posted January 28, 2013
A trio of Aerospace scientists has developed highly successful methods for growing graphene, characterizing it, and applying it toward reducing the weight of space batteries.
“Graphene is a really unique material because it comprises a single layer of carbon atoms,” said Dr. Gouri Radhakrishnan, a senior scientist in the Materials Science Department. “It has some exceptional properties, which make it useful for applications in electronics, in batteries, and in solar cells.”
Of particular interest is the fact that graphene could be used as the negative electrode (the anode) in lithium-ion batteries.
Many people are familiar with lithium-ion batteries in the form of coin cells, which are used in cameras and watches. The anode is usually made of carbon, and graphene, which is a very light form of carbon with a large surface area, is a good candidate for an anode.
“Our goal was to make high-quality graphene and see if we could convert it into a low-mass battery anode,” Radhakrishnan said.
Radhakrishnan has headed the project and the growth of the material, while Paul Adams has been instrumental in characterizing it through transmission electron microscopy, and Dr. Joanna Cardema has been evaluating the material in lithium-ion test cells.
Naturally, there are some difficulties in making graphene, which is 10,000 times thinner than a human hair. The previous method for making graphene was not ideal.
“When it was originally fabricated in 2004, the scientists took a piece of graphite and they peeled it sequentially layer by layer by layer with Scotch tape,” Radhakrishnan said.
The process worked — in fact, it led to a Nobel Prize in 2010. However, it was very tedious and generated only small areas of graphene.
“This is an excellent technique to investigate the fundamental properties of graphene as it produces a single crystalline orientation (or grain) of graphene, but it is not suitable for large-area applications,” Radhakrishnan said. “When we started this two years ago, what we wanted to do was come up with a novel method that would not only be scalable, but also hopefully give us large grains.”
Large grains of graphene would be better because, as Radhakrishnan explained, the larger the grains, the fewer the grain boundaries and hence the fewer the defects in the material.
Radhakrishnan used a method called chemical vapor deposition (CVD), which uses heat to break up a chemical compound.
While CVD is not new, the process developed at Aerospace uses methanol as the source, and this resulted in excellent graphene. In this method, Radhakrishnan used argon gas, which is inert, to transfer methanol from a bubbler into a 1050° C furnace.
This process, which has a patent pending, is much simpler than the tape peeling method, and can be used to create large areas of graphene. Methanol is also inexpensive, readily accessible, and safe.
Unlike other CVD methods used for graphene, this method does not employ hydrogen as one of the process gases, which offers a huge safety advantage.
“All of the process parameters were developed by us. There was nothing at all in the literature that said that methanol could lead to graphene,” Radhakrishnan said.
Using transmission electron microscopy, Adams, a senior scientist in the Materials Science Department, measured the orientation of the graphene lattice from which the size of a single crystalline grain could be determined.
“We’ve been successful in fabricating a single layer of graphene with a significant increase in grain size, which was 10 to 30 times larger than what other people were doing at the time,” Radhakrishnan said.
The Aerospace scientists discovered they can also create one layer or multiple layers of graphene by changing the substrate, which is the material upon which the graphene is deposited.
Radhakrishnan explained that when making coin cells for lithium-ion batteries, the substrate has to be conductive. Therefore, the scientists deposit the graphene directly onto copper or nickel substrates. No binding material is used, eliminating another source of weight.
Using their method, the team has successfully created a graphene anode that is stable, lightweight, and has a measurable high capacity per weight. Cardema, a senior member of the technical staff in the Energy Technology Department, has been testing the electrochemical performance of the coin cells with graphene-based anodes.
“Although small, when we scale the measured capacity of our graphene cells to the capacity of commercial graphite anodes … our simple graphene anode is at least 200 times lighter,” Radhakrishnan said. “This makes us hopeful that we can make further improvements towards better batteries.”
Although the team has achieved impressive preliminary results in the area of batteries, their research also has broader implications.
“As a result of our research, we are developing all these scientific techniques that are enabling graphene applications,” Radhakrishnan said. “While our focus has been on anodes, what we’re really doing is growing high-quality graphene and trying to also push the envelope to hybrid materials so that they can be used for any of the other applications.”
The team is looking forward to how their research can be further improved and used in the future.
“Our research and expertise in graphene technology is really enabling graphene applications and we would like to be in a position—a strong position—in this very quickly developing field to be able to suggest ways that contractors could implement this technology into space systems when it is actually mature for insertion,” Radhakrishnan said.