Unveiling the Influence of Cyanogen Vacancies in Prussian Blue for Sodium-ion Batteries

Prussian blue (PB), recognized as a promising cathode material, has gained significant attention for sodium-ion batteries due to its high theoretical energy density, low cost, and ease of synthesis. However, the influence of anion vacancies on the stability of the PB framework remains controversial, impeding a comprehensive grasp of their precise role in electrochemical performance and the controlled synthesis of PB with smaller anionic vacancies remains challenging due to the limitations inherent in current synthesis strategies. Herein, we present an anion complexation method to synthesize PB materials with tunable cyanide vacancy concentrations. Furthermore, we propose a vacancy-driven mechanism that promotes spin transitions coupled with lattice bending, which are more reversible at low-spin Fe sites, leading to excellent low-spin stability and the design of materials with outstanding electrochemical performance. This anion complexation method not only provides a novel synthetic pathway for PB materials but also advances the understanding of the composition-structure-property relationships between cyanide vacancy configurations and spin transition mechanism, highlighting its potential for future energy storage applications.