Research Team Led by Professor Sung Bong-june of the Department of Chemistry Publishes Paper in the Prestigious International Journal, JACS
A research team led by Professor Sung Bong-june of the Department of Chemistry at this university, in collaboration with a team led by Professor Shinji Saito of the Institute for Molecular Science (IMS) in Japan, has elucidated at the molecular level how lithium ions actually transport within “organic ionic plastic crystals (OIPCs),” which are attracting attention as key materials for next-generation batteries. The findings of this study were published in the Journal of the American Chemical Society, one of the most prestigious academic journals in the field of chemistry.
Most lithium-ion batteries used in cell phones and electric vehicles employ liquid electrolytes, but these liquids are highly flammable, posing a risk of fire and explosion. Solid electrolytes have emerged as an alternative to address this issue, and among them, organic ionic plastic crystals (OIPCs) are unique materials that are rigid solids yet contain molecules that continuously rotate in place. Thanks to this “plastic solid” property, ions can pass through relatively easily, making OIPCs a leading candidate for safe, high-performance next-generation batteries.
So, how do lithium ions, which carry electricity, transport within this rigid solid? For over 25 years, the explanation accepted as the scientific consensus has been the “paddle-wheel mechanism.” According to this theory, large molecules surrounding the ions spin like a paddle wheel, and their rotation pushes adjacent lithium ions to the next position. In other words, the hypothesis was that the “rotation of surrounding molecules” directly causes the “transport of ions.”
The team reexamined this long-standing hypothesis from the ground up using molecular dynamics (MD) simulations performed on a supercomputer, together with a precise analytical tool introduced by the team known as the “hop function” analysis method. The hop function analysis method accurately captures individual “hop” events, specifically when and where a single ion hops from one site to another, in a complex environment where countless molecules are constantly moving and interacting. This enabled the team to examine ion motion on an event-by-event basis, rather than relying solely on averaged measurements as in previous studies.
The results overturned the existing conventional wisdom. The analysis revealed that the movement of the large molecules forming the material’s structural framework was indeed coupled with the rotation of surrounding molecules, consistent with the “paddle-wheel” picture. However, it was discovered that the hopping of lithium ions, the key factor determining battery performance, occurred almost independently of the rotation of surrounding molecules, that is, separately from the “paddle-wheel” mechanism.
Instead, the true secret behind lithium-ion transport lay in the “coordinated rearrangement of the ion cage.” Lithium ions are normally trapped inside a “cage” surrounded by several anions (PF6−); when the surrounding anions, particularly the third, fourth, and fifth nearest ones, simultaneously swap positions, they open the existing cage and form a new one. It is precisely at this moment that the lithium ion escapes from the old cage and moves into the new one. In particular, the team discovered that when the number of anions surrounding the lithium ion temporarily drops to just two, creating an “open-door” state, the ion’s hopping speed increases by as much as 10,000 times.
This study is significant because it provides direct evidence at the molecular level that the “paddle-wheel mechanism,” which has been accepted without question for over 25 years, is not central to lithium-ion transport, at least, and that the “cooperative rearrangement of surrounding ions” takes its place. Furthermore, the hop function analysis method used by the team serves as a powerful analytical framework applicable not only to complex solid electrolytes but also to various plastic crystals and ionic materials. It is expected to serve as a crucial guide for the “principle-based” design of safer and higher-performance solid electrolytes for next-generation batteries.
▶Paper Title: Beyond the Paddle-Wheel Mechanism: Hop Function Analysis of Ion Transport in Organic Ionic Plastic Crystals
▶Journal: Journal of the American Chemical Society (IF 16.6)
▶DOI: https://doi.org/10.1021/jacs.6c04713
[SEO keyword]
Solid Electrolyte, Lithium-ion Battery, Molecular Dynamics
Most lithium-ion batteries used in cell phones and electric vehicles employ liquid electrolytes, but these liquids are highly flammable, posing a risk of fire and explosion. Solid electrolytes have emerged as an alternative to address this issue, and among them, organic ionic plastic crystals (OIPCs) are unique materials that are rigid solids yet contain molecules that continuously rotate in place. Thanks to this “plastic solid” property, ions can pass through relatively easily, making OIPCs a leading candidate for safe, high-performance next-generation batteries.
So, how do lithium ions, which carry electricity, transport within this rigid solid? For over 25 years, the explanation accepted as the scientific consensus has been the “paddle-wheel mechanism.” According to this theory, large molecules surrounding the ions spin like a paddle wheel, and their rotation pushes adjacent lithium ions to the next position. In other words, the hypothesis was that the “rotation of surrounding molecules” directly causes the “transport of ions.”
The team reexamined this long-standing hypothesis from the ground up using molecular dynamics (MD) simulations performed on a supercomputer, together with a precise analytical tool introduced by the team known as the “hop function” analysis method. The hop function analysis method accurately captures individual “hop” events, specifically when and where a single ion hops from one site to another, in a complex environment where countless molecules are constantly moving and interacting. This enabled the team to examine ion motion on an event-by-event basis, rather than relying solely on averaged measurements as in previous studies.
The results overturned the existing conventional wisdom. The analysis revealed that the movement of the large molecules forming the material’s structural framework was indeed coupled with the rotation of surrounding molecules, consistent with the “paddle-wheel” picture. However, it was discovered that the hopping of lithium ions, the key factor determining battery performance, occurred almost independently of the rotation of surrounding molecules, that is, separately from the “paddle-wheel” mechanism.
Instead, the true secret behind lithium-ion transport lay in the “coordinated rearrangement of the ion cage.” Lithium ions are normally trapped inside a “cage” surrounded by several anions (PF6−); when the surrounding anions, particularly the third, fourth, and fifth nearest ones, simultaneously swap positions, they open the existing cage and form a new one. It is precisely at this moment that the lithium ion escapes from the old cage and moves into the new one. In particular, the team discovered that when the number of anions surrounding the lithium ion temporarily drops to just two, creating an “open-door” state, the ion’s hopping speed increases by as much as 10,000 times.
This study is significant because it provides direct evidence at the molecular level that the “paddle-wheel mechanism,” which has been accepted without question for over 25 years, is not central to lithium-ion transport, at least, and that the “cooperative rearrangement of surrounding ions” takes its place. Furthermore, the hop function analysis method used by the team serves as a powerful analytical framework applicable not only to complex solid electrolytes but also to various plastic crystals and ionic materials. It is expected to serve as a crucial guide for the “principle-based” design of safer and higher-performance solid electrolytes for next-generation batteries.
▶Paper Title: Beyond the Paddle-Wheel Mechanism: Hop Function Analysis of Ion Transport in Organic Ionic Plastic Crystals
▶Journal: Journal of the American Chemical Society (IF 16.6)
▶DOI: https://doi.org/10.1021/jacs.6c04713
[SEO keyword]
Solid Electrolyte, Lithium-ion Battery, Molecular Dynamics