Battery Power - November 2011 - (Page 28)

research & develoPmeNt Conducting Polymer Enables the Use of Low-Cost, High-Energy Silicon for Next Generation Lithium-Ion Battery Anodes Lithium-ion batteries are everywhere: in smart phones, laptops, an array of other consumer electronics and the newest electric cars. Good as they are, they could be much better, especially when it comes to lowering the cost and extending the range of electric cars. To do that, batteries need to store a lot more energy. The anode is a critical component for storing energy in lithium-ion batteries. A team of scientists at the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has designed a new kind of anode that can absorb eight times the lithium of current designs, and has maintained its greatly increased energy capacity after over a year of testing and hundreds of charge-discharge cycles. The secret is a tailored polymer that conducts electricity and binds closely to lithium-storing silicon particles, even as they expand to more than three times their volume during charging and then shrink again during discharge. The new anodes are made from low-cost materials, compatible with standard lithium-battery manufacturing technologies. High-Capacity Expansion At left, the traditional approach to composite anodes using silicon (blue spheres) for higher energy capacity has a polymer binder such as PVDF (light brown) plus added particles of carbon to conduct electricity (dark brown spheres). Silicon swells and shrinks while acquiring and releasing lithium ions, and repeated swelling and shrinking eventually break contacts among the conducting carbon particles. At right, the new Berkeley Lab polymer (purple) is itself conductive and continues to bind tightly to the silicon particles despite repeated swelling and shrinking. “High-capacity lithium-ion anode materials have always confronted the challenge of volume change, swelling when electrodes absorb lithium,” said Gao Liu of Berkeley Lab’s Environmental Energy Technologies Division (EETD), a member of the BATT program (Batteries for Advanced Transportation Technologies) managed by the Lab and supported by DOE’s Office of Vehicle Technologies. “Most of today’s lithium-ion batteries have anodes made of graphite, which is electrically conducting and expands only modestly when housing the ions between its graphene layers. Silicon can store 10 times more, it has by far the highest capacity among lithium-ion storage materials, but it swells to more than three times its volume when fully charged,” said Liu. This kind of swelling quickly breaks the electrical contacts in the anode, so researchers have concentrated on finding other ways to use silicon while maintaining anode conductivity. Many approaches have been proposed; some are prohibitively costly. One less-expensive approach has been to mix silicon particles in a flexible polymer binder, with carbon black added to the mix to conduct electricity. Unfortu- At top, spectra of a series of nately, the repeated swelling polymers obtained with soft x-ray absorption spectroscopy at the and shrinking of the silicon Advanced Light Source’s beamparticles as they acquire and line 8.0.1 showed a lower “lowest unoccupied molecular orbital” for release lithium ions eventually push away the added car- PFFOMB (red), indicating better potential for conductivity than bon particles. What’s needed other polymers (purple). Here the is a flexible binder that can peak on the absorption curve reveals the lower key electronic conduct electricity by itself, state. At bottom, simulations diswithout the added carbon. close the virtually complete, two“Conducting polymers stage electron charge transfer aren’t a new idea,” said Liu, when lithium binds to the “but previous efforts haven’t new polymer. worked well, because they www.BatteryPowerOnline.com Continued on page 30 28 Battery Power • November/December http://www.BatteryPowerOnline.com

Table of Contents for the Digital Edition of Battery Power - November 2011

Battery Power - November/December 2011
Contents
Vishay Intertechnology Introduces Powdered-Iron- Based Rx Coil for Wireless Charging
GenSet Eliminator, Designed to Reduce Fuel Consumption and Promote Safety.
The Challenges of Manufacturing Lithium-Ion Batteries for the Electric Vehicle Industry
Considerations for Customized Battery Packs
Efficient Power Protection Means Less Cost, Not Less Reliability
Safety and Preventing Thermal Run Away in Large Li-Ion Batteries
ZigBee Resource Guide
Batteries
Charging & Testing
ICs and Semiconductors
Components
Power Supplies
Industry News
Research & Development
Marketplace
Calendar of Events

Battery Power - November 2011