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Posted: Apr 17, 2013
MXenes - novel two-dimensional materials to improve battery technology
(Nanowerk News) Researchers at Drexel University recently reported on the discovery of a new family of two-dimensional materials called “MXenes.” The materials’ structures are similar to graphene, with which they share many properties, including good electrical conductivity and potential applications in energy storage. Now, in a new paper in Nature Communications ("Intercalation and delamination of layered carbides and carbonitrides"), Drexel researchers have demonstrated several new possible avenues for practical applications of MXenes.
MXenes are transition metal carbides and nitrides, created by selectively removing aluminum from layered ternary carbides known as MAX phases. Through this exfoliation process, the carbide layers are separated into two MXene sheets just a few atoms thick. MXenes can accommodate various ions and molecules between their layers by a process known as intercalation, which is sometimes a necessary step in order to exploit the materials’ unique properties. For example, placing lithium ions between MXene sheets has been shown to render them promising materials for both lithium-ion batteries and electrochemical capacitors.
Computational studies have suggested that fully exfoliating, or delaminating, certain MXenes would yield layers with exceptional charge capacities for use in battery anodes. To date, however, large-scale delamination had not been achieved. In “Intercalation and Delamination of Layered Carbides and Cabonitrides,” the Drexel team reports on successful intercalation of MXenes with several organic molecules, including dimethyl sulfoxide (DMSO), which allowed them to fully exfoliate stacked layers into MXene sheets and ultimately create MXene “paper” by filtering flakes from solution. This flexible and electrically conductive “paper” showed a lithium ion capacity of four times that of typical MXene material, with extremely high charging rates and a cyclability superior to graphite, which is used in commercial lithium-ion batteries. Critically, this work demonstrates that such material can be synthesized on a large scale.
Much attention has recently been drawn to two-dimensional - in other words, atomically thin - materials for which the sheet width is about 10,000 times larger than its thickness. Graphene is just one representative of a large group of two-dimensional solids, and MXenes add a dozen new members to the family that have unusual properties dictated by their structure and presence of various transition metals: for example, combining metallic electrical conductivity with hydrophylicity (good wetting). This new finding further expands the potential uses of the new materials.
“By demonstrating chemical intercalation of organic molecules between MXene layers, we have substantially altered properties of MXenes,” says Yury Gogotsi, whose Nanomaterials Group led the research in partner with Dr. Michel Barsoum. “By separating MXene sheets via intercalation, we produced excellent materials for electrodes of batteries and electrochemical capacitors. We are currently exploring several other exciting applications and we firmly believe that this is just the beginning of an exciting road towards discovery of new MXene structures and finding applications in which they can outperform other materials.”
The researchers note that successful delamination of MXenes also creates opportunities in composites, catalysis, sensors, and sorption applications.s intensity the height of the tunnelling barrier decreases and the shape of the barrier changes qualitatively. First calculations have given hints that in this regime the tunnelling times become relevant again and may be determined experimentally.