Researchers use Common Salt to Develop Better Energy Storage Devices

High-quality energy storage materials can be obtained by growing them with as much surface area as possible. Similar to baking, this process requires an appropriate blend of a specific amount of ingredients prepared in a particular order at a suitable temperature, in order to develop a thin material sheet with completely correct chemical consistency that can be used to store energy. Recently, a research team from Drexel University, Huazhong University of Science and Technology (HUST) and Tsinghua University invented a way to enhance the recipe, increase the size of the resulting materials, and soak up energy by just adding salt.

The findings, recently featured in Nature Communications, highlight that salt crystals used as a template to develop thin sheets of conductive metal oxides allows to materials to become larger and purer chemically. This makes them extremely appropriate to collect ions and storing energy.

The challenge of producing a metal oxide that reaches theoretical performance values is that the methods for making it inherently limit its size and often foul its chemical purity, which makes it fall short of predicted energy storage performance. Our research reveals a way to grow stable oxide sheets with less fouling that are on the order of several hundreds of times larger than the ones that are currently being fabricated.

Jun Zhou, Professor, Wuhan National Laboratory for Optoelectronics, HUST

In energy storage devices, such as a capacitor or a battery, energy is contained while chemically transferring ions from an electrolyte solution to thin conductive material layers. The increasing development of these devices enables them to become smaller and capable enough to hold an electric charge for a longer time without needing a recharge. This enhancement has come from researchers who focus on fabricating materials that are efficiently equipped, chemically and structurally, to gather and disburse ions.

Theoretically, the materials best suited for the job need to be thin metal oxide sheets, because their high surface area and chemical structure enables the ions to be attached easily. This demonstrates the occurrence of energy storage. However, metal oxide sheets fabricated in labs have fallen short of all their theoretical capabilities.

Zhou, Tang and the HUST team point out that the issue exists in the development process of the nanosheets, which requires either a chemical etching or a gas deposition process, often leaving trace chemical residues that contaminate the material and prevent ions from merging with it. Besides this, the materials developed with this method are often only a few square micrometers in size.

When used as a substrate to grow the crystals, salt crystals permit them to scatter and create a bigger sheet of oxide material. This is similar to preparing a waffle, by pouring batter into a pan versus pouring the batter into a large waffle iron. A big, strong product can be obtained by getting the solution, whether a chemical compound or batter, to evenly spread on the template and then stabilize it in a uniform manner.

This method of synthesis, called ‘templating’ — where we use a sacrificial material as a substrate for growing a crystal — is used to create a certain shape or structure. The trick in this work is that the crystal structure of salt must match the crystal structure of the oxide, otherwise it will form an amorphous film of oxide rather than a thing, strong and stable nanocrystal. This is the key finding of our research — it means that different salts must be used to produce different oxides.

Yury Gogotsi, PhD, University and Trustee Chair Professor, College of Engineering, Drexel

Researchers have utilized a wide range of objects, polymers, compounds and chemicals as growth templates for nanomaterials. But this invention highlights the significance of obtaining a template that matches the structure of the material being developed. Salt crystals are considered the suitable substrate to produce oxide sheets of tungsten, molybdenum and magnesium.

The precursor solution is used to coat the salt crystals’ sides as the oxides begin to develop. After solidification, the salt is dissolved in water, resulting in nanometer-thin two-dimensional sheets created on the sides of the salt crystal, and also leaving minimal traces of any contaminants that may be a barrier in the energy storage performance. By developing oxide nanosheets in this manner, the only element that restricts their growth is the salt crystals’ size and the quantity of precursor solution that is used.

“Lateral growth of the 2D oxides was guided by salt crystal geometry and promoted by lattice matching and the thickness was restrained by the raw material supply. The dimensions of the salt crystals are tens of micrometers and guide the growth of the 2D oxide to a similar size,” the researchers write in the paper. “On the basis of the naturally non-layered crystal structures of these oxides, the suitability of salt-assisted templating as a general method for synthesis of 2D oxides has been convincingly demonstrated.”

Based on earlier predictions, the increased size of the oxide sheets also equalized the increased ability to gather and give out ions from an electrolyte solution, which is the final test that decides the potential of being used in energy storage devices. The results that have been reported in the paper highlight that the these materials can be used to develop an aluminum-ion battery capable of storing more charge than the efficient lithium-ion batteries currently used in mobile devices and laptops.

Since 2012 Gogotsi, together with his students in the Department of Materials Science and Engineering, has been working with Huazhong University of Science and Technology to investigate different materials suitable for energy storage application. The lead author of the Nature Communications article, Xu Xiao, and co-author Tiangi Li, both Zhou’s doctoral students, came to Drexel as exchange students to gain insights about the University’s supercapacitor research. These visits started a collaboration, which was supported by Gogotsi’s annual trips to HUST. While the partnership has already produced five joint publications, Gogotsi believes that this work is only beginning.

The most significant result of this work thus far is that we’ve demonstrated the ability to generate high-quality 2D oxides with various compositions. I can certainly see expanding this approach to other oxides that may offer attractive properties for electrical energy storage, water desalination membranes, photocatalysis and other applications.

Yury Gogotsi, PhD, University and Trustee Chair Professor, College of Engineering, Drexel