Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

How electrodes charge and discharge

03.04.2014

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!
Further information:
http://www.mit.edu

Further reports about: MIT batteries coating electrode electrodes electrolyte equation findings materials phosphate processes reaction reactions voltage

More articles from Power and Electrical Engineering:

nachricht Trojan Transit Rolling Out
27.03.2015 | University of Arkansas at Little Rock

nachricht Ultra-Thin Silicon Films Create Vibrant Optical Colors
25.03.2015 | University of Alabama Huntsville

All articles from Power and Electrical Engineering >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Experiment Provides the Best Look Yet at 'Warm Dense Matter' at Cores of Giant Planets

In an experiment at the Department of Energy's SLAC National Accelerator Laboratory, scientists precisely measured the temperature and structure of aluminum as...

Im Focus: Energy-autonomous and wireless monitoring protects marine gearboxes

The IPH presents a solution at HANNOVER MESSE 2015 to make ship traffic more reliable while decreasing the maintenance costs at the same time. In cooperation with project partners, the research institute from Hannover, Germany, has developed a sensor system which continuously monitors the condition of the marine gearbox, thus preventing breakdowns. Special feature: the monitoring system works wirelessly and energy-autonomously. The required electrical power is generated where it is needed – directly at the sensor.

As well as cars need to be certified regularly (in Germany by the TÜV – Technical Inspection Association), ships need to be inspected – if the powertrain stops...

Im Focus: 3-D satellite, GPS earthquake maps isolate impacts in real time

Method produced by UI researcher could improve reaction time to deadly, expensive quakes

When an earthquake hits, the faster first responders can get to an impacted area, the more likely infrastructure--and lives--can be saved.

Im Focus: Atlantic Ocean overturning found to slow down already today

The Atlantic overturning is one of Earth’s most important heat transport systems, pumping warm water northwards and cold water southwards. Also known as the Gulf Stream system, it is responsible for the mild climate in northwestern Europe. 

Scientists now found evidence for a slowdown of the overturning – multiple lines of observation suggest that in recent decades, the current system has been...

Im Focus: Robot inspects concrete garage floors and bridge roadways for damage

Because they are regularly subjected to heavy vehicle traffic, emissions, moisture and salt, above- and underground parking garages, as well as bridges, frequently experience large areas of corrosion. Most inspection systems to date have only been capable of inspecting smaller surface areas.

From April 13 to April 17 at the Hannover Messe (hall 2, exhibit booth C16), engineers from the Fraunhofer Institute for Nondestructive Testing IZFP will be...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

World Conference On Regenerative Medicine 2015: Registration And Abstract Submission Now Open

25.03.2015 | Event News

University presidents from all over the world meet in Hamburg

19.03.2015 | Event News

10. CeBiTec Symposium zum Big Data-Problem

17.03.2015 | Event News

 
Latest News

Two Most Destructive Termite Species Forming Superswarms in South Florida

27.03.2015 | Agricultural and Forestry Science

ORNL-Led Team Demonstrates Desalination with Nanoporous Graphene Membrane

27.03.2015 | Materials Sciences

Coorong Fish Hedge Their Bets for Survival

27.03.2015 | Ecology, The Environment and Conservation

VideoLinks
B2B-VideoLinks
More VideoLinks >>>