Professor David Cahill (left) and graduate student Rosy Huang (right). Credit: The Grainger College of Engineering Materials Research Laboratory
Researchers from the University of Illinois Urbana-Champaign have demonstrated how to study lithium-ion battery cells utilizing the Peltier effect. The findings could influence future lithium-ion battery design by allowing researchers to measure the entropy of the electrolyte within a battery.
“Our work is about understanding the fundamental thermodynamics of dissolved lithium ions, information that we hope will guide the development of better electrolytes for batteries,” said David Cahill, a University of Illinois materials science & engineering professor. “Measuring the coupled transport of electric charge and heat in the Peltier effect allows us to deduce the entropy, a quantity that is closely related to the chemical structure of the dissolved ions and how they interact with other parts of the battery.”
Despite being well studied in solid-state systems, the Peltier effect remains unexplored in ionic systems because the differences created by Peltier heating and cooling are smaller than other effects. To overcome the challenges presented by these small changes, the researchers employed a measurement system that could resolve one hundred-thousandth of a degree Celsius. With this, the team calculated the entropy of the lithium-ion electrolyte in the cell by measuring the heat at either end.
“We’re measuring a macroscopic property, but it still reveals important information about the microscopic behavior of the ions,” said Rosy Huang, a graduate student in Cahill’s research group. “Measurements of the Peltier effect and the solution’s entropy are closely connected to the solvation structure. Previously, battery researchers relied on energy measurements, but entropy would provide an important complement to that information that gives a more complete picture of the system.”
In the study, published in Physical Chemistry Chemical Physics, the team documented how Peltier heat flow changed with the concentration of lithium ions, electrode material, solvent type, and temperature. Additionally, in all observations, the Peltier heat flow was opposite the ionic current in the solution.
“An underappreciated aspect of battery design is that the liquid electrolyte is not chemically stable when in contact with the electrodes,” said Cahill. “It always decomposes and forms something called a solid-electrolyte interphase. To make a battery stable over long cycles, you need to understand the thermodynamics of that interphase, which is what our method helps to do.”