Dissimilar Diffusion Mechanisms of Li<sup>+</sup>, Na<sup>+</sup>, and K<sup>+</sup> Ions in Anhydrous Fe-Based Prussian Blue Cathode
Dan Ito, Seong‐Hoon Jang, Hideo Ando, Toshiyuki Momma, Yoshitaka Tateyama
Abstract
High Resolution Image Download MS PowerPoint Slide 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.