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In science, there is a surprisingly long list of things we still haven’t exactly figured out yet but still use because they work. This unexpectedly has been the case for lithium-ion batteries—a power source for electric vehicles and various portable electronics—where scientists knew what the mechanism was but weren’t sure exactly how it worked.
Fortunately, MIT scientists have found the answer. For a Science paper published October 2, researchers describe a model that illustrates how coupled ion-electron transfer (CIET), a process in which an electron travels to the electrode with an ion, in this case a lithium ion, may explain the life source of a lithium-ion battery. The insight could “guide the design of more powerful and faster charging lithium-ion batteries,” according to the researchers.
A cascade of molecules
A typical lithium-ion battery works via a chemical mechanism called intercalation. Essentially, during battery discharge, lithium ions dissolved in an electrolyte solution insert themselves inside of a solid electrode. When the ions “de-intercalate” and return to the electrolyte, the battery charges.
The rate of intercalation governs everything from a battery’s net power to its charging speed—the reason the researchers found it imperative to better understand the underlying mechanisms, the paper explained.
Previously, scientists believed that lithium intercalation in a battery electrode was driven by a model describing how quickly lithium ions could diffuse between the electrolyte and the electrode. However, actual experiments hadn’t quite matched what that model predicted, suggesting to researchers that there may be another option.
A traveling pair
For the new study, the researchers prepared more than 50 combinations of electrolytes and electrodes to straighten things out once and for all. Like previous experiments, they found sizable inconsistencies between actual data and the model. So instead, the team came up with several alternatives that could explain what they were seeing.
Finally, they decided on a model based on the assumption that a lithium ion could only enter an electrode if it travels with an electron from an electrolyte solution—coupled ion-electron transfer. This electrochemical pairing makes it easier for intercalation to occur, the researchers explained, and the mathematics behind CIET fits the data well.
“The electrochemical step is not lithium insertion, which you might think is the main thing, but it’s actually electron transfer to reduce the solid material that is hosting the lithium,” Martin Bazant, study co-author and a mathematician at MIT, told MIT News. “Lithium is intercalated at the same time that the electron is transferred, and they facilitate one another.”
Not only that, but the researchers also accidentally discovered that switching up the composition of electrolytes influenced intercalation rates. Follow-up investigations could uncover more efficient ways for creating stronger, faster batteries, they explained.
“What we hope is enabled by this work is to get the reactions to be faster and more controlled, which can speed up charging and discharging,” Bazant said.
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Gayoung Lee
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