Thursday, October 29, 2015

Lithium-Air battery research shows potential paths to next-gen batteries

But a commercially viable Li-air battery is still “at least a decade away."
by - Oct 29, 2015 11:23am PDT

Scanning electron microscopy images of the electrode in its pristine state, after the battery is discharged, and after the battery is charged again.
On Thursday, a group of researchers from Cambridge University released a paper showing that they had developed a laboratory model of a lithium-air battery that solved several of the problems associated with batteries of similar chemistry. Their lithium-air battery had a high energy density, and it was capable of being recharged “more than 2,000 times.” The battery was theoretically more than 90 percent efficient in its energy use, as well.
It is the great hope of scientists that lithium-air batteries will one day replace the class of lithium-ion bricks we currently use. “The lithium-ion rechargeable battery is approaching its 25th anniversary,” Professor Clare P. Grey of the University of Cambridge’s chemistry department told a handful of journalists in a phone call on Wednesday. A quarter of a century ago, that new battery composition helped pave the way for the host of portable electronics that we carry with us today—relatively light and compact, Lithium-ion batteries are better-suited for consumer tech than their predecessors were.
But no chemist or engineer would claim that the lithium-ion battery is perfect. As electric vehicles become more popular, researchers are especially excited about lithium-air batteries because they would ideally be much lighter than anything we have powering cars today, and lighter cars mean a longer driving range before the battery runs out. That’s not to mention that the lithium-air batteries would ideally have a higher energy density.
In a press release (PDF), the University of Cambridge scientists admitted that a commercially viable lithium-air battery was still “at least a decade away,” but their research showed that some of the big roadblocks to developing such batteries can be tackled.
So far, previous research has been able to create lithium-air batteries that can hold a charge but can not be cycled frequently enough for commercial use (think of how many times you can recharge your smartphone before the battery kicks the bucket). Or, their laboratory batteries are too unstable for the real world because the oxygen will create unwanted chemical reactions inside the battery.
Whereas several earlier experiments on Li-O2 batteries have cycled by creating a chemical reaction that results in lithium peroxide (Li2O2), the Cambridge researchers built their battery to produce lithium hydroxide (LiOH) as a discharge product. The battery was composed of a lithium metal anode, a graphene oxide electrode, and a lithium iodide (LiI) additive, which acts as a mediator of redox reactions, as well as a dimethoxyethane solvent. The result was that the researchers were able to form and then remove lithium hydroxide during charge and discharge. (They also found that adding a little bit of water(!) to the battery helped.)
The researchers noted that "The cells tolerate high concentrations of water, water being the dominant proton source for the LiOH.” However, the battery can only cycle in pure oxygen, which is less than ideal for a true lithium-air battery (because air, of course, contains more than just oxygen).
As the battery discharged, lithium hydroxide built up in the graphene oxide electrode. That material was chosen among other possible materials because graphene electrodes are "light, conductive, and have a large pore volume that can potentially lead to large capacities,” the researchers write.
In many lithium-air battery prototypes, scientists have struggled to figure out how to most effectively fill and then flush the receiving electrode of the batteries’ discharge products. How well an electrode accepts discharge products and then flushes them when the battery is being recharged affects how many cycles the battery can be used for, as well as its overall efficiency.
Because of graphene oxide’s porousness and because of the way the lithium hydroxide particle built up in the graphene oxide, the researchers estimate that this battery can be cycled more than 2,000 times. They write that the lithium hydroxide discharge products can be quite large compared to the lithium peroxide that's found in other Li-air batteries, but despite their size, they fit into the porous graphene oxide much better than the doughnut-shaped lithium peroxide molecules (sort of how you'll have more room in your suitcase if you fold your clothes rather than just ball them up and throw them in there). "The large LiOH agglomerates efficiently fill up the pore volume available in the [...] electrode, leading to much larger capacities," the researchers write.
The researchers were also able to reduce the “voltage gap” in their Li-air battery, a measure of efficiency that has been difficult for earlier experiments to achieve. With this new chemistry, researchers were able to reduce the voltage gap to 0.2V, a number closer to what is seen in a Li-ion battery.
In their press conference, the researchers noted that their new battery had a theoretical energy density 3,350Wh/kg of electrode. That’s quite impressive, given that current Li-ion batteries can run between 140-250Wh/kg. (But maybe not as impressive as research from 2012 that predicted a Li-air battery with a 13,500Wh/kg capacity.)
This is all well and good, but naturally there are still some issues. The voltage gap decrease and the graphene oxide electrode’s large capacity only hold true for very specific rates of charge and discharge, and the researchers noted in a press conference that the lithium metal anode in their battery can sometimes form dendrites that hinder the battery’s performance. As we noted before, other compounds in air besides O2 could also potentially cause negative chemical byproducts.
All these problems mean you won’t be able to buy a Li-air battery for your phone any time soon. It’s easy to think of new battery technology as vaporware, but instead it’s just painstaking, diligent science. “If we can really understand the nature as to how lithium hydroxide is formed [in the battery], that can give us some clues,” Grey said on Wednesday. Other teams will be able to build off the Cambridge research in the future, and maybe soon scientists will be able to “get at some of the fundamental mechanisms of what’s going on,” the professor added.

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