Reversing the Chemical and Structural Changes of Prussian White After Exposure to Humidity to Enable Aqueous Electrode Processing for Sodium-ion Batteries

Abstract

Prussian White is a promising active material for the positive electrode of sodium-ion batteries as it is comprised of Na, Mn, Fe, C, and N and thus offers high sustainability and low cost. However, exposure of Prussian White to moisture results in chemical changes due to the formation of surface contaminants, as well as structural changes due to the absorption of water into the bulk crystal structure. Here we report an analysis of the formation rate of surface contaminants and bulk water absorption by weight tracking, infrared spectroscopy, and X-ray diffraction over extended periods of storage in high relative humidity air for fully sodiated Na1.8Mn0.8Fe0.2[Fe(CN)6]0.9 and partially sodiated Na1.3Mn0.8Fe0.2[Fe(CN)6]0.9. Fully sodiated Prussian White gains almost 20% in mass due to the formation of interstitial water during 20 h of storage in 100% relative humidity at 25 °C. Surface hydroxides and carbonates are found after storage and a structural change from the rhombohedral to a monoclinic crystal structure is observed. It is found that vacuum drying of Prussian White powder or electrodes at 150 °C can remove the majority of interstitial water and restore the rhombohedral crystal structure, but not remove surface contaminants. Prussian White immersed in water during aqueous electrode processing also shows interstitial water and a monoclinic crystal structure, but no surface contaminants. This suggests that aqueous electrode processing of Prussian White is feasible when effective drying strategies are employed. Indeed, Prussian White electrodes made from H2O-based slurries with CMC/NaPAA binder vacuum-dried at 150 °C show higher specific capacity and similar capacity retention in full cells as Prussian White electrode made from NMP-based slurries with PVDF binder.