In-situ Study of Solid Electrolyte Interphase on Silicon Electrodes using PeakForce Tapping Mode AFM in Glove-box

Transcription

1 General Motors R&D BROWN UNIVERSITY In-situ Study of Solid Electrolyte Interphase on Silicon Electrodes using PeakForce Tapping Mode AFM in Glove-box A. Tokranov 1, X. Xiao 2, C. Li 3, S. Minne 3 and B. W. Sheldon 1 1 School of Engineering Brown University Providence, RI USA 2 General Motors Global R& D Center Mound Road Warren, MI USA 3 Bruker Nano Surfaces Santa Barbara, CA Sponsored by and GM/Brown CRL on Computational Materials Science, NSF under awards CMMI , DMR , KIST, and GAANN fellowship (US. Dept. of Education).

2 Lithium Ion Batteries for Electrical Vehicles Sustainability Energy independence 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

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

4 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

5 Chemical composition in SEI layer Pallavi Verma, Pascal Maire1, Petr Novák, Electrochimica Acta 55 (2010) Much less work done to understand mechanical properties of SEI layer.

6 Mechanical degradation of electrodes Efficiency Cycle number to 80% capacity 99.9% % % 2300 M. W. Verbrugge and Y.-T. Cheng, J. Electrochem. Soc., 156: A927 (2009). 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). 6

7 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. Martin Winter, Z. Phys. Chem. 223 (2009) 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?

8 PeakForce Tapping TM in the AFM System --Topography --Friction --Topography --Phase --Topography --Elastic Modulus --Deformation (Hardness) --Adhesion --Energy Dissipation Interaction Sensing Contact Mode Tapping Mode PeakForce Tapping Bruker Webinar SEI on Si using PeakForce Tapping Mode

9 Quantitative Nanomechanical Mapping Simultaneously obtain quantitative data: Deformation Topography DMT Modulus ~1MPa 100GPa Adhesion Energy Dissipation Deformation Bruker Webinar SEI on Si using PeakForce Tapping Mode 9

10 The Setup at Work Bruker Webinar SEI on Si using PeakForce Tapping Mode

11 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

12 EC Cell & AFM Probe Holder Chemically Compatible ---Easy Assembly---Closed Cell AFM Probe Holder Glass cover plate EC Cell Kalrez O-ring Teflon / Kel-F cell bodies Sample Closed Cell When Engaged 8/9/2013 Bruker Webinar SEI on Si using PeakForce Tapping Mode

13 The EC Cell Lithium Foil as CE/RE HOPG Anode Ni wire connecting lithium foil A small sample glued to the small sample adaptor with Tor-seal Assembled EC Cell Bruker Webinar SEI on Si using PeakForce Tapping Mode

14 SEI on Silicon Problem with Si large expansion during lithiation (up to 420%) Stable SEI would be hard to form Difference in surface chemistry 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.

15 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) SEI on Silicon SEI on Sliding islands Expanding island new SEI during cycling Large number on unanswered questions Problems with mechanical degradation for both Silicon and its SEI

16 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).

17 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

18 Lithography impedes sliding Artifact on the edge due to deposition on photoresist sidewall Minimized with e-beam Lithography Procedure a) Prepare Wafer b) Apply Photoresist c) Align Photomask d) Expose to UV light Cu Substrate PR Cu Substrate Ni glass PR Cu Substrate Ni glass PR Cu Substrate islands g) Remove remaining PR Cu f) Sputter PR Cu e) Develop PR Cu Substrate Substrate Substrate

19 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 Silicon Copper

20 SEI formation on copper current collector 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) Results: SEI formation early during the cycle Total thickness ~20-25 nm, does not change during cycling A thick Alumina layer prevents Li diffusion and act as an insulator

21 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 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

22 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

23 Cross-section TEM Results: SEI formation early during the cycle Total thickness ~20-25 nm, does not change during cycling Pt SEI Si Cu SEI Cu SEI Cu Pt Si Pt Si

24 Pt SEI Si Cu

25 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

26 SEI roughness LiPF6 Both electrolytes level off to the same roughness Cu SEI does not change and is very smooth Preference for certain surface morphology? LiClO4 Difference in roughness depending on proximity to the edge

27 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

28 In-situ SEI formation ~1 minute after the previous scan Thickness continues to increase Roughness is becoming very significant Surface area growing Scan direction

29 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

30 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 < σ > h = σ = film stress 1 2 h = film thickness M s H s * L* C α * Mo 6 M s = biaxial modulus of the substrate H s = thickness of the substrate L = substrate to camera distance α = incidence angle MDS = % difference in spot spacing To computer CCD detector Electrochemical cell Potentiostat Metal ring to press on the window MOSS Setup Laser Etalon Li Use of ksa MOS System for Stress and teflon Thickness Monitoring during CVD Growth ( ) E. Chason Steel O-ring Electrochemical cell Steel screws glass Sample Sample contact Curvature-based Techniques sample for Real-Time Stress separator Measurement During Thin Film Growth ( ) J. Floro and E. Chason Li Contact

31 Supporting Stress data SEI formation begins at 0.6V 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, (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, (2011).

32 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?

33 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

34 Mechanical properties (preliminary results) LiClO4 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

35 Data on SEI Si expansion ~360% (~180nm) Si irreversible expansion ~140% (~70nm) Cu SEI (beginning of cycling) ~20nm 1M LiPF6 EC:DMC 1:1 Slow cycle: SEI total height: SEI thickness: 1M LiClO4 EC:DEC 1:2 Slow cycle: SEI total height: SEI thickness: ~230nm ~70nm ~350nm ~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

36 Diffusion in Si 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 Scan Direction Scan Direction Sample at 0.1V Sample at 0.3V

37 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

38 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