Water effect on sodium mobility in zinc hexacyanoferrate during charge/discharge processes in sodium ion-based battery

Abstract This study discusses the effect of the water removal on the behavior of Na 2 Zn 3 [Fe(CN) 6 ] 2 ·xH 2 O as cathode material for sodium ion-based battery. For comparison, electrochemical and structural data were recorded for both, hydrated and anhydrous phases. The structural changes in zinc hexacyanoferrate, from rhombohedral to monoclinic structure, as consequence of the water molecules removal increase the cation-framework interaction and modify its mobility inside the porous network. These phases show two faradic processes, which are related to the existence of two strength levels (potentials) for the cation–framework interaction. One of them corresponds to the cation subtraction from the material with maximum cation occupation per cavity (two per formula unit). The second effect was ascribed to the formation of a solid solution, (Na) 2(1 − x) Zn 3 [(Fe III ) x Fe II (1 − x) (CN) 6 ] 2 ·xH 2 O. This solid solution is formed when the cation population within the cavities decreases during the extraction (oxidation) process. This proposal is supported by cyclic voltammetry and impedance electrochemical spectroscopy results. Galvanostatic intermittent titration technique and galvanostatic experiments reveal that the monoclinic structure exhibits an improvement for the charge/discharge process and a higher rate capability, related to a greater cationic mobility within the framework. This is associated to higher sodium diffusion rate (evaluated by GITT) for the monoclinic phase compared with the observed for the rhombohedral one.