From smartphones to electric vehicles, lithium-ion batteries sit at the very heart of the energy systems driving modern society. However, the batteries currently in widespread use rely on flammable liquid electrolytes, carrying with them a persistent risk of fire and explosion. To fundamentally eliminate this safety concern, researchers around the world have been focusing intense attention on the development of all-solid-state batteries that employ non-flammable "solid electrolytes."
Among these, organic ionic plastic crystals (OIPCs)—solids in which the constituent molecules and ions actively rotate in place—have attracted enormous interest as "soft solids" capable of combining flexibility with high ionic conductivity. But how, exactly, do lithium ions manage to slip through the tightly bound lattice of a crystal? A long-standing theory that has dominated the field of solid-state ionics for a quarter century—the idea that "paddle-wheel-like blades push ions forward"—is now being quietly overturned by the scalpel of computational science.
The 25-Year-Old Dominant Hypothesis and the Contradiction of the "Soft Solid"
The most fundamental challenge in developing solid electrolytes lies in overcoming a trade-off: how to achieve liquid-like, smooth ion transport while maintaining the stability of the crystal lattice. In ordinary rigid inorganic crystals, ions move through narrow gaps in a fixed lattice with the help of thermal vibration, making it difficult to achieve high conductivity. When the gaps are narrow, collisions between moving ions increase, and so does the associated energy loss.
Organic ionic plastic crystals (OIPCs), by contrast, exhibit distinctive behavior. While the material as a whole maintains a long-range ordered crystal structure and takes the form of a solid, the organic molecules and ions that make up the crystal's framework rotate vigorously at their respective lattice sites. Scientists have long believed that this "soft solid" character strongly promotes lithium-ion movement within the solid. The rare physical trait of being crystalline yet carrying a hint of fluidity was seen as the very source of excellent conductivity.
The framework used to explain this phenomenon—and the dominant theory in the field for more than 25 years—is the "paddle-wheel mechanism." According to this theory, the rotational motion of the large organic ions or molecules surrounding a lithium ion itself acts like the blades of a paddle wheel, physically pushing the neighboring lithium ion into its next site. Researchers worldwide have widely supported this hypothesis, believing without question that increasing the rotational speed of the matrix ions was the sole key to improving lithium-ion conductivity.
However, this dominant theory has long harbored a significant gap in empirical verification. Conventional experimental methods and standard measurements can only observe collective "average values" for the material as a whole, such as diffusion coefficients or correlation functions. Exactly when, where, and how each individual lithium ion escapes from a crystalline trap and hops to the next stable site has been extremely difficult to precisely capture. Without microscopic mechanical proof that rotating molecules were truly pushing lithium ions directly, the paddle-wheel model was passed down for years as an unquestioned premise.
Hop-Function Analysis Exposes the Illusion of the Paddle-Wheel Model
A strong challenge to this long-standing premise has come from an international collaborative research team led by Professor Bong June Sung of the Department of Chemistry at Sogang University in South Korea, together with Professor Shinji Saito of the Institute for Molecular Science, National Institutes of Natural Sciences, and the Graduate University for Advanced Studies (SOKENDAI) in Japan. The research team employed large-scale molecular dynamics simulations run on a supercomputer. After faithfully reproducing the trajectories of atoms and ions within OIPCs based on the laws of physics, they introduced an advanced mathematical technique called "hop-function analysis," succeeding in comprehensively tracking the microscopic motion within this complex system.
Hop-function analysis is a method that precisely extracts and identifies individual transition events—cases where a specific ion jumps from one stable position to another—from among the vast number of molecules and ions oscillating in complex thermal motion. By clearing away the fog of conventional averaged statistics and isolating the microscopic reaction coordinates, this approach made it possible to pinpoint the exact dynamics occurring at the very moment a lithium ion moves. It represents an innovative approach for getting at the heart of a phenomenon that cannot be seen unless one breaks it down and observes individual hopping events.
When the team applied this analytical method to the massive simulation data for OIPCs, a clear fact emerged that ran counter to years of intuition. First, it was confirmed that the movement and rearrangement of the large matrix ions forming the material's framework were strongly correlated with the rotational motion of surrounding molecules. This much aligns perfectly with the overall picture painted by the conventional paddle-wheel mechanism.
However, when the team extracted the hopping events of lithium ions—the events directly relevant to a battery's charge-discharge performance—the results looked completely different. The correlation between lithium-ion hops and the rotational motion of the surrounding large molecules or ions was essentially zero. No matter how fast the surrounding paddle-wheel blades spun, the lithium ion at the center received no propulsive force from them. Here, it was theoretically proven that the paddle-wheel mechanism, believed for more than 25 years, is not the direct driving force behind the rapid movement of lithium ions—at the very least.
| Comparison Item | Conventional Theory (Paddle-Wheel Mechanism) | Findings of This Study (Cooperative Cage Opening/Closing Model) |
|---|---|---|
| Period of acceptance | Over 25 years | 2026 (latest theoretical proof presented by this study) |
| Primary driving force of ion movement | Physical propulsion from the rotational motion of surrounding large molecules or constituent ions | Cooperative rearrangement and opening/closing of the "ion cage" formed by surrounding anions |
| Structure surrounding the lithium ion | Continuously surrounded by constituent ions, pushed outward as they rotate | Normally confined by multiple anions, but the anion coordination number temporarily drops to as few as 2 |
| Determinant of movement speed | Rotational speed and frequency of matrix ions | Frequency of cage opening/closing and hop rate in the "open state" (increases by up to roughly 10,000-fold) |
The "Ion Cage's" Cooperative Opening and Closing—Boosting Speed by 10,000-Fold
If spinning paddle wheels don't actually move lithium ions forward, what is transporting the ions to their next site? The true protagonist that the team pinpointed through precise analysis was the cooperative rearrangement of the "ion cage" (a birdcage-like confining structure) formed by the anions surrounding each lithium ion.
Within the crystal, the positively charged lithium ion is normally attracted strongly by electrostatic forces to multiple negatively charged anions, and is confined within a stable cage. In this closed, normal state, the electrostatic attraction acts as a wall, so the lithium ion merely vibrates thermally around its lattice site. No matter how much the surrounding molecules rotate, the ion cannot escape the cage as long as this electrostatic mesh remains closed.
However, tracking the simulation traps down to extremely small time intervals revealed moments when the shape of the cage changes dramatically. The anions positioned "fourth and fifth closest" to the center of a given lithium ion undergo a cooperative motion in which they swap positions with each other, almost like changing seats. At this moment, a "gap" opens up like a lid in the wall of the old cage that had previously confined the lithium ion. At precisely the same time, a new cage forms in the adjacent space to receive the lithium ion. It is only during this brief instant of cage opening and closing that the lithium ion escapes the confinement of the old cage and hops to the neighboring site.
Furthermore, the team discovered that during this rearrangement process, there exists a temporary "open state" in which the number of anions surrounding the lithium ion drops from its normal value to as few as "two." At the moment this door opens, the hop rate at which the lithium ion leaps to the next site was amplified by up to roughly 10,000-fold compared to the normal, confined state. An accelerator for conduction on a scale utterly incomparable to the simple excitation of molecular rotation had been hidden within the microscopic gaps of the crystal structure.
The true identity of high-speed conduction turned out to be a cooperative door-opening process, in which the surrounding anions work in concert to clear the way and open a path. This clarification of the structural mechanism represents a major turning point that fundamentally rewrites the conventional understanding of ion transport in solid electrolytes.
Molecular Design for All-Solid-State Batteries and Remaining Experimental Verification
The significance of this research achievement is not limited to the basic-science accomplishment of correcting a long-held theory. It also contains practical and reliable guidance that fundamentally transforms the strategy for material development aimed at next-generation all-solid-state batteries. A rewriting of basic theory directly implies a wholesale renewal of the chemical synthesis approaches used in laboratories.
In conventional materials exploration, the design principle for solid electrolytes tended to be "how to make molecules within the crystal rotate as actively as possible." This study, however, has clearly shown that the main battleground for lithium-ion movement lies in "the ease with which anions can cooperatively open and close their cages." Going forward, designing local structures that allow the fourth and fifth neighboring anions surrounding a lithium ion to swap positions with minimal energy, and searching for molecular frameworks in which the coordination number readily and temporarily drops to two, will serve as reliable indicators for developing high-performance electrolytes.
Moreover, the hop-function analysis that proved so powerful in this study is a versatile method applicable not only to OIPCs but broadly to a wide range of inorganic and organic solid electrolytes—including sulfide-based and oxide-based systems—as well as to composite materials with complex interfaces. It opens a path toward rational, predictable material design grounded in molecular-level mechanics, for a field of battery development that has long relied on empirical rules and trial-and-error screening. This new yardstick presented by computational science is likely to become a driving force that significantly accelerates the pace of materials development worldwide.
At the same time, the picture presented by this achievement is a theoretical conclusion derived from molecular dynamics simulations using idealized crystal models on a supercomputer. Inside a real all-solid-state battery, there exist grain boundaries between polycrystals with differing crystal orientations, as well as complex interfaces where electrodes and electrolytes meet, with stress and potential gradients constantly changing. Can the cooperative opening and closing of anion cages maintain a 10,000-fold hop-rate increase, just as in the simulation, even under the harsh conditions of an actual battery cell? And can this local structure actually be deliberately controlled through real chemical synthesis? As science moves beyond a 25-year-old theory toward the practical realization of next-generation batteries, researchers' challenge now advances from theoretical proof into a new phase of experimental verification in the laboratory.