Quantitative Analysis of the Initial Restructuring Step of

Article pubs.acs.org/cm

Quantitative Analysis of the Initial Restructuring Step of Nanostructured FeSn2‑Based Anodes for Li-Ion Batteries Mohamad Chamas,†,§ Moulay-Tahar Sougrati,†,§,∥ Corine Reibel,‡ and Pierre-Emmanuel Lippens†,§,∥,* †

Equipe Agrégats, Interfaces et Matériaux pour l’Energie and ‡Plateau Technique, Institut Charles Gerhardt, UMR 5253 CNRS, Université Montpellier 2, Place Eugène Bataillon, 34095 Montpellier cedex 5, France § Alistore, European Research Institute, 33 rue Saint-Leu, 80039 Amiens Cedex, France ∥ Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR 3459 CNRS, France ABSTRACT: The first discharge of a nanostructured FeSn2 based negative electrode for Li-ion batteries has been studied by combining operando 119Sn Mössbauer spectroscopy and ex situ magnetic measurements. A modified Swagelok-type cell has been designed to perform in situ Mössbauer measurements, which allowed us to quantitatively follow the first discharge. The electrochemical mechanism consists in a conversion reaction that transforms FeSn2 into Li7Sn2. The Mössbauer spectrum at the end of the first discharge has been analyzed from first principles calculations of the Mössbauer parameters. The observed differences with bulk Li7Sn2 have been explained by the small size of the electrochemically formed particles. The magnetic measurements of the electrode material at the end of the discharge show the existence of rather pure superparamagnetic iron nanoparticles with an average diameter in the range 2−3 nm as evaluated from three different methods. The electrode saturation magnetization increases during the discharge, due to the increasing number of formed iron nanoparticles, but unexpected two-step variations were observed. They are interpreted by changes of the FeSn2 magnetization caused by interactions with iron nanoparticles. KEYWORDS: Li-ion batteries, anodes, intermetallics, Mössbauer spectroscopy, superparamagnetism ratio.15 Thus, both the microstructure of the pristine material and the first discharge play a crucial role in the electrochemical performances of tin intermetallic based anodes. Cobalt has been used with tin and carbon to form nanostructured alloys with high capacity and rather long cycle life, as shown by different studies16−18 and the commercialization of the Nexelion battery by Sony.19 It seems to be one of the best transition metal (M) for the Sn−M−C composites, but its replacement by iron, even partially, might reduce the cost of the electrode material and improve the environmental impact.20−22 Although the electrochemical behavior of iron−tin compounds and alloys have been studied in the past,23−30 there is still a need for a fundamental and quantitative analysis of the electrochemical mechanisms, which is one of the key to improve the electrode performances. Many aspects of the behavior of electrode materials have been explained by the development of new experimental tools allowing in situ and operando analyses using, for example, X-ray diffraction (XRD),31,32 X-ray absorption spectroscopy (XAS),33 or Fourier transform infrared spectroscopy (FTIR).34,35 Mössbauer spectroscopy has proved to be highly convenient and efficient for in situ and operando studies of Mössbauer-active nuclides containing materials used in Li-ion batteries.25,36−39

1. INTRODUCTION New anode materials with larger specific and volumetric capacities than carbon are required to increase the energy density of Li-ion batteries. Tin based materials have attracted much attention in recent years due to both higher capacity and better safety than graphite. The highest capacity is expected with metallic tin, βSn, which forms alloys with lithium giving a maximum theoretical specific capacity of 990 Ah kg−1, or volumetric capacity of 7234 Ah L−3, for 4.4 Li/Sn. However, alloying and dealloying reactions are responsible for large volume variations leading to structural instabilities and loss of electrical contacts within electrodes resulting in capacity fade.1 Different strategies based on the dispersion of electrochemically active tin species have been proposed in order to minimize such effects. They are mainly based on the dispersion of particles within the pristine material2−5 or on in situ dispersion obtained during the first electrochemical lithiation of the electrodes containing, for example, tin oxides or sulfides,6,7 tin composite oxides (TCO),8−10 or tin intermetallic based compounds.11−14 In the latter case, the metal atoms are extruded from the pristine material during the first discharge. They form electrochemically inactive metallic particles that are expected to buffer the volume variations and limit the growth and coalescence of lithium−tin particles during the electrochemical cycling. In addition, the electrochemical performances are improved by the use of small tin intermetallic particles because of their small diffusion path length and their high reactivity due to large surface to volume © XXXX American Chemical Society

Received: January 21, 2013 Revised: April 16, 2013

A

dx.doi.org/10.1021/cm400253a | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

lithiated electrodes were dried in a glovebox and transferred under argon to the SQUID chamber to prevent any oxidation. Such self-standing electrodes are more reliable than powdered electrodes for measurements that require an accurate evaluation of the active mass since there is no loss of electrode materials during the different steps of the experimental procedure. 2.3. Mö ssbauer Spectroscopy. The 119Sn Mössbauer spectra were recorded in transmission geometry and constant accelerator mode at room temperature with 119mSn in a CaSnO3 matrix as the source. The velocity scale was calibrated using the magnetic six-line spectrum of a high-purity iron foil absorber as a standard and using 57Co(Rh) as the source. The hyperfine parameters were determined by fitting Lorentzian lines to the experimental data with a nonlinear least-squares method.44 The values of the isomer shift are given with respect to BaSnO3. A Swagelok-type cell has been modified in order to perform in situ and operando measurements. In that way, both the difference of potential between the two electrodes of the cell and the Mössbauer spectra were recorded simultaneously. The body of the cell is made of the commercially available Swagelok tube union made of perfluoroalkoxy polymer (Figure 1a). The internal extremities of the stainless steel end

This technique provides information on the local environment of the Mössbauer isotope and allows to identify poorly crystallized and/or nanosized phases when XRD fails. The aim of this paper is to provide a detailed and quantitative study of the first discharge of nanostructured FeSn2 based anodes for Li-ion batteries. 119Sn Mössbauer spectroscopy has been used in operando mode in order to quantitatively follow the electrochemical reactions from changes in tin local environment. Although different types of electrochemical cells were considered for in situ Mössbauer experiments,25,39−42 we propose here a modified version of the Swagelok-type cells that are commonly used for electrochemical tests of Li-ion batteries. Such electrochemical cells have been developed because of their easily achieved proper installation, excellent gastight sealing, and low cost. The experimental data have been interpreted from first principles calculations of isomer shift, δ, and quadrupole splitting Δ. The variations of the relative amounts of Sn based products in the electrode have been evaluated from their contributions to the Mössbauer spectra including the in situ determination of the Lamb-Mössbauer factors. The nature of the metallic particles formed during the discharge, their average size evaluated by three different methods, and the variations of the magnetization during the discharge were obtained from magnetic measurements of the electrodes. The synthesis and the characterization of the nanostructured FeSn2 and the experimental and theoretical methods including the new electrochemical cell used for Mössbauer operando measurements are described in section 2. The quantitative analysis of the first discharge based on Mössbauer and magnetic experimental data are presented and discussed in section 3.

2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. Synthesis and Characterization. Nanostructured FeSn2 was prepared in two steps. First, microsized particles of FeSn2 were obtained from stoichiometric amounts of Sn (Sigma-Aldrich, 99.5% purity) and Fe (Sigma-Aldrich, 99.5% purity) in an alumina crucible under controlled Ar/H2 (5%) atmosphere, heated to 470 °C for 5 h before air-quenched. The microparticles were then ball milled (Retsch PM 100) with stainless steel balls and grinding jars in argon inert atmosphere. A powder charge of 3 g was milled with five balls of 5 g each for 72 h but stopped every 15 min active milling during 15 min for the cooling of the powder. The purity and the crystallinity of the powdered materials were controlled by XRD with a PHILIPS X’Pert MPD equipped with the X’celerator detector. The XRD patterns were recorded using Cu Kα radiation (λ = 1.5418 Å). The BET surface area was determined from nitrogen adsorption measurements with a Micromeritics ASAP 2020. 2.2. Electrochemistry. The electrochemical tests were carried out with Swagelok-type two-electrode cells assembled inside an argon-filled glovebox. A lithium foil was used as counter electrode and the working electrode was made up of 80 wt % pristine material, 10 wt % polytetrafluoroethylene (PTFE) binder, and 10 wt % carbon black (SP) as conductive additive. The composite was pressed into pellets of 7 mm diameter. The electrolyte was composed of 1 M LiPF6 dissolved in ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC), EC/PC/DMC 1:1:3, v/v. A glass microfiber paper (Whatman) was used as separator. For ex situ XRD measurements the electrode material was extracted from the cell within the glovebox after lithiation performed with a Maccor Series 4000 battery test system under galvanostatic conditions at C/10 regime (1 Li per FeSn2 in 10 h) between 0.01 and 1.2 V vs Li+/Li0. For accurate magnetic measurements, we used self-standing electrodes as described in ref 43 with the same composition as for the electrochemical tests except that binder consisted of 60% PTFE dispersed in water. This gave a paste that was spread into sheets with thickness of several tens of micrometers. Circular electrodes were cut and dried at 120 °C and then put in the Swagelok cells. The

Figure 1. Modified Swagelok electrochemical cell for in situ Mössbauer measurents. The cell (a) is composed of PFA cell body (1), nut (2), PFA sealing ferrules (3), and stainless steel plunger (4). The junction around the electrodes (b) is formed by PMMA windows (5), lithium disc (6), Be based connector (7), Whatman separator (8), and active material (9).

member tubes are formed by PMMA γ-transparent windows glued using Araldite. The electrochemical cell is placed between the two windows (Figure 1b). The anode is made of a thin lithium foil of high quality, which is in contact with the stainless steel tube. Three configurations were tested for the cathode: a beryllium foil used as a connector and placed between the electrode material (film or powder) and the PMMA window, a copper grid instead of the beryllium, and finally, an aluminum foil when the electrode material presents high potential, e.g., LiMnPO4, in order to prevent connector corrosion. The first configuration was considered in the present study. The cell was placed between the γ-rays source and the detector. The measurements were performed under the same galvanostatic conditions as the electrochemical tests: at C/10 regime between 0.01 and 1.2 V vs Li+/Li0 with a Biologic SP-150 potentiostat. The Mössbauer data were recorded during 2 h and saved, and then, the memory was cleaned and so on. Thus, each spectrum was collected during the reaction with 0.2 Li/FeSn2. This means that data were collected continuously during the first discharge. B



Article

2.4. Magnetic Properties. The magnetization of the electrode materials was measured with a Superconducting Quantum Interference Design (SQUID) magnetometer MPMS XL7, in the temperature range 2−300 K and range of magnetic fields 0−5 T. The temperaturedependent 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. 2.5. Theoretical Method. The calculations of the electronic density and electric field gradients at nucleus (EFG) were performed with the augmented plane wave method based on density functional theory (DFT) including local orbitals (APW+lo) as implemented in the WIEN2k code.45 Exchange and correlation effects were treated using the generalized gradient approximation (GGA) and the Perdew, Burke, and Ernzerhof functional.46 The muffin-tin radii used in the calculations were Rmt (Sn) = 2.6 au, Rmt (Fe) = 2.2 au, and Rmt (Li) = 2.0 au. To improve the energy linearization, the basis set was extended with Sn 4d, Fe 3p, and Li 1s local orbitals. In the interstitial region, the wave functions were expanded in plane waves with wavenumbers K such as min(Rmt)·max(K) = 7 and the charge density was expanded in a Fourier series with Gmax = 15. The irreducible wedge of the Brillouin zone was sampled using 1470 and 1350 points for FeSn2 and Li7Sn2, respectively. The experimental values of the lattice constants were used for the calculations but the internal atomic positions were optimized in order to minimize the internal atomic forces. The self-consistency was achieved with an energy tolerance of 10−4 Ry and a force tolerance of 10−3 Ry bohr−1. The EFG at the nucleus were obtained from the lattice harmonic expansion of the potential within the muffin tin spheres.47 They were evaluated after the optimization of the internal atomic positions with an accuracy better than 1019 V m−2.

Sherrer’s law, is of about 10 nm. The BET area surface is of about 5 m2 g−1. 3.2. 119Sn Mö ssbauer Effect Study. The electrochemical potential curve obtained with the modified Swagelok cell for 119 Sn Mössbauer operando measurements is similar to that obtained for electrochemical tests in unmodified Swagelok cell (Figure 3). About 8.5 Li/FeSn2 were inserted in the electrode

Figure 3. Voltage (vs Li+/Li0) profiles as a function of the number of Li per FeSn2, x, for the first discharge at C/10: in situ 119Sn Mössbauer measurements (blue solid line), electrochemical test in Swagelok cell (black dash-dotted line) and ex situ magnetic measurements at different depths of the first discharge (red dashed line).

during the first discharge. There is a small plateau at about 0.9 V vs Li+/Li0 for the first 0.3 Li that can be attributed to the formation of the surface electrolyte interphase (SEI) at the carbon additive surface,49 followed by a continuous decrease of the potential. The Mössbauer spectra obtained during the first discharge show a progressive transformation from a doublet at the beginning of the discharge to a single peak at the end of the discharge. The first Mössbauer spectrum, obtained for 0.2 Li per FeSn2, is similar to the spectrum of the pristine nanostructured FeSn2 used in the electrochemical cell (Figure 4). This means that the electrode preparation and the insertion of 0.2 Li in the electrode do not change the local environments of Fe and Sn in FeSn2. This spectrum is similar to previously reported 119Sn Mössbauer spectra for nanostructured FeSn225,50 but differs from that of bulk FeSn2 which exhibits magnetic splitting.51 As observed by XRD, this could be due to the small size of the particles or to structural disorder, both due to mechanical ball milling. Previous studies indicate that such nanostructured FeSn2 particles are superparamagnetic.52 The Mössbauer spectrum of bulk FeSn2 is more complex because of the 119Sn transferred hyperfine field of 23.4 kG, which is due to the antiferromagnetic behavior of this compound at 295 K.51,52 However, similar values of δ (and Δ) have been reported for nanostructured and bulk FeSn2 (Table 1). In order to evaluate the influence of the magnetic behavior on δ and Δ, DFT calculations of the hyperfine parameters of FeSn2 were performed by considering three different configurations: nonmagnetic (calculation without spin polarization), ferromagnetic (spin polarized calculation), and antiferromagnetic (spin polarized calculation with Fe spin up/ spin down inversion along the crystallographic axis a as reported in 52). The 119Sn Mössbauer isomer shift was evaluated from the calculated electron density at the Sn nucleus, ρ(0), by considering the relation

3. RESULTS AND DISCUSSION 3.1. Structural Characterization of the Pristine Material. FeSn2 has a CuAl2-type structure with the tetragonal space group I4/mcm. The published experimental values of the lattice parameters at 295 K are a = 6.533(1) Å and c = 5.320(2) Å.48 The unit cell contains four Fe atoms at the Wyckoff positions 4a (0, 0, 1/4) and eight Sn atoms at positions 8h (x, x + 1/2, 0) with x = 0.1611. The Fe atoms are bonded to two Fe atoms at 2.66 Å and to eight Sn atoms at 2.79 Å forming quadratic antiprisms with basal planes in the bc plane. Each Sn atom has four Fe firstnearest neighbors at distances of 2.79 Å forming a square based pyramid and eleven Sn atoms at different distances between 2.98 and 3.47 Å. The XRD pattern of nanostructured FeSn2 shows broadened Bragg peaks (Figure 2). The peaks were indexed on the basis of the tetragonal cell providing lattice parameters: a = 6.52(1) Å and c = 5.34(1) Å close to those previously published for bulk material. The average size of the coherent domains, which was evaluated from the line width of the XRD peaks by using the

δ = α(ρ(0) − ρref (0))

Figure 2. XRD pattern of nanostructured FeSn2. C

(1)



Article

η=

Table 1. 119Sn Mössbauer Isomer Shift Relative to BaSnO3 (δ) and Quadrupole Splitting (Δ) of FeSn2a DFT-nm DFT-fm DFT-afm FeSn2 x = 0.2 Li FeSn2-nm25 FeSn2-nm50 FeSn2-afm51

Δ(mm s−1)

Vzz (1021 V m−2)

η

2.1 2.1 2.1 2.18 2.20 2.12 2.20 2.21

0.85 0.87 0.82 0.89 0.90 0.83 0.86 0.86

−11.54 −11.55 −11.06

0.02 0.44 0.27

(3)

The three components VXX, VYY, and VZZ of the diagonalized EFG tensor are defined by |VZZ| ≥ |VYY| ≥ |VXX|. The theoretical isomer shifts evaluated for the three magnetic configurations are identical (δ ≈ 2.1 mm s−1) within calibration and computational errors (≈0.1 mm s−1) (Table 1). This confirms the weak influence of the magnetic interactions on δ. These theoretical values agree with the experimental values previously published and those obtained in the present work (Table 1). The isomer shift of FeSn2 is lower than that of βSn (2.56 mm s−1), which reflects the electronic transfer from Fe to Sn due to Fe 3d−Sn 5p bonds. The theoretical values of Δ are also almost independent of the FeSn2 magnetic configuration and are close to the experimental values (Table 1). As shown by eq 2, Δ strongly depends on VZZ which is nearly constant for the three magnetic configurations. The observed variations of η, from about 0 to 0.44, have only a weak effect on Δ (≈3%). The theoretical determination of the main directions of the EFG (X, Y, Z) indicates that the Z axis is perpendicular to the basal plane formed by the four Fe atoms and the rather high value of VZZ reflects the anisotropy of the Sn 5p electron population between this direction and the perpendicular plane. It is worth noting that the theoretical values of the Fe magnetic moment evaluated within the Fe muffin-tin sphere for the two magnetic configurations (ferromagnetic 1.8 μB and antiferromagnetic 1.9 μB) are close to the experimental value obtained from neutron diffraction (1.70 ± 0.05 μB).52 The 119Sn Mössbauer spectrum obtained at the end of the first discharge is formed by a single and broad peak and was first fitted with an unresolved doublet (Table 2). In this case, the

Figure 4. Selected in situ 119Sn Mössbauer spectra at different depths of the first discharge. The spectra were fitted to three doublets corresponding to Sn in FeSn2 (blue line) and Sn(1) (red line), Sn(2) (green line) in Li7Sn2, respectively.

δ (mm s−1)

|VYY | − |VXX | |VZZ|

Table 2. 119Sn Mössbauer Isomer Shift Relative to BaSnO3 (δ) and Quadrupole Splitting (Δ) of Li7Sn2a DFT Sn(1) Sn(2) EOD (1doublet) EOD D(1) (2 doublets) D(2) Li7Sn256 D(1) D(2) Li7Sn257 D(1) D(2)

a The results obtained from DFT calculations for different magnetic configurations: nonmagnetic (DFT-nm), ferromagnetic (DFT-fm), and antiferromagnetic (DFT-afm) are reported, including the main component of the EFG tensor, VZZ, and the asymmetry parameter η. The experimental data obtained in this work for nanostructured FeSn2 and the lithiated electrode at the beginning of the discharge (x = 0.2 Li) are given as well as previously published experimental results at room temperature (uncertainties of ±0.01 mm s−1).

δ (mm s−1)

Δ(mm s−1)

Vzz (1021 V m−2)

η

1.79 1.91 1.92 1.86 1.96 1.90 1.95 1.84 1.96

0.25 1.10 0.51 0.20 0.70 0.40 1.33 0.28 1.13

3.77 16.09

0.07 0.48

The theoretical values (DFT) of δ and Δ as well as the main component of the EFG tensor, VZZ, and the asymmetry parameter, η, are given for the two crystallographic sites Sn(1) and Sn(2). The experimental values (uncertainties of ±0.01 mm s−1) obtained in this work for the electrode at the end of the first discharge (EOD) are reported for the two fitting procedures with 1 and 2 doublets, rexpectively. In the latter case, D(1) and D(2) denote the two doublets with low and high quadrupole splitting, respectively. Previously published experimental data for bulk Li7Sn2 are given for comparison. a

where ρref(0) is the electron density at the Sn nucleus of the reference material CaSnO3 and α only depends on the characteristics of the 119Sn nucleus. Both of them can be considered as calibration constants and were evaluated from the linear correlation between the experimental values of δ and the calculated values of ρ(0) for a series of tin based compounds.53,54 The 119Sn Mössbauer quadrupole splitting is given by the relation Δ=

1/2 ⎛ η2 ⎞ 1 eQVZZ ⎜1 + ⎟ 2 3⎠ ⎝

Mössbauer parameters are similar to those previously obtained for CoSn2 based electrode at the end of discharge.39 The average value of δ can be correlated with the composition of the Li−Sn compounds.56,57 In the present case, the observed value, δ = 1.9 mm s−1, indicates the existence of a Li-rich compound at the end of discharge. A better agreement between the experimental data and the fitted curve has been obtained by considering two

(2)

where e is the electron charge, Q = 10.5 fm255 is the nuclear quadrupole moment of the first excited nuclear state of 119Sn (I = 3/2), VZZ is the main component of the diagonalized EFG tensor, and η is the asymmetry parameter defined by D



Article

between the values obtained for Sn(1) (0.2 mm s−1) and Sn(2) (1.1 mm s−1) in bulk Li7Sn2. Since the strong increase of Δ from Sn(1) to Sn(2) is mainly due to the existence of Sn(2)−Sn(2) bonds, the value of 0.7 mm s−1 reflects a decrease in the number of these bonds in the electrochemically formed Li7Sn2. This can be due to (i) the small size of the Li7Sn2 particles which tends to increase the ratio of surface to bulk atoms or to (ii) a larger number of isolated Sn atoms within the particles due to structural or chemical disorder. It is worth noting that the quadrupole splitting of Sn(2) in the fully lithiated FeSn2 microparticle based electrode increases from 0.7 to 1.1 mm s−1 during cycling.29 Based on the present analysis, this can be related to the increasing number of Sn−Sn bonds in Li7Sn2 due to the growth of the particles in agreement with (i). To conclude this part, the analysis of the first and last 119Sn Mössbauer spectra measured during the first discharge indicates that nanostructured FeSn2 is transformed into Li7Sn2 nanoparticles. To investigate this transformation, we have fitted all the 119Sn Mössbauer spectra of the first discharge by considering three components: one doublet for FeSn2 and two doublets for Li7Sn2. The values of δ and Δ were obtained from the first (FeSn2) and the last (8.5 Li, Li7Sn2) spectra and were considered as fixed parameters in the fitting procedure. The relative contributions of the two Sn crystallographic sites within Li7Sn2 were constrained to be identical as well as the linewidths of the three components. Only the values of the identical linewidths and the relative contributions of the FeSn2 and Li7Sn2 subspectra to the total spectra were allowed to vary. As a main result, the FeSn2 contribution decreases and the Li7Sn2 contribution increases with increasing number of lithium per FeSn2, x, as shown on some selected Mössbauer spectra (Figure 4). The relative areas of the subspectra assigned to FeSn2 and Li7Sn2, respectively, vary nonlinearly with x (Figure 5).

doublets (Figure 4). In that case, the 119Sn Mö ssbauer parameters are close to those reported previously for both fully lithiated nanostructured and microsized FeSn2 based electrodes from ex situ 119Sn Mössbauer spectroscopy and can be attributed to the two Sn crystallographic sites of Li7Sn2.29 Thus, within the present experimental conditions of the electrochemical tests, the specific capacity of FeSn2 is lower than the maximum theoretical value expected with the formation of Li22Sn5 by about 20%. As shown in Table 2, the Mössbauer parameters are close to those of bulk Li7Sn2 except for the highest of the two values of Δ that has a significantly lower value (0.7 mm s−1) than in bulk Li7Sn2 (≈1.1 mm s−1). In order to explain the origin of this difference, DFT calculations of the 119Sn Mössbauer parameters were performed for bulk Li7Sn2. The structure of this compound is orthorhombic with the space group Cmmm and the experimental values of the lattice parameters at 295 K are a = 9.80 Å, b = 13.80 Å, and c = 4.75 Å.58 The unit cell contains 28 Li atoms at the Wyckoff positions 2a (0, 0, 0), 2c (0.5, 0, 0.5), 4g (0.359, 0, 0), 4j (0, 0.179, 0.5), 8p (0.187, 0.154, 0), and 8q (0.349, 0.165, 0.5). The 8 Sn atoms of the unit cell are at the positions 4i (0, 0.3127, 0) and 4h (0.153, 0, 0.5) for the two crystallographic sites Sn(1) and Sn(2), respectively. Both Sn(1) and Sn(2) have local environments formed by Sn−Li bonds with lengths distributed in the range 2.8−3.5 Å but Sn(2) is also bonded to another Sn(2) at about 3 Å. A good agreement is obtained between the theoretical and experimental (bulk FeSn2) values of δ for Sn(1) and Sn(2) within the accuracy of the present calculations (Table 2). The agreement is even better for Δ, which allows to assign the low and high values to the crystallographic sites Sn(1) and Sn(2), respectively. The values of δ (≈1.9 mm s−1) are very close for the two crystallographic sites and can be attributed to the oxidation state Sn0. However, the difference between these values and that of βSn (δ = 2.56 mm s−1) is higher than the difference between FeSn2 (δ = 2.2 mm s−1) and βSn. This is due to the Sn 5p electron population, which is higher in Li7Sn2 than in FeSn2, leading to a lower value of δ for the former compound than for the latter one, as expected from the relation between the isomer shift and the Sn 5s/5p electron populations.59 This reflects a stronger electronic transfer from Li to Sn than from Fe to Sn, as expected from electronegativity differences. The low value of Δ for Sn(1) comes from the small values of both VZZ and η (Table 2) and can be explained by the rather isotropic Sn 5p electron density due to the large number of Sn(1)−Li bonds within the bond length range 2.8−3.5 Å. The local environment of Sn(2) is also formed by numerous Sn−Li bonds within the same range of bond lengths, but there is an additional Sn(2)−Sn(2) bond. The directions of the EFG main axes are the eigenvectors of the EFG tensor. For Sn(2), the results of the DFT calculations indicate that the Z axis is along the Sn(2)−Sn(2) bond. The rather high value of VZZ for Sn(2), compared to that of Sn(1), is due to the anisotropy of the Sn 5p electrons caused by this bond. This is the origin of the observed difference between the two values of Δ for Sn(1) and Sn(2) in bulk Li7Sn2. The theoretical value of Δ assigned to Sn(1) is consistent with the smallest value (0.2 mm s−1) obtained for the in situ spectrum of the FeSn2 based electrode at the end of the discharge. The other observed value (0.7 mm s−1) is significantly lower, by about 0.4 mm s−1, than that of Sn(2) in bulk Li7Sn2. Such difference was previously observed for the Li7Sn2 particles formed during the first discharge of CoSn2 based electrodes39 or FeSn2 microsized particles29 but not for Cu6Sn5,60 Ni3Sn4,61,62 and NbSn2.63 The present experimental value of Δ for Sn(2) (0.7 mm s−1) is

Figure 5. Relative areas of the 119Sn Mössbauer subspectra of FeSn2 (blue squares) and Li7Sn2 (brown triangles) as a function of the number of Li.

The relative amounts of Sn in FeSn2 and Li7Sn2 can be evaluated from the relative areas of the three Mössbauer subspectra and the Lamb−Mössbauer factors. In principle, the Lamb−Mössbauer factors can be evaluated from temperature dependent Mössbauer measurements for each compound. However, it is easier and more accurate to extract such information from in situ experiments, which is possible here because of the existence of only two tin based phases, and by assuming that the Lamb−Mössbauer factors of the two Sn E



Article

crystallographic sites in Li7Sn2 are identical. Within the thin absorber approximation, the spectrum area for a single crystallographic site is proportional to the Lamb−Mössbauer factor and the site concentration.64 The relative amounts of FeSn2, nFeSn2, and Li7Sn2, nLi7Sn2, are given by −1 ⎛ fFeSn ALi 7Sn2 (x) ⎞ 2 ⎜ ⎟ nFeSn2(x) = ⎜1 + fLi Sn AFeSn2 (x) ⎟⎠ ⎝ 7 2

(4)

nLi 7Sn2(x) = 1 − nFeSn2(x)

(5)

where f FeSn2 and f Li7Sn2 are the averaged Lamb−Mössbauer factors for FeSn2 and Li7Sn2 within the electrode, respectively, AFeSn2 is the relative area of the FeSn2 subspectrum and ALi7Sn2 is the sum of the relative areas of the Sn(1) and Sn(2) subspectra in Li7Sn2. Equation 4 shows that only the ratio between the Lamb− Mössbauer factors of FeSn2 and Li7Sn2 is needed for the evaluation of the relative amounts of these two phases. The total area of the Mössbauer spectra reflects the contributions of both FeSn2 and Li7Sn2: A(x) = A 0(fFeSn nFeSn2(x) + fLi Sn nLi 7Sn2(x)) 2

7

2

Figure 7. Relative amounts of FeSn2 (blue squares) and Li7Sn2 (brown triangles) as a function of the number of Li.

FeSn2 + 7Li → Fe + Li 7Sn2

The present analysis of in situ 119Sn Mössbauer experiments indicates that nanostructured FeSn2 is directly transformed into Li7Sn2 nanoparticles without intermediate Li−Sn phases as observed for tin or tin oxide based electrodes. Thus, by considering both the total and relative areas of the Mössbauer spectra, which is only possible with operando measurements, it has been possible to quantitatively follow the conversion reaction. At the beginning of the discharge (x < 2 Li), the 119Sn Mössbauer spectra do not change significantly (Figures 5 and 7) and the total area is constant (Figure 6). Since the Lamb− Mössbauer factor of Li7Sn2 is lower than that of FeSn2, a small amount of Li7Sn2 (of several percents) could be formed but not detected by Mössbauer spectroscopy. In addition, the absence of significant variations of the Mössbauer parameters also excludes Li insertion into FeSn2. Thus, no reaction (conversion or insertion) of Li with FeSn2 was observed at the beginning of the discharge. As previously described for electrodes containing FeSn2 microsized particles29 or other tin intermetallics such as Ni3Sn4,62 the beginning of the discharge can be related to the formation of a solid electrolyte interphase (SEI) at the surface of carbon and tin intermetallic particles. In the case of microparticles, the SEI formation is clearly correlated to potential variations and can be easily distinguished from the conversion reaction which is characterized by a well-defined plateau. This is not the case here, and in general for nanostrutured pristine electrodes, where the potential curve continuously decreases as a function of the number of lithium although two-phase reaction is unambiguously observed by Mössbauer spectroscopy. 3.3. Magnetic Properties. The Mössbauer experiments have shown that among the different possible Li−Sn phases only Li7Sn2 nanoparticles were found in the electrode at the end of the first discharge. This can be related to the conversion reaction given by eq 8, which furthermore suggests that iron particles are formed during the discharge. In order to check the existence of these particles and to obtain additional information, the mass magnetization, M, of the electrode at the end of the discharge was measured as a function of the applied magnetic field, H, at different temperatures (Figure 8). The variations of M(H) show the existence of an hysteresis loop at 2 K which disappears for temperatures higher than 30 K. This suggests a change from ferromagnetic to superparamagnetic behavior with a blocking temperature in the range 2−30 K due to the formation of

(6)

where A0 is a constant that can be evaluated from the area of the spectrum obtained at the beginning of the FeSn2→Li7Sn2 transformation. The experimental variations of A(x) are plotted in Figure 6. A(x) is constant for the two first Li and then linearly

Figure 6. Total area of the in situ 119Sn Mössbauer spectra as a function of the number of Li.

decreases until the end of the discharge at 8.5 Li. Thus, the transformation begins at about x = 2 Li and A0 = A(x = 2). This means that the reaction of (x − 2) Li with FeSn2 gives (x − 2)/7 Li7Sn2. The total area given by eq 6 can be rewritten ⎡ ⎤ ⎛ fLi Sn ⎞ 1⎜ 7 2⎟ ⎢ ⎥ A(x) = A(x = 2) 1 − ⎜1 − ⎟(x − 2)⎥ ⎢ 7 f ⎝ ⎠ FeSn ⎣ ⎦ 2

(8)

(7)

This relation shows that A(x) decreases linearly with x if the ratio (f Li7Sn2)/( f FeSn7) is lower than 1. A linear regression to the experimental data gives a ratio of 0.47 (Figure 6). The relative amounts of FeSn2 and Li7Sn2 were evaluated by considering this value and eqs 4 and 5. The observed linear variations (Figure 7) show that the FeSn2→Li7Sn2 transformation can be assigned to the conversion reaction F



Article

where T is the temperature of the sample, Ms(T) is the saturation magnetization at temperature T, μ0 is the vacuum permeability, kB is the Boltzmann constant, and m is the magnetic moment of the nanoparticles given by

m = MSbulk ρV

(10)

where and ρ are the saturation magnetization and the density of bulk α-Fe, respectively, and V is the average nanoparticle volume. The variations of M/Ms as a function of μ0H/T in the superparamagnetic regime are plotted in Figure 9. Mbulk S

Figure 8. Magnetization of the nanostructured FeSn2 based electrode at the end of the first discharge as a function of the applied magnetic field for different temperatures. The inset shows the magnification image around μ0H = 0 T.

nanoparticles during the first discharge. At 2 K, the saturation magnetization, Ms, evaluated for H = 5 T is of about 220 A m2 kg−1. This value is close to that of bulk α-Fe (222 Am2kg−1), which is ferromagnetic with a Curie temperature of 1043 K.65 Thus, the magnetic nanoparticles observed at the end of the first discharge contain only Fe atoms and have a structure close to that of α-Fe. A saturation magnetization close to that of α-Fe bulk material was also previously observed for iron nanoparticles synthesized by an organometallic approach66 or by solvated metal atom dispersion method.67 However, lower values have been reported in a majority of systems that have been explained by amorphization, shape anisotropy, surface morphology, the nature of the surface layer, or interactions between particles.65,68−70 The present value of Ms, which is close to the bulk value, suggests that the stabilizing layer (SEI), observed at the nanoparticle surface from impedance measurements71 and XPS,29 does not affect the particle magnetization. This also indicates that (i) neither oxide nor amorphization layer was formed at the iron nanoparticle surface, (ii) the iron nanoparticles have a good crystallinity, and (iii) magnetic interactions between iron nanoparticles are weak, otherwise the magnetization would be reduced. The observed remanent ratio, Mr/Ms, defined as the ratio between the remanent magnetization, Mr, and the saturation magnetization, is equal to 0.22. This is lower than the theoretical value of 0.5 obtained for randomly oriented spherical particles with single domain and uniaxial anisotropy.72 Low values of the remanent ratio were observed for weak interactions between nanoparticles that are magnetostatically coupled excluding strong exchange coupling.73 In addition, the coercive field of 30 mT is higher than the value of 5 mT reported for bulk Fe.74 These results suggest that iron nanoparticles did not aggregate during the first discharge and were rather uniformly dispersed within the electrode.75 The average diameter of the iron nanoparticles in the fully discharged electrodes was evaluated from the magnetic measurements in three different ways by using: the Langevin function, the Bloch law, and the ZFC-FC data, respectively. In the superparamagnetic regime, a universal curve of M/Ms as a function of H/T is expected for monosized nanoparticles with single domain and negligible anisotropy energy and is given by the Langevin function:76 ⎡ ⎛ mμ H ⎞ kT ⎤ M(H , T ) = Ms(T )⎢coth⎜ 0 ⎟ − B ⎥ ⎢⎣ ⎝ kBT ⎠ mμ0 H ⎥⎦

Figure 9. Reduced magnetization M/Ms as a function of the reduced magnetic field μ0H/T at different temperatures for the nanostructured FeSn2 based electrode at the end of the first discharge.

The deviations from a universal curve are small and have been neglected for simplicity. A single Langevin function, as given by eq 9, was used to fit the magnetization curves for T > 30 K with Ms(T) and m as fitting parameters. The average diameters 2 −1 evaluated from eq 10 by considering Mbulk S = 222 A m kg and ρ −3 = 7870 kg m are within the range 1.8−2.2 nm depending on the temperature. These values are well below the critical diameter for the transition between single domain and multiple walls, which is in the range 12−20 nm.77−79 The second evaluation of the iron nanoparticle average size is based on the temperature dependence of the saturation magnetization by considering the modified Bloch law80 given by 1−

M s (T ) = αT β Ms(0 K)

(11)

where α and β are the Bloch constant and exponent, respectively, which both depend on the nanoparticle size but tend to asymptotic values for the bulk material. In the case of bulk α-Fe, the values are α = 3.4 × 10−6 K−3/2 80 and β = 3/2, the latter value being typical of a bulk ferromagnet.81 In the case of nanoparticles, the values of the Bloch parameters can be evaluated from the variations of the experimental values of 1 − Ms(T)/Ms(0 K) as a function of temperature, plotted in a log−log graph (Figure 10). The observed linear variations confirm that the Bloch law given by eq 11 holds for the iron nanoparticles in the fully lithiated electrode. A linear fit to the experimental data gives α = 5 × 10−4 K−1.2 and β = 1.2. The Bloch exponent of the nanoparticles is slightly lower than that of bulk α-Fe while the Bloch constant is about 2 orders of magnitude larger. A value of α higher than that of bulk material can be due to larger fluctuations of the surface moments compared to those of the bulk atoms and is related to surface to volume ratio.70 The variations of Bloch parameters as a

(9) G



Article

where kB is the Boltzmann constant and Keff is the effective magnetic anisotropy constant. By considering the anisotropy constant of bulk α-Fe: Keff = 4.8 × 104 J m−3,83 eq 12 with TB = 20 K gives d = 6 nm. However, Keff increases with decreasing nanoparticle size at low temperature due to symmetry breaking at the nanoparticle surface.83 Previously reported values for iron nanoparticles of 2−3 nm diameter are in the range 2−5 × 105 J m−3.66,83−85 This is about ten times higher than the value of Keff for bulk α-Fe. In the present case, the average diameter obtained from eq 12 with Keff = 5 × 105 J m−3 and TB = 20 K is of about 3 nm. To conclude, the three different analyses of the magnetic measurements of the fully lithiated electrode material give an average diameter of about 2−3 nm for the iron nanoparticles. The magnetic moment is close to that of bulk α-Fe, which suggests that iron nanoparticles are well crystallized, without significant impurities and coated with a nonmagnetic layer. This observed small nanoparticle size suggests that iron atoms extruded during the lithiation of FeSn2 form iron nanoparticles with a constant size of several hundreds of atoms. This excludes the growth or coalescence of the metallic particles during the overall discharge. In order to analyze further the formation of iron nanoparticles during the first discharge, measurements of the field dependence of the magnetization at 2 K were performed for the electrode materials extracted from the Swagelok cells at different depths of discharge (Figure 12). All the curves exhibit a hysteresis loop due

Figure 10. log−log plot of 1 − Ms(T)/Ms(2 K), where Ms(T) is the temperature dependent saturation magnetization of the nanostructured FeSn2 based electrode at the end of the first discharge. The line is a linear fit to the experimental data.

function of diameter were previously published for Mg coated iron nanoparticles that have the same high saturation magnetization as the iron nanoparticles obtained in the present work.67 Our values of the Bloch constant and exponent are both consistent with the reported values for the average nanoparticle diameter of about 3 nm. Finally, the iron nanoparticle average diameter was also evaluated from ZFC-FC measurements. The ZFC magnetization curve shows a maximum at about 20 K while the FC magnetization curve coincides for temperatures greater than about 25 K and slightly increases with decreasing temperature below 25 K (Figure 11). These ZFC/FC curves are typical of

Figure 12. Magnetization as a function of the applied magnetic field at 2 K for the nanostructured FeSn2 based electrode at different depths of discharge.

to ferromagnetic behavior. The saturation magnetization increases with the increasing number of lithium from 25 A m2 kg−1 for x = 0 Li to 220 A m2 kg−1 for x = 9.6 Li (Figure 13). No significant changes have been observed at the beginning of the discharge until x ≈ 1.5 Li, then, Ms increases quasi-linearly until about 4.5 Li where a change in the slope of the line is observed. The beginning of the discharge concerns interfacial reactions, including the formation of the SEI. Both the absence of changes in nanostructured FeSn2, as observed by 119Sn Mössbauer spectroscopy, and the constant value of Ms until about 1.5 Li exclude the formation of iron nanoparticles at the beginning of the discharge. Unlike bulk FeSn2, which is antiferromagnetic with a Néel temperature of 378 K,52 small FeSn2 particles obtained from elements by ball milling have been found to be superparmagnetic.50 In the present case, the absence of magnetic splitting in the 119Sn Mössbauer spectrum at room temperature

Figure 11. Zero-field cooled ZFC and field cooled (FC) magnetization as a function of the temperature for nanostructured FeSn2 based electrodes at different depths of discharge including the end of discharge (EOD).

metallic nanoparticles and the maximum of the ZFC curve corresponds to the change from blocked to superparamagnetic regime with increasing temperature. Neglecting, for simplicity, the size distribution, the nanoparticle diameter, d, can be related to the blocking temperature, TB, by the relation:82

⎛ 150kBTB ⎞1/3 d=⎜ ⎟ ⎝ πKeff ⎠

(12) H



Article

the local environments of volume and surface sites. Thus, the increase in the FeSn2 saturation magnetization during the first discharge could be explained from increasing spin imbalance. This could be due to the increasing surface-to-volume ratio with the decreasing size of FeSn2 particles as expected from the progressive reaction with lithium during the first discharge (eq 8). However, this could also be due to changes in the FeSn2 magnetic properties due to interactions with the progressively formed iron nanoparticles. Indeed, the reaction of lithium with FeSn2 causes the extrusion of Fe atoms that agglomerate to form nanoparticles. Such a process is expected to develop close to the FeSn2 particles. The iron nanoparticles have a size of several nanometers and are smaller than FeSn2 particles. During the first discharge, the number of iron particles increases around the bigger FeSn2 particles leading to an increase of the surface magnetism of FeSn2. The low temperature magnetization of FeSn2 first increases until the particle surfaces are almost fully coated by iron nanoparticles and then decreases (or is almost constant) with the decreasing size of FeSn2. As a consequence, the variations of the saturation magnetization of the electrode are mainly due to the increasing number of iron nanoparticles. However, the observed deviations from the linear law can be attributed to changes in the FeSn2 magnetization due to interactions with iron particles and to the decreasing number of FeSn2 particles. Strong changes in the ZFC/FC curves were observed during the discharge, but they all show a maximum corresponding to the change from blocked to superparamagnetic regime with a blocking temperature well below the room temperature (Figure 11). As the number of lithium increases, the blocking temperature decreases from about TB = 100 K (x = 0−1 Li) to TB = 20 K (x = 9.6 Li) and the magnetization at the temperature TB increases. Both FeSn2 and iron particles contribute to the ZFC/FC curves. As shown above, the blocking temperature of iron nanoparticles is 20 K. Since these particles have a nearly constant size, they only contribute to the low temperature part of the curves. On the other hand, the blocking temperature of nanostructured FeSn2 is higher than 20 K and is expected to decrease with the increasing number of lithium since the average size of the FeSn2 particles decreases during the first discharge. This qualitatively explains the observed shift of the blocking temperature in the ZFC curves. The observed increase of the magnetization at the ZFC maxima and in the low temperature region of the FC curves with increasing number of lithium can be related to the increasing number of iron nanoparticles but is also consistent with the increase of the FeSn2 magnetization, which explains the nonlinear variations of the electrode saturation magnetization.

Figure 13. Saturation magnetization as a function of the number of Li. The lines are provided as a guide to the eye: linear interpolations to the experimental data (solid lines) and between x = 1.5 Li and x = 9.6 Li (dashed line).

shows that FeSn2 is not antiferromagnetic at this temperature. The saturation magnetization of 25 A m2 kg−1 at 2 K and the existence of a maximum in the ZFC curve at about 100 K for x = 1 Li are consistent with the existence of superparamagnetic FeSn2 particles (Figure 11). However, a small amount of Fe based magnetic impurities could partially contribute to the magnetization of the pristine electrode material. In addition, FeSn2 and the electrode material lithiated with x = 1 Li have the same coercive field of 170 mT, which is higher than that of iron nanoparticles (30 mT). Such a high coercivity should be related to a complex magnetic reversal mechanism involving magnetic moment transferred from Fe to Sn as observed in antiferromagnetic bulk FeSn251,52 and to surface effects. We have shown, from 119Sn Mössbauer study, that during the first discharge (for x > 2 Li), the amount of FeSn2 linearly decreases with the increasing number of lithium in agreement with eq 8. This equation also indicates that the amount of iron yielded by this electrochemical reaction is expected to increase linearly. Then, we have found, from the magnetization measurements, that the fully lithiated electrode contains rather pure iron nanoparticles of several nanometer diameter. Such very small sizes indicate there is no growth of the iron particles during the lithiation but an increase in their number. Thus, these results suggest that the number of iron nanoparticles should linearly increase with the number of lithium during the first discharge (for x > 2 Li). If we consider, for simplicity, that all the iron nanoparticles formed during the first discharge have the same saturation magnetization, then, the saturation magnetization of the electrode should increase linearly with the number of iron particles and therefore with the number of lithium, between Ms = 25 A m2 kg−1 and Ms = 220 A m2 kg−1 (dashed line in Figure 13), which is clearly not the case here (solid lines in Figure 13). The observed electrode saturation magnetization is greater than that expected for a conversion reaction involving particles with constant magnetization. This means that the variations of the electrode magnetization cannot be only related to the increasing number of iron particles but also to changes in the particle magnetization. Since the saturation magnetization of the iron nanoparticles is close to that of bulk α-Fe and is not expected to vary along the discharge, this increase can be assigned to that of the FeSn2 magnetization. Bulk FeSn2 is antiferromagnetic and the hysteresis loop observed here for nanostructured FeSn2 at 2 K can be attributed to spin imbalance due to differences between

4. CONCLUSION A swagelok cell, commonly used in the electrochemical tests of electrode materials for Li-ion batteries, has been modified for operando Mössbauer experiments. This cell has been used to quantitatively follow the conversion reaction that transforms a nanostructured FeSn2 based electrode into Li7Sn2 and Fe during the first discharge at C/10. These two products have been characterized in the electrode material at the end of the first discharge. Our analysis of the 119Sn nuclear quadrupole effect based on 119Sn Mössbauer experiments and DFT calculations is consistent with the nanosize of the Li7Sn2 particles. The superparamagnetism of the iron particles, observed from magnetic measurements, is a clear evidence of their small size. The average diameter, evaluated from three different analyses of I



Article

(22) Nwokeke, U. G.; Chadwick, A. V.; Alcántara, R.; Alfredsson, M.; Tirado, J. L. J. Alloys Compd. 2011, 509, 3074. (23) Mao, O.; Dahn, J. R. J. Electrochem. Soc. 1998, 145, 4195. (24) Mao, O.; Dunlap, R. A.; Courtney, I. A.; Dahn, J. R. J. Electrochem. Soc. 1999, 146, 414. (25) Dunlap, R. A.; Mao, O.; Dahn, J. R. Phys. Rev. B 1999, 59, 3494. (26) Mao, O.; Dunlap, R. A.; Dahn, J. R. Solid State Ionics 1999, 118, 99. (27) Nwokeke, U. G.; Alcántara, R.; Tirado, J. L.; Stoyanova, R.; Yoncheva, R.; Zhecheva, E. Chem. Mater. 2010, 22, 2268. (28) Nwokeke, U. G.; Alcántara, R.; Tirado, J. L.; Stoyanova, R.; Zhecheva, E. J. Power Sources 2011, 196, 6768. (29) Chamas, M.; Lippens, P. E.; Jumas, J. C.; Boukerma, K.; Dedryvère, R.; Gonbeau, D.; Hassoun, J.; Panero, S.; Scrosati, B. J. Power Sources 2011, 196, 7011. (30) Huo, H.; Chamas, M.; Lippens, P. E.; Ménétrier, M. J. Phys. Chem. C 2012, 116, 2390. (31) Roberts, G. A.; Stewart, K. D. Rev. Sci. Instrum. 2004, 75, 1251. (32) Ibarra-Palos, A.; Strobel, P.; Proux, O.; Hazemann, J. L.; Anne, M.; Morcrette, M. Electrochim. Acta 2002, 47, 3171. (33) Morcrette, M.; Chabre, Y.; Vaughan, G.; Amatucci, G.; Leriche, J. B.; Patoux, S.; Masquelier, C.; Tarascon, J. M. Electrochim. Acta 2002, 47, 3137. (34) Johnson, C. S.; Kropf, A. J. Electrochim. Acta 2002, 47, 3187. (35) Burba, C. M.; Frech, R. Electrochim. Acta 2006, 52, 780. (36) Heinen, M.; Chen, Y. X.; Jusys, Z.; Behm, R. J. Electrochim. Acta 2007, 52, 5634. (37) Lippens, P. E.; Jumas, J. C. Nanocomposites, Ionic Conducting Materials, and Structural Spectroscopies; Knauth P., Schoonman J., Eds.; Springer: New York, 2007; Vol. 10, p 247, ISBN: 978-0-387-33202-4. (38) Lippens, P.E.; Jumas, J.C. (Guest Ed.)It’s all about the battery.... Mössbauer Eff. Ref. Data J. 2010, 33 (2). (39) Bousquet, C. M.; Lippens, P. E.; Aldon, L.; Olivier-Fourcade, J.; Jumas, J. C. Chem. Mater. 2006, 18, 6442. (40) Dunlap, R. A.; Obrovac, M. N.; Li, J.; Smith, A.; Hatchard, T. D.; Sanderson, R. J.; Dahn, J. R. Mössbauer Eff. Ref. Data J. 2010, 33, 32. (41) Wattiaux, A.; Fournès, L.; Delmas, C. Mössbauer Eff. Ref. Data J. 2010, 33, 37. (42) Ariyoshi, K.; Ohzuku, T. Mössbauer Eff. Ref.Data J. 2010, 33, 43. (43) Ionica-Bousquet, C. M.; Munoz-Rojas, D.; Casteel, W. J.; Pearlstein, R. M.; Girishkumar, G.; Pez, G. P.; Palacin, M. R. J. Power Sources 2010, 195, 1479. (44) Ruebenbauer, K.; Birchall, T. Hyperfine Interact. 1979, 7, 175. (45) Blaha, P.; Schwarz, K.; Madsen, G. K. H.; Kvasnicka, D.; Luitz, J. WIEN2K An Augmented Plane Wave Plus Local Orbitals Program for Calculating Crystal Properties; Vienna University of Technology: Vienna, 2001. (46) Perdew, J. P.; Burke, S.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (47) Blaha, P.; Schwarz, K.; Herzig, P. Phys. Rev. Lett. 1985, 54, 1192. (48) Armbrüster, M.; Schmidt, M.; Cardoso-Gil, R.; Borrmann, H.; Grin, Y. Z. Kristallogr. NCS 2007, 222, 83. (49) Winter, M.; Bessenhard, J. O. In Lithium Ion Batteries; Wakihara, M.; Yamamoto, O., Eds.;Wiley-VCH: Berlin, 1998; p 135. (50) Le Caër, G.; Matteazzi, P.; Fultz, B. J. Mater. Res. 1992, 7, 1387. (51) Le Caër, G.; Malaman, B.; Venturini, G.; Fruchart, D.; Roques, B. J. Phys. F 1985, 15, 1813. (52) Venturini, G.; Malaman, B.; Le Caër, G.; Fruchart, D. Phys. Rev. B 1987, 35, 7038. (53) Wiebel, A.; Bouchet, R.; Savin, S. L. P.; Chadwick, A. V.; Lippens, P. E.; Womes, M.; Knauth, P. ChemPhysChem 2006, 7, 2377. (54) Yalha, H.; Boukra, A.; Belhakem, M.; Lippens, P. E. Solid State Com. 2009, 149, 2202. (55) Lippens, P. E.; Olivier-Fourcade, J.; Jumas, J. C. Hyperfine Interact. 2000, 126, 137. (56) Dunlap, R. A.; Small, D. A.; MacNeil, D. D.; Obravac, M. N.; Dahn, J. R. J. Alloys Compd. 1999, 289, 135. (57) Robert, F.; Lippens, P. E.; Olivier-Fourcade, J.; Jumas, J. C.; Gillot, F.; Morcrette, M.; Tarascon, J. M. J. Solid State Chem. 2007, 180, 339. (58) Frank, U.; Muller, W. Z. Naturforsch B 1975, 30, 316.

the magnetic data of the fully lithiated electrode, is about 3 nm. The observed nonlinear variations of the electrode magnetization during the discharge have been interpreted from the magnetic interactions between FeSn2 and iron particles. The latter particles have constant size and are continuously formed within the electrode during the discharge. Thus, the first discharge of nanostructured FeSn2 based electrode is an important restructuring step that transforms the pristine electrode into α−Fe/Li7Sn2 nanocomposite, which should be considered as the real electrode material for cycling.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was carried out in the framework of Alistore European Research Institute. The authors are grateful to the European Community and the “Région Languedoc Roussillon” for financial support.

■

REFERENCES

(1) Beaulieu, L. Y.; Eberman, K. W.; Turner, R. L.; Krause, L. J.; Dahn, J. R. Electrochem. Solid State 2001, 4, A137. (2) Xu, Y.; Guo, J.; Wang, C. J. Mat. Chem. 2012, 22, 9562. (3) Zheng, Y.; Yang, J.; Nuli, Y. N.; Wang, J. L. J. Power Sources 2007, 174, 624. (4) Aboulaich, A.; Mouyane, M.; Robert, F.; Lippens, P. E.; OlivierFourcade, J.; Willmann, P.; Jumas, J. C. J. Power Sources 2007, 174, 1224. (5) Mouyane, M.; Womes, M.; Jumas, J. C.; Olivier-Fourcade, J.; Lippens, P. E. J. Power Sources 2012, 204, 139. (6) Courtney, I. A.; Dahn, J. R. J. Electrochem. Soc. 1997, 144, 2045. (7) Robert, F.; Lippens, P. E.; Olivier-Fourcade, J.; Jumas, J. C.; Morcrette, M. J. Power Sources 2005, 146, 492. (8) Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Science 1997, 276, 1395. (9) Courtney, I. A.; Dunlap, R. A.; Dahn, J. R. Electrochem. Acta 1999, 45, 51. (10) Robert, F.; Morato, F.; Aldon, L.; Lippens, P. E.; OlivierFourcade, J.; Jumas, J. C.; Simon, B.; Biensan, P. J. Power Sources 2003, 119, 581. (11) Beaulieu, L. Y.; Larcher, D.; Dunlap, R. A.; Dahn, J. R. J. Electrochem. Soc. 2000, 147, 3206. (12) Naille, S.; Ionica, C. M.; Lippens, P. E.; Robert, F.; Morato, F.; Olivier-Fourcade, J. J. Power Sources 2007, 174, 1091. (13) Todd, A. D. W.; Mar, R. E.; Dahn, J. R. J. Electrochem. Soc. 2006, 153, A1998. (14) Naille, S.; Mouyane, M.; El Amraoui, M.; Lippens, P. E.; Jumas, J. C.; Olivier-Fourcade, J. Hyperfine Interact. 2008, 187, 19. (15) Wang, X. L.; Han, W. Q.; Chen, J.; Graetz, J. Appl. Mater. Interfaces 2010, 2, 1548. (16) Ferguson, P. P.; Rajora, M.; Dunlap, R. A.; Dahn, J. R. J. Electrochem. Soc. 2009, 156, A204. (17) Li, J.; Le, D. B.; Ferguson, P. P.; Dahn, J. R. Electrochem. Acta 2010, 53, 2991. (18) Ferguson, P. P.; Todd, A. D. W.; Dahn, J. R. Electrochem. Commun. 2010, 12, 1041. (19) http://www.Sony.net/SonyInfo/News/Press/200502/05-006E. Accessed June 14, 2013. (20) Ferguson, P. P.; Liao, P.; Dunlap, R. A.; Dahn, J. R. J. Electrochem. Soc. 2009, 56, A13. (21) Zhang, R.; Uprti, S.; Whittingham, M. S. J. Electrochem. Soc. 2011, 158, A1498. J



Article

(59) Lippens, P. E. Phys. Rev. B 1999, 60, 4576. (60) Naille, S.; Dedryvère, R.; Leroy, S.; Martinez, H.; Lippens, P. E.; Jumas, J. C.; Gonbeau, D. J. Power Sources 2007, 174, 1086. (61) Naille, S.; Lippens, P. E.; Morato, F.; Olivier-Fourcade, J. Hyperfine Interact. 2006, 167, 785. (62) Ehinon, K. K. D.; Naille, S.; Dedryvère, R.; Lippens, P. E.; Jumas, J. C.; Gonbeau, D. Chem. Mater. 2008, 20, 5388. (63) Naille, S.; Mouyane, M.; El Amraoui, M.; Lippens, P. E.; Jumas, J. C.; Olivier-Fourcade, J. Hyperfine Interact. 2008, 187, 19. (64) Vértes, A.; Korecz, L.; Burger, K. Mössbauer Spectroscopy, Studies in Physical and Theoretical Chemistry 5; Elsevier Scientific Publishing Company: Philadelphia, 1979; p 33. (65) Huber, D. L. Small 2005, 5, 482. (66) Lacroix, L. M.; Lachaize, S.; Falqui, A.; Blon, T.; Carrey, J.; Raspaud, M.; Dumestre, F.; Amiens, C.; Margeat, O.; Chaudret, B.; Lecante, P.; Snoeck, E. J. Appl. Phys. 2008, 103, 07D521. (67) Zhang, D.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Phys. Rev. B 1998, 58, 14167. (68) Grinstaff, M. W.; Salamon, M. B.; Suslick, K. S. Phys. Rev. B 1993, 48, 269. (69) Kura, H.; Takahashi, M.; Ogawa, T. J. Phys. Chem. C 2010, 114, 5835. (70) Gangopadhyay, S.; Hadjipanayis, G. C.; Dale, B.; Sorensen, C. M.; Klabunde, K. J.; Papaefthymiou, V.; Kostikas, A. Phys. Rev. B 1992, 45, 9778. (71) Chamas, M.; Lippens, P. E.; Jumas, J. C.; Hassoun, J.; Panero, S.; Scrosati, B. Electrochim. Acta 2011, 56, 6732. (72) Stoner, E. C.; Wohlfarth, E. P. Philos. Trans. R. Soc. London Ser. A 1948, 240, 599. (73) Carvell, J.; Ayieta, E.; Gavrin, A.; Cheng, R.; Shah, V. R.; Sokol, P. J. Appl. Phys. 2010, 107, 103913. (74) Xiao, G.; Chien, C. L. J. Appl. Phys. 1988, 63, 4252. (75) Hohl, G. F.; Hihara, T.; Sakurai, M.; Konno, T. J.; Sumiyama, K.; Hensel, F.; Suzuki, K. Appl. Phys. Lett. 1995, 66, 385. (76) Charles, S. W.; Popplewell, J. Ferromagnetic Materials; Wohlfarth, H., Ed.; North Holland Publishing Co.: Amsterdam, 1982; Vol. 2. (77) Kittel, C. Phys. Rev. 1946, 70, 965. (78) Gong, W.; Li, H.; Zhao, Z.; Chen, J. J. Appl. Phys. 1991, 69, 5119. (79) Skomski, R. J. Phys: Condens. Matter. 2003, 15, R841. (80) Argyle, B. E.; Charap, S. H.; Pugh, E. W. Phys. Rev. 1963, 132, 2051. (81) Bloch, F. Z. Phys. 1930, 61, 206. (82) Banerjee, S.; Roy, S.; Chen, J. W.; Chakravorty, D. J. Magn. Magn. Mater. 2000, 219, 45. (83) Bodker, F.; Morup, S.; Linderoth, S. Phys. Rev. Let. 1994, 72, 282. (84) Margeat, O.; Respaud, M.; Amiens, C.; Lecante, P.; Chaudret, B. B. J. Nanotechnol. 2010, 1, 108. (85) Yoon, M.; Kim, Y. M.; Kim, Y.; Volkov, V.; Song, H. J.; Park, Y. J.; Vasilyak, S. L.; Park, I. W. J. Magn. Magn. Mater. 2003, 265, 357.

K


Quantitative Analysis of the Initial Restructuring Step of

Recommend Documents