Research looks to better methods of generating, storing power
Cooperative research may lead to researchers discovering ways to improve lithium ion batteries.
Rutgers invited Jianmin Qu to speak at the Fiber Optic Materials Research Building on Busch campus last Wednesday about his research on structural fracturing in multi-chemical batteries.
Normally, lithium batteries use graphite to create energy, but Qu’s research focuses on replacing it with silicon.
“The idea is that having a second chemical changes the behaviors of the host chemical,” said Qu, a professor at Northwestern University. “You are dealing with a mixture that has its own mechanical properties.”
His objective is to figure out the relationship among chemical reactions, diffusions and mechanical stress, he said. Their interactions change the mathematics of designing batteries considerably.
Qu believes if researchers do not improve methods for generating and storing electricity, nuclear energy will be the only alternative to gasoline.
He said the governing equations relating these properties include continuity equations of the host chemical and a slightly altered diffusion equation. The diffusion equation is tailored to represent stress-dependency.
“The diffusion that occurs in a battery causes concentration change, which causes volumetric change, which induces stress, and then stress influences diffusion,” he said.
He said the efficiency of fuel cells becomes so sensitive to stress that if they are not designed properly, they could stop working completely. This occurs when the rate of chemical reaction cannot compete with the rate of diffusion.
Stress, in their research, results from the volumetric expansion of combining lithium and silicon, he said. Silicon expands about four times in volume in the presence of lithium. Trying to operate the battery with this stress results in structural fracturing in the positive side of the battery.
“This is a technology problem that a lot of people have just started working on,” he said. “One precaution taken to prevent fracture is to handle the mixture so as to release the mechanical stress.”
Increasing the temperature for a uniform material is a way to expand volume without stress, Qu said. But if the temperature is not uniformly distributed, or the material is contained in some way, it could still create stress.
“We found one problem that we couldn’t use traditional framework to explain,” he said. “Where one expected a gradual decrease in lithium concentration, we found a core-shell structure.”
This core-shell structure involvs a concentration in the core that is nearly fully saturated with silicon and a shell mostly comprised of lithium, Qu said.
“It is clear that diffusion-driven processes aren’t happening here,” he said. “There is no equal distribution.”
Currently, no truly reliable model exists to determine how stress changes in the presence of a second chemical, he said.
He said their model divides the traditional core-shell model into three zones: one with the host chemical, one with the secondary chemical and one in which the chemical reactions and diffusions occur.
Their mathematical model includes ways to predict electron pathways in the presence of fractures as well, Qu said. This equation applies elastic strain energy to all of the traditional equations.
“The lithium ion battery is the chemistry of choice today — we don’t have any better battery in terms of energy density,” he said.
He said the ability to store energy is limited by the ability to store ions in the device. Of all the known materials now, silicon has the best storage for lithium ions.
George Weng, a professor in the Department of Mechanical and Aerospace Engineering, said multidisciplinary research yields innovation and productivity. A downside is that each member of a multidisciplinary group must be in the forefront of their discipline.
Assimina Pelegri, a professor in the Department of Mechanical and Aerospace Engineering, appreciates how Qu’s research is multidisciplinary.
“The job that he’s doing — micromechanics and continuum mechanics — is one of the important ones because people have only been looking at it from a chemistry perspective,” she said. “These ideas have only been developed in the last 5 to 10 years.”