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.

中文翻译：

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钠对普鲁士蓝类似物中水动力学的影响


普鲁士蓝类似物 （PBA） 是钠离子电池的有趣阴极材料，尤其是铁基 [Fe（CN）₆]n⁻ 无空位 PBA Na_2–xFe[Fe（CN）₆]·zH₂O.然而，水的存在在 PBA 作为电极材料的应用中起着相反的作用：水提供结构稳定性，确保在钠提取和插入过程中的体积变化最小，但是，水会与电解质反应，导致不需要的副反应。因此，必须用另一种相容的小分子代替水，以确保最佳性能。为了实现这一目标，深入了解水的动态至关重要。采用准弹性中子散射 （QENS） 研究了组成为 Na_1.90（9）Fe_0.90（7）²⁺Fe_0.10（3）³⁺[Fe²⁺（CN）₆]·2.12（2）H₂O 和 Na_0.34（5）Fe³⁺[Fe^2.66（5）+（CN）₆]·0.360（4）H₂O 的两个样品。结果表明，水动力学在很大程度上取决于钠含量。发现水在多孔框架的球形空腔内扩散，高钠样品的半径为 2.6 Å，低钠样品的半径为 1.8 Å，与晶体结构中的孔径一致。除了水在孔隙内扩散外，还发现一小部分水表现出嘎嘎作响或旋转运动，这表明这些水与钠离子强烈相互作用并结合。对于高钠样品，这种嘎嘎声或旋转运动转化为 75 K 以下水的量子旋转隧穿。 这些结果为水在 PBA 中的作用提供了新的基本见解，为用另一种与非水电池系统兼容的小分子替代水奠定了基础，同时也确保了结构稳定性。