Impact of Sodium on the Water Dynamics in Prussian Blue Analogues

Prussian blue analogues (PBAs) are interesting cathode materials for sodium-ion batteries, especially the iron-based, [Fe(CN)₆]ⁿ⁻ vacancy-free PBA Na_2–xFe[Fe(CN)₆]·zH₂O. However, the presence of water has an opposing role in the application of PBAs as electrode materials: the water provides structural stability ensuring minimum volume changes during sodium extraction and insertion, however, water can react with the electrolyte leading to unwanted side reactions. Therefore, water must be replaced with another compatible small molecule to ensure optimal performance. To achieve this, insights into the dynamics of water are crucial. Two samples with compositions of Na_1.90(9)Fe_0.90(7)²⁺Fe_0.10(3)³⁺[Fe²⁺(CN)₆]·2.12(2)H₂O and Na_0.34(5)Fe³⁺[Fe^2.66(5)+(CN)₆]·0.360(4)H₂O were investigated using quasi-elastic neutron scattering (QENS). The results show that the water dynamics strongly depend on the sodium content. The water was found to diffuse within a spherical cavity in the porous framework with a radius of 2.6 Å for the high sodium-containing sample and 1.8 Å for the low sodium-containing sample consistent with the pore sizes in the crystal structures. In addition to the water diffusing within the pores, it was found that a small fraction of the water exhibits a rattling or rotational motion suggesting that this water strongly interacts and binds to the sodium ions. For the high sodium-containing sample, this rattling or rotational motion transforms into quantum rotational tunneling of the water below 75 K. These results give new fundamental insight into the role of water in PBAs, laying the groundwork for substituting water with another small molecule compatible with nonaqueous battery systems while also ensuring structural stability.