A. Tokranov1, X. Xiao2, C. Li3, S. Minne3 and B. W. Sheldon1

BROWN UNIVERSITY

1School of Engineering Brown University Providence, RI 02912 USA

2General Motors Global R& D Center 30500 Mound Road

Warren, MI 48090 USA

In-situ Study of Solid Electrolyte Interphase on Silicon Electrodes using PeakForce Tapping®

Mode AFM in Glove-box

General Motors R&D

Sponsored by and GM/Brown CRL on Computational Materials Science, NSF under awards CMMI-1000822, DMR-0520651, KIST, and GAANN fellowship (US. Dept. of Education).

3Bruker Nano Surfaces Santa Barbara, CA

Lithium Ion Batteries for Electrical Vehicles

• Higher Energy and Power Density • Higher Cell Voltage (2 to 3X over Ni-X) • High charge rates available • Low Self discharge rate (1-5%/month) • Life can exceed tens of thousands cycles

• Sustainability • Energy independence

Chemical degradation – Unstable SEI (solid electrolyte interphase) layer formed on

electrode surface whch traps Li leading to capacity loss – Gas generated due to electrolyte decomposition on the

electrode surface. – Mn dissolves from positive electrodes and plates on

negative electrode surface

Mechanical degradation – Cyclic expansion/contraction during charging/discharging

leads to fatigue, cracking, and structural changes

Challenges

Most of degradation mechanisms are related with the failure of solid electrolyte interphase, leading to low current efficiency and short battery life

• A determinant factor on the performance: affecting the cycle life, power capability, shelf life, and safety.

• The formation of appropriate SEI layer is an essential and critical step in optimizing the combination of anode-electrolyte-cathode for lithium ion batteries.

Report on the Basic Energy Sciences Workshop on Electrical Energy Storage, April 2-4, 2007

Chemical composition in SEI layer

Pallavi Verma, Pascal Maire1, Petr Novák, Electrochimica Acta 55 (2010) 6332–6341

Much less work done to understand mechanical properties of SEI layer.

Mechanical degradation of electrodes

6

M. W. Verbrugge and Y.-T. Cheng, J. Electrochem. Soc., 156: A927 (2009).

Efficiency Cycle number to 80% capacity

99.9% 225 99.95% 450 99.99% 2300

• Mechanical degradation is typically coupled with Chemical degradation. • High current efficiency is critical for meeting requirements of 5000 cycles and 10

years life for lithium ion batteries used in electrical vehicles (from USABC).

What is the desirable SEI for Silicon Based Electrode Si has 10 times higher capacity than graphite, however, up to 300% volume expansion and contraction during the cycle makes most of SEI unstable, therefore, leading to lower current efficiency and shorter battery life comparing to graphite based electrode.

How to control SEI layer which can stand such huge volume change is still a grand challenge: • Understand the failure modes of SEI layer on Si • Correlate the current efficiency with mechanical properties • Develop appropriate artificial SEI layer to:

• accommodate or constrain the volume expansion?

Martin Winter, Z. Phys. Chem. 223 (2009) 1395-1406

Interaction Sensing

PeakForce TappingTM in the AFM System

--Topography --Friction

--Topography --Phase

--Topography --Elastic Modulus --Deformation (Hardness) --Adhesion --Energy Dissipation

Contact Mode Tapping Mode PeakForce Tapping

Bruker –Webinar SEI on Si using PeakForce Tapping Mode

Bruker –Webinar SEI on Si using PeakForce Tapping Mode 9

Simultaneously obtain quantitative data:

Topography DMT Modulus

~1MPa – 100GPa Adhesion Energy Dissipation Deformation

Quantitative Nanomechanical Mapping

Deformation

The Setup at Work

Bruker –Webinar SEI on Si using PeakForce Tapping Mode

ICON EC Setup

Scanner Head

Fluid Probe Holder

EC Cell

ICON EC Chuck w/Heater RT~65°

Bruker –Webinar SEI on Si using PeakForce Tapping Mode

EC Cell & AFM Probe Holder Chemically Compatible ---Easy Assembly---Closed Cell

8/9/2013

Glass cover plate

Kalrez O-ring

Teflon / Kel-F cell bodies

Sample

AFM Probe Holder

EC Cell

Closed Cell When Engaged

Bruker –Webinar SEI on Si using PeakForce Tapping Mode

The EC Cell

A small sample glued to the small sample adaptor with Tor-seal

HOPG Anode

Lithium Foil as CE/RE

Ni wire connecting lithium foil

Assembled EC Cell

Bruker –Webinar SEI on Si using PeakForce Tapping Mode

• Problem with Si large expansion during lithiation (up to 420%)

• Stable SEI would be hard to form • Difference in surface chemistry

SEI on Silicon

Possible problems:

• Thick organic SEI which might accommodate the expansion does not appear have good passivating characteristics.

• Thin inorganic SEI is unlikely to withstand large strains – failure of this layer leads to more SEI.

SEI on Silicon

• Large number on unanswered questions

• Problems with mechanical degradation for both Silicon and its SEI

SEI on Sliding islands Expanding island new SEI during cycling

Sliding islands: Soni, S. K. et al. Stress Mitigation during the Lithiation of Patterned Amorphous Si Islands. Journal of The Electrochemical Society 159, A38 (2012)

Current Efforts Use of thin Film Configuration to Study the Surface Reactions

(1) Motivation: Thin films provide a well-controlled configuration for fundamental studies of SEI formation.

(2) Facilitates in situ studies of stress evolution due to SEI formation (done at Brown).

(3) In situ AFM

(4) Complimentary information obtained from TEM (recently initiated), SIMS / XPS (at GM), coin cells (at GM).

Approach / Experimental Setup • Using lithography to create pattern

structures that allow to 3 features:

• the Cu layers as a reference

• Si as the electrode material

• Edge (in this case immobile)

• 2 electrolytes

• 1M LiPF6 EC:DMC (GM)

• 1M LiClO4 EC:2DEC (Mixed)

• ALD coated sample prevent SEI formation

• Patterned Cu as a reference

Substrate

Cu

a) Prepare Wafer

Substrate

Cu

b) Apply Photoresist

PR

Substrate

Cu

c) Align Photomask

PR

glass Ni

d) Expose to UV light

Substrate

Cu PR

glass Ni

e) Develop

Substrate

Cu PR

f) Sputter

Substrate

Cu PR

g) Remove remaining PR

Substrate

Cu islands

Lithography Procedure

• Lithography impedes sliding

• Artifact on the edge due to deposition on photoresist sidewall

• Minimized with e-beam

Outline of the results

1. Irreversible Amorphous Silicon Expansion

2. SEI formation

• Cu SEI

• thickness + roughness

3. SEI mechanical properties (trend lines)

4. Si diffusion data

Copper

Silicon

SEI formation on copper current collector Results: • SEI formation early during the cycle • Total thickness ~20-25 nm, does not

change during cycling

Fabrication: Ti bonding layer – 10nm Cu current collector – 200nm Lithography Instead of Silicon, 50 nm Copper is sputtered, followed by reactive sputtering of Alumina (5nm) A thick Alumina layer prevents Li diffusion and act as an insulator

Sample Details: Current Collector Ti-10nm, Cu-200nm Si islands (40um xy dimension) 50nm high Al2O3 – 10nm on top deposited by reactive sputtering Electrolyte 1M LiPF6 EC:DMC 1:1

Irreversible Si expansion vs SEI

Results: • Total height of Si at full lithiation

~180nm ~360% of the original volume

• Fully delithiated height is ~ 70nm 140% of the original volume

• The irreversible volume change of the fully delithiated material is likely due to change in amorphous structure of the materials

• There is also possible void space in the material

SEI formation on Si with 1M LiPF6 in EC:DMC

SEI starts forming at 0.6V, the change in thickness is very significant At this point the surface also becomes rougher Difference in surface profile near the edge, but average thickness is very similar

Cross-section TEM

Cu

Si

SEI

Pt

Cu

Si

SEI

Pt

Cu

Si

SEI

Pt

Results: • SEI formation early during

the cycle • Total thickness ~20-25

nm, does not change during cycling

Cu Si SEI Pt

SEI formation on Si with 1M LIClO4 in EC:DEC (1:2)

SEI formation also starts at 0.6V Much more rapid SEI growth Dominant thickness effect SEI seems to dissolve at 1.5V after then end of the first cycle, resulting thickness much less then at the end of 0.6V Larger thickness possible due to homemade electrolyte

SEI roughness

LiPF6

LiClO4

Both electrolytes level off to the same roughness • Cu SEI does not change and is very

smooth

• Preference for certain surface morphology?

• Difference in roughness depending on proximity to the edge

In-situ SEI formation Scan

direction

0.9V -> 0.6V transition occurred right before the start of the scan Scan time was 8:30 (510 seconds) The height slowly increased during the scan as well as the roughness

In-situ SEI formation

Scan direction

~1 minute after the previous scan Thickness continues to increase Roughness is becoming very significant Surface area growing

In-situ SEI formation

Both areas have the same trends Rougher area has much more variation The larger surface area might result in more SEI formation Smoother area use for analysis • Easier interpretation • Will provide higher stress

value

Multi Beam Optical Stress Sensor (MOSS)

Wafer curvature measurement Parallel laser beams initially The curvature of the wafer causes

reflection angle to differ Allows to measure average stress

based on beam spacing

Laser

Sample

Etalon CCD detector

To computer

Electrochemical cell

Potentiostat

MOSS Setup

Li teflon Steel

O-ring Steel screws

sample separator

glass Metal ring to press on the window Sample

contact

Li Contact

Electrochemical cell

MC oLHMh ss ***61 2 ασ =><

σ = film stress

h = film thickness

Ms = biaxial modulus of the substrate

Hs = thickness of the substrate

L = substrate to camera distance

α = incidence angle

MDS = % difference in spot spacing

Curvature-based Techniques for Real-Time Stress Measurement During Thin Film Growth (06-26-02) J. Floro and E. Chason Use of kSA MOS System for Stress and Thickness Monitoring during CVD Growth (05-17-00) E. Chason

SEI formation begins at 0.6V

Supporting Stress data

Significant stress response observed

SEI stress: Mukhopadhyay, A., Tokranov, A., Xiao, X. & Sheldon, B. W. Stress development due to surface processes in graphite electrodes for Li-ion batteries: A first report. Electrochimica Acta 66, 28–37 (2012). Si Stress: Soni, S. K., Sheldon, B. W., Xiao, X. & Tokranov, A. Thickness effects on the lithiation of amorphous silicon thin films. Scripta Materialia 64, 307–310 (2011).

In-situ SEI formation

The stress response is very similar to the thickness increase Almost a direct correlation ~5GPa-nm in 28min ~60nm growth in the same time ~80MPa reasonable value possible for a dense organic material

Current is rather large for the surface area ~2.7 µA/cm^2 Increasing with time • More surface area?

Mechanical properties (preliminary results)

The 0.9V 0.6V drop for LiPF6 electrolyte Modulus decreases

Deformation increases The applied force is 1nN more will damage the SEI

Scan direction

Mechanical properties (preliminary results)

Changes in 1M LiClO4 (EC:2DEC) electrolyte: • Modulus increased when the

potential goes to 50mV • Inorganic SEI?

• Does not change after the

potential is increased

LiClO4

Si expansion ~360% (~180nm) Si irreversible expansion ~140% (~70nm) Cu SEI (beginning of cycling) ~20nm

Data on SEI

1M LiPF6 EC:DMC 1:1 Slow cycle: SEI total height: ~230nm SEI thickness: ~70nm

1M LiClO4 EC:DEC 1:2 Slow cycle: SEI total height: ~350nm SEI thickness: ~190nm

Mechanical properties 1M LiPF6 in EC:DMC Significant organic SEI at 0.6V 1M LiClO4 EC:2DEC Increase in surface modulus at low potential

Diffusion in Si

Scan Direction

Scan Direction

Back to the alumina coated silicon sample Alumina prevents Li diffusion The only way to get Lithium inside is

through the defect on the edge

Tracking the diffusion front and profile gives diffusion information

Li source needs to be well controlled (sharp interface)

Possible effects due to the Si-Cu and Si-Al2O3 interface

Sample at 0.3V

Sample at 0.1V

Diffusion in Si

Front position vs. time is shown on the right Interface is diffusion limited

Sqrt(X) fit

Sharp transition at the interface Consistent with phase

transformation D in the xy plane

~2.5x10^-10 cm^2/s

Conclusion Results of this work: SEI formation on battery electrodes

Expansion of the electrodes during cycling

Observe the differences between electrolytes This should work for additives as well

Mechanical properties of the surface region

Diffusion information for the material

Scan direction