. S. Morphological and Chemical Tuning of High-Energy-Density Metal Oxides for Lithium Ion Battery Electrode Applications. ACS Energy Lett. 2017, 2, 1465−1478. (16) Lewis, C. S.; Li, Y. R.; Wang, L.; Li, J.; Stach, E. A.; Takeuchi, K. J.; Marschilok, A. C.; Takeuchi, E. S.; Wong, S. S. Correlating Titania Nanostructured Morphologies with Performance as Anode Materials for Lithium-Ion Batteries. ACS Sustainable Chem. Eng. 2016, 4, 6299−6312. (17) Wang, L.; Zhang, Y.; Scofield, M. E.; Yue, S.; McBean, C.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S.; Wong, S. S. Enhanced Performance of “Flower-like” Li4Ti5O12 Motifs as Anode Materials for High-Rate Lithium-Ion Batteries. ChemSusChem 2015, 8, 3304−3313. (18) Xiong, Q. Q.; Tu, J. P.; Lu, Y.; Chen, J.; Yu, Y. X.; Qiao, Y. Q.; Wang, X. L.; Gu, C. D. Synthesis of Hierarchical Hollow-Structured Single-Crystalline Magnetite (Fe3O4) Microspheres: The Highly Powerful Storage versus Lithium as an Anode for Lithium Ion Batteries. J. Phys. Chem. C 2012, 116, 6495−6502. (19) Su, D. H.; Munroe, J.; Ahn, P.; Ranjbartoreh, H.; Wang, A. R. G. Polyhedral Magnetite Nanocrystals with Multiple Facets: Facile Synthesis, Structural Modelling, Magnetic Properties and Application for High Capacity Lithium Storage. Chem. - Eur. J. 2012, 18, 488− 497. (20) Su, D.; Ahn, H.-J.; Wang, G. One-Dimensional Magnetite Fe3O4 Nanowires as Electrode Material for Li-Ion Batteries with Improved Electrochemical Performance. J. Power Sources 2013, 244, 742−746. (21) Zhang, W.; Bock, D. C.; Pelliccione, C. J.; Li, Y.; Wu, L.; Zhu, Y.; Marschilok, A. C.; Takeuchi, E. S.; Takeuchi, K. J.; Wang, F. Insights into Ionic Transport and Structural Changes in Magnetite during Multiple-Electron Transfer Reactions. Adv. Energy Mater. 2016, 6, 1502471. (22) Ma, F.-X.; Hu, H.; Wu, H. B.; Xu, C.-Y.; Xu, Z.; Zhen, L.; Lou, X. W. Formation of Uniform Fe3O4 Hollow Spheres Organized by Ultrathin Nanosheets and Their Excellent Lithium Storage Properties. Adv. Mater. 2015, 27, 4097−4101. (23) Martinez-Mera, I.; Espinosa-Pesqueira, M. E.; PerezHernandez, R.; Arenas-Alatorre, J. Synthesis of Magnetite (Fe3O4) Nanoparticles Without Surfactants at Room Temperature. Mater. Lett. 2007, 61, 4447−4451. (24) Guyodo, Y.; Mostrom, A.; Lee, P. R.; Banerjee, S. K. From Nanodots to Nanorods: Oriented Aggregation and Magnetic Evolution of Nanocrystalline Goethite. Geophys. Res. Lett. 2003, 30, 1512. (25) Blanco-Andujar, C.; Ortega, D.; Pankhurst, Q. A.; Thanh, N. T. K. Elucidating the Morphological and Structural Evolution of Iron
ORCID
Luyao Li: 0000-0002-2922-0686 Esther S. Takeuchi: 0000-0001-8518-1047 Kenneth J. Takeuchi: 0000-0001-8129-444X Amy C. Marschilok: 0000-0001-9174-0474 Stanislaus S. Wong: 0000-0001-7351-0739 Author Contributions
C.L.M. and L.W. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS All of the work described in these studies was funded as part of the Center for Mesoscale Transport Properties, an Energy Frontier Research Center supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award #DE-SC0012673. Research characterization was performed in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract DE-SC0012704. E.S.T. acknowledges the generous support of William and Jane Knapp through the Knapp Chair in Energy and the Environment.
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REFERENCES
(1) Luo, J.; Liu, J.; Zeng, Z.; Ng, C. F.; Ma, L.; Zhang, H.; Lin, J.; Shen, Z.; Fan, H. J. Three-Dimensional Graphene Foam Supported Fe3O4 Lithium Battery Anodes with Long Cycle Life and High Rate Capability. Nano Lett. 2013, 13, 6136−6143. (2) Bruck, A. M.; Gannett, C. N.; Bock, D. C.; Smith, P. F.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S. The Electrochemistry of Fe3O4/Polypyrrole Composite Electrodes in Lithium-Ion Cells: The Role of Polypyrrole in Capacity Retention. J. Electrochem. Soc. 2017, 164, A6260−A6267. (3) Bruck, A. M.; Cama, C. A.; Gannett, C. N.; Marschilok, A. C.; Takeuchi, E. S.; Takeuchi, K. J. Nanocrystalline Iron Oxide Based Electroactive Materials in Lithium Ion Batteries: The Critical Role of Crystallite Size, Morphology, and Electrode Heterostructure on Battery Relevant Electrochemistry. Inorg. Chem. Front. 2016, 3, 26− 40. (4) Bock, D. C.; Kirshenbaum, K. C.; Wang, J. J.; Zhang, W.; Wang, F.; Wang, J.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S. 2D Cross Sectional Analysis and Associated Electrochemistry of Composite Electrodes Containing Dispersed Agglomerates of Nanocrystalline Magnetite, Fe3O4. ACS Appl. Mater. Interfaces 2015, 7, 13457−13466. (5) Zhang, W.; Bock, D. C.; Pelliccione, C. J.; Li, Y.; Wu, L.; Zhu, Y.; Marschilok, A. C.; Takeuchi, E. S.; Takeuchi, K. J.; Wang, F. Insights into Ionic Transport and Structural Changes in Magnetite during Multiple-Electron Transfer Reactions. Adv. Energy Mater. 2016, 6, 1502471. (6) Zhu, S.; Marschilok, A. C.; Takeuchi, E. S.; Takeuchi, K. J. Crystallite Size Control and Resulting Electrochemistry of Magnetite, Fe3O4. Electrochem. Solid-State Lett. 2009, 12, A91−A94. (7) Komaba, S.; Mikumo, T.; Yabuuchi, N.; Ogata, A.; Yoshida, H.; Yamada, Y. Electrochemical Insertion of Li and Na Ions into Nanocrystalline Fe3O4 and α-Fe2O3 for Rechargeable Batteries. J. Electrochem. Soc. 2010, 157, A60−A65. (8) Bruck, A. M.; Cama, C. A.; Gannett, C. N.; Marschilok, A. C.; Takeuchi, E. S.; Takeuchi, K. J. Nanocrystalline iron oxide based electroactive materials in lithium ion batteries: the critical role of crystallite size, morphology, and electrode heterostructure on battery relevant electrochemistry. Inorg. Chem. Front. 2016, 3, 26−40. 4811
DOI: 10.1021/acsaem.9b00456 ACS Appl. Energy Mater. 2019, 2, 4801−4812
Article
ACS Applied Energy Materials Oxide Nanoparticles Formed by Sodium Carbonate in Aqueous Medium. J. Mater. Chem. 2012, 22, 12498−12506. (26) Vauthier, C.; Cabane, B.; Labarre, D. How to Concentrate Nanoparticles and Avoid Aggregation? Eur. J. Pharm. Biopharm. 2008, 69, 466−475. (27) Uyeda, H. T.; Medintz, I. L.; Jaiswal, J. K.; Simon, S. M.; Mattoussi, H. Synthesis of Compact Multidentate Ligands to Prepare Stable Hydrophilic Quantum Dot Fluorophores. J. Am. Chem. Soc. 2005, 127, 3870−3878. (28) Kovalenko, M. V.; Scheele, M.; Talapin, D. V. Colloidal Nanocrystals with Molecular Metal Chalcogenide Surface Ligands. Science 2009, 324, 1417. (29) Wang, C. W.; Moffitt, M. G. Surface-Tunable Photoluminescence from Block Copolymer-Stabilized Cadmium Sulfide Quantum Dots. Langmuir 2004, 20, 11784−11796. (30) Chen, Y.; Rosenzweig, Z. Luminescent CdSe Quantum Dot Doped Stabilized Micelles. Nano Lett. 2002, 2, 1299−1302. (31) Mariño-Fernández, R.; Masunaga, S. H.; Fontaíña-Troitiño, N.; Morales, M. P.; Rivas, J.; Salgueirino, V. Goethite (α-FeOOH) Nanorods as Suitable Antiferromagnetic Substrates. J. Phys. Chem. C 2011, 115, 13991−13999. (32) Hosseini, F.; Sadjadi, M. S.; Farhadyar, N. Fe3O4 Nanoparticles Modified with APTES as the Carrier for (+)-(S)-2-(6-methoxynaphthalen-2-yl) Propanoic Acid (Naproxen) and (RS) 2-(3-benzoylphenyl)-propionic Acid (Ketoprofen) Drug. Orient. J. Chem. 2014, 30, 1609−1618. (33) Zhu, J.; Wei, S.; Lee, I. Y.; Park, S.; Willis, J.; Haldolaarachchige, N.; Young, D. P.; Luo, Z.; Guo, Z. Silica Stabilized Iron Particles Toward Anti-corrosion Magnetic Polyurethane Nanocomposites. RSC Adv. 2012, 2, 1136−1143. (34) Bini, R. A.; Marques, R. F. C.; Santos, F. J.; Chaker, J. A.; Jafelicci, M. Synthesis and Functionalization of Magnetite Nanoparticles with Different Amino-Functional Alkoxysilanes. J. Magn. Magn. Mater. 2012, 324, 534−539. (35) Ban, C. M.; Wu, Z. C.; Gillaspie, D. T.; Chen, L.; Yan, Y. F.; Blackburn, J. L.; Dillon, A. C. Nanostructured Fe3O4/SWNT Electrode: Binder-Free and High-Rate Li-Ion Anode. Adv. Mater. 2010, 22, E145−E149. (36) Bock, D. C.; Pelliccione, C. J.; Zhang, W.; Wang, J. J.; Knehr, K. W.; Wang, J.; Wang, F.; West, A. C.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S. Dispersion of Nanocrystalline Fe3O4 within Composite Electrodes: Insights on Battery-Related Electrochemistry. ACS Appl. Mater. Interfaces 2016, 8, 11418−11430. (37) Wang, L.; Li, Y. R.; Li, J.; Zou, S.; Stach, E. A.; Takeuchi, K. J.; Takeuchi, E. S.; Marschilok, A. C.; Wong, S. S. Correlating Preparative Approaches with Electrochemical Performance of Fe3O4-MWNT Composites Used as Anodes in Li-Ion Batteries. ECS J. Solid State Sci. Technol. 2017, 6, M3122−M3131. (38) Bock, D. C.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S. Deliberate Modification of The Solid Electrolyte Interphase (SEI) During Lithiation of Magnetite, Fe3O4: Impact on Electrochemistry. Chem. Commun. 2017, 53, 13145−13148. (39) Nguyen, C. C.; Lucht, B. L. Comparative Study of Fluoroethylene Carbonate and Vinylene Carbonate for Silicon Anodes in Lithium Ion Batteries. J. Electrochem. Soc. 2014, 161, A1933−A1938.
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