Insight into Lithium Diffusion in Conversion-Type Iron Oxide Negative

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Insight into Lithium Diffusion in Conversion-Type Iron Oxide Negative Electrode ́ Bingbing Tian, Jolanta Swiatowska,* Vincent Maurice, Catarina Pereira-Nabais, Antoine Seyeux, and Philippe Marcus Chimie ParisTech - CNRS, Institut de Recherche de Chimie Paris, PSL Research University, 11 rue Pierre et Marie Curie, 75005 Paris, France ABSTRACT: In situ electrochemical (by electrochemical impedance spectroscopy, EIS) and ex situ surface (by time-of-flight secondary ions mass spectrometry, ToF-SIMS) analysis were applied to investigate solid-state diffusion coefficient (DLi) into conversion-type α-Fe2O3 negative electrode for Li-ion batteries. DLi values obtained from EIS were in the range of 10−16 to 10−15 cm2 s−1 for electrodes partially and fully lithiated, respectively, showing that pulverization of the converted material promotes Li-ion migration. ToF-SIMS ion depth profiling performed after partial lithiation enabled discriminating the surface solid electrolyte interphase (SEI) region, a converted electrode region (Li2O/Fe0 matrix) of slow diffusion (DLi = 6 × 10−16 cm2 s−1) and an unconverted region (intercalated Fe2O3 matrix) of faster diffusion (DLi = 2 × 10−13 cm2 s−1) ahead of the conversion front. Comparison of the ex situ and in situ results indicates that the electrode conversion kinetics is limited by Li-ion diffusion in the converted matrix and suggests a hindering effect of the passivating SEI layer. ToF-SIMS depth profile analysis appears as a most appropriate and direct methodology to measure Li-ion diffusion solely in electrode materials, excluding SEI layer effects. technique (PRT).19 However, for conversion-type material (e.g., iron oxide), ionic migration has been rarely studied due to lack of physical model accounting for the phase transition associated with the (de)conversion (delithiation) reaction. It is generally believed that the kinetic regime is a conversioncontrolled rather than a diffusion-controlled process. In our previous work, conversion was shown to proceed mostly in the outer part of the iron oxide thin film electrode during the first lithiation owing to mass transport limitation,20 which was also observed for conversion-type Cr2O3 thin films.21 In this case, Li+ ions migration primarily through the SEI and the converted matrix (Li2O/Fe0), and through the unconverted bulk oxide material (Fe2O3) ahead of the conversion front, can be assumed as a primarily one-dimensional diffusion process. Here, we report on the in situ EIS and ex situ time-of-flight secondary ions mass spectrometry (ToF-SIMS) methods employed for evaluating the apparent diffusion coefficients of lithium (DLi) in order to better understand the lithiation kinetics of conversion-type iron oxide. α-Fe2O3 thin film electrodes were prepared by thermal oxidation of pure iron substrate. The application of thin film electrodes with large surface-to-volume ratio and without carbon and polymeric binder additives can obtain clearer insight into ionic transport in iron oxide electrode material and thus also bulk and

1. INTRODUCTION The application of transition metal oxides as negative electrode materials in lithium-ion batteries (LiBs) depends on their electrochemical performances, which are associated with electrode reactions, interphase chemistry and ion mobility (i.e., lithium diffusion).1,2 As nanosized particles, such materials (CoO, Co3O4, NiO, CuO, Cu2O, and FeO) can exhibit reversible capacities up to three times higher than commercially used graphite anodes as reported by Poizot et al.3−5 Among them, hematite iron oxide (α-Fe2O3) is one of the most interesting and important candidate, for its high theoretical capacity (1007 mAh g−1), abundance and low cost, low toxicity, and environmental friendliness. Since reported as a conversiontype material (Fe2O3 + 6Li ↔ 3Li2O + 2Fe),6,7 it has been shown to suffer from poor electronic/ionic conductivity,1,8 the main obstacle for improving rate capability which is one of a primary demands of LiBs. Relatively little attention has been devoted to understanding the diffusion processes in which Li+ ions migrate through the solid electrolyte interphase (SEI) layer and into the bulk electrode material during the discharge/charge process and the determination of the apparent diffusion coefficient of lithium (DLi). Li+ ions migration into graphite (intercalation-type) and silicon (alloying-type) anodes has been studied by various techniques including electrochemical impedance spectroscopy (EIS),9−14 cyclic voltammetry (CV),12,15,16 potentiostatic intermittent titration technique (PITT),10,12,17 galvanostatic intermittent titration technique (GITT),12,18 and potential relax © XXXX American Chemical Society

Received: October 11, 2014 Revised: December 11, 2014

A

DOI: 10.1021/jp510269e J. Phys. Chem. C XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL SECTION Iron foil (Goodfellow, purity, 99.5 wt %; thickness, 0.05 mm) was used as a substrate and polished with diamond spray down to 0.25 μm, then successively rinsed in acetone, ethanol and Millipore© water (resistivity >18 MΩ cm) with ultrasonic cleaning for 2 min, and dried in a compressed air flow. This asprepared iron foil sheet was annealed at 300 °C for 5 min under air atmosphere and then the reaction tube was quenched with 0 °C water.20 Then the oxidized sheet was cut into 8 × 8 mm2 squares providing six identical samples for later use. The average thickness of the oxide thin films was 125 nm as evaluated by profilometry (Veeco Dektak 150 Profilometer) after ToF-SIMS depth profiling on the pristine sample. Raman spectroscopy (Horiba Xplora system, Ar+ laser, λ = 532 nm) was employed for phase identification of the thermal oxide. The electrochemical measurements (galvanostatic discharge−charge, EIS) were performed in a glovebox (Jacomex) under Ar atmosphere with H2O and O2 contents lower than 1 ppm. A three-electrode glass half-cell was used with the iron oxide film as working electrode and Li foil (Sigma-Aldrich) as reference and counter electrodes. All potentials hereafter are given versus Li/Li+ reference electrode. The electrochemical measurements were performed at room temperature using an Autolab (AUT30) electrochemical workstation. The working electrode area was delimited to 0.28 cm2 by an O-ring. The electrolyte was 1 M LiClO4 in propylene carbonate (1 mol L−1 LiClO4/PC, Sigma-Aldrich). Galvanostatic discharge−charge was performed at a current density of 10 μA cm−1 in the potential range of 3.0−0.01 V. The frequency range of the EIS measurements was 100 kHz to 10 mHz with a low ac voltage amplitude of 5 mV. The cell was kept at selected potential values of the first discharge/charge cycle and the EIS was performed at steady state once the voltage change was less than 0.01 V in 10 min. For ToF-SIMS analysis, the thin film electrode was electrochemically treated at the potential of interest. Then the cell was disassembled, the sample was rinsed with acetonitrile (99.8%, Sigma-Aldrich), dried with Ar flow, and transferred under Ar atmosphere from the glovebox to the ultrahigh vacuum ToF-SIMS analysis chamber. Ion depth profiles were acquired using a ToF-SIMS 5 (Ion Tof - Munster, Germany) operating at about 10−9 mbar. A pulsed 25 keV Bi+ primary ion source was employed for analysis, delivering 1.2 pA current over a 100 × 100 μm2 area. Depth profiling was carried out using a 1 keV Cs+ sputter beam giving a 70 nA target current over a 300 × 300 μm2 area. The Ion-Spec software was used for data acquisition and processing.

Figure 1. (a) Raman spectra, (b) XP survey spectra, and (c) XP Fe 2p spectra before and after oxidation of pure iron foil.

However, the higher O intensity and complete attenuation of the sharp low binding energy Fe 2p peak corresponding to metallic iron (Fe0) observed for the thermally oxidized sample (Figure 1c) indicate modification of the surface by growth of a thick oxide layer (125 nm as measured by profilometry). ToF-SIMS negative ion depth profiles of the sample after oxidation are shown in Figure 2a. The intensity of selected secondary ions is plotted in logarithmic scale versus the sputtering time. The Fe-containing secondary ions (FeO− and FeO2− characteristic for the oxide and Fe2− characteristic for the metal) distinguish three regions: the iron oxide thin film bulk region (mostly Fe2O3 as shown by Raman), the metallic iron substrate region, and the interface region between oxide film and metal substrate. Electrode modifications induced by lithiation (Figure 2 b) are discussed hereafter. 3.2. Galvanostatic Discharge−Charge. In the first discharge (lithiation process), the slope between ∼1.9 and

3. RESULTS AND DISCUSSION 3.1. Structure and Composition. Structure and surface composition of the iron foil samples before and after oxidation are shown in Figure 1. Figure 1a shows the Raman spectra in the range of 100−2000 cm−1. There is no detectable crystallized oxide phase on the metal iron surface before oxidation. After oxidation, the surface oxide was crystallized to α-Fe2O3 (hematite) with a small amount of Fe3O4 (magnetite) and FeOOH as discussed previously.20,22−24 The XPS survey spectra (Figure 1b) before and after oxidation both demonstrate that only Fe, O, and C elements are present. B

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the open circuit potential (OCP), the potential at the slope of first discharge (1.2 V), the potential at the plateau of first discharge (0.84 V, also selected for ToF-SIMS analysis), the end point of first galvanostatic discharge (0.01 V), the potential at the plateau of first charge (1.7 V), and the end point of first galvanostatic charge (3.0 V). 3.3. Diffusion from in Situ EIS Analysis. EIS was performed at all selected potentials indicated in Figure 3 but due to difficulties related to nonstationary conditions at OCP, 1.2 and 3.0 V, the EIS results obtained at these potentials were discarded from analysis. Only the three spectra obtained for partial lithiation to 0.84 V, full lithiation to 0.01 V, and partial delithiation to 1.7 V during the first discharge/charge shown in Figure 4a were analyzed. As already discussed in the

Figure 2. ToF-SIMS negative ion depth profiles of (a) the pristine sample after oxidation, and (b) the sample discharge to 0.84 V after 5591 s of lithiation.

∼0.9 V corresponds to the early stage of lithiation leading to formation of the LixFe2O3 (0 < x ≤ 2) intermediate by lithium intercalation and the plateau at ∼0.84 V to both conversion of iron oxide to Li2O/Fe0 and to formation of the SEI layer by reductive decomposition of electrolyte (Figure 3).6,7,20 In the

Figure 4. (a) EIS spectra at different lithiation stages after potential stabilization; (b) relationship between real impedance (Z′) and radial frequency (ω−1/2) at different lithiation stages of the first discharge− charge cycle.

literature,26−29 different regions can be distinguished in electrochemical impedance spectra as a function of frequency: the high frequency region (semicircle) relates to the resistance corresponding to Li+ migration through SEI layer, the medium frequency region (semicircle) corresponds to the charge transfer resistance between SEI layer and electrode, and the low frequency region is attributed to Warburg impedance (diffusion of Li+ in the bulk electrode) and insertion capacitance (accumulation of Li+ in the electrode). The ohmic resistance (RΩ = Rel + Rct) increases as a function of discharge potential (from 0.84 to 0.01 V), which is clearly observed by the appearance of a semicircle (Figure 4a inset).

Figure 3. First galvanostatic discharge and charge cycle of the iron oxide thin film electrode obtained at a current density of 10 μA cm−2 in a potential window of 3.0−0.01 V.

charge (delithiation process), the region from ∼1.0 to ∼2.0 V corresponds to the reversible oxidation of Fe0 to Fe3+. These results are consistent with previously reported in detailed CV measurements.20,25 The points on the discharge/charge curves mark the different stages of lithiation/delithiation selected for the EIS measurements presented hereafter. They correspond to C

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are similar to those previously reported from EIS data obtained on carbonaceous materials,10,27,28,34−36 silicon oxycarbide,37 silicon films,38,39 Cu6Sn5,40 and metal oxides (i.e., nanosized rutile TiO241 and β-MnO242). EIS can be considered as an in situ method for evaluation of kinetics processes. However, it must be emphasized that EIS analysis can be affected by other phenomena including formation of the SEI layer, as shown by comparison with the ToF-SIMS data presented hereafter. 3.4. Diffusion from Ex Situ ToF-SIMS Analysis. ToFSIMS has already been applied to study Li transport in the SEI layer formed on the graphite negative electrode43,44 in silicon negative electrode material 45 and in positive LiFePO 4 electrode.46 In the present work, the diffusion coefficient of Li ions in host Fe2O3 was estimated from the in-depth variation of Li-ion concentration in the thin film iron oxide electrode discharged to 0.84 V (after 5591 s of lithiation). Figure 2b shows the ToF-SIMS negative ion depth profiles for this partially lithiated (converted) electrode. The first region (marked “SEI + interface”) corresponds to the SEI layer grown by reductive decomposition of the electrolyte on the electrode surface. In this region, the intensities of the Li− and LiO− ions increase up to a maximum corresponding to the interface (at ∼60 s of sputtering) with the partially converted iron oxide thin film. The high intensity of the Li− ions at the SEI/electrode interface may originate from increasing concentration of products of electrolyte decomposition containing Li-rich compounds in the inner SEI layer as well as from the Li2O product of the conversion reaction.6,7,20 It is obvious that the iron signal (Fe2−) is not detected in this region, showing full coverage on the electrode by the SEI layer. The second region, extending from ∼60 to ∼400 s of sputtering, corresponds to the lithiated iron oxide thin film. In this region, the intensities of the Li− and LiO− ions decrease from their maxima at the interface with the SEI to reach a plateau before reaching the interface with the substrate while those of the FeO− and FeO2− ions increase to reach their maximum. This defines a converted outer part of the oxide film, assigned to Li2O/Fe0, and an unconverted inner part, assigned to LixFe2O3 (0 < x ≤ 2), which is in agreement with previous results20 and confirms partial conversion of the thin film electrode in these conditions of lithiation. Beyond ∼400 s of sputtering, the iron metal substrate region is reached. The increase of the sputtering time to reach this region is indicative of the volume expansion of the converted electrode also in agreement with previous data.20 The iron substrate is characterized by the fall of the FeO− and FeO2− ion intensities and a maximum intensity plateau of the Fe2− ion intensity, like for the nonlithiated pristine electrode (Figure 2a). Figure 5a shows the depth profile of Li− ion retrieved from Figure 2b (linear scale). The same regions are marked. In order to get the in-depth variation of concentration as a function of sputtering depth (Figure 5b), the sputtering time was converted into depth using a calibrated value of the sputtering rate (∼0.37 nm s −1). The first ∼60 s of sputtering corresponding to the Li ions profile in the SEI layer regions were excluded and the point 0 nm (in Figure 5b) was set to the maximum Li− ion intensity measured at the SEI/electrode interface (at ∼60 s of sputtering in Figure 5a). The Li concentration was normalized to the maximum Li− ion intensity at the SEI/electrode interface.

This can be assigned to the thickness and morphology variation of the thin film electrode (electrode volume expansion) and the formation of the SEI layer, which is in agreement with previous results.20,29 However, it should be also noted that the impedance variations can be also influenced by the changes in surface chemical composition, like formation of more resistive/conductive (inorganic/organic) layer as a function of potential decrease. A large depressed semicircle (observed in Figure 4a inset at lower potentials) can be attributed to a nonuniform, porous microstructure30 of the electrode surface or to a formation of a multilayer SEI structure. During charge to 1.7 V, the semicircle decreases, which is in agreement with the reversible decrease of electrode volume and thickness of the SEI layer observed previously. In order to obtain the diffusion coefficient of Li ions (DLi) into the iron oxide electrode, only the low frequency region attributed to Warburg impedance was considered for analysis. The lithium diffusion coefficient can be deduced from the Warburg impedance, ZW, as follows Z W = σw(1 − j)ω−1/2

(1)

where ω is the radial frequency, and σw is the Warburg impedance coefficient. Figure 4b shows measurement of the real impedance (Z′) plotted versus radial frequency (ω−1/2) allowing determining σw. The diffusion coefficient of the lithium ions (DLi) into the electrode materials is then given by31−33 ⎛ RT ⎞2 ⎛ 1 ⎞2 ⎟ = 4.52 × 10−13⎜ D = 0.5⎜ 2 ⎟ ⎝ σwC ⎠ ⎝ AF σwC ⎠

(2)

where R is the gas constant, T is the absolute temperature, C is the molar concentration of Li+ ions (C (mol cm−3) = nLi (mol)/V (cm3) = [(I × t) C/1.6 × 10−19 C/6.02 × 1023 mol−1]/V (cm3) with V = 3.5 × 10−6 cm3), A is the apparent surface area (here the geometric electrode area, 0.28 cm2, is used for simplicity), and F is the Faraday constant. Relation 2 is valid only if semi-infinite diffusion conditions are fulfilled (ω = 2πf ≫ 2DLi/L2 where L is the finite length). The values of σw, C, and DLi at various electrode potentials are listed in Table 1. Table 1. Values of σw, C, and DLi at Various Electrode Potentials As Determined from EIS Data in the First Discharge−Charge Cycle potentials (V)

σW (Ω s−1/2)

C (mol cm−3)

DLi (cm2 s−1)

discharge to 0.84 V discharge to 0.01 V charge to 1.7 V

865.4 117.4 678.9

0.04112 0.09893 0.05328

3.6 × 10−16 3.4 × 10−15 3.5 × 10−16

With the increase of the Li concentration, an increase of DLi can be observed. This should be related to the volume expansion of the electrode during lithiation confirmed by ToF-SIMS, which probably causes pulverization of the thin film electrode and promotes Li-ion migration thus accounting for the observed variation of the diffusion coefficient. During the charge process, the DLi again decreases with Li extraction (delithiation). Similar DLi values were obtained at the lithiation (0.84 V) and delithiation (1.7 V) stages, suggesting a similar state of the thin film electrode regarding lithium quantity, volume modification (expansion/shrinkage), and surface passivation. The DLi values obtained from EIS in this study show variations as a function of lithiation/delithiation potential that D

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In the third innermost region of the thin film electrode, called “Li trace” and assigned to the interfacial region between the iron oxide thin film and the iron substrate, the best fit (green curve) is obtained with a much higher diffusion coefficient (DLi = 2.0 × 10−13 cm2 s−1). Because the iron oxide matrix is unconverted in this region, the much faster diffusion process would be related to the intercalation reaction of lithium into the α-Fe2O3 matrix ahead of the conversion front. In the second region, called “Li poor” and intermediate between the Li rich and Li trace regions, no good fit of the experimental profile can be obtained with a diffusion profile. This is assigned to the formation of a transition region between the Li rich (converted iron oxide) and Li trace (intercalated iron oxide) regions. Most likely, the kinetics of Li migration in this transition region is not only controlled by processes of diffusion (in the converted and unconverted iron oxide matrices) but also by the advancement of the conversion reaction front. In a homogeneous matrix, the concentration profile should yield one chemical diffusion coefficient independently of the position along the profile. However, in the present case the experimental data clearly require different values of DLi for fitting the different regions of the concentration profile. In the outermost converted part of the electrode, the extracted DLi value (6 × 10−16 cm2 s−1) is much lower than that in the innermost intercalated and nonconverted inner part of the electrode, indicating a slower diffusion process. The distinction between the two diffusion processes was possible most likely because the rate of the conversion reaction is faster than migration of the Li ions in the converted electrode. The methodology based on the ToF-SIMS analysis of partially lithiated thin film electrode material appears thus as decisive for making distinction between the different types of diffusion as a function of the lithiation penetration depth. Such a clear discrimination between different diffusion regions/processes is not possible in a case of calculation of diffusion coefficient by means of classical electrochemical methods. The ToF-SIMS measurement is a direct physical method of the ions concentration profiles in a thin film electrode, which enables to clearly identify different regions, and thus to exclude the surface passivation layer (SEI layer) from the data analysis. It can be then concluded that this method allows for a precise estimation of reaction kinetics in the electrode material. The DLi value (6 × 10−16 cm2 s−1) obtained in the first outermost region, called Li rich, from ToF-SIMS is consistent with that (3.6 × 10−16 cm2 s−1) obtained from EIS at the same lithiation potential (discharge to 0.84 V). The slightly lower value calculated from EIS may be explained by the presence of the SEI layer, which presence would hinder the ionic transport and influence the electrode kinetics. It appears thus that the SEI layer, although much thinner than the electrode film,49 should not be neglected in Li-ion diffusion evaluation due to its poor transport properties. This quite close DLi value obtained from the EIS measurement and ToF-SIMS profile at the same lithiation potential can be also explained by the fact that even the presence of other than an innermost and a middle region when doing EIS data acquisition, the slowest diffusion through the outermost region determines the speed of the overall process.

Figure 5. (a) ToF-SIMS depth profile of Li− ion of the sample discharge to 0.84 V after 5591 s of lithiation and (b) normalized indepth variations of the Li-ion concentration and fitting with diffusion profiles.

The apparent Li diffusion coefficient (DLi) in the thin film electrode could be derived from the infinite integration of Fick’s second law for one-dimensional diffusion47,48 CS − C(x , t ) CS − C 0

⎛ x ⎞ ⎟⎟ , with = erf⎜⎜ ⎝ 2 DLi t ⎠

x 2 DLi t

(3)

( −1)n z 2n + 1 (2n + 1)n!

(4)

z=

with error function erf(z) =

2 π

∫0

z

2

e −t dt =

2 π

∞

∑ n=0

considering the following boundary conditions: C (x = 0) = Cs = 1, constant, fixed; C (x = +∞) = C0 ≈ 0, corresponding to the original concentration of Li existing in the bulk phase. C0 remains constant in the far bulk phase at x = +∞. A development of the resolution of Fick’s second law was presented in details in previous paper.45 Using eqs 3 and 4, diffusion profiles were plotted to fit the experimental concentration profile obtained by ToF-SIMS (Figure 5b). The results show that a single diffusion profile cannot fit the experimental values. In the first outermost region, called “Li rich”, the best fit (red curve) with the experimental profile is obtained with DLi = 6 × 10−16 cm2 s−1, indicating a nearly ideal diffusion process. Because this outer region corresponds to the converted Li2O/Fe0 matrix, it can be concluded that the diffusion coefficient measured in this region is that of lithium in the converted electrode material. E

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(6) Larcher, D.; Masquelier, C.; Bonnin, D.; Chabre, Y.; Mason, V.; Leriche, J. B.; Tarascon, J.-M. Effect of Particle Size on Lithium Intercalation into α-Fe2O3. J. Electrochem. Soc. 2003, 150, A133−A139. (7) Larcher, D.; Bonnin, D.; Cortes, R.; Rivals, I.; Personnaz, L.; Tarascon, J.-M. Combined XRD, EXAFS, and Mössbauer Studies of the Reduction by Lithium of α-Fe2O3 with Various Particle Sizes. J. Electrochem. Soc. 2003, 150, A1643−A1650. (8) Gillot, F.; Boyanov, S.; Dupont, L.; Doublet, M.-L.; Morcrette, M.; Monconduit, L.; Tarascon, J.-M. On the Electrochemical Reactivity and Design of NiP2 Negative Electrodes for Secondary Liion Batteries. Chem. Mater. 2005, 17, 6327. (9) Thevenin, J. Passivating Films on Lithium Electrodes. An Approach by Means of Electrode Impedance Spectroscopy. J. Power Sources 1985, 14, 45−52. (10) Levi, M. D.; Aurbach, D. Diffusion Coefficients of Lithium Ions during Intercalation into Graphite Derived from the Simultaneous Measurements and Modeling of Electrochemical Impedance and Potentiostatic Intermittent Titration Characteristics of Thin Graphite Electrodes. J. Phys. Chem. B 1997, 101, 4641−4647. (11) Shin, H. C.; Cho, W., II.; Jang, H. Electrochemical Properties of the Carbon-Coated LiFePO4 as a Cathode Material for Lithium-Ion Secondary Batteries. J. Power Sources 2006, 159, 1383−1388. (12) Levi, M. D.; Aurbach, D. Frumkin Intercalation Isotherm - A Tool for the Description of Lithium Insertion into Host Materials: A Review. Electrochim. Acta 1999, 45, 167−185. (13) Guo, J.; Sun, A.; Chen, X.; Wang, C.; Manivannan, A. Cyclability Study of Silicon−Carbon Composite Anodes for Lithium-Ion Batteries Using Electrochemical Impedance Spectroscopy. Electrochim. Acta 2011, 56, 3981−3987. (14) Chen, L.; Wang, K.; Xie, X.; Xie, J. Effect of Vinylene Carbonate (VC) as Electrolyte Additive on Electrochemical Performance of Si Film Anode for Lithium Ion Batteries. J. Power Sources 2007, 174, 538−543. (15) Levi, M. D.; Levi, E.; Gofer, Y.; Aurbach, D.; Vieil, E.; Serose, J. Dilute Graphite-Sulfates Intercalation Stages Studied by Simultaneous Application of Cyclic Voltammetry, Probe-Beam Deflection, In situ Resistometry, and X-Ray Diffraction Techniques. J. Phys. Chem. B 1999, 103, 1499−1508. (16) Levi, M. D.; Aurbach, D. The Mechanism of Lithium Intercalation in Graphite Film Electrodes in Aprotic Media. Part 1. High Resolution Slow Scan Rate Cyclic Voltammetric Studies and Modeling. J. Electroanal. Chem. 1997, 421, 79−88. (17) Levi, M. D.; Levi, E. A.; Aurbach, D. The Mechanism of Lithium Intercalation in Graphite Film Electrodes in Aprotic Media. Part 2. Potentiostatic Intermittent Titration and In Situ XRD Studies of the Solid-State Ionic Diffusion. J. Electroanal. Chem. 1997, 421, 89−97. (18) Huang, H.; Kelder, E. M.; Chen, L.; Schoonman, J. Electrochemical Characteristics of Sn1−xSixO2 as Anode for LithiumIon Batteries. J. Power Sources 1999, 81−82, 362−367. (19) Wang, Q.; Li, H.; Huang, X.; Chen, L. Determination of Chemical Diffusion Coefficient of Lithium Ion in Graphitized Mesocarbon Microbeads with Potential Relaxation Technique. J. Electrochem. Soc. 2001, 148, A737−A741. ́ (20) Tian, B.; Swiatowska, J.; Maurice, V.; Zanna, S.; Seyeux, A.; Klein, L. H.; Marcus, P. Aging-Induced Chemical and Morphological Modifications of Thin Film Iron Oxide Electrodes for Lithium-ion Batteries. J. Phys. Chem. C 2013, 117, 21651−3547. ́ (21) Li, J. T.; Maurice, V.; Swiatowska-Mrowiecka, J.; Seyeux, A.; Zanna, S.; Klein, L.; Sun, S. G.; Marcus, P. Time-of-Flight-SIMS and Polarization Modulation IRRAS Study of Cr2O3 Thin Film Materials as Anode for Lithium Ion Battery. Electrochim. Acta 2009, 54, 3700− 3707. (22) Reddy, M. V.; Yu, T.; Sow, C. H.; Shen, Z. X.; Lim, C. T.; Subba Rao, G. V.; Chowdari, B. V. R. α-Fe2O3 Nanoflakes as an Anode Material for Li-Ion Batteries. Adv. Funct. Mater. 2007, 17, 2792−2799. (23) Zheng, Z.; Chen, Y.; Shen, Z. X.; Ma, J.; Sow, C. H.; Huang, W.; Yu, T. Ultra-Sharp Alpha-Fe2O3 Nanoflakes: Growth Mechanism and Field-Emission. Appl. Phys. A: Mater. Sci. Process. 2007, 89, 115−119.

4. CONCLUSIONS In situ electrochemical (EIS) and ex situ surface (ToF-SIMS) analysis were combined to evaluate Li-ion diffusion into αFe2O3 conversion-type negative electrodes for LiBs. DLi values obtained from both techniques for thin film electrodes partially lithiated or delithiated were in the range 10−16 cm2 s−1. The higher value (in the range 10−15 cm2 s−1) obtained from EIS for fully lithiated electrode was explained by pulverization of the converted material promoting Li-ion migration. The Li-ion concentration profile obtained from ToF-SIMS for the partially converted electrode allowed evidencing (i) an outermost converted Li2O/Fe0 matrix region of slowest diffusion (DLi = 6 × 10−16 cm2 s−1), (ii) an innermost region of faster diffusion (DLi = 2.0 × 10−13 cm2 s−1) ahead of the conversion front and where Li-ions are intercalated in the Fe2O3 matrix, and (iii) an intermediate region between the outermost converted and innermost intercalated regions. In this transition region, the Li-ion concentration profile could not be fitted by a solid-state diffusion profile, which was assigned to mixing of the converted and unconverted iron oxide matrices owing to the advancement of the conversion reaction front. The discrimination of these three regions suggests that the kinetics of advancement of the conversion front is faster than that of Li-ion diffusion in the converted composite matrix but slower than that of intercalation in the unconverted Fe2O3 matrix. Comparison of the ex situ and in situ data indicates that the electrode conversion kinetics is limited by Li-ion diffusion in the converted matrix. In order to obtain precise information about Li-ion diffusion solely in electrode materials, surface processes (like SEI formation) should be discriminated in which case ToF-SIMS depth profiling appears as the most appropriate and direct methodology.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +33(0) 144278026. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Region Ile−de−France is acknowledged for partial support for the XPS and ToF−SIMS equipment. Bingbing Tian thanks the China Scholarship Council (CSC) for financial support.

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REFERENCES

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DOI: 10.1021/jp510269e J. Phys. Chem. C XXXX, XXX, XXX−XXX