・ We have theoretically elucidated the molecular-level mechanism by which lithium ions migrate within organic ionic plastic crystals, which are attracting attention as solid electrolytes for next-generation batteries.
・ We verified the "paddlewheel mechanism"--which has been regarded as the primary mechanism of ion conduction for over 25 years--using molecular dynamics simulations and hopping function analysis.
・ The results showed that while the "paddlewheel mechanism" was related to the movement of large ions constituting the crystal, it had almost no correlation with lithium-ion movement; instead, lithium-ion movement is governed by the cooperative rearrangement of "ion cages" formed by surrounding anions.
・ These findings provide new guidelines for designing safe, high-performance solid electrolytes for next-generation batteries at the molecular level.
Summary
An international collaborative research group led by Professor Bong June Sung of the Department of Chemistry at Sogang University and Professor Shinji Saito of the Institute for Molecular Science (IMS), National Institutes of Natural Sciences (NINS), and the Graduate University for Advanced Studies, SOKENDAI has elucidated, at the molecular level, how lithium ions move within organic ionic plastic crystals (OIPCs)--which are attracting attention as solid electrolytes for next-generation batteries.
In OIPCs, although the material is solid, molecules and ions within the crystal rotate actively in situ; consequently, the "paddlewheel mechanism"--in which this rotational motion pushes lithium ions forward--has long been considered the primary mechanism of ion conduction. In this study, the researchers theoretically re-examined this hypothesis using molecular dynamics simulations on a supercomputer and hop-function analysis, which precisely extracts individual ion jumps. The results revealed that the movement of lithium ions is not directly driven by the surrounding rotational motion, but rather occurs through the cooperative rearrangement of an ion cage formed by the surrounding anions. In particular, we theoretically demonstrated that when the number of anions surrounding a lithium ion temporarily decreases--causing the cage to "open"--the movement of the lithium ion increases significantly.
This study was published online in the Journal of the American Chemical Society (JACS) on June 1, 2026.
1. Research Background
Many lithium-ion batteries, which are widely used in smartphones and electric vehicles, employ liquid electrolytes. However, liquid electrolytes are flammable, and the risk of fire or explosion remains a major concern. To address this issue, research is underway to develop highly safe solid electrolytes as a promising solution.
Among these, organic ionic plastic crystals (OIPCs) are attracting attention as "soft solids"--materials that, despite being solid, allow their internal molecules and ions to rotate actively in situ. Such molecular motion has the potential to facilitate ion migration within the solid, and OIPCs are expected to serve as materials that combine safety with high ionic conductivity.
In lithium-ion batteries, charging and discharging occur as lithium ions move through the electrolyte inside the battery. Therefore, in the development of solid electrolytes--including OIPCs--understanding how and how quickly lithium ions can move within the electrolyte is crucial for designing high-performance, safe next-generation battery materials.
Ion conduction in OIPC has primarily been explained by the "paddlewheel mechanism." In this mechanism, the rotational motion of surrounding large molecules and ions is thought to act like the blades of a waterwheel, pushing lithium ions to their next position. However, it was not fully understood when, where, and how individual lithium ions actually move.
2. Research Results
In this study, we combined molecular dynamics (MD) simulations with hop-function analysis to investigate ion motion within OIPC at the molecular level. Hop-function analysis is a method for extracting and elucidating "jump events"--in which a specific ion moves from one stable position to another--from among a large number of molecules and ions undergoing complex motion. This method has made it possible to analyze ion motion--which was previously treated as an average collective event, such as a diffusion coefficient or correlation function--at the level of individual movement events.
The analysis revealed that the movement of large ions constituting the material's framework is correlated with the rotational motion of surrounding molecules and ions. This result is consistent with the previously proposed paddlewheel mechanism. On the other hand, the jumps of lithium ions--which are directly related to battery performance--showed almost no correlation with the surrounding rotational motion. This indicates that the primary mechanism of lithium-ion conduction cannot be explained by a simple paddlewheel mechanism.
So, what drives the movement of lithium ions? This study revealed that the movement of lithium ions is governed by the cooperative rearrangement of "ion cages" formed by surrounding anions. Lithium ions are typically confined within cages surrounded by multiple anions. However, we discovered that when the anions nearest to the lithium ion--specifically the fourth and fifth closest ones--move cooperatively to swap positions, the old cage opens and, simultaneously, a new cage is formed to accommodate the lithium ion. At the moment this cage opens and closes, the lithium ion escapes from its original cage and moves into the adjacent new cage.
Furthermore, we found that in the "open" state--where the number of anions surrounding the lithium ion temporarily decreases to as few as two--the lithium ion's hopping speed increases by up to approximately 10,000 times. This indicates that the high-speed movement of lithium ions depends not merely on the rotation of surrounding molecules, but also on the cooperative opening of the local structure that confines the lithium ion. This study provides a new molecular-level understanding of lithium-ion conduction in OIPC that goes beyond the conventional paddlewheel mechanism.
3. Future Directions and the Social Significance of This Research
This study has revealed that, in lithium-ion conduction within OIPC, the cooperative opening and closing of ion cages formed by anions is more important than the rotational motion of surrounding molecules and ions themselves. This finding provides new guidelines for designing local structures that facilitate lithium-ion movement and materials in which these structures can open and close easily. Furthermore, the hopping function analysis used in this study is a versatile analytical method that can be widely applied not only to OIPCs but also to complex solid electrolytes and ionic materials. It is expected that applying this method to various solid electrolytes in the future will reveal the molecular-level factors governing ion conduction. This is expected to enable not only empirical material screening but also material design based on molecular-level principles, leading to the development of safer, higher-performance next-generation batteries.
Glossary of Terms
(1) Solid-State Electrolyte: A solid material capable of conducting ions, essential for creating next-generation batteries (e.g., all-solid-state batteries). They eliminate the safety risks associated with flammable liquid electrolytes while offering higher energy density.
(2) Organic Ionic Plastic Crystal: A class of crystalline materials composed of organic ions that exhibit long-range translational order but possess significant rotational disorder. This "soft solid" nature allows for high ionic conductivity while remaining mechanically stable.
(3) Paddle-Wheel Mechanism: A transport theory where the rotational motion of neighboring ions or molecules facilitates the translation of a target ion, effectively "pushing" it to the next site. It has been a central mechanistic assumption in solid-state ionics for over 25 years.
(4) Molecular Dynamics (MD) Simulation: A computational method used to track the individual trajectories of atoms and molecules based on physical laws. It allows for the observation of microscopic behaviors and transition states that are often impossible to capture through experimental means alone.
(5) Hop Function Analysis: A powerful framework for resolving individual hopping events in heterogeneous systems. It enables researchers to identify microscopic reaction coordinates and disentangle complex transport pathways that are hidden in average statistical measurements.
(6) Ion Cage: A local coordination environment where a target ion (like Li⁺) is surrounded and confined by multiple neighboring ions. The temporary breaking or reorganization of this "cage" is the critical step for an ion to hop between sites.
5. Publication Information
Journal: Journal of the American Chemical Society
Title: "Beyond the Paddle-Wheel Mechanism: Hop Function Analysis of Ion Transport in Organic Ionic Plastic Crystals"
Authors: Hyungshick Park, Shinji Saito, and Bong June Sung
Publication Date: June 1, 2026 (Published Online)
DOI: 10.1021/jacs.6c04713
6. Research Groups
Soft Matter Computational Chemistry Laboratory (led by Professor Bong June Sung), Department of Chemistry, Sogang University
Saito Group, Institute for Molecular Science (IMS), National Institutes of Natural Sciences
