Novel acetic acid induced Na-rich Prussian blue nanocubes with iron defects as cathodes for sodium ion batteries

Abstract

The Prussian blue cathode has great potential for use in sodium-ion batteries in view of its high gravimetric capacity, facile synthetic procedure and low cost. The main challenges for Prussian blue are the structural degradation caused by [Fe(CN)₆] vacancies and coordinated water in its lattice and low average voltage due to insufficient activation of low-spin Fe^LS(C) redox-couple reactions. Here, Na-enriched Prussian blue with low coordinated water and free [Fe(CN)₆] vacancies has been successfully synthesized by defect engineering, using acetic acid as an iron defect inducer. In particular, Na-rich Na_3.27Fe_0.35[Fe(CN)₆]·0.85H₂O nanocubes with hole centres, low amounts of coordinated water and free [Fe(CN)₆] vacancies exhibit a high specific capacity, impressive cycling stability and good coulombic efficiency. This Na-rich material shows a low charge-transfer resistance (201.1 Ω), a high Na⁺ apparent diffusion coefficient (3.56 × 10⁻¹¹ cm² s⁻¹) and an additional capacity contribution at approximately 4.1 V, demonstrating the sufficient activation of low-spin Fe^LS(C) redox couples in Na-involved reactions. The Na_3.27Fe_0.35[Fe(CN)₆]·0.85H₂O cathode undergoes a reversible redox reaction, which converts the structure from cubic Na₂Fe_0.35[Fe(CN)₆] to rhombohedral Na_3.24Fe_0.35[Fe(CN)₆]. More significantly, this work for the first time realizes the rational composition and architecture design of Prussian blue materials by defect engineering for a broad range of potential applications.