Metal Organic Frameworks to Improve Lithium Sulfur Battery

Pacific Northwest National Laboratory (PNNL) researchers have developed a nickel-based metal organic framework to hold onto polysulfide molecules in the cathodes of lithium-sulfur batteries and extend the batteries’ life spans.

A promising new battery chemistry is the lithium-sulfur battery, which can hold as much as four times more energy in a given mass than typical lithium-ion batteries. Lithium sulfur chemistry would enable far more available energy from a single charge, as well as help store more renewable energy. The down side of lithium-sulfur batteries, however, is they have a much shorter lifespan because they can’t currently be charged as many times as lithium-ion batteries.

The PNNL researchers added the nickel based powder, a kind of nanomaterial called a metal organic framework, to the battery’s cathode to capture problematic polysulfides that usually cause lithium-sulfur batteries to fail after a few charges. A paper describing the material and its performance was published online April 4 in the American Chemical Society journal Nano Letters.

Materials chemist Jie Xiao of the Department of Energy’s Pacific Northwest National Laboratory said, “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. Our metal organic framework may offer a new way to make that happen.”

Of particular interest is today’s electric vehicles that are typically powered by lithium-ion batteries. But the native chemistry of lithium-ion batteries limits how much energy they can store. As a result, electric vehicle drivers are often anxious about how far they can go before needing to recharge.  Metal organic frameworks in the cathodes would enable electric vehicles to drive farther on a single charge, as well as help store more renewable energy.

How the metal frameworks would improve lithium sulfur comes from how batteries work.  Most batteries have two electrodes: one is positively charged and called a cathode, while the second is negative and called an anode. Electricity is generated when electrons flow through a wire that connects the two. At the same time controlling the electrons, positively charged atoms shuffle from one electrode to the other through another path inside the battery: the electrolyte solution in which the electrodes are mounted.

The lithium-sulfur battery’s main problem comes from unwanted side reactions that cut the battery’s life short. The side reactions start on the battery’s sulfur-containing cathode, which slowly disintegrates and forms molecules, called polysulfides, that dissolve into the liquid electrolyte.  Some of the sulfur – an essential part of the battery’s chemical reactions – never returns to the cathode. As a result, the cathode has less material to keep the reactions going and the battery quickly dies.

Researchers worldwide are trying to improve materials for each battery component to increase the lifespan and mainstream the use of lithium-sulfur batteries. For this research, Xiao and her colleagues honed in on the cathode to stop polysulfides from moving through the electrolyte.

Many materials with tiny holes have been examined to physically trap polysulfides inside the cathode. Metal organic frameworks are porous, but the added strength of PNNL’s material is its ability to strongly attract the polysulfide molecules.

The framework’s positively charged nickel center tightly binds the polysulfide molecules to the cathodes. The result is a coordinate covalent bond that, when combined with the framework’s porous structure, causes the polysulfides to stay put.

PNNL electrochemist Jianming Zheng explains, “The metal organic framework’s highly porous structure is a plus that further holds the polysulfide tight and makes it stay within the cathode.”

Metal organic frameworks nanomaterial- also called MOFs – are crystal-like compounds made of metal clusters connected to organic molecules, or linkers. Together, the clusters and linkers assemble into porous 3-D structures.  The MOFs can contain a number of different elements.  PNNL researchers chose the transition metal nickel as the central element for this particular MOF because of its strong ability to interact with sulfur.

During lab tests, a lithium-sulfur battery with PNNL’s MOF cathode maintained 89% of its initial power capacity after 100 charge-and discharge cycles. Having shown the effectiveness of their MOF cathode, PNNL researchers now plan to further improve the cathode’s mixture of materials so it can hold more energy. The team also needs to develop a larger prototype and test it for longer periods of time to evaluate the cathode’s performance for real-world, large-scale applications.

“MOFs are probably best known for capturing gases such as carbon dioxide,” Xiao said. “This study opens up lithium-sulfur batteries as a new and promising field for the nanomaterial.”

Eighty nine percent at 100 cycles is a huge improvement even though not being a truly marketable solution.  But the PNNL team may be closer than we know for now.  Back in January, a Nature Communications paper by Xiao and some of her PNNL colleagues described another possible solution for lithium-sulfur batteries other side: developing a hybrid anode that uses a graphite shield to block the polysulfides.

More research sure to come.