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Aging Processes in Lithiated FeSn Based Negative Electrode for Liion Batteries: a New Challenge for Tin Based Intermetallic Materials Mohamad Chamas, Abdelfattah Mahmoud, Junlei Tang, Moulay Tahar Sougrati, Stefania Panero, and Pierre-Emmanuel Lippens J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11302 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 5, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Aging Processes in Lithiated FeSn2 Based Negative Electrode for Li-ion Batteries: a New Challenge for Tin Based Intermetallic Materials

Mohamad Chamas1,7, Abdelfattah Mahmoud2,3, Junlei Tang1,4, Moulay Tahar Sougrati2,7, Stefania Panero5,7, Pierre-Emmanuel Lippens*,2,6,7

1

2

School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, China

Institut Charles Gerhardt, UMR 5253 CNRS, Université de Montpellier, Place Eugène Bataillon, 34095

Montpellier cedex 5, France

3

LCIS/ GREENMAT, Institute of Chemistry B6, Liège University, Allée de la Chimie 3, B-4000 Liège,

Belgium

4

Sichuan Provincial Key Laboratory of Oil /gas-field Applied Chemistry, Chengdu 610500, China

5

Dipartemento di Chimica, Universita di Roma « La Sapienza », 00185 Rome, Italy

6

Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR 3459 CNRS, France

7

ALISTORE-ERI, FR CNRS 3104, 80039 Amiens Cedex, France

1 ACS Paragon Plus Environment


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ABSTRACT Tin based intermetallic compounds proposed as negative electrode materials for Li-ion batteries not only suffer from capacity fade during cycling due to volume variations but also from aging phenomena in lithiated states. By using FeSn2 as a model compound, we propose an analysis of this process by combining electrochemical potential measurements,

119

Sn and

57

Fe Mössbauer spectroscopies, magnetic measurements and impedance spectroscopy. We

show that the Fe/Li7Sn2 composite obtained at the end of the first discharge is progressively transformed during the aging process occurring within the electrochemical cell in open circuit condition. The Fe nanoparticles are stable while the Li7Sn2 nanoparticles are progressively delithiated with time leading to Sn-rich LixSn nano-alloys without observable back reaction with Fe. The deinserted lithium atoms react with the electrolyte and modify the surface electrode interphase (SEI) by increasing its thickness and/or decreasing its porosity.


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1. INTRODUCTION New negative electrode materials are required to increase the energy density of Li-ion batteries. Metals like tin or silicon have been widely investigated because they can reversibly form Li-rich alloys, leading to high specific capacities compared to commonly used carbon or titanate based electrode materials1,2. However, alloying reactions involve large volume changes during cycling that lead to the loss of electrical contacts, to particle agglomeration and electrolyte degradation due to surface electrode interphase (SEI) instabilities resulting in capacity fade and low coulombic efficiency3–6. The use of intermetallic compounds reduces these effects by forming in situ, during the first discharge, a composite containing Li-rich alloys and metallic nanoparticles. Although the metallic particles are expected to buffer the volume variations of the LixSn nano-alloys during cycling, such materials still suffer from aging phenomena that limit the battery performances. The SEI instabilities due to volume variations and the electrochemically cracking of the particles during cycling both induce increasing cumulative loss of capacity and consumption of lithium that strongly reduce the cycle life. Although, many works have focused on this aspect7–9, there are less studies about the self-discharge of batteries containing tin based intermetallic compounds as negative electrode materials. FeSn2 based negative electrode materials for Li-ion batteries have been previously investigated to evaluate the electrochemical performances10–13 elucidate the electrochemical reaction mechanisms8,14,15, test different formulations16 or composites6,17–19 and analyze electrode/electrolyte reactions including aging during cycling20,21. It is worth noting that Mössbauer spectroscopy is of particular interest to study this electrode material that contains 57

Fe and

119

Sn Mössbauer isotopes22. The stability of nanostructured FeSn2 based electrodes



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in coin cells at the end of first charge obtained in galvanostatic regime at low current was previously investigated8,23. The Sn based species obtained at the end of charge (delithiated electrode) with these experimental conditions contained a small amount of FeSn2, arising from back reaction of Fe with Sn nanoparticles, that grew when maintaining the potential value for charge state. However, the reaction mechanisms occurring in the discharge state (lithiated electrode) have not been previously investigated for this material. The present paper concerns fully lithiated FeSn2 based electrode materials in the open circuit regime that corresponds to the charge state of full cells. In addition to the interest to study such self-discharge mechanism for potential applications in commercial batteries, following the electrochemical behavior is also interesting for fundamental studies of tin based intermetallic electrode materials that are often characterized at different steps of lithiation. This will provide some indications for ex situ experiments in order to avoid or reduce some modifications of the investigated samples caused by their instability. There are different parameters that affect such aging phenomena as the composition and morphology of the electrochemically active material, the electrode formulation or the type of electrochemical cell. In the present work, we have considered commonly used cell configurations, as described in the experimental section. Aging tests were performed with different electrode formulations and experimental conditions but showed similar behaviors except for reaction kinetics. Thus, we only present in this paper in situ aging processes of electrode materials, i.e., within the electrochemical cell, characterized by different in situ experiments (Mössbauer and impedance spectroscopies) and ex situ magnetic measurements. Such a combination of experimental tools has allowed us to determine the main features of the aging process of fully lithiated FeSn2 based electrodes.


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2. EXPERIMENT 2.1. Synthesis method. FeSn2 was prepared directly from Sn (Sigma–Aldrich, 99.5% purity) and Fe (Sigma–Aldrich, 99.5% purity) in an alumina crucible under controlled Ar/H2 (5%) atmosphere, heated at 470°C for 5 h before air-quenched. The purity and the crystallinity of the powdered materials were controlled by X-ray diffraction (XRD). 2.2. Electrode preparation. The electrochemical tests and impedance measurements were carried out with two and three-electrode Swagelok type cells, respectively, assembled inside an argon-filled glove box. A lithium foil was used as counter electrode. The electrodes were prepared as a thin film on a copper foil current collector by doctor-blade deposition of a slurry composed of 80 wt.% pristine material, 10 wt.% polymer binder and 10 wt.% carbon black (SP) as conductive additive. Polyvinylidene fluoride (PVDF) was used as binder. For Mössbauer measurements, powdered samples pressed into pellets were used with the same formulation except for binder that was polytetrafluoroethylene (PTFE). For accurate magnetic measurements, we used self-standing electrodes with the same composition except that binder consisted of PTFE dispersed in water24. The electrolyte was composed of 1 M LiPF6 with ethylene carbonate (EC) and dimethyl carbonate (DMC), EC:DMC 1:1. A glass microfiber paper (Whatman) was used as separator. 2.3. Electrochemistry. The electrochemical discharge (lithiation) was performed by using Biologic Lab test system under galvanostatic conditions at C/3 ( 1 Li per FeSn2 in 3 h) with a cutting low potential of 0.01 V vs Li+/Li0. Aging process of the lithiated electrode was performed in the electrochemical cell and in the glove box to avoid oxygen infiltration during the long periods of time. 2.4. Mössbauer spectroscopy. The

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Sn and

57

Fe Mossbauer spectra were recorded in

transmission geometry and constant acceleration mode at room temperature with

119m

Sn in a



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CaSnO3 matrix and

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Co(Rh) as sources, respectively. The Mössbauer parameters: isomer

shift, δ, quadrupole splitting, ∆, line width at half maximum, Γ, and relative sub-spectra area, A, were determined with a non-linear least-square method, using the program GM5SIT25 and Lorentzian profiles. The values of the isomer shift are given with respect to BaSnO3 and α-Fe for the

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Sn and

57

Fe Mossbauer measurements, respectively. The spectrometer was

calibrated at room temperature with the magnetically split sextet spectrum of a high-purity αFe foil as the reference absorber. The previously described modified Swagelok electrochemical cell15 was used for in situ Mössbauer measurements at different steps of aging process without extracting the electrode material to avoid contamination. 2.5. Magnetic measurements. The magnetic measurements were performed with a Superconducting Quantum Interference Device (SQUID) magnetometer MPMS XL7 in the temperature range 2−300 K and magnetic field range 0−5 T. The temperature dependent magnetization was measured using DC procedure. For the zero-field-cooled (ZFC) measurements, the sample was first cooled to 2 K under zero magnetic field. Then, a low magnetic field (50 mT) was applied and the data were collected from 2 to 300 K. Field Cooled (FC) measurements were performed with an applied field of 50 mT when cooling the sample. The fully lithiated electrodes were dried in a glovebox transferred under argon to the SQUID chamber to prevent oxidation. 2.6.

Electrochemical

impedance

spectroscopy.

The

electrochemical

impedance

spectroscopy measurements were performed by applying a 10 mV amplitude signal in the 10 mHz–100 kHz frequency range with an Ametek Versastat instrument. The spectra were analyzed by the non-linear least-square (NLLSQ) fit software developed by Boukamp26,27. The R(RQ)(RQ)Q equivalent circuit, where R denotes a resistance and Q a constant-phase element (CPE), was considered for all the spectra. The quality of the fit was confirmed by the value of the chi-square (χ2) factor of the order of 10−4, which is considered as acceptable for 6 ACS Paragon Plus Environment

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the validation of the proposed equivalent circuit model.

3. RESULTS AND DISCUSSION The potential curve of the first discharge of FeSn2 based electrode (in Swagelok cell) obtained in galvanostatic regime at C/3 shows a continuous decrease with a shoulder at 0.8 V followed by a large plateau at 0.15 V for a total insertion of about 8 Li per FeSn2 (Figure 1a). The beginning of the discharge reflects the SEI formation at the surface of carbon black and FeSn2 particles. The low potential plateau is related to the conversion reaction that transforms the FeSn2 particles into Li-rich LixSn nano-alloys with x = 3.5 and Fe nanoparticles13. This potential curve differs from that obtained for nanostructured FeSn2 based electrode that shows a continuous and smooth decrease of the potential instead of a well-defined plateau, but in the two cases the fully lithiated electrode materials are similar13,15. The mechanism responsible for this transformation is now well established and can be described by the conversion reaction: FeSn2 + 7 Li → Fe + Li7Sn2

(1)

In contrast with β-Sn based electrodes, there is a direct transformation of FeSn2 into Li7Sn2 without intermediate LixSn compounds. Such mechanism was also observed for CoSn228,29, Ni3Sn43,29 and MnSn230. This explains that the fully lithiated electrode contains one type of LixSn alloy, with x = 3.5. The SEI is mainly formed at the beginning of discharge as previously observed by X-ray photoelectron spectroscopy (XPS) for different tin based intermetallic compounds3,29,30. It is generally composed of different products arising from electrolyte decomposition as the electrode voltage decreases from the open circuit potential vs Li+/Li0 (OCP) at the beginning of discharge. This includes phosphates, lithium fluorides, carbonates, alkyl carbonates and other inorganic and organic species3,30. 7 ACS Paragon Plus Environment


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At the end of the first discharge, the open circuit condition was applied and OCP was measured during 480 h (Figure 1b). The variations of OCP vs time show that the potential increases up to about 0.5 V after 170 h, then is constant during 150 h and continuously increases up to 1.1 V after 480 h. This profile is similar to that obtained for the first charge of FeSn2 based electrode obtained in galvanostatic mode at rates in the range C/10-C/50, as shown for example for the C/20 regime by adjusting the time scales in order to have profile matching (Figure 2a). This suggests that both aging and charge mechanisms are very similar. This is even clearer from the comparison between the differential capacities that both show three oxidation peaks at close positions (Figure 2b). Such peaks were previously observed for the charge of FeSn2 and can be attributed to the formation of different LixSn phases with decreasing x during the delithiation14. The profile matching allows to roughly evaluate that about 6 Li per FeSn2 were deinserted during the aging process, suggesting that the electrode material was not fully delithiated. The first galvanostatic charge of FeSn2 nanoparticles based electrodes was previously interpreted as the progressive delithiation of the electrode material with the successive formation of different LixSn compounds from Li-rich to Sn-rich compounds, followed by back reaction with Fe at the end of charge8. Such mechanism, based on Gibb’s composition triangle for the Fe-Sn-Li system, was identified in galvanostatic regime with a very low imposed current, as required for the reformation of FeSn2 since significant differences occur at higher intensity due to kinetic limitations13. In the present case, the similarity of the two potential profiles (Figure 1a) and of the differential capacity curves (Figure 1b) suggests that aging process consists in the progressive formation of LixSn phases with decreasing x. The differential capacity curve exhibits three oxidation peaks A, B and C that can be compared to the plateaus occurring during the first charge of β-Sn based electrode in order to identify the Li-Sn alloying reactions31, 32. The peaks A and B are similar to those previously reported and interpreted for the charge of FeSn2 based electrodes14. The


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peak A of the aging process at 0.42 V reflects the transformation of Li-rich LixSn alloys with x = 3.5 into intermediate LixSn alloys with x ≈ 2.5. The latter value is in the range of the three Li-rich LixSn compounds with close compositions: Li13Sn5, Li5Sn2, Li7Sn3 that are expected to be formed almost simultaneously by the delithiation of Li7Sn2 due to their close cohesive energies8. The main peak B at 0.56 V reflects the transformation of intermediate LixSn alloys with x ≈ 2.5 into Li-poor LixSn alloys with x ≈ 1. There is also a small peak C at 0.69 V that could be due to the transformation of LixSn alloys from x ≈ 1 to x ≈ 0.4, but neither β-Sn (expected at about 0.8 V) nor back reaction with Fe nanoparticles (expected at about 0.75 V) were observed. Additional characterization techniques were used to have further insights into the aging mechanism. It is now rather well established that electrochemical reactions involving tin intermetallic compounds lead to nanosized and poorly crystallized particles along the first lithiation-delithation cycles and only partial information can be obtained from XRD while techniques that are sensitive to local atomic order are more powerful8,29. Thus, 119Sn and 57Fe Mössbauer spectroscopies were used to characterize tin and iron based species formed at the beginning of the aging process (end of first discharge) and after more than 1 month of aging in the electrochemical cell with open circuit condition. For this experiment, a powdered FeSn2 based electrode was used in a modified Swagelok cell for in situ measurements15. The

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Sn

Mössbauer spectrum of the fully lithiated electrode material obtained at the beginning of the aging process was fitted with two doublets (Figure 3a). The values of the Mössbauer parameters (Table 1) correspond to the two Sn crystallographic sites 4i and 4h previously observed for Li7Sn2 nanoparticles13. It is worth noting that the rather small quadrupole splitting of the Sn 4i site (∆ = 0.7 mm s-1), compared to the crystalline phase (∆ = 1.1 mm s-1), is indicative of the poor crystallinity and the nanosize of the electrochemically formed particles15. The rather small value of the linewidth (Γ = 0.9 mm s-1) indicates that Li7Sn2 is the 9 ACS Paragon Plus Environment


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only type of alloys formed at the end of discharge as expected from Eq. (1). The

57

Fe

Mössbauer spectrum at the beginning of aging process was fitted with one doublet (Figure 3b). The values of the isomer shift and quadrupole splitting are reported in Table 2. They are close to those found previously at the end of first discharge for nanostructured FeSn2 based electrode material15 and can be attributed to Fe(0) in superparamagnetic iron nanoparticles. This confirms the conversion reaction given by Eq. (1) and the formation of both Li7Sn2 and Fe nanoparticles at the end of discharge. There are no other tin or iron based species detected by these two techniques such as unreacted FeSn2, βSn, other LixSn alloys or tin and iron oxides. The 119Sn Mössbauer spectrum obtained after aging of 900 h consists of a broad band that reflects the existence of different tin species (Figure 3a). The measured OCP of 0.55 V is close to that of the main potential plateau observed from 170 to 320 h in electrochemical experiment with a film instead of powder for the electrode (Figure 1). The observed difference between the aging times for the two types of cells indicates that aging kinetics are different for these two electrode preparations (film or powder) and formulations that modify the electrolyte impregnation, diffusion phenomena or species migration.

The

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Sn

Mössbauer spectrum was fitted with two doublets. The values of the isomer shift for these two components are higher than those of Li7Sn2 (Table 1). The Mössbauer isomer shift originates from the electron density at the nucleus and can be correlated to the number of Sn 5s, and to a lesser extent, to the number of Sn 5p valence electrons33. It mainly reflects electron transfers due to chemical bonds involving Sn, i.e., Sn-Sn and Sn-Li bonds in the case of LixSn. The direct correlation between the average value of δ and x, as confirmed experimentally for the LixSn crystalline compounds34, allows to evaluate x even for poorly crystallized materials. In the present case the two measured values δ = 2.05 mm s-1 and δ = 2.45 mm s-1 correspond to x ≈ 2.5 and x ≈ 0.5, respectively. The former value is close to the values of the three Li-rich LixSn compounds with close compositions: Li13Sn5, Li5Sn2, Li7Sn3 (δ = 2.00-2.02 mm s-1)34 10 ACS Paragon Plus Environment

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that are expected to be formed by the delithiation of Li7Sn2 at close potentials. The higher value is in the range of the two Sn-rich compounds LiSn and Li2Sn534. By neglecting the variations of the recoil-free fraction of the 119Sn nuclei in LixSn alloys with x, we can consider that the relative areas of the two sub-spectra give directly the relative amounts of these two types of LixSn alloys (Table 1), leading to the average global composition x ≈ 1.7 Li. This indicates that about 3.5 Li per FeSn2 were deinserted from the fully lithiated electrode material, which is consistent with the values expected from the comparison between the potential profiles (Figure 2) by considering the OCP obtained for this Mössbauer experiment (0.55 V). This agreement indicates that similar mechanisms were obtained for both electrochemical and Mössbauer experiments although the reaction kinetics are different. The 57

Fe Mössbauer spectrum did not change noticeably with time, even after 2200 h (Figure 3b).

There is no typical 6-line splitting caused by hyperfine magnetic field as previously observed after cycling13, which means that iron based particles did not grow significantly during the aging of 2200 h and were still of nanometer size. The Mössbauer parameters obtained by fitting the experimental data with one doublet have the same values as those obtained at the end of discharge (Table 2), which confirms the stability of the iron nanoparticles during aging. An improved fit was obtained by considering an additional Lorentzian component to take into account the observed weak asymmetry. In that case, the relative contribution of the doublet assigned to the Fe nanoparticles is 85%. The isomer shift of the remaining component that contributes to the spectrum by about 15% (δ =0.18 mm s-1) strongly differs from that of FeSn2 (δ = 0.5 mm s-1), which excludes the back reaction of Fe nanoparticles with Sn-rich LixSn formed during the aging process. This value is typical of Fe(0) in Fe nanoparticles and the spectrum asymmetry could be an indication of a weakly asymmetrical distribution of the Mössbauer parameters caused by size distribution of the Fe nanoparticles. Thus, the aging process observed at the end of discharge clearly differs from that observed at the end of 11 ACS Paragon Plus Environment


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charge that shows the partial reformation of FeSn28. This result is consistent with the observed differences between the two potential curves obtained for these two aging processes. To confirm the progressive delithiation during aging, the

119

Sn Mössbauer spectrum was

measured after 170 h (Figure 3a). The spectrum shows a broad band that was fitted with three doublets, two of them being constrained to have the same relative areas. The values of the Mössbauer parameters of these two doublets agree with those of Li7Sn2 nanoparticles formed at the end of discharge and their relative contributions (74%) show they formed the main tin species within the electrode material (Table 1). The Mössbauer parameters of the third doublet are similar to those found for one component of the 900 h aging electrode material and can also be attributed to LixSn nano-alloys with x ≈ 2.5. This result clearly indicates that Li7Sn2 nanoparticles formed during the first discharge are first transformed into nano-alloys with intermediate compositions Lix≈2.5Sn, and finally into Sn-rich LixSn (LiSn and then Li2Sn5) during the aging process. This progressive delithiation of Li7Sn2 to form LixSn nano-alloys with intermediate compositions close to those of the crystalline compounds is similar to the mechanism observed for the first charge of FeSn2 nanoparticles based electrode8. The ex situ magnetic measurements were performed at three steps of the aging process by considering three self-standing electrode materials in different Swagelok cells. The preparation of the electrode materials and of the Swagelok cells, the electrochemical galvanostatic discharges, the aging process, the extraction of the lithiated electrode materials from the cells and the transfer to evacuated sealed tubes were all done in a glovebox under argon atmosphere to avoid oxidation and moisture contamination. The variations of the magnetization at the beginning of the aging process (end of first discharge) show a hysteresis loop, which indicates a ferromagnetic behavior at low temperature (Figure 4). The saturation magnetization (205 A m2 kg-1) is slightly lower than that of bulk α-Fe (222 A m2 kg-1) while the coercive field (38 mT) and the remanent ratio (0.23) both differ from bulk α-Fe35,36, in 12 ACS Paragon Plus Environment

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line with previously obtained results at the end of the first discharge of nanostructured FeSn2 based electrode15. The shape of the ZFC/FC curves is typical of superparamagnetic particles with the maximum of the FC curve giving the blocking temperature TB = 23 K (Figure 5). The average diameter, d, of the particles can be evaluated from the relation: ݀=൬

ଵହ଴௞ಳ ்ಳ గ௄೐೑೑

ଵൗ ଷ

൰

(2)

where kB is the Boltzmann constant and Keff is the effective magnetic anisotropy constant. As shown previously, Keff strongly increases with decreasing d for iron nanoparticles and is about ten times higher than that of bulk α-Fe37. The value Keff = 5 105 J m-3 considered here leads to d = 3.1 nm. Thus, the magnetic measurements confirm that the fully lithiated electrode material contains only Fe nanoparticles in line with the results of

57

Fe Mössbauer

spectroscopy. In addition, these results show that the average size is of several nanometers and the particles are rather well-dispersed in the electrode material, avoiding significant magnetic interactions between them. Finally, it is worth noting that surface effects did not affect significantly the saturation magnetization in agreement with our previous results15, but in contrast with the lower values usually observed for iron nanoparticles that are explained by the existence of an oxide layer or to disorder38. The magnetization curves obtained after 700 and 1500 h of aging are both similar to the curve obtained at the beginning of the process (Figure 4). The saturation magnetizations obtained after 700 h (215 A m2 kg-1) and 1500 h (196 A m2 kg-1) are both close to the value measured at the beginning of the aging process. The magnetization of nanostructured FeSn2 measured at 2 K was found to be lower than 30 A m2 kg-1 15 and the formation of such species during aging should strongly reduce the magnetization of the electrode material. In addition, the values of the coercive field (≈ 36 mT) and the remanent ratio (≈ 0.21) are also close to the values of the initial step of aging. This means that FeSn2 was not detected from the 13 ACS Paragon Plus Environment


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magnetization curves vs aging time. In the same way, the ZFC/FC curves are almost identical for the two aged samples (Figure 5). The blocking temperatures (≈ 20 K) are close to that of the initial step of aging (23 K), leading to the same average diameter (3 nm). Thus, the magnetic measurements confirm the

57

Fe Mössbauer results concerning the reactional

inactivity of the Fe nanoparticles during aging and the absence of back-reaction with Sn atoms. Impedance spectroscopy was used to investigate the reaction mechanism of lithium arising from the delithiation of the electrode material during aging. The preparation and formulation of the electrode materials were identical for both impedance measurements and electrochemical tests. The potential curves obtained during the aging process by these two techniques are very similar and the different stages of aging considered for the impedance measurements are reported in figure 1b. The Nyquist plots of the impedance spectra clearly show two semicircles that are progressively broadened during the aging process (Figure 6). The first impedance spectrum, measured for the initial step of aging, i.e., just after the first galvanostatic discharge, shows a first semicircle at high frequency attributed to the passive film and another semicircle at intermediate frequencies due to the charge transfer, while the linear slope at low frequency is due to diffusion and the cell capacitance. The analysis of the spectra with the R(RQ)(RQ)Q equivalent circuit (Figure 7) allows to evaluate the resistances of the electrolyte, Rel, of the charge transfer, Rct, and of the SEI, RSEI. The variations of the values of these three resistances vs aging time are shown in Figure 8. The electrolyte resistance is constant during the aging process, confirming that electrolyte did not change significantly. Its value, Rel ≈ 5 Ω, is low and similar to that found previously for the same type of electrolyte21,39. There are no significant variations during aging, which indicates that electrolyte degradation did not significantly affect its ionic properties. The SEI resistance is almost constant during the first 100 h with the value RSEI ≈ 20 Ω and then continuously 14 ACS Paragon Plus Environment

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increases up to RSEI ≈ 60 Ω after about 500 h. This indicates that the SEI did not noticeably change at the beginning of the aging process while the potential strongly increased. Then, its thickness increased or its porosity decreased from 100 to 500 h, which corresponds to the potential plateau at 0.5 V and the potential increase at the end of the process. This differs from the galvanostatic charge mechanism, as observed for example for MnSn2 based electrodes, that shows almost constant RSEI39. In the latter case, this indicates that lithium ions extracted from the electrode material during the galvanostatic charge were mainly collected by the cathode, which constitutes the main reaction mechanism compared to side reactions modifying the SEI. In the case of the aging process, there is no ionic current through the electrolyte due to the open circuit condition and the extracted lithium react with the electrolyte and form degradation species that are included in the SEI, resulting in the increase of RSEI. The variations of the charge transfer resistance show two regions with constant values: Rct ≈ 15 Ω during the first 100 h and Rct ≈ 45 Ω after 150 h. This suggests that internal diffusion of lithium was higher during the first 100 h corresponding to the increase of the potential and then smaller during the remaining aging process. This could also be correlated to the variations of RSEI since the value of this resistance is low and almost constant during the first 100 h and then increases. The charge transfer at the electrode/electrolyte interface decreases as the SEI thickness increases. Thus, the evolutions of Rct and RSEI vs aging time indicate that lithium atoms provided by the delithiation of Li7Sn2 react with the electrolyte to increase the amount of SEI products by forming a thicker or more homogeneous stable film at the electrode/electrolyte interface.

4. CONCLUSIONS In this paper, we have shown from electrochemical,

119

Sn Mössbauer and magnetic

measurements that aging process of fully lithiated FeSn2 based electrode in Swagelok cells 15 ACS Paragon Plus Environment


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Page 16 of 32

consists in the progressive delithiation of Li-rich LixSn nano-alloys formed during the discharge. Such process leads to the successive formation of LixSn nano-alloys with intermediate (x ≈ 2.5) and weak (x ≈ 0.5) contents of lithium. The

57

Fe Mössbauer

spectroscopy and magnetic measurements both indicate there is no significant back reaction between the Li-poor LixSn nano-alloys and the Fe nanoparticles with the experimental protocols considered in this work. This means that instability of the lithiated FeSn2 based electrode is due to the metastability of the LixSn nano-alloys caused by their small size and subsequently their high interface area with electrolyte while Fe nanoparticles are rather stable. Finally, the electrochemical impedance spectroscopy has shown that lithium atoms originating from the delithiation of the electrode material react with the electrolyte and modify the SEI by increasing its thickness and/or decreasing the porosity. The in situ aging process of the lithiated FeSn2 based electrode (powder or film) has shown significant changes after a number of days but then continue during typical time of several months. The reaction kinetics is even faster for ex situ aging, suggesting that ex situ characterizations of tin intermetallic materials at different steps of lithiation should be preferentially performed on fresh materials.

AUTHOR INFORMATIONS Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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The authors would like to thank the Région Languedoc-Rousillon (France) and the National Natural Science Foundation of China under grant no 21650110463 for financial support.



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Page 22 of 32

Tables

Table 1: Values of the

119

Sn Mössbauer parameters of FeSn2 based electrode materials at

different steps of the aging process: isomer shift (δ), quadrupole splitting (∆), linewidth at half maximum (Γ) and relative areas of the sub-spectra (A). The 119Sn isomer shifts are given with respect to BaSnO3 and the uncertainties on the Mössbauer parameters are typically of 0.01 mm s-1. The two 119Sn components of the spectra at the end of discharge are labeled by the Sn site symmetry in Li7Sn2.

Aging time 0h

Attribution Lix≈3.5Sn - Sn(4i) Lix≈3.5Sn - Sn(4h)

δ (mm s−1)

∆ (mm s−1)

Γ (mm s−1)

1.96 1.89

0.73 0.04

0.90* 0.90*

A (%) 50 50

170 h

Lix≈3.5Sn - Sn(4i) Lix≈3.5Sn - Sn(4h) Lix≈2.5Sn

1.95 1.89 2.04

0.73 0.04 0.98

0.91* 0.91* 0.91*

37 37 26

900 h

Lix≈2.5Sn Lix≈0.5Sn

2.05 2.45

1.14 0.95

0.99* 0.99*

60 40

* constrained to be equal


Page 23 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2: Values of the

57

Fe Mössbauer parameters of FeSn2 based electrode materials at

different steps of the aging process: isomer shift (δ), quadrupole splitting (∆), linewidth at half maximum (Γ) and relative areas of the sub-spectra (A). The 57Fe isomer shifts are given with respect to α−Fe and the uncertainties on the Mössbauer parameters are typically of 0.01 mm s1

. For the aging time of 2200 h, the spectrum was fit with one (a) and two components (b).

Aging time 0h

Attribution Fe(0)

δ (mm s−1)

∆ (mm s−1)

Γ (mm s−1)

0.24

0.48

0.46

A (%) 100

2200 h (a)

Fe(0)

0.24

0.47

0.44

100

2200 h (b)

Fe(0) Fe(0)

0.26 0.18

0.48 0

0.40* 0.40*

85 15

*constrained to be equal



Figures

2.0

2.0 (a) First discharge in galvanostatic

Potential vs Li+/Li (V)

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Page 24 of 32

(b) Aging after first discharge

regime at C/3 1.5

1.5

1.0

1.0

0.5

0.5

0.0

0

5

10

15

20

25

0.0

0

100

Time (h)

200

300

400

500

Time (h)

Figure 1. Potential curves vs time of FeSn2 based electrode material in Swagelok cell, (a) first discharge in galvanostatic mode at C/3 (lithiation process) and (b) aging process obtained in open circuit condition after the first discharge. In the latter case, the green dots denote the different stages of aging considered for the impedance measurements. The origin of time is taken at the beginning of each process, which corresponds to the end of the first discharge for the aging process.


Page 25 of 32

Number of deinserted Li/FeSn2 1.2

0

1

2

3

4

5

6

B

(b)

Differential capaciy (arb. u.)

(a)

Potential vs Li+/Li0 (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.8

First charge at C/20 0.6 0.4

Aging process 0.2

Aging process

First charge

A C

0.0 0

100

200

300

Time (h)

400

500

0.0

0.2

0.4

0.6

0.8

1.0

Potential (V vs Li /Li ) +

0

Figure 2. Comparison between (a) the potential curves and (b) between the differential capacity curves obtained during the aging process and during the first charge in galvanostatic mode at C/20.



119

Sn Mössbauer spectroscopy Lix~0.5Sn

1.00 0.99 0.98

Lix~2.5Sn

900 h

(b) 57Fe Mössbauer spectroscopy

0.97

1.00

Lix~2.5Sn

1.00

0.99

0.98

Relative transmission

(a)

Relative transmission

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

Li7Sn2 0.96

170 h 0.94 1.00 0.98

Li7Sn2

2160 h

0.97 0.96 0.95 0.94 1.00 0.99 0.98 0.97

0.96

0 day

0.96

0h 0.94 -6

0.98

0.95

-4

-2

0

2

4

6

-4

-3

Velocity (mm/s)

-2

-1

0

1

2

3

4

Velocity (mm/s)

Figure 3. 119Sn (a) and 57Fe (b) Mössbauer spectra of FeSn2 based electrode material obtained at different steps of the aging process.


Page 27 of 32

200 150

M (A m2 / kgFe)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


T=2K

100 50 0 -50 -100

0h 700 h 2200 h

-150 -200 -4

-2

0

2

4

H(T)

Figure 4. Magnetization curves of FeSn2 based electrode materials obtained at different steps of the aging process at 2 K.



10 0h 700 h 2200 h

8

M (A m2/ kgFe)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 32

6

4

2

0 0

50

100

150

200

250

T (K)

Figure 5. ZFC/FC curves of FeSn2 based electrode material obtained at different steps of the aging process.


Page 29 of 32

150

Aging time (h) 125

-Zimag (Ω cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 30 72 144 192 310 360 510

100

75

50

25

0 0

25

50

75

100

125

150

Zreal (Ω cm2)

Figure 6. Nyquist diagrams of impedance measurements of FeSn2 based electrode material at different steps of the aging process.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 32

Figure 7. Model used for the analysis of the impedance spectra and Nyquist plot. The various elements are defined in the text.


Page 31 of 32

8 6 4

Re

2 0 90

Resistance (Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


75 60

Rct

45 30 15 0 90 75 60

R

45

SEI

30 15 0

0

100

200

300

400

500

Time (h)

Figure 8. Variations of the resistances of electrolyte (Re), charge transfer (Rct), and surface electrolyte interphase (SEI) (RSEI) vs aging time. The solid lines are guides to the eye.



TOC graphic

-Zimag (Ω cm2)

50

1h

144 h 310 h 510 h

SEI ↗

25

Potential

0 0

25

50

75

100

125

150

Zreal (Ω cm2)

-6

-4

-2

0

2

Velocity (mm/s)

4

6

Lix↘Sn

Relative Transmission

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 32

1.00 0.99 0.98 0.97 -6

-4

-2

0

2

4

6

Velocity (mm/s)

Aging time


A New Challenge for Tin Based Intermetallic ... - ACS Publications

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