Earlier this year, researchers at the Department of Energy's Pacific Northwest National Laboratory (PNNL) developed a nanomaterial powder called a metal organic framework (MOF) that could be added to the cathode of lithium-sulfur batteries, promising to hold a charge longer and increase the number of charge-discharge cycles.
Now, in joint research between the University of Cambridge and the Beijing Institute of Technology, an MOF powder has been used to improve the cathodes of lithium-sulfur batteries but in a different way. The new wrinkle to this research was that the team wrapped the sulfur-carbon energy storage unit in a thin sheet of flexible graphene, resulting in faster transport of electrons and ions.
In work that was published in the journal APL Materials, the international team used the MOF powder as a template for creating a conductive porous carbon cage in which sulfur acts as the host and each sulfur-carbon nanoparticle behaves as an energy storage unit where electrochemical reactions occur.
“Our carbon scaffold acts as a physical barrier to confine the active materials within its porous structure,” explained Kai Xi, a research scientist at Cambridge, in a press release. "This leads to improved cycling stability and high efficiency."
The researchers report that the graphene in this design is used as a kind of bridge to form interconnected networks that can reduce the internal resistances of each component. The graphene and MOF-derived microporous carbons form a composite structure—sort of a porous scaffold—that has conductive connections, making it a promising electrode structure design for rechargeable batteries.
Xi notes in the release that this work provides, “a basic, but flexible, approach to both enhance the use of sulfur and improve the cycle stability of batteries.” He added that, “Modification of the unit or its framework by doping or polymer coating could take the performance to a whole new level.”
The researchers believe that this novel design, in which energy storage and an ion-electron framework are integrated, could open the door to creating high-performance energy storage systems that are non-topotactic (meaning that the chemical reactions don’t cause structural changes to the crystalline solids involved).