Metal
mix boosts batteries
By
Kimberly Patch,
Technology Research NewsA truly good battery should be made of relatively inexpensive materials, store a significant amount of electricity, and discharge this energy as quickly as an electrical device needs it. And in a world that's increasingly contaminated by the residues of technology, it should be rechargeable and nontoxic.
The common lithium batteries that power portable electronic devices like laptop computers and cell phones use lithium metal oxide electrodes. Five years ago, scientists discovered a cheaper, nontoxic lithium electrode -- lithium iron phosphate. But initial promise turned to disappointment when the material turned out to be a bad conductor, and so could not discharge electricity at rates high enough to be useful.
Researchers from the Massachusetts Institute of Technology have now shown that doping, or mixing, lithium iron phosphate with positive ions of another metal can drastically boost the material's conductivity. Ions are atoms that have fewer or more electrons than electrically neutral atoms and so have a positive or negative charge.
The doping metal increased the conductivity of the lithium iron phosphate by 100 million times, making it an even better conductor than standard lithium metal oxide electrodes, according to Yet-Ming Chiang, a professor of materials science and engineering at the Massachusetts Institute of Technology.
The raw materials that go into the compound are only about one-quarter the cost of those that make up lithium metal oxide electrodes and the compound is nontoxic, Chiang said. The material gives a battery an extremely high rate of charge and discharge, "while at the same time being low in materials cost and very safe," he said.
Lithium iron phosphate batteries could bring on a new class of devices that would bridge the gap between super capacitors, which deliver short bursts of high power, but can only store limited amounts of total energy, and batteries, which have the opposite trade-off, said Chiang.
The material promises to improve batteries for electric and hybrid cars, backup power for implantable medical devices, and fuel cells, according to Chiang.
Batteries generate electricity when the pair of materials that make up the bulk of the battery react chemically, with one material giving up electrons and the other material gaining electrons. Rather than flowing directly from one material to the other, however, the electric current leaves the battery through one electrode and returns through another.
Connecting an electronic device between a battery's electrodes, which act as gatekeepers that determine how quickly the electricity flows, powers the device.
Batteries made with the researchers' new electrode material would deliver voltage similar to conventional lithium batteries, but the material's better conductivity allows for much higher power density, or rate of charge and discharge, said Chiang. A cell containing the new electrode could be charged or discharged in as little as three minutes, while typical batteries might require a half-hour or more, he said.
This is important for electric vehicles because they need a high rate of energy to accelerate and because they need to store electricity quickly in order to reuse breaking energy, said Chiang. "Battery power density is required for rapid acceleration and also to accept the regenerative breaking energy when someone slams on the brakes," he said.
The material could also eventually be used as electrodes for electrochemical applications like fuel-cells and membranes for separating hydrogen gas, according to Chiang. "These are other applications that require rapid electron transport as well as ion transport -- in these cases the ion is hydrogen rather than lithium as in the battery," he said.
The team synthesized more than 50 different mixtures by adding different metals and baking the samples at temperatures as high as 850 degrees Celsius in order to change the crystal structure of the material to improve its conductivity, said Chiang. The metals included magnesium, aluminum, titanium, zirconium, niobium, and tungsten. The challenges were getting the additive to be uniformly distributed in the crystal lattice of the lithium iron phosphate at the right positions in the lattice to have the necessary effect on conductivity, he said.
They knew they were on to something when something strange happened. "Lithium iron phosphate is normally medium gray color, not surprising for an electronic insulator," Chiang said. When one of the samples came out jet-black, "we realized that something special had happened," he said. "Highly conductive materials are usually either metallic in luster -- gold, silver, copper, aluminum -- or black in color -- carbon, oxide superconductors, magnetic ferrites."
The material has a nanoporous crystal structure, said Chiang. Nanoporous materials contain holes nearly as small as atoms. "The nanoporous structure allows for rapid lithium transport into the electrode without impeding the electronic conductivity," he said.
The formulation has proven very stable in abuse tests. This is "especially important for batteries that pack a lot of energy and will be used under a wide range of temperatures and electrical conditions," Chiang said.
Lithium iron phosphate is also the basic formulation of a mineral found in the earth's mantle. Both this and the dopants the researchers added are considered nontoxic compared to nickel-cadmium or lead acid batteries, said Chiang. "We expect no environmental issues concerned with disposal," he said.
The new type of lithium iron phosphate "looks like a major breakthrough," said John Owen, a reader in electrochemistry at the University of Southampton in England for. "This discovery will certainly bring forward the arrival of a new type of lithium battery in its the next year or two," he said.
Once the scope and mechanism of the effect are fully understood, "the way will be open to use a similar technique to improve many other materials in the field of energy conversion, [including] fuel cells and solar cells," he said.
If the result turns out to be reliable, then this is most certainly interesting, said Josh Thomas a professor of solid-state electrochemistry at Uppsala University in Sweden, and director of the university's Advanced Battery Centre. The material "is at the absolute front line... in implementing new materials for developing better, cleaner, more powerful batteries for ever larger applications -- ultimately for traction applications in electric and electric/hybrid vehicles," he said.
The researchers are currently working to understand exactly how the material conducts so well, said Chiang. "We want to understand the crystal chemistry and mechanisms of conduction in this material at a deeper level, to know where the atoms and electrons are and why the conduction is as high as it is," he said. The researchers are also planning to investigate similar compounds, he said.
Because batteries based on the new electrode can use existing materials for the rest of the battery, the material could find its way into products within two years, Chiang said.
Chiang is co-founder of A123Systems, which has licensed the technology from MIT and is working to commercialize it, according to Chiang.
Chiang's research colleagues were Sung-Yoon Chung and Jason T. Bloking. They published the research in the September 22, 2002 issue of Nature Materials. The research was funded by the Department of Energy (DOE).
Timeline: 2 years
Funding: Government
TRN Categories: Energy; Materials Science and Engineering
Story Type: News
Related Elements: Technical paper, "Electronically Conductive Phopho-Olivines as Lithium Storage Electrodes," Nature Materials, September 22, 2002.
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October 2/9, 2002
Page One
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