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