11-09 :: September 2011
nanotimes News in Brief
35
Batteries // Better Batteries with a Revolutionary Conducting Polymer © Text: Paul Preuss / LBL
T
he anode is a critical component for storing energy in lithium-ion batteries. A team of sci-
entists at the U.S. 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 many hundreds of charge- discharge cycles.
The secret is a tailored polymer that conducts elec- tricity and binds closely to lithium-storing silicon par- ticles, 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. The research team reports its findings in Advanced Materials, now available online.
“High-capacity lithium-ion anode materials have always confronted the challenge of volume change – swelling – when electrodes absorb lithium,” says Gao Liu of Berkeley Lab’s Environmental Energy Techno- logies Division (EETD), a member of the BATT pro- gram (Batteries for Advanced Transportation Techno- logies) managed by the Lab and supported by DOE’s Office of Vehicle Technologies.
Says Liu, “Most of today’s lithium-ion batteries have anodes made of graphite, which is electrically con- ducting and expands only modestly when housing the ions between its graphene layers. Silicon can store 10 times more – it has by far the highest ca- pacity among lithium-ion storage materials – but it swells to more than three times its volume when fully charged.”
This kind of swelling quickly breaks the electrical contacts in the anode, so researchers have concen- trated 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. Unfor- tunately, the repeated swelling and shrinking of the silicon particles as they acquire and release lithium ions eventually push away the added carbon parti- cles. What’s needed is a flexible binder that can con- duct electricity by itself, without the added carbon.
“Conducting polymers aren’t a new idea,” says Liu, “but previous efforts haven’t worked well, because they haven’t taken into account the severe reducing environment on the anode side of a lithium-ion