Nanowire Lithium-Ion Batteries as Advanced Electrochemical Energy Storage Yi Cui Department of Materials Science and Engineering & Geballe Laboratory for Advance Materials Stanford University Importance of Energy Storage Portable Electronics Vehicle Electrification Tesla Roadster Storage for Renewable Energy and Grid Implantable Devices Solar Wind Energy Storage Technologies Capacitor Supercapacitor (Electrochemical capacitor) + - + - + - + - solution + - + - + - + - Metal Metal + - + - Electrical double layer dielectrics 1 E = CV 2 2 Batteries (Ag-Zn) Ag + + e− ⎯⎯→ Ag Battery voltage Zn − 2e− ⎯⎯→ Zn2+ 2Ag + + Zn ⎯⎯→ Ag + Zn2+ ,ΔG = −2FV Reaction free energy Faraday constant Fuel Cells http://en.wikipedia.org/wiki/Fuel_cell Comparison of Energy Storage Technologies Capacitors 106 105 104 103 Supercapacitors 102 Batteries Fuel cells 10 Specific power (w/kg) 1 10-2 10-1 1 10 102 103 Specific energy (wh/kg) Important parameters: - Energy density (Energy per weight or volume) - Power density (Power per weight or volume) - Cycle life and safety - Cost Why Li Ion Batteries? Li-related batteries have larger energy density than other batteries. J.-M. Tarascon & M. Armand. Nature 414, 359 (2001). Existing Li Ion Battery Technology Graphite: 370 mAh/g LiCoO2: 140 mAh/g The energy density can not meet the application needs. 1. Energy density: - Anode and cathode Li storage capacity - Voltage 2. Power density: - Li ion moving rate - Electron transport 3. Cycle, calendar life and safety: strain relaxation and chemical stability. 4. Cost: Abundant and cheap materials Electrode Materials Anode: low potential Cathode: high potential J.-M. Tarascon & M. Armand. Nature. 414, 359 (2001). Two Types of Electrode Materials Li Li Existing Tech. Future Tech. New Materials Mechanism Intercalation Displacement/alloy Volume change Small Large Li diffusion rate Fast Slow Specific capacity Low High We work on the future generation of battery materials. C. K. Chan, Y. Cui and co-workers, Nano Letters 7, 490 (2007). C. K. Chan, Y. Cui and co-workers, Nano Letters 8, 307 (2007) C. K. Chan, R. Huggins, Y. Cui and co-workers Nature Nanotechnology 3, 31 (2008) Nanowires as Li Battery Electrodes What nanowires can offer: - Good strain relaxation: new materials possible - Large surface area and shorter distance for Li diffusion - Interface control: (better cycle life). - Continuous electron transport pathway. Example: Si as Anode Materials C anode: the existing anode technology. C6 LiC6 Theoretical capacity: 372 mA h/g Si anode Si Li4.4Si Theoretical capacity: 4200 mA h/g Problem for Si: 400% volume expansion. Vapor-Liquid-Solid (VLS) Growth of Si Nanowires Au nanoparticles SiH4 400-500 ºC chemical vapor deposition Metal substrate Au Nanoparticles: Si Nanowires Scanning Electron Micrograph Scanning Electron Micrograph 5 μm Structure of Si Nanowires High Resolution Transmission Electromicrograph 10 nm 10 nm - Single crystal - 1-3 nm amorphous SiO2 Nanowire Battery Testing Beaker Cell Flat Cell Measured parameters: current, voltage, time. Ultrahigh Capacity Si Nanowire Anodes At C/20 rate • Si nanowires show 10 times higher capacity than the existing carbon anodes. • Si nanowires show much better cycle life than the bulk, particle and thin film. C. K. Chan, R. Huggins, Y. Cui and co-workers Nature Nanotechnology 3, 31 (2008) Power Rate-Dependence Diameter Change of Si Nanowire Anodes Before After The diameter changes to 150% but nanowires don’t break. Length Change of Si Nanowire Anodes EDX mapping Before Li-cycling After Li-cycling Structure Change of Si Nanowire Anodes X-ray diffraction Li insertion Structure Change of Si Nanowire Anodes HRTEM Li insertion progression Pristine 100 mV 50 mV 10 mV Acknowledgement Candace K. Chan Prof Robert Huggins.