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Prussian blue analogues (PBAs), A_xM[M’(CN)₆]_1–y·zH₂O, are a highly functional class of materials with use in a broad range of applications, such as energy storage, due to their porous structure and tunable composition. The porosity is particularly important for the properties and is deeply coupled to the cation, water, and [M’(CN)6]n– vacancy content. Determining internal porosity is especially challenging because the three compositional parameters are dependent on each other. In this work, we apply a new method, positron annihilation lifetime spectroscopy (PALS), which can be employed for the characterization of defects and structural changes in crystalline materials. Four samples were prepared to evaluate the method’s ability to detect changes in internal porosity as a function of the cation, water, and [M’(CN)₆]^n– vacancy content. Three of the samples have identical [M’(CN)₆]^n– vacancy content and gradually decreasing sodium and water content, while one sample has no sodium and 25% [M’(CN)₆]^n– vacancies. The samples were thoroughly characterized using inductively coupled plasma-optical emission spectroscopy (ICP-OES), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Mössbauer spectroscopy as well as applying the PALS method. Mössbauer spectroscopy, XRD, and TGA analysis revealed the sample compositions Na_1.8(2)Fe²⁺_0.64(6)Fe^2.6+_0.36(10)[Fe²⁺(CN)₆]·2.09(2)H₂O, Na_1.1(2)Fe²⁺_0.24(6)Fe^2.8+_0.76(6)[Fe^2.3+(CN)₆]·1.57(1)H₂O, Fe[Fe(CN)₆]·0.807(9)H₂O, and Fe[Fe(CN)₆]0.75·1.5H₂O, confirming the absence of vacancies in the three main samples. It was shown that the final composition of PBAs could only be unambiguously confirmed through the combination of ICP, XRD, TGA, and Mössbauer spectroscopy. Two positron lifetimes of 205 and 405 ps were observed with the 205 ps lifetime being independent of the sodium, water, and/or [Fe(CN)₆]^n– vacancy content, while the lifetime around 405 ps changes with varying sodium and water content. However, the origin and nature of the 405 ps lifetime yet remains unclear. The method shows promise for characterizing changes in the internal porosity in PBAs as a function of the composition and further development work needs to be carried out to ensure the applicability to PBAs generally.

Prussian white (PW, Na2Fe [Fe(CN)(6)] center dot zH(2)O) is a promising cathode material for use in sodium-ion batteries for large-scale energy storage applications, which demand long cycling life-times. However, for non-aqueous battery applications PW must not contain any water, and yet dehydration induces a large volume change destabilizing the structure and reducing the cycling life. The material undergoes multiple phase transitions as a function of both the sodium and water content, however, the mechanism behind is poorly understood. Here, we use neutron diffraction to explore the influence of water on the structure of PW. For the first time, two structures for a single composition of PW were observed near room temperature independent of the synthesis method. These structures differ in the FeN6 and FeC6 octahedral tilting configurations, which is connected to the ordering of water in the framework. The removal of water modulates the magnitude of pre-existing structural distortions, if it is itself disordered within the structure, rather than modifying the nature of the distortions. These results provide a robust fundamental understanding of the chemical driving force impacting the nature and magnitude of structural distortions in Prussian blue analogues. The insights provide guidance for designing tilt-engineering ultimately enabling new materials with enhanced long-term electrochemical performance in battery applications.

The sodium iron hexacyanoferrate compound with chemical formula Na_2.04(2)Fe[Fe(CN)₆]·2.24(2)H₂O, also known as Prussian white (PW), contains disordered and dynamic water molecules that have a dualistic effect on its battery performance. Furthermore, the material exhibits severe strain when dehydrated, which over time diminishes the performance. To understand the complex role of water on the sodium ion conduction and the structural changes happening upon dehydration, local structural characterization is needed. Here, we report the first neutron total scattering study of PW. Reverse Monte Carlo (RMC) fitting reveals that local octahedral distortion of the nitrogen-bound iron octahedra contributes to the disorder of the framework. The strain observed in the dehydrated material comes from a combination of the Fe–N bond elongation and a disordered distribution of sodium throughout the larger structure. In the hydrated material, the sodium exhibits more order due to the presence of water, which constrains the sodium movement. However, the sodium ordering affects the orientation of the water molecules. In the low temperature P2₁/n phase, sodium orders into planes with the oxygen atoms in the water molecules being in the plane, while the hydrogen atoms are pointing away from the sodium plane. In the room temperature R phase, the sodium and water are less ordered despite similar frameworks. Sodium can take a wide range of positions, especially if no water molecule blocks its way, to obtain optimal bonding conditions. These results show that the relationship between sodium and water is co-dependent, and demonstrate that the local structure of framework materials has a crucial link to their properties.

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.

The Prussian blue analogues (PBAs) Na_2–xFe[Fe(CN)₆]·zH₂O (x,z = 0-2) exhibit many phase transitions as a function of the sodium and water content, which involves large volume changes that can negatively affect its energy storage performance in a battery. However, the presence of water helps stabilize the PBA framework and thus diminishes these volume changes. To improve the material for its desired applications, a deeper fundamental understanding of the interactions between water, sodium, and the PBA framework is needed. Here, the local structure and vibrational dynamics of water were studied using inelastic neutron scattering (INS), neutron diffraction, and theoretical calculations. When the sodium content is high, the material exhibits well-defined water environments that become less defined when the sodium content is lower. It was shown that the positions of sodium and water are more complex than suggested by previous diffraction and computational studies. Most of the water in the high sodium sample occupies the center of the PBA subcube, while only a small fraction is located close to the window site of the subcube. For the low sodium sample, the results suggest that a large distribution of local water environments is present. These results lay the groundwork for unraveling the ionic transport in PBAs and the development of improved energy storage materials.

Prussian white (PW), Na_2−xFe[Fe(CN)₆], is an attractive cathode material for sodium-ion batteries due to its porous framework enabling fast sodium-ion extraction and insertion, environmentally safe elements, scalable synthesis, and performance comparable to current lithium-ion technologies. However, PW suffers from large volume changes between rhombohedral and cubic phases during cycling which is suggested to be detrimental over time because of structural degradation and increased ion insertion resistance. In particular, studies on PW hard carbon full cells revealed that most of the capacity is lost from the lower potential plateau, where this phase transition occurs. It is proposed that cycling in a restricted potential range, where the phase transition is avoided, could benefit the cycle lifetime and capacity retention. Here, we show an operando X-ray diffraction study aiming at determining how the structure evolves after prolonged cycling in different restricted potential ranges and how this impacts the cycling stability and capacity fade in PW. No signs of structural degradation were observed independently of the pre-cycling conditions used. In addition, more of the rhombohedral phase and capacity were recovered in the discharged state when a more restricted potential range had been applied. Thus, it was shown that the phase transition and corresponding volume changes have little impact on the capacity fade. Instead, the main source for capacity fade was proved to be sodium inventory loss, especially during the initial cycles, in combination with, to a lesser extent, polarization. This study gives a new perspective on PW-based batteries in that neither volume changes nor phase transitions are detrimental to battery performance. These results aid the development of improved cycling protocols and battery systems comprised of PW where the lifetime of the material is prolonged.