Electrochemical performance of NaFeFe(CN)₆ prepared by solid reaction for sodium ion batteries

Commercially available Na₄Fe(CN)₆ and Fe₄[Fe(CN)₆]₃ are two cheap compounds that have ever been investigated as positive electrode materials for sodium-ion batteries. However, poor electronic conductivity of Na₄Fe(CN)₆ and sodium deficiency of Fe₄[Fe(CN)₆]₃ prevent these two materials from being used in practical rechargeable sodium-ion batteries. In this paper, a NaFeFe(CN)₆ cathode material was synthesized by ball milling the Fe₄[Fe(CN)₆]₃/ Na₄Fe(CN)₆ mixture. The obtained NaFeFe(CN)₆ demonstrated a single cubic phase indexed to Fm3m space group similar to Fe₄[Fe(CN)₆]₃ but with larger lattice parameter due to the existence of Na⁺ in the lattice framework. The NaFeFe(CN)₆ electrode delivered first desodiation capacity of 119.4 mAh g⁻¹ and first sodiation capacity of 153.6 mAh g⁻¹ at 0.05C rate. The NaFeFe(CN)₆ showed excellent cycling stability with reversible capacities of 118.2 mAh g⁻¹ and 96.8 mAh g⁻¹ at 0.1C and 1C rate, respectively. In-situ XRD analyses demonstrated a single cubic phase process during charge-discharge of NaFeFe(CN)₆ electrode. Low water content benefited from the solid reaction method and the homogenous single phase process during charge/discharge assured the stable long term cycling performance of the NaFeFe(CN)₆ product.