Extending a battery's lifetime with heat

October 1, 2015
Naked-eye observation of amorphous/fractal lithium dendrites. Credit: Asghar Aryanfar

Don't go sticking your electronic devices in a toaster oven just yet, but for a longer-lasting battery, you might someday heat them up when not in use. Over time, the electrodes inside a rechargeable battery cell can grow tiny, branch-like filaments called dendrites, causing short circuits that kill the battery or even ignite it in flames. But thanks to new experiments and computer simulations, researchers from the California Institute of Technology have explored in detail how higher temperatures can break down these dendrites—and possibly extend battery lifetimes.

A cell consists of a positive and negative electrode, called the cathode and . As the battery produces electrical current, electrons flow from the anode through a circuit outside the battery and back into the cathode. Having lost the electrons that are generating the current, some of the atoms in the anode—an electrically conductive metal like lithium—become ions that then travel to the cathode, moving through a conductive liquid medium called an electrolyte.

Recharging the battery reverses the process, and the ions travel back and stick onto the anode. But when they do, the ions don't attach evenly. Instead, they form microscopic bumps that eventually grow into long branches after multiple recharging cycles. When these dendrites reach and contact the cathode, they form a short circuit. Electrical current now flows across the dendrites instead of the external circuit, rendering the battery useless and dead.

The current also heats up the dendrites, and because the electrolyte tends to be flammable, the dendrites can ignite. Even if the dendrites don't short circuit the battery, they can break off from the anode entirely and float around in the electrolyte. In this way, the anode loses material, and the battery can't store as much energy.

"Dendrites are hazardous and reduce the capacity of rechargeable batteries," said Asghar Aryanfar, a scientist at Caltech, who led the new study that's published this week on the cover of The Journal of Chemical Physics, from AIP Publishing. Although the researchers looked at lithium batteries, which are among the most efficient kind, their results can be applied broadly. "The dendrite problem is general to all ," he said.

A naked-eye observation of in-situ lithium dendrites created at i = 2mA. cm?2 for t = 24hrs (a) before and (b) after submerging in the oil bath of T = 78C for t = 48hrs. The filled area fraction has been reduced from 64 percent and 41 percent. Credit: Asghar Aryanfar

The researchers grew lithium dendrites on a test battery and heated them over a couple days. They found that temperatures up to 55 degrees Celsius shortened the by as much as 36 percent. To figure out what exactly caused this shrinkage, the researchers used a computer to simulate the effect of heat on the individual lithium atoms that comprise a dendrite, which was modeled with the simple, idealized geometry of a pyramid.

The simulations showed that increased temperatures triggered the atoms to move around in two ways. The atom at the tip of the pyramid can drop to lower levels. Or, an atom at a lower level can move and leave behind a vacant spot, which is then filled by another atom. The atoms shuffle around, generating enough motion to topple the dendrite.

By quantifying how much energy is needed to change the structure of the dendrite, Aryanfar said, researchers can better understand its structural characteristics. And while many factors affect a battery's longevity at high temperatures—such as its tendency to discharge on its own or the occurrence of other chemical reactions on the side—this new work shows that to revitalize a battery, all you might need is some extra heat.

Explore further: Research aims to improve lithium-based batteries

More information: "Annealing kinetics of electrodeposited lithium dendrites," by Asghar Aryanfar, Tao Cheng, Agustin J. Colussi, Boris V. Merinov, William A. Goddard III and Michael R. Hoffmann. The Journal of Chemical Physics October 1, 2015 DOI: 10.1063/1.4930014

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4 comments

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barakn
1 / 5 (3) Oct 01, 2015
If I have a failing battery, why wouldn't I try sticking it in a toaster oven? What have I got to lose, provided I pay attention and don't let it overheat?
shavera
5 / 5 (4) Oct 01, 2015
namely that without really solid experience in the matter, you're entirely likely to start a lithium fire doing so. It would be ludicrous to recommend people actually do it. If it is truly helpful, then they'll need to create the kind of technology that would allow it to be done safely by people who will never read an instruction manual or safety warning.

Edit: also, it appears they removed the lithium anode from the cell before annealing it under oil. Again, do not try this at home, since you must do so in an environment free of oxygen and water vapor. (they did it in a glove box filled with argon)
barakn
1 / 5 (1) Oct 02, 2015
Since the lithium is packed inside a battery, its access to oxygen and water is extremely limited. Also, the dendrite-shrinking temperature they used is only 55 degrees C, a temperature which failing lithium batteries approach anyways. "Note that United Nations (UN) and Underwriters Laboratories (UL) tests for lithium-ion batteries discussed below require cells exhibit long-term thermal stability in the range of 70 to 75°C (158 to 167°F)." From "Lithium-Ion Batteries Hazard and Use Assessment, Final Report" http://www.nfpa.o...sessment The safety precautions exercised by the researchers were in part due to the fact that they removed the anode from the battery before, during and after the heating process. Of course it's going to be more dangerous if exposed. Thanks for the FUD.
xponen
not rated yet Oct 04, 2015
Restoring the capacity could reduce battery price by allow resales of refurbished battery. However, I don't see regular consumer doing this unless its as easy as heating the battery under the sun.

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