Dissimilar Diffusion Mechanisms of Li, Na, and K Ions in Anhydrous Fe-Based Prussian Blue Cathode.

Prussian Blue (PB, AFe[Fe(CN)], where A = Li, Na, K, ), 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)]-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.