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Published: September 27, 2011 r2011 American Chemical Society 22040 dx.doi.org/10.1021/jp206829q | J. Phys. Chem. C 2011, 115, 2204022047 ARTICLE pubs.acs.org/JPCC Study on Microstructural Deformation of Working Sn and SnSb Anode Particles for Li-Ion Batteries by in Situ Transmission X-ray Microscopy Sung-Chieh Chao, Yen-Fang Song,* ,Chun-Chieh Wang, Hwo-Shuenn Sheu, Hung-Chun Wu, § and Nae-Lih Wu* ,Department of Chemical Engineering, National Taiwan University Taipei 106, Taiwan, R.O.C. National Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan, R.O.C. § Materials Research Laboratories, Industrial Technology Research Institute Chutung, Hsinchu 310, Taiwan, R.O.C. b S Supporting Information 1. INTRODUCTION Lithium-ion batteries, which have already been ubiquitous in high-end consumer electronic products, are widely regarded as the choice power source for future electric vehicle applications. To meet this demand, the specic capacities of both cathode and anode have to be signicantly increased from the state-of-the-art materials. The currently predominant anode material is graphite, which has a theoretical specic capacity of 370 mAh/g. Several elements, such as Sn, 1,2 which form alloys with Li are potential anode materials with far greater theoretical lithiation capacities. These Li-alloying materials are known to be subject to signicant volumetric deformations, namely expansion and contraction, during electrochemical lithiation and delithiation due to change in density, and these cyclic dimensional variations tend to cause structural instability of the electrode, leading to fast capacity fading. Understanding the dynamics of these deformation pro- cesses can provide valuable information to researchers for establishing viable high-energy anodes based on these materials. Devising an analytic tool that is capable of in situ monitoring of the behaviors of active materials within a working Li-ion battery is very challenging, considering the extreme sensitivity to humidity and oxygen of the various components within the battery. Outstanding works in revealing the dynamics of dimen- sional variation of working Li-alloying anodes have been demon- strated in earlier years with a few in situ techniques, including dilatometry, 3 atomic force microscopy, 4,5 and atmospheric scan- ning electron microscopy. 6 These analyses, however, detect only the dimension of the active materials but are not capable of revealing the microstructural evolution concurrently taking place within the bulk of the anode particles during the deformation processes. Recent progress in X-ray microscopy (TXM) 7 and transmission electron microscopy (TEM) 8,9 for battery research has oered the opportunity to directly image the interior micro- structures of electrode active materials during the course of electrochemical lithiation/delithiation (L/D). In our previous TXM study, 7 Sn particles are dispersed within the matrix of a free-standing graphite electrode, which is subse- quently assembled with a Li counter electrode into a half-cell. Received: July 18, 2011 Revised: September 17, 2011 ABSTRACT: Sn-containing compounds are potential high- capacity anode materials for Li-ion batteries. They, however, suer from signicant dimensional variations during electro- chemical lithiation and delithiation, causing cycling instability. Understanding the dynamics of these deformation processes may provide valuable information in the establishment of viable high-energy anodes. In this paper, the evolution of interior microstructures of two types of Sn-containing particles, includ- ing Sn and SnSb, during initial cycles of electrochemical lithiation/delithation has been revealed by in situ synchrotron transmission X-ray microscopy, complemented by in situ synchrotron X-ray diraction to provide phase information. The microstructures and deformation rates are shown to depend on particle composition, size, and alloy stoichiometry with Li. During rst lithiation, both particles exhibit core (metal)shell (lithiated compounds) interior structures. Initial formation of a dense surface layer containing Li x Sn phases of low Li-stoichiometry on the Sn particle hinders further lithiation kinetics, resulting in delayed expansion of large particles. In contrast, Sb in SnSb is readily lithiated to form a porous Li-rich (Li 3 Sb) surface layer at higher potential than Sn, which enables the acceleration of lithiation and removal of the size dependence of the lithiation process. Both lithiated particles only partially contract upon delithiation, and their interiors evolve into porous structures due to metal recrystallization. Such porous structures allow for fast lithiation and mitigated dimensional variations upon subsequent cycles. Neither of the two anode particles pulverize upon cycling.

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Published: September 27, 2011

r 2011 American Chemical Society 22040 dx.doi.org/10.1021/jp206829q | J. Phys. Chem. C 2011, 115, 22040–22047

ARTICLE

pubs.acs.org/JPCC

Study onMicrostructural Deformation ofWorking Sn and SnSb AnodeParticles for Li-Ion Batteries by in Situ Transmission X-ray MicroscopySung-Chieh Chao,† Yen-Fang Song,*,‡ Chun-Chieh Wang,‡ Hwo-Shuenn Sheu,‡ Hung-Chun Wu,§ andNae-Lih Wu*,†

†Department of Chemical Engineering, National Taiwan University Taipei 106, Taiwan, R.O.C.‡National Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan, R.O.C.§Materials Research Laboratories, Industrial Technology Research Institute Chutung, Hsinchu 310, Taiwan, R.O.C.

bS Supporting Information

1. INTRODUCTION

Lithium-ion batteries, which have already been ubiquitous inhigh-end consumer electronic products, are widely regarded asthe choice power source for future electric vehicle applications.To meet this demand, the specific capacities of both cathode andanode have to be significantly increased from the state-of-the-artmaterials. The currently predominant anode material is graphite,which has a theoretical specific capacity of 370 mAh/g. Severalelements, such as Sn,1,2 which form alloys with Li are potentialanode materials with far greater theoretical lithiation capacities.These Li-alloying materials are known to be subject to significantvolumetric deformations, namely expansion and contraction,during electrochemical lithiation and delithiation due to changein density, and these cyclic dimensional variations tend to causestructural instability of the electrode, leading to fast capacityfading. Understanding the dynamics of these deformation pro-cesses can provide valuable information to researchers forestablishing viable high-energy anodes based on these materials.

Devising an analytic tool that is capable of in situ monitoringof the behaviors of active materials within a working Li-ionbattery is very challenging, considering the extreme sensitivity to

humidity and oxygen of the various components within thebattery. Outstanding works in revealing the dynamics of dimen-sional variation of working Li-alloying anodes have been demon-strated in earlier years with a few in situ techniques, includingdilatometry,3 atomic force microscopy,4,5 and atmospheric scan-ning electron microscopy.6 These analyses, however, detect onlythe dimension of the active materials but are not capable ofrevealing the microstructural evolution concurrently taking placewithin the bulk of the anode particles during the deformationprocesses. Recent progress in X-ray microscopy (TXM)7 andtransmission electron microscopy (TEM)8,9 for battery researchhas offered the opportunity to directly image the interior micro-structures of electrode active materials during the course ofelectrochemical lithiation/delithiation (L/D).

In our previous TXM study,7 Sn particles are dispersed withinthe matrix of a free-standing graphite electrode, which is subse-quently assembled with a Li counter electrode into a “half-cell”.

Received: July 18, 2011Revised: September 17, 2011

ABSTRACT: Sn-containing compounds are potential high-capacity anode materials for Li-ion batteries. They, however,suffer from significant dimensional variations during electro-chemical lithiation and delithiation, causing cycling instability.Understanding the dynamics of these deformation processesmay provide valuable information in the establishment of viablehigh-energy anodes. In this paper, the evolution of interiormicrostructures of two types of Sn-containing particles, includ-ing Sn and SnSb, during initial cycles of electrochemicallithiation/delithation has been revealed by in situ synchrotrontransmission X-ray microscopy, complemented by in situ synchrotron X-ray diffraction to provide phase information. Themicrostructures and deformation rates are shown to depend on particle composition, size, and alloy stoichiometry with Li. Duringfirst lithiation, both particles exhibit core (metal)�shell (lithiated compounds) interior structures. Initial formation of a densesurface layer containing LixSn phases of low Li-stoichiometry on the Sn particle hinders further lithiation kinetics, resulting indelayed expansion of large particles. In contrast, Sb in SnSb is readily lithiated to form a porous Li-rich (Li3Sb) surface layer at higherpotential than Sn, which enables the acceleration of lithiation and removal of the size dependence of the lithiation process. Bothlithiated particles only partially contract upon delithiation, and their interiors evolve into porous structures due to metalrecrystallization. Such porous structures allow for fast lithiation and mitigated dimensional variations upon subsequent cycles.Neither of the two anode particles pulverize upon cycling.

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High-energy X-rays were allowed to pass through the entire cellas well as the individual Sn particles so as to give transmissionimages of their interior microstructures with a resolution down toa few tenths of a micrometer.

In this work, we have combined in situ TXM with in situ syn-chrotron X-ray diffraction (XRD) to study the evolution of theinterior microstructures, complemented with information ofphase compositions, of two Sn-based anode particles, includingSn and SnSb, during electrochemical L/D. These particles exhibitdifferent classes of electrochemical behaviors. An in-depth reviewon the electrochemical aspects of Sn-based anodes can be foundin ref 10. In brief, for Sn, the maximum theoretical lithiationstoichiometry in crystalline form is Li22Sn5, which gives a theo-retical capacity of 990 mAh/g-Sn and a theoretical volumeexpansion by ca. 350% when compared with Sn. For SnSb, bothconstituent elements are capable of undergoing reversible lithia-tion reactions but at different potentials, giving a maximumcapacity of 825 mAh/g-SnSb. The results obtained in this workprovide valuable information for the understanding of themicrostructure-cycling behaviors of these anode materials andmay pave the way to better design of Li-alloying anode materialsagainst capacity fading.

2. EXPERIMENTAL SECTION

2.1. Samples and Electrode Preparations. Sn powder wasused as received (Aldrich) without any treatment. For preparingSnSb, powders of Sn (with 99% <10 μm) and Sb (�325 mesh)were mixed with stoichiometric ratio (1:1), and the mixtureswere ball-milled with a planetary high-energy ball milling ma-chine. The milling process was carried out for 12 h in Ar with arotation speed of 400 rpm and a ball-to-powder weight ratio 5:1.Electrochemical measurements were carried out by using the

2032-type coin-cells. The working electrode consisted of the Sn-containing active particles, conductive additives and binder withweight ratios of 10: 82: 8. The conductive additives include gra-phitic flakes (KS6, 3 μm, Timcal) and nanosize carbon black(Super P, 40 nm, Timcal) with a weight ratio of 5:1. The binderwas a mixture of styrene-butadiene-rubber (SBR; L1571, AsahiChemicals) and sodium-carboxyl-methyl cellulose (SCMC; WS-C, Cellogen, DKS International, Inc.) with 1:1 weight ratio. Theworking electrode was made by coating the slurry mixture con-taining the above-mentioned solid ingredients on a polymer film(Mylar) to form a continuous top layer. Once dried, the top layerwas removed from the substrate to obtain a free-standing film.The film was then roll-compressed to a final thickness of ca.50 μm, and the working electrode disks of 13 mm in diameterwere punched off from the film. Each electrode contained ca.

5 mg of the active and conductivematerials. The counter electrodewas a Li disk (13-mm diameter, 0.3-mm thick), and the elec-trolyte was 1 M solution of LiPF6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (Zhangjiagang Guotai Ronghua)(1:2 vol. %) with 2 wt % vinylene carbonate (VC). The covers onboth sides of the cell were perforated and sealed with Kaptontapes in order to allow the X-ray beam to pass through the cell.2.2. Electrochemical Analysis. The electrochemical L/D

processes conducted during the TXM and XRD analyses arethe same. The lithiation process includes first a constant-current(0.2 A/g for Sn and 0.05 A/g for SnSb) step from 1.5 to 0.001 V,followed by a constant-voltage step (0.001 V) with a cutoff cur-rent equal to one-tenth of the constant-current value at the firststep. The delithiation process is carried out only with a constant-current step from 0.001 to 1.5 V.2.3. Synchrotron Analyses. The TXM analysis utilizes the

beamline #01B1 facility of the National Synchrotron RadiationResearch Center (NSRRC) in Taiwan, R.O.C. The experimentalsetup is schematically shown in Figure 1. The light sourceoperates with photon energy ranging between 8 and 11 keV.X-rays are generated by a superconducting wavelength shifter(SWLS) source and further focused by a toroidal shaped focus-ing-mirror with a focal ratio close to 1:1. Monochromatic X-rayswith specific photon energy are obtained after the white radia-tions pass through a double crystal monochromator using a pairof Ge (111) crystals. After passing the focusing mirror and GeDCM, the X-rays are further focused on the electrode inside thecoin cell by a capillary condenser and a pinhole. The transmittedX-rays further go through a zone plate optical system and a phasering to form the image. The field of view of a single image is 15�15 μm for the first order diffraction mode of the zone plate. Thephase ring positioned at the back focal plane of the zone plateresults in a recording of the phase contrast images at the detector.

Figure 1. Schematics of the experimental setup of in situ TXM.

Figure 2. SEM micrographs of the adopted Sn-containing particles:(a) Sn and (b) SnSb.

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A mosaic micrograph covering a greater area can be constructedfrom neighboring single-view images collected by shifting thesample stepwise with a motor. The electrochemical test wascarried out with a potentiostat connected with the coin cell.The synchrotron XRD study was conducted by utilizing the

facility of the NSRRC, beamline #01C2. The wavelength of theincident X-ray was 1.0223 Å. Each XRD spectrum was obtainedwith an acquisition time of 240 s (consisting of exposing time andscanning time, 132 and 108 s, respectively). The data wascollected using mar345 imaging plate detector. By using FIT2Dsoftware, the 2D Debye�Scherrer rings were integrated to theequivalent of 2θ scan and were corrected for polarization andtangent geometry. The scattering angle was calibrated by stan-dard sample, a mixture of silver behenate and silicon powders.

3. RESULTS

3.1. Starting Materials. Figure 2 shows the morphologies ofthe anode particles employed in the present study. Dense Snparticles exhibit spherical morphology with a very wide sizedistribution from less than 0.2 μm to as large as 20 μm. Thesynthesized SnSb particles are nonporous and exhibit polyhedralmorphology.3.2. Sn Electrode.The voltage vs time plot of the Sn(graphite)

electrode under TXM analysis during the first L/D cycle andsubsequent rest period is shown in Figure 3a. During this course,a selected Sn particle has been continuously monitored. Thedimension of this particle has also been plotted vs time inFigure 3a. Figure 3b shows a series of snapshots of this Snparticle; their corresponding shooting times and voltages aremarked on Figure 3a. A video showing part of the expansion is inthe Supporting Information, S1. During the first lithiation, thevoltage profile shows a kink at ca. 0.37 V, which can be attributedto the formation of LiSn.10,11 As reported in our preliminarypaper,7 TXM observations over particles of different sizes haveshown that the Sn particles in general do not expand at the sametime nor with the same speed. Rather, the smaller particles tendto expand earlier, i.e., at higher voltages, and faster. For theparticle shown in Figure 3b, although a thin light (lithiated) layeris quickly formed when the cell voltage is below 0.37 V (frame 1,Figure 3b), it does not readily expand with continued lithiation.Rather, it first remains essentially the same size and then showsburst expansion (frames 3�5, Figure 3b) during the rather latestage of the entire lithiation process.As shown by frames 2�5 of Figure 3b, during expansion, the

particle exhibits a multizone structure consisting of a light-contrast shell and a dark core with a diffuse interface in between.The reduction of local electron density arising from lithiationresults in increasing lightness in the lithiated region within theparticle. The gradient contrast at the core�shell interface indi-cates the present of materials of varied Li stoichiometries. Withincreasing depth of lithiation, the shell first forms along theperiphery of the particle and continues to expand in all directions.As a result, the dark core progressively contracts, while the overalldimension of the particle increases.Acceleration of expansion is seen to occur shortly after cracks

starting to appear along the periphery (between frames 3 and 4,Figure 3b). The cracks occur exclusively within the shell regionand extend along the radial direction toward the center. Thisphenomenon is consistent with the fact that the LixSn inter-metallic compounds are brittle,12,13 in contrast to ductile Snmetal, and hence easier to crack under sufficient stress. The radial

extension of the cracks suggests the existence of tensile stressalong the angular direction. Cracks extending in angular direc-tion (e.g., frame 4) also emerge at the core�shell interfaces aslithiation reaches certain high conversion.During the first delithiation, only ∼60% of capacity was

recovered. A complementary synchrotron XRD study (see later)has confirmed that only very small amount of a lithiated Sn phaseexists at the end of delithiation, and it is in too small of an amountto account for the capacity loss. Rather, the capacity loss musthave mainly been caused by other reasons, such as the expectedformation of solid-electrolyte-interphase (SEI) layer on both Snand graphite and unexpected leakage of humidity due to non-perfect sealing of the tested cell.At the end of delithiation, the particle contracted less than

halfway back (Figure 3a and frame 6, Figure 3b), in spite ofdisappearance of essentially all the lithiated phases. During thesubsequent rest period, the interior of the particle continues toreconstruct to develop into a net-like image over a period ofnearly 3 h (frames 7 and 8, Figure 3b). The net-like image in factsuggests the formation of porous microstructure within Snparticle. This has been confirmed by an ex-situ 3D-tomographytechnique (see the Supporting Information, S2). The net-likeimage exhibits backbones ca. 0.2 μm in thickness showing in-creasing darkness and regions surrounded by the backbonesbecoming more transparent. These contrast changes suggestlocal segregation of Sn metal into dense backbones creating in-terior pores.Figure 4 shows in situ XRD patterns acquired from another

cell tested under the same L/D and rest procedures. The voltageprofile is essentially the same as that shown in Figure 3a. A seriesof lithiated Sn compounds, including Li2Sn5, LiSn, and Li22Sn5,with increasing Li contents have been detected during the courseof lithiation. Only a very small amount of Li2Sn5 is left at the endof delithiation. During the rest of the period, the reflectionintensities of pure Sn increase with time. The data evidence thatthe evolution into a porous interior structure of the delithiated Snparticle is accompanied by local recrystallization of Sn. Theformation of the pores is a consequence of vacancy condensation,which results in the release of the stored energy associated withthe binding of Sn atoms to vacancies that are created by removalof Li ions. The low melting-point nature of Sn allows forrecrystallization to take place even at room temperature.The expansion/contraction process of the Sn particle de-

scribed above can be schematically summarized in Figure 5. Orig-inally, the spherical Sn particle is a dense particle (Figure 5a).During the course of lithiation, the particle exhibits a core�shellinterior structure consisting of a lithiated shell zone containingLi22Sn5, a diffuse zone interface containing LixSn phases, a pureSn core, and cracks (Figure 5b) until it fully expands (Figure 5c).During delithiation, the particle only partially contract and theinterior of the particle continues to reconstruct to give a porousinterior structure via a short-ranged recrystallization process(Figure 5d).Upon the second lithiation cycle, lithiation takes place homo-

geneously within the porous interior (frames 9 and 10, Figure 3b)without forming the core�shell structure. The pores are gradu-ally filled up and the overall contrast of the particle turns lighterand more homogeneous. The interior pore serves as an openspace for alleviating the volume change during lithiation. As aresult, the overall dimensional variation during the entire secondcycle has been mitigated as compared with that during thefirst cycle.

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Figure 3. (a) Voltage vs time plot of the Sn electrode and particle dimension vs time plot of the Sn particle shown in (b) during the firstlithiation�delithiation�rest cycle; (b) the snapshots of a selected particle during the same period. The points and numbers shown in (a) mark themoments at which the snapshots of corresponding numbers have been taken. The arrow in the frame 1 in (b) indicates the direction along which theparticle dimensions have been measured.

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3.3. SnSb Electrode. As shown in Figure 6a, the expansionprocess, as well as the evolution of the interior microstructure, ofthe SnSb particle can be characterized by two stages. The firststage takes place over a short plateau at ca. 0.8 V (pts. 2�5,Figure 6a). A rather sharp zone-interface first forms at the parti-cle surface and then moves toward the center (frames 2�5,Figure 6b; also see video in S3). The particle exhibits a nearlysquare core�shell interior structure throughout this stage.Although the interface advances, the dimension of the particlesimultaneously increases—an expansion process similar to thatof the Sn particle shown in Figure 3b. The interior structure stopschanging momentarily between 0.8 and 0.56 V, below which thesecond-stage expansion starts. The particle continues to expandwithout the presence of any core�shell structure, and the con-trast within the particle changes quite homogeneously (frames 6and 7. Figure 6b).In situ XRD analysis (Figure 7) detects Li3Sb as the sole

lithiated Sb phase, along with Sn, upon continuous disappearing

of SnSb. This is consistent with previous studies15,16 which show thelithiation reaction of SnSb to produce Li3Sb and Sn at ca. 0.85 V:

SnSb þ 3Liþ þ 3e� f Sn þ Li3Sb ð1ÞIt is inferred that this reaction accounts for the first-stage expansiondescribed above. The resulting Sn is subsequently lithiated to give

Figure 4. Synchrotron X-ray diffraction data of the Sn electrodeacquired during the first lithiation�delithiation�rest cycle. The numbersshown to the right end of the curves are the same as those in Figure 3b.

Figure 5. Schematics of the expansion/contraction process of Snparticle during the first cycle of lithiation/delithiation: (a) beforelithiation, (b) core�shell structure occurring during lithiation, (c) fulllithiation, and (d) porous structure developed after delithiation.

Figure 6. (a) Voltage vs time plot of the SnSb electrode and particledimension vs time plot of the SnSb particle shown in (b) during the firsttwo lithiation�delithiation cycles; (b) the snapshots of a selectedparticle during the same period. The points and numbers shown in(a) mark the moments at which the snapshots of correspondingnumbers in (b) have been taken.

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mainly Li22Sn5 (Figure 7). Due to fast diffusion of Li+ in the already

expanded particle, this reaction takes place rather homogenouslythroughout the entire particle.During delithiation, plateaus are observed at ca. 0.6 and 1.0 V.

The former corresponds to delithiation from the alloyed Sncompounds, while the latter from Li3Sb. No significant change ineither interior structure or dimension takes place upon delithia-tion passing the 0.6 V plateau (frames 8 and 9, Figure 6b). On theother hand, significant interior structure variation and contrac-tion occurs over the 1.0 V plateau and subsequent rest period.XRD indicates the formation of SnSb during these periods(Figure 7). The interior of the particle eventually evolves tocontain slit-shaped pores. The delayed reconstruction processseen here is similar to that of the Sn particle shown in Figure 3and is presumably due to slow recrystallization of SnSb.

Upon the second cycle, the microstructural change is quitereversible. The slit-shaped pores are gradually filled up, alongwith particle expansion, upon lithiation, and recovered upondelithiation after a rest period (not shown).Figure 8 displays the schematic illustration of expansion/

contraction process of SnSb during the first cycle. The synthe-sized SnSb exhibits a dense interior structure (Figure 8a). Duringlithiation, the particle exhibits a core�shell structure. Li3Sb isfirst formed from the outer shell of the particle during the first-stage expansion, along with segregation of Sn (Figure 8b). Then,Sn reacts with lithium (Figure 8c) during the second-stageexpansion homogeneously throughout the entire particle. Final-ly, upon removal of Li, slit-shaped pores are formed (Figure 8d).

4. DISCUSSION

During the course of the first lithiation, both Sn and SnSbparticles exhibit core�shell interior structures upon expansion.The former, however, exhibits a very diffuse core�shell interface,while the latter shows a sharp one. The difference can beattributed to their varieties of lithiated compounds that areformed during lithiation. Compounds of different Li-stoichiom-eties are expected to form at the interface between the metal coreand the lithiated shell during lithiation, as schematically illu-strated in Figure 9. The coexistence of several lithiated com-pounds, which are formed at different potentials, tends to give athicker and diffused interface layer. Sn has been shown by XRDto form a series of Li-alloying compounds, including Li2Sn5,LiSn, and Li22Sn5. On the other hand, only a single Li-rich inter-metallic phase of Sb, namely Li3Sb, is formed for SnSb particle.

The difference in the varieties of lithiated compounds is alsobelieved to cause different expansion rates observed betweenthese two particles. As mentioned earlier, a thin lithiated layer isquickly formed on the Sn particle when the cell voltage is belowthe lithiation voltage (0.37 V) during the first lithiation (frame 1,Figure 3b). However, the particle does not readily expand untilthe very late stage, where the particle exhibits burst expansion(frames 3�5, Figure 3b). This is in great contrast to the SnSbparticle, which rapidly expands when the cell voltage reaches theformation voltage (0.8 V) of Li3Sb (frames 2�5, Figure 6b). Theburst expansion of the Sn particle takes place shortly after cracksstart to appear along the periphery of the particle. It is alsonoticed that lithiation at the SnSb surface immediately forms aporous surface layer, whereas the thin lithiated layer initially onthe Sn particle is dense. It is inferred that the delayed expansion,and hence the delayed lithiation of the large Sn particle, is due tohindered diffusion of Li within the dense surface layer thatcontains compounds of low Li stoichiometry, and the associated

Figure 7. Synchrotron X-ray diffraction data of the SnSb electrodeacquired during the first lithiation�delithiation cycle. The numbersshown to the right end of the curves are the same as those in Figure 6b.

Figure 8. Schematics of the expansion/contraction process of SnSbparticle during the first cycle of lithiation/delithiation: (a) beforelithiation, (b) formation of a porous Li3Sb shell region containing Snparticles during the first stage of expansion,(c) lithiation of Sn particlesduring the second stage of expansion, and (d) porous interior structureafter delithiation.

Figure 9. Schematics of the core�shell interior structure showing theformation of layers of Li alloys of different Li stoichiometries at thecore�shell interface.

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high Li-diffusion resistance gives rise to a large potential gradientnear the particle surface. Expansion remains slow until a certainthreshold amount of high Li-content alloy(s) is reached, resultingin sufficient stress to cause crack formation. Beyond this point,electrolyte easily gets access to the interior of the particle throughcracks and pores, and the lithiation rate is greatly accelerated.This process is autocatalytic and therefore results in burstexpansion of the Sn particle as observed. In contrast, the firstlithiation step of SnSb directly produces Li3Sb, and the associatedlarge volume expansion immediately results in a porous structurethrough crack formation. Therefore, electrolyte can easily reachthe rim of the unreactedmetal core, and hence there is no delay inlithiation/particle expansion.

The strong size-dependence shown by the expansion of Snparticles in our previous report7 can also be reasoned along thisline. Smaller particles have larger surface-to-volume ratios andhence possess higher overall Li diffuse-in rates per unit mass ofthe metal. It therefore takes less time for the smaller particles toreach the threshold fraction of lithiation that causes crackformation. In addition, considering the potential gradient withinthe particle, larges particles will require greater polarization attheir surfaces in order for complete lithiation. In contrast, the sizedependence was not observed for the SnSb particles becauseready formation of a porous lithiated surface layer enables fastlithiation of all particles regardless of their sizes.

One common phenomenon in the expansion/contractionprocess of these anode particles is that the dimensional variationsof the particles are not reversible during the first L/D cycle butbecome so, along with greatly reduced expansion, during thesecond and subsequent cycles. Table 1 summarizes themaximumexpansion of the first and second cycles for the two particlesstudied. Besenhard et al.17 once noted irreversible thicknessvariations of Li-alloying anodes during the first L/D cycle intheir dilatometric study. The present study confirms that suchdimensional irreversibility is intrinsic to the Sn-based individualparticles. Both resulting Sn and SnSb particles contain internalpores within crystalline skeletons. The porous nature of theparticles accelerates the lithiation process during the subsequentcycles. It is also worth noting that none of the particles pulverizes.This is due to the ductile nature of the resulting metal skeleton inboth cases.

It has been reported18�26 that the cycling stability of Li-alloying anodes can be significantly enhanced if intermetallic orcomposite hosts are employed instead of pure metal. It has beensuggested18,19,24�26 that the component which is less active oreven inactive can serve as a buffer matrix, mitigating the extent ofexpansion of the active material. The data in Table 1 indicate thatthe buffering effect due to the presence of Sb is significant for thefirst cycle, whereas their expansions in the subsequent cycles are

essentially the same. Formation of porous Sn-based particles canbe realized via solution L/D processes.27 Using delithiatedporous particles as the staring electrode material may be a wayto get away with the large expansion during the first L/D cycle soas to improve the cycle life of the Li-alloying anodes.27

’CONCLUSION

The evolution of interior microstructures of Sn and SnSbparticles during initial cycles of electrochemical lithiation/de-lithation has been studied by the combination of in situ synchro-tron TXM and XRD. It has been found that, during firstlithiation, both particles exhibit interior structures containing alithiated shell and metal core. Initial formation of a dense surfacelayer containing LixSn phases of low Li stoichiometry, such asLi2Sn5 and LiSn, slows lithiation kinetics, resulting in delayedexpansion of large particles. In contrast, Sb in SnSb is readilylithiated to form a porous Li3Sb layer at a higher potential thanSn, which enables the acceleration of particle lithiation andremoval of the size dependence. Both lithiated particles onlypartially contract upon delithiation, and their interiors evolveinto porous structures. Such porous structures allow for fastlithiation and mitigated dimensional variations upon subsequentcycles.

’ASSOCIATED CONTENT

bS Supporting Information. A video showing expansion of aSn particle; a video showing 3-D tomography of a porous Snparticle; a video showing expansion of a SnSb particle. Thismaterialis available free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected] (N.-L.W.); [email protected](Y.-F.S., for TXM Instrumentation).

’ACKNOWLEDGMENT

This work is financially supported byNational Science Council(NSC-3114-E-002-012), National Taiwan University and Indus-trial Technology Research Institute.

’REFERENCES

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Table 1. Maximum Linear Expansions of the First TwoCycles for the Sn-Containing Particles

expansiona

material first cycle second cycle

Sn 54% 15%

SnSb 36% 16%a Expansion� (Lf� Li)/Li, where Li and Lf are respectively the particlesizes before and after a cycle consisting of consecutive lithiation,delithiation, and rest steps.

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