Oscillating electric field drastically reduces battery recharge time

Wait, there's more!

(PhysOrg.com)

-- Part of the headache of having to constantly recharge batteries is not just how often they need to be charged, but also the time it takes to charge them. In a new study, researchers have proposed a charging method that could greatly reduce the charging time of lithium-ion batteries, which are used in everything from electronic devices to electric vehicles. The new method uses an additional oscillating electric field (besides the charging field) that should be capable of charging a lithium-ion battery in a fraction of the time compared with traditional methods.

Researchers Ibrahim Abou Hamad from Mississippi State University and coauthors have developed the new charging method thanks to revolutionary developments in molecular dynamics simulations. In their study, the researchers simulated the lithium-ion battery-charging process by simulating the intercalation (i.e. “insertion”) of lithium ions into the battery’s graphite anode. Although intercalation is just one part of the charging process (along with diffusion), it dominates the charging time.

In the charging process, lithium ions first diffuse within the battery’s electrolyte until they reach the graphite anode. At this interface, ions must overcome an energy barrier in order to be intercalated into the anode.

In their simulations, Hamad and his team found that an additional oscillating electric field can lower this energy barrier, enabling lithium ions to intercalate more quickly into the anode. The oscillating field also increases the diffusion rate, which helps further reduce the overall charging time, albeit to a lesser extent.

Specifically, when the scientists applied an oscillating square-wave field with a frequency of 25 GHz and an amplitude of 5 kCal/mol to the graphite sheets in the anode, the lithium ions intercalated into the graphite sheets within an average time of about 50 nanoseconds. By changing the amplitude of the oscillating wave, the researchers found that they could further improve charging time by lowering the energy barrier and speeding up intercalation. Their simulations showed that the dependence of the intercalation time on the amplitude is exponential, meaning that a small increase in amplitude leads to a large increase the intercalation speed, which offers the potential for very fast charging times.

In the future, the researchers plan to further investigate the new method, including analyzing how changing the frequency of the oscillating field effects the charging time. They noted that the new method might provide an increase in battery power densities, as well.

More information: Ibrahim Abou Hamad, M. A. Novotny, D. Wipf, and P. A. Rikvold. “A new battery-charging method suggested by molecular dynamics simulations.” Available at arxiv.org. Doi: 10.1039/b920970k.

Engineers at MIT discover a way to charge batteries in seconds news

this could be big...

domain-b.com:

16 March, 2009

Engineers at MIT have discovered a way to move en­er­gy faster in lithium ion batteries, an advance that could pos­sibly pav­e the way for smaller, light­er bat­ter­ies for cell phones and other devices, that re­charge in sec­onds rath­er than hours.

The work could also allow for the quick recharging of batteries in electric cars, although that particular application would be limited by the amount of power available to a homeowner through the electric grid.

The work, led by Gerbrand Ceder and Richard Simmons Professor of Materials Science and Engineering and reported in the March 12 issue of Nature said that because the material involved is not new, the researchers have simply changed the way they make it and could make it into the marketplace within two to three years.

State-of-the-art lithium rechargeable batteries have very high energy densities and they are good at storing large amounts of charge. The tradeoff is that they have relatively slow power rates, they are sluggish at gaining and discharging that energy.

It may change the way lithium ion batteries are recharged, which is widely used in cell phones, laptops, portable gaming systems and many other devices, but of prime importance will be, electric cars.

Consider current batteries for electric cars. "They have a lot of energy, so you can drive at 55 mph for a long time, but the power is low. You can't accelerate quickly," Ceder said.

In the case of a car, both the charging and discharging abilities are important since currently, a hybrid-electric car to accelerate, relies heavily on its internal combustion engine.

Why the slow power rates? Traditionally, scientists have thought that the lithium ions responsible, along with electrons, for carrying charge across the battery simply move too slowly through the material.

About five years ago, however, Ceder and colleagues made a surprising discovery. Computer calculations of a well-known battery material, lithium iron phosphate, predicted that the material's lithium ions should actually be moving extremely quickly.

"If transport of the lithium ions was so fast, something else had to be the problem," Ceder said.

Further calculations showed that lithium ions can indeed move very quickly into the material but only through tunnels accessed from the surface. If a lithium ion at the surface is directly in front of a tunnel entrance, there's no problem: it proceeds efficiently into the tunnel. But if the ion isn't directly in front, it is prevented from reaching the tunnel entrance because it cannot move to access that entrance.

Ceder and Byoungwoo Kang, a graduate student in materials science and engineering as well as a coauthor of the Nature paper, devised a way around the problem by creating a new surface structure that does allow the lithium ions to move quickly around the outside of the material, much like a beltway around a city. When an ion traveling along this beltway reaches a tunnel, it is instantly diverted into it.

Using their new processing technique, the two went on to make a small battery that could be fully charged or discharged in 10 to 20 seconds (it takes six minutes to fully charge or discharge a cell made from the unprocessed material).

Ceder notes that further tests showed that unlike other battery materials, the new material does not degrade as much when repeatedly charged and recharged. This could lead to smaller, lighter batteries, because less material is needed for the same result.

"The ability to charge and discharge batteries in a matter of seconds rather than hours may open up new technological applications and induce lifestyle changes," Ceder and Kang conclude in their Nature paper.

This work was supported by the National Science Foundation through the Materials Research Science and Engineering Centers program and the Batteries for Advanced Transportation Program of the U.S. Department of Energy. It has been licensed by two companies.