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 Flexible OLED applications arrive
28.06.2016 | Fraunhofer-Institut für Organische Elektronik, Elektronenstrahl- und Plasmatechnik FEP

nachricht Energy from Sunlight: Further Steps towards Artificial Photosynthesis
24.06.2016 | Universität Basel

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: Flexible OLED applications arrive

R2D2, a joint project to analyze and development high-TRL processes and technologies for manufacture of flexible organic light-emitting diodes (OLEDs) funded by the German Federal Ministry of Education and Research (BMBF) has been successfully completed.

In contrast to point light sources like LEDs made of inorganic semiconductor crystals, organic light-emitting diodes (OLEDs) are light-emitting surfaces. Their...

Im Focus: Unexpected flexibility found in odorant molecules

High resolution rotational spectroscopy reveals an unprecedented number of conformations of an odorant molecule – a new world record!

In a recent publication in the journal Physical Chemistry Chemical Physics, researchers from the Max Planck Institute for the Structure and Dynamics of Matter...

Im Focus: 3-D printing produces cartilage from strands of bioink

Strands of cow cartilage substitute for ink in a 3D bioprinting process that may one day create cartilage patches for worn out joints, according to a team of engineers. "Our goal is to create tissue that can be used to replace large amounts of worn out tissue or design patches," said Ibrahim T. Ozbolat, associate professor of engineering science and mechanics. "Those who have osteoarthritis in their joints suffer a lot. We need a new alternative treatment for this."

Cartilage is a good tissue to target for scale-up bioprinting because it is made up of only one cell type and has no blood vessels within the tissue. It is...

Im Focus: First experimental quantum simulation of particle physics phenomena

Physicists in Innsbruck have realized the first quantum simulation of lattice gauge theories, building a bridge between high-energy theory and atomic physics. In the journal Nature, Rainer Blatt‘s and Peter Zoller’s research teams describe how they simulated the creation of elementary particle pairs out of the vacuum by using a quantum computer.

Elementary particles are the fundamental buildings blocks of matter, and their properties are described by the Standard Model of particle physics. The...

Im Focus: Is There Life On Mars?

Survivalist back from Space - 18 months on the outer skin of the ISS

A year and a half on the outer wall of the International Space Station ISS in altitude of 400 kilometers is a real challenge. Whether a primordial bacterium...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

ERES 2016: The largest conference in the European real estate industry

09.06.2016 | Event News

Networking 4.0: International Laser Technology Congress AKL’16 Shows New Ways of Cooperations

24.05.2016 | Event News

Challenges of rural labor markets

20.05.2016 | Event News

 
Latest News

Scientists explain unusual and effective features in perovskite

28.06.2016 | Physics and Astronomy

ChemCam findings hint at oxygen-rich past on Mars

28.06.2016 | Earth Sciences

Previously unknown global ecological disaster discovered

28.06.2016 | Earth Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>