N.C. State researchers are one step closer to making more efficient lithium-ion batteries. 

Textile Engineering, Chemistry and Science Assistant Professor Philip Bradford and Fiber and Polymer Science Associate Professor Xiangwu Zhang led a team of researchers who are seeking to improve battery performance.

By varying the mix of graphite, lithium and silicon, researchers found they could increase the power-absorbing lithium with additional silicon.

“There’s tons and tons of battery research going on in different areas: Anode structures, cathode structures and separators—how do you design the actual circuits for the batteries?”  Bradford said. “And,so, ours is just kind of a small piece of that pie, but it all goes towards creating more efficient, better batteries that can hold more energy. That’s our goal, even though we’re pretty far from it.”

The lithium capacity of a battery, as well as the cycle life – the total number of times you can charge a battery and still have a useable capacity – are the most important properties to look at when examining battery efficiency, according to Zhang.

“It can hold ten times the amount of lithium per weight basis, so for every gram of silicon it can hold ten times the amount of lithium that every gram of carbon can hold,” Bradford said.

One drawback of making these high-energy capacity batteries is their instability, according to Bradford and Zhang.

“The problem with silicon is it doesn’t last because of the energy,” Zhang said. “When you charge a battery, you allow lithium into that battery and it’s going to expand. So this one has very high energy and expands by 400 percent…if it’s rubber material, like a rubber band it’s okay, but silicon is very brittle material. So basically the structure fails.”

Silicon is not conductive, which presents a problem for battery designers.

“It has to be touching something that’s electrically conductive. So you can mix it with carbon or you can coat it with carbon,” Bradford said. “You can do lots of different things and our approach is to use what are called ‘carbon nanotubes,’ w0hich are basically extremely small diameter fibers made of carbon,” Bradford said.

The composition is similar to current batteries, which is a mix carbon powder with a binder to form an anode structure. But the difference lies in the “morphology,” Bradford said.  

“The purpose of our research is to figure out how we can combine silicon and carbon together in a composite structure so that we can have an efficient battery structure that has high capacity, but solves the problems that are associated with silicon,” Bradford said.

To work around the problem, Bradford and Zhang created nanotubes with a 30 nanometer silicon outside layer.

“Then in this case, it’s a very thin sheet, like paper, and when it expands and contracts it’s not going to break easily because it’s so thin,” Zhang said. 

To make the nanotubes, the researchers used an iron-based catalyst to form an array, according to Zhang. The sheets then were stacked in a horizontal fashion to connect their electrons.

Bradford said that after they coat the material with silicon, they punch it into a circular shape that fits in the battery and then perform the same procedure with the cathode.  The separator lies in between them, and a special battery press crimps the coins and material together.

 “This material is relatively easy to make, but it is new work,” Zhang said. “It’s a long way to go before it can be produced in the industry scale.”

This type of structure is also expensive to produce, according to Zhang.

Future applications for the research are numerous because the batteries would be able to hold more energy in the same amount of mass, according to Bradford.

In electric vehicles, higher energy storage could reduce the number of times needed to charge the vehicle, Zhang said.  

Solar and wind energy are also not always reliable and require a form of energy storage for days when there is no wind, or minimal sunlight available, according to Zhang

Smart grid technology is another potential application.

“Today’s power grid in the U.S. was built maybe 50 to 100 years ago, so it is very old and there’s no control at all… if there is a storm or something it’s broken very easily,” Zhang said.