Wednesday, 5 October 2011

A simple way to boost battery storage
30 Sep 2011

Lithium-ion battery electrodes bound together by a new highly conductive material have a much greater storage capacity--a development that could eventually increase the range of electric cars and the life of smart-phone batteries without increasing their cost. Unlike many high-capacity electrodes developed over the last few years, these can be made using the equipment already found in today's battery factories.

The key is a stretchy, highly conductive polymer binder that can be used to hold together silicon, tin, and other materials that can store a lot of energy but that are ordinarily unstable. Researchers at the Lawrence Berkeley National Laboratory painstakingly engineered this new polymer binder and used it to make a silicon anode for a rechargeable lithium-ion battery with a storage capacity 30% greater than those on the market today. It's also more stable over time than previously developed electrodes.

When a lithium-ion battery is charged, lithium-ions are taken up by one of the electrodes, called the anode. The more lithium the anode can hold, the more energy the battery can store. Silicon is one of the most promising anode materials: it can store 10 times more lithium than graphite, which is used to make the anodes in the lithium-ion batteries on the market today. "Graphite soaks up lithium like a sponge, holding its shape, but silicon is more like a balloon", says Gao Liu, a researcher at the Berkeley Lab's Environmental Energy Technologies Division.

However, because the silicon anodes swell and shrink, changing in volume by three or four times as they're charged and discharged, the capacity of the battery fades over time. "After a few rounds of charge and discharge, pretty soon the silicon particles are not in touch with each other", which means the anode can't conduct electricity, says Liu.

One approach to the problem is to structure these anodes in a totally different way, for example growing shaggy arrays of silicon nanowires that can bend, swell, and move around as lithium enters and exits. This approach is being commercialised by Amprius, a startup in Palo Alto, California. But growing nanowires requires new processes that aren't normally used in battery manufacturing.

Today's anodes are made by painting a solvent-based slurry of graphite particles held together with a binder, a simple process that keeps costs low. The Berkeley researchers believe the key to making new battery materials like silicon work is to stick with this manufacturing process. That meant coming up with a rubbery binder that would stick to silicon particles, remain highly conductive in the harsh environment of the anode, and stretch and contract as the anode swells and deflates.