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Wednesday, 8 February 2012

Revealing how a battery material works: Why the hottest new material for rechargeable batteries works

MIT News
Feb 8, 2012

The molecular structure of lithium iron phosphate (LiFePO4)Credit: MIT

Since its discovery 15 years ago, lithium iron phosphate (LiFePO4) has become one of the most promising materials for rechargeable batteries because of its stability, durability, safety and ability to deliver a lot of power at once. It has been the focus of major research projects around the world, and a leading technology used in everything from power tools to electric vehicles. But despite this widespread interest, the reasons for lithium iron phosphate’s unusual charging and discharging characteristics have remained unclear.

Now, research by MIT associate professor of chemical engineering and mathematics Martin Z. Bazant has provided surprising new results showing that the material behaves quite differently than had been thought, helping to explain its performance and possibly opening the door to the discovery of even more effective battery materials.

The new insights into lithium iron phosphate’s behavior are detailed in a paper appearing this week in the journal ACS Nano, written by Bazant and postdoc Daniel Cogswell. The paper is an extension of research they reported late last year in the journal Nano Letters.

When it was first discovered, lithium iron phosphate was considered useful only for low-power applications. Then, later developments — by researchers including MIT’s Yet-Ming Chiang, the Kyocera Professor of Ceramics — showed that its power capacity could be improved dramatically by using it in nanoparticle form, an approach that made it one of the best materials known for high-power applications.

But the reasons why nanoparticles of LiFePO4 worked so well remained elusive. It was widely believed that while being charged or discharged, the bulk material separated into different phases with very different concentrations of lithium; this phase separation, it was thought, limited the material’s power capacity. But the new research shows that, under many real-world conditions, this separation never happens.

Bazant’s theory predicts that above a critical current, the reaction is so fast that the material loses its tendency for the phase separation that happens at lower power levels. Just below the critical current, the material passes through a new “quasi-solid solution” state, where it “doesn’t have time to complete the phase separation,” he says. These characteristics help explain why this material is so good for rechargeable batteries, he says.

The findings resulted from a combination of theoretical analysis, computer modeling and laboratory experiments, Bazant explains — a cross-disciplinary approach that reflects his own joint appointments in MIT’s departments of chemical engineering and mathematics. 
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