Researchers measure reaction rates in electrodes
MIT researchers have found a way to directly measure electrochemical reactions, the fundamental charge transfer rate, inside the porous electrodes of batteries, which in the past has not been made possible. The study found that the Butler-Volmer (BV) equation, usually used to describe reaction rates in electrodes, is inaccurate, especially at higher voltage levels. Instead, a different approach, called Marcus-Hush-Chidsey charge-transfer theory, provides more realistic results, revealing that the limiting step of these reactions is not what had been thought.
The findings could help engineers design better electrodes to improve batteries' rates of charging and discharging, and provide a better understanding of other electrochemical processes, such as how to control corrosion. The work is described this week in the journal Nature Communications by MIT postdoc Peng Bai and professor of chemical engineering and mathematics Martin Bazant.
Previous work was based on the assumption that the performance of electrodes made of lithium iron phosphate, widely used in lithium-ion batteries, was limited primarily by how fast lithium ions would diffuse into the solid electrode from the liquid electrolyte. But the latest analysis shows that the critical interface is actually between two solid materials: the electrode itself, and a carbon coating used to improve its performance.
Bai and Bazant's analysis shows that both transport steps in solid and liquid, ion migration in the electrolyte, and diffusion of "quasiparticles" called polarons, are very fast, and therefore do not limit battery performance. "We show it's actually electrons, not the ions, transferring at the solid-solid interface," Bai said, that determine the rate.
Bazant said researchers had not suspected, despite extensive research on lithium iron phosphate, that the material's electrochemical reactions might be limited by electron transfer between two solids. "That's a completely new picture for this material; it's not something that has even been mentioned before," he stated.
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