Anion rotation found to boost lithium-ion movement in solid-state batteries

Researchers have identified a new way to improve the performance of solid-state batteries, which eliminate the explosion risk by replacing flammable liquid electrolytes with solid ones. The key finding: anions rotating in place inside a solid electrolyte help accelerate the movement of lithium ions.

The Korea Basic Science Institute announced Monday that a research team led by Dr. Lee Young-ju of its Strategic Research Division for Intelligent Secondary Batteries had uncovered the mechanism by which anion rotation and lithium-ion migration are coupled inside solid electrolytes.

Solid-state batteries improve safety by using solid electrolytes instead of liquid ones, which carry a fire risk. However, lithium ions have difficulty moving freely through solid materials, making higher ionic conductivity a central challenge in boosting battery performance.

The team focused on borohydride (BH4-) anions contained in argyrodite-type solid electrolytes, which provide pathways for lithium-ion movement. Borohydride consists of one boron atom bonded to four hydrogen atoms and can rotate in place — changing its orientation — even within the crystal structure of a solid electrolyte.

Earlier research had shown that adding borohydride could raise ionic conductivity, but the precise effect of anion rotation on lithium-ion movement had not been clearly established.

The team used solid-state nuclear magnetic resonance spectroscopy and molecular dynamics simulations to analyze the correlation between the two types of motion.

As a result, they found that the repeated coupling of borohydride rotation with lithium-ion migration promotes the long-range movement of lithium ions.

Particularly telling was a simulation in which restricting borohydride rotation caused lithium-ion diffusivity to drop by nearly half. The result confirmed that anion rotation is not merely incidental movement but a significant factor in facilitating lithium-ion transport.

The positioning of anions also affected lithium-ion movement. When borohydride occupied two distinct sites within the crystal structure simultaneously, the available space for lithium-ion migration became more evenly distributed, allowing ions to move more smoothly from one site to another.

The study is significant for proposing a new design principle: developing solid electrolytes requires considering not only their chemical composition but also the arrangement and motion of internal anions.

The findings are expected to help researchers more efficiently develop high-performance solid electrolytes with fast lithium-ion transport and reduce trial and error in the materials design process.

"This research establishes how the rotation of surrounding anions inside a solid electrolyte assists lithium-ion movement," Lee said. "It can serve as a new design criterion for improving the ionic conductivity of solid electrolytes for all-solid-state batteries."

The findings were published in Small, an international journal in the field of materials science.