Nano-scale research and technology leads to advanced materials development

Your next long-distance flight might be on an aircraft made almost entirely of plastic.

Yes, it’s true. But before you begin booking your next trip on a train, consider the science that’s gone into materials development and the role spectroscopy has played.

Models confirmed by Tip-Enhanced Raman Spectroscopy (TERS) and Atomic Force Raman Microscopy (AFM Raman) explorations helped to confirm key models and hypotheses with innovative research techniques, sophisticated instrumentation, and inventive minds.

Alexei Sokolov holds a Governor’s Chair position at the University of Tennessee and Oak Ridge National Laboratory (ORNL), where he leads the Soft Materials and Membranes group. ORNL is close to Knoxville Tennessee.

He noted conventional TERS technology can easily investigate a material from the bottom, and this was the direction taken by most groups in the world. But this technology works only if the sample and its substrate are transparent.

“We said, well, for most nanostructures, you usually have a non-transparent substrate like silicon, or some metal substrate, and samples might not be transparent either. So, looking from below will not work, and looking from above, you cannot see what happened under the tip,” Sokolov said.

It was a matter of turning the problem on its side.

“The only way to look effectively in this case is to look from the side,” he said. “At that time, when we did it in the early 2000s, people were very skeptical because it's very unusual optics. With a normal microscope, you have to look from below or from the top, and we said, ‘no, let's look from the side.’ We did some simulations and we did some analysis and we demonstrated that, yes, you can work like that. And after that technology started to emerge, companies like HORIBA and others developed the technology. So in that respect, I believe we took a risk, and we got funding from the National Science Foundation for that. We took a risk, we put in our efforts and we demonstrated that, yes, that's a way to go.”

When you look at the materials on the nanoscale level from the side, you can see the chemistry, crystallographic structures, and many other parameters, which Raman and fluorescence spectroscopy can reveal.

He and his team looked at solid electrolyte interfaces (SEI) in Li ion batteries. Because when you cycle all our batteries, these will form very thin layers of electrochemical reactions on electrodes. This was the key to protect battery against failure later, because this thin layer stops the electrochemical reaction, and then the battery is stable. Researchers suspected that this SEI is formed from heterogeneous regions with different chemistries, but the size of this heterogeneity was not clear, and no technique could analyze this length scale.

“And that's what we did,” Sokolov said. “With the AFM Raman experiment, we had to prepare every sample and do all the TERS measurements in a glove box, because once the samples are exposed to air, the SEI layer will be damaged.

These were challenging measurements, but for the first time, researchers demonstrated the SEI heterogeneity with the scale of ~ 20 - 30 nanometers, consisting of different chemistry.”

Why is that important? Well, for one thing, it informs engineers what happens at the electrode-electrolyte interface, and how to design this interface better.

And that leads to better battery efficiency.

His paper published in 2019 clearly revealed heterogeneous chemistry in SEI layer with characteristic size ~10-30 nm. “Earlier researchers have seen mixture of different chemistries in the SEI. And the suspicion was from the beginning that it's not a homogeneous mixture for these chemistries. There will be one chemistry here, another chemistry there, but there was no experimental technique that could resolve this nanoscale heterogeneity. Using TERS, enabled us to reveal this heterogeneity and to estimate its characteristic length scale.”

This example demonstrates the power of AFM Raman/TERS technology in analysis of chemistry in a tiny location, and in nanoscale heterogeneity in this chemistry.

How does the knowledge of that heterogeneity extend to better design? His group didn’t go in that direction. His arena was basic research, and he achieved that mission.

“As you know, in United States, to do science, you need funding. So, because I don't have specific funding for these kinds of research, we did not go any further, except that we analyzed some Silicon-based electrodes also when we discovered some totally new kinds of chemistry, which people did not expect existed. But with this particular SEI layer analysis, we just demonstrated that we can do it. The next logical step would be to use different electrolytes and analyze these to show how layers are formed or what is formed by using different electrolytes.”

And that will help to develop better batteries in the sense that makers can focus on forming most efficient SEI layers. If they know how this layer is formed and what this layer is, it will inform design.

In fact, researchers in Sokolov’s group are working on electrolytes and developing solid state batteries. There’s been a good amount of success in this endeavor. Sokolov believes that we will soon have commercial solid state batteries. Prototypes already exist. He says it will store twice more energy per kilogram, meaning that your electrical car can run two times longer per single charge.

And this is very timely development for Tennessee, because the electric Ford F150 Lightning will be produced here in addition to already existing Volkswagen. Also Microvast is looking to build “Giga-factory” not far from Knoxville.

What’s the significance of solid state battery? It’s different than conventional batteries because a solid state battery doesn’t have liquids. Instead, it features solid electrolytes. All these liquid electrolytes are flammable, toxic and can leak, leading to many safety concerns with conventional batteries. Replacing liquid electrolytes with solid electrolyte will provide much safer battery with higher energy density.

Sokolov also uses AFM Raman also to analyze nanostructures and heterogeneity in nano-composite materials. “We use a lot of plastics polymers and to make them strong we usually put some nanoparticles in them. You want to see how all these particles are arranged and how they change properties of the polymeric materials around them. And they usually change properties only on scale of a few nanometers. And that's what we also trying to analyze,” Sokolov said.

“For long time, there was a discussion that this interfacial layer propagates for one hundred nanometers or even more into the polymer matrix. Our studies clearly demonstrate that this interfacial layer is only a few nanometers thick.”

These nanocomposite materials surround us in everyday life, starting with car tires and many interior and exterior parts. “We are working for example now on doing carbon fiber reinforced plastics. And we are trying to make them also recyclable to minimize the effect of the planet pollution by plastics.“ Adding nanoparticles to a polymer strongly improves its mechanical properties. They become comparable to traditional metal strength while significantly reducing the weight.

A good example is the Boeing 787 Dreamliner. Eighty percent of it by volume is constructed with nanocomposite plastics. The wings and body of this airplane are plastic and much lighter than if it was made using aluminum, but are as strong as aluminum alloys. That means the airplane is much lighter. It consumes less fuel, a significant cost savings. And it will travel farther, and carry more cargo and passengers.

Wind turbines serve as another example of nanocomposite polymers. If the blades of the turbines were made of metal, they would be much heavier, and less efficient in producing electricity. Blades made of plastic reinforced with nanomaterials are lighter and produce energy more efficiently. It adds strength to the blades.

“For nanocomposites, we now know well how to predict what will happen with materials properties when I put particular nano-fillers in a particular polymer,” Sokolov said.

So skip the train, enjoy the reduced fares, and know that nanocomposite materials developed by scientists will have even broader use in many technologies and everyday life, and will be recyclable to enable sustainable future.

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