Preparation of iron–manganese hexacyanoferrate from insoluble salts and mechanistic insights into enhanced sodium storage cycle stability via electrolyte modification

Abstract

Prussian blue analogues (PBAs) are promising cathode materials for sodium-ion batteries (SIBs). However, conventional co-precipitation synthesis using soluble salts often results in rapid reaction kinetics, leading to lattice vacancies and residual coordinated water in PBAs. Although chelating agents can slow reaction rates, they increase production costs. Additionally, pure iron-based PBAs suffer from low capacity, while pure manganese-based PBAs exhibit poor cycling stability. To address these challenges, this study focuses on iron–manganese-based PBAs (Fe–MnHCF) synthesized via the solubility product principle. By selecting insoluble salts (FeC₂O₄/MnC₂O₄) to control reaction kinetics without chelators, the method reduces lattice defects and coordinated water content of Fe–MnHCF materials. Building upon this technology, MnCl₂ is incorporated into the electrolyte to suppress Mn dissolution from Fe–MnHCF materials during the electrochemical sodium-storage cycling process. Through a combination of experimental characterization, theoretical analysis, and numerical simulation, this study systematically investigates the effects of synthesis parameters, component optimization, and electrolyte modification of the fabricated materials. The optimized Fe–MnHCF electrode delivers a high capacity of 115.7 mAh g⁻¹ and retains 90.5% of its reversible capacity after 1500 cycles at 10C in the electrolyte containing 0.5 wt% MnCl₂, demonstrating superior rate capability and long-term stability. This work provides a cost-effective strategy for defect engineering in PBAs and offers valuable insights into electrolyte optimization for high-performance SIBs.