Lithiation of Magnetite - ACS Publications

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C: Energy Conversion and Storage; Energy and Charge Transport 3

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Lithiation of Magnetite (FeO): Analysis Using Isothermal Microcalorimetry and Operando X-ray Absorption Spectroscopy Matthew M. Huie, David C. Bock, Lei Wang, Amy C. Marschilok, Kenneth J. Takeuchi, and Esther S. Takeuchi J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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The Journal of Physical Chemistry

Lithiation of Magnetite (Fe3O4): Analysis Using Isothermal Microcalorimetry and Operando X-ray Absorption Spectroscopy Matthew M. Huiea, David C. Bockb, Lei Wangc, Amy C. Marschiloka, b, c,*, Kenneth J. Takeuchia, c,

a

*, Esther S. Takeuchia, b, c,*

Department of Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, NY 11794, USA b

Energy Sciences Directorate, Brookhaven National Laboratory, Upton, NY 11973 USA c

Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA

ABSTRACT

Conversion electrodes, such as magnetite (Fe3O4), offer high theoretical capacities (>900 mAh/g) due to multiple electron transfer per metal center. Capacity retention for conversion electrodes has been a challenge in part due to the formation of an insulating surface electrolyte interphase (SEI). This study provides the first detailed analysis of the lithiation of Fe3O4 using isothermal microcalorimetry (IMC).

The measured heat flow was compared to heat

contributions predicted from heats of formation for the Faradic reaction, cell polarization, and

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entropic contributions. The total measured energy output of the cell (7,260 J/g Fe3O4) exceeded the heat of reaction predicted for full lithiation of Fe3O4 (5,508 J/g). During initial lithiation (3.0 V to 0.86 V) the heat flow was successfully modeled using polarization and entropic contributions. Heat flow at lower voltage (0.86 V to 0.03 V) exceeded the predicted values for iron oxide reduction, consistent with heat generation attributable to electrolyte decomposition and surface electrolyte interphase (SEI). Operando x-ray absorption spectroscopy indicated the oxidation state of the Fe centers deviated from predicted values beginning at ~0.86 V supportive of SEI onset in this voltage range. Thus, these combined results from electrochemistry, IMC and XAS indicate parasitic reactions consistent with SEI formation at moderate voltage and illustrate an approach for deconvoluting faradaic and non-faradaic contributions to heat which should be broadly applicable to the study of energy storage materials and systems.

1. Introduction

Lithium-ion batteries (LIBs) are the dominant energy storage system used to power portable electronics. Broader application of LIBs is also taking place with their adoption in electrified vehicles and by pairing with renewable energy generations systems and the electric grid.1 While the majority of lithium ion batteries focus on insertion materials with limited electrons transferred per active center, conversion electrodes are an intriguing material class as they provide multiple electron transfer opportunity and thus, deliver greater capacity.2 Magnetite (Fe3O4) is an attractive conversion electrode material because of its low cost, natural abundance and low toxicity.3 However, challenges such as poor capacity retention during cycling and large voltage hysteresis must be overcome.4 Studies show that appropriate design of crystallite size, particle dispersion, and local environment can positively impact the resultant electrochemistry.4


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Fe3O4 undergoes intercalation and conversion processes during reduction with a theoretical eight molar equivalents of lithium per Fe3O4.5 The intercalation and conversion mechanisms were previously examined using several techniques including electrochemistry6, 7, transmission electron microscopy (TEM)5, x-ray diffraction (XRD)6, 8, magnetic measurements6 and x-ray absorption spectroscopy (XAS).5, 6 Notably, the reported capacity of Fe3O4, often exceeds the theoretical value where the additional capacity has been typically ascribed to electrolyte reduction to form a surface electrolyte interphase (SEI).9-11 Direct evidence for formation of the SEI is challenging as it is a thin ( 0.95. During the entropy measurements the OCP values were measured over the course of 68 hrs after each discharge interval. The voltage profiles recorded over 68 hrs were used to extrapolate from 68


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hrs to 500 hrs of rest time to estimate the fully relaxed potentials and were similar to those previously reported for Fe3O4.49

The extrapolated OCP values were used to calculate

polarization contributions to heat flow during discharge. Polarization heat flow was calculated at each increment as the difference between loaded voltage and OCP multiplied by discharge current.

2.2.3 Electrochemical Impedance Spectroscopy (EIS)

Electrochemical impedance spectroscopy (EIS) was measured at 0.2 and 4.0 electron equivalents during discharge using a Bio-logic potentiostat in the frequency range of 1 MHz-10 mHz with a 5 mV sinusoidal amplitude at 30 °C. The spectra were fit using an equivalent Randles circuit and Zview software.

2.2.4 Scanning Electron Microscopy (SEM)

Discharged Fe3O4/PVDF electrodes were recovered from coin cells to observe structural and morphology changes with a high-resolution scanning electron microscope (JEOL 7600F) operating at an accelerating voltage of 10 kV. Electrodes were discharged to 4.0 and 10.0 electron equivalents. Discharged samples were prepared by disassembling each cell in an Arfilled glovebox. The electrodes were washed with anhydrous dimethyl carbonate (DMC) to remove lithium salts from the electrolyte. Electrodes were directly attached to adhesive carbon tape then transferred to the SEM vacuum chamber.


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2.2.5 Operando X-ray Absorption Spectroscopy

A pouch-type cell was fabricated with a Fe3O4/PVDF (90/10) working electrode, polyethylene separator, Li metal anode and 1 M LiPF6 in EC/DMC (30/70) electrolyte. The cell was discharged at a rate of 64 mA/g to 0.03 V. The rate was determined for the XAS measurements based on time available for the synchrotron experiment. Figure S2 provides comparisons of the discharge profiles for the higher areal loading coating in the pouch and coin cell at 64 mA/g to verify that the pouch cell discharged similarly to a coin cell. Figure S2 also overlays the discharge profile and heat flows from the lower areal loading coating used in coin cells for IMC tests discharged at 40 mA/g. Operando X-ray absorption spectroscopy (XAS) measurements at the Fe K-edge (7112 eV) in transmission mode measurements were collected on the cell during discharge at beamline 8-ID at the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory. All samples were measured with a Fe metal foil reference simultaneously for correct energy alignment of individual spectra during data analysis. XAS spectra were collected at 17 points along the first lithiation of the Li/Fe3O4 cell. XAS spectra were aligned, averaged, and normalized using Athena.50, 51 The built-in AUTOBK algorithm was used to limit background contributions below Rbkg = 1.0 Å. Linear combination fitting (LCF) was performed using Athena. Three reference spectra were used to perform LCF: the undischarged cell, the cell lithiated to 1.9 Li per mole of Fe3O4 (Li1.9Fe3O4) and Fe metal foil. The oxidation state of Fe during lithiation was calculated at each point as the sum of the fractions of each phase multiplied by their respective oxidation states.


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Normalized spectra were fit utilizing Artemis with theoretical structural models created with FEFF6.51,

52

EXAFS modeling was conducted based on the inverse-spinel Fe3O4,53 rock-salt

FeO54 and body-centered cubic (bcc) Fe metal55 crystal structures. A k-range of 2 – 9 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 – 3.5 Å was used to fully encompass the first and second shells of |χ(R)| (Fourier transform of χ(k)).


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3. Results and Discussion

3.1 Electrochemistry and Isothermal Microcalorimetry (IMC)

Lithium based electrochemical cells containing magnetite (Fe3O4) with a crystallite size of 12 nm (determined by Rietveld refinement of the powder XRD patterns, Figure S3) were discharged under constant current. IMC heat flow was monitored during the electrochemical reduction of the Fe3O4 and the subsequent voltage recovery. Figure 1 shows the cell potential and IMC heat flow versus time for the lithiation of Fe3O4. Duplicate sets of data are shown in Figure S4 to note the reproducibility of the experiment. The reaction for lithiation of Fe3O4 is described below in the following steps.5

(1) Reaction of the first lithium equivalent where Li+ inserts into an interstitial octahedral (16c) site.

(‫଼)݁ܨ‬௔ (‫݁ܨ‬ଶ )ଵ଺ௗ ܱସ + ‫ ݅ܮ‬ା + ݁ ି → (‫)݁ܨ݅ܮ‬ଵ଺௖ (‫݁ܨ‬ଶ )ଵ଺ௗ ܱସ

(1)

(2) Reaction of the second lithium equivalent where the Li+ ion inserts into the Fe3O4 lattice, displacing the Li+ already situated in 16c sites where the two Li+ are redistributed between 8a, 48f and 8b interstitial tetrahedral sites.

(‫)݁ܨ݅ܮ‬ଵ଺௖ (‫݁ܨ‬ଶ )ଵ଺ௗ ܱସ + ‫ ݅ܮ‬ା + ݁ ି → (‫݅ܮ‬ଶ )଼௔/ସ଼௙/଼௕ (‫݁ܨ‬ଷ )ଵ଺௖/ଵ଺ௗ ܱସ

(2)


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(3) Reaction of the third and fourth lithium equivalents resulting in the formation of a Li2OFeO and Fe0 metal:

(‫݅ܮ‬ଶ )଼௔/ସ଼௙/଼௕ (‫݁ܨ‬ଷ )ଵ଺௖/ଵ଺ௗ ܱସ + 2‫ ݅ܮ‬ା + 2݁ ି → 2‫݅ܮ‬ଶ ܱ ∙ ‫ ܱ݁ܨ‬+ ‫ ݁ܨ‬଴

(3)

(4) Reaction of the final four lithium equivalents resulting in conversion to iron metal and Li2O:

2‫ ܱ݁ܨ‬+ 4‫ ݅ܮ‬ା + 4݁ ି → 2‫݅ܮ‬ଶ ܱ + 2‫ ݁ܨ‬଴

(4)

Thus, the overall reaction for lithium and Fe3O4 is (5):

‫݁ܨ‬ଷ ܱସ + 8‫ ݅ܮ‬ା + 8݁ ି → 3‫ ݁ܨ‬଴ + 4‫݅ܮ‬ଶ ܱ

(5)

Based on this reaction, the total changes in enthalpy (∆Hrxn), entropy (∆Srxn) and Gibbs free energy (∆Grxn) were estimated from standard heats of formation (Hf) and standard entropies (Sf) for Fe3O4 (s), Fe(s), Li(s) and Li2O(s) where the standard values used were taken from NIST.56 The method anticipates a release of 5,500 J per gram of Fe3O4 for full reaction of Fe3O4 with eight molar equivalents of lithium. The total experimental energy released from the cell can be calculated by computing the electrical work (W) and the total heat output measured by the microcalorimeter (Q). Electrical work was determined by integrating the cell potential with respect to capacity. Total heat output


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was computed by integrating heat flow with respect to time. The sum of heat and work exceeded the theoretical change in Gibbs free energy based on the calculated heats of reaction. Thus, the IMC data indicate that there are other processes generating heat in addition to lithiation of Fe3O4, Table 1. The observation that the sum of heat and work was greater than the theoretical change in Gibbs free energy for the Fe3O4 lithiation process is consistent with the electrochemical discharge data where the delivered capacity exceeded the theoretical value of eight molar equivalents of lithium per mole of Fe3O4.

Figure 1. (A, upper) Potential of a Li/ Fe3O4 cell under 40 mA/g discharge to 0.03 V followed by potential recovery during rest. (A, lower) Heat flow during discharge in units of mW per gram


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of Fe3O4. The inset is an enlargement of the heat flow during the first 7 hrs of discharge. (B) Li/ Fe3O4 cell potential with respect to electron count.

“Exo” label notes that negative values are

exothermic and heat is leaving the battery.

Theoretical Capacity (mAh/g) 926 Capacity (mAh/g) 1110

Capacity (Li equivalents) 8.0 Capacity (Li equivalents) 9.6

∆Hrxn (J/g)

T∆Srxn (J/g)

∆Grxn (J/g)= ∆Hrxn-T ∆Srxn

4.844

-5508

Q, Heat (J/g)

W, Work (J/g)

∆HT (J/g)= Q-W

-4162

3099

-7261

-5503 Measured

Table 1. Theoretical capacities are based on the complete reaction of Fe3O4 and Li0 to form Fe metal and Li2O. Enthalpy (∆Hrxn), entropy (∆Srxn) and Gibbs free energy (∆Grxn) of the reaction were calculated based on the heats of formation for Fe3O4, Fe, and Li2O from NIST values. Data listed in the bottom of the table shows the measured experimental results.

Interpretation of the heat flow measured by IMC is based on the thermodynamics of a battery. The first law of thermodynamics governs batteries where changes in internal energy over time (∆U/dt) produce heat (Q̇) and electrical work (Ẇ) through electrochemical reactions, phase changes and mixing processes.

∆௎ ௗ௧

=

∆ு೟೚೟ ௗ௧

= ܳሶ − ܹሶ

(6)

An ideal battery converts all chemical energy into electrical work, but practical batteries have inherent heat losses. The change in internal energy (∆U) is equal to the change in enthalpy (∆Htot) for a system with constant volume, such as a sealed coin cell. The general energy balance


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for batteries was further developed to show ∆Htot/dt consists of contributions from enthalpies of reactions, enthalpies of mixing, phase changes and heat capacity.57, 58 In many thermal analyses of batteries the enthalpy of mixing and enthalpy of phase change are considered negligible, and the energy balance can be simplified to only include the enthalpy of reaction and heat capacity terms.57 The maximum amount of work that a cell can produce is the reversible work, which would occur if the cell was discharged at an infinitely small current. Reversible work can be estimated as the cell’s open-circuit potential (Eocp) during discharge multiplied by the current (I). Entropic heat transfer can be determined from the change in Eocp with respect to temperature multiplied by the temperature of the cell and current. For the IMC experiments reported here, temperature (T) is held constant, so there is no need to consider heat capacities, thus the overall energy balance can be written as38:

ௗா೚೎೛ ܳሶ = ‫ܫ‬൫‫ܧ‬௟௢௔ௗ − ‫ܧ‬௢௖௣ ൯ + ‫ ܫ‬ቀܶ ቁ + ܳ௣ሶ ௗ்

(7)

In this equation, there are three main contributions to heat flow: polarization heating, entropic changes and side reactions. Polarization heating arises from the internal battery resistance and is observed by the cell overpotential (Eload - Eocp) where the contributors to cell resistances are ohmic losses, charge-transfer overpotential, and mass-transfer limitations. Entropic heating ‫ܶ(ܫ‬

ௗா೚೎೛ ௗ்

) arises from changes in entropy as a result of the reactions. The final term, Q̇p, is heat

flow from side or parasitic reactions at the anode and/or cathode. This heat contribution is typically a result of SEI formation or other parasitic reactions.

These contributions are

considered in analysis of the results.


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3.3 Faradaic Reaction

The observed potential plateaus of the Li/Fe3O4 cell corresponded to distinct reduction processes. The potential range from open circuit potential to 0.86 V encompasses the first two Li+ insertion steps represented by reactions (1) and (2). A closer inspection of the heat flows over the potential range from OCP to ~1.5 V revealed a bimodal peak, which indicates a two-step process (inset Figure 1a). This observation is consistent with the reported lithiation mechanism where the first Li+ inserts into 16c sites until a saturation limit is reached, followed by migration of (Fe)8a ions to also occupy 16c sites. Thus, it is reasonable to attribute the two processes observed in the heat flow to lithium insertion and the subsequent Fe migration.5 Following the bimodal peak in heat flow there was a trough in heat flow which represents the conclusion of the first lithation and the start of the next step in the reaction. When the cell potential reached the plateau at ~1.1 V, the heat flow increased to a maximum of ca. -40 mW/g with three inflections in the heat flow profile (Figure 1 inset from 2-7 hrs). This portion of discharge was consistent with the mechanism in reaction (2) where additional Li+ ions insert into tetrahedral (8a) sites where the already present Li+ ions vacate their octahedral (16c) positions and redistribute between tetrahedral (8a/48f/8b) sites. The migration of Li+ from 16c sites allows additional Fe ions to redistribute between 16d and 16c sites. The potential plateau at ~0.86 V is attributed to the conversion of Li2Fe3O4 to Fe metal and Li2O in reactions (3) and (4). Equation (3) describes this section as the insertion of Li+ to form FeO·Li2O and Fe0 domains.

3.4 Heat Flow due to Polarization


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Cells were discharged in short increments at a rate of 40 mA/g where cell overpotential and entropic changes were measured after each increment. Polarization heating was calculated as the current (mA/g) multiplied by overpotential (Eload-Eocp). Voltage profiles and polarization heat flow are shown in Figure 2. The data presented in Figure 2 are the averages of two cells and the profiles for each individual cell are provided in Figure S5. Polarization was responsible for the majority of the total heat flow output from the cells with heat flows that ranged between 10 and 40 mW/g. Conversion electrodes are often noted for their large overpotentials due to the slow reaction kinetics of the systems.59 This leads to a significant difference in the loaded voltage between lithiation and delithiation described as voltage hysteresis. While polarization may be minimized at low rates, it is not eliminated. The cells for this study were discharged at a relatively slow rate of C/25, yet polarization remained a significant factor. Prior electrochemical studies have reported that conversion electrodes have an inherent overpotential (~0.65 V) even at very small currents.60, 61 For example, first-principles studies of a spinel based CuTi2S4 conversion electrode showed the source of the large hysteresis is primarily caused by the difference in the mobility between Li+ and the transition metal ions.62 In that study the copper diffusion coefficient was approximately ten orders of magnitude lower than lithium diffusion and the migration barrier for Cu (~0.9 eV) was significantly higher than Li (0.3-0.4 eV). This study suggests that the observed heat dissipation for lithiation of Fe3O4 at C/25 can be attributed to the slow mobility and greater migration barriers of Fe ions and is consistent with prior observations of conversion electrodes.

3.5 Entropic Heat Flow


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Entropic contributions to heat flow during discharge were determined by the temperature dependence of OCP (Figure 2, bottom). The entropic term was calculated as I*T*dE/dT, where I is current (40 mA/g), T is temperature (303.15 K) and dE/dT is the linear change in OCP with temperature. An example set of data illustrating the changes in OCP with respect to temperature and the fitting method used is shown in Figure S1. This entropy value is the partial derivative of the total entropy of the Li/Fe3O4 cell with respect to the amount of charge passed. The negative entropy value indicates that the transfer of an incremental amount of charge decreases the possible distribution of energy states.63 While the entropy is always negative during lithiation, the value is nonlinearly dependent on depth of discharge (DOD). In the pristine state the empty Fe3O4 lattice should be well ordered (few possible energy states). Initially, entropy became less negative during the potential region (3.0 V to 1.5 V) consistent with more possible arrangements with the introduction of Li+ in interstitial sites. Entropy then became more negative during the second lithiation step (1.5 V to 1.1 V) indicating fewer energy state configurations which is consistent with the previously reported phase change reaction mechanism.5 However, over the second half of discharge the entropy steadily becomes less negative as Fe3O4 approaches full lithiation.


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Figure 2. Cells were discharged in increments to determine changes in polarization and entropy during discharge. (Top) cell potential during discharge. Upticks in potential indicate potential recovery under zero current. (Middle) Polarization is the difference in loaded and open circuit potential multiplied by current (40 mA/g). (Bottom) Entropic contribution to heat.

3.5 Consideration of Heat Flow from Side Reactions

Polarization and entropic contributions to heat flow were summed and overlayed with heat flow data measured by IMC, Figure 3. The entropy and polarization heat flow data in Figure 3 delivered greater capacity than the IMC data due to the long intermittent rest steps (68 hr)


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required for the entropy and polarization measurements. Entropic and polarization contributions to heat flow closely resembled the experimental data measured from the IMC during the first 2 molar equivalents of lithiation, usually recognized as the Li+ intercalation stage of reduction (Equations 1 and 2). Polarization and entropic contributions deviate from IMC data at ~0.86 V (2 molar equivalents of lithiation). The difference between the measured and computed heat flows persists throughout the ~0.86 V plateau and then the separation between the two curves increases during the potential decay to 0.03 V (~6.5 molar equivalents).

Figure 3. Comparison of experimental data from the microcalorimeter (red, solid line) and modeled data based on polarization and entropic changes during discharge (blue, connected-dot line).

IMC heat flow and potential during delithation (charge) are provided in Figure S6. Exothermic heat flows were produced throughout charge culminating in a maximum in heat flow as the cell approached 3.0 V. Polarization and entropic contributions to heating during delithiation were plotted versus the corresponding charging voltage profile in Figure S7, while the sum of the polarization and entropic heating contributions were plotted versus the total heat flow observed via IMC in Figure S8. We observed that summing the entropic and polarization heating did not


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account for total heat evolved, particularly during the early stages (98%), suggesting minimal conversion to Fe metal between 1.9-2.7 electrons. Between 2.7-8.3 electrons, the emergence of the Fe metal phase was observed and concurrent with a decrease in Li1.9Fe3O4 phase intensity. The average Fe oxidation state was calculated based on the compositions determined by LCF and were overlayed with the discharge voltage profile in Figure 7B. Through the first 1.9 electron equivalents of discharge, the data followed the predicted trend with Fe reducing linearly with measured electrochemical electron transfer. However, from 1.9-2.7 electrons the average Fe oxidation state remains at a constant oxidation state of about +2. Notably, at this same region, the IMC heat flow deviated from the predicted heat flow (sum of polarization and entropic contributions). The unchanging position of the Fe K-edge supports the premise that reactions initiating at 1.9 electrons at ~0.86 V proceeded with minimal reduction of Fe and suggest the onset of electrolyte reduction. Upon further lithiation from 2.7-5.4 electrons, Fe oxidation state decreased as the Li1.9Fe3O4 phase was converted to Fe0 and Li2O. In this region the difference between IMC and predicted heat flow remained relatively consistent, suggesting that Fe2+→Fe0 and electrolyte reduction occurred simultaneously. In the final region of the discharge, from 5.48.3 electrons, the iron oxidation state continued to decrease as additional Fe metal was generated. Meanwhile the IMC heat flow reached its peak (-50 mW/g), deviating from the polarization and entropic contributions by >20 mW/g. This increase in heat flow may also be attributed to side reactions with the electrolyte in this region.


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Figure 7. A) Phase composition at each point along lithiation determined LCF of XANES data. B) Operando cell potential as a function of capacity overlaid with the average Fe oxidation state calculated from LCF. The grey dashed line indicates a linear change in Fe oxidation state where one electron would reduce one iron center by one oxidation state and 8 electron equivalents reduces Fe3O4 to Fe0.

In addition to the LCF analysis of the XANES spectra, EXAFS spectra were modeled using Artemis to rigorously determine changes in the local atomic structure of Fe atoms during the lithiation process. Details of the fitting procedures are given in the experimental section. Results are presented in Figure 8 as the normalized amplitude of Fe3O4, FeO, and Fe0 phases determined


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from the spectral fits. Normalized amplitudes were calculated by dividing the coordination number determined from the fitting result by the expected coordination number in the bulk phase. Full EXAFS theoretical modeling results are presented in the supplemental information (Tables S1 – S5 and Figures S10 – S12).

The reported amplitudes of each phase were

significantly lower than the expected amplitudes for the bulk material due to small crystallite size with high ratios of surface to bulk neighboring atoms, which results in lower observed average coordination number from EXAFS modeling.67 The initial EXAFS spectra for the non-discharged state were fit using a theoretical model of the inverse spinel Fe3O4 structure.67, 68 This model produced excellent fits with (R-factor < 2.0) for spectra corresponding to lithiation states through 1.1 electrons. During this initial lithiation region, there was a clear increase in the fitted interatomic distances between neighboring atoms, particularly between octahedral-coordinated Fe and neighboring O atoms (from 1.97 ± 0.02 Å to 2.04 ± 0.03 Å, Figure S12), consistent with volumetric expansion as Li+ ions intercalate into the crystal structure. By 1.5 electron reduction, the EXAFS spectrum could no longer be modeled using the inverse spinel Fe3O4 crystal structure, rather an FeO crystal model with all Fe atoms in octahedral coordination sites was adopted to produce fits with sufficiently low R-factor. Thus, by 1.5 electron equivalents of reduction, the lithiated iron oxide phase was more similar to that of a FeO-like structure, in good agreement with previous finding that by 2.0 electrons all tetrahedral-coordinated Fe atoms have migrated to octahedral sites69 (Equation 2). Upon additional lithiation through 3.1 electron equivalents, there was a decrease in the amplitude of the FeO-like phase that may indicate a reduction in particle size. Notably, the formation of Fe0 metal was not observed from 1.9 - 3.1 electron equivalents, in good agreement with the LCF results that showed minimal change in oxidation state in this region and suggesting that initial formation


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of the solid electrolyte interphase accounted for the electrochemical reduction in this region. At 4.2 electron equivalents, the first formation of Fe0 metal phase was observed from the EXAFS modeling. For the remainder of the lithiation process to 0.03 V, the Fe0 phase consistently increased in intensity and coincided with the loss of FeO phase intensity. The growth of Fe0 metal as the primary phase also corresponded to the large increase in heat generation starting at ca. 5.4 electrons. The Fe0 metal nanograins were anticipated to be on the order of 1 – 2 nm in size based on previous TEM results.67,

69

Notably, nanosized metal particles formed from

reduction of metal oxides have been reported to be catalytic in nature.70-73 Thus, it is possible that the formed Fe nanoparticles may facilitate decomposition of the electrolyte, contributing to the high heat flows at low voltage.

Figure 8. Normalized amplitude of Fe3O4, FeO, and Fe0 phases determined from EXAFS fit results of operando XAS measurements performed on a Li/Fe3O4 cell discharged at 64 mA/g to 0.03 V. Normalized amplitudes were calculated by dividing the coordination number determined from the fitting result by the expected coordination number in the bulk phase.


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The IMC, SEM, EIS, XANES, and EXAFS analyses reported here are consistent with SEI formation initiating at the 0.86 V plateau (~2.0 electron equivalents), causing the differences between the predicted and measured heat flow. Further, the data indicate that as the conversion reaction proceeds and Fe0 metal becomes the principal phase in the discharged electrode, the heat flow increased, resulting in a large deviation from the predicted heat based on entropy and polarization contributions. The electrochemical and thermodynamic analyses suggest that side reactions contribute to the observed capacity and heat flow during the first lithiation of Fe3O4. Notably, previous studies indicate SEI formation in conversion electrodes initiates ~0.8 V vs Li/Li+10,

11, 74

consistent with the observations here.

Based on previous literature, it is

hypothesized that these side reactions are primarily due to electrolyte reduction at the surface of the Fe3O4 (or its reduced form) with the corresponding formation of a solid electrolyte interphase (SEI) layer.9, 75-77 The related IMC and XAS data presented here provide clear evidence of the SEI contribution to capacity in this voltage region.

4. Conclusion

Heat flow during lithiation of Fe3O4 was measured using an isothermal microcalorimeter (IMC) and compared to predicted heat contributions from heat of formation, polarization and entropy. The total heat and work produced from lithiation of Fe3O4 (7,260 J/g Fe3O4) exceeded the heat of reaction calculated from standard heats of formation (5,508 J/g Fe3O4) and is reflected by a delivered capacity (9.6 molar lithium equivalents) in excess of the theoretical 8 molar lithium equivalents for full lithiation of Fe3O4. The heat flow from initial lithiation of Fe3O4 (3.0 V to 0.86 V) could be successfully modeled using only polarization and entropic contributions.


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The heat from over the voltage range of 0.86 V to 0.03 V could not be modeled using only polarization and entropic contributions. In order to gain direct insight on the reduction process, operando x-ray absorption spectroscopy (XAS) was used to probe the lithiation of Fe3O4. Prior to 0.86 V the Fe oxidation state decreased linearly in agreement with coulombs transferred as would be predicted if the reduction of the iron centers was the dominant electrochemical mechanism. Once the cell voltage reached 0.86 V, the decrease in Fe oxidation state deviated from the predicted line indicating an alternative concomitant reduction process. Notably, the IMC cell reached its highest heat flow (50 mW/g) over the potential region from the 0.80 V to 0.03 V. This potential region showed the greatest deviation from heat flow modeled by entropy and polarization. The additional capacity and heat flow are thus ascribed to electrolyte reaction with associated SEI formation initiating at 0.86 V. Operando XAS supported the onset of a secondary reaction at 0.86 V. The use of IMC in conjunction with operando XAS was able identify the onset of SEI formation and the associated increase in heat output.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Supplemental equations and derivations of battery heat flow; voltage recovery profile and temperature dependence of open-circuit potential (Figure S1); electrochemistry comparison of various cell types and discharge currents used for XAS and IMC measurements (Figure S2); Xray diffraction pattern of synthesized Fe3O4 (Figure S3); IMC and electrochemistry data for duplicate cells (Figure S4); polarization and entropic heating contributions during lithiation measured from two cells (Figure S5); IMC and electrochemistry data for delithiation of fully


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lithiated Fe3O4 (Figure S6); polarization and entropic heating contributions measured during delithiation (Figure S7); comparison of IMC and sum of polarization and entropic heating during delithiation (Figure S8); voltage recovery over 500 hr after cells charged to various delithaited states (Figure S9); coordination numbers determined from EXAFS fit with nearest neighboring Fe-Fe paths (Figure S10) and Fe-O paths (Figure S11); interatomic distances determined from EXAFS fit (Figure S12); combined R-factor from EXAFS fits (Table S1); fitted E0 from EXAFS fits (Table S2); Debye-Waller factor and E0 EXAFS fits (Table S3); interatomic distances determined by EXAFS fits (Table S4); coordination numbers for Fe3O4 determined from EXAFS fits (Table S5)

Corresponding Authors *Email address: [email protected] *Email address: [email protected] *Email address: [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.

Acknowledgement


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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. M.M.H. is supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. 1109408. This research used resources of beamline 8-ID Inner-Shell Spectroscopy (ISS) 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. This research 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.

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