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Structural engineering of 2D-layered energy materials

(Nanowerk Spotlight) Two-dimensional (2D) energy materials have outstanding physical and chemical properties in contrast to their bulk counterparts. This is particularly true for charge storage devices such as lithium-ion batteries and supercapacitors.
Unfortunately, when directly applying these 2D nanostructured materials for energy storage, there is still a significant challenge as they may have serious self-restacking leading to decreased active surface areas and sluggish ion transport kinetics.
An international team of researchers has now developed an effective interlayer engineering strategy to improve sodium ion transport in 2D nanosheets via controlled organic intercalation. They designed an effective interlayer engineering strategy to improve the sodium ion transport in 2D nanosheets via controlled organic intercalation.
This interlayer engineering strategy shows the capability to optimize the intercalation chemistry between VOPO4 and sodium ions, and significantly reduces the sodium ion diffusion energy barriers and thus improves the sodium ion transport and storage properties.
As the scientists reported in Nano Letters ("Effective Interlayer Engineering of Two-Dimensional VOPO4 Nanosheets via Controlled Organic Intercalation for Improving Alkali Ion Storage"), they used VOPO4 nanosheets as a model material to demonstrate this idea and studied triethylene glycol and tetrahydrofuran as intercalants for effective tuning of the interlayer distances of VOPO4 nanosheets, leading to reduced energy barriers of sodium ion transport and thus improving the overall sodium ion storage and transport properties.
Illustration of the interlayer engineering strategy in VOPO4 nanosheets
Illustration of the interlayer engineering strategy in VOPO4 nanosheets, the simulated energy barrier profiles and the overall sodium ion storage properties. (Image: UT Austin) (click on image to enlarge)
"The general interlayer engineering strategy reported in this work will bring a unique perspective in structural design of energy storage electrode materials for enabling future generation of large-scale energy storage systems beyond Li+-based energy storage devices," Professor Guihua Yu at the Texas Materials Institute, University of Texas at Austin, tells Nanowerk.
Previous reports have shown poor sodium ion storage properties in layered VOPO4 bulk materials and improved sodium ion storage properties in ultrathin VOPO4 nanosheets.
However, the long-term stability and rate capability especially at high rates of VOPO4 nanomaterials are not quite outstanding due to the self-restacking of the exfoliated VOPO4 nanosheets.
"To address this issue, we proposed the effective interlayer engineering strategy by intercalating organic molecules to expand the interlayer distance of VOPO4 nanosheets," explains Yu. "VOPO4 has been explored as host materials for studying intercalation chemistry involving various organic molecules, and it shows a large degree of control in the interlayer distances. We demonstrated in this work the intercalation method is a novel and effective approach to solve the serious self-restacking issue of 2D nanosheets and improve their ion storage properties for those beyond-lithium ions such as Na+, K+, Mg2+, Zn2+ and Al3+;"
As the team demonstrates, when the interlayer distance is expanded by intercalating small organic molecules TEG and THF, more interlayer surfaces are activated for ion intercalation and storage. The figure below shows typical kinetic analysis to characterize the intercalation chemistry between the host materials and charges.
Because of the optimized intercalation chemistry between VOPO4 and sodium ions, and the decreased energy barriers for sodium ion diffusion, the intercalated VOPO4 nanosheets show much improved sodium ion transport kinetics and greatly enhanced rate capability and cycling stability for sodium ion storage.
Kinetics analysis of the 2D TEG intercalated VOPO4 nanosheets for sodium ion storage
Kinetics analysis of the 2D TEG intercalated VOPO4 nanosheets for sodium ion storage. a) CV curves at various scan rates for TEG intercalated VOPO4 nanosheets. b) b-value evaluation using the relationship between peak current and scan rate. c) Separation of the capacitive and diffusion currents at scan rate of 0.5 mV s-1. d) Contribution ratio of the capacitive and diffusion-controlled charge at various scan rates. (Image: UT Austin) (click on image to enlarge)
The team already is exploring different organic molecule intercalation into VOPO4 nanosheets to tune their physical and chemical properties. They also aim to extend this general interlayer engineering strategy to other materials system.
"We are taking this research direction even further by looking into the transport and storage properties for beyond-Li alkali ions, such as Mg2+ and Zn2+," concludes Yu. "An important challenge is to optimize the interlayer distance of the intercalated VOPO4 nanosheets, which may enable fast ion diffusion through the VOPO4 layers, as well as keep searching for optimal intercalants, which provide best possible interlayer distance and are stable in the VOPO4 layers during the divalent ion transport (Mg2+ and Zn2+) and cannot restrict the ion diffusion."
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk
 

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