Journal of The Electrochemical Society, 162 (12) A2281-A2288 (2015) A22810013-4651/2015/162(12)/A2281/8/$33.00 © The Electrochemical Society

The Mechanism of SEI Formation on Single Crystal Si(100),Si(110) and Si(111) ElectrodesU. S. Vogl,a,b S. F. Lux,a,b,∗ P. Das,b A. Weber,b,∗ T. Placke,a,∗ R. Kostecki,b,∗,z

and M. Wintera,c,∗∗,z

aWestfalische Wilhelms-Universitat Munster, MEET Battery Research Center, Institute of Physical Chemistry,48149 Munster, GermanybEnvironmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley 94611,California, USAcHelmholtz Institute Munster (HI MS), 48149 Munster, Germany

Subsequent to our previous studies on the SEI formation mechanism on the single crystal silicon (100) surface, here we reporton complementary studies of the SEI formation on Si surfaces with the crystal orientations (111) and (110). The differencesin electrochemical behavior of the different crystal orientations are discussed - especially with regard to the effect of the SEIforming electrolyte additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) added to ethylene carbonate (EC)/diethylcarbonate (DEC) based electrolytes. Fourier transform infrared spectroscopy (FTIR) of the SEI during early stages of SEI formationand physico-chemical investigations (wetting behavior) indicate a strong dependence of the chemical composition of the SEI on thesurface orientation and the electrolyte composition during the early stages of lithiation of Si. However, at a higher lithiation degreeless difference in the chemical composition of the SEI can be observed. These findings are in agreement with those made for the SEIformation on the Si(100) surface.© 2015 The Electrochemical Society. [DOI: 10.1149/2.0361512jes] All rights reserved.

Manuscript submitted May 11, 2015; revised manuscript received July 31, 2015. Published September 1, 2015.

Graphite is the current state-of-the-art anode material in lithium-ion batteries.1–4 However, silicon is thought to replace graphite dueto its higher specific capacity of 3,500 mAh/g5 and because of itshigh natural abundance.6 However, silicon is encumbered with seriousdrawbacks hindering its application, such as large volumetric changesof ±300% during the lithiation/de-lithiation processes, poor intrinsicelectronic conductivity of silicon, and a high interfacial instability ofSi electrodes in organic electrolytes.7–14 The consequences of thesehuge volume changes are cracking and disintegration of the siliconelectrode surface resulting in a loss of electronic contact and rapidfading of reversible capacity.7,15,16

The performance of silicon anodes in the cell depends on thecharacteristics of the solid electrolyte interphase (SEI) – regularlyconsisting of decomposition products of the electrolyte salt, which istypically LiPF6,17 and the organic carbonate solvents, which are usu-ally a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC)and/or diethyl carbonate (DEC),18,19- which is formed within the firstcycles during the charge reaction by electrolyte decomposition. TheSEI is essential for a good performance of any anode, including car-bonaceous and intermetallic lithium ion battery anodes.20–25 On siliconelectrodes, the electrolyte decomposition of LiPF6-based electrolytesstarts at a potential of 1.8 V vs. Li/Li+.26 Between 1.8 V vs. Li/Li+ and0.6 V vs. Li/Li+, the decomposition products of the electrolyte startthe formation of films on the silicon surface. At 0.4 V vs. Li/Li+, thelithiation of silicon starts and goes along with the formation of newSEI compounds and new Si compounds, e.g. Si-Li, Si-F, F-Si-Li.26

Electrolyte additives like vinylene carbonate (VC), and fluoroethy-lene carbonate (FEC) are known to affect the composition and phys-ical properties of the SEI layer on the silicon surface resulting in animprovement of the SEI formation process, the electrode cycling sta-bility, and consequently, battery lifetime and Coulombic efficiency ofsilicon anodes.27–30

In our previous work,31 the SEI on a model silicon surface withthe single crystal orientation (100) was investigated. Using this modelsystem with a silicon wafer of a defined surface orientation excludesinfluencing factors coming from the composite electrode composi-tion, e.g. of the binder, the current collector, or the conductive agents.Our studies of the SEI formation and composition in dependence ofdifferent charging (= lithiation) cut-off potentials and various elec-trolyte compositions, i.e., the addition of VC or FEC to the standard

∗Electrochemical Society Active Member.∗∗Electrochemical Society Fellow.

zE-mail: r_kostecki@lbl.gov; martin.winter@uni-muenster.de

electrolyte 1M LiPF6 EC:DEC [3:7] led to various novel findings. Wecould manifest different morphologies and chemical compositions ofthe SEI after addition of FEC and VC at a potential of 500 mV vs.Li/Li+, i.e., when the silicon is only partially lithiated, by means ofSEM, EDX, IR and XPS. Remarkably, the SEI formed in differentelectrolytes, i.e., with and without the FEC and VC additive, showsthe same composition and morphology on the crystal orientation 100when the silicon becomes fully lithiated at a potential of 10 mV vs.Li/Li+. This could be explained by the huge impact of the volume ex-pansion and the transition of crystalline into amorphous silicon whichleads to cracking of the pristine SEI and subsequent repair by newSEI products32–34 at an operation state where the electrolyte additiveshave been to a large part consumed and the base electrolyte compo-nents, e.g. EC and DEC solvents and the electrolyte salt, are the majorcontributor in the SEI formation process.

In this continuing study, we additionally focus on different crystalorientations of the Si anode, i.e., in addition to the Si(100) surface, wewill also regard the Si(110) and Si(111) surfaces. Figure 1 displaysthe unit cells of these different surfaces as well as the view alongthe different directions of the diamond cubic lattice in which theblack marked silicon atoms belong to a particular plane. The surfaceorientations strongly differ in their atomic density and surface energy.For instance, the Si(111) plane exhibits the highest atomic densityand the lowest surface energy among them, whereas the Si(100) planehas the lowest atomic density and the highest surface energy. Table Ilists the atomic density, spacing and surface energy (unrelaxed status)of the three different surface orientations.35,36 From Figure 1, it canbe observed that the crystalline silicon structure exhibits relativelylarge interstitial spaces between the atoms along the (110) direction,which are larger than those along the (100) or (111) directions. Leeet al. reported that lithium ions enter crystalline silicon preferentiallythrough the (110) channels during the initial stages of lithiation, thuscausing a volume expansion along this direction.37 In this study wewill focus on the differences in the SEI formation mechanism at thedifferent silicon surfaces.

Experimental

Silicon wafers were obtained from Silicon Quest International,San Jose, CA, in the crystal orientations (100), (110) and (111). Allsilicon wafers are single-side polished, 500–550 μm thick, n-typedoped with phosphorus and have a resistivity range of 5–10 ohm · cm.After thorough cleaning and dicing into smaller pieces of 1 × 1 cm2,

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A2282 Journal of The Electrochemical Society, 162 (12) A2281-A2288 (2015)

Figure 1. Unit cells of Si(100), Si(110) and Si(111) (upper part) and view along the different directions of the diamond cubic lattice (lower part). The blackmarked silicon atoms belong to a particular plane.

the silicon wafers were heated under vacuum at 195◦C for 48 hoursin order to dry the surface.38

All electrolytes were prepared in a glove-box (MBraun LabmasterUnilab) under helium atmosphere (O2 and H2O < 1 ppm). 1 mol/Llithium hexafluorophosphate, LiPF6 (Sigma Aldrich, battery grade)was dissolved in the solvent mixture consisting of ethylene carbonate,EC (Sigma Aldrich, battery grade) and diethyl carbonate, DEC (SigmaAldrich, battery grade) of a ratio 3:7 in weight in order to receive thebase electrolyte 1M LiPF6 EC:DEC [3:7].

The electrolyte additives fluoroethylene carbonate, FEC (Solvay,battery grade) and vinylene carbonate, VC (Sigma Aldrich, batterygrade) were mixed with the carbonate mixture EC:DEC [3:7] in theratios FEC: EC: DEC [10:27:63] and VC:EC:DEC [2:29.4:68.6], re-spectively, to receive a content of 10 wt.% FEC and 2 wt.% VC in theelectrolytes. Finally, LiPF6 was dissolved in these mixtures to obtainthe target compositions of 1M LiPF6 FEC:EC:DEC [10:27:63] and1M LiPF6 VC:EC:DEC [2:29.4:68.6].

Electrochemical investigations were conducted on a Gamry In-struments Reference 600 Potentiostat/Galvanostat/ZRA. From theopen circuit potential (OCP) the potential was decreased to 500mV and 10 mV vs. Li/Li+ , respectively, with a sweep rate of 0.025mV/s. The cell set-up (see previous publication31) was a three elec-trode beaker cell (reference electrode (RE) and counter electrode(CE): metallic lithium) operated in the prior mentioned glove-boxenvironment.

The silicon single crystal electrodes were polarized to the desiredpotentials of 10 mV or 500 mV vs. Li/Li+, respectively. After celldisassembly, the electrodes were washed three times with DEC in theglove-box and finally dried under vacuum for 5 minutes. The infraredspectra (FTIR) were recorded in an N2-filled environmental chamberwith a Bomem MB102 spectrometer equipped with CsCI windows intransmission mode with a baseline correction of the pristine siliconwafer.

Table I. Atomic density, spacing and surface energy of the siliconsurface orientations (100), (110) and (111).35,36

Atomic properties (100) (110) (111)

Atomic density (1014 cm−2) 6.78 9.59 15.66Spacing (Å) 5.43 3.84 3.13

Surface energy (J m−2) 2.39 2.04 1.82

The drop shape analysis, i.e., the contact angle investigation onthe different silicon surfaces, was performed on a modified automatedGoniometer (rame-hart model 290). The liquid-droplet profile imageswere taken using a CCD camera with 7 mm × 5 mm field of view (640× 480 pixels) at half-second time intervals. The droplets were backlitwith a diffused 150 W halogen lamp. Due to the use of anti-vibrationstages and protection of the aperture from any vibration effects ofthe surrounding, it was ensured that the droplet released only due togravity. Droplets of a volume of 10 μL were applied manually withan Eppendorf Research plus pipette.

Results and Discussion

Figure 2 displays the linear sweep voltammetry results of sili-con wafers with the surface orientation (100), (110) and (111) forthe (a) standard electrolyte 1M LiPF6 EC:DEC [3:7], (b) in 1MLiPF6 VC:EC:DEC [2:29.4:68.6] and (c) in 1M LiPF6 FEC:EC:DEC[10:27:63]. The decomposition of the standard electrolyte starts forall three surface orientations at an on-set potential of 1.84 vs. Li/Li+.In each voltammetric profile one main peak is identifiable. After amajor electrolyte decomposition peak – at > 500 mV vs. Li/Li+ - theformation of lithium silicides starts, which is indicated by a re-rise ofthe current density (Figure 2a). However, differences in the maximumcurrent density, peak profile and the peak potentials are remarkablefor the different surface orientations (Table II). The lowest peak maxi-mum of the electrolyte decomposition is observed on the (100) surfacefollowed by the (110) surface. The (111) surface orientation displaysthe highest current density. Whereas the (100) surface just exhibitsone distinguishable peak in the potential profile, for the (110) and(111) surface orientation adjacent peak shoulders of the main peakcan be clearly identified (cf. Table II). Additionally, the consumedcharge, i.e., the number of electrons consumed in the overall process(LixSi and SEI formation), also indicates differences in electrolytedecomposition on the different surface orientations.

This finding was not unexpected, as similar studies on Sn singlecrystals39 and on graphite2,40,41 have shown major differences in theSEI formation on different crystal surface orientations.

The different amounts of consumed charge indicate that the elec-trolyte decomposition is strong on the (111) surface, followed bythe (110) and last by the (100) surface (Table II). This can eitherindicate that the (111) surface shows a higher reactivity with elec-trolyte, thus leading to more electrolyte decomposition and a thickerSEI surface layer, or that the decomposition process follows differ-ent decomposition reaction mechanisms depending on the surface

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Journal of The Electrochemical Society, 162 (12) A2281-A2288 (2015) A2283

0.5 1.0 1.5 2.0 2.5

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c) 1M LiPF6 in FEC:EC:DEC [10:27:63]

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Figure 2. Charge profiles of the silicon wafers with the crystal orientations(100), (110) and (111) in (a) the standard electrolyte 1M LiPF6 EC:DEC[3:7], (b) in 1M LiPF6 VC:EC:DEC [2:29.4:68.6] and (c) in 1M LiPF6FEC:EC:DEC [10:27:63] in a beaker cell with Li as CE,RE. The currentdensity of a 0.317 cm2 surface in μA cm−2 is plotted against the potential inV vs. Li/Li+.

orientation. In the latter case, the decomposition mechanism on the(111) surface may be a multi-electron reaction which consumes moreelectrons than the mechanism(s) on the other surface orientations(110) and (100).

Considering the VC- and FEC-containing electrolytes (Figure 2band 2c), it can also be clearly observed that there are strong differ-ences in the electrolyte decomposition reaction on the different silicon

surfaces. While for the standard electrolyte the on-set potential of elec-trolyte decomposition is very similar on the different Si surfaces, i.e.,ca. 1.8 V vs. Li/Li+ (Table II), those of the VC- and FEC-containingelectrolytes differ remarkably for different surfaces. For example, forboth additive-containing electrolytes, the decomposition on the (110)surface starts at a lower potential compared to the (100) and (111) sur-faces. For the VC-based electrolyte, the decomposition starts at 1.81 Vvs. Li/Li+ on the (111) surface, at 1.71 V vs. Li/Li+ on the (100) andat 1.68 V vs. Li/Li+ on the (110) surface and for the FEC-based elec-trolyte, the decomposition starts at 1.85 V vs. Li/Li+ on the (111)surface, at 1.97 V vs. Li/Li+ on the (100) and at 1.58 V vs. Li/Li+

on the (110) surface (Table II). Overall, the highest current densitiesfor both the VC- and FEC-containing electrolytes can be observed forthe (100) surface orientation. Furthermore, the highest current densi-ties from all electrolytes are obtained for the FEC-based electrolyte,which may be related to the high amount of FEC (10 wt.%). The totalconsumed charge also displays the highest values for the FEC-basedelectrolyte on the (100) and (111) surfaces.

As a major part of the SEI formation has been accomplished andformation of Li silicides has not started at 500 mV vs. Li/Li+, weinvestigated the SEI on the different silicon surface orientations bymeans of FTIR at this potential. Figure 3 shows the IR peak profiles ofthe surfaces (100), (110) and (111) obtained in the standard electrolyte1M LiPF6 EC:DEC [3:7].

For each surface orientation, the typical peaks of the electrolyte de-composition peaks have been found. The peaks have been previouslyreported by Tsubouchi et al. and Profatilova et al.33,42

Components like LiF, PF3 or P(OR)3 originating from LiPF6 saltdecomposition can be observed in the spectra. In particular, there is abroad peak at a wavenumber of 500 cm−1 occurring from the ν(Li-F)of LiF in the SEI and a peak at 842 cm−1 occurring from ν(P-F) ofPF3. At 982 cm−1, 905 cm−1 and 730 cm−1, the peaks reflecting theν(P-O-C) stretching mode can be found.

Additionally, peaks of solvent decomposition products, i.e., ofthe carbonates EC and DEC are clearly visible. The broad peak at1775 cm−1 originates from the ν(C=O) stretching modes of DECand EC. This broad peak is an overlay to the decomposition peakof DEC (responsible for right side shoulder) and the decompositionpeak of EC (responsible for the left side shoulder). Furthermore, DECdecomposition is confirmed by the symmetric mode ν(C=O) at 1300cm−1. The decomposition of EC can be recognized by the asymmetricstretching mode ν(C=O) at 1484 cm−1. Typical ring stretching modesν(C-O) of EC decomposition products are the two peaks at 1200 cm−1

and 1086 cm−1. The peak at 1410 cm−1 can be attributed to ν(C-H)of CH3/CH2 bonds.

However, the spectra in Figure 3 display remarkable differencesbetween the different surfaces with respect to peak heights and shapes,indicating not only a different contribution of IR-active species in theSEI, but overall different compounds present.

On the (111) surface orientation, the SEI consists mainly ofcarbonate-based decomposition products. The peak at 1775 cm−1,which is the highest one, reflects the ν(C=O) stretch modes of DECand EC. In line with this discovery, the broad LiF peak at 500 cm−1

is very low, indicating less salt decomposition products. In general,the peaks of the SEI on the Si(111) surface orientation are very sharpcompared to the other surface orientations, which may indicate thatthe species in the SEI are very defined compounds.

On the (110) surface, less sharp peaks in the spectrum indicate amore “diverse” spectrum of decomposition species in the SEI com-pared to the (111) surface. There is also a large peak for carbonatedecomposition visible at 1775 cm−1, but in contrast to the (111) sur-face, the broad LiF peak at 500 cm−1 is almost of the same height.We can assume that the decomposition products of the salt and thecarbonate solvents contribute almost equally to the SEI on the (110)surface.

On the (100) surface, the peaks start to be very broad which meansthat there is a bigger fraction of chemically diverse decompositionspecies in the SEI. The ratio of the peaks of the decomposed saltand solvents, respectively, is more balanced than at the (111) surface.

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Table II. Profile characteristics and total charge consumed during a cathodic linear voltammetric sweep (Figure 2) of a Si(100), Si(110) andSi(111) electrode in 1M LiPF6 EC:DEC [3:7], 1M LiPF6 VC:EC:DEC [2:29.4:68.6] and 1M LiPF6 FEC:EC:DEC [10:27:63] electrolytes. The on-setpotential of electrolyte decomposition is related to a specific current of 50 μA cm−2.

ElectrolyteSurface

orientation

Maximum current density(μA cm−2) and peak

potential (V vs. Li/Li+)

Adjacent peak current density(μA cm−2) and peak

potential (V vs. Li/Li+)

On-set potential ofelectrolyte decomposition

(V vs. Li/Li+)

Total consumedcharge (integrated

area) in As

1M LiPF6

EC:DEC [3:7](100) 330 (at 1.42) —– 1.79 4.38(110) 384 (at 1.45) 345 (at 1.60) 1.81 5.54(111) 592 (at 1.30) 393 (at 1.55) 1.80 8.14

1M LiPF6

VC:EC:DEC[2: 29.4: 68.6]

(100) 480 (at 0.88) 462 (at 0.71) 1.71 5.52(110) 181 (at 1.33) 138 (at 0.76) 1.68 3.15(111) 273 (at 1.58) 254 (at 1.32) 1.81 5.00

1M LiPF6

FEC:EC:DEC[10:27:63]

(100) 920 (at 0.50) 842 (at 0.35) 1.97 11.88(110) 222 (at 1.25) 210 (at 0.95) 1.58 3.51(111) 720 (at 1.38) 637 (at 1.60) 1.85 11.54

Likewise to the (110) surface, the IR results indicate the same amountsof salt decomposition and carbonate decomposition.

The results of our previous work31 proved a severe potential depen-dence of the SEI formation and composition. At 500 mV vs. Li/Li+,the SEI is formed on the crystalline silicon phase. At 10 mV vs. Li/Li+,the silicon is fully lithiated and has become amorphous because of thephase transition during the lithiation process. The IR spectra of thefully lithiated phase at 10 mV vs. Li/Li+ are shown in Figure 4 forthe surface orientations (100), (110) and (111). The spectra differsignificantly from those taken from Si polarized to at 500 mV vs.Li/Li+.

On the (111) surface the previously sharp peaks have becomebroader, indicating more decomposition species. The salt decomposi-tion products in general dominate the spectrum, for instance, the peaksat 500 cm−1 and at 842 cm−1 represent LiF and PF3, respectively. Thepeaks representing the ν(P-O-C) stretching mode, visible at 982 cm−1,905 cm−1 and 730 cm−1 have become more intense.

The spectra on the (110) surface show similar peak profiles, butthe LiF peak at 500 cm−1 almost completely disappeared.

As for the spectra obtained on the (111) surface orientation, thepeaks indicating salt decomposition increased strongly for the inves-tigations on the (100) surface. Mainly the LiF peak at 500 cm−1,

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Figure 3. Normalized FTIR spectra of SEI on the silicon wafers chargedto 500 mV vs. Li/Li+ in the standard electrolyte 1M LiPF6 EC:DEC [3:7].Spectrum above: crystal orientation (100). Spectrum in the middle: crystalorientation (110). Spectrum below: crystal orientation (111).

but also the peaks at 842 cm−1 representing PF3 and the peaks forthe ν(P-O-C) stretching mode visible at 982 cm−1, 905 cm−1 and730 cm−1 gained in intensity.

In summary, on the (111) and (100) surface in parallel to thetransition of the crystalline into the amorphous silicon phase a hugeincrease of the LiF amount in the SEI indicated by the biggest broadpeak at 500 cm−1 can be observed. At contrast, the previously broadLiF peak on the (110) surface seems to be almost completely overlaidby the peak representing carbonate decomposition, as the LiF peak at500 cm−1 almost disappeared in this spectrum.

Surprisingly, after the different crystalline phases have been con-verted to the amorphous phase on each wafer, there are even strongerdifferences in the SEI composition than for the initial SEI formed onthe crystalline silicon phase. In the previous manuscript,31 the influ-ence of the electrolyte additives VC and FEC on the SEI at the siliconsurface orientation (100) was revealed. Here, the diverse surface ori-entations cause already differences in the SEI composition for thestandard electrolyte 1M LiPF6 EC:DEC [3:7]. We therefore expectthat the presence of VC and FEC in the electrolyte will result in evengreater differences.

The spectra plotted for the different crystal orientations polarizedto 10 mV vs. Li/Li+ in the VC containing electrolyte, i.e., 1M LiPF6

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Figure 4. Normalized FTIR spectra of SEI on the silicon wafers charged to 10mV vs. Li/Li+ in the standard electrolyte 1M LiPF6 EC:DEC [3:7]. Spectrumabove: crystal orientation (100). Spectrum in the middle: crystal orientation(110). Spectrum below: crystal orientation (111).

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Journal of The Electrochemical Society, 162 (12) A2281-A2288 (2015) A2285

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Figure 5. Normalized FTIR spectra of SEI on the silicon wafers charged to10 mV vs. Li/Li+ in the VC containing electrolyte 1M LiPF6 VC:EC:DEC[2:29.4:68.6]. Spectrum above: crystal orientation (100). Spectrum in themiddle: crystal orientation (110). Spectrum below: crystal orientation (111).

VC:EC:DEC [2:29.4:68.6], are displayed in Figure 5. It is obviousthat VC causes a different SEI formation on all three surfaces. Theimpact of VC is in fact even more intense on wafers polarized to500 mV vs. Li/Li+ (see previous publication31), but interestingly thesedifferences are still very prominent at 10 mV vs. Li/Li+.

On the (111) surface, the LiF peak at 500 cm−1 is dominating,indicating that salt decomposition is favored over carbonate solventdecomposition. Also, on the (110) surface, the LiF peak has almostdouble the intensity of the other peaks. The SEI on this surface orien-tation mainly consists of LiF. At contrast, the SEI on the (100) surfaceis slightly dominated by carbonate decomposition products.

Figure 6 displays, the spectra of the different Si surface orien-tations obtained in the FEC-containing electrolyte, i.e., 1M LiPF6

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Figure 6. Normalized FTIR spectra of an SEI on the silicon wafers charged to10 mV vs. Li/Li+ in the FEC containing electrolyte 1M LiPF6 FEC:EC:DEC[10:27:63]. Spectrum above: crystal orientation (100). Spectrum in the middle:crystal orientation (110). Spectrum below: crystal orientation (111).

FEC:EC:DEC [10:27:63]. Similarly to the VC-containing elec-trolyte, enormous differences in the spectra are noteworthy.

On the (111) surface, FEC leads to a LiF-rich SEI as well. Ad-ditionally, all peaks of other salt decomposition products increasedwhereas the carbonate decomposition peaks almost disappeared. Onthe (110) surface, the same trend can be identified but the peaks areless intense. At contrast, the SEI on the (100) surface is dominated bycarbonate solvent decomposition products.

As described in our previous work,31 the IR spectra of the SEI layeron the (100) surface at 10 mV vs. Li/Li+ are the same for the VC- andFEC-containing electrolytes as well as the electrolyte without any ad-ditive. Due to cracking and subsequent repair the “final” SEI (formedon the lithiated Si at 10 mV vs. Li/Li+) consists of decompositionproducts that stem from the main electrolyte components, i.e., theorganic carbonate solvents and the electrolyte salt. VC or FEC, whichhave been added only in small amounts, have no significant impacton this SEI layer, as they have been consumed to a major part in theprevious SEI formation processes. The overall conclusion was thatVC and FEC have no measurable influence on the SEI compositionon the (100) surface after the complete first charge process at 10 mVvs. Li/Li+, which is in contrast to the other crystal orientations. Onthe (110) surface, the presence of VC and FEC results in slight differ-ences in the SEI composition. This trend is even more striking for theSEI on the (111) surface, where almost no carbonate decompositionproducts can be found when FEC is present in the electrolyte (seeFigure 7).

These results clearly prove that the impact of the electrolyte addi-tive depends on the surface orientation. Whereas on the (100) surfacethe use of film forming electrolyte additives is quite ineffective withregard to the composition of the “final” SEI, enormous differences inthe SEI composition could be identified on the (111) surface. Fourpossible explanations for this behavior will be contemplated:

1) The SEI on the (100) surface is unstable. It cracks fast, easilyresulting in a huge amount of “repair SEI film” on a larger Si sur-face after expansion. The lithiated Si surfaces, which are exposedto the electrolyte without any SEI protection after cracking, arehighly reactive and thus react with any electrolyte componentin vicinity: In other words, there is no specific selectivity vs.a preferred reaction with the electrolyte additives. In addition,

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1M LiPF6 EC:DEC [3:7]

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Figure 7. Normalized FTIR spectra of SEI on the silicon (111) wafer chargedto 10 mV vs. Li/Li+ in the standard electrolyte 1M LiPF6 EC:DEC [3:7](spectrum below), in the VC-containing electrolyte 1M LiPF6 VC:EC:DEC[2:29.4:68.6] (spectrum in the middle) and in the FEC-containing electrolyte1M LiPF6 FEC:EC:DEC [10:27:63] (spectrum above).

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A2286 Journal of The Electrochemical Society, 162 (12) A2281-A2288 (2015)

100

100

111

111

Figure 8. Contact angle dependency on the crystal orientation of silicon wafer(100) and (111) with 1M LiPF6 EC:DEC [3:7] above and with water as sampleliquid below.

since compared to the other electrolyte components, the amountof additive is relatively small, VC and FEC may be more or lessconsumed after the “first SEI” has been formed.

2) The (100) surface favors an SEI formation mechanism involvingcarbonate solvent decomposition in the later stages of the SEIformation process, resulting in a similar SEI for all electrolytesolutions.

3) The physical characteristics of the various silicon surfaces are dif-ferent with regard to the lithiation behavior. Preferred lithiationof a particular surface orientation, i.e. of the (110) surface, mightbe favored leading to higher lithiation degrees thus lower elec-trode potentials, therefore more pronounced volume expansionand SEI cracking.

4) The SEI formed on the (110) and (111) surfaces is more successfulin protecting the silicon and the lithiated amorphous phase vs.reaction with the electrolyte, most likely due to the differentchemical composition (or physical properties, e.g. elasticity andtensile strength) of the SEI prior to the stage where lithiationoccurs.

With regard to the explanations 3 and 4, it should be noted, that thedensity of Si atoms and related surface groups, such as silicon oxideand silanol surface groups is the highest on the Si (111) and the loweston the Si (100) plane.[28]

To better understand the origin of the different SEI layers of thesurface orientations (100), (110) and (111), in particular in view ofthe possible explanations 3 and 4, the wetting ability of the sur-faces was investigated by contact angle measurements. The betterthe interaction between the respective surface orientation and thedrop of the respective liquid electrolyte, the lower is the contactangle.43

Figure 8 depicts the images of the droplets on the various sil-icon wafers. In the upper panel, drops of the standard electrolyte1M LiPF6 EC:DEC [3:7] on the (100) and the (111) surface ori-entations are shown. In the bottom, water droplets as reference onthe same surfaces are depicted. Whereas differences for the elec-trolyte droplet are hard to find, the water droplet seems to behavehydrophobic on the (100) surface, whereas it is hydrophilic on the(111) surface. Figures 9a and 9b show the contact angle values forwater and the three electrolyte systems, i.e., 1M LiPF6 EC:DEC [3:7],1M LiPF6 FEC:EC:DEC [10:27:63] and 1M LiPF6 VC:EC:DEC[2:29.4:68.6]. The interaction of water with the surface is in line withthe surface tensions of the silicon wafers depending on the surfaceorientation.

The (100) surface has the highest surface energy with 2.39 J m−2

and therefore the largest contact angle value with the water droplet.The (110) surface has a value of 2.04 J m−2 and the (111) surfaceexhibits 1.82 Jm−2 in an unrelaxed status (see Table I).35 The highsurface energy with water can be explained by the interaction of waterwith silanol groups via hydrogen bridge bonds. The Si(111) plane hasthe highest surface density of atoms, thus also the highest amount ofsilanol groups, whereas the Si(100) plane has the lowest amount.36

100 110 111

30

60

90

100 110 111

20

25

30

b)

cont

act a

ngle

in °

silicon crystal orientation

Water 1M LiPF6

EC:DEC 1M LiPF6

VC:EC:DEC 1M LiPF6

FEC:EC:DEC

a)

silicon crystal orientation

Figure 9. Contact angle dependency on the crystal orientation of the sil-icon surface. Black square: water, red sphere: 1M LiPF6 EC:DEC [3:7],green triangle: 1M LiPF6 FEC:EC:DEC [10:27:63], blue triangle: 1M LiPF6VC:EC:DEC [2:29.4:68.6].

The more surface silanol groups, the more hydrogen bridge bondsof the surface groups with water molecules are possible causing anattractive interaction and a low contact angle.

However, a look at an enlarged view in Figure 9b reveals that theinteraction energy trend is not proportional to the surface energy ofthe crystals for the organic solvent electrolytes. For all electrolytes,the (110) surface has the lowest contact angle, followed by the (100)surface. The highest contact angle was always noticed on the (111)surface orientation. Additionally, we could observe a time dependentspreading of the droplet of the electrolyte whereas the water dropletsappeared stable in shape and the contact angle did not change withtime (Figure 10).

Consequently, the interaction mechanism between the droplet andthe surface seems to be different for a protic, e.g. water-containingsystem and aprotic systems as the investigated electrolytes, where theorganic compounds in the electrolyte are not able to generate hydrogenbridge bonds with the silanol surface groups.

0 50 100 150 200 250 300

15

20

25

30

35

time in s

1M LiPF6 FEC:EC:DEC [10:27:63]

Water

cont

act a

ngle

in °

Figure 10. Time dependency of the contact angle of the droplets on the sili-con surface (111). Black dots: water, blue triangle: 1M LiPF6 FEC:EC:DEC[10:27:63].

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Journal of The Electrochemical Society, 162 (12) A2281-A2288 (2015) A2287

Table III. Qualitative statement about SEI composition in dependency of electrode potential, electrolyte mixture and Si surface orientation.

SEI composition on Si surfaces

Charging potential (mV vs. Li/Li+) Electrolyte (100) (110) (111)

500 1M LiPF6 EC:DEC [3:7] Carbonate species& fluoride species

Carbonate species& fluoride species

Carbonate species >>>fluoride species

1M LiPF6 VC:EC:DEC[2: 29.4: 68.6]

Carbonate species >fluoride species

N/A N/A

1M LiPF6 FEC:EC:DEC[10:27:63]

Carbonate species <fluoride species

N/A N/A

10 1M LiPF6 EC:DEC [3:7] Carbonate species <fluoride species

Carbonate species >>fluoride species

Carbonate species <<fluoride species

1M LiPF6 VC:EC:DEC[2: 29.4: 68.6]

Carbonate species >fluoride species

Carbonate species <<fluoride species

Carbonate species <fluoride species

1M LiPF6 FEC:EC:DEC[10:27:63]

Carbonate species >fluoride species

Carbonate species <<<fluoride species

Carbonate species <<<<<fluoride species

Finally, there is a second trend. The pure standard electrolyte hasthe lowest contact angle and has therefore the best wetting ability withall silicon surfaces. The VC-containing electrolyte has slightly highercontact angles, followed by the FEC-containing electrolyte with thehighest ones. Obviously, the interaction decreases due to the additionof VC and even more due to the addition of FEC.

Conclusions

An effective SEI formation is the most important pre-requisite forthe long-term stability of reactive anodes, such as lithium metal andlithium-ion battery anodes. It is well known, that there are significantdifferences in the SEI formation processes between metallic lithium,lithium storage metals, like silicon and graphite/carbon. Followingup our previous publication on the SEI formation on the (100) crystalfacet of Si, we extended our investigations to the (110) and (111) facetsand investigated the influence of different surface orientations on thecomposition of the SEI. As we concluded from our previous studyon the (100) surface orientation, the composition of the SEI dependson the polarization potential and therefore the degree of lithiation. Inthis work, we verified that the SEI composition depends on the cut-offpotential (i.e., the lithiation degree) for all three surface facets ((100),(110), (111)).

After polarizing the different Si crystals in the standard electrolyte1M LiPF6 EC:DEC [3:7] to a potential of 500 mV vs. Li/Li+, i.e.,before lithiation takes place, the SEI exhibits a different compositionon each surface orientation indicated by a different ratio of salt andsolvent decomposition products. A qualitative overview of the salt andsolvent ratios within the SEI layer at different operating conditionsand for the different electrolyte mixtures and Si surfaces is depictedin Table III. Despite of the remarkable differences in the “early” SEIcomposition at 500 mV vs. Li/Li+, the SEI at 10 mV vs. Li/Li+ ispractically the same on each Si surface orientation when polarized inthe standard electrolyte. Lithiation leads to transition of the crystallinesilicon phase to amorphous silicon and causes a cracking of the earlySEI layer due to the large volume expansion. The repair of the crackedSEI on the meanwhile lithiated amorphous silicon leads to a similarSEI chemical composition on all surface orientations at 10 mV vs.Li/Li+ for the standard electrolyte.

However, the situation changes, when the electrolyte additivesVC or FEC are present in the electrolyte mixture. Not only the SEIformed at 500 mV vs. Li/Li+, but also the SEI layers formed at10 mV vs. Li/Li+ display remarkable differences between the sur-face orientations (100), (110) and (111) in presence of VC or FEC inthe electrolyte. Whereas on the surface orientation (111), the biggestdifferences between the VC-containing, the FEC-containing and thestandard electrolyte are observable in the SEI composition at 10 mVvs. Li/Li+; the differences are less intense on the (110) surface(Table III). Finally, the fully lithiated Si(100) facet hardly shows any

differences in the SEI formed in the three electrolytes. In consequence,several conclusions can be drawn:

1) Electrolyte decomposition is different on each surface orienta-tion.

2) Considering an influence of the SEI composition on the anodeperformance, the Si(100) surface orientation has less influenceon the cycling behavior of Si in FEC and VC based electrolytescompared to the other surface orientations.

3) The SEI formation on the (100) surface mainly depends on thesurface properties of the crystal and the influence of the elec-trolyte additive diminishes with a higher lithiation degree. TheSEI on the (100) surface might be less stable and cracks easier.In addition, it might be assumed that the (100) surface leads tofaster lithiation than the (111) surface.

4) The SEI formation on the (110) and (111) surfaces stronglydepends on the electrolyte composition, especially whenlithiated.

Furthermore, the physicochemical properties of the surface orien-tations are different as exhibited by the different wetting ability withwater as well as with the different organic electrolytes. The wettingability of water is in line with the different amount of silanol surfacegroups on the different silicon wafer surfaces. The surface with thehighest amount of silanol groups (111) shows the strongest interac-tion. The wetting interaction of the three electrolytes is based on adifferent mechanism than for water but follows the same trend (withinthe electrolytes). The surface facet (110) exhibits the strongest inter-action for all electrolytes, whereas the (111) facet exhibits the smallestinteractions. Furthermore, a second trend on each surface orientationis remarkable. The best wetting ability on each surface is obtained inthe standard electrolyte. The addition of VC and -even more evidently– the addition of FEC to the electrolyte decrease the wettability of theelectrolyte on all surface orientations.

In summary, the mechanism of SEI formation on Si and, in turn,the SEI composition, thickness and its properties depend on numer-ous factors, including the Si crystal orientation, the electrolyte com-position, as well as the lower electrode potential, charging currentand the LixSiy lithiation degree (see Figure 11). The most commonelectrolyte additives FEC and VC display a significant different elec-trochemical behavior in contact with the single crystals, ranging fromalmost zero influence at the (100) surface to major differences inthe chemical composition of the SEI at the (111) crystal facet. Infurther studies it should be evaluated how a certain SEI formationstrategy may influence and how it can be optimized to enhance thelong-term cycling stability of silicon-based anodes for lithium-ionbatteries.

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A2288 Journal of The Electrochemical Society, 162 (12) A2281-A2288 (2015)

Figure 11. Overview of factors influencing the mechanism of SEI formation on the Si surface as well as SEI composition, thickness and properties.

Acknowledgments

This work was supported by the Assistant Secretary for EnergyEfficiency and Renewable Energy, Office of Vehicle Technologiesof the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 under the Batteries for Advanced Transportation Tech-nologies (BATT) Program. The authors also would like to thankSolvay for kindly providing the FEC. The Advanced Light Sourceis supported by the Director Office of Science, Office of Basic En-ergy Sciences, of the U.S. Department of Energy under Contract No.DE-AC02-05CH11231.

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