Correlation between structure, microstructure and electrochemical properties of Prussian white cathode material for sodium-ion batteries

Sodium hexacyanoferrate, or Prussian white (PW), is a cheap cathode material for rechargeable sodium-ion batteries, exhibiting high capacity and charge/discharge rate. Tailoring structure and microstructure of PW to further improve its properties, in particular, cycling stability, is an important task to achieve for successful commercial applications of sodium-ion batteries. Here, the relation of grain refinement and thermal treatment with crystal structure, structural phase transition sequences and electrochemical properties of the PW are studied. The ball-milling process and subsequent drying at high temperatures induce the transformation of the initial rhombohedral structure of the pristine commercial Prussian White powder into a multiphase compound of dehydrated rhombohedral and cubic structures with different average crystallite sizes. The increased surface area of the effectively milled cubic phase promotes its transition into the dehydrated rhombohedral Prussian White at temperatures below 100 °C. Electrodes based on the milled Prussian White powder demonstrate a capacity of ~ 110 mAh·g⁻¹ (compared to ~ 80 mAh·g⁻¹ for the non-milled commercial powder) at 10 C cycling rate and better cycling stability, retaining a larger part of their initial specific capacity after 300 charge/discharge cycles than those based on non-milled material. It is shown that high-spin and low-spin Fe redox processes in Prussian white-based electrodes are also affected by ball milling.