Silicon Nanowires for Rechargeable Li-Ion Batteries

1. Silicon Nanowires for Rechargeable Li-Ion Batteries OnurErgen, 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 Technology Landscape Battery Basics Lithium Ion Battery State of the Art

4. Motivation: Batteries and Life Nanowire Batteries

5. How does a battery work?

6. History of Batteries

7. Lithium-ion Batteries • 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 J.-M. Tarascon& M. Armand. Nature. 414, 359 (2001).

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

11. Silicon: an optimal anode material • Graphite energy density: 372 mA h/g • Silicon energy density: 4200 mA h/g • C6 LiC6 • Si Li4.4Si

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

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

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 Li metal V Electrolyte Si NW on Stainless steel

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

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

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

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

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

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

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

22. Technological Comparison Fuel Cells: Smithsonian Institution, 2008 Li-ion batteries have proved optimal for most mobile electronics and competitive for hybrid and electric vehicles

23. Technological Comparison Supercapacitors: Maxwell Technologies, 2009 Li-ion batteries have proved optimal for most mobile electronics and competitive for hybrid and electric vehicles

24. Technological Comparison Piezoelectric nanogenerators: Wang, ZL, Adv. Funct. Mater., 2008 Li-ion batteries have proved optimal for most mobile electronics and competitive for hybrid and electric vehicles

25. Technological Comparison • 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 Li-ion batteries have proved optimal for most mobile electronics and competitive for hybrid and electric vehicles

26. Economic Perspective Market Analysis Future Outlook Conclusion

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

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

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”

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

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

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”

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