Scientists have long known that understanding how charged particles move through solid materials is critical to building better batteries. Until now, they have lacked a reliable way to watch it happen.
A research team led by scientists at the University of Chicago Pritzker School of Molecular Engineering and Delft University of Technology in the Netherlands has developed a new technique that reveals ion movement inside solid materials in far greater detail than previous methods allowed. Their findings were published in Nature Communications.
According to Phys.org, the researchers used what they call a tracer exchange technique, which works similarly to the isotope tracking methods scientists use to follow individual atoms through chemical reactions or biological processes. By tracing the paths of sodium and lithium ions moving through solid lithium iron phosphate, the team mapped out what actually happens inside a battery material at the nanoscale.
What they found complicated the standard picture. Some ion movement matched the traditional model of diffusion, known as Fickian diffusion. But the team also found regions where ions were forced to travel in single file through one-dimensional channels, regions where chemical reactions or structural changes in the material accelerated ion movement, and other dynamics that existing models had not accounted for.
"When we think about diffusion processes in liquids, you drop ink in water and then you see how the ink will spread," said UChicago PME associate professor Chong Liu. "There are a lot more constraints in solids than in liquid environments, so diffusion is not as well-studied."
Graduate and co-first author Gangbin Yan, who completed his Ph.D. at UChicago PME in 2025, explained why older models fall short. "Those assumptions break because the self-exclusion and the cross-channel hopping are different for highly confined materials," Yan said. "You cannot describe this just using the traditional diffusion model."
TU Delft professor Marnix Wagemaker, a co-corresponding author on the study, said the findings open new questions about a process that researchers thought they understood. "This work demonstrates that something as apparently well-known and described as diffusion of ions is much more intricate and can transition between different modes," Wagemaker said. "These fundamental insights as well as the methods developed to establish this represent building blocks for better understanding of these processes that are relevant for materials for electrochemical energy storage and conversion."
The research team also included scientists from MIT and the University of Illinois at Urbana-Champaign. Co-first author Pierfrancesco Ombrini, a Ph.D. candidate at TU Delft, said the technique could help researchers distinguish between different types of limitations inside battery materials. The team believes the tracer exchange method could be applied well beyond batteries, with potential uses in designing electronics, chemical catalysts and membranes built to filter pollutants or recover valuable materials from water.
