Mechanistic Insights into Sodium Storage in Prussian Blue Analogues: Linking Redox Chemistry, Electronic Structure, and Fast Ion Diffusion

Abstract

Prussian Blue analogues (PBAs) are promising cathode materials for sodium-ion batteries due to their open framework, structural robustness, and favorable ion transport properties. However, a unified understanding of the interplay between electronic structure, redox chemistry, and sodium-ion diffusion across different sodiation states remains lacking. In this work, we perform a comprehensive first-principles investigation of Berlin Green (BG), Prussian Blue (PB), and Prussian White (PW) using both GGA and meta-GGA exchange–correlation functionals. We provide a quantitative atomistic description of a site-selective redox mechanism, in which sodium insertion sequentially reduces Fe–C followed by Fe–N centers. This process governs the structural evolution, drives the metal–semiconductor–metal transition, and gives rise to two voltage steps corresponding to sequential redox processes. Magnetic moment analysis provides direct confirmation of this sequential reduction, establishing a clear microscopic picture of the redox process. Sodium-ion transport is found to be intrinsically fast, with very low migration barriers and high diffusivity, arising from the combined effect of the open framework and redox-induced modulation of the local electronic environment. The close agreement between CI-NEB and AIMD results further validates the predicted diffusion behavior. Overall, this work establishes a direct coupling between electronic structure, redox activity, and ion transport, providing a unified mechanistic framework for sodium intercalation in PBAs and offering key insights for the rational design of high-performance sodium-ion battery cathodes.