Acid-assisted synthesis of low-defect potassium hexacyanoferrate cathodes for long-lifetime potassium-ion batteries

Abstract

Potassium hexacyanoferrate (KFHCF) stands out among Prussian blue analogues (PBAs) as a promising cathode for potassium-ion batteries (PIBs) due to its high capacity and stable crystal structure. However, conventional coprecipitation synthesis often suffers from a rapid and disordered crystallization process, during which the hydrolysis of the ${\left[\text{Fe}{\left(\text{CN}\right)}_6\right]}^{4‐}$ ligand introduces numerous lattice vacancies and coordinated water, severely impairing cycling stability and rate performance. To address this, we introduce a simple acid-assisted strategy by incorporating hydrochloric acid into the reaction system. The acidic environment modulates the nucleation and growth kinetics, promoting a dynamic recrystallization process that produces acid-assisted KFHCF, hereafter referred to as H-KFHCF, with high crystallinity and low defect density. This kinetic control not only enhances the structural stability of the crystal framework during cycling but also facilitates potassium-ion diffusion. As a result, H-KFHCF synthesized via this acid-assisted route exhibits excellent cycling stability, retaining a capacity of 70.28 mAh g⁻¹ after 700 cycles at 0.5C, which corresponds to an ultra-low capacity decay rate of only 0.053% per cycle. This work demonstrates a facile and effective acid-assisted approach that provides valuable insights into the synthesis of high-performance, low-defect PBAs cathode materials.