MIT Invents Simple Formula That Could Guide Faster Charging and Longer-Lasting Battery Design

The core of all lithium-ion batteries is a simple reaction: during the discharge of the battery, lithium ions dissolved in the electrolyte solution "embed" into the solid electrode. When they are extracted and return to the electrolyte, the battery charges.

This process occurs thousands of times during the entire lifespan of a battery. The power output and charging speed of the battery depend on the speed of this reaction. However, little is known about the exact mechanism of this reaction and the factors that control its rate.

Image source: Journal "Science"

According to foreign media reports, in a study published in the journal "Science," researchers from the Massachusetts Institute of Technology (MIT) measured the lithium intercalation rates in various battery materials and used this data to develop a new model to control the reaction. The model indicates that lithium intercalation is governed by a process known as coupled ion-electron transfer, in which electrons are transferred to the electrode along with lithium ions.

Researchers say that the insights gained from this model can guide the design of more robust and faster-charging lithium-ion batteries. "We hope that through this study, we can make the reactions faster and more controllable, thereby accelerating the charging and discharging speed," said Martin Bazant, Chevron Professor of Chemical Engineering and Professor of Mathematics at MIT.

The new model may also help scientists understand why adjusting the electrodes and electrolytes in certain ways can improve energy, power, and battery life— a process that has primarily been achieved through trial and error.

"In this paper, we begin to unify the observed reaction rates of different materials and interfaces into a coupled electron and ion transfer intercalation theory, thereby consolidating previous research findings on reaction rates," said Yang Shao-Horn, Professor of Engineering at MIT and Professor of Mechanical Engineering, Materials Science and Engineering, and Chemistry.

Lithium flow modeling

For decades, scientists have assumed that the insertion rate of lithium ions into the electrodes of lithium-ion batteries depends on the speed at which lithium ions diffuse from the electrolyte to the electrode. They believe that this reaction is governed by the Butler-Volmer equation, a model originally developed nearly a century ago to describe the charge transfer rate in electrochemical reactions.

However, when researchers attempt to measure the rate of lithium intercalation, the measured results do not always align with the rates predicted by the Butler-Volmer equation.

Furthermore, obtaining consistent measurement results between different laboratories has always been challenging, with the measurement results of the same reaction reported by different research teams differing by as much as a billion times.

In this new study, the research team at MIT used an electrochemical technique to measure the lithium insertion rate, which involves applying repeated short voltage pulses to the electrode.

Researchers conducted these measurements on more than 50 combinations of electrolytes and electrodes, including lithium nickel manganese cobalt oxide commonly used in electric vehicle batteries, as well as lithium cobalt oxide used in most smartphones, laptops, and other portable electronic devices.

The measured rates for these materials are much lower than previously reported and do not match the predictions of the traditional Butler-Volmer model.

Researchers proposed another theory regarding the mechanism of lithium embedding at the electrode surface based on these data. This theory is based on the assumption that in order for lithium ions to enter the electrode, electrons from the electrolyte solution must simultaneously transfer to the electrode.

"The electrochemical step is not the insertion of lithium (which might be considered the main step), but rather the transfer of electrons, with the aim of reducing the solid material that carries lithium," Bazant said. "The insertion of lithium and the transfer of electrons occur simultaneously and promote each other."

This kind of Coupled Ionic Electron Transfer (CIET) reduces the energy barrier that must be overcome for intercalation reactions to occur, making them more likely to happen. The mathematical framework of CIET enables researchers to predict reaction rates, which have been validated by experiments and differ significantly from the predictions of the Butler-Volmer model.

Faster charging speed

In this study, the researchers also demonstrated that they could adjust the insertion rate by altering the composition of the electrolyte. For instance, swapping different anions can reduce the energy required for transferring lithium and electrons, thereby increasing the efficiency of the process.

Shao-Horn stated, "By altering the electrolyte to regulate the embedding kinetics, there is a great opportunity to enhance the reaction rate and modify the electrode design, thereby improving the battery's power and energy."

Shao-Horn's lab and its collaborators have been using automated experiments to create and test thousands of different electrolytes, which are used to develop machine learning models to predict functionally enhanced electrolytes.

These findings can also help researchers design batteries with faster charging speeds by accelerating the lithium intercalation reaction. Another goal is to reduce side reactions that may lead to battery performance degradation when electrons detach from the electrode and dissolve into the electrolyte.

"If you want to do this rationally, rather than just through trial and error, you need some theoretical framework to understand which important material parameters you can adjust," Bazant said. "That's exactly what this paper aims to provide."