Salt May Be the Secret for Improving Energy Storage Materials

As researchers look for better ways to store energy, pseudocapacitors (“super-fast batteries”) have proven to be of great interest thanks to their fast charging and discharging, high power density, and excellent cycling stability. The technology is likely to be beneficial to many potential applications ranging from personal (think portable electronics) to nationwide grid energy storage. Typically, transition metal oxides are used in electrodes of batteries and these pseudocapacitors.

Today, scientists are experimenting with growing nanosheets made of two-dimensional atomic crystals that are stacked to form electrodes. The two-dimensional atomic crystals are ideal for energy storage because nearly all of the atoms can be exposed to the electrolyte. However, the oxides used are key to the performance of these nanosheets. Many of the oxides used today don’t perform as well as they could due to their lack of a layered structure. Additionally, efforts to make the sheets larger and achieve theoretical performance have often resulted in chemical contamination that decreases their quality, since ions are prevented from bonding to them.

For optimum performance, energy storage materials should be created with as much surface area as possible. Researchers are experimenting today to get a number of factors right: the materials, the mixture, and the temperatures. The ultimate goal is to produce a thin sheet of material of just the right chemical composition to maximize energy storage. A team of researchers from Drexel University, Huazhong University of Science and Technology (HUST), and Tsinghua University has found a way to make the nanosheets bigger and better at storing energy. Their trick is using salt as a template for growing thin sheets of conductive metal oxide that are optimized for gathering ions and storing energy while remaining chemically pure.

One older method of making oxide sheets just a few atoms in thickness is deposition from vacuum. This method produces good films, but only for a limited number of applications: electronic and some optical purposes, as a single film of oxide is produced on a small area substrate and the cost per gram of material deposited is very high, according to Professor Yury Gogotsi of the Department of Materials Science and Engineering at Drexel University and coauthor of the research.

“The methods of making 2D oxide crystals in larger quantities rely on taking layered oxide particles (like clay) and breaking them into single layers,” he told Design News. “This usually requires intercalation (insertion of atoms and molecules between oxide layers to weaken bonds between them), use of surfactant, and intense ultrasonic treatment, which breaks oxide sheets into very small pieces, invisible to naked eye. Not all oxides we need are available in layered structures, so the method is limited to a few layered solids.”

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In addition, with more traditional methods of production, contaminants in the resulting product hinder energy storage with the sheets, so his team has been looking for a better way to make a purer product.

“Control over the chemical composition is critical in many applications, especially energy storage, as impurities may lead to parasitic electrochemical reactions and limit the voltage window (energy stored) and decrease the lifetime of batteries,” said Gogotsi. “Two-dimensional materials have a large surface area, so surfactants and other impurities stick really well to their surfaces. These may be difficult to remove and may affect their properties.”

Using salt, the research team has been able to grow stable oxide sheets that are several hundreds of times larger than with more traditional processes, and with less fouling. When salt crystals are used as a substrate for growing the crystals, they are better able to spread out and form a larger sheet of oxide material. The result is a material that is simply better at collecting and disbursing ions, thereby improving energy storage.

The oxides crystals produced by this process are unlikely to be large in lateral dimensions: they are flakes only several micrometers across, and very thin. But for batteries and other applications, large particles aren’t needed, according to Gogotsi. It is, however, critical to get the salt composition right so it can act as the perfect substrate for growing oxide sheets of magnesium, molybdenum, and tungsten.“With the new process, it’s critical that the salt crystal lattice match that of the oxide, so that oxide can grow on salt,” Gogotsi said. “So, different salts have to be selected for growing different oxides. Otherwise, no crystals will grow -- only thin amorphous films.

With new transition metal oxides that are nanometer thin, lithium and the ions can diffuse between the layers faster than in bulk crystals, resulting in faster charging of batteries. In the article detailing the research, the authors indicate that their method could be used to create an aluminum-ion battery that could store more charge than the best lithium-ion batteries that are used today in mobile devices.

The team’s work is targeting energy storage applications, but Gogotsi notes that catalysis, optical coatings, and other applications should be explored, as well.

The team's findings were published in the journal, Nature Communications.

[image via Mister GC/freedigitalphotos.net]

Tracey Schelmetic graduated from Fairfield University in Fairfield, Conn. and began her long career as a technology and science writer and editor at Appleton & Lange, the now-defunct medical publishing arm of Simon & Schuster. Later, as the editorial director of telecom trade journal Customer Interaction Solutions (today Customer magazine) she became a well-recognized voice in the contact center industry. Today, she is a freelance writer specializing in manufacturing and technology, telecommunications, and enterprise software.