Electrochemical Presodiation for Boosting Sodium-Ion Storage of Fe-Based Prussian Blue with Synthesis-Induced Sodium Deficiency

Iron-based Prussian blue (PB) shows great potential as a cathode material for sodium-ion batteries (SIBs) because of its high theoretical capacity, long cycle life, and cost-effectiveness. However, liquid-phase-synthesized PB often has a severe sodium deficiency from sodium dissolution in water. Here, an electrochemical presodiation strategy is proposed to enhance the sodium storage of PB with synthesis-induced sodium deficiency. The increased Na content in PB reduces vacancies and crystal water, enhancing electrochemical activity and Na+-diffusion kinetics. During Na+ insertion/extraction, the presodiation product (denoted as PB–W-Na) undergoes a highly reversible two-step phase transition (monoclinic ↔ cubic ↔ tetragonal) with a minimal change in lattice parameter “a”. Consequently, the PB–W-Na electrode exhibits a high capacity and excellent cycling stability. At 0.1 A g–1, it delivers a high capacity of 90.1 mAh g–1 after 300 cycles, retaining 90.3% of initial capacity. Even at a high current density of 1 A g–1, it maintains an impressive first discharge capacity of 97.5 mAh g–1 with 73.5% retention after 1500 cycles. This electrochemical presodiation strategy can be applied to other SIB electrode materials with sodium deficiency issues.