study of the series ti1−ynbysnsb with 0 ≤ y ≤ 1 as anode material for li-ion batteries

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Journal of Power Sources xxx (2012) 1e6

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Study of the series Ti1�yNbySnSb with 0 � y � 1 as anode material for Li-ionbatteries

Cyril Marino, Moulay Tahar Sougrati, A. Darwiche, Julien Fullenwarth, Bernard Fraisse,Jean Claude Jumas, Laure Monconduit*

Institut Charles Gerhardt Montpellier (ICGM-AIME)-UMR 5253 CNRS, ALISTORE European Research Institute (3104 CNRS), Université Montpellier 2,Place Eugène Bataillon, 34095 Montpellier Cedex 5, France

h i g h l i g h t s

< Ti1�yNbySnSb (y ¼ 0, 0.25, 0.5, 0.75, 1) / Single phase.< Mechanism is very similar for all the phases.< Electrochemical performances are quite similar.< MSnSb system, a route to improve intermetallic performances?

a r t i c l e i n f o

Article history:Received 20 September 2012Received in revised form16 November 2012Accepted 19 November 2012Available online xxx

Keywords:Li-ion batteryNegative electrodeConversion materialHigh performance

* Corresponding author.E-mail address: [email protected]

0378-7753/$ e see front matter � 2012 Elsevier B.V.http://dx.doi.org/10.1016/j.jpowsour.2012.11.061

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a b s t r a c t

TiSnSb shows an excellent behavior as negative electrode for Li-ion batteries. However the role of Ti isstill unclear in the mechanism. To better understand the role played by the transition metal on both themechanism and the performance of TiSnSb, a progressive substitution of Ti by Nb has been achieved. Afull study focuses on the electrochemical mechanisms of the Ti1�yNbySnSb systemwith 0 � y � 1 vs Li bycombining in situ XRD and Mössbauer, and EXAFS analyses. The electrochemical mechanism is found tobe a reversible conversion mechanism: MSnSb þ 7Li 4 Li3Sb þ 1/2 Li7Sn2 þ M0. The lithium is found toreact simultaneously with both Sn and Sb. Nb-rich alloys have been found also to be very promisingnegative electrode demonstrating the versatility of the MSnSb electrode material family for Li-ion batteryapplication. Using the appropriate electrode formulation, the good cycling life of TiSnSb is not affected byits substitution by Nb.

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1. Introduction

Many efforts have been expended in the last decade to improveLi-ion batteries. Although significant improvements in their perfor-mances were obtained (increase in energy density and rate capa-bility), the exploration of new materials and the understanding oftheir electrochemical mechanisms are still primary to better controlthe electrode properties in order to meet the market requirements.

The use of intermetallics or composite active/inactive materialsinstead of a pure metal seems to be an effective way to control thevolume changes of the alloys based electrodes that is a majordrawback of these electrode materials during the cycling of thebattery [1]. This is due to the so-called buffering effect of theinactive component. On the other hand the alloys such as SnSb

r (L. Monconduit).

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react with Li at different potentials in the charging/dischargingprocess, thus the volume change occurs in a stepwise manner andthe unreacted component can accommodate the strain yielded bythe reacted phase. However the cycling stability of SnSb is stilllimited, even when used as nanosized morphology or as nano-composites with carbonaceous materials [2e4].

Our recent study has demonstrated that TiSnSb based electrodecan reach high performance with maintain of 90% of the specificcapacity after 90 cycles at C rate [5]. From operando XRD andMössbauer spectroscopy measurements, a conversion mechanismhas been observed leading simultaneously to the formation ofLi3Sb and Li7Sn2 alloys. Upon charge, a re-conversion has beendemonstrated leading to an alloy structurally close to the pristinematerial TiSnSb. The good electrochemical performance of TiSnSb(compared to Sn, Sb or SnSb) would be correlated to the presence ofthe non-active metal (vs Li) i.e. Ti.

Until now the role of Ti stays unclear in the mechanism oflithiation of TiSnSb. The substitution of Ti by another transition

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C. Marino et al. / Journal of Power Sources xxx (2012) 1e62

metal can help to get better insight into its role in the electro-chemical mechanism and associated performance.

Among the rare ternary phases in the system M/Sn/Sb(M ¼ transition metal) [6,7], only NbSnSb [22] presents both thesame stoichiometry than TiSnSb and a close structure. We decidedto explore the new system Ti1�yNbySnSb with 0 � y � 1.

119Sn Mössbauer spectroscopy, XRD and EXAFS analyses havebeen combined to characterize the various Ti1�yNbySnSb samplesand a comparative study of the electrochemical results has beenundertaken in term of capacity retention at various cycling rates.

2. Experimental

2.1. Synthesis procedure

Ti1�yNbxSnSb alloys with 0 � y � 1 have been prepared by ballmilling starting from commercial powdered metallic precursors ofhigh purity (>99.9%) using the planetary Ball Mill Retsch PM 100.For all our preparations, 1 g of metallic powders was used in a 50mlgrinding jar with 6 balls (5 g each). Both the jars and the balls aremade on hardened stainless steel. An active-milling time of 28 h

Fig. 1. Typical SEM images for TiSnSb (a) and NbSnSb (b). Illustration

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was necessary to obtain a complete reaction of the precursors forthe studied compositions. In some cases, minor amount ofunreacted Sn is observed. For XRD analysis the as preparedpowders were heated up to 450 �C (at 1 �C min�1 rate) undervacuum in sealed silica tubes and annealed at this temperature forone week before being cooled in air. The as prepared powders(without annealing) were used for the investigation of the elec-trochemical performances, the annealed ones were exclusivelyused for XRD analyses.

2.2. Chemical analysis and structure determination

The as prepared samples were observed by scanning electronmicroscopy on Hitachi S4800 with secondary mode) in order tocharacterize both their morphology and particle size. X-ray dif-fractograms were recorded on the annealed powders using a Pan-alytical X’pert pro MPD with Cu-Ka radiation in the range10 < 2q < 90�. The FULLPROF software [8] has been used for therefinement of data. In situ XRD analysis has also been used to followthe structural changes accompanying the lithium insertion-deinsertion.

of the crystallographic structures of TiSnSb (c) and NbSnSb (d).

rces (2012), http://dx.doi.org/10.1016/j.jpowsour.2012.11.061

C. Marino et al. / Journal of Power Sources xxx (2012) 1e6 3

To characterize the nature of tin environments the selective119Sn Mössbauer spectroscopy has been used. Room temperaturespectra were collected with a constant acceleration spectrometerusing a Ca119mSnO3 source in the transmission geometry. Theywerefit to combinations of Lorentzian lines. The absorbers used for thedetermination of the hyperfine parameters contained 10 mg cm�2

of the Ti1�yNbySnSb powders. In order to optimize the countingtime for the in situ measurements, the absorber quantity weredoubled and hence it was possible to record good quality spectrawithin 1 h each.

EXAFS analyses have been carried out at Soleil Laboratory on theSamba station in an energy range of 18,950e19,100 eV in both insitu and ex situ transmission modes on the Nb K-edge(18,985 eV). The incident beam was monochromatized using a Si(220) fixed exit, double crystal monochromator. A Nb foil wasincorporated for energy calibration. Data were extracted withestablished methods using the Athena software package [23]. Foreach ex situ sample, 2 spectra were collected in 1 h 30. Theabsorbers were prepared by mixing powder of active material withcarbon black, the mixture is sandwiched between two sheets ofKapton. The amount of material was adjusted to obtain a reason-able edge jump for transmission XAS. For in situ measurements, 1spectrum was collected every 13 min.

2.3. Electrochemical lithium insertionedeinsertion

Electrochemical lithium insertion/extraction tests were carriedout with (Li/LiPF6 1M (EC:PC:3DMC þ 1% VC)/Ti1�yNbySnSb) two-electrode SwagelockTM-type cells assembled inside an argon-filled glove box. For in situ Mössbauer, XRD and XAS measure-ments, a specific cell [9] has been used and the electrochemicaltests have been recorded in a C/9 (1 inserted Li in 9 h) rate forMössbauer and XRD and in a C/5 rate for XAS analysis. Powderedelectrodes were prepared by mixing 60 wt.% of pristine materialswith 20 wt.% carbon black (CB) and 20 wt.% poly(vinylidene fluo-ride) (PVDF) to obtain a pellet. Preliminary cycling life tests havebeen carried out using the formulation process with carbon blackand carboxymethyl cellulose (CMC) described in a previous study

Fig. 2. XRD patterns (left) and Mössbauer spectra (right) of Ti1�yNbySnSb samples(y ¼ 0, 0.25, 0.5, 0.75, 1). The Mössbauer fitted curves are TiSnSb, NbSnSb and Sn in red,blue and olive respectively. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

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[10]. Electrochemical discharge/charge curves were recorded ona multichannel VMP system under galvanostatic conditions atvarious rates and at room temperature.

3. Results and discussion

3.1. Morphological, chemical and structural analysis

The SEM observations show (Fig. 1a and b) that for theTi1�yNbySnSb samples (y¼ 0, 0.25, 0.5, 0.75,1) the obtained powderis made of homogenous shapeless agglomerates with a broaddistribution of average diameters; 1e10 mm (for more claritythe SEM pictures are given only for y ¼ 0 and y ¼ 1). Thecrystallographic structures are illustrated in Fig. 1c and d for theend-members i.e. TiSnSb and NbSnSb that crystallize in the ortho-rhombic (F ddd) and tetragonal (I 4/mcm) systems respectively.

XRD patterns of the annealed powders are given in left panel ofFig. 2. Tin appears systematically as impurity (indicated by stars).The poor crystallinity of the samples does not allow performingboth Rietveld analysis and phase quantification of each phase insamples. It was however found that the lattice parameters do notsignificantly vary with the Ti/Nb ratio. For the end-memberscompounds, the patterns and cell parameters are in agreementwith the previously reported data for TiSnSb and NbSnSb thatcrystallize in the F ddd orthorhombic and I 4/mcm tetragonal spacegroups respectively [5,11] (Fig. 2). For the intermediate alloys, theXRD patterns can be seen as a combination of the end-memberspatterns suggesting a priori the formation a mixture of the TiSnSband NbSnSb phases with unchanged cell parameters. Note that nosolid solution of Nb in TiSnSb, or Ti in NbSnSb are expected sincethey crystallize in different space groups.

119Sn Mössbauer spectra have been recorded for theTi1�yNbySnSb samples at room temperature and showa progressiveevolutionwith the Nb content. For TiSnSb and NbSnSb, the data arefitted with one main component (blue and red doublets) withdifferent isomer shifts and quadrupole splittings as expected sinceSn local environments are different for these two alloys (Table 1 insupplementary data). Note that TiSnSb exhibits higher isomer shiftand quadrupole splitting compared to NbSnSb. The difference inthe isomer shift is explained by the different electronic configura-tions (3d24s2, 4d35s2 for Ti and Nb respectively) whereas thedifference of quadrupole splitting indicates that Sn local environ-ments are more distorted in the case of TiSnSb. For the Ti/Nbphases, theMössbauer spectra have been first fitted by constrainingthem to a combination of the end-members spectra but the fitswere not satisfactory. Better fits (Fig. 2, right panel) are obtained

Fig. 3. Variation of the quadrupole splittings of the two components used to fit theexperimental data.


Fig. 4. Nb K-edge XANES spectra of Ti1�yNbySnSb powders (a) and their pseudoradial distribution functions (not corrected for phase shifts) derived from the EXAFS signals ofTi1�yNbySnSb (b).


when the parameters of the quadrupole splitting of the twocomponents (blue and red) are allowed to vary with Ti amountstarting from the values of the end-members spectra (y ¼ 0 and 1).Interestingly, the quadrupole splitting was found to vary linearlywith Ti amount for both the components as illustrated in Fig. 3. Thisindicates the powders are not simple mixtures of the two TiSnSband NbSnSb alloys. This point will be discussed below in the light ofelectrochemical behavior. The presence of some metallic tinimpurity is confirmed (green component).

Fig. 5. Galvanostatic profiles (left) and their corresponding derivatives (right) forTi1�yNbySnSb series.


To get better insight into the knowledge of the Ti1�yNbySnSbsamples, all the series were studied by ex situ X-ray AbsorptionSpectroscopy (XANES and EXAFS). In order to compare quantita-tively the intensity of absorption features in various compounds,the experimental K-edge spectra were normalized. The zero energyis taken with respect to the first inflection point of the metalniobium in the derivative spectrum at 18,992 eV, which defines thethreshold or onset of the photoelectron of the 1s electron inniobium metal. As seen in Fig. 4a, the normalized K-edge XANESspectrum of all Ti1�yNbySnSb samples is characterized by anabsorption peak between 18,985 and 19,003 eV, followed byoscillations at a higher energy in the EXAFS region. The normalizedK-edge spectra for all samples were quite identical, implying thatNb remains roughly in the same electronic state in all samples. Thepseudoradial distribution derived from EXAFS signals (Fig. 4b)confirms the absence of noticeable changes in atomic distances,implying that Nb remains in the same state whatever the samplecomposition. A more accurate observation shows that the mainpeak is centered around 2.69�A for Ti1�yNbySnSb with y ¼ 0.25 and0.5 and around 2.74 �A for Ti1�yNbySnSb with y ¼ 0.75 and 1. Thistrend is consistent with the MeM and the MeSn/Sb distancesslightly shorter for M¼ Ti than forM¼Nb if and only if we considerthe existence of a single phasemade up of tetragonal (NbSnSb type)and orthorhombic (TiSnSb type) crystallographic domains to beconsistent with the Mössbauer and electrochemical (see below)results.

Fig. 6. Derivative obtained for the electrode made by mixing equal amounts of TiSnSband NbSnSb.


Fig. 7. Cycling performances for Ti1�yNbySnSb series at C rate for y ¼ 0 and 1.

C. Marino et al. / Journal of Power Sources xxx (2012) 1e6 5

3.2. Electrochemical performances

The electrochemical behavior of the samples Ti1�yNbySnSb hasbeen investigated at room temperature. The electrodes have beenformulated as described in references [5,12]. The galvanostaticprofiles and the derivative curve for the first discharge obtainedbetween 0.02 and 1.5 V are shown in Fig. 5. Regarding the firstlithiation, a clear tendency is observed; more the alloy is Nb-richlower is the potential of the main pseudo plateau (Fig. 5).

If Ti1�yNbySnSb samples were simple mixtures of TiSnSb andNbSnSb, the potential profiles of corresponding electrodes wouldbe addition of the individual profiles. A mixture of the TiSnSb andNbSnSb (50/50 at.%) has been tested and shows two potentialplateaus clearly identified by two incremental peaks on the deriv-ative curve at 0.09 V and 0.23 Vwhich correspond to the addition ofTiSnSb and NbSnSb derivative curves (Fig. 6). This behavior

Fig. 8. Voltage profile for y ¼ 0.75 (A), the corresponding XRD patter

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supports strongly the fact that Ti/Nb phases are not made of simplemixtures of the end-members phases. It is worth noting thebehavior of these materials during the first lithiation is fullyconsistent with Mössbauer and XAS analysis suggestion; a singlephase with tetragonal and orthorhombic crystallographic domainspresenting coherence domains large enough to give X-ray diffrac-tion peaks in the patterns (Fig. 2).

Fig. 7 summarizes the cycling performances of TiSnSb andNbSnSb samples at C (1Li/1 h) rate. As it can be expected from themolar mass difference, Ti phase exhibits higher capacity(w500 mAh g�1) than Nb phase (w430 mAh g�1) while the othersamples capacities are comprised between 430 and 500 mAh g�1

and can be maintained for more than 70 cycles. Cycling tests per-formed at 4C-rate (4Li/1 h) give similar capacities and capacityretentions. Interestingly, before its fading an increase of thecapacity is systematically observed suggesting the cracking of thepolymeric film. Finally, the coulombic efficiency for all thesecompounds has been found to be better than 98%.

3.3. Li reaction mechanism

To gain further insights into the mechanism of lithium reactionwith the studied alloys, operando analyses have been carried outusing X-ray diffraction (XRD) and spectroscopic techniques (119SnMössbauer and XAS). Typical XRD patterns obtained while cyclingTi0.25Nb0.75SnSb are shown in Fig. 8B. As reported previously forTiSnSb, XRD data show that the alloy is progressively converted toLi3Sb during the lithiation. During the delithiation, Li3Sb diffrac-tion peaks decrease continuously until their complete vanishing atthe end of the delithiation. At this stage, the XRD patterns becomecompletely flat and no more helpful to follow the reaction owingto the total amorphization of the electrode material. Note that noinformation concerning the LieSn can be extracted from the XRD

ns (B) and Mössbauer spectra (C) obtained during the first cycle.


Fig. 9. Fourier transforms of XAS spectra for the pristine, fully discharged and fullycharged NbSnSb.


data. Mössbauer spectra (Fig. 8C) evidence the conversion of thestarting material to form LieSn alloys. At the end of the lithiationthe Mössbauer parameters are close to those of Li7Sn2 as previ-ously reported for many Sn-based materials [13e15]. During thedelithiation, LieSn alloys are progressively converted to a Sn-based alloy. The exact nature of the formed phase could not bedetermined form the current results. However, from the Möss-bauer data we can exclude the formation metallic Sn or SnSb alloysince their Mössbauer signatures are known.

As previously demonstrated for intermetallic alloys, the tran-sition metal edge is usually only little modified during thelithiationedelithiation processes [16,17]. Here the Nb K-edge (notshown here) is quite unchanged during the discharge/charge of theNbSnSb based electrode. Crystallographic data for the NbSnSbstructure indicate two characteristic distances; NbeNb at 2.87 �A,and NbeSn/Sb at 2.93�Awhereas for metallic Nb (Im-3m, ICSD 002-1108) the shorter NbeNb distance is 2.86 �A. The pseudoradialdistribution function is presented Fig. 9 for NbSnSb during thecycling of the battery. The broad peak (in black) centered at 2.73 �Acorresponds to the NbeNb, NbeSn and NbeSb bonds in thestarting NbSnSb. It disappears to give a broad peak centered at2.68 �A that could correspond to the NbeNb bond in tetragonalmetallic niobium phase. A first fit of the experimental EXAFSspectrum recorded at the end of the discharge with crystallo-graphic data of cubic Nb did not give complete satisfaction. Afternumerous fit attempts, the best result was obtained by adding a Sb(or a Sn) atom in the first coordination sphere of Nb in the cubicniobium cell. The phase diagram of Nb/Sb exhibits a phase with 4%atom of Sb inclusion in the Nb structure [18]. Such Sb (or Sn)dissolution in metallic nanoparticles in a nanosized convertedelectrode material after a full discharge has been recently shown inthe in situ XAS study of NiSb2 [19] and is consistent with the solidsolution of few % of antimony in cubic nickel given in the phasediagram Ni/Sb [20,21]. The lack of oscillation in the spectrum athigher energy (in the EXAFS region) is probably due to anamorphization of the material after the first discharge andprevents to get any information on the other coordination shells.After the charge, the broad peak centered at 2.68�A is difficult to fit,the average distance is again close to those of the nearest neigh-bors of Nb in the NbSnSb as well as in metallic Nb.

4. Conclusion

Focusing on the role of the transition metal in the good electro-chemical features of TiSnSb, the five Ti1�yNbySnSb compositionswith


0� y� 1havebeen successfully prepared. CombinedXRD,Mössbauerspectroscopy, XAS and electrochemical analyses have demonstratedthat single phases with TiSnSbeNbSnSb crystallographic domains invarious amount function of the composition have been obtained. Theelectrochemical mechanisms vs lithium have been shown to be closeto that of TiSnSbwith amore or less reversible conversionmechanismMSnSb þ 7Li 4 Li3Sb þ 1/2 Li7Sn2 þ M0 even though the “MSnSb”phase restructured after charge is different from the pristinematerial.Interestingly a small reactivity of Sb (and/or Sn) with the nanosizedmetallic particles of niobium has been identified in the fully dis-charged electrode. The good electrochemical performance ofTi1�yNbySnSb has demonstrated the versatility of the composition ofMSnSb electrode material concerning the Li-ion application.

Acknowledgments

Pr. L. Stievano and S. Belin (local contact on SAMBA line) arewarmly thanked for their help in the XAS run conducting. Wewould like to thank SOLEIL Synchrotron (project no 20111011) andthe ADEME for its financial support through the PhD grants of C.M.This research was performed in the framework of the ALISTOREEuropean Research Institute and its Mössbauer platform.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2012.11.061.

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study of the series ti1−ynbysnsb with 0 ≤ y ≤ 1 as anode material for li-ion batteries

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