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Journal ofMaterials Chemistry A

PAPER

aICMPE, CNRS-UMR 7182, 2-8, rue Henri DbLRCS, CNRS-UMR 7314, 33 rue Saint Leu,cSOLEIL, L'Orme des Merisiers, Saint-Aubin

E-mail: [email protected]

Cite this: J. Mater. Chem. A, 2013, 1,4706

Received 13th December 2012Accepted 4th February 2013

DOI: 10.1039/c3ta01482g

www.rsc.org/MaterialsA

4706 | J. Mater. Chem. A, 2013, 1, 47

XAS investigations on nanocrystalline Mg2FeH6 used asa negative electrode of Li-ion batteries

Junxian Zhang,*a Warda Zaıdi,b Valerie Paul-Boncour,a Karine Provost,a

Alain Michalowicz,a Fermın Cuevas,a Michel Latroche,a Stephanie Belin,c

Jean-Pierre Bonnetb and Luc Aymardb

In the frame of research on new metallic hydrides as conversion reaction materials for negative electrodes

of Li-ion batteries, the MgFe2H6 complex hydride has been investigated in and ex situ using XRD, XAS and

magnetic measurements at different states: initially ball milled complex hydride (CH), electrochemically

desorbed (ED) and thermally desorbed (TD). Fe–H bonding is clearly evidenced by EXAFS in the CH

sample. It is also observed that the electrochemical reaction leads to a nanocrystalline state that needs

local probe analyses to be fully investigated and interpreted. From the XAS and magnetic data, the

different routes (ED and TD) for dehydrogenation of the complex hydride are compared. For both

electrochemically and thermally driven reactions, the hydrogen depletion from Mg2FeH6 hydride leads

to decomposition into its constituting elements Mg and Fe. However, different Fe structures are

observed: bulk a-Fe and amorphous Fe nanoparticles for TD and ED samples, respectively.

I Introduction

The development of advanced rechargeable lithium ionbatteries for efficient energy storage to satisfy on board orstationary applications is one of the most important challengesof the next decades. Following the study of conversion reactionswith oxides, suldes, nitrides, phosphides or uorides,1–7 theuse of metallic hydrides as negative electrodes for Li-ionbatteries has been recently proposed and has attracted stronginterest.8–10 Amongmetal hydrides, MgH2 is very promising withfour times the capacity of graphite electrodes. The reactivity ofmagnesium hydride with lithium ions proceeds through areversible lithium-controlled conversion reaction leading to theformation of magnesium metal and lithium hydride.11,12 Thisconversion mechanism is not limited to MgH2 and can begeneralized to many binary and ternary hydrides as long as theyare less stable than LiH.8

It has been shown that such conversion reactions canproceed outstandingly more efficiently with nanosized metallichydrides.8,12 Nanoscaled materials constitute, therefore, apromising class of compounds to enhance the performance oflithium-ion batteries due to their intrinsic physical and chem-ical properties caused by downsizing. Signicant advantagescompared to bulk compounds such as a faster charge/dischargerate due to a short diffusion path for lithium, a high surface-to-

unant, 94320 Thiais, France

80039 Amiens, France

– BP 48, 91192 Gif-sur-Yvette, France.

06–4717

volume ratio and an improvement of electron carrying abilitycan be expected. As a consequence, high specic capacities andgood reversibility can be potentially obtained.

Pursuing the exploration of compounds with high storagecapacities, Mg2TMHx hydrides (TM ¼ Fe, Co, Ni; x ¼ 6, 5 and 4,respectively) have been recently studied as negative electrodesfor lithium ion batteries.13 They were produced in a nano-crystalline state by reactive ball milling.14 Among these threehydrides, a full conversion reaction has been obtained forMg2FeH6 leading to the largest electrochemical capacity. Thein situ XRD patterns collected during electrochemical dischargeshow a progressive vanishing of the diffraction peaks of thecomplex hydride and the formation of a poorly crystallizedmaterial.13 Moreover, Mossbauer experiments showed theoccurrence of a magnetically disordered phase for the fullydischarged sample.

The aim of this study is to ascertain whether hydrogendepletion from Mg2FeH6 hydride by electrochemical reactionwith lithium leads either to the decomposition of the Mg–Feintermetallic into the constituting elements, as expected fromequilibrium thermodynamics, or to any metastable Mg–Femetallic phases. Contrary to thermally driven hydride decom-position, lithium reaction with hydrides may lead to theformation of metastable phases due to low atomic mobility atroom temperature. For instance, we have reported on theformation of the metastable TiH phase during the reaction oftitanium dihydride with lithium.10 In order to determine thestructural evolution of Mg2FeH6 hydride during electrochemicaldischarge and to identify the end products, in situ X-rayabsorption spectroscopy (XAS) experiments have been

This journal is ª The Royal Society of Chemistry 2013

Paper Journal of Materials Chemistry A

performed at the Fe–K edge during electrochemical discharge ofa Mg2FeH6-based electrode. These results have been completedby magnetic measurements before and aer the electro-chemical reaction.

II Experimentala Synthesis

Mg2FeH6 complex hydride (sample CH) has been synthesized byreactive ball milling of a mixture made of MgH2 powder (Sigma-Aldrich, hydrogen storage grade, #50 mm) and an appropriateamount of Fe powder (Neyco 99.9%, #44 micron) under astarting hydrogen pressure of 8 MPa. A typical powder weight of5 g was used. The milling conditions are described in ref. 14.

For the sake of comparison between electrochemical andsolid–gas routes, a thermally desorbed sample (sample TD) hasbeen obtained by thermal programmed desorption (TPD) fromroom temperature to 723 K at a heating rate of 2 K min�1. ASievert-type manometric system with calibrated and thermo-stated volumes was used for this purpose. All sample manipu-lations were done in an argon lled and puried glove box.

In addition, an electrochemically discharged sample (sampleED) has also been prepared ex situ before the XAS experimentsby using a dedicated electrochemical cell15 with a mixture of 26mg of Mg2FeH6 and 8 mg of carbon SP. A one-lithium in 20 hrate was used for the discharge. At the end of the process, 6 Liatoms had reacted. For the in situ XRD measurements, theMg2FeH6 was ball milled for 5 hours in a SPEX 8000 mixer-millwith 10% of pre-ground graphite.16 This treatment enhances theelectric conductivity of the material and allows the fulldischarge of the electrode. The samples used for in situ XAS andmagnetic measurements have not been milled with carbon toprevent any contamination of additional Fe coming from theball milling tools.

b X-ray diffraction

The CH and TD samples have been characterized by X-raydiffraction (XRD) using a Bruker D8 Advance diffractometerwith Cu Ka radiation. To prevent any reaction with air, a specialair-tight sample holder from Bruker was used. Phase analysisand structural determination were done by using the full-proletting program FULLPROF based on Rietveld's method.17 Tofollow the electrochemically driven reaction, in situ XRD patternacquisition at various stages of the electrochemical dischargewas done and the results have been published recently.13

c XAS measurements

Ex situ XAS experiments have been rst carried out for CH, TDand ED samples. In addition, in situ XAS experiments wereconducted on the pristineMg2FeH6 compound to determine thestructural evolution during discharge.

For the ex situ XAS experiments, the complex hydride (CH)and the thermally desorbed (TD) samples were mixed withboron nitride and compressed into pellets in a glove box. Thequantities were calculated in order to obtain an edge jump Dmxnear 1, with a total absorbance aer edge mx(E) < 2 with


ABSORBIX, XAFS soware included in the MAX package.18 Ironmetal foil of 4 mm was used as the reference sample. For the EDsample, ex situ XAS experiments were performed on the dis-charged electrode (see Section IIa), kept inside the electro-chemical cell. For the in situ XAS experiments, a mixture of26 mg of Mg2FeH6 and 8 mg of carbon SP was introduced into aspecially designed electrochemical cell dedicated to in situstudies.15 The cell was discharged in galvanostatic mode at acurrent rate of three Li in 10 hours with a cut-off potential of 5mV versus Li+/Li0.

X-ray absorption data were collected at the Fe–K-edge intransmission mode on the SAMBA beam line19 at the Soleilsynchrotron source using a xed sagitally focusing Si(111)double crystal monochromator. Each spectrum was recordedthree times to minimize statistical errors. The EXAFS spectrawere extracted using standard procedures available in MAX-CHEROKEE,18 including polynomial absorption backgrounddetermination, energy-dependent normalization, fast FourierTransform (FT) and ltering of the rst-coordination spheresignal. The initial value of the edge position E0 ¼ 7119 eV,chosen for all the sample spectra, corresponds to the inexionpoint for the Fe foil. A unique interval of Dk ranging from 0.7 to10.6 A�1 was used for obtaining the FT of all EXAFS spectra.

All ts were performed with the MAX-RoundMidnight code18

by tting the experimental spectra with the EXAFS standardformula.20 For MgFe2H6, where the 2–4 A�1 domain is useful tocharacterize low Z atom signals, the total tting range is set to 2–10.6 A�1. In contrast,we restricted thet range to4–10.6 A�1 fora-Fe, TD and ED samples since EXAFS spectra only displaymetalliccontributions and a systematic error was found in the theoreticalFEFF model around 3.5 A�1 for the pure metal spectrum. In thisstudy, only single scattering paths of the rst neighbours weretaken into account to t the rst-coordination sphere. We usedtheoretical amplitudes and phases calculated by the FEFF8code,21,22 on the basis of the Mg2FeH6 (ref. 23–25) and the a-Fecrystallographic structures. The inelastic losses are modelled byan electron reduction factorS0

2 set to 1 andaphotoelectronmeanfree path l set to the standard empirical formula:20

l ¼ 1

G

�k þ

�hk

�4�;

where G and h are constants (G ¼ 1 and h ¼ 3). These standardparameters lead to correct ts for the two reference compoundsof known crystal structure (Fe andMg2FeH6). For these samples,we have adjusted the distance R and rened a unique energythreshold (DE0) and the Debye–Waller factor (s2) for all shells.For the ED sample, it was necessary to rene the number ofneighbours (N) and to introduce cumulants (C3, C4)26 in order totake into account the asymmetry of the distance distribution. Inthe cumulant expansion, the EXAFS oscillation function of eachscattering path is given by the equation

cðkÞ ¼ s02e�2C1=l

C12

j f ðk;pÞjNexp

�� 2k2C2 þ 2

3k4C4 þ.

�

� sin

�2kC1 � 4

3k3C3 þ.þ FðkÞ

�

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Journal of Materials Chemistry A Paper

where C1 ¼ R is the average effective distance, C2 ¼ s2 thedistance variance (or the Debye–Waller factor), C3 measures thedistance distribution asymmetry and C4 its atness. Statisticalerrors on the average spectra (#0.015 A�1) and error bars for thetted parameters were evaluated as recommended by the IXSStandard and Criteria Subcommittee.27 According to theserecommendations, the goodness of t or quality factor is QF ¼Dcmin

2/n, where Dcmin2 is the minimum value of the statistical

Dc2 and n ¼ Nind � Npar is the degree of freedom. Nind is thenumber of independent points and Npar is the total number oftted parameters. Since a k3-weighting is used, the statisticalDc2 is calculated as proposed by Vlaic et al.28 The XANES spectraof the electrochemical in situ experiments were treated as alinear combination of CH and ED samples with the programMAX-StraightNoChaser.29 The procedure to determine theerror bars and the quality factor is the standard denition forlinear t.30

d Magnetic measurements

The magnetic properties of CH, TD and ED samples weremeasured using the Physical Properties Measurements System(PPMS) device from quantum design. The magnetization curvesversus temperature were measured in the Zero Field Cooled(ZFC) and Field Cooled (FC) modes with an applied eld of 300Oe and versus eld up to 9 T at 2 K and 300 K. The AC suscep-tibilities were measured with a eld of 5 Oe and a frequency of5 kHz for all samples and for frequencies between 10�2 and10 kHz for the ED sample.

III ResultsXRD results

The XRD patterns of samples CH, TD and ED are presented inFig. 1. The complex metal hydride CH crystallizes in the cubicFm�3m space group with a ¼ 6.442 A. Upon heating at 723 K(sample TD), it decomposes into a-Fe and Mg metal particles.

Fig. 1 X-ray diffraction patterns (Cu-Ka radiation) of the complex hydride (CH),thermo-desorbed sample (TD) and electrochemical discharged sample (ED). Thedifferent phases are labeled with symbols.

4708 | J. Mater. Chem. A, 2013, 1, 4706–4717

Aer full discharge in the electrochemical cell (sample ED), theXRD pattern shows only a wavy background, which can beexplained either by an amorphisation process or by the forma-tion of very small nanocrystalline particles. The presence of atiny a-Fe peak in sample ED is attributed to milling toolcontamination during the milling with carbon before intro-ducing it into the electrochemical cell.13

XAS results

(a) Comparison of the experimental data. The XANESspectra at the Fe–K-edge are displayed for all samples (CH, TD,ED and Fe foil) in Fig. 2. Edge spectra of samples TD, ED and Fefoil are quite similar with a main shoulder at 7115 eV and themaximum at 7131 eV. The XANES spectra of sample TD and Fefoil are identical. The ED spectrum is characterized by asmoother maximum, whereas the CH sample spectrum is verydifferent with a shi of the shoulder and maximum of 5 and4.2 eV, respectively.

The EXAFS spectra and the corresponding Fourier Trans-form (FT) of the four samples are compared in Fig. 3a and b,respectively. One can observe that the signals of sample TD inboth k and R spaces are very close to those of the Fe foil. Thelower amplitude of sample TD can be explained by smallinhomogeneities in the Fe powder distribution within theFe/BN pellet.31

Moreover, the EXAFS signals of the CH and ED samplesexhibit signicantly lower intensities than those of Fe metal.The difference in the oscillation frequencies also indicatesdistinct distances and chemical nature of the neighbour atoms.This is conrmed by the Fourier transform: the main peak of EDis shied to shorter distances (DR¼�0.16 A) and shows a largerintensity (+80%) than for sample CH. In addition, the EDsample has almost no contribution for R > 3 A, whereas somecontributions are clearly observed in sample CH between 3.5and 6 A. This indicates the occurrence of either a disorderedstructure or very small nanoparticles in the ED sample.

To quantify these differences, a renement of the EXAFSspectra has been undertaken.

Fig. 2 Comparison of the XANES spectra at the Fe–K-edge of Mg2FeH6 complexhydride (CH), Fe metal foil (Fe), thermally desorbed Mg2FeH6 (TD) and electro-chemically discharged Mg2FeH6 (ED). The dashed lines are only guides to the eyeto highlight the shoulder (7115 eV) and maximum (7131 eV) positions in the Fefoil spectrum.


Fig. 3 (a) Normalized EXAFS kc(k) data and (b) the moduli of the Fourier transform k3c(k) of Mg2FeH6 complex hydride (CH), Fe metal foil (Fe), thermally desorbedMg2FeH6 (TD) and electrochemically discharged Mg2FeH6 (ED). The vertical dashed line in (b) is a guide to the eye to show the maximum of the main FT peak in the Fefoil spectrum.


(b) Renement of the EXAFS dataMg2FeH6 (sample CH). The FT of sample CH has been ltered

in the R range 0.72–3.14 A. The ltered spectrum can be wellrened starting from the known crystal structure of Mg2FeH6

(ref. 32) in which Fe atoms are surrounded by one shell of 6 Hatoms at R ¼ 1.51 A and one shell of 8 Mg atoms at R ¼ 2.74 A.The results are detailed in Table 1 and the rened lteredEXAFS data are presented in Fig. 4. To conrm the contributionof Fe–H scattering paths to the EXAFS signal, the spectrum hasbeen rened with only the Fe–Mg shell. In this case, the qualityfactor QF increases to 1.1 (for 8 Fe–Mg only) compared to 0.7(for 6 Fe–H and 8 Fe–Mg). The renement with only Fe–Mgsignal allows only a partial reproduction of the EXAFS signal:the main peak in R-space is well tted, but the signal around 1.2A is not reproduced at all (Fig. 5a). On the other hand, a t withonly 6 Fe–H nicely describes the signal in R-space below 1.7 Aand centered at 1.2 A (Fig. 5b). These results conrm that the FTpeak around 1.2 A is not due to data treatment, but due to thecontribution of H atoms around the Fe atoms in the Mg2FeH6

structure.Thermally desorbed Mg2FeH6 (sample TD). Fig. 6 shows the

experimental and rened EXAFS signals (k3c(k)) and the corre-sponding FT for sample TD. Best t values are given in Table 2.The EXAFS signal of the desorbed sample is well reproduced onthe basis of a-Fe crystal structure with the space group Im�3m, inagreement with XRD results.

Electrochemically discharged Mg2FeH6 (sample ED). For sampleED, the absence of peaks in the XRD pattern suggests thatelectrochemical discharge transforms Mg2FeH6 into either ahighly disordered or a low nanocrystalline state. Considering

Table 1 Results of the EXAFS refinement of sample CH (Mg2FeH6) andcomparison with XRD data obtained in this work

Atom N R (A) (EXAFS) R (A) (XRD)

Shell 1 H 6 1.51(2) 1.528Shell 2 Mg 8 2.74(1) 2.790s2 (A2) 0.0137(5)DE0 (eV) �0.5(5)QF 0.7


that XAS probes the Fe local order, we have assumed differentmodels to rene the experimental data: (i) Fe forms nano-crystalline particles with the same local order as that of a-Fe; (ii)Fe is amorphous with a large distribution of Fe–Fe distances,(iii) Fe participates in the formation of a metastable Fe–Mgphase (there are no stable Fe–Mg phases in the binary phasediagram).

The experimental spectrum has been ltered by the mainpeak of the FT between 1.46 and 3.14 A. The experimental andrened signals with different models are presented in Table 3and Fig. 7. First, the renement with the crystal structure of a-Fe(Model 1), i.e. with two Fe shells, gives a quality factor of 4.84,which is large compared to that obtained for sample CH. Inaddition, the distance of the second Fe shell is shied to 2.71 Acompared to 2.86 A, a difference that is beyond the experi-mental error. The signals in both k- and R-spaces are not welltted (Fig. 7a and b). In R-space, the calculated signal obtainedwith this model presents a shoulder on the right of the mainpeak, due to an interference effect between the two Fe layers,that is completely absent in the experimental signal. Finally, theenergy shi of 7.5 eV is very large compared to that obtained forthe two previous CH and TD samples. These results indicatethat the discharged sample presents a different local order thancrystalline Fe, in which two separate distances are observed.

Then, we considered a t (Fig. 7c and d) with only one Felayer (Model 2a). The quality of the t is improved (QF ¼ 2.67),the value of energy shi is close to zero, the number of Feneighbours is small (N ¼ 6) and the Fe–Fe distance is short(R ¼ 2.47 A), but the main peak presents an asymmetry at largerdistances. To improve the t quality (Fig. 7e and f), we haveintroduced C3 and C4 cumulants (Model 2b) which allowstaking into account an asymmetric distance distribution and adeviation from a Gaussian distribution, respectively. Thisapproach has already been used to describe amorphous Co andFe lms.33 The introduction of C3 and C4 cumulants improvesQF by a factor 2 and leads to a larger Fe–Fe mean distance of2.53 A, whereas N increases with C4. These results, which areexpected for a large and asymmetric distance distribution andthe absence of a signicant signal at larger distances in the FTsignal, can be interpreted as the formation of an amorphousFe phase.

J. Mater. Chem. A, 2013, 1, 4706–4717 | 4709

Fig. 4 Refinement of EXAFS data for sample CH (Mg2FeH6): (a) k3c(k) and (b) modulus and imaginary parts of the Fourier transform (k3c(k), Dk ¼ 2–10.6 A�1). Circles

indicate experimental data and the red line indicates the fit.

Fig. 5 Refinement of FT signals for sample CHwith one-shell models: (a) FT (modulus and imaginary part) with only the Fe–Mg shell and (b) FTwith only the Fe–H shell.Filled and open circles indicate real and imaginary parts, respectively. The red line corresponds to the fit.

Fig. 6 Refinement of EXAFS data for sample TD (desorbed Mg2FeH6): (a) k3c(k) and (b) modulus and imaginary parts of the Fourier transform (k3c(k),Dk ¼ 4–10.9 A�1). Circles indicate experimental data and the red line indicates the best fit.

4710 | J. Mater. Chem. A, 2013, 1, 4706–4717 This journal is ª The Royal Society of Chemistry 2013


Table 2 Results of the EXAFS refinement of sample TD and comparison withXRD data obtained in this work

Atom N R (A) (EXAFS) R (A) (XRD)

Shell 1 Fe 8 2.463(5) 2.479Shell 2 Fe 6 2.833(7) 2.860s2 (A2) 0.006(1)DE0 (eV) �1(1)QF 0.7


Several attempts have also been made to rene the EXAFSsignal with one Fe and one Mg shell to check the possibleformation of a metastable Fe–Mg alloy (Models 3a and 3b). InMg2FeH6, besides the closer H-atoms, Fe is surrounded by 8 Mgand 12 Fe atoms at 2.79 and 4.56 A respectively. Furthermore,no crystal structures of metastable Mg2Fe or Mg–Fe phases havebeen described in the literature. At this point, it is worthconsidering crystal structures of closely related Mg2TM phasesthat are thermodynamically stable, such as Mg2Ni and Mg2Cu.34

For tetragonal Mg2Ni, the Ni atoms are surrounded by 2 Ni at2.67 A and 8 Mg at 2.72 A. In orthorhombic Mg2Cu, the Cuatoms are surrounded by 2 Cu at 2.62 A and 8 Mg at distancesbetween 2.72 and 2.77 A. Therefore, it is reasonable here to startwith a model based on 2 Fe and 8 Mg atoms, varying thedistances around those observed for Mg2Ni and Mg2Cu alloys(Model 3a). The renement of the experimental data with oneshell made of 2 Fe at 2.46 A and one shell of 8 Mg at 2.70 A givesa quality factor of 8.29. The signals in both k- and R-spaces arenot well tted (Fig. 7g and h). In R-space, the calculated main

Table 3 Results of the EXAFS refinement for the electrochemically discharged samgiven in parentheses for the refined parameters

Model

Model 1: a-Fe

Atom N R (A) EXAFS R (A) XRD

Shell 1 Fe 8 2.51(2) 2.479Shell 2 Fe 6 2.71(2) 2.860s2 (A2) 0.011(1)DE0 (eV) 8(2)C3 —C4 —QF 4.84

Model

Model 3 Mg–Fe alloy

(a) Metastable Mg2Fe alloy (N xed)

Atom N xed

Shell 1 Fe 2Shell 2 Mg 8s2 (A2) 0.011(2)DE0 (eV) �7(2)QF 8.29


peak is shied to a higher R value compared to the experimentalone. Consequently, the hypothetical formation of a metastableMg2Fe alloy can be excluded there. Assuming the possibleformation of a Mg–Fe alloy, both the number and distance ofneighbouring atoms have been rened. Using this model(Model 3b), the best t is obtained with 6.9 Fe at 2.46 A and 8.2Mg at 2.63 A with a quality factor of 2.85 (Fig. 7i and j). Althoughthe t is better than with xed N, the quality is still worse thanusing only one Fe shell and adding two cumulants (Model 2b).The strongly negative value of the energy shi suggests that thephases used in this model are not appropriate to t thisexperimental spectrum and consequently that there are actuallyno Mg atoms around Fe atoms.

To conclude, the best t is obtained when the ltered EXAFSsignal is tted with only one layer of 8.6 Fe at 2.53 A and anasymmetric distance distribution, corresponding to the forma-tion of amorphous Fe. This is supported by the signicantimprovement of the quality factor compared to those obtainedfor the other models (a-Fe and Mg–Fe alloys). The absence ofsignal in the FT for R > 3 A is also a conrmation of a stronglydisordered material.

(c) In situ analysis. In order to follow the evolution ofMg2FeH6 during the electrochemical discharge, XANES spectrahave been collected in situ. Ninety-one XANES spectra wererecorded over 15 h.35 Some characteristic spectra for different Licontents are presented in Fig. 8. At the end of discharge, thenumber of lithium atoms that reacted with Mg2FeH6 reached 4Li/Mg2FeH6 (Fig. 10a). The observed spectra were interpreted asa linear combination of the starting CH and ending ED ones(Fig. 9). All spectra can be described in this way showing that

ple ED. The XRD values are deduced from the structure of Fe. The error bars are

Model 2: amorphous Fe

(a) With Gaussian Rdistribution

(b) With asymmetric Rdistribution

N R (A) EXAFS N R (A) EXAFS

6(1) 2.47(1) 8.6(7) 2.53(1)— — — —0.012(2) 0.019(1)0(2) 3.7(9)— 0.0010(1)— 0.0002(1)2.67 1.2

(b) Metastable Mg–Fe alloy

R (A) EXAFS N tted R (A) EXAFS

2.46(3) 7(1) 2.46(1)2.70(4) 8(1) 2.63(3)

0.014(1)�8(1)2.85

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Fig. 7 Refinement of EXAFS data for the electrochemically discharged sample (ED) k3c(k) and FT using different models (see Table 3): a-Fe (a and b), amorphous Fewith Gaussian R distribution (c and d), amorphous Fe with asymmetric distance distribution (e and f), metastable Mg2Fe alloy (g and h) and metastable Mg–Fe alloy (iand j). Circles indicate experimental data and the red line indicates the fit.

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Fig. 8 Evolution of selected XANES spectra of Mg2FeH6 during in situ discharge.For each spectrum, the number of reacted lithium atoms per Mg2FeH6 formulaunit is given in the legend.

Fig. 9 Linear combination (red line) of samples CH (75%) and ED (25%)accounting for the experimental XANES spectrum (open circles) recorded at 2.2 Liduring in situ discharge. The difference spectrum corresponds to the differencebetween the experimental spectrum (open circles) and the one obtained from thelinear combination (red line).

Fig. 10 Evolution of (a) electrode potential versus Li+/Li; (b) phase amounts as afunction of the number x of lithium during the in situ electrochemical discharge ofMg2FeH6, calculated from the linear combination of XANES spectra of samples CHand ED. The error bars are presented for each measurement (�0.5%).


Mg2FeH6 transforms progressively into amorphous Fe. The verygood quality factors obtained for all spectra in the linearcombination show that, at any stage of the reaction, the XANESspectra can be interpreted using only starting CH and endingED spectra and to conclude that no intermediate phase isobserved. The percentage of each phase versus reacted Li isreported in Fig. 10b. At the end of the experiment, only 46% ofamorphous Fe is formed, whereas 66% is expected according tothe partial 2/3rd advancement of the reaction.

Magnetic measurements

The ZFC and FC magnetization curves versus temperature M(T)for the three samples CH, ED and TD are presented in Fig. 11a.Samples CH and ED display almost no difference between theZFC and FC curves from 300 K down to 30 K. Below the lattertemperature, a large irreversibility is observed and peakmaximaofMZFC are noticed at TB¼ 20 K for samples CH and ED. SampleTD exhibits a signicant difference between ZFC and FC curvesbelow 300 K which increases continuously as the temperaturedecreases without a noticeable maximum of MZFC.


The real part (c0) of the AC susceptibilities is shown for thethree samples in Fig. 11b. Both CH and ED samples show a peakat low temperature. This peak maximum Tmax occurs at lowertemperatures and is narrower for sample ED than for the CHone. Sample TD exhibits only a smooth decrease of (c0) oncooling from 300 to 2 K.

For sample ED, c0 was also measured at different frequenciesy ranging between 10 and 104 Hz (Fig. 12). As the frequency y

increases, we observed a shi of Tmax toward larger values.According to the Neel–Brown model for particles withoutinteractions, the relaxation time follows an Arrhenius law:

sm ¼ soexp(EB/kTmax) (1)

where sm (1/y) is the relaxation time of the measurement, EB theenergy barrier and k the Boltzmann constant. From ourmeasurements, we found that log(sm) increases linearly versus1/Tmax with a slope EB/k ¼ 415 K.

Themagnetization curves versus eldM(H) at 2 and 300 K arereported in Fig. 13. The saturation magnetizations (MS) ofsample CH are 0.26mB and 0.19mB at 2 K and 300 K, respectively.For sample ED, the saturation is not reached at either of the twotemperatures. The magnetization at 9 T is close to 1.0mB at 2 Kand 0.41mB at 300 K. For sample TD, the saturation is reached at

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Fig. 11 (a) ZFC (up triangles)-FC (down triangles) magnetization measurements at 0.03 T (b) and the real part (c0) of the AC susceptibility at a frequency of 5 kHz forsamples CH, ED and TD. The blocking temperature (TB) and the temperature maxima (Tmax) for the ZFC and AC susceptibilities respectively are given in the figure forsamples ED and CH.


2 T withMS¼ 1.95 and 1.84mB at 2 and 300 K, respectively. Thesevalues are close to the Fe moment in a-Fe (M ¼ 2.2mB). A smallhysteresis is observed in the M(H) curves at 2 K (Hc ¼ 0.022,0.006, and 0.008 T for CH, TD and ED samples, respectively).

The observation of maxima for samples CH and ED in boththe MZFC and the c0 curves indicates the presence of Fe nano-particles in a superparamagnetic state above the blockingtemperature TB. In contrast, the absence of a maximum at lowtemperature for sample TD shows that the blocking tempera-ture is above 300 K, and therefore the average particle size ismuch larger.36 For samples CH and ED, the value of TB dependson the experimental conditions (applied eld, heating rate, andmeasuring time) and the volume distribution of the particles. Asthe three samples were measured in the same experimentalconditions, the relative variation of TB reects the difference ofparticle size.

Fig. 12 AC susceptibility of sample ED at various frequencies. Inset: frequencyvariation of the maximum temperature Tmax for sample ED.

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The AC susceptibilities reect the dynamic properties of themagnetic Fe nanoparticles, as the relaxation time is relatedto the frequency.37 As expected, the maximum of c0 is largerthan TB.

The smaller values of Tmax for sample ED indicate the pres-ence of Fe nanoparticles with a smaller particle size than forsample CH. According to the small value of TB (20 K), the meanparticle size of the ED sample should be around 5 nm.

The behaviour of the FC curves at low temperature dependson the interaction between the Fe particles: the increase of MFC

as T decreases below 30 K in sample ED reveals smaller inter-actions than in samples CH and TD for whichMFC is constant ordecreases when the temperature is reduced.38

Sample ED contains a larger amount of superparamagneticFe nanoparticles with a smaller size than sample CH. However,the difference between the ZFC and FC curves between 50 and300 K is attributed to a small contribution of Fe particles with alarger volume. The magnetization at 2 K shows a hysteresis witha coercive eld of 70 � 10�5 T. Its asymmetric shape is alsocharacteristic of the presence of Fe particles with different sizes.

IV Discussion

As observed by XRD, the nanocrystalline starting material CHprepared here by reactive ball milling crystallizes in the samestructure as the compound prepared by other methods usingdifferent pressures and temperatures.23–25,32,39 Local orderstudied by EXAFS at the Fe edge conrms that iron atoms have 8Mg neighbours at 2.79 A but in addition shows the neigh-bouring H atoms at a short distance. It must be emphasizedthat a previous study on YMn2H6, which crystallizes in a cubicstructure close to that of Mg2FeH6, has already shown thepossibility to observe such M–H bonds by EXAFS.40 Thisevidence of a Fe–H shell is attributed to the fact that (i) the 6rst H neighbours are located at a small distance (R ¼ 1.53 A),(ii) the 8 second Mg neighbours are situated at a much largerdistance than H (R ¼ 2.79 A) allowing a good separationbetween the two signals, (iii) the bonding between Fe and Hatoms is covalent with highly localized electrons around the


Fig. 13 Magnetization versus field at (a) 2 K and (b) 300 K for samples CH, TD and ED.


hydrogen atoms. Moreover, the XANES spectrum of Mg2FeH6

exhibits distinct structures as compared to Fe metal with theposition of the edge (rst inexion point) shied to higherenergy (E ¼ 7113 eV). This is expected according to the differentvalence states of Fe in these two compounds (FeII in Mg2FeH6

compared to Fe0). However, magnetic measurements reveal thepresence of superparamagnetic Fe particles in sample CH,whereas, according to Mossbauer spectroscopy,41 Fe isdiamagnetic in Mg2FeH6. This fact can be related either to aremaining contribution of non-reacted Fe nanopowder used asa precursor in the ball milling synthesis of Mg2FeH6 or to asmall contamination by the stainless steel milling tools duringprocessing. In any case, from the value of MS at 2 K, themaximum percentage of free Fe is estimated to be around10 wt%.

During the electrochemical discharge, the phase evolutionwas followed in situ by XAS. The at voltage plateau (Fig. 10a)reects a two-phase system. The in situ XANES spectra can betted by the linear combination of the starting sample (CH) andthe fully discharged one (ED). Besides, previous in situ XRDresults13 show the coexistence of the pristine hydride phase andan amorphous halo, without any structural or microstructuralchange in the hydride phase during electrochemical discharge.These facts indicate that upon discharge, the sample progres-sively transforms from crystalline Mg2FeH6 (CH) into the elec-trochemically discharged state (ED) with no intermediate phasetransition. This two-phase behaviour is similar to the hydrogenrelease from Mg2FeH6 by thermal desorption, though the Fephase is crystalline when prepared by the solid–gas reaction,whereas it is amorphous when prepared by the electrochemicalroute. In the electrochemical environment, two-phase equilib-rium reactions have been widely reported such as, for instance,the intercalation of Li in anatase TiO2 (ref. 42 and 43) andLiMnO2.44 At the end of the in situ experiment (x ¼ 4 Li), 50% ofthe complex metal hydride is consumed. Such reaction yield isbelow what one can expect. By comparison with our recentin situMossbauer experiments,41 80% of the hydride phase wereconverted at x ¼ 4 Li. Such difference can be explained by someinhomogeneous reactions within the electrochemical cell as theEXAFS beam probes only a tiny surface (a few mm2) compared tothat of Mossbauer experiments (mm2). Such disparities in the


reaction advancement have already been observed for LiFePO4

electrode materials and were attributed to poor surface homo-geneities within the electrochemical surface cell.15,35 It is verylikely that the same phenomenon takes place here.

At the end of the discharge, the XRD pattern of sample EDshows a wavy background except for a tiny peak attributed tocrystalline Fe. This small contribution of iron appears straightaer the ball milling of Mg2FeH6 with carbon and is constantduring discharge. It is once again attributed to some toolcontamination occurring during the milling preparation butnot to any electrochemical reaction. The XANES spectrum ofsample ED is much less structured than that of pure Fe orsample TD. However, the edge position dened by the rstmaximum of the derivative is similar for both samples (E0 ¼7111 eV). This conrms that Fe in the ED sample remains in ametallic state, in agreement with H depletion from Mg2FeH6

and its reaction with Li to form LiH. Indeed, the EXAFS analysisperformed on the ED sample shows that the best renement isobtained for only one Fe shell with an asymmetric distancedistribution. Previous EXAFS studies of amorphous Fe areavailable. Long et al.45 have studied amorphous Fe (30 nm)prepared by the sonochemical method and the EXAFS rene-ment of this sample also showed a single asymmetric Fe shell.However, their distance distribution model was different thanours and can hardly be compared. Chandesris et al.33 havereported the EXAFS analysis of a 20 A amorphous Fe lm, forwhich they found 10(1) Fe neighbours at 2.55(2) A using twocumulants, C2 ¼ 18 � 10�3 A2 and C3 ¼ 2.4 � 10�3 A3. Finally,Bellissent et al.46 have found by neutron diffraction that amor-phous Fe prepared by the sonochemical method has 8.7 Feneighbours at a distance around 2.56 A.

Our results for the ED sample (8.6 Fe at 2.53 A, s2 ¼ C2 ¼1.7 � 10�3 A2 and C3 ¼ 1.0 � 10�3 A3) compare well withChandesris' and Bellissent's results, which allows conrmationof the amorphous state of Fe in our sample. However, thequestion of the Fe particle size remains. For Fe nanoparticles,we expect a reduced average neighbour atom number due to thelarger ratio of surface atoms to bulk ones. However, thereduction of EXAFS coordination number for an amorphouscompound is not sufficient to conclude on the nanometer sizeof the particles. This point will therefore be discussed in the

J. Mater. Chem. A, 2013, 1, 4706–4717 | 4715


next paragraph dealing with magnetic experiments. Massbalance considerations imply that pure Mg nanodomains arealso formed. These domains are also probably amorphous sinceno crystalline Mg peaks are observed in the XRD patterns.

The magnetic property analysis of ED shows that the samplehas a superparamagnetic behaviour. It contains a much largeramount of superparamagnetic Fe nanoparticles than sampleCH with smaller and different sizes and probably a largervolume. The low values of the magnetization at 2 and 300 K, forsample ED compared to TD (which is expected to decomposeinto large Fe particles), can be explained by a large amount of Featoms at the surface and the amorphous state of these latterparticles. The low interaction between the nanoparticles indi-cates that they are probably dispersed in a matrix made of Mgand LiH. This again agrees with our previous Mossbauerstudy,41 which showed a doublet at room temperature due toeither superparamagnetic Fe nanoparticles or the formation ofa Mg1�xFex amorphous solid solution. As the EXAFS resultsallow the exclusion of the formation of such a Mg1�xFex alloy, itis worth concluding that the ED sample contains nanoparticlesmade of amorphous Fe. According to the small value of theblocking temperature TB (20 K), the mean Fe particle size isestimated to be around 5 nm. Our data do not allow us to fullycharacterize the remaining segregated magnesium or the LiHstates, but it is reasonable to assume that they are also in a non-crystalline state.

Finally, it is worth comparing samples ED and TD that haveundergone different decomposition routes: either electro-chemically or thermally driven ones, respectively. The XANESpattern and the EXAFS spectra of sample TD are very similar tothose of the Fe foil, indicating the decomposition of the hydrideduring thermal desorption into Fe and Mg particles, in agree-ment with XRD and magnetic results. As the thermodesorptionwas performed at 350 �C, both hydrogen desorption andgrowing of the Fe and Mg particles can take place leading tocrystalline elements. Therefore, the thermally desorbed sampleis composed of the same phases as the ED one (except LiH) butwith higher crystallinity. It can then be stated that the electro-chemical discharge of the Mg2FeH6 complex hydride leads tothe formation of LiH and decomposition into its constitutingelements Mg and Fe in the amorphous state, without theformation of any metastable Mg–Fe metallic phases.

V Conclusions

The compound Mg2FeH6 has been investigated at differentstates of reaction from the ball milled initial one to electro-chemically or thermally desorbed ones. The structural param-eters have been followed during discharge by means of localprobe measurements. Both electrochemical and thermal routeslead to the decomposition of the hydride into its constituentelements without the formation of any metastable intermetallicphase. The electrochemical reaction leads to amorphous phasesthat have been characterized by means of XAS and magneticmeasurements, whereas the thermal desorption process leadsto the formation of crystalline Mg and Fe phases.

4716 | J. Mater. Chem. A, 2013, 1, 4706–4717

Acknowledgements

The present work was undertaken within the ANR programSTOCK-E under the contract “NANOHYDLI”, ANR-09-STOCK-E-06-01.

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