Rechargeable batteries—most notably the lithium-ion batteries used in smartphones—are a foundational technology that underpins modern society. Among them, all-solid-state batteries are attracting significant attention as a next-generation energy-storage device expected to combine enhanced safety with high energy density. However, a longstanding challenge is that the ion migration, which is essential for charging and discharging, is much slower inside solids than in liquids, limiting further performance improvements.
Over the past decade, numerous solid electrolyte materials have been developed. Remarkably, the best-performing fast-ion conductors share a common feature: when one ion moves, it triggers the sequential motion of neighboring ions, creating a chain reaction similar to falling dominos. This phenomenon, known as concerted ion migration, allows many ions to participate efficiently in conduction and simultaneously lowers the energy barrier for ion migration—making it a key mechanism that directly governs the performance of solid-state battery materials.
In this study, the research team performed molecular dynamics simulations of solid electrolytes and succeeded in visualizing concerted ion migration as it actually occurs during the simulations. This was achieved using a newly developed directed-graph analysis, which connects the displacement vectors of individual ions to reveal concerted migration pathways. The team further demonstrated that the ionic conductivity calculated from these directed graphs matches the results obtained from rigorous statistical-mechanical formulas. This discovery provides a direct picture of “how many ions move together” during conduction and establishes a new theoretical framework that quantitatively links collective migration with ionic conductivity.
These findings offer a fundamental physicochemical basis for controlling concerted ion migration and improving battery performance. The approach is expected to contribute to the rational design of next-generation solid electrolytes with higher ionic conductivity.
Time evolution of concerted ion migration in 2D ion conductor, Na2Ni2TeO6, visualized by directed graph and calculate ion conductivity.
Papers
Journal: Chemistry of Materials
Title: Visualizing Concerted Ion Migration of Superionic Conductors via Directed Graphs
Authors: Ryuhei Sato*, Yasunobu Ando, Kartik Sau, Yasushi Shibuta
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