For more than 20 years, the Computational Chemistry and Materials Science (CCMS) internship program at Lawrence Livermore National Laboratory (LLNL) has offered students the opportunity to develop and apply computational methods to predict, analyze and optimize the properties of materials for a broad range of applications.
"Many of our interns come back to LLNL as a postdoc and later become a staff scientist, while others become professors or researchers in industry," said LLNL staff scientist and student mentor Tuan Anh Pham. Pham is a group leader in the Materials Science Division and was once a CCMS summer intern himself. "The program not only brings students here to work with us, but we connect them with external collaborators and lecturers who represent many different areas of materials science, showing them a broad view of the field and all of the different research possibilities."
Carbon nanotube defects
Among this year's 15 CCMS interns was Golam Azom, a Ph.D. student studying chemistry at Louisiana State University. For his LLNL summer research, Azom studied the transport physics of ions and protons through nanofluidic channels such as carbon nanotubes. These tiny, cylindrical structures made of carbon atoms have applications in different areas, including water purification and chemical separation.
Specifically, he looked at how potassium ions and protons move through these carbon nanotubes and what happens to the material's transport properties when surface defects are introduced. These surface defects, which include chemical compounds on the outside of the nanotube, can affect the channel's ability to open or close (pore gating).
Earlier experiments showed that adding defects to carbon nanotubes reduces the movement of potassium ions but does not affect proton movement. Azom's work focused on performing computer simulations to model and explain these experimental observations at the molecular level.
He found that the defects make the tubes want to stay closed, blocking the potassium ions; however, the protons could still get through because they move differently, hopping from one water molecule to another instead of needing to flow through the channel.
"In our simulations, we also observed that opening the pore requires a significant amount of energy," Azom said. "Without this energy, the pore wants to stay closed, and the potassium ions can't get through."
Even though this research came with challenges and learning curves, Azom said: "What I love about LLNL is that the people are always ready to help and they're full of ideas about what to try next - it's been a really great experience."
Lithium-ion battery defects
Like Azom, Rachel Gorelik, a Ph.D. student studying materials science and engineering at Arizona State University, also studied defects for her CCMS summer research, but with a focus on lithium-ion batteries.
Lithium-ion batteries are a common type of battery composed of a cathode, an anode and a liquid electrolyte between them.
"The cathode and electrolyte can sometimes react and form something called a CEI, which is a cathode electrolyte interphase," Gorelik said. "The reason that we care about this is because its composition can significantly affect battery performance."
Over the summer, Gorelik studied a specific cathode type made of lithium-nickel-oxide and an ethylene carbonate electrolyte. Using LLNL's high-performance-computing resources, she modeled how different defects affect the battery's rate of electrolyte decomposition, studying a total of eight defect combinations.
For example, lithium vacancies, which are formed during delithiation, were introduced into the battery structure. While delithiation is a normal process of how a battery operates - i.e., charging and discharging - the presence of these empty spaces or "vacancies" on the cathode surface can affect how the electrolyte decomposes to form the CEI, and thus how the battery performs.
Gorelik also simulated how manganese and cobalt substitutions on the surface (replacing some of the original nickel atoms) influence electrolyte decomposition.
"Manganese is often seen as a stabilizer on the surface, but we also want to know if it could have another effect, such as making the electrolyte more likely to decompose," she said.
Compared to her Ph.D. research, which is typically quite broad in terms of materials discovery, Gorelik's LLNL research allowed her to zero in on a new area of energy research that she hadn't studied before.
"Here [at LLNL], I get to focus on just one material system," she said. "I get to know it really well and study it in precise detail, which allows me to truly understand the kind of direct, real-world applications of this specific battery system."
-Shelby Conn
