Tuesday, July 29, 2008
A new way to make advanced lithium-ion battery materials addresses one of their chief remaining problems: cost. Arumugam Manthiram, a professor of materials engineering at the University of Texas at Austin, has demonstrated that a microwave-based method for making lithium iron phosphate takes less time and uses lower temperatures than conventional methods, which could translate into lower costs.
Lithium iron phosphate is an alternative to the lithium cobalt oxide used in most lithium-ion batteries in laptop computers. It promises to be much cheaper because it uses iron rather than the much more expensive metal cobalt. Although it stores less energy than some other lithium-ion materials, lithium iron phosphate is safer and can be made in ways that allow the material to deliver large bursts of power, properties that make it particularly useful in hybrid vehicles.
Indeed, lithium iron phosphate has become one of the hottest new battery materials. For example, A123 Systems, a startup based in Watertown, MA, that has developed one form of the material, has raised more than $148 million and commercialized batteries for rechargeable power tools that can outperform conventional plug-in tools. The material is also one of the types being tested for a new electric car from General Motors.
But it has proved difficult and expensive to manufacture lithium iron phosphate batteries, which cuts into potential cost savings over more conventional lithium-ion batteries. Typically, the materials are made in a process that takes hours and requires temperatures as high as 700 °C.
Manthiram's method involves mixing commercially available chemicals--lithium hydroxide, iron acetate, and phosphoric acid--in a solvent, and then subjecting this mixture to microwaves for five minutes, which heats the chemicals to about 300 °C. The process forms rod-shaped particles of lithium iron phosphate. The highest-performing particles are about 100 nanometers long and 25 nanometers wide. The small size is needed to allow lithium ions to move quickly in and out of the particles during charging and discharging of the battery.
To improve the performance of these materials, Manthiram coated the particles with an electrically conductive polymer, which was itself treated with small amounts of a type of sulfonic acid. The coated nanoparticles were then incorporated into a small battery cell for testing. At slow rates of discharge, the materials showed an impressive capacity: at 166 milliamp hours per gram, the materials came close to the theoretical capacity of lithium iron phosphate, which is 170 milliamp hours per gram. This capacity dropped off quickly at higher discharge rates in initial tests. But Manthiram says that the new versions of the material have shown better performance.
It's still too early to say how much the new approach will reduce costs in the manufacturing of lithium iron phosphate batteries. The method's low temperatures can reduce energy demands, and the fact that it is fast can lead to higher production from the same amount of equipment--both of which can make manufacturing more economical. But the cost of the conductive polymer and manufacturing equipment also needs to be figured in, and the process must be demonstrated at large scales. The process will also need to compete with other promising experimental manufacturing methods, says Stanley Whittingham, a professor of chemistry, materials science, and engineering at the State University of New York, at Binghamton.
Manthiram has recently published advances for two other types of lithium-ion battery materials and is working with ActaCell, a startup based in Austin, TX, to commercialize the technology developed in his lab. The company, which last week announced that it has raised $5.58 million in venture funding, has already licensed some of Manthiram's technology, but it will not say which technology until next year.