Analysis probes reactions in porous battery electrodes for the first time
The electrochemical reactions inside the porous electrodes of batteries and fuel cells have been described by theorists, but never measured directly. Now, a team at MIT has figured out a way to measure the fundamental charge transfer rate — finding some significant surprises.
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 new 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 new analysis shows that the critical interface is actually between two solid materials: the electrode itself, and a carbon coating used to improve its performance.
Limited by electron transfer
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 says, that determine the rate.
Bazant says 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 says.
While coating the electrode surface with a thin layer of carbon or graphene had been shown to improve performance, there was no microscopic and quantitative understanding of why this made a difference, Bazant says. The new findings will help explain a number of apparently conflicting results in the scientific literature, he says.
Unexpectedly low reaction rates
For example, the classical equations used to predict the performance of such materials have indicated that the logarithm of the reaction rate should vary linearly as voltage is increased — but experiments have shown a nonlinear response, with the uptake of lithium flattening out at high voltage. The discrepancies have been significant, Bazant says: "We find the reaction rate is much lower than what is predicted."
The new analysis means that to make further improvements in this technology, the focus should be on "how you engineer the surface" at the solid-solid interface, Bai says.
Bazant adds that the new understanding could have implications far beyond electrode design, since the fundamental processes the team uncovered apply to electrochemical processes including electrodeposition, corrosion, and fuel cells. "It's also important for basic science," he says, since the process is both ubiquitous and poorly understood.
The BV equation is purely empirical, and "doesn't tell you anything about what's going on microscopically," Bazant says. By contrast, the Marcus-Hush-Chidsey equations — for which Rudolph Marcus of the California Institute of Technology was awarded the 1992 Nobel Prize in chemistry — are based on a precise understanding of atomic-level activity. So the new analysis, Bazant maintains, could lead not only to new practical solutions, but also to a deeper understanding of the underlying mechanisms.
Written by David Chandler, MIT News Office
Andrew Carleen | EurekAlert!
Nano-scale process may speed arrival of cheaper hi-tech products
09.11.2018 | University of Edinburgh
Nuclear fusion: wrestling with burning questions on the control of 'burning plasmas'
25.10.2018 | Lehigh University
Biochips have been developed at TU Wien (Vienna), on which tissue can be produced and examined. This allows supplying the tissue with different substances in a very controlled way.
Cultivating human cells in the Petri dish is not a big challenge today. Producing artificial tissue, however, permeated by fine blood vessels, is a much more...
Faster and secure data communication: This is the goal of a new joint project involving physicists from the University of Würzburg. The German Federal Ministry of Education and Research funds the project with 14.8 million euro.
In our digital world data security and secure communication are becoming more and more important. Quantum communication is a promising approach to achieve...
On Saturday, 10 November 2018, the research icebreaker Polarstern will leave its homeport of Bremerhaven, bound for Cape Town, South Africa.
When choosing materials to make something, trade-offs need to be made between a host of properties, such as thickness, stiffness and weight. Depending on the application in question, finding just the right balance is the difference between success and failure
Now, a team of Penn Engineers has demonstrated a new material they call "nanocardboard," an ultrathin equivalent of corrugated paper cardboard. A square...
Physicists at ETH Zurich demonstrate how errors that occur during the manipulation of quantum system can be monitored and corrected on the fly
The field of quantum computation has seen tremendous progress in recent years. Bit by bit, quantum devices start to challenge conventional computers, at least...
09.11.2018 | Event News
06.11.2018 | Event News
23.10.2018 | Event News
14.11.2018 | Materials Sciences
14.11.2018 | Health and Medicine
14.11.2018 | Life Sciences