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At the core of all lithium-ion batteries lies a straightforward reaction: Lithium ions dissolved in an electrolyte solution “intercalate” or embed themselves into a solid electrode during battery discharge. When they de-intercalate and go back to the electrolyte, the battery recharges.

This process occurs thousands of times over the lifespan of a battery. The quantity of energy that the battery can produce, as well as the speed at which it charges, depends on how quickly this reaction occurs. Nonetheless, limited information exists regarding the precise mechanism of this reaction or the factors influencing its rate.

In a recent investigation, MIT researchers have assessed lithium intercalation rates across various battery materials and utilized that information to construct a new model illustrating how the reaction is regulated. Their model proposes that lithium intercalation is controlled by a mechanism known as coupled ion-electron transfer, wherein an electron is relayed to the electrode simultaneously with a lithium ion.

Insights derived from this model could steer the design of more potent and rapidly charging lithium-ion batteries, the researchers suggest.

“What we aspire to achieve through this work is to make the reactions quicker and more regulated, which can expedite both charging and discharging,” asserts Martin Bazant, the Chevron Professor of Chemical Engineering and a mathematics professor at MIT.

The new model may furthermore aid scientists in comprehending why modifying electrodes and electrolytes in specific manners leads to enhanced energy, power, and battery longevity — a process predominantly conducted through trial and error.

“This is one of those papers where we begin to unify the observations of reaction rates that we observe with distinct materials and interfaces, into one theory of coupled electron and ion transfer for intercalation, building upon prior work related to reaction rates,” remarks Yang Shao-Horn, the J.R. East Professor of Engineering at MIT and a professor of mechanical engineering, materials science and engineering, and chemistry.

Shao-Horn and Bazant are the senior authors of the document, which is published today in Science. The primary authors include Yirui Zhang PhD ’22, who is currently an assistant professor at Rice University; Dimitrios Fraggedakis PhD ’21, who is now an assistant professor at Princeton University; Tao Gao, a former MIT postdoctoral researcher who is now an assistant professor at the University of Utah; and MIT graduate student Shakul Pathak.

Modeling lithium movement

For numerous decades, researchers have theorized that the speed of lithium intercalation at a lithium-ion battery electrode is dictated by how swiftly lithium ions can diffuse from the electrolyte into the electrode. This reaction, they believed, was governed by a framework known as the Butler-Volmer equation, initially formulated nearly a century ago to characterize the rate of charge transfer during an electrochemical reaction.

However, when scientists attempted to gauge lithium intercalation rates, the results they obtained did not consistently align with the rates forecasted by the Butler-Volmer equation. Additionally, acquiring consistent measurements across laboratories has proven challenging, with different research teams reporting varying measurements for the same reaction by a factor of up to 1 billion.

In the recent study, the MIT team evaluated lithium intercalation rates utilizing an electrochemical method that involves applying rapid, short bursts of voltage to an electrode. They generated these measurements for over 50 combinations of electrolytes and electrodes, including lithium nickel manganese cobalt oxide, frequently utilized in electric vehicle batteries, and lithium cobalt oxide, commonly found in the batteries that power most smartphones, laptops, and other portable devices.

For these materials, the observed rates are significantly lower than previously reported, and they do not coincide with predictions made by the conventional Butler-Volmer model.

The researchers utilized the findings to formulate an alternative hypothesis regarding how lithium intercalation transpires at the electrode surface. This hypothesis is predicated on the idea that in order for a lithium ion to penetrate an electrode, an electron from the electrolyte solution must be transmitted to the electrode concurrently.

“The electrochemical phase is not lithium insertion, which one might assume is the primary focus, but rather it’s the electron transfer to reduce the solid material that hosts the lithium,” Bazant explains. “Lithium is intercalated concurrently with electron transfer, and they enhance each other’s efficacy.”

This coupled-electron ion transfer (CIET) diminishes the energy barrier that must be surpassed for the intercalation reaction to proceed, making it more probable. The mathematical framework of CIET enabled the researchers to make predictions concerning reaction rates, which were corroborated by their experiments and significantly diverged from those predicted by the Butler-Volmer model.

Enhanced charging

In this research, the investigators also demonstrated that they could adjust intercalation rates by modifying the composition of the electrolyte. For instance, substituting different anions can decrease the energy required to transfer the lithium and electron, enhancing the efficiency of the process.

“Tuning the intercalation kinetics through electrolyte variations presents significant opportunities to boost reaction rates, modify electrode designs, and thereby elevate the battery’s power and energy,” Shao-Horn remarks.

Shao-Horn’s lab and their collaborators have been implementing automated experiments to create and evaluate thousands of different electrolytes, which are used to develop machine-learning models to predict electrolytes with improved functions.

The discoveries could also assist researchers in creating batteries that charge more rapidly, by accelerating the lithium intercalation reaction. Another objective is to minimize the side reactions that can lead to battery deterioration when electrons detach from the electrode and dissolve into the electrolyte.

“If you wish to approach this methodically, rather than simply through trial and error, you require some form of theoretical framework to identify the critical material parameters that you can manipulate,” Bazant asserts. “That’s the goal of this paper.”

The research received funding from Shell International Exploration and Production and the Toyota Research Institute through the D3BATT Center for Data-Driven Design of Rechargeable Batteries.

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