Wednesday, October 25, 2006

3M's Higher-Capacity Lithium-Ion Batteries

New electrodes and electrolytes could mean higher energy and less danger from lithium-ion batteries.

By Kevin Bullis

By the end of next year, engineers at 3M, based in St. Paul, MN, expect to have ready for battery makers new materials and manufacturing methods that will add 30 percent more capacity to lithium-ion batteries. These new methods will also address safety concerns surrounding the use of such batteries in laptops.

The recent recalls of Sony's lithium-ion laptop batteries, due to fears that the batteries could catch fire, included those used in some Dell and Apple computers and could extend to as many as 9.6 million laptop batteries. So it's no surprise that, while Sony says changes have been made at factories that should take care of the problem, many manufacturers are scrambling to find safer technology. But alternatives to conventional lithium-ion batteries tend to present trade-offs, such as increased costs or decreased energy-storage capacity (see "Safer, Higher Capacity Batteries" and "How Future Batteries Will Be Longer-Lasting and Safer").

3M's advance includes new electrolytes and electrode materials. Although both materials will cost more than conventional lithium-ion batteries, the added energy capacity of the electrode materials should make up for the expense by lowering the key measure for battery price, cost per watt hour, says 3M research specialist Mark Obrovac.

The company is addressing battery safety by improving the electrolytes, the liquid inside lithium-ion batteries that conducts lithium ions but blocks electrons, forcing them to travel through an external circuit to power a device. Under certain conditions, such as when a battery is overcharged, overheated, or has an internal short circuit caused by damage or manufacturing problems, the electrolyte can chemically react with materials in the battery electrodes. In some cases, the battery could explode, spraying electrolyte into the surrounding air where it can ignite "like a flamethrower," Obrovac says.

The company has developed additives for existing electrolytes, as well as new electrolytes that will not react with the electrodes. Indeed, when subjected to an open flame, the safer electrolytes do not catch fire. As an added bonus, says 3M's battery-research technical manager, Doug Magnuson, the new chemistries work better at extremely cold temperatures, such as minus 40 degrees Celsius, at which other electrolytes block ion flow and effectively reduce battery capacity by 80 to 90 percent. This capacity loss is now a key impediment to using lithium-ion batteries in hybrid vehicles, which could be exposed to these conditions. The new electrolytes would allow ions to flow more freely at these temperatures, potentially limiting the losses to about 40 percent of capacity, Obrovac estimates.

3M engineers also say that new electrode materials will improve battery-energy capacity by 30 percent. For example, the company is replacing the current anode materials, based on graphite, with a silicon-based anode that should double the amount of lithium ions the anode can store. The capacity of lithium-ion batteries is limited by the amount of lithium that can be stored in electrodes. Graphite anodes can require six carbon atoms to store just one lithium ion. Electrodes containing metals and metalloids such as tin or silicon can hold many more lithium ions--nearly four ions for each silicon atom, for example--by forming alloys.

But such electrodes have been impractical because the material can swell to three times its original size as it incorporates lithium ions. Such dramatic changes in size wreak havoc on a cell, shortening its useful life.