Measurement of mechanical and fracture properties of solid electrolyte interphase on lithium metal anodes in lithium ion batteries

Journal Pre-proof Measurement of Mechanical and Fracture Properties of Solid Electrolyte Interaphase on Lithium Metal Anodes in Lithium Ion Batteries Insun Yoon, Sunhyung Jurng, Daniel P. Abraham, Brett L. Lucht, Pradeep R. Guduru PII:

S2405-8297(19)31005-0

DOI:

https://doi.org/10.1016/j.ensm.2019.10.009

Reference:

ENSM 952

To appear in:

Energy Storage Materials

Received Date: 2 September 2019 Revised Date:

9 October 2019

Accepted Date: 10 October 2019

Please cite this article as: I. Yoon, S. Jurng, D.P. Abraham, B.L. Lucht, P.R. Guduru, Measurement of Mechanical and Fracture Properties of Solid Electrolyte Interaphase on Lithium Metal Anodes in Lithium Ion Batteries, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2019.10.009. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V. All rights reserved.

Measurement of Mechanical and Fracture Properties of Solid Electrolyte Interaphase on Lithium Metal Anodes in Lithium Ion Batteries Insun Yoon1, Sunhyung Jurng2, Daniel P. Abraham3, Brett L. Lucht2, Pradeep R. Guduru1* 1

School of Engineering, Brown University, 184 Hope st. Providence RI 02912

2

Department of Chemistry, University of Rhode Island,140 Flagg Rd. Kingston RI 02881

3

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass. Ave. Argonne IL 60439

*E-mail: [email protected]

1

Measurement of Mechanical and Fracture Properties of Solid Electrolyte Interaphase on Lithium

2

Metal Anodes in Lithium Ion Batteries

3 4

Abstract Mechanical integrity of the solid electrolyte interphase (SEI) plays an essential role in determining

5

the life and performance of lithium-ion batteries. Fracture and continued formation of the SEI contribute

6

to consumption of lithium, drying of electrolyte, increase in impedance, and growth of dendrites resulting

7

in capacity fade and premature failure. Electrolyte additives such as fluoroethylene carbonate (FEC) have

8

been known to improve performance, but the underlying reasons have been elusive. Despite its

9

importance, reliable methods for mechanical characterization of SEI have been lacking. Here, we present

10

a new experimental technique that combines atomic force microscopy and membrane-bulge configuration

11

to accurately measure the stress-strain behavior of SEI, including the onset of inelastic response and

12

evolution of fracture. We characterize the SEI formed with two ethylene carbonate-based electrolytes,

13

without and with fluoroethylene carbonate (FEC) additive. The measurements show a striking contrast;

14

SEI with FEC additive has 80% higher elastic modulus and a vastly higher resistance to fracture. These

15

findings offer a mechanical-behavior based rationale to understand how SEI controls battery performance.

16

Moreover, the experimental technique offers a robust diagnostic tool to design electrolytes that can form

17

SEI with the desired mechanical properties for optimal battery performance.

18 19

Keywords: elastic modulus, yield strength, fracture resistance, electrolyte additives, fluoroethylene

20

carbonate, atomic force microscopy

21 22

1. Introduction

1

23

The increasing use of LIB for transportation demands further advances in their energy density,1 which

24

has attracted interest in Li-alloying materials (e.g. Si, Sn, Ge) and renewed interest in Li metal as

25

anodes.2-5 However, insufficient life and reliability have limited their use in practice.6-9 Recent

26

investigations reveal the central role of mechanical deformation and fracture of SEI in the failure

27

mechanisms of these anodes.6,10-12 The SEI is a thin layer, consisting primarily of electrolyte reduction

28

products, that forms on anode surfaces.13 As the lithium alloying anode particles undergo large volume

29

changes (up to ~300%) during electrochemical cycling, the SEI on their surface is subjected to

30

correspondingly large cyclic tensile/compressive strains.12,14,15 Fracture of SEI induced by the large strains

31

causes continued formation of SEI, which consumes the available lithium and the electrolyte, increases

32

the internal cell impedance, and eventually degrades cycle-life.10,16,17 SEI formed on lithium metal anodes

33

is locally strained due to non-uniform lithium plating and the consequent surface roughness as a result of

34

its inhomogeneous ionic-conductivity.10 Fracture of SEI intensifies non-uniform plating, leading to the

35

formation of dendrites, “dead” lithium and a “mossy” electrode.2,10,18 Consequently, the mechanical

36

properties and fracture resistance of SEI has been widely acknowledged to be an important determining

37

factor for LIB performance and reliability; its significance has been emphasized in several recent

38

articles.8,9,12,19

39

However, despite its importance, there have not been reliable experimental methods for accurate

40

mechanical characterization of SEI. Prior experimental approaches aimed at measurement of elastic

41

modulus of SEI are mostly limited to indentation using atomic force microscopy (AFM)20-22 or

42

measurement of acoustic wave velocity.23 These approaches are prone to uncertainties such as substrate

43

influence, uncertain indenter-sample contact area, or assumptions on the gravimetric density of SEI.

44

Consequently, the reported SEI elastic modulus ranges from tens of MPa to tens of GPa, a spread of three

45

orders of magnitude. Moreover, these approaches measure only the elastic modulus of SEI, while other

46

relevant properties for SEI failure such as the yield strength and inelastic response have not been reported.

2

47

Here, (i) we describe a new experimental technique for accurate mechanical characterization of SEI

48

and (ii) provide a basis to understand the role of electrolyte composition on cell performance in terms of

49

the difference in mechanical properties of the corresponding SEI. The experimental technique employs a

50

micron-scale membrane bulge configuration integrated with an atomic force microscope (AFM). The

51

membrane bulge configuration24-28 consists of a free-standing film subjected to an increasing lateral

52

pressure while the resulting deflection is measured. An analysis of the pressure-deflection relation yields

53

the residual stress, the plane strain elastic modulus, the yield strength, and the inelastic response.24,26-30

54

We present the results for SEI formed with two model electrolytes: (i) 1.2 M LiPF6 in ethylene carbonate

55

(EC) and (ii) 1.2 M LiPF6 in EC+ FEC (8:2 by weight). The main difference between the two electrolytes

56

is the FEC additive, which has been broadly observed to improve the cycling performance of LIB

57

anodes.31-38 The results reveal that the presence of FEC increases the elastic modulus by about 80% and

58

vastly improves the resistance to fracture during inelastic deformation. Although the molecular and meso-

59

scale origins of the observations are unknown at this time, the findings suggest that enhancing the

60

mechanical properties of SEI through electrolyte design could be a way forward in improving the life and

61

reliability of emerging LIB technologies.

62 63

2. Experimental

64

Membrane bulge test

65

The deflection of a free-standing narrow rectangular membrane with constrained edges, subjected to

66

lateral pressure (see Fig. 1) is determined by the geometry of the membrane (thickness, t, width, 2a, and

67

length), the residual stress in the membrane σ0, and the plane strain modulus of the membrane material

68

Here,

69

large aspect ratio of the rectangular membrane (length-to-width ratio of greater than about 6), the

70

influence of the membrane length on the pressure-deflection relation can be neglected for regions



= /(1 −

), where E is the Young’s modulus and ν is the Poisson’s ratio. For a sufficiently

3

.

71

sufficiently away from the ends (Fig. 1a).24 When the membrane remains elastic, the pressure q and the

72

maximum deflection

73

=

+



(shown in Fig. 1b) are related through the following equation.

Equation (1)

74

The pressure-deflection relation deviates from the above equation when the stress/strain level is large

75

enough to induce inelastic deformation of the membrane. Hence, the onset of inelastic deformation can be

76

determined from the stress at which the pressure-deflection relation deviates from the cubic relation

77

expressed by Eq. 1 (see Fig. 1c for a schematic illustration). In the inelastic regime, the strain can be

78

evaluated by approximating the shape of the deformed membrane with a circular arc of radius

=

as shown in Fig. 1b. The membrane can then be approximated to be a section of a thin-walled

79 80

cylinder under uniform internal pressure; the corresponding hoop stress and hoop strain can be calculated

81

through,

82

=

!"

"

, # = #$ + arcsin

"

− 1

Equation (2) $/

+ . The accuracy of the above approximation, even in the

83

where εo is the residual strain, equal to

84

elastic range, has been verified through experiments and computational modelling,29 and is widely used to

85

characterize the mechanical properties and inelastic response of thin-film materials.29,30,39-42

4

(a)

(c)

(b)

(d)

Figure 1. (a) Schematic illustration of a rectangular free-standing membrane with constrained edges and (b) the deflected membrane under a uniform lateral pressure q. The pressure-deflection ( – $ ) relation is governed by the width 2a, the thickness t, the residual stress σ0, and the plane strain modulus , of the membrane material. (c) Schematic of the pressure-deflection curve. When the membrane is elastic, it is a cubic relation (solid curve, Eq. 1). Deviation from the cubic relation indicates onset of inelastic response (dashed line). (d) Schematic of the hoop stress-strain ( -# ) relation, which is obtained by approximating the deflected shape of the membrane as a circular arc and using Eq. 2. 86 87

Specimen fabrication and experimental setup

88

Fig. 2 illustrates the specimen fabrication steps and a photograph of the experimental setup. First, a

89

free-standing silicon nitride (SiN) membrane is fabricated on a silicon wafer (double side polished, <100>

90

orientation, ~10 mm × 10 mm, thickness of 500 µm) through standard micro-nano fabrication steps (Fig.

91

2a). The free-standing SiN membrane has approximate dimensions of 20 – 40 µm in width, 400 µm in

92

length and 20 nm in thickness. See Section S1 of the supporting information for a detailed description of

93

the fabrication process. Next, an ultra-thin polydimethylsiloxane (PDMS) film is deposited on the SiN

94

film through the following steps. PDMS base elastomer and the curing agent are mixed at a ratio of 10:1

95

and the mixture is diluted by hexane at a ratio of 1:15. The solution is spin-coated on the substrate at 6000 5

96

rpm for 2 minutes and cured at room temperature for two hours, followed by curing at 90 °C for twelve

97

hours. The SiN layer in the free-standing area is etched by CF4 reactive ion etching, which creates an

98

ultrathin (~240 nm) free-standing PDMS (Fig. 2b) membrane. Such free-standing PDMS membranes

99

have been shown to be highly compliant and resilient under large deformation.43,44 The specimen is

100

attached to an annular ceramic (Macor®) disc for subsequent assembly.

101 102

Next, a lithium thin film is deposited on the PDMS layer in a custom-made physical vapor deposition (PVD) chamber located in an argon-filled glovebox (Fig. 2c). Prior to loading into the

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2. (a) A schematic and an SEM image of a free-standing SiN membrane. A long rectangular silicon nitride membrane (20 - 40 µm × 400 µm) is fabricated on a Si substrate. (b) Schematic steps of fabricating a freestanding ultra-thin PDMS membrane and an optical microscope image of it. The PDMS layer (~240 nm) is created by spin-coating PDMS:hexane (1:15) solution on the SiN layer. The SiN layer in the free-standing area is removed by CF4 reactive ion etching. (c) A schematic and a photograph of the specimen after lithium deposition. Prior to lithium deposition, the specimen is epoxy bonded to an annular disc for cell assembly (shown in the photo) and a small PDMS mask is placed near the edge to create a substrate-lithium step (highlighted in the dotted square). (d) A schematic and a photograph of the specimen after lithium-to-SEI conversion. The thin film lithium is completely converted to SEI by letting it react with the electrolyte, and the substrate-lithium step becomes a substrate-SEI step. (e) A schematic of the final specimen assembled in a custom pressure cell. The pressure cell design enables controlled lateral pressure to be applied on the specimen. Note that the film thicknesses shown are not to scale. (f) A photograph of the micro-bulge test setup integrated with an AFM located in an argon-filled glovebox. Accurately regulated compressed argon is supplied to the pressure cell. The AFM measures the SEI thickness and the resulting SEI/PDMS bulge topography.

6

103

deposition chamber, a small PDMS mask (2 mm × 2 mm × 500 µm) is placed near the edge of the

104

specimen as illustrated in Fig. 2c (highlighted by a dashed square). Following Li deposition, the PDMS

105

mask is removed to create a PDMS-Li step on the specimen surface, which facilitates the measurement of

106

the SEI thickness as described below. Li deposition is carried out at a nominal source temperature at 830

107

K and a chamber pressure of lower than 10-4 Pa.45 Further, prior investigations have established that

108

PDMS is inert to lithium.45-47

109

Next, the entire lithium film is converted into SEI by letting it react with the electrolyte (see

110

Section 2 of the supporting information). As discussed in the introduction, two electrolytes are prepared –

111

1.2 M LiPF6 in EC (EC electrolyte) and 1.2M LiPF6 in EC + 20 wt% FEC (EC+FEC electrolyte). In each

112

case, ~0.2 mL of the electrolyte is dropped on the specimen surface immediately after taking it out of the

113

deposition chamber. The specimens are placed in an argon-tight container (to prevent evaporation of the

114

electrolyte) for approximately 48 hours to completely convert the lithium thin-film to SEI. The sample is

115

then rinsed with anhydrous dimethyl carbonate (DMC) to remove any residual lithium salt (Fig. 2d). It is

116

worth noting the salient features of the final specimens (Fig. 2e); (i) the PDMS-Li step on the specimen is

117

converted to a PDMS-SEI step, which is measured by AFM to determine the SEI thickness accurately

118

(top left of Fig. 2e), and (ii) creation of a narrow rectangular free-standing SEI-PDMS membrane, which

119

would be subjected to a bulge test.

120

In order to determine the mechanical behavior of the SEI from the bulge test on the SEI-PDMS

121

double-layer membrane, it is necessary to subtract the response of the PDMS layer. First, the membrane

122

bulge test was carried out on bare PDMS films (no SEI on them) and their plane strain modulus and the

123

residual stress are determined to be 6.6 MPa and 0.15 MPa respectively (see Section S3 of the supporting

124

information for details). These values are similar to those reported by Thangawng et al.44 With a thickness

125

of 240 nm, the stretch stiffness of the PDMS film as measured by the product of its modulus and

126

thickness (referred from here onwards as the modulus-thickness) is 1.65 N/m; it would be subtracted from

127

combined stiffness of the SEI-PDMS membrane to extract the properties of the SEI. However, it will be 7

128

seen in the next section that it is only a small fraction, about 5%, of the combined stretch stiffness. Thus,

129

the pressure-deflection relation for the SEI/PDMS double-layer membrane is determined almost entirely

130

by the mechanical behavior of the SEI layer. Similarly, the residual stress – thickness product of the

131

PDMS layer is 0.038 N/m, which is subtracted to determine the residual stress in the SEI. It will be seen

132

in the next section that, similar to the stretch stiffness, the residual stress-thickness of the PDMS layer is

133

small compared to that of the SEI layer.

134

The specimens are assembled in a custom-designed pressure cell as illustrated in Fig. 2e. The

135

bottom chamber of the pressure cell can be filled with argon of controlled pressure at a resolution of 70

136

Pa (Fig. 2e), subjecting the SEI-PDMS membrane to lateral deflection. The pressure cell is integrated

137

with an AFM (Fig. 2f), which measures the SEI thickness, the resulting membrane bulge deflection and

138

the SEI surface topography evolution. The lithium deposition chamber and the AFM are located in a

139

single argon-filled glovebox (O2, H2O < 0.1 ppm); the samples remain in the same inert atmosphere

140

during fabrication, electrochemical reaction to form SEI and the pressure-deflection measurements in the

141

AFM.

142 143

Results and discussion

144

When the Li film is exposed to the electrolyte, the ensuing reactions convert it to a translucent SEI layer.

145

The composition of the resulting SEI may not be identical to that of the SEI formed under electrochemical

146

cycling, hence the SEI investigated here should be viewed as model SEI. Moreover, past investigations

147

have used the method of direct reaction of Li metal with the electrolyte in order to form SEI and

148

investigate its composition48,49. The method employed here is consistent with this prior body of work.

149

For the EC-based electrolytes used in the current investigation, the freestanding membrane remains

150

taut during SEI formation, which implies that the residual stress in the SEI, if any, is tensile. This is in

151

contrast to the case of the ionic liquid (IL) electrolyte reported previously45 where the residual stress in 8

152

the SEI is compressive, resulting in buckling and wrinkling of the underlying PDMS surface. The

153

membranes are subjected to lateral pressure in increments of about 700 Pa in the elastic range while the

154

AFM measures the resulting bulge topography at each pressure. Beyond the elastic limit, the pressure

155

increment is increased to 3500 Pa. In the inelastic regime, in addition to the bulge topography, a high-

156

resolution surface image (20 µm × 20 µm) is also measured at the center of the membrane at each

157

increment.

158

Fig. 3 presents a summary of the results for the two electrolytes considered; Figs. 3a-c (designated as

159

SEIEC) are for the EC electrolyte and Figs. 3d-f (designated as SEIEC+FEC) are for the EC+FEC electrolyte.

160

Figs. 3a and 3d show the deflected shapes of the membrane at a few representative values of the applied

161

pressure (for the sake of clarity, the ordinate scale is magnified by an order of magnitude). The SEI

162

thickness is determined by measuring the PDMS-SEI step height illustrated in Fig. 2e (see Section S4 of

163

the supporting information for details) and the width of the membrane is measured from the bulge

164

topography. The membrane width (2a) remains constant throughout the experiment for all cases

165

considered here, which implies that the interface between PDMS and the Si substrate does not delaminate

166

for the range of pressures applied in the experiments. The SEIEC sample in Figs. 3a-c has an SEI thickness

167

of 118 nm and a membrane width of 31.7 µm. The corresponding values for the SEIEC+FEC sample in Figs.

168

3d-f are 116 nm and 22.3 µm respectively. Figs. 3b and 3e show the pressure vs. deflection curves for the

169

SEI+PDMS. In order to analyze the pressure vs. deflection curves to extract the mechanical response of

170

the SEI, Eqs. 1 and 2 need to be modified to account for the double layer structure of the SEI+PDMS

171

membrane. The modified equations, details of the analysis and how the contribution of the PDMS layer is

172

subtracted to obtain the SEI response are shown in Section S4 of the supporting information. The results

173

of the analysis are shown in Figs. 3c and 3f. For the sake of clarity, in Fig. 3, the elastic range is shown in

174

blue and the inelastic range is shown in orange. It is worth noting that, as observed in Section 2, the

175

contribution of the PDMS layer to the response of the SEI+PDMS double layer is only a small fraction of

176

that of the SEI layer. The combined elastic modulus-thickness, + ∗ -, and the residual stress-thickness of 9

177

the SEIEC+PDMS membrane, σ ∗ -, are ~30.7 N/m and ~0.52 N/m respectively; these values for the

178

SEIEC+FEC/PDMS membrane are ~52.7 N/m and ~0.52 N/m. The corresponding values for the PDMS

179

layer alone are 0.038 N/m and 1.65 N/m. These are approximately 5% and 7% respectively of the

180

SEIEC/PDMS membrane, and 3% and 7% respectively of the SEIEC+FEC/PDMS membrane. Hence, it is

181

clear that the measured pressure-deflection behavior of the specimens is dominated by the response of the

182

SEI.

183

In the double layer membrane, the responses of the SEI and the PDMS layers are elastic initially. A

184

fit of the pressure vs. deflection data by assuming elastic response of the constituent layers is shown in

185

Figs. 3b and 3e (blue dashed curves). The fit agrees with the data initially, but it begins to deviate at

186

higher deflections (i.e. higher strains). It is well known that PDMS remains linear elastic up to a strain of

187

50% 50, i.e., the PDMS layer remains elastic in the strain range applied in this investigation. Hence, the

188

deviation between the elastic fit and the experimental data indicates the onset of inelastic deformation in

189

SEI (as noted schematically in Fig. 1b). From the elastic fits and following the procedure described in

190

Section S5 of the supporting information for the subtracting the contributions of the PDMS layer, the

191

residual stress, plane strain modulus, and the yield hoop stress of the SEI in each case are obtained. From

192

multiple repeated experiments, the plane strain modulus, elastic limit and residual stress of SEIEC are

193

measured to be approximately 240 MPa, 9 MPa and 4.5 MPa respectively; the corresponding values for

194

the SEIEC+FEC are 430 MPa, 9 MPa, and 3.9 MPa. These values are summarized in Table 1 along with the

195

estimated uncertainty/scatter.

196

Based on the modified form of Eq. 2 in Section S6 of the supporting information, the (hoop) stress vs.

197

strain relations for SEIEC and SEIEC+FEC are constructed and presented in Figs. 3c and 3f. Recall that Eq. 2

198

is based on approximating the bulged membrane to be a segment of a thin-walled cylinder. The accuracy

199

of this approximation in the elastic range is demonstrated in Section S6 of the supporting information.

200

Recently, Tanaka et al.51 suggested inelastic deformation behavior of SEI is essential to understand and

201

investigate the failure mechanisms52. The results presented in Figs. 3c, f show that the SEI deformation 10

202

response can be reasonably approximated as elastic – perfectly plastic. However, it is important to

203

recognize that the measured inelastic response is likely to have contributions from plasticity as well as

204

cracking, so the elastic-perfectly plastic description is strictly valid beyond the elastic limit until cracks

205

begin to appear. The deformation behavior investigated here can provide realistic property parameters for

206

models aimed at understanding Li dendrite-SEI interactions or the mesoscale structure of SEI53,54.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 3. (a-c) Evolution of the bugle topography, pressure-deflection response, and hoop stress vs. strain curve for SEIEC and (d-f) those for SEIEC+FEC. (a, d) Initially flat membranes deflect with progressively increasing applied pressure. (b, e) The pressure-deflection relation initially follows the elastic relation (blue dashed line represents the elastic fit) before deviating from it. The deviation implies onset of inelastic deformation of the membrane. (c, f) Hoop stress vs. hoop strain of the SEI in each case, which gives the residual stress, elastic limit, plane strain modulus and inelastic response. The dashed lines are linear fits in the elastic range. 207 208

Table 1 Measured residual stress, plane strain modulus and yield hoop stress of SEIEC and SEIEC+FEC.

SEIEC

Residual stress

Plane strain modulus

Yield hoop stress

4.5 ± 0.8 MPa

238 ± 7.8 MPa

9 ± 1.0 MPa

11

SEIEC+FEC

3.9 ± 1.3 MPa

429 ± 10.0 MPa

9 ± 1.7 MPa

209 210

A striking observation is that the addition of FEC increases the plane strain elastic modulus by

211

approximately 80% (~240 MPa to ~430 MPa). It appears not to substantially alter the residual stress and

212

elastic limit. The increase in the elastic modulus may arise from difference in the composition55,56 and the

213

nano-structure34,57 of the SEI.

214

There have been several recent investigations aimed at measuring the elastic modulus of SEI

215

using AFM indentation approach: (i) Young’s modulus of SEI formed on highly oriented pyrolytic

216

graphite (HOPG) anode using 1 M LiPF6 in EC/DMC (1:1) was reported to be 3.8 ± 5.6 GPa.21 (ii) The

217

measurements on SEI formed on silicon thin film anodes using the same electrolyte composition showed

218

Young’s modulus varying between tens of MPa to several GPa.22 (iii) Mean values of Young’s modulus

219

of SEI formed on MnO electrode using 1 M LiPF6 in EC/propylene carbonate (PC) electrolyte were ~16

220

MPa and ~540 MPa for the inner layer and the outer layer respectively.20 Besides the AFM indentation

221

approach, Zhang et al.23 used laser acoustic wave technique to measure the Young’s modulus of SEI

222

formed on silicon electrode using 1M LiPF6 in EC/DMC (1:1). They assumed the gravimetric density of

223

SEI to be that of fully dense Li2CO3 to yield Young’s modulus of ~50 GPa and ~70 GPa at formation

224

potentials of 0.4 V and 0.2 V respectively. Although a direct comparison between the prior reports and the

225

present investigation cannot be made due to substantial uncertainties or assumptions in the former, the

226

plane strain moduli measured here (240 MPa for SEIEC and 430 MPa for SEIEC+FEC) are within the broad

227

range measured by the AFM indentation approaches. However, it should be emphasized that the approach

228

used in this study eliminates several uncertainties involved in the indentation technique such as the

229

influence of substrate and inaccurate contact area. Thus, the elastic moduli measured here are expected to

230

be precise and capture the influence of electrolyte composition. At the same time, the method presented

231

here yields thickness-averaged properties/parameters; in its current form, it cannot capture the variations

232

along the thickness. Our previous report using elastic buckling-based metrology yielded a value of 12

233

1.6GPa for the plane strain modulus of SEI formed with lithium and bis(trifluoromethylsulfonyl)imide

234

(TFSI) anion based ionic liquid (IL) electrolyte reactions.45 This value is approximately 3-5 times higher

235

than that of the SEI formed with carbonate-based electrolytes measured here. The difference may be

236

attributed to the compositional difference: IL electrolytes are suggested to form mostly inorganic SEI

237

components,58,59 while those of carbonate-base electrolytes are both organic and inorganic materials.9,19

238

Fig. 4 presents detailed surface topographies of SEIEC near the elastic limit and at several strain

239

values in the inelastic regime. Fig. 4a shows the points on the stress-strain curve that correspond to the

240

AFM images in Figs. 4b-f. The AFM images are obtained near the center of the membrane, in both the

241

length and width directions. Fig. 4b shows the AFM image of the SEIEC at a strain of ~3.6% (point b in

242

Fig. 4a), which shows the grainy structure of the SEI, with “grain” sizes of 70 - 120 nm and a roughness

243

Ra of ~6.2 nm (See Fig. S7 in the supporting information for high resolution local surface images).

244

Similar grainy structures were observed in previous reports using AFM60 and TEM.34 Fig. 4c shows the

245

surface at an inelastic strain of ~4.0% (point c in Fig. 4a), which shows the appearance of isolated cracks

246

in SEIEC. The incipient cracks are not obvious at the scale of Fig. 4c (a larger image is provided in the

247

supporting information, Fig. S8); Fig. 4d shows a magnified view of a few representative cracks in Fig. 4c

248

(the magnified regions are marked by dashed rectangles in Fig. 4c). Recall that the membrane has a

249

narrow rectangular geometry with an aspect ratio of greater than ten. In this geometry, the membrane is

250

subjected to hoop straining in the width direction (indicated by red arrows in Fig. 4d), whereas the strain

251

in the longitudinal direction (indicated by black arrows in Fig. 4d), is zero (for regions sufficiently far

252

away from the ends). As one would expect, the initial cracks in Fig. 4d are generally aligned with the

253

longitudinal direction.

13

(a)

(b)

(c)

(e)

(f)

(d)

Figure 4. Fracture/failure evolution in SEIEC. (a) SEIEC stress vs. strain curve showing the points that correspond to the AFM topography images shown. (b, c) Detailed surface topographies at strains of (b) ~3.6% and (c) ~4.0%. (d) Magnified local images revealing a crack broadly aligned with the length direction (black arrow direction). The red arrows indicate the hoop (width) direction. (e, f) Detailed surface topography at strains of ~4.6% and ~5.6%. Blue arrows and dashed lines are drawn to highlight the failure morphology. 254

14

(a)

(f)

(b)

(c)

(d)

(e)

Figure 5. Crack evolution in SEIEC+FEC. (a) Stress vs. strain curve showing the points where presented membrane surface topography is measured. (b-e) Detailed surface topography at strains of ~2.3%, ~3.8%, ~4.8% and ~6.2%. (f) A magnified local surface topography of (d) indicated as dashed rectangle which contains a possible crack. 255 256

Figs. 4e and 4f correspond to strains of 4.6%, 5.6% respectively; the corresponding

257

failure/fracture patterns evolve into complex 2D patterns. It appears that SEIEC starts to crack at an early

258

stage of inelastic deformation. At a strain of ~4.6% (Fig. 4e), initiation of lateral failure (highlighted in

259

blue dashed line) is observed. The failure morphology shown in Fig. 4f at a strain of ~5.6% corresponds

260

to a further evolved fragmentation pattern. The faint lateral failure in Fig. 4e becomes obvious in Fig. 4f

261

as highlighted in the blue dashed lines at the same location. Notice that the failure patterns appear as

262

bright lines indicating local elevation of topography, which is attributed to local bulging of the PDMS in

263

the cracked regions due to the applied pressure. The AFM topography images without the highlighting

264

lines are shown in the supporting information, Fig. S9. Fig. 5 presents the surface topography evolution of 15

265

SEIEC+FEC with increasing strain. Fig. 5a shows the points on the stress-strain curve that correspond to the

266

AFM images in the figure, which correspond to strains of ~2.3%, ~3.8%, ~4.8% and ~6.2% respectively.

267

SEIEC+FEC also shows grainy surface structure with grain size varying from 50 nm to 90 nm and a

268

roughness Ra of 6.2 nm (Fig. S6 in supporting information). The average grain size of SEIEC+FEC is smaller

269

than that of SEIEC, which implies an FEC-induced difference in the substructure. A striking feature of the

270

SEIEC+FEC samples is that cracking does not appear until a strain of ~4.8% (compared to 3.6% for SEIEC),

271

and the crack density is dramatically smaller compared to that of SEIEC at higher strains (a larger image is

272

provided in the supporting information, Fig. S10). The cracks are almost invisible at a strain as high as of

273

6.2% (Fig. 5e), whereas the SEI is completely fragmented at a strain of 5.6% in Fig. 4e. The absence of

274

cracking in SEIEC+FEC suggests that the inelastic response in Fig. 5a is predominantly due to plasticity.

275

Contrasting Figs. 4 and 5 reveals that in the presence of FEC, the SEI cracks very little and retains its

276

mechanical integrity even at high inelastic strains. The contrast in the crack evolution between SEIEC and

277

SEIEC+FEC is direct evidence of improved mechanical properties and integrity of the of SEI formed in

278

presence of FEC in the electrolytes.

279

To gain further insights, the membrane topographies are measured after fully releasing the

280

applied pressure, which are presented in Fig. 6 As expected from the crack evolution, the surface

281

topography of SEIEC has web-like crack pattern over the entire area of the membrane (Fig. 6a); in contrast,

282

the surface of SEIEC+FEC appears smooth and continuous due to the absence of cracking (Fig. 6b). The

283

shapes of the unloaded membranes for the two cases in Figs. 6c and 6d show that the membranes do not

284

recover the initial flat topography due to inelastic deformation. Whereas the inelastic deformation of

285

SEIEC has significant contribution from crack evolution, the case of SEIEC+FEC is almost entirely due to

286

plastic deformation.

16

Figure 6. AFM topography of the membranes after completely releasing the applied pressure. (a) Web-like failure morphology can be observed in SEIEC. (b) The surface of SEIEC+FEC is smooth due to the absence of cracks, indicating improved mechanical stability. (c, d) Cross-section of AFM topographies presented in (a, b) respectively. Also plotted is the cross-section of as-prepared flat membranes. The membranes do not recover the initial flat topography due to inelastic deformation of SEI. 287 288

Clearly, the difference in the mechanical properties of the SEI in the two cases is due to the difference in

289

the composition and the micro/nano-structure of the SEI as a result of FEC. Determining the precise

290

micro/nano-structure of the SEI is a formidable problem, which has been an active topic of research. FEC

291

as an electrolyte additive has been studied for several years and its influence on the composition of SEI

292

has been reported. Numerous reports suggest that the FEC additive increases the fraction of polymeric

293

species.32,33,36,56,61 Additionally, the reduction products of FEC are thought to improve the cross-linking of

17

294

polymeric species in SEI.62-64 FEC is also reported to promote formation of LiF in SEI.55,65,66 To

295

complement the mechanical measurements, we have characterized SEIEC and SEIEC+FEC using X-ray

296

photoelectron spectroscopy (XPS), and the details are shown in Section S11 of the supporting information.

297

The key differences in the spectra due to the addition of FEC in the electrolyte are as follows: (i)

298

relatively larger amount C-C, C-H containing (polymeric) species, (ii) increased amount of LiF, and (iii)

299

less amount of carbonate (Li2CO3) species. These findings, as well as the spectra profiles, are in

300

agreement with those reported in the literature.32,37,56,61 Qualitatively, the high elastic modulus of LiF (~65

301

GPa)67 among the SEI components could account for the measured increase in the modulus reported here,

302

however it is not conclusive since the measured values are about two orders of magnitude smaller than

303

that of LiF. Differences in the nanostructure of the SEI may have as much influence on the mechanical

304

properties as the molecular species present, as suggested by Brown et al.34 As noted in the introduction

305

section, the mechanical properties of SEI are expected to play a significant role in determining the

306

electrochemical performance. SEI fracture or SEI delamination expose the electrode surface to the

307

electrolyte, resulting in more SEI formation and loss of acive lithium. FEC is known to result in better

308

electrochemical performance and our investigation reveals that FEC increases the fracture strain

309

substantially. Hence, one can reasonably conclude that FEC enhances the performance by endowing the

310

SEI with superior mechanical properties. It is possible to extend such reasoning by suggesting that further

311

enhancements in electrochemical performance can be achieved by choosing electrolyte and additive

312

combinations that would incrase the fracture strain of SEI and adhesion strength of SEI to the underlying

313

electrode surface. A fundamental understanding of these issues requires a systematic investigation of the

314

meso-scale structure of the SEI and how it is influenced by electrolyte composition, which remains open

315

for future research.

316 317

Conclusions

18

318

We have developed a new experimental technique for accurate mechanical property and fracture

319

characterization of SEI in which freestanding SEI+PDMS membranes are subjected to lateral pressure

320

while the corresponding deflection is measured with atomic force microscopy. The technique is

321

distinguished by its unprecedented accuracy in determining the SEI properties. The technique is used to

322

characterize SEI formed with two electrolytes, 1.2 M LiPF6 in EC and 1.2 M LiPF6 in EC/FEC (8:2) to

323

reveal the influence of the FEC additive. The key conclusions from this investigation are as follows. (i)

324

The plane strain elastic modulus of SEIEC is measured to be ~240 MPa and that of SEIEC+FEC is measured

325

to be ~430 MPa. (ii) The residual stress and the elastic limit of SEIEC are approximately 4.5 MPa and 9

326

MPa respectively; the corresponding values for SEIEC+FEC are approximately 3.9 MPa and 9 MPa. (iii)

327

Presence of FEC in the electrolyte appears to increase the elastic modulus by approximately 80% while

328

no substantial influence on the residual stress and the yield stress is observed. (iv) The strain at the elastic

329

limit of SEIEC is ~3.8%; cracking begins soon after entering the inelastic regime and the SEI severely

330

degrades by a strain of ~5.6%. In stark contrast, only a few cracks are present in SEIEC+FEC at a strain as

331

high as of 6.2%, which demonstrates dramatically enhanced resistance to fracture and mechanical

332

integrity. (v) The stress-strain response of SEIEC and SEIEC+FEC can be characterized as approximately

333

elastic – perfectly plastic. In the case of SEIEC, cracking begins shortly after entering the inelastic regime;

334

hence the inelastic regime seen in the stress-strain curve is due to a combination of plastic deformation

335

and crack evolution. However, it is predominantly due to plastic deformation in the case of SEIEC+FEC.

336

The findings suggest that the superior mechanical properties observed in the presence of FEC can

337

partly explain the widely observed performance and cycle life improvement of Li ion battery anodes with

338

FEC containing electrolytes. An important and unresolved question is – how does FEC (and other

339

additives) change the mechanical properties so substantially? At this stage, one can only point to the

340

increased fraction of LiF in the SEI as a contributing factor for the higher elastic modulus. A satisfactory

341

explanation requires determination of the precise nanostructure of the SEI, which is beyond the scope of

342

this investigation.

19

343

The approach and the experimental technique presented here can be extended to study deformation

344

and failure of SEI formed with other relevant electrolyte compositions and artificial SEI; it can be a useful

345

tool in developing engineered SEI with the desired mechanical properties to enhance stability and life of

346

high energy-density lithium batteries.

347 348

Acknowledgments:

349

The authors gratefully acknowledge financial support from the United States Department of Energy

350

EPSCoR Implementation award (grant # DE-SC0007074). The micro and nanofabrication work in this

351

investigation was carried out at the Center for Nanoscale Systems (CNS), which is part of the National

352

Nanotechnology Coordinated Infrastructure (NNCI).

353

Data availability

354

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical

355

or time limitations.

356

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522 523 524 525 526 527 528 529

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530

Graphical abstract

531 532 533

A new experimental method provides a route for systematic investigations on the influence of electrolyte compositions on the mechanical stability of solid electrolyte interphase.

534

25

Supporting information Measurement of Mechanical and Fracture Properties of Solid Electrolyte Interaphase on Lithium Metal Anodes in Lithium Ion Batteries S1. Detailed steps in the fabrication of free-standing silicon nitride membranes (a)

(b)

(c)

(d)

Figure S1. Schematic illustration of fabrication of free-standing silicon nitride membranes. The schematic is not to scale.

(a) A non-stoichiometric silicon nitride (SiN) film (~20 nm) is deposited by low pressure chemical vapor deposition (LPCVD) technique on a Si wafer (double side polished, <100> orientation, ~10 mm x 10 mm, thickness of 500 µm). (b) A Photoresist layer is patterned to expose a rectangular area of the substrate with nominal dimensions of 1200 µm × 720 µm. (c) The SiN layer in the exposed region is removed by CF4 reactive ion etching (RIE) at chamber pressure of ~ 100 mTorr and the radio frequency (RF) power of ~ 100 W. The SiN layer in the photoresist masked area remains intact. The photoresist layer is cleaned by a series of acetone, methanol, isopropanol and distilled water baths. (d) The substrate is wet-etched in 30 wt% potassium hydroxide in water solution heated at ~90 °C. The SiN layer is not etched by KOH solution1 and plays a role of a stable mask. Thus, the anisotropic etching only progresses in the exposed Si substrate area. The resulting sample has a free-standing SiN rectangular membrane with dimensions of 20-40 µm × 400 µm.

S2. Evidence for the complete conversion from Li film to SEI.

(b)

(a)

Figure S2. Photographs of a lithium thin film on a bulk PDMS substrate before and after conversion to SEI. (a) Electrolyte drops are placed on the lithium film immediately after taking the sample out of the deposition chamber. (b) Lithium film in contact with the electrolyte drops converts to transparent SEI layer.

S3. Measurement of residual stress and plane strain modulus of ultrathin PDMS membrane The plane strain modulus and the residual stress of a PDMS layer are measured to analyze their contribution to the pressure-deflection behavior of the SEI+PDMS double layer membranes. (i)

The PDMS membranes used in the experiments have a thickness of ~240 nm (Fig. S3a). However, these have proven to be too compliant for our instrumentation (the extremely high compliance results in AFM scan instability under pressurization). Instead, we have characterized thicker (~790 nm) PDMS free-standing membranes (Fig. S3b) and assumed that the properties are independent of thickness in this range.

(ii)

The PDMS membrane is subjected to lateral pressure of up to ~1100 Pa in increments of approximately 100 Pa. The corresponding pressure vs. deflection curve is shown in Fig. S3c.

(iii)

The pressure-deflection curve is analyzed to obtain the plane strain modulus the residual stress



and

of PDMS, which are measured to be ~6.6 MPa and ~0.15 MPa

respectively (Fig. S4d). The corresponding modulus-thickness residual stress-thickness

∗



∗

and

values for the thinner PDMS layer (~240 nm) in

the SEI/PDMS membrane samples are ~1.58 N/m and ~0.036 N/m respectively. (iv)

These values suggest that PDMS layer contributes ~5% and ~3% of modulus-thickness of SEIEC/PDMS and SEIEC+FEC/PDMS membranes; the contribution of PDMS on the residual stress-thickness is approximately 7% for both SEIEC and SEIEC+FEC.

(a)

(b)

(c)

(d)

Figure S3. (a, b) AFM scans to measure the thickness of PDMS layers. (a) ~240 nm thick PDMS membranes to support SEI layers. (b) ~790 nm thick PDMS membranes for measurement of elastic modulus and residual stress. (c) Pressure deflection curve and (d) the hoop stress-strain curve of the PDMS membrane. As discussed in the manuscript, the relative stiffness of the PDMS support layer is very small compared to that of SEI in the SEI+PDMS double layer membranes.

S4. Determination of the thickness of the SEI layer

Figure S4. AFM scan of the SEI-substrate step. The step height represents the SEI thickness as shown in the schematic.

S5. Analysis of the pressure-deflection relation of double-layer SEI+PDMS membrane and subtraction of the PDMS layer contribution S5.1 Pressure-deflection relation of a double-layer membrane Eq.1 of the manuscript describes the pressure-deflection relation for a single layer membrane. It can be extended for a double-layer membrane in a straightforward way as shown below. For the specimens used in this investigation, the pressure-deflection curve results predominantly from the stretching of the membrane; the bending contribution can be neglected.2, 3. Following the work of Vlassak and Nix4, the equilibrium equation and boundary/symmetry conditions of the membrane can be written as =− ±

+ =0

0 =0

! ±

=0

Equation (S1) Equation (S2) Equation (S3) Equation (S4)

Here, u(x) and w(x) are displacements in x and y directions respectively (coordinates are shown in Fig. S5a). Superscripts ‘SEI’ and ‘PDMS’ indicate SEI layer and PDMS layer respectively. The hoop strain in each layer is

"

="

#

+

="

"

+

#

+

%

$ %

=" %

$

+%

+"

=

()*+, - *+,

+"

&'

&'

="

=

Equation (S5) ()./0* - ./0*

Equation (S6)

where "

&' =

#

%

$

+%

Here, "

=

respectively; "

(1*+, - *+, &'

Equation (S7)

and "

=

(1./0* - ./0*

are the residual strains in the SEI layer and the PDMS layer

is the strain induced by the deflection, which is the same in each layer. Integration of

Eq. S1 and applying the boundary condition (Eq. S4) yields, !=

2

% ()*+, 3 *+, 4()./0* 3 ./0*

=

62

()*+, 3 *+, 4()./0* 3 ./0*

%

− 5%

5

Equation (S8)

Equation (S9)

The maximum deflection w0 is at x=0, from which w =

2

% ()*+, 3 *+, 4()./0* 3 ./0*

Substituting #

="

&'

−

%

Equation (S10)

in Eq. S7 and using Eq. S9,

%

2

()*+, 3 *+, 4()./0* 3 ./0*

5%

Equation (S11)

Integrating Eq. S11 and applying boundary/symmetry conditions (Eqs. S2, S3), "

&'

−

2

8 ()*+, 3 *+, 4()./0* 3 ./0*

%

=0

Equation (S12)

Rewriting the second term on the left-hand side of Eq. S12 in terms of w0 and a using Eq. S10, "

&'

=

% 1 9:

Equation (S13)

From Eqs. S5 and S6, = -

= -

"

&'

+

"

&'

Equation (S14) +

Equation (S15)

Substituting Eqs. S14 and S15 in Eq. S10, w =

%

2 - *+, ;<=> 4(1*+, 3 *+, 4 - ./0* ;<=> 4(1./0* 3 ./0*

%

Equation (S16)

Combining Eqs. S13 and S16 and rearranging the terms results in the following expression. =

% (1*+, 3 *+, 4(1./0* 3 ./0* :

! +

?



*+, *+, ./0* ./0* 3 4 3

9: @

!9

Equation (S17)

Thus, the effective residual stress-thickness and modulus-thickness of the double-layered membrane are simply the sum of those values of each layer. S5.2 Measurement of the elastic limit (i.e., the yield stress) Dividing each side of Eq. S17 by w0 and re-definition of variables yield following linear equation. A=

% (1*+, 3 *+, 4(1./0* 3 ./0* :

+

?



*+, *+, ./0* ./0* 3 4 3

9: @

B, ℎEFE, A =

2

1

GH B = ! % Equation (S18)

A plot of the pressure-deflection data in this form is presented in Figs. S5 b, c. The onset of inelastic deformation is determined to the point where the data deviates from linearity. S5.3 Subtraction of PDMS layer contribution The pressure-deflection data in elastic range are fit to the following cubic equation. = I ∙ ! + K ∙ !9

Equation (S19)

where I=

% (1*+, 3 *+, 4(1./0* 3 ./0* :

and K =

?



*+, *+, ./0* ./0* 3 4 3

9: @

The resulting elastic fits are shown as blue dashed curves in Figs. S5 d, e, yielding the coefficients A and B. Using the membrane width 2a, the coefficients A and B are rearranged as follows. :

∙I=

9 ?

?

%

∙K =

+

The stress-thickness

Equation (S20) +



Equation (S21) ∗

and modulus-thickness



∗

values of the PDMS layer

are measured to be 0.036 N/m and 1.58 N/m respectively, as described in Section S3 above. The corresponding PDMS contribution is shown as black dashed curves in Fig. S5 d, e, which are subtracted from Eqs. S20 and S21 to calculate the residual stress

and the plane strain modulus



of SEI. It

can be seen from Figs. S5 d, e that the pressure-deflection behavior is dominated by the response of the SEI layer.

(a)

(b)

(c)

(d)

(e)

Figure S5. (a) Schematic of the specimen showing x, y coordinates. (b, c) A = !20 vs. β= 2 ) plots for SEIEC and SEIEC+FEC to assess the elastic range and the yield point. (d) Pressure 1

q vs. deflection w0 plot of SEIEC/PDMS membrane and (e) that of SEIEC+FEC/PDMS membrane. Also presented as black dashed curves are the contribution of PDMS layer on the pressure-deflection response. Detailed procedures to account the PDMS influence as well as to obtain residual stress and plane strain modulus of SEI are described in the supporting information text.

S6. Accuracy of the geometric approximation, i.e., approximating the bulged membrane to a section of a thin-walled cylinder Note that the analysis described in the preceding section is valid in the elastic range only. Beyond the elastic range, the bulged membrane is approximated to be a section of a thin-walled cylinder and the

corresponding stress and strain are described by Eq. 2. Here, we refer to this approximation as the geometric approximation. The geometric approximation is useful in constructing the stress-strain curve in the inelastic range where an analytical solution is unavailable. Here, we demonstrate that the geometric approximation is quite accurate in describing the experimental data over the entire measurement range, including the elastic range. First, we construct the hoop stress and strain relation in the SEI layer based on the double layered membrane analysis in Section 5. Rearranging Eqs. S6 and S10, +-

= +

"MNO =

Equation (S22)

2:

Equation (S23)

% 1

Substituting PDMS stress and "MNO terms in Eq. S23 from Eqs. S14 and S22 and rearranging the terms results in an equation for the hoop stress in the SEI layer. Note that the PDMS layer contribution is being subtracted. =P

2:

% 1

−P

+-

% 1 :

Q

Q/



Equation (S23)

From Eq. S6, the hoop strain in the SEI layer is given as "

=

(1*+, - *+,

+

% 1 9:

Equation (S24)

The above equations are valid in the elastic range. Now, we present equations for the hoop stress and strain in the SEI layer by approximating the deflected membrane to be a segment of a thin walled cylinder (i.e., the geometric approximation). From equilibrium, +

= S

Rearranging Eq. S25 and Eq. S22 and recalling that S =

Equation (S25) 1 4: % 1

,

=P

2

1 4:

% 1

+-

−P

% 1 :

Q

Q/



Equation (S26)

Under the geometric approximation, the hoop strain in the SEI layer is given by (recall Eq. 2 in the main text), "

=

(1*+, - *+,

T :

+ sin6$

: T

−1

Equation (S27)

Notice that the only difference between the expressions for the hoop stress in the SEI layer from the geometric approximation (Eq. S26) and the elastic analysis in Section S5 (Eq. S23) is the presence of ! % in the first term of the right-hand side. Therefore, the maximum difference in the hoop stress between the two cases is only ~2%. Figs. S6a and S6b show the hoop strain as a function of deflection from Eqs. S24 (red) and S27 (blue) for the experimental data presented in Fig. 3 in the main text. The two curves in each graph almost overlap on each other, indicating that the calculated strain calculated from the elastic analysis and the geometric approximation are nearly the same. Figs. S6c and S6d compare the (hoop) stress vs. strain plots constructed from the analysis in Section 5 and the geometric approximation for SEIEC and SEIEC+FEC respectively. The proximity of the two curves to each other in each case demonstrates the accuracy of the geometric approximation in the elastic range. Further, the geometric approximation continues to be valid beyond the elastic range since its derivation does not rely on the assumption that the material response be elastic.

(b)

Hoop strain εθ

Hoop strain εθ

(a)

Deflection w0 (nm)

Deflection w0 (nm)

Hoop stress σθ (Pa)

(d) Hoop stress σθ (Pa)

(c)

Hoop strain εθ

Hoop strain εθ

Figure S6. (a, b) Hoop strain vs. deflection constructed based on the analysis in Section S5 (red) and the geometric approximation (blue) for (a) SEIEC and (b) SEIEC+FEC. (c, d) Hoop stress vs. strain constructed based on Section S5 analysis (red) and the geometric (thin-walled cylinder) approximation (blue) for (c) SEIEC and (d) SEIEC+FEC.

S7. Surface structure and roughness of SEI

(a)

(b)

Figure S7. Detailed surface topography (2 µm × 2 µm) of (a) SEIEC and (b) SEIEC+FEC. Each topography shows a granular surface structure with grain size of 70 - 120 nm and 50 – 90 nm for SEIEC and SEIEC+FEC respectively. The surface roughness (Ra) is nominally similar in the two cases, with a value of 6.2 nm. These quantities are measured using the built-in AFM software (Bruker, Nanoscope Analysis ver. 1.9).

S8. Magnified SEI surface image containing cracks

Figure S8. Magnified image of Fig. 4c (at a strain of 4.0%) for enhanced visualization of the cracks in SEI. Some of the representative cracks are highlighted by arrows.

S9. SEI surface evolution without annotations

Figure S9. Fig.4 without annotations or overlaid lines.

S10. Magnified SEIEC+FEC surface image

Figure S10. Magnified image of Fig. 5d (at a strain of 4.8%.) for enhanced visualization of

a possible crack (highlighted by an arrow)

S11. XPS measurements on SEIEC and SEIEC+FEC An identical batch of specimens to those used for mechanical characterization is prepared to investigate the chemical compositions of SEI by X-ray photoelectron spectroscopy (XPS). The XPS samples are transferred from the glovebox to the XPS chamber using a vacuum transfer module designed for environment-sensitive samples (Thermo Scientific K-Alpha XPS system). XPS measurements are conducted using Al Kα radiation source (kν = 1486.68 eV) at a chamber pressure of less than 1×10-7 Pa. Sputter etching is performed at an ion beam energy of 200 eV. The binding energies are calibrated by referencing the lithium fluoride (LiF) position to be at 685 eV. Fig. S11 summarizes the detailed XPS spectra in C 1s O 1s and F 1s regions as well as the atomic percent as a function of etch time for SEIEC and SEIEC+FEC. The species at each binding energy is identified primarily referencing a review paper written by Verma et al.5 along with several supplementing references6-8. The following key observations can be made from each spectrum and the depth profile. Fig. S11 (a, d) C 1s: Note that the C 1s spectra of SEIEC are scaled to 300% to improve the clarity. Carbonate species (290 eV), C-O (286.5 eV) and C-C / C-H (285 eV) containing species are identified in this region. Absence of FEC increase the fraction of inorganic carbonate species, while the presence of FEC increase the fraction of organic species. The observation is in good agreement with the report by Veith et al.9. They observed that 5% of FEC additive in 1.2 M the fraction of polymeric species in SEI. The observations reported here also agree with those made by Aurbach’s group10-14. Fig. S11 (b, e) O 1s: Note that the O 1s spectra of SEIEC+FEC are scaled by 300%. The two spectra commonly consist of a broad peak at 532 eV spreading to 533 eV. These are the characteristics of carbonate species and C-O containing species respectively. Stronger peak associated to carbonate species in the O 1s spectra of SEIEC is in good agreement with the observation made from C 1s spectra.

Fig. S11 (c, f) F 1s: Both spectra have main peak at 685 eV with a small bump at 687 eV. These are the characteristics of LiF and LixPFyOz or LiPF6. These spectra suggest that the majority of fluorine in the SEI is in the form of LiF. Fig S11 (g) Atomic percent of SEIEC and SEIEC+FEC: The atomic percent of carbon is close between SEIEC and SEIEC+FEC. This suggests that the absolute amount of carbonate species is larger in SEIEC, while that of polymeric species are larger in SEIEC+FEC. The observation further supported by higher percent of oxygen in SEIEC. The amount of fluorine is smaller in SEIEC compared to SEIEC+FEC. This suggests increased formation of LiF in SEI by addition of FEC. This observation is consistent with the report by Nie et al.7 which characterized chemical compounds SEI on Si formed with either EC electrolyte or FEC electrolyte using TEM, XPS and NMR. (b)

(c)

(e)

(f)

(g)

(a)

(d)

Figure S11. (a)-(f) Detailed XPS spectra in C 1s, O 1s, F 1s regions for SEIEC and SEIEC+FEC and (g) atomic percent profile of each element. SEIEC appears to contain more carbonate species, while SEIEC+FEC appears to contain more polymeric species and LiF.

Supporting Information References 1. Williams, K. R.; Gupta, K.; Wasilik, M., Etch rates for micromachining processing-Part II. Journal of microelectromechanical systems 2003, 12 (6), 761-778.

2. Timoshenko, S. P.; Woinowsky-Krieger, S., Theory of plates and shells. McGraw-hill: 1959. 3. Freund, L. B.; Suresh, S., Thin film materials: stress, defect formation and surface evolution. Cambridge University Press: 2004. 4. Vlassak, J.; Nix, W., A new bulge test technique for the determination of Young's modulus and Poisson's ratio of thin films. J. Mater. Res. 1992, 7 (12), 3242-3249. 5. Verma, P.; Maire, P.; Novák, P., A review of the features and analyses of the solid electrolyte interphase in Li-ion batteries. Electrochimica Acta 2010, 55 (22), 6332-6341. 6. Zhang, Q.; Xiao, X.; Zhou, W.; Cheng, Y. T.; Verbrugge, M. W., Toward High Cycle Efficiency of Silicon‐Based Negative Electrodes by Designing the Solid Electrolyte Interphase. Advanced Energy Materials 2015, 5 (5), 1401398. 7. Nie, M.; Abraham, D. P.; Chen, Y.; Bose, A.; Lucht, B. L., Silicon solid electrolyte interphase (SEI) of lithium ion battery characterized by microscopy and spectroscopy. The Journal of Physical Chemistry C 2013, 117 (26), 13403-13412. 8. Nie, M.; Chalasani, D.; Abraham, D. P.; Chen, Y.; Bose, A.; Lucht, B. L., Lithium ion battery graphite solid electrolyte interphase revealed by microscopy and spectroscopy. The Journal of Physical Chemistry C 2013, 117 (3), 1257-1267. 9. Veith, G. M.; Doucet, M.; Sacci, R. L.; Vacaliuc, B.; Baldwin, J. K.; Browning, J. F., Determination of the Solid Electrolyte Interphase Structure Grown on a Silicon Electrode Using a Fluoroethylene Carbonate Additive. Scientific reports 2017, 7 (1), 6326. 10. Markevich, E.; Salitra, G.; Chesneau, F.; Schmidt, M.; Aurbach, D., Very stable lithium metal stripping–plating at a high rate and high areal capacity in fluoroethylene carbonate-based organic electrolyte solution. ACS Energy Letters 2017, 2 (6), 1321-1326. 11. Markevich, E.; Salitra, G.; Fridman, K.; Sharabi, R.; Gershinsky, G.; Garsuch, A.; Semrau, G.; Schmidt, M. A.; Aurbach, D., Fluoroethylene carbonate as an important component in electrolyte solutions for high-voltage lithium batteries: Role of surface chemistry on the cathode. Langmuir 2014, 30 (25), 7414-7424. 12. Etacheri, V.; Haik, O.; Goffer, Y.; Roberts, G. A.; Stefan, I. C.; Fasching, R.; Aurbach, D., Effect of fluoroethylene carbonate (FEC) on the performance and surface chemistry of Si-nanowire Li-ion battery anodes. Langmuir 2011, 28 (1), 965-976. 13. Markevich, E.; Salitra, G.; Talyosef, Y.; Kim, U.-H.; Ryu, H.-H.; Sun, Y.-K.; Aurbach, D., High Performance LiNiO2 Cathodes with Practical Loading Cycled with Li metal anodes in Fluoroethylene Carbonate Based Electrolyte Solution. ACS Applied Energy Materials 2018. 14. Salitra, G.; Markevich, E.; Afri, M.; Talyosef, Y.; Hartmann, P.; Kulisch, J.; Sun, Y.-K.; Aurbach, D., High Performance Cells Containing Lithium Metal Anodes, LiNi0. 6Co0. 2Mn0. 2O2 (NCM 622) Cathodes and Fluoroethylene Carbonate Based Electrolyte Solution with Practical Loading. ACS applied materials & interfaces 2018.