Deliberate Modification of Fe3O4 Anode Surface Chemistry: Impact on


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Deliberate Modification of Fe3O4 Anode Surface Chemistry: Impact on Electrochemistry Lei Wang, Lisa M. Housel, David C. Bock, Alyson Abraham, Mikaela Dunkin, Alison McCarthy, Qiyuan Wu, Andrew Kiss, Juergen Thieme, Esther S. Takeuchi, Amy C. Marschilok, and Kenneth J. Takeuchi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21273 • Publication Date (Web): 01 May 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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Deliberate Modification of Fe3O4 Anode Surface Chemistry: Impact on Electrochemistry Lei Wang1, Lisa M. Housel1, David C. Bock2, Alyson Abraham1, Mikaela R. Dunkin3, Alison H. McCarthy3, Qiyuan Wu2, Andrew Kiss4, Juergen Thieme4, Esther S. Takeuchi1,2,3, Amy C. Marschilok1,2,3, and Kenneth J. Takeuchi1,3* 1

Department of Chemistry, State University of New York at Stony Brook, Stony Brook, NY 11794-3400

2

Energy Sciences Directorate, Interdisciplinary Sciences Building, Building 734, Brookhaven National Laboratory, Upton, NY 11973

3

Department of Materials Science and Chemical Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2275

4

National Synchrotron Light Source II, Brookhaven National Laboratory, Building 743, Upton, NY 11973-5000

*corresponding author: [email protected] Keywords: Fe3O4, Surface Chemistry, Surface-treatment ligands, Anodes, Li-ion Batteries.

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Abstract

Fe3O4 nanoparticles (NPs) with an average size of 8-10 nm have been successfully functionalized with various surface-treatment agents to serve as model systems for probing surface chemistry-dependent electrochemistry of the resulting electrodes. The surface-treatment agents used for the functionalization of Fe3O4 anode materials were systematically varied to include aromatic or aliphatic structures: 4-mercaptobenzoic acid (MBA), benzoic acid (BA), 3mercaptopropionic acid (MPA), and propionic acid (PA). Both structural and electrochemical characterizations have been used to systematically correlate the electrode functionality with the corresponding surface chemistry. Surface treatment with ligands led to better Fe3O4 dispersion, especially with the aromatic ligands. Electrochemistry was impacted where the PA and BA-treated Fe3O4 systems without the -SH group demonstrated higher rate capability than their thiolcontaining counterparts and the pristine Fe3O4. Specifically, the PA system delivered the highest capacity and cycling stability amongst all samples tested. Notably, the aromatic BA system outperformed the aliphatic PA counterpart during extended cycling under high current density, due to the improved charge transfer and ion transport kinetics as well as better dispersion of Fe3O4 NPs, induced by the conjugated system. Our surface engineering of the Fe3O4 electrode presented herein, highlights the importance of modifying the structure and chemistry of surface-treatment agents as a plausible means of enhancing interfacial charge transfer within metal oxide composite electrodes without hampering the resulting tap density of the resulting electrode.

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1. Introduction

Lithium ion batteries (LIB) represent the battery system of choice for a wide range of applications spanning from portable electronics to electric vehicles.1-5 The development of LIBs utilizing lower cost and more earth abundant materials embodies a highly desirable objective. The ubiquitous use of the traditional graphite anode is limited by the undesired lithium deposition and dendrite formation onto the anode surface, thereby leading to poor cycling stability. As an alternative, magnetite (Fe3O4) with an inverse spinel structure represents a promising candidate for an anode material in LIB, due to its (i) significantly larger reversible capacity (i.e. 926 mAh g-1), (ii) plentiful earth abundance, and (iii) relative non-toxicity.6 Challenges associated with this material still exist. For example, iron nanocrystals tend to form and become dispersed in a Li2O matrix during discharging, which often lead to dramatic volume variation upon electrochemical cycling, thereby resulting in undesired crumbling and cracking of electrodes as well as to a loss of electrical connectivity with the current collector.7-8 At high current densities, additional degradation in electrode performance takes place, associated with sluggish charge transfer and ionic diffusion kinetics as well as from the only moderate electrical conductivity of Fe3O4.9-10 Two common strategies have been employed to circumvent the aforementioned limitations with the goal of generating Fe3O4 anodes with improved rate capability and cycling stability. One protocol embodies the size optimization of Fe3O4 to improve the Li-ion diffusion and electron transport within the nanoparticles.6, 11-12 The other main approach has been to introduce conductive additives, such as carbon nanofibers, graphene, carbon nanotubes (CNTs) and conducting polymers13, in order to not only enhance the overall electrical conductivity but 3 ACS Paragon Plus Environment

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also to accommodate the large volume change during the cycling of electrodes. Despite the improved the electron and ion conductivity induced by the incorporation of carbon additives, the volumetric capacity of the composite electrode remains low. The void spaces within the carbon materials significantly reduce the density of the particles, leading to an undesirable and rather low volumetric capacity of 330–430 mAh cm−3.14 Recently, there has been a growing interest in considering the surface properties of electrodes as critical factors for understanding and optimizing electrochemistry.15-18 Surface treatment based on inorganic coating compounds such as AlPO419 and MOx (M= Zr, Al, Ti, B)2021

has demonstrated improvements in the energy density and cycle performance of layered

LiCoO2 and spinel LiMn2O4 cathodes, due to the improvement of thermal stability, as well as suppression of electrolyte decomposition, strain formation during phase transition, and Co/Mn dissolution in the electrolyte. Although the coating of inorganic materials on the surface of metal oxides/phosphates improves the electrochemical performance and safety properties of batteries, the microstructure of the coating layers and the mechanism of improvement are not fully understood. In addition, limited by the conventional coating methods, it is still very hard to obtain a conformal and multifunctional inorganic coating layer without jeopardizing the electron and ion conductivity within the electrodes. In addition to the inorganic surface coatings, there have been extensive studies on dispersion of metal oxides within electrodes using organic compounds as surface capping agents, or surfactants.22 For instance, a previous report noted the aggregate size-controlled dispersion of Fe3O4 within conductive carbon matrix using an organic oleic acid capping agent on the surface.23 The collective spectroscopic and electrochemical characterizations suggested that the oleic acid capping layer significantly reduced aggregation of Fe3O4 within the conductive carbon 4 ACS Paragon Plus Environment

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black matrix. However, the long-chain, insulating capping layer impeded the kinetics of electron transfer and ion diffusion within the composite electrode, and had to be removed through a heat treatment in order to circumvent the detrimental effects of the insulating ligands on the resulting composite electrode. In separate yet relevant studies on inducing improvements in ionic and electronic transport through the strategic control of the ‘connection’ between the constituent metal oxide motifs such as Li4Ti5O1224 and Fe3O425 and the adjoining CNTs, organic ligands with smaller size were used as bridging agents, which were initially coated on the surface of the metal oxides, and subsequently attached to the CNTs through a judicious selection of physio-chemical attachment strategies. Specifically, when using an amine-functionalized linker, 3-aminopropyl triethoxysilane (APTES), the metal oxide and MWNTs were adjoined through a covalent attachment protocol. In addition, a non-covalent π-π interaction strategy connected the CNTs and the metal oxides through the mediation of π-bonds between the aromatic, 4-mercaptobenzoic acid (4-MBA) linker and the underlying CNTs. Electrode tests have suggested that the composites generated by π-π interactions using 4-MBA linker outperformed the analogous heterostructures utilizing APTES linker, due to smaller charge transfer resistance and faster effective Li-ion diffusion.24 The differences clearly highlighted the significant effect of the physio-chemical structure of the different surface-treatment agents upon the resulting electrochemical properties.26 It is worth noting that although the two previous studies unambiguously suggested the importance of the surface ligands in governing the resulting electrochemistry, the incorporation of additional CNTs was likely to decrease the tap density and volumetric capacity of the final electrode. In addition, the electrochemical behavior observed previously was indeed a synergistic effect of both the bridging ligands and the conductive CNTs. 5 ACS Paragon Plus Environment

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Hence, a more in-depth study is needed to deconvolute the combination effect and delineate merely the impact of surface chemistry governed by the ligands on the resulting electrochemical behaviors of the functionalized metal oxide electrodes. Herein, the focus of our work is to generate surface-treated Fe3O4 electrode materials without the additional incorporation of CNTs, with the goal of i) improving the Fe3O4 dispersion within the electrode without hampering the kinetics of charge transfer and ion diffusion, ii) maintaining the tap density and volumetric capacity of the resulting electrode, and iii) delineating the impact of the surface chemistry, governed by ligands, on the resulting electrochemical behavior of the electrodes. We aim at gaining fundamental insights into electrochemical behavior across nanoscale interfaces within the surface-treated Fe3O4 electrodes as a function of the chemical nature of four mediating ligand molecules, i.e. MBA, BA, MPA, and PA. The ligand molecules we have deliberately chosen here are relatively small organic compounds, with terminal -COOH group, possessing a strong affinity for Fe sites or the -OH groups on the Fe3O4 surface. These ligands are either aliphatic or aromatic compounds with or without the pendant SH functional group. In so doing, our work should shed light upon the impact of the surface chemistry, i.e. the presence of -conjugated/alkyl carbon system as well as the -SH/-COOH pendant group within the surface-treatment agents, on the resulting electrochemical behaviors of the surface-treated Fe3O4 electrodes. 2. Experimental

Synthesis of Fe3O4 nanoparticles. Fe3O4 nanoparticles (NPs) measuring 8-10 nm in diameter (Figure 1A-B) were synthesized using a co-precipitation method similar to that described in a previous report.27 Briefly, a solution of iron (II) chloride tetrahydrate 6 ACS Paragon Plus Environment

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(FeCl2.4H2O) and iron (III) chloride hexahydrate (FeCl3.6H2O) were added to a solution containing triethylamine [N(CH2CH3)3] as the capping agent under a nitrogen atmosphere. The isolated powder samples were extensively washed with DI water and acetone and further dried prior to subsequent characterization. Synthesis of surface-treated Fe3O4 nanoparticles. As-synthesized Fe3O4 NPs were initially dispersed in a deoxygenated ethanolic solution of the four linker molecules, (i.e. MBA, BA, MPA and PA). The mixture is subsequently stirred at 40°C for 18 h to facilitate bidentate coordination bonds between the terminal carboxylic acid groups within the four ligands and the corresponding Fe sites localized on the Fe3O4 NP surface. The surface-treated Fe3O4 were subsequently collected through centrifugation and further washed with ethanol in order to remove the unbound, freestanding ligands. The resulting products were dried under vacuum. Structural characterizations. X-ray Diffraction (XRD). Fe3O4 samples were characterized using a Rigaku SmartLab X-ray powder diffractometer. Cu Kα radiation was utilized with a BraggBrentano focusing geometry. The full width at half maximum (FWHM) of the (311) peak was determined using the Peak Fit software. Crystallite sizes were determined using the Scherrer equation28 after correcting for instrumental broadening using a lanthanum hexaboride (LaB6) standard. Electron microscopy. As-synthesized Fe3O4 NPs were dispersed in ethanol and sonicated to ensure a uniform dispersion. One drop of the solution was evaporated onto a 300 mesh Cu grid, which was coated with a lacey carbon film. Transmission electron microscopy (TEM) characterization was performed on a JEOL JEM-1400 instrument, equipped with a fieldemission electron gun operating at 120 kV. High resolution TEM characterization results of the pristine and ligand-treated Fe3O4 NPs, were acquired with a JEOL JEM 2100F instrument, equipped with a field-emission electron gun operating at 200 kV and a high-resolution pole-piece 7 ACS Paragon Plus Environment

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possessing a 0.19 nm point-to-point resolution. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) mapping were performed using a JEOL 6010 Plus instrument equipped with a tungsten filament electron gun. Thermo-gravimetric Analysis (TGA). Data were acquired using a TGA Q500 instrument over a temperature range spanning from 30 to 800°C under an air atmosphere, using a set heating rate of 10°C/min. It is previously confirmed that Fe3O4 with a diameter smaller than 300 nm oxidizes to γ-Fe2O3 around 220°C, and further transforms to thermodynamically stable α-Fe2O3 above 250°C.29 To deconvolute the weight increase associated with Fe3O4 air oxidation and the weight loss due to the burning off of surface-treatment agents between 200°C to 450°C, the weight percentage the of surfacetreatment agents was estimated by calculating the initial Fe3O4 weight based on the final Fe2O3 weight at 450°C, then subtracting the surface water weight loss together with the Fe3O4 weight from the initial total weight. Fourier transform infrared spectroscopy (FT-IR). Relevant data were obtained on a Thermo Scientific™ Nicolet™ iS50 spectrometer equipped with a diamond ATR accessory. Specifically, solid samples were placed onto a diamond crystal. Measurements were obtained in reflectance mode. Particle size analysis. The aggregate size and associated size distribution of the various Fe3O4 samples dispersed in the N-methyl-2-pyrrolidone (NMP) solvent were measured in a fraction cell using a Horiba LA-950S2 laser diffraction particle size analyzer. Contact angle measurements. Contact angle measurements were performed using a Kyowa Dropmaster Model DM-501 using the Sessile Drop method on pressed composite electrodes of 70% Fe3O4 (pristine or treated)/20% SuperP/10% PVdF using 1 M LiPF6 in EC/DMC (3:7). The following Fe3O4 treatments were measured: MBA, BA, MPA, PA and pristine. FAMAS software was used for contact angle analysis using a half angle method. Values were averaged for nine measurements per condition and reported with one standard deviation. All contact angle measurements were performed in a dry room at 2% relative humidity to minimize water uptake during measurement. AC-SECM 8 ACS Paragon Plus Environment

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measurements. Alternative current scanning electrochemical microscopy (AC-SECM) was performed using a commercial SECM instrument (M470, BioLogic). An ultramicroelectrode (UME) with radius a = 10 µm was used as the working electrode. An Ag/AgCl standard electrode and a Pt sheet were used as reference electrode and counter electrode respectively. Tap water with conductivity of ~200 µS was used as electrolyte. The following conditions were used for each sample type: 0V (vs. open circuit potential) DC-bias, 100 mV AC-bias, and 100kHz AC-frequency. The reported AC-impedance magnitude for each sample is the average value of the scanned area (300 µm x 300 µm with 5 µm scan step, totaling 3600 measurements for each sample).

Electrochemical methods. Electrodes were assembled using 70% pristine or surfacetreated Fe3O4, 20% ketjen black carbon and 10% polyvinylidene fluoride (PVDF) binder. Coin cells were assembled with the Fe3O4 electrode, Li metal anode, and 1 M LiPF6 in EC/DMC (3:7) as electrolyte for the voltammetry studies. Voltammetry studies was conducted between 0.05 and 3.0 V at a scan rate of 0.1 mV s-1. Cycling tests were conducted on coin type cells using a Maccor Battery Tester at 30°C. All galvanostatic charge and discharge experiments were performed between 3 V and 0.05 V at various current densities. Electrochemical impedance spectroscopy (EIS) data were collected over a frequency range of 1 MHz to 10 mHz with a 10mV amplitude. Synchrotron-based X-ray microfluorescence (µ-XRF) mapping and X-ray absorption spectroscopy (XAS) measurements. µ-XRF mapping images of various Fe3O4 electrodes recovered from coin cells after 20 cycles (dis)charge at 400 mA/g were acquired using Beamline 5-ID in the National Synchrotron Light Source II (NSLS II) at Brookhaven National Laboratory. Upon completion of cycling, coin cells were dissembled inside the glovebox and electrodes were rinsed with dimethyl carbonate, dried and sealed between two layers of 50 µm

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thick polyimide film to prevent environmental contamination from air and water. A focused spot of approximately 2 µm x 2 µm with energy tunable via a double crystal monochromator was achieved by using Kirkpatrick-Baez (KB) mirrors. Samples were oriented approximately 45° to the incident beam and were rastered in the beam path using an XY-stage. X-ray fluorescence was detected using a 3-element Vortex ME3 silicon drift detector. The monochromator was calibrated using Fe metal foil and μ-XRF maps were collected at 7200 eV to excite the Fe Kedge (7112 eV). High resolution maps were collected from a 200 µm x 200 µm sample area using a step size of 2 µm and a 0.1s acquisition time. μ-XRF maps were generated by fitting the fluorescence spectra using PyXRF Python-Based X-ray Fluorescence Analysis Package.30 XAS measurements were conducted at specified points selected from the μ-XRF maps with a 2 µm x 2 µm spatial resolution using fluorescence geometry at the Fe edge. Specifically, in the pre-edge region (-100 to -15 eV below the edge), the incident beam energy was scanned using 10 eV steps and in the post edge region (30 eV to 350 eV above the edge), the step size was gradually increased from 1.0 eV to 5.0 eV. Across the edge, a 0.5 eV step size was used for enhanced resolution. A 0.1s acquisition time was used at each data point. Fe metal foil was utilized for initial energy calibration. XAS data were aligned and normalized using Athena software.31 3. Results and discussion

3.1 Structural insights in ligand-functionalization of Fe3O4 NPs As-prepared Fe3O4 NPs were measured 8.8 ± 1.3 nm on average in diameter from TEM (Figure 1A). The corresponding XRD pattern (Figure 1B) was subsequently acquired, which 10 ACS Paragon Plus Environment

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agreed well with a cubic inverse spinel Fe3O4 structure (JCPDS 19-0629), with a crystallite size measuring 8.5 nm according to the Scherrer equation.

Figure 1. TEM image (A) and XRD pattern (B) of the as-prepared Fe3O4 NPs, as well as the structures, names and abbreviations of the four ligands of interest (C). The presence of the various terminated moieties, i.e. MBA, BA, MPA and PA (Figure 1C), on the Fe3O4 surfaces expected after ligand functionalization, was confirmed by FT-IR analysis, as displayed in Figure 2. The corresponding peak assignments are summarized in Table 1. In the four surface-treated Fe3O4 systems, the peak associated with C=O stretch (1680-1700 cm-1) from free carboxylic acid group disappeared or decreased in intensity, which was replaced

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with two more distinctive COO- stretching modes, located at 1517/1400 cm-1, 1516/1403 cm-1, 1531/1433 cm-1, and 1533/1426 cm-1, for the MBA, BA, MPA, and PA-treated Fe3O4, respectively, corresponding to the asymmetric and symmetric COO- stretching modes of the attached carboxylate ligands.32 The separation between the two stretching modes [Δν=ν (COO−)as− ν (COO−)s] can assist in the assignment of the binding mode for carboxylic acidbased ligands to metals and metal oxide surface.32-34 The ΔνMBA, ΔνBA, ΔνMPA, and ΔνPA were calculated to be 117, 113, 98, and 97 cm-1, indicating binding through bidentate chelating or bridging between COO- group and Fe sites on the Fe3O4 surface.32, 35 With respect to the thiolcontaining ligands, the absence of S–H stretching vibration at 2600–2500 cm−1 in the IR spectra of MBA and MPA-treated Fe3O4 (Figure 2c and e) suggested that the S–H bond in MBA and MPA ligands was covalently bound to Fe on the Fe3O4 surface, through the formation of Fe-S bond.36-37 Hence, our overall FT-IR results suggested that in the BA and PA systems, the ligand functionalization of the Fe3O4 surface was achieved only by the bidentate interaction between COO- and Fe, while in the thiol-containing MBA and MPA systems, a second interaction between Fe and S also existed.

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Figure 2. FT-IR spectra of the as-prepared pristine Fe3O4 (black), BA-Fe3O4 (blue), MBA-Fe3O4 (red), PA-Fe3O4 (green), and MPA-Fe3O4 (pink). The labels for a – u correspond to positions and assignments of the peaks labeled in FT-IR spectra in Table 1.

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Table 1. Positions and assignments of the peaks labeled in FT-IR spectra. Samples

Wavenumbers (cm-1)

Peak assignments

MBA-Fe3O4

1681 (a)

C=O stretching

1590,1489 (b and c)

C=C stretching

1517 and 1400 (d and e)

COO- asymmetric and symmetric stretching

1324 (f)

C-O stretching

1115 (g)

C-S stretching

1597, 1493 (h and i)

C=C stretching

1516 and 1403 (j and k)

COO- asymmetric and symmetric stretching

1304 (l)

C-O stretching

2977 (m)

C-H stretching

1531 and 1433 (n and o)

COO- asymmetric and symmetric stretching

1267 (p)

C-O stretching

1090 (q)

C-S stretching

2975, 2924 (r and s)

C-H stretching

1523 and 1426 (t and u)

COO- asymmetric and symmetric stretching

BA-Fe3O4

MPA-Fe3O4

PA-Fe3O4

Thermogravimetric analysis (TGA) was subsequently conducted in an air atmosphere to further confirm the successful surface functionalization of the Fe3O4 NPs, as well as to gauge the loading ratios of ligand molecules on the surface (Figure S1). The observed weight losses before 200°C were associated with the surface absorbed water. Subsequent weight losses between 200°C to 450°C can be ascribed to the burning off and removal of the linker molecules. 14 ACS Paragon Plus Environment

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Additional weight losses after 550°C were noticed in the MBA and MPA system, which might be the decomposition associated with the surface Fe-S bond. The weight percentages of surfacebound ligands were calculated to be 8.5%, 7.3%, 7.4%, and 6.8% for the MBA, BA, MPA and PA systems using the method described previously, corresponding to a ligand layer thickness of 5.7Å, 5.0Å, 5.2 Å, and 5.8Å, respectively, indicating a similar ligand coverage density on the Fe3O4 surface. The thickness of the ligand layer was estimated based on a uniform coating of the organic ligand on the surface of a 10 nm Fe3O4 nanosphere (Figure S2). The detailed calculation steps were demonstrated in the supporting information. To provide direct visualization of various ligands on the surface of Fe3O4 NPs, HRTEM images were acquired, which unambiguously highlighted the layers of surface ligands present in the MBA, BA, MPA, and PA systems, as indicated by the arrows in Figure 3.

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Figure 3. HRTEM images of (A) as-prepared pristine Fe3O4, (B) MBA-Fe3O4, (C) BA-Fe3O4, (D) MPA-Fe3O4, and (E) PA-Fe3O4. The surface ligands are highlighted in red. 16 ACS Paragon Plus Environment

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To assess the effect of surface-treatment agents on the dispersion of the resulting functionalized Fe3O4, particle size analysis was performed in NMP, which was the solvent used to generate the slurry for the Fe3O4 electrodes. The sizes of Fe3O4 agglomerates were summarized in Table 2 and the corresponding distribution of particle sizes was displayed in Figure S3. Our results clearly suggested that the surface treatment through ligands gave rise to i) a narrower size distribution of the Fe3O4 aggregates and ii) a decreased percentage of large size aggregates, especially with the two aromatic ligands, i.e. MBA and BA, possibly due to the higher polarity of these ligands compared to their aliphatic counterparts, which leads to higher solubility in the polar NMP solvent. The MPA ligand seemed to be the least effective amongst the four ligands, in terms of dispersing the Fe3O4 NPs in NMP. The narrower size distribution of the Fe3O4 aggregates might be attributed to the surface ligands forming stabilizing layers to lower surface energies and thus avoiding particle-particle aggregation. To further investigate the distribution of Fe3O4 aggregates within the as-prepared electrode coatings, SEM images and EDS elemental maps were acquired (Figure 4). The aggregate sizes were measured to be 4.2 ±1.9, 2.8 ±1.4, 2.6 ± 0.9, 3.8 ±1.8, and 3.0 ± 1.1 μm for the as-prepared pristine Fe3O4, MBAFe3O4, BA-Fe3O4, MPA-Fe3O4, and PA-Fe3O4, respectively, based upon the measurements of iron elemental mapping results, highlighting the similar general trend observed in the previous particle size analysis. Contact angle measurements were performed to provide additional information regarding the surface energetics in the coating electrodes containing pristine and surface-treated Fe3O4 particles as used in the electrochemical tests. In a sample size of nine measurements per condition, the following average contact angles were obtained on Fe3O4 treated with MBA, BA, MPA, PA,

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and untreated using the electrolyte containing 1 M LiPF6 in EC/DMC (3:7): 29.5(4.4), 25.6(1.1), 19.2(1.4), 27.1(2.0), and 29.7 (1.9). Representative images of contact angle measurements on treated and untreated Fe3O4 coatings are shown in Figure S4. The untreated Fe3O4 sample is within the range of contact angle measurements reported in the literature for deposited thin films of magnetite,38 noting that the incorporation of conductive additive and binder did not alter the contact angle outside of the range expected for the pristine material. The higher contact angle (29.7) associated with the pristine Fe3O4 suggests a lesser degree of wetting when compared to all the ligand-treated Fe3O4 electrodes. The most notable difference was observed between the MPA and untreated electrodes, with a difference of 10.5, as shown in Figure 5. The higher contact angle (29.7) associated with the untreated Fe3O4 suggests a lesser degree of wetting when compared to the MPA, which is aliphatic and contains both -SH and -OH end groups. There was not a notable distinction between the contact angle differences between aromatic compounds (MBA and BA) compared to aliphatic compounds (MPA and PA), or when comparing thiol containing groups, as the two thiol containing functionalities (MBA and MPA), had differences in contact angle (29.5 and 19.2 respectively), with the MBA treated sample being closer to the pristine. To complement the contact angle measurement, the average surface AC-impedance magnitude of different samples was measured and is also summarized in Figure 5. A typical AC-impedance magnitude map was shown in Figure S5. It can be seen clearly that the change in surface AC-impedance magnitude was fairly minor throughout the scanned area indicating homogeneous distribution of the electrode composite. The average surface AC-impedance magnitudes for MBA, BA, MPA, PA and pristine samples were 17.28 ± 0.10, 16.64 ± 0.09, 15.98 ± 0.10, 17.29 ± 0.10, and 17.62 ± 0.57 kΩ, respectively. Interestingly, the surface impedance magnitude shared the same general trend in the contact angle of different samples. The MPA sample has the smallest surface AC-impedance magnitude, followed by the BA sample. MBA and PA have surface AC-impedance magnitude closest to that of the pristine Fe3O4. 18 ACS Paragon Plus Environment

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This general trend also agreed with the trend in R1 in the EIS validating the fitting results of EIS data (Table 3).

Table 2. Aggregate sizes of the pristine and surface-treated Fe3O4 in NMP solvent. D(v, 0.1) (μm)

D(v, 0.5) (μm)

D(v, 0.9) (μm)

pristine-Fe3O4

0.72

4.01

13.96

MBA-Fe3O4

0.85

1.95

4.38

BA-Fe3O4

0.80

1.77

4.60

MPA-Fe3O4

1.21

4.45

9.16

PA-Fe3O4

0.88

2.50

5.37

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Figure 4. SEM images (black and white) and EDS based carbon maps (red), iron maps (green), as well as merged carbon + iron maps (red and green) of coating electrodes containing active materials consisting of as-prepared pristine Fe3O4 (A1-4), MBA-Fe3O4 (B1-4), BA-Fe3O4 (C1-4), MPA-Fe3O4 (D1-4), and PA-Fe3O4 (E1-4), respectively.

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Figure 5. Measured average contact angle (orange) and SECM AC-impedance magnitude (blue) of MBA, BA, MPA, PA, and Pristine Fe3O4 coatings. Error bars are ± one standard deviation.

3.2. Electrochemical behaviors of surface-functionalized Fe3O4 Cyclic voltammetry. In order to probe the implications of different surface chemistry governed by surface-treatment agents on the Fe3O4 surface, cyclic voltammetry (CV) data of the pristine and surface-treated Fe3O4 samples were collected and analyzed focusing on Li+ insertion/extraction as well as of Fe3O4 conversion (Figure 6). In the initial cycle, all five samples gave rise to three distinct reduction peaks located at i) ~1.55 V, corresponding to the initial insertion of the first Li+ ion into the interstitial octahedral sites of the cubic-close-packed Fe3O4 structure and reduction of one electron equivalent of Fe3+ to Fe2+, ii) ~1.00 V, associated with the second electron equivalent reduction and migration of Fe ions from tetrahedral to octahedral sites and transition from inverse-spinel Fe3O4 to a FeO-like rock-salt structure, as well as iii) ~0.65 V, attributed to the conversion from FeO to Fe and the formation of a solid electrolyte interphase (SEI) and Li2O.39 In the anodic process, two overlapping peaks centered at 21 ACS Paragon Plus Environment

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approximately 1.6 and 1.8 V were observed, which can be ascribed to the conversion of Fe0 to a FeO-like structure and delithiation of carbon black.23 In the subsequent cycles, both reduction and oxidation peaks were shifted to more positive voltages, thereby indicating an increased polarization of the electrode materials in the initial cycles.40 Only a single reduction peak at ~0.8V evinced, associated with the FeO to Fe0 metal conversion process. It is worth noting that although the pristine Fe3O4 sample (Figure 6A) demonstrated a sharp reduction peak in the initial cycle, the current showed a drastic and consecutive fading in the subsequent four cycles. By contrast, the BA and PA-treated Fe3O4 samples (Figure 6C and E) demonstrated less fading, with the BA system exhibiting minimal fading from cycle 2 to 5, and the PA sample delivering the highest current (1474 mA/g) in cycle 5. In the second and subsequent cycles, the MPA-treated Fe3O4 (Figure 6D) demonstrated very different CV profiles as compared with all the other counterparts. Specifically, in the anodic peak region, there were minimal if any distinct features corresponding to the Fe oxidation process, suggesting a detrimental effect on the electrochemical function of the electrode. The better capacity retention noted in the BA and PA systems suggested that the thiol functional group within the MBA and MPA systems might have detrimental impact on the resulting electrode behaviors of our Fe3O4 system, possibly due to the strong electron affinity of -SH group, which withdraws electron from Fe3O4, thus rendering Febased redox reactions less reversible.

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Figure 6. CV results of various Fe3O4 electrodes for cycles 1–5 at a scan rate of 0.1 mV/s using active materials of: (A) as-prepared pristine Fe3O4, (B) MBA-Fe3O4, (C) BA-Fe3O4, (D) MPAFe3O4, and (E) PA-Fe3O4. Rate capability test. Rate capability measurements were conducted on the pristine and surface-treated electrodes at a series of current densities (Figure 7A and B). A total of 100 cycles using sequential (dis)charge current densities of 50, 100, 200, 400, 50, 600, 800, 1000, 1600 and 600 mA/g were tested for 10 cycles under each current. With respect to the discharge capacities (Figure 7A), the PA-Fe3O4 sample unambiguously demonstrated the most outstanding rate capability at all rates amongst the five Fe3O4 samples, delivering discharge capacities of 1045, 981, 909, 797, 568, 489, 430, and 306 mAh/g, at 50, 100, 200, 400, 600, 800, 1000, and 1600 mA/g, in cycle 2, 11, 21, 31, 51, 61, 71, and 81, respectively. After the extended cycling at various current densities, the PA-Fe3O4 electrode maintained 987 mA/g capacity in cycle 50, denoting a capacity retention of 94.5% from cycle 2 to 50. In comparison, the as-prepared

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pristine Fe3O4 delivered 943 mAh/g and 729 mAh/g in cycle 2 and 50 (77.3% retention), tested under 50 mA/g. Even at a relatively high current density of 600 mA/g, the PA-Fe3O4 electrode still delivered a capacity of 486 mAh/g in cycle 100, corresponding to 85.5% retention rate from cycle 51 to 100. The aromatic carboxylate ligand, i.e. BA-treated Fe3O4 delivered similar capacities as those of the pristine Fe3O4 system at low current density of 50 mA/g and 100 mA/g in the initial 20 cycles. From cycle 21 and above, the BA system constantly delivered higher capacities than those from the pristine Fe3O4 under the more vigorous testing conditions at higher current densities. By contrast, the thiol-containing counterparts only delivered higher capacities during the initial 10 cycles at a low current density of 50 mA/g. However, a drastic capacity fading was noticed for both the MBA and MPA systems from cycle 11, with the MPA-Fe3O4 delivered little if any capacity after cycle 50. The corresponding charge capacity of the various electrodes were displayed in Figure 7B, which suggested a similar overall trend as that of the discharge capacity presented in Figure 7A, indicating that the PA-Fe3O4 electrode delivered the highest charge capacities amongst all the electrodes at all current densities tested from cycle 11 to 100. The initial coulombic efficiencies were 59.0%, 67.4%, 59.3%, 60.3%, and 58.7% for the as-prepared pristine Fe3O4, MBA-Fe3O4, BA-Fe3O4, MPA-Fe3O4, and PA-Fe3O4, respectively, which were calculated based on the average results from two cells of each electrode type, that were cycled at 50 mA/g for 10 cycles (Figure S6). The irreversible capacity loss can be attributed to the solid electrolyte interphase (SEI) layer formation during the first charging–discharging process. It is interesting to note that the thiol-containing ligands, i.e. MBA and MPA, demonstrated higher initial coulombic efficiencies than their non-thiol counterparts as well as the pristine Fe3O4, possibly due to the unbound carboxylate group in both ligands, which interacted with the Li2CO3-based SEI formed

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by decomposition of the carbonate electrolyte, through the -COOLi and/or -OLi bonds on the surface of Fe3O4. This chemically bonded SEI may diminish further access of solvent molecules to the surface layer of the underlying Fe3O4, and hence reduce the amount of irreversible capacity losses during the first reduction process.41 By contrast, in the BA and PA systems, all the carboxylate groups were bound to the Fe sites on the surface. Hence, no beneficial interfacial interaction between the freestanding carboxylate group and carbonate-based SEI existed, resulting in a lower initial coulombic efficiency. The coulombic efficiency increased to around 91% in the second discharge-charge cycle, and subsequently stabilized at ~93% at cycle 10 and beyond for all the cells.

Figure 7. Specific discharge (A) and charge (B) capacity versus cycle number for electrodes using as-prepared pristine Fe3O4 (black), MBA-Fe3O4 (red), BA-Fe3O4 (blue), MPA-Fe3O4 (pink), and PA-Fe3O4 (olive). Voltage profiles. Figure 8 depicts the representative discharge-charge voltage profiles of the pristine and surface-treated Fe3O4 electrodes acquired at current densities of 50 mA/g, 200 mA/g, 400 mA/g, 800 mA/g, and 1600 mA/g in cycle 1, 21, 31, 61, 81, within a cutoff voltage 25 ACS Paragon Plus Environment

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window of 0.05–3 V. The galvanostatic initial discharge process was characterized by three discharge plateaus at 1.5V, 1.1V, and 0.8 V, which was consistent with the three reduction peaks evinced in the previous CV measurements (Figure 6). The initial voltage decreased to the first plateau at 1.5V was abrupt in the pristine Fe3O4 (Figure 8A), while in the ligand-functionalized Fe3O4 systems (Figure 8B-E), a smoother voltage decrease was observed. As the (dis)charge rate increased from 50 mA/g to 1600 mA/g, the voltage plateau for all electrodes correspondingly decreased, with the PA-Fe3O4 system demonstrating the least polarization. Specifically, the PA system showed the most distinct discharging plateaus at 0.86V, 0.94V, 0.91V, and 0.81V at a sequential current density of 50 mA/g, 200 mA/g, 400 mA/g, and 800 mA/g, respectively, which were higher than those from the other four counterparts. Notably, even at 1600 mA/g, use of the PA-Fe3O4 electrode led to a discharge curve with a capacity of 306 mAh/g and a voltage plateau at 0.56 V.

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Figure 8. Voltage profiles of (A) as-prepared pristine Fe3O4, (B) MBA-Fe3O4, (C) BA-Fe3O4, (D) MPA-Fe3O4, and (E) PA-Fe3O4, at 50, 200, 400, 800, and 1600 mA/g, respectively. Cycling stability test and electrochemical impedance spectra (EIS). To further access the cycling stability of the various Fe3O4 electrodes, cells were subsequently cycled for additional 100 cycles at a current density of 100 mA/g (Figure 9A). At cycle 200, the as-prepared pristine Fe3O4, MBA-Fe3O4, BA-Fe3O4, and PA-Fe3O4 electrodes delivered a capacity of 442, 300, 497, and 568 mAh/g, corresponding to a capacity retention of 69%, 60%, 70%, and 68% from cycle 101 to 200, as well as 51%, 30%, 56%, and 58%, from cycle 11 to 200. The MPA system delivered no capacity from cycle 101 to 144. Another set of cells were cycled under a higher current density of 400 mA/g (Figure 9B), in order to further assess the high rate capability of the various cells. The capacity retention rates were 44%, 38%, 52%, 17%, and 35% for the pristine, MBA, BA, MPA, and PA electrodes, from cycle 2 to 100, indicating a more drastic capacity decrease for all the cells in comparison with the results obtained under 100 mA/g. In fact, in cycle 100, the remaining capacities were 450, 328, 509, 179, and 379 mAh/g, for the pristine, MBA, BA, MPA, and PA electrodes. It is worth noting that the aromatic BA system demonstrated both higher capacities and better cycling stability than the other four counterparts, while the aliphatic PA system demonstrated a more significant capacity decrease, under the more vigorous cycling conditions. Furthermore, the thiol-containing systems (MBA and MPA) yielded much lower coulombic efficiency during the extended cycling, when compared with systems without the -SH pendant group (BA and PA). To further investigate the reaction kinetics and provide a better understanding of the capacity decrease in the PA system and improved electrochemistry of the BA system under the relatively high (dis)charge current density of 400 mA/g, EIS data were collected before and after 27 ACS Paragon Plus Environment

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100 galvanostatic cycles, as displayed in Figure 9C and D. The impedance spectra before and after cycling were fit to the equivalent circuit models displayed in Figure S7A and B, respectively. In both models, R1 is 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 (Equation 1), as displayed in Figure S8 and S9. The effective lithium-ion diffusion coefficient (𝐷𝐿𝑖 + ) was determined from Equation 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 3. Zre   ω -1/2

(Eq. 1)

DLi+ = (R2T2) / ( 2A2n4F4C2 σ2 )

(Eq. 2)

Before cycling (Figure 9C), similar charge transfer resistances were noted for all five cells, measuring 9.7 Ω, 9.8 Ω, 26.5 Ω, 13.6 Ω, and 12.1 Ω. With respect to lithium-ion diffusion efficiency, the aliphatic ligands (MPA and PA) exhibited higher values of effective DLi+ of 2.4 × 10-10 cm2/s and 2.1 × 10-10 cm2/s than those of their aromatic counterparts, indicating a favorable ion transport behavior initially. After cycling for 100 cycles under the current density of 400 mA/g, the charge transfer resistance of all cells increased, especially for the MPA-Fe3O4 system, denoting the largest Rct of 758.9 Ω, which was accompanied by the lowest value of effective DLi+ of 1.9 × 10-15 cm2/s, collectively leading to a detrimental impact on the resulting Fe3O4 electrode. Similar to MPA, the aliphatic PA system also evinced a drastic increase in charge transfer 28 ACS Paragon Plus Environment

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resistance to 112.6 Ω, which resulted in the fading of this system after cycling at 400 mA/g (Figure 9B). Overall, the fitted EIS data suggested that after the extended cycling under relatively fast (dis)charge rate, the aromatic ligands showed i) less charge transfer resistance (Rct) as compared with their alkyl analogues, i.e. Rct-MBA < < Rct-MPA, Rct-BA < Rct-PA, and ii) more efficient Li-ion diffusion kinetics, i.e. DLi-MBA >> DLi-MPA, DLi-BA > DLi-PA, possibly due to the beneficial conjugated π-π system, facilitating efficient charge transfer kinetics.42 In addition, ligands without –SH pendant group exhibited less charge transfer resistance (Rct) as compared with their –SH containing counterparts, i.e. Rct-BA < Rct-MBA, Rct-PA DLi-MBA, DLi-PA >> DLi-MPA. The detrimental effect of thiol group might be attributed to the i) strong electron affinity of –SH, calculated to be 2.23,42 which can potentially withdraw more charge from Fe3O4, leading to sluggish charge transfer behavior of the electrode materials, as well as ii) weaker binding strength of the lone pair electrons in sulfur atoms to Li+, which resulted in sluggish ion transport. Compared to the pristine Fe3O4, surface functionalization through aromatic carboxylate ligands in absence of -SH group, can lead to improved interfacial interaction and by extent, beneficial charge transfer and ion transport kinetics. The thin surface coating of ligands can serve as an artificial SEI layer, where carboxylate groups can react with Li+ from the electrolyte, forming surface Li carboxylic salt (– COOLi), which can be chemically bonded to the Li2CO3-based SEI formed by decomposition of EC and DMC-based electrolytes. The chemically bonded SEI (CB-SEI) can in turn reduce the irreversible capacity by diminishing access of solvent molecules and preventing electrochemical decomposition of the electrolyte.43 In addition, the CB-SEI enabled from the surface-treatment layer can accelerate the ion-transfer rate by coordinating with Li+ using the lone pairs from the oxygen functional group, which can provide more transport pathways than those in the

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traditional thick SEI.44 During extended cycling, the DLi was larger for the MBA, BA, and PA systems compared with that of the pristine Fe3O4, suggesting the better ion transport kinetics in these surface-treated systems.

Figure 9. Discharge capacity (solid sphere) and the corresponding coulombic efficiency (hollow sphere) of the pristine and surface-treated Fe3O4 electrodes after extended cycling under current densities of 100 mA/g (A) and 400 mA/g (B). EIS Nyquist plots of pristine and surface-treated Fe3O4 cells before (C) and after (D)100 galvanostatic cycles under 400 mA/g.

Table 3. EIS circuit parameters of pristine and surface-treated Fe3O4 samples, both before and after 100 cycles under a current density of 400 mA/g. 30 ACS Paragon Plus Environment

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Samples Pristine-Fe3O4 MBA-Fe3O4 BA-Fe3O4 MPA-Fe3O4 PA-Fe3O4

Condition before after before after before after before after before after

R1 (Ω) 1.6 2.3 1.7 2.1 1.7 2.3 1.3 1.6 1.8 1.9

Rct (R2+R3) (Ω) 9.7 109.7 9.8 82.5 26.5 57.5 13.6 758.9 12.1 112.6

DLi+ (cm2/s) 7.9 × 10-11 4.3 × 10-13 9.3 × 10-11 6.2 × 10-13 7.5 × 10-11 9.2× 10-13 2.4 × 10-10 1.9 × 10-15 2.1 × 10-10 5.3 × 10-13

Structural evolution of post-cycling electrode– XRF mapping and sub-micron resolution X-ray absorption near-edge structure (XANES) spectra. To further unravel the significant capacity fading noted in the MPA, MBA, and PA electrodes cycled at 400 mA/g, as well as to provide a better understanding of structural evolution after cycling at high current density, the various Fe3O4 electrodes were cycled at 400 mA/g for 20 cycles and subsequently investigated by SRX measurements at NSLS-II. The corresponding discharge and charge capacities of these cells were provided in Figure S10. Similar to our observation in Figure 9B, the BA-Fe3O4 electrode demonstrated the best cycling stability and highest capacities amongst all systems tested, which was followed by the PA system, delivering the second highest capacities, but accompanied by a drastic fading after cycle 10. Upon cycling at 400 mA/g for 20 cycles, a 200 x 200 µm Fe K-edge elemental map of the discharged electrodes was collected for each Fe3O4 system, which were displayed in Figure 10. Regions of higher (color yellow) and lower (color blue) fluorescence intensity within the acquired areas were noted in each sample, with the high intensity areas corresponding to more densely packed Fe3O4 agglomerates. A notably better dispersion of Fe3O4 NPs was observed in the BA system, which demonstrated an average aggregate size of 6.91 µm, 31 ACS Paragon Plus Environment

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with 86% of the overall aggregates measuring less than 10 µm (Figure S11), while more significant agglomeration and higher agglomerate packing density were apparent in the pristine, MPA, and PA systems, which might lead to the increase of charge transfer impedance after cycling observed in Figure 9D, resulting in fast fading of the discharge capacities. XANES spectra of both discharged and charged electrodes after 20 cycles were collected at the selected spots (2 x 2 μm) with high fluorescence intensity from the XRF maps. For the discharged electrodes (Figure 11A), all spectra shifted from an initial edge position of ca. 7125 eV in the undischarged state to a metallic-like edge energy of ca. 7113 eV, which aligned well with edge position and profile of the Fe0 foil reference. Upon charging to 3.0V (Figure 11B), spectra of both pristine and MBA-Fe3O4 systems remained in the reduced state, showing similar edge position as Fe metal foil, which suggested that the two electrodes did not successfully charge to an oxidized state due to the poor electrochemical reversibility. In the MPA and PA systems, a small edge shift was noted upon charge, indicating a large amount of capacity loss after 20 cycles. By contrast, a more discernable shift evinced in the charged BA system, which possessed an edge position more oxidized than Fe metal, yet not quite reaching the undischarged Fe3O4 reference material, demonstrating a higher degree of reversibility in this system. Linear combination fitting was performed on all the charged XANES spectra, which was displayed in Table S1. The BA system showed the highest Fe3O4 percentage after 20 cycles, which confirmed a higher degree of reversibility in this system under high current density.

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Figure 10. Fine resolution Fe K-edge elemental maps of electrodes, which were discharged to 0.05V at 400 mA/g after 20 cycles, using as-prepared pristine Fe3O4 (A), MBA-Fe3O4 (B), BAFe3O4 (C), MPA-Fe3O4 (D), and PA-Fe3O4 (E) as active materials.

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Figure 11. Overlay of XANES spectra of various pristine and surface-treated Fe3O4 electrodes in the discharged (A) and charged (B) states after 20 cycles at a current density of 400 mA/g, together with those of the Fe0 foil and the undischarged 10 nm Fe3O4 NPs. 4. Conclusion

In this study, various surface-treated Fe3O4 materials using MBA, BA, MPA, and PA organic ligands as surface-treatment agents have been successfully generated through bidentate bonding via carboxylate group and/or Fe-S bond between the surface ligands and the underlying Fe3O4 NPs. Both structural and electrochemical characterization protocols have been utilized to subsequently correlate the surface effect induced by different ligands to the corresponding electrode behaviors. The particle size analysis together with SEM-EDS elemental mapping results have collectively suggested a better dispersion of Fe3O4 NPs within the electrodes after the surface treatment through ligand functionalization, especially with the aromatic ligands such as MBA and BA.

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In terms of the rate capability of the resulting electrodes, both PA and BA-treated systems demonstrated better electrochemistry than that of their thiol-containing counterparts (MPA and MBA) as well as the pristine Fe3O4 system, indicating that the thiol functional group has detrimental effect on the resulting electrode behaviors of our Fe3O4 system, in part due to the strong electron affinity of -SH group, which withdraws electron from Fe3O4. With respect to the cycling stability, it is interesting to note that although the aliphatic PA system demonstrated outstanding rate capability for the initial 100 cycles under various current densities, and good cycling stability in the extended cycles from cycle 101-200 at a low current density of 100 mA/g, when the current density increased to 400 mA/g, the aromatic BA system started to deliver more capacities and show better cycling stability due the improved charge transfer property and ion transport efficiency. To further provide insights into the improved electrochemistry of the BA system at fast cycling rates, synchrotron-based µ-XRF mapping and XANES measurements were conducted, which suggested better dispersion of the Fe3O4 NPs and higher degree of electrochemical reversibility after cycling in this system. The compilation of both structural and electrochemical results showed that (i) the presence of a -conjugated carbon framework within the surface-treatment agents themselves and (ii) the electron affinity of the pendant groups collectively played important roles in the resulting interfacial charge transfer and ion transport from Fe3O4 to the underlying carbon networks. Specifically, the short aromatic ligands without thiol group can potentially improve the electrochemistry of the Fe3O4 anodes, especially during extended cycling under fast (dis)charge rates, due to the enhanced electron and ion conductivity. In addition, due to the small volume of the ligands and the thin coating layer, the tap density can be improved when comparing to those electrodes containing large amount of carbon nanotubes or conducting polymers. Hence, our 35 ACS Paragon Plus Environment

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work on surface engineering of the Fe3O4 electrode presented herein, reinforced the importance of modifying the structure and chemistry of surface ligand molecules as a plausible means of enhancing charge transfer within analogous metal oxide composite electrodes without hampering the resulting tap density of the electrode. Additional future experiments are needed to corroborate the role of ligand loading ratio on the resulting electrode behaviors. 5. Supporting Information Additional structural characterizations of surface-treated Fe3O4 NPs, electrochemical characterization data of various Fe3O4 electrodes, and calculations of weight percentage as well as coating thickness of the surface-treatment agents.

6. Acknowledgements 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. This research used resources of beamline 5-ID, submicron resolution X-ray spectroscopy (SRX) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. The electron microscopy measurements used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. A.A., M.R.D., and A.H.M. acknowledge the Graduate Assistance in Areas of National Need Fellowship (GAANN). E.S.T. acknowledges the William and Jane Knapp Chair in Energy and the Environment. 36 ACS Paragon Plus Environment

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