Nanomaterials Keep Pushing Lithium-Sulfur Battery Capabilities for EVs

Photo: Pacific Northwest National Laboratory

Researchers at the Department of Energy's Pacific Northwest National Laboratory (PNNL) have developed a nanomaterial powder that can be added to the cathode of lithium-sulfur batteries to capture problematic polysulfides that usually cause them to fail after a few charges.

The nanomaterial powder is a metal organic framework (MOF) in which metal ions are coordinated with rigid organic molecules to form a porous material that can be one-, two-, or three-dimensional. The research paper was published in the journal Nano Letters.

"Lithium-sulfur batteries have the potential to power tomorrow's electric vehicles, but they need to last longer after each charge and be able to be repeatedly recharged," said materials chemist Jie Xiao at PNNL in a press release. "Our metal organic framework may offer a new way to make that happen."

Recent nanomaterial research in batteries has been turning away from both lithium-ion (Li-ion) batteries and their anodes to alternative lithium-sulfur batteries and their cathodes. The reason for this is that lithium-sulfur batteries can hold as much as four times more energy per mass than lithium-ion batteries, making possible the kind of driving ranges for all-electric vehicles (EVs) that Li-ion batteries have failed to deliver thus far.

The most recent demonstration of this research trend has been work out of the University of Southern California where researchers developed a graphene oxide coating that improved the sulfur cathode's capacity to 800 mAh/g for 1000 charge-discharge cycles, which is more than five times the capacity of commercial cathodes. Also, researchers at Stanford University developed a nanoparticle made up of an inner core of sulfur surrounded by an outer layer of porous titanium-oxide, which claimed to have established a world record for storage capacity.

In this most recent work, the PNNL researchers focused in on stopping polysulfides getting through the cathode and circulating through the electrolyte. While many porous materials are able to trap these polysulfides, the key distinguishing property of the MOF developed by the PNNL researchers is its ability to attract polysulfide molecules. This attractive feature is caused by the positively charged nickel at the center of the MOF that tightly binds the polysulfide molecules to the cathodes.

In experiments, the researchers demonstrated that a lithium-sulfur with the MOF in the cathode maintained 89 percent of its initial power capacity after 100 charge-and discharge cycles.

In future research, the PNNL team intends to improve the cathode's mixture of materials so it can store more energy, develop a larger prototype and test it for longer periods of time to evaluate the cathode's performance for real-world, large-scale applications.

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