多孔性結晶中のNaイオンの高速拡散機構を新たに提唱～次世代ナトリウムイオン電池の新規正極の開発を加速～

2025-06-30 東京科学大学

図１.（a）イオン二次電池の模式図。負極から正極へと、リチウムイオンなどのプラスの電荷を帯びたイオン（濃いピンク色の丸）が移動することで、電流が流れる。（b）正極材料の一種であるプルシアンブルー（PB）結晶。（c）PB結晶の孔の中で拡散するA⁺（= Li⁺、Na⁺、K⁺）イオンとそのサイズの比。

＜関連情報＞

• https://www.isct.ac.jp/ja/news/9wryndf41fk8
• https://www.isct.ac.jp/plugins/cms/component_download_file.php?type=2&pageId=&contentsId=1&contentsDataId=1781&prevId=&key=77a571f2db146b5967b9fe6af8208ca1.pdf
• https://pubs.acs.org/doi/10.1021/jacs.5c05274

無水Fe系プルシアンブルー正極におけるLi+、Na+、K+イオンの相異なる拡散機構 Dissimilar Diffusion Mechanisms of Li+, Na+, and K+ Ions in Anhydrous Fe-Based Prussian Blue Cathode

Dan Ito,Seong-Hoon Jang,Hideo Ando,Toshiyuki Momma,and Yoshitaka Tateyama
Journal of the American Chemical Society  Published June 30, 2025
DOI:https://doi.org/10.1021/jacs.5c05274

Abstract

Prussian Blue (PB, AFe[Fe(CN)₆], where A = Li, Na, K, etc.), a three-dimensional (3D) metal–organic framework (MOF), emerges as a promising cathode material, particularly for next-generation Na- and K-ion batteries. However, the microscopic occupation positions and diffusion behaviors of A⁺ ions in the unit cell have been inadequately elucidated. This study systematically compares the diffusion mechanisms of multiple Li⁺, Na⁺, and K⁺ ions using density functional theory calculations. We clarified the new stable occupation sites for Li⁺ and Na⁺ ions: the face-centered (FC) 24d and off-FC 48g sites, respectively. The smaller ionic radii of Li⁺ and Na⁺ ions contribute to their enhanced Coulombic attractions from CN^– anions. Li⁺ ions are more self-diffusive than Na⁺ at high temperatures; however, at room temperature, Na⁺ ions have comparable self-diffusivities and lower activation energies than Li⁺ ions. This is attributed to the smaller tilting of [Fe(CN)₆]-octahedra induced by Na⁺ ions’ transfers, resulting in a shallower potential energy landscape than for Li⁺ ions. These results demonstrated that the anhydrous Fe-based pristine PB crystal is an excellent Na⁺-ion conductor. Meanwhile, K⁺ ions prefer the conventional body center (8c site) and exhibit negligible self-diffusivities without anionic defects. Surprisingly, they show anisotropic diffusion along anion vacancy channels in the defective crystal, in contrast with the isotropic pathways for Li⁺ and Na⁺ ions. These findings update the fundamental chemistry of the diffusivity correlation with the electronic orbital interactions and framework distortion within general MOF materials.