Gel-Confined Crystallization Unlocks Defect-Minimized Prussian Blue Cathode for High-Performance Sodium-Ion Batteries

Metal hexacyanoferrates (MHCFs) have emerged as promising cathodes for sodium-ion batteries. However, conventional wet-chemistry-derived MHCFs inevitably contain substantial Fe(CN) 6 vacancies and crystal water, resulting in an undesirable Na-storage performance. Herein, a gel-confined crystallization strategy is developed to prepare highly crystalline MHCFs. In a typical polypyrrole (PPy) gel, the cross-linked network effectively restricts the movement of internal ions through steric hindrance and attractive/repulsive interactions, leading to slow crystal growth and formation of highly crystalline MHCFs. Specifically, iron hexacyanoferrate (FeHCF), with only 1% Fe(CN) 6 vacancy and 2.0 wt% crystal water, has been formed in situ within a PPy gel via this gel-confined crystallization process. The highly crystalline FeHCF coupled with an interconnected PPy framework enables the hybrid cathode to exhibit enhanced activity of low-spin Fe sites, long cycling life, and good rate capability.