XPS and ToF-SIMS Study of Electrode ... - ACS Publications

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XPS and ToF-SIMS Study of Electrode Processes on Sn
Ni Alloy Anodes for Li-Ion Batteries Jun-Tao Li,† Jolanta Swiatowska,‡ Vincent Maurice,*,‡ Antoine Seyeux,‡ Ling Huang,† Shi-Gang Sun,*,† and Philippe Marcus*,‡ †

School of Energy Research, Sate Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China ‡ Laboratoire de Physicochimie des Surfaces, CNRS (UMR 7045)
Chimie ParisTech (ENSCP), 11 rue Pierre et Marie Curie, 75005 Paris, France ABSTRACT: The characterization of electrode processes induced by lithiation/delithiation of Sn
Ni alloy films electroplated on a copper substrate is presented. Galvanostatic discharge/charge measurements were combined with X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS shows the buildup of a solid electrolyte interphase (SEI) layer formed by reductive decomposition of the electrolyte at the surface of the Sn
Ni anode during the first discharge. The SEI layer is constituted of a mixture of Li2CO3, ROCO2Li, Li2C2O4, and/or ROLi whose balance is not markedly modified upon cycling. ToF-SIMS depth profiling evidences an incomplete initial alloying process of lithium ion with Sn and the resulting partition of the Sn
Ni layer alloy into a fully lithiated outer part and a partially lithiated inner layer during the first discharge. After the first cycle, the volume expansion/shrink associated with the alloying/dealloying reaction, also evidenced by ToF-SIMS, irreversibly cracks and divides the Sn
Ni alloy into island-like morphology with gaps filled by the SEI layer. Multicycling (tested up to 9 cycles) amplifies the division of the Sn
Ni alloy layer and the related penetration of the SEI layer as indicated by the increase of trapped lithium and chlorine but with no apparent loss of active material or drop of capacity.

’ INTRODUCTION Lithium-ion batteries (LIBs) are one of the important electrochemical power sources for low- or zero-emission hybrid electrical and electrical vehicles, energy-efficient cargo ships and locomotives, and aerospace and power-grid applications.1
3 It was reported that the development of LIBs has progressed much slower than that of other areas such as electronics due to the lack of suitable electrode materials and electrolytes, and to difficulties in mastering the electrode/electrolyte interface.4 To promote a development of LIBs, it is essential not only to find new materials with high electrochemical performance but also to understand the electrode processes occurring at the electrode/electrolyte interface and in the bulk electrode material that are responsible for the electrochemical features and safety issue of LIBs. The principal electrode processes of LIBs are the insertion/extraction of lithium ions into the host material, inducing changes in the host material, and accompanied with decomposition of the electrolyte, which leads to the formation of a solid electrolyte interphase (SEI) layer. Besides the traditional electrochemical methods, various experimental techniques can be applied to understand the electrode processes, such as Fourier transform infrared reflection spectroscopy (FTIRs),5,6 Raman spectroscopy,7 X-ray photoelectron spectroscopy (XPS),8
15 secondary ion mass spectrometry (SIMS),15
18 differential electrochemical mass spectrometry (DEMS),19 X-ray diffraction (XRD),20,21 X-ray absorption spectroscopy (XAS),21 r 2011 American Chemical Society

nuclear magnetic resonance (NMR),22 electrochemical quartz crystal microbalance (EQCM),23 and laser scanning confocal microscopy.24,25 XPS has been applied to investigate the surface chemical composition of electrode materials including the SEI layer of positive or negative electrodes by analysis of the valence band region and the related core levels.16 As this technique provides information on the first few nanometers at the electrode surface, depth profiling analysis is also necessary for the characterization of the cycled bulk materials because some of the interesting issues also relate to the bulk composition. Time-of-flight SIMS (ToF-SIMS) is a highly sensitive surface analytical technique where a pulsed primary ion beam (e.g., Biþ) is used to extract secondary ions that are analyzed by time-of-flight spectrometry. Interlaced with a sputtering ion beam (e.g., Csþ), compositional depth profiles with excellent depth resolution (monolayer) and high sensitivity (ppb) can be readily obtained. ToF-SIMS has been applied to cycled electrode materials providing information about the SEI layer and the depth and lateral distribution of species in the bulk electrode materials.15,16,26 Throughout the search for alternatives to the currently used LIB carbonaceous anodes, much research effort has been devoted Received: November 8, 2010 Revised: March 8, 2011 Published: March 21, 2011 7012

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Figure 1. Typical SEM image and EDX analysis of the Sn
Ni alloy film electroplated on the Cu substrate.

Figure 2. Galvanostatic discharge/charge of Sn
Ni anode in the first nine cycles performed between 2.5 and 0.01 V in 1 mol L
1 LiClO4/PC.

to Sn alloys due to their higher specific capacity than graphite and better cycling performance than pure tin.27
31 The inactive component in the Sn alloys, such as Co, Ni, and Fe, form a matrix that buffers the volume changes caused by the lithiation/ delithiation processes. The commercial application of Sn alloy materials is still hindered due to a high irreversible capacity during the initial cycle and capacity fading during successive cycling. The initial large irreversible capacity is mainly attributed to the formation of the SEI layer. The volume variations may lead to stress-induced degradation,32,33 which not only destabilizes the SEI layer but also cracks and disrupts the active material and the current collector, and eventually can lead to the collapse of the active material structure. Detailed examination of these phenomena induced by the lithiation/delithiation processes is necessary for the application of Sn alloy materials. The Sn
Ni alloy is a good potential candidate as anode for LIBs.34
41 Electroplated Sn
Ni alloy thin films are very suitable for investigation by XPS and ToF-SIMS, as they are binder free, carbon black free, and can be easily depth profiled. Here, we present a combined investigation of an electroplated Sn
Ni alloy anode cycled in 1 mol L
1 LiClO4/PC using these techniques. The main objective is the characterization of the electrode processes related to the formation and modification of the SEI layer during cycling, the identification of the morphological and chemical changes of the electroplated layer related to the alloying/dealloying reaction, and the determination of the distribution of lithium ions in the Sn
Ni alloy anode.

1 min. Scanning electron microscopy (SEM) observation and energy dispersive X-ray spectroscopy (EDS) analysis are presented in Figure 1. The alloy layer presents a dense and granular microstructure of submicrometer particles and uniformly covers the copper substrate. The Sn:Ni atomic ratio measured by EDS is about 2:1. On the basis of the data reported by Zhang et al. for electroplated Sn
Ni films,41 the thickness of the alloy layer in the present work can be estimated to be ca. 3 μm. Electrochemical Testing. Galvanostatic discharge/charge was performed in a glove box with an Autolab (AUT30) potentiostat/ galvanostat at a current density of 50 mA cm
1 using a conventional three-electrode glass cell with the Sn
Ni alloy as working electrode and Li foil as reference and counter electrodes. The electrolyte was 1 mol L
1 Li perchlorate in propylene carbonate (1 mol L
1 LiClO4/PC, Aldrich). All potentials given hereafter are referred to the Li/Liþ reference electrode. After electrochemical treatment, the specimens were removed from the electrolyte and rinsed with anhydrous CH3CN. XPS and ToF-SIMS Characterization. XPS analysis was carried out with a VG ESCALAB 250 spectrometer equipped with a UHV preparation chamber directly connected to the glovebox10 and using conditions previously described.16,18 The binding energies were corrected by setting the C1s hydrocarbon (
CH2
CH2
bonds) peak at 285.0 eV. Data processing was performed with the Advantage software using a Shirley background. Compositional depth profiling of the pristine and cycled samples was obtained using a ToF-SIMS 5 spectrometer (IonTof) running at an operating pressure of ∼1  10
9 mbar. A pulsed 25 keV Biþ primary ion source was employed for analysis, delivering 1.8 pA of target current over a 100 μm  100 μm area. Sputtering was done using a 2 keV Csþ beam giving a 100 nA target current over a 250 μm  250 μm area. Data acquisition and post-processing analyses were performed using the Ion-Spec software. Negative ion profiles were recorded and analyzed. The intensity is reported using a logarithmic scale in order to magnify the low intensity signals.

’ EXPERIMENTAL SECTION Preparation of Sn
Ni Alloy Film. The electroplated Sn
Ni alloy films were prepared on copper foils (1.3  1 cm2, thickness = 0.1 mm) in a bath containing 40 g L
1 SnCl2 3 2H2O, 15 g L
1 NiCl2 3 6H2O, 350 g L
1 K4P2O7 3 3H2O, 8 g L
1 C4H4O6KNa 3 4H2O, and 8 g L
1 C2H5NO2. The bath temperature was kept at 45 °C, and its pH was adjusted to 8.5 by the addition of NH3 3 H2O. Electroplating was performed in a two-electrode system with a titanium foil used as counter and reference electrode. A constant current density of 1.0 A dm
2 was applied for 5 min leading to the deposition of around 1.7 mg of alloyed film. After electroplating, the prepared electrode was transferred to an anhydrous ethanol bath and rinsed in an ultrasonic bath for

’ RESULTS AND DISCUSSION Electrochemical Performance. The charge
discharge curves of the as-deposited Sn
Ni alloy anode are shown in Figure 2. In the initial discharge curve, the electrode potential 7013

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Figure 4. ToF-SIMS negative ion depth profiles of a pristine Sn
Ni alloy sample.

Figure 3. XP spectra of Sn3d, Ni2p, O1s, and C1s core levels of (a) a pristine Sn
Ni alloy, (b) a half cycled Sn
Ni alloy anode (cutoff potential 0.01 V), (c) a one-cycled Sn
Ni alloy anode (cutoff potential, 2.5 V), and (d) a nine-cycled Sn
Ni alloy anode (cutoff potential, 2.5 V) in 1 mol L
1 LiClO4/PC.

dropped to ca. 0.25 V immediately after starting the test, then stabilized, and decreased again gradually down to 0.02 V, which corresponds to the formation of LixSn alloys of varying stoichiometry. The charge potential plateau appears at ca. 0.5 V. The initial discharge capacity was measured to be 482 mA h g
1, and the corresponding charge capacity was 430 mA h g
1, yielding a Coulombic efficiency of 89%. The theoretical capacity of the electroplated Ni
Sn (66 at. % Sn) film is calculated to be 805 mA h g
1 assuming that Li4.4Sn is formed. An initial specific discharge capacity smaller than the theoretical value is generally reported on electroplated Sn
Ni anodes.11,34,42 It is caused by a limited consumption of the active material by the alloying reaction, which is confirmed by the ToF-SIMS data presented below. After some fluctuations, the discharge capacity of the fifth to ninth cycles is stable at about 380 mA h g
1, also showing the limitation of the alloying reaction. XPS Characterization. The XP Sn3d, Ni2p, C1s, and O1s core level spectra for the pristine and cycled samples are shown in Figure 3. The Sn3d and Ni2p core levels display 5/2 (∼485
487 eV)
3/2 (∼493
495 eV) and 3/2 (∼853
856 eV)
1/ 2(∼871
874 eV) spin orbit doublets, respectively. The Sn3d5/2A at 485.0 eV and Sn3d5/2B at 486.8 eV are assigned to metallic (Sn(0)) and oxidized (Sn(IV)) tin, respectively.12,13 The oxide component dominates with a relative intensity of 84.6%. The Ni2p3/2 core level displays three components. The first one observed at 852.9 eV is attributed to metallic nickel. The

two other smaller components at 856.1 and 861.7 eV are ascribed to the Ni(II) oxide/hydroxide.13 The higher relative intensity of the Sn peak corresponding to Sn(IV) shows that the Sn
Ni alloy surface is covered by a native oxide layer enriched in tin as a result of the sample exposure to atmosphere. The Sn3d and Ni2p signals are completely attenuated on the cycled samples due to the formation on the alloy surface of the SEI layer, at least ∼10 nm thick. On the pristine sample, the C1s signal originates from a contamination always detected on the extreme surface. The decomposition of the C1s region shows three components at 285.0, 286.5, and 288.6 eV assigned to
CH2
CH2
bonds, C atoms bonded to one O atom (C
O bonds), and C atoms bonded to two O atoms (O = C
O bonds), respectively.10 A modified C1s core level is observed on the cycled Sn
Ni alloy samples that show four components, C1sA, C1sB, C1sC, and C1sD, with increased intensities at 285.0, 286.5 ( 0.1, 288.7, and 289.8 ( 0.1 eV, respectively. The C1sA peak at 285.0 eV is ascribed to the C
H bonds in the SEI layer in addition to contamination. The new peak C1sD at 289.8 ( 0.1 eV is assigned to C atoms bonded to three O atoms in carbonate species, corresponding to Li2CO3 and/or lithium alkyl carbonate ROCO2Li43 that have been described as the main components of the SEI layer.44,45 The C1sB (carbon atoms bonded to one oxygen atom) at 286.7 eV corresponds then to the lithium alkyl carbonates species (ROCO2Li) and/or lithium alkoxides (RCH2OLi).12,13,43 The C1sC component at 288.6 ( 0.1 eV assigned to C atoms bonded to two O atoms can be explained by the presence of lithium oxalates (Li2C2O4).46 The O1s core level peak on the pristine sample shows a major component at 530.6 eV assigned to the oxide ions of the oxide surface layer on the Sn
Ni material exposed to ambient air. The shoulder at 532.1 eV is attributed to surface contamination.47 The O1s core levels of the cycled samples consist of different peaks. The component at 530.5 eV is fully attenuated by the growth of the SEI layer, in agreement with the attenuation of the Sn and Ni peaks. The component at 531.8 ( 0.1 eV becomes the major component, and it is assigned to the carbonate species.15 The contribution of the Li2CO3 species would be superimposed to that of the C
O bonds observed on the pristine sample. A new O1s component observed at 533.4 ( 0.1 eV on the cycled samples can be assigned to Li
alkyl carbonates (ROCO2Li) and/or Li
alkoxides (R
CH2OLi).48,49 Thus, XPS shows that 7014

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Figure 7. ToF-SIMS negative ion depth profiles of a Sn
Ni alloy anode after nine cycles in 1 mol L
1 LiClO4/PC (cutoff potential: 2.5 V). Figure 5. ToF-SIMS negative ion depth profiles of the Sn
Ni alloy anode after a half cycle in 1 mol L
1 LiClO4/PC (cutoff potential: 0.01 V).

Figure 6. ToF-SIMS negative ion depth profiles of a Sn
Ni alloy anode after one cycle in 1 mol L
1 LiClO4/PC (cutoff potential: 2.5 V).

the SEI layer is constituted of a mixture of Li2CO3, ROCO2Li, Li2C2O4, and/or ROLi, and the unchanged relative intensities of the core level components indicates that the balance of the SEI constituants does not markedly vary upon cycling. ToF-SIMS Characterization. Figures 4, 5, 6, and 7 display the negative ion depth profiles on the pristine, half-, one-, and ninecycled Sn
Ni samples, respectively. The intensity variation of the selected ions with sputtering time reflects the in-depth compositional changes of the electroplated layer, but the intensity is also dependent on the matrix from which the ions are emitted. The 16O
signal was easily saturated; therefore, the isotopic 18O
signal was used to profile oxygen that mainly originates from the metal oxide and the SEI layer.50 The 118Sn
, 58
63 Ni , Cu
, and 7Li
signals were selected to characterize the distribution of related species in the bulk electrode material. The 23 LiO
signal is also provided to investigate the SEI. Figure 8 shows a variation of the Sn:Cu and Sn:Li intensity ratios. The 12
C and 35Cl
signal is mostly characteristic of the SEI layer. The 35Cl
ions depth profiles are presented in Figure 9. On the pristine Sn
Ni sample (Figure 4), the plateaus observed for the 118Sn
and 58Ni
ion profiles are characteristic of the deposited Sn
Ni thin film. The parallel and nearly

Figure 8. ToF-SIMS Sn:Cu and Sn:Li intensity ratios as indicated for the pristine (a), half-cycled (b), one-cycled (c), nine-cycled (d) samples.

constant intensity profiles measured until ∼1050 s of sputtering are indicative of a homogeneous in-depth composition of the electroplated Sn
Ni alloy film. In this region, one can notice a significant signal of the 63Cu
ions, which suggests the presence of pinhole defects and/or cavities in the Sn
Ni layer. After 1050 s, the 118Sn
and 58Ni
ion profiles decrease in intensity, whereas the 63Cu
ion profile increases (see also the sharp decrease of the Sn:Cu intensity ratio in Figure 8), marking the beginning of the alloy/substrate interfacial region. After ∼1300 s, the bulk substrate region is reached with a Sn:Cu intensity ratio dropping below 0.1 (Figure 8). The alloy/substrate interfacial region is marked at 1150 s by the humps in the 18O
and 12C
ion profiles caused by surface contamination of the substrate before electroplating. Surface contamination by oxygen and carbon of the electroplated thin film is also observed during the first ca. 300 s of sputtering due to the surface roughness. Some carbon contamination of the bulk of the electroplated film is evidenced. 7015

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Figure 9. ToF-SIMS depth profiles of the 35Cl
ions for the pristine, half-, one-, and nine-cycled Sn
Ni samples.

Figure 8 shows that the sputtering time at which the 118Sn
to Cu
intensity ratio is 0.5 (value chosen as a marker of the beginning of the interfacial region) is about 4480, 1580, and 2580 s on the half-, one-, and nine-cycled samples, respectively, instead of about 1150 s on the pristine sample. This evidences the volume expansion/shrink of the anode material caused by lithiation/delithiation. The much longer time on the lithiated sample (half-cycled) evidences the volume expansion caused by the alloying reaction (a factor of 4 if one assumes no matrix effect on the sputtering time). The shorter time on the one-cycled sample than half-cycled sample is consistent with a volume shrink caused by delithiation. The longer time on the nine-cycled delithiated sample than the one-cycled sample indicates no loss of active electrode material with cycling but instead an increasing trapping of lithium in the bulk electrode. Figure 5 shows the negative ion depth profiles of the halfcycled lithiated Sn
Ni anode. Three regions can be distinguished prior to the alloy/substrate interfacial region. The first region (marked as SEI þ Li2O) is characterized by a peak of the 12
C signal, an increasing intensity of the 7Li
signal, and a stable intensity of 18O
signal, indicating that it is mainly composed of the reduction products of electrolyte (SEI layer). The low intensities of the metal ions (118Sn
and 58Ni
) in this region confirm the XPS data. Figure 8 shows that this region extends to ∼260 s of sputtering where the Sn:Li ratio becomes stable. The 23 LiO
ion profile suggests the presence of lithium oxide (Li2O) at the extreme surface of the alloy. LiO2 may be formed by conversion of the tin-enriched surface oxide covering the electroplated alloy. It may also originate from the decomposition of electrolyte that produces the SEI layer.51 On lithium and graphite anodes, Kanamura et al. have suggested that the SEI consists of several layers.52 Close to the electrode, the SEI would consist of thermodynamically stable anions such as oxides and halides, whereas close to the electrolyte, the SEI would contain partially reduced organic material. Our data suggest that the SEI layer also presents a layered structure on the Sn-based anode. The sharp decrease of the 12C
ion profile below the extreme surface is consistent with organic species predominating in the outer part of the SEI layer and the concomitant slight increase of the 23LiO
ion intensity after ∼100 s of sputtering time points to oxide species predominantly formed in the inner part of the SEI layer. The next region (marked as lithiated alloy in Figure 5) is characterized by a pronounced increase of 118Sn
and 58Ni
ion 63

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signals and a much slower increase of the 7Li
ion intensity, evidencing the lithiation of the outer part of the electroplated alloy. This region extends to ca. 2000 s of sputtering time as determined from the 118Sn
to 7Li
ratio in Figure 8, in which the 118Sn
to 7Li
intensity ratio remains fairly stable. Assuming no matrix effect on the sputtering yield, this indicates that the Sn: Li atomic ratio shows no in-depth dependence in the lithiated part of the electrode. The 12C
ion intensity is higher in this region of the half-cycled sample than on the pristine sample, which suggests that the products of electrolyte decomposition penetrate the lithiated alloy layer and fills cracks generated by volume expansion of the material. This is confirmed by the 23 LiO
ion profile, parallel to 7Li
. The parallel 118Sn
and 58
Ni ion profiles, like for the pristine sample, indicate that unreacted Sn and Ni remain alloyed, and thus there is no trapping of the dealloyed Ni component in the lithiated part of the electrode. After 2000 s of sputtering, the intensities of 118Sn
and 58Ni
ions stabilize after a hump while that of the 7Li
ions decreases (see also the drastic change of the Sn:Li intensity ratio in Figure 8). This marks an alloy region much less lithiated, where Li has diffused but where the alloying reaction has not propagated, most likely limited by mass transport of the dealloyed product (Ni) of the reaction. This partition of the initially homogeneous Sn
Ni alloy layer in two sub-layers reflects an incomplete alloying reaction despite the full penetration of lithium ion in the bulk alloy and explains the specific capacity measured to be lower than theoretically expected in the initial discharge process. A similar partition was also observed after the lithiation of Sn
Co alloy anodes.53 Figures 6 and 7 show the negative ion depth profiles of oneand nine-cycled Sn
Ni anodes. They have similar shapes and show several differences in comparison with those of the pristine and half-cycled samples. A first major difference is the nearly constant intensity of all ion profiles beyond the SEI region, after about 150 s of sputtering, as also shown by the stable intensity ratio in Figure 8. This shows the departition of the electroplated alloy after delithiation. A second major difference is the high intensity of the 63Cu
ion profiles in the delithiated alloy region, consistent with the volume shrink of the electrode. This suggests the formation of cracks and voids in the delithiated alloy layer. Thus, after the volume expansion/shrink caused by the lithiation/delithiation cycle, the electroplated Sn
Ni alloy layer most likely becomes divided by gaps and cracks and presents an opened morphology exposing the substrate. The 7Li
and 12
C ion profiles, parallel in the alloy region and peaking at the extreme surface, show that most lithium ions are extracted from the delithiated alloy and localized in the SEI layer at the surface. One also notices that the 12C
, 18O
, and 7Li
ion profiles decrease more slowly in the SEI/delithiated alloy interfacial region for the nine-cycled sample than for the one-cycled sample. This is consistent with a deeper penetration of the SEI layer associated with further division of the Sn
Ni alloy layer upon cycling, in agreement with SEM observations of the division of multicycled Sn
Ni anodes.41,54 As lithiation/delithiation of the Sn
Ni is performed in a Li perchlorate electrolyte, it is also worth profiling Cl ions. Figure 9 compares the 35Cl
depth profiles for the pristine, half-, one-, and nine-cycled Sn
Ni samples. On the pristine sample, one observes pronounced 35Cl
peaks at the surface of Sn
Ni alloy layer and at the interface with the Cu substrate, which are assigned to the preparation of the samples in Cl
-containing solution. On the half-cycled specimen, the intensity of 35Cl
ions 7016

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The Journal of Physical Chemistry C markedly increases in the bulk region of the alloy layer. This is consistent with the penetration of the electrolyte following the division of the alloy layer caused by its volume expansion/shrink and the formation of the SEI layer on the newly exposed interfaces. On the one-cycled sample, Cl is not rejected from the delithiated alloy, showing that its trapping is concomitant with that of lihtium. Upon multicycling, the intensity of the 35Cl
ions is observed to further increase in the bulk alloy region. This is consistent with further penetration of the electrolyte/SEI layer caused by the continuous division of the Sn
Ni alloy up to the nine cycles tested and confirms the conclusion drawn from the 12
18
C , O , and 7Li
ion profiles regarding the accumulation of electrolyte products in the delithiated electrode material.

’ CONCLUSIONS Sn
Ni alloy anodes were prepared by electroplating on a copper substrate, and the electrode processes associated with lithiation/delithiation were investigated by combining electrochemical measurements in 1 M LiClO4/PC with analysis by XPS and ToF-SIMS. The pristine Sn
Ni alloy (∼66 at% Sn) specimens present a homogeneous in-depth composition and are covered by a Sn
rich oxide layer on the extreme surface. A relatively thick (g10 nm) solid electrolyte interphase (SEI) layer is built up at the surface after the first discharge half-cycle as evidenced by the intense C1s and O1s core level peaks measured by XPS as well as the total attenuation of Sn3d and Ni2p core levels. The SEI layer is constituted of a mixture of Li2CO3, ROCO2Li, Li2C2O4, and/or ROLi whose balance does not significantly vary upon cycling. Depth profiling suggests a layered structure of the SEI layer with the predominance of partially reduced organic species in the outer part close to the electrolyte solution and the predominance of lithium oxide in the inner part close to the electrode. ToF-SIMS depth profiling evidences the volume expansion/ shrink associated with the lithiation/delithiation of the Sn
Ni electrode material. Because of mass transport limitation through the thick (∼3 μm) Sn
Li layer, an incomplete initial alloying process of lithium with tin is observed upon the first discharge despite the penetration of lithium throughout the whole layer. It leads to the partition of the electroplated layer into a highly lithiated outer part where no marked variation of the Sn:Li ratio is observed and an essentially nonlithiated inner part where lithium only diffuses. This partition limits the specific capacity of the initial discharge (ca. 430 mA h g
1). Upon charge, a single layered film is rebuilt by delithiation; however, it has a divided morphology of alloy particles caused by cracking of the initially homogeneous structure. The SEI layer builds up at the new surfaces (alloy and copper substrate) exposed to the electrolyte by the division of the cycled electrode, which contributes to trap lithium and chlorine in the delithiated electrode. Multicycling (up to nine cycles tested) amplifies the particle division and the SEI buildup within the electrode layer without any apparent loss of alloy material or drop of the capacity (ca. 380 mA h g
1). It also amplifies the volume expansion of the delithiated electrode as indicated by the increase of trapped lithium and chlorine in the divided electrode. ’ AUTHOR INFORMATION Corresponding Author

*E-mail:vincent
maurice@chimie
paristech.fr (V.M.); sgsun@ xmu.edu.cn (S.-G.S.);philippe-marcus@chimie
paristech.fr (P.M.).

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’ ACKNOWLEDGMENT This work was partially supported by the NSFC (Grant No. 21003102, 20833005, 21021002) and Special Funds for Major State Basic Research Project of China (Grant No. 2009CB220102). Region Ile
de
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