Ed: Woods Seminar: Nanomaterials

Posted by: Max Dunn on November 24, 2008 11:57:35

Designing Nanomaterials for Energy Storage Batteries and Supercapacitors

Speaker: Yi Cui, Department of Material Science, Stanford

Yi Cui is the researcher that is looking at silicon nanowires as anodes for Li-ion batteries which could increase 10 times how much energy a Li-ion battery can store. However, this technology is still about 5 to 7 years away from commercialization, and one of the issues is that it is only getting a lifetime of a hundred cycles because of problems with the electrolyte interacting with the silicon nanowires.

Introduction

• Needs for energy storage: consumer devices, EVs, energy storage especially for renewables
• Ideally like power and energy density as high as possible and cost as low as possible
• Capacitors need dielectric width of about a micron (1000 nm), so can’t store much energy
• Supercacitors have electrolyte solution in order of nano meters and have double layer
• While capacitors rely on surface to store charge, batteries rely on electrolyte oxidation state change in “bulk storage” so it can store more energy

Nano wires

• If battery electrode made out of nano particles, then charge can flow more easily
• Nanowires reduce distance ions and need to travel and provide large surface area. Also conduct electrons well, much better than particles that need to jump from particle to particle
• Same for supercapacitor
• Nanowires can be made conical, ribbon, spiral and other shapes
• His research focuses on nanowires: energy storage, solar cells, memory and bioprobes

Li Ion Battery

• Have anode (negative) carbon, cathode of Al oxide shaped as a jelly foil rolled up
• Most critical battery is now energy density which is determined by how much electrons can be stored in the electrodes
• Anode: Graphite 370 mAh/g
• Cathode: 150-170 mAh/g and 560 Wh/kg
• Li ion technology is improving only 8% per year with standard materials
• Nanowires can have a lot of volume expansion without breakage
• Si has been studied as anode material and has a 4200 mAh/g theoretical limit, but the 400% volume expansion causes problem
• Vapor-Liquid-Solid (VLS) is used to grow Si nanowires
• Si nanowires shows 10 times higerh capacity at over 3000 mAh/g
• Discharge and charge rate at 1C still has 5x improvement over carbon
• 95% of capacity retained after 185 cycles at C/5
• As Li first comes in, wires change from crystaline to amorphous
• However, when discharged, it doesn’t go back to original size but stays large

• Did test of growing crystalline Si of 20 nm and then adding amorphous shell of 100nm
• The crystaline core is only about 2% of the weight
• Limit discharge so it doesn’t go under 150mv so that crystaline stays that way and provides backbone for the wires
• Can do 100 cycles
• If got to 10 mV cutoff, crystaline core turns amorphous

Battery Summary

• Mechanical breaking is solved using Si nanowires
• Only metallurgical grade Si (not solar grade is needed
• Mature semiconductor processing can be used
• Si might be safer
• Carbon has problem with lithium dendrite formation if overcharged, Si has a higher potention (0.2-01.V) so it reduces this problem
• When Si burns it produces glass, when C burns it can explode
• Estimate 5 to 7 years to commercialization
• Still doesn’t know how much it will cost

Battery life

• Only getting 100 or so cycles
• Problem is electrolyte interface to to Si and this needs to be studied more

Supercapacitor

• Amorphous nanowires are nanoporous so provides larger surface area
• Pore number increases up to 6 cycles and settle on a size of about 6 nm
• Energy density increases by 10x compared to commercial porous carbons

Other

• Grew some conical nanowires and found it absorbs many wavelengths (appears black) so it is good for PV cells