Unveiling the Influence of Water Molecules on the Structural Dynamics of Prussian Blue Analogues

Abstract 3D‐framework Prussian blue analogues (PBAs) are appealing as a cost‐effective, sustainable cathodes for Na‐ion batteries. However, the aqueous‐based synthesis of PBAs inherently introduces three different forms of water molecules (surface, interstitial and crystal) into the structure. Removal of water molecules causes phase transformation from monoclinic (M) to rhombohedral (R). This work presents the effects of water molecules on the structure before the phase transformation temperature, employing two promising PBA cathodes, Na 2 FeFe(CN) 6 ·1.69H 2 O and Na 2 MnFe(CN) 6 ·1.76H 2 O. Specifically, the water molecules impact the molecular interactions at the local structure and the electrochemical properties. This work has performed calculations on low‐vacancy Na 2 MFe(CN) 6 PBAs (where M = Mn, Fe, Co, Ni and Cu) to understand the dehydration energy. Employing in situ high‐temperature X‐ray diffraction and Raman spectroscopy, this work observes that water removal induces negative thermal expansion and stronger interactions between C≡N and Na ions, resulting in biphasic reactions with sluggish kinetics. Additionally, water molecules play a role in maintaining the open 3D tunnels and facilitating a solid‐solution like insertion of Na ions. Calculated phonon‐Raman spectra provide insights into cyanide group deformations, revealing the interactions between water molecules, alkali‐ions, and transition‐metal ions. This study enhances the understanding of the relationship among electronic, vibrational, and electrochemical properties.