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Posted: Aug 08, 2011
Energy storage: Nanodiscs stack up
(Nanowerk News) Two-dimensional materials such as graphene have proved invaluable to battery researchers because their sheet-like surfaces can absorb large amounts of charge. Sheet-like transition metal chalcogenides in particular have recently emerged as attractive energy storage materials because the sheets formed by such compounds can readily intercalate lithium ions. Jinwoo Cheon and colleagues from Yonsei University and Seoul National University in Korea have now discovered a way to turn the chalcogenide zirconium disulfide (ZrS2) into two-dimensional nanoscale discs that can boost lithium charge–discharge capacities by over 200% relative to bulk samples ("Ultrathin Zirconium Disulfide Nanodiscs").
Schematic illustration of ZrS2 'nanodiscs' that can stack together and intercalate high densities of lithium ions.
Taking ZrS2 from a bulk, layered material down to nanoscale sheets can lead to increased energy storage by opening up more surface area and enhancing the diffusion of lithium ions. However, peeling apart these strata is challenging because of the unstable, 'dangling' bonds that dot the edges of the two-dimensional surface.
Cheon and his co-workers took a colloid-based approach to resolve this problem. By confining ZrS2 into two-dimensions through the use of a surfactant called oleylamine, the researchers were able to isolate the compound as thin, circular sheets (see image). Transmission electron microscopy showed that this simple procedure gave single-crystalline ZrS2 discs with an average diameter of 20 nm. Prolonging the reaction time enabled the team to produce larger, 60 nm discs while retaining an ultrathin width of 1.6 nm.
Although the researchers initially isolated the ZrS2 nanodiscs as a colloidal dispersion, they soon found that immersing the materials in a polar solvent provoked a unique self-assembly reaction. Attractive forces between the protective surfactant molecules caused the discs to stack up into well-defined cylinders, reminiscent of a roll of coins.
Between each stacked disc sat a relatively empty gap of 1.5 nm — perfect for accommodating a variety of molecules and ions, according to Cheon. Using the layered substances as anodes in lithium-ion batteries revealed a pronounced nanoscale effect: smaller discs held more ions and had quicker charge–discharge rates than wider discs, a critical finding for the development of next-generation batteries.
"The multiple layers of our nanodiscs make them extremely useful for energy storage, catalysis and separations," says Cheon. The team's next goal is to extend the versatility of their system by exploring solar energy conversion applications.