Like turning straw into gold, MIT researchers have transformed a relatively common material, lithium iron phosphate, into one with handsome potential for the next generation of rechargeable batteries in electric cars and other devices. Among other advantages, the material could make such batteries cheaper and safer.
By essentially spiking the original material with tiny amounts of metal and processing it under special conditions, the MIT researchers overcame the main obstacle to its use in batteries. They boosted its electronic conductivity by eight orders of magnitude - some 10 million times better than the starting material - making it conductive enough to use in high-power batteries.
"Low electronic conductivity reduces the power capability of the [battery] cell," explained Michael Thackeray of Argonne National Laboratory in a commentary about the work for the October issue of Nature Materials. By solving the conductivity problem, "this finding has exciting implications for the future of [this material] in a new generation of [rechargeable] batteries," he wrote.
The MIT researchers, led by Professor Yet-Ming Chiang of the Department of Materials Science and Engineering (MSE), reported their results in the same issue of Nature Materials. Chiang's co-authors are Sung-Yoon Chung and Jason T. Bloking, an MSE postdoctoral associate and graduate student, respectively.
For several years, researchers in the battery community have been interested in finding a replacement for one of the materials key to state-of-the-art rechargeable batteries. That compound, composed of lithium, cobalt and oxygen, works well in general but is very expensive. Safety factors also limit the size of the battery that can be made with the material. "It's highly reactive with other battery components when charged, which can lead to overheating," explained Chiang, the Kyocera Professor of Ceramics.
In 1997, a team at the University of Texas at Austin identified a potential replacement. Among other attributes, lithium iron phosphate was cheap, environmentally friendly and safe. However, it also had a major limitation: very low electronic conductivity. That meant that in a rechargeable battery, it "could only be used at very low charge/discharge rates, which kept it from being practical," Chiang said.
Since then, many researchers have worked to solve the problem. "The prevailing approach has been to apply conductive coatings to the material, because it was widely accepted that this material would always be an electronic insulator," Chiang said. In other words, tinkering with the material itself wouldn't work.
The MIT approach, based on Chiang's years of work with conductive ceramic materials, went against that conventional wisdom. "We succeeded in altering the chemistry of the compound," he said.
Concluded Thackeray, "These results will spark much interest in the lithium battery community, which will undoubtedly want to repeat the experiments quickly to verify these very significant increases in electronic conductivity."
The research was sponsored by the Department of Energy. The team used shared experimental facilities at MIT supported by the National Science Foundation.
A version of this article appeared in MIT Tech Talk on October 30, 2002.