1. Silicon Nanowires for Rechargeable Li-Ion Batteries Onur Ergen, Brian Lambson, Anthony Yeh EE C235, Spring 2009

2. Overview  Battery Technology Landscape  Battery Basics  Lithium Ion Battery  State of the Art  Silicon Nanowire Anode  Why Silicon Nanowires?  Experimental Results  Technical Comparison  Economic Perspective  Market Analysis  Future Outlook  Conclusion

3. Battery Basics Lithium Ion Battery State of the Art Battery Technology Landscape

4. Nanowire Batteries Motivation: Batteries and Life

5. How does a battery work?

6. History of Batteries

7. Lithium-ion Batteries J.-M. Tarascon& M. Armand. Nature. 414, 359 (2001).  How do Li-ion batteries work?  Battery Parameters  Energy density: cathode and anode  E (Wh) = voltage x capacity  Power density: ion intercalation and electron transport  Cycle life: strain relaxation  Advantages of Li-ion batteries  High cell voltage  Superior energy and power density  High cycling stability  Low self-discharge  No memory or lazy battery effect  100% depth of discharge possible

8. What we have in daily technology

9. How can we improve from here?  Using silicon nanowires as anode  Energy capacity  Peak power  Endurance  Manufacture cost

10. Why Silicon Nanowires? Experimental Results Technical Comparison Silicon Nanowire Anode

11. Silicon: an optimal anode material  Graphite energy density: 372 mA h/g  Silicon energy density: 4200 mA h/g C 6 LiC 6 SiLi 4.4 Si 11

12. Why haven’t we been using Si anodes? Lithiation of silicon has one major problem – it is accompanied by a 400% volume increase! Chan et. al, Nature Nanotech, 2007 12

13. Solution: Silicon Nanowires  10 x energy density of current anodes  Structurally stable after many cycles Chan et. al, Nature Nanotech, 2007 13

14. Experimental Technique  NW growth on stainless steel by vapor-liquid-solid (VLS) technique  Crystalline Si  Core-shell (core = crystalline Si, shell = amorphous Si)  Test current-voltage characteristics over many charge/discharge cycles using cyclic voltammetry C Si NW on Stainless steel Li metal Electrolyte V 14

15. Experimental Results Chan et. al., Nature Nanotech, 2007 Charge and discharge capacity per cycle 15

16. Experimental Results Chan et. al., Nature Nanotech, 2007 Charge and discharge capacity per cycle Dramatic (~10x) improvement in charging capacity over graphite! 16

17. Experimental Results Chan et. al., Nature Nanotech, 2007 Charge and discharge capacity per cycle No decrease in capacity beyond first charge cycle! 17

18. Experimental Results Cui et. al., Nano Letters, 2009 Core-shell nanowires may improve performance after first cycle 18

19. Experimental Results Cui et. al., Nano Letters, 2009 Core-shell nanowires may improve performance after first cycle Amorphous shell thickness as a function of growth time Crystalline core thickness 19

20. Experimental Results Chan et. al., Nature Nanotech, 2007 Study of reaction dynamics: Near capacity charging at high reaction rates 20

21. Experimental Results Chan et. al., Nature Nanotech, 2007 Study of reaction dynamics: Near capacity charging at high reaction rates Even one hour cycle time is much better than a fully charged graphite anode! Graphite 21

22. Technological Comparison Li-ion batteries have proved optimal for most mobile electronics and competitive for hybrid and electric vehicles TechnologyPower densityEnergy density LifetimeEfficiency Fuel cellsLow/moderateHighLow/moderateModerate SupercapacitorsVery highLowHigh NanogeneratorsVery lowUnlimitedUnknownLow Li-ion w/ graphiteModerate High Li-ion w/ Si NWModerateHighUnder investigation High Fuel Cells: Smithsonian Institution, 2008 22

23. Technological Comparison Li-ion batteries have proved optimal for most mobile electronics and competitive for hybrid and electric vehicles TechnologyPower densityEnergy density LifetimeEfficiency Fuel cellsLow/moderateHighLow/moderateModerate SupercapacitorsVery highLowHigh NanogeneratorsVery lowUnlimitedUnknownLow Li-ion w/ graphiteModerate High Li-ion w/ Si NWModerateHighUnder investigation High Supercapacitors: Maxwell Technologies, 2009 23

24. Technological Comparison Li-ion batteries have proved optimal for most mobile electronics and competitive for hybrid and electric vehicles TechnologyPower densityEnergy density LifetimeEfficiency Fuel cellsLow/moderateHighLow/moderateModerate SupercapacitorsVery highLowHigh NanogeneratorsVery lowUnlimitedUnknownLow Li-ion w/ graphiteModerate High Li-ion w/ Si NWModerateHighUnder investigation High Piezoelectric nanogenerators: Wang, ZL, Adv. Funct. Mater., 2008 24

25. Technological Comparison Li-ion batteries have proved optimal for most mobile electronics and competitive for hybrid and electric vehicles TechnologyPower densityEnergy density LifetimeEfficiency Fuel cellsLow/moderateHighLow/moderateModerate SupercapacitorsVery highLowHigh NanogeneratorsVery lowUnlimitedUnknownLow Li-ion w/ graphiteModerate High Li-ion w/ Si NWModerateHighUnder investigation High  Energy and power density  Only fuel cells and batteries can be primary power supply  Among those, Si NW batteries are optimal  Lifetime and efficiency  Batteries last about as long as typical electronic components  Energy efficiency of electrochemical devices is generally high 25

26. Market Analysis Future Outlook Conclusion Economic Perspective

27. Portable Electronics Lighter Phones Longer-lasting Laptops More powerful PDAs P. Agnolucci, “Economics and market prospects of portable fuel cells” 27

28. Hybrid/Electric Vehicles  Emerging market for H/EV batteries  Batteries are the main roadblock  Energy density (range)  Power density (acceleration)  Li-ion poised to be biggest contender http://www.chemetalllithium.com/index.php?id=56 28

29. Competing Technologies  Other battery technologies  NiMH  NiCd  other Li-ion  Fuel cells  5/8/09 (CNET News) – “DOE to slash fuel cell vehicle research” “[...] many years from being practical.”  Portable fuel cells  Supercapacitors  <30 Wh/kg  Li-ion: <160 Wh/kg P. Agnolucci, “Economics and market prospects of portable fuel cells” 29

30. Economics of Nanowire Batteries  Silicon is abundant and cheap  Leverage extensive silicon production infrastructure  Don’t need high purity (expensive) Si  Nanowire growth substrate is also current collector  Leads to simpler/easier battery design/manufacture (one step synthesis)  Nanowire growth is mature and scalable technique  J.-G. Zhang et al., “Large-Scale Production of Si- Nanowires for Lithium Ion Battery Applications” (Pacific Northwest National Laboratory)  9 sq. mi. factory = batteries for 100,000 cars/day GM-Volt.com, “Interview with Dr. Cui, Inventor of Silicon Nanowire Lithium-ion Battery Breakthrough” K. Peng et al., "Silicon nanowires for rechargeable lithium-ion battery anodes," Applied Physics Letters, 2008 30

31. Can you really get 10x? Si nanowire anode ~3541 Ah/kg Adjust anode/cathode mass ratio Capacity Issues J.-M. Tarascon, M. Armand, "Issues and challenges facing rechargeable lithium batteries" Cathode materials Lithium Cobalt Oxide Lithium Iron Phosphate

32. Lifetime Issues  Initial capacity loss after first cycle (17%)  Cause still unknown?  Capacity stable at ~3500 Ah/kg for 20 cycles  Can’t yet maintain theoretical 4200 Ah/kg  Crystalline-Amorphous Core-Shell Nanowires (2009)  Energy Density: ~1000 Ah/kg (3x) 90% retention, 100 cycles  Power Density: ~6800 A/kg (20x) Y. Cui, “High-performance lithium battery anodes using silicon nanowires” Y. Cui, “Crystalline-Amorphous Core-Shell Silicon Nanowires for High Capacity and High Current Battery Electrodes” 32

33. Conclusion  Summary  Motivation  Technology landscape  Silicon nanowire battery advantages  Market  Prospects  Time to market  ~5 years (Cui) 33