Investigation of SEI Layer Formation and Electrochemical Reversibility

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C: Energy Conversion and Storage; Energy and Charge Transport

Investigation of SEI Layer Formation and Electrochemical Reversibility of Magnetite, FeO, Electrodes: A Combined XAS and XPS Study 3

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David C. Bock, Gordon H. Waller, Azzam N. Mansour, Amy C. Marschilok, Kenneth J. Takeuchi, and Esther S. Takeuchi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01970 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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The Journal of Physical Chemistry

Investigation of SEI Layer Formation and Electrochemical Reversibility of Magnetite, Fe3O4, Electrodes: A Combined XAS and XPS Study David C. Bock1‡, Gordon H. Waller2‡, Azzam N. Mansour2*, Amy C. Marschilok1,3,4, Kenneth J. Takeuchi3,4, and Esther S. Takeuchi1,3,4* 1

Energy Sciences Directorate, Brookhaven National Laboratory, Upton, NY 11973 2

Naval Surface Warfare Center, Carderock Division, West Bethesda, MD 20817 3

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

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

Keywords: Li ion batteries, Fe3O4, solid electrolyte interphase (SEI), fluoroethylene carbonate (FEC), XPS, XAS

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Abstract Magnetite (Fe3O4) is a promising electrode material for the next generation of Li-ion batteries with multiple electron transfers per metal center and a theoretical capacity of 924 mAh/g. However, multiple phase conversions during (de)lithiation of Fe3O4 and formation of a surface electrolyte interphase (SEI) contribute to capacity fade. In this study, X-ray Absorption Spectroscopy (XAS), and X-ray Photoelectron Spectroscopy (XPS) were used to determine the surface chemistry, redox chemistry, and the impact on the electrochemical reversibility in the presence and absence of fluoroethylene carbonate (FEC) solvent. With FEC improved capacity retention and enhanced reversibility are observed. In contrast, electrodes cycled with no FEC exhibit decreased reversibility where the active material remains as reduced Fe0. XPS results reveal LiF and lower quantities of oxygen containing species, especially carbonates at the electrode surface tested in FEC. The improvement in electrochemical reversibility with FEC is attributed to the formation of a solid electrolyte interphase which forms prior to initiation of the conversion reaction limiting SEI growth on the reduced products, Fe0 and Li2O. In contrast, ECbased carbonate electrolyte forms SEI at a potential where formation of Fe0 and Li2O has already initiated resulting in SEI formation on Fe0 nanograins.

Introduction Materials with the capability of providing multiple electron transfers (METs) per cation center have attracted considerable research interest as next generation lithium ion battery electrodes.1-2 The significantly higher energy density of these materials compared to currently utilized intercalation electrodes provide the opportunity to power applications such as electric vehicles and grid level energy storage.2 One particularly intriguing MET compound is magnetite,


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Fe3O4, due to its high theoretical lithium storage capacity, high natural abundance, low cost and low toxicity. Fe3O4 has an inverse spinel structure consisting of a cubic close packed O2- anion framework with Fe3+ ions partially occupying tetrahedral (8a) sites and both Fe3+ and Fe2+ ions occupying octahedral (16d) sites.3 Lithiation of the material proceeds via initial intercalation into octahedral sites, followed by cation displacement/ reordering transformation to a rock-salt-like phase, and ultimately conversion of the lithiated metal oxide to metallic Fe0 and Li2O.4 Full reduction of all the Fe cation centers results in transfer of 8 electron equivalents, corresponding to 924 mAh/g of lithium storage capacity. While the ccp structure and multiple electron transfer capability of Fe3O4 provide exceptional energy density, these same attributes have also complicated practical use of the material. The dense crystallographic structure does not facilitate lithium ion diffusion, making full electrochemical utilization difficult to achieve at high rates.5

Furthermore, the phase

transformations that occur during cycling can result in particle pulverization, loss of electrical contact1, and instability of the solid electrolyte interphase (SEI) formed on the surface of Fe3O4 particles.10,11 As highlighted in a recent review5, three major strategies have been implemented to improve the performance of Fe3O4 electrodes: (1) nanosizing to decrease the path length for Li+ ion diffusion, (2) modifying the morphology to alleviate stress caused due to volume change, including development of hollow nanospheres,6-7 and (3) engineering composite electrode heterostructures containing nanostructured Fe3O4 and carbon that promote electronic conductivity among particles during cycling. Although the above approaches are critical for reducing polarization and improving capacity utilization, fully overcoming the consequences of SEI formation on the particle surfaces remains an area of inquiry.8 Ideally, after initial formation of the SEI, the active material surface


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is protected from further reaction with the electrolyte while permitting Li-ion transport. In the case of conversion electrodes, poor SEI stability can result from the structural and volumetric changes that occur during cycling. The structural transformations occurring during (de)lithiation result in a dynamic electrode surface in contact with the electrolyte, where new active sites for additional electrolyte reduction are repeatedly exposed in each cycle.9-10 Additionally, in the case of metal oxides, the reduced nanocrystalline metal particles in the discharged state have been noted as catalytic towards decomposition of electrolyte.10-13 Thus, continuous decomposition of electrolyte occurs, consuming lithium ions and solvent, while increasing SEI layer thickness that causes ionic and electronic isolation of the active material manifests as capacity loss.9, 14-15 One strategy for improving the stability of the formed SEI on conversion anode materials is the use of electrolyte additives or co-solvents.16

These solvents reduce at higher potentials

relative to EC-based carbonate based solvents (such as ethylene carbonate, dimethyl carbonate, diethyl carbonate) and alter the chemistry of reduction products in the SEI layer. An electrolyte additive previously reported with electrode materials showing significant volume changes, including Si17-27, Sn28-30, Co3O431, and NiO32, is fluoroethylene carbonate (FEC). Multiple mechanisms have been proposed for the FEC modification of the SEI chemistry to improve electrochemical function, with researchers dissenting about how the additive affects the formation of LiF as a decomposition component, and whether the existence of LiF is favorable for stability of the interphase layer.33 Despite the lack of consensus in the literature regarding the mechanism, most studies do indicate that FEC modified SEI results in a considerable improvement in cycling stability. Recently the use of FEC for stabilization of the solid electrolyte interphase formation has been extended to Fe3O4 based anodes.34-35 When cycled with 1.33 M FEC in the electrolyte,


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Fe3O4 electrodes exhibited a 280% increase in capacity relative to EC-based electrolyte after 100 cycles.34 Reduction of the FEC was confirmed to occur at voltages higher than other carbonate components in the electrode, and post cycling the presence of LiF was confirmed on the electrode surfaces for those cycled in FEC.34 The improvement in electrochemical performance was attributed to a modified solid electrolyte interphase (SEI) formed in the presence of FEC. In the current work, we explore the chemical composition of the SEI formed in the presence of FEC (1M LiPF6 30:70 fluoroethylene carbonate (FEC):dimethylcarbonate(DMC)) versus SEI formed in EC-based electrolyte (1M LiPF6 30:70 ethylene carbonate (EC):dimethylcarbonate(DMC)) using X-ray photoelectron spectroscopy (XPS). Results show clear differences in both organic and inorganic species formed on the electrode surfaces. Furthermore, we determine the effect of the solvent on the reversibility of the active material both electrochemically and through use of XAS measurements performed at several stages of cycling. Based on the XAS results and SEI formation at a higher voltage in the presence of FEC, a mechanistic basis for the performance enhancement is proposed. These results are expected to have broad scientific impact for Fe3O4 as well as other conversion electrode materials where utilization of FEC as an electrolyte component may be anticipated to improve electrochemical reversibility.

Experimental Synthesis and Characterization The preparation of ca. 40 nm Fe3O4 nanoparticles was adapted from a previously reported method.36 Briefly, iron (II) chloride hexahydrate aqueous solution was dropped into an aqueous solution of ammonium hydroxide and stirred under air. The Fe3O4 product was isolated by centrifugation and dried under vacuum.


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X-ray diffraction data was collected using a Rigaku Miniflex diffractometer using Cu Kα radiation and a D/teX area detector. The synthesized material was compared with the reported Fe3O4 reference pattern (PDF #01-088-0315). Rietveld analysis was performed using GSAS-II software using a structural model of Fe3O4. Electrochemical Methods Electrodes were prepared on copper foil substrates with Fe3O4, carbon black conductive additive, and PVDF binder in a ratio of 70:20:10. Coin type cells were used for electrochemical tests with lithium metal counter electrodes and polypropylene separators. The electrolytes consisted of 1 M lithium hexafluorophosphate (LiPF6) in solvent mixtures of either 30:70 EC:DMC or 30:70 FEC:DMC. Galvanostatic cycling tests were performed using a C/2 rate (calculated based on the active mass Fe3O4 in the electrode based on a theoretical capacity of 924 mAh/g) between 0.1 and 3.0 V vs. Li/Li+ at 30°C. EIS measurements were collected using a Biologic VSP multi-channel potentiostat with a 5 mV sinus amplitude in the frequency range of 10 mHz – 1 M Hz. Electrodes were cycled to various states for further analysis by XPS and XAS. Specifically, electrodes for XPS were prepared in the following states: as-prepared, cycled 10x – discharged (to 0.1V), cycled 10x – charged (to 3.0 V), cycled 50x – discharged, and cycled 50x – charged. Additional cells containing 80% carbon black, 20% PVDF electrode composition were cycled at equivalent current density for 50 cycles, with both discharged and charged states evaluated. Electrodes for XAS were prepared in as-prepared, cycled 1x-discharged, cycled 1xcharged, cycled 10x-discharged, cycled 10x-charged, cycled 50x-discharged, and cycled 50xcharged states. X-ray Photoelectron Spectroscopy


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Samples were analyzed using a Versaprobe II Scanning X-ray Photoelectron Spectrometer (XPS) Microprobe from Physical Electronics USA, Inc. The Versaprobe II features a monochromatic Al-Kα x-ray source with enhanced lateral resolution, enabling precise analysis on a desired sample region. Samples for XPS were rinsed with dimethyl carbonate (DMC) and dried after electrochemical cycling, mounted into an inert transfer vessel inside an Ar filled glovebox with a moisture and oxygen content below 1 ppm, then directly loaded into the XPS with no exposure to air. Non-conductive, low volatility double sided tape was used to electronically isolate the composite electrode samples and a dual neutralization approach utilizing low energy electrons (~ 1 eV) and low-energy Ar ions (< 10 eV) was applied. The binding energy scale was calibrated with respect to the literature established values of 685.0 eV for LiF (F1s) or in the case of the pristine electrodes 284.4 eV for carbon black (C1s). Fresh, uncycled samples were analyzed to obtain a reference spectrum for the composite electrode, while samples in both the charged and discharged states were analyzed after 10 and 50 charge and discharge cycles. Survey scans were collected on every sample, followed by multiplex measurements in the C1s, F1s, O1s, Li1s/Fe3p and P2p photoelectron regions. The spectra were collected at an electron-take-off angle of 45o. The analyzed region was set to 1000 microns x 200 microns and the 200 micron size X-ray beam was electronically rastered over the analyzed region to minimize damage due to X-ray exposure. The analyzer pass energy was set to 117.4 eV for survey spectra and 23.5 eV for multiplex spectra, which correspond to analyzer resolution of 1.76 and 0.35 eV, respectively. Samples were sputtered for a period of 30 minutes using 500 V Ar ions, which corresponds to a 0.66 nm/min sputter rate on a SiO2 standard, to evaluate the composition distribution within the SEI region. The sputter rate for the SEI layer in this study, which contains mostly carbon, oxygen, lithium, and fluorine, is expected to be somewhat lower


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than the sputter rate calculated for SiO2. Based on our Ar-ion depth profile analysis of 20 and 100 nm thick films of carbon and SiO2 deposited on Si substrate, respectively, the sputtering rate for carbon was estimated to be about half that of SiO2 under similar sputtering conditions. Depth analysis was conducted on the electrode side facing the lithium metal counter electrode. The composition and deconvolution of multiple chemistries were performed using the MutiPak Software version 9.6.0.15 employing a combined Gaussian-Lorentzian line shape and an iterated Shirley model for the background portion of the spectrum. The peak fitting process involved selecting the starting peak shape and optimizing the peak parameters (position, FWHM, area) optimized to obtain a minimum chi-squared value. Doublet peaks were constrained such that they were optimized together when appropriate. Results of all the photoelectron regions were compared to ensure that they were consistent with each other (.i.e. i.e. the O1s content should reflect any oxygen containing assignments in other regions like C1s or Li1s/Fe3p). The relative surface concentrations of various elements on the samples were determined on the basis that the area under each elemental peak is proportional to the product of the element concentration and the sensitivity factor. The area is measured experimentally using the XPS manufacture's peak fitting software (MultiPak) and the sensitivity factor for each XPS line is provided by the manufacturer which includes instrumental corrections. Relative surface concentration of each element is calculated using the following general formula: =

( / ) ∑ /

(1)

Where is the relative atomic percentage of element , is the peak area for the given element, and is the sensitivity factor.


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Ex-Situ X-ray Absorption Spectroscopy: Electrodes were removed from coin cells and sealed between polyimide tape. Prior to measurement, the samples were stored under Ar atmosphere. XAS spectra were collected at sector 12-BM at the Advanced Photon Source at Argonne National Laboratory, IL. All measurements were collected at the Fe K-edge (7.112 keV) in transmission mode using a Si (111) double crystal monochromator. Incident and transmission ion chambers were filled with 100% N2. All samples were measured with a Fe metal foil reference simultaneously for correct energy alignment of individual spectra during data analysis. XAS spectra were aligned, averaged, and normalized using Athena.37-38 The built-in AUTOBK algorithm was used to limit background contributions below Rbkg = 1.0 Å. Normalized spectra were fit utilizing Artemis with theoretical structural models created with FEFF6.38-40 EXAFS modeling was conducted based on the inverse-spinel Fe3O4,41 rock-salt FeO42 and bodycentered cubic (bcc) Fe metal43 crystal structures, as appropriate. A k-range of 2 – 11 Å-1 and Hanning window (dk = 2) were used as fourier transform parameters, and fitting was performed in k, k2 and k3 k-weights simultaneously. An R-range of 1.0 – 3.7 Å or 1.0 – 3.0 Å was used to fully encompass the first and second shells of |χ(R)| (Fourier transform of χ(k)).

Results Materials Synthesis and Characterization Magnetite, Fe3O4 was synthesized using a previously reported coprecipitation method.36 The prepared material was determined to be phase pure by XRD and comparison to the literature reference pattern (PDF #01-088-0315). The isotropic crystallite size of the material was determined to be 40 nm by Rietveld analysis of the XRD data (Figure S1).


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Electrochemistry Coin type half cells with lithium anodes were galvanostatically discharged at a C/2 rate from 3.0 V and 0.1 V in the presence and absence of FEC as an electrolyte component. Figure 1A shows representative cycle 1 discharge and charge profiles for electrodes cycled in 1M LiPF6 30:70 EC:DMC (EC-based) or 1M LiPF6 30:70 FEC:DMC (FEC-based) electrolyte, where capacities are calculated based on the Fe3O4 mass in the electrodes. The first discharge capacities are ca. 1375 and 1425 mAh/g for electrodes cycled in EC-based and FEC-based electrolytes, respectively. The higher than theoretical first cycle capacities are associated with solid electrolyte interphase (SEI) formation on both the Fe3O4 particles and the carbon black.14, 44-46 Between voltages of 2V and 1.1 V, there are significant differences in the voltage profiles and delivered capacity depending on electrolyte composition, with electrodes in EC-based or FECbased electrolytes delivering capacities of 66 mAh/g or 160 mAh/g, respectively. The additional capacity in the 2.0 – 1.1 V range is attributed to the reduction of FEC on the Fe3O4 and carbon black components of the electrodes.34 Galvanostatic cycling results through 50 cycles at the C/2 rate are presented in Figure 1B. The electrode cycled in EC-based electrolyte delivered ca. 410 mAh/g at 50 cycles, corresponding to ca. 42 % capacity retention between cycles 2 - 50. Replacement of EC with FEC significantly improves capacity retention, with a 50th cycle capacity of 907 mAh/g (92% capacity retention), corresponding to a 121% increase relative to the EC-based electrolyte. The contribution of carbon black to the enhanced capacity retention was assessed by testing carbon black / PVDF electrodes under the same cycling conditions (Figure S2). The carbon black/ PVDF electrodes cycled in FEC-based electrolyte exhibited only a ca. 15 mAh/g capacity


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improvement over the electrodes cycled in the EC-based electrolyte. Thus, the lower capacity fade of the Fe3O4/C electrodes in the presence of FEC can be attributed to the Fe3O4 active material.

Figure 1. (A) Cycle 1 discharge/charge profiles and (B) charge (open symbol)/discharge (closed symbol) capacities of Fe3O4/C electrodes cycled in either 1M LiPF6 30:70 EC:DMC or 1M LiPF6 30:70 FEC:DMC electrolyte at C/2 from 0.1 to 3.0 V. (C, D) Nyquist plots of AC impedance response of cells with 1M LiPF6 30:70 EC:DMC or 1M LiPF6 30:70 FEC:DMC electrolyte (C) before cycling and (D) after cycling 50x . The insets highlight the high to middle frequency regions of the impedance spectra.


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Electrochemical Impedance Spectroscopy Electrochemical Impedance Spectroscopy (EIS) was used to investigate the as prepared and cycled cells after 1 cycle, 10 cycles, and 50 cycles in the charged state. Results are presented as Nyquist plots in Figure 1C, D and Figure S3 with the figure insets showing the high to middle frequency region. Before cycling, both cell types exhibit an extended symmetrical semicircular feature as well as a steeply sloping linear tail at lower frequencies; the magnitude of the semicircle diameter is larger for the cell utilizing the FEC-based electrolyte. After cycling, the features evolve such that the semicircular arc is less defined and has similar magnitude for electrodes cycled in either electrolyte, and the slope of the low frequency tail decreases. The asymmetry of the semicircular feature after cycling suggests multiple overlapping impedance contributions in the same frequency range. All impedance spectra were fit to an equivalent circuit model to enable a quantitative comparison of the data.47-48 The equivalent circuit is a version of the Randles circuit49 consisting of a resistor in series with two R/CPE elements and a Warburg element (Figure S4). The resistor, R1, represents electrolyte resistance as well as ohmic resistances of the cell components and is fit to the high frequency intercept of the impedance with the real axis. R2 and CPE1 in the first parallel resistor-constant phase element component are assumed to correspond to the charge transfer resistance and double layer capacitance at the Fe3O4 electrode, while R3 and CPE2 represent the ionic resistance and capacitance of the surface film formed on the Fe3O4/C electrode after cycling. These processes have similar relaxation times and thus both R/CPE elements are used in combination to fit the high to middle frequency depressed semicircular arc. Similar equivalent circuit models have previously been used to model charge transfer resistance and surface film resistance in Fe3O4 composite electrodes.50-52 In the as prepared cells only one


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semicircle is visible and thus only the first R/CPE element is utilized in the model. After cycling the arc becomes broadened, indicating the presence of overlapping semicircles and the growth of the SEI component. The sloping line in the low frequency region is fit with a generalized finite Warburg element,53 and is associated with lithium diffusion in the electrode. Full equivalent circuit fit results are tabulated in Table S1. The low value of R1 for both electrolyte cell types indicates that both have similar ohmic resistance. In as prepared cells, the R2 values are 15 ± 1 Ω and 61 ± 1 ohm for cells utilizing EC-based and FEC-based electrolyte respectively, suggest that the initial resistance associated with charge transfer is higher in the presence of FEC vs. EC. However, after one cycle, the charge transfer resistance of the cell utilizing FEC significantly decreases from 61 ± 1 to 14 ± 2 Ω , compared to 15 ± 1 to 18 ± 2 Ω for the EC cell. Upon further cycling to 10 and 50 cycles, R2 and R3 values for cells with either electrolyte are < 20 Ω. Fitted R3 values indicate that the resistance associated with the film formation on the electrodes is similar for either electrolyte at each cycled state, and increases by only ca. 2 Ω from cycle 1 to cycle 50. It is noted that interpretation of the impedance data after extended cycling is complicated by the phase evolution of the iron oxide as a function of cycle number and electrolyte type, as will be discussed in the subsequent XAS section.

X-ray Absorption Spectroscopy – XANES X-ray Absorption Spectroscopy (XAS) was used to characterize the crystalline, nanocrystalline and amorphous phases of iron in the Fe3O4/C electrodes cycled in EC-based or FEC-based electrolytes. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions of the XAS spectra at the Fe K-edge were analyzed to provide detailed local electronic and atomic structural data. An as prepared electrode and


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electrodes cycled at a C/2 rate in both the charged and discharged state after 1 cycle, 10 cycles, and 50 cycles were measured. The XAS results are presented in Figure 2A, B as XANES. In the non-discharged state, the XANES spectrum has an edge energy (defined as the maximum of the first derivative of xµ(E)) of 7126.3 eV, in good agreement with previous reports.4, 54 After the first discharge, the edge position of electrodes cycled in either electrolyte shifts to that of Fe metal (ca. 7112 eV), indicating that the oxidation state of the iron has reduced to Fe0. Upon recharging to 3.0 V, the edge positions shifts to an energy of ca. 7125.5 eV, a slightly lower energy than the pristine edge position, suggesting that after charge the oxidation state of Fe is lower than that of the pristine material. Furthermore, the white line is shifted (at ca. 7132.3 in pristine material and ca. 7130.2 for cycle 1 charged electrode) with significantly reduced intensity after charge, indicating that the electrodes have different local atomic arrangements in the pristine versus charged state. In the 10th cycle, reversibility in oxidation state between discharge and charge is observed for electrodes cycled in either electrolyte. The XANES of the electrode cycled in EC-based electrolyte exhibits a slight shift (ca. 0.7 eV) to lower energy in the charged state, suggesting the iron centers in the electrode are reduced relative to the cycle 1 charged state. After cycling to cycle 50, significant dissimilarities in the XANES profiles are observed between electrodes cycled in the two different electrolytes. The electrode cycled in FEC-based electrolyte exhibits good reversibility between the discharge and charge, with the oxidized and reduced states having edge positions of ca. 7124.8 and 7112 eV, respectively. In contrast, the XANES profile of the electrode cycled in EC-based electrolyte shifts only ca. 0.3 eV between the oxidized and reduced states. The edge position in the charged state is ca. 7112.3 eV, indicating that the material remains primarily in a reduced state. Thus, after 50 cycles, the iron in the electrode cycled in


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EC-based electrolyte remains as Fe metal regardless of discharge or charge, while the electrode cycled in FEC-based electrolyte retains significant reversibility between Fe metal and an oxidized iron phase.

Figure 2: (A, B) XANES comparison and (C, D) k2 weighted|χ(R)| comparison of electrodes cycled in (A, C) 1M LiPF6 30:70 EC:DMC or (B, D) 1M LiPF6 30:70 FEC:DMC electrolyte during the 1st cycle (undischarged, full discharge and charge), 10th cycle (discharge and charge) and 50th cycle (discharge and charge).


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X-ray Absorption Spectroscopy – EXAFS The extended X-ray absorption fine structure (EXAFS) region of the XAS results was also analyzed to provide atomic structural information as a function of dis(charge) state. Results are presented in Figure 2C, D as R-space plots (Fourier transforms of k2|χ(R)|). Note that distances shown in the R-space plot are not corrected for phase shifts and thus are 0.3 – 0.4 Å less than the true interatomic distances from EXAFS modeling results. The non-discharged composite electrode exhibits an EXAFS spectrum which consists of major peaks centered at 1.4 Å (contributions from neighboring oxygen atoms for octahedrally and tetrahedrally coordinated Fe atoms) and 2.6 Å (contributions primarily from neighboring iron atoms).

In the fully

discharged state (1st discharge), the EXAFS spectra primarily consist of a single broad peak centered at 2.1 Å indicating phase transformation to Fe0. The atomic structure of the recharged iron is significantly different than the initial discharged material, and further evolution of the spectra occurs upon cycling. The EXAFS spectra were theoretically modeled using Artemis to rigorously determine the local atomic environment of Fe atoms as a function of cycling. Details of the fitting procedures are given in the experimental section. Results are presented as interatomic distances and number of near neighbors for Fe-Fe and Fe-O paths in Figures S5 and S6. Full EXAFS theoretical modeling results are presented in the supplemental information (Tables S2 – S8). For electrodes in the undischarged state, a theoretical model of Fe3O4 with 8 distinct scattering contributions from both octahedral and tetrahedral Fe atomic positions in the inverse spinel structure54 was used to fit the EXAFS spectra, with R-factor < 1.0. In the cycled, first discharged state, the theoretical model used to fit the data consists primarily of a scattering path from


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metallic Fe55 at ca. 2.5 Å and 2.85 Å as well as an Fe-O path at ca. 1.9 Å. The presence of an oxygen path at ca. 1.9A suggests that the metallic Fe nanoparticles which form are partially oxidized, though the contribution of the oxide phase is relatively low, as indicated by the small average number of near neighbors (< 1.0). No significant differences are observed in the fitted parameters between electrodes discharged in EC-based or FEC-based electrolyte, suggesting that minor differences in first cycle discharge capacity (ca. 1375 mAh/g for EC-based electrolyte vs. 1425 mAh/g for FEC-based electrolyte) are caused by differences in electrolyte reduction. After charge to 3.0 V, the EXAFS spectra consist of two primary peaks that are shifted in position and intensity relative to those of undischarged Fe3O4. Based on previous work which has shown that charge of Fe + Li2O results in a rock-salt type FeO like structure4, the spectra were fit using a theoretical model based on FeO, with Fe-O and Fe-Fe paths centered at ca. 2.0 Å and ca. 3.1 Å. The charged material which forms is nanocrystalline in nature, with significantly fewer near neighbors than would be expected for bulk FeO (ca. 2 Fe-O near neighbors vs. 6 near neighbors for bulk FeO). An additional long oxygen path at ca. 2.95 Å was needed to produce a good fit and is hypothesized to be due to the nanocrystalline nature of the re-oxidized iron oxide. Second-shell Fe-O paths have also been reported in the charged state of other spinel-type conversion materials CuFe2O4 and ZnFe2O4.56-57 Thus, after the first charge, EXAFS modeling results for electrodes discharged in either electrolyte indicate that Fe atoms no longer reside in the inverse spinel structure of Fe3O4 but are instead associated with a FeO like structure, in good agreement with a previous report.4 In the 10th cycle, both electrode types show reversibility of local atomic structure between the two states with conversion between the Fe0 and FeO-like local atomic structures. However, by cycle 50, significant differences in the atomic environments are observed which


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correlate with the electrochemical findings. The electrode cycled in FEC-based electrolyte converts between the Fe0 atomic structure in the discharged state and the FeO-like structure in the charged state exhibiting electrochemical reversibility, with a capacity of 910 mAh/g at cycle 50. The greater capacity loss of the electrode cycled in EC-based electrolyte (reversible capacity of 410 mAh/g at cycle 50) is reflected in the EXAFS modeling results. The electrode remains in the metallic Fe0 discharged state, regardless of being electrochemically discharged or charged on the 50th cycle. This suggests that the majority of Fe centers within the electrode are no longer electrochemically active and do not participate in the lithium insertion/removal process.

X-ray Photoelectron Spectroscopy Pristine electrodes containing 70 wt% Fe3O4 / 20 wt% conductive carbon / 10 wt% PVDF, or only 80 wt% carbon / 20 wt% PVDF (no Fe3O4) were analyzed prior to electrochemical cycling to identify spectral features in the C1s, F1s, O1s, and Fe3p/Li1s regions and were found to be highly dynamic during SEI formation, with the results presented in the Supporting Information (Figures S7, S8, and Table S9). Electrodes were characterized by XPS after 10x and 50x electrochemical cycles in both the charged and discharged state. In Figure 3, the atomic concentrations of carbon, fluorine, oxygen, and lithium for the electrodes analyzed in each condition are shown. Regardless of number of cycles, the electrodes cycled in EC-based electrolyte show substantially more carbon to oxygen bonding (Figure 3A, 3C), and are consistent with the formation of an SEI with a substantial concentration of CO3, C=O, and C-O species. In contrast, the electrodes cycled in FEC-based electrolyte show more fluorine (Figure 3B) and Li (Figure 3D), which is consistent with the formation of an SEI containing a substantial amount of LiF. Details of the XPS peak analysis for electrodes cycled 10x are provided in the


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next section. Electrodes cycled 50x followed qualitatively the same trends observed for the electrodes cycled only 10x. In Tables 1 and S10 we summarize the composition and peak fitting results for the electrodes cycled 10x and 50x, respectively. A complete discussion of the 50x cycled electrodes is provided in the Supporting Information. Notably, the LiF concentrations for electrodes cycled in the FEC-based electrolyte containing electrolyte was higher after 50 cycles compared to 10 cycles, which may indicate formation of a thicker LiF film near the top of the electrode surface with increased cycling.

Sample

10X EC/DMC Discharge

10X FEC/DMC Discharge

10X EC/DMC Charge

10X FEC/DMC Charge

XPS Line

BE (FWHM) At%

BE (FWHM) At%

BE (FWHM) At%

BE (FWHM) At%

F 1s

685.0 (1.55) 9 687.2 (2.05) 4

685.0 (1.67) 13 688.1 (2.06) 13

685.0 (1.53) 5 687.4 (1.99) 4

685.0 (1.65) 16 688.1 (2.08) 11

Chemistry

LiF Li PF O , x

y

z

PVDF C 1s

283.2 (1.38) 2 284.8 (1.35) 15 286.4 (1.84) 10 288.7 (1.34) 2 290.2 (1.45) 7

O 1s

Li 1s

530.6 (1.46) 5 531.9 (1.62) 19

282.8 (1.36) 1 284.9 (1.65) 10 286.6 (1.53) 4 288.0 (1.77) 3 289.6 (1.58) 1 290.7 (1.58) 1

533.6 (1.72) 6

531.8 (1.87) 9 532.6 (1.62) 11 533.9 (1.97) 4

55.4 (1.8) 21

55.67 (1.62) 24

283.7 (1.26) 5 284.9 (1.16) 8 286.4 (2) 10 289.1 (2) 4 290.3 (1.43) 7

530.7 (1.69) 3 531.9 (1.56) 21

283.4 (1.06) 1 284.9 (1.58) 11 286.6 (1.67) 5 288.2 (1.7) 2 289.6 (1.7) 1 290.8 (1.5) 1

533.5 (2.01) 7

531.5 (1.92) 3 532.5 (1.93) 15 534.2 (1.78) 3

55.5 (1.76) 26

55.7 (1.59) 26

Li-C C-C, C-H C-O, PVDF O-C-O, C=O O-C=O Li CO 2

3

PVDF ROLi Li CO , C=O 2

3

FEC C-O LiF, Li CO3, 2

ROLi

Table 1. XPS peak assignments for electrodes cycled 10x in FEC and EC containing electrolytes.


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Figure 3. Graphical summaries of C (A), F (B), O (C), and Li (D) content for Fe3O4 electrodes cycled in FEC and EC containing electrolytes for 10 and 50 cycles.


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After 10 electrochemical cycles, dramatic changes were observed in the C1s region when comparing the electrodes cycled in the two electrolyte types. As shown in Figure 4, five distinct peaks are assigned to the electrode cycled in EC-based electrolyte while six peaks are assigned to the electrode cycled in FEC-based electrolyte.

Figure 4. C1s Spectra for Fe3O4 electrodes cycled 10x without (A, C) and with (B, D) FEC added. We attempted to model the C1s region with a total of five unique peaks in both cases but the simulation for the electrodes cycled in the FEC containing electrolyte resulted in one of the


21


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peaks being very broad, which is not consistent with any of the chemistries relevant to this study. Hence, we believe we are justified in including an additional peak in the case of electrodes cycled in the FEC-based electrolyte. The C1s region for all electrodes shows a contribution with a binding energy near 285 eV, indicating the presence of a polymeric material, which is rich with hydrocarbon chemistry. Furthermore, all electrodes show signatures of carbon oxygen type bonding which can be assigned to C-O (286.5 eV), O-C-O, C=O, and O-C=O (288 to 289 eV), and CO3 species (~290 eV)58-60. The C-H2 and C-F2 bonding assigned to PVDF (286 and 291 eV, respectively) with nominally equal intensity observed in the uncycled reference (undischarged) samples (Figures S7 and S8) is possibly present in small quantities in the case of the electrodes cycled in the FEC containing electrolyte, but the SEI growth makes this assignment difficult to confirm. Previous XPS results of Fe3O4 composites indicate that the presence of binder can significantly affect the observed spectra.61 An intense peak is observed at high binding energy (~290 eV) for the electrodes cycled in EC-based electrolyte (Figure 4A/4C), which overlaps the C-F bonding expected for PVDF and is assigned to lithium carbonates (Li2CO3) and lithium alkyl carbonates.21,

62

This assignment is also supported by the O1s

component with binding energy near ~532 eV (Figure 5) . A small shoulder peak is observed at 1.0 V, an FEC-modified film has already formed on the surface of the lithiated Fe3O4 particle. Continued lithiation of Fe3O4 results in formation of small (~ 1 nm) nanograins of Fe0 particles embedded in a Li2O matrix, consistent with a recent mechanistic study.4 Thus, the initial SEI formation on the surface of the active material particle would form a protective barrier mitigating further electrolyte decomposition where SEI growth on the Fe0 and Li2O domains would be limited and allow for conversion to iron oxide phase on charge. In contrast, for the EC-based electrolyte, the formation of SEI takes place in the same voltage range as the conversion reaction, which would result in exposure of the Fe0 nanograins (as well as Li2O domains) to the electrolyte with significant film growth. The SEI on the Fe0 nanograins would impede conversion to iron oxide during charge, resulting in trapped active material in the Fe0 oxidation state. With each additional cycle, additional Fe0 nanograins would become electrochemically isolated, such that by 50 cycles significant capacity fade is observed.


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Figure 10. Schematic of SEI formation on Fe3O4 electrodes as a function of discharge voltage for 1M LiPF6 30:70 FEC:DMC and 1M LiPF6 30:70 FEC:DMC electrolytes.

Conclusions The effect of FEC as a co-solvent on the electrochemical performance, evolution of oxidation state and local atomic structure, and surface chemistry of Fe3O4 anodes was investigated using complimentary XAS, and XPS measurements. The substitution of FEC for EC in alkyl carbonate based electrolyte solutions results in a 121% increase in capacity retention after 50 charge/discharge cycles. In good agreement with the electrochemical results, analysis of the oxidation state and local atomic structure via XAS reveals that Fe3O4 electrodes cycled in the FEC-containing electrolyte are reversible between a FeO-like phase and Fe0 in the charged and discharged state, respectively. In contrast, electrodes cycled in electrolyte with no FEC exhibit a reduced iron oxidation state with increasing cycle number, where the active material becomes electrochemically isolated in the reduced phase as Fe0. XPS results reveal several important properties of the surface chemistry of the SEI formed in the FEC-containing electrolyte compared to EC-based electrolyte, including higher concentration of LiF and lower concentration of oxygen containing species, especially carbonates. Thus, the improvement in electrochemical behavior for Fe3O4 anodes cycled in FEC is attributed to a solid electrolyte interphase which forms on the surface of the active Fe3O4 particles prior to the full reduction reaction, limiting further SEI growth on Fe0 and Li2O domains and allowing conversion to an iron oxide phase during charge. In contrast, in EC-based electrolyte SEI forms on the fully reduced material, Fe0 and Li2O nanograins, impeding oxidation during charge resulting in electrochemical isolation of the active material in the reduced state.


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Acknowledgements This work was supported as part of the Center for Mesoscale Transport Properties, an Energy Frontier Research Center supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, under award #DE-SC0012673. XAS measurements used the resources of the Advanced Photon Source, a DOE, Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC0206CH11357. A.N. Mansour and G.H. Waller collected the XPS data and were funded by the Inhouse Laboratory Independent Research (ILIR) program, Program Element 0601152N, managed by the NSWC Carderock Division Director of Research for the Office of Naval Research.

ASSOCIATED CONTENT Supporting Information. The supporting information file includes XRD of synthesized Fe3O4, impedance spectroscopy results, EXAFS fitting results, XPS of pristine electrodes, and XPS discussion of 50x cycled electrodes. AUTHOR INFORMATION Corresponding Authors *Esther S. Takeuchi ([email protected]) *Azzam Mansour ([email protected] ) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.


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TOC Figure:


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Investigation of SEI Layer Formation and Electrochemical Reversibility

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