Electrochemical Performance of NaFeFe(CN)₆Prepared by Solid Reaction for Sodium Ion Batteries

Abstract

Commercially available Na<sub>4</sub>Fe(CN)<sub>6</sub> and Fe<sub>4</sub>[Fe(CN)<sub>6</sub>]<sub>3</sub> are two cheap compounds that have ever been investigated as positive electrode materials for sodium-ion batteries. However, poor electronic conductivity of Na<sub>4</sub>Fe(CN)<sub>6</sub> and sodium deficiency of Fe<sub>4</sub>[Fe(CN)<sub>6</sub>]<sub>3</sub> prevent these two materials from being used in practical rechargeable sodium-ion batteries. In this paper, a NaFeFe(CN)<sub>6</sub> cathode material was synthesized by ball milling the Fe<sub>4</sub>[Fe(CN)<sub>6</sub>]<sub>3</sub>/ Na<sub>4</sub>Fe(CN)<sub>6</sub> mixture. The obtained NaFeFe(CN)<sub>6</sub> demonstrated a single cubic phase indexed to Fm3m space group similar to Fe<sub>4</sub>[Fe(CN)<sub>6</sub>]<sub>3</sub> but with larger lattice parameter due to the existence of Na<sup>+</sup> in the lattice framework. The NaFeFe(CN)<sub>6</sub> electrode delivered first desodiation capacity of 119.4 mAh g<sup>-1</sup> and first sodiation capacity of 153.6 mAh g<sup>-1</sup> at 0.05C rate. The NaFeFe(CN)<sub>6</sub> showed excellent cycling stability with reversible capacities of 118.2 mAh g<sup>-1</sup> and 96.8 mAh g<sup>-1</sup> at 0.1C and 1C rate, respectively. In-situ XRD analyses demonstrated a single cubic phase process during charge-discharge of NaFeFe(CN)<sub>6</sub> 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)<sub>6</sub> product.