In Situ Balanced Synthesis of High-Activity Low-Spin Iron Cathode Prussian Blue for Enhanced Sodium-Ion Storage

The growing market for sodium-ion batteries has stimulated interest in research on Prussian blue-type cathode materials. Iron hexacyanoferrate (FeHCF) is considered a desirable Prussian blue-type cathode, but the incomplete electrochemical property of its low-spin iron sites hinders its further practical application. In this paper, carboxymethyl cellulose is demonstrated to have an appropriate binding energy through DFT calculations, synthesize Prussian blue in situ, balance Fe³⁺ and water in FeHCF, and introduce Fe^III vacancies to activate low-spin Fe sites. Thus, at a 1 C rate, it achieves an initial discharge capacity of 154.7 mAh g^-1 with an energy density of 470.8 Wh kg^-1. The capacity retention is 70.2% after 4000 cycles at a rate of 100 C. This work provides a simpler way to develop more cost-effective, faster, and more durable cathode materials for sodium-ion energy storage.