At the Heart of a Conversion Reaction: An Operando X-ray Absorption

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Article

At the Heart of a Conversion Reaction: an Operando Xray Absorption Spectroscopy Investigation of NiSb, a Negative Electrode Material for Li-ion Batteries 2

Cyril Marino, Bernard Fraisse, Manfred Womes, Claire Villevieille, Laure Monconduit, and Lorenzo Stievano J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5097579 • Publication Date (Web): 11 Nov 2014 Downloaded from http://pubs.acs.org on November 17, 2014

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The Journal of Physical Chemistry

At the Heart of a Conversion Reaction: An Operando X-ray Absorption Spectroscopy Investigation of NiSb2, a Negative Electrode Material for Li-ion Batteries

Cyril Marino1, Bernard Fraisse1,2, Manfred Womes1, Claire Villevieille1,3, Laure Monconduit1,2 and Lorenzo Stievano1,2,*

1

ICGM (UMR 5253), AIME, Université Montpellier 2, CC 1502 Place Eugène Bataillon, 34095 Montpellier Cedex 5, France 2

3

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

Paul Scherrer Institute, Electrochemical Energy Storage Section, CH-5232 PSI Villigen, Switzerland

Abstract The reaction mechanism of NiSb2 with lithium was studied in detail by operando Ni K-edge X-ray absorption spectroscopy (XAS). The redox activity of the Ni centres was followed during the first two lithiation/delithiation cycles, showing that a reversible conversion takes place, with the formation of Ni metal nanoparticles at the end of lithiation and the reversible formation of a Ni-Sb phase close to pristine NiSb2 at the end of the delithiation process. The comparison of operando results with ex situ measurements on cycled NiSb2 materials emphasises the importance of real-time investigation tools for the study of battery materials, in particular in the case conversion reactions. These results can be considered as a reference for the study of other compounds of this family of reactions, where the formation of very reactive and/or unstable species is observed during electrochemical cycling. In fact, such reactive species, which can be identified only by using operando techniques, are one of the keys of the good cycling properties of such materials together with an appropriate formulation strategy.

Keywords: Li-ion batteries, Anode materials, Conversion Reaction, X-Ray Absorption Spectroscopy, Operando analyses.

*

To whom correspondence should be addressed. E-mail: [email protected]

ACS Paragon Plus Environment

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Introduction

The research of new high performance electrode materials for Li-ion batteries requires a better understanding of the electrochemical reaction mechanisms taking place during the charge/discharge process. For this purpose, ex situ studies of electrode materials are now extensively completed by in situ measurements using complementary tools such as diffraction and spectroscopic techniques allowing both structural and electronic characterisations of materials under electrochemical cycling conditions. An in situ approach allows one to avoid several drawbacks due to the sample transfer needed for typical ex situ measurements. Alteration of airor moisture-sensitive species is avoided, as well as the occurrence of relaxation reactions which might show up when the electrical circuit is open, inducing a transformation of the initial cycled material. The effects of sampling deviations are also eluded since the sample remains in the same position during the whole measurement series. Finally, the whole study can be performed on a single test cell suppressing the effects of uncontrolled differences in a set of cells which are needed for a stepwise ex situ study of the electrochemical mechanism. An outstanding case for the application of such techniques is that of electrode materials undergoing a so-called conversion reaction, formerly reviewed by Cabana et al.1 A conversion reaction, which was first verified for transition metal oxides2, occurs between lithium and a binary compound containing a transition metal (M = Ti, Mn, Fe, Co, Ni, etc.) and a group p element (X = O, P, Sb, Sn, etc.), according to the following general equation: MaXb + (b · n) Li a M + b LinX

(1)

Such materials allow reversible capacities as high as 1500 mAh g−1, largely exceeding that of current commercial lithium-ion batteries based on graphite and LiCoO2. Therefore, they are looked with interest as a possible alternative for the future development of high energy storage devices needing a long cycling life, with applications extending from laptop computers to cell phones and electric vehicles.1-2 Recent studies have shown that for conversion reactions, due to the formation of nanosized species, the composites obtained at the end of discharge are particularly unstable, and therefore the use of operando techniques for the study of reaction mechanisms is essential.3-4 Transition metal pnictogenides, a large family of compounds made of a transition metal and of an element of the phosphorous group, have been the object of an intensely growing interest in the last decade because of their reactivity against lithium. Among them are antimonides of general formula MaSbb, which provide capacities between 450 and 600 mAh.g-1. These materials, even


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without an appropriate formulation, can easily stand up to about 20 cycles at stable capacity before fading. The very large volume expansion (of about 300%) experienced during the reaction with lithium is probably at the origin of the rapid fading, causing the pulverisation of the active material particles, with further degradation of the electronic wiring at high rate and agglomeration of the active mass at low rate.5 Several methods were used to improve the cycling life of antimonides, such as nanostructuration of the electrodes,6 carbon coating and optimisation of the formulation.7 The latter applied to NiSb2 has shown a remarkable enhancement of the capacity retention.5 TiSnSb is another interesting case for which electrode formulation together with a tailored electrolyte composition produced a spectacular improvement of the performances, with a stable capacity maintained over several hundred cycles.8 MSbx compounds are expected to react with lithium by forming a matrix of Li3Sb embedding nanoparticles of the transition metal M. Actual reaction mechanisms, however, are usually more complex and often dependent of the specific compound. For instance, several studies on conversion pnictogenides such as InSb,9 FeSb2,10 or MnSb,11 indicate that other intermediate phases, such as ternary insertion phases of general formula LixMaSbb, are formed before starting the true conversion reaction. Moreover, additional phases could also form throughout the whole electrochemical cycle. An example of a complicate and still not completely elucidated reaction mechanism is that of NiSb2, as it was shown by previous studies of our group.5,

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NiSb2 is expected to react

reversibly with lithium to form nickel metal and Li3Sb: NiSb2 + 6 Li Ni + 2 Li3Sb

(1)

providing a theoretical capacity of 532 mAh g-1 (volumetric capacity 4150 mAh cm-3). First of all, a slight shift of the X-ray diffraction reflections of NiSb2 during the first part of the discharge was attributed by Villevieille et al.12 to the possible formation of an intermediate ternary LixNiSb2 solid solution. Secondly, the back-conversion reaction was found to be incomplete, with the formation of some Sb metal, and of an ill-defined Sb-depleted NiSb2 polymorph, which was supposed to crystallise with the structure of a high pressure NiSb2 phase.15 Finally, the formation of Ni nanoparticles at the end of discharge, which are expected for typical conversion reactions,1 could never be proved with certainty. Operando Ni K-edge X-ray absorption spectroscopy (XAS) was used in the present work to specifically address these issues, in order to specifically determine the role of Ni in the electrochemical mechanism in NiSb2. XAS is an element-sensitive tool providing detailed


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information of the short-range structural properties of a large variety of materials. It has been already largely used for the investigation of intercalation mechanisms in electrode materials for lithium batteries,16 but less often for the study of conversion reactions. It must be stressed that conversion-type electrode materials undergo a drastic phase change and are highly sensitive to kinetics. Each interface is highly reactive, and thus the application of an external potential may result in metastable amorphous states different from those expected from the thermodynamic equilibrium. Once the external constraint of an electrochemical potential is lifted, chemical relaxation towards more thermodynamically stable phases may occur before one could investigate the electrode with ex situ techniques. The comparison of operando results with some ex situ measurements on cycled NiSb2 materials was thus performed, showing the importance of realtime investigation tools for the study of conversion reactions. These results will thus be of large interest to the lithium battery community, and they can be used as a reference for other compounds of this family.

2 2.1

Experimental section Materials.

The orthorhombic marcasite NiSb2 phase was synthesised by heating stoichiometric amounts of nickel metal (Ni Alfa Aesar, 350 mesh, 99.9%) and antimony (Sb Alfa Aesar, 350 mesh, 99%) powders in a sealed evacuated silica tube at 600 °C for 4 days, as described in detail by Villevieille et al.12 The tube was placed into a furnace, and the temperature was raised to 600°C with a ramp of 5 °C min−1, held to this temperature for 4 days, and finally air-quenched down to room temperature. The purity of the obtained sample was checked by powder X-ray diffraction (not shown), which confirmed the presence of orthorhombic NiSb2 only (Pnnm space group, lattice parameters: a = 5.18 Å, b = 6.31 Å and c = 3.84 Å).17 2.2

Characterisation.

Operando electrochemical tests: Ni K-edge XAS measurements cannot be performed using typical Cu current collectors due to both the strong absorption of the copper metal, which would decrease strongly the measured signal, and the partial overlapping of the EXAFS oscillations with the absorption at the Cu K-edge, which would reduce the available EXAFS range to values of k below about 10 Å-1. Therefore, self-supported NiSb2 electrodes were used for these experiments, implying a lower electronic conductivity of the electrode which was partially compensated by using larger amounts of carbon additives (26 %) than that (18 %) commonly used for preparing ACS Paragon Plus Environment

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CMC-based electrodes.7 Therefore, operando XAS electrochemical tests were carried out using a specifically designed in situ cell18 on pressed pellets containing 24 wt.% active material (NiSb2), 26 wt.% acetylene black (SN2A Y50, CB) and 50 wt.% PVDF-HFP (Sigma-Aldrich). This formulation was necessary to obtain homogeneous conductive pellets of appropriate mechanical properties and thickness for the XAS measurement. Half-cells vs. Li metal were assembled using a glass fiber separator (Whatman) in an argon-filled glove box according to the following configuration: Li | 1M LiPF6 in EC : DMC (1 : 1 ratio) | NiSb2-pellet with EC = ethylene carbonate and DMC = dimethyl carbonate. Electrochemical cycling tests were performed in the galvanostatic mode using a VSP™ control system (Bio-Logic) at C/2.5 rate (C/n expressed as 1 mole of Li+ per mole of NiSb2 reacted in n hours) between 2.0 and 0.0 V vs. Li+/Li0. The use of this low rate was necessary to obtain a sufficient number of XAS spectra with an acceptable signal-to-noise ratio (about 7 spectra collected per hour). All potentials are given with respect to a Li+/Li reference.

Ex situ reference samples were also prepared by cycling pellets containing 85 wt.% of NiSb2 and 15 wt.% of CB in the galvanostatic mode vs. Li metal using common SwagelokTM cells at the same rate used for the operando experiments, and stopping the reaction at specific potentials during electrochemical cycling. These cells were then disassembled in a glove box and the electrode pellets recovered and sandwiched between two Kapton foils to protect them from reaction with open air. In order to avoid further contamination and/or degradation, the samples were sealed in coffee-bag containers and opened only short before the measurement of the XAS spectra. In this way, the possible permeability of the kapton foils to air and/or moisture (which is known to occur if the samples are kept in air over several days) was reduced as much as possible. Two XAS spectra were measured for each sample, showing no evolution of the materials with time. X-ray absorption spectroscopy (XAS): XAS measurements at the Ni K-edge were performed in the transmission mode at the SAMBA beamline of Synchrotron SOLEIL in Gif-sur-Yvette (France). A Si (111) double crystal monochromator with an energy resolution of 0.5 eV at 8 keV was used. The intensity of the monochromatic X-ray beam was measured by three consecutive ionisation detectors. The in situ electrochemical cell18 was placed between the first and the second ionisation chambers, and the homogeneity of the sample was checked before running the electrochemical test using a phosphorescent screen.


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In the XANES (X-ray absorption near-edge structure) region of Ni K-edge (8333 eV), equidistant energy steps of ∆E = 0.1 eV were used. For all the measured spectra, the exact energy calibration was established with simultaneous absorption measurements on a 5 µm thick Ni metal foil placed between the second and the third ionisation chamber. The first inflection point of the XAS pattern of Ni (8333 eV) was used for the energy calibration. The absolute energy reproducibility of the measured spectra was ±0.05 eV. EXAFS (Extended X-ray Absorption Fine Structure) spectra were collected up to k = 18 Å-1. The spectra were analysed using the IFEFFIT software package.19 Fourier transform of EXAFS oscillations with different k weights was carried out in k-range from 3.0 to 12.5 nm−1. Fitting was performed in R-range from 1 to 4 Å using k2 and k3 weights. EXAFS amplitudes and phase-shifts were calculated by FEFF720 starting from the lattice parameters of NiSb217 and Ni metal available in the literature. Interatomic distances (R) and the Debye-Waller factors were calculated for all paths included in the fits.

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Results and discussion

The galvanostatic cycling curve of NiSb2 in the in situ cell obtained during the measurement of the XAS spectra is shown in Fig. 1. The in situ discharge curve is only slightly different from the one obtained by cycling under usual conditions in Swagelok or coin cells12: in particular, a new plateau accounting for 2 reacted Li appears at about 0.8 V, and a long tail at low voltage is visible after the end of the conversion reaction. These differences lead, for NiSb2 in the in situ cell, to an increase of both the specific capacity at the end of the first discharge and the irreversible capacity loss at the end of the first cycle. Both effects can, however, be related to the increased amount of PVdF binder (50 %) and carbon black (26 %) used for preparing an optimised pellet for the XAS measurements. In particular, large amounts of carbon black are expected to produce an important irreversible degradation of the electrolyte at low voltages leading to the formation of a solid electrolyte interphase (SEI), in line with the higher amount of irreversibly reacted Li at 0.8 V. In addition, also the increased temperature in the cell during the in situ experiment due to the exposition of the pellet to an intense X-ray beam could have favoured possible parasite decomposition reactions of the electrolyte.


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Figure 1. Galvanostatic cycling curve of NiSb2 vs. Li obtained in the in situ cell during the XAS experiment. The arrow indicates the start of the first discharge. 3.1

3.1 Evolution of Ni species during first lithiation (discharge)

The evolution of the Ni K-edge during lithiation under operando conditions is shown in Fig. 2. The spectrum of pristine NiSb2 shows a well-defined step-shaped absorption edge, and several marked oscillations following it. During lithium insertion, the edge is gradually modified and, at the end of lithiation, the step-shape has completely changed in shape, while the oscillations phase is somewhat smoothed and practically inversed compared to the initial one. This observation is in line with a gradual change of the chemical environment around the Ni centres, leading to a complete transformation of initial marcasite NiSb2 structure. The final spectrum, even though quite different from that of bulk metallic nickel, is quite similar to that obtained for well reduced Ni nanoparticles supported on carbon21, or to that of nickel nanoparticles obtained from the decomposition of nickel carbonyls trapped in zeolites.22 The oscillation features in the spectra of nanoparticles, when not superimposed to the signal of oxidised nickel usually formed at the surface, is in fact rather different from that of bulk Ni metal, with most oscillations and edge features strongly weakened and almost flattened out.


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Normalised Absorption

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Figure 2. Operando Ni K-edge evolution during the first galvanostatic discharge of NiSb2 vs. Li metal. Evolution with lithiation is showed on going from darker to brighter spectra. The evolution of the EXAFS signal as well as of the envelope of the corresponding Fourier Transforms (FT) during reaction with lithium is shown in Fig. 3. It is worth noticing that the signalto-noise ratio of the EXAFS signal is satisfying up to about 12.5 Å-1; this value has thus been selected as the upper limit of the FT window, the lower limit being set at 3.0 Å-1.The phaseuncorrected FT signal of the starting NiSb2, shown on the right side of Fig. 3, exhibits a main contribution with a dominant peak at about 2.4 Å and a second smaller peak slightly below 2 Å, and a second contribution with a dominant peak at 4.2 Å. During lithiation, the first contribution slightly decreases in intensity and is gradually replaced by a main peak pointing at about 2.2 Å, whereas the peak at 4.2 Å gradually disappears. 2

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Figure 3. Operando evolution of the Ni K-edge EXAFS spectra (left) and corresponding phaseuncorrected FT signals (right) during the first galvanostatic lithiation of NiSb2 vs. Li metal. Evolution with lithiation is showed on going from darker to brighter spectra (only selected spectra are shown for the sake of clearness).


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The first and the last spectrum, corresponding to the pristine and to the fully lithiated material, were fitted starting from the crystal structures of NiSb2 and Ni metal, respectively. Same neighbours at similar distances, however, were grouped in shells in order to reduce the number of independent fitting parameters and decrease at best the correlations among them. In this way, slight differences in distance are reflected by the increase in Debye-Waller factor. This strategy, which is commonly used when fitting spectra of complex structures with many coordination shells, produced the best fits shown in Fig. 4. The first contribution in the spectrum of pristine NiSb2 corresponds to the signal of the first coordination shell, which can be fitted with six Sb atoms at a distance of 2.547(8) Å. Indeed, in the marcasite structure of NiSb2, the Ni centres are surrounded by six Sb nearest neighbours with a slightly distorted octahedral arrangement, including two Sb atoms at 2.537 Å and four additional ones at 2.569 Å.17 The calculated value is thus in very good agreement with the weighted average Ni–Sb distance of 2.547 Å. The second contribution at 4.2 Å can be fitted with a shell of 6 Sb atoms at 4.27(2) Å, in line with the existence of two Sb neighbours at 4.203 Å and four ones at 4.284 Å in the original marcasite structure. 2

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Figure 4. Experimental (left) and simulated (right) phase-uncorrected FT signals (and corresponding imaginary part) of the Ni K-edge EXAFS spectra of pristine and fully lithiated NiSb2. The EXAFS spectrum of the fully lithiated material was fitted using twelve Ni nearest neighbors at 2.47(1) Å, corresponding to the number of neighbors classically found in Ni metal. This result agrees well with the Ni-Ni distance of 2.491 Å in the fcc lattice of nickel metal. Such a fitting, however, gives an amplitude reduction factor S02 = 0.34; i.e., about half of the usually observed value. Since the amplitude reduction factor and the coordination number are directly correlated one to the other, such a low value of S02 indicates that most probably the number of 12 nearest neighbors is largely overestimated, and that the effective number of Ni nearest neighbors is much smaller, in line with the presence of very small Ni centers with a significant fraction of


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surface atoms. Such reduced coordination numbers were observed, for instance, in the case of copper during electrochemical cycling of Cu0.1V2O5, a positive electrode material.23 A similar result was also often obtained for supported metal nanoparticles in heterogeneous catalysts with sizes below about 2 nm.24 The nanosized nature of the nickel particles is also confirmed by the absence of the following coordination shells in the FT signal of the fully lithiated sample. The presence of Ni nanoparticles with very small sizes at the end of lithiation is in agreement with the expected conversion mechanism for this material.1 0.025 0.7

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Figure 5. Evolution of the amplitude reduction factor S02 (left) and of respective Debye-Waller factors σ2 (right) of the Ni-Sb and Ni-Ni fitting shells during the first discharge. The spectra during lithiation were fitted using a combination of the two Ni-Sb shells (two Sb atoms at 2.537 Å and four at 2.569 Å) and of the Ni-Ni one (12 Ni atoms at 2.491 Å) obtained from the fit of the spectra of pristine and fully lithiated NiSb2 (nickel nanoparticles), respectively. During the fitting of the series of spectra, the goodness-of-fit coefficient of determination “R” was always below 0.02 (not shown). The evolution of the Debye-Waller factors during the first discharge for the different coordination shells is shown in Fig. 5, together with the values of S02 fitted for the two phases. The gradual transformation of NiSb2 into Ni metal with the amount of reacted Li is slow at the beginning, when part of the Li is expected to react with the carbon black to form a SEI, and becomes faster and almost linear with advancing lithiation, within the experimental error, down to about 8 Li. After 8 Li, corresponding to a potential of about 0.3 V, the reaction slows down again, possibly due to a second parasitic reaction consuming part of the reacted lithium. Almost identical variations in the transformation rate of NiSb2 can be observed when doing a linear combination fitting of the absorption edges using the spectrum of the pristine material and that of Ni nanoparticles corresponding to the end of lithiation (see Supplementary Information). The slope between 0.3 and 0 V, corresponding to the region between 8 and 11 reacted Li (cf. Fig.


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1), was described by Tarascon and coworkers25 for the conversion of transition metal oxides as the portion of lithiation corresponding to the growth of a polymer/gel-like film promoted by the highly reactive metallic nanograins formed during the conversion. This polymer phase is expected to dissolve once the potential is raised again during the following delithiation. The interaction of such a gel with the metal particles was evidenced in the case of CoO by Grugeon et al.,26 who also evidenced the catalytic role of Co metal nanoparticles to form such polymer species. The EXAFS spectra collected during the lithiation of NiSb2, however, do not show the presence of any kind of Ni-O neighbour close to the surface of the Ni particles at the end of the discharge. The addition of a Ni-O shell to the fit in the region between 1.8 and 2.5 Å, representing the possible passivation and/or binding of the Ni nanoparticles to oxygen species of such a gel phase, did not provide any realistic result and did not improve the quality of the fit. Anyway, since the complete disappearance of the initial NiSb2 is observed only at the very end of the lithiation (at a potential of 0 V), the possible formation of such a gel during the first discharge can only occur together with the completion of the reaction of NiSb2 with lithium, and does not involve the formation of any stable detectable species on the surface of Ni metal nanoparticles. The Debye-Waller factor of the first shell of NiSb2 remains almost constant during lithiation, indicating that a progressive reaction of pristine NiSb2 with lithium is observed, without strong variations in the local arrangement around the Ni centres. A decrease of the crystallinity and/or particle sizes is witnessed by the strong increase of the Debye-Waller factor of the second Ni-Sb shell at 4.2 Å. Eventually, after the reaction of about 8 Li per unit formula, this contribution is so low in intensity that it cannot be distinguished anymore from the experimental error and thus fitted correctly. Since almost no variation is observed in the bond distances related to the two NiSb fitting shells during lithiation (not shown), the occurrence of specific distances relative to the formation of the initial solid solution LixNiSb2 at the beginning of lithiation (after the reaction of about 3 Li) could not be evidenced. 3.2

Evolution of Ni species during the first delithiation

The evolution of the Ni K-edge during delithiation is presented in Fig. 6. During this process, the shape of the absorption edge corresponding to nickel nanoparticles gradually retransforms back into that of pristine NiSb2. However, the spectrum obtained at the end of delithiation remains quite different from that of the starting material. A linear combination fitting of the absorption edge measured at the end of delithiation performed by using the experimental spectra of pristine NiSb2 and of the fully lithiated sample gives a ratio of 20% Ni nanoparticles and 80% NiSb2,


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indicating that the delithiation is not complete at the end of the charge process. This observation is in line with the reversible formation of NiSb2 from Ni nanoparticles and Li3Sb, both formed at the end of the first discharge. It must be noted that, however, the shape of the linear combination does not reproduce exactly that of the experimental spectra (see Supplementary Information, Fig. S2). This result suggests that the phase obtained at the end of delithiation is not exactly identical to pristine NiSb2. In particular, some oscillations in the edge features are somewhat weaker than those of bulk NiSb2. Such modifications could result from either the nanostructuration of this new electrochemically formed NiSb2 (as already observed in the case of Ni nanoparticles compared to bulk Ni metal, vide supra) or from differences in its crystal structure/composition compared to that of the marcasite structure of NiSb2, in line with the formation of the high pressure form of NiSb2 at the end of charge already suggested by Villevieille et al.12, or from both. 1.0


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Figure 6. Operando Ni K-edge evolution during the first galvanostatic delithiation of NiSb2 vs. Li metal. Evolution with increasing delithiation is showed on going from darker to brighter spectra. The spectrum of pristine NiSb2 is shown in red for comparison. The evolution of the EXAFS signal as well as of the envelope of the corresponding Fourier Transforms (FT) with increasing delithiation is shown in Fig. 7. As in the case of the first discharge, the limits of the FT window were set from 3.0 Å-1 to 12.5 Å-1, since the signal-to-noise ratio remains largely satisfying between these two values even during delithiation. At the beginning of delithiation, no change in the spectrum is observed for the deinsertion of about 1 Li (corresponding to a potential of about 0.6 V). This process may correspond to a partial dissolution of the SEI layer, as recently observed in other conversion materials such as TiSnSb.27 After this period, the behaviour of the spectra is practically the opposite of what is observed during lithiation, with the exception that the initial spectrum is not completely recovered. In particular, it is important to notice that the second shell of neighbours of NiSb2, peaking at 4.2 Å in the pristine ACS Paragon Plus Environment

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material, does not appear again at the end of the charge process. This observation is in line with the lack of long-range order in the material, and thus with the expected nanostructuration of the conversion material during charge. In a previous study,12 the X-ray diffraction pattern at the end of the delithiation showed the appearance of very large diffraction profiles, which were tentatively attributed to the formation of a high pressure NiSb2 phase.15 In this XAS study, the very weak peaks appearing in the FT signal between 4 and 6 Å are of the same intensity of the noise in the spectrum, and tend to disappear when the FT window is restricted to 11 Å-1. Since the first shell distances and the coordination polyhedra are virtually identical for both marcasite and highpressure polymorphs, it is thus impossible to obtain any information from EXAFS concerning the structural type of the phase obtained at the end of the delithiation. 2

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Figure 7. Operando evolution of the Ni K-edge EXAFS spectra (left) and corresponding phaseuncorrected FT signals (right) during the delithiation of fully lithiated NiSb2 vs. Li metal. Evolution with delithiation is shown on going from darker to brighter spectra. The spectrum of pristine NiSb2 as well as its uncorrected FT signal are shown in red for comparison. The EXAFS spectra corresponding to the delithiation process were fitted using two simple shells: a Ni-Ni shell including twelve Ni nearest neighbors at 2.47(1) Å and a Ni-Sb shell including six Sb neighbours at a distance of 2.546(8) Å, corresponding to the signal of Ni metal nanoparticles which are dominant at the beginning of the delithiation and to their gradual transformation into NiSb2, respectively. The evolution of the values of S02 fitted for the two phases is shown in Fig. 8, together with the values of σ2 fitted for the two phases. Similarly to the XANES, the spectra during the first part of delithiation (between 11 and 9.5 Li) remain almost unchanged, and some NiSb2 starts appearing only after some additional Li is deinserted. The back-conversion reaction proceeds then almost linearly, and is somewhat slowed down starting from about 5 Li (corresponding to the end of the plateau at a potential of 1.1 V, cf. Fig. 1), where the


13


transformation of the Ni nanoparticles is probably accompanied by an additional parasite reaction. At the end of delithiation, however, some Ni metal is still present, corresponding to about 10 % of the total amount of Ni. The incomplete transformation of the Ni metal into NiSb2, which is confirmed with slightly different figures by both the fittings of XANES (20 % of Ni) and of EXAFS (10 % of Ni) data, is in line with the presence, at the end of the charge process, of some Sb metal which could be clearly identified by both X-ray diffraction and

121

Sb Mössbauer spectroscopy in

this material.12 0.015 0.7

NiSb2

0.6

Ni

Ni-Sb(1) Ni-Ni

0.5

σ (Ang )

0.010

2

2

0.4 0.3

2

S0

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0.2

0.005

0.1 0.0 0.000 0

2

4

6

8

10

12

0

2

4

6

8

10

12

+

+

Li (x)

Li (x)

Figure 8. Evolution of the amplitude reduction factor S02 (left) and of respective Debye-Waller factors σ2 (right) of the Ni-Sb and Ni-Ni fitting shells during the first charge. 3.3

Reversibility of the conversion mechanism

The in situ measurements could be continued for a second complete charge/discharge galvanostatic cycle of NiSb2 vs. Li. The spectra obtained at the end of the second lithiation delithiation are shown in Fig. 9. As it can be straightforwardly deduced from the figures, the spectra obtained at the end of the two processes are identical to those obtained during the first cycle. Similarly, no visible differences are found for the EXAFS spectra corresponding to the same measurements (not shown). Globally, the phases formed during the first cycle are those that do cycle reversibly during the following lithiation/delithiation processes, indicating that the reaction mechanism is fully reversible.


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1.0

0.5

NiSb2 st

End 1 lithiation nd End 2 lithiation

0.0 8320

8330

8340

8350

8360


1.0


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0.5

NiSb2 st

End 1 delithiation nd End 2 delithiation

0.0

8370

8320

8330

Energy (eV)

8340

8350

8360

8370

Energy (eV)

Figure 9. Ni K-edge of NiSb2 at the end of the second lithiation (left) and delithiation (right). The spectra are compared to those obtained at the end of the first lithiation and delithiation, respectively, and to that of pristine NiSb2. 3.4

Comparison of in situ and ex situ measurements

In parallel to operando measurements, additional ex situ spectra were measured for samples that were cycled vs. lithium about five days prior to the measurements. Such studies are particularly important to investigate whether the phases obtained at the end of the lithiation/delithiation processes are stable, or whether they continue to chemically evolve during open-circuit voltage (OCV) relaxation. In fact, differently from insertion compounds, where more stable species are supposed to form, converted phases formed at the end of the lithiation process may contain highly reactive and/or metastable species undergoing a partial conversion reaction, and thus leading to a rapid and steady increase of the potential. Such a behaviour was already observed for FeP3 and - very recently – for TiSnSb by solid state NMR and Mössbauer spectroscopy.4 In the specific case of NiSb2, if the lithiation is ended at 0 V and the circuit is opened, the potential raises rapidly to about 0.5 V in about 10 hours and grows to 0.8 V after 5 days (see Supplementary Information). On the other hand, if the delithiation is stopped at about 2 V, the potential rises slowly, after a very rapid drop to a lower value, to a stable potential of about 2.1 V after five days. The spectra obtained for the two samples relaxed for 5 days at the end of lithiation and delithiation are shown in Fig. 10, were they are compared to those of the respective samples at the end of both processes in the in situ cell. The spectrum of fully lithiated NiSb2 after relaxation is very similar to a linear combination of those of pristine NiSb2 and of Ni nanoparticles obtained at the end of the lithiation. Such modification is in line with the instability of the Ni-containing species formed at the end of the lithiation process and with the very large variation of the reaction


15


potential observed during relaxation. In the case of the delithiated sample, a modification of the adsorption edge is also observed, even though much less marked than that observed at the end of lithiation. This result indicates that also the species obtained at the end of delithiation are reactive and should be studied in situ, even though they appear less reactive than those obtained after lithiation. It must be noted that a partial modification of the experimental spectra of these two ex situ samples could also be attributed to the partial reaction of the converted species with oxygen. In fact, the kapton windows used for protecting the sample from air are known to be, over several days, permeable to oxygen. However, every precaution was taken in order to expose the samples to air only just before the measurement of the XAS spectra, and to avoid their decomposition at much as possible.

1.0


1.0


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0.5

Ex situ In situ

0.0 8320

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8340

8350

8360

8370

0.5

Ex-situ In-situ

0.0 8320

8330

Energy (eV)

8340

8350

8360

8370

Energy (eV)

Figure 10. Ex situ Ni K-edges of NiSb2 at the end of the first lithiation (left) and of the first delithiation (right) after 5 days of relaxation. The spectra are compared to those obtained at the end of the first lithiation and delithiation, respectively, in the in situ cell. In any case, the strong differences between ex situ and the in situ spectra stress the importance of performing experiments under operando conditions, in particular in the study of conversion mechanisms, and therefore also in the case of alloy systems. In fact, these systems are expected to produce unstable and/or metastable species which tend to decompose under OCV conditions. At the same time, the formation of such species can be detrimental for a possible effective application of such materials in commercial systems. Nevertheless, a recent study on TiSnSb showed that such metastability is not necessarily irreversible, and that, in spite of the loss of charge, a perfect cycling behaviour can be recovered after a long relaxation at the end of lithiation.4


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4

Conclusions

In this paper, we have presented an example of the potential of X-ray absorption spectroscopy in the detailed study of the electrochemical mechanism of an electrode material during galvanostatic cycling. The importance of such a short range technique has been underlined in the specific case of a material undergoing a conversion reaction, i.e., NiSb2, for which the information gathered by other long-range techniques such as X-ray diffraction are incomplete due to the general amorphisation of the electrode material during the reaction with Li. The redox activity was followed during the first two lithiation/delithiation cycles, indicating that a reversible conversion takes place, with a recovery of a Ni-Sb phase close to the initial one, in a nanosized form. The reversibility of the conversion reaction is most probably at the basis of the good electrochemical cycling properties observed for this compound. Recent developments including the use of appropriate electrode formulation strategies were effective in improving even further the cycling stability of the studied composites, leading to state-of-the-art electrode materials with a hopefully viable application in commercial Li-ion batteries. Finally, the comparison of in situ and ex situ spectra of the same material in the electrochemical cell and after relaxation underlined the importance of performing in situ measurements to get a realistic view of the reaction mechanism of battery materials. In fact, especially in the case of conversion-type materials, such investigations can be very complex because the species formed in cycling electrodes are usually very reactive and/or unstable.

5

5. Acknowledgements

Synchrotron SOLEIL (Gif-sur-Yvette, France) is gratefully acknowledged for providing beamtime (project 20090795). Stéphanie Belin, Emiliano Fonda and Valérie Briois are gratefully acknowledged for technical help and expert advice on beamline operation. Alan Chadwick is gratefully acknowledged for helpful discussion on data treatment and interpretation. Supporting Information is available free of charge via the Internet at http://pubs.acs.org.


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Figure captions

Figure 1. Galvanostatic cycling curve of NiSb2 vs. Li obtained in the in situ cell during the XAS experiment. Figure 1. The arrow indicates the start of the first discharge. Figure 2. Operando Ni K-edge evolution during the first galvanostatic discharge of NiSb2 vs. Li metal. Evolution with lithiation is showed on going from darker to brighter spectra. Figure 3. Operando evolution of the Ni K-edge EXAFS spectra (left) and corresponding phaseuncorrected FT signals (right) during the first galvanostatic lithiation of NiSb2 vs. Li metal. Evolution with lithiation is showed on going from darker to brighter spectra (only selected spectra are shown for the sake of clearness). Figure 4. Experimental (left) and simulated (right) phase-uncorrected FT signals (and corresponding imaginary part) of the Ni K-edge EXAFS spectra of pristine and fully lithiated NiSb2. Figure 5. Evolution of the amplitude reduction factor S02 (left) and of respective Debye-Waller factors σ2 (right) of the Ni-Sb and Ni-Ni fitting shells during the first discharge. Figure 6. Operando Ni K-edge evolution during the first galvanostatic delithiation of NiSb2 vs. Li metal. Evolution with increasing delithiation is showed on going from darker to brighter spectra. The spectrum of pristine NiSb2 is shown in red for comparison. Figure 7. Operando evolution of the Ni K-edge EXAFS spectra (left) and corresponding phaseuncorrected FT signals (right) during the delithiation of fully lithiated NiSb2 vs. Li metal. Evolution with delithiation is shown on going from darker to brighter spectra. The spectrum of pristine NiSb2 as well as its uncorrected FT signal are shown in red for comparison. Figure 8. Evolution of the amplitude reduction factor S02 (left) and of respective Debye-Waller factors σ2 (right) of the Ni-Sb and Ni-Ni fitting shells during the first charge. Figure 9. Ni K-edge of NiSb2 at the end of the second lithiation (left) and delithiation (right). The spectra are compared to those obtained at the end of the first lithiation and delithiation, respectively, and to that of pristine NiSb2. Figure 10. Ex situ Ni K-edges of NiSb2 at the end of the first lithiation (left) and of the first delithiation (right) after 5 days of relaxation. The spectra are compared to those obtained at the end of the first lithiation and delithiation, respectively, in the in situ cell.


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At the Heart of a Conversion Reaction: An Operando X-ray Absorption

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